Use Of Microwave Heating Research Articles

The use of microwave radiation for obtaining activated carbons enriched in nitrogen

The activated carbons were synthesized from hay with CO2 as an activating agent. Furthermore hay and the chars obtained by pyrolysis were treated with urea, in order to introduce nitrogen to their structure, and then subjected to physical activation. The porous structure of activated carbons before and after impregnation was characterised by nitrogen adsorption using BET and BJH methods. The thermal properties of the samples were investigated by thermogravimetric (TG) analysis and differential thermogravimetry (DTG).The final products were activated carbons of low surface area varying from 84 to 196m2/g and showing basic character of the surface. The impregnation of hay and chars with urea leads to a decrease in activated carbon surface area and pore volume. However, the materials modified by urea show a very high content of nitrogen (from 8.1 to 12.8wt.%), which could be achieved thanks to the use of microwave heating as this method of heating permits a considerable reduction of the temperatures of pyrolysis and activation and a shortening of the time of activation. Following the character of TG and DTG curves, the nitrogen functional groups introduced into the activated carbon structure have relatively low thermal stability, as evidenced by the distinct loss of mass above the temperature of approximately 400°C, observed both for unmodified carbon and the carbon enriched in nitrogen.

마이크로웨이브를 이용한 폐수 내 고농도 암모니아성질소 제거

Abstract Industrial use of microwave heating as an alternative to conventional heating is becoming popular mainly due to dramatic reductions in reaction time. Therefore, this work experimentally determined the effect of microwave irradiation on ammonia nitrogen removal in wastewater. The effects of air flow rate (0.3 ~ 0.9 L/min), treatment temperature (70 ~ 100°C), and initial pH (9 ~ 11) were characterized. As the air flow rate increased from 0.3 to 0.9 L/min, the ammonia removal rate constant ( k ) increased from -0.6642 to -1.0755 min -1 . As the temperature increased from 70 to 100°C, k increased -0.0338 to -1.0755 min -1 . As the pH increased from 9 to 11, k increased -0.2443 to -1.0755 min -1 . Ammonia removal was strongly dependent on temperature and pH rather than air flow rate. The results show that microwave irradiation is effective in ammonia nitrogen removal in wastewater due to advantages of fast and effective processing. Key words : Air flow rate, Ammonia nitrogen, Microwave, pH, Temperature

Effect of microwave pre-treatment on the magnetic properties of iron ore and its implications on magnetic separation

The use of microwave heating is a potential means of energy savings and is a more effective alternative to the conventional thermal processing of minerals. The effect of microwave treatment on Egyptian iron ore samples was investigated. Changing the microwave exposure time and power as well as their reflection on the magnetic properties of iron ore before and after microwave pretreatment was analyzed using different techniques. Microwave treatment was found to be more efficient at larger particle sizes and longer exposure times for a fixed value of the applied power intensity. Magnetic separation of microwave treated and untreated iron ore indicated that iron recovery increased from 39.54% in the untreated sample to 97.95% in the microwave-treated sample. The results indicated that microwave radiation has a significant effect on the magnetic properties of hematite through the formation of new and more magnetic phases that facilitate their separation from non-magnetic minerals, thereby obtaining high recovery, reaching ca. 98%.

Microwave-assisted cobinamide synthesis.

We present a new method for the preparation of cobinamide (CN)2Cbi, a vitamin B12 precursor, that should allow its broader utility. Treatment of vitamin B12 with only NaCN and heating in a microwave reactor affords (CN)2Cbi as the sole product. The purification procedure was greatly simplified, allowing for easy isolation of the product in 94% yield. The use of microwave heating proved beneficial also for (CN)2Cbi(c-lactone) synthesis. Treatment of (CN)2Cbi with triethanolamine led to (CN)2Cbi(c-lactam).

