Influence of template content on selective synthesis of SAPO-18, SAPO-18/34 intergrowth and SAPO-34 molecular sieves used for methanol-to-olefins process

Abstract

SAPO-18, SAPO-18/34 intergrowth and SAPO-34 molecular sieves have been selective synthesized using different amount of template (tetraethyl ammonium hydroxide, TEAOH). The samples were characterized by XRD, XRF, TG-DSC, ¹³C (²⁹Si) CP/MAS NMR and NH₃-TPD. The synthesized products switched from SAPO-5/18, to SAPO-18, SAPO-18/34, and finally to SAPO-34 with the increasing of TEAOH amount. The TEAOH in SAPO-18 cages mainly acted the role of space filling. While in SAPO-34, the TEAOH was mainly used to compensate the negative charge of the framework. SAPO-34 possessed much more Si(4Al) structure compared to SAPO-18 and SAPO-18/34. For methanol to olefins reaction, SAPO-18 yielded the highest propene and C₄ than that over SAPO-18/34 and SAPO-34, SAPO-34 had the highest ethene yield among the three catalysts. The working lifetimes of SAPO-18/34 and SAPO-34 were significantly longer than that of SAPO-18.