New mesoporous molecular sieves

07-0-01 - Ordered mesoporous carbon molecular sieves by templated synthesis the structural varieties 

Ryoo (a), S.H. Joo (a), S. Jun (a), T. Tsubakiyama (b) and O. Terasaki (b,c) a Korea Advanced Institute of Science and Technology, Taejon, Korea, rrvoo(a)jnaiL kaist. ac. kr. b Department of Physics and CIR, Tohoku University, Sendai, Japan c CREST, JST Corporation, Tohoku University, Sendai 980-8578, Japan 

Ordered mesoporous carbons (OMC) of various structures, designated as CMK-1 5, have been synthesized by carbonization of sucrose, furfuryl alcohol or other carbon sources inside silica or aluminosilicate mesopores that are interconnected into three-dimensional networks such as in MCM-48, SBA-1 and SBA-15. The mesoporous carbon molecular sieves, obtained after template removal, show TEM images and patterns characteristic of the ordered arrangement of uniform mesopores. The OMC, which are opening up a new area of the nanoporous materials, exhibit high BET specific surface areas, excellent thermal stability in inert atmospheres and strong resistance to attack by acids and bases. 

07-0-02 - One-pot synthesis of phenyl functionalized porous silicates with hexagonal and cubic symmetries 

Phenyl functionalized silicates with cubic Pm3n and 2D p6m symmetries have been prepared by direct reaction of a mixture of phenyltriethoxysilane and tetraethoxysilane with an aqueous solution of cetyltrimethylammonium bromide, under acidic or basic conditions. Their structural characterization by XRD using synchrotron radiation, multinuclear MAS-NMR (including 2-dimensional H homonuclear correlation) and N2 adsorption-desorption isotherms will be reported in order to better describe the localization of the phenyl groups in the silicate framework. 

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In this review we will cover the synthesis and sorptive properties of these new mesoporous molecular sieves. 

Olson, D.H., Sheppard, E.W., McCullen, S.B., Higgins, J.B. and Schlenker, J.L. (1992) A new family of mesoporous molecular-sieves prepared with liquid-crystal templates. Journal of the American Chemical Society, 114, 10834-10843. 

The development of composite micro/mesoporous materials opens new perspectives for the improvement of zeolytic catalysts. These materials combine the advantages of both zeolites and mesoporous molecular sieves, in particular, strong acidity, high thermal and hydrothermal stability and improved diffusivity of bulky molecules due to reduction of the intracrystalline diffusion path length, resulting from creation of secondary mesoporous structure. It can be expected that the creation of secondary mesoporous structure in zeolitic crystals, on the one hand, will result in the improvement of the effectiveness factor in hydroisomerization process and, on the other hand, will lead to the decrease of the residence time of products and minimization of secondary reactions, such as cracking. This will result in an increase of both the conversion and the selectivity to isomerization products. 

In 1992, Mobil reported a novel family of molecular sieves known as the M41S materials that established an entirely new area in nanoporous solids (168,169). The M41S materials expanded the range of pore sizes into the mesoporous domain (20-300 A). The impact of this discovery on molecular sieve research has been profound. In 1998, an international conference solely devoted to mesoporous molecular sieves was founded (170). The journal Microporous Materials even re-... 

Two major new classes of molecular sieve type materials were reported in the 1990s (i) microporous frameworks based on mixed octahedral-tetrahedral frameworks in contrast to the previously described tetrahedral frameworks and (ii) mesoporous molecular sieves with pore sizes ranging from about 2 nm to greater than 10 nm. 

F Cot, P. J. Kooyman, A. Larbot, E. Prouzet, in L. Bonneviot, F. Beland, C. Danumah, S. Giasson, S. Kaliaguine (Eds ) Mesoporous Molecular Sieves 1998, Vol. 117, Elsiever Science, Amsterdam, Lausanne, New York, Oxford, Shannon, Singapore, Tokyo 1998, p. 231. 

In 1995, Tanev and Pinnavaia [1] have reported the synthesis of a new type of mesoporous molecular sieve designated as the hexagonal mesoporous silica (HMS). Instead of using the ionic inorganic precursor and surfactant as in the case of MCM-41 [2], HMS is manufactured by hydrolysis reaction between a neutral inorganic precursor, tetraethyl-orthosilicate (TEOS) and a neutral primary amine surfactant (8-18 carbons). HMS possesses numerous favourable characteristics, but, like MCM-41, its synthesis process can only be concluded by the removal of the surfactant. This was reportedly done either by calcination at 630°C or by warm ethanol extraction [1]. 

Since the first synthesis of TS-1 in 1983 [1], considerable efforts have been devoted to the synthesis of titanium-containing zeolites [2, 3]. Recently, Ti-beta, a large-pore molecular sieve, has been extensively studied [4, 5]. Owing to its unique large-pore channel system, Ti-beta seems to be more active than the medium-pore TS-1 catalyst for the oxidation of cyclic and branched alkenes with aqueous hydrogen peroxide. Under the usual synthesis conditions, however, Ti-beta crystallizes with some Al as a framework constituent [4], This leads to the presence of acid centers, which may have a detrimental effect on the activity or selectivity of this type of catalyst. Since 1992, the discovery of a new family of mesoporous molecular sieves has received much attention [6,7], Because of their mesoporous nature (20-100A), the Ti-MCM-41 zeolites may be useful as oxidation catalysts for larger molecules [8], In this... 

Recently, there has been a growing interest into niobium- and tantalum-containing molecular sieves. The introduction of niobium into mesoporous molecular sieves has been studied by Ziolek et al [3,4], while Antonelli and Ying reported the synthesis of mesoporous niobium oxide [5], The synthesis and characterization of niobium- and tantalum-containing silicalite-1 (NbS-1 and TaS-1) was published recently [6,7,8] and some evidence has been presented for isomorphous substitution [6,8] of Nb and Ta into the silicalite-1 framework. The synthesis of NbS-2 (MEL) [9] and a new molecular sieve named NbAM-11 have been reported as well [10],... 

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

New directions in the preparation of framework structures of different chemical composition and of large-pore molecular sieves include the development of phosphate-containing molecular sieves and mesoporous molecular sieves. 

New materials consisting of amorphous silica with regular pore structure and therefore called mesoporous molecular sieves have recently been described (Kresge et al., 1992b). The incorporation of titanium has been attempted by performing the synthesis in the presence of Tilv compounds, such as titanium alkoxides (Corma et al., 1994c Tanev et al., 1994 Blasco et al., 1995 Zhang et al., 1966 Liu et al., 1996). 

All these aspects were thoroughly discussed by lecturers and participants during the round table organized during the Poitiers School on The Future Trends in Zeolite Applications . Special emphasis was placed on the role played by the sites at the external surface (pockets, etc.) or at the pore mouth, by mesopores, extraframework aluminum species, as well as by the polarity of reactant and product molecules. Other important topics dealt with the remarkable catalytic properties of BEA zeolites for fine chemical synthesis, the potential of mesoporous molecular sieves, zeolitic membranes and the role of combinatorial catalysis in the development of zeolite catalysts. It is our hope that the fruits of these discussions will appear in the literature or even better as new and environmentally friendly products or processes. 

With the growing interest in nanochemistry (Ozin, 1992), many attempts have been made in recent years to prepare porous crystals having larger channel widths than those in the aluminosilicate zeolites (i.e. 0.8 nm). In particular, great excitement was generated by the preparation of the wide-pore aluminophosphate, VPI-5, by Davis and his co-workers in 1988 and again when the first member of a new group of mesoporous molecular sieves , MCM-41, was prepared by Mobil scientists in 1992. 

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