Formate to Oxalate: A Crucial Step for the Conversion of Carbon Dioxide into Multi-carbon Compounds

Article abstract of DOI:10.1002/cctc.201600765

The efficient conversion of formate into oxalate could enable the industrial-scale synthesis of multi-carbon compounds from CO₂ by C?C bond formation. We found conditions for the highly selective catalytic conversion of molten alkali formates into pure solid oxalate salts. Nearly quantitative conversion was accomplished by calcination of sodium formates with sodium hydride. A catalytic mechanism proceeding through a carbonite intermediate, generated upon H₂ evolution, was supported by density functional theory calculations, Raman spectroscopy, and the observed changes in the catalytic performance upon changing the nature of the base or the reaction conditions. Whereas the conversion of formate into oxalate by using a hydroxide ion catalyst was previously studied, hydride ion catalysis and the chain reaction mechanism for the conversion involving a carbonite ion intermediate are reported herein for the first time.

Full text of DOI:10.1002/cctc.201600765

Accepted Article
Title: Formate to Oxalate: A Crucial Step for Conversion of CO2 into
Multi-carbon Compounds
Authors: Prasad Lakkaraju, Mikhail Askerka, Heidie Bayer, Charles T
Ryan, Tabbetha Dobbins, Christopher Bennett, Jerry J. Kaczur,
and Victor Batista
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201600765
Link to VoR: http://dx.doi.org/10.1002/cctc.201600765
A Journal of
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ChemCatChem
10.1002/cctc.201600765
COMMUNICATION
Formate to Oxalate: A Crucial Step for Conversion of CO into
2
Multi-carbon Compounds
Prasad S. Lakkaraju*, Mikhail Askerka, Heidie Beyer, Charles T. Ryan, Tabbetha Dobbins, Christopher Bennett,
Jerry J. Kaczur, and Victor S. Batista*
[
11]
Abstract: Efficient conversion of formate into oxalate could enable
the industrial-scale synthesis of multi-carbon compounds from CO
by C-C bond formation. We find conditions for highly selective
catalytic conversion of molten alkali formates into pure solid oxalate
salts. Nearly quantitative conversion is accomplished by calcination
of sodium formates with sodium hydride. A catalytic mechanism
electrochemical reduction of CO2 has been demonstrated.
2
Specifically, potassium and sodium formates are produced with
2
Faradaic efficiencies >60% in 100 cm cells, with current densities
-
2
[11,12]
as high as 120 mA cm and cell potentials lower than 4 Volts.
The alkali metal formate salts may be converted to formic acid by
either electrodialysis or electrochemical acidification (Scheme 1).
2
through a carbonite intermediate, generated upon H evolution, is
supported by density functional theory (DFT) calculations, Raman
spectroscopy and the observed changes in catalytic performance
upon changing the nature of the base, or reaction conditions. While
the formate to oxalate conversion using hydroxide ion catalyst has
been studied before, the hydride ion catalysis and the chain reaction
mechanism for the conversion involving carbonite ion intermediate
are being reported for the first time.
The production of chemicals and fuels from CO
2
could provide
viable solutions to challenges of global relevance. Reducing the
amount of CO
2
in the atmosphere could mitigate global warming.
into fuels could produce carbon-neutral renewable
Converting CO
2
fuels for energy storage, while synthesizing multi-carbon chemicals
from CO could enable the sustainable production of precursors or
feed stocks for organic synthesis.
interest in the conversion of CO
2
[
1,2]
Therefore, there is great
into formate and oxalate
2
[
3-10]
products.
However, economically viable and environmentally
benign methods are yet to be established. Outstanding challenges
include Faradaic efficiency, electrocatalyst stability, product
selectivity, isolation and purification of the products.
2
Scheme 1. Conversion of CO into value-added products.
