J. Phys. Chem. 1981, 85,1947-1951
strength and therefore the contention that counterion condensation does provide a component of the polarization. This same behavior has been observed in DNA, where, however, the observations are made by varying the effective charge density by adjusting the concentration of Mg2+ under conditions where the ion atmosphere is held constant with an excess of Na+.15128 In the experiments on the A-U and A.2U species, the only significant counterion present is Mg2+.
Conclusions The results of these experiments, therefore, demonstrate the following: (1) The insensitivity of the polarization energy of linear polyelectrolytes in an electric field to the formal charge density as predicted for the contributions (28)D.C. Rau and E. Charney, manuscript in preparation.
1947
of both the condensed counterion layer and the diffuse ion atmosphere, and (2) the existence of a strong dependence of the polarization energy on the ionic strength of the diffuse ion atmosphere as predicted by the Debye-Huckel treatment.15 The observation that the dichroism of the two species of poly(rA) and poly(rU) complexes is not linearly displaced (the parallelism of the exponential curves of Figure 2) is new evidence for the existence of a component of the polarization which is independent of ionic strength.1°J2 Because of uncertainties in the compositional dependence of the A.U and A-2U samples on ionic strength, the existence of this component of the polarization, due largely to the condensed counterions, can only be cautiously inferred from the present data. Acknowledgment. We thank H. T. Miles and D. C. Rau for very helpful discussions.
Refractive Indices of Molten KN03-NaN02 Mixtures and Electronic Polarizabilities of Potassium and Nitrite Ions Y. Iwadate, K. Kawamura,' Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, O-okayam, Meguro-ku, Tokyo 152, Japan
and J. Mochlnaga DepartImnt of Synthetic Chemistry, Faculty of €nglnwrlng, Chlba Unlverslty, Yayokho, Chlba 260, Japan (Received: December 29, 1980; In Flnal Form: March 11, 1981)
Refractive indices of the binary molten KN03-NaN02 mixtures were measured with visible light at 12 wavelengths with the goniometer method. The refractive indices of these molten mixtures were represented by empirical formulae as functions of both temperature and wavelength. The molar refractivities were calculated from these data. The refractive indices at infinite wavelength were estimated by use of Cauchy's approximate relation for dispersion, from which it was found that Fajans' theory was not applicable to the description of the electronic polarizabilities of these molten mixtures or to the molar refractivities. The electronic polarizabilities of K+ and NOz- at 340 "C were calculated to be 1.03 X and 3.24 x cm3, respectively.
Introduction Measurement of the refractive indices is one of the useful and convenient approaches for an estimation of the electronic polarizabilities of molecules or ions in a medium. The electronic polarizabilities of individual ions have so far been evaluated in various states such as gases,lS2 aqueous solution^,^-^ organic solutions~'crystals,g1oand liquid crystals.'l Numerous investigations on the refractive indices of molten salts have been performed and these results have been summarized.12 In recent years, excellent accurate (1)L. Pauling, Proc. R. SOC. London, Ser. A, 114,181 (1927). (2)M.Born and W. Heisenberg, 2.Phys., 23,388 (1924). (3)C.J. F. Bottcher, Recl. Trau. Chim. Pays-Bas., 62,325,503(1943); 65,19,91 (1946). (4)K. Fajans and G. Joos, 2.Phys., 23, 1 (1924). (5)J. K. Baird, H. R. Petty, J. A. Crumb, V. E. Anderson, and E. T. Arakawa, J. Phys. Chem., 81,696 (1977). (6) C. J. F. Bottcher, Physica, 9,945 (1942). (7)J. D. Olson and F. H. Horne, J. Chem. Phys., 58, 2321 (1973). (8)A. R. Ruffa, Phys. Rev., 130, 1412 (1963). (9)E.Kordes, 2.Elektrochem., 59, 551 (1955). (10)J. R. Tessman, A. H. Kahn, and W. Shckley, Phys. Reu., 92,890 (1953). (11)P. Adamski and A. D. Gromiec, Mol. Cryst. Liq. Cryst., 35, 337 (1976). (12)G. J. Jam, "Molten Salb Handbook, Academic Press, New York, 1967, p 89. 0022-365418112085-1947$01.25/0
