Microwave‐Assisted Reactions on Graphite

Abstract

for the processing of polymers and composites in which carbon black or graphite particles or fibers are included in the material [9].The MW-promoted cracking of organic molecules in the presence of silica-supported graphite [10 a] or activated charcoal [10 b] has also been reported.Graphite, the most stable of the three allotropic forms of carbon, has two structures, a (hexagonal form) and b (rhombohedral form), which interconvert easily [11]. In a graphite layer, each carbon atom is strongly bonded to three other carbon atoms in a planar configuration (sp 2 hybridization), and the remaining p electrons (one per carbon) are delocalized. The resulting carbon±carbon bonds are very strong (477 kJ mol ± 1 ). The interlayer bonds, in contrast, are weak (17 kJ mol ±1 ), giving rise to the mechanical (lubricant) and chemical (intercalation) properties of graphite. Electronically, graphite is a semimetal of high electrical and thermal conductivity [11]. As for other semimetallic materials, the electronic current (s) is the main factor in the graphite±MW interaction [5]. The rate of heating of a MW-irradiated material has been estimated to be DT/t = s|E| 2 /rC, where E is the electric field, r is the density, and C is the specific heat capacity of the material [5]. Compared with other dielectric solids graphite has an unusually high thermal conductivity (a weak C, 0.63 kJ kg ±1 K ±1 at room temp.) [11]. This thermal conductivity, which decreases exponentially with increasing temperature, is a determining factor in the high rate of heating of graphite on MW irradiation, although other types of MW interaction, e. g. the excitation of weak interlayer bonds, and, especially in graphite powder, eddy currents or localized plasma effects, can also lead to very rapid dissipation of energy in graphite [5].Because of its strong coupling with MW, its good adsorbent properties towards organic molecules [12], and its layer structure which enables it to form intercalated compounds [13], graphite has great potential in MW-assisted synthetic applications in organic chemistry, despite its weak fractal dimension (D & 2) [14].Before the works of Laurent et al. [15,16], which described several examples of MWassisted organic syntheses in the presence of graphite other studies were reported in this field. Literature results are, therefore, very recent. They are presented here in two parts. The first concerns reactions in which graphite behaves as an energy converter (or ªsensitizerº) capable of conveying the energy carried by MW radiation to the chemical reagents. The second includes results which reveal surprising catalytic activity of some metal inclusions of graphite. Graphite as a SensitizerThis section covers reactions in which graphite is a sensitizer, without participation of its metal inclusions as possible catalysts, although a catalyst can be added to the graphite. The amount of graphite can be varied. It is generally at least equal to and most often greater than that of the reagents, resulting in a graphite-supported/microwave (GS/M...

Advantages and Limitations of Microwave Reactors: From Chemical Synthesis to the Catalytic Valorization of Biobased Chemicals

“…Microwave-assisted heterogeneous catalysis was reviewed by multiple groups in gas phase , and most recently and thoroughly in 2009 by Durka, Gerven, and Sankiewicz and in the liquid phase, ,,− ,, most recently in 2018 by the group of Lopez-Sanchez focusing on catalytic conversion involving renewables, such as hydrolysis, (trans)­esterification, oxidation, hydrogenation, or dehydration and in 2014 briefly by Horikoshi and Serpone . Highlighting the 2018 review, higher yields and/or shorter times were reported in cellulose, glucose, fructose, and sucrose dehydration to 5-hydroxymethylfurfural, − although comparison of dehydration of xylose to furfural in a water/NaCl/HCl solution in a SiC vessel (“conventional heating”) vs microwave by Xiouras and co-workers showed similar reaction kinetics between the two reported heating methods although a MW-based run required 30% less power to perform the reaction in comparison with conventional heating.…”

“…Of special interest within the field of renewables and heterogeneous catalysis could be the use of carbon-based materials as especially suited for microwave-assisted catalysis. ,,− These are connected to the recent reviews on conversion and treatment of biomass, which is composed of carbon to a big extent, possibly carried out in biorefineries. ,,− Further reviews on specific reactions that are many times performed over heterogeneous catalysts, such as hydrolysis, production of biodiesel, or pyrolysis, ,,,− are available.…”

“…They can be advantageous for the conversion of biomass and byproducts, − but also waste processing and reactions, especially using heterogeneous catalysts, − both of which can be selectively heated by microwaves. , Pyrolysis, − torrefaction or carbonization, − bioactive compound extraction, , hydrolysis, ,− or biodiesel production are just a few examples of how many renewable sources could be valorized with the help of microwaves. Biomass and many of the compounds derived from it are susceptible to microwaves, but catalysts and especially those based on carbon can further enhance the absorption of MW. ,,− …”

Microwave Flash Pyrolysis

“…In a microwave oven, graphite 4 and carbon nanotubes [5][6][7] absorb radiation with high efficiency and heat rapidly to high surface temperatures. Applications of graphite as a thermal susceptor or "sensitizer" in microwave chemistry have been reviewed by Laporterie, who developed this method, 8 and more recently by Besson. 9 At present, few reported graphite sensitized reactions venture toward the more extreme regime of flash vacuum pyrolysis.…”

Microwave-Assisted Carbohydrohalogenation of First-Row Transition-Metal Oxides (M = V, Cr, Mn, Fe, Co, Ni, Cu) with the Formation of Element Halides

“…For the reduction of element oxides, different kinds of carbon sources can be applied. Because of its very attractive microwave absorption coefficient, the use of graphite gives the best results . For the use of different carbon sources with a lower microwave absorption coefficient, graphite might be admixed to the oxide/carbon mixture to reach reaction temperatures of about 1000–1400 °C.…”