11047
J. Phys. Chem. 1993,97, 11047-1 1052
Location and Adsorbate Interactions of Cu(I1) Ion in Cu(I1)-Exchanged K-L Zeolite by Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopy Jong-Sung Yu Department of Chemistry, Han Nam University, Taejon, Korea 300- 791
Jean-Marc Comets and Larry Kevan’ Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: May 6, 1993; In Final Form: August 11, 1993”
The interaction of Cu(I1) with deuterated adsorbates in Cu(I1)-exchanged K-L channel-type zeolite was studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. It was found that in the hydrated zeolite the major Cu(I1) species is octahedrally coordinated to six water molecules with the complex located in the main channels. Evacuation at room temperature was sufficient to remove three of these water molecules, leaving the Cu(I1) coordinated to three water molecules and anchored to the zeolite lattice by coordination to three zeolitic oxygens. A minor Cu(I1) species was also detected with reverse gvalues characteristic of trigonal bipyramidal coordination. Dehydration at 500 OC produces a Cu(I1) species which is inaccessible to the main channels based on a lack of broadening of its ESR lines by oxygen. Adsorption of molecules such as water, alcohols, and ethylene causes changes in the ESR spectrum of the Cu(II), indicating migration into cation positions in the main channels where adsorbate coordination can occur. Cu(I1) forms complexes with two molecules of methanol and ethanol and one molecule of ethylene based on ESEM data.
Introduction
Experimental Section
Zeolite molecular sieves containing transition metal ions have received considerableattention.’-’ The importance of zeolites as catalysts mainly arises from (i) their thermal stability and homogeneity in composition, (ii) their well-defined reaction sites, and (iii) 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 the catalytic center is paramagnetic, as in the case of Cu(II), electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies have proved very effective for probing the paramagnetic Parameters obtained from ESR can be used to deduce the local symmetry of the transition-metal ions, and analysis of ESEM signals yields information about the number of surrounding adsorbate nuclei (N), their interaction distance ( R ) , and their weak isotropic hyperfine coupling (AiMI).18J9 Most of the ESR characterization on zeolite-supported copper catalysts has focused on the use of the cage-type zeolites X, Y, and A as supports.2”30 More recently, ESR studies on transitionmetal ions supported on mordenite and ZSM-5, which are channeltype zeolites, have appeared.”-35 ZSM-5 exchanged with Cu(I1) is active for the decomposition of nitrogen monoxide.31 The characterization of Cu(I1) exchanged into ZSM-5 and mordenite by ESR and ESEM spectroscopies has also been reported.3639 Zeolite L is a channel-type zeolite which has a main channel diameter of about 0.75 nm. Little work has been reported on transition-metal ions in L eol lite.^.^^ It is thus of interest to examine the location and coordination geometry of Cu(I1) in L zeolite by studying the complexes formed between Cu(I1) and various adsorbates. In this work, the interactions between Cu(11) and oxygen, water, methanol, ethanol, and ethylene adsorbates are examined for the first time in K-L zeolite by ESR and ESEM spectroscopies.
Sample Preparation. Synthetic K-L (&Na&Si27072*21H20r type ELZ-L, Lot no. 4 14048B)was obtained from Union Carbide Corp. This L zeolite contained iron impurities easily detectable by an ESR signal near g = 4.3 for Fe(II1). The zeolite was washed with a 0.1 M solution of potassium acetate (Matheson Coleman and Bell) at 70 OC for 12 h to remove some of this iron and to ensure the K-exchanged form. The exchange was carried out four to five times to obtain maximum cation exchange. After this exchange the g 4.3 ESR signal intensity was reduced severalfold. The filtered zeolite was washed repeatedly with distilled water to ensure the removal of acetate ions and then dried in air at room temperature. This K-L zeolite was then further exchanged at room temperature for 12 h by dropwise addition of 10 mM cupric nitrate (Alfa Products) to form a slurry of 1.0 g of the zeolite in 100 mL of deionized water. The copperexchanged K-L was filtered, washed four times with hot deionized water, and dried in air at room temperature. Such a preparation is denoted as a fresh, hydrated sample. Copper exchange was in the range 0 . 1 4 4 wt 5% K-L. SampleTreatment. The zeolite sample was placed on a porous sintered glass disk inside an electrically heated 10-mm4.d. Pyrex reactor of total internal volume of about -30 cm3. In order to monitor the intensity and migration of Cu(I1) species by ESR in the system treated under different conditions, the reactor was connected to a Suprasil quartz ESR tube (2-mm i.d. by 3-mm 0.d.). The zeolite was heated in a stream of flowing oxygen while the temperature was slowly increased to some maximum temperature, usually 500 OC at which heating was continued for 16 h followed by cooling and evacuation at room temperature to remove excess oxygen. The reaction temperature was monitored by a thermocouple in a thermowell located at the center of the reactor tube. In another series of experiments,the zeolite sample was loaded directly into a Suprasil quartz ESR tube which was connected to a vacuum and gas handling line. Dehydration of the zeolite was carried out by first evacuating the zeolite at room temperature followed by heating to 500 OC over an 8-h period. Following this dehydration procedure the zeolite was exposed to
* Abstract published in Aduunce ACS Absrrucrs, October 1, 1993.
