J . Phys. Chem. 1989, 93, 4669-4674 of polarization charges u.calculated from eq 4 with Do = 78 and Do = 03 agrees within a few percent with the analytical solution. A somewhat different formulation for duj has been given in ref 18. In this work both methods of correction as well as their combination were used. The resulting P M F for solvent separated ions
4669
is within a few percent of that given by Coulomb's law (which could be expected, as this law is exactly correct only for separations that are large compared to the size of the ions) and will not be discussed in detail. For two ions forming one cavity in water, the correction was carried out by normalization of polarization charges as described at the beginning of this section (see ref 12 for details).
Electron Spin-Echo and Electron Spin-Resonance Studies of Cupric Ion-Adsorbate Interactions in Hydrogen, Sodium, Potassium, and Calcium Mordenite Candace E. Sass and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: September 6 , 1988; In Final Form: December 2, 19881
The interaction between cupric ion and both polar and nonpolar adsorbates in mordenite-type zeolite was studied as a function of the cocation. Cu2+was able to coordinate to three molecules of either water or ammonia in H-mordenite but to only two molecules of each in Na-, K-, or Ca-mordenite. These results suggest that Cu2+is located in a site off the main channel in H-mordenite but in a highly twisted 8-ring site in the other forms of mordenite. Evidence based on adsorption and activation studies also suggests that some Cu2+is located in an additional, unidentified site in H-mordenite. These results are compared to the results of similar studies of cupric ion in zeolite ZSM-5.
Introduction Many chemical processes are catalyzed by zeolites.' Their versatility is due in part to the fact that the properties of a particular zeolite can be altered greatly by using simple ion-exchange techniquesS2 The effect of the cocation upon the location of various catalytically active ions such as Cu2+ has been studied by electron spin resonance (ESR) and electron spin-echo modulation (ESEM)3-7spectroscopy. However, the majority of these investigations have been limited to cation effects in cage-type zeolites such as X ~ e o l i t e . ~ Recently, the effect of the cocation (H', Na', K+, or Ca2+) upon Cuz+ in the channel-type zeolite ZSM-5 was examined by ESR and ESEM.8,9 ZSM-5 is a pentad zeolite that consists of a three-dimensional channel system.I0 Cupric ion was found to form larger complexes with C H 3 0 H and CzHSOHin H-ZSM-5 but could not coordinate to as many adsorbate molecules of this size when the cocation was Na+, K+, or Ca2+. N o cocation effect was noted for small ligands such as H 2 0 and NH3. It was concluded that, in all cases, the complexes formed between Cuz+ and the adsorbates studied could only be accommodated at the intersection of the straight and sinusoidal channels, where there is a large increase in free space.1° In contrast to ZSM-5, mordenite consists of a large main channel that is intersected by a series of smaller channels." These small channels, or "pockets", effectively serve to isolate rather than ( I ) Derouane, E. G. In Catalysis by Zeolites. Imelik, B., Naccache, C., Ben Taarit, Y., Vedrine, J. C., Couduner, G., Praliaud, H., Eds.; Studies in Surface Science and Catalysis 5; Elsevier: Amsterdam, 1980; p 5. ( 2 ) Smith, J. V. In Zeolite Chemistry and Catalysis; Rabo, J. A,, Ed.; American Chemical Society: Washington, DC, 1976; Chapter 1. (3) Ichikawa, T.; Kevan, L. J . A m . Chem. SOC.1983, 105, 402. (4) Ichikawa, T.; Kevan, L. J . Chem. Sot., Faraday Trans. I 1981, 77, 2567. ( 5 ) Ichikawa, T.; Kevan, L. J . Am. Chem. Sot. 1981, 103, 5355. (6) Anderson, M. W.; Kevan, L. J . Phys. Chem. 1986, 90,3206. (7) Anderson, M. W.; Kevan, L. J. Phys. Chem. 1987, 91, 2926. (8) Sass, C. E.; Kevan, L. J . Phys. Chem. 1988, 92, 5192. (9) Anderson, M. W.; Kevan, L. J. Phys. Chem. 1987, 91, 4174. ( I O ) Olson, D. H.; Kokotailo, G. T.; Lawton, L. S.; Meier, W. M. J . Phys. Chem. 1981, 85, 2238. ( 1 I ) Meier, W. M. Z . Kristallorg., Kristallgeom., Kristallphys., Krisfallthem. 1961, 115, 439.
