Adsorption of Tetrachloroethylene on Cationic X and Y Zeolites

CsX and CsY samples present the lowest micropore volume, because the exchange cation has a larger ionic diameter. The decrease in micropore volume, an...
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Adsorption of Tetrachloroethylene on Cationic X and Y Zeolites: Influence of Cation Nature and of Water Vapor Marianne Guillemot, Je´ roˆ me Mijoin, Samuel Mignard, and Patrick Magnoux* Laboratoire de Catalyse en Chimie Organique, UMR CNRS 6503, UniVersite´ de Poitiers, 40 aVenue du Recteur Pineau, 86022 Poitiers Cedex, France

In this work, the removal of tetrachloroethylene (PCE) over faujasite-type zeolites exchanged with various cations was investigated under static and dynamic conditions. The nature of the non-framework cation appeared as a predominant parameter, in terms of adsorbate-adsorbent interactions as well as adsorbent micropore volume. Indeed, a molecular simulation has highlighted the interaction between cations and Cl atoms. Moreover, it was observed that the nature of the cation strongly influenced the adsorbent behavior in the PCE removal process. Cationic zeolites appeared as efficient materials for the removal of PCE with good adsorption capacities, in particular, the sodium-exchanged X and Y zeolites. The presence of steam in the gas feed was determined to have a strong negative effect on the performance of the cationic zeolites. However, the addition of 3A sieve in a double bed with NaY zeolite appeared to be a good system. The complete regeneration of the system was obtained at relatively low temperature (180 °C), which allowed it to be still performing after several adsorption/desorption cycles. Introduction The better understanding of the health and environmental hazards of many chemicals has led to the development of elimination processes to reduce atmospheric pollution. Because of their high persistence, some chlorinated solvents that are used in chemical industry contribute to the reduction of the stratospheric ozone layer and to global warming. Moreover, volatile organic compounds (VOC) and nitrogen oxides (NOx) are the major contributors to the formation of photochemical ozone.1-3 Chlorinated ethylenes, such as tetrachloroethylene (PCE) or trichloroethylene (TCE), are used in dry cleaning industry and are known to be harmful for human health and the environment. In 1999, European countries decided to reduce their emissions to 6.6 Gt per year by 2010.4 In such a context, adsorption is an interesting alternative to eliminate chlorinated compounds from industrial waste gases when the objective is to collect and reuse the solvents after desorption. Adsorption on activated carbon is the most widely used technique, because these materials have good adsorption capacities, thanks to their high micropore volume, generally in the range of 0.45-0.65 cm3/g.5-10 Currently, activated carbon adsorbents remain the main choice for air treatment processes, because they are cost-effective materials. However, their efficiency is limited by their sensibility to high temperatures and their difficult regeneration. For example, in the case of methyl ethyl ketone (MEK) and toluene adsorption over activated carbon impregnated with phosphoric acid, a temperature of 300 °C is needed to remove VOC from the adsorbent.11 In this context, some studies were conducted on others adsorbents, such as zeolites of various types (mainly faujasite (FAU), ZSM-5, and silicalite) in their acidic or basic forms.12-14 These solids have gained interest in removal techniques of chlorinated compounds, such as chloroalkenes (trichloroethylene, tetrachloroethylene) and chloroalkanes (chloroform, carbon tetrachloride, dichloromethane, 1,2-dichloroethane, 1,1,1-trichlo-

roethane) from industrial effluents, because of their stability at high temperatures and good adsorption properties. Zeolites also have the advantage to offer a large range of polarities, which is interesting since VOC molecules not only differ in their size but also in their polarities.15 Several studies have shown that zeolites can be effective, depending on the type of compound adsorbed and on the application conditions.16-20 Moreover, some authors observed that the most efficient adsorbents in the lowpressure range are those that have pore dimensions close to VOC molecule dimensions.9 This is generally the case when zeolitetype adsorbents are used for VOC removal. Two types of parameters are involved during the chlorinated VOC adsorption process: interactions between the framework and the Cl atoms of the adsorbate and unspecific interactions coming from the relation between the pore volume and the adsorbate molecule dimensions, which is probably the main factor. Zeolites can be divided into three families, depending on their pores size: zeolites with 8, 10, and 12 oxygen atom apertures, which are also respectively called small pore zeolites, zeolites of intermediate pore size, and large pore zeolites. In this work, faujasite-type zeolites were chosen because they offer a large microporosity, thanks to their 12 O atom apertures. Besides, alkaline cation-exchanged zeolites are able to create strong interactions with chlorinated molecules, since the basicity of the zeolite increases as the cation electropositivity increases. Both the enthalpies and adsorption capacities were determined via a static study of tetrachloroethylene adsorption. The modeling of PCE adsorption on sodium-exchanged X zeolite was also performed. In addition, the performances of basic X and Y zeolites were investigated for PCE removal under dynamic conditions, in dry and wet gas streams. To complete this study, the performances of a double bed 3A/NaY were evaluated under humid conditions, in terms of adsorption capacity and regeneration. Experimental Section

