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On the Adsorption of Gaseous Mixtures of Hydrocarbons on High Silica Zeolites Vittoria Sacchetto, Diana F. Olivas Olivera, Geo Paul, Giorgio Gatti, Ilaria Braschi, Leonardo Marchese, and Chiara Bisio J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11577 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 4, 2017
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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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The Journal of Physical Chemistry
ON THE ADSORPTION OF GASEOUS MIXTURES OF HYDROCARBONS ON HIGH SILICA ZEOLITES V. Sacchetto†, Diana F. Olivas Olivera†, G. Paul†,⊥, G. Gatti†,⊥, I. Braschi⊥,‡, L. Marchese†,⊥ and C. Bisio†,⊥, ,§*
†
Department of Sciences and Technological Innovation and ⊥Interdisciplinary Nano-SiSTeMI
Centre, University of Eastern Piedmont A. Avogadro, viale T. Michel, 11, 15121 Alessandria (Italy); ‡
Department of Agricultural Sciences, University of Bologna, viale G. Fanin 44, 40127 Bologna
(Italy); §
ISTM-CNR Istituto di Scienze e Tecnologie Molecolari, via G. Venezian 21, 20133 Milano (Italy)
ABSTRACT: An experimental study of the interactions of an equimolar binary gaseous mixture of toluene and n-hexane, model molecules of aromatic and aliphatic fuel-based pollutants, with two dealuminated high silica zeolites is here presented for the first time. Zeolites Y and ZSM-5 with diverse textural and surface properties were chosen as adsorbents and the effects of their physicochemical features (predominantly the pore size architecture and silanol content) on sorption capacity were studied. The host−guest (i.e. sorbent-molecules) interactions were studied by FTIR and SS-NMR spectroscopies. IR optical adsorption isotherms of both toluene and n-hexane coadsorbed on the zeolites allowed the determination of the concentration of the adsorbed molecules. Variable temperature SS-NMR spectroscopy allowed the description of the mobility of the pollutant 1 ACS Paragon Plus Environment
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molecules when co-adsorbed at the interface of the zeolites micropores. Finally, it was enlightened how the proposed innovative approach can be of general use to determine the selectivity of adsorbent materials for mixture of hydrocarbons.
1. INTRODUCTION
The competitive adsorption of hydrocarbons is a critical aspect in many engineering and environmental processes, such as petroleum recovery, removal and extraction of fuel-based pollutants from water and gas phase. Among all the hydrocarbons, toluene and alkanes are peculiar since they are solvents commonly used for both industrial and scientific applications.1 These organic compounds can end up in groundwater where industrial areas, especially oil refineries or petrochemicals plants, are located. Thus, in recent years the effective removal of these pollutants has become a relevant environmental issue and for this reason great research efforts on the optimization of adsorption processes and the development of novel adsorbent materials for remediation and separation of organic pollutants from groundwater have been carried out.2 The adsorbent materials are generally selected depending on the type of contaminants to be removed from polluted water: for instance, active carbons or zeolites are appropriate for removal of nonpolar and polar organic molecules, whereas reactive materials (i.e. zero valent iron (ZVI))3 are effective to remove compounds such as chlorinated solvents, heavy metals, pesticides and aromatic nitro compounds. The most promising sorbent materials used for groundwater remediation are granular active carbons (GAC), synthetic resins, clay materials, phosphate compounds (such as mineral apatite Ca5(PO4)3) and zeolites.4,5 Specifically, zeolites own structures with well-defined micropores where adsorption and reaction sites are located: their selectivity and activity can be modulated depending on zeolite structure and chemical composition. In addition, the selective adsorption on zeolites is commonly considered the most economical industrial process for molecular separation and a valid solution for depollution purposes.6,7,8 The definition of the host-guest interactions 2 ACS Paragon Plus Environment
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occurring in confined space between porous solids and embedded hydrocarbons is an important task to improve adsorption properties of materials and essential to optimize effective sorbents for contaminant removal from polluted water.9 More specifically, the adsorption of aromatic and alkane molecules on FAU and MFI zeolites has been extensively investigated as single-component, 8,10,11,12,13
whereas studies on competitive adsorption of organic molecules mixture with different
polarity and chain length from water and gas phase are limited.
14,15,16
On the other hand,
understanding the competition of organic contaminants for a specific adsorbent is relevant from an environmental point of view in that, under real conditions, pollutants are generally present in groundwater as complex mixtures. In this context, the co-adsorption at the gas-zeolite interface of specific n-hexane and toluene binary mixtures has not been so far investigated to our knowledge, whereas their adsorption as single pollutants has been already reported by our group.17 In the present study, two dealuminated high silica zeolites, i.e. Y and ZSM-5, were selected on the basis of their pore dimension, hydrophobicity, and textural properties with the purpose of investigating on the zeolites selectivity toward the adsorption of a toluene and n-hexane mixture in dry conditions. In particular, the hostguest (i.e. the sorbent-pollutant) interactions were studied by using a multidisciplinary experimental approach, which allowed a quantitative estimation of the amount of each component of the gaseous mixture along with the mobility inside the zeolite micropores.
2. EXPERIMENTAL METHODS
A. Materials. Toluene and n-hexane were purchased from Sigma-Aldrich both with a purity of 99.9%. Highly dealuminated (high-silica) zeolite Y (here after named HSZ-Y) powder in protonated form with a 200 SiO2/Al2O3 molar ratio (code HSZ-390HUA) was purchased from Tosoh Corporation (Japan). Highly dealuminated zeolite ZSM-5 powder with a 280 SiO2/Al2O3 molar ratio of was purchased from Zeolyst (code CBV28014). 3 ACS Paragon Plus Environment
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B. Textural analysis. The specific surface area (SSA) and pore size distribution (PSD) of the zeolites were determined by means on nitrogen adsorption at liquid nitrogen temperature (-196°C) using an Autosorb-1-MP (Quantachrome Instruments). Details on samples pre-treatments and data elaboration are reported in our previous paper.17
C. FTIR Spectroscopy. Infrared spectra were collected on a Thermo Electron Corporation FT Nicolet 5700 spectrometer with 4 cm−1 resolution. Self-supporting pellets of the HSZ-Y and ZSM-5 zeolites were obtained with a mechanical press at ca. 7 tons cm−2 and placed into an IR cell equipped with KBr windows permanently attached to a vacuum line (residual pressure ≤ 1 × 10−4 mbar), allowing all treatments and pollutants adsorption/desorption experiments to be carried out in situ. Spectra of pollutants adsorbed on zeolites were collected at beam temperature (ca. 35 °C) on sorbents previously dehydrated (under vacuum at beam temperature). In order to investigate on the zeolites selectivity toward the adsorption of an equimolar toluene/n-hexane gaseous mixture (in dry conditions), specific experiments were carried out. First of all, the gas mixture was prepared in a glass flask. The pressures considered were always lower than the vapor pressure at room temperature (r.t.) of each pollutant to avoid condensation (at 25°C, the toluene and n-hexane vapor pressure is 37.9 and 204.0 mbar, respectively)18. The gaseous mixture was then left in the glass flask for ca. 7 days to make the mixture homogeneous. Then, the binary mixture of pollutants was admitted (maximum pressure of 30 mbar) on dehydrated zeolites, and after waiting for an appropriate time (ca. 60 min) for reaching the equilibrium, the mixture pressure was progressively decreased until the vacuum conditions (p