0022-3654/89/2093-4669$01.50/0
connect the main channels. It is of interest to examine the effect of the cocation upon the Cu2+location in mordenite by studying the complexes formed between Cuz+ and various adsorbates and comparing the results to those obtained in ZSM-5. In this study, the interactions between Cuz+ and adsorbates such as water, methanol, ethanol, ammonia, and ethylene in H-mordenite (H-M), Na-mordenite (Na-M), K-mordenite (K-M), and Ca-mordenite (Ca-M) were characterized. The ESR and ESEM results revealed that Cu2+coordinates to two molecules of H 2 0 or NH3 in the Na, K, and Ca forms of mordenite but to only one molecule of methanol. Cu2+coordinated to three molecules of H 2 0 or NH3 in H-M but two distinct Cu2+species were formed with methanol. Experimental Section Synthetic sodium mordenite was supplied by Dr. J. A. Rabo of Union Carbide Corp. The material was washed repeatedly with a 1 M solution of sodium acetate to remove substantial iron impurities. Manganese was also present as an impurity in the zeolite. H-M was prepared by exchanging Na-M repeatedly with 1.0 M ammonium nitrate, followed by calcination in flowing air at 400 OC for 20 h. Na-M was exchanged five times with 1 M solutions of CaCI2 or KCI to produce Ca-M or K-M. All samples were doped with Cu2+by adding 10 mL of M C U ( N O ~to) ~ 1 g of mordenite and 100 mL of triply distilled water. The samples were filtered and washed thoroughly with hot triply distilled water and then allowed to air dry. Samples prepared in this manner are referred to as "fresh". The samples were subjected to atomic absorption analysis, and the unit-cell contents for each were determined to be
with the level of Cu2+ doping.equa1 to 1 Cu2+/30 unit cells. Samples were activated in 2-mm-i.d. X 3-mm-0.d. Suprasil quartz tubes that were attached to a vacuum line. The temper-
0 1989 American Chemical Society
4670
The Journal of Physical Chemistry. Vol. 93, No. I I , I 9'89
Sass and Kevan
( c )EVAC. 90"
(d)EVAC. 150'
-. L .
L
-
~-&.
d
~
-1
L- _-1
I - I - . L . d
ESR of CuNo-M 01 7 7 K
200G ti c-+
1;
I1
,1
Figure 1. ESR spectra at 77 K from CuNa-mordenite: (a) of fresh sample; (b) after evacuation at room temperature; (c) after evacuation at 75 "C; (d) after evacuation at 105 "C; (e) after evacuation at 400 "C.
ature was raised slowly over an 8-h period to 400 "C. The samples were exposed to 400 Torr of ultrahigh-purity oxygen to reoxidize any Cu2+ that may have been reduced during the activation process. After 2 h, the oxygen was pumped off to a pressure of 1 X 1 0-5 Torr and the samples were cooled to room temperature. D 2 0 and C 2 H 5 0 D (Aldrich), C H 3 0 D , CD,OH, and ND, (Stohler), and C2H4 (Linde) were used as adsorbates after purification by several freeze-pumpthaw cycles. The samples were equilibrated with the saturated room-temperature vapor pressure of the liquids, while 200 Torr of C2H4 and 500 Torr of ND3 were used. After equilibration, the samples were frozen in liquid nitrogen and sealed off. First-derivative ESR spectra were recorded on a Varian E-4 spectrometer at both room temperature and 77 K. Second-derivative spectra were recorded at 77 K on a Bruker ESP 300. Several scans (four to eight) per sample were averaged. ESEM spectra were recorded at 4 K on a home-built spectrometer that has been described.12 The three-pulse, stimulated echoes were recorded with a ~ / 2 ~, / 2 ~, / pulse 2 sequence in which the echo intensity is measured as a function of time, T, between the second and third pulses. The value for T , the time between the first and second pulses, was selected to minimize modulation from 27AI nuclei contained in the zeolite lattice. Phase cycling was employed to correct for the two pulse glitches that occur at T = T and T = 2T.13 The ESEM spectra were simulated with a ratio analysis procedure that has been described in detail e1~ewhere.l~A Tektronix 4502 computer interfaced with a plotter was employed to carry out the simulations. The value for N , the number of nearest equivalent nuclei around the paramagnetic center, was constrained to be integral. I n general, N can be determined uniquely, while the value for R, the distance from the paramagnetic center to the nuclei, is determined to f0.01 nm.
Results The ESR spectra shown in Figure 1 were obtained from a sample of CuNa-M. These spectra are also typical of those recorded from CuK-M and CuCa-M samples. At 77 K, the fresh samples contained a single Cu2+species with ESR parameters of g l = 2.386-2.407, A , = (140-150) X cm-I, and g, = (12) Narayana, P. A.; Kevan, L. Magn. Reson. Rec. 1983, 7. 234. ( 1 3 ) Fauth. J. M.; Schweiger, A.; Brauschweiler. L.; Forrer, J.; Ernst, R . R . J . Magn. Reson. 1986, 66, 74. (14) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwjartz. R. N.. Eds.: Wiley-Interscience: New York. 1979; Chapter 8.
-
ESR OF CUH-M AT 77K 20OG h t---t--
ki '\III Y
Figure 2. ESR spectra at 77 K from CuH-mordenite: (a) of fresh sample; (b) after evacuation at room temperature; (c) after evacuation at 90 "C; (d) after evacuation at 150 OC; (e) after evacuation at 400 "C.
