Density Functional Theory Study On The MoS₂-based Active Phase Structures And Hydrodesulfurization Reaction Mechanisms

Under the current situation,the strict environmental legislation and the increasing demands for ultralow sulfur transport fuels provide the direct propellant forces for optimizing the catalytic hydrodesulfurization?HDS?processes.The great challenge for ultra deep removal of sulfur is the decomposition of the highly refractory aromatic sulfides such as dibenzothiophene?DBT?and 4,6-dimethyldibenzothiophene?4,6-DMDBT?.However,the conventional Co-promoted or Ni-promoted Mo S₂/Al₂O₃catalysts exhibit lower HDS efficiencies in the removal of these macromolecular S-compounds.In order to improve the performances of the Mo-based catalysts,an atomic-scale understanding of the nature of the active sites involved in different HDS reactions is significant for developing special functional catalysts with high activity.In this thesis,the HDS mechanisms over Mo-based catalysts were studied in detail through density functional theory?DFT?calculations.The structure-activity relationship was analyzed based on the theoretical and experimental researches.Firstly,the formation mechanism of sulfur vacancies in the Corner sites of Mo S₂ catalyst system under the reaction conditions was studied by using DFT calculation.Secondly,the DDS mechanisms of DBT and 4,6-DMDBT catalyzed by the Corner sites of NiMo S and Co Mo S active phases were studied in comparison.Thirdly,the HDS mechanisms of S-containing compounds with different molecular sizes catalyzed by Brim and CUS active sites of Co Mo S were investigated.Finally,based on the DFT calculations,a novel HDS catalyst NiMo/?-SBA-16 was designed and applied into the evaluation of HDS activity of DBT.The main research contributions and conclusions are as follows:?1?The formation mechanisms of sulfur vacancies in the Corner sites of Mo S₂,NiMo S and Co Mo S active phases under real HDS conditions were studied.The results showed that the formation of one sulfur vacancy in pure Mo S₂ phase followed the order:Corner vacancy?S-edge?<Edge vacancy?S-edge?<Corner vacancy?M-edge?<Edge vacancy?M-edge?.The NiMo S active phase also followed the order of Corner vacancy?S-edge?<Edge vacancy?S-edge?.On the Co Mo S active phase,the formation of one sulfur vacancy followed the order:Corner vacancy?M-edge?<Edge vacancy?S-edge?<Edge vacancy?M-edge?<Corner vacancy?S-edge?.For NiMo S active phase,two vacancies could be formed at low H₂S pressure(10^-5 MPa)while they were relatively difficult over pure Mo S₂ and Co Mo S active phases.?2?The direct desulfurization?DDS?mechanisms of DBT and 4,6-DMDBT at different Corner sites of NiMo S and Co Mo S active phases were studied.The results confirmed that the presence of one Corner vacancy could proceed DDS reaction of DBT,while two Corner vacancies favored to the adsorption and the scission of C-S bonds of4,6-DMDBT collectively.However,according to DFT calculations,the formation energy of the two vacancies was relatively high,and the amount of vacancies was relatively low under the HDS reaction condition,leading to the low DDS activity of4,6-DMDBT.Compared to Co Mo S,it could also be found that the Corner vacancy of NiMo S showed more superiorities in the C-S bond cleavages of DBT and 4,6-DMDBT due to the low formation energy of 17.99 kcal/mol over the NiMo S active phase.?3?The HDS mechanisms of thiophene,BT and DBT with different molecule sizes on the Brim and CUS sites were investigated using density functional theory calculations.Based on the systematic calculations,the dominant reaction pathways of different molecules on different active sites were clearly discussed.By comparison,it could be seen that thiophene HDS took place on the Brim site without the extra creation of traditional sulfur vacancies.In the BT HDS mechanism,HYD pathway initiated by BT?-mode adsorption on the Brim site was more beneficial for the occurrence of the HDS reaction.Different from the thiophene and BT HDS mechanisms,both Brim and CUS sites were considered to participate in the DBT HDS process.The former provided the hydrogenation active sites and the latter was beneficial for the scission of C-S bonds.The relative research strongly supported the two-site model by proving the synergistic contribution of Brim and CUS sites for the HDS reactions from the theoretical calculation.?4?The strength of B acid produced by the substitution of Al atom in the channel of?zeolite was studied.Based on DFT calculation results,a new composite material?-SBA-16 was synthesized and applied into the development of HDS catalyst.The results of activity evaluation showed that the HDS efficiency of DBT was the highest?97.3%?for NiMo/?-SBA-16 catalyst at WHSV of 20 h^-1.In addition,the HDS efficiency of NiMo/?-SBA-16 at 150 h^-1 was 1.5 times than those of the other two catalysts,which was closely related to the acidity of the support.Accordingly,the Py-FTIR spectra exhibited that NiMo/?-SBA-16 had the highest total acid amount and higher B/L acid ratio.It could be deduced that the introduction of the?zeolite precursor promoted the formation of B acid sites in SBA-16 materials on the base of theoretical and experimental results.