Zeolites Containing Mixed Cations for Air Separation by Weak

Aug 15, 1996 - NaX zeolite was ion-exchanged to obtain LiX and AgX zeolites. ... desorption of N2 on AgX indicated some degree of weak π-complexation...
0 downloads 0 Views 183KB Size
Download PDF
Ind. Eng. Chem. Res. 1996, 35, 3093-3099

3093

Zeolites Containing Mixed Cations for Air Separation by Weak Chemisorption-Assisted Adsorption R. T. Yang,*,†,‡ Y. D. Chen,†,§ J. D. Peck,‡ and N. Chen† Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, and Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260

NaX zeolite was ion-exchanged to obtain LiX and AgX zeolites. The LiX form was further exchanged to replace 20% of the Li+ cations by Ag+, to obtain a LiAgX zeolite. Equilibrium adsorption isotherms of pure-component N2 and O2 were measured at 25 and 50 °C on these four zeolites. AgX was stable since the N2 isotherm was not affected after prolonged exposure of the zeolite to air at 350 °C. Bonding of N2 was substantially stronger on AgX than on the other zeolites. The high isosteric heat of adsorption (8.4 kcal/mol) and the relatively slow desorption of N2 on AgX indicated some degree of weak π-complexation, which was substantiated by molecular orbital calculation results using model systems. Binary N2/O2 selectivity (or separation factor, R) was calculated by using the ideal adsorbed solution theory. The high N2/ O2 selectivities at low total pressures for AgX will result in difficult N2 desorption; therefore, AgX is not suitable for air separation. LiX is presently employed in industry as the sorbent for air separation by pressure-swing adsorption. Comparing LiX with LiAgX, the N2/O2 selectivities were higher for LiAgX at high total pressures and lower for LiAgX at lower pressures, due to a (relative) selectivity reversal. This result, combined with the higher N2 capacity for LiAgX, led to the conclusion that LiAgX can be superior to LiX for air separation. Introduction Nitrogen and oxygen are the man-made chemicals that are produced in, respectively, the second and third largest quantities today. The traditional technology for producing them is cryogenic distillation of air. Since the development of synthetic zeolites and pressureswing adsorption (PSA) cycles, adsorption has been playing an increasingly important role in industrial air separation (Yang, 1987). The sorbents used for air separation were mainly CaA and NaX zeolites. The N2 molecule, because of its larger quadrupole moment, adsorbs more strongly than O2, and the cation in the zeolite plays a crucial role in the N2/O2 selectivity. It has been found recently in this laboratory and elsewhere that Li+ ion-exchanged X (and A) zeolites have higher N2/O2 selectivities as well as higher N2 capacities (Baksh et al., 1992; Chao, 1989; Coe et al., 1992; Chao et al., 1992). This is the result of the smaller ionic radius of Li+, i.e., higher charge density, and hence higher polarizing power. Consequently, PSA using LiX zeolites is a highly efficient technology for air separation and is being used in industry. Separations with new sorbents by exploiting the principle of π-complexation have been proposed recently (Yang and Kikkinides, 1995; Chen and Yang, 1995). These sorbents were prepared by dispersing or ionexchanging cations of the d-block transition metals on the surfaces. A unique characteristic of these cations is the ability of their d orbitals to form a type of bonding (with olefin molecules, for example) broadly referred to as π-complexation. The best known example for π-complexation is Ag+C2H4, where the bonding is contributed by two parts: * To whom correspondence should be addressed at the University of Michigan. Telephone: (313) 936-0771. FAX: (313) 763-0459. E-mail: [email protected]. † University of Michigan. ‡ State University of New York at Buffalo. § Current address: The BOC Group, Inc., Murray Hill, NJ 07974.