2.07-2.08. The room-temperature spectrum showed a very broad anisotropic signal with ESR parameters of gl, = 2.385, All = 137 X cm-I, and g, = 2.08 for the CuNa-M sample, which are nearly identical with the values of the Cu2+species observed in the 77 K spectrum. Room-temperature evacuation led to the formation of two Cu2+ species in the CuNa-M and CuCa-M samples. The ESR parameters of the new Cu2+ species in CuNa-M were gll = 2.382 and All = 145 X IO" cm-I, while the additional species in CuCa-M cm-l. An additional Cu2+ had gll = 2.331 and All = 150 X species was not noted in the CuK-M ESR spectrum at this point. After evacuation at an average temperature of 75 OC, all three samples contained the species with gll = 2.323-2.347 and Ail = 0.01 694.170 cm-', in addition to the Cu2+species in fresh samples. The Cu2+species in fresh samples almost completely disappeared after evacuation at 150 OC. Only the species with ESR parameters of gll = 2.323-2327, All = (163-174) X lo4 cm-I, and g, = 2.06 was detected after activation at 400 OC. The Cu2+species observed in the fresh sample was completely regenerated when the activated samples were exposed to 20 Torr of triply distilled water. In contrast, the fresh sample of CuH-M contained two distinct species as shown in Figure 2. The dominant Cu2+ species has g values that match those of the Cu2+species observed in the fresh Na-M, K-M, and Ca-M samples, while the minor species has g,, = 2.422 and A,, = 142 X lo4 cm-I. The two species could not be distinguished in the room-temperature ESR spectrum. Upon evacuation at room temperature, the intensity of the latter signal decreased drastically and the g values of the other species shifted slightly. This signal was not observed after the sample was evacuated at 50 OC. Only the Cu2+ species having the ESR parameters of gll = 2.417, All = 136 X lo4 cm-', and g, = 2.08 remained in the sample until a temperature of 150 O C was reached. At this point, two new signals of nearly equal intensity were noted having ESR parameters of gll = 2.387, All,= 151 X lo4 cm-I and g, = 2.328, A l l = 175 X IO4 cm-I, respectively. These two species were both present after evacuation at 400 "C. As can be seen in Figure 2, the latter species dominated the spectrum. Exposure of the activated sample to 20 Torr of triply distilled H 2 0 completely restored both the Cu2+ species originally present in the fresh sample. The adsorption of ND3 onto CuNa-M, CuK-M, CuCa-M, or CuH-M produced a new Cu2+species with g,, = 2.241-2.249, All = 179 X cm-I, and g , = 2.05. Figure 3 shows the ESR spectra obtained from CuNa-M and CuH-M samples after ND, adsorption Seven nitrogen hyperfine lines separated by 1 I .7 G
The Journal of Physical Chemistry, Vol. 93, No. 11, 1989 4671
Cupric Ion-Adsorbate Interactions in Mordenite
CuCa-M + CH30D
rz
3 PULSE ESEM R = 0 26 nm A,so=OIMH~ N =I
I
(a)CuH-M+ ND3
3
m a
Y l
-__
CALC
(b) CuNa-M + ND3
/e, I
, ,
117G
Figure 3. Second-derivativeESR spectra at 77 K of mordenite after adsorption of ND3: (a) CuH-M; (b) CuNa-M. The features marked with t are due to Mn.
'"1 n
CuNa-M/CD30H 3 PULSE ESEM
SHELL
12
L
3
R.034 A,.,=O2 N=3
3 PULSE ESEM R = 0.20 nm Aiso=0.25 MHz N = 4.0
m n
5
1
>-
c m
0.42nm 00 MHz 3
---CALC EXPT
-
z W
+ z
Q
0
(b) CuH-M + D20
z W
I
~
3
i
i
i
A
3
2
4
5
T, PS
3 PULSE ESEM R.028 AIs,=030MHz N.6 ---CALC -EXPT
m
I
Figure 6. Three-pulse ESEM spectrum at 4 K of CuNa-M with adsorbed CD30H.
k
T,ps Figure 4. Three-pulse ESEM spectra at 4 K of (a) CuNa-M and (b) CuH-M samples after D,O was adsorbed.
