Synthesis and Characterization of CoNiAl Layered Double Hydroxides and a Co-Substituted Polyoxometalate for the Selective Allylic Oxidation of Cyclohexene with Molecular Oxygen
Date of submission: 4th of September, 2011
Lina Norberg Samuelsson (
[email protected]) Supervisors: Prof. Lars J. Pettersson (KTH, The Royal Institute of Technology, Stockholm, Sweden) and Prof. Can Li (State Key Laboratory of Catalysis, Dalian, China) Master's thesis, the degree program in chemical science and chemical engineering at the Royal Institute of Technology (KTH), Stockholm, Sweden.
Abstract
An industrially very important process is the oxidation of ole ns to epoxides, carbonyls and alcohols. High selectivity and e cient catalyst recovery are essential for green processes and one catalytical system that could meet these requirements during oxidation of ole ns is layered double hydroxides (LDH) intercalated with polyoxometalates (POM). This master's thesis work aimed at synthesizing a heterogeneous LDH-POM catalyst that could convert cyclohexene to cyclohexenone with a selectivity higher than 70%. An attempt was made to synthesize a cobalt substituted 9-tungstosilicate polyoxometalate that could be intercalated into a heterogeneous CoNiAl layered double hydroxide specie. CoNiAl-nitrate, CoNiAl-terephthalate and CoNiAl-adipate were synthesized by conventional methods to nd the most suitable LDH precursor for the intercalation of POM. The attempt to create CoNiAlhydroxide by reconstruction of thermally decomposed CoNiAl-nitrate was unsuccessful due to spinel formation during decomposition at 300◦ C. XRD, ICP, IR, TEM and SEM were used to characterize the synthesized CoNiAl species. The synthesis of the Co-substituted polyoxometalate failed, which was con rmed by ICP. Synthesis of the precursor, an α−Keggin structured 9-tungstosilicate, showed that preparation pH and silicate structure are essential for successful synthesis. The CoNiAl-nitrate species exhibited oxidation activities with turnover numbers up to 2.6 h−1 and the highest cyclohexenone selectivity was estimated to 67-75%. Repeated reaction experiments are needed to con rm these results. Sammanfattning
En industriellt mycket viktig process är oxidation av ole ner till epoxider, karbonyler och alkoholer. Hög selektivitet och eektiv intern katalysatoråtervinning är nödvändigt för att skapa en miljövänlig process. Ett katalytiskt system som skulle kunna möta dessa krav vid oxidering av ole ner är anjoniska leror (LDH) interkalerade med polyoxometalater (POM). Målet med det här examensarbetet var att syntetisera en heterogen LDH-POM katalysator som under milda förhållanden kan oxidera cyclohexen till cyclohexenon med en selektivitet över 70%. Ett försök gjordes att syntetisera en coboltsubstituerad 9-tungstosilikat POM som kunde interkaleras i en heterogen LDH bestående av Co, Ni och Al. För att nna den lämligaste prekursorn för interkalering av POM framställdes CoNiAl-nitrat, CoNiAl-terephthalat och CoNiAl-adipat med beprövade metoder. CoNiAl-hydroxid kunde inte framställas genom återskapande av termiskt nedbruten CoNiAl-nitrat eftersom spineller bildades vid 300◦ C. XRD, ICP, IR, TEM och SEM användes för karakterisering av de syntetiserade lerorna. Syntesen av den coboltsubstituerade polyoxometalaten misslyckades, vilket påvisades med ICP. Syntes av dess prekursor, en polyoxometalat med α−Keggin struktur, visade att pH samt silikatstrukturen är avgörande för huruvida syntesen lyckas. De framställda CoNiAl-nitrat batcherna uppvisade omvandlingstal med värden upp till 2.6 h−1 . Den högsta selektiviteten för cyclohexenon uppskattades ligga mellan 67-75%. Upprepade experiment krävs för att styrka dessa resultat.
i
Contents 1 Introduction 1.1 Overview of this study . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 The LDH and POM catalysts . . . . . . . . . . . . . . . . . . 1.1.2 Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.3 Choice of preparation methods . . . . . . . . . . . . . . . . . 1.1.4 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Heterogeneous catalysts for allylic oxidation of cyclohexene . . . . . 1.2.1 Layered double hydroxides intercalated with polyoxometalates 2 Materials and Methods 2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Catalyst preparation . . . . . . . . . . . . . . . . . . 2.2.1 Preparation of LDH precursors . . . . . . . . 2.2.2 Preparation of POM and Co-POM . . . . . . 2.3 Catalyst characterization and data analysis methods 2.4 Catalyst activity . . . . . . . . . . . . . . . . . . . . 2.4.1 Oxidation catalyzed by CoNiAl-nitrate . . . . 2.4.2 Oxidation catalyzed by Co-POM . . . . . . .
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3 Results 3.1 Catalyst characterization . . . . . . . . . . . . 3.1.1 LDH precursors . . . . . . . . . . . . . 3.1.2 POM and Co-POM . . . . . . . . . . . 3.2 Catalytic activity . . . . . . . . . . . . . . . . 3.2.1 Oxidation catalyzed by CoNiAl-nitrate 3.2.2 Oxidation catalyzed by Co-POM . . .
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4 Discussion 4.1 Synthesis and catalyst characterization 4.1.1 LDH . . . . . . . . . . . . . . . 4.1.2 POM and Co-POM . . . . . . . 4.2 Catalytic activity . . . . . . . . . . . .
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5 Conclusions
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6 Future work and recommendations
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7 Acknowledgments
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References
60
A Appendix A.1 Other heterogeneous catalytic systems for allylic oxidation of cyclohexene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.2 Operation parameters during GC analysis . . . . . . . . . . . . . . . A.3 Calculation of the molar mass of CoNiAl-nitrate . . . . . . . . . . . . A.4 Calculation of the charge density for CoNiAl-nitrate . . . . . . . . . A.5 Estimation of the mass CoNiAl-nitrate (LDHa2) formed during coprecipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.6 Details about the ion exchange experiments . . . . . . . . . . . . . . A.7 Color dierences between the LDH batches . . . . . . . . . . . . . .
67
ii
67 70 70 71 71 72 72
A.8 Calculation of turnover numbers and selectivities . . . . . . . . . . .
iii
74
1
Introduction
One of the most important tasks for today's society is to develop green solutions, both for the industry and for daily life applications. One part of this task is the development of new systems, but an equally important part is to improve already existing processes. An industrially very important process is the oxidation of ole ns to epoxides, carbonyls and alcohols [13].
Although this eld has experienced some large technical improvements
in recent years there is still large potential for improvements [1].
For example, allylic
oxidation is still mainly achieved using oxides of chrome, selenium and manganese in stoichiometric amounts, in the presence of homogeneous catalysts [4, 5].
This is not in
line with the concept of green chemistry [6] and the processes are coupled to both negative environmental impact and technical di culties. Some of the current goals for the industry are to develop more selective oxidation catalysts, decrease the number of process steps, use oxygen as oxidant instead of toxic and waste producing compounds, minimize or eliminate the use of solvents and decrease the energy input, preferably by developing catalysts that work at ambient temperature and pressure [13, 6, 7]. To reach these goals and improve the current processes, a range of heterogeneous oxidation catalysts have been developed (see Table 1 on page 16 and 14 on page 68) and some systems show high activities and selectivities. Layered double hydroxides (LDH) intercalated with polyoxometalates (POM) is one of the promising heterogeneous catalytic systems developed for the selective oxidation of ole ns. The rst LDH-POM system was prepared 1988 [8] and since then a range of dierent systems have been studied. LDHs are easily synthesized by co-precipitation of metal salts under mild conditions and according to literature, LDHs can be produced in large scale at reasonable costs with low environmental impact [9]. A range of metal ions can be used and the resulting positively charged layered structure can be balanced by many dierent kinds of anions. Polyoxometalates is one group of suitable anions.
These versatile metal oxide clusters
have, as well as some LDHs, proved to possess oxidation activity [10,11] and there are some research projects that are currently investigating the possibility to use POMs as oxidation catalysts for alkenes [1, 2]. Due to POM's high activity and ability to activate molecular oxygen [12], POMs are considered as green oxidation catalysts.
But, since POMs are
soluble in polar solvents (water, alkohol, ketones etc) [12, 13] and many catalytic POM systems are homogeneous [14, 15], the use of POMs is coupled to di cult catalyst recovery resulting in waste streams and increased environmental impact. By intercalation of POMs into LDHs heterogeneous oxidation catalysts with interesting activities and selectivities are the result [1621].
The great number of dierent LDH-POM systems that can be
synthesized by changing the composition of LDH and POM increases the possibility to nd highly selective catalytical systems. Finding highly selective, heterogeneous oxidation catalysts that can operate under green process conditions was the objective for this study. The simple preparation procedures for LDH-POM catalysts, together with the environmentally friendly materials used during synthesis, makes this system advantageous over more complex catalytic systems.
Fur-
thermore, most of the studied LDH-POM catalysts have been developed to increase the selectivity to the epoxide and there seems to be no reports concerning LDH-POM catalysts that give the allylic ketone as the main product (See Table 1 on page 16). This study thus aimed at developing a heterogeneous LDH-POM catalyst with a selectivity to the allylic ketone higher than 70%. To reach this goal an attempt was made to synthesize
1
Figure 1:
Oxidation of cyclohexene catalyzed by CoNiAl-carbonate. The selectivities
and reaction conditions have been reported by Dr. Yang [23].
a POM catalyst known to have 100% selectivity to cyclohexanone [22] that could be intercalated into a CoNiAl-LDH catalyst described by [23] (Entry 11, Table 14 on page 68). Cyclohexene was chosen as substrate since it has been widely used in the literature as a model substrate. With the principles of green chemistry in mind, the system studied in this project uses molecular oxygen as oxidant, no solvent and oxidation at relatively low reaction temperature. This also makes the system attractive from an economic perspective. Table 14 on page 68 shows that there are heterogeneous catalytical systems with high activities, and selectivities above 70% for the unsaturated ketone, but these systems are not only more complex than LDH-POM catalysts, they also either use the oxidant tert-butylhydroperoxide or solvents such as dichloromethane. Finally, a study such as this one can give valuable information about how the oxidation of ole ns proceeds using LDH-POM catalysts and the results can thus be used for future research in this area.
1.1 Overview of this study As mentioned above, this study aimed at examining the possibility to combine two oxidation catalysts to create a green, heterogeneous system with a selectivity higher than 70% for the unsaturated ketone.
1.1.1 The LDH and POM catalysts The LDH catalyst that lead to the idea for this project had previously been synthesized by Dr. Yang [23]. The synthesized specie is a CoNiAl−CO3 LDH catalyst reported to give 70% selectivity to cyclohexenone during oxidation of cyclohexene with molecular oxygen ◦ at atmospheric pressure. It was further reported that reaction during 24 h at 60 C gave practically complete conversion without the use of solvent. [23] The reaction conditions and product selectivities can be seen in Figure 1. The POM catalyst that was aimed to be used in this project is a cobalt substituted 910− tungstosilicate (α − [SiW9 O37 (Co(H2 O))3 ] ), hereafter denoted Co-POM, that has been ∗ known in literature for more than 20 years [24]. It has an α−Keggin structure and a surface similar to that of metal oxides [25]. This heteropolyoxometalate has recently been reported to give 100% selectivity to cyclohexanone in the oxidation of cyclohexanol using ◦ molecular oxygen. After 12 h of reaction at 100 C the reported conversion was 62%. [22] The reaction is illustrated in Figure 2 on the next page. ∗
See Section 1.2.1 on page 9.
2
Figure 2:
Oxidation of cyclohexanol catalyzed by a Co-substituted polyoxometalate.
The selectivities and reaction conditions have been reported earlier [22].
Figure 3:
The suggested reaction for the combined LDH-POM catalyst.
1.1.2 Hypothesis The hypothesis that this project aimed to examine is that the combination of the above mentioned LDH and Co-POM catalysts would work together as a heterogeneous catalyst in which Co-POM selectively converts the produced allylic alcohol to cyclohexenone. The selectivity to cyclohexenone in such a system should be higher than 70%. The hypothetical reaction is illustrated in Figure 3.
1.1.3 Choice of preparation methods The methods chosen to prepare the catalysts in this study are all conventional methods referred to in literature.
Four dierent LDH precursors that could be used for the in-
tercalation of Co-POM were to be prepared (See Figure 7 on page 20 for a schematic overview).
The LDH precursors are; (A)
CoNiAl − NO3
(denoted LDHa) prepared by
co-precipitation at low supersaturation with preparation conditions according to [26] and [23], (B)
CoNiAl − adipate (LDHb)
CoNiAl − NO3
prepared by ion-exchange between nitrate in
and adipate using the method described in [27], (C)
CoNiAl − adipate
(LDHc) prepared by ion-exchange between LDH-OH and adipic acid as described in [22, 2830] and (D) LDH-terephthalate (denoted
LDHd)
prepared by direct synthesis, i.e.
co-precipitation of LDH with terephthalate present in the system, as described in [31]. The nitrate form of the LDH, instead of the
CoNiAl − CO3
described by Dr. Yang, was
prepared to enable intercalation of POM (See Section 1.2.1, page 6 for a summary of the ion-exchange capabilities of LDH). The preparation conditions used during synthesis of LDHc (See Section 2.2, page 22), were modi ed (compared to the conditions used in [22, 2830]) to better suit the LDH specie used in this study. The method chosen to prepare the cobalt-substituted POM (Co-POM) has been described in [25].
The precursor to the Co-POM specie, i.e.
an
α-Keggin
structured POM was
prepared according to [32]. For the intercalation of Co-POM into CoNiAl two methods were to be tested to nd the most suitable one.
The chosen methods are (i) simple ion exchange between Co-POM
and nitrate in CoNiAl-nitrate (LDHa) and (ii) ion exchange between Co-POM and the
3
organic-anion (swelling agent) in LDH precursor B, C and D. The rst method was to be tested due to its simplicity [33]. The second method, especially using precursor LDHc, was chosen due to the crystalline products that have been reported to be obtained using this kind of LDH precursor [22, 28, 29].
1.1.4 Limitations In this project cyclohexene was the only substrate used to evaluate the oxidation activity for the synthesized species. Additionally, optimization of the preparation methods and oxidation reaction conditions was outside the scope of this study.
1.2 Heterogeneous catalysts for allylic oxidation of cyclohexene As mentioned in the introduction, there are many heterogeneous catalytic systems developed for the allylic oxidation of ole nes. Some of the recently developed catalytic systems, using cyclohexene as substrate, have been summarized in Table 14 on page 68. Note that Table 14 does not include LDH-POM systems. For a summary of LDH-POM catalysts used for the oxidation of cyclohexene, see Table 1 on page 16.
Table 1 only includes
systems with allylic oxidation activity. It is seen that the highest selectivity to the allylic ketone achieved with a LDH-POM systems is 45%. This section will give an overview of LDH-POM systems.
1.2.1 Layered double hydroxides intercalated with polyoxometalates Layered double hydroxides intercalated with polyoxometalates are inorganic species composed of positively charged layers (LDH) balanced by anionic polyoxometalates (POM) situated between the layers. LDH-POM catalysts are sometimes referred to as pillared (anionic) clays [10, 34] and they have been widely examined for the epoxidation of cyclohexene [16, 17, 20, 35, 36] and other related substrates (See [10, 34, 37] and references therein). The research on LDH-POM systems for allylic oxidation is comparatively scarce.
Al-
lylic oxidation and epoxidation are often competitive reactions, especially in catalytic systems involving cyclic ole ns [38]. Knowledge about the mechanisms involved during epoxidation may give understanding about how to increase the degree of allylic oxidation. Therefore, this section will also present some results from the investigations of epoxidation of cyclohexene. This section will rst describe the properties of the two constituents of the LDH-POM system.
After that follows a description of the characteristics of the combined system
together with a review of some results from this area.
Layered double hydroxides (LDH)
Layered double hydroxides (LDH) is a group of
materials that can be found naturally or be synthesized [39, 40]. The structure of synthesized LDH-materials corresponds to that of the naturally occuring mineral hydrotalcite, which has lead to the use of the word hydrotalcite-like materials or hydrotalcite-like LDH [41]. As the name implies, LDHs have a layered structure and a schematic illustration of the structure is found in Figure 4 on the facing page.
4
Figure 4:
A schematic illustration of the structure of layered double hydroxides. The lay-
ers are composed of divalent and trivalent metal ions octahedrally coordinated to hydroxides. In the picture: red spheres=oxygen, white spheres=hydrogen, green spheres=anions. Blue respectively yellow octahedra represents the two dierent oxidation states for the metal cations situated in the centers of the octahedras. Reprinted with permission. [42]
The anions between the layers can be exchanged for other anions and LDHs are therefore sometimes referred to as anionic clays. LDHs constitute the only known group of positively charged layered materials with ion exchange capability [17] and a range of metals can be used to prepare LDHs.
A description of the structure and properties of LDHs can be
found in the following subsections.
General structure
The composition of LDHs can be described with the following gen-
eral formula
[M(II)1−x M(III)x (OH)2 ]x+ An− x/n · (H2 O)m [34]. M(II) and M(III) are cations such as 2+ 2+ 2+ 2+ Mg , Ni , Cu , Co , Zn2+ , Mn2+ , Fe3+ , Al3+ and Cr3+ that can form hydroxides with n− a brucite-like structure (M(OH)2 ). A is an anion situated in the interlayer space, balancing the positive charge of the sheets (see description below). The interlayer space is also occupied by loosely bound water molecules. The anion is either directly bound to the LDH hydroxyl groups via hydrogen bonding or bound to the layers via intermediate water molecules. The layers are stacked on top of each other in rhombohedral or hexagonal symmetry [43] and they are composed of metal cations and hydroxides. The cations are arranged as in brucite (Mg(OH)2 ), i.e.
they are situated in the middle of an octahedra (6-fold
coordinated) with the edges occupied by hydroxyl groups.
These octahedra share the
same edges and form layers, held together by hydrogen bonding. The net positive charge 3+ in the brucite-like layers results from the trivalent cations (e.g. Al ). The ratio between the di- and tri-valent cations aects the concentration of anions in the interlayer space as well as the purity (a hydrotalcite-structure is de ned as pure) of the system [39, 40]. The optimal values for
x
(see the general formula above) have been shown to be between
0.2 and 0.33. This means that, in order to get pure LDH structures, the optimal ratio between the di- and tri-valent ions is 2 to 4. Lower
5
x
values give formation of
M(II)(OH)2
and higher
x
values can lead to the formation of
M(III)(OH)3 .
Both of these phenomena
gives poorly crystallized samples and high values has been shown to give irregular stacking of the brucite-like layers. [39] The
x
value also aects the basal spacing, de ned as the sum of the thickness of one ∗ and the interlayer space). When increases the charge density of the
x
brucite layer
sheets increases (since the structure contains more M(III)) and both the thickness of the brucite layer and the basal spacing decreases. The thickness of the brucite layer decreases less than the interlayer space (gallery height) when
x
increases. [44] The decrease in gallery
height is a result of the stronger electrostatic attraction between the interlayer anions and the sheets [39]. This trend is not seen for the nitrate ion. For nitrate, when the gallery height also increases. For high
x
x
increases,
This is a result of the low charge density of nitrate.
(high positive charge density of LDH) nitrate needs to pack closely and the
repulsion between the anions increases the interlayer distance. Examples of some basal 2− − spacings, in MgAl LDHs, for dierent anions are: 0.755 nm (OH ), 0.765 nm CO3 and
0.879 nm NO− 3
. [44] The basal spacing also depends on the water content between the
layers. Increased water content increases the gallery height to some extent. [28, 45]
Anions
The anion present between the positive sheets in natural hydrotalcite is usually
carbonate but this anion can be exchanged, without destroying the structure of LDH, for a range of anions able to balance the positive charge of the sheets [39]. The anions need to have a charge density high enough to balance the positive charge. Low charged anions (or anions with low charge density) need to be closely packed in between the layers which increases the energy in the system. Multiple charged anions and anions with high charge density are thus ion exchanged into LDH in preference over anions with low charge density [19, 44, 46]. This limits the number of anions that can be ion exchanged into LDH. The ease of ion exchange has been shown to decrease according to the following trend: 2− − − − − − − CO2− 3 > SO4 > OH > F > Cl > Br > NO3 > I [44, 45]. One problem associated with the preparation of LDH with anions other than carbonate in the interlayer space is that these structures often are contaminated by carbonates forming from carbon dioxide in the air during preparation. This gives unpure materials with lower degree of crystallinity [39, 47].
