Application Of Microwave Irradiation Research Articles

Accelerated generation of intracrystalline mesoporosity in zeolites by microwave-mediated desilication

Microwave-mediated desilication was successfully used to fabricate hierarchically structured zeolites combining micro- and mesoporosity. The application of microwave radiation during alkaline treatment of commercial ZSM-5 samples in aqueous NaOH solution at 338 K spectacularly accelerates the development of intracrystalline mesoporosity compared to the standard treatment (conventional heating). This is attributed to the more efficient transfer of thermal heating to the zeolite solution, thus enhancing the rate of silicon extraction. Very short exposure times (3-5 min) resulted in hierarchical zeolites with mesopore surface areas up to 230 m(2) g(-1) and pores centred around 10 nm, while preserving the crystallinity of the parent sample. The benefits of this treatment can be extrapolated to other zeolite structures.

Application of microwave irradiation in the removal of polychlorinated biphenyls from soil contaminated by capacitor oil

The applicability of microwave irradiation in the removal of polychlorinated biphenyls (PCBs) from the soil heavily contaminated by capacitor oil was studied. Granular activated carbon (GAC) was added into the soil as a microwave-absorbing material to rapidly increase the temperature of the reaction system. Effects of sodium hypophosphite (SHP) and iron powder (IP) as reductants on PCB removal, as well as the effects of water content in soil, microwave power and the amount of GAC added were investigated. The results indicated that in the presence of SHP and GAC, about 80% of PCBs in soil were effectively removed by 10 min microwave irradiation, and the further addition of IP could raise the average removal efficiency to a higher level, more than 95%. In the removal of PCBs with the microwave/GAC/IP/SHP system, an appropriate amount of water proved to be helpful in promoting the removal of PCBs from soil by steam distillation and in acting as a co-solvent of SHP and H-donor of IP.

Effect of pretreatment with microwaves on mechanical extraction yield and quality of vegetable oil from Chilean hazelnuts (Gevuina avellana Mol)

The effect of microwave (MW) radiation on hazelnut seed ( Gevuina avellana Mol) was studied as a substrate pretreatment prior to oil extraction by pressing. Samples were MW-treated at a frequency of 2450 MHz using a microwave oven. Six MW pretreatments were established, combining two potencies (400 W and 600 W) and three times of pretreatment (120, 180 and 240 s). Extraction oil yield increased with MW pretreatments of hazelnut seed with respect to untreated seeds, as a control. Conditions of 400 W and 240 s were selected (45.3% of extraction oil yield). Observations under light microscopy showed that the microstructure of treated samples to 400 W and 240 s, was modified comparing with that of untreated samples, thereby improving the extraction efficiency. The quality characteristics ( e.g. acid value, peroxide value), oil composition ( e.g. fatty acids, α-tocotrienol content) and oil oxidative stability ( e.g. induction time) were measured. These results were compared to those of an untreated oil sample. MW pretreatment had a positive effect on oxidative oil stability (induction time of 23.9 h) with respect to untreated oil (8.8 h). Chilean hazelnut ( Gevuina avellana Mol) is the southernmost Macadamieae species of the family Proteaceae that grows mainly in the southern part of Chile and Argentina. The oil is composed mainly of unsaturated fatty acids, which represent 93% of the total. Its main components are oleic and palmitoleic acids, which represent 70% fatty acids. Conventional vegetable oil extraction is carried out by pressing or solvent extraction. Solvent oil extraction is the most efficient method; however, its application presents some industrial disadvantages such as plant security problems, emissions of volatile organic compounds into atmosphere, high operation costs and poor quality products caused by high processing temperatures. Mechanical pressing oil extraction is technically less extensive and less labor-intensive than the extraction solvent method . The safety and simplicity of the whole process is advantageous over the more efficient solvent extraction equipment. Furthermore, materials pressed out generally have better preserved native properties, end products are free of chemicals and it is a safer process. However, extraction by just pressing the seeds is relatively inefficient. It is advisable to research new methodologies for pretreating substrates that also allow for better retention and availability of desirable plant metabolites. Within these pretreatments, the radiation microwave is included. There is not much information in the literature about the application of microwave radiation as a pretreatment for Chilean hazelnut and its effect on the microstructure of the substrate, extraction yield and quality of oil. The aim of the present study was to investigate the impact of pretreatment by microwave radiation prior to the oil extraction by pressing on the microstructure, recovery of oils and oil quality of Chilean hazelnut seeds ( G. avellana Mol).

