J. Phys. Chem. 1985, 89, 4965-4961
4965
An Approach to the Surface Characteristics of the H+ and H+-La3+ Forms of Cation-Exchange Resins by Measurement of the Heat of Immersion Takashi Suzuki* Department of Applied Chemistry, Yamanashi Uniaersity, Takeda-4, Kofu 400, Japan
and Takayoshi Uematsu Department of Synthetic Chemistry, Chiba University, Yayoi- 1, Chiba 260, Japan (Received: November 14, 1984)
Surface characteristics of H+ and its multivalent cation-exchanged resins, which have been used as catalysts, were probed by measurement of the heats of immersion in 1-nitropropane, n-hexane, and water. It was found that the electrostatic field strengths (F)calculated from the heats of immersion in 1-nitropropane and n-hexane increased with increasing ratios of the exchanged multivalent cation (La3+)in the univalent form (H') cation-exchange resin. This tendency was also observed in the differences in F between the La3+exchanged resins and H+ form of the resin by using the calorimetric data obtained from the heats of immersion in water. These results suggest that the exchanged La3+ion does not homogeneously interact with three univalent anionic sites (SO3-)of the cation-exchange resin, but interacts with only two SO3- ions, that is, the La3+ion is localized on the surface of the resin. The difference in F obtained from the heats of immersion into water was found to be useful as a simple and rapid criterion of the surface characteristics of the cation-exchange resins.
Introduction
Ion-exchange resins are used in the various fields, for example, as the recovery agents' of precious metal ions from plating waste water, catalysts2q3or adsorbents4 in chemical processes, and even disinfectantss7 for water treatment. During investigations of the surface characteristics of ion-exchange resins, we have founds,9 that H+ ions of a macroreticular sulfonic acid cation-exchange resin, which can be used as an acid catalyst or a recovery agent for noble metal ions such as rhodium and nickel from acidic plating waste water, exist in concentrations of 5-6 M on the surface of the resin. If the concentrated H+ ions are fully utilized, it seems that they can be used as a highly effective catalyst or can devitalize microorganisms in water. The actual behavior of the resin, however, is very complicated, especially when the H+ ions are exchanged with multivalent cations. Namely, an interesting behavior of the H+ form of the resin is the nonlinear dependence of the reaction rates on the H+ ion concentration as found by in butanol and formic acid dehydration and by Uematsu1*J3in butene isomerization. These results suggest that the H+ ions on the resins are not always homogeneous, as indicated by Felfferi~h,'~ but heterogeneous. Thus, the purpose of this work is to determine the surface characteristics of the H+ form and its multivalent cation-exchanged resins by means of a simple and elegant method using the cal(1) T. Suzuki, T. Hatsushika, Y . Hayakawa, N. Ayuzawa, and Y. Matsumura, Conserv. Recycl., 4, 239 (1981). (2) T. Suzuki, M. Sen6, and T. Yamabe, J . Chem. SOC.Jpn., 88, 1141 (1967). (3) T. Uematsu, K. Tsukada, M. Fujishima, and H. Hashimoto, J . Catal., 32, 69 (1974). (4) T. Suzuki,Y. Hayakawa, and Y . Matsumura, J. Chem. SOC.,Faraday Trans. I, 77, 2901 (1981). (5) L. R. Fina and J. L. Lambert, Proc. World. Congr. Water Resour. 2nd 1975, 2, 53. (6) T. Suzuki and L. T. Fan, J . Ferment. Technol., 57, 578 (1979). (7) T. Suzuki, S. Goto, and T. Tanaka, Denki Kaguku, 52, 272 (1984). (8) N. Ayuzawa, T. Suzuki, and Y . Hayakawa, Denki Kagaku, 49, 227 (1981). (9) T. Sumki, Y. Iwata, S . Goto, T. Sato, and T. Tanaka, Abstracts of the 36th Japanese Annual Meeting on Colloid and Surface Chemistry, Nishinomiya, Oct 8-10, 1983, p 296. (10) B. C. Gates and G . M. Schwab, J . Catal., 15, 430 (1969). (11) R. Thornton and B. C. Gates, J . Catal., 34, 275 (1974). (12) T. Uematsu, Bull. Chem. SOC.Jpn., 45, 3329 (1972). (13) T. Uematsu, T. Suzuki, and H. Kobayashi, J . Chem. SOC.Jpn., 4,475 (1977). (14) F. Helfferich, "Ion Exchange", McGraw-Hill, New York, 1962, p 519.
