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Langmuir 1997, 13, 3584-3586
Adsorption Studies of Neopentane in Y and ZSM-20 Zeolites J. Pires* and M. Brotas de Carvalho Faculty of Sciences of Lisbon, Department of Chemistry and Biochemistry, R. da Escola Polite´ cnica, 58, 1250 Lisbon, Portugal Received January 23, 1997. In Final Form: April 16, 1997
Introduction Due to their regularity, high crystallinity, and wellknown structures, zeolites are tempting materials for modeling and better understanding the adsorption mechanisms in microporous solids. Frequently, however, the interpretation of results based only on structural parameters is not sufficient, since effects resulting from the surface chemistry, mainly due to the compensating cations and OH groups, have to be taken into account. On the other hand, since zeolites play a relevant role in the field of separation processes by adsorption, it is important to understand the adsorption mechanisms in order to better correlate the data and predict the adsorption under different pressure and temperature conditions. In recent years, new zeolitic materials have been synthesized that possess structures with larger pores than the more “traditional” A or Y zeolites, as, for instance, ZSM-201 and VPI-52 zeolites or the MCM type solids,3 which have a potential interest in adsorption that justifies the studies of adsorption mechanisms in these materials. Y zeolite4 is a well-known material used as the base of cracking catalysts and presents an FAU structure.5 ZSM20 zeolite, patented initially in the middle seventies by Ciric,1 is a material active, for instance, in hydrocracking of C5 hydrocarbons6 and is an intergrowth of 30% FAU and 70% EMT5 structure. In EMT structure the supercages have five apertures (only four in FAU). Two of them are approximately circular, with a diameter of 0.71 nm,5 and are aligned in a way that originates channels in the [001] direction. The remaining three apertures are elliptical, about 0.65 × 0.74 nm. Therefore, EMT supercages are elliptical with free dimensions near 1.3 × 1.4 nm,7 that is, higher than FAU supercages, which are approximately spherical with a diameter of 1.25-1.3 nm. Textural and surface chemistry properties of sodic and protonic forms of ZSM-20, in parallel with the same properties in Y zeolite, have been studied by different techniques and by using the adsorption of distinct probe molecules as nitrogen, n-hexane, 3-methylpentane, carbon dioxide, pyridine, and xenon.8a-e The results obtained with these probe molecules and namely the differences between ZSM-20 and Y zeolites can be interpreted in terms (1) Ciric, J. U.S. Patent 3,972,983, 1976, assigned to Mobil Oil Corp. (2) Davis, M. E.; Saldarriaga, C.; Montes, C.; Garces, J.; Crowder, C. Zeolites 1988, 8, 362. (3) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S.; Nature (London) 1992, 359, 710. (4) Breck, D. W. U.S. Patent 3,130,007, 1964. (5) Meier, W. M.; Olson, D. H. Atlas of Zeolite Structures. In Zeolites 1992, 12, 96. (6) La Pierre, R. B.; Weekman, V. W. U.S. Pat. 4,247,386, 1981, assigned to Mobil Oil Corp. (7) Martens, J. A.; Jacobs, P. A. In Zeolite Microporous Solids: Synthesis, Structure and Reactivity; Deroune, E. G., et al., Eds.; Kluwer Academic Publishers: Dordrecht: The Netherlands, 1992; p 511. (8) Pires, J.; Brotas de Carvalho, M.; Ribeiro, F. R.; Derouane, E. G. (a) In Adsorption: Science and Technology; Rodrigues, A. E., Le Van, D., Tondeur, D., Eds.; Kluwer: Dordrecht, The Netherlands, 1989; p 79. (b) Appl. Catal. 1989, 53, 273. (c) Zeolites 1991, 11, 345. (d) J. Mol. Catal. 1993, 85, 295. (e) Appl. Catal. 1993, 95, 75.
