Specific Microwave Effect Research Articles

Development of a microwave chemical cell coupled with post-wall waveguide for in situ/operando soft x-ray absorbance spectroscopy under microwave irradiation

In this study, we developed a microwave chemical cell for total electron yield soft x-ray absorption spectroscopy (XAS) using synchrotron radiation under microwave irradiation. In addition, in situ XAS measurements of ammonia borane were performed using the device developed. The device comprises a post-wall waveguide housing a sample holder formed within it. Electrons emitted from the sample during soft x-ray irradiation pass through the metal plates of the waveguide and are measured using a picoammeter. Integration of the microwave irradiation structure into a synchrotron radiation beamline, which is a challenge in soft x-ray analysis requiring measurements in a high-vacuum atmosphere, was achieved by miniaturizing the device using 24.125 GHz ISM band microwaves, instead of the typical 2.45 GHz microwaves. Performance of the device was thoroughly assessed through temperature distribution simulation, temperature measurement of water, and evaluation of frequency characteristics. When the sample holder was filled with water and irradiated with 3 W microwaves, the temperature increased to 97 °C, which is close to the boiling point. The in situ XAS measurements performed using this device with ammonia borane/graphite (2:1) powder at the B-K edge during microwave irradiation showed that peaks associated with hydrogen desorption intensified with prolonged microwave exposure. This result indicates that microwave heating in a vacuum and soft x-ray analysis during microwave heating were achieved. The developed microwave chemical cell emerges as a powerful tool, facilitating advancements in our understanding of elementary chemical processes and elucidating microwave-specific effects such as local heating during microwave irradiation.

Microwave Inhibition of the Hydrogenation of CO2 for Methane Formation

The complex equilibria associated with the hydrogenation of CO2 to form valuable products are of considerable interest for the remediation of CO2. Due to the potential usefulness of these reactions, alternative methods of driving them more favorably using alternative energy sources, such as microwave radiation, are also of interest. We report here a study of the methanation reaction that yields CH4, which was studied under microwave and conventional convective heating conditions. An essential part of the science that arises from such studies is characterizing the microwave-specific effects on the reaction. From the determined equilibrium constants, we found that microwave radiation strongly inhibited the formation of CH4. The effective thermodynamic parameters obtained from a van’t Hoff plot reflect this inhibition. The enthalpy under microwave conditions is −38.63 kJ/mol, compared to −180.02 kJ/mol under conventional heating methods. Similarly, the free energy across the temperature range is less negative, suggesting a decrease in spontaneity under microwave conditions. We also observed a significant difference in the entropy, with entropy of −52 J/mol under microwave conditions and −206 J/mol under conventional conditions. This is consistent with prior observations that microwaves do not accelerate exothermic reactions. As discussed, this difference may arise from the microwave-driven dissociation of the reactant before CH4 bonds are made. Analysis of this system is of interest in understanding how the microwave inhibits the formation of the products of exothermic reactions. These results are of significance for understanding the broad area of microwave driven heterogeneous catalysis.

マイクロ波特異効果はあるのか？ 有機合成反応における実験的検証

Highly polar compounds are generally heated by absorbing microwave energy. Since 1986, microwave irradiation has been applied to various organic syntheses as an efficient heating method. On the other hand, an additional effect called “microwave specific effect”, that cannot be explained by the simple heating effect, has been observed. In a solvent that absorbs less microwave, microwave energy can be directly provided to the reaction substrate, and microwave-specific effects can be expected. Some microwave-assisted asymmetric synthetic reactions have been observed to accelerate while maintaining enantioselectivity. Based on this working hypothesis, we experimentally confirmed the microwave-specific effect in several organic synthetic reactions.

Experimental Investigation of Microwave-Specific Effect on Nazarov Cyclization

Abstract A copper-catalyzed Nazarov cyclization using an aryl vinyl ketone derivative containing a 1,3-dicarbonyl moiety was carried out under microwave irradiation conditions. The Nazarov cyclization was dramatically accelerated and the kinetic rate of the microwave irradiation was 5.8 times faster than that by conventional heating conditions. A theoretical interpretation of the microwave-specific effect as physical phenomena of the molecules based on chemical kinetics and dynamics was attempted. The microwave-specific effect in terms of the pre-exponential factor in the Arrhenius equation is discussed, thus a hypothesis of the conformational equilibrium activation by microwaves is theoretically supported. To further investigate the microwave-specific effect, a catalytic asymmetric Nazarov cyclization was finally performed. As the enantioselectivity is defined as the function of the reaction temperature, observation of the same enantioselectivity implies the same reaction temperature. Interestingly, when the asymmetric Nazarov cyclization was conducted using a chiral copper catalyst, the Nazarov reaction was subsequently enhanced by the microwave irradiation without any loss of the enantioselectivity. It is suggested that the drastic enhancement with the retention of the enantioselectivity was caused not by a thermal effect, but by a microwave-specific effect on the enantioselective reaction.

