Synthesis of SAPO-18/34 intergrowth zeolites and their enhanced stability for dimethyl ether to olefins

In order to investigate the effect of phase composition in the silicoaluminophosphate (SAPO) zeolites on their catalytic performance for the dimethyl ether (DME) to olefin (DTO) reaction, four well crystalline SAPO zeolites with different SAPO-18/34 ratios were synthesized hydrothermally by using a mixed template of diethylamine (DEA) and N, N'-diisopropylethylamine (DIEA). The effects of the composition of the mixed template on the physicochemical properties of the zeolites were investigated by XRD, SEM, TEM, XRF, FT-IR and NH3-TPD techniques. The results revealed that the phase composition, morphology, particle size, chemical composition and surface acidity of the zeolites were greatly affected by the template composition. Catalytic tests showed that both SAPO-18 and SAPO-34 presented relatively faster coking rates compared with that of SAPO-18/34 intergrowth zeolites, and pure SAPO-18 with the smallest crystallite size and weakest acidity exhibited the highest amount of coke deposition. The DME conversion and product distribution were similar over all catalysts, but the selectivities of light olefins showed a strong correlation to the cage size. That is, the catalysts with a higher content of slightly wider AEI cages favor the formation of propylene and butylene, and catalysts with a higher content of relatively narrower CHA cages benefit the formation of ethylene.