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Langmuir 2000, 16, 1436-1439
Notes Calorimetric Determinations of Ba2+ and La3+ in the Ion-Exchange Process Involving Pure and Modified r-Titanium Phosphates Claudio Airoldi* and Liliane M. Nunes Instituto de Quı´mica, Universidade Estadual de Campinas, Caixa Postal 6154, 13083-970 Campinas, Sa˜ o Paulo, Brazil Received March 10, 1999. In Final Form: September 7, 1999
Introduction Inorganic phosphates containing tetravalent metals with general formula M(HPO4)2‚nH2O (M ) Zr, Ti, Ce, Sn, etc.) are well-known crystalline compounds, presenting self-organized lamellar structure. These compounds are synthesized as insoluble acid salts and are also employed as inorganic ion exchangers, where the pioneer zirconium phosphate played an important part in this field. In general, this class of exchangers hold high exchange capacity,1-4 varying normally from 4 to 8 mequiv g-1, and they act not only as ionic exchangers but also as important matrixes for a huge variety of intercalation reactions.2,4 A significant amount of investigations are devoted to ion-exchange reactions, with the great majority focused on structural features of these exchangers, probable in exploring the excellent chemical and thermal stabilities and also the facility in developing favorable ion-exchange properties.2,4 Thus, the ion-exchange behavior of those materials has been applied with the complete series of alkaline metals and calcium, barium, and strontium with the matrixes of titanium and zirconium hydrogen phosphate in R or γ crystalline structures.5-14 However, some thermodynamic data for the ion-exhange reactions are reported for R- or γ-titanium and R- or γ-zirconium phosphates in investigations involving alkaline metals6-8,11,12 and transition metals Co(II) and Ni(II) with γ-titanium
hydrogen phosphate,15 for which the corresponding ionic isotherms at distinct temperatures were obtained. The same exchange process was followed through calorimetric determinations with crystalline R-titanium phosphate and crystalline or amorphous zirconium phosphate.10,16-20 While the exchange behavior of R-titanium hydrogen phosphate has been investigated, only few studies are found with other kinds of this modified matrix. However, those exchangers containing sodium and n-butylammonium intercalated forms were recently explored.10,13 From the collected data involving such kinds of exchangers suspended in water a mechanism for the ionexchange process is proposed. This proposal is based on the initial diffusion of nonhydrated or partially hydrated cations into the cavities. Thus, the proximity of the cation to be exchanged with the proton bonded to the acidic P-OH center of the phosphate groups occurs without any disturbance of the interlamellar distance. However, over half of the protons available are exchanged, and the expansion of the lamella can start to occur. The subsequent diffusion of water into the crystal lattice rehydrates the cations and also makes easy the exchange of the remaining protons attached to the phosphate inorganic layers.2,4 The aim of this publication is to investigate the behavior of the ion exchange involving hard barium and lanthanum