4798
J. Phys. Chem. B 1997, 101, 4798-4802
Adsorption of p-Xylene/m-Xylene Gas Mixtures on BaY and NaY Zeolites. Coadsorption Equilibria and Selectivities Ve´ ronique Cottier, Jean-Pierre Bellat,* and Marie-He´ le` ne Simonot-Grange UMR 5613, UniVersite´ de Bourgogne-CNRS, Laboratoire de Recherches sur la Re´ actiVite´ des Solides, BP 400, 21011 Dijon Cedex, France
Alain Me´ thivier Institut Franc¸ ais du Pe´ trole, 92852 Rueil Malmaison Cedex, France ReceiVed: December 6, 1996; In Final Form: April 12, 1997X
Coadsorption of the gases p-xylene and m-xylene on BaY and NaY was studied at 150 °C in the range of pressure 10-2-3 hPa. For BaY coadsorption isotherms and single-component adsorption isotherms are perfectly superimposed in the whole range of investigated pressure. The capacity of adsorption of BaY is 3 molecules R-1 for each single component as well as for each mixture under 3 hPa. For NaY coadsorption isotherms lie between those of single p-xylene and m-xylene. The capacity of adsorption of NaY for mixtures is in proportion to the composition of the initial adsorptive mixture and varies from the capacity of adsorption for single p-xylene (3.3 molecules R-1) to that for single m-xylene (3.6 molecules R-1) under 3 hPa. The selectivity of the Y zeolite depends on the filling, the composition of the mixture, and the exchangeable cation. Two selective adsorption processes are discerned according to the filling of R-cages. For filling lower than 2 molecules R-1, BaY and NaY exhibit the same behavior toward the coadsorption of p-xylene and m-xylene and adsorb preferentially the more abundant isomer in initial adsorptive mixture: the selectivity depends only on the composition of the mixture. For filling higher than 2 molecules R-1 as the last molecules of xylenes are adsorbed in the R-cages, BaY is selective for p-xylene whereas NaY is selective for m-xylene whatever the composition of the adsorptive mixture: the selectivity depends on the exchangeable cation. The dependence of the composition of adsorbate on the selectivity shows a nonideal behavior of the adsorbate.
Introduction The knowledge of p-xylene and m-xylene coadsorption equilibria on faujasite type zeolite has much interest in the improvement of separation processes of C8 aromatic hydrocarbons by selective adsorption.1,2 The separation of xylene isomers is presently performed in the liquid phase at 150 °C with K- or Ba-exchanged faujasite zeolite as the selective adsorbent. It is well-known now that the faujasite zeolite is selective for one isomer or the other according to the exchangeable cation. However the competitive adsorption process and the parameters that govern the selectivity are not clearly defined. Several studies on the coadsorption of p-xylene and m-xylene in liquid or gas phase on NaY, KY, BaX, and KBaY are cited in the literature.3-12 The greater difference between liquid and gas phase adsorption is the rate-limiting step leading, in the liquid phase, to slower kinetics of diffusion. However, at the equilibrium, the maximal amounts adsorbed and the selectivities measured at high filling are of the same order of magnitude in both cases. The works achieved at total filling of Y zeolites showed that NaY is selective for m-xylene, while KY and KBaY are selective for p-xylene. Santacesaria et al.5 attributed this reversal of selectivity to the heats of adsorption, which were greater with m-xylene than with p-xylene for NaY, whereas the reverse result is observed for KY. Nevertheless, they remarked in agreement with the results of Guth et al.3 that the deviations in heats of adsorption between p-xylene and m-xylene being very small, the entropies of adsorption must be taken into account to explain the selectivity. Moreover the Raman spectroscopy measurements of Guth et al.3 showed that the m-xylene molecules were more “frozen” on their adsorption sites X
Abstract published in AdVance ACS Abstracts, May 15, 1997.
S1089-5647(96)04003-5 CCC: $14.00
than the p-xylene molecules. Measurements of selectivity at low filling were performed only by Ruthven et al.7 on KY with a p-xylene/m-xylene gas equimolar mixture. Their results showed that the selectivity of the KY depends on the filling of R-cages and becomes favorable to p-xylene only at a high filling coefficient (θ > 0.6). Concerning the diffusion of xylenes in faujasite zeolites, Goddard et al.,8 studying sorption kinetics of single p-xylene or m-xylene on NaX and natural faujasite, showed that the uptake kinetics were essentially controlled by intracrystalline diffusion and that there was only little difference in corrected diffusivity between p-xylene and m-xylene. As regards the effect of the Ba2+ exchangeable cation on the selectivity, no data have been found in the literature for BaY. The adsorption of the p-xylene/m-xylene mixture was studied only on BaX.12 Experiments, restricted to filling close to saturation (θ > 0.95) and mixtures rich either in p-xylene or in m-xylene, showed that BaX is selective for p-xylene. Our previous works on the adsorption of single-gas p-xylene or m-xylene on NaX, BaX, NaY, KY, and BaY13-17 showed that the adsorption properties of the faujasite zeolite for p-xylene and m-xylene were nearly the same. The deviations in capacities of adsorption and heats of adsorption between p-xylene and m-xylene were small, and the more outstanding differences were observed at total filling of R-cages. Thus for NaY the capacities of adsorption and the heats of adsorption were slightly higher with m-xylene than with p-xylene, while for BaY they were the same with both isomers. Moreover the isosteric entropies of adsorption, which were higher with p-xylene than with m-xylene, showed that at total filling the adsorbate had a physical state close to that of the solid xylene and seemed less mobile in R-cages with m-xylene than with p-xylene as observed by Guth et al.3 On the other hand, it was shown that the © 1997 American Chemical Society
Adsorption of p-Xylene/m-Xylene on Zeolites
Figure 1. Experimental setup.
adsorption affinity of the Y zeolite for xylenes was stronger with Ba2+ than with Na+ or K+ as exchangeable cations. In view of our results and those of the literature it seemed that the entropic effects linked to the arrangement of the xylene molecules in the cavities played an important part in the selective adsorption process. Therefore it was interesting to study experimentally the adsorption of p-xylene and m-xylene mixtures on the faujasite zeolite and especially on BaY in order to know the selective adsorption process and to determine the parameters that govern this selectivity. This paper is then devoted to the coadsorption of gas p-xylene and m-xylene on NaY and BaY at 150 °C in a large range of composition of adsorptive mixture and filling of adsorbent. Experimental Section Experimental Setup. The coadsorption of gas xylenes was studied by means of a volumeter coupled with a GC analyzer.18 The amount of each component adsorbed and the adsorption selectivity were determined from pressures and compositions of adsorptive mixture measured before and after each adsorption experiment. The specific experimental device is described in Figure 1 and is composed of (i) an adsorption cell of volume VA ) 61.6 cm3, containing the zeolite, surrounded by a furnace; (ii) two bulbs filled with single-liquid xylene previously outgassed and dehydrated in situ by means of hydrophilic zeolite; (iii) a flask of volume VF ) 2490 cm3 in which the gas mixture of a given composition was prepared by introduction of gas xylenes one after the other, controlling the pressure; (iv) two pipes of volume Vp1 ) 75.6 cm3 and Vp2 ) 895 cm3 connecting the flask to the adsorption cell; (v) a vacuum line (primary vacuum pump and cryogenic adsorption pump; (vi) a vacuum gauge G and pressure gauges P1 (10-1-100 hPa) and P2 (10-2-10 hPa); (vii) a bypass with sample loop of volume VS ) 0.6 cm3 to take a small part of the adsorptive mixture for GC analysis; and (viii) a gas chromatograph with FID detector and polar capillary column (HP FFAP, L ) 0.2 mm, length ) 50 m). The initial mixture was homogenized by heating the flask. The difference of temperature between the adsorption cell (150 °C) and the pipe (25 °C) was great enough to homogenize the adsorptive mixture by thermal convection. Experimental Conditions. The NaY parent zeolite was a pure crystalline powder produced by Linde-Union Carbide and having a Si/Al ratio of 2.43. BaY was prepared by cation exchange from NaY. The degree of exchange is 99.5%. The chemical formulas for the dehydrated zeolites are Na56(AlO2)56(SiO2)136 and Ba28(AlO2)56(SiO2)136.13 The mass of the sample was about 500 mg. Before each experiment, the zeolite was activated in situ at 400 °C under 10-4 hPa during 12 h. The adsorption temperature was 150 °C, and the pressure ranged
