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.
5-hydroxymethylfurfural (HMF) can be produced from the acid-catalyzed dehydration of fructose but its yield is limited due to subsequent HMF degradation to side products. A reactive adsorption process is proposed to improve the yield to HMF. Separate experimental singlecomponent isotherms of fructose, HMF, formic acid, and levulinic acid on carbon BP2000 and reaction kinetics of the fructose dehydration to HMF in aqueous solution of HCl are presented to develop empirical isotherms and kinetic rate constants, respectively. These sub models are subsequently integrated in an adsorptive reactor at a range of temperatures (100-180°C) with different loadings of adsorbent. It is shown that the adsorbent improves HMF yield compared to the single solution phase (adsorbent-free case). Low temperatures and high adsorbent loadings improve HMF yield. Under certain conditions both reactive adsorption and the commonly used reactive extraction can result in a similar improvement in HMF yield. HMF recovery from the solid adsorbent has been identified as a major challenge that can be ameliorated through
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