Single-Mode Microwave Reactor Used for Continuous Flow Reactions

Research Note pubs.acs.org/IECR

Single-Mode Microwave Reactor Used for Continuous Flow Reactions under Elevated Pressure Masateru Nishioka,*,† Masato Miyakawa,† Yohei Daino,† Haruki Kataoka,‡ Hidekazu Koda,‡ Koichi Sato,† and Toshishige M. Suzuki*,† †

National Institute of Advanced Industrial Science and Technology, AIST, 4-2-1, Nigatake, Miyagino-ku, Sendai, 983-8551, Japan Shinko-Kagaku Co. Ltd., 1544-19, Masumori, Koshigaya-shi, Saitama, 343-0012, Japan

‡

S Supporting Information *

ABSTRACT: We present a flow type single-mode microwave (MW) reactor that forms a uniform electromagnetic field along a tubular reactor (quartz glass, i.d. 1.5 mm × 100 mm) located in the center of a cylindrical MW cavity. The temperature of liquid flow in the reactor tube was controlled precisely by a resonance frequency autotracking function. This MW reactor system is useful for rapid heating of liquid flow at pressures up to 10 MPa. Continuous flows of polar solvents including water, ethylene glycol, and ethanol were heated instantaneously beyond their boiling points by application of pressure. Acceleration of the reaction was exemplified in continuous synthesis of Cu nanoparticles by elevation of the reaction temperature beyond the boiling point of solvent (ethylene glycol) at 2 MPa.

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INTRODUCTION Microwave (MW) dielectric heating, an efficient heating method, has contributed greatly to improvement of chemical processes including inorganic/organic syntheses,1−15 pharmaceutical drug syntheses,16 and nanoparticle fabrication.17−20 Advantages of MW-induced heating over conventional heating with an external heat source have been described often in the literature. Particularly, MW-induced heating effectively accelerates the reaction, thereby reducing reaction times and engendering dramatic improvement in productivity.10,12 Widely diverse MW reactors have been developed, many of which are now commercially available.21−40 However, most are batch-type systems with a multimode cavity in which an electromagnetic field is distributed in a spatially disordered fashion. Continuous-flow type reactors with a single-mode cavity are scarce. The MW penetration depth is only a few centimeters in most solvents. Therefore the temperature distribution is often inhomogeneous in batch reactor vessels that generate superheated solvents. In contrast, continuous processing in a tubular reactor is a means to avoid this penetration limit of MW. We originally designed an MW flow reactor system that forms a uniform electromagnetic field in a cylindrical singlemode MW cavity.41−46 Consequently, a homogeneous heating zone is guaranteed in the reactor tube mounted in the center of the MW cavity. Although the increase of temperature invariably increases the rate of chemical reactions, the boiling point of solvents determines the upper limit of the reaction temperature. Increased pressure will result in a rise of boiling points of liquids. Therefore the reaction temperature can be elevated considerably, leading to acceleration of the reaction rate.47 We present a single-mode MW reactor available for homogeneous heating of liquid flow under application of pressure. Such a single-mode MW reactor system that enables flow reactions under elevated pressure has not been reported to date. The performance of the present MW reactor was examined with © 2013 American Chemical Society

respect to synchronized response of temperature and resonance frequency and temperature stability of pressurized liquid flow including water, ethylene glycol, and ethanol. Additionally, we demonstrated the flow synthesis of Cu nanoparticles in ethylene glycol (bp, 195 °C) at 210 °C under 2 MPa pressure. The MW-assisted flow reactor system equipped with an elevated-pressure attachment is portrayed in Figure 1. The present MW reactor system is characterized by a variablefrequency MW generator (2.5 GHz ± 200 MHz, 100 W; IDX

Figure 1. Experimental setup of the present MW-assisted flow reactor system and a photograph of the MW cavity part. The quartz reactor tube is connected with the PEEK sheath, which is designed to resist up to 10 MPa. Received: Revised: Accepted: Published: 4683

January 20, 2013 February 28, 2013 March 8, 2013 March 8, 2013 dx.doi.org/10.1021/ie400199r | Ind. Eng. Chem. Res. 2013, 52, 4683−4687