Mizoroki–Heck Reactions Catalysed by (N-Heterocyclic carbene)PdCl2(Et3N) Complexes

Complexes of general formula (NHC)PdCl₂(Et₃N) were used as pre-catalysts in Mirozoki–Heck couplings of activated aryl chlorides and deactivated aryl bromides as electrophiles, without the need of an inert atmosphere and in relatively short reaction times. The protocol was extended to the use of microwave heating, reducing further the reaction times with no detriment in the yields of the desired products.

ChemInform Abstract: Use of Microwave Heating in the Synthesis of Heterocycles from Carbohydrates.

ChemInform Abstract: Use of Microwave Heating in the Synthesis of Heterocycles from Carbohydrates.

Efficient pretreatment of lignocellulose in ionic liquids/co-solvent for enzymatic hydrolysis enhancement into fermentable sugars

Ionic liquids (ILs) have been widely used as alternative solvents for biomass pretreatment, however, efficient methods that enable economically use of ILs at large scale have not been established. In this study, a new method in which ILs and polar organic solvents (ILs/co-solvent systems) was proposed for efficient pretreatment of lignocellulosic materials. The combination use of appropriate ILs and organic co-solvents can significantly enhance the solubility of lignocellulose due to the lower viscosity of ILs/co-solvent mixture as compared to those of pure ILs while the hydrogen bond basicity was maintained. In addition, the solubility of lignocellulosic materials in ILs/co-solvent system was found to be correlated with the Kamlet-Taft solvent parameters. Moreover, the use of microwave heating also enhances the efficiency of lignocellulose pretreatment. For example, the microwave-assisted [Emim][OAc]-DMSO (1:1 volume ratio) treated-rice straw could be hydrolyzed at least 22 times faster than that of untreated-rice straw by cellulase from Trichoderma reesei. This enhancement was attributed by several factors including more efficient lignin extraction, less crystalline cellulose and lower residual ILs in treated-rice straw. The produced sugars can be effectively fermented by Pichia stipitis for ethanol production. Moreover, [Emim][OAc]-DMSO mixture could be reused at least 5 times without significantly decrease in effectiveness demonstrated that the use of ILs/co-solvent was potential alternative method for large-scale biomass pretreatment.

Use of Microwave Heating in the Synthesis of Heterocycles from Carbohydrates

This review concerns the role of microwave irradiation in the synthesis of heterocycles starting from carbohydrates. This topic has received increasing interest owing to the wide variety of heterocyclic compounds as biologically significant chiral natural products or drugs, and products for industrial application, which cover nearly all aspects of carbohydrate chemistry. In particular, the possibility of employing microwave irradiation as a source of energy together with simple low-molecular-weight carbohydrates, as attractive chiral raw materials, has led to the production of low-cost and eco-friendly heterocycles within the framework of a green chemistry approach. Moreover, the employment of small amounts of solvents or solvent-free systems with solid supports, enables reaction completion in much shorter times under more environmentally clean conditions and preparation of the target compounds on the scale of grams, without the risk of any major hazards. The many possible combinations of skeletal, building block, and stereochemical diversity elements make this a highly attractive synthetic approach. This review covers the period from the first utilization of microwave irradiation until 2012 and includes the syntheses of (i) heterocycles linked directly or via a tether to carbohydrates, (ii) heterocycles annulated with carbohydrates, (iii) heterocycles from carbohydrates via hetero-Diels-Alder (HDAR) and Diels-Alder (DAR) reactions, or via 1,3-dipolar cycloadditions, with particular regard to the “click reaction” (CuAAC), (iv) iminosugars, and (v) 5-hydroxymethylfurfural. Keywords: Carbohydrates, click reactions, cycloaddition reactions, green chemistry, heterocycles, iminosugars, microwave.

