Showing papers in "Chemical Reviews in 1995"

Environmental Applications of Semiconductor Photocatalysis

Abstract: The civilian, commercial, and defense sectors of most advanced industrialized nations are faced with a tremendous set of environmental problems related to the remediation of hazardous wastes, contaminated groundwaters, and the control of toxic air contaminants. For example, the slow pace of hazardous waste remediation at military installations around the world is causing a serious delay in conversion of many of these facilities to civilian uses. Over the last 10 years problems related to hazardous waste remediation have emerged as a high national and international priority.

Palladium-catalyzed cross-coupling reactions of organoboron compounds

Abstract: B. Other Catalyti; Process by Transition-Metal Complexes IV. Cross-Coupling Reaction A. Coupling of 1 -Alkenylboron Derivatives: Synthesis of Conjugated Dienes 6. Coupling of Arylboron Derivatives: Synthesis of Biaryls C. Coupling of Alkylboron Derivatives D. Coupling with Triflates E. Synthesis of Vinylic Sulfides F. Coupling with lodoalkanes: Alkyl-Alkyl CouDlino G. Coupling with Other Organic Halides and Boron Reagents V. Head-to-Tail Coupling VI. Carbonylative Coupling VII. Alkoxycarbonylation and Dimerization VIII. Conclusion 2457 2458 2458

Photocatalysis on TiO2 Surfaces - Principles, Mechanisms, and Selected Results

Abstract: In 1972, Fujishima and Honda discovered the photocatalytic splitting of water on TiO{sub 2} electrodes. This event marked the beginning of a new era in heterogeneous photocatalysis. Since then, research efforts in understanding the fundamental processes and in enhancing the photocatalytic efficiency of TiO{sub 2} have come from extensive research performed by chemists, physicists, and chemical engineers. Such studies are often related to energy renewal and energy storage. In recent years, applications to environmental cleanup have been one of the most active areas in heterogeneous photocatalysis. This is inspired by the potential application of TiO{sub 2}-based photocatalysts for the total destruction of organic compounds in polluted air and wastewaters. There exists a vast body of literature dealing with the electron transfer and energy transfer processes in photocatalytic reactions. A detailed description of these processes is beyond the scope of this review. Here, the authors tend to focus on interfacial processes and to summarize some of the operating principles of heterogeneous photocatalysis. In section 2, the authors first look at the electronic excitation processes in a molecule and in a semiconductor substrate. The electronic interaction between the adsorbate molecule and the catalyst substrate is discussed in terms of the catalyzed ormore » sensitized photoreactions. In section 3, thermal and photocatalytic studies on TiO{sub 2} are summarized with emphasis on the common characteristics and fundamental principles of the TiO{sub 2}-based photocatalysis systems. In section 4, they address the research effort in the electronic modification of the semiconductor catalysts and its effect on the photocatalytic efficiency. Several representative examples will be presented including the Schottky barrier formation and modification at metal-semiconductor interfaces. Some concluding remarks and future research directions will be given in the final section. 160 refs.« less

Light-Induced Redox Reactions in Nanocrystalline Systems

Abstract: A review with 156 refs. on interfacial electron transfer reactions in colloidal semiconductor solns. and thin films and their application for solar light energy conversion and photocatalytic water purifn. Some of the topics discussed include; optical and electronic properties of colloidal semiconductor particles, quantum size effects in the photoluminescence of colloidal semiconductors, light-induced charge sepn., dynamics of interfacial charge transfer processes, properties and prepn. of nanocryst. semiconductor electrodes, energetics and operations of the nanoporous solar cell.

Inorganic Solid Acids and Their Use in Acid-Catalyzed Hydrocarbon Reactions

Abstract: It is possible to say that solid acid catalysis involves the largest amounts of catalysts used and the largest economical effort in the oil refining and chemical industry. In this review the author has tried to describe perhaps the most important solid acids based on inorganic oxides, going from their preparation procedures and characterization, to their catalytic activity for a series of hydrocarbon reactions. The review starts with an introductory part in where the nature of the acid sites and their physicochemical characterization is described. Then the classification to the different catalysts is initiated with the older amorphous silica-alumina and aluminum phosphates and followed by catalysts with more interest at present which are discussed in order of increasing acid strength: zeolites, heteropoly acids, and sulfated metal oxides. The aim of this review is to present an extended summary of the state of the art and the current and the future tendencies in the field. 720 refs.