S0888-5885(95)00783-4 CCC: $12.00

the donation of the olefin π-bond electrons to the empty s-orbital of Ag+ and the backdonation of the d-orbital electrons of Ag+ (the 4d orbitals of Ag+ are filled with 10 electrons) to the empty antibonding π orbitals of the olefin molecule. The Ag+-olefin bonds are generally weak, with bond energies near 10 kcal/mol. As discussed above, all known sorbents for air separation are based on van der Waals forces, with bond energies (to N2 and O2) typically 3-6 kcal/mol. The basic idea of this work was to prepare new sorbents with enhanced N2 adsorption capacity and N2/O2 selectivity by exploiting the nature of weak π-complexation bonds. Ag+ was the obvious first choice. Indeed, literature data exist that show strong adsorption of N2 by Ag+ ionexchanged zeolites (Habgood, 1964; Huang, 1974). The bond energies of N2 on these zeolites are considerably higher than those with other cations, and the isotherms are more rectangular in shape. Due to the high amounts of N2 adsorbed at low pressures, Ag+exchanged zeolites are not desirable sorbents for PSA (i.e., pressure-swing adsorption) air separation. Consequently, the strategy of this work was to prepare mixed Ag+/Li+ type X zeolite that contains only a small fraction of Ag+ to take advantage of the strong bonding with N2 while retaining the same degree of linearity in the isotherm pertaining to LiX. Cation Exchange The chemistry of ion exchange in zeolites is well documented (Breck, 1973). For both type A and X zeolites, the starting forms contain Na+. The Ag+-Na+ ion exchange isotherms showed the possibility of 100% exchange by Ag+ in both type X (Sherry, 1966) and type A (Breck, 1974) zeolites. Complete exchange for Li+, however, is difficult. The selectivity for ion exchange in type X zeolite follows the sequence Ag >> Na > Li. Strong exchange conditions are needed for Na+ f Li+ exchange. For the mixed Ag+-Li+ form, a sequential exchange procedure was adopted in which Ag+ exchange followed Li+ exchange. The detailed exchange conditions are given below. © 1996 American Chemical Society

3094 Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996

All exchanges were performed batchwise. The starting material was 13X powder (binderless), from Linde Division, Part No. MS-1328, Lot No. 94508406002. All exchange solutions were aqueous. LiX. A total of 2.0 g of 13X was used in five exchanges, each with 33 mL of a 2.2 M LiCl solution at 100 °C, in a 125 mL Erlenmeyer flask, on a magnetic stirrer for 12 h. The sample was vacuum filtered and washed with distilled/deionized water. Calcination of the sample was conducted sequentially in a muffle furnace for 4 h at 100 °C, for 4 h at 200 °C, and for 12 h at 400 °C. Compared to the original cation-exchange capacity (CEC), each solution contained 7-fold Li+ equivalents. LiAgX (by Sequential Exchange). A total of 0.1 g of previously exchanged LiX was used in one exchange by 10 mLof a 0.01 M AgC2H3O2 solution at room temperature (approximately 20 °C), in a 125 mL Erlenmeyer flask, on a shaker bath for 18 h. The sample was vacuum filtered and washed with distilled/deionized water. Calcination was conducted sequentially in a muffle furnace for 4 h at 100 °C, for 4 h at 200 °C, and for 12 h at 350 °C. Compared to the original CEC, the solution contained 0.2-fold Ag+ equivalents. This procedure ensured that 20% of the cations in the final product was Ag+. AgX. A total of 0.2 g of 13X was used in two exchanges each with 80 mL of 0.05 M AgC2H3O2 at room temperature in a 250 mL Erlenmeyer flask, in a shaker bath for 24 h. The sample was vacuum filtered and washed with distilled/deionized water. Runs 63-66 represent half of the sample which was calcined in He up to 400 °C. Run 67 denotes the other half, which was calcined in a muffle furnace for 4 h at 100 °C, for 4 h at 200 °C, and for 12 h at 350 °C. Compared to the original CEC, each solution contained 4-fold Ag+ equivalents. This procedure ensured 100% Ag+ exchange. Isotherm Measurements Equilibrium isotherms were measured gravimetrically using a Cahn 2000 System 113 microbalance. The gases used were N2 (high-purity grade, 99.99% purity, 3 ppm maximum H2O), O2 (extra-dry grade, 99.6%, 10 ppm maximum H2O), and He (as the carrier gas, highpurity grade, 99.995%, 5 ppm maximum H2O). All gases were supplied by Linde Division. Each gas was first passed through a bed (one for each gas) containing 13X zeolite in order to remove water vapor and trace impurities. The beds were fitted with heating tapes and periodically regenerated. Control of gas flow rates was facilitated by Linde flowmeters, calibrated by a 25 mL bubble flowmeter. Samples were contained in a quartz cup suspended from a platinum wire, attached to one arm of the electrobalance. A counterweight, of the same mass as the wire and cup assembly, was attached to the other arm. The electrobalance and counterweight were isolated by a Pyrex glass casing, while the sample cup was suspended in a quartz hang-down tube. The hang-down tube was enveloped by a resistance coil furnace. The temperature of the sample was ascertained by an axially located type-K thermocouple, the tip of which was located just beneath the sample cup. The bottom of the quartz hang-down tube was filled to the level of the sample cup with 3A zeolite, to remove trace amounts of residual H2O. The exhaust from the TGA (thermogravimetric analyzer) microbalance was connected to a fume hood.

