molecules Article
Gold Nanoparticles Deposited on Surface Modified Carbon Xerogels as Reusable Catalysts for Cyclohexane C-H Activation in the Presence of CO and Water Ana Paula da Costa Ribeiro 1 , Luísa Margarida Dias Ribeiro de Sousa Martins 1,2, *, Sónia Alexandra Correia Carabineiro 3, *, José Luís Figueiredo 3 and Armando José Latourrette Pombeiro 1 1 2 3
*
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal;
[email protected] (A.P.C.R.);
[email protected] (A.J.L.P.) Chemical Engineering Department, Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa, R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal Laboratório de Catálise e Materiais, Laboratório Associado LSRE-LCM, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal;
[email protected] Correspondence:
[email protected] (L.M.D.R.S.M.);
[email protected] (S.A.C.C.); Tel.: +351-21-831-7226 (L.M.D.R.S.M.); +351-22-041-4907 (S.A.C.C.)
Academic Editor: Georgiy B. Shul’pin Received: 12 February 2017; Accepted: 3 April 2017; Published: 9 April 2017
Abstract: The use of gold as a promotor of alkane hydrocarboxylation is reported for the first time. Cyclohexane hydrocarboxylation to cyclohexanecarboxylic acid (up to 55% yield) with CO, water, and peroxodisulfate in a water/acetonitrile medium at circa 50 ◦ C has been achieved in the presence of gold nanoparticles deposited by a colloidal method on a carbon xerogel in its original form (CX), after oxidation with HNO3 (-ox), or after oxidation with HNO3 and subsequent treatment with NaOH (-ox-Na). Au/CX-ox-Na behaves as re-usable catalyst maintaining its initial activity and selectivity for at least seven consecutive cycles. Green metric values of atom economy or carbon efficiency also attest to the improvement brought by this novel catalytic system to the hydrocarboxylation of cyclohexane. Keywords: gold nanoparticles; C-H activation; hydrocarboxylation; cyclohexane; catalyst recycling; water; green metric
1. Introduction The single-pot carboxylation of Cn alkanes to Cn+1 carboxylic acids by CO is a particularly attractive alkane functionalization procedure [1–17], in view of the increasing industrial demand for carboxylic acids and of the drawbacks of their current synthetic methods [18–21]. However, catalytic carboxylation of saturated hydrocarbons, such as alkanes, requiring C-H activation, is a considerable chemical challenge, in particular for the least reactive lower alkanes (1 to 6 carbon atoms). Fujuwara et al. [1,2,4] found that a cyclohexane undergoes carboxylation to cyclohexanecarboxylic acid (4.3% yield relative to the substrate) with CO and peroxodisulfate in trifluoroacetic acid (TFA) at 80 ◦ C, catalysed by a Pd(II)/Cu(II) system. The strongly acidic medium is required due to the inertness of the alkane. In recent years, intensive research has been focused on the improvement of alkane carboxylation towards future sustainable carboxylic acid production [10,11,15,17,22], namely regarding the use of greener and safer solvents. Hydrocarboxylation of cyclohexane to cyclohexanecarboxylic acid with CO Molecules 2017, 22, 603; doi:10.3390/molecules22040603
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and water (72% yield), in the presence of peroxodisulfate oxidant, in water/acetonitrile medium at ◦ C and in the presence of a tetracopper(II) catalyst has been achieved [10]. In this improved circa 50 50 at circa °C and in the presence of a tetracopper(II) catalyst has been achieved [10]. In this improved system, water plays playsthe theroles rolesofofboth bothreactant reactant and solvent [10]. In contrast to carboxylation the carboxylation in system, water and solvent [10]. In contrast to the in TFA TFA [4,6–8], the carboxylation of cyclohexane by CO in the H O/MeCN/K S O system proceeds 2 2 2 8 proceeds to some [4,6–8], the carboxylation of cyclohexane by CO in the H2O/MeCN/K 2S2O8 system to someinextent in the absence any metal catalyst, leading to the formation (uptoto12% 12% yield) yield) of extent the absence of anyofmetal catalyst, leading to the formation (up of cyclohexanecarboxylic more efficiently in the presence of aof metal (V, Mn, cyclohexanecarboxylicacid. acid.However, However,ititcan canproceed proceed more efficiently in the presence a metal (V, Fe orFe Cu) [10,11,22], leading to higher yieldsyields of carboxylic acid. acid. Mn, orpromotor Cu) promotor [10,11,22], leading to higher of carboxylic In farfar any tested homogeneous catalytic systems [22][22] havehave the In spite spite of ofthe theabove aboveachievements, achievements,soso any tested homogeneous catalytic systems drawback of not being re-usable, thus the search for a more efficient and eco-friendly heterogeneous the drawback of not being re-usable, thus the search for a more efficient and eco-friendly processes for the synthesis such industrially important commodities continues. Goldcontinues. catalysts heterogeneous processes forofthe synthesis of such industrially important commodities are currently “hot topic” of research, showasapplication in many reactions industrial Gold catalystsaare currently a “hot topic”as ofthey research, they show application in manyofreactions of and environmental importance [23–28]. Several variables have been considered as important factors industrial and environmental importance [23–28]. Several variables have been considered as influencing the structure, reactivity, and catalytic activity.and Among them activity. are the method preparation, important factors influencing the structure, reactivity, catalytic Amongofthem are the the nature the support, particularly, the gold nanoparticle size method of of preparation, theand nature of the support, and particularly, the[23–28]. gold nanoparticle size [23–28]. Herein, report the use of gold as promotors ofascyclohexane Herein, wewe report the use nanoparticles of gold nanoparticles promotorshydrocarboxylation. of cyclohexane We have chosen the We above-mentioned protocol [10] and theprotocol use of gold as athe metal hydrocarboxylation. have chosen the above-mentioned [10] and use promotor of gold asina n Bu N][AuCl ], Au n view of the ability of [ C-scorpionate gold complexes, and Au nanoparticles 4 4 metal promotor in view of the ability of [ Bu4N][AuCl4], Au C-scorpionate gold complexes, and Au to catalyze thetoperoxidative of cyclohexane to cyclohexane KA oil (cyclohexanol cyclohexanone nanoparticles catalyze theoxidation peroxidative oxidation of to KA oiland (cyclohexanol and mixture) [29,30]. Moreover, gold nanoparticles are supported carbon xerogels different cyclohexanone mixture) [29,30]. Moreover, gold nanoparticles areon supported on carbonwith xerogels with treatments, in order toinprovide recyclable forcatalysts the one-pot hydrocarboxylation of cyclohexane different treatments, order to provide catalysts recyclable for the one-pot hydrocarboxylation of to cyclohexanecarboxylic acid (Schemeacid 1). (Scheme 1). cyclohexane to cyclohexanecarboxylic
Scheme Hydrocarboxylation of of cyclohexane cyclohexane to cyclohexanecarboxylic Scheme 1. 1. Hydrocarboxylation cyclohexanecarboxylic acid catalysed by gold gold nanoparticles nanoparticles supported supported on on carbon carbon xerogels. xerogels.
