Direct conversion of glycerol into formic acid via water stable Pd(II) catalyzed oxidative carbon-carbon bond cleavage

Article abstract of DOI:10.1016/j.tetlet.2013.06.041

Using our tridentate NHC-amidate-alkoxide Pd(II) complex, we developed a catalytic method for oxidative C-C bond cleavage of glycerol. The glycerol was degraded exclusively to formic acid and CO₂. Two possible degradation pathways were proposed through ¹³C labeled studies.

Full text of DOI:10.1016/j.tetlet.2013.06.041

Tetrahedron Letters 54 (2013) 4463–4466
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Tetrahedron Letters
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Direct conversion of glycerol into formic acid via water stable Pd(II)
catalyzed oxidative carbon–carbon bond cleavage
⇑
Prasanna Pullanikat, Joo Ho Lee, Kyung Soo Yoo, Kyung Woon Jung
Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, CA 90089, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Using our tridentate NHC-amidate–alkoxide Pd(II) complex, we developed a catalytic method for oxida-
tive C–C bond cleavage of glycerol. The glycerol was degraded exclusively to formic acid and CO2. Two
possible degradation pathways were proposed through 13C labeled studies.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 15 April 2013
Revised 4 June 2013
Accepted 11 June 2013
Available online 18 June 2013
Keywords:
NHC amidate ligand
Palladium catalyst
Degradation of glycerol
Formic acid
Oxidative catalysis
Glycerol, also called glycerin, is a main by-product of biodiesel
production and traditional soap manufacturing processes.1 A rapid
increase in biomass conversion has produced a massive stockpile
of glycerol, and its transformation to value-added chemicals has
been in great demand. A number of methods have been reported
to provide C2–C3 chemical products such as glyceraldehyde, gly-
ceric acid, hydroxypyruvic acid, tartronic acid, glycolic acid, and
oxalic acid.2 Because these C2–C3 products have been short of prac-
tical use, the formation of C1 products such as formic acid has at-
tracted much attention for future energy applications.3 There are
few examples that have been introduced through hydrothermal
oxidation, heterogeneous catalysts, and electrocatalytic oxidation.4
One noteworthy application of formic acid is the DFAFC (direct
formic acid fuel cell), which has been of increasing popularity com-
pared with hydrogen and methanol based fuel cells because of
their ease of refuelling, efficiency, and safety. As an emerging tech-
nology, DFAFC is currently being tested by major producers of por-
table electronics in phones, laptops, and computers.5 In an effort to
find a potential source of formic acid, we embarked on the devel-
opment of new oxidative carbon–carbon bond cleavage methods
of glycerol mainly because the previously reported conditions
failed to degrade glycerol as shown in Scheme 1.
tion, these procedures converted ketoses into both formic and
glycolic acids (2), while glycolic acid was resistant to further deg-
radation. These shortcomings prompted us to undertake studies on
oxidative degradation using organometallic catalysts. (see
Scheme 2)
Recently, we reported that NHC–palladium complexes includ-
ing 4 were highly stable and still reactive enough to facilitate C–
OH
H2O2
No reaction
No reaction
HO
HO
OH
KOH or NH4OH
1
O
H2O2
OH
KOH or NH4OH
2
Scheme 1. Limitations of known conditions on glycerol degradation.
Pd catalyst
1
2
+
HCO2H
3
+
CO2
H2O2
Representative examples included Isbell’s alkaline hydrogen
peroxide and our hydrogen peroxide/ammonia water conditions.6
Under these conditions, various aldoses were oxidatively trans-
formed to formic acid whereas glycerol (1) barely reacted. In addi-
O
N
PdCl2
N
N
Pd
Me
Pd(OAc)2
Cl
O
4
CH3
⇑
Corresponding author. Tel.: +1 213 740 8768; fax: +1 213 740 6270.
E-mail address: kwjung@usc.edu (K.W. Jung).
Scheme 2. Glycerol degradation in the presence of Pd(II) catalysts.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.06.041

