Effect of acidity in TS-1 zeolites on product distribution of the styrene oxidation reaction

Applied Catalysis A: General 258 (2004) 1–6

Effect of acidity in TS-1 zeolites on product distribution of the styrene oxidation reaction Jianqin Zhuang a , Ding Ma a , Zhimin Yan a , Xiumei Liu a , Xiuwen Han a , Xinhe Bao a,∗ , Yihua Zhang b , Xinwen Guo b , Xiangsheng Wang b a

b

State Key Laboratory of Catalysis, Chinese Academy of Sciences, Dalian Institute of Chemical Physics, 457 Zhongshan Road, Dalian 116023, Liaoning, China State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, Liaoning, China Received in revised form 11 April 2003; accepted 11 June 2003

Abstract The role of Brønsted acidity of titanium silicalite zeolite (with different ratios of Si/Ti) in oxidation reactions of styrene has been investigated and discussed. For zeolites with Si/Ti > 42, most of the titanium is in the zeolite framework. These framework titanium species, which act both as the isolated titanium centers and as Brønsted acidity centers (together with the Brønsted acidity produced by the tetrahedral aluminum impurity introduced during synthesis), can catalyze both the epoxidation and the succeeding rearrangement reactions, thus promoting the formation of phenylacetaldehyde. With an increase in the titanium content of the zeolite, titanium will tend to stay outside the zeolite lattice, except for the TiOx nanophases which can be occluded in the zeolite channels or on the external surface. These non-framework titanium species are favorable for the carbon–carbon bond scission, leading to the production of additional benzaldehyde. The catalytic performances of these zeolites with different Si/Ti ratios are correlated here with their structural information by using solid-state NMR and UV-Vis methods. © 2003 Elsevier B.V. All rights reserved. Keywords: Acidity; Zeolites; Styrene

1. Introduction The titanium-containing molecular sieve TS-1, which is a zeolite of the pentasil family, was first synthesized by Tarahasso in 1983 [1]. Owing to its excellent shape selective as well as its environmentally benign character, it plays an outstanding role in the partial oxidation of many organic compounds, such as olefin epoxidation, phenol hydroxylation, alcohol oxidation, and alkane oxyfunctionalization [2–5]. With aqueous hydrogen peroxide (30% H2 O2 ) as the oxidant under mild conditions, the oxidation of styrene catalyzed by TS-1 zeolite has also been reported [6]. If one compares with the oxidation of other light olefin hydrocarbons, the main product from the oxidation of styrene is phenylacetaldehyde, while no styrene oxide, which should be the primary product expected from the epoxidation of the double bond in the styrene side chain, has been detected. Uguina and Fu

reported that, in addition to the oxidation centers due to isolated titanium atoms, the weak acidic centers from the zeolite can catalyze the rearrangement of the primary product of styrene oxidation to produce phenylacetaldehyde [7,8]. Another secondary product, benzaldehyde, which was formed by a C–C bond cleavage in the styrene side chain, was also detected [8]. In the present study, by correlating the catalytic data with the structural information of a series titaniumsilicate zeolites with different Si/Ti ratios measured through UV-Vis and solid-state MAS NMR methods, we identified different titanium species. Their roles in the styrene oxidation reaction are investigated and discussed.

2. Experimental 2.1. Synthesis of TS-1 zeolite

∗

Corresponding author. Tel.: +86-411-4686637; fax: +86-411-4694447. E-mail address: [email protected] (X. Bao). 0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.06.002

A series of samples was synthesized by the following procedure. TiCl4 as titanium source was dissolved in alcohol

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J. Zhuang et al. / Applied Catalysis A: General 258 (2004) 1–6

