Electrochemical cross-coupling of biogenic di-acids for sustainable fuel production

Article abstract of DOI:10.1039/c8gc03745k

Direct electrocatalytic conversion of bio-derivable acids represents a promising technique for the production of value-added chemicals and tailor-made fuels from lignocellulosic biomass. In the present contribution, we report the electrochemical decarboxylation and cross-coupling of ethyl hydrogen succinate, methyl hydrogen methylsuccinate and methylhexanoic acid with isovaleric acid. The reactions were performed in aqueous solutions or methanol at ambient temperatures, following the principles of green chemistry. High conversions of the starting materials have been obtained with maximum yields between 42 and 61% towards the desired branched alkane products. Besides costly Pt electrodes also (Ru_xTi_1-x)O₂ on Ti electrodes exhibited a notable activity for cross-Kolbe electrolysis. As some of the products are insoluble in water, easy product isolation and reuse of the reaction solvent is enabled via phase separation. Several side products have been identified to evaluate the efficiency of the reaction and to elucidate the factors influencing the product selectivity. The yielded alkanes and esters were assessed with regard to their potential as fuels for internal combustion engines. While the longer alkanes constitute promising candidates for the compression-ignition engine, the smaller ester represents an interesting option for the spark-ignition engine.

Full text of DOI:10.1039/c8gc03745k

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Electrochemical cross-coupling of biogenic
di-acids for sustainable fuel production†
Cite this: DOI: 10.1039/c8gc03745k
d
F. Joschka Holzhäuser,a Guido Creusen,b Gilles Moos,a,c Manuel Dahmen,
a
Andrea König, e Jens Artz, a Stefan Palkovitsa and Regina Palkovits
*
Direct electrocatalytic conversion of bio-derivable acids represents a promising technique for the pro-
duction of value-added chemicals and tailor-made fuels from lignocellulosic biomass. In the present con-
tribution, we report the electrochemical decarboxylation and cross-coupling of ethyl hydrogen succinate,
methyl hydrogen methylsuccinate and methylhexanoic acid with isovaleric acid. The reactions were per-
formed in aqueous solutions or methanol at ambient temperatures, following the principles of green
chemistry. High conversions of the starting materials have been obtained with maximum yields between
42 and 61% towards the desired branched alkane products. Besides costly Pt electrodes also (RuxTi1−x)O2
on Ti electrodes exhibited a notable activity for cross-Kolbe electrolysis. As some of the products are in-
soluble in water, easy product isolation and reuse of the reaction solvent is enabled via phase separation.
Several side products have been identified to evaluate the efficiency of the reaction and to elucidate the
factors influencing the product selectivity. The yielded alkanes and esters were assessed with regard to
their potential as fuels for internal combustion engines. While the longer alkanes constitute promising
candidates for the compression–ignition engine, the smaller ester represents an interesting option for the
spark-ignition engine.
Received 30th November 2018,
Accepted 25th February 2019
DOI: 10.1039/c8gc03745k
rsc.li/greenchem
available from biorefineries, they have received much attention
in recent decades.5 Other acids such as methyl succinic acid
Introduction
Today’s world energy supply and basic organic chemical pro- are easily obtainable based on itaconic acid.6,7 In addition, ali-
duction are mainly based on fossil resources like coal, crude phatic C2–C6 acids, e.g. acetic or isovaleric acid, become avail-
oil and natural gas. The depletion of these resources has fos- able from biomass via fermentation.8–10 In the last decades,
tered research on the use of renewable carbon sources such as research focussed on efficient processes for the production of
lignocellulosic biomass, which displays a rich variety of chemi- alternative fuels and other value-added chemicals with the
cal compounds.1–3 Besides furans, sugars and others, the US aforementioned acids as feedstocks.2,11,12 Since most studies
Department of Energy (DoE) highlighted organic acids as valu- dealt with thermo-catalytic biomass transformations, first
able bio-based platform chemicals.4 For instance, the DoE examples of electricity driven reactions instead of heat have
named levulinic, succinic and itaconic acid as three of twelve only recently been reported.13–18 These studies are also motiva-
future major platform molecules derived from biomass feed- ted by the fact that electrical energy production continues to
stocks. Since these carboxylic acids are readily and abundantly shift from fossil to renewable resources such as wind and solar
energies. This shift is inherently accompanied by an intermit-
tent energy supply following temporal, seasonal and geo-
aInstitut für Technische und Makromolekulare Chemie, RWTH Aachen University,
Aachen, Germany. E-mail: palkovits@itmc.rwth-aachen.de
bInstitut für Makromolekulare Chemie, Albert-Ludwigs-Universität Freiburg,
Freiburg, Germany
cCurrently at: Max Planck Institute for Chemical Energy Conversion,
Stiftstraße 34-36, 45470 Mülheim an der Ruhr, Germany
dInstitut of Energy and Climate Research IEK-10: Energy Systems Engineering,
Forschungszentrum Jülich, Jülich, Germany
graphical fluctuations. This translates into a local and tem-
poral deviation of electricity demand and supply. Accordingly,
new technologies to transform surplus electricity into a stor-
able form are required.19
Besides water splitting and battery technologies, a new
approach uses the electrical energy from renewable resources
for electrochemical production of compounds with an
enhanced calorific value and the potential to serve as fuel
candidates.13–15,20–23 First examples of the opportunities
offered by electrocatalysis were provided by the groups of
Schröder and Li, who reported the electroreduction of levulinic
eAachener Verfahrenstechnik - Process Systems Engineering,
RWTH Aachen University, Aachen, Germany
†Electronic supplementary information (ESI) available: NMR spectra of the pro-
ducts, additional experimental results, calculations, photography’s. See DOI:
10.1039/c8gc03745k
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acid in aqueous acidic solutions at room temperature. The goes back more than 180 years, when Faraday first reported
selectivity towards valeric acid or γ-valerolactone was con- the formation of hydrocarbons for the electrolysis of an
trolled by using Pb or glassy carbon electrodes, respect- aqueous acetate solution in 1834.28 Roughly 15 years later in
ively.13,14,20,21 The obtained valeric acid could be converted to 1849, Hermann Kolbe studied the electrochemical decarboxyl-
n-octane in a consecutive step via Kolbe electrolysis either in ation of organic acids and described the formation of an oily
an aqueous or organic solvent.13,14
product (n-octane) at the anode upon electrolysis of a valeric
In 2017 Yin et al. reported the successful synthesis of three- acid solution.29 According to the reaction mechanism of Kolbe
dimensional membrane electrodes (Co3O4 nanowires on Ti) electrolysis proposed by Schäfer (Scheme 1),24 the deproto-
for the selective oxidation of alcohols.25 Via stable nano-V2O5/ nated organic acid transfers an electron to the anode resulting
Ti membrane electrodes cyclohexane could be efficiently oxi- in a carboxyl radical. A fast decomposition of this species into
dized into cyclohexanol and cyclohexanone only one year an alkyl radical and carbon dioxide is followed by a dimeriza-
later.26 Another example for the successful electrochemical tion of two radicals, to yield the so-called Kolbe product.
upgrading of bio-derived compounds into value-added pro- Besides this, the alkyl radical can also disproportionate into a
ducts is the electroreduction of furfural and 5-(hydroxymethyl) saturated and unsaturated compound. Another electron
furfural towards the respective furanes or even to 2,5-hexane- abstraction from the alkyl radical leads to a carbocation, which
dione presented by Choi and co-workers.15,27 The group of reacts to the corresponding alcohol in the presence of water
Schröder reported the production of olefin/ether mixtures and (Hofer–Moest reaction). A multitude of products is accessible
olefins with diesel-like properties via non-Kolbe electrolysis of through this reaction by substituting water by other nucleo-
fatty acids and triglycerides.16 A very recent study dealt with philes. The formation of esters can be explained by the reac-
the synthesis of drop-in fuels or more specifically of alkanes tion of a carbocation with a deprotonated acid. Moreover, by
and alcohols from biomass through fermenting corn beer into eliminating a proton from the cationic species, an unsaturated
different carboxylic acids.22 The authors successfully demon- compound is obtained (non-Kolbe electrolysis). Recently, we
strated extraction of the respective acids using a pertraction demonstrated the Kolbe and non-Kolbe electrolysis of succinic
system with mineral oil (3 wt% trioctylphosphine oxide) and acid towards valuable monomers. Additionally, a cheap and
an alkaline back solution and the coupling of the obtained facile synthesis of a Ru/TiO2-anode was established replacing
acids via Kolbe electrolysis into alkanes.22
the Pt-anode as the most common working electrode (WE) for
Noticeable, Kolbe electrolysis presents a key reaction to Kolbe electrolysis.30 Despite several studies on Kolbe electroly-
access alternative fuels from biomass-based carboxylic acids. sis of carboxylic acids31 and also other coupling techniques,
The reaction itself is well understood nowadays. Its history such as C–H aryl-dimerization,32 strategies to access promising
molecular structures with suitable fuel properties are not yet
available.
The present study proposes a new methodology for the pro-
duction of branched and long-chained alkanes as well as
esters from bio-organic mono- and diacids. The concept is
simple: Kolbe electrolysis is the required tool for the chosen
biogenic acids in order to cross-connect them similar to bricks
in a construction kit (Scheme 2). The most important benefit
of this electrochemical approach lies in the facile methodology
to synthesize fuels and fuel additives with unique and tailor
made properties. The employed different bio-derived diacids
and mono-acids depict model molecules to demonstrate the
proposed methodology. Because of the bifunctional character
Scheme 1 Reaction pathway of Kolbe electrolysis and main side
reactions.24
Scheme 2 The cross-coupling of HESA, MMSA, MHO and IVA into potential fuel and diesel compounds. The carboxylic acids can be selected like
small bricks (different colors) and connected to each other at the α-carbon atom.
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of the diacids, significantly more combinations are possible of MMSA with IVA follow a similar trend (see ESI Fig. 1a†).
than for carboxylic acids with only one acid-head function. In Therefore, the discussion herein focusses mainly on HESA as a
a first step, mono-ethyl succinic acid (HESA) or α-methyl substrate. At first, the optimal substrate ratio of HESA or
methylsuccinic acid (MMSA) was cross-coupled with isovaleric MMSA to IVA was determined. Screening experiments were
acid (IVA) to yield a branched ester. In a second step, the conducted within HESA–IVA ratios of 1 : 1 up to 1 : 16 (Fig. 1
resulting 5-methylhexanoic acid (MHO) was used as a model and Fig. ESI 1a†). According to previous findings that higher
substrate for further Kolbe cross-coupling with IVA or itself. current densities and low temperatures favour Kolbe dimeriza-
We studied the influence of various parameters on the trans- tion, all experiments were conducted at 100 mA cm−2 and
formations and confirmed the performance of the derived 0 °C.24 In addition, the use of an ice bath ensured that all reac-
compounds and mixtures as fuel compounds.
tions were carried out at constant temperatures. Preliminary
tests showed good performance for a solvent mixture of 80%
methanol and 20% water. An excessive amount of IVA was used
to promote the formation of cross-coupled product. Though, it
cannot completely suppress the dimerization of HESA or
MMSA. Side products such as the acrylates (EA and MMA), tert-
butanol, EP, TBI and MIB were in the same order of magnitude
contributing between 1 and 5% in total for all parameter vari-
ations. The yield of the cross-coupled product EMH decreases
with lower HESA–IVA ratios. While the yield of EMH reached
22% at a 1 : 1 ratio of both acids, a final yield of approximately
50% was achieved increasing the ratio to 1 : 8 or 1 : 16. For DH
an analogue behaviour was observed and for lower HESA–IVA
the yield increased to approximately 30%. As expected, the
yield of DEA decreases with lower concentrations of HESA, as
coupling with IVA becomes more likely. The results regarding
MMSA and IVA showed a similar trend. The desired cross-
coupling product yields of methyl 2,5-dimethylhexanoate
(MDH) and DH decreased with increasing ratios of MMSA to
IVA, whereas the yield of dimethyl 2,5-dimethylhexanoate
(DDH) increased. Nevertheless, all product yields with MMSA
were slightly lower compared to HESA as a substrate, yielding a
maximum of 37% MDH and 21% DH, respectively. This behav-
iour was observed for all reactant ratios and most likely caused
by a higher steric hindrance of MMSA in comparison to HESA.
According to the state of the art, the carboxylates are most
probably cleaved via an irreversible 1 e-transfer to the alkyl
radical and CO2. However, for saturated alkyl radicals the
dimerization takes place after desorption from the electrode.
This is indicated by the statistical recombination of the alkyl
radicals due to the radical mechanism of Kolbe electrolysis.24
Related to our study, there is a high probability that a radical
species of the short-handed substrate couples with a radical of
the substrate in excess. Furthermore, self-coupling is more
likely for the radicals of the substrate in excess. For the follow-
ing reactions, a ratio of 1 : 4 was chosen as technically feasible
proportion. Furthermore, at this ratio similar yields for the
cross-coupling product of HESA/MMSA with IVA were achieved
as found for lower HESA/MMSA-IVA ratios. To assure good con-
ductivity of the reaction solution, an electrolyte is generally
added for Kolbe electrolysis. Although often KOH is used as
electrolyte for Kolbe coupling reactions, in this report, NEt3
was chosen. NEt3 has the capability to deprotonate the starting
material and simultaneously serve as an electrolyte. Contrary
to KOH, it is not strong enough to hydrolyse ester functional-
ities under the applied reaction conditions, thus suppressing
the formation of undesired side-products. From an economic
Results and discussion
The first step: HESA/MMSA coupling with IVA
The main products of both cross-coupling reactions investi-
gated within this study are depicted in Scheme 3. Further
potential side-products such as isobutyl isovalerate, propene,
propane or iso-octane were either not observed or detected in
trace amounts in the liquid phase. The compounds which can
be potentially derived by cross-coupling of HESA or MMSA
with IVA are of high industrial interest. The hydrolysed forms
of diethyl adipate (DA)33 and dimethyl 2,5-dimethyl-
hexanedioate (DDH)17 as well as the acrylates34,35 are known
building blocks in polymer industry. Even the by-product tert-
butanol is known as antiknock agent and can thus be used as
fuel additive or as solvent.36 Moreover, ethyl propionate (EP)
and methyl isobutyrate (MIB) may for instance serve as sol-
vents for cellulose ethers and esters.37
The only exception is tert-butyl isovalerate (TBI) for which
there are no reported applications yet to the best of our knowl-
edge. In order to optimize the electrochemical oxidative coup-
ling of the monoester acids HESA and MMSA with the mono-
acid IVA, various reaction parameters were separately investi-
gated. All experiments were conducted with the consumption
of one farad equivalent for all substrates exactly. One farad
equivalent represents the charge amount which is required to
electrochemically convert all substrates into the desired pro-
ducts (see ESI 1. Calculations†). The results for cross-coupling
Scheme 3 Schematic illustration of the different observed products
after the electrochemical oxidation of HESA/MMSA and IVA. The colored
molecule parts of the substrates are also depicted in the end-products.
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Fig. 1 Variation of different ratios of HESA with IVA with 0.1 M NEt3 (left) and the electrolyte concentration with 0.33 M HESA and 1.3 M IVA (right).
Conditions: 0 °C, MeOH : H2O 80 : 20, 1 farad equivalent, 100 mA cm−2, WE: Pt, CE: Ti. Yield of EMH/DEA related to HESA, DH related to IVA (total
volume 5 mL).
and ecological point of view, the amount of added electrolyte
should be as small as possible (Fig. 1 and ESI Fig. 1a + 2a†).
The obtained results suggest that the cross-coupling of HESA/
MMSA with IVA were almost independent of the electrolyte
concentration. Surprisingly, large amounts of NEt3 reduce
both the yields of EMH/MDH and DEA/DDH, whereas the yield
of DH stays nearly constant. This might be due to the lower
acidity of IVA in comparison to HESA and MMSA. A lower
acidity would require a stronger deprotonating base in order to
achieve a fast oxidation and according radical formation.24
These results coincide with the trends described by the group
of Harnisch stating that an increase of the reactant concen-
trations facilitates Kolbe electrolysis rather than adding elec-
trolytes.38 Beside the electrolyte concentration, the compo-
sition of the electrolysis solvent has a decisive impact on the
success of Kolbe electrolysis. Often methanol is reported as
one of the best solvents for Kolbe electrolysis, as high faradaic
efficiencies can be achieved. In addition, it also shows notable
fuel properties and might be used as a blend component.39 On
the other hand, water possesses ecological and economic
advantages, as it is non-toxic and cheap.24 In this regard, the
Fig. 2 Variation of the solvent mixture in vol%. Conditions: 0 °C, 1 farad
equivalent, 100 mA cm−2, WE: Pt, CE: Ti, 0.33 M HESA, 1.3 M IVA, 0.1 M
cross-coupling reaction was studied in different water :
NEt3 (for 100% water: 0.1 M HESA, 0.4 M IVA). Yield of EMH/DEA related
methanol compositions (Fig. 2 and ESI Fig. 3a†). For both sub-
to HESA, DH related to IVA (total volume 5 mL).
strates (HESA and MMSA), the lowest conversion of IVA and
product yields were observed in pure water as a solvent. In
addition, due to the low solubility of the organic acids in
water, the acid concentration had to be reduced to only one-
third. Stang et al. raised the question whether the low solubi-
lity of organic acids and the insolubility of alkanes in water
might be the key reason for decreased yields.38 In this regard,
it cannot be completely excluded that the low yields in com-
parison with the experiments in solely methanol are caused by
the lower concentration of the starting materials or/and by the
insolubility of the yielded alkanes and esters. Accordingly, by
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increasing the volume percentage of methanol in the solvent
composition, the yields of EMH, MDH and DH can be
increased. For instance, 59% of EHM or 51% of MDH were
obtained using pure methanol as solvent. This leads to the
conclusion that approximately two to three times more
product can be obtained by switching from water to methanol.
Following this data, already small amounts of methanol (20
vol%) are sufficient to boost the reaction, as the yield of EMH
or MDH was increased by 10 or 20%, respectively.
Raising the percentage of alcohol to a 50 : 50 mixture did
not improve the yield substantially. Even higher methanol
ratios (80 : 20 or 90 : 10) further increased the yield of EMH or
MDH and DH. Less than 10 vol% of water does not affect the
formation of the desired product, while even small amounts of
water seem to partly inhibit the formation of DH. Schäfer24
and Vijh40 reported that the adsorption of alkoxy or alkyl rad-
icals on the electrode surface is a crucial factor for the suppres-
sion of water oxidation and the successful transformation of
acids into Kolbe dimers. Thus, the addition of organic acids to
a conductive aqueous solution shifts the onset potenial from
the water oxidation potential to higher voltages, where the
Kolbe decarboxylation takes place. Nevertheless, water oxi-
dation poses a very challenging competitive side reaction and
it already starts to lower the current efficiency and overall
product yield for water amounts below 5 vol%.24
Fig. 3 Screening of (RuxTi1−x)O2 on Ti plates. Conditions: 0 °C, 1 farad
equivalent, 100 mA cm−2, MeOH as solvent, CE: Ti, 0.33 M HESA, 1.3 M
IVA, 0.1 M NEt3. Yield of EMH/DEA related to HESA, DH related to IVA
(total volume 2 mL).
The solvent mixture does not affect the formation of tert-
butanol, EA, EP, MIB and MMA as more or less constant 53% was achieved after 1 farad equivalent, whereas for
values were found for those compounds. Despite the lower (Ru0.75Ti0.25)O2 40% were achieved. The plain RuO2 showed
efficiency in aqueous solutions, using water or alcohol/water lower efficiency compared to (Ru0.75Ti0.25)O2. These results
mixtures as a solvent system remains a promising approach: were also observed when using MMSA as the substrate. The
the coupling reaction of the acids leads to the formation of a trend suggests that the activity of sole RuO2 and (RuxTi1−x)O2
water-insoluble organic phase mainly consisting of alkanes in the rutile structure is not only influenced by RuO2 alone but
and esters. In this manner, addition of at least 50 vol% of also by the amount of TiO2 respectively. According to the raw
water to the reaction solution enables facile isolation of the material costs, (RuxTi1−x)O2 electrodes are clearly advan-
products by phase separation.
Generally, smooth Pt electrodes are used for the Kolbe elec- trode while on the other hand only 0.8–1.6 mg cm−2 of Ru was
trolysis, as it is a highly active electrode material for the coated on
Ti electrode. The new anode materials
tageous. Approximately 500 mg cm−2 Pt was used as bulk elec-
a
electrochemical oxidative coupling. One major drawback of Pt undercut the cost of solid Pt electrodes by several orders of
electrodes is their comparably high price, especially when it magnitude. Additionally, the new electrodes have
comes to larger setups, e.g., in industrial plants.
been used several times for the same reaction and thus
Recently, we have shown that using readily synthesized can be considered as stable under the applied reaction
(RuxTi1−x)O2 on Ti plates as working electrodes for the self- conditions.
coupling of HESA into DEA leads to comparable yields of the
The conversion for HESA lies between 91 and 97%, for
coupled HESA.30 As the best results were achieved for (Ru1Ti0) MMSA between 89 and 96%. Although there are several identi-
O2, (Ru0.75Ti0.25)O2 and (Ru0.5Ti0.5)O2, we tested these promis- fied side products such as ethyl propionate and acrylate or
ing electrodes also for the cross-coupling of HESA or MMSA methyl isobutyrate and methacrylate, respectively, the mass
with IVA (Fig. 3 and ESI Fig. 4a†).
balances are not fully closed. In the worst cases, a mass loss of
Full analytic descriptions of the freshly synthesized electro- around 45% is found (e.g., for (Ru0.5Ti0.5)O2) whereas the
des are given in our previous study.30 Although (RuxTi1−x)O2 results for Pt had a maximum mass loss of around 25%. The
electrodes of high Ru content also work in aqueous solutions, reason for the broad gap between yield and conversion might
we chose methanol as a solvent to achieve high yields in the be due to the evaporation of volatile products (e.g., ethyl acry-
cross-coupling reactions. The obtained results show that ruthe- late), the over-oxidation into CO2, several unidentified pro-
nium–titanium dioxide coatings as working electrodes are suit- ducts in lower quantities and low extraction values from the
able alternatives to neat Pt. Although efficiency decreases aqueous phase to the organic n-heptane phase. A closed
slightly when lowering the Ru content, the results are in the system and an optimized extraction procedure is believed to
same order of magnitude as Pt. For a full Pt plate a yield of overcome these challenges.
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with stronger bases as electrolytes are possible. Judging from
the yield obtained when using NEt3 or KOH and a Pt plate as
working electrode, the stronger base decreases the activity. The
opposite case is observed when (RuxTi1−x)O2 on Ti electrodes
are used. The stronger base KOH leads to a general increase
in yield for the cross-coupling products DMO, DMD and
DH. Reasons for this trend can be found in the low
oxygen evolution activity of RuO2. The group of Hu reported
that RuO2 coated Ti anodes show a much lower activity for
oxygen formation in alkaline solutions while acidic solutions
do not seem to suppress the formation of oxygen.41 In
addition, it should not be neglected that the pH can have a
large influence on adsorption and desorption processes. Hu
Scheme 4 Illustration of the different observed products after the
electrochemical oxidation of MHO alone and together with IVA. The
coloured molecule parts of the substrates are also depicted in the end-
products.
reported
a lower adsorption/desorption peak value in
CV-measurements. Transferring these findings to the results
reported in this work, undesired side reactions like the for-
mation of over-oxidized products or the oxygen evolution reac-
tion are suppressed while Kolbe electrolysis is favoured
leading to higher yields of the cross-coupled products, when
strong bases such as KOH are used. In case of Pt instead, the
higher pH causes a shift towards non-Kolbe and over-oxidized
products.
Production of branched alkanes from MHO
In the second step, the hydrolysed form of ethyl-5-methyl-
hexanoate (EMH) was used as starting material for the syn-
thesis of the final products. One approach consisted of the
coupling of IVA and MHO into 2,7-dimethyloctane while the
second showed that MHO can also be coupled with itself to
form 2,9-dimethyldecan (Scheme 4).
Fig. 4 depicts the results of a systematic screening of
(RuxTi1−x)O2 on Ti plates and Pt plates together with NEt3 or
KOH as electrolyte.
This assumption becomes more evident studying (RuxTi1−x
)
O2 on Ti electrodes for Kolbe electrolysis with MHO as single
substrate. The yield of DMD can be increased by almost 10%
when changing the electrolyte NEt3 (33%) to KOH (41%) and
using RuO2 on Ti (Fig. 5). The group of Harnisch reported that
they could not observe any effect due to a change in the pH
while the group of Bandarenka stated a strong correlation
between the pH and the oxidation activity of Pt for hydrogen
and oxygen.42
The conversion of MHO reaches between 79 and 90%.
Since there were no challenges with ester groups, experiments
In our study the obtained results do neither confirm the
importance of a high pH for the oxidation activity nor the
independence of the activity on the pH.
Fig. 4 Screening of (RuxTi1−x)O2 on Ti and Pt plates with different elec-
trolytes. Left: Using 0.1 M NEt3 as electrolyte and base. Right: Using 0.1
M KOH as electrolyte and base. General conditions: 0 °C, 1 farad equi-
valent, 100 mA cm−2, MeOH as solvent, CE: Ti, 0.33 M MHO, 1.3 M IVA.
Yield of DMD/DMO related to MHO, DH related to IVA (total volume
2 mL).
Fig. 5 Screening of (RuxTi1−x)O2 on Ti and Pt plates with different elec-
trolytes. General conditions: 0 °C, 1 farad equivalent, 100 mA cm−2
MeOH as solvent, CE: Ti, 1 M MHO (total volume 2 mL).
,
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Fuel assessment
value for DCN means higher auto-ignition tendency, whereas a
higher value for RON means higher auto-ignition resis-
Electrochemical conversion of MMSA and HESA can store elec- tance.49,61 We employed the structural group contribution
trical energy in a chemical form. From an energetic point of model proposed by Dahmen and Marquardt48 to predict DCN
view, the branched esters and alkanes obtained in this study of the branched alkanes and esters obtained in this study.
may thus be envisioned as renewable fuel components for use Furthermore, DCN was converted into RON by applying the
in internal combustion engines. To make a first assessment of correlation formula derived by Perez and Boehman.57 As can
their suitability as tailor-made fuels, we consider two pro- be seen from Table 1, TBI stands out with its high predicted
perties of paramount importance to smooth engine operation: RON. This suggests that TBI exhibits high knock-resistance.
auto-ignition quality and volatility.43 Table 1 compares the Due to its relatively high boiling point, we do not expect TBI to
normal boiling points of the synthesized compounds to the be a pure compound fuel, not even in a tailor-made SI combus-
boiling ranges of typical fossil-based gasoline and diesel fuels. tion system.43,62 However, TBI could be attractive as an octane
In spark-ignition (SI) engines, low fuel volatility can lead to booster component for blending with fossil or renewable base
problems when starting the engine at cold conditions. fuels. Small esters, specifically methyl acetate and ethyl
Moreover, high boiling point components can cause engine oil acetate, have been found to effectively increase the RON of
dilution and excessive deposit formation.44,45 Generally, com- gasoline.63 Furthermore, TBI/methanol blends might offer a
pression–ignition (CI) engines can burn less volatile fuel, but promising fuel option both from a process and a product per-
again, too heavy fuel components can result in cylinder wall spective. Methanol is already used as a solvent in the syn-
impingement and hence oil dilution, particularly in case of thesis, thus, instead of removing it, it could be left as a blend
low-temperature combustion strategies.46,47 By promoting the component. This potentially leads to a more facile product
evaporation of injected liquid fuel, higher fuel volatility is separation and purification thereby improving an overall
thought to improve mixture homogeneity and thus to yield process. With respect to CI engines, the larger alkanes DMD,
lower soot emissions.44 Fuel auto-ignition quality can be used DMO, TEMO and TMO constitute promising candidates
to differentiate between fuels for SI and CI engines.43,48,49 because they are associated with diesel fuel-like DCNs (>40)49
Because in SI engines the ignition event is controlled by the and volatilities. In fact, branched medium-sized alkanes like
spark plug, the fuel/air mixture has to withstand auto-ignition DMD, DMO, TEMO and TMO constitute a significant share of
in the so-called end gas, i.e., those parts of the charge that fossil-based diesel fuel.64 This structural similarity together
have not yet been reached by the flame. The phenomenon of with the favourable DCNs and boiling points suggest that
undesired auto-ignition in SI engines is referred to as engine DMD, DMO, TEMO and TMO can be blended with convention-
knock and severely limits SI engine efficiency.50 On the con- al fuels in substantial percentages without the need to change
trary, the operating principle of the CI engine is based on fuel existing combustion systems or fuel distribution infrastruc-
auto-ignition occurring briefly after injection of the fuel into tures. Finally, predicted DCNs and RONs of DH, EMH and
compressed hot air.
MDH are poorly suited to the requirements of SI and CI
CI engines therefore require fuel that is prone to auto- engines. Adding these compounds to either gasoline or diesel
ignition. The fuel’s readiness to auto-ignite is often expressed fuel would deteriorate ignition quality. The rather high boiling
as derived cetane number (DCN)59 or research octane number points of EMH and MDH also hinder blending with gasoline.
(RON).60 DCN and RON are inversely correlated, i.e., a higher From a fuels perspective, the new synthetic approach thus is con-
Table 1 Physico-chemical properties of relevant products compared to those of typical fossil-based gasoline and diesel fuels. Derived cetane
number (DCN) was estimated with the model proposed by Dahmen and Marquardt.48 Research octane number (RON) has been computed from
DCN with the equation proposed by Perez and Boehman57
Compound
DCN
RON
TBoil [°C]
Assessment
2,5-Dimethylhexane (DH)
tert-Butyl isovalerate (TBI)
34.9
15.9
51.4
99.3
10851
154–15652
Potential blend component for
SI engine fuels (octane improver)
Ethyl 5-methylhexanoate (EMH)
Methyl 2,5-dimethylhexanoate (MDH)
26.7
20.5
71.2
87.7
181–18253
190a
2,9-Dimethyldecane (DMD)
2,7-Dimethyloctane (DMO)
2,5,6,9-Tetramethyldecane (TEMO)
2,4,7-Trimethyloctane (TMO)
59.6
47.8
54.3
44.5
−5.0
21.9
7.1
200a
16054
245a
177a
Potential blend components for
CI engine fuels
29.6
Petroleum diesel55,56
Petroleum gasoline55
40–45
—
—
91
180–360
25–210
a Normal boiling point was predicted with Joback’s method.58
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fied and their potential applications discussed. Furthermore, a
first assessment of the obtained products with regard to their
use in internal combustion engines has been conducted. This
yielded candidates for blending applications targeting both
spark-ignition and compression–ignition engines.
Notably, the proposed strategy follows the rules of green
chemistry. Mild reaction conditions have been applied, only
water and methanol were used as solvents and waste was mini-
mized by using a heterogeneous electrode. The reactions con-
ducted in aqueous solutions showed a phase separation of the
main products and the reaction mixture. This might facilitate
purification processes and enable the reuse of the reaction
solvent. In addition, (RuxTi1−x)O2 on Ti electrodes are effective
for the anodic decarboxylation and cross-coupling reaction
with the promise to lower the content of precious metals from
107 mg cm−2 Pt (50 μm thick electrode) to up to 1.6 mg cm−2
Ru, while Ru in general is the less expensive metal to this date.
This proof of concept study confirmed cross-coupling via
Kolbe electrolysis as a promising tool for the development of a
completely new sets of tailor made chemicals and fuels. In the
concept, we combine renewable carbon feedstocks with renew-
able energy providing a potential future technology for an elec-
trified biorefinery.
Fig. 6 Concept of a continuous cell for production of fuels from
organic acids and the separation of H2 and CO2.
sidered most promising with regard to its ability to yield either
small, compact esters like TBI as octane improvers for SI engine
fuels or medium-sized alkanes for blending with CI engine fuels.
In future studies, an experimental assessment of a wider range of
physico-chemical properties of different blends, followed by
investigations in combustions engines are in the focus.
Future prospects
Experimental
Besides the presented branched alkanes, the electrochemical
system produces pure CO2 as by-product of Kolbe electrolysis
and H2 on the cathode (Fig. 6). In this regard, the discussed
Kolbe electrolysis could present an interesting alternative to
oxygen evolution usually accompanying the electrochemical
hydrogen generation. Also the released CO2 bears a high poten-
tial for further applications. For instance, it can be used in
methanation processes65 or be recovered in an electrocatalytic
reductive CO2 electrolysis.66–68 Particularly interesting is the
idea of using H2 and CO2 as gaseous mixture in a molten car-
bonate fuel cell. Levy et al.69 reported in the report of the
Department of Energy (DoE, USA) of 1980 that almost 50% of
the necessary energy consumption of Kolbe electrolysis could
be regained by applying this technique. In addition, two major
advantages of molten fuel cell lie in the flexibility towards the
used fuel (natural gas, hydrogen and CO) and that no Pt metal
catalyst has to be employed. In this regard, a molten carbonate
cell might be used to recover parts of the energy for the Kolbe
electrolysis but in case of a lower demand of biofuels the cell
can produce electrical energy with alternative fuels.69,70
Electrochemical setup
In all electrolysis experiments, we used a potentiostat–galvano-
stat (Metrohm PGSTAT302N) as power supply. It was possible
to choose a two electrode configuration in this work, as the
focus lied on a constant current instead of an exact measure-
ment of the potential. Throughout the electrolysis the poten-
tial was recorded (Metrohm Nova) and adapted in order to
keep the current at a constant level. An undivided cell was
used. Furthermore, smooth Pt or RuO2/TiO2 on Ti plates were
used as working electrode (anode), whereas the counter elec-
trode (CE) (cathode) was made from Ti material. The electro-
des were gently washed with water and ethanol after every use.
No decrease in activity could be observed after several runs
with the same electrode. The electrolysis experiments were
conducted in a 5 mL glass vial with an inhouse 3D-printed
cover, holding the electrodes in place (see ESI†). All reported
current densities are normalized to the geometric surface area
of the working electrode (surface = 2 cm2).
All chemicals and materials within this work if not stated
otherwise were used as purchased.
General reaction procedure
Conclusion
The substrates and the electrolyte (triethylamine (NEt3 or
We could demonstrate a versatile electrochemical strategy to KOH) were dissolved in methanol and/or water. The amount of
access tailored fuel compounds from biomass-based carboxylic solvent was adapted to obtain a total volume of 2 or 5 mL. In
acids. As case study, cross-coupling of ethyl hydrogen succinate, order to maintain a constant low temperature the electro-
methyl hydrogen methylsuccinate and methyl hexanoic acid chemical cell was placed in an ice-bath. A current of 100 mA
together with isovaleric acid by Kolbe electrolysis allows access cm−2 was applied, and a total charge of 1 farad equivalent was
to different esters and alkanes. The side products were identi- passed (calculations see ESI†). After electrolysis, n-heptane
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(2 mL) was added and the two-phasic system was intensely agi- vial. The reaction mixture was stirred under microwave
tated. After phase separation, the n-heptane phase was ana- irradiation (100 W) at 120 °C for 10 min in a microwave syn-
lyzed by GC using ethyl heptanoate as internal standard.
thesis oven (CEM Discover SP). After the reaction was finished,
a phase separation and color change of clear to orange-brown
was observable. The upper phase was washed with an aqueous
Metal oxide coated titanium electrodes
Mixed metal oxide on Ti electrodes are prepared following a Na2CO3 solution before the pH was adjusted to 8 by adding
modified literature procedure.71 The grade
Ti sheets further Na2CO3 solution. The organic phase was dried over
2
(0.5 mm) are cut, sanded with 400 grit paper and rinsed with MgSO4 and remaining traces of tert-butanol were removed
acetone and water prior to coating. (RuxTi1−x)O2 coatings are under reduced pressure to yield in an orange-brown oil.
prepared for x = 1.00, 0.75, 0.50. Stock solutions of 1.0 M TiCl3 Purity = crude product, determined by NMR.
and 1.0 M RuCl3 both in 10% aqueous HCl are prepared,
1H NMR (400 MHz, (CD3OD)): δ = 2.10 (d, J = 6.4 Hz, 2H),
mixed in the appropriate ratio and diluted with isopropanol 2.04 (m, 1H), 1.47 (s, 9H), 0.96 (d, J = 6.5 Hz, 6H). MS (70 eV);
2 : 1 by volume. Precursor solutions are applied to the surface m/z (%) = 129 (1) [M+ − CH3, –CH3], 103 (20) [M+ − C2H5], 85
and dried in an oven at 80 °C for 10 minutes prior to calcina- (95), 60 (22), 57 (100), 43 (17), 41 (45).
tion at 470 °C for 10 min. This procedure is repeated four
Isolation of ethyl 5-methylhexanoate (EMH)
times, and the last coating was calcined at the same tempera-
ture for 4 h.
1.2 g of HESA (8.3 mmol, 1 eq.) were combined with 3.4 g IVA
(33.4 mmol, 4 eq.), 170 mg NEt3 (4% of total acid amount),
3.8 mL methanol and 16.7 mL water. The reaction solution
Synthesis of MMSA
The MMSA was synthesized following a method of Parker and was portioned into five 5 mL electrochemical cells, and the
coworkers with small modifications.72 20 g of itaconic acid reaction conditions were the following: 0 °C, 200 mA, 2 cm2,
dimethyl ester (127 mmol) were dissolved in a mixture of 1.0 farad equivalent. After electrolysis, the (upper) product
72 mL formic acid, 8 mL water and 10 mL methanesulfonic phase was separated from the water methanol solution and
acid. The reaction mixture was heated to 100 °C for 10 min submitted to liquid chromatography. As stationary phase silica
and then poured into 200 mL of ice-water, followed by an was used, while the eluent mixture was n-hexane and ethylace-
extraction with ca. 400 mL dichloromethane. The organic tate (9 : 1). After solvent evaporation a colorless liquid was
phase was dried over MgSO4 and the solvent removed under obtained.
reduced pressure. The obtained itaconic acid mono-ethyl ester
was dissolved in 14 mL methanol and reduced under 60 bar
Rf (TLC, n-hexane/ethylacetate 9 : 1) = 0.5
1H NMR (400 MHz, (CD3OD)): δ = 3.66 (s, 6H), 2.47–2.35
H2 at 60 °C for 90 min in a steel autoclave, using 300 mg of Pd (m, 2H), 1.71–1.57 (m, 2H), 1.48–1.32 (m, 2H), 1.14 (d, J =
on carbon (5% Pd). After the reduction, the catalyst was 7.0 Hz, 6H). MS (70 eV); m/z (%) = 101 (15), 87 (15), 74 (100),
removed by filtration and the solvent was evaporated. The 69 (25), 59 (42), 55 (22), 43 (60), 41 (47), 39 (25).
crude MMSA was dissolved in water, the pH adjusted to 8 by
Isolation of dimethyl 2,5-dimethylhexanedioate (DDH)
addition of NaOH and then washed with Et2O. The pH-value
of the aqueous phase was then adjusted to 2.5 and the product 2.37 g of MMSA (16.2 mmol) were combined with 100 mg NEt3
was extracted with Et2O. The solution was dried over MgSO4 (6% of amount MMSA), 1.5 mL methanol and 6 mL water. The
and the solvent removed under reduced pressure to yield in a reaction solution was portioned into two 5 mL electrochemical
colorless liquid.
cells, and the reaction conditions were the following: 0 °C,
1H NMR (400 MHz, (CDCl3)): δ = 3.69 (d, J = 1.7 Hz, 3H), 200 mA, 2 cm2, 1.5 farad equivalent. After electrolysis, the
2.96–2.85 (m, 1H), 2.83–2.41 (m, 2H), 1.23 (dd, J = 7.2, 1.9 Hz, upper product phase was separated from the water–methanol
3H).
solution and submitted to liquid chromatography. As station-
ary phase silica was used, and the eluent mixture was n-hexane
and ethylacetate (4 : 1). After solvent evaporation a colorless
Synthesis of HESA
30 g mono-ethyl sodium succinate (178 mmol) were dissolved liquid was obtained.
in 50 mL water and the pH was adjusted to 2 with an aqueous
hydrogen chloride solution. The solution was extracted with di-
Rf (TLC, n-hexane/ethylacetate 2 : 1) = 0.5
1H NMR (400 MHz, (CD3OD)): δ = 3.66 (s, 6H), 2.47–2.35
ethylether and the organic phase was dried over MgSO4, before (m, 2H), 1.71–1.57 (m, 2H), 1.48–1.32 (m, 2H), 1.14 (d, J =
the solvent was removed under reduced pressure. The product 7.0 Hz, 6H). MS (70 eV); m/z (%) = 171 (5) [M+ − OCH3],
is a clear colorless oil.
143 (20) [M+ − CHO].
1H NMR (400 MHz, (CDCl3)): δ = 4.09 (q, J = 7.2 Hz, 2H),
2.62 (m, 2H), 2.55 (m, 2H), 1.20 (t, J = 7.2, 3H).
Isolation of methyl 2,5-dimethylhexaneoate (MDH)
0.73 g of MMSA (5 mmol, 1 eq.) were combined with 1.63 g IVA
(16 mmol, 4 eq.), 80 mg NEt3 (10% of amount MMSA) and
Synthesis of tert-butyl isovalerate
A solution of IVA (91 mmol, 9.3 g, 1 eq.), tert-butanol 7.2 mL methanol. The reaction solution was portioned into
(182 mmol, 13.5 g, 2 eq.) and 98% fuming sulfuric acid (1.2 g, four 2 mL electrochemical cells, and the reaction conditions
5 wt% of solution) were combined in a microwave reaction were the following: 0 °C, 100 mA, 1 cm2, 1.0 farad equivalent.
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After electrolysis the mixture was concentrated and dried
under high vacuum. The material was purified by distillation
(25–150 °C) and the distillate was clear but still contaminated
oil.
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