Showing papers on "Catalysis published in 1999"

When Gold Is Not Noble: Nanoscale Gold Catalysts

Abstract: While inert as bulk material, nanoscale gold particles dispersed on oxide supports exhibit a remarkable catalytic activity. Temperature-programmed reaction studies of the catalyzed combustion of CO on size-selected small monodispersed Aun (n ≤ 20) gold clusters supported on magnesia, and first-principle simulations, reveal the microscopic origins of the observed unusual catalytic activity, with Au8 found to be the smallest catalytically active size. Partial electron transfer from the surface to the gold cluster and oxygen-vacancy F-center defects are shown to play an essential role in the activation of nanosize gold clusters as catalysts for the combustion reaction.

Large Scale CVD Synthesis of Single-Walled Carbon Nanotubes

Abstract: The synthesis of bulk amounts of high quality single-walled carbon nanotubes (SWNTs) is accomplished by optimizing the chemical compositions and textural properties of the catalyst material used in the chemical vapor deposition (CVD) of methane A series of catalysts are derived by systematically varying the catalytic metal compounds and support materials The optimized catalysts consist of Fe/Mo bimetallic species supported on a novel silica−alumina multicomponent material The high SWNT yielding catalyst exhibits high surface-area and large mesopore volume at elevated temperatures Gram quantities of SWNT materials have been synthesized in ∼05 h using the optimized catalyst material The nanotube material consists of individual and bundled SWNTs that are free of defects and amorphous carbon coating This work represents a step forward toward obtaining kilogram scale perfect SWNT materials via simple CVD routes

Highly Active Palladium Catalysts for Suzuki Coupling Reactions

Abstract: Mixtures of palladium acetate and o-(di-tert-butylphosphino)biphenyl (4) catalyze the room-temperature Suzuki coupling of aryl bromides and aryl chlorides with 0.5−1.0 mol % Pd. Use of o-(dicyclohexylphosphino)biphenyl (2) allows Suzuki couplings to be carried out at low catalyst loadings (0.000001−0.02 mol % Pd). The process tolerates a broad range of functional groups and substrate combinations including the use of sterically hindered substrates. This is the most active catalyst system in terms of reaction temperature, turnover number, and steric tolerance which has been reported to date.

Transition Metal Catalyzed Coupling Reactions under C-H Activation.

Abstract: C−C coupling by transition metal catalyzed C−H activation has developed into a diverse area of research. The applicable catalysts are manifold, and the variety of products obtained range from basic chemicals to pharmaceuticals and building blocks for carbon networks. One reaction, in which several C−C bonds are formed under C−H activation of a methyl group, is the conversion of ortho-iodoanisole according to Equation (1).

Biodiesel production via acid catalysis

Abstract: Vegetable oils and animal fats can be transesterified to biodiesel for use as an alternative diesel fuel. Conversion of low cost feedstocks such as used frying oils is complicated if the oils contain large amounts of free fatty acids that will form soaps with alkaline catalysts. The soaps can prevent separation of the biodiesel from the glycerin fraction. Alternative processes are available that use an acid catalyst. The objective of this study was to investigate the effect of process variables on acid-catalyzed transesterification. The molar ratio of alcohol, reaction temperature, catalyst amount, reaction time, water content, and free fatty acids were investigated to determine the best strategy for producing biodiesel. Food grade soybean oil was used to prepare esters using excess methanol and sulfuric acid as a catalyst. To compare the effect of different alcohol types on ester formation, methanol, ethanol, 2-propanol, and n-butanol were compared. The American Oil Chemists’ Society Method Ca 14-56 was used to measure the biodiesel’s total glycerin amount as an indicator of the completeness of the reaction. It was found that acid catalysis can provide high conversion rates but much longer times are required than for alkaline catalysts. The acid catalyst also requires the concentration of water to be less than 0.5%, which is about the same as is required for alkaline catalysts. Water formed by the esterification of free fatty acids limited their presence in the oil to 5%.

