Acetone aldolization and further self-deoxygenation (cracking on Brønsted acid site) has been widely studied to produce isobutene (ATIB) on zeolite catalysts, on which coking typically causes rapid deactivation of the catalysts. Recently, we demonstrated that a unique Lewis acid-base pair could be generated over ZnxZryOz catalysts, on which a highly active and stable cascade acetone aldolization and self-deoxygenation was achieved to produce high purity isobutene (zero other C4 isomers and aromatics typically coproduced on Brønsted acid site) in the presence of co-fed water. Despite of the fact that the roles of water have been extensively studied in the cascade ATIB reaction over zeolite catalysts, which could be attributed to: a) a weakened surface acidic sites and thus an attenuation of the side cracking reactions; b) a mitigated oligomerization, which subsequently prevents coke formation; c) H2O-mediated β-scission of acetone dimers to produce isobutene. So far, the critical role(s) of water in the Lewis acid-base pairs (i.e., ZnxZryOz) catalyzed ATIB reaction remains elusive. In this contribution, we combine in situ DRIFTS-pyridine, in situ 1H-NMR, reaction kinetics, DRIFTS-MS, isotopic labelling study, and density functional theory (DFT) to elucidate the cooperative roles of Lewis acid-base pairs and cofed water, as well as the detailed reaction mechanism in ATIB on ZnxZryOz catalysts. Our results reveal that the ATIB reaction follows the acetone-diacetone alcohol-isobutene pathway. Isobutene is produced through an intramolecular rearrangement of the 8-membered ring intermediate formed via the adsorption of diacetone alcohol on the Lewis acid-base pairs. OH adspecies generated by the co-fed water on the catalyst surface, were found to distort diacetone alcohol’s hydroxyl functional group toward its carbonyl functional group and facilitate the intramolecular rearrangement of diacetone alcohol to form isobutene. In the absence of water, diacetone alcohol binds strongly to the Lewis acid site, e.g., at a Zr4+ site, via its carbonyl functional group, leading to its dramatic structural distortion and further dehydration reaction to form mesityl oxide as well as subsequent polymerization reactions and the formation of coke.

Cooperative roles of water and Lewis acid-base pairs in cascade C-C coupling and self-deoxygenation reactions

ACS Spring 2022

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