MICROPOROUS and mesoporous inorganic solids (with pore diameters of ≤20 A and ∼20–500 A respectively)1 have found great utility as catalysts and sorption media because of their large internal surface area. Typical microporous materials are the crystalline framework solids, such as zeolites2, but the largest pore dimensions found so far are ∼10–12 A for some metallophosphates3–5 and ∼14 A for the mineral cacoxenite6. Examples of mesoporous solids include silicas7 and modified layered materials8–11, but these are invariably amorphous or paracrystalline, with pores that are irregularly spaced and broadly distributed in size8,12. Pore size can be controlled by intercalation of layered silicates with a surfactant species9,13, but the final product retains, in part, the layered nature of the precursor material. Here we report the synthesis of mesoporous solids from the calcination of aluminosilicate gels in the presence of surfactants. The material14,15 possesses regular arrays of uniform channels, the dimensions of which can be tailored (in the range 16 A to 100 A or more) through the choice of surfactant, auxiliary chemicals and reaction conditions. We propose that the formation of these materials takes place by means of a liquid-crystal 'templating' mechanism, in which the silicate material forms inorganic walls between ordered surfactant micelles.

Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism

The synthesis, characterization, and proposed mechanism of formation of a new family of silicatelaluminosilicate mesoporous molecular sieves designated as M41S is described. MCM-41, one member of this family, exhibits a hexagonal arrangement of uniform mesopores whose dimensions may be engineered in the range of - 15 A to greater than 100 A. Other members of this family, including a material exhibiting cubic symmetry, have ken synthesized. The larger pore M41S materials typically have surface areas above 700 m2/g and hydrocarbon sorption capacities of 0.7 cc/g and greater. A templating mechanism (liquid crystal templating-LCT) in which surfactant liquid crystal structures serve as organic templates is proposed for the formation of these materials. In support of this templating mechanism, it was demonstrated that the structure and pore dimensions of MCM-41 materials are intimately linked to the properties of the surfactant, including surfactant chain length and solution chemistry. The presence of variable pore size MCM-41, cubic material, and other phases indicates that M41S is an extensive family of materials.

http://dns2.asia.edu.tw/~ysho/ysho-english/1000 wc/pdf/j ame che soc114, 10834.pdf

A new family of mesoporous molecular sieves prepared with liquid crystal templates

It is possible to say that zeolites are the most widely used catalysts in industry They are crystalline microporous materials which have become extremely successful as catalysts for oil refining, petrochemistry, and organic synthesis in the production of fine and speciality chemicals, particularly when dealing with molecules having kinetic diameters below 10 A The reason for their success in catalysis is related to the following specific features of these materials:1 (1) They have very high surface area and adsorption capacity (2) The adsorption properties of the zeolites can be controlled, and they can be varied from hydrophobic to hydrophilic type materials (3) Active sites, such as acid sites for instance, can be generated in the framework and their strength and concentration can be tailored for a particular application (4) The sizes of their channels and cavities are in the range typical for many molecules of interest (5-12 A), and the strong electric fields2 existing in those micropores together with an electronic confinement of the guest molecules3 are responsible for a preactivation of the reactants (5) Their intricate channel structure allows the zeolites to present different types of shape selectivity, ie, product, reactant, and transition state, which can be used to direct a given catalytic reaction toward the desired product avoiding undesired side reactions (6) All of these properties of zeolites, which are of paramount importance in catalysis and make them attractive choices for the types of processes listed above, are ultimately dependent on the thermal and hydrothermal stability of these materials In the case of zeolites, they can be activated to produce very stable materials not just resistant to heat and steam but also to chemical attacks Avelino Corma Canos was born in Moncofar, Spain, in 1951 He studied chemistry at the Universidad de Valencia (1967−1973) and received his PhD at the Universidad Complutense de Madrid in 1976 He became director of the Instituto de Tecnologia Quimica (UPV-CSIC) at the Universidad Politecnica de Valencia in 1990 His current research field is zeolites as catalysts, covering aspects of synthesis, characterization and reactivity in acid−base and redox catalysis A Corma has written about 250 articles on these subjects in international journals, three books, and a number of reviews and book chapters He is a member of the Editorial Board of Zeolites, Catalysis Review Science and Engineering, Catalysis Letters, Applied Catalysis, Journal of Molecular Catalysis, Research Trends, CaTTech, and Journal of the Chemical Society, Chemical Communications A Corma is coauthor of 20 patents, five of them being for commercial applications He has been awarded with the Dupont Award on new materials (1995), and the Spanish National Award “Leonardo Torres Quevedo” on Technology Research (1996) 2373 Chem Rev 1997, 97, 2373−2419

From Microporous to Mesoporous Molecular-Sieve Materials and Their Use in Catalysis

"Space—the final frontier." This preamble to a well-known television series captures the challenge encountered not only in space travel adventures, but also in the field of porous materials, which aims to control the size, shape and uniformity of the porous space and the atoms and molecules that define it. The past decade has seen significant advances in the ability to fabricate new porous solids with ordered structures from a wide range of different materials. This has resulted in materials with unusual properties and broadened their application range beyond the traditional use as catalysts and adsorbents. In fact, porous materials now seem set to contribute to developments in areas ranging from microelectronics to medical diagnosis.