Oscillatory Kinetics in Heterogeneous Catalysis

Abstract: Practically by definition, heterogeneous catalytic reactions represent systems far from thermodynamic equilibrium, and therefore one can observe in such systems rate oscillations, spatiotemporal patterns and chaos--a group of phenomena which has been denoted ``dissipative structures`` by Prigogine. Although oscillatory kinetics in a heterogeneous chemical reaction system had been discovered quite early, it was only about 25 years ago that such phenomena were also found in heterogeneous catalysis by the group of Wicke, who observed rate oscillations in catalytic CO oxidation. Since then, oscillatory surface reactions have developed into a field of very active research. The purpose of the present paper is not to give a full account of all the experimental and theoretical work on oscillatory surface reactions, but to demonstrate the main lines in the development of the whole field. The paper is organized such that first the phenomenology and the mechanisms which give rise to oscillations are presented before the more complicated aspects of spatiotemporal self-organization and chaotic dynamics are discussed. 360 refs.

Photofunctions of intercalation compounds

Abstract: In this article, the authors review the studies on the photofunctions of intercalation compounds. (The structures and properties of host materials which have been used for immobilizing photoactive species have been summarized in the following section.) some of these studies are for the purpose of characterizing the properties of host materials and host-guest systems, and others are for the purpose of contributing to future practical applications. The well-defined layered structures as well as the ability to accommodate guest species on the surface of the layers are very useful for organizing photoactive species to evaluate and control the photofunctions. Table 1 summarizes the characteristics of typical host-guest systems studied for immobilizing photoactive species. Attention is mainly focused on the role of layered structure on the organization of photoactive species; the photofunctions of intercalation compounds are discussed only in connection with the microscopic structures. 321 refs.

Bridged polysilsesquioxanes. highly porous hybrid organic-inorganic materials

Abstract: This contribution reviews a new family of inorganic-organic hybrid materials that are assembled by sol-gel polymerization of polyfunctional molecular building blocks. These bridged polysilsesquioxanes are three-dimensional network materials that are distinguished by incorporation of an organic fragment as an integral component of the network. The intimate association of the organic and inorganic phase, a true molecular composite, coupled with the variability of the organic component, permits engineering of both chemical and physical properties of the material. The paper reviews bridged polysilsesquioxanes, arylene-bridged polysilsesquioxanes, alkylene-bridged polysilsesquioxanes; and their applications.

Heterogeneous Basic Catalysis

Abstract: Heterogeneous acid catalysis attracted much attention primarily because heterogeneous acidic catalysts act as catalysts in petroleum refinery and are known as a main catalyst in the cracking process which is the largest process among the industrial chemical processes. In contrast to these extensive studies of heterogeneous acidic catalysts, fewer efforts have been given to the study of heterogeneous basic catalysts. The types of heterogeneous basic catalysts are listed in Table 1. Except for non-oxide catalysts, the basic sites are believed to be surface O atoms. The studies of heterogeneous catalysis have been continuous and progressed steadily. They have never been reviewed in the chemical Reviews before. It is more useful and informative to describe the studies of heterogeneous basic catalysis performed for a long period. In the present article, therefore, the cited papers are not restricted to those published recently, but include those published for the last 25 years. The paper first describes the generation of basic sites before describing methods used in the characterization of basic surfaces. These are indicator methods, temperature programmed desorption (TPD) of CO{sub 2}, UV absorption and luminescence spectroscopies, TPD of H{sub 2}, XPS, IR of CO{sub 2}, IR of pyrrole, and oxygen exchange betweenmore » CO{sub 2} and the surface. The paper then discusses studies on the catalysis by heterogeneous basic catalysts. Some of these reactions are dehydration, dehydrogenation, hydrogenation, amination, alkylation, ring transformation, and reactions of organosilanes. Catalysts discussed are single component metal oxides, zeolites, non-oxide types, and superbasic catalysts. 141 refs.« less

Homogeneous Hydrogenation of Carbon Dioxide

Abstract: Carbon dioxide (CO{sub 2}) is of the greatest interest as a C{sub 1} feedstock because of the vast amounts of carbon which exist in this form and because of the low cost of bulk CO{sub 2}. Currently, toxic carbon monoxide, the main competitor for many processes, is used in industry instead because CO{sub 2} is perceived to be less reactive and its efficient catalytic conversion has remained elusive. Because CO{sub 2} is a highly oxidized, thermodynamically stable compound, its utilization requires reaction with certain high energy substances or electroreductive processes. Catalytic hydrogenation is one of the most promising approaches to CO{sub 2} fixation. Recent research has shown that high catalytic efficiency, yields, and rates of reaction can be obtained from CO{sub 2} with optimum conditions and catalysts. This review will describe the simplest and most studied reactions of CO{sub 2}: the catalytic reactions with H{sub 2} in the presence or absence of other reactive species. The mechanisms of homogeneously catalyzed reactions will be emphasized. Subjects which will not be covered, aside from brief mentions, include stoichiometric reactions of CO{sub 2} with complexes, the reverse water gas shift reaction, hydrosilylation, and electrochemical or photochemical reductions of CO{sub 2}. 132 refs.

