Sustainable Co-Synthesis of Glycolic Acid, Formamides and Formates from 1,3-Dihydroxyacetone by a Cu/Al2O3 Catalyst with a Single Active Sites

Article abstract of DOI:10.1002/anie.201814050

Glycolic acid (GA), as important building block of biodegradable polymers, has been synthesized for the first time in excellent yields at room temperature by selective oxidation of 1,3-dihyroxyacetone (DHA) using a cheap supported Cu/Al₂O₃ catalyst with single active Cu^II species. By combining EPR spin-trapping and operando ATR-IR experiments, different mechanisms for the co-synthesis of GA, formates, and formamides have been derived, in which ^.OH radicals formed from H₂O₂ by a Fenton-like reaction play a key role.

Full text of DOI:10.1002/anie.201814050

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Sustainable co-synthesis of glycolic acid, formamides and
formates from 1,3-dihydroxyacetone by a Cu/Al2O3 catalyst with
single active sites
Authors: Xingchao Dai, Sven Adomeit, Jabor Rabeah, Carsten
Kreyenschulte, Angelika Brückner, Hongli Wang, and Feng
Shi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201814050
Angew. Chem. 10.1002/ange.201814050
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http://dx.doi.org/10.1002/ange.201814050

10.1002/anie.201814050
Angewandte Chemie International Edition
COMMUNICATION
Sustainable co-synthesis of glycolic acid, formamides and
formates from 1,3-dihydroxyacetone by a Cu/Al2O3 catalyst with
single active sites
Xingchao Dai,+[a, c] Sven Adomeit,+[b] Jabor Rabeah,[b] Carsten Kreyenschulte,[b] Angelika Brückner,*[b]
Hongli Wang,[a] and Feng Shi*[a]
Abstract: Glycolic acid (GA) as important building block of
biodegradable polymers has been synthesized for the first time in
excellent yields at room temperature by selective oxidation of 1,3-
dihyroxyacetone (DHA) using a cheap supported Cu/Al2O3 catalyst
with single active CuII species. By combining EPR spin-trapping and
operando ATR-IR experiments, different mechanisms for the co-
synthesis of GA, formates and formamides have been derived, in
which •OH radicals formed from H2O2 via a Fenton-like reaction play
a key role.
amounted to 159 h-1 (Entry 1). After 24 h, the desired GA could
be obtained in up to 94% yield (Entry 10). However, only 41%
yield of formic acid (FA) was obtained due to further oxidation to
CO2. Quantitative analysis of the gas phase after reaction
revealed that 58% of FA was oxidized to CO2. When bare Al2O3
or even no catalyst was applied, 83-85% DHA conversion was
obtained but the yield of GA was only 24 % (Entries 12-13). This
might be due to the non-selective oxidation of DHA by peroxide
radicals generated from H2O2. 1H-NMR analysis revealed 6% FA
and 58% hydrated DHA while 18% of FA was oxidized to CO2.
-
o
ese
ro uc
t
Bi di
l by p
d
There is a rising demand for biodegradable polymers to be used,
e. g., for packaging[1] and in biomedicine.[2] Especially
polyglycolic acid (PGA) has attracted much attention. It is
usually produced by carbonylation of formaldehyde (FAL)[3] or
selective oxidation of ethylene glycol[4], yet these processes
suffer from high pressure, corrosion of equipment as well as
difficult workup. It would be very attractive to produce PGA by
condensation of glycolic acid (GA) obtained from renewable
biomass-based platform molecules such as 1,3-dihyroxyacetone
(DHA). This is easily available from glycerol[5] being a by-product
of biodiesel production with about 3 million tons per year.[6]
Recently, the synthesis of glycolate from DHA has been
reported[7], yet excess amounts of base are required and the
glycolate needs to be acidized before further use, leading to
much inorganic waste. With O2 as oxidant, total oxidation of GA
dominates.[8] Therefore, direct synthesis of GA from DHA with
good enough yield would be a very attractive alternative.
H
O
O
N
R1
R2
OH
+
o o ca
bi l gi
l
N
HO
R1
H
R2
ermen a on
f
t ti
O
O
u
C /Al2O3
HO
OH
OH
OH
+
+
COO
H
HO
HO
OH
R2
O
O
O
R1
R1
R2
Scheme 1. One-pot synthesis of GA, formamides and formates with 1,3-
dihydroxyacetone as a versatile building block.
