Catalytic Production of Alanine from Waste Glycerol

Article abstract of DOI:10.1002/anie.201912580

Chemical synthesis of amino acids directly from biomass feedstock is rare. Reported here is a one-step protocol to convert crude glycerol, from the biodiesel industry, into 43 % alanine over a Ru₁Ni₇/MgO catalyst. The multifunctional catalytic system promotes glycerol conversion into lactic acid, and then into alanine. X-ray absorption spectroscopy and scanning transmission electron microscopy revealed the existence of bimetallic RuNi species, whereas density-functional theory calculations suggested Ni-doped Ru substantially decreased the E_a of C?H bond dissociation of lactate alkoxide to form pyruvate, which is the rate-determining step. The catalytic route established in this work creates new opportunities for glycerol utilization and enriches the substrate scope of renewable feedstock to access value-added amino acids.

Full text of DOI:10.1002/anie.201912580

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Accepted Article
Title: Catalytic production of alanine from waste glycerol
Authors: Yunzhu Wang, Shinya Furukawa, Song Song, Qian He,
Hiroyuki Asakura, and Ning Yan
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201912580
Angew. Chem. 10.1002/ange.201912580
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Catalytic production of alanine from waste glycerol
Yunzhu Wang, Shinya Furukawa, Song Song, Qian He, Hiroyuki Asakura and Ning Yan*
Abstract: Chemical synthesis of amino acids directly from biomass
feedstock is rare. Herein, we report a one-step protocol to convert
crude glycerol from the biodiesel industry into 43% alanine over a
used organic amines as the nitrogen source, or suffered from
various other issues such as low selectivity and/or the
[
7]
requirement of multiple steps. Recently, transformation of
biomass-derived hydroxyl acids into corresponding amino acids
1 7
Ru Ni /MgO catalyst. The multifunctional catalytic system promotes
[8]
glycerol conversion into lactic acid, and then to alanine. X-ray
absorption spectroscopy and scanning transmission electron
microscopy revealed the existence of bimetallic RuNi species,
whereas density functional theory calculations suggested Ni-doped
has been reported, opening a path towards amino acids.
[5]
Considering that the conversion of glycerol to lactic acid and the
conversion of lactic acid to alanine[ have both been achieved,
an opportunity arise to combine the two steps in a single catalytic
system comprising both base and metal functionalities so that
glycerol is directly converted into alanine (Figure 1). A major
challenge of the new route is the identification of proper catalytic
systems that enable efficient, sequential dehydrogenation,
dehydration, rearrangement, and amination steps.
8a]
a
Ru substantially decreased the E of C–H bond dissociation of lactate
alkoxide to form pyruvate, which is the rate-determining step. The
catalytic route established in this work creates new opportunities for
glycerol utilization and enriches the substrate scope of renewable
feedstock to access value-added amino acids.
Glycerol is the major by-product in biodiesel industry soon
reaching an annual scale of 3.7 million metric tons.[1] Glycerol on
its own is a low-value chemical whose production far exceeds the
current demand. Extensive research activities have been devoted
to converting glycerol to value-added chemicals, following
oxidation/dehydrogenation, dehydration, hydrogenolysis, steam
reforming and transesterification pathways.[2] In the oxidation
pathway, glycerol can be converted into tartronic acid, glyceric
acid, dihydroxyacetone or others depending on the
regioselectivity and/or degree of oxidation. The dehydration
pathway is useful to generate C=C bond-containing chemicals
such as acrolein.[1a] Hydrogenolysis is applied to partially or fully
remove oxygen from glycerol, offering propanediols, propanols,
propylene glycol, et al. as products.[4] Chemicals with more
complex functionalities, such as lactic acid, may be produced
when two or more catalytic functionalities are introduced into the
same catalyst.[5]
Figure 1. An illustration of one-step conversion of waste glycerol to alanine.
