Controlled Heating of Palladium Dispersed Porous Alumina Tube and

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Controlled Heating of Palladium Dispersed Porous Alumina Tube and Continuous Oxidation of Ethylene Using Frequency-Variable Single-Mode Microwave Reactor Masateru Nishioka,*,† Koichi Sato,† Ayumi Onodera,‡ Masato Miyakawa,† David A. Pacheco Tanaka,§ Makoto Kasai,† Akira Miyazawa,† and Toshishige M. Suzuki*,† †

National Institute of Advanced Industrial Science and Technology, AIST, 4-2-1, Nigatake, Miyagino-ku, Sendai, 983-8551, Japan Tohoku Gakuin University, 1-13-1, Chuo, Tagajo, Miyagi, 985-8537, Japan § Tecnalia, Paseo Mikeletegi, 2, E-20009, Donostia - San Sebastian, Spain ‡

S Supporting Information *

ABSTRACT: A frequency-variable single-mode microwave (MW) reactor system was designed and applied for selective heating of a palladium dispersed layer deposited on a porous alumina tube. This MW reactor system automatically detects the suitable resonance frequency and provides the optimum MW irradiation conditions in the cylindrical cavity via a power feedback loop. The temperature program ensured simultaneous MW power response and fast heating of the palladium dispersed layer of the reactor tube. The high-power MW amplifier was oscillated using a semiconductor instead of a conventional magnetron-type oscillator. The semiconductor device provides a narrower distribution of frequencies, resulting in an intense and sharp heating zone along the tubular reactor. High reaction conversion with efficient energy use was demonstrated via continuous oxidation of ethylene by focusing the electric field along the palladium dispersed reactor tube.

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INTRODUCTION Microwave (MW) dielectric heating is recognized as a promising methodology and is widely utilized for inorganic/ organic syntheses,1−15 nanoparticle fabrication, and material processing.16−19 Various MW-assisted reactors are commercially available for applications ranging from laboratory-scale syntheses to pilot plant-scale production.20−31 In conventional MW reactors, the electromagnetic field distribution is spatially disordered causing inhomogeneous heating of the reactor. When the diameter of the cylindrical cavity is designed to the multiple of the irradiated wavelength (λ), the progressing wave and the wave reflected from the cavity wall overlap to generate a harmonic resonance frequency. This MW state known as the “single-mode” can be generated at the center of the cavity by controlling the cavity diameter. However, there were a limited number of examples for the use of continuous flow-type MW reactors with single-mode cavities.32−36 We originally designed a MW flow reactor system that generates a uniform electromagnetic field in a cylindrical TM010 single-mode cavity in which a homogeneous heating zone is formed along the central axis of the cavity.32−36 The temperatures of flowing liquids, e.g., water, ethanol, and ethylene glycol, in the reactor tube at the cavity center were precisely controlled via the resonance frequency autotracking function.36 Continuous-flow synthesis of metal nanocolloids and metal complexes in polyols at ambient pressure and elevated pressure has been successfully achieved using our “smart” MW reactor system.34−36 MW-assisted chemical reactions are not limited to liquid-phase reactions, but various solid-phase reactions have been examined.37−46 For example, Mingos’ group developed MW reactors comprising a magnetron MW generator coupled with a cylindrical MW cavity; these © 2013 American Chemical Society

were applied to the synthesis of superconducting ceramics, metal-powder processing, and heterogeneous catalytic reactions.37−40 Application of a single-mode MW reactor system to the heterogeneously catalyzed gas phase flow reactions remains scarce.44,46 This work aims at applying our frequency-variable singlemode MW reactor to the controlled heating of a supported metal catalyst and continuous gas-flow reactions over solid catalysts. We installed a palladium dispersed tubular reactor in the center of the cylindrical TM010 single-mode MW cavity. The palladium particles dispersed in the porous Al2O3 tube can be selectively heated by focusing the electric field along the tubular reactor. The temperature of the reactor surface was controlled by automatic tracking of the resonance frequency depending on the changes in the system and generation of optimum MW irradiation to maintain the required temperature. We used a semiconductor as the high-power MW oscillator to generate intense and narrower frequency distribution than a conventional magnetron oscillator. Continuous oxidation of ethylene, which is a volatile organic compound, was performed as a model gas-phase reaction by passing an ethylene/air mixture through the MW-heated palladium dispersed porous alumina tube.

