Electrocatalytic Processes for the Valorization of CO 2 : Synthesis of Cyanobenzoic Acid Using Eco-Friendly Strategies

: Carbon dioxide (CO 2 ) is a known greenhouse gas, and is the most important contributor to global warming. Therefore, one of the main challenges is to either eliminate or reuse it through the synthesis of value-added products, such as carboxylated derivatives. One of the most promising approaches for activating, capturing, and valorizing CO 2 is the use of electrochemical techniques. In the current manuscript, we described an electrocarboxylation route for synthesizing 4-cyanobenzoic acid by valorizing CO 2 through the synergistic use of electrochemical techniques (“green technology”) and ionic liquids (ILs) (“green solvents”)—two of the major entries in the general green chemistry tool kit. Moreover, the use of silver cathodes and ILs enabled the electrochemical potential applied to be reduced by more than 0.4 V. The “green” synthesis of those derivatives would provide a suitable environmentally friendly process for the design of plasticizers based on phthalate derivatives. mechanism proposal for the electrochemical reduction of 1 for both media. CO 2 atmosphere using silver as a cathode.


Introduction
Fossil fuels in the form of coal, oil, and natural gas account for 80% of the world's energy use and have caused increases in the Greenhouse Gas (GHG) concentrations of the atmosphere. These have led to global warming, climate change, and ozone layer depletion, with destructive impacts on human society and the economy. In the last four decades, despite global attempts to mitigate emissions, the world saw more than 100% growth in annual CO 2 emissions, which surpassed 32 billion tons in 2011. These global CO 2 emissions will continue to increase and are projected to reach 36 billion tons in 2020, and double that by 2050 if appropriate climate change mitigation measures are not put in place [1][2][3]. In this sense, different research strategies are currently being developed for the activating, capturing, and valorizing of CO 2 . Hence, CO 2 uses are generally classified into different categories, such as direct use and/or conversion to chemicals and energy [4][5][6][7][8][9][10][11].
Carboxylic acids and esters are important classes of chemicals that are widely found in pharmaceuticals, polymers, agrochemicals, natural products, and biological systems, and they have been widely applied as versatile building blocks in organic synthesis [12][13][14]. For instance, the utilization of CO 2 as a C1 synthon for incorporation with lithium phenolate, or Grignard reagents for the synthesis of carboxylic acids have been well known for more than a century. On the other hand, metal-catalyzed direct insertion of CO 2 into different compounds is another valuable method for the preparation of carboxylic acids with high selectivity. This approach is widely used to obtain carboxylic analogues from carbon-halide, carbon-boron, carbon-oxygen, aromatic and alkyl reagents, etc. [15][16][17][18][19][20][21][22]. Moreover, the development of new methods that take advantage of the abundant and inexpensive CO 2 without catalysts (carbon monoxide, phenols, and others), for the transformation of aryl halides into their corresponding aryl carboxylic acid provides an attractive option for their assembly [23,24].
In this sense, the CO 2 valorization by electrochemical routes is receiving increasing attention as a way to obtain chemicals with added value, and as a promising option to chemically store renewable energy from intermittent sources like solar or wind, thus reducing our reliance on fossil fuels. Although the benefits of using electrochemical approaches to perform classic chemical reactions in the design of greener and more suitable processes are well established, the main drawbacks of performing electrochemical reactions are the use of non-volatile organic polar compounds, which are well-known hazardous substances, as well as the use of supporting electrolytes in high concentration [25][26][27][28][29]. In this sense, the replacement of electrolytes based on an organic solvent with ionic liquids (ILs), which are considered "green solvents" would solve this problem. Ionic liquids (ILs) are a family of solvents with unique properties that have led to their consideration as interesting alternatives and more effective solvents in many applications, including electrochemistry [30][31][32] and CO 2 storage and capture [33][34][35].
Therefore, the growing interest in the electrochemical valorization of CO 2 has resulted in different innovative attempts, including the use of ILs, in order to improve the performance of these electrochemical approaches.
