Hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene over NiMo and CoMo sulfide catalysts: kinetic modeling approach for estimating selectivity.

The hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) over two synthesized unsupported CoMo (CMS) and NiMo (NMS) sulfide catalysts was investigated in a batch reactor. The HDS reactions proceeded through two parallel-consecutive reaction pathways: direct desulfurization (DDS) and hydrogenation (HYD), in which two main intermediates-biphenyl or 3,3'-dimethylbiphenyl and partially hydrogenated DBT or 4,6-DMDBT-were involved. Kinetic models were developed to account for the partial contributions of these intermediates in the overall reaction network as well as for their selectivities. The models allow a precise estimation of the apparent rate constants of all steps in the reaction network. In the HDS reactions of 4,6-DMDBT, the HYD pathway was more pronounced than the DDS route over the NMS catalyst. Different selectivities in terms of yield fraction (percentage ratio of HYD/DDS) were observed for these two catalysts. The kinetic results according to these models show that the partially hydrogenated 4,6-DMDBT intermediate transformed mainly to 3,3'-dimethylcyclohexylbenzene. Besides, this intermediate is suggested to be partly transformed to 3,3'-dimethylbiphenyl. The rate of sulfur removal from the partially hydrogenated 4,6-DMDBT intermediates was an order of magnitude slower than that from the partially hydrogenated DBT. The NiMo sulfide catalyst was more active than the CoMo sulfide catalyst for the HDS of 4,6-DMDBT. The results are discussed in the context of proposed HDS reaction networks.