To prevent this from happening LDHs are
often prepared under carbon dioxide-free conditions (usually nitrogen atmosphere) [10]. Carbonate has been shown to be more di cult to avoid in systems with more basic LDH (such as magnesium and aluminium based) than systems of less basicity (LDH containing zinc) [48].
This is related to the solubility of carbon dioxide.
Another problem with
carbonate is that it is very di cult to exchange for other anions by ion exchange, as seen from the trend above. The nature of the anion (size, charge density, orientation between the layers, concentration and hydrated diameter) aects the distance between the positive layers. [39]
Preparation
Co-precipitation, followed by thermal treatment, is the most common
way to prepare LDH [47, 49]. Other methods are the sol-gel method [50] and hydrolytic reconstruction.
The hydrolytic reconstruction is based on the memory eect and that
method will be described on page 8. ∗
Often assumed to be 0.48 nm even though it depends on the cations [33].
6
Co-precipitation can be performed during conditions of low and high supersaturation [47]. Both methods operate at constant pH. Precipitation of the metal hydroxides results when the pH of the solution, containing the metal salts, is adjusted to the solubility level of the hexaaquo complexes formed by the cations in the solution. For most metal hydroxides this level is reached at pH 8-10. Too high and too low pH may give dissolution of some complexes and the pH must therefore be properly controlled. To get a uniform composition of cations in the LDH structure the pH needs to be adjusted so that both types of cations (sometimes three) precipitate simultaneously. During coprecipitation at low supersaturation, two diluted streams (concentration
0.5 − 2M)
are
added simultaneously to a single container where the metal hydroxides precipitates. One stream contains the metal salts and the other stream contains the precipitation agent (e.g NaOH,
Na2 CO3 ). [39, 47] The anion that one wishes to incorporate into the layers of LDH
should be present in excess during co-precipitation in order to reduce the incorporation of other ions [47]. LDHs containing organic anions can be prepared by addition of the organic specie to the co-precipitation mixture (refered to as direct synthesis) [31]. Co-precipitation during conditions of low supersaturation usually gives products with higher degree of crystallinity than co-precipitation during high supersaturation conditions. This is due to the fact that during low supersaturation the rate of crystal growth is higher than that of nucleation. [39, 47] Thermal treatment processes are generally of two types; those with temperatures above 100◦ C and those with temperatures below 100◦ C. The purpose of thermal treatment is to make small precipitation crystals grow larger and more regular. The temperature during thermal treatment, as well as during co-precipitation, is important since it aects the purity (composition and crystallinity) of the nal specie [51, 52]. If the temperature is less than
100◦ C
the process is often called aging.
Aging follows
directly after the co-precipitation step and it consists of letting the precipitate slurry rest at constant, elevated temperatures without stirring. The duration of the thermal process varies between a few hours and a few days.
Generally, the crystallinity increases with
time and temperature but at too high temperatures the structure decomposes. [39]
Thermal decomposition
Thermogravimetric analyses of LDH generally give curves ◦ with four weight loss steps. The rst weight loss (below 150 C) results from the removal ◦ of water from the external surface of LDH. In the next stage, ranging from about 150 C ◦ to 280 C, interlayer water and physisorbed anions are removed. After the second stage of weight loss some of the hydroxides in the LDH structure are decomposed and interlayer anions are transformed to gaseous compounds that leave the structure.
The tempera-
ture at which dehydroxylation (decomposition) begins depends on factors such as type of cations present, their ratio, anions present and treatment process (aging, drying). Depending on the cations present in LDH, total dehydroxylation is reached at a temperature ◦ around 400 C. [9, 36, 39, 50, 53] Thermal decomposition of LDH is said to begin after the second stage of weight loss (from 280◦ C to around 350◦ C) since the layered structure is not aected by the rst two stages. The only structural change before decomposition of LDH commences is a decrease in the gallery height due to removal of water and physisorbed anions [9, 36, 39, 53]. Which phases result from thermal decomposition depends on the decomposition temperature.
Amorphous oxides (M(II)O, M(III)O) are rst formed and, as the temperature
7
increases, they are transformed into crystalline oxides. [9, 39, 41, 49, 50, 53] The properties of thermally decomposed LDHs depend on a range of factors, such as the original LDH composition (anions and cations), LDH preparation conditions , decomposition procedure as well as presence of impurities in the system [39].
Two properties important
during preparation of LDH-POM catalysts (See page 17) are the basicity and a property called memory eect.
Basicity 2−
O
The basicity of calcined LDHs originates from three types of surface sites, i.e. − (strong basicity), O situated near a hydroxyl group (medium basicity) and OH-
groups (weak basicity) [39, 54, 55]. The basic properties of decomposed LDHs are ascribed mostly to the basic character of MgO but other oxides, such as e.g. ZnO and
ZnAl2 O4 ,
also exhibit basic properties [41]. Non-calcined (fresh) LDHs also exhibit surface basicity due to the basicity of metal hydroxides [24, 28, 55]. The basicity of LDHs (both calcined and fresh) is related not only to the cations in the structure but also to the crystallinity of the material, since the crystallinity determines the area and chemistry of the surface. The basicity of calcined LDHs is also a function of the type of anions present in the structure [40, 50]. For fresh LDHs the interlayer anions do not aect the basicity and acidity since the anions are present in between the layers [55]. The number and strength of the basic sites are important for the oxidation activity and product selectivity of LDH catalysts [40]. For example, higher basicity has been suggested to increase the activity during oxidation of cyclohexene with hydrogen peroxide and POM [33, 46]. Besides acting as a base during oxidation, LDHs have been shown to be active in the oxygen transfer step [56].
Memory eect
The memory eect that LDHs exhibit is the ability to regain the layered
structure, after thermal decomposition, by intercalation of anions and water. This means that calcined LDH (mixed oxides) can be reconstructed by addition of water. [39] Addition of water gives a LDH specie balanced by hydroxide. Since the original anions are removed during decomposition this method can be used to create systems with anions other than those incorporated during co-precipitation [47]. The possibility to reconstruct the original LDH structure from the decomposed mass has been shown to decrease with increased decomposition temperature [39, 48]. In order to reconstruct a layered structure from an amorphous oxide the metal cations need to be octahedrally coordinated in the oxides. If the temperature during calcination is high enough, spinels will form and the metal cations will become tetrahedrally coordinated. [39] Furthermore, reconstruction is only possible if the thermally decomposed LDH contained volatile ions such as carbonate.
Other interlayer anions, for example sulfate, may be
irreversibly grafted to the structure during thermal decomposition and reconstruction is not possible. One drawback with hydrolytic reconstruction is that the gallery height generally is lower for the reconstructed sample than for the fresh sample.
By exchanging the interlayer
hydroxides with larger anions the gallery height can be increased. [22, 39]
Applications
LDHs possess catalytic activity in a range of organic reactions [5, 39, 47]
among which epoxidation of ole ns is one of the most studied [10, 57]. Other application
8
Figure 5:
A Keggin structured polyoxometalate illustrated in two dierent angles. The
structure consists of twelve octahedra and can be described with the following general n− formula, [XM12 O40 ] . Reprinted from [40] with permission.
areas for LDHs are in
NOx
abatement, as llers to scavenge acid byproducts from PVC,
as part of Fischer-Tropsch reactions, steam reforming of ethanol and in cracking of waste plastics [41, 49, 58]. Since calcined LDHs generally exhibit conciderably higher catalytic activities than fresh LDH, have larger surface area [55] and higher structural stability, many of the applications of LDH uses the calcined form of LDH. For low temperature applications of basic catalysts it has been suggested that the fresh LDH could be a better alternative. [53]
Polyoxometalates (POM and HPOM)
As the name suggests, polyoxometalates are
species consisting of several oxometalates. Examples of oxymetalates are permanganate and chromate and an oxometalate can thus be described as an anionic specie having a metal core coordinated to oxygen ligands. Another way to describe polyoxometalates is as an early transition-metal oxide anionic cluster [59, 60].
General structure
Metals that form polyoxometalates are those from group ve and
six in the d-block, usually tungsten, molybdenum and vanadium. The cations have high oxidation state (IV-VI) and constitute the cores of linked polyhedra, with the general formula
MOx .
The metal oxide polyhedra (MOx ) share edges, corners and faces.
A
schematic illustration of a Keggin structured polyoxometalate consisting of 12 octahedra is seen in Figure 5. There are two types of polyoxometalates i.e. isopolyoxometalates and heteropolyoxometalates. In isopolyoxometalates, which only contain one type of metal ion, each metal ion is usually coordinated to six ligands in an octahedral structure. In the other, larger group of polyoxometalates, called heterpolyoxometalates (HPOM, but often just denoted POM) ∗ the structure consists of one or more central heteroatom(s) . [6163] These heteroatoms, which are species from the p-block, are tetrahedrally coordinated to octahedrally coordinated metal ions (called addenda atoms). These addenta atoms can be exchanged for other metal atoms to give new structures (See page 10). [12] ∗
The heteroatoms in HPOMs are Si, B, P, S, Ge, As, Se, Sb, Te and I.
9
Figure 6:
Ball-and-stick representations of a Keggin and a Wells-Dawson structured
POMs. Reprinted from [12] with permission.
The metal-oxygen bonds (dπ -pπ ) in the octahedra (both in isopolyoxometalates and in the metal-oxo units in HPOM) are directed outwards and they can be described as short terminal bonds. [62] As a result, polyoxometalates have structural properties similar to that of metal oxides [25, 59].
In addition to the terminal M=O oxygen atoms, the
surface also has oxygens bridged between metal ions (M-O-M). The terminal oxygens
O = M [O5 ]
(described as
or
cis − O2 M [O4 ]
in the octahedra) are less negative than the
bridged oxygens. The bridged oxygen atoms are therefore more readily protonated than the terminal ones. [62]
Preparation binuclear
∗
POM can be synthesised to have a range of dierent nuclearities. Small
species exists as well as macromolecules such as
[(MoO3 )176 (H2 O)80 H32 ]
[62]. The nuclearity of the specie is related to the pH in which
the specie is stable. The higher the nuclearity the more acidic is the specie. For example, 2− the stability of tungstates changes according to the following; WO4 is stable at a pH > 8, 12− W12 O12− 42 at pH 7.8 and W12 O39 is stable at pH 5.7. POM can thus be synthesized from oxometalates by decreasing the pH. [34, 40] This property (nucleation is a function of pH) can be used when intercalating polyoxometalates into LDH (See page 17).
In systems
with LDH this pH dependence of POMs has given problems since the POMs tend to depolymerize due to hydrolysis reactions caused by the basic LDH [48].
HPOM structures
Besides the possibility to change nuclearity of POM, the structure
and the composition can also be changed. For example, species of HPOM can be synthesized using more than 70 dierent elements [62]. The most studied HPOM structure n− is the Keggin anion with the general formula [XM12 O40 ] . is the central atom (from the p-block),
n
is the charge and
M
X
is either W(VI) or Mo(VI). As the formula suggests,
the Keggin structure consists of twelve metaloxygen octahedra. As can be seen in Figure 6, these octahedra form a practically spherical shell that surrounds a tetrahedrally coordinated heteroatom [40]. ∗
Mononuclear species are not de nied as POM.
10
Modi cation of the Keggin anion by removing one, or several, metal centra (M=O) from ∗ an edge octahedra gives a so called lacunary (defect) Keggin structure . These defect structures can be linked by transition metal ions to give so called sandwich-like heteropolyanions. These structures have proved to be active catalysts in oxidation reactions [62]. HPOMs can also be modi ed by transition metals without linkage [18, 22, 25]. One exam10− ple of this is the cobalt substituted 9-tungstosilicate (α − [SiW9 O37 (Co(H2 O))3 ] ) [22]. Furthermore, POMs can be modi ed by the addition of organic molecules to the inorganic framework [37, 60, 62].
General properties
Polyoxometalates have a range of interesting features such as tun-
able solubility, strong acidity and they are e cient oxidants [12]. The high oxidation state of the metals also makes POMs relatively resistant to oxidation [22]. The solubility of polyoxometalates can be changed by chosing dierent types of counterions to the anionic complex.
POM can thus be made soluble in both aqueous and organic
phases. [12] The acid forms of polyoxometalates are very soluble in polar solvents (water, alkohols, ketones etc.) but generally not in nonpolar solvents [13]. Keggin acids (e.g
H3 PW12 O40 and H4 SiW12 O40 ) are strong Brønsted acids with an acidity HNO3 , HCl). Additionally, the
conciderably higher than that for mineral acids (H2 SO4 ,
acid strength of Keggin acids is higher for species containing tungsten than molybdenum. [13, 64] The thermal stability of HPOMs is generally low and depends on the type of HPOM [64]. POM's stability to hydrolysis is also specie dependent. Keggin-type POMs are found less stable to hydrolysis than Dawson-type POMs [30, 33].
Applications
The diverse properties of POM have lead to applications in a range of
elds such as catalysis (e.g.
oxidation catalysts [7, 11, 40]), medicine, photochromism
and electrochemistry [12, 59, 61, 62]. Regarding catalysis, POM are well known oxidation catalysts used both in homogeneous and heterogeneous systems and they can activate both oxygen and hydrogen peroxide [11, 12, 40]. Most of the applications as catalysts are homogeneous and the problems coupled to such systems has lead to research in how to heterogenize POM [14, 15, 22]. POMs also have applications as catalysts in photocatalytical decomposition of aromatic and chlorinated hydrocarbons [17].
The LDH-POM system
As mentioned in the introduction, intercalation of POM
into LDH is an attempt to combine the high oxidation activity of POM with the catalytic properties of LDH and the advantages of heterogeneous systems. The combined systems usually show dierent activities and selectivities compared to that of the two original catalysts (See Section 1.2.1) and interesting features such as shape selectivity have been reported (See Section 1.2.1 on page 14). Some ndings related to the characteristics and preparation of LDH-POM catalysts for oxidation of cyclohexene are presented below. ∗
When one metal ion is removed half an octahedra in the structure is destroyed and the resulting
lacunary POM will thus have one less oxygen atom than the original POM structure.
11
Activity of LDH-POM catalysts
It has been reported that immobilization of POMs
into LDHs in many cases give systems with lower activities than that of the original homogeneous POM catalysts [14].
Zn3 Al − LDH
An example of this is
K8 SiW11 O39
immobilized in
[19]. In that investigation, the conversion during oxidation of cyclohexene
using hydrogen peroxide was drastically lowered when POM was immobilized.
When
molecular oxygen was used though, the decrease in activity was not as large.
It was
noticed that
Zn3 Al − LDH,
without POM, was catalytically active when oxygen was the
oxidant but not when hydrogen peroxide was used. This explains the lower decrease in activity of the oxygen system compared to the hydrogen peroxide system. It also suggests that the oxidation activity of LDH depends on the oxidant in the system. The decrease in activity was explained to result from sterical hindrance due to LDH, decreasing the substrates possibilities to reach POM. It seems that the usual result when POM is intercalated into LDH is an increased activity, compared to that for the homogeneous POM (See [8, 14, 17, 18, 22]). The reasons for the increased activity have been examined and one explanation is the presence of mixed salts, resulting from reactions between LDH and POM [17].
It was shown that the activity
of the mixed salt impurity was high only if the salt was dispersed on the surface of LDH. This means that even though the increased activity of pillared systems is not due to a cooperation between LDH and POM (but due to a new phase) LDH is still vital for the activity due to the surface area it provides. It has proved di cult to determine if the increase in activity of LDH-POM systems, compared to free POMs, is due to immobilization of POM or due to the presence of a catalytically active impurity phase. [16] The impurity phase results from acid-base reactions between the basic LDH and the acidic POM during the intercalation of POM. The acid-base reactions destroy part of the brucite structure in LDH and metal ions become solvated. The metal ions released can form hydrolyzed species that react with POM to create an amorphous salt phase. This gives LDH-POM species containing amorphous impurity phases and dierent approaches have been tested to minimize the degree of this (See page 17 for a description of dierent preparation methods). It has been shown that the salt phase is created on the external surface of the LDH particles and not in the interlayer space. [28] The impurity phase has been detected by many researchers as a broad re ection around 1.1 nm in XRD diraction patterns [16, 19, 28, 33]. Other explanations for the broad peak near 1.1 nm is that POM has been grafted onto the LDH surface [29], that POM has been depolymerized [21] or that the interlayer consists of a mixture of POM and small anions [48]. LDHs behave as base catalysts during oxidation reactions and the basic strength is correlated to the number and strength of the basic sites and to the cations present (See page 8). In LDH-POM systems there are also acidic and electron accepting sites due to the presence of POM [55]. The acid-base strength in LDH-POM species can thus be changed by varying the concentration of the intercalated POM [40]. Some of the acidic character displayed by LDH-POM systems has been shown to result from hydrolyzed POM species, and not from pillared POM [55]. Furthermore, the basic strength of LDH-POM catalysts has been shown to aect the acitivity to epoxidize cyclohexene. Systems with higher basic strength show higher activity [33, 46]. Some studies of how the metals in POM and LDH aect the activity have also been performed. It was shown that the activity for transition metal substituted HPOM species, n− with the following formula XW11 Z(H2 O)O39 (X=Si, P), intercalated into LDH containing 12
magnesium and aluminium was high enough to give conversions above 80% when Z=Co, Cu, Mn and below 10% when Z=Fe, Ni, W. The activity also depended on X. HPOMs containing P gave higher conversions. [18] In the case of LDH, nickel seems to be more active in the epoxidation of cyclohexene than LDH containing zinc or magnesium but since the reaction times and oxidants used are not the same in the compared studies no general conclusions can be drawn [19, 21]. Finally, the activity of LDH-POM systems during epoxidation of cyclohexene using hydrogen peroxide has also been shown to depend on the solvent used during reaction [21, 36].
Structure and stability of LDH-POM catalysts
Immobilization of POM in LDH
not only changes the catalytic activity of the system but also the structure. As described above both POM and LDH may be decomposed by acid-base reactions during intercalation. This results in chemical as well as morphological changes. If the intercalation is successful (i.e. both POM and LDH are unchanged to a large degree) the intercalation of POM into LDH gives rise both to micro and interparticle mesopores and the surface area for the system is thus increased. Results have shown that the increase in surface area is not as high as theoretically possible. One explanation to this is that the impurity phase created during intercalation blocks the pores and thus hinders access to the interlayer region. [16] The micropores result from the so called pillaring of POM between the layers in LDH. Pillaring results in systems with access to the interlayer crystalline surface. [8,22,28,33,36] LDHs intercalated by small anions (gallery heights around 0.3-0.4 nm) do not possess microporosity since nitrogen used during measurements of the surface area can not enter the interlayer region [36]. The microporosity can be varied by varying the size of the intercalated POM. Larger POM species give larger pores and decreased microporous surface area. Furthermore, by varying the size of POM the gallery height (distance between the brucite-like layers) is changed. For spherical POMs the gallery height is nearly the same as the diameter of the POM species but when non-spherical POMs are intercalated the gallery height depends on the orientation of the anions.
POMs will adjust their orientation to maximize the
hydrogen-bonding with the surrounding. [28, 65] The pillaring in uences the activity. Close pillaring isolates the POM species from the substrate and the conversion is decreased [17]. It is thus important to get enough lateral spacing of the POMs to allow the substrate to diuse into the micropores. A well ordered gallery of POM in the interlayer region can be achieved with LDH species that have uniform charge density.
Crystalline LDHs are thus desired.
The crystallinity of LDH
depends of the preparation procedure of LDH (See page 6). The distance between the POM species can be governed with the charge density of POM and LDH. Increased charge densities of POM increases the lateral separation due to electrostatic repulsion between the intercalates. [8, 28, 65] The charge densities of POM and LDH determine the distance between the brucite-like layers, as explained on page 5. The thermal stability for LDH-POM systems has proved to be relatively low but it depends on the type of LDH respectively POM. Reported temperatures that give thermal ◦ ◦ ◦ decomposition of LDH-POM catalysts are 120 C [17], 200 C [16,28] and 300 − 400 C [36]. The drying temperature used to dry the LDH-POM catalyst after intercalation and washing must be adjusted not to destroy the catalyst. LDH-POM systems dried in air at room
13
temperature lost their ordered structure during reaction and increased temperatures during drying are thus needed [17].
