Conversion of isobutane over solid acids?A sensitive mechanistic probe reaction

Amorphous solid acids, such as SiO2Al2O3 and halided-Al2O3, convert isobutane at elevated temperatures (700–920 K) to a limited number of products. These solid acids crack isobutane 30 to 400 times faster than a blank reactor, and yield a product consisting primarily of methane, propene, butenes, and hydrogen. This product distribution is similar to that obtained thermally. Conversion rates over amorphous solid acids display a first-order isobutane pressure dependency. In contrast to amorphous solid acids, a highly active, ultrastable Y, faujasite exhibits a paraffinic rather than an olefinic product. Hydrogen is not produced in significant quantities over faujasite but is incorporated into the conversion products. Isomerization to n-butane is a major path, rather than a trace reaction, alkylation reactions yielding C5(+) hydrocarbons readily occur and methane production is reduced. The rates of formation of cracking fragments, n-butane, and C5(+) products are second-order in isobutane. The product distribution and kinetics lead to the conclusion that amorphous solid acids convert isobutane by a different mechanism than faujasite. Over amorphous solid acids, the data are not consistent with the presence of “free” carbonium ion intermediates whereas with faujasite it is. Radical cations are suggested as initial intermediates over all solid acid systems and differences in their subsequent reactions account for the observed variation in products and kinetic orders.