2652
J. Phys. Chem. 1992, 96, 2652-2651
been stripped of its solvent shell and is brought close to the basal surface oxygen atoms it forms strong coordinate bonds to these oxygen atoms. These bonds are not broken when water is reintroduced. Presumably the Cu(I1) recesses within the pseudohexagonal sites in the basal oxygen surfaces. Further evidence for the binding of Cu(I1) to the beidellite layers comes from the two-pulse data. For samples that were vacuum-dried no modulation is observed in the two-pulse ESEM after rehydration at 100% relative humidity or after water-soaking. In contrast, distinct 27Almodulations were observed for samples that were first dehydrated at 100 OC before rehydrating. In fact these modulations were most intense for the water-soaked sample initially dehydrated at 100 OC for which it would be expected that the layers would be separated by a distance e x d i n g 2 nm. Apparently the energy bamer that needs to be surmounted in order to drive exchangeable Cu(I1) ions in beidellite into the pseudohexagonal cavities is not high. In fluorohectorite and hydroxyhectorite more energy is required to solvate the exchangeable cations than is required for the cations to migrate into the pseudohexagonal cavities since even before heating Cu(I1) cations bind to the surface of these smectites. When the fluorohectorite is dehydrated at 100 OC,a better resolution of the copper hyperfine in the g , region is observed and a third water molecule coordinates the Cu(I1) cation. With this exception the ESR and ESEM datas of the fluorohectorite and hydroxyhectorite are not significantly different. This demonstrates that the substitution of hydroxyl groups for fluorine atoms located on the edges of the octahedrons of the fluorohectorite does not modify the interactions between the basal oxygen surface of the clay, the exchangeable cations, and the interlayer water and, thus, no explanation to the affmity of the Cu(I1) cations for the hectorite layers can be provided.
Conclusions In the present study, the solvation of Cu(I1) in four smectite
clay minerals with differing surface charge distributions has been examined. Dehydration of the smectites under mild conditions followed by rehydration either by equilibrating at 100% relative humidity or by soaking with water generates Cu(I1) that exhibits distinctly different interactions with surrounding water molecules and with the smectite layer surfaces. This interaction is also dependent on the pretreatment conditions. The interplay between exchangeable cation charge and the characteristics of the layer charge distribution are factors affecting the type of exchangeable cation-water complex formed. An important conclusion of the present study is that the behavior of Cu(I1)-exchanged into four different types of smectites is remarkably similar in that in all four smectites the Cu(I1) cations irreversibly bind to the basal oxygen surface after dehydration at 100 OC. Although this has been realized for montmorillonites that are heated beyond 200 O C it has not been shown to be the case for montmorillonites treated under the mild conditions used in this study. Even more important is the fact that, in similar fashion to montmorillonite, Cu{II) binds also to the surfaces of beidellite, fluorohectorite, and hydroxyhectorite after only mild dehydration. The surface-bound Cu(I1) retains the ability to coordinate to some adsorbed water in beidellite, fluorohectorite, and hydroxyhectorite but loses this ability in montmorillonite. Finally, it is important to note that mild thermal treatment such as heating to 100 O C may be sufficient to induce quite pronounced changes in the microstructure of interlayer water in smectites. This underscores the need to carefully consider the process used to dehydrate a smectite clay when attempting to gain information on either water structure or exchangeable cation positions.
Acknowledgment. This research was supported by the Robert A. Welch Foundation, the Texas Advanced Research Program, and the National Science Foundation. The assistance of Xinhua Chen in the implementation of software used for the simulations is gratefully acknowledged.
Study of Cu(I1) Locatlon and Adsorbate Interaction in CUH-SAPO-34 Molecular Sieve by Electron Sp4 Resoname and Etectron Spln Echo Modulath Spectroscopies Maggie Zamadies, Xinbua Chen, and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: August 26, 1991)
H-SAPO-34 doped with Cu(1I)ions is studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM) techniques. In the hydrated sample Cu(I1) was determined to be octahedrally coordinated to three framework oxygens and three water molecules. The most likely location for this copper ion complex to reside is at a site displaced from the hexagonal window into the ellipsoidal cavity. Upon dehydration at 400 OC two distinct Cu(I1)complexes are generated. The ESR parameters for one of the Cu(I1)complexes formed on dehydration is suggestive of trigonal symmetry, where the complex is formed in the plane of the six-membered ring or displaced from the six-membered ring. One type of Cu(I1)complex is observed with the adsorption of water, ammonia, ethylene, dimethyl sulfoxide, ethanol, and 1-propanol while two different Cu(I1) complexes are observed with the adsorption of methanol. From ESEM it is determined that Cu(I1) interacts with three equivalent ammonia molecules,one ethylene molecule, and one dimethyl sulfoxide molecule. Through ESEM spectroscopy the Cu(I1)cation is determined to interact with three molecules of ethanol, two of which interact at a shorter distance than the third.