Initially, a standard 2 wt% Ru/MgO catalyst, prepared by co-
precipitation (CP) method followed by calcination in air and
[
3]
reduction in H
Table S1). Little alanine (0.7%) was observed in the presence of
NaOH and 10 bar H at 220 °C for 4 h (Figure 2a). Then, a series
of bimetallic Ru 10/MgO (M = Fe, Co, Ni and Zn) catalysts were
prepared using the CP method, with Ru to second metal molar
ratio fixed at 1:10 (denoted as Ru 10/MgO, Table S1). A general
trend is that the Ru 10/MgO catalysts exhibited much-improved
activity than the single-metallic Ru/MgO catalyst. In particular,
Ru Ni10/MgO offered 48 times more alanine (34%) than Ru/MgO
2
both at 500 °C, was used in conversion of glycerol
(
2
1
M
1
M
1
M
All the strategies mentioned above only offer oxygen-
containing chemicals from glycerol. Converting glycerol to
1
under the same condition. Of note, pure Ni catalysts, such as
Ni/MgO (20 wt%) and Raney® Ni presented essentially no activity
towards alanine, indicating Ru, instead of Ni, is the main
catalytically active species.
organonitrogen chemicals, which often have higher commercial
value,[6] has received much less attention. Limited reports either
[*]
Dr. Y. Wang, Dr. S. Song, Prof. Dr. N. Yan
Department of Chemical and Biomolecular Engineering
National University of Singapore
A series of Ru
and CeO using wet-impregnation (W) method, and ZnO and
Al using CP method were prepared and evaluated. Despite
being slightly less effective than the CP-derived Ru Ni10/MgO
catalyst, Ru Ni10/MgO-W and Ru Ni10/CNT-W were much better
1 2
Ni10 catalysts supported on MgO, CNT, TiO
2
4
Engineering Drive 4, Singapore 117585, Singapore
2 3
O
E-mail: ning.yan@nus.edu.sg
Prof. Dr. S. Furukawa
Institute for Catalysis, Hokkaido University, N-21, W-10, Sapporo
1
1
1
than catalysts on other supports (Figure 2b). Considering MgO
and CNT are stable under highly basic conditions while others are
not, the observed support effect is attributed to the support
stability under the reaction conditions. As the Ni:Ru ratio
increased from 3 to 7 on MgO using CP method (Table S2), the
alanine yield increased gradually to 36%. Meanwhile, the lactic
acid yield displayed a reversed trend decreasing from 26% to 10%
0
01-0021, Japan.
Elements Strategy Initiative for Catalysis and Battery, Kyoto
University, Kyoto Daigaku Katsura, Nishikyo-ku, Kyoto, Japan, 615-
8510.
Prof. Dr. Q. He,
Department of Materials Science and Engineering
National University of Singapore
9
Engineering Drive 1, Singapore 117575, Singapore
Prof. Dr. H. Asakura
(
Figure 2c). Further increasing the Ni:Ru ratio to 10 did not induce
a significant increase in alanine yield. Within the first 0.5 h over
the optimum Ru Ni /MgO catalyst (reduced at 600 °C, as
discussed below), lactic acid was rapidly produced reaching 71%
Department of Molecular Engineering, Graduate School of
Engineering, Kyoto University, Kyotodaigaku Katsura, Nishikyo-ku,
Kyoto 615-8510, Japan
Supporting information for this article is given via a link at the end of
the document.
1
7
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yield, whereas alanine yield was only 4.5% with small amount of
glycine detected (Figure 2d and Table S3). The alanine yield
progressively increased to 45% in the next 3.5 h, together with the
declining yield of lactic acid to less than 17%. These observations
are in line with the designed two-step strategy in which lactic acid
is the key intermediate. Below 200 °C the activity is very low
whereas at 240 °C or above, side-reactions become significant
of the Ru K-edge for Ru
1
Ni
7
/MgO suggested a Ru-Ni coordination
number of 2.9, providing evidence of Ru-Ni alloy structure (Table
S4 and Figure S7). Selected catalysts were also studied using
aberration-corrected scanning transmission electron microscopy
(STEM). Figure 3(c) shows a representative high angle annular
1 7
dark-field (HAADF) image of the Ru Ni /MgO catalyst. From the
atomic number (Z) contrast, metal particles less than 10 nm were
found uniformly dispersed on the MgO support. The compositions
of these particles were further studied using energy-dispersive X-
ray spectroscopy (X-EDS). A representative HAADF image, the
corresponding chemical map and elemental composition of
selected area were shown in Figure 3(d), 3(e-f) and Figure S8
respectively, indicating that the nanoparticles are Ru-Ni alloys.
