8646
J. Phys. Chem. 1993, 97, 8646-8649
Adsorption of Primary Alcohols in Cu(I1)-Doped Mg(I1)-Exchanged Smectite Clays Studied by Electron Spin Resonance and Electron Spin Echo Modulation Jean-Marc Comets, Xinbua Cben, and Larry Kevan' Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: February 5, 1993; In Final Form: May 26, 1993
Electron spin resonance (ESR) and electron spin echo modulation (ESEM) have been used to study the adsorption of short chain length primary alcohols in Cu(I1)-doped Mg(I1)-exchanged synthetic beidellite, fluorohectorite, and hydroxyhectorite layered smectite clays. These results are compared with previous results for the adsorption of alcohols in Cu(I1)-doped Mg(I1)-exchanged natural montmorillonite. In montmorillonite, Cu(I1) cations form square-planar complexes with four molecules of methanol and ethanol in the middle of the interlayer space of the smectite but coordinate with only three propanol molecules. In beidellite, the Cu(I1) species are coordinated to only three alcohol molecules and to presumably three oxygen atoms of the interlayer surface. In the two hectorites, only one alcohol molecule directly coordinates to Cu(I1) cations which are recessed inside pseudohexagonal cavities bordering the interlayer space.
Introduction
Experimental Section
The study of the adsorption of organic compounds in smectite clay minerals is important to understand soil mechanisms and to predict their catalytic activity. This adsorption capability is due to the swelling property of the clay layers and the presence of cations as Lewis acid sites in the interlayer region.I-) The structure of water adsorbed on the surface of smectites has been studied's and reviewedugMore recently using electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies,IOthe adsorption of water in natural and synthetic smectites has been analyzed. When dehydrated at room temperature, Cu(I1) species exchanged in natural montmorillonite and synthetic beidellite coordinate to six water molecules whereas Cu(I1) exchanged in synthetic fluorohectorite and hydroxyhectorite coordinates only to one or two water molecules. In contrast, Cu(I1) in montmorillonite forms square-planar complexes with methanol, ethanol, and propanol." Powder X-ray diffraction studies on montmorillonite,on which these alcohols have been adsorbed,I2-l5 have shown that on exposure to high vapor pressure, or on immersion in alcohol, characteristic dm1 spacings airse corresponding, in each case, to two molecular layers of the alcohol in the interlayer region. It has also been established, from infrared studies, that such treatment with excess alcohol results in replacement of any water in the interlayer region by alcohol, including those water molecules in the primary solvation shell of an exchangeable cation.') In the present work, we apply ESR and ESEM spectroscopies to the study of the adsorption of primary alcohols to recently synthesized Cu(I1)-doped, Mg(I1)-exchanged smectites such as beidellite, fluorohectorite,and hydroxyhectorite,and we compare these results with those of alcohols adsorbed in Cu(I1)-exchanged, Mg(I1)-doped natural montmorillonite. Beidellite is a dioctahedral smectite in which layer charge is created by the substitution of Al(II1) for Si(1V) in the tetrahedral sheet bordering the interlayer space. Fluorohectorite and hydroxyhectorite are trioctahedral smectites in which layer charge is created by the substitution of Li(1) for Mg(I1) in the octahedral sheet in the center of each layer. Fluorohectorite contains fluorine atoms on the edges of the octahedrons while in hydroxyhectorite those sites are occupied by hydroxyl groups. The influence of the location and density of the layer charge of these clays on their adsorption geometries with alcohols is assessed.
