Mechanistic Insight Into The Thiophene Hydrodesulfurization On Molybdenum Sulfide Catalysts

Crude oil with sulfur-containing compounds is harmful to the environment and industrial reproduction.Hydrodesulphurization?HDS?technology is an effective approach to remove sulfur atoms from sulfur-containing compounds in crude oil and to produce clean energy.Nowadays,molybdenum sulfides have been widely applied in the industrial HDS processes due to its high catalytic activity.However,the knowledge about the HDS mechanisms on molybdenum sulfide catalyst is still inadequate,especially for the competitions of the hydrogenation and desulfurization elementary reactions,the effect of active sites on reaction routes and product selectivities,and the promotion effects caused by catalyst modifications.Thus,it is necessary to investigate the essencial relationship between molybdenum sulfide catalysts and HDS reaction mechanisms.The related theoretical calculations can deepen the understanding of the microscopic HDS reaction mechanism,and provide new insight into the design and screening of the efficient HDS catalysts.In this thesis,we constructed three different types of molybdenum sulfide catalysts based on previous experimental and theoretical studies,and thiophene was selected as the research object since it is the simple cyclic sulfur-containing compound.We focused on the HDS mechanism on different active sites by density functional theory?DFT?calculations combining with the analysis methods,such as adsorption geometric structure,electronic structure characteristics,thermomechanical analysis,and reaction kinetics and thermodynamics.The overall research can be divided into three parts.The first part investigated the competition mechanism between initial hydrogenation and direct desulfurization on perfect MoS₂?100?surface.The second part focused on the formation competition of the coordinatively unsaturated active site?CUS?.The last part part investigated the role of Co atoms in the promoted molybdenum sulfide catalyst.Our results can be shown as follows:?1?On perfect MoS₂?100?surface,the“flat”adsorption of thiophene is more stable than the“vertical”adsorption.After the“flat”adsorption,the thiophene C^??S bonds are elongated,which is beneficial for the S removing.For the two possible direct desulfurization reactions,the elementary steps of the first C?S bond scission are the rate-controlling steps.During the initial hydrogenation reactions,C^?atoms are easier to bond to H.By contrast,due to the lower energy barrier,direct desulfurization is more favorable among the HDS initial reactions on perfect MoS₂?100?surface.?2?On 50%S-coverage MoS₂ cluster,the formation of the corner CUS is much easier than that of edge CUS and the single CUSs may restrain the formation of other vacancies.We can reduce the CUS formation energies through raising the H₂ partial pressure and temperature.Due to the smaller steric hindrance,the“flat”adsorption is more stable on the corner CUS,while the“vertical”adsorption is more favorable on the edge CUS.During the HDS reaction on the corner CUS,the HYD route is more competitive than the DDS route.The desulfurization product of HYD is 2-butene,but the sulfur-containing impurity tetrahydrothiophene may generate at the same time.On the edge CUS,both HYD and DDS routes are possible with the product of butane and butadiene respectively.The DDS route is more competitive because of the lowest energy barrier of the rate-controlling step,indicating the edge CUS is a great HDS active site.?3?Co-doped MoS₂ cluster is beneficial for thiophene adsorption,because the d_z2 orbital contribution of the active site is greater and the steric hindrance is smaller.There is no“flat”adsorption on both corner and edge Co-doped site.In the most stable adsorption configurations,the C² and C³ bond to Co,and the S bond to Mo.The most stable adsorption energy of thiophene on the edge Co-doped MoS₂ cluster reaches up to-2.12 eV.On the corner Co-doped site,HYD is the only possible route and the2-butene is the main desulfurization product.The introduction of Co atom lower the energy barrier of the rate-controlling step,meaning the higher HDS efficiency.On the edge Co-doped site,there is also HYD route with the product of 2-butene.Although the rate-controlling step barrier has small change compared with that on the edge CUS,the significantly boosted thiophene adsorption energy is beneficial for HDS process.