Insight into stepwise dealumination in acid-treatment of IM-5 for catalytic cracking of n-heptane and Fischer-Tropsch naphtha

Abstract

10-membered ring IM-5 zeolite has shown excellent performance in many catalytic reactions. The efficient preparation of high silica IM-5 by situ synthesis remains huge challenge. Here, IM-5 with SiO2/Al2O3 ratio (SAR) of 49–116 was acquired by HNO3 treatment. An interesting stepwise dealumination behavior in IM-5 was discovered during the whole acid-treatment process (based on XRF characterization). Extra-framework Si (EF-Si) compensated for the defects in IM-5 framework, thereby postponed the removal of Al species at the initial period of acid treatment, as indicated by 27Al, 29Si, 1H NMR and density-functional theory (DFT) calculations. The dealuminated IM-5 were revealed to retain more than 83 % crystallinity with little loss in micropore volume. Bimolecular reaction was predominant in n-heptane cracking reaction resulted from decreased acid amount and strength in dealuminated IM-5. Meanwhile, high silica IM-5 improved overall yields to light olefins and liquid petroleum gas (LPG) and less coke production in Fischer-Tropsch naphtha (FT-naphtha) cracking reaction.