On the importance of simultaneous infrared/fiber-optic temperature monitoring in the microwave-assisted synthesis of ionic liquids

Abstract

The temperature profiles obtained from both an external infrared and internal fiber-optic sensor were compared for heating and synthesizing the ionic liquid 1-butyl-3-methylimidazolium bromide (bmimBr) under microwave conditions. Utilizing a single-mode microwave reactor that allows simultaneous infrared/fiber-optic temperature measurements, significant differences between the two methods of temperature monitoring were revealed. Due to the strong microwave absorptivity of ionic liquids and the delay experienced in monitoring temperature on the outer surface of a heavy-walled glass vial, external infrared temperature sensors can not be used to accurately control the temperature in the heating of ionic liquids under microwave conditions. The use of internal fiber-optic probes allows the monitoring and control of the heating behavior in a much better way. In order to prevent the strong exotherm in the synthesis of bmimBr under microwave conditions the use of a reaction vessel made out of silicon carbide is the method of choice. Because of the high thermal conductivity and effusivity of silicon carbide, the heat generated during the ionic liquid formation is efficiently exchanged with the comparatively cool air in the microwave cavity via the silicon carbide ceramic.

Rapid, microwave-assisted thermal polymerization of poly(ethylene glycol) diacrylate-supported ionogels

“…Microwave heating of several common ionic liquids has been well-studied; 14,15,[17][18][19] however, to date, there have been no known reports of the use of microwave irradiation to create a free-standing, solid ionogel electrolyte.…”

Microwave in Ionic Liquid Synthesis

“…36 O sensor de infravermelho mede a temperatura da parede do recipiente da reação, logo a precisão da medida depende da condutividade térmica material do reator. Uma alternativa para evitar o superaquecimento causado pela condução iônica em líquidos iônicos envolve o uso de recipientes de reação, os quais apresentam alta absortividade à radiação e alta condutividade térmica, como carbeto de silício (SiC) e que promovem a igualdade das temperaturas interna e da parede do reator.…”

“…42 Obermayer e Kappe utilizaram rampa de aquecimento e controle de temperatura por fibraótica para atenuar o superaquecimento gerado pela liberação de calor na alquilação com bromo-butano (80 para 160 ºC em poucos minutos). 36 Os autores obtiveram C 4 mimBr (7, X= Br) em ótimos rendimentos em menos de 30 minutos de irradiação empregando o reator Monowave 300 da Anton Paar, que opera somente com sistema fechado (Tabela 2, entradas 1 e 2). Entretanto, Aupoix e colaboradores obtiveram C 4 mimBr (7, X = Br), em bom rendimento, através de condições mais brandas e sem rampa de aquecimento, mas não há informações sobre o monitoramento da temperatura (entrada 3).…”

Ionic Liquids as Doping Agents in Microwave Assisted Reactions

“…While these are great solvents for performing the reaction, the subsequent workup procedure is difficult to remove them due to their high boiling point and miscibility with the product [53]. Thus, in certain situations, it is convenient to use solvents which are less polar such as THF, TOL and HEX [14,58,[59][60]. However, it is necessary to use a heating agent for the reactions carried out in solvents with low absorption in the MW irradiation.…”

“…A reaction medium with a high tan δ at the standard operating frequency of a MW synthesis reactor (2.45 GHz) is required for good absorption and, consequently, for efficient heating. Solvents used for MW synthesis can be classified as high with tan δ > 0.5, medium with tan δ 0.1 -0.5, and low MW absorbing with tan δ < 0.1 (Table 1) [14,58]. In general, the reactions which used solvents with a high tan δ have a good absorption of MW irradiation and, accordingly, an efficient heating [8,59,60].…”