ARTICLE pubs.acs.org/IECR
Surface Characterization of Iron-Modified Sepiolite by Inverse Gas Chromatography astvan,† Antonije Onjia,‡ Jugoslav Krstic,§ Djordje Janackovic,† and Slavica Lazarevic,*,† Ivona Jankovic-C † Rada Petrovic †
Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia The Vinca Institute, POB 522, 11001 Belgrade, Serbia § Department of Catalysis and Chemical Engineering, Institute of Chemistry, Technology and Metallurgy, Njegoseva 12, Belgrade, Serbia ‡
ABSTRACT: Inverse gas chromatography at infinite and finite surface coverages was applied to evaluate the surface properties of an iron-modified sepiolite sorbent, Fe-sepiolite. The adsorptions of different nonpolar and polar probes were measured in the infinite-dilution regime, in the temperature range 483513 K. For the organic probes used, the thermodynamic parameters (free energy, enthalpy, and entropy) of adsorption and the dispersive and specific components of the free energy of adsorption were determined and are discussed in terms of specific reactions that can occur on the adsorbent surface. Following the approach of Gutmann (The DonorAcceptor Approach to Molecular Interactions; Plenum Press: New York, 1978), acid/base surface characteristics were determined. The adsorption isotherms of n-hexane, benzene, chloroform, and tetrahydrofuran were used to estimate the specific surface areas, isosteric heats of adsorption, and adsorption energy distributions for the adsorption of organic molecules.
1. INTRODUCTION In inverse gas chromatography (IGC), the interactions of gaseous probe molecules with a stationary phase immobilized within a column results in a characteristic retention time, tR, that can be translated into a number of important surface properties. The techniques of IGC provide information on the thermodynamic properties, surface energy, morphological parameters (such as surface area and porosity), and reaction kinetics associated with gassolid adsorption and catalytic reactions. Furthermore, IGC is a rapid method that can be applied over a wide range of temperatures and for various adsorbates (molecular probes), which can be selected for their structure and intermolecular interactions. Two IGC techniques can be distinguished: infinite-dilution IGC and finite-solute-concentration IGC. In case of infinitedilution IGC, very small amounts of probes are injected onto the column, which permits the lateral interactions between the adsorbed molecules to be neglected and results in retention volumes that are independent of the injection sample size. In the finite-concentration IGC mode, measurable amounts of solutes are injected, leading to skewed chromatographic peaks. The retention volumes are dependent on the adsorbate concentration in the gas phase. From the peak shapes, adsorption isotherms and adsorption energy distributions can be calculated. IGC has been widely used to study surface properties of various materials, such as polymers,1 foods,2 carbons,35 fibers,6 clays,79 alumina,10 silica,11 calcium carbonate,12 zeolites,13 and catalysts.14 Sepiolite is a natural hydrated magnesium silicate with a theoretical half-unit-cell formula of Si12Mg8O30(OH)4(OH2)4 3 8 H2O and a wide range of industrial applications derived mainly from its adsorptive properties. The size of the fibers is variable, but an average of 800 25 4 nm is typical, thus resulting in a solid with an external surface area of the same order of magnitude r 2011 American Chemical Society
as the area of the micropores. The structure of sepiolite, in some aspects, is similar to those of other 2:1 trioctahedral silicates, such as talc, but it has discontinuities and inversions of the silica sheets that give rise to structural tunnels and blocks. Each structural block is formed of two tetrahedral silica sheets enclosing a central octahedral magnesia sheet. In the octahedral sheet, Mg2+ ions occupy two different structural positions: on the edges of the octahedral sheets, completing their octahedral coordination by bonding to two molecules of water (coordinated water), and in the interior of the blocks linked to hydroxyl groups. In the inner blocks, all corners of the silica tetrahedra are connected to adjacent blocks, but in the outer blocks, some of the corners are Si atoms bound to hydroxyl groups (SiOH).15 The periodic inversion of the SiO4 tetrahedra after every six units is the origin of structural channels with dimensions of 0.37 1.06 nm running parallel to the length of the fibers. These channels are occupied by zeolitic water and account in large part for the high surface area and excellent adsorptive properties of sepiolite once the zeolitic water has been removed by thermal treatment that does not exceed 200 °C. This unique structure allows the penetration of organic and inorganic species into the structure. Based on such structural features, its high surface area, and its high physicochemical activity, sepiolite is used in numerous applications such as catalysts and catalyst supports,16 membranes for ultrafiltration,17 adsorption, decolorization, and molecular sieving.18 The use of particles of clayiron oxide systems to solve environmental problems has received considerable attention in Received: March 24, 2011 Accepted: August 20, 2011 Revised: July 18, 2011 Published: August 21, 2011 11467
dx.doi.org/10.1021/ie200595n | Ind. Eng. Chem. Res. 2011, 50, 11467–11475
Industrial & Engineering Chemistry Research recent years. Such clayiron oxide systems are synthesized by adding natural clay to an iron nitrate or iron chloride solution under strongly basic conditions. These systems are able to improve catalytic characteristics and sorb higher concentrations of species because of the presence of the iron oxides, which are also active sorbents.1924 Iron oxides have relatively high surface areas and surface charges and can be applied for the treatment of heavy metals or organic matters from waste waters. In a previous study, the sorption of Ni2+ onto an iron-sepiolite solid system was described.25 Because of the presence of the Fe spaces located on external sepiolite sites, the new material is characterized by the presence of additional active sites (FeOH) that are able to react with organic and/or inorganic spaces. The aim of this study was to analyze the influence of the modification of sepiolite, that is, the formation of the sepioliteiron system, on the interaction with organic molecules from the gaseous phase obtained by IGC at zero and finite surface coverage. In the infinite-dilution IGC mode, the surface characteristics of the modified sepiolite (the dispersive components of the surface free energies, enthalpies, and entropies and the acid/base constants) were determined in the temperature range of 483513 K. Inverse gas chromatography at finite surface coverage was used to determine the adsorption isotherms, isosteric heats of adsorption, and adsorption energy distributions for the adsorptions of hexane, benzene, chloroform, and tetrahydrofuran on Fe-sepiolite.
2. EXPERIMENTAL DETAILS 2.1. Materials. The Fe-sepiolite (Fe-SEP) adsorbent was synthesized by following the method described by Doula.23 The system was prepared by mixing 20.0 g of sepiolite, 200 cm3 of freshly prepared 0.5 mol/dm3 FeCl3, and 360 cm3 of 1 mol/dm3 NaOH solution. The addition of NaOH solution was rapid under stirring. The suspension was diluted to 1 dm3 with water (18 MΩ obtained from a Milli-Q water purification system) and kept in a closed polyethylene flask at 343 K for 48 h. The precipitate was then centrifuged and washed with distilled water until Cl-ion-free. The chemical composition and the results of differential thermal analysis (DTA), X-ray diffraction, and FTIR analysis of the sample were reported previously.25 Nitrogen adsorptiondesorption isotherms were determined using a Thermo Finnigan/Sorptomatic 1990 instrument at 240 °C. Before the measurements, the Fe-sepiolite sample was degassed at 240 °C (the highest IGC experimental temperature) for 18 h under reduced pressure (