Table 1: GA yields and TOF values for different catalysts.[a]
Entry
Cat.
Y
[%]
TOF
[h-1]
Entry
Cat.
Y
[%]
TOF
[h-1]
1
2
3
Cu/Al2O3
Ru/Al2O3
Pd/Al2O3
85
68
42
159
269
121
9
Cu/C
34
94
72
59
57
Here we report for the first time the catalytic one-pot
synthesis of GA and co-synthesis of formamides and formates
from DHA as versatile building blocks without any added base
(Scheme 1). With a non-noble metal Cu/Al2O3 catalyst and H2O2
as a clean oxidant, the C-C bond in DHA could be selectively cut
to form GA in high yields at room temperature (RT).
10[b]
11[b][c]
Cu/Al2O3
Cu/Al2O3
91/86[d]
85[e]/88[f]
4
5
6
7
Pt/Al2O3
Cu/Fe2O3
Cu/Ni2O3
Cu/TiO2
--
--
12
--
24
24
20
57
--
Several supported catalysts with different metals and
supports were tested in the solvent-free synthesis of GA from
65
74
41
108
138
81
13
Al2O3
Cu/Al2O3
--
o
DHA at 25 C (Table 1). Highest GA yield was obtained with Cu
14[g]
15
--
supported on Al2O3 (Entries 1, 10 and 11), known to be active in
selective C-C bond cleavage.[9] Therefore, we focus only on
Cu/Al2O3 in due course. With this catalyst, TOF values
Cu/Al2O3-
1%
22
8
Cu/SiO2
46
86
16
Cu/Al2O3-
H2O
58
115
[a]
[b]
Xingchao Dai,[+] Hongli Wang and Prof. Dr. Feng Shi, State Key
Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences,
No.18, Tianshui Middle Road, Lanzhou, 730000, China; E-mail:
fshi@licp.cas.cn
[a] Reaction conditions: 1 mmol 1,3-dihydroxyacetone, 3 equiv., H2O2, 25 mg
catalyst, 25 oC, 8 h. [b] 24 h. [c] 1 equiv., H2O2. [d] Isolated yield. [e] Catalyst
re-used in 2nd and [f] 3rd run. [g] The catalyst was removed by filtration after 3 h
use and then the reaction was performed for another 5 h.
Sven Adomeit,[+] Dr. Jabor Rabeah, Dr. Carsten Kreyenshulte and
Prof. Dr. Angelika Brückner, Leibniz-Institut für Katalyse e.V. an der
Universität Rostock (LIKAT), Albert-Einstein-Str. 29a, 18059
Rostock, Germany, E-mail: angelika.brueckner@catalysis.de
University of Chinese Academy of Sciences, No. 19A, Yuquanlu,
Beijing, 100049, China
These authors contributed equally to this work
Supporting information for this article is given via a link at the end of
the document.
Based on the proposed mechanism in Scheme 2a (vide
infra) and the stoichiometric balance (Fig. S1), conversion of 1
mmol DHA to GA requires at least 2 mmol of H2O2 (1 equiv.).
Noteworthy, with 1 equiv. of H2O2 a similar GA yield like with 3
equiv. H2O2 was achieved (cf. Entry 10 and 11 in Table 1). The
[c]
+
1
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10.1002/anie.201814050
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COMMUNICATION
desired GA could be easily isolated in high purity by removing
water via simple distillation in up to 86% yield (Table 1, Entry 11
and Fig. S2). Estimation of residual H2O2 (SI, section 6)[10]
indicated an H2O2 utilization efficiency of 91%. Pre-reduction of
the catalysts had no positive effect since similar or lower GA
yields were obtained in comparison to the active Cu/Al2O3
catalyst (cf. Table S1 and Table 1, entry 1).
can be seen (Fig. S5), suggesting that Cu is mainly present as
single sites. This is evidenced, too, by the EPR spectrum of
Cu/Al2O3 (Fig. S7) showing an axial signal of isolated CuII ions
with spin hamiltonian parameters of g||=2.349, g⊥=2.078 and
A||=142 G[11] without significant superposition of a broad singlet
expected for CuxOy clusters, as observed for similarly prepared
catalyst, yet with 1 wt.% of Cu deposited on Al2O3 (vide infra).
