Bruce C. Gates

Hydrodesulfurization of methyl-substituted dibenzothiophenes catalyzed by Co-Mo/gamma-Al2O3

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Reactivities in deep catalytic hydrodesulfurization: challenges, opportunities, and the importance of 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene

Abstract The organosulfur compounds present in fossil fuels vary widely in their reactivities in catalytic hydrodesulfurization. In naphtha, thiophene is so much less reactive than the thiols, sulfides, and disulfides that the latter can be considered to be virtually infinitely reactive in practical high-conversion processes. Similarly, in gas oils and petroleum residua, the reactivities of (alkyl-substituted) 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene are much less than those of other sulfur-containing compounds. Consequently, in deep hydrodesulfurization, the conversion of these key substituted dibenzothiophenes largely determines the required conditions. Because of the growing technological importance of deep desulfurization of heavy feedstocks, we infer that 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene are the most appropriate compounds for investigations of candidate catalysts and reaction mechanisms.

Monomolecular and bimolecular mechanisms of paraffin cracking: n-butane cracking catalyzed by HZSM-5

Abstract The cracking of n -butane catalyzed by the zeolite HZSM-5 has been characterized by measurements of the conversion determined with a flow reactor at temperatures of 426–523°C and n -butane partial pressures of 0.01–1.00 atm. The primary products, each formed in a first-order reaction, are H 2 + butenes; methane + propylene; and ethane + ethylene. In the limit approaching zero conversion, each compound in each stated pair was formed at approximately the same rate as the other. Propane and a small amount of isobutane were formed as secondary products in second-order reactions. The results are consistent with the occurrence of two simultaneous mechanisms: (1) a monomolecular mechanism proceeding through the pentacoordinated carbonium ion formed by protonation of the n-butane at the two position and (2) a bimolecular hydride transfer proceeding through carbenium ion intermediates. The former proceeds almost to the exclusion of the latter in the limit approaching zero n -butane conversion. The limiting product distribution characterizes the intrinsic selectivity of the collapse of the carbonium ion; at 496°C, the relative rates of decomposition of the carbonium ion to give H 2 + butenes, methane + propylene, and ethane + ethylene are 30 ± 6, 36 ± 4, and 34 ± 5, respectively, with the corresponding activation energies all being approximately 140 kJ/mol. These results provide the first demonstration of stoichiometric dehydrogenation accompanying paraffin cracking.

Catalysis by gold dispersed on supports: the importance of cationic gold

There are many examples of catalysis in solution by cationic complexes of gold, and recent results, reviewed here in this critical review, demonstrate that cationic gold species on oxide and zeolite supports are also catalytically active, for reactions including ethylene hydrogenation and CO oxidation. The catalytically active gold species on supports are evidently not restricted to isolated mononuclear gold complexes, but include gold clusters, which for at least some reactions are more active than the mononuclear complexes and for some reactions less active. Fundamental questions remain about the nature of cationic gold in supported catalysts, such as the nature of the cationic gold clusters and the nature of gold atoms at metal-support interfaces (88 references).

Observation of ligand effects during alkene hydrogenation catalysed by supported metal clusters

Homogeneous organometallic catalysts and many enzymes activate reactants through coordination to metal atoms; that is, the reactants are turned into ligands and their reactivity controlled through other ligands in the metals coordination sphere. In the case of supported metal clusters, catalytic performance is influenced by the support and by adsorbed reactants, intermediates or products. The adsorbates are usually treated as ligands, whereas the influence of the supports is usually ascribed to electronic interactions, even though metal clusters supported on oxides and zeolites form chemical bonds to support oxygen atoms. Here we report direct observations of the structure of supported metal clusters consisting of four iridium atoms, and the identification of hydrocarbon ligands bound to them during propene hydrogenation. We find that propene and molecular hydrogen form propylidyne and hydride ligands, respectively, whereas simultaneous exposure of the reactants to the supported iridium cluster yields ligands that are reactive intermediates during the catalytic propane-formation reaction. These intermediates weaken the bonding within the tetrahedral iridium cluster and the interactions between the cluster and the support, while replacement of the MgO support with γ-Al2O3 boosts the catalytic activity tenfold, by affecting the bonding between the reactant-derived ligands and the cluster and therefore also the abundance of individual ligands. This interplay between the support and the reactant-derived ligands, whereby each influences the interaction of the metal cluster with the other, shows that the catalytic properties of supported metal catalysts can be tuned by careful choice of their supports.

