Michael P. Rosynek

Partial oxidation of methane to carbon monoxide and hydrogen over a Ni/Al2O3 catalyst

Abstract Partial oxidation of methane occurs in the temperature range 450–900°C by reaction of an oxygen-deficient CH 4 /O 2 mixture over a 25 wt% Ni/Al 2 O 3 catalyst. Carbon monoxide selectivities approaching 95% and virtually complete conversion of the methane feed can be achieved at temperatures >700°C. The oxidation state and phase composition of the catalyst were characterized using X-ray photoelectron spectroscopy and X-ray powder diffractometry. This study revealed that, under operating conditions, the previously calcined catalyst bed consists of three different regions. The first of these, contacting the initial CH 4 /O 2 /He feed mixture, is NiAl 2 O 4 , which has only moderate activity for complete oxidation of methane to CO 2 and H 2 O. The second region is NiO + Al 2 O 3 , over which complete oxidation of methane to CO 2 occurs, resulting in an exotherm in this section of the bed. As a result of complete consumption of O 2 in the second region, the third portion of the catalyst bed consists of a reduced Ni/Al 2 O 3 phase. Formation of the CO and H 2 products, corresponding to thermodynamic equilibrium at the catalyst bed temperature, occurs in this final region, via reforming reactions of CH 4 with the CO 2 and H 2 O produced during the complete oxidation reaction over the NiO/Al 2 O 3 phase.

Catalytic Properties of Rare Earth Oxides

Abstract The rare earth (lanthanide) oxides, although constituting a closely related group of compounds, exhibit a rich variety of characteristic behaviors and solid-state properties, including several features that make them interesting subjects for Catalytic studies. Abundances and costs of these materials parallel those of the rare earth metals themselves, and range from common and fairly inexpensive (e.g. La2O3 and CeO2) to scarce and relatively costly (e.g., Tm2O3 and Lu2O3). All are quite refractory and nonvolatile, having melting points in excess of 2000° C, and some display a complex array of nonstoichiometric compositions. Most of the oxides are paramagnetic, with stable cationic electron configurations containing as many as seven unpaired 4f-electrons, and, as escheated from their periodic location, all are quite strongly basic. However, although the catalytic and surface properties of alkali, alkaline-earth, and other basic oxides have been extensively studied and documented [1], equivalent inf...

Catalytic conversion of methane to benzene over Mo/ZSM-5

Dehydroaromatization of methane to benzene occurs over a 2 wt% Mo/ZSM-5 catalyst at 700‡C under non-oxidizing conditions. Following an initial induction period, during which CH4 reactant reduces the original Mo6+ ions in the zeolite to Mo2C and deposition of coke occurs, a benzene selectivity of ∼ 70% at a CH4 conversion of 8–10% could be sustained for more than 16 h. X-ray photoelectron spectroscopy and X-ray powder diffraction measurements indicate that the reduced Mo is highly dispersed in the channels of the zeolite. Initial activation of CH4 reactant occurs on Mo2C sites, leading to the formation of C2H4 as the primary product. The latter then undergoes subsequent oligomerization reactions on acidic sites of the zeolite to form aromatic products.

Preparation and characterization of catalytic lanthanum oxide

Surface parameters and thermal dehydration/rehydration behavior of the La(OH)3/La2O3 system have been examined with a view toward establishing suitable methods for preparing and characterizing catalytic forms of the rare earth oxides. Relatively nonporous La(OH)3, prepared by hydrolysis of the pure oxide, undergoes thermal dehydration in two distinct stages. A well-defined oxyhydroxide (LaOOH) intermediate, formed by dehydration of the hydroxide at 200 °C, decomposes in a second step at 300 °C to generate the sesquioxide. A surface carbonate layer on the resulting oxide is only removed completely by subsequent thermal treatment at 700–800 °C. Exposure of the oxide to water vapor at <200 °C causes complete rehydration to the trihydroxide, rather than mere formation of surface hydroxyl groups, and results in complete recovery of surface area lost by hightemperature sintering. Spectroscopic studies reveal the existence of two structurally dissimilar kinds of bulk hydroxide ions in La(OH)3, differing in their relative extents of hydrogen bonding and giving rise to infrared bands at 3610 and 3590 cm−1. The latter band corresponds to the less numerous and more strongly bound type of OH− that is removed during second-stage dehydration of the oxyhydroxide.

Effect of cobalt source on the reduction properties of silica-supported cobalt catalysts

Abstract The bulk and surface reduction properties of silica-supported cobalt catalysts are influenced by the identity of the cobalt salt employed in catalyst preparation. Temperature-programmed reduction (TPR), X-ray powder diffraction (XRPD), and X-ray photoelectron spectroscopy (XPS) have been used to characterize the reduction, calcination, and catalytic behaviors of a series of 6 wt.% Co/SiO 2 catalysts prepared from nitrate, chloride, and acetate precursors. TPR profiles in hydrogen of the uncalcined catalysts reveal that reduction of Co(NO 3 ) 2 /SiO 2 occurs via an initial reductive decomposition of the nitrate ions, producing CoO x SiO 2 surface species that are much more difficult to reduce to metallic cobalt than is the unsupported nitrate salt. Complete reduction of the silica-supported acetate is also markedly inhibited compared to that of the unsupported salt. By contrast, reduction of CoCl 2 /SiO 2 occurs in a single step that is virtually unaffected by the presence of the silica support. XRPD analysis confirms that precalcination of the three catalysts at 500°C prior to reduction leads to the formation of Co 3 O 4 with the nitrate- and chloride-derived catalysts, but not with the acetate-derived material. TPR profiles and XPS spectra indicate that isothermal reduction in hydrogen at 400°C is much less complete for the uncalcined catalysts than for the calcined materials, particularly for the nitrate and acetate precursors. Exposure of the uncalcined, hydrogen treated catalysts to a H 2 /CO reaction mixture at 250°C results in further reduction of Co 2+ to Co 0 for the nitrate- and acetate-derived catalysts, which had been only slightly reduced by the prior hydrogen treatment, and partial re-oxidation of Co 0 to Co 2+ for the chloride-derived material, which had been largely reduced by the hydrogen treatment.

