Synthesis of Titanium Silicalite-1 with Small Crystal Size by Using

Jun 15, 2011 - ABSTRACT: Titanium silicalite-1 (TS-1) with small crystal size was synthesized in a TPABr-ethylamine system using the mother liquid of ...
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Synthesis of Titanium Silicalite-1 with Small Crystal Size by Using Mother Liquid of Titanium Silicalite-1 As Seed Yi Zuo, Xiangsheng Wang, and Xinwen Guo* State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116023, P. R. China ABSTRACT: Titanium silicalite-1 (TS-1) with small crystal size was synthesized in a TPABr-ethylamine system using the mother liquid of nanosized TS-1 as seed. The as-synthesized small-crystal TS-1, the size of which was about 600 nm  400 nm  250 nm was characterized with X-ray diffraction (XRD), Fourier transform-infrared (FT-IR), ultravioletvisible (UVvis), UV-Raman, N2 sorption, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Its catalytic performance was evaluated in the epoxidation of propylene and the hydroxylation of phenol. The conversion of hydrogen peroxide and selectivity of propylene oxide (PO) in the propylene epoxidation reached 92 and 98 mol %, respectively. In the hydroxylation of phenol, smallcrystal TS-1 also resulted in a high conversion of phenol.

1. INTRODUCTION Propylene oxide is an important chemical intermediate for producing polyether polyol polymers. The commercial manufacture routes contain the chlorohydrin route and the Halcon route.1 The chlorohydrin route produces a lot of pollution, while the Halcon route is capital intensive and its economics depends on those of the coproducts.2 The epoxidation of propylene catalyzed by TS-1 using hydrogen peroxide as the oxidant provides an environmental friendly and economically viable route.25 Titanium silicalite-1 with MFI topology was first hydrothermally synthesized by Taramasso et al.3 in 1983. The unique catalytic performance of TS-1/H2O2 in selective oxidation reactions, such as the epoxidation of propylene has attracted the attention of many researchers.46 The classical synthesis method provided by Taramasso et al.3 using tetrapropylammonium hydroxide (TPAOH) as the template can produce TS-1 with very small crystal size (100200 nm), but the product is difficult to separate from the mother liquid. Moreover, TPAOH is expensive, which is one of the major hurdles for the application of TS-1 in industry.7 Therefore, it is important to develop an economically viable synthesis method. Instead of TPAOH, M€uller and Steck successfully obtained TS-1 using tetrapropylammonium bromide (TPABr) as the template,8 which could decrease the synthesizing cost, but the crystal size was larger than that from the synthesis with TPAOH. Since then, many studies have been performed using TPABr as the template to synthesize TS-1,9,10 but it is very difficult to generate TS-1 crystallites smaller than 1 μm. As a consequence, the activity of the catalysts and selectivity of PO, both of which are heavily influenced by diffusion restriction, are very low.11 Zhang et al.12 synthesized TS-1 with a crystal size of 3 μm  2 μm, using TiCl4 and colloidal silica, in the TPABr-NH3 3 H2O system. They found that the addition of powdered nanosized TS-1 (∼200 nm) as the crystallization seed could accelerate the crystallization, decrease the induction period, and render a higher crystallinity of the resulting TS-1. Mao et al.13 modified microsized TS-1 (∼1 μm) with TPAOH solution. TEM images showed that some large irregular porosity r 2011 American Chemical Society

appeared after modification, which facilitated the diffusion of the reactants and/or the main product, thus improved the catalytic performance. However, this modification requires extended preparation time, and the process still needs to use TPAOH. Wang et al.14 invented a method for the fast synthesis of nanosized (∼200 nm) TS-1. In this method, a suspension of nanosized TS-1 in the mother liquid was obtained after crystallization, which did not precipitate for a long time. The separation of the crystals from the mother liquid was difficult because of the small size of the crystals. Furthermore, there was excessive TPAOH which was about 0.1 mol/L in the mother liquid. In the present work, we studied the synthesis of TS-1 using the mother liquid of nanosized TS-1 as seed in a TPABr-ethylamine hydrothermal system. Combining synthesis with modification, we were able to shorten the preparation period and obtained small-crystal TS-1 with a size of about 600 nm  400 nm  250 nm. The appearance of the small-crystal TS-1 is significantly different from microsized TS-1, and its catalytic performance for propylene epoxidation and phenol hydroxylation is excellent.