Furthermore, the Cu/Al2O3 catalyst showed no deactivation
in 3 subsequent runs after centrifugation, washing and drying
(Table 1, Entry 11). Copper loadings of 0.17, 0.13, 0.14 and 0.13
wt.% (± 10% error) were detected in fresh Cu/Al2O3 and after 1st,
2nd and 3rd use. This shows that, apart from slight Cu leaching
during the first run, the catalyst is stable. To test if the leached
copper is responsible for the reaction, the catalyst was removed
by filtration after 3 h use and only 10% conversion of DHA and 9%
yield (≈ 90% selectivity) of GA were measured. When the
reaction was run for another 5 h after catalyst removal, 95%
conversion of DHA but only 20% yield of GA were obtained
(Table 1, Entry 14). This result is similar to that of the blank test,
indicating that supported copper is the active and selective
species.
Table 3: Co-production of GA and formates from DHA and different alcohols.[a]
R1
O
O
O
O
O
O
n
O
O
,
c
b
[
=
=
=
,
]
b
[ ]
n
n
n
94% 86%
[
]
1 85% 90%
[
]
]
]
R2
,
3 71% 80%
83% 95%
[ ]
[
[
=
=
=
=
=
=
=
,
,
96% 96%
R1 H R2
H
,
[
]
6 96% 71%
=
,
H
,
R1
M
R2
1
e
,
8 % 90%
[
]
]
=
,
77
R1 H R2
M
4
e
% 8 %
[
=
,
,
=
R1
M
R2 H 7
,
=
r
B
e
O
0% 83%
7
[
[ ]
]
]
,
R1 H R2
7
Cl 9 % 9%
,
,
80% 0%
R1 H R2
7
[
[a] 5 mmol alcohols, 1 mmol DHA, 1.5 equiv., H2O2, 25 mg Cu/Al2O3 catalyst,
80 oC, 24 h. [b] 50 oC. [c] 5 mL CHCl3 as solvent. GA yield in brackets.
In the Cu K-edge EXAFS spectra, only a peak for a Cu-O
coordination between 1.2 and 1.8 Å but no typical peak for a Cu-
Cu shell between 2.2 and 2.8 Å was observed, neither in the
fresh nor in the used catalysts (Fig. S8). This confirms clearly
the dominance of CuII single sites attached to O atoms of the
Al2O3 support surface.[12]
To find out whether it is indeed these Cu single sites that
govern catalytic activity, we prepared a 1 wt.% Cu/Al2O3 catalyst
by the same method, which showed a GA yield of only 57%,
despite its five times higher Cu content (cf. Table 1, entry 1 and
15). In the EPR spectrum of this catalyst, CuxOy clusters are
clearly visible by the broad background signal superimposed on
the hfs signal of the single CuII sites (Fig. S7), supporting clearly
that it is indeed the single CuII species which are active in GA
synthesis from DHA.
Besides acetone, also water was used as solvent in the
impregnation procedure for catalyst synthesis. However, due to
its low volatility, it had to be removed by rotary evaporation at 80
oC. This may have led to clustering of Cu sites. Only 58% GA
yield was obtained in this case (Table 1, entry 16). Therefore,
this synthesis method was disregarded.
To explore the mechanism of GA formation from DHA, EPR
spin trapping experiments and operando ATR-IR investigations
were performed, in which conversion of DHA to GA is monitored
by decreasing ν(C–OH) and ν(C=O) bands of DHA and rising
ν(C=O) and ν(C-OH) bands of GA. EPR spectra in the presence
of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trap have
been recorded since formation of radicals during reaction was
assumed.[13] Addition of H2O2 to Cu/Al2O3 + spin trap in aqueous
solution leads to DMPO-OH and DMPO-OOH spin adducts (Fig.
1a and S9), indicating formation of 62.6% •OH and 37.4% •OOH
radicals[14], derived by spectra simulation.
