Electrochimica Acta 49 (2004) 397–403
Electrochemical investigations of the oxidation–reduction of furfural in aqueous medium Application to electrosynthesis P. Parpot a , A.P. Bettencourt a , G. Chamoulaud b , K.B. Kokoh b , E.M. Belgsir b,∗,1 b
a Departamento de Quimica, Universidade do Minho, Largo do Paço, 4719 Braga Codex, Portugal Laboratoire de Catalyse en Chimie Organique, Equipe Electrocatalyse, Université de Poitiers, UMR 6503, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France
Received 19 December 2002; received in revised form 6 May 2003; accepted 16 August 2003
Abstract The present study concerns the electrochemical properties of furfural in aqueous medium on noble (Au and Pt) and non-noble (Pb, Cu and Ni) metal electrodes. The anodic and cathodic reactions are investigated by cyclic voltammetry on Au, Pt and Ni electrodes and during prolonged electrolyses on Pt, Pb and Cu in order to find the optimum conditions for a paired electrosynthesis. Anodic reactions are controlled by diffusion in the range of the stability of the solvent (water). Beside these limits, the gas evolution competes with the conversion of furfural. The best conditions for preparative electrooxidation (Ni anode, 0.5 M NaOH, j = 0.8 mA cm−1 ) gave furoic acid in a 80% yield and furfuryl alcohol was obtained by electroreduction in a 55% yield on Cu cathodes at pH 10 and 30 mA cm−2 . © 2003 Elsevier Ltd. All rights reserved. Keywords: Electrosynthesis; Platinum; Furfurylic alcohol; Furfural; Furoic acid
1. Introduction Furfural is at an intermediate oxidation state between the corresponding alcohol and acid. Therefore it can be reduced into furfurylic alcohol or oxidized into furoic acid:
In aqueous medium, the electrooxidation may be straightforward. Under controlled conditions, the aldehyde is hydrated and the oxidation consists of simple dehydrogenation process:
∗ Corresponding author. Tel.: +33-549-454-881; fax: +33-549-470-876. E-mail address:
[email protected] (E.M. Belgsir). 1 ISE member.
0013-4686/$ – see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.08.021
The electroreduction of furfural is more complex and can lead to varied products such as alcohols, hydrocarbons or pinacols (electrohydrodimerisation). In principle, the protonation of the carbonyl makes the furfural more easily reducible, however the primary product, i.e. furfurylic alcohol is readily resinified in acidic media. Among a wide variety of furfural derivatives furfurylic alcohol and furoic acid are very interesting intermediates in pharmaceutical industry, perfumery or polymer industry [1]. There are very few reports concerning the electrochemistry of furfural particularly on noble metal electrodes [2]. Much emphasis has been placed on investigating the electrooxidation of furfural on nickel anodes [3–7]. The only preparative experiments were reported by Schäfer [6] on nickel anode in K2 CO3 aqueous solution with a moderate yield (62%) and good selectivity (84%). The reduction of furfural was investigated on Pt/Pd, Zn and Cd cathodes (W.O. Reichsfeld, et al. and M.E. Manzhelei, et al. as cited in [8]). Yields were moderate to satisfactory but we failed to reproduce these interesting results. In the present study, the electrochemical behaviour of furfural is investigated on gold, platinum, nickel, copper and lead electrodes. The reaction kinetics and the mechanisms
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are evaluated by cyclic voltammetry and prolonged electrolyses experiments.