Figure 1. Equilibrium adsorption isotherms at 298 K on cationexchanged type X zeolites for nitrogen (filled symbols) and oxygen (open symbols). The lines are fitted by eqs 4 and 5, with parameters given in Table 2.

Experiments were performed at atmospheric pressure, which was recorded prior to each run by a spring barometer. Introduction of a step change in the feed concentration was conducted in the following manner. First, flow was switched from the TGA to the hood by means of a three-way valve. Next, the appropriate adjustments were made to the needle valves on the corresponding flowmeters. Finally, after allowing sufficient mixing time of the helium carrier and adsorptive gas (either nitrogen or oxygen), the direction of the three-way valve was switched back to the TGA. Experiments were performed with aluminum “blanks” in order to account for buoyancy and viscous drag effects originating from changes in the feed gas composition. These blanks represented the baseline correction for each step change in adsorptive concentration. Output from the TGA microbalance was recorded simultaneously by a strip chart and a digital voltmeter. Readings were obtained at a voltage resolution which, in terms of the amount adsorbed, converted to below (0.01 mmol/g. Prior to each experimental run, the samples were dehydrated in situ with He up to 400 °C. The isotherms measured were reversible. The total volumetric flow rate of the helium/adsorptive mixture was kept constant at approximately 300 sccm. Results and Discussion Equilibrium Isotherms. Results of the equilibrium isotherms at 298 K for N2 and O2 are summarized on Figure 1 for the four zeolites: NaX (13X), LiX, AgX, and LiAgX. Isotherms on the first three zeolites (NaX, LiX, and AgX) are available in the literature and can be compared. Since Na+ is more selective than Li+ for ion exchange on X zeolite, complete exchange for Li+ is difficult. Comparing the N2 isotherm in Figure 1 with those found in the literature (Chao, 1989; Coe et al., 1992), the degree of exchange of our sample was approximately 85% Li+, with the balance being Na+. A comparison of the isotherms on AgX with previous studies (Habgood, 1964; Huang 1974) is shown in Figure 2. All three nitrogen isotherms exhibit the same shape,

Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 3095

Figure 2. Comparison of equilibrium isotherms of nitrogen on AgX with data from the literature (Habgood, 1964; Huang, 1974). All isotherms were measured at 298 K. The lines are fitted by eqs 4 and 5.

Figure 4. Equilibrium isotherms for nitrogen (circles) and oxygen (squares) on LiAgX zeolite. Filled and open symbols represent data taken at 25 and 50 °C, respectively. The lines are fitted by eqs 4 and 5.

Figure 3. Equilibrium isotherms for nitrogen (circles) and oxygen (squares) on AgX zeolite. Filled and open symbols represent data taken at 25 and 50 °C respectively. The lines are fitted by eqs 4 and 5.

