Chain Mail for Catalysts

Encapsulating transition‐metal nanoparticles inside carbon nanotubes (CNTs) or spheres has emerged as a novel strategy for designing highly durable nonprecious‐metal catalysts. The stable carbon layer protects the inner metal core from the destructive reaction environment and thus is described as chain mail for catalysts. Electron transfer from the active metal core to the carbon layer stimulates unique catalytic activity on the carbon surface, which has been utilized extensively in a variety of catalytic reaction systems. Here, we elaborate the underlying working principle of chain mail for catalysts as well as the key factors that determine their catalytic properties, and provide insights into the physicochemical nature of such catalyst architectures for further application of the strategy in rational catalyst design.     

Colloids for Catalysts: A Concept for the Preparation of Superior Catalysts of Industrial Relevance

Compared to conventional preparation methods for supported heterogeneous catalysts, the use of colloidal nanoparticles (NPs) allows for a precise control over size, size distribution, and distribution/location of the NPs on the support. However, common colloidal syntheses have restrictions that limit their applicability for industrial catalyst preparation. We present a simple, surfactant‐free, and scalable preparation method for colloidal NPs to overcome these restrictions. We demonstrate how precious‐metal NPs are prepared in alkaline methanol, how the particle size can be tuned, and how supported catalysts are obtained. The potential of these colloids in the preparation of improved catalysts is demonstrated by two examples from heterogeneous catalysis and electrocatalysis.     

金催化剂及其在有机反应中的应用研究进展*

本文综述了金催化剂在选择氧化、选择加氢、不对称醇醛缩合、C-N键生成等多相和均相催化反应中的应用研究,并讨论了各种影响金催化剂催化活性的因素,最后展望了金催化剂的应用前景.     

Surfactant‐Encapsulated Polyoxometalates as Immobilized Supramolecular Catalysts for Highly Efficient and Selective Oxidation Reactions

A type of interesting immobilized supramolecular catalysts based on surfactant‐encapsulated polyoxometalates has been developed for oxidation reactions. Through a sol‐gel process with tetraethyl orthosilicate, hydroxyl‐terminated surfactant‐encapsulated polyoxometalate complexes have been covalently and uniformly bound to a silica matrix with unchanged complex structure. The formed hybrid catalysts possess a defined hydrophobic nano‐environment surrounding the inorganic clusters, which is conducive to compatibility between the polyoxometalate catalytic centres and organic substrates. The supramolecular synergy between substrate adsorption, reaction, and product desorption during the oxidation process has been found to have an obvious influence on the reaction kinetics, with the activity of the catalyst being greatly improved. The supramolecular catalysts performed effectively in the selective oxidation of several different kinds of organic compounds, such as alkenes, alcohols, and sulfides, and the main products were the corresponding epoxides, ketones, sulfoxides, and sulfones. More significantly, the catalyst could be easily recovered by simple filtration, and the catalytic activity was well retained for at least five cycles. Finally, the present strategy has proved to be a general route for the fabrication of supramolecular hybrid catalysts containing common polyoxometalates suitable for various purposes.     

共浸渍制备高效的Ru/AC氨合成催化剂

通过钌的络合物前驱体和硝酸钡的共浸渍制备的Ru Ba K/AC催化剂氨合成转化效率高,其氨合成转化频率在0.87～1.30 s-1之间,与氯化钌制备的Ru/AC催化剂相比,其转化频率提高幅度在26%～88%。共浸渍法制备的催化剂氨合成转化效率高,其主要原因可能是共浸渍法制备的催化剂钌粒子粒径分布区间较窄,易形成更多的活性位;钌表面氢的吸附受到抑制,氮更易活化,因而催化效率更高。     

An Overview of Heterogeneous Transition Metal-Based Catalysts for Cyclohexene Epoxidation Reaction

