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Langmuir 1997, 13, 6550-6554
N2 Adsorption in LiX Zeolite: Isotopic Transient Analysis Imre-Georges Bajusz and James G. Goodwin, Jr.* Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261 Received August 6, 1997. In Final Form: September 11, 1997X Steady-state isotopic transient kinetic analysis (SSITKA) is a very powerful technique used in heterogeneous catalysis to measure coverages and reactivities of surface intermediates. This paper reports the results of a study by SSITKA of N2 adsorption behavior (physisorption) in a LiX zeolite around room temperature. By varying the partial pressure of N2 in the feed stream, N2 adsorption isotherms at temperatures between 303 and 338 K were able to be measured at equilibrium conditions with SSITKA using isotopic switches between 14N2 and 15N2. The amount of N2 adsorbed per total Li cation content varied between (1 and 6.0) × 10-2 molecules/cation. The isosteric heats of adsorption of N2 as a function of coverage were calculated from the measured isotherms using the Clausius-Clapeyron equation. The obtained values (around 5.0 kcal/mol) are in good agreement with ones obtained from data collected using traditional techniques. On the basis of the results of this study, application of SSITKA to the investigation of adsorption from multicomponent sorbing mixtures offers exciting possibilities.
Introduction Physical adsorption measurements are widely used to characterize adsorbents. Various procedures have been developed to determine the amount of gas adsorbed in a porous material. Adsorption isotherms of gases on solids are generally determined by either static (volumetric or gravimetric) or dynamic methods (chromatographic).1,2 However, the common disadvantage of most of the techniques so far is their limitation in terms of measuring adsorption characteristics for multicomponent mixtures. For practical purposes, many research efforts have recently been devoted to developing new investigation methods allowing a better understanding of adsorption behavior of multicomponent mixtures.3,4 Rynders et al. have shown how the isotopic exchange technique (IET) can be used to measure pure and multicomponent gas adsorption equilibria and kinetics resolving some of the problems encountered by conventional measurements.5 In the last 2 decades, a powerful technique, steadystate isotopic transient kinetic analysis (SSITKA), has emerged allowing in-situ measurements of the concentrations and average lifetimes of surface intermediates for heterogeneously catalyzed reactions.6-9 The basic principle of this technique is to make switches between isotopically labeled reactants during reaction without changing the partial pressures of reactants or disturbing steady-state reaction. The isotopic switches generate transient responses of labeled compounds at the outlet of the reactor which are recorded using a mass spectrometer. The analysis of these measured transient responses provides the information necessary to calculate average * To whom all correspondence should be addressed: e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, November 1, 1997. (1) Oz´cik, J. Adsorption; Cooper, I. L., Translation Ed.; John Wiley & Sons: New York, 1982; Chapter 3. (2) Conder, J. R.; Young, C. L. Physicochemical Measurements by Gas Chromatography; John Wiley & Sons: New York, 1979; Chapter 1. (3) Sircar, S. Adsorption 1996, 2, 327. (4) Sircar, S. J. Chem. Soc., Faraday Trans. I 1983, 79, 2085. (5) Rynders, R. M.; Rao, M. B.; Sircar, S. AIChE J, in press. (6) Bennett, C. O. In Catalysis under Transient Conditions; Bell, A. T., L. I., Eds.; ACS Symp Ser 178; American Chemical Society: Washington, DC, 1982. (7) Happel, J. Chem. Eng. Sci. 1978, 33, 1567. (8) Biloen P. J. Mol. Catal. 1983, 21, 17. (9) Shannon, S. L.; Goodwin, J. G., Jr. Chem. Rev. 1995, 95, 677.