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where f, a, μr, and εr respectively denote the resonance frequency, inner radius of the cavity, relative MW permeability, and the relative dielectric constant of fluid inside of cavity. As Figure 2 presents, the resonance frequency always increased continuously with the stepwise increase of temperature. Because the relative MW permeability of water is regarded as 1.0, the relative dielectric constant of water shifts slightly to a smaller value by the increase of temperature.50 The present frequency feedback system can track the correct resonance frequency within 1 s when the dielectric constant of fluid changes. Then the oscillation frequency compensates the shift of resonance frequency automatically in real time, bringing in precise temperature control. Similar to the water case, a rapid temperature response of ethylene glycol (bp 195 °C) and ethanol (bp 78 °C) under a flow rate of 10 mL h−1 was demonstrated as shown in Figure 3

Co. Ltd.), pressure pump and a TM010 single-mode cavity in which a quartz tubular reactor is mounted along the central axis. The key feature of our MW reactor system is the automatic detection and compensation of the resonance frequency shift (Supporting Information, Figures S1 and S2).41 The temperature of the reaction solution is measured using a radiation thermometer through the open slit (width, 8 mm) of the cavity (Supporting Information, Figure S3). The reaction fluid is pressurized by a pump (Nanospace SI-2; Shiseido Co. Ltd.) and is guided to the quartz reactor tube inlet (i.d. 1.5 mm × 100 mm). The pressure is controlled manually using a pressure regulator (P-880; Upchurch Scientific Inc.); it is measured using the pressure gauge. The pump can operate at flow rates of 0.06−180 mL h−l. The quartz reactor tube is connected with the PEEK sheath as designed to tolerate up to 10 MPa (Taiatsu Techno Corp.) and is sealed with either a Viton or Kalrez O-ring (Supporting Information, Table S1). In the MW heating process, polar solvents with high dielectric loss property are frequently chosen because they absorb MW energy efficiently and convert it rapidly to thermal energy.47,48 The continuous flow of polar solvents, that is, water, ethylene glycol, and ethanol was heated by MW under application of pressure. Figure 2 presents a time profile of the

Figure 2. Time profile of the temperature, applied MW power, and the resonance frequency for the stepwise heating of water at the flow rate of 10 mL h−l. The temperature was raised from 100 to 200 °C under 2 MPa.

Figure 3. Time profile of the temperature and applied MW power for the stepwise heating of ethylene glycol (boiling point 195 °C) and ethanol (boiling point 78 °C) at the flow rate of 10 mL h−l: (a) ethylene glycol (180−240 °C, 2 MPa) and (b) ethanol (120−190 °C, 6 MPa).

temperature, applied MW power, and resonance frequency for the heating of water flow (10 mL h−l). The water temperature was raised stepwise up to 200 °C without boiling under application of pressure (2 MPa). The outlet temperature of the reactor was followed with good time response (less than 10 s) at 100−200 °C without significant temperature overshoot. Upon input of the temperature program, the simultaneous response of the MW power takes place along with the instantaneous elevation of the fluid temperature because of the resonance frequency autotracking function. The resonance frequency of the TM010 single-mode cavity is expressed as eq 1:49 11.5 f= a · με (1) r r

panels a and b. These solvents were heated steadily to 240 and 190 °C, respectively, by application of 2 and 6 MPa. It is noteworthy that the subcritical fluidal condition of ethanol was realized by elevation of temperature and pressure. The critical temperature and pressure of ethanol are 241 °C and 6.1 MPa, respectively. Good temperature response was maintained in ethylene glycol, with an increase of the flow rate to 50 mL h−1 (Supporting Information, Figure S4). By application of the maximum MW power, the steady heating (160 °C) of ethylene glycol flow up to 400 mL h−l was confirmed (Supporting Information, Table S2).44 Even much higher flow rate can be possible when the MW power increased more than 100 W. Increase of throughput was possible by accumulation of multiple MW reactors in a series as shown in Supporting Information, Figure S5. Solvents of low MW absorbing nature 4684

dx.doi.org/10.1021/ie400199r | Ind. Eng. Chem. Res. 2013, 52, 4683−4687

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using ethylene glycol and diethylene glycol as solvents.51−55 As presented in Table 1, precedent Cu nanoparticle synthesis required long reaction times. We attempted to accelerate the reaction by increasing the reaction temperature. A mixed solution of 10 mM copper(II) acetate ([Cu(ac)2]) and 3 wt % PVP (Mw = 10 000) dissolved in ethylene glycol was introduced continuously to the reactor tube. Figure 5a compares the UV−

(low dielectric constants) like toluene and hexane can be heated by the present reactor system because MW energy is focused along the reactor tube and efficiently converted to thermal energy (Supporting Information, Table S3). Figure 4 panels a and b exhibit the long-term constancy of the temperature and the applied MW power for continuous

Figure 5. (a) UV−vis spectra of the synthesized Cu−ethylene glycol solutions prepared at different reaction temperatures and times. [Cu(ac)2] = 10 mM, [PVP] = 3 wt %. (b) Typical TEM image of Cu nanoparticles (temperature = 210 °C, reaction time = 30 s). Nanoparticles of 63 nm average sizes were obtained with the relative standard deviation of 0.30, as shown in the particle size histogram (inset).

