J. Phys. Chem. 1990, 94, 8589-8593
8589
Sorption Properties of EU-1 Zeolites G. N. Rao, P. N. Joshi, A. N. Kotasthane, and V. P. Shiralkar* Catalysis Group, Physical Chemistry Division, National Chemical Laboratory, Pune 41 I 008, India (Received: November 8, 1989; In Final Form: June 8, 1990) Various features of the sorption of benzene, n-hexane, and cyclohexane, H20,n-butylamine on the protonic forms of EU-1 zeolites of varying Si/AI ratio (SAR = 57.5-262.5) are reported. Equilibrium sorption uptake at PIP, = 0.8 and 298 K for benzene (8.50 f 0.50 wt %), n-hexane (8.25 f 0.20 wt %), and cyclohexane (1.55 f 0.25 wt %) was found to be unaffected by varying Si/AI ratio. Equilibrium uptake for water was found to decrease (5.89-3.95%) with the increase in Si/AI ratio, indicating enhanced hydrophobic character of the zeolites on increasing Si/AI ratio. The BET surface area was also found to decrease from 385 to 290 m2/g with increasing Si/AI ratio. Sorption isotherms for n-butylamine (n-BA) in the temperature range 298-423 K were found to be of Langmuir type. The number of n-BA molecules sorbed per unit cell of EU-I zeolites decreases with the increase in Si/AI ratio. n-BA sorption data was satisfactorily represented by Dubinin, BET, and Langmuir isotherm equations. However, the Sips equation, the Koble-Corrigan equation, and the Langmuir and Volmer coefficients (KLand Kv) failed to represent n-BA sorption data in EU-1 zeolites. Chemical affinity curves for n-BA sorption exhibited very sharp decrease with the coverage and showed the highest chemical affinity for the higher aluminum-content (Si/AI = 57.5) sample. lsosteric heats also showed decrease in lower coverage region with complicated behavior in the higher coverage region. The sample with higher content of aluminum @/AI = 57.5) showed higher Q,,values. Introduction
TABLE I: Unit Cell Composition (on Anhydrous Basis) of EU-1
Recent developments in the area of zeolite synthesis has brought about many novel and catalytically high potential silica zeolites, for which no natural counterparts exist. Hydrothermal synthesis for one such member EU-I was first described in European patent applications.’v2 It is believed3 that EU-1 is the first high-silica zeolite to be synthesized with bis(quaternary ammonium) compound (hexanel ,6-bis(trimethylammonium) ions). Subsequently, a range of bis(quaternary ammonium) compounds was used4 as templating species for a variety of high-silica zeolites including EU-I. Studies on crystallization kineticssv6as well as physicochemical characteri~ation’-~ of zeolite EU- 1 have also been reported. Sorption and the catalytic data suggested that zeolite EU-I possesses channels bound by a IO-membered ring and its effective pore width is in the range of that of ZSM-12. Recently disclosedI0 framework topology confirms space group to be , C with measured lattice parameters a = 13.695 A, b = 22.326 and c = 20.178 A, havin a unidirectional 10-ring channel system bound by 5.8 X 4.1 in the [ 1001 direction. However, within these channels, there are side pockets of 6.8 X 5.8 A cross section and 8.1 A depth. In the present paper the sorption properties of EU-1 zeolites with different %/AI ratios are reported. In particular, sorption isotherms with n-butylamine (n-BA) as probe molecule are described. Sorption characteristics are evaluated in terms of various thermodynamic parameters, isotherm equations, and the physical state of a sorbed phase.
