3206
J. Phys. Chem. 1986, 90, 3206-3212
Electron Spin Echo Study of Cu2+-Doped Zeolite K-ZK4: Adsorbate Interaction
Cation Location and
Michael W. Anderson and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Received: March 14, 1986)
An electron spin echo (ESE) study on a suite of Cu2+-doped K-ZK4 samples enables the effect of changing Si/AI ratio to be evaluated on Cu2+location and coordination in A-type zeolites. In samples with more than ten K+ cations per unit cell the Cu2+migrates upon dehydration from site S2* in the a-cage to site S2 and eventually to site S2' in the @-cage. As a result of steric hindrance, caused by the K+ cations located in the octagonal windows, Cu2+in site S2* is coordinated to only one water molecule. In the initially hydrated zeolite some Cu2+ is also in the &cage at site S2' coordinated to three water molecules. Samples with less than ten K+ cations per unit cell exhibit the increasing dominance of a Cu2+at site S3 or S2* in the a-cage coordinated to two waters at the expense of Cuz+coordinated to one water. This is explained by additional room in the a-cage afforded by the lower K+ content per unit cell. In K-ZK4 with the lowest number of K+ studied adsorption of methanol forms a new large complex of Cu2+coordinated to three methanols. This species gives rise to an ESR spectrum with reversed g values (g I > g,,). Based on the space available, the formation of such a species indicates that the K+ cations are preferentially located in six-ring sites rather than in eight-ring sites in zeolite K-ZK4.
Introduction The importance of zeolites as catalysts arises from (i) their well-defined reaction sites and (ii) their molecular sieving properties which provides the basis for shape-selective catalysis. In order to tailor a zeolite to give desired catalytic properties, an understanding of both the cation location and adsorbate interaction at the catalytic center is necessary. When that catalytic center is paramagnetic, as is the case with Cu2+,electron spin resonance (ESR) coupled with electron spin echo (ESE) spectroscopy has proved very revealing, yielding accurate coordination numbers and geometries.' Hitherto, the Cu2+-doped zeolite X, Y , and A systems have received the most a t t e n t i ~ n . ~ - Comparisons '~ have been made between the Cuz+location in the isostructural zeolites X and Y , and the differences have been attributed to changes in the adsorption energy with the Si/AI ratios4 This work attempts to make a similar comparison between the isostructural zeolites A and ZK4. Zeolite A has a Si/AI ratio of one while a zeolite with a higher Si/AI ratio is termed ZK4. The potassium form of the zeolite was chosen because work on CuK-A is well-documented in the literature. In the present work a new Cu2+species coordinated to two water molecules is observed, predominantly in zeolites with a higher Si/AI ratio. The formation of large complexes with methanol allows us to infer the distribution of K+ cocations in zeolite CUK-ZK4. Experimental Section Three samples of zeolite ZK4 with differing Si/AI ratios were prepared according to the method of Jarman et. a l l 4 Prior to cation exchange the zeolites were calcined at 550 O C in flowing dry air in order to remove the clathrated tetramethylammonium cation. The resulting H-ZK4 was exchanged with a 0.1 M KOH solution followed by two exchanges with a 1 M KOH solution. Finally, Cuzt was introduced by exchanging with 10 cm3 of M C U ( N O , ) ~ . ~ H ,and O 100 cm3 of water per gram of zeolite. (1) Kevan, L.; Narayana, M. ACS Symp. Ser. 1983, No. 218, 283. (2) Ichikawa, T.; Kevan, L. J . Am. Chem. SOC.1983, 105, 402. (3) Narayana, M.; Kevan, L. J . Chem. Phys. 1983, 78, 3573. (4) Ichikawa, T.; Kevan, L. J . Phys. Chem. 1983, 87, 4433. (5) Herman, R. G.; Flentge, D. R. J . Phys. Chem. 1978, 82, 720. (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) Ichikawa, T.; Kevan, L. J . Chem. Soc., Faroday Trans. 1981, 77, 2561. (9) Ichikawa, T.; Kevan, L. J . Am. Chem. SOC.1981, 103, 5355. (IO) Narayana, M.; Kevan, L. J . Chem. Phys. 1981, 75, 3269. (1 1) Goslar, J.; Wieckowski, A. B. J . Solid State Chem. 1985, 56, 101. (12) Narayana, M.; Kevan, L. J . Chem. Soc., Faraday Trans I 1986,82,
,
213. (13) Narayana, M.; Kevan, L. J. Phys. C 1983, 16, 361. (14) Jarman, R. H.: Melchior, M. T.; Vaughan, D. E. W. ACS Symp. Ser. 1983. No. 218. 267.
