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The electrochemical oxidation of tetrahydrofuran in sulphuric acid solution
Electrochimica Acta 44 (1999) 3295±3301
The electrochemical oxidation of tetrahydrofuran in sulphuric acid solution Christiana Avgousti a, Nickos Georgolios a, George Kyriacou b, George Ritzoulis a,* a Laboratory of Physical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 540 06 Thessaloniki, Greece Laboratory of Inorganic Chemistry, Department of Chemical Engineering, Aristotle University of Thessaloniki, 540 06 Thessaloniki, Greece
b
Received 20 July 1998; received in revised form 7 December 1998
Abstract The electrochemical behaviour of THF on Pt in H2SO4 using cyclic voltammetry was examined. The products were identi®ed with GC±MS. The two basic products were found to be g-butyrolactone and 2-hydroxytetrahydrofuran with current eciencies up to 54.4 and 41.2% correspondingly. In addition, the UPD of Tl and Cu were shown to inhibit THF oxidation, since the deposits probably block active sites on the electrode surface. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Tetrahydrofuran; g-Butyrolactone; Succinic acid; Electrochemical oxidation; GC±MS; Underpotential deposition
1. Introduction The electrochemical oxidation of small organic molecules as CH3OH, HCOOH and HCHO on noble metal electrodes has been extensively studied [1±3]. Small cyclic ethers do not easily oxidize [4]. Tetrahydrofuran (THF) is a typical cyclic ether which is soluble in aqueous solutions. In addition, it is a common solvent in chemical reactions. Its chemical behaviour and especially its chemical oxidation has been extensively studied [5±7]. Dierent results appear so far in the literature concerning the electrochemical
* Corresponding author. Fax: +30-31-997-844. E-mail address: [email protected] (G. Ritzoulis)
behaviour of THF [8±11]. Although the main oxidation product seems to be dierent, it is generally accepted that the oxidation products are generated after adsorption of intermediates and indirect reactions following them. Wermeckes et al. [8] found that a selective oxidation of THF takes place and 2-hydroxytetrahydrofuran (THF±OH) is formed when high current densities were used. Only traces of other products as g-butyrolactone (g-BL) and succinic acid were detected. Kirsanova et al. [9] showed that using potentiodynamic methods THF oxidation on Pt led to g-BL. Finally, Horanyi et al. [10] found using potentiostatic polarization measurements and radiotracer adsorption, that on a platinized platinum electrode in aqueous acidic medium, THF is oxidized to succinic acid.
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 0 5 1 - 1
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Fig. 1. Cyclic voltammograms for a Pt electrode in 0.5 M H2SO4 for three dierent THF concentrations; Curves: a for 0.1 M, b for 0.5 M, c for 1 M. Sweep rate u=100 mV sÿ1.
Concerning the catalytic eect of foreign metal deposits on the oxidation of small organic molecules (as HCOOH, CH3OH and HCHO) on Pt a lot of work has been carried out and several theories have been proposed to explain the electrocatalytic properties of these modi®ed electrodes [3,12]. In the present paper the electrochemical oxidation of THF, in the presence of sulphuric acid, was studied using cyclic voltammetry. The oxidation of g-BL was also studied, since it is assumed to be an intermediate product of THF oxidation. 2. Experimental The polycrystalline Pt electrode was a wire of active area 0.181 cm2. Before each experiment the electrode was heated to redness in a gas-oxygen ¯ame and quenched with ultra pure water. Thus, it was isolated from the ambient by a residual ®lm of water and could
easily be transferred to the electrochemical cell without contamination. The cell and the other glassware used were cleaned before each experiment by soaking overnight in a dilute solution of KMnO4 in concentrated sulphuric acid. The counter electrode was a Pt grid electrode and the reference electrode was platinized Pt gauze in the same solution (0.5 M H2SO4) (RHE). All potentials in this paper are referred to the RHE. All solutions were prepared from sulphuric acid (Merck p.a.) using ultrapure water supplied by a Millipore system. The salts Tl2SO4 and CuSO4 used in the experiments were of reagent grade quality. THF was supplied by Fluka (Puriss p.a.) and was distilled before each experiment. All solutions were degassed by bubbling in Ar for about 20 min; an Ar stream was passed over the solutions during the measurements. The cyclovoltammetric set-up included a Bank Elektronik Wenking Potentioscan POS 73 and a Linseis LY 1400 x±y recorder.