* To whom correspondence should be addressed. Tel.: +33-5-4934-98. Fax: +33-5-49-37-79. E-mail address: [email protected].

Adsorbents Preparation. The Cs-, K-, and Li-exchanged zeolites were prepared from a NaFAU zeolite by ion exchange

10.1021/ie0616390 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/24/2007

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Figure 1. Spectra of pyridine adsorption-desorption at 150 °C followed by infrared (IR) spectroscopy over (a) LiX, (b) NaX, (c) KX, and (d) CsX.

through the following steps. Three consecutive exchanges of NaFAU with the nitrate salt corresponding solution (0.5 mol/ L) are performed during period of 3 h under slow agitation, at room temperature. The zeolite was separated by filtration, washed with deionized water, and dried overnight at 100 °C, before it was subjected to a new exchange. Zeolites were then calcinated under a dry air flow at 450 °C for 12 h, with a 1-h step at 100 °C. Characterization of Adsorbents. Elementary analyses of the samples were performed by the analysis center of CNRS in Vernaison, France. Nitrogen adsorption-desorption measurements were performed at a temperature of -196 °C (77 K) with an ASAP 2010 (Micromeritics) gas adsorption system. Samples were previously pretreated under vacuum at 350 °C overnight. The “t-plot” method was applied to obtain an estimation of the micropore volume. The acidity of the catalysts was characterized by pyridine adsorption, followed by infrared (IR) spectroscopy. IR spectra were recorded with a Fourier transform infrared (FTIR) ThermoNicolet spectrometer (NEXUS), using thin pellets (515 mg/cm2) activated in situ in the IR cell in air (60 mL/min) at 450 °C for 12 h. The IR spectra were recorded on the sample at room temperature after activation and after pyridine thermodesorption under vacuum (2 × 10-3 Pa) for 1 h at 150 °C. The concentration of Brønsted and Lewis sites able to retain pyridine adsorbed at 150 °C were determined from the adsorbance surface area of the bands at 1545 and 1454 cm-1, respectively, using the extinction coefficients previously determined.21 Adsorption Apparatus. A DSC 111 calorimeter (Setaram) equipped with a volumetric vacuum line was used for microcalorimetric and volumetric study of tetrachloroethylene adsorption. The physical properties of PCE are reported in Table 1. Each sample (50 mg) was pretreated overnight at 350 °C under vacuum (10-6 bar). Adsorption was performed at 25 °C, by admitting successive doses of PCE and recording the thermal effect. The equilibrium pressure after each adsorption was measured using pressure gauges, which permit to measure pressures of 0-12 mbar. Adsorption isotherms and heats of adsorption were determined simultaneously.

Table 1. Tetrachloroethylene (PCE) Physical Characteristics characteristic

value

molar mass density at 25 °C saturated vapor pressure at 25 °C heat of vaporization, ∆H°vap at 25 °C boiling temperature, Tb