-
0022-365419312097-11047$04.00/0 0 1993 American Chemical Society
Yu et al.
11048 The Journal of Physical Chemistry, Vol. 97,No. 42, 1993 Fresh Hydrated CUK-L
A
aL \I glB=2.157
x8
-
Llf-)
gIlA=2.412
A
V
gl(B=1.943
200 G
Figure 1. ESR spectrum of fresh, hydrated CuK-L zeolite recorded (a) at room temperature and (b) at 77 K.
high-purity dry oxygen for 4 h at 500 OC in order to oxidize any copper species that had been reduced during the heating period. Finally, the oxygen was pumped off at room temperature under a lO-s-Torr vacuum. This heat-treated sample is termed a dehydrated zeolite sample. After dehydration, adsorbates were admitted at room temperature to the sample tubes and left for several hours to equilibrate. Deuterated adsorbates such as D20, CD30H, CH3OD, CHsCHzOD, and C2D4 were obtained from Aldrich, Stohler Isotope Chemicals, and Cambridge Isotope Laboratories and used after repeated freezepumpthaw cycles. Spectroscopic Measurements. K-L zeolite samples after Cu(11) exchange and calcination were examined by powder X-ray diffraction (XRD) with a Philips PW 1840diffractometer. FSR spectra were measured at 77 K on a modified Varian E-4 spectrometer interfaced to a Tracor Northern TN-17 10 signal averager. Each spectrum was obtained after multiple scans to achieve a satisfactory signal-to-noise ratio. Each acquired spectrum was transferred from the signal averager to an IBM PC/XT compatible computer for analysis and plotting. The magnetic field was calibrated with a Varian E-500 gaussmeter. The microwave frequency was measured by a Hewlett Packard HP 5342A frequency counter. ESEM spectra were recorded with a Bruker ESP 380 pulsed ESR spectrometer. Three-pulse echoes were measured by using a 9O0-~-9Oo-T-9O0 pulse sequence with the echo measured as a function of T. In order to maximize the modulation depth from deuterium and eliminate the modulation from zeolitic 27Al,the time between the first two pulses, T, was set at 0.28 ps. Both the theory and methods used for simulation of the data are described in detail elsewhere.'*
dTf 5O0C12h
glD=2.05 200 G
7
Figure 2. ESR spectra at 77 K of CuK-L zeolite after evacuation (a) at room temperature for 3 h, (b) at 107 O C for 1 h, (c) at 200 OC for 16 h, and (d) at 350 OC for 2 h.
a
EvaculOxidized gp2.334
C
€IllD
Results
Electron Spin Resonance Measurements. Figure 1 shows the ESR spectra of fresh hydrated CuK-L at room temperature and 77 K. In the fresh sample measured at 77 K, two species, denoted as species A and B, are observed as shown in Figure 1b. Species A has the ESR parameters of 81 = 2.41, All = 137 X 1 V cm-l, and gl = 2.08. Interestingly, species B,which is a minor species, shows ESR parameters with reversed g values of 81= 1.94, All =97 X lVcm-l,andgl =2.16. Atroomtemperature,however, the ESR spectrum shows an almost isotropic signal at gh = 2.1 7 as shown in Figure la. Figure 2 shows the trend in the ESR spectra upon dehydration of CuK-L. Evacuation at room temperature produces species C which shows little change in the ESR profile at 77 K except for a shift in gll from 2.41 to 2.39 and a decrease in the All coupling. After about 1-3 h evacuation at room temperature, complete disappearance of the isotropic signal is observed.