Figure 5. Effect of calcination gas and history on N2 adsorption on AgX at 298 K: (O) AgX calcined in He at 400 °C (Run 63); (0) AgX calcined in air at 350 °C (Run 67); (]) repeat of Run 67, immediately; (4) repeat of Run 67, 24 h later.

with the data collected by Habgood showing slightly higher capacity and the data of Huang slightly lower. The original data taken from Habgood (1964) was reported on a “mmol/g NaX” basis. The density of 13X (Na+) is 0.64 times that of AgX. Thus, Habgood’s result was obtained by dividing the original data, for a “mmol/g AgX” basis, by a correction factor of 0.64. Heats of Adsorption and Stability of Ag+. Isosteric heats of adsorption of N2 on AgX and LiAgX zeolites were obtained from the temperature dependence of the isotherms. The isotherms at two temperatures (25 and 50 °C) for these two zeolites are shown in Figures 3 and 4 . The isosteric heats of adsorption were, for N2 on AgX (in kcal/mol), 8.43 (at 0.1 mmol/g), 7.18 (at 0.2 mmol/g), 6.48 (at 0.3 mmol/g), and 6.07 (at 0.4 mmol/g). The isosteric heats of adsorption of N2 on LiAgX were (in kcal/mol) 6.89 (at 0.1 mmol/g), 6.48 (at 0.2 mmol/g), 6.24 (at 0.3 mml/g), and 6.09 (at 0.4 mmol/ g). Habgood (1964) reported overall (nonisosteric) heats of adsorption from chromatographic measurements. The values of 7.4 and 7.8 kcal/mol were reported for N2 on

two AgX samples. Our values of isosteric heats of adsorption are in agreement with that of Habgood’s. The stability of the Ag+ ions in the AgX zeolite was studied by treating the sample under strong oxidizing conditions as an accelerated test. As described in the foregoing, after ion exchange the AgX sample in Run 63 was calcined in He at stepped temperatures with a final step of 400 °C for 12 h, whereas the sample in Run 67 was calcined in air at a final temperature of 350 °C for 12 h. The equilibrium isotherms of N2 on these samples are shown in Figure 5. Repeated isotherm measurements on the air-calcined sample are also included in the figure. The isotherms shown in Figure 5 were within minimal scatter, which was caused by experimental factors such as fluctuations in flow rates and ambient pressure. The same buoyancy and viscous drag corrections were used for all runs shown in Figure 5. Figure 5 shows that that exposure of the AgX zeolite to air at 350 °C had no effect on the N2 isotherm. This result ensures the stability of the AgX zeolite as a sorbent for air separation. Nature of the Bonds. Adsorption of N2 and O2 on type A and X zeolites containing different alkali and