cations, which differ in charge with near ionic radii,21 being 135 and 115 pm, respectively, with the inorganic matrixes of titanium hydrogen phosphate in R (TPH), sodium (TPNa), and intercalated with n-butylammonium (TPBA) forms. The ion exchange of these stable matrixes containing distinct exchangeable monovalent cations into the lamellar cavity can give explanation about this process, mainly when associated to the energetic of the system. For this purpose, from calorimetric titration of a suspension of the exchanger in water with the select cations, the thermochemical data of the reactions are explored in order to evaluate the influence of the charge of the cations on ion exchange. Experimental Section
(1) Kanzaki, Y.; Mitsuo, A. Bull. Chem. Soc. Jpn. 1991, 64, 1846. (2) Clearfield, A. In Inorganic Ion Exchange Materials; CRC Press: Boca Raton, FL, 1982. (3) Hasegawa, Y., Akimoto, T.; Kojima, D. J. Incl. Phenom. 1995, 20, 1. (4) Alberti, G., Bein, T., Eds. In Comprehensive Supramolecular Chemistry, 1st ed.; Elsevier: New York, 1996; Vol. 7. (5) Salvado´, M. A.; Pertierra, P.; Garcı´a-Granda, S.; Sua´rez, M.; Rodrı´guez, M. L.; Llavona, R.; Garcı´a, J. R.; Rodrı´gues, J. J. Mater. Chem. 1996, 6, 415. (6) Gonza´lez, E.; Llavona, R.; Garcı´a, J. R.; Rodrı´guez, J. J. Chem. Soc., Dalton Trans. 1989, 829. (7) Sua´rez, M.; Rodrı´guez, M. L.; Llavona, R.; Garcı´a, J. R.; Rodrı´guez, J. Thermochim. Acta 1995, 249, 367. (8) Trobajo, C.; Sua´rez, M.; Llavona, R.; Garcı´a, J. R.; Rodrı´guez, J. Thermochim. Acta 1991, 186, 253. (9) Llavona, R.; Sua´rez, M.; Garcı´a, J. R.; Rodrı´guez, J. Inorg. Chem. 1989, 28, 2863. (10) Nunes, L. M.; Airoldi, C. Thermochim. Acta 1999, 328, 297. (11) Kullberg, L.; Clearfield, A. J. Phys. Chem. 1981, 85, 1585. (12) Sua´rez, M.; Garcı´a, J. R.; Rodrı´guez, J. J. Phys. Chem. 1984, 88, 159. (13) Llavona, R.; Garcı´a, J. R.; Alvarez, C.; Sua´rez, M.; Rodrı´guez, J. Solvent Extr. Ion Exch. 1986, 4, 567. (14) Tegehall, P. E. Acta Chem. Scand. 1989, 43, 322.
Preparation of Crystalline Titanium Hydrogen Phosphate in the r Form (TPH). This compound was obtained through the reaction of oxidation of titanium trichloride (Carlo Erba) with some modifications of the original method.22 This synthetic crystalline compound was prepared by reacting 0.126 mol of 15% titanium trichloride with 0.50 mol of 85% phosphoric acid during 4 days in a polyethylene flask at 333 K, with periodical stirring. The solid was separated by centrifugation and washed (15) Mene´ndez, F.; Rodrı´guez, M. L.; Trobajo, C.; Sua´rez, M.; Garcı´a, J. R.; Rodrı´guez, J. Solvent Extr. Ion Exch. 1995, 13, 179. (16) Roca, S.; Airoldi, C. J. Chem. Soc., Dalton Trans. 1997, 2517. (17) Roca, S.; Airoldi, C. Thermochim. Acta 1996, 284, 289. (18) Clearfield, A.; Kullberg, L. H. J. Phys. Chem. 1974, 78, 152. (19) Kullberg, L.; Clearfield, A. J. Phys. Chem. 1980, 84, 165. (20) Sua´rez, M.; Garcı´a, J. R.; Rodrı´guez, J. J. Phys. Chem. 1984, 88, 157. (21) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. In Inorganic Chemistry: Principles of Structure and Reactivity; Harper & Row: New York, 1972. (22) Bortun, A.; Jaimez, E.; Llavona, R.; Garcia, J. R.; Rodrı´guez, J. Mater. Res. Bull. 1995, 30, 413.
