Variations In Catalytic Properties Research Articles

Nitrogen-doped carbon confined cobalt nanoparticles as the steric acid-base multifunctional catalysts for Knoevenagel condensation

Nitrogen-doped carbon confined cobalt nanoparticles (Co-N-C) were synthesized through multi-step pyrolysis method. The Co-N-C samples exhibit excellent catalytic performance in the Knoevenagel reaction of benzaldehyde and malononitrile, of which the yield can be as high as 93.7% within 2.5 min with outstanding stability and recyclability. The catalytic performance may be primarily ascribed to the richness in nitrogen and cobalt in the form of the solid Frustrated Lewis Pair (FLP), i.e. the Co ion acts as a Lewis acid site to activate the carbonyl oxygen of benzaldehyde molecules, and nitrogen in carbon acts as a Lewis base site to activate the methylene hydrogen of malononitrile. From the perspective of intrinsic catalytic activity, variations in catalytic properties are attributed to the electron density distribution on the surface of the catalyst.

Efficient low-temperature hydrogenation of acetone on bimetallic Pt-Ru/C catalyst

Carbon supported Pt-Ru nano-alloy with multiple reactive sites exhibits unprecedentedly efficient performance in acetone hydrogenation at room temperature. Catalytic properties can be controllably regulated and optimized via performing the specific annealing treatment for the catalyst. The essential correlation between the variations in catalyst’s annealing conditions and the resultant variation in catalytic properties has been explored and discussed as a structure-function relationship. The combination of Pt and Ru sites exhibits enhanced catalytic activity compared with monometallic ones. Larger PtRu alloy particle size favors the rapid CO hydrogenation and the improvement of catalytic selectivity to isopropanol. Isopropyl ether generation is related to the Pt defect sites on the surface, which can be blocked though the catalyst’s oxidation treatment. The presence of surface metal-oxide species is beneficial to the performance enhancement, resulting from the synergetic behavior between metal-oxide sites and metallic sites on respectively activating the reactants. Moreover, 3-pentanone is used as a probe molecule to verify the structure-function relationship of catalytic low-temperature CO hydrogenation on PtRu/C.

Functional and structural characterization of synthetic cardosin B-derived rennet.

The potential of using a synthetic cardosin-based rennet in cheese manufacturing was recently demonstrated with the development and optimization of production of a recombinant form of cardosin B in Kluyveromyces lactis. With the goal of providing a more detailed characterization of this rennet, we herein evaluate the impact of the plant-specific insert (PSI) on cardosin B secretion in this yeast, and provide a thorough analysis of the specificity requirements as well as the biochemical and structural properties of the isolated recombinant protease. We demonstrate that the PSI domain can be substituted by different linker sequences without substantially affecting protein secretion and milk clotting activity. However, the presence of small portions of the PSI results in dramatic reductions of secretion yields in this heterologous system. Kinetic characterization and specificity profiling results clearly suggest that synthetic cardosin B displays lower catalytic efficiency and is more sequence selective than native cardosin B. Elucidation of the structure of synthetic cardosin B confirms the canonical fold of an aspartic protease with the presence of two high mannose-type, N-linked glycan structures; however, there are some differences in the conformation of the flap region when compared to cardosin A. These subtle variations in catalytic properties and the more stringent substrate specificity of synthetic cardosin B help to explain the observed suitability of this rennet for cheese production.

Characterization of bifunctional PtSn/H[Al]ZSM5 catalysts: a comparison between two impregnation strategies

Bifunctional monometallic Pt/H[Al]ZSM5, Sn/H[Al]ZSM5 and bimetallic PtSn/H[Al]ZSM5 catalysts were prepared and characterized by means of XPS, TEM and toluene hydrogenation. For the bimetallic samples, co-impregnation of both metals and sequential impregnation (where Sn was added to monometallic Pt catalysts) were used with two tin atomic fractions, X Sn = 0.46 and 0.28. Based on XPS data, it was found that the surface composition of the Pt–Sn bimetallic catalysts depends on the impregnation method employed: in the co-impregnated samples alloys form between both metals, while in the sequentially impregnated catalysts no alloy formation occurs but both Pt and Sn are easily reduced. Also, TEM analysis showed that the co-impregnated samples have similar particle sizes as the monometallic Pt catalysts, but that the particles in the sequentially impregnated samples are of larger sizes, closer to those seen in monometallic Sn catalysts. It is suggested here that while in the former case the particle size is not affected by the formation of Pt–Sn alloys, in the latter the tin covers the Pt species, and therefore increases the overall particle size. Finally, it was found that some hydrogenating activity could be detected with the co-impregnated catalysts but not with the sequentially impregnated samples, even though the latter had only a small fraction of atomic tin. In general, the results obtained in this study show a significant variation in catalytic properties with the method of preparation used for these bifunctional PtSn/H[Al]ZSM5 catalysts.

