Article pubs.acs.org/IECR
Synthesis of TS‑1 from an Inorganic Reactant System and Its Catalytic Properties for Allyl Chloride Epoxidation Meng Wang,† Jicheng Zhou,*,† Guiyue Mao,† and Xianglan Zheng† †
Key Laboratory of Green Catalysis and Chemical Reaction Engineering of Hunan Province, School of Chemical Engineering, Xiangtan University, Xiangtan 411105, P. R. China ABSTRACT: TS-1 has been synthesized from an inorganic reactant system using Ti(SO4)2 as the titanium source, TPABr as the templating agent, and ammonia as the base to provide the alkalinity needed for crystallization. The effects of preparation parameters, such as titanium sources, crystallization temperature and time, SiO2/TiO2, TPABr/SiO2, ammonia/SiO2 and seed crystals on the physicochemical and catalytic properties of TS-1 were investigated in detail. The TS-1 samples were characterized by XRD, FT-IR, UV−vis, SEM, and ICP-AES, and the catalytic performance of TS-1 was evaluated in the epoxidation of allyl chloride. The results show that the catalytic performance of TS-1 synthesized using Ti(SO4)2 was excellent, and the crystal size of the catalyst was small. The synthesis conditions had a great influence on the properties of TS-1. The optimum synthesis conditions for the present system should employ a gel composition of SiO2/TiO2/NH3/TPABr/H2O = 1:0.022:2.5:0.15:35 and carry out the crystallization at 443 K for 3 days; the addition of seed in the synthesis gel played an important role in the crystallization of TS-1. Moreover, the stability and reusability of TS-1 prepared from this system was fine.
1. INTRODUCTION Since it was first successfully synthesized by EniChem in 1983,1 titanium silicalite-1 (TS-1) has attracted much research attention for its remarkable catalytic properties to the selective oxidation using H2O2 as green oxidant, such as aromatic hydroxylation,2 oxidation of alcohols and alkanes,3,4 ammoximation of ketone, and epoxidation of alkenes.5,6 Originally, TS1 zeolite was synthesized from organic reactant system by using tetraethyl orthosilicalite (TEOS) and tetraethyl orthotitanate (TEOT) as the silicon and titanium resource, respectively, and using a large amount of tetrapropylammonium hydroxide (TPAOH) as the template.1 However, these organic materials such as TPAOH and TBOT are expensive;7 the ethanol formed from the hydrolysis of TEOS requires a troublesome step of eliminating; the TS-1 is too small (100−200 nm) to separate from the mother liquid or the reaction solution. Therefore, the preparation cost of TS-1 is high, and its further commercial applications are delayed. To reduce the cost of TS-1, researchers studied the synthesis of TS-1 by using cheap template, inorganic titanium, and silicon sources. Muller and Steck reported a kind of TS-1 prepared from the TPABr-NH3·H2O system,8 respectively, titanium tetraisopropoxide (TPOT) and colloidal silica as titanium and silicon sources, and the synthesis of TS-1 with TPABr and other organic amines as the template has also been reported.9−11 Xia and Gao adopted a mixture of TEACl and TBACl as the templating agents and TiCl3 as the titanium source to synthesize TS-1,12 but a certain amount of the seeds and a concentrated ammonia solution must be used. Recently Wu et al.13 successfully prepared TS-1 with the cooperative effects of active seeds and the structure-directing agent hexamethyleneimine (HMI) for a long crystallization time (6 days). Tuel et al.14 reported the synthesis of TS-1 by using TiF4 as the titanium source, while Zhou used TiCl3 as the titanium source and TPABr as template agent.15 Gabelica et al.16 prepared TS-1 using the media of TPABr-TiCl4-HF-H2O2; the © 2012 American Chemical Society