Remediation of hydrocarbon-contaminated soils by ex situ microwave treatment: technical, energy and economic considerations

In this study, the remediation of diesel-polluted soils was investigated by simulating an ex situ microwave (MW) heating treatment under different conditions, including soil moisture, operating power and heating duration. Based on experimental data, a technical, energy and economic assessment for the optimization of full-scale remediation activities was carried out. Main results show that the operating power applied significantly influences the contaminant removal kinetics and the moisture content in soil has a major effect on the final temperature reachable during MW heating. The first-order kinetic model showed an excellent correlation (r2 > 0.976) with the experimental data for residual concentration at all operating powers and for all soil moistures tested. Excellent contaminant removal values up to 94.8% were observed for wet soils at power higher than 600 W for heating duration longer than 30 min. The use of MW heating with respect to a conventional ex situ thermal desorption treatment could significantly decrease the energy consumption needed for the removal of hydrocarbon contaminants from soils. Therefore, the MW treatment could represent a suitable cost-effective alternative to the conventional thermal treatment for the remediation of hydrocarbon-polluted soil.

Microwave heating of carbon-based solid materials

As a part of the electromagnetic spectrum, microwaves heat materials fast and efficiently via direct energy transfer, while conventional heating methods rely on conduction and convection. To date, the use of microwave heating in the research of carbon-based materials has been mainly limited to liquid solutions. However, more rapid and efficient heating is possible in electron-rich solid materials, because the target materials absorb the energy of microwaves effectively and exclusively. Carbon-based solid materials are suitable for microwave- heating due to the delocalized pi electrons from sp2-hybridized carbon networks. In this perspective review, research on the microwave heating of carbon-based solid materials is extensively investigated. This review includes basic theories of microwave heating, and applications in carbon nanotubes, graphite and other carbon-based materials. Finally, priority issues are discussed for the advanced use of microwave heating, which have been poorly understood so far: heating mechanism, temperature control, and penetration depth.

Inorganic materials synthesis in ionic liquids

The field of "inorganic materials from ionic liquids" (ILs) is a young and dynamically growing research area for less than 10 years. The ionothermal synthesis in ILs is often connected with the preparation of nanomaterials, the use of microwave heating and in part also ultrasound. Inorganic material synthesis in ILs allows obtaining phases which are not accessible in conventional organic or aqueous solvents or with standard methods of solid-state chemistry or under such mild conditions. Cases at hand include "ligand-free" metal nanoparticles without added stabilizing capping ligands, inorganic or inorganic-organic hybrid solid-state compounds, large polyhedral clusters and exfoliated graphene from low-temperature synthesis. There are great expectations that ILs open routes towards new, possibly unknown, inorganic materials with advantageous properties that cannot (or only with great difficulty) be made via conventional processes.

New prospects in pretreatment of cotton fabrics using microwave heating

As microwaves are known to give fast and rapid volume heating, the present study is undertaken to investigate the use of microwave heating for pretreatment cotton fabrics to reduce the pretreatment time, chemicals and water. The onset of the microwave heating technique on the physicochemical and performance properties of desized, scoured and bleached cotton fabric is elucidated and compared with those obtained on using conventional thermal heating. Combined one-step process for desizing, scouring and bleaching of cotton fabric under microwave heating was also investigated. The dual effect of adding urea, (as microwave absorber and hydrogen peroxide activator) has been exploiting to accelerate the pretreatment reaction of cotton fabric. DSC, FT-IR and SEM have been used to investigate the onset of microwave on the morphological and chemical change of cotton cellulose after pretreatment and bleaching under microwave heating. Results obtained show that, a complete fabric preparation was obtained in just 5min on using microwave in pretreatments process and the fabric properties were comparable to those obtained in traditional pretreatment process which requires 2.5–3h for completion.

Impact of Plasticization by Microwave Heating on the Total Deformation of Beech Wood

This paper reports on the total deformation and loading force after plasticizing beech wood by microwave heating. There have been few studies devoted to the examination of microwave heating for plasticizing of beech wood. Therefore, a procedure was developed to verify the use of microwave heating for the purpose of plasticizing. Total deformation and loading force were investigated on beech samples immediately after plasticizing by microwave heating. The samples were loaded with pressure applied parallel to the grain. Measured results served the purpose of quantifying total deformation. The investigated factors (wood moisture and plasticizing time) had significant influences on the loading force and total deformation of beech wood plasticized by microwave heating. Increasing initial wood moisture increased total deformation. Increasing plasticizing time decreased the total deformation of wood because of a larger loss of wood moisture. Loading force had contrary behavior: increasing wood moisture decreased the loading force, and increasing plasticizing time increased the loading force.

