Tuning the arrangement of atoms in SAPO-34 molecular sieves for a deeper understanding of their performance in the MTO reaction

Abstract

The catalytic properties of SAPO-34 molecular sieves synthesized using two different organic structure-directing agents (OSDAs), triethylamine (TEA) and tetraethylammonium (TEA⁺), were investigated in the methanol-to-olefin (MTO) reaction. Although both OSDAs lead to the same CHA-type structure, the two samples exhibit distinct physicochemical properties inducing different performances in the MTO reaction. The amount of incorporated silicon is higher in the SAPO-34 synthesized with TEA (sample N-C), while a higher concentration of Si(4Al) species is obtained when TEA⁺ (sample P-C) is used. Additionally, the size of the crystals decreased significantly when using TEA⁺. The differences observed in activity are mainly due to different textural properties and silicon distribution; TEA⁺ favours the formation of nanocrystals where the dispersion of Si gives rise to a high concentration of Brønsted acidic sites while TEA favours the clustering of Si in ‘islands’ in micron-sized crystals, influencing the catalytic performance of SAPO-34 in the MTO reaction. The N-C sample exhibits high initial selectivity toward light olefins (C₂–C₄) and strong hydrogen transfer activity, but undergoes rapid deactivation due to its larger crystal size that promotes coke formation. In contrast, the P-C sample shows slightly lower initial selectivity but superior stability and lifetime, enabled by its nanoscale crystal size, larger external surface area, and higher acid site density, which together enhance diffusion, limit coke deposition, and sustain methanol conversion.