Energetics of Ammonia Sorption in Alkali Metal Exchanged

J. Phys. Chem. B 2001, 105, 10637-10647

10637

Energetics of Ammonia Sorption in Alkali Metal Exchanged Analogues of Linde Type X Zeolites U. D. Joshi,† P. N. Joshi,‡ S. S. Tamhankar,‡ V. V. Joshi,§ and V. P. Shiralkar*,‡ Catalysis DiVision, National Chemical Laboratory, PUNE 411 008, India, Netaji Subhash Chandra Bose College, NANDED 431 601, India, and Science College NANDED 431 601, India ReceiVed: April 9, 2001; In Final Form: August 17, 2001

The parent zeolite NaX was prepared from hydrogel system 3.6 Na2O:3 SiO2:Al2O3:144 H2O at 368 K for 8 h under static condition. Postsynthesis modification was carried out using the conventional ion-exchange technique to obtain different ion-exchanged forms. The parent as well as exchanged samples were characterized by chemical analysis, IR, SEM, powder XRD, and low temperature nitrogen sorption. The ammonia sorption isotherms in parent NaX (Si/Al ) 1.15) and its modified forms with a nearly equal degree of exchange by K+, Rb+, and Cs+ cations have been measured in the temperature range of 303-453 K up to 500 Torr. The equilibrium sorption uptake at 50 Torr and especially at temperatures above 363 K was found to follow the sequence NaX > NaK(53)X > NaRb(53)X > NaCs(58)X. The analysis of the ammonia sorption data in terms of different isotherm equations revealed satisfactory representation by Langmuir, Dubinin, BET, and Sips equations and statistical models of Langmuir and Volmer. However, the Freundlich equation failed to represent ammonium sorption data at higher pressures. All of the parent and exchanged samples were compared in terms of chemical potential of ammonia sorption. Isosteric heat of ammonia sorption data revealed the higher heterogeneity of sorbent surface of Cs+-exchanged samples than those of other analogues. The variations in the charge density of the extraframework cations were found to influence the ammonia sorption energetics via alteration in Lewis acid-base character.

1. Introduction Zeolite X, an aluminum-rich end member of the faujasite family, has been extensively used in the industrial applications such as adsorption, ion exchange, and catalysis1, in consequence of its open framework (12 MR), large number of exchangeable cations, and higher framework charge. Postsynthesis modification of zeolite type X by ion-exchange especially with alkali metal cations with varying sizes is expected to alter the strength of Lewis base centers in addition to their usual Lewis acidic properties. Generally, Lewis acid-base character of the zeolites arises because of the formation of Lewis acid-base pairs when the basic sites (framework oxygen) are associated with the Lewis acid sites (cations). Therefore, by keeping framework Si/Al ratio constant in a given structure, the intrinsic framework basicity of the zeolite can be increased by decreasing the intermediate electronegativity (Sint) by exchanging the extraframework cations with low electronegative cations such as K+, Rb+, and Cs+. Thus, the Lewis acid-base character of the zeolite can be subtly tuned by the proper choice of the nature and population of the extraframework cations, keeping the framework composition (Si/Al ratio) undisturbed. Such nonprotonic forms of zeolite X have an added advantage in adsorption processes wherein the chances of any catalytic transformation of the adsorbate on acid sites is low. Various cationic forms of the X-type zeolite and others have been reported2,3 as potential adsorbents for practical applications and/or fundamental studies. * To whom correspondence should be addressed. Fax: +91-20-5893761. E-mail: [email protected]. † Netaji Subhash Chandra Bose College NANDED. ‡ National Chemical Laboratory. § Science College NANDED.

The steric as well as energetic properties are of the prime importance to decide the best fit of any adsorbent for a given adsorptive process. These properties are being increasingly used as model substances for studying the interactions of the adsorbed molecules with solid surfaces. Even though several studies on the influence of type4-6 and concentration7-14 of the exchanged cations in faujasite on the sorption properties have been disclosed using different sorbates, no systematic work has been reported in relation to energetics of ammonia in zeolite X with different Lewis acid-base character that resulted from exchanging it with an almost identical population of monovalent cations with different charge density. In the present studies, the ammonia sorbate was selected as a probe molecule on account of its (1) large dipole moment, (2) size small enough to access the sodalite cavities,15 (3) pronounced stabilization effect,16 tendency to produce diffusion block,17,18 and (4) basic nature. In this paper, we report the results on ammonia sorption in parent NaX and its modified form viz NaKX, NaRbX, and NaCsX, wherein the number of exchanged cations per unit cell are almost identical. The effect of different Lewis acid-base character (cationic and framework oxygen charge densities) of these sorbents on the equilibrium sorption capacities, application of various isotherm equations, physical state of sorbate, free energies and isosteric heats of sorption will be discussed. 2. Experimental Section 2.1. Materials. Zeolite NaX was prepared from the hydrogel system with the composition in terms of oxide as 3.6 Na2O:3 SiO2:Al2O3:144 H2O by hydrothermal crystallization at 368 K for 8 h. The raw materials used for the preparation of the

10.1021/jp0113174 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/03/2001

10638 J. Phys. Chem. B, Vol. 105, No. 43, 2001

Joshi et al.

