1850
J . Phys. Chem. 1987, 91, 1850-1856
of RIS parameters, and we report results with the former. For alkanes in isotropic media ("free chains") the conformational energy consists of two contributions, E D A and E N B . The dihedral angle energy E D A is a sum of the local RIS energies, E, and E,, and the nonbonded energy E N B is calculated for all atom pairs separated by four or more bonds; a Lennard-Jones 6-12 potential with united atoms44is used (Table IV). E N 8 ensures that energetically unfavorable local sequences are appropriately weighted (e.g., g f g F pairs are prohibited, the so-called "pentane effect") and removes longer ranged (excluded volume) overlaps. As the ensemble average that we employ to simulate quadrupolar splittings exhibited by the alkane solutes involves an average over complex angular variables, we first demonstrate that the alkane geometry and RIS parameters used herein are adequate for reproducing conformationally averaged properties. In order to evaluate them, we repeat a classic test of the RIS approximation: we compute the dipole moment of the apdibromo-n-alkanes, Br-(CH2-)m-Br.4s In these substituted alkanes, the RIS parameters differ from the conventional values at the dihedral angles a2and a,,,, and the reported values for the dihedral angle energy suggest E, - E, = O.& In recent calculations of the Kerr constant and dipole moment of these dibromoalkanes, Khanarian and T ~ n e l l conclude i~~ that ag*at the antepenultimate bonds are in the range of 80-100°; we employ a value of @gf = 80°. Following earlier modeling of the dipole moments, the energy of each conformation U(n)consists of three internal energy
u(n)= EDA(n) + ENB(n) + Edd(n) ('41) The third term E d d is the electrostatic interaction energy between (44) Gibson, K. D.; Scheraga, H. A. Proc. Nail. Acad. Sei. U.S.A. 1967, 58, 420. (45) Leonard, N. J.; Jernigan, R. L.; Flory, P. J. J. Chem. Phys. 1965,43, 2256. (46) Khanarian, G.; Tonelli, A. E. J . Chem. Phys. 1981, 75, 5031.
the intramolecular C-Br bond dipoles (and induced dipoles). As suggested in ref 46, we assume that the C-Br bond dipole is accompanied by an induced dipole moment along the nearestneighbor C-C bonds with magnitudes for C-Br and C-C bond dipoles p = 1.73 D and p = 0.49 D, respectively. The point dipole approximation with each dipole located at the midpoint of the respective bonds gives a value for Edd(n) via
where t = 2.274 is the dielectric constant of the solvent (benzene) used in e x p e r i m e n t ~and ~ ~ ,a~is~the vector connecting the point dipoles at bonds i = 1, 2 and j = m, m 1. The rms dipole / 2 calculated by introducing the squared magmoment ( p u 2 ) 1 is nitude of the dipole moment ( p ( n ) = pl + p2 + pm + p m + l ) into the general expression for a statistical mechanical average:
+
= Z - l U n ) exp[-U(n)/kT] n
(A3)
Figure 7 shows the calculated rms dipole moments together with experimental values. The agreement is good both in the magnitude and the subtle variation in the conformationally averaged dipole moment with alkyl chain length (Le., the steep increase from m = 3 to m = 5 and the smoother changes in the higher homologues are reproduced). The calculations clearly show sensitivity to RIS parameters, in particular, to the value of E,. Calculations employing an enhanced value of E underestimate ( p 2 ) . These results indicate that the geometry and the conventional RIS parameters we are using (agf= 120°, E, = 2.092 kJ mol-') reproduce the observed averaged dipole moments of the dibromoalkanes with a reasonable degree of accuracy. Registry No. D2. 7782-39-0. (47) Hyaman, H. J. G.; Eliezer, I. J . Chem. Phys. 1961, 35, 644.
