619
J. Phys. Chem. 1993, 97, 619-624
COz Sorption Isotherms in LTL Zeolitest
P. N. Joshi and V. P. Shiralkar. National Chemical Laboratory, Pune 41 I 008, India Received: December 12, 1991; I n Final Form: October 19, 1992
Isotherms of C02 sorption in KL, HKL, Na(l)KL, and Na(2)KL in the temperature range 273-393 K have been measured. The equilibrium sorption capacity a t 400 Torr was found to increase in the order HKL C KL < Na( l)KL < Na(2)KL. The sorption data were analyzed in terms of different models of sorption isotherm equations. Dubinin, Langmuir, and Freundlich isotherm models could represent the sorption data satisfactorily, yielding linear plots. BET and Sips sorption models showed limited applicability with linear plots only in the low-coverage region. The chemical potential of sorption also followed the same sequence as that of equilibrium sorption. The rate of drop of chemical potential, however, showed the reverse order. The isosteric heat of C02 sorption, calculated by using the Clausius-Clapeyron equation, was found to be temperature independent and ranged between 27 and 46 kJ/mol. Zeolite KL offered the most homogeneous surface for C02 sorption.
TABLE I: Unit Cell Compositions (on Dry Basis) of LTL zeolites
It has been shown’ that zeolites act both as acidic as well as basic catalysts. Basic probe molecules like ammonia2v3 and n-butylamine(n-BA)4characterize the acid centers in the zeolites and sorption of an acidic probe molecule like carbon dioxides*6 that interacts with base centers in the zeolites. In addition to the basic and acidic characters, the polarity (dipole and quadrupole) and the size of the probe molecule contribute significantly to the sorption energetics. A comparatively smaller (critical diameter = 3.2 A) carbon dioxide molecule with appreciable quadrupole moment has been best suited’ as the acidic probe to characterize the base properties of the zeolites. Zeolite LTL, an interesting catalytic material, provides a pore system having cancrinite cages (1 1-hedra) alternating with hexagonal prisms (8-hedra) stacked in columns parallel to the c a x k 8 The channelsare circumscribed by a nearly planer 12-membered ring having a free diameter of -7.8 A. Therefore, although the framework structure of the LTL zeolite is regarded as being as open as faujasite, its total intracrystalline volume estimated from water sorption is little more than half of that in faujasite. Although a number of sorption studies in different ion-exchanged LTL zeolites of hydrocarbons8 and ammoniag and carbon dioxidelOJ1are reported, we have carried out carbon dioxide sorption in different LTL zeolites of varying base character. The results are reported in the present paper.
Experimental Section KL, Na( l)KL, and Na(2)KL samples of type LTL zeolites were prepared by hydrothermalcrystallization in accordancewith the method described in our earlier communication.12 HKL was prepared by treating KL with a 0.3 M NH4Cl solution at room temperature for 6 h. The exchanged zeolite was washed until the filtrate was free from C1-ions. The ammonium form so obtained was deammoniated at 673 K for 4 h to get the protonic form. Both the filtrate and solid were analyzed for their potassium content in order to estimate the degree of ion exchange. The chemical composition of the sample was determined by wet gravimetric analysis; sodium and potassium were estimated by an atomic absorption spectrometer (Hitachi-8000), flame photometer, and induced coupled plasma spectrometer (Jobin Yvon, f
To whom all the correspondence should be addread. NCL Communication No. 5315.
0022-3654/58/2097-0619$04.00/0
sample HKL KL Na(1 )KL Na(2)KL
unit cell composition
Si/AI
H3.6Ks.1[(A102)a.7o(Si0z)z71.)01 3.139 I b . 7 3 [(AlO~)a.70(Si02)27.301 3.139 K7.9zNa1.31[(AlO2)9.11(sioz)z6.891 2.953 K7.1Na2.38[(AI02)9.u(Si0~)~6.s6] 2.8 13
Al/(Al+ Si) 0.241 0.241 0.253 0.262
France). Unit cell compositions of all the four samples are listed in Table I. The adsorption isotherms were measured by the method described elsewhereS using all glass volumetric units connected to the high vacuum system. The sorption isotherms were measured up to 400 Torr in the temperature range 273-393 K. In order to check the reversibility of the sorption, desorption measurements were also carried out. After each isotherm, the sample was evacuated at 773 K at 10-6 Torr for several hours. X-ray diffractograms were recorded for each sample before and after the sorption measurements to check the structure stability.
