Transforming atmospheric CO2 into alternative fuels: A metal-free approach under ambient conditions

Article abstract of DOI:10.1039/c8sc03581d

This work demonstrates the first-ever completely metal-free approach to the capture of CO₂ from air followed by reduction to methoxyborane (which produces methanol on hydrolysis) or sodium formate (which produces formic acid on hydrolysis) under ambient conditions. This was accomplished using an abnormal N-heterocyclic carbene (aNHC)-borane adduct. The intermediate involved in CO₂ capture (aNHC-H, HCOO, B(OH)₃) was structurally characterized by single-crystal X-ray diffraction. Interestingly, the captured CO₂ can be released by heating the intermediate, or by passing this compound through an ion-exchange resin. The capture of CO₂ from air can even proceed in the solid state via the formation of a bicarbonate complex (aNHC-H, HCO₃, B(OH)₃), which was also structurally characterized. A detailed mechanism for this process is proposed based on tandem density functional theory calculations and experiments.

Full text of DOI:10.1039/c8sc03581d

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Transforming atmospheric CO2 into alternative
fuels: a metal-free approach under ambient
Cite this: DOI: 10.1039/c8sc03581d
conditions†
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Samaresh Chandra Sau,b Rameswar Bhattacharjee,
Pradip Kumar Hota,b
a
Pavan K. Vardhanapu,b Gonela Vijaykumar,
R. Govindarajan,
Ayan Datta
b
b
a
*
b
*
and Swadhin K. Mandal
This work demonstrates the first-ever completely metal-free approach to the capture of CO2 from air
followed by reduction to methoxyborane (which produces methanol on hydrolysis) or sodium formate
(which produces formic acid on hydrolysis) under ambient conditions. This was accomplished using an
abnormal N-heterocyclic carbene (aNHC)–borane adduct. The intermediate involved in CO2 capture
(aNHC-H, HCOO, B(OH)3) was structurally characterized by single-crystal X-ray diffraction. Interestingly,
the captured CO2 can be released by heating the intermediate, or by passing this compound through an
ion-exchange resin. The capture of CO2 from air can even proceed in the solid state via the formation of
a bicarbonate complex (aNHC-H, HCO3, B(OH)3), which was also structurally characterized. A detailed
mechanism for this process is proposed based on tandem density functional theory calculations and
experiments.
Received 12th August 2018
Accepted 29th November 2018
DOI: 10.1039/c8sc03581d
rsc.li/chemical-science
catalysts.20–25 These reports utilized commercially available
pure CO2 gas from a cylinder. However, it remains difficult to
Introduction
capture CO2 present in the ambient atmosphere at extremely
low concentrations and to perform reduction reactions to
obtain fuels under ambient conditions. Only a very few
compounds26–32 have demonstrated selective CO2 absorption
from air, but these compounds are not capable of reducing the
captured CO2 to generate methanol. Recently, Olah and
coworkers combined two compounds with the aim of capturing
CO2 from air followed by reduction into methanol. However,
this process required the use of a rare and expensive Ru-based
Carbon dioxide is an attractive, economical, and renewable C1
source for the production of value added chemicals and fuels.1–5
Many fuel-related C1 products, such as methane (CH4), meth-
anol (CH3OH) and formic acid (HCOOH), can be obtained by
treating CO2 with a reducing agent as a hydride source.6 Among
the various possible CO2 hydrogenation products, CH3OH and
formic acid are the most attractive candidates. Methanol can be
used as a direct replacement for gasoline in internal combus-
tion engines and also in direct methanol fuel cells.7 In addition,
formic acid has applications in the textile industry in dyeing
processes, as a food preservative and as a hydrogen storage
material.8–10 During the last 20 years, numerous reports have
described the reduction of CO2 by various transition metal
based compounds.11–17 The reduction was also accomplished
catalytically without a transition metal based compound18 or by
direct reduction with NaBH4 without requiring any catalyst.19
The reduction of CO2 has been achieved even with metal-free
ꢀ
species at 155 C under harsh reaction conditions (50 bar H2
pressure).33 At present, there are no methods for the capture of
CO2 from the atmosphere and the consecutive reduction of this
CO2 to produce methoxyborane (which can be hydrolyzed to
methanol) under ambient conditions using a single chemical
system without any metal-based compounds. Moreover, there is
currently only a very limited understanding of the mechanistic
pathways for such key chemical transformations based on
isolating and characterizing various intermediates. Density
functional theory (DFT) calculations along with several anal-
yses, including structure determination by single-crystal X-ray
diffraction (SC-XRD), were employed in this work to gain better
insights into the mechanistic details of the conversion. The
present study describes metal-free capture of CO2 from air
and its reduction under ambient conditions (Fig. 1) using an
abnormal N-heterocyclic carbene.