measurements of the refractive indices of molten salts have been carried out by Gustafsson et al.,13-15in which the refractive indices have been measured up to seven significant figures with wave-front shearing interferometry. Nevertheless, few data on the refractive indices of molten salts are available to estimate the refractive indices extrapolated to infinite wavelength by which the electronic polarizabilities of ions in molten salts can be determined. The purpose of this work is to obtain the temperature coefficients of the refractive indices needed to estimate the thermal conductivities of these molten ionic mixtures by wave-front shearing interferometry16-18and, a t the same time, to measure the electronic polarizabilities of ions in ionic melts. In the present work, the refractive indices of molten KNO3-NaNOZ mixtures were measured with visible light (13)L.W. Wendelov, S.E. Gustafsson, N. Halling, and R. A. Kjellander, 2.Naturjorsch. A, 22, 1363 (1967). (14)S.E. Gustafsson and E. Karawacki, Appl. Opt., 14,1105 (1975). (15)E.Karawacki and S. E. Gustafsson, 2.Naturforsch. A, 31,956 (1976). (16)0.Odawara, I. Okada, and K. Kawamura, J. Chem. Eng. Data, 22, 222 (1977). (17)S. E.Gustafsson, 2.Naturforsch. A, 22, 1005 (1967). (18)S. E.Gustafsson, N. 0. Halling, and R. A. E. Kjellander, Z. Nuturforsch. A, 23,44,682 (1968).
0 1981 American Chemical Society
Iwadate et al.
The Journal of Physical Chemistty, Vol. 85, No. 73, 798 7
1948
TABLE I: Refractive Index Equations of the Binary KN0,-NaNO, System" h /nm
X(NaNO,)/ mol %
440 460 480 500 520 540 560 580 600 620 632.8 650 temp range, "C
"n =a- b X range from 0.66 X
0.00 24.84 50.43 52.41 74.92 100.00 1.4647 1.4821 1.4754 1.4843 1.4806 1.4781 1.241 1.576 1.500 1.692 1.587 1.487 1.4789 1.4714 1.4597 1.4799 1.4791 1.4804 1.191 1.583 1.478 1.662 1.628 1.652 1.4592 1.4726 1.4651 1.4744 1.4791 1.4759 1.470 1.375 1.273 1.572 1.690 1.593 1.4532 1.4720 1.4613 1.4735 1.4712 1.4708 1.158 1.521 1.331 1.554 1.529 1.589 1.4526 1.4677 1.4625 1.4714 1.4730 1.4724 1.200 1.456 1.419 1.616 1.618 1.638 1.4612 1.4525 1.4704 1.4677 1.4665 1.4698 1.257 1.561 1.431 1.522 1.614 1.521 1.4713 1.4653 1.4670 1.4645 1.4591 1.4487 1.181 1.455 1.417 1.574 1.676 1.496 1.4657 1.4650 1.4664 1.4653 1.4574 1.4446 1.101 1.510 1.402 1.594 1.555 1.515 1.4650 1.4616 1.4637 1.4638 1.4571 1.4442 1.427 1.124 1.548 1.499 1.567 1.454 1.4624 1.4597 1.4637 1.4619 1.4583 1.4446 1.163 1.475 1.497 1.575 1.528 1.427 1.4618 1.4605 1.4630 1.4612 1.4570 1.4475 1.280 1.481 1.479 1.575 1.521 1.468 1.4617 1.4590 1.4606 1.4617 1.4553 1.4447 1.219 1.515 1.446 1.531 1.536 1.446 342.5-422.3 314.8-365.8 295.7-396.1 310.4-399.5300.1-389.1292.4-353.0
t , ( t / " C ) . In each column, the top line gives the a value, the bottom line b , and the standard errors to 6.25 X
a t 12 wavelengths by the goniometer method. Experimental Section Chemicals and Melt Preparation. The chemicals KNOB and NaNOz used were of analytical reagent grade. They were roughly dried by heating in vacuo to the temperatures just below their respective melting points for 7 h and then melted. A small amount of water was removed by bubbling dry argon gas into the melts for a few hours. The argon gas was purified beforehand by passing through a titanium sponge heated to 1000 OC. Such pretreatment is indispensable since only a slight amount of water contained in the melts is corrosive toward fused silica and makes it difficult to measure the refractive indices. Apparatus and Procedure. The experimental method and the schematic diagram of the apparatus have been described in detail p r e v i o ~ l y . ' ~The hollow prismatic cell used was made of optical fused silica. The volume of a melt needed for one experimental run was only 2 cm3 and it took a few hours to perform one run. In order to estimate the refractive indices, we measured the angles of the minimum deviations with a goniometer to within a precision of 1'. The relation between the refractive index and the angle of minimum deviation is expressed in the form nh = sin [(6, A)/2]/sin (A/2)
+
where nh, Ljh, and A are the refractive index at wavelength A, the angle of minimum deviation at wavelength X and the apex angle of the prism, respectively. The apex angle of the prism was calibrated by use of NaN03 as a reference material, since the refractive indices of the pure NaN03 melts have already been accurately measured by Gustafsson and Karawacki.14 The temperature of the melts was measured by directly inserting a sheathed chromel-alumel thermocouple into the melts and recorded with the precision of 0.1 "C during the experiment. As seen from Figure 1,the refractive indices of pure ~~
(19)J. Mochinaga and Y. Iwadate, Denki Kagaku, 47, 345 (1979).
