Microwave Flow Chemistry as a Methodology in Organic Syntheses, Enzymatic Reactions, and Nanoparticle Syntheses

Abstract

Several studies have used microwaves as a heat source for carrying out various types of reactions employing circulation reaction vessels. The microwave flow chemical synthesis methodology is most appropriate in the use of microwaves in chemical syntheses. It can attenuate the problem of microwave heating (non-uniform heating and penetration depth) and maximize the benefits (rapid heating and first temperature adjustments). In this brief review, we examine and explain some of the relevant features of microwave heating with applicative examples of the usage of microwave flow chemistry equipment in carrying out organic syntheses, enzymatic reactions, and (not least) nanoparticle syntheses. Keywords: Microwave heating, Microwave organic flow synthesis, Microwave-assisted catalyzed flow reactions, Microwave flow synthesis of biomaterials, Microwave flow synthesis of nanoparticles e * ¼ e 0 Àje 00 ð1Þwhere e' and e'' are the dielectric constant and the dielectric loss, respectively, and j ¼ ffiffiffiffiffiffi ffi À1 p. The ratio of the dielectric loss to the dielectric constant is given by the loss tangent and is expressed by eqn. 2:[a] S. Horikoshi

Microwave-Promoted Continuous Flow Systems in Nanoparticle Synthesis—A Perspective

“…Owing to the heterogeneous mixing in batch reactors, conversion and selectivity are compromised when the speed of mixing is lower than the rate of reaction . Hence, the reactions taking place in a batch system even under MWH would require constant, rigorous stirring to ensure homogeneous heating, which would greatly reduce the benefits of using microwaves …”

“…A solution to these problems was initially proposed in 1990 in the form of the first-ever prototype microwave flow reactor. Since then, microwave heating has received the limelight it deserved and the advantages have become apparent in organic, ,,, inorganic, , and biological synthesis. As the technology is suitable for high-throughput and can be optimized for inexpensive preparation of inorganic nanomaterials, numerous researchers utilized microwave heating to increase the efficiency of various synthetic processes .…”

“…Horikoshi et al compared the two types of reactors experimentally, according to the throughput of the products from Heck reaction. For the batch reaction, a 10 mL vial was heated at 190 °C via microwave irradiation for 5 min at a power of 175 W, followed by a purification step using flash column chromatography.…”

“…5 Another key limitation is the inadequate depth of penetration of microwave irradiation into the absorbing medium, and this can be a major concern when using batch synthesis for mass production. 12 In most large-sized batch reactors, microwaves fail to reach the center of the mixture since most mediums have a penetration depth of no more than a few centimeters. For instance, microwaves, known to have a penetration depth of up to 1.8 cm in the water at 25 °C, will not heat water uniformly in a vessel with a 10 cm diameter.…”

Ultrasound and microwave irradiation: contributions of alternative physicochemical activation methods to Green Chemistry

“…Its use for chemical syntheses can attenuate the problem of MW heating (nonuniform heating and penetration depth) and maximize the benefits (rapid heating and first temperature adjustments). 101 The process intensification involving MW is currently an important challenge to develop green processes on larger scales. Interestingly, some researchers combined the effects of both, US and MW in the same system to highlight some synergetic effects.…”