aSchool of Chemical Sciences, Indian Association for the Cultivation of Science, 2A and
2B Raja S. C. Mullick Road, Jadavpur 700032, Kolkata, West Bengal, India. E-mail:
spad@iacs.res.in
bDepartment of Chemical Sciences, Indian Institute of Science Education and Research
Kolkata, Mohanpur 741246, Nadia, West Bengal, India. E-mail: swadhin.mandal@
iiserkol.ac.in
† Electronic supplementary information (ESI) available: Experimental section,
NMR data, a video clip, computational details, XRD details with CIF les and
NMR images. CCDC 1583077–1583080. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c8sc03581d
An abnormal N-heterocyclic carbene (aNHC; 1) was prepared
according to a literature procedure,34 following which the
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Fig. 2a. When a solution of 2 in deuterated benzene was le
exposed to the ambient air overnight, a distinct color change
from light yellow to green was observed. The formation of a new
1
product (3, Fig. 2b) was also conrmed by H NMR spectros-
copy. The conversion of the original compound during this
process was 66% and the isolated crystal yield of 3 was 40%. The
1H NMR spectrum of 3 in deuterated chloroform showed two
singlets at d ¼ 8.53 and 8.55 ppm with a 1 : 1 signal intensity
ratio, attributed to the formate anion36 and C5–H of the azolium
cation,37 respectively. The presence of formate anions indicated
that CO2 in the ambient air may have been xed during the
transformation to provide these species. The 13C NMR spectrum
of 3 contains signals at d ¼ 169.2 ppm (assigned to the C]O of
formate) and 125.4 ppm (the C5 of azolium), which further
substantiates the incorporation of a formate anion in 3.24,36 The
fate of the B atom in the 9-BBN backbone of 3 during this
transformation was determined using 11B NMR spectroscopy,
which showed a singlet at d ¼ 20.8 ppm, attributed to free boric
acid {B(OH)3}. Based on these data, a structure for 3 was
developed (Fig. 2b).This compound was successfully crystallized
into colorless crystals from a benzene/n-hexane mixture in open
atmosphere. Single-crystal XRD analysis conrmed the
proposed structure (Fig. 2b), in which a CO2 molecule is xed as
a formate anion by reaction with the aNHC–borane adduct 2.
To ascertain the reactivity of the captured CO2, we treated 3
with a boron hydride. Upon reaction of 3 with 10 equiv. of the
hydroborane 9-BBN in the presence of air, the corresponding
methoxyborane product (CH3OBBN) was obtained with complete
consumption of 3 within 6 h in deuterated benzene (Scheme 1a,
methoxyborane). The formation of CH3OBBN was conrmed by
1H NMR (d ¼ 3.44 ppm) and 11B NMR (d ¼ 57 ppm) spectroscopy
in deuterated benzene and by comparing our results to previous
literature data.20 During the course of the reaction, gas evolution
was observed (see the video le in the ESI†), which was conrmed
to be H2 by 1H NMR spectroscopy (d ¼ 4.47 ppm).38 Additionally,
upon introducing dry CO2 from a cylinder (99.995% pure) and
performing the same reaction under moisture-free pure CO2
instead of air, 3 exhibited catalytic activity, which may be
explained based on a mechanism previously proposed by our
group.24 Under these conditions, CO2 was catalytically converted
to the corresponding trimethoxyboroxine21,39 with 67%
Fig. 1 Metal-free CO2 fixation from air followed by reduction to
methoxyborane or sodium formate under ambient conditions.