can be seen clearly in the second-derivative spectrum of CuH-M (Figure 3a), while only five hyperfine lines were detected in the ESR spectrum of CuNa-M (Figure 3b) and of the other samples. The remaining lines are attributed to Mn impurities in the samples. When methanol was adsorbed onto samples of CuNa-M, CuK-M, or CuCa-M, the Cu2+ species formed had gll = 2.390-2.414, All = (136-140) X cm-I, and g, = 2.07-2.08. In contrast, the adsorption of methanol onto CuH-M resulted in the formation of two distinct CuZf species of approximately equal ESR intensity. The ESR parameters of one species were gll = 2.382 and A,, = 145 X IO4 cm-I, while the other had gl1= 2.440 cm-I. and A l l = 114 X The adsorption of ethanol onto all samples proceeded slowly and took approximately twice as long to equilibrate, based on ESR spectral changes, as when methanol was adsorbed. Two Cu2+ species were observed in samples of Na-M, K-M, and Ca-M with g,l = 2.392-2.395, A , l = 140 X cm-' and gll = 2.330-2.331, AI, = (164-165) X cm-I. When ethanol was adsorbed onto CuH-M, the same two species were detected that had been observed upon methanol adsorption, in addition to one with gll = 2.307 and A l l = 162 X cm-'. Ethylene was adsorbed onto samples of CuCa-M, CuK-M, and CuH-M to determine the interaction of Cu2+ with nonpolar molecules. Adsorption onto CuCa-M resulted in the formation of two new Cu2+ species, one with gll = 2.395 and A l l =J 18 X IO4 cm-' and the other with g,l = 2.340 and All = 143 X 10 cm-I. The ESR intensity of the two species was nearly equal. The same
two species were noted in the sample of CuK-M after exposure to C2H4,although the former species was clearly dominant. Two species were present in CuH-M: The first matched the major species in the CuK-M spectrum, while the second had gll = 2.332 and All = 169 X cm-I. Table I lists the ESR parameters of all cupric ion species that were observed in these samples. Figure 4a shows the three-pulse ESEM spectrum that was obtained from a sample of CuNa-M containing D 2 0 as the adsorbate. The spectrum was simulated by four deuterium nuclei at a distance of 0.28 nm and Ai, = 0.25'MHz. Similar results were obtained from CuK-M and CuCa-M samples rehydrated with DzO. These results indicate that Cu2+is directly coordinated to two water molecules in Na-, K-, and Ca-M. The three-pulse ESEM spectrum shown in Figure 4b was recorded from a sample of CuH-M with adsorbed D20.The best fit to the experimental spectrum was found to be N = 6 at R = 0.28 nm with Aeo= 0.30 MHz, indicating that Cuz+ coordinates to three water molecules in H-M. Since the fresh sample of CuH-M contained two Cu2+species (see Figure 2), ESEM spectra were recorded from CuH-M samples containing D 2 0 that had been evacuated at room temperature and at 50 "C. Very little difference was observed in the ESEM spectra of these samples, although the spectrum from the sample evacuated at 50 "C was much noiser. The same parameters were used to simulate these spectra ( R = 0.28, N = 6). Because evacuation at room temperature almost completely destroys the Cu2+ species with gll.= 2.422 and All = 142 X lo4 cm-I, it is clear that the Cu2+species with the ESR parameters of gll = 2.382, All = 151 X lo4 cm-I, and g, = 2.07 is that which is coordinated to three water molecules. The three-pulse ESEM spectrum that was obtained from a sample of CuCa-M after CH,OD adsorption is shown in Figure 5 . The spectrum was simulated with one deuterium nucleus interacting with Cu2+ at a distance of 0.26 nm with Aiso = 0.1 MHz. Similar results were obtained from a sample of CuCa-M
4612
~
TABLE 1: ESR Parameters for Cupric Ion at 77 K in Mordenite for Various Sample Pretreatment Conditions and Adsorbates ( A X lo4 cm-I) sample pretreatmentQ H+ freshb gll = 2.394 A l l = 140 g, = 2.08 fresh gll = 2.382 All = 151 gll = 2.422 All = 142 evac, RT gll = 2.413 All = 141 g, = 2.08
evac, 75 'C
evac, 150 'C
N D3 CH,OD
C,H,OD
CJ-4
Sass and Kevan
The Journal of Physical Chemistry, Vol. 93, No. I I 1989
gll= 2.417 All = 136 gL = 2.08 gl, = 2.387 All = 151 gll= 2.328
g l , " =2.325 A , , = 163 glI"=2.249 All = 179 g, = 2.05 gll = 2.440 All = 114 gii = 2.382 All = 145 gl, = 2.382 All = 151 g,l = 2.447 A,, = 119 gl,= 2.307 Ali = 162 g,l = 2.408 A , = 121 g,l = 2.332 All = 169
TABLE 11: Number of Nuclei ( N ) and Interaction Distances ( R ) Obtained from Simulations of ESEM Spectra of Cupric Ion in Mordenite"
cocation
cocation Na+
K+
adsorbate
Ca2+
parameter
Na+
K+
CaZ+
4 0.28 0.25 1 0.29 0.25 3 0.34 0.20 3 0.42 0.0
4 0.27 0.30 1 0.27 0.15 3 0.34 0.10
4 0.27 0.28 1 0.26 0.1 3 0.34 0.10
~~
g,l = 2.385 A l l = 137 g, = 2.08 g,l = 2.399 A i l = 141 g, = 2.07
= 2.382 All = 139 g, = 2.08 = 2.407 A,l = 150 g, = 2.08
gll
D2O
