SAPO-18 with the AEI topology is a highly promising catalyst for the methanol-to-olefins reaction, but its synthesis is highly sensitive to the properties of the precursor gel. In this work, we systematically compared the conventional synthesis route (ST) with three modified routes in which H₂O and the OSDA were fixed as the initial components, and investigated the effects of phosphoric acid addition sequence and crystallization time on the formation and catalytic performance of SAPO-18. Although the AEI framework could be obtained through all four routes, namely ST, APS, PAS, and ASP, the PAS route was identified as the most favorable strategy. This is because adding H₃PO₄ to the DIPEA+H₂O mixture before introducing the Al source establishes a more homogeneous precursor environment with a mild pH of approximately 7, thereby promoting the formation of a uniform aluminophosphate gel. The results show that the PAS route not only accelerates the formation of the AEI structure, but also improves framework integrity, increases the proportion of isolated Si species, reduces Si-island-related structural environments, and optimizes the distribution of Brønsted acid sites. Among all samples, S18 PAS (120 h) exhibits the best overall properties in terms of framework integrity, isolated Si incorporation, acidity, and diffusion performance, achieving a light olefin selectivity as high as 84.69% together with the best catalytic stability. These findings highlight the decisive role of precursor gel chemistry in regulating SAPO-18 crystallization and provide a practical strategy for the rational design of high-performance AEI-type SAPO catalysts.
 

SAPO-18; AEI zeolite; precursor gel; silicon distribution; methanol-to-olefins