to obtain solution A, which was then added dropwise to the desired amount of silicate gel (concentration 30 wt.%) under vigorous stirring, and the resulting mixture was stirred at room temperature for about 1 h. Then TPABr as the template, NH4 OH as base, and distilled water were added in turn. The gels with a TiO2 /SiO2 ratio of 0.01, 0.02, 0.03, or 0.05 were stirred at the same temperature for 2 h. Each gel was then transferred into a stainless steel autoclave and crystallized at 150–180 ◦ C for 3–5 days under stirring. After crystallization, all samples were recovered by centrifugation, washed several times with distilled water and dried in an oven. Occluded TPABr species were removed by calcination in air at 833 K for 6 h. 2.2. Physicochemical characterization The structure of the obtained zeolites was checked with a D/max-␥b type X-ray diffractometer (Rigaku) using monochromatic Cu K␣ radiation (40 kV and 100 mA), with a scan speed of 5◦ /min in 2θ. The XRD measurements confirmed that the synthesized materials had a MFI structure. UV-Vis spectra were recorded on a JASCO V-550 spectrometer with a scan range of 190–490 nm. The samples were treated at 813 K for 5 h before the detection. Chemical analysis was performed using a SRS 3400 X-ray fluorescence spectrometer. 2.3. NMR measurements With a special device [9], the TS-1 samples were heated at 673 K in vacuum (below 10−2 Pa) for 20 h for dehydration. The adsorption of trimethylphosphine (Acros Organics) was performed by exposing the dehydrated sample to its saturated vapor pressure at room temperature for 30 min. Each sample was then evacuated for 20 min to remove the TMP physisorbed on the surface. After the treatment, the sample was filled in situ into an NMR rotor, then sealed without exposure to air. The spectra were obtained at room temperature on a Bruker DRX-400 spectrometer with a BBO MAS probe using 4 mm ZrO2 rotors. 29 Si MAS NMR spectra with high-power proton decoupling were recorded at 79.5 MHz using 1.6 ␮s pulse, 4 s repetition time and 2048 scans. 1 H → 29 Si CP/MAS NMR experiments were performed with a 4 s repetition time, 4096 scans and contact time of 1.5 ms. All 29 Si spectra were recorded on samples spun at 4 kHz and reference was to DSS. 27 Al MAS NMR spectra were recorded with high-power proton decoupling at 104.3 MHz using a 0.75 ␮s pulse, 2 s repetition time and 2048 scans. 1 H MAS NMR spectra were obtained at 400.1 MHz by means of single-pulse experiments, with a 3 ␮s pulse, 4 s repetition time and 100 scans. All 1 H and 27 Al MAS NMR spectra were recorded with samples spun at 8 kHz, and chemical shifts were referenced to saturated aqueous solutions of DSS and 1% aqueous Al(NO3 )3 , respectively. 31 P MAS NMR spectra with high-power proton

decoupling were obtained at 161.9 MHz, using a 2.0 ␮s pulse, 2 s repetition time and 2048 scans, and the samples were spun at 6 kHz and referenced to 85% H3 PO4 . 2.4. Catalytic reaction Styrene oxidation tests were performed in a laboratory scale device. A standard reaction was carried out as follows. The catalyst and the reagents of acetone, styrene and hydrogen peroxide were all transferred into a 250 ml stainless steel autoclave reactor fitted with a magnetic stirrer and a thermometer, and then sealed (styrene 10 ml, catalyst 1 g, acetone 20 ml, 30 wt.% H2 O2 4 ml). The solution was then heated to the reaction temperature. An aliquot was removed after the reaction, and the products were analyzed by gas chromatography (1102G, Shanghai Analysis Corp.).

3. Results and discussion 3.1. Physico-chemical characterization According to the XRD analysis, all samples have a MFI structure of good crystallinity, and no diffraction peaks of non-zeolitic phases were detected. Table 1 lists the results of XRF measurements. The overall Si/Ti ratios of these samples are 75, 42, 28, 18 (from sample A to D), respectively. At the same time, considerable amounts of aluminum were found in all of the four samples, while their Si/Al ratios remained almost constant. The aluminum detected here, which is considered to be the origin of the acidity in TS-1 zeolite, was due to impurities of the silicate gel. The UV-Vis diffuse reflectance spectra of samples A–D shown in Fig. 1 have provided significant information regarding the titanium species of the samples. All of them show a strong transition centered at 210 nm, which was originated from the electronic transfer of the p␲–p␲ transitions between titanium and oxygen in the framework titanium species, the Ti–O–Si in the zeolite [10]. This is characteristic of the isolated tetrahedral-coordinated Ti4+ cations [11]. Applying the Pauling rule of coordination, an upper limit of the ratio of Ti/(Ti + Si) in the TS-1 zeolite was reported to be 2.5% [12]. Interestingly, in the present case, when the Si/Ti ratio has reached 42, where the Ti/(Ti + Si) is 2.8%, a broad shoulder at about 240–260 nm in the UV-Vis spectra appeared, which was indicated to be TiOx nanophases [11]. Table 1 XRF analysis of TS-1 zeolites with different Si2 O2 /TiO2 ratios Sample

