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).     

C-H bond functionalization in complex organic synthesis

Direct and selective replacement of carbon-hydrogen bonds with new bonds (such as C-C, C-O, and C-N) represents an important and long-standing goal in chemistry. These transformations have broad potential in synthesis because C-H bonds are ubiquitous in organic substances. At the same time, achieving selectivity among many different C-H bonds remains a challenge. Here, we focus on the functionalization of C-H bonds in complex organic substrates catalyzed by transition metal catalysts. We outline the key concepts and approaches aimed at achieving selectivity in complex settings and discuss the impact these reactions have on synthetic planning and strategy in organic synthesis.     

Dynamic control of chiral space in a catalytic asymmetric reaction using a molecular motor

Enzymes and synthetic chiral catalysts have found widespread application to produce single enantiomers, but in situ switching of the chiral preference of a catalytic system is very difficult to achieve. Here, we report on a light-driven molecular motor with integrated catalytic functions in which the stepwise change in configuration during a 360° unidirectional rotary cycle governs the catalyst performance both with respect to activity and absolute stereocontrol in an asymmetric transformation. During one full rotary cycle, catalysts are formed that provide either racemic (R,S) or preferentially the R or the S enantiomer of the chiral product of a conjugate addition reaction. This catalytic system demonstrates how different molecular tasks can be performed in a sequential manner, with the sequence controlled by the directionality of a rotary cycle.     

新型直链淀粉手性固定相拆分戊唑醇对映体

采用新型直链淀粉-三(5-氯-2-甲基苯基氨基甲酸酯)涂敷型手性固定相在反相色谱条件下拆分戊唑醇对映体,评价了流动相组成(甲醇/水,乙腈/水)及柱温(5～45℃)对对映体分离的影响。结果表明：以甲醇/水为流动相时,在甲醇含量100%至40%范围内,戊唑醇对映体均无分离趋势;而用乙腈/水为流动相时,在乙腈含量100%至40%范围内,戊唑醇对映体均可实现基线分离;在5～45℃,V(乙腈)∶V(水)=1∶1,对映体的保留因子(k)与分离因子(α)随温度升高而降低,分离度(RS)却先升后降,基于线性Van't Hoff曲线的热力学参数证明戊唑醇对映体的分离受焓驱动。     

Homogeneous catalysis in supercritical fluids

Supercritical fluids (SCFs), compounds heated and pressurized beyond the critical point, have many unusual properties. Homogeneous molecular catalysts, which have far greater control over selectivity than heterogeneous solid catalysts, are now being tested in SCFs, and early results show that high rates, improved selectivity, and elimination of masstransfer problems can be achieved. As industry moves away from toxic or environmentally damaging solvents, supercritical carbon dioxide may be an ideal replacement medium for nonpolar or weakly polar chemical processes. More than simply substitutes for nonpolar solvents, SCFs can radically change the observed chemistry. Supercritical carbon dioxide is also an excellent medium for its own fixation, as demonstrated by studies of its hydrogenation.     

Renewable hydrogen from ethanol by autothermal reforming

Ethanol and ethanol-water mixtures were converted directly into H2 with approximately 100% selectivity and >95% conversion by catalytic partial oxidation, with a residence time on rhodium-ceria catalysts of <10 milliseconds. Rapid vaporization and mixing with air with an automotive fuel injector were performed at temperatures sufficiently low and times sufficiently fast that homogeneous reactions producing carbon, acetaldehyde, ethylene, and total combustion products can be minimized. This process has great potential for low-cost H2 generation in fuel cells for small portable applications where liquid fuel storage is essential and where systems must be small, simple, and robust.     

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.     

Antibody-based bio-nanotube membranes for enantiomeric drug separations

Synthetic bio-nanotube membranes were developed and used to separate two enantiomers of a chiral drug. These membranes are based on alumina films that have cylindrical pores with monodisperse nanoscopic diameters (for example, 20 nanometers). Silica nanotubes were chemically synthesized within the pores of these films, and an antibody that selectively binds one of the enantiomers of the drug was attached to the inner walls of the silica nanotubes. These membranes selectively transport the enantiomer that specifically binds to the antibody, relative to the enantiomer that has lower affinity for the antibody. The solvent dimethyl sulfoxide was used to tune the antibody binding affinity. The enantiomeric selectivity coefficient increases as the inside diameter of the silica nanotubes decreases.     

Reaction-driven restructuring of Rh-Pd and Pt-Pd core-shell nanoparticles

Heterogeneous catalysts that contain bimetallic nanoparticles may undergo segregation of the metals, driven by oxidizing and reducing environments. The structure and composition of core-shell Rh(0.5)Pd(0.5) and Pt(0.5)Pd(0.5) nanoparticle catalysts were studied in situ, during oxidizing, reducing, and catalytic reactions involving NO, O2, CO, and H2 by x-ray photoelectron spectroscopy at near-ambient pressure. The Rh(0.5)Pd(0.5) nanoparticles underwent dramatic and reversible changes in composition and chemical state in response to oxidizing or reducing conditions. In contrast, no substantial segregation of Pd or Pt atoms was found in Pt(0.5)Pd(0.5) nanoparticles. The different behaviors in restructuring and chemical response of Rh(0.5)Pd(0.5) and Pt(0.5)Pd(0.5) nanoparticle catalysts under the same reaction conditions illustrates the flexibility and tunability of the structure of bimetallic nanoparticle catalysts during catalytic reactions.     

