12436
J. Phys. Chem. 1994, 98, 12436-12441
Cu(I1)-Adsorbate Interactions in Cu(I1)-Exchanged K-L Zeolite Jong-Sung Yu Department of Chemistry, Han Nam University, Taejon, Chungnam, Korea 300-791
Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: June 22, 1994; In Final Form: August 22, 1994@
The interaction of Cu(I1) in Cu(II)-exchanged K-L channel-type zeolite with adsorbates containing coordinative nitrogen (ammonia, pyridine, hydrazine, and nitrogen monoxide) and carbon monoxide, benzene, and dimethyl sulfoxide was studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. Adsorption of ammonia produces a complex containing four mulecules of ammonia based on resolved nitrogen superhyperfine and analysis of deuterium modulation. Equilibrium with pyridine also results in a stable complex containing three molecules of pyridine based on resolved nitrogen superhyperfine. However, no resolved nitrogen superhyperfine between hydrazine and Cu(I1) and nitrogen monoxide and Cu(I1) is seen. Adsorption of nitrogen monoxide on hydrogen-reduced CuK-L samples produces an unstable Cu+-NO complex with reversed g value (gl > gll) ESR parameters and results in some reoxidation of reduced copper species back to cupric ions. Adsorption of carbon monoxide, benzene, or dimethyl sulfoxide causes changes in the ESR spectrum of Cu(II), indicating migration of Cu(I1) into cation positions in the main channels where adsorbate coordination can occur. Cu(I1) forms complexes with one molecule of benzene and dimethyl sulfoxide, respectively, based on ESEM data.
Introduction Transition metal ions on zeolite surfaces are increasingly being exploited as controllable catalytically active This requires knowledge of both the location and adsorbate interactions of the transition metal ion. Various experimental techniques have been used in the elucidation of these proper tie^.^-'^ Electron spin resonance (ESR) has been one such technique, widely used in delineating information concerning the number of different species, oxidation state, and coordination environment of a paramagnetic metal center such as C U ( I I ) . ' ~ - A ~ ~type of pulsed ESR, known as electron spin echo modulation (ESEM) spectroscopy, can provide additional quantitative information concerning the number of surrounding adsorbate nuclei and their interaction distance.20-2s The structural features of the host zeolite matrix have a direct influence on the site location of paramagnetic species and their coordination geometry with adsorbates, thus altering their catalytic properties. The majority of these investigations have been limited to the cage-type zeolites X, Y, and A.26-34 It has been reported that ZSM-5 zeolite exchanged with Cu(II) is active for the decomposition of nitrogen monoxide.35 More recently, the characterization of Cu(I1) exchanged into ZSM-5 and m ~ r d e n i t e ~ ~and, - ~ l most recently, the characterization of Cu(I1) in silicoaluminophosphate (SAPO)-5 and -11 molecular sieve^,^^-^^ which all have channel-type structures, have appeared. In recent previous the location of Cu(I1) in hydrated and dehydrated CuK-L and coordination structure of Cu(I1) with adsorbates such as water, alcohols, and ethylene was studied for the first time by ESR and ESEM spectroscopies. Zeolite L is also a channel-type zeolite with a main channel diameter of about 0.75 nm.47-51 Here, we extend the characterization of Cu(1I) in zeolite L to adsorbates containing coordinative nitrogen such as b o n i a , pyridine, hydrazine, and nitrogen monoxide with which direct nitrogen hyperfine interac@
Abstract published in Advance ACS Absrrucfs, November 1, 1994.
