Experimental and DFT studies of initiation processes for butane isomerization over sulfated-zirconia catalysts

Reaction kinetics studies were conducted of isobutane and n-butane isomerization at 423 K over sulfated-zirconia, with the butane feeds purified of olefins, Dihydrogen evolution was observed during butane isomerization over fresh catalysts, as well as over catalysts selectively poisoned by preadsorbed ammonia. Butane isomerization over sulfated-zirconia can be viewed as a surface chain reaction comprised of initiation, propagation, and termination steps. The primary initiation step in the absence of feed olefins is considered to be the dehydrogenation of butane over sulfated-zirconia, generating butenes which adsorb onto acid sites to form protonated olefinic species associated with the conjugate base form of the acid sites. Quantum-chemical calculations, employing density-functional theory, suggest that the dissociative adsorption of dihydrogen, isobutylene hydrogenation, and dissociative adsorption of isobutane are feasible over the sulfated-zirconia cluster, and these reactions take place over Zr-O sites. (C) 1998 Academic Press.