CATALYSIS IN DIRECT COAL-LIQUEFACTION BY SULFATED METAL-OXIDES

The physicochemical properties and catalytic activities of an intriguing class of compounds based on iron and tin oxides treated with varying amounts of sulfate anion and employed for the direct liquefaction of three Argonne coals of varying ranks are reported in this paper. The sulfated transition-metal oxides have become a topic of interest partly because of their unusual properties, one of these being their so called "superacidity". The physicochemical properties of the sulfated oxides before reaction as determined by BET, XRD, TGA/DTA, SEM/TEM, and the types of active phases formed under liquefaction conditions as determined by XRD, STEM, EXAFS, and Mossbauer spectroscopy are correlated with their apparent activities for hydrocracking of coal. The sulfated iron oxide, Fe2O3/SO4(2-) was found to be an effective catalyst for coal liquefaction when used in small concentrations (< 0.4 wt % iron); its use resulted in an 86 wt % (maf basis) conversion of Illinois No. 6 coal at 400-degrees-C and 1000 psig of hydrogen (initial) with more than 50 wt % of the products consisting of oils (n-pentane solubles). Addition of elemental sulfur to the same catalyst (at 0.35 wt % Fe) enhanced the overall conversion to 90.3 wt % with more than 60% of products consisting of oils. Similar results for coal conversion were obtained for a solid superacid catalyst made from tin, SnO2/SO4(2-), in the presence of sulfur. These conversions were considerably higher than those obtained in a thermal run under the same reaction conditions (% conversion = 62, wt % oils = 28). For both iron and tin oxides, their sulfated forms containing between 1.5 and 6 wt % of SO4(2-) groups were more active than their respective unsulfated forms. Significant hydrodenitrogenation (> 70%) and hydrodesulfurization (> 90%) were obtained with the sulfated metal oxide catalysts. Very small amounts of nitrogen (< 0.5 wt %) and sulfur (<0.28 wt %) were found in the methylene chloride soluble products obtained from the liquefaction runs. The effects of the sulfate group in these oxides are likely due to an increase in catalyst dispersion; their superacidity may play a part or some other mechanism (radical ions) may be involved. The sulfate group probably inhibits agglomeration of the metal oxide catalysts at high temperatures.