Role of pentahedrally coordinated titanium in titanium

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Cite this: RSC Adv., 2015, 5, 17897

Role of pentahedrally coordinated titanium in titanium silicalite-1 in propene epoxidation† Yi Zuo,a Min Liu,a Ting Zhang,a Luwei Hong,a Xinwen Guo,*a Chunshan Song,b Yongsheng Chen,*b Pengyu Zhu,b Cherno Jayec and Daniel Fischerc Two titanium silicalite-1 samples with different crystal sizes were synthesized in the tetrapropylammonium bromide (TPABr) and tetrapropylammonium hydroxide (TPAOH) hydrothermal systems. The small-crystal TS-1 with a size of 600 nm was then treated with different organic bases. These TS-1 samples were evaluated in the epoxidation of propene, and characterized by ultraviolet-visible diffuse reflectance (UVvis), X-ray absorption near edge structure (XANES) and Raman spectroscopies. The Ti L-edge absorption spectra show that a new Ti species, pentahedrally coordinated Ti, appears in some of the samples. This pentahedrally coordinated Ti species is correlated with the catalytic oxidation activity of TS-1, closely. Tetrahedrally coordinated Ti in TS-1 is the primary active center for selective oxidation reactions, but the existence of a small amount of pentahedrally coordinated Ti can further improve the catalytic activity. A

Received 6th January 2015 Accepted 5th February 2015

high molar ratio of Si/Ti (n(Si/Ti)) in the synthesis process (n(Si/Ti) ¼ 92.78) was beneficial for the generation of pentahedrally coordinated Ti. The improved catalytic activity of the TPAOH treated TS-1 is mainly due to the increasing amount of pentahedrally coordinated Ti, besides the elimination of diffusion

DOI: 10.1039/c5ra00194c

limitation. Slowing down the crystallization rate can also increase the content of pentahedrally

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coordinated Ti.

1. Introduction Since it was rst hydrothermally synthesized by Taramasso et al.,1 titanium silicalite-1 (TS-1) has attracted much attention due to its excellent catalytic performances for selective oxidation reactions, such as oxidation of alkanes,2 epoxidation of alkenes,3 hydroxylation of aromatics4 and ammoxidation of ketones.5 The hydroxylation of phenol over TS-1 using methanol and acetone as the solvents at 353 K to produce benzenediol was rst commercialized by Enichem in 1986.6 The conversion of phenol reached 25% in the Enichem route, which was much higher than those in the Rhone-Poulenc route (5%) and Brichima route (10%). Epoxidation of propene is another important application of TS-1. In recent years, the hydrogen peroxide

a

State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China. E-mail: [email protected]; Fax: +86 411 84986134; Tel: +86 411 84986133

b

EMS Energy Institute, PSU-DUT Joint Center for Energy Research, Department of Energy & Mineral Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USA. E-mail: [email protected]; Tel: +1 814 8659834

c

to propene oxide route (HPPO) has become a mature technology and been commercialized by BASF/Dow Chemical and Evonik/ Uhde in Belgium and Korea, respectively.7,8 The industrial applications of TS-1 are more and more attractive. However, the true active center for the oxidation reactions in TS-1 is still controversial. Most researchers believe that there are three types of Ti species in TS-1. Tetrahedrally coordinated titanium, which is called framework Ti, is usually considered as the active center for the oxidation reactions.9 The tetrahedral Ti can form a ve member ring (5MR) with hydrogen peroxide and alcohol (see Scheme 1).10 This 5MR, which was reported by Clerici et al., can oxidize propene to prepare propene oxide. The 5MR mechanism well explains the effect of alcohol solvents in the propene epoxidation. Thus, it is widely accepted. In this mechanism, a pentahedrally coordinated Ti intermediate (species iv in Scheme 1) was provided and was considered as an inert state for the oxidation reactions, due to the negative charge of the species. The other two types of Ti species are octahedrally coordinated Ti (usually called extra-framework Ti) and anatase

