Electrosynthesis of amino acids from biomass-derivable acids on titanium dioxide

Article abstract of DOI:10.1039/c9cc07208j

Seven amino acids were electrochemically synthesized from biomass-derivable α-keto acids and NH₂OH with faradaic efficiencies (FEs) of 77-99% using an earth-Abundant TiO₂ catalyst. Furthermore, we newly constructed a flow-Type electrochemical reactor, named a "polymer electrolyte amino acid electrosynthesis cell", and achieved continuous production of alanine with an FE of 77%.

Full text of DOI:10.1039/c9cc07208j

Volume 55 Number 98 21 December 2019 Pages 14703–14858
ChemComm
Chemical Communications
rsc.li/chemcomm
ISSN 1359-7345
COMMUNICATION
Takashi Fukushima and Miho Yamauchi
Electrosynthesis of amino acids from biomass-derivable
acids on titanium dioxide

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Electrosynthesis of amino acids from
biomass-derivable acids on titanium dioxide†
Cite this: Chem. Commun., 2019,
55, 14721
Takashi Fukushimaa and Miho Yamauchi
*
ab
Received 14th September 2019,
Accepted 17th October 2019
DOI: 10.1039/c9cc07208j
rsc.li/chemcomm
Seven amino acids were electrochemically synthesized from biomass- two steps, namely, (i) condensation between a carbonyl compound
derivable a-keto acids and NH2OH with faradaic efficiencies (FEs) of and a nitrogen source, e.g., NH3 and NH2OH, to afford a nitroge-
77–99% using an earth-abundant TiO2 catalyst. Furthermore, we newly nated intermediate, e.g., imines and oximes, and (ii) subsequent
constructed a flow-type electrochemical reactor, named a ‘‘polymer reduction of the intermediate. It should be noted that recent
electrolyte amino acid electrosynthesis cell’’, and achieved continuous thorough studies advanced the production of several a-keto acid
production of alanine with an FE of 77%.
derivatives from lignocellulosic biomass as a non-food-competing
chemical feedstock.10,11 For example, a-hydroxyl acids, which can be
Amino acids are key building blocks in nature and thus have easily converted into the corresponding a-keto acids by simple two-
many potential uses, e.g., animal feed additives, flavor enhancers, electron oxidation,12,13 such as lactic acid,14–21 glycolic acid,22,23 and
pharmaceuticals and cosmetics, and therefore the global demand a-hydroxyglutaric acid,24 are obtainable via the hydrothermal degra-
for amino acids has spurred both academic and industrial dation of cellulose. Possible routes for the preparation of a-keto acids
research to develop more environmentally-friendly and lower- are summarized in the ESI† (Fig. S2). We then envisaged that
cost processes for their production. Currently, amino acids are electrochemically driven reductive amination of a-keto acids would
mainly produced through a microbial fermentation process from offer a simple method for amino acid synthesis. Another merit of the
sugar- or starch-based feedstock.1–3 Although the process has electrochemical process is the availability of water and electricity
made continuous advances over several decades for the production produced from renewable energies as hydrogen and energy sources,
of 20 proteinogenic amino acids, the production of some amino respectively. A limited number of works dealing with the electro-
acids still suffers from low efficiency. Besides, the fermentation synthesis of amino acids from a-keto acids have already been
process has critical drawbacks such as high energy and time reported.25–29 Most of these works, however, employ toxic metals
consumption for microbial culturing and complicated processes such as Pb and Hg or precious Pt as electrocatalysts.
for product isolation and purification. Chemical synthesis is a
On another front, nontoxic, earth-abundant and durable TiO2,
simple and efficient way to produce amino acids, and the most which exhibits highly selective electroreduction of carbonyl groups
conventional approach is the Strecker reaction,4–6 in which alde- due to its relatively large overpotential for hydrogen production,
hydes, ammonia and hydrogen cyanide react to produce amino- is a candidate as an electrocatalyst for synthesizing organic
nitriles that can easily undergo hydrolyzation to form amino acids. compounds in aqueous media.30–34 Recently, we reported that a
However, the use of hazardous cyanide and nonrenewable aldehydes Ti mesh electrode covered with anatase TiO2 (TiO2/Ti mesh), which
does not match the scope of this powerful reaction. Meanwhile, the is prepared through a hydrothermal treatment of Ti mesh,35–37
reductive amination of a-keto acids is a potential candidate as a next- catalyses the electrochemical reduction of a-keto acids to produce
generation synthetic process for amino acid production without any the corresponding a-hydroxyl acids with a remarkably high selec-
consumption of toxic reagents (Fig. 1).7–9 This process consists of tivity under highly acidic conditions where hydrogen is usually a
major product.37 We, herein, report an efficient electrocatalytic
system for the reductive amination of a-keto acids on a TiO2/Ti
a International Institute for Carbon-Neutral Energy Research (WPI-I2CNER),
Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan.
E-mail: yamauchi@i2cner.kyushu-u.ac.jp
mesh electrode in the presence of NH3 or NH2OH as the nitrogen
source to produce the corresponding amino acids. We demon-
strate the electrochemical synthesis of 7 types of amino acids with
considerably high FE values. Electrochemical syntheses of aspartic
acid, phenylalanine, and tyrosine are realized in this study for
the first time. Furthermore, we perform continuous synthesis of
b Department of Chemistry, Faculty of Science, Kyushu University, Motooka 744,
Nishi-ku, Fukuoka 819-0395, Japan
† Electronic supplementary information (ESI) available: Materials and methods,
supplementary figures and tables, and NMR data. See DOI: 10.1039/c9cc07208j
This journal is ©The Royal Society of Chemistry 2019
Chem. Commun., 2019, 55, 14721--14724 | 14721

View Article Online
Communication
ChemComm
Fig. 1 The possible role of reductive amination in a sustainable society. Reductive amination can convert biomass-derived a-keto acids into amino acids,
which are building blocks of human and animal bodies, in the presence of nitrogen sources, i.e., NH3 and NH2OH.
alanine using a flow-type reactor, named a ‘‘polymer electrolyte alanine in NH2OH and the production of lactic acid in NH3
amino acid electrosynthesis cell (AAEC)’’.
solution.
We next optimized the reaction conditions, such as applied
We firstly examined the TiO2/Ti mesh catalyzed electro-
chemical reduction of pyruvic acid in the presence of NH3 potential, temperature, pH, concentration of pyruvic acid, and
and NH2OH to produce alanine. Cyclic voltammetry (CV) curves amount of nitrogen source, for alanine electrosynthesis (Fig. 2e
of the TiO2/Ti mesh electrode recorded in 1.5 M NH3/(NH4)2SO4 and Fig. S5, S6, Tables S1, S2, ESI†). When NH3 was introduced
buffer with 30 mM pyruvic acid (pH 10) and in 0.20 M H2SO4 as the nitrogen source, the reaction conditions did not greatly
(aq.) with 160 mM pyruvic acid and 80 mM (NH2OH)2ÁH2SO4 affect the product selectivity, and the maximum FE for alanine
(pH 0.53) are shown in Fig. 2b and c, respectively. Compared to production was 29% (Fig. S5 and Table S1, ESI†). By contrast,
that in the absence of pyruvic acid, the current density in the the product selectivity in the presence of NH2OH was strongly
presence of pyruvic acid increased at potentials lower than affected by NH2OH amount, pH, and temperature (Fig. 2e and
À0.10 and À0.20 V vs. RHE in Fig. 2b and c, respectively, clearly Fig. S6, Table S2, ESI†). When the NH2OH amount was higher
indicating the reduction of pyruvic acid or its nitrogenated than 1.2 equiv. relative to pyruvic acid, the FE for alanine
species, i.e., the imine or oxime. Accordingly, we performed the production decreased with increasing NH2OH amount (Fig. 2e(i)),
electroreduction of pyruvic acid at applied potentials of À0.32 which is possibly attributable to the electroreduction of resi-
and À0.40 V in the presence of NH3 and NH2OH, respectively, dual NH2OH. At pH values lower than 3, protonation of the
using a two-compartment electrochemical cell separated by a nitrogen atom on the pyruvic oxime molecule occurs, which
Nafion membrane (Fig. 2a). After the 2 hour electroreduction, possibly accelerates alanine production (Fig. 2e(ii)); cf. the pKa
the electrolyte solution in the cathode chamber was analyzed by for the oxime protonation is 1.3.38 Indeed, it is well known for
high-performance liquid chromatography (HPLC) and 1H NMR. proton-coupled reduction reactions such the oxime reduction
When we used NH2OH as the nitrogen source, alanine was that protonation of the substrates increases their redox potentials
produced with a 78% FE, whereas the corresponding reduction and facilities the progress of the reduction.39,40 Considering that
conducted in NH3 buffer resulted in less selective production of oximes undergo hydrolysis upon heating in acidic media and
alanine, i.e., 28 and 24% of FEs for the production of alanine afford the original ketone and NH2OH,41,42 a temperature increase
and lactic acid, respectively (Fig. 2d). As is evident from can induce lactic acid formation (Fig. 2e(iii)). Finally, we achieved
1H NMR measurements for pyruvic acid in D2O solution con- an FE of 99% for alanine production under the optimum
taining NH2OH (Fig. S3, ESI†) and NH3 (Fig. S4, ESI†), pyruvic conditions, i.e., applied potential of À0.50 V, pH 0.19, tempera-
acid almost quantitatively reacted with NH2OH to form pyruvic ture of 0 1C, pyruvic acid concentration of 160 mM, NH2OH
oxime, whereas pyruvate imine could not be observed even with amount of 1.2 equiv. relative to pyruvic acid.