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Fig. 2. Cyclic voltammograms of 0.5 M THF on Pt in 0.5 M H2SO4 for dierent positive potential limits. Sweep rate u=100 mV sÿ1.
Fig. 3. Cyclic voltammogram of 0.5 M g-BL on Pt in 0.5 M H2SO4. Sweep rate u=100 mV sÿ1.
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Fig. 4. Chromatogram from the analysis of the analyte of THF oxidation at 2.1 V for 24 h.
The identi®cation of products was performed using a Hewlett Packard HP5890 gas chromatograph combined with a HP5989 MS engine using a NHP-5MS 30 m length, 0.25 mm diameter, after extraction from the aqueous layer by ether. The temperature was programmed between 35 and 2808C. The acids were identi®ed with their retention time using a DIONEX 4500i ion chromatograph with a PRP-X300 anion exclusion column with 0.01 N H2SO4, at a ¯ow rate of 0.5 ml/ min. 3. Results and discussion The electrochemical oxidation of THF on a Pt polycrystalline electrode in 0.5 M H2SO4 for three dierent THF concentrations, is shown in Fig. 1. All three cyclic voltammograms display two oxidation regions, one during positive sweep with peaks at ca. 0.9 and 1.3 V and another during negative sweep at 0.6 V. Since an increase of the THF concentration leads to a respective increase of the peaks mentioned above, it is obvious that these oxidation waves are due to THF oxidation. In order to elucidate the oxidation process of THF the cyclic voltammograms of THF (C = 0.5 M, in 0.5 M H2SO4) with dierent positive potential limits were ®rst taken (Fig. 2). By decreasing the positive potential limit, a relative decrease on the oxidation wave at 0.6
V, during negative sweep, was observed. In addition, its Ep was shifted to more positive potentials. This wave almost disappeared when the positive potential limit was 0.9 V. Moreover, varying the start potential up to 0.3 V, no eect was observed on the oxidation peaks of THF. As it is well known, at potentials between 0.8 and 1.5 V the platinum surface is covered with PtOH and PtOx species [13±15]. At this potential region, a possible adsorption of THF occurs forming complexes with the PtOx species, as (PtO±THF)s [8]. During negative sweep the oxides (PtOx) reduction takes place and a large oxidation wave at 0.6 V appears. There is a similarity between the THF oxidation and other small organic molecules as HCOOH, where during a cathodic scan a large oxidation wave also appears [16]. Pt oxides formed during the anodic scan probably block active sites on the Pt surface and prevent THF oxidation until they are totally oxidised at more positive potentials, where the oxidation wave lies at 1.3 V. Hence, it is clear that THF oxidation is dependent on intermediate formation occurring at this potential region. This behaviour has been observed during oxidation of other small organic molecules such as HCOOH and HCHO [1,16,17]. As mentioned [8], g-BL is a probable product in the THF oxidation process. Hence, its electrochemical behaviour under the same conditions was studied. Its
C. Avgousti et al. / Electrochimica Acta 44 (1999) 3295±3301
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Table 1 Current eciency of the main products of THF oxidation at a Pt anode for electrolysis time 24 h, H2SO4 0.5 M Anodic potential (V versus RHE)
2.0 2.1 2.2 2.3
Current density (mA cmÿ2)
1.25 10.0 17.5 70.0
cyclovoltammetric behaviour is shown in Fig. 3. It is seen, that g-BL is oxidized in an analogue way with THF. During negative sweep there is an oxidation wave at ca. 0.5 V. By reducing the positive potential limit, a relative decrease of the current peak was observed. This behaviour shows that the wave at 0.5 V is also dependent on the anodic potential limit. This analogue cyclic voltammetric behaviour indicates a similar oxidation mechanism for g-BL. In order to identify the products, analysis was carried out using GC±MS. A typical gas chromatogram of a solution of THF after electroxidation at 2.1 V for
Current eciency THF±OH
g-butyrolactone
succinic acid
3.9 9.1 37.1 41.2
0.7 48.2 34.8 54.4
nd 0.5 1.1 2.3
24 h, is shown in Fig. 4. The mass spectra indicated that the peak at 7.5 min is ascribed to 2-hydroxytetrahydrofuran (THF±OH) and the peak at 12.69 min to g-BL, with a matching quality greater than 90%. The area ratio of these two peaks is about 4:1. The peak at 11.33 min is attributed to butanodial. The peaks after 21 min, because of their molecular weight, are probably due to dimerization products of tetrahydrofuran. The other peaks appearing in the chromatogram were impossible to identify, due to their low concentration. Furthermore, succinic acid and formic acid were detected by ion chromatography comparing their
Fig. 5. Cyclic voltammograms for a Pt electrode in 0.5 M H2SO4 +0.5 M THF in the presence of 10ÿ3 M Tl+ (- - -) and 10ÿ5 M Tl+ (ÐÐÐ). Sweep rate u=100 mV sÿ1.