National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

† Electronic supplementary information (ESI) available: Standard Ti L-edge XANES spectra of different coordinated Ti, and the characterization of samples by X-ray powder diffraction, Fourier-transform infrared spectroscopy, nitrogen physisorption and transmission electron microscopy, accompanied with the calculation process of different coordinated Ti contents and their TOFs in small-crystal TS-1. See DOI: 10.1039/c5ra00194c

This journal is © The Royal Society of Chemistry 2015

Scheme 1

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TiO2. They are considered inert in oxidation reaction.11 Moreover, anatase TiO2 can even lead to decomposition of hydrogen peroxide. Therefore, efforts have been to increase the content of framework Ti and decrease the extra-framework Ti proportion.12 However, the Ti content in the TS-1 framework was reported to be limited and the maximum was 2.5 wt%,13 due to the expansion of lattice constant. If the amount of titanium added to the hydrothermal synthesis system exceeds 2.5 wt%, it will exist as extra-framework Ti. Thus, the molar ratio of Si/Ti (n(Si/Ti)) is usually more than 33 in the synthesis gel. In order to increase the Ti content in the framework, researchers have explored another preparation method called gas–solid substitution method. Many silanol nests are created in the dealuminum of ZSM-5 or deboron of B-ZSM-5 by this method, and Ti atoms are then planted in these nests to obtain Ti-ZSM-5. However, higher framework Ti content does not bring a higher catalytic activity, probably due to the difference of Ti locations in TS-1 and Ti-ZSM-5. In addition, it was also reported that octahedrally coordinated extra-framework Ti with a Ti–O–Ti bond was the true active center for the oxidation reactions, according to the characterization of ultraviolet-visible diffused reectance and ultraviolet resonance Raman spectroscopy.14,15 Therefore, it is very important to recognize the true active center for further improving the activity of TS-1. In our previous work, small-crystal TS-1 (crystal size of 600 nm 400 nm 250 nm) was synthesized in a tetrapropylammonium bromide (TPABr)–ethylamine (EA) hydrothermal system by using the mother liquor of nano-sized TS-1 (200 nm) as the seed.16 The treatment of small-crystal TS-1 with organic bases further improved its catalytic activity in phenol hydroxylation.17 In the present contribution, small-crystal TS-1 and nano-sized TS-1 with different catalytic oxidation activities were characterized by ultraviolet-visible diffuse reectance (UV-vis) spectroscopy, X-ray absorption near edge structure (XANES) spectroscopy and Raman resonance spectroscopy. Via investigating the relationship between Ti coordination structure and the catalytic activities, we nd a new Ti coordination structure in TS-1, which is pentahedrally coordinated Ti. This pentahedral Ti is more active than tetrahedral one in oxidation reactions.

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(TMAOH), and tetrapropylammonium hydroxide (TPAOH), was carried out in a Teon lined autoclave with a solid/liquid ratio of 1 g : 10 mL at 443 K for 48 h.17 The concentration of the base solution was 0.06 mol L1. The solid was then washed and calcined at 813 K for 6 h. The samples treated with EA, DEA, TMAOH and TPAOH were denoted as TS-1–EA, TS-1–DEA, TS-1–TMAOH and TS-1–TPAOH, respectively. The synthesis of nano-sized TS-1 was carried out according to ref. 19. Tetraethyl orthosilicate and tetrabutyl orthotitanium were used as silicon and titanium sources, and TPAOH, the template. The molar composition of the mixture was: SiO2 : TiO2 : TPAOH : H2O ¼ 1 : 0.025 : 0.4 : 10. The posttreatment of nano-sized TS-1 was the same as that of smallcrystal TS-1. The as-synthesized TS-1 was denoted as TS-1–NA. The TS-1–AC was prepared by adding ammonium carbonate in the synthesis gel with an ammonium/silicon molar ratio of 0.3.12 The other synthesis conditions were the same as that of TS-1–NA.

