G.C.A. Schuit

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 CoOue5f8MoO3γ-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.

Hydrodesulfurization of thiophenic compounds: the reaction mechanism

Abstract It has often been assumed that catalytic hydrodesulfurization of thiophene and related compounds proceeds via a one-point “end-on” adsorption involving bonding of the sulfur atom with Mo ions at an anion vacancy on the catalyst surface. This interpretation is inadequate, failing to account for hydrogen-deuterium exchange results, the reactivities of benzothiophene and dibenzothiophene, and the small steric effects of methyl substituents on the adsorption and reaction of compounds in the thiophene, benzothiophene, and dibenzothiophene families of homologs. An alternative mechanism, presented here, involves a multipoint adsorption of the reactant, with a Cue5fbC bond interacting with a Mo cation, and the S atom of the reactant interacting with a S ion on the surface. The new mechanism accounts for the observed deuterium exchange and hydrodesulfurization reactions and is consistent with the observed steric effects.

In situ measurements of the electrical conductivity of bismuth molybdate catalysts in operation for oxidative dehydrogenation of butene

Abstract In situ measurement of electrical conductivities were performed on MoO 3 , Bi 2 Mo 2 O 9 , and Bi 2 MoO 6 (koechlinite), the latter either as such or doped with small amounts of Mo or Bi; the reaction applied was the oxidative dehydrogenation of butene to butadiene. The reaction was performed in continuous flow or by pulsing butene over the catalyst, often followed by the addition of O 2 pulses. The catalysts were characterized by surface area, XRD, and XPS. The latter measurements showed that surface Bi Mo ratios were often different from those in the bulk; reduction by butene at temperatures around 673 K sometimes led to considerable changes in the surface Bi Mo ratio that ran parallel with changes in activity. Pulsing butene in a He carrier stream over the catalyst strongly increased the electrical conductivity while pulsing O 2 decreased it again; the conductivity is almost entirely located in the surface layers. The koechlinite-type catalysts, when heated in He, showed a considerably higher conductivity than the other catalysts. This is ascribed to a dissociation of O 2 presumably from the Bi 2 O 2 layers. Subsequent pulsing of butene first increased the conductivity but for catalysts with Bi Mo bulk ratios >1, the conductivities became constant while reduction continued. The first process is supposed to be connected with a stripping of the oxygen in the surface layer, the second with a migration of O 2− from bulk to surface and of electrons from surface to bulk. A model was presented for the surface band.

Surface co-ordination of oxygen on oxygen-deficient TiO2 and MoO3 as revealed by e.s.r.-measurements

E.s.r. spectra were obtained from TiO2(anatase) and MoO3 after evacuation at 500°C and subsequent addition of O2 at room temperature. The spectra observed for TiO2 are quite complex: by varying the degree of reduction, the O2-pressure, the microwave power and the temperature for e.s.r. recording they were separated in a number of components. On degassed TiO2 two of the components are assigned respectively to an F-center and to Ti3+, both at the surface. A third signal remained unidentified. Addition of O2 causes the emergence of at least four components. Three of these were assigned to O2-surface co-ordination complexes, viz., the L-form (similar to CN–co-ordination), the P-form (similar to C2H2-co-ordination) and the A-form (similar to H2O2). The fourth signal is believed to be connected with O– formed by H-abstraction from OH– presumably by the peroxo-complexes.On MoO3 after evacuation four component signals were observed, one of which is assigned to an F-centre, a broader and more intense one to surface Mo5+, the remaining two signals remaining unidentified. On O2-addition none of the signals observed for TiO2 were seen to occur here, the interaction with O2 being almost solely restricted to a decrease in intensity of the F-center and Mo5+-signals. The Bi2O3+MoO3 system after evacuation is similar to MoO3 except for the absence of the Mo5+-signal. On NiO no signals could be found either in the evacuated or in the “reoxidized” form. The possible significance of the presence of absence of the co-ordinated O2-species for selective oxidation catalysis is discussed.

Carbon monoxide and propene oxidation by iron oxides for auto-emission control

Activities of iron-based materials for the simultaneous total oxidation of CO and C/sub 3/H/sub 6/ were measured under conditions that were typical of automotive operation: space velocity of 35,000 h/sup -1/; temperatures between 373 and 873 K; atmospheric pressure; feed composition of 2.5% CO, 1.7% O/sub 2/, 0.5% H/sub 2/, 0.05% C/sub 3/H/sub 6/, and, optionally, 0.004% SO/sub 2/ in He. In the absence of SO/sub 2/, activity decreased in the order Fe/sub 2/O/sub 3//Al/sub 2/O/sub 3/ > Fe/sub 2/O/sub 3//TiO/sub 2/ approx. Fe/sub 2/O/sub 3/ >> FeSbO/sub 4/ > FePO/sub 4/ > Fe/sub 2/(MoO/sub 4/)/sub 3/. CO and C/sub 3/H/sub 6/ removal followed apparent first-order kinetics and the data showed a compensation law effect. Oxidation was inhibited when SO/sub 2/ was present; temperatures for CO conversion over Fe/sub 2/O/sub 3/ were raised about 160 K, while the comparable rise for C/sub 3/H/sub 6/ oxidation was about 80 K. Inhibition was less with Fe/sub 2/O/sub 3/ on 35 m/sup 2//g TiO/sub 2/ than with Fe/sub 2/O/sub 3/ on 350 m/sup 2//g Al/sub 2/O/sub 3/ or with unsupported 5 m/sup 2//g Fe/sub 2/O/sub 3/. Both FeSbO/sub 4/ and FePO/sub 4/ showed good activity for the conversion of C/sub 3/H/sub 6/, butmorexa0» not of CO, when SO/sub 2/ was absent. Material balances indicate that the partial oxidation product acrolein inhibits CO oxidation over these binary selective oxidation catalysts. Collectively, the data suggest that an inhibitor is created by oxidation of a precursor and its oxidation can be inhibited by the product of another feed component through control of the size of reactive ensembles on the catalyst surface.«xa0less

