(608h) Olefin Methylation Reactions over Iron Zeolites: Increasing Reaction Rates at Lower Temperatures and Shifting the Selectivity Towards Desired Products

The methanol to hydrocarbons (MTH) reaction is a pathway for converting methanol or dimethyl ether (DME) into fuels and chemicals. The reaction is catalyzed by a variety of acidic zeolites. With the large-pore zeolite H-[Al]Beta, the products are predominantly isobutane and triptane. The alkanes are formed via hydrogen transfer reactions of the isostructural olefins formed through successive methylation of small olefins in the olefin cycle of the MTH reaction. The hydride donors (other olefins) undergo further reactions to give aromatic products such as n-methylbenzenes; these side products are not valuable as a fuel and lead to catalyst deactivation. Zeolite catalysts containing framework iron reduce the rate of hydrogen transfer reactions due to their weaker Brønsted acid strength. At the same time, the methylation rates on Fe-zeolites are reduced thus, reaction temperature must be higher to get similar rates leading to rapid deactivation, for example at 400 . Alternatively, olefins can be added to the reactor feed to increase reaction rates at lower temperatures. This talk will describe research on understanding the kinetics and selectivity effects of olefin additives on Fe-zeolites. Over H-[Fe]Beta adding Isobutene at a DME:Isobutene ratio of 15:1 at 300 °C doubles the DME consumption rate while shifting the carbon selectivity towards C5, C6, and C7 olefins. These trends continue as the Isobutene loading is decreased; however, further addition of isobutene leads to reductions in DME consumption rates. When a medium pore zeolite H-[Fe]ZSM-5 is used, the DME consumption rate increases, and the selectivity to smaller olefin cracking products and C8 increases. Co-feeding isobutene overcomes low catalytic rates by increasing the DME consumption rate at lower reaction temperatures. The methylation of other molecules including C5-C8 olefins and cyclic alkenes over Fe-zeolites is also reported.