2.2. Characterization of TS-1 Ti L2,3-edge (453.8 and 460.2 eV) XANES spectroscopy was performed at the National Institute of Standards and Technology U7A beamline at National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The storage ring was operated with an electron beam energy of 800 MeV and an average current of 600 mA. The XANES spectra were collected in a partial electron yield mode with low energy electron compensation to minimize charging effect for poor conductors. Additionally a clean gold coated mesh was used to monitor I0, the incident beam intensity. The data were processed using Athena.20 The Raman spectra were recorded on a Bruker laser Raman spectrometer, using a 532 nm laser as the excitation source. The UV-vis spectra were collected on a Jasco UV-550 spectrometer from 190 to 500 nm, and pure BaSO4 was used as a reference. A Perkin Elmer OPTIMA 2000DV Optical Emission Spectrometer provided the elemental composition (ICP).

2.3. Epoxidation of propene

2.

Experimental

2.1. Preparation of TS-1 Small-crystal TS-1 with different n(Si/Ti) was hydrothermally synthesized according to ref. 18, using 30 wt% colloidal silica and titanium tetrachloride as silicon and titanium sources, respectively, and TPABr as the template. The molar composition of the synthesis gel was: SiO2 : TiO2 : TPABr : H2O ¼ 1 : (0.0125– 0.050) : 0.4 : 12. The seed/SiO2 weight ratio was 0.06. The gel was crystallized for 48 h, and then the obtained solid was separated from the mother liquor, washed, dried, and calcined at 813 K for 6 h. The small-crystal TS-1 with the feeding n(Si/Ti) of 20, 50 and 80 were denoted as samples TS-1-20, TS-1-50 and TS-1-80, respectively. Treatment of TS-1 with the organic bases, including EA, diethylamine (DEA), tetramethylammonium hydroxide

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The epoxidation of propene was carried out in a 400 mL stainless-steel batch reactor. In a typical run, 0.2 g TS-1, 32 mL methanol and 30 wt% of H2O2 were added to the reactor; propene was then charged to reach 0.4 MPa. The initial concentration of H2O2 was 3.5 mol L1. Aer heating the mixture with stirring at 313 K for 1 h, the residual H2O2 was checked by iodometric titration. The products were analyzed on a Tianmei 7890F gas chromatograph with a ame ionization detector (FID) and a PEG-20M capillary column (30 m 0.25 mm 0.4 mm). Propene oxide (PO) was the main product, and propene glycol (PG) and its monomethyl ethers (MME) were the by-products. The conversion of H2O2 (X(H2O2)), selectivity of PO (S(PO)), utilization of H2O2 (U(H2O2)) and turnover frequency (TOF) were calculated using eqn (1)–(4), respectively: X(H2O2) ¼ (n0(H2O2) n(H2O2))/n0(H2O2)

(1)


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S(PO) ¼ n(PO)/(n(PO) + n(MME) + n(PG))

(2)

U(H2O2) ¼ (n(PO) + n(MME) + n(PG))/(n0(H2O2)$X(H2O2)) (3) TOF ¼ n0(H2O2)$X(H2O2)/(t$n(Ti))

(4)

where n0(H2O2) and n(H2O2) are the initial and nal molar concentrations of H2O2, n(PO), n(MME) and n(PG) represent the molar contents of PO, MME and PG, t is the reaction time, which is 1 h in this paper, and n(Ti) is the molar number of Ti, determined by ICP in the 0.2 g TS-1.

3.

Results and discussion

Fig. 1

Raman spectra of small-crystal TS-1 with different n(Si/Ti).

