Manifesto for the routine use of NMR for the liquid product analysis of aqueous CO2 reduction: From comprehensive chemical shift data to formaldehyde quantification in water

Article abstract of DOI:10.1039/c9dt04749b

CO₂ reduction research is at a critical turnaround since it has the potential to partially or even substantially fulfil future clean energy needs. CO₂-to-CO electrochemical conversion is getting closer from industrial implementation requirements. Efforts are now more and more directed to obtain highly reduced products such as methanol, methane, ethylene, ethanol, etc., most of them being liquids. Gas-phase products (e.g., CO, CH₄) are typically detected and quantified by well-defined gas chromatography (GC and GC/MS) protocols. On the other hand, NMR, GC-MS, HPLC have been used for the liquid phase characterization, but no routine technique has yet been established, mainly due to lack of versatility of a single technique. Additionally, except NMR and GC-MS, classical techniques cannot distinguish ¹³C from ¹²C products, although it is a mandatory step to assess products origin. Herein, we show the efficiency and applicability of ¹H NMR as routine technique for liquid phase products analysis and we address two previous shortcomings. We first established a comprehensive ¹H and ¹³C NMR chemical shifts list for all ¹²CO₂ and ¹³CO₂ reduction products in water ranging from C₁ to C₃. Then we overcame the difficulty of identifying aqueous formaldehyde intermediate by ¹H NMR through an efficient chemical trapping step, along with isotopic signature study. Formaldehyde can be reliably quantified in water with a concentration as low as 50 μM.

Full text of DOI:10.1039/c9dt04749b

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DOI: 10.1039/C9DT04749B
ARTICLE
Manifesto for the routine use of NMR for the liquid product analysis of aqueous
CO2 reduction: from comprehensive chemical shift data to formaldehyde
quantification in water
Received 00th January 20xx,
Accepted 00th January 20xx
a,
*,a
Tamal Chatterjeea, , Etienne Boutin and Marc Robert

DOI: 10.1039/x0xx00000x
CO2 reduction research is at a critical turnaround since it has the potential to partially or even substantially fulfil future clean
energy needs. CO2-to-CO electrochemical conversion is getting closer from industrial implementation requirements. Efforts
are now more and more directed to obtain highly reduced products such as methanol, methane, ethylene, ethanol, etc.,
most of them being liquids. Gas-phase products (e.g., CO, CH4) are typically detected and quantified by well-defined gas
chromatography (GC and GC/MS) protocols. On the other hand, NMR, GC-MS, HPLC have been used for the liquid phase
characterization, but no routine technique has yet been established, mainly due to lack of versatility of a single technique.
Additionally, except NMR and GC-MS, classical techniques cannot distinguish 13C from 12C products, although it is a
mandatory step to assess products origin. Herein, we show the efficiency and applicability of 1H NMR as routine technique
for liquid phase products analysis and we address two previous shortcomings. We first established a comprehensive 1H and
13C NMR chemical shifts list for all 12CO2 and 13CO2 reduction products in water ranging from C1 to C3. Then we overcame the
difficulty of identifying aqueous formaldehyde intermediate by 1H NMR through an efficient chemical trapping step, along
with isotopic signature study. Formaldehyde can be reliably quantified in water with a concentration as low as 50 µM.
importance from the fuel and chemical industry perspective. As an
example, methanol market size is expected to grow to ca. 190 million
Introduction
Through the concept of ‘Artificial Photosynthesis,’ electro-
metric tons by 2030, with a promising compound annual growth rate
(CAGR) between 7 and 9 %.10 Upon considering C≥2 reduction
products, the list of possible compounds expands and most of them
are also generated in liquid state. As the CO2RR field is evolving at a
rapid pace, it is important to develop and adopt a standard method
for product analysis and further evaluation of CO2RR catalysts. Gas
chromatography is established as the routine technique for gas-
phase product analysis and along with MS detector, it allows for
assessing product origin (from CO2 or not) via labelled experiments.
