Synthesis of nano ZSM-5 molecular sieve by seed-assisted and pre-crystallization and its application in aromatization of propane
In the aromatization reaction of propane,the conventional molecular sieve catalyst is prone to carbon deposition and deactivation,resulting in unsatisfactory catalytic performance and service life.Nanometer ZSM-5 molecular sieve(ZSM-5-Nano)was synthesized with pure silica molecular sieve Silicalite-1(S-1)as seed by combination with seed method and pre-crystallization method.The structure of ZSM-5-Nano was characterized by scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD)and N2 physical adsorption/desorption.The results show that ZSM-5-Nano is composed of small particles with a particle size of about 50 nm,its specific surface area is up to 395 m2/g,and it is rich in intergranular mesopores.The catalyst Zn-ZSM-5-Nano was prepared by loading Zn with ZSM-5-Nano,and then the catalytic performance of the catalyst for the aromatization reaction(n(N2):n(C3H8)= 7:3,space velocity of 3000 h-1 and reaction temperature of 550 ℃)of propane was evaluated in a fixed bed reactor.The results show that Zn/ZSM-5-Nano has relatively mild acid properties,and shows good catalytic performance in the aromatization reaction of propane.The conversion rate of propane and the selectivity of light aromatics(benzene,toluene and xylene)within 5 h are as high as 88.4％and 86.7％,respectively,and the activity is stable within 5 h.The catalytic performance is better than that of the micrometer ZSM-5 molecular sieve(hydrothermal method)supported Zn catalyst and the nanometer ZSM-5 molecular sieve(pre-crystallization method)supported Zn catalyst.The results of structure activity analysis show that the better catalytic performance of Zn/ZSM-5-Nano is the result of the combination of morphology and acidity,and the shorter diffusion distance and milder acidity effectively delay the formation of carbon deposition.
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