Mechanistic role of protonated polar additives in ethanol for selective transformation of biomass-related compounds

Protonated polar additives control selectivity and stabilize intermediates in acid catalyzed reactions. [Display omitted] •Polar additives in ethanol catalytically protonate as DMSO > GVL > ACN > Acetone > THF.•Protonation of polar additive proportional to its Kamlet-Taft (KT) π* value.•High KT π* value of polar additive gives high (82 %) 5-HMF selectivity from fructose.•Water product oxygen atom originates from 5-HMF hydroxyl group in etherification step.•Protonated polar additives in ethanol inhibit side-reactions in Fischer esterification. We report on a combined experimental, spectroscopic and theoretical study of acid catalysed dehydration-etherification of fructose in ethanol for understanding the mechanistic role of polar solvent additives and product selectivity. Herein, we show that polar solvent additives (e.g. tetrahydrofuran, acetone, acetonitrile, gamma-valerolactone, dimethyl sulfoxide) protonated with a common solid acid catalyst in ethanol allow transformation of biomass-related compounds into desired dehydration or etherification products. Fructose in ethanol with DMSO additive is selectively transformed into 5-hydroxymethylfurfural with negligible formation of 5-ethoxymethylfurfural due to preferential DMSO protonation according to its polarity. Spectroscopic methods and density functional theory show that additives having higher polarity than ethanol are readily protonated and act as the key catalytic protonation species and as the key stabilization species for reaction intermediates. Understanding the mechanism of protonated polar additives in reaction systems allows one to tailor selectivity in acid-catalyzed dehydration-etherification schemes and to develop sustainable chemistry for biomass resources.