J. Phys. Chem. B 2005, 109, 17239-17244
17239
Effect of Preadsorbed Water on the Adsorption of p-xylene and m-xylene Mixtures on BaX and BaY Zeolites Jean-Charles Moı1se† and Jean-Pierre Bellat*,‡ Laboratoire de Thermodynamique des Milieux Polyphase´ s, Ecole Nationale Supe´ rieure des Industries Chimiques, Institut National Polytechnique de Lorraine, 1 rue GrandVille, BP 451, 54001 Nancy Cedex, France, and Laboratoire de Recherches sur la Re´ actiVite´ des Solides, UMR 5613 CNRS, UniVersite´ de Bourgogne, 9 aVenue A. SaVary, BP 47870, 21078 Dijon Cedex, France ReceiVed: May 16, 2005; In Final Form: July 8, 2005
Adsorption of an equimolar p-xylene/m-xylene mixture on partially hydrated barium-exchanged X and Y zeolites is studied at 423 K in the pressure range 10-2-8 hPa by differential calorimetry coupled with manometry and chromatography. Results are consistent with structural studies and Monte Carlo simulations of the literature. The presence of preadsorbed water in supercages increases the adsorption selectivity toward p-xylene to the detriment of the adsorption capacity and the adsorption affinity, as indicated by a sharp decrease of the Henry constants. However, the coadsorption heats at zero filling are not influenced by the presence of water. Therefore, entropic effects seem to play an important role in the coadsorption process. Two adsorption sites whose energy differs are identified by calorimetry. The more energetic site could correspond to the p-xylene or m-xylene molecule in interaction with the compensation cation located in sites II in the supercage, the less energetic to the adsorption of p-xylene molecule in the 12-ring window joining two supercages. The presence of this second site for p-xylene could be at the origin of the selectivity.
Introduction In the petrochemical industry, the p-xylene is recovered from the C8 aromatic cut by selective adsorption on zeolites. The adsorbents commonly used in the simulated moving bed separation units, as for example, the Parex1 or Eluxyl2 processes, are faujasite-type zeolites exchanged with potassium or barium. The improvement of these separation processes requires a lot of experimental data and a better knowledge of separation mechanisms in order to optimize the formulation of the selective adsorbent and the operating conditions. The adsorption mechanisms of xylene in the supercages of faujasite zeolites are very complex. Indeed, we have shown in previous papers3-7 that, although the adsorption isotherms and adsorption heats of single xylenes are very close, the faujasite zeolite exhibits an adsorption selectivity for one isomer or the other according to the nature of the compensation cation. For example, NaY is selective for m-xylene, while KY and BaY are selective for p-xylene. Moreover, the selectivity only appears at high filling, when the last xylene molecule is adsorbed in the supercage. It is now admitted that the key parameter which governs the selectivity is the compensation cation. Studies performed at a microscopic level by X-ray and neutron diffraction8-11 as well as by bias grand canonical Monte Carlo simulation12-16 have shown that the adsorption selectivity of p-xylene toward m-xylene is dependent on the extraframework cation distribution on the different sites available in the supercage17 (site II at the center of the hexagonal window of the sodalite cage and site III around the dodecagonal window of the supercage). According to the nature of the compensation cation, three adsorption sites have been localized near site II, between sites II and III and in the * Author to whom correspondence should be addressed. E.mail:
[email protected]. † Laboratoire de Thermodynamique des Milieux Polyphase ´ s. ‡ Laboratoire de Recherches sur la Re ´ activite´ des Solides.