2. Experimental All the experiments were carried out in aqueous solutions using ultrapure water (18 M cm, Barnted E-pure system). The Analytical grade supporting electrolytes were purchased from Merck. Furfural (99%) and its derivatives were supplied by Aldrich. Before each experiment, furfural was distilled under vacuum and the solutions were degassed by bubbling “ultrapure” argon (Air Liquide). An argon stream was kept over the solution during the experiments. The voltammograms were recorded in a thermostated three electrodes cell using a waveform generator and a potentiostat (HI-TEK) and a XY recorder (Philips). The reference electrode was a saturated calomel electrode (SCE). The counter electrode was a Pt/Ir (10%) grid. Discs (∼1.5 mm2 ) of nickel, copper and lead, and spheres (∼0.1 mm2 ) of platinum and gold were used as working electrodes. The active area was estimated from the hydrogen adsorption for the platinum electrode and from the reduction of the oxygenated species for the gold electrode. For the non-noble electrodes, the current densities are given versus the geometric area. As it is known, it is not easy to carry out CV experiments on nonnoble metal electrodes to investigate electrosynthetic reactions. For example the very active PbO2 layer obtained on Pb electrodes during the positive sweep oxidize furfural into CO2 . The oxidation waves of such processes (formation of PbO2 and furfural oxidation) are superimposed. Pb and Cu were used only as cathodes and the electrolysis current or potentials were applied immediately after the electrodes were in contact with the solution as to prevent any anodic corrosion. Before each experiment the noble metals electrodes were annealed and the non-noble electrodes were polished or pretreated in an acidic solution (HNO3 , 5%). The laboratory scale electrolyses were performed in a two compartments (0.1 dm3 ) batch cell separated by an cation exchange membrane (IONAC MC-3475 from Sybron Chemicals) and in a FM01-LC flow cell (ICI). In the later case, the anolyte and the catholyte separated by a cation exchange membrane (IONAC MC 3470) were pumped by a Watson–Marlow peristaltic pump. Electrochemical instrumentation consisted of a galvanostat/potentiostat AMEL 2055 monitored by a PC equipped with an AD/DA converter (PCI-MIO-16E-4, National Instrument) under Labview (National Instrument) environment. An automatic pH titrator (Black Stone) was used to maintain the pH at the same value during the long term electrolysis. Analysis of the composition of the reaction mixture was carried out by HPLC which consisted of a pump (PU 980, Jasco) and two detectors settled online (UV 975 and RI 1530, Jasco). The partition was performed on a RP-18 column (Lichrosorb, Merck, water/acetonitrile 85/15 + trifluoroacetic acid (TFA)
0.2%, 0.6 ml min−1 ) and the quantitative analyses were carried out using the “external standard” method. The aqueous solutions were also extracted using an accelerated one-step extractor and analysed by GC and GC–MS coupling (INCOS 500 and ITS 40, Varian). 3. Results and discussion 3.1. Redox activity of noble metal electrodes All the experiments were carried out at pH 9.4. In acidic media during the negative sweep, we have observed a relative deactivation of the electrodes due to the resinification reactions. 3.1.1. Gold electrode Furfural is weakly oxidized on gold under the standard conditions in NaHCO3 /Na2 CO3 buffer medium. At 273 K two small anodic peaks are developed during the positive scan (Fig. 1, peaks A and B). The second peak quoted B is not only connected to the so called “oxygen region” of gold, it is also attributed to the oxidation of furfural (see the effect of the concentration in Fig. 4). During the negative sweep and after the reduction of the surface, furfural is oxidised according to peak C. As expected, furfural is not reduced on gold in the range of the stability of the solvent. At higher temperature (353 K), the most apparent differences are the 16-fold increase in current density and the positive shift (+0.27 V) of the maximum (Fig. 1, peak A ). However, it is of interest to note that the oxidation wave always starts before the so-called “oxygen region” and that the maximum is reached when the metal surface begins to be oxidized after what the anodic signal decreased with the formation of AuO (Fig. 2). We conclude that the furfural oxidation on gold does not involve the strongly adsorbed oxygen species; on the contrary the metal oxides seem to act as inhibitors.
C'
2 mA cm
-2
B' A'
80˚C
C
-0,4
B
A
19˚C
0,0
0,4
0,8
1,2
E / V(SCE) Fig. 1. Effect of the temperature on the voltammogram of gold recorded in NaHCO3 /Na2 CO3 buffer (pH 9.4) at 50 mV s−1 and in the presence of 50 mM furfural.
