Seed-assisted synthesis of hierarchical SAPO-18/34 intergrowth and SAPO-34 zeolites and their catalytic performance for the methanol-to-olefin reaction

Hierarchical silicoaluminophosphate SAPO-18/34 intergrowth and SAPO-34 zeolites were synthesized by adding SAPO-34 seed crystals to a SAPO-18 zeolite precursor in an aminothermal system, in which triethylamine was used as both the solvent and the template. The prepared samples were characterized by XRD, SEM, TEM, ICP-OES, N2 adsorption–desorption, NMR, TPD, TGA and GC–MS and a possible formation mechanism was proposed. The XRD and NMR results indicated that as the amount of SAPO-34 seed crystals increased, the phase of the products changed from SAPO-18 to SAPO-18/34 intergrowth and then to pure-phase SAPO-34. SEM and TEM results showed that the SAPO samples are the aggregates of many nanosized plate-like nanoparticles containing mesopores and/or macropores. These samples had higher external surface areas and mesopore volumes than the single-phase SAPO-18. The NH3-TPD results showed that the amount and strength of the strong acid sites in the SAPO-18/34 intergrowth and SAPO-34 samples were influenced by the amount of added SAPO-34 seed crystals, but all values were higher than those of pure SAPO-18. The SAPO-18/34 intergrowth and SAPO-34 zeolites with the hierarchical structure and appropriate acidic properties exhibited excellent catalytic properties for the methanol to olefin reaction. Moreover, the selectivities of light olefins varied with the cage type, the acidic properties and hierarchical structure of SAPO-18/34 intergrowth and SAPO-34 zeolites.