In the present work, a description is presented of an electrocarboxylation route for synthesizing 4-cyanobenzoic acid by valorizing CO 2 through the synergistic use of electrochemical techniques and ILs (two of the major entries in the general green chemistry tool kit). The "green" synthesis of 4-cyanobenzoic acid is produced through the electrochemical cleavage of a carbon-iodide bond and the subsequent capture of CO 2 (Scheme 1). It was decided to perform the electrocarboxylation process from 4-iodobenzonitrile, since less energy is required to break it down than the C-F bond, the C-Cl bond, and even the C-Br bond. Moreover, electrocatalytic effects related to the nature of the cathodes and the use of ILs will also be investigated for reducing the electrochemical potential needed. The optimization of electrocarboxylation conditions would open a suitable environmentally friendly process for obtaining 4-cyanobenzoic derivatives, which can be potentially useful for the designing of "green" plasticizers based on phthalate derivatives [12]. Scheme 1. Electrocarboxylation of 4-iodobenzonitrile.

Results and Discussion
To establish the optimal experimental conditions for the optimal electrochemical capture of CO 2 , the electrochemical behavior of 4-iodobenzonitrile (1) was studied in different solvents and electrodes under an inert atmosphere.

Electrochemical Reduction of 4-Iodobenzonitrile under an Inert Atmosphere
Electrochemical Reduction Mechanism of 4-Iodobenzonitrile on Carbon and Silver Cathodes Cyclic voltammograms (CVs) of a 10 mL solution of 1 in dimethylformamide (DMF) using 0.10 M of tetrabutylammonium tetrafluoroborate (TBA BF 4 ) were recorded at different scan rates (from 0.10 a 1.0 V s −1 ) using glassy carbon (GC, dotted line) and Ag (solid lines) as working electrodes under a N 2 atmosphere, and are depicted in Figure 1. The same general trend was observed in both cases, a first two-electron irreversible wave followed by a second reversible one-electron wave. This second electron transfer corresponded to the electrochemical reduction of benzonitrile, which is in agreement with previous studies reported in the literature under similar experimental conditions [36]. A closer look at the CVs revealed that the reduction potential value of 1 was reduced by at least 0.46 V (from −1.83 V to −1.37 V), when Ag was used as cathode material instead of GC. The reduction of the over-potential showed a well-defined electrochemical signal ( Figure 1). Hence, the use of silver as a cathode introduced electrocatalytic properties for the current reduction processes performed in an aprotic organic electrolyte, such as DMF + 0.10 M TBA BF 4 . electron transfer corresponded to the electrochemical reduction of benzonitrile, which is in agreement with previous studies reported in the literature under similar experimental conditions [36]. A closer look at the CVs revealed that the reduction potential value of 1 was reduced by at least 0.46 V (from −1.83 V to −1.37 V), when Ag was used as cathode material instead of GC. The reduction of the overpotential showed a well-defined electrochemical signal ( Figure 1). Hence, the use of silver as a cathode introduced electrocatalytic properties for the current reduction processes performed in an aprotic organic electrolyte, such as DMF + 0.10 M TBA BF4. The same electrocatalytic effect was observed when DMF was replaced by ionic liquids (ILs). Thus, when 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM]TFSI) and 1methyl-1-propylpiperidinium bis(trifluoromethylsulphonyl)-imide ([PP13]TFSI) were used as electrolytes, the reduction potential value of 1 was reduced by approximately (ca.) 0.5 V ( Figure 2). Note that ILs can also act as a co-catalyst when silver is used as a working electrode, since the reduction potential value of 1 is ca. 0.1 V lower compared to DMF + 0.10 M TBABF4 (Table 1). Hence, the solvation process of 1 − appeared to be more effective in the case of using a high concentration of cations (ILs) than tetrabutylammonium salts at low concentrations (0.1 M TBABF4 in DMF). Besides, it seemed that the presence of pyrrolidinium cations in the IL composition led to stronger coulombic interactions, which rendered better solvation of 1 − . The same electrocatalytic effect was observed when DMF was replaced by ionic liquids (ILs). Thus, when 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM]TFSI) and 1-methyl-1-propylpiperidinium bis(trifluoromethylsulphonyl)-imide ([PP13]TFSI) were used as electrolytes, the reduction potential value of 1 was reduced by approximately (ca.) 0.5 V ( Figure 2). Note that ILs can also act as a co-catalyst when silver is used as a working electrode, since the reduction