products, however, were complex, and the crystal size was very large (16−60 μm). Other researchers also reported the synthesized TS-1 by using (NH2)2TiF6 and titanium dioxide as titanium sources.17,18 Moreover, Lu et al.19 used amorphous SiO2 as the silicon source and TPABr as the template to prepare TS-1. It is well-known that there is a close relationship between the crystal size and the catalytic activity of TS-1,20 the addition of seeds is useful to decrease the crystal sizes, promote the nucleation, and enhance the crystallization rate in the synthesis of TS-1.21,22 Epichlorohydrin (ECH) is an important fine chemical product and organic chemical raw material; it can be used to synthesize epoxy resins, chlorohydrine rubber, and glycerin. In current industrial processes, there are two main manufacturing routes - the method via allyl acetate and the high temperature chlorination of propylene, which all produce a large quantity of calcium chloride byproduct and halogen-containing wastewater. So a greener technique to synthesize ECH is urgently required. Compared with the other routes, the direct epoxidation of allyl chloride (ALC) to ECH catalyzed by TS-1 using H2O2 as oxidant is a green and low cost process.23−25 In this method, the byproducts have three main sources under the catalysis of TS-1: the solvolysis of ECH with methanol, the hydrolysis of ECH, and the reoxidation of ECH (see Scheme 1).25,26 In this paper, TS-1 was prepared by hydrothermal synthesis using TPABr, colloidal silica, and Ti(SO4)2 as the template, silicon source, and titanium source, respectively, instead of costly TPAOH, Ti-alkoxide, and Si-alkoxide, which has not yet been reported in the literature. The effects of various parameters on the synthesis of TS-1 were investigated in Received: Revised: Accepted: Published: 12730
November 3, 2011 April 1, 2012 September 6, 2012 September 6, 2012 dx.doi.org/10.1021/ie202524t | Ind. Eng. Chem. Res. 2012, 51, 12730−12738
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MFI zeolite. The sample with the largest integrated intensities of diffraction peaks was selected as the reference and considered as the 100% crystalline. FT-IR spectra were recorded on a Nicolet 380 spectrometer from 4000 to 400 cm−1, and the KBr pellet technique was used. The UV−vis measurements were performed on the Shimadzu UV-2550 spectrometer by using the diffuse reflectance technique in the range of 200−500 nm and BaSO4 as the reference. 2.4. Epoxidation of Allyl Chloride. The epoxidation of allyl chloride (AC) was carried out in a 250 mL glass flask equipped with a condenser. In a typical run, 7.01 g of allyl chloride, 15.65 g of methanol solvent, 7.65 g of H2O2 (30 wt % aqueous solution), and 1.0 g of catalyst were mixed in the reactor. The molar ratio of AC/H2O2 was 1.35/1. The reaction was carried out at 303 K for 40 min under magnetic stirring. The products of reaction were analyzed on an Aglient 7890N gas chromatograph with a FID and a DB-17 capillary column (30 m × 0.32 mm × 0.5 μm), and the residual H2O2 was checked by iodometric titration. The conversion of H2O2 (X(H2O2)), the selectivity to epichlorohydrin (S(ECH)), the yield to epichlorohydrin (Y(ECH)), and the utilization of H2O2 (U(H2O2)) were calculated as follows (selectivity and yield were calculated based on H2O2):
Scheme 1
detail, the catalytic activity and the stability and reusability of TS-1 have also been tested.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Seeds. The TS-1 seeds was prepared by hydrothermal synthesis using colloidal silica (30 wt %) as the silicon source, Ti(SO4)2 as the titanium source, and TPAOH as the template. Ti(SO4)2 and colloidal silica were hydrolyzed in an aqueous solution of TPAOH and ammonia (25 wt % aqueous solution), and a clear solution with a molar composition of SiO2/TiO2/TPAOH/H2O/NH3= 1:0.022:0.10:30:1.0 was obtained. The gel was placed into an autoclave and heated at 443 K for 3 days. The solids obtained were filtered, washed with distilled water until the filtrate reached pH = 7, then dried at 393 K for 12 h, and calcined at 823 K for 5 h in air. 2.2. Preparation of TS-1. TS-1 was prepared by using colloidal silica (30 wt %) as the silicon source, Ti(SO4)2 as the titanium source, TPABr as the template, and ammonia (25 wt % aqueous solution) as the base to adjust the pH value of the matrix gel. A typical synthesis of TS-1 was as follows: first, TPABr aqueous solution and ammonia were added into the colloidal silica together, then Ti(SO4)2 was dissolved in distilled water, and the obtained solution was added into the above gel under stirring. The mixture was stirred for an hour to obtain a gel with a molar composition of SiO2/TiO2/NH3/TPABr/H2O = 1:0.011−0.066:1.0−5.0:0.03−0.15:35. Finally, the seeds were added; the