(349d) Alkane Cracking and Electron Transfer Processes in Acid Zeolites

We investigate the effect of thermal dehydroxylation on the catalytic reactivity and selectivity of acid ZSM-5 zeolites on the cracking of propane and isobutane. Samples of H-ZSM-5 are investigated before and after pre-treating the samples at 780°C/He. The catalytic tests are conducted at low conversion and low concentration of hydrocarbons to ensure that only monomolecular cracking reactions are observed. We find that the thermal treatment reduces the number of Brønsted acid sites as determined by ammonia TPD and by IR spectroscopy by a factor of at least 50%. The reduction in Brønsted acid sites, however, does not lead to a reduction in the reaction rate and changes selectivity from a condition where cracking of propane into ethylene and methane is the predominant reaction to a condition in which propane cracking and dehydrogenation occur at about the same rate. In the case of isobutane the observations are very different. First, the reaction rate of isobutane is enhanced by a factor of at least four after thermal treatment. Second, methane and hydrogen formation occur at the same rate on the pristine zeolite but on the treated zeolite methane formation is the predominant reaction by a factor also of about four.

These observations can be rationalized if an electron-deficient site is produced upon thermal dehydroxylation of the zeolite. This electron-deficient site is not acidic and abstracts an electron from adsorbe alkanes forming radical cations that rapidly decompose leading to both methane and hydrogen loss. As observed in the zeolites, in mass-spectrometry the cracking patterns of propane and isobutane radical cations are very different and show that propane radical cations decompose forming methane and hydrogen at similar rates while for isobutane the cracking leads predominantly to the formation of methane and propene. Studies of naphthalene radical cation formation at room temperature and of the H/D isotope effect of the decomposition of deuterated propane are also consistent with this interpretation.