Table 2: Co-production of GA and formamides from DHA and different
amines.[a]
O
O
HN
O
O
N
HN
O
O
N
N
N
HN
3
3
,
c
c
]
b
[
]
[
96% 84%
[
91% 98%
[
96% 75%
[
84% 71%
[
99% 89%
[
99% 99%
]
]
]
]
]
[
]
H
H
H
N
H
N
O
O
N
N
O
O
O
N
Cl
97% 86%
90% 93%
95% 96%
]
98% 90%
99% 88%
[ ]
[
]
[
]
[
[
]
[a] 5 mmol amines, 1 mmol DHA, 1.5 equiv., H2O2, 25 mg Cu/Al2O3, 2 mL H2O,
reactions run at 25 oC for 24 h, respectively at 50 oC for 12 h. GA yields shown
in brackets. [b] Dimethylamine gas as starting material. [c] 5 mL H2O.
Together with GA, FA was formed (Scheme 1, determined
by 1H NMR), but only in 41 % yield, which might be due to over-
oxidation to CO2. Hence, to avoid waste of the carbon source
and raise atom efficiency, a protocol for the co-production of GA
and formamides or formates has been developed by trapping FA
with amines or alcohols (Scheme 1). For this purpose, DHA was
reacted with different amines in the presence of Cu/Al2O3 (Table
2). Remarkable formamide yields of 84-99% have been obtained
with both aromatic and aliphatic amines, while GA was still
generated in high yields (71-99%).
Besides amines, various alcohols were used as efficient
trapping agents for FA (Table 3), forming up to 97% formates
and 70-96% GA. GA and formates could be easily separated by
simple distillation, leading to isolated GA yields of 71-81% with
ethanol and benzyl alcohol.
To explore the correlation between structure and catalytic
performance, the active Cu/Al2O3 catalyst was characterized
comprehensively. The Cu loading dropped marginally from 0.17%
in the fresh to 0.13% in the used catalyst, while BET surface
area, pore volume and mean pore diameter remained constant
at 146 m2g-1, 43.5 cm3 g-1 and 6 nm, respectively, confirming that
the catalyst is stable during reaction (Fig. S3). As expected for
such low Cu contents, no signals of Cu species were observed,
neither by XRD (Fig. S4) nor by XPS (not shown). HAADF-
STEM (Fig. S5) of Cu/Al2O3 do not show any Cu containing
particles although the EDX analysis in Fig. S6 points to the
presence of Cu. If any, then only few very small CuOx moieties
After adding DHA, a new EPR signal from a DMPO-R spin
adduct appeared (Fig. 1a, top), indicating formation of a C-
centered radical intermediate (•R ≈ 20% after 30 min). Probably
•OH radicals attack DHA, leading to a drop of DMPO-OH from
62.6 to 40.1% while the amount of DMPO-OOH remained
almost constant (Fig. 1b). In due course, DMPO-OH is formed
again due to excess H2O2 while DMPO-R species decreased,
2
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10.1002/anie.201814050
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suggesting that •R is converted into EPR silent species probably
due to product formation.
rapidly reacts with H2O2 to FA while H2O is liberated. This is also
evident from the rising intensity of the ν(O–H) vibration of H2O at
1647 cm-1 (Fig. 2a). Finally, in excess H2O2, a part of FA can be
further oxidized to CO2 and H2O.
Fig. 1. a) Normalized EPR spectra of DMPO spin adducts (cf. Fig. S9) with
Cu/Al2O3 and H2O2 before (bottom) and 90 min after addition of DHA (top). Hfs
parameters: AN ≈ AH =14.9 G for •OH, AN =14.3, AβH = 11.4, AγH =1.2 G for
•OOH, and AN =15.9, AH =22.7 G for •R. b) Relative amount of the DMPO-X
spin adducts before (t=0) and after adding DHA.
Operando ATR-IR spectra were recorded in D2O to avoid
overlap of the ν(C=O) bands with the δ(O–H) vibration of H2O
(Fig. 2a, cf. Fig. S10). Initially, two ν(C–OH) bands of DHA at
1004 and 1064 cm-1 are seen, while the ν(C=O) band of DHA
falls at 1736 cm-1. After adding H2O2 these bands drop as DHA
is consumed and new ν(C=O) and ν(C-OH) bands of GA appear
at 1724 and 1092 cm-1 (cf. reference spectrum in Fig. S11).
