Direct electrochemical addressing of immobilized alcohol dehydrogenase for the heterogeneous bioelectrocatalytic reduction of butyraldehyde to butanol

Article abstract of DOI:10.1002/cctc.201402932

Modified electrodes using immobilized alcohol dehydrogenase enzymes for the efficient electroreduction of butyraldehyde to butanol are presented as an important step for the utilization of CO₂-reduction products. Alcohol dehydrogenase was immobilized, embedded in an alginate-silicate hybrid gel, on a carbon felt (CF) electrode. The application of this enzyme to the reduction of an aldehyde to an alcohol with the aid of the coenzyme nicotinamide adenine dinucleotide (NADH), in analogy to the final step in the natural reduction cascade of CO₂ to alcohol, has been already reported. However, the use of such enzymatic reductions is limited because of the necessity of providing expensive NADH as a sacrificial electron and proton donor. Immobilization of such dehydrogenase enzymes on electrodes and direct pumping of electrons into the biocatalysts offers an easy and efficient way for the biochemical recycling of CO₂ to valuable chemicals or alternative synthetic fuels. We report the direct electrochemical addressing of immobilized alcohol dehydrogenase for the reduction of butyraldehyde to butanol without consumption of NADH. The selective reduction of butyraldehyde to butanol occurs at room temperature, ambient pressure and neutral pH. Production of butanol was detected by using liquid-injection gas chromatography and was estimated to occur with Faradaic efficiencies of around 40%.

Full text of DOI:10.1002/cctc.201402932

DOI: 10.1002/cctc.201402932
Full Papers
Direct Electrochemical Addressing of Immobilized Alcohol
Dehydrogenase for the Heterogeneous Bioelectrocatalytic
Reduction of Butyraldehyde to Butanol
S. Schlager,*[a] H. Neugebauer,[a] M. Haberbauer,[b] G. Hinterberger,[a] and N. S. Sariciftci*[a]
Modified electrodes using immobilized alcohol dehydrogenase
enzymes for the efficient electroreduction of butyraldehyde to
butanol are presented as an important step for the utilization
of CO2-reduction products. Alcohol dehydrogenase was immo-
bilized, embedded in an alginate–silicate hybrid gel, on
a carbon felt (CF) electrode. The application of this enzyme to
the reduction of an aldehyde to an alcohol with the aid of the
coenzyme nicotinamide adenine dinucleotide (NADH), in anal-
ogy to the final step in the natural reduction cascade of CO2 to
alcohol, has been already reported. However, the use of such
enzymatic reductions is limited because of the necessity of
providing expensive NADH as a sacrificial electron and proton
donor. Immobilization of such dehydrogenase enzymes on
electrodes and direct pumping of electrons into the biocata-
lysts offers an easy and efficient way for the biochemical recy-
cling of CO2 to valuable chemicals or alternative synthetic
fuels. We report the direct electrochemical addressing of im-
mobilized alcohol dehydrogenase for the reduction of butyral-
dehyde to butanol without consumption of NADH. The selec-
tive reduction of butyraldehyde to butanol occurs at room
temperature, ambient pressure and neutral pH. Production of
butanol was detected by using liquid-injection gas chromatog-
raphy and was estimated to occur with Faradaic efficiencies of
around 40%.
Introduction
Carbon dioxide emissions are rising and have reached the level
of 400 ppm in the atmosphere.[1,2] This increase is expected to
create global warming, which may cause severe meteorological
events. To curb this development, a carbon capture and se-
questration (CCS) in deep rocks is suggested for reducing the
emissions. This approach is costly, makes no use of CO2, and
has the risk of sudden eruptions of stored CO2. In our ap-
proach we follow the carbon capture and utilization (CCU)
strategy, in which useful chemicals and alternative synthetic
fuels are produced by photochemically or electrochemically re-
cycling CO2, for example, by using solar or wind energy. Nowa-
days, many scientists consider CO2 as the next chemical feed-
stock and as an alternative to the petrochemical refinery.[3–5]
However, wind and solar energy are supply-driven energy
sources and it is difficult to adjust production and consump-
tion in an effective way. Thus, large-scale and transportable
energy storage, such as the storage for synthetic fuels or valu-
able chemicals, is required. In particular, the conversion of CO2
into other chemicals by means of carboxylation or reduction is
promising.[6,7] Carbohydrate products, such as methane or dif-
ferent acids and alcohols, are of high interest for the substitu-
tion of fossil fuels.[5] Higher alcohols, such as butanol, are par-
ticularly attractive owing to high energy density and direct ap-
plicability to existing combustion engines.
