1846
Langmuir 1998, 14, 1846-1852
Isotopic Transient Study of Multicomponent N2 and O2 Adsorption on CaX Zeolite Imre-Georges Bajusz and James G. Goodwin, Jr.* Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
Douglas Galloway and Nanette Greenlay UOP, LLC, 25 East Algonquin Road, P.O. Box 5017, Des Plaines, Illinois 60017 Received October 1, 1997. In Final Form: January 16, 1998 Steady-state isotopic transient kinetic analysis, previously used extensively for the study of surfacecatalyzed reactions, was used for the first time to compare the adsorption behavior of pure N2 and O2 with the adsorption of binary N2/O2 mixtures on a CaX zeolite. By variation of the partial pressure of N2 or O2 in the feed stream, N2 and O2 adsorption isotherms at temperatures between 303 and 338 K were able to be measured at equilibrium conditions using isotopic switches between 14N2 (or 16O2) and 15N2 (or 18O2). The N2 uptake was about 1 order of magnitude higher than the O2 uptake on CaX, mainly due to the higher quadrupole moment of N2. N2 adsorption isotherms in the presence of a constant partial pressure of O2 in the gas phase were also measured between 303 and 338 K. The N2 uptake in the presence of O2 was significantly smaller than the one measured for pure N2. However, the isosteric heats of adsorption of N2 calculated from the N2/O2 data were very similar to those obtained from the pure N2 data (around 6.3 kcal/mol). On the basis of these observations, it is suggested that O2 does not affect the interactions between N2 molecules and the Ca2+ adsorption sites. On the other hand, when the O2 partial pressure was varied at constant N2 partial pressure, a systematic decrease in N2 uptake could be observed with increasing O2 uptake. Thus, the smaller N2 uptake in the presence of O2 would appear only to be due to the competitive adsorption of N2 and O2 molecules. Additionally, N2 and O2 uptakes were measured from mixtures having a constant N2 to O2 ratio of 4/1 where only the overall adsorbate pressure (PN2 + PO2) was varied. While the amount of N2 adsorbed increased with an increase in PN2, no significant change in nitrogen selectivity, RN2, was able to be observed. This suggests, that the competition for adsorption between N2 and O2 depends solely on the PN2/PO2 ratio rather than on the absolute value of PN2.
Introduction The growing interest in applying adsorption techniques for air separation has been mainly generated by the recent developments in the use of molecular sieves. It has been found that alkaline earth X zeolites are very selective for N2 adsorption due to the strong interactions between the quadrupole moment of N2 and the cation positioned in the zeolite supercage.1 The calcium form, mainly due to its high charge density, exhibits the highest nitrogen capacity, isosteric heat of adsorption, and nitrogen/oxygen selectivity among all alkaline earth exchanged X zeolites.2 In fact, the nitrogen capacity and nitrogen/oxygen selectivity for CaX are among the highest reported in the literature for any zeolitic adsorbent.3 Until now, the adsorbents used for air separation have mainly been characterized in terms of adsorption behavior of a single component (oxygen and nitrogen). However, to develop a complete understanding of the gas separation behavior of these materials, it is necessary to determine the adsorption characteristics of multicomponent mixtures. Numerous theoretical models have been proposed in the literature to predict the adsorption behavior of
multicomponent mixtures from pure component adsorption data. The main models used so far are the extended Langmuir theory,4 the ideal adsorbed solution theory (IAST),5 the vacancy solution theory (VST),6,7 and the simplified statistical thermodynamic theory (SSTT).8,9 The extended Langmuir theory is often preferred over models such as IAST, VST, and SSTT due to its mathematical simplicity. However, the major drawback of these models is that they simulate adsorption of mixtures based on data collected from pure component systems. Unfortunately, measurement of adsorption characteristics in multicomponent systems has been in the past very tedious and time-consuming. Recently, research efforts have been devoted to develop simpler techniques for measuring in situ adsorption characteristics of multicomponent mixtures. Rynders et al.10 have shown how to describe gas adsorption equilibria and kinetics from multicomponent adsorption data collected using the isotopic exchange technique (IET). In a previous article,11 it was demonstrated how another isotopic technique, steady-state isotopic transient kinetic analysis (SSITKA) used to study catalytic reactions on
* To whom all correspondence should be addressed. (1) Jasra, R. V.; Choudary, N. V.; Bhat, S. G. T. Ind. Eng. Chem. Res. 1996, 35, 4221. (2) Coe, C. G.; Parris, G. E.; Srinivasan, R.; Auvil, S. R. In New Developments in Zeolite Science and Technology-Proceedings of the 7th International Zeolite Conference, Tokyo; Iijima, A., Ward, J. W., Murakami, Y., Eds.; Elsevier: New York, 1986. (3) Coe, C. G. In Process Technology Proceedings 8, Gas Separation Technology; Vansant, E. F., Dewolfs R., Eds.; Elsevier: Amsterdam, 1990.