potential value of 1 is ca. 0.1 V lower compared to DMF + 0.10 M TBABF 4 (Table 1). Hence, the solvation process of 1 − appeared to be more effective in the case of using a high concentration of cations (ILs) than tetrabutylammonium salts at low concentrations (0.1 M TBABF4 in DMF). Besides, it seemed that the presence of pyrrolidinium cations in the IL composition led to stronger coulombic interactions, which rendered better solvation of 1 − . Table 1 summarizes the electrochemical data obtained for the electrochemical reduction of 1 using several solvents and cathode materials. In all the cases a fast, irreversible, two-electron reduction wave was observed, whereas the electron transfer seemed to be faster in silver than in glassy carbon, since the peak width value was reduced from ca. 100 mV (slow electron transfer) to ca. 60 mV (fast electron transfer) [37][38][39][40].
To fully establish the electrochemical reduction mechanism of 1 in ionic liquids, a controlled potential electrolysis after the first reduction wave (at a potential value of ca. 0.1 V more negative than the E pc ) was performed under an inert atmosphere using either carbon graphite or silver cathodes. The analysis of the sample after the electrolysis showed that, in all the cases, benzonitrile, 2, was the only product formed after the passage of 2F (C mol −1 ) ( Figure 3). The electrochemical processes were monitored by cyclic voltammetry.  Table 1 summarizes the electrochemical data obtained for the electrochemical reduction of 1 using several solvents and cathode materials. In all the cases a fast, irreversible, two-electron reduction wave was observed, whereas the electron transfer seemed to be faster in silver than in glassy carbon, since the peak width value was reduced from ca. 100 mV (slow electron transfer) to ca. 60 mV (fast electron transfer) [37][38][39][40].  To fully establish the electrochemical reduction mechanism of 1 in ionic liquids, a controlled potential electrolysis after the first reduction wave (at a potential value of ca. 0.1 V more negative than the Epc) was performed under an inert atmosphere using either carbon graphite or silver cathodes. The analysis of the sample after the electrolysis showed that, in all the cases, benzonitrile, 2, was the only product formed after the passage of 2F (C mol −1 ) (Figure 3). The electrochemical processes were monitored by cyclic voltammetry.    Taking into account the cyclic voltammetry and controlled potential electrolysis experiments, it is possible to conclude that 1 followed the Electron transfer, Chemical reaction, Electron transfer (ECE) mechanism. Hence, in a first electrochemical step (E) the radical anion, 1 − , was generated. A chemical reaction (C) coupled to this first electron transfer led to a benzonitrile radical and an iodide anion through a C-I bond cleavage reaction. Finally, the radical was reduced to an anion at the Taking into account the cyclic voltammetry and controlled potential electrolysis experiments, it is possible to conclude that 1 followed the Electron transfer, Chemical reaction, Electron transfer (ECE) mechanism. Hence, in a first electrochemical step (E) the radical anion, 1 − , was generated. A chemical reaction (C) coupled to this first electron transfer led to a benzonitrile radical and an iodide anion through a C-I bond cleavage reaction. Finally, the radical was reduced to an anion at the electrode surface (E), which later protonated by reacting with the solvent (Scheme 2). Note that the electrochemical generation of benzonitrile anion, which acts as a nucleophile leading to benzonitrile, opened the door to incorporate CO 2 via nucleophile-electrophile reaction through the electrochemical reduction of 1. Taking into account the cyclic voltammetry and controlled potential electrolysis experiments, it is possible to conclude that 1 followed the Electron transfer, Chemical reaction, Electron transfer (ECE) mechanism. Hence, in a first electrochemical step (E) the radical anion, 1 − , was generated. A chemical reaction (C) coupled to this first electron transfer led to a benzonitrile radical and an iodide anion through a C-I bond cleavage reaction. Finally, the radical was reduced to an anion at the electrode surface (E), which later protonated by reacting with the solvent (Scheme 2). Note that the electrochemical generation of benzonitrile anion, which acts as a nucleophile leading to benzonitrile, opened the door to incorporate CO2 via nucleophile-electrophile reaction through the electrochemical reduction of 1. Scheme 2. ECE mechanism proposal for the electrochemical reduction of 1 for both media. Scheme 2. ECE mechanism proposal for the electrochemical reduction of 1 for both media.

