Heterogeneous catalysis: some recent developments

In recent years major progress has been made in the area of heterogeneous catalysis by metals. Much has been learned about the nature of metal catalysts and of catalytic phenomena on metals. Characteristic patterns of catalytic behavior among the metallic elements have been established for certain classes of reactions, and these patterns provide a first step toward a more comprehensive understanding of catalytic specificity. Studies on metal alloys and related bimetallic catalysts have revived interest in a geometric factor in surface catalysis to complement the traditional electronic factor. Closely related to this geometric factor is the discovery that selectivity, rather than activity alone, is a major factor in reactions on bimetallic catalysts. Concurrent with progress in understanding how catalysts work, advances are also being made in the development of new catalyst systems, examples of which are the bimetallic (or polymetallic) cluster catalysts. Research in this area provides an example of how advances in catalyst technology can be realized within a framework of fundamental research on catalytic phenomena (38).     

Asymmetric electrophilic fluorination using an anionic chiral phase-transfer catalyst

The discovery of distinct modes of asymmetric catalysis has the potential to rapidly advance chemists' ability to build enantioenriched molecules. As an example, the use of chiral cation salts as phase-transfer catalysts for anionic reagents has enabled a vast set of enantioselective transformations. Here, we present evidence that a largely overlooked analogous mechanism wherein a chiral anionic catalyst brings a cationic species into solution is itself a powerful method. The concept is applied to the enantioselective fluorocyclization of olefins with a cationic fluorinating agent and a chiral phosphate catalyst. The reactions proceed in high yield and stereoselectivity, especially considering the scarcity of alternative approaches. This technology can in principle be applied to the large portion of reaction space that uses positively charged reagents and reaction intermediates.     

Isocyanate Intermediates in Ammonia Formation over Noble Metal Catalysts for Automobile Exhaust Reactions

Isocyanate species have been detected on the surface of noble metal catalysts during the reactions of carbon monoxide with nitric oxide. The intensity of the surface isocyanate infrared band correlates with the known ammonia-forming tendencies among the noble metals. The discovery of this isocyanate species suggests a new mechanistic pathway to the ammonia formed during catalytic reduction of nitrogen oxides in automobile exhaust.     

Iron catalysts for selective anti-Markovnikov alkene hydrosilylation using tertiary silanes

Alkene hydrosilylation, the addition of a silicon hydride (Si-H) across a carbon-carbon double bond, is one of the largest-scale industrial applications of homogeneous catalysis and is used in the commercial production of numerous consumer goods. For decades, precious metals, principally compounds of platinum and rhodium, have been used as catalysts for this reaction class. Despite their widespread application, limitations such as high and volatile catalyst costs and competing side reactions have persisted. Here, we report that well-characterized molecular iron coordination compounds promote the selective anti-Markovnikov addition of sterically hindered, tertiary silanes to alkenes under mild conditions. These Earth-abundant base-metal catalysts, coordinated by optimized bis(imino)pyridine ligands, show promise for industrial application.     

Chemical and spectroscopic evidence for an FeV-oxo complex

Iron(V)-oxo species have been proposed as key reactive intermediates in the catalysis of oxygen-activating enzymes and synthetic catalysts. Here, we report the synthesis of [Fe(TAML)(O)]- in nearly quantitative yield, where TAML is a macrocyclic tetraamide ligand. Mass spectrometry, M?ssbauer, electron paramagnetic resonance, and x-ray absorption spectroscopies, as well as reactivity studies and density functional theory calculations show that this long-lived (hours at -60 degrees C) intermediate is a spin S = 1/2 iron(V)-oxo complex. Iron-TAML systems have proven to be efficient catalysts in the decomposition of numerous pollutants by hydrogen peroxide, and the species we characterized is a likely reactive intermediate in these reactions.     

Mechanism of the selective catalytic reduction of nitric oxide by ammonia elucidated by in situ on-line fourier transform infrared spectroscopy

The selective catalytic reduction reaction of nitric oxide bv ammonia over vanadia-titania catalysts is one of the methods of removing NOx pollution. In the present study, it has been possible to identify the reaction mechanism and the nature of the active sites in these catalysts by combining transient or steady-state in situ (Fourier transform infrared spectroscopy) experiments directly with on-line activity studies. The results suggest a catalytic cycle that consists of both acid and redox reactions and involves both surface V-OH (Br?nsted acid sites) and V=O species. A fundamental microkinetic model is proposed, which accounts for the observed industrial kinetics performance.     