165.8 g/mol 1.61 g/cm3 2.48 × 103 Pa 39.7 kJ/mol 121.1 °C

Dynamic Adsorption of Tetrachloroethylene (PCE). Adsorption experiments were conducted at a constant temperature (50 °C) and atmospheric pressure in a fixed-bed column. The Pyrex column tube had an inner diameter of 5 mm and was 70 cm long. The adsorbent is deposited on a glass wool layer. The zeolites were pelletized, crushed, and then sieved to grains with a diameter of 0.2-0.4 mm. Each adsorbent was pretreated under dry nitrogen at 350 °C overnight and then cooled down to the adsorption temperature (50 °C). The mass of the dry zeolite was 100 mg. The gaseous feed was obtained by passing nitrogen through two saturatorssone at room temperature, containing water, and the other one, containing PCE, was placed in a thermostated bath. The mixture that contained PCE (573 or 1146 ppm) and water (17 500 ppm, i.e., 50% of relative humidity) was then passed through the adsorbent bed. For dry experiments, the water bubble flask was disconnected. The inert flow was 50 or 100 mL/min, which gave global gas hourly space velocity (GHSV) values of 15 300 or 30 600 h-1, respectively. Inlet and outlet PCE levels were measured on-line, using a model 3400 gas chromatograph (Varian) that was equipped with a CP-SIL 5 capillary column linked to a flame ionization detection (FID) device. Breakthrough was defined as the time when the PCE concentration in the outlet stream reached 20 ppm. Saturation was defined as the point where the inlet and outlet PCE concentrations merged. The amount of PCE adsorbed (in units of mol/g), before breakthrough and at saturation, was determined from the amount of PCE adsorbed deduced from breakthrough curves at a given relative pressure of PCE and temperature. It was then possible to convert that value to unit of grams of PCE per gram of zeolite, using the molar weight of PCE. The convertion in molecules of PCE per supercage was then obtained via the number of

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Table 2. Adsorbents Characterization acidity (µmol/g)b formulaa

absorbent LiY NaY KY CsY LiX NaX KX CsX

Na31Li25Al56Si136O384 Na56Al56Si136O384 Na4K52Al56Si136O384 Na18Cs38Al56Si136O384 Na46.5Li41.5Al88Si104O384 Na88Al88Si104O384 Na17K71Al88Si104O384 Na44.3Cs43.7Al88Si104O384

exchange ratio (%) 45 94 67 47 81 50

Si/Al

Brønsted

Lewis

micropore volumec (cm3/g)

2.4 2.4 2.5 2.5 1.2 1.2 1.2 1.2

0 0 0 0 0 0 0 0

n.d.d 525 n.d.d n.d.d n.d.d 1358 n.d.d n.d.d

0.352 0.345 0.306 0.221 0.297 0.291 0.247 0.181

a Determined from elemental analysis. b Determined by pyridine adsorption-desorption at 150 °C followed by IR spectroscopy. c Determined by the t-plot method. d Not determined.

supercages per gram of zeolite, known from the molar weight of the zeolite. Results and Discussion Characterization of Zeolite Samples. Faujasite zeolites were chosen because they generally present high micropore volumes, thanks to the 12-oxygen aperture supercages. Table 2 reports the physicochemical characteristics of the zeolites. As expected, Na-, Li-, K-, and Cs-exchanged zeolites do not exhibit Brønsted acidity. The Lewis acidity in the case of NaX and NaY samples is due to pyridine adsorption over Na+ cations, since bands at 1445, 1491, and 1596 cm-1, corresponding to pyridine coordinated to Na+ cations, are observed.22 As shown on Figure 1, the wavenumbers of absorption bands of pyridine over cations shift to lower values from LiX to CsX, that is to say, when the electropositivity of the exchange cation increases. The charge density on the oxygen atoms increases as the compensating cation electropositivity increases, leading to a more basic zeolite. Also note that pyridine is retained at 150 °C by alkaline cations. Consequently, cationic zeolites can be expected to be good adsorbents for the PCE elimination process. Among X and Y faujasite zeolites, Li- and Na-exchanged adsorbents are the two that have the highest microporosity. CsX and CsY samples present the lowest micropore volume, because the exchange cation has a larger ionic diameter. The decrease in micropore volume, and, consequently, pore diameter consecutive to cationic exchanges, could increase the heat of adsorption by increasing the adsorbate-adsorbent interactions. On the other hand, this could also hinder the adsorption of voluminous molecules, from a steric point of view. In the case of Li- and Cs-exchanged faujasite, the exchange rate is quite low. Because of its low ionic radius, the Li cation is strongly solvated in aqueous solution, which can make the exchange of Na+ cations by Li+ cations unfavorable. Concerning the Cs+ cation, its large size probably limits its access to the supercages. Moreover, because cations located in sites III (i.e., at the entrance of the supercages) are the first to be exchanged by cesium, the main part of sites II (located inside the supercages) remains inaccessible and, therefore, are occupied by Na+ cations. That could explain the lower exchange rate of CsX, compared to that of CsY. Their large size also prevents Cs+ cations from reaching sites I in the sodalite cages. Heats of Tetrachloroethylene (PCE) Adsorption on X Zeolites. One aspect of VOC adsorption on zeolites and, in particular, in the case of chlorocarbons, is the interaction between the framework and the extra framework species as exchange cations with the polarizable Cl atoms.23 The modeling of the adsorption of one PCE molecule on NaX zeolite allowed us to see the adsorption mechanism involved.

