Synthesis Of Nanocrystal HZSM-5 Zeolites And Their Performance In MTP Reaction

Methanol to propylene(MTP)reaction is an important way to obtain propylene,it means a lot for increasing propylene yield while reducing the dependence on petroleum routes.Nano-size HZSM-5 is considered to be one of the most important catalysts because of its advantages,such as small size,short channels and rational distribution of acid sites.In this study,nano-size HZSM-5 samples were hydrothermally synthesized.These samples were characterized by X-ray diffraction(XRD),field emmission scanning electron microscope(FE-SEM),FTIR of adsorbed pyridine,N2 adsorption measurements,temperature programmed ammonia desorption(NH3-TPD)and inductively coupled plasma-atomic emission spectrosemetry(ICP-AES),in order to study the effect of SiO2/Al2O3 ratio,templates and mixed templates on the synthesis of nano-size HZSM-5 zeolites.These samples were tested under 450℃,0.1MP and WHSV 7.6h-1 in MTP reaction to observe the catalytic performance.The experimental results showed that using tetrapropylammonium hydroxide(TPAOH)as template,the crystal size of zeolites was 60-80nm and zeolites were similar in crystal morphology but significantily different in acidity as zeolites with different SiO2/Al2O3 ratios of 200,400,600,800,1000,1200.When SiO2/Al2O3 ratio was equal to 1000,the nano-size HZSM-5 zeolite showed suitable acid distribution and the optimal catalytic performance with the highest selectivity of propylene(49.17%).Using tetrapropyl bromide(TPABr)as template to synthesize HZSM-5 zeolite,the crystal size of zeolite was 100 nm,the BET surface area and pore volume of zeolites were less than the zeolites synthesized with TPAOH and the strength of acid was weaker.Using ethylenediamine(EDA)as template,the crystal size was larger,about 450 nm,the relative crystallinity and the catalytic performance in MTP reaction were poor,the selectivity of propylene was only 27%.When TPAOH was mixed with EDA equimolarly,the crystal size of HZSM-5 was 180nm,the crystallinity was better and the catalytic performance of zeolite was similar to zeolites prepared by pure TPAOH.Introducing EDA moderately and mixed with TPAOH as template can reduce the content of TPAOH when synthesize nano-size HZSM-5 zeolites.