aNHC–9BBN adduct, 2, was synthesized in a 75% isolated yield
by reacting 1 and 9-BBN in a 1 : 1 equivalent ratio in toluene at
40 ꢀC for 2 h (Fig. 2a). Because a higher yield and crystals
suitable for XRD analysis were obtained, this reaction protocol
represents an improvement over the previously reported
procedure by Crudden and coworkers using an aNHC salt, in
which the XRD structural analysis of the adduct 2 was not
carried out.35 Crystals of 2 suitable for XRD analysis were
successfully grown from a toluene/hexane mixture under an
inert atmosphere and the resulting structure is presented in
Fig. 2 Fixation of CO2 in air. (a) Synthesis of an abnormal N-hetero-
cyclic carbene–9-borabicyclo(3.3.1)nonane adduct (aNHC–9BBN, 2)
and an ORTEP drawing of the molecular structure of 2. (b) Fixation of Scheme 1 Reduction of CO2 to methoxyborane and sodium formate
CO2 from air with the formation of 3 and an ORTEP drawing of 3.
in air.
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conversion, using a low cost borane (BH3$SMe2) as the hydride 6.76%). The DTA data in Fig. 3 (blue line) exhibit an exotherm
source (Scheme 2). The feasibility of applying 3 to the synthesis of in agreement with the loss of CO2. Based on the NMR evidence
formic acid via the formation of sodium formate was also and the TG-DTA analysis,30,42–44 we may conclude that heating 3
assessed. Adding 3 to 5 mL of a 2 M sodium hydroxide solution at 150 ꢀC releases the captured CO2 that is present as a formate
and 0.5 mL tetrahydrofuran at room temperature in air resulted anion. It was also found possible to replace the formate anion in 3
in complete consumption of the compound (Scheme 1b, sodium with a chloride ion simply by passing it through a Dowex ion-
formate). The formation of sodium formate (HCOONa) was exchange resin. This procedure generated compound 4 with
conrmed by 1H NMR spectroscopy (d ¼ 8.38 ppm in deuterated a 50% isolated yield (Fig. 4).45 The formation of 4 was conrmed
water) based on previous literature data.11 We subsequently by NMR analysis and single-crystal XRD. The 1H NMR spectrum
attempted to remove the CO2 captured in 3 by heating. Raising of the resulting compound in deuterated chloroform contained
the temperature of 3 to 150 ꢀC for 12 h in the solid state was found only one singlet at d ¼ 8.84 ppm, while the 13C NMR spectrum did
to promote the loss of the formate anion, leading to an abrupt not display any signals beyond 145.1 ppm, conrming the
color change from colorless to brown, and forming a new product absence of formate ions in 4. Finally, 4 was obtained as colorless
as determined by 1H NMR spectroscopy.40,41 As noted previously, 3 crystals from methanol, and single-crystal XRD conrmed the
1
showed two singlets in its H NMR spectrum at d ¼ 8.53 and expected structure (Fig. 4), in which the formate anion was
8.55 ppm with a 1 : 1 intensity ratio in deuterated chloroform. replaced by a chloride ion.45 Based on the above ion exchange
1
ꢀ
Aer heating 3 at 150 C for 12 h, the H NMR spectrum of the chromatographic experiment, it was possible to regenerate the
resulting compound contained only a new singlet (d ¼ 8.35 ppm) aNHC salt (4). Interestingly, when 2 was exposed to air in the solid
while the other original singlet was absent. This observation state for three days as a ne powder in a Petri dish, the formation
indicates that the formate anion might have been lost during the
heating process. To further support this assumption, a 13C NMR
spectroscopic experiment was performed. Following heating, the
singlet in the downeld region of the 13C NMR spectrum acquired
in deuterated chloroform (at d ¼ 169.2 ppm) disappeared
completely, conrming the loss of the formate counter anion.
Thermogravimetric-differential thermal analysis (TG-DTA)
ꢀ
analyses were also performed by heating 3 from 27 to 300 C at
a rate of 4 ꢀC minꢁ1. The TG data in Fig. 3 (black line)
ꢀ
demonstrate mass loss beginning at 95.5 C and continuing up
to 160.2 ꢀC, for a total loss of 7.22% (going from a molecular
mass of 651.4 to 603.9). This may be attributed to the loss of
a CO2 molecule from 3 on heating (the theoretical mass loss is
Fig. 4 Substitution of a formate anion with a chloride ion upon
passing 3 through an ion-exchange resin, and an ORTEP drawing of
the molecular structure of 4.
