Multimode microwave heating

Fig. 2.7 Temperature profiles for a 30mL sample of 1-methyl-2-pyrrolidone heated under open-vessel microwave irradiation conditions [19]. Multimode microwave heating at different maximum power levels for 6 min with temperature control using the feedback from a fiber-...

Fig. 5.2 Temperature profile for a 30 ml sample ofwater heated under sealed-vessel conditions. Multimode microwave heating with 100 W maximum power for 8 min with temperature control using the feedback from a f ber-optic probe ramp within 120 s to 70 °C hold for 120 s at 70 °C ramp within 120 s to 100 °C hold for 120 s at 100 °C.

Another series of pyrroles, structurally related to amino acids, was obtained in a microwave-assisted solvent-free condensation of a-amino acid methyl esters with chloroenones, which provided the four-carbon unit of the pyrrole. The reaction was carried out by mixing the reagents on silica gel and irradiating for 2-6 min inside a multimode microwave cavity (Scheme 7). The authors reported higher yields and cleaner products when microwaves were used instead of conventional heating [34],... 

In conclusion, metal nanoclusters in DMF interact strongly with microwaves. In reactions catalysed by these clusters, the microwave heating may be tantamount to preferentially heating the catalytic site, which can lead to more effective catalysis. Such cluster-catalysed reactions can be in principle screened in parallel in multimode m/w ovens reducing both time and operational costs. However, the ovens must be adapted so that the parallel reactors are uniformly heated. 

Other microwave-assisted parallel processes, for example those involving solid-phase organic synthesis, are discussed in Section 7.1. In the majority of the cases described so far, domestic multimode microwave ovens were used as heating devices, without utilizing specialized reactor equipment. Since reactions in household multimode ovens are notoriously difficult to reproduce due to the lack of temperature and pressure control, pulsed irradiation, uneven electromagnetic field distribution, and the unpredictable formation of hotspots (Section 3.2), in most contemporary published methods dedicated commercially available multimode reactor systems for parallel processing are used. These multivessel rotor systems are described in detail in Section 3.4. 

The authors showed that it was possible to perform this reaction in a multimode microwave oven [19] in a few minutes on a large scale in water containing a slight excess of potassium hydroxide but without cosolvent. Under the action of classical heating the major problem with these syntheses is the instability of the thiophene o-amino acids, which readily decarboxylate at room temperature to give aminothio-phenes which are themselves unstable [20 a] and have to be used as soon as they are prepared. With large quantities of reactants, moreover, the hydrolysis step is not easy to perform because of the low reactivity of thiophene carboxylates 39 and 42 [20 b]. 

Antimicrobial oxazolidinones were successfully synthesized with the help of single-mode microwave heating on a polystyrene resin. In this case, the use of commercial multimode ovens was associated with inconsistent yields and purities, presumably due to the nonhomogeneity of the heating and a lack of sufficient temperature and pressure controls. A representative reaction is presented in Scheme 61. These solid-supported reactions proceeded smoothly in 5-10 minutes, with the boronic acid added in six equivalents and a small library with variations in both the N-acyl and the biaryl functionalities was created [145]. 

The integration of microwave heating and fluorous technologies has generated a powerful solution to address both the reaction and separation issues in parallel and combinatorial synthesis. With the further development of multimode microwave reactors for plate reactions and F-SPE for plate-to-plate separations, microwave-assisted fluorous synthesis will play an even more important role in compound library synthesis. 

Purification of carbon nanotubes has been performed in multimode [31] and monomode [32] systems. Prato purified HIPCO carbon nanotubes in a multimode oven. Common impurities are amorphous carbon and iron particles. The raw material was soaked in diethyl ether to obtain a more compact material. After evaporation of the solvent the flask was subjected to microwave heating (80 W) and an immediate weight loss occurred (5 s). This process was then repeated. Results from iron analysis were 16% (iv/iv) for the first run and 9% wjw) after the second heating cyde. It is clear in this instance that the microwave heating is selectively directed to the iron particles. It can be seen from Fig. 5.12 that, although the quality of the tubes remained similar to the original material, most of the iron spots had disappeared. 

When performing a reaction on a small scale using a monomode microwave unit, a temptation is to heat the reaction mixture using the full 300-400 W available. However, this can sometimes lead to uncontrollable temperature and pressure rises and, as a result, vessel failure. When scaling up reactions using larger multimode microwave units, the ability to heat the reaction mixture as intensively is much reduced since the power density is significantly lower. 

If the product has an irregular shape with no rectangular cross section, the multimode microwave cavity will provide more uniform heating. 

Recently, a wide range of organic reactions have been promoted by microwave irradiation," but in the field of Heck chemistry only a limited number of papers have appeared. " " "" Two types of microwave heating equipment have been used, a multimode reactor or a monomode reactor.The latter is more expensive but allows the placement of the reaction mixture at a fixed position of much higher continuous electric field strength than can be obtained in a multimode reactor." This is particularly important with Pd-catalyzed reactions since the reaction mixture must be heated to a high temperature in a reproducible and homogeneous fashion. 

Incorporating two new pieces of equipment will move this research program towards achieving controlled synthesis of catalytically active carbide and nitride nanolayers on early transition metals. Whereas the current microwave is multimode, like all domestic microwave ovens, a single axial-mode cylindrical cavity microwave oven will be acquired. This apparatus provides uniform continuously variable microwave heating that will eliminate the hot and cold spots that likely exist in the current oven. 

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