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

Abstract

Zeolites-microporous crystalline aluminosilicates-are widely used in petrochemistry and fine-chemical synthesis 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 limitations 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 diffusion. This has been realized by synthesizing zeolite nanocrystals, by exfoliating layered zeolites, and by introducing mesopores in the microporous material through templating strategies or demetallation processes. 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.

Hierarchy concepts: classification and preparation strategies for zeolite containing materials with hierarchical porosity

“…This finally leads either to mesoporous zeolite crystals 61 or a layered assembly of zeolitic nanosheets. 76 For example, depending on the choice of the surfactant and targeted zeolite framework type, very thin zeolitic MFI-type nanosheets with intercrystalline mesopores were obtained in the presence of a specially designed diquarternary ammoniumtype surfactant. 61,76,77 In this approach (during the synthesis), the hydrophilic ammonium groups directed the formation of the microporous zeolite structure, whereas the hydrophobic surfactant chains were used to limit the further growth of the zeolite network along the b-direction.…”

Dimethyl Ether Carbonylation to Methyl Acetate over Nanosized Mordenites

“…12 Reducing the crystalline size from the typical micrometer scale to the nanometer level shortens the diffusion pathway and improves the efficiency of molecule transportation. For example, decreasing the size of HZSM-5 to about 100 nm was found to markedly enhanced the catalytic stability in n-hexane cracking, 13,14 acetone to olefin, 15,16 methanol to gasoline, 17,18 and methanol to DME. 19 Nanosized Beta zeolites (80−100 nm) also exhibited activity and stability superior to those of the conventional zeolites with large particle sizes (2−4 μm) in acylation of anisole and toluene.…”

Methanol-to-olefin conversion over H-MCM-22 catalyst

“…The catalyst stability is estimated by using the reaction time for complete methanol conversion at a certain WHSV according to Refs. [11,[27][28][29][30][31][32][33], and the catalyst lifetime is different from one another. As the Si/Al ratio increased from 10.8 to 35.1, the catalytic lifetime increased from 23 to 48 h. However, with the Si/Al ratio further increasing to 49.8, the catalytic lifetime dramatically decreased to 17 h, which may be due to the presence of lower amounts of acid sites, especially for the amount of the Lewis acid sites in the sample (Table 2).…”