Introduction In 1982 Wilson et al. at Union Carbide reported the synthesis of aluminophosphate molecular sieves (AIP04) which are composed of aluminum and phosphorus tetrahedral 0xides.I Shortly after the success of this APO4 synthesis, different classes of new microporous structures were prepared by isomorphic substitution (1) Wilson,S.T.; Lok, B. M.; Messina, C. A.; Cannon, T. R.; Flanigen, E.M.J . Am. Chem. SOC.1982, 104, 1146.
0022-365419212096-2652$03.00/0
of several different elements into the AlP04 frameworka2 SubStitUtiOn Of Silicon into the Alp04 framework results in SiliCOaluminophosphate molecular Sieves (SAPO). Like the -4 class Of mOleCUlar Sieves, SAPO molecular Sieves have frameworks StrUCtUrally related to zeolites, as well as novel structure^.^ (2) Flanigen, E. M.; Lok, B. M.; Patton, R. L.; Wilson, S.T. Pure Appl. Chem. 1986, 58. 1352. ( 3 ) Lok, B. M.; Messina, C. A,; Patton, R. L.;Gajek, R. T.; Cannin, T. R.; Flanigen, E. M. J. Am. Chem. SOC.1984, 106, 6092.
0 1992 American Chemical Society
Cu(1I) Location in CuH-SAPO-34 Molecular Sieve This work examines Cu(I1) ions doped into H-SAPO-34. SAPO-34 has a structure similar to the naturally occurring zeolite, chabazite. By incorporating cupric ions into the cationic positions one may manipulate the adsorption and catalytic properties of the molecular sieve. The potential use of CuH-SAPO-34 as a catalyst may be evaluated from the Cu(I1) cation location and the adsorbate interactions with Cu(I1) within the molecular sieve lattice. Electron spin resonance (ESR) and electron spin echo modulation (ESEM) have been employed to investigate the local environment of the cupric ions within molecular sieve^.^^^ From ESR spectroscopy the geometry of the paramagnetic species is determined,6 while ESEM probes the local environment of paramagnetic species yielding information on the coordination number and interaction distance.' In this study the interaction between Cu(I1) and adsorbates, such as water, ammonia, methanol, ethanol, 1-propanol, ethylene, and dimethyl sulfoxide, in H-SAPO-34 is investigated by ESR and ESEM. One Cu(I1) complex is observed in samples with adsorbed water, ammonia, ethylene, ethanol, 1-propanol, and dimethyl sulfoxide, while in a sample with adsorbed methanol two distinct Cu(I1) complexes are formed. The ESR and ESEM results suggest that Cu(I1) in the hydrated sample coordinates to three framework oxygens and to three water molecules. The ESEM simulations indicate that Cu(I1) is complexed to three equivalent ammonia molecules, three ethanol molecules, one ethylene molecule, and one dimethyl sulfoxide molecule.