3.1. Different molar ratio of Si/Ti Small-crystal TS-1 with different n(Si/Ti) was synthesized by adding different amount of titanium tetrachloride in the synthesis gel. Three TS-1 samples were characterized and evaluated in the epoxidation of propene to study the relationship between the Ti coordination structure and catalytic activity. All the small-crystal TS-1 show a similar crystal size (600 nm 400 nm 250 nm), regardless of their n(Si/Ti) and treatment with different organic bases (cf. Fig. S9 and S10†). The weight contents of silicon and titanium in the samples obtained from ICP was settled in Table 1 with their catalytic performances. The actual n(Si/Ti) slightly differed from the feeding one. Fig. 1 shows the Raman spectra of the three samples. The vibration bands at 144, 390, 516 and 637 cm1 are assigned to the characteristic bands of anatase TiO2,21 and the peak at 380 cm1 corresponds to ve-member-ring structure of MFI topology.22 The most anatase TiO2 appears in TS-1-20. The characteristic band of MFI topology (380 cm1) is almost covered by that of anatase TiO2 (390 cm1). This suggests that the amount of the framework titanium is limited.13 When the amount of titanium added to the synthesis gel past the maximum, extra titanium will exist as anatase TiO2 or octahedrally coordinated extraframework Ti. In recent years, many researchers tried to increase the amount of framework Ti. Fan et al. found that the addition of ammonium salts in the hydrothermal system could not only decrease the basicity and the rate of crystallization, but also change the crystallization mechanism. They obtained TS-1

Table 1

with a n(Si/Ti) of 33, and almost no anatase TiO2 was present aer being washed by HCl solution.23 The Raman bands at 390, 516 and 637 cm1 nearly disappear and that at 144 cm1 becomes weaker for TS-1-50, indicating that the content of anatase TiO2 decreases sharply when the n(Si/Ti) increases from 18.82 to 52.00. It can be inferred that titanium exists mainly as tetrahedrally coordinated framework Ti. Further increase of the n(Si/Ti) to 92.78 results in the presence of only Raman band at 380 cm1, which is the characteristic band of MFI topology, demonstrating that anatase TiO2 is not present in TS-1-80. Evidently, the content of anatase TiO2 in small-crystal TS-1 decreases with the decrease of the amount of titanium added to the synthesis process. The existence of anatase TiO2 may lead to the decomposition of H2O2 and the coverage of the active framework Ti centers.24 Therefore, the mitigation of anatase TiO2 generation is benecial for the oxidation reactions. Based on the Raman spectra, we believe that a high n(Si/Ti) may have a positive effect on the catalytic reactions because it results in a low anatase TiO2 content. Raman spectroscopy provided us the information about anatase TiO2, but it did not tell us how the content of framework Ti changed with the variation of n(Si/Ti). UV-vis spectroscopy, one of the rst spectral techniques used for the detection of Ti coordination states in titanium silicalites, provides important information.25 Fig. 2 shows the UV-vis spectra of small-crystal

Catalytic performances of propene epoxidation and IH/L of TS-1 samplesa

Cat.

Si/wt%

Ti/wt%

n(Si/Ti)

X(H2O2)/%

S(PO)/%

U(H2O2)/%

TOF/(mol H2O2/(h mol Ti))

IH/Lb

TS-1-20 TS-1-50 TS-1-80 TS-1–EA TS-1–DEA TS-1–TMAOH TS-1–TPAOH TS-1–NA TS-1–AC

43.58 43.50 46.01 44.97 44.94 44.96 44.86 45.43 45.20

3.97 1.50 0.85 2.04 2.09 2.04 2.21 1.59 1.89

18.82 52.00 92.78 37.79 36.86 37.78 34.80 48.98 40.99

21.8 26.2 26.9 52.0 52.8 52.1 58.3 29.1 52.0

82.0 78.7 83.7 84.4 83.6 85.8 90.2 87.7 91.1

87.1 88.2 89.6 86.2 85.1 84.3 88.5 87.4 85.6

156.8 498.8 903.8 728.0 721.5 729.4 753.4 522.7 785.8

0.92 1.59 2.43 1.34 1.31 1.46 1.86 2.15 3.32

a Reaction conditions: catalyst 0.2 g, methanol 32 mL, H2O2 3.5 mol L1, propene pressure 0.4 MPa, 313 K, 1 h. b IH/L stands for the relative intensity of the higher energy peak of L3 edge to the lower one in XANES.


