Homogeneous hydrogenation of CO2 to methyl formate utilizing switchable ionic liquids

Article abstract of DOI:10.1021/ic501378w

Combined capture of CO₂ and subsequent hydrogenation allows for base/methanol-promoted homogeneous hydrogenation of CO₂ to methyl formate. The CO₂, captured as an amidinium methyl carbonate, reacts with H2 with no applied pressure of CO₂ in the presence of a catalyst to produce sequentially amidinium formate, then methyl formate. The production of methyl formate releases the base back into the system, thereby reducing one of the flaws of catalytic hydrogenations of CO₂: the notable consumption of one mole of base per mole of formate produced. The reaction proceeds under 20 atm of H2 with selectivity to formate favored by the presence of excess base and lower temperatures (110 °C), while excess alcohol and higher temperatures (140 °C) favor methyl formate. Known CO₂ hydrogenation catalysts are active in the ionic liquid medium with turnover numbers as high as 5000. It is unclear as to whether the alkyl carbonate or CO₂ is hydrogenated, as we show they are in equilibrium in this system. The availability of both CO₂ and the alkyl carbonate as reactive species may result in new catalyst designs and free energy pathways for CO₂ that may entail different selectivity or kinetic activity.

Full text of DOI:10.1021/ic501378w

Article
pubs.acs.org/IC
Homogeneous Hydrogenation of CO2 to Methyl Formate Utilizing
Switchable Ionic Liquids
Mahendra Yadav, John C. Linehan, Abhijeet J. Karkamkar, Edwin van der Eide, and David. J. Heldebrant*
Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, United States
S
* Supporting Information
ABSTRACT: Combined capture of CO2 and subsequent hydrogenation allows for base/methanol-promoted homogeneous
hydrogenation of CO2 to methyl formate. The CO2, captured as an amidinium methyl carbonate, reacts with H2 with no applied
pressure of CO2 in the presence of a catalyst to produce sequentially amidinium formate, then methyl formate. The production of
methyl formate releases the base back into the system, thereby reducing one of the flaws of catalytic hydrogenations of CO2: the
notable consumption of one mole of base per mole of formate produced. The reaction proceeds under 20 atm of H2 with
selectivity to formate favored by the presence of excess base and lower temperatures (110 °C), while excess alcohol and higher
temperatures (140 °C) favor methyl formate. Known CO2 hydrogenation catalysts are active in the ionic liquid medium with
turnover numbers as high as 5000. It is unclear as to whether the alkyl carbonate or CO2 is hydrogenated, as we show they are in
equilibrium in this system. The availability of both CO2 and the alkyl carbonate as reactive species may result in new catalyst
designs and free energy pathways for CO2 that may entail different selectivity or kinetic activity.
Milstein, Sanford, and Leitner to yield methanol.14−17,35 Nearly
all reactions currently require moderate to high pressures of
INTRODUCTION
■
Carbon dioxide has been widely recognized as a potent
greenhouse gas and is linked to global warming.1−3 As a result,
CO2.
recycling of CO2 has been a topic of intense research4−11 and a
There are two primary methods employed to make the
hydrogenation of CO2 into formic acid more favorable:
subject of discussion not only from a scientific but also from an
ecological−political point of view. Because CO2 is a highly
oxidized, thermodynamically stable compound, its utilization
trapping formic acid as formate salts and using highly polar
solvents. As CO2 hydrogenations to produce formic acid are
endergonic, the easiest method to force the reaction to become
requires reaction with high-energy substances. Also, while CO2
exergonic is to add base to produce formate salts.24 Solvent
may be abundant, its separation, purification, and compression
are energy-intensive processes. Combining the chemical
stabilization has also been shown to promote the hydro-
genation of CO2, where more polar solvents such as water or
capture and conversion of CO2 into useful products into one
ionic liquids stabilize formic acid and formate salts.34 The
step removes large inefficiencies of separate capture and
conversion, but also entails potentially new reactivity that
may enable faster and more economical catalytic conversions of
CO2. Catalytic hydrogenation is one approach to CO2 fixation.
degree of stabilization of formic acid and formate salts follows
the trend of ionic liquid > water > alcohols, ethers.