0022-3654/86/2090-3206$01.50/0
This amounted to a doping of approximately one Cu2+cation per 45 unit cells. The zeolite was then washed with hot triply distilled water, filtered, and dried in air at room temperature. Crystallinity and purity were monitored by X-ray diffraction using a Philips diffractometer with a vertical goniometer, scanning at 1O of 29 per minute. Chemical analysis of the zeolite framework was performed by 29Sihigh-resolution solid-state nuclear magnetic resonance with magic angle spinning, (MASNMR) according to the method of Englehardt et aI.l5 Measurements were made on a 200-MHz Bruker instrument fitted with a spinning probe. Five-second intervals were allowed between pulse sequences, and spectra were recorded at 39.47 MHz. Samples that were not to be fully dehydrated were repeatedly exchanged with D,O in a vacuum desiccator before trahsferring, under dry nitrogen, to a sealed 3-mm-0.d. Suprasil quartz sample tube. The extent of D 2 0 / H 2 0exchange was estimated from the Fourier-transformed two-pulse electron spin echo spectrum. In all cases the proton contribution was negligible. Samples were dehydrated under vacuum to an ultimate pressure of Torr followed by heating to 400 "C over an 8-h period. The zeolites were then exposed to 400 Torr of dry, high-purity oxygen in order to oxidize any copper(1) or copper (0) that had been formed during the dehydration process. The oxygen was then pumped off for 2 h at 400 OC. Deuterated adsorbants D20, CD,OH, CH30D, CH3CH20D, and C2D4 were obtained from Stohler Isotope Chemicals and Aldrich and were used without any further purification. ESR spectra were obtained at 77 K on a Varian E-4 spectrometer. ESE spectra were recorded at 4 K on a home-built spectrometer described el~ewhere.'~*"Three-pulse, stimulated echoes were recorded to observe modulation from both ,'AI and D. For observation of deuterium modulation without interference from aluminum modulation T , the time between the first and second pulses, was kept between 0.24 and 0.28 pus. To observe *'AI modulation, samples containing H,O instead of D,O were used and T was fixed a t 0.4 ps. A complete description of both the theory and simulation of ESE data is given e 1 ~ e w h e r e . l ~ ~
Results Compositional data on the zeolites studied are given in Table I. The range of Si/A1 ratios covered was from 1 to 2.4 which (15) (a) Englehardt, G.; Lohse, V.; Lippmaa, E.; Tarmak, M.; Magi, M. Z . Anorg. Allg. Chem. 1981, 482, 49. (b) Ramadas, S.; Thomas, J. M.; Klinowski, J.; Fyfe, C. A,; Hartman, J. S . Nature (London) 1981, 292, 228. (16) Ichikawa, T.; Kevan, L.; Narayana, P. A. J . Phys. Chem. 1979, 83, 3378. (17) (a) Narayana, P. A,; Kevan, L. Photochem. Photobiol. 1983, 37, 105. (b) Narayana, P. A,; Kevan, L. Magn. Reson. Reu. 1983, I , 234. (c) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R.N., Eds.; Wiley-Interscience, New York, 1979; Chapter 8.
0 1986 American Chemical Society
ESE Study of CuZ+-DopedZeolite K-ZK4
The Journal of Physical Chemistry, Vol. 90, No. 14, 1986 3297
TABLE I: Composition and Unit Cell Parameters of Zeolites Studied sample compsn designation %/A1 by per unit cell in text MASNMR" K12(A102)12(SiOf)I* K-A 1 1.34 KIo.,(A102)lo,a(S102)Ip,7 K-ZK4( 1.94 K8.2(A102)8.2(Si02) 15.8 K-ZK4(2) 2.40 K7.1 (A10217.1 (si02) 16.9 K-ZK4(3)
no. of K+ cations/ unit cell
unit cell parameter, nm
12
1.23 1.22 1.209
10.3 8.2 7.1
1.205
Magic angle spinning nuclear magnetic resonance. bContains 3-4% metasilicate impurity.
CuK-ZW2) SVAI = 1.94
CuK- ZK4(3) W A l = 2.40
-.. c--t--t
1
,
Figure 2. ESR spectra at 77 K of zeolites dehydrated at 400 OC and oxidized: (a) CuK-A, (b) CuK-ZK4(1), (c) CuK-ZK4(2], and (d) CuK-ZK4(3).
Figure 1. ESR spectra of CuK-A recorded at 77 K: (a) hydrated, (b) evacuated at room temperature, (c) evacuated at 120 OC, and (d) evacuated at 265 OC.
translates to a K+ content per unit cell from 12 to about 7. The nomenclature used to describe these samples is also given in Table I. All the synthesized zeolites were highly crystalline by X-ray diffraction with lattice parameters in good agreement with those reported by Jarman et al.14 Table I1 gives g values and hyperfine splittings for all the samples studied after a number of different treatments. Values for the hyperfine splitting of the perpendicular component are generally not given because in most cases this was not well-resolved. The effect of dehydration on CuK-A is shown in Figure 1 and is similar to that reported previously.'z The fully hydrated material exhibits two species, one corresponding to tetrahedrally coordinated Cu2+bound to three zeolitic oxygens, 0,, and one water molecule (Cu,). The subscript on Cu gives the humber of water ligands. The other species corresponds to octahedrally coordinated Cuz+ bound to three 0, and three water molecules (CU~II). Upon evacuation a t room temperature the CuI species disappears and a Cull species appears. The CUII species gives rise to an ESR spectrum with reversed g values (gl > gll) which is attributed to a trigonal-bipyramidal complex with three equatorial ligands to 0, and two axial ligands to water molecules. Heating to 115 OC converts the Cui11 species into another unknown species, and further heating to 265 O C produces a Cuo species which is trigonally bound to three 0,. A similar series of ESR spectra, not shown, are found for CuK-ZK4(1). There are three species present in the hydrated sample although the ESR spectrum is not well-resolved. Upon dehydration at room temperature a small signal with reversed g values is observed, and the overall profile in the g,,region is very similar to that for zeolite CuK-A dehydrated at room temperature. Further heating to 80 OC produces another signal with gIl = 2.321 and All = 0.0176 cm-I which becomes the only species present after dehydration at 180 O C . After complete dehydration at 400 O C a second species arises shown in Figure 2b, with gll = 2.357
-
CUK- Z W 3 ) Si/AI = 2.40
200G H
Figure 3. ESR spectra of CuK-ZK4(3) recorded at 77 K: (a) hydrated, (b) evacuated at 40 OC, and (e) evacuated at 115 OC.
