Electrochemistry for the Generation of Renewable Chemicals: One-Pot Electrochemical Deoxygenation of Xylose to Δ-Valerolactone

Article abstract of DOI:10.1002/cssc.201700209

In this study, the electrochemical conversion of xylose to δ-valerolactone via carbonyl intermediates is demonstrated. The conversion was achieved in aqueous media and at ambient conditions. This study also demonstrates that the feedstock for production of renewable chemicals and biofuels through electrochemistry can be extended to primary carbohydrate molecules. This is the first report on a one-pot electrochemical deoxygenation of xylose to δ-valerolactone.

Full text of DOI:10.1002/cssc.201700209

Accepted Article
Title: Electrochemistry for the generation of renewable chemicals: One-
pot electrochemical deoxygenation of xylose to δ-valerolactone
Authors: Olusola Oladele James, Waldemer Sauter, and Uwe Schröder
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To be cited as: ChemSusChem 10.1002/cssc.201700209
Link to VoR: http://dx.doi.org/10.1002/cssc.201700209
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ChemSusChem
Electrochemistry for the generation of renewable chemicals: One-pot electrochemical
deoxygenation of xylose to δ-valerolactone
Olusola O. James,1,2 Waldemer Sauter,1 Uwe Schröder1
1 Institute of Environmental and Sustainable Chemistry, Technical University, Braunschweig,
Hangenring 30, 38106, Braunschweig, Germany
2Chemistry Unit, Kwara State University, Malete, P.M.B. 1530, Ilorin, Nigeria
Abstract
Electrochemical method is now widely recognized as a green and sustainable route for
conversion of biomass to fuels and chemicals. However, most electrochemical biomass
conversions that involve deoxygenation depend on intermediates from thermo-chemical and
biological processes. At present, electrochemical deoxygenation of primary biomass
feedstocks is rare. Thus, it is desirable to develop electrochemical methods for deoxygenation
of primary biomass feedstocks into fuels and chemicals. In this study, we demonstrate the
electrochemical conversion of xylose to δ-valerolactone via carbonyl intermediates. The
conversion was achieved in aqueous media and at ambient conditions. We showed that a
tandem anodic and cathodic conversion is a viable strategy for achieving electrochemical
deoxygenation of biomass substrates. This is the first report on a one-pot electrochemical
deoxygenation of xylose to δ-valerolactone.
Keywords: Xylose, δ-valerolactone, oxidation, cathodic reduction, deoxygenation
1
Introduction
Although fossil fuels remain the dominant primary energy resources that power socio-
economic activities in our modern society, increasing the share of renewable electricity
generation in the global energy mix is an active pursuit towards curtailing climate change and
global warming.[1-2] Due to intermittent of nature solar and wind electricity generation,
integration into convention electrical energy installations requires storage buffers.[3-6]
Rechargeable batteries are an established technology for storing electricity as chemical
energy. The battery technologies depend on selection of appropriate reversible redox
reactions for different applications. Lithium affords the most promising battery chemistry and
it is presently the leading battery technology.[7] Lithium battery products range from small to
medium scale storage applications (such as from laptop to electric cars). [8-10] However, there
are still design constraints for grid scale storage applications.[11] Fossil fuels which we
currently depend on are products of energy storage of captured solar energy in covalent bonds
during photosynthesis.[12] Thus, taking inspiration from nature, transformation leading to
formation of covalent bonds can affords large scale storage of renewable in portable forms
such fuels and chemicals, especially when renewable energy is exploited to transform carbon
dioxide into fuels and chemicals.[13-14] This area is currently witnessing intense research
activities,[15-17] and it has the potential of helping to achieve the goals of energy storage and
curtailment of global warming.