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Fig. 2

UV-vis spectra of small-crystal TS-1 with different n(Si/Ti).

TS-1 with different n(Si/Ti). Peak deconvolutions were performed using the PeakFit program with the Gaussian tting method.26 In the spectra of small-crystal TS-1, there are usually three major absorption bands, which are centered in 200–210, 240–290 and 310–330 nm. The band at 200–210 nm is assigned to tetrahedrally coordinated Ti, while that at 310–330 nm corresponds to anatase TiO2.27 The contribution of the band at 240–290 nm is controversial. Blasco et al. claimed that the band belonged to an octahedrally coordinated Ti containing Ti–O–Ti bond,28 while Geobaldo et al. attributed it to isolated Ti species that coordinated with two water molecules.29 In our opinion, more than one kind of Ti species appear between the band of 230 and 290 nm in the UV-vis spectra of TS-1. The band at about 250–290 nm is assigned to the octahedrally coordinated Ti species, which is inactive for the oxidation reactions, and that at 220–250 nm is isolated Ti species with less coordination number of oxygen (such as pentahedral Ti). The undercoordinated Ti has a higher energy than the octahedral one, so its band shis to shorter wavelength (hypsochromic shi). The contents of tetrahedral Ti, octahedral Ti and anatase TiO2 increase with the decrease of n(Si/Ti), and the increase of anatase TiO2 is much stronger than those of the others, indicating that the introduction of Ti in the framework is limited. There is no absorption at 300–310 nm in TS-1-80, which demonstrates that anatase TiO2 is absent. This is consistent with its Raman spectrum. A new band appears at 235 nm in TS-1-80, which is considered to be pentahedrally coordinated Ti (proved by XANES). This band does not appear in other samples.

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X-ray absorption spectroscopy (XAS) has been used by many researchers to examine the local structure of active centers.30,31 Ti K-edge XANES is oen used to determine Ti coordination structure in titanium silicalites and oxides.32–35 Several in situ Ti K-edge XAS studies revealed that active Ti center was present in tetrahedral coordination.36,37 Ti L-edge XANES is also used for studying the coordination of Ti in Ti-silicate glasses.38 The Ti Ledge XANES spectrum consists of two sets of doublets (see Fig. 3), which correspond to the 2p1/2 and 2p3/2 transitions of the 3d0 to 2p53d1 states.39 The L2 edge is at higher energy (462– 470 eV) and the L3 edge is at lower energy (455–462 eV). The splitting of each edge is attributed to the teg and eg symmetry of the d orbital. The higher energy peak of L3 edge (459–462 eV) consists of two peaks in some samples, a main peak and a shoulder one, which is on the lower energy side (460 eV, b) for rutile TiO2 and the higher energy side (461 eV, c) for anatase TiO2.38 The intensities of the two peaks are reversed for the substances that they represent. For example, the intensity of peak b is higher than that of peak c in the spectrum of anatase TiO2. The tetrahedrally coordinated Ti is characterized by the absence of splitting of the peak at 459–462 eV and a relatively weaker intensity of the peaks at lower energy of both L2 and L3 edges (peaks a and d) than those at higher energy (peaks b and e). Pentahedrally coordinated Ti was rst discovered in fresnoite, whose coordination structure is that a Ti atom centered in a tetragonal pyramid, formed by ve oxygen atoms. The pentahedral Ti is characterized by a slight shi to higher energy and a drastic decrease of peak a, a shi of peak e to lower energy and the appearance of a shoulder peak on peak d. TS-1-50 contains primarily tetrahedrally coordinated Ti, because the relative intensity of peak a is obviously lower than that of peak b. Anatase TiO2 is observed in TS-1-20 and TS-1-50, which is proved by the slight splitting of peaks b and c. The content of anatase TiO2 in TS-1-20 is more than that in TS-1-50, illustrated by the relatively weaker intensity of peak b to peak a (denoted as IH/L). This result conrms that the content of Ti in the framework is limited, and the excessive Ti will exist as anatase TiO2. The peak intensity of TS-1-80 is much weaker than those of the others, which is due to the least Ti content.