Study of Cu2+ Location in Zeolites Na-A and K-A by Electron Spin Resonance and Electron Spin Echo Spectroscopies Michael W. Anderson and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: September 29, 1986)
Electron spin resonance (ESR) and electron spin echo (ESE) spectroscopies are used to determine the cation site location of Cu2+ in hydrated and dehydrated forms of Cu2+-dopedzeolites Na-A and K-A. This is achieved by partial exchange of the Na-A and K-A zeolites by Cs+ cations which reside in well-defined crystallographicsites. Monitoring the weak hyperfine interaction between the Cuz+ and Cs+ cations using ESE makes it possible to determine the cation siting of Cu2+relative to the matrix of Cs' cations. At low Cuz+loadings the Cu2+favor sites on the threefold axis close to a zeolite six-ring. The larger the number of water molecules coordinated to the Cu2+the more the displacement of the Cu2+from the center of the six-ring face-except in the case of a trigonal-bipyramidalCu2+complex coordinated to one water molecule in the a-cage, one in @-cage,and three equatorial lattice oxygens which is located in site S2. An octahedrally coordinated Cu2+ complex in both zeolites, Na-A and K-A, coordinated to three H 2 0 molecules is displaced 0.09 nm into the @-cagewhile a tetrahedrally coordinated Cu2+complex bound to only one water molecule in K-A is displaced 0.02 nm into the @-cage. After complete dehydration the Cu2+ moves almost into the plane of the six-ring to give the strongest possible cation-zeolite interaction with a Cu2+-Oz bond length of 0.23 nm (0, = zeolite oxygen).
Introduction Transition-metal-exchanged zeolites are useful for a wide variety of catalytic reactions.' In particular Cu2+ has received much attention2-+' which, being paramagnetic, may be studied by electron (1) Maxwell, I. E. Aduan. Coral. 1982, 31, 1. (2) Mochida, I.; Hayata, S.;Kato, A.; Seiyama, T. J . Caral. 1970, 19,405. (3) Tsuruya, S.; Tsukamoto, M.; Watanak, M.; Masai, M. J. Catai. 1985, 93, 303.
0022-3654/87/209 1- 1850$01.50/0
spin resonance (ESR) technique^.^-^ ESR provides information concerning the number of different species and, from the g values and hyperfine splittings, their stereochemistry. When ESR is (4) Benn, F. R.; Dwyer, J.; Estahami, A.; Evmerides, N. P.; Szczepura, A. K. J . Catal. 1977, 48, 60. ( 5 ) Hathaway, B. J.; Billing, D. E. Coord. Chem. Rev. 1970, 5, 143. (6) Herman, R. G. Inorg. Chem. 1979, 18, 995. (7) Conesa, J. C.; Soria, J. J . Chem. Soc., Faraday Trans. 1 1978, 74,406. (8) Herman, R. G.; Flentge, D. R. J. Phys. Chem. 1978, 82, 720.
0 1987 American Chemical Society
Cu2+ Location in Na-A and K-A Zeolites
The Journal of Physical Chemistry, Vol. 91, No. 7, 1987 1851
coupled with electron spin echo (ESE) spectroscopy additional, quantitative, information may be obtained concerning the number and relative orientation of ligand molecule^.^-^^ The cation site location, however, very often remains somewhat speculative. For instance, sites S2' and S2* in zeolite A, which lie on the threefold axis passing through a zeolite six-ring displaced into the &cage and into the a-cage, respectively, cannot be distinguished. X-ray and neutron diffraction are not sufficiently sensitive at low Cu2+loadings or when there are several different Cu2+ species present to determine this. In this present work samples of Na-A and K-A are partially exchanged with Cs+ cations. These cesium cations are located in well-defined crystallographic sites. Cu2+ is then introduced into this matrix of Cs+ cations and the weak hyperfine interaction between the unpaired electron on the Cu2+and the nuclear spin on the Cs+ is monitored by ESE spectroscopy. By determining the position of the Cu2+with respect to the Cs+ cations the site location of the Cu2+ may in turn be determined. Owing to the nature of the experiment it is often possible to isolate a specific Cu2+ species when more than one Cu2+ species is present. A similar study was performed recently on CsH-Rho;I4 however, owing to the closer proximity of the Cs+ cations to the Cu2+in zeolite A, the technique is more sensitive to changes in the Cu2+ location in this latter zeolite. In both hydrated zeolites, Na-A and K-A, an octahedrally coordinated Cu2+complex is located at site S2' in the &cage. Also a tetrahedrally coordinated Cu2+ complex in K-A is located in the &cage but much closer to the plane of the six-ring. After complete dehydration the Cu2+ migrates to site S2 in a six-ring where it is trigonally coordinated to three lattice oxygens.