Based on the evidence from XANES, XPS and STEM analysis,
and the fact that NiO can be reduced to metallic Ni in the presence
(
Figure S1).
of Ru under the reaction condition (10 bar H
2
at 220 °C) owing to
the hydrogen spillover capability of Ru to Ni,[ we postulate
9]
bimetallic RuNi species to be the key for the exceptional activity.
Figure 2. Catalytic conversion of glycerol to alanine in a batch reactor over (a)
Ru 10/MgO, Ru/MgO, Ni/MgO and Raney® Ni catalysts, (b) Ru Ni10 on
different supports (W indicates catalysts prepared by wet-impregnation method),
c) MgO supported RuNi catalyst with varied Ru:Ni ratio, and (d)
conversion/yield profile of Ru Ni /MgO catalyst as a function of time. Reaction
conditions: 100 mg glycerol, 200 mg NaOH, 67 mg catalysts or 30 mg Raney®
Ni (Ru:substrate molar ratio = 0.012), 2 mL NH O (25 wt%), 10 bar H , 220 °C,
h. Catalysts were reduced at 500 °C for (a-c) and 600°C for (d).
1
M
1
(
x
1
7
3
H
2
2
4
The effect of calcination temperature in the range of 300 °C to
00 °C was investigated (Figure S2). As suggested by X-ray
9
powder diffraction (XRD) analysis (Figure S3), 500 °C is the
minimum temperature required for the complete generation of
MgO from its precursor. H
2
-temperature programmed reduction
Ni /MgO and Ru/MgO catalysts
(
H
2
-TPR) profiles of the Ru
1
7
exhibited reduction peaks at around 450-475 °C (Figure S4).
Correspondingly, the alanine yield has a positive correlation with
the catalyst reduction temperature in the range of 300 °C to
Figure 3. (a-b) XANES spectra of Ru
Ru Ni /MgO-spent catalysts. (c) A representative HAADF image of Ru
catalyst, (d-f) EDS mapping of particles in the Ru Ni /MgO catalyst. The
1
Ni
7
/MgO, Ru/MgO, Ni/MgO and
6
00 °C, hinting that the metallic species are the active sites
1
7
1
Ni /MgO
7
(
Figure S5). From these studies, 500 °C and 600 °C were selected
1
7
as calcination and reduction temperature, respectively.
simultaneously obtained HAADF signal, Ni K-peak map and Ru L-peak map are
shown in (d), (e) and (f) respectively.
X-ray absorption near edge structure (XANES) revealed that
the valent state of Ru in Ru/MgO is between Ru4+ (RuO
as
reference) and Ru0 (Ru foil as reference) (Figure 3a). With
addition of Ni, the K-edge absorption of Ru in Ru Ni /MgO
2
13
C nuclear magnetic resonance (NMR) spectrum of the
13
1
7
3
product mixture using glycerol- C as the starting material was
exhibited a red-shift towards that of the metallic Ru. X-ray
photoelectron spectroscopy (XPS) provided similar information
recorded. The spectra of the product mixture was relatively simple
throughout the reaction duration of 1 h (Figure S9) or 4 h (Figure
1 7
that Ru in Ru Ni /MgO is closer to metallic state than in Ru/MgO
4
a). Lactic acid and alanine are the main species identified,
(
Figure S6a). In both XPS and XANES, Ni is oxidized due to the
substantiating lactic acid to be the only stable, abundant
exposure to air before measurements (Figure 3b and Figure S6b).
Extended X-ray absorption fine structure (EXAFS) fitting results
intermediate in the system. A small amount of C
C and formamide- C) and C species (e.g. glyceric acid- C )
2 2
1
(e.g. formic acid-
13
13
13
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Angewandte Chemie International Edition
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COMMUNICATION
were detected, representing the only identifiable side reactions.