gll = 2.389 All = 134 g, = 2.08 gll = 2.386 Ail = 140 g, = 2.07
Ais:
CH30D
CD30H gll= 2.399 AI, = 141 gll= 2.382 Ail = 145 giI= 2.380 All = 144 gll = 2.340 A l l = 169 gll = 2.370 4 ,11 = 139 gll = 2.324
gll = 2.378 All = 134 g, = 2.07
N Rb N R Aiso
N,
gll = 2.390
gl, = 2.404 All = 146 gll = 2.347 A l l = 170 gIl = 2.36 All = 155 g, = 2.07
All = 140 g l i= 2.331 All = 150 gIl= 2.382 A l l = 145 g,i= 2.323 All = 170 gll = 2.335 A,l = 164 g, = 2.08
g; = 2.06 g, = 2.06
g, = 2.06
gll = 2.249 All = 179 g, = 2.05 gll = 2.414 All = 136 g, = 2.08
gl, = 2.241 All = 179 g, = 2.05 gll = 2.390 All = 140 g, = 2.07
gll = 2.245 All = 179 g i = 2.05 gll = 2.390 All = 140 g, = 2.07
gll= 2.392 4 ,11 = 140 gll = 2.330 Ail = 165
gll = 2.393 All = 140 g, = 2.330 Ail = 165
gIl= 2.395 All = 140 g, = 2.331 AIi = 164
gll = 2.395 All = 118 gll = 2.340 All = 143
gli = 2.395 AI, = 118 gll = 2.340 Ap = 143
'RT = room temperature, evac = evacuated. b E S R a t room temperature instead of 77 K.
upon C H 3 0 D adsorption, but the CuNa-M/CH,OD spectrum could only be simulated with one deuterium nucleus at a longer distance of 0.29 nm with A,,, = 0.25 MHz. These results were confirmed by adsorbing C D 3 0 H onto CuNa-M, CuK-M, and CuCa-M and carrying out three-pulse ESEM studies. The spectra for CuCa-M and CuK-M were both fit with a single shell of three interacting deuterium at R = 0.34 nm with A,, = 0.10 MHz. A single shell of deuterium nuclei was insufficient to adequately simulate the ESEM spectrum obtained from the CuNa-M/CD30H sample. The spectrum, which is shown in Figure 6, contains strong modulation at short T, which is characteristic of short-range interactions, as well as modulation that persists to long T , which is indicative of longer range interactions. The beginning of the spectrum was fit well by using N = 3 at R = 0.34 nm with A,, = 0.20 MHz. This shell was multiplied by a second shell of three deuterium nuclei at the longer distance of R = 0.42 nm with A,,, = 0.0 MHz. This procedure resulted in a good fit to the experimental spectrum. The A,,, parameters are all small and of little physical significance. Both CH,OD and C D 3 0 H were adsorbed onto CuH-M, and deuterium modulation was recorded. However, no simulations were carried out because no portion of the spectrum was isolated that had nonoverlapping contributions from the two Cu2+species. Very weak deuterium modulation was detected when samples of CuNa-, K-, Ca-, or H-M containing C2H50Dwere studied by ESEM. Because all samples contained two overlapping Cu2+ species, simulation3 were not attempted.
ESEM spectra were all recorded at 4 K. R values are given in nanometers. 'A,,, values are given in megahertz. Selected cation locations in mordenife
C
t
b
Figure 7. Cation locations I-VI in mordenite on a projection of the bc plane where the lines represent framework oxygens and the intersections are Si or Ai. Site I is the center of the small channels, site VI is the center of the large main channels, and sites 11-IV are in the interconnecting channels. Adapted from ref 16.
Table I1 summarizes the three-pulse ESEM results that were obtained in this study.
Discussion Several crystallographic studies have been done to determine the temperature-dependent location of various cations in both hydrated and dehydrated forms of m ~ r d e n i t e . ' ~ -Figure '~ 7 shows a projection of the bc plane of mordenite16 with the cation sites labeled. In hydrated Ca-M, half of the calcium ions are located in site VI, which is the center of the main channel.16J7 These CaZ+ are completely surrounded by seven water molecules. The remaining Ca2+are found in site I, where they are bonded to two waters and six oxygens of the zeolite framework. The cation distribution is somewhat different in hydrated K-M.ls The K+ are primarily located in site IV where they are coordinated to three molecules of water and four lattice oxygens. In hydrated Na-M, it has been established that there are four Na+ in site I,'5s'9 but the positions of the other four sodium ions have not been determined. The ESEM results indicate that cupric ion is coordinated to two water molecules in fresh samples of Na-M, K-M, and Ca-M. The ESR parameters are characteristic of Cu2+-( H20)*complexes in zeolites,20where the cupric ion is also coordinated to the zeolite ~~
( 1 5) Mortier, W. J. Compilation of Extra Framework Sites in Zeolites;
Butterworth: Guilford, Surrey, U.K., 1982; p 54. (16) Mortier, W. J. J. Phys. Chem. 1977, 81, 1334. (17) Elsen, J.; King, G. S. D.; Mortier, W. J. J . Phys. Chem. 1987, 91, 5800. (18)
Mortier, W. M.; Pluth, J . J.; Smith, J . V . In Natural Zeolites; Sand, L. B., Mumpton, F. A,, Eds.; Pergamon Press: Oxford, 1978; p 53. (19) Schlenker, J . L.: Pluth, J. J.; Smith, J. V. Mater. Res. Bull. 1979, 14, 751.