0, adsorption
11
2000
Figure 3. ESR spectra at 77 K of CuK-L zeolite (a) dehydrated by evacuation at 500 OC for 4 h, oxidized with dry oxygen at the same temperature for 4 h in a static reactor, and evacuatedat room temperature, (b) exposed to 200 Torr of oxygen at room temperature after (a), and (c)dehydrated by flowing oxygenover thezeoliteat 500 OCandevacuation at room temperature.
Further dehydration near 100 OC produces another species, denoted as D, with ESR parameters of = 2.335 and All = 161 X 1 V cm-1. This species becomes the dominant species as evacuation continues at higher temperature as shown in Figure 2c,d. After complete dehydration by heating from 350 to 500 OC only species D remains. Figure 3 shows the ESR spectra of Cu(I1) in CuK-L after different dehydration treatments. Figure 3a shows the ESR
Cu(I1) Ion in Cu(I1)-Exchanged K-L Zeolite a. Methanol
The Journal of Physical Chemistry, Vol. 97, No. 42, 1993 11049
TABLE I: ESR Parameters at 77 K of Cu2+in CuK-L Zeolite treatment'
species
freshe fresh
A
evac R T
C
evac 100 OC
C
M
evac 200 OC
D D
B
C
g1(-2.382
evac 350 OC dehydrated
D D
+HzO
E A
B
c. Ethylene
+CH3OH +CH$H2OH +C,?H4
probable confignd
gLb
CUVl
2.166r 2.412 1.943 2.391 1.943 2.390 2.336 2.335 2.390 2.335 2.334 2.389 2.412 1.943 2.394 2.382 2.343
B
b. Ethanol
A~c
gf
137 97 131 98 131 160 160 130 161 159 134 137 97 134 134 159
2.08 2.157 2.07 2.157 2.07 2.05 2.05 2.07 2.05 2.05 2.08 2.157 2.08 2.08 2.07
evac = evacuatedat; RT = room temperature. Estimatcduncertainty is f0.003. The unit of Ai is l(r cm-I, and the estimated uncertainty is i 5 X l(r cm-I. The subscripts for the probable configuration of Cu(I1) indicate the number of coordinated waters. Multiple species are listed in order of decreasing concentration. cESR recorded at room temperature. f g ,value. @
gl(=2.343
200 G
I--t-,
w
Figure 4. ESR spectra at 77 K of dehydrated CuK-L zeolite (a) after adsorption of methanol, (b) after adsorption of ethanol, and (c) after adsorption of 300 Torr of ethylene at room temperature.
spectrum after evacuation at 500 OC for 4 h, exposure to 400 Torr of dry oxygen for 4 h at the same temperature and then evacuation at room temperature in a static system. The ESR spectra clearly show two species, D and E. The new species E with ESR parameters of gl = 2.39 and All = 134 X 10-4 cm-1 appears as a minor species after oxidation. Species E shows line broadening by the addition of oxygen as shown in Figure 3b. The zeolite pretreatment is uspally dehydration in a stream of flowing oxygen while the temperature is slowly increased to some maximum temperature, usually -500 OC, at which heating is maintained for 8-10 h. The ESR spectrum obtained is identical to that from a static system treatment but usually has less intensity of species E (Figure 3a). Sometimes species D predominates as shown in Figure 3c. After the dehydration treatments, complete rehydrationby exposureof the dehydratedzeolite to the saturation vapor pressure of water at room temperature regenerates the original species A and B found in fresh CuK-L (Figure 1). Adsorption of methanol on a dehydrated sample produces an ESR spectrum with 81 = 2.39 and All = 134 X 10-4cm-l (Figure 4a). Upon adsorption of ethanol, a similar ESR spectrum is observed with 811 = 2.38 and ,411 = 135 X 10-4 cm-'. The ESR spectrum at room temperature is the same as that recorded at 77 K. After adsorption of 300 Torr of ethylene to the dehydrated zeolite, the gll value shows a shift from 2.335 to 2.343 with about the same hyperfine splitting ,411= 159 X 10-4 cm-1. Table I summarizes the ESR parameters of Cu(I1) in CuI(-L zeolites observed after various treatments. Electron Spin Echo Modulation Measurements. The threepulse ESEM spectrum at 4 K of CuK-L with adsorbed D20 is shown in Figure Sa with a corresponding ESR spectrum recorded at 77 K. The arrows on the inserted ESR spectrum correspond to the fields where ESEM measurements were made. The simulation shown indicates that Cu(I1) is interacting with 12 neighboring deuterium nuclei, i.e., six water molecules with a Cu(I1)-D distance of 0.30 nm (Figure 5a). This indicates that six water molecules are coordinated to the Cu(I1) with a Cu(11)-0 distance of 0.23 nm, which indicates direct coordination. Figure 5b shows the three-pulse ESEM spectrum at the field corresponding to the gl component of species B with reversed g
U
I
Y
0
1
'
2
4
3 T, PS
5
-
Figure 5. Experimental (-) and simulated (- -) three-pulse ESEM spectra recorded at 4 K of dehydrated CuK-L with adsorbed D2O (a) a t H = 3190 G and (b) at H = 3417 G. In the ESR spectral insert the arrows correspond to the fields for the ESEM measurements.
