A novel phased array antenna system for microwave-assisted organic syntheses under waveguideless and applicatorless setup conditions

Abstract

A novel phased array antenna system was tested for use in microwave-assisted organic syntheses under waveguideless and applicatorless setup conditions with the synthesis of the ionic liquid 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) as an example; 5.8 GHz microwaves were used to irradiate the sample through free space a distance of 5 m from the antenna system.

---

Introduction

Microwave-assisted organic syntheses (MAOS) were reported for the first time by Gedye and Giguere in 1986 using domestic microwave ovens.^1,2 By the end of the 1900s, microwaves had reached the status as a material heating source for chemical reactions. So it is not unusual to find microwave chemical synthesis equipment in various organic laboratories. Several different organic chemical reactions have now been reported in a number of excellent technical books.³ However, scaling up microwave reactors to accommodate several liters of reagents from small-scale laboratory units of several milliliters has not kept pace with the number of microwave-assisted reactions because of the need to resolve a few issues connected with microwave usage. These issues have retarded the development of microwave-assisted chemistry at the industrial level. Nonetheless, several studies have focused on the possibility of scaled-up microwave chemistry.^4,5 Typically, microwaves are diffused in free space and therefore have had to be sheltered with metallic elements. Studies into the optimization of waveguides and applicators have continued to resolve this problem.⁶

A microwave single-mode applicator can be used very efficiently to irradiate samples with microwaves. However, only samples smaller than the wavelength of the microwaves (2.45 GHz = 12.24 cm) can be heated uniformly because the single mode applicator acts as a kind of resonator. Accordingly, any notion of scale-up with a single mode applicator requires the use of a flow-type reactor. By contrast, there are no limits as to the size of the samples when using microwaves with a multi-mode applicator.

Microwave usage in syntheses typically requires the reaction sample to be stirred or else the sample must be exposed uniformly to the microwaves using a turntable as currently found in domestic microwave ovens. Research into setting up a large-scale installation is of greater relevance than the scale-up of the microwave sample. If the microwaves are concentrated to mimic a laser beam so as to selectively microwave-irradiate the sample, then it is imperative to consider various ideas of novel equipment and applications. In this regard, a microwave semiconductor generator for microwave heating was recently reported as an alternative microwave source to a magnetron generator.⁷ In contrast to the latter, a semiconductor generator provides a much narrower microwave spectrum, is compact, and has a longer lifetime.⁸

Historically, the idea of a phased array antenna was proposed in radar development in 1958 at the Lincoln Laboratory;⁹ such antennas are currently well established in radar technology. Beam control of the microwaves is possible with semiconductor generators in performing microwave-assisted syntheses with such phased array antennas. Accordingly, the principal objective of the present study was to test the use of 5.8 GHz microwaves synthesized in free space with a phased array antenna to irradiate a reaction sample with no need for a waveguide (i.e., waveguideless) nor for an applicator (i.e., applicatorless). The synthesis of the room-temperature ionic liquid 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) was chosen as an example of an organic synthesis.

Experimental section

Microwave irradiation system with a phased array antenna

Photographs and a diagram of the microwave irradiation system with a phased array antenna are displayed in Fig. 1a–c. The microwave irradiation system was set on a mobile rack and consisted of a phased array antenna with circular-polarized 256 patch antenna elements, a 5.8 GHz microwave semiconductor generator and amplifiers (total maximum power, 1900 W) with a water cooling system, and a computerized beam control unit.¹⁰ The 256 antenna elements with a module containing a GaN amplifier (7 W max.) and a 5 bit MMIC phase shifter each made up the phased array for the 5.8 GHz microwave band. The size of each antenna element was 33 × 33 mm, each being installed on a plane (height, 630 mm; width, 740 mm); the 256 elements made up the 880 mm (width) by 950 mm (height) unit. The half power azimuth and elevation beamwidths were, respectively, 3.6 degrees and 4.2 degrees. For safety reasons, the synthesis was carried out in an anechoic chamber whose walls were covered with a suitable microwave-radiation absorbent material.

Fig. 1 (a) Photograph and (b) diagram of the microwave irradiation system with the phased array antenna; (c) front end photograph of the 256 antenna elements.

