Microwave Flash Pyrolysis

Abstract

In a microwave reactor, graphite heats rapidly to high surface temperatures; applications of graphite thermal "sensitization" have been described previously. We report here that microwave thermal sensitization with graphite, carbon nanotubes, or silicon carbide can be used to carry out reactions more typically accomplished by flash vacuum pyrolysis (FVP) and which usually require temperatures much higher than the nominal limit of a microwave reactor. The graphite-sensitized microwave reaction of azulene in the solid phase at temperatures of 100 to 300 degrees C affords rapid rearrangement to naphthalene, a reaction typically observed by FVP at 700-900 degrees C. Multiwall carbon nanotubes give similar results when used as a thermal sensitizer. Other graphite-sensitized reactions that we have observed include the following: conversion of 2-ethynylbiphenyl to phenanthrene, fragmentation of phthalic anhydride to benzyne, cleavage of iodobenzene to phenyl radical, aryl-aryl bond cleavage, and a variety of cycloaromatizations. An advantage is seen for less volatile substrates. Rearrangement of azulene and generation of benzyne from phthalic anhydride have also been observed on powdered silicon carbide. Because of the high temperature, rapid heating, and frequent ejection of material from the irradiation zone, we refer to this general method as microwave flash pyrolysis (MFP).

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

“…[3] A variety of SiC materials in the form of powders, granules, and vessels (crucibles) have been used for some time in high-temperature microwave processing applications taking advantage of the extremely good microwave absorption properties, chemical resistance, and high thermal conductivity. [4][5][6][7][8][9] Several studies have described the use of SiC as socalled microwave susceptor or sensitizer in material sciences, [4,5] aiding for example in the rapid microwave-assisted synthesis of superconducting MgB 2 at temperatures close to 800 8C, [6] or in the microwave sintering of ZrO 2 ceramics. [7] In fact, because of its high melting point and chemical resistance, SiC crucibles have been used for the microwave melting of aluminum, copper, and nuclear waste glass.…”

Annulation Cascade of Aryl Alkynes Induced by Alumina‐Mediated CF Bond Activation

“…Due to the large thermodynamic driving force associated with alkyne hydroannulation, [ 14 ] CC represents a useful functionality for the synthesis of nanographenes (NGs). Thus, it has been used in benzannulation induced by transition metals, [ 15 ] Brønsted acids, [ 16 ] flash‐vacuum pyrolysis, [ 17 ] photochemical excitation, etc. However, all these methods enable only single CC bond formation per triple bond.…”

Azulene–Naphthalene‐Type Rearrangements in Benz[a]azulene and Cyclohepta[b]indole

“…(1)] has intrigued organic, physical and computational chemists for nearly 70 years . Although the reaction is usually achieved by high‐temperature flash, flow, or static pyrolysis, recent variations include the graphite‐sensitized, microwave‐induced rearrangement and a room temperature cationic reaction promoted by BF 3 ⋅ Et 2 O …”