TABLE 1: Unit Cell Composition (on Anhydrous Basis) and Related Details of Cation Exchanged Type X Zeolites sample designation NaX NaK(53)X NaRb(53)X NaCs(58)X

unit cell composition

unit cells/ g x 1019

Sint

framework oxygen charge (δO)

Na 89.4 [(AlO2)89.4 (SiO2)102.6] Na42.02 K47.38 [(AlO2)89.4 (SiO2)102.6] Na42.02 Rb47.38 [(AlO2)89.4 (SiO2)102.6] Na37.55 Cs51.85 [(AlO2)89.4 (SiO2)102.6]

4.46 4.22 3.66 3.13

3.23 3.12 3.08 3.00

0.417 0.441 0.448 0.464

homogeneous hydrogel were a water glass solution (28.5% SiO2, 8.5% Na2O, 63.0% H2O), sodium aluminate (43.65% Al2O3, 39% Na2O, 17.35% H2O), sodium hydroxide (AR grade, LOBA), and distilled water. The solid product was separated and thoroughly washed with hot water till free from excess alkali and other soluble species. The product was then dried for 8 h in an air oven maintained at 383 K. The post-synthesis modification of NaX was carried out by conventional ionexchange technique, using 5 wt % of aqueous solutions of K, Rb, and Cs chloride salts. The aqueous salt solution was taken in the proportion of 15 mL/g of zeolite, for each exchange experiment. The different degrees of exchange were achieved by repeating the cation exchange treatments at 368 K. The solid was separated by suction filtration and then washed with distilled water till the wash water was free from chloride. The solid was then dried in an air oven maintained at 383 K for 8 h. From the various modified forms, four samples were selected on the basis of almost identical degree (55.5 ( 2.5%) of exchange by K+, Rb+, and Cs+ cations. Highly purified ammonia (purity > 99.9%) was used after passing it over freshly ignited calcium oxide, potassium hydroxide pellet, and 3 Å activated molecular sieves. 2.2. Methods. The chemical compositions of parent and exchanged samples were determined by conventional wet gravimetric methods in conjunction with atomic absorption spectrometry (Hitachi, Z 8000) and inductively coupled plasma emission spectroscopy. In the case of exchanged samples, the amount of sodium was also estimated by an atomic absorption spectrometer from the filtrate collected after ion-exchange treatment to confirm the degree of exchange. Framework IR spectra of all of the samples were recorded on a Pye-Unicam SP-300 spectrometer using a Nujol mull technique. The crystallite size and morphology of all of the samples under investigation were examined by SEM (model JEOL, JSM-5200, Japan). The phase purity and crystallinity of the as-synthesized and exchanged forms were examined by powder X-ray diffraction. The powder XRD patterns were collected over a 2θ range of 5-40° using Ni filtered Cu KR (λ ) 1.54041 Å) radiation using a Rigaku D Max-III VC X-ray diffractometer. Surface area measurements were carried out by low temperature (78 K) nitrogen sorption using a BET volumetric apparatus following the procedure describe earlier.1 Ammonia sorption isotherms were measured at an interval of 30 K in the temperature range of 303-453 K on an all-glass gravimetric apparatus using a McBain-Baker type Silica Spring (sensitivity ≈ 50 cm/g) balance described elsewhere.19 A 70 mg sample in binder-free pellet form was degassed in a vacuum (10 -6 Torr) at 673 K for 10 h. After degassing to a constant weight, the balance case containing the sample was maintained at the isotherm temperature for at least 2 h before the commencement of the measurement. The temperature accuracy throughout the sorption was maintained within (1 K using an Aplab temperature controller. The sorbate pressure was measured with a cathetometer, and the amount sorbed was measured accurately from the gain in the weight of the sample after equilibrating for 2 h, at each equilibrium pressure. The sorption

isotherms were obtained from the data collected by a progressive increase in the sorbate vapor pressure up to 500 Torr and the corresponding gain in the amount sorbed at a desired temperature. Similar steps were followed for obtaining another isotherm at sequentially lower temperature after subjecting to the same degassing procedure prior to the sorption measurement. The XRD pattern was also recorded for each sample after the sorption measurements to confirm the structural damage if any. Sanderson’s electronegativity equalization principle was used to evaluate the intermediate electronegativity and the partial charges on the atoms. 3. Result and Discussion Unit cell composition, number of unit cells/g, intermediate electronegativity (Sint), and the framework oxygen charge (δO) of the parent and cation exchanged samples used in the present studies are tabulated in Table 1. The samples were designated in such way that the type of cation being exchanged in NaX is denoted along with their degree of exchange in the bracketed figure. The number of exchanged extraframework cations estimated in the solid by chemical analysis was found to be in close agreement with the number of sodium cations estimated from the effluent collected after ion exchange followed by washing. It is seen from the Table 1 that, depending on the nature and concentration of the exchanged cations, the variation in the number of unit cell/g was found to be in the range of 3.13-4.46 × 1019. Moreover, the type and contribution of different cations in the total number of charge balancing cations [M/(Na+M) where M ) K or Rb or Cs] was found to be operative in the alteration of intermediate electronegativity (Sint) and the partial charge (δ) on framework and nonframework atoms. It can be clearly seen from Table 1 that the partial charge on the framework oxygen increases with the decrease in the charge density of the exchanged cation. The mid-infrared region of the framework IR spectra (2001300 cm-1) is useful to provide structural information as it contains the fundamental vibrations of the Si(Al)O4 groupings. The obtained IR spectrum of the parent sample is in close agreement with that reported in the literature.20 The framework of the IR spectra of the modified samples revealed no shift to higher frequency in the band at 974 and 673 cm-1, assigned20 to stretching modes, which are sensitive to framework Si/Al composition. Thus, it seems that no dealumination has occurred during the ion-exchange procedure. It was, therefore, concluded that the framework composition (Si/Al ratio) remained unaltered during the cation exchange process followed in the present studies. Chemical analysis data also support this conclusion. The SEM photographs of the parent and all modified forms have shown the crystallites of 2-3 µm size and nearly of spherical shape. The powder XRD profiles of parent and modified samples essentially showed that the characteristic peaks closely match with those reported in the literature.21 The absence of impurity peaks and amorphous halo22 region indicated highly pure and

