Photocatalytic Reverse Semi-Combustion Driven by Ionic Liquids

Article abstract of DOI:10.1002/cssc.201802974

The simple photolysis of CO₂ in aqueous solutions to generate CO and/or hydrocarbons and derivatives in the presence of a catalyst is considered to be a clean and efficient approach for utilizing CO₂ as a C1 building block. Despite the huge efforts dedicated to this transformation using either semiconductors or homogeneous catalysts, only small improvements of the catalytic activity have been achieved so far. This article reports that simple aqueous solutions of organic salts—denominated as ionic liquids—can efficiently photo-reduce CO₂ to CO without using photosensitizers or sacrificial agents. The system relies on the formation of the [CO₂]^.? intermediate through homolytic C?C bond cleavage in a cation–CO₂ adduct of imidazolium-based ionic liquids (ILs). The system continuously produced CO up to 2.88 mmol g^?1 of IL after 40 h of irradiation by using an aqueous solution of 1-n-butyl-3-methylimidazolium-2-carboxylate (BMIm.CO₂) IL, representing an apparent quantum yield of 3.9 %. The organophotocatalytic principles of our system may help to develop more simple and efficient organic materials for the production of solar fuels from CO₂ under mild conditions, which represents a real alternative to those based on semiconductors and homogeneous metal-based catalysts.

Full text of DOI:10.1002/cssc.201802974

Accepted Article
Title: Photocatalytic Reverse Semi-combustion Driven by Ionic Liquids
Authors: Jairton Dupont, Muhammad Irfan Qadir, Marcileia Zanatta,
Eduarda S Gil, Hubert K Stassen, Paulo Gonçalves, Brenno
A.D Neto, and Paulo E.N de Souza
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
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To be cited as: ChemSusChem 10.1002/cssc.201802974
Link to VoR: http://dx.doi.org/10.1002/cssc.201802974
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ChemSusChem
10.1002/cssc.201802974
FULL PAPER
Photocatalytic Reverse Semi-Combustion Driven by Ionic Liquids
[
a]
[a]
[a]
[a]
[a]
MuhammadꢀI.ꢀQadir, ꢀMarcileiaꢀZanatta, ꢀEduardaꢀS.ꢀGil, ꢀHubertꢀK.ꢀStassen, ꢀPauloꢀGonçalves, ꢀBrennoꢀ
[
b]
[c]
[a]
A.ꢀD.ꢀNeto, ꢀPauloꢀE.ꢀN.ꢀdeꢀSouza ꢀandꢀJairtonꢀDupont*
Abstract: The simple photolysis of CO
2
in aqueous solutions to
drive the simultaneous processes of water oxidation at the
[
5]
generate CO and/or hydrocarbons and derivatives in the presence of
a catalyst is considered a clean and efficient approach for the use of
this gas as a C1 building block. Despite the huge efforts dedicated to
this transformation, either using semiconductors or homogeneous
catalysts, only small improvements on the catalytic activity have
been achieved so far. We report that simple aqueous solutions of
organic salts – denominated ionic liquids – can efficiently photo-
valence band and CO
2
reduction at the conduction band.
activation, side
However, the high-energy barrier for CO
2
reactions such as hydrogen evolution, and high rates of
electron-hole pair recombination of the employed photocatalysts,
mainly based on semi-conductors and hybrids materials, still
[
6, 7]
remain unsolved challenges to be faced.
advances have been achieved to enhance the CO
the catalytic activity and/or selectivity is still very low.
photocatalytic reduction of CO can also be performed with
Although promising
2
conversion,
[
3, 8-10]
reduce the CO
sacrificial agents. The system relies on the formation of the [CO
intermediate; via homolytic C-C bond cleavage in a cation-CO
adduct of imidazolium-based ionic liquids (ILs). The system
2
to CO without the use of photosensitizers or
The
•
-
2
]
2
2
homogenous based metal catalysts employing appropriate
sensitizers and sacrificial electron donors. However, much
remains to be done to generate more durable, simple, recyclable,
-
1
continued producing CO up to 2.88 mmol.g IL after 40 h of
irradiation and represented apparent quantum yield of 3.9%, when
an aqueous solution of 1-n-butyl-3-methylimidazolium-2-carboxylate
[
9, 11-13]
and efficient photocatalytic systems.
