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Synthesis of TS-1 using amorphous SiO2 and its catalytic properties for hydroxylation of phenol in fixed-bed reactor
Applied Catalysis A: General 293 (2005) 153–161 www.elsevier.com/locate/apcata
Synthesis of TS-1 using amorphous SiO2 and its catalytic properties for hydroxylation of phenol in fixed-bed reactor Hong Liu a,b, Guanzhong Lu a,*, Yanglong Guo a, Yun Guo a a
Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, PR China b College of Environmental and Chemical Engineering, Shanghai University, Shanghai 200072, PR China Received 29 March 2005; received in revised form 10 June 2005; accepted 9 July 2005 Available online 22 August 2005
Abstract Titanium silicalite-1 (TS-1) has been synthesized using amorphous SiO2 as silicon source and tetrapropylammonium bromide (TPABr) as template. The effects of preparation parameters, such as silicon sources, crystallization temperature and time, aging time, H2O/SiO2, SiO2/ TiO2, TPABr/SiO2 and n-butylamine (NBA)/SiO2, and nonionic surfactants 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, ICP-AES and N2 adsorption. In the fixed-bed reactor, the catalytic property of TS-1 for the phenol hydroxylation was tested. The studies show that the catalytic performance of TS-1 synthesized using amorphous SiO2 is close to that of the samples prepared with tetraethyl orthosilicalite (TEOS) for the phenol hydroxylation with H2O2. The crystallinity of the sample increases with an increase of the crystallization temperature, crystallization time, the ratio of SiO2/ TiO2, SiO2/H2O and NBA/SiO2. TS-1 with smaller crystals can be obtained by increasing aging time, H2O/SiO2 and NBA/SiO2, and using the nonionic surfactants. Moreover, adding the nonionic surfactants in the matrix gel can increase the amount of Ti incorporated in framework of zeolite and reduces the amount of TiO2 in an extra framework. # 2005 Elsevier B.V. All rights reserved. Keywords: Titanium silicalite-1; Amorphous SiO2; Hydroxylation of phenol; Fixed-bed reactor
1. Introduction Titanium silicalite-1 (TS-1) has attracted much attention recent decade, because of its unique catalytic properties for the selective oxidation reactions using hydrogen peroxide as oxidant, such as aromatic hydroxylation [1], epoxidation of alkenes [2], ammoximation of cyclohexanone [3] and oxidation of alkanes and alcohols [4,5]. Generally, the synthesis of TS-1 needs large amount of alkali-free tetrapropylammonium hydroxide (TPAOH) as template, tetraethyl orthosilicalite (TEOS) as silicon source and tetrabutyl orthotitanate (TBOT) as titanium source. Therefore, TS-1 synthesized by classical method is very expensive and it is difficult to its commercial application. To reduce the cost of TS-1, some researchers investigated the synthesis of * Corresponding author. Fax: +86 21 64253703. E-mail address: [email protected] (G. Lu). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.07.021
TS-1 with the cheap template, silicon source and titanium source. For example, Muller and Steck [6] adopted tetrapropylammonium bromide (TPABr) as a template and ammonia as a base to synthesize TS-1. Xia and Gao [7] synthesized TS-1 by using binary mixture of tetraethylammonium chloride (TEACl) and tetrabutylammonium chloride (TBACl) as the structure directing agents, but a certain amount of the TS-1 crystalline seeds and concentrated ammonia solution must be used. The synthesis of TS-1 with TPABr + methylamine [8], TPABr + hexanediamine [9], TPABr + n-butylamine, or TPABr + ethylenediamine [10] as a template has also been reported. Jorda and Tuel [11] reported the synthesis of TS-1 with TiF4 as a titanium source, and Gao and co-worker [7] used TiCl3 as a titanium source. Shibata and Gabelica [12] used the media of TPABr-TiCl4-HF-H2O2 to prepare TS-1, but the preparation procedure of TS-1 was very complex and the crystal size of TS-1 prepared was very large (16–60 mm). Gontier and Tuel
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[13] used amorphous SiO2 as a silicon source and TPAOH as a template to prepare TS-1, but its crystal size was also larger (2–19 mm) and its catalytic performance was not reported. In this paper, TS-1 was prepared by hydrothermal synthesis using TPABr, amorphous SiO2 and TBOT as a template, silicon source and titanium source, respectively, and instead of costly TPAOH and Si-alkoxide, which has not yet been reported in literature. Recently, the phenol hydroxylation catalyzed by the TS1/diatomite catalyst in the fixed-bed reactor operated continuously has been developed [14]. Compared with the batch process, the continuous process has many advantages, such as free from tiresome operations of the catalyst filtration and makeup, and operation in large scale. Here, the hydroxylation of phenol by hydrogen peroxide as an oxidant in the fixed-bed reactor system was used as a model reaction to test the performance of the TS-1 catalyst.
2. Experimental 2.1. Synthesis of TS-1 TS-1 was prepared by using amorphous SiO2 (the BET surface area 128 m2/g, average particle size 0.1 mm, Shanghai Wujing Chemical Industry Co. Ltd.) as a silicon source, TBOT as a titanium source, TPABr as a template and nbutylamine (NBA) as a base to adjust the pH value of the matrix gel. 2.28 g TPABr and 3.5 ml NBA were dissolved in 35 ml distilled water, and then 2.84 g amorphous SiO2 was added under stirring until a homogenous gel was formed. 0.495 g TBOT in 10 ml absolute isopropyl alcohol was added dropwise to the gel under stirring. The composition of the matrix gel is SiO2:0.03 TiO2:0.73 NBA:0.175 TPABr:41 H2O. After heated to 353 K and kept for 3 h to remove isopropyl alcohol, the gel was transferred into a stainless steel autoclave with PTFE liner and crystallized at 453 K for 3 days under static condition. The solid obtained was filtered, washed with distilled water, dried overnight at 393 K and calcined at 823 K for 6 h. Then the particles of TS-1 prepared were pressed and crushed 40–60 mesh to obtain the TS-1 catalyst. In order to understand the effects of silicon source on the properties of TS-1 prepared, the colloidal silica (30 wt.%), TEOS and diatomite as a silicon source were used to synthesize TS-1. 2.2. Characterization of the sample The X-ray diffraction (XRD) analysis was performed on the Rigaku D/MAX-2400 diffractometer using Cu Ka radiation and graphite monochromator. The relative crystallinity of TS-1 was estimated by comparing the intensities of five characteristic diffraction peaks (2u = 228–258) of the MFI zeolite. The sample with the largest integrated intensities of diffraction peaks was selected as the reference and considered as the 100% crystalline.
