Introduction

Conversion of carbon dioxide (a main component of natural photosynthesis) as a renewable C1 feedstock to value-added compounds (e.g., methane, methanol, carbon monoxide and sugar) has attracted considerable attention due to its significance in chemical industry, geopolitics and carbon recycling within the ecosystem1,2,3,4,5,6. In nature, the capture, concentration and conversion of atmospheric CO2 is realized by metalloenzymes in photosynthetic organisms such as plants, algae and cyanobacteria that convert CO2, water and solar energy to sugars for the plant and oxygen for Earth's atmosphere. Usually, artificial conversion of extremely-inert CO2 require its catalytic activation by transition-metal catalysts with multiple redox states and subsequently integrating to reduction reactions via multi-electron transfer coupled with protons to avoid high energy intermediates.

Recent development in the field of C1 chemistry involves the emergent applications of metal-free organocatalysts, such as frustrated Lewis pairs (FLPs), carbenes, bicyclic amidines and ionic liquids (ILs) as chemical coordination substrates for the binding and activation of CO2 at room temperature and atmospheric pressure(Fig. 1)7,8,9,10. For example, FLPs were illustrated to catalyze CO2 reduction to methanol and methane11. N-heterocyclic carbene (NHC) converts CO2 to CH3OH via formation of zwitterionic NHĊCO2 adducts as key intermediates in the reductive deoxygenation of CO2 with diphenylsilane as a stoichiometic reductant12. Very recently, Rosen et al. has demonstrated the promoted electrochemical reduction of CO2 to CO at overpotential of only 0.17 V by using 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid as the CO2 coordinating substrates in water13.

Figure 1
figure 1

Metal-free organocatalysts for CO2 activation by coordinating with frustrated Lewis pair (a), N-heterocyclic carbene (b), bicyclic amidine (c) and ionic liquid (d).

Full size image

ILs are room temperature molten salts, formed by the weak combination of a large organic ion and a charge-delocalized inorganic/organic anion, with versatile structural and functional variations14. The scientific and technological importance of ionic liquids is their wide applications in lubricants, electrolytes, catalysts and as gas capturer15,16,17. Of particular interest is the promising application of ILs as green solvents with a number of important properties, such as negligible volatility, high stability, high ionic conductivity, high polarity and solubility with many compounds18,19,20,21,22.

Room temperature ILs (RTILs) represent a highly versatile and tunable platform for the development of reversible CO2 capture systems with high adsorption capacities. Amine-functionalized task-specific ILs (TSILs) have been demonstrated to show a gravimetric capability of 7% (0.5 mol CO2 per mol of the TSIL) for CO2 capture at ambient pressure, but with extreme viscosity that limits kinetics23. In addition, the synthesis of amine-functionalized TSILs requires several synthetic and purification steps. Recently, Dai's group developed basic and superbase-derived ILs that showed promise in rapid and switchable CO2 capture with an equimolar absorption capability24. The combination of ILs with alkanolamines has also been developed as a viable approach to achieve high levels of reversible CO2 capture in IL solvents25. Interestingly, in the above-mentioned carbon capture systems, the formation of carbamates is evident by binding of CO2 either to the amines tethered to cations (or anions) of ILs or to the amino groups. This way is similar to the first step of plant photosynthetic cycle, in which the CO2 molecule is initially bonded to nitrogen atoms, making reactive carbamate intermediates in the biology system26. We are therefore inspired to link the unique coordination chemistry of ILs to artificial photosynthesis for promoting carbon fixation.

Although the uses of ILs to promote CO2 capture and conversion are already evident and has been proposed for future carbon photo-fixation in high pressure biphasic ILs-CO2 (liquid) systems13,23,24,25,27,28, the merging of IL chemistry with photoredox organocatalysis to achieve gas CO2 fixation with visible light under ambient conditions is rarely covered. Herein, we delineate the application of ILs to facilitate CO2 capture under ambient conditions and then integrating to a classic photoredox catalytic cycle for efficient CO2 conversion to CO29. This new, IL-promoted CO2 photoconversion protocol was first examined in a photochemical tandem system that contains a catalyst combination of [Ru(bpy)3]Cl2(bpy = 2,2′-bipyridine) and CoCl2·6H2O as a light sensitizer and an electron mediator, respectively, along with triethanolamine (TEOA) as an electron donor and a visible light source. The system cooperatively works with ILs to accelerate CO2 photochemical reduction in various solvents at mild conditions.

