Microwave Catalytic Reactors

magnetron 3, stirring bar 4, aluminum plate 5, magnetic stirrer 6, IR pyrometer 7, on-ofFswitch 8, water cooler. 

Domestic ovens can be inexpensively and safely modified, however this almost eliminates these disadvantages and enables independent temperature measurement and reasonable temperature control. For temperature measurement an IR thermometer or, better, a fiber-optic thermometer [75-77] has been recommended. Such a batch microwave reactor made by modification of a domestic microwave oven is depicted in Fig. 13.1 and has been described elsewhere (Refs. [51, 75-77, 141-144] and references cited therein). 

A complementary, more advanced, laboratory-scale microwave batch reactor for synthetic and kinetic studies has been developed by Strauss et al. (Fig. 13.2) [145]. 

The reactor is equipped with magnetic stirrer, microwave power, and temperature control by computer and can operate under pressure. Although it was developed for homogeneous organic synthetic reactions it can also be used for heterogeneous catalytic reactions in the liquid phase. 

The first continuous-flow reactor was developed by Strauss [144—148] and has recently been commercialized. It consists of microwave cavity fitted with a tubular coil (3 m X 3 mm) of microwave-transparent, inert material (Fig. 13.3). The coil is attached to a metering pump and pressure gauge at the inlet end and to a heat exchanger and pressure-regulating valve at the effluent end. Temperature is monitored outside the cavity at the inlet and the outlet. 

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For gas phase heterogeneous catalytic reactions, the continuous-flow integral catalytic reactors with packed catalyst bed have been exclusively used [61-91]. Continuous or short pulsed-radiation (milliseconds) was applied in catalytic studies (see Sect. 10.3.2). To avoid the creation of temperature gradients in the catalyst bed, a single-mode radiation system can be recommended. A typical example of the most advanced laboratory-scale microwave, continuous single-mode catalytic reactor has been described by Roussy et al. [79] and is shown in Figs. 10.4 and... 

Thomas JR, Faucher FJ (2000) Thermal modeling of microwave heated packed and fluidized bed catalytic reactors. Microw Power Electromagn Energy 35 165-174... 

Plazl, I., Pipus, G. and Koloini, T., Microwave heating of the continuous flow catalytic reactor in a nonuniform electric field, AIChE J., 1997, 43, 754. 

Figure 18 Yields of hydrogen cyanide in catalytic and noncatalytic microwave-heated reactors. (From Ref. 23.)...

Sample Preparation. Cobalt catalysts were prepared by subliming Co2(C0)g into the pores of dehydrated NaX zeolite in a vacuum line at pressures of 1 x 10- f torr. Argon was flowed over the metal loaded zeolite sample at a pressure of 0.3 torr. A microwave plasma was induced with a static gun and the decomposition of the metal carbonyl precursor occurred for two hours. After total decomposition of the metal carbonyl which can be determined by the color of the plasma, the argon flow was stopped and the sample was sealed off by closing the Teflon stopcocks at both ends of the reactor. The sample was then brought into a drybox and loaded into catalytic reactors or holders for spectroscopic experiments. Further details of this procedure can be found elsewhere (11, 25). Iron samples were prepared in a similar fashion except ferrocene was used as a metal precursor. 

By means of an ETHOS MR oven, Nuchter et al. [33] accomplished scaling-up of a microwave-assisted Fischer glycosylation to the kilogram scale with improved economic efficiency. In batch reactions, carbohydrates (o-glucose, o-mannose, d-galactose, butyl o-galactose, starch) were converted on the 50-g scale (95-100% yield, from 95 5 to 100%) with 3-30-fold molar excesses of an alcohol (methanol, ethanol, butanol, octanol) in the presence of a catalytic amount of acetyl chloride under pressure (microwave flow reactor, 120-140 °C, 12-16 bar, 5-12 min) or without applying pressure (120-140 °C or reflux temperature, 20-60 min). Furano-sides are not stable under these reaction conditions. 

The synthesis of pyrido[2,3-d]pyrimidin-7(8H)-ones has also been achieved by a microwave-assisted MCR [87-89] that is based on the Victory reaction of 6-oxotetrahydropyridine-3-carbonitrile 57, obtained by reaction of an Q ,/3-unsaturated ester 56 and malonitrile 47 (Z = CN). The one-pot cyclo condensation of 56, amidines 58 and methylene active nitriles 47, either malonitrile or ethyl cyanoacetate, at 100 °C for benzamidine or 140 °C for reactions with guanidine, in methanol in the presence of a catalytic amount of sodium methoxide gave 4-oxo-60 or 4-aminopyridopyrimidines 59, respectively, in only 10 min in a single-mode microwave reactor [87,88]... 

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. 

In a related study, Srivastava and Collibee employed polymer-supported triphenyl-phosphine in palladium-catalyzed cyanations [142]. Commercially available resin-bound triphenylphosphine was admixed with palladium(II) acetate in N,N-dimethyl-formamide in order to generate the heterogeneous catalytic system. The mixture was stirred for 2 h under nitrogen atmosphere in a sealed microwave reaction vessel, to achieve complete formation of the active palladium-phosphine complex. The septum was then removed and equimolar amounts of zinc(II) cyanide and the requisite aryl halide were added. After purging with nitrogen and resealing, the vessel was transferred to the microwave reactor and irradiated at 140 °C for 30-50 min... 

The most important apphcation of this metal is as control rod material for shielding in nuclear power reactors. Its thermal neutron absorption cross section is 46,000 bams. Other uses are in thermoelectric generating devices, as a thermoionic emitter, in yttrium-iron garnets in microwave filters to detect low intensity signals, as an activator in many phosphors, for deoxidation of molten titanium, and as a catalyst. Catalytic apphcations include decarboxylation of oxaloacetic acid conversion of ortho- to para-hydrogen and polymerization of ethylene. 

The catalytic reactions were performed either on the hydrated solids (equilibrated with the relative humidity -about 55%- of the atmosphere) or on the dehydrated catalysts (heated at 160°C, during 3 hours). An intimate mixture of the inorganic solid (100 mg) and the oxime in solid state (20 mg) was introduced in a Pyrex glass reactor. Thus, the reaction was carried out in "dry media" conditions, i.e. without any solvent. The mixtures were either activated with a microwave oven or heated at 100, 130 or 160°C in a conventional oven, during variable times (in the standard procedure 1 hour). The microwave oven used is a domestic (2450 MHz) Moulinex model FM 460, carrying out the experiments at 600 W of power and introducing a unic vessel in the oven in each experiment. The reaction products were extracted by treatment with a large excess (5 ml) of an appropriate solvent (methanol or chloroform), and the extracts were analyzed by GC. 

These standard and nonstandard reactors mentioned above have been widely used for promotion of various organic and inorganic reactions and processes [717-722] dehydration of crystal hydrates [723-726], optimization of catalytic processes [704], activation of elemental metals [720], synthesis of inorganic compounds, materials [719,727a], nanoparticles [727b], etc. From the point of view of the author of Ref. 728, microwave radiation has become a catalyst for chemical reactions. Microwave use for the preparation of some coordination and organo-... 

Earlier, the group of Laporterie reported on another prototype CF microwave reactor [80]. Solvent-free Friedel-Crafts reactions have been successfully carried out employing only catalytic amounts of the FeCl3 catalyst (Scheme 19). At a flow rate of 20-22 mL min 1 the corresponding substrates have been circulated in a molar scale (2 1 ratio) in the apparatus. Thus, 150-250 g products could be isolated. Excess substrates have been recovered by evaporation and recycled in the process. 

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