Effects of chloride ions in acid-catalyzed biomass dehydration reactions in polar aprotic solvents

Abstract

The use of polar aprotic solvents in acid-catalyzed biomass conversion reactions can lead to improved reaction rates and selectivities. We show that further increases in catalyst performance in polar aprotic solvents can be achieved through the addition of inorganic salts, specifically chlorides. Reaction kinetics studies of the Brønsted acid-catalyzed dehydration of fructose to hydroxymethylfurfural (HMF) show that the use of catalytic concentrations of chloride salts leads to a 10-fold increase in reactivity. Furthermore, increased HMF yields can be achieved using polar aprotic solvents mixed with chlorides. Ab initio molecular dynamics simulations (AIMD) show that highly localized negative charge on Cl − allows the chloride anion to more readily approach and stabilize the oxocarbenium ion that forms and the deprotonation transition state. High concentrations of polar aprotic solvents form local hydrophilic environments near the reactive hydroxyl group which stabilize both the proton and chloride anions and promote the dehydration of fructose.

Monosaccharides Dehydration Assisted by Formation of Borate Esters of α-Hydroxyacids in Choline Chloride-Based Low Melting Mixtures

“…These results follow the same trend as highlighted in the work of Mellmer et al ( 2019 ). The author studied D-fructose dehydration in γ-valerolactone with Bronsted acid and different salts.…”

Glycosidic C–O Bond Activation in Cellulose Pyrolysis: Alpha Versus Beta and Condensed Phase Hydroxyl-Catalytic Scission

“…This ordering of hydroxyl groups around active centers to stabilize transition states appears to be qualitatively similar to unique ordering of solvents around active centers that has been observed in recent studies on acid-catalyzed dehydration reactions in solvent mixtures. 56,57 These AIMD results suggest that even in a solid/molten cellulosic matrix, which is not physically similar to a conventional homogeneous solvent system, local hydroxyl groups tend to arrange around charged O centers as nested sites to attain the increased stability offered by such H-bonding networks.…”

Fast predictions of liquid-phase acid-catalyzed reaction rates using molecular dynamics simulations and convolutional neural networks

“…33, since chloride can impact reaction kinetics. 6 The test set results show that SolventNet performs well for DMSO-water mixtures with an RMSE of 0.43. This result is somewhat surprising because DMSO is more basic than the cosolvents used for training 3 and is known to compete against water for binding sites around hydroxyl groups on hydrophilic reactants.…”

“…5 In the past decade, ab initio quantum chemical methods have been used to quantify solvent effects on barriers to elementary steps for biomass conversion reactions. 1,5,6 Using ab initio molecular dynamics (MD) simulations, Mellmer et al found that the inclusion of organic cosolvents increases biomass conversion reaction kinetics by lowering the activation energy due to changes to the solvation environment around the acidic proton catalyst, the reactant, and charged transition states (Fig. 1b).…”