ChemCatChem最新文献

We studied the influence of the distance of olefin metathesis catalysts from the inner surface of a mesoporous support on macrocyclization and Z-selectivity under confinement. For these purposes, the cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) catalysts [Mo(N-(2-^tBu-C₆H₄)(1-mesityl-3-(3-trimethoxysilylprop-1-yl)-imidazol-2-ylidene)(CHCMe₂Ph)(MeCN)Br⁺ B(Ar^F)₄⁻] Mo2, [Mo(N-(2-^tBu-C₆H₄)(1-mesityl-3-(3-trimethoxysilylprop-1-yl)-imidazol-2-ylidene)(CHCMe₂Ph)(MeCN)OTf⁺ B(Ar^F)₄⁻] Mo3, [Mo(N-(2,6-Me₂-C₆H₃)(1-mesityl-3-(3-trimethoxysilylprop-1-yl)-imidazol-2-ylidene)(CHCMe₂Ph)(MeCN)Br⁺ B(Ar^F)₄⁻] Mo5, and [Mo(N-(2,6-iPr₂-C₆H₃)(1-mesityl-3-(3-trimethoxysilylprop-1-yl)-imidazol-2-ylidene)(CHCMe₂Ph)(MeCN)⁺Br B(Ar^F)₄⁻] Mo7 (B(Ar^F)₄ = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), all containing a trimethoxysilylpropyl tether, were selectively immobilized inside the mesopores of SBA-15. Under confinement, both macro(mono)cyclization (MMC) and Z-selectivity were higher than in solution but lower than with catalysts directly bound to the surface of the mesoporous supports. These findings are in agreement with existing theoretical models on substrate and product distribution in mesopores, which suggest that the highest substrate concentration is found at the pore wall and that it increases with decreasing pore diameter.

In this work, we described a general monometallic cobalt heterogeneous catalyst for the hydroformylation of olefins, achieving good yields and recyclability up to five times with no loss in catalytic activity. These catalysts were prepared through the pyrolysis of the well-defined metal–organic framework ZIF-67. The addition of steam during the pyrolysis did not affect the final phase composition of the cobalt particles; nonetheless, it resulted in an increase of the cobalt particle size and the partial removal of the carbonaceous matrix. The materials were extensively characterized by several techniques, and it was observed that the N-doped carbon matrix played a crucial role in terms of activity and stability. Different liquid olefins, including internal, terminal, and cyclic were successfully tested in our hydroformylation protocol. Aldehydes yields of 48%–77% for different liquid olefins were achieved with the optimal catalyst. No leaching of the active sites was observed over five catalytic cycles. The high stability of the catalyst is attributed to the presence of stabilizing nitrogen atoms bearing the cobalt sites.

Alloying and solid-solution formation is a powerful technique that enhances and adds properties through elemental mixing, but unfortunately, some elements simply cannot mix as their chemical nature prevents a thermodynamically stable structure. For example, the inherent nobility of platinum group metals does not favor bond formation and precludes their incorporation into higher (boron-rich) metal borides. However, we demonstrate that when using five or more constituents, the higher mixing entropy will overcome these chemical limitations and form a stable high-entropy alloy, demonstrating the formation of new compounds with substituents that are seemingly impossible with a traditional metal alloying approach. The high-entropy boride (HEB) Al_0.2Nb_0.2Pt_0.2Ta_0.2Ti_0.2B₂ was synthesized, where platinum was forced to occupy a 12-coordinate site, sandwiched between honeycomb borophene sheets. In addition to the unusual coordination, the boron serves as a poison panacea. Pure platinum is strongly susceptible to sulfur poisoning by adsorption, rendering a platinum catalyst ineffective. Boron is known to be resistant to sulfur poisoning. The boron sheets present in the HEB shield the platinum from sulfur while maintaining high catalytic activity. This is confirmed with the facile hydrogenation of thiol-containing nitro compounds, where the HEB resists sulfur poisoning while retaining its high catalytic activity.

Homogeneous electrocatalysts typified by transition-metal complex show transcendent potency in efficient energy catalysis through molecular design. For example, metal complexes with elaborate design performed wonderful activity and selectivity for electrocatalytic CO₂ reduction. Primary coordination sphere of metal complexes plays a key role in regulating its intrinsic redox properties and catalytic activity. However, the overall reduction efficiency of CO₂ is also bound up with the substrate activation process. Transition-metal complexes are hoped to exhibit reasonable redox potential, reactive activity, and stability, while binding and activating CO₂ molecules to achieve efficient CO₂ reduction. Construction of second coordination sphere, especially hydrogen-bonding network of transition-metal complexes, is reported to be the “kill two birds with one stone” strategy to realize efficient CO₂ reduction catalysis via systematic catalyst properties modulation and substrate activation. Herein, we present recent progress on the construction of hydrogen-bonding network in the second coordination sphere of metal complexes by ligand modification or the introduction of exogenous organic ligand, and the resulted productive enhancement of the catalytic performance of metal complexes by the improvement of adsorption capacity and activation of CO₂, proton transfer rate, and stability of reaction intermediates, and so forth.