3096 Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996

alkaline-earth cations has been studied extensively (McKee, 1964; Habgood, 1964; Chao, 1989; Baksh et al., 1992; Coe et al., 1992; Chao et al., 1992). The effects of different cations are qualitatively understood. The main reason for the stronger adsorption of N2 than O2 on these zeolites is the stronger ion-quadrupole interaction (Razmus and Hall, 1991), due to the larger quadrupole moment of N2 than O2. For CaA zeolite, Monte Carlo simulation results (Razmus and Hall, 1991) indicated that the potential energy for the ion-quadrupole interaction for Ca2+-N2 was approximately 4 times that of Ca2+-O2. For zeolites with different alkali and alkalineearth ions, the strengths of adsorption for both N2 and O2 are well correlated with the charge densities (charge/ ionic radius) of the cations. N2 adsorbs most strongly on Li+ zeolites because Li+ has the smallest ionic radius, hence, the distance is the shortest between the nucleus of Li and the center of mass of the N2 molecule. A similar situation exists for other adsorbate molecules such as CH4 (Talu et al., 1993). The heats of adsorption on these zeolites are in the range 4-6 kcal/mol for N2 and 3-4 kcal/mol for O2. The heats of adsorption of N2 on AgX are considerably higher than those on the alkali zeolites. The ionic radius of Ag+ is 1.26 Å, considerably larger than that of Li+ (0.68 Å). Thus, the strong bonds between N2 and Ag+ cannot be attributed to the ion-quadrupole interaction. It was also observed in our experiments that the desorption of N2 from AgX zeolite was slower than that from the alkali X and alkaline-earth zeolites. At 25 °C, desorption of N2 from AgX upon switching from N2 to He required a minimum of 1.5 min. The relatively slow desorption and the high heats of adsorption (of up to 8.4 kcal/mol) indicated the possibility of involvement of weak π-complexation bonds. The adsorption of olefin molecules (with π electrons) on Cu+ and Ag+ cations has been studied recently (Yang and Kikkinides, 1995; Chen and Yang, 1995). The basic concept for π-complexation was described first by Dewar (1951). The outer shell orbitals for Cu(I) and Ag(I) consist of an empty s orbital and filled d orbitals (by 10 electrons). The π-complexation is formed by a σ-bond from the overlap of the empty s orbital of the metal with a filled 2pπ orbital of the olefin (donation of π electrons from olefin to the metal) and by the overlap of filled d orbitals of the metal with a vacant 2pπ* antibonding orbital of the olefin (that is, backdonation of electrons to the empty olefin antibonding orbital). Our molecular orbital calculations showed that, for the C2H4-Ag+ system, the π-σ donation was the major contribution to the π-complexation, whereas the backdonation of the d-orbital electrons from the metal made only a minor contribution. For the example of C2H4 on AgSO3C6H5, the σ-donation accounted for 84% of the bond, whereas the d-π* backdonation accounted for only 16%. We have also studied the weak π-complexation of acetylene on Ni2+ (Yang et al., 1996; Yang and Foldes, 1996). The bonding between C2H2 and Ni2+ is similar to that in olefin-Ag+. The heats of adsorption of C2H6 and C3H8 on Ag+ ion-exchanged resin were nearly 5 kcal/mol, and those for C2H4 and C3H6 on Ag+-resin were approximately 10 kcal/mol. The heat of adsorption of C2H2 on Ni2+ was 9.3 kcal/mol. The outer orbital structure for the N2 molecule is:

[(σ2s)2(σ*2s)2(π2py)2(π2pz)2(σ2px)2(π*2py)0(π2pz)0] The antibonding π orbitals are empty (and are included above for illustrative purposes), whereas the 2py

Table 1. Total Atomic Charges and Energies of Adsorption (∆E) total atomic charges adsorbate O2 N2 C2H4 AgCl O2-AgCl N2-AgCl C2H4-AgCl

Ag

Cl

∆E, kcal/mol

0.6571 0.5773 0.5556 0.5420

-0.6571 -0.6576 -0.6541 -0.6637

8.85 10.60 16.00

0.0000 0.0000 0.0000 0.040 (for O) 0.0492 (for N) 0.0608 (for CH2)

and 2pz π orbitals are filled, each with two electrons. When interacting with the Ag+ ion, it is entirely feasible to form a weak π-complexation bond similar to that between olefin and Ag+. Here a σ bond can be formed by the donation of the π electrons from N2 to the empty 5s orbital of Ag+, while the 4d electrons (filled with 10 electrons) of Ag+ would backdonate to the empty antibonding π orbitals of N2. (The O2 molecule, on the other hand, has partially filled antibonding π orbitals, thus prohibiting the π-complexation formation.) The heat of adsorption of 8.4 kcal/mol for N2 on AgX is in line with the possibility of the formation of a weak π-complexation bond. Molecular Orbital Calculation. In order to understand the interactions between N2 and Ag+, molecular orbital calculations based on quantum mechanics were performed. We have reported molecular orbital results for the interactions between olefins and Ag+ (and Cu+) using a semiempirical method (Chen and Yang, 1995). In this work, the more rigorous ab initio method Gaussian 94 program (Frisch et al., 1995) was used. The systems studied in this work included the interactions between one of the group N2, O2, and C2H4 (as the sorbate) and one of the group AgF, AgCl, AgI, and a Ag-zeolite model (as the sorbent). The results on AgCl as the sorbent are illustrative and will be summarized here. The detailed results of the calculations will be published elsewhere. The structures of the adsorbates, adsorbents, and the adsorbed systems (i.e., adsorbate + adsorbent) were optimized geometrically. The interaction energy, ∆E, was calculated from E1 (i.e., energy of adsorbate), E2 (energy of AgCl), and E3 (energy of adsorbate + AgCl) by the following formula:

∆E ) E1 + E2 - E3

(1)

The interaction energies can be considered as the theoretical heats of adsorption. The orbital energies of the orbitals, the occupancies of the orbitals, and the atomic charges (i.e., charges of all atoms) were calculated by employing the Natural Bond Orbital (NBO) method (NBO Version 3.1 in Frisch et al., 1995). When the adsorbate molecule is brought close to AgCl, the atomic charges will change. The amount of change indicates the extent of interaction. The total atomic charges are summarized in Table 1. From the results, it was seen that the charges of the adsorbate molecules always increased, indicating a net transfer of electrons from the adsorbate molecules to Ag. The changes of total atomic charges followed the order:

C2H4-AgCl > N2-AgCl > O2-AgCl

(2)

which indicated that the interactions were strongest for C2H4 and weakest for O2. Moreover, the interactions were considerably stronger for N2 than O2. The energy changes upon adsorption, ∆E, are also included in Table

Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 3097

1. The experimental heat of adsorption for C2H4 on Ag+resin was 10 kcal/mol (Yang and Kikkinides, 1995), in reasonable agreement with the theoretical value. This was the only experimental data available for systems considered in this study. It is meaningful to compare the differences of energy levels between the HOMO (highest occupied molecular orbital) of the adsorbate and the LUMO (lowest unoccupied molecular orbital) of the adsorbent. A smaller gap between these two energy levels would indicate easier formation of the π-complexation bond (Chen and Yang, 1995). The orbital energy gaps are shown in Figure 6. It is seen that the largest gap was for O2 and Ag, indicating the difficulty in forming a bond, and it was considerably easier for bond formation between N2 and Ag. N2/O2 Selectivity for Separation. For air separation, two important factors in selecting sorbents are capacity and selectivity (Yang, 1987). Capacity refers to the equilibrium amount adsorbed of the stronger component (i.e., more preferred component), whereas selectivity (or separation factor) determines the separation. The selectivity or separation factor for N2/O2 is defined by:

XN2/XO2 RN2/O2 )

YN2/YO2

Figure 6. Orbital energy gaps between the lowest unoccupied molecular orbital (LUMO) of the adsorbent and the highest occupied molecular orbital (HOMO) of the adsorbate.

(3)

where R is the selectivity, X is the mole fraction in the adsorbed phase, and Y is the mole fraction in the gas phase. Binary mixture data are needed for calculating the selectivities. The ideal adsorbed solution (IAS) theory of Myers and Prausnitz (1965) was used for calculating binary adsorption from the pure-component data. The accuracy of prediction by IAS theory varies depending on the gas-solid system, as discussed by Yang (1987; Chapter 3). It is quite accurate for N2/O2 on zeolites (below 5% deviation between theory and experiment for 5A zeolite). The pure-component adsorption data were correlated by the modified Langmuir isotherm (eq 4) or the linear isotherm (eq 5):

q)

qsbPn 1 + bPn

q ) HP

(4) (5)

where q is the amount adsorbed at partial pressure P, qs is the saturated amount, and b and H are temperature-dependent constants. The pure-component data were fitted by nonlinear regression, and the fitted parameters are given in Table 2. The fitted curves are shown in Figure 1. Since vacuum-swing adsorption (VSA) is the standard technology used in industry for air separation (i.e., adsorption at 1 atm and desorption at a partial vacuum), only isotherms below 1 atm are of interest. The binary N2/O2 isotherms on LiAgX at 25 °C for different total pressures up to 1 atm are shown in Figure 7. The N2/O2 selectivity results are shown in Figure 8. It is seen that the N2/O2 selectivity increases toward lower gas-phase concentrations of N2, and for the same gas-phase composition, it is higher at lower total pressures. These qualitative features are similar to other Li-containing X zeolites (Chao et al., 1992). This result

