Skeletal Isomerization of n-Butenes Catalyzed by Medium-Pore Zeolites and Aluminophosphates

The recent literature related to selective skeletal isomerization of n -butenes catalyzed by medium-pore zeolites and Me-aluminophosphates is reviewed. In the presence of medium-pore molecular sieve catalysts, n -butenes are selectively transformed into isobutylene via a monomolecular mechanism. This is an example of restricted transition state shape selectivity, whereby the space available around the acidic site is restricted, constraining the reaction to proceed mainly through a monomolecular mechanism. Coking of the catalyst that leads to poisoning of the acidic sites located on the external surfaces and to a decrease in the space around the acidic sites located in the micropores renders the catalyst more selective.