Results and Discuaoion
Isotherm Analysis. Figure 1illustrates the familiesof sorption isotherms of COz on different cationic forms of LTL zeolites up to 400 Torr. Sincethe number of unit cells per gram of dehydrated zeolite may vary with the different cationic species and the a-ibility of the sorbatemolecule toward thedifferent polyhedra is determined by the size of the sorbate molecule, the amount sorbed is expressed as molecules per unit cell. Figure 1 shows that especially at low temperature, the sorption isotherm a p proximates to a type I isotherm (Langmuir isotherm) according to Kiselev’s~1assification.I~ Although the decreasein equilibrium sorption capacity at 400 Torr with an increase in temperature is logical in accordance with thermodynamic expectations, the extent of the drop in the sorption capacity becomes more severe in the order Na(2)KL < Na( 1)KL < KL < HKL. In other words, in the case of HKL, only about 10% sorption capacity is retained at 393 K to that compared at 273 K,whereas up to 25% sorption capacity is retained at 393 K as compared to that at 273 K in Na(2)KL. This behavior may be related to the variation in adsorption strength of the basic sites in the zeolites with change in cationicspecies in LTL zeolites. Figure 2 systematicallyshows that the equilibrium sorption capacity at 400 Torr increases at all the sorption temperatures in the order HKL < KL < Na(1)KL < Na(2)KL. Taking into consideration that C02 is an acidic molecule, it seems that the base character increases from Ca 1993 American Chemical Society
Joshi and Shiralkar
The Journal of Physical Chemistry, Vol. 97, No. 3, 1993
620
L
e
10.0
1
0
I
0
0
1 5
O
I
/ /
O 0 I
HKL
KL
I
I
Na( I ) K L
Na(2)KL
Figure 2, Equilibrium sorption capacities at 400 Torr and at (1) 273, (2) 303, (3) 333, (4) 363, and (5) 393 K. 100 200
300
400
100
200
300 400
P,mm
Figure 1. COz sorption isotherms in (A) HKL, (B)KL, (C) Na( l)KL, and (D)Na(2)KL at ( 1 ) 273, (2) 303, (3) 333, (4) 363, and (5) 393 K.
HKL to Na(2)KL. Acidic protons are present in HKL on account of replacement of K+ by protons. Further, when K+ is replaced by Na+ progressively, the base character of the base centers in the zeolites (Le., framework oxygens) increases. When K+ is partly replaced by Na+ during the synthesis, Table I shows that the resulting zeolite possesses lower framework Si/Al, inviting more extraframework cations to balance the charge on the icnreasing number of aluminum tetrahedra. The ionic radius of K+ is higher than that of Na+,I4 and hence, the higher e / . and higher electronegativity of the latter impart more basic character to framework oxygen species in the zeolite LTL. The basicity of framework oxygens in a zeolite of a particular framework structure is a combined effect of the electronegativity of extraframework cations along with their cation density in the open sites (less screened) by virtue of which a sorbate molecule interacts with them. However, the consideration of electronegativity may be taken as a criterion for the basicity of framework oxygen, only when different extraframeworkspeciespossess almost the same cation density in open sites (-Dnsites in LTL). Therefore, although in principle KL is expected to exhibit more basic character than NaKL, actually on account of the higher cation densityof Na+ than K+ in D sites (open sites, i.e., accessible to a sorbate molecule), NaKL exhibits a higher basic character than KL. At the same time, the strength of these basic sites depends upon the framework AI/(Al Si) ratio (Table I). The increased sorption capacity in the cases of Na( l)KL and Na(2)KL as compared to that in KL may also be partly due to referential'^ occupation of open sites by Na+. Therefore, although the total number of cations in both KL and Na(2)KL are almost identical, the number of cations acting as sorption centers occupying open sites is more in the latter and, hence, enhanced the sorption capacity. Therefore, cation-site selectivity also influences the sorption capacity.