Hydrodesulfurization of thiophene, benzothiophene, dibenzothiophene, and related compounds catalyzed by sulfided CoO-Mo3/gamma-Al2O3 : low-pressure reactivity studies

Hydrodesulfurization experiments were carried out with a sulfided CoOMoO3γ-Al2O3 catalyst in a pulse microreactor operated at atmospheric pressure and temperatures of 350 to 450 °C. The reactants were hydrogen and pure sulfur-containing compounds (or pairs of compounds), including thiophene, benzothiophene, dibenzothiophene, several of their hydrogenated derivatives, and various methyl-substituted benzothiophenes and dibenzothiophenes. The aromatic compounds appeared to react with hydrogen by simple sulfur extrusion; for example, dibenzothiophene gave H2S + biphenyl in the absence of side products. The reactivities of thiophene, benzothiophene, and dibenzothiophene were roughly the same. Each hydrogenated compound (e.g., tetrahydrothiophene) was more reactive than the corresponding aromatic compound (e.g., thiophene). Methyl substituents on benzothiophene had almost no effect on reactivity, whereas methyl substituents on dibenzothiophene located at a distance from the S atom slightly increased the reactivity, and those in the 4-position or in the 4- and 6-positions significantly decreased the reactivity. In contrast to the observation of a near lack of dependence of low-pressure reactivity on the number of rings in the reactant, the literature shows that at high pressures the reactivity decreases with an increased number of rings. The pressure dependence of the structure-reactivity pattern is suggested to be an indication of relatively less surface coverage by the intrinsically more reactive compounds (e.g., thiophene) at low pressures but not at high pressures. The relative reactivities are also suggested to be influenced by differences in the structures of the catalyst at low and high hydrogen partial pressures, which may be related to the concentrations of surface anion vacancies and the nature of the adsorbed intermediates.

Catalytic conversion of compounds representative of lignin-derived bio-oils: a reaction network for guaiacol, anisole, 4-methylanisole, and cyclohexanone conversion catalysed by Pt/γ-Al2O3

The conversion of compounds representative of lignin and lignin-derived bio-oils (guaiacol, anisole, 4-methylanisole, and cyclohexanone), catalysed by Pt/Al2O3 in the presence of H2 at 573 K is described by a reaction network indicating a high selectivity for platinum-catalysed aromatic carbon–oxygen bond cleavage accompanied by acid-catalysed methyl group transfer reactions.

Direct imaging of single metal atoms and clusters in the pores of dealuminated HY zeolite

Zeolites are aluminosilicate materials that contain regular three-dimensional arrays of molecular-scale pores, and they can act as hosts for catalytically active metal clusters. The catalytic properties of such zeolites depend on the sizes and shapes of the clusters, and also on the location of the clusters within the pores. Transmission electron microscopy has been used to image single atoms and nanoclusters on surfaces, but the damage caused by the electron beam has made it difficult to image zeolites. Here, we show that aberration-corrected scanning transmission electron microscopy can be used to determine the locations of individual metal atoms and nanoclusters within the pores of a zeolite. We imaged the active sites of iridium catalysts anchored in dealuminated HY zeolite crystals, determined their locations and approximate distance from the crystal surface, and deduced a possible cluster formation mechanism.

Catalysis by matrix-bound sulfonic acid groups: Olefin and paraffin formation from butyl alcohols

Abstract Catalysis by a poly(styrene-divinylbenzene) matrix containing -SO3H groups was characterized by reaction rates and product distributions for reactions of isopropyl, isobutyl, and s-butyl alcohols at about 100 °C. Infrared spectra of functioning catalyst membranes established the presence of a network of hydrogen-bonded -SO3H groups at low alcohol concentrations and dissociated groups at high concentrations. When partial pressures of alcohol contacting the catalyst exceeded about 0.1 atm, the alcohols (like water) were reaction inhibitors, and the catalyst approached the character of an acid solution. In contrast, as substrate concentration approached zero, s-butyl alcohol gave predominantly trans-2-butene without 1-butene; and isobutyl alcohol gave not only butenes, but isobutane and tar. The high catalytic activity and unusual selectivity of the network incorporating small amounts of alcohols demonstrate that the proton donor-acceptor tendencies of the network are significantly stronger than those of the solvated acid groups. The dehydration reaction mechanisms are suggested to involve concerted proton transfers within cyclic hydrogen-bonded networks incorporating -SO3H groups which surround and conform to alcohol molecules.

Structure and properties of tungstated zirconia catalysts for alkane conversion

Abstract Promoted tungstated zirconia (WZ) catalysts are active and selective for isomerization of light alkanes, offering good prospects for industrial application. This account is an abbreviated summary of what these catalysts are and how they work. WZ containing approximately a monolayer of tungstate covering the zirconia support was prepared by impregnation of zirconia, tested in a flow reactor, and characterized with a variety of spectroscopic methods. The catalytic activity is associated with interconnecting polyoxotungstate clusters on the surface of tetragonal zirconia. The polyoxotungstate species increase the acid strength of the catalyst relative to that of unmodified zirconia, but the acid strength is still less than that of zeolites and far below the superacidic range. Redox properties of WZ characterized by EPR spectroscopy suggest that alkane activation proceeds via homolytic C H bond cleavage, leading to the formation of W 5+ centers and organic radicals, which can be converted to alkenes, initiating catalysis. Unpromoted WZ has a low activity; reaction intermediates remain adsorbed on the surface, where they undergo polymerization and cracking, leading to fast deactivation and poor selectivity. The addition of platinum to the catalyst and H 2 to the feed drastically improves the catalytic activity, selectivity, and stability. The platinum enhances the desorption of reaction intermediates and minimizes the condensation reactions, so that monomolecular isomerization predominates. An additional promotion with iron compounds leads to further improvement in catalytic activity and selectivity.