S0743-7463(97)00887-1 CCC: $14.00
surface lifetimes and surface concentrations of the reaction surface intermediates. This technique has been extensively reviewed elsewhere.9 This paper demonstrates how this technique can be extended as a robust tool to measure fundamental adsorption characteristics such as coverage and isosteric heat of adsorption. In the present study, SSITKA was used to measure adsorption isotherms and isosteric heats of adsorption of N2 in a LiX zeolite. Type X zeolites are extensively used as selective adsorbents for N2 in air separation processes.10-15 The higher selectivity of these zeolites toward N2 has been attributed to electronic interactions between the zeolite cations and N2. It has been shown that the enrichment of N2 in the adsorbed phase of X zeolites is mainly due to its higher quadrupole moment compared to that of oxygen.16,17 X zeolites exchanged with lithium or calcium have been shown to be much more efficient for N2 adsorption than NaX zeolites.18 Methods Materials. The zeolite used for this study was a proprietary, highly exchanged LiX zeolite with a low Si/ Al ratio. It was obtained by aqueous exchange of a NaX commercial zeolite with Li so that >95% of the cations were Li+. The preparation procedure for this exchange is similar to those described in refs 19 and 20. SSITKA System. All the adsorption measurements were done using the system shown in Figure 1. The (10) Yang, R. T. Gas Separation by Adsorption Processes; Butterworths: Boston, MA, 1987. (11) Sircar, S. Preparation of High Purity Oxygen; U.S. Patent 4,756,723, 1988. (12) Sircar, S.; Zondlo, J. W. Fractionation of Air by Adsorption; U.S. Patent 4,013,429, 1977. (13) Sircar, S.; Conrad, R. R.; Ambs, W. J. Binary Ion Exchanged Type X Zeolite Adsorbent; U.S. Patent 4,557,736, 1985. (14) Collins, J. J. Air Separation by Adsorption; U.S. Patent 3,973,931, 1977. (15) Kratz, C. W.; Sircar, S. Production of Oxygen Enriched Air; U.S. Patent 4,685,939, 1987. (16) Breck, D. W. Zeolite Molecular Sieves; R. E. Krieger Publishing: Malabar, FL, 1984. (17) Choudary, V. N.; Jasra, R. V.; Bhat, T. S. G. Ind. Eng. Chem. Res. 1993, 32, 548. (18) Coe, C. G.; Kirner, J. F.; Pierantozzi, R.; White, T. R. Nitrogen adsorption with a Ca and/or Sr Exchanged Lithium X-Zeolite; U. S. Patent 5,152,813, 1992. (19) Coe, C. G.; Parris, G. E.; Srinivasan, R.; Auvil, S. R. In New Developments in Zeolite Science and TechnologysProceedings of the 7th International Zeolite Conference, Tokyo; Iijima, A., Ward, J. W., Murakami, Y., Eds.; Elsevier: New York, 1986.
© 1997 American Chemical Society
N2 Adsorption in LiX Zeolite
Figure 1. SSITKA system: (1) back-pressure regulator; (2) pressure transducer; (3) pneumatic switch valve; (4) reactor and oven assembly; (5) bleed valve; (6) mass spectrometer; (7) differential pump; (8) gas chromatograph.
adsorbent was placed in a quartz micro-reactor with ID of 4 mm. A thermocouple was installed at 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 (14N2 vs 15N2). 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 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 hold-up 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 15N which was obtained from Isotech. Before entering 2 the adsorption system, the gases were further purified using an Alltech Gas Purifier packed with indicating Drierite and 5A molecular sieve. Adsorption Measurements. Adsorption measurements were made using 50-100 mg of adsorbent loaded into the flow microreactor. Before each adsorption measurement the adsorbent was activated to remove the traces of water associated with the cation. Activation of the adsorbent was carried out in flowing helium (purity 99.999%) using a temperature ramping rate of 5 °C/min to 350 °C and holding for 10 h at this temperature. After activation the adsorbent bed temperature was lowered to the desired adsorption temperature, and the feed was switched to the adsorption mixture. The total flow rate was kept at 34 cm3/min for all the experiments. The feed consisted of 4, 6, 8, or 10 cm3/min N2 with the balance being helium. The total pressure was maintained at 1.1 bar (1 bar ) 105 Pa) for all the experiments. Steady-state isotopic transient data were collected at 303, 318, 328, and 338 K after 15 min of time-on-stream to ensure adsorption equilibrium conditions. At the space velocity and the adsorption conditions utilized, mass transport limitations were minimized. The isotopic transients were taken by switching two feed streams where the only difference was the isotopic composition of N2: (20) Coe, C. G.; Kuznicki, S. M.; Srinivasan, R.; Jenkins, R. J. ACS Symp. Ser. 1988, No. 368, 478.
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Figure 2. Normalized transient responses after an isotopic switch from A to A*.
