Kinetics of Homogeneous Brønsted Acid Catalyzed Fructose Dehydration and 5-Hydroxymethyl Furfural Rehydration: A Combined Experimental and Computational Study

Abstract

We perform the first extensive experimental kinetic studies of fructose dehydration and 5-hydroxymethyl furfural (HMF) rehydration at low temperatures over a wide range of conditions (T ∼ 70–150 °C; pH values 0.7–1.6 and initial concentrations of fructose (5–20%w/v) and HMF (2.5–10%w/v)). Guided from insights from our first-principles calculations, we perform kinetic isotope effect (KIE) experiments of labeled fructose to validate the rate-limiting step. Subsequently, we develop the first skeleton model for fructose dehydration and HMF rehydration that integrates the fundamental kinetic experiments and accounts for the KIE, as well as the distribution of fructose tautomers, which changes significantly with temperature, and a direct path of fructose conversion to formic acid. It is shown that the skeleton mechanism of two steps consisting of fast protonation and dehydration followed by intramolecular hydride transfer as the rate-limiting step can capture the experimental kinetics and KIE experiments well. Fructose dehydration is found to result in stoichiometric excess of formic acid relative to levulinic acid, produced directly from fructose. All reactions are shown to be pseudo-first order in both catalyst and substrate. These insights are incorporated in a continuous flow reactor model; higher temperatures improve the optimum yield of HMF, while HMF selectivity at low conversions is less sensitive to temperature.

Influence of the pH Value on the Hydrothermal Degradation of Fructose

“…However, this can be compensated by applying a higher temperature. This might be even beneficial for achieving high maximal HMF yields, as it is reported that the selectivity of HMF formation increases with increasing temperature …”

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

“…[26] At 130 o C, the FA/LA yield is higher but humins remain the dominant product. [26] At 130 o C, the FA/LA yield is higher but humins remain the dominant product.…”

“…[14] Parameter estimation involves fitting the transient profiles of glucose, mannose, fructose, HMF, and LA using the fminunc minimization routine in Matlab R2013a. [26] Results and discussion [26] Results and discussion…”

“…The activation energies of Brønsted-acid catalyzed HMF rehydration to FA/LA and HMF degradation to humins are similar, [7] so the change in selectivity of the HMF consumption paths with temperature underscores that 9 there is a Lewis acid-catalyzed path of humin formation from HMF. [26] Given the high yield to humins, we hypothesize that FA is mainly produced as a byproduct of HMF condensation leading to humins via a Lewis-acid catalyzed path. This situation differs from typical Brønsted acid rehydration chemistry that produces an equimolar amount of FA and LA.…”

Hemicellulose arabinogalactan hydrolytic hydrogenation over Ru-modified H-USY zeolites

“…Furthermore, substantial formation of high molecular weight compounds, that is, aggregates of sugars and humins (polydispersed heterogeneous carbonaceous materials), occurred. Mechanistic aspects of humins formation have recently been addressed [20][21][22][23][24][25]. Their formation mechanism has been related to polycondensation reactions giving a network of furan rings linked by ether or acetal bonds [20].…”