Rapid preparation of α-FeOOH and α-Fe2O3 nanostructures by microwave heating and their application in electrochemical sensors

α-FeOOH (goethite) and α-Fe2O3 (hematite) nanostructures have been successfully synthesized using the microwave-assisted hydrothermal (MAH) method and by the rapid burning in a microwave oven of the as-prepared goethite, respectively. The orthorhombic α-FeOOH to rhombohedralα-Fe2O3 structural transformation was observed by X-ray diffraction (XRD) and Raman spectroscopy results. Plates-like α-FeOOH prepared in 2min and rounded and quasi-octahedral shaped α-Fe2O3 particles obtained in 10min were observed using field emission gun scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The use of microwave heating allowed iron oxides to be prepared with shorter reaction times when compared to other synthesis methods. α-FeOOH nanoplates were incorporated into graphite-composite electrodes, which presented electrocatalytic properties towards the electrochemical oxidation of ascorbic acid in comparison with unmodified electrodes. This result demonstrates that such α-FeOOH nanostructures are very promising chemical modifiers for the development of improved electrochemical sensors.

Mechanistic Investigation into the Accelerated Synthesis of Methacrylate Oligomers via the Application of Catalytic Chain Transfer Polymerization and Selective Microwave Heating

The synthesis of methyl methacrylate (MMA) oligomers by catalytic chain transfer polymerization (CCTP) is demonstrated to be significantly accelerated by the use of microwave heating. The CCTP reactions, which use a cobalt-based catalyst to very efficiently control the molecular weight of the final polymer, were conducted in both a conventional oil bath and a CEM Discover microwave reactor with a target set point of 80 °C. The required reaction time was shown to be reduced from 300 to 3 min, while also retaining control over the polymerization. Additionally, for the first time the bulk temperature of these catalyzed polymerizations was monitored in both heating methods by the use of internal optical fiber sensors. It was demonstrated that, to monitor the temperature of the reaction correctly, it is essential to use an optical fiber sensor rather than the external IR sensor supplied with the reactor. The acceleration in the synthesis during microwave heating was attributed to selective heating of the radical and oligomeric species within the reaction, which lead to both rapid heating of the reaction bulk to reaction temperature and average reaction temperatures that were higher than the chosen set point. However, comparative reactions carried out under conventional heating (CH) conditions at the true reaction temperature of the microwave experiments (MWH) showed that MWH was able to produce significantly greater yields than the CH experiments after only 3 min, indicating the existence of a real selective heating effect during the reaction. Three methods have been investigated to optimize the acceleration achieved in the MWH experiments while retaining control and yield levels within the MWH experiments. These were varying the; solvent concentration, initiator concentration and chain transfer agent concentration. It was demonstrated that by understanding the influence of the microwave heating that it was possible to retain control over the molecular structure of the product polymer at the accelerated rate.

Changes in Temperature and Moisture Content in Beech Wood Plasticized by Microwave Heating

This paper reports on changes in temperature and moisture content after plasticizing beech wood by microwave heating. There is currently no known use of microwave heating for plasticizing wood. Therefore, a proper procedure was developed to verify the use of microwave heating for the purpose of plasticizing. Moisture content was monitored in beech test samples with dimensions of 25 × 25 × 400 mm and 40 × 40 × 600 mm, while temperature was investigated only on samples 25 × 25 × 400 mm. In all cases, the temperature was suitable for the wood bending process. Microwave heating is quite intense, therefore for this kind of heating, it is better to have higher moisture content in the appropriate power device and minimal plasticizing time. The biggest advantage of microwave heating is the short plasticizing time, a few minutes, while steaming requires tens of minutes or even hours.

