Sustainable electroorganic synthesis of lignin-derived dicarboxylic acids

Article abstract of DOI:10.1039/d0gc02210a

The oxidative ring opening of lignin-derived alkylated cyclohexanols to bio-based alkylated dicarboxylic acids is successfully performed by an electrocatalytic conversion. To establish this transformation as a green method, we developed a simple protocol for the anodic oxidation at nickel oxide-hydroxide (NiOOH) foam anodes in caustic soda in both a batch and flow electrolysis approach.

Full text of DOI:10.1039/d0gc02210a

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DOI: 10.1039/D0GC02210A
COMMUNICATION
Sustainable Electroorganic Synthesis of Lignin-derived
Received 00th June Dicarboxylic Acids
2
020,
a
b
a
Anna Lisa Rauen , Frank Weinelt , Siegfried R. Waldvogel
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The oxidative ring opening of lignin-derived alkylated its alkylated derivatives by the nitric acid process has significant
cyclohexanols to bio-based alkylated dicarboxylic acids is disadvantages such as corrosion and safety issues, in particular
1
0
successfully performed by an electrocatalytic conversion. To due to the unavoidable formation of highly toxic NO
x
.
For a
establish this transformation as a green method, we developed a fully sustainable process, the valorisation of biomass goes hand
simple protocol for the anodic oxidation at nickel oxide-hydroxide in hand with the demand for green chemical methods for
(
NiOOH) foam anodes in caustic soda in both, batch and flow further transformations, such as electro synthesis (Figure 1).
electrolysis approach.
With this versatile method, waste producing, toxic, or difficult-
to-handle reagents can be avoided by the direct use of electric
A growing awareness for the environment and limited fossil
resources are calling for sustainable alternatives to the common
feedstock of our actual chemical infrastructure. New sources for
11–15
current as a reagent.
In addition, many individual steps can
be abbreviated or simplified by electrolysis in comparison to
classical synthesis. Furthermore, this method is inherently safe,
as it immediately stops when the current is switched off. Thus,
the danger of thermal run-away-reactions is eliminated.
The NiOOH electrode has proven to be an excellent anode for
the oxidation of alcohols to ketones or carboxylic acids. Schäfer
1
commodities and fine chemicals must be developed urgently.
Lignin is one potential source for renewable chemicals, fuels,
2
and materials. During the pulping process of wood to cellulose,
large amounts of lignin accumulate as a by-product. With the
holistic use of this side stream in mind, lignin represents a
promising source for carbon building blocks instead of the
current use as low-value thermal energy source. For instance,
lignin can be converted into a variety of alkylated cyclohexanols
via catalytic reductive fractionation. Already in 1938 the
catalytic hydrogenolysis of hardwood lignin, was reported by
using copper chromite as catalyst, whereby 4-
3
,4
5
,6
7
propylcyclohexanol was obtained in 11 wt% yield. This
compound and other lignin-derived alkylated cyclohexanols
provide a potential source to biogenic dicarboxylic acids, such
as substitutes for adipic acid, which might be employed as
precursors for alkylated variations of polyesters and
polyamides.8 The conventional production of adipic acid and
,9
a. Johannes Gutenberg University Mainz, Department of Chemistry
Duesbergweg 10-14, 55128, Mainz/D, waldvogel@uni-mainz.de
b.
Evonik Resource Efficiency GmbH, Postal Area 16, Building 1227, Office 213 Paul-
Baumann-Straße 1, 45772 Marl/D
Figure 1: Transformation of lignin to dicarboxylic acids by electro catalytic
conversion.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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DOI: 10.1039/D0GC02210A
Scheme 1: Reaction scheme for the anodic oxidation of 4-methylcyclohexanol (1) to 3-methyladipic acid (2) and shorter by-products 3 and 4.
et al. reported the electrolysis of alcohols in caustic soda.16 To
convert less water-soluble substrates, tert-butanol was added
or petroleum ether was employed in a biphasic mixture. Low-
molecular-weight alcohols could be oxidized to carboxylates at
room temperature in good yields. In a subsequent reaction
during the oxidation of secondary alcohols, 10-15% of the
formed ketones were converted to the corresponding
carboxylic acids by C-C bond cleavage. Petrosyan et al. used this
side reaction to prepare adipic acid from cyclohexanol via
cyclohexanone.17 The electrochemical conversion starting from
3
,3,5-trimethylcyclohexanol at similar conditions yields 2,2,4-
trimethyladipic acid and 2,4,4-trimethyladipic acid by anodic
ring cleavage.18 NiOOH as active anode was successfully
employed in the conversion of renewables.