The montmorillonite used was a natural sample termed STx-1 from The Clay Resource Repository,University of Missouri. Each gram of the crude clay was stirred with 0.05 M HCl to remove carbonate impurities and was filtered and dispersed in 1 M NaCl solution for 12 h. This suspension was filtered and the montmorillonite stirred with 1 M NaCl for about 12 h. After this treatment was repeated twice, the clay suspension was dialyzed to remove excess cations. The synthesisand characterization of beidellite, fluorohectorite, and hydroxyhectorite are described elsewhere.1° The chemical composition of these synthetic clays has been controlled so as to obtain a different charge density on the clay layers. Beidellite with the chemical formula N ~ o . ~ ~ A ~ ~ ( S ~ ~ . ~has ~A~.~~)OZ been synthesized. 27AlNMR was performed on this beidellite by M. Narayana of Shell Research Laboratories and showed that there are approximately 5.5 more aluminums in octahedral sites than in tetrahedral sites which confirms the given chemical formula. Fluorohectorite ( N ~ I ( M ~ ~ . s L ~ o . ~ )and S~B hy-O ~ ~ F ~ ) droxyhectorite (Nal ( M ~ ~ . s L ~ ~ . s )have S ~ been ~ O synthesized ~~F~) using a similar procedure. The four smectites were converted to the Mg(I1)-exchanged form using MgCl2 by a similar method used to exchange Na(1) into montmorillonite. Replacement of some of the Mg(I1) by Cu(I1) was acheived by dispersing a known amount of the Mg(11)-smectite in deionized water and adding a calculated amount of CuC12. The suspension was stirred overnight and then filtered, and the smectite was dialyzed. Thedialyzed smectites were dried in air at about 40 OC. Generally, enough CuC12 was added to exchange only about 5% of Mg(I1) by Cu(I1) to obviate the possibility of spin-exchangeinteractions between Cu(I1) cations. Samples of powdered clay in 2-mm4.d. by 3-mm-i.d. Suprasil quartz tubes were dehydrated by evacuation to -4 X 1 V Torr overnight at room temperature. ESEM studies with adsorbed D20 show that this procedure is sufficient to remove water coordinated to transition metal ions although some water may still be retained by the clay structure. The appropriate alcohols were then distilled, under vacuum, onto the clay samples until excess solvent was visible in the sample tubes. The tubes were allowed to soak for 24 h before ESR and ESEM spectra were recorded. The adsorbates used were as follows: CHpOD, 99.5 atom % D, Aldrich; CD)OH, 99 atom % D, Aldrich; CH~CHZOD, 99.5 atom % D, Aldrich; C H ~ C H Z C H ~ OAldrich. D,
0022-365419312097-8646%04.00/0 0 1993 American Chemical Society
Cu(I1)-Doped Mg(I1)-Exchanged Smectite Clays CH,OH adsorbed in CuMg-mont
CH,OH adsorbed in CuMg-mont
Figure 1. ESR spectrum at room temperature of CuMg-beid soaked in
methanol. X-ray basal spacings were measured on a Phillips Model 1840 diffractometer with a vertical goniometer, using the Cu Ka line. ESR spectra were recorded on a Varian E-4 spectrometer at 77 and 300 K. ESE spectra were recorded at 4 K on a Bruker ESP-380 spectrometer and a home-built spectrometer described previously.16 Three-pulse stimulated echoes were recorded with a 7~/2-7~/2-~/2 pulse sequence, and the echo was detected as a function of T, the time between the second and third pulses. The time between the first and second pulse, T , was chosen to maximize modulation from 27Al,7Li, or 2H. Simulations were made in terms of Nequivalent nuclei at a distanceR and isotropichyperfine coupling A using a spherical approximation.17 ReSults ESR spectra are shown in Figures 1-5. Experimental and simulated ESEM spectra are shown in Figures 2b-5b. ESEM parameters are displayed in Table I. Powder ESR Data. When methanol is adsorbed in CuMgmontmorillonite (CuMg-mont), both an isotropic and an anisotropic ESR signal are observed at room temperature (Figure 1). Clearly, two kinds of Cu(I1)-methanol complexes are present at room temperature; one complex is able to freely tumble, and the other one is hindered by the clay surface. This result is different from that of Brown and Kevan," who observed only an isotropic ESR signal at room temperature. When ethanol and propanol are adsorbed, only anisotropic ESR spectra are obtained at room temperature. This shows that at room temperature the Cu(I1)ethanol or propanol complexes are hindered by the clay surface. For CuMg-beidellite (CuMg-beid), CuMg-fluorohectorite (CuMg-fluorohect), and CuMg-hydroxyhectorite (CuMg-hydroxyhect) soaked with methanol, ethanol, and propanol, only anisotropic powder ESR spectra are observed at room temperature. This suggests that the Cu(I1) centers in these systems are in hindered sites. However, ESR spectra of films of these smectites soaked with primary alcohols did not show any dependence on the film angle relative to the applied magnetic field which contrasts