Electrochemical Attachment of Organic Groups to Carbon Felt

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Electrochemical Attachment of Organic Groups to Carbon Felt Surfaces Estelle Coulon and Jean Pinson* Laboratoire d’Electrochimie Mole´ culaire, Unversite´ Paris 7-Denis Diderot, 75251 Paris Cedex 05, France

Jean-Dominique Bourzat, Alain Commerc¸ on, and Jean Pierre Pulicani* Centre de Recherche de Vitry, Aventis Pharma, 13 Quai Jules Guesdes, BP14, 94403, Vitry sur Seine Cedex, France Received March 29, 2001. In Final Form: July 10, 2001 Electrochemical reduction of diazonium salts and oxidation of carboxylates on high surface felt electrodes permits functionalization of the surface of the felt. The functionalization has been ascertained by different methods including elemental analysis, X-ray photoelectron spectroscopy, scanning microscopy/energydispersive spectrometry, and IR spectroscopy. The loadings which are obtained are of the same order of magnitude as those of commercial ion-exchange resins or of resins used for combinatorial chemistry.

Introduction The surface of carbons can be derivatized by two kinds of reactions. Strong oxidation methods such as the electrochemical oxidation at the decomposition potential of water or treatments in strongly and sometimes hot oxidizing media such as nitric acid or potassium permanganate have been used mainly for the formation of oxygenated functions on the surface of carbon fibers, but in these ways, a large variety of oxygenated functions are formed on the surface and therefore the derivatization is nonspecific. Other methods that have recently been reviewed1 use less drastic methods and lead to the binding of a specific organic group to the surface of the electrode. These methods include the oxidation of primary and secondary amines;2 the oxidation of alcohols,3 of carboxylates,4 and of hydrazides;5 and the reduction of diazonium salts.6 They lead to the strong covalent attachment of -NHR, -OR, -CH2R, -NHR, and -aryl groups, respectively, on the surface of carbons. Different forms of carbon have been derivatized including glassy carbon,2a,b,3,4,5a,b highly oriented pyrolytic graphite (HOPG),6b,g,j carbon fibers,2a,f and glassy carbon spheres (2.05 m2 g-1 packed under high pressure in a chromatographic column2c). Except for carbon fibers, all these carbon materials have relatively low or very low specific surfaces (m2 g-1). However, among the different applications of the derivatized carbon surfaces that have been tested, some of them * To whom correspondence should be addressed. E-mail: pinson@ paris7.jussieu.fr; [email protected]. (1) Downard, A. J. Electroanalysis 2000, 12, 1085. (2) (a) Barbier, B.; Pinson, J.; De´sarmot, G.; Sanchez, M. J. Electrochem. Soc. 1990, 137, 175. (b) Deinhammer, R. S.; Ho, M.; Anderegg, J. W.; Porter, M. D. Langmuir 1994, 10, 1306. (c) Deinhammer, R. S.; Ting, E. Y.; Anderegg, J. W.; Porter, M. D. J. Electroanal. Chem. 1993, 362, 295. (d) Downard, A. J.; bin Mohamed, A. Electroanalysis 1999, 11, 418. (e) Tanaka, H.; Aramata, A. J. Electroanal. Chem. 1997, 437, 29. (f) Antoniadou, S.; Jannakoudakis, A. D.; Jannadoukakis, P. D.; Theodoridou, E. J. Appl. Electrochem. 1992, 22, 1060. (3) (a) Maeda, H.; Yamauchi, Y.; Hoso, M.; Li, T. X.; Yamaguchi, E.; Kasamatsu, M.; Ohmori, H. Chem. Pharm. Bull. 1994, 42, 187. (b) Guo, B.; Anzai, J.; Osa, T. Chem. Pharm. Bull. 1996, 44, 860. (4) Andrieux, C. P.; Gonzalez, F.; Save´ant, J. M. J. Am. Chem. Soc. 1997, 119, 4292. (5) (a) Novall, W. B.; Wipf, D. O.; Kuhr, W. G. Anal. Chem. 1998, 70, 2601. (b) Hayes, M. A.; Kuhr, W. G. Anal. Chem. 1999, 71, 1720.

