Active Sites and Structure–Activity Relationships of Copper-Based Catalysts for Carbon Dioxide Hydrogenation to Methanol

Capping experiments reveal multiple surface active sites in CeO2 and their cooperative catalysis

Understanding of surface active sites (SAS) of CeO₂ is crucial to its catalytic applications. In the present study, we have employed capping experiments, DFT calculations, and spectroscopic characterization to study pristine CeO₂ catalyst. We find that multiple SAS coexist on the CeO₂ surface: oxygen vacancies as redox sites and the coordinately unsaturated Ce cations near the oxygen vacancies and the neighboring oxygen ions as Lewis acid–base sites. Dimethylsulfoxide (DMSO), pyridine, and benzoic acid are utilized to cap the redox sites, Lewis acid sites, and base sites, respectively. Selective capping on the redox site does not have much effect on the acid–base catalysis, and vice versa, indicating the distinct surface proximity and independent catalysis of these SAS. We draw attention to a relationship between the well-known redox sites and the surface Lewis acid and Lewis base pairs on CeO₂ surface, which are responsible for driving various heterogeneous catalytic reactions.

Capping with pyridine, benzoic acid, and DMSO in catalytic reactions reveals the locations of surface active sites of CeO₂.

Effect of Calcination Atmosphere and Temperature on the Hydrogenolysis Activity and Selectivity of Copper-Zinc Catalysts

A series of CuZn catalysts with a Cu/Zn ratio of 1.6 was prepared by the calcination of a single precursor, CuZn-P consisting of an equimolar mixture of aurichalcite and zincian malachite, in three different calcination atmospheres (air, nitrogen, and hydrogen) at three temperatures (220, 350, and 500 °C). All catalysts were characterized by XRD and N2-physisorption to assess their phase composition, crystallite sizes and textural properties and tested in dimethyl adipate (DMA) hydrogenolysis in a batch reactor at 220 °C and 10 MPa H2. The XRD examination of these catalysts proved that both parameters, calcination temperature and atmosphere, affected the resulting phase composition of the catalysts as well as their crystallite sizes. In an oxidizing atmosphere, CuO and ZnO in intimate contact prevailed whereas in inert or reducing atmosphere both oxides were accompanied by Cu2O and Cu. The crystallite size of Cu2O and Cu was larger than the size of CuO and ZnO thus indicating a less intimate contact between the Cu-phases and ZnO in catalysts calcined in nitrogen and hydrogen. Catalysts prepared by calcination at 220 °C and CuZn catalyst calcined in the air at 350 °C significantly outperformed the other catalysts in DMA hydrogenolysis with a 59–78% conversion due to the small crystallite size and intimate contact between the CuO and ZnO phases prior to catalyst reduction. Despite the low DMA conversion (<30%), transesterification products were the main reaction products with overall selectivities of >80% over the catalysts calcined in nitrogen or hydrogen at least at 350 °C. The obvious change in the preferred reaction pathway because of the atmosphere calcination and temperature shows that there are different active sites responsible for hydrogenolysis and transesterification and that their relative distribution has changed.

Coupling Glucose Dehydrogenation with CO2 Hydrogenation by Hydrogen Transfer in Aqueous Media at Room Temperature

Conversion of CO₂ into value-added chemicals and fuels provides a direct solution to reduce excessive CO₂ in the atmosphere. Herein, a novel catalytic reaction system is presented by coupling the dehydrogenation of glucose with the hydrogenation of a CO₂ -derived salt, ammonium carbonate, in an ethanol-water mixture. For the first time, the hydrogenation of CO₂ to formate by glucose has been achieved under ambient conditions. Under the optimal reaction conditions, the highest yield of formate reached approximately 46 %. We find that the apparent pH value in the ethanol-water mixture plays a central role in determining the performance of the hydrogen-transfer reaction. Based on the ¹³ C NMR and ESI-MS results, a possible pathway of the coupled glucose dehydrogenation and CO₂ hydrogenation reactions was proposed.

Correlating ultrasonic impulse and addition of ZnO promoter with CO2 conversion and methanol selectivity of CuO/ZrO2 catalysts

The thermal characteristics of Cu-based catalysts for CO₂ utilization towards the synthesis of methanol were analysed and discussed in this study. The preparation process were varied by adopting ultrasonic irradiation at various impulses for the co-precipitation route and also, by introducing ZnO promoters using the solid-state reaction route. Prepared catalysts were characterised using XRD, TPR, TPD, SEM, BET and TG-DTA-DSC. In addition, the CO₂ conversion and CH₃OH selectivity of these samples were assessed. Calcination of the catalysts facilitated the interaction of the Cu catalyst with the respective support bolstering the thermal stability of the catalysts. The characterisation analysis clearly reveals that the thermal performance of the catalysts was directly related to the sonication impulse and heating rate. Surface morphology and chemistry was enhanced with the aid of sonication and introduction of promoters. However, the impact of the promoter outweighs that of the sonication process. CO₂ conversion and methanol selectivity showed a significant improvement with a 270% increase in methanol yield.

Selective hydrogenation of CO2 to methanol catalyzed by Cu supported on rod-like La2O2CO3

Cu-based catalysts have long been applied to convert CO₂ and H₂ into methanol, and their performances are well known to be markedly influenced by the support and promoter.

Recent progress in improving the stability of copper-based catalysts for hydrogenation of carbon–oxygen bonds

Five different strategies to enhance the stability of Cu-based catalysts for hydrogenation of C–O bonds are summarized in this review.