Experimental Section SAPO-34 was synthesized on the basis of the method described by Xu et a1.* An aluminophosphate gel was made by mixing 6.7 mL of H3P04 (Fisher Chemical) in 10 g of deionized H 2 0with 4.08 g of aluminum oxide (Catapal-A, Vista Chemical) in 50 g of deionized H 2 0 . After a homogeneous gel was formed, 3.2 g of a 40 wt % silica sol (Ludox AS-40, Dupont) was mixed into the aluminophosphate gel and stirred for about an hour. This final synthesis gel contained mole ratios of 1.0 Si, 3.0 Al, and 5.0 P. To this reaction mixture, 14.8 g of triethylamine (Alfa Chemicals) was added and the gel was stirred overnight. After about 14 h of stirring, the pH of the reaction mixture was adjusted to 7.0 by the addition of 40 wt % H F (Aldrich). A Teflon-lined autoclave was filled to 60% capacity and heated at 200 OC. After 7 days the reaction was quenched to room temperature, and the solid product was retrieved by filtration followed by washing with boiling deionized water. Comparison of the literature X-ray diffraction pattern to that published9 indicated that the product was a single phase with good crystallinity. Prior to copper incorporation the organic template, triethylamine, was removed by heating to 600 OC in flowing oxygen for 12 h. Cu(I1) was exchanged into H-SAPO-34 by stirring 1 g of the molecular sieve with a solution composed of 10 mL of a 1 mM CuC12 in methanol and 90 mL of methanol at about 55 'C for 1 h. The exchanged sample was filtered and then washed with warm methanol to remove Cu(I1) ions from the exterior surface. The sample was dried at 150 OC for 2 h and then allowed to adsorb water. The concentration of Cu(I1) ions in the molecular sieve is 0.06 wt % based on the assumption of complete exchange. This is equivalent to approximately one Cu(I1) ion per 38 unit cells. To examine the change in the coordination environment of the Cu(I1) ion as a function of hydration the sample was dehydrated under vacuum to a residual pressure of about 1 X Torr. (4) Kevan, L. In Electron Magnetic Resonance of the Solid State; Weil, J., Ed.; Canadian Society for Chemistry: Ottawa, 1987; pp 281-293. (5) Sass, C. E.; Kevan, L. J . Phys. Chem. 1988, 92, 5192. (6) Goodman, B. A.; Raynor, J. B.Adu. lnorg. Chem. Radiochem. 1970,
13, 135. (7) Kevan, L. In Modern Pulsed and Continuous-Wave Electron Spin Resonance; Kevan, L., Bowman, M. K., Eds.; Wiley: New York, 1990; pp 231-266. (8) Xu, Y.; Maddox, P. J.; Couves, J. W. J . Chem. SOC.,Faraday Trans. 1990, 86, 425. (9) van Ballmoos, R.; Higgins, J. 8.Zeolites 1990, 10, 386.
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 2653
n FRESH
gI1= 2.381
\.r EVAC RT
x4
g,,= 2.336
g,,= 2.382
1
/ I * " ( x4
-U
EVAC400'C
gl, = 2.338
g,,= 2.381
Figure 1. ESR spectra of CuH-SAPO-34 recorded a t 7 7 K: (a) hydrated, (b) evacuated a t room temperature overnight, and (c) evacuated a t 400 'C.
Samples dehydrated at temperatures higher than 400 OC were heated in 760 Torr of oxygen for 2 h and evacuated at that same temperature to remove the oxygen. To study adsorbate interactions the samples were dehydrated at 400 "C in flowing oxygen for 4 h, evacuated, and exposed to adsorbates. Liquid adsorbates were exposed to the dehydrated samples overnight at their room temperature vapor pressure and gaseous adsorbates at 200 Torr were allowed to equilibrate overnight. The adsorbates used were DzO, ND3, 15NH3,C2D4, CH30D, CH3CH20D (Cambridge Isotope Laboratories), and (CD3)*S0(Aldrich). ESR spectra were recorded at 77 K on a modified Varian E-4 spectrometer described previously.1° ESEM signals were recorded on a homebuilt ESE spectrometer at 4 K described elsewhere." In a three-pulse experiment a pulse sequence of 9O0-~9O0-T-9Oo is employed and Tis swept. To suppress modulation from zeolitic aluminum, T was fixed at 0.28 ps.II The ESEM data were transferred to a HCP386 IBM PC compatible computer for data analysis. Simulation of the deuterium modulation observed in the ESEM signals was performed using the analytical expression derived by Dikanov et al.I2 Data were extracted from the modulation pattern by comparing the experimental ESEM signal with the calculated signal. The best fit was found by varying the parameters: the number of interacting nuclei N , the interaction distance R, and the isotropic hyperfine interaction AiW,until the sum of the squared residuals was minimized.
Results ESR. Figure 1 shows the ESR spectra taken at different temperatures which show the progression of the dehydration of CuH-SAPO-34. The Cu(I1) ion displays axial symmetry in all spectra. Only one Cu(I1) species is present in the hydrated sample. The g values and hyperfine coupling constant for this Cu(I1) complex are g,,= 2.381, g, = 2.073 and A,, = 0.0143 cm-'. Overnight evacuation at room temperature produces two Cu( 11) species with ESR parameters gl,= 2.382, All = 0.0135 cm-l and g,,= 2.336, All = 0.0168 cm-I. Two species are present after evacuation at 400 'C. One Cu(I1) complex has ESR parameters ~~
~
~
(IO) Chen, X.;Kevan, L. J . Am. Chem. SOC.1991, 113, 2861. ( 1 1 ) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L.; Schwartz, R. N., Eds.; Wiley Interscience: New York, 1979; pp 279-341. (12) Dikanov, S. A,; Shubin, A. A,; Parmon, V. N. J . Magn. Reson. 1981, 4 2 , 474.