Chemical synthesis using supercritical fluids

Abstract: Introduction: introduction to SCF, history, recent trends basic physical properties, phase behaviour and solubility physical properties as related to chemical reactions. Experimental techniques - reaction equipment design and safety extraction using SCFs precipitation and crystallization techniques surfactants for SCFs. Spectroscopy of SCF solutions: 3.1 IR and Raman spectroscopy in SCF 3.2 NMR spectroscopy in SCF 3.3 other spectroscopic techniques (UV, ESR) 4 reactions in SCF: 4.1 stoichiometric organic synthesis 4.2 stoichiometric inorganic synthesis 4.3 photochemistry 4.4 polymerization in scCO2 4.5 free-radical polymerization in reactive supercritical fluids 4.6 homogeneous catalysis 4.7 heterogeneous catalyis 4.8 enzymatic catalysis 4.9 phase transfer catalysis.

Homogeneous Hydrogenation Catalysis with Monodisperse, Dendrimer-Encapsulated Pd and Pt Nanoparticles.

Abstract: Extraordinarily stable, monodisperse noble metal nanoparticles can be prepared by using dendrimers as both templates and stabilizers. Dendrimer-encapsulated Pd nanoparticles (see the schematic representation) exhibit high catalytic activity for the hydrogenation of alkenes in water. The catalytic activity and selectivity of these materials can be controlled by adjusting the dendrimer generation.

Methanol–steam reforming on Cu/ZnO/Al2O3 catalysts. Part 2. A comprehensive kinetic model

Abstract: Surface mechanisms for methanol–steam reforming on Cu/ZnO/Al 2 O 3 catalysts are developed which account for all three of the possible overall reactions: methanol and steam reacting directly to form H 2 and CO 2 , methanol decomposition to H 2 and CO and the water-gas shift reaction. The elementary surface reactions used in developing the mechanisms were chosen based on a review of the extensive literature concerning methanol synthesis on Cu/ZnO/Al 2 O 3 catalysts and the more limited literature specifically dealing with methanol–steam reforming. The key features of the mechanism are: (i) that hydrogen adsorption does not compete for the active sites which the oxygen-containing species adsorb on, (ii) there are separate active sites for the decomposition reaction distinct from the active sites for the methanol–steam reaction and the water-gas shift reaction, (iii) the rate-determining step (RDS) for both the methanol–steam reaction and the methanol decomposition reaction is the dehydrogenation of adsorbed methoxy groups and (iv) the RDS for the water-gas shift reaction is the formation of an intermediate formate species. A kinetic model was developed based on an analysis of the surface mechanism. Rate data were collected for a large range of conditions using a fixed-bed differential reactor. Parameter estimates for the kinetic model were obtained using multi-response least squares non-linear regression. The resultant model was able to accurately predict both the rates of production of hydrogen, carbon dioxide and of carbon monoxide for a wide range of operating conditions including pressures as high as 33 bar.

Sorption‐enhanced reaction process for hydrogen production

Abstract: A novel concept called Sorption Enhanced Reaction Process (SERP) for hydrogen production by steam-methane reformation (SMR) reaction uses a fixed packed column of an admixture of an SMR catalyst and a chemisorbent to remove carbon dioxide selectively from the reaction zone. The chemisorbent is periodically generated by using the principles of pressure swing adsorption. The SERP process steps allow direct production of high-purity hydrogen (> 95 mol %) at high methane to hydrogen conversion (> 80%) with dilute methane ( 650 C) to achieve the same methane to hydrogen conversion, but produces a much lower purity of hydrogen product ({approximately} 75 mol %) with a large quantity of carbon oxide ({approximately} 20 mol %) impurities. A novel chemisorbent, which reversibly sorbs carbon dioxide in the presence of excess steam at a temperature of 300--500 C, was developed for application in the SERP and the process is experimentally demonstrated in a bench-scale apparatus.

Methanol–steam reforming on Cu/ZnO/Al2O3. Part 1: the reaction network

Abstract: On-board generation of hydrogen by methanol–steam reforming on Cu/ZnO/Al 2 O 3 catalyst is being used in the development of fuel-cell engines for various transportation applications. There has been disagreement concerning the reactions that must be included in the kinetic model of the process. Previous studies have proposed that the process can be modelled as either the decomposition of methanol followed by the water-gas shift reaction or the reaction of methanol and steam, to form CO 2 and hydrogen, perhaps followed by the reverse water-gas shift reaction. Experimental results are presented which clearly show that, in order to explain the complete range of observed product compositions, rate expressions for all three reactions (methanol–steam reforming, water-gas shift and methanol decomposition) must be included in the kinetic analysis. Furthermore, variations in the selectivity and activity of the catalyst indicate that the decomposition reaction occurs on a different type of active site than the other two reactions. Although the decomposition reaction is much slower than the reaction between methanol and steam, it must be included in the kinetic model since the small amount of CO that is produced can drastically reduce the performance of the anode electrocatalyst in low temperature fuel cells.