While formic acid has been explored as a fuel and in applications to
commercial chemistry, the development of viable synthetic routes to
generate C2 and higher order carbon compounds from formate salts
remains an outstanding challenge. In this communication, we report
a highly useful catalytic conversion of formates into oxalates:
Over the past six years, research at Liquid Light Inc. (LL) has
been focused on the development of electrochemical and chemical
methods for the conversion CO
2
into value added products on a
Scheme 1 illustrates some of the pathways of
interest for bulk synthesis of C compounds from CO , relying upon
[
11-14]
large scale.
2
2
cat
Δ
2
NaHCO (l) !!→ Na C O (s) +H (g)
(1)
2
2
2
4
2
conversion of formate to oxalate, including synthetic routes for
production of highly utilized chemicals such as ethylene glycol and
glyoxylic acid. The pathways are particularly attractive since an
efficient method for generation of alkali metal formates by
with high selectivity for C-C coupling chemistry achieved by
calcination with alkali hydroxide, or hydride catalysts. The two main
products
2 2 2 4
H (g) and Na C O (s) are spontaneously separated,
making the synthetic process simple and practical when by-products
such carbon monoxide and carbonates are curtailed, as in a
[
13,14]
recently disclosed integrated process.
[*]
J.J. Kaczur
Liquid Light, Inc.
The conversion of formate into oxalate has been a long-standing
11 Deer Park Drive, Suite 121
[15-24]
[16]
challenge.
Thermal approaches were explored
and factors
Monmouth Junction, NJ, 08852
Email: jkaczur@LLchemical.com
determining the conversion efficiency into oxalates and other by-
products were analysed by using varying amounts of sodium
hydroxide, under various temperatures and partial vacuum
Prof. P. Lakkaraju, Heidie Beyer, Charles T. Ryan
Chemistry & Biochemistry Department
Georgian Court University
[
17]
conditions.
oxalates, CO and CO
analytical analysis of thermal decomposition of LiHCO
Products of decomposition including carbonates,
were determined from dynamic thermo-
, NaHCO
However, conditions for achieving
high product yields have yet to be established. For example,
900 Lakewood Ave, Lakewood, New Jersey 08701
2
Email: Lakkaraju@georgian.edu
2
2
,
[
18]
2 2 2
KHCO , RbHCO and CsHCO .
Dr. Tabbetha Dobbins, Christopher Bennett
Department of Physics, Rowan University
[
19]
201 Mullica Hill Road, Glassboro, NJ 08028
Baraldi
analyzed the IR spectra of twelve metal formates under
heating in air at atmospheric pressure, and under a dynamic vacuum
M. Askerka, Prof. V. S. Batista
-
2
Department of Chemistry, Yale University
of 10 Torr. Formation of oxalate was suggested by IR features
when formate was heated in vacuum at 300°C. However,
decomposition into carbonate was observed at 475°C.
225 Prospect St, New Haven, Connecticut 06520-8107
Email: victor.batista@yale.edu
Supporting information for this article is given via a link at the end of
the document
[
20,21]
Górski
explored the influence of gaseous and solid reactants
on the yields of oxalates and carbonates, during thermal
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ChemCatChem
10.1002/cctc.201600765
COMMUNICATION
decomposition of LiHCO
2
, NaHCO
2
and KHCO
2
. Addition of NaBH
4
In addition, we find that the nature of the alkali metal ion
significantly affects the product yield and optimum reaction
conditions. For comparison shown in Table 2 reports the product
yields obtained with (or without) KOH by calcination at 420-
480°C for various reaction times. Clearly, KOH increases the
yield obtained at 420-440°C, although it requires higher
temperature than NaOH, or NaH, to obtain yields >75%. At
higher temperatures (>500 °C) the reaction yields are not
affected by the addition of a catalyst and the reaction times are
significantly reduced, even without addition of a catalyst base.
to NaHCO
2
resulted in 88% conversion yield, which declined to 0%
conversion when using a 1:1 molar ratio. Thus, reaction conditions
for quantitative yields have not reported. In addition the underlying
reaction mechanism remains uncertain. While Górski proposed
oxalate formation upon decomposition of formate, as follows:
-
2-
2
HCO
2
→ CO
2
+ CO
2
+H
2
(2)
(3)
2
-
2-
CO
2
+ CO
2
2 4
→ C O
It remains to be explored whether such a reaction, or an alternative
mechanism, is consistent with theoretical studies and direct
comparisons with experimental measurements, as reported below.
