J. Phys. Chem. C 2009, 113, 7181–7185
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Study of Higher Selectivity to Styrene Oxide in the Epoxidation of Styrene with Hydrogen Peroxide over La-Doped MCM-48 Catalyst Wangcheng Zhan, Yanglong Guo,* Yanqin Wang, Yun Guo, Xiaohui Liu, Yunsong Wang, Zhigang Zhang, and Guanzhong Lu* Key Laboratory for AdVanced Materials, Research Institute of Industrial Catalysis, East China UniVersity of Science and Technology, Shanghai 200237, P.R. China ReceiVed: NoVember 18, 2008; ReVised Manuscript ReceiVed: March 6, 2009
The epoxidation of styrene with hydrogen peroxide with higher selectivity to styrene oxide was carried out over lanthanum-doped MCM-48 (La-MCM-48) molecular sieves, in which aqueous NaOH solution was used to adjust the pH value of the reaction solution. The results show that the pH value of the reaction solution has a great influence on the catalytic performance of La-MCM-48 and the product distribution. When the pH value of the reaction solution was 11.5, the conversion of styrene reached 54.5% and the selectivity to styrene oxide was 98.8%. UV-vis and EPR spectroscopy was used to characterize the lanthanum species in the framework of La-MCM-48 and the peroxo-lanthanum species by H2O2 reacting with the lanthanum species. It has been found that the conversion of styrene has a close relationship with the amount of the peroxolanthanum species, and the selectivity to styrene oxide has a close relationship with the ratio of La(III)superoxide species to La(III)-hydroperoxide species, which can be controlled by adjusting the pH value of the reaction solution. 1. Introduction Epoxides are important intermediates in the fine chemicals industries for the production of perfume materials, anthelmintics, epoxy resins, plasticizers, drugs, sweeteners, etc. Therefore, the catalytic epoxidation of olefins or substituted olefins is an important reaction in organic synthesis. Titanosilicates, particularly TS-1, have been widely investigated because of their remarkable catalytic performance in the selective oxidation of styrene with hydrogen peroxide under mild conditions.1-6 Several factors including the preparation method, crystallite size, impurities, solvent, etc., could affect the catalytic performances of TS-1 catalysts.7-15 Recently, Ti-OMS-2,16 Ti-HMS-1,17 TMMCM-48 (TM ) Mn, V, Cr),18 Ti-MCM-41,19 CoVSB-5,20 OMS-2 (manganese oxide octahedral molecular sieves),21 and even bimetallic (VTi, NbTi, NbCo, RuCr, and RuNi) ionmodified MCM-4122,23 have been studied as catalysts in the epoxidation of styrene. Many attempts have been made to improve the conversion of styrene and the selectivity to styrene oxide. A high conversion (71%) of styrene with a maximum selectivity to styrene oxide (87%) has been reported by using anhydrous urea-hydrogen peroxide as an oxidant over TS-1,4 in which anhydrous H2O2 with high explosivity must be used, and the usage amount of TS-1 was as high as 20 wt% of styrene. Using monovanadium ion-modified mesoporous material as the catalyst, the conversion of styrene can reach 100%; however, the selectivity to styrene oxide was only 12%, and the selectivity to benzaldehyde was as high as 88%.18 Similarly, a high conversion of styrene has also been obtained over other catalytic materials with a lower selectivity to styrene oxide.20-23 Therefore, it is evident that the higher selectivity to styrene oxide is more difficult to achieve than the high conversion of styrene. We have reported that lanthanum-doped MCM-48 (La-MCM48) molecular sieves were an effective catalyst for the oxidation * Corresponding authors. Fax: +86-21-64252923. E-mail: gzhlu@ ecust.edu.cn (G.Z.Lu) or
[email protected] (Y.L.Guo).