Contrasting with gas-phase products, reports on the liquid phase
products remain rare and not comprehensive since there is no
adequate single technique for the identification of all possible
products. Classical methods include Ionic Chromatography (IC)11
or photo-electro- reduction of CO2 to fuels or useful chemicals has
emerged as one of the most popular approaches for future carbon-
2
neutral energy production.1,
CO2 reduction reaction (CO2RR)
requires catalysts, and a range of gas (e.g., CO, CH4) or liquid products
(e.g., methanol, ethanol, etc.) can be generated.3 Currently, CO2RR is
an extremely active field with communities of scientists, engineers,
and entrepreneurs set together to reach the target of
5
commercialized technologies.4,
Significant progress has been
achieved for the 2 e- and 2 H+ reduction of CO2 to CO,6-8 with key
parameters such as current density, selectivity and cell voltage, now
complying (or being close) with industrial requirement.9 Attention is
more and more given to the development of new catalytic processes
for more reduced products. Some of these products are of immense
which does not allow for alcohol, aldehyde, ether or ester detection
13
;
1H NMR12,
that does not permit formaldehyde and oxalate
a. Université de Paris, Laboratoire d’Electrochimie Moléculaire, CNRS, F-75013 Paris,
France.  contributed equally
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
detection in water; High precision liquid chromatography (HPLC)
coupled with UV-Vis, VWD or RID detector but depending on the
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detector, some compounds may be missed due to absence of signal
or limited detector sensitivity.11, 14 Product analysis with in-situ mass
spectrometry (OLEMS and DEMS) may become intricate when
multiple products are formed.15, 16 LC-MS and GC-MS techniques are
suitable but the former is affected by the signal of the mobile phase
while the latter is subject to lifetime shortening when liquid sample
containing electrolyte are injected.17 Also, NMR and MS techniques
provide a decisive advantage as each analyte shows a unique signal,
allowing for unusual products detection. It could then be very useful
when some minor signals are overlooked and simply attribute to
impurities or catalyst degradation product.
Results and Discussions
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DOI: 10.1039/C9DT04749B
Parameters for 1H NMR studies
To get accurate information, we have taken care of various
parameters, such as correct shimming, pre-saturation, delay time,
and pH of the solution during the NMR measurement. These
parameters have marked effect on the obtained results, as briefly
described below. As previously mentioned, water medium was
1
chosen, thus the following results are discussed on the basis of H
NMR data recorded in water (H2O:D2O, 90:10, w/w, using D2O as lock
solvent). Resulting conclusions may entirely or partially differ from
data recorded in other common organic solvents such as CD3CN,
CDCl3 or d6-DMSO.
NMR is obviously a ubiquitous and versatile tool for the
detection, identification, and quantification of various molecules and
is used routinely for product analysis in research labs, industry,
clinics, etc.18 In connection to aqueous CO2RR liquid phase product
analysis, we provide a comprehensive table of chemical shifts for CO2
reduction products in water along with their distinct coupling
constant values (JC-H). We restricted the analysis to C1, C2 and C3
products having as general formula CxOyHz. Water was chosen as
solvent since CO2RR technological devices need to be developed in
this media. An absolute requirement for CO2RR process is to
distinguish between 12C and 13C compounds, so as to demonstrate
that reduction products are (or are not) issued from CO2. 1H NMR is
one of the convenient techniques to perform such labeled
experiments. Moreover, experimental parameters such as pH of the
solution, pre-saturation of solvent signal or delay time play
important roles for product quantification, which is highlighted with
the help of adequate examples.
Pre-saturation of solvent signal
After performing CO2RR in a typical sample, amount of
protons from the analytes is far below the amount of protons from
1
water solvent. The H NMR spectrum will be dominated by water
proton and water peak suppression is a necessary step before
starting any analysis of possible liquid products. The use of D2O as
solvent cannot cope with this problem since it will make CO2
1
reduction products undetectable in H NMR. Pre-saturation, a well-
known technique in NMR, is employed to get the required solvent-
reduced spectra.21 A composite pulse sequence is used to selectively
saturate the water frequency range and then to excite the
resonances of the analytes under the NMR study. This technique
considerably improves the signal to noise ratio of the spectrum and
helps for precise quantification. A comparison of the 1H NMR spectra
of nitroethane recorded with and without pre-saturation is shown in
As a particular case, formaldehyde (HCHO) formation
remains largely unrevealed in CO2RR. One reason could also be the
unavailability of a convenient method for direct identification in
1
figure 1. Needless to mention that a sharp improvement of the H
resonances is noted with pre-saturation even when baseline
correction is applied to the non-pre-saturated spectra. Apart from
the pre-saturation technique, other methods such as WATERGATE or
WET can be used for the water peak suppression. Comprehensive
details on these methods can be found elsewhere.22, 23
1
water medium.11 The H NMR chemical shift of monomeric HCHO
(9.66 ppm) is hardly distinguishable in water,19 due to complete
hydrolysis into methanediol, which has a 1H NMR signal at 4.91 ppm,
merging with the broad water peak (4.79 ppm).20 Herein, we
establish a simple method for 1H NMR method allowing identification
and quantification of HCHO in water that could be used during the
practical course of CO2RR process. We anticipate that this
contribution will be useful as a ‘finger-print catalogue’ for hassle-free
product analysis and possible exploration of new CO2RR catalysts.
