Current Microwave Chemistry - Volume 8, Issue 1, 2021

Microwave-assisted Claisen rearrangement of allyloxybenzene with a hydroxyl group was conducted in the presence of metal salts. The rearrangement was promoted in the presence of an alkali metal salt, because the reaction substrate was converted into a phenoxide-type ion, which can efficiently absorb microwaves. In contrast, a Lewis acid was strongly coordinated to the ethereal oxygen, and this structure could also absorb microwaves efficiently.

Microwave heating technology, as a new green metallurgical method, is the core technology used in metallurgical engineering. Based on a brief overview of the principle of microwave technology heating and its application in the metallurgical industry, this paper summarizes the latest research progress and development status of the current microwave heating technology in the hydrometallurgy leaching process and the trend of the application of microwave heating technology in metallurgy. A detailed classification and discussion on the leaching process of common metals were made. The purpose is to further improve the application level of the technology and provide technical support for the improvement of the market position of China’s metallurgical industry. Finally, the problems that need to be urgently solved in the hydrometallurgy of microwave-assisted leaching are further discussed and prospects and suggestions are also discussed.

Manganese ore is an important metallurgical raw material that holds an important strategic position in the national economy of China. However, the grade of manganese ore in the country is mostly low, and the utilization efficiency of low-grade manganese ore resources is low, which seriously restricts the healthy and stable development of China’s metallurgical industry. As a new green heating method, microwave is expected to address the problems of conventional methods and realize the effective utilization of low-grade manganese ore. In this paper, the research status of the microwave composite reduction of pyrolusite in recent years is reviewed. Microwave plays an important role in metallurgy, and it is the current direction pursued to improve the research level of microwave heating and extend it to actual industrial processes.

As a new technology for preparing zirconia materials, microwave-assisted technology has been widely concerned in recent years. In this paper, the microwave-assisted method and its application in the preparation of inorganic materials are briefly described. The research progress of the preparation of zirconia by microwave-assisted method is reviewed. Some problems existing in the research status are pointed out, and the application prospect in this field is prospected.

Aims: The objective of this study was to investigate the impacts of combined microwave- convective drying on the quality of dried-pears. Color, bioactive compounds (total phenolics (TP) and antioxidant capacity (AC)) and textural (hardness) properties of convective (50 and 60 °C), microwave (100 and 200 W), and combined microwave-convective (100-50 °C, 200W-50 °C, 100 W-60 °C and 200 W-60 °C) dried pears were compared with the untreated pears. Results and Methods: A Konica Minolta CR-400 Chroma Meter was used to examine the color changes in dried pears. TP was measured using the Folin-Ciocalteu reagent. The free radical scavenging activity of the extract was analyzed using 1,1-dipheyl-2-picylhydrazyl (DPPH). The hardness of dried pear slices with different drying methods was obtained by a texture analyzer. The microwave- convective dried pear slices (especially at the condition of 200 W - 60 °C) showed higher preservation of bioactive compounds in addition to improved textural properties and color compared to other dried pear samples. Conclusion: Overall, combined microwave-convective drying is a promising process, as demonstrated in current research by its capability to better retain dried pear quality in terms of functional and textural properties.

Aim: To use the microwave radiation as the post-treatment method to tune the acidic properties of Fe/ZSM-5 catalyst. Background: ZSM-5 zeolite is a widely used standard catalyst in the methanol conversion to olefins and high-octane gasoline range hydrocarbons. However, the coke deposition and the concentration of acid sites determine its overall catalytic activity. Thus, the concentration of acid sites more precisely, the number of Br&#;d acid sites, is vital in determining the activity of ZSM-5 zeolite and product distribution in the methanol-to-hydrocarbons reaction. Objectives: (1) To modify ZSM-5 using an iron solution to form Fe/ZSM-5 via the impregnation method. (2) To tune the acidic properties of Fe/ZSM-5 using microwave radiation. (3) To check and determine the concentration of acidic sites using n-propylamine temperature-programmed surface reaction. (4) To check the effect of microwave radiation and acidic properties in the methanol conversion (product distribution) via methanol temperature-programmed surface reaction. Methods: Two sets of zeolites were used, with iron being introduced by ion-exchange from ferric nitrate (Fe(NO3)3 9H2O, Sigma Aldrich, ≥ 98% metal) solutions. These two series were designated as Z (0.5FeZ10/0-700) and X (0.5FeX10/0-700) after microwave treatment. The Z and X series possess the Si/Al framework ratio of 30 and 80, respectively. The TPSR studies were then conducted for characterization and catalytic tests. Results: From the C3H9N-TPSR experiments, it was found that the concentration of Br&#;d acid sites decreased with increasing microwave power level. Both X and Z series exhibited high selectivity to propene than ethene. Microwave treated catalysts (0.5FeZ10/280 and 0.5FeX10/462) with decreased concentration of Br&#;d acid sites showed the highest propene/ethene ratios of 1.67 and 5.27, respectively. Conclusion: From the results obtained, it was found that the amount of methane evolved (as a measure of coke deposited) and the concentration of Brønsted acid sites decreased with increasing microwave power level (0-700 Watts). High selectivity to propene was found when using both X series (0.5FeX10/0-700) and Z series (0.5FeZ10/0-700) as catalysts. After decreasing the concentration of Br&#;d acid using microwave treatment, the highest P/E ratios were observed for 0.5FeX10/280, 0.5FeX10/462, 0.5FeZ10/280 and 0.5FeZ10/462 catalysts. It is reasonable to suggest that microwave radiation could be a feasible post-synthesis modification step to produce ZSM-5 based catalysts that exhibit reduced concentration of Br&#;d acid sites, reduced methane formation, increased catalytic activity and selectivity.