J. Phys. Chem. B, Vol. 101, No. 24, 1997 4799 from 10-2 to 3 hPa. As the adsorption occurs at constant volume, the ranges of equilibrium compositions of gas and adsorbate investigated from each initial adsorptive mixture depend on the experimental setup (volume of apparatus, mass of sample, etc.). The compositions of initial adsorptive mixtures and the ranges of equilibrium compositions of gas and adsorbate investigated with our own volumetric device are given in Table 1. The experimental accuracies were (0.5 °C for the temperature, (10-4 hPa for the pressure, (0.1 molecules R-1 for the amount adsorbed, and (0.03 for the mole fraction in gas. Operating Procedure. The coadsorption isotherms were obtained by successive adsorption of small amounts of mixtures (∼0.5 molecules R-1). The desorption was not studied. Each coadsorption experiment was carried out as follows. For the experiment number k (i) an amount of initial adsorptive mixture was introduced into the volume Vp2 and analyzed by GC. The mole fraction of each component i in the adsorptive mixture is yi,p2(k) and the pressure is pp2(k). (ii) The adsorptive mixture was brought into contact with the zeolite by connecting the volume Vp2 with the adsorption cell. Previous CG analyses of one adsorptive mixture as a function of time showed that 3 h later the pressure and the composition of gas no longer varied, meaning that the adsorption equilibrium was reached. (iii) At the adsorption equilibrium, the volume VA was isolated and the adsorptive mixture was analyzed. The mole fraction of each component i in the adsorptive mixture is yi(k) and the pressure is p(k). The amount of each component i adsorbed at the point k of the isotherm, , was calculated in molecules of xylenes per R-cage of activated zeolite (molecules R-1) from the relation
Nai(k) )
[
Vp2 (y , (k) pp2(k) - yi(k) p(k)) + RTp2 i p2 VA MZ + Nai (k-1) (y (k-1) p(k-1) - yi(k) p(k)) RTA i 8m
]
with yi(k-1), p(k-1), and Nai (k-1) the mole fraction of component i in the adsorptive mixture, the equilibrium pressure, and the amount of component i adsorbed at the previous point (k-1) of the isotherm, respectively. Then the mole fraction of component i in the adsorbate is
Nai(k) xi )
a
N
with Na )
∑i Nai (k)
Results and Discussion Capacities of Adsorption. Single Components. The adsorption isotherms of single xylenes at 150 °C are shown in Figure 2. For BaY the adsorption isotherm of p-xylene is superimposed on that of m-xylene in the whole range of investigated pressure. This result means that BaY adsorbs p-xylene and m-xylene in the same way. Despite the differences in shape and size (φk(pX) ) 0.67 nm and φk(mX) ) 0.74 nm) as well as in polarity (µ(pX) ) 0 and µ(mX) ) 0.36 D) between p-xylene and m-xylene, the capacities of adsorption of BaY are exactly the same for both isomers. At the plateau of the isotherm the amount of p-xylene or m-xylene adsorbed under the pressure of 3 hPa is 3 molecules R-1. For NaY the amounts of p-xylene and m-xylene adsorbed under a pressure lower than 0.04 hPa are the same, but above this pressure NaY adsorbs more m-xylene than p-xylene. Under 3 hPa the capacity of adsorption at 150 °C is 3.3 and 3.6 molecules R-1 with p-xylene and m-xylene, respectively. Since in this investigation the relative pressure never exceeds 2 × 10-3, the zeolite is not fully saturated. In our previous
4800 J. Phys. Chem. B, Vol. 101, No. 24, 1997
Cottier et al.
TABLE 1: Compositions of Initial Adsorptive Mixtures and Investigated Ranges of Equilibrium Compositions of Gas and Adsorbate initial adsorptive mixture
y°pX
ypX
xpX
y°pX
ypX
xpX
1 2 3 4 5
0.10 0.26 0.49 0.76 0.87
0.04-0.33 0.11-0.41 0.27-0.49 0.55-0.69 0.63-0.84
0.10-0.12 0.25-0.28 0.48-0.51 0.76-0.81 0.87-0.90
0.20 0.49 0.80
0.31-0.45 0.42-0.67 0.59-0.87
0.17-0.20 0.45-0.50 0.78-0.81
BaY
NaY
Figure 2. Adsorption isotherms of single p-xylene (0) and single m-xylene (O) on BaY and NaY at 150 °C.
