Showing papers on "Selectivity published in 1981"
592 citations
TL;DR: In this article, a series of bimetallic catalysts has been prepared by impregnating a commercial Pt/alumina catalyst (series B catalyst) with solutions containing compounds of tin or lead in aqueous or nonaqueous solvents.
248 citations
TL;DR: In this paper, the selectivity and ethylene oxide yield of polycrystalline silver can be affected significantly by electrochemical oxygen pumping, and a simple model is proposed in order to explain this new phenomenon.
226 citations
TL;DR: In this article, the selectivity of catalysts for CO hydrogenation has been investigated, and the most basic support, MgO, exhibited 90 % selectivity to methanol.
Abstract: Catalytic hydrogenation of CO offers tremendous challenges in catalyst design to control selectivity because of the large variety of hydrocarbons and oxygen-containing species that can be formed; Rh can produce the full range of potential products. Rh on SiO2, Al2O3, TiO2, CeO2 and MgO has been characterized by a complement of techniques and by CO hydrogenation activity. Catalytic activity varied over 200-fold, dependent on the support; Rh/TiO2 was the most active. Selectivity to alcohols vs. hydrocarbons and to methanol vs. ethanol varied over 50-fold. CO hydrogenation selectivity to alcohols varies with the basicity of the support; MgO, the most basic support, exhibited 90 % selectivity to methanol.
124 citations
TL;DR: In this article, remarkable threo selectivity was observed in the reaction of aldehydes with trans-and cis-1-bromo-2-butene with the aid of chromium-II salts.
121 citations
TL;DR: In this paper, the acid-base properties of 12-molybdophosphoric acid (H 3 PMo 12 O 40 ) and its salts have been investigated in relation to the acidic property, the contribution of bulk acid, the effect of steam in the feed gas, thermal stability, the effects of countercations, and the oxidation activity and selectivity.
120 citations
TL;DR: In this paper, the authors evaluated selected data describing complexation (including adsorption) of cadmium and other trace metals by natural solid and soluble soil components, using a consistent format to demonstrate the conditional nature of reported selectivity coefficients.
Abstract: We reevaluated selected data describing complexation (including adsorption) of cadmium and other trace metals by natural solid and soluble soil components, using a consistent format to demonstrate the conditional nature of reported selectivity coefficients. In all cases, the determined selectivity values increased substantially as the concentration of trace metals decreased with respect to the concentration of other competing cations. This dependence upon relative metal concentrations is apparently due to the heterogeneous composition of complexing sites in most soil systems and competition among the various cations present for these sites. Most selectivity coefficients have been obtained after the introduction of metal concentrations well above those normally encountered in natural systems, with little attention paid to the concentration and composition of competing cations. Under such conditions, the metal occupied not only the most specific sites, but a large portion of the more numerous nonspecific sites. The use of selectivity values obtained under such conditions, in predictive models, will greatly underestimate metal complexation in natural systems. This demonstrates the importance of determining selectivity coefficients under conditions comparable to those existing in nature, if results are to be applicable to real systems.
81 citations
TL;DR: A chiral macrocyclic crown ether exhibits an enantiomer-selectivity of 2.6 for α-phenylethylammonium ion when incorporated in solvent polymeric membranes as discussed by the authors.
Abstract: A chiral macrocyclic crown ether exhibits an enantiomer-selectivity of 2.6 for α-phenylethylammonium ion when incorporated in solvent polymeric membranes. The sequence of selectivity of these membranes clearly differs from that of lipophilicity for the different biogenic ammonium ions studied, indicating a significant structural contribution.
51 citations
50 citations
TL;DR: In this paper, several different bonded columns were evaluated for use in reverse phase HPLC of triglycerides, and differences in selectivity were discussed, and selectivity was also modified by adding silver ion to the solvent.
Abstract: Rapid separations of triglycerides by chain length and degree of unsaturation have been made by high performance liquid chromatography (HPLC) using reverse phase columns. Several different bonded columns were evaluated for use in reverse phase HPLC of triglycerides, and differences in selectivity are discussed. Selectivity was also modified by adding silver ion to the solvent, which produces marked changes in selectivity of solute triglycerides.
48 citations
TL;DR: In this paper, the acid-strength distribution of the sites generated at various levels of Na+H+ exchange in synthetic, large-port sodium mordenite has been characterised by measuring the heat of adsorption of NH3 on these sites.
