5-(Hydroxymethyl)furfural (HMF) and levulinic acid production from glucose in a cascade of reactions using a Lewis acid (CrCl3) catalyst together with a Brønsted acid (HCl) catalyst in aqueous media is investigated. It is shown that CrCl3 is an active Lewis acid catalyst in glucose isomerization to fructose, and the combined Lewis and Brønsted acid catalysts perform the isomerization and dehydration/rehydration reactions. A CrCl3 speciation model in conjunction with kinetics results indicates that the hydrolyzed Cr(III) complex [Cr(H2O)5OH](2+) is the most active Cr species in glucose isomerization and probably acts as a Lewis acid-Brønsted base bifunctional site. Extended X-ray absorption fine structure spectroscopy and Car-Parrinello molecular dynamics simulations indicate a strong interaction between the Cr cation and the glucose molecule whereby some water molecules are displaced from the first coordination sphere of Cr by the glucose to enable ring-opening and isomerization of glucose. Additionally, complex interactions between the two catalysts are revealed: Brønsted acidity retards aldose-to-ketose isomerization by decreasing the equilibrium concentration of [Cr(H2O)5OH](2+). In contrast, Lewis acidity increases the overall rate of consumption of fructose and HMF compared to Brønsted acid catalysis by promoting side reactions. Even in the absence of HCl, hydrolysis of Cr(III) decreases the solution pH, and this intrinsic Brønsted acidity drives the dehydration and rehydration reactions. Yields of 46% levulinic acid in a single phase and 59% HMF in a biphasic system have been achieved at moderate temperatures by combining CrCl3 and HCl.

Steady state, isotopic, and chemical transient studies of ethanol dehydration on γ-alumina show unimolecular and bimolecular dehydration reactions of ethanol are reversibly inhibited by the formation of ethanol−water dimers at 488 K. Measured rates of ethylene synthesis are independent of ethanol pressure (1.9−7.0 kPa) but decrease with increasing water pressure (0.4−2.2 kPa), reflecting the competitive adsorption of ethanol−water dimers with ethanol monomers; while diethyl ether formation rates have a positive, less than first order dependence on ethanol pressure (0.9−4.7 kPa) and also decrease with water pressure (0.6−2.2 kPa), signifying a competition for active sites between ethanol−water dimers and ethanol dimers. Pyridine inhibits the rate of ethylene and diethyl ether formation to different extents verifying the existence of acidic and nonequivalent active sites for the dehydration reactions. A primary kinetic isotope effect does not occur for diethyl ether synthesis from deuterated ethanol and only occurs for ethylene synthesis when the β-proton is deuterated; demonstrating olefin synthesis is kinetically limited by either the cleavage of a C β -H bond or the desorption of water on the γalumina surface and ether synthesis is limited by the cleavage of either the C−O bond of the alcohol molecule or the Al−O bond of a surface bound ethoxide species. These observations are consistent with a mechanism inhibited by the formation of dimer species. The proposed model rigorously describes the observed kinetics at this temperature and highlights the fundamental differences between the Lewis acidic γ-alumina and Brønsted acidic zeolite catalysts.

The mechanism and kinetics for ethanol coupling to n-butanol over hydroxyapatite (HAP) were investigated at 573-613 K. In-situ titration experiments show that the active sites for acetaldehyde and butanol formation are different. In combination with FTIR studies, it was found that ethanol dehydrogenation is catalyzed by Ca-O sites, whereas condensation of acetaldehyde is catalyzed by CaO/PO4 3pairs. Measurements of the reaction kinetics at various ethanol (3.5-9.4 kPa) and acetaldehyde (0.055-0.12 kPa) partial pressures reveal that direct condensation involving two ethanol molecules does not play a significant role in butanol formation; instead, n-butanol is formed via a Guerbet pathway. At a constant acetaldehyde pressure, enolate formation is rate-limiting and ethanol inhibits acetaldehyde condensation rates by competitive adsorption. A model of the reaction kinetics consistent with all experimental observations is developed.