Figure 7. N2/O2 binary mixture isotherms on LiAgX zeolite at 298 K and different total pressures calculated by IAS theory from pure-component isotherms. Table 2. Equilibrium Isotherm (Equations 4 and 5) Parameters (25 °C) nitrogen sorbent AgX LiX LiAgX 13X

qm (mmol/g)

b (1/atm)

n

0.980 4.851 1.146

2.919 0.155 1.572

0.649 0.718 0.879

H

oxygen: H (mmol/g/atm)

0.431

0.187 0.140 0.158 0.132

indicated that the adsorption sites (cation sites) were more heterogeneous for N2 than for O2. The binary N2/O2 selectivity (R) is plotted against the total pressure at a fixed gas-phase mole fraction, as shown in Figure 9. Only data for N2 mole fraction 0.8 are shown; the results were essentially the same for other mole fractions. The binary selectivity increased as the total pressure was decreased.

3098 Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996

Figure 8. N2/O2 selectivity or separation factor on LiAgX zeolite at 298 K at different total pressures.

pressures are desirable properties for LiAgX over LiX. Additionally, the N2 capacity for LiAgX is also higher than LiX. For these combined advantages, LiAgX zeolite is superior to LiX for air separation, under proper vacuum swing conditions. When Na+ was exchanged by Li+ or Ag+ in the X zeolite, interactions with both N2 and O2 were increased. This is clearly seen in Figure 1. The N2/O2 selectivities on NaX were well below 10 and were not included in Figure 9. The increased interactions with O2 by Li+ were due to its smaller ionic radius, and those by Ag+ were due to its high polarizability. The results shown in Figure 9 indicate that for LiX, at very low pressures, the enhancement in interactions with Li+ was stronger for N2 than for O2. For AgX at low pressures, the Ag+ enhanced its interactions with N2 much more than with O2. For AgLiX at low pressures, it appeared that Ag+ enhanced its interactions with N2 and O2 more evenly. The reason for this phenomenon is not understood. The most energetic cation sites are responsible for adsorption at very low pressures. A possible reason for the above phenomenon was that the most energetic Ag+ sites in the LiAgX zeolite could have been more accessible to O2 than N2 due to the smaller kinetic diameter of O2. Ag+ would influence the Li+ sites, and the arrangement of the mixed Ag+ and Li+ sites in X zeolite is not known (Herden et al., 1981). Acknowledgment This work was supported by the NSF under Grant CTS-9212279 and partially supported by the BOC Group and Praxair. Literature Cited

Figure 9. Comparison of N2/O2 selectivities on three ionexchanged zeolites at 298 K and gas-phase N2 mole fraction 0.8.

For the application of a sorbent in air separation, it is desirable to have high N2/O2 selectivities at high total pressures and low selectivities at low total pressures. All three zeolites exhibit high selectivities at high pressures. The choice of sorbent is then dependent on the relative selectivities at low pressures. A lower selectivity at low pressures would enable ease of desorption and thereby a better separation. The extraordinarily high selectivities on AgX at low pressures (Figure 9) make the AgX zeolite undesirable for air separation. The interesting comparison is between LiX and LiAgX zeolites. The N2/O2 selectivities were higher for LiAgX at pressures above approximately 0.07 atm. At lower pressures, a reversal in selectivity was observed. The selectivity reversal occurred in the range of total pressure 0.06-0.07 atm for all gas-phase compositions. At pressures below the reversal point, the selectivity on LiX continued to rise sharply toward lower pressures, whereas that on LiAgX increased only slowly. Comparing LiAgX with LiX, the higher N2/O2 selectivities at high pressures and lower selectivities at low

