12004
J. Phys. Chem. 1993,97, 12004-12007
Coordination of Cupric Ions to Water and to Metal Oxide Pillars in Copper(I1)-Doped, Allj- and ZrrPillared Montmorillonite Clays Studied by Electron Spin Echo Modulation Spectroscopy Jean-Marc Comets and Larry Kevan' Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: June 8, 1993; In Final Form: September 23, 1993"
The location and coordination of Cu(I1) cations exchanged into Al13- and Zr4-pillared natural montmorillonite has been studied by electron spin resonance and electron spin echo modulation (ESEM) spectroscopies. For the first time, ESEM spectroscopy was applied successfully to Cu(I1)-doped pillared montmorillonite. When copper cations are exchanged before introduction of the alumina pillars, they are directly coordinated to two water molecules and four oxygens of an alumina pillar. When copper cations are exchanged after introduction of the alumina pillars, they are directly coordinated to one water molecule and five oxygens of an alumina pillar. Thus in both cases the cupric ion is strongly bound to the alumina pillars. In Zrd-pillared montmorillonite, cupric ion directly coordinates to three water molecules and to only three oxygens of a zirconia pillar. Thus the cupric ion binds less strongly to zirconia pillars. Also, in Zr4-pillared montmorillonite, but not in All3pillared montmorillonite, part of the Cu(I1) is freely tumbling between the clay pillars and is suggested to be hexacoordinated to water.
(CEC) of 84.4 mequiv/100 g. Each gram of the crude clay was stirred with 0.05 M HCl to remove carbonate impurities and was Due to their large cavities, pillared clays are of interest for then exchanged with Na+ cations by stirring three times with 1 catalysis.1-4 Catalytic activity may be modified by doping mol dm-3 NaCl solution at room temperature. The clay was then transition-metal cations into pillared montmori1lonite.s It has separated out by centrifugation, resuspended in deionized water, been demonstrated that, in smectiteclays, replacing exchangeable and dialyzed uti1 free of excess cations. The clay suspension was alkali-metal and alkaline-earth-metal cations by transition-metal then allowed to dry at room temperature. cations increases the reaction rate of the Lewis-acid-catalyzed Polymeric Alls7+cations were synthesized by adding dropwise Friedel-Crafts reaction.6 over a 2 h period sufficient 0.1 mol dm-3 Al(N03)9 solution to Two kinds of pillars have been mainly investigated: aluminum give an OH/Al ratio of 2.4. Al13-pillaredmontmorillonite (Aland zirconium oxide pillars which are formed from metal hydroxy mont) was prepared in a similar manner to that reported recently.8 polymeric cations. Their chemical formulas are [A11304(OH)24The purified Na+-montmorillonite was stirred rapidly in deionized (H20)12]'+ and [ Z ~ ~ ( O H ) ~ ( H16]' Z O+,) re~pectively.I.~*~ These water. To the stirred suspension was added 10 times more of the polymeric cations can be introduced into the clay interlayer by Alls7+polymeric cation than was required to satisfy the CEC of ion exchange. The possiblity of chemisorption of the Cu2+cations the clay. The temperature of the stirred suspension was raised on such pillars has been investigated.8-11 to 60 OC for 4 h after which stirring was continued at room The location of transition-metal cations such as Cu(I1) and temperature for a further 12 h. The clay was then dialyzed in Pd(I1) exchanged into A113- and Zr4-pillared montmorillonite deionized water to remove excess cations and dried at room has been studied using electron spin resonance (ESR).8-11 temperature. X-ray diffraction (XRD) analysis