Source of Activity in Other Siliceous Catalysts

Source of Activity in other Siliceous Catalysts.—Although various oxides can be combined with silica to give amorphous, acidic catalysts, the replacement of aluminium in zeolites (specially non-faujasitic zeolites) has proved to be very difficult with any element other than gallium. Materials of ZSM-5 structure with iron or boron in place of aluminium have been claimed recently, but it is not yet certain that either iron or boron is part of the zeolite lattice or that the catalytic activity observed is not due to residual lattice aluminium. 

Tanabe has reviewed the earlier work with silica-magnesia, silica-zirconia, and other amorphous siliceous materials. In a model for binary siliceous oxide catalysts, only the non-siliceous component was considered in terms of proton affinity and co-ordination number. Tanabe and co-workers proposed a general model for mixed oxide catalysts in which acidity is caused by an excess of negative or positive charge in a model structure of the binary oxide. The hypothesis is shown to fit 28 of the 31 binary oxides tested. One of these oxides, 

Tanabe, T. Sumiyoshi, K. Shibata, T. Kiyoura, and J. Kitagaura, Bull. Chem. Sac. Japan, 1974, 47, 1064. 

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