Results

First, 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) as a RTIL was introduced into the photocatalytic system that splits CO2 to CO and [O] in aqueous medium, accompanying by the generation of H2 from the oxidative dehydrogenation of TEOA. Part of the generated H2 (or 2H+ + 2e−) accepted the [O] to form H2O and thus closing the two-electron two proton reduction cycle. The sum reaction is CO2 + 2H+ + 2e−→ CO + H2O. A number of reference experiments were designed to emphasize the role of ILs in the reactions and to optimize the reaction conditions as well. Results are summarized in Table 1. In dark, there is no gas detected in the system. However, when the system was illuminated with visible light, the very stable CO2 molecules were photocatalytically converted into a more reactive CO species (entry 1, table 1) at a reaction rate of 15.5 μmol/h, while releasing H2 at a rate of 2.1 μmol/h. These experiments provide strong evidence of the participation of the dye excited state in the catalytic cycle. The mixture of CO and H2 is the main component of syngas, an important carbon feedstock in chemical industry to make synthetically valuable chemicals (e.g. diesel, methane, methanol and dimethyl ether)30.

Table 1 Study of reaction conditions[a]
Full size table

Control experiments showed that no reaction occurred in the absence of either [Ru(bpy)3]Cl2 or TEOA or [EMIM][BF4] (entry 4–6, table 1). To rule out the potential promotional effect of the BF4− anion (note that CO2 is known to form weak complexes with BF4− anion), HBF4 was used to replace [EMIM][BF4] and it was found that neither CO nor H2 could be generated (entry 8, table 1). This is a strong indication that it is the imidazolium cation that significantly influences the reaction process of the CO2 to CO conversion. When Co2+ as an effective electron transport carrier was absent, as expected, this reaction was restrained dramatically, only yielding a small amount of CO and H2 (entry 7, table 1).

Once CO2 was replaced by N2 in the system, evidence was observed that confirmed the participation of CO2 in the reaction, because only H2 gas was detected under the reaction conditions and no CO was found. Evidently, in the absence of CO2, light-induced electrons reduce proton to produce H2. However, in the presence of CO2 captured by the [EMIM][BF4]-TEOA system (termed as *CO2 to differentiate from inert CO2), the overall efficiency of the photochemical reduction process increased greatly. Clearly, the photoinduced electrons are kinetically favorable for the reduction of *CO2 over protons under the experimental conditions. As proposed by Bockris and coworkers, a high over-potential is typically needed to convert CO2 since the first step in CO2 conversion is the formation of a "CO2−" intermediate, with a very negative formation potential in water and in most organic solvents31. [EMIM][BF4] has been reported to greatly lower the free energy of the formation of the CO2− via complexation (with EMIM+), reducing the overall barrier to the reaction13,32. Thus, the IL is favorable not only for CO2 capture but for CO2 activation, acting as a cocatalyst that reduces the potential for formation of the CO2− species.

The role of the [EMIM][BF4]/H2O ratio on the reduction of CO2 was investigated, too. As displayed in the inset of Fig. 2, the production of both CO and H2 increased with increasing ratio of [EMIM][BF4]/H2O which proved that a high concentration of IL is an important role in the enhancement of photocatalytic activities for CO2 reduction. However, a dramatic decrease in the activity for the CO and H2 productions was observed when H2O was removed. Clearly, the reaction is sluggish to start without H2O, as both electrons and holes prefer charged reaction partners. This result indicates that there is a mutual action of IL and H2O, which plays a critical role on the improvement of photocatalytic activity towards CO2 reduction.

Figure 2
figure 2

The promotional effect of ILs on CO2 photofixation in various solvents.

(DMF: N,N-dimethylformamide. MeCN: acetonitrile. THF: tetrahydrofufan. BTF: benzenyltrifluoride). The inset is the effect of [EMIM][BF4]/H2O ratio on the photocatalytic reduction of CO2.

