Characterization of Ti-ZSM-5 Prepared by Isomorphous Substitution of

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Ind. Eng. Chem. Res. 2010, 49, 2194–2199

Characterization of Ti-ZSM-5 Prepared by Isomorphous Substitution of B-ZSM-5 with TiCl4 and Its Performance in the Hydroxylation of Phenol Jian Gao, Min Liu, 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 116012, China

Ti-ZSM-5 has attracted much attention for its unique catalytic properties in selective oxidation. Isomorphous substitution is an alternative synthesis method to obtain Ti-ZSM-5 with low contents of extraframework Ti. However, the nature of the precursor also has a great influence on the catalytic performance of Ti-ZSM-5. In this work, two Ti-ZSM-5 samples were prepared by isomorphous substitution of precursor B-ZSM-5 zeolites with gaseous TiCl4. The effect of the nature of the B-ZSM-5 precursor obtained by different hydrothermal synthesis procedures on the catalytic performance was investigated through characterization by XRD; FT-IR, UV-vis, and UV-Raman spectroscopies; SEM; N2 adsorption; and phenol hydroxylation. The characterization results show that the two Ti-ZSM-5 samples synthesized by gas-solid isomorphous substitution of the B-ZSM-5 precursor contain very little anatase and have similar amounts of active sites. They consist of cuboidlike crystals with different sizes that are agglomerated into different sizes of spherelike particles. Phenol hydroxylation with dilute H2O2 showed that, because of the smaller crystal size of its cuboid particles, TiZSM-5 synthesized with B-ZSM-5 as the precursor at lower crystallization temperature had better catalytic performance and better recyclability. Introduction 1

Since the first synthesis of TS-1 by Tarramasso et al. in 1983, TS-1 has attracted much attention for its unique catalytic oxidation properties, especially in selective oxidation reactions with dilute H2O2 as the oxidant under mild conditions. There are two methods for synthesizing TS-1: hydrothermal synthesis2-8 and gas-solid isomorphous substitution.9-13 TS-1 synthesized by hydrothermal synthesis has been commercialized, but few studies have been published on gas-solid isomorphous substitution, which is a simple way to synthesize titanium silicate zeolite. The key in this synthetic method is the preparation of precursors, such as H-ZSM-5, B-ZSM-5, and Al-ZSM-5,8-10,13-15 and the subsequent titanation treatment. However, the catalytic activities of these samples are usually much lower than those of TS-1 obtained from hydrothermal synthesis. For example, in phenol hydroxylation, under the same reaction conditions, the phenol conversion was lower than 10% over TS-1 prepared by the substitution route,8-11 whereas over TS-1 from the hydrothermal method, it was above 20%.1,8,16 Recently, Liu et al.17,18 reported that Ti-ZSM-5 synthesized by using B-ZSM-5 and adjusting the crystallization temperature had catalytic performance in selective oxidation reactions similar to that of TS-1 obtained from hydrothermal synthesis.19 On the basis of these studies, we investigated the variations between B-ZSM-5 precursors synthesized by different synthesis processes and attempted to find the reasons for the good catalytic activity of Ti-ZSM-5. Experimental Section Sample Preparation. In the synthesis of the B-ZSM-5 precursor, silica sol (aluminum trace, Na2O e 0.3 wt %) was used as the silicon source, tetrapropylammonium bromide was used as the template, and boric acid was used as the boron source [SiO2/B2O3 ) 5 (mol/mol)]. The synthesis materials were mixed as indicated in a previous publication.16 One-half of the mixture * To whom correspondence should be addressed. E-mail: guoxw@ dlut.edu.cn. Tel.: +86-41139893990. Fax: +86-41183689065.