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EXPERIMENTAL SECTION Preparation of Palladium Dispersed Tubular Reactor. A tube (o.d., 10 mm; i.d., 7 mm; length, 150 mm) comprising Received: Revised: Accepted: Published: 1073

October 1, 2013 December 4, 2013 December 30, 2013 December 30, 2013 dx.doi.org/10.1021/ie4032555 | Ind. Eng. Chem. Res. 2014, 53, 1073−1078

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Figure 1. Preparative scheme of palladium dispersed porous alumina tube. Palladium particles (ca. 10 nm) are dispersed in the nanoscale pores of γalumina layer (thickness: 4 μm).

Figure 2. (a) Schematic of tubular reactor and microwave heating unit. The alumina tubular reactor was installed along the central axis of cylindrical cavity. An open slit (8 mm) is present in the center of the cavity for sensing temperature and microwave frequency. (b) Distribution of electric field in the cavity, as simulated using the COMSOL Multiphysics program.

porous α-Al2O3 was purchased from NGK Insulators, Ltd. and used as the support for the palladium dispersed tubular reactor (Figure 1).47,48 This support tube is composed of a microporous γ-Al2O3 outer layer (200 μm thickness) and macropourous α-Al2O3 inner layer (Supporting Information, Figure S1). One end of the tube was closed by connecting it to a dense alumina rod. The surface of the tube was sealed with glass sealant (Nippon Electric Glass 6A-13N/325), leaving 100 mm of the porous portion for palladium deposition. A thin layer of γ-alumina (8−18 nm particles, Kawaken Fine Chemicals Co. Ltd., Japan) was dip-coated over the α-alumina tubular support, followed by calcination at 600 °C. The thickness of the γ-alumina layer was around 4 μm. The palladium particles were dispersed in the γ-alumina layer by dipping the tube into a chloroform solution of palladium acetate (0.2 wt %) and air-dried, followed by reduction with 1

M aqueous hydrazine (Figure S2). This procedure was repeated five times. Palladium particles (ca. 10 nm) were dispersed in the nanoscale pores of the γ-alumina layer.47,48 Microwave Heating Apparatus. Figure 2a shows a schematic drawing of the MW reactor equipped with the palladium dispersed porous alumina tube in a cylindrical aluminum cavity. The MW heating/controlling unit was composed of a variable frequency generator, field intensity monitor, and temperature sensor. The palladium dispersed alumina tube was installed in the quartz outer shell and oriented along the central axis of the aluminum cavity. The diameter of the cylindrical cavity was determined to be 87 mm via simulations based on the wavelength of the incident MW required to form a single standing wave (Figure 2b). MWs were generated by a semiconductor-type generator, which enabled precise control of power (0−100 W) and frequency (2.3−2.7 1074

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the magnetron generator. Moreover, the oscillation frequency is variable, and the response rate is several orders of magnitude larger than that of the magnetron system. These features allow remarkably efficient energy use during MW heating. Measurement and Control of Temperature at Palladium Dispersed Layer. The temperature of the alumina tube surface was measured through the open slit of the cavity using an IR thermometer at a wavelength that penetrates the quartz outer shell. In this system, most of the MW power is delivered to the metal-deposited layer because the loss tangent of the reactant gas and alumina tube is 2−3 orders of magnitude lower than that of the metal catalyst and is thus regarded as MWtransparent. Palladium particles dispersed in the alumina pores selectively absorb the MW energy. Figure 3a shows the surface