such as the preparation of ion exchange materials2f or of catalysts2e,6p,q would find an increased interest through the use of high specific carbon surfaces. Other applications could also be developed such as carbon supports for combinatorial chemistry that could provide an alternative to the commercial resins. The surface modification of carbon blacks with high specific surfaces (230-560 m2 g-1) has previously been achieved by simply mixing the carbon black with a solution of a diazonium salt; however, the surface concentration of the organic groups was found to be approximately 1/10 of what is obtained when the derivatization is performed electrochemically. The modified carbon blacks obtained in this way have been used for the preparation of inks, coatings,7 and aerogels.8 It is the purpose of the present paper to demonstrate that it is possible to functionalize carbon felts of high specific surface and to fully characterize the nature and the surface concentration of the attached molecules. Experimental Section The large specific surface carbon felt Actitex 1500-1 was obtained from Actitex (Levallois-Perret, France). Its nominal specific surface is 1500 m2 g-1. The carbon felt was cut into disks (22 mm diameter, 2 mm thick) that were functionalized in a circulation cell where the electrolysis solution (acetonitrile (ACN) + 0.1 M NBu4BF4 + the substrate at concentrations from 1.3 to 5 mM) percolates (at (6) (a) Delamar, M.; Hitmi, R.; Pinson, J.; Save´ant, J. M. J. Am. Chem. Soc. 1992, 114, 5883. (b) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Save´ant, J. M. J. Am. Chem. Soc. 1997, 119, 201. (c) Bourdillon, C.; Delamar, M.; Demaille, C.; Hitmi, R.; Moiroux, J.; Pinson, J. J. Electroanal. Chem. 1992, 336, 113. (d) Delamar, M.; De´sarmot, G.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Save´ant, J. M. Carbon 1997, 35, 801. (e) Saby, C.; Ortiz, B.; Champagne, G. Y.; Be´langer, D. Langmuir 1997, 13, 6805. (f) Ortiz, B.; Saby, C.; Champagne, G. Y.; Be´langer, D. J. Electroanal. Chem. 1998, 455, 75. (g) Liu, Y. C.; McCreery, R. L. J. Am. Chem. Soc. 1995, 117, 11254. (h) Ray, K., III; McCreery, R. L. Anal. Chem. 1997, 69, 4680. (i) Kuo, T.; McCreery, R. L. Anal. Chem. 1999, 71, 1553. (j) Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 6534. (k) Downard, A. J.; Roddick, A. D. Electroanalysis 1995, 7, 376. (l) Downard, A. J.; Roddick, A. D.; Bond, A. M. Anal. Chim. Acta 1995, 317, 303. (m) Downard, A. J.; Roddick, A. D. Electroanalysis 1979, 9, 693. (n) Downard, A. J. Langmuir 2000, 16, 9680. (o) Dequaire, M.; Degrand, C.; Limoges, B. J. Am. Chem. Soc. 1999, 121, 6946. (p) Liu, S.; Tang, Z.; Shi, Z.; Niu, L.; Wang, E.; Dong, S. Langmuir 1999, 15, 7266. (q) Liu, J.; Cheng, L.; Liu, B.; Dong, S. Langmuir 2000, 16, 7471.

10.1021/la010486c CCC: $20.00 © 2001 American Chemical Society Published on Web 10/05/2001

