Nuclear Magnetic Resonance in Solid-State Chemistry

Various problems of solid-state chemistry can be solved by spectroscopic investigation of the nuclear magnetic resonance (NMR and NQR methods). This progress report presents a survey of the nature of the interactions in the crystal that underlie such measurements, of the observations in the spectrum, and of the information obtainable from it. The field of application of the methods covers both static and dynamic properties of solids.     

Discovery and Optimization of Heterogeneous Catalysts by Using Combinatorial Chemistry

A novel ceramic array microreactor system has been designed and, in conjunction with resonance-enhanced multiphoton ionization (REMPI), used for the discovery of an optimum ternary catalyst composition for the dehydrogenation of cyclohexane to benzene. The catalyst library consisted of 66 ternary combinations of Pt, Pd, and In loaded on γ-Al₂O₃ pellets. The optimum catalyst for the production of benzene had the composition 0.8 % Pt, 0.1 % Pd, and 0.1 % In (see diagram). The preparation and screening of the library of 66 catalysts took about 2.5 days to complete—with conventional methods this would have taken several months!     

Elementary Steps in Heterogeneous Catalysis

Despite the great importance of heterogeneous catalysis, research in this field has long been characterized by its empiricism. Now, however, thanks to the rapid development of methods in surface physics, the elementary steps can be identified at the atomic level and the underlying principles understood. Defined single crystal surfaces are employed as models, based on the analysis of the surfaces of ‘real’ catalysts. Direct images, with atomic resolution, can be obtained using scanning tunneling microscopy, while electron spectroscopic methods yield detailed information on the bonding state of adsorbed species and the influence of catalyst additives (promotors) upon them. The successful application of this approach is illustrated with reference to the elucidation of the mechanism of ammonia synthesis. The catalyst surface is usually transformed under reaction conditions, and, as the processes involved are far-removed from equilibrium, such transformations can lead to intrinsic spatial and temporal self-organization phenomena. In this case, the reaction rate may not remain constant under otherwise invariant conditions but will change periodically or exhibit chaotic behavior, with the formation of spatial patterns on the catalyst surface.     

Inter-Metal Interaction of Dual-Atom Catalysts in Heterogeneous Catalysis

Dual-atom catalysts (DACs) have been a new frontier in heterogeneous catalysis due to their unique intrinsic properties. The synergy between dual atoms provides flexible active sites, promising to enhance performance and even catalyze more complex reactions. However, precisely regulating active site structure and uncovering dual-atom metal interaction remain grand challenges. In this review, we clarify the significance of the inter-metal interaction of DACs based on the understanding of active center structures. Three diatomic configurations are elaborated, including isolated dual single-atom, N/O-bridged dual-atom, and direct dual-metal bonding interaction. Subsequently, the up-to-date progress in heterogeneous oxidation reactions, hydrogenation/dehydrogenation reactions, electrocatalytic reactions, and photocatalytic reactions are summarized. The structure-activity relationship between DACs and catalytic performance is then discussed at an atomic level. Finally, the challenges and future directions to engineer the structure of DACs are discussed. This review will offer new prospects for the rational design of efficient DACs toward heterogeneous catalysis.     