0022-3654/85/2089-4965$01.50/0
TABLE I: Heats of Immersion of the H+-La3+ Form of Cation-Exchange Resins in 1-Nitropropane and R -Hexane'
Hi,,erg/cm2 La3+ exchange molar ratio, % 1-nitropropane n-hexane 0 25 50 15 100
1183 1242 1316 1405 1582
355 355 370 384 414
F
X
lo-' esu/cm2 6.0 6.2 6.6 7.2 8.2
' S = 2.83 X IO6 cm2/g.
orimetric data obtained from the heats of immersion in water. Experimental Section
Materials. Amberlyst-15 (Rohm and Haas), which is a macroreticular sulfonic acid cation-exchange resin, was used. The resin particles, sieved to 28-32 mesh, were pretreated in a column with aqueous solutions of hydrochloric acid (1 mol dm'-3) and sodium hydroxide (1 mol dm-3), and then with ethanol and water. After this treatment, the resin was exchanged to H+ form in excess hydrochloric acid. Repeated washing with water was carried out prior to drying at 80 "C for 3 h. The resins were stored in a desiccator which had been prepared with sulfuric acid and water at 25 "C and had a relative humidity of 30%. The H+ form of the resin was converted to H+-La3+ as follows. Each H+ form of the resin (0.1 g) from the desiccator was mixed and stirred with 30 cm3 of aqueous La(N03), solutions containing various amounts of lanthanum (La3+)ranging from 0.3 X to 10 X mol at 20 "C for 2 h and forms of the resin loaded with various amounts of La3+ions were prepared. The La3+ ion was selected as representative of typical and stable multivalent cations. The loaded sample resins were repeatedly washed with water and dried at 80 "C prior to storage in a desiccator at a relative humidity of 10%. The amount of exchanged La3+was determined by EDTA titration. The solvents used were distilled water and reagent grade solvents such as 1-nitropropane and n-hexane, which were purified by a usual method15 with dehydrating agents and dried with Linde molecular sieve 4A. 1-Nitropropane and n-hexane were selected as appropriate solvent molecules as discussed later because they (15) A. D. Wilks and D. J. Pietrzyk, Anal. Chem., 44, 676 (1972)
0 1985 American Chemical Society
4966
Suzuki and Uematsu
The Journal of Physical Chemistry, Vol. 89, No. 23, 1985
have almost same sizes and different permanent dipole moments (1-nitropropane, 3.57 D; n-hexane, 0 ) . Measurement of the Heat of Immersion. Glass ampules, each containing a sample from the desiccator, were evacuated at 120 O C under 2 X Pa for 4 h. They were sealed in vacuo. One of the sample ampules and one empty ampule were placed in the two cells of the twin conduction type microcalorimeter (MPC-11, Tokyo Rico Co. Japan), together with 3 0 cm3 of a solvent until they reached thermal equilibrium at 25 0.1 OC. Then the ampules were broken in the solvent and the heat liberated was recorded. A diagram of the measurement system is schematically represented in Figure 1. Each datum obtained was actually the average of at least three determinations for each sample. The reproducibility errors were found to be always less than 3%.
