Microwave pyrolysis results

The results shown in Table 21.1 do not imply that microwave pyrolysis is slower than conventional pyrolysis, but confirm the need to consider heat and/or mass transfer limitations because of the particle and size samples used in the experiments [26]. 

Table 21.2 shows various results for product (phases) yields for the degradation of PE at 500 and 600°C along with the results obtained using microwave pyrolysis. As can be seen in the table, in the latter case the increase in temperature caused little difference in the yields of the products. These results, which seem to contradict most previous findings, may be explained by the configuration of the microwave pyrolysis equipment. 

Simple thermal pyrolysis of lignocellulosics with or without additives has been reviewed by several authors (15-17). In contrast to microwave pyrolysis, reasonably extensive conventional pyrolysis product characterization has been conducted for certain types of biomass (15-25) and some of these results will be compared to those cited here. To these authors knowledge, no single study on microwave pyrolysis (plasma or di-electric loss mode) has identified the components of all product fractions nor their relative amounts work reported here has been extended by others (43,44) to include pyrolysis studies of biomass fractions and other types of biomass with the emphasis on detailed product characterization, formation kinetics, and effect of transport rates. 

Chemat et al. have reported several microwave reactors, including systems that can be used in tandem with other techniques such as sonication [68], and ultraviolet radiation [69]. With the microwave-ultrasound reactor, the esterification of acetic acid with n-propanol was studied along with the pyrolysis of urea. Improved results were claimed compared with those from conventional and microwave heating [68]. The efficacy of the microwave-UV reactor was demonstrated through the rearrangement of 2-benzoyloxyacetophenone to l-(2-hydroxyphenyl)-3-phenylpropan-l,3-dione [69]. 

By coupling an ultrasonic probe with a microwave reactor and propagating the ultrasound waves into the reactor via decalin introduced into their double jacket design, Chemat et al. studied the esterification of acetic acid with propanol and the pyrolysis of urea to afford a mixture of cyanuric acid, ameline and amelide (Scheme 9.19)136. Improved results were claimed compared to those obtained under conventional and microwave heating. The MW-US technique was also used to study the esterification of stearic acid with butanol and for sample preparation in chemical analysis137,138. 

Microwave treatment is widely used to prepare various refractory inorganic compounds and materials (double oxides, nitrides, carbides, semiconductors, glasses, ceramics, etc.) [705], as well as in organic processes [706,707] pyrolysis, esterification, and condensation reactions. Microwave syntheses of coordination and organometallic compounds, discussed in this chapter, are presented in a relatively small number of papers in the available literature. As is seen, the use of microwaves in coordination chemistry began not long ago and, due to the highly limited number of results, these works can be considered as a careful pioneer experimentation, in order to establish the suitability of this technique for synthetic coordination chemistry. 

Based on the pyrolysis of the neat hydrochloride salts of imidazolyl tetritols, e.g. 115a-c, which leads to the 4-glycofuranosyl-lH-imidazoles, e.g. 116a-c and their anomer mixtures, Tschamber et al. [80] obtained the same pairs of C-nucleosides in one-pot procedures by microwave irradiation with a domestic Whirlpool MO 100 oven for 1.7-3 min. Mixtures containing formamidine acetate, a few drops of water, and the appropriate hexose or hexulose, i.e. D-fructose (or o-glucose), d-galactose, or L-sorbose, resulted in overall yields of 19-28% (Scheme 12.48). Irradiation for longer periods led to caramelization. 

Carbon/silica adsorbents or carbosils have been prepared by both conventional and microwave heating. These materials were produced by pyrolysis of CH2CI2 on microporous silica gel surfaces for 30 min to 6 h at 550 °C. The resultant materials were hydrothermally treated with steam or liquid water using either a conventional autoclave or a microwave unit. As with the clay materials, hydrothermal treatment using microwave irradiation leads to a significant increase of surface area and total pore volume of the carbosils, as compared to conventional methods. 

Up to now, polymer pyrolysis has been investigated especially to develop ceramic fibers [46,47] and ceramic matrix for ceramic matrix composites [48-50]. More recently studies have been undertaken to exploit this method to develop ceramic thin films [51-53], foams [54], joints [55], and bulk materials [56]. Moreover, noncon-ventional heating systems such as laser [57], microwave heating [53], or even athermal conversion processes such as ion bombardment are just now starting to be applied to the polymer route and the preliminary results are very promising [58-60]. In this chapter we focus on the polymer processing of bulk ceramics obtained by pyrolysis of partially cross-linked preceramic bodies and of thin ceramic films (obtained either by traditional pyrolysis or by the innovative ion irradiation process). 

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