Baksh, M. S. A.; Kikkinides, E. S.; Yang, R. T. Lithium Type X Zeolite as a Superior Sorbent for Air Separation. Sep. Sci. Technol. 1992, 27 (3), 27. Breck, D. W. Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley: New York, 1974. Chao, C. C. Process for Separating Nitrogen from Mixtures Thereof with Less Polar Substances. U.S. Patent 4,859,217, 1989. Chao, C. C.; Sherman, J. D.; Mullhaupt, J. T.; Bolinger, C. M. Mixed Ion-Exchanged Zeolites and Processes for the Use Thereof in Gas Separations. U.S. Patent 5,179,979, 1992. Chen, J. P.; Yang, R. T. A Molecular Orbital Study of Selective Adsorption of Simple Hydrocarbons on Ag+ and Cu+ Exchanged Resins and Halides. Langmuir 1995, 11, 3450. 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. Dewar, M. J. S. A Review of the π-Complex Theory. Bull. Soc. Chim. 1951, 18, C71. Frisch, M. J.; et al. (and 34 names). Gaussian 94, Revision A.1; Gaussian, Inc.: Pittsburgh, PA, 1995. Habgood, H. W. Adsorptive and Gas Chromatographic Properties of Various Cationic Forms of Zeolite X. Can. J. Chem. 1964, 42, 2304. Herden, H.; Einicke,W.-D.; Scho¨llner, R. Investigations of the Arrangement and Mobility of Li Ions in X- and Y-Zeolites and the Influence of Mono- and Divalent Cations on It. J. Inorg. Nucl. Chem. 1981, 43, 2533. Huang, Y. Y. Adsorption in AgX and AgY Zeolites by Carbon Monoxide and other Simple Molecules. J. Catal. 1974, 32, 481. McKee, D. W. Separation of an Oxygen-Nitrogen Mixture. U.S. Patents 3,140,932 and 3,140,933, 1964. Myers, A. L.; Prausnitz, J. M. Thermodynamics of Mixed-Gas Adsorption. AIChE J. 1965, 11, 121. Rasmus, D. M., Hall, C. K. Prediction of Gas Adsorption in 5A Zeolite Using Monte Carlo Simulation. AIChE J. 1991, 37, 769.

Ind. Eng. Chem. Res., Vol. 35, No. 9, 1996 3099 Sherry, H. S. The Ion-Exchange Properties of Zeolites. I. Univalent Ion Exchange in Synthetic Faujasite. J. Phys. Chem. 1966, 70, 1158. Talu, O.; Zhang, S.-Y.; Hayhurst, D. T. Effects of Cations in Methane Adsorption by NaY, MgY, CaY, SrY and BaY Zeolites. J. Phys. Chem. 1993, 97, 12894. Yang, R. T. Gas Separation by Adsorption Processes; Butterworth: Boston, 1987. Yang, R. T., Foldes, R. New Sorbents Based on Principles of π-Complexation: Monolayer Dispersed Ni(II) for Acetylene Separation by π-Complexation. Ind. Eng. Chem. Res. 1996, 35, 1006. Yang, R. T.; Kikkinides, E. S. New Sorbents for Olefin/Paraffin Separations by Adsorption via π-Complexation. AIChE J. 1995, 41, 509.

Yang, R. T.; Kikkinides, E. S.; Foldes, R. New Sorbents for OlefinParaffin and Acetylene Separations by π-Complexation. In Fundamentals of Adsorption; LeVan, M. D., Ed.; Kluwer Academic Publ.: Norwell, MA, 1996.

Received for review December 28, 1995 Revised manuscript received April 2, 1996 Accepted April 3, 1996X IE950783A

X Abstract published in Advance ACS Abstracts, August 15, 1996.