Layered double hydroxides exchanged with tungstate as biomimetic catalysts for mild oxidative bromination

Abstract: The manufacture of a range of bulk and fine chemicals, including flame retardants, disinfectants and antibacterial and antiviral drugs, involves bromination1. Conventional bromination methods typically use elemental bromine, a pollutant and a safety and health hazard. Attempts to develop alternative and more benign strategies have been inspired by haloperoxidase enzymes, which achieve selective halogenation at room temperature and nearly neutral pH by oxidizing inorganic halides with hydrogen peroxide2,3. The enzyme vanadium bromoperoxidase has attracted particular interest4,5 in this regard, and several homogeneous inorganic catalysts mimicking its activity are available6,7,8,9,10,11, although they are limited by the requirement for strongly acidic reaction media. A heterogenous mimic operating at neutral pH has also been reported12, but shows only modest catalytic activity. Here we describe a tungstate-exchanged layered double hydroxide that catalyses oxidative bromination and bromide-assisted epoxidation reactions in a selective manner. We find that the catalyst is over 100 times more active than its homogeneous analogue. The low cost and heterogeneous character of this system, together with its ability to operate efficiently under mild conditions using bromides rather than elemental bromine, raise the prospect of being able to develop a clean and efficient industrial route to brominated chemicals and drugs and epoxide intermediates.

An ultrasafe hydrogen generator: Aqueous, alkaline borohydride solutions and Ru catalyst

Abstract: A novel, simple, convenient, and safe, chemical process generates high purity hydrogen gas on demand from stable, aqueous solutions of sodium borohydride, NaBH,, and ruthenium based (Ru), catalyst. When NaBH, solution contacts Ru catalyst, it spontaneously hydrolyzes to form H, gas and sodium borate, a water-soluble, inert salt. When H, is no longer required, Ru is removed from the solution and H, generation stops. Since this H, generator is safer, has quicker response to H, demand, and is more efficient, than commonly used H, generators, it is ideal for portable applications. INTRODUCTION PEM fuel cells are attractive power sources for providing clean energy for transportation and personal electronics applications where low system weight and portability are important. For powering these systems, H, gas is the environmentally desirable anodic fuel of choice since only water is formed as a discharge product. A major hurdlc is how to gcnerate/store controlled amounts of H, fuel directly without resorting to high temperature reformers with significant heat signatures or bulky, pressurized cylinders. Background of the Borohydride H, Generator Our safe, portable H, generator overcomes these problems by using aqueous, alkaline, sodium borohydride (NaBH,, tetrahydroborate) solutions which are extremely stable. However, as found by Schlesinger et al. ( I ) , in the presence of selected metal (or metal boride) catalysts, this solution hydrolyzes to yield H, gas and water-soluble, sodium metaborate, NaBO,. NaBH, + 2 H,O ----> 4 H, + NaBO, P I catalyst This hydrolysis reaction occurs at different rates depending on the catalyst used and its preparation method. Levy et al. (2) and Kaufman and Sen (3) investigated cobalt and nickel borides as catalysts for practical, controlled generation of H, from NaBH, solutions. We studied ruthenium (Ru) based catalyst supported on ion exchange resin beads. Using Ru is based on the work or Brown and Brown (4). who investigated various metal salts and found that ruthenium and rhodium salts liberated H, most rapidly from borohydride solutions. We chose Ru because of its lower cost. Ru catalysts are not consumed during hydrolysis and are reusable. We have designed our system so that reaction [ I ] is either selfregulating or carefully controllable. To generate H,, NaBH, solution is allowed to flow onto a Ru catalyst, or NaBH, solution is injected onto Ru catalyst. This ensures fast response to H, demand i.e. H, is generated only when NaBH, solution contacts Ru catalyst. When H, is no longer needed, NaBH, solution is removed from Ru catalyst and H, production ceases. With molecular weights of NaBH, (38) and 2 H,O (36), forming 41-1, (8), reaction [ I ] has a I-l,storage efficiency of 8/74 = 10.8%. In addition to H,, the other discharge product, NaBO,, commonly found in laundry detergents, is safe. Unlike phosphates, borates are not environmentally hazardous in water supplies. Table I compares operational and safety features of generating H, via base-stabilized NaBH, solutions and via reactive chemical hydrides. Our generator is considerably saferhore efficient than producing H, via other reactive chemicals. The heat generated by our system (75 kJ/mole H, fomied), is less than what is produced by other hydrides (>I25 kJ/mole H,), and ensures a safe, controllable chemical reaction. The total amount of H, produced by reaction [ I ] depends on NaBH, solution volume and concentration. H, generation rates are primarily a function of Ru catalyst active surface area. H, pressure/flow rates can be accurately controlled and made self-regulating by numerous feedback