of styrene by H2O2 with a main product of benzaldehyde and a small amount of styrene oxide.24 The La content in the catalysts, reaction temperature, reaction time, and solvent greatly affect the catalytic oxidation of styrene. An interesting phenomenon was observed that the conversion of styrene and the selectivity to styrene oxide increase noticeably when a small amount of aqueous NaOH solution is added to the reaction solution. In this paper, the effect of the pH value of the reaction solution, adjusted by aqueous NaOH solution, on the epoxidation of styrene with hydrogen peroxide over La-MCM-48 catalyst was investigated in detail, in order to achieve higher selectivity to styrene oxide. Moreover, the active sites for the epoxidation of styrene and the peroxo-lanthanum species by an interaction between La-MCM-48 and H2O2 were characterized with UVvis and EPR spectroscopy, and the relationship between the peroxo-lanthanum species and its catalytic performance was discussed, which have not yet been reported. 2. Experimental Section 2.1. Preparation of La-MCM-48. La-MCM-48 molecular sieves were synthesized under the hydrothermal conditions as follows: The surfactant cetyltrimethylammonium bromide (CTAB) and sodium hydroxide (NaOH) were dissolved in deionized water under stirring at 50 °C. Then tetraethyl orthosilicate (TEOS) and La(NO3)3 · 6H2O were added to the above template solution under stirring. The composition of the synthesis gel was SiO2:0.02La:0.5NaOH:0.5CTAB:55H2O. After being stirred for 3 h, the synthesis gel was transferred to a Teflon-lined autoclave and allowed to stay at 100 °C for 72 h. Finally the solid products were collected by filtration, dried at 100 °C overnight, and calcined in air at 550 °C for 6 h to remove the surfactant. Pure silica MCM-48 (Si-MCM-48) molecular sieves were prepared with the same procedure as for La-MCM-48 but without La(NO3)3 · 6H2O. 2.2. Characterization of La-MCM-48. Powder XRD patterns were recorded on a Rigaku D/max-2550VB/PC diffrac-
10.1021/jp8101095 CCC: $40.75 2009 American Chemical Society Published on Web 04/03/2009
7182 J. Phys. Chem. C, Vol. 113, No. 17, 2009 tometer with Cu KR radiation (λ ) 0.15406 nm), and the diffraction patterns were taken in the Bragg’s angle (2θ) range from 1 to 70° at room temperature. Elemental analysis of the samples was performed with inductively coupled-plasma atomic emission spectroscopy (ICP-AES) on a TJA Iris Advantag 1000 instrument. UV-vis spectra were recorded on a Varian Cary 500 UV-vis-NIR spectrophotometer in the range of 200-800 nm, and BaSO4 was used as a reference. EPR spectra of samples at 100 K were recorded on a Bruker EMX-8/2.7 spectrometer operating at X-band frequency and 100 kHz field modulation. Before characterization with UV-vis spectroscopy, the LaMCM-48 molecular sieves were pretreated with different solutions. Typically, 100 mg of La-MCM-48 was soaked with different solutions (acetonitrile, H2O, H2O2, or the solution mixture of 4.5 mL of acetonitrile, 0.5 mL of dimethylformamide, and 5 mmol of 30 wt% H2O2) for 5 min at room temperature and then collected by filtration and characterized with a UV-vis-NIR spectrophotometer. In some experiments, the pH value of pretreatment solution was adjusted with 3 wt% aqueous NaOH solution. Before characterization with EPR spectroscopy, 10 mg of LaMCM-48 molecular sieves was completely soaked in 0.45 mL of acetonitrile solvent, and then 0.5 mmol of 30 wt% H2O2 aqueous solution was added. After addition of H2O2, the samples were quenched to 100 K immediately and then EPR spectra were recorded. In some experiments, 3 wt% aqueous NaOH solution was added to adjust the pH value of the solution mixture after addition of H2O2. 2.3. Epoxidation of Styrene. The catalytic performance of La-MCM-48 molecular sieves for the epoxidation of styrene was investigated in a 50 mL flask equipped with a stirrer, thermometer, and reflux condenser. In a typical batch experiment, 100 mg of catalyst, 5 mmol of styrene, 4.5 mL of acetonitrile (MeCN), and 0.5 mL of dimethylformamide (DMF) were first introduced into the flask. After 5 mmol of 30 wt% H2O2 was added, 3 wt% aqueous NaOH solution was used to adjust the pH value of the reaction solution. Then the flask was kept in the water bath at 60 °C under vigorous stirring. After reaction for 24 h, the catalyst was separated from the reaction solution by centrifugation. Then this separated catalyst was dried at 100 °C overnight and calcined in air at 