Table 2. Formate to oxalate conversion yields, obtained by
using KOH as a catalyst for various reaction times at 420-520°C
Reaction Times and Product Yields: We find reaction conditions
for the best yield conversion of formate into oxalate when alkali
formate salts are melted at high temperatures in the presence of a
strong base, such as NaH or NaOH by varying both the catalyst
under a N atmosphere.
2
Sample
Temp
°C
Calcination
Time (min)
KOH
% Yield to
Oxalate
Number
Wt% Added
amount and the temperature. A porous solid, self-rising NaC
2 4
O is
(
±1%)
8
produced with hydrogen gas evolved as by-product. DFT
a
1
2
3
4
5
6
7
8
9
420
420
420
420
420
440
440
440
480
480
520
30
30
30
45
60
30
30
60
30
30
5
0
5
8
8
5
0
8
5
0
2
0
calculations support a mechanism based on a hydride catalytic
pathway (Figure 1), consistent with Raman spectroscopy and the
observed changes in reaction times and product yields produced by
changing the base (e.g., NaH or NaOH) or the reaction conditions. In
particular, the reaction times are much shorter for the stronger base
76
74
74
78
14
(
NaH) than for NaOH at the same temperature. These observations
77
are consistent with rate-limiting step due to the primary
a
78
2
deprotonation of formate upon H evolution, followed by C-C bond
73
formation (vide infra).
1
1
0
1
76
71
Table 1 (NaOH) reports the product yields obtained for different
reaction times with NaH and NaOH. We find nearly quantitative
conversion into oxalate for NaH and yields >80% with NaOH at
Formate to Oxalate Conversion Mechanism: DFT
calculations support a simple mechanism (Figure 1), initiated by
3
90°C but negligible conversion at lower temperatures (330 - 350°C).
2
-
2
hydrogen evolution and formation of the carbonite (CO )
intermediate, upon reaction of formate with the hydride catalyst.
Carbonite reacts with formate to produce oxalate by C-C bond
formation, regenerating the hydride catalyst.
Table 1. Formate to oxalate conversion yields, obtained by
using NaH and NaOH as a catalyst for various experimental
conditions of temperature and reaction times under a
atmosphere.
N
2
O
O
II
CO22-
Catalystꢀ Sampleꢀ Tempꢀ Calcinationꢀ
Catꢀꢀ Oxalateꢀ
Intermediate
O
Numberꢀ
°Cꢀ
Time,ꢀmin.ꢀ Wt%ꢀꢀ
%ꢀ
O
H
Yieldꢀ
(
±1%)ꢀ
92ꢀ
NaHꢀ
1ꢀ
390ꢀ
390ꢀ
390ꢀ
10ꢀ
10ꢀ
8ꢀ
2ꢀ
2ꢀ
2ꢀ
+
HCO H
2
-
H-
2
3
ꢀ
ꢀ
99ꢀ
- H-H
III
93ꢀ
I
ꢀ
1ꢀ
2ꢀ
3ꢀ
5ꢀ
6ꢀ
390ꢀ
390ꢀ
390ꢀ
350ꢀ
350ꢀ
20ꢀ
30ꢀ
40ꢀ
15ꢀ
30ꢀ
2.5ꢀ
2.5ꢀ
2.5ꢀ
0ꢀ
74ꢀ
68ꢀ
86ꢀ
0.9ꢀ
0.5ꢀ
O
+HCO2H
H-
O
O
NaOHꢀ
+
ꢀ
ꢀ
ꢀ
H
+
H
2
-
-
C O
2 4
O
O
O
0ꢀ
Formate
Reactant
Hydride
Catalyst
Product
Oxalate
Shorter reaction times, higher product yields and superior
quality of the oxalate product are obtained when using NaH as a
catalyst (Table 1, NaH). In contrast, NaOH catalyzed reactions
Figure 1. Catalytic conversion of formate to oxalate. Using the
hydride base, one mole of the formate is converted to a
carbonite ion (blue color) that nucleophilically attacks the second
mole of formate, releasing the hydride catalyst.