and All = 0.0154 cm-'.This is to be compared with zeolite CuK-A where only one Cu2+species is present in the dehydrated zeolite. In the hydrated material of CuK-ZK4(2) we observe the same three species as seen in CuK-ZK4( 1). However, we now observe an additional dominant species with gll = 2.401 and All = 0.0143 cm-'. After dehydration at room temperature again the ESR profile is similar to that for both CuK-A and CuK-ZK4(2) samples. Further heating to 115 "C produces the same species that was formed after dehydration at 180 OC of CuK-ZK4( 1) with gll = 2.321 and All = 0.0176 cm-I. Continued dehydration to 400 OC, shown in Figure 2c, produces the same two species that were produced in CuK-ZK4(1) although the relative populations of the two species are different. Figure 3 shows the effect of dehydration on CuK-ZK4(3). In the hydrated material only one species is present with gll and All values identical with the dominant species in hydrated CuKZK4(2). After evacuation at room temperature this species is converted into a different species with gll = 2.378 and All = 0.0145 cm-' with no sign of a species with reversed g values. Another transition takes place on heating to 115 "C producing the same
3208 The Journal of Physical Chemistry, Vol. 90, No. 14, 1986
Anderson and Kevan
CuK- ZrUr(3) fresh sample
8-
b
CuK-ZM(II Si/Al=1.34
r = 0 . 2 8 ~ ~ N.4 r 0.28nm A = 0.4
G-i\
I\ I\
CuK-ZK4(2) SVAI = 1.94
C
CuK-ZK4(3) SVAI =2.40
gL= 2.281
1
OO
I
T,
Figure 4. ESR spectra of zeolites with adsorbed methanol, recorded at 77 K: (a) CUK-A, (b) CuK-ZK4(1), (c) CuK-ZK4(2), and (d) CUK-
CuK-ZK4(2)+ CDsOH H = 3080 G I 0 . 2 8 ~ ~ N =6 r = 0.41nm A A = 0.0
ZK4(3). W
CUK-ZK~(I)+CD~OH r=0 . 2 8 ~ ~ N=6 r = 0.43 nm A = 0.0
2
-
n
I W V I
0
1'2,
1
2
3
4
T, P '
L I
4
t'
Figure 6. Experimental (-) and simulated (- - -) three-pulse ESE modulation spectrum of hydrated CuK-ZK4(3) recorded at 4 K. The decay function used is exp(-0.946 - 0.664T + 0.132p - 0.010T3).
gll = 1.997
SOOG Hc
4
3
2
2
3
CuK-ZK4(2) + CD30H H = 2790 G
>-
cv,
4
z W
T, PS
I-
z
r = 0.36nm
b o I
CUK-ZK~(I)+CHSOD
V W
-
0
1
2
3
4
5
T, p * 8
2
3
4
T, P '
Figure 5. Experimental (-) and simulated (- - -) three-pulse ESE modulation spectra of CuK-ZK4(1) with (a) adsorbed CDJOH and (b) adsorbed CH30D. Spectra were recorded at 4 K. The decay functions used were exp(-0.201 - 0.544T + 0.061p - 0.001 T3)and exp(2.172 0.583T + 0.072p - O.O02P), respectively.
species found in CuK-ZK4(1) heated to 180 OC and in CuKZK4(2) heated to 115 O C . Complete dehydration at 400 "C, shown in Figure 2d, produces two species, one of which is the same as that formed in CuK-ZK4(1,2). The ESR spectra after partial rehydration at room temperature with 16 Torr of H 2 0 indicate that in zeolite CuK-ZK4(1) only one species is observed with giI = 2.351 and All = 0.0162 cm-I. This species is also formed in both CuK-ZK4(2) and CuKZK4(3), but in these latter zeolites it is accompanied by another species with gll = 2.393 and All = 0.0137 cm-'. Complete rehydration of all four zeolites gave ESR spectra identical with those for initially hydrated samples. Adsorption of methanol has no effect on the ESR spectrum of zeolite CuK-A, shown in Figure 4a; however, for CuK-ZK4 samples adsorption of methanol produces an ESR species with gll = 2.401 and All = 0.0121 cm-I. For zeolites CuK-ZK4(2,3) an additional species with reversed g values, g, = 2.291, gll = 2.003, and All = 0.0069 cm-I, is also observed. Figures 5-7 show a selection of ESE spectra, exhibiting deuterium modulation, and their associated simulations. A complete
Figure 7. Experimental (-) and simulated (- - -) three-pulse ESE modulation spectra of CuK-ZK4(2) with adsorbed CD,OH (a) at 3080 G and (b) at 2790 G . Recorded at 4 K. The decay functions used are exp(2.170 - 0.480T + 0.lOlP - 0.008T3) and exp(-0.756 - 0.260T0.139p + 0.025T3),respectively.
