(279a) Control of Isobutane Cracking Selectivity Over H-ZSM-5 by High-Temperature Pretreatments

Zeolites are crystalline materials that have wide application in industry as solid acid catalyst. Zeolites have Brønsted acid sites that are hydroxyl groups bridged between Al and Si (Al-OH-Si) tetrahedra. The acid sites of zeolites are decomposed at high temperatures, usually above 873 K in a process called dehydroxylation. This high temperature condition is commonly found in the fluidized catalytic cracking where catalyst is recycled forth and back between the riser and the regenerator under an oxidative atmosphere. The dehydroxylation is believed to proceed via a dehydration mechanism of the acid sites, however, hydrogen is also formed during the dehydroxylation process⁴. To explain this observation, the decomposition of Brønsted acid through the formation of electron deficient redox sites, such as [AlO₄]⁰, in zeolites are proposed. The activation of small alkanes over acid sites has been investigated extensively because of its relevance to technologically important processes such as fluidized catalytic cracking in petroleum refineries, but also because C-H and C-C bond activation is of fundamental scientific interest. Here, we investigate reactivity and selectivity of newly generated sites using isobutane conversion.

We have found that in addition to the catalytic effect of Brønsted acid catalyst, redox sites, which are generated by high temperature for dehydroxylation, also promote alkane cracking process. The specific reaction rates of dehydroxylated samples increase respect to the pristine samples. When conversion is low, the product distribution is limited to the monomolecular cracking of the C-C bond and dehydrogenation of the C-H bond. For isobutane, the cracking-to-dehydrogenation ratio increases by a factor of ~2 after dehydroxylation treatments. We proposed that the presence of redox sites resulted in radical cation chemistry instead of protolytic chemistry in the isobutane cracking process. In addition there are dramatic differences in selectivity: the dehydroxylated catalysts are much more selective toward the cracking of isobutane into propylene and methane than towards dehydrogenation. This is in contrast to propane, where the dehydroxylated samples show preference towards dehydrogenation. The molecular origin of these differences is being investigated and will be discussed.