was heated in a Teflon-lined stainless steel autoclave at 80 °C for 20 h and then at 165 °C for 72 h. The other half was kept in a Teflon-lined stainless steel autoclave at 165 °C for 72 h. The resulting solids were recovered by filtration, washing, drying, and calcination in static air at 540 °C for 5.5 h. The obtained samples are denoted as S-1 and S-2, respectively. Deboronation by acid treatment and titanation treatment were performed according to the literature.17 Both B-ZSM-5 samples (S-1, S-2) were washed with 2 mol/L hydrochloric acid aqueous solution (HCl) for 2 h at 80 °C to remove boron atoms in B-ZSM5, and afterward, the titanation treatment was performed at 500 °C for 26 h with TiCl4. The Ti-ZSM-5 samples prepared using S-1 and S-2 as the precursors are denoted as S-1-Ti and S-2-Ti, respectively. In the silylation of the catalyst, a certain amount of Ti-ZSM-5 zeolite was suspended in a mixture of cyclohexane and tetraethylorthosilicate (TEOS), after which it was subjected to ultrasonic treatment for 30 min. Finally, the samples were dried at 100 °C and calcined at 540 °C for 5.5 h in dry air. The resulting SiO2 loadings were ∼1, ∼3, and ∼9 wt %. Characterization. X-ray powder diffraction (XRD) was performed on a D/max-2400 diffractometer using Cu KR radiation. The relative crystallinity of the samples was calculated by summation of the XRD peak intensities at 2θ ) 7.8°, 8.8°, 23.2°, 23.8°, and 24.3°. Fourier-transform infrared (FT-IR) spectra were measured on an EQUINOX55 spectrometer. The samples were ground with KBr, pressed into thin wafers, and dried at 120 °C for 1 h in air. UV-vis spectra were recorded on a Jasco V-550 instrument in the range of 190-400 nm using the diffuse-reflectance technique, with the blank board as the reference. UV-Raman spectra were recorded on a Jobin Yvon U-1000 Raman spectrometer. N2 adsorption was carried out on a Quantachrome AUTOSORB-1-MP instrument, and scanning electron microscopy (SEM) was performed on a Hitachi S4800 microscope. The physical adsorption of cyclohexane using a flow-gravimetric method was performed as described in the literature.20 Thermogravimetry/differential thermogravimetric (TG/DTG) analysis was carried out at a rate of 10 °C/min from

10.1021/ie901360y  2010 American Chemical Society Published on Web 02/02/2010

Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010

Figure 1. XRD patterns of the precursors (S-1, S-2) and Ti-ZSM-5 samples (S-1-Ti, S-2-Ti).

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Figure 2. FT-IR spectra of the precursors (S-1, S-2) and Ti-ZSM-5 samples (S-1-Ti, S-2-Ti).

room temperature to 700 °C on a TGA/SDTA851e thermal analyzer (Mettler Toledo) in an air flow of 60 mL/min. Catalytic Reaction. Phenol (PHE) hydroxylation was carried out in a 30 mL glass batch reactor. A mixture of phenol, acetone, H2O2 (30 wt %), and catalyst was fed into the reactor. The reaction was carried out at 80 °C for 6 h with magnetic agitation. The product was analyzed using a GC-7890F gas chromatograph with a flame ionization detector and a capillary column (SE30, 30 m × 0.25 mm). The main products of the reaction were catechol (CAT) and hydroquinone (HQ), and the byproduct was para-benzoquinone (PBQ). Evaluation of the reaction with H2O2 was carried out using the quantities 0 nPHE - nPHE × 100% nPHE

XPHE ) SCAT )

nHQ

SHQ+PBQ )

Figure 3. UV-vis spectra of the Ti-ZSM-5 samples (S-1-Ti, S-2-Ti).

nHQ + nPBQ × 100% nHQ + nCAT + nPBQ

XH2O2 )

eH2O2 )

nCAT × 100% + nCAT + nPBQ

nH0 2O2 - nH2O2 nH0 2O2

× 100%

nHQ + nCAT + nPBQ nH0 2O2XH2O2

× 100%

where XPHE, XH2O2, SCAT, SHQ+PBQ, and eH2O2 denote the conversions of phenol and H2O2; the selectivities to catechol, hydroquinone, and para-benzoquinone; and the H2O2 efficiency, respectively. n0 and n denote the initial and final molar amounts, respectively. Results and Discussion Characterization of Ti-ZSM-5 Samples. The XRD patterns of the samples exhibit the specific diffraction peaks of MFItype zeolite (Figure 1), which means that the framework of the samples was not destroyed during the titanation process. The relative crystallinities of S-1 and S-2 were 100% and 96%, respectively. After the gas-solid isomorphous substitution, both relative crystallinities decreased to around 90%. The FT-IR spectra of the samples are shown in Figure 2. The vibrational band at 550 cm-1 is assigned to the five-

Figure 4. UV-Raman spectra of the Ti-ZSM-5 samples (S-1-Ti, S-2-Ti) excited with the laser line at 244 nm.

membered rings of the pentasil zeolite. The IR peaks at around 1400 and 918 cm-1 are due to tri- and tetracoordinated boron in the framework, respectively. After acid treatment and titanation, these peaks disappeared, indicating that boron was removed from the framework. A new peak appeared at 968 cm-1 for S-1-Ti and S-2-Ti, suggesting the incorporation of Ti4+ into the zeolite framework.21

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Figure 5. Near-UV Raman spectra of the Ti-ZSM-5 samples (S-1-Ti, S-2Ti) excited with the laser line at 325 nm.

