Sintered Silicon Carbide: A New Ceramic Vessel Material for Microwave Chemistry in Single‐Mode Reactors

Abstract

Silicon carbide (SiC) is a strongly microwave absorbing chemically inert ceramic material that can be utilized at extremely high temperatures due to its high melting point and very low thermal expansion coefficient. Microwave irradiation induces a flow of electrons in the semiconducting ceramic that heats the material very efficiently through resistance heating mechanisms. The use of SiC carbide reaction vessels in combination with a single-mode microwave reactor provides an almost complete shielding of the contents inside from the electromagnetic field. Therefore, such experiments do not involve electromagnetic field effects on the chemistry, since the semiconducting ceramic vial effectively prevents microwave irradiation from penetrating the reaction mixture. The involvement of electromagnetic field effects (specific/nonthermal microwave effects) on 21 selected chemical transformations was evaluated by comparing the results obtained in microwave-transparent Pyrex vials with experiments performed in SiC vials at the same reaction temperature. For most of the 21 reactions, the outcome in terms of conversion/purity/product yields using the two different vial types was virtually identical, indicating that the electromagnetic field had no direct influence on the reaction pathway. Due to the high chemical resistance of SiC, reactions involving corrosive reagents can be performed without degradation of the vessel material. Examples include high-temperature fluorine-chlorine exchange reactions using triethylamine trihydrofluoride, and the hydrolysis of nitriles with aqueous potassium hydroxide. The unique combination of high microwave absorptivity, thermal conductivity, and effusivity on the one hand, and excellent temperature, pressure and corrosion resistance on the other hand, makes this material ideal for the fabrication of reaction vessels for use in microwave reactors.

Rapid Wolff–Kishner reductions in a silicon carbide microreactor

“…In chemical synthesis, Kappe and co-workers used a microwave vial made out of SiC to perform high temperature chemical reactions which would corrode typical Pyrex microwave vials. 15 otherwise be impossible in traditional glass or metal reactors, without needing to commit the large amounts of material necessary to operate industrial scale SiC reactors…”

“…1 In general, the transformation involves mixing the substrate with a 15 stoichiometric amount of hydrazine to form water and an intermediate hydrazone that, in the presence of a strong base, liberates nitrogen gas to give the final product. While many procedures for performing Wolff-Kishner reductions exist, the most common is the Huang-Minlon modification, where a high 20 boiling solvent is used and the temperature of the reactor is gradually elevated over the course of several hours.…”

“…12 15 While the Wolff-Kishner reduction uses inexpensive hydrazine as a reducing agent and generates only water and nitrogen gas as waste products, alternative reduction methods are often chosen due to issues with high temperatures, high pH, reactor incompatibility, the need for multiple stages, and potential 20 decomposition of hydrazine.…”

Theoretical Aspects of Microwave Irradiation Practices

“…For the same heating profile using the Pyrex tube, the heating ramp observed for hexane and [Bmim] [PF 6 ] are completely different and the low absorbing microwave solvent (hexane) is unable to reach temperatures above 70 °C (Fig. 1.6) [27,28].…”

Microwave Assisted Reactions in Organic Chemistry: A Review of Recent Advances

“…A single-mode high field-density microwave reactor (2.45 GHz, 850 W) along with internal fiber-optic temperature control had been used in the reactions to monitor the reaction temperature accurately in a small scale(0.5-2 mL) (Obermayer, Gutmann, & Kappe, 2009;Gutmann et al, 2010). Most of the researchers have carried out the microwave-assisted proteomics studies in domestic microwave ovens without reliable reaction temperatures (Vaezzadeh et al, 2010;Hahn et al, 2009;Lesur et al, 2010).…”