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 reaction 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 and the switch was performed instantaneously with the feed streams kept at constant pressure and flow rate, 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 a mass spectrometer. Transient Response Analysis. The most general and accurate parameter obtained from SSITKA is the surface concentration at steady-state of adsorbate A, NA. NA can be obtained from the transient response of A measured after a step change in the isotopic labeling of A (from A to A*)
NA )
[
r (t)
∫0∞ rA(t) - rII,ss rA,ss
]
dt
(1)
where rA(t) and rI(t) represent, respectively, the flow rates of A and the inert tracer I at the outlet of the reactor. rA,ss and rI,ss are the corresponding steady-state flow rates before the isotopic switch. For practical purposes, the transient responses of A and I are generally represented in a normalized form as shown in Figure 2, where FA(t) and FI(t) are the normalized responses of the adsorbate and the inert measured at the outlet of the reactor. The average surface residence time of A, τA,ave corresponding to the hatched area in Figure 2, is calculated with the following expression:
τA,ave )
∫0∞[FA(t) - FI(t)] dt
(2)
Furthermore, it can be shown that NA is related to τA,ave by
∫0∞[FA(t) - FI(t)] dt ) rA,ssτA,ave
NA ) rA,ss
(3)
The average surface residence time, τA,ave, is first calculated
6552 Langmuir, Vol. 13, No. 24, 1997
Bajusz and Goodwin Table 1. SSITKA Parameters for N2 Adsorption in LiX (T ) 303 K and PTot ) 1.1 bar) 303 K
318 K
328 K
338 K
PN2 (bar)
τ N 2a (s)
NN2/Lib (10-2)
τN2 (s)
NN2Li (10-2)
τN2 (s)
NN2/Li (10-2)
τ N2 (s)
NN2/Li (10-2)
0.13 0.19 0.26 0.32
3.3 3.5 3.8 3.3
2.3 3.7 5.3 5.8
2.2 2.2 2.3 2.3
1.5 2.3 3.2 4.0
1.7 1.8 2.0 2.0
1.2 1.9 2.8 3.5
1.4 1.7 1.5 1.3
1.0 1.8 2.1 2.3
a Error ) (0.2 s. b N up-take expressed in adsorbed N molecules 2 2 per total Li+ cations, error ) (0.12 × 10-2.
Figure 4. Effect of bed-loading on τN2 (T ) 303 K, PN2 ) 0.13 bar, PTot ) 1.1 bar).
Figure 3. (a) Normalized transients responses after a switch between 14N2/Ar and 15N2 in R-Al2O3 (T ) 303 K). (b) Normalized transient responses after a switch between 14N2/Ar and 15N2 in LiX (T ) 303 K).
from the normalized transients, and then NA is calculated using eq 3. More details about the analysis can be found in ref 9. It is important at this point to mention some limitations relative to the interpretation of the parameters obtained with SSITKA. The surface coverage obtained from the transient responses is very accurate provided the interactions between inert tracer and the adsorbent are negligible. The calculation of NA requires no other assumption since it is based solely on a mass balance. On the other hand, the interpretation of τA,ave requires much more attention. It can be easily demonstrated that diffusion and readsorption phenomena can have major impacts on τA,ave. Another limitation related to the interpretation of τA,ave comes from the fact that τA,ave is averaged over all molecules of A including those which do not adsorb at all on the adsorbent during flow through the sorbent bed. Results Figure 3 show typical transients which were obtained after switching between 14N2/Ar and 15N2. The transients from Figure 3a were generated using a loading of R-Al2O3 in the microreactor equivalent in volume to the LiX zeolite
studied. These transients constitute the “blank” measurements. In Figure 3b the blank transient for Ar is compared with N2 and Ar transients measured in the presence of 50 mg of LiX zeolite loaded in the microreactor. The impact of N2 partial pressure on the SSITKA parameters for different temperatures is displayed in Table 1. NN2/Li represents the number of N2 molecules adsorbed at equilibrium on the surface of the LiX zeolite per total Li+ content in LiX, determined using NN2 calculated from the N2 transients using eq 1. τN2 is the average surface residence time for all molecules exiting the reactor and was calculated using eq 2. The effects of bed loading on τN2 are shown in Figure 4. A linear increase in the average surface residence time of N2 was observed with increasing zeolite loading in the microreactor. τN2 extrapolates to zero for zero bed loading because it is averaged over all N2 molecules flowing, not because it indicates a zero residence time for a molecule of N2 adsorbing. It can be observed from Table 1, that the variation in partial pressure