550 °C for 2 h, to obtain the regenerated catalyst. The catalytic performance of the regenerated catalyst was investigated as the same procedure as the fresh catalyst. The composition of the epoxidation products was analyzed by a Perkin-Elmer Clarus 500 gas chromatograph equipped with a PE-2 capillary column (25m × 0.32 mm × 1.0 µm) and a FID detector. Cyclooctane was used as an internal standard substance. The temperatures of the injector and the detector were 220 °C, and the temperature of the column oven was programmed to increase from 100 to 180 °C at a rate of 10 °C/ min. The amount of H2O2 was quantitatively analyzed by the conventional iodometry, and then H2O2 efficiency was calculated by the molar ratio of the amount of the oxidative products produced from styrene to the amount of H2O2 consumed. At the end of the epoxidation reaction, the concentration of La ions in the reaction solution was analyzed with ICP-AES to test the leaching of La from La-MCM-48 catalyst. 3. Results and Discussion 3.1. Epoxidation of Styrene. In our earlier work,24 over LaMCM-48 molecular sieves with only lanthanum in the framework, the conversion of styrene increased from 17.9% to 52.8%, the selectivity to styrene oxide increased from 23.4% to 96.1%,
Zhan et al.
Figure 1. Effect of the pH value of the reaction solution on the catalytic performance of La-MCM-48 for the epoxidation of styrene. (Symbols: 9, conversion of styrene; b, selectivity to styrene oxide; 2, H2O2 efficiency. Reaction conditions: 100 mg of catalyst, 5 mmol of styrene, 5 mmol of H2O2, 4.5 mL of MeCN, 0.5 mL of DMF, at 60 °C for 24 h).
and H2O2 efficiency increased from 24.7% to 54.3% when 3 mL of 3 wt% aqueous NaOH solution was added to the reaction solution. However, the conversion of styrene decreased to 2.1%, the selectivity to styrene oxide declined to 83.4%, and H2O2 efficiency decreased to 2.8% when 6 mL of NaOH solution was added. Herein, the effect of the nature of the alkali aqueous solutions, such as NaOH, KOH, K2CO3, and Na2CO3, on the catalytic performance of La-MCM-48 was investigated. The results show that the conversion of styrene, the selectivity to styrene oxide, and the H2O2 efficiency have close relationships with the pH value of the reaction solution, regardless of the nature of the alkali solutions. To gain a better understanding of an alkali effect on the epoxidation of styrene with hydrogen peroxide over LaMCM-48, the relationship between the catalytic performance and the pH value of the reaction solution adjusted by NaOH solution was investigated further. Figure 1 shows the effect of the pH value of the reaction solution on the catalytic performance of La-MCM-48 for the epoxidation of styrene. The pH value of the reaction solution is 5.5 without addition of NaOH solution. With an increase in the pH value from 5.5 to 11.5, the conversion of styrene increases from 17.9% to a maximum of 54.5% and then decreases to 0.6% when further increasing the pH value to 13.0. The H2O2 efficiency has a similar trend to the conversion of styrene. As for the selectivity to styrene oxide, it increases from 23.4% to 96.6% with an increase in the pH value from 5.5 to 10.0 and then stabilizes at more than 95% until the pH value is above 12.0. At a pH value of 11.5, a 54.5% conversion of styrene and the highest selectivity to styrene oxide (98.8%) are achieved. This selectivity to styrene oxide is higher than the maximum (87%) reported previously in the literature.4 Therefore, the pH value of the reaction solution has a great influence on the catalytic performance of La-MCM-48 and the product distribution for the epoxidation of styrene with H2O2. The recycle usage of La-MCM-48 catalyst was carried out in the reaction solution with pH ) 11.5, the results of recycling 10 times were shown in Figure 2. The results show that there is a little loss in the conversion of styrene and the selectivity to styrene oxide after the La-MCM-48 catalyst was used repeatedly for 10 times, in which the conversion of styrene (51.0%) and the selectivity to styrene oxide (96.7%) are still achieved in the 10th recycle usage. The analytical result of La ions in the
Epoxidation of Styrene with Hydrogen Peroxide
Figure 2. Catalytic performance of La-MCM-48 catalyst as a function of the recycling times for the epoxidation of styrene. (Reaction conditions: 100 mg of catalyst, 5 of mmol styrene, 5 of mmol H2O2, 4.5 mL of MeCN, 0.5 mL of DMF, pH ) 11.5 of the reaction solution, at 60 °C for 24 h).