set of data derived from these simulations, and others not shown, is given in Table 111. A comparison is made of the amplitude of *'AI modulation for both hydrated and dehydrated zeolites CuK-A and CuK-ZK4 in Figures 8 and 9. An increase in the Si/AI ratio is associated with a decrease in the modulation depth. Also, the modulation depth decreases upon dehydration. Figures 10 and 11 show field swept ESE spectra from threepulse experiments with 7 = 0.3 1 s and T = 0.4 1 s where 7'is the time delay between the second and third pulses. The derivative profiles may be compared with the associated ESR spectra and indicate that at this value of 7 all paramagnetic species are contributing to the echo. Discussion Zeolite ZK4 is isostructural with zeolite AI8 shown in Figure 12. The lattice is built from /3-cages, or truncated octahedra, linked together via double four-rings to give a simple cubic arrangement. The larger a-cages so formed are accessed through eight-rings with free diameter 0.42 nm while the @-cages are (1 8) Ken,
G.T. Inorg. Chem. 1966, 5, 1537
The Journal of Physical Chemistry, Vol. 90, No. 14, 1986 3209
ESE Study of Cu2+-Doped Zeolite K-ZK4
10
lord
81
CuK-ZK4(1) evacuated room temperature
6l
0
4 r
b
= 0.4 p s
h/l
CUK- ZK4 ( I )
E
0
6
2 W
I-
z
2
0
I
lo-
CuK-ZK4(1) evacuated
86-
12ooc
V W
4-
C
i
T, P’
Figure 8. Three-pulse ESE spectra showing 27Almodulation for the hydrated zeolites (a) CuK-A, (b) CuK-ZK4( l), and (c) CuK-ZK4(2). Recorded at 4 K.
FIELD (GAUSS)
hi
Figure 10. Derivative field swept ESE spectra of CuK-ZK4(1) (a) evacuated at room temperature and (b) evacuated at 120 OC. The absorption spectrum was recorded at 4 K with T = 0.3 ps and T = 0.4 ps.
The derivative was obtained with a computer program which resulted
in the large noise level shown.
CUK-A dehydrated r = 0.4 PS
O
O
I
2 3 T, PS
4
CuK-ZK4(2)
+ CDsOH
A
2400
2600
2000
3000 3200 FIELD (GAUSS)
3400
3600
Figure 11. Field swept ESE spectrum of CuK-ZK4(2) with adsorbed CD30H recorded at 4 K with T = 0.3 ps and T = 0.4 ps. The arrow indicates the field at which the three-pulse ESE modulation spectrum was recorded in order to isolate the species with reversed g values.
Figure 9. Three-pulse ESE spectra showing 27Almodulation for the dehydrated zeolites (a) CuK-A and (b) CuK-ZK4(1). Recorded at 4 K.
accessed through six-rings with free diameter 0.22 nm. Consequently, in the present study only the cations and D 2 0 are small enough to enter the @-cages. The cation locations with which we are concerned are also given in Figure 12. Site S2 is at the center of a six-ring, and sites S2’ and S2* are shifted into and out of the @-cagealong the threefold axis, respectively. Site S3 is located in the a-cage close to the oxygen four-ring, and site S4 is at the center of the a-cage. Finally, site S5 is located at the center and in the plane of the oxygen eight-ring. In hydrated zeolite K-A eight K+ cations are in S2* sites, three are slightly shifted from S5 sites, and one is at site S4.I9 As there are only eight S2* and three S5 sites per unit cell, every six-ring and every eight-ring is “blocked” by a K+ cation in zeolite K-A. Also, in the hydrated zeolite there are 20 water molecules per unit cell. Eight are located in the @age, hydrogen bonded to each other to give a cubic arrangement, and the other twelve are located in the a-cage, hydrogen bonded to lattice oxygens and interacting with K+ cations in sites S5. Upon dehydration two K+ cations shift from sites S2* to S2’ in the @-cage, and the cations near site S5 move exactly into sites S5. The cation which was located in the center of the a-cage a t site S4 probably (19) h u n g , P. C. W.; Kunz, K. B.; Seff, K.:Maxwell, I. E. J. Phys. Chem. 1975, 79, 2157.
Figure 12. Crystal structure of zeolite A showing cation positions. Site S2 is at the center of a six-ring face with sites 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. Site S4, not shown, is at the center of the a-cage.
moves to site S3 to prevent noncoordination.20 Still all six- and eight-rings are ”blocked”. The cation positions for K-ZK4 are not known; however, it is safe to assume that the positions are a perturbation of those found (20) Pluth, J. J.; Smith, J. V. J . Phys. Chem. 1979, 83, 741.
