Control of Microwave-Generated Hot Spots. 6. Generation of Hot Spots

Article pubs.acs.org/IECR

Control of Microwave-Generated Hot Spots. 6. Generation of Hot Spots in Dispersed Catalyst Particulates and Factors That Affect Catalyzed Organic Syntheses in Heterogeneous Media Satoshi Horikoshi,*,† Momoko Kamata,† Tomohiko Mitani,‡ and Nick Serpone§ †

Department of Material & Life Science, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyodaku, Tokyo, 102-8554, Japan ‡ Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto, 611-0011, Japan § PhotoGreen Laboratory, Dipartimento di Chimica, Università di Pavia, Via Taramelli 12, Pavia 27100, Italy ABSTRACT: This article revisits the formation of 4-methylbiphenyl by the Suzuki−Miyaura reaction to examine the formation of hot spots and the factors impacting product yields, such as (i) mass transfer of reactants to the Pd/AC catalyst, (ii) continuous versus pulsed microwave irradiation, (iii) presence of a standing wave versus a nonstanding wave, and (iv) microwave input power levels. Present results indicate that mass transfer and hot spots impact the catalytic process. The rate of stirring of the heterogeneous mixture impinges on the formation of hot spots and product yields. Continuous and pulsed microwaves have little effect, whereas both mass transfer and the presence or absence of a standing wave do affect the yields. Beyond a certain stirring rate (1500 rpm), mass transfer is no longer an issue as yields remain constant; below this value, however, mass transfer, hot spots, and microwave input power levels play a role in the extent of products formed. of microwave irradiation from a domestic microwave oven.5 Related to the latter, in an earlier study we observed, by means of a high-speed camera, the existence of hot spots (microplasma) occurring on the Pd/activated carbon (AC) catalyst surface during the Suzuki−Miyaura cross-coupling reaction taking place in toluene solvent.6 In that study we discovered that the Pd catalyst particles, initially highly dispersed on the surface of the activated carbon support, formed aggregates at positions where hot spots had formed. Intuitively, one would expect to see an increased rate of reaction occurring within the volume wherein such high temperatures are reached at the hotspot locations. In fact, we observed the very opposite. Catalyst activity decreased appreciably, indicating that formation of hot spots had a negative effect on the synthesis. Accordingly, it was imperative that formation of hot spots be controlled whenever they might form, which led us to examine some possible means to control or otherwise minimize/suppress the generation of these microplasmas; these involved (i) exposing the reaction only to the magnetic field (H-field) component of the microwave radiation so as to minimize, or otherwise suppress, the formation of hot spots;6 (ii) making use of carbon microcoils rather than activated carbon particles as supports for the Pd catalyst particles;7 (iii) using 5.8 GHz microwaves rather than the 2.45 GHz microwaves commonly used in nearly all microwave-assisted syntheses in commercial apparatus8 (note that the ISM (industrial, scientific, and medical) bands are defined by the radio communications sector of the International Telecommunication Union; the frequencies used globally

1. INTRODUCTION Many chemical reactions have been carried out with commercially available microwave apparatus in the past 20 years, with which various organic syntheses were carried out and described by de la Hoz and Loupy,1 particularly those that require selective heating, one of the main features of microwave heating not otherwise fulfilled by conventional techniques. Germane to the present study, several other reactions have been carried out in the past two decades that involved solid catalysts for applications of microwave radiation into materials synthesis, for example, the synthesis of acetylene by oxidation of methane using particulate metal catalysts.2 In contrast to microwave heating, catalyst activity stopped when the reaction was only ca. 25% complete when a conventional heating method was used, whereas with microwave heating the reaction proceeded to about 90% completion. However, differential heating of the solid catalyst by the microwaves with surplus power input caused formation of hot spots that have proven deleterious to syntheses. In fact, the use of solid metal catalysts is now forbidden in most commercial microwave synthesis equipment because of the potential for breakage of the glass reactor when generating hot spots on metal catalysts. Accordingly, the advantages presented by the microwave method over conventional heating are significantly reduced whenever solid catalysts are involved in the reactions. Hot spots that occur on metallic catalyst surfaces, when subjected to microwave radiation, have been examined by Kappe’s group,3 as have those phenomena of microwaves observed in metal−solvent mixtures.4 Germane to the present study, nearly a decade ago Basu and co-workers demonstrated that it was possible to perform palladium-catalyzed Suzuki couplings of polyhaloaromatics with phenyl boronic acid on the surface of KF−alumina with the aid © 2014 American Chemical Society