0022-36541941209 8- 12436$04 SO10
tion is observable. Coordination with carbon monoxide, benzene, and dimethyl sulfoxide is also studied. These results are compared with channel-type zeolites and silicoaluminophosphates of similar size. Experimental Section Sample Preparation. Synthetic K-L (K6Na3A19Si2707221H20, type ELZ-L, Lot No. 4140-08B) 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). This was washed with a 0.1 M solution of potassium acetate (Matheson Coleman and Bell) at 70 "C for 12 h to remove some of this iron and to ensure fully K-exchanged L zeolite. 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 by about 3-fold. 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 solution of cupric nitrate (Alfa Products) to an aqueous slurry of 1.O g of the zeolite in 100 mL of deionized water. The copper-exchanged 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 catalyst. Copper exchange was in the range of 0.1-0.4 wt % K-L, assuming complete exchange of cupric ion. Sample Treatment. The zeolite sample was placed on a porous sintered glass disk inside an electrically heated 10 mm i.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 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 10-
-
0 1994 American Chemical Society
Interaction of Cu(I1) in Cu(I1)-Exchanged K-L Zeolite
J. Phys. Chem., Vol. 98, No. 47, 1994 12437
16 h and then cooled and evacuated at room temperature for 1 CUK-UNH, h to remove oxygen. This heat-treated sample is termed a a. Dehydrated/O, Flow/500 ' C dehydrated zeolite. When heated under flowing oxygen, all the Cu(I1) remains as Cu(I1) based on a constant ESR i n t e n ~ i t y . ~ ~ , ~ ~ The reaction temperature was monitored by a thermocouple in a thermowell located at the center of the reactor tube. The dehydrated sample was transferred into the Suprasil quartz ESR tube without exposure to air. After dehydration, adsorbates as gases were admitted at room temperature to the sample tubes. Adsorbates such as ND3, 15NH3, pyridine, hydrazine, NO, 13C0, C&, and (D3C)2SO were obtained from Aldrich, Cambridge Isotope and MSD Isotope Laboratories and used after repeated freeze-pumpthaw cycles. Spectroscopic Measurements. ESR spectra were measured at 77 K on a modified Varian E-4 spectrometer interfaced to a Tracor Northern TN-1710 signal averager. Each spectrum was obtained after multiple scans to achieve a satisfactory signalto-noise ratio. Each acquired spectrum was transferred from the signal averager to an IBM PCKT compatible computer for I analysis and plotting. The magnetic field was calibrated with d2WdH= a Varian E-500 gaussmeter. The microwave frequency was measured by a Hewlett Packard HP 5342A frequency counter. 200 G ESEM spectra were recorded at 4.5 K with a Bruker ESP 380 LLCY pulsed ESR spectrometer. Three-pulse echoes were measured g1 =2.047 by using a 90"-z-90"-T-90" pulse sequence with the echo Figure 1. ESR spectra at 77 K of CuK-L zeolite (a) dehydrated by measured as a function of T. In a typical ESEM study, the flowing 02 over the zeolite catalyst at 500 "C and degassed at room deuterium modulation from a deuterated adsorbate is analyzed temperature, (b) with 300 Torr of I4ND3 added at room temperature to determine its coordination to Cu(I1). In order to maximize after dehydration, and (c) with 300 Torr of 15NH3 added at room temperature after dehydration. (The gl region is expanded as the second the modulation depth from deuterium and eliminate the moduladerivative to more clearly show the hyperfine structure.) tion from zeolite 27Al,the time between the first two pulses, z, was kept between 0.28 and 0.30 pZ4 Both the theory and CuK-Upyridine methods used for simulation of the data are described in detail elsewhere.24 A
-
Results Electron Spin Resonance Measurements. Figure 1 shows the ESR spectra at 77 K of dehydrated CuK-L and dehydrated CuK-L with adsorbed NH3. After dehydration in a stream of flowing oxygen with slowly increasing temperature to 500 "C for 10-16 h, the ESR spectrum shows predominantly one species, denoted as D with gll = 2.336, All = 158 x cm-' and g l = 2.05, A 1 = 20 x cm as shown in Figure la. This species was assigned to Cu(II) in the center of a hexagonal prism in a 6-ring channel in earlier work.46 Figure lb,c shows the ESR spectra after adsorption of NH3 onto dehydrated CuK-L. A new cupric ion species is observed cm-' with ESR parameters of gll= 2.255 andAl~=177 x after NH3 adsorption. CuK-L with adsorbed 15NH3 (15N has a nuclear spin of l/2) shows five 15Nhyperfine lines centered at g l = 2.047 and split by 18 x cm-', which are shown in the expanded second-derivative spectrum. With adsorbed 14NH3(14N has a nuclear spin of l), the ESR spectrum shows more hyperfine lines in the g l region, but they are not clearly resolved. The ESR spectrum of dehydrated CuK-L measured after pyridine adsorption is shown in Figure 2. A new cupric ion species due to complex formation with pyridine is observed with ESR parameters of gll= 2.260 and All = 181 x cm-I and seven hyperfine lines centered at g l = 2.055 and split by 16 x cm-l. When hydrazine is added to a dehydrated sample, a new Cu(I1) species is observed with ESR parameters of gll = 2.355, AdVt = 159 x cm-', and gl = 2.06 (Figure 3b). The cupric ion intensity was slightly decreased after hydrazine
.J
4I
g,,=2.055
Figure 2. ESR spectrum at 77 K of dehydrated CuK-L zeolite equilibrated with pyridine containing 14N at room temperature.