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9DT04749B
Fig. 1 Comparison of the 1H NMR spectrum of nitroethane recorded
in D2O/H2O mixture, (a) without pre-saturation, (b) with pre-
saturation.
Delay time (d1)
Relaxation time (RT) is another critical parameter for quantitative
NMR measurements (Q-NMR) since insufficient longitudinal
relaxation affects signals intensities.24, 25 All the possible resonances
of an analyte should fully relax between the intervals of two pulses
(i.e., relaxation delay d1) to get correct quantification data. Roughly,
relaxation delay (d1) value should be at least five times larger than
T1 time, corresponding to the slowest relaxing resonances of
analytes. The T1 value can be either calculated experimentally26 or
found in the literature. Optimum d1 time for accurate quantification
of an analyte can also be determined experimentally. An example is
illustrated in Fig. 2 where d1 value is progressively increased for a
known concentration of an analyte until accurate quantification is
reached. In this experiment, we recorded a series of 1H NMR spectra
of known concentrations of CH3OH with different delay times (d1).
CH3OH concentration was then back calculated from the
corresponding 1H NMR peak area (3.34 ppm) relative to
dimethylsulfoxide (DMSO) as internal reference. Depending on the
delay time, CH3OH quantification varies from 80 to 100% of the exact
value. Also, the higher the magnet strength, the faster the
relaxation26 so that optimal delay time should be set for each
experimental conditions. Alternatively, the optimal delay time (d1)
can also be minimized by applying a smaller pulse width, but this
method would also minimize the analyte signal intensity and that can
reciprocate errors in the quantification process.
Fig. 2 Accuracy of 1H NMR MeOH quantification as a function of delay
time (d1). (a) CH3OH concentration obtained from 1H NMR related to
DMSO reference as a function of exact CH3OH concentration for
increasing delay time, from 1 s (brown), 2 s (red), 5 s (orange), 15 s
(green) and 25 s (blue). (b) Percentage of detected CH3OH for 1H NMR
analysis as a function of delay time. In both graphs, expected 100%
methanol value is drawn in gray (dotted line).
pH effect on products bearing acid groups
For the identification of CO2RR products bearing acid functional
groups, one should be careful about the pH of the solution, since the
1
acid and its conjugate base show different H NMR chemical shifts.
Figure 3 depicted the effect of pH on the 1H NMR of formic
acid/formate couple (HCOOH/HCOO-; pKa 3.75 in water). In figure 3a,
pH is larger than the pKa of HCOOH/HCOO-, so the signal at  8.44
ppm corresponds to formate, while in Figure 3c (pH below the pKa),
the signal of formic acid at  8.22 ppm is observed. In the pH range
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[pKa -1; pKa +1], the peak position will vary between these two values under labelled conditions, we also provide the 13C NMR chemical
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DOI: 10.1039/C9DT04749B
(Fig. 3b). Important to mention, CO2RR has been reported along all shift of the reported products.
the pH range,3c,12 so it is important that operators remain careful to
the exact pH value.
The hunt for unusual CO2 reduction product identification
is important, since a couple of examples have been reported where
the formation of a complex reduction product was favoured without
the obtention of more simple intermediates. Among these examples,
n-propanol (Table 1) which is issued from a 12e- reduction using CO
as substrate, has been produced with relatively high Faradaic
selectivity (23% FE) at copper adparticle electrode without forming
neither formaldehyde (2e- products) nor methanol (4e- products).27
In another case, CO and methanol (simple CO2 reduction products)
have been reported to be further reduced into the more complex
dimethylcarbonate (Table 1) at Au or Pd electrodes.28 Another
example is related to the production of formaldehyde along with
trace amount of glycolaldehyde (Table 1), enough to start formose
reaction where glycolaldehyde, glyceraldehyde or dihydroxyacetone
(Table 1) are formed without forming more simpler products.29 Apart
from the CO2RR, the above mentioned valuable products may also
be generated from different sources such as biomass or non-
recyclable plastic wastes. Notably, significant research effort is
currently underway for the transformation of plastic waste into
organics such as acetate, glyoxal, glycolate, formate, etc (Table 1).30
Thus, the following table could also be useful for broadening the
scope of various catalytic processes.
pKa
Fig. 3 pH effect on the 1H NMR chemical shift of formic acid/formate
couple (HCOOH/HCOO-). (a) pH = 7, (b) pH= pKa = 3.75, (c) pH = 1.