Zeolites
x
1,
Experimental Section
Samples of zeolite EU-I with different Si/AI ratio were synthesized in accordance with the procedures described in ref 6. The following oxide mole composition of the reactant mixture was used: aR:bSiO2:AI2O3:cNa20:dH20 where R represents dibenzyldimethylammonium cation (DBDM+), values a, c, and d a r e con( I ) Casci. J. L.; Lowe, B. M.; Whittam, T. V. Eur. Pat. Appl. 42226, Imperial Chemical Industries, 1981. (2) Casci, J. L.; Lowe, B. M.; Whittman, T. V. Eur. Pat. Appl. 2077709A, Imperial Chemical Industries, 1981. (3) Casci, J. L.; Whittam, T. V.; Lowe, B. M. Proceedings of the 6th International Conferenceon Zeolites, Reno; Buttenvorths: Guildford, U.K., 1984; 894. (4) Casci, J. L. Proc. 7ih Ini. ConJ Zeolites, Tokyo; 1986, 215-222. (5) Dodwell, D. W.; Denkewicz, R. P.; Sand, L. B. Zeolites 1985,5, 153. (6) Rao, G. N.; Joshi, P. N.; Kotasthane, A. N.; Ratnasamy, P. Zeolites 1989, 9, 483. (7) Kumar, R.;Ernst, S.;Kokotailo, G. T.; Weitkamp, J. froc. Intl. Symp. Innovation Zeolite Mater. Sci. Niewpoort. Belgium. In Siud. Surf. Sci. Catal. 1988, 37, 45 1. (8) Weitkamp, J.; Ernst, S.; Kumar, R. Appl. Coral. 1986, 27, 207. (9) Deweing. J . J. Mol. Cotol. 1984, 27, 25. (IO) Briscoe, N. A.; Johnson, D. W.; Shannon, M. D.; Kokotailo, G. T.; McCusker, L. B. Zeolites 1988, 8, 74.
0022-3654/90/2094-8589$02.50/0
zeolite (Si/AI)
no. of unit
unit cell composition
celk/g x
stants and b is varied in the range 150-600. Typically zeolite EU-1 (Si/AI = 57.5) was synthesized by using dimethylbenzylamine and benzyl chloride as organic templates. A solution of 0.70 g of sodium aluminate [43.6% (w/w) A1203, 39% (w/w) Na20, 17.40% (w/w) HzO] and 1.82 g of NaOH (98%) in 10 g of distilled water was prepared. This was added slowly to an aqueous suspension of 28.4 g of silica (Microsil Silica 11, 95% S O 2 ) in 75 g of distilled water with vigorous stirring. After the mixture was stirred for 1 h at room temperature, a mixture of 5.7 g of dimethylbenzylamine (Fluka) and 5.5 g of benzyl chloride (Fluka) was added. The resulting gel (pH = 13) was then transferred to a stainless-steel autoclave (250 mL capacity) and was crystallized at 423 K for 15 days without agitation. After the crystallization was complete, the contents of the autoclave were quenched under cold water. The crystalline product so obtained was filtered, washed thoroughly with deionized water, and dried at 383 K in air for IO h. EU-I samples with %/AI = 90, 141.5, and 262.5 were prepared in a similar way. The crystalline solid material was calcined at 8 13 K in a flow of air for 16 h in order to remove the organic matter from the zeolite. The unit cell composition of the corresponding products in Na form (after removal of the template by calcination at 8 13 K) are listed in Table I. Four samples with product Si/AI = 57.5, 90.0, 141.5, and 262.5 were calcined at 813 K for 10 h to remove the template by decomposition. Products so obtained in the sodium form were digested repeatedly with 5% ammonium nitrate solution at 368 K till the residual Na+ content was less than 500 ppm. These samples were deammoniated at 813 K for 8 h to obtain protonic forms which were then used for the measurement of sorption properties. n-Butylamine, cyclohexane, and n-hexane used for sorption measurements were of high purity (>99.99%) and were further dried over activated 3A molecular sieve extrudates and purified by freeze-thaw techniques. Double-distilled water was used for the sorption measurements, and high-purity nitrogen was used for the surface area measurements. Methods
Surface area measurements are carried out by low-temperature (77 K) nitrogen sorption using Accusorb-2800 (Micromeritics). 0 1990 American Chemical Society
8590
The Journal of Physical Chemistry, Vol. 94, No. 23, 1990
Rao et al.