typically yielded
a mixture of products, including oxalate,
carbonate, carbon monoxide and hydrogen. The maximum
oxalate yield, obtained with NaOH, was about 80%, including as
much as 10% of carbonate. The reaction times were typically 4
times longer for NaOH when compared to the NaH. When the
formate to oxalate conversion is carried out using NaH or NaOH
at temperatures >420C, charring of the sample occurs affecting
the quality of the product.
We probed the catalytic thermal conversion of formate into
oxalate by Raman spectroscopy at 350°C (i.e., the highest
temperature allowed by our variable temperature system). As
-
1
the reaction proceeded, we monitored the bands at 1357 cm ,
-
1
-1
1
072 cm and 770 cm (see Supplemental material), assigned
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ChemCatChem
10.1002/cctc.201600765
COMMUNICATION
to the CO
2
symmetric stretch, formate C-H bending and CO
2
We first analyzed the thermodynamics of formate to oxalate
conversion in terms of the simplest possible mechanism, the
direct reaction of two formate ions in the absence of the catalyst
base (Table 4, a). We found that while the reaction is certainly
thermodynamically feasible, it is kinetically hindered (with a high-
bending, respectively. Simultaneously, two bands emerged at
-
1
-1
1
457 cm and 884 cm assigned to the oxalate C-O symmetric
stretch and oxalate C-C stretch, respectively. The bands
observed for products were identical to those observed for an
authentic sample of sodium oxalate.
-
1
energy barrier >70 kcal mol ) when leading to H
2
evolution
through a single transition state. Oxalate formation according to
-
1
The experimental Raman peak at 1076 cm was assigned to
the O-C-O symmetric stretch of the carbonite intermediate,
supported by DFT calculations (Supporting Information) and with
the steady-state concentration of carbonite established during
the course of the catalytic reaction upon continuous purging of
[20,21]
(
2) and (3), as suggested by Gorski
, (Table 3, i and j) was
also found to be thermodynamics unflavored due to the
decomposition of formate into carbonite and carbon dioxide
(
Table 3, i). Therefore, alternative reaction pathways were
explored, including the pathway outlined in Figure 1 where the
catalyst plays an important role.
H
2
(g) with N
2
(g) in the variable temperature system. Another
observation that supports the assignment to carbonite is that the
-
1
-1
intensity of the 1076 cm band diminishes upon exposure of the
sample to air or water, enhancing the intensity of the oxalate
Table 3. Reaction free energies (kcal mol ) in aqueous solution
for various possible elementary steps associated with formate to
oxalate conversion.
-
1
bands. The 1076 cm intensity also correlates with hydrogen
evolution during the course of the reaction and increases when
increasing the concentration of the hydride catalyst, as expected
for an increased steady state concentration of carbonite.
Reaction
Δ
G
Na
r
+
+
No
ion
8
--
21
35
K
-
2-
a)
a
2HCO2 → C O +H2
-1
70
2
72
25
35
30
2
4
The formation of oxalate from the reaction of carbonite with
water (Table 3, k) is likely spontaneous because of a large
negative reaction Gibbs free energy predicted by DFT. We are
conducting additional Raman spectroscopy studies on the
TS
-
≠
2-
)
2HCO2 → [TS ] → C O +H2
1
2
4
-
-
b)
c)
d)
HCO
→ OH + CO
21
33
33
2
-
-
HCO2 → H + CO
2
-
-
2-
HO + HCO → H O + CO
2
H + HCO → H + CO
2
2
2
52
32
-
-
2-
2
13
e)
e
15
28
14
26
2
2
reaction using H and C isotope enriched formate samples,
and the results of these investigations will be reported in a future
publication.