of an Al13STx Electron spin echo modulation spectroscopy would also be very sample prepared in this way gave an intense basal reflection with useful, but no electron spin echo signal was observed in these d(0001) = 1.8 nm,confirming the formationofthepillarstructure. It was proposed that the intrinsic Fe3+ in natural XRD experiments were performed on a Philips PW 1840 montmorillonite affects the observation of an electron spin echo diffractometer. modulation after pillaring since an electron spin echo signal is observed in Cu-doped Al13-pillared synthetic laponitel2 which Two sets of A113-pillaredmontmorillonite were prepared. In contains little iron. However, electron spin echo signals have one set, the Cu(I1) cations were exchanged after incorporating been obtained when monovalent cations such as Ag(1) are the alumina pillars (Cu-Al-mont). Some ion-exchangecapacity exchanged into All3-pillared montmorillonite.13 This could be seems to remain after pillaring. In the second set, Cu(I1) cations due to different locations for Ag(1) versus Cu(I1). In this work, were exchanged prior to incorporating the alumina pillars (Alelectron spin echo modulation (ESEM) spectroscopy has been Cu-mont). The procedure followed to dope the clay structure applied successfully for the first time to Cu(I1)-doped Al13-and with Cu(I1) cations is similar to that followed to exchange CuZrrpillared montmorillonite wet with deuterated water. This (11) cations in Mg(II)-smectites.I4 The purpose of these two enables Cu(I1) interactions with deuterium in DzO to be preparations is to investigate whether there is any change in the determined, which can be compared with similar interactions in location of the Cu(I1) cations incorporated before and after unpillared montmorillonite. pillaring since both pillaring and Cu(I1) incorporation are ionexchange processes. After Cu(I1) exchange there are 1016-1017 Experimental Section Cu spins per gram. Zr4-pillared STx was prepared by the method of F i g ~ e r a s . ~ The well-characterized reference clay STx- 1 montmorillonite, M+O.~~S[A~~.O~~M~O.~ used ~ SinFthis ~ ~study .O~ was I S ~ ~ZrOC12 O ~ O ((25 O Hcm3, ) ~ 0.10 . mol dm-3) was added dropwise over 1 h to a stirred suspension of STx (1 g STx/ 100cm3deionized water). obtained from the Source Clay Minerals Repository at the Universityof Missouri and had a stated cation exchangecapacity This was then stirred continuously for 24 h, the suspension was filtered and the clay washed until no chloride ion was detected 0 Abstract published in Aduance ACS Absrracrs. November 1, 1993. in the filtrate by AgN03. The X-ray diffraction analysis gave Introduction
0022-36S4/93/2097-12004$04.00/0 0 1993 American Chemical Society
Cu(I1)-Doped Montmorillonite Clays Studied by ESEM
The Journal of Physical Chemistry, Vol. 97, NO. 46, 1993 12005 a
2.1491'
A 2.1676
b
b
g = 2.358
V FL
g A= = 2.394 136.7x
2.0708
Figure 1. ESR spectra of Cu-Al-mont, soaked with D20, recorded (a) at room temperature and (b) at 71 K. The small sharp signal in a is probably a defect center associated with oxygen.
h
lp A
2.077
m
Figure 3. ESR spectra of Cu-Zr-mont, soaked with DzO,recorded (a) at room temperature and (b) at 77 K.
TABLE I: ESR Parameters for Cu(I1) Exchanged into Pillared Montmorillonites
sample d(001), nm T,K gll Cu-All3-mont, wet D20 17 2.36 1.8
All3-Cu-mont, wet D2O 1.9
Cu-Zr4-mont, wet DzO
b
1.9
A
300 17 300 77 3W
b 2.36
b
All" gl 139 2.07 b 2.08 130 2.09 b 2.09 137 2.08
2.39 2.17 (gw) 2.32 146 2.08
a Unitsare l@cm-Iwithanestimatederrorof*S X 1Vcm-I. Due to paramagnetic impurities,gu and4,11 cannot be determined. Two Cu(I1) species are observed.