10.1021/la9902892 CCC: $19.00 © 2000 American Chemical Society Published on Web 11/24/1999
Notes
Langmuir, Vol. 16, No. 3, 2000 1437
with bidistilled water until the washing part reached the pH range 3.5-4.0. The final product was dried at 313 K and characterized as previously,10 to give an interlayer distance of 760 pm. Preparation of r-Titanium Phosphate in the Sodium Form (TPNa). This modified matrix was prepared through a batch method, following an established procedure.10,13 Briefly, this method consisted in dispersing 0.50 g of TPH in 64.0 cm3 of 0.10 mol dm-3 sodium chloride (Nuclear). The suspension was slowly titrated with 36.0 cm3 of 0.10 mol dm-3 sodium hydroxide (Nuclear) and stirred for 120 h at 298 K to complete the ionic exchange. Then the solid was centrifuged, washed with bidistilled water to eliminate chlorides, and dried for 24 h at 313 K. In agreement with previous results10,12,14,16 the product can be formulated as TiHNa(PO4)2‚4H2O (67%) and Ti(NaPO4)2‚H2O (33%) with interlayer distances of 1040 and 840 pm, respectively. Preparation of r-Titanium Phosphate in the Butylammonium Form (TPBA). This intercalated compound was prepared as previously.10,13 Thus, 1.0 g of TPH was added to 76.0 cm3 of 0.10 mol dm-3 of an aqueous solution of n-butylamine (Aldrich) and stirred for 24 h at 298 K. The solid was centrifuged, washed with bidistilled water, and dried for 24 h at 313 K. The isolated compound10,13 Ti(HPO4)2‚1.05H2N(CH2)3CH3‚1.5H2O exhibed an interlayer distance of 1839 pm. Preparation of Lanthanum Chloride. Lanthanum oxide (Merk) reacted with 6.0 mol dm-3 hydrochloric acid in a porcelain capsule in a water bath. The excess of acid was evaporated in a water bath with constant manual agitation, and distilled water was added and evaporated until the final pH was the same as the water. After drying, the amount of lanthanum in the sample was determined through complexometric titration with EDTA.23 Ion Exchange. From variable concentrations of the desired species in a batch process, the equilibrium of the system was reached after 4 h of stirring at 298 ( 1 K. In this operation, about 0.15 g of the ionic exchanger was suspended in 20.0 cm3 of aqueous solution containing the desired cation, with concentration varying from 1.0 × 10-3 to 0.20 mol dm-3. The concentration of the exchangeable cations in solution was determined by complexometric titration with EDTA.23 The amount of exchanged cations (nf) was determined through the relationship (ni - ns)/m, where ni is the initial number of moles of cation in solution, ns is the number of moles of cations in equilibrium with the solid phase after the exchange process, and m is the mass of the exchanger used. Analytical Procedure. The exchanged materials were characterized by the following techniques: thermogravimetry (TG) with a DuPont model 1090B instrument, at a heating rate of 0.17 K s-1 in argon atmosphere; X-ray diffractometry by using Cu KR radiation with a Shimadzu model XD3A diffractometric apparatus; infrared spectra between 400 and 4000 cm-1 with a BOMEM model MB-series instrument, using the KBr pellet technique. Calorimetry. The calorimetric titration was performed in a LKB 2277 differential isothermic microcalorimetric system. In a stainless steel ampule, about 20.0 mg of the exchanger was suspended in 2.0 cm3 of bidistilled water. The system was stirred with a gold helix and thermostated at 298.15 ( 0.02 K. After stabilization of the baseline, the apparatus was standardized. A microsyringe was coupled to the system, which was connected to a stainless steel needle and through it increments of the metal ion solution were added. The thermal effect caused by the reaction was recorded after each addition. The same procedure was used to follow the thermal effect of the exchanger suspended in water and also the titrant solution added in water.10,13
Results and Discussion The ion-exchange process involving the lamellar inorganic compounds TPH, TPNa, and TPBA, for which exchangers are represented by TPY, has the desired cations inserted into the free cavity space, which are available to exchange with the chosen species in solution. After the equilibrium time conditions were established, (23) Kolthof, I. M.; Elving, P. J. In Treatise on Analytical Chemistry; Intercience: New York, 1963; Vol. 8.
Figure 1. Isotherms of the ion exchange of barium with TPH (b), TPNa (4), and TPBA (9).