+
-
Type Y zeolites in the sodium form with 58 tetrahedral aluminumsin the unit cell of 192tetrahedra sorbed 80 molecules of C02 per unit cell at 400 Torr.s LTL zeolite in the (Na,K) form (with maximum possible Na+) with 9.44 tetrahedral aluminums in the unit cell of 36 tetrahedra sorbed 10 C02 molecules per unit cell at 400 Torr. Almost 5.3 unit cells of LTL zeolite give 1 unit cell of type Y zeolite. Therefore, had the type L zeolite been isostructural with zeolite Y, it would have sorbed 53 molecules of COZper unit cell. Zeolite LTL is more silicous (Si/Al= 2.6-3.45) than zeolite Y (Si/Al = 2.3), and hence, the cation density is less in zeolite L than in zeolite Y. Similarly in both, COZmolecules in the zeolites penetrate only supercages and not polyhedra like cancrinite cages and hexagonal prisms in LTL zeolite and sodalite cages and hexagonal prisms in type Y zeolite. However, of the total extraframeworkcation, occupation of theopensites (to beable tointeract with thesorbatemolecules) is lower in type LTL zeolite15 (3.6 out of 9.7) than in Y zeolite’O (28 out of 58). This then leads to comparatively lower sorption uptake of C02 in LTL zeolite as compared to Y zeolite. Application of Sorption Isotherm E q ~ n t i o ~ D u b i i Equation. Analysis of the sorption data in terms of different isotherm equation models always yields useful information with regard to the nature of the sorption. An attempt is made here to apply the Polanyi potential theory modified by Dubinin and RadushkevichI6for C02 sorption in different cationic forms of LTL zeolite in the temperature range of 273-393 K. The Dubinin-Radushkevich equation is expressed as
Po where Wis the amount sorbed at equilibrium pressure P, WOis the total sorption capacity, B is a constant independent of temperature and characteristic of sorbent pore structure, and /3
The Journal of Physical Chemistry, Vol. 97, NO. 3, 1993 621
CO2 Sorption Isotherms in LTL Zeolites
0.3
0.6 [LOG
I .2
0.9 (+
1.5
11'
Figure 3. Dubinin plots for C02 sorption in HKL at (1) 273, (2) 303, (3) 333, (4) 363, and (5) 393 K.
is the affinity coefficient. The Dubinin plots so obtained were reasonablylinear in all four samplesat all temperatures. Typical Dubinin plots are shown in Figure 3, and they indicated that the C02 sorption data in the LTL zeolite could be satisfactorily expressed by eq 1. The slopes of these plots decrease with an increase in temperature. The values of the saturation capacities and B / b 2obtained from the intercept and slope, respectively, of the linear plots are summarized in Table 11. The saturation capacities increase from 6.03 to 10.47 molecules per unit cell at 273 Kin the order HKL < KL < Na( 1)KL < Na(2)KLand from 1.62 to 3.24 molecules per unit cell at 393 K. These saturation capacities are in close agreement with those obtained experimentally. From the values of B/b2,it is evident that the affinity coefficient fl for all the zeolites decrease3 with an increase in temperature. The affinity coefficient follows the sequenceHKL > KL > Na(2)KL > Na(1)KL at 273 K and HKL > Na(2)KL > Na( l)KL > KL at 393 K. Such a complicated behavior in the affinity coefficient may be due to the heterogeneous character of thezeoliticsurfaceandmay alsobedue tocation-site selectivity. ~ s o r p t i o n I s o t b e r m E q u r t i o sLocalizedsorption . with the monolayer approach and 1:1correspondencebetween sorbate molecules and sorption centers and energetically homogeneous sorption centers are salient features of the Langmuir isotherm equation. When the Langmuir equation was applied to the data of C02 sorption in LTL zeolites in the present studies, linear plots with varying intercepts were obtained. Typical Langmuir plots are shown in Figure 4 for all four zeolites at 273 and 393 K. The linearity of these plots indicates the applicability of the Langmuir approach to the C02 sorptiondata in the LTL zeolites. However, the Langmuir approachfailed to representSC02 sorption data in cation-exchanged Y zeolites. As far as C02 sorption is concerned, the zeolitic surface of cationic forms of LTL zeolites seems to exhibit more homogeneollscharacter than that for cationexchanged Y zeolites.' Probably more silicous zeolites with less
