Diffusion of volatile organic chemicals in porous media. 1. alcohols and aromatics/MCM-41 mesoporous materials

Abstract

The aim of the present paper was to measure the apparent coefficients of diffusion, Knudsen diffusivities, pore diffusivities and activation energies of diffusion at 26-32 oC and to determine the modes of transport of some alcohols (methanol, ethanol, propanol, n-butanol) and aromatics (benzene, ethylbenzene, propylbenzene, toluene, o-xylene, mxylene, p-xylene) into the porous structure of MCM-41 mesoporous material synthesized. The diffusion coefficients of alcohols and aromatics were calculated from the slope of graphs of Mt/M∞ versus t1/2. As the molecular weight of the alcohols and aromatics increased, apparent coefficients of diffusion decreased, the activation energy for diffusion increased. Lower molecular weight alcohols and aromatics had higher coefficients of diffusion compared to those with higher molecular weight alcohols at the same temperatures. The diffusion of isomeric molecules within the mesoporous channels were affected by the position of branching, the deterministic behavior depended on the molecular weight, length of side chain and ortho, meta and para isomerism of the molecule. Increasing the temperature, raised the kinetic energy of the molecules which resulted increases in the coefficients of diffusion of the alcohols and aromatics in the MCM-41 material. Diffusion rate constants of alcohols and aromatics increased with temperature within the range of 26-32 °C, and decreased as the molecular weight of the diffusing chemical increased. The diffusion of alcohols and aromatics in the MCM-41 obeyed the anomalous transport mechanism. Diffusion exponents, n, being in the range of 0.99-1.07 indicated an anomalous diffusion non-Fickian/super-Case II) mechanism for alcohol diffusion. However for the case of aromatics, diffusion exponents from 0.7 to 1.00 indicated that the diffusion mechanisms were either non-Fickian or non-Fickian/super-Case II depending on the substitution to the benzene ring. Activation energies of alcohols and aromatics were also in good agreement with the values of coefficients of diffusion of alcohols and aromatics such that larger activation energies resulted in smaller diffusion coefficients.