Microwave Reactor Research Articles

Microwave-Reactor-Based Preparation of Red Iron Oxide Pigment from Waste Iron Sulfate.

This article presents a two-step method of iron red synthesis based on waste long-term deposited iron(II) sulfate. The first step is the purification of waste iron sulfate, and then the pigment is synthesized by precipitation using a microwave reactor. The newly developed method of purification allows for quick and thorough purification of iron salt. The use of a microwave reactor in the synthesis of iron red makes it possible to reduce the temperature of the goethite-hematite phase transition from 500 °C to 170 °C and skip the calcination process. A temperature reduction in the synthesis decreases the formation of agglomerates of synthesized materials compared to commercial ones. The results of the research showed a change in the physicochemical properties of the obtained pigments depending on the conditions of synthesis. Waste iron(II) sulfate is a promising raw material for the synthesis of iron red pigments. Laboratory pigments are found to be differ from commercial pigments. The difference in properties speaks in favor of synthesized materials.

Facile and green synthesis of Cu/Cu2O composite for photocatalytic H2 generation

Cu/Cu2O composites as photocatalysts were successfully synthesized by a sustainable and eco-friendly method employing microwave-assisted heating. In addition, Citrus aurantium L. has been used as a green reducing agent to replace conventional toxic reducing agents. The crystalline phases, morphology, and absorption properties of the samples were characterized by XRD, HR-SEM, TEM, and UV–Vis DRS respectively. The Cu and Cu2O phases were found in the synthesized composite powders, which show a hollow spherical morphology after the microwave reaction synthesis at a relatively low temperature (80 °C). The Cu phase content in those composites increases with the reaction time of microwave heating, which inhibits the Cu2O phase growth. Furthermore, a higher Cu phase content also enhances the absorption in the visible light region, which is important for photocatalytic hydrogen production. Finally, Cu/Cu2O composites were proven as photocatalysts and showed a 141 μmol/g H2 production rate under ambient conditions and visible light. This study confirms an alternative for designing rapid and low-cost photocatalysts for H2 production.

Effect of Microwave Radiation on the Solvent-free Synthesis of Phthaloylamino Acids

Background: In recent years, microwave radiation has been widely used in organic synthesis, including solvent-free mode. However, the reaction conditions of phthalic anhydride with amino acids under solvent-free microwave activation have not been studied so far. Objective: In the present work, the effect of microwave activation on the interaction of phthalic anhy-dride with amino acids in solvent-free conditions has been studied in detail. Methods: The microwave heating dynamics of phthalic anhydride, glycine and their equimolar mixture under microwave conditions have been investigated, and the dependence of the heating rate on the mi-crowave power is defined. Results: The common conditions for the synthesis of phthaloylamino acids have been determined as continuous heating at a power of 200 W at 130 °C for 5-6 min and additional heating for 5-10 min at a temperature close to the melting point of the corresponding amino acid. We have applied the developed two-step solvent-free microwave reaction protocol successfully for the synthesis of phthaloyl deriva-tives of glycine, alanine, β-alanine, 4-aminobenzoic acid, γ-aminobutyric acid, isoleucine, leucine, phe-nylalanine. Conclusion: Reaction conditions for synthesizing phthaloylamino acids by microwave activation with-out solvent have been established. The solvent-free microwave reaction between phthalic anhydride and amino acid has been found to proceed in the melted phthalic anhydride.

Biosynthesis of silver nanoparticles using extract of banana peel waste assisted by microwave irradiation

Silver (Ag) nanoparticles are among the most desirable nanomaterials for a variety of applications, such as lithium-ion battery, water pollution monitoring, photocatalytic water splitting, and medical devices. Their increased production and integration in environmental applications play an important role for nanoscience and nanotechnology in engineering. The synthesis methods of Ag nanoparticles include physical, chemical, and biological routes. In this study, we reported a rapid, eco-friendly, and cost-efficient method to synthesize Ag nanoparticles using an extract of banana peel waste as a reducing and capping agent under microwave radiation. A comparison between the conventional heating method and microwave radiation revealed that the microwave radiation method for the extraction of banana peel waste and the synthesis of Ag nanoparticles required much shorter extraction and reaction times. The effects of extraction conditions (banana peel content and extraction time) and synthesis conditions (silver nitrate concentration and reaction time) on Ag nanoparticles were investigated. The optimum conditions were found to be for the extraction: 50 g/L banana peel-to-water ratio and microwave extraction time of 140 s at 800 W and for the Ag nanoparticles synthesis: microwave reaction time of 120 s at 800 W and 1 mM silver nitrate solution and. Plasmonic Ag nanoparticles with a surface plasmon resonance peak at 420 nm and an average size of about 149 nm were synthesized.