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retention time with known standards. The electrooxidation of g-BL under the same conditions (2.1 V for 24 h) showed products as formic and succinic acid. Table 1 shows the current eciencies (CEs) for the main oxidation products at dierent anodic potentials. The CE for THF±OH increases with increasing anodic potential from 3.9% at 2.0 V to about 41% at 2.3 V. g-BL was found to be the main oxidation product at 2.3 V with maximum CE 54%. Finally, succinic acid was detected only at anodic potentials higher than 2.1 V, with low CEs as shown in Table 1. The results con-
cerning succinic acid are in accordance with those previously reported by Wermeckes et al. [8,18], but the ratio of CEs between THF±OH and g-BL is dierent. This behaviour could be attributed to dierent experimental conditions (e.g. electrolysis time, structure of electrodes, etc.). Taking into account the experimental results and the mechanisms proposed so far in the literature [8± 10,19,21] a possible path of electrochemical oxidation could be as follows. The ®rst step includes the adsorption of THF on the Pt surface, that has been covered with oxides (Pt±O)s as shown in the following scheme:
This chemisorbed intermediate species (II) is then oxidised after reaction with OH radicals, produced from water oxidation at the electrode surface, and gives THF±OH as the main oxidation product [10].
An alternative oxidation process has been proposed via cation radical formation at lead anodes [20]. THF±OH in sequence is oxidised to g-BL, as it is generally accepted for the alcohol oxidation to ketone [19,22].
As mentioned before, succinic acid was detected in the products both in THF and g-BL oxidation. This fact, which is in accordance with their voltammetric behaviour, indicates that their oxidation follows some common steps. A possible path for the opening of the ring of g-BL is that proposed by Wermeckes et al. [8].
Since butanodial was detected in our experiments, a parallel oxidation path via THF±OH should be accepted, as already proposed [8,10]. This path includes the opening of the ring according to the following scheme:
Finally, the eect of underpotentially deposited Tl adatoms on THF oxidation was also examined. Fig. 5 shows the eect of Tl adatoms on THF oxidation. At 10ÿ3 M Tl+ there is no evidence of THF oxidation, whereas the peaks attributed to the UPD and desorption of Tl and to the system Tl+ _ Tl3+ + 2e appear. At 10ÿ5 M Tl+, the waves of Tl are suppressed, but the oxidation peak of THF during negative sweep, although small enough, is seen. In conclusion, Tl adatoms seem to inhibit THF oxidation at concentrations 10ÿ3 M or higher, whereas at lower concentrations no signi®cant catalytic eect was observed. These adatoms probably block active sites of the Pt surface preventing surface oxide formation and the following adsorption of THF [17]. As accepted, surface oxides act as a source of oxygen in the oxidation process [3]. Since the oxides cannot be formed, the THF oxidation is prohibited. It is obvious that THF oxidation does not occur on the metal already deposited (Tl). A similar behaviour was also observed in the case of Cu adatoms, where the cyclic voltammogramm seems more complicated, since the bulk dissolution of Cu is also involved.