AgN03 and KNOBmelts obtained with this apparatus were in good agreement with those in the l i t e r a t ~ r e . ' ~ J ~ ~ ~ ~ Especially, the straight lines C and D substantially coincide with each other. In the present experiment the fused silica expansion will have little influence on the refractive index since the coefficient of expansion of fused silica is very small, viz., 5.5 x OC-'. The refractive indices of molten KN03-NaN02 mixtures were measured with visible light at wavelengths of 440,460, 480,500,520,540,560,580,600,620,632.8, and 650 nm. Results and Discussion Refractive Indices of the Binary KN03-NaN02System. The refractive indices of molten KN03-NaN02 mixtures and pure KN03 and NaNO, melts at a given wavelength decrease with increasing temperature in the same manner as shown in Figure 1. Gustafsson and K a r a ~ a c k i , ' ~ Wendelov et al.,13 and Karawacki et al.15921have reported that the refractive indices at a fixed wavelength slightly decreased curvilinearly with temperature. By expressing the refractive indices as both linear and quadratic functions of temperature, they have concluded that the quadratic functions are better. In the present experiment, however, the standard errors for the linear functions were almost equal to those for the quadratic ones, that is, some are better fitted by linear functions of temperature and the others by quadratic ones. In all cases, the temperature coefficients of the quadratic term in the refractive indices are too small to affect the values of the refractive indices above experimental errors. Accordingly, only the refractive index equations as linear functions of temperature are tabulated in Table I, where a and b are the constants. As seen from Table I, the temperature coefficients of the refractive index equations are nearly independent of wavelength and mole fraction (20) H. Bloom and D. C. Rhodes, J.Phys. Chem., 60,791 (1956);R. Aronsson and E. Karawacki, Z. Naturforsch. A , 35, 694 (1980). (21)E.Karawacki, Ph.D. Thesis, Chalmers University of Technology, Goteborg, Sweden, 1977.
The Journal of Physical Chemistry, Vol. 85, No. 13, 1981
Refractive Indices of Molten KNOB-NaNO, Mixtures
1.7
tl"C
200
1949
4 AgN03 ..........
c
1.6
.
1.5
.
1
1.4
1.3
D- '
KNO,
5
400
300
tI T
0
Flgure 1. Refractive indices of pure AgNO, and KNOBmelts: (A) this work, n = 1.7115 - 1.664 X 104t, 243-342 OC, 589.3 nm. (B) Bloom and Rhodes,20n = 1.706 - 1.55 X 104t, 260-365 OC, 589.3 nm; (C) this work, n = 1.4618 - 1.521 X 10-4t, 343.4-421.9 OC,632.8 nm; (D) Gustafsson and Karawacki," n = 1.464045 - 1.5635 X 10-4t, 340-420 OC, 632.8 nm.