2. EXPERIMENTAL SECTION 2.1. Preparation of TS-1. Titanium silicalite was prepared in the TPABr-ethylamine hydrothermal system reported in ref 15, using colloidal silica (30 wt %) and titanium tetrachloride as silicon and titanium source, respectively. TPABr was used as the template and aqueous ethylamine (65 wt %) as the base. Molar composition of the gel was

nðSiO2 Þ  nðTiO2 ÞnðTPABrÞnðC2 H5 NH2 ÞnðH2 OÞ ¼ 1 : 0:02 : 0:15 : 1:0 : 16 In contrast, we used nanosized TS-1 powder and its mother liquid added as crystallization seedings in the synthesis of TS-1. Received: February 9, 2011 Accepted: June 15, 2011 Revised: May 19, 2011 Published: June 15, 2011 8485

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Table 1. Material Balance and Synthesis Time of Small-Crystal TS-1 and Nano-Sized TS-1 small-crystal TS-1 feedstock

nanosized TS-1

weight (g)

feedstock

weight (g)

colloidal silica (30 wt %)

60.0

tetraethoxysilane

titanium tetrachloride

1.14

tetrabutyl titanate

16.6 0.79

TPABr ethylamine (65 wt %)

12.0 20.1

TPAOH (25 wt %)

18.7

water

65.0

water

4.7

isopropyl alcohol

4.7

isopropyl alcohol

5.0

seeding

1.05a

seeding

none

obtained TS-1

19.2

obtained TS-1

4.8

yield

98.5%

yield

97.2%

synthesis time

120 hb

synthesis time

60 h

a

The data is the weight of solid nanosized TS-1 in its mother liquid. b The synthesis time contains the time of seed preparation and smallcrystal TS-1 synthesis.

The powder and mother liquid of nanosized TS-1 were prepared according to ref 14. The concentration of the nanosized TS-1 was 0.7 g/mL in its mother liquid, and the mother liquid of the nanosized TS-1 used in the hydrothermal system was 10 vol % of the mother liquid of small-crystal TS-1. The amount of the powdered nanosized TS-1 added into the gel was the same as that contained in the mother liquid of the nanosized TS-1. The material balance of small-crystal TS-1 and nanosized TS-1 was shown in Table 1. The synthesis procedure was as follows: titanium tetrachloride was added into isopropyl alcohol; the obtained solution was added into colloidal silica. Finally, TPABr, ethylamine, seed, and water were added into the colloidal silica gradually. The mixture was stirred for half an hour, then transferred to a Teflon lined autoclave, and crystallized for 36 h at 170 °C. The obtained mother liquid could precipitate after 12 h. The solid was separated from the liquid and dried at 100 °C and then calcined at 540 °C for 6 h to remove the template. In this method, microsized TS-1 was obtained when the nanosized TS-1 powder was used as the seed, while small-crystal TS-1 was obtained when the mother liquid of the nanosized TS-1 was used as the seed. Modification of microsized TS-1 was carried out in a Teflon lined autoclave by adding microsized TS-1 into aqueous TPAOH solution, and the modifying conditions were 48 h at 170 °C according to ref 13. TS-1 was also synthesized using the improved conventional method, which was invented by Thangaraj and Sivasanker, according to ref 16. 2.2. Characterization of TS-1. X-ray diffraction (XRD) characterization was performed on a Rigaku Corporation D/ MAX-2400 using Cu KR radiation. FT-IR spectra were recorded on a Bruker EQUINOX55 spectrometer from 4000 to 400 cm1, and the KBr pellet technique was used. UVvis spectra were obtained on a Jasco UV-550 spectrometer from 190 to 500 nm, and pure BaSO4 was used as a reference. UVRaman spectra were recorded on a UVRaman spectrometer built at the State Key Laboratory of Catalysis (Dalian Institute of Chemical Physics, P. R. China). Nitrogen sorption measurements were performed at liquid nitrogen temperatures on a Quantachrome AUTOSORB-1 physical sorption apparatus. Total surface area and pore volume were calculated according to the BET and BJH method, respectively. The appearance of the crystals was

