SAPO-18分子筛晶化机理及其甲醇制烯烃性能

分别以拟薄水铝石、硅溶胶、磷酸为铝源、硅源、磷源,N,N-二异丙基乙胺为模板,采用水热法制备出不同晶化时间下的SAPO-18分子筛。采用X射线衍射(XRD)、电子扫描显微镜(SEM)、氨气程序升温脱附(NH_3-TPD)、固体核磁共振(²⁹Si MAS NMR,²⁷Al MAS NMR)、傅里叶红外(FT-IR)等对不同晶化时间下的产物进行表征以及甲醇制烯烃(MTO)催化性能检测。表征结果结合MTO催化反应性能表明:水热体系下SAPO-18分子筛生长过程可分为3个阶段:晶化初期(≤2 h),快速生长期(6～12 h),生长稳定期(1～3 d)。其中,在晶化初期只有极少量的硅进入Al(PO)_4类中间体;此后为快速生长期,硅原子进入Al(PO)_4类中间体及SAPO-18晶体取代磷、铝原子,以SMⅡ机制为主,SMⅢ机制为辅;在生长稳定期2种取代机制主次发生了变化。

Crystallization Mechanism and Performance in Methanol to Olefins Reaction of SAPO-18 Molecular Sieve

SAPO-18 molecular sieves with the different crystallization times were prepared by hydrothermal method, which respectively taking the pseudo-boehmite, silica sol and phosphoric acid as the aluminum source, silicon source, phosphorus source, and N,N-diisopropylethylamine as the template. The samples at different crystallization times and its MTO (methanol to olefins) catalytic performance were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), NH3-TPD, 29Si MAS NMR,27Al MAS NMR, FT-IR and other methods. The characterization results combined with the catalytic performance of MTO indicated that the growth process of SAPO-18 molecular sieve in the hydrothermal system could be divided into three stages: initial crystallization (≤2 h), rapid growth (6~12 h) and stable growth (1~3 d). In the early stage of crystallization, only a small amount of silicon enter the Al(PO)4 intermediates. The silicon atoms then enter the Al(PO)4 intermediates and SAPO-18 crystal, and replace phosphorus and aluminum atoms. SMⅡ mechanism is as the priority and SMⅢ mechanism is as the auxiliary pole in the fast growing period. The two substitution mechanisms of growth stabilization are in the opposite order.