SAPO-18 catalysts and their Brønsted acid sites

The incorporation of silicon via direct synthesis into AlPO₄-18 (AEI), which has a framework structure related to, but crystallographically distinct from, that of the well-known solid acid catalyst SAPO-34, was investigated by a range of techniques. Unlike the Si/(Si + Al + P) ratio (typically about 0.10) in SAPO-34, which can be varied only within a very narrow range under normal synthetic conditions, the Si/(Si + Al + P) ratio in SAPO-18 is tunable from O to 0.10 by varying the silicon content in the synthetic gel. ²⁹Si MAS NMR spectroscopy reveals that silicon substitutes for both phosphorus and aluminum in SAPO-18, whereas in SAPO-34, silicon substitutes only for phosphorus. Infrared and ¹H MAS NMR spectroscopies and temperature-programmed desorption (TPD) of ammonia were used to examine the Brønsted acid sites borne by SAPO-18 samples. As expected, the concentration of Brønsted acid sites in all SAPO-18 samples is much less than that in SAPO-34. In contrast to the essentially neutral AlPO₄-18, which catalyzes methanol conversion only to dimethyl ether, SAPO-18 catalytically converts methanol to light olefins with high activity and selectivity. The maximum conversion of methanol to ethene and propene reaches 80% with a 100% conversion of methanol to hydrocarbons. With an optimum framework composition, SAPO-18 retains its catalytic activity and selectivity longer than SAPO-34.