Scheme
conditions.
2 Metal-free catalytic reduction of CO2 under ambient
Fig. 3 Removal of captured CO2 in solid state, as monitored by Fig. 5 Capture of CO2 from air by 2 in the solid state with the
thermogravimetric-differential thermal analysis of compound 3.
formation of the bicarbonate 5, and its ORTEP drawing.
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Fig. 6 Mechanistic scheme for the reduction of CO2 to methoxyborane and sodium formate with 2 in air. For drawings of 9 and 10 see Fig. S3.†
1
of a new product was observed, as evidenced by H NMR spec- corresponding 13C NMR spectroscopic signals at d ¼ 169 ppm
troscopy (Fig. 5).26,30,46
(C]O of bicarbonate) and 125.4 ppm (C5 of azolium).
The 1H NMR spectrum of this new compound in deuterated Furthermore, the 11B NMR spectrum of 5 exhibits a singlet at
chloroform at room temperature contained two singlets at d ¼ d ¼ 21 ppm that can be ascribed to free boric acid. Based on
8.58 and 8.77 ppm with a 1 : 1 intensity ratio. These two signals these data, it is evident that 5 was obtained (Fig. 5). Colorless
(which are different from those produced by 3) were attributed crystals of 5 were grown from a benzene/n-hexane mixture with
to a bicarbonate anion30 and an imidazolium cation,24 respec- a 50% yield and the single-crystal XRD analysis of compound 5
tively. These assignments were further substantiated by the conrmed the expected structure (Fig. 5).
Fig. 7 Computed Gibbs free energy profiles at 25 ꢀC for the conversion of CO2 to methoxyborane or sodium formate with 2 in air. The relative
free energies (in kcal molꢁ1) obtained in solvent were calculated with respect to the energy of the separate reactants {aNHC (1) and 9-BBN}.
*Formation of cyclooctane was confirmed by 1H NMR spectroscopy. #H2 gas evolution was confirmed by 1H NMR spectroscopy and visual
observation (see the video file in the ESI†). For drawings of all compounds see Fig. S3.†
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The mechanism associated with the reduction of the captured deuterated benzene) of the reaction mixture.48 Compound 3 was
CO2 was examined by performing several stoichiometric reac- thoroughly characterized by XRD, NMR spectroscopy and
tions in addition to high level DFT calculations. On the basis of elemental analysis. This compound was also found to react with
the experimental results, a full mechanistic path (Fig. 6) is sodium hydroxide solution to furnish sodium formate (8b), as
proposed together with the energy prole diagram shown in conrmed through NMR analysis.11
Fig. 7. The optimized transition states for the mechanistic path
When sodium hydroxide was added to a solution of 3 in
are presented in Fig. 8. The combination of 1 with 9-BBN in an water/tetrahydrofuran, sodium formate was released with the
equimolar amount in toluene at 40 ꢀC furnishes 2,35 as conrmed formation of intermediate 11 (Fig. S3†). The structure of ꢁ11, as
by NMR and XRD analysis. The electrostatic potential (ESP) shown in Fig. S3,† consists of two fragments (viz. B(OH)4 and
surface and the highest occupied molecular orbital of 2 were also aNHC-H+) that subsequently release a water molecule to form
determined (Fig. S1, ESI†). In the ESP surface diagram, the red boronic acid (12) and free aNHC (1) (Fig. S3†) via TS2 (Fig. 8). The
region around the B–H bond of intermediate 2 indicates higher free energy of activation for water elimination was calculated to
electron density around that area. Thus, 2 could potentially serve be 13.6 kcal molꢁ1. In addition, another 9-BBN molecule reacts
as a hydride donor during the course of the reaction. This can be with 3 to furnish 3a (Fig. S3†), thus regenerating free aNHC (1)
explained by the activation of the B–H bond of 2 as a result of its with the formation of boron formate20 (6) via hydrogen gas
proximity to the strongly s-donating aNHC moiety. In the pres- evolution38 (see the video le in the ESI†) through TS3 (Fig. 8).