UV-vis characterization is a sensitive way to verify the coordination state of titanium species. The UV-vis spectra of S-1Ti and S-2-Ti (Figure 3) exhibit only a strong absorption at 212

nm, due to the charge transfer of the oxygen 2p electron to the empty 3d orbital of Ti4+ ion, the active center in the selective oxidation reactions.22 The fact that no band is observed at ∼330 nm indicates that no anatase is present in either of the Ti-containing samples.23 Ultraviolet (UV) Raman spectroscopy is an effective technique for identifying anatase and framework titanium species. Figure 4 shows the UV-Raman spectra of the Ti-ZSM-5 samples excited by the laser line at 244 nm. The strong bands observed at 490, 520, and 1113 cm-1 are assigned to framework titanium species. The band at 1113 cm-1 is not in agreement with previous literature on TS-1 obtained from the hydrothermal synthesis method, which reports a band at 1125 cm-1.24,25 In the hydrothermal synthesis method, the resonance-enhanced Raman bands at 490, 520, and 1125 cm-1 are attributed to a local [Ti(OSi)4] unit of TS-1, with the band at 1125 cm-1 being attributed to the asymmetric stretching vibration of Ti-O-Si.26,27 It is likely that, for the different synthesis methods, the coordination state of the titanium atoms in the zeolite framework is different. The titanium atoms in Ti-ZSM-5 synthesized by

Figure 6. SEM images of B-ZSM-5 (S-1, S-2) and Ti-ZSM-5 (S-1-Ti, S-2-Ti) samples.


gas-solid isomorphous substitution are relatively flexible and not as strictly fixed as those in TS-1 through tetrahedral coordination. In addition, resonance-enhanced Raman bands at 800 and 960 cm-1 appeared, which is characteristic of silicalite-1 zeolite and is assigned to Si-O-Si species next to the framework Ti-O-Si species.26,27 In the near-UV Raman spectra of S-1-Ti and S-2-Ti excited at 325 nm (Figure 5), the bands at 144, 390, and 637 cm-1 corresponding to anatase24,25 are not present, suggesting that there might be only trace amounts of or even no extraframework titanium species in S-1-Ti and S-2Ti. This is consistent with the UV-vis results, which show only one strong absorption at 212 nm. The Raman band at 374 cm-1 (Figure 5) is also characteristic of silicalite-1 zeolite. In conclusion, the formation of TiO2 species (anatase) can be efficiently avoided in the gas-solid isomorphous substitution process. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) results showed that no residual boron atoms were present in either Ti-ZSM-5 sample and that the amounts of Ti in S-1-Ti and S-2-Ti were 1.48 and 1.40 wt %, respectively. The influence of residual boron atoms on the catalytic properties of Ti-ZSM-5 can thus be neglected. Because anatase was not detected and the content of other extraframework titanium species was low, as confirmed by the UV-vis and UV-Raman characterizations, the two Ti-ZSM-5 samples had almost the same amounts of framework titanium species. The SEM images in Figure 6 show that samples S-1 and S-2 had spherelike shapes. S-1 was smaller than S-2 in crystal size, because low-temperature crystallization always results in smaller crystals with more perfect crystallinity. In the randomly selected SEM images, the numbers of measured spherelike crystallites for S-1 and S-2 were 107 and 136, respectively. As Figure 7 shows, the main spherelike crystallite size of S-1 was within the range of 2.7-3.3 µm, accounting for around 80% of the crystallites, but the main spherelike crystallite size of S-2 was 8-10 µm, accounting for around 85% of the crystallites. At higher magnification, it can be seen that the spheres of S-1 and S-2 consisted of agglomerates of much smaller crystals that had a cuboidlike shape. The statistics of the distributions of these smaller crystals sizes from the SEM images were also obtained. In the randomly selected SEM images, the number of measured cuboidlike particles for S-1 and S-2 was 30-40. As Figure 8 shows, the cuboidlike particle size of S-1 was mainly within the range of 250-310 nm, accounting for around 80% of the crystallites, but the cuboidlike particle size of S-2 was mainly within the range of 1.0-1.2 µm, accounting for around 90% of the crystallites. Obviously, the length of the cuboid crystals of S-1 was much shorter than that of the S-2 crystals. The cuboid crystals are the basic units that form the spherelike crystals of B-ZSM-5 and could contribute to the different catalytic performances of Ti-ZSM5. After acid treatment and titanation, the shapes of S-1-Ti and S-2-Ti were irregular, but the samples still kept the MFI structure as identified by XRD characterization (Figure 1). The results of N2 sorption characterization of the samples are reported in Table 1. The pore volumes and BrunauerEmmett-Teller (BET) surface areas of all samples were almost the same, and their pore diameter calculated by the HK (Horvath-Kawazoe) method was nearly 0.55 nm. The cyclohexane adsorption capacities of the samples are also included in Table 1. Because the kinetic diameter of cyclohexane is 0.6 nm, its diffusion in the channels of Ti-ZSM-5 will be slow.28 Therefore, the adsorption capacity of cyclohexane can serve as an indication of the accessible pore volume. The higher cyclohexane adsorption capacities on S-1 and S-1-Ti mean that

