Adsorption and Diffusion of n -Heptane and Toluene over Mesoporous ZSM-5 Zeolites

ZSM-5 zeolites with different mesoporosities were prepared by alkaline treatment and characterized by powder XRD and nitrogen adsorption. Two C-7 hydrocarbons of n-heptane and toluene were employed as probe molecules to investigate the effects of the introduction of mesopore on the adsorption and diffusion properties of ZSM-5 zeolites by comparing the experimental results of the samples treated and untreated by using NaOH. Adsorption isotherms were measured gravimetrically in the pressure range 0-32 mbar and from 293 to 338 K. The isotherms of microporous and mesoporous ZSM-5 were successfully fitted by using the Langmuir-Freundlich model and the dual-site Langmuir-Freundlich model, respectively. Henry's constants and the initial heats of adsorption calculated from the adsorption isotherms as well as the fitting parameters displayed that the interactions between adsorbent and adsorbate were weakened after the introduction of mesopore, and the interactions between the adsorbates with microporous surface are much stronger than that between them with the mesoporous surface. Diffusion measurements were undertaken using the zero length column (ZLC) technique at partial pressure of p/p(0) < 0.000 15 from 333 to 393 K. The results showed that the effective diffusion constants (D-eff/R-2) of the two C-7 hydrocarbons increased greatly in the presence of mesopores, while the corresponding activation energy decreased due to the reduced diffusion resistance and the shortened diffusion path in the mesoporous zeolites. Also, higher and much more dramatic enhancement of the efficient diffusivities as a function of mesoporous volume for toluene relative to that for n-heptane were found, indicating that the diffusion of n-heptane is controlled by the micropore diffusion and that the diffusion of toluene is exclusively determined by mass transfer through the mesopores.