Aluminum-oxygen bond, zeolite

From the zeolite mordenite, Chen and Smith (82) obtained silica having the same zeolitic skeleton by repeatedly heating the hydrogen form in steam at 538 C and extracting the liberated aluminum with acid. The pores are apparently lined with siloxane oxygen bonds and are hydrophobic, retaining high hydrocarbon sorption capacity. 

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. 

The effect of probe molecules on the 27A1 NMR has attracted some attention recently. In particular, the determination of the quadrupole coupling constant, Cq, is a sensitive means to learn more about the bonding situation at the aluminum in acid sites, and how it reflects the interaction with basic probe molecules. If one of the four oxygen atoms in an AIO4 tetrahedral coordination is protonated, as in a zeolitic acid site, the coordination is somewhat in between a trigonal and a tetrahedral A1 environment [232]. The protonated oxygen decreases its bond order to A1 to approximately half of its size compared to an unprotonated zeolite. 

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...

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