Design Of New Catalysts Research Articles

Combinatorial Computational Chemistry Approach in the Design of New Catalysts and Functional Materials

Three decades ago, J. J. Hanak proposed a new approach for materials exploration that he called the ‘‘multisample concept.’’ The goal was to make materials discovery processes faster with higher efficiency as compared to the standard methods (1). This idea of producing many different compositions at once appeared to be an elegant art that introduced diversity and parallel processing in experiments. More recently, researchers have begun to use a new idea commonly known as the combinatorial approach. At first, this was applied mostly to chemical synthesis and screening of large numbers of organic compounds for drug discovery processes (2). By using the combinatorial approach, researchers have been able to predict and find an optimal composition with the desired characteristics much faster than before. There are many good examples of applications of the combinatorial method in chemistry, including the development of catalytic antibodies (3) and, more recently, the identification of nonpeptide agonists (activators) for different somatostain receptor subtypes (4). Inorganic materials have many more potential applications than organic materials because they exhibit a wide range of physical/chemical properties, many of which are strongly dependent on compositions. The combinatorial approach has great potential in investigating important inorganic materials, and its utility

Structure, Bonding, and Stability of a Catalytica Platinum(II) Catalyst: A Computational Study

Periana et al. [Science 1998, 280, 560] previously reported two catalysts for low-temperature methane activation to methanol: PtCl2(NH3)2 and PtCl2(bpym). It was shown that the ammine catalyst is much more active, but it decomposes rapidly in sulfuric acid to form a PtCl2 precipitate, while the bpym system is long-lived. To have a basis for developing new catalysts that would not decompose, we undertook a study of the structure, bonding, and stability of the PtCl2(NH3)2 and PtCl2(bpym) catalysts, using quantum mechanics (QM) [density functional theory (DFT) at the B3LYP/LACVP**(+) level] including solvation in sulfuric acid via the Poisson−Boltzmann continuum approximation. Critical results include the following: (1) The influence of a trans ligand Y on the Pt−X bond follows the order Cl- > NH3 (bpym) > OSO3H- > □ (empty site). Thus the Pt−N bond length is longer (up to 0.04 Å) and the Pt−N bond is weaker (up to 18 kcal/mol) when trans to a Cl- as compared to trans to OSO3H-. (2) The bpym ligand acts as both a σ-donor and a π-acceptor. As bpym is protonated, the Pt−N bond strength decreases (by up to 51 kcal/mol). Thus, ΔH(soln, 453 K) for Pt(OSO3H)2(bpym) (69.6) > [Pt(OSO3H)2(bpymH)]+ (49.4) > [Pt(OSO3H)2(bpymH2)]2+ (18.7). (3) In sulfuric acid replacing the ammine ligands with bisulfate ligands is thermodynamically favorable [by ΔG(soln, 453 K) = −23 kcal/mol], whereas replacement of bpym with OSO3H- is unfavorable [by ΔG(soln, 453 K) = +16 kcal/mol]. (4) Replacement of chloride ligands with bisulfate ligands is thermodynamically unfavorable [by ΔG(soln, 453 K) = ∼7 kcal/mol for ammine and ∼12 kcal/mol for bpym]. (5) Protonation of PtCl2(bpym) is thermodynamically favorable, leading to [PtCl2(bpymH)]+ as the stable species in sulfuric acid (by 8 kcal/mol). Thus we conclude that in hot concentrated sulfuric acid it is quite favorable for PtCl2(NH3)2 to lose its ammine ligands to form PtCl2, which in turn will dimerize and oligomerize, leading eventually to a (PtCl2)n precipitate and catalyst death. We find that PtCl2(bpym) is resistant to solvent attack, favoring retention of the bpym ligand in hot concentrated sulfuric acid. These results agree with experimental findings. The insights from these findings should help screen for stable new ligands in the design of new catalysts.

Homogeneous and Heterogeneous Catalysis: Bridging the Gap Through Surface Organometallic Chemistry

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Homogeneous and heterogeneous catalysis: bridging the gap through surface organometallic chemistry.

Surface organometallic chemistry is an area of heterogeneous catalysis which has recently emerged as a result of a comparative analysis of homogeneous and heterogeneous catalysis. The chemical industry has often favored heterogeneous catalysis, but the development of better catalysts has been hindered by the presence of numerous kinds of active sites and also by the low concentration of active sites. These factors have precluded a rational improvement of these systems, hence the empirical nature of heterogeneous catalysis. Catalysis is primarily a molecular phenomenon, and it must involve well-defined surface organometallic intermediates and/or transition states. Thus, one must be able to construct a well-defined active site, test its catalytic performance, and assess a structure-activity relationship, which will be used, in turn-as in homogeneous catalysis-to design better catalysts. By the transfer of the concepts and tools of molecular organometallic chemistry to surfaces, surface organometallic chemistry can generate well-defined surface species by understanding the reaction of organometallic complexes with the support, which can be considered as a rigid ligand. This new approach to heterogeneous catalysis can bring molecular insight to the design of new catalysts and even allow the discovery of new reactions (Ziegler-Natta depolymerization and alkane metathesis). After more than a century of existence, heterogeneous catalysis can still be improved and will play a crucial role in solving current problems. It offers an answer to economical and environmental problems faced by industry in the production of molecules (agrochemicals, petrochemicals, pharmaceuticals, polymers, basic chemicals).

