Synthesis And Property Modulation Of Y/SAPO Molecular Sieve And Its Application In Hydrocracking

Hydrocracking is a major process in refineries to produce high quality kerosene and diesel fuel,and it requires catalysts with both good cracking and isomerization activity.The commonly used cracking acidic component of the Y zeolite has the problems of long pore size and strong acidity,which easily leads to excessive cracking and low yield of kerosene and diesel;while the isomerized acidic component of the SAPO-11 molecular sieve has the problems of small pore size,weak acidity,low isomerization and high freezing point of kerosene and diesel.For this purpose,the dissertation proposes the preparation of small crystal Y zeolite with high silicon,the preparation of special morphology SAPO-11 molecular sieve with hierarchical pore structure and the acidity modulation thoughts.To improve the degree of isomerization and selectivity and yield of the middle distillate while maintaining the catalyst cracking activity.Based on this,the thesis focuses on the selective hydrocracking and isomerization reactions of large molecular long-chain alkanes,investigates the structure-activity relationship between the physicochemical properties of molecular sieve materials and catalyst performance,and develops design ideas and preparation methods for hydrocracking catalysts with high yield and high quality middle distillates.Y zeolites with crystal size of 515 mn,317 nm and 220 nm have been synthesized by the seeding method by varying the seed aging temperature.It was found that decreasing the ageing temperature of the seed can reduce the crystal size of Y zeolites effectively.The results of n-hexadecane hydrocracking showed that reducing the crystal size of Y zeolites was beneficial to improve the selectivity and yield of middle fraction products(C₈-C₁₂)of the catalysis.Shortening the residence time of reactants in the Y zeolite pore channel can improve the accessibility of reactants to the catalyst active center and avoid excessive cracking.The highest yield of C₈-C₁₂ product was obtained with the Ni W/（Y3+ASA）catalyst made from Y zeolite with a crystal size of 220 nm,which was44.0%.The dissertation further synthesized Y-x D series zeolites with crystal size of 210nm-160 nm by adding dodecyltrimethylammonium bromide（DTAB）in the preparation of the seed.These zeolites have larger pore volumes,pore sizes and Br（?）nsted acid ratios,and the series of Cat-x D catalysts made with them as the support acid components exhibit higher n-C₁₆ conversions and higher C₈-C₁₂ product yields.Cat-0.15D had the highest n-C₁₆ conversion and C₈-C₁₂ product yield at a reaction temperature of 360 ℃.The n-C₁₆conversion was 10.1 percentage points higher than the Cat-0D catalyst,and the C₈-C₁₂product yield was 54.1%.In order to improve the hydrothermal stability of Y zeolites,enhance the strength of Br（?）nsted acid sites and reduce the acid density,and inhibit the excessive cracking of the catalyst,the dissertation proposes the idea to synthesize the Y zeolites with high Si O₂/Al₂O₃ ratio assisted by H₂O₂.The Y-x H series of high-silica small crystal Y zeolites with crystal size of 158-118 nm and the n（Si O₂/Al₂O₃）ratio of 5.59-6.37 were successfully synthesized.The Cat-x H series catalysts made by using Y-x H as the support acidic components exhibited higher selectivity and yield of C₈-C₁₂ products.At the reaction temperature of 360 ℃,the Cat-0.3H catalyst made from Y-0.3H with an n（Si O₂/Al₂O₃）ratio of 6.17 had the highest C₈-C₁₂ product selectivity,which was 10.7percentage points higher than that of Cat-0H,and the C₈-C₁₂ product yield was 58.3%.The increase in the n（Si O₂/Al₂O₃）ratio leads to the decrease of the n-C16 conversion of the catalyst due to the weakening of the acidity.Therefore,in order to further enhance the number of Br（?）nsted acid sites in high-silica Y zeolite and to modulate the balance between the metal center and the acid center,the dissertation further proposed a new idea of in situ modification of Ni to prepare high-silica small crystal Ni Y zeolite for the purpose of pre-impregnating the Ni in Y zeolites.The synthesized Y-x Ni zeolites have a higher number of Br（?）nsted acid sites,and the Cat-x Ni series catalysts made with them as the support acid components exhibit a higher n-C₁₆ conversion and C₈-C₁₂ product yields due to the higher sulfation of the active metal and the higher dispersion of the Ni WS active phase.At the reaction