Synthesis And Catalytic Performance Of ZSM-5/MCM-41Composite Material

Micro-mesoporous composite materials which combine adjustable mesopore size with high hydrothermal stability and acid strength of zeolite are widely used in the field of catalysis. In this paper, ZSM-5/MCM-41composites were synthesized by surfactant-directed recrystallization and the properties especially the catalytic activity of them were studied in detail.The effects of NaOH concentration, crystallization time and mass fraction of surfactant on ZSM-5/MCM-41composite zeolites were studied. It was found the composites possessed the complete dual structure of micropore and mesopore and excellent physico-chemical property at the point of NaOH solution concentration1.5M, the crystallization time24h and mass fraction of surfactant10%. Zeolite coatings were prepared in the stailess steel tubes (internal diameter2mm) by washcoating method. Catalytic cracking of n-dodecane over zeolite coatings was studied on the inside of a tubular reactor at550C and4MPa. Compared with the ZSM-5and desilicated ZSM-5zeolite coatings, ZSM-5/MCM-41composite coating showed higher catalytic activity and stability.ZSM-5/MCM-41composite zeolites with mesopore size in the range of2.5-4.0nm were prepared by alkyltrimethylammonium bromide with different chain length. XRD and TEM revealed that the composites possessed the characteristics of both ZSM-5and mesoporous MCM-41with hexagonal symmetry. N2-adsorption-desorption,27Al MAS NMR, NH3-TPD and in situ FT-IR results suggested that the sizes of the mesopores and the accessibility of acid sites could be affected greatly by the chain length of alkyltrimethylammonium bromide used. The conversion of n-dodecane increased gradually with increasing the mesopore sizes of ZSM-5/MCM-41composites, implying a positive effect of the mesopores on the diffusion inside pore channels and the accessibility of acid sites by the reactants.The effect of particle size and Si/Al2ratio of ZSM-5zeolite on the structure and catalytic performance of ZSM-5/MCM-41composites was studied. It was found more silicate dissolved and high surface area and large mesopore size were gained when the particle size was larger and Si/Al ratio was higher. However, the amount of acid sites decreased gradually. The catalytic cracking results demonstrated the conversion of n-dodecane over composite coatings increased first and then decreased with the increasing of Si/Al2ratio. When the Si/Al2was the same, the catalytic activity of composite coatings increased with the decrease of particle size.