Sulfidation mechanism and hydrogenation activity regulation of iron oxides sulfurized with elemental sulfur for coal liquefaction catalysts

Abstract

The key to coal hydroliquefaction under mild conditions is the realization of the “catalytic cycle of hydrogen donor solvents (CCHDS)” pathway, which couples solvent hydrogen donation with catalytic solvent hydro-regeneration; here, the catalyst plays a crucial role, and its ability to restore the hydrogen-donating capacity of the spent solvent rapidly is the critical determinant of liquefaction efficiency. In this study, elemental sulfur was employed as the sulfiding agent. Using synthesized γ-Fe₂O₃ as the iron precursor, a series of iron–sulfur catalysts was prepared in a slurry-bed reactor by adjusting the (S/Fe)_atom and sulfidation temperature. The catalytic activity of catalysts obtained under different conditions was evaluated using naphthalene hydrogenation as a probe reaction, and multiple characterization techniques were adopted to systematically analyze the properties of the as-prepared catalysts, with the aim of elucidating correlations between catalyst phase composition, pore structure, acidity, and hydrogenation performance. The results indicated that an excessively high (S/Fe)_atom ratio (>2.5) induces cracking of the liquid medium used during catalyst preparation, generating carbon-containing free radicals and, subsequently, carbon deposition. When the temperature is below 300 °C, the primary adverse effect is carbon deposition on the catalyst surface caused by unreacted-sulfur-radical-assisted formation of carbon-containing free radicals; in contrast, at temperatures above 300 °C, the dominant negative impact is the transformation and loss of active phase components. The hydrogenation activity of the catalysts is closely correlated with the type and content of active phase components, as well as the extent of carbon deposition, considering that different active phase components exhibit significant differences in catalytic activity. Furthermore, it was found that the sulfidation process follows a “sulfur-radical-driven, intermediate synchronous reaction, and autocatalytic formation of Fe_1−xS” mechanism.