Figure 2. ESR spectra of Al-Cu-mont, soaked with D20,recorded (a) at room temperature and (b) at 77 K. The small sharp signal in a is probably a defect center associated with oxygen.
a strong basal reflection with 4001) = 1.9 nm, confirming the formation of the pillar structure. The pillared clays were dried under a dynamic vacuum of 2 mTorr. Then, they were wet by adding about 2 mL of DzO per milligram of clay. ESR spectra were recorded on a Varian E-4 spectrometer at 77 and 300 K. ESE spectra were recorded a t 4 K on a Bruker ESP-380 spectrometer. Three-pulse stimulated echoes were recorded with a r f 2 1f 2-r f 2 pulse sequence, and the echo was detected as a function of T,the time between the second and third pulses. The value of the time between the first and second pulses that gave the maximum intensity of the deuterium modulations was T = 0.28 ps. Simulations were made in terms of Nequivalent nuclei at a distance R with isotropic hyperfine coupling A using a spherical approximation.15 All the ESEM experiments were done at the field positions corresponding to the maximum intensity of the spin echoes. Results 1. Electron Spin Resonance Specwa. The room-temperature and 77 K ESR spectra of pillared montmorillonites are displayed in Figures 1-3. The ESR parameters are given in Table I. (a) Cu-Al-mont. At room temperature, an anisotropic ESR spectrum is observed for Cu-Al-mont soaked with DzO (Figure la). Thisindicates that theCu(1I)-water complexisboundeither to the clay layer within the interlayer space or to the alumina
pillars. The 77 K ESR spectrum indicates that only one Cu(I1) species is present. The ESR parameters (gll and All) indicate that the environment of the Cu(I1) cations is approximately octahedrall6 (Figure 1b); approximately is used since all the ligands are not likely identical. A typical value for tetrahedral coordination is gil > 2.4.16 (b) Al-Cu-mont. At room temperature, an anisotropic ESR spectrum very similar to and with about the same intensity as that of Cu-Al-mont wet with DzO is obtained for Al-Cu-mont wet with DzO (Figure 2a). The same interpretation for the Cu(11) cation location can be made. The Cu(I1) species are bound either to the clay layers or to the alumina pillars. At 77 K, the ESR parameters are also indicative that the environment of the Cu(I1) cations is approximately octahedral (Figure 2b). (c) Cu-Zr-mont. At room temperature, two kinds of Cu(I1) cations are present; one has an isotropic ESR signal with gi, = 2.17, indicative of a freely tumbling Cu(I1) cation, and the second one has an anisotropic ESR signal (Figure 3a). At 77 K, only an anisotropic ESR signal is seen and the ESR parameters are consistent with an approximately octahedral Cu(I1) complex (Figure 3b). 2, Three-Pulse ESEM Data. The ESEM spectra are reported in Figures 4 and 5 , and the simulation parameters are in Table 11.
(a) Cu-AI-mont. A weak electron spin echo signal was obtained (Figure 4a). The ESEM spectrum was best simulated using a two-shell model with N = 2, R = 0.28 nm, A = 0.32 MHz and N = 2, R = 0.33 nm, A = 0.05 MHz. The Cu-D distance of 0.28 nm is characteristic of direct coordination of Cu(I1) to DzO through its oxygen. Thus one water molecule is directly coordinated to the exchangeable Cu(I1) cations, and another water molecule is more weakly coordinated at a further distance.
Comets and Kevan
12006 The Journal of Physical Chemistry, Vol. 97, No. 46, 1993 a
that in Cu-Zr-mont wet with D20 three water molecules are directly coordinated to the interlayer Cu(I1) cations. Discussion
b AI-Cu-mont, wet D z 0
m 0.8
I I
Shell N R. nm Ai,,MHz
U
1
14
0.27
0.30
c 5 0.4 v)
t-
z
p o.2 3 0
1
3
2
4
5
T, 1s
and simulated (- - -) three-pulse ESEM spectra of (a) Cu-Al-mont and (b) Al-Cu-mont, both soaked with DzO.
Figure 4. Experimental (-) F1.0-r
Cu-Zr-mont, wet DzO
2 3
I 1
m 0.8.