Cyclohexane epoxide, which contains highly active epoxy groups, plays a crucial role as an intermediate in the preparation of fine chemicals. However, controlling the epoxidation pathway of cyclohexene is challenging due to issues such as the allylic oxidation of cyclohexene and the ring opening of cyclohexane epoxide during the cyclohexene epoxidation process to form cyclohexane oxide. This review focuses on the structure-activity relationships and synthesis processes of various heterogeneous transition metal-based catalysts used in cyclohexene epoxidation reactions, including molybdenum(Mo)-based, tungsten(W)-based, vanadium(V)-based, titanium(Ti)-based, cobalt(Co)-based, and other catalysts. Initially, the mechanism of cyclohexene epoxidation by transition metal-based catalysts is examined from the perspective of catalytic active centers. Subsequently, the current research of cyclohexene epoxidation catalysts is summarized based on the perspective of catalyst support. Additionally, the differences between alkyl hydroperoxide, hydrogen peroxide (H₂O₂), and oxygen (O₂) as oxidants are analyzed. Finally, the main factors influencing catalytic performance are summarized, and reasonable suggestions for catalyst design are proposed. This work provides scientific support for the advancement of the olefin epoxidation industry.     

Metal–Support Cooperative Catalysts for Environmentally Benign Molecular Transformations

Metal–support cooperative catalysts have been developed for sustainable and environmentally benign molecular transformations. The active metal centers and supports in these catalysts could cooperatively activate substrates, resulting in high catalytic performance for liquid‐phase reactions under mild conditions. These catalysts involved hydrotalcite‐supported gold and silver nanoparticles with high catalytic activity for organic reactions such as aerobic oxidation, oxidative carbonylation, and chemoselective reduction of epoxides to alkenes and nitrostyrenes to aminostyrenes using alcohols and CO/H₂O as reducing reagents. This high catalytic performance was due to cooperative catalysis between the metal nanoparticles and basic sites of the hydrotalcite support. To increase the metal–support cooperative effect, core–shell nanostructured catalysts consisting of gold or silver nanoparticles in the core and ceria supports in the shell were designed. These core–shell nanocomposite catalysts were effective for the chemoselective hydrogenation of nitrostyrenes to aminostyrenes, unsaturated aldehydes to allyl alcohols, and alkynes to alkenes using H₂ as a clean reductant. In addition, these solid catalysts could be recovered easily from the reaction mixture by simple filtration, and were reusable with high catalytic activity.     

Single-Site Heterogeneous Catalysts: From Synthesis to NMR Signal Enhancement

Catalysts with well-defined, single, active centers are of great importance and their utilization allows the gap between homo- and heterogeneous catalysis to be bridged and, importantly, the main selectivity problem of heterogeneous catalysis and the main separation challenge of homogeneous catalysis to be overcome. Moreover, the use of single-site catalysts allows the NMR signal to be significantly enhanced through the pairwise addition of two hydrogen atoms from a parahydrogen molecule to an unsaturated substrate. This review covers the fundamentals of the synthesis of single-site catalysts and shows the new aspects of their applications in both modern catalysis and the field of parahydrogen-based hyperpolarization. The different novel aspects of the formation and utilization of single-site catalysts, along with the possibility of NMR signal enhancement observations are described.     

Mesoporous Molecular Sieves as Advanced Supports for Olefin Metathesis Catalysts

Summary: Siliceous mesoporous molecular sieves MCM-41, MCM-48 and SBA-15 and organised mesoporous alumina represent progressive supports for new heterogeneous catalysts for olefin metathesis and metathesis polymerization. In combination with Mo and Re oxides they provide catalysts of considerably higher activity in comparison with those based on conventional silica and alumina. Immobilization of Mo and Ru alkylidenes on these materials led to the highly active and selective catalysts with negligible leaching of transition metal.     

Kelvin Probe Force Microscopy Study of a Pt/TiO2 Catalyst Model Placed in an Atmospheric Pressure of N2 Environment

A catalyst model comprising platinum nanoparticles deposited on a TiO₂(110) wafer was prepared in a vacuum, transferred in air, and characterized with a Kelvin probe force microscope placed in a N₂ environment. The topography and local work function of individual nanoparticles were observed with single‐nanometer resolution in the N₂ environment of one atmosphere pressure. Some nanoparticle presented positive shifts of work function relative to that of the TiO₂ surface, while the others showed negative shifts. This finding suggests heterogeneous properties of the nanoparticles exposed to air and then N₂. The ability of the advanced microscope was demonstrated in observing the work function of metal nanoparticles on a metal oxide support even in the presence of vapor environments.     