2654 The Journal of Physical Chemistry, Vol. 96, No. 6, 1992
Zamadics et al.
+ METHANOL
-
g,, = 2.373
g,,= 2.412
A = 18.3 G gll = 2.374
ll
Y
Figure 4. ESR spectrum of CuH-SAPO-34 recorded at 77 K after the adsorption of I5NH3.
gll = 2.375
Figure 2. ESR spectra of CuH-SAPO-34 with adsorbed alcohols recorded at 77 K: (a) methanol, (b) ethanol, and (c) propanol.
gll = 2.376
A Figure 5. ESR spectrum of CuH-SAPO-34recorded at 77 K after the adsorption of dimethyl sulfoxide.
Figure 3. ESR spectrum of CuH-SAPO-34recorded at 77 K after the adsorption of ethylene.
of gll = 2.382, All = 0.0127 cm-' while the other species has gll = 2.338 and A, = 0.0154 cm-'. The fully hydrated Cu(I1) complex is regenerated when the activated sample is exposed to water vapor at room temperature. In a sample with adsorbed methanol two distinct Cu(I1) complexes are detected in the ESR spectrum as shown in Figure 2a. The ESR parameters for the major species (-60%) are g, = 2.373, A, = 0.0143 cm-' and for the minor species (-40%) are g, = 2.412, A, = 0.0118 cm-I. Samples with adsorbed ethanol and 1-propanol give similar Cu(I1) ion ESR spectra (Figure 2b,c). In both samples only one type of Cu(I1) complex is observed. The ESR parameters of this Cu(I1) species are gh = 2.375, A, = 0.0145 cm-l for samples with adsorbed propanol and gw= 2.374 and All = 0.0143 cm-' for samples with adsorbed ethanol. Ethylene adsorption gives rise to only one complex as illustrated in Figure 3. The ESR parameters for this complex are gil = 2.392 and A, = 0.0137 cm-I. Equilibrium, which was determined by the absence of changes in the ESR signal with increasing adsorbate exposure time, was reached after 2 days. The detailed kinetic changes in the ESR signal were not analyzed since the focus here is on structure rather than dynamics. Adsorption of I5NH3produced a single species with gll= 2.233 and A, = 0.0177 cm-I as shown in Figure 4. Also shown in Figure 4 is the second-derivative signal which reveals four hyperfine lies with an intensity ratio of 1:3:3:1. This suggests that three equivalent I5Nnuclei are directly coordinated to the Cu(I1) ion. Dimethyl sulfoxide adsorbed onto an activated sample (Figure 5 ) gives an ESR spectrum that shows only one Cu(I1) complex. The gvalue and coupling constant are gll = 2.376 and All = 0.0140 cm-l. ESR parameters for all the Cu(I1) complexes that were studied are summarized in Table I.
TABLE I: ESR Parameters at 77 K for Cu(I1) in H-SAPO-34 Molecular Sieve sample pretreatmenta gob AI: IO4 cm-I g, fresh 2.381 143 2.073 evac, RT (12 h) 2.336 168 d 2.382 135 d evac, 400 "C 2.382 127 d 2.338 154 d 2.233 177 d ND3 2.392 137 2.070 C2D4 (CD312SO 2.376 140 2.062 CH3OD 2.373 143 d 2.412 122 d CHICH20D 2.374 143 2.061 CH3CH2CH20D 2.375 145 2.062 'evac = evacuated. bEstimated uncertainty is 10.005. CEstimated uncertainty is 1 5 X loJ cm-I. dNot resolved. TABLE II: ESEM Simulation Parameters for Cu(I1) in H-SAPO-34 Molecular Sieve R,6 AQ,C no. of adsorbate Na nm MHz adsorbate molecules 0.25 3 DZO 6 0.28 4 0.37 0.05 1 C2D4 0.10 1 (CWW 6 0.41 9 0.30 0.10 3 ND3 0.27 0.4 2 CH3CH20Dd 2 ~~
1
0.32
0.1
a Number of nuclei. * CU(II)-~Hdistance. coupling constant. dTwo-shell model required.