TABLE 11: Somtion Prowrties of EU-1 Zeolites" ~~~
zeolite
(Si/AI) 57.5 90.0 141.5
262.5
~
amount adsorbed, g/IOO g of zeoliteb H20 5.89 (22.0) 5.02 (18.7) 4.52 (16.9) 3.95 (14.7)
surface C6H6 area, m2/g ~ I - C ~ H IC6H12 ~ 385 8.23 (6.44) 1.56 (1.25) 8.04 (6.92) 379 8.20 (6.38) 1.72 (1.37) 8.42 (7.22) 348 8.47 (6.56) 1.90 (1.52) 8.31 (7.16) 290 8.11 (6.32) 1.88 (1.50) 8.51 (7.28)
" p / p 0= 0.8 at 298 K . *Figures in parentheses are sorption capacities in molecules/UC.
The sorption measurements were carried out on an all-glass gravimetric apparatus using a McBain-Baker type silica spring (sensitivity =50 cm/g) balance described earlier." About 70 mg of sample was used in a pellet form. The balance case of the apparatus containing the sample was enclosed in a thermostatic arrangement and the temperature was maintained within *I K by using an Aplab temperature controller. Prior to any sorption measurement, the zeolite sample was degassed at 673 K. The temperature of the sample was raised with the heating rate of 2 K m i d with simultaneous evacuation at lo4 Torr. The sample was degassed at 673 K for IO h and was then cooled to 298 K and was maintained at 298 K for at least 2 h before the commencement of the measurement. The sorbate vapors were then contacted with the sample at a relative pressure of 0.8 at 298 K. The sorption kinetics were then carried out upto 120 min to determine the equilibrium sorption capacity. The same sample was then again activated by degassing under vacuum, as described above, before carrying out the sorption kinetics with other sorbate molecule. The sorption isotherms in the temperature range 298-423 K were measured upto a pressure of 80 Torr following the procedure described elsewhere.I2 About 70 mg of sample in a pellet form was also used in this case and the sample activation procedure was essentially the same as described above. After activation, the balance case containing the sample was maintained at the isotherm temperature for at least 2 h before the commencement of the measurements. The absorbate pressure was measured with cathetometer and the amount sorbed was measured accurately from the change in the weight of the sample after equilibriating for 2 h at each equilibrium pressure. The sorption isotherm was obtained by a progressive increase in the vapor pressure by noting the amount sorbed. Sorption isotherm was obtained initially at the lowest (298 K) temperature and then subsequently at the next higher temperature after carrying out activation procedure. In order to check the reversibility of the sorption, desorption measurements were carried out. After each isotherm the sample was evacuated at IO4 Torr at least for 8 h. The X-ray diffractograms were recorded for each sample before and after the sorption measurements to check the structural stability.
I------
-
Results and Discussion Equilibrium Sorption Capacities and BET Surface Areas. Table I gives the unit cell compositions of different samples of EU-I zeolites used in the present studies for evaluating their sorption characteristics. Prior to the sorption measurements, samples were ammonium exchanged and were subsequently deammoniated to convert them in protonic forms. Table l l summarizes the equilibrium sorption capacities at 298 K, and PIP, = 0.8 and BET surface areas for all the EU-1 samples. The salient features of the Table 11 include almost constant equilibrium sorption capacities for n-hexane, cyclohexane, and benzene, irrespective of the %/AI variation from 57.5 to 262.5. Uptakes of cylindrical n-hexane molecule (molecular diameter = 4.2 A and of benzene (molecular diameter = 5.85 A) molecule are almost the same (around 8.5 0.4 wt %). On the contrary, equilibrium uptake of the comparatively larger and nearly spherical cyclohexane molecule (molecular diameter = 6 . 2 A) is only 1.9%. Uptake of water decreases from 5.89 to 3.95% with increase in
*
I
3
,
, 20
,
, 40
,
, 60
,
,
80
/
,
, 20
,
/
,
,
I
80
P (torr) Figure 1. n-BA sorption isotherms for EU-I zeolites with Si/AI ratio (A) 57.5, (B) 90.0, (C) 141.5, and (D) 262.5 at ( I ) 298 K, (2) 323 K, (3) 348 K, (4) 373 K, (5) 398 K, and (6) 423 K.