3210
The Journal of Physical Chemistry, Vol. 90, No. 14, 1986
Anderson and Kevan
TABLE 11: Magnetic Parameters, Water Coordination, and Probable Sites for Cupric Ion in CuK-A and CuK-ZK4 Zeolites for Various Sample Pretreatments cm-I
g,
water coordn and siteb
2.129 2.070 2.072 2.289 2,070 2,072 2,065 2.065
Cul (S2*) Cull, (S2') Cui;' (S3. S2*) cull(s2) culll (~2,) cull( ~ 3~, 2 ' ) cu0( ~ 2 , ) Cuo (SZ')
A 110-4
sample
treatmenta
gll
CUK-A CUK-A CUK-A CUK-A CUK-A CUK-A CUK-A CUK-A
hydrated 2.479 hydrated 2.345 hydrated 2.390 evac at r.t. 2.004 evac at r.t. 2.349 evac at r.t. 2.390 evac at 265 OC 2.391 evac at 400 "C 2.391
100 166 148 84 154
CuK-ZK4( 1 ) CuK-ZK4( I ) CuK-ZK4( 1) CuK-ZK4(1) CuK-ZK4(1) CuK-ZK4(1) CuK-ZK4(1) CuK-ZK4(1)
hydrated 2.469 hydrated 2.333 hydrated 2.390 evac at r.t. 2.005 evac at r.t. 2.351 evac at r.t. 2.390 evac at 400 OC 2.357 evac at 400 OC 2.321
96 171 148 76 I65 I48 154 176
CuK-ZK4(2) CuK-ZK4(2) CuK-ZK4(2) CuK-ZK4(2) CuK-ZK4(2) CuK-ZK4(2)
hydrated hydrated hydrated evac at r.t. evac at r.t. evac at r.t.
143 171 96 165 148 76
2.40 1 2.333 2.469 2.351 2.390 2.005
148
133 133
2.072 2.293 2.065 2.072 2.075 2.072 2.065 2.072 2.293
A,,/10-4
sample
treatmentD
giI
cm-I
g,
water coordn and siteb
CuK-ZK4(2) evac at 400 "C 2.321 CuK-ZK4(2) . , evac at 400 OC 2.357
176 154
2.075 C U (S2*) ~ cu, (S2)
CuK-ZK4(3) CuK-ZK4(3) CuK-ZK4(3) CuK-ZK4(3) CuK-A
148 145 161 152 100 166 162 165 137 165 137
2.072 Cull, (S3, S2*) 2.060 Cull, (S3, S2') CU,,,, (S3, S2*) (S2') 2.129 Cu, (S2*) 2.070 CulIl (S2') 2.064 C U , (S3, ~ S2') 2.064 Cuom (S3, S2') 2.070 CUI,, (S3, S2*) 2.064 CU,~,(S3, S2*) 2.070 CUII,(S3, S2*)
133 154 100 121 68 121 68
2.065 Cu, (S2') cu,. (S2')
hydrated 2.390 evac at r.t. 2.378 evac at 400 "C 2.343 evac at 400 OC 2.359 rehydrated 2.479 2.345 CU, (S2') CuK-ZK4(1) rehydrated 2.351 Cull, (S2') CuK-ZK4(2) rehydrated 2.350 Culy (S3, S2*) CuK-ZK4(2) rehydrated 2.393 Cull (S2) CuK-ZK4(3) rehydrated 2.350 Cu,,, (S2') CuK-ZK4(3) 2.393 . , rehvdrated , Cui;,' (S3,'S2*) cuo. (S2') CuK-A +MeOH 2.391 Cuw (S3, S2*) CuK-ZK4(1) fMeOH 2.357 CuK-ZK4(1) +MeOH 2.427 Cull, (S3, S2') CuK-ZK4(2) +MeOH 2.401 Culll (S2') CuK-ZK4(2) +MeOH 1.997 Cu, (S2*) CuK-ZK4(3) +MeOH 2.401 Culll (S2') CuK-ZK4(3) +MeOH 1.997 Cull, (S3, S2*) Cull (S2)
cue..
2.08 1 2.281 2.081 2.281
'evac = evacuated; r.t. = room temperature; rehydrated = partial rehydration with 16 Torr of H,O; +MeOH = after adsorption of 90 Torr of methanol. bSpecies given in order of decreasing concentration; in A and ZK4(1) Cul and Culll are of comparable concentration. Subscripts indicate number of coordinated waters. The probable site in the zeolite structure is given in parentheses.
TABLE 111: Distance ( r ) and Number ( N ) of Deuterons Interacting with Cu2+in the Zeolites Shown from ESE Modulation Spectra samule treatmentn field. G r. nm N a;*".M H z CUK-A +CD3OH CUK-A +CH30D CuK-ZK4( 1) e v a c a t 120 "C C UK-ZK4( 1) evac a t 400 OC CuK-ZK4( 1) rehydrated CuK-ZK4( 1) +CD,OH CuK-ZK4( 1) + C H 3 0 D CuK-ZK4( 2) evac at 115 "C CuK-ZK4(2) evac at 400 OC CuK-ZK4(2) +CD30H CuK-ZK4(2) +CD,OH C U K - Z K ~3) ( hydrated CuK-ZK4( 3) +CD,OH
3130 3130 3150 3180 3150 3170 3155 3150 3110 3080' 2790b 3135 3150
0.41 0.39 0.35 0.43 0.43 0.45 0.35 0.41 0.36' 0.2V 0.45
3 1 2 0 2 6 2 2 0 6 9 4 6
0 0 0 0 0 0 0.1 0 0.1 0.4 0
"evac = evacuated; +CD,OH, etc.; after adsorption of 90 Torr of methanol. of t w o species. 'Distance short enough to indicate direct coordination. All other distances are too large for direct coordination.