1
Isotropic hyperfine
ESEM. Three-pulse ESEM signals for samples with adsorbed DzO, C2D4, ND3, CH3CH,0D, and (CD3)2S0 are shown in Figures 6-10. The simulation parameters are listed in Table 11. ESEM signals for DzO, CzD4, ND3, and (CD3)#0 were simulated using a oneshell model. It was necessary to use a two-shell model for fitting the ESEM signal of CH3CH20D. The ESEM signal for adsorbed D 2 0 is shown in Figure 6. The simulation indicates three water molecules with a CU(II)-~H distance of 0.28 nm. This distance indicates that the water molecules are directly coordinated to the Cu(I1) ion through the oxygen, giving a Cu(II)-O distance of about 0.21 nm.
The Journal of Physical Chemistry, Vol. 96, No. 6, 1992 2655
Cu(I1) Location in CuH-SAPO-34 Molecular Sieve CUH-SAPO-34 + D20 N=6 R = 0.28 nm Ais0 = 0.21 MHz
m
a a
CUH-SAPO-34 + (CD3)2SO N=6 R = 0 43 nm Also = 0.14MHz
1 q A
U -0 6 t
a
-0.6
t
k
CI)
+ z
0.4'
80.2
Y 0
1
2
3
4
I
1 0
5
1
2
T, PS
-
Figure 6. Experimental (-) and simulated (- -) three-pulse ESEM spectrum of CuH-SAPO-34 with adsorbed D20. Spectra recorded at 4
K.
-
5
-
CUH-SAPO-34+ CH3CH20D SHELL N R(nm) Aiso(MHz) 1 2 0.28 0.4 2 1 0.32 0.1
U
a
4
Figure 9. Experimental (-) and simulated (- -) three-pulse ESEM spectrum of CuH-SAPO-34 with adsorbed (CD,)2S0. Spectra recorded at 4 K.
CUH-SAPO-34+ ND3 N=9 R = 0.27 nm Ais0 = 0.20 MHz
t
3 T, !AS
0.6 ..
5 0.4 I-
z
z
g 0.2
g 0.2 Y U
n
1
2
3
4
Figure 7. Experimental (-) and simulated (- - -) three-pulse ESEM spectrum of CuH-SAPO-34 with adsorbed ND,. Spectra recorded at 4
K. 1 .o
0
5
1
2
Z 3 0.8
a
a
4
5
Figure 10. Experimental (-) and simulated (- - -) three-pulse ESEM spectrum of CuH-SAPO-34 with adsorbed CH,CH20D. Spectra recorded at 4 K.
CUH-SAPO-34 + C2D4 N=4 R = 0.34 nm Ais0 = 0.12MHz
t
3
T,PS
CUH-SAPO-34 HYDRATED
a -0.6
5-
5I- 0.4,
3
8 0.28 0 3 4 5 T, !AS Figure 8. Experimental (-) and simulated (- - -) three-pulse ESEM spectrum of CuH-SAPO-34 with adsorbed C2D4. Spectra recorded at 4 K. 0
p,.0.21 w It U 0
U
1
2
Figure 7 illustrates the ESEM signal for ND3. The signal is best fit with parameters of N = 9, R = 0.3 nm, and Ais,,= 0.10 MHz. The ESEM signals for adsorbed C2D4and (CD3)2S0are shown in Figure 8 and 9, respectively. Both ESEM signals are fit with parameters which indicate the interaction of one adsorbate molecule with the Cu(I1) cation. Figure 10 shows the experimental and simulated ESEM signals for adsorbed CH3CH20D. The best fit simulation was obtained using a two-shell model. The simulation parameters suggest that two molecules of ethanol are directly coordinated to the Cu(I1)
1
2
3
4
5
T, YS Figure 11. Three-pulse ESEM spectra showing 27AI modulation for hydrated CuH-SAPO-34. Recorded at 4 K.
cation at a distance of 0.27 nm while a third molecule is indirectly coordinated at a larger distance of 0.32 nm. Modulation from zeolitic 27Alwas monitored in a three-pulse ESEM experiment from a hydrated sample with H 2 0instead of D 2 0 by setting T = 0.40 ps, which is the optimal value for recording the deepest 27Almodulation." Pulse widths were increased to 78 ns to eliminate lH modulation. The 27Almodulation that was observed is shown in Figure 11. The observation of *'A1 modulation indicates that the Cu(I1) ion is within 0.5 nm of zeolite framework a l ~ m i n u m . ' ~ (13) Sass, C.E.;Kevan, L. J . Phys. Chem. 1989, 93, 7856.
