Hierarchical mesoporous TS-1 zeolite: a highly active and extraordinarily stable catalyst for the selective oxidation of 2,3,6-trimethylphenolw

The selective oxidation of 2,3,6-trimethylphenol (TMP) is of great importance for the preparation of 2,3,5-trimethyl-pbenzoquinone (TMBQ), a key intermediate in the production of Vitamin E. Various oxidant/catalyst, such as O2 or H2O2/CuCl2 (especially with various co-catalysts) have been employed in this oxidation process on the industrial scale. However, the main drawback of the above-mentioned systems is the use of homogeneous catalysts which leads to the separation and recycling problems of catalysts and purification of products. Therefore, the development of more efficient and ‘‘green’’ catalysts for the selective oxidation is still a great challenge. Until now, Ti-doped microporous zeolites, especially the TS-1, have been widely used as catalysts for selective oxidation in industry due to their high activity and stability. However, these zeolites are inefficient for the selective oxidation of bulky reactants due to the slow diffusion of reactants and/or products in their sub-2 nm sized channel systems and/or deactivation caused by substance blockage in micropores. Therefore, in recent years, mesostructured titanosilicates, for example, TiO2–SiO2 aerogels, Ti-MCM-41, Ti-SBA-15 and Ti-MMM-series et al., have attracted increasing research interest as heterogeneous catalysts for the selective oxidation reactions involving bulky molecules due to their much larger pore size than conventional zeolites. However, their amorphous framework and consequently, low hydrothermal stability have seriously limited their practical applications. In a word, the successful synthesis of new materials combining the advantages of high hydrothermal stability and catalytic activity of zeolites and large pore size of mesoporous materials is highly desired. Up to now, various synthetic procedures have been attempted in the preparation of highly efficient and relatively stable mesostructure catalysts. Xiao et al. used zeolite seeds as framework building units, which could self-organize into mesoporous matrices with triblock polymers, to successfully prepare strongly acidic micro/mesoporous titanosilicates. Unfortunately, however, the composites assembled from zeolite seeds are inherently not hydrothermally stable in a manner similar to that of nanocrystalline zeolites. Very recently, a new TS-1/mesoporous hierarchical structure has been synthesized by Tuel et al. using specially designed and synthesized amphiphilic organosilane, however the material is not suitable for reactions involving hydrophobic reactants because of its hydrophilic character. Moreover, although the hydrothermal stability of the catalysts was tested, few reports have concerned the cyclical catalytic stability, which is very important in practical applications. Herein, we report the synthesis of a new kind of hierarchical mesoporous TS-1 zeolite by a simple steam-assisted crystallization method (SAC), which was developed recently in our group for the synthesis of mesoporous ZSM-5 zeolite. This novel material, which was denoted as c-Ti-TUD-1, possesses high surface area, large pore volume, well-crystallized framework of TS-1 zeolite, and high hydrothermal stability. Moreover, it demonstrates significantly higher catalytic activity and a strongly improved lifetime compared with amorphous titanosilicate in the selective oxidation of TMP to TMBQ. In our approach, tetrabutyl titanate, triethanolamine (TEA) and tetraethylorthosilicate (TEOS) were dissolved in turn into distilled water under continuous stirring. After that, tetrapropylammonium hydroxide (TPAOH, 25% aqueous solution) was added dropwise into the resultant sol. Therein, the molar ratio of the precursors was TiO2:SiO2:TPAOH:TEA: H2O = 1 : 50 : 5 : 50 : 500. The resultant wet gel was dried and then crystallized by the SAC method. Afterwards, the products were washed repeatedly, dried in air and then calcined. Finally white powders were obtained and labelled as c-Ti-TUD-1. Part of the above mentioned wet gel was dried and calcined directly, and the products were named as Ti-TUD-1. Fig. 1 shows the XRD patterns and N2 sorption isotherms of Ti-TUD-1, c-Ti-TUD-1 and conventional TS-1 zeolite. In the XRD pattern of TUD-1, there is no pronounced peak except a broad one around 231 which suggests an amorphous structure of the framework. However, for sample c-Ti-TUD-1, its XRD pattern clearly shows the characteristic peaks of TS-1 in spite of weak intensity, indicating clear crystallization of the material. For the N2 sorption isotherms, Ti-TUD-1 and c-Ti-TUD-1 exhibit typical type-IV sorption isotherms with State Key Lab of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, No. 1295 Ding-xi Road, Shanghai, 200050, P. R. China. E-mail: [email protected]; Fax: +86 21 52413122; Tel: +86 21 52412712 w Electronic supplementary information (ESI) available: Experimental details, FTIR, SEM, N2 sorption, hydrothermal test and selectivity data. See DOI: 10.1039/c0cc00499e