Experimental Section Zeolites Na-A and K-A were obtained from Union Carbide, Linde Division. Each zeolite was exchanged with three different loadings of Cs+ by exchange with 0.05 M CsOH solutions as follows: Na-A was exchanged with 80, 116, and 155 mL per 4 g of zeolite; K-A was exchanged with 107, 143, and 178 mL per 4 g of zeolite. Exchanges were carried out at room temperature for 24 h. These six samples were analyzed for Cs+, Na+, and K+ concentrations by commercial atomic absorption analysis. All the zeolites were then doped with Cu2+ by exchanging with 10 ~ ~ 100 mL H20per 1 g of zeolite which mL of IV3 M C U ( N Oand amounted to an exchange of approximately one Cu2+cation for every 40 unit cells. A higher level of Cu2+ exchange was also achieved by exchanging both Na-A and K-A with 200 mL of M C U ( N O ~per ) ~ 0.5 g of zeolite. Finally, the zeolites 1X were washed with hot, triply distilled water to remove externally adsorbed Cu2+complexes and dried in air at room temperature. A zeolite prepared in such a manner is termed "fresh". Crystallinity of the exchanged zeolites was checked by X-ray diffraction using a Philips diffractometer scanning at 1' of 28 per min. For ESR and ESE measurements the zeolite samples were loaded into 3-mm-0.d. Suprasil quartz sample tubes where they could be evacuated and heated for activation. Samples that were dehydrated at 400 'C were subsequently exposed to 400 Torr of high-purity oxygen in order to oxidize any Cu(1) or Cu(0) formed upon activation. The oxygen was then pumped off for 30 min a t 400 'C and the sample cooled to room temperature. ESR spectra were recorded at both room temperature and 77 K on a Varian E-4 spectrometer. The ESE spectra were recorded at 4 K on a home-built spectrometer, described e l ~ e w h e r e , ' ~ , ' ~
Figure 1. Crystal structure of zeolite A showing cation positions. Site S2 is at the center of a six-ring face with site S2' and S2* displaced into and out of the @-cagealong the triad axis, respectively. Site S3 is adjacent to the four-ring in the a-cage while site S5 is at the center of the octagonal window.
linked to a Nicolet 1280 computer and 293 B pulse programmer. In all cases three-pulse, stimulated echoes were measured as a function of time T between the second and third pulses. Modulation from zeolitic aluminum was suppressed by keeping the time between the first two pulses, T, in the range 0.28 to 0.32 ps. Simulation of the ESE data was performed using methods akin to those described previo~sly.'~The pulse sequence used was 90'-90'-90' and phase cycling was employed to eradicate the two pulse glitches which occur at times T = T and T = 27, owing to imperfect 90' pulses." The following phase sequence is used: [(000) + (naO)] - [(ma) + (OOa)]. This sequence both eliminates the two pulse glitches and corrects for base line drift.
Theory The general equations used to simulate the three-pulse ESE modulation from cesium in these experiments were outlined previo~sly.'~However, in this paper we invoke a model which includes correlation9 between the Cs+ cations. The structure of zeolite A is shown in Figure 1 and consists of P-cages linked together via double four-rings in a cubic arrangement to enclose the larger cy-cage. The cy-cage is accessed by eight-rings and it is at the center of these eight-rings, site S5, where the Cs+ cations are located. The Cu2+will then more than likely be located near a six-ring face of a P-cage. This arrangement can be envisioned as the Cs+ cations positioned at the center of the faces of a cube of edge length 1.21 nm, the unit cell dimension of zeolite A, with the Cu2+cation located on the [ 11 11 unit vector. Therefore, if all the faces of the cube are occupied by Cs+ cations, then this juxtapositioning of cations forms a triangular-based pyramid with the Cu2+at the apex and the Cs+ cations at the corners of the base. The following relationship then exists between the Cu2+-Cs+ distance, r, and the displacement of the Cu2+from the plane of the six-ring into the cy cage, 6: r,, = (0.351 - 0.6176,,
+ 6,m2)1/2
(1)
The 6 parameter is negative if the displacement is into the &cage. Also the angle, 01, between the electron-nuclear vector r and the [ 11 11 direction, which for octahedral-, tetrahedral-, and trigonal-bipyramidal complexes will be the same as the g,,direction, is given by 8, = cos-' t(0.314 - 6)/r]
(2)
(9) Kevan, L. In Time Domain Electron Spin Resonance, Kevan, L., Schwartz, R. N., Eds.; Wiley-Interscience: New York, 1979; Chapter 8. (10) Kevan, L.; Narayana, M. ACS Symp. Ser. 1983, No. 218, 283. (11) Ichikawa, T.; Kevan, L. J . Am. Chem. SOC.1983, 105, 402. (12) Narayana, M.; Kevan, L. J . Cfiem. Phys. 1983, 78, 3573. (13) Ichikawa, T.; Kevan, L. J. Phys. Cfiem. 1983, 87, 4433. (14) Anderson, M. W.; Kevan, L. J. Pfiys. Cfiem. 1986, 90, 6452. (15) Ichikawa, T.; Kevan, L.; Narayana, P. A. J. Phys. Cfiem. 1979,83,
Therefore, for migration of Cu2+ into and out of the @-cage along the threefold axis the constants rand 0, required to calculate the echo modulation may be easily determined by using eq 1 and 2.
