Langmuir 1990,6, 1237-1245
attributed to C02,19and a much increased Clod- band at 1106 cm-1. Also to be noted is the very much diminished 1561-cm-l band ascribed to C=C stretch compared with the normalized difference spectra a t +0.6 V. These findings afford evidence that at +0.8 V the adsorbed BQT undergoes partial oxidation to yield carbon dioxide and other products which still remain to be identified. Comparison of in S i t u a n d ex S i t u Results. Hubbard and co-worker$ have recently reported Auger electron spectra (AES),high-resolution electron energy loss spectra (HREELS), and cyclic voltammetry data for DHT adsorbed on Pt(ll1) from aqueous solutions. I t was concluded on the basis of those results that about 50% of the adsorbed DHT undergoes cleavage generating hydroquinone (which presumably remains in solution phase) and atomic S on the Pt(ll1) surface. In addition to losses at energies higher than 1000 and smaller than 3000 cm-l, these authors reported bands at 1574,1463,and 1171cm-l which, although slightly shifted, compare well with those observed in this work. The in-plane 0-H deformation modes at ca. 1350 cm-l (ref 20) and the two different C-0 stretches could not be resolved by HREELS and hence were not listed in that work. Although HREELS offers a much higher sensitivity than conventional reflection absorption infrared spectroscopy, (20) (a) Silverstein,R. M.; Bassler, G . C.; Morril1,T.C. Spectrometric
Identrfrcation of Organic Compounds; Wiley: New York, 1981; p 114. (b)Bellamy, L J. The Infrared Spectra of Complex Molecules; Chapman and Hall: London, 1975; p 122. (c) Yates, P.; Ardao, M. I.; Fiester, L. F. J . Am. Chem. SOC.1956, 78, 650.
1237
there are at least two issues that may limit the use of this technique in the study of electrochemical interfaces: its intrinsic ex situ character and, perhaps most important, its much poorer resolution, a factor that will preclude the detection of subtle modifications in the position and relative intensities of various spectral features induced by changes in the applied potential. Summary The results obtained in this study have demonstrated the following: (i) In situ FTIRRAS in its present stage of development has enough sensitivity to examine the spectral properties of DHT (and a number of other species irreversibly adsorbed on Au (and Pt) electrodes) as a function of potential. (ii) The overall spectral characteristics of DHT and its quinone derivative (BQT) in the adsorbed state resemble closely those of their solution-phase counterparts. (iii) The relatively large intensity of the signals observed affords strong evidence that D H T and BQT are adsorbed on both Pt and Au with the molecular plane at a small rather than large angle with respect to the normal to the surface. Acknowledgment. This work was supported by the Gas Research Institute. Registry No. DHT, 2889-61-4; BQT, 91751-34-7;Au, 744057-5.
Adsorption of Alkyl-Substituted Phenols onto Montmorillonite: Investigation of Adsorbed Intermediates via Visible Absorption Spectroscopy and Product Analysis Debra D. Sackett and Marye Anne Fox* Department of Chemistry, University of Texas at Austin, Austin, Texas 78712 Received November 9, 1989. In Final Form: February 16, 1990
The mechanism of oxidative adsorption of substituted phenols on H+- and Cu2+-exchangedmontmorillonite has been investigated via visible absorption spectroscopy. A characteristic red absorption maximum at approximately 520 nm appears to derive from a phenoxonium ion. The presence of water increased phenol reactivity by increasing the pH of the clay and thus facilitating phenol oxidation. The products extracted from the clays were identified, and a mechanism was proposed for their formation. Molecular oxygen also increased reactivity,most significantly on H+-exchanged clays. The oxidationpotential and steric bulk of the phenols are clearly correlated to their reactivity at the clay surface. Introduction
A wide variety of organic molecules are known to react at clay surfaces via initial oxidation of the substrate by the clay.' Of these, adsorbed aromatic molecules have been the most widely studied. The products resulting from oxidation of adsorbed substituted benzene molecules tend to be dimers.l.2 In some instances, however, oligomers and polymers are produced.3* The factors which control oligomerization remain unknown, however, and we have 0743-7463/90/2406-1237$02.50/0
sought to determine in this study the relative importance of structural variables in governing observable chemical reactivity profiles on cation-exchangedclays. Such studies would be important contributions to mechanistic surface(1) (a) Voudrias, E. A.; Reinhard, M. In Geochemical Procesees at Mineral Surfaces;Davis, J. A., Hayes, K. F., E&.; ACS SymposiumSeriea 323; American ChemicalSociety: Washington, DC, 1986, pp 462-488. (b) Theng, B. K. G . In International Clay Conference; van Olphen, H., Veniale, F., Eds.;Elsevier: Amsterdam,1982. (c) Laszlo, P.Acc. Chem. Res. 1986, 19, 121.
