Transfer hydrogenation of carbon dioxide and bicarbonate from glycerol under aqueous conditions

Article abstract of DOI:10.1039/c8cc03157f

The transfer hydrogenation of CO₂ from glycerol to afford formic and lactic acid is a highly attractive path to valorizing two waste streams and is a significantly more thermodynamically favorable process than direct CO₂ hydrogenatio

Full text of DOI:10.1039/c8cc03157f

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DOI: 10.1039/C8CC03157F
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Transfer Hydrogenation of Carbon Dioxide and Bicarbonate from
Glycerol Under Aqueous Conditions
Jacob Heltzel, Matthew Finn, Diana Ainembabazi, Kai Wang and Adelina M. Voutchkova-Kostal*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The transfer hydrogenation of CO2 from glycerol to afford formic hydrogenation (TH) from a renewable hydrogen source could
and lactic acid is a highly attractive path to valorizing two waste also afford an additional product in a single process. The
streams, and is significantly more thermodynamically favorable generation of an additional by-product could improve the
process than direct CO2 hydrogenation. We report the first economics of the process, provided the product is more
homogeneous catalyst for this transformation, consisting of water- valuable than the hydrogen donor and is formed selectively, and
soluble Ru N-heterocyclic carbene complex. The catalyst affords could offer additional thermodynamic driving force for the
lactic and formic acid selectively in the presence of base at reaction.
temperatures between 150 and 225 ˚C. Carbonate salts can also be
We thus compared free energies of reaction for direct
utilized in place of CO2, affording the same products at higher rates. hydrogenation and transfer hydrogenation of CO2 in aqueous
media. TH from isopropanol, a common H2-donor, is more
Methods to convert CO2 to formaldehyde, formic acid (FA) and
methanol have been the subject of intense investigation in
recent years, given the abundance of CO2 as a C1 feedstock.1
Among these products, FA is particularly attractive due to its
widespread utility as a chemical feedstock,2 commodity
chemical for food and agriculture,2 fuel3 and hydrogen storage
medium.4 Catalytic methods for CO2 direct hydrogenation
continue to be intensively studied: highly active homogeneous
catalysts with Rh,5 Ru,6 Ir,7 Fe8 and Co9 have been reported,
with turnover numbers (TONs) on the order of 106. Base or
amine additives help stabilize the product as formate, but
favourable by ca. 9.7 kcal/mol compared to direct
hydrogenation of CO2 (Scheme 1, entries 1-2). Catalytic
examples of TH from isopropanol have been reported by Peris
et al1e using water-soluble Ru and Ir NHC complexes.16
Compared to isopropanol, glycerol is a more desirable H2-donor
due to its abundant supply as a renewable by-product of
biodiesel production. Furthermore, using glycerol as hydrogen
donor in TH of CO2 could provide a route to glycerol
valorization.17 However, CO2 TH from glycerol is considered
challenging compared to reactions with isopropanol.18
recent examples demonstrate the feasibility of the aqueous
Scheme 1. Calculated free energies of reaction (Gaq) for the CO2 direct
hydrogenation and transfer hydrogenation (Gaussian16, G3B3, PCM water).
a Reaction includes carbonate base, and was calculated as-stated.
10
reaction in the absence of additives.6c,
Since CO2 is in
−
equilibrium with HCO3 in aqueous media (pKa1 = 6.35),
hydrogenation of bicarbonate has also been demonstrated with
Rh5, 11, Ru12, Ir13 and Fe14 catalysts. However, reported activities
are often lower than those for CO2, despite the fact that the
thermodynamics of bicarbonate hydrogenation in water are
reported to be slightly more favourable than that of CO2.15
Although a number of highly active catalysts for direct
hydrogenation of CO2 have been reported, the ability to directly
utilize a renewable source of hydrogen would be highly
advantageous from a sustainability perspective. Transfer
Gaq
(kcal/mol)
Entry
O
13.4
3.70
12.3
1
CO2
+
+
H2
OH
H
OH
O
O
CO2
+
2
3
H
OH
O
OH
O
+
CO2
CO2
+
+
HO
HO
OH
HO
OH
OH
H
OH
O
OH
+
OH
OH
-9.21
4.44
4
5
H
OH
O
O
-
HCO3 + H2
+ H2O
H
O
a.Chemistry Department, the George Washington University, 800 22nd St NW, Suite
4000, Washington, DC 20052 (USA)
E-mail: avoutchkova@gwu.edu
O
O
OH
-
HO
OH
6 HCO
H
O
+
+
+
H2O
OH
3
-83.9
OH
†
Electronic Supplementary Information (ESI) available: (catalyst synthesis,
characterization, and additional figures/tables). See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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This notion is consistent with our calculations, which show
Transfer hydrogenation reactions were carriedVoieuwtAirnticalehOinglihne-