Since catalytic hydrogenations of CO2 to formate salts can be
made favorable with bases and highly polar media, combining
Hydrogenation of CO2 can lead to a variety of useful
the bases and the polar media such as in a switchable ionic
compounds such as methanol, hydrocarbons, esters, and
ethers.12 Of these, methanol holds a central position, as it is
liquid may lead to further rate enhancements. Switchable ionic
a key petrochemical.13 One path for the homogeneously
liquids (SWILs) are materials that convert from a molecular
relatively nonpolar liquid to a highly polar ionic liquid by the
catalyzed hydrogenation of CO2 to methanol begins with
reversible complexation with CO2.36 SWILs are blends of
hydrogenation of CO2 into formate, which has been studied by
several research groups.14−17 Researchers have developed
alcohols and amidine or guanidine bases that chemically react
catalysts from precious metals14−27 and nonprecious met-
with or capture CO2 to form amidinium or guanidium alkyl
carbonate ionic liquids at low or moderate pressures.36 These
ionic liquids can contain 15 wt % (1:1 mol %) CO2 chemically
fixated in solution at room temperature and 1 atm. In addition,
als28−31 utilizing supercritical fluids,32 water,33 or ionic liquids34
with varying degrees of success. The second step toward
methanol involves esterification reactions of the formate with
an alcohol, which has been demonstrated by direct hydro-
genation of CO2 in the presence of alcohols.14−17 Lastly,
hydrogenations of formate esters have been demonstrated by
Received: June 18, 2014
Published: August 29, 2014
© 2014 American Chemical Society
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SWILs can also physically dissolve additional CO2 under
pressure.37 The available concentration of CO2 in solution is far
greater than other conventional solvents or traditional ionic
liquids at ambient pressure, thus opening new possibilities for
catalytic hydrogenations at lower pressures. This paper
investigates the hydrogenation of the “captured” CO2 to
formate and subsequent esterification of formate to methyl
formate under atmospheric pressure of CO2.
corresponds to the carbonyl carbon of the methyl carbonate
anion. A proton-coupled 13C NMR spectrum (Figure S1, inset)
shows the formate carbon split into its expected doublet with a
1J(C−H) of 180 Hz.18
Crystallographic analysis of the isolated crystals from the
reaction solution confirmed the [DBUH+] formate structure
(see the Supporting Information, Figure S2).
Step 3 in Scheme 1 was found to proceed under the same
conditions of step 2, albeit at increased temperatures (Figure
2). The results of the experiments are tabulated in Table 1.
Performing reaction 1 at 110 °C immediately followed by
heating of the reaction product mixtures to 140 °C for 40 h was
found to convert methyl carbonate to methyl formate.
Alternatively, a direct hydrogenation of methyl carbonate to
methyl formate was performed using the same conditions of
step 2 but heated to 140 °C for 40 h instead of 110 °C at 16 h.
The esterification reaction between the produced [DBUH+]
formate and methanol (the C−O bond cleavage) may be rate
limiting.19 In the absence of the metal catalyst no esterification
of [DBUH+] formate (independently synthesized) was
observed under the reaction conditions used in this study,
suggesting the metal catalyst is necessary for esterification.
We found that ruthenium- and iron-based catalysts were
active, with best activities for ruthenium-based catalysts. The
two heterogeneous catalysts tested showed no hydrogenation
activity under the reaction conditions employed, Table S1.
We performed multiple hydrogenation experiments for
reaction 4 using [DBUH+] methyl carbonate with homoge-
neous catalysts known to hydrogenate CO2 (Table 1).
Additionally, the system was run in SWILs with different
alcohol chain lengths (Table 2), at different catalyst loadings
(Table 3), and with varied CO2 pressures (Table 4). The hexyl
carbonate was found to be least active; the propyl carbonate
was intermediate in activity, while the methyl carbonate was
found to be most active. We believe this is due to the higher
polarity of the methyl carbonate compared to the hexyl
carbonate. We have previously shown that the degree of ionic
conversion (alkyl carbonate concentration) in switchable ionic
liquids is sensitive to polarity,39,40 and we hypothesize that the
free energy of step 2 can be impacted by the inherent polarity
of the ionic liquid. Thus, the more polar methyl carbonate will
allow stronger solvent stabilization of the produced formate
salts compared to the less polar hexyl carbonate.38,41 The best
TON (5100) was achieved using RuCl2(PPh3)3 at substrate-to-
catalyst loadings of 8650.
RESULTS AND DISCUSSION
Scheme 1 outlines a proposed hydrogenation pathway of CO2
to methyl formate in three distinct steps. Step 1 is the capture
■
Scheme 1. Proposed Preparation of Methyl Formate from
CO2 and H2 in DBU−Methanol-Based Switchable Ionic
Liquid
and possible “activation” of CO2 by DBU (1,8-
diazabicyclo[5.4.0]undec-7-ene) and an alcohol to form the
alkyl carbonate ionic liquid. Step 2 is the hydrogenation of the
alkyl carbonate into the [DBUH+] formate. The third step is
esterification of [DBUH+] formate with the previously liberated
alcohol from step 2, making an alkyl formate, liberating base
and water.
To investigate the possibility of this CO2 reduction scheme,
we studied each step along the path. We have previously shown
that methyl carbonate [DBUH+] salts can be formed in close to
quantitative yields at 1 atm, step 1 of Scheme 1.38 Hydro-
genations of [DBUH+] methyl carbonate, [DBUH+] propyl
carbonate, and [DBUH+] hexyl carbonate using ruthenium-
based catalysts known for their ability to hydrogenate CO2 were
investigated as representative systems for step 2. The
hydrogenation of [DBUH+] methyl carbonate to [DBUH+]
formate (Figure 1) was found to proceed in appreciable
Hydrolysis of DBU into a cyclic lactam (Figure 2, confirmed
by 13C{1H} NMR) is caused by the water released during
esterficiation. This hydrolysis appears to be quantitative based
upon the water released. This hydrolysis lactam of DBU is
similar to the lactam of hydrolyzed 1,5-diazabiciclo-[3.4.0]non-
5-ene (DBN), which had been previously synthesized and
characterized by Pereira et al.42 Experiments without a metal
catalyst at 140 °C with [DBUH+] methyl carbonate in
methanol showed negligible hydrolysis (Figure S3); therefore
it is surmised that the hydrolysis was occurring from water
produced during the catalyzed reaction. Attempts to use drying
agents such as molecular sieves, B(OH)3, and MgSO4 in the
catalyzed reactions, in situ, failed to prevent hydrolysis of DBU.
Table 3 shows identical conversion, 60%, in all ratios of
catalyst to substrate, demonstrating that the reaction is not
mass transport limited under these conditions. This may also
demonstrate that the reaction reaches an equilibrium under the
conditions used.
Figure 1. Hydrogenation of [DBUH+] methyl carbonate to [DBUH+]
formate.
conversion with selectivity for [DBUH+] formate salt. The
hydrogenation reactions were performed at 110 °C, 20 atm of
H2, for 16 h, with the only CO2 in the system provided by
[DBUH+] methyl carbonate.
The 13C{1H} NMR spectrum of the reaction mixture after 16
h, collected at 110 °C (Table 1, entry 1), is shown in Figure S1.