0 1990 American Chemical Society
1238 Langmuir, Vol. 6, No. 7, 1990 catalyzed chemistry and to the development of methods of formation of organic/inorganic polymeric composites. One of t h e f i r s t s t u d i e s of this t y p e involved characterization of the intermediates and products formed from benzene adsorption on Cu2+ montmorillonite.'S8 electron spin resonance (ESR), and resonance Infrared (IR), Raman studies indicated that two types of species are formed a t the surface: poly-p-phenylene cation radical (type 11) and the reduced form, poly-p-phenylene (type I).4*8One may readily interconvert type I and I1 species by adding or removing water. The type I1 species exists on dry clay surfaces, while type I exists on hydrated clays. The adsorption and oxidation of monosubstituted benzene onto clay surfaces have also been reported.2*%*9J0 The predominant reaction of these molecules appears to be dimerization via coupling of the cation radical with a neutral molecule (e.g., anisole or aniline). These coupling reactions were found to occur para to the substituent. When l,4-disubstituted benzenes are adsorbed to clays, many were found to be stable to self-condensation,ll e.g., in experiments involving 4,4'-disubstituted biphenyls for which cation radicals and dications (dependent upon substitution) are stable on the clay surface.2J2 Unlike anisole and aniline, which undergo clayinduced dimerization, phenol appears to oligomerize on clay surfacesq2t3Soma et al. have proposed that this oligomerization occurs via a phenoxy radical.I3 Sawhney and co-workersrecently reported that 2,6-dimethylphenolforms oligomers when adsorbed and oxidized on montmorillonite.3 They found that the degree of reaction, as well as adsorption, was affected by the identity and concentration of the exchanged cation. Analysis of the products from these reactions showed that oligomerization occurred via both C-C and C-0 coupling. Thus far, the reactivity of para-substituted phenols on clays has been relatively ~nexplored.~'Such substrates might prove to be excellent probes to define substituent effects and tolerance limits for oligomerization. The increased number of substituents may allow for stabilization of a monomeric reactive intermediate, which would be more easily characterized than the oligomeric material generally obser~ed.~s We therefore have studied the clay-induced oxidations of adsorbed ortho- and parasubstituted phenols on montmorillonite. The reactions of these phenols were probed by visible absorption spectroscopy and by chemical analysis of the products extracted from phenol-treated clays. While useful information would surely also be obtained from ultraviolet (2) Soma, Y.; Soma, M.; Harada, I. J. Phys. Chem. 1985,89,738-742. (3) (a) Sawhney, B. L. Clays Cloy Miner. 1985,33,123-127. (b)Sawh-
ney, B. L.; Kdoki, R K.; Isaacaon, P. J.; Gent, M. P. N. Cloys Cloy Miner. 1984,32,108-114. (c) Isaacson, P. J.; Sawhney,B. L. Clay Minerals 1983, 18,253-265. (4) Soma, Y.; Soma, M.; Harada, I. J.Phys. Chem. 1984,843,30343038. (5) Eaetman, M. P.; Patterson, D. E.;PanneU, K. H. Clays Clay Miner. 1984,32, 327-333. ( 6 ) Soma, Y.; Soma, M.; Harada, I. Chem. Phys. Lett. 1983,99,153156. (7) Rupert, J. P. J. Phys. Chem. 1973, 77,784-790. (8) (a)Fenn, D. B.; Mortland, M. M.; P i n ~ v a i aT. , J. Clays Clay Miner. 1973,21, 315-322. (b) Pinnavaia, T. J.; Hall, P. L.; Cady, S. S.; Mortland, M. M. J.Phys. Chem. 1974,78,994-999. (c) Pinnavaia, T. J.; Mortland, M. M. J. Phys. Chem. 1971, 75,3957-3962. (9) Vansant, E. F.; Yariv, S. J. Chem. SOC.,Faraday Trans. 1 1977, 73,1815-1824. (10) Thompson, T. D.; Moll, W. F., Jr. Clays Clay Miner. 1973,21,337350. (11) Soma, Y.; Soma, M.; Harada, I. Chem. Phys. Lett. 1983,94,475478. (12) Kovar, L.; DellaGuardia, R.; Thomas, J. K. J. Phys. Chem. 1984, 88,3595-3599. (13) Soma, Y.; Soma, M.; Harada, I. J. Contaminant Hydrology 1986, I , 95-106.
