Catalyst Substrate Research Articles

Efficient Transformation of Polymer Main Chain Catalyzed by Macrocycle Metal Complexes via Pseudorotaxane Intermediate

AbstractPost‐synthesis modification of polymers streamlines the synthesis of functionalized polymers, but is often incomplete due to the negative polymer effects. Developing efficient polymer reactions in artificial systems thus represents a long‐standing objective in the fields of polymer and material science. Here, we show unprecedented macrocycle‐metal‐complex‐catalyzed systems for efficient polymer reaction that result in 100 % transformation of the main chain functional groups presumably via a processive mode reaction. The complete polymer reactions were confirmed in not only intramolecular reaction (hydroamination) but also intermolecular reaction (hydrosilylation) by using Pd‐ and Pt‐macrocycle‐catalyzed systems. The most fascinating feature of the both reactions is that higher‐molecular‐weight polymers reach completion faster. Various studies suggested that the reactions occur in the catalyst cavity via the formation of a supramolecular complex between the macrocycle catalyst and polymer substrate like pseudorotaxane, which should be of characteristic of the efficient polymer reactions progressing in a processive mode.

Efficient Transformation of Polymer Main Chain Catalyzed by Macrocycle Metal Complexes via Pseudorotaxane Intermediate.

Post-synthesis modification of polymers streamlines the synthesis of functionalized polymers, but is often incomplete due to the negative polymer effects. Developing efficient polymer reactions in artificial systems thus represents a long-standing objective in the fields of polymer and material science. Here, we show unprecedented macrocycle-metal-complex-catalyzed systems for efficient polymer reaction that result in 100 % transformation of the main chain functional groups presumably via a processive mode reaction. The complete polymer reactions were confirmed in not only intramolecular reaction (hydroamination) but also intermolecular reaction (hydrosilylation) by using Pd- and Pt-macrocycle-catalyzed systems. The most fascinating feature of the both reactions is that higher-molecular-weight polymers reach completion faster. Various studies suggested that the reactions occur in the catalyst cavity via the formation of a supramolecular complex between the macrocycle catalyst and polymer substrate like pseudorotaxane, which should be of characteristic of the efficient polymer reactions progressing in a processive mode.

Enzyme-Mimetic Photo-decarboxylation Based on Geometry-Dependent Supramolecular Association

Natural enzymes rely on their active center for geometric recognition of their substrates, which contributes to their catalytic properties. Achieving enzyme-like binding in artificial systems remains a challenge, particularly for molecular catalysts and substrates that have distinct shapes. Herein, we report the association of amino acids, tethered with a cleavable tricyclic fluorenyl moiety, to the flavin-based photocatalyst of complementary shapes, with aromatic stacking as the driving force, to perform the photocatalytic decarboxylation of amino acids. Our experimental and theoretical results indicate that the stacking of the fluorenyl moiety to flavin resulted in location of the flavin proximal to the α-carbon of the amino acid. As a result, the efficiency of the decarboxylation reactions, which occurred at the α-carbon of the amino acid, was significantly improved, followed by addition of hydroxyl or oxygen under aerobic conditions. In contrast, considerably lower efficiency in decarboxylation reactions was observed when the amino acids were unmodified or modified with a tert-butyl or benzyl moiety, in which the α-carbon was located away from the catalyst, confirming the importance of the flavin–fluorenyl recognition and the proximity effect to the catalytic activity. Moreover, photo-decarboxylation and oxygenation was observed for a short peptide that has a carboxylate at the α-carbon close to the fluorenyl moiety. Our findings provide a method to create catalyst–substrate recognition and may aid in the future design of bioinspired catalytic systems.

Cation Vacancy Clusters in Ti3C2Tx MXene Induce Ultra‐Strong Interaction with Noble Metal Clusters for Efficient Electrocatalytic Hydrogen Evolution