Microwave-assisted green synthesis of silver nanoparticles using extract of mint leaves

Silver nanoparticles are widely used in areas such as biosensing, hydrogen production, electronics, photovoltaics, antimicrobials, and biomedical engineering applications. Biological synthesis is regarded as a safe and non-toxic process, but its main disadvantage is the slow processing time. The mint leaf extract can act as both a reducing and stabilizing agent for the biological synthesis of silver nanoparticles. In this work, a rapid and green method for the microwave-assisted biological synthesis of silver nanoparticles was investigated using an aqueous leaf extract of mint as a biosource of cost-effective, non-hazardous reducing and stabilizing agents. The optimum conditions for microwave extraction of mint leaves were 220 s at 800 W and a 5 g/L mint-to-water ratio. Meanwhile, the optimal conditions for the biological synthesis of silver nanoparticles were as follows: concentration of silver nitrate solution = 2 mM, microwave reaction time = 100 s at 800 W, and pH = 8. The silver nanoparticles obtained by the microwave heating process showed a surface plasmon band centered at 420 nm with a higher peak height than those prepared by the conventional heating process. Dynamic light scattering analysis has shown the synthesized silver nanoparticles to have an average diameter of 54 nm.

Conventional and MW assisted PET glycolysis promoted by titanium based catalyst

Polyethylene terephthalate (PET) flakes from postconsumer bottles were depolymerized in the presence of ethylene glycol (EG) and titanium (IV) phosphate (TiP) as catalyst. Glycolysis was carried out both as conventional heated and under microwave (MW) irradiation. For the former method the reaction conditions were: molar ratio PET repeating unit to EG-1:2.77, without catalyst and with different amounts of TiP at 200 °C. The MW assisted process was performed under similar experimental conditions applying a power range 400–600 W. It was optimizated with respect to the catalyst concentration and yeild of bis(2-hydroxyethyl) terephthalate (BHET). The experiments showed that the optimal result was achieved in a MW reactor using 0.2 wt% TiP (based on PET weight) at 450 W for 45 min reaction time. The degradation product contained the monomer BHET (61.7%), dimers, trimers and low amounts oligomers and EG. The yield of BHET achieved was 87%. The glycolysis products were characterized by a combination of analytical techniques – chromatographic, spectroscopic and thermal methods. The comparison of both approaches showed the advantages of MW assisted degradation in view of efficiency and time saving.

Application of microwave hydrothermal synthesis for the solidification of copper: Effect of heavy metal content and microwave time

The synthesis type and heavy metal content significantly affect the performance of microwave hydrothermal synthesis. This synthesis approach has lower energy consumption and shorter reaction time for consolidating the heavy metals in the tobermorite lattice. The goal of this study was to investigate the influence of microwave reaction time and copper (Cu) content on the preparation of Cu-substituted tobermorite (Cu-tobermorite). Fumed silica, calcium hydroxide and copper chloride dihydrate were used to prepare the Cu-tobermorite under a liquid-solid ratio of 30 mL/g, reaction temperature of 220 ℃, and (Ca+Cu)/Si ratio of 0.83. The effect of reaction time (2 h and 4 h) and Cu content (Cu/Ca ratios are 0, 0.05, 0.1, 0.15 and 0.2) on the microstructure and morphology of the Cu-tobermorite were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetry (TG), and Fourier-transform infrared (FTIR) spectroscopy. Moreover, the solidification efficiency was assessed by inductively coupled plasma-optical emission spectrometry (ICP-OES). The results showed that the main products of microwave synthesis were Cu-tobermorite and tobermorite, while the conventional hydrothermal synthesis products were Cu-tobermorite and gyrolite. Prolonging the elongation of the reaction time did not change the type of hydration product. Both microwave and traditional hydrothermal synthesis of tobermorite solidified Cu with a solidification rate of almost 100%. However, the microwave method provided a 1/7th shorter reaction time and less than 10% of the energy composition (< 10%