Industrial electrochemical processes such as chlor-alkali process, Hall-Heroult process etc.
consume electricity at grid scale level to drive irreversible chemical reactions. These
inorganic electrochemical processes are important for production of basic chemicals (NaOH,
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10.1002/cssc.201700209
ChemSusChem
Cl2, and H2) and aluminium metal respectively. At the turn of the century electricity had been
used to drive industrial production of a number of organic chemicals.[18] While most of the
industrial organic electrochemical processes have been displaced by thermochemical
methods, due to cheap availability of petroleum, there is a renaissance of interest in organic
electrochemical processes using biomass substrates for production of fuels and chemicals as a
means of biomass valorisation and storage of excess renewable electricity. Thus use of
renewable resources (biomass and renewable electricity) for production of chemicals and
fuels is one of the goals of sustainable chemistry.
Generally, the main goal in conversion of a carbohydrate biomass derived substrate into a
fuel compound entails an increase of the hydrogen/oxygen ratio with minimal loss of carbon
atoms of the substrate. For instance, in fermentation of glucose to ethanol: C6H12O6 →
2C2H5OH + 2 CO2, the increase of the hydrogen/oxygen ratio in ethanol is accomplished by a
loss of two carbon atoms per unit of the starting glucose converted. Partial dehydration of
carbohydrates e.g., xylose and glucose is a promising path, but the resulting products
(furfural, hydroxymethylfurfural, levulinic acid) are yet suitable as fuel because they contain
reactive groups (unsaturation, carbonyl and carboxylic acid).[19-23] Further conversion
processes of the partial dehydration intermediates into fuel suitable molecules require high
temperature
and
high
pressure
hydrogenation
process.
Electrochemical
hydrogenation/reduction at ambient conditions is now recognised as an environmentally
benign process for converting furfural, hydroxymethylfurfural and levulinic acid into fuel
suitable molecules.[24-31] However, the electrochemical routes to biofuels are largely as
complementary to thermochemical methods so far as the scope of substrates is limited to the
above intermediates from thermochemical partial oxidation processes. It is desirable to
expand the scope of substrates to primary carbohydrate biomass derived feedstocks and the
robustness of electrochemical conversions as a means of storing excess renewable electricity.
Thus, in this communication we demonstrate deoxygenation of xylose into delta
valerolactone, a valuable chemical intermediate, using a one-pot electrochemical process.
2
Experimentals
2.1 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.
2.2 Electrode materials
Ag/AgCl sat. KCl electrode (SE11, Sensortechnik Meinsberg, Germany, 0.197 vs standard
hydrogen electrode (SHE) as reference. All used electrodes are listed in Table 1 with their
respective purity and surface area.
Table 1. Purity and surface area of electrode materials used
Electrode material
Purity Electrode surface (cm2)
99.999 11
99.99 11
Lead (Pb)A
Zinc (Zn) A
Platinum (Pt) A
99.9
11
Ruthenium based das anode (RUA)B
(A) ChemPUR, Germany B) Umicore Galvanotechnik GmbH, Germany
(B) Ti Anode Fabricators Pvt. Ltd, Chennai - 126, India
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2.3 Electrochemical procedure
All electrochemical reactions were conducted under potentiostatic control using a
potentiostat/galvanostat SP50 (Bio-Logic SAS, Claix, Frankreich). All experiments used a
three-electrode configuration and were stirred continuously with a magnetic stirrer. The
undivided cell is a single 40 ml chamber cyclic glass cell with a 1cm neck window fitted with
a Teflon® stopper bearing the anode and cathode terminals 2 cm apart. The divided cell was
a 40 ml each two-chamber H-type glass cell. The anode and cathode terminals were 10 cm
apart, the anode and the cathode chambers was separated via a cation exchange membrane
(fumasep FKE, Fumatech, Germany)
2.4 Analysis
Qualitative analyses were carried out using gas chromatography-mass spectrometry (GC-MS)
(Trace GC Ultra, DSQ II, Thermo Scientific, Germany) equipped with a TR-Wax MS (30 m
x 0.25 mm ID x 0.25 mm film GC column, Thermo Scientific, Germany). Routine
quantitative analyses were performed by high performance liquid chromatography (HPLC),
by means of a refractive index (IR) detector (Spectrasystem P4000, Finnigan Surveyor RI
Plus Detector, Fischer Scientific) equipped with HyperREZ XP carbohydrate H+8 µm (S/N:
026/H/012-227) column. Sulphuric acid (0.005N, flow rate 0.5 ml min-1) as eluent. The
column was operated at different temperature depending on sample composition, the
refractory index detector was operated at 25oC.
