J. Phys. Chem. 1993,97, 466-469
466
Adsorption of Ammonia and Pyridine on Copper(I1)-Doped Magnesium-Exchanged Smectite Clays Studied by Electron Spin Resonance Jean-Marc Comets and Larry Kevan' Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: July 28, 1992; In Final Form: October 5, 1992
The interaction between N-donor adsorbates such as ammonia and pyridine with Cu( 11)-exchanged montmorillonite, beidellite, fluorohectorite, and hydroxyhectorite into smectite clays has been studied by electron spin resonance. Cu(I1) cations exchanged into smectites coordinate five ammonia or pyridine molecules in beidellite, four ammonia or pyridine molecules in montmorillonite and fluorohectorite independent of variable layer charge in fluorohectorite, and two ammonia or pyridine molecules in hydroxyhectorite. Thus, the Cu(I1) cations bound to the interior surfaces of these smectite clays constitute strong Lewis acid sites.
Introduction Smectite clays are used as catalysts and catalyst supports. Due to their laminar structure and their readily expandable interlayer region, smectite clays offer sites for adsorbed molecules. The mechanism of such adsorption is controlled by the presence of charge balancing cations between the clay layers and the interactions of these cations with interlayer surface sites and adsorbate molecules.14 The strength of the interaction between an adsorbate molecule possessing a coordinative nitrogen and metal ions imbedded in an inorganic support depends on the electron donating capability of the adsorbate molecule and the electron accepting capability of the metal ion (Lewis acidity). Transition metals with partially occupied d-orbitals possess relatively strong coordinating prop erties. Thus the Lewis acid strength of transition metal ions can control their catalytic activity. Several studies have dealt with the interaction of N-donor solvents with metal cations located at exchangeable sites in smectite clays. Farmer and Mortland5 have observed the formation of a square planar complex ion Cu(EtNH2)d2+when gaseous ethylamine is adsorbed on Cu(I1)-exchanged montmorillonite. Greene-Kelly6 suggesteda model for pyridine adsorbed in Na( 1)-montmorillonite in which four pyridine molecules are coordinated to Na(1) to form a 23.3-Aintercalate. Laszlo and Mathy'v2 have shown potential for organic synthesis by natural clay minerals containing transition metal ions in exchange sites. In this study the interactionof Cu(I1)-doped Mg(I1)-exchanged smectites with ammonia and pyridine is investigated by electron spin resonance (ESR) to determine the number of coordinated molecules.
Experimental Section The montmorilloniteused was a natural sampledenoted STx- 1 supplied by The Clay Resource Depository, University of Missouri. Each gram of the crude clay was stirred with 0.05 M HCl to remove carbonate impurities. This suspension was filtered, and the montmorillonitestirred with 1 M NaCl solution for about 12 h to effect complete sodiumcation exchange. After this treatment was repeated twice, the clay suspension was dialyzed to remove excess cations. No impurities were detected by powder X-ray diffraction. This natural clay has been used in several previous studies with consistent results with other synthetic clays, so it does not seem to contain impurities which affect the adsorption res~lts.4.~*8 Thesynthesisandcharacterization of beidellite,fluorohectorite, and hydroxyhectorite are described el~ewhere.~The chemical composition of these synthetic clays has been controlled so as to 0022-3654/58/2097-0466$04.00/0
obtain a different charge density on the clay layers. Beidellite with the chemical formula Na0.s7A4(Si7.,3Alo.~7)o~(oH)4 has been synthesized. 27AlNMR performed on this smectite showed that there are approximately 5.5 more aluminums in octahedral sites than in tetrahedral sites which confirms the given chemical formula. Several fluorohectorites with different negative layer charges of 0.4,0.6,0.8,and 1 per unit cell (Si8020unit) have been synthesized. These were obtained by choosing x = 0.4,0.6,0.8, and 1 in the chemical formula of fluorohectorite: Nax(Mga.o-xLix)Sig0~F4.A similar procedure was followed to obtain hydroxyhectorite (Nal.o(MgS.oLil.o)SisOzo(OH)4). The four smectites were converted to the Mg(I1)-exchanged form using MgCl2 by a method similar to that used to exchange Na(1) into the montmorilloniteas discussedabove. Replacement of some of the Mg(I1) by Cu(I1) was achieved by dispersing a known amount of the Mg(II)-smectite in deionized water and adding a calculated amount of CuC12. The suspension was stirred overnight and filtered, and the smectite was dialyzed. The dialyzed 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 significant spin-exchange interactions between the Cu(I1) cations. Activation of the samplesand exposure to the various adsorbaka was carried out in a glass reactor to which was attached a Suprasil quartz ESR tube so that ESR spectra could be recorded without exposing the sample to the atmosphere. The reactor could be attached to a vacuum line. The adsorbates used were as follows: pyridine, C5D514N,99 at. 96 D, Aldrich; C5H5l5N, 99 at. 4% 15N, Cambridge Isotope Laboratories (CIL); 14NH3, Aldrich; ISNH3, 99.8 at. 96 I5N, Aldrich. The clays were dehydrated under 4 mTorr vacuum at room temperature for about 12 h. For a test of whether adsorbed water was removed by this treatment, some samples were a h activated by heating to 100,200,300,and 400 OC for 2 h followed by heating at the same temperature in flowing oxygen to reoxidize any reduced copper formed during dehydration. These heating treatments did not modify the adsorption results, which indicates that dehydration was reasonably complete. The dehydrated samplewas then exposed to pyridinevapor at its room temperatwe vapor pressure or to 200-500 Torr of ammonia for about 2 h. Following adsorption, excess ammonia or pyridine was removed by evacuation to 4 mTorr. ReSUltS
Ammonia is a good Lewis base and should therefore form strong complexes with Lewisacid siteson the clay mineral surfaces. These are Cu(I1) sites on the surface of the interlayer region of (B
1993 American Chemical Society
Adsorption of Ammonia and Pyridine on Smactite Clays
The Journal of Physical Chemistry, Vol. 97, No. 2, 1993 467
CuMgmont + "NH,
g = 2.036 A = 12.9 x
_j\ 1500
,'
= 2.039 A = 16.3 x
lo4 cm" CuMg-mont + "NH,
m
-
lo4 cm" A
/ 1500
s
,,'
A = 20.6 x 1O4 cm"
g = 2.036 A = 18.9 x
lo4 cm'' pb.rr 1. Adsorption of I4NH3and 15NH3on CuMg-montmorillonite: first (upper) and second (lower) derivative ESR spectra.
the clay since most of the Cu(I1) are located in the interlayer region and not on the edges, which constituteonly a small fraction of the total surface area. According to previous work,+lS the maximum resolution of the superhyperfine splittings due to nitrogen nuclei of adsorbates usually occurs in the g1 region of the ESR spectrum. There are four gl components due to a large Ai hyperfine splitting due to the coppernucleus. This copper hypcrfiie is usually not resolved in the g1 region. Nitrogen superhyperfine splittings also appear sometimes on the high field gl component. In some cases the superhypcrfiielinesarealsovisibleonthelowfieldgl component.1° In the first derivative spectrum when there is a separation of a few Gauss between the g1 component and the high field gw component, the Cu-Ln2+ complex (L = ammonia or pyridine) formed undergoes a rhombic di~tortion.'~In this case the high fieldgl componentappears at a higher field than thegL component; that is, the gl and g1 regions overlap. Usually the tendency to undergorhombicdistortion is more important for a CuLs2+species than for a CuLd2+species.15 ~ M o a t m o r i U o n i t e .The 77 K ESR spectra of CuMgmontmorillonite(CuMg-mont) with adsorbed "NH3 and 15NH3 are shown in Figure 1. The second derivative spectra with adsorbed "NH3 shows nine superhyperfine lines centered at g = 2.036 and split by 12.9 X 10-4 cm-l. The poor resolution is probably due toquadrupolar broadening. Since 14Nhas a nuclear spin of 1, the lines are assigned to four ammonias coordinated to the copper cations by Cu(I1)-nitrogen bonds. The second derivative spectra with adsorbed I5NH3on montmorillonite shows five lines centered at g = 2.036 and split by 18.9 X 10-4 cm-1. Since I5N has a nuclear spin of l/2, the superhyperfine lines indicate four ammonias and support the 14NH3data. In both spectra the superhyperfine lies on the high field component are not resolved. The 77 K ESR spectra of CuMg-mont with adsorbed C5H514N and CsH5ISNare shown in Figure 2. The analysis of both spectra leads to the same conclusion that Cu(1I) is coordinated to four pyridine molecules. In the case of CuMg-mont with adsorbed C J ; H ~ the ~ ~lines N on the high field gl component are well resolved and oonfirm the assignmentof four pyridine molecules coordinated to the Cu(I1) cations. ChMg-MdeUite. Strong superhyperfine structure due to
+
g = 2.028 A = 22.8 x 1O4 cm"
Figure 2. Adsorption of C5Hs1'N and C5H51JN on CuM8montmorillonite: first (upper)andsecond (1ower)derivativeESRspcctra. CuMg-beid + "NH,
v
1== 2.054 12.8 x 1o4 cm-1 id + "NH,
LLfLu g = 2.043 A = 18.4 x
lo4 cm"
Figure 3. Adsorption of "NH3 and 15NH3on CuMg-beidellite: f m t (upper) and second (lower) derivative ESR spectra.