determined on a Hitachi scanning electron microscope (SEM) and Tecnai G220 S-Twin transmission electron microscope (TEM). 2.3. Epoxidation of Propylene. The epoxidation of propylene was carried out in a stainless-steel reactor. At first, 0.4 g of assynthesized catalyst, 24 mL of acetone, 8 mL of methanol, and 30 wt % of hydrogen peroxide were added to the reactor. The concentration of hydrogen peroxide was 1.1 mol/L. Propylene was then charged to the reactor to reach 0.4 MPa. The reaction was finished after heating the mixed solution at 60 °C for 1 h. The residual H2O2 was checked by iodometric titration. The products were analyzed on a 7890F gas chromatograph equipped with a flame ionization detector (FID) and a PEG-20 M capillary column (30 m  0.25 mm  0.4 μm). Propylene oxide (PO) was the main product, and propylene glycol (PG) and its monomethyl ethers (MME) were byproducts. The conversion of H2O2 (X(H2O2)), selectivity of PO (S(PO)), and utilization of H2O2 (U(H2O2)) were calculated as follows: XðH2 O2 Þ ¼ ðn0 ðH2 O2 Þ  nðH2 O2 ÞÞ=n0 ðH2 O2 Þ SðPOÞ ¼ nðPOÞ=ðnðPOÞ + nðMMEÞ + nðPGÞÞ UðH2 O2 Þ ¼ ðnðPOÞ + nðMMEÞ + nðPGÞÞ=ðn0 ðH2 O2 Þ 3 XðH2 O2 ÞÞ

The n0(H2O2) and n(H2O2) stand for the initial and final mole content of H2O2, respectively. The n(PO), n(MME), and n(PG) represent the number of moles of PO, MME, and PG, respectively. 2.4. Hydroxylation of Phenol. The hydroxylation of phenol was carried out in a glass reactor. A total of 0.4 g of as-synthesized catalyst, 8.4 mL of acetone, 4 g of phenol, and 30 wt % of hydrogen peroxide were added to the batch reactor. The molar ratio of phenol/H2O2 was 3/1. The reaction was at 80 °C for 6 h, and the residual H2O2 was checked by iodometric titration. The products were analyzed on a 7890F gas chromatograph with a FID and a SE30 capillary column (30 m  0.25 mm 0.5 μm). The products were catechol (CAT), hydroquinone (HQ), and para-benzoquinone (PBQ). The conversion and selectivity were calculated as follows: XðH2 O2 Þ ¼ ðn0 ðH2 O2 Þ  nðH2 O2 ÞÞ=n0 ðH2 O2 Þ XðPHEÞ ¼ 1  nðPHEÞ=ðnðPHEÞ + nðCATÞ + nðHQ Þ + nðPBQ ÞÞ

SðCATÞ ¼ nðCATÞ=ðnðCATÞ + nðHQ Þ + nðPBQ ÞÞ SðHQ Þ ¼ nðHQ Þ=ðnðCATÞ + nðHQ Þ + nðPBQ ÞÞ SðPBQ Þ ¼ nðPBQ Þ=ðnðCATÞ + nðHQ Þ + nðPBQ ÞÞ The n0(H2O2) and n(H2O2) stand for the initial and final mole content of H2O2, respectively. The n(PHE), n(CAT), n(HQ), and n(PBQ) stand for the number of moles of PHE, CAT, HQ, and PBQ, respectively.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Samples. Small-crystal TS-1 (sample 1-1) was synthesized using the mother liquid of nanosized TS-1 as the seed. To compare its catalytic performance with other TS-1 catalysts, microsized TS-1 (sample 1-2) was synthesized using powdered nanosized TS-1 as the seed. Sample 1-2 8486

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Figure 1. XRD patterns of small-crystal TS-1 (1-1), microsized TS-1 (1-2), microsized TS-1 modified by TPAOH (1-3), and nanosized TS-1 (1-4).