Cupric Ion-Adsorbate Interactions in Mordenite lattice through several oxygens of the framework. A complex such as this is similar to the Na+-, K+-, and Ca2+-H20 complexes found in site I of mordenite. The Cu2+-0 bond length calculated from the ESEM results is 0.22 nm, which agrees with the Ca2+-(0)H20 distance of 0.233 nm1.17Thus, it seems likely that in fresh CuCa-M Cu2+ is located in site I . This species is designated as Cull, where the subscript denotes the numbers of directly coordinated waters. By analogy to the CuCa-M case, Cull is also likely to be in site I in Na-M and K-M. As these fresh samples were dehydrated to a final temperature of 400 "C, a shift in ESR parameters was noted that is characteristic of Cull losing water ligands and coordinating only to the zeolite l a t t i ~ e . ~It*is~ of . ~interest ~ to note that, with the exception of a small amount of another species observed in CuCa-M, the transition from Cull to Cuo appears to proceed without the formation of an intermediate species. The transition was essentially complete after the samples were evacuated for several hours at 175 OC. This correlates with the X-ray results of the dehydration of calcium ions in mordenite obtained by Mortier.I7 Ca2+in site I did not migrate to a different site upon dehydration but merely lost waters and became better coordinated to the oxygens of the framework. This provides further support for the assignment of the fresh Cu2+species to site I in these samples and suggests that it maintains this position upon dehydration. Two Cu2+species were detected in the fresh sample of CuH-M. The dominant species was identified by ESEM studies as Cu2+ coordinated to three water molecules and is designated CuIII. If cupric ion were exchanged into site I, one would anticipate that it would be coordinated to only two water molecules, as found for Cu2+ in Na-M, K-M, and Ca-M and also for hydrated calcium ions in m ~ r d e n i t e . 'These ~ results seem to suggest that the maximum number of water molecules to which an ion in site I can coordinate is 2 and therefore that Cu2+is not located in site I in the H form of mordenite. Crystallographic studies have determined that hydrated potassium ions in mordenite reside in site IV,18 where they are coordinated to three water molecules. On the basis of the number of coordinated waters, site IV seems to be a likely choice for the location of CuIll in CuH-M. Not enough experimental data are available to identify the second Cu2+species in CuH-M. While the ESR parameters do match those of Cull, one would expect that it would contribute to the ESEM spectrum. But, the ESEM spectrum did not change upon evacuation at 50 "C, at which point this species was no longer observed. While it is unclear what this second species is, its presence suggests that there is more than one site in H-M that can accommodate a cupric ion. The behavior of Cu2+in H-mordenite upon dehydration is much more complex than in the other samples that were studied. The major fresh species Culll is replaced at approximately 150 OC by two species of nearly equal ESR intensity (see Figure 2). The g values for one of these Cu2+species are similar to those of the Cu2+species that are present in the other samples after evacuation at 400 "C. The ESR parameters suggest that this species is most likely Cuo. The other Cu2+species has g values close to the Cu2+ species found in the CuK-M sample upon evacuation at 150 OC. Whereas the Cu2+species in K-M becomes Cuo at 400 OC, the Cu2+species in H-M does not change. Again, these results suggest that cupric ion exists in two different sites in this form of mordenite. The adsorption of both polar and nonpolar molecules onto activated samples revealed further differences between Cu2+in H-M and Cu2+in either Na-, K-, or Ca-M. Cupric ion was able to coordinate to three molecules of ammonia in H-M but to only two ammonia molecules in the other samples. The nitrogen hyperfine splitting indicated that Cu2+ is directly interacting with the nitrogen of the ammonia ligand. This has been observed previously with Cu2+-NH3 complexes in zeolites.21 These results (20) (a) Anderson, M. W.; Kevan, L. J . Phys. Chem. 1986,90,6452. (b) Narayana, M.; Kevan, L. J . Chem. SOC.,Faraday Trans. I 1986, 82, 213. (21) Vedrine, J . C.; Derouane, E. G.; Ben Taarit, Y. J . Phys. Chem. 1974,
78. 5 3 1 .