Moreover, the photo-
generated CO can be used for the catalytic functionalization of
[
14]
(
BMIm.CO
principles of our system may help to develop more simple and
efficient organic materials to produce solar fuels from CO under
2
)
ionic liquid was used. The organo-photocatalytic
various substrates.
Ionic liquids (ILs, Figure S1) are known to solubilize and, in
some cases, to activate carbon dioxide by stabilizing
2
[
15, 16]
mild conditions which represents a real alternative to those based on
semi-conductors and homogeneous metal based catalysts.
radical/anionic species
attractive material for CO
theoretically, excess of charge localization/holes (generated
and hence, may constitute an
[
17]
2
photoreduction.
Most important,
[
18]
electrochemically or by radiation) may be formed in ILs.
The objective of the work described herein is to develop IL
photocatalysts that would promote the formation of the activated
Introduction
CO
produce
electrochemically.
butyl-3- methylimidazolium-2-carboxylate (BMIm.CO
2
intermediates, which then react with water photoproducts to
CO akin to those pathways induced
We have chosen the zwitterionic IL 1-n-
) to test
Despite a growing number of climate change mitigation policies,
increasing investments associated with the capture and storage
[19]
2
technologies for CO , the anthropogenic emissions of this gas
2
[
1]
are inexorable growing. Hence, there is a growing interest in
finding large-scale commercially viable end-use opportunities for
whether such a route was feasible. The photoactivation of this
zwitterion may provide reactive species such as the imidazolium
the CO
2
utilization. In the last decade, carbon dioxide photo-
radical cation, which has already been detected under radiolytic
reduction to CO and/or hydrocarbon derivatives (artificial
[20]
degradation conditions from imidazolium cations.
In addition,
[
2, 3]
photosynthesis) has grown into a blooming field of research.
•-
2 2
CO radical anion ([CO ] ) could be evoked by the photolysis of
A simple combination of sun light, aqueous solutions saturated
with carbon dioxide and an appropriate photocatalysts may yield
CO (reverse semi-combustion) and/or hydrocarbon derivatives
[21]
this zwitterion in the presence of photosensitizers.
[
4, 5]
(
reverse combustion).
2
The photocatalytic CO conversion
Results and Discussion
over semiconductors employs photo-excited electrons/holes to
We firstly tested the photoreduction of aqueous BMIm.CO
2
(
Scheme 1) under argon atmosphere employing a Xe lamp (300
[a]
Dr. M. I. Qadir, Dr. M. Zanatta, E. S. Gil, Prof. H. K. Stassen, Prof.
Dr. P. Gonçalves, Prof. Dr. J. Dupont
Institute of chemistry
W). The gaseous products were analyzed and quantified by GC
1
13
(Figure 1a), and the aqueous phase by H and C NMR (see
Figures S8-9).
Universidade Federal do Rio Grande do Sul
Av. Bento Gonçalves, 9500 Porto Alegre 91501-970 RS (Brazil)
E-mail: jairton.dupont@ufrgs.br
[
b]
c]
Prof. Dr. B. A. D. Neto, Prof. Dr. P. E. N. de Souza
Institute of chemistry and Physics
University of Brasília
Campus Universitário Darcy Ribeiro, CEP 70904-970, Brasília, DF,
Brazil
[
Prof. Dr. P. E. N. de Souza
Institute of Physics
University of Brasília
Campus Universitário Darcy Ribeiro, CEP 70904-970, Brasília, DF,
Brazil
Supporting information for this article is given via a link at the end of
the document.
Scheme 1. Possible pathways involved in the photolysis of BMIm.CO
during CO photo-reduction.
