Determınatıon Of The Surface Propertıes OfZsm-5 Zeolıte By Inverse Gas Chromatography

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1 15 th International Conference on Environmental cience an Technology Rhoes, Greece, 31 August to 2 eptember 217 Determınatıon Of The urface Propertıes OfZsm-5 Zeolıte By Inverse Gas Chromatography Bilgiç C. Department of Chemical Engineering, Faculty of Engineering an Architecture, Eskişehir Osmangazi University, 2648 Eskişehir, Turkey Corresponing author: cbilgic@ogu.eu.tr Abstract Inverse gas chromatography (IGC) has been use to measure the surface asorption of some probes (nalkanes on a series (n-c6 to n-c9) on ZM-5 zeolite. The asorption thermoynamic parameters (the stanar enthalpy ( H ), entropy ( ) an free energy of asorption ( G ) an the ispersive component of the surface energy ( ), of zeolite surface were estimate by using the retention time of ifferent non-polar probes at infinite ilution region. Dispersive component of the surface energy of zeolite was calculate between 29 an 32 C. It was observe that, γ values ecrease with increasing temperature. Keywors: Inverse gas chromatography, Asorption thermoynamic parameters, urface energy, ZM-5 zeolite ITRODUCTIO structures with ifferent pore sizes an shapes. s can absorb large amounts of molecules both in the gas an in liqui phases. Moreover, they are nontoxic an have goo thermal an chemical stability. Due to this, they became very popular in ifferent areas of chemical research an are wiely use in inustrial, agricultural, environmental an biological technology. They are applie for water purification, gas rying, an separation of mixtures of ifferent compouns. ZM-5 is one of the more important zeolites ue to its high number of inustrial applications, mainly in the fiel of aci catalysis (Chen, 1989; atterfiel, 1991). It is a synthetic zeolite of meium pore size, an its structure an properties have been wiely stuie an are well known. The crystalline structure consists of two sets of channels: a set of straight channels (elliptical cross-section of about 5.7x5.1 A ) an another set of sinusoial channels (circular cross-section of about 5.4 A iameter) (Dwyer, 1989). s are a large group of minerals that belongs to the class of silicates an have ifferent chemical composition, framework an properties. The term zeolite was introuce in 1765 by weish mineralogist Axel Freerik Cronstet. The name comes Greek an means boiling stones. s have the ability to accumulate so-calle zeolite water in the internal channels. This water can be remove as a result of heating an then reabsorbe or replace by other substances (Piaskowski, K. an Anielak A.M., 2; Karge, et al., 1982). s are microporous, crystalline aluminosilicate minerals. They are ivie into natural (e.g. morenite, clinoptilolite, chabazite, analcime) an synthetic. There are known more than 4 natural zeolites, an synthesize more than 1. The most popular synthetic zeolites are zeolite type A, X, Y (Ruren, et al., 27). s are crystalline aluminosilicates containing pores an cavities of molecular imension. The primary structural units of zeolites are the tetraheral of silicon an aluminum. The framework contains well organize an regular system of channels an cages. Insie these vois are water molecules an small cations which compensate the negative framework charge an can be easily exchange by other ions. In these solis ominate asorption of molecules that fit snugly insie the pores an exclue molecules that are too large. Thus, zeolites act as sieves on a molecular scale an belong to the family of molecular sieves (Van Bekkum, et al., 21). They are very goo matrices for hosting nanosize particles because of large variety of crystalline Inverse Gas Chromatography (IGC) was applie for the examination of surface properties of the aluminosilicates at the ifferent conitions (humiity). IGC is the extension of conventional gas chromatography (Voelkel, A., 24; Belgacem an Ganini, 1999; chreiber, H.P. an loy, D.R., 1989). The wor inverse means that the examine material is place in the chromatographic column an its properties are etermine base on the retention behaviour of carefully selecte test compouns. Dispersive properties of the surface may be stuie by means of IGC. Reverseflow GC (or time-resolve GC ) introuce by Katsanos an Karaiskakis is sophisticate version of IGC technique (Katsanos an Karaiskakis, 24). Time-resolve IGC can be use in examination of such properties as asorption energies (Katsanos, et al., 1999) local monolayer capacities, local isotherms, energy istribution functions, asorption rates with lateral molecular interactions (Katsanos, et al., 22), mass transfer coefficients (Karaiskakis, et al., 1986), surface iffusion coefficients, activity coefficients ( (Katsanos et al., 1985) an other (Katsanos an Karaiskakis, 24; Thielmann, et al., 21; Bogillo et al., 1998; chultz, et al., 1987). In the IGC the molecular sieve zeolite to be characterize is use as the stationary phase, an a solute with wellknown properties is injecte as a probe. everal thermoynamic quantities, incluing surface energy, can thus be erive the retention volume of probe passing CET217_74

