12894
J. Phys. Chem. 1993,97, 1289412898
Effect of Cations on Methane Adsorption by Nay, MgY, Cay, SrY, and BaY Zeolites Orhan Talu,’ Si-Yang Zhang, and David T. Hayhurstt Department of Chemical Engineering, Cleveland State University, Cleveland, Ohio 441 15 Received: July 12, 1993; In Final Form: September 28, 1993’
Methane isotherms on N a y , MgY, C a y , SrY, and BaY zeolites are measured between 25 and 70 OC and upto 6760 kPa. All isotherms are of type I. The initial heat of adsorption for divalent cationic forms decreased with decreasing charge density ( C a y > MgY > S r Y > B a y ) except for MgY. Similar anamoly was observed for methane adsorption on MgX (Zhang, S.Y.; Talu, 0.;Hayhurst, D. T. J . Phys. Chem. 1991, 95, 1722) and it is attributed to incomplete dehydration (activation) at normal activation temperatures. The cation type significantly affected adsorption properties even at loadings as high as 7.5 molecules/cavity. CaY has the largest capacity per weight, but the methane pore density decreased in order of decreasing cationic size ( B a y > SrY > CaY > MgY > N a y ) at 25 OC and 5200 kPa fugacity. This unexpected result is attributed to possible differences in molecular packing around the cations at high loadings. The data were satisfactorily correlated by the virial isotherm model.
Introduction Although the adsorption of gases above their critical temperature have been extensively studied in the past, only limited work exists at high pressures. As more technologies are developed for bulk gas separation by adsorption,’ such as pressure swing adsorption, measurement and correlation of high-pressure equilibria is becoming more important since it is directly related to the efficiency of these processes. In addition to separation processes, recent interest in adsorptive storage of gases, especially of natural gas,Z has sparked many studies investigating supercritical methane adsorption by different approaches such as theoretical molecular simulations,3-sand experimental studies on activated carbons,f-g or on molecular sieve zeolites.1b14 Limited high-pressure experimental work in literature targeted specific solids for study under limited conditions to evaluate the feasibility of a process. As such, the results could not be generalized to provide an understanding of high-pressure supercritical adsorption. In this work, we present a systematic study of high-pressuremethane adsorption on Y-zeolites to complement our previous similar work with X-zeolites.14 Y-zeolites have found a wide range of industrial application (see ref’s for a summary) primarily due to the excellent stability of the crystal structure and a large available pore volume and surface area. In addition to the accessible pore volume, the adsorptivepropertiesof zeolites heavily depend on the size, charge density, and distribution of cations in the porous structure. Voluminous literature on the topic mostly deals with low-pressure adsorption where the effect of cations is most apparent through the impact of cation charge density on the adsorption potential, and hence on the initial heat of adsorption. Early work on the effect of cations on initial heat of adsorption was performed by Dhizigit et a1.16 with X-zeolites which are isostructural with Y-zeolites. The initial heat of adsorption of water was found to increase with decreasingcation size for monovalentcations. Later, Bezus et al.” showed the same trend for nonpolar guest molecules such as ethane and ethylene for monovalent cation exchanged X-zeolites. The effect of cations at high loading which is believed to be primarily due to cation size differences,has not been studied e~tensively.1~ This work investigates the effect of cations at high loading. In addition to the obvious importance in adsorptive natural gas storage application, methane was used in this study due to its t Presently with the University of South Alabama. *Abstract published in Aduance ACS Abstrucrs, November 1 , 1993.
nonpolar and sphericallysymmetricalnature. The effect of cations shown here is, therefore, a result of induced electrostatic interactions which are common for adsorption on all ionic surfaces regardless of the polar or nonpolar nature of the guest molecule.