SiO2 (wt.%)

TiO2 (wt.%)

A12 O3 (wt.%)

SiO2 /TiO2

SiO2 /A12 O3

A B C D

88.88 88.20 86.42 85.03

1.568 2.807 4.190 6.286

0.184 0.193 0.198 0.170

75 42 28 18

821 777 741 850

J. Zhuang et al. / Applied Catalysis A: General 258 (2004) 1–6

Fig. 3. Q3 line of the (D) 18.

Fig. 1. UV-Vis spectra of TS-1 zeolites with different ratios of Si/Ti: (A) 75, (B) 42, (C) 28, (D) 18.

Apparently, more nanophases were formed with a decreasing of the Si/Ti ratio. With further increase in the amount of titanium of the zeolite, TiOx nanophases will aggregate to form the TiO2 anatase phases, leading to a peak at about 300 nm in the UV-Vis spectra [13,14]. 3.2. NMR measurements 29 Si

MAS NMR is a useful method to elucidate structural variations concerning silicon in the zeolite [15]. As demonstrated in Fig. 2, two main peaks at −114.9 and −116.9 ppm are well resolved, together with a hint of a weak peak at −104.6 ppm. The peak at −114.9 ppm is normally assigned to resonance from Q4 , [Si(OSi)4 ] species. The resonance at −104.6 ppm in the 29 Si MAS NMR spectra can be assigned to silanols, in which the terminal hydroxyl groups are connected directly to the Si atoms in SiO4 tetrahedral (Q3 [Si(OSi)3 OH]) species [15]. As shown in the amplified Q3 lines of 29 Si MAS NMR spectra of samples A–D (Fig. 3), the amount of Q3 species decreased when the Si/Ti ratio

Fig. 2. 29 Si MAS NMR spectra of TS-1 with different ratios of Si/Ti: A (75), (B) 42, (C) 28, (D) 18.

29 Si

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MAS NMR spectra: (A) 75, (B) 42, (C) 28,

was changed from 75 to 42, while no significant variation was observed for a further reduction of the Si/Ti ratio. The corresponding 29 Si CP/MAS NMR spectra shown in Fig. 4 confirmed this conclusion, indicating that the Ti species introduced into the zeolite lattice may suppress the existence of the silanol defects. 29 Si MAS NMR spectra of TS-1 also exhibited a broad peak around −116.9 ppm, which has been interpreted as arising from distorted Si–O–Si bonds resulting from the presence of Si–O–Ti linkages [16]. As one might conclude, the intensity of the peak around −116.9 ppm should increase with an increase in the titanium content of the zeolite. However, we find that the amount of species relating to the −116.9 ppm line in our 29 Si MAS NMR spectra is kept at a nearly constant concentration whatever the titanium loading is. Therefore, just as has been pointed out by Tuel, it can be concluded that the presence of the NMR shoulder at −116.9 ppm is another Q4 line and is characteristic of the substituted MFI-type zeolites having an orthorhombic symmetry [17]. As demonstrated in Fig. 4, application of the CP technique can enhance selectively the Si signal (at −104.6 ppm) of the silicon atoms that are coupled with the protons of the hydroxyl groups by dipolar 1 H → 29 Si interaction. Meanwhile, a new signal at −94.1 ppm appeared, suggesting the existence of geminal silanol species (Q2 [Si(OSi)2 (OH)2 ] species) in the TS-1 [18]. No significant change occurred in the geminal silanols species with the titanium loading.

Fig. 4. 29 Si CP/MAS NMR spectra of TS-1 with different ratios of Si/Ti: A (75), (B) 42, (C) 28, (D) 18.

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Fig. 6. 31 P MAS NMR spectra of trimethylphosphine adsorbed on TS-1 with different ratios of Si/Ti: A (75), (B) 42, (C) 28, (D) 18.