Aims: The study aims to synthesize thiocoumarin scaffolds clubbed with pyrimidine and 1,3,4-oxadiazole ring system under microwave irradiation and describe their pharmacological activities. Background: We report herein an efficient and simple Lewis acid-catalyzed procedure for the synthesis of novel series of thiocoumarin clubbed with pyrimidine and 1,3,4-oxadiazole motifs under microwave irradiation. The microwave-assisted technique has many advantages, such as a higher yield, a clean and selective procedure, shorter reaction time, and simple work-up. Objective: The objective of the present study is to design and synthesize thiocoumarin scaffolds clubbed with pyrimidine and 1,3,4-oxadiazole ring system by microwave-assisted heating. Our prime focus is to highlight the synthetic approach developed for the synthesis of heterocyclic moieties of pharmacological interest, and the prominence has also been given to distinct advantages provided by microwave heating. Methods: Thiocoumarin clubbed with pyrimidine, and 1,3,4-oxadiazole motifs was synthesized under microwave irradiation. All the synthesized molecules were evicted by IR, ¹H NMR, ¹³C NMR, and mass spectra. The anti-microbial activity of synthesized compounds was examined against two Gram-negative bacteria (E. coli, P. aeruginosa), two Gram-positive bacteria (S. aureus, S. pyogenes), and three fungi (C. albicans, A. niger, A. clavatus) using the MIC (Minimal Inhibitory Concentration) method, anti-tubercular activity H37Rv using L. J. Slope Method and anti- oxidant activity using DPPH and ABTS bioassay method. Results: The application of microwave technology for the rapid synthesis of biologically significant thiocoumarin analogues is very promising because of its shorter reaction time and higher yield. Some of these new derivatives showed moderate to good in-vitro anti-bacterial, anti-fungal, and anti-tubercular activity. Compounds B4 and B7 appeared to have high radical scavenging efficacies as 35.32 ± 0.446 and 33.97 ± 1.069 μg/mL ± SD of IC50 values in DPPH and ABTS bioassay, respectively. Conclusion: Microwave-assisted synthesis provides an implicit way to discover a promising class of molecular entities for the development of new anti-microbial and anti-oxidant agents. Oxadiazole and pyrimidine bearing thiocoumarin derivatives showed improved anti-microbial, antitubercular, and anti-oxidant activity.

Background: In recent years, 3,4-dihydro-pyrimidin-2-(1H)-ones (DHPMs) have attracted significant attention due to their diverse range of biological properties such as antibacterial, antiviral, antitumor, anti-inflammatory, calcium channel blocking activities, etc. Taking into account, in this present work, a polymer-based solid acid catalyst is employed for the microwaveassisted synthesis of DHPMs. Introduction: Multicomponent reactions (MCRs) are attracting utmost attention as they promote the formation of several bonds in a single process with diverse advantages. Biginelli reaction is the top-ranked example of MCR for the synthesis of dihydropyrimidinones (DHPMs). Solid catalysts are considered a significant tool for MCRs as they are non-toxic, easy to handle, reusable, and have high selectivity and easy separation process. Methods: DHPMs synthesis was carried out using acid-functionalized mesoporous polymer (AFMP) catalyst under microwave irradiation and solvent-free condition. Results: AFMP showed good to excellent DHPMs yield (89-98%) under the optimized reaction conditions: 1:1:1.2 molar ratio of aldehyde/ethyl acetoacetate/urea, catalyst loading of 6 wt.% (with respect to aldehyde), the temperature of 80 °C and microwave power of 500 W. Conclusion: We have successfully utilized microwave-assisted AFMP catalyst for the synthesis of DHPMs under solvent-free conditions via the Biginelli approach. The catalyst is recyclable and reusable in up to 5 consecutive reaction cycles with no significant loss in catalytic activity.