Synthesis of the ionic liquid 1-butyl-3-methylimidazolium chloride ([BMIM]Cl)

The room-temperature ionic liquid 1-butyl-3-methyl-imidazolium chloride ([BMIM]Cl) was synthesized by a solvent-free, one-pot microwave-assisted synthesis method as reported earlier by Horikoshi and coworkers.¹¹ The two reagents 1-methylimidazole (11.0 mL, 0.139 mol) and 1-chlorobutane (15.0 mL, 0.144 mol) were introduced into the quartz test tube reactor (dia., 14 mm; height, 250 mm) under argon-purged conditions, after which the reactor was connected to a reflux condenser (Fig. 2a); reactants were not stirred. Subsequently, the synthesis of [BMIM]Cl (reaction (1)) was carried out under microwave dielectric heating with the reactor being located at a distance of 5 m from the microwave phased array antennas (partially illustrated in Fig. 2b); also shown is the condensed microwave radiation applied to the sample solution from the 256 phased array antennas. The temperature of the reacting solution was monitored using an optical fibre thermometer (FL-2000, Anritsu Meter Co. Ltd).

Fig. 2 (a) Photograph of the reactor with the sample solution, and (b) photographic image of the microwave source with the phased array antennas irradiating the sample solution.

At the conclusion of the synthesis, the solution was added to acetonitrile yielding 1-butyl-3-methyl-imidazolium chloride; this product was repeatedly recrystallized from ethyl acetate solutions resulting in a white crystalline solid subsequently dried in vacuo at 100 °C for a few hours.

Analysis of the C₈H₁₅ClN₂ product gave C, 54.23%; H, 8.96%; N, 16.12% against the expected theoretical values C, 54.31%; H, 9.62%; N, 15.84%. Product integrity was further ascertained using a 500 MHz ¹H-NMR spectrometer (JEOL GX-270; CDCl₃ was the solvent; TMS was the reference standard) and by mass spectral methods using the JEOL JMS-700 FAB-MS spectrometer. The ¹H-NMR spectrum displayed chemical shifts δ at 10.54 (1H, s, NCH̲N), 7.55 (1H, m, CH₃NCHCH̲N), 7.40 (1H, m, CH₃NCH̲CHN), 4.26 (2H, t, J = 7.3 Hz, NCH̲₂(CH₂)₂CH₃), 4.11 (3H, s, NCH̲₃), 1.82 (2H, m, NCH₂CH̲₂CH₂CH₃), 1.30 (2H, m, N(CH₂)₂CH̲₂CH₃), 0.89 (3H, t, J = 7.3 Hz, N(CH₂)₃CH̲₃) in accord with a previous study.¹² The FAB mass spectral analysis (positive ion mode) gave m/z: 313 ({[BMIM]₂Cl}⁻), 139 ([BMIM]⁺).

Results and discussion

Effective microwave power density at the sample position

The total microwave maximum power of the semiconductor generator was 1900 W divided amongst the 256 antenna elements; as such, the incident maximum power for each antenna was ca. 7.42 W. The microwave radiation from the 630 × 740 mm antenna panel was phase-synthesized in space; the highest microwave strength was located at the centre position, with the strength decreasing away from the centre. Irradiating the reaction sample with microwaves was not without some difficulty, however, because of the relatively small size of the reactor vis-à-vis the size of the phased array antenna panel. The microwave power incident at the reactor was determined with a microwave receiving dipole antenna (Fig. 3), whose effective opening area was ca. 3.48 cm² (antenna gain: 2.15 dBi). The cross-sectional area of the sample solution in the reactor was approximately 2.7 cm². The effective applied microwave power levels at the sample are reported in Table 1. Even though the applied microwave power was 1900 W, the effective power at the reactor was some three orders of magnitude lower (only 0.512 W) so that most of the microwaves were not used for heating purposes.

Fig. 3 Photograph illustrating the theoretical measurement of the effective microwave irradiation (electric) power at the sample using the microwave receiving antenna.

Table 1 Irradiation microwave power from the phased array antenna and the theoretical microwave power incident on the sample

Microwave power from the phased array antenna	Theoretical microwave power to the sample
1710 W	0.396 W
1805 W	0.454 W
1900 W	0.512 W

---

Synthesis of the [BMIM]Cl ionic liquid

The temperatures of the solutions during the reaction at the various power levels were also monitored; they are displayed in Fig. 4a, which shows a faster increase of temperature for the 0.512 W power level relative to the others. No temperature differences were seen for the 0.454 W and 0.396 W conditions. At the 30 min mark, the temperatures of the solutions after microwave irradiation were, respectively, 69.3 °C, 71.6 °C and 76.7 °C, in relative accord with the respective yields of [BMIM]Cl (Fig. 4b).

Fig. 4 (a) Reaction temperature profile of the solutions at the various microwave applied power levels. (b) Chemical yields of [BMIM]Cl under various effective microwave applied power levels.

