George Marcelin

Surface phenomena during the oxidative coupling of methane over Li/MgO

This paper details an investigation of the oxidative coupling of methane for reaction temperatures up to 645°C over MgO and Li/MgO catalysts using steady-state isotopic transient kinetic analysis (SSITKA). Oxygen-exchange experiments in the absence of methane resulted in a quantification of the lattice oxygen diffusivity and total oxygen uptake. The catalyst had three more-or-less distinct regions: (1) the physical surface at which exchange between the gas phase and the solid occurred, (2) several subsurface atomic layers readily available for exchange, and (3) the bulk oxide. Using isotopic switches of oxygen and methane under steady-state reaction, the active intermediates along the carbon and oxygen reaction pathways were quantified. Lattice oxygen was found to play a significant role in the oxidation process under steady-state reaction. CO and CO2 appeared to be formed via a multistep surface oxidation pathway while ethane was formed via surface-generated intermediates along a parallel pathway. Sites involved with the generation of intermediates for selective coupling were found to have a lower activity than sites active for the generation of nonselective intermediates.

Acid and catalytic properties of nonstoichiometric aluminum borates

This paper details an investigation on the structure-acidity-catalytic activity relationship in aluminum borate mixed oxides. This aluminum borate mixed oxide is amorphous, is refractory, and exhibits properties which are dependent on the exact stoichiometry of the material. A series of catalysts with BAl ratios ranging from 0 to 1 was prepared and characterized with respect to surface area, pore volume, and thermal stability. Nuclear magnetic resonance spectroscopy of 11B and 27Al was used to determine the local structure. Two boron species were identified: a tetrahedral BO4 and a trigonal BO3. The relative amounts of the two boron species were dependent on the BAl stoichiometry. Three aluminum species were identified: a tetrahedral AlO4, an octahedral AlO6, and an AlO4 with four B next-nearest neighbors. The strength and concentration of acid sites present on the surface of the aluminum borates were determined using adsorbed indicators. The presence and coordination of boron were seen to have a dramatic effect on the acidity of these materials with the acidity being closely related to the relative concentration of BO4. 2-Propanol dehydration was used as a probe reaction to measure the acidity. Results from this reaction have shown these materials to indeed be highly acidic and catalytically active.

Alumina-aluminum phosphate as a large-pore support and its application to liquid phase hydrogenation

Abstract Studies were conducted on the preparation of several compositions of alumina-aluminum phosphate and their resulting physical properties. It was observed that by varying the stoichiometry of the precipitates, surface areas and pore size distributions could be controlled. High alumina content invariably yielded materials with high surface area and small pores while high aluminum phosphate resulted in smaller surface area and larger pores. Some of the large-pore support gels were mixed with nickel salts to produce catalysts which were found to be highly active for the liquid phase hydrogenation of 2-ethylhexenal. The relative activities were successfully correlated with catalyst average pore radii and surface areas.

Study of the mechanism of the cracking of small alkane molecules on HY Zeolites

Cracking of i-butane and n-pentane was studied on HY zeolites. These reactions were initiated by the protonation of C-H and C-C bonds by the Bronsted acid sites. The pentacoordinated carbonium ions thus formed decomposed into carbenium ions and the products of the initiation reactions, viz., H2, CH4, and C2H6. These carbenium ions propagated chain reactions, mainly by isomerization followed by hydride transfer. Disproportionation occurred concomitantly. For these small alkanes the contribution of /gb-scission to product formation was negligible. “Chain length” (the number of chain cycles per initiation) was defined as the ratio of reactant molecules consumed by hydride transfer to those reacted by protonation, i.e., the ratio of rates of bimolecular propagation to unimolecular initiation. Thus, chain length reflected the average lifetime of the carbenium ions. The extent of protonation was found to increase with the strength of acid sites while the chain length remained relatively constant for preparations of a similar nature. The product distribution obtained was therefore critically dependent on the steady-state population of carbonium ions. Finally the chain reactions were terminated by decomposition of carbenium ions into corresponding alkenes. Mass balances derived from these initiation, propagation, and termination steps were in agreement for both the substrates. The product distributions obtained for i-butane and n-pentane cracking were satisfactorily explained on this basis.

Oxidative coupling of methane over antimony-based catalysts

Abstract The conversion of methane to C 2 hydrocarbons by oxidative coupling was studied using a series of potassium-doped bulk and supported antimony oxide catalysts over the temperature range 700–800 °C. A combination of both pulse and flow reaction techniques was used in the study. The use of a tapered reactor design significantly reduced any gas-phase contribution to the reaction. α-Sb 2 O 4 was identified as the only antimony oxide phase which gave high C 2 selectivity under the reaction conditions studied. It was found that addition of potassium to supported antimony oxide resulted in a slight increase in the C 2 yield and C 2 selectivity. Kinetic studies were used to elucidate the reaction mechanism.