nitrogen nuclei is observed on the gL component of the 77 K ESR spectra of beidellite with adsorbed I4NH3in Figure 3. Since the peak corresponding to gl appears at lower field than the high field component of gL,a strong rhombic distortion of the copperamine complex is suggested.lS The second derivativeof the ESR spectrumofbeidellitewithadsorbcd~4NH~shows 11 linesantered at g = 2.05 and split by A = 12.8 X 10-4 cm-l. Superhyperfie splittings on the high field 81 component are not resolved. The secondderivative spectrumof 15NH3adsorption on beidelliteshows six intense lines centered at g = 2.04 and split by 18.4 X 10-4 cm-'. S i x superhyperfie lines can also be tentatively assigned to the high field a component. They are centered at g = 1.994
468 The Journal of Physical Chemistry, Vol. 97, No. 2, 1993
Comets and Kevan
CuMg-fluorohect t “NH,
LLLu+uLy
g = 2.056 A=12.6x1O4cm.’
A
-
= 2.002 19.3 x
LLYY
g = 2.041 A = 20.4 x
-
g = 2.05 A 12.9 x
lo4 cm”
-
lo4 cm“
A
g = 2.039
lo4 cm”
F@N 4. Adsorption of CsHsL4Nand CsHsIsN on CuMg-beidellite: first (upper and second (lower) derivative ESR spectra.
A
18.2 x 1O4 cm“
Figme 5. Adsorption of “NH3 and IsNH3 on CuMg-fluorohedorite (layercharge = 1 per unit all):fiiat (upper)and second (lower)derivative ESR spectra.
and split by 17 X l(r cm-l. These assignmentsshow that Cu(I1) is coordinated to five ammonias in beidellite. The 77 K ESR spectra of CuMg-beidellite (CuMg-beid) with adsorbed C5H5I4N and C5H3l5N are shown in Figure 4. The analysis of these spectra also leads to the conclusion that Cu(I1) is bound to five pyridines. A smaller rhombic distortion of the complex than that in the copper-ammonia complex is observed. The lines due to 15Nnuclei are more intense than those due to I4N nuclei. C h M g - F l ~ ~ ~ h e c t ~d A t eW~ - H y h W k t d k SF tra of CuMg-fluorohectorite (CuMg-fluorohect) (layer charge = 1 per unit cell) with adsorbed ammonia and pyridine are shown in Figures 5 and 6, respectively. These spectra demonstrate that four ammonias or four pyridines are coordinated to Cu(I1) in CuMg-fluorohect. In the case of fluorohectorites possessing different layer charges, similar results are obtained. In Figure 7 the second derivative ESR spectra of the gl region of air-dried CuMg-hydroxyhectorite (CuMg-hydroxyhect) and CuMg-hydroxyhect which was exposed to I4NH3, I5NH3, and C5H5l5N are displayed. Even if the resolution of the superhyperfinesplitting lines is poor, these spectra demonstrate that only two ammonias or pyridines are coordinated to the Cu(1I) species.
