Research Activity on Supercritical Fluids Technology at Kumamoto
UniversityReaction of Aromatic Compounds using Pulsed Discharge Plasma in SupercriticalFluids
Investigation of pulsed discharge phenomena in sub- and supercritical fluids has
been conducted in our laboratory [40-42]. The DC pre-breakdown phenomena andbreakdown voltage characteristics of point-to-plane gap in pressurized CO2 up to
supercritical state has been reported. By using negative DC discharge, corona
discharges proceeding complete breakdown has been observed more clearly in liquidand supercritical state than in gas state of CO2. We also reported that the estimatedcorona discharge onset voltage by streamer theory had good agreement with themeasured one in the density region of 1.7 to 30 kg/m3. In addition, the breakdown
mechanism in liquid state can also be classified into two categories depending onpressure: bubble-triggered and non-bubble-triggered breakdown at lower and higher
pressures, respectively [40]. Moreover, it was also found that negative discharge wasdesired for DC plasma reactor since an active corona appeared more stable insupercritical CO2 under such conditions compared to the positive polarity [42].
Furthermore, it is possible for making ion-rich atmosphere, degrading and reacting
aromatic compounds by using pulsed discharge plasma in sub-critical water. Ion productof water rises to 10-11 at sub-critical condition, while it is 10-14 at atmospheric condition.
Forming plasma discharge in sub-critical water generates active species (H radical, OHradical, ion, free electron) which is unstable molecule and has high reactivity. In thiswork, non-catalytic reaction synthesis of aromatic and phenolic compounds using
pulsed discharge plasma in sub-critical water and supercritical CO2 or argon weredeveloped to establish novel environmentally treatment technology.
The experimental work was conducted in the apparatus with schematic diagram andpicture as shown in Figure 4 (a) and (b), respectively. The apparatus was supplied byAKICO co., Japan. Two saphire windows were installed in the reactor to monitor theperformance of plasma production. The reactor is made of stainless steel (SUS316) withvolume of 900 ml. The maximum temperature and pressure are 300oC and 30 MPa,respectively. For sub-critical water experiment, initially Ar gas was introduced to thereactor to replace air, and then 550 ml of aniline solution was loaded into the reactor
using a pump. While for supercritical CO2 or argon, phenol solution was loaded into thereactor, and then CO2 or argon was introduced to the reactor to reach reaction pressure.
After the experimental temperature and pressure were achieved, the pulse power wasapplied to produce electric discharges at a constant temperature. The electrodeconfiguration consisted of a point (positive electrode) and a planar surface (negativeelectrode). They were made of tungsten and stainless steel, respectively. After a certain
reaction time, the products obtained by these experiments were introduced to methanol
in a flask. The remaining fraction inside the reactor was then washed out with methanol
at 90oC for about 2 h. Each fraction collected with methanol was analyzed by using
GC-MS, HPLC, and MALDI TOF-MS. The surface of the electrode before and after
every treatment was observed by SEM.

(a)
(b)
Figure 4. Experimental apparatus for reaction synthesis of aromatic compounds insupercritical CO2 or argon with discharged plasma. (a) Schematic diagram, 1. reaction
cell, 2. gas outlet, 3. solution outlet, 4. gas inlet; 5. pump, 6. supply vessel, 7. CO2/argoncontainer, 8. current transformer, 9. Magnetic Pulse Compressure (M.P.C.), 10. highvoltage probe, 11. digital oscilloscope; (b) Apparatus photograph.

hydroquinone
Solvent derived

phenolSolvent derived

Unidentified compounds(a)(b)
Figure 5. GC-MS chromatogram of reaction synthesis product of phenol in supercriticalargon with discharge plasma at 40oC and 5 MPa. (a) product, (b) remaining material.
Figure 5 shows a GC-MS chromatogram of product obtained during phenol
reaction in supercritical argon with discharge plasma (a) and remaining material (b) at
40 oC and 5 MPa. The GC-MS chromatograms exhibit the presence of compounds withlower and higher molecular weight. It can be shown that phenol and solvent derived
compounds were detected clearly in the product. However, some phenolic compoundswith higher boiling point than phenol were detected in the remaining material. It could
be explained that phenol radicals allowed alkyl rearrangement and re-polymerization toform heavier cross-linked phenolic char.NH2N N

Figure 6. MALDI TOF-MS
chromatogram of methanol soluble
fraction obtained after pulsed arc
discharge treatment in supercriticalargon.

Figure 7. GC-MS chromatogram of aniline
treated with discharge plasma at 100oC and5 MPa

Figure 6 shows the MALDI spectra of the remaining material after washed out by
using methanol. The results show that the formation of species in low molecular weight(less than 100 amu) occurred due to the scission of an O-H bond of phenol. At the sametime, the higher molecular weight compounds (phenol oligomers; more than 200 amu)
were formed via the association reaction of phenol and its derived compounds. These
results provide evidence that functional groups react with other phenolic structures toform higher molecular weight compounds.
GC-MS chromatogram of aniline treated with discharge plasma at 100oC and 5
MPa is shown in Figure 7. Diphenylazene which consisted of aniline dimers wasobtained in the lipophilic substance. The result provided evidence that functional groups
react with other nitrogen structures to form higher molecular weight compounds(oligomers). It is considered that synthesis reaction of aniline in subcritical water with
discharged plasma probably proceeded through the scission and association, whichleads to the formation of lower or higher molecular weight compounds.
References[40] T. Kiyan, A. Uemura, B. C. Roy, T. Namihira, M. Hara, M. Sasaki, M. Goto, H.
Akiyama, IEEE Trans. Plasma Sci., Vol. 35, 2007, p. 656.
[41] T. Kiyan, M. Takade, T. Namihira, M. Hara, M. Sasaki, M. Goto, H. Akiyama,IEEE Trans. Plasma Sci., Vol. 36, 2008, p. 821.
[42] M. Goto, M. Sasaki, T. Kiyan, T. Fang, B. C. Roy, T. Namihira, H. Akiyama, M.
Hara, J. Physics: Conference Series, Vol. 121, 2008, p. 1.