High Propylene Selectivity in Methanol Conversion over a Small-pore SAPO Molecular Sieve with Ultra-small Cage

Conversion of methanol to olefins (MTO) is an important non-oil alternative route for ethylene and propylene production, which has been industrialized based on a fluidized-bed process with a small-pore SAPO-34 (CHA topology) molecular sieve as the active catalyst component. However, it remains a challenge to effectively regulate the selectivity toward single ethylene or propylene due to the limited catalyst selection and insufficient understanding of the selectivity control principle. Herein, we report the synthesis of a small-pore SAPO-14 molecular sieve (AFN topology) with an ultra-small cage structure and a narrow 8-membered ring (8-MR) channel system, over which the propylene selectivity can rise to as high as 77.3%, representing the highest record of one-pass propylene selectivity in the MTO reaction. The influence of reaction conditions on the catalytic performance was investigated, and the olefin formation mechanism was revealed by combining the analyses of 12C/13C-methanol isotopic switch experiments, confined organics analysis, and reaction-diffusion simulations. This work provides a possibility to subtly control the MTO product distribution by the design and synthesis of the molecular sieve catalyst.