G
aq of CO2 TH from glycerol (entry 3) is 8.6 kcal/mol less pressure Parr reactor and sampled periodDicOaIl:ly10, .m10a3i9n/Cta8iCnCin0g31t5h7eF
favourable than TH from isopropanol when dihydroxyacetone designated CO2 pressure. Products were identified and
1
(DHA) is the glycerol by-product. However, recent reports show quantitated with HPLC (ESI Figure S1) and H NMR (Figure 2)
that glycerol dehydrogenation reactions under basic conditions with an internal standard. Control reaction without catalyst
afford lactic acid,19-20 likely via DHA isomerization, dehydration afford no appreciable conversion of glycerol, while control
and intramolecular Cannizzaro reaction.19a When lactic acid is reactions without CO2 or carbonate afford only lactate. Esters
the glycerol by-product, CO2 TH becomes 22.6 kcal/mol more of glycerol lactate or formate were not observed under basic
favourable than direct hydrogenation (entries 4 and 1). conditions. An extensive series of optimization reactions was
Furthermore, while all CO2 hydrogenation processes are more performed for this process, given that the speciation of CO2 in
favourable in the presence of base, the formation of two acids aqueous solution is pH-dependent, and that catalyst
1 was
in the reaction of CO2 and glycerol provides additional driving suspected to be also capable of facilitating pH-dependent
force under basic conditions (ESI Table S2). Consistent with this, formate dehydrogenation.
the calculated
highly favourable (entry 6).
Given that the TH from glycerol to CO2 or bicarbonate is the temperature and availability of base. In the absence of base,
thermodynamically favourable, we were surprised that catalytic no products are observed from a reaction at 150 C. However,
examples of this reaction have not been more extensively reactions with cat (0.15 mM), 1 M KOH, 1:1 glycerol:water
exploited. In the first of two important advances, Lin et al (conditions optimized previously for glyceroat 46 bar CO2 and
reported use of heterogeneous Pd/C for TH of bicarbonate using 150 C afford 330 turnovers in 24 h, equivalent to ~50 mM of

G
aq for TH of bicarbonate from glycerol is also
Therefore, it was not surprising that the turnovers of lactate
and formate observed with catalyst were found to depend on
1