adsorption. No nitrogen hyperfine interaction was observed on hydrazine adsorption. When 100 Torr of nitrogen monoxide is added to dehydrated CuK-L at room temperature, no ESR change was observed. This is probably because Cu(I1) in the dehydrated zeolite is located in an inaccessible site for interaction with nitrogen monoxide. Thus, interaction between nitrogen monoxide and reduced CuK-L was investigated. When 70 Torr of hydrogen is added to a dehydrated CuK-L at room temperature, no ESR change was observed for -24 h. Even when the sample was heated to 300 "C for 40 min with adsorbed hydrogen, no change of the ESR spectra was seen, confirming that cupric ion species D in dehydrated CuK-L are indeed located in recessed sites.
12438 J. Phys. Chem., Vol. 98, No. 47, 1994
Yu and Kevan
a. Dehydrated/ 505 O C
TEzL-
b. NHZ-NH,
V
gil =2.355
*Igi,=2.341
x4
C.
c
2
x
a
'
(
DSCSOCD,
L gl1=2.392
200 G
200 G gI1= I ,906
Figure 3. ESR spectra at 77 K of CuK-L zeolite (a) dehydrated at 505 "C and degassed at room temperature, (b) exposed to hydrazine (H2NNH2) at room temperature after dehydration, (CI) exposed to 70 Torr of hydrogen and heated to 510 "C for 40 min and degassed at room temperature, and (c2) exposed to 100 Torr of NO at room temperature after the hydrogen reduction in treatment cl. However, the cupric ion intensity was gradually decreased on heating with increasing temperature above -300 "C. Figure 3cl shows the ESR spectrum of dehydrated CuK-L after adsorption of hydrogen and heating at 5 10 "C for 40 min. The ESR intensity of Cu(I1) decreased to about 15% of the value in dehydrated CuK-L. Addition of oxygen at room temperature to this CuK-L sample did not cause any ESR change. So, oxygen does not reoxidize the reduced copper back to cupric ion at room temperature. However, when 100 Torr of NO is adsorbed at room temperature on the reduced CuK-L sample in Figure 3c1, a new ESR signal immediately occurs with reversed ESR parameters of gll 1.906 and g l = 2.030 superimposed on the original Cu(I1) species D as shown in Figure cm-' (239 G) and 3C2. The new species has All = 213 x A 1 = 161 x cm-' (169 G), but the complex was unstable and gradually decreased in intensity. After 1 h the new complex completely disappeared, and only the original Cu(I1) species D was seen. The overall cupric ion intensity initially increased to about twice that in Figure 3cl after nitrogen monoxide adsorption. When carbon monoxide is adsorbed on a dehydrated CuK-L sample (Figure 4a), a new Cu(I1) signal is observed with gll = 2.345 and All = 153 x cm-'. Identical ESR spectra were recorded for adsorption of l2C0 and 13C0,and no 13Chyperfine interaction was observed. Upon adsorption of benzene (Figure 4b), a similar ESR spectrum is observed with gll = 2.341 and All = 161 x cm-'. But after adsorption of dimethyl sulfoxide (Figure 4c), gll shifts to 2.392 with a hyperfine splitting of All = 117 x cm-'. Table 1 summarizes the ESR parameters of Cu(I1) in CuK-L zeolites observed after various treatments. Electron Spin Echo Modulation Measurements. The threepulse ESEM spectra and simulation parameters for Cu(I1) in CuK-L interacting with various deuterated adsorbates are
-
Figure 4. ESR spectra at 77 K of dehydrated CuK-L zeolite (a) after adsorption of 110 Torr of CO, (b) after equilibration with benzene, and (c) after equilibration with dimethyl sulfoxide at room temperature. TABLE 1: ESR Parameters at 77 K of Cu(II) in CuK-L Zeolite Observed after Various Sample Treatments treatmenr dehydrated +ammonia +pyridine +hydrazine +carbon monoxide +benzene +dimethyl sulfoxide
W?