Nevertheless, chemical conversion of substrates like CO2, al-
dehydes, or other educts of such chemicals requires high-
energy inputs. To lower the energy barriers of such reactions,
homogenous and/or heterogeneous catalysts have to be ap-
plied. Besides approaches that use organic[8,9] and metal–or-
ganic[10–13] molecules as catalysts, there has also has been work
on biocatalysis by using microorganisms or enzymes. The
great advantage of biocatalysts is their high selectivity for spe-
cific products as well as their operation at ambient conditions.
In 1976, Ruschig et al. presented work on the enzymatic re-
duction of CO2 to formate by using formate dehydrogenase
and nicotinamide adenine dinucleotide (NADH) as a coen-
zyme.[14] Investigations on the immobilization of three dehy-
drogenase enzymes in sol–gel matrices for the application in
NADH-assisted CO2 reduction to methanol has been reported
by Obert and Dave.[15] Immobilization of those enzymes in algi-
nate matrices is suggested to improve reproducibility.[4] Studies
combining enzymatic CO2 reduction for methanol production
and immobilization of enzymes have also been reported by Xu
et al.[16] The application of enzymes offers great potential for
a highly selective and efficient production of fuels and chemi-
cals by means of reduction reactions.
[a] S. Schlager, Dr. H. Neugebauer, G. Hinterberger, Prof. Dr. N. S. Sariciftci
Linz Institute for Organic Solar Cells
Johannes Kepler University Linz
Altenbergerstraße 69, 4040 Linz (Austria)
E-mail: stefanie.schlager@jku.at
serdar.sariciftci@jku.at
[b] M. Haberbauer
PROFACTOR GmbH
Such biological approaches have also been reported for the
generation of butanol, which is, compared to methanol or eth-
anol, more favorable owing to higher energy densities. Tracy
Im Stadtgut A2, 4407 Steyr-Gleink (Austria)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402932.
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et al. showed the biological pathway of the reduction of CO2
to butanol by using Clostridia cultures. They used the Wood–
Ljungdahl approach with two possible reduction pathways of
CO2 to butanol by multistep enzyme catalysis. In both path-
ways the last step involves the reduction of butyraldehyde to
butanol with the aid of alcohol dehydrogenase as catalyst.[17]
Mꢁller et al. and Chen have also presented pathways on the
formation of butanol from butyraldehyde by using en-
zymes.[18,19]
enzymes on the electrode for longer operational lifetimes. As
an additional advantage compared to homogeneous catalysis,
immobilization of the enzymes facilitates the separation of
product and catalyst and offers the opportunity of reusability
of the catalyst.
Herein, we present successful immobilization of the catalyst
and efficient and selective conversion of butyraldehyde to bu-
tanol directly, without sacrificial electron-donor materials. This
bioelectrocatalytic approach is presented in Scheme 1 and is
compared to the conventional method with NADH.
However, almost all of these previous studies have concen-
trated on the use of NADH as the source of electrons and pro-
tons for CO2 reduction with dehydrogenase enzymes. In a dif-
ferent approach, direct pumping of electrons into biocatalysts
offers an easy and efficient way for biochemical-assisted reduc-
tion processes in the CO2-reduction cascade, avoiding the use
of expensive sacrificial coenzymes.
Results and Discussion
Initially, nonelectrochemical control experiments using alcohol
dehydrogenase-containing alginate gel beads and the coen-
zyme NADH were performed to prove the activity of the en-
zymes. In this experiment samples were taken immediately
after the addition of NADH and butyraldehyde, as well as after
8 h of reaction time. Analysis of the samples was performed by
using liquid-injection gas chromatography.