Cu/SiO₂ catalysts are widely applied to the hydrogenation of dimethyl oxalate and ethylene carbonate to the corresponding ethylene glycol and methanol simultaneously, whereas ethylene glycol and methanol are important bulk commodities and raw materials for the production of oxygen-containing chemicals and fuels. However, Cu particles usually aggregate or sinter to deactivate and the ratio of surface Cu⁰/Cu⁺ species is also difficult to control under the reaction conditions, so that the catalyst activity and stability is still a big challenge. It was found that modification of Cu-based catalysts with certain organic compound inhibited the agglomeration of Cu particles, regulated the ratio of surface Cu⁰/Cu⁺ species, and even generated carbon layers to protect the Cu particles, which definitely improved the stability of the catalyst along with the enhanced catalytic performance. In this review, recent developments in ester hydrogenation over organic compound-modified Cu/SiO₂ catalysts were summarized and the issues to be further clarified are discussed as well.

The Front Cover visualizes competing pathways in ethylene oligomerization including dimerization, isomerization and trimerization, investigated by Jianwen Jiang and co-workers on defective HKUST-1 supported metal hydrides through density functional theory calculations. The microscopic insights would facilitate the rational design of new catalysts based on metal-organic frameworks for selective ethylene oligomerization. More information can be found in the Research Article by J. Jiang and co-workers (DOI: 10.1002/cctc.202400906).

In this study, we report an environment-friendly protocol by integrating the nonnatural catalytic activity of lipase with electrocatalysis for synthesizing C-3 alkylated oxindoles, which are part of many natural and pharmaceuticals products. Gratifyingly, Candida antarctica lipase B (CALB) is found to be highly active and regioselective for catalyzing the nonnatural C-3 alkylation reaction at indole when combined with an electrochemical C-2 oxidation process in the same vessels. Further, the generality and feasibility of the developed protocol are shown by employing several functional groups on the indole moiety and obtaining the desired products in moderate to good yield. Besides, the control experiments are set up along with the molecular docking studies to substantiate the role of the active site of Candida antarctica lipase B (CALB) in carrying out the regioselective C-3 alkylation reaction. In addition, control experiments and cyclic voltammetry are performed to get insight into the electrochemical C-2 oxidation process and as a result, a plausible mechanism for the integrated process is presented.

The Cover Feature depicts the award ceremony of a tournament. Several teams of chemists faced off to find a new organic base to replace the costly 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). Each team investigates one base, displayed on the banners hanging in the room. The ceremony celebrates the winner of the study, i.e., 1,1,3,3-tetramethyl guanidine (TMG). This illustration creatively conveys the essence of the research presented by Coralie Jehanno and co-workers, emphasizing the economic competitiveness and efficiency of the TMG base over the existing TBD system. More information can be found in the Research Article by C. Jehanno and co-workers (DOI: 10.1002/cctc.202400215).

Proton exchange membrane fuel cells (PEMFCs) are crucial for the efficient utilization of hydrogen. Currently, their efficiency is mainly limited by the slow kinetics of the cathode oxygen reduction reaction (ORR) and the poisoning effect between ionomers and catalytic sites, particularly with Pt-based catalysts. Recent works suggest that the emerging porous carbon-supported catalysts hold promise in mitigating these challenges by ensuring fast kinetics while alleviating the poisoning. This review examines porous carbon-supported catalysts for PEMFC cathodes, covering synthesis methods, structure and performance evaluation, and future prospects, with an emphasis on the influence of porous carbon support on PEMFC performance. On one hand, the rational design of pore structure in carbon support can help optimize the location of the active sites and enhance mass transfer. On the other hand, diverse pore structures provide a platform for gaining a deeper understanding of the mechanisms behind microscale mass transfer and reaction at the three-phase boundaries. This review aims to inspire innovative strategies for the precise synthesis of porous carbon-supported catalysts with various pore structures to further boost PEMFC performance.

The recent breakthroughs in heterogeneous catalysis emphasize the need for novel materials capable of driving organic transformations. The present study explores two copper phosphonate hybrid framework materials, 2D Cu₃[(Hhedp)₂(C₄H₄N₂)].2H₂O (Cu(II)Pyz-P) and 3D Cu₃[(H₃hedp)₂(C₄H₄N₂)₄(SO₄)].2H₂O (Cu(I/II)Pyz-P), as single crystals isolated via hydrothermal synthetic strategy using H₄hedp (1-hydroxyethane 1,1-diphosphonic acid) and N-donor secondary ligand (Pyrazine; C₄H₄N₂). Cu(II)Pyz-P contains exclusively +2 oxidation state of copper, while Cu(I/II)Pyz-P is characterized by a mixed oxidation state, where copper +2 phosphonate is embedded within the network formed by copper +1 state. Moreover, magnetic study elucidates the distinct oxidation states of both compounds by showing deviations from each other. The aforementioned compounds are exceedingly effective for catalyzing multicomponent reactions, that is, A³ coupling reaction and click reaction. Cu(I/II)Pyz-P ensures the regioselective synthesis of triazole with high purity, while A³ coupling reaction is facilitated by both the compounds. Solvent- and additive-free efficient multicomponent reactions are explored for the first time in the copper phosphonate hybrid framework structures. The present study reveals the promise of copper-based hybrid phosphonate frameworks as durable and efficient catalysts for organic synthesis, providing cost-effective and sustainable ways for advanced catalytic transformations.