Effects of the Framework and Mesoporosity on the Catalytic Activity of Hierarchical Zeolite Catalysts in Benzyl Alcohol Conversion

Zeolites are crystalline inorganic porous materials formed by TO4 tetrahedra (T=Si, Al, P, etc.) with ordered microporous structures. As a result of their tunable acidities and framework structures, zeolites have been used widely in catalysis and molecular adsorption/separation as well as several other emerging applications. However, the pore size of a zeolite is usually smaller than 2 nm, which often causes severe diffusion limitations for catalytic reactions that involve bulky molecules. The introduction of intracrystalline mesopores within zeolites to form hierarchical porous structures is a promising way to enhance the mass transport properties and catalytic performance of zeolite catalysts. 3] Various synthesis approaches have been developed to fabricate mesoporous zeolites, such as the exfoliation or pillaring of layered zeolites, desilication or demetalation of zeolites by selective dissolution, and softand hard-templating methods. Delaminated zeolites, such as ITQ-2, which result from the delamination of a layered zeolite precursor, MCM-22(P), exhibit thin zeolite sheets (~2.5 nm thick) with a high external surface area (>700 m2g¢1).[4b] However, this method is only applicable for materials composed of layered zeolite precursors. With the use of the soft-templating synthesis strategy, mesopores can be introduced successfully into zeolite crystals through the coherent assembly of zeolite precursors and specially designed structure-directing agents (SDAs). Ryoo and co-workers designed a series of diquaternary ammonium-type surfactants with long hydrophobic hydrocarbon chains to synthesize zeolites with ultrathin nanosheets. Tsapatsis and co-workers reported the one-step synthesis of a self-pillared pentasil (SPP) zeolite composed of orthogonally connected single-unit cell lamellae. Three-dimensionally ordered mesoporous imprinted (3DOm-i) zeolites synthesized using 3DOm carbon as a mesoscale hard template was synthesized using a hard-templating synthesis strategy. Hierarchical zeolites with intracrystalline mesopores can provide easy access to the active sites located within micropores and fast mass transport that can improve the activity, selectivity, and lifetime of zeolite catalysts for a wide range of catalytic reactions. Although a series of breakthroughs have demonstrated the enhanced catalytic properties of hierarchical zeolites, structure–property relationships for hierarchical zeolites have not been fully established yet. There are several critical issues that limit the rational development of these materials for catalytic reactions. These include (1) the lack of facile The catalytic performance of three-dimensionally ordered mesoporous imprinted (3DOm-i) zeolite catalysts with different frameworks (3DOm-i MFI, 3DOm-i BEA, and 3DOm-i LTA) were investigated by the liquid-phase catalytic conversion of benzyl alcohol in mesitylene and compared to that of other microporous and mesoporous catalysts with a high external surface area, which included MCM-22, 300 nm MFI, ITQ-2, and Al-MCM41. The mesoporosity in MFI and BEA zeolites can effectively enhance the catalytic performance of the zeolite catalysts for benzyl alcohol self-etherification catalyzed by both the acid sites on the external and internal surface and the alkylation of mesitylene with benzyl alcohol catalyzed exclusively by the acid sites on the external surface. For 3DOm-i LTA, MCM-22, and Al-MCM-41, only the acid sites on the external surface can be utilized in the catalytic reactions. A distinct difference in the product selectivity was also observed for the microporous and mesoporous catalysts.