Perovskite oxides: materials science in catalysis

In a time of growing need for catalysts, perovskites have been rediscovered as a family of catalysts of such great diversity that a broad spectrum of scientific disciplines have been brought to bear in their study and application. Because of the wide range of ions and valences which this simple structure can accommodate, the perovskites lend themselves to chemical tailoring. It is relatively simple to synthesize perovskites because of the flexibility of the structure to diverse chemistry. Many of the techniques of ceramic powder preparation are applicable to perovskite catalysts. In their own right, they are therefore of interest as a model system for the correlation of solid-state parameters and catalytic mechanisms. Such correlations [See figure in the PDF file] have recently been found between the rate and selectivity of oxidation-reduction reactions and the thermodynamic and electronic parameters of the solid. For commercial processes such as those mentioned in the introduction, perovskite catalysts have not yet proven to be practical. Much of the initial interest in these catalysts related to their use in automobile exhaust control. Current interest in this field centers on noble metalsubstituted perovskites resistant to S poisoning for single-bed, dual-bed, and three-way catalyst configurations. The formulations commercially tested to date have shown considerable promise, but long-term stability has not yet been achieved. A very large fraction of the elements that make up presently used commercial catalysts can be incorporated in the structure of perovskite oxides. Conversely, it is anticipated that perovskite oxides, appropriately formulated, will show catalytic activity for a large variety of chemical conversions. Even though this expectation is by no means a prediction of commercial success in the face of competition by existing catalyst systems, it makes these oxides attractive models in the study of catalytic chemical conversion. By appropriate formulation many desirable properties can be tailored, including the valence state of transition metal ions, the binding energy and diffusion of O in the lattice, the distance between active sites, and the magnetic and conductive properties of the solid. Only a very small fraction of possible perovskite formulations have been explored as catalysts. It is expected that further investigation will greatly expand the scope of perovskite catalysis, extend the understanding of solid-state parameters in catalysis, and contribute to the development of practical catalytic processes.     

Discovery of a Jupiter/Saturn analog with gravitational microlensing

Searches for extrasolar planets have uncovered an astonishing diversity of planetary systems, yet the frequency of solar system analogs remains unknown. The gravitational microlensing planet search method is potentially sensitive to multiple-planet systems containing analogs of all the solar system planets except Mercury. We report the detection of a multiple-planet system with microlensing. We identify two planets with masses of approximately 0.71 and approximately 0.27 times the mass of Jupiter and orbital separations of approximately 2.3 and approximately 4.6 astronomical units orbiting a primary star of mass approximately 0.50 solar mass at a distance of approximately 1.5 kiloparsecs. This system resembles a scaled version of our solar system in that the mass ratio, separation ratio, and equilibrium temperatures of the planets are similar to those of Jupiter and Saturn. These planets could not have been detected with other techniques; their discovery from only six confirmed microlensing planet detections suggests that solar system analogs may be common.     

The breakup of a main-belt asteroid 450 thousand years ago

Collisions in the asteroid belt frequently lead to catastrophic breakups, where more than half of the target's mass is ejected into space. Several dozen large asteroids have been disrupted by impacts over the past several billion years. These impact events have produced groups of fragments with similar orbits called asteroid families. Here we report the discovery of a very young asteroid family around the object 1270 Datura. Our work takes advantage of a method for identification of recent breakups in the asteroid belt using catalogs of osculating (i.e., instantaneous) asteroid orbits. The very young families show up in these catalogs as clusters in a five-dimensional space of osculating orbital elements.     

碱性功能化离子液体在Knoevenagel缩合反应中的应用进展

Knoevenagel缩合反应被广泛应用于合成香豆素、化妆品及药物中间体功能化离子液体的设计、合成为Knoevenagel缩合反应提供了高效、环保的制备途径。以功能化离子液体为催化剂或者溶剂,一系列的醛和酮类化合物与活泼亚甲基化合物的缩合反应可以顺利进行,避免了使用额外的催化剂。文章综述了碱性功能化离子液体在Knoev...     

Intermetallic Compounds of the Type MNi5 as Methanation Catalysts

Catalytic reactions of carbon monoxide with hydrogen have been studied in which intermetallic compounds of the formula MNi(5) (where M is thorium, uranium, or zirconium) have been used as the catalysts. The materials perform effectively as methanation catalysts; ThNi(5) has a specific activity exceeding that of a typical commercial oxide-supported methanation catalyst by a factor of about 5. This material also shows superior resistance to hydrogen sulfide poisoning. Nickel, formed as a decomposition product of the MNi(5) intermetallic compound, is probably the active species, but its properties are influenced by the nature of M in the precursor MNi(5) system.     

Organometallic chemistry in homogeneous catalysis

Many reactions catalyzed by soluble transition metal compounds proceed by way of organometallic intermediates, even though the original catalyst may be a simple salt. This generality is illustrated for three industrial syntheses of acetic acid that use homogeneous catalysts. Some developments in organometallic chemistry that may extend the utility of homogeneous catalysis are photoactivation of catalysts and the recognition of the importance of metallacyclic intermediates.     

Privileged chiral catalysts

One of the most active current areas of chemical research is centered on how to synthesize handed (chiral) compounds in a selective manner, rather than as mixtures of mirror-image forms (enantiomers) with different three-dimensional structures (stereochemistries). Nature points the way in this endeavor: different enantiomers of a given biomolecule can exhibit dramatically different biological activities, and enzymes have therefore evolved to catalyze reactions with exquisite selectivity for the formation of one enantiomeric form over the other. Drawing inspiration from these natural catalysts, chemists have developed a variety of synthetic small-molecule catalysts that can achieve levels of selectivity approaching, and in some cases matching, those observed in enzymatic reactions.     