Based on above and related studies in the literature,[10] a plausible
reaction pathway is proposed (Figure 4b). Sequential
dehydrogenation, dehydration and rearrangement lead to lactic
acid from glycerol. Lactic acid then reacts with ammonia to form
physical mixture of Ru/MgO and Ni/MgO did not afford alanine
with substantial yield, suggesting the importance of the existence
of Ru and Ni in close proximity to promote C-H activation. Of note,
the overall alanine production rate was lower than the lactic acid
amination rate. A plausible explanation is that the unreacted
glycerol inhibit lactic acid formation by strongly coordinate to the
catalyst. Indeed, adding external glycerol decreased the alanine
yield in lactic acid amination reaction (Table S5). Considering the
conversion of glycerol is much faster over the RuNi catalyst
(Figure 2d) than over the Ru catalyst (Figure S11), a second
function of Ni in the system is to accelerate glycerol
transformation into lactic acid although the underlying mechanism
needs further exploration.
alanine following
a
dehydrogenation-imination-hydrogenation
species into C and C products
pathway.[ Hydrogenolysis of C
8a]
3
1
2
are the main side reactions. Metal and base act as active sites for
dehydrogenation/hydrogenation/hydrogenolysis and
dehydration/Cannizzaro reaction, respectively.[11] Base also
promotes metal catalysed glycerol dehydrogenation by
deprotonation,[ and likely displaces lactic acid/alanine from
catalyst surface as sodium lactate/sodium alaninate, which
explains the proportional increase of lactic acid and alanine yields
as a function of NaOH amount (Figure S10).
12]
Table 1. Reaction rates of glycerol to alanine.[a]
Step
Substrate
Glycerol
Solvent
Time
h)
Conv.
(%)
Yield
(%)
Production
rate[
b]
(
(mmol/g/h)
Over
all
NH
O
3
H
2
0.67
15
0.12
0.19
[
e]
I
Glycerol[c]
Lactic acid
H
2
O
0.33
1
11
5.4[f]
5.0[e]
32
II
NH
O
3
H
H
2
2
5.9
5.6
II
Lactic
acid[
NH
O
3
3
6.8
6.2[e]
2.2
d]
[
a] Reaction conditions: 100 mg substrate, 10 mg Ru
NaOH, 2 mL solvent, 10 bar H
alanine or lactic acid) over gram of catalyst over time. [c] 5 mg Ru
d] 10 mg Ru/MgO. [e] Yield of alanine. [f] Yield of lactic acid.
1
Ni
7
/MgO, 200 mg
2
, 220 °C. [b] Calculated by mmol of product
(
[
1
Ni /MgO.
7
Next, DFT study was conducted to further understand the role
of Ni. We considered not only a standard flat slab surface but also
interfacial models with a Ru20 cluster on MgO(100) (Figure 5a).
Two different metal-support interfaces were compared to consider
the structural difference (Figure 5b, A and B sides). The activation
a
and reaction energies (ΔE and E , respectively) of the rate-
determining step—C–H activation of lactate alkoxide to
pyruvate—were calculated on various catalyst structures. For
Ru20–MgO(100), the molecules were placed on the interface so
that the alkoxide/carbonyl and carboxylate moieties bound to Ru
(
metal) and Mg (support), respectively. Based on the phase
Figure 4. (a) 13C NMR spectra of alanine and lactic acid produced from glycerol-
diagram of the Ru–Ni system, where only a small amount of Ni
(<7%) can be dissolved into Ru,[13] one-atom substitution of Ru by
Ni was considered. The position of Ru→Ni substitution was varied
so that the Ni atom bound to different part of the molecules;
13
. Reaction condition: 108 mg glycerol-13
C
3
C
3 1 7
, 67 mg Ru Ni /MgO, 200 mg
NaOH, 2 mL NH
network.
H O (25 wt%), 10 bar H , 220 °C, 4 h. (b) Proposed reaction
3 2 2
2
alkoxide O (Oal), secondary C (C ), eliminated H (Hel), and
carbonyl O (Oca, only for Ru(0001) (Figure 5 and Figures S12-S21
for detailed structures). Ru–NiO interface was also considered by
substituting two Mg cations of Ru20–MgO(100) by Ni.