To the the best best of of our our knowledge, knowledge, this this is is the the first first report report dealing dealing with with hydrocarboxylation hydrocarboxylation of of alkanes alkanes To using gold nanoparticles as catalysts. In fact, the only reports found in literature so far, dealing with using gold nanoparticles as catalysts. In fact, the only reports found in literature so far, dealing with hydrocarboxylation of hydrocarbons using gold catalysts, refer to hydrocarboxylation of alkynes and hydrocarboxylation of hydrocarbons using gold catalysts, refer to hydrocarboxylation of alkynes and to gold gold complexes complexes (not (not gold gold nanoparticles) nanoparticles) [31,32]. [31,32]. Moreover, the only only report report for for hydrocarboxylation hydrocarboxylation to Moreover, the using carbon materials deals with 1,3-butadiene using a Rh(I) complex immobilized on activated activated using carbon materials deals with 1,3-butadiene using a Rh(I) complex immobilized on carbon as the catalyst [33]. Therefore, our work is also the first report of such reaction carried out carbon as the catalyst [33]. Therefore, our work is also the first report of such reaction carried out using using carbon based catalysts. carbon xerogelxerogel based catalysts. 2. Results and Discussion 2.1. Characterisation of Xerogel Supports Supports The carbon carbon xerogel xerogelwas wasused usedasas a support in original its original as prepared oxidised a support in its formform as prepared (CX),(CX), oxidised (-ox), (-ox), and oxidised with nitric acid subsequently and subsequently treated sodium hydroxide (-ox-Na). and oxidised with nitric acid and treated with with sodium hydroxide (-ox-Na). The The characterization details of these samples found Table1 1and andFigure Figure1,1, which which include the characterization details of these samples cancan be be found in in Table textural textural and and surface surface characterisation. characterisation. Table pore size, size, as as expected. expected [29,30,34–37]. Table 11 shows shows that CX is mainly mesoporous and has a large pore By comparing the parameters of the oxidized (CX-ox) samples with those of the parent material (CX), it liquid phase activation slightly decreased the surface area andarea poreand volume, it isisobserved observedthat that liquid phase activation slightly decreased the surface pore probably volume, due to some wallpore collapse to the or presence of numerous oxygen-containing surface groups, probably duepore to some wall or collapse to the presence of numerous oxygen-containing surface groups, which might partially block the access of N2 molecules to the smaller pores [37]. Nitric acid consumes large amounts of carbon atoms and changes the structure of pores, merging some of them together.
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which might partially block the access of N2 molecules to the smaller pores [37]. Nitric acid consumes large amounts of carbon atoms and changes the structure of pores, merging some of them together. Molecules 2017, 22, 603
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Table 1. Description and characterisation of carbon xerogel samples: surface area (SBET ), total pore Table(V 1.pDescription and characterisation of carbonvolume xerogel(V samples: surfacearea area(S(Sexternal BET), total pore volume ), average mesopore width (L), micropore ), obtained micro ), external ◦ volume (V p ), average mesopore width (L), micropore volume (V micro ), external area (S external ), obtained by adsorption of N2 at −196 C, and amounts of CO and CO2 desorbed, as determined by temperature by adsorption of N2 at −196 °C, and amounts of CO and CO2 desorbed, as determined by temperature programmed desorption (TPD). programmed desorption (TPD). SBET SBET (m2 /g) (m2/g) CX CX 604604 CX-ox 570 CX-ox CX-ox-Na 570560 CX-ox-Na 560 Sample
Sample
V Vp 3p (cm /g) 3
(cm /g) 0.91 0.91 0.80 0.80 0.75 0.75
LL (nm)
(nm) 13.7 13.7 18.8 18.8 17.6 17.6
Vmicro Vmicro (cm3 /g)
(cm3/g) ~0 ~0 0.038 0.038 0.036 0.036
Sexternal Sexternal (m2 /g)
(m2/g) 604 604 512 512 496 496
CO CO (µmol/g)
(µmol/g) 492 492 4609 4609 3720 3720
CO2 CO2 (µmol/g)
(µmol/g)
135 135 3774 3774 3793
3793
Figure 1 1shows desorbingin indifferent differenttemperature temperature ranges, Figure showsthe theidentification identificationof of types types of of groups groups desorbing ranges, according to what is already established in the literature [37–41]. Upon oxidation treatment, the amounts according to what is already established in the literature [37–41]. Upon oxidation treatment, the of amounts CO and of COCO (Table 1 and Figure 1). Figure Figure 1). 1a Figure shows 1a theshows CO desorption 2 increase and COenormously 2 increase enormously (Table 1 and the CO profiles. The effect of oxidation treatments is also seen in these data. The largest CO evolution desorption profiles. The effect of oxidation treatments is also seen in these data. The largest CO of evolution the -ox materials starts at around 350at ◦ C, whereas -ox-Naforsamples, the temperature of the -ox materials starts around 350 for °C, the whereas the -ox-Na samples, the is slightly higher.isThat can be due to thecan destruction carboxylic anhydrides (that anhydrides desorb as CO and temperature slightly higher. That be due toofthe destruction of carboxylic (that CO in that temperature range [37–41]) also contributing to the increase of the carboxylate groups, desorb as CO and CO 2 in that temperature range [37–41]) also contributing to the increase of the 2 groups, as proposed in literature [42]. Moreover, theisprofile -ox-Na is a little sharper, ascarboxylate proposed in literature [42]. Moreover, the profile of -ox-Na a littleofsharper, more intense, and more intense, andathas its maximum at a higher temperature of theThis -ox sample. This suggests has its maximum a higher temperature than that of thethan -ox that sample. suggests that phenol that phenol groupsas(that COconverted [37–41]) are converted intowhich phenolates, which are[42]. moreThe stable groups (that desorb CO desorb [37–41])asare into phenolates, are more stable CO2 [42]. The CO 2 desorption profiles (Figure 1b) of the -ox and -ox-Na materials also show a considerable desorption profiles (Figure 1b) of the -ox and -ox-Na materials also show a considerable increase ◦ C, in theofamount of carboxylic acid(which groups decompose (which decompose in the temperature 200– in increase the amount carboxylic acid groups in the temperature range range 200–350 °C, as described in the literature [37–41]) when compared to the original materials. as 350 described in the literature [37–41]) when compared to the original materials. Phenols
Carbonyls/ Quinones
Anhydrides
Carboxylic acids
Anhydrides
CX-ox-Na
CX-ox
Lactones
CX-ox CX-ox-Na
CX
(a)
CX
(b)
Figure1. 1. Temperature Temperature programed programed desorption desorption (TPD) Figure (TPD) profiles profiles for for the thecarbon carbonxerogel xerogelmaterials. materials. Desorption of CO (a) and CO2 (b) is shown, with identification of types of groups desorbing in Desorption of CO (a) and CO2 (b) is shown, with identification of types of groups desorbing in different temperature ranges (the different colour bars are only indicative of the temperature ranges different temperature ranges (the different colour bars are only indicative of the temperature ranges expected for the desorption of different groups, and do not provide any information on their expected for the desorption of different groups, and do not provide any information on their amounts). amounts).