values. The best fit simulation is N = 4, R = 0.31 nm, and A i , = 0.13 MHz, which indicates two directly coordinated water molecules. Figure 6 shows the three-pulse ESEM spectrum of CuK-L with adsorbed D20 after evacuation at room temperature. The spectrum is simulated by N = 6 deuterium at R = 0.30 nm with A h = 0.17 MHz. This indicates that Cu(I1) is coordinated to only three water molecules after evacuation. The three-pulseESEM spectra for adsorbed CDBOHand CH3OD are shown in Figure 7. The simulation for CD30H indicates interaction with six deuteriums, Le., two methanol molecules, with a Cu(I1)-D distance of 0.37 nm. The simulation for CH3OD indicates interaction with two deuteriums, Le., two molecules of methanol, with a Cu(I1)-D distance of 0.28 nm. These results are self-consistent with the geometry of methanol.
Yu et al.
11050 The Journal of Physical Chemistry, Vol. 97, No. 42, I993
CUK-L + D20,evacuated r.i
R = 0.37 nm A = 0.07 MHz
A = 0.17 MHz
0
1
2
3
4
CUK-L (a) + CD30H N=6 R = 0.37 nm A = 0.10 MHZ
1
2
3 T, PS
4
5
N=2
T,
4
5
and simulated (- - -) three-pulse ESEM spectrum recorded at 4 K of dehydrated CuK-L with adsorbed CzD4. Figure 9. Experimental (-)
TABLE Ik ESEM Parameters for Cu(II) in CuK-L Zeolite Where N Is the Number of Deuteriums, R is the Cu(II) to D Distance, and A h Is the Isotropic Deuterium Hyperfine coupling treatmentc Na R,nm A h , MHz +DzO 12 0.30 0.26 4
(b)+ CH30D R
3 T, P
Figure 6. Experimental (-) and simulated (- - -) three-pulse ESEM spectrum at H = 3190 G recorded at 4 K of dehydrated CuK-L with adsorbed DzO followed by evacuation at room temperature.
0
2
1
5
T, FS
= 0.28nm
PS
-
Figure 7. Experimental (-) and simulated (- -) three-pulse ESEM spectra recorded at 4 K of dehydrated CuK-L (a) with adsorbed CD3OH and (b) with adsorbed CHJOD.
T, PS
and simulated (- - -) three-pulse ESEM spectrum recorded at 4 K of dehydrated CuK-L with adsorbed CH3CHzOD. Figure 8. Experimental (-)
The three-pulse ESEM spectrum for adsorption of deuterated ethanol, CH,CH3OD, is shown in Figure 8. The best fit gives N = 2, R = 0.27 nm, and A i , = 0.19 MHz. This indicates that two molecules of ethanol are directly coordinated to Cu(I1) as for methanol. Whendeuterated ethylene, C2D4, is adsorbedontoa dehydrated sample of CuK-L, the ESEM spectrum shown in Figure 9 is observed. This spectrum is simulated by four interacting deuterium nuclei, at R = 0.37 nm and A i , = 0.07 MHz. This indicates that one ethylenemolecule is coordinated to the Cu(I1).
0.31
0.13
+Dz@ 6 0.30 0.17 +CDpOH 6 0.37 0.10 +CH3OD 2 0.28 0.20 +CHoCHzOD 2 0.27 0.19 +CzD4 4 0.37 0.07 Speciesgiven in order of decreasingconcentration. Speciesobtained after evacuation at room temperature. Table I1summarizes the ESEM parameters for Cu(I1) in CuK-L zeolites with various adsorbates.