(4) Markham, E. D.; Benton, A. F. J. Am. Chem. Soc. 1931, 53, 497. (5) Myers, A. L.; Prausnitz, J. M. AIChE J. 1965, 11, 121. (6) Suwanayuen, S.; Danner, R. P. AIChE J. 1980, 26, 68. (7) Suwanayuen, S.; Danner, R. P. AIChE J. 1980, 26, 76. (8) Ruthven, D. M. In Principles of Adsorption and Adsorption Processes; John Wiley and Sons: New York, 1984. (9) Ruthven, D. M.; Loughlin, K. F. Chem. Eng. Sci. 1983, 38, 1517. (10) Rynders, R. M.; Rao, M. B.; Sircar, S. AIChE J, in press. (11) Bajusz, I.-G.; Goodwin, J. G., Jr. Langmuir 1997, 13, 6550.
S0743-7463(97)01082-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/27/1998
N2 and O2 Adsorption on CaX
surfaces, could be successfully employed to measure adsorption isotherms and isosteric heats of adsorption of N2 on a LiX zeolite. Basically, this technique utilizes switches between isotopically labeled adsorbates during adsorption without changing the partial pressures of adsorbates or disturbing equilibrium. The analysis of the transient responses of labeled compounds generated by those switches provides the information necessary to calculate the gas uptake on the sorbent. The pure component adsorption measurements using SSITKA suggested possibilities for the study of more complicated systems, especially multicomponent ones. This paper shows how SSITKA can be used to measure adsorption characteristics of adsorbate mixtures. In the present study, SSITKA was used to measure adsorption behavior of binary mixtures of N2 and O2 on a CaX zeolite. The adsorption characteristics of pure N2 and O2 on CaX were compared with the adsorption behavior of N2/O2 binary mixtures. One of the major strengths of SSITKA demonstrated in this study is its ability to collect data which allows the calculation of heats of adsorption for an adsorbate (N2) in a multicomponent mixture (N2 and O2). Methods Materials. The zeolite used for this study was a highly exchanged CaX zeolite with a low Si/Al ratio (1.2). It was obtained by aqueous exchange of a NaX commercial zeolite. The preparation procedures for the synthesis are similar to those described in refs 2 and 12. The synthesis procedure resulted in exchange of 94% of the sodium. SSITKA System. All the adsorption measurements were done using the system described in ref 11. The adsorbent was placed in a quartz microreactor with 4 mm i.d. A thermocouple was installed on the top of the adsorbent bed. A pneumatic valve operated electronically was used for the switch between two gas streams with the same gas composition and flow rate but different isotopic labeling. The pressure was maintained constant in the two streams to be switched by using two back-pressure regulators. The system had an on-line Leybold Inficon Auditor-2 mass spectrometer (MS) equipped with a high-speed data acquisition system interfaced to a personal computer. A sampling assembly was installed so that the amount of gas leaking into the MS could be controlled precisely by differential pumping. The holdup of the gases in the entire system was minimized. All the gases used for this study were of ultra-high-purity grade from Praxair, except 15N2 and 18O2, which were obtained from Isotech. Before the gases were allowed to enter the adsorption system, they were further purified using an Alltech gas purifier packed with indicating Drierite and 5A molecular sieve. Adsorption Measurements. Adsorption measurements were made using between 50 and 100 mg of adsorbent loaded into the flow microreactor. Before each adsorption measurement, the adsorbent was activated to remove adsorbed water. Activation of the adsorbent was carried out in flowing helium (purity 99.999%) with a temperature ramp of 5 °C/min to 350 °C where the temperature was held for 10 h. After activation, the adsorbent bed temperature was lowered to the desired adsorption temperature and the feed was switched to the adsorption mixture. (i) Adsorption of Pure N2 on CaX. Adsorption isotherms of pure N2 were taken for N2 partial pressures between 0.06 and 0.32 bar (1 bar ) 105 Pa) at 303, 318, 330, and 338 K. Unfortunately, the system did not allow us to take adsorption measurements at the lower nitrogen partial pressures (PN2 < 0.13 bar) keeping the total pressure constant. For the measurements taken between 0.06 and 0.09 bar of N2, the feed consisted of 4 cm3/min N2 diluted by 80, 60, and 50 cm3/min He, respectively. The total pressures were 1.34, 1.26, and 1.20 bar, respectively. For the measurements taken between 0.13 and 0.32 bar, the total flow rate was kept at 34 cm3/min and the total pressure at (12) Coe, C. G.; Kuznicki, S. M.; Srinivasan, R.; Jenkins, R. J. ACS Symp. Ser. 1988, No. 368, 478.
