Microwave dehydration

Durable changes of the catalytic properties of supported platinum induced by microwave irradiation have been also recorded [29]. A drastic reduction of the time of activation (from 9 h to 10 min) was observed in the activation of NaY zeolite catalyst by microwave dehydration in comparison with conventional thermal activation [30]. The very efficient activation and regeneration of zeolites by microwave heating can be explained by the direct desorption of water molecules from zeolite by the electromagnetic field this process is independent of the temperature of the solid [31]. Interaction between the adsorbed molecules and the microwave field does not result simply in heating of the system. Desorption is much faster than in the conventional thermal process, because transport of water molecules from the inside of the zeolite pores is much faster than the usual diffusion process. 

A large class of coordination compounds, metal chelates, is represented in relation to microwave treatment by a relatively small number of reported data, mainly p-diketonates. Thus, volatile copper) II) acetylacetonate was used for the preparation of copper thin films in Ar — H2 atmosphere at ambient temperature by microwave plasma-enhanced chemical vapor deposition (CVD) [735a]. The formed pure copper films with a resistance of 2 3 pS2 cm were deposited on Si substrates. It is noted that oxygen atoms were never detected in the deposited material since Cu — O intramolecular bonds are totally broken by microwave plasma-assisted decomposition of the copper complex. Another acetylacetonate, Zr(acac)4, was prepared from its hydrate Zr(acac)4 10H2O by microwave dehydration of the latter [726]. It is shown [704] that microwave treatment is an effective dehydration technique for various compounds and materials. Use of microwave irradiation in the synthesis of some transition metal phthalocyanines is reported in Sec. 5.1.1. Their relatives - porphyrins - were also obtained in this way [735b]. 

Prothon, E, Ahrne, L., Funebo, T., Kidman, S., Langton, M., and Sjoholm, I. Effects of combined osmotic and microwave dehydration of apple on texture, microstructure and rehydration characteristics, Lebensm. Wiss. TechnoL, 34, 95, 2001. 

Figiel, A., 2009. Drying kinetics and quality of vacuum-microwave dehydrated garlic cloves and slices. J. Food Eng. 94 98-104. 

Physical and sensory properties of vacuum microwave dehydrated shrimp. /. Aquat. Food Prod. Tedmol. 8 41-53. 

Sheinburg et al have explored several evaporative approaches to powder formation from aqueous nitrate solutions in the Gd-Ba-Cu-0 system including microwave drying and decomposition. Simple microwave dehydration of the nitrate solutions yielded fine powders, with continued heating causing some decomposition (though not the Ba(N03)2). The principle problems with... 

Hydration and Dehydration. Succinic anhydride reacts slowly with cold water and rapidly with hot water to give the acid. For this reason it must be carefully stored in anhydrous conditions. Succinic acid can be dehydrated to the anhydride by heating at 200°C, optionally in the presence of a solvent (31). Dehydration can also be performed with clay catalysis in the presence of isopropenyl acetate under microwave irradiation (32) or with his (trichi oromethyl) carbonate at room temperature (33). 

A new synthetic route for functionalized polyhydroxyalkyl-pyrimidines starting from unprotected aldoses and based on montmorillonite K-10 catalysis and solvent-free microwave irradiation conditions, has been reported by Yadav et al,m Thus, reaction of D-glucose and D-xylose with semicarbazide or thiosemicarbazide (186) in the presence of montmorillonite K-10, under microwave irradiation, proceeded via domino cycloisomerization, dehydrazination, and dehydration of the intermediate semi- or thiosemicarbazones (187) to afford l,3-oxazin-2-ones or l,3-oxazine-2-thiones (188) in one single step and in yields between 79% and 85% (Scheme 34). Other mineral catalysts tested, such as silica gel and basic alumina, were far less effective for this transformation and only silica gel was active at all, giving low yields (15-28%) of compounds 188a-d. The l,3-oxazin-2-ones(thiones) thus synthesized were subsequently converted into the target pyrimidines by reaction with aromatic... 