Experimental Section Five forms of the Y-zeolite were used: N a y , MgY, C a y , SrY, and Bay. The base material was monovalent NaY powder (Si/ A1 = 2.2) produced by PQ Corp. (284-366, Lot. No. 728-91-2). Divalent cation exchangeswere performed followingthe protocol outlined by Sherry.ls Table I lists the extent of exchange of samples determined by atomic absorption. The solids were not heated between exchanges;thus, the residual sodium is most likely located in the sodalite cages1*and is not in intimate contact with the guest methane molecules. The freshly ion-exchanged samples were initially air-dried in excess of 3 days. The samples were activated in situ with the protocol established for X-zeolites;141 OC/min heating rate, upto 400 OC, under vacuum (P = 0.01 Pa). The total activation time was 24 h. The samples were cooled to experimental temperature under vacuum after activation. The methane isothermswere measured in a high-pressure Cahn 1000 microbalance. High-purity methane (99.9%) was used. Sample size was about 250 mg. The sampleweight was monitored after each dose, equilibriumwas assumed when the sample weight change was less than 10.01 mg over a 30-min period. Each measurement required 1-2 h. The isotherms were traced by increasing the dose pressure upto 6760 P a . Desorption was also performed by periodically reducing the pressure. Adsorption hysteresis was not observed,as expected. The sampletemperature was controlled by an external water bath and the actual temperature was determined by a thermocouple located 2.0 mm below the sample pan in the microbalance. The accuracies of weight, temperature, and pressure measurements were hO.01 mg, 10.5 OC, and 1 7 P a (except vacuum measurements). The accuracy of amount adsorbed is estimated to be 12%. The raw data were corrected for the bouyancy force and real gas contributions. First, the bouyancy of balance components was experimentallydetermined without any sample in the system. The bouyancy forceon the inpenetrablesolidmatrix wascalculated from (1) the activated sample weight, (2) the solid density as listed by Breck,lgand (3) the gas density. Real gas formulations are used in this work since high pressures are involved. Methane gas-phase propertieswere obtained from the thermodynamic data of Canjar and Manning.20
0022-3654/93/2097- 12894%04.00/0 0 1993 American Chemical Society
Methane Adsorption by Y-Zeolites TABLE I: Effect of Cations on Methane Adsorption in Y-zeolites NaY MgY CaY SrY 0 11 86 89 % exchanged 1.12 0.66 0.99 cationic radius (A) 0.91 4.52 4.11 3.96 4.44 amount adsorbed (25 OC, 5200 kPa, mol/kg) molcculcs/cane 6.36 6.11 1.16 7.35 (25 OC; 5260 kPa) initial heat of adsomtion 18.9 19.5 22.3 18.9 (kJ/mol)
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12895
BaY 13 1.34 3.82 1.45 18.2
Isotherm Data The isotherm data for the five solids at several temperatures upto 6760 kPa pressure (6326 kPa fugacity) are listed in Table 11. All isotherms were found to be of type I in the IUPACZ* classification, Figure 1 shows MgY isotherms as an example. The data show the “surface excess amount adsorbed” by the Gibbs definition of adsorption. As such, no maximum was observed in the isotherms even though the data extend to relatively high pressures. The data presented here compare well with the only Y-zeolite/methane data available in literature for NaY .22 Effect of Cations at High Loading Figure 2 shows adsorption isotherms for all the solids a t 25 OC. Unlike the trends suggested by Hori et al.,23 the surface phase does not approach a homogeneous supercritical fluid phase as loading increases. Our results indicate that there is a distinct difference between the behavior of methane molecules in different pore environments even at high loadings. The cations have an important impact even at high loading in adsorption by zeolites. The CaY has the highest methane adsorption capacity for all temperatures above about 1000 kPa. This finding is similar to our observations with to X-ze01ites.l~ The capacities at 25 OC decrease in the order CaY > MgY > SrY > NaY > BaY as listed in Table I at 5200 kPa. This trend in adsorption capacity is somewhat puzzling since the order is not consistent with the characteristics of the cations, i.e., ionic radii of divalent cations increases in the order Mg < Ca < Sr < Ba as listed in Table I. The lack of any correlation of the amount adsorbed with ionic radii is a result of the commonly used per weight basis for the amount adsorbed. Thedata are converted to molecules per cavity to clarify the effect of cations on methane pore density at high loading; the conversion compensates for the differences in the unit cell formula weights. The numbers of methane molecules per cavity as shown in Figure 3 (and listed in Table I) are in the order of decreasing capacity BaY > SrY > CaY > MgY > N a y . This trend, which was also reported for X-zeolites,14 represents a direct relation between molecules/cavity and ionic radii, i.e., higher methane density with increasing cationic size. If the effect of cations at high loading were purely a freevolume