Full size image

The IL-promoted CO2 photoreduction system was then carried out in various reaction media to determine the generality of the promotional effect of ILs and also to search for a favorable IL-coupler as reaction medium for efficient CO2 photofixation. Various solvents (e.g. DMF, MeCN, THF and BTF) were applied in the CO2 reduction system. The corresponding results are shown in Fig. 2 and Fig. S1. Remarkably, upon adding [EMIM][BF4], all solvent systems displayed enhanced photocatalytic reactivity towards CO2 reduction, to various extends. In the IL-MeCN mixture, the CO evolution rate (CER) and the H2 evolution rate (HER) reached 26.3 and 2.1 μmol/h, respectively. Notably, in contrast to the generation of H2, the yield of CO improved much more after the involvement of IL, again reflecting the promoted kinetics of IL for CO2 activation and conversion. Addition of IL was therefore substantially altering selectivity of system towards CO.

As the ultimate objective of artificial photosynthesis is to link CO2 fixation with water splitting, the carbon fixation is desirable to perform in a reaction medium that contains water. The effect of H2O on the IL-MeCN system was therefore explored. Results revealed that addition of 15% H2O can significantly increase the CER from 26.3 to 64.0 μmol/h (TOF = 6.4/h), whereas the best addition amount of water for promoting HER is 8% (Fig. S2). The addition of water has been reported to have strong effect on the photolabilisation of a bpy ligand from [Ru(bpy)3]2+ to generate the active catalytic species33,34. Additionally, the reaction rate especially CER agrees with the slightly-enhanced conductivities of water containing media (Table S1). Higher conductivity of electrolyte improves the ability of the fluid environment to support the electron transport during the reduction procedure, which may contribute in part to the enhanced activity. The overall apparent quantum yield of this optimized IL-promoted CO2 photoreduction system was estimated to be 12.3% under the monochromatic irradiation at λ = 420 nm.

Studies on the CO/H2 evolution as a function of reaction time showed that the relationship between the amount of CO/H2 produced and the reaction time was non-linear (Fig. 3). After 4 h illumination, the total production of CO and H2 reached a maximum value and thereafter increased slightly. This is consistent with the inherently unstable nature of Ru-based dyes after several turnover numbers in photochemical applications33,34. It is therefore encouraged to use significantly more durable super-molecular and polymeric semiconductors as light energy transducers to couple with ILs for CO2 reduction. Work along this line is in progress in our lab.

Figure 3
figure 3

The amount of CO and H2 produced from the IL-promoted CO2 conversion system as a function of reaction time under visible light illumination.

Full size image

Discussion

The wavelength dependence of CO evolution revealed that the trend of CO production matched well with the optical absorption spectrum of the antenna molecule (Fig. 4). This investigation provides an extra confirmation that the CO2 reduction process relied on charge photogeneration, separation and the subsequent tandem electron transfer.

Figure 4
figure 4

The dependence of the wavelength of incident light on the gas evolution rate of CO and H2 from the CO2 photoreduction system in [EMIM][BF4]-H2O medium.

Full size image

Various ILs (shown in Fig. 5) were applied in the reaction system to gain further insights into the effect of the counterions and the substituents of the organic components on the photochemical reduction of CO2. As elucidated in Table 2, the efficiency of CO2 photoreduction was modulated strongly by the different type of counterions examined. The Tf2N− anion showed the best promotional effect in the photocatalytic production of CO and H2 than other anions such as, L-L−, TfO−, Ac−, DCA− and BF4−. The high structural symmetry of Tf2N− anion is known to endow the system with a low viscosity that is favorable for reaction kinetics35. Both fluoroalkyl group and Tf2N− with a large ionic size are known to increase the interaction of the ILs and CO236. When the substituted alkyl chain on 1-position of the imidazolium ring was extended from ethyl to octyl, the activities for the production of CO decreased sequentially. When the H in the C(2) position of 7 was substituted with methyl group 10, the activity decreased too. Obviously, the decreased yields were due to the increased length of the carbon chain, associating with the increase in van der Waals interactions and molecular weight. These cause an increased viscosity of the solvent, which together with the increased steric hindrance limits catalytic kinetics, leading to low reactivities36. These observations underline the fact that the promotional effect of ILs in the photochemical reduction of CO2 are closely related to both the counterions and the organic functional groups on ILs, which determine the properties of ILs such as viscosity, conductivity, polarity, dielectric constant and acid-base chemistry. Therefore, the wide availability of different cations and anions and their almost unlimited combination has led to a choice of ionic liquids that have a potential synergism between the cation and the anion for supporting CO2 photochemical reduction.