12-membered window joining two supercages. On the first two sites, the xylene molecules directly interact with the compensation cation via their aromatic ring, while not on the third. The difference in adsorption energy between these three sites depends on the compensation cation and the xylene isomer and could explain why the p-xylene/m-xylene selectivity is reversed when sodium is replaced by potassium or barium. However, the separation of xylenes does not result only from differences in adsorbate-adsorbent interactions. At high filling, the xylene molecules are confronted with important steric hindrance that is related to the size, the number, and the location of the compensation cations. Entropic effects probably take a nonnegligible part in the separation process by competitive adsorption. On the other hand, the adsorption selectivity is dependent on the degree of hydration of the zeolite.18,19 Indeed, the presence of preadsorbed water molecules influences strongly the cation distribution and reduces the free space in the supercage, leading thus to an adsorption in favor of p-xylene, probably because this isomer has a smaller size than the other one. So, it seems to us particularly relevant to study from an energetic point of view the coadsorption of xylene isomers on faujasite zeolites in the presence of water. Indeed, to our knowledge, no data on the adsorption heats of xylenes mixture on partially hydrated zeolites are given in the literature. The present paper is then devoted to the study of coadsorption of p-xylene and m-xylene on barium-exchanged faujasite zeolites. The energetic phenomena during the coadsorption of xylene isomers are investigated by differential calorimetry, and a special attention has been paid to the effect of preadsorbed water on the adsorption selectivity. Experimental Experimental Setup. The adsorption of single components and binary mixtures was studied by using a homemade
10.1021/jp0525639 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/19/2005
17240 J. Phys. Chem. B, Vol. 109, No. 36, 2005
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Figure 2. Adsorption isotherms of single p-xylene (open triangles) and m-xylene (filled squares) on the partially hydrated bariumexchanged zeolites at 423 K (solid lines without symbols: single pX or mX on anhydrous zeolites).
Figure 1. Experimental setup and details on the connection of the six-way gas injection valve used for GPC sampling.
manometric apparatus coupled with a heat flow Setaram C80 differential calorimeter (Figure 1). This setup is connected to a HP4890 gas-phase chromatograph with a TCD detector and a Chromapack Bentone 34 packed column. A bypass system connected to the vacuum line (turbomolecular pump) and composed of a six-way gas injection valve (GIV) with sample loop allowed collection of a small amount of gas under low pressure for chromatographic analysis.20 Liquid xylenes are stored in evacuated vessels in the presence of NaX hydrophilic zeolite to trap any residual water or other impurities. The pressure is measured with MKS Baratron pressure sensors (10-4-10 hPa). Experimental Conditions. The BaY and BaX zeolites were prepared by classical cation exchange starting from the pure synthetic NaY and NaX faujasites commercialized by Union Carbide. Their physicochemical features were finely characterized in a previous article.21 The sample weight was about 500 mg. All the experiments were performed at 423 K, and the mole fraction of p-xylene and m-xylene in the initial gas mixture was 0.5. Most of them had a pressure range from 10-2 to 8 hPa. The amount of preadsorbed water on barium-exchanged zeolites was of about 1 molecule per compensation cation. Before each adsorption, the sample was activated in situ at 673 K under 10-4 hPa for 12 h. Operating Procedure. After activation, the sample temperature was brought to 473 K, and preadsorption of water was performed by successive introductions of vapor doses into the adsorption cell, via the flask of volume VF, until the desired water content was reached. The adsorption cell was then isolated, and its temperature was lowered to 423 K in order to adsorb the residual water vapor. By this way, the water vapor pressure in the dead volume of the adsorption cell never exceeded 0.01 hPa. The binary aromatic gas mixture was prepared in the flask of volume VS by introducting xylenes one after the other while controlling the pressure. This initial mixture was homogenized by heating the flask, and its composition was controlled by GPC. Coadsorption isotherms were obtained by successive introduc-