We used CERIUS2 software that was obtained from Biosystem/ Molecular Simulations. Simulations are based on the calculation of minimal energy. The force field used is that developed by Mellot et al.23 These authors have observed that, at low sorbate loadings, the heat of adsorption of chloroform was higher on NaX than on NaY and on siliceous faujasite. These results highlight the importance of the charge-compensating cations. The NaX zeolite (Na88Al88Si104O384) was built by loading 32 cations in sites I/I′, 32 in sites II/II′, and 24 in sites III/III′. It seems that the PCE molecule is adsorbed within two configurations. The first configuration involves two Na+ cations, one in site II and the other one in site III (Figure 2a). The second configuration is more stable, because three Na+ cations are involved; one in site II, the second in site III, and the last in site III′ (see Figure 2b). From these results, and taking into account the large size of the Cl atom, in comparison to the C atom, one could assume that the main interactions between the chlorocarbon and the zeolite involve the Cl atoms of the PCE molecule and the non-framework cations. These two configurations can be separated by their potential energy: the first configuration is observed at 63 kJ/mol and the other configuration is observed at 196 kJ/mol. This high adsorption energy can be explained by the interaction with two cations in sites III and III′, since those sites have higher potential energy than sites I and II.24 These results are consistent with those obtained by Pinard et al., who studied the adsorption of dichloromethane on NaX zeolite.25 They showed that the adsorption of this chlorinated compound on Na+ cations of sites III and III′ has a higher energy than its adsorption on cations of sites II. The experimental results show an average of the two potential energies previously obtained by molecular modeling. On Figure 3, the heats of PCE adsorption, as measured by microcalorimetry, are given as a function of the amount of PCE adsorbed, in terms of molecules per supercage. Calorimetric study was often reported to be a good method to determine the energy distribution of the surface sites of solids. First, we can observe a step at ∼80 kJ/mol, up to a value of 2.8, 3, and 3.5 molecules per supercage for LiX, KX, and NaX, respectively. In the case of CsX zeolite, the plateau is observed at 90 kJ/mol, up to 1.5 molecules per supercage. This plateau corresponds to adsorption on the predominant sites, that is to say, on alkaline cations located inside supercages.12,26 It is generally admitted that 70 kJ/mol is the energy limit between physisorption and chemisorption. On the four samples studied here, the plateau is superior to this value, which means that PCE molecules are chemisorbed and confirms the results of molecular simulation, showing strong interactions between PCE molecules and zeolite compensating cations. The adsorption of PCE on CsX zeolite shows a slightly higher energy than that on other zeolites. This behavior can be ascribed to the more

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Figure 2. Modeling of tetrachloroethylene (PCE) molecule adsorption on Na sites in a faujasite (FAU)-type X supercage: (a) Esim ) 63 kJ/mol and (b) Esim ) 196 kJ/mol. (Distance values are given in angstroms.)

Figure 3. Adsorption enthalpies of PCE as a function of adsorbed amount, expressed in terms of molecules per supercage, at 25 °C, on (4) LiX, (0) NaX, (O) KX, and (×) CsX zeolites.