Approaches following the idea of direct electrochemical ad-
dressing, but using living microorganisms, have been demon-
strated by Hasan et al. who used the electron transfer from an
osmium redox polymer as a mediator to microorganisms,
grown on an electrode.[20,21] Other groups investigated the use
of microbial electrochemical cells for the conversion of CO2 to
acetate,[22] methane,[23] and higher alcohols such as isobuta-
nol.[24]
The amount of NADH added was 7.5 mg, which corresponds
to 1.127ꢂ10ꢀ5 moles. From gas chromatography approximate-
ly 200 ppm of butanol was determined, corresponding to
a concentration of 1.067ꢂ10ꢀ5 moles of butanol in 4 mL of
buffer solution. For each molecule of NADH one molecule of
butyraldehyde is reduced to one molecule butanol, which
gives an efficiency of 96% for the production of butanol with
NADH as an electron and hydrogen donor. Therefore, the re-
sults show an efficient gel bead immobilization of the enzyme.
The chromatograms after the addition of butyraldehyde and
the coenzyme NADH at the beginning of the reaction and
after 8 h of reaction time are shown in Figure 1. The retention
time and amount of butanol were identified by liquid-injection
gas chromatography by using external and internal standards.
A retention time of 3.64 min was determined for butanol. The
small peak that can be observed in the chromatogram record-
ed right after the addition of the coenzyme and butyraldehyde
Using enzymes directly, Kuwabata et al. demonstrated the
electrochemical conversion of CO2 to methanol with formate
dehydrogenase and methanol dehydrogenase as homogenous
catalysts and methylviologen or pyrrolequinolinequinone as
electron mediators.[25] In another study, Reda et al. also showed
the direct electrochemical reduction of CO2 using formate de-
hydrogenase, immobilized on a glassy carbon electrode by
simple adsorption.[26] Through the use of an osmium redox
polymer, application of enzymes to electrodes has been dem-
onstrated by the groups of Yakovleva,[27] McKenzie,[28] and
Rengaraj.[29]
In our investigations we concentrate on the final step of the
biological reduction cascade from CO2 to butanol by direct
electrochemical reduction of butyraldehyde, catalyzed by im-
mobilized alcohol dehydrogenase. The enzyme was immobi-
lized by using an alginate–silicate matrix on a carbon felt (CF)
electrode. This matrix is expected to stabilize the immobilized
Figure 1. Liquid-injection gas chromatograms from a nonelectrochemical
control experiment using enzyme-containing alginate gel beads and NADH
as coenzyme. An intense peak is observed at the retention time of butanol
(3.64 min) after 8h of reaction time.
Scheme 1. Schematic description of the reduction of butyraldehyde to buta-
nol using a) NADH as sacrificial electron donor and b) a bioelectrocatalytic
approach.
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indicates a fast reduction reaction to butanol with NADH as an
electron and proton donor.
These results show the successful employment of alcohol
dehydrogenase immobilized in alginate gel beads for the re-
duction of butyraldehyde to butanol with high efficiency. How-
ever, for this nonelectrochemical control experiment the ex-
pensive proton- and electron-donor NADH is required and is
consumed irreversibly. Therefore, we focused on the use of
bioelectrocatalysis by immobilizing the enzymes on the elec-
trode for direct electron injection without the need for any
sacrificial electron donor.
The comparison of cyclic voltammograms (CV) of an algi-
nate-modified carbon felt electrode with immobilized alcohol
dehydrogenase and an equally prepared alginate-gel-modified
carbon felt electrode without enzyme, at a scan rate of
5 mVsꢀ1, is shown in Figure 2. The CV’s were recorded after
Figure 3. Liquid-injection gas chromatograms before and after electrolysis
with butyraldehyde using a gel-modified electrode with immobilized alcohol
dehydrogenase. At the retention time of 3.64 min a peak appears after elec-
trolysis, indicating butanol generation.
Figure 2. Cyclic voltammograms after the addition of butyraldehyde to the
electrolyte solution, using an alginate–enzyme modified carbon felt (CF)
electrode (red line) and an equally modified electrode without enzyme
Figure 4. Current–time plot of the electrolysis experiment using an alcohol
dehydrogenase-containing gel-modified CF electrode and electrolyte solu-
tion containing butyraldehyde.
(black) at a scan rate of 5 mVsꢀ1
.
the addition of 0.1 mL of butyraldehyde. A reductive current
can be observed, only for the electrode containing the
enzyme, starting at ꢀ400 mV vs. Ag/AgCl. These results indi-
cate that electrons can be injected directly into the immobi-
lized alcohol dehydrogenase to catalyze the reduction of bu-
tyraldehyde to butanol.
lombs (Q) consumed during electrolysis over 8 h at ꢀ600 mV
vs. Ag/AgCl. The number of moles, n, can be determined by
using the following equation:
Q
n ¼
zF
To prove the formation of butanol by the reduction process,
electrolysis experiments were performed for 8 h at ꢀ600 mV
vs. Ag/AgCl. Samples of the electrolyte solution before and
after electrolysis were analyzed by using liquid-injection gas
chromatography. The comparison of the chromatograms
before and after electrolysis is shown in Figure 3. At the reten-
tion time of butanol (3.64 min) a peak is observed according
to the formation of this substance during the reduction pro-
cess (the small peak at 3.59 min is related to GC-sample han-
dling and has no significance on butanol detection). From
quantitative analysis of the GC measurements an amount of
20 ppm butanol in 20 mL electrolyte solution was detected,
which is calculated as 5.4ꢂ10ꢀ6 moles. Analyzing the current
versus time curve (Figure 4), 2400 mAs were calculated from
the area enclosed by the curve, which corresponds to 2.4 Cou-
in which F=Faraday constant (96485.33 Cmolꢀ1) and z=
number of charges (2). The theoretical number of moles of bu-
tanol was calculated to be 1.244ꢂ10ꢀ5. Comparing the experi-
mentally determined and theoretically calculated amount of
butanol a corresponding Faradaic efficiency of around 40%
was found. These results support a reaction mechanism with
direct electrochemical access of the dehydrogenase enzyme
and subsequent reduction of butyraldehyde to butanol.
Control experiments have been performed both with
enzyme-free electrodes in butyraldehyde-containing electrolyte
solutions, as well as with the opposite combination, enzyme-
containing electrodes in butyraldehyde-free solutions. In both
cases, the chromatograms before and after electrolysis did not
show any significant evidence for the generation of butanol.
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Butanol generation after electrolysis was only observed for
electrodes containing alcohol dehydrogenase and after the ad-
dition of butyraldehyde to the electrolyte solution.
Conclusions
The application of carbon felt electrodes modified with immo-
bilized alcohol dehydrogenase suitable for the electrochemical
reduction of butyraldehyde to butanol has been demonstrat-
ed. In control experiments, nonelectrochemical reduction by
using NADH as a sacrificial electron donor has been achieved
with efficiencies up to 96%, which shows successful immobili-
zation without the loss of enzymatic activity. To avoid the con-
sumption of expensive coenzymes and their irreversible oxida-
tion, an electrochemical approach for the direct enzymatically
catalyzed electrochemical reduction of butyraldehyde to buta-
nol was demonstrated. Using this direct reduction method,
Faradaic efficiencies of around 40% could be reached. Efficien-
cies could be improved by suppressing base currents, occur-
ring even without the addition of enzyme or butyraldehyde.
Modification of the alginate-based immobilization matrix with
additional conducting materials, tuning of the electrode mate-
rial, or temperature could be convenient approaches for effi-
ciency improvement. The method of bioelectrocatalytic reduc-
tion, reported in this work, is convenient, inexpensive, and suit-
able for the substitution of NADH as a sacrificial electron
donor. It offers the possibility of producing butanol as a high
energy density chemical. Compared to homogeneous catalysis,
the direct electron injection from the electrode, heterogene-
ously catalyzed by immobilized enzymes, facilitates the easy
separation of the product and catalyst and offers the opportu-
nity of reusability of the catalyst. The method is highly selec-
tive; butanol production is only observed for alginate-gel-
modified electrodes containing alcohol dehydrogenase and
when butyraldehyde is present in the electrolyte solution. Our
results on the reduction of butyraldehyde to butanol with the
aid of alcohol dehydrogenase show the possibility to address
immobilized dehydrogenase enzymes electrochemically and
suggest further use of enzyme immobilization for electrochem-
ical CO2 recycling.