TABLE 11. Molar Refractivities o f the Binary K N 0 , - N a N O , System X(NaNO,)/ mol %
0.00 24.84 50.43 52.41 74.92 100.00 0.00
24.84 50.43 52.41 74.92 100.00
RA,~
cm
RA(add)/
cm3
devn from add./%
340 'c, A = 580 nm 13.39 13.39 12.37 12.41 11.40 11.40 11.32 11.37 10.46 10.43 9.44 9.44
0.00 -0.32 0.00 0.44 0.29 0.00
340 'C, A = 650 nm 13.29 13.29 12.32 12.28 11.31 11.31 11.28 11.24 10.38 10.3 5 9.37 9.37
0.00 -0.32 0.00 0.36 0.29 0.00
of NaN02 over the measured temperature range. Molar Refractivity. The molar refractivity introduced by Lorentz is one of the best parameters that explain the polarization phenomenon. It is obtained from the refractive index data by use of the Lorentz-Lorenz equation RA= l(nx2- U/(n? + 2)1Vm (1) = {(n? - l)/(n? + 2)1(M/d) where R, is the molar refractivity of the molten mixture a t wavelength A, M the mean molecular weight of the mixture defined as M = Mix, + M2X2(Miand Xiare the molar weight and mole fraction of component i, respectively), and d and V , are the density and molar volume of the mixture, respectively. The molar volume data for molten KN03-NaN02 mixtures were taken from the literature.22 The molar refractivities of molten KN03-NaNOz mixtures based on measurements with light of wavelengths 580 and 650 nm at 340 "C are listed in Table 11, where R,(add) refers to the molar refractivity calculated (22) Y. Iwadate, I. Okada, and K. Kawamura, to be submitted to J. Chem. Eng. Data.
v,
=
3.89832 X 10 - 1.10007X - 1.49694 X l o x + 7.98663X3 + (2.40004 X 10" - 2.72564 X lO-'X + 3.41525 X 10-'X2 - 1.87457 X 10-'X3)t
where V,, X, and t are the molar volume, the mole fraction of NaNOz melt, and the absolute temperature, respectively.
3,
Inm
Figure 2. Dispersion in refractive index: (A) pure NaNO, melt (290 OC); (B) molten KNOB(47.59 mol %)-NaNO, (52.41 mol % ) mixture (380 "C); (C) pure KNO, meit (400 OC).
on the basis of the additivity principle. As shown in Table 11, the additivity principle can be applied to the molar refractivities of molten KN03-NaN02 mixtures within experimental errors. The additivity principle holds well even when measuring conditions such as temperature, wavelength, and composition are changed. The molar refractivities of pure KN03 melt were found to be always greater than those of pure NaNOz melt. The molar refractivity generally increases with temperature, although the temperature coefficient is very small. The case is the same with the present mixtures as in other mixtures previously s t ~ d i e d . ' ~The , ~ ~temperature coefficients of the molar refractivities of molten KN03-NaN02 mixtures ranged from 0.50 X lo9 to 1.50 X cm3 'C-l, which are in good agreement with the result by Bloom and Peryer." This result indicates that the molar volume contributes more to the molar refractivity than the quantity (nX2l)/(n? 2) does,25since the latter decreases with an increase of temperature. Dispersion in the Refractive Index. The relations between the refractive indices of molten KN03-NaN02 mixtures and wavelength are illustrated in Figure 2, in which the refractive indices at given temperatures were interpolated with the linear equations in Table I. The refractive indices of the mixtures decreased curvilinearly with increasing wavelength. They show the so-called normal dispersion.26 The refractive index extrapolated to infinite wavelength is needed for the determination of the electronic polarizability. In trying to evaluate it, we have examined the following four equations:
+
+ B/A2 + C/h4 n, = A + B/Az = ( A + B/A2 + C/X4)i/2 n, = ( A + B/A2)i/2
n, = A n,
(2) (3) (4)
(5)
These equations are derived by expanding the dispersion (23) J. Mochinaga and Y. Iwadate, J. Fac. Eng. Chiba Uniu., 30, 213 (1979). (24) H. Bloom and B. M. Peryer, Aust. J. Chem., 18, 777 (1965). (25) Y. Iwadate, K. Igarashi, and J. Mochinaga, Denki Kagaku, 48,97 (1980). (26) M. Born and E. Wolf, "Principles of Optics", Pergamon Press, New York, 1974, Chapter 2.