Cu supported on thin carbon layer-coated porous SiO2 for efficient ethanol dehydrogenation

The designed Cu/C/SiO₂ catalyst combines the favourable properties of carbon and silica, thus showing improved selectivity associated with good stability.

Surprisingly high sensitivity of copper nanoparticles toward coordinating ligands: consequences for the hydride reduction of benzaldehyde

Depending on the ligand, ligand-induced leaching of copper nanoparticles may produce catalytically active species for the reduction of benzaldehyde.

Structure–activity relationships of Cu–ZrO2 catalysts for CO2 hydrogenation to methanol: interaction effects and reaction mechanism

A systematic study on the Cu–ZrO₂ catalysts with different oxygen vacancy concentrations and interaction gives a new approach for understanding the reaction mechanism of CO₂ hydrogenation to methanol.

Hydrogenation of dimethyl malonate to 1,3-propanediol catalyzed by a Cu/SiO2 catalyst: the reaction network and the effect of Cu+/Cu0 on selectivity

1,3-Propanediol was synthesized via the hydrogenation of dimethyl malonate over a Cu/SiO₂ catalyst. The reaction network and active sites were revealed for the first time.

Catalytic consequences of Ga promotion on Cu for CO2 hydrogenation to methanol

The addition of Ga to Cu/SiO₂ generates new active sites increasing selectivity to methanol. The mechanistic implications are studied by in situ DRIFTS and kinetic experiments.

Integrated Experimental and Theoretical Study of Shape-Controlled Catalytic Oxidative Coupling of Aromatic Amines over CuO Nanostructures

We have synthesized CuO nanostructures with flake, dandelion-microsphere, and short-ribbon shapes using solution-phase methods and have evaluated their structure-performance relationship in the heterogeneous catalysis of liquid-phase oxidative coupling reactions. The formation of nanostructures and the morphological evolution were confirmed by transmission electron microscopy, scanning electron microscopy, X-ray diffraction analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, energy-dispersive X-ray spectroscopy, elemental mapping analysis, and Fourier transform infrared spectroscopy. CuO nanostructures with different morphologies were tested for the catalytic oxidative coupling of aromatic amines to imines under solvent-free conditions. We found that the flake-shaped CuO nanostructures exhibited superior catalytic efficiency compared to that of the dandelion- and short-ribbon-shaped CuO nanostructures. We also performed extensive density functional theory (DFT) calculations to gain atomic-level insight into the intriguing reactivity trends observed for the different CuO nanostructures. Our DFT calculations provided for the first time a detailed and comprehensive view of the oxidative coupling reaction of benzylamine over CuO, which yields N-benzylidene-1-phenylmethanamine as the major product. CuO(111) is identified as the reactive surface; the specific arrangement of coordinatively unsaturated Cu and O sites on the most stable CuO(111) surface allows N-H and C-H bond-activation reactions to proceed with low-energy barriers. The high catalytic activity of the flake-shaped CuO nanostructure can be attributed to the greatest exposure of the active CuO(111) facets. Our finding sheds light on the prospective utility of inexpensive CuO nanostructured catalysts with different morphologies in performing solvent-free oxidative coupling of aromatic amines to obtain biologically and pharmaceutically important imine derivatives with high selectivity.

Co-Cu Nanoparticles: Synthesis by Galvanic Replacement and Phase Rearrangement during Catalytic Activation

The control of the phase distribution in multicomponent nanomaterials is critical to optimize their catalytic performance. In this direction, while impressive advances have been achieved in the past decade in the synthesis of multicomponent nanoparticles and nanocomposites, element rearrangement during catalyst activation has been frequently overseen. Here, we present a facile galvanic replacement-based procedure to synthesize Co@Cu nanoparticles with narrow size and composition distributions. We further characterize their phase arrangement before and after catalytic activation. When oxidized at 350 °C in air to remove organics, Co@Cu core-shell nanostructures oxidize to polycrystalline CuO-Co3O4 nanoparticles with randomly distributed CuO and Co3O4 crystallites. During a posterior reduction treatment in H2 atmosphere, Cu precipitates in a metallic core and Co migrates to the nanoparticle surface to form Cu@Co core-shell nanostructures. The catalytic behavior of such Cu@Co nanoparticles supported on mesoporous silica was further analyzed toward CO2 hydrogenation in real working conditions.

Glycerol hydro-deoxygenation aided by in situ H2 generation via methanol aqueous phase reforming over a Cu–ZnO–Al2O3 catalyst

Methanol APR–glycerol HDO reactions were successfully coupled to produce 1,2-propanediol at high yields over an efficient CuZnAl catalyst.

A highly effective and stable CuZn0.3MgxAlOycatalyst for the manufacture of chirall-phenylalaninol: the role of Mg and its hydrotalcite-like precursor

A highly effective and stable CuZn_0.3Mg_0.1AlO_ycatalyst derived from a Cu-rich hydrotalcite-like precursor was prepared for the catalytic hydrogenation ofl-phenylalanine methyl ester tol-phenylalaninol with ~100% ee selectivity.

A study on the precipitating and aging processes of CuO/ZnO/Al2O3 catalysts synthesized in micro-impinging stream reactors

Precipitating and aging processes of CuO/ZnO/Al₂O₃ catalysts were performed more uniformly in micro-impinging stream reactors than in stirred reactors.

Continuous heterogeneous hydrogenation of CO2-derived dimethyl carbonate to methanol over a Cu-based catalyst

Copper content played a significant role in the catalytic performance of Cu/SiO₂ catalysts in dimethyl carbonate hydrogenation to methanol. Optimized hydrogenation activity was achieved over the 40Cu/SiO₂ sample.