Shell N R. nm Al,,MHz 1 16 0.28 0.31
U
9 0.6.
c
0
T, P
and simulated (- - -) three-pulse ESEM spectrum of Cu-Zr-mont soaked with D20. Figure 5. Experimental (-)
TABLE II: ESEM Parameters for Cu(I1) Exchanged into Pillared Montmorillonites simulation parameters shell 1
sample
shell 2
NE R,bnm Ad,C MHz N R, nm A h , MHz
0.32 2 0.33 0.05 Cu-Al,,-mont, wet DzO 2 0.28 0.30 d Al13-Cu-mont, wet D20 4 0.27 Cu-Zrd-mont, wet D20 6 0.28 0.31 d Number of deuterium nuclei to the nearest integer. * Distance between Cu(I1) and deuterium; estimated uncertainty is 10.01 nm. Isotropic hyperfine coupling; estimated uncertainty is f l O W . A satisfactory fit is obtained with only one shell. (b) Al-Cu-mont. The electron spin echo signal was also weak, but distinct deuterium modulations were observed (Figure 4b). The ESEM data could be best simulated with N = 4, R = 0.27 nm, and A = 0.30 MHz. This corresponds to two water molecules directly coordinated to the interlayer Cu(I1) cations. (e) Cu-Zr-moat. A strong electron spin echo signal was obtained (Figure 5 ) . The ESEM data could be best simulated with n = 6, R = 0.28 nm, and A = 0.31 MHz. This indicates
As emphasized in earlier work," no electron spin echo signal has been observed in Cu(I1)-doped pillared montmorillonite. The explanation given is that the large amount of iron(II1) present in theoctahedral sheet lowers theintensity of thespin echo signal.'' The Bruker ESP 380 has a higher sensitivity than the previously used home-built ESE,I8 and weaker echoes can now be observed. In Cu-Al-mont wet with DzO,the ESR data are consistent with those obtained previously in which Kukkadapu and coworkers* observed an anisotropic Cu(I1) species at room temperature and an anisotropic Cu(I1) signal at 77 K with approximate octahedral symmetry. The ESEM data indicate that only one water molecule is directly coordinated to the interlayer Cu(I1) cations and another one is a t a further distance. Therefore, the Cu(I1) species is also bound either to the negatively charged interlayer surface or to oxygens of the alumina pillars. It is not likely that the Cu(I1) is bound to the interlayer surface since on montmorillonite without pillars the exchanged Cu(I1) cations are freely rotating at room temperature and hence are not bound to the interlayer s ~ r f a c e . ~ We ~ J ~suggest that the exchangeable Cu(I1) cations are coordinated to five oxygens on the alumina pillars and to one water molecule in order to achieve approximate octahedral coordination. The detailed geometry of coordination to the pillar is unknown. In Al-Cu-mont wet with DzO, a similar anisotropic ESR spectrum is obtained at room temperature, indicating that the Cu(I1) species are bound to the interlayer surface or to the pillar structure. The 77 K ESR parameters indicate that the Cu(I1) complex is approximately octahedral. The ESEM data show that two water molecules are directly coordinated to the exchangeable Cu(I1) cations. By analogy we suggest that the Cu(I1) cations in Cu-Al-mont are bound to four oxygens of the alumina pillars and to two water molecules. The room-temperature ESR spectrum of Cu-Zr-mont wet with DzO shows that two Cu(I1) species are present: one gives an isotropic ESR signal, and the other gives an anisotropic ESR signal. At 77 K, only one anisotropic species is observed and the ESR parameters indicate that the Cu(I1) complex has approximate octahedral symmetry. As for