Langmuir, Vol. 14, No. 7, 1998 1847 1.1 bar. The feed consisted of 4, 6, 8, and 10 cm3/min N2 with the balance being helium. Steady-state (equilibrium) isotopic transient data were collected at all adsorption temperatures after 15 min of time-onstream where adsorption equilibrium conditions were ensured. At the space velocity and the adsorption conditions utilized, mass transport limitations on adsorption were minimized. The isotopic transients were taken by switching two feed streams where the only difference was the isotopic composition of N2: one stream containing 14N2 and the other 15N2. A trace of argon (5%) was present in the 14N2 stream in order to measure the gas-phase holdup of the entire adsorption system. This was not enough to disrupt the adsorption equilibrium during the isotopic switches. The isotopic switch in the feed stream introduced a step input of the new isotopic label in the adsorbate. Since there were no isotope effects, the switch was performed instantaneously, the two feed streams to be switched were kept at constant pressure and flow rate, and adsorption equilibrium was maintained in the adsorbent bed. The transient responses of the labeled adsorbate and inert tracer were monitored at the system outlet by mass spectrometry. Because of the low activation energy of desorption of physisorbed molecules, total desorption of the previously labeled adsorbed species occurred during the measurement of a transient. (ii) Adsorption of Pure O2 on CaX. An adsorption isotherm of O2 on CaX was taken at 303 K for O2 partial pressures between 0.13 and 0.32 bar. The total flow rate was kept at 34 cm3/min for all the measurements. The feed consisted of 4, 6, 8, or 10 cm3/min O2 with the balance being helium. The total pressure was maintained at 1.1 bar for all the measurements. Analogously to the N2 adsorption measurements, the isotopic transients were taken by switching two feed streams where the only difference was the isotopic composition of O2: one stream containing 16O2 and the other 18O2. A trace of argon (5%) was also present in the 16O stream in order to measure the gas-phase holdup of the 2 entire adsorption system. (iii) Adsorption of N2/O2 Mixtures on CaX. N2 isotherms similar to the ones described in (i) were measured in the presence of 0.58 bar of O2 between 303 and 338 K. An O2 isotherm similar to the one described in (ii) was measured in the presence of 0.13 bar of N2 at 303 K. Additionally, adsorption measurements were taken for a binary mixture with a constant N2/O2 ratio of 4/1. The flow rates of N2 and O2 were kept constant at 16 and 4 cm3/min, respectively. The balance consisted of 0, 8, 12, and 35 cm3/min He, respectively. The total pressure was also maintained at 1.1 bar for all these measurements. For the binary mixtures, the isotopic transients were taken by switching between a stream containing 14N2 and 16O2 and one stream containing 15N2 and 18O2. A trace of argon (5%) was present in the 14N2 for the gas-phase holdup correction. Transient Response Analysis. The two parameters obtained from SSITKA in this study were the surface concentration at equilibrium of adsorbed species A, NA, and the average surface residence time of A, τA.13 For practical purposes, τA is calculated first by integrating the area between the normalized transient responses of A, FA(t), and the one of the inert I, FI(t), following an isotopic switch from A to A* (labeled A), where I is present as a trace in A:
τA )
∫ [F (t) - F (t)] dt ∞
A
0
I
(1)
Furthermore, it can be shown13 that NA is related to τA by
NA ) rA,ss
∫ F (t) dt ) r ∞
0
A
A,ssτA
(2)
where rA,ss is the steady-state flow rate of A before the isotopic switch. The surface coverage obtained from the transient responses is very accurate provided the interactions between the inert tracer and the adsorbent are negligible. Although it is calculated via τA, the calculation of NA requires no assumptions for interpreta(13) Shannon, S. L.; Goodwin, J. G., Jr. Chem. Rev. 1995, 95, 677.