2
adduct
2
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10.1002/cssc.201802974
FULL PAPER
2 2 2 2
Figure 1. (a) Reaction conditions: BMIm.CO IL (5 mg, 0.027 mmol), CO (atmospheric pressure), H O (3 mL), temp. (25 ˚C), Xe lamp (300 W) and BMIm.CO IL
(
126 mg, 0.69 mmol under argon), the error bars have been obtained from at least three experiments, (b) Experimental and simulated ESR spectra of 1-n-butyl-3-
methylimidazolium radical obtained from the photolysis of BMIm.CO aqueous solution and (c) Photo-catalytic reduction of CO by different ILs. Reaction
conditions: IL (5 mg), CO (atmospheric pressure), H O (3 mL), temp. (25 ˚C), and Xe lamp (300 W).
2
2
2
2
-
1
-1
We detected CO (3.9 µmol.g .h ) and CO
products together with traces of H (Figure 1a) after 5 h of
reaction, and the BMIm cation associated with the carbonate
2
as the only gaseous
presented
a noteworthy drop in its amplitude due to its
2
consumption.
When the frozen solution was once more irradiated (254 nm) for
5 minutes, an increase in the amplitude of the signal related to
the radical cation was observed again (Figure S5). The
2
-
anion (CO
phase (Figures S2 and S3). Performing the same reaction under
CO atmosphere, the same products (gaseous and aqueous)
were observed, but with an increase production rate of CO up to
3
) was identified by NMR experiments of the aqueous
•
-
[22]
2
formation of the [CO
well as the imidazolium cation radical could not be evidenced by
Raman spectroscopy in BMIm-CO aqueous solution submitted
2
]
(estimated half-life time of 10 ns)
as
-
1
-1
5
6 µmol.g .h as shown in Figure 1a. The presence of these
2
compounds in both, the gaseous and the aqueous phase (Figure
S2 and S3), suggests that the photolysis proceeds through the
to the same irradiation conditions applied to the ESR
experiments, indicating that this radical anion is rapidly
•
+
[23]
formation of the BMI radical cation ([BMIm] ) and the CO
2
consumed via paths suggested in Scheme 2.
NHCs (N-
•
-
radical anion ([CO
2
] ), formed by homolytic cleavage of the C2—
IL. The origin of CO from the CO
heterocyclic carbenes) have been already evoked to explain
laser flash photolysis results of zwitterionic carboxylate
is formed and, in the presence of
CO bond in the BMIm.CO
2
2
2
a
•
-
photoreduction was verified by performing the reaction with
derivative, in which [CO
2
]
1
3
13
-
CO
2
and the aqueous solution of BMIm. CO
2
IL that produces
water, affords formate [HCO
2
]
without mentioning the
1
3
[21]
CO detected by the appearance of a signal at m/z=29 in the
corresponding radical cation.
.
GC-MS (Figure S4).
Therefore, the possible reaction pathways involved in the CO
2
•
-
The reaction may proceed via two distinct pathways (Scheme 1):
photoreduction occurs via the formation of [CO
with CO to generate the CO
by addition of another [CO
as observed in electrochemical reductions (Scheme 2).
The imidazolium cation is likely to be regenerated by the
2
]
that reacts
•
-
(
i) the homolytic cleavage of BMIm.CO
2
to generate the radical
2
2
dimer radical anion [C
2
O
4
]
that,
•
-
•-
cation and anion ([CO
2
] ) (Scheme 1(A), photo pathway) or (ii)
2
] , produces the CO and carbonate
[
19]
the heterolytic cleavage leading to the unproductive path
Scheme 1(B) (thermal pathway) with the formation of the N-
•
+
heterocyclic carbene (NHC) and CO
By Electron Spin Resonance (ESR), the possible involvement of
radical species during the photolysis of BMIm.CO (Figure 1b)
was verified and the predicted ESR signal at 3365 G of the
2
.
