초임계 CO2내에서 전해카르복실화 및 cyclic voltammetry

박지영, 이윤우, 김재덕, 이창하*, 임종성

한국과학기술연구원 국가지정 초임계유체 연구실,

연세대학교 화학공학과*

Electrocarboxylation and cyclic voltammetry in supercritical CO2 media

Ji-Young Park, Youn Woo Lee, Jae Duck Kim, Chang-Ha Lee*, Jong Sung Lim

National Research Lab. for Supercritical Fluid, KIST

Dept. of Chem. Eng., Yonsei University*

Introduction

Although the first electroorganic reaction in organic synthesis is probably the famous Kolbe electrolysis published in 1849, no other remarkable reactions have been found until the reductive dimerization of acrylonitrile to adiponitrile was developed by Dr. M. M. Baizer of Monsanto Co. in 1964.  Since then, the electroorganic chemistry has been studied with the expectation that it is a new useful tool for finding novel reactions in organic synthesis [1]. By using these electrochemical methods, the wide variety of organic electrochemical trans formations can be carried out rapidly and in good yield, using relatively simple equipment, with the added advantage of avoiding hazardous or toxic reagents or by-products. In addition, not only is it possible to carry out many reactions more efficiently electrochemically than with chemical reagents, there are in fact many reactions that can only be carried out electrochemically [2].  For these reasons, there has been increasing interest in electrochemical methods in organic chemistry. Until now, however, almost all the studies on the electroorganic synthesis and its reaction mechanism have been performed only under an atmospheric pressure [3-5]. Recently, several attempts such as the electrochemical reduction of CO2 [6] and radical reactions [7] to apply the advantages of supercritical fluids to electroorganic reaction have been tried. It has been reported that the reaction in supercritical fluids has an excellent mass transfer and high diffusion rate and is known as an environmentally friendly process.

In this work, we have tried to study electrocarboxylation in supercritical CO2. The electroorganic synthesis of 1-naphthalenecarboxylic acid was performed using 1-chloronaphtalene as a reactant under the supercritical CO2 fluid. In addition, cyclic voltammogram for this reaction was measured. A newly designed high-pressure view-reactor was manufactured for this electroorganic synthesis. The experiment and the results of this reaction are discussed.

Theory

Reaction Mechanism of electrocarboxylation of □-chloronaphthalene  The reaction mechanism of □-naphthalenecarboxylic acid was shown in Schem 1. The reaction was done on two electrodes.  The intermediate (RMgX) of □-naphthal- enecarboxylic acid reacted with Mg2+ on the anodic electrode was poured into 1N HCl. After this procedure, □-naphthalenecarboxylic acid was produced [3, 4]

Pt(-)  Mg(+)  3F/mol

CH3CN + Bu4NBF4

40℃, 85 bar

at anode    (Mg)     Mg → Mg2+  + 2e-

at cathode  (Pt)      RX + CO2 + 2e- → RCO-2 + X- (RX : □-chloronaphthalene)

schem 1. The electrocarboxylation of □-chloronaphthalene

Experimental

Chemicals and preparation of solution Tetra-n-butylammoniumtetrafluoroborate (Bu4NBF4, Merck, Germany) was used as a supporting electrolyte. This was dried in a heating oven at 75℃ for 2 days and used without purification. The purities of 1-chloronaphthalene (Aldrich, USA) was over 90%(remainder 2-chloro

-naphthalene), acetonitrile (Aldrich, USA) was 99.8%, and CO2 (Duck Yang, Korea) was 99.99%. Preparation of electrolytes containing reactant 1-chloronaphthalene (2 mmol) and Bu4NBF4 (2.5/5mmol) were  dissolved  in acetonitrile (100ml).

Experimental apparatus Experimental apparatus was designed to carry out the electrocarboxylation in the temperature up to 353.15 K and pressure up to 300 bar.  The reactor cell was made of 316 stainless steel and had an internal volume of 155 cm3. In the middle part of the cell, Pyrex glass windows of 18 mm thickness was placed to allow the observation for mixing and reaction phenomena. The interior of this reactor was cylindrical and coated with Teflon. A platinum plate  was used as a cathode and a magnesium was used as an anode. A stirrer, rotated at variable speeds by an external magnet, was used to mix the reactant and the electrolyte solution in the high-pressure reactor. A simple thermostatic air bath was used to maintain the system temperature constant within ± 0.1 K. The temperature in the reactor was measured with a K-type thermocouple and a digital indicator (OMEGA Co.) which was calibrated by KRISS with an uncertainty of ± 0.1 K. The pressure in the reactor was measured with a pressure transducer and digital indicator which was calibrated by a dead weight gauge with the measuring accuracy of ±0.01 bar.