Exposed aluminum ions as active sites on γ-alumina

Abstract The EPR spectrum of nitric oxide which is weakly adsorbed on γ-Al 2 O 3 exhibits a strong crystal field gradient and a hyperfine structure due to exposed aluminum ions. Hydrogen sulfide is selectively adsorbed on these exposed aluminum ions, whereas H 2 O and CO 2 are not. The latter may be removed from the aluminum ions by brief evacuation at 25 °C. Quantitative results for the poisoning of the 1-butene isomerization reaction and the decrease in the EPR spectrum of NO strongly suggest that the exposed aluminum ions function as active sites for the isomerization reaction. For one sample of alumina the number of such sites is between 1 × 10 12 /cm 2 as measured from the spin concentration and 5 × 10 13 /cm 2 as measured from H 2 S poisoning experiments.

Steady-state conversion of methane to aromatics in high yields using an integrated recycle reaction system

Methane can be converted in high yields to aromatic products using an integrated recycle system containing both an oxidative coupling (OCM) reactor at 800°C, for conversion of CH4 to C2H4, and a secondary reactor containing Ga/ZSM-5 at 520°C for subsequent conversion of ethylene to aromatics. Using this system, aromatic product yields of >70% at CH4 conversions of ~100%, based on total CH4 added, can be obtained.

Characterization of catalytic lanthanum oxide for double bond isomerization of n-butenes

Abstract Lanthanum sesquioxide, when properly activated, is an extremely active catalyst for double bond migration in the n -butenes. The initial rate of 1-butene conversion on rehydrated La 2 O 3 varies with final calcination temperature, and attains a maximum of 4.2 × 10 20 molecules/ m 2 /min at 0 °C, following catalyst evacuation at 650 °C. In the temperature range 0 to 50 °C, the reaction is zero order in 1-butene for initial reactant pressures of 25 to 200 torr. A cumulative self-poisoning of the catalyst occurs at reaction temperatures due to adsorption of a strongly held, unreactive form of n -butene, but the latter is completely removed and activity restored upon subsequent rehydration and calcination. Initial cis -2-butene/ trans -2-butene ratios are large (7 to 8 at 0 °C), and the rates of reaction of both 2-butene isomers are much lower than that of 1-butene. Direct interconversion of cis - and trans -2-butene occurs very slowly or not at all on La 2 O 3 , and a reaction scheme for the three-component system is proposed. Active sites for 1-butene isomerization involve surface O 2− ions in conjunction with adjacent defect structures such as anion vacancies. A difference of 4.2 kcal/mol between the apparent activation energies of cis - and trans -2-butene appearances suggests that formations of the two 2-butene isomers from the 1-butene reactant may occur on energetically and/or structurally dissimilar sites on La 2 O 3 .

The role of sodium carbonate and oxides supported on lanthanide oxides in the oxidative dimerization of methane

The addition of Na2CO3 to three representative lanthanide oxides, La2O3, CeO2, and Yb2O3, has a marked effect on the catalytic properties of these materials for the oxidative dimerization of methane. The effect is most dramatic for CeO2, which, upon addition of Na2CO3, is transformed from a total oxidation catalyst to one which is reasonably selective for the conversion of CH4 to C2H4 and C2H6. Results obtained by X-ray photoelectron spectroscopy and ion scattering spectroscopy show that a sodium carbonate /sodium oxide phase largely covers the lanthanide oxide surface, thus the catalytic properties are those of the sodium phase, rather than those of the lanthanide oxide. Indeed, the specific activities of the Na+/LnxOy catalysts and of pure Na2CO3 were the same within a factor of 2.5, with Na+/Yb2O3 at the high end of the range. The specific activities of pure lanthanide oxides were considerably greater than those of the modified catalysts. Although Na2CO3 is the principal compound present on the surface of the Na+/LnxOy catalysts, it is probable that Na2O2 is responsible for the activation of CH4.

Characterization of Ga/ZSM-5 for the catalytic aromatization of dilute ethylene streams

Ga/ZSM-5 is an effective catalyst for the conversion of dilute (3%) ethylene-in-methane reactant streams into aromatic hydrocarbons at 500–550°C. A Ga loading as low as 0.5 wt% is sufficient to obtain maximum yields of aromatic products. At 520°C, an ethylene conversion of 93%, with an aromatics selectivity of 81%, was obtained over a 5 wt% Ga/ZSM-5 catalyst. The conversion of ethylene into aromatics over Ga/ZSM-5 catalysts involves a complex sequence of oligomerization, isomerization, cracking, and cyclization reactions that occur on Brønsted acid zeolites in the zeolite. The role of the gallium, which exists as both Ga3+ at zeolitic exchange sites and as Ga2O3 within the channels and on the external surface of the calcined catalyst, is to promote dehydrogenation of the acid-catalyzed oligomerization and cyclization products.