The infrared (FT-IR) spectrum of sample was recorded on the Nicolet Nexus 670 FT-IR spectrometer. The UV–vis measurements were performed on the Varian Cary-500 spectrometer by using the diffuse reflectance technique in the range of 200–500 nm and BaSO4 as the reference. The scanning electron micrographs (SEM) were obtained on the JSM-6360LV microscope. The BET surface area was determined by the Micrometrics ASAP 2010 N2 adsorption apparatus. Chemical composition of the TS-1 samples was analyzed by the ICP-AES spectrometry (TJA IRIS1000). 2.3. Activity test of the catalyst The hydroxylation of phenol was carried out in the continuous flow fixed-bed glass reactor (Ø10 mm). 0.66 g TS1 catalyst was packed in the isothermal region of reactor. The mixture solution of phenol, H2O2 (30 wt.%) and solvent (acetone) was fed by a micropump from the bottom of reactor. The reaction condition is, the reaction temperature 357 K,phenol/H2O2 = 3:1(mol), phenol/acetone=1.25:1 (wt), WHSV = 7.49 h1. The concentration of residual H2O2 was determined by an iodometric titration. The concentrations of products were analyzed by the PE Autosystem XL gas chromatograph with the flame ionization detector and PE-5 capillary column (25 m 0.32 mm 1.0 mm, 5% methyl benzene silicone). The conversion of phenol and H2O2, the selectivity and yield of product are defined as follows: Xphenol ¼
n0phenol nphenol n0phenol
XH 2 O 2 ¼
n0H2 O2 nH2 O2 n0H2 O2
SDHB ¼
nCAT þ nHQ nCAT þ nHQ þ nPBQ
YDHB ¼
nCAT þ nHQ n0phenol
0 YDHB ¼
nCAT þ nHQ n0H2 O2
0 Xphenol, XH2 O2 , SDHB, YDHB and YDHB denote the conversions of phenol and H2O2, the selectivity and yield of dihydroxybenzene based on phenol and H2O2, respectively. n0 and n denote the initial molar amount and the final molar amount, respectively. CAT, HQ and PBQ represent catechol, hydroquinone and p-benzoquinone, respectively.
3. Results and discussion 3.1. Effect of the silicon source The samples 1, 2 and 3 were prepared using amorphous SiO2, colloidal silica and TEOS as a silicon source,
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Fig. 1. XRD patterns of the samples synthesized with amorphous SiO2 (1), colloidal silica (2), TEOS (3) and diatomite (4). Fig. 3. UV–vis spectra of the samples synthesized with amorphous SiO2 (1), colloidal silica (2) and TEOS (3).
Fig. 2. FT-IR spectra of the samples synthesized with amorphous SiO2 (1), colloidal silica (2), TEOS (3) and diatomite (4).
respectively. As seen from Fig. 1, the samples 1, 2 and 3 behave the MFI structures with a high crystallinity, in which the contaminating phases are not found. The FT-IR spectra in Fig. 2 shows that, the samples 1, 2 and 3 have a characteristic absorption peak at about 960 cm1, which indicates that titanium has been incorporated into the framework of TS-1 zeolite [15]. Compared with the samples 2 and 3, the peak at 960 cm1 of the sample 1 is somewhat weaker. This shows that the amount of framework Ti in the sample 1 is lower than that of other two samples. The UV–vis spectroscopy is a sensitive technique to verify the coordination state of titanium species. In the UV– vis spectrum of TS-1, the absorption peak at 210 nm is attributed to the tetrahedral coordinated framework Ti [16].
Fig. 3 shows that the peak intensities at 210 nm increase somewhat in the order of the sample 1 < the sample 2 < the sample 3, which indicates the amount of Ti incorporated in the framework of the sample 1 is little lower than that of the samples 2 and 3. This agrees with the result of FT-IR. There is also a very weak absorption peak at 280–330 nm attributed to the extra-framework TiO2 [17] in the UV– vis spectra of the samples 1 and 3, and the former is little larger than the latter. However, no obvious absorption peak at 280–330 nm can be observed in the spectrum of the sample 2. The chemical compositions and unit cell volumes of the TS-1 samples prepared using different silicon sources are listed in Table 1. The results show that the ratio of Si/Ti in all the TS-1 samples is a little higher than that in the gel, that is, the amount of Ti in TS-1 is lower than that in the gel. As compared with the samples 2 (prepared with colloidal silica) and 3 (TEOS), the unit cell volume of the sample 1 (amorphous SiO2) is little lower. Based on that the unit cell volume increases linearly with the Ti content in the framework [18,19], the Ti amount in the framework of the sample 1 is little lower than that of the samples 2 and 3, which is consistent with the results obtained by the investigations of FT-IR and UV–vis above. This is probably related to dispersion of the Ti species in monomeric silicate species in the precursor gel. Higher the dispersion of Ti in the gel, higher the amount of Ti
Table 1 Effect of Si-source on the physicochemical and catalytic properties of TS-1 ˚ 3) Xphenol (%) Sample Si-source Si/Ti (mol) VCell (A 1 2 3
Amorphous SiO2 Colloidal silica TEOS
Gel
Solid
33 33 33
35.4 34.8 34.4
5383.3 5389.1 5397.9
11.5 14.3 14.7
XH2 O2 (%)
SDHB (%)
YDHB (%)
0 YDHB (%)
98.1 81.4 81.6
94.0 81.5 85.0
10.8 11.7 12.5
46.7 47.9 48.5
Composition of gel: SiO2:0.03 TiO2:0.73 NBA:0.175 TPABr:94 H2O; crystallized at 453 k for 5 days.
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Fig. 4. SEM micrographs of TS-1 synthesized with amorphous SiO2 (1), colloidal silica (2) and TEOS (3).
incorporated in the framework is [13] and larger the volume of the unit cell is. Fig. 4 shows that the sample 1 prepared with amorphous SiO2 has the same shape of hexagonal prism as that of the samples (2 and 3) prepared with colloidal silica and TEOS, but its particles (10–11 mm) are little larger than the particles prepared with colloidal silica (10 mm) and TEOS (8 mm). The sample 1 has a uniform pore size (Fig. 5), and its BET surface area and pore volume is 498 m2/g and 0.228 cm3/g, respectively. For the hydroxylation of phenol, TS-1 prepared with amorphous SiO2 behaves a bit lower activity than that of other two samples synthesized with colloidal silica and TEOS (Table 1). It was known that the framework titanium in TS-1 is the active sites for an oxidation reaction and the catalytic activity of TS-1 decreases with an increase of its
crystal size [1,2,20]. The studies above show that, the amount of framework titanium in the sample 1 prepared with amorphous SiO2 is little lower than that of the other two samples, and the sample 1 has a larger crystal size, so the sample 1 behaves a relative lower catalytic activity. The results in Table 1 also show that, the decomposition of hydrogen peroxide over sample 1 is more serious (higher conversion of H2O2) than over the samples 2 or 3, which is related to the presence of tiny extra-framework TiO2 in the sample 1. However, it is noticed that the dihydroxybenzene selectivity over the sample 1 is higher and the dihydroxybenzene yield over the sample 1 is close to that over the samples 2 and 3. Furthermore, in the hydroxylation of phenol catalyzed by metal oxides or heteropoly compounds and using especially water as solvent, the tarry by-products can be formed easily, but no obvious tarry products occur in this reaction system catalyzed by the sample 1. The results above indicate that amorphous SiO2 can be used as a silicon source to synthesize TS-1 instead of TEOS. We tried to synthesize TS-1 using diatomite as a silicon source instead of amorphous SiO2, but no TS-1 or the ZSM5 crystals were observed in its XRD and FT-IR spectra (Figs. 1 and 2). 3.2. Effects of the crystallization condition
Fig. 5. Pore size distribution of TS-1 synthesized with amorphous SiO2.