extraframework cations may offer comparatively more homogeneous surface. A salient feature of these Langmuir plots is the marginal decrease in the intercept on the Y axis at low temperature and a reasonable decreaseat higher temperature in the sequence HKL > KL > Na( 1)KL > Na(2)KL. The reciprocal of the intercept on the Y axis may be related to the strength of sorption, and as the intercept decreases, more stronger interaction is involved in the sorption process. This was reflected in an increase in base character in the reverse order of the equilibrium sorption capacities reported earlier. The saturation capacities obtained from the reciprocals of the slopes of these linear plots are marginally higher at lower temperature and considerably higher at higher temperature than those observed experimentally. BET Sorption Isotherm Equation. The multilayer sorption approach leads to the type I1 sorption isotherm according to Kiselev's classification. When C02 sorption data were analyzed in terms of the BET approach, linear plots were obtained up to a relative pressure of 0.25 Torr. Typical BET plots are presented in Figure 5. Usually linearity in BET plots is observed up to a relative pressure of 0.2 Torr even in the case of a sorbate like N2, and a deviation from linearity in the higher pressure region may be due to multilayer formation and/or capillary condensation. BET plots display a very small intercept on the Yaxis, indicating a very high value of C, Le., heat of sorption of the first layer of the sorbate on the sorbent surface. The monolayer capacity calculated from the slopes and intercepts of these linear plots was found to be much lower than the experimental values. Perhaps the experimental values represent the sorption capacity which is more than that needed for the formation of the monolayer on the sorbent surface. The C02 sorption data in cation-exchanged Y zeolites failed5 to follow the BET sorption model. The BET approach in the present case yields linear plots up to a pressure of 150 Torr. However, the Langmuir and Dubinin approaches yielded linear plots even up to 400 Torr. Sips Equation. SipsI7 derived a new theoretical isotherm equation for rigorously calculating the distribution of adsorption energies of the sites of a sorbent surface when sorption isotherms were knownand the sorptionwas localized without sorbatbsorbate interactions. The original Sips equation**on linearization takes the form log(&)=logA+clogP where A and c are constants and P is the equilibrium pressure at coverage 8. For calculating 0, the saturation capacities were obtained from the Langmuir plots. It is assumed that adsorption takes place as a chemical reaction between the active centers and sorbate molecules, and if 1:l correspondence is assumed, the Langmuir isotherm results. However, if some tolerance is made for the complicating factor, the above equation results. In order to check the applicability of the Sips equation, the experimental values of 0 and P were substituted in eq 2. The typical plots of log (0/(l- e)) against log P for the four zeolites at 273 and 293 K are presented in Figure 6. It is seen from Figure 6 that at lower temperature, the Sips equation was linear only in the low-pressure region, and at higher pressure, it deviated from linearity. The deviation from linearity was found more in sodium-containingzeolites. Perhaps at lower temperature some complicatingfactors like chemisorption, etc., may be involved in the sorptionenergetics. At higher temperature, however, all four zeolites yielded excellent linear plots which are parallel to each other with almost the same slope. The deviation of the value of c in the Sips equation (the slope of the linear plots) from unity may be takenI7as a measure of the deviation from the Langmuir isotherm. In the present case, the slopes of all four plots in Figure 6 B are almost unity (1.O & 0.04). The value of c decreased with the increase in the temperature and the increase in the degree of
