Synthesis And Performance Of Hierarchical HZSM-5 Zeolite For Catalytic Cracking Of Supercritical Hydrocarbon

Catalytic cracking of hydrocarbons over HZSM-5 zeolite under supercritical condition is one of the effective ways to overcome thermal barrier form hypersonic flight. However, the diffusion of hydrocarbons within micropores of HZSM-5 zeolite was significantly reduced under supercritical condition because of the increased density of hydrocarbons, which led to seriously decreased catalytic activity and stability. Diffusion could be effectively improved through fabricating mesopores in HZSM-5 zeolites. In this paper, hierarchical HZSM-5 zeolites with different micro-mesopore structure were prepared by various methods. Further examination of their catalytic performance using a model reaction of catalytic cracking of n-dodecane was performed to study the effects of micro-mesopore structure on catalytic cracking performance of zeolites.Hierarchical HZSM-5 zeolites were synthesized by desilication with TAAOH in the presence of CTAB. It was found the loss of crystallinity and acid amount as well as the formation of extra-framework Al could be minimized with TAAOH as alkali source. Both the size and amount of mesopores increased with TAAOH chain length, which lead to improved catalytic performance.Hierarchical HZSM-5 zeolites were prepared using cationic polymers of polydiallydimethylammonium chloride(PDDA) as meso-porogen. Pre-crystallization can enhance crystallinity of hierarchical zeolites effectively. Mesopore size was enlarged and its distribution was tailored from 11-13 nm to 11-34 nm by increasing of the PDDA amount, whereas accompanied by a significant decrease of acid sites accessibility and catalytic stability. Besides, intracrystal mesopores can hardly be fabricated with PEI and PVP as meso-porogen.Long-range ordered mesopores with pore size of 3-5 nm were fabricated in HZSM-5 crystals with the aid of amphiphilic organosilane(TPOAC). Monodispersed hierarchical zeolites with high crystallinity and specific surface area can be prepared under moderate crystallization temperature and ethanol amount. Intracrystal mesopore amount and acid properties can be controllably adjusted through changing TPOAC amount. The hierarchical HZSM-5 zeolite with moderate amounts of mesopores and acid exhibited the highest catalytic activity and stability. And its initial conversion increased by 36% compared with the microporous HZSM-5, while the deactivation rate decreased by 57%.Hierarchical HZSM-5 zeolites were prepared by recrystallization of alkali-treated HZSM-5 with TAAOH in the presence of CTAB. The use of short chain quaternary ammonium cations, involving TMA+ and TEA+, induced the creation of core-shell structured zeolites with meso-HZSM-5 as the core and MCM-41 as the shell, and the latter induced a thicker MCM-41 shell with thickness of 58 nm. While a whole MCM-41 shell can not be obtained with TPAOH and TBAOH as alkali source. The hierarchical zeolite with the whole MCM-41 shell showed higher catalytic activity and stability. And its initial conversion increased by 31% compared with the microporous HZSM-5, while the deactivation rate decreased by 58%.The core-shell structured meso-ZSM-5@MCM-41 zeolites with both ordered intracrystal mesopores and ordered mesopores in the shell were synthesized by coating method. Through this process, MCM-41 shell closely grew around the meso-HZSM-5 crystals. The shell thickness can easily be tuned in the range of 33-87 nm by changing the mass ratio of synthetic solution to meso-HZSM-5 powder. And the mesopore size of MCM-41 shell was tailored in the range of 2.5-10 nm by changing CTAB chain length and using TMB as swelling agent. The conversion of n-dodecane increased gradually with increasing the mesopore size of MCM-41 shell.