3.2.1. Crystallization temperature and time Fig. 6 shows the relation of the relative crystallinity of TS-1 synthesized using amorphous SiO2 to the crystallization time and temperature. The sample synthesized at 453 K for 168 h (Fig. 6) was used as the reference and its
H. Liu et al. / Applied Catalysis A: General 293 (2005) 153–161
Fig. 6. Relative crystallinity of TS-1 synthesized vs. the crystallization time at 453 K (&), 443 K (*), 433 K (~) and 423 K (!).
relative crystallinity was defined to 100% (similarly hereinafter). The results show that increasing the crystallization temperature shortens the induction period of TS-1 crystallization and increases its crystallization rate considerably.
As seen from Table 2, the crystal size of the samples increases also with an increase of the crystallization temperature. This is because the growth rate of the crystals is faster than the rate of nucleation. Reddy et al. [21] thought that the relative intensity of the absorption peaks at about 960 cm1 to 550 cm1 (I960/I550) in the FT-IR spectrum increases linearly with an increase of the amount of framework titanium that is considered as the active sites for an oxidation reaction. The results in Table 2 show that crystallization temperature has no obvious influence on the ratio of I960/I550 and the unit cell volume corresponding to the amount of Ti incorporated into the framework. The catalytic activity of TS-1 for the phenol hydroxylation decreases with an increase of the crystallization temperature because of the increase of its crystal size. Increasing the crystallization time can increase the crystallinity, unit cell volume and I960/I550 of the samples, and that their crystal size increases also. But an increase of the crystal size of TS-1 makes its catalytic performance decrease for the hydroxylation of phenol.
Table 2 Effects of the crystallization temperature and time on the physicochemical and catalytic properties of TS-1 ˚ 3) I960/I550 d (mm) Xphenol (%) XH2 O2 (%) Crystallization Crystallization Si/Ti (mol) VCell (A temperature (K) time (h) in solid 423 433 443 453 453
120 120 120 120 72
35.0 34.5 35.2 36.0 37.1
5387.1 5389.6 5388.3 5387.1 5384.1
0.80 0.88 0.86 0.80 0.63
157
5–10 8–13 9–14 10–15 5–8
6.22 4.54 4.06 3.98 8.13
67.4 63.0 58.6 47.4 84.9
SDHB (%)
YDHB (%)
0 YDHB (%)
61.0 50.9 44.9 36.4 81.9
3.79 2.31 1.82 1.45 6.66
38.6 38.0 33.5 26.1 41.8
Composition of gel: SiO2:0.03 TiO2:0.73 NBA:0.175 TPABr:41 H2O.
Fig. 7. SEM micrographs of TS-1 synthesized under aging time 0 h (1), 6 h (2) and 12 h (3).
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Table 3 Effect of the aging time on the physicochemical and catalytic properties of TS-1 ˚ 3) I960/I550 Aging time (h) d (mm) Si/Ti (mol) in Solid VCell (A
Xphenol (%)
XH2 O2 (%)
SDHB (%)
YDHB (%)
0 YDHB (%)
0 6 12
3.98 7.83 8.43
47.4 77.2 79.6
36.4 80.1 78.6
1.45 6.27 6.63
26.1 38.5 40.9
10–15 6 5
36.0 35.8 36.2
5387.1 5388.5 5383.6
0.80 0.79 0.81
Composition of gel: SiO2:0.03 TiO2:0.73 NBA:0.175 TPABr:41 H2O; crystallization at 453 k for 5 days.
Table 4 Effect of H2O/SiO2 on the physicochemical and catalytic properties of TS-1 ˚ 3) I960/I550 VCell (A Relative crystallinity (%) d (mm) H2O/SiO2 (mol)
Xphenol (%)
XH2 O2 (%)
SDHB (%)
YDHB (%)
0 YDHB (%)
94 70 41
12.5 8.65 8.13
98.7 93.5 84.9
95.0 80.8 81.9
11.9 6.99 6.66
48.0 43.1 41.8
0.65 0.63 0.63
5383.3 5385.7 5384.1
90.5 96.5 98.9
3–6 4–7 5–8
Composition of gel: SiO2:0.03 TiO2:0.73 NBA:0.175 TPABr; crystallization at 453 k for 3 days.
3.2.2. Aging time Before crystallization, the matrix gel was kept at room temperature for 0–12 h (aging time) under a static condition. As seen from Fig. 7, the crystal size of TS-1 decreases and its particles becomes more uniform with an increase of aging time, such as increasing the aging time from 0 to 12 h makes the crystal size of TS-1 decrease from 10–15 mm to about 5 mm. But the effect of aging time on the chemical composition, the relative intensity of I960/I550 and the unit cell volume of the samples (Table 3) is little, that is to say, the amount of framework Ti is hardly affected by the aging time. The catalytic performance of TS-1 for the phenol hydroxylation increases with increasing aging time. It attributes to shortening the distance of reactant diffusion inside the channels of catalyst, as the crystal size of catalyst diminish [20]. 3.3. Effect of H2O/SiO2 The effect of H2O/SiO2 (mol) in the matrix gel on the physicochemical and catalytic properties of TS-1 is shown in Table 4. With a decrease of H2O/SiO2, the amount of framework Ti (I960/I550 and VCell) in TS-1 changes little, and the relative crystallinity of sample increases. This is attributed to the faster rate of nucleation in the concentrated matrix gel. The gel diluted by water makes the average crystal size of sample decrease. Therefore, TS-1 prepared in the matrix gel with higher H2O/SiO2 has smaller crystal size and better catalytic performance.
[22]. But the amount of Ti incorporated into framework (I960/I550 and VCell) and the catalytic performance of TS-1 decreases with an increase of SiO2/TiO2. This proves also that the Ti atoms in TS-1 are the active sites for the phenol hydroxylation. 3.5. Effect of TPABr/SiO2 The results in Table 6 show that the crystallinity of the sample increases significantly with an increase of TPABr/ SiO2 from 0.10 to 0.30, but when TPABr/SiO2 is beyond 0.30, the crystallinity of the sample decreases slightly and its particle sizes increase a little. This indicates that adequate concentration of template is favorable to the crystallization of TS-1, and the sample prepared with TPABr/SiO2 = 0.175–0.30 has best crystallinity. It is suggested that a low concentration of template should be used, because of a high cost of template. The data of the unit cell volume of the samples and I960/I550 of the FT-IR spectra show that the concentration of TPABr in the matrix gel has a
3.4. Effect of SiO2/TiO2 The effects of SiO2/TiO2 (mol) in the matrix gel on the physicochemical and catalytic properties of TS-1 are shown in Fig. 8 and Table 5. With an increase of SiO2/ TiO2, the crystallization rate of TS-1 increases and its crystal size decreases gradually, which is similar to the situation of preparing the high-silicon molecular sieves
Fig. 8. FT-IR spectra of TS-1 synthesized. SiO2/TiO2 in the matrix gel: (1) 27, (2) 33, (3) 40, (4) 50.
H. Liu et al. / Applied Catalysis A: General 293 (2005) 153–161 Table 5 Effect of SiO2/TiO2 on the physicochemical and catalytic properties of TS-1 ˚ 3) I960/I550 VCell (A Relative d (mm) SiO2/TiO2 (mol) crystallinity (%) Gel Solid 27 33 40 50
29.0 35.4 42.2 51.0
0.66 0.65 0.56 0.51
5410.8 5383.3 5377.5 5370.8
88.5 90.5 91.9 95.7
4–8 3–6 3–5 2–4
159
Xphenol (%)
XH2 O2 (%)
SDHB (%)
YDHB (%)
0 YDHB (%)
12.9 12.5 7.67 7.31
99.7 98.7 91.7 91.3
98.7 95.0 81.8 73.7
12.7 11.9 6.27 5.39
50.6 48.0 40.2 35.6
Composition of gel: SiO2:0.73 NBA:0.175 TPABr:94 H2O; crystallization at 453 k for 3 days.
Table 6 Effect of TPABr/SiO2 on the physicochemical and catalytic properties of TS-1 TPABr/SiO2 (mol)
I960/I550
VCell ˚ 3) (A
Relative crystallinity (%)
d (mm)
Xphenol (%)
XH2 O2 (%)
SDHB (%)
YDHB (%)
0 YDHB (%)
0.10 0.175 0.30 0.40
0.69 0.63 0.65 0.64
5389.1 5384.1 5387.1 5383.4
75.5 98.9 99.4 92.6
3–6 5–8 6–9 8–10
6.30 8.13 10.1 8.70
96.7 84.9 99.1 99.8
69.5 81.9 92.4 92.5
4.38 6.66 9.33 8.05
34.6 41.8 48.5 47.7
Xphenol (%)
XH2 O2 (%)
SDHB (%)
YDHB (%)
0 YDHB (%)
8.53 8.88 9.39
79.8 80.2 84.8
79.6 81.7 83.0
6.79 7.25 7.79
42.8 44.1 47.3
Composition of gel: SiO2:0.03 TiO2:0.73 NBA:41 H2O; crystallization at 453 k for 3 days.
Table 7 Effect of NBA/SiO2 on the physicochemical and catalytic properties of TS-1 ˚ 3) I960/I550 VCell (A Relative d (mm) NBA/SiO2 (mol) crystallinity (%) 0.73 0.84 1.04
0.55 0.60 0.66
5343.6 5400.8 5410.8
89.9 91.5 96.2
4 3 2.5
Composition of gel: SiO2: 0.03 TiO2:0.175 TPABr:41 H2O; aging at room temperature for 12 h and crystallization at 453 k for 3 days.
slight influence on the amount of Ti incorporated into framework. The SEM micrographs indicate that the crystal size of the sample increases with an increase of TPABr/ SiO2, which is similar to the results reported by Gontier and Tuel [13]. The effect of TPABr/SiO2 on the catalytic performance of TS-1 is similar to that of TPABr/SiO2 on the crystallinity of TS-1.
The base often plays an important role in the crystallization of zeolites. The results in Table 7 show that the molar ratio of NBA/SiO2 affects the physicochemical and catalytic properties of TS-1. Increasing NBA/SiO2 can increase both the crystallinity of TS-1 and the amount of
Fig. 9. XRD patterns of TS-1 prepared with no surfactant (1), Tween 20 (2) and Tween 60 (3).
Fig. 10. FT-IR spectra of TS-1 prepared with no surfactant (1), Tween 20 (2) and Tween 60 (3).
3.6. Effect of NBA/SiO2
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Table 8 Effect of nonionic surfactant on the physicochemical and catalytic properties of TS-1 ˚ 3) I960/I550 d (mm) Xphenol (%) Sample Surfactant VCell (A
XH2 O2 (%)
SDHB (%)
YDHB (%)
0 YDHB (%)
1 2 3
47.4 72.4 90.5
36.4 71.8 86.0
1.45 5.17 9.46
26.1 42.8 48.9
No Tween 20 Tween 60
5387.1 5397.4 5411.8
0.80 0.83 0.84
10–15 8–12 4–9
3.98 7.20 11.0
Composition of gel: SiO2:0.01 Tween:0.03 TiO2:0.73 NBA:0.175 TPABr:41 H2O; crystallization at 453 k for 5 days.
3.7. Effect of nonionic surfactant
Fig. 11. UV–vis spectra of TS-1 prepared with no surfactant (1), Tween 20 (2) and Tween 60 (3).
framework Ti in TS-1, and makes the crystal size decrease slightly and the catalytic performance of TS-1 increase. This indicates that higher alkalinity is favorable to the synthesis of TS-1.
Nonionic surfactant can decrease the surface tension of solution and therefore decrease the concentration of template in the synthesis of zeolite [23]. Two kinds of nonionic surfactant (Tween 20 and Tween 60) were adopted in the synthesis of TS-1 and the effect of nonionic surfactant on the physicochemical and catalytic properties of TS-1 was investigated. The results in Fig. 9 show that all the samples prepared with and without surfactant have the typical MFI topological structure, but the crystallinity of the samples prepared with surfactant decrease slightly. As seen from Fig. 10 and Table 8, the amount of Ti existed in the framework of TS-1 increases slightly after using nonionic surfactant. Fig. 11 shows the UV–vis spectra of TS-1 prepared with and without nonionic surfactant. The peak intensity at 210 nm of the sample prepared with nonionic surfactant is higher than that of the sample prepared without nonionic surfactant, which is consistent with the results measured by FT-IR and XRD. The absorption peak at 280–330 nm of the sample prepared with Tween 20 is weaker than that of the sample prepared without surfactant, and no absorption
Fig. 12. SEM micrographs of TS-1 prepared with no surfactant (1), Tween 20 (2) and Tween 60 (3).
H. Liu et al. / Applied Catalysis A: General 293 (2005) 153–161
peak at 280–330 nm is observed in the sample prepared with Tween 60. These results show that using nonionic surfactant can reduce the amount of extra-framework TiO 2 in TS-1. The SEM micrographs in Fig. 12 show that, the crystal size of the sample prepared using Tween 20 or Tween 60 is smaller than that prepared in absence of surfactant, and small crystal size is propitious to improve the catalytic performance of the TS-1 catalyst for the hydroxylation of phenol (Table 8). As an additive used in the synthesis of TS1, Tween 60 is superior to Tween 20.
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hydroxylation of phenol with H2O2 in the fixed-bed reactor system.
Acknowledgement This project is supported financially by the Commission of Science and Technology of Shanghai Municipality (No. 03DJ14006).