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Figure 7. Spherelike crystallite size distribution of B-ZSM-5 (S-1, S-2) samples.

the samples with smaller crystals have more accessible pore volumes and thus allow more reactants to access the active sites. Phenol Hydroxylation. Phenol hydroxylation with dilute H2O2 was carried out to investigate the catalytic properties of Ti-ZSM5. The results are summarized in Table 2. S-1-Ti showed a high activity for the reaction. The conversions of phenol and H2O2 and the H2O2 efficiency over S-1-Ti were higher than those over S-2Ti. As mentioned above, S-1-Ti and S-2-Ti had the same amounts of active sites; therefore, the number of active sites is not the sole reason that S-1-Ti had a higher activity. According to the SEM images, the crystal sizes of the cuboid crystals that are agglomerated into spherelike particles of different sizes seem to play an important role in the catalytic properties of Ti-ZSM-5 in phenol hydroxylation. The smaller the cuboid crystal size of the precursor B-ZSM-5, the higher the catalytic activity of the resulting Ti-ZSM-5. The adsorption results confirm that S-1-Ti had a higher adsorption capacity than S-2-Ti and that its active sites were more accessible. Therefore, the smaller size of the cuboid crystals of B-ZSM-5 is the other key reason why S-1-Ti had a higher activity in the oxidation reaction. Because small crystals always have more external active sites than larger crystals, the external active sites of S-1-Ti were eliminated by TEOS coating to investigate whether they play an important role in this reaction. TEOS is often used to modify the external surface and pore mouth of ZSM-5 in many shape-selective reactions.29 It has also been used for the elimination of the external active sites in TS-1.30 The activities of Ti-ZSM-5 samples with different SiO2 loadings are reported in Table 3. When the SiO2

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Ind. Eng. Chem. Res., Vol. 49, No. 5, 2010 Table 3. Phenol Hydroxylation over S-1-Ti and S-2-Ti after Treatment with Different Concentrations of TEOSa selectivity (%) SiO2 loading (wt %) XPHE (%)

SCAT

SHQ+PBQ 55 55 56

97 96 97

64 62 59

56 57 59

73 73 63

60 55 56

∼1.0 ∼3.0 ∼9.0

21 20 19

S-1-Ti 45 45 44

∼1.0 ∼3.0 ∼9.0

15 13 12

S-2-Ti 44 43 41

XH2O2 (%) eH2O2 (%)

a Reaction conditions: T ) 80 °C, t ) 6 h, molar ratio of PhOH to H2O2 ) 3, molar ratio of acetone to PhOH ) 2.7, mass ratio of catalyst to PhOH ) 5%.

Table 4. Recycling Results in Phenol Hydroxylation over S-1-Ti and S-2-Tia number of reaction cycles

XPHE(%)

XH2O2 (%)

eH2O2 (%)

1 2 3 regeneration

S-1-Ti 22 20 18 21

93 92 91 91

70 67 61 68

1 2 3 regeneration

S-2-Ti 16 10 9 16

82 67 56 82

58 50 48 58


Figure 8. Cuboidlike crystallite size distribution of B-ZSM-5 (S-1, S-2) samples. Table 1. Adsorption Properties of the Catalysts sample

pore volume (cm3/g)

BET surface area (m2/g)

c-C6 adsorption capacity (wt %)

S-1 S-2 S-1-Ti S-2-Ti

0.35 0.35 0.36 0.37

413 403 427 422

7.5 5.5 7.5 6.3

Table 2. Phenol Hydroxylation over Ti-ZSM-5 Samplesa selectivity (%) sample

XPHE (%)