Ti L-edge XANES spectra of small-crystal TS-1 with different n(Si/Ti). (Peaks a–c are the characteristic peaks of L3 edge, while peaks d and e are those of L2 edge.). Fig. 3


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Moreover, the peak a is very weak and shis to higher energy slightly (from 458.1 eV to 458.4 eV), the peak e shis to lower energy (from 466.0 eV to 465.8 eV), and a weak shoulder peak appears in peak d, conrming the existence of pentahedrally coordinated Ti in TS-1-80. We can provide a possible reason for this phenomenon, according to our previous works.16–18 The crystallization of small-crystal TS-1 starts from the combination of one [TiO4] and several [SiO4], forming Ti[–O–SiO4]x species. This species then combines with each other to form the TS-1 framework under the structure-direct of template. Finally, the le [SiO4] grows along the crystal surface. In other words, the combination of [SiO4] with [TiO4] makes silicon easier to be inserted into framework. Thus, [SiO4] trends to combine with [TiO4] to crystallize in a supersaturating condition. If the n(Si/Ti) was very high (may be more than 80), [SiO4] would show a competitive effect with each other. Therefore, it can be inferred that one titanium atom needs to combine with more than four [SiO4]. At this time, pentahedrally coordinated Ti is easy to be formed. Furthermore, it was reported that octahedrally coordinated Ti was generated by a tetrahedral one with two molecular water. The combination between them was the oxygen atom from water. [SiO4] can also provide the oxygen atom to combine with [TiO4]. Therefore, [TiO4] has the ability to bond with more than four [SiO4], although the bonding may lead to a lattice distortion. The catalytic performances of the small-crystal TS-1 with different n(Si/Ti) were evaluated in the propene epoxidation (Table 1). Although the Ti content in TS-1-20 is the largest, the conversion of H2O2 over it is the lowest among the three samples, indicating that the Ti coordination state is more important than its content for the catalytic activity. The content of Ti in TS-1-80 is the least, but its activity is higher than those of the other two. The highest TOF was obtained over TS-1-80, indicating that the pentahedrally coordinated Ti was the most active species of all the Ti coordination. The lowest TOF were obtained over TS-1-20, indicating that octahedrally coordinated Ti showed only negative effects on the oxidation reaction. Based on the XANES and TOF data, we calculated the contents of different coordinated Ti in the three samples (see Table 2) and the TOF of each coordinated Ti. The process of calculation was provided in ESI.† The activity of TS-1 is not affected only by the Ti coordination, but also by the crystal size and diffusion property. Therefore, the results obtained may be just t for the untreated small-crystal TS-1. The highest content of pentahedrally coordinated Ti was generated in TS-1-80 (0.43 wt%), while that of tetrahedral one in TS-1-50 (1.14 wt%). The content of pentahedrally coordinated Ti decreases and

Table 2

Contents of different coordinated Ti in small-crystal TS-1 Percentage of different coordinated Ti/%

Cat.

Tetrahedral Ti

Pentahedral Ti

Octahedral Ti

TS-1-20 TS-1-50 TS-1-80

42 76 50

0 21 50

58 3 0


octahedral one increases in TS-1-50, compared with those in TS1-80, suggesting that the insertion of Ti into the framework is nearly the maximum in this synthesis condition. Continuing adding of Ti leads to the sharp increase of octahedrally coordinated Ti. The TOFs of pure tetrahedrally, pentahedrally and octahedrally coordinated Ti in small-crystal TS-1 are 373.3, 1434.3 and 0 mol H2O2/(h mol Ti). This results conrms that pentahedral Ti is the most active species among the three coordination structures. It was also reported that pentahedrally coordinated Ti in a homogeneous catalyst showed a high catalytic activity in the epoxidation of cyclohexene.40,41 Moreover, a pentahedrally coordinated Ti formed by tetrahedrally coordinated Ti in Ti-MWW, H2O2, methanol and piperidine was reported and considered to show a sharp increasing activity in the epoxidation of propene, 1-hexene and cyclopentene.42 However, the controllable synthesis of TS-1 with a large amount of pentahedral Ti is still impossible. When more Ti was added to the synthesis gel, the pentahedral Ti would transform to tetrahedral and octahedral ones, due to the losing of pentahedral Ti generation conditions mentioned previously.