A Novel and Versatile Entry to Asymmetrically Substituted Pyrazines

A novel and convenient procedure for the synthesis of asymmetrically tri- and tetrasubstituted pyrazines starting from para-methoxybenzyl-protected 3,5-dichloro-2(1H)-pyrazinones was elaborated. The key step is the conversion of the intermediate para-methoxybenzyl-protected thiopyrazinone upon treatment with MeI/I(2), into a pyrazine, rendering the chlorine in the C5-position susceptible to substitution. This approach entails the orthogonal introduction of the four substituents of the pyrazine scaffold. The application of microwave irradiation during different steps of the sequence has been shown to be highly valuable for speeding up reactions.

Microwave-Mediated Nickel-Catalyzed Cyclotrimerization Reactions: Total Synthesis of Illudinine

Rapid and efficient [2 + 2 + 2] cyclotrimerization reactions were discovered through the application of microwave irradiation in conjunction with a Ni(CO)(2)(PPh(3))(2) catalyst. This enables the facile construction of highly substituted indane, isoindoline, and tetraline core structures. The developed microwave-mediated Ni-catalyzed cyclotrimerization reaction was employed as the key step in a concise synthesis of the isoquinoline natural product illudinine. This represents the first example of a Ni-catalyzed cyclotrimerization reaction in total synthesis.

Microwave‐Assisted Natural Product Chemistry

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Microwave radiation induced collective response in Si/SiGe heterostructures with a 2D electron gas

The collective resonant photoresponse in Si/SiGe structures with a 2DEG under microwave radiation is reported for the first time. The application of microwave radiation of various frequencies results in resonant photoconductivity at magnetic field values that are systematically lower than expected for the cyclotron resonance (CR) in an infinitely large two-dimensional electron system. The analysis of the data shows that the observed microwave radiation induced response is dominated by plasmon excitations.

Advances in Microwave-Assisted Combinatorial Chemistry Without Polymer-Supported Reagents

Combinatorial methodologies have dramatically changed the chemical research and discovery process, offering an unlimited source of new molecule entities to be screened for activity. The application of microwave irradiation in Combinatorial Chemistry and high-throughput synthesis has become increasingly popular. By taking advantage of this energy source, compound libraries for lead generation can be assembled in a fraction of time required by conventional thermal heating. This review focuses on the advances in developing synthetic methodologies in microwave without polymer-supported reagents suitable for combinatorial chemistry, including the advances in microwave-assisted fluorous synthesis technology.

An Efficient Synthesis of Thiopyrano[5,6‐c]coumarin/[6,5‐c]chromones Through Intramolecular Domino Knoevenagel Hetero Diels—Alder Reactions.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Hydrodechlorination of chlorobenzene over supported metal catalysts

The use of supported Pd catalysts, with low and high metal content, for the hydrodechlorination of chlorobenzene is presented in this article. Application of microwave irradiation during preparation of catalysts resulted in the synthesis of large Pd particles at moderate temperatures. The nature of the support played a key role in the formation of cationic Pd species. The extent of interaction of the Pd species with the support, the nature of metal precursor, particularly the residual chlorine on the surface were found to significantly affect the activity of the catalysts. In the case of bimetallic catalysts also microwave heating resulted in creation of bigger particles of Pd compared to those observed in conventionally heated catalysts. Besides, it minimized alloy formation as a result of which the activity of the catalysts in hydrodechlorination was found to be higher compared to that observed on conventionally prepared catalysts. Contrary to the general observation that low dispersed Pd catalysts are preferable for high stability, by means of the deposition-precipitation method adopted for catalyst preparation it was demonstrated that even highly dispersed (low Pd containing) catalysts can exhibit comparable activity and stability. An analysis of the nature of Pd species and its role in the stability of the catalysts is presented.

An efficient synthesis of thiopyrano[5,6- c]coumarin/[6,5- c]chromones through intramolecular domino Knoevenagel hetero Diels–Alder reactions

The synthesis of novel polycyclic thiopyrano coumarin/chromone frameworks through intramolecular domino Knoevenagel hetero Diels–Alder reactions of 4-hydroxy coumarin and its benzo-analogous with S-prenylated aromatic aldehydes was studied. A high degree of chemoselectivity was achieved by the application of microwave irradiation and a solid support.

