Ordered mesoporous materials in catalysis

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.

http://fulltext.calis.edu.cn/nature/nature/461/7265/nature08493.pdf

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

Recent advances in transition metal catalyzed oxidation of organic substrates with molecular oxygen.

State of the art and future challenges of zeolites as catalysts

3. Soluble Metal Oxides 2459 3.1. Polyoxometalates 2459 3.2. Peroxotungstates 2459 3.3. Peroxomolybdates 2460 3.4. Methyltrioxorhenium 2461 3.5. Other Metal Oxides 2461 4. Metal Oxides Generated in Situ 2461 4.1. Selenium and Arsenic Compounds 2461 4.2. Simple Metal Salts 2462 5. Coordination Complexes 2463 5.1. Manganese Porphyrins 2463 5.2. Iron Porphyrins 2464 5.3. Manganese Salen Complexes 2466 5.4. 1,4,7-Triazacyclononane (TACN) Complexes 2466 5.5. Iron and Manganese Pyridyl-Amine Complexes 2468

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Metal-Catalyzed Epoxidations of Alkenes with Hydrogen Peroxide

A novel titanosilicate with MWW topology, Ti-MWW, has been prepared by an acid treatment on a corresponding lamellar precursor which is hydrothermally synthesized with the coexistence of boron and titanium using piperidine (PI) or hexamethyleneimine (HM) as a structure-directing agent. The MWW precursor can be synthesized to have a Si/Ti ratio as low as 10 when the Si/B ratio of the gel is maintained at 0.75. Both the materials synthesized using PI and HM exhibit the crystal form of thin platelets, while the latter shows a larger crystal size. Besides the tetrahedral Ti species, the precursor always contains the octahedral Ti species showing a UV−vis band at 260 nm, regardless of the Ti content. Calcination of the precursor results in a partial condensation of the octahedral Ti species to form the anatase phase, which is hardly removed by the acid treatment. MWW-type titanosilicate nearly free of both anatase and boron, however, is successively prepared by a cyclic treatment on the lamellar precursor, tha...

A Novel Titanosilicate with MWW Structure. I. Hydrothermal Synthesis, Elimination of Extraframework Titanium, and Characterizations

A Novel Titanosilicate with MWW Structure: II. Catalytic Properties in the Selective Oxidation of Alkenes

A Titanosilicate That Is Structurally Analogous to an MWW-Type Lamellar Precursor†

Preparation of B-free Ti-MWW through reversible structural conversion

A novel titanosilicate with the MWW topology hydrothermally synthesized in the presence of boron shows catalytic activity for the liquid-phase oxidation reaction of cyclohexene after removal of the extraframework Ti species by an acid treatment.

Peng Wu

Papers

A Novel Titanosilicate with MWW Structure. I. Hydrothermal Synthesis, Elimination of Extraframework Titanium, and Characterizations

A Novel Titanosilicate with MWW Structure: II. Catalytic Properties in the Selective Oxidation of Alkenes

A Titanosilicate That Is Structurally Analogous to an MWW-Type Lamellar Precursor†

Preparation of B-free Ti-MWW through reversible structural conversion

Hydrothermal Synthesis of a Novel Titanosilicate with MWW Topology