16th International Conference
on Microwave and High Frequency Heating

18-21 September 2017, Delft, The Netherlands
09:45   Process intensification with electromagnetic energy (1)
Chair: Georgios Stefanidis
09:45
15 mins
HYDROGEN PEROXIDE – OXIDATION UNDER MICROWAVE IRRADIATION
Dariusz Bogdal
Abstract: In our work we have successfully used hydrogen peroxide for oxidation of various organic compounds under microwave conditions. First, we considered oxidation of primary and secondary alcohols to corresponding carboxylic acids and ketons as well as N-oxidation reactions. The reactions were performed under phase-transfer catalysis conditions (PTC) in the presence of hydrogen peroxide or urea-hydrogen peroxide complex (UHP) /Na 2 WO 4 /tetrabutyloamonium hydrogensulfate (TBAHS) or hexadecyltrimethyl hydrogensulfate (HDTMHS) as catalysts. Then the reaction systems were modified and hydrogen peroxide encapsulated in silica xerogels was applied as oxidizing agent. The xerogel is readily heated by microwave irradiation could be used as both an oxidizing agent and as solid support for microwave assisted solvent-free oxidation. Finally, Zn-polyoxo- metalated were used as a catalyst primary and secondary alcohols were oxidized to carboxylic acids and ketones, respectively, in short reaction times (ca. 15 min) under microwave-pressurized conditions. Then, we introduced bromine and chlorine atoms in the halooxidation reactions using H 2 O 2 /HX (HX- hydrohalide acid) system and microwave irradiation. This method, in which bromine and chlorine are generated in situ in the reaction of H 2 O 2 and HX, has several advantages over other bromination/chlorination protocols because whole amount of bromine/chlorine used for a reaction is consumed. Moreover, this system is much more easier to handle since bromine transfer and storage facilities are not required. Eventually, we have exploited the method employing the hydrogen peroxide as an oxidant and microwaves irradiation to obtain the epoxy-like compounds from simple alkenes as well as vegetable oils, which in turn we used for the preparation of polyols and polymers. In conclusion, hydrogen peroxide seems to be an very efficient oxidizing agent under the microwave conditions.
10:00
15 mins
MICROWAVE-ASSISTED OXIDATION OF ALCOHOLS TO ALDEHYDES AND KETONES USING ZINC POLYOXOMETALATE CATALYST
S Dworakowska, W Kasprzyk, S Bednarz, Dariusz Bogdal
Abstract: The influence of microwave irradiation on the acceleration of many organic reactions has been described in the literature [1]. Oxidation processes, for instance, could be effectively conducted under microwave conditions especially by using hydrogen peroxide as oxidant [2-7]. The peroxide is a proper oxidizing agent in terms of non-toxic side products (i.e. water) and high oxidation potential, but it requires chemical activation (mainly by d-block metals). Among others, polyoxometalates (POMs) show promising catalytic performance in liquid phase oxidation reactions mediated by hydrogen peroxide [8,9]. Therefore, in the present work we applied microwave irradiation together with zinc polyoxometalate catalyst (POMZn) in the oxidation of various alcohols. Reactions proceeded in biphasic organic-water solvent systems under microwave irradiation conditions and conventionally using high pressure reactors. Hydrogen peroxide (35%) was used as oxidizing agent with a molar ratio of 2:1 with respect to the substrate. Application of POMZn catalyst in the presence of 35% aq. hydrogen peroxide is a promising route to convert alcohols and obtain products with satisfactory yields. Furthermore, the results indicate that microwave irradiation accelerates hydrogen peroxide mediated oxidation of alcohols more efficiently in comparison to conventional heating method.
10:15
15 mins
ON THE MOST PROMISING ADVANCEMENTS AND PERSPECTIVES IN THE USE OF MICROWAVE ENERGY FOR COMBUSTION SYNTHESIS
Roberto Rosa, Lorenzo Trombi, Paolo Veronesi, Cristina Leonelli
Abstract: Aim of this work is to show how microwave-heating characteristics can be proficiently exploited in several combustion synthesis (CS)-based processes occurring between solid-state reactants. Indeed, despite extremely high heating rates, that can modify fundamental combustion parameters, the selectivity and the volumetric nature of dielectric heating together with its energy transfer-based foundation, smooth the way towards a plenty of exciting possibilities in advanced materials processing, including for example the control of the microstructure of high temperature coatings, and the limitation of heat affected zones of the substrate joined by a CS approach. Moreover an extremely high versatility will be also demonstrated in the case of an ideal chemical environment to interact with the electromagnetic energy at the microwave frequencies, that is solution combustion synthesis (SCS), used for the obtainment of different nano-structured materials. In order to gain a comprehensive and trustworthy knowledge of microwave ignited combustion synthesis related phenomena, experiments (both in solid state and in solution) need to be carried out in single mode applicators. In most of the reported experimental applications, the great support offered by a simultaneously performed numerical simulation in estimating the absorbed power, the optimal load configuration, temperature distribution and the reaction rate during the whole combustion synthesis process, is highlighted. The employment of less conventional frequencies as well as the use of new-generation solid state generators is critically discussed also in the framework of scaling-up and microwave reactor design considerations.