Surface science. Bridging gaps and opening windows.

New experimental techniques allow the characterization of reaction patterns across a range of pressures and length scales. As [Jaeger][1] explains in his Perspective, these techniques enable the previous gap between operating conditions of real catalysts and systems accessible to experiment to be bridged. Jaeger highlights the report by [ Sachs et al .][2], who use two different techniques to study the reaction dynamics at atomic resolution and on micrometer scales under the same operating conditions. He charts recent progress toward understanding the dynamics of surface reactions and using the acquired knowledge in the rational design of new catalysts. [1]: http://www.sciencemag.org/cgi/content/full/293/5535/1601 [2]: http://www.sciencemag.org/cgi/content/short/293/5535/1635

Design and characterization of a novel "family-shuffling" technology adapted to membrane enzyme: application to P450s involved in xenobiotic metabolism.

Cytochrome P450 functional diversity and their predominant role in drug and pollutant metabolism and toxicity1 makes these enzymes particularly suitable for the design of new catalysts as well as for structure-function analysis2. Combinatorial molecular evolution (CME) is a powerful approach used for tuning protein functions3;4and for investigation of biochemical mechanisms driving substrate recognition5 or catalysis6. Family-shuffling has proved to accelerate the evolution process7. A low content of mosaic structures was frequently reported in libraries constructed using DNase I fragmentation8. We designed a new strategy for family shuffling in yeast expression vectors. This procedure takes advantage of the association between in vitro 9 and in vivo 10. recombination mechanisms to build a high complexity library containing low levels of parental structures. The use of engineered yeast strains for expression of membrane proteins into an optimized redox environment 11 also allows efficient in vivo bioconversion. The model used is human CYPIAI and CYPIA2 which share 71% identity and have distinct, while overlapping, substrate specificities.

Oxidations by copper metalloenzymes and some biomimetic approaches

This paper illustrates the various aspects of the reactivity of the Cu(II)–Cu(I) system in biological systems, with one example of an enzymatic reaction in which Cu(II) alone is oxidizing enough to carry out the reaction (superoxide dismutase), one example in which a Cu(II)-bound peroxo intermediate is the active species (tyrosinase) and the examples of galactose oxidase and copper amine oxidases in which Cu(II) is associated with a redox active organic cofactor. In some cases, we will show some illustrations of biomimetic approaches developed in our laboratories, aimed at a better understanding of reaction mechanisms and at an original design of new catalysts with potential applications in synthetic chemistry. Some comments are given concerning the respective features of copper and iron.

Fischer--Tropsch, hydrotreating and superClaus catalysts

In modern society the application of catalysts, and hence, research in the field of catalysis, is becoming more and more important. In the strongly increasing chemical industry most reaction processes are performed with the aid of catalysts. Knowledge about the structure of the active sites present at the surface of catalysts will facilitate the design of new catalysts with better performances. Such knowledge can be obtained by Mossbauer spectroscopy, which is an excellent in-situ characterization technique due to the high penetrating power of the γ-radiation used and the sensitivity of the spectral parameters for the chemical state and local environment of the Mossbauer atoms.

Methylamines synthesis: A review

Monomethylamine (MMA), dimethylamine (DMA) and trimethylamine (TMA) are major industrial chemical intermediates. Generally, these compounds are prepared by reaction of methanol and ammonia over a dehydration catalyst such as silica alumina. While thermodynamics favors TMA formation, market demand is for DMA. This has led to the development of highly DMA selective zeolite-based catalysts. In this article, the history of, the reported routes to, the design of new catalysts for, and the mechanism of the synthesis of methylamines are discussed.

Solid-state reaction examination of copper oxide impregnated on pure and La-modified gamma alumina