An Efficient Synthesis of 2,4,5-Trisubstituted and 1,2,4,5-Tetrasubstituted-1H-imidazoles

Since the appearance of the first paper on the application of microwave irradiation in the chemical synthesis the approach has blossomed into a useful technique for a variety of applications in organic synthesis and functional group transformations. In moving towards sustainable technologies the focus has now shifted to less cumbersome solventless methods wherein solid supported microwave synthesis has played an important role. This technique eliminates the use of solvent during the reaction stage but requires an appreciable amount of solvent for the adsorption of reagents and elution of product. The emergence of neat reaction is a recent approach under the new paradigm of green chemistry where the neat reactants undergo facile reaction to provide high yield of pure products thus eliminating or minimizing the use of organic solvents. These no solvent reactions are not only advantageous for environmental reasons but also offer benefits of enhanced reaction rates, greater selectivity and experimental ease of manipulation. The imidazole ring system is of particular interest as it is a component of histidine that produces histamine in metabolic process. The potency and wide applicability of the imidazole pharmacophore can be attributed to its hydrogen bond donor-acceptor capability as well as its high affinity for metals which are present in many protein active sites (eg. Zn, Fe, Mg). Triaryl imidazoles are used in photography as photosensitive compounds. In addition, they are of interest because of their herbicidal, analgesic, fungicidal, antiinflammatory and antithrombotic activities. The original synthesis of imidazole utilized glyoxal, formaldehyde and ammonia and established that the formation of four N–C bonds was a viable route. Although classical methods were derived from this early success, the reactions suffered low yields, mixture of products and lack of generality. Synthetic methodology alternatives are many and varied and have resorted to harsh conditions (eg. the formamide synthesis, which requires excess reagents, H2SO4 as a condensing agent, 150-200 °C, 4-6 h, 40-90%). Also, reagents for these procedures are not readily available, a key deficiency for library synthesis. In continuation to our endeavour towards green chemistry and the biological importance of substituted imidazole derivatives we have developed an ecologically safe strategy for the synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazole derivatives employing neat reaction conditions using MWI. Results and Discussion

Microwave‐Assisted Epoxidation of Simple Alkenes in the Presence of Hydrogen Peroxide

A novel application of microwave irradiation for the epoxidation of some simple alkenes, in which hydrogen peroxide was used as an oxidant together with sodium tungsten and phosphorous acid under phase‐transfer catalytic (PTC) conditions, is described as a new environmentally benign method. In comparison with conventional heating, the microwave process is a very useful alternative for introducing of the oxirane ring into some unsaturated hydrocarbons because of reduction of the reaction time and increase in yield.

Account of microwave irradiation for accelerating organic reactions

This account provides a summary of a chapter on Microwave Irradiation for Accelerating Organic Reactions in Advances in Heterocyclic Chemistry, volume 90, Part II: Six, Sevenmembered, Spiro and Fused Heterocycles by E. S. H. El Ashry, A. A. Kassem and E. Ramadan. Microwave irradiation (MWI) has been used extensively in organic synthesis. Application of MWI leads to many advantages, like the use of non-corrosive and inexpensive reagents, in addition to the eco-friendly green chemistry economical and environmental impacts.

Induction of Micronuclei in Mice Lymphocytes Exposed to Microwave and Toluene

Increasing applications of microwave radiation are of great concern with regard to public health. Several studies have been conducted detect effects of microwave exposure genetic material leading to negative or questionable results. The Micronucleus (MN) assay which is proved to be a useful method for detection of radiation exposure-induced cytogenetic damage was used in the present study to investigate the genotoxic effect of microwave and toluene alone and in combination in Balb/c lymphocytes. The electromagnetic field with two frequencies (980, 950 MHz, 200 KHz Mod), 5 w and 500 ppm Toluene applied for two weeks. Microwave irradiation had no significant effect on the frequency of micronucleus induced, but exposure of animals to toluene alone and in combination with microwave have significantly increased the induced micronucleus (p<0.05). Indeed combination exposure of microwave and toluene showed higher rates of micronucleus in comparison with toluene alone. This study indicated that microwave radiation cannot induce any significant cytogenetic effects but, in combination with toluene could show synergistic effect.

Reactions of 3-formylchromone with active methylene and methyl compounds and some subsequent reactions of the resulting condensation products.

This review presents a survey of the condensations of 3-formylchromone with various active methylene and methyl compounds, e.g. malonic or barbituric acid derivatives, five-membered heterocycles, etc. The utilisation of the condensation products for the synthesis of different heterocyclic systems, which is based on the ability of the γ-pyrone ring to be opened by the nucleophilic attack is also reviewed. Finally, the applications of microwave irradiation as an unconventional method of reaction activation in the synthesis of condensation products is described and the biological activity of some chromone derivatives is noted.

