The reaction chemistry for the conversion of isobutane on pure and flu
orided aluminas was compared to continuous flow experiments. Catalysts
with 0, 2.6, 4.5, and 6.5% F were pretreated in flowing oxygen either
at 500 or 650 degrees C. Only the products of dehydrogenation and dem
ethanation were obtained on pure alumina; the rate of reaction was abo
ut one order of magnitude higher when the catalyst was pretreated at 6
50 degrees C than when it was pretreated at 500 degrees C. Paraffins,
including neopentane, were also produced over fluorided aluminas; on c
atalysts pretreated at 650 degrees C while dehydrogenation and demetha
nation decreased, paraffin formation increased with time on stream (TO
S). Over the catalysts with 4.5 and 6.5% F the rate of hydride transfe
r-producing paraffins exceeded that of initiation (H-2 + CH4 formation
) by a factor of from 2 to 3. All of the catalysts were more active th
an silica-alumina for i-butane conversion. IR spectra from pyridine ad
sorbed on catalysts with 0-4.5% F and pretreated either at 500 or 650
degrees C showed no evidence of a band of 1540-50 cm(-1) assignable to
pyridinium ion formed by interaction between pyridine and Bronsted ac
id sites. On the catalyst with 6.5% F this band appeared when the cata
lyst was pretreated at either 500 or 650 degrees C. Isobutane can be d
ehydrogenated and demethanated over the dual acid-base pair sites of t
he alumina. The increasing paraffin formation with TOS may result from
new and additional Bronsted sites introduced during dehydrogenation.
(C) 1995 Academic Press,Inc.