1

glycerol, but with CO2 only 22 turnovers were obtained in 12 formate and lactate (Table S1 entry 3). Further increase in
hours at 240 oC.18a In a significant advance towards a reaction time afforded more lactic but no more formic acid.
homogeneous process, Aresta et al reported a stoichiometric Doubling the base concentration (2 M KOH) affords similar
reaction of RuCl2(PPh3)2 with CO2 in aqueous glycerol, which TONs in 24 hours, but the initial activity (TON at t=1hr) is
affords glycolic and formic acid at 82 oC.18b The authors propose doubled (Table S1 entry 4). This is attributed to the fact that the
that DHA undergoes dehydrogenation and decarbonylation to initial KOH concentration is higher for the reaction with 2 M
afford glycolic acid and Ru(H)(CO)(formate), and the latter could KOH, resulting in higher initial rate. However, as the vessel is
not eliminate formic acid. This suggests that preventing pressurized, the KOH solubility reaches saturation, which
decarbonylation is important for facilitating catalytic turnover. results in a similar effective base concentration in both
Here were we report the first homogeneous catalyst with reactions. When KOH concentration is decreased to 0.25 M, the
appreciable activity for the TH of glycerol with CO2 and reaction still affords ~50 mM lactate, but significantly less
bicarbonate, selectively affording lactic and formic acids with formate (18 mM, Table S1 entry 1), and a drop in pH to 6.9 is
high turnover numbers and concentrations that show potential observed after 24 hr. The 1-hr TONs are 141 and 71 respectively
for development into a competitive hydrogenation process.
for lactate and formate, with lactate concentration steadily
increasing and that of formate becoming steady (ESI Figure S2-
a). We hypothesized this observation was due to competing CO2
hydrogenation and formic acid dehydrogenation at the lower
Scheme 2
O
OH
O
1
CO2
+
HO
OH
+
pH. Indeed, experiments with cat
1 at 150 C confirm that the
O
H
O
Base
catalyst has pH-dependent activity for decomposition of formic
acid/formate (Figure S4).
OH
Given that maintaining high pH is necessary to minimize
formate decomposition, we opted to increase the effective base
concentration by decreasing CO2 pressure. A reaction with 26
bar CO2 at 150 C and 2 M KOH affords almost double the
activity observed with 48 bar CO2 (~600 turnovers in 24 hr,
Figure 1a and Table S1 entry 7). Thus, lower CO2 pressure is
favourable for both formate and lactate production. When the
Recently we reported that Ir and Ru N-heterocyclic carbene
(NHC) complexes with sulfonate-functionalized wingtips are
highly active for acceptorless dehydrogenation of glycerol to
lactic acid,20 with no detectable formation of decarbonylation
product. When we performed this reaction with carbonate as
base, formate was observed. This prompted us to examine a
series of water-soluble complexes for TH of both bicarbonate
and CO2 from glycerol. This initial report focuses on the most
active Ru compound we have identified, Ru(II) CNC with
same reaction was performed at 180 C, the catalyst affords
1065 turnovers of formate and 1685 of lactate in 24 hr (Figures
1b), corresponding to 166 mM and 262 mM respectively. The
reaction was monitored until 86 hr, at which time TONs of
lactate and formate reached 5046 and 2200 respectively (NMR
spectra in Figure 2). The difference in formate vs lactate
concentration is likely the result of catalytic dehydrogenation
and thermal decomoposition of potassium formate (thermally
propanesulfonate wingtips (
procedure.21
1), synthesized via a literature
decomposes at 167 
C22).
2 | J. Name., 2012, 00, 1-3
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Further increase in reaction temperature exacerbates the observed is consistent with previous observations of
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DOI: 10.1039/C8CC03157F
1 at lower pH (Figure S4). PDO
difference in lactate vs formate produced. In addition, at dehydrogenation activity of cat
temperatures higher than 180 C significant etching is observed
on the glass insert of the autoclave, which necessitates the use
of lower base concentration. For example, at 225 C the


reaction could only be performed with 0.25 M KOH, affording
0.328 M lactate (3160 turnovers), but only 28 mM formate (268
turnovers) in 24 hours (ESI Figure S2).
is only observed under the lower pH conditions, as we
previously reported.[21]
Figure 3. Contour plots showing TONs in 24h for (a) formate and (b) lactate using
cat 1.
Figure 1. Time course for production of formate and lactate from the reaction of
50000
40000
30000
20000
10000
0
4000
3000
2000
1000
0
CO2 and glycerol using catalyst
1 using pCO2 26 Bar, 6.85 M aqueous glycerol and
2.00 M KOH at (a) 150
replicates.
C and (b) 180 C. Error bars based on average of two
LA
FA
0
4
8
12
Time [Hr]
Figure 4. Time course for production of formate and lactate from the reaction of
glycerol using catalyst using pN2 26 Bar, 6.85 M aqueous glycerol, 2.00 M K2CO3,
at 150 C.
16
20
1