The Journal of Physical Chemistry, Vol. 93, No. 11, 1989 4673 Straight channel
Sinusoidal channel Ah
ZEOLITE ZSM-5
@, , (
-Main
Small channels channels
ZEOLITE MORDENITE
Figure 8. Comparison of the mordenite and ZSM-5 channel systems.
are in agreement with the results of Barrer and Townsend22who found that Cu2+ coordinated two to three molecules of N H 3 in the ammonium form of mordenite. When these results are coupled with the ESEM data on the hydrated Cu2+complexes, it becomes apparent that Cu2+can consistently accommodate an additional small ligand in H-M compared to Na-, K-, or Ca-M. The absorption of methanol onto CuNa-M, CuK-M, and CuCa-M resulted in Cu2+ coordination to one molecule of C H 3 0 H . Cu2+is coordinated through the oxygen at a distance of 0.21 nm. The hydroxyl end of the methanol molecule is tilted toward the cupric ion. The Cu2+-methanol complexes formed in K- and Ca-M are virtually identical with each other with respect to ESEM and ESR parameters. The Cu2+-methanol complex formed in Na-M is in a slightly different location, as implied by the existence of a second interacting shell of deuterium at a longer distance. The complex in Na-M is located so that the methyl end of an additional methanol molecule is able to interact weakly with the cupric ion. The adsorption of methanol onto CuH-M produced two distinct Cu2+ species that could not be identified by ESEM. While it is not possible to determine the number of methanol molecules to which Cu2+is coordinated, the observation of deuterium modulation does signify that an interaction between at least one Cu2+species and C H 3 0 D or C D 3 0 H does occur. The results obtained when ethanol was adsorbed onto these samples paralleled the results of methanol adsorption, except that ethanol did not cause all of the Cu2+to complex. In all samples, varying amounts of Cuo were observed in the ESR spectrum. The ESEM data obtained for CuNa-M, CuK-M, and CuCa-M revealed very weak modulation that could not be simulated from one nucleus of deuterium at 0.4 nm. This suggests that the cupric ion is not directly interacting with ethanol in Na-, K-, and Ca-M, presumably because the Cu2+location is inaccessible to ethanol. The formation of two cupric ion species in addition to Cuo in H-M was observed, again suggesting that Cu2+has more sites available to it in H-M than in the other mordenite forms. Ethylene was employed as an adsorbate to examine the interaction between Cu2+and a nonpolar adsorbate. Some Cuo was detected in H-M samples, which indicates that not all of the cupric ions are in sites accessible to ethylene. This lends further support to the idea that cupric ion is located in at least two sites in H-M. All of the Cu2+ in the Ca-M and K-M samples was complexed by ethylene, as indicated by the disappearance of the Cuo ESR signal. Substantial evidence exists, therefore, to demonstrate that Cu2+ is located in at least two sites in H-M. In contrast, cupric ion is located in a single discrete site in the other forms of mordenite that were studied. While it has been shown that cations such as Na', K+, and Ca2+occupy specific sites in mordenite, H+ does not show any preference for a given l ~ c a t i o n . ~This ~ , ~could ~ by why the cupric ion can occupy different sites in H-M. In addition, it is clear from these results that Cu2+does not occupy the same sites in H-M that it occupies in the other samples. This can easily be concluded from the results obtained upon water and ammonia (22) Barrer, R. M.;Townsend, R. J . Chem. Soc., Faraday Trans. I 1976, 72, 2659. (23) Mortier, W. J.; Pluth, J. J.; Smith, J. V. Mater. Res. Bull. 1975, I O , 1319. (24) Schlenker, J. L.; Pluth, J. J.; Smith, J. V. Mater. Res. Bull. 1979, 14, 849.
J . Phys. Chem. 1989,93,4674-4677
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adsorption revealing differences in the number of ligands, as well as the differences in the Cu2+species formed upon exposure to C H 3 0 H , C2HSOH,and C2H4. It is of interest to compare these results with the results obtained in a similar study carried out on Cu2+ in zeolite ZSM-5.8 As mentioned previously, both ZSM-5 and mordenite are channeltype zeolites. Figure 8 illustrates the difference between the two structures. ZSM-5 consists of nearly circular channels of 0.54 X 0.56 nm diameter that are interconnected by sinusoidal channels that are 0.52 X 0.55 nm in diameter.I0 The intersections create a large increase in the available free space. I n a pair of recent ESEM s t u d i e ~ , Cu2+ ~ , ~ was found to coordinate to six water molecules and four molecules of ammonia in all forms of ZSM-5 (H+, Na’, Na+-H+, K+, or Ca2+). Cupric ion was able to add an additional ligand of either methanol or ethanol in H-ZSM-5. While similar results were obtained in the present study, cupric ion coordinates to more adsorbate molecules in ZSM-5 than in
mordenite. The difference can be attributed to the essential difference in the two zeolite structures. The large complexes formed between Cu2+and the adsorbate molecules in ZSM-5 are thought to be accommodated at the channel intersections. Mordenite does not contain these intersections, and this is reflected in these results. Large complexes such as [CU(CH,OH),~’] or [ C U ( C H ~ O H ) ]were ~ ~ +not observed in this study but are formed readily in ZSM-5. These results support the earlier work done on zeolite ZSM-5 and further illustrate the importance of the channel intersections found in ZSM-5 for catalytic applications. Acknowledgment. This research was supported by the National Science Foundation, the Robert A. Welch Foundation, and the Texas Advanced Technology Research Program. We thank Dr. Ravi Kumar Kukkadapu for many helpful discussions. Registry No. Cu, 7440-50-8; H20, 7732-18-5; NH,, 7664-41-7; H*, 12408-02-5; N a , 7440-23-5; K, 7440-09-7; Ca, 7440-70-2.
