ChemCatChem最新文献

The direct conversion of CO₂ into fuels via hydrogenation represents a promising solution to the storage of renewable energy. This reaction generally proceeds through the reverse water gas shift (RWGS) reaction to produce CO and the subsequent CO-Fischer–Tropsch synthesis (FTS). On Co-based catalysts, the introduction of dopants can improve CO₂ activation, enhance the RWGS activity, and decrease the methanation reaction. We reported that Na-promoted Co/TiO₂ catalysts outperform the unpromoted ones in terms of activity and selectivity toward C₂₊. To further improve the catalytic performances, we doped a Na-promoted Co/TiO₂ catalyst with ruthenium, which is known to favor a high degree of cobalt reduction in CO-FTS. The effect of Ru location in relation to cobalt was investigated by using bimetallic and mixtures of monometallic catalysts. This study demonstrates that Ru doping leads to an improvement in catalytic activity. Furthermore, we show that the proximity between Co and Ru has a pronounced effect on the selectivity. The best configuration to achieve higher CO₂ conversion and C₅₊ selectivity is obtained using mixtures of monometallic catalysts separated in two beds. Reducing the Ru loading significantly improved the selectivity toward C₂-C₄ hydrocarbons.

CO₂ hydrogenation to methanol as a value-added chemical or low-carbon fuel is a promising and practical process to utilize CO₂ emission and reduce carbon footprint. However, the reaction's energy-intensive demands at high temperature and pressure are hindering its economic feasibility and commercialization. To lower the energy consumption of the process, a new type of titanate-anchored Cu/ZnO/Al₂O₃(CZA) catalyst is reported for efficient and robust CO₂ conversion to methanol at low temperature (200 °C–220 °C) and low pressure (1–10 bar). Distinct from the pellet-structured commercial CZA catalyst and the previously reported nanoarray-dispersed CZA catalyst, the titanate nanorod forest serves as a unique support to trap and anchor co-precipitated CZA nanoparticles and slow down their thermal sintering. Meanwhile, Cu²⁺ and Cu⁺ species populate with increased amounts due to a strong interaction between the CZA and titanate forest, resulting in increased CO₂ and H₂ absorption and methanol production. As a result, an improved methanol production rate is achieved over the titanate-anchored CZA catalyst, with a high methanol yield of 6 mol.kg⁻¹.h⁻¹ under 220 °C and 10 bar conditions. The active catalyst loading, dip coating solvent, annealing temperature and time, and pressure are found to effectively tailor the methanol production performance based on the titanate forest configuration.

Hemilabile ligands have the potential to tune catalyst reactivity and selectivity in myriad ways. In this study, we explore the role of phosphine donors on the reactivity of zinc catalysts for the polymerization of rac-lactide and ε-caprolactone. We report the synthesis of a series of zinc complexes supported by an amino-phenolate ligand bearing a phosphine arm. Systematic comparisons between complexes with and without the hemilabile phosphine functionality reveal distinct differences in polymerization between rac-lactide and ε-caprolactone. Combined analysis of polymerization data and kinetic studies demonstrates that the phosphine arm provides essential steric bulk to the catalyst to maintain a mononuclear solution-phase structure.

Mechanoenzymology is a green chemistry technology that has emerged in recent years, which can efficiently promote enzymatic hydrolysis reactions through mechanical force under low-solvent conditions. Mechanoenzymatic reactions has the green metrics of reducing solvent usage, minimizing waste generation, potentially improving reaction efficiency, and mild reaction conditions, which conform to the Principles of Green Chemistry. In this review, the focus is on the latest research progress of mechanoenzymatic reactions and the green aspects based on the Principles of Green Chemistry. The challenges and prospects of mechanoenzymology are discussed to further promote its development and application.

Asymmetric hydroformylation (AHF) of prochiral alkenes is an efficient way to synthesize optically active aldehydes, which are versatile chiral building blocks for pharmaceuticals, agrochemicals, and other fine chemicals. The purpose of this review is to take stock of developments in the last decade and shed light on the understanding of the field of AHF. So far, most of the literature methods focused on the use of Rh-based catalysts, due to high catalytic activity and excellent chemoselectivity for the aldehydes. Several chiral phosphorus ligands have been successfully developed for Rh-catalyzed AHF reactions. This review examines the role of the substrate/olefins in AHF. Several different types of “mono-substituted” terminal olefins (functionalized/nonfunctionalized) with a variety of chiral ligands have been investigated, which show high activity and excellent ee of up to 99%. The AHF of “di-substituted” and “tri-substituted” olefins is rarely reported. This review summarizes the evolution of chiral ligands for AHF. It discusses the progress made in desymmetrizing hydroformylation. In addition, it highlights important developments in AHF carried out with and without syngas. These advances span a wide variety of alkenes. Additionally, the review offers future approaches in the field of AHF for the synthesis of optically active aldehydes.

The generation of multi-walled carbon nanotubes (MWCNTs) typically utilizes solid catalyst nanoparticles. These particles often exhibit a liquid-like nature during synthesis and remain encapsulated inside the final MWCNTs. Molten Cu–In has been recently reported to produce high amounts of MWCNT in bubble column reactors during methane pyrolysis for clean H₂ generation. In the present work, nanodroplets are isolated and studied on supports. The droplets are observed to deform into a head and tail geometry and generate bamboo-like MWCNTs. The compositions of 50–70 at.% Cu repeatedly generate dense bundles of MWCNTs, while higher or lower compositions yield little or no MWCNTs. The lower surface tension of the alloy at these compositions reduces the thermodynamic driving force for coalescence, stabilizing small droplets at high temperature. Graphitic structure with some defects is confirmed by transmission electron microscopy and Raman spectroscopy, showing 3.35 +/−0.08 Å interlayer spacing and an I_D/I_G ratio of 0.87 +/−0.13, respectively. Droplets distributed between 10 nm and 1 micron generate MWCNTs 10–400 nm in diameter, suggesting droplets above 400 nm do not generate MWCNTs at any catalyst composition. Bundles of MWCNT exceeding hundreds of microns are observed in reaction times over 1 h.

