Showing papers on "Catalysis published in 1997"
10 Jul 1997
TL;DR: This paper presents a meta-modelling system that automates the very labor-intensive and therefore time-heavy and therefore expensive and expensive process of characterization and activation of Solid Catalysts.
Abstract: Preparation of Solid Catalysts. Characterization of Solid Catalysts. Model Systems. Elementary Steps and Mechanisms. Kinetics and Transport Processes. Deactivation and Regeneration. Special Catalytic Systems. Laboratory Reactors. Reaction Engineering. Environmental Catalysis. Inorganic Reactions. Energy-related Catalysis. Organic Reactions.
4,227 citations
TL;DR: In this article, the adsorption properties and reactivities of gold are summarized in terms of their size dependency from bulk to fine particles, clusters and atoms, and the catalytic performances of gold markedly depend on dispersion, supports, and preparation methods.
3,854 citations
2,035 citations
TL;DR: In this paper, a reversible hydrogen storage system based on catalyzed reactions is proposed, where the catalytic acceleration of the reactions in both directions is achieved by doping alkali metal aluminium hydrides with a few mol% of selected Ti compounds.
1,671 citations
TL;DR: In this article, Fischer-Tropsch synthesis (FTS) catalysts with high cobalt concentration and site density are used for the synthesis of hydrocarbons from CO/H2 mixtures.
Abstract: Catalyst productivity and selectivity to C5+ hydrocarbons are critical design criteria in the choice of Fischer-Tropsch synthesis (FTS) catalysts and reactors. Cobalt-based catalysts appear to provide the best compromise between performance and cost for the synthesis of hydrocarbons from CO/H2 mixtures. Optimum catalysts with high cobalt concentration and site density can be prepared by controlled reduction of nitrate precursors introduced via melt or aqueous impregnation methods. FTS turnover rates are independent of Co dispersion and support identity over the accessible dispersion range (0.01–0.12) at typical FTS conditions. At low reactant pressures or conversions, water increases FTS reaction rates and the selectivity to olefins and to C5+ hydrocarbons. These water effects depend on the identity of the support and lead to support effects on turnover rates at low CO conversions. Turnover rates increase when small amounts of Ru (Ru/Co<0.008 at.) are added to Co catalysts. C5+ selectivity increases with increasing Co site density because diffusion-enhanced readsorption of α-olefins reverses, β-hydrogen abstraction steps and inhibits chain termination. Severe diffusional restrictions, however, can also deplete CO within catalyst pellets and decrease chain growth probabilities. Therefore, optimum C5+ selectivities are obtained on catalysts with moderate diffusional restrictions. Diffusional constraints depend on pellet size and porosity and on the density and radial location of Co sites within catalyst pellets. Slurry bubble column reactors and the use of eggshell catalyst pellets in packed-bed reactors introduce design flexibility by decoupling the characteristic diffusion distance in catalyst pellets from pressure drop and other reactor constraints.
1,366 citations
TL;DR: In this paper, the effect of mixing intensity (Reynolds number = 3,100 to 12,400) and temperature (30 to 70°C) on the rate of reaction were studied while the molar ratio of alcohol to triglycerol (6:1) and the concentration of catalyst (0.20 wt% based on soybean oil) were held constant.
Abstract: Transesterification of soybean oil with methanol was investigated. Three stepwise and reversible reactions are believed to occur. The effect of variations in mixing intensity (Reynolds number=3,100 to 12,400) and temperature (30 to 70°C) on the rate of reaction were studied while the molar ratio of alcohol to triglycerol (6:1) and the concentration of catalyst (0.20 wt% based on soybean oil) were held constant. The variations in mixing intensity appear to effect the reaction parallel to the variations in temperature. A reaction mechanism consisting of an initial mass transfer-controlled region followed by a kinetically controlled region is proposed. The experimental data for the latter region appear to be a good fit into a second-order kinetic mechanism. The reaction rate constants and the activation energies were determined for all the forward and reverse reactions.
1,016 citations
913 citations
TL;DR: Several ruthenium-based olefin metathesis catalysts of the formula (PR3)2X2RuCHCHCPh2 have been synthesized, and relative catalyst activities were determined by monitoring the ring-closing met athesis of the acyclic diene diethyl diallylmalonate as discussed by the authors.