reaction of [BMIm] with a bicarbonate radical anion that was
observed by ESR from the 1-n-butyl-3-methylimidazolium
2
3
hydrogen carbonate (BMIm.HCO ) salt (Figure S6). The
produced basic carbonate/bicarbonate can then deprotonate the
imidazolium cation forming the NHC-carbene that by interaction
•
+ [20]
imidazolium radical cation [BMIm]
was observed after 15
minutes of irradiation at 254 nm (-196 °C). The simulated and
detected ESR signals present excellent agreement (Figure 1b
with CO
2
regenerates the zwitterion BMIm.CO
2
(Scheme 2). The
and
has been estimated as only 19.6 kcal.mol in water,
the formation of BMIm.HCO by the reaction of BMIm.CO and
activation energy for the equilibrium between BMIm.CO
2
•
+
-1
and S4). Afterward, the frozen [BMIm] was thawed to room
temperature and the aqueous solution was then submitted to a
BMIm.HCO
3
3
2
[
24]
CO
96 °C, Figure S5) in the attempt to observe the reverse
reaction, that is, the CO incorporation restoring the zwitterion
BMIm.CO . The signal corresponding to the radical cation
2
flux for a period of 1 minute followed by another freezing (-
2
H O is relatively fast (20 min).
1
13
1
The H and C NMR spectra of the aqueous phase of the
2
-
2
BMIm.CO
2
IL confirms the generation of CO
3
when the reaction
-
2
was performed under argon (Figures S2 and S3). The HCO
3
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(
Figure 1c). For example, 1-n-butyl-3-methylimidazolium
2 2
diacyanimide (BMIm.N(CN) ) IL photo-reduces the CO to 110
-
1
µmol.g of CO (the ESR signal still observable albeit the low
intensity as shown in Figure S11-b) whereas in 1-n-butyl-3-
methylimidazolium hexafluorophosphate (BMIm.PF
6
)
IL no
production of CO has been detected. This phenomenon may be
correlated with the basicity of the anions, which may play an
important role in the CO
2
sorption, and subsequently formation
of the BMIm.CO zwitterion that drives the generation of CO
2
during the photo-reduction process. Therefore, it is plausible to
assume that the most probable limiting step is the formation of
[
26]
2
the imidazolium-CO adduct that is related to anion basicity.
2
Scheme 2. Possible species involved in the CO photo-reduction driven by ILs
(
for details see text).
2
-
and CO
3
species were detected at the end of the photo-
-
reduction (Figure S2-c). Only HCO
3
was observed within the
first 30 min of irradiation under CO atmosphere (Figure S7) with
2
the production of CO indicating that the bicarbonate is
consumed during the reaction, close to estimated time
necessary to attain the equilibrium for the formation of
[
24]
BMIm.CO
2
from the reaction of BMIm.HCO
3
and water.
Note
that by ESI-MS and NMR analysis of the aqueous solution after
the reaction no other species have been observed.
Moreover, the IL containing the basic acetate anion (BMIm.OAc)
also exhibits significant activity toward the photo-reduction of
Figure 2. Schematic overview of the MMIm.CO
barrier is the releases of CO in the ground state (path B). In the excited state
(path A), the first step is the excitation of the MMIm.CO to S1 and subsequent
ISC with T2 that achieve T1 by IC. In the T1 state occurs the release of [CO
2
photo-dissociation. The lower
-
1
2
CO to CO (134 µmol.g IL, Figure 1c) and the typical ESR
2
signal of the imidazolium radical cation was observed (Figure S8
and Tables S1 and S2). In general, ILs containing basic anions
2
•
-
2
]
.
may generate the imidazolium zwitterion (BMIm.CO
2
) when
interacting with CO2, and possibly photo-induce the reduction of
2
-
13
Quantum mechanical calculations were performed to elucidate
the possible pathways involved in the photolysis of BMIm.CO
CO
2
toward CO. Only CO
3
was detected by C NMR analysis
2
.
in aqueous solution of BMIm.OAc IL after irradiation under CO
2
-
For an adequate description of excited states the multi-
configurational methods CASSCF//CASPT2 were used. These
calculations performed on the simpler 1,3-dimethylimidazolium-
(
Figure S9), which corroborates that bicarbonate (HCO
3
) is
generated and consumed during the reaction. The origin of CO
1
3
2 2
from CO was confirmed by the photo-reduction of CO by an
1
3
2 2
CO adduct (MMIm.CO ) corroborate that path B (Scheme 1)
aqueous solution of the BMIm.OAc IL that yields CO (Figure
S10). Of note that no CO was observed when aqueous solution
of BMIm.OAC IL was irradiated under argon atmosphere.