Electrochemical Reduction of 4-Iodobenzonitrile under a Carbon Dioxide (CO 2 ) Atmosphere
Electrochemical Reduction Mechanism of 4-Iodobenzonitrile on Carbon and Silver Cathodes 4-Iodobenzonitrile (1) showed a different electrochemical behavior at GC and silver cathodes in DMF under a CO 2 atmosphere. When GC was used, two reduction peaks were observed at −1.83 V and −2.35 V, the first one corresponding to the electrochemical reduction of 1, and the second one to the electrochemical reduction of benzonitrile (Figure 4a). It is worth noting that in the presence of CO 2 there was an increase in the peak current value of this second reduction peak and a loss of reversibility, which indicated that the benzonitrile was acting as an organic mediator for the reduction of CO 2 through a homogeneous indirect catalytic process [41,42]. In the corresponding anodic counter scan, a new oxidation peak was observed at 0.45 V, which corresponded to the oxidation of the oxalate anion generated in the CO 2 reduction process under a saturated CO 2 atmosphere [43] (Scheme 3). Note that the peaks at higher oxidation potential values corresponded to the oxidation of iodide and triiodide, respectively. Due to the low oxidation potential value of silver, it was not possible to record any anodic counter scan so as to detect the nature of the anionic products formed.
anodic counter scan, a new oxidation peak was observed at 0.45 V, which corresponded to the oxidation of the oxalate anion generated in the CO2 reduction process under a saturated CO2 atmosphere [43] (Scheme 3). Note that the peaks at higher oxidation potential values corresponded to the oxidation of iodide and triiodide, respectively. Due to the low oxidation potential value of silver, it was not possible to record any anodic counter scan so as to detect the nature of the anionic products formed.   oxidation of the oxalate anion generated in the CO2 reduction process under a saturated CO2 atmosphere [43] (Scheme 3). Note that the peaks at higher oxidation potential values corresponded to the oxidation of iodide and triiodide, respectively. Due to the low oxidation potential value of silver, it was not possible to record any anodic counter scan so as to detect the nature of the anionic products formed.   The CV of 1 at the silver cathode under a CO 2 atmosphere in DMF showed three reduction peaks at −2.23 V, −2.09, and −2.40 V vs. SCE. The first and third peaks corresponded to the electrochemical reduction of 1 and benzonitrile, respectively. The second peak corresponded to the electrochemical reduction of CO 2 . This assignment of the second reduction peak to the electrochemical reduction of CO 2 can be confirmed since the peak current value was directly linked to the CO 2 concentration, as it can be seen in Figure 4b. In the case of using silver as a working electrode, the electrochemical processes and their associated reactivity were easily controlled by the applied potential. Note that in this case, since the electrochemical reduction of CO 2 occurred prior to the benzonitrile reduction, no electrocatalytic processes were observed related to the role of benzonitrile as a redox mediator (Scheme 4).