amount of seeds added corresponded to a proportion of 0−9.0 wt % (dry basis) of silica relative to the total amount of silica in the gel. After stirring for five minutes, the gel was transferred to Teflon-lined autoclaves and crystallized under autogenous pressure at 443 K for 3 days. The products were filtered, washed several times with distilled water, dried overnight at 393 K, and calcined in air at 823 K for 5 h. In order to understand the effects of titanium sources on the properties of TS-1 prepared, the TiCl3 (15 wt %) and TBOT as titanium sources were used to synthesize TS-1 using the above method. At last, TS-1 has been synthesized using an organic reactant system described by Thangaraj et al. for comparison.27 2.3. Characterization of TS-1. The scanning electron micrographs (SEM) were obtained on the JSM-6610LV microscope. Chemical composition of the TS-1 samples was analyzed by the ICP-AES spectrometry (TJA IRIS1000). The X-ray diffraction (XRD) analysis was performed on the Rigaku D/MAX-2550 diffractometer using Cu Ka radiation (λ = 0.1542 nm) and a graphite monochromator. The relative crystallinity of TS-1 was estimated by comparing the intensities of five characteristic diffraction peaks (2θ = 7.8, 8.8, 22°−25°) of the
X(H 2O2 ) = (n0(H 2O2 )‐n(H 2O2 ))/n0(H 2O2 )
(1)
S(ECH) = n(ECH)/(n(ECH) + n(other))
(2)
Y (ECH) = n(ECH)/n0(H 2O2 )
(3)
U (H 2O2 ) = Y (ECH)/(S(ECH) × X (H 2O2 ))
(4)
The n0(H2O2) and n(H2O2) stand for the initial and final mole content of H2O2, respectively. The n(ECH) and n(other) represent the number of moles of ECH and its byproduct, respectively. For the regeneration of TS-1, the spent catalysts were separated by centrifugation after reaction, then dried at 373 K, and calcined in air at 823 K for 5 h.
3. RESULTS AND DISCUSSION 3.1. Effect of the Titanium Source. Using TPABr as the template, we synthesized a series of titanium silicalite-1 (TS-1)
Figure 1. XRD patterns of TS-1 samples synthesized with different titanium sources: (a) TiCl3, (b) TBOT, (c) Ti(SO4)2, and (d) TBOT (synthesized from the classic method). 12731
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Table 1. Effect of Ti-Source on the Physicochemical and Catalytic Properties of TS-1a allyl chloride epoxidation (%)b
Ti/Si (mol) 3
Ti-source
gel
solid
I960/I550
VCell (Å )
XH2O2
SECH
UH2O2
YECH
TiCl3 TBOT Ti(SO4)2 TBOTc
0.022 0.022 0.022 0.022
0.020 0.021 0.021 0.021
0.36 0.40 0.44 0.45
5377 5385 5395 5399
93.76 99.29 99.57 100
99.59 99.93 99.97 99.93
93.95 98.50 99.50 99.17
87.73 97.73 99.04 99.10
a
Composition of gel: SiO2:0.15 TPABr:0.022 TiO2:2.5 NH3:35 H2O with seeds (6.6 wt % of silica); crystallized at 443 K for 3 days. bReaction conditions: catalyst 1.0 g, allyl chloride 7.01 g, methanol solvent 15.65 g, H2O2(30 wt %) 7.65 g, 303 K, 40 min. cThis sample was synthesized from the classic method.
synthesized by different titanium sources. As seen from Figure 1, all the samples exhibit the MFI structures without the impurity phase. The unit cell volumes and chemical compositions of the TS-1 samples prepared using different titanium sources are listed in Table 1. The results show that the ratio of Ti/Si in all the TS-1 samples was a little lower than that in the gel, in other words, the amount of Ti in TS-1 was lower than that in the gel. Compared with samples a and b, the unit cell volume of sample c was higher. Because the Vcell increases linearly with the Ti content in the framework,30 the Ti amount in the framework of sample c was a little higher than samples a and b. The FT-IR technique is a useful tool for characterizing framework titanium species. The FT-IR spectra in Figure 2 shows that all samples show a characteristic absorption peak at about 960 cm−1, which indicates that titanium has been incorporated into the framework of TS-1 zeolite.28,33 Reddy et al.29 reported that the relative intensity of the absorption peaks at about 960 cm−1 to 550 cm−1 (I960/I550) in the FT-IR spectrum increases linearly with the increase of Ti content in the framework which is the active site for an oxidation reaction. The data in Table 1 show that the I960/I550 ratio of sample c is nearly the same as sample d and higher than the other two, so the amount of framework Ti in this sample