Microwave-Assisted Synthesis: Can Transition Metal Complexes Take Advantage of This "Green" Method?

Microwave-assisted synthesis is considered environmental-friendly and, therefore, in agreement with the principles of green chemistry. This form of energy has been employed extensively and successfully in organic synthesis also in the case of metal-catalyzed synthetic procedures. However, it has been less widely exploited in the synthesis of metal complexes. As microwave irradiation has been proving its utility as both a time-saving procedure and an alternative way to carry on tricky transformations, its use can help inorganic chemists, too. This review focuses on the use of microwave irradiation in the preparation of transition metal complexes and organometallic compounds and also includes new, unpublished results. The syntheses of the compounds are described following the group of the periodic table to which the contained metal belongs. A general overview of the results from over 150 papers points out that microwaves can be a useful synthetic tool for inorganic chemists, reducing dramatically the reaction times with respect to traditional heating. This is often accompanied by a more limited risk of decomposition of reagents or products by an increase in yield, purity, and (sometimes) selectivity. In any case, thermal control is operative, whereas nonthermal or specific microwave effects seem to be absent.

Effect of microwave-assisted processing on polyphenol oxidase and peroxidase inactivation kinetics of açai-berry (Euterpe oleracea) pulp

Microwave heating has been considered a promising technology for continuous flow thermal processing of fluid foods due to better retention of quality. Considering the importance of açai-berry pulp and its perishability, the inactivation kinetics of peroxidase (POD) and polyphenol oxidase (PPO) were investigated under conventional and microwave heating. First-order two-component model was well fitted to the data, indicating the presence of at least two fractions with different resistances. POD was more thermally resistant (90% inactivation for 40 s at 89 °C) and could be considered as a processing target. Inactivation curves dependency on heating technology suggests specific effects of microwaves on the protein structure. Additionally, the dielectric properties of açai-berry pulp were evaluated at 915 and 2,450 MHz for temperatures up to 120 °C. Power penetration depth dropped with temperature at 915 MHz (from 29 to 11 mm), but was less affected at 2,450 MHz (between 8 and 11 mm).

Microwave irradiation versus conventional heating assisted free-radical copolymerization in solution

Microwave (MW) irradiation has arisen as a more sustainable alternative to conventional heating (CH) for chemical reactions because it provides non-contact, volumetric equal and fast heating. However, in free-radical polymerization, the MW effect is still quite controversial, probably due to the lack of reliable, comparable experimental data of both CH and MW assisted polymerization processes. In this work, taking advantage of technically superior MW reactor design, similar temperature profiles and conditions of CH and MW assisted polymerization reactions were enabled. Copolymerization of various monomer couples with different polarity and dielectric properties was studied in solution of different organic solvents in the presence of different initiators. As a result, it was concluded that the interaction of the reaction components with the MW irradiation is essential and will determine if any effect of MW irradiation on the reaction and products occurs. On the contrary to some reported studies, for the typical monomers used in free radical polymerization studied here (acrylates, methacrylates, styrene) no difference in reaction rates, copolymer composition and properties were observed between the MWH and CH processes, independently of the solvent and initiator used. However, in the case of the presence of an organometallic monomer within the monomer couple, an observable reaction rate enhancement was obtained under MW irradiation, along with changes in the reactivity ratios and differences in the copolymer composition. This effect is considered a specific microwave effect, for the first time demonstrated in free-radical solution polymerization process and was explained by the principles of selective heating of the organometallic reaction components in solution.

Microwaves and Heterogeneous Catalysis: A Review on Selected Catalytic Processes

Since the late 1980s, the scientific community has been attracted to microwave energy as an alternative method of heating, due to the advantages that this technology offers over conventional heating technologies. In fact, differently from these, the microwave heating mechanism is a volumetric process in which heat is generated within the material itself, and, consequently, it can be very rapid and selective. In this way, the microwave-susceptible material can absorb the energy embodied in the microwaves. Application of the microwave heating technique to a chemical process can lead to both a reduction in processing time as well as an increase in the production rate, which is obtained by enhancing the chemical reactions and results in energy saving. The synthesis and sintering of materials by means of microwave radiation has been used for more than 20 years, while, future challenges will be, among others, the development of processes that achieve lower greenhouse gas (e.g., CO2) emissions and discover novel energy-saving catalyzed reactions. A natural choice in such efforts would be the combination of catalysis and microwave radiation. The main aim of this review is to give an overview of microwave applications in the heterogeneous catalysis, including the preparation of catalysts, as well as explore some selected microwave assisted catalytic reactions. The review is divided into three principal topics: (i) introduction to microwave chemistry and microwave materials processing; (ii) description of the loss mechanisms and microwave-specific effects in heterogeneous catalysis; and (iii) applications of microwaves in some selected chemical processes, including the preparation of heterogeneous catalysts.