Thermodynamic and Kinetic Data for Macrocycle Interaction with Cations, Anions, and Neutral Molecules

Abstract: Cation-macrocycle interaction was treated in three earlier Chemical Reviews articles (1974, 1985, and 1991), anion-macrocycle interaction was treated in 1991, and neutral molecule-macrocycle interaction was treated in 1992. The present review updates this material and includes data which were inadvertently omitted earlier. In general, this review does not repeat the data included in the earlier reviews. As in the past reviews, particular effort has been made to include literature from countries of the former USSR, Eastern Europe, and the Peoples Republic of China. Thermodynamic and kinetic data are brought together to provide a quantitative base for understanding the effect of macrocycle and guest parameters on the thermodynamic and kinetic stabilities of the resulting complexes. In turn, this understanding can lead to the intelligent design of new macrocycles and to predictions of their effectiveness in forming complexes of desired stability with guest species. In this review, emphasis will be placed on the presentation of thermodynamic and kinetic data for the period from 1991--1993. Data will be included where appropriate. The complete data set is included in the supporting information. 478 refs.

Design and Synthesis of Macrocyclic Ligands and Their Complexes of Lanthanides and Actinides

Abstract: A review article which covers the various design and synthetic strategies developed to synthesize macrocyclic complexes of lanthanides and actinides, their structural features, quantitative studies on the stabilities of these complexes, their applications, and the structure-reactivity principle would be an asset for those who are actively engaged in this area of research. This review is also purported to give a comprehensive view of the current status of this area of research to the beginners and to highlight the application of this chemical research to emerging nonchemical applications to lure the potential workers. The coordination template effect provides a general strategy for the synthesis of a wide variety of discrete metal complexes. The principal conceptual and experimental development that have established and exploited this strategy are briefly outlined. A brief review of the coordination template effect and subsequent developments in the design of macrocyclic complexes of alkali, alkaline earth, and transition metal ions is presented as an essential basis for the rational design of new macrocyclic complexes of lanthanides and actinides. The exciting aspect of this chemistry is that in the majority of cases the molecules meet the design criteria very well. It is evident that in an increasing number ofmore » cases the driving force behind the synthetic effort is the desire to create a molecule which will enable the user to make specific applications. 506 refs.« less

Aspects of Methane Chemistry

Abstract: As the simplest hydride of carbon and the major constituent of natural gas, methane has attracted increased attention in recent years. An important factor has been the recognition of the significance of world natural gas reserves to energy and chemicals production in the 21st century. This expectation is lined to the gradual depletion of oil reserves and to the possible influence of greenhouse global warming effect on energy policy. The combustion of fossil fuel has caused a rise in the CO{sub 2} level in the atmosphere from an estimated preindustrial level of 280 ppm to the current 360 ppm. If current climate models are correct, this may cause a global warming trend in the next few decades. If public resistance to nuclear energy remains strong, natural gas is likely to become a more important energy source. The low C:H ratio of CH{sub 4} means that on combustion it can furnish a much larger amount of energy per CO{sub 2} molecule released than can oil (approximate ratio, CH{sub 2}) or coal (approximate ratio, CH). This article reviews the occurrence, production, and origin of natural gas and methane. The physical properties, structure, and chemical reactivity of methane are also reviewed. 180 refs.