1H and 13C NMR chemical shift data
1H and 13C NMR shifts for C1 to C3 CO2RR reduction product
are provided in Table 1. Unstable or gaseous products are included
in the list of possible products (C1 to C3) which is given in supporting
information (S6). For each product, chemical shift of all detectable
protons is provided with multiplicity. In the case of acids, pH range is
also provided. When more than one acid group are present in the
product, only the fully protonated and fully deprotonated forms are
reported. For aldehydes, more number of signals are occasionally
observed and their origin is attributed to the hydrolysed form of the
corresponding aldehyde product in water. When one form is
predominant in water, only that one is reported in the tables.
Furthermore, JC-H coupling constant values were obtained using the
satellite peaks originated from the 1% of 13C naturally presents in the
sample:
To assess the method accuracy, we further performed tests
with various CO2RR products. With our specific set of NMR
parameters (see SI for details), we plotted the percentage of
detected products using NMR quantification method for various
known concentration of analytes, as illustrated in Fig.
4
(concentration of compound being converted into actual
concentration of detected protons). Error bars drawn in colours
indicate the standard deviation as a function of equivalent proton
concentrations, providing an estimation of the error introduced by
the NMR quantification method. As an example, standard deviation
on quantification accuracy is about 5% for a 15 µM acetone solution
due to the 6 protons borne by the molecule. At the same time, the
standard deviation will be almost equivalent for a 150 µM of formate
that contains only one detectable proton. For higher concentration,
standard deviation remains in the range of 1 to 2%.
푱퐂 ― 퐇 (퐇퐳)
푱퐂 ― 퐇 (퐩퐩퐦) =
퐒퐩퐞퐜퐭퐫퐨퐦퐞퐭퐞퐫 퐟퐢퐞퐥퐝 퐬퐭퐫퐞퐧퐠퐭퐡 (퐌퐇퐳)
Due to strong hydrogen bonding interaction in water, none of the O-
H protons are distinguishable from the broad water signal at ( 4.79
ppm). To provide complementary characterization of the products
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9DT04749B
Fig. 4 Percentage of detected product vs. equivalent concentration
of protons in compounds: acetone (red), methanol (orange) and
formate (green). Black dots represent measured data while coloured
traces indicate error bars.
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DOI: 10.1039/C9DT04749B
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Table 1 1H and 13C NMR chemical shifts for C1, C2 and C3 compounds issued from CO2RR in water as solvent.
