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Microwave Synthetic Technology
Published in Banik Bimal Krishna, Bandyopadhyay Debasish, Advances in Microwave Chemistry, 2018
Biswa Mohan Sahoo, Bimal Krishna Banik, Jnyanaranjan Pa
Traditionally, organic reactions are heated using an external heat source, such as an oil bath, and, therefore, heat is transferred by conductance. This is a comparatively slow and inefficient method for transferring energy into the system because it depends on the thermal conductivity of the various materials that must be penetrated and on the results in the temperature of the reaction vessel being higher than that of the reaction mixture. By contrast, microwave irradiation produces efficient internal heating by direct coupling of microwave energy with the polar molecules (for example, solvents, reagents and catalysts) that are present in the reaction mixture. Materials respond to microwave radiation differently and some may not be susceptible to microwave heating. Based on their response to microwaves, materials can be broadly classified as follows:1.Materials that are transparent to microwaves, e.g. sulphur2.Materials that reflect microwaves, e.g. copper3.Materials that absorb microwaves, e.g. water
Synthesis and Characterization of Metal–Organic Frameworks
Published in T. Grant Glover, Bin Mu, Gas Adsorption in Metal-Organic Frameworks, 2018
Typically, conventional solvothermal methods require a day or longer periods to prepare MOFs, which could prevent large-scale MOF production per time, but this challenge can be overcome using microwave-assisted synthesis.45 Microwave irradiation induces polar molecules to rotate and generate thermal energy that drives the chemical reactions. The frequencies of microwaves rang from 300 MHz to 300 GHz, in which the frequency of 2.45 GHz (12.2 cm) is commonly used due to the appropriate penetration depth for laboratory-scale reactions. As long as the reaction mixture absorbs microwave energy, it is possible to heat the reaction mixture higher than 100°C. Common reaction conditions used during the solvothermal MOF synthesis process are also suitable for the microwave-assisted synthesis, since DMF, NMP, and water show moderate response to the irradiation, and methanol and ethanol absorb microwave energy. Due to the direct heating of the reactants, the nucleation rate is accelerated, leading to drastically shorter MOF formation time (<1 h) compared to a conventional solvothermal method. However, when the applied microwave energy is too high (i.e., 800–1000 W), MOFs are often obtained as a fine crystalline powder form (or poorly crystalized powder sample) which may not be the best form for practical applications. Therefore, irradiation power and reaction time should be optimized for controlling of the particle size and crystallinity of MOF materials.46 Typical experimental conditions are described in Section 2.5.
Graphene-Inorganic Hybrids (II)
Published in Ling Bing Kong, Carbon Nanomaterials Based on Graphene Nanosheets, 2017
Ling Bing Kong, Freddy Boey, Yizhong Huang, Zhichuan Jason Xu, Kun Zhou, Sean Li, Wenxiu Que, Hui Huang, Tianshu Zhang
A potential mechanism has been proposed to explain the formation of the 3D Pd-E-PG hybrid, with the presence of the nanoholes, as schematically shown in Fig. 4.28 [91]. Microwave irradiation heats a material by inducing the polar molecules in it to move and rotate. Firstly, microwave exfoliated graphene oxide (MEGO) with layered structure was produced as the graphite oxide was irradiated. Then the MEGO was mixed with palladium acetate [Pd(O2CCH3)2] and ethanol (C2H5OH) in order to form a homogeneous suspension. It was then heated to completely evaporate the C2H5OH, leading to a powder. The powder was treated with low power microwave irradiation (700 W) for 30 s, during which Pd NPs were obtained due to the composition of Pd(O2CCH3)2. The Pd NPs were deposited on the graphene nanosheets through the interaction with the oxygen-containing functional groups, thus forming Pd-D-G. After that, the Pd-D-G was further irradiated for a longer time (60 s), during which agglomeration of the Pd NPs occurred. After microwave irradiation at a high power of 900 W for 60 s, the agglomerated Pd NPs in the Pd-D-G diffused inside the graphene layers, generating nanoholes from the outer layers of the few-layered graphene nanosheets. The agglomeration of the Pd NPs was attributed to the high temperatures caused by the high power microwave irradiation. Finally, a 3D structure was obtained, in which large Pd particles were embedded in the nanoholes without detachment.