Abstract: Several ruthenium-based olefin metathesis catalysts of the formula (PR3)2X2RuCHCHCPh2 have been synthesized, and relative catalyst activities were determined by monitoring the ring-closing metathesis of the acyclic diene diethyl diallylmalonate. The following order of increasing activity was determined: X = I < Br < Cl and PR3 = PPh3 ≪ PiPr2Ph < PCy2Ph < PiPr3 < PCy3. Additional studies were conducted with the catalyst (PCy3)2Cl2RuCH2 to probe the mechanism of olefin metathesis by this class of catalysts. The data support a scheme in which there are two competing pathways: the dominant one in which a phosphine dissociates from the ruthenium center and a minor one in which both phosphines remain bound. Higher catalyst activites could be achieved by the addition of CuCl to the reaction.
631 citations
TL;DR: In this article, the main data appeared in the last years on the oxidative dehydrogenation (ODH) of short chain alkanes on supported vanadium oxide catalysts, and it was concluded that tetrahedral V5+-species are active and selective sites in the ODH of C2-C4 alkanes.
Abstract: This paper summarizes the main data appeared in the last years on the oxidative dehydrogenation (ODH) of short chain alkanes on supported vanadium oxide catalysts. The acid-base character of metal oxide support influences the dispersion of vanadium on the surface of the support, as well as the nature of the vanadium species. The reducibility and structure of surface vanadium oxide species and the acid-base character of catalysts, in addition to their catalytic properties in the ODH of C2–C4 alkanes, strongly depend on the metal oxide used as support and the vanadium loading. In this way, it appears that tetrahedral V5+-species are active and selective sites in the ODH of C2–C4 alkanes. The effect of the coordination number and aggregation state of surface vanadium oxide species, and the presence of acid/base sites on the catalytic behavior of supported vanadium oxide catalysts are discussed. It is concluded that these are important factors that must be considered to develop selective catalysts in ODH reactions.
627 citations
TL;DR: In this paper, the authors describe the development of rare-earth-alkali metal complexes such as LnM3tris(binaphthoxide) complexes (LnMB, Ln = rare earth metal, M = alkali metal), which are readily prepared from corresponding rare earth trichlorides or rare earth isopropoxides, and their application to catalytic asymmetric synthesis.
Abstract: This review focuses on a new concept in catalytic asymmetric reactions that was first realized for the use of heterobimetallic complexes. As these heterobimetallic complexes function as both a Bronsted base and as a Lewis acid, just like an enzyme, they make possible a variety of efficient catalytic asymmetric reactions. This heterobimetallic concept should prove to be applicable to a variety of new asymmetric catalyses. The first part of this review describes the development of rare-earth–alkali metal complexes such as LnM3tris(binaphthoxide) complexes (LnMB, Ln = rare-earth metal, M = alkali metal), which are readily prepared from the corresponding rare-earth trichlorides or rare-earth isopropoxides, and their application to catalytic asymmetric synthesis. By using a catalytic amount of LnMB complexes several asymmetric reactions proceed efficiently to give the corresponding desired products in up to 98% ee: LnLB-catalyzed asymmetric nitroaldol reactions (L = Li), LnSB-catalyzed asymmetric Michael reactions (S Na), and LnPB-catalyzed asymmetric hydrophosphonylations of either imines or aldehydes (P K). Applications of these heterobimetallic catalysts to the syntheses of several biologically and medicinally important compounds are also described. Spectral analyses and computational simulations of the asymmetric reactions catalyzed by the heterobimetallic complexes reveal that the two different metals play different roles to enhance the reactivity of both reaction partners and to position them. From mechanistic considerations, a useful activation of the heterobimetallic catalyses was realized by addition of alkali metal reagents. The second part describes the development of another type of heterobimetallic catalysts featuring Group 13 elements such as Al and Ga as the central metal. Among them, the AlLibis(binaphthoxide) complex (ALB) is an effective catalyst for asymmetric Michael reactions, tandem Michael–aldol reactions, and hydrophosphonylation of aldehydes.
