Synthesis of Low Silica SAPO-34 by In Situ Pretreatment and Its Catalytic Performance in the Methanol-to-Olefins (MTO) Reaction

A series of low-silica SAPO-34 molecular sieves with a tunable silica content were synthesized through a facile in situ pretreatment strategy under different pretreatment condition. The effects of the pretreatment condition on the crystal phases, chemical composition, morphology and acidity of SAPO-34 were characterized by XRD, XRF, SEM, N₂ adsorption-desorption, NH₃-TPD and ²⁹Si MAS NMR. The obtained low-silica SAPO-34 exhibits higher catalytic performance in the MTO reaction than conventional SAPO-34 due to the moderate acidity and larger surface areas. Sample S4 with a Si content of Si/(Si+Al+P)=0.053 showed the prolonged catalytic lifetime of 204 min and remarkably improved selectivity for ethylene and propylene reaching up to 86.58 %, which was 34 % and 4.16 % higher than the reference sample.