2 through the column fille with zeolites. This work was thus unertaken in orer to investigate how IGC coul be applie in estimating ispersive components of surface energies of ZM-5 zeolite. Another purpose was to etermine the variation in asorption thermoynamic parameters of some hyrocarbons on zeolite minerals. 2. MATERIA AD METHOD 2.1. Materials ZM-5 zeolite (CBV2814) was irectly supplie in the ammonium form by Zeolyst International. For the IGC analysis, the polar probes use were n-hexane n-heptane, n-octane, n-nonane. Properties were taken literature of the probes (Coreiro et al., 211). These probes are commonly use for soli surface characterizations by IGC metho. The chromatographic experiments were performe with Agilent 789 gas chromatography equippe with a flame ionization etector (FID). A vacuum pump was use for packing the solis into the columns. High purity nitrogen was use as the carrier gas with a flow rate of about 4 an 6 m/min. The flow rate of carrier gas was correcte for pressure rop an temperature change in the column using James-Martin gas compressibility factor. A stainless steel column (2. m long, 5.35 mm I.D.) previously washe with methanol an acetone was packe with zeolite powers. The two ens of the column were plugge with silane-treate glass wool. Measurements were carrie out in the temperature range of C. The column was stabilize overnight in stream of nitrogen at 35 C. The ea volumes of the columns were etermine by injecting methane. At least three replicate eterminations were use in averaging the net retention volume (V ) Calculations IGC is use to stuy the surface free energy an aci base properties of solis; the stuy is generally performe in the area of infinite ilution where the interactions between the asorbe probe molecules are negligible. The basic value in IGC is the net retention volume. The net retention volume is the volume of carrier gas require to elute a given probe. The net retention volume (V ) is calculate using the following equation: V T a Ta t t.f..j R m where t R is the probe retention time, t m is the retention time of the mobile phase (holup time, ea-time), etermine by methane, F a is volumetric flow rate measure at column outlet an at ambient temperature, T a is ambient temperature (K), T is the column temperature (K) an j is James-Martin gas compressibility correction factor. When asorption takes place in the Henry s law region, the stanar free energy of asorption ( G ), as a function of V, can be etermine : G (1) V P RT ln (2) m where R is the ieal gas constant, m is the mass of asorbent in the column, is the specific surface area of asorbent, P is the asorbate vapor pressure in the gaseous stanar state having a value of 11.3 k/m 2 (1 atm) an is the reference two-imensional surface pressure whose stanar state is arbitrary. The stanar reference state was taken as =.338 mj/m 2 propose by e Boer (Boer, 1953). Therefore, G can be written as: G RT ln + C (3) V At zero surface coverage H is the ifferential heat of asorption of the probe an it is estimate the changes in V with temperature, i.e.: lnv H R (4) (1/ T) The stanar entropy change of asorption of the probe at zero coverage,, can be calculate the following equation: H G (5) T Provie that H is temperature inepenent in the investigate temperature range, Eq. (5) preicts a linear relationship between G an H (Vial et al., 1987, Roriguez et al., 1997, Chappell an Williams, 1989) Generally, surface free energy of a soli ( ) can be split into two components: the ispersive component ( ), an the specific component ( sp ). The ispersive component epens on the van er Waals ispersion forces which are relatively weak, while the specific component contains all the polar forces (Milonjic, 1999, Ansari an Price, 24). There are various methos for calculating the ispersive component of the surface free energy using IGC, but two of these are the most wiely use. These are the Dorris Gray an chultz methos. In both these methos, when calculating, a homologue alkane series in extremely small concentrations (infinite ilution) an in isothermal conitions is injecte into the column in sequence. A single numerical value is calculate the acquire alkane series retention time ata at stuie column temperature (Ylä-Mäihäniemi et al., 28). The calculation of accoring to the chultz metho, using infinite ilution measurements, is one using the following equation (hi et al., 211). RTnV (6) 1/ 2 1/ 2 2.( ). ( ) C (7) where V is the net retention volume of the n-alkane probe, R is the gas constant, T is the absolute column temperature (K), a is the molecular surface area coate with a kin of asorbe alkane, is Avogaro's number, an C is the constant. In this equation, is the ispersive component of the surface free energy of the soli phase, CET217_74