C. Avgousti et al. / Electrochimica Acta 44 (1999) 3295±3301
References [1] S. Campbell, R. Parsons, J. Chem. Soc. Faraday Trans. 88 (1992) 833. [2] T. Zerihun, P. Grundler, J. Electroanal. Chem. 441 (1998) 57. [3] R. Parsons, T. VanderNoot, J. Electroanal. Chem. 257 (1988) 9. [4] V.D. Parker, in: M.M. Baizer (Ed.), Anodic Oxidation of Oxygen-containing Compounds in Organic Electrochemistry, Marcel Dekker, New York, 1973, p. 531. [5] H. Muller, Tetrahydrofuran, in: Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, Vol. A26, 1995. [6] D.G. Lee, M. Van den Engh, Can. J. Chem. 50 (1972) 3129. [7] S. Baskaran, S. Chandrasekaran, Tetrahedron 31 (1990) 2775. [8] B. Wermeckes, F. Beck, H. Schulz, Tetrahedron 43 (1987) 557. [9] A.I. Kirsanova, M.G. Smirnova, Elektrokhimiya 15 (1979) 819. [10] G. Horanyi, E.M. Rizmayer, Electrochim. Acta 30 (1985) 767.
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[11] R.R. Adzic, in: H. Gerischer, C.W. Tobias (Eds.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 13, Wiley, New York, 1984. [12] S.A. Campbell, C. Bowes, R.S. McMillan, J. Electroanal. Chem. 284 (1990) 195. [13] W. Vielstich, Fuel Cells, Wiley, New York, 1970. [14] S. Gilman, in: A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 2, Marcel Dekker, New York, 1967, pp. 112±192. [15] J. Giner, Z. Electrochem. 63 (1959) 386. [16] A. Capon, R. Parsons, J. Electroanal. Chem. 44 (1973) 239. [17] M.D. Spasojevic, R.R. Adzic, A.R. Despic, J. Electroanal. Chem. 109 (1980) 261. [18] B. Wermeckes, F. Beck, Chem. Ber. 120 (1987) 1679. [19] D. Kyriakou, D. Jannakoudakis, in: Electrocatalysis for Organic Synthesis, J. Wiley and Sons, 1986, pp. 57±61. [20] M. Suguwara, M. Sato, T. Osuda, H. Yamamoto, Chem. Abstr. 82 (1975) 97398q. [21] R.R. Adzic, D.N. Simic, A.R. Despic, D.M. Drazic, Z. Phys. Chem. NF 98 (1975) 95. [22] V. B Vassiliev, V.S. Bagotzky, V.A. Korsman, L.S. Grinberg, L.S. Kavesky, V.R. Polishchyak, Electrochim. Acta 27 (1982) 919.
The electrochemical oxidation of tetrahydrofuran in sulphuric acid solution Christiana Avgousti a, Nickos Georgolios a, George Kyriacou b, George Ritzoulis a,* a Laboratory of Physical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 540 06 Thessaloniki, Greece Laboratory of Inorganic Chemistry, Department of Chemical Engineering, Aristotle University of Thessaloniki, 540 06 Thessaloniki, Greece
b
Received 20 July 1998; received in revised form 7 December 1998
Abstract The electrochemical behaviour of THF on Pt in H2SO4 using cyclic voltammetry was examined. The products were identi®ed with GC±MS. The two basic products were found to be g-butyrolactone and 2-hydroxytetrahydrofuran with current eciencies up to 54.4 and 41.2% correspondingly. In addition, the UPD of Tl and Cu were shown to inhibit THF oxidation, since the deposits probably block active sites on the electrode surface. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Tetrahydrofuran; g-Butyrolactone; Succinic acid; Electrochemical oxidation; GC±MS; Underpotential deposition
1. Introduction The electrochemical oxidation of small organic molecules as CH3OH, HCOOH and HCHO on noble metal electrodes has been extensively studied [1±3]. Small cyclic ethers do not easily oxidize [4]. Tetrahydrofuran (THF) is a typical cyclic ether which is soluble in aqueous solutions. In addition, it is a common solvent in chemical reactions. Its chemical behaviour and especially its chemical oxidation has been extensively studied [5±7]. Dierent results appear so far in the literature concerning the electrochemical
* Corresponding author. Fax: +30-31-997-844. E-mail address: [email protected] (G. Ritzoulis)
behaviour of THF [8±11]. Although the main oxidation product seems to be dierent, it is generally accepted that the oxidation products are generated after adsorption of intermediates and indirect reactions following them. Wermeckes et al. [8] found that a selective oxidation of THF takes place and 2-hydroxytetrahydrofuran (THF±OH) is formed when high current densities were used. Only traces of other products as g-butyrolactone (g-BL) and succinic acid were detected. Kirsanova et al. [9] showed that using potentiodynamic methods THF oxidation on Pt led to g-BL. Finally, Horanyi et al. [10] found using potentiostatic polarization measurements and radiotracer adsorption, that on a platinized platinum electrode in aqueous acidic medium, THF is oxidized to succinic acid.
