A low-deactivation-rate Lewis acid zeolite prepared in an alkali metal ion-containing system for alkene epoxidation

Abstract

Titanium silicate-1 (TS-1) is a type of commercial titanium-containing Lewis acid zeolite used as a catalyst in environmentally-benign processes to produce epoxide from alkene. Activity, selectivity of targeted products and catalytic life are three key factors to evaluate the catalytic performance of the TS-1 catalyst. Here, we showed a highly active TS-1 catalyst and thereof low-cost synthetic method with the advantages of improving the active-titanium content, yield and reproducibility. The catalytic life is more than 360 minutes while only less than 20 minutes for TS-1 from conventional methods, leading to conversion of reactants more than twice as high as that of TS-1 synthesized by conventional methods. Both the conversion and catalytic life are the best among the current reported TS-1 zeolites for 1-hexene oxidation under similar reaction conditions.

---

Zeolites are silica-based microporous crystalline materials, which are widely applied in separation, adsorption and catalysis, especially in petroleum refining and fine chemicals.^1–6 Introduction of titanium(IV) into the framework of a high-silica zeolite endowed this titanium-containing zeolite with Lewis acid characteristics that can catalyze numerous industrially important organic reactions,⁷ for example, alkane or benzene oxidation,^8–11 phenol hydroxylation,¹² alkene epoxidation,^13–19 ammoximation of cyclohexanone,^20,21 oxidative desulfurization,^22,23 and so on.^24,25 Titanium silicate-1 (TS-1), a commercial titanium-containing zeolite with MFI-type topology, is one of the most important among all the discovered titanium-containing zeolites, which is known as a milestone in field of zeolite and heterocatalysis research.²⁶ The catalytic performance of TS-1 mainly depends on the amounts of isolated framework titanium, which is proven a possible maximal amount of 2.0 wt% (39 of Si/Ti molar ratio) deduced from the experimental data as well as crystallographic theory.^7,27 In the practical production process, the framework Ti content is difficult to reach this maximal value by current methods. This is because the larger ionic radius of titanium than silicon ion leads to difficulties in the crystallization of raw materials and the incorporation of titanium into the zeolite framework.^26,28 Therefore, the synthesis of titanium-rich TS-1 is still very difficult, but greatly desired.

So as to achieve high content of framework titanium in TS-1 zeolite, strict synthetic conditions and expensive agents were required in conventional processes,^7,26,29 such as subjecting to high crystallization temperature, employing large amounts of organic structure-directing agent, carefully dispersing titanium into silica matrix and using high-pure agents free of alkali metal ions. The strict synthetic conditions especially the usage of high-pure agents free of alkali metal ions cause a dramatic increase in the production cost.^30–32 Therefore, it is significant to reduce the production cost of TS-1 for the commercial benefit. So if we can replace the expensive high-pure agents free of alkali metal ions by alkali-metal-containing low-cost agents, the production cost will significantly decrease, but the adverse effects of alkali-metal ions during synthesis should be firstly solved.

It was a challenging task to solve the adverse influence of alkali metal ions during TS-1 synthesis over a long period in the past.^33,34 It was found that the presence of alkali metal ions during synthesis, even trace amount (0.1 wt%), would greatly affect the final TS-1 product by the reduction of titanium content, decrease in the crystallinity also the incorporated titanium(IV) may lose its catalytic activity etc.^28,33–35 However, for a long time, there was no effective progress to find solutions to eliminate the adverse influence of alkali metal ions during synthesis. Most of reported strategies were only to deal with alkali metal ions at the cost of some key properties of TS-1, such of framework Ti content, activity and so on.^36,37

Recently, one effective method we developed was the introduction of a cation-exchange resin into the TS-1 synthetic system to capture the alkali-metal cations via ion exchange.³⁸ Another method was to employ anionic polymer PAA for controlling the pH value of synthetic system to protect the framework from the adverse influence of alkali metal ion by decreasing the reaction activity between alkali metal ions and silica-based framework.³⁹ Both of them were effective strategies to solve the adverse effects of alkali metal ions during synthesis, however, cation-exchange resin also could remove another important TPA⁺ cation that used as structure-directing agent via ion exchange and the treatment of viscous anionic polymer was also a troublesome job during synthesis. Therefore, it is significant and desired to develop a facile route to synthesized high-performance and titanium-rich TS-1 zeolites from a low-cost alkali metal ion-containing system.