3 an is the ispersive component of the surface free energy of the probe. The plot of RT ln V versus 1/ 2 can be useful. uch a plot is linear an the slope of the lines gives ispersive free energy of the soli phase. The values of 1/ 2 are necessary for the calculations an can be easily foun in the literature. 3. REUT AD DICUIO The chromatographic peaks were symmetric an ha maxima that were inepenent of the amount injecte. It is assume that the asorption occurs in the Henry's law region (at zero surface coverage) where lateral interactions between the asorbates at the surface can be neglecte. The net retention volumes, V, were obtaine the maxima of the chromatographic peaks an the ea time volume. In this stuy, thermoynamic parameters for asorption of polar probes on zeolite were etermine in the infinite ilution region. tanar free energies changes of asorption ( G ) were calculate by Eq. (2). Differential heats of asorption ( H ) were calculate plots of lnv against 1/T. The slopes of the lines are H / R accoring to Eq. (4). tanar entropies of asorption ( ) were calculate by Eq. (5). Thermoynamic parameters (average values) are given in Table 1. The values of H, an G increase with increasing carbon number an the linear increase was obtaine for the n-alkanes. This is ue to the increase in the boiling points of n-alkanes an to the stronger interaction between the solute an asorbent surface. The more negative the heat of asorption, the greater the interaction between the asorbate an the asorbents. Table 1. Thermoynamic parameters for ZM-5 zeolite G o ( kjh o. mol ) ( kj o. mol K ) ( kj. mol ) n-hexane n-heptane n-octane n-onane was observe that values ecrease with temperature. The obtaine values for zeolite were compare to the corresponing values of ifferent clay types reporte by various authors an liste in Table COCUIO IGC metho is base on the stuy of physical asorption of appropriate molecular probes by means of chromatographic (ynamic) experiments. In contrast to static methos, ynamic systems utilize a flowing gas system. The most common flow methos are IGC, gravimetric instruments, an permeability measurement systems. The principle of ynamic gravimetric systems is the measurement of the amount of solute asorbe a flowing gas stream using a microbalance. Dynamic measurements give less accurate results when compare with static methos because they rely on measuring a small ifference between quantities at ifferent temperatures. However, for heats at zero coverage, infinite ilution gas chromatography is a more reliable metho because it requires no extrapolation of ata over a region where the heat can be very sensitive to small changes in coverage. In this stuy, showe that IGC is a powerful analytical technique very useful for stuying the surface properties of ZM-5 zeolite an for monitoring asorption processes. Thermoynamic information on asorption was obtaine the temperature variation of the partition coefficients for probes at zero coverage. The high value of the ispersive component of free energy of asorption probably was relate to structural heterogeneities on the lateral surfaces, as well as to the channels an pores present at C. The ispersive component of the surface free energy,, was etermine by injection of a homologous series of n- alkanes having between 6 an 9 carbon atoms. One of the most commonly measure parameters for the escription of the energy situation on the surface of a soli is the surface energy. The surface energy can affect, e.g. catalytic activity or the strength of particle-particle interaction. The ispersive components of zeolite at experimental temperatures were calculate Eq. (7), (Fig. 1). The variation of calculate as a function of temperature. It CET217_74

4 RTlnV kj/mol Table 2. Comparison of the present an reporte ispersive component of surface energy T ( o C) (mj/m 2 ) 13X (Diaz et al.,24) ay (Aşkın, an Bilgiç 25) 5A Alümina Activate (Diaz et al.,24) (Diaz et al.,24) carbon (Diaz et al., 24) epiolite Kaolinite ZM-5 (azarević et al. 29) (Kubilay et al. 26) This tuy ºC ºC 12 3 ºC 29 ºC a( ).5 (Å 2 mj.5 m -1 ) Figure 1. The ln( V ) RT 1/ 2 graphs for ZM-5 CET217_74

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