Tuning the density of Brønsted acid sites on mesoporous ZSM-5 zeolite for enhancing light olefins selectivity in the catalytic cracking of n-octane

A molecular template (tetrapropylammonium hydroxide, TPAOH) and a phosphorus-containing template (tetrabutylphosphonium hydroxide, TBPOH or tetrabutylphosphonium bromide, TBPBr) were used as dual templates to prepare the mesoporous ZSM-5 zeolites. The physicochemical properties of the as-prepared ZSM-5 catalysts were characterized by means of XRD, N2 adsorption, NH3-TPD, FT-IR spectra of adsorbed pyridine, 27Al MAS NMR, etc., and their performances for the catalytic cracking of the n-octane to light olefins were investigated. The results indicated that the acidity of ZSM-5, especially the concentration of strong Brønsted acid sites (SBAS) can be finely modulated by tuning the molar ratio of TPA+/TBP+. The charge-compensation effect of TBP+ and the phosphorus species played critical roles in regulating the SBAS concentrations of ZSM-5. Consequently, the as-prepared ZSM-5 catalysts with suitable strong Brønsted acid sites density was favorable for enhancing the selectivity to light olefins. At the temperature of 600 °C, the selectivity of propene and light olefins over MFI-9/2.5 catalyst were achieved 34.7% and 70.4%, which were higher than those over conventional ZSM-5 by 2.4% and 5.1%, respectively. Selectivity to C2=∼C4= light olefins was found to be decreased with increasing the concentration of strong Brønsted acid sites due to the reduction of SBAS which inhibited the hydrogen transfer reaction. This work will provide insights and new platforms for the design of high-efficiency zeolite catalysts for important industrial reactions.