Here, based on the theory of pH-value adjusting and chelating properties, we chose a multifunctional small molecule of ethylenediaminetetraacetic acid (EDTA) that possessed functions of chelating ability for capturing alkali metal ions and acidic property for adjusting pH value to overcome the challenges in the synthesis of highly active TS-1 in the presence of high concentration of alkali-metal ions. We found that EDTA is more effective than polymers and resins to protect the framework titanium(IV) from the adverse influence of alkali metal ions, achieving a highly active TS-1 at a relative small added amount of EDTA.

TS-1 zeolites were synthesized by the hydrothermal treatment of an aqueous mixture with the molar composition of 1.0SiO₂/0.025TiO₂/0.36TPAOH/0.1KOH/0.01NaOH/0.13 EDTA/35H₂O in an autoclave at 170 °C. The pH value was controlled close to 9. The white powder formed was separated by centrifugation, dried and calcined at 550 °C to obtain the final product, designated as TS-1(EDTA). The sample synthesized via conventional method as described in the literature,³⁹ designated as of TS-1-Con. The detailed method of preparation was described in the ESI.† The yield of TS-1(EDTA) was nearly 100% based on the SiO₂ and TiO₂ in starting materials while only ∼70% in yield of TS-1-Con. And the Ti content in the final product of TS-1(EDTA) was 2.1 wt% (determined by inductively coupled plasma optical emission spectrometer). Powder X-ray diffraction pattern (PXRD) indicated that the synthesized white powder of TS-1(EDTA) was crystalline with pure MFI-type topology, which is the structure of TS-1 zeolite (Fig. 1).

Fig. 1 Powder X-ray diffraction pattern (PXRD) of synthesized TS-1 zeolite.

Epoxidation of alkene is of great important processes for producing fine chemicals. Lewis acids and noble metal nanoparticles were widely used as catalysts in homogenous or heterogeneous reaction system.^40–42 As a type of microporous heterogeneous catalyst of TS-1 zeolites, only framework Ti species are proven to provide the catalytic activity in alkene oxidation,^7,26 therefore it is important and necessary to detect the framework Ti in TS-1. The absorbance peak at about 210 nm in the diffuse reflectance ultraviolet-visible (DRUV/vis) spectrum attributed to Ti⁴⁺O²⁻ → Ti³⁺O⁻ ligand-to-metal charge transfer is assigned to the framework Ti species. As shown in Fig. 2, there are three absorbance bands for synthesized TS-1 zeolites while no obvious absorbance in the range of 200–600 nm for Ti-free silicalite-1 with the same MFI-type structure as TS-1. The strong absorbance band at 210 nm indicated the presence of a large amount of framework Ti in the final product. The weak bands at 260 nm and 320 nm suggested a very small amount of extra-framework Ti species and anatase-like TiO₂, respectively.

Fig. 2 Diffuse reflectance ultraviolet-visible (DRUV/vis) spectrum of (a) synthesized TS-1 zeolite and (b) Ti-free silicalite-1.

The band at 960 cm⁻¹ in IR and Raman spectra is also widely accepted as a proof of isomorphous substitution of Ti in the TS-1 framework although there is a controversy about the origin of this band. The controversial issue is because there is also an absorbance band at 960 cm⁻¹ for Si–OH vibration. As shown in Fig. 3A, there is a band at 960 cm⁻¹ for Ti-free pure-silica silicalite-1. However, the band of 960 cm⁻¹ is much lower than that of Ti-containing TS-1 zeolite, especially in Raman spectrum (Fig. 3B). So, the 960 cm⁻¹ band is probably an overlap peak attributed to Si–O–Ti and Si–OH in TS-1 zeolites. Considering low intensity of 960 cm⁻¹ band in silicalite-1, this band in TS-1 is mainly contributed by the vibration of Si–O–Ti, suggesting the framework Ti in TS-1 zeolite. In addition, the presence of a Ti 2p peak at 460 eV in XPS spectrum due to framework Ti (Fig. S1†) and a shoulder peak at −116 ppm in ²⁹Si MAS NMR attributed to arise from the distorted silicon environment in tetrahedral containing Si–O–Ti bonds (Fig. 4) is also the evidences of the tetrahedral framework Ti in TS-1 zeolite.^43–46 In addition, the ratio of the peak at −116 ppm and peak at −113 ppm increased from 0.30 of TS-1-Con to 0.32 of TS-1(EDTA), suggesting the higher content of framework Ti in TS-1(EDTA) than that in TS-1-Con (Fig. 4).^44–46

Fig. 3 (A) Infrared spectra and (B) Raman spectra of (a) synthesized TS-1 zeolite and (b) Ti-free silicalite-1.