Received: Revised: Accepted: Published: 14941

May 28, 2014 August 19, 2014 September 3, 2014 September 3, 2014 dx.doi.org/10.1021/ie502169z | Ind. Eng. Chem. Res. 2014, 53, 14941−14947

Industrial & Engineering Chemistry Research

Article

are 2.45, 5.8, and 24 GHz while other frequencies are established by each country, so that the 5.8 GHz frequency is available for microwave heating); and (iv) controlling the formation of hot spots on Pd/AC particles by using a hybrid internal/external heating method.9 The mechanism by which hot spots are formed on the activated carbon during the Suzuki−Miyaura cross-coupling reaction was described earlier.10 In that study, hot spots were generated on the activated carbon particle surface when irradiated by the microwaves’ electric field component. This resulted in particles being attracted to each other in the direction of the electric field owing to the microwave-induced polarization of the particles. When the interval (distance) between the AC particles was set at about 10 μm, the electric field (E-field) was concentrated on the space between the particles, and caused the formation of hot spots in that space/volume. Hot spots also formed under strong microwaves’ H-field radiation as a result of the generation of eddy currents. It became thus apparent that the distance between particles is an important parameter in the formation of hot spots. The present article revisits the formation of hot spots and their influence in the heterogeneous catalyzed Suzuki−Miyaura cross-coupling reaction by examining several possible factors that impinge on their formation and thus on the synthesis: (i) the size of the reactor; (ii) the extent to which the Pd/AC solid catalyst particles are dispersed within the reaction volume through changes in stirring rates, that is, their dispersiveness; (iii) irradiation with continuous versus pulsed microwaves; (iv) when irradiating with microwaves the generation of a standing wave or its absence (nonstanding wave conditions); and (v) the microwaves’ input power levels.

Figure 1. (a) Sketch showing details of the experimental setup in the single-mode microwave apparatus. (b) Photograph of the single-mode microwave resonator and the quartz reactor within the cavity.

λ=

λo 1 − (λo /2b)2

(1)

where λ is the wavelength in the waveguide, λo = 12.24 cm is the wavelength in a vacuum given by c/f {c is the speed of light, 2.9979 × 1010 cm s−1, and f is the microwave frequency used, 2.45 × 109 s−1}, and b is the length of the waveguide, 10.92 cm. The maximal position of the E-field from the iris occurred at 3/ 4 the wavelength of the standing wave in the waveguide, namely at 11.04 cm.12 The microwave radiation was transmitted to the sample employing a short plunger, a three-stub tuner, an iris, and a power monitor. The semiconductor microwave generator used was an Agilent Technologies N5183A MXG system (radiation range, 100 kHz to 32 GHz) whose emitted microwaves were amplified using a GA0827-4754-R power amplifier (R&K Co. Ltd.) and introduced into the waveguide through a coaxial cable. The frequency distribution of the irradiating microwaves was monitored with an Agilent Technologies N9010A EXA signal analyzer. Temperatures of the solutions were measured at 3 s intervals with a ceramic-type optical fiber thermometer (FL2000, Anritsu Meter Co. Ltd.) fixed at the center of the sample. Under conditions wherein the phase of the traveling wave (TW) and that of the reflected wave (RW) coincided, a standing wave formed between the short plunger and the iris. On the other hand, under conditions with which the phases of the TW and RW waves do not coincide, a nonstanding wave is formed and the heating efficiency is then decreased significantly. To examine the influence of the phase of the microwaves’ TW and RW on the formation of hot spots in the present study, we used experimental conditions such that a standing wave formed, by shifting the TW by 2 mm with the short plunger. To generate a standing wave, the resonance of the