References 4. Conclusions TS-1 is easily synthesized by using amorphous SiO2 and TPABr instead of costly TPAOH and Si-alkoxide. The catalytic performance of TS-1 prepared using amorphous SiO2 is close to those of the sample prepared with TEOS for the hydroxylation of phenol with H2O2 in the fixed-bed reactor system. The synthesis conditions have a great influence on the physicochemical and catalytic properties of TS-1. The crystallinity of the sample increases with increasing crystallization temperature, crystallization time, SiO2/ TiO2, SiO2/H2O or NBA/SiO2. An increase of the aging time, H2O/SiO2, SiO2/TiO2 or NBA/SiO2 is in favor of gaining the sample with smaller crystal size. On the contrary, increasing the crystallization temperature and time, or TPABr/SiO2 can increase the crystal size of sample. The amount of Ti4+ incorporated into framework of TS-1 zeolite is affected hardly by the crystallization temperature, aging time, H2O/SiO2 or TPABr/SiO2. However, the high ratio of NBA/SiO2 or TiO2/SiO2 in the matrix gel promotes Ti incorporating into the framework of TS-1. Adding nonionic surfactant as an additive in the synthesis gel, can increase the amount of framework Ti, and reduce the crystal size of sample and the amount of TiO2 outside framework. Smaller crystal size and more amount of framework Ti are favorable to increase the catalytic performance of TS-1 for the
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Synthesis of TS-1 using amorphous SiO2 and its catalytic properties for hydroxylation of phenol in fixed-bed reactor Hong Liu a,b, Guanzhong Lu a,*, Yanglong Guo a, Yun Guo a a
Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, PR China b College of Environmental and Chemical Engineering, Shanghai University, Shanghai 200072, PR China Received 29 March 2005; received in revised form 10 June 2005; accepted 9 July 2005 Available online 22 August 2005
Abstract Titanium silicalite-1 (TS-1) has been synthesized using amorphous SiO2 as silicon source and tetrapropylammonium bromide (TPABr) as template. The effects of preparation parameters, such as silicon sources, crystallization temperature and time, aging time, H2O/SiO2, SiO2/ TiO2, TPABr/SiO2 and n-butylamine (NBA)/SiO2, and nonionic surfactants 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, ICP-AES and N2 adsorption. In the fixed-bed reactor, the catalytic property of TS-1 for the phenol hydroxylation was tested. The studies show that the catalytic performance of TS-1 synthesized using amorphous SiO2 is close to that of the samples prepared with tetraethyl orthosilicalite (TEOS) for the phenol hydroxylation with H2O2. The crystallinity of the sample increases with an increase of the crystallization temperature, crystallization time, the ratio of SiO2/ TiO2, SiO2/H2O and NBA/SiO2. TS-1 with smaller crystals can be obtained by increasing aging time, H2O/SiO2 and NBA/SiO2, and using the nonionic surfactants. Moreover, adding the nonionic surfactants in the matrix gel can increase the amount of Ti incorporated in framework of zeolite and reduces the amount of TiO2 in an extra framework. # 2005 Elsevier B.V. All rights reserved. Keywords: Titanium silicalite-1; Amorphous SiO2; Hydroxylation of phenol; Fixed-bed reactor
1. Introduction Titanium silicalite-1 (TS-1) has attracted much attention recent decade, because of its unique catalytic properties for the selective oxidation reactions using hydrogen peroxide as oxidant, such as aromatic hydroxylation [1], epoxidation of alkenes [2], ammoximation of cyclohexanone [3] and oxidation of alkanes and alcohols [4,5]. Generally, the synthesis of TS-1 needs large amount of alkali-free tetrapropylammonium hydroxide (TPAOH) as template, tetraethyl orthosilicalite (TEOS) as silicon source and tetrabutyl orthotitanate (TBOT) as titanium source. Therefore, TS-1 synthesized by classical method is very expensive and it is difficult to its commercial application. To reduce the cost of TS-1, some researchers investigated the synthesis of * Corresponding author. Fax: +86 21 64253703. E-mail address: [email protected] (G. Lu). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.07.021
TS-1 with the cheap template, silicon source and titanium source. For example, Muller and Steck [6] adopted tetrapropylammonium bromide (TPABr) as a template and ammonia as a base to synthesize TS-1. Xia and Gao [7] synthesized TS-1 by using binary mixture of tetraethylammonium chloride (TEACl) and tetrabutylammonium chloride (TBACl) as the structure directing agents, but a certain amount of the TS-1 crystalline seeds and concentrated ammonia solution must be used. The synthesis of TS-1 with TPABr + methylamine [8], TPABr + hexanediamine [9], TPABr + n-butylamine, or TPABr + ethylenediamine [10] as a template has also been reported. Jorda and Tuel [11] reported the synthesis of TS-1 with TiF4 as a titanium source, and Gao and co-worker [7] used TiCl3 as a titanium source. Shibata and Gabelica [12] used the media of TPABr-TiCl4-HF-H2O2 to prepare TS-1, but the preparation procedure of TS-1 was very complex and the crystal size of TS-1 prepared was very large (16–60 mm). Gontier and Tuel
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[13] used amorphous SiO2 as a silicon source and TPAOH as a template to prepare TS-1, but its crystal size was also larger (2–19 mm) and its catalytic performance was not reported. In this paper, TS-1 was prepared by hydrothermal synthesis using TPABr, amorphous SiO2 and TBOT as a template, silicon source and titanium source, respectively, and instead of costly TPAOH and Si-alkoxide, which has not yet been reported in literature. Recently, the phenol hydroxylation catalyzed by the TS1/diatomite catalyst in the fixed-bed reactor operated continuously has been developed [14]. Compared with the batch process, the continuous process has many advantages, such as free from tiresome operations of the catalyst filtration and makeup, and operation in large scale. Here, the hydroxylation of phenol by hydrogen peroxide as an oxidant in the fixed-bed reactor system was used as a model reaction to test the performance of the TS-1 catalyst.
2. Experimental 2.1. Synthesis of TS-1 TS-1 was prepared by using amorphous SiO2 (the BET surface area 128 m2/g, average particle size 0.1 mm, Shanghai Wujing Chemical Industry Co. Ltd.) as a silicon source, TBOT as a titanium source, TPABr as a template and nbutylamine (NBA) as a base to adjust the pH value of the matrix gel. 2.28 g TPABr and 3.5 ml NBA were dissolved in 35 ml distilled water, and then 2.84 g amorphous SiO2 was added under stirring until a homogenous gel was formed. 0.495 g TBOT in 10 ml absolute isopropyl alcohol was added dropwise to the gel under stirring. The composition of the matrix gel is SiO2:0.03 TiO2:0.73 NBA:0.175 TPABr:41 H2O. After heated to 353 K and kept for 3 h to remove isopropyl alcohol, the gel was transferred into a stainless steel autoclave with PTFE liner and crystallized at 453 K for 3 days under static condition. The solid obtained was filtered, washed with distilled water, dried overnight at 393 K and calcined at 823 K for 6 h. Then the particles of TS-1 prepared were pressed and crushed 40–60 mesh to obtain the TS-1 catalyst. In order to understand the effects of silicon source on the properties of TS-1 prepared, the colloidal silica (30 wt.%), TEOS and diatomite as a silicon source were used to synthesize TS-1. 2.2. Characterization of the sample The X-ray diffraction (XRD) analysis was performed on the Rigaku D/MAX-2400 diffractometer using Cu Ka radiation and graphite monochromator. The relative crystallinity of TS-1 was estimated by comparing the intensities of five characteristic diffraction peaks (2u = 228–258) of the MFI zeolite. The sample with the largest integrated intensities of diffraction peaks was selected as the reference and considered as the 100% crystalline.
