Enabling the selective conversion of biomass-derived oxygenates to C4-C5 dienes

Abstract

The catalytic conversion of biomass-derived saturated furans over zeotype solid acids affords a potentially renewable route to access conjugated C4-C5 dienes — commodity monomers in tires, plastics, adhesives, and resins. A lack of fundamental understanding of reaction mechanisms and pathways coupled with existing trial-and-error catalyst design approaches have limited diene yields to <60%. Poor catalyst lifetimes, attributed to rapid coking typical for oxygenate conversion reactions, have also remained a challenge. Improving the diene yields and mitigating catalyst deactivation are the first key steps to engender industrial interest in the resulting process technology. In this dissertation,we first highlight the mechanistic details of the tandem-ring opening and dehydration of tetrahydrofuran (THF) to butadiene on the aluminosilicate H-ZSM-5, which enable the formulation of the relative ratio of C-O to C-C scission rates as the diene selectivity descriptor. By considering aluminum-, and boron-substituted zeolites in 2-methyltetrahydrofuran (2-MTHF) dehydration to pentadienes, we demonstrate the weakening of solid acid strength as a strategy to tune this descriptor towards dienes’ production. By exploiting the thermodynamic stability of the desirable C5 conjugated diene (1,3-pentadiene), we further explicate strategies harnessing diffusional hurdles to suppress the production of its non-conjugated isomer (1,4-pentadiene). Combined, these insights lead to ~30% improvement in 1,3-pentadiene yield. Having discovered the utility of mild solid acids, we focus the rest of the dissertation on investigating the broad implications of weak surface binding in dehydration catalysis. Using two distinct classes of solid acid zeotype materials with weak Brønsted acidity (namely, borosilicates, and phosphorous-modified zeosils), we detail how these materials can potentially improve dehydration selectivity and stability, albeit often at a cost of lower overall turnover rates. Tying this discussion back to renewable dienes production on these materials, we conclude this work by underscoring the technological and economic improvements still required to achieve competitive diene prices from this process technology.