Selective Production of Light Olefins from Catalytic Cracking of n-Hexane over OSDA-Free Beta Zeolites

摘要

The catalytic cracking of n-hexane for producing light olefins on Beta zeolite synthesized under organic structure-directing agent free conditions (OFB) was carried out as a model reaction of naphtha cracking. The Al-rich H-form OFB (HOFB) catalyst exhibited a short catalytic life and a low selectivity to propylene even at the lower reaction temperature of 773 K, due to its high acid amount. Dealumination of HOFB by ammonium hexafluorosilicate (AHFS) treatment and followed by nitric acid treatment resulted in the increase in the catalytic life and propylene selectivity, regardless of the reaction temperatures ranging from 773 to 923 K. This could be attributed to the lower amount of the acid sites especially the Lewis acid sites, which suppressed the secondary reactions of propylene and butenes and the hydride transfer to form coke. For example, the HOFB-AHFS(0.5 M)-HNO3(6 h) catalyst showed a very high selectivity of ca. 41 C-% to propylene even at nearly 100 % n-hexane conversion, at the higher reaction temperature of 923 K.