The infrared (FT-IR) spectrum of sample was recorded on the Nicolet Nexus 670 FT-IR spectrometer. The UV–vis measurements were performed on the Varian Cary-500 spectrometer by using the diffuse reflectance technique in the range of 200–500 nm and BaSO4 as the reference. The scanning electron micrographs (SEM) were obtained on the JSM-6360LV microscope. The BET surface area was determined by the Micrometrics ASAP 2010 N2 adsorption apparatus. Chemical composition of the TS-1 samples was analyzed by the ICP-AES spectrometry (TJA IRIS1000). 2.3. Activity test of the catalyst The hydroxylation of phenol was carried out in the continuous flow fixed-bed glass reactor (Ø10 mm). 0.66 g TS1 catalyst was packed in the isothermal region of reactor. The mixture solution of phenol, H2O2 (30 wt.%) and solvent (acetone) was fed by a micropump from the bottom of reactor. The reaction condition is, the reaction temperature 357 K,phenol/H2O2 = 3:1(mol), phenol/acetone=1.25:1 (wt), WHSV = 7.49 h1. The concentration of residual H2O2 was determined by an iodometric titration. The concentrations of products were analyzed by the PE Autosystem XL gas chromatograph with the flame ionization detector and PE-5 capillary column (25 m 0.32 mm 1.0 mm, 5% methyl benzene silicone). The conversion of phenol and H2O2, the selectivity and yield of product are defined as follows: Xphenol ¼
n0phenol nphenol n0phenol
XH 2 O 2 ¼
n0H2 O2 nH2 O2 n0H2 O2
SDHB ¼
nCAT þ nHQ nCAT þ nHQ þ nPBQ
YDHB ¼
nCAT þ nHQ n0phenol
0 YDHB ¼
nCAT þ nHQ n0H2 O2
0 Xphenol, XH2 O2 , SDHB, YDHB and YDHB denote the conversions of phenol and H2O2, the selectivity and yield of dihydroxybenzene based on phenol and H2O2, respectively. n0 and n denote the initial molar amount and the final molar amount, respectively. CAT, HQ and PBQ represent catechol, hydroquinone and p-benzoquinone, respectively.
3. Results and discussion 3.1. Effect of the silicon source The samples 1, 2 and 3 were prepared using amorphous SiO2, colloidal silica and TEOS as a silicon source,
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155
Fig. 1. XRD patterns of the samples synthesized with amorphous SiO2 (1), colloidal silica (2), TEOS (3) and diatomite (4). Fig. 3. UV–vis spectra of the samples synthesized with amorphous SiO2 (1), colloidal silica (2) and TEOS (3).
Fig. 2. FT-IR spectra of the samples synthesized with amorphous SiO2 (1), colloidal silica (2), TEOS (3) and diatomite (4).
respectively. As seen from Fig. 1, the samples 1, 2 and 3 behave the MFI structures with a high crystallinity, in which the contaminating phases are not found. The FT-IR spectra in Fig. 2 shows that, the samples 1, 2 and 3 have a characteristic absorption peak at about 960 cm1, which indicates that titanium has been incorporated into the framework of TS-1 zeolite [15]. Compared with the samples 2 and 3, the peak at 960 cm1 of the sample 1 is somewhat weaker. This shows that the amount of framework Ti in the sample 1 is lower than that of other two samples. The UV–vis spectroscopy is a sensitive technique to verify the coordination state of titanium species. In the UV– vis spectrum of TS-1, the absorption peak at 210 nm is attributed to the tetrahedral coordinated framework Ti [16].
Fig. 3 shows that the peak intensities at 210 nm increase somewhat in the order of the sample 1 < the sample 2 < the sample 3, which indicates the amount of Ti incorporated in the framework of the sample 1 is little lower than that of the samples 2 and 3. This agrees with the result of FT-IR. There is also a very weak absorption peak at 280–330 nm attributed to the extra-framework TiO2 [17] in the UV– vis spectra of the samples 1 and 3, and the former is little larger than the latter. However, no obvious absorption peak at 280–330 nm can be observed in the spectrum of the sample 2. The chemical compositions and unit cell volumes of the TS-1 samples prepared using different silicon sources are listed in Table 1. The results show that the ratio of Si/Ti in all the TS-1 samples is a little higher than that in the gel, that is, the amount of Ti in TS-1 is lower than that in the gel. As compared with the samples 2 (prepared with colloidal silica) and 3 (TEOS), the unit cell volume of the sample 1 (amorphous SiO2) is little lower. Based on that the unit cell volume increases linearly with the Ti content in the framework [18,19], the Ti amount in the framework of the sample 1 is little lower than that of the samples 2 and 3, which is consistent with the results obtained by the investigations of FT-IR and UV–vis above. This is probably related to dispersion of the Ti species in monomeric silicate species in the precursor gel. Higher the dispersion of Ti in the gel, higher the amount of Ti
Table 1 Effect of Si-source on the physicochemical and catalytic properties of TS-1 ˚ 3) Xphenol (%) Sample Si-source Si/Ti (mol) VCell (A 1 2 3
Amorphous SiO2 Colloidal silica TEOS
Gel
Solid
33 33 33
35.4 34.8 34.4
5383.3 5389.1 5397.9
11.5 14.3 14.7
XH2 O2 (%)
SDHB (%)
YDHB (%)
0 YDHB (%)
98.1 81.4 81.6
94.0 81.5 85.0
10.8 11.7 12.5
46.7 47.9 48.5
Composition of gel: SiO2:0.03 TiO2:0.73 NBA:0.175 TPABr:94 H2O; crystallized at 453 k for 5 days.
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Fig. 4. SEM micrographs of TS-1 synthesized with amorphous SiO2 (1), colloidal silica (2) and TEOS (3).
incorporated in the framework is [13] and larger the volume of the unit cell is. Fig. 4 shows that the sample 1 prepared with amorphous SiO2 has the same shape of hexagonal prism as that of the samples (2 and 3) prepared with colloidal silica and TEOS, but its particles (10–11 mm) are little larger than the particles prepared with colloidal silica (10 mm) and TEOS (8 mm). The sample 1 has a uniform pore size (Fig. 5), and its BET surface area and pore volume is 498 m2/g and 0.228 cm3/g, respectively. For the hydroxylation of phenol, TS-1 prepared with amorphous SiO2 behaves a bit lower activity than that of other two samples synthesized with colloidal silica and TEOS (Table 1). It was known that the framework titanium in TS-1 is the active sites for an oxidation reaction and the catalytic activity of TS-1 decreases with an increase of its
crystal size [1,2,20]. The studies above show that, the amount of framework titanium in the sample 1 prepared with amorphous SiO2 is little lower than that of the other two samples, and the sample 1 has a larger crystal size, so the sample 1 behaves a relative lower catalytic activity. The results in Table 1 also show that, the decomposition of hydrogen peroxide over sample 1 is more serious (higher conversion of H2O2) than over the samples 2 or 3, which is related to the presence of tiny extra-framework TiO2 in the sample 1. However, it is noticed that the dihydroxybenzene selectivity over the sample 1 is higher and the dihydroxybenzene yield over the sample 1 is close to that over the samples 2 and 3. Furthermore, in the hydroxylation of phenol catalyzed by metal oxides or heteropoly compounds and using especially water as solvent, the tarry by-products can be formed easily, but no obvious tarry products occur in this reaction system catalyzed by the sample 1. The results above indicate that amorphous SiO2 can be used as a silicon source to synthesize TS-1 instead of TEOS. We tried to synthesize TS-1 using diatomite as a silicon source instead of amorphous SiO2, but no TS-1 or the ZSM5 crystals were observed in its XRD and FT-IR spectra (Figs. 1 and 2). 3.2. Effects of the crystallization condition
Fig. 5. Pore size distribution of TS-1 synthesized with amorphous SiO2.
