Silicon-based zeolite

Rauter and her coauthors Xavier, Lucas, and Santos (Lisbon) present here a detailed overview of the potential for heterogeneous catalysts in useful synthetic transformations of carbohydrates. Such silicon-based catalysts as zeolites are easy to handle and recover, are nontoxic, and can offer interesting possibilities for exercising stereo- and regio-control in many established carbohydrate transformations. 

ZEOLITES AND OTHER SILICON-BASED PROMOTERS IN CARBOHYDRATE CHEMISTRY ... 

Another silicon-based porous solid which has been used in heterogeneous catalysis of organic reactions is silica gel. However, unlike zeolites and clays, silica gel is... 

During development, evaluation of the consolidated materials was based primarily on two criteria, leachability and the concentration factor, i.e., the concentration of waste oxides on a volume basis. The concentration factor is directly affected by the residual porosity in a consolidated waste as well as by the dilution caused by the addition of consolidation aids. This factor can be as high as 1.2 g/cm3 for a fully dense ( 5 g/cm ) titanate waste prepared from the projected Barnwell plant solution composition. The factor is slightly lower for a titanate waste containing silicon and zeolite additions, which has a typical density of U.2 g/cm3. The leachability was determined by an "instantaneous leach test developed for fast, comparative evaluations of materials, the details of which are described elsewhere (l6). 

Among the various materials are crosslinked PAN, polyphosphazenes, polyphe-nylenesulfide, polyetheretherketone, and various polymer blends [28-31]. Particularly interesting is the use of zeolites as filler in organic polymers, which aims at improving the performance of (silicone-based) membranes for separations in nonpolar solvents, by adding more cross-links to the membrane material [32, 33]. 

A high degree of hydrophobic character is an almost unique characteristic of silicon-rich or pure-silica-type microporous crystals. In contrast to the surface of crystalline or amorphous oxides decorated with coordinatively unsaturated atoms (in activated form), the silicon-rich zeolites offer a well-defined, coordinatively saturated sur ce. Such surfrces, based on the strong covalent character of the silicon-oxygen bond and the absence of hydrophilic centers, display a strong hydrophobic character unmatched by the coordinativeiy unsaturated, imperfect surfaces. Also, hydrophobic zeolite crystals have been reported to suppress the water affinity of transition metal cations contained in the zeolite pores. This property permits the adsorption of reactants such as carbon monoxide or hydrocarbons in the presence of water. 

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