SCAT

SHQ+PBQ

XH2O2 (%)

eH2O2 (%)

S-1-Ti S-2-Ti

22 16

48 46

52 54

93 82

70 58


loading was increased, there was a slight decrease in the conversion of phenol and an increase in the proportion of para products. This indicates that most of the active sites are located inside the pore channels, and thus, the greater accessibility of the active sites in S-1-Ti makes S-1-Ti more active in phenol hydroxylation. Recycling Test. Table 4 compares the recyclability characteristics of S-1-Ti and S-2-Ti in phenol hydroxylation. After each reaction, the samples were just dried at 100 °C without any other treatment. After three reactions, a reduction was observed in both the conversion of phenol and the efficiency of H2O2 over both

catalysts. Meanwhile, S-2-Ti deactivated more strongly than S-1Ti. With longer channels in S-2-Ti, the residence time of reactants and products in the channel is prolonged. This makes byproduct generation easier and the diffusion of some bulky products more difficult. Then, these bulky products can be further converted into coke. Therefore, the activity of S-2-Ti decreased sharply (Table 4). However, after regeneration (calcination at 540 °C for 6 h), the byproducts were removed, and the catalytic properties of the Ti-ZSM-5 samples were recovered to those of the fresh catalysts. Figure 9 shows the TG/DTG curves of the two Ti-ZSM-5 samples after phenol hydroxylation. The weight loss of S-1-Ti and S-2-Ti mainly occurred when the decomposition temperature was 350-450 °C. This means that many reaction products still resided in the zeolite pores. There was also a weight loss in the range of 150-200 °C, which can be assigned to water molecules in the zeolite. Thus, it can be concluded that, when the channels become longer, more reaction substances can be occluded in the channels and lead to a reduction of the catalytic activity of S-2-Ti. Conclusion Ti-ZSM-5 was synthesized using B-ZSM-5 as the precursor according to the gas-solid isomorphous substitution method. A low crystallization temperature resulted in a small crystal size of the B-ZSM-5 precursor and, hence, affected the activity of Ti-ZSM-5 in phenol hydroxylation. The characterization results showed that the two Ti-ZSM-5 samples had negligible amounts of extraframework Ti species and nearly the same amounts of active framework Ti4+ species. The smaller crystals of Ti-ZSM-5 had a higher adsorption capacity because of the increase in more accessible pores, making the internal active sites be more accessible. In addition, the shorter diffusion length in the smaller crystals can diminish the formation of byproducts and the


Figure 9. TG/DTG curves of Ti-ZSM-5 samples after phenol hydroxylation one time: (a) S-1-Ti, (b) S-2-Ti.

presence of residual reaction substances in the pore channels, thus improving the recyclability of Ti-ZSM-5. Acknowledgment This work was financially supported by the Program for New Century Excellent Talents in University (NCET-04-0268) and the 111 Project. We also thank Prof. Chunshan Song and Prof. Roel Prins for helpful discussions. Literature Cited (1) Taramasso, M.; Perego, G.; Notari, B. Preparation of porous crystalline synthetic material comprised of silicon and titanium oxide. U.S. Patent 4, 410,501, 1983. (2) Notari, B. Titanium SilicalitesA New Selective Oxidation Catalyst. Stud. Surf. Sci. Catal. 1991, 60, 343. (3) Thangaraj, A.; Sivasanker, S. An Improved Method for TS-1 Synthesis: 29Si NMR Studies. Chem. Commun. 1992, 2, 123. (4) Mu¨ller, U.; Steck, W. Ammonium-based Alkaline-free Synthesis of MFI-type Boron and Titanium Zeolites. Stud. Surf. Sci. Catal. 1994, 84, 203. (5) Gao, H. X.; Suo, J. S.; Li, S. B. An Easy Way to Prepare Titanium Silicalite-1 (TS-1). Chem. Commun. 1995, 8, 835. (6) Tuel, A. Crystallization of Titanium Silicalite-1 (TS-1) from Gels Containing Hexanediamine and Tetrapropylammonium Bromide. Zeolites 1996, 16 (2-3), 108. (7) Shibata, M.; Gerard, J.; Gabelica, Z. Rapid Synthesis of MFI Titanosilicates Using in Situ Seeding Method. Microporous Mater. 1997, 12, 141.

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ReceiVed for reView August 29, 2009 ReVised manuscript receiVed December 17, 2009 Accepted January 20, 2010 IE901360Y

Characterization of Ti-ZSM-5 Prepared by Isomorphous Substitution of

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