Infrared spectra of dinitrogen adsorbed on bimetallic lanthanide (Eu or Yb)-Ni/SiO2 catalysts

Bimetallic lanthanide (Ln: Eu or Yb)–Ni/SiO2 catalysts prepared by the use of dissolution of lanthanide metals in liquid ammonia have been studied by infrared spectroscopy for dinitrogen adsorption. The infrared spectra were measured at 133–300 K using Ln–Ni/SiO2 obtained when the Eu or Yb metal dissolved in liquid ammonia reacted with 20 mass% Ni/SiO2 in different ratios. Infrared spectra for Eu–Ni/SiO2 showed absorption bands at 2336, 2265, 2254 and 2227 cm−1 at 133 K, which disappeared upon evacuation. The adsorbed state was found to be all molecular from the isotope shift using 28N2 and z30N28. The bands at 2254 and 2227 cm−1 of them were assigned to new adsorbed dinitrogen species resulting from synergetic interactions between the europium and nickel metal. The concentration of adsorbed dinitrogen on Eu–Ni/2 varied markedly with the Eu/Ni ratios, and particularly, it increased in the region of high Eu content. Upon introduction of ytterbium onto nickel, new bands at 2254 and 2226 cm−- similarly appeared. However, the dependence of dinitrogen adsorption as a function of Yb content in Yb–Ni/SiO2> was somewhat different from that for Eu–Ni/SiO2. The effects of lanthanide on the surface of Ln–Ni/SiO2 were discussed in connection with the variation in catalytic properties.

Metal‐Support interaction in Silica‐Supported Cobalt Catalyst

AbstractThe variations in catalytic properties for CO hydrogenation and adsorptive properties for H2 and CO of silica‐supported cobalt catalyst as a function of reduction temperature (400 to 700 °C) were investigated. A mild sintering of cobalt metal from 130 Å to 145 Å occurred only when the reduction temperature was further increased from 600 to 700 °C. However, a monotone decrease in the activity (per gram Co) for CO hydrogenation was observed with increasing reduction temperature. This is accompanied with a decrease in methane and an increase in olefin formation. The decrease in activity can not be accounted for by the variation of crystallite size of cobalt metal. A concurrent suppression in the amount of hydrogen and CO adsorptions was observed after reduction temperature was raised to 700 °C. The turnover frequencies, based on cobalt dispersion estimated from H2 and CO chemisorption, remained, more or less, similar with increasing reduction temperature from 400 to 600 °C, but increased ca. 3 and 1.5 times, respectively, at 700 °C. The relative amount of subcarbonyl species (2062 cm−1) with respect to the linear adsorbed CO (2010 cm−1) was found to increase as well. Furthermore, reduction of silica surface at 700 °C was indicated by the presence of ˜Si‐H, which was detected by infrared spectroscopy at 2294 cm−1. This shows the presence of metal‐support interaction for silica‐supported cobalt systems after reduction treatment at 700 °C.

ChemInform Abstract: Lamellar Perovskites MI (An-1BnO3n+1) Catalysts for Oxidative Coupling of Methane.

Abstract The aim of this work concerns the study of new lamellar perovskites M I (A n −1 B n O 3 n +1 ) as catalysts for the oxidative coupling of methane. The main results show that: Substitution in an octahedral site (B) strongly modifies both activity and selectivity. In this site niobium (an element of Group V) leads to a more selective catalyst. Substitution either of the A cation in the perovskite site or of the interlamellar cation (M I ) modifies the selectivity more than the activity. An increase in the thickness ( n ) of the layer decreases both activity and selectivity. It can be considered as a structural effect in the oxidative coupling of methane. Variations of catalytic properties can be correlated to the covalent character of the B-O bond. The reactional center can be described as a polaron created at a structural defect, the existence of very localized centers being less probable at such high temperatures.