S0743-7463(97)00074-7 CCC: $14.00
that the adsorption properties of ZSM-20 zeolite are mainly ruled by the EMT structure. As no current theory can well describe adsorption in micropores,9 the aim of this work is to model the adsorption of neopentane, a globular molecule with a Pauling diameter of 0.7 nm, in two closely related materials that have industrial application, as is the case of Y zeolite, or potential application, as the ZSM-20 zeolite. For this purpose, a model based on the statistical thermodynamic principles, formulated for zeolites by Ruthven and Goddard,10,11 is used. Recently, the same model was applied to small molecules in A and X zeolites at high pressures.12 Experimental Section Y zeolite was from Union Carbide (LZY52). ZSM-20 was obtained as described in the literature13 with a Si/Al ratio of 4.8. Other properties of this zeolite can be found elsewhere.8c,13 Protonic forms were obtained from sodic forms by ion exchange, with ammonium nitrate (2 M) for Y zeolite and ammonium chloride (2 M) for ZSM-20 zeolite, followed by calcination under a flow of dry air at 773 K for 8 h. Neopentane from UCAR (99%) was purified by freezingvacuum-thaw. Adsorption isotherms were obtained by the volumetric method in a Pyrex made apparatus equipped with greaseless stopcocks (from ACE Inc.). Pressure readings were made with a transducer Datametrics type 650a for pressures until 1 Torr (133 Pa) and type 600a for higher pressures. Due to the high rectangular character of the adsorption isotherms, the studies were performed only at relatively low pressures, below 15 Torr (2 kPa), which corresponds to the most informative part of the isotherm. Regarding equilibrium times, these were about 12 h for each of the first two or three points and the subsequent data were determined over a period of 2 days. Samples were outgassed for 4 h at 623 K under a dynamic vacuum of 10-4-10-5 Torr (10-2-10-3 Pa) and adsorbed amounts are expressed by weight of outgassed sample. Adsorption temperatures (273, 298, and 318 K) were maintained with a water bath ((0.05 K).
Results and Discussion The adsorption data, expressed in molecules/cavity considering an average value of 4.8 × 1019 cavities/g, are registered for sodic and protonic forms in Tables 1 and 2, respectively for Y and ZSM-20 zeolites, at 273, 298, and 318 K. The parameters of the statistical model were obtained as described in ref 11. Briefly, the values of ξ, which are related to the grand partition function, Ξ, by ξM ) Ξ were obtained from eq 1 after numerical integration of the adsorption isotherm, n(p) ) f(p), between 0 and a given pressure value (p).
∫
ξ ) exp[ n(p) dp/p]
(1)
From geometrical considerations only about three molecules of neopentane can fit in an FAU or EMT supercage. In this case, according to Ruthven and Goddard,11 ξ can be approximated to
ξ ) 1 + Kp + (Kp)2A2/2 + (Kp)3A3/6
(2)
(9) Rouquerol, J.; Avnir, D.; Fairbridje, C. W.; Everett, D. H.; Haynes, J. H.; Pernicone, N.; Ramsay, J. D. F.; Sing, K. S. W.; Unger, K. K. Pure Appl. Chem. 1994, 66, 1739. (10) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley: New York, 1984. (11) Ruthven, D. M.; Goddard, M. Zeolites 1986, 6, 275. (12) Vernesse, J.; Vidal, D.; Malbrunot, P. Langmuir 1996, 12, 4190. (13) Dewaele, N.; Maistriau, L.; Nagy, J. B.; Gabelica, Z.; Derouane, E. G. Appl. Catal. 1988, 37, 273.