3.2.1. Crystallization temperature and time Fig. 6 shows the relation of the relative crystallinity of TS-1 synthesized using amorphous SiO2 to the crystallization time and temperature. The sample synthesized at 453 K for 168 h (Fig. 6) was used as the reference and its
H. Liu et al. / Applied Catalysis A: General 293 (2005) 153–161
Fig. 6. Relative crystallinity of TS-1 synthesized vs. the crystallization time at 453 K (&), 443 K (*), 433 K (~) and 423 K (!).
relative crystallinity was defined to 100% (similarly hereinafter). The results show that increasing the crystallization temperature shortens the induction period of TS-1 crystallization and increases its crystallization rate considerably.
As seen from Table 2, the crystal size of the samples increases also with an increase of the crystallization temperature. This is because the growth rate of the crystals is faster than the rate of nucleation. Reddy et al. [21] thought that the relative intensity of the absorption peaks at about 960 cm1 to 550 cm1 (I960/I550) in the FT-IR spectrum increases linearly with an increase of the amount of framework titanium that is considered as the active sites for an oxidation reaction. The results in Table 2 show that crystallization temperature has no obvious influence on the ratio of I960/I550 and the unit cell volume corresponding to the amount of Ti incorporated into the framework. The catalytic activity of TS-1 for the phenol hydroxylation decreases with an increase of the crystallization temperature because of the increase of its crystal size. Increasing the crystallization time can increase the crystallinity, unit cell volume and I960/I550 of the samples, and that their crystal size increases also. But an increase of the crystal size of TS-1 makes its catalytic performance decrease for the hydroxylation of phenol.
Table 2 Effects of the crystallization temperature and time on the physicochemical and catalytic properties of TS-1 ˚ 3) I960/I550 d (mm) Xphenol (%) XH2 O2 (%) Crystallization Crystallization Si/Ti (mol) VCell (A temperature (K) time (h) in solid 423 433 443 453 453
120 120 120 120 72
35.0 34.5 35.2 36.0 37.1
5387.1 5389.6 5388.3 5387.1 5384.1
0.80 0.88 0.86 0.80 0.63
157
5–10 8–13 9–14 10–15 5–8
6.22 4.54 4.06 3.98 8.13
67.4 63.0 58.6 47.4 84.9
SDHB (%)
YDHB (%)
0 YDHB (%)
61.0 50.9 44.9 36.4 81.9
3.79 2.31 1.82 1.45 6.66
38.6 38.0 33.5 26.1 41.8
Composition of gel: SiO2:0.03 TiO2:0.73 NBA:0.175 TPABr:41 H2O.
Fig. 7. SEM micrographs of TS-1 synthesized under aging time 0 h (1), 6 h (2) and 12 h (3).
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Table 3 Effect of the aging time on the physicochemical and catalytic properties of TS-1 ˚ 3) I960/I550 Aging time (h) d (mm) Si/Ti (mol) in Solid VCell (A
Xphenol (%)
XH2 O2 (%)
SDHB (%)
YDHB (%)
0 YDHB (%)
0 6 12
3.98 7.83 8.43
47.4 77.2 79.6
36.4 80.1 78.6
1.45 6.27 6.63
26.1 38.5 40.9
10–15 6 5
36.0 35.8 36.2
5387.1 5388.5 5383.6
0.80 0.79 0.81
Composition of gel: SiO2:0.03 TiO2:0.73 NBA:0.175 TPABr:41 H2O; crystallization at 453 k for 5 days.
Table 4 Effect of H2O/SiO2 on the physicochemical and catalytic properties of TS-1 ˚ 3) I960/I550 VCell (A Relative crystallinity (%) d (mm) H2O/SiO2 (mol)
Xphenol (%)
XH2 O2 (%)
SDHB (%)
YDHB (%)
0 YDHB (%)
94 70 41
12.5 8.65 8.13
98.7 93.5 84.9
95.0 80.8 81.9
11.9 6.99 6.66
48.0 43.1 41.8
0.65 0.63 0.63
5383.3 5385.7 5384.1
90.5 96.5 98.9
3–6 4–7 5–8
Composition of gel: SiO2:0.03 TiO2:0.73 NBA:0.175 TPABr; crystallization at 453 k for 3 days.
3.2.2. Aging time Before crystallization, the matrix gel was kept at room temperature for 0–12 h (aging time) under a static condition. As seen from Fig. 7, the crystal size of TS-1 decreases and its particles becomes more uniform with an increase of aging time, such as increasing the aging time from 0 to 12 h makes the crystal size of TS-1 decrease from 10–15 mm to about 5 mm. But the effect of aging time on the chemical composition, the relative intensity of I960/I550 and the unit cell volume of the samples (Table 3) is little, that is to say, the amount of framework Ti is hardly affected by the aging time. The catalytic performance of TS-1 for the phenol hydroxylation increases with increasing aging time. It attributes to shortening the distance of reactant diffusion inside the channels of catalyst, as the crystal size of catalyst diminish [20]. 3.3. Effect of H2O/SiO2 The effect of H2O/SiO2 (mol) in the matrix gel on the physicochemical and catalytic properties of TS-1 is shown in Table 4. With a decrease of H2O/SiO2, the amount of framework Ti (I960/I550 and VCell) in TS-1 changes little, and the relative crystallinity of sample increases. This is attributed to the faster rate of nucleation in the concentrated matrix gel. The gel diluted by water makes the average crystal size of sample decrease. Therefore, TS-1 prepared in the matrix gel with higher H2O/SiO2 has smaller crystal size and better catalytic performance.
[22]. But the amount of Ti incorporated into framework (I960/I550 and VCell) and the catalytic performance of TS-1 decreases with an increase of SiO2/TiO2. This proves also that the Ti atoms in TS-1 are the active sites for the phenol hydroxylation. 3.5. Effect of TPABr/SiO2 The results in Table 6 show that the crystallinity of the sample increases significantly with an increase of TPABr/ SiO2 from 0.10 to 0.30, but when TPABr/SiO2 is beyond 0.30, the crystallinity of the sample decreases slightly and its particle sizes increase a little. This indicates that adequate concentration of template is favorable to the crystallization of TS-1, and the sample prepared with TPABr/SiO2 = 0.175–0.30 has best crystallinity. It is suggested that a low concentration of template should be used, because of a high cost of template. The data of the unit cell volume of the samples and I960/I550 of the FT-IR spectra show that the concentration of TPABr in the matrix gel has a
3.4. Effect of SiO2/TiO2 The effects of SiO2/TiO2 (mol) in the matrix gel on the physicochemical and catalytic properties of TS-1 are shown in Fig. 8 and Table 5. With an increase of SiO2/ TiO2, the crystallization rate of TS-1 increases and its crystal size decreases gradually, which is similar to the situation of preparing the high-silicon molecular sieves
Fig. 8. FT-IR spectra of TS-1 synthesized. SiO2/TiO2 in the matrix gel: (1) 27, (2) 33, (3) 40, (4) 50.
H. Liu et al. / Applied Catalysis A: General 293 (2005) 153–161 Table 5 Effect of SiO2/TiO2 on the physicochemical and catalytic properties of TS-1 ˚ 3) I960/I550 VCell (A Relative d (mm) SiO2/TiO2 (mol) crystallinity (%) Gel Solid 27 33 40 50
29.0 35.4 42.2 51.0
0.66 0.65 0.56 0.51
5410.8 5383.3 5377.5 5370.8
88.5 90.5 91.9 95.7
4–8 3–6 3–5 2–4
159
Xphenol (%)
XH2 O2 (%)
SDHB (%)
YDHB (%)
0 YDHB (%)
12.9 12.5 7.67 7.31
99.7 98.7 91.7 91.3
98.7 95.0 81.8 73.7
12.7 11.9 6.27 5.39
50.6 48.0 40.2 35.6
Composition of gel: SiO2:0.73 NBA:0.175 TPABr:94 H2O; crystallization at 453 k for 3 days.