Thermally decomposed LDH-POM catalysts can be
rehydroxylated (reconstructed) to regain the layered structure to some degree but the pillaring can not be restored [40]. The stability of the system during reaction and recycling has also been investigated [22,33, 46]. The results show that the activity and stability of LDH-POM is practically unchanged even after several uses and LDH-POM systems are considered as truely heterogeneous since no leaking of POM has been observed.
Selectivity in LDH-POM systems
In ideal LDH-POM systems, i.e. systems with
homogeneous pillaring, the reactions take place in the interlayer space.
Ever since the
synthesis of the rst LDH-POM system in 1988 [8] it has been suggested that this kind of system exhibit shape selectivity. The idea with shape selectivity is that, by changing the gallery height (by intercalation of e.g.
small POM species), one can control which
substrate that can react (enter the interlayer region) and which substrate that can not, due to steric hindrance.
In the rst investigation of this property, 1992, cyclohexene
and hexene were used as substrates [20]. It was shown that the oxidation of hexene was favored over that of cyclohexene in the reaction catalyzed by a LDH-POM catalyst. In a homogeneous system (only POM) the oxidation for cyclohexene was favored. When a larger POM specie was intercalated the dierence in selectivity between the two substrates decreased, which further supported the hypothesis that the substrate selectivity originated from shape selectivity. In 1998 these results were questioned since it was found that the preparation conditions used by [20] probably gave a LDH-POM catalyst consisting mostly of decomposed POM species [17] and that molecules larger than nitrate were restricted by size to enter the interlayer space [16]. It was concluded that the selectivity found in the previous study was not due to pillaring and size selectivity but more probably a result of selective adsorption of the substrate on the catalyst surface. In a later investigation done by another group (See [19]) the activity of a LDH-POM system was found to decrease with increased ring size for three cyclic ole ns. For the free POM no such trend was seen. The shape selectivity for the LDH-POM catalyst was explained to result both from a lower diusivity for the larger substrates and from size restrictions between the layers in the catalyst. Besides possible shape selectivity for the substrate, LDH-POM systems have also been shown to give dierent product selectivities than homogeneous POM and LDH alone. Especially the selectivity for the epoxide and the diol are changed. The epoxide selectivity is generally higher for the intercalated system than for homogeneous POM species [16, 19, 20, 33].
This change in selectivity is explained by the ability of the basic LDH to
suppress hydrolysis of the epoxide by the acidic POM [16, 19, 20].
For systems using
hydrogen peroxide as an oxidant another explanation has been presented. Results showed that since hydrolysis of the epoxide was happening in the solution and not on the POM surface, hydrogen peroxide gave the increased selectivity to the diol [33]. As mentioned on page 4 epoxidation and allylic oxidation are often competitive reactions during oxidation of alkenes. Oxidation of alkenes can thus give epoxides, dioles or, if allylic oxidation, unsaturated ketones and alkohols. A majority of the LDH-POM systems studied give epoxides as the main product but some systems show dierent product selectivities. Some results from previous studies of oxidation of cyclohexene are summarized in Table 1 on page 16. There are some other LDH-POM systems with interesting selectivity
14
towards the allylic oxidation products but since they involve POM with organic modi cations [37] and LDH intercalated with tungstates [15, 46] these systems are not presented here. The data presented in Table 1 only include the catalysts that show high activity or selectivity during allylic oxidation. It is seen that LDH consisting of magnesium, zinc and nickel have been studied.
The POM species studied are both isopolyoxometalates and
heteropolyoxometalates. Table 1 includes one reference that have studied Co-substituted POM (Entry 1 and 2, [18]). The selectivity to the allylic oxidation products is relatively high for these systems. The same reference also examined POMs substituted with Mn, Fe, Ni and Cu. The selectivity trend achieved with these metals was similar to that with Co (decrease in selectivity in the order allylic alkohol, allylic ketone, epoxide, diol) but the activity was lower. The product distribution was not dependent on the heteroatom in HPOM (Si or P) but when P was used the selectivities achieved with the dierent transition metals diered more than for HPOM containing Si. Most of the systems presented in Table 1 use hydrogen peroxide as oxidant. When hydrogen peroxide was used with the catalysts from [18], low conversion resulted due to decomposition of the oxidant early during reaction. It was concluded that the reaction mechanism giving allylic oxidation products was dierent from that using hydrogen peroxide.
The proposed reaction mechanism consisted of transfer of molecular oxygen via
the transition metal in POM and transfer of lattice oxygen in LDH. During oxidation of cyclic ole ns, allylic oxidation and epoxidation reactions usually occur simultaneously [38].
During heterogeneous oxidation, the substrate can be subject to
either electrophilic oxidation mechanisms or nucleophilic oxidation mechanisms. The rst step in the oxidation of ole ns, irrespective of the mechanism, is the adsorption of the substrate on the surface by hydrogen bonding between the substrate's hydrogen atoms and 2− the metal oxides surface groups (OH and O ). Activated oxygen, in the form of molecular or atomic radicals, can then either do an electrophilic attack on the double bond (leading to epoxide) or abstract a hydrogen atom, leaving a carbocation. The carbocation can then be attacked by a nucleophilic oxide ion and allylic products result. During nucleophilic oxidation oxygen acts as nucleophile and the oxide surface is the oxidant. Furthermore, allylic oxidation has been shown to be favored when transition metals with low oxidation states are present on the surface.
They can act as Lewis acids that interact with the
double bond of the substrate to create a surface complex. Stong basic OH-groups on the surface can then do a nucleophilic attack in the
α-position.
Epoxidation is favored in the
presence of oxometallic species. [38, 66] In order to create systems with high selectivity the catalyst must be designed so that the rate of the desired step is increased while undesired mechanisms are suppressed. One LDH-POM system with 100% selectivity to the saturated ketone is Mg3 Al(OH)8 − [SiW9 O37 (Co(H2 O))3 ]10− [22]. The substrate is cyclohexanol and the oxidant is molecular oxygen. The conversion was 67%. With other transition metals (i.e. Cu, Fe, Ni and Cr) the selectivity was practically 100% but the conversions were substansially lower. Since no double bond is present in the substrate, no epoxide is formed. Oxidation of the ketone, to the carboxylic acid, was expected but no acid was detected. Table 1 also illustrates that POM containing Mo, instead of W, seems to give slightly increased selectivity towards allylic products (Entry 5, 10-12). Finally, the selectivity has been shown to depend not only on the composition of the LDH-POM catalyst but also on the solvent used during reaction [21].
15
[18]
Ref.
1
Entry
[46]
[19]
7
6
5
3
2
[20]
4
[17]
[16] 10
9
8
[21] 11 12
LDH-POM
Mg3 Al(OH)8 − SiW11 Co(H2 O)O39 Mg3 Al(OH)8 − PW11 Co(H2 O)O39 Zn3 Al(OH)8 − SiW11 O39 Ni2 Al(OH)6 − SiW12 O40 Mg3 Al(OH)8 − (Mo7 O34 )0.024 Mg23.3 Al10 (OH)66.6 − W12 O41 6− g Zn2 Al(OH)6 − H2 W12 O40 6− g Mg2 Al(OH)6 − W7 O24 14− g Zn2 Al(OH)6 − NaP5 W30 O110 Ni0.68 Al0.32 (OH)2 − (Mo7 O24 )0.05 Mg0.60 Al0.39 (OH)2 − (Mo7 O24 )0.065 Mg0.60 Al0.39 (OH)2 − (Mo7 O24 )0.065
a No pressure data given.
C]
Temp.
o
Solvent
70
70
[ none
70
40
b
none
b
TBP
Dioxane
TBP
TBP
TBP
TBP
55
55
70
73
73
70
35
Isobutyraldehyde+DCM
b
Dioxane
55
DMF
Dioxane-Butyl maleate
b Tributyl phosphate (TBP), Dichloromethane (DCM), Dimethylformamide (DMF) c Conversion [percent] and Turn Over Frequency. d Selectivity [percent] e Reaction time. f (-) No data given.
Oxidant
O2 a O2 a H2 O 2 O2 a H2 O 2 H2 O 2 H2 O 2 H2 O 2 H2 O 2 H2 O 2 H2 O 2 H2 O 2
87.5 / - (10)
(t [h])
-
29.3
31.9
Ketone
-
-
63.5
62.3
Alkohol
17.5
major
major
3.9
3.4
Epoxide
major
major
-
none
minor
3.3
2.5
Diol
d
91.7 / - (10)
-
4.4
none
minor
c
- / 14 (3)
-
S
- / 25 (10)
-
-
e
X / TOF
21.9 / - (3)
-
-
-
-
95.2
-
major
-
major
none
f
22 / - (3)
- / 21.8 (3)
17 / - (3)
-
-
trace
71.0
-
-
trace
49.8
4.8
28.2
trace
85 / - (4) 78 / - (4)
45.8
- / 182 (4)
75 / - (4)
A summary of some LDH-POM systems used for the oxidation of cyclohexene. Note that the conversions indicated above
g Catalyst precursor material
Table 1:
are achieved in systems with dierent ratios of catalyst and oxidant, dierent reaction times and temperatures. Furthermore, this table
only includes data for reactions using fresh catalysts ( rst run catalysts). For a description of the catalyst systems, see references.
16
Preparation methods for LDH-POM catalysts able for the preparation of LDH-POM catalysts.
There are several methods avail-
They all have their advantages and
drawbacks. The most challenging problems during LDH-POM preparation are probably the destructive acid-base reactions between the two precursors and the pH-dependent structure of POM. The preparation methods that will be mentioned in this section are all based on ion exchange. Ion exchange has been widely used in the preparation of LDH-POM catalysts and it was the method used to prepare the rst LDH-POM system [8].
Ion exchange is a rather
limited method. As mentioned on page 6, multiple charged anions and anions with high charge density are ion exchanged into LDH in preference over anions with low charge density.
For polyoxometalates it has been shown that species with a negative charge
lower than ve can not be ion exchanged into LDH [19, 67]. The charge density for such POM species is generally not high enough to balance that of LDH. The dierent ion exchange methods that will be discussed here are: (i) ion exchange of simple inorganic ions and (ii) ion exchange using swelling agents. (i) The simplest form of ion exchange used to prepare LDH-POM catalyst is to let the POM specie undergo exchange with the interlayer anions that were incorporated into LDH during co-precipitation. A range of LDH-POM catalysts synthesized using simple ion exchange have been reported [8, 18, 19, 29, 33, 36]. Generally, a solution of POM (or LDH) is slowly added to a solution of LDH (or POM). The driving force for the ion exchange is the preference for the highly charged POM anion over that of the inorganic anion (usually nitrate or chloride).
This method has recently been used to intercalate
Keggin- and Sandwich-type POMs into LDHs containing Mg, Zn and Al [33]. The POM species were intercalated in preference of other anions present in the POM solution and no puri cation of the POM solution was thus required. The same investigation also showed that POM was intercalated without degradation. It was concluded that the method was more suitable for Sandwich-type POM, than for Keggin-type POM, since the Sandwichtype POM specie is stable at higher pH than the Keggin-type POM. If POMs stable at low pH are to be incorporated successfully into LDH, the pH of the LDH-POM mixture needs to be kept low (at the pH where POM is stable) in order to avoid hydrolysis of POM. Simple anion exchange is thus often accompanied by addition of acid, which may give undesired dissolution of the basic LDH structure. LDH dissolution needs to be avoided since it may change the charge density of the structure, which in turn aects the pillaring (See page 13) By chosing a less basic LDH specie hydrolysis of POM during ion exchange can be minimized [17]. Ion exchange between POM and carbonate has been investigated and shown to be unsuccessful, probably due to LDHs's high a nity for carbonate.
Destructive acid-base
reactions between POM and LDH resulted and no pillaring was detected. [22] (ii) To increase the rate of ion exchange and minimize the degree of acid-base reactions organic molecules, referred to as swelling agents, are intercalated into LDH whereafter they are ion exchanged for POM. Examples of swelling agents are terephthalic acid [17, 1921, 31], adipic acid [22, 2830] and para-toluene sulfonic acid [46]. The swelling agent is either added to a slurry of LDH and allowed to ion exchange with the interlayer anion (nitrate has been exchanged for terephthalate and hydroxide has been exchanged for adipic acid (adipate)) or intercalated into LDH during co-precipitation of LDH (direct synthesis) [31]. During ion exchange the gallery height increases due to the bulky nature of the swelling agent and this facilitates the intercalation of the large POM species in the
17
following intercalation step. During intercalation of POM, either a solution of HPOM or, if LDH is to be intercalated with isopolyoxometalates, a solution of POM monomers 2− (e.g MoO4 ) is added to the preswelled LDH slurry. When monomers are used the pH of the solution containing monomers and the LDH precursor is slowly decreased. This gives 2− 6− polymerization of POM (from e.g MoO4 to Mo7 O24 [17]). The terephthalate anions in LDH are protonized in the acidic media and the resulting charge neutral molecules are easily exchanged for the polymerized, highly charged POM species. When adipic acid is used, LDH has rst been thermally decomposed and reconstructed to give LDH balanced by hydroxide. By addition of the adipic acid to the LDH-OH slurry, the acid will react with the interlayer hydroxide and diuse as adipate into the structure [28]. Further studies have shown that the best results using adipic acid is achieved if the adipic acid is added in 2-fold excess of the anion exchange capacity (AEC) of LDH. POMs should not be added in an excess larger than 5% since this lowers the crystallinity of the nal pillared LDH. [30] There are several comparative studies performed to investigate how the structure of the nal LDH-POM catalyst depends on the intercalation method (See [22, 28, 29]).
The
compared methods are; a) Addition of POM to decomposed LDH (a mixed oxide solution), b) ion-exchange between POM and LDH-OH, c) ion-exchange between POM and LDH-adipate, d) ion-exchange between carbonate and POM and e) ion-exchange between nitrate and POM. Among the compared methods the best pillaring (most crystalline catalyst) was achieved using LDH-adipate as a precursor during intercalation of POM. Adipic acid (adipate) is easily exchanged for POM ions and the rate of the ion exchange process is thus increased, which minimize the degree of destructive acid-base reactions. Furthermore, POM is less likely to undergo hydrolysis in the presence of adipate. [22] X-Ray diraction patterns showed that all methods were accompanied by acid-base reactions. The degree of acid-base reactions could be minimized by increasing the temperature to 100◦ C during ion exchange between POM and the intercalated adipate. Partial dissolution of magnesium ions from the LDH framework was con rmed by elemental analysis and the dissolution increased with increased intercalation temperature. [28] In all studies, hydrolytic reconstruction in the presence of POM (addition of POM to a mixed oxide solution) gave practically no intercalation. Other preparation methods used for the preparation of LDH-POM catalysts are direct synthesis (co-precipitation of the metal salts with POM present in the precipitation liquid), methods using microwave and ultrasonics and phase-transfer methods (For information about these methods see [34, 40, 46] and references therein.).
18
2
Materials and Methods
The experimental work performed in this study can be divided into three parts; (i) preparation of catalysts (LDH precursors and Co-POM), (ii) characterisation of the synthesized species and (iii) activity study with the prepared catalysts.
2.1 Materials The chemicals used during catalyst synthesis and activity evaluation were all used as recieved.
Na2 WO4 · 2H2 O (≥ 99.5%) and anhydrous Na2 SiO3 (≥ 99.0%) were purchased from Shenyang Chemical Company. Al(NO3 )3 · 9H2 O (≥ 99.0%) and Ni(NO3 )2 · 6H2 O (≥ 98.0%) were purchased from Shanghai Zhenxin Chemical Reagent Factory and Shanghai Hengxin Chemical Reagent Company respectively. Tianjin No. 1 Chemical Reagent Factory provided diethyl ether (≥
99.5%).
Ethanol (≥
99.7%),
1,4-dioxane (≥
99.5%)
and cyclohex-
ene (≥
98.0%) were purchased from Sinopharm Chemical Reagent Company. Tianjin Kermel Chemical Reagent Company provided Na2 SiO3 · 9H2 O (≥ 99.0%), HNO3 (65 − 68%), Co(NO3 )2 · 6H2 O (≥ 99.0%), acetonitrile (≥ 99.5%), Co(CH3 COO)2 · 4H2 O (≥ 99.5%), KCl (≥ 99.5%) and NaOH (≥ 96.0%). Phthalic acid (≥ 98.5%) was purchased from Beijing Jinlong Chemical Reagent Company and HCl (36-38%) was purchased from Anshan Chemical reagent company. Inc.
n-decane was purchased from Shanghai Chemical Reagent
of Chinese Medicine group.
Organics, cyclohexanone (≥
Cyclohexenone (≥
99.0%)
97.0%)
was purchased from Acros
was purchased from Alfa Aesar and cyclohexenol
(≥
95.0%) was purchased from Aldrich. The rest of the chemicals, i.e. anhydrous Na2 CO3 (≥ 99.8%), ethyl acetate (≥ 99.5%) and cyclohexanol (≥ 98.0%) were purchased from Tianjin Bodi Chemical Company. All the water used during synthesis was destilled at the laboratory using a quartz double pure water distiner (ISO 9001:2000).
2.2 Catalyst preparation 2.2.1 Preparation of LDH precursors This section describes the preparation of the four LDH precursors (LDH a-d) that were to be used for intercalation of Co-POM. The four preparation methods are illustrated in Figure 7 on the next page.
The precursors to be prepared were chosen since they
have been reported to be suitable (give crystalline LDH-POM species) for intercalation of POM (See Section 1.2.1 on page 17). Two methods (i.e. B and C) were tested to prepare CoNiAl-adipate. Method C was chosen due to the good results achieved with this method (See [22, 2830]). Method B was chosen due to the simplicity of the method. All the water used during synthesis was boiled before use to remove carbonate.
The
handling of the prepared slurries were done trying to minimize the contact with air as much as possible.
19
Figure 7:
Schematic overview of the preparation pathways used to prepare the four LDH
precursors CoNiAl-NO3 (LDHa), CoNiAl-adipate (LDHb), CoNiAl-adipate (LDHc) and CoNiAl-terephthalate (LDHd).
CoNiAl-nitrate (LDHa)
The nitrate form of the CoNiAl LDH (denoted LDHa) was
prepared by co-precipitation at low supersaturation and at a pH around 9. The pH was held around 9 to get simultaneous precipitation of the metal ions and thus achieve a homogeneous LDH structure. This pH was chosen from literature (See [39, 68, 69]) and ∗ also con rmed as suitable by analytical simulations with Medusa . The nitrate metal salt solution (1.7 M) and the co-precipitation agent NaOH (2.3 M) were added in ratios † to give a LDH with the composition Co0.36 Ni0.36 Al0.28 (OH)2 (x ≈ 2.6 ). The preparation procedure was based on the descriptions by [23, 26]. Three batches of
CoNiAl − NO3
were
prepared (LDHa1, LDHa2, LDHa3) using the same preparation method but with some minor dierences in preparation conditions. The preparation conditions and the resulting weights of the dry masses are summarized in Table 2 on the next page. The three batches were characterized with XRD, IR and ICP. Batch 1 and 2 were characterized with TEM and batch 2 and 3 were characterized with SEM.
Co(NO3 )2 · 6H2 O (36 3 mmol), Ni(NO3 )2 · 6H2 O (36 mmol) and Al(NO3 )3 · 9H2 O (28 mmol) in 0.06 dm of water 3 and pouring the solution into a dropping funnel. A NaOH solution (0.1 dm , 2.1 M) was
The rst step in the preparation of LDHa consisted of dissolving
prepared, containing more than stoichiometric amount of hydroxide (i.e. more than 0.20 mol), and poured into another dropping funnel. The molar excess of NaOH was needed to obtain a nal LDH slurry with a pH between 9 and 10. The metal salt solution and the NaOH solution were added dropwise, simultaneously to a vigorously stirred three necked 3 3 round bottom ask containing either 0.05 dm (LDHa1, LDHa2) or 0.02 dm (LDHa3) of ◦ water. The co-precipitation was performed under argon atmosphere, at 20 C (oil bath). The pH of the solution was checked continuously with pH test paper (range 1-14). During pH-testing, the system was opened and a glass rod was dipped into the slurry. By opening the system the possibility that carbon dioxide entered increased. The use of argon, instead ∗
Make Equilibrium Diagrams Using Sophisticated Algorithms by Ignasi Puigdomenech, Inorganic Chem-
istry, KTH, Sweden.