Furthermore, a new νas(COO-) band of carboxylate rises at
1590 cm-1, suggesting partial deprotonation of the formed
carboxylic acids. Due to overlap in the ν(C=O) region, different
carbonyl compounds cannot be discerned in the absorption
spectrum. However, the 2nd derivative shows at least two other
ν(C=O) bands besides that of GA at 1709 cm-1 and 1693 cm-1
(Fig. S13). The former might arise from some small fraction of
FA (cf. Fig. S11), the presence of which was also confirmed by
1H-NMR (Fig. S15). Moreover, the formation of another C=O
containing compound is evident from the slope of the peak
heights at 1724,1092 and 1064 cm-1 in Fig. 2a (cf. Fig. S14). The
band at 1724 cm-1, reflecting GA + some other carbonyl species,
rises faster than the GA band at 1092 cm-1 while the drop of the
DHA band at 1064 cm-1 parallels the rise of the former.
GC-MS analysis detected only small amounts of FA,
suggesting its formation via an intermediate such as FAL or
formyl species. In aqueous media, FAL also shows a ν(C=O)
band around 1730 cm-1.[15] In view of up to 94% yield of GA
(Table 1, entry 10), its conversion to FA can be basically
excluded. Instead, the C3 skeleton of DHA may undergo
selective C-C cleavage to generate one C2 product (GA) and
one C1 product (FA). Surprisingly, only 41% yield of FA was
obtained. Given that H2O2 is in excess, this can be explained by
oxygenolysis of FA as 0.58 mmol CO2 was observed after
reaction. The reaction of H13COOH with H2O2 under the same
conditions confirmed this transformation (Scheme S2), in which
13CO2 was detected (Fig. S16).
Fig. 2. Operando ATR-FTIR spectra during conversion of DHA to GA
performed in D2O a) without amines and b) with amines. (For detailed
stepwise addition of H2O2 see Fig. S12).
Inspired by this mechanism, we assumed that FA can react
in situ with amines and co-produce GA and formamides, yet
surprisingly dibutylamine and FA did not react to N,N-
dibutylformamide but only to the ammonium salt of FA, while up
to 99 % yield of N,N-dibutylformamide was obtained with FAL
(Fig. S17). This suggests that FAL is a reaction intermediate.
1
Furthermore, no FA has been detected by GC-MS and H-NMR
during reaction of DHA with H2O2 in the presence of N,N-
dibutylamine (DBA), which implies that no •CH2OH was
generated in this case. Thus, FA can be safely excluded as
intermediate in the co-production of GA and formamides from
DHA and amines.
Based on all of these results, a reaction mechanism can be
proposed (Scheme 2a). Cu/A2O3 activates H2O2 to form •OH
radicals, which split a C-C bond in DHA resulting in the radical
intermediates •COCH2OH and •CH2OH. The latter react with
•OH to the desired GA and a short-lived acetal intermediate that
To obtain more insight into the mechanism of formamide
production, the reaction of DHA with H2O2 in the presence of
3
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DBA was monitored by operando ATR-IR spectroscopy in D2O
(Fig. 2b). The ν(N-H) band of DBA at 1589 cm-1 increases with
time reaching steady state after about 11 h (Fig. 2b). This is due
to the fact that initially DBA forms a separate layer on top of the
D2O/H2O2 solution and dissolved in the latter only with time
during reaction. Then a broad feature rises at 1640 cm-1. From
the 2nd derivative of the spectrum two bands at 1647 and 1638
cm-1 can be discerned (Fig. S13, right). While the former stems
from δ(OH) of the formed H2O, the latter is close to ν(C=O) of a
tertiary formamide, i. e., the desired product.[16] Simultaneously,
the small ν(C=O) band of DHA at 1736 cm-1 vanishes but no
other carbonyl bands appeared, suggesting that the formed
carboxylic acid intermediate reacts completely with the amine
present in excess.
Based on these findings, we propose another reaction
mechanism for the co-production of GA and formamides from
DHA (Scheme 2b) which involves radical intermediates, too, as
confirmed by in situ EPR measurements (Fig. 3).
H
2O
a
O
b
84-99%
CO2
O
R2R1N
H
[Cat.]
