Effect of framework topology of SAPO catalysts on selectivity and deactivation profile in the methanol-to-olefins reaction

- Small differences in the size and shape of 8R strongly affect the deactivation rate of SAPO.
- The wider 8-ring and medium-sized cage of STA-7 favor high methanol conversion capacity.
- Light branched alkanes can block narrow pores, causing fast deactivation of SAPO-35.
- Larger 8R openings of STA-7 and SAPO-42 enhance production of C4+ hydrocarbons.
- Large cages in SAPO-56 boost the fast formation of heavy polyaromatic hydrocarbons.

Catalytic performance in the methanol-to-olefins reaction was studied on four 8-ring silicoaluminophosphate (SAPO) molecular sieves with different framework topologies and cavity-windows of various dimensions: SAPO-35 (LEV), SAPO-56 (AFX), STA-7 (SAV), and SAPO-42 (LTA). The four catalysts were compared with the widely investigated SAPO-34 (CHA) and showed remarkable differences in lifetime and products selectivity. In particular, the STA-7 catalyst gave higher selectivity to C3-C4 olefins, while it had methanol conversion capacity similar to that of a SAPO-34 catalyst with similar crystal size and acidity. Organic species retained in the cavities of partially deactivated catalysts were analyzed at different times on stream. The changes in composition of trapped compounds were correlated with those of the effluent products to elucidate the reaction mechanism and the deactivation pathway. The differences in effluent product distribution and trapped coke species suggest that framework topology has a great influence not only on the stability of the materials but also on the products distribution.

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