Michel Daage

Optical absorption and catalytic activity of molybdenum sulfide edge surfaces

Abstract The optical and catalytic properties of molybdenum sulfide edge planes have been studied in single crystals and disordered powders. We observe an infrared optical absorption due to the edge planes in the single crystal platelets. This absorption is proportional to the hydrodesulfurization catalytic activity in unsupported powders which were annealed at various temperatures to change their degree of crystallinity. This provides a direct correlation between the electronic structure of these materials as measured by optical absorption and their catalytic activity. This result shows that the optical and catalytic properties of the edge sites are similar in the amorphous and crystalline materials and we derive turnover numbers and absorption cross sections for these sites.

Synthesis and activity of hydrotreating catalysts prepared via promotion by low-valent transition-metal complexes

A new family of hydrotreating catalysts is developed via low-temperature chemical reaction of the edge of preformed MoS2 particles. Specifically, an “edge decoration” is achieved via reaction of MoS2 with low-valent organometallic complexes such as Co2(CO)8. This approach is suggested by the reactivity of similar low-valent organometallics with molecular complexes whose structure contains fragments that resemble those in transition-metal sulfides. Activity tests with pure compounds and commercial feedstocks have shown that these catalysts are active for hydrodesulfurizatlon.

Isomerization and cracking of 13C-labeled hexanes over H-mordenite: I. Labeling of the products and general reaction scheme

Abstract The catalytic conversion of branched hexanes over H-mordenite at 170 °C has been studied with a constant hydrocarbon pressure in a flow system. Under these conditions, isomers are desorbed first from the catalyst and cracked products (isobutane and isopentane exclusively) appear later on in the gas phase. Extrapolation of product distribution at zero conversion gives evidence for the occurrence of a direct interconversion between 3-methylpentane and 2,3-dimethylbutane and shows that 2-methylpentane is the only isomer yielding cracked fragments as primary products. The use of 13 C-labeled 3-methylpentanes demonstrates that isobutane, isopentane, and part of the hexanes result from the random association of 13 C and 12 C carbon atoms and that two types of isotopic isomer are formed. A detailed reaction scheme, involving the β-fission of a 2-methylpentenium-4 cation as initial step, is proposed to explain the formation of cracked products. It is suggested that sites of two different locations are involved in the reactions of hexanes on H-mordenite.

Structure-Function Relations in Layered Transition Metal Sulfide Catalysts

Abstract Chemical and physical studies of MoS 2 single crystals, microcrystals and powders were carried out with the goal of understanding the relation between the edge plane properties and catalytic properties. The bulk electronic structure of MoS 2 is directly related to the catalytically active surface states. The catalytic states, created by edge termination or probably sulfur vacancies, lie just above the dz 2 band near the Fermi level. Optical and electron spectroscopic measurements on geometrically well defined catalysts were used to obtain a “HDS edge site turnover frequency”, which does not suffer from an ambiguity in dispersion. Furthermore, the catalytic activity of unsupported molybdenum disulfide is related to the existence of two different sites and their relative concentration is directly dependent upon the morphology of the particle and is controlled by the stacking of the layers.

Isomerization and cracking of 13C-labeled hexanes over H-mordenite: II. Intramolecular rearrangements of the hexyl cations

Abstract The intramolecular rearrangements of the branched hexyl cations which take place at 170 °C over H-mordenite have been studied by using 13C labeling techniques. Each individual step of these rearrangements including alkyl shifts (1,2-ethyl shift, 1,3-methyl shift, and 1,2-methyl shift), branching rearrangements (interconversions between the two methylpentanes and 2,3-dimethylbutane), and interchanges between internal and external carbon atoms has been isolated. Alkyl shifts proceed through alkyl bridged cations as transition states. Their relative rates are in the sequence: 1,2-ethyl shift > 1,3-methyl shift > 1,2-methyl shift. This result is interpreted by differences in the stabilities of the alkyl bridged structures. The branching rearrangements and the carbon interchange reaction are best explained by considering equilibration between corner- and edge-protonated alkyl cyclopropanes and alkyl bridged structures.