Measurement of intracrystalline diffusivity of benzene within MFI-type zeolite from bulk benzene/cyclohexane liquid phase

Abstract

A new method is proposed for measuring the diffusivities of hydrocarbons within solid catalysts in the liquid phase using Raman spectroscopy. This method was applied to measuring the intracrystalline diffusivity of benzene within silicalite-1 (SL) and γ-Al 2O 3 in cyclohexane solution at a temperature range from 323 to 413 K. From Raman spectra of benzene/cyclohexane solution, the proportional relationship between the benzene concentration and the intensity ratio of benzene to cyclohexane was obtained, which enables us to measure the benzene concentration in the solution. From this relationship, the transient change in benzene concentrations in the solution due to its adsorption and diffusion within the catalysts can be measured. The intracrystalline diffusivity ( D ad) in the liquid phase was calculated using theoretical equations, and the experimental results of the concentration change with time. The Arrhenius plots of the diffusivity of benzene within SL (micro pore region) in the gas and liquid phase were correlated with a single line, regardless of the existence of solvent in the liquid phase. This result indicates that in the structure-sensitive diffusion region (diffusion of benzene within the micropore of SL), the diffusion mechanism in the gas phase is the same as that in the liquid phase. In contrast, the diffusivity of benzene within macro-pores of γ-Al 2O 3 in the liquid phase was approximately 10 7 times higher than that within SL. The diffusivity of benzene within γ-Al 2O 3 was almost the same as that of the theoretical molecular diffusion coefficient in cyclohexane solution calculated from the Wilke–Chang equation.