TL;DR: In this paper, a study was made of the heterogeneous gas-phase hydrogenation of benzene to cyclohexene and cycloencane in a continuous-flow apparatus over non-supported Ru, at 0.11 MPa total pressure, in the range of temperatures from 273 to 320 K.
Abstract: A study has been made of the heterogeneous gas-phase hydrogenation of benzene to cyclohexene and cyclohexane in a continuous-flow apparatus over non-supported Ru, at 0.11 MPa total pressure, in the range of temperatures from 273 to 320 K. When working with highly purified reactants, no formation of cyclohexene is detected. On adding water vapour to the feed a selectivity to cyclohexene is gradually built up over a period of more than 10 h. If Ru is pre-covered with adsorbed water, the selectivity to cyclohexene is effected immediately, and on drying the feed to zero partial water-vapour pressure, the selectivity towards cyclohexene falls away rapidly.At 318 K the maximum cyclohexene selectivity arrived at is 42 %, at 20 % conversion. Reduction of spent Ru catalysts with hydrogen at 473 K leads to total recovery of activity and selectivity. Alloying of Ru with 1 at. % Au improves the catalyst stability but suppresses the cyclohexene selectivity.The preparation of Ru powders with a high surface purity is described. Characterization of the various samples is performed by hydrogen chemisorption, nitrogen physisorption and X.p.s./A.e.s. surface analysis. From adsorption isotherms between 290 and 340 K the enthalpy of adsorption of water on Ru, in the presence and in the absence of chemisorbed hydrogen, is calculated, and the result is compared with the enthalpy of H2O adsorption on Pt.A tentative explanation is presented of the beneficial influence of adsorbed water on the selectivity towards cyclohexene formation.
TL;DR: In this paper, the selectivity effects of a water-methanol-tetrahydrofuran ternary system are discussed with respect to the separation of vanillin and syringaldehyde and that of eight cinnamic acid derivatives.
TL;DR: In this paper, a Co2(CO)8-phosphine system was used as an efficient catalyst for the homogeneous water-gas shift reaction, which is also active for the hydration of ethene.
Abstract: The hydrocarbonylation of ethene with CO and H2O produces diethyl ketone with a high selectivity of 99±1%(propanal 1±1%); a Co2(CO)8–phosphine system is used as an efficient catalyst, which is also active for the homogeneous water-gas shift reaction The effects of the reaction parameters(CO pressure, temperature, and ethene and H2O concentrations) on the selectivity and activity are examined for the Co2(CO)8–1,2-bis(diphenylphosphino)ethane (diphos) system; the selectivity to yield the ketone is fairly high under the variety of conditions studied here Evidence suggests that the active species which catalyze the water-gas shift reaction participate in the hydrocarbonylation of ethene
TL;DR: In this article, it was shown that hydrogen atoms extracted from the sub-surface layer of the membrane catalyst participate in the hydrogenation process. But the authors did not consider the effect of the amount of hydrogen in the membrane layer on the selectivity of the catalysts.
Abstract: The methods of controlling a catalyst's selectivity by holding its surface (and in special cases its sub-surface) layer in non-stationary states towards the reagents are discussed.This may be achieved by the use of (a) a membrane catalyst; (b) a fluid bed; (c) a riser reactor; (d) periodical changes of the process parameters or catalyst circulation between the reactor and the regenerator; (e) a chromatographic regime; (ƒ) a self-oscillating or stochastic regime. The membrane catalyst produces a non-stationary state of the catalyst surface more easily than do the other methods and for a longer period of time. This propensity of the membrane catalyst is especially important for small-scale industrial installations and for producing ultrapure substances. The transfer of one reagent through the catalyst, for example through a palladium-based septum, increases the selectivity of the hydrogenation of triple bonds into double bonds and of, say, one double bond in a cyclic diene in comparison with normal hydrogenation by the same catalyst. Selectivity of the hydrogenation is a function of hydrogen content in the membrane catalyst. It is shown that hydrogen atoms extracted from the sub-surface layer of the membrane catalyst participate in the hydrogenation process.
TL;DR: In this article, the influence of the type of anchored ligand on the catalytic performance of rhodium-based catalysts was discussed, and it was shown that adding a small amount of triphenylphosphine to catalyst with chemically anchored ligands raises the selectivity for n-butyraldehyde formation.