NMR study of reactions of alcohols on solid acids

Abstract Contradictory evidence currently exists concerning the stability of small aliphatic carbenium ions on the surfaces of various zeolites. This question has been investigated using triphenylmeth-anol- 13 COH, 2-propanol-2- 13 C and propene-2- 13 C on a silica-alumina catalyst and H-Y, H-ZSM-5, and H-M (mordenite) zeolites using the 13 C MASNMR technique. The stable (C 6 H 5 ) 3 C − cation was used as a calibration standard and a test was devised (bleaching with NH 3 or H 2 O) to discriminate between carbenium ions and other nonionic species present. The data indicate that whereas carbenium ions have evidently been formed from 2-propanol and propene, they are not stabilized on these materials. Instead they react with olefin released from other sites to produce polymeric residues, some of which may be ionic.

Effect of dealumination on the catalytic activity of acid zeolites for the gas phase synthesis of MTBE

Abstract The gas phase MTBE synthesis reaction was studied on a number of zeolite catalysts, in order to obtain a better understanding of the impact of acidity on zeolite activity for MTBE formation away from thermodynamic equilibrium limitations. The catalysts investigated included a series of dealuminated HY zeolites with different acid properties. In addition, H-ZSM-5, an amorphous silica-alumina, and Amberlyst-15 resin were investigated for comparison. An increase in acidity of the HY zeolites produced by dealumination was found to result in a significant enhancement in the intrinsic activity for MTBE formation. The ratio of extra-lattice to lattice Al appears to be an important parameter for determining the catalytic behavior of zeolites for this reaction.

The gas-phase hydrogenation of benzene using phosphate-supported nickel catalysts

The gas-phase hydrogenation of benzene was studied on a series of nickel catalysts supported on alumina-aluminum phosphate (AAP), magnesia-alumina-aluminum phosphate (MgAAP), SiO/sub 2/, and kieselguhr. Measurements of the reaction rates at 110/sup 0/C 3.5 atm, and GHSV = 42,000 showed the phosphate-supported Ni catalysts to be less active than catalysts supported on SiO/sub 2/ or kieselguhr. Additionally, the relative rate on Ni/MgAAP was dependent on the reduction temperature employed during pretreatment. Comparison of the hydrogenation rates and the sulfur capacity toward thiophene poisoning versus the irreversible hydrogen uptake of the various catalysts indicated that the reduced activity of phosphate-supported Ni was due to metal-support effects and that only sites which exhibited strong hydrogen chemisorption were active for benzene hydrogenation.

Oxidative coupling of methane I. Behavior and characteristics of PbMgO catalysts

The role of the lead active species has been studied for PbMgO catalysts in the oxidative coupling of methane. In general, the effect of adding lead to MgO was to increase the rate of total methane conversion. The C2 selectivity exhibited a volcano-type pattern with a maximum of 51% being achieved at about 0.4 at.% Pb. This volcano-type behavior is interpreted in terms of an “isolated site”-type mechanism. Addition of PbO in small amounts to a relatively inert MgO matrix results in the formation of a number of isolated strong oxidizing sites which can generate the methyl radicals more efficiently. Since these sites are isolated, the methyl radical has little likelihood of being further oxidized and can desorb into the gas phase where it can couple to form ethane. Surface and bulk characterization studies of the catalysts suggest that at low loadings the lead is indeed highly dispersed on the surface of the catalyst.

Oxidative coupling of methane: II. Formation of active sites by lead and tin oxides on MgO

Abstract The oxidative coupling of methane has been studied over SnMgO catalysts with varying amounts of Sn. The addition of small amounts of tin oxide to MgO increased both the rate of methane conversion and the C 2 selectivity. A maximum C 2 selectivity of 40% was obtained over 0.4 at.% SnMgO at 780°C at a methane conversion of 20%. The maximum in Cz selectivity can be explained by considering an “isolated site” type mechanism in methane oxidation. According to this mechanism, an active and selective catalyst should have highly oxidizing sites isolated from each other by relatively inactive sites. Such an arrangement would allow a high methane conversion rate and the highest Cz selectivity by limiting the total oxidation of intermediates on the catalyst. A simple kinetic model based on the isolated site mechanism, which explains most of the catalytic features of the SnMgO catalytic system, has been developed. The model predicts an optimum selectivity for C 2 hydrocarbons as a function of Sn content.