Attachment of Organic Groups to Carbon Felt room temperature) through the felt. This type of cell has been described by Moinet and al.9 The disks are used as the working electrode; they are pressed in the central compartment (up to eight disks). A contact is made to the disks by a graphite paper ring that also serves to render the cell leakproof. The two carbon counter electrodes (one upstream and one downstream) are separated from the working electrode by ion exchange membranes (anionic for the oxidation of aryl acetates and cationic for the reduction of diazonium salts). A Ag/AgCl reference electrode is placed in the compartment of the working electrode. A potentiostat maintains the potential of one of the counter electrodes (current I1 flows through this electrode) so as to obtain the desired potential difference between the working and the reference electrode; the current of the second counter I2 is maintained by a galvanostat under automatic control by I1 so that I1/I2 ) 1.9 where I1 and I2 are the currents flowing to or from the upstream or downstream counter electrode, respectively.9 At the end of the electrolysis, the disks were rinsed as described in the text. Microanalysis, IR spectrometry, and energy-dispersive spectrometry (EDS) were performed by the analysis department of Aventis (Centre de Recherche de Vitry-Alfortville). The scanning electron microscopy images were obtained with a field emission secondary microscope JEOL JSM6300F equipped with a secondary electron detector and an EDS Link/Oxford with a germanium crystal and an ATW window. The mass percent of the elements is normalized to 100% (% of the elements ) 100% of the sample). Infrared spectra were obtained by mechanically reducing the felt to a powder and mixing this powder with KBr to produce a pellet. The spectra were recorded on a Nicolet 510 FTIR spectrometer. X-ray photoelectron spectroscopy (XPS) analyses were recorded with an Escalab MK1 spectrometer using a magnesium anode producing X-ray photons KR (Mg) of 1253.6 eV. Elemental percentages xi were obtained from xi ) ((Ai/si)/ ∑i(Ai/si) where Ai and si are respectively the peak area of the detected element and the sensitivity coefficient deduced from the study of stoichiometric compounds. GC/MS spectra with electronic impact (70 eV) were obtained with a Hewlett-Packard instrument. Surface concentrations were calculated in the following way: taking -C6H4CH2Cl (M ) 125.5 g/mol) as an example (Table 3, line 2), the % Cl being x, for 100 g of modified felt the mass of organic groups is morg ) (x/35.5) 125.5 and that of the carbon mcarb ) 100 - morg from which one obtains the surface coverage assuming a specific surface of 1500 m2 g-1: (x/35.5)/(mcarb 1500 × 104) mol cm-2. In the same example, the loading is calculated as for commercial Merrifield resins: 10x/35.5 mol Cl/g of modified felt. The solvent used for the electrolysis was ACN Merck spectroscopic grade containing 0.1 M NBu4BF4 (Fluka purum). 4-Bromobenzenediazonium tetrafluoroborate, 1, and 4-(bromomethyl)phenylacetic acid, 3, were obtained from Aldrich. (7) (a) Belmont, J. A.; Amici, R. M.; Galloway, P. Patent PCT Int. Appl. WO 96 18688 A1 (to Cabot Corp.). (b) Belmont, J. A. Patent PCT Int. Appl. WO 96 18690 A1 (to Cabot Corp.). (c) Belmont, J. A.; Reed, T. F. Patent PCT Int. Appl. WO 96 18674 A1 (to Cabot Corp.). (d) Belmont, J. A.; Johnson, J. E.; Adams, C. E. Patent PCT Int. Appl. WO 96 18695 A1 (to Cabot Corp.). (e) Belmont, J. A. Patent US 5672198 (to Cabot Corp.). (f) Johnson, J. E.; Belmont, J. A. Patent PCT Int. Appl. WO 97 47692 A1 (to Cabot Corp.). (g) Belmont, J. A.; Adams, C. E. Patent US. 57 13988 A (to Cabot Corp.). (h) Mahmud, K.; Belmont, J. A.; Adams, C. E.; Foster, J. K. Patent PCT Int. Appl. WO 97 47698 A1 (to Cabot Corp.). (i) Reed, T. F.; Mahmud, K. Patent PCT Int. Appl. WO 98 34960 A1 (to Cabot Corp.). (j) Adams, C. E.; Belmont, J. A.; Amici, R. M. Patent US 56 98016 A (to Cabot Corp.). (k) Belmont, J. A.; Amici, R. M.; Galloway, C. P. Patent US 5851280 (to Cabot Corp). (l) Belmont, J. A.; Adams, C. E. Patent PCT Int. Appl. WO 9907,794 (to Cabot Corp.). (m) Adams, C. E.; Belmont, J. A. Patent US 5885355 (to Cabot Corp.). (n) Whitehouse, R. S.; Devonport, W.; Warley, R. L.; Rawalpally, T. R., Tu, H. Patent PCT Int Appl. WO 99 23,174 (to Cabot Corp.). (o) Palumbo, S. P. Patent PCT Int. Appl. WO 0053681A1 (to Cabot Corp.). (p) Cooke, J. M.; Galloway, C. P.; Bissell, M. A.; Adams, C. E.; Yu, M. C.; Belmont, J. A.; Amici, R. M. Patent US 61 109994 A (to Cabot Corp.). (8) Smith, D. M.; Maskara, A.; Boes, U. J. Non-Cryst. Solids 1998, 225, 254. (9) (a) Landaez-Machado, H. J.; Darchen, A.; Moinet, C. Electrochim. Acta 1980, 25, 1321. (b) Jacob, G.; Moinet, C. Bull. Soc. Chim. Fr. 1983, 291. (c) Peltier, D.; Moinet, C. Bull. Soc. Chim. Fr. 1968, 2657. (e) Ng, P. K. J. Electrochem. Soc. 1981, 128, 792. (f) Pollard, R.; Trainham, J. A. J. Electrochem. Soc. 1983, 130, 1531.