The peak at 162.8 ppm corresponds to the protonated
amidinium carbon in [DBUH+], while the peak at 166.9 ppm
corresponds to the formate product and the peak at 158.4 ppm
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Table 1. Catalytic Hydrogenation of [DBUH+] Methyl Carbonate Using Different Homogeneous Catalysts
c
c
catalyst
DBU-MeOH ratio
sub/cat
temp (°C) (time)
conv (%) (MF)
conv (%) (DF)
total conv (%) (MF + DF)
TON
a
RuCl2(PPh3)3
2:1
2:1
1:3
1:3
1:3
1:3
1:3
1:3
2160
2160
2160
2080
1130
1130
1130
960
110 (16 h)
140 (40 h)
140 (40 h)
140 (40 h)
110 (16 h)
110 (40 h)
140 (40 h)
140 (40 h)
0
33
45
42
7
24
1
24
34
61
50
24
26
48
20
510
a
RuCl2(PPh3)3
750
b
RuCl2(PPh3)3
16
8
1300
b
RuHCl(PPh3)3
RuCl(OAc)(PMe3)4
RuCl(OAc)(PMe3)4
RuCl(OAc)(PMe3)4
FeCl2(dmpe)2
1060
b
17
20
28
2
270
b
6
300
b
20
540
b
18
200
a
Catalytic conditions: 514 mg (2.25 mmol) of [DBUH+] methyl carbonate, 0.34 mL (2.25 mmol) of DBU, 1 mg (0.001 mmol) of RuCl2(PPh3)3, 20
b
atm of H2. Catalytic conditions: 514 mg (2.25 mmol) of [DBUH+] methyl carbonate, 0.18 mL (4.5 mmol) of methanol, 1 mg [0.001 mmol of
RuCl2(PPh3)3, 0.001 mmol of RuHCl(PPh3)3, 0.002 mmol of RuCl(OAc)(PMe3)4, or 0.002 mmol of FeCl2(dmpe)2] of catalyst, 20 atm of H2.
c
Conversion is determined by NMR analysis of the reaction solutions; see Experimental Section. Relative uncertainties are estimated at 5% based
upon reproduscibility of duplicate runs. DBU = 1,8-diazabicycloundec-7-ene. DF = [DBUH+] formate. MF = methyl formate. MeOH = methanol.
Table 4. Effect of CO2 Pressure on the Catalytic
Hydrogenation of [DBUH+] Methyl Carbonate Using
a
RuCl2(PPh3)3 Catalyst
Figure 2. Conversion of DBU methyl carbonate to formate and
methyl formate.
conv
(%)
conv
(%)
(DF)
total conv
(%) (MF +
DF)
PH2
PCO2
catalyst
(atm) (atm) (MF)
TON
Table 2. Catalytic Activity of RuCl2(PPh3)3 in Different
Switchable Ionic Liquids
RuCl2(PPh3)3
RuCl2(PPh3)3
RuCl2(PPh3)3
20
20
20
0
13
20
45
56
59
16
13
11
61
69
70
1300
1490
1510
conv
(%)
conv
(%)
(DF)
total conv
(%) (MF +
DF)
a
Catalytic conditions: 514 mg (2.25 mmol) of [DBUH+] methyl
carbonate, 0.18 mL (4.5 mmol) of methanol, 1 mg (0.001 mmol) of
RuCl2(PPh3)3, reaction temp 140 °C, reaction time 40 h. Relative
uncertainties are estimated at 5%.
temp
SWIL
(°C) (MF)
TON
a
DBU-MeOH (1:3)
110
140
110
140
110
140
3
45
0
11
16
20
9
14
61
20
35
10
23
310
a
1300
DBU-n-propanol (1:3)
DBU-n-hexanol (1:3)
440
770
230
510
26
0
10
5
tolerant PEEK NMR cell (Figures S4−6) to identify the
speciation of CO2 under reaction temperatures (25−110 °C).43
In these experiments, the 13C{1H} NMR spectrum showed 25%
of the methyl carbonate remained in solution, with no
detectable CO2 in the liquid phase, indicating 75% of the
CO2 was in the gas phase (Figure S6). This experiment
demonstrated that alkyl carbonates do not fully decompose at
reaction temperatures and that physical solubility of CO2 is too
low to be detected via NMR under reaction conditions. In the
reaction conditions, 2.25 mmol of [DBUH+] methyl carbonate
was used in a 10 mL cell at 110 °C. Assuming ideal behavior
(and that 25% of the carbonate remains intact), we calculate at
most a CO2 partial pressure of 4.4 atm may be generated. Such
a low partial pressure of CO2 is not sufficient enough to force
appreciable solubility of CO2 in solution at 110 °C, as it is
known that CO2 physical solubility decreases with increases in
temperature. The CO2 solubility at 110 °C would be essentially
negligible under reaction conditions. Such a concentration
discrepancy would suggest that hydrogenation of the alkyl
carbonate would be more likely, but we cannot disprove any
direct CO2 hydrogenation.
18
a
Catalytic conditions.: 2.25 mmol of [DBUH+] alkyl carbonate, 4.5
mmol of alcohol, 1 mg (0.001 mmol) of RuCl2(PPh3)3, 20 atm of H2.
Relative uncertainties are estimated at 5% based upon reproducibility
of duplicate runs.
Table 3. Catalysis Profile of RuCl2(PPh3)3 at Different Metal
a
Loading
conv
(%)
conv
(%)
total conv
(%) (MF +
DF)
sub/
cat
b
catalyst
(MF)
(DF)
TON
640
RuCl2(PPh3)3
RuCl2(PPh3)3
RuCl2(PPh3)3
RuCl2(PPh3)3
1080
2160
4330
8650
46
1
1
2
1
13
1
1
2
2
59
61
60
59
1
1
2
1
5
45
45
42
16
15
17
1300 10
2600 50
5100 50
a
Catalytic conditions: 514 mg (2.25 mmol) of [DBUH+] methyl
carbonate, 0.18 mL (4.5 mmol) of methanol, an appropriate amount of
b
RuCl2(PPh3)3, 20 atm of H2, 140 °C, 40 h. Average of two runs.
Absolute uncertainties are the difference between the two runs.
The presence of additional CO2 with H2 leads to higher
conversion to [DBUH+] formate and methyl formate (Table
4). Without additional CO2 pressure total conversion was 61%
to [DBUH+] formate and methyl formate, which can be
increased up to 70% at a 20 atm pressure of CO2. These results
suggest that the availability of CO2 in the reaction mixture may
be catalytically acted upon, or alternatively the CO2 gas may
lead to a steady state of methyl carbonate once the original
methylcarbonate is consumed in catalysis. A series of 13C{1H}
NMR experiments were performed using a custom pressure-
As alkyl carbonates and CO2 are in equilibrium in SWILs, it
is impossible to decouple the two species to decipher reactivity.
Thus, to try to identify the reactive species, analogous systems
where a carbonate or CO2 was the sole species in solution were
needed. Bicarbonate salts were chosen to elucidate the
reactivity, as they are similar to alkylcarbonates. NaHCO3 and
KHCO3 were not chosen, as they thermally disproportionate
into CO2, H2O, and Na2CO3 and K2CO3, respectively, at
temperatures used in this study (Table S2),44 thus effectively
providing both bicarbonate and CO2. For this reason CsHCO3
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was chosen due to its higher decomposition temperature (∼175
°C).44
designs and reactive pathways for CO2 that may have different
selectivity or kinetic activity.
Hydrogenations of CsHCO3 using RuCl2(PPh3)3 under
comparable conditions showed modest conversion to CsHCO2
with an equivalent amount of CsCH3OCO2 (Figure S7). All
carbon-containing species were identified via 13C{1H} NMR
spectroscopy (170.9 (CsHCO2), 168.2 (CsHCO3), 161.4
(CsCH3OCO2), and 53.9 (CsCH3OCO2)). The presence of
CsCH3OCO2 suggests that the bicarbonate was converted into
the methyl carbonate, and the hydrogenation may proceed via
the methylcarbonate salt of cesium. Attempts to make and
isolate CsCH3OCO2 from CsHCO3 were unsuccessful; thus no
direct hydrogenation studies of CsCH3OCO2 could be
performed. The activity of CsHCO2, albeit indirectly, suggests
the combination of both alcohol and base may be required for a
hydrogenation of CO2 to methyl formate.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were performed
under an N2 atmosphere unless stated otherwise. The methyl formate,
triphenylphosphine, NaHCO3, KHCO3, CsHCO3, dmpe (1,2-bis-
(dimethylphosphino)ethane), and depe (1,2-bis(diethylphosphino)-
ethane) were purchased from Aldrich and used without further
purification. DBU (Aldrich), methanol (Aldrich), propanol (Aldrich),
and hexanol (Aldrich) were dried over sodium or 4 Å sieves. The
syntheses of RuCl2(PPh3)3,45 RuHCl(PPh3)3,46 RuCl(OAc)-
48
(PMe3)4,47 and FeCl2(dmpe)2 were previously reported elsewhere.