was higher than samples a and b. UV−visible spectroscopy is also an effective method for characterizing the coordination environment of titanium in zeolite frameworks. Figure 3 shows the UV−visible spectra of TS-1 samples. All the samples exhibited a band at 210 nm, which is attributed to charge transfer from oxygen atoms to Ti4+, characteristic of tetrahedrally coordinated Ti in the framework.11,32 In the spectrum of samples a and b, an obvious absorption peak at 330 nm which is attributed to the extraframework TiO2 was found.11 Compared with samples a and b, the UV−vis spectra of sample c was more smooth and the peak at 330 nm was much smaller, which indicated the amount of extra-framework TiO2 of this sample was less than the other two. This agrees with the result of FT-IR and Vcell, and even contrasting with sample d, the peak intensity of sample c at 330 nm was low. The SEM microscopy of different samples is shown in Figure 4. The pellet appearance of sample d was spherical or elliptical, and its diameter was small (100−200 nm). Sample a was in the form of a rectangular plank, and the size (10.7 μm × 6.6 μm × 1.7 μm) was much larger than samples b and c. Additionally, some structures referring to twin crystals and epitaxy growth on the surface of the big crystallites can be clearly observed in sample a, and the surface of sample a was rougher than the others. Samples b and c exhibited a quite different morphology, in which the crystals showed a form of hexagonal prism (Figure 4 b, c). Furthermore, the crystal size of sample c (2.0 μm × 1.1
Figure 2. FT-IR spectra of TS-1 samples synthesized with different titanium sources: (a) TiCl3, (b) TBOT,(c) Ti(SO4)2, and (d) TBOT (synthesized from the classic method).
Figure 3. UV−vis spectra of TS-1 samples synthesized with different titanium sources: (a) TiCl3, (b) TBOT, (c) Ti(SO4)2, and (d) TBOT (synthesized from the classic method).
with TiCl3 (sample a), TBOT (sample b), and Ti(SO4)2 (sample c) as titanium sources, respectively. For comparison, TS-1 (sample d) was also synthesized by an organic reactant system using TBOT, TEOS, and TPAOH as raw material.27 Figure 1 presents the XRD patterns of TS-1 samples 12732
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Figure 4. SEM micrographs of TS-1 synthesized with different titanium sources: (a) TiCl3, (b) TBOT, (c) Ti(SO4)2, and (d)TBOT (synthesized from the classic method).
exhibited a relative higher catalytic activity (Y(ECH) = 99.04% and U(H2O2) = 99.50%). What is more, the useless decomposition of hydrogen peroxide over sample a was less (higher utilization of H2O2) than over the other samples, which was related to the least presence of extra-framework TiO2 and the lower reaction temperature. Compared with sample d, the sample synthesized from Ti(SO4)2 had a similar activity. However, for the catalyst separation after reaction, the TS-1 prepared with the classic method needed a long period of high speed centrifugation, while TS-1 synthesized from Ti(SO4)2 with an inorganic route can be separated easily by filtration which is much easier to achieve and has a low cost in industrial production. When we select the titanium source used for the sythesis of TS-1, the role we must follow is that the hydrolytic rate of titanium source must match with the hydrolytic rate of silicon source, which is closely bound up with the amount of Ti atoms incorporated into the framework of TS-1. From the above investigation, we found that the hydrolytic rate of titanium source can match with the hydrolytic rate of silicon source well when we used Ti(SO4)2 as titanium source. For the hydrolysis of TBOT and TiCl3, a high basicity (pH = 12.0−13.5) is required;12 the pH value of the gel synthesized using ammonia as the base (pH = 10.5−11.5) is too low to match the hydrolysis of TBOT and TiCl3 with the hydrolysis of colloidal silica. Moreover, the incomplete hydrolysis of the titanium source promoted the formation of extra-framework TiO2. The results above indicate that Ti(SO4)2 can be used as the titanium source to synthesize TS-1 instead of TBOT.
Figure 5. XRD patterns of TS-1 samples synthesized from Ti(SO4)2 with different crystallization times: (a) 12, (b) 24, (c) 36, (d) 48, (e) 60, and (f) 72 h.