proton, sodium, and n-butylammonium cations attached to the inorganic layer of the suspended matrixes in aqueous solution were exchanged with barium and lanthanum cations, as represented as Mn+. Then, the expected ionexchange equilibrium process can be summarized as the following:
nTPY(aq) + Mn+(aq) H (TP)nM(aq) + nY+(aq) The isotherms were adjusted to the modified Langmuir equation, which model was previously shown to be applied to these systems involving ion exchange,16,17,24 as can be represented by eq 1
Cs Cs 1 ) s + s nf n b n
(1)
where Cs (mol dm-3) is the concentration of the supernatant cation, nf (mol g-1) is previously defined, ns is the maximum amount of solute exchanged per gram of exchanger (mol g-1), and b is a parameter related to the thermodynamic constant by the expression (K × MM)/F, where MM and F are the mass and density of the solvent, respectively.25 Considering these values, the free energy of the system can be calculated (∆bG), where the b index is used to distinguish from the same value as obtained by calorimetry. The value of parameter b is obtained from the linear coefficient. The isotherms for the modified matrixes presented a close degree of exchanging, which contrasted to that obtained for the original lamellar compound, as shown for barium exchange with these three exchangers in Figure 1. This behavior clearly demonstrated that a previous increase in the interlamellar distance favored the exchange. In this process the cations can naturally diffuse inside the lamella and, consequently, cause an increase in the number of cation exchanged,11,15-17 as is expected for the general ion-exchange mechanism. For comparison, the maximum value of barium exchanged for the TPH matrix is 0.66 mmol g-1, which was increased to 2.78 and 2.77 mmol g-1 for the TPNa and TPBA exchangers, respectively. The maximum ion-exchange capacities calculated for acid, sodium, and n-butylammonium matrixes are 7.75, 6.19, and 5.82 mmol g-1. On the basis of these values, the degree of exchanging reached 17.0, 90.0, and 95.0% for (24) Airoldi, C.; Roca, S. J. Mater. Chem. 1996, 6, 1963. (25) Airoldi, C.; Alcaˆntara, E. F. C. J. Chem. Thermodyn. 1995, 27, 623.
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Notes
Table 1. Number of Intercalated Cations (nf), Integral Enthalpy of the Ion Exchange for Formation of a Monolayer per Unit Mass of Exchanger (∑∆monoH), Enthalpy of Exchange (∆exchH), Free Energy for the Exchange Processes through Calorimetry (∆G), Free Energy Obtained through a Batch Process (∆bG), and Entropy of the System (∆S) exchanger TPH TPNa TPBA
cation
nf (mmol g-1)
∆monoH (J g-1)
∆exchH (kJ mol-1)
-∆G (kJ mol-1)
-∆bG (kJ mol-1)
∆S (J mol-1 K-1)
Ba2+ La3+ Ba2+ La3+ Ba2+ La3+
0.66 0.070 2.78 1.62 2.77 1.62
0.76 ( 0.02 0.39 ( 0.01 2.20 ( 0.02 8.49 ( 0.07 -10.74 ( 0.06 12.29 ( 0.23
1.51 ( 0.09 7.00 ( 0.13 0.85 ( 0.02 14.84 ( 0.60 -4.07 ( 0.09 21.45 ( 1.60
18.89 ( 0.05 33.05 ( 0.26 27.84 ( 1.15 31.34 ( 0.31 28.33 ( 0.74 28.65 ( 0.62
22.64 ( 0.26 32.89 ( 0.54 21.93 ( 0.89 30.56 ( 0.37 25.89 ( 0.92 25.47 ( 0.63
68 ( 1 134 ( 1 96 ( 1 155 ( 1 81 ( 1 168 ( 1
barium and 2.7, 79.0, and 83.0% for lanthanum, respectively. These data suggested that the percentages in exchanging for both matrixes can be related with the increase in the interlamellar distance. The entrance of sodium or n-butylammonium into the lamelllar cavity caused an expansion of the interlamellar distance, which favored the increase in the exchanging process. The complexity of the ion-exchange process in the acidic form matrix was previously demonstrated for zirconium phosphate. In that case the most effective exchange can be improved by modifying the exchange conditions,26,27 by using high temperatures, together with an increase in contact time, or by using an equivalent mixture of the chloride and hydroxide of the desired cation. An alternative method to increase the number of cations exchanged used in this investigation consisted in enhancing the interlamellar distance by modifying the exchanger with sodium or by introducing an organic spacer into the lamellar gallery.28,29 