3.2. Treatment with organic bases Although zeolites are widely used in industry, one problem is the diffusion limitation by zeolite channels. Compared with those molecular sieves with mesopores or macropores, the hydrothermal stability of zeolite is excellent, but the diffusion resistance in channels is quite signicant. Therefore, improving diffusion within the channels is one way to improve performance of zeolites. Many studies were focused on the treatment of zeolites with organic bases, because the treatment could generate some mesopores in the zeolite crystals,43–46 which may decrease the diffusion resistance. In a previous work, we treated small-crystal TS-1 with different organic bases forming hierarchical structures.17 The catalytic performances of phenol hydroxylation over treated samples were improved to different extents, partly due to the unblocking of channels by the dissolution of silicon in the crystals. In this work, we characterize the treated samples with Ti L-edge XANES, and nd that the generation of pentahedrally coordinated Ti can be another reason for the increasing activity of TPAOH treated TS-1. The Ti L-edge XANES of the treated samples are shown in Fig. 4. The IH/L can be considered to be positive related with the percentage of the pentahedrally coordinated Ti in all kinds of Ti coordination states. The larger the IH/L is, the higher pentahedral Ti contains. The IH/L decreased to different extents aer the treatment except that of TS-1–TPAOH (Table 1). The IH/L of TS-1–TPAOH is higher than those of the other treated samples, indicating that pentahedral Ti was generated aer being treated with TPAOH. A shoulder peak appears in the spectra of TS-1–DEA and TS-1–TMAOH on the higher side of the higher energy peak of L3 edge, but it is very small in TS-1–EA and TS-1– TPAOH. The treatment with organic bases leads to the dissolution of silicon and titanium. The silicon and titanium will recrystallize in the present of a template.47 The recrystallization rate depends on the structure-directing effect of the base. The strong structure-directing effects of EA and TPAOH cause a

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3.3. Nano-sized TS-1

Fig. 4 Ti L-edge XANES spectra of organic bases treated small-crystal TS-1. (Peaks a–c are the characteristic peaks of L3 edge, while peaks d and e are those of L2 edge.)

Fig. 5

Raman spectra of organic bases treated small-crystal TS-1.

faster rate than those of DEA and TMAOH. Furthermore, TPAOH has a protection effect for framework Ti, which may prevent the dissolution of framework titanium.17 Therefore, the contents of anatase TiO2 in TS-1–EA and TS-1–TPAOH are less than those in TS-1–DEA and TS-1–TMAOH. Fig. 5 shows the Raman spectra of the samples treated with different organic bases. Aer the treatment, all the samples show weaker bands than the untreated ones due to the decrease of the amount of anatase TiO2. Some part of the TiO2 is probably dissoluted in the treatment or converted to pentahedrally coordinated Ti (such as TS-1–TPAOH). This conversion to pentahedral Ti is benecial for the oxidation reactions. The TOFs of the treated samples are all higher than the untreated TS-1-50. However, the reasons for the increased TOF may not be the same. TS-1–EA, TS-1–DEA and TS-1–TMAOH show a smaller IH/L than TS-1-50. Thus, the unblocking of channels is considered as the primary factor for the improvement of activity. The lower activity over TS-1–DEA than that over TS-1–EA is due to the weaker structure-directing effect of the former, leading to a slower recrystallization rate. The highest TOF, which can be inferred, is obtained over TS-1–TPAOH, accounted for the high IH/L demonstrating a relatively high pentahedral Ti content.