We propose a mechanism for PDO formation either via (i)
dehydration, transfer hydrogenation, or (ii) dehydrogenation to
DHA or glyceraldehyde, then dehydration and transfer
hydrogenation. The only proposed intermediate that has been
directly observed is minute amounts of pyruvaldehyde (Figure
2).
Figure 2. Sections of 1H NMR spectra corresponding to t=0 (1), t=1h (2), t=8h;
t=15h (3), t=19h (4), t=24h (5) and t=86 h (6) for reaction of CO2 and glycerol using
catalyst 1 (pCO2 26 Bar, 6.85 M aqueous glycerol and 2.00 M KOH at 180 C). Key:
▲
Lactate; ✸ Formate; ▿Pyruvaldehyde; ◎TSP (standard).
The effect of temperature, base and CO2 pressure are
Scheme 3. Proposed route to lactic acid and 1,2-PDO from glycerol transfer
hydrogenation.
summarized by contour plots based on temperature and the
“effective” base concentration (estimated as [KOH]/p(CO2)
(Figure 3). Thus, optimal equimolar production of formate and
lactate occurs when temperature and effective base
concentration are maximized. However, higher lactate
production can be achieved with higher reaction temperature
and lower base concentration. The conditions can thus be used
to “tune” the ratio of formate and lactate produced.
Given that CO2 in aqueous base forms carbonate and
bicarbonate, we performed a reaction with 2.0 M K2CO3 and 26
bar N2 at 150
C. Carbonate was selected over bicarbonate in
We considered potential formation of lactate and formate
esters as side reactions that could interfere with product
quantitation. Thus, formate and lactate esters of glycerol were
synthesized under acidic conditions and characterized by NMR
(ESI Figure S5). These esters were not observed by NMR in the
reactions under the current conditions.
Mechanistically, we postulate that the TH likely proceeds
via coordination of glycerol, followed by deprotonation by base.
Subsequent -hydride elimination likely occurs at the secondary
order to increase initial pH. The initial pH of 12.1 drops to 10.8
over 20 hours of reaction. The theoretical ratios of HCO3 :CO3
-
2-
at pH 12.1 and 10.8 are 2:98 and 40:60 respectively, which is
important because bicarbonate is likely the only species that
can undergo transfer hydrogenation.15 In 20 hours the reaction
affords 42610, 3588 and 5649 turnovers respectively of lactate,
formate and 1,2-PDO (Figure 4 and additional visualization in
Figure S4). The formate yield based on carbonate is 25% (505
mM formate). The higher lactate vs formate concentration
position of glycerol, affording Ru-H and DHA. DHA is converted
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W.-H. Wang; J. T. Muckerman; E. Fujita; Y. Himeda, Inorg.
to lactic acid via isomerization, dehydration and intramolecular
Cannizzaro reaction.19b Bicarbonate next binds to the catalyst
and undergoes a hydroxide elimination, which has been
previously proposed by DFT calculations for the Ru-catalysed
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8
9
hydrogenation of bicarbonate.15 The resulting H-Ru-CO2
23
complex undergoes insertion15,
to generate ruthenium
formate. The final formate dissociation is likely facilitated by the
polar aqueous reaction medium, which allows excellent
solvation of formate anions, making the dissociation
energetically possible in water. 15
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Conclusions
Here we demonstrate a homogeneous catalytic process for CO2
and bicarbonate transfer hydrogenation (TH) from glycerol with
a water-soluble Ru N-heterocyclic carbene complex (cat 1) and
base. The reaction selectively affords two value-added
products: potassium formate and lactate. Calculations show
that TH of CO2 and bicarbonate are made thermodynamically
more favourable by the ultimate conversion of glycerol to lactic
acid. As a result, CO2 TH from glycerol becomes significantly
more favorable than direct hydrogenation under basic
conditions. Equimolar amounts of lactate and formate are
afforded (~600 turnovers) at 150 ˚C, while greater lactate than
formate production is observed at T > 150 ˚C. At 180 ˚C cat
1
affords 1685 and 1065 turnovers respectively of lactate and
formate in 24 h. Carbonate salts can also be utilized in place of
CO2, affording 42610, 3588 and 5649 turnovers respectively for
lactate, formate and 1,2-propanediol. A preliminary mechanism 14 C. Federsel; A. Boddien; R. Jackstell; R. Jennerjahn; P. J.
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is proposed, but further experimental and theoretical
investigations of mechanism, as well as comparable activity of
other catalyst precursors, are under investigation.
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Conflicts of interest
There are no conflicts to declare.
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