Figure 4. Adsorption isotherms of PCE at 25 °C, on (4) LiX, (0) NaX, (O) KX, and (×) CsX zeolites.

basic character of this zeolite, which is due to the higher electropositivity of the exchange cation. Simonot-Grange et al. made a similar study that involved the adsorption of 1,2dichloroethane on silicalite.27 The lower adsorption energy can be explained by the weaker interactions between the chlorinated VOC and the zeolite framework. The drastic decrease in the heats of adsorption corresponds to the end of the filling of micropores and to PCE adsorption at the external surface. Since the enthalpy of a multilayer adsorption is lower than the heat of adsorption in micropores, the enthalpy decreases continuously. It can be noticed that, over CsX zeolite, the number of adsorbed PCE molecules per supercage is less than that over NaX. This result is in agreement with the lower pore volume obtained on CsX (see Table 2), which is only due to the presence of the bigger Cs+ cations, which leads to a limitation of PCE access to all cationic sites. As shown by the molecular simulation, the adsorption of the PCE molecule on NaX zeolite involves interactions between exchange cations and Cl atoms. We can expect that, in the adsorption process, there are two types of influence. With the study of the heats of adsorption, we saw the interactions between the framework and the Cl atoms of the adsorbate. However, the main factor is probably the unspecific interactions that lead to a relationship between pore volume and adsorbate molecule dimensions. Adsorption Isotherms of Tetrachloroethylene (PCE) on X Zeolites. The adsorption isotherms of PCE on LiX, NaX,

KX, and CsX zeolites (Figure 4) show that, unexpectedly, the LiX zeolite has a lower adsorption capacity than the NaX zeolite. The exchange rate of LiX zeolite is 47% and its micropore volume (0.297 cm3/g) is similar to that of NaX (0.291 cm3/g), so similar adsorption capacities were expected. These results show that there are, most probably, one or several parameters that limit the adsorption capacity of LiX, and one of them could be the adsorption energy of PCE on LiX. The Li+ cation is known to be very hydrophilic, and, consequently, it exhibits low interactions with nonpolar molecules, such as PCE. Consequently, in the case of PCE adsorption over the LiX zeolite, probably weaker interactions between adsorbate molecules and zeolite framework are involved, which can explain the obtained results. The lowest adsorption capacity is observed for the CsX zeolite. Actually, this adsorbent has the lowest micropore volume, compared to the other three zeolites, because of the large diameter of the Cs cation. We can then assume that the micropore volume is one of the limiting parameters for PCE adsorption on FAU-type adsorbents. The volume of one supercage of NaX is of 814 Å3. It was estimated from the total micropore volume determined by nitrogen adsorption, since N2 can only penetrate into supercages,28 and the number of supercages per gram of zeolite (3.58 × 1020 supercages/g). The volume of one PCE molecule was then determined from the total amount of PCE adsorbed on NaX (2.061 mmol/g, corresponding to 1.24 × 1021 molecules/g) and the PCE liquid density (making the assumption that PCE was in the liquid phase): it is equivalent to 170 Å3. It is then

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Figure 5. Breakthrough curves of PCE (1146 ppm) at 50 °C on (a) (4) LiX, (0) NaX, (O) KX, and (×) CsX zeolites and (b) (4) LiY, (]) NaY, (O) KY, and (×) CsY zeolites. (GHSV ) 15 300 h-1.)

theoretically possible to adsorb 4.8 molecules per supercage, which is greater than the experimental value of 3.5 molecules per supercage. Two hypotheses can be proposed to explain this difference. First, it was assumed that PCE was in the liquid phase at the loading of 3.4 molecules per supercage (i.e., P/Psat ) 0.41), but as reported by Goulay et al.28 this could not be the case here. However, as shown on Figure 3, the adsorption isotherm plateau is already attained at a relative pressure of 0.41, meaning that saturation has been attained. One then could also assume that, at saturation, all of the free volume of the NaX supercage is not filled by PCE molecules, because of steric hindrance. Dynamic Adsorption of PCE on Cationic FAU-Type Zeolites in Dry Conditions. PCE dynamic flow adsorption was performed over various faujasite-type X and Y zeolites. The experiments were performed with a concentration of PCE of 1146 ppm and a gas hourly space velocity (GHSV) of 15 300 h-1. The breakthrough curves give the evolution of the C/C0 ratio as a function of time, where C is the concentration of PCE at the outlet of the column and C0 is the concentration of PCE at the inlet. The adsorption capacity before breakthrough corresponds to the amount of PCE adsorbed so long that the outlet concentration is inferior to 20 ppm. The saturation of the zeolite is obtained when C/C0 equals 1. Figure 5 shows that the NaX and NaY zeolites have almost the same breakthrough time, corresponding to adsorption capacities of 30.1 and 30.6 wt %, respectively. It could be expected that NaX exhibited higher adsorption capacity than NaY, because of its higher number of adsorption sites (88 cations per supercage for NaX versus 56 for NaY). However, this is not the case. We can then assume that, here, two parameters are opposed: on one side, the number of adsorption sites and on the other side, the micropore volume. Since the micropore volume decreases as the number of nonframework cations increases, in the case of NaX zeolite, its lower micropore volume (0.291 cm3/g versus 0.345 cm3/g for NaY) is the limiting parameter for PCE adsorption. Then, concerning NaY, its higher micropore volume does not provide it a higher adsorption capacity, probably because of the limited number of adsorption sites. A similar study was conducted on Y and X zeolites exchanged with Li+, K+, and Cs+ cations. As shown in Figure 5, we obtained various behaviors. In both cases, the Na+-exchanged sample has the best adsorption capacity, before breakthrough and at saturation (see Table 3). The evolution of the adsorption capacities with the exchange cations is quite similar to that observed for PCE adsorption under static conditions. The adsorption capacities decrease as the