71-99%
O
O
HO
less
possible
HO
94%
OH
H
2O2
H
2O2
HO
H
2O
OH
H
2O2
H
41%
•
•
OH
OH
[Cat.]
O
•
O
O
•
O
H
2O
H
2O2
HO
OH
HO
NR1R2
HO
OH
HO
•
NHR1R2
1R2RN
•
•
•
OH
OH
CH2OH
[Cat.]
H
H
2O2
H
2O2
•
H
2O
OH
2O
HO
OH
•
OOH
Scheme 2. Proposed mechanism for the synthesis of GA from DHA in the absence (a) and in the presence (b) of amines .
occur preferentially on the phase boundary. When a sample of
the organic phase is mixed after 4.5 h with DMPO, the EPR
spectrum shows signals from different DMPO adducts and an
N–centered radical (Fig. 3), which reflect species proposed in
the mechanism of Scheme 2b, namely •OH, •OOH,
•C(O)CH2OH and •NR1R2. •OH and •OOH are formed from
H2O2 over Cu/A2O3 via a Fenton’s type reaction already without
any substrate (Fig. 1a, bottom).[7b, 17] •OH can react with DHA in
the aqueous phase to GA (Fig. 1a, top and Scheme 2a) and
with the amine on the phase boundary to •NR1R2 radicals (Fig.
3). These react with DHA on the phase boundary to the
respective formamide (Scheme 2b), possibly via a hemiaminal
radical that, however, cannot be discerned by EPR from other
C-centered R radicals. Thus, the R component in Fig. 3
reflects all C-centered radicals postulated in Scheme 2.
In summary, the biomass-based platform molecule DHA
has been catalytically activated and directly converted with
amines and alcohols to highly valuable GA, formamides and
formates in excellent yields at room temperature by a simple
supported non-noble metal Cu/Al2O3 catalyst. To the best of our
knowledge, this is the first demonstration that DHA can behave
as a versatile building block to synthesize selectively various
valuable molecules. The new concept of effectively converting
DHA to value-added molecules by a simple and inexpensive
supported Cu/Al2O3 catalyst could open new opportunities for
the valorization of other biomass-based platform molecules and,
thus, makes a significant contribution to sustainable chemical
production.
Fig. 3. EPR sum spectrum (top) of the organic phase, taken after 4.5 h from
a reaction mixture containing Cu/Al2O3, H2O2, DHA and DBA after adding
DMPO. The simulated spectrum was obtained by superimposing spectra of a
•NR1R2 radical (AN =15.4 G) and DMPO adducts of •OH (AN =12.1 G, AH
=14.2 G), •OOH (AN =14.7 G, AβH =13.1 G, AγH =2.1 G) and •R (AN =16.0 G,
AH =22.9 G).
Since the reaction mixture of the Cu/Al2O3 catalyst, H2O2,
DHA and DBA consists of an aqueous phase containing mainly
H2O2 and DHA and an organic amine phase, the reaction might
4
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Acknowledgements
We acknowledge financial support by the National Key
Research
and
Development
Program
of
China
(2017YFA0403100), NSFC (21633013) Key Research Program
of Frontier Sciences of CAS (QYZDJ and SSW-SLH051). We
thank Prof. Dr. Lirong Zheng (Institute of High Energy Physics,
Chinese Academy of Sciences) for helping us to perform the
EXAFS analysis.
Keywords: glycolic acid • 1,3-dihydroxyacetone • formamides
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10.1002/anie.201814050
Angewandte Chemie International Edition
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COMMUNICATION
Clean and green by radicals
Xingchao Dai, Sven Adomeit, Jabor Rabeah,
Carsten Kreyenshulte, Angelika Brückner,*
Hongli Wang, and Feng Shi*
1,3-dihydroxylacetone, a biomass-based
platform molecule has been oxidized with
100% carbon atom efficiency at room
temperature to valuable glycolic acid,
formamides and formates by a cheap non-
noble metal catalyst. Radical-based
mechanisms were discovered by in situ
EPR with spin traps and operando ATR-IR
spectroscopy.
Page No. – Page No.
Sustainable co-synthesis of glycolic acid,
formamides and formates from 1,3-
dihydroxyacetone by a Cu/Al2O3 catalyst
with single active sites
6
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