Jeongnam Kim

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

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.

Directing Zeolite Structures into Hierarchically Nanoporous Architectures

Bifunctional surfactants are used to synthesize zeolites with multiple scales of porosity and enhanced catalytic activity. Crystalline mesoporous molecular sieves have long been sought as solid acid catalysts for organic reactions involving large molecules. We synthesized a series of mesoporous molecular sieves that possess crystalline microporous walls with zeolitelike frameworks, extending the application of zeolites to the mesoporous range of 2 to 50 nanometers. Hexagonally ordered or disordered mesopores are generated by surfactant aggregates, whereas multiple cationic moieties in the surfactant head groups direct the crystallization of microporous aluminosilicate frameworks. The wall thicknesses, framework topologies, and mesopore sizes can be controlled with different surfactants. The molecular sieves are highly active as catalysts for various acid-catalyzed reactions of bulky molecular substrates, compared with conventional zeolites and ordered mesoporous amorphous materials.

High temperature treatment of ordered mesoporous carbons prepared by using various carbon precursors and ordered mesoporous silica templates

The effect of high temperature treatment of ordered mesoporous carbons (OMCs) under neutral atmosphere is studied for OMCs prepared by using different carbon precursors (furfuryl alcohol, sucrose, acenaphthene and mesophase pitch) and different ordered mesoporous silica (OMS) templates (MCM-48 and KIT-6). The OMS-templated carbons were thermally treated at various temperatures ranging from 900 °C to 2400 °C to study changes in their porosity and framework crystallinity. The KIT-6 silica was synthesized at different conditions to control the size of primary mesopores and interconnecting complementary pores. The use of MCM-48 silica as template afforded the carbon replicas, CMK-1, which underwent a structure transition from Iad to I41/a. The use of the KIT-6 silica template, depending on the size of complementary pores, afforded a faithful inverse replica, CMK-8, as well as the CMK-1-type structure that underwent the aforementioned symmetry transition. The XRD patterns for the carbons studied showed that their thermal treatment led to a gradual deterioration of the carbon structure, which was associated with structure shrinking and pore walls fracturing. Particularly significant changes in the structural properties of the carbons studied occurred for those heated (graphitized) at 2400 °C, which manifested itself in a partial or complete loss of the pore volume. It was found that the CMK-1-type graphitized carbons exhibited better thermal stability, which is reflected by the presence of residual mesopores and/or nanostructure ordering. The degree of graphitization for the carbons heated at 2400 °C depended insignificantly on the type of carbon precursor; however, the precursor effect became more pronounced with decreasing temperature of the thermal treatment.