effect, the number of molecules/cavity should decrease with increasing cation size, contrary to the experimental observation. Clearly, the experimental trend which is significantly beyond experimental error cannot be explained by only volume reduction. Figure 4, where molecules/cavity is shown as a function of ionic radii, has a slope of about 0.98 indicating almost a linear relation between methane density and cationic size. If volume exclusion is the only effect, the slope should be negative in Figure 4. One possible explanation for increasing capacity with increasing cation size is differences in the packing of molecules around the chargecenters (cations) at high loading. The inducedelectrostatic potential experienced by a molecule is related to inverse sixth power of its distance from the cations. A single methane molecule can get closer to the smaller cations; therefore adsorption potential is higher for small cations as indicated by the larger initial heat of adsorption at infinite dilution. On the other hand, the “total” potential experiencedby several molecules depends on the distance
of each molecule from the charge centers, or on the methane density distribution function in the cavity. The total potential is largely determined by the number of molecules in the “first shell” neighborhood of the charge centers, since the induced potential experienced by the “second shell” neighbors are much lower, being roughly one molecular diameter further away (about 3.82 A for methane). Small cations impose a strong potential but only for a small number of molecules in the first shell. On the other hand, large cations may impose a somewhat lower potential but for a larger number of molecules in the first shell. The differences in molecular packing around cations can result in the higher affinity of the solid with larger cations at high loading as observed in the experiments. The above argument which is based on “shielding” of small cations by the very first few molecules in a cavity is a viable explanation for the experimental observation of increasing capacity with increasing cation size. This postulate can be tested by other techniques such as molecular simulations and/or experimental methods based on molecular spectroscopy which are beyond the scope of the present work. It should be noted that this analysis is performedat high loadings reaching 7.5 molecules/ cavity, but not at the capacity limit. The theoretical capacity of these zeolites is about 12 methane molecules/cavity at infinite pressure. At full capacity, methane loading should decrease with increasing cation size due to volume exclusion. Henry’s Law Constant and Initial Heat of Adsorption Accurate data in the Henry’s law range is extremely important to determine the vertical interaction between guest molecules and the surface at infinite dilution. The data presented here covers more than two decades in pressure, lowest loading of about 0.15 molecules/cavity represents about 1% of capacity which is well within the Henry’s law range. Such low loading data lead to accurate determination of the Henry’s law constant, and hence the initial heat of adsorption which is related to “isolated pair” potential between the surface and the guest molecules. The Henry’s law constants were determined from the intercepts of the logarithm of fugacity over amount adsorbed versus amount adsorbed as shown on Figure 5 for MgY. The coordinates used in the figure enable accurate determination of experimental Henry’s law constants by easy extrapolation to zero loading at all experimental temperatures. The experimental Henry’s law constants are then cross plotted in Figure 6 against the inverse temperature in a vant Hoff diagram where the slope is related to the initial heat of adsorption. The initial heat of adsorption values determined from the data are listed in Table I. The initial heat of adsorption of methane on the only monovalent form, N a y , is 18.9 kJ/mol which is close to the published data of 18.1 kJ/mo1.22 The initial heat of adsorption in kJ/mol for divalent cationic forms follow the order CaY(22.3) > MgY(19.5) > SrY(18.9) > BaY(18.2). Sameorderwasobservedfor X-zeolites in our previous work.14 As expected, these values are about 2 kJ/mol lower than X-zeolites due to the lower alumina content of the Y-zeolite. Lower alumina content translates to smaller number of cations per unit cell. The initial heat of adsorption increases with decreasing cation size (increasing chargedensity) except for the MgY. Even though Mg cation has the smallest size, the methane initial heat of adsorption is larger on CaY than on MgY. The same phenomena was observed for the X-ze01ites.I~ This anomaly is attributed to the incomplete dehydration (activation) of MgY. Mg cation charge density is so strong that it is not possible to completely remove the hydroxyl groups without risking structural damage by applying very high temperature during activation. This conclusion is consistent with the results of Coe et al.25who studied the effect of residual water on adsorption properties.
Talu et al.