GHz). The temperature at the central part of the reactor tube was measured through an 8-mm slit in the cavity using an infrared (IR) thermometer with a wavelength that penetrates the quartz outer shell. An IR imaging camera (TH-5100, NEC Sanei Instruments, Ltd.) was used to measure the temperature distribution along the reactor tube. The temperature of the reactor tube surface obtained with IR radiation was calibrated with that measured by a thermocouple. A rod shaped heater was inserted into the reactor tube and electrically heated, and the surface temperature of the reactor tube was measured by direct contact of a thermocouple wire. The calibration plots of temperatures measured by two methods gave good linearity, and the temperature correction was made with this calibration curve. MW-Assisted Heating and Catalytic Oxidation of Ethylene. The palladium dispersed porous alumina tube with an open inlet and closed end was installed in the quartz outer shell and oriented along the central axis of the cylindrical cavity. A gaseous sample comprising nitrogen gas with 50 ppm ethylene and 20% oxygen was continuously supplied to the reactor tube from the open inlet (“gas inlet” in Figure 2a). The flow rate of the gas was controlled using the mass-flow controller. The sample gas was catalytically oxidized via contact with the palladium particles during permeation through the pores of the reactor tube. The reactant and product gas that were emitted from the support tube (“gas outlet” in Figure 2a) were analyzed using a Fourier-transform infrared (FTIR) spectrometer (Nexus-470; Thermo Fisher Scientific Inc.) equipped with a 2-m gas cell. For comparison, a study involving conventional heating by an electric furnace was also conducted under the same reaction conditions with MW heating. Conventional Heating. The palladium dispersed alumina tube was installed in the quartz outer shell (20 cm), which was placed in the center of a jacket type mantle heater of 20 cm length. The temperature of the reactor was measured by use of a thermocouple directly contacting the reactor tube. Sample gas supply, flow rate control, and analysis of the product were conducted with a similar procedure to that described for the MW-assisted reaction.

Figure 3. (a) Surface temperature of the palladium dispersed tube as a function of applied MW power obtained using a semiconductor power generator (■) and magnetron generator (⧫). The temperature was monitored using an IR thermometer. (b) Temperature distribution profile along the reactor tube from a semiconductor power generator, as observed by an IR imaging camera.

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temperature of the palladium particles dispersed in the porous alumina tube versus the applied power for the semiconductoramplified and magnetron-generated MW. It is evident that the semiconductor device attains the same temperature with less power input than that required by the magnetron-generated MW. Figure 3b exhibits the temperature distribution along the reactor tube when a semiconductor power amplifier was used: the temperature gradient was ∼8 °C/cm. A distinct arcing was not visible within the resolution (0.5 mm square) of the IR imaging camera. It is known that arcing or sparks tend to occur as the particle size increases.49 In the present case, most of the palladium particles are around 10 nm because the particles are confined in the nanosize space of the γ-Al2O3 layer.47 Although it appeared that the reactor tube is homogeneously heated in the camera image resolution, the local temperature of the palladium nanoparticles and γ-Al2O3 support might not be equal. Highly conductive palladium particles are heated much more efficiently than alumina support. Consequently, the temperature of palladium particles can be higher than that of the macroscopic temperature determined by the IR thermometer. In this system, temperature changes and shifts of the dielectric constants of the reactor tube surface are automatically

RESULTS AND DISCUSSION Single-Mode Microwave Reactor System. The design and control of a single-mode cavity for a MW reactor is complicated because the resonance frequency of the cavity depends on the cavity size, dielectric properties of the reaction materials, and temperature. We developed a variable-frequency MW reactor with a cylindrical cavity that undergoes a frequency shift depending on the dielectric properties of the materials and temperature, which was compensated by using an automatic detection and frequency feedback loop. Namely, the temperature detected by IR thermometer was fed back to the variable attenuator and then the MW output was automatically adjusted by monitoring with the field sensor.36 This system automatically detects the suitable resonance frequency and shifts to generate the optimum MW irradiation conditions. Another feature of this MW reactor system is the use of a semiconductor as the MW power oscillator instead of the commonly used magnetron. The spectrum bandwidths for magnetron and semiconductor power generators are 10 and 0.3 MHz, respectively (Figure S3, Table S1). By concentrating the MW frequency, the semiconductor power generator forms a more intense and sharp heating zone along the tubular reactor than 1075