Temperature Assessment Of Microwave-Enhanced Heating Processes

In this study, real-time and in-situ permittivity measurements under intense microwave electromagnetic fields are proposed as a powerful technique for the study of microwave-enhanced thermal processes in materials. In order to draw reliable conclusions about the temperatures at which transformations occur, we address how to accurately measure the bulk temperature of the samples under microwave irradiation. A new temperature calibration method merging data from four independent techniques is developed to obtain the bulk temperature as a function of the surface temperature in thermal processes under microwave conditions. Additionally, other analysis techniques such as Differential Thermal Analysis (DTA) or Raman spectroscopy are correlated to dielectric permittivity measurements and the temperatures of thermal transitions observed using each technique are compared. Our findings reveal that the combination of all these procedures could help prove the existence of specific non-thermal microwave effects in a scientifically meaningful way.

Microwave-specific Effect on Enantioselective Reactions

Microwave-specific Effect on Enantioselective Reactions

High-Speed but Not Magic: Microwave-Assisted Synthesis of Ultra-Small Silver Nanoparticles.

Reaction procedures have been improved to achieve higher yields and shorter reaction times: one possibility is the usage of microwave reactors. In the literature, this is under discussion, for example, nonthermal effects resulting from the microwave radiation are claimed. Especially for the synthesis of nanomaterials, it is of crucial importance to be aware of influences on the reaction pathway. Therefore, we compare the syntheses of ultra-small silver nanoparticles via conventional and microwave heating. We employed a versatile one-pot polyol synthesis of poly(acrylic acid)-stabilized silver nanoparticles, which display superior catalytic properties. No microwave-specific effects in terms of particle size distribution characteristics, as derived by small-angle X-ray scattering and dynamic light scattering, are revealed. Because of the characteristics of a closed system, microwave reactors give access to elevated temperatures and pressures. Therefore, the speed of particle formation can be increased by a factor of 30 when the reaction temperature is increased from 200 to 250 °C. The particle growth process follows a cluster coalescence mechanism. A postsynthetic incubation step at 250 °C induces a further growth of the particles while the size distribution broadens. Thus, utilization of microwave reactors enables an enormous decrease of the reaction time as well as the opportunity of tuning the particle size. Possibly, decomposition of the stabilizing ligand at elevated temperatures results in reduced yields. A compromise between short reaction times and high yields can be found at a temperature of 250 °C and a corresponding reaction time of 30 s.

Direct microwave sintering of alumina in a single mode cavity: Magnesium doping effects

Unexpected results have been obtained when comparing the densification behaviour of undoped and MgO-doped α and γ alumina powders in conventional and direct microwave heating. Although the densification starts at lower temperature under microwaves, the addition of MgO in the raw powder slows down the densification under microwaves. This leads to samples with a lower final density compared to undoped or conventionally heated samples after similar thermal cycles. These original results have been obtained realizing microwave sintering experiments by direct heating, in a 2.45GHz single-mode cavity. The furnace allowed for an accurate control of the thermal cycle and for a continuous measurement of specimen dimensional changes. Direct comparison with conventional sintering has been achieved. These results are explained by the combination of the MgO effect on the alumina sintering behavior at low temperature generally reported under conventional heating and of a specific microwave effect linked to a ponderomotive force.

Specific microwave effect on Sn- and Ti-MFI zeolite synthesis

A specific microwave effect, demonstrated by the differences on Sn- and Ti-MFI zeolite preparation and characterization, provides a new direction for investigating microwave effects.

Microwave Specific Effect on Catalyticatropo-Enantioselective Ring-Opening Reaction of Biaryl Lactones

AbstractThe microwave specific effect on the catalytic atropo-enantioselective ring-opening reaction of biaryl lactones was investigated. Under strictly controlled temperature conditions, the reaction was accelerated by microwave irradiation without any loss of the enantioselectivity. Also, the racemization rate of the atropo-optically active biaryl lactone was enhanced by the microwave irradiation.

Microwave Specific Effect on Catalytic Enantioselective Conia-Ene Reaction

Microwave irradiation was applied to the enantioselective Conia-ene reaction catalyzed by Cu(OTf)2 with a chiral amine. Under strictly controlled temperature conditions, microwave irradiation impro...

