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.     

Simple Preparation and Application of TEMPO‐Coated Fe3O4 Superparamagnetic Nanoparticles for Selective Oxidation of Alcohols

The organic oxidant TEMPO (2,2,4,4‐tetramethylpiperdine‐1‐oxyl) was immobilized on iron oxide (Fe₃O₄) superparamagnetic nanoparticles by employing strong metal‐oxide chelating phosphonates and azide/alkyne “click” chemistry. This simple preparation yields recyclable TEMPO‐coated nanoparticles with good TEMPO loadings. They have excellent magnetic response and efficiently catalyze the oxidation of a wide range of primary and secondary alcohols to aldehydes, ketones, and lactones under either aerobic acidic Mn^II/Cu^II oxidizing Minisci conditions, or basic NaOCl Anelli conditions. The nanoparticles could be recycled more than 20 times under the Minisci conditions and up to eight times under the Anelli conditions with good to excellent substrate conversions and product selectivities. Immobilization of the catalyst through a phosphonate linkage allows the particles to withstand acidic oxidizing environments with minimal catalyst leaching. Clicking TEMPO to the phosphonate prior to phosphonate immobilization, rather than after, ensures the clicked catalyst is the only species on the particle surface. This facilitates quantification of the catalyst loading. The stability of the phosphonate linker and simplicity of this catalyst immobilization method make this an attractive approach for tethering catalysts to oxide supports, creating magnetically separable catalysts that can be used under neutral or acidic conditions.     

Nanoparticle Growth in Supported Nickel Catalysts during Methanation Reaction—Larger is Better

A major cause of supported metal catalyst deactivation is particle growth by Ostwald ripening. Nickel catalysts, used in the methanation reaction, may suffer greatly from this through the formation of [Ni(CO)₄]. By analyzing catalysts with various particle sizes and spatial distributions, the interparticle distance was found to have little effect on the stability, because formation and decomposition of nickel carbonyl rather than diffusion was rate limiting. Small particles (3–4 nm) were found to grow very large (20–200 nm), involving local destruction of the support, which was detrimental to the catalyst stability. However, medium sized particles (8 nm) remained confined by the pores of the support displaying enhanced stability, and an activity 3 times higher than initially small particles after 150 h. Physical modeling suggests that the higher [Ni(CO)₄] supersaturation in catalysts with smaller particles enabled them to overcome the mechanical resistance of the support. Understanding the interplay of particle size and support properties related to the stability of nanoparticles offers the prospect of novel strategies to develop more stable nanostructured materials, also for applications beyond catalysis.     

Design and Synthesis of Copper–Cobalt Catalysts for the Selective Conversion of Synthesis Gas to Ethanol and Higher Alcohols

Combining quantum‐mechanical simulations and synthesis tools allows the design of highly efficient CuCo/MoO_x catalysts for the selective conversion of synthesis gas (CO+H₂) into ethanol and higher alcohols, which are of eminent interest for the production of platform chemicals from non‐petroleum feedstocks. Density functional theory calculations coupled to microkinetic models identify mixed Cu–Co alloy sites, at Co‐enriched surfaces, as ideal for the selective production of long‐chain alcohols. Accordingly, a versatile synthesis route is developed based on metal nanoparticle exsolution from a molybdate precursor compound whose crystalline structure isomorphically accommodates Cu²⁺ and Co²⁺ cations in a wide range of compositions. As revealed by energy‐dispersive X‐ray nanospectroscopy and temperature‐resolved X‐ray diffraction, superior mixing of Cu and Co species promotes formation of CuCo alloy nanocrystals after activation, leading to two orders of magnitude higher yield to high alcohols than a benchmark CuCoCr catalyst. Substantiating simulations, the yield to high alcohols is maximized in parallel to the CuCo alloy contribution, for Co‐rich surface compositions, for which Cu phase segregation is prevented.     

Solvent-Dependent Growth and Stabilization Mechanisms of Surfactant-Free Colloidal Pt Nanoparticles

Understanding the formation of nanoparticles (NPs) is key to develop materials by sustainable routes. The Co4Cat^TM process is a new synthesis of precious metal NPs in alkaline mono-alcohols well-suited to develop active nanocatalysts. The synthesis is ‘facile’, surfactant-free and performed under mild conditions like low temperature. The reducing properties of the solvent are here shown to strongly influence the formation of Pt NPs. Based on the in situ formation of CO adsorbed on the NP surface by solvent oxidation, a model is proposed that accounts for the different growth and stabilization mechanisms as well as re-dispersion properties of the surfactant-free NPs in different solvents. Using in situ and ex situ characterizations, it is established that in methanol, a slow nucleation with a limited NP growth is achieved. In ethanol, a fast nucleation followed by continuous and pronounced particle sintering occurs.     