Ai?
.?la
2.336 2.255 2.260 2.355 2.345 2.341 2.392
158 177 181 159 153 161 117
2.05 2.05 2.06 2.06 2.05 2.05 2.06
a Estimated uncertainty is f0.007. The unit of All is 1 x and the estimated uncertainty is f 4 x cm-I.
cm-l,
c 1.0ri
CUK-L
5
z
+
ND3
N s12
0.4
R I0.28 nm Also = 0.28 MHz
I-
0.2
0
1
2
3 T, PS
4
5
Figure 5. Experimental (-) and simulated (- - -) three-pulse ESEM recorded at 4 K of dehydrated CuK-L with adsorbed 1 4 N D 3 . shown in Figures 5-7. The ESEM spectrum recorded at 4.5 K of dehydrated CuK-L with adsorbed ND3 is shown in Figure 5. The simulation shows 12 interacting deuterium nuclei at a distance of 0.28 nm with Aiso = 0.28 MHz. This indicates that four ammonia molecules are directly coordinated to Cu(I1). Figure 6 shows the three-pulse ESEM spectrum of dehydrated CuK-L with adsorbed C a 6 . The spectrum is best simulated by six interacting deuterium nuclei at a distance of 0.41 nm with Aiso = 0.14 MHz. This indicates one directly coordinated benzene molecule. Figure 7 shows deuterium modulation observed for dehydrated CuK-L with adsorbed deuterated dimethyl sulfoxide.
Interaction of Cu(I1) in Cu(I1)-Exchanged K-L Zeolite
J. Phys. Chem., Vol. 98, No. 47, 1994 12439
T, PS
Figure 6. Experimental (-) and simulated (- - -) three-pulse ESEM spectrum recorded at 4 K of dehydrated CuK-L with adsorbed C a 6 .
CUK-L
+
1
I
I
SO(CD312
U I 0.37 nm A i i o i0.02 YHZ
R
I-
0 0.2 I
0
1
2
3
4
5
T?Ps
Figure 7. Experimental
(-) and simulated (- - -) three-pulse ESEM
spectrum recorded at 4 K of dehydrated CuK-L with adsorbed (D3C)2-
so.
TABLE 2: ESEM Parameters for Cu(I1) in CuK-L Zeolite Including Number of Deuterium Nuclei (N), Cu(II) to D Distance (R),and Isotropic Hyperfine Coupling ( A b ) treatment +m3
+(CD3)2SO
N 12
R," nm
A,,,? MHz
0.28
0.28
6
0.41 0.37
0.14
6
0.02
Estimated uncertainty is f 0.01 nm. Estimated uncertainty is f
10%.