Electrocarboxylation procedure In our previous work [8], the experimental procedure of the electrocarboxylation from 1-chloronaphthalene was introduced.

Cyclic voltammetry measurement Electrochemical measurements were performed using an Potentiostat/Galvanostat (EG&G PRINSTON Applied Research) which was controlled by 270M electrochemistry software. Platinum wire spiral was used as a reference electrode and platinum plate electrode was used as a counter electrode and magnesium plate electrode was used as a working electrode.

Results and discussion

Electrocarboxylation of 1-chloronaphthalene was carried out using 1-chloro- naphthalene as a reactant and acetonitrile solution containing Bu4NBF4 as a supporting electrolyte in the conditions of varying current densities and supporting electrolyte concentration. Reaction temperature was 40℃, and the pressure was 85 bar. A platinum plate was used as a cathode and a magnesium rod was used as an anode. The product mixture was analyzed by HPLC. The final powder product was confirmed to be the 1-naphthalene- carboxylic acid by the NMR and IR spectrum distribution. Table 1 lists the experimental results of electrocarboxylation under supercritical CO2 condition in this work. The electrocarboxylation in constant voltage state was carried out at 8 volt (entry 1 in Table 1) and 25 volt (entry 2 in Table 1) and that in constant current state was performed at I = 40mA (entry 3 in Table 1), and I = 80mA (entry 4 in Table 1), respectively. In the case of constant voltage state, the current density was not constant and the current did not flow well because of the deposition on the surface of the electrodes from the beginning of the reaction. But in the case of the constant current state, unlike the constant voltage state, the current density was constant and the current flowed well for a long time.  As expected, the yield of 1-naphthalenecarboxylic acid and the reduction rate of 1-chloronaphthalene by the constant voltage state were better than those by the constant current state. Among these experiments, the highest yield of 1-naphthalenecarboxylic acid (23.27%) and the highest conversion of 1-chloro- naphthalene (73.36%) were obtained when the concentration of supporting electrolyte was 2.5 mmol, the constant current was 40mA, and the current density was 10mA/cm2. Cyclic voltammetry was performed in both cases of the oxidation and reduction. The shape of cyclic voltammograms obtained during electrocarboxylation was shown in Fig. 1 (a)-(b). Experimental condition in acetonitrile solution with 2.5 mmol Bu4NBF4 and 2 mmol 1-chloronaphthalene was 20 mV/s of sweep rate and 2.0 mV sweep increment. Fig. 1 (a) is a result of reduction process occurring on the surface of the platinum plate electrode.  It indicates that the electrode potential region was from - 0.2 V to - 4.0 V  versus platinum reference electrode. From this cyclic voltammogram, this reaction is considered as a reversible reaction. Fig. 1 (b) is a result of oxidation process on the surface of the magnesium plate electrode. The electrode potential region was from 2.0 V to 3.0 V  versus platinum reference electrode. By this cyclic voltammogram, this reaction is considered as a irreversible reaction.

Conclusion

The electrocarboxylation of 1-chloronaphthalene was performed at a platinum cathode with a magnesium anode as the active metal in supercritical CO2 and cyclic voltammogram was recorded in both reactions of oxidation and reduction.

The final goal of this study is to obtain the optimal condition for achieving a high yield by electrocarboxylation in supercritical CO2. So, the studies about electroorganic synthesis to obtain a high yield and to find out its reaction mechanism are currently under investigation.

Table 1. Yield of □-naphthalenecarboxylic acid

Constant voltage
No.	Bu4NBF4 conc.	Current	Cell voltage	Current density	Yielda %	conversionb%
1	5 mmol	-	8 volt	-	13.58	39.76
2	5 mmol	-	25 volt	-	3.52	12.58
Constant current
No.	Bu4NBF4 conc.	Current	Cell voltage	Current density	Yielda %	conversionb%
1	5 mmol	80 mA	-	20 mA/cm2	20.44	69.45
2	2.5 mmol	40 mA	-	10 mA/cm2	23.27	73.36

a, bDetermined by HPLC as % of the peak area ; Uv-detection at  283 nm, mobile phase water (60 ml) and acetonitrile (40 ml), flow rate 1ml·min-1, and injection volume 20□l.

(a)                                    (b)

Fig. 1. The cyclic voltammogram of 1-chloronaphthalene on a magnesium electrode in acetonitrile solution with supporting electrolyte B4NBF4 : (a) reduction process, (b) oxidation process

Acknowledgement

This work has been supported by the National Research Laboratory (NRL) Program for Supercritical Fluid and the authors would like to thank to the Ministry of Science and Technology, Korea.

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