μm × 0.4 μm) was smaller than that of sample b (5.0 μm × 2.0 μm × 0.5 μm). Table 1 shows the catalytic properties of TS-1 synthesized from different titanium sources. It was reported that the framework Ti of TS-1 is the active site for an oxidation reaction and the catalytic activity of TS-1 increases with the decrease of its crystal size.3,20 Sample c had a higher framework Ti content and a smaller crystal size than samples a and b, so this sample 12733
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Table 2. Effects of the Crystallization Time on the Physicochemical and Catalytic Properties of TS-1 Synthesized from Ti(SO4)2b allyl chloride epoxidation (%)c 3
crystallization time (h)
relative crystallinity(%)
VCell (Å )
pH value of gel
12 24 36 48 60 72
22.91 44.88 94.95 96.97 97.92 99.30
5313 5327 5378 5380 5387 5397
10.88 10.74 10.64 10.91 11.01 10.98
a
XH2O2
SECH
UH2O2
YECH
4.05 96.25 97.84 98.43 99.23 99.57
67.53 98.55 99.09 99.81 99.92 99.97
78.97 97.77 98.01 98.30 99.45 99.50
2.16 92.74 95.02 96.57 98.61 99.04
a
The pH value of the gel after crystallization. bComposition of gel: SiO2:0.022 TiO2:0.15 TPABr:2.5 NH3:35 H2O with seeds (6.6 wt % of silica); crystallized at 443 K. cReaction conditions: same with Table 1.
Table 3. Effects of the Crystallization Temperature on the Physicochemical and Catalytic Properties of TS-1 Synthesized from Ti(SO4)2a allyl chloride epoxidation (%)b crystallization temperature (K)
I960/I550
relative crystallinity (%)
VCell (Å3)
XH2O2
SECH
UH2O2
YECH
423 433 443 453
0.37 0.40 0.43 0.36
51.48 94.02 99.30 94.75
5374 5382 5395 5389
99.24 98.86 99.57 99.17
99.01 99.43 99.97 99.70
95.70 98.07 99.50 98.14
94.03 96.43 99.04 97.07
a
Composition of gel: SiO2:0.022 TiO2:0.15 TPABr:2.5 NH3:35 H2O with seeds (6.6 wt % of silica); crystallized for 3 days. bReaction conditions: same with Table 1.
Table 5. Effect of TiO2/SiO2 on the Physicochemical and Catalytic Properties of TS-1 Synthesized from Ti(SO4)2a TiO2/SiO2 (mol)
allyl chloride epoxidation (%)b
gel
solid
I960/ I550
VCell (Å3)
XH2O2
SECH
UH2O2
YECH
0.011 0.022 0.044 0.066
0.011 0.021 0.042 0.062
0.28 0.41 0.41 0.43
5381 5386 5387 5389
98.76 98.51 99.27 99.43
99.65 99.96 99.91 99.89
95.61 98.98 97.07 93.58
94.09 97.47 96.27 93.15
a
Composition of gel: SiO2:0.10 TPABr:2.5 NH3:35 H2O with seeds (6.6 wt % of silica); crystallized at 443 K for 3 days. bReaction conditions: same with Table 1.
prepared from Ti(SO4)2. Figure 5 shows the XRD patterns of TS-1 samples crystallized at 443 K for 12−72 h. Few TS-1 crystals were formed when the crystallization time was 24 h. However, the crystallinity increased quickly by extending the crystallization time; the relative crystallinity increased to 94.95% after 36 h and reached a maximum (99.30%) at 72 h. The pH value of the gel did not have any obvious change for the buffering of ammonia; this is different with the crystallization of TS-1 using TPAOH as the template agent. The nucleation of TS-1 synthesized with the inorganic reactant
Figure 6. XRD patterns of TS-1 samples synthesized from Ti(SO4)2 with TPABr/SiO2 molar ratios of (a) 0.03, (b) 0.05, (c) 0.10, and (d) 0.15.
3.2. Effects of the Crystallization Condition. 3.2.1. Crystallization Time. A series of experiments was carried out to examine the effect of crystallization time on crystallinity of TS-1
Table 4. Effect of TPABr/SiO2 on the Physicochemical and Catalytic Properties of TS-1 Synthesized from Ti(SO4)2a allyl chloride epoxidation (%)b TPABr/SiO2 (mol)
I960/I550
relative crystallinity(%)
VCell (Å3)
XH2O2
SECH
UH2O2
YECH
0.03 0.05 0.10 0.15
0.36 0.40 0.41 0.43
58.46 83.80 90.60 99.30
5374 5378 5386 5395
90.32 98.93 98.51 99.57
99.06 99.21 99.96 99.97
93.34 98.05 98.98 99.50
83.51 96.23 97.47 99.04
a
Composition of gel: SiO2:0.022 TiO2:2.5 NH3:35 H2O with seeds (6.6 wt % of silica); crystallized at 443 K for 3 days. bReaction conditions: same with Table 1. 12734
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Figure 7. UV−visible spectra of TS-1 samples synthesized from Ti(SO4)2 with TiO2/SiO2 molar ratios of (a) 0.011, (b) 0.022, (c) 0.044, and (d) 0.066.
