Behaviour of tetrahydroquinoline in temperature-staged liquefaction of subbituminous coal

Tetrahydroquinoline (THQ) is well known as a solvent and hydrogen donor in coal liquefaction. The separate roles of dissolution and hydrogen donation have been demonstrated by comparing the liquefaction behaviour of Deitz (Montana, USA) subbituminous coal in naphthalene - THQ mixtures as the solvent with two impregnated hydrogenation catalysts, sulphided ammonium molybdate and a novel catalyst in which THQ is incorporated. The addition of THQ to the solvent results in increased conversion and increased oil to asphaltene (O:A) ratios relative to otherwise similar experiments without THQ. The addition of THQ which is 'locked' into the catalyst provides no increase in total conversion, but gives a significant improvement in the O:A ratio. The increased conversion and O:A ratio with free THQ in the solvent is a result of the THQ attacking the macromolecular structure of the coal and solvating small fragments. THQ locked in the catalyst cannot participate in the attack and dissolution of the coal structure and thus provides no increase in conversion, but functions very effectively as a hydrogen donor on the catalyst surface, resulting in an improved O:A ratio. This work suggests that in liquefaction using impregnated catalysts, but in the absence of good hydrogen donor solvents (as, for example, in dry liquefaction), liquids formed by the breakdown of the coal structure must migrate to the catalyst particles, where they are then hydrogenated, effecting the conversion of asphaltenes to oils.