TS
-
-
≠
2-
2-
)
H + HCO → [TS ] → H + CO
35*
-26
26
-18
69
2
2
2
2
4
-
2-
-
2-
f)
g)
h)
i)
HCO2 + CO2 → H + C O
-18
16
-16
50
-14
16
-14
53
2
4
-
-
-
OH + 2HCO2 → H O + H + C O
2
2
-
-
H O + H → H + OH
2HCO2 → CO + CO +H2
2
2
The temperature dependence of the reaction rates, shown in
Figure 2, allowed for estimation of the activation enthalpy and
activation entropy, according to the Eyring-Polanyi equation. The
slope of the plot (Figure 2) yields an activation enthalpy of 41
-
2-
2
2
2-
2-
j)
k)
CO + CO2 → C O
4
-61
-95
-51
-67
-49
-58
2
2
2
-
2-
-
2 2 4
2CO2 + 2H O → C O + 2OH + H2
-
1
The proposed mechanism is consistent with the various
outcomes under different experimental conditions. We note that
a small amount of oxalate is obtained at 420°C (8% yield) even
without addition of any kind of catalyst base, and higher yields
kcal mol and the intercept corresponds to activation entropy of
-
1
-1
-
6 cal K mol . As discussed below, these results are fully
consistent with the mechanism introduced in Figure 1, as
supported by DFT calculations of the reaction free energy profile
and direct comparisons to experimental data.
(
(
>70%) are produced without catalyst at higher temperatures
>480°C) (Table 2). However, at the lower temperatures (e.g.,
4
20°C), obtaining significant yields required addition of a base.
Also, the resulting product yields are quite different when using
NaH, NaOH or KOH (Tables 1 and 2). These observations are
all consistent with generation of the base catalyst in situ at the
higher temperatures (>480°C). In particular, the catalyst base
-
OH can be generated by thermal decomposition of formate
(
Table 3, b). The reaction involves a free energy change of 21
-
1
and 25 kcal mol for sodium and potassium salts, respectively.
ꢀ
ꢀ
Formation of hydride by thermal decomposition of formate is
ruled out since it is thermodynamically a lot more demanding
ꢀ
ꢀ
(
Table 3,c). However, the hydride catalytic species are readily
ꢀ
-
generated from OH , upon reaction with formate (Table 3, g).
These results are thus consistent with generation of the hydride
catalyst base in situ from OH- at high temperatures. Formate to
oxalate conversion proceeds according to Figure 1 (Table 3, e
ꢀ
ꢀ
ꢀ
Figure 2. Eyring-Polanyi graph for thermal conversion of sodium
formate to sodium oxalate in the 330 - 420°C temperature range,
using sodium hydride as a catalyst base (2.5% by mass).
ꢀ
-
and f), after generation of the H catalyst. A detailed mechanism
is described by the DFT free energy profile, shown in Figure 3.
Figure 3 (top panel) shows that the first step (I
electrostatic complexation of NaH (or KH) with formate. Then, H
evolves upon nucleophilic attack of hydride on formate (I →I ).
The transition state TS is reached through rotation of the
sodium (or potassium) hydride fragment, which relaxes into the
weakly bound H + CO Na intermediate, I . The H fragment
1 2
→I ) involves
DFT exploration of low free energy pathways supports the
proposed reaction mechanism, shown in Figure 1, as indicated
by the thermodynamics of elementary steps summarized in
Table 3 (details of the DFT methodology are given in the
Supporting information).
2
2
3
1
2
2
2
3
2
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ChemCatChem
10.1002/cctc.201600765
COMMUNICATION
spontaneously exchanges with formate (I
4
), forming the more
support a simple mechanism through a carbonite intermediate,
stable intermediate I
5
.