AbstractMXenes are promising substrates for supported noble metal electrocatalysts. Yet, it is a significant challenge to modulate the metal–support interaction (MSI) for enhancing catalytic performance. Herein, employing a facile HF etching method, the cation vacancy structures in Ti3C2Tx MXenes are controllably tuned, producing nearly vacancy‐free (Ti3C2Tx‐V0), single Ti atom vacancy (Ti3C2Tx‐VS), or Ti vacancy cluster (Ti3C2Tx‐VC) engineered MXenes. Ruthenium atomic clusters, as a model catalyst, successfully anchor on all MXene substrates. Different from the terminal O/F coordination groups on routine Ti3C2Tx MXene surfaces, the Ti vacancy clusters in Ti3C2Tx‐VC create unique lattice carbon ligand environment toward Ru species, which induces ultra‐strong MSI. As a result, compared to Ti3C2Tx‐V0 and Ti3C2Tx‐VS, the Ti3C2Tx‐VC modulated Ru clusters (Ru@Ti3C2Tx‐VC) exhibit the optimized balance of H2O adsorption/dissociation and OH/H desorption, thereby delivering superior electrocatalytic performance in the alkaline hydrogen evolution reaction (HER). Within the wide range from laboratory‐level (90 mA cm−2) to industrial‐level (1.5 A cm−2) current density, Ru@Ti3C2Tx‐VC outperforms commercial Pt/C in terms of overpotential and mass activity. Moreover, as a universal substrate for noble metal catalysts, Ti3C2Tx‐VC can also anchor Ir/Pt/Rh atomic clusters and enable excellent HER catalytic activity. This work expands the scope of the MSI between MXene and noble metal catalysts.

Amido-ene(amido) Ni(II)-Catalyzed Highly Enantioselective Transfer Hydrogenations of Ketone: Dual Functions of the Ene(amido) Group

Catalytic asymmetric transfer hydrogenation (ATH) reactions of ketones have become an important tool for producing enantiomerically pure alcohols that are widely used in the syntheses of bioactive compounds. However, the high toxicities and low abundances of traditionally employed precious metals serving as catalyst centers have impeded widespread application of precious metal-based catalysts in asymmetric synthesis. Development of catalysts from abundant and less toxic 3d metals is highly desirable but still challenging. We report herein the development of structurally well-defined (R,R)-amido-ene(amido) nickel catalysts for ATH of ketone substrates, facilitating access to a wide range of secondary (S)-configuration alcohols, many of which are synthetic building blocks of biologically relevant molecules, with up to 95% enantiomeric excess (e.e.) and broad scope of functional group tolerance. Catalytic intermediate isolation and density functional theory calculation ascribed the catalytic activity to stabilization of the electron-rich nickel center with the ene(amido) group via a π-backdonation interaction within the catalytic intermediates, while the stereoinduction mainly arises from a catalyst–substrate π–π interaction in the transition state for enantioselective hydride transfer.

Two-photon nanolithography of positive photoresist of AZ 5214E with a spatial resolution at nanoscale

The authors investigated two-photon nanolithography (TPNL) of positive AZ 5214E photoresist thin film with a spatial resolution at nanoscale. A continuous trench with average width of 101 nm was obtained, and a trench with several break points but feature width of 19 nm was fabricated by further increasing the scanning speed. Hole arrays composed of holes with diameters of about 451 nm and period of 800 nm were also fabricated with this photoresist. These results not only reveal that the TPNL of positive photoresist can obtain similar spatial resolution to that of negative photoresists, but also have extensive application prospection in fabrications of integrated circuits, microfluidic devices, surface-enhanced Raman scattering (SERS) substrates, catalyst, and biosensor substrates.

Iron-Catalyzed Coupling of Alkenes and Enones: Sakurai-Michael-type Conjugate Addition of Catalytic Allyliron Nucleophiles.

The iron-catalyzed coupling of alkenes and enones through allylic C(sp3)-H functionalization is reported. This redox-neutral process employs a cyclopentadienyliron(II) dicarbonyl catalyst and simple alkene substrates to generate catalytic allyliron intermediates for 1,4-addition to chalcones and other conjugated enones. The use of 2,4,6-collidine as the base and a combination of triisopropylsilyl triflate and LiNTf2 as Lewis acids was found to facilitate this transformation under mild, functional group-tolerant conditions. Both electronically unactivated alkenes as well as allylbenzene derivatives could be employed as pronucleophilic coupling partners, as could a range of enones bearing electronically varied substituents.

Catalytic Asymmetric Imine Cross-Coupling Reaction

Catalytic asymmetric cross-coupling of imines constitutes a particularly desirable method for the synthesis of chiral vicinal diamines directly from readily available achiral precursors. The potential of this method lies in the possibility of utilizing a variety of imines as reacting partners. However, the realization of highly stereoselective cross-coupling of two different imines proved to be a formidable challenge. Herein we report an unprecedented catalytic asymmetric cross-coupling reaction that tolerates a variety of ketimines and aldimines as nucleophiles and electrophiles, respectively. The realization of this reaction resulted from the development of a new chiral ammonium catalyst, which was guided by insights from studies of catalyst-substrate interactions. With a 0.5 mol % loading of an organocatalyst, this reaction proceeded in a highly diastereo- and enantioselective manner to afford a diverse range of chiral vicinal diamines as nearly single stereoisomers. This catalytic reaction establishes a new approach for the asymmetric synthesis of chiral vicinal diamines.