Table 2 The effect of various ILs on the photocatalytic conversion of CO2 to CO under mild conditions
Full size table
Figure 5
figure 5

Chemical structures of various ionic liquids corresponded to those in Table 2 for the photocatalytic conversion of CO2 to CO under mild conditions.

Full size image

In summary, a combination of CO2 binding and activation by ILs with photoredox catalysis has been developed to achieve the conversion of CO2 to CO at mild, environmental conditions. The promotional effect of ILs in CO2 photochemcial reduction is strongly related to the chemical properties of counterions and the organic functional groups on the imidazolium cation. IL chemistry thereby continues to facilitate new strategies for the creation of valuable chemicals from CO2 by organocatalysis12, here more specifically photoredox organocatalysis. This new photochemical cascade reaction offers a new protocol to split and convert chemically inert CO2, by the extension of IL chemistry to artificial photosynthesis.

Methods

Chemicals

All the solvents including N,N-dimethylformamide (DMF, anhydrous, 99.8%), Tetrahydrofufan (THF, anhydrous, 99.9% ), Acetonitrile (MeCN, anhydrous, 99.8%) and Benzenyltrifluoride (BTF, anhydrous, ≥99%) are purchased from China Sinopharm Chemical reagent Co. and stored over molecular sieve, which were used directly without further purification. Tris(2,2′-bipyridyl)ruthenium(II) chloride hexahydrate (TBR, >98.0%) purchased from Tokyo Chemical Industry Co. Ionic liquids are also commercially supplied (≥98% Shyfhx Co.). Other reagents used were analysis grades without further purification.

CO2 photoreduction studies

All experiments were performed in a Schlenk flask (80 ml) under an atmosphere pressure of CO2 (1 atm). In the Schlenk flask, Tris (2, 2′-bipyridyl) ruthenium(II) chloride hexahydrate (10 μmol), CoCl2·6H2O (1 μmol) were dissolved in 6 ml mixture of solvent/ILs/TEOA (3:1:1 by volume). This mixture system was subjected to vacuum degassing and backfilling with pure CO2 gas. This process is repeated (three times) and after the last cycle the flask is backfilled with CO2. Then the system was irradiated with a non-focus 300 W Xe-lamp with a 420 nm cut-off filter under vigorous stirring at 30°C as controlled by a water-cooling system. The produced gases (CO, H2) were detected using gas chromatography (Shimadzu 8A) equipped with a packed molecular sieve column (TDX-1 mesh 42/10). Ar was used as the carrier gas of the GC.

IR characterization

The measured solution was obtained by saturating [EMIM][BF4] with CO2. IR spectra were measured by sandwiching a thin film of the solution between two CaF2 substrates by a Nicolet Magna 670 FT-IR spectrometer. Pure [EMIM][BF4] is used as reference sample.

Conductivity measurements

The conductivity was determined with a conductivity meter produced by Shanghai Precision & Scientific Instrument Co. (DDSJ-318) that is equipped with DJS-1C conducting electrodes. The instrument was calibrated with KCl solution. In the experiment, the solution and the electrode were sealed in a glass tube, which was placed in a constant temperature water bath (298±0.05 K). Each measurement was repeated three times and the average values were calculated.

Quantum yield measurements

The apparent quantum yield (AQY) for CO/H2 generation was measured using the same photochemical experimental setup, but with a LED lamp (low-power 420 nm-LED, 3 W, Shenzhen LAMPLIC Science Co. China) as a light source. The intensity of LED irradiation was measured as 20.6 mW/cm2 (Newport 842-PE) and the irradiated area was controlled at 1.0 cm2.