with CuMg-mont. This suggests that square-planar complexes of Cu(I1) are not formed in these other smectites. Three-Pulse ESEM Data. CuMg-Montmorillonite. The ESEM results demonstrate that when methanol and ethanol are adsorbed on CuMg-mont, the Cu(I1) interlayer cation coordinates to four alcohol molecules (Table I). This result is the same as that of Brown and Kevan." But, when propanol is adsorbed, only three propanol molecules coordinate to the Cu(I1) species, while Brown and Kevan found four. CuMg-Beidellite, ESR and ESEM spectra of CuMg-beid soaked in CD30H and CH3CH20Dare shown in Figures 2 and 3. The ESEM spectrum of CuMg-beid soaked with CD30H recorded at 31 5 5 G was best simulated with N = 9, R = 0.37 nm, and A = 0.09 MHz, and the ESEM spectrum of CuMg-beid soaked with CHsCHzOD recorded at 3 197 G was best simulated
The Journal of Physical Chemistry, Vol. 97, No. 33, 1993 8647
with N = 3, R = 0.28 nm, and A = 0.21 MHz. The CzHsOD and C3H7OD spectra were also best simulated with three deuterium nuclei at approximately 0.28 nm (Table I). These data show that, in beidellite saturated with primary alcohols, that only three molecules of alcohol directly coordinate to the Cu(I1) ions in the interlayer region. CuMg-Fluorohectorite and CuMg-Hydroxyhectorite. The 77 K ESR spectra of CuMg-fluorohect soaked in CD30H and CzH5OD are shown in Figures 4a and 5a. In CuMg-fluorohect soaked in CDsOH, ESEM data taken at 3144 G could be best simulated using a two-shell model with N = 3, R = 0.37 nm, A = 0.09 MHz and N = 3, R = 0.43 nm, A = 0.02 MHz (Figure 2b). These data indicate that in CuMg-fluorohect the Cu(I1) cations have only one directly coordinated methanol molecule and interact with a second, more distant, methanol molecule. The ESEM data of CuMg-fluorohect soaked in CzHsOD, recorded at 3170 G , are shown in Figure 5b. The best fit was obtained using a two-shell model with N = 1, R = 0.28, A = 0.30 MHz and N = 1, R = 0.32 nm, A = 0.05 MHz. The ESEM parameters of CuMg-fluorohect soaked in CH30D and C3H7OD are similar to those of CuMg-fluorohect soaked in C~HSOD. The ESEM parameters of CuMg-hydroxyhect soaked in alcohols are also not significantlydifferent from thoseof CuMg-fluorohect. These data indicate that in fluorohectorite and hydroxyhectorite soaked in primary alcohols only one alcohol molecule directly coordinates to the interlayer Cu(I1) cations. Three-pulse ESEM was performed at 4 K on the four smectites soaked with C~HSOH.The values of the interpulse time were varied so as to maximize the modulations due to 27Alor 7Linuclei. In CuMg-mont, only proton modulations are observable. In CuMg-beid, proton and aluminum modulations are observable. In CuMg-fluorohect and CuMg-hydroxyhect, proton and lithium modulations are detected.
Discussion When methanol and ethanol are adsorbed in CuMg-mont, square-planar Cu(I1)-alcohol complexes form in the middle of the clay interlayer." In the case of methanol, two kinds of Cu(II)-(methanol)d complexes can be distinguished: one is free to tumble in the interlayer region of the clay, and the other one is hindered by the clay surface. Three-pulse ESEM data indicate that only three propanol molecules coordinate to the interlayer Cu(I1) cations. In the case of beidellite, since anisotropic ESR spectra are obtained at room temperature with any alcohol, the Cu(I1)alcohol complex does not freely tumble as it does in CuMg-mont with adsorbed methanol. Though enough interlayer space exists for this complex to freely tumble based on the dm1 spacing of CuMg-beid with adsorbed CH30H which is similar to that of CuMg-mont, it seems that the Cu(I1) complex is hindered by the clay surface. This is suggested to be a consequence of the greater layer charge bordering the interlayer space in beidellitecompared to montmorillonite (also see below). Three-pulse ESEM data of CuMg-beid soaked with CH3OH show both 27Al and lH modulations. 1H modulations are to be expected since CH30H contains protons. The 27Al molecules indicate that the Cu(I1) species are within 6 A of tetrahedral Al(II1) cations which exist in the tetrahedral sheet bordering the interlayer space, and are likely coordinated to interlayer surface oxygens.18 Three-pulse ESEM data of CuMg-beid soaked with any of the three short chain length primary alcohols studied show that the Cu(I1) species are coordinated to only three alcohol molecules through Cu(II)-oxygen bonds. We suggest that the Cu(II)-(alcohol)3 complex is also bound to three additional oxygens of the interlayer surface through Cu(II)-oxygen bonds of a tetrahedron containing A1 which is supported by the observation of Z7Al modulation (Figure 6). Analogous behavior has been observed for Ag(1)-exchanged beidellite soaked with
8648 The Journal of Physical Chemistry, Vol. 97, No. 33, 1993
Comets et al.