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 0 5 1 - 1
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C. Avgousti et al. / Electrochimica Acta 44 (1999) 3295±3301
Fig. 1. Cyclic voltammograms for a Pt electrode in 0.5 M H2SO4 for three dierent THF concentrations; Curves: a for 0.1 M, b for 0.5 M, c for 1 M. Sweep rate u=100 mV sÿ1.
Concerning the catalytic eect of foreign metal deposits on the oxidation of small organic molecules (as HCOOH, CH3OH and HCHO) on Pt a lot of work has been carried out and several theories have been proposed to explain the electrocatalytic properties of these modi®ed electrodes [3,12]. In the present paper the electrochemical oxidation of THF, in the presence of sulphuric acid, was studied using cyclic voltammetry. The oxidation of g-BL was also studied, since it is assumed to be an intermediate product of THF oxidation. 2. Experimental The polycrystalline Pt electrode was a wire of active area 0.181 cm2. Before each experiment the electrode was heated to redness in a gas-oxygen ¯ame and quenched with ultra pure water. Thus, it was isolated from the ambient by a residual ®lm of water and could
easily be transferred to the electrochemical cell without contamination. The cell and the other glassware used were cleaned before each experiment by soaking overnight in a dilute solution of KMnO4 in concentrated sulphuric acid. The counter electrode was a Pt grid electrode and the reference electrode was platinized Pt gauze in the same solution (0.5 M H2SO4) (RHE). All potentials in this paper are referred to the RHE. All solutions were prepared from sulphuric acid (Merck p.a.) using ultrapure water supplied by a Millipore system. The salts Tl2SO4 and CuSO4 used in the experiments were of reagent grade quality. THF was supplied by Fluka (Puriss p.a.) and was distilled before each experiment. All solutions were degassed by bubbling in Ar for about 20 min; an Ar stream was passed over the solutions during the measurements. The cyclovoltammetric set-up included a Bank Elektronik Wenking Potentioscan POS 73 and a Linseis LY 1400 x±y recorder.
C. Avgousti et al. / Electrochimica Acta 44 (1999) 3295±3301
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Fig. 2. Cyclic voltammograms of 0.5 M THF on Pt in 0.5 M H2SO4 for dierent positive potential limits. Sweep rate u=100 mV sÿ1.
Fig. 3. Cyclic voltammogram of 0.5 M g-BL on Pt in 0.5 M H2SO4. Sweep rate u=100 mV sÿ1.
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Fig. 4. Chromatogram from the analysis of the analyte of THF oxidation at 2.1 V for 24 h.