The same general electrochemical trend was observed when DMF was replaced by [PP13]TFSI under a N 2 and a CO 2 atmosphere at the GC and silver electrodes. However, when using [EMIM]TFSI, the electrochemical reduction of benzonitrile was not detected in any of the cases due to the fact that the reduction of the imidazolium cation appeared at less negative potentials either in inert or CO 2 atmospheres [44].
CO2 can be confirmed since the peak current value was directly linked to the CO2 concentration, as it can be seen in Figure 4b. In the case of using silver as a working electrode, the electrochemical processes and their associated reactivity were easily controlled by the applied potential. Note that in this case, since the electrochemical reduction of CO2 occurred prior to the benzonitrile reduction, no electrocatalytic processes were observed related to the role of benzonitrile as a redox mediator (Scheme 4). The same general electrochemical trend was observed when DMF was replaced by [PP13]TFSI under a N2 and a CO2 atmosphere at the GC and silver electrodes. However, when using [EMIM]TFSI, the electrochemical reduction of benzonitrile was not detected in any of the cases due to the fact that the reduction of the imidazolium cation appeared at less negative potentials either in inert or CO2 atmospheres [44].

Electrochemicarboxylation of 4-Iodobenzonitrile
Once the electrochemical reduction mechanism was disclosed, electrocarboxylation processes were performed using a carbon graphite rod and a silver foil electrode after its first reduction wave (ca. −1.9 V) under a CO2 atmosphere. The results are summarized in Table 2. It is important to note that, when silver was used as a cathode, the electrochemical potential required was 0.5 V lower. A closer look at the data when carbon electrode was used indicates that when the electrocarboxylation process was performed in [EMIM]TFSI, there was a loss of efficiency and no electrocarboxylation products were obtained (entry 1). Those facts can be easily explained by taking into account the IL electrochemical window and the acidity of the C2-H of the imidazolium moiety. The replacement of the carbon cathode for silver allowed the efficiency of the electrosynthesis to increase, since the reduction potential applied was less negative, although no electrocarboxylated product was observed, again due to the acidity of the IL cation (entry 4).

Electrochemicarboxylation of 4-Iodobenzonitrile
Once the electrochemical reduction mechanism was disclosed, electrocarboxylation processes were performed using a carbon graphite rod and a silver foil electrode after its first reduction wave (ca. −1.9 V) under a CO 2 atmosphere. The results are summarized in Table 2. It is important to note that, when silver was used as a cathode, the electrochemical potential required was 0.5 V lower. A closer look at the data when carbon electrode was used indicates that when the electrocarboxylation process was performed in [EMIM]TFSI, there was a loss of efficiency and no electrocarboxylation products were obtained (entry 1). Those facts can be easily explained by taking into account the IL electrochemical window and the acidity of the C 2 -H of the imidazolium moiety. The replacement of the carbon cathode for silver allowed the efficiency of the electrosynthesis to increase, since the reduction potential applied was less negative, although no electrocarboxylated product was observed, again due to the acidity of the IL cation (entry 4). The use of an IL composed of non-acidic cations enabled electrocarboxylated products to be obtained. When [PP13]TFSI was used, around 10% of electrocarboxylated products (p-cyanobenzoic acid (3) and benzoic acid (4)) were obtained using carbon electrodes (Scheme 5, entries 2 and 3). The use of a high reduction potential value caused the appearance of an unexpected carboxylation product, 4, which should be related to the further electrochemical reactions of compound 3 that take place under these electrochemical conditions. Note that when the charge passed to the system increased, the only product obtained was 4 in a 12% yield (entry 3). To confirm this hypothesis, a controlled potential electrolysis of 3 was performed under the same electrochemical and chemical conditions previously described in entry 3, revealing the formation of benzoic acid in a ca. 25% yield.