TABLE I: Simulation of Three-Pulse ESE Deuterium Modulation Data for CuMg-Smectites with Adsorbed Alcohols simulation parameters shell 1 sample CuMg-mont CHsOD CDjOH EtOD nPrOD CuMg-beid CHiOD CDaOH EtOD nPrOD CuMg-fluorohect CHjOD CDaOH EtOD nPrOD CuMg-hydroxyhect CHjOD CD3OH EtOD nPrOD
shell 2
field0 (G)
Nb
fl (nm)
Ad (MHz)
3110 3149 3166 3134
4 12 4 3
0.28 0.38 0.27 0.28
0.31 0.09 0.33 0.30
3191 3155 3197 3161
3 9 3 3
0.28 0.37 0.28 0.27
0.25 0.09 0.21 0.26
3103 3144 3170 3120
1
3 1 1
0.28 0.37 0.28 0.28
3111 3192 3127 3136
1 3 1 1
0.28 0.37 0.27 0.28
N
R (nm)
A (MHz)
0.30 0.09 0.30 0.32
1 3 1 1
0.32 0.43 0.32 0.34
0.05 0.02 0.05 0.05
0.28 0.09 0.32 0.26
1 3 1
0.36 0.46 0.34
0.04 0.02 0.06
a Field values corrected for a frequency of 9.2MHz. Number of deuterium nuclei to the nearest integer. Distance between Cu(I1) and deuterium; estimated uncertainty is kO.01 nm. Isotropic hyperfine coupling; estimated uncertainty is 110%.
CuMg-beid wet CD,OH
CuMg-beid wet CH,CH,OD
b
b F'.Or\
z
3 oj
0.8 .
H=3155G ~ = 0 . 2 7 p
H
= 3197 G T = 0.27 PS
n:
a0.6 cv)
j
5 0.4
water in which Ag(1) is directly coordinated to two water molecules
and two oxygens of the interlayer surface.l9 As in the present study, this result is opposite to that of Ag(1)-exchanged montmorillonite in which four water molecules form a square-planar complex in the middleof the interlayer space. Recent electrostatic potential calculations have demonstrated that there is a greater electrostatic field flux above the interlayer surface oxygens of beidellite than the corresponding oxygens of montmorillonite.2° The results of this study together with those of previous workslOJ8 confirm the idea that the interlayer surface oxygens in beidellite have greater Lewis base character than the correspondingoxygens in montmorillonite. The three-pulse FSEM data indicate that, in CuMg-fluorohect and CuMg-hydroxyhect, only one water molecule directly
coordinates to the Cu(I1) species. Also, three-pulse ESEM experiments performed on CuMg-fluorohect and CuMg-hydroxyhect soaked with C2H7OH with I = 0.28 ps to optimize 'Li modulation indicate that the Cu(I1) species are within about 4 Aof the7Licationslocatedin theoctahedral~heet.'~Modulations due to 7Linuclei have also been observed when two-pulse FSEM was performed on Cu(I1)-exchanged hectorites.10 Also, the migration of Ag(1) into pseudohexagonal cavities of the interlayer surface of fluorohectorite when the clay is dehydrated under vacuum has been deduced from the observation of fluorine modulations.19 Therefore, we suggest that, in fluorohectorite and hydroxyhectorite, the Cu(I1) cations are recessed inside pseudohexagonal cavities borderingthe hectorite interlayer space
Cu(I1)-Doped Mg(I1)-Exchanged Smectite Clays
The Journal of Physical Chemistry, Vol. 97, No. 33, 1993 8649
CuMg-lluorohect wet CD,OH Si04 tetrahedron of upper tetrahedral sheet
A104 tetrahedron
of lower tetrahedral sheet
b H = 3144 G t = 0.27 PS
0
o
AI(I11) ion Si(1V) ion
0 Oxygen atom 8 Cu(l1) cation
% G
0
I
1
,
2
,
3
,
,
4
,
5
T, 1.1s
Figure 4. (a) ESR spectrum at 77 K of CuMg-fluorohect soaked in CDsOH. (b) Experimental (-) and simulated (- -) three-pulse ESEM spectrum of the sample from (a), recorded at 4 K.