%/AI ratio from 57.5 to 261.5. Uptake of water is always taken as a measure of hydrophobicity/hydrophilicitywhich is dependent on AI content of the zeolite. BET surface area is also found to decrease from 384 to 290 m2/g with increase in %/AI ratio. It seems, therefore, that crystallization of EU-I with higher Si/AI ratio is operative in decreasing the available void volume on account of increase in crystallite size from 2-4 to 5-8 pm. Figure 1 shows families of isotherms of sorption of n-butylamine in EU-1 zeolites with different Si/AI ratios. Most of the isotherms exhibit type 1 (Langmuir type) isotherm according to Kiseleve's classification.i3 Almost 75% of the total sorption takes place over a very narrow range of relative pressures. Initially, basic n-BA molecules interact with the sorption centers of higher strength (usually acidic hydroxyls in this case) and thus get solvated, and then the volume-filling phenomenon may be operative. Sorption isotherms show that equilibrium sorption capability (molecules/UC) (UC = unit cell) is found to decrease continuously with the increase in Si/AI in the zeolite. This seems to be reasonable if we assume that acidic protons (hydroxyls) obtained via deammoniation and surfce silanol groups and residual sodium ionst4 act predominantly as sorption centers for basic n-BA molecules. The number of acidic hydroxyls is proportional to the framework aluminum and thus as Si/AI increases (i.e., AI decreases), the number of acidic protons also decreases and hence the decrease in sorption capacity. Figure 2 shows systematically a typical dependence of decrease in sorption capacity with the decrease in aluminum content in EU-1 zeolites over the entire range of isotherm temperature (298-423 K). Although points for lower aluminum content are not available on account of inability of crystallizing EU-1 zeolite framework with Si/AI > 600, the decreasing trend of each of the curves in Figure 2 is apparent and each curve passes through the origin. Application of Isotherm Equations: Dubinin Isotherm Equation. Analysis of the sorption data in terms of various isotherms equations always yields useful information. An attempt is made here to apply Polanyi's potential theory modified by for n-BA sorption over the entire Dubinin and Rad~shkevichl~ range of temperature (298-423 K) in zeolites EU-1 with Si/AI ~~
( I I ) Shiralkar, V. P.;Kulkarni, S. B. Z . Phys. Chem. ( k i p r i g ) 1984, 265, 313. (12) Shiralkar. V . P.; Kulkarni. S. B. Zeolites 1984, 4 , 329.
,
,
60
40
~~
(13) Kiselev, A. V . Discuss. Faruduy SOC.1965, 40, 205. (14) Kulkarni, S.J.; Kulkarni, S. B. Indian J . Chem. 1989, 28A, 6. ( 1 5) Dubinin, M. M.;Radushkevich, L. V. Proc. Acod. Sci , USSR 1974, 55, 321
Sorption Properties of EU- 1 Zeolites
The Journal of Physical Chemistry, Vol. 94, No. 23, 1990 8591
TABLE HI: Saturation Capacities‘ and B/B’ for Eu-1 Zeolites from Dubinin Plots 298 K 323 K 348 K 373 K zeolite B/S2 SC B/S2 SC B/S2 SC B/S2 SC (Si/AI) 57.5 90.0 241.5 262.5
0.144 0.188 0.190 0.201
0.097 0.158 0.184 0.189
4.45 4.15 3.65 2.67
3.30 3.19 3.00 2.17
0.086 0.107 0.156 0.178
2.74 2.41 2.25 1.59
0.062 0.099 0.115 0.152
398
2.30 2.00 1.69 1.10
K
423
K
B/S2
SC
B/S2
SC
0.055 0.081 0.112 0.114
1.94 1.69 1.38 0.83
0.053 0.069 0.100 0.110
1.54 1.40 1.20 0.57
“SC, expressed in molecules/UC. I
I
10
P
PP I,
x lo3
PIS
lo3
Figure 4. BET plots for n-BA sorption at (A) 298 K and (B) 423 K in EU-I zeolites with Si/AI ratio (1) 262.5, (2) 141.5, (3) 90.0, and (4) 57.5. a is the amount sorbed in molecules/UC. 0
0.5
1.5
I
AI/
2 .0
U.C
Figure 2. Sorption capacity as a function of AI/UC at ( I ) 298 K, (2) 323 K, (3) 348 K , (4) 373 K, (5) 398 K, and (6) 423 K. .7
P (torr )
P(torr)
Figure 5. Langmuir plots for n-BA sorption at (A) 298 K and (B) 423 K in EU-1 zeolites with %/AI ratio ( I ) 262.5, (2) 141.5, (3) 90.0, and (4) 57.5. a is the amount sorbed in molecules/UC.
