Zeolite chemistry pore shape selectivity

Phosphates having these types of open structures can act as shape-selective acid catalysts, for example, for the cracking and isomerization of hydrocarbons. For examples of lamellar materials, see Section 5.3 and see Intercalation Chemistry). Microporous catalysts are described above and in (see Porous Inorganic Materials and Zeolites). Mesoporous AlPO materials have larger pores within a matrix of amorphous A1P04. ... 

Acetic acid is known to undergo a vapor-phase ketonization reaction with formation of acetone on Brpnsted acids in general, and on proton-zeolites in particular. On large-pore zeolites in their proton form, the ketonization reaction is followed by acid-catalysed self-condensation amounting to mesitylene, mesityl oxide and phorone as main products [1], the chemistry being essentially identical to that in mineral acids. In H-pentasil zeolites with suitable acid site density, phorone isomerises to isophorone, which is cracked to yield 2,4-xylenol [1]. With propionic acid a similar chemistry occurs, but the formation of phenolics is severely suppressed by transition-state shape-selectivity effects... 

In the area of heterogeneous acid catalysis, zeolites (microporous, crystalline aluminosilicates) have had a massive impact on the conversion of small molecules, usually at high temperatures, and their role in the conversion of crude oil to smaller, typically unsaturated molecules has been a cornerstone of chemistry for almost half a century. This relies on a combination of tunable (and potentially very strong) acidity, shape selectivity due to small and very uniform pores, and exceptional thermal stability, which allows regeneration to be carried out by burning off heavy byproducts. 

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