Table 6 Effect of TPABr/SiO2 on the physicochemical and catalytic properties of TS-1 TPABr/SiO2 (mol)
I960/I550
VCell ˚ 3) (A
Relative crystallinity (%)
d (mm)
Xphenol (%)
XH2 O2 (%)
SDHB (%)
YDHB (%)
0 YDHB (%)
0.10 0.175 0.30 0.40
0.69 0.63 0.65 0.64
5389.1 5384.1 5387.1 5383.4
75.5 98.9 99.4 92.6
3–6 5–8 6–9 8–10
6.30 8.13 10.1 8.70
96.7 84.9 99.1 99.8
69.5 81.9 92.4 92.5
4.38 6.66 9.33 8.05
34.6 41.8 48.5 47.7
Xphenol (%)
XH2 O2 (%)
SDHB (%)
YDHB (%)
0 YDHB (%)
8.53 8.88 9.39
79.8 80.2 84.8
79.6 81.7 83.0
6.79 7.25 7.79
42.8 44.1 47.3
Composition of gel: SiO2:0.03 TiO2:0.73 NBA:41 H2O; crystallization at 453 k for 3 days.
Table 7 Effect of NBA/SiO2 on the physicochemical and catalytic properties of TS-1 ˚ 3) I960/I550 VCell (A Relative d (mm) NBA/SiO2 (mol) crystallinity (%) 0.73 0.84 1.04
0.55 0.60 0.66
5343.6 5400.8 5410.8
89.9 91.5 96.2
4 3 2.5
Composition of gel: SiO2: 0.03 TiO2:0.175 TPABr:41 H2O; aging at room temperature for 12 h and crystallization at 453 k for 3 days.
slight influence on the amount of Ti incorporated into framework. The SEM micrographs indicate that the crystal size of the sample increases with an increase of TPABr/ SiO2, which is similar to the results reported by Gontier and Tuel [13]. The effect of TPABr/SiO2 on the catalytic performance of TS-1 is similar to that of TPABr/SiO2 on the crystallinity of TS-1.
The base often plays an important role in the crystallization of zeolites. The results in Table 7 show that the molar ratio of NBA/SiO2 affects the physicochemical and catalytic properties of TS-1. Increasing NBA/SiO2 can increase both the crystallinity of TS-1 and the amount of
Fig. 9. XRD patterns of TS-1 prepared with no surfactant (1), Tween 20 (2) and Tween 60 (3).
Fig. 10. FT-IR spectra of TS-1 prepared with no surfactant (1), Tween 20 (2) and Tween 60 (3).
3.6. Effect of NBA/SiO2
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Table 8 Effect of nonionic surfactant on the physicochemical and catalytic properties of TS-1 ˚ 3) I960/I550 d (mm) Xphenol (%) Sample Surfactant VCell (A
XH2 O2 (%)
SDHB (%)
YDHB (%)
0 YDHB (%)
1 2 3
47.4 72.4 90.5
36.4 71.8 86.0
1.45 5.17 9.46
26.1 42.8 48.9
No Tween 20 Tween 60
5387.1 5397.4 5411.8
0.80 0.83 0.84
10–15 8–12 4–9
3.98 7.20 11.0
Composition of gel: SiO2:0.01 Tween:0.03 TiO2:0.73 NBA:0.175 TPABr:41 H2O; crystallization at 453 k for 5 days.
3.7. Effect of nonionic surfactant
Fig. 11. UV–vis spectra of TS-1 prepared with no surfactant (1), Tween 20 (2) and Tween 60 (3).
framework Ti in TS-1, and makes the crystal size decrease slightly and the catalytic performance of TS-1 increase. This indicates that higher alkalinity is favorable to the synthesis of TS-1.
Nonionic surfactant can decrease the surface tension of solution and therefore decrease the concentration of template in the synthesis of zeolite [23]. Two kinds of nonionic surfactant (Tween 20 and Tween 60) were adopted in the synthesis of TS-1 and the effect of nonionic surfactant on the physicochemical and catalytic properties of TS-1 was investigated. The results in Fig. 9 show that all the samples prepared with and without surfactant have the typical MFI topological structure, but the crystallinity of the samples prepared with surfactant decrease slightly. As seen from Fig. 10 and Table 8, the amount of Ti existed in the framework of TS-1 increases slightly after using nonionic surfactant. Fig. 11 shows the UV–vis spectra of TS-1 prepared with and without nonionic surfactant. The peak intensity at 210 nm of the sample prepared with nonionic surfactant is higher than that of the sample prepared without nonionic surfactant, which is consistent with the results measured by FT-IR and XRD. The absorption peak at 280–330 nm of the sample prepared with Tween 20 is weaker than that of the sample prepared without surfactant, and no absorption
Fig. 12. SEM micrographs of TS-1 prepared with no surfactant (1), Tween 20 (2) and Tween 60 (3).
H. Liu et al. / Applied Catalysis A: General 293 (2005) 153–161
peak at 280–330 nm is observed in the sample prepared with Tween 60. These results show that using nonionic surfactant can reduce the amount of extra-framework TiO 2 in TS-1. The SEM micrographs in Fig. 12 show that, the crystal size of the sample prepared using Tween 20 or Tween 60 is smaller than that prepared in absence of surfactant, and small crystal size is propitious to improve the catalytic performance of the TS-1 catalyst for the hydroxylation of phenol (Table 8). As an additive used in the synthesis of TS1, Tween 60 is superior to Tween 20.
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hydroxylation of phenol with H2O2 in the fixed-bed reactor system.
Acknowledgement This project is supported financially by the Commission of Science and Technology of Shanghai Municipality (No. 03DJ14006).
References 4. Conclusions TS-1 is easily synthesized by using amorphous SiO2 and TPABr instead of costly TPAOH and Si-alkoxide. The catalytic performance of TS-1 prepared using amorphous SiO2 is close to those of the sample prepared with TEOS for the hydroxylation of phenol with H2O2 in the fixed-bed reactor system. The synthesis conditions have a great influence on the physicochemical and catalytic properties of TS-1. The crystallinity of the sample increases with increasing crystallization temperature, crystallization time, SiO2/ TiO2, SiO2/H2O or NBA/SiO2. An increase of the aging time, H2O/SiO2, SiO2/TiO2 or NBA/SiO2 is in favor of gaining the sample with smaller crystal size. On the contrary, increasing the crystallization temperature and time, or TPABr/SiO2 can increase the crystal size of sample. The amount of Ti4+ incorporated into framework of TS-1 zeolite is affected hardly by the crystallization temperature, aging time, H2O/SiO2 or TPABr/SiO2. However, the high ratio of NBA/SiO2 or TiO2/SiO2 in the matrix gel promotes Ti incorporating into the framework of TS-1. Adding nonionic surfactant as an additive in the synthesis gel, can increase the amount of framework Ti, and reduce the crystal size of sample and the amount of TiO2 outside framework. Smaller crystal size and more amount of framework Ti are favorable to increase the catalytic performance of TS-1 for the
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