586 citations
TL;DR: In this paper, the formation of coke during the steam reforming of light hydrocarbons results mainly from catalytic reactions, and it is believed that the process is endemic to steam reforming in that the same intermediates are involved in the main reaction as are involved by coking.
TL;DR: In experiments modeling the reactions of the reductive acetyl-coenzyme A pathway at hydrothermal temperatures, it was found that an aqueous slurry of coprecipitated NiS and FeS converted CO and CH3SH into the activated thioester CH3-CO-SCH3, which hydrolyzed to acetic acid.
Abstract: In experiments modeling the reactions of the reductive acetyl–coenzyme A pathway at hydrothermal temperatures, it was found that an aqueous slurry of coprecipitated NiS and FeS converted CO and CH 3 SH into the activated thioester CH 3 -CO-SCH 3 , which hydrolyzed to acetic acid. In the presence of aniline, acetanilide was formed. When NiS-FeS was modified with catalytic amounts of selenium, acetic acid and CH 3 SH were formed from CO and H 2 S alone. The reaction can be considered as the primordial initiation reaction for a chemoautotrophic origin of life.
TL;DR: The influence of the preparation methods on the catalytic activity for CO oxidation was markedly large for Au-TiO2 and negligible for Pt- TiO2 catalysts as discussed by the authors, and the most active catalysts for both Pt and Au were prepared by deposition-precipitation (DP), photodeposition (FD), and impregnation (IMP).
Abstract: The influence of the preparation methods on the catalytic activity for CO oxidation was markedly large for Au-TiO2 and negligible for Pt-TiO2 catalysts. Platinum and gold were deposited on TiO2 by deposition-precipitation (DP), photodeposition (FD) and impregnation (IMP). The DP method gave the most active catalysts for both Pt and Au. Gold catalysts prepared by DP were active at temperatures below 273 K and showed a much greater activity than Pt catalysts.
TL;DR: In this paper, a catalytic asymmetric epoxidation method for olefins using potassium peroxomonosulfate (Oxone, Dupont) as oxidant and a fructose-derived ketone (1) as catalyst is described.
Abstract: This article describes a highly effective catalytic asymmetric epoxidation method for olefins using potassium peroxomonosulfate (Oxone, Dupont) as oxidant and a fructose-derived ketone (1) as catalyst. High enantioselectivies have been obtained for trans-disubstituted and trisubstituted olefins which can bear functional groups such as tributylsilyl ether, acetal, chloride, and ester. The enantiomeric excesses for cis-olefins and terminal olefins are not high yet. The current epoxidation shows that the catalyst efficiency is enhanced dramatically upon raising the pH. Mechanistic studies show that the epoxidation mainly proceeds via a spiro transition state, which provides a model for predicting the stereochemical outcome of the reaction. The planar transition state is likely to be the main competing pathway. The extent of the involvement of the planar mode is subject to the steric effect of the alkyl groups on the olefins.
TL;DR: The most important manufacturing processes for aniline and other bulk arylamines are based on the continuous catalytic hydrogenation of nitro compounds, employing heterogeneous copper, nickel or platinum-group metals as discussed by the authors.
Patent•
ARCO1
TL;DR: In this paper, highyl active double metal cyanide (DMC) catalysts are disclosed. The catalysts polymerize propylene oxide at a rate in excess of about 1 kg PO/g Co/min.
Abstract: Highyl active double metal cyanide (DMC) catalysts are disclosed. The catalysts comprise a DMC complex, and organic complexing agent, and from about 5 to about 80 wt.%, based on the amount of catalyst, of a polyether having a number average molecular weight less than about 500. The catalysts polymerize propylene oxide at a rate in excess of about 1 kg PO/g Co/min. at 100 ppm catalyst, based on the weight of finished polyether, at 105 °C. The catalysts, which are easy to prepare, give polyether polyols with exceptionally low unsaturation levels.
TL;DR: In this article, the gas phase photocatalytic degradation of 17 VOCs over illuminated titanium dioxide was investigated using a plug flow reactor with the following experimental conditions: 200 ml min−1 flow rate, 23% relative humidity, 21% oxygen and an organic compound concentration range of 400-600 ppmv.
