Induced catalytic activity of fluorided alumina in the reactions of isobutane

The reaction chemistry for the conversion of isobutane on pure and fluorided 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 demethanation were obtained on pure alumina; the rate of reaction was about one order of magnitude higher when the catalyst was pretreated at 650 degrees C than when it was pretreated at 500 degrees C. Paraffins, including neopentane, were also produced over fluorided aluminas; on catalysts pretreated at 650 degrees C while dehydrogenation and demethanation decreased, paraffin formation increased with time on stream (TOS). Over the catalysts with 4.5 and 6.5% F the rate of hydride transfer-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 than silica-alumina for i-butane conversion. IR spectra from pyridine adsorbed 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 acid sites. On the catalyst with 6.5% F this band appeared when the catalyst was pretreated at either 500 or 650 degrees C. Isobutane can be dehydrogenated and demethanated over the dual acid-base pair sites of the alumina. The increasing paraffin formation with TOS may result from new and additional Bronsted sites introduced during dehydrogenation. (C) 1995 Academic Press,Inc.