The simulation indicates six interacting deuterium nuclei at a distance of 0.37 nm from Cu(II) with Aiso = 0.02 MHz. This indicates one interacting dimethyl sulfoxide molecule. Table 2 summarizes the ESEM parameters for Cu(I1) CuK-L zeolite with the deuterated adsorbates. Discussion Zeolite L is a synthetic aluminosilicate zeolite that has not yet been found to occur naturally. The common SUA1 ratio in zeolite L is 3.0, which indicates an ordered Si, AI distribution. The crystal structure of zeolite L is based on the €-cages of the 18 tetrahedral units found in cancrinite, which are formed by five 6-rings and six 4 - r i n g ~ . " ~ -The ~ ~ €-cagesare linked through their two nearly planar 6-rings, forming hexagonal prisms with the planes of the 6-rings normal to the c axis. Thus, L zeolite consists of a series of columns along the c axis where €-cages and hexagonal prism units alternate. These columns are linked to each other as shown in Figure 8, producing large channels of 12-rings parallel to the c axis. These main channels have free diameters of 0.71-0.78 nm. The largest internal diameter estimated midway between the 12-rings is about 1.3 nm. The walls of the main channels consist of 8-rings and 4-rings. The main channels are connected with each other through nonplanar 8-rings. There are five types of cation sites located for Na+ and K+ as indicated in Figure 8.48,50 Site A is located in the center of
Figure 8. Schematic representation viewed approximately perpendicular to the c axis of zeolite L showing large channels parallel to the c axis and possible extraframework cation sites as letters (upper); projection viewed parallel to the c axis (lower). Adapted from ref 47.
the hexagonal prism. A cation in site A forms distorted octahedral coordination with six framework oxygens, three above and three below the cation. Site B is in the center of the €-cage. Site C is located midway between the centers of two adjacent €-cages. Site D, the only cation position found in the main channel, is best understood by reference to Figure 8. The cation in site D coordinates to four oxygens in the nonplanar 8-ring. Site E is located midway between adjacent A sites. After dehydration in a stream of flowing oxygen at 500 "C, the ESR spectrum shows predominantly one Cu(I1) species D which was assigned to site A in the center of a hexagonal prism in earlier work."6 This was further verified by hydrogen adsorption on dehydrated CuK-L. Heating CuK-L with adsorbed hydrogen to 300 OC did not cause any change in the ESR spectra, confirming that Cu(II) species D in a dehydrated sample is indeed located in recessed sites consistent with the center of a hexagonal prism. The changes in the ESR parameters of dehydrated CuK-L zeolite after adsorption of various adsorbate molecules indicate that the adsorbates interact with the Cu(I1) ion. Adsorption of NH3 on dehydrated CuK-L zeolite results in complex formation with NH3. This indicates that Cu(II) species D migrates from recessed sites to sites accessible for coordination with ammonia. The second-derivative spectrum with adsorbed " 4 3 clearly shows five hyperfine lines due to nitrogen hyperfine interaction (Figure 1C). Since 15N has a nuclear spin of l/2, the five lines indicate four ammonias coordinated to the Cu(I1). The analysis of the three-pulse ESEM spectrum of CuK-L with adsorbed ND3 (Figure 5 ) also indicates four ammonia molecules directly coordinated to Cu(I1) with an average Cu(I1)-D distance of 0.28 nm and supports the ESR data. Tetracoordinated cupric complexes such as Cu& (X = C1, NH3, etc.) generally prefer a square-planar c o n f i g u r a t i ~ n . ~Thus, ~ ~ ~ ~in this case the Cu(I1) species is suggested to be located in the center of the 12-ring main channel coordinating to four ammonias in a squareplanar geometry. The ESR parameters of complexes formed
