Enabling Selective Catalytic Conversion of Biomass-Derived Saturated Furans to C4-C5 Dienes

Conjugated C4-C5 dienes are key industrial monomers, currently recovered from naphtha crackers, with ethylene as the main product. Gradual shift towards cheap light feedstocks has resulted in refineries decoupling the production of ethylene and larger fractions including C4-C5 dienes, accentuating the need to produce this fraction from alternate routes. Previous reports have highlighted a thermochemical route to these dienes from the vapor-phase ring-opening dehydration (dehydra-decyclization) of biomass-derived saturated furans on solid Brønsted acid catalysts including zeolites. However, in addition to a fundamental lack of understanding of the reaction mechanisms and pathways, the diene yields have remained moderately low (50-65 % C basis).

First, combining results from kinetic experiments, DFT calculations, and microkinetic modeling, we highlighted the dehydra-decyclization and the Retro-Prins pathways for THF conversion to butadiene and propene, respectively, on H-ZSM-5. This study established that i) C-O cleavage exhibited the highest degree of rate-control, and ii) the selectivity control descriptor was ratio of C-O/C-C scission rates. We then undertook a detailed catalytic evaluation of zeolites with varying Brønsted acid strength (namely, Al-, and B-substituted zeolites) in different micropore environments (BEA, MFI, and MWW) to provide new insights on the role of heteroatom identity and confining environments in tuning C-O/C-C scission rates for 2-methyltetrahydrofuran (2-MTHF) dehydra-decyclization. While weakly acidic borosilicates selectively catalyzed desirable dehydra-decyclization pathways, micropore environments influenced diene distributions independent of heteroatom identity. Combining these learnings, we reported an unprecedented ~86% (per C basis) 1,3-pentadiene yield on B-MWW.A following techno-economic analysis of proposed furfural-to-butadiene process concluded that the process cannot currently compete with existing petrochemical routes due to high butadiene selling price ($5.43/kg), but we detailed the technological improvements and economic conditions to achieve butadiene selling price below $1.5/kg.