Zamadics et al.
2656 The Journal of Physical Chemistry, Vol. 96, No. 6, 1992
TABLE III: ESR Parameters at 77 K for Cu(I1) in Several Dehydrated Molecular Sieves molecular dehydration sieve temp. O C H-SAPO-34 400
Ca-X Na-Y
\
\
' q// /'
Figure 12. Schematic representation of the unit cell of SAPO-34. Each open circle represents an Al, P, or Si atom and the solid circles represent oxygens. The Roman numerals indicate cation sites. (Adapted from ref 14.)
Lliscussion Structure. SAPO-34 molecular sieve has a framework similar to the naturally occurring zeolite, chabazite. The framework of SAPO-34 consists of distorted hexagonal prisms linked together by four-membered rings to form a large ellipsoidal cavity. Each ellipsoidal cavity is interconnected to six like cavities through an eight-membered oxygen ring with an opening diameter of about 0.4 nm. Although no X-ray crystallographic studies have identified specific sites occupied by charge compensating cations in SAPO-34, an estimate of probable locations can be made from those sites previously determined in its structural analogue, chabazite. The designations of these sites are given according to Calligaris et al.I4 Site I is displaced from the six-ring into the ellipsoidal cavity; site I1 is located near the center of the ellipsoidal cavity; site I11 is found in the center of the hexagonal prism, and site IV is near the eight-ring window. Figure 12 illustrates the unit cell of SAPO-34 along with the designated cation sites. Dehydration Process. The ESR parameters of the hydrated sample are oharacteristic of an octahedrally coordinated Cu(I1) ~omplex.'~,'~ Simulationof the ESEM signal indicates that Cu(11) ion is directly coordinated to three water molecules. Modulation from 27Alis also observed which suggests that the cupric ion is specifically associated with aluminum in the lattice. A complex such as CU(O~)~(H~O)~, where OFrefers to the framework oxygen, is most likely. A site within the molecular sieve lattice where this complex could exist is site I. Upon evacuation overnight at room temperature two cupric species are observed with ESR parameters different from those in the hydrated sample. Thus prolonged evacuation at room temperature is sufficient to remove some H20ligands. It is not possible to deduce the nature of the Cu(I1) complexes from ESEM spectroscopy, since there is spectral overlap of the two species. Evacuation of a sample with adsorbed D20 does show deuterium modulation in the ESEM signal; thus, at least one Cu(I1) species is complexed with water after overnight evacuation. As the hydrated sample is dehydrated to a final temperature of 400 OC two distinct Cu(I1) species are formed as indicated from the two sets of parameters determined from the ESR signal. No deuterium modulation is observed from samples with previously adsorbed D20and then dehydrated to a temperature of 400 OC. The species giving rise to the g value of 2.382 can be assigned, by comparison to the ESR parameters for Cu(I1) doped into Na-YI6 and Ca-X," to be trigonally coordinated. These parameters are listed in Table III.16317 The only location within the (14) Calligaris, M.; Nardin, G. Zeolites 1982, 2, 200. (15) Anderson, M. W.; Kevan, L. J . Phys. Chem. 1987, 91, 4174. (16) Conesa, J. C.; Soria, J. J. Chem. Soc. Forudoy Trow I 1979, 74,406. (17) Narayana, M.; Kevan, L. J. Chem. Phys. 1983, 78, 3573.
