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.

We conduct a combined experimental and computational study to reveal the kinetics of tandem glucose isomerization and fructose dehydration to 5-hydroxymethylfurfural (HMF) over a

INTRODUCTIONFermentation-derived butanol is a potential renewable fuel that has several advantages over other biofuels, including a higher energy density than ethanol and better solubility with current hydrocarbon fuels because of its relative nonpolarity. However, the recovery of the product from ABE fermentation broths is complicated by the low concentrations inherently yielded by the fermentation process because of the cytotoxicity of Clostridium acetobutylicum strains. 1 Because the normal boiling point of butanol is higher than that of water, the traditional recovery method of distillation requires high energy inputs from water boilup and is further complicated by a heterogeneous azeotrope at 365 K. 2 Overall, the energy demand is of comparable magnitude to the energy content of the product itself. 3 Common alternatives to distillation include pervaporation, liquidÀliquid extraction, osmosis, and adsorption-based techniques. 1 Butanol recovery by adsorption is predicted to have an average energy demand that is ∼10% of the heat of combustion of butanol and is easily implemented. 1 Common adsorbents typically investigated are activated carbons, 3 zeolites, 4 and polymers. 5 These adsorbents are generally inexpensive and have good capacity but are limited in their ability to be modified systematically on the molecular length scale. The alternative approach pursued here is to synthesize solid materials modified with organic groups that act as adsorption sites. Cavitands (intrinsically microporous organic molecules such as calixarenes and cyclodextrins) immobilized on solid supports make up one subclass of such materials that has been used in sensing, chromatography, and the separation of contaminants from wastewater 6À12 but remains relatively unexplored in the context of biofuel separations. Advantages to this approach of materials design are that (i) the structure and number of adsorption sites are, in principle, known a priori and tunable, (ii) understanding the adsorption behavior of the hybrid material is aided by solution and solid-state cavitand/ guest complexation, and (iii) sites can be optimized for adsorption strength or selectivity independent of the optimization of the support for robustness and surface area.Herein is presented the synthesis, characterization, and analysis of a class of adsorbents consisting of hydroxycalixarenes immobilized directly onto chlorinated silica surfaces. 7 Although calixarene-based adsorbents or separation media are known, the materials described here are distinguished by the lack of a flexible organic tether or polymer chain typically employed to attach the calixarene to the surface. 13 These directly attached calixarenes ABSTRACT: Materials were synthesized for the recovery of nbutanol from dilute aqueous solutions, as may be useful for applications in biofuelÀwater separations. These materials are composed of hydrophobic, cavity-containing calixarenes covalently bound directly to porous, hydrophilic silica supports through a Si linker atom rather than a flexible orga...

A kinetic model for the tandem conversion of glucose to 5-hydroxymethylfurfural (HMF) through fructose in aqueous CrCl3-HCl solution was developed by analyzing experimental data. We show that the coupling of Lewis and Brønsted acids in a single pot overcomes equilibrium limitations of the glucosefructose isomerization leading to high glucose conversions and identify conditions that maximize HMF yield. Adjusting the HCl/CrCl3 concentration has a more pronounced effect on HMF yield at constant glucose conversion than that of temperature or CrCl3 concentration. This is attributed to the interactions between HCl and CrCl3 speciation in solution that leads to HMF yield being maximized at moderate HCl concentrations for each CrCl3 concentration. This volcano-like behavior is accompanied with a change in the rate-limiting step from fructose dehydration to glucose isomerization as the concentration of the Brønsted acid increases. The maximum HMF yield in a single aqueous phase is only modest and appears independent of catalysts' concentrations as long as they are appropriately balanced. However, it can be further maximized in a biphasic system. Our findings are consistent with recent studies in other tandem reactions catalyzed by different catalysts.