Rui Lin

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.

A fluorogeochemical model for coal macerals

Abstract A model is proposed based on the concept that the fluorescence displayed by coal macerals is due to the activity of π-electrons. The delocalization of π-electrons which accompanies polymerization results in a shift towards the red in the spectral fluorescence of macerals during coalification. At the same time, fluorescence quenching is progressively enhanced by virtue of polymerization and accompanying removal of nonfluorophoric media, causing a decrease in fluorescence intensity. Estimations of maceral fluorescence properties using the molecular orbital theory produced the same trends. The build-up in fluorescence intensity and slow-down in the spectral red shift of bituminous vitrinites are exceptions to these trends due to the development of a mobile phase at this rank level. The model is further supported by fluorescence studies of some coal-precursor and coal-derived chemical systems.

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.

Observations concerning the nature of maceral fluorescence alteration with time

Abstract Distinctive features of the alteration of maceral fluorescence intensity with upon blue-light or UV excitation are related to the level of organic maturation. Alteration has been measured in different media; the results demonstrate clearly that the negative component of the alteration phenomenon is due to photochemical oxidation. Highly weathered bituminous coals do not display negative alteration because the potential for oxidation quenching has been reduced. The positive component of alteration is thought to be due to the creation of additional absorption centers and increased molecular rigidity as a result of photochemical oxidation and the enhancement of cross-linking among mobile phase and network molecules.

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.

Optical properties of coals and liquefaction residues as indicators of reactivity

Abstract The trend of vitrinite fluorescence intensity within the bituminous rank range, peaking in high volatile A coals, is thought to be due mainly to the development of the mobile or molecular phase with thermal maturation. A strong parallelism exists between fluorescence intensity, solvent yield and the thermoplastic behaviour of coals and hydrogenated coals. Determination of the reflectance distribution of hydrogenated coals can provide a measure of competing hydrogenation and retrogressive reactions in catalysed and non-catalysed runs, conducted both dry and with a solvent vehicle. The type of vitroplast generated and the anisotropy of the unextracted products give further insight into the size and mobility of the molecules generated by hydroprocessing. The relative reactivities of vitrinite and sporinite were reversed at different rank levels. The alteration of fluorescence intensity with time is another rank-dependent property; the phenomenon is seen clearly to be due to photochemical oxidation on irradiation. Highly weathered bituminous coals respond in a unique manner, similar to those of low rank coals which lack the negative alteration component due to the presence of the mobile phase. This may be due to the creation of absorption centres and enhanced molecular rigidity through cross-linking reactions.

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.

Dry catalytic liquefaction of a subbituminous coal: structural inferences☆

Abstract The catalytic liquefaction of a subbituminous coal has been studied in the absence of solvent using an impregnated sulphided Mo catalyst. Reactions were conducted for various times at temperatures in the range 300–400 °C. The yield and composition of gaseous and chloroform-soluble products and insoluble residue were followed as a function of the reaction conditions, using a number of techniques. The activity of the catalyst in promoting the production of liquids is dependent upon temperature. The main function of the catalyst may be to dissociate molecular hydrogen rather than directly participate in bond-cleavage reactions. At 300 °C, the catalyst had little effect on liquids production although it appears to cause some reduction in cross-link density due to the cleavage and stabilization of weaker bonds. At higher temperatures, the presence of the catalyst is necessary to obtain high liquid yields. The liquids first produced (up to ≈ 10%) are highly aliphatic and appear to be held within the coal structure by weak bonding or physical entrapment. At higher yields, the liquids are more aromatic and derive substantially from the breakdown of the coal network structure. Sharp changes in composition on going from low to high yield provide a basis for estimating the mobile phase content of the coal (10–20%). Associated changes in the structure and composition of the chloroform-insoluble residue have been further investigated by petrographic analysis and by measurements of swelling in pyridine. The high-swelling ratios measured for the parent coal and for the residues at low liquid conversions ( 20%) are consistent with the elimination of functional groups and an increase in cross-link density caused by condensation reactions.