Catalytic oxidation of carbon monoxide on monodispersed platinum clusters: Each atom counts

Abstract: Nanoclusters open fascinating opportunities for quantum engineering because quantum-size effects become dominant in determining catalytic,1-3 optical,4 electronic,5 and magnetic6 properties. We succeeded in the controlled production of low-energy and high-flux monodispersed cluster beams, which allow for a systematic study of their reactivity after deposition onto a chemically inert substrate. We investigated the catalytic reaction that is the oxidation of CO on platinum and observed a distinct atom by atom size dependency for monodispersed platinum clusters on thin MgO(100) films. These results clearly show that the efficiency of a heterogeneous catalytic reaction can be tuned by the judicious choice of particle size.

Phonon- Versus Electron-Mediated Desorption and Oxidation of CO on Ru(0001)

Abstract: Heating of a ruthenium surface on which carbon monoxide and atomic oxygen are coadsorbed leads exclusively to desorption of carbon monoxide. In contrast, excitation with femtosecond infrared laser pulses enables also the formation of carbon dioxide. The desorption is caused by coupling of the adsorbate to the phonon bath of the ruthenium substrate, whereas the oxidation reaction is initiated by hot substrate electrons, as evidenced by the observed subpicosecond reaction dynamics and density functional calculations. The presence of this laser-induced reaction pathway allows elucidation of the microscopic mechanism and the dynamics of the carbon monoxide oxidation reaction.

Palladium(II)-Catalyzed Oxidation of Alcohols to Aldehydes and Ketones by Molecular Oxygen.

Abstract: A novel combination of Pd(OAc)2/pyridine/MS3A catalyzes the aerobic oxidation in toluene of a variety of primary and secondary alcohols into the corresponding aldehydes and ketones in high yields. Various substituents and protecting groups are compatible with this oxidation. The ca. 2:3 ratio of O2 uptake to product yield is observed, whereas in the absence of MS3A, the ratio is ca. 1:1, suggesting the in situ formation of H2O2 and its decomposition by MS3A into water and oxygen. A catalytic cycle including the formation of a Pd(II)-alcoholate followed by β-elimination of a Pd(II)H species and a carbonyl compound and then the formation of a Pd(II)OOH species is proposed.

Oxidation of Methanol on 2nd and 3rd Row Group VIII Transition Metals (Pt, Ir, Os, Pd, Rh, and Ru): Application to Direct Methanol Fuel Cells

Abstract: Using first principles quantum mechanics [nonlocal density functional theory (B3LYP)], we calculated the 13 most likely intermediate species for methanol oxidation on clusters of all 2nd and 3rd row Group VIII transition metals for all three likely binding sites (top, bridge, and cap). This comprehensive set of binding energies and structures allows a detailed analysis of possible reaction mechanisms and how they change for different metals. This illustrates the role in which modern quantum chemical methods can be used to provide data for combinatorial strategies for discovering and designing new catalysts. We find that methanol dehydrogenation is most facile on Pt, with the hydrogens preferentially stripped off the carbon end. However, water dehydrogenation is most facile on Ru. These results support the bifunctional mechanism for methanol oxidation on Pt−Ru alloys in direct methanol fuel cells (DMFCs). We find that pure Os is capable of performing both functionalities without cocatalyst. We suggest that...