Langmuir, Vol. 17, No. 22, 2001 7103 4-Chloromethylbenzenediazonium tetrafluoroborate, 2, was obtained from 4-aminobenzyl chloride hydrochloride that is obtained from 4-aminobenzyl alcohol in a one-pot reaction. In a round-bottom flask, 1 g of 4-aminobenzyl alcohol (8.13 × 10-3 mol, 1 equiv) and 1.48 g of tetramethylammonium chloride (58.94 × 10-3 mol, 1.1 equiv) are placed in 10 mL of concentrated (36%) HCl (0.12 mol, 14.4 equiv). The orange solution that is obtained is stirred at room temperature, in the dark and under argon for 24 h. A white precipitate is formed. The reaction medium is cooled to 0 °C, and 10 mL of a cooled (0 °C) aqueous solution of tetrafluoroboric acid (34%) (4.76 × 10-2 mol, 5.9 equiv) is added. Stirring is maintained for 15 min, and 0.62 g of sodium nitrite (8.99 × 10-3 mol, 1.1 equiv) dissolved in the minimum amount of demineralized water is added; the white precipitate dissolves, while a new beige precipitate appears. Stirring is maintained for another 20 min, and the solution is placed overnight in the refrigerator. After filtration, the precipitate is washed with a 5% solution of sodium tetrafluoroborate, methanol, and diethyl ether to give 0.9 g of a pale yellow powder (47% yield). 1H NMR, 200 MHz, δ ppm: 5.01 (s, 2H), benzylic protons; 8.02 (d, 2H, J ) 8.8 Hz) and 8.68 (d, 2H, J ) 8.8 Hz), aromatic protons β to the diazonium function and to the benzylic carbon, respectively. To prepare the carboxylate 3 that is the starting product of the electrolysis, a solution of the acid was neutralized with 1 equiv of tetramethylammonium hydroxide (25% solution in methanol).

Results and Discussion Whatever the use of the modified carbon felt as an ion exchange material, as a support for combinatorial chemistry, or as a catalyst, the larger the specific area, the larger the loading and the more efficient the material. We therefore choose Actitex 1500-1, a commercial carbon felt with a large specific area of 1500 m2 g-1. As shown in Figure 1, the scanning electron microscopy of the carbon felt reveals a material made of approximately 10 µm carbon fibers. All these fibers have the same morphology; they are composed from smaller threads of approximately 1 µm that are welded together. An empty canal sometimes appears in the middle of the fiber, and the number of threads varies somewhat from one fiber to the other; a typical elemental analysis of the felt is shown in Table 1. We have performed a large number of these analyses, and the sum of all elements never reaches 100%; besides, there is some variation (about 4%) from one analysis to the other. Besides carbon, one can observe nitrogen, hydrogen, and oxygen that probably stem from surface oxygenated functions as well as some fluorine of unknown origin. The IR spectrum presents two large and strong bands at 1560 and 1140 cm-1 that correspond to the vibrations of the carbon structure (Figure 2a). The EDS analysis of the surface of the fibers is presented in Table 2; some variation in the analysis is observed from one spot to the other of the same sample (about 3%). The surface of the felt does not show any halogen signal; as the modifications that will be described below will often involve the attachment of halogens to the surface, it will be possible to link any such EDS signal to the result of the surface modification. Although the intimate structure of the carbon fibers which compose the felt has not been investigated as this is not the subject of this paper, they are likely composed, as other fibers, of small graphitic domains with many defects in the hexagonal planes and in the edges. The derivatization of the carbon felt (Scheme 1, where C1, C2, and C3 represent modified carbon surfaces whether glassy carbon or carbon felt) was achieved with three different reactions: (i) the electrochemical reduction of 4-bromobenzenediazonium (reaction 1) and (ii) of 4-chloromethylbenzenediazonium (reaction 2) and (iii) the electrochemical oxidation of 4-bromomethylphenylacetate (reaction 3). Reactions 2 and 3 would lead to carbon