Figure 2. FT-IR spectra of small-crystal TS-1 (1-1), microsized TS-1 (1-2), microsized TS-1 modified by TPAOH (1-3), and nanosized TS-1 (1-4).

was then modified with TPAOH solution to get modified microsized TS-1 (sample 1-3). The nanosized TS-1 (sample 1-4) was synthesized following the improved conventional method.16 Figure 1 shows the X-ray diffraction (XRD) patterns of the four samples. All samples have the MFI topology, the characteristic peaks of which are sited at the 2θ of 7.8, 8.8, 23.0, 23.9, and 24.4°. After modification with TPAOH, the relative crystallinity decreased from 93.7% to 91.0% (the relative crystallinity which was calculated by comparing the total intensity of the characteristic peaks mentioned above). The possible reason is that the sample partially dissolved and then recrystallized during the modification.17 Sample 1-1 has the highest relative crystallinity of all (95.0%) and is not affected by the TPAOH in the mother liquid of nanosized TS-1. The lowest relative crystallinity was acquired on sample 1-4, which was only 84.4%. This may be due to its small size and short crystallization time (12 h). Fourier-transform infrared (FT-IR) spectra are shown in Figure 2. The bands at 550 and 800 cm1 were considered as the framework characteristic bands.18 The intensity of the peaks for sample 1-4 at 550 and 800 cm1 is very low, indicating its low relative crystallinity, which coincides with the XRD patterns. The introducing of titanium into the framework leads to a new band at 960 cm1. This band is due to stretching vibration of [SiO4] units strongly influenced by titanium ions in the neighborhood coordination place,19 and it does not exist in pure silicate molecular sieves. The spectra indicate that all the four samples contained tetrahedral Ti in the framework. The band intensity in

Figure 3. UVvis spectra of small-crystal TS-1 (1-1), microsized TS-1 (1-2), microsized TS-1 modified by TPAOH (1-3), and nanosized TS-1 (1-4).

the spectrum of sample 1-1 was strongest, implying that it contained the most framework Ti, which could be the reason of its highest catalytic activity (see section 3.2). Ultraviolet-visible diffuse reflectance (UVvis) spectroscopy is one of the first spectral techniques used for the detection of coordination states of Ti in titanium silicalites. Figure 3 shows the UVvis spectra of the four samples. Multibands were deconvoluted by the Peak Fit program using the Gaussian fitting method. The deconvolution results were also shown in Figure 3. There are three major bands for all samples, sited at 200210, 240260, and 300310 nm. The band at about 200210 nm, indicating the existence of framework Ti, from sample 1-1 is significantly higher than those from the others, which demonstrates the highest framework Ti content in sample 1-1.20,21 The band at 250280 nm is considered as a charge-transfer process in isolated [TiO4] or [HOTiO3] units, which is usually called nonframework Ti, while the band at 300310 nm is assigned to anatase TiO2. Comparing the bands of sample 1-2 with those of sample 1-3, it can be seen that the modification by TPAOH transformed some nonframework titanium, such as octahedral coordinated Ti, to anatase TiO2 which may accelerate the decomposition of hydrogen peroxide. Sample 1-1 contained a 8487

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Figure 4. UVRaman spectra of small-crystal TS-1 (1-1), microsized TS-1 (1-2), and microsized TS-1 modified by TPAOH (1-3). The excitation wavelength is 244 nm.