Hydrodeoxygenation of 1-naphthol: Activities and stabilities of molybdena and related catalysts

Hydrodeoxygenation (HDO) of 1-naphthol catalyzed by Moγ-Al2O3 and Ni-Moγ-Al2O3 was investigated with a flow reactor operated at 225 °C and 120 atm. The oxidic forms of the catalysts were more active than the sulfidic forms. The catalysts were deactivated during operation, evidently by water formed in the HDO reaction. The selectivity changed markedly during the deactivation, the HDO reactions becoming markedly slower and the hydrogenation reactions only slightly slower. The deactivated oxidic and sulfidic catalysts could be reversibly regenerated by treatment with H2 and H2S in H2, respectively. The catalyst surfaces are suggested to incorporate two kinds of catalytic sites, one catalyzing direct HDO, the other aromatic ring hydrogenation. These sites are suggested to be anion vacancies, susceptible to inhibition by bonding of H2O.

Hydrogenation of aromatic compounds catalyzed by sulfided CoOMoO3γ-Al2O3

Abstract Hydrogenation of aromatic compounds catalyzed by sulfided CoOue5f8MoO 3 γ- Al 2 O 3 was investigated with batch and flow reactors at 275–350 °C and pressures near 100 atm. The reactants included benzene, biphenyl, dibenzothiophene, naphthalene, 2-phenylnaphthalene, and benzo-[ b ]naphtho[2,3- d ]thiophene. The reactivity for ring saturation was about an order of magnitude greater for the latter three [(substituted) naphthalenes] than for the others [(substituted) benzenes]. Sulfur in a reactant molecule slightly increased the reactivity of a neighboring ring for hydrogenation. Dibenzothiophene hydrogenation was not inhibited by H 2 S, but biphenyl hydrogenation was moderately inhibited by H 2 S and dibenzothiophene hydrogenolysis was strongly inhibited by H 2 S. These and other results suggest that the reactant to be hydrogenated is π-bonded at exposed Mo cations, where H 2 S undergoes weak competitive adsorption and bases like acridine undergo strong competitive adsorption.

Paramagnetic platinum and oxygen species on supported platinum

EPR was used to determine the paramagnetic platinum and oxygen species which are present at various stages in the genesis of a supported platinum catalyst and to determine the effect of the support on the formation of such species. Paramagnetic platinum centers, believed to be due to Pt 8+, are formed when Pt(NHa)4+2/A1203 is calcined in 02; for low metal contents a large fraction of the platinum is in the form of these species. Catalysts prepared by the same techniques with SiO2 as a support show no paramagnetie platinum signals. Adsorption of O2 on the Pt/A120a catalysts results in the formation of a 3-g 02- signal and a second 2-g signal due to O- and results in a corresponding reduction in the platinum signal. Again Pt/SiO2 did not show comparable behavior. The results are interpreted in terms of electron transfer to the AlcOa.

Selective oxidation catalysts containing antimony for the conversion of 1-butene to butadiene: II. Selective oxidation of 1-butene and CO

Abstract The selective oxidation of 1-butene to butadiene and the oxidation of CO over a series of MSbO 4 , with M = Fe, Al, Cr, Co, and Rh, were studied using a fixed-bed, integral reactor system. Activity and selectivity measurements were performed using catalysts which had been prepared by precipitation or impregnation and which had previously undergone extensive characterization. The most selective catalyst for butadiene production was determined to be antimony-impregnated FeSbO 4 . Post-reaction characterization revealed that the catalysts were stable for prolonged reaction times, the Fe-Sb-O system being the only one able to be reduced under reaction conditions.

Selective oxidation catalysts containing antimony for the conversion of 1-butene to butadiene: I. Preparation and characterization

Abstract FeSbO4 catalysts containing “excess” surface antimony are active for the selective oxidative dehydrogenation of 1-butene to butadiene. In this paper, the synthesis and characterization of a series of MSbO4, with M = Fe, Al, Cr, and Rh, and CoSb2O6 catalysts are reported. Synthesis techniques included precipitation from slurry, solid-state reaction, and impregnation. Characterization of the catalysts has been performed using X-ray diffraction, X-ray fluorescence, and X-ray photoelectron Spectroscopy.