The Role of Salts and Brønsted Acids in Lewis Acid‐Catalyzed Aqueous‐Phase Glucose Dehydration to 5‐Hydroxymethylfurfural

Abstract

The effect of salts and Brønsted acids on the Lewis acid (CrCl3⋅6 H2O)‐catalyzed glucose dehydration to 5‐hydroxymethylfurfural (HMF) in aqueous media are described. We show that the reaction with bromide salts in place of chlorides leads to higher HMF yields. The influence of salts can be attributed to the anions in solution, specifically to the bromide anions enhancing the fructose dehydration step. Additionally, we demonstrate that the reaction kinetics are governed strongly by acidity. Although the fructose dehydration step is accelerated by the addition of Brønsted acids, even on a catalytic scale, a significant retardation of the glucose conversion rate results in a substantial drop in HMF yields. The suppression in glucose‐to‐fructose isomerization rate with increasing acidity is ascribed to the restrained formation of the chromium–glucose chelate complex during the reaction.

Microwave-enhanced aqueous biphasic dehydration of carbohydrates to 5-hydroxymethylfurfural

“…The positive effect of halide ions on HMF yields decreased in the order Br > Cl > I s F, which is in agreement with previous results using organic solvents or comparable biphasic system under conventional heating. 5,17 Notably, the use of MeCN or biomass-derived gvalerolactone (GVL), the latter of which has been extensively studied as a renewable solvent for biomass conversion, 26 in place of traditionally used MIBK/2-BuOH, THF, DMF or alcoholic solvents, such as 2-BuOH and i-PrOH, as an organic phase resulted in faster fructose conversion rate and better HMF selectivity and yields. The high HMF yield of 85% and 84% obtained with KBr/MeCN and KBr/GVL systems are comparable to those of obtained in ionic liquids and high-boiling organic solvents, 4 and considerably better than obtained in aqueous monophasic solutions.…”

“…17 The strong retardation of the glucose-to-fructose isomerization, arising from the presence of Brønsted acid, resulted in lower substrate conversions, HMF selectivity and yield. According to our results, this tendency can be avoided by conducting the reaction rapidly at high temperature under MW irradiation.…”

“…17 This was ascribed to the Cl À and Br À anions inu-encing the reaction by enhancing the fructose dehydration step (the reaction proceeds through fructose intermediate). 5 Also, in aqueous biphasic systems the addition of an excess of salt is Scheme 1 Fructose conversion to HMF and its transformation to valuable products.…”

Tandem Lewis/Brønsted homogeneous acid catalysis: conversion of glucose to 5-hydoxymethylfurfural in an aqueous chromium(iii) chloride and hydrochloric acid solution

“…Glucose primarily isomerizes to fructose at short times. [14,23] To explain this phenomenon, we proposed that the ionic species Cr(H2O)5OH 2+ is the active species for the isomerization and the addition of HCl shifts the hydrolysis reaction reducing the concentration of the active centers. Humins also form.…”

“…[14,15] The Davis group obtained HMF yields in excess of 50% in a one-pot reactor containing Lewis acidic Sn-Beta and HCl. [15] Following Davis' group pioneering work, many other tandem Lewis/Brønsted acid catalysts were investigated, [16] including aluminosilicate zeolites, [17] activated carbons, [18] and homogeneous metal chlorides, [19] such as CrCl3 [14,[20][21][22][23] and AlCl3. [24] For the homogeneous Lewis acid CrCl3/Brønsted acid HCl single pot catalysis, we revealed rather complex and unexpected interactions that render understanding experimentally challenging.…”