1H NMR
chemical
shifts
JC-H
value
(Hz)
chemical
formula
pH
range
13C NMR chemical shifts
compounds
multa
(ppm)
(ppm)
C1 compounds
methanol
CH3OH
HCHO
HCOOH
HCOO-
s
s
s
s
3.34
142
-
49.5 (CH3OH) 31
84 (CH2(OH)2) 19
165.6 (HCOOH) 32
171.1 (HCOO-) 32
formaldehyde
formic acid
formate
See next section
8.22
8.44
219
195
< 2.75
> 4.75
C2 compounds
CH3CH2OH
CH3CH2OH
CH2(OH)CH2(OH)
CH3CHO
t
q
s
d
q
d
q
d
s
s
-
1.17
3.64
3.65
2.22
9.66
1.31
5.24
4.80
2.08
1.90
-
126
143
143
128
176
126
ND
ND
130
127
-
-
ethanol
17.47 (-CH3); 58.05 (-CH2OH) 31
63.17 (-CH2OH) 31
-
ethylene glycol
acetaldehyde
-
-
30.83 (-CH3); 207.40 (-CHO) 33
CH3CHO
-
CH3CH(OH)2
CH3CH(OH)2
(CHO)2
-
acetaldehyde b
23.92 (-CH3); 88.96 (-CH(OH)2) 33
-
glyoxal 34, 35
acetic acid
acetate
-
91.2 (-CH(OH)2)
21.03 (-CH3); 177.21 (-COOH) 31
24.0 (-CH3); 181.8 (-COO-) 35
162.9 (-COOH) 36
CH3COOH
CH3COO-
< 3.76
> 5.76
oxalic acid
oxalate
HOOCCOOH
O-OCCOO-
< 0.2
> 5.30
-
-
-
173.4 (-COO-) 37
HCOOCH3
s
s
s
s
t
8.13
3.75
9.61
4.41
5.04
3.50
4.20
3.93
229
149
178
142
ND
143
144
143
-
methyl formate
glycolaldehyde
glycolaldehyde b
52.11 (-CH3); 165.12 (-HCOO-)
ND
HCOOCH3
-
CHOCH2OH
CHOCH2OH
CH(OH)2CH2OH
CH(OH)2CH2OH
CH2OHCOOH
CH2OHCOO-
-
-
-
65.30 (-CH2OH)b; 90.51 (-CH(OH)2)
d
s
s
-
glycolic acid
glycolate
glyoxylic acid b
glyoxylate b
< 2.83
> 4.83
60.06 (-CH2OH); 176.98 (-COOH)
61.96 (-CH2OH);180.45 (-COO-)
86.93 (-CH(OH)2); 173.85 (-COOH)
93.7 (-CH(OH)2); 182.8 (-COO-) 38
CH(OH)2COOH
CH(OH)2COO-
H3COCH2OH
H3COCH2OH
s
s
s
s
5.34
5.06
3.39
4.69
165
ND
ND
ND
< 2.30
> 4.30
-
-
methoxymethanol 39
56.6 (-OCH3); 90.1 (-CH2OH)
C3 compounds
CH3CH2CH2OH
CH3CH2CH2OH
CH3CH2CH2OH
CH3CH(OH)CH3
CH3CH(OH)CH3
CH3CH(OH)CH2OH
CH3CH(OH)CH2OH
CH3CH(OH)CH2OH
t
m
t
0.89
1.54
3.56
125
125
140
127
144
126
141
141
propanol
-
-
-
10.22 (-CH3); 25.26 (-CH2-); 64.30 (-CH2OH) 40
24.38 (-CH3); 64.88 (-CHOH-) 31
d
1.15
isopropanol
1,2-propanediol
se.
d
4.01
1.12
m
dd
3.87
18.70 (-CH3); 67.37 (-CH2OH); 68.68 (-CHOH-)
3.43, 3.53
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OHCH2CH2CH2OH
OHCH2CH2CH2OH
t
3.67
1.78
143
126
ND
1,3-propanediol
-
34.4 (-CH2-);D5O9I.:310(-.1C0H3V29iOe/wCH9A)Dr4tiT1c0le4O7n4l9inBe
qu.
dd
OHCH2CH(OH)CH2OH
3.54, 3.64
glycerol
-
-
63.2 (-CH2OH); 72.7 (-CHOH-)
55.55 (-OCH3); 97.53 (-CH2-)
OHCH2CH(OH)CH2OH
CH3OCH2OCH3
CH3OCH2OCH3
CH3CH2CHO
m
s
3.77
3.37
4.63
1.04
2.55
9.69
0.90
1.60
4.96
2.21
2.30
5.25
1.19
ND
143
ND
dimethoxymethane
s
t
127
126
23, 174
127
ND
propionaldehyde
CH3CH2CHO
qd
t
-
-