Vitrified Dilute Aqueous Solutions. 3. Plasticization of Water’s H-Bonded Network and the Glass Transition Temperature’s Minimum Klaus Hofer, Andreas Hallbrucker, Erwin Mayer,* Institut f u r Anorganische und Analytische Chemie. Universitat Innsbruck, A-6020 Innsbruck, Austria
and G . P. Johari Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada (Received: June 29, 1988; In Final Form: October 18, 1988)
The glass transition temperature, T,, of hyperquenched dilute (-0.4-6 mol %) binary aqueous solutions of LiCI, ethylene glycol, and ethanol and its composition dependence have been studied by differential scanning calorimetry. On the initial addition of the solutes, Tgdecreases. Further addition causes it to reach a minimum value and thereafter T, increases. Thus the initial addition of ionic or nonionic solutes weakens the H-bonded network against its resistance to relaxation. The minimum in the T, with increasing concentration is likely to be associated with the structural changes of the H-bonded network on the hydration of ions, stability of ion pairs, H bonding with alcohols, hydrophobic interactions, etc., but which of these processes predominate is not certain. Nevertheless, our results do make it possible to identify a new hydration number in the deeply supercooled state of water.
Introduction Micrometer-size water droplets on hyperquenching, or rapid cooling at rates > I O s K s-l, produce glassy or vitreous water,’-3 which has recently been shown by differential scanning calorimetry (DSC) to have a reversible glass-liquid transition with an onset temperature, or T,, of 136 f 1 K.4 Dilute aqueous solutions can be similarly completely vitrified by hyperquenching of micrometer-size droplets, and their glass transition and other properties can now be studied in the completely vitrified state for the first time. The infrared spectra of vitrified aqueous dilute alkali-metal nitrate and perchlorate solutions3 and the thermal behavior of hyperquenched NaCI-H20 and ethylene glycol (EG)-H20 glassesShave already shown several interesting features. We report a study of the T s of hyperquenched dilute aqueous solution glasses as a function of the concentration of one structure-forming electrolyte, and two H-bonding, one monohydroxy (ethanol) and the second a dihydroxy (EG) alcohol, and show that the initial addition of the solutes lowers the T, of glassy water, or plasticizes ( I ) Mayer, E. J. Appl. Phys. 1985, 58, 663. (2) Hallbrucker, A.; Mayer, E. J. Phys. Chem. 1987, 91, 503. (3) Mayer, E. J. Phys. Chem. 1986, 90, 4455. (4) Johari, G. P.; Hallbrucker, A.; Mayer, E. Nnture 1987, 330, 552. ( 5 ) Hallbrucker, A.; Mayer, E. J. Phys. Chem. 1988, 92, 2007.
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it, an effect similar to that of plasticization of polymer^.^ With increasing concentrations of the solute T, increases, and this plasticization seems to effectively vanish at or near the T, minimum. The new data for hyperquenched dilute solutions are then connected with the T, of the “glass-forming composition regions’’ of their concentrated solution^^-'^ and discussed in terms of the changes in the mobility of water in the H-bonded structure.
Experimental Section Dilute solutions were vitrified by a procedure described in earlier reports.’-5 Briefly, aerosol droplets were transferred through a ~~~~~
~
~
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(6) Ferry, J. D. Viscoelnstic Properties of Polymers; Wiley: New York, 1980. (7) Angell, C. A,; Sare, E. J. J. Chem. Phys. 1968, 49, 4713; 1970, 52, 1058. (8) Angell, C. A,; Sare, E. J.; Donnella, J.; MacFarlane, D. R. J . Phj’s. Chem. 1981, 85, 1461. (9) Angell, C. A,; Tucker, J. C. J. Phys. Chem. 1980, 84, 268. (IO) Kanno, H. J . Phys. Chem. 1987, 91, 1967. ( 1 1) Luyet, B.; Rasmussen, D. H. Biodynnmica 1968, I O , 167. (12) Rasmussen, D. H.; MacKenzie, A. P. J. Phys. Chem. 1971, 75,967. (13) Boutron, P.; Kaufmann, A. Cryobiology 1979, 16, 83. (14) Angell, C. A. In Wafer,A Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1982; Vol. 7, Chapter I , p 19, with references.
Q 1989 American Chemical Society