1950
Iwadate et al.
The Journal of Physical Chemistry, Vol. 85, No. 13, 1981
TABLE 111: Approximations of the Refractive Indices o f the Binary KN0,-NaNO, System on the Basis of Cauchy's Relation n(t,h) = ( p + q / A 2 + r / A ' ) + (Pt + q t / h 2 + r t / h 4 ) t ; ( t r C ) X(NaNO, ) 0.00 24.84 50.43 52.41 74.92 100.00 P 4 r Pt qt
't
std error
1.4308 1.5306 X lo4 -1.0862 X l o 9 -1.0025 X -3.0550 X 10 3.6995 X l o 6 2.46 X
1.4224 1.8345 X 10' -1.3960 X l o 9 -6.7328 X -4.3525 X 10 5.1621 X l o 6 3.82 x 10-4
1.4597 -2.1458 X lo' 1.3326 X l o 9 -1.7982 X 1.6213 X 10 -2.7233 X l o 6 3.44 x
formula into a power series of wavelength on the basis of Cauchy's relation under the condition of constant temperature. Marcoux2' has made use of eq 4 in order to interpret the dispersion of the refractive indices of inert gases such as Ar, Kr, and Xe. By comparing the standard errors on fitting to these approximate equations, eq 2 is found to be the best applicable to the experimental values as is the case with the refractive indices of molten rare earth chloride-alkali chloride mixture^.'^^^^ Since the refractive indices are dependent on both temperature and wavelength, it may be more convenient to express the empirical equations as functions of temperature and wavelength in the following form:25 n(t,X) = ( p + q / X 2
+ r/X4) + (pt + qt/X2 + rt/X4)t
1.4563 -3.7340 X 10' 1.0403 X lo9 -1.6987 X 1.3606 X 10 -2.1958 X l o 6 2.59 x 10-4
1.4661 -8.9133 X l o 3 2.0676 X lo9 -2.1142 X 3.8904 X 10 -5.2263 X l o 6 2.08 x 10-4
TABLE IV: Electronic Polarizabilities for the Binary KN0,-NaNO, System at 340 "C X(NaNO,)/ mol %
0.00 24.84 50.43 52.41 74.92 100.00
CY-/
cm3 5.12 4.76 4.39 4.36 4.01 3.62
(8)
1
where Ni and ai are the number of ith ions in the unit volume and the electronic polarizability of ith ion, respectively. The refractive indices at infinite wavelength (27) J. E. Marcoux, Can. J.Phys., 48, 1947 (1970).
0.00 0.21 0.69 0.69 0.25 0.00
5.12 4.75 4.36 4.33 4.00 3.62
NO,-, K',
t/"C C Y , ( K N O , ) / ~ Ocm3 -'~ NO,- )/lo- a4 cm3 C Y , ( K + ) / ~ O - ' cm3 ~
CY -(
340
360
5.12 4.09 1.03
5.15 4.10 1.05 t/"C
+
(nm2- l)/(nm2+ 2) = (4a/3)(CNiai)
@,(add)/ devn from cm3 add./%
TABLE V : Electronic Polarizabilities of NO;, and Na' Ions and KNO, and NaNO,
(6)
The constants from p to rt for the molten KN03-NaN02 mixtures were determined by a least-squares method. The results are shown in Table 111. By use of eq 6, not only the refractive index extrapolated to infinite wavelength (p ptt) but also the refractive index and its temperature coefficients a t an arbitrary temperature and wavelength are obtainable. In applying eq 6 to the original data, attention must be paid to anomalous dispersion, since the refractive indices are changed drastically near the absorption bands. Electronic Polarizabilities of Ions. It is well-known that polarization can be classified into three types, that is, an orientation polarization, an ionic one, and an electronic one. The induced dipole moment is the product of the polarizability and the so-called local field. In general, the polarizability is denoted in the form of 3 X 3 tensor in discussing it in the solid state. In liquids including molten salts it becomes scalar as the averaged value of its tensor. In the present work, the ionic polarization and the orientation one are negligible because neither can follow the rapid change of the alternating electric field at the visible wavelength region (about 5 X 1014 Hz), and only the electronic one remains undisturbed by the thermal motions of the individual ions a t high temperature. The polarizability discussed here is, therefore, reduced to only the electronic one. The electronic polarizabilities for the molten mixtures are defined by the semiclassical Clausius-Mossotti equation analogous to the Lorentz-Lorenz equation a, = (3/(4~Iv))((n,~ - l ) / ( n m 2+ 2)1Vm (7) where N is Avogadro's number and the subscript ~0 refers to infinite wavelength. An additional expression has been introduced for the purpose of further interpretation of the polarization phenomenon