Mechanocatalytic Depolymerization of Dry (Ligno)cellulose As an Entry Process for High-Yield Production of Furfurals

Driven by mechanical forces, the acid-catalyzed depolymerization of solid biomass completely overcomes the problems posed by the recalcitrance of lignocellulose. The solid-state reaction leads to water-soluble oligosaccharides, which display higher reactivity than cellulose and hemicellulose. Here, we show that water-soluble oligosaccharides are useful feedstock for the high-yield production of 5-hydroxymethylfurfural (HMF) and furfural in biphasic reactors. This is because they readily undergo hydrolysis upon microwave heating, selectively forming monosaccharides as intermediates in the aqueous phase. Short reaction times are possible with the use of microwave heating and limit the extent of degradation reactions. This work provides an ionic-liquid-free approach to process lignocellulosic substrates into HMF and furfural with high yields. In fact, starting this novel approach with α-cellulose, yields of HMF of 79% and furfural of 80% at 443 K for 9 min were obtained. The processing of real lignocellulose (e.g., beechwood and sugar cane bagasse) also achieved high yields of HMF and furfural. Thereby, the current results indicate that the process limitation lies no longer in the recalcitrance of lignocellulose, but in the extraction of highly reactive HMF and furfural from the aqueous phase in the biphasic reactor.

Microwave Heating in the Solid‐Phase Synthesis of N‐Methylated Peptides: When Is Room Temperature Better?

AbstractThe N‐methylation of backbone amides in synthetic peptides is an important method for improving properties such as bioavailability, stability, as well as structural preferences, and is thus an attractive design strategy. However, the synthesis of N‐methylated peptides can be challenging as the nucleophile in the acylation step is a sterically hindered secondary amine. In this study, we have systematically evaluated the use of microwave heating, different coupling conditions and the role of steric effects on coupling yields. First, the coupling reagent providing the highest yields was identified by a series of test N‐acylation reactions on a selected peptide sequence. Secondly, a comparison of the coupling temperature, that is, room temperature versus microwave heating, with the optimal coupling reagent was performed. Finally, under the optimal conditions, a comprehensive study employing a set of sterically hindered N‐methylated amino acids was conducted. Interestingly, incorporation of Fmoc‐Arg(Pbf)‐OH proceeded with higher yields at room temperature than at elevated temperature as a result of intramolecular Arg lactam formation, which competes with intermolecular amide bond formation. Fmoc‐Arg(Boc)2‐OH provided even lower coupling yields, due to the complete formation of the Arg lactam product. Our studies have led to a robust, rapid, easy and high‐yielding microwave‐assisted method using COMU for the incorporation of Fmoc amino acids into challenging, sterically hindered N‐methylated amino acid residues in a peptide sequence without the use of harsh reagents.

Simultaneous Ultrasensitive Subpopulation Staining/Hybridization in Situ : Multiplex Analyses of Transcriptionally Active HIV-1 Intact Cell Reservoirs

Simultaneous Ultrasensitive Subpopulation Staining/Hybridization <i>in Situ</i> : Multiplex Analyses of Transcriptionally Active HIV-1 Intact Cell Reservoirs

Effect of Residual Water and Microwave Heating on the Half‐Life of the Reagents and Reactive Intermediates in Peptide Synthesis

Precise microwave heating has changed the way many small molecules are being synthesized and, currently, the field of solid-phase peptide synthesis is undergoing dramatic changes owing to the use of microwave heating. To fully reap the benefits of precise microwave heating for the formation of amide bonds in peptide synthesis, it is important to understand the kinetics of formation and break-down of activated esters and their N-acylation of the nascent peptide chain at elevated temperatures. Herein, we present systematic studies of, first, the rate of formation of activated esters by NMR spectroscopy and, second, their N-acylation during peptide synthesis. A study of the amount of residual water in the solvents revealed a significant effect on electrophilic reagents and intermediates. This observation was expanded into a general study of microwave heating in peptide synthesis.