A combined experimental and density functional theory computational study was performed to understand the reaction mechanism of hydrodeoxygenation of phenol on the Pt(111) surface. Partial hydrogenation of phenyl ring reduces the barrier of deoxygenation. The intermediate formed by adding 5 H atoms to the phenyl ring and with α-C adsorption on Pt is identified as the key intermediate responsible for the formation of different products with mild barriers: deprotonating the hydroxyl to cyclohexanone at 0.38 eV, hydrogenation at α-C to cyclohexanol at 0.56 eV, and deoxygenation at 0.76 eV followed by dehydrogenation to benzene. Microkinetic parameter analysis indicates that the hydrogenation steps are fast and reversible while deoxygenation steps are slow and almost irreversible, which is consistent with the experimental observation that hydrogenation products are the major primary products at low conversions while deoxygenation product dominates at high conversions, at 523 K and ambient H2 pressure. H2 pressure plays an essential role on surface coverage of H and available adsorption sites, modulating the competition between hydrogenation and deoxygenation reactions and, thereby, the product distributions.

Conversion of propionic acid to gasoline-range molecules was investigated at 350 °C on a series of ZSM-5 catalysts with varying density of Brønsted acid sites (BAS), achieved by ion exchange of proton with Na+. Ketonization of propionic acid to 3-pentanone is the primary reaction, with the sequential aldol condensation to dipentanone alcohol being the secondary. The major reaction pathway for forming the aromatics involves dehydration, cyclization, dehydration and hydride transfer from dipentanone alcohol, leading to the formation of C10 aromatics before being dealkylated to lighter aromatics. Temperature programmed desorption of propionic acid indicates that the reaction initiates with acylium cation formation on BAS through dehydration. Comparing the turnover frequencies of ketonization and aldol condensation on ZSM-5 with varying density of BAS indicates that BAS is the active site for both reactions. The propionic acid feed deactivates the catalyst faster than the 3-pentantone feed due to a stronger adsorption of propionic acid on the acid sites of ZSM-5.Highlights•Conversion of propionic acid and 3-pentanone to hydrocarbons is achieved on HZSM-5.•Ketonization is the primary reaction while aldol condensation is the secondary.•Brønsted acid sites are the active sites for both ketonization and aldol condensation.•Ketonization is initiated with acylium cation formation through dehydration.Download high-res image (226KB)Download full-size image

Conversion of CO2 and CH4 to value-added products will contribute to alleviating the green-house gas effect but is a challenge both scientifically and practically. Stabilization of the methyl group through CH4 activation and facile CO2 insertion ensure the realization of C–C coupling. In the present study, we demonstrate the ready C–C coupling reaction on a Zn-doped ceria catalyst. The detailed mechanism of this direct C–C coupling reaction was examined based on the results from density functional theory calculations. The results show that the Zn dopant stabilizes the methyl group by forming a Zn–C bond, thus hindering subsequent dehydrogenation of CH4. CO2 can be inserted into the Zn–C bond in an activated bent configuration, with the transition state in the form of a three-centered Zn–C–C moiety and an activation barrier of 0.51 eV. The C–C coupling reaction resulted in the acetate species, which could desorb as acetic acid by combining with a surface proton. The formation of acetic acid from CO2 and CH4 is a reaction with 100% atom economy, and the implementation of the reaction on a heterogeneous catalyst is of great importance to the utilization of the greenhouse gases. We tested other possible dopants including Al, Ga, Cd, In, and Ni and found a positive correlation between the activation barrier of C–C coupling and the electronegativity of the dopant, although C–H bond activation is likely the dominant reaction on the Ni-doped ceria catalyst.