of N2 had no significant impact on the surface average time of N2. On the other hand, at constant pressure, τN2 decreased systematically with increasing temperature. Some of the variability in τN2 is possibly due to changes in the fraction of the N2 molecules in the gas stream which adsorb as well as due to changes in surface residence time of those molecules which do adsorbsthe true surface residence time of an adsorbed molecule will decrease with temperature. The adsorption isotherms measured at different temperatures are displayed in Figure 5 (the lines are the best fitting Langmuir curves). For all the temperatures, the amount of N2 adsorbed increased systematically with PN2. As expected, the maximum coverage (0.058 N2 molecules/ LiTotal) was observed at the lowest temperature (303 K)
N2 Adsorption in LiX Zeolite
Langmuir, Vol. 13, No. 24, 1997 6553
Figure 5. Adsorption isotherms of N2 in LiX obtained from SSITKA. Table 2. N2 Up-Take for Different LiX Loadings (T ) 303 K, PN2 ) 0.13 bar, PTot ) 1.1 bar) bed loading (mg)
NN2/Lia (10-2)
49.0 74.8 103.0
2.3 2.2 2.3
a N up-take expressed in adsorbed N molecules per total Li+ 2 2 cations, error ) (0.12 × 10-2.
and highest N2 partial pressure (0.32 bar). In addition, the calculated coverage of N2 per total Li+ content was found to be independent of the bed-loading (Table 2). The latter result is a good way to double check the reproducibility of the up-take measurements. Discussion From Figure 3a, it can be observed that the argon and 14 N2 transients are essentially superimposed. This is due to the fact that the up-take of these gases on R-Al2O3 is insignificant under these conditions. Although Ar can also be adsorbed in LiX, a comparison of the transients for Ar (blank) and Ar (LiX) in Figure 3b confirms that the up-take of Ar in LiX can be considered to be negligible under these conditions and at its low partial pressure. Thus, it serves as a means to delineate gas-phase holdup. It is important at this point to underline the limitation of the technique already mentioned earlier. The most accurate SSITKA parameter is the N2 up-take, NN2, since it is simply based on a mass balance around the adsorbent bed. However, τN2 is difficult to interpret meaningfully, since it is averaged over all the N2 molecules flowing through the reactor, including those which readsorb and those which do not adsorb at all. When the partial pressure of N2 was increased at constant temperature, no significant variation in τN2 was observed (Table 1). This suggests that under these conditions, the competition between N2 molecules for adsorption sites is negligible. This seems to be due to the fact that the adsorption sites are isolated from each other and are far from being saturated under these conditions. The equilibrium isotherms measured by SSITKA and given in Figure 5 are comparable with the ones reported
in the literature.21-23 From Table 1, it is clear that only a very small fraction of the Li+ cations is adsorbing N2 molecules at any time. The highest measured coverage corresponds to only 5.8% of the Li+ sorbing N2. This relative low occupancy of Li+ sites is due in part to the fact that not all the Li+ present in the crystal structure of the exchanged X zeolite are accessible to N2. N2 can only sorb on cations in sites II and III since they are located within the supercage. The positions of cations in an 80% exchanged LiNaX zeolite24 have been determined by 7Li-NMR. All the Na cations were located in site III in the supercage. It was found that 32 Li+ ions/unit cell were located in site I′, 32 in site II, and 4 in site III. Sites I and II′ were not occupied by any cation. It was also shown that in the site II position, Li+ because of its smaller size (0.68 Å) compared to Na+ (0.95 Å) was located almost in the plane of the six-member ring. This position is, thus, less advantageous for direct interaction with N2. Thus, the best Li+ position for N2 adsorption in LiX zeolite is site III which starts to be populated with Li+ at high levels of exchange (g80%). The measurement of adsorption isotherms at different temperatures with SSITKA permits the calculation of the heat of adsorption as a function of surface coverage. It is important to note that there are several definitions for heat of adsorption, depending on the measurement method. When experimental data are reported as a set of adsorption isotherms for a particular gas-adsorbent system, the differential isosteric heat of adsorption, ∆Hst, is generally calculated. ∆Hst corresponds to the heat released when 1 mol of adsorbate is adsorbed by an infinite amount of solid without a change in the fraction of the surface covered by the adsorbate. The isosteric heat of adsorption can be obtained from a series of isotherms using the Clausius-Clapeyron relation.25 It is assumed that the volume change accompanying the phase change from gas to adsorbed phase is approximately equal to the volume of the gas phase. Assuming ideal gas law behavior, the Clausius-Clapeyron relation can be written as