3378. (16) (a) Narayana, P.A.; Kevan, L. Photochem. Photobiol. 1983, 37, 105. (b) Narayana, P.A.; Kevan, L. Mugn. Res. Reu. 1983, I , 234.
(17) Fauth, J.-M.; Schweiger, A.; Braunschweiler, L.; Forrer, J.; Emst, R. R. J . Magn. Reson. 1986, 66, 74.
1852 The Journal of Physical Chemistry, Vol. 91, No. 7 , 1987
0.1nm into
a-cage
Anderson and Kevan TABLE I: Composition of Zeolites Studied As Determined by Commercial Atomic Absorotion sample composition/unit cell designation in text
0.05nm
into a-cage
52 0.05nm
-
,
O
2
into P-cage
;
into P-cage
W V
0
I
4
3
2
5
T, ps
Figure 2. Theoretical three-pulse ESE spectra for Cu2+cation located on the triad axis near site S2 and interacting with three nearest-neighbor
cesium cations. The average spin echo intensity ( V,,,/mod)3 for interaction with three Cs' cations obtained by using a correlated model will then be given by
g,,=2335
where Bo is the angle between the field Ho and the glldirection and 4 is the azimuthal angle about z between the z,x plane and the z,Ho plane (with gllaligned along 2). As in previous workI4 integration is only performed over two dimensions of disorder as the ESE experiment only excites spins within a small range of
200G
r
g,=2065
H
I------+--,
v g,,= I 996
Figure 3. ESR spectrum of fresh CuCs2,Na-A recorded at 77 K .
80.
If all the eight-rings are not occupied by Cs' cations then the one, ( Vd)l, average modulation for interaction with two, ( Vmod)2, and zero Cs+ cations will be given by
CuCsp 2 Na-A fresh N.17
1\ tg =I,997
9.2 055
g = 2.267 (bl g = 1.997 (c)
(vmlmod)O = 1
For an exchange level of 3 Cs' cations per unit cell all the eight-rings will be occupied and the spin echo modulation will be given by eq 3. For an exchange level of less than 3 Cs' cations per unit cell then the overall spin echo modulation will be given by a combination of eq 3 and 4. If we assume a random distribution of c s + cations among the available eight-ring sites then the probability that a Cu2+ cation interacts with n Cs+ cations Cu(nCs) will be given by the following binomial distribution PC"(3CS) = Pcs3 PCU(2CS) = 3Pcs2(1 - P a ) PCu(lCS) = 3Pcs(l - Pcs)2 PCU(0CS) = (1
-PcsS
(5)
where pcs is the probability that an eight-ring adjacent to a Cu2+ cation is occupied by a Cs+ cation. This is given by PCS = N / 3
(6)
where N is the number of Cs' cations per unit cell and must be less than or equal to three. The total spin echo modulation at will an exchange level of N Cs' cations per unit cell, ( V,,),, then be given by 3
(Vm/mod),v = Cpcu(nCs)( V m d ) n t7=0
(7)
On the basis of eq 7 the theoretical spin echo modulation for Cu2+
9.2.055 (d) ._
0
I
2
3
4
g=2.449(e)
, .
5
T, p Figure 4. (a) Field-swept ESE spectrum of fresh CuCs2,,Na-A. Experimental (-) and simulated (---) three-pulse ESE spectra of fresh CuCs2,Na-A recorded at 4 K at (b) g = 2.266, (c) g = 1.997, (d) g = 2.055, and (e) g = 2.449. Distances in brackets indicate displacement of Cu2+from S2.