Palladium Nanoparticles Immobilized on Individual Calcium Carbonate Plates Derived from Mussel Shell Waste: An Ecofriendly Catalyst for the Copper‐Free Sonogashira Coupling Reaction

The conversion of waste into high‐value materials is considered an important sustainability strategy in modern chemical industries. A large volume of shell waste is generated globally from mussel cultivation. In this work, mussel shell waste (Perna viridis ) is transformed into individual calcium carbonate plates (ICCPs) and is applied as a support for a heterogeneous catalyst. Palladium nanoparticles (3–6 nm) are deposited with an even dispersion on the ICCP surface, as demonstrated by X‐ray diffraction and scanning electron microscopy. Using this system, Sonogashira cross‐coupling reactions between aryl iodides and terminal acetylenes were accomplished in high yields with the use of 1 % Pd/ICCP in the presence of potassium carbonate without the use of any copper metal or external ligand. The Pd/ICCP catalyst could also be reused up to three times and activity over 90 % was maintained with negligible Pd‐metal leaching. This work demonstrates that mussel shell waste can be used as an inexpensive and effective support for metal catalysts in coupling reactions, as demonstrated by the successful performance of the Pd‐catalyzed, copper‐free Sonogashira cross‐coupling process.     

CO2 Hydrogenation over Oxide‐Supported PtCo Catalysts: The Role of the Oxide Support in Determining the Product Selectivity

By simply changing the oxide support, the selectivity of a metal–oxide catalysts can be tuned. For the CO₂ hydrogenation over PtCo bimetallic catalysts supported on different reducible oxides (CeO₂, ZrO₂, and TiO₂), replacing a TiO₂ support by CeO₂ or ZrO₂ selectively strengthens the binding of C,O‐bound and O‐bound species at the PtCo–oxide interface, leading to a different product selectivity. These results reveal mechanistic insights into how the catalytic performance of metal–oxide catalysts can be fine‐tuned.     

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.     

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.     

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。     

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.     

Bridging the Materials Gap in Catalysis: Entrapment of Molecular Catalysts in Functional Supports and Beyond

Rising sun of the materials world : Tremendous efforts are being made to combine the potential of molecular catalysts with that of functional supports. An approach towards unifying homogeneous and heterogeneous catalysis is the entrapment of organometallic catalysts in a metal matrix, which leads to well‐defined composites that are suitable as heterogeneous catalysts for hydrogenation of styrene and diphenylacetylene.

     

Polymeric Carbon Nitride/Mesoporous Silica Composites as Catalyst Support for Au and Pt Nanoparticles

Small and homogeneously dispersed Au and Pt nanoparticles (NPs) were prepared on polymeric carbon nitride (CN_x)/mesoporous silica (SBA‐15) composites, which were synthesized by thermal polycondensation of dicyandiamide‐impregnated preformed SBA‐15. By changing the condensation temperature, the degree of condensation and the loading of CN_x can be controlled to give adjustable particle sizes of the Pt and Au NPs subsequently formed on the composites. In contrast to the pure SBA‐15 support, coating of SBA‐15 with polymeric CN_x resulted in much smaller and better‐dispersed metal NPs. Furthermore, under catalytic conditions the CN_x coating helps to stabilize the metal NPs. However, metal NPs on CN_x/SBA‐15 can show very different catalytic behaviors in, for example, the CO oxidation reaction. Whereas the Pt NPs already show full CO conversion at 160 °C, the catalytic activity of Au NPs seems to be inhibited by the CN_x support.     

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.     

TEMPO supported on magnetic C/Co-nanoparticles: a highly active and recyclable organocatalyst

TEMPO was grafted on graphene-coated nanobeads with a magnetic cobalt core by using a general applicable "click"-chemistry protocol. The new heterogeneous CoNP-TEMPO emerged as a highly active catalyst for the chemoselective oxidation of primary and secondary alcohols using bleach as terminal oxidant. The outstanding stability of the C/Co nanoparticles enables the nanopowder to tolerate several TEMPO-mediated iterative oxidation reactions without any significant loss in catalyst activity. Furthermore, the excellent magnetic properties enable the rapid separation and quantitative recycling of CoNP-TEMPO out of the reaction mixture by simple magnetic decantation. The recovered nanoparticles can be subsequently reused without any further purification.     

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.