Fig. 5. 27 Al MAS NMR spectra of TS-1 with different ratios of Si/Ti: A (75), (B) 42, (C) 28, (D) 18.

During the synthesis of the TS-1 zeolites, a limited amount of aluminum as impurity was inevitably incorporated into the zeolites. 27 Al MAS NMR spectra of TS-1 zeolites given in Fig. 5 show only one signal at 54.0 ppm, indicating the presence of Al in a Td symmetry [19], while octahedral Al was not observed. Moreover, it is clear from Fig. 5 that the intensities of the 54 ppm peaks for all of the four samples are similar, which agrees well with the XRF analysis results. Thus, it can be concluded that a small amount of Al was present in the framework of the zeolite, which serves as a kind of Brønsted acid site in the samples. 3.3. Acidity in TS-1 zeolites 31 P

has the advantage over 15 N of greater NMR sensitivity due to a higher magnetogyric ratio and the abundance of the phosphor 31 isotope is 100%. Therefore, many compounds containing 31 P have been used as probe molecules to determine the zeolite acidity. Especially, trimethylphosphine, with a pKb of 5.35, has been widely used for the characterization of acidity in zeolites such as H-Y and USY [20,21]. In this work, the acidity of TS-1 was studied using trimethylphosphine as the probe molecule. The 31 P MAS NMR spectra of the TMP adsorbed on different TS-1 samples shown in Fig. 6 present a resonance at −4.4 ppm, which has been well-documented and is assigned to a [(CH3 )3 PH]+ arising from the interaction of (CH3 )3 P with Brønsted acid sites of the zeolite. The peak at −33.2 ppm is attributed to the Lewis-bound trimethylphosphine [20]. As demonstrated in the spectra, the intensity at −4.4 ppm experienced a sharp increase when the ratio of Si/Ti decreased from 75 to 42. Considering the fact that the Brønsted acid sites normally come from the Si–OH–Al species, which means that one

tetrahedral framework aluminum species corresponds to one Brønsted acid site, our present experimental data suggest the existence of another kind of Brønsted sites in our samples, namely, the titanium introduced into the zeolite framework, which is evidenced by the present observation. When the Si/Ti ratio varies from 75 to 42, more titanium (in +4 valence) enters into the zeolite lattice and results in an increase of the amount of Brønsted sites. On the other hand, a further lowering of the Si/Ti ratio will not cause additional titanium to enter the zeolitic framework, according to the Pauling rule of coordination and previous UV-Vis results. Thus, the amount of Brønsted sites (−4.4 ppm) remains constant, as can be seen from Fig. 6B–D. In our previous paper, we have presented convincing results that the Lewis acid sites relating to resonance at −33.2 ppm arise from coordinately unsaturated as well as tetrahedrally coordinated Ti ions [22]. It is known that the upper limit of titanium content in the framework is 2.5%, so there should be a constant concentration of titanium in the framework when the titanium loading is more than 2.5%. However, it was observed that the intensity of the Lewis acid sites in the TS-1 zeolite increased with an increasing of the titanium content up to 6.3% from the 31 P MAS NMR spectra. Therefore, we can infer that non-framework titanium such as TiOx nanophases in these zeolites is also contributing to the Lewis acidity. 3.4. Catalysis of TS-1 zeolites Many researchers have discovered that phenylacetaldehyde and benylaldehyde are the main products of TS-1 catalyzed oxidation of styrene under mild conditions, while the speculated product due to epoxidation of styrene has not been observed. Uguina and Reddy reported that phenylacetaldehyde was a rearrangement product of epoxide catalyzed by acidic species. Thus, the acid capacity of the TS-1 zeolite is equally important for this reaction. Table 2 gives the results of the oxidation of styrene at 333 K on the TS-1

J. Zhuang et al. / Applied Catalysis A: General 258 (2004) 1–6 Table 2 The catalytic performances of TS-1 with different Si/Ti ratios on the oxidation of styrene Sample

Conversion of styrene (%) Selectivity of PADH (%) Selectivity of BADH (%)

A (75)

B (42)

C (28)

D (18)

4.7 67.9 32.1

9.4 81.8 18.2

8.0 76.5 23.5

7.3 70.7 29.3

Note: Catalyst 1 g; styrene 10 ml; H2 O2 (30 wt.%) 4 ml; solvent, CH3 COCH3 20 ml; reaction temperature 333 K. PADH: phenylacetaldehyde; BADH, benylaldehyde.

