Adsorption of paracresol in silicalite-1 and pure silica faujasite. A comparison study using molecular simulation L. Narasimhan, Pascal Boulet *, Bogdan Kuchta, Christelle Vagner, Oliver Scha ¨ f, Renaud Denoyel Laboratoire Chimie Provence, UMR 6264, Unviversite ´ Aix-Marseille I, II and III, CNRS, Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France 1. Introduction Zeolites are microporous crystalline materials with the diame- ter pores size of the order of the nanometer [1]. A common application of zeolites is for purification purposes [2–4]. Another possible application is the removal of uremic toxins during the hemodialysis process: zeolites would be used for the storage of the toxins instead of conventionally used polymer membranes [5]. The necessity of the zeolite usage in dialysis arises from the inefficient removal of paracresol (1,4-methylphenol) in conven- tional dialysis method. Contrary to water bound solutes (e.g. urea and creatinine), protein bound solutes (e.g. paracresol) are not efficiently removed in conventional dialysis. The high toxicity of paracresol (hereafter called p-cresol) is responsible for the intoxication of patients, ultimately causing heart attack because of its increased serum concentration during renal failure [6–9]. The need to gather detailed insights into the behavior of zeolite with the sorbate molecules on the molecular scale has influenced the usage of molecular simulation studies. The Monte Carlo techniques have been used to complement experimental informa- tion on adsorption problems [10–16]. The present work is devoted to the adsorption of p-cresol and water in pure silica faujasite and silicalite-1 zeolites. Although pure silica faujasite is unstable; this work has been carried out to better understand properties of both zeolites in view of possible applications in dialysis. The report is categorized as follows. The methodology used in the simulation studies is first emphasized which includes the molecular models for adsorbates and frame- works. In the further part of the report, we discuss the results on the adsorption of p-cresol and water in silicalite-1 and siliceous faujasite, and the development of a new adsorption isotherm that accounts for the effect of the solvent during the adsorption of the toxin. 2. Simulation details 2.1. Zeolite models Only pure siliceous silicalite-1 and faujasite models of zeolite were used in our simulations. The structure of silicalite-1 has been built according to IZA database [17]. The silicalite-1 zeolite ((SiO 2 ) 96 ) belongs to orthorhombic, Pnma space group [18] with the lattice parameters a = 20.02 A ˚ ´ , b = 19.98 A ˚ ´ and c = 13.38 A ˚ ´ . Two straight channels intercept two sinusoidal channels forming a cavity larger in size than the channels. The porous network of faujasite is made up of cuboctahedral sodalite cages which are linked together in a tetrahedral manner by six oxygen atoms forming cavities called supercages. These supercages are interconnected in the same tetrahedral manner by windows. A single unit cell of faujasite consists of 192 SiO 2 units having eight supercages and eight sodalite cages belonging to Fd3m space group with the lattice parameters a = b = c = 25.03 A ˚ ´ . 2.2. Simulation techniques 2.2.1. Monte Carlo simulations All the Monte Carlo (MC) simulations were carried out using the simulation package ‘‘Towhee version 5.21-6.0.2’’ [19]. In this study Applied Surface Science 256 (2010) 5470–5474 ARTICLE INFO Article history: Available online 4 January 2010 Keywords: Adsorption in porous materials Co-adsorption Monte Carlo simulations Paracresol Uremic toxins Zeolites ABSTRACT This paper presents the results on the Grand-Canonical Monte Carlo simulations of the adsorption of the paracresol uremic toxin and water into the silicalite-1 and pure silica faujasite zeolites. The co- adsorption of water and paracresol seems to proceed along a cooperation effect between the toxin and the solvent. A model of adsorption that accounts for the effect of the solvent has been elaborated and verified using experimental isotherms. The model is based on the Langmuir isotherm in which an apparent adsorption enthalpy is used that changes with the concentration of the solute. The new expression for the isotherm reproduces the experimental isotherm with good accuracy and physical interpretation is given to justify the model. ß 2010 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +33 491 63 71 17. E-mail address: pascal.boulet@univ-provence.fr (P. Boulet). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.12.142