The identi®cation of products was performed using a Hewlett Packard HP5890 gas chromatograph combined with a HP5989 MS engine using a NHP-5MS 30 m length, 0.25 mm diameter, after extraction from the aqueous layer by ether. The temperature was programmed between 35 and 2808C. The acids were identi®ed with their retention time using a DIONEX 4500i ion chromatograph with a PRP-X300 anion exclusion column with 0.01 N H2SO4, at a ¯ow rate of 0.5 ml/ min. 3. Results and discussion The electrochemical oxidation of THF on a Pt polycrystalline electrode in 0.5 M H2SO4 for three dierent THF concentrations, is shown in Fig. 1. All three cyclic voltammograms display two oxidation regions, one during positive sweep with peaks at ca. 0.9 and 1.3 V and another during negative sweep at 0.6 V. Since an increase of the THF concentration leads to a respective increase of the peaks mentioned above, it is obvious that these oxidation waves are due to THF oxidation. In order to elucidate the oxidation process of THF the cyclic voltammograms of THF (C = 0.5 M, in 0.5 M H2SO4) with dierent positive potential limits were ®rst taken (Fig. 2). By decreasing the positive potential limit, a relative decrease on the oxidation wave at 0.6
V, during negative sweep, was observed. In addition, its Ep was shifted to more positive potentials. This wave almost disappeared when the positive potential limit was 0.9 V. Moreover, varying the start potential up to 0.3 V, no eect was observed on the oxidation peaks of THF. As it is well known, at potentials between 0.8 and 1.5 V the platinum surface is covered with PtOH and PtOx species [13±15]. At this potential region, a possible adsorption of THF occurs forming complexes with the PtOx species, as (PtO±THF)s [8]. During negative sweep the oxides (PtOx) reduction takes place and a large oxidation wave at 0.6 V appears. There is a similarity between the THF oxidation and other small organic molecules as HCOOH, where during a cathodic scan a large oxidation wave also appears [16]. Pt oxides formed during the anodic scan probably block active sites on the Pt surface and prevent THF oxidation until they are totally oxidised at more positive potentials, where the oxidation wave lies at 1.3 V. Hence, it is clear that THF oxidation is dependent on intermediate formation occurring at this potential region. This behaviour has been observed during oxidation of other small organic molecules such as HCOOH and HCHO [1,16,17]. As mentioned [8], g-BL is a probable product in the THF oxidation process. Hence, its electrochemical behaviour under the same conditions was studied. Its
C. Avgousti et al. / Electrochimica Acta 44 (1999) 3295±3301
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Table 1 Current eciency of the main products of THF oxidation at a Pt anode for electrolysis time 24 h, H2SO4 0.5 M Anodic potential (V versus RHE)
2.0 2.1 2.2 2.3
Current density (mA cmÿ2)
1.25 10.0 17.5 70.0
cyclovoltammetric behaviour is shown in Fig. 3. It is seen, that g-BL is oxidized in an analogue way with THF. During negative sweep there is an oxidation wave at ca. 0.5 V. By reducing the positive potential limit, a relative decrease of the current peak was observed. This behaviour shows that the wave at 0.5 V is also dependent on the anodic potential limit. This analogue cyclic voltammetric behaviour indicates a similar oxidation mechanism for g-BL. In order to identify the products, analysis was carried out using GC±MS. A typical gas chromatogram of a solution of THF after electroxidation at 2.1 V for
Current eciency THF±OH
g-butyrolactone
succinic acid
3.9 9.1 37.1 41.2
0.7 48.2 34.8 54.4
nd 0.5 1.1 2.3
24 h, is shown in Fig. 4. The mass spectra indicated that the peak at 7.5 min is ascribed to 2-hydroxytetrahydrofuran (THF±OH) and the peak at 12.69 min to g-BL, with a matching quality greater than 90%. The area ratio of these two peaks is about 4:1. The peak at 11.33 min is attributed to butanodial. The peaks after 21 min, because of their molecular weight, are probably due to dimerization products of tetrahydrofuran. The other peaks appearing in the chromatogram were impossible to identify, due to their low concentration. Furthermore, succinic acid and formic acid were detected by ion chromatography comparing their
Fig. 5. Cyclic voltammograms for a Pt electrode in 0.5 M H2SO4 +0.5 M THF in the presence of 10ÿ3 M Tl+ (- - -) and 10ÿ5 M Tl+ (ÐÐÐ). Sweep rate u=100 mV sÿ1.
