HYDRODESULFURIZATION OF THIOPHENE OVER MILDLY DEACTIVATED COAL-LIQUIDHYDROTREATMENT CATALYST - EFFECT OF TEMPERATURE

The hydrodesulfurization (HDS) of thiophene was carried out over mildl y deactivated NiMo/alumina catalysts used for the hydrotreatment of co al-derived liquids. Arrhenius plots were obtained covering a wide temp erature range, 180-540-degrees-C. The observed rate constant is found to level off at the highest temperatures. This phenomenon cannot be ex plained by further catalyst deactivation, catalyst desulfiding, extern al mass transfer control, formation of an inert external shell or ther modynamic equilibrium for the HDS of thiophene. A modified two-step re action mechanism is suggested to explain these results. Step 2 has a h igher activation energy and controls the reaction rate at lower temper atures. Step 1 has a low activation energy and controls the rate at th e highest temperatures. The proposed mechanism successfully predicts t he experimental observations. Increasing deactivation affects the effe ctive diffusivity to a greater extent than it does the intrinsic react ion rate constants.