†
x+
[M (II)1−x M (III)x (OH)2 ]
20
An− x/n ·mH2 O,
see Section 1.2.1, page 5.
Batch
Final slurry volume
Aging time
Washed volume
Drying
Dry weight
[dm3 ]
[h]
[dm3 ]
[◦ C] and [h]
[g]
LDHa1
0.21
21
0.05
100/22
n.m.b
LDHa2
0.21
16
0.05
80/24
3.7c
LDHa3
0.18
21
0.03
80/6a
1.5
a The slurry was placed in a petri dish instead of dried as batch 1 and 2 in centrifugation asks.
b (n.m.) Not measured. c The high mass (higher than estimated in Section A.5) is probably due to withdrawal of settled slurry.
Table 2:
The preparation conditions for the three CoNiAl-nitrate batches and the re-
sulting weights of the dry masses. The
nal slurry volume
is the volume of the slurry in
the three-necked ask after completion of the co-precipitation step. The
washed volume
is the slurry volume withdrawn for drying. The nal slurry volume is lower for batch 3 since less water was added to this batch (See text).
of nitrogen, may protect the slurry from carbon dioxide somewhat better than nitrogen, due to argons higher density. Batch 1 had large uktuations in pH during co-precipitation (pH 5-11) but the other two batches were prepared with relatively good pH control (pH about 8 to 10) for a large part of the process. After complete addition of the metal salt solution (this took about 2 h), the precipitate slurry was stirred for 1 h. The temperature ◦ was then increased to 80 C and the solution was aged during 16-21 h (See Table 2) without stirring.
After aging, the slurry had a light beige (LDHa1, LDHa2) or light pink color (LDHa3). The liquid above the settled slurry was transparent and had a pH of about 9.
The
three necked ask was gently stirred to get a homogeneous slurry before one part of the slurry was withdrawn with a pipette. The withdrawn slurry was washed with water and centrifuged several times until the pH of the clear liquid was neutral.
The washed slurry was dried in air at during 6-24 h (See Table 2).
80◦ C
(LDHa2 and LDHa3) or
100◦ C
(LDHa1)
Batch LDHa1 and LDHa2 were dried in their respective
centrifugation asks while LDHa3 was smeared out in a petri dish before drying.
The
resulting mass was grinded to a ne powder and used for characterization (XRD, IR, ICP) and activity tests. The remaining, unwashed part of the slurry was left in the sealed three necked ask until used for preparation of LDH precursors. 49 days after the preparation of batch LDHa2 a second slurry sample was removed from the three-necked ask. This ◦ sample was washed and dried using the same method as before (air, 80 C). It was dried in the centrifugation ask during 37 h and the resulting powder is denoted LDHa2-2. The rst powder achieved from batch LDHa2 is denoted LDHa2-1.
The dried masses had dierent colors and consistency. Batch LDHa1 was black, very hard and glass-like after drying. LDHa2-1 and LDHa2-2 were soft as dried mud and had the original beige color.
LDHa3 also kept its original pink color and, possibly since it was
dried in a more e cient manner than the two previous batches, it was soft and easy to grind. A photo of the slurries and powders from the three LDHa batches can be seen at page 73.
21
Batch
LDH precursor
Slurry volume [ml]
LDH in slurrya [g]
Adipic acid [g]
NaOH [g]
pH
Time [h]b
LDHb1
LDHa2
40
2.1
0.83
-
4
136
LDHb2
LDHa3
45
3.0
1.1
0.63
7
144
a Estimation based on ICP data for batch LDHa2 (See Section A.5). b The ion exchange time between the organic swelling agent and nitrate.
Table 3:
The preparation conditions for the two CoNiAl-adipate batches. LDHb2 was
prepared at neutral pH while LDHb1 was prepared by direct addition of adipic acid. The
slurry volume
is the volume of LDHa. The
LDH in slurry
is the estimated mass of LDHa
in the slurry volume.
CoNiAl-adipate (LDHb)
The preparation of CoNiAl-adipate was based on the de-
scription in [27] and consisted of anion exchange between nitrate and adipate (swelling agent). Two batches were prepared (LDHb1, LDHb2). The preparation conditions and the volumes and masses used for the two batches are found in Table 3. 40-45 ml of the previously prepared
CoNiAl − NO3 -slurry
was added to a two necked
round bottom ask containing 20 ml (0.83 g adipic acid) adipic acid solution (LDHb1) or 20 ml (1.1 g adipic acid and 0.63 g NaOH) sodium adipate solution (LDHb2). 20 ml of water was chosen since that gave full dissolution of the adipic acid powder.
Batch
LDHb1 was prepared to study the result of decomposition of the layered structure due to acid-base reactions. The pH of the adipate solution used for batch LDHb2 was set to 7 to avoid the probable incorporation of hydroxide in the LDH structure and to protect LDH from decomposition. The mass of LDH in the CoNiAl-nitrate slurries were estimated to be 2.1 g (40 ml LDHa2) respective 3.0 g (45 ml LDHa3) (total AEC equal to 4.6 mmol respective 6.6 mmol) after doing ICP on batch LDHa2-1 (See Section A.5 for a description of the etimation method used). The adipic acid solution respective the sodium adipate solution were thus prepared to contain more than two times the amount of negative charge needed to balance the positive charge in LDH, as recommended in literature [27, 30]. The mixture of LDHa and swelling agent was gently stirred during 136 respective 144 ◦ h at 45 C. The ask was coupled to argon ow but the ow was stopped from time to ∗ time due to external factors . When the ow was stopped, valves were closed to prevent gas from entering the system through the pipes leading to the ask. The pH after anion exchange was 4 (LDHb1) respective 9 (LDHb2). Batch LDHb1 had a pink solution over the settled green gray slurry while the slurry in batch LDHb2 was pink and had a clear liquid. The slurry was washed with water several times and centrifuged until the liquid ◦ above the slurry had a neutral pH. 20 ml of the slurry was dried at 80 C in air during 13 h in a centrifugation ask (LDHb1, resulted in a glasslike, black phase) or during 2.5 h in a petri dish (LDHb2, resulted in a soft, clay like phase with the original pink color). After drying, the masses were grinded to ne powders that were used for characterization (XRD, IR, SEM, ICP).
CoNiAl-adipate (LDHc)
The preparation of CoNiAl-adipate was based on the pro-
cedures described in [22, 2830] and the preparation steps are summarized in Figure 8 on page 24. Two batches (LDHc1, LDHc2) were prepared with some dierent preparation conditions, as described below. Only the rst batch (LDHc1) was nished. ∗
The laboratory was crowded and other students needed to use gas.
22
Batch
Calcination time [h]
Heating rate [◦ C/min]
Calcination temperature [◦ C]
N2 ow rate [ml/min]
LDHc1
15
4.31
400
25 ± 5
LDHc2
15
4.73
300
25 ± 5
Table 4:
The run parameters used during decomposition of the CoNiAl-nitrate powder
(LDHa2) that resulted in batch LDHc1 and LDHc2. The heating rate and nitrogen ow were chosen according to [23].
After washing and drying of the CoNiAl-nitrate slurry (LDHa2), as described in the ◦ previous section, 1.0 g of the powder was calcined in a quartz tube at 400 C ( LDHc1) ◦ and 1.2 g was calcined at 300 C (LDHc2). The calcination parameters are found in Table 4.
The lower temperature was used after realizing that the higher temperature ◦ gave spinel formation (See Figure 13 on page 39 for the XRD pattern). 400 C was chosen after consulting literature that gave thermogravimetric data for CoNiAl LDHs (See [69]). The literature data showed that the weight loss during thermal analysis became zero ◦ at about 400 C and since the purpose with calcination of LDH in the preparation of LDH-OH is to remove nitrate and carbonate from the interlayer space this temperature was assumed to be suitable for decomposition.
During decomposition the temperature
should not be high enough to allow formation of spinels (See Section 1.2.1, page 8) and ◦ according to [70] CoAl LDHs undergo spinel formation at temperatures around 400 C. Mass spectrometry data from [71] show that carbonate and nitrate is removed from CoAl ◦ ◦ and NiAl LDHs between 250 C and 400 C but IR data from [70] and [72] indicate that ◦ carbonate is removed from CoAl and NiAl LDHs at 450 − 550 C. Since no articles were ◦ found that give the corresponding data for CoNiAl LDHs, 400 C was rst chosen as the decomposition temperature. The decomposition time, 15 h, was taken directly from literature describing the preparation of LDH-adipate [22] and the heating rate was chosen according to [23].
After calcination the powders had lost 40% of their original weight and they were black. The next step in the preparation of CoNiAl-adipate was the reconstruction of the decomposed LDH to get the CoNiAl-OH. 0.40 g of batch LDHc1 and 0.60 g of batch LDHc2 3 3 were placed in two separate two-necked asks and 0.10 dm respective 0.11 dm of boiled water was added.
The rest of the calcined powders were used for analysis (XRD, IR).
The black mixture was gently stirred during 120 h (LDHc1) respective 124 h (LDHc2) at ◦ ambient temperature (12 − 16 C). LDHc1 was reconstructed in air while LDHc2 had an argon atmosphere during reconstruction. During reconstruction of LDHc2 the argon ow was shut o from time to time (due to external factors described above) and the system 3 ◦ was then closed. After reconstruction 0.066 dm of the LDHc1 slurry was heated to 50 C 3 and 0.093 dm of lukewarm adipic acid solution (0.73 g in water) was added all at once to the gently stirred slurry. The mixture was stirred during 1.5 h in air. After 1.5 h half of the CoNiAl-adipate slurry was removed from the ask, washed and centrifuged until the pH was neutral. The CoNiAl-OH slurry that had not been used for ion exchange with adipic acid (adipate) was washed using the same procedure. The washed masses were ◦ dried in vacuum at 60 C during 18 h. No adipic acid was added to batch LDHc2 since the XRD pattern for the decomposed powder (LDO2) showed that spinels had formed (See Figure 13 on page 39).
23
Figure 8:
Schematic overview of the preparation steps and preparation parameters in
the synthesis of CoNiAl-adipate (LDHc).
CoNiAl-terephthalate (LDHd)
Two batches of CoNiAl-terephthalate (LDHd1, LDHd2)
were prepared according to the preparation procedure described in [31]. Two batches were prepared since the dropping funnel containing the metal salts, used during preparation 3 of LDHd1, began to leak during co-precipitation (approximately 0.02 dm was lost from the system). Since that aected the amount of slurry formed, batch LDHd1 could not be ∗ used for intercalation of Co-POM . Batch LDHd2 was thus synthesized. The experimental setup for LDHd synthesis was the same as for the preparation of LDHa.
Co(NO3 )2 · 6H2 O (36 mmol), Ni(NO3 )2 · 6H2 O (36 mmol) and Al(NO3 )3 · 9H2 O (28 mmol) 3 3 were dissolved in 0.06 dm of water and poured into a dropping funnel. A 0.12 dm solution containing 0.27 mol NaOH and 0.028 mol phthalic acid was then poured into another dropping funnel. The amount of phthalic acid was chosen to 28 mmol since that corresponds to an amount 2 times the AEC of the formed LDH slurry (See Section A.4), assuming that all the added metal salts formed LDH. The added amount of NaOH gave full deprotonation of the added acid and it was enough to give a nal slurry with a pH around 9. The three-necked ask that the two dropping funnels were connected to contained 10 ml (LDHd1) respective 20 ml (LDHd2) of water before the co-precipitation started. The initial water volume in batch LDHd2 was larger than in batch LDHd1 since 10 ml gave a slurry too thick to be able to stirr properly. The co-precipitation was performed during ◦ vigorous stirring at 25 C under argon atmosphere. For batch 1 the pH was uktuating between about 7 and 12 for a large part of the process while batch 2 was performed with pH 8-10 for almost the whole process. The addition of the two salt solutions took about ◦ 90 minutes and after total addition the temperature was raised to 74 C and the slurry was aged during 23 h (LDHd1) or 19 h (LDHd2) without stirring. After aging batch LDHd1 had a light gray color and batch LDHd2 had a blueish, gray color. The slurries also had dierent pH. Batch LDHd1 had a pH around 10-11 while batch 3 LDHd2 had pH 9 after aging. 0.03 dm of each slurry was washed and centrifuged until the pH of the water was neutral. The batches were dried in their respective centrifugation ◦ asks in air at 80 C during 37 h (gave LDHd1-1 and LDHd2-1). This resulted in hard, glasslike, black masses. Batch LDHd2 had kept its original color inside the black crust ∗
If the LDH concentration is unknown then a suitable amount of Co-POM can not be added.
24
but it was very hard. Since the samples were very di cult to grind, two new samples were removed from the three-necked asks, washed and dried (denoted LDHd1-2 and LDHd2◦ 2). The washed powders were dried at 80 C during a total of 19 h. After 13 h the masses were removed from the centrifugation asks and grinded. The masses had a consistensy similar to para n and after grinding the masses were put in petri dishes and dried until the consistency was similar to dry clay.
The dried powders (LDHd1-2 and LDHd2-2)
were characterized by XRD, ICP, SEM and IR and the remaining slurries were left in the sealed three-necked asks.
2.2.2 Preparation of POM and Co-POM POM the
The rst step in the preparation of the Co-POM catalyst is the preparation of
α-Keggin
POM (Na10
[SiW9 O34 ]).
Chemically, the synthesis of the
α-Keggin
POM
can be described with the following formula [32]:
9WO4 2− + SiO3 2− + 10H+ + 10Na+ −→ Na10 [α − SiW9 O34 ] + 5H2 O
(1)
The preparation procedure that was used during synthesis is here cited:
Procedure Sodium tungstate (182 g, 0.55 mol) and sodium metasilicate (11 g, 50 mmol) are dissolved in 200 o mL of hot water (80-100 C ) in a 1-L beaker containing a magnetic stirring bar. To this solution is added dropwise 130 mL of 6 M HCl in until the volume is
∼300
∼30
min with vigorous stirring. The solution is boiled
mL. Unreacted silica is removed by ltration over a ne frit or over
Celite or by centrifugation. Anhydrous sodium carbonate (50 g) is dissolved in 150 mL of water in a separate beaker. This solution is slowly added to the rst solution with gentle stirring. A precipitate forms slowly. It is removed by ltering, using a sintered glass lter, after 1 h. The solid is stirred with 1 L of 4 M NaCl solution and ltered again. It is then washed successively with two 100-mL portions of ethanol and 100 mL of diethyl ether and dried under vacuum. Yield:
∼110
g (85%). [32]
Ten batches of POM were synthesized but only six of them gave a product that could be dried and used for characterization with IR and ICP. The other batches (i.e. batch 3, 7, 8 and 10) gave no precipitate that could be dried. Four of the batches were used for the preparation of Co-POM. The ten batches were prepared with the following percentages of the amounts used in the literature description (starting with batch 1): 30%, 30%, 30%, 30%, 50%, 50%, 25%, 10%, 10% and 10%. Ten batches were prepared since there was no success in preparing the POM and Co-POM catalysts (See Section 3 for the results). Table 5 on the next page summarizes the amounts of the chemicals used during synthesis of the POM batches, the time used to add some of the chemicals, the pH after addition of HCl, the drying parameters, the resulting yields, the characterization methods used and wether or not the batches were used for synthesis of Co-POM. From Table 5 it can be seen that only batch 9 was prepared according to the litterature on every point. Batch 1 and 2 were prepared with too little silicate (11.6 mmol instead of 15 mmol). The wrong amount was used since the mass given in the literature description (and not the mole) was used as basis during preparation.
25
It was later found that the
Batch
Na2 WO4
Na2 SiO3
HCl
#
[g]
[g]
[M]
Add HCl [o C]b
pH after
Add
Drying
Yield
Co-POM
add
Na2 CO3
[o C]
[%]
batch #
[mmol]
[mmol]
[ml]
[h]
HCl
[h]
[h]
6.0
n.m.c
n.m.
n.m.
60
56
1, 2
IR, ICP
n.m.
n.m.
80
37
3
IR, ICP
-
-
-
2.6
-
IR, ICP
38
4
IR, ICP
4.0
-
IR, ICP
1
54.6
3.30
186
11.6
39
n.m.
2
54.6
3.30
6.7
80
186
11.6
39
0.50
n.m.
22
6.7
90
-d
-
39
0.67
3
54.6
1.83
186
15.0a
4
54.6
1.83
6.7
85
186
15.0a
39
0.75
5
90.7
3.05
6.0
100
65
n.m.
275 6
90.7
25.0a 3.05
6.0
80
65
0.50
1.53
6.0
88
138
12.5a
33
0.42
18.1
1.42
6.0
85
55.0
5.00
14
n.m.
18.1
1.42
6.0
80
55.0
5.00
13
n.m.
18.1
1.42
6.0
80
55.0
5.00
3.0
n.m.
275
25.0a
7
45.4
8 9 10
Charact.
24
n.m.
n.m.
0.9
n.m.
n.m.
60 24 16 n.m.
n.m.
n.m.
16 n.m.
4-5
0.75
-
-
-
-
4
2.0
-
-
-
-
48
5, 6
IR, ICP
-
-
-
7-8
2.0
16 n.m.
9-10
-
-
a Anhydrous sodium silicate was used. In the other batches metasilicate nonahydrate was used. b Temperature in the solution when began to add HCl and time used to add HCl. c (n.m.) Not measured. d (-) Not performed.
Table 5:
Overview of the dierent preparation parameters used during synthesis of POM.
26
metasilicate refered to in the consulted literature contained ve to six crystal water. The metasilicate used during preparation of batch 1 and 2 contained nine crystal water. Batch 3-7 were prepared using anhydrous metasilicate instead of silicate containing crystal water. Batch 8-10 used the metasilicate nonahydrate but with the correct molar amount. Furthermore, batch 2-4, 8 and 10 were prepared with wrong molar amount of HCl. The water volume removed during boiling of the liquid was collected and measured for all the batches except batch 1 and 2. For batch 1 and 2 the removed volume was roughly estimated visually. The rst four batches were dried in vacuum at elevated temperatures. The rest of the batches were dried under ambient conditions. According to [73] the solution of a freshly prepared
α-Keggin POM (Na10 [SiW9 O34 ]) has a pH of 9-10.
This was the case for batch 7
after addition of anhydrous carbonate but batch 7 resulted in no precipitate. Batch 8-10 were prepared with pH as a preparation variable. Batch 8 gave no precipitate. Since the amount of HCl needed to reach pH 9-10 was only 3 ml (instead of the 13 ml that should be added according to [32]), batch 10 was not completed. During preparation of batch 8-10 it was noted that the pH was constant during the larger part of the HCl addition and then changed rapidly from 7 to 4-5 by addition of just 1 ml of HCl. The yields achieved varied from 2.6% to 56%. It was noticed that if anhydrous silicate was used, no or very little precipitate was achieved. If the nonahydrate metasilicate was used the yield was considerably higher. However, none of the batches had a precipiate after one hour as described in the consulted literature. The yield for batch 5 was relatively high 3 since around 0.06 dm water was removed by evaporation. Before the removal of water the solution contained practically no precipitate. The dried precipitates were white, soft ∗ and had a texture resembling that of styrofoam .
Co-POM
Six batches of Co-POM (α
− [SiW9 O37 (Co(H2 O))3 ]10− )
cording to the procedure described in [25].
were prepared ac-
Batch 2, 4 and 5 were tested in oxidation
reactions. The preparation of Co-POM was not successful and the preparation procedure used is therefore here cited:
α g,
− [SiW9 O37 (Co(H2 O))3 ]10− , Potassium salt. To a solution of Co(CH3 CO2 )2 · 4H2 O (12.00 3 0.048 mol) in water (600 cm ) was added α − SiW9 (44.65 g, 0.016 mol) in small portions
with vigorous stirring. Since the latter salt is insoluble the temperature was slowly raised during ◦ the addition. At about 80 C the solution changed from red to brown and the dissolution of SiW9 3 became more apparent. Addition of water (400 cm ) resulted in almost complete dissolution. ◦ After the solution had been stirred for 1 h at 80 C it was allowed to cool and passed through a cation-exchange column to remove unreacted cobalt. Addition of a solution of KCl (13 g) in water 3 (50 cm ) to the eluate resulted in immediate precipitation of a microcrystalline brown solid (dark brown when dry). Yield ca. 40%. [25]
The six Co-POM batches (starting with batch 1) were prepared using the following percentages of the amounts in the description above; 30%, 15%, 15%, 10%, 10% and 2.5%. The amount of POM added was based on the assumption that POM contained 18 crystal water [25]. For each batch, the result after addition of POM and dilution with water was a red solution with a precipitate. ∗
There was no color change from red to brown when
Before the dry lter cake could be grinded it had to be divided into smaller pieces with a spoon.