c I
0
2
4
6
8 2
10
I
12
[Log P,/p] Figure 3. Dubinin plots for n-BA sorption in EU-I zeolite (Si/AI = 57.5) at ( I ) 298 K, (2) 323 K, (3) 348 K,(4) 373 K,(5) 398 K, and (6) 423 K. a is the amount sorbed in molecules/UC.
ratio ranging from 57.5-262.5. The Dubinin-Radushkevich equation is expressed as log W = log Wo - B/2.303p2[T log Po/P]’ (1) where W is the amount sorbed at equilibrium pressure P, W, is the total sorption capacity, B is a constant independent of temperature and characteristic of sorbent pore structure, and p is the affinity coefficient. The Dubinin plots so obtained are reasonably linear and they deviate from linearity with increasing Si/AI. Since
the sorption capacity decreases with the decrease in aluminum content in the zeolite, the slight deviation of the linearity of Dubinin plots may be probably due to an experimental error. Typical Dubinin plots for n-BA sorption in EU-1 zeolites are shown in Figure 3. The saturation capacities and B / b 2 (6 is affinity coefficient) obtained from intercept on the Y axis and slopes respectively of these linear plots are tabulated in Table 111. Saturation capacities obtained from Dubinin plots are, in general, in close agreement with those obtained experimentally, indicating that the data on sorption of n-BA in EU-I zeolites (Si/AI < 141.5) would be satisfactorily represented by the Dubinin-Radushkevich equation. Saturation capacities decrease with the increasing temperature and also with the Si/AI ratio in the zeolite. The affinity coefficient, on the other hand, is found to increase with increase in temperature and decrease in Si/AI ratio. BET Isotherm Equation. The application of BET equation to the sorption of n-BA in EU-1 zeolites yields linear plots and typical plots are shown in Figure 4. The saturation capacities (monolayer capacities) obtained from the slopes of these linear plots (Table IV) are in good agreement with those obtained by the Dubinin equation. The linearity of the plots indicates that the n-BA sorption data for EU-1 zeolites can be described by the BET sorption model. All the linear plots pass through the origin and therefore the constant C in BET equation is very high. Langmuir Isotherm Equation. We tried to fit n-BA sorption data to the Langmuir isotherm equation based on monolayer
8592
TABLE IV: Comparison of Saturation Capacities of EU-1Zeolites temp, K 298 323 348 373 398 423
Rao et al.
