Closed-vessel microwave systems

Table 3.23 gives an overview of the vessel types in use for microwave applications. It is especially important to distinguish between open vessel (as used in Sox wave ) and closed vessel (pressurised) microwave heating systems (as in MAE). Both open-vessel and closed-vessel microwave systems use direct absorption of microwave radiation through essentially microwave transparent vessel materials (Teflon, PC). 

Recently, an extension of the above-described focused single-closed vessel microwave-heated system was developed that allows the simultaneous treatment of up to six samples in a gas-pressurized metal chamber while controlling and measuring the pressure in one vessel. Pressures up to 130 bar and temperatures up to 320°C can thus be reached [32]. 

Microwave digestion systems have become very popular for decomposing samples. The photo shown is a closed-vessel microwave digestion system for high-pressure digestions. A microwave oven with a built-in fume exhaust system is shown along with sample trays that contain up to 1 2 samples. Teflon sample vessels can be operated at temperatures up to 2300 C and 625 psi. 

Usually, multi-mode systems use closed type vessels and focused systems use open vessels. /. Closed-Vessel Microwave Devices... 

A closed vessel microwave digestion system (MLS-1200 mega, Mileston s.r.l, Italy) was used for the extraction of the standard-spiked PUF and filter samples... 

The first completely re-engineered laboratory-focused microwave system was introduced by Prolabo in 1986. Most commercial open-vessel microwave systems manufactured since then are of the focused-microwave type, i.e., they use the waveguide as a single-mode cavity. Since their introduction, they have widely been used for sample extraction, substituting in most cases the closed-vessels systems, which as of now are used mainly for carrying out sample digestions. 

FIGURE 11.33 Closed-vessel microwave-assisted extraction system (reprinted with permission from H. M. Kingston and L. B. lassie, eds.. Introduction to Microwave Sample Preparation, American Chemical Society, 1998). 

Although microwave-heated organic reactions can be smoothly conducted in open vessels, it is often of interest to work with closed systems, especially if superheating and high-pressure conditions are desired. When working under pressure it is strongly recommended to use reactors equipped with efficient temperature feedback coupled to the power control and/or to use pressure-relief devices in the reaction vessels to avoid vessel rupture. Another potential hazard is the formation of electric arcs in the cavity [2], Closed vessels can be sealed under an inert gas atmosphere to reduce the risk of explosions. 

Microwave heating has proven to be of benefit particularly for reactions under dry media (e.g., solvent-free conditions) in open vessel systems (i.e., in the absence of a solvent, on solid support with or without catalysts) [4]. Reactions under dry conditions were originally developed in the late 1980 s [51], but solventless systems under microwave conditions offer several additional advantages. The absence of solvent reduces the risk of explosions when the reaction takes place in a closed vessel. Moreover, aprotic dipolar solvents with high boiling points are expensive and difficult to remove from the reaction mixtures. During microwave induction of reactions under dry conditions, the reactants adsorbed on the surface of alumina, silica gel, clay, and other mineral supports absorb microwaves whereas the support does not, and transmission of microwaves is not restricted. Moreover, microwaves can interact directly with reagents and, therefore, can more efficiently drive chemical reactions. The possible accelerations of such reactions are expected... 

The basic components of a microwave system include a microwave generator (magnetron), a waveguide for transmission, a resonant cavity, and a power supply. For safety and other reasons, domestic microwave ovens are not suitable for laboratory use. There are two types of laboratory microwave units. One uses closed extraction vessels under elevated pressure the other uses open vessels under atmospheric pressure. Table 3.12 lists the features of some commercial MAE systems. 

Closed-vessel units were the first commercially available microwave ovens for laboratory use. A schematic diagram of such a system is shown in... 

The 1,2,4-triazine core is a synthetically important scaffold because it could be readily transformed into a range of different heterocyclic systems such as pyridines (Sect. 3.1) via intramolecular Diels-Alder reactions with acetylenes. 1,2,4-Triazines have been synthesized by the condensation of 1,2-diketones with acid hydrazides in the presence of NH4OH in acetic acid for up to 24 h at reflux temperature. Microwave dielectric heating in closed vessels allowed the reaction to be performed at 180 °C (60 °C above the boiling point of acetic acid). As a result, the reaction time was reduced to merely 5 minutes. Subsequently, a 48-membered library of 1,2,4-triazines was generated from diverse acyl hydrazides and a-diketones [139]. Two thirds of the desired heterocycles precipitated from the reaction mixture upon cooling with > 75% purity, while the remaining part of the library was purified by preparative LCMS (Scheme 56). 

The CMR and MBRs provided the basis for modern commercial microwave reactors, including robotically operated automated systems that are now widely employed in synthetic research and pilot-scale laboratories in academia and industry [13]. Since 2000, commercial microwave reactors have become available. Batch systems, produced by three major companies in Italy and Germany, Sweden and the United States, typically operate on a scale from 0.5 mL up to 2 L. Other companies based in Austria, Poland and Japan have also recently entered the market. Systems possessing either multimodal or monomodal cavities are produced with one recent addition being a single unit capable of performing in either mode as required. Microwave reactors are employed extensively in chemical discovery where successive reactions can be performed rapidly in parallel or sequentially. One manufacturer recently estimated that about 10000 reactions per week were performed in its systems alone. This indicates the extent to which microwave chemistry in closed vessels has dramatically influenced approaches to synthesis. 

Scale-up as defined for this chapter covers batch reactions in closed vessels at the > 50 mL scale, flow systems employing flow cells > 5 mL, and SF containers of > 50 mL volume. Microwave-mediated scale-up reactions under open vessel conditions are not discussed in detail as up to date only a few examples have been published [33-37] and most of the beneficial rate enhancements have been reported under sealed vessel conditions. 

Microwave assisted wet digestion has attracted considerable attention and has been successfully applied to plant material. Both open and closed vessels have been used, but the most popular approach is the sealed bomb method (Kingston and Jassie, 1988 Sulcek and Povondra, 1989 Matusiewicz, 1991). Karanassios et al. (1991) describe microwave stopped flow digestion systems that can give rapid (ie, less than 5 min) reproducible extractions of elements of environmental concern from plant samples. 

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