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P. Pullanikat et al. / Tetrahedron Letters 54 (2013) 4463–4466
Table 1
Catalytic oxidative carbon–carbon bond cleavage of glycerol with various oxidizing
agents in the presence of Pd catalysts at room temperaturea
Entry
Catalyst
Oxidant
Yieldd (%)
2/3e (Â10À5 mol)
1
2
3
PdCl2
Pd(OAc)2
H2O2
H2O2
H2O2
t-BuO2H
Oxone
K2S2O8
O2
Trace
6
41
Trace
10
—
Trace/trace
0.16/0.20
1.50/8.05
—/—
0.39/1.52
—/—
4
4
4
4
4
4
5b
6
(A)
(B)
7c
—
—/—
a
All reactions were performed with glycerol (10 mg, 10.8 Â 10À5 mol), Pd cata-
lyst (5 mol %). Entry 1–3: 30% H2O2 (0.4 mL) in H2O (0.1 mL) was added. In entry 4,
t-BuOOH (70% in H2O, 0.4 mL) was used. Entries 5 and 6: 10.8 Â 10À5 mol of oxidant
(entry 5: oxone, entry 6: K2S2O8) in 0.3 mL of H2O was used. Entry 7: O2 was
bubbled in 0.3 mL of H2O solution. All reactions were performed at room temper-
ature for 6 h.
b
Run in H2O (0.4 mL).
Continuous flow with O2.
c
d
Conversion yield of glycerol.
e
The amount of each product was determined by 1H NMR spectral analysis using
MeOH as the internal standard.8
Figure 1. 1H NMR spectra for the oxidative degradation reactions of 1,3-13C-
H activation of relatively unreactive hydrocarbons.7 Their high sta-
bility in nucleophilic solvents such as water and alcohol could al-
low for conditions amenable to oxidative carbon–carbon bond
cleavage of glycerol. These processes could provide C1–C2 products
encompassing carbon dioxide, formic acid (3), and glycolic acid (2).
Using known palladium catalysts and our Pd(II) catalyst 4, we eval-
uated the feasibility of oxidative degradation of glycerol (Table 1).
Regarding oxidative degradation by hydrogen peroxide, com-
mercially available Pd complexes including PdCl2 and Pd(OAc)2
did not offer meaningful improvement over KOH or NH4OH (en-
tries 1 and 2).6 However, NHC–Pd complex 4 exhibited significant
consumption of glycerol at room temperature to furnish formic
acid as the major product (entry 3). We also noticed that the ratio
of formic acid to glycolic acid produced was much higher than that
of the reaction with Pd(OAc)2 despite its low yield. These results
might indicate our catalyst degraded both glycerol and glycolic
acid unlike other Pd salts or basic conditions. We screened other
oxidants such as tert-butyl peroxide, oxone, K2S2O8, and molecular
oxygen, most of which were ineffective (entries 4–7). Similarly to
hydrogen peroxide, oxone provided a higher ratio of formic acid
to glycolic acid compared to the Pd(OAc)2 case (entries 2 and 5).
In the catalytic processes using 4, one equivalent of glycerol can
produce either three equivalents of formic acid or one equivalent
of formic acid and glycolic acid each. To understand how many
equivalents of formic acid can be produced from glycerol, it was
necessary to understand degradation pathways. In this context,