Energetics of Ammonia Sorption

J. Phys. Chem. B, Vol. 105, No. 43, 2001 10639

TABLE 2: Surface Area and Void Volumes of the NaX and Its Exchanged Forms Dubinin micropore sample designation

surface area (m2/g) BET Langmuir

NaX NaK(53)X NaRb(53)X NaCs(58)X

925 834 721 587

998 912 799 648

volume (mL/g) 0.350 0.321 0.280 0.230

crystalline nature of all of the phases under investigation. However, the change in the relative intensities was observed in the modified samples without any shift in the respective peak positions when compared with those of the parent NaX sample. The variation in the charge-to-size ratios of extraframework cationic species, which are operative in altering the scattering power of the X-rays and in framework distortion to some extent, are responsible for the change in the relative intensities. The results of powder XRD, framework IR, SEM, and chemical analysis clearly indicate that the modification by ionexchange technique has not resulted in any change as far as framework composition (Si/Al ratio), phase purity, crystallinity, and crystallite size of parent NaX and its exchange forms are concerned. The textural characteristics such as surface area and micropore volume were evaluated from the low temperature nitrogen adsorption isotherm data and are summarized in Table 2. The specific surface areas obtained by applying the Langmuir approach were found to be rather higher than those obtained by applying the conventional BET approach. In view of the linearity of Langmuir plots over wide range of relative pressures

compared to BET plots, the surface area obtained by the Langmuir approach probably represents a more realistic estimate. Despite having identical crystallite size and shape for all of the samples, the increased value of the surface area and the micropore volume are consistent with the reduction in the sizes of the nonframework-exchanged cation. The decreasing trend in both the specific surface area and micropore volume as a function of increasing size of exchanged cation indicated that all of the exchanged cations are accessible to nitrogen molecules and the extent of electrostatic interaction energy with the cations is found to be commensurate with the cationic charge density. 3.1. Ammonia Sorption Isotherms. Families of isotherms of ammonia sorption are presented in Figure 1 for parent NaX and its modified forms with almost identical population (49 ( 2 per unit cell) of K, Rb, and Cs cations. The samples were selected on the basis of identical degree of exchange so as to study the influences of cationic charge, electronegativity and nature on ammonia sorption. It is clearly seen from the figure that the shape of the isotherms approximates to the type I (Langmuir type) according to Kiselev’s classification.21 It is interesting to note that, at lower (303 K) temperature, all of the samples have shown almost 70-80% of the total uptake over a very narrow range (within 50 Torr), whereas in the same pressure range, only 25-55% of the total uptake has been exhibited at higher (453 K) temperature. The extent of uptake was found to depend on both the nature of the extraframework cation and the temperature at which the isotherms were measured. The equilibrium sorption uptake at 50 Torr and especially at higher temperatures (>363 K) follows the sequence NaX > NaK (53) X > NaRb (53) X > NaCs (58) X. A similar trend was reported23 on the basis of the number of ammonia

Figure 1. Isotherms for ammonia sorption in (A) NaX, (B) NaK(53)X, (C) NaRb(53)X, and (D) NaCs(58)X.


Joshi et al.

Figure 2. Langmuir plots for ammonia sorption in (A) NaX, (B) NaK(53)X, (C) NaRb(53)X, and (D) NaCs(58)X.

molecules required to saturate one-eighth of a unit cell. Eventhough, the occupancy of the different exchange positions in the parent and modified forms is not known, it is believed that cations are distributed over the highest charged sites in such a way that the free energy of the structure is minimized. Because sites I, I′, and II′ are inaccessible to ammonia molecules on account of steric hindrance, the above equilibrium ammonia sorption sequence suggests that the exchanged extraframework cations may be located only in site II (in a supercage adjacent to the 6 ring outside a sodalite cage) and in site III (in a supercage in the neighborhood of 4 ring). Furthermore, the above trend also indicates that cations with lower charge density will have lower extent of electrostatic interactions with the sorbate molecules. However, cations located within the supercage may experience lower electric field around themselves on account of partial shielding, and thus, the electrostatic and induction interactions are likely to be lower than those of an isolated one. Because ammonia molecules interact mainly with the extraframework cations and framework oxygen, the dispersion forces acting on the sorbate molecule will also be altered to different extents depending upon the charge density of the extraframework cations and framework oxygen as well. At higher pressures and lower temperatures, the sequence in ammonia sorption capacities of the parent and modified X zeolites samples become rather complicated on account of simultaneous variations in contributions due to different energy components such as dispersion and repulsion energies between ammonia and the zeolite, polarization energy, field dipole energy, field gradient-quadrupole energy, and self-potential. 3.2. Application of Isotherm Equations. 3.2.1. Langmuir Sorption Model. A Langmuir sorption isotherm equation has been derived on the assumption of localized monolayer sorption

on the sorption centers of equal energy and 1:1 correspondence between sorption centers and sorbate molecules. When ammonia sorption data in NaX and its K+-, Rb+-, and Cs+-exchanged forms were fit to a Langmuir model, almost linear plots were obtained over the entire range of pressure and temperature (303-453 K) as illustrated in Figure 2. It is clearly seen from the figure that the linearity of these plots indicates the applicability of the Langmuir equation to describe the ammonia sorption in samples with an almost identical level of population of different monovalent alkali metal cations. The excellent linearity of the Langmuir plots suggests that ammonia molecules are localized on the sorption center. Similarly, n-butylamine sorption in EU-124, Fe 3+ exchanged Y type zeolites25 did yield linear plots, when the Langmuir equation was applied. On the contrary, the Langmuir equation failed to yield linear plots for CO226 and NH327 sorption in a series of Y type zeolites exchanged with La3+, Ca2+, and H+. This suggests that the structural and compositional components of a sorbent and characteristics of the sorbate play an important role to confirm the validity of the Langmuir equation for their sorptive behavior. The reciprocal of the intercept made by these plots on the ordinates does often represent the magnitude of the constant “C” in the Langmuir equation. The value of constant C may qualitatively be looked upon, in one way or other, related to the heat of sorption during the monolayer formation. The higher the value of C (lower the intercept), the stronger the interaction is between the sorption center and sorbate molecules. It is observed that, as the isotherm temperature increases, the value of the intercept made by these plots on the ordinate also increases; thus, the value of C decreases. Therefore, as temperature increases from 303 to 453 K, interaction between the sorption center and sorbate molecule decreases. Furthermore,