thermogravimetric study13,14 the maximal capacities of adsorption of BaY and NaY at 150 °C were estimated at 3 and 3.3 molecules R-1 with p-xylene and 3 and 3.6 molecules R-1 with m-xylene, respectively. Therefore at 150 °C under 3 hPa the total filling of R-cages is almost reached. Mixtures. The coadsorption isotherms Na ) f(p)T and the partial adsorption isotherms Nai ) f(p)T at 150 °C are plotted and compared with the single-component adsorption isotherms in Figures 3 and 4. For BaY whatever the composition of initial adsorptive mixture, the coadsorption isotherms are superimposed on the single component adsorption isotherms in the whole range of investigated pressure (Figure 3). At constant temperature and pressure, the total amount adsorbed does not vary with the composition of the initial adsorptive mixture. At 150 °C under a maximal pressure of 3 hPa, BaY adsorbs 3 aromatic molecules per R-cage for each adsorptive mixture for both single xylenes (Figure 3). Then, the amount of each component adsorbed varies as a linear function of the composition of initial adsorptive mixture (Figure 5). This result agrees with the correlation observed by Lewis et al.19 for the adsorption of unsaturated hydrocarbons on silica gel or active carbons and whose expression is
NapX NapX *
NamX +
NamX *
)1
The fact that such a correlation is checked suggests that the molecular volumes of xylenes in the adsorbate mixture are the same as in the single adsorbates. In contrast to BaY the coadsorption isotherms of NaY above 0.04 hPa are not superimposed on the single-component adsorption isotherms but lie between them according to the composition of the initial adsorptive mixture (Figure 4). Figure 5 shows that the Lewis correlation is also checked for NaY and that under 3 hPa the capacity of adsorption of NaY at 150 °C is a linear function of
Figure 3. Coadsorption isotherms on BaY at 150 °C [(b) total amount of mixture; (9) partial amount of p-xylene; (4) partial amount of m-xylene; (0) single p-xylene; (O) single m-xylene].
Figure 4. Coadsorption isotherms on NaY at 150 °C [solid line, single component; dashed line, mixtures].
the composition of the initial adsorptive mixture and varies from 3.3 to 3.6 molecules R-1. Selectivities. The adsorption selectivity of p-xylene with respect to m-xylene RpX/mX of the Y zeolite depends on the filling, the composition of the initial adsorptive mixture, and the exchangeable cation (Figures 6 and 7). For both zeolites and whatever the composition of the initial adsorptive mixture, two selective adsorption processes are discerned according to the range of filling.
0.5 < Na < 2 molecules R-1 For filling ranging from 0.5 to 2 molecules R-1 and for each adsorptive mixture the selectivity RpX/mX of BaY and NaY does
Adsorption of p-Xylene/m-Xylene on Zeolites
J. Phys. Chem. B, Vol. 101, No. 24, 1997 4801
Figure 7. Dependence of the filling of R-cages on the selectivity RpX/mX of NaY at 150 °C. Figure 5. Dependence of the composition of the initial adsorptive mixture on the amounts adsorbed [(O) total; (9): p-xylene; (4) m-xylene].
Figure 6. Dependence of the filling of R-cages on the selectivity RpX/mX of BaY at 150 °C.
not change with the filling. With the mixtures 1 and 2 rich in m-xylene, the selectivity is lower than 1, meaning that the zeolite adsorbs preferentially m-xylene. With the equimolar mixtures 3, the selectivity is equal to 1: the adsorption process is not selective. With the mixtures 4 and 5 rich in p-xylene, the selectivity is higher than 1: the zeolite adsorbs more p-xylene than m-xylene. Thus for filling lower than 2 molecules R-1, the Y zeolite is selective for the more abundant isomer in the initial adsorptive mixture whatever the exchangeable cation. In this case the selectivity depends only on the composition of initial adsorptive mixture.
