Swelling Pretreatment of Coals for Improved Catalytic Temperature-Staged Liquefaction

Two coals, a Texas subbituminous C and a Utah high-volatile A bituminous, were used to examine the effects of solvent swelling and catalyst impregnation on liquefaction conversion behavior in temperature-staged reactions for 30 min each at 275 and 425 °C in H₂ and 95:5 H₂:H₂S atmospheres. Methanol, pyridine, tetrahydrofuran, and tetrabutylammonium hydroxide were used as swelling agents. Molybdenum-based catalyst precursors were ammonium tetrathiomolybdate, molybdenum trisulfide, molybdenum hexacarbonyl, and bis(tricarbonylcyclopentadienylmolybdenum). Ferrous sulfate and bis(dicarbonylcyclopentadienyliron) served as iron-based catalyst precursors. In addition, ion exchange was used for loading iron onto the subbituminous coal. For most experiments, liquefaction in H₂–H₂S was superior to that in H₂, regardless of the catalyst precursor. The benefit of the H₂S was greater for the subbituminous, presumably because of its higher iron content relative to the hvAb coal. Tetrabutylammonium hydroxide was the only swelling agent to enhance conversion of the hvAb coal significantly; it also caused a remarkable increase in conversion of the subbituminous coal. The combined application of solvent swelling and catalyst impregnation also improves liquefaction, mainly through increased oil yields from the hvAb coal and increased asphaltenes from the subbituminous. A remarkable effect from use of ammonium tetrathiomolybdate as a catalyst precursor is substantial increase in pristane and phytane yields. Our findings suggest that these compounds are, at least in part, bound to the coal matrix.