1848 Langmuir, Vol. 14, No. 7, 1998
Figure 1. Normalized transient responses of Ar after a switch between 14N2/Ar and 15N2 over R-Al2O3 and CaX, respectively.
Figure 2. Normalized transient responses of N2 after a switch between 14N2/Ar and 15N2 over CaX. tion since it is based solely on a mass balance. On the other hand, the interpretation of τA requires much more attention. It can be easily demonstrated that diffusion and readsorption phenomena can have major impacts on τA. Another limitation related to the interpretation of τA comes from the fact that τA is averaged over all molecules of A including those which do not adsorb at all on the adsorbent. A detailed description of the methodology used for data analysis in SSITKA can be found in refs 11 and 13.
Results Figures1-3 represent typical isotopic transients which were obtained after switching between 14N2/Ar and 15N2 or between 16O2/Ar and18O2. In Figure 1, a transient of the inert tracer over CaX is compared with one over R-Al2O3. The argon transient over R-Al2O3 constitutes the “blank” transient and was generated using a loading of R-Al2O3 in the microreactor equivalent in volume to the CaX studied. From Figure 1, it can be observed that the argon transient over R-Al2O3 is essentially superimposed over the argon transient generated over CaX. Thus, Figure 1
Bajusz et al.
Figure 3. Normalized transient responses of O2 after a switch between 16O2/Ar and 18O2 over CaX.
Figure 4. Adsorption isotherms of N2 on CaX obtained from SSITKA.
confirms that, although Ar can adsorb on CaX, its uptake is essentially negligible under the conditions of this study and at its low partial pressure. Consequently, it serves as an accurate means to delineate gas-phase hold-up in the entire system. (i) Adsorption of Pure N2. The adsorption isotherms of pure N2 on CaX at different temperatures calculated using NN2 obtained by SSITKA are displayed in Figure 4. The amount of N2 adsorbed (Table 1) was directly calculated from the measured transients using eq 2 whereas τA was calculated using eq 1. The lines are the best fitting Langmuir curves obtained from the following Langmuir isotherm equation
ΘN2 )
KN2PN2 1 + KN2PN2
(3)
where ΘN2 is the N2 coverage and KN2 the adsorption equilibrium constant of N2 on CaX. The KN2 values corresponding to the best fitting Langmuir curves are displayed in Table 2.
N2 and O2 Adsorption on CaX
Langmuir, Vol. 14, No. 7, 1998 1849 Table 3. SSITKA Parameters for O2 Adsorption on CaXa
Table 1. SSITKA Parameters for N2 Adsorption on CaX 303 K PN2 τN2 (bar) (s)
b
319 K
330 K
338K
NN2 τ N2 NN 2 τ N2 NN 2 τ N2 NN 2 (µmol/g) (s) (µmol/g) (s) (µmol/g) (s) (µmol/g) c
0.13 0.19 0.26 0.32
6.0 5.2 4.5 4.0
305 396 457 508
(a) CaXa (PTot ) 1.10 bar) 4.2 213 3.3 168 3.8 290 3.0 229 3.4 346 2.7 274 2.9 368 2.5 318
3.1 2.6 2.4 2.1
158 198 244 267
0.06 0.08 0.09
7.1 8.1 9.0
(b) CaXd (PTot ) 1.20-1.34 bar) 193 4.6 125 3.3 90 220 5.3 144 4.2 114 244 5.8 158 5.7 155
2.7 3.3 3.7
73 90 100
Reactor loading ) 53 mg. b Error ) (0.2 s. c Error ) (7 µmol/g CaX. d Reactor loading ) 100 mg. a
Table 2. Adsorption Equilibrium Constant Determined for N2 on CaX temperature (K)
KN2 (µmol/g‚bar)
temperature (K)
KN2 (µmol/g‚bar)
303 319
3785 2226
330 338
1497 1264
Figure 5. Adsorption isotherms of N2 and O2 on CaX obtained from SSITKA at 303 K in the absence of the other.