Discussion After adsorption of ammonia or pyridine in the vapor phase, Cu(I1) is bound to five ammonia or pyridine moleculesin CuMgbeid, four ammonia or pyridine molecules in CuMg-fluorohect and CuMg-mont, and two ammonia or pyridine molecules in CuMg-hydroxyhect. These adsorptions occur at room temperature. These results show the ability of different Cu(I1)exchanged smectites to coordinate different numbers of N-donor molecules and c o n f i i their strong Lewis acid character. Several studies1b20have reported the strong ability of smectite clays to intercalate neutral molecules in the interlayer region. In receht work in this laboratoryZ1athe ability of ammonia and pyridine tointeract withCu(I1) located inside the latticeof fluorohectorite through the pseudohexagonal cavities has been demonstrated, and thus the coordination to Cu(I1) located in the interlayer region should be cvcn easier. Montmorillonite is a dioctahedralsmcctitein which layer charge is m a t e d by substitution of Mg(I1) for Al(II1) in the octahedral
+ 2.031
1= 20.3 x lo4 cm” I
FtgpcL Adsorptionof C ~ H Sand ~ ~CsHsL5N N on CuMg-fluorohectorite (layercharge = 1 per unit cell): first (upper) and second(lower)derivative ESR spectra.
sheet. Beidellite is also a dioctahedral smectite, but in thiacuu layer charge is created by the substitution of Al(II1) for Si(1V) in the tetrahedral sheet. Fluorohectorite and hydroxyhectorite are triactahedral smoctites in which layer charge is created by the substitution of Li(1) for Mg(I1) in the octahedral sheet. Fluorohectorite contains fluorine atoms on the edges of the octahedrons, and in hydroxyhectoritethose sites are occupied by hydroxyl group instead of fluorine atoms. Table I indicates the charge per unit cell and the number of ammonia or pyridine molecules coordinated to the Cu(I1) species for each smectitc. Fluorohectorites with various layer charger and montmorillonite which have the layer charge in an octahedral sheet adsorb the same number of ammonia8 and
Adsorption of Ammonia and Pyridine on Smectite Clays
U+U
L+I
The Journal of Physical Chemistry, Vol. 97. No. 2, 1993 469 interlayer region of the clays with a layer spacing of 13 A and planar aromatic compounds give a minimum spacing of 12.5 A. Experiments on oriented films demonstrate that the copperammonia or copper-pyridine complexes have their main axis oriented perpendicular to the clay layers in fluorohectorites, montmorillonite, and beidellite.z6 These results suggest that, in fluorohectorites and montmorillonite, copper and the four ammonias or pyridines form a square planar complex in the middle of the interlayer region of the clays. In beidellite, the geometry of the copper(I1) complex coordinated to five ammonias or pyridines is not well understood. If the smectites are heated at temperatures higher than 100 OC before adsorbing ammonia or pyridine, the same number of molecules bind to the interlayer Cu(I1) cations as when the clays are dehydrated at room temperature. This result shows that considering the interaction between the support, the interlayer cations and the solvent, the interaction between the cations and the solvent is stronger if the solvent is an N-donor adsorbate such as ammonia or pyridine than if the solvent is an 0-donor adsorbate such as water.
g = 2.061 A = 13.8 x 1O4 cm”
g = 2.053 A=19x104cm”
50 G
C~CldOM
y g = 2.049 +A = 22.8 x lo4 cm“
Figure 7. Second derivative ESR spectra of air-dried CuMg-hydroxyhectoriteand CuMg-hydroxyhectorite with adsorbed “NH,, *WHs.and CsH5”N.
TABLE I: Number of Coordinrted Ammonii dPyridine Molecllks v
m the Layer Cbuge per Si&
Unit Cell in
smectite clays
ammonia molecules
clay type
negative layer charge
kidelite montmorillonite fluorohectorites
0.65
5
0.85
4
1 0.8
4
hydroxyhectorite
0.6
4 4
0.4 1
4 2
pyridine
molecules 5 4 4 4 4 4
Cu(I1) cations exchanged into the interlayer space of smectite clays are strong Lewis acid sites with respect to the adsorption of N-donor Lewis acid bases. Thus, Cu(I1)-exchanged smectites have potentially good catalytic properties. The coordination number of Cu(I1)-exchanged smectites for N-donor molecules seems to be affected by the location of the negative layer charge in octahedral versus tetrahedral sheets. However, the magnitude of the layer charge in octahedral sheets does not seem to affect the coordination number.