Figure 5. UVRaman spectra of small-crystal TS-1 (1-1), microsized TS-1 (1-2), and microsized TS-1 modified by TPAOH (1-3). The excitation wavelength is 325 nm.

slightly stronger band at ∼300 nm than sample 1-2, which may be caused by residual TPAOH in the mother liquid of nanosized TS-1. Sample 1-4 contains the most nonframework Ti and anatase TiO2, maybe because the titanium had not been inserted into the framework completely in such a short crystallization time. However, the utilization of H2O2 was not affected by the anatase TiO2 obviously. UVRaman spectroscopy is a sensitive technique for detecting the chemical environment of metal cations, such as titanium.22 Figures 4 and 5 are the UV-Raman spectra of samples 1-1, 1-2, and 1-3, measured with excitation wavelength of 244 and 325 nm, respectively. In Figure 4, the 490, 530, and 1125 cm1 bands are considered to be due to framework Ti.23 The band at 380 cm1 is a character of five-member ring framework. Smallcrystal TS-1 had the highest relative intensity ratio I1125/I380, which indicates that it contains the largest amount of framework Ti.24 This agrees with the finding of UVvis spectra (see Figure 3). In Figure 5, the bands at 390, 516, and 637 cm1 are the proof of the presence of anatase TiO2.23 Apparently, there are framework and nonframework Ti in the three samples. In other words, it is difficult to avoid nonframework Ti in a hydrothermal synthesis. Figure 6 shows the nitrogen sorption isotherms of samples 1-1, 1-2, and 1-3. There are hysteresis loops for both the small-crystal TS-1 (sample 1-1) and the modified microsized TS-1 (sample 1-3). For sample 1-1, this may be due to the mesopores between the crystals, but for sample 1-3 it is likely because of the

Figure 6. N2 sorption isotherms of small-crystal TS-1(1-1), microsized TS-1 (1-2), and microsized TS-1 modified by TPAOH (1-3).

Table 2. Surface Area and Pore Volume of Small-Crystal TS1(1-1), Micro-Sized TS-1 (1-2) and Micro-Sized TS-1 Modified by TPAOH (1-3)a SBET

SBET,micro

pore volume

pore volumemicro

cat.

(m2/g)

(m2/g)

(cm3/g)

(cm3/g)

1-1

402

302

0.32

0.14

1-2

387

274

0.25

0.12

1-3

375

264

0.35

0.11

a

The SBET and SBET,micro stand for the total and micropore surface areas, respectively. The pore volume and pore volumemicro stand for the total and micropore pore volumes, respectively.

mesopores inside the crystals (see Figure 8). The BET surface area and pore volume (Table 2) of the small-crystal TS-1 are larger than the corresponding values for the microsized TS-1, which would be one of the reasons that higher catalytic activity was obtained on the small-crystal TS-1. It is interesting that after modification with TPAOH, the surface area decreased slightly but the pore volume increased. We believe that the modification generated mesopores inside the crystals by combining the micropores as indicated by the pore volume values in Table 2. The scanning electron microscope (SEM) images of the samples are shown in Figure 7. The improved conventional method produced nanosized TS-1, whose size was about 150 nm (sample 1-4). When using the powdered nanosized TS-1 as the seed in TPABr-ethylamine, the crystal size became bigger, which was about 1 μm. The TPAOH modification had little influence 8488

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Figure 7. SEM images of small-crystal TS-1 (1-1), microsized TS-1 (1-2), microsized TS-1 modified by TPAOH (1-3), and nanosized TS-1 (1-4).

on the surface of the crystals. It has been proposed that the modification improves catalytic performance by generating mesopores inside the crystals.13 However, the appearance of smallcrystal TS-1 is entirely different from that of the others. Its size is about 600 nm  400 nm  250 nm. Furthermore, unlike the smooth surface of the microsized TS-1, a rough surface emerged on the small-crystal TS-1, which would provide more active centers on the surface. Figure 8 shows transmission electron microscope (TEM) images of samples 1-1, 1-2, and 1-3. No irregular mesopores were observed inside the small-crystal TS-1, and its pore is uniform. Therefore, the hysteresis loop (shown in Figure 6) in the N2 sorption isotherm for small-crystal TS-1 may be due to the mesopores between the crystals. Many irregular pores appeared in microsized TS-1 modified by TPAOH (sample 1-3), which was considered to be the reason for high catalytic performance. 3.2. Catalytic Performance Measurements. Two probe reactions, the epoxidation of propylene and the hydroxylation of phenol, were carried out to evaluate the catalytic performance of the four samples. In Table 3, the results of the epoxidation of propylene were presented. The conversions of H2O2 were almost the same on the small-crystal TS-1 and the microsized TS-1 modified by TPAOH and significantly higher than that on the microsized TS-1 and the nanosized TS-1. Modification with