Discussion Zeo1ite.L is a synthetic aluminosilicate zeolite that has not yet been found to occur naturally. The common Si/A1 ratio in zeolite L is 3.0, which indicates an ordered Si, A1 distribution. The crystal structure of zeolite L is based on the e-cages of the 18tetrahedra unit found in cancrinite which are formed by five six-membered and six four-membered rings.4245 The t-cages are linked through two nearly planar six-membered rings, forming hexagonal prisms with the planes of the six-membered rings normal to the c axis. Thus, L zeolite consists of a series of columns along the c axis where t-cages and hexagonal prism units alternate. These columns are linked to each other as shown in Figure 10, producing wide channels of 12-membered rings parallel to the c axis. These wide channels, usually called main channels, have free diameters of 0.714.78 nm. The largest internal diameter of the 12-membered rings is about 1.3 nm. The walls of the main channels consist of eight-membered and four-membered rings. The main channels are connected with each other through nonplanar eight-membered rings. There are five types of cation sites located for Na+ and K+ as indicated in Figure Site A is located in the center of the hexagonal prism. This site is comparable to an SI site in the middle of a hexagonal prism in X zeolite. The cation in site A forms distorted octahedral coordination with six framework oxygens, three above and three below the cation. Site B is the center of the c-cage. Site C is located midway between thecenters of two adjacent t-cages. Site D, the only cation position found in the main channels, is best understood by reference to Figure 10. The cation in site D coordinates to four framework oxygens in the nonplanar eight-membered ring. Site E is located midway between adjacent A sites. The K-L zeolite after Cu(I1) ion exchange is highly crystalline by X-ray diffraction. Little loss of crystallinity of the L zeolite was observed after ion exchange and calcination. The fresh hydrated CuK-L zeolite gives an ESR spectrum mainly consisting of a broad isotropic line at ambient temperature
Cu(I1) Ion in Cu(I1)-Exchanged K-L Zeolite
.'
0 h.
'-
F e 10. Schematic representationviewed approximatelyperpendicular to the c axis of potassium zeolite L showing large channels parallel to the caxis and possibleextraframeworkcationsitesas letters (upper);projection viewed parallel to the c axis (lower). Adapted from ref 41. (Figure la). Such a broad ESR signal at room temperature is indicative of a mobile species which is rotationally unrestricted on the ESR time scale. Analysis of the three-pulse ESEM spectrum (Figure 5a) of zeolite K-L, which has been rehydrated with D2O and which exhibited an identical ESR spectrum to the fresh zeolite, indicates 12 nearest, approximately equivalent, coupled deuterium nuclei. This corresponds to a water solvation number of six around Cu(II), Le., [Cu(H@)6l2+. Such an octahedral hydrated copper species has been observed by ESEM analysis for Cu(I1) in several other zeolites.27J7J9 However, hydrated Cu-mordenite, which is also a channeltype zeolite, did not show such an isotropic ESR signal at room temperature, probably due to smaller main channels of 0.65 X 0.70 nm.36 X and Y zeolites are cage types and isostructural. Interestingly, hydrated CuK-Y zeolite shows an isotropic ESR signal27 while hydrated CuK-X zeolite does not at ambient temperature.26J8 This probably reflects the lower cation density in Y zeolite. L zeolite has 12-ring main channels whose diameter is about 0.75 nm. These are the only possible locations for accommodation of a hexaaquo complex in L zeolite. This hexaaquo cupric ion species will be referred to as CUVI,where the subscript indicates the number of coordinated water ligands. At 77 K this [Cu(H2O)6l2+complex becomes immobilized and gives rise to an asymmetric spectrum as shown in Figure 1b. The ESR spectrum at 77 K shows predominatelyspecies A. Minor species B is also observed with < gl. The parameter 811is less than gl for Cu(I1) when the unpaired spin has a 13z2- r2)orbital as the dominant part of the ground-state wave function.4 This occurs when Cu(I1) has some unusual geometries4749such as (a) tetragonally or rhombically compressed octahedron, (b) cisdistorted octahedron, (d) trigonally compressed tetrahedron, (d) linear configuration, and (e) trigonal-bipyramidalconfiguration. From a consideration of the symmetry expected in A and Y zeolites, Hermanl2 suggested that a Cu(I1) species with reversed g values has a trigonal bipyramidal configuration with three zeolitic oxygens (0,)and two ligand molecules. The analysis of the deuterium modulation obtained for species B indicates that Cu(I1) interacts with four deuteriumscorrespondingto two water molecules (Figure 5b). Neither octahedral nor tetrahedral symmetry around Cu(I1) including coordination to two water
