Sulfated-zirconia catalysts show rapid deactivation (deactivation constants
near 0.02 min(-1)) during isomerization of n-butane when olefins are prese
nt in the feed. Removal of olefins from the feed decreases the rate of cata
lyst deactivation over sulfated-zirconia (to values near 0.007 min(-1)). Th
e rates of deactivation are slower during isobutane isomerization (deactiva
tion constants near 0.003 min(-1)) than during n-butane isomerization over
sulfated-zirconia catalysts in the presence of feed olefins. Deactivation o
f the catalysts during n-butane isomerization appears to be caused by the p
roduction of coke on the catalyst from straight-chain olefinic species eith
er present in the feed or produced on the catalyst under reaction condition
s. Butane isomerization over sulfated-zirconia can be viewed as being a sur
face chain reaction comprising initiation, propagation, and termination ste
ps. The primary initiation step in the absence of feed olefins is the dehyd
rogenation of butane over sulfated-zirconia, generating butenes which adsor
b onto acid sites. Quantum-chemical calculations, employing density functio
nal theory, suggest that the dissociative adsorption of dihydrogen, isobuty
lene hydrogenation, and dissociative adsorption of isobutane are feasible o
ver the sulfated-zirconia cluster, and these reactions take place over the
Zr-O sites. (C) 1999 Elsevier Science B.V. All rights reserved.