TPAOH might increase the conversion by unblocking the channel so that the reactants can diffuse into the crystals more easily.13 The high activity of small-crystal TS-1 may be accounted for by its smaller size and rougher surface, which provided a larger surface area and more active sites. The low conversion of H2O2 on the nanosized TS-1 may be due to its low crystallinity. The selectivity of propylene oxide was only 91% on the microsized TS-1, probably because its larger size obstructed the diffusion of the main product, which thus continued to react with methanol or water to form a byproduct. The microsized TS-1 modified with TPAOH generated some irregular mesopores which could diminish the influence of the internal diffusion limitation, so the selectivity of PO increased sharply. However, no mesopores were observed by TEM in crystals of sample 1-1. Thus, its high selectivity of PO should not be due to mesopores, but to the smaller size which is beneficial to diffusion of product. High selectivity of PO for sample 1-4 was also accounted for by its small size. The utilization of H2O2 for the samples are almost the same and all above 95%; in other words, the small amount of anatase TiO2 in the catalysts hardly influences the decomposition of H2O2. The results of phenol hydroxylation are shown in Table 4. The conversions of H2O2 and phenol are similar on samples 1-1, 1-3, and 1-4, and higher than that on sample 1-2. The molecular size of phenol is larger than that of propylene, so that the diffusion 8489

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Figure 8. TEM images of small-crystal TS-1(1-1), microsized TS-1 (1-2), and microsized TS-1 modified by TPAOH (1-3).

Table 3. Catalytic Performances of the Samples for Epoxidation of Propylenea cat.

XH2O2/%

SPO/%

UH2O2/%

1-1

92

98

97

1-2

86

91

95

1-3

91

95

95

1-4

82

95

93

a

Reaction conditions: catalyst 0.4 g, acetone 24 mL, methanol 8 mL, H2O2 1.1 mol/L, propylene pressure 0.4 MPa, 333 K, 1 h.

Table 4. Catalytic Performances of the Samples for Hydroxylation of Phenola

a

cat.

XH2O2/%

XPHE/%

SCAT/%

SHQ/%

SPBQ/%

1-1

97

22

53

43

4

1-2

94

16

47

44

9

1-3

97

23

52

46

2

1-4

98

23

47

44

9

Reaction conditions: catalyst 0.4 g, acetone 8.4 mL, n(phenol)/ n(H2O2) = 3, phenol 4.0 g, 353 K, 6 h.

limitation is more serious. The irregular mesopores in sample 1-3 can supply enough space for phenol, which can lead to a high catalytic activity of the catalyst.13 The small-crystal TS-1 has no mesopore in the crystals according to the TEM images. The reasons that the small-crystal TS-1 and the nanosized TS-1 performed well are due to their large surface area and small crystal size.

4. CONCLUSIONS Small-crystal titanium silicalite (TS-1) has been synthesized in a TPABr-ethylamine system using the mother liquid of nanosized TS-1 as the crystallization seed. Using this method, we cut down the preparation period of the catalyst and obtained small-crystal TS-1 with a size of about 600 nm  400 nm  250 nm. The precipitation of the mother liquid of small-crystal TS-1 is relatively quick, which led to an easy separation. The as-synthesized small-crystal TS-1 has a good catalytic performance both for the epoxidation of propylene and for the hydroxylation of phenol. This is attributed to the following factors: (a) larger surface area and rougher surface provide more active sites for the reactions so that reactants conversions are higher and (b) smaller size of crystals and larger pore size than 8490