2.2. Characterisation 2.2. CharacterisationofofGold GoldCatalysts Catalysts The nominal gold loading Table 2, The nominal gold loadingwas was3% 3%wt. wt.(see (seeMaterials Materialsand andMethods). Methods). However, However, as as shown shown in in Table only CX showed an actual loading near that value (2.8%). Regarding the functionalized samples, it has 2, only CX showed an actual loading near that value (2.8%). Regarding the functionalized samples, it been reported that surface oxygen groups can act as anchoring sites for metallic precursors [43,44]. has been reported that surface oxygen groups can act as anchoring sites for metallic precursors However, in the particular case of Au on xerogels by theby colloidal method usedused in this work [43,44]. However, in the particular caseloaded of Au loaded on xerogels the colloidal method in this (see Materials and Methods), it seemsit that thethat presence of oxygenated groupsgroups is detrimental for Au work (see Materials and Methods), seems the presence of oxygenated is detrimental for Au loading, as much less gold was loaded on CX-ox (1.4%) and even less on CX-ox-Na (0.5%), as depicted in Table 2.
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loading, as much less gold was loaded on CX-ox (1.4%) and even less on CX-ox-Na (0.5%), as depicted in Table 2. Molecules 2017, 22, 603
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Table 2. Average gold nanoparticle size and dispersion (calculated from TEM measurements) and gold Table(calculated 2. Average by gold nanoparticle sizespectroscopy) and dispersionon(calculated from TEM xerogel measurements) and loading atomic absorption the different carbon materials. gold loading (calculated by atomic absorption spectroscopy) on the different carbon xerogel materials. Sample AuAverage AverageSize Size(nm) (nm) Metal (%) Gold Sample Au Metal Dispersion Dispersion (%) GoldLoading Loading(%) (%) Au/CX 16.6 6.9 ** 2.8 Au/CX 16.6* * 6.9 2.8 Au/CX-ox 14.2 8.1 1.4 Au/CX-ox 14.2 8.1 1.4 Au/CX-ox-Na 13.7 8.4 0.5 Au/CX-ox-Na 13.7 0.5 *—calculated thespherical sphericalnanoparticles, nanoparticles, nanorods. *—calculatedtaking takinginto intoaccount account only only the notnot thethe nanorods.
Figure 2 showssome someselected selected transmission transmission electron (TEM) images of the It Figure 2 shows electronmicroscopy microscopy (TEM) images of samples. the samples. can be seen that gold nanoparticles are deposited mostly in the form of nanorods on CX (Figure 2a,b). It can be seen that gold nanoparticles are deposited mostly in the form of nanorods on CX (Figure 2a,b). Although spherical nanoparticles are usually obtained with the colloidal method [30,45–47], Although spherical nanoparticles are usually obtained with the colloidal method [30,45–47], nanorods nanorods are also often reported in literature [48,49]. They are usually formed through a seedare also often reported in literature [48,49]. They are usually formed through a seed-mediated method, mediated method, which includes the formation of “seed” nanoparticles and the growth of such which includes the formation of “seed” nanoparticles and the growth of such seeds into rods [49]. seeds into rods [49]. Also other agglomerates of particles are seen (Figure 2b). Au on CX-ox and CXAlso other agglomerates of particles are seen (Figure 2b). Au on CX-ox and CX-ox-Na (Figure 2c,d, ox-Na (Figure 2c,d, respectively) show more regular sphere-like particles, which are larger on CX-ox respectively) (Figure 2c).show more regular sphere-like particles, which are larger on CX-ox (Figure 2c).
(a)
(b)
(c)
(d)
Figure 2. TEM images carbon xerogel samples: CXCX-ox (a,b),(c)CX-ox (c) and CX-ox-Na (d). Gold Figure 2. TEM images carbon xerogel samples: CX (a,b), and CX-ox-Na (d). Gold nanoparticles arespots, seen as darker spots,(a,b), withor rod-like (a,b), or spherical/obliquous arenanoparticles seen as darker with rod-like spherical/obliquous (c,d) shapes. (c,d) shapes.
Table 2 shows a summary of the values of the average gold nanoparticle size and dispersion. It can be seen that the average size is larger for CX (16.6 nm, calculated only for spherical nanoparticles, as nanorods larger than 100 nm are also observed—Figure 2a,b). 14.2 nm was found for CX-ox and
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Table 2 shows a summary of the values of the average gold nanoparticle size and dispersion. It can be seen that the average size is larger for CX (16.6 nm, calculated only for spherical nanoparticles, as nanorods larger than 100 nm are also observed—Figure 2a,b). 14.2 nm was found for CX-ox and 13.7 nm for CX-ox-Na. In both cases, there was agglomeration of gold nanoparticles (one example is shown in Figure 2c). Consequently, dispersion is smaller on CX and larger on CX-ox-Na, although the value of 8.4% can still be considered low. In a previous work of ours, dealing with gold nanoparticles on several carbon materials, including xerogels [30], for 1% Au loading, spherical nanoparticles of ca. 4.4 nm were obtained with a metal dispersion of 26.2%. Most likely, the larger loading used in this work promoted agglomeration as well as nanorods formation in the case of CX. Although apparently detrimental for gold loading, the presence of surface oxygenated groups seems beneficial for the formation of spherical, less agglomerated nanoparticles. As stated above, it was previously reported that surface oxygen groups can act as anchors for the metallic precursors [43,44] and that can result in smaller and better dispersed nanoparticles (at least compared with the unfunctionalized support). 2.3. Catalytic Results Gold nanoparticles supported on different carbon xerogel samples exhibited different catalytic activities (Figure 3). The desired cyclohexanecarboxylic acid was achieved with up to 54.5% yield with Au on CX-ox-Na (entry 1, Table 3). However, KA oil (cyclohexanol and cyclohexanone mixture) and cyclohexane-1,2-diol were also obtained, although in much lower ( CX-ox > CX (Figure 3). This can be related with the smaller gold nanoparticle size found on CX-ox-Na and CX-ox, compared to that of CX (Table 2), which is expected to affect catalytic activity [23–28]. It is also well known that the presence of alkali metals enhances the activity of gold catalysts [50–52]. Thus, the presence of sodium carboxylate and phenolate groups might also be beneficial to the catalytic activity (although the supports alone, without gold, revealed no catalytic activity). Higher activities (for the same Au amount and reaction conditions) were found for gold nanoparticles deposited on carbon xerogel samples, when compared to HAuCl4 ·3H2 O used in homogeneous medium, i.e., in aqueous solution (Figure 3, Table 3). The catalytic activity of gold nanoparticles on xerogels is also dependent on the reaction conditions. It was found that high pressures of CO do not enhance the production of carboxylic acid, the best being the 1:1 molar ratio of CO relative to cyclohexane (compare entries 1 and 2 of Table 3). In all hydrocarboxylation systems known to date, a 10:1 molar excess of CO relative to substrate is required (see conditions of Table 4) [22]. This is a very important advantage for our system, in terms of the environment and in process safety. Table 3. Selected data a for cyclohexane hydrocarboxylation promoted by Au/CX-ox-Na.