occurs in the ground state with energetic barriers around 10
kcal/mol, confirming that this path is the major (Figure S12 of
Supporting Information) for CO
2
release. We also observed that
release in the S1 excited state
When the imidazolium cation has the C2 position blocked as is
the case in 1-n-butyl-2,3-dimethyl imidazolium acetate
the reaction coordinate for CO
2
possesses a very high-energy barrier (See Figure S13 of
Supporting Information). However, the excited S1, T1, and T2
states present very similar energies, and thus; we suppose that
(
BMMIm.OAc), the production of CO was slightly less than the
-
1
BMIm.OAc IL during the first 3 h of reaction yielding 132 µmol.g
IL (Figure 1c). In this case, the formation of the zwitterion can
occur only at the less favorable C-4 and C-5 positions of the
•
-
the release of [CO
energy barrier lower than for CO
Figure S14 of Supporting Information). A schematic overview of
the MMIm.CO photo-dissociation is illustrated in Figure 2.
2
] occurs in the triplet state T1, presenting the
[
25]
release in the S1 state (See
imidazolium ring.
However, even in this case, the radical
2
cation was observed by ESR analysis. The radical is localized
closer to one of the N-imidazolium positions (Figure S11-a), and
the expected multiplicity of the observed signal confirms the
formation of the radical cation at C-4 (or C-5).
2
1
3
The S1 state has nπ* character, whereas T1 is of nπ*
character. Hence, intersystem crossing (ISC) between these two
states should be negligible according to the El-Sayed rules.
Other ILs that are less prone to the formation of the zwitterion
BMIm.CO
2 2
are less active as CO photo-reduction catalysts
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2 2 2 2
Figure 4. The recycling test of CO photo-reduction by aqueous solution of BMIm.CO IL. Reaction conditions: BMIm.CO IL (5 mg, 0.027 mmol), CO (1 bar),
2
H O (3 mL), temp. (25 ˚C) and Xe lamp (300 W).
However, the ISC may occur between S1 and T2, which
due to the augmentation of carbonate and bicarbonate in the
aqueous solution corroborating the reaction paths suggested in
3
possesses a ππ* character. This transition is allowed according
-
1
to the El Sayed rules. At least, the T1 state is achieved by
internal conversion (CI) from the T2 state. In the T1 state, the
Scheme 2. The CO productivity attained by the IL (2.88 mmol.g
-
1
-1
IL (72.0 µmol.g .h )) after 40 h is within the same range and
often higher when compared with rather sophisticated semi-
conductors decorated with noble metal nanoparticles that are
usually not recyclable (for comparison see Table S3-4) or with
homogenous metal based catalysts that operate in the presence
•
-
dissociation of [CO
2
]
is evolved, although a considerable
energy barrier must be exceeded (around 28 kcal/mol). The spin
density (Figure 3) in the T1 state is concentrated at the C2 of the
imidazolium ring and the carbon of the CO
homolytic cleavage of the C2-CO bond. Therefore, path A
Scheme 2) is the minor one, but the generated [CO
consumed to yield CO. Finally, there are two major problems
during the photo-reduction of CO , a) absorption of CO and b)
activation of CO . These problems can be solved by the
introduction of imidazolium-based ILs. ILs make imidazolium-
adduct (BMI.CO ), these imidazolate-adduct not only facilitates
CO adsorption but also promotes CO activation by reducing
the potential for the formation of CO
and the overall barrier of Conclusions
the redox reaction, in this case the homolytic cleavage of the Im-
2
moiety, indicating
[
9]
2
of photo-sensitizers and sacrificial electron donors. Note that
the turnover number (TON) for the CO conversion is larger than
•
-
(
2
] is rapidly
2
1 after 40 h, indicating that this photo-reaction can be also
catalytic in the IL. Moreover, the obtained quantum yield of 3.9%
is the same range of those usually observed in photolysis by
semi-conductors (for references see Table S3).
2
2
2
2
2
2
-
•
2
•
-
CO
2
bond generating the reactive [CO
2
] that rapidly yields CO
In summary, simple imidazolium salts associated with basic
anions, such as acetate, furnishing cation-CO adducts can be
(
Scheme 2).