P. Parpot et al. / Electrochimica Acta 49 (2004) 397–403
399
0.16 6
0.08 2
j / mA cm
j / mA cm
-2
4
-2
0.12
0.04 0 0.00 -0.8
-0.4
0.0
0.4
0.8
1.2
E / V(SCE) Fig. 2. Voltammograms of gold recorded in NaHCO3 /Na2 CO3 buffer (pH 9.4) at 50 mV s−1 . (- - -) The supporting electrolyte only, (—) the presence of 50 mM furfural.
To discuss the apparent energy of activation involved during peaks A and C, we made use of the fundamental equation: -2
log (j / mA.cm )
∂ log jp
H ∗ =− ∂(1/T)E,C 2.3R
0.0
1.0
1.5
2.0
2.5
-1
log (v / mV.s )
Fig. 3. Logarithm of the maximum current density j vs. logarithm of the variation of the potential sweep rate on gold electrode in the presence of 50 mM furfural in NaHCO3 /Na2 CO3 buffer (pH 9.4). (䊊) Peak A, (䊐) peak C.
0.80 C
0.60 -2
evaluated for peaks A and C are reported in Table 1. There is a good agreement with the results obtained on the effect of the temperature. Increasing the initial concentration of furfural results in an increase of all the current densities particularly during the reverse peak (Fig. 4).
-1.0
-1.5 0.5
j / mA cm
The effect of the temperature was investigated from 278 to 353 K and the results are reported in Table 1. On looking at the high value of H∗ (80.6 kJ mol−1 ), the processes involved during the peak A seem to be controlled by adsorption. During the reverse scan (Fig. 1, peak C ) the oxidation of furfural is rather controlled by diffusion ( H ∗ = 30.6 kJ mol−1 ). We have also investigated the variation of the sweep rate and found that the logarithms of the current densities, j, are directly proportional to the logarithms of the sweep rates (Fig. 3). The slopes: ∂ log(j) ξ= ∂ log(v) E,C,T
-0.5
0.40 B
0.20 A
Table 1 Electrocatalytic oxidation of furfural at a gold electrode in a carbonate buffer (pH 9.4) solution
Peak A Peak C
H∗ (kJ mol−1 )
∂ log(j)/δ log(v)
rds
80.6 30.6
0.90 0.50
Adsorption Diffusion
Apparent energy of activation and kinetic parameters.
0.00 -0.20 -0.8
-0.4
0.0
0.4 E / V(SCE)
0.8
1.2
Fig. 4. Effect of the variation of the furfural concentration on the voltammogram of gold recorded in NaHCO3 /Na2 CO3 buffer (pH 9.4) at 50 mV s−1 . (- - -) 10−2 M furfural, (—) 0.25 M furfural.
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P. Parpot et al. / Electrochimica Acta 49 (2004) 397–403 0.6 -0.5
-2
log (j / mA cm )
0.4
j / mA cm
-2
-1.0
-1.5
0.2 0.0 -0.2
-2.0
-0.4 -3.5
-3.0
-2.5
-2.0 -1.5 log (C/M)
-1.0
-0.5
Fig. 5. Plot of log j vs. the logarithm of furfural concentration at a gold electrode.
-0.5
∂ log(j) =p ∂ log(c) In the range of concentrations lower than 10−2 M, p is unity (1.03), and the current density is directly proportional to the concentration of furfural. 3.1.2. Platinum electrode As can be observed in Fig. 6, furfural is adsorbed on platinum during the positive sweep from the lower potential limit (lpl). The so called “hydrogen region” is decreased and the organic adsorbates inhibit the electrode activity towards the adsorption/desorption of protons and up to the “oxygen region”. Then a weak oxidation peak is observed on the platinum oxides region and the oxygen evolution is shifted towards more positive potentials compared to the voltammogram recorded in the supporting electrolyte (Fig. 6, dotted line). During the negative sweep and conversely to the gold electrode, no oxidation peak is noticed after the reduction of the surface. However furfural is reduced from −0.67 V(SCE). When it was studied as function of the lpl, the reduction processes involving furfural did not display any voltammetric peak because of the hydrogen evolution (competitive processes).