The Journal of Physical Chemistry, Vol. 94, No. 23, I990
method exptl BET Langmuir exptl BET Langmuir exptl BET Langmuir exptl BET Langmuir exptl BET Langmuir exptl BET Langmuir
saturation capacities (molecules/UC) for zeolites with %/AI 57.5 90 141.5 262.5 4.25 3.87 3.36 2.62 3.47 3.30 2.38 4.09 3.88 3.37 2.63 4.25 3.00 2.78 2.20 3.15 2.68 2.01 2.70 2.87 3.00 2.80 2.21 3.15 2.35 2.07 1.58 2.67 2.10 2.02 1.42 2.53 2.36 2.08 1.60 2.69 2.28 1.98 1.61 1.10 2.02 1.77 1.45 I .01 2.30 1.98 1.63 1.10 1.93 1.69 1.33 0.82 1.76 1S O 1.21 0.70 1.94 1.61 1.34 0.82 I .52 I .36 I .25 0.57 I .44 1.30 1.03 0.51 1.15 0.58 1.53 1.36
approach. Langmuir plots so obtained were linear for all the samples at all the isotherm temperatures and typical plots are shown in Figure 5. Again saturation capacities (Table IV) obtained from the s l o p of these plots are in close agreement with those obtained from the Dubinin equation and the BET equation. Both the Langmuir and the BET equations could yield linear plots for sorption of an acidic molecule like C 0 2 in cation-exchanged Y type zeolites;I2and also the sorption data of basic molecule like n-BA in ferric-exchanged Y zeolites when analyzed by applying Langmuir isotherm equation gave linear p10ts.I~ It was also suggested l4 that n-BA sorption is localized and a basic molecule like n-BA possesses a strong interaction with acidic protons along with other acidic species. In the present studies also, on account of localized sorption of n-BA and strong interaction with acidic protons in EU-I zeolites, both Langmuir and BET isotherm equations yield linear plots and they represent n-BA sorption satisfactorily. Sips' equationI6 based on localized sorption with sorbatesorbate interaction was satisfacorily applied to CO2I2 and NH3I7 in cation-exchanged Y zeolites. However, Sips' equation was found not applicable in the present studies to the n-BA sorption in EU-1 zeolites. Sips' equation usually takes care of any deviation from the Langmuir approach. If the sorption is assumed to be chemical reaction between sorption centers and sorbate molecules, Langmuir equation results with I : ] correspondence; and if some tolerance is made for the complicating factors, the Sips equation results. At low pressure, the Sips isotherm reduces to the Freundlich isotherm equation and it also did not yield linear plots of n-BA sorption in EU-I zeolites. This means that there is no complicating factor in n-BA sorption in EU-I zeolites, which causes it to deviate from the Langmuir approach (localized sorption with 1:l correspondence between sorption center and sorbate molecule). Similarly, the Koble-Corrigan18 isotherm equation based on the exact solution for dissociative sorption of sorbate molecules on two active centers could represent sorption of CO2I2in ion-exchanged Y zeolites, but the same approach could not represent the n-BA sorption data in EU- I zeolites. This then clearly indicates that n-BA sorption in EU-I zeolites does not allow the assumption made during the derivations of the Koble-Corrigan equation of dissociative sorption of sorbate molecules on two active centers. Physical models, such as those of Langmuir and Volmer, for the sorbed state describe the idealized systems and may not be applicable to the real systems on account of deviations arising from surface heterogeneity, multilayer formations, and mutual interactions between sorbed molecules. Nevertheless, these equations have been to give a satisfactory analysis of equilibria (16) Sips, R. J . Chem. Phys. 1948,16, 491. (17) Shiralkar, V . P.: Kulkarni. S. B. J . Colloid Interface Sci. 1985,108,
I
I
2
4
3
I
I
n -EA, Molecules / U
2
4
C
Figure 6. Chemical affinity curves for n-BA sorption a t (A) 298 K and (B) 423 K in EU-I zeolites with Si/AI ratio ( I ) 262.5, (2) 141.5, (3) 90.0, and (4) 57.5.