400 600
E,,
2.382 2.338
2.379 2.380
A,,, lo4 cm-l
127 154 133 140
ref this work 17 16
molecular sieve lattice where a coordination of this type could occur is at site I, where the Cu(I1) ion is coordinated to three framework oxygens near the plane of the six-membered ring. This location has been identified by single crystal X-ray diffraction for Cu(I1) cations in dehydrated Cu-chabazite.I* The location of the second Cu(I1) complex generated by dehydration has not been unambiguously identified. One possible location for this Cu(I1) species is in the hexagonal prism (site 111). Mn(I1) cations have been identified by single crystal X-ray measurements to occupy site I11 in dehydrated Mn-~habazite.'~ Adsorbate Interaction. Adsorption of ammonia results in the formation of a complex involving three molecules of ammonia which is verified by the hyperfine components in the ESR spectrum and analysis of deuterium modulation in the ESEM signal. The ESR signal is suggestive of an axially symmetric Cu(I1) complex.2o A complex in which Cu(I1) is coordinated to three framework oxygens and three ammonia molecules seems most probable. Site I would provide the conditions necessary for a complex of this type to reside. The adsorption of methanol produces two distinct Cu(I1) complexes. Neither of these complexes has ESR parameters similar to the two Cu(I1) species in a dehydrated sample; thus, the appearance of both Cu(I1) species can be associated with the methanol adsorbate. One set of the Cu(I1) ion ESR parameters resembles those determined for a sample after exposure to ethanol, where the Cu(I1) ion was determined to coordinate to three ethanol molecules. The second set of ESR parameters vary significantly from the other values found in this study. It is unclear at this point why two different Cu(I1) complexes are generated by methanol adsorption. Ethanol and 1-propanol were used as adsorbates to gain an insight as to why two Cu(I1) complexes are produced after methanol adsorption. These alcohols yielded only one type of Cu(I1) complex after adsorption which contrasts with the results for methanol adsorption. While methanol is slightly more polar than ethanol and propanol, a more significant difference between these adsorbates is their relative size. Thus, steric effects are a probable contributing factor for the absence of the Cu(I1) species with the larger g value that is observed in the sample with adsorbed methanol. Adsorption of dimethyl sulfoxide produces a single Cu(I1) species as indicated by the ESR spectrum. Thus, the diffusion of dimethyl sulfoxide into the ellipsoidal cavity appears to influence the location of one or both of the two types of Cu(I1) species in a dehydrated sample. The ESR parameters and ESEM simulation results determined in this study show the CU(II)-(CD~)~SO complex to be of axial symmetry which is similar to that obtained for CuKTl-X zeolite.21 Ethylene was employed to examine the influence of a relatively nonpolar adsorbate on the location of the Cu(I1) cations in SAPO-34. The size of the ethylene molecule should not hinder its diffusion into the ellipsoidal cavity but the polarity of the molecular sieve could retard this diffusion process. Thus the prolonged equilibration time can be attributed to at least two factors: slow diffusion of ethylene into the ellipsoidal cavity and the adsorbate's inability to facilitate a rapid migration of the (18) Pluth, J. J.; Smith, J. V.; Mottier, W . J. Moter. Res. Bull. 1977, 12, 1001. (19) Calligaris, M.; Mezzetti, A,; Nardin, G.; Randaccio, L.Zeolites 1985, 5, 317. (20) Hathaway, B. J.; Tomlinson, A. A. G. Coord. Chem. Rev. 1970, 5, I. (21) Lee, H.; Narayana, M.; Kevan, L.J . Phys. Chem. 1985,89, 2419.
J. Phys. Ckem. 1992, 96,2657-2668 Cu(I1) cation. The weak interaction between the cupric cation and ethylene, as indicated by the CU(II)-~Hinteraction distance of 0.37 nm, is evidence of the C2D4 inefficiency to create expedient migration of Cu(I1) cation in SAPO-34. The observation of longer equilibration times with nonpolar adsorbates was also found in CuNaH-ZSM-5 and C ~ H - m o r d e n i t e . ' ~ , ~ ~ The orientation of the ethylene molecule can be determined from the deuterium modulation observed in the ESEM signal. The interaction of Cu(I1) with four equivalent deuterons indicates that C2D4 interacts with the Cu(I1) cation with its molecular plane perpendicular to a line toward the Cu(I1) ion. A similar ethylene orientation is found in Cu(I1)-doped zeolite A.23 In this study we observed the behavior of Cu(I1) cations in H-SAPO-34 when subjected to various treatments to be comparable to the behavior of Cu(I1) ions in zeolites. One similarity between Cu(I1) ions in these materials is the migration of the Cu(I1) cation upon thermal treatment. The appearance of two Cu(I1) species after dehydration indicates the existence of two different site preferences within the molecular sieve. This difference was not observed in dehydrated SAPO-5I0 or SAPO-1lZ4 with similar Cu(I1) ion loadings. This site preference suggests that the distribution of silicon atoms varies for each unit cell. A nonhomogeneous distribution of framework atoms has been reported in SAPO-2025and SAPO-47.26 From this study we also observe a change in the ESR parameters of dehydrated samples after the adsorption of various ~~~
~
(22) Sass, C. E.; Kevan, L. J . Phys. Chem. 1989, 93,4669. (23) Ichikawa, T.; Kevan, L. J . Am. Chem. SOC.1981, 103, 5355. (24) Lee, C. W.; Chen, X.; Kevan, L. J. Phys. Chem. 1991, 95, 8626. (25) Hasha, D.; Saldarriaga, L. S. de; Saldarriaga, C.; Hathaway, P. E.; Cox, D. F.; Davis, M. E. J . Am. Chem. SOC.1988, 110, 2127. (26) Pluth, J. J.; Smith, J. V . J . Phys. Chem. 1989, 93, 6516.