Heterogeneous catalysts 5 wt % Ru/C, 8.8 wt % Pd/Al2O3, and 4 wt %
Pd/MCM-41 were prepared by using the wet impregnation and
subsequent hydrogen reduction method. The supercritical fluid grade
CO2 (Praxair, SFE grade, 99.999%, H2O <0.5 ppm) and ultrahigh pure
grade H2 gases were purchased from Oxarc and used as received. The
gas was delivered to the tube directly from the tank through a stainless
steel gas manifold line at specified pressures.
We proposed to close the cycle by hydrogenating methyl
formate to methanol (Scheme 2). Hydrogenations of methyl
Acquisition of NMR Spectra. Proton and 13C NMR spectra were
collected on either 300 or 500 MHz Varian instruments. The 13C
spectra were collected using a 45 deg pulse, 0.865 s acquisition time,
and a 1 s recycle time transient average set at 256. These parameters
were found to yield highly reproducible results through testing with
known concentrations of carbonate- and formate-containing solutions.
Chemical shifts were referenced both externally to TMS and internally
to residual DBU. Coupling constants are reported in hertz (Hz). The
PEEK high-pressure NMR tubes were designed and built at Pacific
Northwest National Lab.
Scheme 2. Proposed Hydrogenation of Methyl Formate to
Methanol from a Switchable Ionic Liquid
formate to methanol have recently been shown by Sanford,
Milstein, and Leitner to proceed with Bronsted acid and metal
cocatalysts.14−17,35 Our attempts to hydrogenate methyl
formate directly to CH3OH were performed under varied
conditions; however no detectable conversion to methanol was
observed using 13C{1H} NMR. The lack of activity is likely due
to this system being base-promoted, which, to our knowledge,
has yet to be demonstrated (Scheme 2). Attempts to identify
conditions or catalysts that can promote a base-promoted
hydrogenation of methyl formate are the focus of current work
in our laboratory.
Synthesis of [DBUH+] Methyl Carbonate. Dried DBU (3 mL,
20 mmol), methanol (0.8 mL, 20 mmol), and a stir bar were charged
into a dried stainless steel vessel and pressurized to 1 atm with dry
CO2 at room temperature. A white powder was obtained, which was
used for further hydrogenation reaction.
Hydrogenation of [DBUH+] Methyl Carbonate to [DBUH+]
Formate and Methyl Formate. A 514 3 mg (2.25 mmol) amount
of [DBUH+] methyl carbonate and 0.34 mL (2.25 mmol) of DBU or
0.18 mL (4.5 mmol) of methanol and 1 mg [0.001 mmol of
RuCl2(PPh3)3, 0.001 mmol of RuHCl(PPh3)3, 0.002 mmol of
RuCl(OAc)(PMe3)4, or 0.002 mmol of FeCl2(dmpe)2] of catalyst
were mixed in a stainless steel tube reactor. The catalyst was delivered
from a stock solution (5
0.1 mg/5 mL) in CH2Cl2 to yield an
CONCLUSIONS
■
appropriate amount of catalyst. The CH2Cl2 was evaporated prior to
addition of other reagents. The SS reactor, composed of a
commercially available nominally 10 mL sample cylinder (total
measured volume of 11 mL) equipped with a stir bar, was charged
with 20 atm of H2 at room temperature. The reaction mixture was
heated to 110 or 140 °C while being stirred (200 rpm) for 16 or 40 h
at reaction temperature. After the reaction, the autoclave was cooled
with ice water and the pressure was slowly released after complete
cooling. The reactor was opened in a glovebox, and the product was
characterized via NMR analysis of the neat solution. [DBUH+] propyl
and hexyl carbonate were produced in situ by adding 1 atm of CO2 for
30 min with stirring a 1:3 mixture of DBU and the alcohol and then
removing the excess CO2 under mild vacuum. NMR analysis of similar
solutions showed almost complete conversion (of DBU) to the alkyl
carbonate.