located at various points on the threefold axis and interacting with 3 Cs+ cation is shown in Figure 2. As can be seen, this technique is fairly sensitive to migration of Cu2+cation from site S2' in the @-cagethrough the six-ring to site S2* in the a-cage. Results All the exchanged zeolites were highly crystalline to X-rays and all exhibited a unit cell parameter of 1.21 nm. The unit cell contents as determined by commercial atomic absorption are given in Table I as are the designations by which each zeolite will be referred to in the text. The ESR spectrum of hydrated CuCs2,,Na-A is shown in Figure 3. It exhibits two major species one of which has reversed g values, g, > gl,. The ESR parameters for the two species are g, = 2.065, gll = 2.335, and All = 1.66 X cm-I and g, = 2.264, gll = 1.996, and All= 74 X cm-I. These parameters are very similar to those reported previously'* for Cu2+in partially
Cu2+ Location in Na-A and K-A Zeolites
The Journal of Physical Chemistry, Vol. 91, No. 7, 1987 1853
TABLE II: Location of Cu2+ in A Zeolites Obtained from ESEM Measurements sample
treatment
CuCs I ,9Na-A CuCs I ,,Na-A CuCsl,,Na-A CuCsI,,Na-A CUC~~,~N~-A CUCS,,~N~-A CUC~~,~N~-A CUC~~,~N~-A CUCS,,~N~-A CUC~~,~N~-A CUCS,,~N~-A CuCs,,,Na-A CuCsz,,Na-A CuCs,,,Na-A CuCs2,,Na-A CuCs3,oK-A CuCs3,oK-A CuCs3,oK-A CuCs3,oK-A CuCs3,oK-A CuCs3,oK-A CuCs3,oK-A
hydrated hydrated hydrated hydrated hydrated hydrated hydrated hydrated dehydrated dehydrated dehydrated hydrated hydrated hydrated hydrated hydrated evacuated rt evacuated rt evacuated rt evacuated rt dehydrated dehydrated
Cst/unit cell
1.2 1.2 1.2 1.2 1.7 1.7 1.7 1.7 1.7 1.7 1.7 2.0 2.0 2.0 2.0 1.5 1.5 1.5 1.5 1.5 1.5 1.5
N"
species
e,, deg
Cu2+locationbVc 0.08 nm into 0-cage 0.08 nm into @-cage s2 s2 0.09 nm into 0-cage 0.09 nm into @-cage s2 s2 0.02 nm into 0-cage 0.02 nm into 0-cage 0.02 nm into @-cage 0.10 nm into 0-cage 0.10 nm into @-cage s2 0.02 nm into 0-cage 0.02 nm into 0-cage 0.05 nm into @-cage 0.05 nm into @-cage s2 s2 0.01 nm into a-cage 0.01 nm into a-cage
90 0 90
cull1 cull1 cull CUI1 CUI11 cull1 CUI1 CUI1 CUO CUO CUO CUI11 CUI11 CII cull CUI CUI11 CUllI CUI1 cull CUO CUO
0
90 0 90 0 90 0 0/32* 90 0 90 0 90 90 0 90 0 90 0
"Effective number of Csc/unit cell in the unit cell containing cupric ion. bError in site location is fO.O1 nm. cDisplacement is given from site S2 in the six-ring plane. dSimulation based on superposition of two hyperfine components, each given equal weighting. CuCs2.2 No-A dehydrated at 400" C
CuCs2.2No-A dehydrated ot 4 0 0 ° C
-I g,,=2.396
(a)
VI 0
2
I
3
4
5
T, ps
-
XX)G H
Figure 5. ESR spectra of C U C ~ ~ , ~ Ndehydrated ~-A at 400 OC and recorded at 77 K (a) derivative recording; (b) integrated ESR spectrum.
dehydrated zeolite CuNalz-A. The assignment given in that work for these two species respectively was octahedrally coordinated Cu2+,designated CullI, bound to three water molecules and three lattice oxygens and trigonal bipyramidal Cu2+,designated CUII, bound to two water molecules and three lattice oxygens. Both the field-swept ESE and three-pulse ESE spectra are shown in Figure 4. The ESE has been recorded at four different fields corresponding to the gll and g, components of the two species present. Similar ESR and ESE spectra, not shown, were recorded for the CuCsl,9Na-A and CuCs2,,Na-A samples which suggest that these levels of Cs+ exchange have little effect on the Cu2+ site location and complexation. A complete list of all ESE simulation parameters is given in Table 11. Figure Sa shows the ESR spectrum for CuCs2,zNa-A zeolite Torr. One after complete dehydration at 400 " C and well-defined species is observed with narrow line widths and ESR parameters g, = 2.064, gll = 2.396, and All = 132 X lo4 cm-I. Again these are similar to the values reported previously for CuNa12-A1*after complete dehydration. This species is termed Cuo indicating no water ligands. Also shown in Figure 5b is the
Figure 6. (a) Field-swept ESE spectrum of CuCsz.,Na-A dehydrated at 400 OC. Experimental (-) and simulated (---) three-pulse ESR spectra of C U C S , , ~ N ~ -dehydrated A at 400 OC and recorded at 4 K at (b) g = 2.066, (c) g = 2.337, and (d) g = 2.425. C U C S ~ , ~ K fresh K-A
