Acid-Catalyzed and Bifunctional Isomerization

Reviews on solid acids289,290and heteropoly compounds, applied in both the heterogeneous289-292 and homogeneous phases,293 briefly discuss the use of these materials as catalysts in various isomerization processes. 

According to an early report, sulfated zirconia promoted with 1.5% Fe and 0.5% Mn increased the rate of isomerization of n-butane to isobutane by several orders of magnitude at modest temperature (28°C).299 This reactivity is surprising, since the isomerization of n-butane in strong liquid acids takes place at a rate much lower than that of higher alkanes, which is due to the involvement of the primary carbocationic intermediate. In addition, other solid acids, such as zeolites, did not show activity under such mild conditions. Evidence by isotope labeling studies with double-labeled n-butane unequivocally shows, however, that the isomerization of 

M-butane proceeds via an intermolecular mechanism with 2-butene involved intermediately.300-303 The role of the transition metal promoters such as Fe and Mn was shown to increase the surface concentration of the intermediate butene 304 The formation of butene is speculated to occur through an oxidative dehydrogenation on the metal site305 or by one-electron oxidation.306 

Isobutane formation is accounted for by alkylation to form C8+ surface intermediate species followed by P scission.300,305 One possible pathway is depicted as follows  

Kinetic analysis and the identification of the C3 + C5 coproducts gave additional support to this mechanism. 

---