12896 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
TABLE 11: Metbane Isotherm Data on Y-Zeolites f(kPa) N (mol/kg) f ( w a ) N(moI/kg) f (Wa) N (mol/kg)
f (kPa)
N (mol/kg) f(kPa)
25 "C 1746.58 1125.65 2508.67 2947.28 3330.02 3827.99
2.868 3.054 3.238 3.407 3.559 3.69
4 35 2.2 9 4773.5 5119.31 5584.68 6041.8
3.806 3.908 3.943 4.012 4.068
40 OC 2168.5 2547.1 2883.4 3266.2 3618.7 3987.6
2.734 2.926 3.066 3.231 3.318 3.424
4387.1 4602.9 4929.6 5276.4 5711.6 6135.4
3.521 3.591 3.670 3.738 3.815 3.882
2436.6 2745.6 3058.1 3424.3 3774.8 4122.1
2.593 2.746 2.905 3.039 3.147 3.257
4527.2 4885.1 5282.0 5744.9 6202.5
3.361 3.446 3.584 3.671 3.747
0.226 0.46 0.789 1.166 1.516 1.856
60 OC 2112.56 2504.56 2867.22 3315.76 3728.55 4105.92
2.132 2.344 2.565 2.762 2.926 3.053
4554.31 4967.49 5371.11 5728.14 6219.84
3.183 3.283 3.384 3.456 3.56
0.226 0.420 0.696 1.006 1.285 1.546
70 OC 2224.1 2617.8 2950.0 3344.4 3717.2 4058.2
1.780 1.993 2.184 2.354 2.508 2.634
4389.8 4640.6 5028.2 5413.2 5825.4 6306.0
2.736 2.816 2.939 3.036 3.143 3.236
123.85 233.51 356.39 614.14 969.9 1347.67
0.311 0.661 1.003 1.589 2.156 2.561
110.1 240.5 445.2 729.9 1011.9 1430.7 1844.1
0.280 0.556 0.951 1.429 1.778 2.200 2.514
309.0 568.1 872.5 1241.4 1626.5 2040.4
0.581 0.982 1.364 1.747 2.076 2.359
137.68 302.31 554.89 907.39 1277.22 1703.48 185.8 398.3 711.9 1084.1 1240.4 1453.7
41.3 96.4 172.0 247.6 336.7 473.5 676.6 1016.6
0.174 0.369 0.543 0.732 0.851 1.096 1.421 1.879
103.3 172.1 309.3 493.9 711.9 1023.4
0.238 0.364 0.594 0.8 12 1.026 1.376
50 'C
my 20.68 61.99 96.37 168.44 27 1.64 4 17.64 546.51 65.4 103.3 172.0 274.8 41 1.5 568.1 751.0 27.6 68.9 134.3 213.3 336.7 555.3 745.8
0.09 0.254 0.397 0.634 0.955 1.417 1.692 0.173 0.261 0.431 0.629 0.904 1.163 1.433 0.047 0.113 0.226 0.339 0.549 0.818 1.041
25 OC 748.9 1036.54 1367.73 1665.3 2005.1 2350.28 2766.44 45 OC 1027.1 1361.4 1699.0 2014.5 2339.6 2803.2 3254.2 65 OC 1030.2 1373.3 1680.7 2006.1 2401.7 2800.5 3215.4
2.075 2.439 2.812 3.102 3.348 3.547 3.745 1.798 2.158 2.465 2.708 2.921 3.181 3.396 1.357 1.678 1.964 2.187 2.431 2.681 2.897
3231.82 3755.61 4186.27 4663.06 5073.37 5364.67 5903.2 3718.7 4183.7 4648.7 5156.3 5502.3 5904.1
3646.7 4016.8 4477.9 4928.4 5320.5 5837.6
3.916 4.067 4.202 4.312 4.413 4.416 4.521 3.616 3.734 3.883 4.027 4.091 4.174
3.079 3.232 3.396 3.533 3.658 3.779
CaY 27.56 55.1 96.37 185.58 281.36 410.84 627.64
0.205 0.371 0.57 0.949 1.227 1.561 1.951
25 OC 883.04 1242.13 1485.64 1817.92 2197.4 2508.67 2822.7
N (mol/kg) f(kPa) N (mol/kg) CaY
NaY
2.341 2.756 2.98 3.232 3.461 3.622 3.78
3262.56 3683.17 4055.7 4399.46 4744.48 5033.12 5285.1 1