we carried out the oxidative cleavage reaction using 1,3-13C-la-
beled glycerol and 2-13C-labeled glycerol in the presence of
NHC–Pd complex 4 and hydrogen peroxide at 60 °C for 6 h, under
which conditions we tried to consume most of glycerol (cf. Table 2,
entry B).
glycerol (A) and 2-13C-glycerol (B).
As shown in Figure 1, the reaction of 1,3-13C-labeled glycerol (5)
afforded both 13C-labeled formic acid (6) and unlabeled formic acid
(3) in a 2 to 1 ratio, as well as a small amount of 13C-labeled gly-
colic acid on the b-carbon (7). In the case of 2-13C-labeled glycerol
(8), a 1 to 2.5 ratio of 13C-labeled formic acid (6) to unlabeled for-
mic acid (3), as well as 1-13C-labeled glycolic acid (9) was ob-
served. In addition, 13C NMR analysis further confirmed the
assignment by 13C–12C coupling for 13C-labeled glycolic acid (7)
(d = 60.6 Hz at 176.0 ppm). Therefore, these results indicated that
the formic acid produced contained both the secondary and pri-
mary carbons of glycerol. On the other hand, the carbonyl carbon
of glycolic acid would stem only from the secondary carbon of
glycerol while the carbinol carbon would originate from the pri-
mary carbons.
Since the observed products did not satisfy the mass balance,
we suspected that we lost some carbons in the form of carbon
dioxide, and examined such possibility (Scheme 3). In the presence
of NHC–Pd complex 4, both acids gradually disappeared over time
to form carbon dioxide. For example, glycolic acid led to formic
acid in 10% yield after 3 h while formic acid gave no detectable
products except carbon dioxide. In addition, unlabeled formic acid
(3), but no 13C-labeled formic acid (6) was detected when (1-13C)
glycolic acid (9) was reacted with hydrogen peroxide. It was evi-
dent that 2-12C and 1-13C in glycolic acid were incorporated into
formic acid and 13CO2, respectively. These results were consistent
with the aforementioned glycol oxidation patterns.
These labeled experiments suggested two possible degradation
pathways. If formic acid was derived equally from all three carbons
in gylcerol, the ratio of 13C-labeled formic acid (6) to unlabeled for-
mic acid (3) should be 2 to 1 in Figure 1 (A), and 1 to 2 in Figure 1
(B). Even though the observed ratios were close to the theoretical
Table 2
Yields of reactions in Figure 3
Entry
Glycerola (%)
Glycolic acidb (%)
Formic acidc (%)
O
B
C
D
E
Trace
Trace
19
11
20
28
0
39
50
57
61
4
HO 13C
3 (10%)
H2O2, 60 o
C
OH
9
0
4
a
b
c
Remaining glycerol/added glycerol  100.
3
CO2
H2O2, 60 o
C
Moles of glycolic acid/moles of added glycerol  100.
(Moles of formic acid/moles of glycerol  100)/3, assuming one mole of glycerol
produced 3 mole of formic acid. Moles of each product were calculated by using
methanol as an internal NMR reference.
Scheme 3. Potential degradation of glycolic acid and formic acid in the presence of
NHC–Pd(II) catalyst 4.

