ELECTROCHEMICAL METHOD FOR PRODUCING VALERIC ACID

Abstract

The invention is directed to a method of electrochemically producing valeric acid.

The method of the invention comprises contacting a solution of levulinic acid with an anode and a cathode in an electrochemical cell; and electrochemically reducing levulinic acid at the cathode to form valeric acid,
wherein the cathode comprises one or more materials selected from the group consisting of cadmium, zinc, and indium.

Claims

1. A method for electrochemically producing valeric acid, said method comprising contacting a solution of levulinic acid with an anode and a cathode in an electrochemical cell; and electrochemically reducing levulinic acid at the cathode to form valeric acid, wherein the cathode comprises one or more materials selected from the group consisting of cadmium, zinc, and indium.

2. The method according to claim 1, wherein the cathode comprises indium.

3. The method according to claim 1, wherein the anode comprises one or more materials selected for the group consisting of iridium oxide, platinum oxide, ruthenium oxide, tantalum oxide, and titanium.

4. The method according to claim 1, wherein said electrochemical cell is an undivided electrochemical cell.

5. The method according to claim 1, wherein said electrochemical cell is a divided electrochemical cell.

6. The method according to claim 1, wherein said method has a conversion of levulinic acid of 50% or more, such as 60% or more, or 70% or more, 80% or more, 90% or more, or 95% or more.

7. The method according to claim 1, wherein said method has i) a selectivity towards valeric acid of 80% or more, such as 85% or more, or 95% or more; and/or ii) a selectivity towards -valerolactone of 10% or less, preferably 7% or less, more preferably 5% or less.

8. The method according to claim 1, wherein the aqueous solution of levulinic acid further comprises one or more selected from the group consisting of sulphuric acid, sodium sulphate, perchloric acid, and alkylsulphonic acid.

9. The method according to claim 1, which method is carried out at a temperature of 20-80 C., such as 30-70 C., or 40-60 C.

10. The method according to claim 1, wherein the pH of the aqueous solution is kept within the range of 1.2 to 2.0, preferably 1.0 to 1.5, more preferably 0.7 to 1.2, even more preferably 0.3 to 1.0.

11. The method according to claim 1, wherein a current is imposed on the electrochemical cell, said current having a current density of 50-500 mA/cm.sup.2, preferably 100-350 mA/cm.sup.2, more preferably 120-250 mA/cm.sup.2.

12. The method according to claim 1, wherein said aqueous solution further comprises a strong acid, such as one or more selected from the group consisting of sulphuric acid and alkylsulphonic acid.

13. The method according to claim 1, wherein said method further comprises phase separating a valeric acid rich phase from a levulinic acid rich phase.

14. The method according to claim 1, further comprising esterifying the valeric acid to produce valerates, such as one or more valerates selected form the group consisting of methyl valerate, ethyl valerate, propyl valerate, butyl valerate, and pentyl valerate.

15. The method according to claim 1, said method further comprising converting valeric acid into octane at an anode of the electrochemical cell.

Description

EXAMPLES

[0042] Various materials were selected based on their high overpotential for the hydrogen evolution reaction. The applicability was assessed by preparative electrolysis (H-cell) performed at constant current (150 mA/cm.sup.2) and at a temperature of 50 C. Analysis was performed by high performance liquid chromatography (HPLC), .sup.1H-NMR and inductively coupled plasma mass spectrometry (ICP-MS). The H-cell was a two-compartment cell containing 150 ml of catholyte (1 M H.sub.2SO.sub.4 and 0.5 M levulinic acid), and 150 ml of anolyte (1 M H.sub.2SO.sub.4), separated by a Nafion 117 membrane. The H-cell contained a wire cathode of 70 cm length and an iridium oxide anode. All solutions were removed from the cell, and the cell was rinsed with demineralised water. All solutions were collected, put together and analysed.

[0043] The electrolysis results for various cathode materials are shown in Table 1 below. In table 1, charge denotes the relevant charge added to levulinic acid, E.sub.av is average potential of the reference electrode, X.sub.LA is conversion of levulinic acid, S.sub.VA is the selectivity towards valeric acid, S.sub.gVL is the selectivity towards -valerolactone, CE.sub.VA is the current efficiency, and M is the concentration of metal in solution at the end of electrolysis. The indicated selectivity values are based on HPLC analysis.

TABLE-US-00001 TABLE 1 Electrolysis results for different cathode materials cathode charge E.sub.av X.sub.LA S.sub.VA S.sub.gVL CE.sub.VA M Ex. material [F/mol] [V vs. SCE] [%] [%] [%] [%] [mg/l] 1 Pb 8.2 1.77 99 92 2.2 47 27.8 (9.2) .sup.d 2 Cd 8.3 1.77 98 89 3.8 45 18.7 3 In 8.3 1.58 90 99 0 45 107 4 Zn 8.3 1.41 56 95 1.7 27 61.1 5 Al 8.3 2.11 14 71 22 5.6 1924 6 Ni 8.2 0.96 7.3 74 21 2.7 6.5 7 Ga .sup.a 8.2 .sup.1.97 .sup.c 0 0 0 0 96.0 8 Ag 8.2 1.12 0 0 0 0 0.12 9 Ti .sup.b 3.9 1.32 0 0 0 0 0.12 10 Sn 8.2 1.22 0 0 0 0 2.1 .sup.e .sup.a a gallium pool electrode (15.9 cm.sup.2) .sup.b 22 cm wire .sup.c corrected for iR drop .sup.d total Pb concentration, between brackets Pb as precipitate .sup.e some precipitates noticed in solution and not taken into account

[0044] Table 1 clearly shows that next to lead and mercury, also indium, cadmium and zinc are able to reduce levulinic acid to valeric acid with high selectivity. Other materials with a high overpotential for the hydrogen evolution reaction (Al, Ga, Ti and Sn) are however, not able to reduce levulinic acid. The obtained valeric acid and -valerolactone (as identified by HPLC) selectivity for lead is similar to reported in literature (Nilges et al., Energy & Environmental Science 2012, 5(1), 5231-5233; Xin et al., ChemSusChem 2013, 6(4), 674-686; Qiu et al., Green Chemistry 2014, 16(3), 1305-1315; Dos Santos et al., RCS Advances 2015, 5(34), 26634-26643; each based on HPLC analysis). Conversion of levulinic acid and selectivity towards valeric acid is confirmed by .sup.1H-NMR analysis, as shown in FIG. 2 (a .sup.1H-NMR overlay (0.7-1.6 ppm) showing impurity (red circle) in samples obtained at lead, cadmium, zinc cathodes and not at the indium cathode). Indium and zinc exhibit the highest selectivity towards valeric acid. In addition, reduction of levulinic acid occurs at a higher potential at indium and zinc. This is shown in FIG. 3, which displays the electrode potential of various cathode materials during electrolysis of 0.5 M levulinic acid in 1 M H.sub.2SO.sub.4 at 50 C.

[0045] In an additional experiment, the formation of a two-phase system is shown (FIG. 4) for zinc, cadmium and lead cathode materials. This figure shows the concentration of levulinic acid versus the supplied charge. Initially, the concentration of levulinic acid follows Faradays law (four electron reduction to valeric acid) to approximately 25% conversion, i.e. a concentration of about 0.25 mol/l valeric acid at about 100% selectivity. After this point, a two phase system develops resulting in an additional decrease of levulinic acid due to its dissolution in the valeric acid phase. The point of formation of the second phase is somewhat lower than the solubility of valeric acid, viz. 0.39 M at 50 C. (Romero et al., J. Solution Chem. 2009, 38(3), 315-320). This might be explained by: i) the salting out effect due to the presence of 1 M of sulphuric acid, and/or ii) the presence of an additional solute, levulinic acid. The solid black line in FIG. 4 depicts the theoretical decrease of levulinic acid based on the supplied amount of electrons to the system (Faraday's law). The grey line depicts the effect of the formation of a second phase on the concentration dependency, taking into account a solubility of 0.25 mol/l valeric acid and a distribution coefficient of 25 for levulinic acid.