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Microwave-Assisted Chemistry: Synthetic Applications for Rapid Assembly of Nanomaterials and Organics
Citation:
Gawande, M., S. Shelke, R. Zboril, AND R. Varma. Microwave-Assisted Chemistry: Synthetic Applications for Rapid Assembly of Nanomaterials and Organics. Accounts of Chemical Research. ACS Publications, Washington, DC, 47(4):1338-1348, (2014).
Impact/Purpose:
Sent for publication to American Chemical Society (ACS) Journal, Accounts of Chemical Research.
Description:
The magic of microwave (MW) heating technique, termed as the Bunsen burner of the 21th Century, has emerged as valuable alternative in synthesis of organics, polymers, inorganics, and nanomaterials. Important innovations in MW-assisted chemistry now enable chemists to prepare catalytic materials/nanomaterials and desired organic molecules selectively in almost quantitative yields and with greater precision. By controlling the specific MW parameters (temperature, pressure and ramping of heat) and choice of solvents, researchers can now move into the next generation of advanced nanomaterial design and development. Microwave-assisted chemical reactions are now well-established practices in the laboratory setting although some controversy lingers as to how MW irradiation is able to enhance, or influence, the outcome of chemical reactions. Much of the discussion has focused on whether the observed effects can, in all instances, be rationalized by purely thermal Arrhenius-based phenomena (thermal microwave effects), i.e. importance of the rapid heating and high bulk reaction temperatures that are achievable using MW dielectric heating in sealed reaction vessels, or whether these observations can be explained by so-called ‘non-thermal’ or ‘specific microwave’ effects. Silicon carbide (SiC) reactors appear to be good alternatives to MW transparent glass (Pyrex), because of their high microwave absorptivity, and as serve as valuable tools to demystify the claimed magical MW effects. These reactors can be utilized at elevated temperatures because of their higher melting point (2700 oC) and low thermal expansion coefficient. This enables one to evaluate the influence of the electromagnetic field on the specific chemical reactions, under truly identical conventional heating conditions, wherein temperature is measured accurately by fiber optic (FO) probe. This account describes the current status of MW-assisted synthesis highlighting the introduction of various prototypes of equipment and classes of organic reactions pursued using nanomaterials as nanocatalysts. Microwave irradiation, as a nonclassical energy source, has become increasingly useful for the synthesis of unique and multifunctional nanomaterials, and nanocatalysts; the ensuing nanomaterials possess zero-dimensional to three-dimensional shapes such as spherical, hexagonal, nanoprisms, star-shaped and nanorods, etc. The synthesis of well-defined nanomaterials and nanocatalysts is an integral part of nanotechnology and catalysis science, as it is imperative to control their size, shape, and compositional engineering for unique deployment in the field of nanocatalysis and organic synthesis. MW-assisted methods have been employed for the convenient and reproducible synthesis of well-defined noble and transition core–shell metallic nano-particles with tuneable shell thicknesses. Some of the distinctive attributes of MW selective heating in the synthesis and sustainable applications of magnetic nano-catalysts in organic synthesis under benign reactions conditions are highlighted.