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Figure 1. Scanning microscope images of Actitex 1500-1 carbon felt. Table 1. Elemental Analysis of Actitex 1500-1 Carbon Felt %C

%H

%N

%O

%F

86.5

0.64

0.98

3.04

0.20

analogues of the Merrifield’s resin used in combinatorial chemistry (chloromethylated styrene/divinylbenzene copolymer). These reactions were first tested on glassy carbon electrodes, and the electrolysis potentials were determined from the voltammetric curves: 1 and 2 present at 0.2 V s-1 a broad irreversible reduction wave at -0.03 and -0.35 V/SCE (saturated calomel electrode), respectively6b (reduction of the diazonium salt and grafting of the aryl radical). These waves decrease and then disappear upon repetitive scanning as usually observed during the grafting reaction of diazonium salts or of carboxylates.6 3 is oxidized along two irreversible waves which both correspond to the oxidation of the carboxylate and the grafting of the decarboxylated moiety.4 The modified glassy carbon electrodes C1, C2, and C3 obtained upon electrolysis of 1, 2, and 34,6 were examined. C1 presents an irreversible reduction wave at -2.5 V/SCE which corresponds to the cleavage of the C-Br bond. C2 presents an irreversible reduction wave at -2.65 V/SCE, located at a potential close to that of the reduction wave of 4-chloromethylbenzene (-2.24 V/SCE), both being related to the cleavage of the C-Cl bond. Grafting of the glassy carbon plate (C2) was ascertained by XPS; two peaks (2.6%, see experimental part) were observed at 200 and 202 eV that can

Figure 2. Ir spectra of (a) Actitex 1500-1, (b) after derivatization with 4- bromobenzene diazonium, and (c) 4-bromotoluene. Table 2. EDX Analysis of Actitex 1500-1 Carbon Felt %C

%O

%P

Na

87.8

11.4

1.5

0.1

be assigned to Cl2p3/2 and Cl1/2, respectively. The modified glassy carbon electrode C3 shows an irreversible reduction wave at -1.77 V/SCE which is related to the cleavage of the C-Br bond, and the XPS spectrum evidences two peaks at (1.6%) at 70 and 71 eV corresponding to Br3d5/2 and Br3/2, respectively, along with a minute amount of Br- at 68 and 69 eV. We then attempted the modification of the carbon felt with the same reaction using the circulation cell described in the Experimental Section. The conditions that have been used are more than equivalent to those that permitted the attachment of aryl groups to HOPG surfaces and should therefore permit the derivatization of both edges and planes of the graphitic domains of the fibers. The results of the optimized grafting experiments are given in Table 3. The chemical analysis shows that the attachment of the molecules does indeed take place. The surface

Attachment of Organic Groups to Carbon Felt Scheme 1

concentrations that can be calculated are about 1/10 of what is observed on HOPG ((12-16) × 10-10 mol cm-2)6b,e,g,h most likely indicating that part of the surface (microporosity) is not accessible to the molecules. However, the capacity of the material can be favorably compared with that of commercial Merrifield’s resins where the loading is 1-4 mmol g-1. In the case of reactions 1 and 2, it was observed that even without potential, some grafting of the surface occurred; the amount of bromine was found to be 7.50% (see also Table 4) and that of chlorine was 3.15%, that is, approximately half of what is obtained by electrochemical reduction; this is in line with the findings of Belmont et al.7 who found it possible to attach aryl radicals to the surface of carbon black without electrochemistry. The reaction was performed at different potentials, and it was found that the % Br (from 10.8 to 14.6) measured from the modified carbon felt did not show any significant variation with the potential. For both reaction 1 and reaction 3, increasing the electrolysis time above 2 h did not increase the amount of bromophenyl groups attached to the surface. A very important point in the process is the homogeneity of the surface modification. For this purpose, we have analyzed in the case of reaction 1 (reduction of a diazonium salt) and reaction 3 (oxidation of an aryl acetate) the different disks to see if the amount of grafting did vary along with the position of the disk in the stream and within the disk; the results are summarized in Tables 4 and 5. In the case of reaction 1, two disks were placed in the cell and we analyzed different places of the disks close to the border (external), in the center (internal), and between (intermediate) for both upstream and downstream disks. Table 4 also includes the results of two different experiments to give an idea of the reproducibility of the grafting reaction. This table shows that the grafting is homogeneous over the entire surface of the disk; the difference in the percent of Br never exceeds 10%. We also performed the same type of experiment using reaction 3 (oxidation of 4-bromophenylacetate). In this case, six disks were pressed in the cell and we analyzed each of them. Table 5 reports the percent of Br for these disks. The experiment was repeated with 16 disks (using a longer cell), and the grafting was found to be homogeneous over all the disks. Under the conditions of Table 3, the % Br of three different experiments varied from 6.32 to 7.01%.