Abstract: The gas-phase photocatalytic destruction of 17 VOCs over illuminated titanium dioxide was investigated using a plug flow reactor with the following experimental conditions: 200 ml min−1 flow rate, 23% relative humidity, 21% oxygen and an organic compound concentration range of 400–600 ppmv. At steady state, high conversion yields were obtained for trichloroethylene (99.9%), isooctane (98.9%), acetone (98.5%), methanol (97.9%), methyl ethyl ketone (97.1%),t-butyl methyl ether (96.1%), dimethoxymethane (93.9%), methylene chloride (90.4%), methyl isopropyl ketone (88.5%), isopropanol (79.7%), chloroform (69.5%) and tetrachloroethylene (66.6%). However, the photodegradation of isopropylbenzene (30.3%), methyl chloroform (20.5%) and pyridine (15.8%) was not so efficient. Carbon tetrachloride photoreduction was investigated in the presence of methanol as an electron donor. It was observed that the presence of methanol results in higher degradation rates. No reaction byproducts were detected for all VOCs tested under the experimental set-up and conditions described. Also, long-term conversion was obtained for all tested compounds. Catalyst deactivation was detected with toluene only, but the activity was restored by illuminating the catalyst in the presence of hydrogen peroxide. The capacity of the process to destroy different classes of volatile organic compounds present in the atmosphere was demonstrated.
TL;DR: In this article, a 2 wt% Mo/ZSM-5 catalyst was used for the dehydroaromatization of methane to benzene in the absence of an added oxidant.
TL;DR: In this article, the selectivity pattern and the formation rates of the reaction products are used to characterize both structural (dispersion) as well as chemical properties (acid base and redox) on supported oxide catalysts, especially for molybdenum-based systems supported on silica and vanadia on titanium oxide.
Abstract: The goal of this review is to present some aspects of the use of a test reaction, i.e., methanol oxidation, as a tool to characterize oxidation catalysts. The selectivity pattern and the formation rates of the reaction products are used to characterize both structural (dispersion) as well as chemical properties (acid-base and redox) on supported oxide catalysts, especially for molybdenum-based systems supported on silica and vanadia on titanium oxide. This highly sensitive technique which gives information on the catalytically active sites at the molecular level characterizes a catalyst at work and is particularly well-adapted to the study of supported catalysts.
TL;DR: In this article, the first catalytic asymmetric 1,4-addition reactions of organometallic reagents with complete stereocontrol were shown to be possible with complete enantioselectivity.
Abstract: Although efficient catalysts for a number of asymmetric carbon carbon formations are known to date, a highly enantioselective catalytic version of the conjugate addition of organometallic reagents to enones is lacking. Recently chiral catalysts based on Cu, Ni, Zn, or Co complexes of a variety of ligands have shown enantioselectivities up to 90 % in 1,4additions of Grignard, organolithium, or dialkylzinc reagents. The results so far have not revealed, however, the key elements for realization of complete stereocontrol but do reveal the rather complex nature of some of these chiral catalytic systems. Previously we have demonstrated that copper complexes of chiral phosphorus amidites show relatively high ee values for the 1,4-adducts of R2Zn reagents and acyclic as well as cyclic enones. In this communication both the first catalytic asymmetric 1,4-addition reactions of organometallic reagents with complete
TL;DR: Nickel(II) dibromide complexes containing para-and unsubstituted aryl α-diimine ligands in combination with modified methylaluminoxane (MMAO) are highly active and efficient catalysts for the oligomerization of ethylene to linear α-olefins.
TL;DR: In this article, the authors showed that the CuO CeO 2 catalysts exhibit high catalytic activity in CO oxidation, showing markedly enhanced catalytic activities due to the combined effect of copper oxide and cerium dioxide.