Fig. 4 ²⁹Si MAS NMR spectra of calcined TS-1(EDTA) and TS-1-Con.

The nitrogen adsorption characterization (Fig. 5) displayed a typical type I adsorption–desorption isotherms, indicating a microporous structure. The surface area is 387 m² g⁻¹ and micropore volume is 0.15 cm³ g⁻¹. As a microporous catalyst, molecular diffusion restriction in its microporous channels can greatly affect its catalytic performance. The short diffusion channel will be beneficial to fast mass transportation and high reaction rates.^47,48 So, the crystal size of TS-1 plays an important role in catalytic performance. SEM and TEM images (Fig. 6) showed that the synthesized TS-1 crystal had plate-like morphology with the uniform thickness of ∼400 nm, which were expected to have less mass transportation restriction in this dimension. All these characterizations proved that the synthesized TS-1 product is a potential high-performance catalyst in alkene epoxidation combined with H₂O₂ as oxidant, a very important environmentally-benign process to produce epoxides.

Fig. 5 Nitrogen adsorption–desorption isotherms of calcined TS-1 zeolite.

Fig. 6 (a) SEM image (b) TEM image of synthesized TS-1 zeolite.

Table 1 and Fig. 7 showed the catalytic performance for 1-hexene oxidation with H₂O₂ by using TS-1(EDTA). And here the sample of TS-1-Con synthesized via conventional method in the absence of alkali metal ions and TS-1-alkali via conventional method in the presence of alkali metal ions, were used as control experiment.

Table 1 Catalytic oxidation of 1-hexene with H₂O₂ by using TS-1 zeolites synthesized by different methods^a

Catalyst	Si/Ti (mol/mol)	Si/K (mol/mol)	Si/Na (mol/mol)	Conversion (%)	Sepoxide (%)	TON (mol/mol-Ti)	Rinitial^b (×10^−3/s^−1)	SH2O2^c (%)
a Reaction conditions: catalyst (25 mg), methanol (5 mL), 1-hexene (5 mmol), H2O2 (5 mmol), temperature 60 °C, 6 h.b Initial reaction rate per Ti calculated based on the conversion at 10 min.c Selectivity in the use of H2O2 toward the oxidation of 1-hexene.d Reaction time was 24 h.e Third use of the catalyst.f Calcination at 550 °C after the third use of the catalyst. The abbreviation of n.d. means not determined.
TS-1-Con	51	—	—	15.0	95.3	94	103	72
TS-1-alkali	57	45	194	1.5	97.5	11	<0.1	27
TS-1(EDTA)	37	223	>800	35.3	97.4	162	43	97
TS-1(EDTA)-24 h^d	37	223	>800	40.0	94.1	184	43	60
TS-1(EDTA)-3rd^e	37	n.d.	n.d.	24.3	98.6	112	n.d.	n.d.
TS-1(EDTA)-3rd-c^f	37	n.d.	n.d.	34.0	98.2	156	n.d.	n.d.

---

Fig. 7 The catalytic performance for 1-hexene oxidation with H₂O₂ by using (a) TS-1-Con from conventional method, (b) TS-1-alkali from conventional method in the presence of alkali metal ions and (c) TS-1-TS-1(EDTA). Reaction conditions: catalyst (25 mg), methanol (5 mL), 1-hexene (5 mmol), H₂O₂ (5 mmol), temperature 60 °C.