Another important feature on the exchange process is related to the number of ions exchanged (nf) by barium, which is greater in all the matrixes. This behavior can be attributed to three reasons. The exchange property depends on the (a) radius of the cation, (b) the interlayer distance, and (c) the low degree of hydration of this cation. This last factor favors the mobility of the cations into the lamellar space and consequently makes easy the interaction within the lamella.11,28,29 This same behavior was observed before for potassium and calcium10 with these matrixes, evidencing that the volume of hydration of these cations is a decisive property in the ionic exchange. The volumes of hydration30 for sodium, potassium, barium, calcium, and lanthanum are 109.0, 94.4, 146.7, 156.7, and 208.3 cm3 mol-1, respectively. On the other hand, the proposed argument is supported by nf values listed in Table 1, where lanthanum with the largest volume of hydration gave a smaller nf value, as observed for all three matrixes. For example, TPNa exchanged 2.78 and 1.62 mmol g-1 for barium and lanthanum cations, respectively. The X-ray diffraction technique was normally used to obtain additional information on the interlamellar distance of these exchangers. After exchange with Ba2+ and La3+, the original distance of 760 pm in the TPH matrix was maintained. The low degree of ion exchange favors a distribution of these cations into the gallery space without disturbing the inorganic layer of the matrix. However, with TPNa there occurred a contraction from the original lamellar distance of 1040 pm to 849 and 1004 pm for barium and lanthanum, respectively. This same behavior was observed for the TPBA matrix, where the original distance of 1839 pm is reduced to 1162 and 1577 (26) Llavona, R.; Sua´rez, M.; Garcı´a, J. R.; Rodrı´guez, J. Inorg. Chem. 1989, 28, 2863. (27) Clearfield, A.; Kalnins, J. M. J. Inorg. Nucl. Chem. 1976, 38, 849. (28) Rosenthal, G. L.; Caruso, J. J. Solid State Chem. 1991, 93, 128. (29) Alberti, G.; Bertrami, R.; Casciola, M.; Costantino, U.; Gupta, J. P. J. Inorg. Nucl. Chem. 1976, 38, 843. (30) Marcus, Y. In Ion Solvation; John Wiley: Chichester, U.K., 1985.
Figure 2. Calorimetric titration of the exchanger TPH in aqueous solution with lanthanum chloride. The points in the experimental curve are the sum of the thermal effects ∑∆tith (b), ∑∆dilh (9), and ∑∆rh (4).
pm for barium and lanthanum, respectively. The values obtained with these modified matrixes are consistent with the almost complete substitution of sodium or the nbutylammonium cations from the interlamellar space, to cause a rearrangement of the inorganic layers in adjusting to a new situation of equilibrium with contraction of the lamella. In all cases, the original cations sodium and n-butylammonium also remained in the lamella after exchanging and the entering cations have a distinct volume of hydration, which confer to the final matrixes the differences in interlamellar distances. To obtain better information on the energetics of the intercalation processes, a series of calorimetric titrations were carried out for all studied systems, based on the following general equation:
nTPY(aq) + Mn+(aq) ) (TP)nM(aq) + nY+(aq)
∆rh
The data for the calorimetric titrations of ion exchange of lanthanum with the TPH matrix are shown in Figure 2. The general thermal effects (∑∆h) of titration, dilution, and the resulting curve are represented as a function of the volume added (Vad). As the thermal effect of the hydration of the matrixes was null, the resultant thermal effect of the reaction (∑∆rh) can be calculated through the thermal effects of titration and dilution by means of the following expression: ∑∆rh ) ∑∆tith - ∑∆dilh. In the consideration of those data from the enthalpy of reaction (∑∆RH), the integral enthalpy of the ion exchange for a monolayer formation per unit of mass of exchanger (∑∆monoH) can be obtained by using the previous data adjusted to the modified Langmuir eq 2.31
∑X 1 ∑X + ) ∑∆RH (K - 1)∆monoH ∆monoH (31) Airoldi, C.; de Oliveira, S. F. Struct. Chem. 1991, 2, 41.