Figure 3. UV-vis spectra of (a) La-MCM-48 and La-MCM-48 pretreated with (b) MeCN, (c) H2O, (d) the solution mixture of 4.5 mL of MeCN, 0.5 mL of DMF, and 5 mmol of H2O2, and (e) H2O2.
reaction solution after the epoxidation reaction shows that about 0.4% La leaches from La-MCM-48 catalyst. If the recycled La-MCM-48 catalyst was only dried at 100 °C overnight and not calcined at 550 °C, the conversion of styrene decreases to 42.5% and the selectivity to styrene oxide is only 84.3% in the second recycle usage, which is probably ascribed to coverage of the surface of La-MCM-48 catalyst by the aromatic species. After the recycled La-MCM-48 catalyst was calcined at 550 °C for 2 h, the aromatic species adsorbed on the surface of the catalyst can be removed, resulting in the recovery of its catalytic performance. 3.2. UV-vis Spectroscopy. Figure 3 shows the UV-vis spectra of La-MCM-48 pretreated with different solutions, such as MeCN, H2O, H2O2, or the solution mixture of 4.5 mL of MeCN, 0.5 mL of DMF, and 5 mmol of 30 wt% H2O2. When La-MCM-48 was pretreated with MeCN or H2O, there is little change in the absorption band at 230-280 nm which is ascribed to the presence of the La3+ ions with tetracoordination in the framework.24 However, when La-MCM-48 was pretreated with H2O2 or the solution mixture of MeCN, DMF, and H2O2, there are strong absorption bands at 210-360 nm or 202-310 nm. Similar to the formation of peroxo-Ti species by H2O2 adsorbed on titanosilicate,4,25-29 the interaction between La-MCM-48 and H2O2 leads to the formation of the peroxo-lanthanum species (Scheme 1), such as La(III)-hydroperoxide species and La(III)-
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Figure 4. UV-vis spectra of La-MCM-48 pretreated with the solution mixture of 4.5 mL of MeCN, 0.5 mL of DMF, and 5 mmol of H2O2 at different pH values adjusted by aqueous NaOH solution.