The structure properties of control samples were shown in ESI of Fig. S2 and S3.† TS-1-Con has uniform cubic crystal with 100–200 nm in size. And TS-1 (TS-1-Alk) has the similar particles surrounded with some amorphous small particles as shown Fig. S2b.† The presence of framework Ti in TS-1-Con and TS-1-alkali were confirmed by DRUV-vis tests (Fig. S3†). The content of framework Ti in both TS-1-Con and TS-1-alkali was less than that of TS-1(EDTA) as compared the intensity of 960 cm⁻¹ band in IR spectra (Fig. S3D†), which was consistent with the results of ²⁹Si MAS NMR characterization (Fig. 4).

As shown in Table 1, there was almost no catalytic activity of TS-1-alkali observed for 1-hexene oxidation with H₂O₂, indicating the presence of alkali metal ions during synthesis, which could not produce an active TS-1 catalyst, which was consistent with the previous reported studies.^33,38,39 As compared with TS-1-Con, the conversion of 1-hexene catalyzed by TS-1(EDTA) was less than that catalyzed by TS-1-Con during the starting 20 minutes. However, the conversion of 1-hexene catalyzed by TS-1(EDTA) kept rising to 35.3% until to more than 360 minutes, while the conversion of 1-hexene catalyzed by TS-1-Con displayed no obvious increase after 20 minutes.

This indicates less deactivation rate of TS-1(EDTA) than that of TS-1-Con, leading to the conversion of TS-1(EDTA) (35.3%) more than twice as high as that of TS-1-Con (15.0%) during 360 minute reaction. Further prolonging the reaction time, the conversion only slightly increased with low H₂O₂ efficiency, suggesting that a large amount of H₂O₂ would decompose after a long-time reaction. To the best of our knowledge, the conversion of 35.3% here is the highest conversion related to the reported TS-1 catalysts. Due to framework Ti species are proven to provide the catalytic activity in alkene oxidation, the reason for the improvement of the catalytic performance is mainly attributed to the high content of framework titanium species in TS-1(EDTA) as confirmed by DRUV-vis, IR and ²⁹Si NMR characterizations. In addition, alkali metal ions would be contained in the TS-1 catalysts after synthetic process (Table 1), and these alkali metal ions in TS-1 could greatly influence its catalytic performance. A large amount of alkali metal ions present in TS-1-alkali led to no catalytic activity for 1-hexene oxidation as shown in Table 1. So, the low content of alkali metal ions in the TS-1(EDTA) products was another important reason for the improvement of catalytic performance.

Further increasing the reaction time to 24 hours, as shown in Table 1, the conversion of 1-hexene only slightly increased, but the selectivity of epoxide decreased from 97.4% to 94.1%. In addition, the H₂O₂ efficiency greatly decreased from 97% to 60%, indicating the decomposition of H₂O₂ after long-time reaction. Catalyst of TS-1(EDTA) showed significant decrease of conversion after third use by separated from reaction system and simply washed by methanol. After characterizations of third-recycled TS-1(EDTA) by using XRD (Fig. S4†), TG (Fig. S5†) and nitrogen adsorption–desorption analysis (Fig. S6†), we found the left of some absorbed substance in the micropores of TS-1 samples to block the contact of reactants and Ti catalytic centers would provable a main reason for deactivation of this samples based on the less BET surface area (34 m² g⁻¹), low porous volume (0.01 m³ g⁻¹) and 7.6 wt% weight loss at the range of 150–500 °C in TG test. Therefore, the catalytic activity could be restored (34.0% vs. 35.3% of fresh TS-1 catalyst) via calcination at 550 °C to remove the adsorbate in the micropores in TS-1 (Table 1), confirming the recyclable catalyst.

In conclusion, low-cost, titanium-rich and plate-like TS-1 zeolites with exceptionally long catalytic life were synthesized in an alkali metal ion-containing system. The synthetic strategy effectively solved the problem of the adverse effects of alkali metal ions during synthesis processes via introducing EDTA, resulting in a feasible synthesis of highly active TS-1 zeolites by using low-cost tetrapropylammonium hydroxide (TPAOH) as a starting reagent that contains alkali-metal cations such as Na⁺ and K⁺ and is much cheaper than the high-purity TPAOH currently used in conventional routes. The synthesized TS-1 catalyst showed excellent catalytic activity and long catalytic life for alkene oxidation to produce epoxide. The catalytic life is more than 360 minutes while only less than 20 minutes for TS-1 from conventional method, leading to a conversion of reactant more than twice as high as that of TS-1 synthesized by conventional method. Both the conversion and catalytic life are the best among the current reported TS-1 zeolites for 1-hexene oxidation under the similar reaction conditions. This low-cost synthetic method with the advantages of improving the active-titanium content, yield and reproducibility has great potential for commercialization.