The use of an IL composed of non-acidic cations enabled electrocarboxylated products to be obtained. When [PP13]TFSI was used, around 10% of electrocarboxylated products (p-cyanobenzoic acid (3) and benzoic acid (4)) were obtained using carbon electrodes (Scheme 5, entries 2 and 3). The use of a high reduction potential value caused the appearance of an unexpected carboxylation product, 4, which should be related to the further electrochemical reactions of compound 3 that take place under these electrochemical conditions. Note that when the charge passed to the system increased, the only product obtained was 4 in a 12% yield (entry 3). To confirm this hypothesis, a controlled potential electrolysis of 3 was performed under the same electrochemical and chemical conditions previously described in entry 3, revealing the formation of benzoic acid in a ca. 25% yield. Scheme 5. Electrocarboxylation of 1 acid under CO2 atmosphere using graphite as a cathode.
The use of silver cathodes allowed working under milder conditions, and as a consequence, not only were the electrocarboxylation yields almost three times higher, but also the selectivity of the increases, and only 3 was obtained as a carboxylation product in [PP13]TFSI (entries 5-7). Only compounds 2 and 3 were obtained in 70% and 30% yields, respectively, when exhaustive controlled potential electrolysis was performed. These results can also be reproduced using two different ILs consisting of non-acidic cations, such as 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulphonyl)imide ([BMPyr]TFSI) and N-trimethyl-N-butylammonium methylimidazolium bis(trifluoromethylsulphonyl)imide ([N1114]TFSI), entries 8 and 9 respectively. In all the cases, conversion rates ca. 30% were obtained (Scheme 6). Scheme 6. Electrocarboxylation of 1 under CO2 atmosphere using silver as a cathode. The use of silver cathodes allowed working under milder conditions, and as a consequence, not only were the electrocarboxylation yields almost three times higher, but also the selectivity of the increases, and only 3 was obtained as a carboxylation product in [PP13]TFSI (entries 5-7). Only compounds 2 and 3 were obtained in 70% and 30% yields, respectively, when exhaustive controlled potential electrolysis was performed. These results can also be reproduced using two different ILs consisting of non-acidic cations, such as 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulphonyl)imide ([BMPyr]TFSI) and N-trimethyl-N-butylammonium methylimidazolium bis(trifluoromethylsulphonyl)imide ([N1114]TFSI), entries 8 and 9 respectively. In all the cases, conversion rates ca. 30% were obtained (Scheme 6).
The use of an IL composed of non-acidic cations enabled electrocarboxylated products to be obtained. When [PP13]TFSI was used, around 10% of electrocarboxylated products (p-cyanobenzoic acid (3) and benzoic acid (4)) were obtained using carbon electrodes (Scheme 5, entries 2 and 3). The use of a high reduction potential value caused the appearance of an unexpected carboxylation product, 4, which should be related to the further electrochemical reactions of compound 3 that take place under these electrochemical conditions. Note that when the charge passed to the system increased, the only product obtained was 4 in a 12% yield (entry 3). To confirm this hypothesis, a controlled potential electrolysis of 3 was performed under the same electrochemical and chemical conditions previously described in entry 3, revealing the formation of benzoic acid in a ca. 25% yield. Scheme 5. Electrocarboxylation of 1 acid under CO2 atmosphere using graphite as a cathode.
The use of silver cathodes allowed working under milder conditions, and as a consequence, not only were the electrocarboxylation yields almost three times higher, but also the selectivity of the increases, and only 3 was obtained as a carboxylation product in [PP13]TFSI (entries 5-7). Only compounds 2 and 3 were obtained in 70% and 30% yields, respectively, when exhaustive controlled potential electrolysis was performed. These results can also be reproduced using two different ILs consisting of non-acidic cations, such as 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulphonyl)imide ([BMPyr]TFSI) and N-trimethyl-N-butylammonium methylimidazolium bis(trifluoromethylsulphonyl)imide ([N1114]TFSI), entries 8 and 9 respectively. In all the cases, conversion rates ca. 30% were obtained (Scheme 6). Scheme 6. Electrocarboxylation of 1 under CO2 atmosphere using silver as a cathode. Scheme 6. Electrocarboxylation of 1 under CO 2 atmosphere using silver as a cathode.

