Altering C2H4/C3H6 yield in methanol to light olefins over HZSM-5, SAPO-34 and SAPO-34/HZSM-5 nanostructured catalysts: influence of Si/Al ratio and composite formation

Abstract

A series of nanostructured catalysts composed of HZSM-5 zeolite with various ratios of Si/Al (120, 140, 180 and 220) was successfully synthesized by a hydrothermal method. The HZSM-5 sample with optimum ratio of Si/Al = 180 was then added into SAPO-34 synthetic gel to synthesize SAPO-34/HZSM-5 composite. The physicochemical properties and catalytic performance of SAPO-34/HZSM-5 composite were then compared to HZSM-5 (Si/Al = 180) and SAPO-34 catalysts. The nanostructured catalysts were characterized by XRD, FESEM, EDX, BET and FTIR techniques. The XRD patterns indicated that the extent of crystallization and crystallite size were dependent on the Si/Al ratio and type of structure phase (CHA and/or MFI). The EDX images show high distribution of Si for the SAPO-34/HZSM-5 composite. The highest BET surface area (388 m² g⁻¹) was obtained for HZSM-5 zeolite catalyst with Si/Al ratio of 220. For the MTO reaction, comparing to the HZSM-5 synthesized with different Si/Al ratios, the HZSM-5 (Si/Al = 180) sample possesses highest crystallinity and therefore exhibits the highest selectivity to ethylene. The SAPO-34/HZSM-5 composite is an active catalyst with high selectivity toward light olefins of around 80%.