The Journal of Physical Chemistry, Vol. 97, No. 42,1993
11051
molecules is expected in the L zeolite framework. A linear configuration is also improbable in the main channels. Thus, trigonal bipyramidal coordination is the most probable for species B with reversedgvalues. One possible location for Cu(I1) forming trigonal bipyramidal coordination is site B, the center of an e-cage where Cu(I1) can coordinate to three oxygens between two connected 4-rings in the cage and to two water molecules, one above and the other below the cation in thecage. Another possible location is the center of a 6-ring in the ring of an ecage where Cu(I1) can coordinate to three oxygens in the ring and to one water molecule in the e-cage and another water molecule near site C. Species B is a diaquo complex [Cu(Oz)j(H20)2] and is referred to as CUII. Such Cu(I1) species with reversed g values were observed and reported in other zeolites.8~9J2J6~6This geometry was confirmed by ESEM studies.8~gJ6 When the samples are evacuated at room temperature, the isotropic components of the ESR signal recorded at ambient temperature decrease. After 1-3 h of evacuation, the ESR signal is no longer broadened at room temperature and shows another species C. This is indicative of the copper losing some water ligands and becoming immobilized by coordination to several lattice oxygens. The three-pulse ESEM results (Figure 6) indicate that Cu(I1) is now coordinated to only three water molecules. This species is referred to as CUIII. With continued dehydration the ESR spectrum shows the development of another species D, which becomes predominant at higher temperature while species C decreases (Figure 2b-d). After this treatment the ESR intensity of Cu(I1) is decreased severalfold. With dehydration at 350 OC or higher only species D remains, which is not coordinated to any water molecules and is designated CN. This is verified from a three-pulse ESEM spectrum of a sample of CuK-L, which has been dehydrated to 450 OC, fully rehydrated with D20, and then dehydrated again to 400 OC. No deuterium modulation is observed, indicating no coordinationto residual water molecules or to OD groups formed by hydrolysis upon dehydration. A sample evacuated at 500 OC was exposed to oxygen and heated at the same temperature to reoxidize any copper ion reduced during evacuation. After degassing at room temperature, the ESR intensity of Cu(I1) was recovered and the ESR spectrumshowed the formation of another minor species E in addition to major species D (Figure 3a). The decrease in the intensityof E when CuK-L was dehydrated under flowing oxygen and the broadening of the ESR lines of species E upon adsorptionof oxygen (Figure 3c) in contrast to no intensity change in species D indicate that species E is located in positions accessible to the main channel while species D is located in recessed or inaccessible sites to the main channel. Some cations in site D are found to withdraw from the channel to a more symmetric coordination position on dehydration.42-46 Thus, species E is assigned to site E at the center of double 8-rings. Cu(I1) locates at the center of a hexagonal prism in dehydrated X and Y zeolites because Cu(I1) can form an energetically stable octahedral complex with six framework oxygens. And species D is assigned to site A in the center of a hexagonal prism. Adsorption of methanol on CuK-L zeolite produces a single species (Figure 4a) and results in a complex involving two molecules of methanol from an analysis of the ESEM spectrum (Figure 7). This indicates that species D Cu(I1) migrates from recessed sites to sites available for coordination with methanol. From the Cu(I1)-D distances obtained from ESEM of 0.28 nm for the hydroxyl deuterium and0.37 nm for the methyl deuterium, it is apparent that the methanol is complexed through the hydroxyl oxygen. Adsorption of ethanol gives ESR parameters very similar to those for adsorption of methanol. The ESEM spectrum also indicates that Cu(I1) is coordinated to two molecules of ethanol (Figure 8). Since both methanol and ethanol are too bulky to enter the ecage, Cu(I1) is believed to migrate into the main
Yu et al.