Using SWILs, CO2 can be captured and chemically converted
to an alkyl carbonate, hydrogenated to formate, and then
esterified to methyl formate. The hydrogenations of alkyl
carbonates are base-tolerant and proceed under the conditions
of 110 °C and 20 atm of H2. Raising the temperature to 140 °C
promotes esterification of the produced DBU formate (with the
liberated alcohol from the hydrogenation step), providing
alkylformate and water, and releases the base. Hydrolysis of
DBU observed during the esterification indicates bases that are
susceptible to hydrolysis should be avoided for the esterification
step in future studies. Speciation studies under reaction
conditions suggest methyl carbonates are present in millimolar
concentrations, far higher than undetectable concentrations of
any evolved CO2 from the carbonate that may be physically
dissolved in solution at 110 °C. At this time the reactive species
(i.e., alkyl carbonates or CO2) cannot be identified, as the
alkylcarbonate and CO2 are in equilibrium in this system.
Hydrogenations of CsHCO3 to CsHCO2 were successful in
methanol, possibly via the observed CsCH3OCO2. The results
from this study indicate that, at least with respect to SWILs,
alkyl carbonates may be a substrate for hydrogenation, in
addition to free CO2. Availability of either CO2 or the
carbonate as the active species opens new doors for catalyst
The 13C{1H} signals of the neat product solution for the bridgehead
carbon on DBU and protonated DBU ([DBUH+]) appear at the same
position. The peak area of this peak was added to the peak area of the
carbonyl carbon of any hydrolyzed DBU to give the total amount of
DBU in the system. The conversion of [DBUH+] methyl carbonate to
[DBUH+] formate was calculated by dividing the peak area for the
formate carbon by the DBU total. Due to the high volatility of methyl
formate (bp = 32 °C), some methyl formate was always lost during
venting. Fortunately, for this analysis, the hydrolysis of DBU is
complete under these reaction conditions, allowing the use of the peak
area of the carbonyl carbon of the hydrolyzed DBU in place of methyl
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formate. The remaining methyl carbonate can be either directly
measured in the 13C{1H} NMR spectrum or calculated by difference.
The 13C{1H} NMR signals of the carbonyl carbons for formate,
methyl formate, hydrolyzed DBU, and methyl carbonate, in addition
to the signals for the bridgehead carbons for DBU and [DBUH+], all
are nonprotonated and show similar relaxation times, allowing direct
comparison of the peak areas under the NMR conditions used. This
was tested using known amounts of methyl formate, [DBUH+]
formate, DBU, and [DBUH+] carbonate and found to be correct
within 10%.
Hydrogenation of NaHCO3, KHCO3, or CsHCO3 to Cs
Formate. A 189 mg (2.25 mmol) portion of NaHCO3, 225.3 mg
(2.25 mmol) or 436.3 mg of CsHCO3 (2.25 mmol), 0.9 mL (22.5
mmol) of methanol, and 1 mg (0.001 mmol) of RuCl2(PPh3)3 were
added into a stainless steel tube reactor. Next, the tube reactor was
filled with 20 atm of H2 at room temperature. The reaction mixture
was heated to 110 °C and then was stirred (200 rpm) for 16 h at
reaction temperature. After the reaction time, the autoclave was cooled
with ice water and the pressure slowly released. The reactor solution
was opened in a glovebox, and the product was characterized via NMR
analysis as for the [DBUH+] methyl carbonate.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Tables and supporting NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: david.heldebrant@pnnl.gov.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Research by M.Y., A.J.K., and D.J.H. was funded by the
Laboratory Directed Research and Development Program at
Pacific Northwest National Laboratory. Research by J.C.L. and
E.F.v.d.E. was supported by the U.S. Department of Energy
Basic Energy Sciences, Division of Chemical Sciences, Geo-
sciences & Biosciences. Pacific Northwest National Laboratory
is operated by Battelle for the U.S. Department of Energy. The
authors thank Dr. Roger Rousseau for guidance and support.
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