in energetically homogeneous systems and provide a basis for discussion of the extent of deviations in energetically heterogeneous systems. However, in the present studies on the sorption of n-BA in EU-I zeolites, plots of KL,Kv In KL,and In Kv against the coverage were nonlinear. The KLvs 8 plots were curved, showing increase upto a coverage of 0.8 then passing through a maximum followed by sharp decrease upto 8 = 1.0. In KL vs 8 plots resulted in a characteristic curve with a gradual increase upto 8 = 0.9 and thec a very sharp increase upto 8 = 1. Nonlinearity of these plots indicates virtual absence of sorbate-sorbate interaction in the localized sorption and these data also do not indicate mobile sorption of n-BA in EU-1 zeolites with or without sorbatesorbate interaction. Therefore, no meaningful deductions were obtained from these plots. Chemical Affinity and the Selectivity of the Sorbed Phases. A decrease in chemical affinity takes place when a gas at a standard pressure Po is transformed reversibly and isothermally into an infinite amount of sorbent-sorbate mixture over which the equilibrium pressure is P. The chemical affinity, when the nonideality of the sorbate is neglected, may be expressed as Ap =
R T In ( P / P , )
(2)
The value of -Ap is often takenIg as the quantitative measure of the chemical affinity of the sorbate for the sorbent. The plots of -Ap against amount sorbed also serve as useful criteria for the comparison of the sorption affinities of probe molecules in the lattices of various cation-exchanged zeolites. Typical chemical affinity curves for n-BA sorption in EU-1 zeolites are shown in Figure 6. The Figure 6 shows that there is a continuous decrease in the chemical potential as the coverage increases and the decrease becomes more and more sharp as the temperature of the sorption increases. The decrease in the chemical affinity also becomes comparatively more sharp as the Si/AI ratio in the zeolite decreases (aluminum content of the zeolite increases). Therefore, the affinity follows the sequence as EU-1 (57.5) > EU-1 (90) > EU-1 (141.5) > EU-I ( 2 6 2 . 5 ) , over the entire coverage within the isotherm temperature range 298-423 K. Isosteric Heats of Sorption (QJ. The isosteric heat of sorption ( Q S t is ) derived from sorption isosteres by applying the Clausi-
I.
(18) Koble, R. A,; Corrigan, T. E. Ind. Eng. Chem. 1952,4 4 , 383.
3
(19) Shiralkar, V . P.;Kulkarni, S. B. Zeolires 1985,5 , 37.
The Journal of Physical Chemistry, Vol. 94, No. 23, 1990 8593
Sorption Properties of EU-I Zeolites
TABLE V: Isosteric Heats (Q,) of n-BA Sorption on EU-1Zeolites zeolite (Si/AI)
Q,,,kJ mol-'" 0.50
57.5 90.0 141.5 262.5
1.oo
0.75
14.5
1.25 119.0 57.0
51.5
49.0 41 .O
39.0
41.5
1.50 54.0 39.0 52.0 49.0
1.75 61.5 36.5 42.5 48.5
2.00 46.0 52.0 49.5 50.0
3.25 40.0 63.5 36.0
2.50 49.0 65.0 34.0
'At coverages (n-BA molecules/UC).
-
NH3 sorption and in Fe3+-exchanged zeolites of type Y14 during n-BA sorption. Q,,curves for ammonia sorption in cation-exchanged Y zeolites20showed continuous but steady decrease over the entire coverage. However, in the present case of n-BA sorption in EU-1 zeolites, Q,, decreased in the lower coverage region and in the higher coverage region maxima and minima were observed. This indicates that excepting the few protons of higher acid strength most of the protons (generated upon deammoniation of NH,-EU-1) are of medium acid strength.
-"
c -
D
C
I -
, 2 5
I
I
29
33
I
25
2 9
33
Figure 7. lsosteres for n-BA sorption in EU-1 zeolites with Si/AI ratio (A) 57.5. (B) 90.0, (C) 141.5, and (D) 262.5. Figures on the isosteres indicate the amount sorbed in molecules/UC.
