Designing SAPO-18 with energetically favorable tetrahedral Si ions for an MTO reaction.

The silicon site location in silicoaluminophosphate zeolites has significant influences on their acidic and catalytic properties. Herein, AEI analogue silicoaluminophosphates with controlled tetrahedral silicon centers (T sites) were synthesized using specially designed amines as expected organic structure-directing agents (OSDAs). DFT calculations show that the OSDAs with different electronegativity can direct Si atoms into the T sites with more favorable energy advantages. Their catalytic performances in a methanol-to-olefin (MTO) reaction also reflected that OSDAs controlled the Si location in the framework, and the T3 sites had better performance than T1 sites. This finding provides evidence that OSDAs are capable of guiding the Si ions into more favorable T sites, achieving desirable catalytic properties as solid acid catalysts.