© 1997 American Chemical Society
Notes
Langmuir, Vol. 13, No. 13, 1997 3585
Table 1. Adsorbed Amounts of Neopentane, in Molecules/Cavity, at the Respective Relative Pressures and Temperatures, as Well as the Deviations (in Percent) in Relation to the Amounts Predicted by the Statistical Model in the Sodic and Protonic Forms of Y Zeolite 273 K p/p°
mol/cav
298 K dev (%)
p/p°
318 K
mol/cav
7.47 × 10-5 1.20 × 10-4 1.75 × 10-4 3.04 × 10-4 7.79 × 10-4 2.32 × 10-3
1.05 1.39 1.77 2.13 2.50 2.73
4.1 6.6 -1.1 -2.3 -0.8 1.6
1.38 × 10-4 1.86 × 10-4 2.51 × 10-4 3.31 × 10-4 4.44 × 10-4 6.09 × 10-4 1.87 × 10-3
NaY 0.882 1.15 1.42 1.74 2.00 2.24 2.63
7.89 × 10-5 1.71 × 10-4 2.47 × 10-4 4.13 × 10-4 5.14 × 10-4 1.05 × 10-3 1.97 × 10-3 2.10 × 10-3
0.915 1.21 1.54 1.94 2.11 2.34 2.50 2.50
1.8 1.2 8.9 -0.6 -4.7 -3.0 -1.2 1.5
1.59 × 10-4 2.05 × 10-4 2.84 × 10-4 4.00 × 10-4 5.74 × 10-4 7.11 × 10-4 2.63 × 10-3
HY 0.874 1.08 1.30 1.61 1.89 2.03 2.50
dev (%) -5.9 -7.2 -5.7 -7.6 -5.7 -4.1 3.5
0.8 -1.2 1.5 -1.9 -3.0 -3.4 2.0
p/p°
mol/cav
dev (%)
4.13 × 10-5 6.77 × 10-5 1.63 × 10-4 2.01 × 10-4 2.52 × 10-4 2.88 × 10-4 3.29 × 10-4 3.59 × 10-4 4.94 × 10-4 9.51 × 10-4 1.96 × 10-3 4.88 × 10-4
0.166 0.277 0.608 0.752 0.931 1.07 1.22 1.35 1.63 2.10 2.40 2.65
3.6 5.2 14.8 11.0 6.6 1.7 -3.3 -8.1 -10.4 -13.3 -12.5 -9.3
1.10 × 10-4 1.68 × 10-4 2.35 × 10-4 3.05 × 10-4 7.29 × 10-4 2.10 × 10-3 4.96 × 10-3
0.348 0.584 0.834 1.08 1.82 2.24 2.45
-22.5 -34.9 -41.1 -45.3 -43.9 -26.8 -14.6
Table 2. Adsorbed Amounts of Neopentane, in Molecules/Cavity, at the Respective Relative Pressures and Temperatures, as Well as the Deviations (in Percent) in Relation to the Amounts Predicted by the Statistical Model in the Sodic and Protonic Forms of ZSM-20 Zeolite 273 K p/p°
mol/cav
298 K dev (%)
6.65 × 10-5 1.33 × 10-4 2.11 × 10-4 4.19 × 10-4 1.55 × 10-3 5.46 × 10-3 2.13 × 10-2
0.769 1.26 1.66 2.13 2.56 2.81 3.03
12.4 17.7 9.7 1.9 1.37 1.5 -2.5
9.48 × 10-5 1.29 × 10-4 1.86 × 10-4 2.83 × 10-4 3.87 × 10-4 6.66 × 10-4 1.32 × 10-3 1.77 × 10-3 5.67 × 10-3
0.719 0.868 1.03 1.31 1.60 1.97 2.31 2.45 2.73
-4.1 2.5 11.5 11.4 4.3 1.0 -0.1 -1.4 0.9
p/p°
318 K dev (%)
p/p°
mol/cav
dev (%)
9.26 × 10-5 1.36 × 10-4 1.86 × 10-4 2.60 × 10-4 3.36 × 10-4 4.39 × 10-4 5.72 × 10-4 1.45 × 10-3 6.71 × 10-3
NaZSM-20 0.598 0.768 0.948 1.20 1.43 1.68 1.89 2.35 2.76
mol/cav
-14.8 3.1 12.8 14.1 11.1 6.7 3.7 0.7 0.8
7.11 × 10-5 1.03 × 10-4 1.47 × 10-4 2.18 × 10-4 3.77 × 10-4 5.88 × 10-4 6.79 × 10-4 8.85 × 10-4 2.04 × 10-3 5.25 × 10-3
0.365 0.402 0.568 0.756 1.10 1.46 1.65 1.79 2.27 2.58
-33.3 -0.6 8.7 23.6 28.2 18.7 10.1 8.4 -1.6 -2.6
1.46 × 10-4 2.80 × 10-4 3.77 × 10-4 5.07 × 10-4 7.01 × 10-4 1.29 × 10-3 3.73 × 10-3 8.36 × 10-3
HZSM-20 0.566 0.877 1.09 1.34 1.60 2.01 2.44 2.63
46.2 50.7 39.2 25.9 13.8 -0.2 -6.5 -5.3
1.47 × 10-4 2.27 × 10-4 2.83 × 10-4 3.63 × 10-4 5.80 × 10-4 9.06 × 10-4 1.38 × 10-3 2.82 × 10-3 4.53 × 10-3
0.389 0.540 0.688 0.842 1.16 1.47 1.75 2.15 2.34
-19.2 -15.9 -20.9 -21.2 -18.5 -12.6 -5.5 3.7 7.1