Cu(I1) exchanged into beidellite,14 some Cu(I1) ions can freely tumble as ions hexacoordinated to water in the middle of the interlayer space of the clay, while other Cu(I1) ions attach to a pillared clay surface. For the same reasons as in alumina-pillared montmorillonite, it is not likely that the Cu(I1) cations are bound to the interlayer surface. We suggest that the bound Cu(I1) ions are coordinated to three oxygens of the zirconia pillars and to three water molecules. This result is different from that of Kukkadapu and Kevan? who worked with C a S T x - 1 instead of N a S T x - 1 and used a lower zirconia pillar concentration. Conclusions
The successful electron spin echo experiments performed for the first time on Cu(I1)-doped pillared montmorillonites open the possibility for better structural information on layered clay materials. In both hydrated Al-Cu-mont and Cu-Al-mont with a high pillar concentration, the interlayer Cu(I1) cations are directly coordinated to oxygens of the alumina pillars, although thedetailed geometry isunlmown. Exchanging theCu(I1) cations before or after introduction of the A1137+ pillaring ion does not change the interaction of the Cu(I1) with pillars, but the Cu(I1) interacts more strongly with the pillar when the pillar is incorporated first, as shown by the ESEM results. The ESEM results also show that the Cu(I1)-pillar interaction is stronger for alumina pillars than for zirconia pillars. Therefore, the nature
Cu(II)-Doped Montmorillonite Clays Studied by ESEM and the concentration of the pillars are determinant for pillar interaction of interlayer transition-metal ions such as Cu(I1) cations. Acknowledgment. This research was supported by the Robert A. Welch Foundation and the National Science Foundation. References and Notes (1) Pinnavaia, T. J. Science 1983, 220, 365. (2) Pinnavaia, T. J. In Heterogeneous Catalysis; Shapiro, B. L., Ed.; Texas A & M University Press: College Station, TX, 1984; p 142. (3) Vaughan, D. E. W.; Lussier, R. J. In Proceedings of the 5th
International Conference on Zeolites; Rea, L. V., Ed.; Heyden: London, 1980; p 94. (4) (a) Figueras, F. Catal.Rev.-Sci. Eng. 1988,30,457. (b) Figueras, F.; Mattrod-Bashi, A,; Fetter, G.;Thrienn, A.; Zanchetta, J. V. J. Card. 1989, 119, 91.
The Journal ofPhysical Chemistry, Vol. 97, No. 46, 1993 12007 ( 5 ) Cerrado, K. A,; Suib, S.L.; Skoularikis, N. D.; Coughlin, R. W. Inorg. Chem. 1986,25,4217. (6) Laszlo, P.; Mathy, A. Helv. Chim. Acta 1987, 70, 577. (7) Muha, J. M.; Vaughan, P. A. J. Chem. Phys. 1960, 33, 194. (8) KukkadaDu. R. K.: Kevan. L. J. Phvs. Chem. 1988. 92.6073. (9j KukkadGu; R. K.; Kevan;L. J. Chim. Soc. Faraday Trans. 1990, 86, 691. (10) Luca, V.; Kukkadapu, R. K.; Kevan, L. J. Chem.Soc.Faraday Trans. 1991, 87, 3083. (1 1) Kevan, L. Pure Appl. Chem. 1992, 64, 78 1. (12) Kukkadapu, R. K.; Kevan, L. J. Phys. Chem. 1989, 93, 1654. (13) Brown, D. R.; Luca, V.; Kevan, L. J. Chem. Soc. Faraday Trans. 1991, 87, 2749. (14) Comets, J.-M.; Luca, V.; Kevan, L. J. Phys. Chem. 1992,96,2645. (15) Kevan, L.; Bowman, M. K.; Narayana, P. A.; Boeclunan, R. K.; Yudanov, V. F.; Tsvetkov, Yu. D. J. Chem. Phys. 1975, 63, 409. (16) Hathaway, B. J.; Billing, D.E. Coord. Chem. Reo. 1970, 5, 143 (17) Finel, C.; Kevan, L. J. Chem. Soc. Faraday Trans. 1993,89. (18) Narayana, P. A,; Kevan, L. Magn. Reson. Rev. 1983, I , 234. (19) Brown, D. R.; Kevan, L. J. Phys. Chem. 1988, 110, 2743.