2. EXPERIMENTAL SECTION 2.1. Microwave Multimode System. Pyrex tube reactors were used in a microwave chemical reaction apparatus with a multimode cavity (μ-Reactor Ex, Shikoku Instrumentation Co. Inc.) from which the microwaves uniformly irradiate the sample while being stirred with a microwave mode stirrer. The input power levels of the continuous microwave radiation were 550 and 1000 W. In addition, microwave irradiation was performed either by the continuous wave method (CW) or by the pulsed wave method (PW). Note that proportional−integral−differential (PID) control, which automatically adjusts the microwaves’ input power according to the sample temperature, was not used in this experiment. Temperatures of the heterogeneous Pd/AC dispersions were measured at 3 s intervals with a ceramic-type optical fiber thermometer (FL-2000, Anritsu Meter Co. Ltd.) with the tip positioned at the center of the sample. In comparative experiments, the same conditions of heating rate, reactor type, optical fiber thermometer, and a stirrer bar were used whenever heating involved the oil-bath conventional method. 2.2. Microwave Single-Mode Setup. The microwave irradiation setup with the TE103 single-mode cavity is illustrated in Figure 1. The heterogeneous Pd/AC dispersion was introduced into the quartz tube reactor (internal diameter, 17 mm) located at the maximal position of the microwave electric field (E-field maximum) in the waveguide. The wavelength of the propagation of the microwaves in the TE103 mode within the waveguide was ca. 14.7 cm, estimated using expression 1:11 14942

dx.doi.org/10.1021/ie502169z | Ind. Eng. Chem. Res. 2014, 53, 14941−14947


Article

heterogeneous Pd/AC catalyst particles was monitored through the hole on the microwave cavity side using a Casio high-speed camera (EX-ZR1000).

microwaves in the waveguide, as measured by the Agilent Technologies 8720C Network Analyzer, can change when positioning the reactor in the waveguide of the microwave semiconductor generator setup. Accordingly, appropriate adjustments were made using the short plunger and the three-stub tuner. In addition, the dielectric parameters of the sample can change with temperature. Although a slight reflected wave occurred by a change of the dielectric parameters of the sample, it could be controlled immediately by the plunger. 2.3. Synthesis of 4-Methylbiphenyl. The synthesis of 4methylbiphenyl by the Suzuki−Miyaura cross-coupling process (reaction 1) was carried out so as to compare the results with our earlier study.6

3. RESULTS AND DISCUSSION 3.1. Influence of Reactor Size. The influence of hot spots on the synthesis of 4-MB (4-methylbiphenyl) was examined using two different Pyrex reactors having different internal diameters: the sizes were 11 mm (narrow reactor) and 25 mm (wide reactor). The photograph of Figure 2a displays both

The Pd/AC catalyst (0.18 g; mesh size of AC support, 0.95 mm; “AC” refers to the activated carbon particles), phenylboronic acid (0.96 mmol; 0.12 g), 1-bromo-4-methylbenzene (0.72 mmol; 0.12 g), K2CO3 as the base (1.4 mmol; 0.20 g), and the toluene/1-hexanol mixed solvent (6 mL; volume ratio, 1:1) were mixed and subsequently added under an Ar atmosphere to the cylindrical reactor. Note that pure toluene was used as the solvent in our earlier study. However, because phenylboronic acid does not dissolve in toluene solvent, we added 1-hexanol to solubilize the acid. The relative dielectric loss factor of a sample solution without the Pd/AC catalyst particles was εr″ = 0.54 at 25 °C, by comparison significantly smaller than that of pure distilled water (εr″ = 8.7 at 25 °C); the toluene solvent was slightly heated by the microwave radiation. Therefore, even though we added 1-hexanol in toluene solvent, the main source of heating by the microwaves originated from the dispersed Pd/AC catalyst particles used to carry out the reactions.7 A condenser was connected to the microwave cylindrical reactor. The heterogeneous sample solution was mixed with a stirring magnetic bar (9 mm). The Pd/AC catalyst was prepared using the following procedure: the activated carbon particles (ACs; 5 g) were washed with ultrapure water and quantities of NaOH (2 M, 50 mL) for 24 h under ambient temperature conditions, after which the ACs were washed once again with ultrapure water and dried for a few hours at 100 °C. The so-washed ACs (1 g) were then introduced into an aqueous PdCl2 (0.034 g) and HCl (1 M) solution (50 mL), following which the solution was brought to pH 14 by addition of NaOH. Subsequently, NaBH4 (0.016 M) was added to the solution and stirred for 3 h; stirring was continued for an additional 2 h at 60 °C. Finally, the colloidal Pd/AC particles in the solution were filtered, washed with ultrapure water, and then dried at 100 °C overnight. The quantity of Pd on the activated carbon particulate support (diameter, 0.65 mm) was ca. 1.5 wt % Pd ascertained by atomic emission spectroscopy using the Shimadzu ICPE-9000 apparatus. Reaction yields of 4-methylbiphenyl were determined by gas chromatographic analyses (Shimadzu Model 2014 equipped with a Shimadzu GLC Ultra alloy-1 capillary column; carrier gas, helium; column temperature, 100−260 °C; heating rate, 20 °C min−1) from samples suitably prepared from the various dispersions; a pure sample of 4-methylbiphenyl (Wako Pure Chemical Industries, Ltd., 100% GC standard) was used to calibrate the chromatograph. Generation of hot spots on the

