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RSC Advances
Page 2 of 5
DOI: 10.1039/C5RA19289G
COMMUNICATION
Journal Name
In this work, we aimed to develop a novel electrochemical this concept, the carboxylated product may be obtained in a
carboxylation system for CO2 fixation to organic compounds higher productivity.
using a microreactor. We have successfully demonstrated that
a
microreactor is extremely useful for controlling
electrochemical carboxylations involving unstable carboxylate
ions even without the need for sacrificial anodes. As a model
reaction, we chose electrochemical carboxylation of 1-
(chloroethyl)benzene
for the model reaction consists of two regions: an electrolysis
region for the generation of carboxylate ion and an
acidification region for its rapid reaction with HCl. The
1 (Fig. 1). The microreactor fabricated
B
electrochemical generation of carboxylate ion
the reaction between carbon nucleophile
B
capitalizes on
generated
A
cathodically from the substrate 1 and CO2.
Table 1. Electrochemical carboxylation of
1
using a
conventional batch-type reactor and a microreactora
Figure 2. Effect of the cathode material on electrochemical
carboxylation of . Experimental conditions: anode, Pt plate
(1
3 cm2); current density, 18 mA cm-2; solvent, DMF;
substrate 10 mM of ; supporting electrolyte, 100 mM of
1
×
1
Bu4NBF4; charge passed, 2.5 F mol-1 for the case with Ag
cathode, 5.0 F mol-1 for the case with Pt cathode, 10 F mol-1 for
the cases with graphite, stainless steel, and GC cathodes.
Yieldc
(%)
Entry
Reactor type
Cathode
Anode
The cathode material also plays an important role in
selecting the course of an electrochemical reduction process
such as electrochemical carboxylation, and in controlling the
efficiency of the process.15 Therefore, we next investigated the
effect of the cathode material on the model reaction. As
1
2
3
Batch type reactor
Batch type reactor
Flow microreactorb
Pt
Pt
Pt
Mg
Pt
88
49
90
Pt
shown in Figure 2, the electrochemical carboxylation of
proceeded very smoothly to provide the carboxylated product
in excellent yields with the use of glassy carbon (GC) and
1
Experimental conditions: current density, 18 mA cm-2; charge
a
2
passed, 5.0 F mol-1; solvent, DMF; substrate, 10 mM of
1
; supporting
platinum (Pt) cathodes. In particular, the glassy carbon
electrode gave the best results among all tested cathode
materials. On the other hand, compared with Pt and GC,
graphite, silver (Ag), and stainless steel cathodes gave lower
b
electrolyte, 100 mM of Bu4NBF4. Electrode distance, 20 µm; flow
rate, 0.6 mL min-1. c Determined by HPLC.
First, the model reaction was carried out using
a
yields. It should be noted here that the yield of
2 was relatively
conventional batch-type reactor and a microreactor (Table 1).
With the former, the use of a sacrificial Mg anode gave the
low when a Ag cathode was used, although Ag cathodes have
electrocatalytic activity for the reduction of organic halides.16
This may be ascribed to the fact that the competing CO2
reduction took place simultaneously at the cathode since at Ag
cathodes the reduction potential of CO2 is lower than that of
organic halides. In fact, we observed CO gas evolution from the
reactor in this case.
corresponding carboxylated product
2 in excellent yield
because the Mg ions generated from the anode stabilize the
unstable carboxylate ions (Table 1, entry 1). In contrast, with a
Pt anode (non-sacrificial), the yield of the carboxylated product
2
remarkably decreased due to decomposition of carboxylate
ions (Table 1, entry 2). On the other hand, the model
carboxylation in the microreactor gave the highest yield of
B
From the above considerations, it can be expected that the
residence time of an unstable intermediate such as
2
even without the use of a sacrificial Mg anode (Table 1, entry
3), which indicates that the electrogenerated carboxylate ions
carboxylate ion
B in the reactor greatly influences the product
yield. In this electrosynthetic flow system, the residence time
can be easily controlled by changing the electrode distance
while the electricity is maintained the same. Therefore, we
next examined the effect of the electrode distance on the
B
immediately undergo subsequent acidification in the
microreactor before decomposition can take place.
In this demonstration, one microreactor unit produced only
a
small amount of the desired carboxylated product
model reaction (Figure 3). The yield of carboxylated product
2
(productivity of 51 mg h-1). However, it is well known that a
microreactor system with multiple unit cells, a so-called
“numbering-up” system, can produce a large amount of
product via simultaneous production.14 Therefore, by applying
decreased with an increase in the electrode distance. This
result apparently indicates that the longer residence time led
to side reactions or decomposition of the unstable carboxylate
ions B.
2 | J. Name., 2012, 00, 1-3
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