2
photoactivated to generate CO without the use of any
photosensitizers and sacrificial agents. The ESR experiments
clearly verify the involvement of the imidazolium cation radical
indicating that the activation path occurs via homolytic cleavage
of the imidazolium cation-CO
anion that undergoes reduction to CO by the reaction paths akin
to those reported for electrocatalytic CO reductions. Our results
open a new avenue of opportunities for the development of
simpler and more efficient organocatalysts for CO
2 2
adduct to yield the CO radical
2
2
photoreduction when compared to those based on
homogeneous catalysts or semi-conductors. One of the key
points is the formation of cation-CO
2
adducts that can be
photoactivated to generate both, the cation radical and the CO
radical anion.
2
•
+
•-
Figure 3. Spin density of the diradical ([MMIm] + [CO
2
] ) formed in T1 state.
Experimental Section
General. The ionic liquids 1-n-butyl-3-methylimidazolium-2-carboxylate
It was also evaluated the long-time stability of the BMIm.CO
for CO photo-reduction through the cycling test. After each
cycle of five hours, the reactor was degassed with argon (Figure
). Then the reactor was filled with CO and the IL solution was
re-used for the next run. It can be observed that the BMIm.CO
maintains its activity, becoming eventually increased with time
2
IL
[
27]
[28]
(
BMIm.CO
n-butyl-2,3-methylimidazolium
methylimidazolium bicarbonate
methylimidazolium hexafluorophosphate (BMIm.PF
according to literature procedures. Whereas,
methylimidazolium dicyamide (BMIm.N(CN)
2
), 1-n-butyl-3-methylimidazolium acetate (BMIm.OAc), 1-
2
[28]
(BMMIm.OAc),
(BMIm.HCO
1-n-butyl-3-
1-n-ehtyl-3-
3
)
and
29]
[
4
2
6
)
were prepared
2
1-n-butyl-3-
2
) IL was purchased from
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FULL PAPER
Sigma-Aldrich. All the ILs were dried under vacuum and argon for 2 days
gas-tight syringe from the reactor’s headspace in desired time intervals
and analyzed by GC. After the photocatalysis, the liquid phase was
analyzed by NMR. For the evolutions of intermediates by NMR analyses
all the reactions were performed with 120 mg of IL under our standard
conditions.
prior to use. All ESR measurements were performed in
a Bruker
spectrometer (Bruker EMXplus, Germany), equipped with an X-band (9
GHz) high sensitivity cavity (Bruker ER 4119HS, Germany) using frozen
samples inside a quartz finger Dewar filled with liquid nitrogen. 400
microliters of the aqueous ionic liquid solution (0.22 molar for each ionic
liquid) were collected and transferred to a 1 mL de-capped syringe and
frozen in liquid nitrogen. The frozen cylindrical samples were transferred
to a quartz finger Dewar (Noxygen, Germany) filled with liquid nitrogen,
placed inside the resonator and their ESR spectra were recorded at -
Computational details
Quantum mechanical calculations have been carried out on a single 1,3-
2
dimethyl-2-carboxyl-imidazolium zwitterion (MMIm.CO ). The theoretical
1
96 °C. This procedure ensured identical volumes for all samples,
calculations were performed using the MS-CASPT2//CASSCF
allowing the quantitative comparison among the recorded ESR spectra.
The instrumental settings were 2 mW microwave power, 10 G amplitude
modulation, 100 kHz modulation frequency, 1000 G sweep width, 3365 G
central field and 50s sweep time. The peak-to-peak amplitude, that is the
difference between the lowest and the highest amplitudes in the first
derivative spectrum, was used to detect signal quantification. NMR
[30]
protocol. The geometry optimizations of the excited states and ground
states were carried out using the state-average complete active space
self-consistent field level (SA-CASSCF), and for these geometries the
energies were computed using the multi-state complete active space
second-order perturbation theory (MS-CASPT2). The convergence
criterion with respect to the energy change and the norm of the gradient
analyses were performed on
a
Bruker Avance 400 spectrometer,
ꢂꢃ
ꢂꢄ
was the Baker criteria (1.0ꢀꢁ and 3.0ꢀꢁ with respect to the energy
equipped with a BBO 5 mm probe with z- gradient operating at 400 MHz
[31]
change and the norm of the gradient, respectively).