27
80◦ C
was reached. Batch 1, 2, 4 and 6 had a brown precipitate while batch 3 and 5 had
a more slurry-like, pink, precipitate. For batch 3 and 6, the temperature was raised to 90◦ C to see if dissolution of the precipitate was possible but without success. Batch 4 3 ◦ and 5 were both diluted with 0.04 dm of 80 C water and the temperature was raised to 85◦ C respective 87◦ C but the precipitate was not dissolved. For batch 6, water was added with 200% excess but no dissolution resulted. During preparation of batch 6 the POM powder was added during 4.5 h but the result was still a mixture of a slurry-like brown precipitate and a red solution. Since the solutions contained precipitates the cation exchange step was coupled to some di culties.
Cation exchange was only performed for batch 1 and 3.
It was performed
since, according to [22], Co-POM was intercalated into LDH as its acid form (and not as its sodium salt). The other Co-POM batches were not subjected to ion exchange since the main target was to evaluate the oxidation activity and not to do intercalation in the rst place. To enable ion exchange some dierent methods were tested. Data about the cation exchange experiments are given in Section A.6. To enable ion exchange batch 1 was diluted with water (to a volume of 1
dm3 )
and the
solution was heated with a hair dryer while passed through the ion exchange column. The ion exchange was performed during 8 h.
No KCl was added after ion exchange.
The resulting red eluate was concentrated to a sticky paste by rotary evaporation. The ◦ mass (denoted Co-POM1-IE) was dried in air at 80 C over night and characterized by IR and ICP. Before ion exchange some of the precipitate was removed from the ask, ◦ centrifuged and dried in vacuum at 80 C over night. The precipitate (denoted Co-POM1) was analyzed with IR and XRD. Batch 3 had a more slurry-like precipitate and it was assumed that the slurry consisted of unreacted POM. The solution was therefore ltered prior to ion exchange. After ion exchange a slightly yellow, acidic eluate resulted. Addition of KCl gave no precipitate. The red slurry, removed by ltration, was not analyzed. The precipitates in batch 2, 4 and 5 were centrifuged and dried (Co-POM2, Co-POM4 ◦ and Co-POM5). Batch 2 was dried in vacuum at 50 C over night while batch 4 and 5 were ◦ dried in air at 80 C during 1.3 hours. Since the literature reports that Co-POM is soluble in water 20 ml of the red solution above the slurry from batch 4 and 5 was concentrated to a sticky paste by rotary evaporation, dried in a similar way as the precipitates and analyzed with IR (Co-POM4-liq, Co-POM5-liq).
Batch 6 was not analyzed since the
resulting mixture had large amounts of precipitate even though it had been diluted with water.
2.3 Catalyst characterization and data analysis methods The prepared catalysts were characterized by XRD, FT-IR, TEM, SEM and ICP.
XRD
The XRD powder diraction patterns were collected on a Rigaku RINT D/Max ◦ ◦ 2000 system (40 kV, 200 mA), using continuous scanning 2θ from 2 to 90 with a step ◦ of 0.02 and monochromatic Cu − kα-radiation. XRD measurements were performed to ∗ determine the lattice parameters (basal spacing), and for the dierent LDH species .
d
a
c
It was also used in an attempt to characterize Co-POM. ∗
For a description of the crystal structure of LDH see [43]. A general overview of crystal structures and
XRD is found in [74].
28
The basal spacing for LDH is given from the position of the peak at the lowest diraction
.
angle (assumed to be a re ection from the (003) basal plane) in the XRD spectra [33, 43]
After visually determining the position (2θ -value) of the (003) peak, the basal spacing was calculated using Bragg's equation:
d003 = where
nλ 2sinθ
(2)
n
is an integer (n=1, 2 and 3 for crystal plane (003), (006) respective (009).) ◦ and λ [nm] is the x-ray wavelength (for Cu − kα radiation λ = 0.154178 nm). 2θ [ ] is the
diraction angle, i.e. the dierence between the incident and the diracted x-rays. [74] It
l
should be noted that all of the (00 ) peaks (
l
is one of the three Miller indices [74]) can
be used for the calculation of the basal spacing since Bragg's equation includes factor XRD patterns can also give information about the lattice parameters
a
and
c.
n.
By as-
suming that the prepared samples were all 3-layer polytypes with rhombohedral structure [39, 69, 75], lattice parameter
c
can be calculated as [2, 43, 69]
c = 3d003 Lattice parameter
c
(3)
is a function of the columbic forces between the layers and the inter-
calated anions [46, 69]. The calculated
c -values should therefore be correlated to the ratio
between the di- and trivalent cations in the structure (i.e. the charge density of LDH). Lattice parameter
a
is a measure of the distance between two cations and the value should 2+ therefore be higher for samples containing a higher percentage of Co . This since the 2+ 2+ ionic radius for Co is slightly larger than that for Ni . For a rhombohedral structure,
a
is calculated as
a = 2d110 where
d110
is calculated from the peak near
2θ = 60◦∗
(4) (peak (110)). [43, 69]
If the XRD patterns have relatively unde ned peaks (broad, weak, asymetrical) then only the basal spacing may be estimated with reasonable accuracy [43].
FT-IR
FT-IR spectra were collected on a Thermo-Nicolet Nexus 470 infrared spectrom−1 eter. KBr tablets were used and a resolution of 4 cm . The spectra were used to identify the prepared POM and Co-POM species and to analyze the anions in the prepared LDH precursors.
TEM
TEM micrographs were taken using a FEI Tecnai
G2
Spirit transmission elec-
tron microscope. The apparatus was equipped with a eld emission gun and the working voltage was 120 kV. Other data: Point resolution 0.35 nm, line resolution 0.20 nm, magni cation range 18.5 times to 740k times. TEM was used to study the surface morphology of some of the prepared LDHs. ∗
Valid for
Cu − kα radiation. 29
Figure 9:
A schematic illustration of the experimental setup used during oxidation
reaction experiments.
SEM
SEM micrographs were collected using a FEI Quanta 200F unit with an accelera-
tion voltage capability of 0.5-30 kV. The point resolution was 2 nm and the magni cation range was 28 times to 300k times. SEM was used to study the morphology of some of the prepared LDH precursors.
ICP
ICP analyses were performed with an ICPS-8100 Shimadzu unit after dissolving
the samples in
HNO3 (65 − 68%) and water.
The samples were prepared to contain 5 ppm
(weight) of the least frequent element that was to be analyzed. The molar masses for POM and Co-POM used when preparing the ICP samples are 2780 g/mol [25] respective 3059 ∗ g/mol . The molar masses used for the three LDH batches (LDHa, LDHb and LDHd) can be found in Table 9 on page 35. ICP gave the elemental compositions of the prepared LDH, POM and Co-POM species.
ICP was also used to estimate the molar mass and
anion exchange capability for the prepared LDH species (See Section A.3 and A.4 for the calculation procedures used to determine these values). The method used to estimate the molar masses has not been encountered in literature.
2.4 Catalyst activity A schematic illustration of the experimental setup used during oxidation reactions is seen in Figure 9. † points .
The setup was based on that used by [23] but it was modi ed on several
All the oxidation reactions were carried out in heated, magnetically stirred two-necked round bottom asks (10 ml) coupled to oxygen ow at atmospheric pressure.
Most of
the reaction experiments were carried out with two reactions running simultaneously, as illustrated in Figure 9.
During some of the experiments the gas leaving the reaction
asks was bubbled through oil to check that the gas was owing through both of the ∗
It was assumed that the prepared POM and Co-POM species both contained 18 crystal water.
†
The
cooling temperature was lowered, the mixing was improved and a rotameter and gas indicators were installed.
30
systems (denoted
gas indicator
in Figure 9). The glass vessels were equipped with re ux
condensers (countercurrent cooling) and the oxygen volumetric ow rate was adjusted ◦ manually with a gas owmeter (rotameter, range 0-60 ml/min, calibrated for air at 20 C). The reaction system was prepared for reaction by adding the catalyst, in the form of a ne powder, to the substrate in the ask and the air in the system was exchanged for oxygen during approximately 10 minutes, depending on the volumetric ow rate of oxygen. During this process, the reactor was contacted with air at ambient temperature to avoid premature reaction. The volumetric ow rate of air was di cult to keep constant and the purity of the oxygen used is not known. The reaction was initiated by lowering the reaction vessel into a stirred, heated oil bath equipped with a temperature controller. When the reaction time had elapsed, the vessel was removed from the oil bath and the oxygen ow was turned o. The re ux condensers were left on while the vessels cooled down to room temperature.
Gas chromatography
was used to analyze the reaction products and determine the turnover numbers (TON) and product selectivities.
n-decane was used as an internal standard.
In the cases no
solvent was added to the reaction mixture, the GC samples were prepared by diluting the reaction mixture with ethyl acetate (it was added at the top of the re ux condenser). In ∗ each case, the reaction mixture was ltered through a column with silica gel to remove the catalyst particulates. GC spectra were collected on an Agilent 6890N GC with an HP19091G-B213 capillary column. The GC operation parameters can be found in Table 16 on page 70. The selectivities and TONs were determined by preparing reference samples containing known amounts of the products (cyclohexenone, cyclohexenol and cyclohexanone) and the internal standard. No reference sample could be prepared for the two products epoxide and diol.
GC analyses of the prepared reference samples gave information needed to
determine the amounts of products formed during reaction. This method had to be used to determine TON since cyclohexene evaporated with relatively high rates. For a detailed description of how the TONs and the selectivities were calculated, see Section A.8.
2.4.1 Oxidation catalyzed by CoNiAl-nitrate To evaluate the oxidation activity for the synthesized LDHa catalysts, eight oxidation reactions and two blank reactions (no catalyst) were performed.
The blank reactions
were carried out in an attempt to determine the evaporation rate of cyclohexene and the plausible degree of auto oxidation. The reaction conditions, catalyst and substrate amounts are given in Table 6 on the following page.
All oxidation experiments except
experiment 1 and 2 (Entry 1 and 2 in Table 6) were performed with two reactions running simultaneously as illustrated in Figure 9 on the preceding page.
That means that e.g.
reaction 3 and 4 (Entry 3 and 4 in Table 6) were run simultaneously, with reaction 3 as reaction vessel nr. 1 (See Figure 9). For reaction 1-3 and 5-8 the added mass of LDHa corresponds to 754 mmol of LDH (See Section A.4 on page 71 for a description of the calculations.). This amount was chosen since it can balance 0.02 mmol of Co-POM, which was the amount of Co-POM added during oxidation reactions (See next section). The calculations performed to determine the ion exchange capability and molar mass for the LDH species were based on ICP ∗
No physical data available.
31
Entry
Substrate
Solvent
Internal standard
O2
Reaction
Cooling
#
batch
Catalyst [mg]
[mmol]
[mmol]
[mmol]
[ml/min]
[h]
[◦ C]
1
LDHa2-1
77.9
10
-a
-
n.m.b
8
n.m.
2
LDHa2-1
77.9
10
95
-
25
8
n.m.
10c
4
n.m.
4
n.m.
5
-2
5
-2
4
-7
4
-7
3
LDHa2-1
89.2
10
-
1.0
4
LDHa3
89.2
10
-
1.0
10c
5
LDHa1
176
50
-
1.0
7c
6
-
-
50
-
1.0
7c 10c
7
LDHa2-1
89.2
1.0
95
1.0
8
LDHa2-2
126
1.0
95
1.0
10c
9
LDHa2-2
13.0
50
-
1.0
10
24
-7
10
-
-
50
-
1.0
10
24
-7
a (-) Not used. b (n.m.) Not measured.
c Oxygen was bubbled through the reaction mixture with a syringe.
Table 6:
Catalysts, substrate amounts, reaction conditions and other related data for the
oxidation experiments performed with LDHa catalysts. The substrate was cyclohexene. n-decane was added as an internal standard and acetonitrile was used as solvent.
analyses. Reaction 4 was performed with 580 mmol of LDH, instead of the intended 754 mmol, since the added mass was the same as that for reaction 3. Later ICP results showed that the molar masses for the two batches (LDHa2 and LDHa3) were dierent. Reaction 9 was performed with 13.0 mg catalyst which corresponded to that used previously by Dr. Yang [23]. The reaction temperature was
60◦ C
and the substrate was cyclohexene.
Dierent sub-
strate amounts were used. 10 mmol of substrate was used according to [22] while 50 mmol was used to enable longer reaction times and 1.0 mmol to enable complete reaction. The stirring during reaction was kept low to minimize the evaporation of cyclohexene. This gave poor mixing between the substrate and the catalyst powder. By not placing the reaction vessel in the center of the oil bath the mixing became turbulent and the problem with settling catalyst powder was avoided to some degree. This modi cation was made for reaction 3-10. The reaction time was varied from 4 to 24 h.
For reaction 3 and 4 the reaction was
terminated after 4 h since almost all of the substrate had evaporated from reaction vessel nr 2 at that time. Reaction 9 was run for 24 h since that was the reaction time that had been used by Dr. Yang [23]. The volumetric ow rate of oxygen was chosen to 10 ml/min for reaction 3-10, since that ow rate was used for the oxidation reactions with Co-POM. The oxygen volumetric ow rate for reaction 1 was not measured and the ow rate for reaction 2 was 25 ml/min. Oxidation experiment 3-8 had bubbling oxygen which resulted in increased evaporation rates of the substrate. Cooling with ethanol was installed for reaction 3-10 in an attempt to decrease the amount of cyclohexene that left the system with the gas ow. The other reactions had owing tap water as coolant. Since the re ux condensers were installed in series it is probable that the coolant inlet temperature to reaction system 2 was slightly higher than that for reaction system 1. The blank reaction was therefore coupled to reaction system 2.
32
A solvent (acetonitrile) was used for reaction 2, 7 and 8.
In reaction 2 it was used to
allow for higher stirring rates, and thus improve the mixing, since it was belived that the evaporation rate of cyclohexene was decreased with addition of solvent. In reaction 7 and 8 solvent was added to enable longer reaction times with only 1.0 mmol of substrate. Complete conversion was desired in an attempt to determine how much of the substrate that left the system with the gas ow.
The reactions were however terminated after 4
hours due to technical problems with the gas ow.
It was noticed that gas only ew
through reaction vessel 2 (no bubbles in reaction vessel 1) and as a result reaction 9 and 10 were thus equipped with gas indicators as seen in Figure 9 on page 30. With the use of gas indicators it was noted that if the two gas valves positioned after the rotameter were fully opened the gas ow rate was not equal in the two systems. By slightly closing one of the gas valves during reaction, the ow rate could be set equal. For reaction 3-8 it is highly plausible that the oxygen gas ow rate was dierent in the two reaction systems. All the GC analyses were performed the same day as the reaction experiments were performed. The results from the analysis can be found in Section 3.2.1.
2.4.2 Oxidation catalyzed by Co-POM Five oxidation experiments with Co-POM as catalyst were run. Reaction nr 2-5 (Entry 2-5 in Table 7) were performed according to [22], with 0.02 mmol of Co-POM (i.e. 0.06 mmol of Co), 10 mmol of cyclohexanol and 100 mmol of 1,4-dioxane. The molar mass of Co-POM was calculated assuming that the dried powder contained 18 crystal water per mole of Co-POM [25]. Reaction nr 1 was performed with 10 mmol of cyclohexene as substrate. The reactions were performed before elemental analysis of POM and Co-POM had been performed and ICP later showed that the added Co-POM amounts did not correspond to 0.02 mmol of Co-POM. The reaction conditions, volumetric oxygen ow rates and solvent amounts can be found in Table 7 on the following page. The oxygen volumetric ow rate was adjusted to around 10 ml/min instead of 5 ml/min used by [22]. Reaction 4 and 5 had bubbling oxygen. The reaction time ranged from 4 to 8 hours.
According to [22], 62% conversion was reached after 12 h and it was thus
assumed that, if the synthesized catalyst possessed oxidation activity, four hours would be long enough to detect a conversion with GC analysis. Flowing tap water was used as cooling medium in the re ux condensers. All the GC analyses were performed the same day as the reaction experiments were performed. The results from the analyses can be found in Section 3.2.2.
3
Results
This study aimed at developing a heterogeneous LDH-POM oxidation catalyst with a selectivity to the allylic ketone higher than 70%.
Since the Co-POM catalyst was not
prepared successfully this goal was not reached and the hypothesis could not be tested. The results from the characterizations and activity tests are presented in this section.
33
Entry
Catalyst
Solvent
Internal standard
O2
#
batch
[mmol]
[mmol]
[ml/min]
1
Co-POM2
-a
-
5
60
8
2
Co-POM2
100
-
5
100
8
3
Co-POM2
100
-
20
100
4
10b
100
8
100
8
4
Co-POM4
100
1.0
5
Co-POM5
100
1.0
10b
Reaction [◦ C]
[h]
a (-) Not used. b Oxygen was bubbled through the reaction mixture with a syringe.
Table 7:
The reaction conditions for the ve oxidation reactions catalyzed by Co-POM.
The solvent was 1,4-dioxane and n-decane was the internal standard.
Batch
ICP
XRD
IR
Elemental compositiona
[g/mol]b
Basal spacing [nm]b
Detected anions
LDHa1
Co0.31 Ni0.40 Al0.29
234
0.819
NO− 3
LDHa2-1
Co0.33 Ni0.38 Al0.29
129
0.867
NO− 3
LDHa2-2
Co0.31 Ni0.39 Al0.30
167
c -
LDHa3
Co0.37 Ni0.36 Al0.27
154
0.835
NO− 3
LDHb1
Co0.24 Ni0.38 Al0.38
138
1.37
NO− 3 , adipate
LDHb2
Co0.36 Ni0.37 Al0.27
187
0.779
NO− 3 , adipate
LDHd1-2
Co0.35 Ni0.37 Al0.28
208
1.43
NO− 3 , terephthalate
LDHd2-2
Co0.36 Ni0.37 Al0.27
192
1.43
NO− 3 , terephthalate
NO− 3
a The numbers have been rounded o to two signi cant values. b The numbers have been rounded o to three signi cant values. c (-) Not performed.
Table 8:
Characteristics of the prepared LDH precursors.
3.1 Catalyst characterization 3.1.1 LDH precursors An overview of the characteristics of the LDH catalysts (LDHc1 and LDHc2 excluded) can be seen in Table 8.
The data come from ICP, XRD and IR analyses.
from these analyses are presented in the following subsections.
The results
ICP analyses showed
that co-precipitation at low supersaturation gave the desired composition. XRD patterns showed that the syntheses were successful in all cases except for batch LDHb2 that had no increased basal spacing after anion exchange between nitrate and adipate. IR was used to analyze the interlayer anions and con rmed the results from the XRD analyses. No carbonate was found by IR analyses in any of the samples. A photograph of the prepared LDH slurries and dried powders, intended to show the dierent colors of the LDH species, is found in Section A.7.
ICP
ICP analyses were performed to determine the elemental composition and estimate
the molar mass for the prepared LDH species. The elemental composition for all of the LDH batches except LDHc1 and LDHc2 can be
34
Batch
Chemical formula
Molar mass [g/mol]
LDHa
− [Co0.36 Ni0.36 Al0.28 (OH)2 ] (CO2− 3 )0.04 (NO3 )0.23 (H2 O)0.50
110
LDHb
[Co0.36 Ni0.36 Al0.28 (OH)2 ] (CO2 (CH2 )4 CO2 )0.14 (H2 O)0.50 2− [Co0.36 Ni0.36 Al0.28 (OH)2 ] [p − (CO2 )2 C6 H4 ]0.14 (H2 O)0.50
113
LDHd
Table 9:
116
Analytical molar masses for the prepared LDH speciescontaining nitrate, adi-
pate and terephthalate. The amounts of water, nitrate and carbonate are based on literature data for CoNiAl-nitrate [68].
found in Table 8 on the facing page. During the co-precipitation processes (synthesis of LDHa and LDHd), the metal salts were added in ratios to give the elemental composition
Co0.36 Ni0.36 Al0.28 .
