(344l) Olefin Methylation and Chain Initiators in the Methanol to Hydrocarbon Reaction over Iron Zeolites

The methanol to hydrocarbons (MTH) reaction is a pathway for taking methanol or dimethyl ether (DME) and converting it into fuels and chemicals. The reaction is catalyzed by various acid zeolites and using the large-pore zeolite H-[Al]Beta, the products contain predominantly triptane and isobutane. The alkanes are formed via hydrogen transfer reactions of the isostructural olefins formed through the successive methylation of small olefins in the olefin cycle of the MTH reaction. The hydride donors (other olefins) then 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 (instead of aluminum) reduce the rate of hydrogen transfer reactions due to their weaker Brønsted acid strength but also reduce methylation rates: on Fe-zeolites thus, reaction temperature must be relatively high also leading to rapid deactivation. To reduce reaction temperatures, a chain initiator can be added to the feed to jumpstart chain growth and the olefin cycle. We have found, for example, that pure DME is essentially unreactive at 240 °C over H-[Fe]Beta, but after the addition of isobutene, the consumption rate of DME increased significantly. After stopping the isobutene flow, DME consumption rate decreased back to its initial low value. Co-feeding isobutene thus overcomes low catalytic rates by increasing the DME consumption rate at otherwise low reaction temperatures where the precursors to stimulate the formation of the carbon pool, do not form. These results suggest that, over Fe-zeolites, isobutene can initiate the methylation of olefins and the olefin cycle. This allows for the carbon from shale gas to be incorporated into larger molecules more suited for uses as fuels and chemicals then traditional MTH reaction products. The methylation of other molecules, including pentenes and cyclic alkenes, over Fe-zeolites is also reported.