Francis J. Derbyshire

The influence of surface functionality on the activity of carbon-supported catalysts

The aim of this research is to investigate how the presence of surface functional groups can influence the activity of carbon-supported MoS2 catalysts for coal asphaltene hydrogenation. Porous carbons were subjected to various chemical treatments in order to introduce oxygen and nitrogen surface functionalities prior to impregnation with ammonium tetrathiomolybdate. Supports and catalysts were examined by FTIR. Preoxidation of polymer-derived carbons lowered catalyst activity whereas preoxidation of a carbon black composite support increased it. Similar mixed results have been reported in the literature. The modified catalytic activity cannot be unequivocally attributed to surface oxygen. Qualitatively, it is considered that these groups exert some influence, but it is the structure of the carbon which is ultimately the controlling factor. In some cases, oxidation may introduce sites which promote interaction with metal species and, in others, oxidation may modify or destroy favourable sites which are already extant. Prenitriding was found to have a distinct influence in enhancing catalyst activity. The presence of nitrogen-containing surface groups may provide preferential sites for the adsorption of Mo species.

Effects of surface and structural properties of carbons on the behavior of carbon-supported molybdenum catalysts

Previous work on carbon-supported hydrodesulfurization (HDS) catalysts has led to the general realization that the nature of the support has a very significant influence on catalytic activity. The present investigation attempts to provide a more focused understanding of the exact nature of this influence. Mostly one support, a commercial carbon black, was subjected to oxidative and/or thermal treatment to modify its surface and structural properties. These were thoroughly examined using temperature-programmed desorption, X-ray diffraction, titrations, and electrophoresis. The various carbon-supported molybdenum catalysts were prepared by equilibrium adsorption and incipient wetness impregnation using four different catalyst precursors. The catalytic activity in thiophene HDS and Fischer-Tropsch synthesis was determined in fixed-bed flow reactors connected on-line to gas chromatographs. The catalysts were characterized by X-ray photoelectron spectroscopy. The heretofore mostly neglected knowledge of carbon surface chemistry was shown to provide the necessary framework for the understanding of the variations in catalytic activity. It is concluded, however, that two conflicting requirements complicate the preparation of highly active (i.e., highly dispersed) molybdenum species on carbon surfaces. On one hand, the introduction of oxygen functional groups provides anchoring sites for catalyst precursor adsorption and thus the potential for its high initial dispersion. On the other hand, this also renders the support surface negatively charged over a wide range of pH conditions. At very low pH conditions, below the isoelectric point of the support, when the attractive forces prevail between the Mo anions and the positively charged carbon surface, Mo polymerization is thought to contribute to catalyst agglomeration. Final catalyst dispersion (i.e., catalytic activity) is also influenced by the thermal stability of the oxygen functional groups on the carbon surface. No significant correlation between structural parameters of the support and catalytic activity was found.

Coal liquefaction by molybdenum catalyzed hydrogenation in the absence of solvent

Abstract Dry catalytic hydrogenation, using an impregnated molybdenum catalyst, has been used to explore the processes of coal liquefaction and their relation to coal structure. It was found that for high activity the molybdenum must be present as MoS 2 and that the conditions used for impregnation can strongly influence catalyst dispersion and activity. At temperatures of 400°C and lower, a subbituminous coal was more readily converted to liquid products and gases, than a coal of bituminous rank. The low-rank coal products were comparatively richer in hydrogen, possessed higher oil to asphaltene ratios and were more aliphatic than the bituminous coal liquids. It is considered that the higher reactivity of the low-rank coal is attributable to the initial reactions being conducted under conditions which promote hydrogenation while minimizing condensation and cracking reactions. Examination of reacted coals by reflected fluorescent light microscopy showed a close correlation between the yield of extractable liquids and the maximum intensity of vitrinite fluorescence. The fluorescence appears to be directly related to the liberation of relatively low molecular weight species within the coal structure. Coal pretreatment by low-temperature dry hydrogenation, prior to higher temperature liquefaction in the presence of solvent, improved the overall conversion and product selectivity of both bituminous and subbituminous coals. It is suggested that subtle control of the initial conversion reactions can lead to more efficient liquefaction of coals of different rank.

Vitrinite secondary fluorescence: Its chemistry and relation to the development of a mobile phase and thermoplasticity in coal

Abstract Coal fractions obtained by a progressive solvent extraction scheme and HPLC (high-performance liquid chromatography) fractionation have been examined by fluorescence microscopy. The results show that aromatic oligomers, polar compounds and a small portion of aliphatics in the mobile phase are actively and highly fluorescent. It is concluded that these compounds are fluorophors responsible for the secondary fluorescence displayed by some vitrinite. The overall secondary fluorescence of vitrinite is a complex overlapping of the fluorescence signals of these compounds with intermolecular and interphase interactions. These interactions cause a certain degree of fluorescence quenching through radiationless processes. The aromatic network system within the vitrinite is apparently too condensed to display a visible fluorescence at a rank higher than subbituminous C/B as a result of self-quenching. It is suggested that the development of a ‘mobile phase’ during the bituminous stage of coalification is responsible for the secondary fluorescence of the vitrinite, as well as its thermoplastic behavior.