-
CuMg-fluorohect wet CH,CH,OD
I
j
H=3170G ~ = 0 . 2 7 ~ ~
0.8 0.6
t 5 zj
0.4
I-
&
1
z? 0.2
G
0
Figure 6. Schematic model of the adsorption geometry of alcohol molecules on CuMg-beid.
(11)-exchanged smectiteclays is somewhat similar to that of water in Cu(I1)-doped or Ag(1)-doped Mg(I1)-exchanged smectites. Due to the different locations of the layer charge in Mg(I1)exchanged montmorillonite versus beidellite, in the octahedral and tetrahedral sheets, respectively, the density of charge on the clay surface is greater in beidellite than in montmorillonitewhich leads to different coordinationnumbers of alcohol adsorbates. In fluorohectorite and hydroxyhectorite the coordination number of alcohol adsorbates is even less due to recession of the cupric ion into hexagonal cavities bordering the interlayer space.
Acknowledgment. This research was supported by the Robert A. Welch Foundation and the National Science Foundation. The assistanceof Dr. Victor Luca in the synthetic procedures and Dr. M. Narayana for the NMR data is gratefully acknowledged.
b
I
Alcohol molecule
I1 .2 *
3
.
4
.5 ,
T, PS
Figure 5. (a) ESR spectrum at 77 K of CuMg-fluorohect soaked in C2H5OD. (b) Experimental (-) and simulated (- - -) three-pulsc ESEM spectrum of the sample from (a), recorded at 4 K.
and can only directly coordinate to one alcohol molecule due to steric constraints.
Conclusions This study demonstrates that the mechanism of the adsorption of short chain length primary alcohols in Cu(I1)-doped, Mg-
References and Notes (1) Laszlo,P. Science 1987, 235, 1473. (2) Laszlo,P.;Mathy, A. Helv. Chim. Acta 1987, 70, 577.
(3) Newman, A. C. D. In Chemistry of Clays and Clay Minerals; Newman, A. C. D., Ed.; Wiley-Interscience: New York, 1987; Chapter 5. (4) Clementz, D. M.; Pinnavaia, T. J.; Mortland, M. M. J . Phys. Chem. 1973, 77, 196. (5) McBride, M. B.; Mortland, M. M. Soil Sci. Soc. Am. Proc. 1974, 48,408. (6) McBride, M. B.; Pinnavaia, T. J.; Mortland, M. M. J. Phys. Chem. 1975, 79,2430. (7) McBride, M. B. Clays Clay Miner. 1976, 24, 211. (8) McBride, M.B. Clays Clay Miner. 1982,30,200. (9) Sposito, G.; Prost, R. Chem. Rm. 1982,82,553. (10) Comets, J.-M.; Luca, V.;Kevan, L. J. Phys. Chem. 1992,96,2645. (11) Brown, D. R.; Kevan, L. 1.Am. Chem. Soc. 1988, 110, 2743. (12) Brindley, G. W.; Ray, S . Am. Mineral. 1964, 49, 106. (13) (a) Dowdy, R. H.; Mortland, M. M. Clays Clay Miner. 1967, 15, 259. (b) Dowdy, R. H.; Mortland, M. M. SoilSci. 1968, 105, 36. (14) Brindley, G. W.; Wiewora, K.; Wiewora, A. Am. Mineral. 1%9,54, 1635. .... (15) McBride, M. B.; Mortland, M. M. Soil Sci. Soc. Am. Proc. 1974, 38,408. (16) Kevan, L.; Bowman, M. K.; Narayana, P. A.; Boeckman, R. K.; Yudanov, V. F.;Tsvetkov, Yu. D. J. Chem. Phys. 1975,63,409. (17) Narayana, P.A.; Kevan, L. Magn. Rkon. Rev. 1983, 1, 234. (18) Kevan, L. In Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R. N., W.; Wdey-Interscience: New York, 1979; Chapter 8. (19) Luca, V.;Brown D. R.; Kevan, L. J . Phys. Chem. 1991,95,10169. (20) Bleam, W. F. Clays Clay Miner. 1990,38,527.
