Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts

Abstract

Zeolites — microporous crystalline aluminosilicates — are widely used in industry as size- and shape-selective catalysts. But the very micropores that make this catalytic activity possible also cause diffusion limitations. Choi et al. now show that the problem can be overcome by synthesizing zeolites in the presence of bifunctional surfactants, which simultaneously direct the formation of micropores and limit the growth of the zeolite crystal to that of a ''nanosheet'' with a thickness of only one unit cell. These structural features render the ultrathin zeolites highly active for the catalytic conversion of large organic molecules; they also minimize the adverse effects of diffusion limitations, as illustrated by drastically reduced coke deposition and catalyst deactivation during methanol-to-gasoline conversion. Zeolites — microporous crystalline aluminosilicates — are widely used in industry as size- and shape-selective catalysts, but the micropores that enable this catalytic activity also cause diffusion limitations that adversely affect it. This can be overcome by reducing the thickness of the zeolite crystals and thus improving molecular diffusion. Here it is shown that bifunctional surfactants can direct the formation of zeolite structures that are only one unit cell thick. Zeolites—microporous crystalline aluminosilicates—are widely used in petrochemistry and fine-chemical synthesis1,2,3 because strong acid sites within their uniform micropores enable size- and shape-selective catalysis. But the very presence of the micropores, with aperture diameters below 1 nm, often goes hand-in-hand with diffusion limitations3,4,5 that adversely affect catalytic activity. The problem can be overcome by reducing the thickness of the zeolite crystals, which reduces diffusion path lengths and thus improves molecular diffusion4,5. This has been realized by synthesizing zeolite nanocrystals6, by exfoliating layered zeolites7,8,9, and by introducing mesopores in the microporous material through templating strategies10,11,12,13,14,15,16,17 or demetallation processes18,19,20,21,22. But except for the exfoliation, none of these strategies has produced ‘ultrathin’ zeolites with thicknesses below 5 nm. Here we show that appropriately designed bifunctional surfactants can direct the formation of zeolite structures on the mesoporous and microporous length scales simultaneously and thus yield MFI (ZSM-5, one of the most important catalysts in the petrochemical industry) zeolite nanosheets that are only 2 nm thick, which corresponds to the b-axis dimension of a single MFI unit cell. The large number of acid sites on the external surface of these zeolites renders them highly active for the catalytic conversion of large organic molecules, and the reduced crystal thickness facilitates diffusion and thereby dramatically suppresses catalyst deactivation through coke deposition during methanol-to-gasoline conversion. We expect that our synthesis approach could be applied to other zeolites to improve their performance in a range of important catalytic applications.