Microwave-assisted synthesis, antioxidant activity, docking simulation, and DFT analysis of different heterocyclic compounds

In this investigation, pressure microwave irradiation was used to clarify the activity of 1-(2-hydroxyphenyl)-3-(4-methylphenyl)prop-2-en-1-one (3) towards several active methylene derivatives utilized the pressurized microwave irradiation as green energy resource . Chalcone 3 was allowed to react with ethyl cyanoacetate, acetylacetone, and thioglycolic acid; respectively, at 70 °C with pressure under microwave reaction condition to afford the corresponding 2-hydroxyphenylcyanopyridone, 2-hydroxyphenyl acetylcyclohexanone, and thieno[2,3-c]chromen-4-one derivatives respectively. Moreover, the reaction of chalcone 3 with hydrogen peroxide with stirring affords the corresponding chromen-4-one derivative. All the synthesized compounds were confirmed through spectral tools such as FT-IR, 1HNMR, 13CNMR, and mass spectrum. Furthermore, the synthesized heterocycles were exhibited excellent antioxidant activity and comparable with vitamin C, where the presence of the OH group increases the scavenger radical inhibition. Furthermore, the biological activity of compound 12 was demonstrated through molecular docking stimulation using two proteins, PDBID: 1DH2 and PDBID: 3RP8, which showed that compound 12 possesses greater binding energy and a shorter bond length comparable with ascorbic acid. Also, the compounds were optimized through DFT/B3LYP/6-31G (d,p) basis set and identification of their physical descriptors, whereas the compound 12 was confirmed through X-Ray single structure with Hirsh field analysis of the compound to know the hydrogen electrostatic bond interaction, and correlated with the optimized structure by comparing their bond length, bond angle, FT-IR, and NMR, which gave excellent correlation.

Synthesis of Amidophosphates Based on 1-Ethynyl-1-Aminocyclohexane Upon Microwave Activation and their Biological Activity

The researches for developing the methods for synthesizing new functionally substituted derivatives of alicyclic dialkylamidophosphates, establishing their structure, and searching for promising biologically active compounds in their series, are modern and relevant. The given study aimed to develop the synthesis of amidophosphates based on the example of phosphorylation of 1-ethynyl-1- aminocyclohexane with dialkyl phosphites in the presence of triethylamine in carbon tetrachloride in the Atherton-Todd reaction conditions using microwave irradiation of the reaction mixture. The reaction of 1-ethynyl-1-aminocyclohexane with dialkyl phosphites was carried out in a CCl4 excess at an equimolar ratio of reagents, triethylamine was used as an HCl acceptor. The highest yields of dialkyl- N-(1-ethynylcyclohexan-1-yl)amidophosphates were achieved by adding equimolar amounts of dialkyl phosphites to the mixture of 1-ethynyl-1-aminocyclohexane, triethylamine, and CCl4 followed by the subsequent activation of the mixture in a microwave reactor (MAS-II Plus MW) (MW 102 W, 115 °C, microwave irradiation time was 3‒5 min). The composition and structure of the obtained compounds were proved by elemental analysis, IR, 1H, 13C, 31P NMR spectroscopy, and X-ray diffraction analysis (XRD). The biological tests carried out in laboratory and field conditions allowed identifying diethyl-N-(1-ethynylcyclohexan-1-yl) amidophosphate among the synthesized compounds, which produces a high growth-regulating effect on the growth, development, and yield of agricultural crops.