Spherical nickel doped cobalt phosphide as an anode catalyst for oxygen evolution reaction in alkaline media: From catalysis to system

Anion exchange membrane water electrolysis (AEMWE) uses a zero-gap membrane-electrode-assembly (MEA) consisting of a polymer electrolyte membrane and non-precious metal-based catalysts. However, anode catalysts for the oxygen evolution reaction (OER), which is the rate-determining-step in WE, need to be intensively developed to achieve superior electrocatalytic activity and stability. Herein, we report the synthesis and characterization (experimental and simulation) of spherical Ni-doped (5–15 at%) cobalt phosphide (Ni-CoP) as the anode catalyst for the OER. Specifically, the DFT calculation exhibited that the addition of Ni in CoP might increase the energy density in the Fermi level of pristine CoP, enhance the charge transfer rate during the OER, and reduce the energy barrier for the formation of OOH* , thereby boosting the OER activity. The prepared Ni-doped CoP catalyst was directly loaded onto a foam-type Ni-based gas diffusion layer for effective application in AEMWE. It was found that the electrocatalytic activity of the AEMWE depends on the porosity of the NFs as catalyst substrates. The unit cell containing the membrane-electrode-assembly fabricated with NCP-10 delivered a high current density of 1.12 A cm−2 at 1.8 V and a low reduction rate of 0.64 mA cm−2 h−1 for 250 h.

Handily etching nickel foams into catalyst–substrate fusion self‐stabilized electrodes toward industrial‐level water electrolysis

AbstractThe key challenge of industrial water electrolysis is to design catalytic electrodes that can stabilize high current density with low power consumption (i.e., overpotential), while industrial harsh conditions make the balance between electrode activity and stability more difficult. Here, we develop an efficient and durable electrode for water oxidation reaction (WOR), which yields a high current density of 1000 mA cm−2 at an overpotential of only 284 mV in 1 M KOH at 25°C and shows robust stability even in 6 M KOH strong alkali with an elevated temperature up to 80°C. This electrode is fabricated from a cheap nickel foam (NF) substrate through a simple one‐step solution etching method, resulting in the growth of ultrafine phosphorus doped nickel‐iron (oxy)hydroxide [P‐(Ni,Fe)OxHy] nanoparticles embedded into abundant micropores on the surface, featured as a self‐stabilized catalyst–substrate fusion electrode. Such self‐stabilizing effect fastens highly active P‐(Ni,Fe)OxHy species on conductive NF substrates with significant contribution to catalyst fixation and charge transfer, realizing a win–win tactics for WOR activity and durability at high current densities in harsh environments. This work affords a cost‐effective WOR electrode that can well work at large current densities, suggestive of the rational design of catalyst electrodes toward industrial‐scale water electrolysis.

Direct electrification of Rh/Al2O3 washcoated SiSiC foams for methane steam reforming: An experimental and modelling study

Electrified methane steam reforming (eMSR) is a promising concept for low-carbon hydrogen production. We investigate an innovative eMSR reactor where SiSiC foams, coated with Rh/Al2O3 catalyst, act as electrical resistances to generate the reaction heat via the Joule effect. The novel system was studied at different temperatures, space velocities, operating pressures and catalyst loadings. Thanks to efficient heating, active catalyst and optimal substrate geometry, complete methane conversions were observed even at a high space velocity of 200000 Nl/h/kgcat. A specific energy demand as low as 1.24 kWh/Nm3H2, with an unprecedented energy efficiency of 81%, was achieved on a washcoated foam with catalyst density of 86.3 g/L (GHSV = 150000 Nl/h/kgcat, S/C = 4.1, ambient pressure). A mathematical model was validated against measured performance indicators and used to design an intensified eMSR unit for small scale H2 production.

