Adsorption and diffusion of n-heptane and toluene over mesostructured ZSM-5 zeolitic materials with acidic sites

Abstract

In order to understand the effects of the surface acid sites in mesoporous zeolites on the adsorption and diffusion of hydrocarbons, the adsorption and diffusion of n-heptane and toluene (two C7 hydrocarbons) over the mesoporous HZSM-5 zeolites (SiO2/Al2O3 = 36) were investigated. The adsorption isotherms of two C7 hydrocarbons over the mesoporous HZSM-5 zeolites showed the experimental data good fitting to dual-site Langmuir-Freundlich model (DSLF) similar to mesoporous NaZSM-5. Henry's constants and the initial heats of adsorption displayed stronger interactions in HZSM-5 than in NaZSM-5, especially for n-heptane, due to the presence of surface acid sites. The influence of acidity to the adsorbate-adsorbent interactions decreases with increasing of the mesoporosity. The investigation on the diffusion of n-heptane and toluene in mesoporous HZSM-5 samples by the zero-length column (ZLC) method showed the strong interaction of adsorbates and HZSM-5 samples by effective diffusivity and activation energy. Moreover, the diffusions of adsorbates in zeolites were advanced with the introduction of mesopores in the zeolitic samples, though the dramatic influence of mesopore to diffusion is not arisen in mesoporous NaZSM-5, reflecting larger influence of acid sites on diffusion in mesoporous HZSM-5 samples.