Lamellar perovskites MI (An−1BnO3n+1) catalysts for oxidative coupling of methane

Abstract The aim of this work concerns the study of new lamellar perovskites M I (A n −1 B n O 3 n +1 ) as catalysts for the oxidative coupling of methane. The main results show that: Substitution in an octahedral site (B) strongly modifies both activity and selectivity. In this site niobium (an element of Group V) leads to a more selective catalyst. Substitution either of the A cation in the perovskite site or of the interlamellar cation (M I ) modifies the selectivity more than the activity. An increase in the thickness ( n ) of the layer decreases both activity and selectivity. It can be considered as a structural effect in the oxidative coupling of methane. Variations of catalytic properties can be correlated to the covalent character of the B-O bond. The reactional center can be described as a polaron created at a structural defect, the existence of very localized centers being less probable at such high temperatures.

Hydrogenation of carbon monoxide on carbon-supported cobalt rare earth catalysts

A study of carbon monoxide hydrogenation over cobalt—lanthanum and cobalt—cerium catalysts supported on carbon was carried out for the purpose of identifying the effects of rare earth elements. It was observed that these promoters give rise to a 100-fold increase in specific activity and turnover frequency, which is not a result of the inhibition of carbon monoxide or hydrogen adsorption. Moreover, the selectivity for the C 2 C 4 fraction increased from 4% to about 40% and this fraction is essentially olefinic. Such variations in catalytic properties, especially activity, are a strong indication of the formation of new sites. In comparison with the results of recent work, one may conclude that carbon monoxide and hydrogen hydrocondensation needs redox centres that could easily be formed with metal-rare earth catalysts.

CO hydrogenation over well-dispersed carbon-supported iron catalysts

It became apparent that carbon-supported iron systems were quite interesting and a broader base was required for comparison, so catalysts composed of iron dispersed on graphitized carbon, carbon blacks, and activated carbons were prepared. In contradiction to earlier studies of Fe/carbon CO hydrogenation catalysts, these carbon-supported iron samples were very active, and they typically had higher activities and olefin/paraffin ratios than unpromoted Fe/Al/sub 2/O/sub 3/ catalysts. In addition, an aqueous impregnation technique was developed which allowed the preparation of very highly dispersed iron on certain carbons. These iron/carbon catalysts could be placed in two general groups: one with high dispersion (HD) and one with low dispersion (LD). These reduced catalysts exhibited extremely interesting chemisorption behavior at 300 K - hydrogen chemisorption either did not occur or was very low while CO chemisorption always occurred, with very large CO uptakes existing in some cases indicating the presence of very small iron particles (i.e., high dispersions). In addition to these chemisorption properties, significant differences existed in catalytic behavior which could be correlated with the crystallite size of the iron. This note describes these variations in catalytic properties.

Polymerization of propylene with highly active titanium-magnesium catalysts

A study has been made of the effect of the composition of highly active, titanium-magnesium catalysts obtained by reacting TiCl 4, TiCl 3 or the TiCl 4· nC 6H 5COOC 2H 5 complex with anhydrous, finely dispersed magnesium chloride, on their activity and stereoregulating ability in polymerization of propylene. The isotacticity of the polypropylene is increased by use of the TiCl 4·C 6H 5COOC 2H 5 complex, by heating the titanium-magnesium catalysts, replacing the AlEt 2Cl cocatalyst by AlEt 3 or by adding ethyl p-methoxybenzoate to the reaction mixture. By varying these factors it is possible to increase the isotacticity of polypropylene from 40% to 95%, while preserving high activity at a level more than ten times the activity of δ TiCl 3·O·33AlCl 3. The polypropylene obtained in the presence of the titanium-magnesium catalysts is of low molecular weight. The structure and substructure of these catalysts has been investigated and suggestions are made about the composition of the active component and the cause of the variation in catalytic properties.

Alpha-amylase from five strains of Bacillus amyloliquefaciens: evidence for identical primary structures

The alpha-amylases from five strains of Bacillus amyloliquefaciens were compared to determine whether differences in primary structure are responsible for variations in catalytic properties previously reported among the enzymes. Amino acid analysis established virtually identical compositions for the proteins. Reaction with dimethylaminoaphthylene sulfonylchloride indicated the amino-terminal amino acid of each amylase to be valine. Carboxyl termini of the enzymes have been determined by digestion with carboxypeptidase A. The resulting kinetic data indicate tyrosine as the carboxyl terminus and leucine as the penultimate residue for all five proteins. Isoelectric focusing of the enzymes yielded isoelectric points in the pH range of 5.09 to 5.18. Tryptic digests of the enzymes chromatographed on a cation-exchange column showed identical elution patterns. It is concluded that the primary structure of the amylase from the five strains is identical or exhibits only conservative substitutions.