12440 J. Phys. Chem., Vol. 98,No. 47, 1994
Yu and Kevan
TABLE 3: ESR Parameters at 77 K of Cu(I1)-Ammonia Complexes in Various Molecular Sieves matrix
complex
glf
AI? type:sizec
L zeolite [ C U ( N H ~ ) ~ ] ~2.255 + 177 ch12-ring [Cu(NH33)3I2+ 2.249 179 ch: 12-ring H-mordenite Na,K-mordenite [Cu(NH3)2I2+ 2.245 179 ch:12-ring ZSM-5 [Cu(NH3)4I2+ 2.246 183 ch:lO-ring SAPO-Sd [Cu(NH3)3]*+ 2.245 204 ch:12-ring SAPO-1Id [Cu(NH3)4l2+ 2.226 190 ch:lO-ring X zeolite [Cu(NH3)4I2+ 2.228 178 ca:1.3 nm Y zeolite [Cu(NH3)4I2+ 2.235 175 ca:1.3 nm rho [Cu(NH3)4I2+ 2.239 175 ca:1.2 nm
TABLE 4: ESR Parameters at 77 K in Various Matrices with Adsorbed NO
ref
matrix
this work 40a 40a 40b, 41,54 42 43.44 59 60 61
Cu+-NO in L zeolite Cu+-NO in Y zeolite Cu+-NO in Y zeolite I4NO on MgO I4NO on ZnO I4NO on NaY I4NO on decationated Y
cm-l, a Estimated uncertainty is f0.007. The unit of All is 1 x and the estimated uncertainty is f 5 x cm-I. ch represents a channel-type zeolite, and ca represents a cage-type zeolite; 1.3 and 1.2 nm are the a-cage intemal diameters. SAPO-n represents silicoaluminophosphate molecular sieves.
gif
gi"
AI?
1.91 2.030 239 1.99 2.034 238 1.89 2.009 240 1.89 1.996 1.94 1.979 1.86 1.898 1.95 1.996
Aib
ref
169(Cu)c 177(Cu) 190(Cu) 33(N) 30(N) 28(N) 14(N)
this work 69 66 63 64 65 65
Estimated uncertainty is f0.007. The unit of All and A1 is gauss, and the estimated uncertainty is f 5 G. ( ) represents the nucleus for which hyperfine is observed.
adsorption. However, the ESR spectra of CuK-L with adsorbed hydrazine were quite different from those with adsorbed ammonia (see Figures l b and 3b). No superhypefine splitting was observed due to nitrogen after hydrazine adsorption. The between Cu(II) and ammonia in various zeolites and molecular ESR parameters of CuK-L measured after adsorption of sieves are tabulated in Table 3 for comparison. hydrazine are rather similar to those after adsorption of ethylene. Mordenite and ZSM-5 are similar to zeolite L, in that various From an analysis of the three-pulse ESEM spectrum of CuK-L channels i n t e r s e ~ t Mordenite . ~ ~ ~ ~ ~has~ a~ 12-ring main channel, with adsorbed C2D4 studied in previous work,& it was shown while ZSM-5 has a 10-ring main channel. Interestingly, that one ethylene was weakly coordinated to the Cu(I1) ion near however, Cu(I1) exchanged into Z S M J has more coordinated ligands than does Cu(1I) exchanged into m ~ r d e n i t e . ~This , ~ , ~ ~ an 8-ring in the main channel. Thus, one hydrazine molecule is suggested to be similarly, weakly coordinated to Cu(I1) in is because Cu(I1) in mordenite forms a coordination complex site D in the main channel. in sites off the main channel, while Cu(I1) in ZSM-5 forms a When NO is adsorbed on a hydrogen-reduced CuK-L complex at the wider intersection of two channels.40a*bIt is sample, a complex with NO occurs with ESR parameters of gll also interesting to compare SAPO (si1icoaluminophosphate)-5 gl. The ESR parameters of this complex are quite different and SAPO-1 1 molecular sieves which are also channel types. from the g values of normal Cu(I1) ions in CuK-L zeolite. Since SAPO-5 (12-ring) has a greater channel size than SAPOSimilar ESR spectra have been