In the context of preparing analogues of chiral l,2-dimethyl-3-(2-naphthyl)-3-hy-droxy-pyrrolidines, which are known non-peptide antinociceptive agents, Collina and coworkers have reported the solvent-free dehydration of hydroxypyrrolidines to pyrrolines under microwave conditions (Scheme 6.141) [278]. In a typical experiment, the substrate was adsorbed onto a large excess of anhydrous ferric(III) chloride on silica gel and then irradiated as a powder under microwave conditions for 30 min at 150 °C. The microwave method leads to dehydration without racemiza-tion and provides higher yields in considerably shorter times than the conventionally heated process. 

One of the most common approaches to pyrrole synthesis is the Paal-Knorr reaction, in which 1,4-dicarbonyl compounds are converted to pyrroles by acid-mediated dehydrative cyclization in the presence of a primary amine. The group of Taddei has reported a microwave-assisted variation of the Paal-Knorr procedure, whereby a small array of tetrasubstituted pyrroles was obtained (Scheme 6.181) [342], The pyrroles were effectively synthesized by heating a solution of the appropriate 1,4-dicarbonyl compound in the presence of 5 equivalents of the primary amine in acetic acid at 180 °C for 3 min. The same result was obtained by heating an identical mixture under open-vessel microwave conditions (reflux) for 5 min. Interestingly, the authors were unable to achieve meaningful product yields when attempting to carry out the same transformation by oil-bath heating. 

A set of heterocyclic ketones including 1,3,4-oxadi azole-linked compounds were synthesized via a dehydrative cyclization using the Burgess reagent in a single-mode microwave <2003BML3909>. 

The reaction of benzo-2-furancarboxylic acid with thiosemicarbazide under microwave conditions enables the use of an equivalent amount of phosphorus oxychloride and a short reaction time <2003SC2891 >. Moreover, solvent-free conditions are achieved when acidic alumina is used as dehydrating agent under microwave heating in the reaction of alkyl carboxylic acids with thiosemicarbazide <2000SC3031>. 

Microwave-assisted (0.1 or 0.2 Wg-1) convection drying was also applied to osmotically dehydrated blueberries, leading to dried berries that were comparable to freeze-dried ones in much shorter time (Venkatachalapathy and Raghavan, 1998). Frozen blueberries were also dried in a microwave and spouted bed combined dryer (MWSB) after a pretreatment using ethyl oleate and a NaOH dipping solution followed by sucrose osmotic treatment (Feng et al., 1999). Osmotic dehydration prevented the blueberries from... 

Erie, U. and Shubert, H. 2001. Combined osmotic and microwave-vacuum dehydration of apples and strawberries. J. Food Engineer. 49, 193-199. 

Torringa, E., Esveld, E., Scheewe, I., van den Berg, R., and Bartels, P. 2001. Osmotic dehydration as a pre-treatment before combined microwave-hot-air drying of mushrooms. J. Food Engineer. 49,... 

Venkatachalapathy, K. and Raghavan, G.S.V. 1998. Microwave drying of osmotically dehydrated blueberries. J. Microwave Power Electromagn. Energy 33, 95-102. 

Mixtures of organic solvent and water have also been studied (Scheme 11). hi this context, Watanabe and coworkers studied the catalytic dehydration of fructose to HMF at 150°C in acetone-water mixtures and in the presence of a cation-exchange resin catalyst (Dowex 50wx8-100) [92]. The use of acetone-water (70 30 w/w) as reaction medium resulted in a yield of HMF of 73% at 94% conversion. Moreover, under these conditions, the catalyst was stable for at least five catalytic runs. Assistance of microwave not only increased the selectivity to HMF but also had a beneficial effect on the reaction rate. In this context, Gaset et al. studied the activity of Lewatit SPC-108 (cation-exchange resin) in a mixture of organic solvent (MIBK or diethyl ketone or benzonitrile or butyronitrile or dichlor-oethylether or nitropropane) and water (from 1/7 to 1/12 by volume) at a temperature around 85-90°C Under these conditions, HMF has been obtained with a yield of 70-80% [93, 94]. 

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. 

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