a large excess amount of NH3 in the sample solution. These
We further examined the electrosynthesis of other amino
results clearly explain the higher FEs for the production of acids having various types of functional groups in their residues.
14722 | Chem. Commun., 2019, 55, 14721--14724
This journal is ©The Royal Society of Chemistry 2019

View Article Online
ChemComm
Communication
Fig. 3 Electrochemical production of various amino acids from the
corresponding a-keto acids and NH2OH. FEs for amino acid production
are presented. Detailed conditions for each electroreduction are summarized
in Table S3 in the ESI.†
acid, phenylalanine, and tyrosine for the first time in this study, and
achieved the highest FE values for the electrochemical synthesis
of alanine and glutamic acid, i.e., 99 and 97%, as summarized in
Table S4 (ESI†).
We finally investigated the continuous electrochemical
production of alanine from pyruvic acid and NH2OH using an
electrolyser, called an AAEC. Fig. 4a shows the structure of the
AAEC. We employed Ti felts covered with anatase TiO2 (TiO2/Ti
felt) and nanoscale IrO2 as a cathode and anode, respectively,
which we originally developed.35,36 The TiO2/Ti felt (2 Â 2 cm2)
and a membrane electrode assembly (MEA) prepared by hot
pressing of a Nafion membrane bearing an IrO2 deposited layer
(2 Â 2 cm2) and porous Ti paper were sandwiched between
the cathode and anode current collectors with sample flow
channels and sealed with silicone gaskets. Alanine electro-
synthesis with the AAEC was performed at various applied
potentials in the range from À2.2 to À2.8 V with continuous
flows of water (1 mL minÀ1) and 0.2 M H2SO4 (aq.) containing
Fig. 2 Electrochemical reduction of pyruvic acid on a TiO2/Ti mesh
electrode in the presence of NH3 and NH2OH to produce alanine.
(a)
A schematic diagram of the electrochemical system for alanine
160 mM pyruvic acid and 96 mM (NH2OH)2ÁH2SO4 (0.5 mL minÀ1
)
electrosynthesis using a two-compartment glass cell. (b) CV curves of
the TiO2/Ti mesh electrode in 1.5 M NH3/(NH4)2SO4 buffer (pH 10) at 40 1C
with (red line) and without (blue line) 30 mM pyruvic acid. (c) CV curves of
the TiO2/Ti mesh electrode in 0.2 M H2SO4 (aq.) at 25 1C with (red line,
pH 0.53) and without (blue line, pH 0.62) 160 mM pyruvic acid and
80 mM (NH2OH)2ÁH2SO4. (d) FEs for alanine (red bar) and lactic acid
(blue bar) production via pyruvic acid (160 mM) reduction at À0.32 V in
1.5 M NH3/(NH4)2SO4 buffer (40 1C, pH 10) and at À0.4 V in 0.2 M H2SO4 aq.
with 80 mM (NH2OH)2ÁH2SO (25 1C, pH 0.53). (e) FEs for alanine (red bar) and
lactic acid (blue bar) production in the presence of NH2OH under varying
(i) NH2OH amounts, (ii) pH values, and (iii) temperatures. Detailed conditions
for each electrochemical run are summarized in Table S2 in the ESI.†
in the anode and the cathode, respectively. The amperometric i–t
curves acquired at a series of potentials reveal the constant nature
of the current densities during the AAEC operation, suggesting the
good stability of the catalysts (Fig. 4b). The conversion of pyruvic
acid increased as the applied potential increased and ultimately
reached 89% at an applied potential of À2.8 V (Fig. 4c). Reasonably
high FEs of 73–77% for alanine production were obtained at a
potential range of À2.2 to –2.8 V (Fig. 4d).