As expected, the N2 uptake increased systematically with the N2 partial pressure and decreased with temperature. At constant N2 partial pressure, τN2 decreased systematically with temperature. Additionally, a slight decrease of τN2 with increasing PN2 can be observed from Table 1a. The small changes in total pressure (Table 1b) have a minute impact on the average surface residence time due to the slight change in frequency of collisions with and adsorption on adsorption sites. (ii) Adsorption of Pure O2. The adsorption isotherm of pure O2 at 303 K is displayed and compared to the N2 isotherm at the same temperature in Figure 5. The O2 uptake (Table 3) is significantly lower than the N2 uptake at those conditions as was expected. No O2 isotherm was determined at higher temperatures due to the limit in detectability of the small amounts of O2 adsorbed under those conditions. The impact of O2 partial pressure on the average surface residence time of O2, τO2, is displayed in Table 3. As opposed to τN2 (Table 1), τO2 did not vary with increasing O2 partial pressure within experimental error. (iii) Adsorption of N2/O2 Mixtures. The SSITKA
PO2 (bar)
τO2b (s)
NO2c (µmol/g)
0.13 0.19 0.26 0.32
1.0 0.8 0.9 1.0
27 32 48 67
a T ) 303 K, P b Tot ) 1.1 bar, reactor loading ) 101 mg. Error ) (0.2 s. c O2 uptake, error ) (7 µmol/g of CaX.
Table 4. SSITKA Parameters for N2 Adsorption on CaX in the Presence of O2a 303 K PN2 τN2 (bar) (s)
b
0.13 0.19 0.26 0.32 c
9.6 8.4 7.9 7.3
319 K
332 K
343K
N N2 τ N2 NN 2 τ N2 NN 2 τ N2 NN 2 (µmol/g) (s) (µmol/g) (s) (µmol/g) (s) (µmol/g) c
260 341 428 494
6.8 6.1 5.8 5.5
184 248 314 372
5.5 5.0 4.2 3.7
149 203 227 250
3.4 3.4 3.3 3.2
92 138 179 217
a P b O2 )0.58 bar, reactor loading ) 101 mg. Error ) (0.2 s. Error ) (7 µmol/g of CaX.
Figure 6. Adsorption isotherm of N2 for pure N2 and a N2-O2 mixture at 303 K (PO2 ) 0.58 bar).
parameters obtained from the measurements of N2 adsorption on CaX in the presence of 0.58 bar of O2 at different temperatures are displayed in Table 4. As observed previously in the case of pure N2, the N2 uptake increased systematically with the N2 partial pressure and decreased with temperature. In terms of the average surface residence time, a consistent decrease in τN2 with increasing PN2 was evident for all the temperatures except 343 K, where τN2 remained constant within experimental error. The N2 adsorption isotherm in the presence of O2 at 303 K is plotted in Figure 6 along with the adsorption isotherm of pure N2 at the same conditions. Even in the presence of O2, a reasonable fit of a Langmuir adsorption curve through the N2 isotherm was obtained. The adsorption equilibrium constant, KN2, was determined to be 2448 µmol/g‚bar. On the other hand, in the presence of 0.58 bar of O2, the uptake of N2 was systematically smaller than for the case where N2 was the only adsorbate. The impact of O2 partial pressure on the amount of adsorbed O2 and adsorbed N2 at constant N2 partial pressure (as determined by SSITKA) is displayed in Table 5 and Figure 7. From Table 5 it can be seen that the average surface residence time of O2 was consistently 1 order of magnitude smaller than that of N2. Between
1850 Langmuir, Vol. 14, No. 7, 1998
Bajusz et al.
Table 5. SSITKA Parameters for Adsorption of N2/O2 Mixtures on CaXa PO2 (bar)
τO2b (s)
NO2c (µmol/g)
τN2b (s)
NN2c (µmol/g)
0.13 0.26 0.39 0.52
0.7 0.6 0.6 1.0
19 32 72 106
12.6 12.6 12.1 11.5
335 335 321 305
a T ) 303 K, P N2 ) 0.13 bar, PTot ) 1.1 bar, reactor loading ) 101 mg. b Error ) (0.2 s. c Error ) (7 µmol/g of zeolite.
