Propylene oxide (PO) is a versatile chemical, mainly used in the synthesis of polyurethane plastics. Propylene epoxidation using molecular oxygen could replace the tedious current synthesis protocols, which use expensive H 2 O 2 or organic peroxides as oxidants. This review focuses on the propylene epoxidation reaction using molecular oxygen in the gas phase over copper-and silver-based catalysts. Silver is a proven and industrially used ethylene epoxidation catalyst. However, it initiates allylic hydrogen stripping (AHS) in propylene epoxidation, shifting the selectivity toward unwanted acrolein and total oxidation. Nevertheless, silver has been extensively studied to determine if AHS could be mitigated by targeted active site design and various doping strategies. Copper-based catalysts have been less extensively studied but have been experimentally proved as well as theoretically confirmed that their PO selectivity is on par with that of silver. In this review, different catalyst modification strategies have been analyzed and the achieved improvements discussed. Theoretical approaches aimed at understanding the mechanism and predicting catalytic performance on the basis of electronic states (density functional theory calculations) are also reviewed. We conclude with a future outlook on how the current state of the art knowledge of active site modification and reaction engineering approaches could leverage the PO selectivity toward industrial requirements, thus enabling a breakthrough in gas-phase propylene epoxidation using O 2 .

The methane activation and methane dry reforming reactions were studied and compared over 4 wt % Ni/CeO2 and 4 wt % Ni/CeZrO2 (containing 20 wt % Zr) catalysts. Upon the incorporation of Zr into the ceria support, the catalyst exhibited a significantly improved activity and H2 selectivity. To understand the effects of the Zr dopant on Ni and CeO2 during the dry reforming of methane (DRM) reaction and to probe the structure–reactivity relationship underlying the enhanced catalytic performance of the mixed-oxide system, in situ time-resolved X-ray diffraction (TR-XRD), X-ray absorption fine structure (XAFS), and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) were employed to characterize the catalysts under reaction conditions. TR-XRD and AP-XPS indicate that ceria–zirconia supported Ni (Ni/CeZrO2) is of higher reducibility than the pure ceria supported Ni (Ni/CeO2) upon the reaction with pure CH4 or for the methane dry reforming reaction. The active state of Ni/CeZrO2 under optimum DRM conditions (700 °C) was identified as Ni0, Ce3+/Ce4+, and Zr4+. The particle size of both nickel and the ceria support under reaction conditions was analyzed by Rietveld refinement and extended XAFS fitting. Zr in the ceria support prevents particle sintering and maintains small particle sizes for both metallic nickel and the partially reduced ceria support under reaction conditions through a stronger metal–support interaction. Additionally, Zr prevents Ni migration from the surface into ceria forming a Ce1–x Ni x O2–y solid solution, which is seen in Ni/CeO2, thus helping to preserve the active Ni0 on the Ni/CeZrO2 surface.

No abstract

No abstract

Bio-based levulinic acid (LA), furfural (FF), and hydroxymethylfurfural (HMF) represent key chemical intermediates when biorefining biomass resources, i.e., either cellulose, glucose, hexoses, etc. (HMF/LA), or hemicellulose, xylose, and pentose (FF). Despite their importance, their online in situ detection by process analytical technologies (PATs), solubility, and its temperature dependence are seldom available. Herein, we report their solubility and temperature dependence by examining n-hexane, cyclohexane, benzene, toluene, 1,4-dioxane, diethyl ether, dichloromethane, tetrahydrofuran, ethyl acetate, acetone, dimethylformamide, acetonitrile, dimethyl sulfoxide, formic acid, n-butanol, n-propanol, ethanol, methanol, and water. These solvents were selected as they are the most common nonpolar, polar aprotic, and polar protic solvents. Fourier-transform infrared (FTIR) spectroscopy was applied as a fast, accurate, and sensitive method to the examined solutions or mixtures. The latter also enables operando monitoring of the investigated compounds in pressurized reactors. Selected temperatures investigated were chosen, as they are within typical operating ranges. The calculated thermodynamic data are vital for designing biorefinery process intensification, e.g., reaction yield optimization by selective compound extraction. In addition to extracting, upstream or downstream unit operations that can benefit from the results include dissolution, crystallization, and precipitation.

Dry reforming of methane (DRM) is a promising way to convert methane and carbon dioxide into H 2 and CO (syngas). CeO 2 nanorods, nanocubes, and nanospheres were decorated with 1−4 wt % Ni. The materials were structurally characterized using TEM and in situ XANES/EXAFS. The CO 2 activation was analyzed by DFT and temperature-programmed techniques combined with MS-DRIFTS. Synthesized CeO 2 morphologies expose {111} and {100} terminating facets, varying the strength of the CO 2 interaction and redox properties, which influence the CO 2 activation. Temperature-programmed CO 2 DRIFTS analysis revealed that under hydrogen-lean conditions mono-and bidentate carbonates are hydrogenated to formate intermediates, which decompose to H 2 O and CO. In excess hydrogen, methane is the preferred reaction product. The CeO 2 cubes favor the formation of a polydentate carbonate species, which is an inert spectator during DRM at 500 °C. Polydentate covers a considerable fraction of ceria's surface, resulting in less-abundant surface sites for CO 2 dissociation.

No abstract

CeO2, V2O5 and CeVO4 were synthesised as bulk oxides, or deposited over activated carbon, characterized by XRD, HRTEM, CO2-TPO, C3H8-TPR, DRIFTS and Raman techniques and tested in propane oxidative dehydrogenation using CO2. Complete oxidation of propane to CO and CO2 is favoured by lattice oxygen of CeO2. The temperature programmed experiments show the ~ 4 nm AC supported CeO2 crystallites become more susceptible to reduction by propane, but less prone to re-oxidation with CO2 compared to bulk CeO2. Catalytic activity of CeVO4/AC catalysts requires a 1–2 nm amorphous CeVO4 layer. During reaction, the amorphous CeVO4 layer crystallises and several atomic layers of carbon cover the CeVO4 surface, resulting in deactivation. During reaction, V2O5 is irreversibly reduced to V2O3. The lattice oxygen in bulk V2O5 favours catalytic activity and propene selectivity. Bulk V2O3 promotes only propane cracking with no propene selectivity. In VOx/AC materials, vanadium carbide is the catalytically active phase. Propane dehydrogenation over VC proceeds via chemisorbed oxygen species originating from the dissociated CO2. Graphic Abstract