(
)
δ ln p δ(1/T)
v
)
∆Hst R
(4)
∆Hst can be easily obtained by evaluating the slope of a plot of ln p vs 1/T at any value of v or respectively at any coverages. The plots of ln p versus 1/T at three different coverages are shown in Figure 6. Only three were plotted in order to be easier to see. The isosteric heats of adsorption at different coverages were calculated and are displayed in Figure 7. The obtained values are in fair agreement with the ones obtained using a microbalance for the same adsorbate-substrate system (4.3 kcal/mol).23 It can be observed from Figure 7 that the heat of adsorption did not vary significantly with the coverage. The approximately constant heat of adsorption with increasing coverage is probably synonymous of a balance between the degree of heterogeneity of gas-solid interactions and the strength of cooperative gas-gas interactions in the studied loading range as denoted by Dunne et al.26 (21) Yang, R. T.; Chen, Y. D.; Peck, J. D.; Chen, N. Ind. Eng. Chem. Res. 1996, 35, 3093. (22) Gaffney, T. R. Curr. Opin. Solid State Mater. Sci. 1996, 1, 69. (23) Baksh, M. S. A.; Kikkinides, E. S.; Yang, R. T. Sep. Sci. Technol. 1992, 27, 277. (24) Herden, H.; Einicke, W.-D.; Scho¨llner, R. J. Inorg. Chem. 1987, 43, 2538. (25) Smith, J. M.; Van Ness, H. C. In Introduction to Chemical Engineering Thermodynamics; McGraw-Hill, Inc.: New York, 1987. (26) Dunne, J. A.; Mariwala, R.; Rao, M.; Sircar, S.; Gorte, R. J.; Myers, A. L. Langmuir 1996, 12, 5888.
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Bajusz and Goodwin
ising for the study of more complicated systems, especially multicomponent ones. Currently, our laboratory is developing the methodology permitting the measurement of the adsorption behavior of mixtures of N2 and O2 and the calculation of single component isosteric heats of adsorption in this multicomponent system. Conclusions
Figure 6. Plots of ln pN2 vs 1/T at three different coverages.
Figure 7. Isosteric heat of adsorption of N2 in LiX.
On the basis of the results of this investigation, adsorption measurements using SSITKA are very prom-
In this study, the application of SSITKA for adsorption measurements was successfully demonstrated for N2 adsorption in LiX zeolite. It was shown that around 1-6% of the Li+ in LiX was involved in sorbing N2 at any time for PN2 ) 0.1-0.3 bar and T ) 30-65 °C. These extremely low coverages were caused in part by the fact that most of the Li+ cations in the LiX zeolite were not easily accessible to N2 and in part by the low equilibrium coverage at these relatively high physisorption temperatures. The average surface residence time of N2 on LiX was found to be independent of N2 partial pressure for a given temperature. This was suggested to be due to minimal competition between the molecules in the gas phase for adsorption sites due to the low site coverage. The isosteric heats of adsorption of N2 in LiX at different coverages were calculated using the Clausius-Clapeyron relationship. The values obtained (around 5 kcal/mol) were similar to those reported in the literature for the same adsorbate-adsorbent system (4.3 kcal/mol) determined by Baksh et al.23 No variation of the isosteric heat of adsorption with N2 coverage could be observed. This was suggested to be caused by a balance between the degree of heterogeneity of adsorption sites and attractive interactions between the gas molecules. Since use of multiple isotopic labels are no more difficult than use of a single label, these results suggest that SSITKA offers an excellent means to study the adsorption behavior of multicomponent mixtures. It is especially interesting as a means to determining isosteric heats of adsorption of the various adsorbing species. Acknowledgment. The support of this study by the National Science Foundation (NSF Grant No. CTS9312519) is gratefully acknowledged. LA970887L