Ordered porous materials for emerging applications

Hollow tubular structures of molecular dimensions may offer a variety of applications in chemistry, biochemistry and materials science. Concentric carbon nanotubes have attracted a great deal of attention, while the three-dimensional tubular pore structures of molecular sieves have long been exploited industrially. Nanoscale tubes based on organic materials have also been reported previously. Here we report the design, synthesis and characterization of a new class of organic nanotubes based on rationally designed cyclic polypeptides. When protonated, these compounds crystallize into tubular structures hundreds of nanometres long, with internal diameters of 7-8 A. Support for the proposed tubular structures is provided by electron microscopy, electron diffraction, Fourier-transform infrared spectroscopy and molecular modelling. These tubes are open-ended, with uniform shape and internal diameter. We anticipate that they may have possible applications in inclusion chemistry, catalysis, molecular electronics and molecular separation technology.

Self-assembling organic nanotubes based on a cyclic peptide architecture

The framework topology of zeolite beta

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.

Nanometer-sized helium droplets, each containing about 10 4 helium atoms, were used as an inert substrate on which to form previously unobserved, spin-3/2 (quartet state) alkali trimers. Dispersed fluorescence measurements reveal that, upon electronic excitation, the quartet trimers undergo intersystem crossing to the doublet manifold, followed by dissociation of the doublet trimer into an atom and a covalently bound singlet dimer. As shown by this work, aggregates of spin-polarized alkali metals represent ideal species for the optical study of fundamental chemical dynamics processes including nonadiabatic spin conversion, change of bonding nature, and unimolecular dissociation.

Photoinduced Chemical Dynamics of High-Spin Alkali Trimers

Framework topology of AIPO4-8: the first 14-ring molecular sieve

The synthesis of molecular sieves with framework densities (FD) lower than those currently attainable is an ongoing challenge in molecular sieve science. Currently, the minimum FD is 12.5 for CoAPO-501 which translates to a void space that occupies approximately half the crystal volume. Brunner and Meier2 have shown that there is a correlation between the minimum FD and the smallest ring size within the structure [minimum ring (MINR)]. If the correlation of Brunner and Meier is correct, then structures with three-membered rings (3MR) must be synthesized in order to achieve porosities greater than 50%. We have initiated a project aimed at finding and exploiting a suitable gel chemistry for the synthesis of 3MR-containing materials. Beryllosilicates do promote 3MR formation as evidenced by the microporous mineral lovdarite3.4 and the dense minerals phenakite and euclase .5 A synthetic analogue of lovdarite has also been reported.6 However, many beryllium compounds are highly toxic and thus make it an undesirable element to work with. A synthetic aluminosilicate zeolite, ZSM-18, contains 3MR.7 Aluminosilicates, however, do not prefer the narrow T-0-T angles ca. 130" which are necessary to form 3MR. It is not likely that a general synthetic route to 3MR-containing frameworks will employ aluminosilicate chemistry. We have observed that zincosilicate analogues of euclase and phenakite, namely clinohedrite and willemite ,8 respectively, do exist. A synthetic non-microporous zincosilicate has also been reported.9 Thus, zinc appears to be a suitable substitute for beryllium in the formation of 3MRcontaining materials. We report here VPI-7$ the first microporous zincosilicate material to contain 3MR. VPI-7 is synthesized by heating a reaction mixture of composition 0.08 TEABr: 0.44 Na20: 0.28 ZnO: SiO2: 44 H 2 0 for 12 days at 200°C. The addition of TEABr (tetraethylammonium bromide) serves only to speed the rate of crystallization and is not incorporated into the product VPI-7. The Si02 : ZnO ratio of this sample is 3.3 : 1. VPI-7 crystals are typically in the form of fans composed of thin rectangular plates; the f_ans range in size from -10 to 50 pm. VPI-7 was exchanged with ammonium ions by refluxing in a 0.67 mol dm-3 ammonium hydroxide-0.33 mol dm-3 ammonium acetate solution for 4 h and the experimental X-ray powder diffraction pattern of NH4-VPI-7 is shown in Fig. l(a). The VPI-7 framework is tetragonal with maximum toplogical symmetry of Z4rn2. Ammonium exchanged VPI-7 has unit-cell dimensions of a = 7.179(1) and c = 40.62(1)A, and has a FD = 17.2. The c dimension of VPI-7 is the largest framework repeat in any known molecular sieve topology. The unit-cell dimensions of VPI-7 are similar to those of lovdarite (a = 39.72, b = 6.94, c = 7.15 A, orthorhombic basis).6 The Na cations present in as-synthesized VPI-7 are exchangeable, like in other molecular sieves, for such cations as Li+, K+ or NH4+. The VPI-7 framework exhibits reversible H20 adsorption. At PH20 = 20 Torr and T = 25"C, K-VPI-7 reversibly adsorbs H20 with an adsorption capacity of 0.19 g

/pdf/vpi-7-the-first-zincosilicate-molecular-sieve-containing-2gi1e09va3.pdf

John B. Higgins

Papers

The framework topology of zeolite beta

The framework topology of zeolite beta

Photoinduced Chemical Dynamics of High-Spin Alkali Trimers

Framework topology of AIPO4-8: the first 14-ring molecular sieve

VPI-7: The First Zincosilicate Molecular Sieve Containing Three-membered T-Atom Rings