Entry
Au/µmol
P(CO)/atm
Temperature/◦ C
1 2 3 4 5
2 2 2 2 20
2 20 2 2 2
50 50 30 80 50
a
Total TON 375 311 111 245 23
Yield/% c CyCOOH Cy-H =O CyOH Cy-H (OH)2
b
54.5 28.3 12.0 24.1 19.9
9.1 19.4 5.3 14.4 12.3
7.1 14.5 4.9 9.5 12.5
4.3 3.9 0.8 0.9 0.5
Reaction conditions: cyclohexane (1.00 mmol), p(CO) = 2–20 atm, K2 S2 O8 (1.50 mmol), catalyst (2–20 µmol), H2 O (3.0 mL)/MeCN (3.0 mL), 30–80 ◦ C, 6 h in an autoclave (13.0 mL capacity); b Turnover number = moles of products per mole of catalyst; c Moles of product per 100 mol of cyclohexane; Cy = C6 H11 .
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60
60
6060 60 60
50
50
5050 50 50
20
Products yield /%
Products Productsyi yiel eldd /% /% Products yield /%
10
10
1010 10 10
0
0
40 30 20
Products yield /%
Products yield /%
Moreover, the Au/CX-ox-Na/CO/K2 S2 O7 /H2 O/MeCN system exhibits its maximum performance at the mild temperature of 50 ◦ C (compare entries 1, 3 and 4, Table 3) requiring a significantly (up to2017, 16603 times) lower systems Molecules 2017, 22, 603 amount of metal promoter than in the previously reported 6 12 of 12 Molecules 2017, 22, 603 Molecules 2017, 22, 603 6of of 124). Molecules 22, 603 6 (Table of612 Molecules 2017, 22, Molecules 2017, 22, 603 6 of 12 6 of 12 In fact, considering the cyclohexanecarboxylic acid yield per amount of metal promotor (Table 4) the ox-Na system exhibits performance on cyclohexane hydrocarboxylation relative ox-Na system exhibits significantly better performance cyclohexane hydrocarboxylation relative ox-Na system exhibits significantly better onon cyclohexane hydrocarboxylation relative ox-Na system exhibits significantly better performance on cyclohexane hydrocarboxylation relative system exhibits significantly better performance onperformance cyclohexane hydrocarboxylation relative ox-Naox-Na system exhibits significantly better performance on cyclohexane hydrocarboxylation relative Au/CX-ox-Na system exhibits significantly better on cyclohexane hydrocarboxylation to the formerly tested promotors [10] (cyclohexanecarboxylic acid yield is 1.5 times higher than theformerly tested promotors [10](cyclohexanecarboxylic (cyclohexanecarboxylic acid yield istimes 1.5 higher than that tototested the tested acid isyield 1.5 times higher than that torelative the formerly promotors [10] (cyclohexanecarboxylic yield istimes 1.5 higher than thatthat the formerly tested promotors [10]promotors (cyclohexanecarboxylic acidacid yield is yield 1.5 higher than that to the to formerly promotors [10] (cyclohexanecarboxylic acid yield is 1.5 times higher than toformerly thetested formerly tested [10] (cyclohexanecarboxylic acid istimes 1.5that times higher than ofofthe the literature 34}]O) (BOH) 4entries 224,,]2entries 4 and 16, respectively, of the literature best catalyst [OCu 43{N(CH 2(BOH) CH O) } 4(BOH) , entries 4 and 16, respectively, of literature best catalyst 42{N(CH ][BF 4] ] entries 4 and 16, respectively, ofof ofthat the literature best catalyst [OCu 4[OCu {N(CH 22CH 24O) 32}O) 42(BOH) 4][BF 44]][BF 2, and 4 and 16, respectively, of the literature best catalyst 4{N(CH CH O)32}CH 44432][BF 4(BOH) ] 2,44][BF entries 4 and 16, respectively, of of theof literature best catalyst [OCu 4[OCu {N(CH 2[OCu CH 2O) }44{N(CH (BOH) 4][BF , entries 4 16, respectively, of of the literature best catalyst CH } ][BF ] , entries 4 and 16, respectively, 2 2 3 4 4 4 2 Table 4). Table 4). Table 4). 4). 4). Table Table 4). Table of Table 4).
40 30
4040 40 40 3030 30 30 2020 20 20
0
00 0
cyclohexanol cyclohexanone cyclohexane-1,2-diol cyclohexanecarboxilic cyclohexanol cyclohexanone cyclohexane-1,2-diol cyclohexanecarboxilic acid cyclohexanol cyclohexanone cyclohexane-1,2-diol cyclohexanecarboxilic acid cyclohexanol cyclohexanone cyclohexane-1,2-diol cyclohexanecarboxilic cyclohexanol cyclohexanone cyclohexane-1,2-diol cyclohexanecarboxilic acid acid acid cyclohexanol cyclohexanone cyclohexane-1,2-diol cyclohexanecarboxilic acid no rom oter llC44 uu XX AX A XXu-ox XXN-oxNN aaN a u-oxXN/C-ox u /C Xa-oxno rom /C /C -ox /C -oxno poter u /C XuuA-ox u /CAANXuuA-oxp rom u oter H CA lu4 H/CAACXuuAl4AHHu uCCH /C A /C u /C -ox A/C a/C no p rom no oter Arom uno Hppoter Cp lrom 4A oter AAlXu4u/C/CXAAX-ox uAA/C X/C aAX -ox-
Figure 3.3.Products Products yields from cyclohexane hydrocarboxylation: metal-free (), Figure yields from cyclohexane hydrocarboxylation: metal-free ( ),promoted promoted Figure Products yields from cyclohexane hydrocarboxylation: metal-free (), promoted Figure 3. Products yields cyclohexane hydrocarboxylation: metal-free (), promoted byby 3. 3. Products yields from cyclohexane hydrocarboxylation: metal-free promoted 3. Products yields from cyclohexane hydrocarboxylation: metal-free (), (), promoted by by by FigureFigure 3. Figure Products yields from cyclohexane hydrocarboxylation: metal-free (), promoted by HAuCl 4.3H2O in homogeneous conditions () and by Au NPs deposited on different carbon xerogels: HAuCl 4 .3H 2 O in homogeneous conditions () and by Au NPs deposited on different carbon xerogels: HAuCl .3H O in homogeneous conditions ( ) and by NPs deposited on different carbon HAuCl 4 .3H 2 O in homogeneous conditions () and Au NPs deposited on different carbon xerogels: HAuCl 4 .3H 2 O in homogeneous conditions () and by Au NPs deposited on different carbon xerogels: HAuCl 4 .3H 2 O in homogeneous conditions () and by Au NPs deposited on different carbon xerogels: HAuCl4.3H2O in homogeneous conditions () and by Au NPs deposited on different carbon xerogels: 4 2 as prepared (CX) (), treated with nitric acid (CX-ox) oxidized with nitric acid asprepared prepared (CX) (), treated with nitric acid ()and and oxidized with nitric acidand and as (CX) (), treated with nitric acid (CX-ox) () and oxidized with nitric acid and prepared (CX) (treated ),with treated with nitric acid (CX-ox) ((CX-ox) ) oxidized and oxidized with nitric acid and subsequently as as prepared (), treated with nitric acid (CX-ox) () and oxidized with nitric acid and as prepared (CX)(CX) (), with nitric acid (CX-ox) () and() oxidized with nitric acid and as prepared (CX) (), treated nitric acid (CX-ox) () and with nitric acid and subsequently treated with sodium hydroxide (CX-ox-Na) (). subsequently treated with sodium hydroxide (CX-ox-Na) (). subsequently treated with sodium hydroxide (CX-ox-Na) subsequently treated with sodium hydroxide (CX-ox-Na) subsequently treated with sodium hydroxide (CX-ox-Na) (). ().(). treated with sodium hydroxide (CX-ox-Na) ( ). subsequently treated with sodium (CX-ox-Na) (). a for the cyclohexane hydrocarboxylation to Table 4.promotors Metal promotors performance comparison afor a the a for a for Table 4.Metal Metal promotors performance comparison forthe the cyclohexane hydrocarboxylation to 4. promotors performance comparison cyclohexane hydrocarboxylation Table 4. 4.Metal promotors performance comparison foraa cyclohexane the cyclohexane hydrocarboxylation 4.Table Metal performance comparison hydrocarboxylation to to toto Table Table 4. Metal promotors performance comparison the cyclohexane hydrocarboxylation to Metal promotors performance comparison for the cyclohexane hydrocarboxylation Table cyclohexanecarboxylic acid. cyclohexanecarboxylic acid. cyclohexanecarboxylic acid. cyclohexanecarboxylic cyclohexanecarboxylic cyclohexanecarboxylic acid. acid.acid. cyclohexanecarboxylic acid.