Electrochemical Experiments
An electrochemical conical cell was used for the set-up of the three-electrode system. For cyclic voltammetry (CV) experiments, the working electrode was a silver disk with a diameter of 1.6 mm and a glassy carbon disk with a diameter of 1 mm. It was polished using a 1 mm diamond paste. The counter electrode was a Pt disk <1 mm in diameter. All of the potentials were reported versus an aqueous saturated calomel electrode (SCE) isolated from the working electrode compartment by a salt bridge. The salt solution of the reference calomel electrode was separated from the electrochemical solution by a salt bridge ended with a frit, which was made of a ceramic material, allowing ionic conduction between the two solutions and avoiding appreciable contamination. Ideally, the electrolyte solution present in the bridge is the same as the one used for the electrochemical solution, in order to minimize junction potentials. The error associated with the potential values was less than 5 mV. The ohmic drop can be one of the main sources of error when ILs are used as solvents, since they are more resistive media than polar aprotic solvents with 0.10 M concentration of supporting electrolytes.
Due to the low conductivity and larger viscosity of RTILs (Room Temperature Ionic Liquids), in order to facilitate the direct comparison between the cyclic voltammograms in the different media, the current values obtained were first normalized by the concentration and the scan rate (Ip/cv 1/2 ). Later, those values were also normalized and the relationship between the four solvents can be expressed by a total normalization coefficient. This coefficient can be expressed as a ratio between DMF/[EMIM]TFSI, DMF/[PP13]TFSI, DMF/[N1114]TFSI, and DMF/[BMPyr]TFSI 3.0, 1.7, 6.0, and 2.7, respectively.
For electrolysis experiments, the disc working electrodes were replaced by a graphite carbon bar and a silver foil. The counter (Pt bar) and reference (SCE) electrodes were separated from the electrochemical solution by a salt bridge filled by the electrolyte solution. The salt bridge ended with a frit, which was made of a ceramic material, avoiding appreciable contamination. Compounds were electrolyzed at a negative potential of 0.10 V more negative than the E pc potential value under nitrogenor carbon-saturated solutions. When the reaction was completed, the mixture was extracted into water/ether mixtures. The organic layer was dried with Na 2 SO 4 and evaporated to yield a residue. All products obtained were characterized by Proton Nuclear Magnetic Resonance ( 1 H-NMR) and Gas Chromatography (GC) by comparison with their respective pure commercially available analogues.

Determination of the CO 2 Concentration in ILs
A thermal mass flow meter of modular construction with a 'laboratory style' pc-board housing (EL-FLOW ® Mass Flow Meter/Controller, Bronkhorst Hi-Tec, Ruurlo, Netherlands) was used to monitor the CO 2 concentrations in the solution [28]. Control valves are integrated to measure and control a gas flow from the lowest range of 0.2-10 mL/min.

Conclusions
This manuscript describes a new "environmentally-friendly" electrochemical approach for producing highly valuable compounds using CO 2 as a building block in ILs. The selection of the solvent and the nature of the cathode material are crucial, since it helps to overcome the main drawbacks associated with previous processes reported in the literature, in which organic solvents, redox mediators and large quantities of supporting electrolytes are employed. The use of silver instead of carbon cathodes enables p-cyanobenzoic acid to be obtained from the electrochemical reduction of 4-iodobenzonitrile under mild experimental conditions with moderate yields and conversion rates under a CO 2 atmosphere. The use of non-acidic ILs also makes the electrocarboxylation process more environmentally friendly, since it is not only able to avoid the use of organic solvents and large quantities of supporting electrolytes, but also the solvation process of 1 − appears to be more effective. Hence, the use of ILs (high concentration of cations) instead of tetrabutylammonium salts at low concentrations (0.1 M TBABF4 in DMF) enables the electrochemical potential applied to be also reduced. This study opens the door to develop new strategies for the synthesis of similar derivatives, such as phthalates, by following this "greener" route.