tions of small amounts of gas mixture stored in bulb Vs into the adsorption cell by means of flask VF, previously evacuated, and controlling the pressure. The initial pressure of xylenes in the volume VF was never lower than 0.01 hPa in order to avoid any desorption of water consecutive to a depression when the cell was put in communication with the flask. The amount of each component i adsorbed (Nai) and its mole fraction in the adsorbate (xi) were calculated by performing a mass balance on the gas phase from the total pressure (p) and the mole fraction of component i in the gas phase (yi), measured before and after each adsorption experiment. In addition, the calorimetric measurements leads to the molar coadsorption heats. The calculation of adsorption heats, amounts adsorbed, and selectivity has been described exhaustively in refs 5 and 6. Thus, this technique allowed determination simultaneously of the total and partial adsorption isotherms, (Na or Nai ) f(p)T), the coadsorption heats (∆adsH˙ m ) f(Na)T), and the adsorption selectivity of p-xylene with respect to m-xylene (RpX/mX ) (xpX‚ymX)/(xmX‚ypX) ) f(Na)T with xi and yi the mole fractions of component i at the equilibrium in the adsorbed phase and in the gas phase, respectively) as a function of the filling. The amounts of xylenes or water adsorbed are expressed in number of molecules per unit cell (molec‚uc-1). We recall that the unit cell of faujasite zeolite contains 8 supercages, and xylenes are adsorbed only in supercages, while water can be adsorbed in both supercages and sodalite cages. The amount of water preadsorbed in the zeolites (about 1H2O/Ba2+) was equivalent to 30 and 43.2 molec‚uc-1 for BaY and BaX, respectively. The experimental accuracies were about 0.1 K for the temperature, 10-4 hPa for the pressure, 0.8 molec‚uc-1 for the amount adsorbed, 0.03 for the mole fraction in gas, and less than 5 kJ‚mol-1 for the adsorption heats. Results and Discussion Adsorption of Single Xylenes. The adsorption isotherms of single p-xylene and m-xylene on partially hydrated BaYh and BaXh are shown on Figure 2 in comparison with those obtained on the anhydrous zeolites BaYa and BaXa in previous works.3-6 As observed with the anhydrous forms, for pressure lower than
Adsorption of p-Xylene and m-Xylene on Zeolites
J. Phys. Chem. B, Vol. 109, No. 36, 2005 17241
TABLE 1: Henry Constants for the Adsorption of Single p-Xylene and m-Xylene on Anhydrous and Partially Hydrated BaY at 423 K
TABLE 3: Experimental Data of Coadsorption Equilibria of p-Xylene and m-Xylene on Anhydrous BaY and BaX Zeolites at 423 Ka
KH/(molec‚uc-1‚hPa-1) sample
BaYa
p-xylene m-xylene
p
BaYh
136 136
BaXa
16 16
BaXh
960 960
BaYa
16 16
TABLE 2: Amount of Single p-Xylene and m-Xylene Adsorbed on the Anhydrous and Partially Hydrated Barium-Exchanged Faujasites at 423 K under 3 HPaa sample
BaYa
BaYh
BaXa
BaXh
adsorbate
Na
Va
Na
Va
Na
Va
Na
Va
pX mX water
22.5 22.5 0
5.34 5.26 0
20.0 17.5 30.00
4.75 4.10 1.01
26.0 26.0 0
6.18 6.08 0
20.0 18.5 43.2
4.75 4.32 1.45
a The amount of water preadsorbed on zeolites is also reported. Na is expressed in molec‚uc-1 and Va in nm3‚uc-1.
1 hPa, the adsorption isotherms of p-xylene and m-xylene are superimposed. At very low pressure (p < 10-2 hPa), the adsorption follows Henry’s law. The values of Henry constants on partially hydrated and anhydrous zeolites are given in Table 1. These values are lower for partially hydrated zeolites. This means that the presence of water molecules weakens the adsorption affinity of the faujasite zeolites for xylenes. At the plateau of adsorption isotherms, for pressure higher than 1 hPa, the amounts of single xylenes adsorbed on the two zeolites are lower in the presence of water. The volumes of single xylenes adsorbed on the anhydrous and partially hydrated barium-exchanged zeolites at 423 K and under 3 hPa are collected in Table 2. The volume of water preadsorbed on each zeolite before adsorption of aromatic compounds is also reported. These adsorbate volumes are determined by taking the density equal to 0.7411, 0.7531. and 0.89125 g‚cm-3 for p-xylene, m-xylene, and water, respectively. These densities are calculated by assuming that, for temperature above the boiling point, the adsorbed phase is like a pressurized gas, as suggested by Nikolaev and Dubinin.22 It may be noticed that the decrease in volumes of adsorbed xylenes caused by the presence of water is of the same order of magnitude as the volume of preadsorbed water. Because the xylene isomers are only