Kinetics of sulfur model molecules competing with H2S as a tool for evaluating the HDS activities of commercial CoMo/Al2O3 catalysts

The relative-volume activities (RVAs) for real feedstocks HDS of four commercial CoMo/Al2O3 catalysts have been compared to the rates for thiophene and dibenzothiophene conversion. The reaction of thiophene competing with H2S was studied in flow microreactors under a wide range of conditions: 300-400 degrees C, overall pressure 0.1 or 3 MPa, thiophene pressure 8-125 kPa, H2S content 0-15 mol%. The reaction of dibenzothiophene (DBT, 2 wt% in decaline) was carried out in a batch reactor at 335 degrees C and 4 MPa. The conversion of the two model molecules proceeds through the same mechanism with a preliminary dearomatization step followed by parallel hydrogenolysis and hydrogenation. From kinetic modeling, the global rates and the contribution of the hydrogenation and hydrogenolysis routes to HDS were determined. Under pressure, hydrogenolysis was predominant. In that case, thiophene and DBT behaved similarly and their initial relative rates did not correlate the RVA. Industrial HDS is controlled by hydrogenation as evidenced by the good correlation between RVA and the rates of dearomatization of thiophene at atmospheric pressure and hydrogenation of the product biphenyl from DBT under pressure. It is concluded that the reaction of model molecules under selected conditions can appraise rapidly industrial HDS. (C) 1998 Elsevier Science B.V. All rights reserved.