Figure 9. XRD patterns of TS-1 samples synthesized from Ti(SO4)2 with different amount of seeds: (a) without seeds, (b) 3.0 wt %, (c) 6.0 wt %, and (d) 9.0 wt %.
temperature of TS-1 was 443 K. The nucleation and crystallization rates of TS-1 were slow when the crystallization operation was conducted at a low temperature, and a higher temperature would promote the degradation of a template agent. So both higher and lower crystallization temperatures were beneficial to the formation of extraframework Ti, which can reduce the utilization of H2O2. 3.3. Effect of TPABr/SiO2. The mole ratio of TPABr/SiO2 is a major factor to the cost of TS-1, and Tuel reported every unit cell of TS-1 has four TPA+ and the minimum mole ratio of TPABr/SiO2 is 0042.11 Figure 6 shows the XRD patterns of TS-1 synthesized from Ti(SO4)2 with different TPABr/SiO2 molar ratios. The results show that the crystallinity of the sample increased significantly with the increase of TPABr/SiO2, indicating that adequate template concentration is favorable to the crystallization of TS-1. As shown in Table 4, the data of unit cell volume and I960/I550 of the FT-IR spectra show that the concentration of TPABr in the matrix gel was conducive to the incorporation of Ti into the framework. For a lower ratio (0.03), XRD analysis and IR spectra show that TS-1 was obtained with a low crystallinity (58.46%); this is consistent with the report of Tuel. Furthermore, the catalytic properties of TS-1 also increased significantly with the increasing concentration of TPABr. 3.4. Effect of TiO2/ SiO2. Titanium content in the framework is an important factor to the catalytic activity of TS-1, so we investigated the influence of different TiO2/SiO2 molar ratios in the synthesis of TS-1 using Ti(SO4)2. Millini reported that the maximum amount of Ti incorporated in TS-1
Figure 8. XRD patterns of TS-1 samples synthesized from Ti(SO4)2 with NH3/SiO2 molar ratios of (a) 1.0, (b) 2.5, (c) 3.5, and (d) 5.0.
system is much slower compared to the classical method because of the weak alkalinity. In addition, prolonging the crystallization time can increase the unit cell volume and the crystallinity of the samples, so the catalytic performance of TS-1 also increased with the extension of crystallization time (Table 2). 3.2.2. Crystallization Temperature. Table 3 shows the effect of the crystallization temperature on the TS-1 synthesized using Ti(SO4)2 as the titanium source. The ratio of I960/I550, the catalytic property of allyl chloride epoxidation, the relative crystallinity, and the unit cell volume all reached the maximum at 443 K, which indicated the optimum crystallization
Table 6. Effect of NH3/SiO2 on the Physicochemical and Catalytic Properties of TS-1 Synthesized from Ti(SO4)2a allyl chloride epoxidation (%)b NH3/SiO2 (mol)
I960/I550
relative crystallinity (%)
pH value of gel
VCell (Å3)
XH2O2
SECH
UH2O2
YECH
1.0 2.5 3.5 5.0
0.36 0.40 0.33 0.29
77.04 99.30 96.71 68.03
10.29 11.05 11.27 11.43
5380 5395 5384 5375
97.85 98.51 98.55 95.49
99.86 99.96 99.89 98.67
91.22 98.98 95.40 89.82
89.13 97.47 93.91 84.63
a
Composition of gel: SiO2:0.15 TPABr:0.022 TiO2:35 H2O with seeds (6.6 wt % of silica); crystallized at 443 K for 3 days. bReaction conditions: same with Table 1. 12735
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Figure 10. SEM micrographs of TS-1 synthesized from Ti(SO4)2 with different amount of seeds: (a) without seeds, (b) 3.0 wt %, (c) 6.0 wt %, and (d) 9.0 wt %.
