Direct synthesis of HZSM-5 from natural clay

Abstract

Highly crystalline HZSM-5 zeolite was directly synthesized through the steam-assisted conversion (SAC) approach using kaolin as the raw material, where NH₃·H₂O was used as an alkaline vapor source. The catalytic performance of the obtained HZSM-5 with SiO₂/Al₂O₃ ratios from 25.67 to 80 was investigated for converting methanol to light olefins (MTO). The results indicated that NH₃·H₂O provided alkalinity to dissolve the raw materials and NH₄⁺ cations to balance the charge of the framework. Highly crystalline HZSM-5 with crystal size in the range of 0.8–1 μm was obtained within 1.0 h at 190 °C. The obtained HZSM-5 possessed high S_BET (above 440 m² g⁻¹) and high total pore volume (above 0.3 cm³ g⁻¹). The acidity of the HZSM-5 was affected by the SiO₂/Al₂O₃ molar ratio and residual metallic elements (such as Fe, Ti, Ca, P, etc.) in the leached metakaolin. The existence of the metallic elements similarly affected HZSM-5 modified with metal cations. HZSM-5 with a higher SiO₂/Al₂O₃ molar ratio was favourable for the formation of C2–C4 olefins. The selectivity of C2–C4 olefins reached 68% when the SiO₂/Al₂O₃ molar ratio was 68.79. This productive and facile method is inexpensive because the ammonium and calcination ion exchange processes are unnecessary.