5.3 (-CH3); 37.1 (-CH2-); 209.4 (-CHO) 35
8.3 (-CH3); 30.2 (-CH2-); 92.6 (CH(OH)2) 35
CH3CH2CHO
CH3CH2CH(OH)2
CH3CH2CH(OH)2
CH3CH2CH(OH)2
CH3COCH3
t
Propionaldehyde b
acetone
m
t
ND
s
128
130
ND
-
-
30.89 (-CH3); 215.94 (-CO-) 31
CH3COCH(OH)2
CH3COCH(OH)2
CH3C(OH)2CH(OH)2
s
b, 42
methylglyoxal
25.4 (-CH3); 90.6 (-CH(OH)2; 209.9 (-CO-)
s
s
ND
b, 42
methylglyoxal
-
22.2 (-CH3); 92.7 (-CH(OH)2); 96.0 (-C(OH)2-)
CH3C(OH)2CH(OH)2
CHOCH2CHO
CHOCH2CHO
CHOCH2CHO
CHOCH2CHO
CH3CH2COOH
s
d
t
d
t
4.30
8.49
5.69
8.64
5.30
1.07
ND
ND
ND
ND
ND
129
< 3.00
> 5.00
malonaldehyde 43, 44
ND
t
propionic acid
propionate
< 3.88
> 5.88
9 (-CH3); 27 (-CH2-); 180 (-COOH) 45
10 (-CH3); 31 (-CH2-); 185 (-COO-) 45
CH3CH2COOH
CH3CH2COO-
q
t
2.38
1.04
129
127
CH3CH2COO-
HOOCCH2COOH
O-OCCH2COO-
CH2CHCH2OH
CH2CHCH2OH
CH2CHCH2OH
CH2CHCHO
q
s
2.16
3.51
127
132
128
ND
malonic acid
malonate
< 1.83
> 6.69
42 (-CH2-); 172 (-COOH) 46
49 (-CH2-); 178 (-COO-) 46
s
3.10
qd
m
td
dd
m
d
5.17, 5.25
5.99
allyl alcohol
acrolein
154
143
ND
-
-
62.8 (-CH2OH); 115.5 (=CH2); 136.7 (=CH-) 35
137 (=CH-); 142 (=CH2); 199 (-CHO) 47
4.10
6.52, 6.67
6.40
CH2CHCHO
ND
CH2CHCHO
9.47
177
ND
CH2CHCOOH
CH2CHCOOH
CH2CHCOO-
CH2CHCOO-
CHCCH2OH
dd
m
dd
m
t
5.98, 6.41
6.16
acrylic acid
acrylate
< 3.25
> 5.25
128.43 (=CH-); 133.38 (=CH2); 171.01 (-COOH)
127.19 (=CH-); 134.41 (=CH2); 175.97 (-COO-)
ND
5.64, 6.00
6.12
ND
ND
2.82
49; 251
propargyl alcohol
-
50.4 (-CH2OH); 73.8 (≡CH); 82.0 (≡C-) 48
CHCCH2OH
CHCCOOH
CHCCOO-
d
s
s
4.22
3.54
ND
149
259
ND
propiolic acid
propiolate
< 0.89
> 2.89
ND
ND
CH3CH(OH)CH(OH)2
CH3CH(OH)CH(OH)2
CH3COCH2OH
d
m
s
1.16
3.66
2.13
4.36
1.41
4.38
1.31
4.10
127
ND
lactaldehyde b
hydroxyacetone
lactic acid
-
16.5 (-CH3); 69.8 (-CHOH-); 92.6 -CH(OH)2) 49
25.54 (-CH3); 68.25 (-CH2OH); 213.21 (-CO-)
20.01 (-CH3); 67.13 (-CHOH-); 179.27 (-COO-)
22.7 (-CH3); 71.1 (-CHOH-); 184.9 (-COO-) 50
129
143
129
ND
-
CH3COCH2OH
s
CH3CH(OH)COOH
CH3CH(OH)COOH
CH3CH(OH)COO-
CH3CH(OH)COO-
d
q
d
q
< 2.86
> 4.86
128
ND
lactate
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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ARTICLE
Journal Name
CH2OHCH2CHO
CH2OHCH2CHO
t
q
t
3.95
2.75
9.72
3.70
1.85
5.18
3.85
2.61
3.78
2.42
ND
ND
ND
View Article Online
46 (-CH2-); 55 (-DCHO2I:O1H0.)1;023097/C(9-CDHT0O4)7;49B
reuterin 47
-
-
CH2OHCH2CHO
ND
CH2OHCH2CH(OH)2
CH2OHCH2CH(OH)2
CH2OHCH2CH(OH)2
OHCH2CH2COOH
t
ND
reuterin b 47
q
t
ND
40 (-CH2-); 58 (-CH2OH); 89 (-CH(OH)2)
ND
t
147
127
145
127
ND
3-hydroxypropionic
acid
< 3.87
> 5.87
37.08 (-CH2-); 57.66 (-CH2OH); 176.78 (-COOH)
