Synthesis of Meso-microporous Y Zeolite and Its Application in Diesel Ultra-deep Hydrodesulfurization

Author:Zhou Wen Wu

Supervisor:zhou ya song wei qiang


Degree Year:2018





Design and preparation of hydrotreating catalysts with excellent catalytic performances is the key for ultra-deep hydrodesulfurization(HDS)of inferior diesels.The performances of the HDS catalysts are always affected by the composition and properties of the raw materials,the physicochemical properties of the HDS catalysts and the hydrotreating conditions.Among all these qualifications,the physicochemical properties of HDS catalyst are the most important,and the physicochemical properties of the HDS catalyst are always closely related to the physicochemical properties of the support materials.Thus it is crutial to design and synthesis support materials for HDS catalysts with excellent performances,which can build the foundations for designing and preparing HDS catalysts with high HDS activity for inferior diesels.This dissertation is based on the technical difficulties of inferior diesels ultra-deep HDS over Al2O3 supported hydrotreating catalysts.After that,the kinetic behaviours of highly refractory sulfur containing compounds,DBT and 4,6-DMDBT,HDS over NiMo/MA-S catalyst were studied,then the fundamental reasons for the difficulty in the HDS of highly refractory organosulfur compounds were uncovered.Moreover,the most efficient technical route to solve this problem was put forward and the proper support materials for the corresponding inferior diesel HDS catalysts were developed,based on which,the methodology of designing and preparing catalysts suitable for inferior diesel ultra-deep HDS was proposed.Finally,catalyst with high HDS performances for inferior diesels was successfully prepared.It is disclosed that the organosulfur compounds which are difficult to remove in inferior diesels exhibit the following two structural characteristics:the one is they always have high unsaturated numbers,the other is there are always substituted alkyl groups on the beta C atoms.Due to their bulky molecules,Al2O3 with narrow dispersed wide-open mesopores was synthesized to eliminate the internal diffusion limitations,based on which,the corresponding NiMo supported catalysts was prepared and the kinetic behaviors of both DBT and 4,6-DMDBT HDS reactions over the corresponding catalyst were studied.The results show that DDS pathway is the main pathway for DBT HDS over NiMo/MA-S catalyst while HYD pathway is the main pathway for4,6-DMDBT HDS over the same catalyst;the rate constant of HYD pathways for DBT and 4,6-DMDBT HDS are similar but the rate constant of DDS pathway for4,6-DMDBT is much smaller than that of DBT,moreover,the cleverage of C-S bond of the hydrogenation intermediantes of 4,6-DMDBT is much harder than that of DBT and its hydrogenation intermediantes;the rate-determining step of 4,6-DMDBT HDS is the cleverage of C-S bond.Thus to consolidate the hydrogenolysis activity of the HDS catalyst is the key to realize ultra-deep HDS of inferior diesels.In this work,Y zeolites with both high Si/Al ratio and small crystallite size were successfully synthesized,the synthesized Y zeolites was used as a co-support for HDS catalyst to enhance its hydrogenolysis activity.The result show that the HDS performances enhanced a lot over the corresponding Y zeolite containing catalyst,and the kinetic study show that the great increase in the catalytic activity is mainly own to the DDS pathway.4,6-DMDBT can transfer through four different pathways over the corresponding catalyst:DDS pathway and HYD pathway,which were the same as those over the Al2O3 supported catalysts;the isomerization pathway(ISO pathway),in which4,6-DMDBT firstly isomerized into 3,6-DMDBT and 3,7-DMDBT,which are further transferred mainly through the DDS pathway;the transmethylation pathway,in which even more refractory organosulfur compounds,trimethyldibenzothiophene and tetramethyldibenzothiophene,would be produced;the ISO pathway is the main pathway for 4,6-DMDBT HDS over the zeolite containing catalysts.More importantly,we also found that the selectivity of ISO pathway is closely related to the anount of Br?nsted acid sites and the selectivity of TRM pathway is closely related to the anount of Br?nsted acid sites.To avoid the TRM pathway,modification of Y zeolites with Ga and P were also studied,the results show that modification of Y zeolite with P can promote the HDS of almost all kinds of organosulfur compound except for 4,6-DMDBT;modification of Y zeolite with Ga can promote the HDS of all kinds of organosulfur compounds including 4,6-DMDBT,and the synergistic effect was also observed between these two modification methods.The intraparticle diffusion of both DBT and 4,6-DMDBT were observed over Y zeolite crystals during the HDS reaction,to solve this problem,meso-microporous Y zeolite with narrow dispersed mesopore was successfully synthesized through fine design of SiO2@CTAB mesostructured micelle and the pore diameter of mesopores can be adjusted between 4-10 nm.The adsorption results of DBT and 4,6-DMDBT over different meso-microporous Y zeolite show that the diffusion limitation of DBT disappeared when the pore diameter of mesopores reached 4 nm and the diffusion limitation of 4,6-DMDBT disappeared when the pore diameter of mesopores reached 6nm.The meso-microporous Y zeolites with different Ga/Al ratio were also synthesized to investigate the synergistic effect between the mesopore structure and the surface physicochemical properties of Y zeolites in 4,6-DMDBT HDS reactions.The results show that on the one hand,modification of Y zeolites can modulate the physicochemical properties of Y zeolite to prevent the TRM pathway of 4,6-DMDBT HDS,on the other hand,incorporation of mesopores can eliminate the introparticle diffusion effect of the refractory organosulfur compounds.Moreover,4,6-DMDBT can transferred into 4-MDBT and DBT through direct demethylation pathway(DDM pathway)over the corresponding catalysts,which can benefit the ultra-deep HDS of inferior diesels a lot.Based on the fundamentals of chemical transformation of 4,6-DMDBT HDS over the related catalysts,the gallium containing meso-microporous Y zeolites and boehmite with mass ratio of 1:9 was mixed and extruded to form a three-leaf shape with diameter of 1.5 mm,and the corresponding HDS catalysts was prepared by an incipient-wetness impregnation method,then the catalytic performances of the catalysts were evaluated.The HDS results of the inferior mixed diesel show that diesel fractions with sulfur content less than 10μg?g-1 can be produced under moderate reaction conditions(360oC,5 MPa,2.0 h-1)over all the NiMo/MY-xGa-A series catalysts.Among the NiMo/MY-xGa-A series catalysts,diesel fractions with sulfur content less than 10μg?g-1 can be produced under even more moderate reaction conditions(350oC,5 MPa,2.0 h-1)over catalysts NiMo/MY-0.5Ga-A,NiMo/MY-1Ga-A and NiMo/MY-2Ga-A.