Biomass Catalytic Pyrolysis with Ni Based Catalyst to Produce Hydrogen Rich Gas

Hydrogen rich gas was produced using rice husk as biomass material on the continuous biomass pyrolysis apparatus which consisted of continuous pyrolysis reactor and secondary catalytic cracking reactor. Ni based catalysts of different Ni/Al mass ratio and calcined temperature were prepared by impregnating method. The catalysts were characterized by X-ray diffraction (XRD), scan electron microscope (SEM) and FT-IR Spectrometer (FT-IR). Ni based catalyst showed good selectivity for H2 production from biomass. Catalysts prepared under different conditions had little influence on the yields of three states products when used at the same cracking temperature. Ni/Al mass ratio played an important role in products selectivity. However, the content of NiO increased further when Ni/Al mass ratio values reached 0.7 : 10, and the yield of H2 slightly increased. Hydrogen yield was greatly impacted by calcined temperature. Catalyst calcined at 550℃ performed best. When the catalyst was calcined at high temperature, NiO in the catalyst transformed into NiAl2O4, and the acid site also changed, which caused the deactivation of the catalyst. The hydrogen yield increased with the cracking temperature. The highest stable yield of hydrogen was about 30% without increasing with the cracking temperature.     

A simple, multidimensional approach to high-throughput discovery of catalytic reactions

Transition metal complexes catalyze many important reactions that are employed in medicine, materials science, and energy production. Although high-throughput methods for the discovery of catalysts that would mirror related approaches for the discovery of medicinally active compounds have been the focus of much attention, these methods have not been sufficiently general or accessible to typical synthetic laboratories to be adopted widely. We report a method to evaluate a broad range of catalysts for potential coupling reactions with the use of simple laboratory equipment. Specifically, we screen an array of catalysts and ligands with a diverse mixture of substrates and then use mass spectrometry to identify reaction products that, by design, exceed the mass of any single substrate. With this method, we discovered a copper-catalyzed alkyne hydroamination and two nickel-catalyzed hydroarylation reactions, each of which displays excellent functional-group tolerance.     

Intercalated clay catalysts

Recent advances in the intercalation of metal complex cations in smectite clay minerals are leading to the development of new classes of selective heterogeneous catalysts. The selectivity of both metal-catalyzed and proton-catalyzed chemical conversions in clay intercalates can often be regulated by controlling surface chemical equilibria, interlamellar swelling, or reactant pair proximity in the interlayer regions. Also, the intercalation of polynuclear hydroxy metal cations and metal cluster cations in smectites affords new pillared clay catalysts with pore sizes that can be made larger than those of conventional zeolite catalysts.     

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.     

Solvent-free oxidation of primary alcohols to aldehydes using Au-Pd/TiO2 catalysts

The oxidation of alcohols to aldehydes with O2 in place of stoichiometric oxygen donors is a crucial process for the synthesis of fine chemicals. However, the catalysts that have been identified so far are relatively inactive with primary alkyl alcohols. We showed that Au/Pd-TiO2 catalysts give very high turnover frequencies (up to 270,000 turnovers per hour) for the oxidation of alcohols, including primary alkyl alcohols. The addition of Au to Pd nanocrystals improved the overall selectivity and, using scanning transmission electron microscopy combined with x-ray photoelectron spectroscopy, we showed that the Au-Pd nanocrystals were made up of a Au-rich core with a Pd-rich shell, indicating that the Au electronically influences the catalytic properties of Pd.     

Organic Compounds in Meteorites: They may have formed in the solar nebula, by catalytic reactions of carbon monoxide, hydrogen, and ammonia

Organic compounds in meteorites seem to have formed by catalytic reactions of CO, H2, and NH3 in the solar nebula, at 360 degrees to 400 degrees K and (4 to 10) x 10-6 atm. The onset of these reactions was triggered by the formation of suitable catalysts (magnetite, hydrated silicates) at these temperatures. These reactions may be a source of prebiotic carbon compounds on the inner planets, and interstellar molecules.     

High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt

The prohibitive cost of platinum for catalyzing the cathodic oxygen reduction reaction (ORR) has hampered the widespread use of polymer electrolyte fuel cells. We describe a family of non-precious metal catalysts that approach the performance of platinum-based systems at a cost sustainable for high-power fuel cell applications, possibly including automotive power. The approach uses polyaniline as a precursor to a carbon-nitrogen template for high-temperature synthesis of catalysts incorporating iron and cobalt. The most active materials in the group catalyze the ORR at potentials within ~60 millivolts of that delivered by state-of-the-art carbon-supported platinum, combining their high activity with remarkable performance stability for non-precious metal catalysts (700 hours at a fuel cell voltage of 0.4 volts) as well as excellent four-electron selectivity (hydrogen peroxide yield <1.0%).     

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.     

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.     

Molecular recognition in the selective oxygenation of saturated C-H bonds by a dimanganese catalyst

Although enzymes often incorporate molecular recognition elements to orient substrates selectively, such strategies are rarely achieved by synthetic catalysts. We combined molecular recognition through hydrogen bonding with C-H activation to obtain high-turnover catalytic regioselective functionalization of sp3 C-H bonds remote from the -COOH recognition group. The catalyst contains a Mn(mu-O)2Mn reactive center and a ligand based on Kemp's triacid that directs a -COOH group to anchor the carboxylic acid group of the substrate and thus modify the usual selectivity for oxidation. Control experiments supported the role of hydrogen bonding in orienting the substrate to achieve high selectivity.