We also investigated the effect of pentahedrally coordinated Ti in nano-sized TS-1. Nano-sized TS-1 usually needs to be synthesized with the presence of a very expensive structuredirecting template, TPAOH. The substitution of TPAOH by TPABr oen makes the crystal size larger, leading to a poorer activity.48 Therefore, further improving the activity of nanosized TS-1 to reduce its unit synthesis cost is signicant. Increasing the content of framework Ti by adding ammonium salts is one of the methods.23 It was considered that the slowing down of crystallization rate was benecial for the incorporation of Ti into the TS-1 framework. The framework Ti is accepted by most researchers as the active center for oxidation reactions. Therefore, the addition of ammonium salts can improve the catalytic activity. In our opinion, its effect on crystallization rate may be one of the reasons. The two samples with and without adding of ammonium carbonate show a similar crystal size (Fig. S9†), indicating that the size is not the main factor affecting their catalytic performances. From the Ti L-edge XANES of nano-sized TS-1 (Fig. 6), it can be seen that the IH/L of TS-1–AC is higher than that of TS-1–NA (3.32 versus 2.15), indicating that the amount of pentahedrally coordinated Ti in the former is more than the latter. The UV-vis spectra of these two samples (Fig. 7) conrms that the coordination state of Ti between 230 and 290 nm in TS-1–AC is mainly pentahedrally coordinated Ti (239 nm), but it is octahedrally coordinated Ti (261 nm) in TS-1–NA. The inhibition of anatase TiO2 formation with the addition of (NH4)2CO3 is probed by UV-vis spectroscopy. A very sharp decrease of the amount of anatase TiO2 was observed in the spectrum of TS-1–AC. However, the amount of framework tetrahedrally coordinated Ti was not improved, but decreased slightly. This is due to the slowing down of crystallization rate by the addition making the insertion of Ti to the framework slower. The Ti in the liquid phase, thus can be combined with more than four [SiO4], forming pentahedral Ti. In other words, the tetrahedral Ti, extra-framework octahedral Ti and anatase TiO2 were transferred to the framework pentahedral Ti by the effect of ammonium carbonate. In Section 3.1, we found that a small amount of pentahedrally coordinated Ti could show a very high activity. Therefore, the increasing pentahedral Ti amount in TS-1–AC could also

Fig. 6

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Ti L-edge XANES spectra of nano-sized TS-1.


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program for the New Century Excellent Talent in University (Grant NCET-04-0268) and the Plan 111 Project of the Ministry of Education of China.

Notes and references

Fig. 7 UV-vis spectra of nano-sized TS-1.

lead to an obvious increase of activity. The catalytic performances of the two nano-sized TS-1 samples exhibit the same trend with what we supposed according to the XANES and UV-vis spectra. TS-1–AC shows a much higher TOF than TS-1–NA. The increase of the content of pentahedrally coordinated Ti, but not tetrahedrally coordinated Ti, is considered as the main reason for the improvement of catalytic activity.

4. Conclusions Three sets of TS-1 were evaluated and characterized. A new type of Ti species, pentahedrally coordinated Ti was discovered in TS-1. Tetrahedrally coordinated Ti is the main active center in TS-1, but pentahedrally coordinated Ti is considered to be more active than tetrahedral one. Treated with TPAOH can improve the catalytic performance of TS-1, partly due to the formation of pentahedrally coordinated Ti. A low content of Ti in the synthesis gel and the slowing down of crystallization rate are benecial for the generation of pentahedrally coordinated Ti. This study may be helpful for synthesizing TS-1 with a high catalytic activity.

Disclaimer Certain commercial names are mentioned in this manuscript only for the purpose of illustration and do not represent an endorsement by the National Institute of Standards and Technology.

Acknowledgements This work was partly nancialized by the Fundamental Research Funds for the Central Universities (2342013DUT13RC(3)704), China Postdoctoral Science Foundation (2014M551094), the


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