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molecules. This observation indicates the coordination of the adsorbate to the Cu(I1) cation and hence a migration of one or both of the Cu(I1) species to a position in order to acquire the best coordination. A position near or at site I will allow for the maximum coordination number and is the most accessible location for complexation to occur. We also observed in this work an unusual behavior of the Cu(I1) ion after exposure to methanol. The reason why two species are formed after adsorption of methanol in not clear at this time. Energetically both Cu(I1)-methanol complexes formed should be equally favorable; if not we would expect the migration of the Cu(I1) ion into any one location as was observed with the other adsorbates.
Conclusions In a hydrated sample Cu(I1) is octahedrally coordinated to three zeolitic oxygens and three water molecules. This species most likely resides in site I, which is displaced from the six-membered ring into the ellipsoidal cavity. Dehydration to 400 OC produces two distinct Cu(I1) species. Ammonia adsorption also results in an octahedral complex in which the Cu(I1) is coordinated to three zeolitic oxygens and to three ammonia molecules through the nitrogen. This complex is likely located in site I. Adsorption of methanol results in the formation of two distinct Cu(I1) species. Propanol and ethanol adsorption gives rise to only one Cu(I1) complex. The Cu(I1) complexes with one molecule of ethylene and one molecule of dimethyl sulfoxide. Acknowledgment. This research was supported by the National Science Foundation, the Robert A. Welch Foundation, and the Texas Advanced Research Program.
Electrochemistry at @-Hydroxy Thiol Coated Electrodes. 3. Voltage Independence of the Electron Tunneling Barrier and Measurements of Redox Kinetics at Large Overpotentials Anne M. Becka and Cary J. Miller* Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742 (Received: August 7, 1991)
Self-assembled monolayers of w-hydroxy thiols on Au electrodes are investigated as electron tunneling barriers allowing the measurement of heterogeneous electron kinetics of solution species over a wide range of electrode potentials without mass transport limitations. From the dependence of the electron-transfer rate of a series of redox couples on the thickness of the monolayer film, a more precise tunneling coefficient, 0,of 1.08 0.20 per methylene unit in the w-hydroxy thiol was measured independent of the redox couple and was found to be nearly independent of the electrode potential. The heterogeneous electron-transfer rates for a series of facile redox couples measured at w-hydroxy thiol monolayer coated Au electrodes display a pronounced sigmoidal dependence on the electrode overpotential, which is predicted by the Marcus theory. Reorganization energies and preexponential factors for a series of redox couples are extracted from current/voltage curves. The level of defects within these w-hydroxy thiol monolayers is probed by several electrochemical measurements, which indicate that defects do not significantly perturb the kinetic measurements.
*
Introduction The understanding of redox reactions and the characterization of redox-active molecules are of fundamental importance in chemistry. Electrochemical methods have many advantages over homogeneous measurements for the determination of kinetic parameters. The electrode, by virtue of its continuously variable potential, behaves as a universal oxidant or reductant. In a single voltammetric experiment, the electrode current gives a direct measure of the rate of the electron transfer, allowing the reactivity of a given redox molecule to be probed over a continuous range of potentials. The major problem with these heterogeneous electron-transfer reactions is that the redox molecules must be 0022-3654/92/2096-2657$03.00/0
brought to the electrode surface prior to their oxidation or reduction. The rate at which redox species can be transported to the electrode surface sets a limit on the size of heterogeneous electron-transfer rate constants which can be measured. In addition to the problem of diffusion limitations, the possible presence of potential drops and specific adsorption sites at the electrode surface complicates the description of the heterogeneous electron-transfer rate.' The concentration, structure, and reactivity ( 1 ) Delahay, P. In Double Layer and Electrode Kinetics: Advances in Electrochemistry and Electrochemical Engineering; Delahay, P., Tobias, C. W., Eds.; Wiley Interscience: New York, 1965; Chapter 3.
0 1992 American Chemical Society