Abstract: Copper oxide supported on cerium dioxide ( CuO CeO 2 ) catalysts were prepared and used for carbon monoxide oxidation in stoichiometric carbon monoxide and oxygen. The catalysts were characterized by means of XRD, H2-TPR and CO-TPD studies. The CuO CeO 2 catalysts exhibit high catalytic activity in CO oxidation, showing markedly enhanced catalytic activities due to the combined effect of copper oxide and cerium dioxide. The activity of the CuO CeO 2 (15%) catalyst prepared by impregnation is higher than that prepared by co-precipitation. CeO2 promotes the hydrogen reduction activity of copper, so that CuO CeO 2 catalysts show a different behavior with respect to pure CuO. Two reducible copper species were observed in all CuO CeO 2 catalysts. CO-TPD experiments revealed that CuO CeO 2 catalysts can adsorb CO, while pure CuO and CeO2 cannot. Combining the results of TPR, TPD study, and the catalytic activity measurements, it is proposed that the well dispersed CuO which can adsorb CO and which is reducible at low-temperature is responsible for low-temperature CO oxidation. The bulk CuO which cannot adsorb CO and which is reducible at high-temperature contributes little to the oxidation activity.
TL;DR: Heterogeneous asymmetric catalysis on chirally modified supported platinum catalysts has undergone significant development recently based on the knowledge gained on the mechanism of the asymmetric hydrogenation of α-ketoesters, catalyzed by platinum modified with cinchona alkaloids.
Abstract: Heterogeneous asymmetric catalysis on chirally modified supported platinum catalysts has undergone significant development recently Based on the knowledge gained on the mechanism of the asymmetric hydrogenation of α-ketoesters, catalyzed by platinum modified with cinchona alkaloids, new efficient chiral auxiliaries have been developed and the scope of reactants has been extended The research strategy which led to this progress and the results achieved are reviewed
TL;DR: In this paper, the authors presented a shorter and more efficient approach to synthesize macrolides (R)-(+)-ricinelaidic acid lactone (6) and (−)-gloeosporone (7), a fungal germination self-inhibitor.
Abstract: Total syntheses of the macrolides (R)-(+)-ricinelaidic acid lactone (6) and (−)-gloeosporone (7), a fungal germination self-inhibitor, are presented, which are distinctly shorter and more efficient than any of the previous approaches to these targets reported in the literature. Both of them benefit from the remarkable ease of macrocyclization of 1,ω-dienes by means of ring-closing olefin metathesis (RCM) using the ruthenium carbene 1a as catalyst precursor. The diene substrates are readily formed via the enantioselective addition of dialkylzinc reagents to aldehydes in the presence of catalytic amounts of Ti(OiPr)4 and bis-triflamide 18 and/or the stereoselective allylation of aldehydes developed by Keck et al. using allyltributylstannane in combination with a catalyst formed from Ti(OiPr)4 and (S)-(−)-1,1‘-bi-2-naphthol. Comparative studies show this latter procedure to be more practical than the stoichiometric allylation reaction employing the allyltitanium−α,α,α‘,α‘-tetraaryl-1,3-dioxolane-4,5-dimethan...
TL;DR: In this paper, the interaction of methane with unsupported and supported molybdenum compounds (Mo, MoO2, Mo3, Mo2C, and MoC(1−x)) has been investigated at 973 K. ZSM-5 was used as a support.
TL;DR: In this paper, it was suggested that the difference in the catalytic performances of copper and Group VIII metals was attributed to the reactivity of HCHO species formed in the course of the reactions.
TL;DR: In this article, a series of solid-acid catalysts, such as γ-Al2O3, H-ZSM-5, amorphous silica-alumina, as well as titania modified zirconia, were studied for the catalytic conversion of methanol to dimethyl ether (DME).
Abstract: The catalytic conversion of methanol to dimethyl ether (DME) has been studied over a series of solid-acid catalysts, such as γ-Al2O3, H-ZSM-5, amorphous silica-alumina, as well as titania modified zirconia. All the catalysts are active and selective for DME formation. The apparent activation energy for DME formation over γ-Al2O3 is ca. 25 kcal/mol, a value that increases to ca. 37 kcal/mol upon the addition of 23 Torr of H2O to the reagent. The rate of methanol dehydration decreases with increasing acidity (silica content) over the amorphous silica-alumina catalysts. Although H-ZSM-5 with Si/Al = 25 is the most active among the catalysts tested, the DME selectivity is only 20% at 280°C, a typical temperature used in the syngas-to-methanol process. An amorphous silica-alumina catalyst with 20 wt.-% silica content (SIRAL20) exhibits the best catalytic performance of those tested at 280°C.