1.4400 -7.3170 X 10' 1.0802 X l o 9 -1.4150 X 1.3925 X 10 -2.0819 X l o 6 2.25 X
a,(NaN0,)/10-24 cm' a,(Na+)/10-24cm3 ~ , ( N 0 , - ) / 1 0 - 2cm' 4
3 20
340
3.62 0.38 3.24
3.62 0.38 3.24
were calculated from the equations at Table 111. The results for molten KN03-NaN02 mixtures at 340 "C are given in Table IV, in which a,(add) is the electronic polarizability estimated on the basis of the additivity principle. As seen from Table IV,the electronic polarizabilities for the molten mixtures decrease linearly with increasing mole fraction of NaN02 and the additivity principle can be applied to the electronic polarizabilities as well as the molar refractivities. Fajam% has reported in a series of papers that the deviations from additivity occur in the presence of electrical interactions among particles such as molecules and ions, and in cases of less symmetrical distribution of positive charges. It is generally ascertained by the molecular dynamics simulation that there exist not only the electrical interactions between ions but also the less symmetrical distribution of cations in molten salts. Accordingly, electronic polarizabilities cannot correlate to electrical interactions and lesser symmetricity. It is pointed out from the present work that Fajans' theory is not suited to electronic polarization in the molten salt mixtures. We tried to evaluate the electronic polarizabilities of the K+ ion a t 340 and 360 "C and of the NO2- ion at 320 and 340 OC on the basis of eq 8. For this calculation, electronic polarizability data for the NOs- ion and the Na+ ion are necessary and were taken from those by Murakami et al.= In the estimation for the NOz- ion, the electronic polarizability of the Na+ ion was calculated by subtracting that (28) K. Fajans and 0. Johnson, Trans. Electrochem. Soc., 82, 273 (1942); K.Fajans, Trans. Faraday Soc., 23, 357 (1927). (29) K. Murakami, K. Igarashi, and J. Mochinaga, "Proceedings of the Symposium of the Electrochemical Society of Japan (Denki-KagakuKyokai Shuki-Nenkai Koen-Yokoshu)", Tokyo, Oct 1-2, 1980.
J. Phys. Chem. 1981, 85,1951-1956
of the NO3- ion from that of NaN03 and then the polarizability of the NO2- ion was estimated with the value of Na+ ion, since LiNOz is easy to decompose on melting and it was impossible to measure the refractive indices and the molar volumes of LiN02, that is, to evaluate the electronic polarizability of the NOz- ion directly even if that of Li+ ion was assumed to be 0.029 X cm3, as determined by Pauling.' In the estimation for the K+ ion, a similar procedure was also used. The results are listed in Table V. It is worthy to note that the electronic polarizabilities of the K+ and NO3- ions are slightly dependent on the temperature, but those of the Na+ and NOz- ions are almost independent of it. This result indicates that small ions have small electronic polarizabilities for which temperature coefficients are negligibly small. Since the NO2- ion is less bulky than the NO3- ion, the electronic polarizability of the NOz- ion is smaller than that of the NO3- ion.
1951
These results are in good agreement with the values of cm3 for the K+ ion and 3.4 X 10-24-4.0 X lo-" 1.20 X cm3 for the NO3- ion reported by Tessman et al.'O It is also found that the cations K+ and Na+ are less polarizable than the anions NO, and NO2-. This fact is attributed to the following reason that the relatively increased nuclear charge attracts the electron shells when one atom becomes cationic. From the above results and other data previously reit is concluded that the factors which govern the electronic polarization in the molten salt are the ionic radius, the effective nuclear charge, and the temperature. These relations are, however, left to be solved quantitatively. Acknowledgment. The calculations were carried out with the computers (HITAC M-180 and M-200H) at the Tokyo Institute of Technology.