zeolites with different ratios of Si/Ti. Two types of products were detected: phenylacetaldehyde and benzaldehyde. The catalytic activity of the TS-1 zeolite increased sharply when the Si/Ti ratio decreased from 75 to 42. Then, with a further increase of titanium content, the catalytic activity of TS-1 decreased slightly. By considering the upper limit of the titanium that can be incorporated into the framework, one would expect non-framework titanium species to be formed when the Si/Ti ratio reached to 42. These non-framework titanium species might lead to the decomposition of hydrogen peroxide. As a result, a lowering of the catalytic activity will be observed. As demonstrated in Table 2, the highest selectivity of phenylacetaldehyde on the TS-1 zeolite was obtained when the Si/Ti ratio was equal to 42. But it decreased with a further increase of the titanium content. On the other hand, the selectivity of benzaldehyde experienced a converse trend with the variation of the Ti content. The non-framework titanium species such as the TiOx nano-particles favored the carbon–carbon bond cleavage [23]. Therefore, with a Si/Ti ratio lower than 42, the increase in the amount of these TiOx nano-particles will promote the bond scission reaction, leading to the formation of more benzaldehyde over samples C and D. To confirm the role of acidity on the product distribution of the styrene oxidation reaction, we added sodium hydroxide as a base to the oxidation system to neutralize the acidity of these samples. When much sodium hydroxide was added into the system, the active sites (framework Ti) were covered and poisoned, which led to the lower conversion of styrene. Table 3 shows the variation in product selectivities of the TS-1 (Si/Ti = 18) samples with the amount of Table 3 The effect of base on the variations of product selectivities of styrene oxidation reaction Amount of sodium hydroxide (ml)

Conversion of styrene (%) Selectivity of PADH (%) Selectivity of BADH (%) Selectivity of hypnone (%)

0

0.1

0.2

0.4

7.3 70.7 29.3 0

7.3 56.5 43.5 0

6.8 40.0 50.6 9.4

3.8 31.8 54.4 13.8

Note: Catalyst (Si/Ti = 18) 1 g; styrene 10 ml; H2 O2 (30 wt.%) 4 ml; solvent, CH3 COCH3 20 ml; reaction temperature 333 K; sodium hydroxide 3 wt.%. PADH, phenylacetaldehyde, BADH, benylaldehyde.

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sodium hydroxide added into the catalysts before the reaction. Upon the addition of the sodium hydroxide, a decrease in the formation of phenylacetaldehyde was observed. This result indicates that the formation of phenylacetaldehyde is actually an acid-involving process. Suppressing the acidity by the addition of a base results in a decrease in the selectivity towards phenylacetaldehyde. At the same time, with the adding of the sodium hydroxide, the selectivity of benzaldehyde was enhanced. Clerici has reported that weak acids were produced during the process of the reaction, which could dissociate into a proton and an anion.

Basic compounds can shift the equilibrium to increase the concentration of the anions, so that their oxidation abilities are strong enough to cleave the carbon–carbon bond to form benzaldehyde [11,24]. Therefore, both of these factors contributed to the observed variation of the product distribution. Meanwhile, we should mention that with the addition of sodium hydroxide, a little amount of hypnone was also detected, which was another rearrangement product of epoxide.

4. Conclusions Phenylacetaldehyde and benzaldehyde were the major products from the oxidation of styrene catalyzed by the TS-1 zeolite, while a speculated product, styrene epoxide, was not detected. By means of UV-Vis, 27 Al MAS NMR and 31 P MAS NMR with TMP as a probe, it can be concluded that the formation of these products are closely related to the different titanium species or acid sites in the zeolite system. The Brønsted acid sites originating from Si–OH–Al and framework titanium species catalyze the rearrangement of the intermediate, leading to the formation of phenylacetaldehyde, while non-framework titanium species such as TiOx nano-particles promote the carbon–carbon bond cleavage, thus resulting in the formation of benzaldehyde.

Acknowledgements We are grateful for the support of the National Natural Science Foundation of China and The Ministry of Science and Technology of China.

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