Kevin J. Smith

A Review of Molybdenum Catalysts for Synthesis Gas Conversion to Alcohols: Catalysts, Mechanisms and Kinetics

Recent literature on synthesis gas conversion to higher alcohols over Mo-based catalysts is reviewed. Density functional theory calculations show that Mo-CO adsorption is weakened by C, P, or S ligands and this facilitates CO dissociation, either directly on Mo2C, or by H-assisted dissociation on MoS2, Mo2C, and MoP. Consequently, Mo-based catalysts have high hydrocarbon selectivity unless they are promoted with alkali metals and/or Group VIII metals. Promoted MoS2 and MoP have alcohol selectivities of ∼80 C atom % (CO2-free basis) at typical operating conditions (5–8 MPa, H2/CO = 2–1, 537–603 K), whereas on promoted Mo2C, alcohol selectivities are ∼60%. The kinetics of the synthesis gas conversion reactions over Mo-based catalysts have mostly been described by empirical power law models and the alcohol and hydrocarbon product distributions are consistent with a CO insertion mechanism for chain growth.

A comparison of bulk metal nitride catalysts for pyridine hydrodenitrogenation

A comparison of various group IV–VIII bulk metal nitride catalysts identified Co4N and Fe3N as having higher pyridine hydrodenitrogenation activity per unit area than Mo2N. Formation of the metal nitrides was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy, and all the nitrides were prepared from metal oxide precursors using the same temperature-programmed reaction technique. In general, the specific activity of the metal nitrides decreased with increased heat of formation of the metal nitride.

Hydrodeoxygenation of Phenolic Model Compounds over MoS2 Catalysts with Different Structures

Several MoS 2 catalysts of different structure, prepared by in situ decomposition of ammonium heptamolyb- date (AHM) and molybdenum naphthenate (MoNaph), and by MoS2 exfoliation (TDM), were characterized by BET, X-ray diffraction (XRD), Energy Dispersive X-ray (EDX) and transmission electron microscopy (TEM). The analy- sis showed that MoS2 structure was dependant upon the preparation procedure. The activity of the catalysts was de- termined by measuring the hydrodeoxygenation (HDO) of phenol, 4-methylphenol and 4-methoxyphenol using a batch autoclave reactor operated at 2.8 MPa of hydrogen and temperatures ranging from 320-370°C. By comparing the conversion, the reactivity order of the catalysts was: AHM>TDM-D>MoNaph>thermal>MoS2 powder> TDM-W. Also, the effect of reaction temperature on the HDO conversion was explained in terms of equilibrium of reversible reaction kinetics. The main products of the HDO for phenolic compounds were identified by gas chro- matography/mass spectrometry (GC/MS). The results showed that the product distribution and the HDO selectivity were correlated with the reaction temperature. Two parallel reaction routes, direct hydrogenolysis and combined hy- drogenation-hydrogenolysis, were confirmed by the analysis of the product distribution. High temperature favored hydrogenolysis over hydrogenation for HDO of phenol and 4-methoxyphenol, whereas for 4-methylphenol the re- verse was true. Keywords ammonium heptamolybdate derived MoS2, structure effect, characterization, hydrodeoxygenation, re- activity, product distribution

An experimental study of heavy oil ultrafiltration using ceramic membranes

The ultrafiltration of Cold Lake heavy oil using asymmetric, single-tube ceramic membranes (length 25 cm, o.d. 1 cm, average pore diameter 0.02-0.1 μm) was investigated in a batch ultrafiltration unit operated with partial recycle of retentate. Experiments were conducted at a transmembrane pressure (ΔP) of ∼600 kPa, temperatures (T) in the range 80–160°C and cross-flow velocities (Ucf) in the range 2–10 m s−1. Both permeate flux and asphaltene content were monitored as a function of time on-stream during each experiment. Ultrafiltration resulted in rapid fouling of the membranes, which significantly reduced the permeate flux but increased the retention of asphaltenes. For the 0.1 μm membrane operated at ΔP = 600 kPa, T = 120°C and Ucf = 7 m s−1, the permeate mass flux decreased from an initial value of 660 kg m−2day−1 to ∼60 kg m−2day−1 after 6 h on-stream. However, the asphaltene retention increased from < 1% to 80% over the same period. The decrease in asphaltene content of the permeate compared with the feed was correlated with decreases in permeate density, viscosity and Ni and V contents. The effect of membrane pore diameter and cross-flow velocity on permeate flux and asphaltene retention provided evidence that membrane fouling occurred initially by a pore-restriction mechanism, the formation of a gel layer being important near the end of the run.

Methane homologation and reactivity of carbon species on supported Co catalysts

Abstract The effect of catalyst support on the activity of Co catalysts has been investigated for the two-step CH 4 homologation reaction. The total amount of CH 4 per mole of surface Co decomposed at 450°C, in a reaction time of 1 or 3 min, was greater on the Co/Al 2 O 3 catalyst than the Co/SiO 2 catalyst. In the subsequent hydrogenation step at 100°C, more of the carbonaceous deposit was hydrogenated on the Co/SiO 2 than the Co/A1 2 O 3 . The selectivity to C 2 + hydrocarbons was also higher on the Co/ SiO 2 catalyst. These results are discussed in terms of the effect of migration of the carbonaceous deposit from the metal to the support, that apparently occurs more readily on the Co/A1 2 O 3 catalyst than on the Co/SiO 2 catalyst.

