Highly Stable and Reusable Multimodal Zeolite TS‐1 Based Catalysts with Hierarchically Interconnected Three‐Level Micro–Meso–Macroporous Structure

Abstract

Microporous titanosilicates, such as TS-1, are a class of important catalysts with high activities and selectivities coupled with environmentally benign catalytic performance, and play a vital role in a series of catalytic oxidation reactions with H2O2. [1] However, an important drawback of these titanosilicate catalysts is that their pores are too small to be accessed by bulky reactants, and this hinders their use in the fine-chemical and pharmaceutical industries. Nanosized TS1 materials were initially considered as a potential approach to improving the accessibility of such catalysts because, owing to their larger external surface areas, they have more active sites than conventional zeolites. However, complex processes for their separation from reaction products and the ease of aggregation of the nanosized zeolites during synthesis and catalytic reactions limit seriously their development. Recent progress in the field has seen the incorporation of titanium ions into the framework of mesoporous materials and grafting of a titanocene complex onto mesoporous silica. These ordered mesoporous titanosilicates have pore diameters of 2–8 nm and exhibit catalytic properties for the oxidation of bulky reactants under mild conditions. Unfortunately, when compared with TS-1, their catalytic activity, for example, that of Ti-MCM-41, is relatively low. This is attributed to the difference in the titanium coordination environment (amorphous nature of the mesoporous wall). A series of ordered mesoporous titanosilicates have been synthesized by assembly of preformed titanosilicate zeolite precursors with triblock copolymers, and showed good activity in the oxidation of small molecules such as phenol and styrene as well as bulkier molecules like trimethylphenol. However, calcination leads to a significant reduction in catalytic activity towards both small and bulky molecules, due to the relatively low stability of catalytically active fourcoordinate titanium sites in these materials compared to those in TS-1. The relatively low stabilities of both the titanium species and the structure in catalytic processes may be related to imperfectly condensed mesoporous walls. Possibly, the degree of crystallization of the mesoporous walls should be enhanced. Therefore, mesoporous titanosilicates with an fully crystalline structure are highly desirable. Novel 3D crystalline metallosilicates with expanded pores were recently synthesized from 2D Ti-MWW (MWW-type titanosilicate) precursors, according to a strategy of inserting a monomeric Si source into the interlayer spaces. The resultant materials showed expanded pore apertures, high crystallinity, and outstanding redox catalytic properties towards bulky molecules. Consequently, increasing the pore size has been one of the goals of structural control, to permit the penetration of large molecules into the host porous structure. Macroporous titanosilicates with crystalline structure are particularly interesting, due to their improved transport properties. Well-defined macroporous arrays should show optimal fluxes, whereby diffusion is not a limiting issue. Therefore more efficient catalysts could be targeted through the controlled design of hierarchically meso–macroporous titanosilicates with crystalline structure, principally by introducing the multipore system evenly throughout the framework. The ideal hierarchically porous structure in efficient titanosilicate catalysts should contain a macropore system to enhance mass transport, mesopores for precise selectivity, and microporous zeolitic structure to provide the catalytically active sites. More attractive applications could be developed if new titanosilicate catalysts could be constructed with hierarchical micro–meso–macropore systems yet still be composed of the same highly active [*] Dr. L.-H. Chen , Dr. Y. Li, Dr. X.-Y. Yang , Prof. B.-L. Su State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology Luoshi Road 122, Wuhan 430070 (China) E-mail: xyyang@whut.edu.cn baoliansu@whut.edu.cn