Quantum-Chemical Study of the Physical Characteristics of AI3+, AIOH2+, and AI(OH),+ Zeolites S. Beran,' P. Jhu, and B. WichterlovC The J. Heyrovskj Instirute of Physical Chemistry and Eiectrochemistry, Czechoslovak Academy of Sciences, 121 38 Prague 2, Mchova 7, Czechoslovakla (Received: December 9, 1980)
Models of faujasite zeolites with AI3+,Al(OH)2+,and Al(OH)2+cations localized in the Sn and Si cationic positions were studied by the CNDOIP method. Calculations indicate that the A1 cations introduced into the zeolite structure exhibit strong Lewis acidity with the trend A13+ > A1(OH)2+> A1(OH)z+and that the OH groups bonded to the A1 ions are less acidic than the skeletal hydroxyls. The presence of these cations in the zeolite cavities has no substantial effect on the charge distribution in the skeleton, but the cation-skeleton bond strength and weakening of the S i ( A l 4 bonds of the zeolite skeleton are more marked for these cations than for univalent Na+ cations. The final effect is, however, an increase in the stability of the zeolite structure.
Introduction Faujasite-type zeolites with simultaneously exchanged Me"+ and H+ ions have recently attracted attention because of their high carbonogenic activity and relatively high structure stability.'-3 Ion-exchange zeolites A1H-Y4 have been studied in our laboratory in connection with a study of the structure and catalytic activity of stabilized H-Y zeolites. Because of the ready hydrolysis of A13+,A1H-Y zeolites probably contain A1(OH)2+and A10H2+ions in addition to A13+ions. The experimentally determined high catalytic activity of these zeolites compared with H-Y (deuterium exchange and ethylene oligomerization) has been attributed to the presence of both electron-acceptor sites (Lewis sites) localized on the A1 ions in the cation positions and proton-donor sites (Bronsted sites), represented by acid skeletal groups. Earlier studies by Wang and Lunsford2s3 indicate increased catalytic activity in the disproportionation of toluene and structural stability of A1H-Y zeolites compared with H-Y. However, the ESR spectra of adsorbed NO and 02-and IR studies of skeletal OH group^^,^ of (1) 1977. (2) (3) (4)
P. A. Jacobs, "Carbogenic Activity of Zeolites", Elsevier, New York,
K. M. Wang and J. H. Lunsford, J. Catal., 24, 262 (1972). K. M. Wang and J. H. Lunsford, J.Phys. Chem., 75,1165 (1971). B. Wichterlovi, J. Nov&ovi, L. Kubelkovi, and P. Jiru, Proc. Int. Conf. Mol. Sieues, 5th, 373 (1980). 0022-365418112085-195 1$01.25/0
these zeolites did not satisfactorily explain the function of the Al cations in the cation positions of the zeolites. The identity of the mentioned ESR spectra for A1H-Y and H-Y zeolites indicates2J that adsorption of NO and 0 2 on A1H-Y also occurs on the tricoordinated skeletal A1 similarly to dehydroxylated H-Y zeolite. As the presence of A P , A1(OH)2+,and A1(OH)2+ions in A1H-Y-type zeolites makes a considerable contribution to their catalytic activity and increased structural stability, this work was also devoted to study of the coordination of the above cations in the cation positions of the skeleton, their physicochemical properties, and the effect of these cations on the character of the skeletal hydroxyls and stability of the zeolite structure. Model and Method Coordination of the A13+,A1(OH)2+,and A1(OH)2+cations in the zeolite skeleton was investigated by using the cluster model of the zeolite. Especially for ionic crystals, this method of modeling the solid phase represents a considerable simplification as it does not include electrostatic effects in real crystals. A number of works"18 using (5) I. D. Mikheikin, I. A. Abronin, G. M. Zhidomirov, and V. B. Kazanskii, J. Mol. Catal., 3, 435 (1978). (6) I. D. Mikheikin, I. A. Abronin, G. M. Zhidomirov, and V. B. Kazanskii, Kinet. Katal. 18, 1580 (1977). (7) V. I. Lygin and V. Seregina, Vestn. Mosk. Unzu., Khim., 515 (1976).
0 1981 American Chemical Society