Turning down the heat: design and mechanism in solid-state synthesis

Solid-state compounds have historically been prepared through high-temperature solid-solid reactions. New mechanistic understanding of these reactions suggests possible routes to metastable compositions and structures as well as to thermodynamically stable, low-temperature phases that decompose at higher temperatures. Intermediate-temperature synthetic techniques, including flux and hydrothermal methods, as well as low-temperature intercalation and coordination reactions, have recently been developed and have been used to prepare unprecedented materials with interesting electronic, optical, and catalytic properties. The trend in modern solid-state synthesis resembles increasingly the approach used in small-molecule chemistry, in the sense that attention to reaction mechanism and the use of molecular building blocks result in an ability to prepare new materials of designed structure.     

Mass spectrometry of large, fragile, and involatile molecules

Desorption ionization makes it possible to obtain mass spectra of molecules whose vaporization by heating may lead to thermal degradation. Several methods are in use, but in general desorption is achieved by particle or photon bombardment of the sample and the mass spectra obtained by different methods are fundamentally similar. Desorption ionization techniques have been used to obtain mass spectra of biomolecules, including peptides, antibiotics, and oligosaccharides, for which normal mass spectral methods have been of limited power. Several examples are given of recent applications of these new techniques, and prospects for their further evolution are discussed.     

采用SOAP方法解决电子阅览室网络故障的应用实践

SOAP方法具有主观感觉、客观事实、分析、制定方案4个核心要素,能够解决电子阅览室存在的网络故障。首先,介绍了SOAP方法的具体内容,然后,分析了此方法解决网络故障的思路,最后,通过实践来证明此方法在使用过程中的实用性,从而保障电子阅览室网络的正常运行。     

基于质谱的代谢组学数据分析技术研究进展

代谢组学是系统生物学研究的重要组成部分,是一种对特定条件下生物体内所有内源性小分子代 谢物进行全面定性和定量分析的技术。质谱和核磁共振系统的不断更新迭代推进了代谢组学技术的迅猛发展, 其中质谱技术因其能同时检测出数千个生物流体、细胞和组织中的代谢物,且所需前处理步骤简单,已发展为 当前代谢组学研究中应用最广泛的技术,开发基于质谱的代谢组学数据分析方法也因此成为过去 10 年代谢组学 研究的热点领域。对基于 GC-MS 和 LC-MS 的代谢组学数据预处理、代谢组学数据统计分析、代谢途径富集分析, 以及未知代谢物鉴定 4 个方向取得的研究进展进行系统总结,详细介绍常用的数据分析策略和分析软件;并重 点综述了包括基于数据库、分子网络算法、人工智能算法等未知代谢物鉴定的前沿方法,最后展望了基于质谱 的代谢组学数据分析的未来发展方向,在已知生化反应和分子网络分析的基础上再整合代谢物合成的遗传位点 等信息,有望进一步提高代谢物的鉴定数量和准确度。全面综述基于质谱的代谢组学数据分析技术,将为开发 新的代谢组学分析方法和挖掘代谢组学数据的生物学意义提供重要的参考和思路。     

Asymmetric hydrogenation of unfunctionalized, purely alkyl-substituted olefins

Asymmetric hydrogenation of olefins is one of the most useful reactions for the synthesis of optically active compounds, especially in industry. However, the application range of the catalysts developed so far is limited to alkenes with a coordinating functional group or an aryl substituent next to the double bond. We have found a class of chiral iridium catalysts that give high enantioselectivity in the hydrogenation of unfunctionalized, trialkyl-substituted olefins. Because these catalysts do not require the presence of any particular functional group or aryl substituent in the substrate, they considerably broaden the scope of asymmetric hydrogenation.     

植物基因组学与种质资源新基因发掘

种质资源是基因的载体,如何从种质资源中发掘出新的目标基因是21世纪植物育种所面临的最大挑战之一。近年来,随着植物基因组学的迅猛发展,新的基因发掘方法和技术不断涌现,新基因发掘也产生了各种新的思路和策略。笔者在回顾基因发掘历史的基础上,对中国的新基因发掘现状和问题进行了分析,并针对各种基因发掘方法和策略进行了评述,提出了开展新基因发掘的发展方向。     

In situ interfacial mass detection with piezoelectric transducers

The converse piezoelectric effect, in which an electric field applied across a piezoelectric material induces a stress in that material, has spurred many recent developments in mass measurement techniques. These methods commonly rely on the changes in the vibrational resonant frequency of piezoelectric quartz oscillators that result from changes in mass on the surface of the oscillator. The dependence of frequency on mass has been exploited extensively for mass measurements in vacuum or gas phase, for example, thickness monitors for thin-film preparation and sensors for chemical agents. Advances in piezoelectric methodology in the last decade now allow dynamic measurements of minute mass changes (< 10(-9) grams per square centimeter) at surfaces, thin films, and electrode interfaces in liquid media as well. Mass measurements associated with a diverse collection of interfacial processes can be readily performed, including chemical and biological sensors, reactions catalyzed by enzymes immobilized on surfaces, electron transfer at and ion exchange in thin polymer films, and doping reactions of conducting polymers.