Diverse Supports for Immobilization of Catalysts in Continuous Flow Reactors

The rapid development of continuous flow processes is driving innovations in various chemical syntheses and industrial productions. Immobilizing catalysts in flow reactors allows transformations with high-efficiency and excludes the subsequent separation procedures. This concept outlines the approaches to incorporate catalysts within flow reactors, with particular focus on the application of additional supports including inorganic materials like silica, zeolite and reduced graphene oxide, polymeric materials like polymer packings, monoliths, cross-linked gels and polymer brushes, and other materials for specific conditions like transparent glass fibers and glass beads. Furthermore, advanced methods to develop ordered micro-/nanoarrays from internal walls of flow channels for immobilization of catalysts as well as application of innovative vortex fluidic devices are discussed to inspire new designs of supports for novel fluidic reactors with broad applications.     

Pd基催化剂在分解甲酸析氢中的研究进展

甲酸是最简单的羧酸,无色、低毒,在室温下便于运输和储存。最近,甲酸作为一种最有前景的储氢材料,在室温下采用异相催化剂分解甲酸制氢气引起了科研工作者的广泛关注。和其他催化剂相比,Pd基催化剂在温和条件下催化甲酸分解制备氢气方面表现出优良的性能,是一种非常理想的非均相催化剂。本文介绍了多种Pd基催化剂的性能特点、制备方法和其在催化甲酸分解制备氢气领域的研究进展,并对其未来研究发展方向进行展望。     

How Strain Affects the Reactivity of Surface Metal Oxide Catalysts

Highly dispersed molybdenum oxide supported on mesoporous silica SBA‐15 has been prepared by anion exchange resulting in a series of catalysts with changing Mo densities (0.2–2.5 Mo atoms nm^?2). X‐ray absorption, UV/Vis, Raman, and IR spectroscopy indicate that doubly anchored tetrahedral dioxo MoO₄ units are the major surface species at all loadings. Higher reducibility at loadings close to the monolayer measured by temperature‐programmed reduction and a steep increase in the catalytic activity observed in metathesis of propene and oxidative dehydrogenation of propane at 8 % of Mo loading are attributed to frustration of Mo oxide surface species and lateral interactions. Based on DFT calculations, NEXAFS spectra at the O‐K‐edge at high Mo loadings are explained by distorted MoO₄ complexes. Limited availability of anchor silanol groups at high loadings forces the MoO₄ groups to form more strained configurations. The occurrence of strain is linked to the increase in reactivity.     

Development of a Continuous‐Flow System for Asymmetric Hydrogenation Using Self‐Supported Chiral Catalysts

Well‐designed, self‐assembled, metal–organic frameworks were constructed by simple mixing of multitopic MonoPhos‐based ligands ( 3 ; MonoPhos=chiral, monodentate phosphoramidites based on the 1,1′‐bi‐2‐naphthol platform) and [Rh(cod)₂]BF₄ (cod=cycloocta‐1,5‐diene). This self‐supporting strategy allowed for simple and efficient catalyst immobilization without the use of extra added support, giving well‐characterized, insoluble (in toluene) polymeric materials ( 4 ). The resulting self‐supported catalysts ( 4 ) showed outstanding catalytic performance for the asymmetric hydrogenation of a number of α‐dehydroamino acids ( 5 ) and 2‐aryl enamides ( 7 ) with enantiomeric excess (ee) ranges of 94–98 % and 90–98 %, respectively. The linker moiety in 4 influenced the reactivity significantly, albeit with slight impact on the enantioselectivity. Acquisition of reaction profiles under steady‐state conditions showed 4 h and 4 i to have the highest reactivity (turnover frequency (TOF)=95 and 97 h^?1 at 2 atm, respectively), whereas appropriate substrate/catalyst matching was needed for optimum chiral induction. The former was recycled 10 times without loss in ee (95–96 %), although a drop in TOF of approximately 20 % per cycle was observed. The estimation of effective catalytic sites in self‐supported catalyst 4 e was also carried out by isolation and hydrogenation of catalyst–substrate complex, showing about 37 % of the Rh^I centers in the self‐supported catalyst 4 e are accessible to substrate 5 c in the catalysis. A continuous flow reaction system using an activated C/ 4 h mixture as stationary‐phase catalyst for the asymmetric hydrogenation of 5 b was developed and run continuously for a total of 144 h with >99 % conversion and 96–97 % enantioselectivity. The total Rh leaching in the product solution is 1.7 % of that in original catalyst 4 h .     