Acknowledgment. This research was supported by the Robert A. Welch Foundation and the National Science Foundation. The assistance of Dr.Xinhua Chen in the implementation of the software used for the simulations and of Dr.Victor Luca in the synthetic procedure is gratefully acknowledged.
2
pyridines. This suggests that there is no correlation between the total layer charge of the Cu(I1)-exchanged smectites and the ability of Cu(I1) to coordinate to N-donor molecules. With this hypothesis one expects that hydroxyhectorite will coordinate the same number of ammonia molecules. It is therefore surprising that only two ammonias coordinate to the interlayer Cu(I1) cations. This result may be due to the lower crystallinity of hydroxyhectorite which is evidenced by larger line widths for the reflections in the X-ray powder diffraction pattern. Beidellite which has its layer charge in a tetrahedral sheet adsorbs more ammonias or pyridine than the other smectites with layer charge in an octahedral sheet. This suggests that the ability of the Cu(11)-exchanged smectitesto adsorb N-donor molecules is stronger if the layer charge is located in the tetrahedral sheet rather than in the octahedral sheet. The ESR parameters, gl and Ai, are characteristic of the environmentalsymmetryof thecupric ion. In thecase of ammonia or pyridine adsorbed on fluorohectorites, montmorillonite, and beidellite, the typical ESR parameters were = 2.23 and A! = 190 X 1O%rl. These parameterssuggestthat in fluorohectorites and montmorillonite the four ammonia or pyridine molecules and the copper cation form a square planar or tetragonal complex. Several early~tudies*~~ show that if theclay layersarecompletely dehydrated, normal amines form monolayer complexes in the
References d Notes (1) Laszlo, P.Science 1987, 235, 1473.
(2) Laszlo, P.; Mathy, A. Helv. Chim, Acta 1987, 70, 511. (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) Comets, J.-M.; Luca, V.; Kevan, L. J. Phys. Chem. 1992,96,2645. ( 5 ) Farmer, V. C.; Mortland, M.M.J. Phys. Chem. 1965,69,683. (6) GreenaKeUy, R. J. Chem. Soc., Faraday Trans. 1956,2,226. (7) Brown, D. R.; Kevan, L. 1. Phys. Chem. 1988, 92, 1971. (8) Luca, V.; Kukkadapu, R.; Kevan, L. J. Chem. Soc., Faraday Trans. 1991, 87, 3083. (9) Bonomo. R. P.; Riggi, F. Chem. Phys. &ti. 1982, 93,99. (10) Bereman, R. D.; Koaman, D. J. J . Am. Chem. Soc. 1977,99,7322. (11) Cleveland, L.; Coffman, R. E.; Coon, P.; Davis, L. Biochemistry 1975, 14, 1108. (12) Vedrine, J. C.;Derouane, E. G.; Ben Taarit, Y.J . Phys. Chem. 1974, 78, 531. (13) Leith, I. R.; Leach H. F. Proc. R. Soc. London A 1972,330, 247. (14) Wiemma, A. K.; Windle, J. J. J. Phys. Chem. 1964, 68, 2316. (15) Siddiqui, S.; Shepherd, R. E. Imrg. Chem. 1986, 25, 3869. (16) Rupert, J. P. J. Chem. Phys. 1973, 77,184. (17) Pinnavaia, T. J.; Mortland, M.M. J . Phys. Chem. 1971, 75,3957. (18) Comet, I. J. Chem. Phys. 1943,11,217. (19) Mortland, M. M. Science 1969, 166, 1406. (20) Slabaugh, W. H.; Siegel, R. H. J. Phys. Chem. 1956,60, 1105. (21) Luca, V.; Chen. X.; Kevan. L. Chem. Mater. 1991, 3, 1073. (22) Luca, V.; Kevan. L. 1. Phys. Chem. 1992,96, 3391. (23) MacEwan, D. M.C. 1. Chem. Soc., Faraday Trans. 1948,44,349. (24) Greene-Kelly, R. J. Chem. Soc., Faraday Trans. 1955,51,412. (25) Grcene-Kelly, R. J. Chem. Soc., Faraday Trans. 1956, 52, 1281. (26) Brown, D. R.; Kevan, L. J . Am. Chem. Soc. 1968,110, 2743.