Table 7. Effect of Seeds Amount on the Physicochemical and Catalytic Properties of TS-1 Synthesized from Ti(SO4)2a allyl chloride epoxidation (%)b
a
3
seeds amount (wt %)
crystallization time (h)
I960/I550
relative crystallinity (%)
VCell (Å )
XH2O2
SECH
UH2O2
YECH
0 3.0 6.0 9.0
120 72 72 72
0.33 0.35 0.43 0.42
50.82 62.48 99.30 100
5357 5373 5395 5397
69.49 98.44 99.57 99.61
99.43 99.71 99.89 99.88
91.32 96.76 99.58 99.58
63.10 94.97 99.04 99.07
Composition of gel: SiO2:0.15 TPABr:0.022 TiO2:2.5 NH3:35 H2O; crystallized at 443 K for 3 days. bReaction conditions: same with Table 1.
framework is limited to 2.5 mol %.30 For a lower TiO2/SiO2, the Ti active site is not great enough and the catalytic activity of TS-1 is not high; for an excessive TiO2/SiO2, the Ti cannot be inserted into framework completely, and the extraframework Ti species can catalyze the decomposition of H2O2. The chemical compositions of gel and solid zeolites are summarized in Table 5; the TiO2/SiO2 ratio in the zeolites reflected quite well the gel composition. The presence of framework and extraframework Ti species in samples was confirmed by UV−vis spectroscopy (Figure 7). All the samples with different Ti contents showed the absorption band at 220 nm. When the TiO2/SiO2 ratio was lower than 0.025, the band at 330 nm was not obvious, indicating that the samples had a few anatase phases. The obvious adsorption appeared at 330 nm for the samples synthesized with high TiO2/SiO2 values (>0.025), suggesting the vast formation of extraframework Ti. In Table 5, the ratio of I960/I550 and the Vcell of the TS-1 increased with the increasing Ti content in the gel when the mole ratio of TiO2/SiO2 was lower than 0.025. However, the
I960/I550 and Vcell had no obvious change after the TiO2/SiO2 was higher than 0.025. Based on the UV−vis and FT-IR analysis, we conclude that the maximum Ti content in the framework of TS-1 is also limited to less than 2.5 mol % in the present synthesis system. The catalytic properties of TS-1 in the allyl chloride epoxidation are also listed in Table 5. Both the conversion of H2O2 and the utilization of H2O2 increased with the increase of the Ti content in TS-1 when the mole ratio of TiO2/SiO2 was less than 0.025. The amorphous TiO2 can block the pore of the TS-1 and accelerate the decomposition of H2O2. So for a TiO2/ SiO2 higher than 0.025, the conversion of H2O2 became higher, while the utilization of H2O2 became lower than the sample with a TiO2/SiO2 ratio of 0.022. 3.5. Effect of NH3/SiO2. The base plays an important role in the crystallization of the TS-1. The effect of NH3/SiO2 in the matrix gel on the physicochemical and catalytic properties of TS-1 synthesized with Ti(SO4)2 as the titanium source is shown in Figure 8 and Table 6. First, the crystallinity of zeolites 12736
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than 0.35 in the gel.31 However, we cannot get a high-quality TS-1 product if the base in the gel is excessive in the organic system, but we obtained the TS-1 at a high NH3/SiO2 for the alkalescence of ammonia by an inorganic method. Therefore, increasing the base amount in a suitable range was favorable to the formation of crystal nucleus. The effect of NH3/SiO2 on the catalytic performance of TS-1 was similar to that of NH3/SiO2 on the crystallinity of TS-1. 3.6. Effect of Seed Crystals. Addition of seeds can shorten the crystallization time and decrease the crystal size of the TS1.22,27 The effect of seeds on the physicochemical and catalytic properties of TS-1 prepared using Ti(SO4)2 as the titanium source is shown in Figure 9, Figure 10, and Table 7. With an increase of the additional seeds in the preparation of TS-1, the TS-1 crystal formation was facilitated, and the crystallinity of zeolites and the amount of framework Ti (I960/I550 and VCell) were also increased. However, all these increases were inconspicuous after the amount of seeds was more than 6.0 wt %. Furthermore, the size of the crystals obtained was decreased observably. The size of TS-1 synthesized without seeds was as large as 20 μm × 13.1 μm × 4.0 μm, and there was some amorphous material together; we obtained a small crystal (2.0 μm × 1.1 μm × 0.4 μm) from the TS-1 synthesized using 6.0−9.0 wt % seeds. The catalytic properties of these samples increased with the decrease of crystal size significantly (Table 7). 3.7. Stability and Reusability of TS-1. The stability and reusability of TS-1 synthesized from an inorganic reactant system were tested. The XRD patterns of TS-1 with different regenerate times is shown in Figure 11. The main diffraction peaks of all the samples were the same as the typical TS-1 zeolite, indicating the regeneration did not destroy the MFI structure of TS-1. The I960/I550 ratio and the VCell of the regenerated sample were a bit lower than the fresh catalyst (Table 8), indicating a slight decrease of the framework Ti or Si contents in the regenerated catalyst. The catalytic performance of regenerated catalysts for the allyl chloride epoxidation was tested, and the results are presented in Figure 12. The selectivity of epichlorohydrin over the catalyst was still more than 99% after nine times of regeneration, and the conversion of H2O2 decreased slightly. Although the utilization of H2O2 and the yield of epichlorohydrin decreased obviously with the increasing regenerate times, the catalytic performance of regenerated catalyst was still fine after nine-times regeneration. The results obtained in our work indicated that the stability and reusability of TS-1 catalysts synthesized from this inorganic system are good.