3
Results and discussions
3.1 Conversion of hydroxyl to carbonyl in divided cell
Carbohydrates possess a carbonyl group and several hydroxyl groups. Thus, deoxygenation
of carbohydrates would entail transforming its carbonyl group and hydroxyl groups to a
methylene bridge. The exploitation of Clemenson-type cathodic reductions of aldehyde and
ketone groups is well documented. Also, hydroxyl group(s) at alpha position(s) to a carbonyl
group can be converted to methylene bridge.[32-34] But direct reduction of a normal hydroxyl
group and a carboxylic group are electrochemically inaccessible in aqueous media. Hence the
level of deoxygenation that can be achieved in a carbohydrate substrate by direct cathodic
reduction is limited. To achieve higher level of deoxygenation, a complementary anodic
selective conversion of the hydroxyl groups to carbonyl becomes imperative. Anodic
oxidation of polyols has been an active area of research for fuel cell applications where the
target is exhaustive or complete oxidation of substrate to carbon dioxide. However, selective
anodic oxidation of polyols is just receiving attention of late. Some of reported electrodes for
selective anodic oxidation of secondary alcohols include Ru, Bi, Sb promoted platinum.[35-37]
While these electrodes possess desired selectivity, they suffer from stability issues which
render them unsuitable for use in an undivided cell. Leaching and migration of Ru from
RuO2-Pt anode to the cathode had been reported in a fuel cell system.[38-39] Bi-Pt and Sb-Pt
anodes are likely to be prone to similar stability challenges. Our attention is drawn to
hypochlorite (HClO) as a selective oxidant for conversion of alcohol group to carbonyl
group(s).[40] According to the Pourbaix diagram of aqueous chlorine, hypochlorite is stable in
the pH range 3-6. The standard organic chemistry hypochlorite recipe is NaClO/CH3COOH,
and NaClO can be generated via electrolysis of NaCl. So we opted to explored in-situ
generation of active oxidant from a suitable electrolyte.
Platinum is a stable anode for chloride ion oxidation to chlorine and carbonyl to methylene
conversion had been demonstrated over lead electrode. So we investigated in-situ generation
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10.1002/cssc.201700209
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of HClO using an undivided cell consisting of
a
mixture of NaCl and
CH3COOH/CH3COONa buffer as electrolyte, platinum and lead as anode and cathode
respectively. During the electrolysis, we observed deposition of lead oxide on the platinum
anode. The same experiment with just NaCl without acetate buffer showed no lead oxide
deposition at the platinum anode. Since lead acetate is very soluble in water, acetate ion can
promote transport of lead (II) ions to the anode where they get further oxidised and deposited
as lead oxide. In addition to leaching of lead cathode, presence of acetate in the electrolyte
can also affect the selectivity of the electro-generation of the active oxidant. To avoid this
unwanted effects of the acetate ion, we proceeded with just NaCl solution as electrolyte.
3.2 Conversions of model substrates in divided cell
Common carbohydrate substrates such as xylose and glucose are multi-hydroxyl substrates
and can assume open chain or ring structure in solution. Monitoring the selective conversion
of the hydroxyl groups in xylose and glucose to carbonyl group will be challenging.