Transition metal oxides dispersed in support oxides have been studied as catalysts for various chemical processes and the removal of environmental pollution. Recently, copper-exchanged zeolites including ZSM-5 have been extensively studied for high potential use as exhaust de-NOx catalysts in automobile engines and factory generator systems [1]. However, the aluminosilicates decompose under thermally hard conditions above 900 °C and following solid-state reactions lead to a large degredation of catalytic activity to remove NOx species. The study of the thermal behaviour and stability of catalytic compositions gives important information for the materials design of new catalysts and the improvement of their thermal durability. Copper oxide impregnated on gamma alumina (7-A1203) has been traditionally examined for oxidation reactions at low temperatures, however, the thermal behaviour at temperatures above 900 °C has rarely been reported. Highly dispersed copper on an alumina support is considered as a potential alternative material for a de-NOx catalyst if its stability can be improved at elevated temperatures. In this letter, we describe the solid-state reaction of highly dispersed copper on pure and La-modified y-A1203. The difference in thermal behaviour of copper oxide between pure and La-modified y-A1203 support and the phase evolution of these systems should be emphasized for the purpose of proposing basic data to develop improved catalysts. La-modified y-A1203 powders with 0 to 10 mol % LaOl.s were prepared by an impregnation procedure, as reported in a previous paper [2]. Powdered alumina with surface area of 100 m2/g was impregnated using an aqueous solution of lanthanum nitrate, and then dried at 110 °C for 10 h and heated at 600 °C for 3 h in air. The impregnation of 5 to 20 mol % Cu to alumina supports was performed by aqueous copper nitrate, and then dried at 110 °C for 10 h and heated at 500 °C for 3 h in air. The samples were heated at temperatures of 800 °C to 1100 °C for 3 h in air. The evolution phases were examined by an X-ray diffraction (XRD) technique. Here we described the case of catalysts having 10 mol % Cu which are supported by pure and 10 mol% Laadded y-Al203 in order to examine the typical effect of La-modification on an alumina support. Fig. 1 shows the XRD patterns of 10mol% Cu-impregnated pure y-A1203 followed by heat treatment temperatures of 800 to 1000 °C for 3 h in air. Pure y-type alumina transforms to 0-type at the temperature range of 800 °C to 1000 °C and changes to 0:-type above 1050 °C. However, the data in Fig. 1 indicate that Cu-impregnated 7-A1203 transforms not to 0-type but to y-type modified with Cu at a temperature range of 800 °C to 900 °C. The characteristics of this Cu-modified y-phase were shown by diffraction peaks at 0.268 and 0.196 nm (20 = 33.4 ° and 46.3 ° in Fig. 1) in addition to the usual y-type pattern [3]. The data are believed to characterize the

Trans-effect and trans-influence series for phosphanes in octahedral environment: A guide for design of new catalysts

By a kinetic approach, the trans-effects and trans-influences of nineteen tertiary phosphane ligands were evaluated for a hexacoordinated d 6-low spin metal center. Plots of specific rate constants versus formal potentials for Ru(III)/Ru(II) couples provide a simple, reliable and efficient method to predict phosphane reactivity and are useful as a guide for design of new catalysts.

Antibody Catalyzed Cationic Cyclization

Two major goals for the design of new catalysts are the facilitation of chemical transformations and control of product outcome. An antibody has been induced that efficiently catalyzes a cationic cyclization in which an acyclic olefinic sulfonate ester substrate is converted almost exclusively (98 percent) to a cyclic alcohol. The key to the catalysis of the reaction and the restriction of the product complexity is the use of antibody binding energy to rigidly enforce a concerted mechanism in accord with the design of the hapten. Thus, the ability to direct binding energy allows the experimenter to dictate a reaction mechanism which is an otherwise difficult task in chemistry. New catalysts for cationic cyclization may be of general use in the formation of carbon-carbon and carbon-heteroatom bonds leading to multi-ring molecules including steroids and heterocyclic compounds.

Vanadium(V) complexes of sulfonated 1,5,10-tris(2,3-dihydroxybenzoyl)-1,5,10-triazadecane as catalysts for the Stretford process

The reaction of the V(V) complex of sulfonated 1,5,10-tris(2,3-dihydroxybenzoyl)-1,5,10-triazadecane (3,4 LICAMS) with H 2S has been studied as a model for the Stretford process. Oxidation of the hydrogen sulfide is found to occur in two steps. The first involves the pH dependent formation of a V(V) LICAMS-H 2S intermediate while the second is a pH independent electron transfer to produce elemental sulfur and V(IV) LICAMS. Rate constants have been measured and the significance of these data to the design of new catalysts is discussed.

The structural relationships between the various phases of bismuth molybdates with special reference to their catalytic activity

Abstract Including those reported here for the first time. there are at least seven distinct phases of bismuth molybdate: Bi2O3·nMoO3, with n = 3, n = 2 and n = 1 (three variants); 3Bi2O3·2MoO3; and 19Bi2O3·7MoO3. Hitherto most of these phases have been interpreted to possess quite different, and seemingly unrelated, crystallographic structures. We advance arguments, based largely on high-resolution electron microscopy, as well as electron and X-ray diffraction studies. that the majority of these phases can be derived from the fluorite structure, a conclusion which offers new understanding and strategic scope in the design of new catalysts for the selective oxidation of hydrocarbons.

Theoretical model for exploration of catalytic activity of enzymes and design of new catalysts: CO2 hydration reaction

AbstractThe quantum chemical model for predicting the optimal molecular environment exerting the highest catalytic activity on transition complex has been proposed. It was applied for the CO2 hydration reaction at the nonempirical LCAO MO SCF level. The possible use of the resulting optimal charge distribution of environment for design of new synthetic catalysts and explanation of the carbonic anhydrase action was discussed. In addition, a simplified approach based on the difference electrostatic molecular potentials evaluated within the many center multipole expansion is presented. It may enable us to perform corresponding estimations even by means of a programmable calculator.