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°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°C than when it was pretreated at 500°C. Paraffins, including neopentane, were also produced over fluorided aluminas; on catalysts pretreated at 650°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 (H2 + 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°C showed no evidence of a band of 1540-50 cm-1) assignable to pyridinium ion formed by interaction between pyridine and Brønsted acid sites. On the catalyst with 6.5% F this band appeared when the catalyst was pretreated at either 500 or 650°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 Brønsted sites introduced during dehydrogenation.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry