Highly Selective Cross-Etherification of 5-Hydroxymethylfurfural with Ethanol

Etherification of 5-hydroxymethylfurfural (HMF) and ethanol to yield ethoxymethylfurfural (EMF) is an important reaction for producing diesel blendstocks from biomass resources. This aqueous-phase etherification reaction occurs with higher selectivity on H-BEA (95%) than on Amberlyst-15 (76%), H-FAU (64%), H-MFI (88%), and H-MOR (63%) at high conversions (>60%). This selectivity toward EMF is not driven by transport limitations or Bronsted acid site concentrations; instead, a kinetic preference for etherification on BEA leads to such high yields. Nonidealities associated with the liquid-phase reaction are addressed by using UNIFAC-derived activities rather than concentrations, and reaction kinetics measurements indicate that the reaction proceeds on a surface saturated by ethanol with rates that are first order with respect to HMF activity and negative order with respect to ethanol activity. Gas-phase density functional theory calculations are unified with aqueous liquid-phase kinetics measurements by using partial pressures of a hypothetical gas phase calculated from UNIFAC-derived activities. DFT calculations indicate that etherification occurs in a single concerted step-rather than through a two-step sequential pathway-in which HMF is protonated and dehydrated to form a relatively stable methoxyfurfural carbocation (in contrast to ethanol dehydration). This methoxyfurfural carbocation is stabilized by resonance in contrast to ethyl carbocations formed from ethanol protonation and dehydration, and it also leads to the observed high selectivity for cross-etherification (to EMF) vs self-etherification of either HMF or ethanol. This rigorous mechanistic investigation uses gas-phase DFT calculations to offer insights into aqueous-phase catalysis, thereby elucidating an important reaction in biomass upgrading.