Search

[Display omitted] • Direct reduction of ester to ether requires Brønsted acid site close to metal site. • Strong Brønsted acid sites promote cleavage of the C O bond of ester. • Kinetically relevant C O bond cleavage on Brønsted acid sites for ether formation. Esters represent an important class of reagents and intermediates for the production of fine chemicals and polymers. In prior studies of homogeneous catalysis, molecular complexes have been reported to selectively cleave either carbonyl C O or acyl C O bonds of ester to ether or alcohols, respectively. In contrast, reactions of esters and H 2 upon heterogeneous catalysts typically cleave acyl C O bonds and produce alcohols. Here, we demonstrate that the proximity of Brønsted acid sites to Pd nanoparticles and the thermodynamic strength of these Brønsted acid sites influence the rates and selectivities toward C O bond cleavage pathways either to ethers or alcohols during reactions of esters over bifunctional solid catalysts. The combined results from rate measurements as functions of H 2 pressure, ester conversion, and methods to combine Pd nanoparticles and acidic supports; kinetic isotope effects; in situ Brønsted acid site titrations; and calorimetric assessments of Brønsted acid strength provide evidence for bifunctional pathways that require proximity between Brønsted acid sites and Pd atoms to form ethers. These findings suggest that Brønsted acid sites near Pd nanoparticles and with lower deprotonation energies promote the direct reduction of esters to ethers by cleaving the carbonyl C O bond. Taken together, these data indicate that direct reduction of esters to ethers involves the hydrogenation of the ester reactant to form hemiacetal at Pd nanoparticles, followed by dehydration of the hemiacetal at proximal acid sites, and subsequent hydrogenation of the enol ether. In comparison, hydrogenolysis of acyl C O bonds of the ester reactant involves the reaction of a hydrogenated intermediate (plausibly hemiacetal) upon Pd nanoparticles to form the corresponding alcohol and aldehyde products. Among the materials examined, Pd nanoparticles supported on WO 3 catalyze the direct reduction of esters and lactones to corresponding ethers and remain stable over extended periods of time on stream (∼42 h). These findings offer a foundation for further design and improvement of heterogeneous catalysts for the selective reduction of esters and other challenging hydrogenations. [ABSTRACT FROM AUTHOR]

The reaction mechanism of glycerol hydrogenolysis to 1,2-propanediol over a spinel CuCr2O4 catalyst was investigated by using DFT calculations. Theoretical models were developed from the results of experimental characterization. Adsorption configurations and energetics of the reactant, intermediates, final product, and transition states were calculated on Cu(1 1 1) and CuCr2O4(1 0 0). Based on our DFT results, we found that the formation of acetol is preferred to that of 3-hydroxypropionaldehyde thermodynamically and kinetically on both surfaces. For glycerol hydrogenolysis to 1,2-propanediol, the CuCr2O4 surface is less exothermic but more kinetically favorable than the Cu surface. The low activation barrier during the reaction on the CuCr2O4 surface is attributed to the unique surface structure; the cubic spinel structure provides a stable adsorption site on which reactants are allowed to be dehydrated and hydrogenated easily with the characteristic adsorption configuration. The role of the Cu and Cr atoms in a CuCr2O4 surface were revealed. The results of reaction tests supported our theoretical calculations. [ABSTRACT FROM AUTHOR]

Hierarchical porous catalysts have recently attracted increasing interest because of the enhanced accessibility to active sites on such materials. In this context, previously reported hierarchically mesoporous ASN and ASPN materials are evaluated by applying them to the dehydration of glycerol, and demonstrate excellent catalytic performance. In addition, a comprehensive understanding of the effects of pore structures and the acid properties on the reaction through comparative studies with microporous HZSM-5 and mesoporous AlMCM-41 is provided. [ABSTRACT FROM AUTHOR]

Abstract: Samples of nanocrystalline copper aluminate (CuAl2O4) were prepared using various calcination temperatures and surfactant concentrations of citric acid via a sol–gel method for the hydrogenolysis of glycerol. A variety of techniques, including TPR, H2-TPD, XRD and XPS, were used to characterize the samples. Crystallized CuAl2O4 was initially observed after calcination at the temperature of 600°C and only peaks corresponding to CuAl2O4 spinel crystal were observed at 800°C. The citric acid concentration influenced the size of the CuAl2O4 crystal (ca. 10–30nm) and the electron state of Cu after reduction, which optimized the ratio of metal precursor cations and citric acid with 1 to 2. Catalytic activities of the prepared catalysts were examined for the hydrogenolysis of glycerol to 1,2-PDO. CuAl2O4 catalyst calcined at 800°C, which contains only the CuAl2O4 crystalline phase, showed the highest catalytic performance with over a 90% conversion and selectivity. This can be attributed to the exceptional reducibility of Cu species and hydrogen adsorption/desorption ability among the catalysts studied. [Copyright &y& Elsevier]

The reaction mechanism of glycerol hydrogenolysis to 1,2-propanediol over a spinel CuCr 2 O 4 catalyst was investigated by using DFT calculations. Theoretical models were developed from the results of experimental characterization. Adsorption configurations and energetics of the reactant, intermediates, final product, and transition states were calculated on Cu(1 1 1) and CuCr 2 O 4 (1 0 0). Based on our DFT results, we found that the formation of acetol is preferred to that of 3-hydroxypropionaldehyde thermodynamically and kinetically on both surfaces. For glycerol hydrogenolysis to 1,2-propanediol, the CuCr 2 O 4 surface is less exothermic but more kinetically favorable than the Cu surface. The low activation barrier during the reaction on the CuCr 2 O 4 surface is attributed to the unique surface structure; the cubic spinel structure provides a stable adsorption site on which reactants are allowed to be dehydrated and hydrogenated easily with the characteristic adsorption configuration. The role of the Cu and Cr atoms in a CuCr 2 O 4 surface were revealed. The results of reaction tests supported our theoretical calculations., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)