P. Pullanikat et al. / Tetrahedron Letters 54 (2013) 4463–4466
4465
OH
OH
[O]
O
HO
HO
HO
OH
10
1
Pd
H2O2
HCO2H + H2O
HCO2H + H2O
O
O
[O]
HO
OH
2
H
11
Pd
H2O2
2 H2O
+ CO2
Pd
H2O2
HCHO
HCHO
[O]
[O]
HCO2H
HCO2H
Scheme 4. Potential oxidative degradation pathways of glycerol.
H
O
O
[Pd]
HO
13
[Pd]
HO
12
H2O2
Figure 3. 1H NMR study for the degradation pathway of glycerol: (A) starting
glycerol, (B) 0.4 mL H2O2 for 6 h at 60 °C, (C) 6 h slow addition of 0.4 mL H2O2 at
60 °C, (D) 3 h slow addition of 0.4 mL H2O2 at 0 °C and 8 h stirring at room
temperature, (E) 3 h slow addition of 0.3 mL H2O2 at 0 °C and 3 h slow addition of
0.2 mL H2O2 at 60 °C.
- 2 H2O
HO
HO
HO
O
H
OH
H
HCO2H
+ H2O
H2O2
[O]
OH
H
through the glycolic acid intermediate. As the incipient product,
Pd-glycerol adducts could be generated and oxidized to aldehyde
10, which would undergo rapid C–C bond cleavage to release for-
mic acid and another aldehyde 11. Further oxidative cleavage of
11 could afford the second equivalent of formic acid and formalde-
hyde, which eventually would generate the third equivalent of for-
mic acid. Meanwhile, the first oxidative C–C bond cleavage product
11 can be oxidized to yield glycolic acid 2, which subsequently can
be degraded to formaldehyde and CO2, ultimately furnishing one
equivalent of formic acid. In summary, dual mechanistic pathways
would contribute to our catalytic processes.
In the reaction mixture, it was unable to detect any aldehyde
(10, 11, or formaldehyde). This could indicate that aldehyde com-
pound was not completely released from Pd complex but under-
went C–C bond cleavage to form formic acid and another
aldehyde. Even though aldehyde was released from Pd, it could
be oxidized quickly by the oxidant to form acid (2 or formic acid).
Therefore the following catalytic cycle could be proposed
(Scheme 5). Glycerol adduct 12 could be oxidized to form 13. Sub-
sequently, one equivalent of H2O2 could generate one equivalent of
formic acid and 14. Following third oxidation, formaldehyde ad-
duct 15 could be formed while releasing another equivalent of for-
mic acid. The last formic acid could be produced from this
formaldehyde.
O
H
O
H2O2
[Pd]
[Pd]
HCO2H
15
14
HO
HCO2H + H2O
Scheme 5. Catalyic pathway for the formation of formic acid.
ones, these ratios were still different from the expected ones by
10–25%. Thus, we assumed a major pathway would oxidize all
three carbons to formic acid. Since a small amount of glycolic acid
was detected, we considered that another pathway through gly-
colic acid was active concomitantly. In fact, both C-1 and C-3 in
glycerol could be converted into formic acid whereas the C-2
would fail to give formic acid and instead furnish CO2 as aforemen-
tioned. As a consequence, one could expect more 13C-labeled for-
mic acid than 12C-formic acid in Figure 1 (A) and more 12C-
formic acid than 13C-labeled one in Figure 1 (B), respectively.
Based on these results and previously reported studies, two
possible degradation pathways can be proposed as in scheme 4.
3,9–11
One mechanistic pathway would form three equivalents of
formic acid from each glycerol molecule while another mechanism
would lead to two equivalents of formic acid and one part of CO2
Figure 2. Concentration changes versus volume of H2O2 (A), reaction temperature (B), and time (C) for the oxidative degradation of glycerol: glycerol (red square), formic acid
(blue diamond), and glycolic acid (green triangle).

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P. Pullanikat et al. / Tetrahedron Letters 54 (2013) 4463–4466
To seek optimal conditions, we investigated various factors
References and notes
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decreased at higher temperatures (Fig. 2B) presumably due to
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(Fig. 2C). In addition, when the amount of Pd (4) was increased
to 10% or 15% from 5%, overall yield of formic acid as well as con-
sumption of glycerol dropped due to the degradation of formic acid
and hydrogen peroxide by the catalyst.
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Acknowledgments
12. Representative reaction: A mixture of glycerol (10 mg, 0.1 mmol) and catalyst
4 (5 mol %) in 0.2 mL of water was placed in a rubber stoppered vial. To this
mixture, 0.3 mL of 30% H2O2 was added for 3 h at 0 °C by using a syringe pump.
After complete addition, the reaction mixture was heated to 60 °C, then 0.2 mL
of 30% H2O2 was added for 3 h. Wet 1D NMR of the crude reaction mixture
showed the complete conversion of glycerol to give 0.196 mmol of formic acid.
Wet 1D:d = 8.1 (s, 1H).
We acknowledge generous financial support from the Hydro-
carbon Research Foundation and the National Institute of Health
(S10 RR025432).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.06.
041.