CyCOOH Yield CyCOOH Yield CyCOOH Yield CyCOOH CyCOOH YieldYield CyCOOH Yield Carbon Atom b/μmol Atom Carbon Atom Carbon Atom Carbon Carbon Atom Atom Carbon CyCOOH Yield Entry Metal Promoter (%) of M b b b b b c d Entry Metal Promoter(%) /μmol (%) /μmol of Entry Metal Promoter (%) of MM c Efficiency/% Entry Metal Promoter /μmol of M Promoter (%) (%) /μmol of/μmol M Entry Entry Metal Metal Promoter of M Carbon Atom Economy/% b c c d d c d c d d Economy/% Economy/% Efficiency/% Entry Metal Promoter (%) /µmolEfficiency/% of MEfficiency/% Efficiency/% Economy/% Economy/% Economy/% Efficiency/% Promotor c d Efficiency/% Economy/% Promotor Promotor Promotor Promotor Promotor Promotor HAuCl .3H O2Oee e e 22O e 442.3H eHAuCl HAuCl 4.3H 5.2 7.7 7.7 5.2 7.7 .3H O 1 1 11 1 HAuClHAuCl 24O 5.2 5.2 5.2 7.7 7.7 7.7 1 4.3HHAuCl 2O4.3H 5.2 e 2O e 1 HAuCl · 3H 5.2 7.7 2 Au/CX 6.9 9.3 4 e e e e e 2 Au/CX 6.9 9.3 Au/CX 6.9 9.3 Au/CX 6.9 9.3 2 2 22 6.9 9.3 2 Au/CXAu/CX 6.9 9.3 e 33.5 Au/CX e 6.9 9.3 33.5 33.5 33.5 33.533.5 33 Au/CX-ox 12.7 18.4 e e e e e 33.5 e Au/CX-ox 12.7 18.4 3 Au/CX-ox 12.7 18.4 Au/CX-ox 3 3 Au/CX-ox 3 Au/CX-ox 12.7 12.7 12.7 12.718.4 18.4 18.4 18.4 3 Au/CX-ox e 4 Au/CX-ox-Na 27.3 37.9 e ee e e eAu/CX-ox-Na 27.337.9 37.9 37.937.9 37.9 Au/CX-ox-Na Au/CX-ox-Na 4 4 444 Au/CX-ox-Na Au/CX-ox-Na 27.3 27.3 27.327.3 4 Au/CX-ox-Na 27.3 37.9 5 Cr(OH)3·2.5H2O [10] 0.4 0.6 3 ·2.5H 2 O [10] 0.4 0.6 5 Cr(OH) 3 ·2.5H 2 O [10] 0.4 0.6 5 Cr(OH) 3·2.5H 2O [10] Cr(OH) ·2.5H 2O [10] 0.4 0.4 0.4 0.6 0.6 0.6 0.6 5 5 Cr(OH) 3·2.5H 23O [10] 0.4 5 Cr(OH) Cr(OH) ·2.5H 2 O [10] 65 K2Cr23O 7 [10] 1.0 3.9 6 K 2 Cr 2 O 7 [10] 1.0 3.9 6 K 2 Cr 2 O 7 [10] 1.0 3.9 6 K 2 Cr 2 O 7 [10] 1.0 3.9 6 K 2 Cr 2 O 7 [10] 1.0 3.9 6 K2Cr2O7 [10] 1.0 6 K2 Cr2 O7 [10] 1.0 3.9 3.9 7 MoO 3 [10] 0.00.0 0.0 0.0 3 [10] 0.0 0.0 MoO 0.0 0.0 3MoO [10]3 [10] 0.0 0.0 7 7 777 MoOMoO 3 [10] 0.0 0.0 7 MoO3 [10] 0.0 0.0 3 8 HH 4[PMo 11VO 40]·34H 2O O [10] 0.40.4 1.1 [PMo VO ]O ·34H [10] 0.4 1.1 4411 11 40 [PMo 11 VO 402]·34H 22O [10] 0.4 1.1 1.1 8 H11 H 4[PMo 1140 VO 40]·34H 2O [10] HVO ]·34H [10] H440[PMo 4[PMo 11 40 ]·34H 2O [10] 0.4 0.4 0.4 1.1 1.1 1.1 8 8 H848[PMo VO ]·34H 2VO O [10] 8 99 MnO 2 [10] 0.5 1.4 MnO [10] 0.5 1.4 22 [10] 0.5 1.4 1.4 MnO 2MnO [10]2 [10] 2 [10] 0.5 0.5 0.5 1.4 1.4 1.4 9 9 99 [10] MnO 0.5 9 MnO2 MnO 10 Fe(OH) [10] 1.6 10 Fe(OH) 3·0.5H 2O [10] 1.01.0 1.6 3 ·0.5H 2O 33.5 10 Fe(OH) 32·0.5H 2O [10] 1.0 1.6 Fe(OH) ·0.5H 2O [10] 1.0 1.6 Fe(OH) 3·0.5H O [10] 1.0 1.6 10 10 10 Fe(OH) ·0.5H 2O3[10] 1.0 1.6 10 Fe(OH) 3·0.5H 23O [10] 1.0 1.6 33.5 11 Co(acac) [10] 0.9 33.5 33.5 33.5 33.533.5 11 Co(acac) 33 [10] 0.60.6 0.9 3 [10] 0.6 0.9 11 Co(acac) 3 [10] 0.6 0.9 Co(acac) 3 [10] 0.6 0.9 Co(acac) 3 [10] 0.6 0.9 11 11 11 Co(acac) 3 [10] 0.6 0.9 11 Co(acac) 12 Zn(NO )2 [10] 0.8 12 Zn(NO 3)32 [10] 0.50.5 0.8 2 [10] 0.5 0.8 0.8 12 Zn(NOZn(NO Zn(NO 23)[10] 3)2 )[10] Zn(NO 3)Zn(NO 2 [10] 0.5 0.5 0.5 0.8 0.8 0.8 12 12 12 3)2 [10] 0.5 12 13 Cu(NO 1.0 3.3 23·)2.5H 2 O [10] 13 Cu(NO3)32·2.5H 2O [10] 1.0 3.3 14 [Cu(H tea)(N [10] 2.0 3.3 13 Cu(NO )22·2.5H O[10] [10] 1.0 3.3 3.3 3)23[10] ·2.5H 2O Cu(NO 3)2·2.5H O [10] 13 13 13 Cu(NO 3)2Cu(NO 2O ·2.5H 2O 1.0 1.0 1.0 3.3 3.3 3.3 32)] 13 Cu(NO 3)2·2.5H [10] 1.0 14 [Cu(H 2tea)(N3)] [10] 2.04.9 3.3 15 [Cu (tpa)] 2nH 3.8 2 (H 2 tea) 22tea)(N 2 O [10]2.0 [10] 2.0 3.3 3.3 14 [Cu(H 2tea)(N 3n )]3·)] [10] 2tea)(N 3)] [10] [Cu(H 23tea)(N 3)] [10] 2.0 2.0 2.0 3.3 3.3 3.3 14 14 14 tea)(N )][Cu(H [10] 14 [Cu(H2[Cu(H 15 [Cu 2(H2tea) 2(tpa)] 2O ][BF [10]4 ]2 [10] 4.9 3.8 [OCu {N(CH CH O)3 }n4·2nH (BOH) 18.1 7.1 16 4 2 2 4 15[Cu [Cu 2(H tea) 2n (tpa)] n2·2nH 2O [10]4.9 4.9 3.8 3.8 2(tpa)] (H 22tea) 2[10] (tpa)] ·2nH 2O [10] [Cu 2[Cu (H tea) (tpa)] ·2nH O [10] 15 15 2(H 2tea) n ·2nH 2O n[10] 4.9 4.9 4.9 3.8 3.8 3.8 15 [Cu15 2(H 2tea) 2(tpa)] n2·2nH 2O [OCu 4{N(CH2CH2O)3}4(BOH)4][BF4]2 [10] 18.1 7.1p(CO) = 20 atm, K S O a 16 Typical (unless otherwise stated) cyclohexane 2 2 8 {N(CH 2CH }4(BOH) 418.1 ][10] 2 [10] 18.1 18.1 18.17.1 (1.00 7.1 16[OCu 43(BOH) 4][BF 4]2conditions: 18.1 16 [OCu 4[OCu CH O) 3}24O) (BOH) 44][BF 4]42][BF [10] 7.1 7.1 164{N(CH 4{N(CH 22O) 3}224CH ]2reaction [10] 7.1mmol), 16 [OCu 2CH 2{N(CH O)2CH 34}{N(CH 44(BOH) 4(BOH) ][BF 42]O) 234}][BF [10] 16 [OCu ◦ a (1.50 mmol),(unless H2 O (3.0 mL)/MeCN (3.0 mL), 50conditions: C, 6 h incyclohexane an autoclave(1.00 (13.0mmol), mL capacity); Moles Typical otherwise stated) reaction p(CO) =b 20 atm, of a Typical a (unless a Typical a Typical a Typical (unless otherwise stated) reaction conditions: cyclohexane (1.00 mmol), p(CO) = 20 c Typical (unless otherwise stated) reaction conditions: cyclohexane (1.00 mmol), p(CO) = 20 atm, (unless otherwise stated) reaction conditions: cyclohexane (1.00 mmol), p(CO) = 20 atm, otherwise stated) reaction conditions: cyclohexane (1.00 mmol), p(CO) = 20 atm, (unless otherwise stated) reaction conditions: cyclohexane (1.00 mmol), p(CO) = 20 atm, cyclohexanecarboxylic 100mL)/MeCN mol of cyclohexane; C6 H of carbon in CyCOOH per atm, total b K2S2O8 (1.50 mmol),acid H2Oper (3.0 (3.0 mL),Cy 50=°C, 611h; inAmount an autoclave (13.0 mL capacity); d Molecular weight of desired product per combined molecular bweightb of starting b b b carbon in reactants × 100%; 2 S 2 O 8 (1.50 mmol), H 2 O (3.0 mL)/MeCN (3.0 mL), 50 °C, 6 h in an autoclave (13.0 mL capacity); K 2 S 2 O 8 (1.50 mmol), H 2 O (3.0 mL)/MeCN (3.0 mL), 50 °C, 6 h in an autoclave (13.0 mL capacity); K 2 S 2 O 8 (1.50 mmol), H 2 O (3.0 mL)/MeCN (3.0 mL), 50 °C, 6 h in an autoclave (13.0 mL capacity); K 2 S 2 O 8 (1.50 mmol), H 2 O (3.0 mL)/MeCN (3.0 mL), 50 °C, 6 h in an autoclave (13.0 mL capacity); K K2S2O8 (1.50 mmol), H2O (3.0 mL)/MeCN (3.0 mL), 50 °C, 6 h in an autoclave (13.0 mL capacity); c Moles of cyclohexanecarboxylic e p(CO) = 2 atm. acid per 100 mol of cyclohexane; Cy = C6H11; Amount of carbon in materials × 100%; c c c c c Moles cyclohexanecarboxylic acid per100 ofcyclohexane; cyclohexane; Cy =C carbon ofofcyclohexanecarboxylic acid per mol ofCy 11carbon ;11; Amount ofof Moles of cyclohexanecarboxylic acid per 100 mol ofmol cyclohexane; =Cy C 116C ;H Amount of carbon in inin ofMoles cyclohexanecarboxylic acid per 100 of100 cyclohexane; =;Cy CAmount 6H 11 ;6=HAmount ofAmount carbon incarbon Moles Moles of cyclohexanecarboxylic acid per 100 mol ofmol cyclohexane; = CCy 6H11 of6H in d Molecular weight of desired product per combined CyCOOH per total carbon in reactants d× 100%; d Molecular d Molecular d weight d Molecular weight of desired product per combined CyCOOH pertotal total carbon ×100%; 100%; weight of desired product per combined CyCOOH per ininreactants × Molecular Molecular weight of desired product per combined CyCOOH per total carbon in× reactants × 100%; weight of desired product per combined CyCOOH percarbon total carbon incarbon reactants ×reactants 100%; of desired product per combined CyCOOH per total in reactants 100%; molecular weight of starting materials × 100%; ee p(CO) = 2 atm. eatm. e×p(CO) e100%; atm. molecular weight starting = =2 2atm. molecular weight ofof starting materials p(CO) = 2 atm. molecular weight of starting × 100%; = p(CO) 2e p(CO) atm. molecular weight of starting materials ×materials p(CO) =×100%; 2100%; molecular weight of starting materials ×materials 100%;
In addition, the present catalytic system was evaluated by green metrics such as atom economy addition, thepresent present catalytic system was evaluated green metrics such asatom atom economy InInaddition, the catalytic system was byby green metrics as economy In the addition, the present catalytic system was evaluated by green metrics such as atom economy In addition, the present catalytic system was evaluated by green metrics such as such atom economy In addition, present catalytic system was evaluated by evaluated green metrics such as atom economy (molecular weight of desired product per combined molecular weight of starting materials) or carbon (molecular weight desired product percombined combined molecular weight starting materials) carbon (molecular weight ofofdesired product molecular weight ofofstarting materials) ororcarbon (molecular of desired product per per combined molecular of starting materials) or carbon (molecular weight of desired product per combined molecular weight of starting materials) or carbon (molecular weight of weight desired product per combined molecular weight of weight starting materials) or carbon efficiency (amount of carbon in CyCOOH per total carbon in reactants). Such metrics were not efficiency (amount ofcarbon inCyCOOH CyCOOH per total inreactants). reactants). Such metrics were not efficiency (amount of in per total carbon inSuch Such metrics were efficiency (amount ofincarbon in CyCOOH total carbon in reactants). Such metrics were not not efficiency (amount of carbon incarbon CyCOOH per per total carbon incarbon reactants). Such metrics were not efficiency (amount of carbon CyCOOH per total carbon in reactants). metrics were not included in the previously reported systems. Thus, to compare our system with the previous ones included the previously reported systems. Thus, compare ourthe system with theprevious previous ones included ininthe previously reported systems. totocompare our system included in previously the previously reported systems. Thus, to our compare system with the the previous included in the reported systems. Thus, toThus, compare our our system with thewith previous onesonesones included in the previously reported systems. Thus, to compare system with previous ones