In summary, we have presented highly efficient electrochemical
conversion of a-keto acids into amino acids, i.e., with 77–99% FE
values, on TiO2/Ti mesh electrodes using NH2OH as a nitrogen
Glycine, aspartic acid, glutamic acid, and leucine were synthesized source, which is due to the exceptionally high selectivity for the
from the corresponding a-keto acids and NH2OH (Fig. 3) with FE hydrogenation of preferentially-formed oximes on the TiO2 catalyst
values higher than 90%, whereas the FE values for the production even in aqueous media. The continuous electrosynthesis of
of phenylalanine and tyrosine were slightly lower, i.e., 87 and 77%, alanine from pyruvic acid and NH2OH using the AAEC
respectively, which is attributable to the low concentration of the equipped with a TiO2/Ti felt was demonstrated, and 89% con-
starting a-keto acid due to their solubility limit. To the best of our version of pyruvic acid and 77% FE for alanine production were
knowledge, we demonstrated electrochemical synthesis of aspartic achieved at applied potentials of À2.8 and À2.4 V, respectively.
This journal is ©The Royal Society of Chemistry 2019
Chem. Commun., 2019, 55, 14721--14724 | 14723

View Article Online
Communication
ChemComm
Notes and references
1 M. Ikeda and S. Takeno, in Corynebacterium Glutamicum: Biology and
Biotechnology, ed. H. Yukawa and M. Inui, Springer Berlin Heidelberg,
Berlin, Heidelberg, 2013, p. 107.
2 J. Becker and C. Wittmann, Curr. Opin. Biotechnol, 2012, 23, 718.
3 M. D’Este, M. Alvarado-Morales and I. Angelidaki, Biotechnol. Adv.,
2018, 36, 14.
4 A. Strecker, Justus Liebigs Ann. Chem., 1850, 75, 27.
5 H. Groger, Chem. Rev., 2003, 103, 2795.
6 J. Wang, X. H. Liu and X. M. Feng, Chem. Rev., 2011, 111, 6947.
7 K. Harada and K. Matsumoto, J. Org. Chem., 1967, 32, 1794.
8 H. Kuang, M. L. Brown, R. R. Davies, E. C. Young and M. D. Distefano,
J. Am. Chem. Soc., 1996, 118, 10702.
9 S. Ogo, K. Uehara, T. Abura and S. Fukuzumi, J. Am. Chem. Soc.,
2004, 126, 3020.
10 M. Besson, P. Gallezot and C. Pinel, Chem. Rev., 2014, 114, 1827.
11 C. H. Zhou, X. Xia, C. X. Lin, D. S. Tong and J. Beltramini, Chem. Soc.
Rev., 2011, 40, 5588.
12 A. Subash, V. S. Malik, S. S. Shafi and S. S. Mansoor, Pharma Chem.,
2015, 7, 299.
13 A. Garcia-Raso, P. M. Deya and J. M. Saa, J. Org. Chem., 1986, 51, 4285.
14 W. Deng, P. Wang, B. Wang, Y. Wang, L. Yan, Y. Li, Q. Zhang, Z. Cao
and Y. Wang, Green Chem., 2018, 20, 735.
15 X. Lei, F.-F. Wang, C.-L. Liu, R.-Z. Yang and W.-S. Dong, Appl. Catal., A,
2014, 482, 78.
16 C. Guarin, L. Gavila, M. Constanti and F. Medina, Chem. Eng. Sci.,
2018, 179, 83.
17 F.-W. Wang, Z.-B. Huo, Y.-Q. Wang and F.-M. Jin, Res. Chem.
Intermed., 2011, 37, 487.
18 L. Kong, G. Li, H. Wang, W. He and F. Ling, J. Chem. Technol.
Biotechnol., 2008, 83, 383.
19 F. F. Wang, C. L. Liu and W. S. Dong, Green Chem., 2013, 15, 2091.
20 X. Yang, L. Yang, W. Fan and H. Lin, Catal. Today, 2016, 269, 56.
21 Y. Wang, W. Deng, B. Wang, Q. Zhang, X. Wan, Z. Tang, Y. Wang,
C. Zhu, Z. Cao, G. Wang and H. Wan, Nat. Commun., 2013, 4, 2141.
22 J. Zhang, X. Liu, M. Sun, X. Ma and Y. Han, ACS Catal., 2012, 2, 1698.
23 Y. Chen, X. Ren and Q. Wei, Bioresour. Technol., 2017, 228, 47.
24 E. Sjostrom, Biomass Bioenergy, 1991, 1, 61.
25 E. A. Jeffery, O. Johansen and A. Meisters, Aust. J. Chem., 1978, 31, 79.
26 E. A. Jeffery and A. Meisters, Aust. J. Chem., 1978, 31, 73.
27 A. Anne, S. Daninos, J. Moiroux and C. Bourdillon, New J. Chem.,
1994, 18, 1169.