reported in Cu(1)-Y zeolites 11 (10-ring), Cu(I1) in SAPO-5has generally more coordinated after NO a d s ~ r p t i o n . ~ These ~ - ~ ~ signals were assigned to a ligands than does Cu(I1) in SAPO-11?2-4s The same is true Cu+-NO complex where the observed hyperfine structure came for Pd-exchanged SAPO-5 and Pd-exchanged SAPO-11.5s-s8 from the interaction of an unpaired electron in JG*antibonding But Cu(I1) in SAPO-1 1 coordinates with four ammonias, while Cu(I1) in SAPO-5 coordinates with only three a m m ~ n i a s . ~ * - ~ orbitals in the complex." The measured g values and hyperfine splittings in various other matrices treated with NO are tabulated Interestingly, Pd(1) in SAPO-11 also coordinates with four in Table 4. The g values of a Cu+-NO complex are similar to ammonias, while Pd(1) in SAPO-5 coordinates with only two the g values of NO absorbed on MgO, ZnO, Nay, and a m m ~ n i a s . ~ ~It- ~is*suggested that the size of the 10-ring in decationated Y. They all have gll < gl. Interestingly, no SAPO-11 is such that the nitrogens of the four ammonia nitrogen hyperfine coupling was observed in the Cu+-NO molecules can coordinate to the metal ions in the center of the complexes formed in zeolites. channel with the hydrogens of the ammonia molecules being The cupric ion intensity increased about 2-fold after nitrogen able to interact favorably, perhaps through a type of hydrogen monoxide adsorption. The increase of cupric ion intensity is bonding, with the oxygens of the 6-ring faces of the 10-ring attributed to reoxidation of some reduced copper species to channels. In cage-type zeolites such as X, Y, and rho zeolites, cupric ions. Similar reoxidation processes were reported in Cu(n) usually forms a square-planar complex in the a - ~ a g e . ~ ~ - ~ l Cu-Y and Pd-Y zeolites when NO was added to reduced The ESR spectrum of CuK-L with adsorbed pyridine is Cu(1)-Y and Pd(0)-Y zeolite^.^^^^^ It was suggested in Pd-Y similar to that with adsorbed NH3. The second-derivative zeolite that some NO decomposed according to the reaction 2N0 spectrum shows seven hyperfine lines. Since 14N in pyridine N20 0'. The 0. species was suggested to be the oxidizing has a nuclear spin of 1, the seven lines indicate three pyridine agent for the Cu(1) reoxidation process. Similarly, in zeolite molecules directly coordinated to the Cu(I1). Since pyridine is L, an 0' species produced from NO decomposition may be too bulky to enter the €-cage, the cupric ions are suggested to involved in the reoxidation of reduced copper to Cu(I1). migrate into the main channel, most probably toward site D, Upon adsorption of CO, benzene, and dimethyl sulfoxide, where pyridines can coordinate with Cu(1I). In this case the gll values of Cu(I1) species all shift to lower field as shown Cu(I1) does not form a square-planar geometry, probably due in Figure 4. When 13C0 was added to a dehydrated CuK-L, to the bulkiness of pyridine, which may cause steric hindrance no 13C0 hyperfine interaction was observed in the ESR in the main channel. However, Cu(II) in X and Y zeolites forms spectrum. CO has been shown to reduce Cu(I1) ion in Cu(I1)a square-planar complex, [Cu(CsH3N)4l2+,with four pyridines exchanged zeolite^.^^,^,^^ But there is no indication of reduction in a large a - ~ a g e . ~ ~ , ~ * of Cu(I1) in dehydrated K-L zeolite. It was reported in Upon adsorption of hydrazine on dehydrated CuK-L, the gll Pd(I1)-exchanged K-L zeolite that two CO molecules were value shifts to lower field. It was reported in Pd-exchanged directly coordinated to Pd(1) based on resolved I3C hyperfine SAPO (si1icoaluminophosphate)-5molecular sievess7 that the intera~tion.'