The synthesis of the ionic liquid was carried out under microwave irradiation of the reacting mixture for 30, 60 and 120 min periods; the resulting yields of [BMIM]Cl at various effective microwave power levels are displayed in Fig. 4b. After 30 min of irradiation, the chemical yields of [BMIM]Cl were 11%, 13% and 15% for an effective power level of 0.396 W, 0.454 W and 0.512 W, respectively, whereas after 120 min the chemical yields of [BMIM]Cl increased to 17%, 20% and 28% for 0.396 W, 0.454 W and 0.512 W. On a per watt basis, the relative yields at the microwave power levels incident on the reactor (0.396, 0.454 and 0.512 W) were 43% W⁻¹, 44% W⁻¹, 55% W⁻¹, respectively; though the increase in yield for the effective microwave power of 0.512 W may not be spectacular, it is nonetheless significant. The reactor most suitable for this novel type equipment could be increased significantly, so that it is possible to improve the yields.

Concluding remarks

This brief article has described a microwave chemical synthesis equipment that uses neither a waveguide nor an applicator but a phased array antenna system to deliver microwaves to a reactor some distance away (5 m) and has demonstrated its usefulness in driving the synthesis of the room-temperature ionic liquid [BMIM]Cl. Although preliminary yields have not been spectacular, the efficiency and the size of the microwave antenna system, relative to the size of the reactor, are two factors that necessitate further studies so as to optimize and demonstrate the potential utility of this new microwave irradiation apparatus in the syntheses of products and materials processing at the industrial scale. Additional future studies should also examine such other factors as reactor orientation, variations in the distance of the reactor from the antenna assembly, and suitability of stirring (among others).

Acknowledgements

We are grateful to the Japan Society for the Promotion of Science (JSPS) for financial support through a Grant-in-aid for Scientific Research (JSPS KAKENHI Grant No. 16K14856). We also thank Sophia University for a grant from the Sophia University-wide Collaborative Research Fund, and Advanced Materials (ADAM) and Microwave Energy Transmission Laboratory (METLAB) at the Research Institute for Sustainable Humanosphere, Kyoto University to S. H. One of us (N. S.) is grateful to Prof. Albini of the University of Pavia (Italy) for his continued hospitality in his PhotoGreen Laboratory.

Notes and references

1. R. Gedye, F. Smith, K. Westaway, H. Ali, L. Baldisera, L. Laberge and J. Rousell, Tetrahedron Lett., 1986, 27, 279–282.
2. R. J. Giguere, T. L. Bray, S. M. Duncan and G. Majetich, Tetrahedron Lett., 1986, 27, 4945–4948.
3. Microwaves in Organic Synthesis, ed. A. de la Hoz and A. Loupy, Wiley-VCH Verlag, GmbH, Weinheim, Germany, 3rd edn, 2012, vol. 1 and 2.
4. J. D. Moseley, P. Lenden, M. Lockwood, K. Ruda, J.-P. Sherlock, A. D. Thomson and J. P. Gilday, Org. Process Res. Dev., 2008, 12, 30–40.
5. N. G. Patil, F. Benaskar, E. V. Rebrov, J. Meuldijk, L. A. Hulshof, V. Hessel and J. C. Schouten, Org. Process Res. Dev., 2014, 18, 1400–1407.
6. B. Smith and M. H. Carpentier, The Microwave Engineering Handbook, Microwave Systems and Applications, Springer Science+Business Media, Dordrecht, 1993.
7. Microwaves in catalysis – methodology and applications, ed. S. Horikoshi and N. Serpone, Wiley-VCH, Weinheim, Germany, 2015.
8. S. Horikoshi and N. Serpone, Microwaves in Organic Synthesis, ed. A. de la Hoz and A. Loupy, Wiley-VCH Verlag, GmbH, Weinheim, Germany, 3rd edn, 2012, ch. 9, pp. 377–423.
9. A. J. Fenn, D. H. Temme, W. P. Delaney and W. E. Courtney, Lincoln Laboratory Journal, 2000, 12, 321–340.
10. Y. Homma, T. Sasaki, K. Namura, F. Sameshima, T. Ishikawa, H. Sumino and N. Shinohara, IMWS-IWPT2011 Proceedings, 2011, pp. 59–62.
11. S. Horikoshi, T. Hamamura, M. Kajitani, M. Yoshizawa-Fujita and N. Serpone, Org. Process Res. Dev., 2008, 12, 1089–1093.
12. http://www.rsc.org/suppdata/gc/b9/b916882f/b916882f.pdf, accessed August 13, 2016.

---