TL;DR: In this article, vanadium-containing catalysts [α-Al2-xVxO3(x= 0-0.2), TixV1O2-o2(x = 0 − 0.2) and SnxV1-oO2(X = 0−0.1) ] have been studied to discover the extent to which conductivity or electron-exchange effects are important in determining catalyst selectivity in the oxidation of methanol.
Abstract: Model vanadium-containing catalysts [α-Al2–xVxO3(x= 0–0.2), TixV1O2–x(x= 0–0.2) and SnxV1–xO2(x= 0–0.2)] have been studied to discover the extent to which conductivity or electron-exchange effects are important in determining catalyst selectivity in the oxidation of methanol. By cross-comparison of the catalytic behaviour and solid-state data for the three catalyst systems it is concluded that the high selectivity to partial oxidation products is correlated with relatively facile electron exchange between active-site cations. The host lattice is shown to have an important role in that the “electronic isolation” of the active sub-lattice seems to be important.
TL;DR: In this article, the effect of adsorbed sulphur on the performance of buta-1,3-diene isomerization was investigated at ruthenium sites congested by CO.
Abstract: Previously published models, which have interpreted selectivity characteristics of metallic hydrogenation catalysts in terms of (i) molecular congestion at active sites, (ii) surface contamination and (iii) hydrogen occlusion in metals, are tested and elaborated.But-1-ene isomerisation has been investigated at ruthenium sites congested by CO. The catalyst was prepared by impregnation of Ru6C(CO)17 into silica. After a minor displacement of CO it showed behaviour characteristic both of the parent cluster (activity in hydrogen–deuterium exchange) and of conventional supported ruthenium (product composition in the ethene–deuterium exchange) and of conventional supported ruthenium (product composition in the ethene–deuterium reaction). Preferential cis-but-2-ene formation from but-1-ene occurred at these CO-congested sites, whereas uncongested sites provided preferential trans-but-2-ene formation.Adsorption of sulphur on catalytically active metal by decomposition of H2S modifies selectivity in that it converts the major process in buta-1,3-diene hydrogenation from 1:2-addition to 1:4-addition. Adsorption of H2S has been measured on evaporated films of Cr, Mn, Fe, Co, Ni, Mo, Pd, W, Re and Pt and on Co-powder and Ni/silica. The selectivity changes are recorded and interpreted in terms of the electronic effect of adsorbed sulphur on metal atoms remaining exposed at the surface and, for Fe, Co, Ni and Pd, the effect of subsequent incorporation of sulphur into the surface. The behaviour of Pt was anomalous, the effect of adsorbed sulphur on selectivity being less than expected.Finally, the cavity theory of hydrogen occlusion in metals correctly predicts the formulation or conditions of preparation required to translate iridium from its traditional position as the least selective Group VIII metal for buta-1,3-diene hydrogenation (to butene) to a new position as one of the most selective.
TL;DR: In this article, the influence of the solvent and activation temperature of the zeolite on the reaction of t-butanol and dioxygen has been studied and the results on other supported Co-catalysts showed that selectivity and activity strongly depend on the support applied and on the size of the alkyl groups.
TL;DR: In this paper, it was shown that Rhodium catalysts are about one order of magnitude more active for ethane hydrogenolysis when supported on alumina or titania than when they were supported on silica when reduction is carried out at low temperature.
Abstract: Rhodium catalysts are about one order of magnitude more active for ethane hydrogenolysis when supported on alumina or titania than when supported on silica when reduction is carried out at low temperature. However, the difference in activity for ethane hydrogenolysis between low-(573 K) and high-(773 K) temperature reduced Rh on titania is much greater. When the selectivity between dehydrogenation and hydrogenolysis of cyclohexane is compared at low and high temperature, a modest increase in selectivity is found to accompany the increased reduction temperature. A more substantial effect on selectivity is evident when Rh–Ag on silica and titania, both reduced at low temperature, are compared. The direct effect of rhodium–titania interaction (varied reduction temperature) and the indirect effect of support (changed Rh–Ag interaction) appear to have a common origin. It is proposed that in both the direct and indirect support interactions there may be preferential interaction with the smallest particles in the distribution.
TL;DR: In this article, the chiral formyl anion equivalent p-tolyl p-tlylthiomethyl sulphoxide (+)-(S)-(1) to the cyclopentenone derivative (2a) leads to the adduct (3a) in 45% yield with high diastereoselectivity.