J. Phys. Chem. B, Vol. 105, No. 43, 2001 10641 TABLE 3: Freundlich Sorption Isotherm for NH3 Sorption in Alkali Metal Exchanged Type X Zeolite sample designation NaX NaK(53)X NaRb(53)X NaCs(58)X

Figure 3. Typical Freundlich plots for ammonia sorption in different sorbents at 453 K.

the extent of increase in the intercept as a function of temperature also was found to increase as cationic size of the exchanged cations increases. Thus, not only the temperature but also the charge densities of exchanged cations were found to influence the extent of interaction between sorption center and sorbate molecule. It is interesting to note that, at any given temperature, the overall trend observed in the extent of interaction between sorption center and sorbate molecule was as

NaX > NaK(53)X > NaRb(53)X > NaCs(58)X As expected, the monolayer capacities (expressed as ammonia molecules sorbed /unit cell) obtained from the reciprocal of the slopes of the linear plots were found to be rather higher than those obtained experimentally in the pressure range of 450500 Torr in all the samples. 3.2.2. Freundlich Isotherm Equation. All of the ammonia sorption isotherms exhibited relatively rapid uptake in the lowpressure region followed by slow uptake in the high-pressure region. The Freundlich isotherm equation was applied to the sorption data to represent the variation of the number of ammonia molecules sorbed per unit cell. The Freundlich sorption equation may be written in the form

θ ) APc

(1)

where θ is the fractional coverage at equilibrium pressure P, A and c are the empirical constants which depends on the nature of the sorbent and sorbate and on temperature. This may be expressed in a logarithmic form as

log θ ) c log P + log A

(2)

Analyses of ammonia sorption data in parent NaX and on its K+-, Rb+-, and Cs+-exchange forms were used to represent Freundlich plots as depicted in Figure 3. The fractional coverage θ was calculated by using the monolayer saturation capacities obtained by the Langmuir equation descried as earlier.28 Although the requirements of the equation were met satisfactorily at the lower pressures, at high pressures, the experimental points curve away from the straight lines indicating that this equation does not have general applicability in describing the sorption of ammonia by these samples. Because the Freundlich sorption isotherm equation approximates a two-dimensional film of the sorbate on the sorbent surface, the linearity of these plots in the low-pressure region may form a basis for assuming a two-dimensional monolayer film of ammonia molecules on the

temp.

Y ) kP1/n

453 K 303 K 453 K 303 K 453 K 303 K 453 K 303 K

Y ) 114.8(P)0.44 Y ) 5.50(P)0.34 Y ) 38.02(P)0.47 Y ) 5.01(P)0.27 Y ) 15.13(P)0.60 Y ) 2.04(P)0.22 Y ) 11.48 (P)0.81 Y ) 1.90 (P)0.34

sorbent surface. The nonlinearity at higher coverage partly supports the earlier findings regarding the complicated sequence in ammonia sorption capacities of parent and modified X zeolite samples at higher pressures. From the typical linear Freundlich plots, the constants c and A were calculated from the slope and from the ordinate intercept, respectively, and are tabulated in Table 3. It is clearly evident from the table that the Freundlich sorption model takes a different form depending on the temperature and nature of the nonframework cations. Because the Freundlich sorption equation approximates a two-dimensional film of the sorbate on the sorbent surface, in the lowpressure region a two-dimensional monolayer film of ammonia molecules on the parent and exchanged forms of X type zeolitic surface can be assumed. However, the n-butylamine sorption data in Al and Fe analogues of beta zeolites,29 Al and Ga analogues of MFI type zeolites,30 Fe -exchanged Y zeolites,27 and CO2 sorption in LTL zeolites31 could satisfactorily be represented by the Freundlich equation, which, however, failed to represent n-butylamine sorption in EU-1 zeolites24 and TS-1 molecular sieves.32 3.2.3. Sips Equation. If it is assumed that sorption takes place as a chemical reaction between an active center and sorbate molecules with 1:1 correspondence and if some tolerance is made for the complicating factor, the Sips equation results. The Sips equation33 is based on localized sorption, which usually takes care of the sorbate-sorbent interaction and any deviation from the Langmuir approach. The original Sips equation when linearized takes the form

log[θ/(1 - θ)] ) log A + c log P

(3)

where A and c are constants, P is the equilibrium pressure, and θ is the coverage. In the low-pressure region, the equation reduces to a Freundlich type isotherm. The plots of log[θ/(1 θ)] against log P were constructed using the experimental values of P and respective coverage θ in order to check the applicability of the Sips equation. The fractional coverage θ was calculated by using the monolayer saturation capacities obtained by the Langmuir equation descried as earlier.28 Figure 4 shows the typical Sips plots for ammonia sorption in NaX and its K+-, Rb+-, and Cs+-exchange forms in the temperature range of 303453 K. The linearity of these plots shows the applicability of the Sips equation. From the typical linear Sips plots, the constants c and A were calculated from the slope and the intercept on the ordinate, respectively, and are tabulated in the Table 4. As the constant c evaluated from the linear plot approaches to unity, the Sips equation reduces to the Langmuir model, and the deviation of c from unity may be taken as a measure of a deviation from Langmuir isotherm. It is clearly seen from the Table 4 that, depending upon the sorption temperature and charge density of the nonframework cations in the sample, the value of c changes in the range of 0.65-0.99. The only regular decreasing trend in c values with the increase in temperature


Joshi et al.