Figure 4. Time profile of temperature and applied MW power for continuous heating of water and ethylene glycol at 10 mL h−l under 10 MPa of pressure: (a) water (180 °C, 5 h) and (b) ethylene glycol (210 °C, 1 h).

heating of water (180 °C) and ethylene glycol (210 °C) at the flow rate of 10 mL h−l (10 MPa). The temperatures of these fluids were maintained steadily for 5 h (water) and 1 h (ethylene glycol) with high precision (±1 °C) by the continuous supply of ca. 15 W MW power. We examined the continuous synthesis of Cu nanoparticles as an example of the reaction using the present MW reactor system. Because Cu nanoparticles are easily oxidized by exposure to atmosphere, quick and one-step synthesis of Cu nanoparticles without separation and redispersion is significantly demanded particularly in the metal ink process in the fabrication of printed circuit assembly. Earlier reports describe MW-assisted synthesis of Cu nanoparticles in batch processes

vis spectra of Cu-ethylene glycol solutions prepared either at 180 °C under atmospheric pressure or at 210 °C under 2 MPa. In both cases, the initial blue-green solution of Cu(II) changed to yellow with the progress of the reaction. The yellow color that appeared at 450 nm was assigned to a plasmon resonance absorption of Cu2O generated as the reaction intermediate.54 Upon further heating, the solution became an intense red color (590 nm) associated with the formation of Cu nanoparticles.54 Although formation of a small amount of Cu nanoparticles was apparent at 180 °C, it required very long residence time (600 s;

Table 1. Batch and Flow MW Synthesis of Cu Nanoparticles in Polyolsa

a

entry

polyols

heating type

1 2 3 4 5 6 7

EG EG EG DEG EG EG EG

batch batch batch batch batch flow flow

temp (°C)

time (s)

particle size (nm)

Cu precursor

other materials

ref

ca. 10 ca. 15 100−250 154 ca. 2

CuSO4·5H2O Cu(ac)2·H2O CuSO4·5H2O Cu(ac)2·H2O CuCl2 Cu(ac)2 Cu(ac)2

NaH2PO2·H2O PVP

197 197 200 185 180 210

300 540 900 3000 1800 600 30

51 52 53 54 55 this work this work

63

acrylamide PVP NaOH PVP PVP

EG and DEG respectively denote ethylene glycol and diethylene glycol (bp, 245 °C). 4685

dx.doi.org/10.1021/ie400199r | Ind. Eng. Chem. Res. 2013, 52, 4683−4687

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flow rate, 0.5 mL h−l). The reaction was apparently accelerated (residence time, 30 s; flow rate, 10 mL h−l) by elevation of temperature to 210 °C (2 MPa). A typical TEM image shown in Figure 5b portrays the formation of nanoparticles of 63 nm average diameter with the relative standard deviation of 0.3. The main difficulty of Cu nanoparticles arises from their easy oxidation during long synthetic procedures. Our MW reactor system is a promising tool for on-site syntheses of unstable or short-life chemicals such as Cu nanoparticles.

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CONCLUSION In conclusion, we presented a flow type single-mode MW reactor system with a elevated-pressure option. The MW power was controlled by a temperature feedback module and resonance frequency autotracking function, by which the fluid temperature was controlled precisely. A flow of liquids having high dielectric property was instantaneously heated in the reactor tube, enabling a continuous chemical reaction process. Application of pressure raised the boiling point of the solvents, leading to expansion of the operational temperature window of the MW-assisted chemical reactions. The reaction rate can be increased by elevation of temperature and pressure. This can improve the productivity scale which is a common limitation of microflow reactors. This MW reactor system can provide a significantly beneficial methodology that is widely applicable to organic and inorganic syntheses and to material processing.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and additional data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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