Diffusion of alcohols and aromatics in a mesoporous MCM-41 material

Official URL: http://dx.doi.org/10.1016/j.fluid.2014.09.009

The aim of the present paper was to measure the apparent diffusivities, Knudsen diffusivities, pore diffusivities and activation energies of diffusion at 26–32 °C and to determine the modes of transport of some alcohols (methanol, ethanol, propanol, n-butanol) and aromatics (benzene, ethylbenzene, propylbenzene, toluene, o-xylene, m-xylene, p-xylene) into the mesoporous structure of MCM-41 synthesized. As the molecular weight of the alcohols and aromatics increased, apparent diffusivities decreased and the activation energy for diffusion increased. Lower molecular weight alcohols and aromatics had higher diffusivities 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 in an increase in the diffusivities of the alcohols and aromatics in MCM-41. Diffusion rate constants of alcohols and aromatics increased with increasing temperature within the range of 26–32 °C, and the rate decreased as the molecular weight of the diffusing chemical increased. The diffusion of alcohols and aromatics in 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 diffusivities of alcohols and aromatics such that larger activation energies resulted in smaller diffusivities. Alcohols and aromatics with greater solubility parameters were found to have greater diffusivities.