Curvature effect on graphene-based Co/Ni single-atom catalysts

Graphene has been widely utilized as a substrate for metal-based catalysts due to its unique structural and material properties. Despite the numerous strategies proposed for designing graphene-based catalysts, the effect of curvature on their performance has received relatively little attention. In this work, we construct novel curved graphene models by compressing lattice constant and curving C–C bonds. Curved graphene-based single-atom catalysts (SACs) are obtained by doping Co, Ni, or CoNi atoms with pyridine nitrogen coordination on the curved graphene. Through first-principles calculations, we investigate the effect of curvature on both graphene and graphene-based SACs in terms of their structure and catalytic performance for the hydrogen evolution reaction, the oxygen evolution reaction, and the oxygen reduction reaction. Our results show that tuning the curvature of graphene can effectively modulate the catalytic performance and stability of graphene-based SACs, providing novel design guidance for these materials.

Nickel-Catalyzed Asymmetric Hydrogenation of α-Substituted Vinylphosphonates and Diarylvinylphosphine Oxides.

Chiral α-substituted ethylphosphonate and ethylphosphine oxide compounds are widely used in drugs, pesticides, and ligands. However, their catalytic asymmetric synthesis is still rare. Of the only asymmetric hydrogenation methods available at present, all cases use rare metal catalysts. Herein, we report an efficient earth-abundant transition-metal nickel catalyzed asymmetric hydrogenation affording the corresponding chiral ethylphosphine products with up to 99% yield, 96% ee (enantiomeric excess) (99% ee, after recrystallization) and 1000 S/C (substrate/catalyst); this is also the first study on the asymmetric hydrogenation of terminal olefins using a nickel catalyst under a hydrogen atmosphere. The catalytic mechanism was investigated via deuterium-labelling experiments and calculations which indicate that the two added hydrogen atoms of the products come from hydrogen gas. Additionally, it is believed that the reaction involves a Ni(II) rather than Ni(0) cyclic process based on the weak attractive interactions between the Ni catalyst and terminal olefin substrate.

Nickel‐Catalyzed Asymmetric Hydrogenation of α‐Substituted Vinylphosphonates and Diarylvinylphosphine Oxides

AbstractChiral α‐substituted ethylphosphonate and ethylphosphine oxide compounds are widely used in drugs, pesticides, and ligands. However, their catalytic asymmetric synthesis is still rare. Of the only asymmetric hydrogenation methods available at present, all cases use rare metal catalysts. Herein, we report an efficient earth‐abundant transition‐metal nickel catalyzed asymmetric hydrogenation affording the corresponding chiral ethylphosphine products with up to 99 % yield, 96 % ee (enantiomeric excess) (99 % ee, after recrystallization) and 1000 S/C (substrate/catalyst); this is also the first study on the asymmetric hydrogenation of terminal olefins using a nickel catalyst under a hydrogen atmosphere. The catalytic mechanism was investigated via deuterium‐labelling experiments and calculations which indicate that the two added hydrogen atoms of the products come from hydrogen gas. Additionally, it is believed that the reaction involves a NiII rather than Ni0 cyclic process based on the weak attractive interactions between the Ni catalyst and terminal olefin substrate.

Computational Understanding of Dual Gold and Photoredox-Catalyzed Regioselective Thiosulfonylation of Alkenes.

Herein, a computational work was carried out to gain mechanistic insights into dual gold and photoredox-catalyzed regioselective thiosulfonylation of alkenes with PhSO2SCF3. Computational results suggest that it is more favorable for the complex of Au(I) with PhSO2SCF3 (INT1), instead of an Au(I) catalyst or individual substrates, to quench the excited *[Ru]II photocatalyst in a single-electron oxidative manner to afford [Ru]III. The complexation of the Au(I) catalyst with PhSO2SCF3 could lead to a substantially lowered energy level of the lowest unoccupied molecular orbital, which may be mainly responsible for the feasibility of INT1 in quenching the excited photocatalyst. The resultant single-electron reduced complex, subsequently, is ready to undergo a S-S bond cleavage to form an Au(I)-SCF3 species and a benzenesulfonyl radical. Next, the yielded Au(I)-SCF3 species could undergo single-electron oxidation by [Ru]III to afford an Au(II) intermediate. Subsequently, the binding with an alkyl radical for the formed Au(II) species could occur to further convert to an Au(III) species, from which the final product can be furnished by a reductive elimination step and the Au(I) catalyst is regenerated. Thus, an Au(I)/Au(II)/Au(III)/Au(I) catalytic cycle is suggested to mainly account for the regioselective thiosulfonylation of alkenes.