SCHEME 1: Formation of Peroxo-Lanthanum Species by an Interaction between La-MCM-48 and H2O2
superoxide species. So the strong absorption bands in the curves d and e of Figure 3 may result from the overlap of the absorption bands corresponding to the peroxo-lanthanum species with pentacoordination and the residual lanthanum species with tetracoordination in the framework. To investigate whether the peroxo-lanthanum species are the active sites for the epoxidation of styrene, 100 mg of La-MCM-48 was pretreated with the solution mixture of 4.5 mL of MeCN, 0.5 mL of DMF, and 5 mmol of 30 wt% H2O2. After being dried at room temperature for 4 h, pretreated La-MCM-48 was mixed with styrene at 60 °C without addition of any oxidant to the reaction solution. After reaction for 4 h, the products of benzaldehyde and styrene oxide could be detected. This indicates that the lanthanum species in the framework of La-MCM-48 can react with H2O2 to form the peroxo-lanthanum species which are the active sites for the epoxidation of styrene. Figure 4 shows the UV-vis spectra of La-MCM-48 pretreated with the solution mixture of 4.5 mL of MeCN, 0.5 mL of DMF, 5 mmol of 30 wt% H2O2, and 3 wt% aqueous NaOH solution used to adjust the pH value of the solution. The pH value of the solution mixture is 5.5 without adding aqueous NaOH solution. With an increase in the pH value of the pretreatment solution from 5.5 to 11.5, the strong absorption band of the peroxo-lanthanum species shifts to high wavenumber gradually and the absorbance intensity obviously increases. This indicates that the amount of the peroxo-lanthanum species increase, which agrees well with the increase trend in the conversion of styrene over La-MCM-48. However, when La-MCM-48 was pretreated with the solution mixture of pH g 12.0, a continuous absorption band at 300-400 nm can be observed, which indicates a change of the geometry of some peroxo-lanthanum species. Figure 5 shows the XRD patterns of La-MCM-48 used for 24 h in the reaction solution with different pH values adjusted by 3 wt% aqueous NaOH solution. With an increase in the pH value of the reaction solution, the intensities of the diffraction peaks of La-MCM-48 obviously weaken, which indicates that the basicity of the reaction solution can erode the silica structure and make the order of the mesostructures of La-MCM-48
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Figure 5. XRD patterns of La-MCM-48 used for 24 in the reaction mixture with different pH values adjusted by aqueous NaOH solution.
Zhan et al. favor of the production of benzaldehyde, which is similar to the situation of titanosilicate catalyst.26,27,30 As seen from Figure 1, the selectivity to styrene oxide increases from 23.4% to 96.6% with an increase in the pH value of the reaction solution from 5.5 to 10.0, and the selectivity to styrene oxide can be kept at more than 95% when the pH value of the reaction solution is 10-12. When the data in Figure 1 are associated with the results in Figures 4 and 6, it can be found that the conversion of styrene has a close relationship with the amount of the peroxo-lanthanum species, and the selectivity to styrene oxide has a close relationship with the ratio of La(III)-superoxide species to La(III)-hydroperoxide species. Furthermore, the ratio of La(III)-superoxide species to La(III)-hydroperoxide species increases significantly with an increase in the pH value of the reaction solution from 5.5 to 10.0, keeps nearly constant at the pH value of 10-12, and obviously decreases when the pH value is more than 12. 4. Conclusions
Figure 6. EPR spectra of La-MCM-48 pretreated with the solution mixture of MeCN and H2O2 at different pH values adjusted by aqueous NaOH solution.