Abstract: Conjugate addition of the chiral formyl anion equivalent p-tolyl p-tolylthiomethyl sulphoxide (+)-(S)-(1) to the cyclopentenone derivative (2a) leads to the adduct (3a) in 45% yield with high diastereoselectivity. Reduction of (3a) gives the dithioacetal (4a), an interesting optically pure intermediate in the synthesis of 11-deoxy-ent-prostanoids. The high selectivity, contrasting with a lower selectivity observed using cyclopentenone, is to be ascribed to more severe steric demands in the transition state.
TL;DR: In this paper, a high selectivity as high as 89% was achieved by using PC6P as the ligand for methanol-to-ethanol conversion using an α,ω-bis(diphenylphosphino)alkane (PCnP) mixture.
Abstract: Cobalt iodide modified by α,ω-bis(diphenylphosphino)alkane (PCnP; n=1–6) efficiently catalyzes the homologation of methanol to ethanol. The ethanol selectivity as high as 89% was achieved by using PC6P as the ligand.
TL;DR: In this article, it was shown that carbon-13 selectivity increases when the excitation frequency is slightly red-shifted from the v 1 band centre at R(18).
Patent•
28 Apr 1981
TL;DR: In this paper, the authors proposed to obtain the titled compound in high selectivity and yield stable for a long term by oxidizing isobutylene with molecular oxygen in the presence of a specific catalyst containing a component capable of preventing the deterioration in activity thereof, e.g. Mo, Tl and Ni, Fe, etc.
Abstract: PURPOSE:To obtain the titled compound in high selectivity and yield stable for a long term, by oxidizing isobutylene with molecular oxygen in the presence of a specific catalyst containing a component capable of preventing the deterioration in activity thereof, e.g. Mo, Tl and Ni, Fe, etc. CONSTITUTION:Isobutylene is oxidized with molecular oxygen in the presence of a catalyst having a composition of the formula (X is Cu, Nb or Sm; a-g are the numbers of moles based on 12mol Mo; a is 0.2-6; b is 0.2-6; a+b is 1-10; c is 0.1-4; d is 0.1-3; f and g are 0.1-5; h is the number of oxygen atoms satisfying the other valences present) at 350-450 deg.C under ordinary pressure to 2atm to give methacrolein with suppressed formation of isobutyl aldehyde, methacrylic acid, etc. as by-products. The components Fe, Ni or X prevent the deterioration in the activity, and the component Tl improves the selectivity. Furthermore, the component Pb imparts the high activity and maintains the high selectivity.
Patent•
09 Mar 1981
TL;DR: In this article, a process for the preparation of a benzene-monocarboxylic acid by oxidation of a monoalkyl-benzene compound in the liquid phase with the aid of a gas containing molecular oxygen in the presence of a catalyst composed of a cobalt and a manganese compound, both of which are soluble in the reaction mixture.
Abstract: A process for the preparing of a benzene-monocarboxylic acid by oxidation of a monoalkyl-benzene compound in the liquid phase with the aid of a gas containing molecular oxygen in the presence of a catalyst composed of a cobalt and a manganese compound, both of which are soluble in the reaction mixture, with a manganese-cobalt atomic ratio between 1:500 and 1:100,000, thus improving the selectivity of the reaction to form the desired benzene-monocarboxylic acid.
Patent•
16 Apr 1981
TL;DR: In this article, the authors proposed a new catalytic gas phase oxidizer for methacrylic acid in the presence of a nitrogen containing heterocyclic compound, which is called PaMoVcXdYeOf.
Abstract: PURPOSE: To obtain a catalyst excellent in the selectivity and the yield ratio of methacrylic acid by adding Na or the like and Cu or the like to a heteropoly- acid having an almost salt structure consisting of P, Mo, V and O prepared in the presence of a nitrogen containing heterocyclic compound. CONSTITUTION: As the oxidizing catalyst for preparing methacrylic acid by subjecting 4C unsaturated hydrocarbon or the like to catalytic gas phase oxidation, one wherein a heteropoly-acid is molybdophosphoric acid or a partial substitution thereof and has an almost salt crystal structure whose 2θ is about 26.2°, 10.5°, 21.3° and 30.3° in X-ray pattern and a general formula is PaMoVcXdYeOf wherein X is Na, K or the like, Y is Cu, Sb or the like, when b is 12, a is 0.1W 3, c is 0W6, d is 0.05W10, e is 0.01W5 is used. This catalyst prepares methacrylic acid in high selectivity and a high yield ratio. COPYRIGHT: (C)1982,JPO&Japio