(2)
Notes
Figure 3. Isotherm of the integral enthalpy of exchange (∆RH) versus molar fraction (∑X) obtained from a calorimetric titration of the exchanger TPH with lanthanum chloride. The straight line is the linearized form of the isotherm.
In this equation, ∑X is the sum of the molar fraction of the cation in solution after exchanging, where X can be calculated for each point of the titrand addition, by using the modified Langmuir equation, which was previousty shown to be a good adjustable model.24,25 ∆RH is the integral enthalpy of exchange (J g-1), obtained by dividing the resultant thermal effect of the reaction ∑∆rh by the mass of the exchanger, and K is a constant of proportionality that also includes the equilibrium constant. By using the angular and linear coefficient values from the ∑X/∆Rh versus ∑X plot, it was possible to obtain ∆monoH value as shown in Figure 3. Then, the enthalpy of exchange ∆exchH could be calculated by means of the expression ∆exchH ) ∆monoH/nf, where nf is the number of fixed cations after calorimetric equilibrium is reached. Calorimetry has been shown to be a useful technique to determine the energetics involving the intercalation or ion-exchange processes.10,16-18,24,25,31, The enthalpies of ion exchange of lanthanum are endothermic in nature for all the matrixes examined. The values are 7.00 ( 0.13, 14.84 ( 0.60, and 21.45 ( 1.60 kJ mol-1 for the TPH, TPNA, and TPBA matrixes, respectively, while for the exchange with Ba2+ the enthalpies are endothermic for matrixes TPH and TPNa and exothermic for TPBA, showing values of 1.51 ( 0.09, 0.85 ( 0.02, and -4.07 ( 0.09 kJ mol-1 for the same sequence, as listed in Table 1. The resultant endothermic values for the majority of these ion-exchange processes could be directly influenced by the endothermic enthalpic exothermic value of dehydration of these cations, which presented enthalpic values of hydration32 of -4648 and -2166 kJ mol-1 for lanthanum and barium, respectively.
Langmuir, Vol. 16, No. 3, 2000 1439
The increase in the endothermicity of the ion-exchange values for lanthanum should be related to the cation charge, which demands three active sites to promote the exchange. In this process sodium and n-butylammonium cations must be removed, causing an increase in the endothermic effect of the reaction, when compared with barium values. Some of the published data are connected to the exchanger γ-titanium phosphate and cations such as barium, strontium, and potassium.7-9 The respective exchange enthalpy values are 30.01, 9.31, and -4.39 kJ mol-1. Another example involves the exchanger zirconium phosphate with the cations rubidium, potassium, and lithium. The exchange enthalpic values were -13.46, -10.46, and -9.62 kJ mol-1, respectively.11,19 By comparison of our results with those data, some difficulties are inherent due to the fact that distinct exchangers and techniques were to obtain the thermodynamic data. In the previous study the enthalpy values were calculated from a series of isotherms at various temperatures, which procedure differs considerably from our calorimetric methodology. The Gibbs free energy was calculated from the expression ∆G ) -RT ln K, for which constant values were obtained from the calorimetric data. These values are quite close to those obtained from batch isotherm data. Both methods yielded exothermic values for all systems, indicating that the reactions are spontaneous in nature. The entropic values listed in the Table 1 were calculated from the general expression ∆G ) ∆H - T∆S, which are also consistent with the argument that the reactions are entropically favored. This can be accounted by considering the fact that during the exchange process the metallic cation loses molecules of water of hydration to bond to the active acidic center of the matrixes. This transference of water molecules from the hydration sphere to the medium of the reaction promotes the deorganization of the system and, consequently, leads to an increase in the entropy in all ion-exchange reactions.16,18,31,33 Acknowledgment. The authors are indebted to the FAPESP for financial support and gratefully acknowledge the CNPq and CAPES for fellowships. LA9902892 (32) Krestov, G. A. In Thermodynamics of Solvation; Ellis Harwood: Chichester, U.K., 1991. (33) Airoldi, C.; Chagas, A. P. Coord. Chem. Rev. 1992, 119, 29.