us-Clapeyron equation at constant sorbate loading using the equation
-AH = Q,, = R(T2TI/T2 - TI) In ( P 2 / P I )
(3)
If Q,, is temperature independent, the plots of In (P) against l / T should be linear. In the present studies isosteres for n-BA sorption in EU-I zeolites were found to be linear as shown in Figure 7. The linearity of these isosteres therefore justifies the assumptions of independence of Q,,on temperature. Table V lists the isosteric heats of n-BA sorption in EU-I zeolites. Since this is an indirect method of evaluation of isosteric heat, Table V shows the variation in Q,, values for all the zeolites over the wide range of coverage. At the coverage of 1. I 1 molecules/UC, the sequence in the isosteric heat is EU-I (57.5) [I30 kJ mol-I] > EU-I (90) [60 kJ mol-I] > EU-I (141.5) [53 kJ mol-I] > EU-I (262.5) [42 kJ mol-']. This indicates that isosteric heats (QSJ at lower coverage decrease with the decrease in aluminum content (increase in Si/AI ratio) in the zeolite. Similarly Table V shows that initially the Q,,values decrease with the coverage, Le., n-BA molecules initially interact with acidic framework hydroxyls of higher strength and once these hydroxyls get solvated by n-BA molecules they interact with framework hydroxyls of lower strength. The initial decrease in the Q,, becomes sharper as the aluminum content increases (Le., Si/AI decreases). Similarly, the sharp fall in Q,, over a very narrow range of coverage indicates that there are very few acidic protons of higher strength. In the higher coverage region the sequence becomes complicated and it may be partly due to surface heterogeneity on account of protons obtained via deammoniation of the ammonium zeolite. Such surface heterogeneity was also observed in case of La3+-exchanged zeolites of type Y Z 0during (20) Shiralkar, V. P.; Kulkarni,S. B. J . Colloid Inregace Sci. 1986, 109,
1 IS.
Conclusions Equilibrium sorption capacities in the protonic forms of EU- 1 zeolites for almost nonpolar sorbate molecules like benzene, cyclohexane, and n-hexane were found to be unaffected by variation of the Si/AI ratio in the zeolite. Volume-filling phenomenon seems to be operative in case of sorption of these molecules and they do not interact strongly with the nonframework cations associated with the framework alumina tetrahedra. In spite of having 10membered ring pore opening in all the EU-1 and ZSM-5, -22, -23, and -48 zeolites, cyclohexane sorption capacity is higher in ZSM-48 and considerably higher in ZSM-5 than in the rest of the zeolites. Pore opening of a more elliptical nature in EU-1 zeolite does not seem to be accessible to the cyclohexane molecule which thus gets sorbed on the external surface only. This leads to the lower and nearly constant cyclohexane sorption in EU- 1 zeolites, contrary to that in ZSM-5 which decreases with the increase in Si/AI ratio in the zeolite. Based on the sorption capacities of n-hexane and benzene, the void volume in EU-1 zeolite seems to be lower than in ZSM-5 (due to dual channel system and channel intersection) and comparable to that in ZSM-22, -23, and -48 zeolites (having unidirectional channels with side pockets in EU-1) with the same Si/Al ratio. The decrease in equilibrium sorption capacity of polar molecules like water and n-BA (from sorption isotherms) with the increase in Si/AI ratio was found to be consistent with decrease in aluminum content in the zeolite. Both these polar molecules, especially basic n-BA molecules, interact strongly with protons associated with negatively charged alumina tetrahedra. Hence the decrease in framework aluminum content in the zeolite resulted in decrease in sorption capacities of both the molecules. BET surface area also decreased from 385 to 290 m2 g-' with increase in Si/Al ratio from 57.5 to 262.5 in the zeolite. BET, Langmuir, and Dubinin isotherm equations were found to applicable to n-BA sorption isotherm data in EU-I zeolites. Nonlinear plots for KL,In KL and Kv and In Kv against n-BA coverage indicated nonapplicability of mobile sorption model and absence of sorbate-sorbate interaction. Chemical affinity curves showed sharp decrease with coverage. Isosteric heats (Q,,) showed decrease in the lower coverage region with complicated behavior in the higher coverage region. Higher aluminum containing zeolite showed higher Q,, values. Acknowledgment. G.N.R. thanks CSIR for the award of fellowship. Thanks are also due to Dr. Paul Ratnasamy for his constant encouragement. This work was partly funded by UNDP. Registry No. n-BA, 109-73-9; benzene, 71-43-2; n-hexane, 1 10-54-3; cyclohexane, 1 10-82-7; water, 7732-18-5.