Mesoporous Mn- and La-Doped Cerium Oxide/Cobalt Oxide Mixed Metal Catalysts for Methane Oxidation

New precious-metal-free mesoporous materials were investigated as catalysts for the complete oxidation of methane to carbon dioxide. Mesoporous cobalt oxide was first synthesized using KIT-6 mesoporous silica as a hard template. After removal of the silica, the cobalt oxide was itself used as a hard template to construct cerium oxide/cobalt oxide composite materials. Furthermore, cerium oxide/cobalt oxide composite materials doped with manganese and lanthanum were also prepared. All of the new composite materials retained the hierarchical long-range order of the original KIT-6 template. Temperature-programmed oxidation measurements showed that these cerium oxide/cobalt oxide and doped cerium oxide/cobalt oxide materials are effective catalysts for the total oxidation of methane, with a light-off temperature (T50%) of ∼400 °C observed for all of the nanostructured materials.

K promotion of Co catalysts for the two-step methane homologation reaction

The promotion of Co catalysts with K has been examined for the two-step CH4 homologation reaction. The effect of K was strongly influenced by the catalyst support. In the case of the SiO2 supported catalyst, addition of K increased the CH4 decomposition activity and decreased the second-stage hydrogenation activity while the C2+ selectivity increased from 14 C at% to 36 C at%. With the Al2O3 support, addition of K increased CH4 decomposition activity but the C2+ selectivity increased only marginally. These results are discussed in terms of Co dispersion, support effects and the effect of K on the reactivity of the carbon species deposited during CH4 decomposition.

Activity and kinetics of ruthenium supported catalysts for sodium borohydride hydrolysis to hydrogen

Ru–RuO2/C prepared by galvanic replacement has high catalytic activity for sodium borohydride hydrolysis. In the present study, a series of Ru–RuO2/C catalysts, Ru–RuO2/C reduced, RuO2/C and Ru supported on Ni foam (Ru/Ni foam) are prepared and characterized. Results show that RuO2 on Ru–RuO2/C is formed from both the consumption of the parent Ni and NiO nanoparticles and the disproportionation of RuCl3 with epitaxial growth of Ru species. The quantity of RuO2 with oxygen vacancies in Ru–RuO2/C determines the hydrolysis activity for sodium borohydride. In contrast to Ru–RuO2/C, Ru/Ni foam without oxygen vacancies has the lower hydrolysis activity. Results of kinetics calculation further confirm that without mass transfer limitation, Ru–RuO2/C has lower intrinsic activation energy and correspondingly higher catalytic activity due to existence of oxygen vacancies than those from Ru–RuO2/C reduced, RuO2/C, Ru/Ni foam and catalysts from the literature.

A study of synthesis gas conversion to methane and methanol over a Mo6P3 cluster using density functional theory

Synthesis gas (CO+H2) conversion to CH4 and CH3OH over a MoP catalyst has been examined using density functional theory and a Mo6P3 cluster model of the MoP surface. A model of synthesis gas conversion was developed by calculating adsorption energies of all possible arrangements of stable surface intermediates on Mo6P3. For CH4 formation, the potential energy surface (PES) followed the route (Had addition at each step is assumed but not shown) COad → CHOad → CH2Oad → CH2OHad → CH2.ad+H2Oad → CH3.ad+H2Oad → CH4+H2O and CH3OH followed COad → CHOad → CH2Oad → CH2OHad → CH3OHad. The activation energy for the formation of CH3OH from hydroxymethyl (100.9 kcal/mol) is higher than for the formation of methylene and water (40.3 kcal/mol), suggesting that CH4 rather than CH3OH will be produced from synthesis gas over MoP catalysts.

XPS studies of the nitridation of MoO3 thin films on alumina and silica supports

Abstract X-ray photoelectron spectroscopy has been used to characterize thin films of MoO3 formed on planar oxidized supports, namely AlOx (from aluminum strip) and SiO2 (from silicon wafer). Comparisons are made with behaviour for MoO3 on metallic Mo substrate. It is observed that on calcination at 450°C, the Mo 3d spectral features shift to higher binding energy for MoO3/AlOx and to lower binding energy for MoO3/SiO2. On nitridation by heating in NH3, it is found that the samples on the oxide supports show easier O–N replacement compared with the MoO3/Mo system. In general, the nitridation behaviour for MoO3/AlOx is similar to that of MoO3/Mo, but Mo species in MoO3/SiO2 seem to be more easily reduced (Mo(0) is detected for the SiO2 system but not for AlOx). Comparisons of heating rates for the second nitridation step from 350 to 450°C were made for the MoO3/Mo and MoO3/AlOx samples. Differences between the high heating rate (100 K/h) and the low heating rate (40 K/h) are incremental but definite. The lower heating rate is favourable both for the O–N replacement and for the metal reduction. For example, more Mo(+3) is present after the nitridation to 450°C when the low-heating rate regime is used, compared with that formed when the heating is done at the higher rate.