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In addition, the present catalytic system was evaluated by green metrics such as atom economy (molecular weight of desired product per combined molecular weight of starting materials) or Molecules efficiency 2017, 22, 603(amount of carbon in CyCOOH per total carbon in reactants). Such metrics 7were of 12 carbon not included in the previously reported systems. Thus, to compare our system with the previous (reported in [10]), carbon efficiency and the and atomthe economy were determined (Table 4). Although ones (reported in the [10]), the carbon efficiency atom economy were determined (Table 4). presenting the same atom economy value, our gold systems exhibit markedly higher Although presenting the same atom economy value, our gold systems exhibit markedly higher carbon carbon efficiency values values (Table (Table 4), 4), which which is is aa significant significant improvement improvement in in terms terms of of the the sustainability sustainability of of the the efficiency hydrocarboxylation reaction. reaction. hydrocarboxylation Another important the possibility of of being recycled andand reAnother important advantage advantageofofthe thepresent presentsystems systemsis is the possibility being recycled used. Recycling of the best catalyst, Au/CX-ox-Na, was tested on up to seven consecutive cycles. On re-used. Recycling of the best catalyst, Au/CX-ox-Na, was tested on up to seven consecutive cycles. completion of each stage, thethe products were analyzed and On completion of each stage, products were analyzed andthe thecatalyst catalystwas wasrecovered recoveredby by filtration, filtration, thoroughly washed, and then reused for a new set of cyclohexane hydrocarboxylation experiments. thoroughly washed, and then reused for a new set of cyclohexane hydrocarboxylation experiments. The filtrate filtratewas wastested tested a new reaction (by addition of reagents), fresh reagents), no oxidation was The in in a new reaction (by addition of fresh and no and oxidation was detected. detected. Figurethe 4 shows the excellent recyclability of theAu/CX-ox-Na: system Au/CX-ox-Na: in the second, third, Figure 4 shows excellent recyclability of the system in the second, third, fourth, fourth, fifth, sixth, and seventh run, the observed activity was 99.8%, 99.7%, 98.4%, 98.3%, 97.7% and fifth, sixth, and seventh run, the observed activity was 99.8%, 99.7%, 98.4%, 98.3%, 97.7% and 97.5% of 97.5% of the initial one, being the selectivity maintained. the initial one, being the selectivity maintained.
4. Effect Figure 4. Effect of of the the catalyst catalyst recycling recycling on the yield of cyclohexanecarboxylic acid obtained by hydrocarboxylation of cyclohexane cyclohexane catalyzed catalyzed by byAu/CX-ox-Na. Au/CX-ox-Na. Reaction conditions: conditions: cyclohexane cyclohexane 8 8(1.50 2O2 O (3.0 mL)/MeCN (3.0(3.0 mL), 50 (1.00 mmol), p(CO)= p(CO)= 22 atm, atm, K K22SS2O (1.50mmol), mmol),catalyst catalyst(2(2μmol), µmol),HH (3.0 mL)/MeCN mL), 2O 50 C, and hours in an autoclave (13.0 capacity). °C,◦and six six hours in an autoclave (13.0 mLmL capacity).
3. Materials and Methods 3.1. Reagents All the reagents and solvents were purchased from commercial commercial sources sources and and used used as as received. received. The water used for for all all reactions reactions and and analyses analyses was was double double distilled distilled and and deionised. deionised. 3.2. Carbon Materials Materials Preparation 3.2. Carbon Preparation Carbon a Carbon xerogel xerogel (CX) (CX) was wasprepared preparedby bypolycondensation polycondensationofofresorcinol resorcinoland andformaldehyde, formaldehyde,using using pH ofof6,6,according a pH accordingtotoa aprocedure proceduredescribed describedelsewhere elsewhere[29,30,34–37]. [29,30,34–37].ItItwas wasused usedin in its its original original form form (CX), oxidized (-ox), and oxidized with nitric acid and subsequently treated with sodium (CX), oxidized (-ox), and oxidized with nitric acid and subsequently treated with sodium hydroxide hydroxide (-ox-Na). CX-ox was was obtained obtained by by refluxing refluxing CX CX with with 75 75 mL mL of of aa 55 M (-ox-Na). CX-ox M nitric nitric acid acid solution, solution, per per gram gram of of carbon material, for 3 h, then separated by filtration and washed with deionized water until neutral carbon material, for 3 h, then separated by filtration and washed with deionized water until neutral
pH, similarly to what was reported earlier [29,35–37]. CX-ox-Na was obtained by treating CX-ox with 75 mL of a 20 mM NaOH aqueous solution, per gram of carbon material, in reflux for 1 h, as reported in the literature [29,35,36]. This material was also separated by filtration and washed until neutral pH.