The Influence of Loadings and Substrates on the Performance of Nickel‐Based Catalysts for the Oxygen Evolution Reaction

AbstractEfficient and durable catalysts for the oxygen evolution reaction (OER) are of great importance for energy storage and conversion devices. However, an objective evaluation and fair comparison of different catalysts remain challenging due to the different catalyst loadings and substrates for OER measurements. In this work, we investigated NiFe layer double hydroxide and commercial Ni/NiO catalysts with different loadings and substrates of glassy carbon (GC), porous nickel foam (NF), and carbon paper (CP). The activity, cycling stability, and potentiostatic stability of the catalysts are compared with respect to the loading and substrate. Catalyst loading exhibits a volcano trend with OER activity, while it has little impact on stability. The 3D substrates NF and CP significantly improved the OER activity of the catalysts compared to GC, especially at higher loadings. The consistent degradation trend of the catalysts confirms the validity of using NF or CP as substrates for the stability test.

A Si(111)-(7 × 7) surface as a natural substrate for identical cluster catalysts

We for the first time demonstrate that the Si(111) surface with 7 × 7 reconstruction, a commonly available material in the laboratory, is an ideal substrate to prepare subnanometer identical clusters for catalytic applications.

Aerobic oxidative C-C bond cleavage and functionalization for the synthesis of value-added chemicals.

The aerobic oxidative cleavage of C-C bonds is an attractive and sustainable route for constructing valuable molecules such as esters, nitriles, and amides. Traditionally homogeneous catalytic systems for C-C bond cleavage required harsh conditions, stoichiometric oxidants, and noble metal catalysts to overcome the thermodynamic and kinetic barriers of C-C bonds, imposing environmental concerns of the transformation. Therefore, developing efficient, low-cost, and environmentally benign methods for C-C bond cleavage is of great importance and a cutting-edge area in modern chemistry. This feature article summarizes the sustainable aerobic oxidative C-C bond cleavage method developed by our group in the past 5 years. Fundamental principles in catalyst design, substrate scope, and mechanism for C-C bond cleavage are also discussed.

Six metal-organic architectures from a 5-methoxyisophthalate linker: assembly, structural variety and catalytic features.

A methoxy-functionalized isophthalic acid, 5-methoxy isophthalic acid (H2mia), was used a versatile linker for assembling six new metal(ii) compounds under hydrothermal conditions. The obtained products were [Cu2(μ2-mia)2(phen)2(H2O)2]·2H2O (1), [Mn(μ3-mia)(phen)]n (2), [Co(μ2-mia)(2,2'-bipy)(H2O)]n·nH2O (3), [Co(μ3-mia)(μ2-4,4'-bipy)]n·nH2O (4), [Co(μ3-mia)(py)2]n (5), and [Cd(μ2-mia)(py)(H2O)2]n·nH2O (6), where phen(1,10-phenanthroline), 2,2'-bipy(2,2'-bipyridine), 4,4'-bipy(4,4'-bipyridine) or py(pyridine) were incorporated as auxiliary ligands. The crystal structures of 1-6 range from 0D (1) and 1D (2, 3, 5, 6) CPs to a 2D network (4) with a variety of topological types. The catalytic behavior of 1-6 was studied in the cyanosilylation reaction between trimethylsilyl cyanide and aldehydes, resulting in up to 99% yields of products under optimized conditions. Various reaction parameters as well as catalyst recycling and substrate scope were investigated. This study widens the use of H2mia as a versatile dicarboxylate linker for assembling a diversity of functional metal-organic architectures with remarkable structural features and catalytic properties.

Effect of Mg Powder's Particle Size on Structure and Mechanical Properties of Ti Foam Synthesized by Space Holder Technique.

Titanium foam has been the focus of special attention for its specific structure and potential applications in purification, catalyst substrate, heat exchanger, biomaterial, aerospace and naval industries. However, the liquid-state foaming techniques are difficult to use in fabricating Ti foam because of its high melting temperature and strong chemical reactivity with atmospheric gases. Here, the fabrication of Ti foams via the powder metallurgy route was carried out by utilizing both magnesium powders and magnesium particles as spacer holders, and Ti powders as matrix metal. The green compacts containing Ti powder, Mg powder and Mg particles were heated to a certain temperature to remove the magnesium and obtain the Ti foam. The results show that the porosities of the obtained Ti foam are about 35-65%, and Young's modulus and yield strength are found to be in the ranges of 22-126 MPa and 0.063-1.18 GPa, respectively. It is found that the magnesium powders play a more important role than the magnesium particles in the deformation and the densification of the green compact during the pressing, and the pore structure of Ti foam depends on the amount and the size of the magnesium spacer holders after sintering.