Table 3. Adsorption Capacities of PCE Qads Before Breakthrough

At Saturation

adsorbent

molecules/ R-cage

g/ga

molecules/ R-cage

g/ga

LiX NaX KX CsX LiY NaY KY CsY

2.3 3.0 1.9 1.2 1.8 2.9 1.5 1.5

0.24 0.30 0.18 0.09 0.19 0.31 0.14 0.13

3.3 3.7 3.5 1.5 2.4 3.5 2.2 2.1

0.34 0.37 0.32 0.11 0.26 0.37 0.22 0.19

a

Grams of PCE per gram of zeolite.

diameter of the exchange cation increases. However, as observed previously, Li-exchanged zeolites exhibit particular results. Although the micropore volumes of LiX and LiY are quite similar to those of NaX and NaY, respectively, the adsorption capacity of LiX and LiY before breakthrough is much lower. In the case of the LiX sample, the adsorption capacity at saturation is similar to that of the NaX zeolite. Therefore, approximately the same amount of PCE can be adsorbed on both adsorbents, but the PCE breaks through more rapidly on LiX than on NaX. These results merge those obtained over the same adsorbents under static conditions. As mentioned previously, this could mean that fewer adsorbate-adsorbent interactions are involved on LiX than on NaX, because of the lower electropositivity of the exchange cation. The K-exchanged zeolites exhibit lower adsorption capacities, despite the higher electropositivity of the K+ cation. One could assume that, in this case, the micropore volume is the limiting factor, because that of the KX zeolite is less than that of the NaX zeolite. We also observed a large difference between the CsX and CsY behaviors. Indeed, the CsY zeolite adsorbs more PCE than CsX, probably because of its higher micropore volume. This can be explained by the localization of Cs+ cations in both zeolites: In CsY, no cations are located in sites III, contrary to CsX zeolite. The sites III are at the entrance of the supercages, so that cations localized in these places probably decrease the micropore volume and hinder the access of PCE molecules to the supercage. Dynamic Adsorption of PCE on Cationic FAU-Type Zeolites in the Presence of Water Vapor. Because of the fact that industrial effluents that contain chlorinated hydrocarbons generally also contain water vapor, it is important to know the adsorbent behavior in the presence of steam. To examine this point, water vapor was added to the inlet gas, which already contained 573 ppm of PCE. The relative humidity of the effluent

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Figure 6. Breakthrough curves of PCE (573 ppm) at 50 °C on (a) (4) LiX, (0) NaX, (O) KX, and (×) CsX zeolites, and (b) (4) LiY, (]) NaY, (O) KY, and (×) CsY zeolites. (RH ) 50%, and GHSV ) 30 600 h-1.) Table 4. Adsorption Capacities of PCE over the 3A/NaY System Qads Before Breakthrough run

breakthrough time (min)

molecules/ R-cage

g/ga

molecule/ R-cage

g/ga

1 2 3 4

41 39 41 39

2.9 2.7 2.9 2.7

0.30 0.28 0.30 0.28

3.9 3.6 3.8 3.7

0.40 0.37 0.39 0.38

a

Figure 7. Breakthrough curves of PCE (1146 ppm) at 50 °C on a 3A/NaY double bed (RH ) 50%, GHSV ) 30 600 h-1): (0) 150 mg of 3A, (O) 500 mg of 3A, and (4) 750 mg of 3A.