3.938 4.095 4.124 4.312 4.404 4.462 4.553
45 OC 1393.4 1660.7 2006.1 2359.0 2735.3 3189.6 3678.7
2.284 2.568 2.841 3.085 3.225 3.404 3.601
4054.9 4496.7 4834.7 5190.1 5547.4 5880.4 6275.7
3.754 3.851 3.993 4.101 4.160 4.256 4.356
65 OC 1346.5 1687.4 2045.8 2395.1 2813.5 3254.1
1.699 1.968 2.203 2.454 2.687 2.918
3697.9 4099.2 4590.5 5102.8 5518.1 5856.0
3.1 14 3.299 3.431 3.578 3.690 3.783
25 OC 688.33 1016.56 1341.08 1681.57 1992.23 2318.49 2710.06
2.022 2.454 2.768 2.977 3.135 3.326 3.482
3274.84 3755.61 4079.5 4440.64 4894.95 5324.95
3.689 3.825 3.951 4.0 13 4.121 4.187
45 o 1020.4 1354.8 1685.9 2014.3 2333.1 2726.3 3178.4
1.881 2.245 2.469 2.723 2.918 3.104 3.327
3656.3 4085.1 4526.8 4933.4 5264.1 5780.4 6122.5
3.496 3.609 3.779 3.825 3.903 3.991 4.051
1.486 1.779 2.057 2.369 2.509 2.791 2.941
3454.1 3902.3 4364.7 4672.0 5077.9 5444.1 5764.1
3.105 3.221 3.393 3.490 3.608 3.652 3.741
SrY 34.41 68.87 110.12 171.88 240.35 342.77 485.55
0.269 0.464 0.602 0.857 1.096 1.352 1.656
27.6 62.0 110.1 172.0 274.8 411.5 547.7 744.3
0.156 0.317 0.467 0.616 0.824 1.066 1.284 1.558
34.5 82.7 144.6 275.0 412.0 548.5 752.6
0.125 0.250 0.407 0.648 0.878 1.006 1.202
c
65 "C 1023.4 1292.8 1614.0 2045.8 2224.1 2683.1 3027.9 BaY
34.45 68.86 106.7 168.45 271.11 410.85 553.3
0.27 1 0.459 0.652 0.895 1.207 1.539 1.832
25 OC 735.45 1016.56 1360.83 1681.57 1979.35 2318.49 2697.52
2.103 2.396 2.655 2.842 3.006 3.13 3.271
3133.2 3501.27 3990.12 4470.02 4923.8 5302.19 5601.51
3.407 3.519 3.62 3.687 3.784 3.839 3.881
27.5 68.9 102.0 172.0 274.8 391.0 568.1
0.146 0.380 0.512 0.701 0.955 1.229 1SO7
45 o 751.0 1020.1 1354.8 1692.5 2007.7 2404.3 2822.4
1.757 2.053 2.384 2.598 2.762 2.934 3.083
3298.4 3818.3 4312.8 4745.8 5 174.3 5549.8 5951.0
3.233 3.368 3.488 3.562 3.632 3.686 3.732
34.5 62.0 137.7 236.8 350.4 473.5 684.7
0.134 0.207 0.416 0.606 0.830 1.028 1.299
65 OC 1030.2 1373.3 1694.0 2039.2 2408.2 2859.1 3350.8
1.664 1.951 2.198 2.387 2.582 2.762 2.963
3838.5 4345.8 4728.3 5227.5 5696.6 6026.8
3.070 3.184 3.251 3.345 3.421 3.481
c
Methane Adsorption by Y-Zeolites
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12897
1
7.6
/I
0 0
1000
2000
3000
4000
Fugacity (kPa)
,
1
C 1 .e I 3
-8-
1
I .5
Cation Rodius (A)
Figure 4. Dependence of methane molecular density in supercages of Y-zeolites on cationic radii at 25 O C and 5200 kPa fugacity.
Figure 1. Adsorption isotherms of methane on MgY zeolite. 4.8
.5
6000
5000
7.5
1
NoY
* MpY CaY
% .--
U
+ SrY + BOY + 65 0
900
1800
2700
3600
4500
5400
6300
5
Fugacity (kPa)
0
Figure 2. Methane adsorption isotherms at 25 OC on different cationic forms of Y-zeolite.
