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Research paper : Establishment of compact processes (A. Suzuki et al.)−152−Synthesiology - English edition Vol.3 No.2 (2010) microreactor can be taken advantage of more efficiently. Currently, the technology for a microreactor that can withstand high-temperature or high-pressure has not been established.On the other hand, the supercritical fluid is defined as a fluid above its critical point (endpoint of saturated vapor pressure curve), and is called the fourth fluid that does not belong to the three phases of substance: solid, liquid, or gas. However, it is not very special, but is a non-condensible fluid that does not liquidize even when compressed to high density. The density of the supercritical fluid can be changed continuously and at great range from gas to liquid equivalent by changing the temperature and pressure, and the transport properties such as viscosity and diffusion coefficient, and solvent properties such as dielectric constant and ion product greatly change accordingly[2][3]. Particularly, the dielectric constant of the supercritical water, which is a state over its critical point (374 ºC, 22 MPa), is like an organic solvent, and is considered to be the only stable reaction solvent at high temperature. Also, the ion product can be increased to 10-10, and the supercritical water is expected to take the role of acid or base catalyst. Such properties imply the application of the supercritical water in high-speed chemical reaction, and the technology using supercritical water is expected to become the core technology of the low-volume distributed production.2 Integration of the microreactor and supercritical waterUntil about 2002, the common knowledge was that in the chemical process using supercritical water or high-temperature and high-pressure water, the decomposition of organic compounds (by hydrolysis and pyrolysis) was possible, while synthesis was not[4]. In fact, although the acid and base properties, which were not present in regular water, were observed in the supercritical water from physicochemical or spectroscopic studies, the results always produced none or very low yield of the target substance when organic synthesis experiments under supercritical water condition were conducted using the batch reaction device[5]-[7]. From these results, the use of supercritical water in organic synthesis was thought to be extremely difficult, and the research for the application of supercritical water fell into stagnation (the valley of death) for a while. Research funds declined and we had no alternative but to continue to conduct reactions by self fabricating a lab-scale flow reactor using old pumps for liquid chromatography and used-up high-pressure tubes. Suddenly, we found that the yield increased. When we observed closely, we understood that the reason that the target product could not be obtained before was because the breakdown and side reactions of the raw material or the target product occurred in the heating range (cooling range) if long heating (or cooling) time was taken to achieve the reaction temperature, even though the reaction time at the reaction temperature was controlled carefully. From this moment, our research moved forward rapidly[8]. Figure 1 shows the conceptual diagram of the importance of the rapid heat exchange in this reaction. The example of the reaction that prologued the organic synthesis under supercritical water condition will be described below.The synthesis of -caprolactam, a material for nylon, is conventionally done by the Beckmann rearrangement reaction of cyclohexanone oxime using concentrated sulfuric acid as an acid catalyst. However, in this synthesis, the concentrated sulfuric acid must be neutralized by ammonia, which generates large amount of ammonium sulfate, and its disposal is a major environmental and economic issue. We suggested a method of Beckmann rearrangement reaction using the acid catalyst property of the supercritical water[5][8]. The result of the Fig. 1 Points in the development of organic synthesis using supercritical water (need of rapid heat exchange)Since the supercritical water has high reactivity, side reactions and breakdown reactions occur and inhibit the main reaction if too much time is spent on heating or cooling. Rapid introduction and withdrawal from the reaction field is necessary. TemperatureMain reactionReaction time < several secTimeSide and breakdown reactions occur due to slow heating speedSide and breakdown reactions occur due to slow cooling speedBatch reactionConventional continuous processTemperatureTimeRealization of highly controlled, high-temperature and high-pressure reactionRapid heating in 0.01 sec or lessReaction time < several secMain reactionRapid cooling in 0.01 sec or lessTable 1 Synthesis of -caprolactam using supercritical water (experiment result)The yield was low in the batch reaction, but high yield was achieved in the continuous microreaction. Difference due to reaction time (including heating time) was significant.83.00.62540400Continuousmicroreaction1.918040400Batch reactionYield (%)Reaction time(sec)Reactionpressure (MPa)Reactiontemperature (ºC)Apparatus

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