Tin-Containing Catalysts,Modified with Alkaline Earth Metals in the Oxidative Dimerization of Methane

The effect of addition alkaline earth metals on the physico-chemical properties of M-Sn-O/-Al₂O₃ and M-Pb-Sn-O/-Al₂O₃ catalysts is studied. The formation of metal metastannates in the M-Sn-O/-Al₂O₃ system facilitated the growth of selectivity of the catalysts. Addition of alkaline earth metals into the Pb-Sn-O/-Al₂O₃ composition resulted in stabilization of lead in the 4-valent state with formation of metal ortho-plumbates and a decrease in the selectivity for C₂ hydrocarbons. The linkage between the basicity of the surface and the catalytic properties of the samples was established.     

Highly Effective Ru/BaCeO3 Catalysts on Supports with Strong Basic Sites for Ammonia Synthesis

BaCeO₃‐a and BaCeO₃‐b, with strong basic sites, were synthesized by using a co‐precipitation method at different calcination temperatures, and used as supports to evaluate their performance in ammonia synthesis. The ammonia synthesis rate with the 1.25 % Ru/BaCeO₃‐a catalyst is 24 mmol g^?1 h^?1, which is higher than that of 1.25 % Ru/BaCeO₃‐b catalyst (18 mmol g^?1 h^?1) at 3 MPa and 450 °C. Moreover, the performance of the 4 % Cs‐1.25 % Ru/BaCeO₃‐a catalyst was further improved to 28 mmol g^?1 h^?1, and no sign of deactivation was observed after a reaction time of 120 h. The XPS and H₂ temperature‐programmed reduction analyses indicated that the Ru/BaCeO₃‐a catalyst has more oxygen vacancies than the Ru/BaCeO₃‐b catalyst. In addition, the average Ru particle size of the Ru/BaCeO₃‐a catalyst is closer to 2 nm than the Ru/BaCeO₃‐b catalyst, which promotes the generation of B₅‐type sites (the active site for N₂ dissociation). The CO₂ temperature‐programmed desorption analysis indicates that BaCeO₃‐a has a high basic density, which is beneficial for electron transfer to Ru and further facilitates the dissociation of N≡N bonds.     

新型后过渡金属烯烃聚合催化剂－－镍系烯烃聚合催化剂*

镍系烯烃聚合催化剂是近年来受到广泛关注的一类新型催化剂,是配位催化研究的热点之一。这类催化剂具有高催化活性、单活性中心和良好的分子剪栽性,可以在分子层次上实现烯烃聚合的分子设计与组装。本文介绍了镍系烯烃聚合催化剂的发展和研究概况,并评述了聚合特性及最新研究进展。     

手性自负载催化剂研究新进展

催化剂的负载和回收再利用是提高其使用效率、降低反应成本和减少金属离子对产物污染的一条有效途径。与传统的负载模式不同, 手性自负载催化剂通过含双或多官能团的手性配体与金属通过自组装形成一类有机-无机聚合物,因此无需使用任何载体,即能够有效地实现手性催化剂的回收和再利用。近年来,手性自负载催化剂作为一种新的负载模式,已经成功地应用于一些非均相催化的不对称反应中。本文概述了手性自负载催化剂的在一些不对称催化反应研究中取得的新进展。     

新型后过滤金属烯烃聚合催化剂——镍系烯烃聚合催化剂

镍系烯烃聚合催化剂是近年来受到广泛关注的一类新型催化剂，是配位催化研究的热点之一。这类催化剂具有高催化活性、单活性中心和良好的分子剪栽性，可以在分子层次上实现烯烃聚合的分子设计与组装。本文介绍了镍系烯烃聚合催化剂的发展和研究概况，并评述了聚合特性及最新研究进展。