•Co/NaY was prepared by impregnation of NaY with ethanol-cobalt nitrate solution.•Co/NaY was applied in the ethyl lactate hydrogenation.•100% conversion and 96% selectivity to 1,2-PDO was achieved.•The structural characteristics were affected by the condition of preparation.•Conversion of ethyl lactate correlates linearly with number of surface metallic Co.Co/NaY catalysts prepared by impregnation of NaY with a cobalt nitrate-ethanol solution were used to catalyze liquid-phase hydrogenation of ethyl lactate to 1,2-propanediol (1,2-PDO). To elucidate the effect of calcination conditions and cobalt loading, the catalysts were characterized using XRD, N2 physical adsorption, H2-TPR, XPS, H2-TPD, and TEM. The results show that both high calcination temperature and high amount of Co loading result in relatively large Co3O4 particles. The reduced cobalt catalysts consist of three cobalt species, including metallic Co, cobalt oxides and cobalt species interacting with zeolite (cobalt aluminate/silicate), due to incomplete reduction of cobalt species. The absence of the characteristic peak of metallic Co in the XRD pattern confirms its high dispersion on NaY, in agreement with the TEM results. H2-TPD effectively quantifies the number of surface metallic Co sites, which correlates linearly with the ethyl lactate conversion. This correlation indicates that metallic Co is the active site for ethyl lactate hydrogenation. A complete conversion of ethyl lactate with a selectivity of 96% to 1,2-PDO has been achieved over the 6Co/NaY-673-5 catalyst.

The adsorption, dissociation, and hydrogenation of phenol on the Pt(111) and Pd(111) surfaces have been studied using density functional theory slab calculations. The results show that phenol favors adsorption through a mixed σ–π interaction on both surfaces through its phenyl ring, with the hydrogen atoms and hydroxyl tilted away from the surface. The dissociation of phenol to phenoxy is both thermodynamically and kinetically favored on Pd but not on Pt. The phenoxy adsorbs on Pd through both the phenyl ring and the oxygen atom, whereas the O atom points away from the surface on Pt. On Pt, the barrier for adding one hydrogen atom to the adsorbed phenol is 0.49 eV lower than the overall barrier for phenol dissociation to phenoxy followed by adding the hydrogen atom to its phenyl ring, resulting in direct hydrogenation of the adsorbed phenol to cyclohexanol as the dominant reaction pathway. In contrast, on Pd, the barrier for direct hydrogenation (1.22 eV) is higher than the overall barrier of dissociation followed by the hydrogenation process (0.85 eV), resulting in hydrogenation of the adsorbed phenoxy to cyclohexanone as the major reaction pathway. Microkinetics analysis confirms that hydrogenation of the adsorbed phenol is the dominant pathway on Pt, whereas phenoxy hydrogenation drives the turnover on Pd. These results are consistent with the experimentally observed selectivity of phenol hydrogenation on Pd and Pt catalysts.Keywords: cyclohexanol; cyclohexanone; DFT; Pd; phenol; Pt; selective hydrogenation

•Selective deoxygenation of m-cresol to toluene is achieved on bimetallic Ni–Re.•Re increases Ni dispersion and results in Ni–Re surface alloy formation.•Re breaks the Ni surface into small ensembles, with Ni being electron-deficient.•C and O adsorb onto Ni and Re neighboring sites, which facilities deoxygenation.•CC hydrogenolysis is inhibited by both geometric and electronic effects.Ni–Re/SiO2 bimetallic catalysts were prepared using a co-impregnation method and tested in vapor phase hydrodeoxygenation of m-cresol at 300 °C and 1 atm H2. In contrast to the use of unselective monometallic Ni/SiO2 for catalyzing deoxygenation, hydrogenation, and CC hydrogenolysis reactions, bimetallic 5%Ni–2.5%Re/SiO2 improved the intrinsic reaction rate of the hydrodeoxygenation reaction by a factor of 6, with the turnover frequency for selective deoxygenation to toluene increased by four times, while that for CC hydrogenolysis to methane was reduced by one-half. Characterization results from X-ray diffraction, Raman, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy of CO adsorption, H2 temperature-programmed reduction, and CO chemisorption indicate that adding Re increased Ni dispersion and resulted in Ni–Re surface alloy formation after reduction. The presence of Re in the surface alloy breaks the continuous Ni surface into smaller ensembles (geometric effect) and reduces the d-band electron density of Ni (electronic effect). Results from density functional theory calculations indicate that the Ni–Re neighboring site is the active site for breaking the CO bond by adsorbing the O atom on Re and the phenyl ring on the neighboring Ni atoms, which facilitates deoxygenation to toluene. The reduced Ni ensemble size inhibits the hydrogenolysis of the CC bond by destabilizing the transition state, whereas the reduction of the electronic density in d states of Ni weakens the adsorption of the phenyl ring, and both contribute to the greatly reduced methane production from successive CC hydrogenolysis.Download high-res image (64KB)Download full-size image