Conversion of glycerol to lactic acid (step I) and amination of
lactic acid to alanine (step II) were evaluated separately over the
Ru catalysts (Table 1). Step II is 5 times slower than step I over
the Ru
fact, Ru
step II) that differentiated it from less active catalysts (Table S5).
1
Ni
7
/MgO catalyst under the same reaction condition. In
a
Figure 5c summarizes the overall relationship between E and
ΔE. A rough linear correlation was observed, indicating that the
reaction mainly follows the Bønsted–Evans–Polanyi principle.
1
Ni
7
/MgO provided the highest rate in lactic acid amination
(
Considering C-H bond cleavage at α-carbon is the rate-
determining step (r.d.s.) in lactic acid amination,[8a] and that
amination is much slower than glycerol to lactic acid, C-H bond
activation is the r.d.s. in glycerol transformation into alanine. A
a
The interfacial sites (A and B) gave much lower E than the
metallic terrace sites, probably because of the less atomic
displacement during the C–H activation (Figures S12 and S15).
Although these are only a part of numerous possible sites, the
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Angewandte Chemie International Edition
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metal–support interface is likely to work as an effective reaction
site for the C–H activation of lactate. For the Ru20–MgO(100)A
TEM analysis. The valance states of Ru and Ni were almost
unchanged (Figure 3a-b) but the size of the NPs slightly increased
(Figure S23), the latter of which might account for the loss of
activity.
series, Ru→Ni substitution at Oal significantly lowered E
a
(circles
to triangles), while that at C increased E . Similar trends were
2
a
observed in other series (circles to squares), but there was no
obvious trend for the substitution at Hel sites. These results
indicate that the doped Ni in Ru facilitates the reaction when the
alkoxide oxygen binds to Ni. On the contrary, Mg→Ni substitution
In summary, glycerol has been converted in a single-step to
alanine over a RuNi bimetallic catalyst in aqueous ammonia.
Lactic acid was determined as the key intermediate, and the
amination of lactic acid to alanine was the rate-controlling step.
The addition of Ni remarkably enhanced the C-H activation over
Ru in the lactic acid amination step and the transformation from
(
Figure 5c, filled to open symbols) did not show a specific trend,
suggesting that Ru–NiO interface does not strongly contribute to
the catalytic performance. Thus, the DFT study suggests the
significant role of the Ru–Ni alloy phase on the enhanced
catalysis, in agreement with the experimental observations
1 7
glycerol to lactic acid. The optimized Ru Ni /MgO catalyst
afforded value-added alanine directly from crude glycerol and is
reusable. The novel reaction route from “waste” of the biodiesel
plant to produce value-added amino acid adds new possibilities
in biorefinery.
(
Figure 2c) that the major function of Ni is to promote Ru catalysed
conversion of lactic acid to alanine.
Acknowledgements
This work was supported by the National University of Singapore
Flagship Green Energy Program (R-279-000-553-646, and R-
2
79-000-553-731). Q.H. thanks the support from the National
Research Foundation, Singapore (NRF-NRFF11-2019-0002).
XAS experiments was performed at a public beamline, BL01B1 at
SPring-8 with the approval of JASRI (Proposal Nos. 2019A1447).
Keywords: Glycerol • Alanine • Ruthenium-Nickel •
Heterogeneous catalysis • Amino acids
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a
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[
[
4]
5]
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glycerol include water, methanol, base and fatty acid salts. Under
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Angewandte Chemie International Edition
10.1002/anie.201912580
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Direct conversion: 43% alanine was
achieved from crude glycerol over a
Ru Ni /MgO catalyst. Ni-doped Ru
1 7
Yunzhu Wang, Shinya Furukawa, Song
Song, Qian He, Hiroyuki Asakura, Ning
Yan*
remarkably promoted lactic acid
amination, a key step in the reaction.
Page No. – Page No.
Catalytic production of alanine from
waste glycerol
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