Mechanism of catalysed solvent-mediated coal liquefaction

This thesis describes a study of the control of the reaction chemistry of the coal dissolution stage of a two-stage coal liquefaction process through the use of catalysts to produce coal liquids more amenable to secondary upgrading, particular emphasis being placed on investigating the feasibility of using "spent" or "deactivated" catalysts from, for example, upgrading of coal-derived liquids, to promote solvent-mediated coal dissolution in an inert atmosphere, such systems having an appreciable impact on process economICS. The coal dissolution reaction was carried out using a tubing bomb micro-reactor, primarily, with British Point of Ayr coal in recycle solvent using a range of Mo based catalysts, both fresh and "spent", under a variety of reaction conditions. Initially the influence of the catalysts on single model solvents, such as tetralin, 9, I O-dihydroanthracene and phenanthrene, and binary and ternary blends of these model solvent was investigated in order to establish whether the catalysts had any effect on the solvent itself and/or on any interaction between them. This work demonstrated that the catalysts did indeed exert an effect on the reaction chemistry of the solvents. In all instances the catalysts were effective in promoting the dehydrogenation of the solvents and this reaction can be used as a base of ranking various catalysts. It has been found from the coal dissolution investigation that the coal dissolution under the reaction conditions used, is primarily a thermolytic process. The temperature for significant coal dissolution with a IS min reaction period was 3S0°C and the reaction was complete within 30 min at 42S°C. Increase in temperature above 3S0°C increased coal dissolution and evidence was obtained to indicate some merit in operating at a temperature as high as 4S0°C, but at short reaction time. The influence of the catalyst was to enhance the yield of low molecular weight materials in the dissolution products and in this way benefited the first stage of a two-stage coal liquefaction process. The catalytic activity was maintained at low concentration of Mo 0.01 wt% (daf coal). The rate of coal dissolution is highly coal rank dependent with the higher rank coals showing little propensity for dissolution. On the other hand, limited evidence was obtained indicating that the blending of coals, which are susceptible to dissolution, may be advantageous with some synergistic effects