Figure 2. (a) Photograph of narrow Pyrex tube reactor (internal diameter, 11 mm) and of wide Pyrex tube reactor (internal diameter, 25 mm) containing the heterogeneous sample. (b) Photograph of heterogeneous catalytic dispersion in the narrow tube reactor stirred at 1000 rpm with a stirring magnetic bar.

reactors into which equivalent amounts of the samples were added. Even under the same stirring rate, the efficiency with which the Pd/AC catalyst particles were dispersed in the narrow reactor was lower than that in the wide reactor. The heterogeneous media in both reactors were subjected to continuous microwave irradiation (input power, 550 W) while dispersing the reactor contents at 1000 rpm using stirring bars (9 mm) of the same size. In the present case, the microwave system used was the μ-Reactor Ex (Shikoku Instrumentation Co. Inc.). The synthetic yields of 4-MB are reported in Figure 3. The synthesis yield of 4-MB with the narrow reactor

Figure 3. Temporal changes of product yields of 4-methylbiphenyl (4MB) in toluene/1-hexanol solvent: (a) by microwave heating method (550 W); (b) by oil-bath heating method. Unless noted otherwise, the stirring rate was 1000 rpm.

increased slightly after 30 min of irradiation, whereas the synthesis yield of 4-MB increased in the wide reactor after this period by nearly 2.2 times compared to the narrow reactor. The strong (intense) hot spots in the narrow reactor compared to the wide reactor were frequently observed with the high-speed camera. The hot spots occurred owing to the concentration of the E-field whenever an activated carbon particle approached another to within a distance of ca. 10 μm.10 Therefore, hot spots can be generated efficiently from the viewpoint of the 14943



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levels of 550 and 1000 W using a 25 mm Pyrex reactor and a 9 mm stirring magnet bar. For the case where the dispersions were not stirred (0 rpm), there was but a negligible difference in the product yields with regard to microwave heating at 550 and 1000 W and to oil-bath heating. When the heterogeneous dispersions were stirred at 500 rpm, the yield of 4-MB increased significantly compared with the nonstirring case (0 rpm). We ascribe this to a greater efficient contact between the substrates and the Pd/AC catalyst particulates (mass transfer factor) and to a better uniform distribution of the solid catalyst throughout the reaction volume thereby enhancing the catalyzed Suzuki−Miyaura crosscoupling process; that is, the mass transfer efficiency was increased under the stirring conditions. However, it was rather surprising that the yields of 4-MB observed for the microwave method were lower than those obtained by the oil-bath heating method. We infer that this was caused by the inactivation of the Pd catalyst as a result of the formation of hot spots on the Pd/ AC surface under the influence of the microwaves. Moreover, the greater yields of 4-MB with changes in the microwave power level from 550 to 1000 W, which are ca. 20% greater in the latter case, are attributed to the greater MW power and to a more efficient mass transfer throughout the dispersions as the stirring rate increased from 500 to 1500 rpm. Thus, the yields of 4-MB are determined by three principal factors: mass transfer, formation of hot spots, and the microwave input power. Interestingly, the yields of 4-MB at stirring rates of 1500 and 2000 rpm are not affected by the stirring and consequently the influence of mass transfer is no longer a factor under very fast stirring conditions beyond 1500 rpm; only the formation of hot spots and the microwave input power determine the synthesis yields of 4-MB. The three photographs reported in Figure 5 illustrate the mixing of the Pd/AC catalyst particles and the substrates at various stirring rates (500, 1000, and 1500 rpm). For the dispersion being stirred at 500 rpm, the Pd/AC particles tended to remain at the bottom of the reactor, hence there was

dispersiveness of the Pd/AC particulates. The reason(s) for this behavior was (were) examined from the perspective of the microwaves’ penetration depth into the reaction samples. The penetration depth (Dp) is the depth at which the microwaves pervade the material when the power flux falls to 1/e (=36.8%) of its surface value; the magnitude of Dp can be estimated from eq 2.13 ⎡ ⎤1/2 λ ⎢ 2 ⎥ Dp = 4π ⎢⎣ ε ′( 1 + (ε ″ /ε ′)2 − 1) ⎥⎦ r r r

