Selective Participation of Methane in Olefin Upgrading over Pd/ZSM-5and Ir/ZSM-5: Investigation using Deuterium Enriched Methane

摘要

The upgrading of olefins under methane environment is a promising process. Not only is the olefin content reduced, which is critical for the storage and transportation of oil products, but also are high value-added products produced from methane. The study on the reaction intermediates during this process is crucial for a better understanding of the involved catalytic reaction mechanism as well as catalyst development. Catalysts that displayed high selectivity towards olefin upgrading including Ir/ZSM-5 and Pd/ZSM-5 in our previous research are studied in this work. CD4 is employed as the methane source in the reaction to track the evolution of hydrogen in the methane molecule. The significant change of the allylic hydrogen peak on 1HNMR spectra upon CD4 introduction demonstrates that the hydrogen sites on the carbon atoms adjacent to the double bond moieties are selectively occupied by deuterium atoms, suggesting that these sites are preferred by methane hydrogen atoms and may play a key role in the reaction pathway. The evolution of methane and the interaction between methane and olefin over the catalyst are observed in the DRIFT spectra. The IR resonance due to O-D stretching of the terminal silanol hydroxyl group at 2753 cm~(-1) is observed under CD4 environment, while the O-D stretching of bridging hydroxyl group is not detected, indicating the interaction between methane and the catalyst is highly selective, which may contribute to the high selectivity in methane hydrogen evolution.