in zeolite K-A since in zeolite K-A all reasonable cation locations are occupied. This restricts the possible permutations for the distribution of K+ cations among the various cation sites in zeolite ZK4. In earlier studies on Cu2+in zeolite K-A8,9712.14 the copper cation has been located at sites S2, S2', and S2* depending upon the state of hydration or type of adsorbate. It is reasonable to assume that Cu2+will also occupy these sites in zeolite ZK4. Indeed, in reported crystal structures of copper-loaded zeolite A Cu2+always occupies a six-ring site.21 These sites have the highest coordination number and shortest coordination distance of all the possible cation sites in the zeolite A structure and are therefore favored energetically. Both the copper species, CuI and CuII1, reported to occur in hydrated zeolite CuK-Al2 also exist in hydrated zeolite CuKZK4(1). However, there is an additional species with gll= 2.390 and A , l = 0.0148 cm-'. This has not been reported in CuK-A; however, on closer inspection of the ESR spectrum of hydrated CuK-A (see Figure la) we notice weak shoulders probably due (21) Lee, H. S.; Seff. K. J . Phys. Chem. 1981, 85, 397
to a small concentration of this additional species. In hydrated zeolite CuK-ZK4(2) this additional species becomes dominant, and in hydrated CuK-ZK4(3) it becomes the only species present. Its concentration appears to be a function of the Si/Al ratio or K+ content. The ESE modulation spectrum of deuterated CuK-ZK4(3) in Figure 10 identifies this species as Cu2+coordinated to two water molecules with a Cu-D distance of 0.28 nm. This translates into a Cu-0 distance of 0.21 nm assuming the negative end of the water dipole to be pointing toward the Cuz+ cation. We shall designate this species Cull,. The location of this species is not definite however; its greater preponderance in samples with lower K+ content suggests that it is prevented from forming due to steric reasons. This implies that the Cull,species is in the a-cage at either site S2* or site S3 since all the K+ cations are also located in the a-cage. Upon partial dehydration of zeolite CuK-A by evacuation at room temperature Cu1 disappears and a Cull species with reversed g values corresponding to a trigonal-bipyramidal complex appears. This occurs when Cu2+as Cul migrates along the threefold axis from site S2* to site S2 where it picks up another water molecule from inside the P-cage to form Cull. Further dehydration at 120 "C leaves a species with g,, = 2.374, g, = 2.067, and A,, = 0.0147 cm-l and a small amount of Cull. This former species has similar g values to the Cull, tetrahedral complex. In a similar manner dehydration of CuK-ZK4(1) by evacuation at room temperature removes all CuI leaving CuIII,Cull,, and Cull. Further dehydration at 80 O C produces Cull, as the major species while CUI,, and Cull diminish in concentration. The same trends are observed for CuK-ZK4(2) except that CulIt is the major species in the hydrated sample, and upon evacuation at room temperature a large concentration of CuIl is formed. In CuK-ZK4(3) Cull, is the only hydrated Cu2+species, and upon dehydration this converts directly to a Cuo species. As the Si/Al ratio of the zeolites increases, the temperature at which Cuo forms decreases. Over 250 'C is required in zeolite CuK-A while only 115 O C is required in CuK-ZK4(3). This is a reflection of the decreasing hydrophilicity of the zeolite as the lattice aluminum content decreases.22 (22) Anderson, M. W.; Klinowski, J . J . Chem. SOC.,Faraday Trans. 1, in press.
ESE Study of Cu2+-Doped Zeolite K-ZK4
The Journal of Physical Chemistry, Vol. 90, No. 14, 1986 3211
n positions Cu, complex in a-cage
SITE S2'
Figure 13. Schematic representation of the CUII~ complex with CuZt located in site S2' using van der Waals radii. The diagram shows CuZt coordinated to one 03-type lattice oxygen and one water molecule which
is easily accommodated within the 0-cage. The other ligands are not shown for clarity. In order to rationalize the data discussed so far, it is necessary to reinterpret some previous results. It is well-known that Cu2+ is reluctant to take up a tetrahedral geometry.23 The reason that it does so in zeolites is most probably due to steric factors which prevent the formation of larger complexes with more ligands or the more favorable square-coplanar configuration. The CuI species in CuK-A has previously been assigned13 to site S2' of the p-cage for the following reasons: (i) ESE studies of i33Csmodulation in hydrated CuCs7Na5-A zeolite were interpreted to indicate that Cu2+ is located in the @-cageand not in the a - ~ a g e . ' ~ (ii) - ' ~ Inside the &cage there is less room for an octahedrally coordinated species than in the a-cage. (iii) CuI is the major species whenever the cocation is bulky, such as NH4+, Cs+, Rb', or K+, which encourages the Cu2+to move into the @-cage. (iv) CUIIIwould be expected to form if the Cu2+was at site S2* in the a-cage. (v) Upon rehydration of CUK-A C U ~ I I is formed in preference to CuI because the Cu2+cation is forced into the a-cage upon dehydration by K+ cations which move irreversibly into the @-cage. We now conclude that these postulates need to be reinterpreted as follows. (i) The simulation of i33Csmodulation10suggests that in hydrated CuCs7NaS-A the Cu2+interacts with one Cs+ at a distance of 0.50 nm. However, subsequent calculations which place Cu2+at site S2' in the @-cagewere based on a unit cell parameter of 1.04 nm which should have been 1.23 nm. Reevaluation of these results leaves the location of CuI ambiguous. (ii) Inside the @-cagethere is sufficient room to accommodate an octahedral CUIII species with Cu2+ in site S2'. The Cuz+ is coordinated to three 0 3 oxygens, which are the oxygens shared between a six- and a four-ring, and the water molecules project across the p-cage toward the opposite three square faces bounded by 03-type oxygens. Figure 13 shows a schematic representation of one water molecule stretching across the ,@cage. The water molecules could be hydrogen bonded to two 0 3 oxygens of the opposite wall with a bond distance of about 0.27 nm. (iii) CuI would be preferred in the presence of a bulky cocation even if the Cuz+ remained in the a-cage. (iv) In hydrated CuK-A it would be difficult to form an octahedral species with Cu2+at site S2*. In this site Cu2+ is bound to three 03-type oxygens and the water molecules would project almost directly toward K+ cations located in sites S5 in the center of the octagonal windows. Figure 14 shows a schematic representation of this complex which reveals that the hydrogen atoms of the water molecules could nominally overlap or come quite close to a K+ cation. Such an interaction would not be favorable. A CuI complex with Cu2+in site S2* would have one (23) Hathaway, B. J.; Billing, D. E. Coord. Chem. Reo. 1970, 5 , 143.