Methods for Characterizing Zeolite Acidity

Abstract: The review will be presented in three sections. The first section, ``Experimental Methods in Investigations of Zeolite Acidity``, will be a guide to the literature on standard approaches to the quantification of Broensted acidity in zeolites. Most of these experimental methods have been recently reviewed; therefore, the authors believe it is most valuable for the reader to first cite leading references which can be consulted for a detailed look at the technique and its history, and then add a few comments from experience with the application of each technique to standard materials like H-ZSM-5. In the next section, ``Stoichiometric Adsorption Complexes at Acid Sites in Zeolites``, the authors will document results from their laboratories utilizing a combination of these approaches that have led them to conclude that carefully prepared H-ZSM-5 can be profitably described as a collection of isolated, uniform (within the power of any of these techniques to discriminate), proton-donor sites. The final section, ``Structure/Activity Models Starting from Stoichiometric Complexes,`` will discuss how that description of the nature of H-ZSM-5 as a solid acid, along with the proton affinity concept, can then be used as a starting point for potential energy descriptions of acid-catalyzed reactions in zeolites. The authorsmore » will show how it is especially well suited to link recent progress in NMR of reaction intermediates, with rapidly improving theoretical approaches, to intrazeolite chemistry. 125 refs.« less

Reactivity Theory of Zeolitic Broensted Acidic Sites

Abstract: The unique environment of the Bransted acidic protons in the micropores of zeolites controls the overall catalytic behavior of zeolites to a significant extent. One of the most obvious consequences for reactivity is the resulting stereoselectivity of zeolitic catalysts. Molecules too large to enter the zeolite will not be converted. Also, large molecules that are formed in the microcavities of the zeolite but are unable to pass through the smallest micropore diameter on their path outward of a zeolite particle will not appear as a product. Molecules that move through the micropores of a zeolitic particle can have a significant interaction with the walls of the zeolite micropores. The strength of this interaction can be deduced from heats of adsorption measurements. I t appears to be a sensitive function of zeolite micropore diameter or shape.lj2 The interaction energy tends to increase as the zeolite micropores get smaller. The smaller the micropore diameters, the shorter the average distance between the molecular adsorbate atoms and the micropore

Methods for Preparation of Catalytic Materials

Abstract: To establish guidelines for the development of a scientific basis for catalyst preparation is perhaps a very ambitious goal. One would re required first to answer the following rhetorical questions: what are the properties which determine the performance of a catalytic material; how can these properties be introduced, developed, and/or improved during preparation? The answer to these questions involves a comprehensive discussion of the theories of catalysis, which is beyond the scope of this review. The authors will attempt, instead, to provide a rationale for each reader to answer these questions on the basis of his/her own interests. They start the discussion by describing the fundamental steps in producing bulk catalysts and/or catalyst supports. The fundamental processes involved are those derived from traditional three-dimensional chemistry. The topic areas will include single-component and multicomponent metal oxides. Unsupported metallic catalysts are formed by transformations involving physical or chemical processes, and the preparation methods for this class of materials will be discussed next. Attention will then turn to the preparation of supported catalytic materials. The main topics to be discussed will be those related to the interaction between the support and the active phase when they are put together to generate the catalyst.more » In this approach, the authors exploit the virtually unexplored field of surface, or two-dimensional, physical chemistry. The materials considered include dispersed metals and alloys and composite oxides. 366 refs.« less

Transition Metal-Catalyzed Oxidation of Sulfur(IV) Oxides. Atmospheric-Relevant Processes and Mechanisms

Abstract: The transition metal-catalyzed oxidation of sulfur(IV) oxides has been known for more than 100 years. There is a significant lack of information on the actual role of the transition metal-catalyzed reactions, and much of the earlier work was performed without a detailed knowledge of the chemical system. For this reason attention is focused on the role of transition metal ions in the oxidation of sulfur(IV) oxides in terms of the coordination chemistry involved, as well as the stability and chemical behavior of the various participating species. The oxidation process of sulfur(IV) oxides plays an important role in atmospheric chemistry (e.g. acid rain formation) as well as industrial processes (e.g. desulfurization of plume gases and ore). The present report deals with the mechanism of the transition metal-catalyzed oxidation of sulfur(IV) oxides with the aim to discuss this in terms of atmospheric and chemical processes. In addition, the authors would like to emphasize the key role of oxygen in these processes. 1,076 refs.

Actinide Carbonte Complexes and Their Importance in Actinide Environmental Chemistry

Abstract: In the last decade scientists have dramatically increased their understanding of the chemistry of actinide elements with a potent emphasis on relevance to the environment. This flourishing chemistry of the 5f elements was stimulated by many factors, including inorganic chemists` interest in structural diversity, new synthetic methods, new chemical separations, and a need to understand the fate and transport properties of actinides in natural aquifer systems. The purpose of this review is to present the motivation behind environmentally important actinide carbonate research and to provide a modern reference in the area of actinide carbonate chemistry that reflects the developments and achievements in the field since Newton and Sullivan`s thorough review of actinide carbonate solution chemistry. 234 refs.