Silica molecular sieves, isomorphous

An extremely versatile catalyst for a variety of synthetically useful oxidations with aqueous hydrogen peroxide is obtained by isomorphous substitution of Si by Ti in molecular sieve materials such as silicalite (the all-silica analogue of zeolite ZSM-5) and zeolite beta. Titanium(IV) silicalite (TS-1), developed by Enichem (Notari, 1988), was the progenitor of this class of materials, which have become known as redox molecular sieves (Arends et al., 1997). 

Breck has reviewed the early literature where Ga3+, P5+, and Ge4 were potentially incorporated into a few zeolite structures via a primary synthesis route (2). Evidence has also been presented to show that the small amounts of Fe3+, typically present in both natural and synthetic zeolites, are located in framework tetrahedral positions (3). A more recent review of "isomorphic substitution" in zeolites, via primary synthesis methods, speculates on the potential Impact of such substitutions on catalysis (4). The vast majority of work has been related to the high silica zeolites, particularly of the ZSM-5 type. Another approach to substitution of metal atoms into the open frameworks of zeolite structures has been to replace the typical silica alumina gel with gels containing other metal atoms. This concept has resulted in numerous unique molecular sieve compositions containing aluminum and phosphorus 5 silicon, aluminum and phosphorus (6) and with... 

New materials consisting of amorphous silica with regular pore structure, therefore called mesoporous molecular sieves, have recently been described [7]. Isomorphous substitution of Si by Ti has been attempted by performing the synthesis in the presence of titanium compounds. Ti-MCM have been tested for oxidation of hydrocarbons in liquid phase, using HjOj or hydroperoxides as oxidants [8-10]. 

Powder X-ray profiles of silicalite-1 prepared by various research workers, during their preparation of molecular sieves by isomorphously substituting the T element by titanium, tin, zirconium, vanadium, chromium, molybdenum, differ in their crystallinity (80-95%) and their unit cell volume values (Table 1). It is obvious from the Table that whenever a metal ion is being substituted, a metal free all silica polymorph should also be prepared for deciphering the unit cell expansion. The variation in the unit cell volume obtained by various workers is an indication of the inherent problems in synthesising zeolites and molecular sieves with repeatable metal substitution in the framework. 

The synthesis of molecular sieves containing transition metals in the framework represents a reliable route for preparing materials with novel catalytic properties. In spite of the large number of papers and patents claiming the incorporation of several transition metal ions in different microporous silica frameworks, unambiguous evidence in favor of the isomorphous substitution of silicon has been achieved only for Ti and Fe, for the former limited to few framework types. Some evidence exists also for V and Cr, but the data available do not allow definite conclusions to be drawn in this regard. 

The most efficient catalysts in liquid-phase oxidation of organic compoimds were crystalline mked oxides [1]. They are ionic mixed oxides or mixed oxides containing oxides supported on oxides. In the latter case, the catalytic activity of the oxide support is increased by adding one or more metal components or is obtained by immobilization of metal oxides on inactive oxide support. Metal ions were isomorphously substituted in framework positions of molecular sieves, for example, zeolites, silicalites, silica, aluminosilicate, aluminophosphates, silico-aluminophosphates, and so on, via hydrothermal synthesis or postsynthesis modification. Among these many mixed oxides with crystalline microporous or mesoporous structure, perovskites were also used as catalysts in liquid-phase oxidation. 

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