Figure 4. Sips plots for ammonia sorption in (A) NaX, (B)NaK(53)X, (C)NaRb(53)X, and (D) NaCs(58)X.

TABLE 4: Constants of Sips Equation for NH3 Sorption sample designation

c

NaX NaK(53)X NaRb(53)X NaCs(58)X

0.92 0.95 0.84 0.87

303 K A × 102 9.56 9.13 8.11 7.87

c 0.80 0.85 0.85 0.85

333 K A × 102 9.26 9.11 6.81 4.29

c

363 K A × 102

0.72 0.84 0.85 0.94

was observed in the case of the NaX sample. However, at any given temperature above 363 K, the increase in the c values was observed with the increase in the size of the extraframework exchanged cation. Therefore, it seems likely that some complicating factors are involved in ammonia sorption in compositionally different X type zeolites causing the deviation from the Langmuir approach. Despite having such deviation, the validity of the Langmuir equation to describe the ammonia sorption in samples with almost equal level of population of different monovalent cations was observed and discussed earlier. Thus, it can be believed that either the value of c in the range of 0.65-0.99 is tolerable as far as the applicability of the Langmuir approach is concerned or the deviation may be taking care of any other complicating factor. If the constant A obtained from the intercept could be looked upon qualitatively representing the strength of adsorption, the higher value for NaX than any other modified forms suggests the stronger adsorption in the former and is in accordance with the conclusion drawn earlier from the experimental isotherms. It is clearly seen from the Table 4 that the value of A decreases with the increase in the temperature. However, the extent of such a decrease was found to vary depending upon the nature of nonframework cation. The following trend was observed in the drop of A values (as indicated in square-bracketed figure)

9.15 6.62 6.19 1.90

c 0.67 0.74 0.84 0.88

393 K A × 102 8.85 5.34 4.38 1.13

c 0.66 0.79 0.85 0.98

423 K A × 102 7.09 4.21 1.68 0.45

c 0.65 0.81 0.84 0.99

453 K A × 102 5.82 1.75 1.02 0.23

in the temperature range of 303-453 K:

NaX [60.87%] > NaK(53)X [19.16%] > NaRb(53)X [12.57%] > NaCs(58)X [2.92%] Furthermore, decrease in A with the increase in cationic size has indicated the decreased sorption on account of decreased charge density of the cation and increased framework oxygen charge. It is interesting to note that, the decrease in A value as a function of cationic charge density has exhibited the dependence on temperature. The drop in the A values was found minimum (9.56-7.87) at 303 K, whereas it was maximum (5.82-0.23) at 453 K when NaX and NaCs(58)X were compared. Furthermore, to obtain the desired value of A, it was found that a higher temperature is needed for the samples containing equal population of smaller extraframework cations. As an illustrative example, to obtain a value of 7.87 (A × 102), NaCs(58)X will require an isotherm temperature on the order of 303 K, whereas NaRb(53)X will require a temperature higher than 303 K but less than 333 K, NaK(53)X will require the temperature higher than 333 K but less than 363 K, and NaX will require the temperature higher than 393 K but less than 423 K to obtain the same value of A. Thus, temperature and the nature of the nonframework cation were found to be the



Figure 5. BET plots for ammonia sorption in (A) NaX, (B) NaK(53)X (C) NaRb(53)X, and (D) NaCs(58)X.

key parameters for governing the strength of ammonia sorption in the present studies. 3.2.4. BET Equation. The BET isotherm equation is based on the multilayer assumption by assigning a much higher value of the heat of sorption for the formation of the first layer than for the formation of successive layers. Figure 5 shows the BET plots obtained from the ammonia sorption data of the parent NaX and its K+-, Rb+-, and Cs+-exchanged forms. The linearity of these plots also indicated that the ammonia sorption data in X zeolites can be satisfactorily described by the BET model. The saturation capacities (monolayer capacities) obtained from the slope and the intercept of these plots were found to be lower than those of obtained from the Langmuir approach and from the sorption experiment as well. Although, assumption-wise the Langmuir equation is based on the monolayer sorption and the BET is based on the multilayer sorption, if one takes into consideration the limitations and the constraints on the formation of more than one layer in the cavities of most of the zeolite, essentially the BET equation reduces to the Langmuir equation. If the derivation of the BET sorption isotherm equation is taken into consideration, the first layer of the sorbate has a very high value of the heat of adsorption compared to that for the successive layers; as far as only the first layer is operative, it behaves similarly to the Langmuir equation. The BET plots for the parent and cation exchanged samples show considerable values of the intercept on the ordinate depending on the nature of the nonframework cations and the sorption temperature. The constant C in the BET equation is related to the heat of sorption for the first layer and is proportional to the reciprocal of the intercept on the ordinate. Thus higher cationic diameter and sorption temperature seems

to be operative in the weakening in the bond between the ammonia molecule and the sorption center on the sorbent surface. 3.2.5. Dubinin Equation. Analysis of the sorption data in terms of a different isotherm equation model always provides useful information in identifying the nature of sorption centers and the physical state of sorbed molecules. An attempt has been made here to apply the Polanyi potential theory modified by Dubinin and Radushkevich34 for ammonia sorption in NaX and its exchanged forms. The Dubinin-Raduschkevich relation is expressed as

log(W) ) log(Wo) - (B/2.303β2)[T log(Po/P)]2

(4)

where W is the amount sorbed at equilibrium pressure P, Wo is the total sorption capacity, B is a constant independent of temperature and is characteristic of a sorbent pore structure, and β is the affinity coefficient. Dubinin plots were constructed by plotting [log(PO/P)]2 against log W for the NaX and its exchanged form in the temperature range of 303-453 K as shown in Figure 6. Almost all of the linear plots were obtained in all of the samples indicating ammonia sorption in these samples can be satisfactorily represented by the Dubinin-Radushkevich equation. The saturation capacities and B/β2 were obtained from the intercept on the Y axis and slopes of the linear plots, respectively. The values obtained for saturation capacities were found to be lower from those obtained using Langmuir models but higher than those obtained from the BET model. A typical comparison between the experimental saturation capacities and those obtained by the Langmuir, BET, and Dubinin models is presented in Figure 7. The magnitude of the affinity coefficient


Joshi et al.