ence of air, a CO2 molecule is selectively incorporated into the The spatial orientation of TS3 is shown in Fig. 8, in which the
B–H bond36,47 of 2 to form 2a via reactant complex RC1 (Fig. S3†) distance between the two hydrogen atoms is 0.82 A. The free
˚
and transition state TS1 (Fig. 8). During the reaction of 2 with dry energy barrier for this hydrogen evolution reaction was calcu-
air, we observed the capture of CO2 as formate ions36 as lated to be 27.7 kcal molꢁ1. In addition, 6 can be reduced to its
conrmed by the corresponding 13C NMR signal at d ¼ acetal form (H2C(OBBN)2; 7) in the presence of 9-BBN via TS4
168.2 ppm (Fig. S25†). The activation barrier for this elementary (Fig. 8), through reactant complex RC2 (Fig. S3†).20,22 The free
step is much lower (DG‡ ¼ 8.2 kcal molꢁ1). The formation of 2a is energy barrier for this hydroboration is 8.5 kcal molꢁ1 and this
thermodynamically favorable, as this particular step involves particular step is highly exothermic (DG ¼ ꢁ 37.6 kcal molꢁ1).
a heat release of 18.5 kcal molꢁ1. As a result of atmospheric The formation of 7 was conrmed from its characteristic
moisture, 2a is hydrolyzed to 3, which stabilizes the compound as chemical shi (d ¼ 5.34 ppm in deuterated benzene) upon
1
a zwitterion.47 During this process, the 9-BBN fragment is analysis of the reaction mixture by H NMR spectroscopy.20
hydrolyzed to boric acid with the elimination of a cyclooctane Finally, 7 is reduced to the methoxide derivative 8a in the pres-
1
molecule, as conrmed by H NMR analysis (d ¼ 1.55 ppm in ence of 2 and 9-BBN (Fig. 7), through reactant complex RC3
(Fig. S3†) and product complex PC1 (Fig. S2†).20 The formation of
8a is associated with the elimination of 9 and 10 (Fig. S3†), and
the free energy change (DG) for this exothermic elementary step
was determined to be 20.6 kcal molꢁ1
.
In summary, in this work we synthesized an abnormal N-
heterocyclic carbene (aNHC) supported 9-BBN adduct (2) for
CO2 capture from air with subsequent conversion to methox-
yborane or sodium formate under ambient conditions. The
intermediate (aNHC-H, HCOO, B(OH)3; 3) was structurally char-
acterized to unequivocally establish the capture of atmospheric
CO2. It was further shown that CO2 present in low concentrations
(such as in ambient air) could be captured by this compound in
the solid state. This represents the rst-ever demonstration of the
possibility of CO2 capture from air followed by its reduction to
methanol without employing any metal under very mild condi-
tions. A detailed mechanistic pathway for this fascinating trans-
formation was proposed based on tandem experimental and
computational studies.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Fig. 8 Optimized structures of the transition states involved in the
potential energy surface of CO2 reduction to methoxyborane and
sodium formate with 2 in air. Important bond distances (in A˚) are
shown. Only relevant H atoms are provided in the structures.
S. K. M. thanks the IISER Kolkata for nancial support. S. C. S.
thanks the UGC for a research fellowship and Invictus Oncology,
Delhi, for providing a research scientist position. P. K. V. and
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G. V. K. thank the UGC, New Delhi, for a research fellowship. P. K. 24 S. C. Sau, R. Bhattacharjee, P. K. Vardhanapu,
H. thanks the IISER Kolkata for a research fellowship. R. G.
thanks the SERB for a National Postdoctoral Fellowship. R. B.
G. Vijaykumar, A. Datta and S. K. Mandal, Angew. Chem.,
Int. Ed., 2016, 55, 15147–15151.
thanks the CSIR India for a research fellowship, and A. D. thanks 25 T. Wanga and D. W. Stephan, Chem. Commun., 2014, 50,
the INSA, DST and BRNS for partial funding and providing access 7007–7010.
to a CRAY supercomputer. The authors gratefully acknowledge 26 M. Yamashita, K. Goto and T. Kawashima, J. Am. Chem. Soc.,
Dr Tapan Kanti Paine, Dr Bikash Shaw and Mr Dinesh Mullangi 2005, 127, 7294–7295.
for their help in acquiring DTA data and Dr Soumik Mandal for 27 U. R. Pokharel, F. R. Fronczek and A. W. Maverick, Nat.
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