Evolution behavior of active phase of oil-soluble iron catalysts during direct liquefaction of Shangwan coal

Abstract Oil-soluble catalyst is a promising alternative to non-oil-soluble catalyst due to excellent dispensability in coal-oil slurry and relatively high activity. To reveal evolution behavior of active phase of the oil-soluble Fe-based catalysts in direct liquefaction of Shangwan coal, three oil-soluble Fe-based catalysts (FeLA, FeOA, FeSA) were synthesized via saponification method with linoleic acid, oleic acid and stearic acid as organic ligands, and were compared with two traditional iron oxide catalysts (Fe 2 O 3 , FeOOH). The evolution of active compositions during different temperatures was studied by XRD, SEM, XPS and HRTEM. Results demonstrated superior stability and catalytic activity of oil-soluble catalyst during direct liquefaction. TG and FTIR analyses confirmed the decomposition at lower temperatures, and formed active phases more readily than Fe 2 O 3 and FeOOH catalysts. The oil yields were significantly improved up to 62.82 wt%, 64.38 wt% and 66.36 wt% for FeLA, FeOA and FeSA, which were higher than those of Fe 2 O 3 and FeOOH catalysts. FeOA and FeLA catalysts particularly excelled in promoting the formation of cycloalkanes, thereby improving oil quality. The characterizations by XRD and XPS analyses of the samples indicated that these oil-soluble catalysts rapidly form Fe 0.875 S at lower temperatures, avoiding intermediate oxide phases. Smaller crystallite sizes were produced for oil-soluble catalysts compared than Fe 2 O 3 and FeOOH. Especially, substantial amounts of Fe 0.875 S were generated for FeSA during initial coal pyrolysis, which activated hydrogen to stabilize coal macromolecular radicals and enhanced coal conversion and yield. This work provides a guide for the development of highly efficient catalysts in direct coal liquefaction.