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An important point of the process is the rinsing of the felt after derivatization, first to be certain that the organic groups are strongly attached to the surface and second to be sure that no substrate, solvent, or supporting electrolyte, which could interfere during further reactions, remains adsorbed on the surface. We found that the best procedure involves rinsing of the felt with acetonitrile, followed by rinsing in an ultrasonic bath for 30 min with successively acetonitrile, chloroform, acetone, and finally diethyl ether; the sample is then placed in an oven at 40 °C for 24 h. Longer rinsing did not further decrease the amount of bromine (reactions 1 and 3) or chlorine (reaction 2) indicating that the molecules are strongly attached to the surface as previously observed with other carbon material6 and that there is no abrasion of the surface during the ultrasonic treatment. The use of an ultrasonic bath makes the operation shorter; indeed, 10 h was necessary with a Soxhlet apparatus. Final rinsing with diethyl ether is mandatory as chloroform remains adsorbed on the surface even after 24 h in the oven but is completely removed by ether. Elemental analysis is the most convenient method to ascertain the presence of the attached organic group on the surface provided it possesses an atom different from C, H, N, and O which are present on the carbon surface. As shown in Tables 3, 4, and 5, due to the large specific area of Actitex 1500-1 the % Br or Cl attached to the surface is well above the sensitivity limit of the method and permits one to follow the yield of the grafting reactions as the parameters are changed. To further demonstrate the attachment of organic groups to the surface, we examined the modified surfaces with XPS, IR spectrometry, and scanning microscopy/EDS. The results of the XPS analysis are gathered in Table 6. They clearly confirm the attachment of the halogenated molecules to the carbon. Such a confirmation can also be obtained by scanning microscopy/EDS. First, the morphology of the fibers does not change significantly from the underivatized felt indicating that no thick polymeric layer is attached to the surface; then one does not observe any halogen on the plate of the microscope as is the case when the products are only adsorbed on the carbon surface and desorb under the high vacuum of the microscope. The EDS analysis confirms the presence of the halogens as shown in Table 7. However, some Cl is detected in the samples with 4-bromobenzene diazonium and 4-bromomethylphenylacetate; it is likely strongly attached to the surface as no contamination of the microscope plate is observed. By comparison with untreated carbon felt (Figure 2a) and 4-bromotoluene (Figure 2c), the infrared spectroscopy of the derivatized carbon felt (Figure 2b) permits observation of not only the halogen but also the aromatic rings attached to the surface (Figure 2b). The results are reported in Table 8. It is possible to observe the benzylic CH2 for C2 and the aromatic rings for both C1 and C2. It is also interesting to observe the vibrations characteristic of a 1,4-disubstitution indicating that the substitution on the molecule has not been perturbed during the attachment process; this rules out the formation of a polymeric layer by further substitution on the free (meta) position of the phenyl ring of C1 and C2. At the end of the electrolysis of 4-bromophenylacetate, the solution becomes yellow, and we decided to analyze this solution. After evaporation and washing of the yellow solid with an aqueous acidic solution, two products are isolated and analyzed by mass spectrometry. The first one (M ) 296) corresponds to a cyclic dimer of 4-bro-

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Table 3. Results of the Grafting Experimentsa starting chemical, concentration (mM)

reduction/ oxidation

E (V vs SCE)

electrolysis time (h)ours

elemental analysis

1010 Γc,d (mol cm-2)

loading (mmol g-1)d

1, 3.10 2, 1.36 3,b 4.99

reduction reduction oxidation

-0.8 -0.8 +1.2

2 2 2

% Br, 14.64 % Cl, 6.13 % Br, 7.01

1.71 1.47 0.70

1.83 1.73 0.88

a Eight disks of Actitex 1500-1 carbon felt pressed together as the working electrode; the flow of the peristaltic pump was 0.1 mL min-1. As the tetramethylammonium salt (see the Experimental Section). c Surface concentration. d The method for obtaining these numbers is given in the Experimental Section.