Figure 6. Dubinin plots for ammonia sorption in (A) NaX, (B) NaK(53)X, (C) NaRb(53)X, and (D) NaCs(58)X.

Figure 7. Correlationship between experimental saturation capacities and saturation capacities obtain from different models for NaX. Inset: correlationship between B/β2 and intermediate electronegativity (Sint).

β is inversely proportional to the square root of the factor B/β2 obtained from the slope. The slope of these linear plots, in other words B/β2, was found to increase with the decrease in sorption temperature. Thus, the decrease in affinity coefficient β (increase in B/β2) with an increase in sorption temperature indicates reduced affinity of the sorption center toward the ammonia molecules at higher temperature. Furthermore, the affinity coefficient (β) follows the sequence at a given sorption temperature in the range from 303 to 453 K, as

NaX > NaK(53)X > NaRb(53)X > NaCs(58)X The sorption affinity of a sorbate molecule toward the sorption

centers on the surface may be partly associated with intermediate electronegativity of the sorbents. The Figure 7 inset represents the correlation established between B/β2 and the intermediate electronegativity of these samples. It is clearly evident from the figure that, as the charge density of extraframework cation increases, there is an increase in the sorption affinity of ammonia molecule toward the sorption center. 3.3. Applications of Statistical Models of Langmuir and Volmer. The surface heterogeneity, multilayer formation, and mutual interaction between sorbed molecules are some the dominant factors which are responsible for deviating the real systems from the idealized one. The statistical model equations



TABLE 5: Test of Isotherm Equations35 isotherm equation

plot against

(I) P ) KL(θ/1 - θ) (II) P ) KBW(θ/1 - θ) exp(Zωθ/RT)

P(1 - θ/θ) ) KL ln(1 - θ/θ) ) ln KL

(III) P ) KV(θ/1 - θ) exp(θ/1 - θ) (IV) P ) KVW(θ/1 - θ) exp[(θ/1 - θ) - (Rθ)]

P(1 - θ/θ) exp(-θ/1 - θ) ) KV ln[P(1 - θ/θ) exp(-θ/1 - θ)] ) ln KV

of Langmuir and Volmer are often applied to such systems to yield information on the extent of deviation. The simplest isotherm equation describing localized sorbed molecules on the sites, which are independent of each other and are energetically equivalent, is that of Langmuir, expressed as follows:

P) KL (θ/1 - θ)

(5)

Modified forms of the Langmuir equation have been used21,28,35,36 to explain the physical states of sorbed phase.37-39 The resulting equations and method of testing them are summarized in Table 5. The coverage θ was calculated by dividing the amount sorbed at a given pressure by the saturation sorption capacities obtained by applying the Langmuir approach to ammonia isotherm data. The plots of ln KL against θ for ammonia sorption at various temperatures in the range of 303-453 K for all of the samples approximate to linearity but with +Ve slope up to a coverage of 0.7. The plots for ln KV for ammonia sorbed on all of the samples have exhibited a gentle curvature and hence no meaningful deductions could be drawn. Therefore, in general, all of the systems approximate better to a model describing localized sorption with sorbate-sorbate repulsion as compared to a mobile model without interactions. Thus, localized sorption exhibiting strong intermolecular repulsion seems to be the most likely physical environment experienced by the ammonia molecules. These curves have also exhibited the deviation from the ideality, the extent of which depends on the nature of the extraframework cations. NaX to a greater extent appears to be homogeneous sorbent, and NaCs(58) X has shown the most heterogeneous character. Even though the large dipole moment of ammonia induces the coordination ability for cations, the charge density of the cation, its population at a particular site, and the average charge on the framework oxygen charge are expected to be controlling factors for the ammonia interactions within the sorbed phase and with the zeolitic surface. 3.4. Chemical Affinity and the Selectivity of a Sorbed Phase. The chemical affinity of ammonia for sorbents having the same topology but different chemical composition, especially differing in charge density and size of extraframework cations, is one of most significant parameters governing the extent of decrease in free energy which accompanies a given sorption process. Normally, this parameter takes account of any chemical consideration, which may operate in favor of or against the sorption process. Thus, the decrease in chemical potential was considered as a quantitative measure of the chemical affinity when the gas is transferred reversibly and isothermally from the gas phase at a standard pressure Po into an infinite amount of sorbent-sorbate mixture over which the equilibrium pressure is P. If the nonideality of the gaseous sorbate like ammonia is neglected, the chemical affinity of sorption may be estimated as a function of amount sorbed by using the equation

∆µ ) RT ln(P/Po)

(6)