Cobalamin-dependent aryl methyl ether O-demethylase is a multi-component enzyme system that converts O-methylated aromatic compounds into demethylated phenolics. The central enzyme of the system is a cobalamin-dependent protein that interacts with methyltransferase enzymes for transferring the methyl group between O-methyl groups of aryl methyl ethers and various methyl acceptors. Besides their role in energy metabolism of certain anaerobic bacteria, O-demethylases possess high potential for biocatalysis, including lignin valorization and use in organic synthesis for reversible (de)methylation reactions. An increasing number of cobalamin-dependent O-demethylase enzyme systems from various bacteria, including gut microorganisms, with different substrate scopes and regioselectivity profiles have been identified in the recent decade. Moreover, biocatalytic studies have been carried out on O-demethylase systems demonstrating their potential in synthetic applications. In this review, we provide a comprehensive overview of the cobalamin-dependent aryl methyl ether O-demethylase systems identified to date in various microorganisms. We present the mechanism, biological function, substrate scope and selectivity of the studied systems and discuss their potential for biocatalytic applications.

Hydrogen peroxide (H₂O₂) is a versatile chemical, valued as both a promising energy carrier and a widely used oxidizing agent in disinfection and organic synthesis. The light-driven catalytic oxygen reduction reaction (ORR) using carbon nitrides (CN_x) is based on the conversion of solar into chemical energy and thus offers a sustainable pathway for decentralized H₂O₂ production. This study presents a novel synthetic strategy for producing ionic derivatives of CN_x, specifically poly(heptazine imides) (PHIs) with higher specific surface areas, using an ordered mesoporous silica material (SBA-15) as a template. The templated PHIs exhibit enhanced porosity, controlled incorporation of transition metals, improved visible-light absorption, tunable hydrophilicity and more efficient charge separation compared to bulk CN_x. PHIs containing iron, cobalt or nickel accelerate H₂O₂ decomposition, whereas templated potassium PHI (KPHI) achieves a 2.1-fold increase in H₂O₂ production with ethanol as a hole scavenger under visible light irradiation (455 nm, 50 mW·cm⁻²) compared to bulk KPHI (KPHI_b). A high H₂O₂ production rate of 19.0 mmol·L⁻¹·h⁻¹ (i.e., 76.2 mmol·g⁻¹·h⁻¹) under the same irradiation condition is achieved with KPHI in a 90 vol.% methanol solution and an optimal photonic yield of 12.8% is obtained with KPHI at 365 nm.

The use of phosphine-based transition metal complexes in hydrogen borrowing (HB) reactions at low catalyst loading and milder conditions is challenging due to their air-sensitive nature. In order to address this issue, an air-stable Ru(II) complex, [(p-cymene)RuCl₂(PPh₂Cbz)], (Ru-PCbz) (Cbz = N-carbazolyl), has been synthesized from diphenyl-N-carbazolyl phosphine (PPh₂Cbz) and [Ru(p-cymene)(μ-Cl)Cl]₂, and characterized using various spectroscopic and analytical techniques. The molecular structure of the complex has been determined through single crystal X-ray diffraction analysis. Catalytic investigations reveal that Ru-PCbz is both versatile and highly efficient in performing transfer hydrogenation of ketones, dehydrogenation of secondary alcohols, and N-methylation of anilines. In order to unravel the role of a carbazolyl substituent in the observed catalytic activity of Ru-PCbz, catalytic studies were also conducted with triphenyl phosphine (PPh₃) bearing Ru(II) complex [(p-cymene)RuCl₂(PPh₃)] (Ru-PPh) as the control. The results reveal that Ru-PCbz significantly outperforms Ru-PPh for all the three catalytic transformations investigated. By performing other appropriate control experiments, most plausible mechanistic pathways for these reactions using Ru-PCbz have been proposed, often by trapping the intermediates using various spectroscopic techniques.

Electrocatalytic co-reduction of nitrate (NO₃⁻) and carbon dioxide (CO₂) for urea synthesis offers an eco-friendly solution to mitigate nitrate contamination and reduce the energy demands. The investigation of the catalyst geometric and electronic structures is critical to elucidating the reaction mechanisms for the design of high-performance catalysts. Herein, this work systematically studied the cobalt (Co) atom geometric configurations in Co-based spinel oxides for C−N coupling reaction. It demonstrated that C−N coupling is more likely to take place at octahedral Co (Co_Oh) sites instead of tetrahedral Co (Co_Td) sites and the Co_Oh sites in spinel structures facilitating both electron and ion transport. Leveraging the synergistic effect between Co_Oh and Co_Td sites, Co₃O₄ achieved the highest Faradaic efficiency and urea yield. Meanwhile, isotope labeling experiment confirmed the urea formation through the C−N coupling of CO₂ and NO₃⁻. By replacing inactive tetrahedral cobalt atoms with copper atoms, the catalytic performance was further enhanced. This study provides key design principles for high-performance metal oxide catalysts for C−N coupling reactions.