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pH, similarly to what was reported earlier [29,35–37]. CX-ox-Na was obtained by treating CX-ox with 75 mL of a 20 mM NaOH aqueous solution, per gram of carbon material, in reflux for 1 h, as reported in the literature [29,35,36]. This material was also separated by filtration and washed until neutral pH. 3.3. Carbon Materials Characterisation The carbon materials were characterised by N2 adsorption at 77 K in a Quantachrome Nova 4200e apparatus (Boynton Beach, FL, USA), using the Brunauer-Emmett-Teller (BET) theory for total surface area determination, Barrett-Joyner-Halenda (BJH) for pore size distribution and Boer’s t-method for micropore volume and external surface area. Their surface chemistry was characterised by temperature programed desorption (TPD) using an Altamira AMI-300 apparatus (Pittsburgh, PA, USA), with a coupled Ametek Dycor DyMaxion quadrupole mass spectrometer (Pittsburgh, PA, USA). 3.4. Gold Loading Gold (nominal 3% wt) was loaded on the xerogel supports by the colloidal method [30,45–47], which consists of dissolving the gold precursor, HAuCl4 ·3H2 O (Alfa Aesar, Karlsruhe, Germany), in water, adding polyvinyl alcohol (Aldrich, Darmstadt, Germany) and NaBH4 (Aldrich), resulting in a ruby red solution to which the xerogel support was added under stirring. After a few days, the solution of CX started to lose colour, as Au was deposited on the support. The colourless solution was filtered, the catalyst washed thoroughly with distilled water until the filtrate was free of chloride and dried at 110 ◦ C overnight. Solutions of CX-ox and CX-ox-Na took more time and even so the deposition was not complete (as found out later when the amount of gold present was determined), as these solutions never turned colourless. However, the same filtering and washing procedures were followed as with CX. The organic scaffold was removed from the supports by heat treatment under N2 flow for 3 h at 350 ◦ C (shown by elemental analysis to be efficient for this purpose), and then, the catalyst was activated by further treatment under hydrogen flow for 3 h also at 350 ◦ C. 3.5. Gold Catalysts Characterisation The Au/xerogel samples were imaged by transmission electron microscopy (TEM). The analyses were performed on a Leo 906E apparatus (Austin, TX, USA), at 120 kV. Samples were prepared by ultrasonic dispersion in hexane and a 400 mesh formvar/carbon copper grid (Agar Scientific, Essex, UK) was dipped into the solution for TEM analysis. The average gold particle size was determined from measurements made on about 300 particles. The metal dispersion was calculated by DM = (6ns M)/(ρNdp ), where ns is the number of atoms at the surface per unit area (1.15 × 1019 m−2 for Au), M is the molar mass of gold (196.97 g mol−1 ), ρ is the density of gold (19.5 g cm−3 ), N is Avogadro’s number (6.023 × 1023 mol−1 ) and dp is the average particle size (determined by TEM, assuming that particles are spherical). In order to determine the loading of gold, samples were incinerated at 600 ◦ C and the resulting ashes were dissolved in a concentrated HNO3 and H2 SO4 mixture. The resulting solution was diluted and analysed by atomic absorption spectroscopy (AAS) using a Unicam 939 atomic absorption spectrometer (Kent, UK) and a hollow cathode lamp Heraeus 3UNX Au. 3.6. Catalytic Tests The single-pot reactions were carried out in stainless steel autoclaves, by reacting, at typical temperatures of 30–80 ◦ C and in a water/acetonitrile medium with cyclohexane, carbon monoxide (pressures from 2 to 20 atm), gold catalyst (2–20 µmol) and potassium peroxodisulfate. The reaction mixture was stirred for 3–6 h (typically 6 h) using a magnetic stirrer and an oil bath, whereupon it was cooled in an ice bath, degassed, opened, and the contents transferred to a Schlenk flask. Diethyl ether (9.0–11.0 mL) and 90 mL of cycloheptanone (GC internal standard) were added. The obtained mixture was vigorously stirred for 10 min, and the organic layer was analysed typically by gas chromatograph (GC). A Fisons Instruments GC 8000 series gas chromatograph (Agilent Technologies, Santa Clara, CA,
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USA) with a DB-624 (J&W) capillary column (flame ionization detector) and the Jasco-Borwin v.1.50 software (Jasco, Tokyo, Japan) were used. GC-MS analyses were performed in a Perkin Elmer Clarus 600 C GC-MS instrument (Shelton, Connecticut, USA) equipped with a 30 m × 0.22 mm × 25 µm BPX5 (SGE) capillary column, using He as the carrier gas. The internal standard method was used to quantify the organic products, since the desired cyclohexanecarboxylic acid was not isolated from the reaction mixture. Blank tests (i) without any catalyst; (ii) only with CX, CX-ox and CX-ox-Na; and (iii) using only one of the solvents (H2 O or NCMe) were also performed, to assess if the carboxylation reactions proceeded in the absence of the metal catalyst, and the importance of each support and solvent. Moreover, aqueous solutions of the gold precursor were also tested for comparison (homogenous medium). 4. Conclusions Gold nanoparticles were successfully deposited on carbon xerogel, as prepared and with different treatments: with nitric acid; and oxidized with nitric acid and subsequently treated with sodium hydroxide. The catalytic activity of the said materials was assessed for the single-pot hydrocarboxylation of cyclohexane, in H2 O/MeCN, under mild conditions (50 ◦ C, 2 atm of CO). Au/CX-ox-Na exhibited the best performance, yielding cyclohexanecarboxylic acid up to 54.5% yield, and excellent recyclability, maintaining 97.5% of the initial activity after seven consecutive catalytic cycles. Green metric values of carbon efficiency also confirmed the improvement brought by this novel catalytic system to the hydrocarboxylation of cyclohexane. These results have an important implication on the design of gold catalysts and are of potential significance for the sustainable production of carboxylic acids. Acknowledgments: This work has been partially supported by the Fundação para a Ciência e a Tecnologia (FCT), Portugal, and its projects PTDC/QEQ-ERQ/1648/2014, PTDC/QEQ-QIN/3967/2014 and UID/QUI/00100/2013. This work is a result of project “AIProcMat@N2020—Advanced Industrial Processes and Materials for a Sustainable Northern Region of Portugal 2020”, with the reference NORTE-01-0145- FEDER-000006, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF) and of Project POCI-01-0145FEDER-006984—Associate Laboratory LSRE-LCM funded by ERDF through COMPETE2020—Programa Operacional Competitividade e Internacionalização (POCI)—and by national funds through FCT—Fundação para a Ciência e a Tecnologia. SACC acknowledges Investigador FCT program (IF/01381/2013/CP1160/CT0007), with financing from the European Social Fund and the Human Potential Operational Program. Authors thank Pedro Tavares and Lisete Fernandes (UME/CQVR/UTAD) for assistance with the TEM analyses. Author Contributions: S.A.C.C. prepared and characterized the catalysts. A.P.C.R. and L.M.D.R.S.M. conceived and designed the experiments; A.P.C.R. performed the experiments; A.P.C.R., L.M.D.R.S.M. and S.A.C.C. analysed the data; L.M.D.R.S.M. and S.A.C.C. wrote the paper; L.M.D.R.S.M., J.L.F. and A.J.L.P. provided the means needed for the realization of this work. All authors read and approved the manuscript. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
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