Na > 2 molecules R-1 BaY becomes selective for p-xylene Above 2 molecules (Figure 6), whereas NaY becomes selective for m-xylene (Figure R-1
7). As the filling increases, the selectivity remains almost unchanged with mixture 4 rich in p-xylene for BaY and with mixture 2 rich in m-xylene for NaY. However it may be noticed for BaY that when the initial adsorptive mixture is exceedingly rich in p-xylene (mixture 5), the selectivity decreases slightly as the complete filling of R-cages is reached, meaning that in the adsorbate some p-xylene molecules are replaced by m-xylene molecules. Thus the selective adsorption process of p-xylene with respect to m-xylene on Y zeolite occurs only at high filling, as the last molecules of xylenes are adsorbed in the R-cages. In this case the selectivity depends only on the exchangeable cation. These results are in good agreement with those found on the KY zeolite by Ruthven et al.7 Dependence of Equilibrium Composition. The dependences of the equilibrium composition of adsorbate on the equilibrium composition of gas and on the selectivity are shown in Figures 8 and 9. The selectivity diagram ypX ) f(xpX)T,p must be defined at constant temperature and pressure. For BaY, as all coadsorption isotherms are superimposed on one another, the total amount adsorbed at a given pressure is the same whatever the composition of adsorbate. In this particular case, the selectivity diagram is also defined at constant filling of adsorbent. For NaY, the total amount adsorbed at constant temperature and pressure is a function of the composition of adsorbate. Therefore it may be not overlooked, analyzing the selectivity diagram of NaY defined at constant pressure, that the filling varies with the composition of adsorbate. The experimental selectivity diagram has two different shapes according to the filling of adsorbent (Figure 8). At low filling (Na < 2 molecules R-1) BaY and NaY are selective for the more abundant isomer in the adsorbate. Then the selectivity increases as the mole fraction of p-xylene in the adsorbate increases (Figure 9). A reversal of selectivity occurs as the mole fractions of the adsorptive mixture and adsorbate at the equilibrium are close to 0.45. It is noteworthy that at low filling the selectivities of BaY and NaY have about the same values and vary in the same way with the composition of adsorbate. At low filling both zeolites exhibit the same behavior toward the coadsorption of xylenes. At high filling (Na > 2 molecules R-1), BaY is selective for p-xylene in the whole range of composition of adsorbate (Figure 8), and its selectivity decreases when the adsorbate becomes rich in p-xylene (Figure 9). Inversely, NaY is selective for m-xylene, and its selectivity is almost unchanging with the composition of adsorbate (Figure 9). Conclusion The objective of this work is to show that the selective adsorption process on the Y zeolite depends on the composition
4802 J. Phys. Chem. B, Vol. 101, No. 24, 1997
Cottier et al. selectivity that occurs at high filling of R-cages is probably the result of an important change of molecular interactions in which the exchangeable cation has a large part. Indeed at high filling the adsorption of xylene molecules occurs with strong stresses and the adsorbate undergoes a molecular rearrangement. At total filling the molecules aggregate and form a cluster in the R-cages, as was observed by Fitch et al.20 in the case of the adsorption of benzene in NaY. According to the size, the charge, and the location of the exchangeable cation in the cavities, the free space available for the adsorption of the last molecule in the R-cage is more or less great. Therefore the formation of this cluster will occur in favor of the adsorption of one isomer or the other according to the free space available in the R-cage and in such a way that the total energy of the system will be minimized.
Figure 8. Selectivity diagrams at 150 °C [low filling: Na ) 1.5 molecules R-1 under 0.07 hPa for BaY and 0.03 hPa for NaY; high filling: Na ) 3 molecules R-1 under 2.7 hPa for BaY and 3.3 < Na < 3.6 molecules R-1 under 3 hPa for NaY].
Abbreviations Used T p m MZ Na Nai *
Nai φk µ y°i Figure 9. Dependence of the composition of adsorbate at equilibrium on the selectivity RpX/mX at 150 °C [low filling: Na ) 1.5 molecules R-1 under 0.07 hPa for BaY and 0.03 hPa for NaY; high filling: Na ) 3 molecules R-1 under 2.7 hPa for BaY and 3.3 < Na < 3.6 molecules R-1 under 3 hPa for NaY].