empty
E 3
sampl e, ( 0.1 g )
glass ampoule
*
Results and Discussion In Table I are summarized the heats of immersion of the H+-La3+ form of the resins in 1-nitropropane and n-hexane, respectively, together with the field strengths (column 4) which will be fully discussed in the following sections. As indicated in the table, the heats of immersion of the H+-La3+ form of the resins in polar solvent (1-nitropropane) were large and increased with increasing exchange ratios of La3+ions in the resins, while the heats of immersion in nonpolar solvent (n-hexane) were small and almost constant irrespective of the ratios of the La3+ions. Since the ionic exchange site of the cation-exchange resin and the resin matrix have been found16 to be hydrophilic and lipophilic, respectively, n-hexane is considered to mainly interact with the resin matrix and 1-nitropropane with both the matrix and the ionic site. Thus, the large difference between the heats of immersion in 1-nitropropane and those in n-hexane suggests that the ionic sites rather than the resin matrixes govern the surface characteristics of the ion-exchange resins. To elucidate the surface characteristics of the ionic exchange sites further, the following method was applied. In general
reference
sample
side side Figure 1. Schematic diagram of the measurement system.
i
(1) A",, = AH,,, + AH,,, where AH,,,, is the heat of immersion, AHH,,,the heat of solvation of the resin ionic sites, and AH,,, the heat of interaction of the solvent with the resin matrix. Applying eq 1 to systems of M form of the cation-exchange resins in 1-nitropropane (A) and n-hexane (B), we have respectively
+
AH~M,(A)= A H ~ , ( A ) AH:~(A)
(2)
(3) AH,M,(B) = A H ~ , ( B )+ AH;~(B) Since molecules A and B have almost same lipophilic characteristics and sizes, AHfur(A) and AHfu,(B) can be considered almost identical. Thus
AH$(A) - A H k D ) * AHZLA) - AHEdB) According to Fowkes" and Takahashi et a1.I' AH,,, = E P + E" + Ed E!- = -nFcL
(4)
+ Ld'exchange molar ratio for H+ form resin ("lo)
(5)
Figure 2. Electrostatic field strengths (F)of the H+-La3+ form of cation-exchange resins and the difference in the electrostatic field strengths between the H+-La3* form of the resins and the H+form of the resin.
(6) where E' is the energy of interaction between the electrostatic field F of the resin ionic sites and the permanent dipole of the solvent, E" the polarization energy, Ed the dispersion energy, n the number of the adsorbed solvent molecules per unit area (cm2), and p the permanent dipole moment of the solvent. As clearly shown in eq 5 and eq 6 , the heat of solvation of the resin ionic site in the solvent with p = 0 is decided by E" and Ed, while p becomes larger and F stronger, E' becomes dominant and causes higher heat values. (The p values of A and B are 3.57 and 0, respectively.) Moreover, E"(A) + Ed(A) can be considered almost identical with E"(B) + od(B), because molecules A and B have almost same structures and sizes as mentioned above. Thus, the large difference between the immersional heats in A (16) T. Suzuki and Y . Hayakawa, J . Phys. Chem., 83, 1178 (1979). ( 1 7) F. M. Fowkes, 2nd. Eng. Chem., 56, 40 (1964). (18) K. Tsutsumi and H . Takahashi, J . Phys. Chem., 74, 2710 (1970).
and B is considered to be attributable to that between E@(A)and E@(B) . Taking the arguments into account, we can obtain the following equation: = - AHiM,(A) - AH,f",(B) ~AMA
(7)
The specific surface area of the resin ionic sites, which is needed for calculating AH;,,,, was obtained by assigning one water molecule (cress-sectional area of 10.8 A2) to one resin ionic site. The value of the area was 2.83 X lo6 cm2 g-I. This value is more reasonable than the value of 0 . 5 3 X lo6 cm2 g-' obtained by applying the BET method at the liquid-nitrogen temperature. Because the resin shrinks appreciably at the liquid-nitrogen temperature, the surface area obtained at this low temperature is not considered to be realistic. Furthermore, since the average distance between two adjacent ionic sites has been estimated to
Surface Characterization of Cation-Exchanged Resins
The Journal of Physical Chemistry, Vol. 89, No. 23, 1985 4967
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50
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+La3+exchange molar ratio for H+ form resin ("lo)
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Figure 3. Heats of immersion of the H+-La3+ form of cation-exchange resins in water and the difference in the electrostatic field strengths between the H+-La3+ form of the resins and the H+ form of the resin obtained from the heats of immersior in water.