Joshi and Shiralhr
622 The Journal of Physical Chemistry, Vol. 97, No. 3, 1993
TABLE II: Saturation Capacities and Affioity Coefficients of LTL Zeolites saturation capacities, molecules/U.C. sample temp, K Langmuir BET Dubinin experimental 3.837 6.025 5.713 6.519 HKL 273
KL
Na(1)KL
Na(2)KL
2.957 1.722 1.249 0.548 4.075 3.127 2.5 1 1.814 0.689 5.82 3.754 3.160 2.007 1.32 1 6.835 3.905 3.43 2.343 1SO7
5.33 4.246 3.234 1.878 8.33 6.66 6.25 5.35
303 333 363 393 273 303 333 363 393 273 303 333 363 393 27 3 303 333 363 393
8.68 6.63 5.82 4.57 4.10 10.06 6.887 5.833 5.599 4.708
4.168 2.8 18 1.905 0.758 7.58 5.49 4.17 2.63 1.62 8.414 6.025 4.57 3.31 1 2.187 10.471 6.607 4.897 3.63 3.236
affinity coeff loS(B/@*) 7.24 9.47 12.17 12.67 10.97 7.6 6.5 10.37 10.62 17.41 9.88 9.05 11.62 10.57 12.13 7.71 8.26 9.71 10.07 12.05
3.933 2.567 1.697 0.704 7.488 5.24 3.74 2.53 1.42 8.153 5.824 4.284 2.954 1.955 9.90 6.281 4.576 3.286 2.246
A
45 60
0.18
-
30 -
-
0 15-
a
a
12
0.12
> 0.09
0
20
0.06
4 5 00
0.03
300
0.I
0.2
0.3
0.4
0.5
0.6
P / P*
Figure 5. BET plots for C02 sorption at 273 K i n (1) HKL, (2) KL, (3) Na(l)KL, and (4) Na(2)KL.
L
I
50
I
100
I
I50
I
200
I
250
I
300
I
350
I 400
P, Torr
Figure 4. Langmuir plots for C02 sorption at (A)273 and (B)393 K in (1) HKL, (2) KL, (3) Na(l)KL, and (4) Na(2)KL.
La3+exchange for COZsorption in Y-type zeolite^.^ If constant A could be looked upon qualitatively as representing the strength of sorption, then the increase in A with sequence HKL < KL < Na(1)KL < Na(2)KL and increase in A with the increase in temperature in Figure 6 are in accordance with experimental observation and thermodynamic expectation. An increased value ofA in thecaseofNa(2)KL thanthat in thecaseoftheremaining zeolite samples suggests stronger sorption in the former than in the latter ones. Freundlich SorptionIsotherm Model. In a low-pressure region, the Sips equation reduces to a Freundlich-type isotherm. In the low-coverage region, the Freundlich isotherm approximates a two-dimensional film of the sorbate on the sorbent surface. Analysis of the C02 data in the different cationic forms of the LTL-type zeolites in terms of the Freundlich isotherm model yielded linear plots. Typical Freundlich plots for C02 sorption
in all four zeolites at 273 and 393 K are shown in Figure 7. The excellent linearity of these plots confirms the applicability of the Freundlich isotherm equation to the C02 sorption data in the cationic forms of the LTL zeolites. n-BA sorption data in Fe3++exchanged Y-type zeolites were also satisfactorily represented19 by linear plots; however, the Freundlich sorption model failed to represent n-BA sorption in Eu-14 zeolites and titanosilicates.20 Chemical Affinity and Selectivity of the Sorbed Phse. When a gas is transferred reversibly and isothermally from the gas phase at a standard pressure PO(760 Torr) into an infinite amount of sorbent-sorbate mixture under equilibrium pressure P,there is a decrease in the potential. Neglecting the nonideality of the sorbate, the chemical affinity may be expressed’ as A p = RT In (PIP,) (3) The value of AN may be taken as the quantitative measure of the chemical affinity of the sorbate for the sorbent. The plots of -AN against the amount sorbed also serve as useful criteria for the comparison of sorption affinities of probe moleculesin the lattices of various cationic forms of zeolites. Typical chemical affinity plots for CO2 sorption in all four cationic forms of zeolite LTL at 273 and 393 K are given in Figure 8. The chemical affinity sequence over the entire temperature range was HKL < KL