For the LDHa and LDHd batches, the elemental compositions seen
in Table 8 are all very close to the desired composition.
This is somewhat surprising
since it was very di cult to keep the pH-value near 9 with the applied method.
In
consulted literature, pH is often strictly controlled (e.g pH=9±0.2 [26]) and it has been reported [47] that the use of automatic pH controlling devices during co-precipitation gives better results than by manually controlling the pH. That statement is not con rmed by these results. Batch LDHa1 and LDHa2 both have lower amounts of Co than batch LDHa3. This is explained to result from the fact that the two rst LDHa batches were prepared with a cobalt nitrate salt that during storage had adsorbed signi cant amounts of water. The 2+ amount of Co added during co-precipitation was thus lower than intended. The two LDHb-batches were subjected to anion exchange reactions and the resulting elemental compositions are in accordance with theory.
It is seen that for LDHb1, in
which adipic acid was added directly to the LDHa slurry, cobalt and some nickel were dissolved from the structure. The red, clear liquid above the slurry that resulted after anion exchange supports this result. For LDHb2 no metals seems to have been leaking from the structure. The calculated molar masses are found in Table 8. Compared to analytical molar masses, presented in Table 9, it is seen that the method used to estimate the molar massed can be used only to give a very rough estimation. It should be noted that the values calculated in Table 8 are based on only one prepared ICP sample for each LDH batch and it was assumed that the dried powders used to prepare the ICP-samples consisted of pure LDH phase. It is recommended that the molar mass is determined by other methods than ICP since the applied method is highly dependent on the accuracy of the scale used during preparation of the ICP samples and the water amounts in the samples. Water contents can be estimated based on thermogravimetrical (TG) analyses [26, 69].
GC [75] and MS [68] can be used to determine carbonate and
nitrate amounts.
TEM
TEM micrographs were taken of the slurry of LDH precursor LDHa1 and LDHa2.
The micrographs are presented in Figure 10 on the following page. The micrographs show that the two analyzed batches both have ake-like crystals. The akes have a diameter of about 20 nm and, regarding the thickness of the akes, they consist of several sheets stacked on top of eachother. Compared to TEM micrographs for MgAl (See [33] and [16, 33]) the crystals in LDHa1 and LDHa2 are small.
35
(a)
Figure 10: XRD
(b)
CoNiAl-NO3 (LDHa1)
CoNiAl-NO3 (LDHa2-1)
TEM micrographs of the two CoNiAl-NO3 precursors LDHa1 and LDHa2.
XRD is one of the most common ways to analyze LDH [39] despite the fact
that the XRD patterns from LDH often have broad and asymmetrical peaks, leading to di culties extracting reliable information from the patterns. XRD patterns for the synthesized LDH batches are found in Figure 11 (LDHa), 12 (LDHb), 13 (LDHc) and 14 (LDHd). The lattice parameter
d
has been calculated with
Bragg's equation, according to the description in Section 2.3 on page 28. The other lattice parameters could not be determined with reasonable accuracy, due to the broad and asymetrical nature of the XRD peaks. The three LDHa batches show similar XRD patterns. The patterns have the typical LDH appearance (See [39, 43, 75]) that is characterized by more intense, sharp and symmetrical peaks at lower angles.
The patterns are very similar to CoNiAl LDHs prepared
using similar preparation procedures [26, 69].
By comparison with literature data, the
peaks from left to right probably correspond to diraction from basal plane (003), (006), (009)/(012) and (110) [26, 39, 43, 69, 75]. The peaks are quite broad which, especially in
l
the case of the more symmetrical (00 ) peaks, indicates that the crystals in the samples are small [43, 70, 74].
The small crystal sizes were con rmed by TEM micrographs of
LDHa1 and LDHa2. Broad, asymetrical peaks, like that at 35
◦
in Figure 11, is usually a result of irregular
stacking of the layers [43, 75]. The (015) and (018) peaks that, for CoNiAl LDHs appears ◦ ◦ around 40 to 50 [26, 69] are not distinguishable from the XRD patterns in Figure 11. This is probably also a result of poor stacking of the layers. Regarding the relative peak intensities for one and the same sample, peak (003) has higher intensity than peak (006). This is the usual case for LDH species containing anions with low scattering abilities (low Z number) [43] and it is reasonable since the intensities of the transmitted x-rays decay exponentially with travelled distance [74].
l
For (01 ) peaks, the so called shark's n peaks have been analyzed [43, 75] and found to be due to stacking faults resulting from a mixture of rhombohedral and hexagonal layers which is common for synthetic LDH species. The mixing of crystal structures gives
36
Figure 11:
XRD patterns for the three CoNiAl-NO3 (LDHa) batches.
little ordering of the cations in the stacking direction and the incoming x-rays are thus scattered with several dierent angles. Stacking faults are thought to be related to the intercalated anion, that may prevent the layers from ideal stacking. [43] The gallery height depends on the interlayer anion and to some extent also on the cations and their ratios in the brucite-like layer. As seen in Table 8 on page 34, the calculated basal spacings are in agreement with, but slightly lower than, those found in the literature for LDH that contains nitrate (0.879 nm [44]). The XRD-patterns for the two LDHb batches are seen in Figure 12 on the next page. It is seen that the (003) peak for LDHb1 is strong and at lower angles than the (003) peak for precursor LDHa2 containing nitrate. The XRD pattern for LDHb2 is practically unchanged from that for LDHa3.
The hypothesis when preparing batch LDHb1 was
that the addition of adipic acid would decompose the layered structure due to acid base reactions but this is not seen from the XRD patterns. Instead a high intensity peak ◦ ◦ appeared at 6.5 together with one new peak at about 13 . Additionally, something that ◦ could be one very broad or several weak peaks appeared around 26 . By using Bragg's equation and assuming that the rst peak in the XRD pattern from LDHb1 is due to diraction from the (003) basal plane, the basal spacing is calculated to 1.37 nm. This value is in good agreement with previously determined basal spacings for LDH containing adipate, i.e. 1.40 nm [76]. This means that the adipate molecules are arranged perpendicular to the brucite-like layers. The second peak is presumably due to diraction from the (006) basal plane, and not from another LDH phase. This conclusion is drawn after calculating the basal spacing for the three rst peaks assuming that they are (003), (006) and (009).
This gave the values 1.36 nm (006) and 1.33 nm (009). If ◦ instead assuming that the second peak (at 13 ) is a re ection from the (003) basal plane of LDH−CO3
d
equals 0.68 nm. The basal spacing for a CoNiAl LDH specie containing
carbonate has previously been estimated to 0.77 nm [69] which further implies that the second peak is the (006) peak for CoNiAl-adipate.
37
Figure 12:
XRD patterns for the two CoNiAl-adipate (LDHb) batches. For comparison,
the XRD spectra for the two CoNiAl-nitrate precursors used to prepare the LDHb batches have been included in the gure.
The increased intensity for LDHb1 compared to the precursor LDHa2 is probably related to the leakage of cobalt from the structure. It has previously been shown that the crystallinity for CoNiAl LDH increases with increased Ni content [26, 69]. Regarding the XRD pattern for LDHb2 it is clearly seen that the intercalation of adipate was unsuccessful with the experimental conditions tested. The XRD patterns for the decomposed LDHa precursors are practically identical, as seen in Figure 13 on the facing page. All the peaks from the original LDHa precursors have dissappeared and several thin, intense peaks have formed. These peaks have been ◦ reported for CoNiAl LDHs calcined in air at 500 C [26] and were then identi ed as cobalt ◦ spinels. CoAl LDH species decomposed in air at 550 C [72] gave the same XRD pattern which indicates that the peaks seen in Figure 13 probably are due to spinels of cobalt and aluminium. Figure 14 on the next page gives the XRD patterns for the two prepared LDHd species.
l
The patterns have no peaks that coincide with the (00 ) peaks for the CoNiAl-nitrate. Assuming that the new peaks are due to scattering from basal plane (003), (006), (009) and (0012) Bragg's equation gives the following basal spacings; 1.43 nm, 1.45 nm, 1.40 nm and 1.45 nm. The calculated values strongly suggest that the peaks are due to one single phase, and not several dierent LDH species. The calculated values are similar to that found for MgAl LDH intercalated with terephthalate, i.e. 1.44 nm [31] and 1.41 nm [77]. The corresponding gallery height, achieved by substracting the thickness of one brucite layer from the basal spacing is 0.98 nm. This indicate that the terephthalate anions are stacked vertically to the brucite-like layers (the size of the anion is 0.72 nm [77]). The probable reason for the broad and asymetrical peaks was dicussed above. The intensity for diractions from higher order planes such as (006) are generally lower than for (003) planes, as mentioned above. This pattern is seen for LDHd2 but not for
38
Figure 13:
XRD patterns for the two decomposed LDHa (denoted LDO1 and LDO2),
the reconstructed LDO1 (denoted LDH-OH1) and LDHc1. For comparison, the XRD pattern for the CoNiAl-nitrate precursor has been included in the gure.
The peaks
formed after decomposition are due to spinel formation.
Figure 14:
XRD patterns for the two CoNiAl-terephthalate (LDHd) batches and the
black phase (LDHd1-1) that resulted after prolonged drying times. For comparison, the XRD pattern for CoNiAl-nitrate have been included in the gure.
39
(a)
LDHa2-2
(b)
Figure 15:
LDHa3
SEM micrographs of two of the CoNiAl-NO3 species (LDHa2-2 and LDHa3).
The scale in the two pictures to the left is 500.0 nm and it is 5.0
µm
in the pictures to
the right.
batch LDHd1. A discussion about the inversed intensities can be found in Section 4 at page 54.
SEM
SEM micrographs were collected for LDH batch LDHa2-2, LDHa3, LDHb1, LDHb2,
LDHd1, LDHd2-1 and LDHd2-2. The SEM analyses were performed to study the morphology of the samples since the surface area aects the catalytic activity and probably also the intercalation process. Large crystals should need longer intercalation times due to the longer diusion distances for the anions. The morphology also gives some information about the preparation process. The SEM micrographs are presented in Figure 15, 16 and 17. It is seen that batch LDHa2-2 contains small akes, similar to those seen in the TEM pictures (See page 36). This kind of crystals were only found in batch LDHa2-2 but batch LDHa3 and LDHd2-2 contained something similar but in much lower amounts. The other batches all looked very similar, having a relatively smooth surface built up of large thick akes. Note that the analyzed batches had been stored as slurries during unequally long time.
Batch LDHa2-2 had been stored in the three-necked ask during 47 days before
40
Figure 16: µm
LDHb1
(b)
LDHb2
SEM micrographs of the two CoNiAl-adipate species prepared with method
B. The scales are 1.0 and 5.0
(a)
µm (Figure a,
left), 5.0
µm (Figure a,
(Figure b, right).
41
right), 2.0
µm (Figure b,
left)
(a)
Figure 17: 2.0
LDHd1
(b)
LDHd2-1
(c)
LDHd2-2
SEM micrographs of the two CoNiAl-terephthalate species. The scales are
µm (Figure a, left), 10.0 µm (Figure a, right), 2.0 µm (Figure b, left), 10.0 µm (Figure nm (Figure c, left) and 5.0 µm (Figure c, right).
b, right), 500.0
42
Figure 18:
IR spectra for the LDH batches containing nitrate (LDHa).
drying. Synthesis of LDHb1 began after 45 days storage of LDHa2. The other batches were dried directly after the aging time had elapsed. Finally, the morphology is very dierent from the disk-like crystals seen in SEM micrographs of MgAl-POM and ZnAl-POM prepared in similar way [33].
IR
Infrared spectroscopy analyses were performed on all the prepared LDH batches to
identify the interlayer anions. The spectra can be found in Figure 18 (LDHa), 19 (LDHb), 20 (LDHc) and 21 (LDHd). A large part of the spectra were measured with small amounts of sample in the KBr tablet and the peak intensities in these spectra are therefore very low. In some spectra it is therefore di cult to distinguish the peaks resulting from the samples from those caused by disturbances during the measurements. This makes the results from the IR analyses less reliable.
Furthermore, since the KBr tablets were prepared with
unequal amounts of sample the absorbances can not be used to compare the amount of anions in the dierent samples.
Finally, the sometimes positive, sometimes negative −1 peaks between 2300 and 2400 cm are due to increased carbon dioxide concentrations in the air, present because of breathing while loading the samples. The IR spectra for the CoNiAl-NO3 species are seen in Figure 18 and they all have the same appearance.
−1 and the weak peak at 1627 cm are due to nitrate re−1 −1 −1 spective water [78]. The other peaks (839 cm , 613 cm , 426 cm ) could not be −1 −1 identi ed. No peaks due to carbonate (from literature: 1370 cm (strong) [78], 1363 cm −1 −1 −1 (strong) [69], 1414-1420 cm and 818-833 cm [72] and 670-690 cm [39]) could be iden-
The strong peak at 1384
cm−1
ti ed. Furthermore, batch LDHa2 seems to have stayed unchanged during the 49 days of storage in the three-necked ask (compare LDHa2-1 and LDHa2-2). The hypothesis was that carbonat could have entered the structure during storage. The IR spectra for the LDHs containing adipate are seen in Figure 19. The two spectra contain the same peaks but with dierent relative intensities. It is seen −1 that both of the samples contains a signi cant amount of nitrate (1384 cm ) but LDHb1 −1 −1 also shows two intense peaks at 1554 cm and 1409 cm . These peaks are visible in
43
Figure 19:
IR spectra for the LDH batches containing adipate (LDHb). The inserted cm−1 .
box is a magni cation of the peaks at 1700-1300
Figure 20:
IR spectra for the thermally decomposed LDH species (LDO).
the spectrum for LDHb2 but they are weak compared to the nitrate peak. These two −1 −1 peaks are probably due to the asymetric (1560cm [79], 1557 cm [80]) and symmetric −1 −1 [79], 1398 cm [80]) vibrations of the carboxyl group in adipate. The spectra (1414 cm are in good agreement with the XRD patterns for LDHb1 and LDHb2. Since IR reveals that nitrate is present in LDHb1 and the XRD pattern showed no re exion from a phase containing nitrate, nitrate must be present in between the layers together with adipate. The XRD for LDHb2 showed no peaks resulting from a structure containing adipate but according to IR analysis adipate is present. The IR spectra for the two decomposed LDH-NO3 species, seen in Figure 20, do not give much valuable information due to the low absorbances. It is seen that LDO2 has a more prominent nitrate peak than LDO1.
This is in good
accordance with theory since more nitrate should be present after thermal decomposition −1 at lower temperatures. The peak at 1625 cm is due to water, as mentioned above. Since the peaks for water coincide with those for hydroxide it is not possible to decide wether the samples have been subjected to dehydroxylation or not. It is most probable
44
Figure 21:
IR spectra for the LDH batches containing terephthalate (LDHd).
that, since water leave the structure before dehydroxylation begins, the structures have not been fully dehydroxylated. The three spectra for the terephthalate intercalated LDHs are seen in Figure 21. As for all the other LDH precursors, the LDH batches contains nitrate.
Since XRD
showed that the gallery height had increased, nitrate must be present in between the −1 layers together with the organic anion. The following peaks (cm ) in the spectra for the ∗ LDHd species are probably due to terephthalate; 1571, 1502, 1014, 821 and 746 [81] .
3.1.2 POM and Co-POM The preparation of POM was coupled to a lot of experimental errors. Attempts to synthesize POM with anhydrous silicate was made and too low amounts of metasilicate nonahydrate and HCl were used. The results from the experiments show that the amount of HCl added during synthesis and the structure of silicate are both essential for the formation of POM. If the wrong amount of HCl was added, none or very little precipitate formed. The suitable pH was pH=7±0.5 (measured by pH indication paper ) and it was reached by following the preparation procedure in [32]. As mentioned in Section 2.2.2, if anhydrous silicate was added during synthesis nothing precipitated from the solution after addition of HCl. This observation suggests that the structures of anhydrous silicate and metasilicate are not the same.
ICP
The results from the ICP analyses are summarized in Table 10 on the following
page. Table 10 also includes the desired compositions of POM and Co-POM. It is seen that only one of the prepared POM species (i.e.
POM9) has a composition similar to
the desired one. It is not surprising that only the ninth batch seems to have succeeded. POM9 was the only batch prepared according to the literature on every point. showed that the Co-POM synthesized from POM9 (i.e.
ICP
Co-POM5) did not have the
desired composition. The results from oxidation reactions (See Section 3.1.2 and 3.2.2) ∗
Peaks (cm
−1
) reported to stem from vibrations of terephthalate are; 1566, 1504, 1393, 1015, 825 and
743 [81] and 1567 and 1390 [82].
45
Table 10:
Batch
Na
Si
W
Co
Desired POM
10
1
9
-
POM1
8.0
1
49
-
POM2
7.9
1
56
-
POM4
291
1
0.018
-
POM5
16
1
35
-
POM6
61
1
0.20
-
POM9
10
1
9.7
-
Desired Co-POM
10
1
9
3
Co-POM1-IE
10
1
61
1.3
Co-POM2
9.8
1
61
2.9
Co-POM4
4.1
1
0.026
5.2
Co-POM5
5.3
1
0.025
4.0
Molar ratios of the elements Na, Si, W and Co in the prepared POM and
Co-POM batches analyzed with ICP.
Batch
Soluble in
POM1
HNO3
Colorless liquid
POM2
HNO3
Colorless liquid
POM4
H2 O
POM5
HNO3
POM6
H2 O
The powder became yellow and insoluble
POM9
H2 O
The powder kept its white color but was insoluble
Co-POM1-IE
HNO3
The powder became white before it was dissolved and the solution became colorless
Co-POM2
HNO3
The powder was red before it was dissolved and the solution became light pink
Co-POM4
HNO3
The powder was red before it was dissolved and the solution became light pink
Co-POM5
HNO3
The powder was red before it was dissolved and the solution became light pink
Table 11:
Observation when adding HNO3
The powder hissed, became yellow and insoluble Colorless liquid
The solubility for POM and Co-POM and observations done during prepara-
tion of ICP samples.
also indicate that the Co-POM5 batch was unsuccessful.
The results from IR analysis
however suggests that the batch was successful and contained the desired specie. Several interesting observations were made during preparation of the ICP samples. They are summarized in Table 11. It is seen that some of the POM samples were not soluble in nitric acid, while all of the Co-POM batches were soluble.
IR
The rst part in the synthesis of Co-POM was to synthesize POM. The IR spectra
for the POM batches are presented in Figure 22 (POM1 and POM2), and 23 (POM batch 4-6 and POM9) . The sometimes positive, sometimes negative peaks between 2300 and −1 2400 cm are due to increased carbon dioxide concentrations in the air, present because of breathing while loading the samples. The IR spectra for POM1 and POM2 are very similar and they both agree well with literature values from [32]. The conclusion drawn after seeing the IR spectra is that the preparations of POM1 and POM2 were successful. This is in contradiction with the ICP results.
46
(a)
(b)
Figure 22:
(a) IR spectra for POM1 and POM2. (b) An enlargment of the IR spectra
seen in Figure (a) together with vertical lines (literature values) marking the desired positions for the peaks for POM [32].
47
The IR spectra for POM batch 4-6 and 9 are seen in Figure 23 on the next page. Among these batches POM9 and POM5 agrees best with the literature data (See Figure 23 (b) and (c)). POM4 has no peaks that match with the literature values for POM while POM6 has some peaks that match. POM5 and POM9 match literature data equally well as the IR spectra for POM1 and POM2 but there are one fact that distinguishes POM9 from the other three samples. That is, the spectrum for POM9 practically only has peaks due to vibrations from POM. Regarding the spectra for POM4, ve of the strong peaks, and three of the medium strong ∗ peaks, agree well with IR data for tungstate, silicate and carbonate . This supports the observations done during preparation of the ICP sample (See Table 11 on page 46). The hissing probably came from carbon dioxide produced in the reaction between nitric acid and sodium carbonate and the yellow color came from formed tungstic acid. After synthesis of POM some of the batches were used to synthesize Co-POM. The IR spectra for the prepared Co-POM batches are seen in Figure 24 on page 50. Comparison of the IR spectra for Co-POM1 and Co-POM2 with the IR spectra for POM1 (Figure 24 (a)) reveals that the POM powder added during Co-POM synthesis was transformed into something else. It is seen that Co-POM5 has the best match with the vertical lines representing literature data (peak positions for Co-POM). This result supports the suggestion that POM batch 9 was successfully prepared (POM9 was used to prepare Co-POM5). Co-POM4 also shows a spectrum that agrees well with literature data. Furthermore, the IR analyses reveal that the liquid above the precipitate in batch 4 and 5 did not contain the desired Co-POM. The strong peak at 1415
cm−1
present in all the Co-POM spectra is probably due to ? acetate, as suggested by comparison with literature IR data for adipate . The peak at −1 1552 cm in the spectrum for Co-POM1 also matches with IR data for acetate (See footnote). This peak is absent in Co-POM1-IE which can not be explained with the data −1 available at this point. The strong peak at 1579 cm could not be identi ed.