Microwave-carbon fiber cloth co-ignited catalytic degradation of waste plastic into high-yield hydrogen and carbon nanotubes

Environmental pollution and waste of resources caused by the massive accumulation and landfill of plastic waste has been prompting the research considering its reclamation. In this work, a microwave-carbon fiber cloth (CFC) co-ignited cracking strategy is proposed for highly-efficient conversion of polyethylene to hydrogen and high-quality carbon nanotubes conducted in a simple multimode microwave reactor. When FeAlOx@C is used as the catalyst, a hydrogen yield of 64.5 mmol g−1plastic is achieved while biochar powder as the ignitor only give yield to 30.6 mmol H2 g−1plastic, demonstrating the superiority of the CFC strategy where the carbon ignitor has little contact with the reactants. Further, FeAlOx@C treated by air plasma with rich oxygen vacancies is used as the catalyst. The yield is enhanced significantly, especially at low microwave power, resulting in a hydrogen yield of 67.3 mmol g−1plastic at a microwave power of 900 W, which was the highest as far as we know in the literature under direct catalytic cracking processes. Notably, the integrity of the CFC piece is maintained well, and is highly recyclable. Numerical simulation suggests that the position of CFC plays a key role in initiating the enhanced catalytic cracking process enabled by direct microwave heating.

CFD simulation of a shell and multiple tubes condensing heat exchanger in a modified microwave plant applied for reprocessing End of Life Tires (ELTs)

AbstractThe condensation heat exchanger has a critical role in the microwave reduction process to separate and capture valuable by‐products after the microwave reactor. This study aims to perform a computational fluid dynamics (CFD) simulation of a condenser to assess the heat transfer performance of the heat exchanger comprehensively. The type of heat exchanger used for this study is based upon an existing industrial‐scale condenser taken from a conventional thermal pyrolysis end of tires (ELT) reprocessing plant and repurposed for integration into a 1500 kg/h ELT microwave reduction process. The condenser is a shell‐and‐multiple‐tube heat exchanger in which the syngas passes through the tubes while cold water from a cooling tower is placed inside the shell. After the simulation, the effects of inlet temperatures and mass flow rates of the gas and water are investigated. The results show that the heat transfer rate is 58 kW for the inlet air velocity of 0.25 m/s and increases due to the convective heat transfer by 32% and 55% when the air velocity rises to 0.5 and 1 m/s, respectively, for the inlet gas and water temperatures of 80 and 15°C, respectively. Additionally, because the outlet air temperature and the inlet water temperature are strongly correlated with convective heat transfer, the outlet air temperature is equivalent to 17.2, 22.3, and 29.5°C when the inlet water temperature is 15, 20, and 25°C, respectively.

Microwave-assisted co-gasification of mixed plastics and corn stover: A synergistic approach to produce clean hydrogen

Co-gasification of waste plastic and corn stover into hydrogen is an effective way of reducing hydrogen cost and carbon footprint. However, current gasification technologies are energy intensive and cannot accommodate flexible feedstock. Microwave gasification offers several advantages including higher H2 yield and enhanced selectivity to syngas over tars. This work presents parametric optimization study of microwave co-gasification of mixed plastics and corn stover for hydrogen-rich syngas. High hydrogen yields of 30.5 mmolH2/gfeed at 700 °C were obtained using a microwave reactor, as opposed to 0.9 mmolH2/gfeed at 700–950 °C with conventional heating. Hydrogen yields are >98% of the theoretical extractable hydrogen from feedstock. Microwaves enhanced plastic and corn stover synergy and reduced tar yields. H-poor carbon formation resulting from the dehydrogenation of tar and coke, and light olefins decomposition, triggered by the external microwave electric fields, allowed further cracking of intermediate hydrocarbons to produce additional H2.

Plant-wide modeling and techno-economic analysis of a direct non-oxidative methane dehydroaromatization process via conventional and microwave-assisted catalysis