The small ANO
[32]
1
13
for H, and 100 MHz for C. The spectra were obtained at 298 K unless
otherwise specified. Chemical shifts are reported in parts per million
basis set (ANO-S-VDZP) was employed in all calculations carried out
[33]
with the OpenMolcas program.
Excited state geometries and
(
(
ppm, δ) referenced to D
2 6
O and DMSO-d as an external reference
configurations along the reaction path were optimized with an active
space including 14 electrons and 10 orbitals and state-averaged over
four states (SA4-CASSCF(14,10)). The active space has been spanned
by seven occupied (six bonding π orbitals and a non-bonding orbital at
the oxygens) and three virtual orbitals of π * type. This active space was
maintained during the subsequent geometry optimizations. All the orbitals
included in the active space are shown in Figure S20. For the correction
capillary). The UV-Vis spectra of the ILs are presented in Figure S15.
1
13
The NMR spectra ( H and C) before and after photolysis using
BMMIm.OAc and BMIm.N(CN) are presented in Figures S16 and S17,
2
respectively. The generated gases during the photocatalytic reaction
were quantified by gas chromatography (GC) using an Agilent 6820
equipped with a Porapak Q 80-100 Mesh column and argon as carrier
gas. The gaseous products were simultaneously analyzed with a thermal
conductivity detector (TCD) and a flame ionization detector (FID, Figure
S18). Aliquots of 100 μL from the gas phase were removed from the
head of the photo-reactor reactor in desired time intervals and injected
with a syringe containing a Hamilton sample lock valve. In order to detect
of energy of the states, the CASPT2 was averaged over four states
[34]
(
MS4-CASPT2). Orbitals have been rendered by the IboView program.
1
3
the generated CO, a mass spectrometer (QIC 20®-Hiden Analytical)
configured with the ionization of 70 eV was used.
Acknowledgements
The authors are thankful to CAPES (158804/2017-01),
FAPERGS (16/2552-0000), and CNPq-INCT (465454/2014-3
and 465259/2014-6) for financial support and J. D. Alves
1
3
2
Preparation of BMIm. CO ionic liquid
1
3
[27]
2
BMIm. CO IL was prepared according to the reported literature.
(
University of Nottingham) for helpful discussions.
Typically, 1-n-butyl-3-methyl imidazolium chloride (6.32 g, 0.036 mol)
was dissolved in dry DMF (25 mL). In a stainless-steel reactor (100 mL)
containing glass (Parr Series 5000 Multiple Reactor System), 3.83 g
Keywords: Carbon dioxide • Ionic liquids • photocatalysis •
(
0.036 mol) of dry Na
2
CO
3
was added under argon atmosphere. After
carbon monoxide • solar fuels
1
3
closing, the reactor was filled with CO
2
(10 bar) and the reaction
mixture was heated to 110 ° C for 36 h. The reactor was cooled and
depressurized. The solution was filtered and the solvent was evaporated
under reduced pressure. Recrystallization from acetonitrile produced 4.7
g of a light-yellow solid, which was characterized by NMR (Figure S19)
as a mixture of 1-n-butyl-3-methylimidazolium-2-carboxylate, 1-n-butyl-3-
methylimidazolium-4-carboxylate and 1-n-butyl-3-methylimidazolium-5-
carboxylate.
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Simple aqueous solutions of
ionic liquids can efficiently
Muhammad I. Qadir, Marcileia
Zanatta, Eduarda S. Gil,
Hubert K. Stassen, Paulo
Gonçalves, Brenno A. D.
Neto, Paulo E. N. de Souza
and Jairton Dupont*
photo-reduce the CO
without the use of
2
to CO
photosensitizers or sacrificial
agents. The system relies on
•
-
2
the formation of the [CO ]
intermediate; via homolytic
C-C bond cleavage in a
Page No. – Page No.
Photocatalytic Reverse
Semi-Combustion Driven by
Ionic Liquids
2
cation-CO adduct of
imidazolium-based ionic
liquids (ILs).
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