Figure 14. Schematic representation of the CulIl complex with CuZt located in site S2* using van der Waals radii. The two possible Kt cation positions shown depend on the displacement of the potassium from site S5. In one case the CulIlcomplex would physically not fit in the available space while in the other case a strong unfavorable interaction between water and the Kt cation would occur.
water molecule projecting toward the center of the 0-cage where the distance between the positive end of the water dipole and the K' cation at site 54 would be more than 0.1 nm. (v) Upon rehydration a Cui11 species would be no less likely to form in the a-cage than in the &cage because all the K+ cations at sites S5 which cause the steric hindrance are still present. On the basis of the above reasoning, we conclude that the CuI complex consists of tetrahedrally coordinated Cu2+located at site S2* in the a-cage with one water molecule projecting toward the center of the a-cage. The CUI~Icomplex is an octahedrally coordinated Cu2+cation located in site S2' in the p-cage with ligands to three water molecules which project across the 0-cage toward the opposing four-rings. Upon dehydration of zeolite CuK-A, CuIl is formed at the expense of Cui. This is probably due to CuI at site S2* migrating along the triad axis to site S2 where it "picks up" a second water molecule located inside the p-cage, forming the more stable trigonal-bipyramidal complex. Upon further dehydration some CUII~species is formed probably as the K+ cations migrate into the @-cage,leaving room for the Cull, to form in the a-cage. Such a migration of K+ cations is known to occur from crystallographic data.Ig After complete dehydration the Cu2+is then located in site S2', contrary to earlier interpretation.I2 This is substantiated by the fact that Cuo in zeolite CuK-A does not interact directly with methanol as indicated by the large Cu-D distances of 0.41 nm for C D 3 0 H and 0.39 nm for C H 3 0 D (see Table HI). With increasing Si/A1 ratio more Cull, is present. The dominance of this species over the CuIll species suggests that the concentration of K+ cations, all of which are locatd in the a-cage in zeolite K-A, influences the site location of Cu2+. At low K+ concentration the hydrated Cu2+ also occupies positions in the a-cage. Unlike dehydrated zeolite CuK-A which contains only one Cu, species, all the CuK-ZK4 samples exhibit two out of three distinct species, Cue., Cuwt,and Cuojt,(see ESR spectra in Figure 3). In all CuK-ZK4 samples the first Cuo species to appear upon dehydration at temperatures between 110 and 180 OC is CuV. Then at 400 OC in CuK-ZK4(1) the dehydrated complex CuV,appears. Adsorption of methanol on this sample causes the ESR lines of only CuV to disappear, indicating interaction of methanol with CuVbut not with Cuw,. This indicates that Cu,, is located in the a-cage and Cuw,in the &cage which is inaccessible to methanol. See Table I1 for the distribution of the various Cu, species among the other zeolites. The Cu complex which only occurs in CuK-ZK4(3) also disappears on methanol adsorption, indicating that it is in the a-cage. Verification that none of these dehydrated Cu2+ species have ligands to hydroxyl groups derived from hy-
,..
3212 The Journal of'Physica1 Chemistry, Vol. 90, No. 14, 1986
Figure 15. Arrangement of three methanol molecules (shaded)and three lattice oxygens of the six-ring about Cu2+to give a distorted octahedral complex.
