Diffusion of alcohols and aromatics in a mesoporous MCM-41 material Asli Nalbant Ergün a , Züleyha Özlem Kocabaş a , Alp Yürüm b , Yuda Yürüm a, * a Material Science and Nano Engineering Program, Faculty of Engineering and Natural Sciences, Sabanci University, Tuzla, Istanbul 34956, Turkey b Sabanci University Nanotechnology Research and Application Center, Tuzla, Istanbul 34956, Turkey A R T I C L E I N F O Article history: Received 11 June 2014 Received in revised form 20 August 2014 Accepted 4 September 2014 Available online 16 September 2014 Keywords: MCM-41 Mesoporous Diffusivity Knudsen Transport mechanism A B S T R A C T 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. ã 2014 Elsevier B.V. All rights reserved. 1. Introduction Diffusion is the random transfer of molecules or small particles, occurring due to thermal energy. A better understanding of this phenomenon will aid in optimizing separation, kinetics, and catalytic processes industrial applications. For example, in separation processes, the necessity to comprehend the diffusion phenomena is obvious. In addition, membrane-based separations also rely on the diffusion properties of the utilized membrane. Therefore, to advance in practical applications, diffusion must be precisely understood. Zeolites and related materials are microporous crystalline solids of special interest in the chemical and the petroleum industries, which are used as catalysts and sorbents [1]. For these applications, migration or diffusion of sorbed molecules through the pores and cages within the crystals plays a dominant role. Various techniques for the measurement of intracrystalline diffusion have been developed [2–7], which widely vary in scope, degree of experimental and theoretical sophistication, and range of applicability. For a large number of indirect methods, the diffusivity is calculated from the external measurement of pressure, concentration, or sample weight. Seferinoglu and Yürüm [8] measured the diffusivities of pyridine in raw and acid-washed low-rank coals. The method they used was simple and precise for the measurement of diffusivities of solvents in coals. Ritger and Peppas [9] and Howell and Peppas [10] studied diffusion processes to describe the transport kinetics for pyridine in coal. Bludau et al. [11] studied the uptake of pyridine into mordenite and H-ZSM-5. Their data evaluation was based on the solution of Fick’s second law, using diffusivities for the whole process. Dyer and White [12] studied cation diffusion in a natural zeolite called clinoptilolite and compared three different approaches to determine diffusivities, including Fick’s second law of diffusion, which was found to produce similar results with other approaches. The applicability of various models for the determination of ion exchange diffusivities in clinoptilolite was examined in another study [13]. Marecka and * Corresponding author. Tel.: +90 533 6333340. E-mail address: yyurum@sabanciuniv.edu (Y. Yürüm). http://dx.doi.org/10.1016/j.fluid.2014.09.009 0378-3812/$ – see front matter ã 2014 Elsevier B.V. All rights reserved. Fluid Phase Equilibria 382 (2014) 169–179 Contents lists available at ScienceDirect Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid