Electrochemistry for the generation of renewable chemicals: Electrochemical conversion of levulinic acid

Article abstract of DOI:10.1039/c4ra16303f

The oxidative and reductive electrochemical conversion of levulinic acid to its primary products valeric acid, γ-valerolactone, 2,7-octanedione, 4-hydroxy-2-butanone and 3-buten-2-one is studied in detail. The reactions were performed in aqueous solutions and at ambient temperature, following the principles of green chemistry. The obtained primary reaction products were studied with respect to the oxidative and reductive electrochemical formation of secondary products, such as n-octane, 1-butanol and 1,3-butanediol. It is shown that the choice of electrolyte composition, educt concentration and the nature of the electrode material has a strong influence on the selectivity of product formation. For instance it is demonstrated that in alkaline solutions γ-valerolactone can be gained from levulinic acid at iron electrodes with similar Coulombic efficiency (～20%) but higher selectivity (S = 70%) than on lead (S = 50%). Furthermore, for the first time the electrochemical two-step reaction of levulinic acid to 1-butanol via 4-hydroxy-2-butanone is reported. For some of the reaction pathways the main product is water insoluble, which allows a direct separation of the product and the potential electrolyte reuse in a semi-continuous process. Especially the use of the electrocatalytic hydrogenation may provide a path for the storage of electricity into liquid organic fuels as shown by a basic energetic assessment of all electrochemical conversions.

Full text of DOI:10.1039/c4ra16303f

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Electrochemistry for generation of renewable chemicals:
Electrochemical conversion of levulinic acid
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Tatiane Regina dos Santosa,b, Peter Nilgesa, Waldemar Sautera, Falk Harnischb, Uwe
Schrödera*
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a Institute of Environmental and Sustainable Chemistry, Technische Universität Braunschweig,
Hagenring 30, 38106 Braunschweig, Germany,
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b Department of Environmental Microbiology, UFZ-Helmholtz Centre for Environmental Research,
Permoserstrasse 15, 04318, Leipzig, Germany
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*author of correspondence: e-mail: uwe.schroeder@tu-bs.de, phone: +49 5313918425; fax: 49
5313918424
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Abstract
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The oxidative and reductive electrochemical conversion of levulinic acid to its primary products
valeric acid, γ-valerolactone, 2,7-octandione, 4-hydroxy-2-butanone and 3-buten-2-one is studied in
detail. The reactions were performed in aqueous solutions and at ambient temperature, following
the principles of green chemistry. The obtained primary reaction products were studied with respect
to the oxidative and reductive electrochemical formation of secondary products, such as n-octane, 1-
butanol and 1,3-butanediol. It is shown that the choice of electrolyte composition, educt
concentration and the nature of the electrode material has a strong influence on the selectivity of
product formation. For instance it is demonstrated that in alkaline solutions γ-valerolactone can be
gained from levulinic acid at iron electrodes with similar coulombic efficiency (~20 %) but higher
selectivity (S = 70 %) than on lead (S = 50 %). Further, for the first time the electrochemical two-step
reaction of levulinic acid to 1-butanol via 4-hydroxy-2-butanone is reported. For some of the reaction
pathways the main product is water insoluble, which allows a direct separation of the product and
the potential electrolyte reuse in a semi-continuous process. Especially the use of the electrocatalytic
hydrogenation may provide a path for the storage of electricity into liquid organic fuels as shown by
a basic energetic assessment of all electrochemical conversions.
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Keywords:
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electroorganic synthesis, levulinic acid, green chemistry, electrochemical decarboxylation,
electrofuel, electrochemical hydrogenation, electrocatalytic hydrogenation
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Table of contents
Levulinic acid as a platform for electrochemical synthesis: Electrochemical conversion of levulinic
acid and its primary products – via oxidation and reduction pathways - is presented as a promising
alternative for the generation of renewable chemicals and biofuels and for energy storage.
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1. Introduction
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Levulinic acid (LA), produced from lignocellulosic biomass, is considered to be an important precursor
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for renewable chemicals and liquid biofuels. The synthesis of levulinic acid from saccharose was
reported already in 1840 4 and since then was studied from different perspectives. Generally, LA can
be produced in high yields from carbohydrates and from lignocelluloses by acid catalyzed
hydrolysis. 5-15 The review by Rackemann and Doherty 16 outlines the chemistry of levulinic acid
synthesis and discusses current and potential technologies for producing levulinic acid from
lignocellulosics biomass. Its molecular structure, comprising two functional groups (a keto and a
carboxyl group), makes levulinic acid an ideal platform chemical. Thus, the lab-scale synthesis of bio-
chemicals, polymers, pharmaceuticals, flavor agents, solvents, antifreeze agents, biofuels,
plasticisers, and herbicides from LA has been reported. 2, 17-23 The respective syntheses are generally
based on homogeneous and heterogeneous catalysis, whereas electrochemical conversions of LA are
only rarely described. The electrochemical formation of γ-valerolactone (gVL) from LA was described
by Tafel, when he studied the electrochemical reduction of levulinic acid in alkaline solution at a lead
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cathode.
Further examples are the oxidative formation of 5-acetyl-2,9-decanedione in a
methanol/water solution 27 and recently the works of Xin et al. and Qiu et al. reporting on
electrocatalytic processing of levulinic acid to produce biofuels. 25, 28
By means of an electrochemical hydrogenation coupled with a Kolbe reaction for the conversion of
levulinic acid to n-octane, with valeric acid (VA) as intermediate product, we recently proposed the
use of electrochemistry for the production of liquid biofuels (also denominated as "electrofuels").
The high selectivity (>95 %) of the electrochemical hydrogenation of levulinic acid to valeric acid
achieved in this study in batch-reactions was recently confirmed by Xin et al. for a flow cell process,
yielding 90 % valeric acid and 5 % γ-valerolactone. 25
In this work we investigate promising primary as well as secondary electrochemical conversion
processes of levulinic acid, comprising oxidative as well as reductive pathways (Scheme 1). This study
considerably expands the portfolio of chemicals that can be obtained from the electrochemical
conversion of LA 29, as it introduces the electrochemical synthesis of not only valeric acid, and n-
octane but also γ-valerolactone, 2,7-octandione, 4-hydroxy-2-butanone and 3-buten-2-one (also
known as methyl-vinyl-ketone) as well as n-octane, 1-butanol, 1,3-butanediol and methyl-
tetrahydrofuran.
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Scheme 1. Schematic illustration of the pathways for the primary and secondary electrochemical conversion of
levulinic acid. A) Electrochemical reduction of levulinic acid to: (I) γ-valerolactone or (II) valeric acid, B)
electrochemical oxidation from levulinic acid to: (III) 2,7-octadione, (IV) 4-hydroxy-2-butanone or (V) 3-buten-2-
one and C) electrochemical conversion of the primary reaction products to: (VI) 2-methyltetrahydrofuran
(electrochemical conversion was not reproducibly possible) (VII-VIII) n-octane, (IX) 1-butanol or (X) 1,3-
butanediol.
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Scheme 1 summarizes the primary and secondary oxidation as well as reduction reactions
investigated in this study. The derived products may either find direct application as fuels or solvents
or may serve as intermediates for further processes – which are summarized in Table 1.
As Table 1 also shows, most of the conventional synthesis routes for the conversion of levulinic acid
are high-temperature and high-pressure processes. Electrochemical synthesis may provide an
alternative route, as the respective reactions can be carried out in aqueous solutions and at room
temperature and thus fulfilled major criteria of green chemistry.30
Additionally to the interesting product spectrum and its exploitation for (bio)chemical synthesis , the
direct use of electric current for chemical conversions may even allow the storage of energy from
renewable resources like wind power and photovoltaic in the form of chemical potential energy into
compounds with higher energy density. For serving as a high energy density fuel molecule, oxygen
functionalities have to be removed from LA, for which new processes are needed. For some of the
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conversions from LA to a (more reduced and thus higher energy dense) target molecule consecutive
reactions, i.e. an oxidation followed by a reduction or vice versa, are necessary. Such a combined
oxidation and reduction can, in principle, be achieved in one electrochemical cell.
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As demonstrated in a principle energetic assessment of the described transformation routes, the
electrochemical conversion of levulinic acid provides a promising platform.
With the results in the present manuscript, we aim to demonstrate that some well-known
electrochemical reactions like the Kolbe and non-Kolbe reaction and the electrocatalytic
hydrogenation, are well suitable for the conversion and generation of green platform chemicals.
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Table 1. Relevance of the compounds potentially derived from levulinic acid via electrochemical conversion: Applications and conventional production
methods.
Products
Applications
Production methods
valeric acid
(VA)
Intermediate to valeric fuels and additives.
For example by catalytic hydrogenation of levulinic acid to valeric acid, typically
performed in a two-step reaction at 150-350 °C, under hydrogen pressure of 10-
40 bar and at platinum metal based catalysts. 3, 31, 32
γ-valerolactone
(gVL)
Intermediate for butene, valeric acid, and 5-nonanone production. gVL can be
used as renewable solvent.
Catalytic hydrogenation using different transition metal based catalysis at high
temperature (70-265 °C), under hydrogen atmosphere (5-150 bar). 33-45
2,7-octandione
(2,7-OD)
The occurring 2,7-octandione can be used in a intramolecular aldol reaction 46, 47 Usually achieved by complex catalytic routes, e.g, based on Au catalyzed reactions of
or intramolecular pinacol coupling
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the di-acetylene at 120 °C 49, Ir-complex-catalyzed addition of water to 1,7-octadiyne
at 70 °C 50 or the Pd catalyzed Wacker oxidation of olefins at 80 °C under oxygen
(6 atm) in DMA. 51
4-hydroxy-2-butanone 4-hydroxy-2-butanone is used as a platform chemical for further syntheses, like
the dehydration to 3-buten-2-one 52 and the reduction to 1,3 butanediol 53 or to
1-butanol (electrochemical path shown below).
4-hydroxy-2-butanone is industrially produced by the coupling of fossil source based
propanone and formaldehyde in basic media. 54 The Wacker-type oxidation of 3-
buten-1-ol, 55 selective catalytic dehydrogenation of 1,3-butanediol, 56-58 selective
heterogeneous catalytic oxidation of 1,3-butanediol with H2O2 in CH3CN, 59, 60 and
reduction of a dicarbony (1,3-butanone) at 130 °C over metallocene complexes 61 are
(4H2B)
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examples for the complex formation of 4-hydroxy-2-butanone.
Technically, 3-buten-2-one is built from acetone with formaldehyde and diethylamine
3-buten-2-one
(MVK)
Used in the production of gas-tight plastics, a intermediate in the production of
pharmaceutical, fungicides63, vitamin A und vitamin E 64. Important compound in hydrochloride (Mannich-reaction) or by Aldol-condensation of acetone and
Michael-Additions, Robinson-Annulations, Diels-Alder-Reactions, Baylis-Hillman- formaldehyde. 63 The catalytic oxidation of levulinic acid over CuO/CeO2 and
Reactions, radical additions heterocycle-synthesis; often found in natural
material synthesis (polyketides, macrolactone, terpenes, pheromons and
alkaloids). 63
CuO/Al2O3 gives 2-buten-3-one in 15.5 % yield at 175 °C 65 or 67.5 % at 300 °C over
CuO2 66, it is also patent-registered. 67
1,3-butanediol
(1,3-BDO)
n-octane
Polyester plasticizers, humectants.
n-octane is used as solvent, cleaning agent, fuel additive or as reaction agent like Fractional distillation and refining of petroleum or the selective electrolytic reductive
oxidation by cobalt 68 aromatization 69 is described also.
homocoupling of alkyl iodides at Ag-Pd cathodes in dipolare solvents. 70
2-Methyltetrahydrofuran is produced by Ni-catalyzed hydrogenation of 2-
methylfuran. This reaction can be performed in the liquid phase underH2 pressures of
2 MPa or in the vapor phase at atmospheric pressure with common supported Ni
catalysts.73
methyl-
tetrahydrofuran
(MTHF)
2-Methyltetrahydrofuran is mainly used as a higher boiling substitute for
tetrahydrofuran as specialty solvent. 2-Methyltetrahydrofuran can also be used
in the electrolyte formulation for secondary lithium electrodes and as a
component of alternative fuels. 71, 72
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2. Experimental section
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Chemicals
All chemicals used in this study were of analytical grade. For qualitative and quantitative analysis,
reference materials and solvents were used as purchased, without purification. 2,7-octandione was
produced by Kolbe-electrolysis and was purified by re-crystallization in heptane and its purity was
confirmed by in-house NMR (1H and 13C) measurements.
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Electrode materials
As anode or cathode material were used: graphite foil (chemPUR, Germany), polycrystalline graphite
sheet (SGL Carbon GmbH, Germany), graphite felt (SGL Carbon GmbH, Germany), lead sheet
(99.999 %, chemPUR, Germany), copper-sheet (99.999 %, chemPUR, Germany), iron sheet (99.5 %,
chemPUR, Germany), nickel sheet (99.5 % PHYWE Systeme GmbH & Co.KG, Germany), and platinum
sheet (99.9 %, chemPUR, Germany).
A pretreatment procedure was applied to remove contaminants from the electrode surfaces. The
lead electrodes were rinsed in 25 % HNO3, treated with sand paper (grain size 8000) and washed with
deionized water. Graphite felt electrodes were washed with deionized water and used directly.
Platinum electrodes were pretreated by flame annealing and washed with deionized water. All the
others electrodes were washed in HCl, treated with sand paper (grain size 8000) and washed with
deionized water.
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Electrochemical procedures
All electrochemical reactions were conducted under potentiostatic control using
a
potentiostat/galvanostat (AMEL 7050 (Amel srl, Milano, Italy) or a SP50 potentiostat (Bio-Logic SAS,
Claix, France)). A three electrode configuration is used with EWE standing for the potential applied at
the working electrode and ECE for potential measured at the counter electrode versus reference
electrode, for which Ag/AgCl sat. KCl electrodes (SE11, Sensortechnik Meinsberg, Germany, 0.195 V
vs. standard hydrogen electrode (SHE)) were used throughout the study. All electrode potentials in
this study are reported versus Ag/AgCl sat. KCl, but normalized to standard hydrogen electrode for
energetic calculations.
As electrochemical cells for batch reaction one chamber undivided glass cells with 50 mL reaction
volume and two-chamber H-type electrochemical glass cells with 40 mL anode and cathode
chambers separated via a cation exchange membrane (fumasep® FKE, Fumatech, Germany), was
used, with a geometric electrode area varying from 5 to 24 cm2. All reported current densities are
normalized to the geometric surface area.
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The electrochemical flow cells were a custom-made Plexiglas® cell with 21 mL reaction volume and
11 cm2 anode and cathode area and a microflowcell (Micro Flow Cell, ElectroCell, Denmark) with
10 mL volume and 10 cm2 for anode and cathode. The cells were operated in single-pass or
recirculation mode at flow rates between 0.36 and 10 mL min-1 (thus the hydraulic residence time
varies from 6 to 3600 s).
The duration of the electrolysis experiments varied according with initial concentration and achieved
current. A detailed overview on all performed reactions is shown in the tables SI1 to SI3 in the
Supplementary Information.
Analysis
Qualitative and quantitative analysis was carried out using gas chromatography–mass spectrometry
(GC-MS) (Trace GC Ultra, DSQ II, Thermo Scientific, Germany) equipped with a TR-5MS (30 m x
0.25 mm ID x 0.25 µm film GC column Thermo Scientific, Germany) or TR-Wax MS (30 m x 0.25 mm
ID x 0.25 mm film GC column, Thermo Scientific, Germany).
Quantitative analysis was further carried out by gas chromatography (GC) with
flame ionization detection (FID) (Hewlett Packard Series II 5890, Hewlett Packard, USA) equipped
with a DB-5 (30 m x 0.25 mm ID x 0.25 mm film GC Column from Agilent JW Scientific, USA).
External calibration curves with different concentration levels in a range from 0 to 600 ng µL-1 were
used for quantification.
Routine substance quantification was obtained by high performance liquid chromatography (HPLC),
by means of a refractive index (RI) detector (Spectrasystem P4000, Finnigan Surveyor RI Plus
Detector, Fisher Scientific) equipped with a HyperREZ XP Carbohydrate H+ 8 μm (S/N: 026/H/012-
227) column. Sulfuric acid (0.005 N, flow rate 0.5 mL min-1) served as the eluent. The column was
operated at different temperatures depending on samples composition; the refractory index
detector was operated at 40 °C.
Quantitative analysis was further realized with a HPLC-UV/VIS system (Hewlett Packard Series 1050,
Hewlett Packard, United States of America) with a diode array detector (DAD) and equipped with a
HyperREZ XP Carbohydrate H+ 8 μm (S/N: 026/H/012-227). Phosphoric acid (0.05 %, flow rate 0.5 mL
min-1) served as the eluent. The column was kept at room temperature. Educt and product
concentrations were determined using calibration curves in a range from 0 to 0.2 M.
For details on raw data (SI1 for primary and SI2 for secondary reactions) as well as calculations of
coulombic efficiency (CE) and selectivity (S) (SI3) see supplementary information.
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3. Results and Discussion
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3.1 Primary reductive reactions: Reduction of levulinic acid to valeric acid and γ-valerolactone
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Scheme 2 shows the electrochemical reduction pathways of LA. Selected conditions have already
been investigated by several authors, e.g., 25. In the present study, we systematically studied the
impact of a variety of electrode materials, the electrolyte composition and the educt concentration
on the LA reduction.
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Scheme 2. Reaction pathways of the electrochemical reduction of levulinic acid to (I) γ-valerolactone and (II)
valeric acid (Part A of Scheme 1).
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Figure 1 summarizes the results of the levulinic acid reduction as a function of initial educt
concentration. Changing the educt concentration does not affect the selectivity of the product
formation: at a Pb electrode in 0.5
M H2SO4 electrolyte solution, and an applied working electrode
potential of -1.8 V, valeric acid is always the main product. The increase of the initial educt
concentration has, as expected, a positive impact to the Coulombic efficiency (CE) (see SI S3 for
definition). At sufficiently high levulinic acid concentration the hydrogenation process is the
dominating reaction; whereas at low educt concentrations hydrogen evolution via water electrolysis
as the main side reaction prevails, leading to low Coulombic efficiency and LA hydrogenation rate.
Consequently, when using a LA starting concentration of 0.5
conversion rate increases from 3.3 mg cm-2 h-1 to 29.5 mg cm-2 h-1 and the achieved Coulombic
efficiency increases from 6% to 60%. A LA concentration of 0.5 represents an optimum value, since
M instead of 0.05 M, the average
M
a further increase of the starting concentration does not lead to a further significant increase of
conversion rate and CE.
Beside the Coulombic efficiency aspect, it is of special interest to achieve high product
concentrations for the conversion of levulinic acid to valeric acid (Scheme 2 - route II), since
spontaneous product separation can be achieved when the saturation concentration of valeric acid
of 0.39
M is exceeded (see inset picture in Figure 1).
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40
20
0
formation of organic
product phase
Conversion
Selectivity VA
Selectivity gVL
Coulomb efficiency
0.05
0.10
0.50
0.75
1.00
Initial concentration LA [M]
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Figure 1. Electrocatalytic hydrogenation of levulinic acid at Pb-electrode in a divided H-Cell at different initial
concentration of levulinic acid after 4-8 hours electrolysis at a potential of -1.8V. The formation of the organic
product phase is shown in the inset picture. Details of all reactions can be found in Table SI 1 entry 1 to 17.
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Electrochemical conversions are heterogeneous processes, and as such, their mechanisms are
decisively determined by the nature of the electrode material. Figure 2 summarizes major
parameters of the levulinic acid reduction as a function of the used electrode material and nature of
electrolyte. In 0.5
M sulfuric acid, lead stands out clearly over the other electrodes in terms of educt
conversion to valeric acid and Coulombic efficiency. This performance may be explained by the
higher overpotential of the hydrogen evolution reaction at Pb-cathodes, since at Cu, Fe and Ni the
water electrolysis commences already at an electrode potential of -1.0 V.
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Conversion
Selectivity VA
Selectivity gVL
Coulombic efficiency
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60
40
20
0
C (a)
Cu (a)
Fe (a)
Fe (b)
Ni (a)
Pb (a)
Pb (b)
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Figure 2. Electrocatalytic hydrogenation of levulinic acid in a divided H-Cell as a function of the nature of
electrode material and electrolyte:(a) 0.5 H2SO4 and (b) 1 NaOH. The data are based on 2-4 hours
electrolysis at an electrode potential of -1.8 V; the initial LA concentration was 0.5
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M
M
M
.
When switching the cathode material from lead to carbon in acidic electrolyte, the hydrogenation of
levulinic acid leads almost exclusively to the formation of -valerolactone (gVL). A conversion of 40 %
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γ
was obtained, which represents a ten-fold improvement as compared to the previous work by Xin et
al. 25. The selectivity of the gVL formation is 68 %. Whereas gVL is formed in sulfuric acid solutions at
Cu and Ni electrodes, no formation of valeric acid was observed in alkaline solution at this electrode
materials. In alkaline solution gVL is the major product only when Pb and Fe electrodes are used. For
the electrochemical conversion of LA to gVL at Pb electrode a Coulombic efficiency of 21 % can be
achieved, whereas the use of Fe cathode results in a slightly lower Coulombic efficiency (18 %), but a
higher selectivity. For this reason, and especially with regard to the sustainability of the process, Fe
and C are very promising alternatives as electrode material for further investigations. It is very likely,
that the Coulombic efficiency can be enhanced when using higher initial educt concentrations or
when operating in a continuous system.
In summary, VA and gVL can both be obtained from LA with good selectivity and acceptable
Coulombic efficiency.
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3.2
Primary oxidative reactions: Oxidation of levulinic acid to 2,7-octanedione, 4-hydroxy-2-
butanone and 3-buten-2-one
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Whereas the reductive conversion of levulinic acid may potentially be directly used for energy
storage processes (see also 3.4.), the oxidation of levulinic acid is of primary interest for chemical
production. Levulinic acid can be oxidized either to a keto-alkyl radical (one electron oxidation) or to
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a carbenium ion (two-electron oxidation) (see Scheme 3). Thereby the Kolbe product
of LA, the
dimer 2,7-octanedione, yields from the radical and its subsequent dimerization (route III).29 The
carbenium ion, product of the two-electron oxidation, on the other hand, may react further either
with nucleophiles like hydroxide ions (Non-Kolbe-reaction or Hofer-Moest-reaction 78-80), yielding 4-
hydroxy-2-butanone (route IV), or may undergo an elimination of H+, forming 3-buten-2-one (route
V).81
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Scheme 3. Reaction pathways of the electrochemical oxidation of levulinic acid.
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One-electron oxidation: formation of 2,7-Octandione (route III)
In accordance with our previous study, 29 2,7-octanedione can be gained with good selectivity and a
good Coulombic efficiency at platinum electrodes in methanol solution. In the present work we show
that water can also be used as an alternative solvent. A major disadvantage, however, is a decrease
of the selectivity and a significant impact on the Coulombic efficiency – decreasing from 86 % in
methanol to 5 % in aqueous solution (see SI, Table SI 1, entry 13 and 14). This was not unexpected,
since in aqueous solution the current density was higher, promoting a stronger solvent
decomposition. An increase of the initial levulinic acid concentration could not compensate this
effect. Furthermore, aqueous solutions stabilize ionic species like the two-electron oxidation product
carbenium ion, which leads to the formation of multiple side products. The formation of side
products explains the lower selectivity (27 %) in comparison to methanol (60 %) even at higher
conversions of levulinic acid (74 %).
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Two-electron oxidation: formation of 4-Hydroxy-2-butanone (route IV) and formation 3-buten-2-one
(route V)
The formation of the carbenium ion by the two-electron oxidation of LA can be generally enhanced
by the use of a graphite anode instead of platinum, as at Pt radical adsorption promotes
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dimerization. The reactions were first conducted in a batch reactor in an aqueous 0.2
M NaOH
electrolyte solution using a carbon sheet anode. In alkaline solution carbenium ions are captured by
hydroxide ions, leading to the formation of 4-hydroxy-2-butanone (route IV). At an electrode
potential of 6 V a selectivity of 20 % and a Coulombic efficiency of 5 % was obtained. Noteworthy, to
our knowledge it is the first time that the direct production of this compound from levulinic acid is
presented. In addition to this non-Kolbe product only minor amounts of side products including the
Kolbe product (2,7-octandione) and 2-butanone were identified. When the above reaction is
performed in alcoholic solution, 82, 83 alkoxide ions serve as nucleophile; thus the reaction product is
the respective ether. We have confirmed the formation of 4-methoxy-2-butanone and 4-ethoxy-2-
butanone in methanol or ethanol electrolyte solution (data not shown).
Interestingly, when performing the reaction in a membrane-separated flow cell, the major oxidation
product is 3-buten-2-one (Scheme 3, route V). This finding (See Figure S1) can so far not be explained
and has to be studied further.
282
3.3
Secondary electrochemical conversion steps (C)
283
284
285
286
287
288
289
290
The products of the primary LA conversions can be used as educts for secondary electrochemical
conversion steps (see Scheme 1 – secondary conversions). Some of these reactions, as the
production of n-octane by the reduction of 2,7-octandione and the Kolbe-coupling of valeric acid
have been already reported.29 The following section presents further details of the Kolbe and the
non-Kolbe reaction of valeric acid. Further, the hydrogenation of γ-valerolactone (Scheme 1, route
VI) is very appealing, but unfortunately could not be experimentally proven.
Electrochemical hydrogenation of 4-hydroxy-2-butanone (route IX and X)
291
292
Scheme 4. Reaction pathways for the electrochemical hydrogenation of 4-hydroxy-2-butanone.
293
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295
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298
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301
302
303
304
305
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307
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309
The electrocatalytic hydrogenation of 4-hydroxy-2-butanone (4H2B) was performed at Pb electrodes
in aqueous 0.5 sulfuric acid electrolyte solution. After 3.5 h of electrolysis, 85 % conversion of 4-
M
hydroxy-2-butanone was achieved and 1-butanol was gained with 84 % selectivity. At 100 %
conversion 1-butanol is gained with 75 % yield and with a Coulombic efficiency of 29 %. 1,3-butandiol
was found as the only side product. For the use of other electrode materials both conversion rate
and Coulombic efficiency were lower (comparable to the results found for the hydrogenation of
levulinic acid) and 1,3-butandiol was produced only with low selectivity (not higher than 10 %).
Similar as for the hydrogenation of levulinic acid, the Coulombic efficiency of the 4H2B
hydrogenation can be significantly improved by higher initial educt concentration. Thus, the
Coulombic efficiency of 1-butanol formation increased from 13 % for a 0.08
a 0.5 4-hydroxy-2-butanone solution.
M solution to 52 % using
M
In order to achieve a higher selectivity of 1,3-butanediol formation, the hydrogenation of 4H2B was
studied using different electrolytes and different electrode materials. Here, the use of phosphate
buffer solution (0.5 M, pH 7) and iron electrodes yielded the highest 1,3-butanediol selectivity of 34 %
(see SI, Table SI 2, entry 1 to 7).
310
311
Oxidation of valeric acid to n-octane (route VII)
312
313
Scheme 5. Reaction pathway for the electrochemical oxidation of VA to n-octane.
314
315
316
317
318
319
320
321
322
323
324
325
29
The Kolbe electrolysis of valeric acid to n-octane was already presented in our previous work
,
where the production of n-octane by the one-electron oxidation of valeric acid was performed in an
undivided cell, at a Pt anode and in aqueous as well as methanolic solutions.
As known from literature, the Kolbe reaction proceeds preferably at high current density and high
educt concentration – both factors promoting the formation of high keto alkyl radical concentrations
at the electrode surface and thus to the production of the dimer – in this case n-octane. Additional to
n-octane, this organic phase contains the side products butyl-valerate and 1-butanol 29. Wadhawan
et al. proposed a possible mechanism for the production of esters during the Kolbe electrolysis in
84
aqueous solution (Scheme 6). Scheme 6 illustrates that butyl-valerate is formed, when only one
CO2 is cleaved, whereas the formation of n-octane requires the complete elimination of both CO2.
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-e-
x 2
-CO2
-
-
R-CO2
R-CO2 OCO-R
R-CO2
R-CHO2
R-CO2 OCO-R
R-CO2-R
R-R
ads
ads
ads
-2 CO2
326
327
328
Scheme 6. Potential mechanism for the formation of n-octane and butyl-valerate during Kolbe electrolysis
(derived from Wadhawan 84).
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
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347
348
349
350
351
352
353
354
355
356
The side product 1-butanol is generated via a Hofer-Moest reaction by a two electron oxidation of
valeric acid and the formation of a carbenium ion that reacts with a hydroxide ion to form 1-butanol.
The side product formation is dependent on the starting concentration of valeric acid and on the
reactor configuration. In order to suppress this kind of side-reactions, experiments in a flow reactor
were performed. Compared with our previous results for batch reactors, the Coulombic efficiency
could not be improved with this setup, but even for a non-optimized system a CE of 50 % was
reached. A further improvement can be achieved, when using a flow reactor, as the selectivity of n-
octane formation increases from 51 to 81 %. The higher selectivity may result particularly from the
lower residence time, which was for batch systems at least 4 hours, whereas a residence time not
longer than 5 minutes, preventing further reactions, was realized in flow-through reactors.
Two-electron oxidation and the Non-Kolbe-Reaction of valeric acid
Within this investigation a non-Kolbe reaction based decomposition of valeric acid (Scheme 7) was
studied and the difficulty to control the product formation is demonstrated.
When the electrochemical conversion of short chain fatty acids, like valeric acid, is performed at
carbon instead of platinum electrodes, the reaction leads to the formation of carbenium cations via a
two-electron oxidation (route VII). These cations can then be attacked by a respective nucleophile (in
water the hydroxide ion). Thus, the electrolysis of valeric acid at a carbon anode in aqueous
environment (0.5
M H2SO4) leads to the formation of alcohols. Depending on the location of the
positive charge at the carbenium ion the hydroxide ion can attack either at position 1 (to form 1-
butanol) or at position 2 (to form 2-butanol). At the applied positive anode potential a further
oxidation of these products takes place: 1-butanol is oxidized to butanal, 2-butanol is oxidized to 2-
butanon. Whereas ketones are inert to further oxidation, the aldehyde can be oxidized to carboxylic
acids – which again can be decarboxylated in a further oxidation cycle. Thus – as illustrated in
Scheme 7 – a successive shortening of the carbon chain occurs, leading to carbon dioxide as the final
product.
In order to control the reaction and to prevent the successive decomposition, a charge transfer from
a primary to secondary carbenium ion is necessary. The secondary carbenium ions will react with
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358
359
360
361
362
363
hydroxide ion to a secondary alcohol and thus no further oxidation than to the ketone is possible.
Such control of the reaction did not succeed in full extent and the use of large electrolyte ions, such
-
79
as ClO4 and HCO32-, as described in literature did not lead to the desired success. This cannot be
fully explained, but the effect was mainly described for Pt-electrodes, where anion can adsorb thus
prevent dimerization. In our study, we used graphite, at which anion adsorption may not, or only to a
minor extent, take place.
364
365
Scheme 7. Anodic degradation of valeric acid at graphite electrodes.
366
367
368
369
As depicted in Figure 3, the proposed in Scheme 7 degradation can be experimentally confirmed.
Depending on the reaction time (and thus the degree of valeric acid conversion), the major reaction
products and intermediates could be found.
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butyric acid
propionic acid
acetic acid
formic acid
2-butanone
1-butanal
valeric acid
reaction time = 0 h
10.0
7.5
5.0
2.5
0.0
two-electron oxidation
products
reaction time = 11 h
25
50
75
100
20
30
40
50
time [min]
conversion VA [%]
370
371
372
373
374
Figure 3. HPLC – Chromatograms and the quantitative product analysis of a valeric acid (50 mM) Non-Kolbe
electrolysis (performed at a graphite electrode in 50 mM NaOH solution, at a potential of 2.5 V and a current
density of 2.5 mA/cm2
375
376
377
378
379
380
381
382
The experiment also confirms the accumulation of 2-butanone – the product of the secondary
carbenium ion. Interestingly, no acetone was detected throughout the experiments, which indicates
that the primary propyl carbocation very quickly reacts to the primary alcohol, before the secondary
carbenium ion is formed. The addition of other electrolyte species, however, did not have any
significant impact on the course of the reaction (see Table SI 3).
3.4 Energetic considerations for electrochemical levulinic acid conversions
383
384
385
386
387
388
Alternatively to their exploitation as platform chemicals (see Table 1) the electrochemical
conversions of LA can be used for the storage of electric energy. The derived products may serve as
alternative “green” fuels for combustion engines 85. To be exploited as an alternative fuel two key
properties of a chemical have to be considered: i) its ignition properties and ii) its calorific value. Both
properties are strongly influenced by the oxygen content of a substance. Dependent on the nature of
the functional group, oxygen can be removed either reductively (via electrocatalytic hydration), or
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390
391
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400
401
402
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404
405
406
407
408
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410
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419
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423
oxidatively (e.g., via Kolbe reaction). Certainly, a reduction reaction may represent a direct way to
store electric energy into a chemical compound, whereas a (partial) oxidation lowers the absolute
energy content with respect to the educt/ starting material. Yet, the electrochemical oxidation
reaction of levulinic acid (and its primary products) not only allows removing functional groups like
carboxylic groups from a given educt (thus improving its ignition properties), but it also allows
increasing the mass related energy density of the product by removing the oxygen containing
functional groups. For instance the energy density of levulinic acid – a compound that is not suitable
to be used in combustion engines due to the carboxylic group – of HLHV = 20.8 MJ kg-1 can be
increased to that of 1-butanol (HLHV = 36 MJ kg-1) and of n-octane (HLHV = 44.4 MJ kg-1) by
electrochemical oxidation (see Table SI 4). (For the determination of the respective lower heating
value, HLHV , the method according to Boie 86 was used (see SI 3 for details)).
In order to assess the energy efficiency of the reactions shown in Scheme 1, a calculation of the
“energy storage efficiency”, i.e. the difference of the lower heating values of educt and product
divided by the energy needed for the electrochemical reaction, for the different compounds and the
input energy for the reaction is present in Table 2. Thereby the theoretical and experimentally
electric energy required to drive the reaction(s) was compared to the increase in terms of energy
density of the product molecules (See SI S3 for details of definitions and all calculations.). Since in our
study all reactions were performed in half-cell experiments, the energy calculation was based on the
respective half-reaction, considering the needed average working electrode potential (vs. standard
hydrogen electrode, SHE). The average working electrode potential for the reduction reactions was -
1.8 or -1.5 V (vs. Ag/AgCl, corresponding respectively to a potential of -1.5 and -1.3 V vs standard
hydrogen electrode, SHE). For the oxidation reactions the applied working electrode potential varied
from 2 to 10 V (vs. Ag/AgCl, corresponding respectively to a potential of 2.2 and 10.2 V vs standard
hydrogen electrode, SHE), please note that the higher oxidation potentials were only applied for
systems using organic solvents.
Table 2 shows the maximum and so far achieved energy storage efficiencies. Regarding the one step
reactions, only the reductions provide the possibility for energy storage (the values for the oxidations
are shown in Table SI 5). Most of the reduction reactions exhibit a potential for energy storage
efficiency higher than 70%. Here, for the theoretically most favourable reduction of levulinic acid into
γ-valerolactone, (with a maximum energy storage efficiency of 92%), only 16% are achieved in our
experiments. However, clearly closer matches of the theoretical and achieved energy storage
capacity are shown, e.g. for route II and route IX. For the two-step reactions the theoretical
maximum values are lower and the achieved energy storage efficiencies are maximum ca. 10% (route
II combined with route VII). These calculations show that electrochemical conversions, here of
levulinic acid and its primary products, can provide an interesting and potentially energetically and
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thus economically competitive alternative not only for chemical production, but also for fuel
generation and energy storage.
- Table 2 -
Conclusions
We have reported the electrochemical transformation of levulinic acid to its primary products valeric
acid, γ-valerolactone, 2,7-octandione, 4-hydroxy-2-butanol and 3-buten-2-one. In a secondary step,
the obtained products were subjected to further electrochemical oxidations and reductions. The
presented electrochemical approach represents an energetical and thus potentially economical
alternative to the conventional catalytic processes. Thereby, the selectivity for the reaction products
can be tailored by electrolyte composition, electrode material and current density. The use of
aqueous electrolyte solutions, ambient pressure and room temperature allows obeying the rules of
green chemistry. Therefore, the results demonstrate that renewable electricity could be used for the
sustainable synthesis of a variety of renewable chemicals from levulinic acid and for the storage of
electric energy in the form of organic compounds, partially being already suitable as drop-in biofuels.
440
441
Acknowledgement
442
443
444
445
F.H. acknowledges support by the BMBF (Research Award “Next generation biotechnological
processes – Biotechnology 2020+”) and the Helmholtz-Association (Young Investigators Group). This
work was supported by the Helmholtz-Association within the Research Programme Renewable
Energies. T.R.S acknowledges support by Albert und Anneliese Konanz-Stiftung (HS Mannheim).
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446
447
Table 2. Estimation of the energy storage efficiency based on heating values and the molar electric energy for reduction conversions and combination of the two-step reactions
(details to reaction pathways at Scheme 1 and detailed data for oxidation reactions at Table SI 5).
achieved b
theoretical maximum
molar
Δ molar
heating
values
average
electrode CE
potential
molar
electric
energy
energy
storage
efficiency potential
average
energy
storage
efficiency
energy density a
(HLHV
route
z c
electrode CE
electric
energy
)
/ kJ mol-1
/kJ mol-1
/ kJ mol-1
/ kJ mol-1
/ V
/ %
/ %
/ V
/ %
/ %
educt
product
2650
I
levulinic acid → γ-valerolactone
2420
230
2
-1.5
20
1447
16
-1.3
100
251
92
II
levulinic acid → valeric acid
γ-valerolactone → 2-methyl-THF
2,7-octandione → n-octane
2420
2751
4323
2840
2863
5047
420
112
724
4
4
8
-1.5
--
60
--
965
--
44
--
-1.3
-1.3
-1.3
100
100
100
502
502
84
22
72
VI
VIII
-1.5
10
10526
7
1003
IX
X
4-hydroxy-2-butanone → 1-butanol
4-hydroxy-2-butanone → 1,3-butanediol
levulinic acid → 2-methyl-THF
levulinic acid → n-octane
2083
2083
2420
2420
2420
2420
2420
2446
2273
2863
5047
5047
2446
2273
363
190
443
207
391
26
4
2
-1.3
-1.3
--
58
2
865
12543
--
42
2
-1.3
-1.3
100
100
100
100
100
100
100
502
251
753
743
1245
984
733
72
76
59
28
31
3
I + VI
II + VII
--
--
1795
11071
11285
22963
11.53
3.53
0.23
-0.64
III + VIII levulinic acid → n-octane
IV + IX
IV + X
levulinic acid → 1-butanol
levulinic acid → 1,3-butanediol
-147
-20
448
449
a estimated according to Boie 86 b achieved in this study, c number of electrons
,
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