11052 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993 channel, most probably toward site D where the alcohols can coordinate with Cu(I1). Equilibration with ethylene produces a single species (Figure 4c). Ethyleneis a relativelysmall moleculewith a kinetic diameter of 0.39 nm,50which can diffuse through the main channels of CuK-L. Analysisof the ESEM spectrum with adsorbed ethylene indicates that Cu(I1) interacts with one molecule of ethylene at 0.37 nm with its molecular plane perpendicular to a line toward Cu(I1) since four equivalent deuteriums are observed. From the known geometry of ethylene the distance from Cu(I1) to the center of the C-C double bond is deduced to be 0.34 nm. The ethylene plane has a half-thicknessof 0.17 nm due to its T bonds.5’ To this one adds 0.07 nm as a maximum effective half-thickness of Cu(I1) at site D in the main channel. This leaves 0.10 nm as the displacement of Cu(I1) from site D toward site E. This indicates weak coordination between Cu(I1) and ethylene. It seems that the nonpolar ethylene molecule does not cause Cu(I1) to migrate to site D in the main channel. Very similar results to those in channel zeolite L are observed for Cu(I1) complexes in the ZSM-5 channel zeolite. In hydrated Cu-exchanged ZSM-5, Cu(I1) forms a similar hexaaquo com~ l e x . 3Dehydration ~ at room temperature decreases the Cu(I1) coordinationto three water moleculeswhile completedehydration produces one Cu(I1) species which is inaccessible to the main channel as in L zeolite. However, Cu(I1) ion in ZSM-5 coordinatesto three molecules of methanol or ethanol at its channel intersections37 which is greater than the two molecules found for L zeolite. Mordenite consists of a large 12-ring main channel that is intersected by a series of smaller channels. Mordenite does not contain such large channel intersections as found in ZSM-5 zeolite. Probably due to the absence of such large channel intersections, Cu(I1) ion coordinates fewer water molecules and only one methanol molecule in mordenitewhich is less than in L and ZSM-5 zeolites.36
Conclusions The combination of ESR and ESEM measurements has provided fairly good information on ligand coordination and probable locations of Cu(I1) ions in CuK-L zeolite. The cupric ion in hydrated CuK-L is an octahedrally coordinated hexaaquo species, [Cu(H20)6]2+. This species resides in the main channel and has considerable freedom of movement at room temperature. A minor species observed with a reversed g factor coordinates to three zeolitic oxygens and two water molecules to form trigonal bipyramidal geometry. Upon partial dehydration at room temperature, the fully hydrated cupric ion loses some of its coordinated water and becomes anchored to the zeolite lattice by partial coordination to zeolitic oxygens. When completely dehydrated, cupric ions are mainly located in cation sites recessed from the main channels. Adsorption of molecules such as water, alcohols, and ethylene causes migration of Cu(I1) from these recessed sites into the main channel and coordination with the adsorbates. Cu(I1) forms complexes with two molecules of methanol, two of ethanol, and one of ethylene.
Ackwwledgmeat. This research was supported by the National Science Foundation and the Robert A. Welch Foundation. References and Notes (1) Maxwell, I. E. Ado. Curd 1982, 31, 1.
