Experimental And Theoretical Studies Of Mechanism Of Hydrodesulfurization Of Model Compounds

Environmental regulations have been tightened in many countries to reduce the sulfur contents of transportation fuels to ultra-low level,and it has been a great challenge for world's oil refining industry to produce the low sulfur or ultra low sulfur oil.Nowadays,hydrodesulfurization,selective adsorption desulfurization,as well as reactive adsorption desulfurization,are the major desulfurization technologies in industry,but hydrotreating technology is still the most effective method in large-scale for the low-sulfur diesel production,while as the traditional hydrotreating catalyst cannot meet the strict requirement of oil deep hydrodesulfurization.Therefore it's important to develop novel catalysis materials and catalysts to realize the deep hydrodesulfurization?HDS?of diesel for clean diesel production.However,its mechanism remains unclarified,and the existing limitations and difficulties in revealing HDS mechanism experimentally is still a challenging task,which affects the developments of novel HDS catalysts.In this thesis,we focused on the HDS process of thiophene and benzothiophene at different edges of MoS₂.We combined the calculations of quantum chemistry based on density functional theory?DFT?and hydrodesulfurization experiments,and first attempted to give the HDS mechanism of thiophene and benzothiophene on the surface of MoS₂.We hope these works can provide theoretical guidance for the design and preparation of novel HDS catalysts.In this thesis,the mechanism of the sulfur vacancies formation at different MoS₂ edges was firstly studied.It cantains the scission and adsorption of H₂,the transfer of H atom,the formation of H₂ S and its desorption.And at S-edge the formation of a second S vacancy has also been studied because of its high S coverage.The results showed that at Mo-edge it was more beneficial for the formation of one Mo-H bond and one S-H bond than two S-H bonds after the H-H bond scission.Different with the results at Mo-edge,there was no frequency of Mo-H bond at S-edge after the calculation of imaginary frequency.Compared with the two processes at different edges,it was found that the formation of S vacancies at Mo-edge was much easier than that at S-edge,where reaction barriers were much lower.Through the results of the DFT calculations,it was proved that the formation of S vacancies at S-edge was also been favored of thermodynamics and dynamics,hence which could be contained in the processes of HDS reactions.Moreover,the HDS mechanism at S vacancy of different MoS₂ edges was further studied.Thiophene and benzothiophene were chosen as the model compounds and the S vacancy as the initial active site to study the HDS mechanism.The results showed that the S vacancy at Mo edge was more beneficial for the DDS pathway,and thiophene was more likely desulfurized by direct S-C bond scission at this active site.The S vacancy at S edge was also an active site for the HDS of thiophene molecule,especially for the HYD route.2,3-dihydrothiophene and 2,5-dihydrothiophene cannot be further hydrogenated to form the tetrahydrothiophene through HYD pathway at S edge.And 2,5-dihydrothiophene can only be formed through HYD pathway at S edge.1-butene can be formed through HYD and DDS pathway,but 2-butene can only be formed through HYD pathway.In the benzothiophene DDS route,it was found that the C-S bond near the benzene ring was easier to be broken.In order to verify the DFT calculation,a series of Beta-SBA-16 micro-mesoporous materials and its HDS catalysts were synthesized,and were used in the evaluation of benzothiophene HDS reaction.Combined with the product analysis and the results of activity evaluation,we can speculate the HDS mechanism,considering the DFT calculation,these results corroborated with each other.Firstly,the synthesis conditions of BS series materials were optimized,and the effects of different Si/Al ratio?30,40,50 and 60?and crystallization time?24h,36 h,48h and 60h?on the physico-chemical properties of materials were investigated.The synthesized materials were characterized by XRD,IR,SEM and TEM.The characterization results showed that the synthesized material had typical SBA-16 structure.The evaluation results showed that when the Si/Al ratio was 50 and the crystallization time was 36 h,the synthesized catalysts had the highest activity.The highest efficiency of BS catalysts is 99.2% for HDS.The HDS results of benzothiophene showed that the DDS route was the main reaction path,which was well proved by DFT calculation.Therefore,the combination of experiments and calculations provided us a new insight for better understanding of the HDS mechanism of the model componds on MoS₂,which was the basis of developing more novel or efficient HDS catalysts.