1 .e
.8
2.4
3.2
4
C
8
Amount Adsorbed (mol/kg)
Figure 5. MgY isotherms in virial coordinate system for the calculation of experimental Henry’s law constants. 7
X
0
6.5
d
6 h
c
v
4 NoY
Y
+ MqY 4
5.5
COY
+ sn
P
t BOY
I
5 900
I800
2700
3600
4500
5400
C
Fugacity (kPa)
Figure 3. Methane molecular density in supercages of Y-zeolites at 25 OC.
Correlation of Isotherm Data
The data were curve fitted to several isotherm equations to provide correlations for future use. Three isotherm models were considered: (1) the virial isotherm equation,26(2) the Dubinin2’ equation with hypothetical liquid methane properties estimated as outlined in R. T. Yang,’ and (3) the simplified statistical model of RuthvenS28 Although the latter two are commonly used for high-pressure adsorption, the agreement with data was not as good as the results with the virial equation in this study.
4.:
I
3
3.1
3.2
3.3
4
1000/T(K)
Figure 6. Henry’s law constants at different temperatures for the calculation of initial heat of adsorption.
The success of the virial equation in fitting the data covering a large range in pressure (and loading) must be partly attributed to the flexibility of the model. Conceptually,the virial equation is exact only in the limit of zero surface density, similar to its counterpart for the gas phase. On the other hand, if correlation of data rather than the physical significance of parameters is the primary concern, the thermodynamically c0nsistent2~virial model
12898 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
TABLE III: Virial Isotherm Eauation Parameters’ NaY ~~~
MRY
CaY
SrY
BaY
13.2789 -2342.58 0.27046 -1 1.6545 -0.08974 20.4580 -0.00743 5.82416
13.7826 -2679.57 3.58252 -902.137 -2.37321 696.101 0.35324 -103.770
12.2121 -2277.97 2.1 1523 -329.275 -1.39572 342.883 0.20710 -51.0079
11.9619--2185.32 0.89422 -49.5026 -0.58214 134.490 0.08241 -15.4430
~
ko
ki bo
bl co CI
do
di
13.2955 -2273.64 1.80964 -556.324 -0.56805 202.176
-
For eq 1 and 2,fin kPa, N in mol/kg, Tin K. is very versatile in representing isotherm data covering a wide range in pressure, amount adsorbed and temperature as in this study. The virial isotherm equation is given as
+
+
+
j= Nexp(K(T) B ( T ) N + C(T)N* D ( T ) N ~ --) (1) where f is the fugacity (Wa) and N is the amount adsorbed (mol/kg). K(T)is the exponentialof Henry’s law constant, and B( T), C(T),etc., stand for two-body, three-body, etc. interactions between guest molecules. During data reduction, the virial coefficients and the Henry’s law constant were expanded in inverse temperature as B(T) = 6, + (b,/T), etc. (2) K(T) = ko + ( k , / T ) It should be noted that the temperature dependencies differ from our previous use14 in that the first constant term on the right-hand side of eq 2 stands for the quadratic and higher order terms in the infinite series expansion in inverse temperature. We found that it is not possible to resolve the higher order temperature terms with statistical significancegiven the small variation (40 “C) in temperature. A stepwise regression technique which maximized the F-statistics was used in data analysis where all isotherms at several temperatures were simultaneously fitted in a single regression. This approach imposes the correct temperature depedency of the parameters. Use of F-statistics assures that regression model is not overspecified since the F-statistics measures the goodness of fit with consideration to the number of parameters in the model. The F-statistics reached maximum with the K( T),B( T), C( T), and D( T ) terms for all systems except for NaY where only two virial constants were necessary. These were expected due to the shape of ln(flN) vs N plots for these systems (shown in Figure 5 for MgY). The temperature-independent isotherm equation constants are listed in Table 111. The accuracy of the virial correlation can be evaluated in Figures 1, 2, 3, 5, and 6 where the model is shown as solid curves.
Conclusions Adsorption isotherms were measured for methane on N a y , MgY, Cay, SrY, and BaY at temperatures ranging from 25 to 70 OC and at pressures upto 6760 Wa. Hysteresis and maxima in the “Gibbs surface excess amount” were not observed. The experimental data covering 2.5 decades of pressure were satisfactorily correlated with the Virial adsorption model.