Sackett and Fox
absorptions, significant absorption and light scattering by the clay itself made quantitative spectral monitoring of this region impossible. 2,4,6-Trimethylphenol (2,4,6TMP) was used as the principal substrate. 2,6-Di-tertbutyl-4-methylphenol (2,6-DB-4-MP),and 2,4,6-tri-tertbutylphenol (2,4,6-TBP) were studied to determine the effect of the steric bulk of the alkyl substituents. 33Dimethylphenol (3,5DMP) was also studied since its para position is sterically hindered, rather than directly substituted. The reactivity of these substrates could then be compared with the reactivity of 2,6-dimethylphenol(2,6DMP), for which adsorptive reactivity on clay has been previously de~cribed.~ Adsorption studies were performed on H+- and Cu2+exchanged clays. Cu2+-exchanged clays are known to oxidize organic molecules,' whereas the oxidizing ability of H+-exchanged clays has not been determined. In addition, we sought to correlate the redox potential with the reactivity of the substituted phenols and to determine the effect of clay hydration on its oxidative reactivity. Montmorillonite, a 2:l layered expanding clay particularly suited to adsorption and intercalation of organic materials,14was used exclusively. Because organic molecules readily penetrate the clay layers, montmorillonite allows one to probe the effect of the steric bulk of substituents on the extent of adsorption and reactivity of phenols.
Results 1. Visible Absorption Spectroscopy. Cu2+-and H+exchanged clay films were formed by drying aqueous clay suspensions onto a microscope slide. Two methods for phenol adsorption were employed: (1)dipping of films into a hexane solution of the substituted phenol (aereated conditions) or (2) exposure of the film to the substituted phenol vapor in a controlled-atmosphere chamber (anaerobic conditions). Absorption spectra were obtained of the substituted phenols intercalated into these films. Two techniques were employed since each method had a characteristic inherent deficiency. Adsorption from dilute solution under an inert atmosphere was inconvenient since it requires large quantities of several solvents to be effectively degassed. On the other hand, vapor-phase adsorption of phenols in the presence of air failed to effect significant reaction. Even after several weeks of exposure of the film to 2,6-DMP, the most reactive phenol, the intensity of the 440-nm band (Cu2+ exchange) never exceeded 0.01 absorbance unit. Both techniques made quantitative mass balance in product isolation studies difficult to establish, a complication which has also been noted by previous investigators.' A. Phenol Adsorption on Clay Films in the Presence of Oxygen. Upon exposure to 2,6-DMP, both H+- and Cu2+-exchangedfilms turned bright yellow within minutes, Figure 1. 2,4,6-TMP formed a colored species more slowly, with each exchanged film turning red within 2 h. The absorption maxima were found at 520 and 522 nm for the H+ and Cu2+ films, respectively. With 2,6DB-4-MP, formation of the colored species was exceedingly slow; a faint pink color was evident after 72 h of exposure to Cu2+- or H+-exchanged montmorillonite. Although 2,4,6-TBP and 3,5-DMP caused slight coloration of the clay, the attainable optical density was too low to obtain a welldefined absorption spectrum. Absorption characteristics of the composite clay/phenol films are summarized in Table I. It was observed that the rate of formation of the (14) van Olphen, H. An Introduction to Clay Colloid Chemistry, ed. 2; Wiley: New York, 1977; Chapter 5.
