Elucidating the Cooperative Roles of Water and Lewis Acid–Base Pairs in Cascade C–C Coupling and Self-Deoxygenation Reactions

Abstract

Water plays pivotal roles in tailoring reaction pathways in many important reactions, including cascade C–C bond formation and oxygen elimination. Herein, a kinetic study combined with complementary analyses (DRIFTS, isotopic study, 1 H solid-state magic angle spinning nuclear magnetic resonance) and density functional theory (DFT) calculations are performed to elucidate the roles of water in cascade acetone-to-isobutene reactions on a Zn x Zr y O z mixed metal oxide with balanced Lewis acid–base pairs. Our results reveal that the reaction follows the acetone–diacetone alcohol–isobutene pathway. Isobutene is produced through an intramolecular rearrangement of the eight-membered ring intermediate formed via the adsorption of diacetone alcohol on the Lewis acid–base pairs in the presence of cofed water. OH adspecies, formed by the dissociative adsorption of 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 Zr 4+ 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. The present results provide insights into the cooperative roles of water and Lewis acid–base pairs in catalytic upgrading of biomass to fuels and chemicals.

Confined dual Lewis acid centers for selective cascade C–C coupling and deoxygenation

“…Details of the synthesis and related experiments can be found in ESI. † The obtained catalysts, with similar surface area and pore volume (Table S1 † ), were evaluated in acetone-to-isobutene reaction via cascade C–C coupling, self-deoxygenation, and C–C bond cleavage reactions ( Scheme 1 ), 11 and the results are shown in Fig. 1 .…”

“… 3 However, the observed inactivity in the NaSnBeta catalyst suggests that Sn species, lacking protons that could be titrated by Na + , are incapable of efficiently catalyzing one or multiple reaction steps in these cascade reactions, such as enolization, C–C coupling, decomposition, and ketonization. 11 …”

“…6,9,10 The synergy between water and balanced Lewis acid-base pairs (e.g., Zn-O-Zr) generates an appropriate local environment, enabling the production of isobutene from acetone (Scheme 1) with desirable catalyst stability and selectivity. 11 This reaction can also be catalyzed by Brønsted acidic zeolites, however, accompanied with catalyst deactivation even when water is present. 12 Among these studies, co-fed water is critical to mitigate the severe deactivation of catalysts utilized in the acetone-to-isobutene conversion.…”

“…12 Among these studies, co-fed water is critical to mitigate the severe deactivation of catalysts utilized in the acetone-to-isobutene conversion. 9,11,13 Due to the high temperature (473-723 K) needed for this reaction, necessitating signicant energy for water heating and potential post-treatment, it would be desirable to conduct this reaction without co-fed water. However, no known catalytic materials selectively catalyze this reaction without water, and stabilizing the isobutene precursor (e.g., diacetone alcoholderived intermediates) without water remains an unknow challenge.…”

“…3 However, the observed inactivity in the NaSnBeta catalyst suggests that Sn species, lacking protons that could be titrated by Na + , are incapable of efficiently catalyzing one or multiple reaction steps in these cascade reactions, such as enolization, C-C coupling, decomposition, and ketonization. 11 Basic or amphoteric metal oxides (e.g., ZnO, CeO 2 ) have been reported to accelerate enolization of acetone, 16 C-C coupling followed by decomposition of generated dimers, 17 and ketonization reactions. 18 To improve the performance of SnBeta catalysts, Ce species were introduced and evaluated under the same conditions.…”