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.

http://science.sciencemag.org/content/sci/333/6040/328.full.pdf

Directing Zeolite Structures into Hierarchically Nanoporous Architectures

The discovery of mesoporous molecular sieves, MCM-41, which possesses a regular hexagonal array of uniform pore openings, aroused a worldwide resurgence in this field. This is not only because it has brought about a series of novel mesoporous materials with various compositions which may find applications in catalysis, adsorption, and guest-host chemistry, but also it has opened a new avenue for creating zeotype materials. This paper presents a comprehensive overview of recent advances in the field of MCM-41. Beginning with the chemistry of surfactant/silicate solutions, progresses made in design and synthesis, characterization, and physicochemical property evaluation of MCM-41 are enumerated. Proposed formation mechanisms are presented, discussed, and identified. Potential applications are reviewed and projected. More than 100 references are cited.

Advances in Mesoporous Molecular Sieve MCM-41

The framework topology of zeolite beta

The presence of basic centers in some oxides has been recognized for a long time as being important in catalysis [1-4]. Usually both basic and acid sites exist simultaneously. The two centers may work independently or in a concerted way. For instance, in alcohol transformation, dehydration is favored on acidic sites and dehydrogenation on basic centers [3,5]. A large variety of materials are cited as having basic character. They include single-metal oxides (MgO, CaO, ZnO), supported alkali metals (Na/MgO, K/K2CO3), mixed-metal oxides (MgO-A12O3, ZnO-SiO2, MgO-TiO2), zeolites (X and Y saturated with alkaline cations of low electronegativity), hydrotalcite-type anionic clays, asbestoslike materials, carbon-supported basic catalysts, and basic organic resins. ∗ Present address: 16 rue Francois Gillet, 69003 Lyon, France.

Basic zeolites : Characterization and uses in adsorption and catalysis

Zeolite beta, first described in 19671, is an active catalyst and a useful sorbent1 Sorption1,2 and catalytic data3,4 suggest that the zeolite could possess a three-dimensional 12-ring pore system Such a pore system suggests technological potential similar to that of faujasite framework materials, but until now the structure of this zeolite has eluded determination Powder X-ray diffraction patterns comprise both sharp and broad features, indicative of an extensively faulted structure Here we determine the structure of zeolite beta by high-resolution electron microscopy, electron diffraction and computer-assisted modelling The zeolite is an intergrown hybrid of two distinct but closely related structures Both are constructed from the same centrosymmetrlcal tertiary building unit arranged in layers, and both possess three-dimensional 12-ring pore systems One end member, polymorph A, forms an enantiomorphic pair, with symmetries P4122 and P4322, with a = 124 A and c = 265 A Polymorpb B, in which the stacking of layers alternates in handedness, is achiral with space group P1 ¯, and a ≃ b = 124 A, c = 145 A, α ≃ β = 73°, γ ≃ 90° The high density of stacking faults in zeolite beta materials arises because successive layers must interconnect in either a left- or a right-handed fashion, and both modes of linkage occur with almost equal probability

Two new three-dimensional twelve-ring zeolite frameworks of which zeolite beta is a disordered intergrowth

The tetrahedral framework structure of zeolite beta is disordered along (001). The disordered structure is related by a/3 and/or b/3 displacements on (001) planes to three ordered polytype structures with triclinic, monoclinic, and tetragonal symmetry. Three mutually perpendicular 12-ring channel systems are characteristic of the three ordered polytypes and the disordered beta structure. The proposed framework structures are consistent with the known diffraction, sorption an cation exchange properties of zeolite beta.

The framework topology of ZSM-12: A high-silica zeolite

The framework topology of ZSM-48: A high silica zeolite

The crystal structure of a high silica zeolite ZSM-39, unit cell composition excluding residual water and occluded materials—(Na, tetramethylammonium ion, tetraethylammonium ion)0.4 (AlO2)0.4 (SiO2)135.6, was determined by X-ray powder diffraction. The framework is pseudoface-centred, pseudocubic with a = 19.36±0.02 A and ideal symmetry Fd3m. The framework consists of a space-filling arrangement of pentagonal dodecahedra and hexakaidecahedra and is isostructural with the 17 A cubic gas hydrate. The ZSM-39 framework is composed entirely of five- and six-rings which limits its sorptive and exchange properties. However, the large fraction of five-rings and the high Si/Al ratio (>40) impart a high thermal stability. ZSM-39 containing no aluminium constitutes the end member composition. Although ZSM-39 is the only synthetic zeolite analogue of a gas hydrate, we propose here two related hypothetical frameworks containing pentagonal dodecahedral cages.

W.J. Rohrbaugh

Papers

The framework topology of zeolite beta

The framework topology of zeolite beta

The framework topology of ZSM-12: A high-silica zeolite

The framework topology of ZSM-48: A high silica zeolite

Crystal structure of a synthetic high silica zeolite—ZSM-39