adsorbed in supercages, this suggests that water molecules would not be all adsorbed in sodalite cages, as it could be expected. A large part of preadsorbed water molecules would also be located in the supercages and would not be replaced by xylenes when the filling increases. This result is consistent with the neutron diffraction data of Pichon and al.8 and the Monte Carlo simulations of Buttefey,23 who found that the water preadsorbed on BaX is located near the barium on site I′ inside the sodalite cage, but also close to the barium on site II in the supercage. Moreover, it is observed that the partially hydrated BaYh adsorbs more p-xylene than m-xylene. Indeed, the amounts of p-xylene and m-xylene adsorbed under 3 hPa are 20 and 17.5 molec‚uc-1, respectively, whereas they are the same on anhydrous BaYa (22.5 molec‚uc-1). The preadsoption of water leads to a decrease of the adsorption capacity of BaY of about 11% for p-xylene against 22% for m-xylene. The presence of water molecules in the cavities creates a steric hindrance unfavorable to the adsorption of m-xylene, which is less symmetric and more bulky than p-xylene. This difference between the adsorption capacities of single xylenes is less noticeable with the partially hydrated BaXh. Indeed, the amounts of p-xylene and m-xylene adsorbed on this zeolite differ by less than 1.6 molec‚uc-1. This is probably due to the fact that the supercage is greater in X zeolite
BaXa
0 0.008 0.022 0.059 0.086 0.117 0.330 0.354 0.415 0.594 0.831 1.168 2.556 3.533 0 0.001 0.003 0.005 0.013 0.016 0.036 0.042 0.104 0.607 1.720 2.742 3.307
ypX 0.44 0.43 0.46 0.43 0.36 0.28 0.29 0.29 0.29 0.29 0.29 0.29 0.29 0.38 0.40 0.46 0.47 0.42 0.40 0.46 0.41 0.35 0.40 0.40 0.40
Natotal 0 1.3 4.1 8.1 11.8 15.7 17.0 17.5 18.2 18.8 19.3 20.9 21.2 21.5 0 3.2 6.1 8.9 12.3 15.1 17.4 19.7 22.5 24.8 25.5 26.3 26.6
xpX 0.48 0.48 0.48 0.48 0.48 0.48 0.50 0.51 0.52 0.52 0.52 0.52 0.52 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.50 0.51 0.52 0.53 0.54
|∆adsH˙ m,pX+mX| 115.0 114.0 118.0 114.6 112.1 90.0 78.0 78.3 83.1 86.5 83.3 71.2 60.0 58.0 120.0 118.0 119.6 121.3 123.6 124.5 124.9 119.5 109.3 87.7 60.3 54.6 49.0
a p is given in hPa, Natotal in molec‚uc-1, and ∆adsH˙ m,pX+mX in kJ‚mol-1. Values in italics are obtained by extrapolating to zero filling.
than in Y zeolite (0.830 nm3 for BaX and 0.756 nm3 for BaY from estimations using crystallographic data). Adsorption of Equimolar Mixture. For the two partially hydrated zeolites, the total and partial adsorption isotherms of p-xylene/m-xylene mixture NapX+mX or Nai ) f(p)T and the dependence of the filling on the adsorption selectivity of p-xylene with respect to m-xylene RpX/mX ) f(Na)T and on the coadsorption heats ∆adsH˙ m ) f(Na)T are shown in Figures 3, 4, and 5. Values of coadsorption experimental data are collected in Tables 3 and 4. Adsorption Isotherms. The adsorption isotherms (Figure 3) show that the two zeolites do not have the same behavior toward preadsorbed water when the filling in aromatic mixture increases. For BaYh zeolite, the amount of preadsorbed water is constant and equal to 30 molec‚uc-1, i.e., around 1 molecule per barium cation. No partial water desorption is observed during the adsorption of the xylene mixture. On the contrary, for BaXh zeolite, water is partially desorbed when the amount of adsorbed xylenes increases. This desorption is observed at a filling in xylenes higher than 16 molec‚uc-1. Then, at higher filling in xylenes, above 6 hPa, the amount of preadsorbed water becomes constant and is equal to 38 molec‚uc-1, i.e., 0.9 molecule of water per cation. This slight partial desorption is attributed to the fact that BaX exhibits a weaker adsorption affinity for water than BaY, as we have shown in a previous paper.21 Under 8 hPa, when the filling of supercages is almost complete, both zeolites adsorb more p-xylene than m-xylene. Adsorption SelectiVity. As observed with the anhydrous zeolites,6 the adsorption selectivity of partially hydrated zeolites depends on the filling (Figure 4). Some values of the adsorption selectivity at different fillings are given in Table 5. For Na < 16 molec‚uc-1, the selectivity is constant and slightly higher than that for the anhydrous forms. Above this filling, as the last molecule of xylenes is adsorbed, the adsorption
17242 J. Phys. Chem. B, Vol. 109, No. 36, 2005
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TABLE 4: Experimental Data of Coadsorption Equilibria of p-Xylene and m-Xylene on Partially Hydrated BaY and BaX Zeolites at 423 Ka p BaYh
