Metal-Free Synthesis of HMF from Glucose Using the Supercritical CO₂–Subcritical H₂O–Isopropanol System

Abstract

5-Hydroxymethylfurfural (HMF) is a promising building block for the production of biofuels and fine chemicals. While HMF can be derived from biomass, specifically sugars, its synthesis typically requires the use of metal-based catalysts and ionic liquids. In this study, the supercritical CO 2 −subcritical H 2 O−isopropanol system was proposed as a green reaction medium for the conversion of glucose to HMF. The response surface methodology based on the Box-Behnken design was used to investigate the effect of temperature, pressure, and isopropanol concentration on the yield of HMF. From the quadratic models, the highest HMF yield was predicted to be 38.3% with a corresponding glucose conversion of 87.5% at 200 °C, 16 MPa, and 8 vol % isopropanol. A mechanistic study based on the density functional theory was also performed and the results revealed that isopropanol can facilitate the various hydride shifts that occur in the conversion of glucose to HMF.

Production of furans from C5 and C6 sugars in the presence of polar organic solvents

“…30 Protic solvents, like isopropanol and ethanol, have been successfully used both in HMF and furfural synthesis. [89][90][91][92][93] The use of a protic solvent is justified because of the beneficial suppressing effect on the formation of humic byproducts by sugar-furan condensation. [89][90][91][92][93] In fact, the in-situ formation of an alkyl-glucoside has been hypothesized to protect the sugar, suppressing parallel condensation reactions that result in the formation of humic by-products (Scheme 3).…”

“…[89][90][91][92][93] The use of a protic solvent is justified because of the beneficial suppressing effect on the formation of humic byproducts by sugar-furan condensation. [89][90][91][92][93] In fact, the in-situ formation of an alkyl-glucoside has been hypothesized to protect the sugar, suppressing parallel condensation reactions that result in the formation of humic by-products (Scheme 3). 89 Köchermann et al have shown that, while side reactions seem to be suppressed, the presence of ethanol below 50 vol% does not influence the rate of xylose consumption.…”

“…89,90 This approach was also successfully applied using heterogeneous and homogeneous catalysis conditions, but the most promising results in terms of sugar-to-furan selectivity have been observed under homogeneous catalysis conditions. [89][90][91][92][93] Specifically, about 75 mol% xylose-to-furfural selectivity was obtained using a water-ethanol (1:1 v-v) mixture, using H2SO4 as acid catalyst. 89 Among the alcohols, n-butanol has the specific characteristic of forming a biphasic system with water at T < 80 o C, transitioning into a monophasic system at T > 80 o C. 95 The use of different organic solvents, like formic acid, as additives to the aqueous phase, has also been reported.…”

Furfural manufacture from xylose : routes towards high selectivity and novel processes

“…29 Protic solvents, like isopropanol and ethanol, have been successfully used both in HMF and furfural synthesis. [64][65][66][67] The use of a protic solvent is justified because of the beneficial suppressing effect on the formation of humic byproducts by sugar-furan condensation. 64,67 In fact, the in-situ formation of an alkylglucoside has been hypothesized to protect the sugar, suppressing parallel condensation reactions that result in the formation of humic by-products (Scheme 2.3).…”

“…64 This approach was also successfully applied using heterogeneous and metal-free catalysis conditions, but the most promising results in terms of sugar-to-furan selectivity have been observed under homogeneous catalysis conditions. [64][65][66][67] Specifically, about 75 mol% xylose-to-furfural selectivity was obtained using a waterethanol (1:1 v-v) mixture, using H2SO4 as acid catalyst. 64 Among the alcohols, n-butanol has the specific characteristic of forming a biphasic system with water at T < 80 o C, transitioning into a monophasic system at T > 80 o C. 68 Scheme 2.3 Possible reaction pathway for the formation of the ethyl-xyloside and 2-(diethoxymethyl)-furan in a water-ethanol solvent system.…”