Moreover, because of the limitation of mimicking the established recipe for in-situ generation
of HClO; we investigated the selectivity of the electrogenerated oxidant from NaCl solution
on simpler substrates: 2,3-butanediol, 1,2-propanediol & glycerol. Figure 1 shows the
conversion and the selectivity of the oxidation of 2,3-butanediol by the electro-generated
oxidant. The substrate in scheme 1 contains only a secondary hydroxyl group, and the results
in Figure 1 indicates absence of side reactions or deviations from the proposed pathway of
oxidation of 2,3-butandiol in scheme 1. Thus, substrates containing only secondary hydroxyl
groups are promising candidates for selective electrochemical deoxygenation via tandem
selective oxidation and Clemenson-type cathodic reduction.
2,3-Butanedione
Acetoin
Conversion
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
2
4
6
8
10
Time (h)
Figure 1: Conversion and selectivity of oxidation of 0.2M 2,3-Butanediol in a divided cell using Pt anode and
cathode using 0.5M NaCl (potential 2.5V Saturated AgCl/Cl, temperature 25oC)
Scheme 1: Proposed pathway of electrooxidation of 2,3-butanediol by the electrogenerated oxidant from NaCl
solution
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Scheme 2 shows proposed pathways of the electrooxidation of 1,2-propandiol. The dash line
divides the desired products (above) from the undesired products (below). Figure 2 shows the
monitored oxidation products of 1,2-propandiol with the electrogenerated HClO. This
substrate has a primary and a secondary hydroxyl group, hence, the product profile can give
information about the selectivity of the electrogenerated oxidant on the type of hydroxyl
groups. Figure 2 showed that the oxidant displayed preferential oxidation of the secondary
over the primary hydroxyl group of the substrate. However, the sum of selectivities of
monitored products does not account for the conversion of the substrate. The decrease in the
material balance with time suggests that the main product undergoes other reactions outside
the scheme. The carbonyl products (acetol and lactaldehyde) are prone to undesired aldol
condensation reactions. The products are also prone to myriads of ester formations via their
hydroxyl, aldehyde and acid groups.[41-44]
Scheme 2: Proposed pathway of electrooxidation of 1,2-Propanediol by the electrogenerated oxidant from NaCl
solution
Pyruvic acid
Lactic acid
Acetol
Pyruvaldehyde
Lactaldehyde
Conversion
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
2
4
6
8
10
Time (h)
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Figure 2: Conversion and selectivity of oxidation of 0.25M 1,2-Propanediol in a divided cell using Pt anode and
cathode using 0.5M NaCl. (potential 2.5V Saturated AgCl/Cl, temperature 25oC)
To ascertain that the observed product selectivities are due to the electrogenerated oxidant,
the oxidation of 1,2-propandiol was repeated using HCl and H2SO4 as electrolyte. The
product profile obtained with HCl is nearly identical with that of NaCl but different from that
obtained using H2SO4 as electrolyte (supplementary information). This shows the anodic
reaction is independent of the nature of cation and different mechanism accounts for the
electrooxidation of 1,2-propandiol in chloride and sulphate media. Chloride media displayed
preferential oxidation of the secondary over the primary hydroxyl group of the 1,2-
propandiol, but the reverse is the case in sulphate. Since sulphate ion is electrochemical
refractory in aqueous media electrooxidation of 1,2-propandiol takes place on the platinum
surface. But oxidation of chloride ion is possible on platinum and the resulting chlorine
species, probably HClO, is the active oxidant in the oxidation of 1,2-propandiol in NaCl and
HCl solution.
Scheme 3 depicts proposed pathways of electrooxidation of glycerol, a substrate with two
primary and a secondary hydroxyl groups. Electrooxidation of glycerol was investigated in
NaCl, HCl and H2SO4 as electrolytes. The three electrolytes displayed poor material balance
with respect to the monitored products (supplementary information). The products profile
obtained in NaCl and HCl/H2SO4 are similar to reported product selectivities of glycerol
oxidation over platinum in alkaline and acidic media respectively.[45] This shows that
oxidation of glycerol occurred on the platinum surface. This suggests increasing number of
hydroxyl group of the substrate increasing the strength of adsorption which promotes
oxidation on the platinum instead of desired oxidation in the electrolyte through
electrogenerated chlorine oxidant.
Scheme 3: Proposed pathway of electrooxidation of Glycerol by the electrogenerated oxidant from NaCl
solution
3.3 Conversions of model substrates in an undivided cell
To achieve the intended deoxygenation objective, it is desirable to couple anodic oxidation
and cathodic reduction in an undivided cell. It is anticipated that this arrangement will take
advantage of the possibility of a combined transformation of carbonyl and alpha hydroxyl
groups to methylene bridges to achieve high level of substrate deoxygenation. The results in
section 3.2 satisfy the selective oxidation requirement, thus providing the reducible carbonyl
intermediates for the cathodic half reaction. However, the cathodic conversion of the
carbonyl compound in aqueous media is complicated due to keto-enol and hydration-
dehydration equilibria.[46] The keto-enol equilibria result in different reduction paths to
different products. Species containing an alpha sp3 hydrogen atom to a carbonyl group are
prone to keto-enol equilibria.[47] Presence of an alpha hydroxyl group to a carbonyl group
favours aldo-keto isomerisation as in the case of acetol-lactaldehyde equilibria.[48] Theoretical
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studies had also suggested that HOCl, the most probable chlorine oxidant species in the HCl
electrolyte, is a potential catalyst of keto-enol tautomerism.[49] Also in aqueous media,
carbonyl compounds react with water forming geminal diols. The geminal diol group is
cathodic inactive.[50-51] The hydration-dehydration equilibria favours hydration and the rate
of the dehydration is usually slow.[52-54] So the conversion of a hydrated carbonyl species at
the cathode follows a CE mechanism, a pre-chemical dehydration of a germinal diol before
electron transfer at the cathode. Thus, the results of in Figure 3 can be explained by selective
oxidation 1,2-Propanediol to carbonyl intermediates and their subsequent transformations at
the cathode presented in scheme 4. The cathodic reduction can follow a four protons-four
electrons path or a two protons-two electrons path. Acidic media favour dehydration and the
four protons-four electrons reduction path that leads to the desired deoxygenation.
A similar electrolysis of 2,3-Butanediol gave very small amount reduced products (Figure 4)
and the Pb cathode transformed into PbO2. This is a signature of reaction of chlorine with Pb
catalysed by presence of oxygen.[55] Cathodic reduction of 2,3-Butanedione had been
demonstrated using Hg cathode;[56] and paired electrochemical conversion of 2,3-Butanediol
to butanone in a divided flow cell using NaBr as electrolyte and Pb/Hg as cathode had been
reported by Baizer et al.[57] As illustrated in Figure 4, we carried out the electrochemical
reduction of the products (2,3-Butanedione Acetoin & Butanone) using a Pb cathode in a
divided cell. The products are readily reduced at the Pb electrode and a similar cathodic
reduction paths as proposed in the case of 1,2-propanediol may be expected for the oxidized
intermediate of 2,3-Butanediol (supplementary information Figure 4b, Scheme 5). So we
inferred that the lack of reduced product in the 2,3-Butanediol electrolysis was due to HClO
chlorine attack on the Pb cathode. While 1,2-Propanediol or its oxidized intermediates act as
inhibitor of HClO attack on the Pb cathode.
Propanal
1-Propanol
Lactic acid
Acetol
100
90
80
70
60
50
40
30
20
10
0
80
70
60
50
40
30
20
10
0
Acetone
Lactaldehyde
Conversion
2
4
6
8
10
Time (h)
Figure 3: Conversion and selectivity of oxidation of 0.25M 1,2-propandiol in an undivided cell using Pb as
working electrode and Pt counter cathode using 0.5M HCl. (potential -1.8V Saturated AgCl/Cl, temperature
25oC)
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Scheme 4: Proposed pathway of the cathodic transformations at a Pb electrode during electrolysis of 1,2-
Propanediol (Pb as working electrode and Pt counter cathode using 0.5M HCl, -1.8V Saturated AgCl/Cl,
temperature 25oC)
2,3-Butanedione
Acetoin
Conversion
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
2
4
6
8
10
Time (h)
Figure 4: Conversion and selectivity of the oxidation of 0.25M 2,3-butandiol in an undivided cell using Pb as
working electrode and Pt counter cathode using 0.5M HCl. (potential -1.8V Saturated AgCl/Cl, temperature
25oC)
3.4 Electrochemical deoxygenation of xylose
The results from the conversions of the model substrates suggests that electrochemical
deoxygenations are more probable for primary biomass substrates that have no primary
hydroxyl or minimal ratio of primary/secondary hydroxyl group via a sequential oxidation
and reduction steps in a divided cell. Among the primary carbohydrate molecules xylose
comes close to meeting these requirements. Xylose exists predominantly as xylopyranose in
solution. xylopyranose is a six-membered ring, with an intramolecular hemiacetal formed
between a hydroxyl (on C5 carbon) and the aldehyde carbon (C1). The remaining three
secondary hydroxyl groups that can be selectively converted to ketones, and a two-step
electro-deoxygenation of xylose to δ-valerolactone was envisaged. Xylose and δ-
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10.1002/cssc.201700209
ChemSusChem
valerolactone are the two extremes of a continuum of possible oxidised and reduced
intermediates (see Scheme 4 in supplementary information). Since selective anodic oxidation
is the critical step of the electrochemical deoxygenation, obtaining reduced intermediates is
an evidence of successful selective oxidation step. Moreover, hydrophobicity of the
intermediates increasing with increasing extent of reduction, therefore, extraction into ethyl
acetate will allow identification of reduced intermediates using GC-MS.
Anodic oxidation of xylose in a divided cell using Pt working electrode (2.5V vs Ag/AgCl
reference electrode) in 0.5M HCl, followed by cathodic reduction using Pb working
electrode at (-1.8V vs Ag/AgCl reference electrode) gave no δ-valerolactone. Then we
carried out xylose conversion in an undivided cell containing 0.25M xylose, 0.5M HCl, a Pt
working electrode and Pb counter electrode at 2.5V vs Ag/AgCl reference electrode),
followed cathodic reduction in a divided cell at -1.8V, Pb working electrode and Pt counter
electrode. GC-MS analysis of ethyl acetate extract of the sequential electrochemical oxidation
and reduction of xylose showed no compound. We interpreted the absence of reduced
intermediates as non-selective oxidation of xylose. Anodic polarisation brings about surface
oxide formation on platinum, and as explained in the case of glycerol oxidation, three
hydroxyl groups of xylose will lead to strong interaction with the surface oxide on platinum.
This strong adsorption of xylose on platinum surface may not only inhibits chloride ion
oxidation but also promote xylose non-selective oxidation on the platinum surface. In order
minimise non-selective oxidation we carried out the oxidation step at high current density and
reduced reaction time (1h), followed by stirring for 2h before the reduction step. GC-MS
analysis of ethyl acetate extract of the cell content after the reduction indicates a trace of δ-
valerolactone. This confirms that the platinum is not sufficiently chlorine selective in the
presence of xylose, thus, a chlorine selective anode will lead to successful deoxygenation of
xylose.
Dimensionally stable anode (DSA) anodes are platinum group metal oxide (PGM) coated
titanium electrocatalysts. They are widely used in the chlor-alkali industry. They are more
chlorine selective than platinum. So we explored a commercial ruthenium based DSA anode
(RUA) in the two step electrochemical deoxygenation of xylose. GC-MS analysis of ethyl
acetate extract confirmed dexygenation of xylose to δ-valerolactone. Encouraged by δ-
valerolactone formation, we attempted to integrate the two step into one-pot reaction. But
taking into consideration the earlier experience of the problem of electrogenerated chlorine
oxidant attack of the cathode when the undivided cell was used (with 2 cm separation
between the anode and the cathode), we allowed sufficient gap between the anode and the
cathode so as minimise the unwanted dissolution of the cathode by the oxidant. We used the
divided cell without the membrane separation. This afforded a 10 cm gap between the anode
and the cathode. The separation will create time gap for chlorine oxidant to react with xylose
thereby preventing or minimising it migration to the cathode. A significant improvement of
δ-valerolactone formation was obtained in the one-pot deoxygenation in the undivided two-
chamber H-cell (Figure 5). δ-valerolactone is a chemical intermediate used in production of
polyesters. A conservative coulomb efficiency calculated with respect to reduction of three
carbonyl groups to methylene groups is 18.4 %. Although coulomb efficiency obtained is
rather low, the process is attractive from green chemistry perspective since the conversion
was achieved in aqueous media and at ambient conditions and there is no harzardous waste
generation. Improvement of the coulomb efficiency may be possible by exploring ways to
increase the rate of the cathodic half reaction such as high surface area by use of porous lead
electrode. Use of other hydrogen over-potential metal electrode such as cadmium or lead-
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10.1002/cssc.201700209
ChemSusChem
cadmium alloy, optimisation of educt and electrolyte concentration, may be explored for
improving the cathodic half of the conversion. A proposed simplified pathway of the one-pot
electro-deoxygenation of xylose to δ-valerolactone is shown in scheme 6.
10
9
8
δ-Valerolactone
Furan-2(5H)-one
7
6
5
4
3
2
1
0
0
2
4
6
8
Time (h)
Figure 5: One-pot electrochemical deoxygenation of xylose
Scheme 6: Simplified pathway of electro-deoxygenation of xylose to δ-valerolactone
Furan-2(5H)-one is also obtained as a side-product in the one-pot deoxygenation of xylose.
The exact elementary steps from xylose to furanone is unclear now, the unsaturation suggests
a dehydration step during oxidation in the HCl electrolyte. Furan-2(5H)-one is recently
implicated as starting chemical for synthesis of insecticide,[58] hence this study demonstrates
possibility of using exclusive electrochemical routes for production of valuable renewable
chemicals and fuels. Oxidation of hydroxyl groups in starch to carbonyls has been exploited
for producing modified starch for different applications.[59] This suggests similar direct one-
pot electrodeoxygenation of starch to biofuels or chemicals could be possible. Also, the one-
pot experiment is clearly a pointer to the possibility of integration of the separate
electrochemical processes in a continuous membrane chlor-alkali type cell with HCl
electrolyte. This will prevent unwanted competing dissolution of the cathode by the oxidant
and to ultimately optimized energy efficiency of electrodeoxygenation reaction.
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Conclusion
We have demonstrated of use electrochemistry for achieving deoxygenation of carbohydrate
substrates into value-added chemicals intermediates and biofuels. The deoxygenation was
achieved in a one-pot electrochemical process: partially separated selective oxidation via
electrogenerated chlorine oxidant and cathodic reduction. Successful deoxygenation of a
substrate depends on selective conversion of the hydroxyl groups in the substrate to
corresponding carbonyl groups. Aldehyde is readily convertible into carboxylic acid by the
oxidant, hence, primary hydroxyl groups are a selectivity weak link of the overall
deoxygenation process. Where complete absence of primary hydroxyl groups is not possible,
minimal ratio of primary/secondary hydroxyl groups is a desirable criterion for selection of
substrates for this electrodeoxygenation process. The electrochemical deoxygenation can be
further optimized using a continuous integrated process in a chlor-alkali type reactor system.
While fulfilling the requirement for green chemistry, this study also adds to the
electrochemistry options for storage of excess renewable electricity via production of
renewable chemicals and biofuels.
Acknowledgements
Olusola O. James acknowledged Alexander von Humbolt Foundation, Germany, for Goerg
Forster Postgraduate Fellowship Award. O. O. James is also grateful to Mr. V. P. Jekakumar,
Managing Director, Ti Anode Fabricators Pvt. Ltd, Chennai - 126, India, for providing the
DSA anode used in this study at zero cost.
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