of the adsorptive mixture and the exchangeable cation. Two different selective adsorption processes are observed according to the filling. Below 2 molecules R-1, the zeolite does not govern the selectivity, the role of the adsorbent is only to store the adsorbate in a condensed state, and the selectivity depends only on the composition of adsorptive mixture. Above 2 molecules R-1, the selectivity is governed by the zeolite, especially by the exchangeable cation. Such results are of relevant interest for the modeling and the simulation of the separation processes that operate in the liquid phase at temperatures close to 150 °C, i.e. at saturation of R-cages of the zeolite (3 < Na < 4 molecules R-1). The fact that the selectivity varies with the composition of adsorbate in the complete range of filling (except for NaY at high filling) means that the adsorbate is not ideal. Therefore it will be not possible to predict the coadsorption equilibria from the single-component adsorption isotherms with the usual thermodynamic models based on the ideality of the adsorbate. It may be noticed that at high filling of NaY the adsorbate exhibits probably an ideal behavior because the selectivity is changed only slightly with the composition of adsorbate. The more outstanding result is that xylenes are adsorbed selectively by BaY despite the fact that its single-component adsorption isotherms are superimposed. In view of such results added to those of the previous works found in the literature, it seems clear now that the molecular sieving effects, the adsorbateadsorbent interactions, and the mass transport phenomena cannot explain the selectivity because no sufficiently great differences in capacities of adsorption, heats of adsorption, and diffusivities between p-xylene and m-xylene have been observed. The
yi xi RpX/mX ) xpXymX/xmXypX
temperature of adsorption pressure of gas at the equilibrium mass of dehydrated adsorbent molar mass of anhydrous zeolite amount of mixture adsorbed (molecules R-1) amount of single xylene adsorbed at the same temperature and pressure as for the mixtures (molecules R-1) amount of component i adsorbed (molecules R-1) kinetic diameter dipolar moment mole fraction of component i in the initial adsorptive mixture mole fraction of component i in the gas at equilibrium mole fraction of component i in the adsorbate at equilibrium selectivity of p-xylene with respect to m-xylene
References and Notes (1) Broughton, D. B.; Neuzil, R. W.; Pharis, J. M.; Brearly, C. S. Chem. Eng. Prog. 1970, 66 (9), 70. (2) Seko, M.; Miyake, T.; Inada, K. Ind. Eng. Prod. Res. DeV. 1979, 18 (4), 263. (3) Guth, J. L.; Jacques, P.; Stoessel, F.; Wey, R. J. Colloid Interface Sci. 1980, 76 (2), 298. (4) Santacesaria, E.; Morbidelli, M.; Danise, P.; Mercenari, M.; Carra`, S. Ind. Eng. Chem. Process Des. DeV. 1982, 21, 440. (5) Santacesaria, E.; Gelosa, D.; Picenoni, D.; Danise, P. J. Colloid Interface Sci. 1984, 98 (2), 467. (6) Santacesaria, E.; Gelosa, D.; Danise, P.; Carra`, S. Ind. Eng. Chem. Process Des. DeV. 1985, 24, 78. (7) Ruthven, D. M.; Goddard, M. Zeolites 1986, 6, 275. (8) Goddard, M.; Ruthven, D. M. Zeolites 1986, 6, 283. (9) Li, M. H.; Hsiao, H. C.; Yih, S. M. J. Chem. Eng. Data 1991, 36 (2), 244. (10) Hulme, R.; Rosensweig, R. E.; Ruthven, D. M. Ind. Eng. Chem. Res. 1991, 30, 752. (11) Hsiao, H. C.; Yih, S. M.; Li, M. H. Adsorpt. Sci. Tech. 1989, 6 (2), 64. (12) Allain, X. Thesis, Universite´ de Bourgogne, Dijon, France 1993. (13) Bellat, J. P.; Simonot-Grange, M. H.; Jullian, S. Zeolites 1995, 15, 124. (14) Bellat, J. P.; Simonot-Grange, M. H. Zeolites 1995, 15, 219. (15) Mellot, C.; Simonot-Grange, M. H.; Pilverdier, E.; Bellat, J. P.; Espinat, D. Langmuir 1995, 11 (5), 1726. (16) Pilverdier, E. Thesis, Universite´ de Bourgogne, Dijon, France, 1995. (17) Simonot-Grange, M. H.; Bertrand, O.; Pilverdier, E.; Bellat, J. P.; Paulin, C. J. Therm. Anal. in press. (18) Cottier, V. Thesis, Universite´ de Bourgogne, Dijon, France, 1996. (19) Lewis, W. K.; Gilliland, E. R.; Chertow, B.; Cadogan, W. P. Ind. Eng. Chem. 1950, 42 (7), 1319. (20) Fitch, A. N.; Jobic, H.; Renouprez, A. J. Phys. Chem. 1986, 90, 1311.