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retention time with known standards. The electrooxidation of g-BL under the same conditions (2.1 V for 24 h) showed products as formic and succinic acid. Table 1 shows the current eciencies (CEs) for the main oxidation products at dierent anodic potentials. The CE for THF±OH increases with increasing anodic potential from 3.9% at 2.0 V to about 41% at 2.3 V. g-BL was found to be the main oxidation product at 2.3 V with maximum CE 54%. Finally, succinic acid was detected only at anodic potentials higher than 2.1 V, with low CEs as shown in Table 1. The results con-
cerning succinic acid are in accordance with those previously reported by Wermeckes et al. [8,18], but the ratio of CEs between THF±OH and g-BL is dierent. This behaviour could be attributed to dierent experimental conditions (e.g. electrolysis time, structure of electrodes, etc.). Taking into account the experimental results and the mechanisms proposed so far in the literature [8± 10,19,21] a possible path of electrochemical oxidation could be as follows. The ®rst step includes the adsorption of THF on the Pt surface, that has been covered with oxides (Pt±O)s as shown in the following scheme:
This chemisorbed intermediate species (II) is then oxidised after reaction with OH radicals, produced from water oxidation at the electrode surface, and gives THF±OH as the main oxidation product [10].
An alternative oxidation process has been proposed via cation radical formation at lead anodes [20]. THF±OH in sequence is oxidised to g-BL, as it is generally accepted for the alcohol oxidation to ketone [19,22].
As mentioned before, succinic acid was detected in the products both in THF and g-BL oxidation. This fact, which is in accordance with their voltammetric behaviour, indicates that their oxidation follows some common steps. A possible path for the opening of the ring of g-BL is that proposed by Wermeckes et al. [8].
Since butanodial was detected in our experiments, a parallel oxidation path via THF±OH should be accepted, as already proposed [8,10]. This path includes the opening of the ring according to the following scheme:
Finally, the eect of underpotentially deposited Tl adatoms on THF oxidation was also examined. Fig. 5 shows the eect of Tl adatoms on THF oxidation. At 10ÿ3 M Tl+ there is no evidence of THF oxidation, whereas the peaks attributed to the UPD and desorption of Tl and to the system Tl+ _ Tl3+ + 2e appear. At 10ÿ5 M Tl+, the waves of Tl are suppressed, but the oxidation peak of THF during negative sweep, although small enough, is seen. In conclusion, Tl adatoms seem to inhibit THF oxidation at concentrations 10ÿ3 M or higher, whereas at lower concentrations no signi®cant catalytic eect was observed. These adatoms probably block active sites of the Pt surface preventing surface oxide formation and the following adsorption of THF [17]. As accepted, surface oxides act as a source of oxygen in the oxidation process [3]. Since the oxides cannot be formed, the THF oxidation is prohibited. It is obvious that THF oxidation does not occur on the metal already deposited (Tl). A similar behaviour was also observed in the case of Cu adatoms, where the cyclic voltammogramm seems more complicated, since the bulk dissolution of Cu is also involved.
C. Avgousti et al. / Electrochimica Acta 44 (1999) 3295±3301
References [1] S. Campbell, R. Parsons, J. Chem. Soc. Faraday Trans. 88 (1992) 833. [2] T. Zerihun, P. Grundler, J. Electroanal. Chem. 441 (1998) 57. [3] R. Parsons, T. VanderNoot, J. Electroanal. Chem. 257 (1988) 9. [4] V.D. Parker, in: M.M. Baizer (Ed.), Anodic Oxidation of Oxygen-containing Compounds in Organic Electrochemistry, Marcel Dekker, New York, 1973, p. 531. [5] H. Muller, Tetrahydrofuran, in: Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, Vol. A26, 1995. [6] D.G. Lee, M. Van den Engh, Can. J. Chem. 50 (1972) 3129. [7] S. Baskaran, S. Chandrasekaran, Tetrahedron 31 (1990) 2775. [8] B. Wermeckes, F. Beck, H. Schulz, Tetrahedron 43 (1987) 557. [9] A.I. Kirsanova, M.G. Smirnova, Elektrokhimiya 15 (1979) 819. [10] G. Horanyi, E.M. Rizmayer, Electrochim. Acta 30 (1985) 767.
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