Contribution to the understanding of the reaction chemistry of isobutane and neopentane over acid catalysts, I

Abstract

The reaction chemistry for the cracking of neopentane was compared with that for isobutane in steady-state flow experiments over silica-alumina catalysts and HY zeolites. The activity for isobutane cracking changed only slightly as it lined out. In contrast, the neopentane reaction was self-poisoning; coking was nearly 10 times as fast as that with isobutane. In addition, the principal reaction pathway with neopentane was the primary unimolecular demethylation step forming CH 4 and C 4 hydrocarbons as products. With both substrates paraffins were formed extensively on the more active catalysts. At low conversions with isobutane this was largely due to H − transfer from the reactant; with neopentane, the surface residues furnished the required hydrogen. Further dehydroxylation of silica-alumina catalysts effected by raising the pretreatment temperatures from 500 to 650'C was found to decrease the conversion rates by about a factor of 2, suggesting that the primary reaction events are Brønsted acid catalyzed. Replacing such sites in Y-zeolites with Na + ions also drastically reduced the catalytic activity, but the observed effects resulted from changes in the strength (intensive factor) of the sites, not just from the decrease in their number (extensive factor). Interestingly, the nearly 10 2 increase in rates obtained on removing Na + was not accompanied by a change in selectivity at constant temperature when the conversion was maintained constant at 2.33% by increasing the space velocity. The effects of adding small amounts of olefins into an isobutane feed stream were also investigated. Ethene inhibited the reaction somewhat, but otherwise had little effect. Propene and isobutene additives enhanced both the conversion and paraffin formation. The way these results amplify our understanding of these simple reactions and make them more useful for catalytic studies is briefly discussed.