XRD
An attempt to use XRD to identify Co-POM1 was made since this method was
used by [22] for the characterization of this specie. The resulting XRD pattern gave no valuable information. The pattern is seen in Figure 25 on page 51. The peaks are irregular and broad which indicate poor crystallization and small crystals. The sample analyzed by XRD in [22] had been recrystalized from hot water and had potassium as counter ion.
3.2 Catalytic activity In short, the CoNiAl-nitrate species possessed oxidation activities much lower than reported [23] and the highest estimated selectivity to cyclohexenone was between 67-75%. The synthesis of Co-POM was unsuccessful and the resulting specie denoted Co-POM5 possessed low oxidation activity with cyclohexanone as the only product. ∗
IR data for sodium metasilicate nonahydrate, sodium tungstate dihydrate, sodium carbonate and
cobalt(II) acetate came from SDBSWeb :
http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of
Advanced Industrial Science and Technology, accessed 2011-06-06)
48
(a)
(b)
(c)
Figure 23:
IR spectra for POM batch 4, 5, 6 and 9. (b) Enlargement of the spectra in
(a) with literature values [32] included a vertical lines in the gure. (c) Enlargement of the spectra for POM9.
49
(a)
(b)
(c)
Figure 24:
IR spectra for (a) Co-POM 1, 2 and POM1, (b) Co-POM 4 and 5 and
(c) an enlargement of the spectra in (b). denotion
IE
The literature values comes from [22].
The
means ion exchange and the batch had been subject to this process before IR
analysis. The denotion
liq
means that the spectra comes from a sample obtained through
evaporation of a solution (the other samples were precipitates collected by centrifugation). 50
Figure 25:
XRD pattern for Co-POM1.
3.2.1 Oxidation catalyzed by CoNiAl-nitrate GC analyses were done on all the reaction mixtures to determine the selectivity and turnover number (TON) for the reactions.
A detailed description of how these values
were determined is found in Section A.8. The results are summarized in Table 12 on the next page. The highest TON
∗
2.6 h−1 . The other −1 (0.28 h ). It is seen
was achieved with catalyst LDHa2-2 and reached
catalysts were up to ten times less active, LDHa1 being least active
that batch LDHa2-1 and LDHa2-2 have the highest conversions and TONs (See entry nr.
3 and 9) but that they are signi cantly dierent.
Batch LDHa2-2 has the highest
TON while LDHa2-1 has the highest conversion. The slightly higher activity for batch LDHa2 compared to LDHa1 and LDHa3 can not be explained by dierences in elemental composition or reaction conditions but may be a result of dierences in morphology as indicated by SEM. Repeated reaction experiments have to be carried out before any conclusions can be drawn about what the dierent activities stems from. Note also that the catalyst amounts used during reactions were based on the estimated molar masses determined by ICP. The TONs presented in Table 12 do not include the product epoxide which means that the actual TONs are higher than those in the table.
The epoxide was not included in
the calculations since no reference sample for that specie was prepared (See Section A.8). Furthermore, the calculations were done ignoring the fact that the substrate was not pure (purity≥
98.0%)
and contained small amounts of the product species.
The TON could only be determined for entry nr. 3-5 and 9 in Table 12. For entry nr. 1 and 2 no internal standard was used and for reaction 7 and 8 practically all substrate evaporated before measurable conversions had been reached. The estimated selectivities for the ketone range from 51 to 75 % (See Section A.8 for a description of how the selectivities were estimated).
The estimated selectivities in
Table 12 do not include the product epoxide since no referense sample for the epoxide could be prepared.
This means that the estimated selectivities for cyclohexenone and
cyclohexenol written in Table 12 are higher than the actual selectivities. If the selectivities were estimated with simple area comparison of the product peaks in the GC diagrams the ∗
TON is de ned as the mmol of product formed per mmol of catalyst and reaction time.
51
Conversiona, reaction
Entry
Catalyst
Composition
#
batch
ICP
[%], [h]
1
LDHa2-1
Co0.33 Ni0.38 Al0.29
-c
2
LDHa2-1
Co0.33 Ni0.38 Al0.29
-c
3
LDHa2-1
Co0.33 Ni0.38 Al0.29
4
LDHa3
5
LDHa1
6
time
TON b
Selectivity ketenone
alkenol
-
-
-
-
-
-
15, 4
0.56
51
49
Co0.37 Ni0.36 Al0.27
8.6, 4
0.31
74
26
Co0.31 Ni0.40 Al0.29
2.1, 5
0.28
75
25
-
-
-
-
-
-
7
LDHa2-1
Co0.33 Ni0.38 Al0.29
-
-
-
-
8
LDHa2-2
Co0.31 Ni0.39 Al0.30
-
-
-
-
9
LDHa2-2
Co0.31 Ni0.39 Al0.30
9.7, 24
2.6
54
46
10
-
-
-
-
-
-
h−1
a De ned as mmol of product formed per mmol of substrate added. b De ned as mmol of product formed per mmol of added catalyst and reaction time. c No internal standard was used.
Table 12:
The calculated turnover numbers (TON) and selectivities for the reactions
catalyzed by LDH-nitrate species. The selectivities are calculated neglecting the formation of epoxide which means that the actual selectivities are somewhat lower than indicated in this table. The TONs do not include the epoxide and are therefore actually higher than in this table. All the values have been rounded of to two signi cant values.
highest selectivity reached 67% (See Table 13 on page 58). Furthermore, it seems to be no connection between the selectivities and the elemental compositions for the catalysts but repeated reaction experiments are needed before any conclusions can be drawn. The two blank reactions showed that the substrate evaporated with 2.5 mmol/h respective 0.42 mmol/h, depending on wether oxygen was bubbled through the reaction mixture or not. As a result of this, the TONs were calculated based on the amount of product formed, and not on the amount of substrate left after reaction. Finally, the conversions and selectivites presented in Table 12 dier a lot from the values presented by Dr Yang (conversion 99% after 24 h with 70% selectivity for cyclohexenone [23]). The dierences are probably due to dierent calculation methods, See Section 4, page 56. This conclusion is drawn after doing ICP analyses (not shown) that showed that the catalyst synthesized by Dr.
Yang had similar elemental composition as the LDHa
batches prepared in this project.
3.2.2 Oxidation catalyzed by Co-POM Five reaction experiments with Co-POM as catalyst were performed. The reaction details are presented in Table 7 on page 34. GC analysis of the reaction mixture catalyzed by Co-POM4 (entry nr. 4, Table 7) revealed that no measurable conversion had taken place. Since an internal standard was used only for the reaction experiment using Co-POM5 as catalyst (entry nr. 5, Table 7) the conversion could be calculated only for that reaction. The TON and the conversion after 8 h of reaction were calculated to
5.7 h−1
respective
9.1 %. No product other than cyclohexanone was detected. The TON and the conversion for the reaction catalyzed by Co-POM5 were calculated using the same method as for LDH (See Section A.8).
52
The measured activity is much lower than that reported in literature (62 % conversion after 12 h and 100% selectivity to cyclohexanone [22]).
This is not a surprising result
since the catalyst used during reaction did not have the correct composition, as seen from ICP analysis (page 46).
4
Discussion
The aim for this project was to synthesize a highly selective oxidation catalyst by combination of a Co-substituted polyoxometalate catalyst with a CoNiAl layered double hydroxide catalyst. Since the synthesis of the polyoxometalate was unsuccessful the hypothesis that the intercalation of Co-POM into LDH would give a selectivity to the unsaturated ketone higher than 70 % could not be tested. This section discusses some of the results presented in the previous section.
4.1 Synthesis and catalyst characterization 4.1.1 LDH ICP analyzes of the CoNiAl-nitrate and CoNiAl-terephthalate species showed that coprecipitation at low supersaturation give the desired elemental composition while TEM, SEM and XRD showed that the prepared samples were poorly crystallized.
Further
examination of the surface characteristics can give information about what the dierences in oxidation activity between the CoNiAl-nitrate species stems from. SEM micrographs of the LDH species revealed that batch LDHa2-2 had a surface morphology consisting of small akes. the other batches.
That kind of morphology was practically absent in
The dierences in morphology can be due to dierences in storage
time. Longer storage time of the slurry before drying should give rise to increased crystallinity. This explanation can not explain the fact that LDHb1 (CoNiAl-adipate) did not exhibit the same surface morphology as LDHa2-2. It is however plausible that the acidic environment during ion exchange destroyed the small crystals. The IR spectra for the prepared LDH species did not indicate that carbonate had been incorporated into the structures. This is surprising since the system was opened frequently during the co-precipitation process and the slurries were dried in air. In the case of the LDHa batches, one possible explanation is that nitrate is intercalated into LDH in pref− erence over the HCO3 ion that predominates at pH 6-10 (given by analytical simulations ∗ with Medusa ). If the pH was slightly higher than 8 in the slurry during co-precipitation the concentration of carbonate was relatively low and incorporation of carbonate should not be a problem. Regarding the absence of carbonate in the LDHd structures, the same explanation can be used, i.e. that terephthalate is intercalated into LDH in preference over the hydrogen carbonate ion. Another explanation not related to the anions present in the system is that the higher density argon gas can protect the slurry from contact with air better than nitrogen can. No examples from the literature have been found that uses argon as a protective gas during co-precipitation and there is therefore no references ∗
Make Equilibrium Diagrams Using Sophisticated Algorithms by Ignasi Puigdomenech, Inorganic Chem-
istry, KTH, Sweden.
53
to compare with.
By performing co-precipitation in air clues about the reason for the
absence of carbonate could be found. CoNiAl-adipate could not be prepared by ion exchange between nitrate and adipate, which was con rmed by XRD analysis. This result is surprising since it has been reported that carboxylates can be successfully intercalated into ZnAl-nitrate LDH species by ion exchange [27]. Since IR analysis did not indicate any presence of carbonate the unsuccessful ion exchange was not due to high amounts of carbonate in the structure. One possibility is that adipate entered the interlayer space but arranged parallell to the layers.
This
explanation also explains the IR result that indicated that adipate was present in the analyzed sample. Repeated intercalation experiments with batch LDHa3 as well as other LDHa batches are needed before any conclusions can be drawn about the possibility to intercalate adipate into CoNiAl-nitrate species. Furthermore, the XRD pattern for batch LDHb1 indicates that CoNiAl-nitrate is a suitable precursor for direct intercalation of acidic POM species since the layered structure was not destroyed even at pH 4.
The
decrease in gallery heigh compared to that for batch LDHa3 can not be explained with available data. One explanation is that hydrogen bonding between carboxylate and LDH compresses the layers. XRD analyses gave some information about the structures of the prepared LDH species but since the XRD patterns consisted of broad and asymetric peaks the analysis of the patterns were di cult.
Altering the preparation and aging conditions could give more
ordered structures and more information can then be achieved by XRD analysis. Even though the XRD patterns were characterized by ill de ned peaks the calculated basal spacings agree well with values reported in literature. The CoNiAl-terephthalate species gave XRD patterns with inversed intensities for the
l
(00 ) peaks (See Figure 14 on page 39). XRD analyses of LDH containing benzoate gave inversed intensities similar to those for the LDHd batches, see [77].
This phenomenon
was explained to result from high electron density between the layers due to high water content. A similar pattern (intensity for peak (0012) > (009)) can be seen in XRD patterns for LDH intercalated with terephthalate [31] but it was not discussed. Results from [82]
l
show XRD patterns for LDH-terephthalate with inversed intensities for the (00 ) peaks. In that investigation the inversed intensities are absent for the wet samples and emerges after drying. This contradicts the results presented by [77] for LDH containing benzoate. If the inversed intensities in Figure 14 are due to a high water content between the layers then the drying time and temperature should aect the peak intensities. The XRD pattern for the black phase (LDHd1-1) that formed during prolonged drying times is inserted in Figure 14. There it is seen that the pattern for LDHd1-1 is practically the same as for the two batches dried for shorter times. The intensity for peak (006) is slightly higher than for (003) but the dierence in peak intensities is not as clear as for batch LDHd1-2. This observation supports the suggestion that the inversed intensity stems from high water content between the layers but it does not explain why batch LDHd2-2 did not give XRD patterns with inversed peak intensities. The only dierence during preparation of batch LDHd1 and LDHd2 was that the water volume in the three-necked ask at start was 0.01 dm3 respective 0.02 dm3 . For batch LDHd1, since 0.02 dm3 of the metal salt solution was not added (due to leaking) that batch actually contained more water per mole of formed LDH than batch LDHd2, which supports the proposed explanation.
l
Inversed intensities for the (00 ) peaks have been reported for LDH intercalated by terephthalate [77]. The explanation for this phenomenon was not increased electron density due
54
to accumulation of water but increased electron density due to regular collapse of every two layers.
That explanation was also given by [83] in an investigation of how water
amount aects the arrangement of terephthalate in LDH. Decreasing the water content gave collapse of some layers (horisontal arrangement of terephthalate) which gave strati ed structures consisting of alternating collapsed and expanded layers. By rehydration of the dried LDH species some of the collapsed layers were re-expanded by change from horisontal to vertical arrangement of terephthalate. The rehydrated, strati ed structure gave XRD patterns with inversed intensities similar to those in Figure 14. In this project, batch LDHd2-1 was dried without interruption while batch LDHd1-1 was removed from the oven while grinded after which it was reinserted (See page 24). The results reported by [77, 83] can explain the appearance of the XRD patterns in Figure 14. Since this kind of strati ed structure can form during repeated dehydration and hydration it also explains why batch LDHd1-1, that was dried without interruption, did not clearly show inversed peak intensities. One fact that indicates that the existence of interstrati ed collapsed and expanded layers
l
is not the actual reason for the inversed intensities seen in Figure 14 is that the (00 ) ◦ peaks in the XRD patterns should be separated by about 4 (2θ) [77, 83]. The rst peak should correspond to a basal spacing of 2.24 nm which is the sum of the thickness of one
l
collapsed and one expanded layer. The (00 ) peaks in Figure 14 are separated by about 6◦ (2θ). Another investigation by [82] showed that by rehydration of a previosly dried LDH specie, the gallery height increased through interstrati ed re-expansion. The inversed intensities for the resulting XRD patterns were present for LDH species with relatively high charge 2+ 3+ density (M /M = 0.5) but not for species with metal ratio M2+ /M3+ = 1. The opposite was found for LDH containing benzoate. Contradicting results are reported by [83] and [77] that found that layer collapse only resulted with relatively low layer charge densities 2+ 3+ (collapse for M /M = 3 but not for M2+ /M3+ = 2). It seems more reasonable that collapse is facilitated by lowering the layer charge density since the sterical hindrance between the intercalated species is lowered with decreased layer charge density. Since the elemental composition for LDHd1 and LDHd2 are practically the same, the dierences in their respective XRD patterns can not be explained by these observations. The XRD patterns for the interstrati ed, re-hydrated LDH specie in [82] are very similar to those achieved for the LDHd batches. Additionally, the XRD peaks presented in [82] ◦ are not separated by 4 (2θ) but have the same positions as the peaks for LDHd1-2 and LDHd2-2. This is in contradiction with the results in [83] and [77].
l
In conclusion, the inversed intensities for the (00 ) peaks seems to be dependent on both water content (drying process) and layer charge density. Since the literature in this area gives dierent explanations further experiments are needed in order to explain the typical appearance of the XRD patterns for the CoNiAl-terephthalates synthesized in this project.
4.1.2 POM and Co-POM The way that pH changed during synthesis of POM batch 8-10 (addition of HCl) can be understod, perhaps somewhat simpli ed, by studying reaction formula 1 on page 25. The formula suggests that the pH is constant as long as POM is produced. When the limiting reactant is consumed, the pH should start to decrease unless byproducts are formed that
55
consume the added oxonium ions. This reasoning explains why the pH stayed practically constant during a large part of the preparation process of POM batch 8-10. Regarding the characterization methods for POM and Co-POM, IR analysis is not a suitable characterization method. Even though the ICP analyses showed that the elemental composition was not correct, the corresponding IR spectra suggested that the samples contained the desired specie. This either means that the prepared batches contained the desired POM structures together with by-products, or that the formed structures gave IR spectra similar to that for the desired polyoxometalates.
When analyzing POM it
is strongly recommended that both IR and ICP analyses are performed.
It is further
recommended that polarography is used since this method has been used previously for characterization of this POM specie [25, 32, 73]. The preparation of Co-POM was unsuccessful but since the preparation consists of only one step the possibility to vary the preparation conditions is very limited. According to the preparation description in [25], after addition of the POM powder to the Co-acetate ◦ solution and increasing the temperature to 80 C the powder should be almost completely dissolved and the solution should have a brown color. The preparation of Co-POM was performed twise using the presumably correct POM (POM9) but without success. Failure to prepare Co-POM suggests that the prepared POM maybe is not the desired specie. This idea is supported by the fact that after complete addition of HCl during synthesis of POM, no precipitate resulted after 1h. Even after waiting one night, the obtained yield was not nearly as high as that given in the preparation description. The recommendation is thus to do polarography on prepared POM species and make sure the desired POM specie is synthesized before preparing Co-POM.
4.2 Catalytic activity The catalytic acitivities estimated in the reactions catalyzed by the CoNiAl-nitrate batches are conciderably lower than those reported by Dr. Yang (practically complete conversion after 24 h with 10 mg of catalyst and 50 mmol of substrate [23]). With similar process conditions the conversion reached in this project was 9.7% (Entry 9, Table 6 and 12). Comparison of the calculation methods used to determine the activities shows that the dierence in activity seems to be due to the choice of calculation method, and not due to actual activity dierences.
∗ In this project, the activity calculations are based on the amount of products formed at the end of reaction instead of the amount of substrate left after reaction. This method was used since the substrate leaves the system with the gas ow and the usual method to calculate conversion, equation 5, could thus not be used.
Conversion =
Initial substrate amount − Final substrate amount Initial substrate amount
(5)
Equation 5 was used by Dr. Yang to calculate the conversion. Since Dr. Yang only used tap water as coolant in the re ux condensers the calculated conversion should be coupled to signi cant errors due to evaporation of the substrate. One result that is not coupled to calculation dierences is that the TON for reaction 9 (LDHb2-2) in Table 12 on page 52 is ten times higher than TON for the other reactions. ∗
Conversion=product amount/initial substrate amount
56
This result presumably stems from dierences between the active surface areas for the systems.
Large amounts of catalyst were used in all reactions except reaction 9.
Poor
mixing probably isolated large parts of the catalyst powder that additionally may have aggregated to some degree in the hydrophobic media. Since the TON is inversely proportional to the catalyst amount low active surface area can result in low TON. This explanation is supported by the results in Table 12 were it is seen that the highest TON was achieved with the lowest catalyst amount.
Calculation of the turnover frequency
(TOF), de ned as amount of product formed per time unit, show the same thing as the conversion, i.e. that catalyst LDHa2-1 was the most active specie. The TOF values for reaction 3, 4, 5 and 9 in Table 12 are; 0.39, 0.21, 0.21 and 0.20 (mmol/h).
Addition-
ally, when comparing the activities for the LDH batches one have to remember that the catalyst amounts added during reaction were based on the estimated molar masses (See Table 8 on page 34). The dierent activities achieved may therefore result from errors in the estimated molar masses.
Since the catalysts were tested under slightly dierent
reaction conditions and without repeated experiments, further investigations are needed to determine wether or not the catalysts are unequally active and in that case what the possible dierences in oxidation activity stems from. The method used by Dr. Yang to estimate the product selectivities consisted of direct comparison of the peak areas for the products in the GC diagrams. The method used in this project showed that two equally large peak areas for cyclohexenone and cyclohexenol did not correspond to a molar amount of 1:1 but to 1.06:1 (See Section A.8). If Dr. Yang's method was used to estimate the product selectivities the ketone selectivity became lower (See Table 13).
This since the epoxide was not included in the selectivity estimations
done in this project (See Section A.8 for an explanation of this).
Table 13 shows that
the epoxide probably formed in signi cant amounts and therefore the epoxide should be included in the selectivity calculations to make it more accurate. Table 13 also reveals that the selectivity achieved with catalyst LDHa3 is very similar to that reported by Dr. Yang. This suggests that the LDH batches prepared in this project behave in the same way as that prepared by Dr. Yang. The signi cant dierences in product selectivity between catalyst LDHa1, LDHa2-1 and LDHa2-2 can not be explained by dierences in elemental composition. Catalyst LDHa1, LDHa2-1 and LDHa2-2 should give similar selectivities if the selectivity is a function of catalyst composition.
No trend in selectivity coupled to elemental composition can be
seen from the data in Table 13. The most surprising result is that catalyst LDHa2-2 gave considerably lower selectivity to the allylic ketone than did catalyst LDHa2-1. Since these two catalysts come from the same batch the dierences in selectivities probably stem from either morphological dierences or measurement errors. Furthermore, no trend can be seen for the selectivities and the dierences in reaction conditions. Additional experiments are needed before any conclusions about what causes the dierences in product selectivity can be made.
5
Conclusions
This master's thesis project has investigated the possibility to synthesize CoNiAl LDH species that can be used for the intercalation of polyoxymetalates. CoNiAl-nitrate and CoNiAl-terephthalate species can be successfully synthesized by co-precipitation and manual pH control is su cient to give the desired composition. CoNiAl-adipate could not be
57
Entry
Catalyst
Composition
#
Table 13:
Selectivity ketenone
alkenol
epoxide
1
LDHa2-1
Co0.33 Ni0.38 Al0.29
64
29
7.0
2
LDHa2-1
Co0.33 Ni0.38 Al0.29
64
29
7.0
3
LDHa2-1
Co0.33 Ni0.38 Al0.29
61
33
6.0
4
LDHa3
Co0.37 Ni0.36 Al0.27
67
28
5.0
5
LDHa1
Co0.31 Ni0.40 Al0.29
66
29
5.0
9
LDHa2-2
Co0.31 Ni0.39 Al0.30
50
35
15
Selectivities determined by Dr. Yang's method based on area comparison in
GC spectra. The entries are the same as in Table 12 on page 52.
prepared by ion exchange with adipate but addition of adipic acid gave a layered, expanded structure with decreased Co content. Thermal decomposition of CoNiAl-nitrate, followed by reconstruction, was unsuitable for CoNiAl-nitrate LDH species since Co-spinels form at relatively low temperatures. For characterization of LDH XRD, IR and ICP are useful tools. Additionally, it was shown that during preparation of
α−Keggin structured 9-tungstosilicate
(POM), pH and the structure of silicate are essential. Anhydrous silicate gave no product formation. IR was unsuitable for the characterization of the synthesized POM and Co-POM species and it is thus recommended that ICP and polarograhy are used together with IR for characterization of these species.
6
Future work and recommendations
To reach the goal stated in the introduction and test the hypothesis that a selectivity higher than 70% for the ketone can be achieved by combination of the Co-substituted POM and the CoNiAl LDH specie the following is recommended:
•
Prepare POM according to the description in [32] and verify that the synthesis is successful with ICP, IR and polarography.
•
When POM has been successfully prepared, try to prepare Co-POM according to [25]. If the synthesis is unsuccessful, try contacting the authours of [22] and ask for suggestions.
•
When Co-POM has been successfully prepared, test the oxidation activity in three ◦ dierent systems; (i) according to [22], (ii) with cyclohexenol as substrate at 60 C ◦ and (iii) with cyclohexene as substrate at 60 C . Reaction (i) should be performed to con rm the results presented by [22], reaction (ii) should be performed to check the synthesized Co-POM's oxidation activity with the unsaturated alcohol and reaction (iii) is needed to check wether or not Co-POM can oxidize the ole n.
•
Perform repeated oxidation reactions catalyzed by CoNiAl-nitrate and determine, accurately, the conversion and product selectivity for this system.
•
Intercalate Co-POM into the prepared CoNiAl species (LDH-nitrate, LDH-adipate and LDH-terephthalate). Check the results with IR, ICP, XRD and oxidation reactions.
58
One interesting aspect of LDH-POM catalysts that has not been found mentioned in literature is the reason why nonpolar substrates diuse into the polar interlayer space of LDH-POM catalysts. No literature concluding that the substrate is actually present in between the layers during reaction have been found during this project. The presence of the substrate between the layers in LDH seems to be an obvious fact in the consulted literature. Since the driving force for the diusion has not been mentioned and no results have been presented to prove this assumption, research is needed on this topic.
7
Acknowledgments
First of all I want to express my thanks to Professor Can Li for inviting me to his laboratory and for the advices he has given me during my time at DICP. I also want to thank Professor Lars J. Pettersson for valuable supervision and help through the whole process of this project. Furthermore, I thank Dr. Zhiwang Yang, Boyu Zhang, Changhao Wang, Dr Yan Liu, Xu Wang, Yinghao Li, Dr Jun Li, Zaihong Guan, Qianru Jin and Dr Shengmei Lu for valuable discussions and guidance in my laboratory work. I thank Rasmus for the support and for reading my drafts and giving constructive feedback. I also thank those who have helped me to do ICP, XRD, IR, SEM and TEM. A special thanks to Peilin Niu for all the help with the administrational work. Additionally I want to express my warmest thanks to all the other members of group 503. You have not only gladly helped me, you have also made my stay at DICP full of joy and made me feel most welcome. Thanks also to: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 16th of May, 2011)
59
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65
Synthesis,
J. Mol. Catal. A: Chem.,
[87] M. Salavati-Niasari, F. Farzaneh, and M. Ghandi. Oxidation of cyclohexene with tertbutylhydroperoxide and hydrogen peroxide catalyzed by alumina-supported manganese(II) complexes.
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Cyclohexene oxidation with tert-
butylhydroperoxide and hydrogen peroxide catalyzed by new square-planar manganese(II), cobalt(II), nickel(II) and copper(II) bis(2-mercaptoanil)benzil complexes supported on alumina. [89] M. tion
Salavati-Niasari, of
cyclohexene
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M.
Shaterian,
M.
R.
Ganjali,
withtert-butylhydroperoxide
and
P.
catalysted
Norouzi. by
host
Oxida(nanocav-
ity of zeolite-Y)/guest (Mn(II), Co(II), Ni(II) and Cu(II) complexes of N,N'bis(salicylidene)phenylene-1,3-diamine) nanocomposite materials (HGNM).
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Silica immobilized salicylaldimine Cu(II) and
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Highly eective catalysts of natural
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[97] Z. Y. Cai, M. Q. Zhu, J. Chen, Y. Y. Shen, J. Zhao, Y. Tang, and X. Z. Chen. Solvent-free oxidation of cyclohexene over catalysts Au/OMS-2 and Au/La-OMS-2 with molecular oxygen.
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66
A
Appendix
A.1 Other heterogeneous catalytic systems for allylic oxidation of cyclohexene Table 14 and 15 on the following pages present some heterogeneous catalytic systems developed for the allylic oxidation of cyclohexene.
67
◦
[84]
[Mn(H4 C6 N6 S2 )]NaY
[VO(bpy)2 ](SO4 )-Al2 O3
(Ru/Co/Ce)-THNO
none
CH2 Cl2
none
C2 H4 Cl2
Ambient
Ambient
Ambient
Ambient
70
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
84.4 (8)
84.6 (8)
87.6 (8)
90.3 (8)
90.3 (8)
95.2 (8)
98.5 (8)
(t [h])
82.9
83.2
84.2
85.3
82.1
87.5
85.8
95.0
Ketenone
-
13.1
14.7
10.5
11.34
11.5
10.1
3.8
Alkenol
3.1
3.7
1.1
4.2
6.56
1.0
4.1
-
Epoxide
S [%]
[85]
[Mn(bpy)2 ]Cl2 -Al2 O3
Ambient
TBHP
67.4 (12)
X [%]
1
[86]
[Mn(mabenzil)]/Al2 O3
Ambient
TBHP
Oxidant
2
[87]
[Mn(Sal-1,3-phen)]NaY
120
T [ C]
3
[88]
[Mn(habenzil)]/Al2 O3
Solvent
4
[89]
CrMCM-48
CH2 Cl2 CH2 Cl2 CH2 Cl2 C6 H5 Cl H2 O
Catalyst
5
[90]
Cu-amp AMPS
Ref.
6
[91]
Co(II)-complex on silica
Entry
7
[92]
99 (24)
44.4
70
18.0
40.2
26
6.7
-
10
0 60
56.5
c
94.5 (7)
a 75.5 (12)
0
70
0 none
70
22
none
78
CoNiAl-CO3 -LDH
none
0
100
Cobalt resinate
65.2
3.7
63 (8)
[23]
CS-Salophen Mn(III)-complex
34.8
11.2
2.5
50 (27)
[94]
61.2 (8)
74.2
12.4
TBHP
11
[95]
70
52.2 (24)
48.5
40.3
60
12
none
70
39.1
a
44.0 -(6)
48.0 (24)
50
a
8
[93]
d
9
a
10
CH3 CN
13 [Ni(Me6 [14]aneN4 )]
none
-NaY
[96] CrMCM-41
2+
14 [4]
15
80
[97]
none
H O2 /O2 2 O2 O2 O2 O2 O2 O2 b H2 O2
Au/La-OMS-2
16 MX-PW
Ambient
[37]
Dioxane
17
a No data given.
b Oxygen pressure 0.4 MPa.
c 18.8 percent 2-cyclohexene-1-hydroperoxide
A summary of investigated heterogeneous catalysts used for allylic oxidation of cyclohexene. In
d Tert-butylhydroperoxide (TBHP)
Table 14:
Furthermore, this table only includes the systems with relatively high values for conversion
the case a mentioned reference has examined multiple heterogeneous catalysts only the most active species are presented.
and/or selectivity. Note that the conversions presented above are achieved in systems with dierent ratios of
catalyst and oxidant, dierent reaction times and temperatures. Furthermore, this table only includes data
for reactions using fresh catalysts ( rst run catalysts). For a description of the catalyst systems, see Table 15 and references. The oxygen pressure is 1 atm if not stated dierently.
68
69
[95]
[96]
[4]
[97]
[37]
13
14
15
16
17
[91]
8
[94]
[90]
7
12
[89]
6
[23]
[88]
5
11
[87]
4
[92]
[86]
3
[93]
[85]
2
9
[84]
1
10
Ref.
Entry
Table 15:
More descriptive names for the catalysts mentioned in Table 14.
Meixnerite intercalated with peroxotungstate acid modi ed with phosphoric acid
Au/La incorporated into manganese oxide molecular sieve
Mesoporous Chromosilicate (molecular sieve)
Nickel(II)-complex encapsulated into the nanopores of zeolite-Y
Chitosan supported Salophen Mn(III)-complex
Cobalt resinate
CoNiAl-CO3 (Layered double hydroxide)
Silica immobilized salicylaldimine Co(II)-complex
Schi base copper complex immobilized on aminopropyl silica
Mesoporous Chromosilicate (molecular sieve)
Alumina-supported manganese complex with tetradentate Schi-base ligand bis(2-hydroxyanil)
Manganese complex with tetradentate Schi-base ligand N,N'-bis(salicylidene)phenylene-1,3-diamine encapsulated into the nanopores of zeolite-Y
Alumina-supported manganese complex with tetradentate Schi-base ligand bis(2-mercaptoanil)benzil
Alumina-supported manganese complex with bipyridine ligands
Manganese complex with macrocyclic tetradentate ligand encapsulated into the nanopores of zeolite-Y
Alumina-supported vanadyl complex with ligands of bipyridine
Trimetallic hybrid nano-mixed oxide
Catalyst
Method 1
Initial oven temperature and hold time Front detector temperature Front inlet temperature
60◦ C, 1 min 260◦ C 260◦ C
Front inlet pressure
69 bar
Ramp #
1
◦ Heating rate [ C/min] ◦ Ramp temperature [ C] Hold time [min] Method 2
Initial oven temperature and hold time Front detector temperature Front inlet temperature
3
4
10
10
10
20
80
100
120
140
1
1
1
1
◦
100 C, 0 min 260◦ C 260◦ C
Front inlet pressure
69 bar
Ramp #
1
◦ Heating rate [ C/min] ◦ Ramp temperature [ C]
5 180
Hold time [min]
Table 16:
2
3
The operation parameters for the GC analyses.
A.2 Operation parameters during GC analysis Two dierent operation methods were used during GC analysis.
These are denoted
Method 1 and Method 2 and the run parameters are summarized in Table 16. For each sample, 1
µl
was manually injected with a syringe.
Method 1 was used for all of the
analyses except for reaction 4 and 5 catalyzed by Co-POM.
A.3 Calculation of the molar mass of CoNiAl-nitrate The following calculations, based on the results from the ICP analysis of batch LDHa2-1, give the experimental molar mass of LDH. The experimental molar mass of LDH,
MLDH
is given by:
mAl,ICP
nAl MAl = mLDH,ICP nLDH
(6)
MLDH
ICP analysis gave the ratios of the cations in the LDH sample (LDHa2-1) (rounded of ):
[Co0.33 Ni0.38 Al0.29 ] The moles of aluminium cations per mole of LDH is thus
mAl,ICP = 3.0239 × 10−4 g MAl
nAl nLDH
=0.29.
(the mass of aluminium in the analyzed ICP-sample)
is the molar mass of aluminium.
mLDH,ICP = 5.0 × 10−3 g
(the weight of the LDH sample used for preparation of the ICP
sample) Insertion of the above values in equation 6 gives the molar mass of LDHa2-1:
MLDH = 129 g/mol 70
A.4 Calculation of the charge density for CoNiAl-nitrate The following calculations were used to determine the charge density of LDH and the amount of LDH that can be balanced by 0.02 mmol Co-POM. The basis for the calculations is 0.02 mmol Co-POM since this is the amount of Co-POM that was used for the oxidation reactions in the literature consulted (See [22]). Each Co-POM has a negative charge of 10. The anion exchange capacity (AEC) that LDH needs in order to intercalate this amount of negative charge is:
AECLDH = ChargeCoPOM × nCoPOM = 0.2 × 10−3
mole
The amount of positive charge needed to balance POM's negative charges can be expressed ∗ as a function of the moles of Al per gram of LDH :
AECLDH = mLDH × where
mLDH
nAl,ICP mLDH,ICP
[mole]
(7)
is the mass of LDH needed to intercalate 0.02 mmol of Co-POM.
The charge density for CoNiAl-NO3 (LDHa2-1) is:
nAl,ICP /mLDH,ICP = 2.2 × 10−3 [mole Al/g LDH] (given by ICP, see Section A.3 on the preceding page) Equation 7 gives the mass of LDHa2-1 needed to balance the charge of 0.02 mmol of Co-POM:
mLDH = 0.09 g
A.5 Estimation of the mass CoNiAl-nitrate (LDHa2) formed during co-precipitation The estimation is based on the assumption that all the nickel ions
†
added during syn-
thesis of batch LDHa2-1 were incorporated in LDH and that the formed slurry had a homogeneous composition with the metal ion ratio given by ICP (i.e. The mass of LDH formed during co-precipitation,
mLDH ,
mLDH = MLDH × where A.3),
[Co0.33 Ni0.38 Al0.29 ]).
can be expressed as follows:
nNi xNi
MLDH (= 129 g/mol) is the molar mass of LDHa2-1 estimated by ICP (See Section nNi (= 3.6 mmol) is the moles of nickel added during co-precipitation and xNi (=
0.38) is the moles of nickel per mole of LDH (given by ICP analysis of batch LDHa2-1). The mass of LDH formed during co-precipitation is thus:
mLDH ≈ 12.2 g This mass was used to estimate the amount of slurry withdrawn during synthesis of the two LDHb batches. ∗
Equation 7 can be understood by remembering that it is the aluminium cations that give LDH its net
positive charge.
†
Nickel limited the amount of LDHa2-1 formed during synthesis.
71
The mass withdrawn with the 40 ml of slurry used for synthesis of LDHb1 was calculated as below: Amount of slurry in the three necked ask at start Mass LDHa2 withdrawn for drying
≈
Mass of LDH remaining in the ask Volume of the remaining slurry
≈
12.2 g
3.7 g (weight dry mass, see Table 2 on page 21)
≈
0.16
Mass of LDH in the withdrawn 0.040
≈
8.5 g
dm3
dm3 slurry ≈ 2.1 g (slurry withdrawn for preparation
of batch LDHb1) Additionally, the assumption that was made during the estimation of the mass of LDH in batch LDHa3 was that batch LDHa3 had the same cation ratio as batch LDHa2. The LDH concentration in batch LDHa3 was thus estimated to be 0.068 g LDH per ml slurry. This concentration was used during preparation of batch LDHb2.
A.6 Details about the ion exchange experiments Two ion exchange experiments were performed to remove the unreacted cobalt from the Co-POM solution and to transform the sodium salt of Co-POM to its corresponding acid − form. A strong acid cation exchange resin (SO3 ) with an ion exchange capacity of 4.2 ∗ mmol/ g resin was used . Prior to ion exchange the resin was washed with water several times before left in a glass beaker (containing a volume of water 3 times that of the resin) for one day. After that, HCl was added to saturate the resin with protons. For batch 1, about 70 g of resin (contained 50 wt% of water according to the manufacturer) was used and the pH of the acid water was 1-2 after one nights saturation process. The proton saturated resin was poured into a glass column and washed with water until the eluate had a pH of 7. After ion exchange the eluate had a pH of 7. For batch 3, a more detailed experimental setup was performed. To 24 g of resin 4.8 ml of concentrated HCl was added. The mixture was left during one night and the resulting resin had a pH below 0.5 (the pH did not change during the night). The cation exchange 3 was performed during 9 h and the nal eluate (0.2dm ) had a pH around 1.5.
A.7 Color dierences between the LDH batches The colors of the prepared LDH batches are seen in Figure 26 on the next page. The color dierences are most obvious for the LDHa batches. The powder of batch LDHa1 is darker than the original slurry since the long drying time resulted in a black phase. The color of the LDHa2 slurry became darker over time as can be seen by comparison of the dried powder and the 3 months old slurry. Batch LDHa3 was 1 month old and the slurry had kept the pink color. The decomposed LDHa2 resulted in black powder (LDO). The LDHd2 batch had been stored 1.5 months when the photograph was taken and the color had changed from light blue to a more gray color. For the LDHd batches the black, glass like phases were included in the picture. The two powders present for LDHd2 are the grinded blue phase (above) and the grinded black phase (below). The LDHc1 powder is dark since the darkened LDHa2 slurry was used to prepare this batch. ∗
The particle size and porosity is unknown.
72
Figure 26:
A photograph of some of the prepared LDH samples intended to show the
color dierences for the batches.
73
A.8 Calculation of turnover numbers and selectivities The TON is de ned as the mmol of product formed per mmol of catalyst and reaction time.
The amount of product formed during reaction was calculated by preparing ref-
erens samples containing 1 mmol of the product (cyclohexenone, cyclohexenol respective cyclohexanone) and 1 mmol of the internal standard (n-decane). Two samples of every type were prepared to correct to some degree for preparation and injection dierences. GC analyses of the reference samples gave the ratios between the product area and the internal standard area. This fraction was denoted
x
and by using the following equation
the product amount formed during reaction could be determined:
nprod Aprod =x AI nI where
Ai
ratios,
x,
is the peak area for
i
and
ni
is the amount of
(8)
i
in the sample.
The area
were determined to 0.68, 0.72 and 0.65 for cyclohexenone, cyclohexenol and
cyclohexanone. The areas were measured manually using the integration tool included in the GC software. Equation 8 was used to determine TON since the two blank reactions (entry nr. 6 and 10 in Table 12 on page 52) showed that the substrate evaporated with 2.5 mmol/h respective 0.42 mmol/h. The selectivities were determined with the following formula:
S1 = were
Si
nprod,1 nprod,1 + nprod,2
[%] is the selectivity for product
i.
The selectivity for the epoxide could not be
determined since no reference sample, necessary to determine
74
x, was prepared.