~Such an interaction is not seen here for Cu(I1). ESR spectra observed after adsorption of hydrazine became similar to those observed after adsorption of ammonia. Since Conclusions hydrazine has two NH2 moieties, Pd(1) was suggested to be coordinated to hydrazine through two N of one hydrazine on ESR and ESEM spectroscopic methods have been used to the basis of the similarity of its ESR signal to that for NH3 investigate adsorbate interactions of Cu(1I) ions in CuK-L
-
+
Interaction of Cu(I1) in Cu(I1)-Exchanged K-L Zeolite zeolite. Upon adsorption of ammonia and pyridine, Cu(I1) forms complexes with direct coordination to four molecules of ammonia and three molecules of pyridine, respectively. However, no nitrogen superhyperfine interaction was observed between Cu(I1) and hydrazine or nitrogen monoxide. Nitrogen monoxide forms a Cu+-NO complex in reduced CuK-L zeolite and reoxidizes some reduced copper species back to cupric ion. Adsorption of other molecules such as carbon monoxide, benzene, and dimethyl sulfoxide causes migration of Cu(II) into the main channel to form complexes with one molecule of benzene and dimethyl sulfoxide, respectively. Acknowledgment. This research was supported by the National Science Foundation, the Robert A. Welch Foundation, and the Basic Science Research Institute Program (1992) of the Korean Ministry of Education. The authors also thank Mr. Jean Marc Comets and Dr. Vadim Kurshev for help in the ESEM measurements. References and Notes (1) Maxwell, I. E. Adv. Catal. 1982, 31, 1. (2) Mochida, I.; Hayata, S.; Kato, A.; Seiyama, T. J . Catal. 1970, 19, 405. (3) Tsuruva.. S.:. Tsakumoto.. M.:. Watanabe.. M.:. Masai. M. J . Catal. 1985, 93, 303: (4) Benn. F. R.; Dwver, J.; Estahami. A.; Evmerides. N. P.; SzczeDura. A. K. J . Catal. 1977, 4 i , 60. ( 5 ) Naccache, C. M.; Taarit, Y. B. J. Catal. 1971, 22, 171. (6) Dimitrov, C.; Leach, H. F. J. Card 1969, 14, 336. (7) Maxwell, I. E.; Downing, R. S.; van Langen, S. A. J. J . Catal. 1980, 61, 485. (8) Gallezot, P.; Ben Tarrit, Y.; Imelik, B. J . Catal. 1972, 26, 295. (9) Maxwell, I. E.; de Boer, J. J. J . Phys. Chem. 1975, 79, 1874. (10) Szostsk. R. Molecular Sieves: Van Nostrand Reinhold: New York. 1976; Chapter 5. (11) Detka, J. Zeolites 1985. I, 145. (12j Packet, D.; Schoonheydt, R. A. Stud. Surf.Sci. Card 1984, 18, 41. (13) Narayana, M.; Contarini, S.; Kevan, L. J . Catal. 1985, 94, 370. (14) Ichikawa, T.; Kevan, L. J . Am. Chem. SOC.1983, 105, 402. (15) Narayana, M.; Kevan, L. J . Chem. Phys. 1983, 78, 3573. (16) Ichikawa, T.; Kevan, L. J . Phys. Chem. 1983, 87, 4433. (17) Herman, R. G.; Flentge, D. R. J . Phys. Chem. 1978, 82, 720. (18) Herman, R. G. Inorg. Chem. 1979, 18, 995. (19) Anderson, M. W.; Kevan, L. J . Phys. Chem. 1987, 91, 1850. (20) Anderson, M. W.; Kevan, L. J . Phys. Chem. 1987, 91, 2926. (21) Narayana, M.; Kevan, L. J . Chem. SOC.,Faraday Trans. 1986,82, 213. (22) Narayana, M.; Kevan, L. Langmuir 1985, I, 553. (23) Narayana, M.; Kevan, L. J . Am. Chem. SOC.1981, 103, 5729. (24) Kevan, L. In Time Domain Electron Spin Resonance; Kevan L., Schwartz, R. N., Eds.; Wiley-Interscience: New York, 1979; Chapter 8. (25) Kevan, L.; Narayana, M. lntrazeolite Chemistry; American Chemical Society: Washington, DC, 1983; ACS Symp. Ser. No. 218, p 283. (26) Gentry, S. J.; Rudham, R.; Sanders, M. K. J . Catal. 1974, 35, 376.
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