Synchrotron X-ray and Neutron Diffraction Studies in Solid-State Chemistry

Neutron diffraction studies, especially with powders, play an important role in structural solid-state chemistry, making possible the precise determination of the location of light atoms, particularly hydrogen, and enabling a distinction to be made between certain neighboring elements in the periodic table that are difficult to distinguish in experiments with X-rays. Neutron diffraction investigations also make a unique contribution in the area of magnetic structure determination. The availability of intense synchrotron X-rays sources, however, is opening up new opportunities to the structural chemist, many of them complementary to the “traditional” strengths of neutron methods. The key features of synchrotron radiation in relation to structural studies are the wavelength tunability, which facilitates the use of resonant diffraction methods, and the high brightness and excellent vertical collimation of the source, which make possible the construction of diffractometers with unparalleled angular and spatial resolution. The following types of experiments are now possible with synchrotron X-ray diffraction: (1) The ab initio determination of structures from powder diffraction data. (2) The differentiation between different oxidation states of an element (valence contrast experiments) based upon the sensitivity of an absorption edge to the valence of the element in question. (3) The differentiation of elements adjacent to each other in the periodic table, which is now feasible with synchrotron X-rays for all elements beyond chromium. (4) Site-selective X-ray absorption spectroscopy. (5) The study of cation occupancies in materials where more than one element occupies a site that is, or may be, partially occupied. (Such problems are important in zeolite chemistry and high-temperature superconductors.) (6) The determination of crystal structures from microcrystals. (7) In situ and rapid, time-resolved diffraction studies. This review examines the roles played by X-ray and neutron diffraction studies in modern solid-state chemistry, and describes some recent examples in which the use of neutron radiation or synchrotron X-rays has been advantageous.     

Combinatorial Chemistry—Challenge and Chance for the Development of New Catalysts and Materials

One should not underestimate the capability of the combinatorial method in solid-state chemistry; this is the opinion of the author. Combinatorial chemistry can provide a large number of new compounds, but once the components that are interesting for a certain application have been successfully selected, the techniques of conventional catalysis and materials research are required. The strengths of conventional chemistry lie in the optimization, systematic modification, and improvement of new lead structures. In contrast, discovery is the potential strength of combinatorial chemistry. Careful design is most important for the synthesis of useful libraries, since the diversity of the periodic table is much too large to be accessed comprehensively or systematically by such large libraries.     

Colloid-Bound Catalysts for Ring-Opening Metathesis Polymerization: A Combination of Homogenous and Heterogeneous Properties

Tailor-made thiols allow a ruthenium-based catalyst for ring-opening metathesis polymerization (ROMP) to be tethered to bulk gold surfaces and to gold colloids (see picture). The functionalized gold colloids combine the properties of homogeneous and heterogeneous catalyst systems. Tethering of the catalyst leads to a pronounced increase in its catalytic activity.     

Our Contributions in Synthesis of Diverse Heterocyclic Scaffolds by Using Mixed Oxides as Heterogeneous Catalysts

This personal account mainly introduces and reviews our recent contributions in developing different catalyst materials involving mixed oxides and their scope as renewable catalysts in multicomponent reactions to synthesize various novel heterocyclic scaffolds under green conditions. The application of various mixed oxides and their composites in the organic synthesis is emphasized through this review, in order to reveal the versatility, scope and importance of mixed oxides and their interactions during the reaction. We have also briefed the limitations of mixed oxides as catalysts, to put forward the broader prospective in the field.     

Water-Soluble Ligands,Metal Complexes,and Catalysts: Synergism of Homogeneous and Heterogeneous Catalysis

Rapid developments in the field of catalysis are leading to an increased demand for tailor-made catalysts. Water-soluble complex catalysts, which are being intensively investigated at the present time, combine the advantages of homogeneous and heterogeneous catalysis: simple and complete separation of the product from the catalyst, high activity, and high selectivity. From the large number of available water-soluble ligands, the appropriate catalysts can be developed for many reactions. The industrial applications in the fields of hydrogenation and hydroformylation have already indicated the wide scope of this type of catalyst. In addition, the annual production of 300 000 tons of butyraldehyde through application of water-soluble rhodium complexes at Hoechst AG in Oberhausen, Germany, has demonstrated the industrial importance of the concept of complex-catalyzed reactions in aqueous two-phase systems. The efficient operation of catalytic processes increasingly requires the loss-free recycling of the noble metal catalyst, and this can be simply and economically realized in two-phase systems. Special applications in biochemical problems open up developments in the field of water-soluble transition metal complexes that far transcend the familiar kinds of homogeneous catalysis. In the near future, the investigation and application of metal complex catalysts that are compatible with the physiological, cheap, and environmentally friendly solvent, water, is likely to become a high priority in catalysis research.     