Acknowledgements

This work is supported by the National Natural Science Funds of China (No. 51602164, 21376125, 2171131, 51402157), and Supported by Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province.

Notes and references

1. M. Choi, K. Na, J. Kim, Y. Sakamoto, O. Terasaki and R. Ryoo, Nature, 2009, 461, 246–249.
2. T. C. T. Pham, H. S. Kim and K. B. Yoon, Science, 2011, 334, 1533–1538.
3. X. Zhang, D. Liu, D. Xu, S. Asahina, K. A. Cychosz, K. V. Agrawal, Y. Al Wahedi, A. Bhan, S. Al Hashimi, O. Terasaki, M. Thommes and M. Tsapatsis, Science, 2012, 336, 1684–1687.
4. P. Guo, J. Shin, A. G. Greenaway, J. G. Min, J. Su, H. J. Choi, L. Liu, P. A. Cox, S. B. Hong, P. A. Wright and X. Zou, Nature, 2015, 524, 74–78.
5. F. Jiao, J. Li, X. Pan, J. Xiao, H. Li, H. Ma, M. Wei, Y. Pan, Z. Zhou, M. Li, S. Miao, J. Li, Y. Zhu, D. Xiao, T. He, J. Yang, F. Qi, Q. Fu and X. Bao, Science, 2016, 351, 1065–1068.
6. Y. Zhou, Y. Jin, M. Wang, W. Zhang, J. Xie, J. Gu, H. Wen, J. Wang and P. Luming, Chem.–Eur. J., 2015, 21, 15412–15420.
7. M. Taramasso, G. Perego and B. Notari, US Pat. 4,410,501, 1983.
8. D. R. C. Huybrechts, L. Debruycker and P. A. Jacobs, Nature, 1990, 345, 240–242.
9. T. Tatsumi, M. Nakamura, S. Negishi and H. Tominaga, J. Chem. Soc., Chem. Commun., 1990, 476–477.
10. A. Bhaumik, P. Mukherjee and R. Kumar, J. Catal., 1998, 178, 101–107.
11. J. G. Wang, Y. L. Zhao, T. Yokoi, J. N. Kondo and T. Tatsumi, ChemCatChem, 2014, 6, 2719–2726.
12. M. Lin, X. T. Shu, X. Q. Wang and B. Zhu, China Pat. 99126289.1, 1999.
13. M. G. Clerici, G. Bellussi and U. Romano, J. Catal., 1991, 129, 159–167.
14. S. B. Kumar, S. P. Mirajkar, G. C. G. Pais, P. Kumar and R. Kumar, J. Catal., 1995, 156, 163–166.
15. L. Wu, S. Zhao, L. Lin, X. Fang, Y. Liu and M. He, J. Catal., 2016, 337, 248–259.
16. M. Lin, C. Xia, B. Zhu, H. Li and X. Shu, Chem. Eng. J., 2016, 295, 370–375.
17. J. Kim, J. Chun and R. Ryoo, Chem. Commun., 2015, 51, 13102–13105.
18. Y. Zuo, M. Liu, T. Zhang, C. Meng, X. Guo and C. Song, ChemCatChem, 2015, 7, 2660–2668.
19. Y. Zuo, M. Liu, T. Zhang, L. Hong, X. Guo, C. Song, Y. Chen, P. Zhu, C. Jaye and D. Fischer, RSC Adv., 2015, 5, 17897–17904.
20. J. LeBars, J. Dakka and R. A. Sheldon, Appl. Catal., A, 1996, 136, 69–80.
21. T. Zhang, Y. Wang, S. Wang, X. Wu, P. Yao, W. Feng, Y. Lin and J. Xu, React. Kinet., Mech. Catal., 2015, 114, 735–752.
22. S. Du, F. Li, Q. Sun, N. Wang, M. Jia and J. Yu, Chem. Commun., 2016, 52, 3368–3371.
23. S. Du, X. Chen, Q. Sun, N. Wang, M. Jia, V. Valtchev and J. Yu, Chem. Commun., 2016, 52, 3580–3583.