Talu et al. It is found that the cations have an important effect on adsorption even at high loading. The CaY has the highest adsorption capacity per weight above 1000 Wa. The methane pore density at highest loading levels decreased in the order BaY > SrY > CaY > MgY > N a y , in the reverse order of the ionic radii. The increasing methane density at high loading with increasing cationic radius is attributed to possible differences in the packing of molecules in the zeolite cavity. The experimental Henry’s law constants and initial heats of adsorption were accuratelydetermined fromdata extending below 1%coverage. As expected, the initial heat of adsorption is found to increase with decreasingcation size except for the MgY. This anomaly which is consistent with our previous work with X-zeolites and findings of other researchers is attributed to the incomplete dehydration of the Mg cation at the normal activation temperatures. Acknowledgment. This project is partly supported by the Research Challenge program of the State of Ohio. Reference and Notes (1) Yang, R. T., Gas Separation by Adsorption Processes; Butterwoths: London, 1987. (2) Talu, 0.Proc 4th Int. Conf. Fundam. Adsorption, Kyoto, Jpn. May 1992. (3) Matranga, K. R.; Stella, A.; Myers, A. L.; Glandt, E. D. Sep. Sci. Technol. 1992, 27(14), 1825. (4) Bojan, M. J.; VanSlooten, R.; Steele, W. Sep. Sei. Technol. 1992, 27(141. -. ,- .,, 1x17 - -- . . (5) Tan, Z.; Gubbins, K. E. J . Phys. Chem. 1990, 94,6061. (6) Otto, K. Altern. Energy Sources 1982, 4(6), 241. (7) Barton, S.S.;Dacey, J. R.;Quinn, D. F. FundamentalsofAdsorption; Eng. Trustees Inc.: New York, 1984; p 65. (8) Chkhaidze, E. V. Izv. Akad. Nauk SSSR Ser. Khim. 1986, 4, 929. (9) Nitta, T.; Nozawa, M.; Kida, S. J. Chem. Eng. Jpn. 1992, 25(2), 176. (10) Munson, R. A. US.Natl. Tech. Inform. Serv. PE Rep. 1971, No. 203892. (1 1) Rolniak, P. D.; Kobayashi, R., AIChE J. 1980, 26(4), 616. (12) Zuech, J. L. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 172. (13) Chkhaidze, E. V.; Fomkin, A. A. Izv. Akad. NaukSSSRSer. Khim. 1985, 5, 974. (14) Zhand, S.Y.; Talu, 0.;Hayhurst, D. T. J . Phys. Chem. 1991,95, 1722. (15) Flanigen, E. M. Zeolites: Science and Technology; Martinus Nijhoff: The Hague, 1984. (16) Dzhigit, 0.M.; Kiselev, A. V.; Mikos, K. N.; Muttik, G. G. J . Chem. Soc. Fraday Trans. I 1971,67,458. (17) Bezus, A. G.; Kiselev, A. V.; Sedlacek, Z.; Du, P. Q. J. Chem. Soc. Faraday Trans. I , 1971,67,468. (18) Sherry, H. S.J . Phys. Chem. 1968, 72,4086. (19) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974. (20) Canjar, L. N.; Manning, F. S. Thermodynamic Properties and Reduced Correlations for Gases; Gulf: Houston, TX, 1967. (21) IUPAC, Pure Appl. Chem. 1986, 57(4), 603. (22) Neddenriep, R. J. J. Colloid Interface Sci. 1968, 28(2), 293. (23) Hori, Y.; Kobayashi, R. Ind. Eng. Chem. Fundam. 1971, 12, 26. (24) Karavias, F.; Myers, A. L. Molecular Simulation 1991,8, 23. (25) Coe, C. G.; Parris, G. E.; Srinivasan, R.; Auvil, S. R., New Developments in Zeolite Science Technology; Kodansha: Japan, 1906. (26) Barrer, R. M. Zeolites and Clay Minerals; AcademicPress: London, 1978. (27) Dubinin, M. M. Chem. Rev. 1960, 60, 235. (28) Ruthven, D. M. Nature 1971, 232, 70. (29) Talu, 0.;Myers, A. L. AIChE J. 1988,31(11), 1887.