The variation in the chemical affinity of the ammonia as a function of the nature of nonframework cations has been estimated by using -∆µ as a quantitative measure. Because

result if model is applicable KL independent of θ straight line with slope ) (Zω/RT) and intercept ) ln KBW KL independent of θ straight line with slope ) -R And intercept ) ln KVW

the influence of the sorbate was thought to manifest itself through the affinity factor, the plots of chemical potential (expressed in J/mole) as a function of NH3 molecules adsorbed/ u.c. were considered for representing the chemical affinity profiles of the sorption system. The comparison of chemical affinity profiles for NH3 sorption in NaX and its cationexchanged analogues are made in Figure 8. It seen from the figure that, in accordance with thermodynamic expectation, all of the sorbents exhibited continuous increase in the amount sorbed (coverage) with the decrease in chemical potential. These plots also indicated the greater extent of decrease in -∆µ at lower coverage as compared to that at higher coverage. In addition to this, these curves also showed a rather sharp drop in the chemical affinity as the sorption temperature and the size of the extraframework cation increased. When all of the samples were compared at 453 K (figure not shown), the decreasing trend in chemical affinity for ammonia was observed as NaX > NaK(53)X > NaRb(53)X > NaCs(58)X. On the contrary, no regular trend in chemical affinity was observed at 303 K on account of the crossover of the curves, indicative of differing saturation capacities of the sorbents. Thus, at lower sorption temperature, the replacement of the monovalent extraframework cation by another monovalent cation but with larger size may give rise rather more complicated sorption phenomenon on account of varying chemical affinity due to simultaneous but unequal variations in the contributions due to energy components such as dispersion and repulsion energies between ammonia and the zeolite, polarization energy, field-dipole energy, field gradientquadrupole energy, and self-potential. 3.5. Isosteric Heat (qst) of NH3 Sorption. The isosteric heat (qst) is equal in magnitude but opposite in sign to differential heat of sorption (∆H1).40 The isosteric heat (qst) of ammonia sorption is derived23,26,41 from the shift of sorption equilibrium with temperature at constant sorbate loading by applying the Clausius Clapeyron equation:

qst ) -∆H h 1 ) R[(T2T1)/(T2 - T1)] ln(P2/P1)

(7)

The isosteric heats of ammonia sorption as a function of amount adsorbed by the parent NaX and exchanged forms are presented in Figure 9. It is clearly evident that the curves for the exchanged samples present more heterogeneous profiles than that for the parent (NaX) sample. Comparatively, the isosteric heat of ammonia sorption in NaX was found to be independent of coverage indicating that in NaX specific interaction is present but to a similar extent for each sorbate molecule. The curves for the NaX, NaK(53)X, NaRb(53)X, and NaCs(58)X correspond with qst at zero coverage respectively to about 40, 36, 35, and 32 kJ/mol. The trend shown by these samples is in close agreement with the reported one.23 However, the lower qst values obtained in the present studies than that of reported-one may be associated with the difference in the framework composition and hence the population of extraframework ions and different degree of exchange. The above observed trend in the qst values indicated that the acidic character decreases with the increase in size of monovalent extraframework cations. In other words,


Joshi et al.

Figure 8. Chemical affinity profiles for ammonia sorption in (A) NaX, (B) NaK(53)X, (C) NaRb(53)X, and (D) NaCs(58)X.

Figure 9. Isosteric heat (qst) for ammonia sorption.

the progressive increase in the basicity of the sorbent occurred with the replacement of sodium by equal amount of K, Rb, and Cs cations without disturbing the crystallinity and framework composition. This has been well reflected in the partial charge on framework oxygen as tabulated in Table 1. Strikingly, the most reduced acidic character, a wide variation in qst values with the amount sorbed in NaCs(58)X, indicated that this sample offered the most heterogeneous surface as compared to the other samples. The humps in qst values were observed in the midcoverage region in all of the curves and may be associated with the differing extent of both the sorbate-sorbent and sorbate-sorbate interactions. It is clearly seen from the figure that the position

of the hump was found to be shifted to lower coverage region upon exchanging with larger cations. A steady decrease in the qst values with the increase in coverage after the hump involves only sorbate-sorbate interactions, which are rather weak in magnitude. Thus, as the NaCs(58) sample has exhibited a larger extent of decrease, it can be concluded that the contribution due to sorbate-sorbate interaction are predominant in this sample as compared to rest of the samples. 4. Conclusions The results of powder XRD, framework IR, SEM, and chemical analysis clearly indicate that the modification by ion-

Energetics of Ammonia Sorption exchange technique has not resulted into any change as far as framework composition (Si/Al ratio), phase purity, crystallinity, and crystallite size of parent NaX and its exchange forms are concerned. The textural characteristics such as surface area and micropore volume were evaluated from the low-temperature nitrogen adsorption isotherm data and showed the dependence on the size of the nonframework-exchanged cation. Families of isotherms of ammonia sorption in parent NaX and its modified forms with almost identical population (49 ( 2 per unit cell) of K-, Rb-, and Cs-exchanged cations up to 500 Torr in the temperature range 303-453 K were found to approximate to the Langmuir type. Ammonia sorption data could be satisfactorily represented by Langmuir, Dubinin, BET and Sips equations and statistical models of Langmuir and Volumer. However, the Freundlich equation failed to represent ammonium sorption data at higher pressures. The values of saturation capacity obtained from the Dubinin-Radushkevich equation were found to be lower than those obtained using the Langmuir model but higher than those obtained from the BET model. The affinity coefficient was found to follow the sequence NaX > NaK(53)X > NaRb(53)X > NaCs(58)X at a given sorption temperature in the studied range. The plots of ln KL and ln KV against θ for ammonia sorption at various temperature in the range 303-453 K indicated that all of the systems approximate better to a model describing localized sorption with sorbatesorbate repulsion as compared to a mobile model without interactions. Thus, localized sorption exhibiting strong intermolecular repulsion seems to be the most likely physical environment experienced by the ammonia molecules. The greater extent of decrease in chemical affinity was observed at lower coverage as compared to that at higher coverage. Furthermore, a rather sharp drop in the chemical affinity was observed as the sorption temperature and the size of the extraframework cation was increased. Simultaneous but unequal variations in the contributions due to energy components such as dispersion and repulsion energies between ammonia and the zeolite, polarization energy, field-dipole energy, field gradientquadrupole energy, and self-potential were found to be operative at lower sorption temperature yielding rather more complicated sorption phenomenon. The isosteric heat of ammonia sorption in NaX was found to be independent of coverage indicating that in NaX a specific interaction is present but to a similar extent for each sorbate molecule. The qst values showed dependence on Lewis acid-base character in different samples having the same framework composition. A wide variation in qst values with the amount sorbed in NaCs(58)X indicated the most heterogeneous surface and greater contribution due to sorbate-sorbate interaction as compared to rest of the samples. References and Notes (1) Joshi, U. D.; Joshi, P. N.; Tamhankar, S. S.; Joshi, V. V.; Shiralkar, V. P. J. Colloid Interface Sci. 2001, 235, 135. (2) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular SieVes; Academic Press: London, 1976.