2
generated upon H evolution by reaction of formate with the
catalytic hydride. The mechanism is consistent with
thermodynamic parameters, obtained from the temperature
dependence analysis and with Raman spectroscopy of the
carbonite intermediate. The reported findings are particularly
relevant for industrial-scale generation of multi-carbon
Our calculations of the complete thermodynamic cycle
Supporting Information, Table S1 and Table S2) show that I
TS is the rate-limiting step for oxalate formation (Table S2), with
(
2
→
1
-
1
a barrier of 41.0 kcal mol in excellent agreement with the
experimental value estimated from the observed temperature
dependence (Figure 2).
compounds from CO
2
since formate can be efficiently generated
by electrochemical reduction of CO
2
.
Figure 3 (top panel) shows that out-of-plane rotation of
Further, we are currently investigating the stability and
formate in I
transition state TS
OOCHCOO)Na complex (I
complex I through a 90 rotation of the O-C-C-O torsion angle
and a slight rearrangement of the Na counter ions. The C-H
bond is then elongated, forming NaH + Na (I ) through the
transition state TS . The separation of these electrostatically
bound species, upon precipitation of Na (s) forms I and
regenerates NaH for the next turn of the catalytic cycle.
5
, brings together the two carbon atoms, forming the
which forms the C-C bond in the
) before relaxing into the isomeric
13
reactivity of carbonite ions by Raman spectroscopy of C and
2
2
H enriched samples, and DFT studies. These studies could
(
3
6
have significant impact on the utilization of formate as a potential
carboxylating reagent via carbonite nucleophile.
o
7
+
2
C
2
O
4
8
3
Experimental Section
2
C
2
O
4
9
Experimental Details. The thermal reactions were explored by using a
Thermo Scientific Thermolyne Benchtop Muffle Furnace that could reach
a maximum temperature of 1200°C. Reactions were performed under a
2 2
flowing N atmosphere by introducing N gas through a vent port since
oxygen lowers the yield of oxalate formed. A series experiments were
designed using reaction temperature, reaction time and the amount of
catalyst as the reaction condition variables to obtain the best possible
yields. A typical bench-scale reaction was conducted using a 4.0 g.
formate sample placed into a 50 mL nickel crucible and calcined between
3
00 - 480°C. All chemicals were reagent grade obtained from Sigma
Aldrich including NaOH, sodium hydride, KOH, sodium and potassium
formate, sodium and potassium oxalate, sulfuric acid, and potassium
permanganate (J. T. Baker). The catalyst (e.g., NaH, NaOH, KOH) in
weighed amounts of 2.5% by mass were mixed thoroughly using a
mortar and pestle in a nitrogen glove box. The quantitative analysis of
oxalate formation was performed by volumetric titrations using
[
22,23]
standardized KMnO
4
solutions
as well as by ion chromatography
methods for the analysis of formate and oxalate.
Acknowledgements
Georgian Court gratefully acknowledges funding from Liquid
Light, Inc. for the undergraduate students who helped conduct
the experimental work for this paper. V.S.B. acknowledges the
AFOSR grant FA9550-13-1-0020 and high performance
computing facilities from NERSC and Yale University.
Keywords: Formate to Oxalate Conversion
• Carbon Chain
Generation • CO Reduction • Density Functional Theory
2
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formate into oxalate can be achieved by simple calcination of
molten formate salts in the presence of NaH. DFT calculations
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10.1002/cctc.201600765
COMMUNICATION
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ChemCatChem
10.1002/cctc.201600765
COMMUNICATION
COMMUNICATION
Author(s), Corresponding Author(s)*
Prasad S. Lakkaraju
Victor S. Batista
Page No. – Page No.
Title
Formate to Oxalate: A Crucial Step for
Conversion of CO
Compounds
2
to Multi-carbon
Text for Table of Contents: Thermal conversion of formate to oxalate accomplished with quantitative yields by calcination with NaH.
2
The reaction pathway through a carbonite intermediate, generated by upon H evolution is supported by Raman spectroscopy and
DFT calculations.
This article is protected by copyright. All rights reserved.