Solvent-Free Regioselective Synthesis of Novel Isoxazoline and Pyrazoline N-Substituted Saccharin Derivatives Under Microwave Irradiation

Novel isoxazoline and pyrazoline derivatives of N-substituted saccharin were synthesized in good yields by 1,3-dipolar cycloaddition of N-crotonoyl- or N-cinnamoylsaccharin as dipolarophile to arylnitrile oxides or nitrile imines using p-HAP300 as catalyst under solventfree microwave conditions. In this process, the yields were significantly improved compared to classical conditions without alteration of the selectivity. The regioselectivity as well as the nonthermal specific microwave effect are discussed.

ChemInform Abstract: On the Existence of and Mechanism for Microwave‐Specific Reaction Rate Enhancement

AbstractReview: 59 refs.

Effect of High Domestic Microwave Radiations at Sub-Lethal Temperature on the Bacterial Content of Raw Milk

In light of the paucity of knowledge surrounding the existence of specific microwave effects on bacteria, the aim of the present study was to investigate the effects of high domestic microwave radiation on the safety of raw milk. Forty samples of raw milk were collected from retail shops in El Menofia governorates, Egypt. Milk samples were subjected to conventional heating in water bath to reach temperature of 40°C and 1100 Watt in domestic microwave for 3 minutes in cooled water jacket to reach the same temperature. Counts of different groups of bacteria in microwave treated samples were compared with those treated by conventional heating and non-treated raw milk samples. Additionally, to investigate the minor effects of such type of microwave treatment on the cell permeability to antibiotics, 10 of our laboratory bacterial strains with known antibiotic resistant patterns were treated by microwave and conventional heating. The zone of inhibition for tested antibiotics were measured after each treatment. Of note, the average total aerobic, Enterobacteriaceae, Pseudomonas spp. and staphylococci counts in milk samples were 6.34, 4.20, 3.42 and 3.72 log cfu/ml, respectively. The bacterial counts were nearly similar in samples treated by conventional heating and microwave. Statistical analysis revealed non-significant effect of microwave treatment at sub-lethal temperature as P values were more than 0.05. The diameter of zone of inhibition of tested antibiotics did not change in milk treated by both methods and non- treated samples. In conclusion, this study provide evidence that high domestic microwave irradiation at sub-lethal temperature have no significant effect on the hygienic quality of milk and not provide any degree of safety to the consumer. Additionally, it confirms the hypothesis that microwave radiation under sub-lethal temperature resulting in a reversible microwave-induced poration of the cell membrane that prevent penetration of antibiotics after few minutes.

A Critical Investigation on the Occurrence of Microwave Effects in Emulsion Polymerizations

The existence of special microwave effects in the emulsion polymerization of styrene and methyl methacrylate with two different initiators is investigated. All the experiments are conducted in either Pyrex or silicon carbide reaction vessels employing a dedicated single‐mode microwave reactor and are monitored with a dual temperature‐measurement system consisting of an external infrared sensor and an internal fiber optical probe. The goal of this study is to study the influence of the electromagnetic field on both the decomposition rate of the radical initiators and on the emulsion polymerization itself. For both types of transformation, no direct influence of the electromagnetic field can be observed, therefore ruling out the occurrence of any type of specific or nonthermal microwave effects in these reactions. image

Parameters affecting the microwave-specific acceleration of a chemical reaction.

Under appropriate conditions, significant microwave-specific enhancement of the reaction rate of an organic chemical reaction can be observed. Specifically, the unimolecular Claisen rearrangement of allyl p-nitrophenyl ether (ApNE) dissolved in naphthalene was studied under microwave heating and conventional convective (thermal) heating. Under constant microwave power, reaching a temperature of 185 °C, a 4-fold rate enhancement was observed in the microwave over that using convective heating; this means that the microwave reaction was proceeding at an effective temperature of 202 °C. Conversely, under constant temperature microwave conditions (200 °C), a negligible (∼1.5-fold) microwave-specific rate enhancement was observed. The largest microwave-specific rate enhancement was observed when a series of 300 W pulses, programmed for 145-175 °C and 85-155 °C cycles, where 2- and 9-fold rate enhancements, over what would be predicted by conventional thermal heating, was observed, respectively. The postulated origins of the microwave-specific effect are purely thermal and arise from selective heating of ApNE, a microwave-absorbing reactant in a nonabsorbing solvent. Under these conditions, excess heat is accumulated in the domains around the ApNE solute so that it experiences a higher effective temperature than the measured temperature of the bulk medium, resulting in an accelerated unimolecular rearrangement.