Fabrication And Catalytic N-Dodecane Cracking Performance Of Hierarchical HZSM-5 Zeolite Coating

Monolithic HZSM-5 zeolite catalyst（HZSM-5 coating）is a promising fuel cracking catalyst for flight systems.But in practical application,the mechanical stability（adhesion）is still poor.HZSM-5 coating prepared by hydrothermal crystallization can improve the adhesion of the coating on the carrier.However,the intergrowth in the coating tends to result in severe pore diffusion limitation,reducing catalytic activity and stability.Therefore,enhancing the diffusion properties of HZSM-5 coating is a key factor for improving the catalytic cracking activity and stability of supercritical hydrocarbons,on the basis of ensuring the good mechanical stability.Hierarchical HZSM-5（meso-HZSM-5）coatings with both the acid sites of microporous zeolites and the diffusion properties of mesoporous materials exhibit the effectively improved accessibility of the acid sites in the coating.Thus,more and more attention has been paid for fabricating the hierarchical zeolites.In this paper,hierarchical HZSM-5 coating was constructed by various methods,and the catalytic cracking of n-dodecane at supercritical condition（500-550°C,4 MPa）was used as a model reaction to evaluate the catalytic performance of the coating.The aim of this study was to illustrate the structural-activity relationship between the coating structure,chemical properties and catalytic cracking performance,and thus to guide the construction of high-performance coatings.Meso-HZSM-5/MCM-41 composite catalyst was constructed via transforming Si O₂binder in the coating into MCM-41 mesoporous structure by alkali-treatment and recrystallization.When HZSM-5 crystals were encapsulated in a continuous MCM-41network,the bond strength of the composite coating was significantly improved.Additional Fe and Al species were added to the MCM-41 skeleton to increase the active sites.The composite coating prepared by introducting Fe and Al into MCM-41 skeleton showed excellent catalytic performance,with the initial conversion rate of n-dodecane increased by 50.6%,the inactivation rate decreases by 64.5%,and the heat sink increases by 14.5%,compared with the coating treated by alkali treatment before recrystallization.This could be ascribed to the cascade catalytic pathways of catalyze precracking,cracking,and dehydrogenation by the acid site of MCM-41,the acid site of meso-HZSM-5,and the redox sites of Fe-MCM-41,respectively.Macro-meso-HZSM-5 with mesoporous crystals along the quasi-c-axis were prepared on a dense,randomly oriented seed layer using TPOAC as a mesoporous agent by an ammonia-assisted secondary growth process.TPOAC induced the formation of intra-crystalline mesoporous pores,and inorganic ammonium salts facilitated the formation of macropores the growth of meso-HZSM-5 crystals along the（00h）&（10h）orientation.Using Si-MFI,Al-MFI or GA-MFI as seed,the chemical composition of the seed layer was adjusted,and the dense substrate was formed through the intergrowth in the seed layer to enhance the mechanical strength.Macro-meso-HZSM-5 coating（MZC-Al-2N）synthesized from Al-MFI seed showed excellent catalytic cracking conversion and stability for supercritical n-dodecane.Its initial reaction rate and the deactivation rate were three times and 52%of those for the conventional HZSM-5 coating,respectively.Compared with MZC-Al-2N,the coating synthesized from Ga-MFI seed exhibited lower activity,but higher heat sink,which was probably due to the endothermic dehydrogenation of the low carbon alkanes by the synergistic effect of B-acid site and extra-framework Ga species.