Fig. 4 Electrosynthesis of alanine from pyruvic acid and NH2OH using the
AAEC. (a) A schematic diagram of the AAEC. The presented building blocks
are stacked to form the AAEC. (b) Current densities, (c) conversions of
pyruvic acid, and (d) FEs for alanine production during the operations of
the AAEC at various applied potentials in the range from À2.2 to À2.8 V
with flows of water (1 mL minÀ1) and 0.2 M H2SO4 (aq.) containing 160 mM
pyruvic acid and 96 mM (NH2OH)2ÁH2SO4 (0.5 mL minÀ1) in the anode and
the cathode, respectively.
28 W. Markle and S. Lutz, Electrochim. Acta, 2008, 53, 3175.
29 S. Yamamoto, T. Nobukuni and M. Matsuda, JP Pat., 1958-001867, 1958.
30 C. Liu, A. Y. Zhang, D. N. Pei and H. Q. Yu, Environ. Sci. Technol.,
2016, 50, 5234.
31 Y. Wang, K. Jia, Q. Pan, Y. D. Xu, Q. Liu, G. W. Cui, X. D. Guo and
X. P. Sun, ACS Sustainable Chem. Eng., 2019, 7, 117.
32 R. Watanabe, M. Yamauchi, M. Sadakiyo, R. Abe and T. Takeguchi,
Energy Environ. Sci., 2015, 8, 1456.
33 M. Yamauchi, N. Ozawa and M. Kubo, Chem. Rec., 2016, 16, 2249.
34 T. Fukushima, S. Kitano, S. Hata and M. Yamauchi, Sci. Technol.
Adv. Mater., 2018, 19, 142.
35 M. Sadakiyo, S. Hata, X. D. Cui and M. Yamauchi, Sci. Rep., 2017, 7, 17032.
36 T. Fukushima, M. Higashi, S. Kitano, T. Sugiyama and M. Yamauchi,
Catal. Today, DOI: 10.1016/j.cattod.2019.03.071.
Electrochemical processes have been believed to play a role in
the origin of life. Recently, various studies have clarified that
fundamental chemicals that make up living organisms are pro-
ducible through abiological pathways; e.g., acetates and pyruvate
can be generated via CO2 reduction.43,44 Thus, our report on the
electrochemical production of amino acids from a-keto acids may
contribute to the elucidation of the mystery of the creation of life.
Assuming that the future of human beings involves living away
from Earth, it will be necessary to conveniently and efficiently
synthesize essential nutrients such as amino acids for maintaining
their life in space. In this context, electrochemical processes using
solar electricity will play a significant role in material synthesis
under space-limited and resource-restricted conditions. We hope
that our approach will provide useful clues for the future construc-
tion of artificial carbon and nitrogen cycles in space.
´
37 M. Sadakiyo, S. Hata, T. Fukushima, G. Juhasz and M. Y. Yamauchi,
Phys. Chem. Chem. Phys., 2019, 21, 5882.
38 D. J. Palling and T. C. Hollocher, J. Org. Chem., 1984, 49, 388.
39 T. Sarma, G. Kim, S. Sen, W. Y. Cha, Z. Duan, M. D. Moore,
V. M. Lynch, Z. Zhang, D. Kim and J. L. Sessler, J. Am. Chem. Soc.,
2018, 140, 12111.
40 T. Teratani, T. Koizumi, T. Yamamoto and T. Kanbara, Inorg. Chem.
Commun., 2011, 14, 836.
41 Y. Y. Xu, Z. H. Li, L. Y. Gao, D. S. Zhang, X. Q. Zhao, S. F. Wang and
Y. J. Wang, Ind. Eng. Chem. Res., 2015, 54, 1068.
42 F. F. Zhao, K. Y. You, C. Peng, S. Tan, R. G. Li, P. L. Liu, J. Wu and
H. A. Luo, Chem. Eng. J., 2015, 272, 102.
We thank Mr Manabu Higashi for his experimental supports.
This work was supported by MEXT KAKENHI Grant Number
JP18H05517 and JP19K22205, and JST-CREST, Japan.
43 N. Kitadai, R. Nakamura, M. Yamamoto, K. Takai, N. Yoshida and
Y. Oono, Sci. Adv., 2019, 5, eaav7848.
44 N. Kitadai, R. Nakamura, M. Yamamoto, K. Takai, Y. M. Li,
A. Yamaguchi, A. Gilbert, Y. Ueno, N. Yoshida and Y. Oono, Sci.
Adv., 2018, 4, eaao7265.
Conflicts of interest
There are no conflicts to declare.
14724 | Chem. Commun., 2019, 55, 14721--14724
This journal is ©The Royal Society of Chemistry 2019