where K is the Henry constant and A2 and A3 are empirical coefficients that account for the interaction between molecules when respectively there are two or three molecules per supercage. These coefficients would be equal to 1 when the interactions are negligible. After the Henry constants are estimated, from the initial slopes of the isotherms, the values of A2 and A3 can be estimated respectively from the intercept and slope of plots of (ξ - 1 - Kp)/(Kp)2 vs Kp. From the coefficients A2 and A3 the adsorbed amounts as a function of the pressure, n(p), can be obtained from the statistical model via eq 3.11 These values are compared with the experimental data in Figure 1, taking the NaY zeolite as an example. The deviations (in percent) between the experimentaldata and the data predicted by the model are registered in Tables 1 and 2 for the samples.
(Kp)3A3 2! n(p) ) (Kp)2A2 (Kp)3A3 1 + Kp + + 2! 3! Kp + (Kp)2A2 +
(3)
A first examination of Figure 1 and Tables 1 and 2 pointed out that the agreement between the estimated and the experimental results is, in general, better for the lower temperatures. It is useful at this point to remember that the statistical model assumes that migration of molecules within a given cage is much more frequent than migration between cages.11 This situation is in principle favored for lower adsorption temperatures. Moreover, the fittings are better for the sodic than for the protonic forms, which in principle can be related with specific interactions with cations. Isosteric heats of
3586 Langmuir, Vol. 13, No. 13, 1997
Notes
Figure 2. Isosteric heats of adsorption of neopentane in the range 273-318 K for the studied zeolites. Estimated errors vary between (2 and 0.3 kJ/mol between the coverages respectively of 0.5 and 2 molecules/cavity.
Figure 1. Adsorption isotherms of neopentane in NaY zeolite at indicated temperatures (closed symbols for experimental and open symbols for model).
adsorption (qst) were estimated from the experimental adsorption results using the Clausius-Clapeyron equation14 and are displayed in Figure 2. It can be seen from this figure that for comparable adsorbed amounts the qst (14) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982.
values in the sodic forms are higher than in the protonic forms, reflecting the higher polarization forces due to the presence of Na+ cations in the zeolitic structure. In this case after an initial decrease, the isosteric heats are reasonably constant between 1 and 2 molecules per cage. A different pattern is observed for the protonic forms where the adsorption heats show a minimum value followed by a steep increase for fillings between 1 and 2 molecules per supercage. It seems that in protonic forms the adsorbateadsorbate interactions are quite relevant, when compared with adsorbate-adsorbent interactions. In this case the migration of the molecules between cages is probably easier when compared with the correspondent situation in the sodic form. In this way it can be admitted that the above mentioned requisite of the statistical model is better fulfilled in the sodic forms. No trend of the coefficients with temperature is noticed, a feature that occurred also in other works.11 Although, the model is capable of reproducing the experimental results, the divergence being lower in the sodic forms and for lower temperatures. The maximum values for A2 can be found for ZSM-20 zeolite, particularly in the sodic forms. From the qst values it can be disclosed that for fillings higher than 1.5 molecules per cage the molecule-molecule interactions are more intense in ZSM-20, which is probably the origin of the existence of higher A values for this zeolite. LA970074X