Figure 7. N2 and O2 uptake on CaX for PN2 ) 0.13 bar and 303 K. Table 6. SSITKA Parameters for Adsorption Measurements at Constant N2/O2 Ratio ()4) on CaXa PO2 (bar)
P N2 (bar)
τO2b (s)
NO2c (µmol/g)
τ N 2b (s)
NN2c (µmol/g)
0.08 0.14 0.16 0.22
0.32 0.55 0.63 0.88
0.3 0.5 0.5 0.5
8 13 13 13
12.6 12.6 12.1 11.5
415 670 766 787
a T ) 303 K, P b Tot ) 1.1 bar, reactor loading ) 102 mg. Error ) (0.2 s. c Error ) (7 µmol/g.
0.13 and 0.26 bar of O2, τO2 and τN2 were both constant within experimental error. Between 0.26 and 0.52 bar of O2, τO2 increased whereas τN2 decreased. Figure 7 shows that with increasing O2 partial pressure, the O2 uptake, NO2, monotonically increased as would be expected. On the other hand, the N2 uptake decreased for partial pressures of O2 > 0.26 bar. Table 6 shows the results of adsorption measurements of binary N2/O2 mixtures taken at 303 K and a constant N2/O2 ratio of 4/1. At such a high N2/O2 ratio, no relevant change in either the average surface residence time or the uptake of O2 was observed when PO2 and PN2 were simultaneously increased. On the other hand, the average surface residence time of N2 slightly decreased and the N2 uptake increased consistently as both O2 and N2 partial pressures were increased. Discussion The difference in the areas between the transient decays of N2 and O2 relative to the transient of argon illustrated in Figures 2 and 3 demonstrates how the difference in adsorption affinities of N2 and O2 on CaX can be easily measured with SSITKA. The results obtained from transient analysis and displayed in Figure 5 confirm the
high sorption selectivity of CaX for N2. This remarkable difference in sorption capacities between N2 and O2 on CaX has been mainly attributed to major differences in the molecular interactions between the quadrupole moments of these adsorbate molecules with Ca2+ cations.14 It can be proven using quantum mechanics that the key element for air separation on CaX zeolite is the significantly higher quadrupole moment of N2 relative to that of O2. The fact that Ca2+ is a relatively small cation and possesses a double charge reinforces its selectivity for N2 and makes Ca zeolites the best N2 adsorbents in terms of absolute uptake. The first parameter which is derived from the isotopic transients is the average surface residence time. It is however very difficult to interpret either τN2 or τO2, since they are averaged over all the N2 or O2 molecules flowing through the reactor, including those which readsorb and those which do not adsorb at all. Thus, the information hidden in this parameter is a combination of (a) adsorption kinetic characteristics, (b) readsorption phenomena, and (c) impact of bypassing (nonadsorbing) molecules. All these three factors can be observed in Table 1. τN2 decreased monotonically with increasing temperature. This phenomenon can be mainly attributed to the increase of adsorption-desorption rates, which causes a constant decrease in the average surface residence time of N2 on CaX. On the other hand the monotonic decrease of τN2 with increasing N2 partial pressure reflects the impact of readsorption and bypassing molecules. At higher PN2, the competition for adsorption increases, resulting in more N2 molecules bypassing the adsorbent bed and less N2 readsorbing. Both of these phenomena contributed to the overall decrease in the average surface residence time of N2 observed in Table 1. However for O2, no decrease in τO2 was observed with increasing PO2 (Table 3). This may be due to the extremely low affinity of the adsorption sites for O2. Under these conditions, the coverage of O2 on CaX is very low, thus the amount of nonoccupied sites is quite high and consequently the competition for adsorption between O2 molecules minimal. The adsorption characteristics of pure N2 and O2 on CaX will now be compared with the adsorption of N2/O2 mixtures under similar conditions. This involves actually the most promising feature of SSITKA in terms of adsorption measurementssnamely its capability for collecting data for multiadsorbate systems in an efficient manner. As can be seen from Table 4, the presence of a constant partial pressure of O2 did not affect the trends in the SSITKA parameters of N2 already observed in Table 1. On the other hand, the N2 uptake was significantly smaller than in the case when N2 was the sole adsorbate present (Figure 6). The relative amount of N2 adsorbed on CaX in the presence of O2 is plotted against the N2/O2 ratio in the gas phase in Figure 8. It can be suggested from this representation that, at constant O2 partial pressure, the effect of O2 on N2 adsorption is directly proportional to the amount of O2 in the gas phase. The decrease in the amount of adsorbed N2 in the presence of O2 may be caused principally by an impact of O2 on the interaction between N2 and cationic sites and/or by the competition for adsorption between N2 and O2 molecules. The interactions between N2 and sorption sites are quantitatively reflected by the isosteric heat of adsorption of N2 on CaX, ∆Hst,N2. The isosteric heat of adsorption can be easily obtained from a series of isotherms using the Clausius-Clapeyron relation.15 The isosteric (14) Choudary, N. V.; Jasra, R. V.; Bhat, S. G. T. Ind. Eng. Chem. Res. 1993, 32, 548.