1
Figure 2: H NMR spectrum (400 MHz, CDCl
3
) of 3-Methyladipic acid (2)
and by-products 3 and 4
1
9,20
In this work, the electro catalytic oxidation of lignin-derived
alkylated cyclohexanols to bio-based dicarboxylic acids is
reported in a batch and a flow electrolyzers. To establish this
transformation as a green method, we developed a simple
protocol for the anodic oxidation at NiOOH foam anodes in
caustic soda by using 4-methylcyclohexanol as a test
compound. Several essential reaction parameters were
systematically studied in the batch approach with regard to
their influence onto product yield: (i) electrode material, (ii)
current density, (iii) amount of applied charge, (iv) temperature,
and (v) electrolyte composition.
product can be obtained. To enhance the performance of the
Ni-electrodes tremendously, a preceding anodic activation of
the electrode in a buffered NiSO /NaOAc/NaOH solution is
4
carried out to deposit a thin and porous layer of NiOOH onto
16
the electrode surface following a protocol from Schäfer et al.
NiOOH is an efficient oxidizer, which is formed and continuously
regenerated in alkaline media onto the anodic surface. Thus,
the surface structure plays an important role. Therefore, we
compared different morphologies such as foam, sheet and net.
Since the electro catalyst is formed and active at the electrode
surface, the larger surface of the foam electrode resulted in the
best yield of 51% 3-methyladipic acid (2) by applying the same
geometrical current density. The high reactivity and successful
regeneration of NiOOH could also be demonstrated with a
graphite electrode, which is coated by a thin NiOOH layer. In
contrast, a pristine graphite electrode is not capable for the
The electrolysis of 4-methylcyclohexanol (1) was conducted in
an undivided beaker-type glass cell equipped with two
electrodes at constant current conditions. In addition to 3-
methyladipic acid (2) as main product, shorter diacids such as
succinic (3) and glutaric acid (4) derivatives were also formed in
minor quantities (Scheme 1). Since test reactions showed, that
the dicarboxylic acid is not further oxidized at the experimental
conditions, the formation of these by-products can be assumed
by partial oxidative decomposition of the cyclohexanone ring in
2
1
an early step of the electrolytic conversion. The workup of the
electrolyte involves an extraction with methyl tert-butyl ether
of the alkaline mixture to remove unreacted starting material
and a subsequent extraction of the acidified reaction mixture at
pH 1. The acidic extract contains the dicarboxylic acids, which
can be analysed without further modification by HPLC/MS and
1
NMR. The product can be quantified by H NMR by comparing
the integrals of the methyl group of 3-methyladipic acid (Figure
2
) and 13C NMR using 1,3,5-trimethoxybenzene as internal
standard.
Figure 3: Effect of anode material onto the electrochemical synthesis of 2.
Electrolytic conditions: stainless steel cathode, 0.1 M starting material, 1 M
2
Initial experiments were carried out to find a suitable anode
material (Figure 3). With a Ni-based electrode, the desired
NaOH, H O, 5 mA/cm (geometric current density), 8 F, 20 °C.
2
2
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desired oxidation reaction. Following previous work in our lab
where active MOOH-electrodes were used, we studied further
transition metals for the oxidation of cyclohexanol.
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DOI: 10.1039/D0GC02210A
2
2,23
Vanadium and molybdenum showed no conversion of the
starting material 1 at all. Also, a cobalt electrode and a cobalt
electrode with a previously prepared cobalt oxide-hydroxide
layer to enhance the reactivity according to the activation of the
Ni electrodes, gave almost no conversion to the desired
product. In accordance with the lower effective current density
at the larger electrode surface of the three-dimensional NiOOH
foam electrodes, low geometrical current densities about
2
.5 mA/cm2 were most effective for the transformation.
Figure 4: Effect of applied charge onto the electrochemical synthesis of 2.
Electrolytic conditions: NiOOH (foam) anode, stainless steel cathode, 0.1 M starting
material, 1 M NaOH, H O, 5 mA/cm2 (geometrical current density), 20 °C.
2
However, to reach a decent balance between yield and reaction
time, the transformation was carried out at 5 mA/cm
geometric current density for the optimization reactions. Table
2
1
displays the influence of the solvent and supporting
increase the solubility of 4-methylcyclohexanol is the
temperature. Raising the reaction temperature from 20 °C to
electrolyte onto the electrolysis. The solubility of 4-
methylcyclohexanol in water is very low and especially in 1 M
caustic soda, the starting material is not soluble in appropriate
concentrations and only vigorous stirring provides a reasonable
availability. To reach a higher solubility in the electrolyte,
several organic co-solvents, as well as different supporting
electrolytes, were tested for their suitability in the system. Tert-
butanol in several ratios and with different supporting
electrolyte concentrations as well as dimethyl sulfoxide and
tert-amylalcohol gave no better conversions to the desired
product. Also, lower concentrations of caustic soda than the
initially used 1 M did not result in a better solubility and
conversion of the starting material. Thus, it can be supposed
that the basicity is too low to sufficiently generate the enol
form, which is (according to the mechanism) most likely to be
oxidized.21 As expected, higher concentrations of sodium
hydroxide in the electrolyte gave no higher yield of 3-
methyladipic acid, due to the low solubility of the starting
material. The most effective electrolyte system was 1 M caustic
soda without any further additives in the greenest of all
solvents, water. Another way to
5
0 °C showed no significant influence on the conversion of the
starting material. Higher temperatures than 50 °C decreased
the conversion due to the volatility of the starting material.
Regarding the applied charge, the best yield was achieved with
the slightly higher amount of charge of 8.5 F instead of the
Table 1: Effect of electrolyte composition on the electrochemical synthesis of 2.
Entry Solvent system
v/v)
Supporting
electrolyte
0.1 M NaOH
0.5 M NaOH
1.0 M NaOH
2.0 M NaOH
5.0 M NaOH
Yield
(%)
20
45
51
45
16
3
(
1
2
3
4
5
6
7
8
9
1
1
1
1
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
O
O
O
O
O
O
O
1.0 M K
1.0 M KOH
1.0 M NaOH
2 3
CO
50
1
t
O/ BuOH (3:7)
O/ BuOH (2:1)
O/ BuOH (1:1)
O/PE (1:1)
O/DMSO (1:1)
O/ AmylOH (2:1)
t
0.25 M NaOH 26
t
0
1
2
3
0.18 M KOH
1.0 M NaOH
1.0 M NaOH
1.0 M NaOH
7
16
7
t
27
Electrolytic conditions: NiOOH (foam) anode, stainless steel cathode, 0.1 M
2
starting material, 1 M NaOH, H
PE: petroleum ether.
2
O, 5 mA/cm (geometric current density), 20 °C.
Scheme 2: Optimized reaction conditions and substrate scope.
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theoretically required amount of charge of 8 F. The yield Table 2: Optimisation of flow mode and flow rate for the synthesis of 2.
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DOI: 10.1039/D0GC02210A
increased with increasing applied charge, culminating at 8.5 F
and stagnating for higher charges (Figure 4).
Entry
Mode
Flow rate
Yield 2
%)
0.47 µL/min 51
(
The best electrolytic conditions determined by this screening
were then applied to further lignin-derived 4-alkylated
cyclohexanols (Scheme 2). Only minor adjustments of the
temperature were necessary to increase the yield for some
derivatives with the longer side chains.
1
2
3
4
5
6
7
8
single-pass
cycling
cycling
cycling
cycling
cycling
cycling
cycling
0.1 mL/min
1.0 mL/min
5.0 mL/min
7.5 mL/min
53
56
58
60
10.0 mL/min 62
12.5 mL/min 64
15.0 mL/min 60
Since the scale-up of electrochemical methods in a batch
approach faces some major drawbacks like mixing difficulties,
and a decreasing volume-to-surface ratio from electrolyte to
electrode in large batches, it is favourable to transfer the
Electrolytic conditions: NiOOH (foam) anode stainless steel cathode, 0.1 M
2
starting material, 1 M NaOH, H
2
O, 5 mA/cm (geometrical current density), 20 °C.
2
4
reaction to a flow process. For this reason, a great deal of
effort is currently invested in the design of suitable flow
electrolyzers.2
5–30
Designs for using porous electrodes are
Conclusions
3
1–34
mostly dealing with carbon felt electrodes.
The Waldvogel
A simple and green protocol for the electrochemical oxidation
of lignin-derived alkylated cyclohexanols to alkylated adipic acid
derivatives at a NiOOH foam electrode was established.
electro-conversion can be conducted in both batch and flow
electrolyzers. For the latter, a novel foam electrode device for a
commercial flow cell was developed in order to scale up this
electrosynthesis. The electrolyte only consists of the starting
material, and caustic soda and does not require any additional
additives. This emphasizes the intrinsic sustainability of the
developed method. With this protocol five alkylated
dicarboxylic acids were obtained in yields up to 64%.
Lab has also designed a highly modular flow cell for
electrosynthesis.3
5,36
For the current issue, we developed the
existing system to use foam electrodes in the flow cell by adding
a Teflon spacer that brings the Ni-foam electrode in contact to
a connected Ni-sheet electrode, but keeps it in distance to the
counter electrode. The active anode surface consists of a Ni
foam (20 mm x 60 mm x 6 mm) connected to a 20 mm x 60 mm
nickel sheet placed in a Teflon half-cell (Figure 5). A membrane
pump is used to circulate the electrolyte through the flow cell.
We carried out the electrosynthesis in this modified flow cell in
a single-pass mode and a cycling mode (electrolyte passes the
cell then several times). In both cases, 25 mL of the reaction
mixture, comparable to the batch process, were converted.
The optimised conditions of the batch process could be
Conflicts of interest
consistently transferred to the flow process with the same yield There are no conflicts to declare.
in the single-pass mode (Table 2, entry 1). The advantage of the
operation in cycling mode, is the efficient removal of hydrogen,
the product of the cathodic reaction, so that the electrode Acknowledgements
surface is not blocked by gas bubbles. Operating in cycling mode
This work was funded by the Federal Ministry of Education and
with flow rates between 0.1 mL/min and 15.0 mL/min
increased the yield of 3-methyladipic acid in general. By
pumping the electrolyte with a flow rate of 12.5 mL/min
through the electrolysis cell, the yield of 3-methyladipic acid can
be increased to 64%.
Research, Project APPLE (FKZ 031B0663A).
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