0.9 0.6
-2
0.3 j / mA cm
where j is the current density, n the number of electrons, F the Faraday constant, k the overall constant rate, p the overall reaction order and c the initial concentration of furfural. In Fig. 5, the logarithms of the current densities of peak C are plotted against the logarithms of the concentration of furfural. The overall reaction order, p, is represented by the slope calculated according to:
1.0
Fig. 6. Voltammogram of platinum electrode in carbonate buffer (pH 9.4) recorded at 50 mV s−1 without (· · · ) and with (—) 50 mM furfural.
The effect of the furfural concentration on the anodic activity resumed on the negative scan is interesting to evaluate because the oxidation processes are taking place on a bare surface. The concentration dependence on the current density for heterogeneous reaction rate control is: log(j) = log(nFk) + p log(c)
0.0 0.5 E / V(SCE)
0.0 -0.3 -0.6 -1.0
-0.5
0.0
0.5
1.0
1.5
E / V(SCE)
Fig. 7. Effect of the temperature on the voltammogram of platinum recorded in NaHCO3 /Na2 CO3 buffer (pH 9.4) and at 50 mV s−1 . (· · · ) 293 K, (—) 356 K.
The effect of the temperature on the j = f (E) curves is represented in Fig. 7. The maximum of the oxidation peak is shifted towards more negative potentials as result of the temperature activation and the current density at this maximum increases about three-fold. The value of the apparent energy of activation (23 kJ mol−1 ) suggests a kinetic controlled by diffusion (Table 2). However, the qualitative relationship between the slope (0.75) of the log(j) = f(log(v)) and the rds identification may be hard to derive. 3.2. Redox activity of nickel electrode The voltammogram of nickel in NaHCO3 /Na2 CO3 buffer medium and in the presence of 0.05 mol l−1 furfural, Table 2 Electrocatalytic oxidation of furfural at a platinum electrode in a carbonate buffer (pH 9.4) solution
Peak at 0.37 V(SCE)
H∗ (kJ mol−1 )
∂ log(j)/∂ log(v)
23
0.75
Apparent energy of activation and ∂ log(j)/∂ log(v) slope value.
P. Parpot et al. / Electrochimica Acta 49 (2004) 397–403
the furfural concentration decreased the hydrogen evolution became more competitive. Ninety-six percentage of the initial furfural was consumed after 4 h however only 70% were transformed into furfurylic alcohol. The analyses by GC–MS have shown the presence of other reaction products as:
8
6 -2
j(mA cm )
401
4
• the 2-methyl-furane coming from the total reduction of the carbonyl function:
2
0 -1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
E(V/SCE)
Fig. 8. Voltammogram of a nickel electrode in carbonate buffer (pH 9.4) recorded at 50 mV s−1 without (· · · ) and with (—) 50 mM furfural.
are given in Fig. 8. The voltammogram was recorded at 50 mV s−1 , at room temperature. During the positive variation of potential, two oxidation peaks are observed at 0.95 and at 1.2 V(SCE). The oxidation of furfural occurs in NiOOH region (j = 3.8 mA cm−2 ) near to the upper potential limit of the solvent. The increase of temperature leads to an enhancement of the overall current densities. At the same time, the oxidation peaks on the anodic sweep shift towards lower potentials. The value of activation energy, i.e. 34 kJ mol−1 , calculated from Arrhenius law suggests that the reaction of the furfural oxidation on nickel is controlled mainly by diffusion. 3.3. Preparative electrolyses 3.3.1. Reduction of furfural Electrolyses of furfural were carried out on Pt, Pb in acid medium and on Cu electrode in alkaline medium in the hydrogen evolution region (Table 3). On platinum cathode, the applied potential was set at −0.36 V(SCE). The current intensity decreased exponentially during the first 7 h (40%) and furfural was reduced selectively to furfurylic alcohol (98%) but the conversion remained very low (7.8%) after 24.5 h of electrolysis. On lead electrode in 0.1 M H2 SO4 , the reduction was more effective than on platinum. The electrolysis was performed at −1.5 V(SCE). Surprisingly, during operation the current intensity and the quantity of electricity were increasing while the faradaic yield was decreasing. In fact, when
• the 1,5-pentanediol coming from the opening of the furanic cycle:
• the hydroxyfuroin coming from electrohydrodimerization processes:
The reduction of furfural was also investigated on a copper electrode (64 cm2 ) using a filter press flow cell (FM01-LC from ICI) in 0.1 M NaOH electrolyte. Constant current electrolysis performed at −30 mA cm−2 allowed consuming 0.5 mol furfural in 4 h. However, the yield in furfurylic alcohol remained very low (16%). It was found that the main reaction products were coming from electrodimerization processes (see above). It was possible to reduce selectively (80%) furfural into furfuryl alcohol at a constant potential (0.65 V(SCE)) on Cu cathodes in a carbonate buffer (pH 10). The control of the electrolysis potential and the pH of the electrolyte allowed to obtain a good chemical yield (conversion and selectivity) (Table 3).In methanolic solution (0.1 M KOH) and at constant current intensity (I = −50 mA) the selectivity was found to be strongly dependent on the initial concentration of furfural (Table 4).
Table 3 Electrocatalytic reduction of furfural E or I Pt Cu Cu Pb
−0.36 V(SCE) −30 mA cm−2 −0.65 V(SCE) −1.5 V(SCE)
Medium 0.1 M HClO4 0.1 M NaOH Carbonate pH 10 0.1 M H2 SO4
Initial concentration of furfural 50 cm3
50 mM in 1 M in 100 cm3 100 mM in 50 cm3 50 mM in 50 cm3
t (h)
τ (%)
Qex /C
τ F (%)
τ C (%)
S (%)
24.5 4 5.5 4
9.1 100 90 96
345 27648 965 434
13 56 72 75
7.8 14 72 63
98 16 80 70
Effect of the nature of electrode material. t: electrolysis duration; τ: conversion yield; Qex : total charge passed; τ F : Faradaic yield vs. furfuryl alcohol; τ C : chemical yield; S: furfurylic alcohol selectivity.
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P. Parpot et al. / Electrochimica Acta 49 (2004) 397–403
Table 4 Reduction of furfural on copper in methanol + 0.1 M KOH
50
[Furfural] (mol dm−3 ) 0.5
50 69 26 2
23 19 63 12
40 30 C(mM)
Conversion yield (%) Selectivity in furfurylic alcohol (%) Selectivity in the main pinacol (%) Other products (%)
0.25
20 10
Effect of the initial concentration on the yields and the selectivity after 2 h of electrolysis.
0 0
50
100
150
t/min
Fig. 10. c = f(t) curves plotted during the electrolysis of 50 mM furfural at 2 mA cm−2 on nickel electrode and in carbonate buffer. (䊏) Furfurylic alcohol, ( ) furfural, (䊊) furoic acid.
C(mM)
30
20
10
0 0
3
6
9
12
15
t/h
Fig. 9. c = f (t) curves plotted during the electrolysis of 35 mM furfural at 0.15 V(SCE) on gold electrode and in carbonate buffer. (䊏) Furfural and (䊊) furoic acid.
The faradaic yields were lower during the electrolyses carried out in the presence of 0.25 mol dm−3 . As expected the electrohydrodimerisation processes were favoured when the initial concentration of furfural was increased while the conversion yield was decreased. 3.3.2. Oxidation of furfural The electrolysis of 35 mM furfural in alkaline solution at a controlled potential was carried out on a gold electrode.
The constant current electrolysis (2 mA cm−2 ) of 50 mM furfural was performed at a nickel anode in a buffer solution (0.5 M NaHCO3 /Na2 CO3 ) in a FM01-LC flow cell from ICI. The selectivity towards furoic acid was excellent (95%) however the conversion of furfural remained very low, i.e. 5% and reached 20% when a stacked meshed electrodes were used instead of the flat plate anodes. In order to increase the conversion, the oxidation of furfural was investigated in a strong alkaline medium. Considering the Cannizzaro reaction, the stability of furfural in 1.5 M NaOH aqueous solution was investigated by HPLC. After 2 h 80% of the initial furfural were proportionated into equal amounts of furoic acid and furfurylic alcohol. We have then investigated the oxidation of furfuryl alcohol under the same conditions. The electrolysis of 50 mM furfurylic alcohol leads to furfural and furoic acid following the kinetic represented in Fig. 10. Furfural is the primary product and seems to be more reactive than furfurylic alcohol:with k2 k1 2H
+
2H H2O
k1 O
CH2OH
-
2e
The electrolysis potential settled at 0.15 V(SCE) during 10 s was repetitively followed by a short plateau during 1 s at 1 V(SCE) as to regenerate the electrode surface [9]. Furoic acid was the only product detected by HPLC. The kinetic of the consumption of furfural and the production of furoic acid during the electrolysis is represented in Fig. 9.After 13 h of electrolysis, 56% of furfural were converted into furoic acid with an excellent selectivity (95%). The apparent reaction order is unity.
O
CHO
+
k2
-
O
COOH
2e
Electrolysis of furfural at constant current (j = 0.8 mA cm−2 ) was performed in 0.5 M NaOH on flat plate nickel anode. A constant pH was maintained during electrolysis using an automatic titrator. The variation of the concentration of furfural and its oxidative derivative is shown in Fig. 11. Furoic acid was obtained with 80% chemical yield and no furfuryl alcohol was detected by HPLC during the electrolysis. In fact, the rate of the electrocatalytic oxidation of
P. Parpot et al. / Electrochimica Acta 49 (2004) 397–403
0.03
C(M)
0.02
0.01
0.00 0
50
100
150
200
t/min
Fig. 11. c = f(t) curves plotted during the electrolysis of 50 mM furfural at 0.8 mA cm−2 on nickel electrode and in 0.5 M NaOH. (䊏) Furfural and (䊊) furoic acid.
furfuryl alcohol is much higher than that of Cannizzaro reaction:
403
nickel anodes. On gold, the two peaks A and C—observed during the positive and the negative sweep—were controlled by diffusion and adsorption, respectively. The best chemical yields of furoic acid were obtained on nickel anodes in strong alkaline medium. Furfural was reduced on Pt, Pb and Cu cathodes. On high hydrogen overvoltage cathodes (Pb) secondary products coming from electrohydrodimerization processes were observed. These reactions were favoured in methanolic solutions and with the highest initial concentration of furfural. On Pt cathodes the selectivity towards the furfuryl alcohol was excellent however the conversion was poor (<20%). The best results were obtained on Cu cathodes in carbonate buffer (pH 10). Acknowledgements This work was support by ICCTI (Portugal) and CNRS (France) through ICCTI/CNRS Cooperation (Project no. 6766). The authors are grateful to ICI Chemicals for loaning the FM01-LC flow cell. References
4. Conclusion The voltammetric results have shown that the rate of furfural oxidation is controlled by diffusion on platinum and
[1] K.J. Zeitsch, The Chemistry and Technology of Furfural and its Many By-Products, Sugar Series 13, Elsevier, Amsterdam, 2000. [2] D.V. Sokolskii, M.S. Erzhanova, N.A. Zibrova, B.B. Kamardinova, Elektrokhimya 12 (1976) 1388. [3] M. Fleishmann, K. Korinek, D. Pletcher, J. Electroanal. Chem. 31 (1971) 39. [4] G. Vértes, F. Nagy, Acta Chim. (Budapest) 71 (1972) 333. [5] M. Amjad, D. Pletcher, C. Smith, J. Electrochem. Soc. 124 (1977) 203. [6] H.J. Schäfer, Top. Curr. Chem. 142 (1987) 101. [7] V.A. Sverev, V.I. Milman, Elektrokhimiya 16 (1980) 1867. [8] M.M. Baizer, in: H. Lund, M.M. Baizer (Eds.), Organic Electrochemistry, Marcel Dekker, New York, 1990, p. 440. [9] E.M. Belgsir, E. Bouhier, H. Essis-Yei, K.B. Kokoh, B. Beden, H. Huser, J.M. Leger, C. Lamy, Electrochim. Acta 36 (1991) 1157.