Figure 11. XRD patterns of TS-1 synthesized from Ti(SO4)2 with different regenerate times: (a) fresh, (b) 5, and (c) 9.
Table 8. Effect of Regeneration on the Physicochemical and Catalytic Properties of TS-1 Synthesized from Ti(SO4)2a,b allyl chloride epoxidation (%)c number of regeneration
I960/ I550
VCell (Å3)
XH2O2
SECH
UH2O2
YECH
0 5 9
0.43 0.42 0.40
5395 5390 5384
98.94 98.28 97.81
99.51 99.47 99.12
98.10 97.76 96.25
96.58 95.57 93.32
a
Composition of gel (the fresh TS-1): SiO2:0.15 TPABr:0.022 TiO2:2.5 NH3:35 H2O with seeds (6.6 wt % of silica); crystallized at 443 K for 3 days. bThe spent catalysts were separated by centrifugation, dried, and then calcined at 823 K under air for 5 h. c Reaction conditions: catalyst 3.0 g, allyl chloride 21.00 g, methanol solvent 47.00 g, H2O2 (30 wt %) 23.00 g, 303 K, 40 min.
4. CONCLUSIONS In our study, TS-1 was synthesized from a cheap inorganic material system using TPABr, Ti(SO4)2, and colloidal silica instead of costly TPAOH, Ti-alkoxide, and Si-alkoxide. The TS1 prepared using Ti(SO4)2 as the titanium source showed a higher catalytic performance than the sample synthesized with TBOT and TiCl3 for the epoxidation of allyl chloride with H2O2, and the crystal size of TS-1 synthesized with Ti(SO4)2 was smaller. Even compared with TS-1 prepared by the classic method, the catalytic activity of TS-1 synthesized from Ti(SO4)2 was not low. The synthesis conditions had a great influence on the physicochemical and catalytic properties of TS-1. The crystallinity of zeolites and the amount of Ti4+ incorporated into the framework increased with increasing crystallization
Figure 12. Effect of regeneration on the catalytic properties of TS-1 synthesized from Ti(SO4)2.
and the amount of Ti incorporated into the framework (I960/ I550 and VCell) increased with the increase of base in the same crystallization time; but the crystallinity and the amount of framework Ti decreased obviously with the increase of NH3/ SiO2 (mol) when the mole ratio of NH3/SiO2 was higher than 2.5. van der Pol found that the TS-1 cannot crystallize completely, and the crystal size is big when the OH−/Si is less 12737
dx.doi.org/10.1021/ie202524t | Ind. Eng. Chem. Res. 2012, 51, 12730−12738
Industrial & Engineering Chemistry Research
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time and TPABr/SiO2. For the conditions such as crystallization time, NH3/SiO2, and TiO2/SiO2, it was favorable to the synthesis of TS-1 when they increase in a special range. From our investigation, the optimum synthesis conditions for the present system should employ a gel composition of SiO2/ TiO2/NH3/TPABr/H2O = 1:0.022:25:0.15:35 and carry out the crystallization at 443 K for 3 days. Furthermore, the seeds played an important role in the crystallization of TS-1. The addition of seeds in the synthesis gel could increase the amount of framework Ti and reduce the crystallization time, the crystal size, and the amount of TiO2 outside framework, and the optimum amount of seeds was 6.6 wt %. The stability and reusability of TS-1 synthesized from inorganic reactant system were also excellent, the MFI structure of TS-1 had no obvious change after nine-times regeneration, and the catalytic performance of TS-1 was still fine.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-731 58298173. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of the Natural Science Foundation of China (20976147), the Key Project of Hunan Provincial Natural Science Foundation of China (09JJ3021), and Aid Program for Science and Technology Innovative Reseacher Team in Higer Education Institutional of Hunan Province.
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REFERENCES
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dx.doi.org/10.1021/ie202524t | Ind. Eng. Chem. Res. 2012, 51, 12730−12738