40.76 (-CH2-); 59.56 (-CH2OH); 181.11 (-COO-)
OHCH2CH2COOH
OHCH2CH2COO-
OHCH2CH2COO-
t
t
3-hydroxypropionate
t
HOCH2CH(OH)CH(OH)2
HOCH2CH(OH)CH(OH)2
HOCH2CH(OH)CH(OH)2
HOCH2CH(OH)COOH
HOCH2CH(OH)COOH
HOCH2CH(OH)COO-
HOCH2CH(OH)COO-
HOCH2COCH2OH
ND
ND
ND
d
t
DL-glyceraldehyde b
ND
143.3
162.1
146
ND
-
63.4 (-CH2OH); 75.5 (-CHOH-); 91.2 (-CH(OH)2)
51
ND
3.87
4.37
3.71, 3.81
4.09
4.40
3.56
glyceric acid
glycerate
< 2.42
> 4.42
64.8 (-CH2OH); 72.8 (-CHOH-); 177.2 (-COOH) 51
65, 75 (-CH2OH, -CHOH-); 180 (-COO-) 38
q
q
S
ND
ND
dihydroxyacetone
dihydroxyacetone b
143
ND
-
-
65 (-CH2OH); 212 (-CO) 52 53
64 (-CH2OH); 95 (-C(OH)2) 52 53
HOCH2C(OH)2CH2OH
s
β-hydroxypyruvic
acid
HOCH2COCOOH
s
3.72
145
< 1.57
ND
β-hydroxypyruvate 54
tartronic acid
tartronate
pyruvic acid
pyruvic acid b
HOCH2COCOO-
HOOCCH(OH)COOH
O-OCCH(OH)COO-
CH3COCOOH
s
s
s
s
s
4.70
4.95
4.32
2.45
1.57
ND
ND
ND
130
128
> 3.57
< 1.85
> 5.85
< 1.40
< 1.40
ND
71.82 (-CHOH-); 172.20 (-COOH)
75.90 (-CHOH-); 177.15 (-COO-)
29 (-CH3); 200 (-CO-); 166 (-COOH) 55
28 (-CH3); 96 (-C(OH)2-); 178 (-COOH)
CH3C(OH)2COOH
pyruvate
pyruvate b
mesoxalic acid b
mesoxalate b
CH3COCOO-
CH3C(OH)2COO-
HOOCC(OH)2COOH
O-OCC(OH)2COO-
CH3OCH2CH2OH
CH3OCH2CH2OH
CH3OCH2CH2OH
CH3CH2OCHO
s
s
-
2.35
1.56
-
130
ND
-
> 3.40
> 3.40
29 (-CH3); 200 (-CO-); 173 (-COO-)
28 (-CH3); 96 (-C(OH)2-); 181 (-COO-)
94.0 (-C(OH)2-); 173.5 (-COOH) 36
93.48 (-C(OH)2-); 176.37 (-COO-)
-
-
-
s
t
3.37
3.55
3.71
1.28
4.24
8.12
3.41
142
ND
140
127
ND
227
144
2-methoxyethanol
ethyl formate
-
-
58.61 (-CH3); 60.91 (-CH2OH); 73.77 (-OCH2-)
14.10 (-CH3); 62.06 (-CH2O-); 164.97 (-OCHO)
t
t
CH3CH2OCHO
q
s
s
CH3CH2OCHO
CH3OCH2COOH
2-methoxyacetic acid
2-methoxyacetate
< 2.83 59.34 (-CH3); 69.48 (-CH2-); 175.04 (-COOH)
CH3OCH2COOH
CH3OCH2COO-
CH3OCH2COO-
CH3COOCH3
s
s
s
s
4.13
3.35
3.85
2.07
145
143
143
130
> 4.83
58.83 (-CH3); 71.40 (-CH2-); 178.04 (-COO-)
methyl acetate
-
-
-
20.83 (-CH3); 52.84 (-OCH3); 175.72 (-COO-)
55.85 (-CH3); 157.77 (-CO-)
CH3COOCH3
OC(OCH3)2
s
s
s
s
t
3.66
3.77
4.22
3.75
3.83
4.29
8.19
5.21
148
149
145
149
143
ND
dimethyl carbonate
methyl glycolate
HOCH2COOCH3
HOCH2COOCH3
HOCH2CH2OCHO
HOCH2CH2OCHO
HOCH2CH2OCHO
(OCH2)3
53.02 (-CH3); 60.29 (-CH2OH); 175.53 (-COO-)
2-hydroxyethyl
formate
t
-
-
60.15 (-CH2OH); 66.31 (-CH2O-); 164.61 (-OCHO)
94.11 (OCH2)3
s
s
230
167
1,3,5-trioxane
asignal multiplicity; s = singlet; d = doublet; t = triplet; q = quartet; qu. = quintet; se. = septet; dd = double doublet; qd = quartet doublet; m =
multiplet; br = broad singlet signal. b hydrolized form of carbonyl group. ND = not determined.