decline. When the pH value of the reaction solution is more than 12.0, the diffraction peaks of La-MCM-48 have disappeared, which indicates that Si-O-La or Si-O-Si bonds of La-MCM-48 are cracked gradually during the reaction process, while the La-OH or Si-OH species are formed on its surface, resulting in an increase in the hydrophilicity of La-MCM-48 and restraining an access of styrene to the peroxo-lanthanum species. 3.3. EPR Spectroscopy. The peroxo-lanthanum species including La(III)-hydroperoxide species and La(III)-superoxide species (Scheme 1) are formed by H2O2 adsorbing on the lanthanum species in the framework of La-MCM-48. For EPR spectroscopy, the diamagnetic La(III)-hydroperoxide species are inactive, and the La(III)-superoxide species are active because of their paramagnetism property. When Si-MCM-48 was pretreated with the solution mixture of MeCN and H2O2, no EPR signal can be detected. Figure 6 shows the EPR spectra of La-MCM-48 pretreated with the solution mixture of 0.45 mL of MeCN and 0.5 mmol of 30 wt% H2O2 at different pH values adjusted by 3 wt% aqueous NaOH solution. With an increase in the pH value of the pretreatment solution of La-MCM-48, the intensity of EPR signal corresponding to the La(III)superoxide species increases obviously to a maximum at the pH value of 11.5 and then decreases significantly at the pH value of 12.0. On the basis of the relationship between the selectivity to styrene oxide and the intensity of EPR signal, it can be found that the La(III)-superoxide species are in favor of the production of styrene oxide, and the La(III)-hydroperoxide species are in
La-MCM-48 is an effective catalyst for the epoxidation of styrene with hydrogen peroxide with higher selectivity to styrene oxide, in which the conversion of styrene and the product distribution are easily controlled by adjusting the pH value of the reaction solution. At the pH value of 11.5, a conversion of styrene of 54.5% and the highest selectivity to styrene oxide of 98.8% can be achieved. The results show that the peroxolanthanum species, formed by the interaction between the lanthanum species in the framework of La-MCM-48 and H2O2, can be controlled by adjusting the pH value of the reaction solution. The conversion of styrene has a close relationship with the amount of the peroxo-lanthanum species, and the selectivity to styrene oxide has a close relationship with the ratio of La(III)superoxide species to La(III)-hydroperoxide species. Acknowledgment. This project was financially supported by National Basic Research Program of China (2004CB719500), International Science and Technology Cooperation Program of China (2006DFA42740), and Commission of Education of Shanghai Municipality (2008CG35). References and Notes (1) Clerici, M. G.; Ingallina, P. J. Catal. 1993, 140, 71. (2) Khouw, C. B.; Dartt, C. B.; Labinger, J. A.; Davis, M. E. J. Catal. 1994, 149, 195. (3) Zhuang, J. Q.; Ma, D.; Yan, Z. M.; Liu, X. M.; Hana, X. W.; Bao, X. H.; Zhang, Y. H.; Guo, X. W.; Wang, X. S. Appl. Catal., A 2004, 258, 1. (4) Laha, S. C.; Kumar, R. J. Catal. 2001, 204, 64. (5) Zhang, P.; Wang, L. F.; Chen, Y. H. Spectrosc. Spectral Anal. (Beijing, China) 2007, 27, 886. (6) Yang, Y. C.; Li, H.; He, D. H.; Wang, Y.; Xu, C. H.; Qiu, F. L.; Ye, Z. X. Acta Chim. Sin. 2006, 64, 1411. (7) Lopez, A.; Tuilier, M. H.; Guth, J. L.; Delmotte, L.; Popa, J. M. J. Solid State Chem. 1993, 102, 480. (8) Bordiga, S.; Coluccia, S.; Lamberti, C.; Marchese, L.; Zecchina, A.; Boscherini, F.; Buffa, F.; Genoni, F.; Leofanti, G.; Petrini, G.; Vlaic, G. J. Phys. Chem. 1994, 98, 4125. (9) Perego, G.; Bellussi, G.; Corno, C.; Taramasso, M.; Buonomo, F.; Esposito, A. Stud. Surf. Sci. Catal. 1986, 28, 129. (10) Millini, R.; Previde Massara, E.; Perego, G.; Bellussi, G. J. Catal. 1992, 137, 497. (11) Huybrechts, D. R. C.; Buskens, P. L.; Jacobs, P. A. J. Mol. Catal. 1985, 71, 129. (12) Bellussi, G.; Carati, A.; Clerici, M. G.; Esposito, A. Stud. Surf. Sci. Catal. 1991, 63, 421. (13) Thangaraj, A.; Kumar, R.; Ratnasamy, P. J. Catal. 1991, 131, 294. (14) Thangaraj, A.; Sivasanker, S.; Ratnasamy, P. J. Catal. 1991, 131, 394. (15) Fan, W. B.; Duan, R. G.; Yokoi, T.; Wu, P.; Kubota, Y.; Tatsumi, T. J. Am. Chem. Soc. 2008, 130, 10150.
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