b

Table 4. Homogeneity of the Grafting for Reaction 1 % Br elemental analysis

internal upstream

intermediate upstream

external upstream

internal downstream

intermediate downstream

intermediate downstream

blanka expt 1b expt 2b

7.75 14.08 15.06

7.35 14.47 14.21

7.48 13.61 13.55

7.53 13.99 13.72

7.51 13.83 13.97

7.37 14.30 13.69

a

Without applied potential. b Two different experiments.

Table 5. Homogeneity of the Grafting for Reaction 3a disk 1 disk 2 disk 3 disk 4 disk 5 disk 6 % Br elemental analysis a

4.61

4.51

4.56

4.50

4.77

4.60

Disk 1 is upstream, and disk 6 is downstream. Table 6. XPS Analysis of the Modified Surfaces

untreated carbon felt

C1

C2

C3

% Cl or Br: a

% Br: 2.3b

% Cl: 1.7

% Br: 0.45

a

: traces. b Mean of three measurements.

surface decreasing the amount of bromine measured by elemental analysis, XPS, or EDS and forming ester bonds on the surface; indeed, characteristic vibrations of esters can be detected by IR spectrometry. A consequence of the formation of these ester bonds shown in Chart 1 is that the surface may become chemically inhomogeneous and as the esters bond are easily cleaved yields of further reactions could be lowered. A possibility would be to decrease the amount of tetramethylammonium hydroxide used for the neutralization of 4-bromophenyl acetic acid, but if the acid is only half neutralized, the % Br decreases from 5.67 to 2.91 (all other conditions being the same).

Table 7. EDS Analysis of the Derivatized Carbon Felt %C %O % Cl % Br

C1a

C2b

C3c

78.7 2.7 0.6 16.9

83 10.2 1.2 d

82.3 14.1 0.5 1.4

a Also 0.9% P. b Also 1.1% P, 0.1% Ca, and 0.1% S. c Also 1% P and 0.5% Si. d : traces.

Table 8. Infrared Spectra of Modified Carbon Felts νaCH2 νsCH2 νCdC aromatic rings δCH aromatic rings para substituted aromatic ring

C1 ν cm-1

C2 ν cm-1

1460 1040, 980 820

2923 2853 1503 1016, 1084 808

Chart 1

mophenylacetic acid, while the second one (M ) 444) corresponds to a trimer (Chart 1). These compounds are formed by nucleophilic substitution of the benzylic bromine by the carboxylate 3. If this reaction occurs in solution, it probably also occurs on the

Conclusion We have shown that it is possible through both the reduction of diazonium salts and the oxidation of phenylacetates to attach organic groups to the surface of high surface carbon felt and to fully characterize these groups. Let us now suppose that we use 1 g of Actitex 1500-1 and that the halogen atom of C1, C2, or C3 is replaced by a molecule of molecular weight 200. Let us also suppose that in a further reaction this molecule is cleaved from the substrate; it is possible to show that 0.37, 0.35, and 0.18 g can be recovered from C1, C2, and C3, respectively (provided that the same coverage as in Table 3 is achieved and that the yield of the reaction is quantitative). These quantities are sufficient for NMR and mass analysis; therefore, these modified felts could be used for solid phase supported combinatorial chemistry, in particular C2, which is an analogue of the Merrifield resin. They could also be used for the depollution of industrial effluents, provided complexing agents are bonded to the support; work is in progress to achieve this goal. Many other applications can be thought of for this material including the preparation of catalysts. Acknowledgment. We are highly indebted to Genevieve Lepinasse and Michel Moreau (Aventis) for the careful interpretation of EDS and FTIR spectra, respectively. We thank Ms Bounine and Kochanek (Aventis) for performing the elemental analysis on the felts. We acknowledge the help of Michel Druet (Laboratoire d’Electrochimie Mole´culaire) for designing and building the electronic equipment and of Carole Bilhem and Pascal Bargiela (Itodys, Universite´ Paris 7) for recording the XPS spectra. LA010486C