Figure 5. Experimental procedure: A) Alginate–silicate hybrid matrix solution
containing alcohol dehydrogenase, B) gelated as beads in 0.2m CaCl2 or
C) soaked with a CF electrode and D) gelated in 0.2m CaCl2 to obtain E) a al-
ginate–enzyme modified carbon felt electrode.
electrode, but without enzyme was prepared. Pt foil was used as
the counter electrode and Ag/AgCl as reference electrode. TRIS-
HCl buffer (0.05m, pH 7.65) was chosen as the electrolyte solution.
Butyraldehyde (0.1 mL) was added to the electrolyte solution for
the reduction reactions. All measurements were done in oxygen-
free (N2 purged) systems. Cyclic voltammograms (CV) were record-
ed for both the immobilized-enzyme electrode and the electrode
without enzyme, between 0 mV and ꢀ600 mV vs. Ag/AgCl with
a scan rate of 5 mVsꢀ1. Electrolysis was carried out for 8 h at
ꢀ600 mV vs. Ag/AgCl for significant butanol production, according
to the observed reduction potentials from cyclic voltammetry
(Figure 2). Samples of the electrolyte solution were taken before
and after electrolysis for product analysis in liquid-injection gas
chromatography (Thermo Fischer, Trace 1310). All electrochemical
measurements were carried out in a two-compartment cell with
separated anode and cathode chamber (2ꢂ20 mL) and electro-
chemical measurements were recorded with a Jaissle Potentiostat-
Galvanostat IMP 88 PC-R.
For comparison and enzyme-activity testing, control experiments
with free-floating alginate beads, (not immobilized on an elec-
trode), and with the aid of NADH as an electron supplier, were also
conducted. For gel bead formation, the enzyme-containing gel
was dropped into 0.2m aqueous CaCl2 solution by a syringe, the
formed beads were washed and transferred into the buffer solu-
tion (see Figure 5A and B). The buffer solution (4 mL) containing
the gel beads was purged with N2, followed by addition and disso-
lution of NADH (5 mg). Subsequently, an excess volume of the bu-
tyraldehyde (0.1 mL) was added and the reaction was conducted
for 8 h. Samples were taken immediately after the addition of bu-
tyraldehyde and after 8 h of reaction time. Liquid sample analysis
and product identification were performed by using liquid-injec-
tion gas chromatography (Thermo Fischer, Trace 1310).
Experimental Section
Acknowledgements
All Chemicals were used as purchased from Sigma Aldrich. Alginic
acid sodium salt was dissolved in pure water (2 mL, 18.2 MW) to
achieve a 2 wt% solution. The solution was mixed vigorously with
tetraethylorthosilicate (725 mL, TEOS). Alcohol dehydrogenase from
Saccharomyces cerevisae (415 umgꢀ1 solid, 4 mg) was dissolved in
aqueous 0.05m Tris(hydroxymethyl)-aminomethane-HCl (TRIS-HCl)
buffer (pH 7.65) and added to the alginate–silicate solution. For im-
mobilization on the electrode, carbon felt (CF), purchased from
SGL Carbon GmbH with a size of 0.6ꢂ3ꢂ0.6 cm, was soaked with
the previously prepared matrix solution. Congelation of the algi-
nate–silicate matrix was performed by dipping the electrode into
0.2m aqueous CaCl2 solution for 20 min. The corresponding steps
of the preparation procedure are depicted in Figure 5A and C–E.
Financial support by the Austrian Science Foundation FWF within
the Wittgenstein Prize and “REGSTORE”. The project “REGSTORE”
was funded under the EU Programme Regional Competitiveness
2007–2013 (Regio 13) with funds from the European Regional De-
velopment Fund and by the Upper Austrian Government.
Keywords: alcohol dehydrogenase · biocatalysis · butanol ·
electrochemistry · reduction
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Received: November 20, 2014
Revised: January 15, 2015
Published online on February 20, 2015
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