Aluminosilicate and Aluminophosphate Molecular-Sieve Catalysts

Aluminosilicates (zeolites) are widely used as acidic and bifunctional catalysts. The formation of carbocationic intermediates is generally ascribed to the protons present in the open zeolite structure. A newer class molecular-sieve catalyst is the aluminophosphates. These may contain sihcon (SAPO) or metal (MeAPO) in their AiP04 frameworks. These framework substitutions in several cases generate protonic acidity that makes SAPO and MeAPO acid catalysts. There is already an extensive literature of this subject (20, 21). 

The following sections are a summary of the characteristics of the zeolites and aluminophosphates that have been investigated as catalysts for skeletal isomerization of n-butenes. 

The general formula of these zeolites in the hydrogen form is 

ZSM-5 incorporates two systems of intersecting channels lying along the a and b crystal axes (22) the straight channels have elliptical apertures with cross-sections of 0.51 X 0.55 nm, and the sinusoidal channels have nearly circular apertures with cross-sections of 0.53 x 0.56 nm. At the channel intersections there are cages about 1 nm in diameter with 0.6-nm diameter entry ports (Fig. 3). 

The ZSM-11 pore structure consists of straight ch annels in two directions, with intersections and interconnected cavities (cages). The cross section of the 10-ring is 0.54 X 0.53 nm (Fig. 4). 

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