Structural Regulation Of ZSM-5 Zeolite And Au Modification For Propylene Production Via Catalytic Cracking

Propylene is one of the important raw materials in petrochemical industry and its demand is growing rapidly.The catalytic cracking process is still being optimized as the second largest source for propylene production.It is well known that ZSM-5 is often used as a catalyst or additive in catalytic cracking to increase the production of light olefin（propylene）.With an attempt to further increase the yield of propylene and to reduce the harsh reaction temperature,a series of post-treatment modifications were made to ZSM-5in this thesis.The focus was put on modifying the pore structure and acid properties of ZSM-5 before incorporating Au nanoparticles.The relationship between the structure features of the studied catalysts and their catalytic performance was investigated.The main conclusions are as follows:（1）Au catalyst supported by external surface modified ZSM-5 zeolite:A series of Au/ZSM-5-T catalysts were firstly prepared by modifying ZSM-5 with external surface regent of trimethoxyoctadecylsilane（TOS）and followed by the addition of Au nanoparticles.The external surface modification of the ZSM-5 reduced the pore size of the ZSM-5 and coverage of the acidic sites on the external surface,thus leading to increased selectivity for propylene.The further introduction of Au nanoparticles with an optimized amount resulted in a more regular pore structure of the ZSM-5,which effectively increased the yield of propylene and also lowered the reaction temperature of catalytic cracking.Among the catalysts studied,the 0.2wt%Au/ZSM-5-1%T catalyst（0.15wt%,actual loading）showed the best feedstock conversion and propylene selectivity.The conversion of n-octane notably increased from 92%to 100%with the selectivity of propylene increasing from 45%to 58%at a reaction temperature of 360°C.In addition,the more catalytic performance was observed for the 0.2wt%Au/ZSM-5-1%T catalyst at360°C,as comparing withthe parent ZSM-5 at 460°C,suggesting 100°C is lowered in the catalytic cracking reaction temperature.（2）Joint of dealumination and Au modification:A series of Au/ZSM-5-F catalysts were prepared by de-aluminizing and silicon supplementing the ZSM-5 with（NH₄）₂Si F₆（AHFS）and then modifying it with Au species.The AHFS effectively transforms the zeolite from"close"Al to"single"Al and maintains the same silica to aluminum ratio of the zeolite framework,thus suppressing the side reactions.The 0.3wt%Au/ZSM-5-F catalyst（0.23wt%,actual loading）showed the best catalytic performance,increasing the propylene selectivity at 460°C from 53%to 63%with complete conversion of n-octane.The catalytic performance of the 0.3wt%Au/ZSM-5-F catalyst at 260°C and 310°C is comparable to that of the parent ZSM-5 at 410°C and 460°C.The cracking temperature is reduced by about 150°C,showing a significant energy saving effect and great potential application in the petrochemical industry.（3）Effect of the size of Au nanoparticles:A series of Au/ZSM-5-S_a catalysts(a is the ratio of n_{sodium citrate}/n_{chloroauric acid})with different sizes of Au particles were prepared by controlling the ratio of sodium citrate and chloroauric acid to synthesize gold sols with different sizes,which were then adsorbed onto ZSM-5.As the proportion of sodium citrate increases,the size of the prepared gold sols becomes smaller.From the catalytic cracking results of n-octane,it was found that the small Au nanoparticles modified ZSM-5 had better catalytic performance.The Au/ZSM-5-S₈ catalyst with an Au nanoparticle size of2.07 nm performed the best catalytic performance with 99%conversion of n-octane and62%selectivity of propylene at 460°C.