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detected, and the resultant resonance-frequency shift is compensated. Figure 4 shows a time profile of the surface

Figure 4. Time profile of the surface temperature, applied MW power, and resonance frequency of the stepwise heating of a palladium dispersed porous alumina tube. The dotted line denotes the programmed temperature, and the solid line is the observed temperature.

temperature, applied MW power, and the resonance frequency for heating of the palladium dispersed layer. The temperature of the tube surface was raised stepwise up to 250 °C without any significant overshoot. In accordance with the temperature program, the response of the MW power and increase in the temperature of the tube surface occur simultaneously, and the surface is then maintained at the target temperature using the resonance-frequency autotracking function. The elevation of the temperature is instantaneous and takes less than 10 s to raise each 50 °C by supply of 100 W MW power (Figure 4). This corresponds to more than 10 times faster than that heated by an electric heater with the same power input. In addition, only 10−20 W of the electric power is sufficient to keep the reactor temperature constant because the reactor tube is directly heated by MW irradiation. In contrast, a conventional furnace requires much greater power to keep the entire reactor room at a constant temperature. Oxidative Decomposition of Ethylene over Palladium Dispersed Tubular Reactor. Ethylene is known as the aging hormone in plants, causes fruit to ripen, and perhaps even causes plants to die. Continuous oxidation of ethylene over the palladium dispersed layer was attempted as an example of a heterogeneous catalytic reaction. Figure 5a shows the ethylene conversion versus the temperature of the reactor surface, as monitored using the IR thermometer. For comparison, ethylene degradation was employed by installing a palladium dispersed tube, followed by heating in the conventional electric furnace. As given in Figure 5a, MW heating resulted in a much higher reaction conversion than the conventional furnace heating within the same temperature range. As discussed above, the apparent temperature of the reactor surface observed by the IR thermometer is the average temperature of the catalyst and support material. The palladium particles are selectively heated by MW irradiation, and the temperature of the microscopic region is much higher than the support because of its high loss tangent compared to that of the alumina tube. The significant increase of reaction conversion is the result of nonuniform heating of palladium particles than the observed temperature. The sample gas contact with the palladium particles of high temperature was catalytically oxidized more efficiently. The

Figure 5. (a) Ethylene oxidation as a function of the surface temperature of palladium dispersed tubular reactor (gas flow rate: 50 mL min−1): (■) microwave heating and (⧫) conventional heating by mantle heater. Temperature was monitored using an IR thermometer. (b) Reaction temperature required for 50% ethylene conversion versus gas flow rate. Here, “surface” and “outlet” denote the temperature of the catalytic tube surface and outlet gas, respectively.

high reaction efficiency of MW heating for many heterogeneous catalytic systems has been attributed to the formation of a local “hot-spot” at the metal catalyst.37,50 The temperature of the gas outlet of the MW cavity was only ∼10 °C higher than that of the gas inlet (room temperature) because of the short contact time of the gas to the selectively heated catalytic zone (Figure 5b).

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CONCLUSION We presented selective heating of a palladium dispersed alumina tube using a frequency-variable MW reactor system installed with a single-mode cavity. High MW energy amplified using a semiconductor was focused along the reactor tube, located in the center of the cavity. The frequency feedback system can track the correct resonance frequency when the dielectric properties shift and then the oscillation frequency compensates the shift of resonance frequency automatically, bringing in the precise temperature control. Rapid response and precise control of the temperature was demonstrated during heating of the palladium dispersed alumina tube. High reaction conversion with efficient energy use during continuous ethylene oxidation was achieved by passing the gas through the catalytic alumina tube. The frequency-variable single-mode MW heating system provides a promising methodology that is applicable not only to liquid flow reactions but also to solid-state processing and gas-flow reactions. 1076

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ASSOCIATED CONTENT

S Supporting Information *

Additional data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interests.

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