Flexural strength of autopolymerizing denture reline resins with microwave postpolymerization treatment

Microwave postpolymerization has been suggested as a method to improve the mechanical strength of repaired denture base materials. However, the effect of microwave heating on the flexural strength of the autopolymerizing denture reline resins has not been investigated. This study analyzed the effect of microwave postpolymerization on the flexural strength of 4 autopolymerizing reline resins (Duraliner II, Kooliner, Ufi Gel Hard, and Tokuso Rebase Fast) and 1 heat-polymerized resin (Lucitone 550). For each material, 80 specimens (64 x 10 x 3.3 mm) were polymerized according to the manufacturer's instructions and divided into 10 groups (n = 8). Control group specimens remained as processed. Before testing, the specimens were subjected to postpolymerization in a microwave oven using different power (500, 550, or 650 W) and time (3, 4, or 5 minutes) settings. Load measurements (newtons) were made at a crosshead speed of 5 mm/min using a 3-point bending device with a span of 50 mm. The flexural strength values were calculated in MPa. Data analyses included 3-way and 2-way analysis of variance and the Tukey Honestly Significant Difference test (alpha = .05). The flexural strengths of resins Duraliner II and Kooliner were significantly increased (P = .0015 and P = .0046, respectively) with the application of microwave irradiation using different time/power combinations. The materials Lucitone 550, Tokuso Rebase Fast, and Ufi Gel Hard demonstrated no significant strength improvement compared to the corresponding control. Only after microwave postpolymerization irradiation for 3 minutes at 550 W did Lucitone 550 show significantly higher flexural strength (P =.001) than Tokuso Rebase Fast and Ufi Gel Hard relining resins. Microwave postpolymerization irradiation can be an effective method for increasing the flexural strength of Duraliner II (at 650 W) and Kooliner (at 550 W and 650 W for 5 minutes).

Application of Microwave Irradiation, Solid Supports and Catalysts in Environmentally Benign Heterocyclic Chemistry

AbstractFor Abstract see ChemInform Abstract in Full Text.

Sintering of metal fibre reinforced glass matrix composites using microwave radiation

The application of microwave radiation to sinter a chopped metal fibre reinforced glass matrix composite without the use of a susceptor material to preheat the powder compact has been demonstrated. The system considered was a soda borosilicate glass powder requiring a relatively low sintering temperature, mixed with 2 or 10 vol.-% Hastelloy X fibres. Single mode microwave heating was performed using radiation of 2·45 GHz in an applicator operating in the TE103 mode. Samples could be sintered in less than 5 min. Samples heated in the maximum of the electric field showed increased pore shrinkage in comparison with samples sintered in the maximum of the magnetic field. The present work demonstrates that metallic additions may be used to eliminate the requirement of a susceptor material (usually SiC) to heat powder glass samples in a short time to a temperature at which it would be able to couple with the microwaves in order for sintering to occur.

Exocytosis sensitivity to growth hormone-releasing hormone in subsets of GH cells in rats under different corticosterone conditions. Ultrastructural study using microwave irradiation for fixation and immunocytochemistry

Growth hormone (GH) cells in the rat anterior pituitary have been morphologically classified into three subtypes: type I (mature) containing large secretory granules about 350 nm in diameter, type II (intermediate) containing a mixture of large and small granules, and type III (immature) containing small granules about 150 nm in diameter. However, the functional implications of morphological heterogeneity, especially the different sensitivities to growth hormone-releasing hormone (GRH) under different corticosteroid conditions have not been elucidated to date. In the present study, by application of microwave irradiation (MWI) for fixation and immunocytochemistry, new findings of the exocytotic response have been revealed among the subsets of GH cells following adrenalectomy (ADX), corticosterone treatment and/or GRH treatment. The MWI gave effective results for fixation, especially for the permeability of the fixative, and showed good results for immunoelectron microscopy using the protein-A gold method. Moreover, the use of MWI greatly shortened the fixation, processing and immunolabeling times without compromising the quality of ultrastructural preservation and the specificity of labeling. The number of exocytotic figures was low in all subtypes of GH cells in the sham-operated control rats. GRH treatment induced a significant increase in exocytosis in each subtype of GH cells, particularly in type I (mature) and type II (intermediate) GH cells in the control rats. GRH injection to rats for 4 days after ADX also showed an increase in exocytosis, but the degree was significantly less in comparison with the GRH injection in the control group. Corticosterone replacement given to ADX rats induced a clear recovery of the exocytotic response to GRH to the control level. Serum GH content measured by radioimmunoassay correlated with these morphological results. These results suggest that the secretion of GH stimulated by GRH is closely related to corticosteroids, and that the sensitivity to GRH differs among the three subtypes of GH cells.