Dry gel conversion as a suitable method for increasing the lifetime of SAPO-18 in MTO process

• SAPO-18 was synthesized by three methods of CHT, SAC and VPT.
• Effect of DGC methods on physiochemical properties of the SAPO-18 was studied.
• DGC changes the acidity by affecting the amount and mechanism of Si incorporation.
• SAC method significantly increases the useful lifetime of SAPO-18 in MTO process.

The crystalline and pure SAPO-18 was synthesized by dry-gel conversion (DGC) methods (including steam-assisted conversion (SAC) and vapor phase transport (VPT)) and by conventional hydrothermal (CHT) method. The physicochemical properties of the catalysts were analyzed by XRD, FESEM, EDXS, and TPD techniques. It was found that the DGC methods have no significant effect on the crystallization time of SAPO-18. The catalysts with smaller crystallite size and in a higher solid yield were achieved by DGC methods. The results also revealed that DGC method changes significantly the acidic properties of SAPO-18 by affecting the amount and mechanism of silicon incorporation into the framework. The catalytic behavior of the samples was investigated in methanol-to-olefins (MTO) reaction. Although no considerable change was observed in light olefin selectivity over the samples, SAC method showed an increase of about 20% in useful lifetime of SAPO-18. It can be due to the smaller crystallite size and desirable acidic characteristic. Finally, the results were compared with those of DGC-synthesized SAPO-34 reported in the literature. It suggests that SAPO-18 synthesized by SAC method can be an acceptable alternative to SAPO-34 by exhibiting a comparable activity and longer lifetime in MTO process.