(2)

The relative dielectric constant (εr′) and the relative dielectric loss factor (εr″) of the sample solution without the Pd/AC particles were experimentally determined: εr′ = 3.22 and εr″ = 0.53. From these values we estimate the penetration depth of the 2.45 GHz microwaves to be ca. 66 mm from the reactor walls fully permeating to the center of the sample in the narrow and wide reactors. That is, the generation of hot spots is related to the dispersiveness of the Pd/AC particulates rather than the penetration depth. A round-bottom flask reactor with larger diameter was also used (200 mm, inside diameter); no increase in the yield of 4-MB was seen relative to the wide reactor (Figure 3a). Examination of the data reported in Figure 3a indicates that at higher stirring rates (from 1000 to 2000 rpm) in the narrow reactor the number of hot spots decreased by increasing the stirring rate as evidenced by the significant increase in the yield of 4-MB after the 2 h irradiation period, as also observed with the high-speed camera. The synthesis of 4-MB under 1000 rpm conditions was also performed with the two types of reactors (narrow and wide) using heating by the conventional oil-bath method (Figure 3b). Within experimental error, there were no changes in the synthesis yields regardless of the size of the reactors (11 mm versus 25 mm). Thus, we infer that the differences in yields seen in Figure 3a, and thus the generation efficiency of hot spots, must be due to differences in the reactor sizes. Note that the generation of 4-MB displayed in Figure 3 followed growth exponential kinetics, which show that the quantities of 4-MB formed reached their respective plateaus even at longer reaction times; that is, the yields would not have changed greatly even at times longer than 2 h. 3.2. Influence of Stirring Rate. The influence of the stirring rate on the synthesis of 4-MB was examined by both the microwave heating method and the conventional oil-bath heating method (Figure 4). The experimental setup was the μReactor Ex microwave system operated at microwave power

Figure 4. Temporal increase of product yields of 4-methylbiphenyl (4MB) in toluene/1-hexanol solvent by microwave heating method (microwave power, 550 and 1000 W) and by oil-bath heating method under different stirring conditions: 0, 500, 1000, 1500, and 2000 rpm.

Figure 5. Photographs for mixture of heterogeneous Pd/AC catalytic solution condition in tube reactor (25 mm) with a magnetic stirrer: (a) 500, (b) 1000, and (c) 1500 rpm. 14944



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Formation of hot spots in the dispersions examined herein under microwave irradiation causes changes to the nature of the Pd catalyst particles supported on the AC surface. These changes were observed by transmission electron microscopy (TEM; Hitachi-high-technologies Co. HF-3300); see Figure 7.

ineffective mixing; at 1000 rpm the catalyst particulates and the substrates began to mix with a significant concentration of the Pd/AC particles at the center of the reactor. At 1500 rpm, we observed a more uniform distribution of the catalyst and substrates, that is, a better, more efficient contact between the reacting partners. The photographs that display the generation of hot spots in the heterogeneous sample at 0 rpm (no mixing) and under conditions of significant mixing (stirring at 1500 rpm) are collected in Figure 6. The hot spots formed under nonstirring

Figure 7. TEM images of aggregated Pd particles on activated carbon surface: (a) 0 min irradiation and (b) after 30 min of microwave irradiation (narrow tube reactor; stirring, 1000 rpm; microwave input power, 550 W).