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by the program for New Century Excellent Talent in University (Grant NECT-04-0268) and the Plan 111 Project of the Ministry of Education of China. The authors also thank Prof. Roel Prins for his thoughtful discussion. ’ REFERENCES (1) Kirk, R. O.; Dempsey, T. J. Kirk-Othmer Encyclopedia of Chemical Technology; Wiley: New York, 1982; Vol. 19, p 246. (2) Wulff, H. P.; Wattimena, F. Olefin epoxidation. U.S. Patent 4,021,454, May 3, 1977. (3) Taramasso, M.; Pergo, G.; Notari, B. Preparation of porous crystalline synthetic materials comprised of silicon and titanium oxides. U.S. Patent 4,410,501, October 18, 1983,. (4) Clerici, M. G.; Bellussi, G.; Romano, U. Synthesis of propylene oxide from propylene and hydrogen peroxide catalyzed by titanium silicalite. J. Catal. 1991, 129, 159. (5) Clerici, M. G.; Ingallina, P. Epoxidation of lower olefins with hydrogen peroxide and titanium silicalite. J. Catal. 1993, 140, 71. (6) Neri, C.; Anfossi, B.; Esposito, A.; Buonomo, F. Process for the epoxidation of olefinic compounds. U.S. Patent 4,833,260, May 23, 1989. (7) Li, G.; Wang, X. S.; Yan, H. S.; Liu, Y. H.; Liu, X. W. Epoxidation of propylene using supported titanium silicalite catalysts. Appl. Catal., A 2002, 236, 1. (8) Muller, U.; Steck, W. Ammonium-Based Alkaline-Free Synthesis of MFI-Type Boron- and Titanium Zeolites. Stud. Surf. Sci. Catal. 1994, 84, 203. (9) Wang, X. S.; Guo, X. W. Synthesis, characterization and catalytic properties of low cost titanium silicalite. Catal. Today 1999, 51, 177. (10) Li, G.; Guo, X. W.; Wang, X. S.; Zhao, Q.; Bao, X. H.; Han, X. W.; Lin, L. W. Synthesis of titanium silicalites in different template systems and their catalytic performance. Appl. Catal., A 1999, 185, 11. (11) Yan, H. S.; Liu, J.; Wang, X. S. Study on solvolysis of propylene oxide. Chin. J. Catal. 2001, 22, 250. (12) Zhang, Y. H.; Wang, X. S.; Guo, X. W. Preparation of titanosilicalite TS-1 using TiCl4 dissolved in alcohol as titanium source. Chin. J. Fuel Chem. Technol. 2000, 28, 550. (13) Mao, J. B.; Liu, M.; Li, P.; Wang, X. S. Effect of Tetrapropyl ammonium hydroxide modification of micron-scale TS-1 on ammoxidation of methyl ethyl ketone. The 15th International Zeolite Conference, Beijing, China, August 1217, 2007. (14) Wang, L. Q.; Wang, X. S.; Guo, X. W. Quick synthesis of titanium silicalite-1. Chin. J. Catal. 2001, 22, 513. (15) Wang, X. S.; Zuo, Y.; Guo, X. W. Quick synthesis of TS-1 with small crystal size in cheap hydrothermal system. CN Patent 201010235977.3, 2010. (16) Thangaraj, A.; Sivasanker, S. An improved method for TS-1 synthesis: 29Si NMR studies. J. Chem. Soc., Chem. Commun. 1992, 2, 123. (17) Wang, Y. R.; Lin, M.; Tuel, A. Hollow TS-1 crystals formed via a dissolutionrecrystallization process. Micro. Meso. Mater. 2007, 102, 80. (18) Vayssilov, G. N. Structural and physicochemical features of titanium silicalites. Catal. Rev.Sci. Eng. 1997, 39, 209. (19) Scarano, D.; Zecchina, C.; Bordiga, S.; Geobaldo, F.; Spoto, G.; Petrini, G.; Leofanti, G.; Padovan, M.; Tozzola, G. Fourier-transform infrared and Raman spectra of pure and Al-, B-, Ti-, and Fe-substituted

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