N2 and O2 Adsorption on CaX
Langmuir, Vol. 14, No. 7, 1998 1851
heats of adsorption of N2 at different coverages in the presence and absence of O2 were calculated and are displayed in Figure 9. The obtained values for pure N2 (around 6.3 kcal/mol) are in fair agreement with the ones obtained using a microbalance also for N2 adsorption on a CaX zeolite (6.8 kcal/mol).16 On the other hand, they are notably higher than the isosteric heats of N2 on LiX (around 5.0 kcal/mol) obtained from SSITKA measurements under the same conditions.11 This is indicative of the stronger interactions between N2 and Ca2+ due to its smaller size and its double charge. It is interesting to note from Figure 9 that there was no significant difference in the isosteric heat of adsorption of N2 whether O2 was present in the gas phase or not. This suggests that O2 has no direct influence on the sorption interactions between the N2 molecules and the zeolite cations. This suggests
that the changes observed for N2 uptake in the presence of O2 come principally from competitive adsorption between N2 and O2. Furthermore, it can be observed from Figure 9 that the isosteric heat of adsorption does not vary significantly with increasing coverage. This is probably due to a balance between the degree of heterogeneity of the gas-solid interactions and the strength of cooperative gas-gas interactions in the studied loading range as denoted by Dunne et al.17 Another reason for the constant heat of adsorption might be that, under the studied conditions, the obtained coverages are relatively low and the adsorbed species are fairly isolated from each other within the supercages. This might minimize attractive or repulsive interactions between the sorbed species. To consider a possible competition effect between O2 and N2, it is useful to analyze the data taken at different O2 partial pressures keeping the N2 partial pressure constant (Table 5 and Figure 7). Between 0.13 and 0.26 bar of O2, the amount of N2 adsorbed did not change at all, whereas the O2 uptake did not vary significantly beyond the experimental error. When the partial pressure of oxygen was further increased (between 0.26 and 0.52 bar), a significant increase in the amount of O2 adsorbed on CaX was accompanied by a systematic decrease in N2 uptake. The increase in O2 uptake does not match exactly the decrease in N2 uptake. Between 0.26 and 0.39 bar of O2, 40 µmol/g O2 was additionally adsorbed whereas NN2 decreased by 14 µmol/g. Between 0.39 and 0.52 bar of O2, 34 µmol/g of O2 were additionally adsorbed whereas NN2 decreased by only 16 µmol/g. However, these observations suggest that the trends observed in Figures 6, 7, and 8 concerning the decrease in N2 uptake for N2 adsorption in the presence of O2 are related to a simple competition between N2 and O2 molecules for the same adsorption sites when O2 is present in a large quantity in the gas phase. The fraction of Ca2+ cations involved in the adsorption process under the conditions studied can be estimated. It has been calculated previously that a CaX zeolite with a similar Si/Al ratio as the one of this study has 43 calcium ions per unit cell.18 Of these 43 cations, 16 are in site I, which is inaccessible for N2 or O2 adsorption. Thus only 27 cations per unit cell (63% of all the Ca2+ cations) should be accessible to N2 or O2. The highest measured coverage in this study was at PO2 ) 0.22 bar and PN2 ) 0.88 bar (Table 6). With respect to the accessible sites, this corresponds to 38% of accessible Ca2+ sorbing N2 and 0.6% sorbing O2. The fact that around 60% of the accessible Ca2+ were not involved in the sorption process under these conditions is probably due to the equilibrium thermodynamic limitations placed on coverage at 303 K. It is interesting to analyze the results from Table 6 where the N2/O2 ratio of 4 simulates an air mixture. For this high fraction of N2 in the gas phase, the SSITKA parameters for O2 were extremely small. The O2 uptake, NO2, was insensitive to changes in PO2. Under these conditions, the uptake of N2 increased systematically with increasing PN2. To investigate more precisely the effect of O2 on N2 adsorption under these conditions, it is useful to consider the selectivity of N2 relative to O2, RN2/O2, defined by the following relationship:
(15) Smith, J. M.; Van Ness, H. C. In Introduction to Chemical Engineering Thermodynamics; McGraw-Hill: New York, 1987. (16) Choudary, N. V.; Jasra, R. V.; Bhat, S. G. T. In Studies in Surface Science and Catalysis; Delmon, B., Yates, J. T., Eds.; Elsevier: Amsterdam, 1994; Vol. 84.