Process intensification of microwave-assisted acetylation reaction of glycerol to synthesize fuel additives using porcine pancreas lipase catalyst
Published in Biofuels, 2023
Rajeswari M. Kulkarni, Bedanta Sarkar, Karan Modhvadiya, Sarthak Patil
Microwave radiation is a non-conventional energy source that has been gaining increasing popularity in recent years. Microwave heating provides a more uniform temperature distribution as compared to conventional heating. Generation of heat using a microwave occurs primarily when ions are accelerated and collide with other molecules or when dipoles rotate rapidly to align with the changing electric field created by electromagnetic radiation [13]. The application of microwave field as a process intensification (PI) technology, has the advantages of volumetric heating, energy saving, and higher selectivity [14,15]. The synergistic use of microwave irradiation coupled with enzymes is an emerging area in the synthesis of value-added chemicals. Low-temperature microwave irradiation technology can reduce or eliminate side reactions caused by severe thermal effects and is applied widely because of its high speed and safety. Enzymatic reactions conducted under microwave irradiation are faster, cleaner and offer better conversion in a shorter time compared to reactions carried out under conventional heating. Microwave-assisted enzymatic reactions have easy work-ups and reduced times, because of which there is a saving of energy, and improved enzyme stability with less solvent consumed [16].
Ionic liquid and microwave irradiation synergism for efficient biodiesel synthesis from waste cooking oil
Published in Biofuels, 2023
However, localized superheating caused by microwaves due to dipole moment and ionic conduction accelerates the reaction more than conventional heating [45,46]. Microwave irradiation acts on polar molecules and ions, resulting in rapid heating. Two species can absorb microwave irradiation in the reaction medium during the transesterification reaction. These are methanol and IL. Because methanol has a high dipole moment, it is selectively heated under microwave irradiation, which could lead to the rapid formation of microzones with temperatures much higher than that of the reaction mixture [47–49]. Thus, methanol heats up fast, quickly reaches the boiling point, and provides energy for the transesterification reaction. Moreover, ILs can rapidly and effectively absorb microwave energy due to their polar and ionic nature [50]. Thus, the reaction medium heats up rapidly, accelerating the reaction. Microwave irradiation can transfer energy directly to the reactant such that the energy transfer is more effective, causing the reaction medium to heat up quickly [51,52]. Unlike microwave heating, in the conventional heating method, the reaction mixture is heated by the inward transfer of heat from the surface, resulting in a longer reaction time and higher energy consumption [49,53].
Highly efficient and selective heterogeneous catalytic reduction of 2-nitroaniline by cerium oxide nanocatalyst under microwave irradiation
Published in Environmental Technology, 2022
Dadu Mal, Esra Alveroglu, Aamna Balouch, Muhammad Saqaf Jagirani, Sagar Kumar
In addition, the microwave method used in the heterogeneous catalytic reaction is far better than conventional heating. The conventional methods require excessive heating and extended reaction time which is not so effective. While utilization of microwaves in heterogeneous reactions is advantageous because it requires shorter reaction time, better production yield, and reduced energy consumption [67]. In green chemistry, microwave radiation is applied to various chemical reactions as an alternative energy source than normal conventional heating. In this method, the liquid or solid material transforms the EMR (electromagnetic radiation) energy into heat to speed up the reaction. Microwave irradiation has innumerable benefits over conventional heating methods because the absorption and production of energy in microwave-based reactions are very different than other heating approaches. Microwave irradiation is considered the most convenient, time-saving, and green approach because no hazardous materials were generated during the reaction. Microwave-based technology has been fluently applied to green synthesis, organic synthesis waste management, polymer synthesis, drug release, and organic decomposition reactions [68–71].