8 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9DT04749B
ARTICLE
Formaldehyde detection and quantification in water
stable for several days. To the best of our knowledge, it is the first
time 1H NMR is used for both HCHO quantification and H13CHO
identification. In a typical experiment, 5 mM of a HCHO solution was
mixed with a 1 M NaHSO3 solution (50:50 v/v) and well stirred to
In CO2RR, HCHO is formed upon 4e- /4H+ reduction. Besides
being a valuable commodity chemical, HCHO also serves as a crucial
intermediate for the formation of other CO2RR products.56 Hence, a
reliable yet simple method for HCHO quantification is mandatory not
only for catalytic mechanism understanding but also for improving
catalytic process performance. Direct detection of HCHO in water
medium is difficult due to the high intrinsic HCHO reactivity. Indirect
quantification of HCHO has been performed for CO2RR, upon
reacting it first with 2,4-Dinitrophenylhydrazine (DNPH) through
Brady’s reaction,57 and further using HPLC to quantify the HCHO-
DNPH adduct. However, DNPH derivatization method requires
organic solvents and the UV-vis detector does not allow distinction
between H12CHO and H13CHO, a severe drawback preventing from
carbon source identification.
1
form adduct (A), giving a H NMR signal at around  4.39 ppm (Fig.
5a), consistent with previous report.60 The NMR peak is well shifted
from the water peak (4.79 ppm) as shown in Fig. 5a.
Scheme 1 Reaction between HCHO and NaHSO3.
To circumvent these difficulties, we have established a
Furthermore, two satellite peaks (4.58 ppm and 4.20 ppm
respectively in 400 MHz spectrometer) are also visible in the spectra,
originating from the presence of 55 µM H13CHO, which is naturally
present in the 5 mM initial HCHO solution (Fig. 5b). The obtained JC-H
value (153 Hz) provides full characterization of HCHO in water.
simple HCHO quantification method using routine 1H NMR. The
method relies on HCHO-bisulfite adduct (A) detection, which is
formed by the reaction between sodium bisulfite (NaHSO3) and
HCHO in water (Scheme 1).58,59 The reaction is complete due to very
favourable thermodynamics and the resulting adduct (A) remains
Fig. 5 1H NMR spectrum of the mixture of 5 mM HCHO and 1 M NaHSO3 solutions (50:50 v/v) versus DMSO2 reference. (a) Full spectrum; (b)
zoom on the peaks corresponding to adduct (A).
Beside HCHO identification, the method turned out as comparing NMR peak area of adduct (A) to an internal standard.
perfectly suited for quantification. It was easily performed upon Note that DMSO is reacting with bisulfite ion, which precludes its use
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Journal Name
as standard (see Fig. S1a and S1b in SI). Dimethylsulfone (DMSO2) Authors do not have any conflict of interest to declare.
View Article Online
DOI: 10.1039/C9DT04749B
was instead employed and it is proved to be stable under our
experimental conditions for at least 2 days (see Fig. S1c and S1d in
Acknowledgements
SI). Figure
6 illustrates the accuracy of the current HCHO
The authors warmly thank Dr. Christine Cordier (Université de Paris)
for her expertise and advices with NMR technique. The work
described in this paper was supported by the French National Agency
for Research (ANR-16-CE05-0010-01). T.C. is grateful for the “Make
Our Planet Great Again” fellowship awarded by the French
Government. Partial financial support to M.R. from the Institut
Universitaire de France (IUF) is also gratefully acknowledged.
quantification method. The introduced HCHO and back calculated
HCHO from NMR signal area (Fig S2) of adduct are in agreement
within a range extending from 50 µM to 5 mM HCHO, although error
remains in the range of 10%. Note that due to the reactivity of
2-
bisulfite and adduct A with CO2 and CO3 respectively61, 62, caution
need to be taken before performing the analysis. The analyte
solution has to be degassed and adjusted to an optimum pH range,
as described in the SI.
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DOI: 10.1039/C9DT04749B
Manifesto for the routine use of NMR for the liquid product analysis of aqueous
CO2 reduction: from comprehensive chemical shift data to formaldehyde
quantification in water
Tamal Chatterjee,a, Etienne Boutina, and Marc Robert *,a
Université de Paris, Laboratoire d’Electrochimie Moléculaire, CNRS, Paris, France.
Email: robert@univ-paris-diderot.fr
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