24. W. Jiao, Y. He, J. Li, J. Wang, T. Tatsumi and W. Fan, Appl. Catal., A, 2015, 491, 78–85.
25. L. H. Chen, X. Y. Li, G. Tian, Y. Li, J. C. Rooke, G. S. Zhu, S. L. Qiu, X. Y. Yang and B. L. Su, Angew. Chem., Int. Ed., 2011, 50, 11156–11161.
26. W. B. Fan, R. G. Duan, T. Yokoi, P. Wu, Y. Kubota and T. Tatsumi, J. Am. Chem. Soc., 2008, 130, 10150–10164.
27. B. Notari, Adv. Catal., 1996, 41, 253–334.
28. G. Bellussi and V. Fattore, Stud. Surf. Sci. Catal., 1991, 69, 79–92.
29. A. Thangaraj, M. J. Eapen, S. Sivasanker and P. Ratnasamy, Zeolites, 1992, 12, 943–950.
30. T. Iwasaki, M. Isaka, H. Nakamura, M. Yasuda and S. Watano, Microporous Mesoporous Mater., 2012, 150, 1–6.
31. Y. L. Ding, Q. P. Ke, T. T. Liu, W. C. Wang, M. Y. He, K. Q. Yang, H. L. Jin, S. Wang and T. D. Tang, Ind. Eng. Chem. Res., 2014, 53, 13903–13909.
32. D. G. Huang, X. Zhang, T. W. Liu, C. Huang, C. H. Chen, C. W. Luo, E. Ruckenstein and C. Z. S. Chao, Ind. Eng. Chem. Res., 2013, 52, 3762–3772.
33. C. B. Khouw and M. E. Davis, J. Catal., 1995, 151, 77–86.
34. P. X. Yao, Y. Q. Wang, T. Zhang, S. H. Wang and X. Y. Wu, Front. Chem. Sci. Eng., 2014, 8, 149–155.
35. T. Tatsumi, K. A. Koyano and Y. Shimizu, Appl. Catal., A, 2000, 200, 125–134.
36. A. Vasile and A. M. Busuioc-Tomoiaga, Mater. Res. Bull., 2012, 47, 35–41.
37. H. C. Guo, L. Zhang, H. L. Li, Y. H. Jia and C. Y. Liu, China Pat. 201110295596.9, 2011.
38. J.-G. Wang, Y. Wang, H. Chen, J. Lim, T. Tatsumi and Y. Zhao, RSC Adv., 2016, 6, 15615–15621.
39. J.-G. Wang, Y. Wang, T. Tatsumi and Y. Zhao, J. Catal., 2016, 338, 321–328.
40. J. Fang, B. Zhang, Q. Yao, Y. Yang, J. Xie and N. Yan, Coord. Chem. Rev., 2016, 322, 1–19.
41. B. Zhang, J. Fang, J. Li, J. J. Lau, D. Mattia, Z. Zhong, X. Jianping and N. Yan, Chem.–Asian J., 2016, 11, 532–539.
42. S. F. Pedersen, J. C. Dewan, R. R. Eckman and K. B. Sharpless, J. Am. Chem. Soc., 1987, 1279–1282.
43. B. Notari, Stud. Surf. Sci. Catal., 1991, 60, 343–352.
44. B. Kraushaar and J. H. C. van Hoof, Catal. Lett., 1988, 1, 81.
45. J. S. Reddy, R. Kumar and P. Ratnasamy, Appl. Catal., 1990, 58, L1–L4.
46. A. Thangaraj, R. Kumar, S. P. Mirajkar and P. Ratnasamy, J. Catal., 1991, 130, 1–8.
47. C. E. Ramachandran, Q. Zhao, A. Zikanova, M. Kocirik, L. J. Broadbelt and R. Q. Snurr, Catal. Commun., 2006, 7, 936–940.
48. A. J. H. P. Van der Pol, A. J. Verduyn and J. H. C. Van Hooff, Appl. Catal., A, 1992, 92, 113–130.

---

Footnotes

† Electronic supplementary information (ESI) available: Details of experimental section, XPS spectra and TG curve, etc. See DOI: 10.1039/c6ra17891j

‡ K. Fu and J. Yao contributed to this paper equally.

---