Carbonization of coker feedstocks and their fractions

Abstract Carbonization studies of petroleum vacuum distillation residua have been carried out on a laboratory scale with the objective of developing a better understanding of the causes and mechanisms of the incidence of shot coke formation in delayed coking. The rates of carbonization of the residua are found to inversely relate to the extent of mesophase development in the resulting semicoke. The asphaltenic fractions of the petroleum stocks carbonized most rapidly and produced cokes containing small anisotropic regions (fine mosaics), similar to that of commercially produced shot coke and consistent with arrested mesophase development. The addition of aromatic petroleum materials enhanced mesophase development commensurate with their reducing the rate of carbonization. While the microstructural features of shot coke are reproducibly produced in the laboratory, the characteristic spherical morphology is not and appears to be determined by the conditions obtaining in large-scale operation. It is suggested that this morphology may arise from the segregation of regions of rapidly carbonizing constituents from a less reactive matrix.

Catalytic hydrogenation of coals in the absence of solvent: 1. Compositional changes of an hvA bituminous coal by chemical analysis and reflected fluorescent light microscopy

Abstract The effect of dry catalytic hydrogenation of an hvA bituminous coal, using an impregnated Mo catalyst, has been examined at temperatures up to 400 °C. Yields of chloroform-soluble extracts of over 30% were obtained with low attendant gas yield. Analyses of the extracts show that although there were no dramatic compositional changes over a wide range of yield, there are some possibly important distinctions which merit further investigation. Fluorescence microscopy showed that the intensity of vitrinite fluorescence increased in parallel with extract yield. With increasing yield, shifts in the fluorescence spectra were indicative of an increase in the concentration of condensed aromatic structures in the extract. It is considered that, at high yield, a considerable proportion of the extract is derived from the breakdown of the vitrinite macromolecular structure.

The effect of sulfur evolution on the properties of high-temperature carbons—I. Thianthrene and thianthrene-anthracene precursors

Abstract The carbonization (525°C) and graphitization (up to 2500°C) of thianthrene, anthracene and a 17% wt thianthrene-83% wt anthracene mixture were investigated. The thianthrene carbons were found to be non-graphitizable and of low density. Upon heat-treatment from 1200–2500°C sulfur was evolved continuously and there was a further decrease in density due to an increase in closed porosity. The carbons produced from the mixture were found to be graphitizable and possessed similar crystal parameters to those carbons derived from anthracene after heat treatment to the same temperature. In contrast to the anthracene carbons, the mixed carbons showed a sharp decrease in density with heat treatment temperature. The principal reduction in sulfur content occurred between 1400–1600°C. The density change appears to be produced through the disruption of crystallite alignment which substantially increases the open pore volume for pores of less than 200 nm diameter. The behavior of the mixed carbon is similar to that reported for high sulfur petroleum cokes which can undergo an irreversible expansion (puffing) during graphitization.

The chemistry of vitrinite fluorescence

Abstract The presence of a mobile phase within the structure of vitrinite is responsible for a major part of the macerals secondary fluroescence. Fluorophoric structures contained in the mobile phase include aromatics, polars and a small proportion of aliphatics. The development of the mobile phase in the bituminous range of coalification enhances vitrinite secondary fluorescence and coal thermoplasticity. The macromolecular network is shown to be too condensed to display visible fluorescence as a result of self-quenching and π-electron delocalization. Dry catalytic hydrogenation has enabled the structure of coal to be related to the fluorescence displayed by the hydrogenated vitrinite. The hydrogenation-induced fluorescence and yield of chloroform-soluble material were progressively enhanced by hydrogenation; however, the residual aromatic network displays no visible fluorescence.

Measurement of active phase dispersion in molybdenum sulfide/carbon catalysts using toluene chemisorption

Abstract A range of carbon-based molybdenum sulfide catalysts has been prepared which have high activities for the hydrotreating of coal asphaltenes into oils. In general, their coking tendency was extremely low. Ammonia pretreatment of some of the carbons enhanced the activity of the catalysts for hydrotreating asphaltenes. In the current work toluene adsorption, complemented by X-ray diffraction, was used as a probe for molybdenum sulfide dispersion. The amount of chemisorbed toluene gave an indication of the extent of molybdenum sulfide dispersion. As determined by this technique, molybdenum sulfide dispersion was qualitatively related to catalytic activity for coal-asphaltene hydrogenation.

The effect of sulfur evolution on the properties of high-temperature carbons. II: The addition of Fe2O3

A thianthrene-anthracene mixture has been used to study the influence of Fe2O3 addition in suppressing the volume expansion of carbons due to sulfur release on heat treatment. For undoped carbons, the particle density began to decrease above about 1200°C and continued to 2500°C. There was an accompanying sharp reduction in sulfur content between 1400–1600°C. With Fe2O3 addition, the sulfur content was higher over the whole temperature range studied. Between 1200–1600°C the carbon density increased slightly, while from 1600–2500°C the density continuously decreased. Below 1600°C, iron sulfide was detected in the cooled reaction mixture, consistent with the presence of an iron-sulfur liquid phase during heat treatment. The liquid phase appears to influence density changes through modifying the mechanisms for sulfur release. Although FeS was no longer evident at higher temperatures, the presence of iron compounds appeared to have an influence in reducing expansion. The addition of Fe2O3 did not affect crystal growth and development of structural order during graphitization. Density decreases for doped and undoped carbons are attributed to disruption of crystallite alignment. Porosity measurements showed that the increase in pore volume is due to the formation of pores smaller than 200 nm diameter.