(2) Mochida, I.; Hayata, S.; Kato, A.; Seiyama, T. J . Curul. 1970, 19, 405. (3) Tsuruya, S.;Tsakumoto, M.; Watanabe, M.; Masai, M. J . Cutul. 1985, 93, 303. (4) Benn, F. R.; Dwyer, J.; Estahami, A,; Evmerides, N. P.; Szczepura, A. K. J. Cutal. 1977, 48, 60. (5) Naccache, C. M.; Taarit, Y. B. J. Cutul. 1971, 22, 171. (6) Dimitrov, C.; Leach, H. F. J . Curd 1969, 14, 336. (7) Maxwell, I. E.; Downing, R. S.; van Langen, S. A. J. J . Cutul. 1980, 61, 485. (8) Ichikawa, T.; Kevan, L. J. Am. Chem. Soc. 1983, 105,402. (9) Narayana, M.; Kevan, L. J . Chem. Phys. 1983, 78, 3573. (10) Ichikawa, T.; Kevan, L. J. Phys. Chem. 1983, 87, 4433. (11) Herman, R. G.; Flentge, D. R. J. Phys. Chem. 1978, 82, 720. (12) Herman, R. G. Inorg. Chem. 1979, 18,995. (13) Anderson, M. W.; Kevan, L. J. Phys. Chem. 1987,91, 1850. (14) Anderson, M. W.; Kevan, L. J. Phys. Chem. 1987, 91, 2926. (15) Narayana, M.; Kevan, L. J. Chem. Soc.,Furuduy Trans. I 1986,82, 213. (16) Narayana, M.; Kevan, L. Lcrngmuir 1985, I, 553. (17) Narayana, M.; Kevan, L. J. Am. Chem. Soc. 1981, 103, 5729. (18) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., Eds.; Wiley-Interscience: New York, 1979; Chapter 8. (19) Kevan, L.; Narayana, M. Intruzeolite Chemistry; ACS Symp. Ser. No. 218; American Chemical Society: Washington, DC, 1983; p 283. (20) Conesa, J. C.; Soria, J. J. J. Chem. Soc., Furuduy Trans. I 1979,75,
406. (21) Gentry, S. J.; Rudham, R.; Sanders, M. K. J . Curd. 1974,35,376. (22) (a) Lee, H.; Kevan, L. J . Phys. Chem. 1986,90,5776. (b) Lee,H.; Kevan,L. J. Phys. Chem. 1986,90,5781. (b) Lee,H.;Narayana, M.;Kevan, L. J. Phys. Chem. 1985,89, 2419. (23) (a) Yu, J. S.; Lee, H.; Kevan, L. In Curulysis 1987; Ward, J. W., Ed.; Elsevier: Amsterdam, 1988; pp 273-279. (b) Yu,J. S.; Kevan, L.In
Microstructure and Properties of Curulysts; Treacy, M. M. J., Thomas, J. M., White, J. M., Eds.; Materials Research Society: Pittsburgh, 1988; pp 245-429. (24) Anderson, M. W.; Kevan, L. J. Phys. Chem. 1986,90, 3206. (25) Uh,Y. S.; Chon, H. J. J . Korean Chem. Soc. 1979,23, 80. (26) Yu, J. S.; Kevan, L. J. Phys. Chem. 1990,94, 5995. (27) Yu, J. S.; Kevan, L. J. Phys. Chem. 1990, 94,7612. (28) Yu, J. S.; Kevan, L. J. Phys. Chem. 1990, 94,7620. (29) Yu, J. S.; Kevan, L. J. Phys. Chem. 1991,95, 3262. (30) Yu, J. S.; Kevan, L. J. Phys. Chem. 1991, 95, 6648. (31) Iwamoto, M.; Furukawa, H.; Mine, Y.; Uemura, F.; Mikuriya, S.; Kagawa, S. J. Chem. Soc., Chem. Commun. 1986,1272. (32) Kucherov, A. V.; Slinkin, A. A. Zeolites 1986, 6, 175. (33) Sendda, Y.; Ono, Y. Zeolites 1986, 6, 209. (34) Anderson, S. L. T.; Scurrell, M. S. J . Cutul. 1981, 71, 233. (35) Elsen, J.; King, G. S. D.; Mortier, W. J. J. Phys. Chem. 1987, 91, 5000. (36) Sass, C. E.; Kevan, L. J. Phys. Chem. 1989, 93,4669. (37) Sass, C. E.; Kevan, L. J. Phys. Chem. 1988, 92, 5192. (38) Sass, C. E.; Kevan, L. J . Phys. Chem. 1988, 92, 14. (39) Anderson, M. W.; Kevan, L. J. Phys. Chem. 1987, 91, 4174. (40) Bass, J. S.; Kevan, L. J. Phys. Chem. 1990,94,4640. (41) Sayari, A.; Morton, J. R.; Preston, K. F. J. Phys. Chem. 1989,93, 2093. (42) Barrer, R. M.; Villiger, H. Z . Kristullogr. 1969, 128, 352. (43) Wright, P. A.;Thomas, J. M.;Cheetham,A. K.;Nowak.A. K. Nuture 1985, 318,611. (44) Newsam, J. M. Mater. Res. Bull. 1986, 21, 661. (45) Breck, D. W. Zeolite Moleculur Sieves;Wiley: New York, 1974; p 113. (46) Abragam, A.; Bleaney, B. Electron Purumugnetic Resonance of Trunsition Ions; Clarendon Press: Oxford, 1970; p 456. (47) Hathaway, B. J.; Billing, D. E. C w r d . Chem. Rea 1970, 5, 143. (48) Slade, R. C.; Tomlinson, A. A. G.; Hathaway, B. J.; Billing, D. E. J . Chem. SOC.A 1968.61. (49) Hoffman, S. K. Acta Phys. Pol. 1976, A49, 253. (50) Breck, D. W. Zeolite Moleculur Sieues; Wiley: New York, 1974; p 636. (5 1) Pauling, L. Nuture of the Chcmicul Bond, 3rd ed.;Cornell University Press: Ithaca, NY, 1960; pp 260, 514, 518.
