Swelling pretreatment of coals for improved catalytic liquefaction

Abstract

Two coals, a Texas lignite and a Utah high volatile C bituminous, were used to examine the effects of solvent swelling pretreatment and catalyst impregnation on conversion behaviour at 275 °C, representative of the first, low-temperature stage in a temperature-staged liquefaction reaction. Iron(II) sulphate, iron pentacarbonyl, ammonium tetrathiomolybdate, and molybdenum hexacarbonyl were used as catalyst precursors. Without swelling pretreatment, impregnation of both coals increased conversion, mainly through increased yields of preasphaltenes. Methanol, tetrahydrofuran, tetrabutylammonium hydroxide, and pyridine were used as swelling agents. In the absence of catalyst, swelling the lignite before reaction improves conversion by enhancing oil and gas yields; the effectiveness of the solvents in enhancing conversion is in the same order as their swelling ratios. Swelling with methanol or pyridine has little effect on reaction of the bituminous coal, but both tetrahydrofuran and tetrabutylammonium hydroxide treatments increase conversion as a result of higher preasphaltene yields. The combined effect of catalyst addition and swelling enhances conversion, as much as two-fold, of the lignite and increases yields of all products. On the other hand, little benefit was obtained by combining catalyst addition and swelling for the bituminous coal, though it is possible to effect changes in the relative amounts of the various products. Tetrabutylammonium hydroxide not removed from the coal after pretreatment appears to decompose to the good solvent tributylamine, suggesting the possibility of using swelling agents as ‘solvent precursors’, first swelling the coal and then thermally decomposing to a strong solvent that is emplaced inside the coal. The action of iron pentacarbonyl is sensitive to the reactive gas atmosphere used; in hydrogen it decomposes to an oxide that mainly facilitates hydrogenation of the heavy products to lighter oils, whereas in hydrogen sulphide/hydrogen mixtures the iron pentacarbonyl is sulphided and mainly facilitates depolymerization of the coal to heavy products. This finding suggests the possibility of tailoring the behaviour of the catalyst by appropriate selection of catalyst precursor and reactive atmosphere.