Detection of Catalytic Activity in Combinatorial Libraries of Heterogeneous Catalysts by IR Thermography

Only about 200 μg of catalyst are required to identify the catalytically active materials on a combinatorial library of heterogeneous catalysts by IR thermography. The procedure is highly sensitive, and temperature differences arising from catalytic activity of hydrogenations and oxidations have been reliably detected down to 0.1 K.     

复相催化反应实验中活性催化剂的探索

为了改善流动法测定催化剂活性的实验,通过原位还原置换法将Ag负载于Raney-Cu表面,制备Ag/Raney-Cu催化剂,并比较了Ag/Raney-Cu与Raney-Cu及传统ZnO/Al_2O_3催化剂对甲醇分解反应的催化性能。研究发现所制备的Ag/Raney-Cu及Raney-Cu均显示了高于ZnO/Al_2O_3催化剂的低温催化活性。在250℃时,Raney-Cu的活性高于Ag/Raney-Cu;随着温度升高,Ag/Raney-Cu的活性显著增强,当温度升高至350℃,其活性高于Raney-Cu的活性。     

金属-有机框架作为异相催化剂的表征方法概述

作为一类具有大的比表面积、高孔隙率、合成方便、骨架规模可变、化学可修饰以及结构组成多样等优点的新型多孔材料,金属-有机框架（MOFs）在光电材料、药物传输、气体吸附分离及催化等领域有着广阔的应用前景,成为近年来研究的热点。异相催化是MOFs最具发展潜力的应用领域之一,各种表征方法和研究手段是开展MOFs异相催化研究的工作基础。本文主要围绕表征MOFs作为异相催化剂的常用技术手段进行介绍,包括X-射线单晶衍射、X-射线粉末衍射、热重分析、红外光谱/拉曼光谱分析、透射/扫描电镜等,旨在为开展相关MOFs催化研究提供一定参考。     

Solid Ion Conductors in Heterogeneous Catalysis

The electrochemical measurement of oxygen activity using ion-conducting solid electrolytes (λ-sensors) has become widely known, at least since the application of three-way catalysts in the postcombustion of exhaust gases from spark-ignition engines. However, the use of solid ion conductors is not limited to control devices. There are various other potential applications and numerous problems which can be studied: the formation of oxides in the course of catalytic reactions on metal surfaces, the improvement of selectivity and yield of catalytic reactions, such as the epoxidation of ethylene on silver catalysts and, finally, the cogeneration of electrical energy during oxidation reactions, such as the partial oxidation of methanol to formaldehyde.     

Zinc Silicates: Very Efficient Heterogeneous Catalysts for the Addition of Primary Alcohols to Alkynes and Allenes

There is an astonishing parallel between the mechanism generally accepted for the addition of water to CO₂ catalyzed by the enzyme carbonic anhydrase and the mechanism calculated for the addition of methanol to allene catalyzed by the naturally occurring zinc silicate hemimorphite. The latter reaction was investigated in detail following the observation that hemimorphite as well as an amorphous zinc silicate prepared in situ are excellent heterogeneous catalysts for the addition of primary alcohols to alkynes and allenes [Eq. (1)].     

How Chemistry and Physics Meet in the Solid State

To make sense of the marvelous electronic properties of the solid state, chemists must learn the language of solid-state physics, of band structures. An attempt is made here to demystify that language, drawing explicit parallels to well-known concepts in theoretical chemistry To the joint search of physicists and chemists for understanding of the bonding in extended systems, the chemist brings a great deal of intuition and some simple but powerful notions. Most important among these is the idea of a bond, and the use of frontier-orbital arguments. How to find localized bonds among all those maximally delocalized bands? Interpretative constructs, such as the density of states, the decomposition of these densities, and crystal orbital overlap populations, allow a recovery of bonds, a finding of the frontier orbitals that control structure and reactivity in extended systems as well as discrete molecules.