Direct non-oxidative methane dehydroaromatization (DHA) process via conventional and microwave (MW)-assisted thermo-catalytic catalysis is studied. Rate models for methane DHA reactions, including the effect of catalyst deactivation, are developed by using the in-house experimental data. Model results for gas concentration profile and catalyst deactivation are in good agreement with the experimental data. This rate model is then used for the development of dynamic multi-scale, multi-physics commercial-scale reactor models. Total number of fixed bed reactors desired for a cyclic steady state process is estimated. Plant-wide models are then developed for conventional and MW-assisted processes for producing products of desired specifications. Techno-economic analysis of the methane DHA process is undertaken. Economics of these methane DHA processes are compared with the typical multi-step natural gas to aromatics production process via methanol synthesis. Sensitivity of internal rate of return (IRR) and net present value (NPV) to various economic and process parameters such as plant scale, desired rate of return, reactor cost, feedstock and utility cost, catalyst variable cost, and MW reactor cost is studied. Electric equivalent efficiency of the conventional methane DHA process is found to be 69.2 % and 67.3 % at 750 °C and 800 °C, respectively, while the MW-assisted methane DHA process has the electric equivalent efficiency of 48.9 % at 800 °C. IRRs of the conventional methane DHA process at 750 °C and 800 °C, and MW-assisted process are 15.2 %, 17.5 %, and 18.8 %, respectively for a methane feed flowrate of 19,782 kg/h, while the IRR of the multi-step natural gas to aromatics production process is estimated to be 0 % for the same plant scale. Impact of change in the methane price, electricity price, and catalyst cost is found to be considerable on the process economics, while the cost of the MW reactor is found to have negligible impact.

A Computationally Fast Method to Simulate Microwave-Heated Monoliths

Microwave-assisted process intensification is a promising technique hindered by the unavailability of design rules for optimization and scale-up. High-throughput computational models allowing the exploration of a vast design space are imperative for the rapid development and deployment of intensified microwave reactors. Recently, structured reactors, especially monoliths, are emerging as a canonical multiphase reactor setup in the microwave-heating community. The vast separation of scales in these reactors leads to a large number of mesh elements, making the simulations time-consuming and challenging to converge. To this end, we employ volume and asymptotic averaging to represent monoliths, a multiphase system consisting of a fluid and a solid phase, as a continuum or effective medium. We rigorously verify the adequacy of the averaging techniques to predict the spatiotemporal distribution of the electric field, electromagnetic power dissipation, and temperature against fully resolved monolith simulations. The developed continuum model can replicate the three-dimensional transient behavior obtained from the multiphase simulations with an order of magnitude lower computational expense. Moreover, the continuum model allows easier mesh generation and convergence of numerical solution than the multiphase model. The developed multiscale framework can be used to simulate microwave-heated monoliths and other multiphase reactors, such as packed beds and open-cell foams.

Use of Bioprinted Lipases in Microwave-Assisted Esterification Reactions

In this study, a comparative evaluation was performed in batch esterification reactions under conventional heating (CH) and assisted by microwave irradiation (MW) using bioprinted lipases. Microwave-irradiation-assisted reactions generally provide higher productivities and improve synthesis performance in terms of increased rate and reduced reaction times, resulting in higher interest yields in less time. Productivity was calculated with the enzymes: Burkholderia cepacia lipase (BCL), Candida rugosa lipase (CRL), and porcine pancreas lipase (PPL) using different fatty acids (lauric acid (12:0), myristic acid (14:0), palmitic acid (16:0), stearic acid (18:0), and oleic acid (18:1)) and alcohols at a molar ratio of 1:8. The microwave reactor was operated at a temperature of 45 °C, and power varied between 50 W and 200 W. Bioprinted BCL (bBCL) showed the highest productivity among the tested lipases. In the reaction with the best result, bBCL with lauric acid under MW, the reaction time decreased from 24 h (CH) to 25 min (MW) and the productivity increased 33 times compared with the reactions under CH. The increase in productivity demonstrates its activation that occurred as a result of conformational changes of the enzyme in the bioprinting process, confirmed by Fourier transform infrared (FTIR) spectrometric analysis, which reduces the content of bBCL α-helix with lauric acid. The biocatalyst showed high operational stability over eight cycles, while losing only 19% of its initial activity with half-life times of 12.8 batches. The storage time was five weeks, maintaining ≈80% activity. The results demonstrate the prospect of a new enzymatic route to obtain hyperactive catalysts, with the use of bioprinted lipases in esterification reactions under microwave irradiation, for the synthesis of esters with a view to large-scale industrial application.