Langmuir, Vol. 6, No. 7, 1990 1239
Adsorption of Phenols onto Montmorillonite
lhVCLENGTH
IAVELE1ISTH (m)
Figure 1. 2,6-DMP adsorbed onto H+-exchangedfilm from hexane (-). 2,6-DMP adsorbed onto Cu2+-exchangedfilm from hexane (- - -). Absorbance units are arbitrary;see Table I for spectral data. Table I. Summary of Visible Absorption Spectral Data for Phenols Adsorbed to H+-and Cua+-Exchanged Montmorillonite exchanged reaction phenol cation ,,A, nm absorbance, au conditions" 2,6-DMP H+ 436 0.26 A cu2+ cu2+ cu2+
2,4,6-TMP
H+ cu2+ cu2+
cu2+
cu2+ 2,6-DB-4-MP
cu2+
cuz+
cu2+ 2,4,6-TBP
cu2+
cuz+ 3,5-DMP
cu2+ cu2+
2,6-DMBQ 2,6-DMHQ
cu2+ cu2+
cu2+
440 446 418 (s) 444 418 (5) 520 520 512 488 512 488 514 488 520 520 446 520 448 516 441 518 443 518 518 444 436 518 440 516 442
(nn)
Figure 2. Cu2+ film exposed to 2,4,6-TMP under an inert atmosphere. Spectra taken immediately after removal from phenol vapor (-), after l-h exposure to air (- - -), and after 72-h exposure to air (-). Absorbance units are arbitrary; see Table I for spectral data.
0.48 0.34 0.76 0.04 0.09 0.11 0.09 0.21 0.20 0.40 0.44 0.02 0.07 0.03 0.10 0.05 0.07 0.05 0.08 0.07 0.09 0.11 0.03 0.02 0.12 0.18
a Reaction conditions: (A) Adsorption of phenol from hexane solution in the presence of air. (B)Adsorption of phenol from the vapor phase under an inert atmosphere. After films were treated by method B they were exposed to air for (C) 1 h, (D) 3 h, (E) 12 h, (F)24 h, (G)48 h, and (H)72 h.
red species decreased in the order 2,4,6-TMP > 2,6-DB4-MP > 2,4,6-TBP. In these experiments, the intensity of the observed adsorption bands was somewhat dependent on the length of time the film was exposed to the phenols. However, it was found that the length of time the films were allowed to "develop" in the air after exposure to the phenol had a greater influence on the observed absorbance. B. Phenol Adsorption onto Clay Films in the Absence of Oxygen. None of the phenols reacted with the H+-exchanged clays under an inert atmosphere, since
,
B
s
w
w
I
B
I
IAVELENGTH h)
Figure 3. Cu2+ film exposed t o 3,5-DMP under an inert atmosphere. Spectrum taken immediately after removal from phenol vapor (-) and after 48-h exposure to air (- - -). Adsorbance units are arbitrary; see Table I for spectral data. no color change could be observed when these films were exposed to the phenols for long periods of time. Cu2+exchanged films very rapidly turned yellow (Ama = 446 nm) when exposed to 2,6-DMP under an inert atmosphere. When this same film was subsequently exposed to the air, the intensity of the absorption maximum continued to increase slowly (Table I). The absorption spectrum for 2,4,6-TMP adsorbed to Cu2+-exchangedfilms under Ar is shown in Figure 2. When exposed to air, the color of the film changed from pink to orange, with a band at 488 nm growing in intensity relative to the 512-nm band. The rate of change in intensity of the bands, as well as the initial rate of their appearance, depended on the water content of the clay, see below. Initially, 3,5-DMP adsorbed to Cu2+-exchanged films showed a single absorption band occurring at 518 nm, Figure 3, which upon exposure to air was accompanied by a short wavelength band at 444 nm which grew in after 2 days. 2,6-DB-4-MP and 2,4,6-TBP showed similar behavior: adsorption to the clay in the absence of air gave a single band at ca. 520 nm, and after several days of exposure to air, a new band at ca. 446 nm grew in (Table I). C. Quinone and Hydroquinone Adsorption into Clay Films in the Absence of Oxygen. To evaluate the possible formation of quinones upon adsorption,3b two related compounds, 2,6-dimethyl-p-benzoquinone (2,6DMBQ) and the corresponding hydroquinone (2,6-
1240 Langmuir, Vol. 6, No. 7, 1990
Sackett and Fox
Table 11. Product Distribution from Reaction of 2,4,6-TMP on Cua+-ExchangedClay entry
reaction conditions,a extraction solvent
2,4,6-TMP
1
A, MeCN
30
1
2 3 4 5
B, MeCN
28
19 27 0 0 1
3
C, MeCN
A, EbO
6
2,6-DMBQ
B, Et20
54 48
C, Et20
29
product distribution, % yieldb 1 2 3 0 0
