Synthesis Of NiMoS Catalysts And Its Hydrodesulfurization Performance

According to International Energy Outlook published by the International Energy Agency（2019）,70%of global energy demand in 2050 will come from fossil energy.While China’s energy structure is short of oil,less in gas and relatively rich in coal,China can obtain the liquid products from the thermal conversion of coal by hydrogenation to balance the shortage of crude oil to a certain extent.The presence of more sulfur-containing organic compounds in the liquids obtained from direct coal liquefaction hinders the direct use of coal-based liquids for saving the ecological environment and even human health.Therefore,the hydrotreatment of coal-based liquids is inevitable to remove the sulfur-containing organic compounds before their practical applications.The main factors influencing the degree of desulfurization are the preparation of highly efficient sulfide catalysts,optimization of hydrodesulfurization reaction conditions,and competition from other hydrogenation reactions.The conventional preparation of sulfide catalyst not only requires energy and hydrogen consumption,but also needs H₂S which is toxic and corrosive.The resultant Ni（Co）Mo（W）type I active phase exhibits inadequate HDS activity for dibenzothiophene（DBT）and 4,6-dimethyldibenzothiophene（4,6-DMDBT）,which makes it difficult to attain the stringent national fuel quality standards（<10 ppm S）.In this dissertation,Ni Mo S/SBA-15 catalysts were prepared by hydrothermal synthesis using SBA-15 as the catalyst carrier,which enhances the formation of efficient Ni Mo S type II active phase showing a weak metal-support interaction（MSI）.The HDS efficiency of the prepared catalysts was studied using DBT and 4,6-DMDBT as model compounds at 320 ^oC and3-6 MPa.The main results and conclusions obtained are as follows.1.Catalyst design and reparation.The hydrothermal synthesis method not only avoids an additional sulfidation step,but also promotes the formation of the better dispersed Ni Mo S active phase with a shorter slab length and more exposed edge active sites.In addition,the formation of inactive nickel sulfide can be inhibited by delaying the contact time between Ni and S source by taking advantage of the chelating effects associated with nickel acetylacetonate.This advantageous hydrodesulfurization catalyst Ni Mo S-S0.3/SBA-15 was prepared at 200 ^oC（10hours）.2.The metal loading method affected the structure of catalyst active phase.Prior to the hydrothermal treatment,the metal precursors of Ni and Mo were loaded onto SBA-15 by two different methods such as equal volume impregnation and physical mixing.The Ni Mo S-S0.3-One Step catalysts produced by physically mixing the metal precursors with SBA-15 show the excellent properties in terms of the number of edge active sites,degree of sulfidation,dispersion and slab length compared to the equal volume impregnation method.3.The number of edge active sites exposed in the active phase of the catalyst and the degree of sulfation influence the HDS performance.Catalysts with more exposed edge sites can be prepared by optimizing the Ni/（Ni+Mo）ratio.The catalyst active phase exposed the highest number of edge active sites when Ni/（Ni+Mo）was 0.3;At a fixed Ni/（Ni+Mo）ratio（0.3）,the increase in sulfation degree from 35.11%to 81.17%increases the HDS activity(k _HDS)of Ni Mo S-0.3/SBA-15 from 0.28 m³/kg_cats to 0.34 m³/kg_cats.4.The reaction conditions influence the HDS reaction path.Although the HDS activity increased with increasing hydrogen initial pressure from 3 to 6 MPa,the selectivity of DDS/HYD showed a decreasing trend.This is because the increasing H₂ concentration in the solvent with pressure is more favorable for the HYD pathway.5.Carrier acidity influences the HDS.It was found that the Br?nsted acid site can catalyze the isomerization of 4,6-DMDBT to produce 3,7-DMDBT,which eliminates the steric hindrance around sulfur,leading to a lower activation energy of the desulfurization reaction,and therefore,it can be easily removed.Based on this,the bimetallic sulfide catalyst Ni Mo S-x/y containing Br?nsted acid sites was prepared by doping HZSM-5 in SBA-15.When the doping amount of HZSM-5 was 10%,the catalyst Ni Mo S-1/9 exhibited high HDS activity with98.6%removal of 4,6-DMDBT and k _HDS of 0.72 m³/kg_cats.