J. Phys. Chem. B, Vol. 105, No. 43, 2001 10647 (3) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley and Sons: New York, 1984. (4) Kiselev, A. V. AdV. Chem. Ser. 1971, 102, 37. (5) Barrer, R. M. AdV. Chem. Ser. 1971, 102, 1. (6) Sing, K. S. W. Colloid Sci. 1973, 1, 1. (7) Barrer, R. M.; Coughlan, B. Mol. SieVe, Soc. Chem. Ind., London, 1968, p 233. (8) Khvoshew, S. S.; Zhdanov, S. P. IsVest. Acad. Nauk S.S.S.R. Ser. Khim. 1970, N11, 2443. (9) Loseva, T. I.; Kochurihin, V. E.; Zelvensky, Y. D. Zh. Fiz. Khim. 1973, 47, 200. (10) Coughlan, B.; Kilmaritin, S. J. Chem. Soc., Faraday Trans. 1 1975, 71, 1809. (11) Khwoshev, S. S.; Zhdanov, S. P.; Sakharova, T. N. Doklady Acad. Nauk S.S.S.R. 1968, 181, 1189. (12) Barrer, R. M.; Cram, P. J. AdV. Chem. Ser. 1971, 102, 105. (13) Coughlan, B.; Carroll, W. M. J. Chem. Soc., Faraday Trans. 1 1976, 72, 2016. (14) Bezus, A. G.; Kiselev, A. V.; Du Pham Quang J. Colloid Interface Sci. 1972, 40, 223. (15) William, L. E.; Paul, O. F.; Atholl, A. V. G.; Jack, H. L. J. Phys. Chem. 1987, 91, 2091. (16) Barrer, R. M.; Bratt, G. C. J. Phys. Chem. Solids 1959, 12, 130. (17) Breck, D. W.; Eversole, W. G.; Milton, R. M.; Reed, T. B.; Thomas, T. L. J. Am. Chem. Soc. 1956, 78, 5963. (18) Rees, L. V. C., Berry, T. Molecular SieVes; Society of Chemical Industry: London, 1968; p 149. (19) Shiralkar, V. P.; Kulkamrni, S. B. Z. Phys. Chem. (Leipig) 1984, 265, 313. (20) Flanigen, E. M.; Khatami, H.; Szymanski, H. A. Molecular Sieve Zeolite I. In AdVances in Chemistry; Gould, R. F., Ed.; American Chemical Society: Washington, DC, 1971; Vol. 101, p 201. (21) Breck, D. W. Zeolite Molecular SieVes; Wiley: New York, 1974; Chapter 2. (22) Joshi, P. N.; Kim, T. H.; Kim, K. I.; Shiralkar, V. P. Adsorp. Sci. Technol. 1999, 17(8), 639. (23) Barrer, R. M.; Gibbons, R. M. Trans. Faraday Soc. 1963, 59, 2569. (24) Rao, G. N.; Joshi, P. N.; Kotasthane, A. N.; Shiralkar, V. P. J. Phys. Chem. 1990, 94, 8589. (25) Kulkami, S. J.; Kulkarni, S. B. Indian J. Chem. 1989, 18A, 6. (26) Shiralkar, V. P.; Kulkami, S. B. Zeolites 1984, 4, 330. (27) Shiralkar, V. P.; Kulkarni, S. B. J. Colloid Interface Sci. 1985, 108, 1. (28) Coughlan, B.; Kilmaritin, S. J. Chem. Soc., Faraday Trans. 1 1975, 71, 1809. (29) Joshi, P. N.; Eapen, M. J.; Shiralkar, V. P. J. Chem. Soc., Faraday Trans. 1994, 90(2), 387. (30) Awate, S. V.; Joshi, P. N.; Eapen, M. J.; Shiralkar, V. P. J. Phys. Chem. 1993, 97, 6042. (31) Joshi, P. N.; Shiralkar, V. P. J. Phys. Chem. 1993, 97, 619. (32) Mirajkar, S. P.; Thangraj, A.; Shiralkar, V. P. J. Phys. Chem. 1992, 94, 3073. (33) Sips, R. J. Chem. Phys. 1948, 16, 491. (34) Dubinin, M. M.; Redushkevich, L. V. Proc. Acad. Sci. USSR 1974, 55, 327. (35) Coughlan, B.; McCntee, J. J. Proc. R. Irish Acad. 1976, 76B, 473. (36) Barrer, R. M.; Stuart, W. I. Proc. R. Soc. London 1959, A249, 464. (37) Shiralkar, V. P.; Kulkami, S. B. J. Colloid Interface Sci. 1986, 109, 115. (38) Flower, R. H. Proc. Cambridage Philos. Soc. 1935, 31, 260. (39) Hill, T. L. Introduction to Statistical Thermodynamics; AddisonWesely: Reading, MA, 1960. (40) Barrer, R. M.; Coughlan B. Mol. SieVe, Soc. Chem. Ind., London 1968, 241. (41) Coughlan, B.; McCann, W. A. J. Chem. Soc., Faraday Trans. 1 1979, 1984.

Energetics of Ammonia Sorption in Alkali Metal Exchanged

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