be approximately 10 A'3s16the assignment of one water molecule (10.8 A2) to one ionic site is considered to be reasonable. nA was 4 X lOI4. This value was calculated by assuming the cross-sectional area" of a 1-nitropropane molecule to be 25 A2. The obtained electrostatic field strengths (F)and the differences (F$-La'+)between the La3+-exchangedresins and H+ form of the resin are shown in Figure 2. increased From the figure, it was found that both F and clearly as the exchanged ratios of La3+were increased, especially when the ratios were increased beyond 50%. This result suggests that the surface of the resin becomes more polar with increasing exchange ratio of the La3+ ion. Notice that the Fvalue of 100%exchanged resin is 8.2 X IO5 esu.cm-2 and the value is larger than the value of 6.0 estimated by Takahashi et aI.'* for the 100% exchanged Na+ form of Y zeolite (inorganic cation exchanger) by the divalent CaZ+ion. In this manner, the obtained electrostatic field strength (F)was found to be useful as an index of the surface characteristics of the resins, but the method for the determination of F was not so simple and rapid. Thus, the following method was applied. Applying eq 5 to the system of the M and N forms of resins by using water, we have respectively H=j(H20) = 1?'*~(H20)+ E"3M(H-jO) EdsM(H20) (8)
+
HE,(HzO) = E'sN(HzO) + E"3N(H20)+ Ed,N(H20) (9) Since water, which has a large p, is the solvent and the M or N form of the resin, which has a large F, is the adsorbent, we have approximately the following relation: EpsM(H20)or Ew."(HzO)> E"9'(H20) + EdlM(H20)N E"3N(H20)+ EdsN(H20)(10) Substituting eq 6 in eq 8 and 9, and taking eq 10 into account, we obtain the following expression for the difference in the electrostatic field strengths between the two resins, FE, as FN - Fhg =
Pi = -
A~,rnN(HzO) - WnlM(H*O) ~H~OPH~O
(11)
I
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,
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( C )
Figure 4. Estimated ionic states on the surface of the cation-exchange resin: (a) original H + form of the resin: (b) H+-La3+ form of the resin: (c) La3+ form of the resin.
F i values obtained from eq 11 are presented in Figure 3 as encircled data, where M is H+, N are the La3+-exchanged resins, nHZ0equals 9.26 X loi4,and pHz0 is 1.85 D. As clearly shown in this figure, the tendency of F; parallels that of Figure 2 (cf. the encircled open circles in Figure 2), that is, it was found that acid H+-La3+ ions on the resin interact with water, which is considered to be a base, in a manner similar to that of 1-nitropropane, which is a typical polar solvent, without special action. From these results, the state of the exchanged La3+ ion on the cation-exchange resin is estimated as follows. The exchanged trivalent La3+ ion does not homogeneously interact with three negative univalent sites (SO3-) of the cationexchange resin, but interacts with only two SO3-ions, that is, the La3+ ion is localized near the two SO3- ions, as shown schematically in Figure 4. Under this situation there exist one positive charge at the filled site and one negative charge at the empty site as clearly shown in Figure 4c and therefore, a strong electrostatic field would result. This estimation is considered to be reasonable, because the ion-exchange resin used has rigid matrix structure and the average distance between the two adjacent negative ionic sites (SO3-) has been estimated to be approximately 10 A, i.e., it is difficult for one trivalent La3+ion to interact with the three negative univalent sites homogeneously. Thus, the FE obtained from the heats of immersion in water was found to be useful as a simple and rapid criterion for studying the surface characteristics of H+ and its multivalent cation-exchanged resins. Acknowledgment. We thank the Ministry of Education of the Japanese Government for financial support (Project 57550497) and Mrs. S. Kat0 for secretarial help in the preparation of this paper. Registry No. La, 7439-91-0; amberlyst 15, 9037-24-5; 1-nitropropane, 108-03-2; n-hexane, 110-54-3: water, 7732-18-5.