was 50%. The GHSV of these reactions was 30 600 h-1. As shown in Figure 6, all the cationic zeolites exhibit the same behavior. First, PCE is adsorbed (C/C0 ) 0), then PCE breaks through more or less rapidly, depending on the nature of the exchange cation. The PCE concentration at the adsorbent outlet increases rapidly, becoming higher than that at the adsorbent inlet. This means that adsorbed PCE molecules are displaced from the adsorbent by water molecules.29 The final loading of PCE is nil when the adsorbent is filled by water. It shows that, in the presence of water vapor, cationic zeolites behave like hydrophilic adsorbents, showing no selectivity toward PCE. To compensate for the influence of steam, the adsorption experiments were performed over a 3A sieve/NaY zeolite double bed. This latter was selected because it appeared to be the best adsorbent among those tested in this work. The 3A sieve is a 3-Å-aperture adsorbent that is able to adsorb only water and no PCE. The 3A bed was placed first in the effluent way, the NaY bed coming next. Consequently, water was expected to be adsorbed on the 3A sieve until the complete filling of NaY by PCE occurred. Various amounts of the 3A sieve were tested, and the corresponding breakthrough curves are reported in Figure 7. With 150 mg of the 3A sieve, water breakthrough on 3A occurs earlier than that for the PCE on NaY, leading to the complete desorption of PCE from NaY before its complete filling. With 500 and 750 mg of the 3A sieve, PCE breakthrough occurs at the same time. In the first case, we observe a desorption peak immediately after the beginning of the breakthrough. We can then assume that water and PCE breakthroughs occur simultaneously. On the contrary, 750 mg of the 3A sieve allows us to separate PCE breakthrough on NaY from that of water on the 3A sieve. The first one occurs at 41 min, and water breakthrough, which corresponds to the desorption peak of PCE, is observed at 84 min. The adsorption capacities of NaY before

At Saturation

Grams of PCE per gram of zeolite.

breakthrough (30 wt %) and at saturation (40 wt %) are then very similar to those obtained without water vapor. To evaluate the stability and the regeneration capabilities of this system, four adsorption/desorption cycles were performed over the same adsorbents couple (3A/NaY). As shown previously, PCE can be completely removed from NaY by water molecules at room temperature. However, it was necessary to heat both the 3A sieve and NaY, to completely eliminate water before the next adsorption. For this reason, the two adsorbent beds were heated under a nitrogen flow at 180 °C with a temperature ramp of 1 °C/min after each adsorption/desorption cycle. Such regeneration conditions are quite moderate, compared to those applied for the regeneration of activated carbon.11 It clearly appeared that NaY did not undergo any loss of adsorption capacity, since PCE breakthrough occurs at the same time during the four cycles, corresponding to the same adsorption capacity (see Table 4). Conclusions The modeling of the adsorption of tetrachloroethylene (PCE) inside the NaX supercage gave us interesting information on the mechanism involved. It seems that, in the case of PCE, cationic sites are those engaged in the adsorption process. This work also showed that cationic zeolites, in particular, Naexchanged faujasite, are good adsorbents for PCE elimination, thanks to their high micropore volume and their strong interactions with adsorbate molecules. A high adsorption capacity before breakthrough (30 wt %) was obtained over the NaY zeolite. However, in the presence of water vapor, we observed that these types of zeolites were not efficient in the removal of PCE, because of their strong hydrophilic character. The addition of a 3A sieve in a double bed with the NaY zeolite allowed the latter to get back the same adsorption capacity as that under dry conditions. Moreover, we showed that this system completely removed PCE from the effluent during at least four adsorption/desorption cycles without any loss of capacity. Hence, NaY and NaX zeolites exhibit high PCE adsorption capacities and can be used to eliminate volatile organic

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ReceiVed for reView December 19, 2006 ReVised manuscript receiVed March 22, 2007 Accepted April 14, 2007 IE0616390