The initial size of the nonirradiated Pd particles on the activated carbon surface ranged from 6 to 10 nm, even though they were stirred at 1000 rpm in the narrow tube reactor (Figure 7a). However, after the Pd/AC catalyst particles were subjected to 550 W microwave irradiation and stirred at 1000 rpm, the TEM image of Figure 7b clearly shows the significant changes that occurred on the AC surface, namely that as a result of the formation of hot spots the Pd particulates formed aggregates with significant increase in size from several tens of nanometers to several hundred nanometers after the 30 min irradiation period. Even though the melting point of bulk Pd metal is 1555 °C, the Pd nanoparticle on the AC surface is less than 10 nm, with the result that the melting point drops to ca. 300−900 °C.16 Therefore, melting of the Pd nanoparticles as a result of the existence of hot spots will take place under those high temperature conditions and lead to the formation of large size particles through the melting and aggregation events occurring on the AC surface. The fiber-optic thermometer coated with a silicone resin cloth was introduced into the reactor for the synthesis of 4-MB under 0 rpm conditions. (Note that the fiber-optic thermometer with a ceramics cloth was used in other experiments.) Upon microwave irradiation the silicone resin cloth on the fiber-optic thermometer melted with the heat of the intense hot spots. Inasmuch as the melting point of the silicone resin cloth is 260 °C, we infer that the temperature at a hot spot is likely greater than 260 °C. On the other hand, a piece of pure Teflon placed into the reactor and then irradiated with microwaves under nonstirring conditions caused no melting of the Teflon. Accordingly, the observations reported in Figure 6 imply that the temperature of the hot spots that formed on the activated carbon surface is likely greater than 930 °C. However, it is also possible that, since the sizes of the hot spots are very small, their calorific capacity is also small so that Teflon did not melt under those conditions. 3.3. Recycling of Catalyst Particles. Once the catalyst (Pd/AC) particulates had been used in the Suzuki−Miyaura cross-coupling reaction, they were recycled after thorough washing to be used again for this reaction (narrow tube reactor; stirring, 1000 rpm; microwave input power, 550 W). The

Figure 6. High-speed-camera photographs of electrical arc discharge occurring on Pd/AC catalyst surface during reaction in the 25 mm wide tube reactor under 550 W microwave irradiation and at stirring rates of (i) 0 , (ii) 0, and (iii) 1500 rpm.

conditions showed up as intense white sparkles (Figure 6i). With the turbulence created by the microwave heating, a large number of orange-colored hot spots were observed over a wide area (Figure 6ii). By contrast, no hot spots displaying the whitish intense sparkles were observed at 500 rpm stirring; only the orange-colored hot spots were seen (not shown in the figure). The number and frequency of formation of the orangecolored hot spots decreased considerably on increasing the stirring rate to 1500 rpm (Figure 6iii); in fact, there was a hint of only one hot spot. The bright white and orange hot spots reflect the blackbody radiation occurring on the microwave-activated AC carbon surface. Generally, white light is at a temperature greater than ca. 1400 °C, while the temperature of the orange light is at about 930 °C.14 We can presume, therefore, that such temperatures have been achieved on the activated carbon surface by concentration of the microwaves’ electric field. In this regard, Menéndez et al.15 examined various graphite specimens placed on an alumina (Al2O3) board and irradiated them with microwaves. They observed a microwave-induced discharge plasma as a ball lightning plasma and as a plasma arc as might occur in a domestic microwave oven. These authors also found that the temperature for the generation of the ball lightning plasma was less than 400 °C, and that the temperature of the plasma arc was in the 400−700 °C range. 14945



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(semiconductor microwave generator). The standing wave can reduce a reflected wave and can generate microwave heat rather efficiently. However, in the absence of a standing wave (i.e., nonstanding wave), the reflected waves increase and so the microwaves cannot generate efficient heating, which will then necessitate higher microwave input power levels to carry out the reaction under nonstanding wave conditions. For standing wave conditions, the microwave TW with an average power of 12.6 W hardly generated a reflected wave. By contrast, under nonstanding wave conditions the traveling wave and the reflected wave occurred at average powers of 49.2 and 28.5 W, respectively, a difference of 20.7 W of microwave power. The product yields in the synthesis of 4-MB under conditions of standing wave versus nonstanding wave and with stirring set at 1200 rpm are given in Figure 9. In the case of the standing

recycled Pd/AC catalyst particles resembled the aggregates displayed in Figure 7b; larger aggregated Pd particles were not observed with TEM observation in the latter case. However, there was a 19% drop in the yield of the 4-MB product (i.e., in the first use of the Pd/AC catalyst the yield was 26.0%, while in the second case the yield of 4-MB was only 21.1%). This decrease in yield is reminiscent of the results reported in an earlier report.6 The synthesis of 4-MB with the recycled Pd/AC catalyst was also carried out with the oil-bath heating method. There was a 9% drop in yield of product when the recycled catalyst particulates were used; that is, the fresh catalyst sample gave a 37.6% yield of 4-MB while for the recycled catalyst the yield was but 34.2%. Clearly, the catalyst activity decreased upon recycling the catalyst that has been subjected to the generated hot spots. 3.4. Influence of Pulsed versus Continuous Waves. The synthesis of 4-MB was carried out by pulsed wave (PW) microwave irradiation and by continuous wave (CW) microwave irradiation using the μ-Reactor Ex device (fixed stirring rates, 0 and 1000 rpm). The microwave input power for the PW and CW microwave irradiation is displayed in Figure 8a,