drolysis of water during dehydration comes from the three-pulse ESE spectrum of dehydrated CuK-ZK4( 1 ) which exhibits no deuterium modulation. Adsorption of methanol on CuK-A results in no direct coordination to the Cuo species; see Figure 4a. Deuterium modulation from ESE measurements after adsorption of CD30H and CH30D, shown in Figure 5, indicates large CuZ+-D distances in accordance with this finding. Methanol is probably hydrogen bonded to the wall of the zeolite in the a-cage with Cuz+ sitting near site S2' in the @-cage. Methanol adsorption on CuK-ZK4( 1) produces a small ESR signal indicating direct coordination (see Figure 4b); however, it is not possible to verify the nature of this species by ESE as methanol interacting indirectly with Cuz+ in the @-cagealso gives rise to strong modulation. Both zeolites CuK-ZK4(2) and -(3) give rise to species with reversed gvalues upon adsorption of methanol with g, = 2.281, g,, = 1.997, and All = 0.0068 cm-I. The only likely geometries that would produce such a spectrum with reversed g values in a zeolite are trigonal-bipyramidal or compressed octahedraLZ3 The unpaired electron in Cu2+which assumes such geometries would reside in a dz2 rather than in a dX2-y;orbital. An ESE experiment was performed on CuK-ZK4(2) with adsorbed C D 3 0 H recorded at the field indicated on the field swept ESE spectrum in Figure 11. This revealed that the spectrum with reversed g values was due to Cu2+interacting with nine deuterium atoms, in other words three methanol molecules, with a Cu-D distance of 0.36 nm; see Figure 7b. Unfortunately, the echo was too weak to establish a geometry for the adsorption of CH30D. However, it appears that Cu2+is bound to either three zeolite oxygens and three methanol molecules, giving a compressed octahedral complex, or to two zeolite oxygens and three methanol molecules, giving a trigonal-bipyramidal complex. This latter complex would be somewhat distorted in order to be accommodated in the a-cage. A compressed octahedral complex, therefore, seems more likely, and this is shown in Figure 15. A complex containing three molecules of methanol has also been reported in CuTI-X zeolite." However, the ESR parameters did not exhibit reversed g values which is indicative of an elongated octahedral complex in that case. In a distorted octahedral complex the methanol molecules must project through the eight-rings requiring that no K+ cations occupy sites S5. This requirement explains why such a species is not formed in zeolite CuK-ZK4( 1 ) and CuK-A. In these zeolites there are ten or more K+ cations per unit cell. The introduction of one Cuz+ ion replaces two K+ cations, leaving at least eight. (24) Narayana, M.; Kevan, L. In Proceedings of the 6th International Zeolire Conference; Bissio, A,, Olsen, D. M., Eds.; Butterworth Scientific: London, 1984; p 774.
Anderson and Kevan Up to seven of the K+ cations may occupy six-ring sites with the eighth site being occupied by Cu2+. The remaining one or more K+ cations will then probably occupy S5 sites. This would prevent the formation of a large complex with methanol. In zeolites CuK-ZK4(2,3) there are less cations per unit cell which can, and indeed must, be accommodated in six-ring sites. Adsorption of ethanol on CuK-ZK4(2) produces no such species giving rise to an ESR spectrum with reversed g values. This is presumably because such a complex would be too large to be accommodated within the zeolite structure. ESE results indicate that Cu2+is interacting with only one molecule of ethanol. The effect of rehydration on these zeolites is sensitive to the time of exposure to water vapor. It is well-knownz5that water diffuses slowly into the @-cagesespecially if the six-ring entrances are essentially blocked by a large cation such as K+. After 4-h rehydration with 22 Torr of HzO all the zeolites return to their original hydrated form. This includes even zeolite CuK-A which has been reported previouslyIz not to re-form the Cu, species. Presumably, the conditions of rehydration in this earlier workI2 were less complete than those employed here. This indicates that K+ cations which migrate into the @-cagesupon dehydration of CuK-A will return, albeit slowly, to the a-cages upon rehydration at room temperature. Partial rehydration of CuK-ZK4( 1) results in one species which is not directly coordinated to the adsorbed water but interacts with one water molecule with a Cu-D distnce of 0.43 nm. This water will probably be hydrogen bonded in the a-cage to the zeolitic oxygens, possibly of type 0 3 , with the Cuz+ located at site S2' in the @-cage. Partial rehydration of both CuK-ZK4(2) and CuK-ZK4(3) produces the species noted above along with Cull.. The 27Al modulation from hydrated zeolites CuK-A and CuK-ZK4 shows a direct correlation between the aluminum content of the lattice and the modulation depth as shown in Figure 8. A simulation of this data was not possible owing to the unknown magnitude, orientation, and asymmetry of the z7Alquadrupole interaction. A similar trend is seen for the dehydrated zeolites in Figure 9. There is a marked decrease in the modulation depth upon dehydration, but it is not possible to tell whether this is due to an increase in the Cu-A1 distance or to a change in the quadrupole interaction. An observation made while experimenting with this series of isostructural zeolites was that the echo intensity for both two- and three-pulse ESE experiments diminished as the Si/Al ratio of the zeolite increased, even though the Cuz+concentration in all the samples was essentially the same, about one CuZt for every 45 unit cells. Conc1usions
This study of Cu*+-doped zeolite K-ZK4 provides a better understanding of the cation locations and adsorbate interactions in CuK-A. Three different coordinative complexes are observed in the zeolite A structure: octahedral, tetrahedral, and trigonal-bipyramidal, The tetrahedral configuration is only preferred when geometrical restrictions prevent a square-coplanar complex or a complex with more ligands. The study of methanol adsorption indicates that the K+ cation is preferentially located in the six-ring sites, either S2* or S2', rather than in the octagonal windows in the zeolite A structure.
Acknowledgment. This work was supported by the Robert A. Welch Foundation and the National Science Foundation. We are grateful to the Energy Laboratory of the University of Houston for equipment support, to Prof. J. Butler for use of an X-ray powder diffractometer, to Dr. W. Rothwell of Shell Development Co. for MASNMR data, and to Dr. D. Goldfarb for many useful discussions. Registry No. Cu, 7440-50-8; MeOH, 6 7 - 5 1 , ( 2 5 ) Barrer, R. M.; Langley, D. A. J . Chem. SOC.1958, 38 I 1
