Silicon-oxygen bond, zeolite

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. 

SILICON-OXYGEN AND ALUMINIUM-OXYGEN BOND LENGTHS ( ) IN ZEOLITE-A, SPACE GROUP Fm3c ... 

The sp -bonded zeolite type structures are obtained from natural silica frameworks by substituting the silicon atoms with carbon and removal of oxygen. We demonstrate that considering n-type doping such structures [46-48] behave very much like diamond. A mid-gap state occurs after a substantial relaxation of the impurity which is localized on a nearby carbon atom. 

Sihcon—oxygen bonds dominate the chemistry of the silicates, silica, and the aluminosihcates.The SiO unit can be linked together to form rings, chains, sheets, and three-dimensional arrays. Asbestos is a generic name for a class of double-chain silicates that, until lately, has been put to a wide variety of uses. SiHca comes in a variety of polymorphs including the optically active a-quartz. Quartz is also piezoelectric. Water glass and siUca gel have a variety of practical appHcations. When aluminum replaces some silicon atoms in a silicate, the aluminosilicates result. These include the zeolites and the micas. 

Figure 1 is an idealized representation of the structure proposed for zeolite 12 by Barrer and Villiger on the basis of x-ray powder data (5). The intersections of the straight lines represent point aluminum or silicon atoms, each of which is bonded to four tetrahedrally disposed oxygen atoms. 

Fig. I Structure of the mineral zeolite chabu/ite is depicted by packing model, tell, and skeletal model, right. The silicon and aluminum atoms lie at ihe corners ol die framework depicted hy solid lines. In this figure die solid lines do nut depict chciniej bonds. Oxygen atoms lie near rhe midpoint ol the lines connecting Irainewnrk corners. Cation sites are shown m three vJillereni toeanons referred io as sues I. II, and III. Courtesy nl Sriewilii Atmoi. im...

If a cation M4 is a substituent for Si4 in a zeolite framework, it is expected to be bonded to four silicon atoms through bridging oxygens in tetrahedral coordination. 

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