Synthesis of SAPO-34 Molecular Sieves and Their Catalytic Performances in Methanol-to-Olefins Reaction

SAPO-34 molecular sieves were synthesized hydrothermally.The effects of templates,template amounts,pH,crystallization temperature and time on the synthesis were investigated.By means of XRD,SEM,BET and NH_3-TPD techniques,the influences of the molecular sieve structure and surface acidity on the catalytic properties were also studied.The results show that compared with DEA,TEA and TMAOH,TEAOH is the most proper template for synthesis of SAPO-34 with large BET surface areas(400 m~2/g),homogeneous cubic crystals of narrow size distribution and proper ratio of strong to weak acid sites,while the SAPO-34 prepared by using DEA as template shows strongger acidity. Using TEAOH as template with its amount in the range of n(TEAOH)∶n(Al_2O_3) of 2.02-1.35,the crystallinity of product SAPO-34 decreases upon reducing the amount of template.Reducing TEAOH amount in the range of n(TEAOH)∶n(Al_2O_3) of 1.35-1.01 leads to SAPO-5 cogenerating with SAPO-34.A directly fast heating is unfavorable for the growth of crystals.The procedure of preheating to 90-150 ℃,keeping this temperature until the transition state becomes stable and then heating to 170-250 ℃ for crystallization is favorable for the synthesis of SAPO-34 with higher crystallinity which shows the best catalytic performances with a 100% methanol conversion,a selectivity of 83.40% for C~=_2-C~=_3,and a life time of 220 min.