(17) Dunne, J. A.; Mariwala, R.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Langmuir 1996, 12, 5888. (18) Mortier, J. In Compilation of Extraframework Sites in Zeolites; Butterworth & Co.: London, 1982.
Figure 8. Impact of O2 on relative N2 uptake at PO2 ) 0.58 bar and 303 K.
Figure 9. Isosteric heat of adsorption of N2 on CaX in the presence (PO2 ) 0.58 bar) and the absence of O2.
1852 Langmuir, Vol. 14, No. 7, 1998
RN2/O2 )
NN2PO2 NO2PN2
Bajusz et al.
(4)
The N2 selectivity based on the data in Table 6 is plotted versus PN2 + PO2 in Figure 10. RN2/O2 increased only slightly with increasing pressures of N2 and O2 for N2/O2 ) 4. This suggests that, for the conditions studied, the “relative” interaction of the Ca2+ with the N2 quadrupole moment is not significantly affected by increasing the pressure of the adsorbing species and simultaneously increasing coverage of N2. The fact that the O2 coverage remained constant even though PO2 increased (see Table 6) may be caused by the high PN2/PO2 ratio which favors preferentially N2 adsorption on CaX. Conclusions In this study the adsorption behavior of pure N2 and O2 was compared with the adsorption behavior of N2/O2 binary mixtures using SSITKA. It was shown that, for the single components, Ca2+ sorbs around 10 times more N2 than O2 for partial pressures of adsorbate between 0.13 and 0.32 bar and at 303 K. It was found that the N2 uptake for a N2 adsorption isotherm taken in the presence of a constant partial pressure of O2 was significantly smaller than the N2 uptake without O2. The isosteric heat of adsorption of N2 in the absence of O2 on CaX (6.3 kcal/mol) was found to be similar to the one obtained for N2 in a binary (N2 + O2) mixture. This suggests that O2 does not have a direct impact on the electronic interactions between N2 molecules and Ca2+ adsorption sites. This is probably due to the fact that, at these adsorption conditions, only around 40% of the accessible Ca2+ cations are involved in the sorption process. Thus the sorbate-cation complexes are fairly isolated from each other at these low coverages, which reduces considerably the possibility of adsorbateadsorbate interactions. In another set of experiments where the O2 partial pressure was varied and the N2 partial pressure was kept constant, a systematic decrease in N2 uptake was observed parallel to an increase in O2 uptake. It is suggested that the decreases in N2 uptake in the presence of O2 observed in this study were mainly caused by competitive adsorption of N2 and O2.
Figure 10. Selectivity of N2 adsorption over that of O2 on CaX at 303 K with PN2/PO2 ) 4/1.
Adsorption measurements were also taken for binary adsorbate mixtures with a N2/O2 ratio of 4/1 in the gas phase (typical of the ratio in air) for various total sorbate pressures (PN2 + PO2). No change in the sorption parameters of O2 with increasing O2 partial pressure was detected under these conditions. This was probably due to the parallel increase in PN2 combined with the high PN2/PO2 ratio used for this set of experiments. It could also be observed that the N2 selectivity did not vary significantly with increasing overall sorbate pressure. This may indicate that the “relative” interaction between N2 molecules and Ca2+ sites compared to that of O2 is not affected by an increase in pressure or surface coverage, at least for surface coverages
