Multi-Dimensional Design And Structure-Performance Relationship Of Metal-Acid Bifunctional Catalysts For Hydrogenation Of Long-Chain Alkanes

There is a great significance in the two-stage hydrogenation route to produce jet fuel,using animal and vegetable oils as raw materials.The hydrocracking/isomerization of long-chain alkanes is the key step affecting the yield and quality of target products.In the reaction,the hydrogenation activity of long-chain alkane and the yield of jet fuel is determined by the properties of supports and the synergistic effect of bifunctional sites,in which the metal site catalyzes the dehydrogenation of alkanes and hydrogenation of alkenes while the acid site catalyzes the cracking and skeletal rearrangement of carbon-carbon bonds.In this paper,the metal-acid bifunctional catalysts with a single variable were designed and prepared from multiple dimensions to explore the effect of acid properties,diffusion performance,and intimacy on the hydrocracking/isomerization of n-C16.Based on the structure-activity relationship,the WHSS@Pt catalyst with weak acid,strong diffusion performance,minor-nanoscale intimacy,and yolk-shell structure was designed and prepared,which executed the efficient performance in the hydrocracking/isomerization of n-C16 to produce jet fuel.Pt/20 wt.%HPA/MCM-41 catalysts with acid strength as a single variable and Pt/X wt.%HPMo/MCM-41 catalysts with acid density as a single variable were prepared to investigate the effect of acid properties on the performance of n-C16hydrogenation.The number of strong B acid sites is linearly related to the selectivity of deep cracking products<C9,while the total B acid amount is positively related to the conversion of n-C16.The yield of jet fuel of 20 wt.%was obtained over Pt/40 wt.%HPMo/MCM-41 with more weak B acid sites.The main reaction network of n-C16hydrogenation was that n-C16 was first isomerized to single-branched isomer,then further isomerized to multi-branched isomer,and finally cracked to jet fuel.Based on the reaction mechanism and pathway,it is concluded that the catalyst with strong isomerization performance is conducive to the hydrocracking/isomerization of n-C16to produce jet fuel.Pt/HPMo/SBA-15 catalysts with pore size as a single variable were prepared to explore the effect of diffusion performance on the long-chain alkane isomerization.The chemisorption of alkane shows that there are obvious differences in diffusion performance over three catalysts.There is a good linear relationship between the conversion rate of n-C16 and the effective diffusion time coefficients measured by the ZLC method.The calculation model of the internal diffusion criterion Weisz-Prater number was optimized by the diffusion-reaction kinetics method.Based on the novel model,it was found that the formation of m-i-C16 was sensitive to internal diffusion.The mesoporous HYE was prepared with HY parent zeolite and the bifunctional catalysts with different intimacy at large nanoscale were prepared with HY-Al₂O₃ or HYE-Al₂O₃ as composite supports.The difference in the accessibility of Pt resulted in the difference in the diffusion paths of the intermediate products,and then affected the isomerization of n-C16.The core-shell structure Pt/HY@m AMS catalyst with the consistent accessibility of Pt was prepared.The intimacy was controlled at minor-nanoscale,which can shorten the diffusion distance of intermediate,reducing the frequency of secondary cracking to promote the isomerization of n-C16.Pt/HY and Pt/Al-MCM-41 catalysts with intimacy at atomic scale were prepared by ion exchange.The environment where Pt sites are surrounded by acid sites resulted in the inevitably overcracking of the intermediates,reducing the selectivity of isomers.Based on the preparation of yolk-shell catalysts with one-pot microemulsion,four catalysts with different acid strengths,diffusion properties,and intimacy were prepared.The WHSS@Pt catalyst with weak acid,strong diffusion,and minor-nanoscale intimacy was gradient optimized.The higher the optimization grade of the catalyst,the higher yield of isomers and jet fuel.It was proved that the catalyst with stronger isomerization performance is favorable for the selective cracking of long-chain alkanes to produce jet fuel.The WHSS@Pt catalyst with weak acidity,strong diffusion performance,and minor-nanoscale intimacy was favorable for the isomerization of n-C16.The yield of 37 wt.%was excellently performed over WHSS@Pt in the hydrocracking/isomerization of n-C16 to produce jet fuel.