Figure 9. Product yields of 4-methylbiphenyl (4-MB) in toluene/1hexamol solvent using the single-mode cavity system under conditions of a standing wave versus nonstanding wave (stirring, 1200 rpm).

wave, the synthesis yields of 4-MB increased linearly with time, whereas under nonstanding wave conditions, the synthesis yields of 4-MB remained constant beyond 1 h into the reaction. The generation of hot spots observed with the high-speed camera indicated the formation of a significant number of hot spots under nonstanding wave conditions, thus impacting on the product yields. By contrast, hot spots hardly formed under conditions where the standing wave was generated. Hence, selective heating of a catalyst can be performed efficiently using standing wave conditions, while formation of hot spots is either minimized or otherwise suppressed.

Figure 8. (a) Microwave applied power for pulsed wave (PW) and continuous wave (CW) radiation. (b) Temperature−time profiles of temperature in heterogeneous solution. (c) Temporal changes of product yields of 4-methylbiphenyl (4-MB) in toluene/1-hexanol solvent by CW and PW methods (mixture stirred at 0 and 1000 rpm).

4. CONCLUDING REMARKS The present study examined the generation of hot spots and their impact on the yields of products in organic syntheses, as demonstrated by the formation of 4-methylbiphenyl from the heterogeneous Suzuki−Miyaura cross-coupling reaction in the presence of Pd/AC catalyst particles. Several factors were considered: the dispersiveness of the catalyst and substrates (i.e., mass transfer), pulsed wave versus continuous wave irradiation, and the presence of standing waves versus nonstanding waves when microwaves were used to carry out the reaction. Generation of hot spots can be clearly controlled by the dispersiveness of the catalyst in solution. However, even though the heterogeneous dispersions were stirred at fairly high speeds (up to 2000 rpm), formation of hot spots could not be suppressed completely. On the other hand, microwave irradiation with pulsed waves versus irradiation with continuous waves appears to have little or no impact on the formation of hot spots. Nonetheless, it was possible to control the formation of hot spots with microwave selective heating of Pd/AC catalyst particulates by power-saving microwave irradiation using standing waves.

whereas the time-dependent profiles of the reaction temperature are illustrated in Figure 8b. The microwave input power was set at an average of 550.3 W for the PW irradiation and 550.1 W for the CW irradiation. The synthesis yields of 4-MB under irradiation by the CW and PW microwaves are reported in Figure 8c under nonstirring (0 rpm) and at 1000 rpm stirring conditions. To the extent that each of the three experimental parameters (stirring rate, microwave input power, and temperature) was constant, the synthetic yields of 4-MB should also show no change, as observed. Accordingly, and in accord with our observations, formation of hot spots also occurred under pulsed wave (PW) irradiation. 3.5. Influence of Standing Wave versus Nonstanding Wave on Synthesis Yields. The dependence of the synthesis yields of 4-MB on the microwaves’ standing wave versus nonstanding wave was examined using the single-mode cavity 14946



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(15) Menéndez, J. A.; Juárez-Pérez, E. J.; Ruisánchez, E.; Bermúdez, J. M.; Arenillas, A. Ball lightning plasma and plasma arc formation during the microwave heating of carbons. Carbon 2011, 49, 346−349. (16) Guisbiers, G.; Abudukelimu, G.; Hourlier, D. Size-dependent catalytic and melting properties of platinum-palladium nanoparticles. Nanoscale Res. Lett. 2011, 6, 396−401.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81-3-3238-4662. Fax: +81-3-3238-3361. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Japan Society for the Promotion of Science (JSPS; Grant-in-Aid for Scientific Research No. C25420820) and from the Ministry of the Environment through the Environment Research and Technology Development Fund (Rehabilitation Adoption Budget) is gratefully appreciated. We are also grateful to the Sophia University-wide Collaborative Research Fund for a grant to S.H. N.S. thanks Prof. Albini of the University of Pavia (Italy) for his continued hospitality during the many winter semesters in his laboratory.

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REFERENCES

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Control of Microwave-Generated Hot Spots. 6. Generation of Hot Spots

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