temperature of 360 ℃,the catalyst Cat-0.2Ni had excellent synergistic effect between the acid center and the metal center,and the n-C₁₆ conversion was increased by 10.2 percentage points compared with Cat-0Ni,and the yield of C₈-C₁₂product was 65.4%.To reduce the SAPO-11 micropores while introducing mesopores to improve the diffusion efficiency of the catalyst and increase the yield of isomers.The dissertation proposes the idea of synthesizing micro-mesoporous and special morphology SAPO-11molecular sieves using CTAB and KF double addition hydrothermal method.A series of flower-like hierarchical pore structural SAPO-11-x B molecular sieves with stable lamellar structure was successfully synthesized.These molecular sieves exhibit fewer micropores and larger mesopores,and the Cat-x B series catalysts made with them as the support acidic components enhance the diffusion rate of reactants and products and reduce the cracking probability of isomeric products.Under the same conditions,it exhibited higher iso-C₁₆product selectivity and yield than the conventional microporous spherical Cat-0B catalyst.The iso-C₁₆ selectivity of the flower-like catalyst Cat-0.2B was21.3 percentage points higher than that of the spherical Cat-0B catalyst at 360 ℃ reaction temperature,and the maximum iso-C₁₆ product yield was 59.4%.The Cat-0.2B catalyst had the highest selectivity of monobranched and dibranched iso-C₁₆ products with 69.3%,accounting for 90.0%of iso-C₁₆ selectivity.The reduction of acid sites in the flower-like SAPO-11 molecular sieve due to the reduction of micropores decreased the n-C₁₆conversion of the catalyst.To further enhance the number of Br（?）nsted acid sites in the flower-like SAPO-11 molecular sieve,enhancing the degree of matching between the metal center and the acid center.A series of SAPO-11-x Ni molecular sieves were synthesized by in situ Ni modification method for the purpose of pre-impregnating Ni into SAPO-11 molecular sieves.The series Cat-S-x Ni catalysts made by using it as the support acid components have higher number of Br（?）nsted acid sites and higher sulfuration degree of active metal and dispersion of active phase,which effectively modulate the balance between acid centers and metal centers and exhibit a higher conversion of n-C₁₆ and yield of iso-C₁₆.The catalyst Cat-S-0.2Ni had the highest iso-C₁₆yield of 71.9%at the reaction temperature of 360 ℃.The highest selectivity of 76.5%for monobranched and dibranched iso-C₁₆products was also obtained for catalyst Cat-S-0.2Ni,which accounted for 91.7%of the iso-C₁₆ selectivity.A middle distillate hydrocracking catalyst with good cracking activity and isomerization activity was prepared for producing high quality middle distillate.The idea of synthesizing Y-SAPO-11 composite molecular sieves and pre-impregnating the active metal Ni was proposed.Improve the matching of cracking and isomerization functions and the synergy between acid and metal centers to increase the proportion of isomeric products in middle distillate oils.The in situ composite x Y-SAPO-11 series composite molecular sieves were successfully synthesized by hydrothermal crystallization method.Compared with mechanical mixtures of the same mass fraction,the Y-SAPO-11composite molecular sieves have larger pore volume,pore size and lower acidity.The series of Cat-x Y-S catalysts made with them as the acidic components of the support exhibited higher C₈-C₁₂ product yields than the mechanical mixture.When the weight fraction of Y zeolite was 61.7 wt%,i.e.,the Cat-8Y-S catalyst had the highest iso-(C₈-C₁₂)product yield of 33.1%（360 ℃）while maintaining a high C₈-C₁₂ product yield due to the suitable cracking activity with isomerization activity.In order to further enhance the catalytic performance,x Ni-YS series molecular sieves were synthesized by in situ Ni modification of 8Y-SAPO-11 composite molecular sieves to achieve the purpose of pre-impregnating Ni into the molecular sieves.The Cat-x Ni-YS series catalysts made with them as the acid components of the support have higher sulfurization of the active metal and Ni WS active phase dispersion,more"type II"Ni WS active phases with high hydrogenation performance and show higher n-C₁₆ conversion and C₈-C₁₂ product yields.At the reaction temperature of 360 ℃,the catalyst Cat-0.3Ni-YS had a high yield of C₈-C₁₂ product of 58.5%.Meanwhile,it had the largest yield of iso-(C₈-C₁₂)product with43.4%,accounting for 74.2%of the C₈-C₁₂ product yield.