Detection and Bioimaging of Glutathione in Cells via a Turn-On Near-Infrared Fluorescent Nanoprobe Composed of Ag2S Quantum Dots and CoOOH Nanosheets

The variations of the microenvironment in cells are always associated with changes in the concentrations of some active species. However, visual detection of these species is still a challenge. In this work, near-infrared (NIR)-emitting fluorescent Ag2S quantum dots (Ag2S QDs) were synthesized via a rapid microwave reaction under normoxic conditions, which were utilized as sensitive fluorescent nanoprobes in sensing and bioimaging of glutathione (GSH) combined with CoOOH nanosheets (CoOOH NSs). The negatively charged Ag2S QDs can easily and effectively attach to the surface of CoOOH NSs to form the Ag2S QD/CoOOH nanoprobe through electrostatic interactions, accompanied by the quenching of the fluorescence of Ag2S QDs at 685 nm via the inner-filter effect (IFE). However, upon the addition of GSH, the fluorescence of Ag2S QDs was recovered due to the reduction of CoOOH NSs into Co2+ by GSH. Meanwhile, the fluorescent emission wavelength shifts slightly, which may result from the ligand exchange reaction between GSH and d-penicillamine (DPA) to form comodified Ag2S QDs. Based on the low toxicity and high tissue penetration depth of the NIR fluorescence of the Ag2S QDs and the good biocompatibility of CoOOH NSs, the nanoprobe composed of Ag2S QDs and CoOOH NSs was successfully applied in bioimaging, which can distinguish the normal cells and cancer cells, the semiquantitative content of endogenous and exogenous GSH. The as-developed strategy not only provides a simple, convenient, cost-effective, and rapid platform for the diagnosis of clinical diseases related to GSH but shows great prospects in fluorescence-assisted surgery.

Investigating the role of microwave thermal and non-thermal effects on WO3-graphene oxide composite synthesis.

The effects of microwave-assisted synthesis on the morphology and crystalline structure of WO3-graphene oxide (GO) composites have been investigated. Using two different microwave reactors, evidence supports that thermal and non-thermal effects significantly influence the properties of the synthesized materials. The findings reveal that the microwave cavity geometry affects how the microwaves are "delivered" to the reactional cavity as a function of time; it also orientates the growth process of the WO3 particles. Consequently, the crystalline structure and morphology are affected. As a result, the WO3-GO composites produced using a CEM reactor exhibit a rounded shape and hexagonal phase of WO3, besides enhanced reduction of GO. Whereas the composites made using an Anton-Paar reactor are composed of sheets and flowers of WO3 with hexagonal, triclinic and/or WO3 hydrate structures and cause a lower reduction on the GO.

A Novel Combined Design of Vessel and Resonant Cavity for Microwave Multi-Frequency Heating Chemical Reactor Using Antennas as Applicators

In this work a new design concept in the field of microwave heating assisted chemical reactors is proposed focusing on two main innovations. The first one consists in combining the resonant cavity and the vessel in the same volume, improving the durability of the system by using a metallic material for the vessel. The second consists of integrating antennas as applicator ports for energy transmission from the magnetron into the cavity, instead of waveguides, allowing greater design flexibility in terms of cavity and system dimensions. In this work, a microwave reactor with four electromagnetic energy emitting antennas is optimized. This innovation is evaluated by means of a simulation model that solves the finite element method (FEM) using the commercial software COMSOL Multiphysics coupling radio frequency and heat transfer physics. Within this work, simulations have demonstrated the microwave heating process in the configured chemical reactor where the implementation of standard waveguides would not be possible due to the incompatible size required in the selected diameter dimension for 106 litres. Therefore, the results achieved lay the foundation for the construction of a microwave reactor intended to drive the industrial process of chemical recycling of polymers on a large industrial scale.

Comparison of carbon dots prepared from collagen peptides using conventional hydrothermal and microwave methods

Differences between CDs prepared by microwave reaction and conventional hydrothermal methods were systematically compared to guide the optimization of the reaction parameters of biomass-derived CDs.

An efficient method for the synthesis of 1,4-dihydropyridine derivatives in a flow microwave reactor

An efficient method for the synthesis of 1,4-dihydropyridine derivatives in a flow microwave reactor