Use of a silicon carbide multi-well plate in conjunction with microwave heating for rapid ligand synthesis, formation of palladium complexes, and catalyst screening in a Suzuki coupling

The preparation of a library of bis-imidazolium salts and corresponding palladium complexes is reported. These complexes are screened as catalysts in the Suzuki reaction between 4-bromoanisole and phenylboronic acid. Each step is performed in parallel using a 24-position silicon carbide plate and microwave heating. The plate allows for use of standard glass vials as reaction vessels. The ease and speed of operation show the potential for microwave heating in conjunction with the silicon carbide plate as a tool for catalyst screening.     

Microwave assisted preparation of Eu2+-doped Åkermanite Ca2MgSi2O7

A rapid and energy efficient microwave assisted solid state preparative route for europium-doped Åkermanite (Ca₂MgSi₂O₇) has been developed. This method reduces the reaction time and energy needed by more than 90%, compared to the preparation carried out in a conventional furnace. The obtained samples are phase pure as has been determined using synchrotron X-ray powder diffraction data and Rietveld analyses. Scanning electron microscopy was employed to investigate the morphology of the microwave prepared compounds whilst energy dispersive X-ray spectroscopy (EDX) was used to verify the elemental composition of the specimens. A systematic investigation of the influence of the utilized microwave setup is presented. Finally, the microwave prepared materials were subject to temperature dependent photoluminescence measurements in order to investigate the thermal quenching of the luminescence.     

超级微波消解-电感耦合等离子体发射光谱（ICP-OES）法测定土壤中18种元素

为提高土壤多元素同时检测的效率,采用超级微波消解-电感耦合等离子体发射光谱法测定土壤中钾、钠、钙、镁、铜、铁、锰、锌、磷、硫、硼、砷、镉、铬、铅、钴、镓、锂等18种元素含量。比较了超级微波消解、常规微波消解和电热板消解的处理效果,采用超级微波消解法对样品进行前处理,并优化了消解条件。在最优条件下,各元素的检出限在0.05~20 mg/kg,加标回收率在86.2%~107.5%,RSD在0.1%~3.0%,方法准确度及精密度可以满足多元素同时测定的需求,且该方法具有简单、快速、成本低、用酸量少、重现性好等特点。     

Atomic emission spectroscopy for the on-line monitoring of incineration processes

A diagnostic measurement system based on atomic emission spectroscopy has been developed for the purpose of on-line monitoring of hazardous elements in industrial combustion gases. The aim was to construct a setup with a high durability for rough and variable experimental conditions, e.g. a strongly fluctuating gas composition, a high gas temperature and the presence of fly ash and corrosive effluents. Since the setup is primarily intended for the analysis of combustion gases with extremely high concentrations of pollutants, not much effort has been made to achieve low detection limits. It was found that an inductively coupled argon plasma was too sensitive to molecular gas introduction. Therefore, a microwave induced plasma torch, compromising both the demands of a high durability and an effective evaporation and excitation of the analyte was used as excitation source. The analysis system has been installed at an industrial hazardous waste incinerator and successfully tested on combustion gases present above the incineration process. Abundant elements as zinc, lead and sodium could be easily monitored.     

Atomic Oxygen Concentration in a Flowing Post-Discharge Reactor

Concentration of neutral oxygen atoms in the flowing post-discharge of a pure oxygen microwave discharge at different experimental conditions was determined with a nickel catalytic probe. The post-discharge reactor was setup for metal surface cleaning. It worked at the pressure between 20 and 100 Pa and at output power of the microwave plasma generator between 80 and 150 W. At those experimental conditions the O-atom density was found to be of the order of 10²¹ m^–3. It increased both with increasing pressure and microwave power. The degree of dissociation of oxygen molecules, on the other hand, decreased with increasing pressure.     

Microwave-assisted asymmetric organocatalysis. A probe for nonthermal microwave effects and the concept of simultaneous cooling

A series of five known asymmetric organocatalytic reactions was re-evaluated at elevated temperatures applying both microwave dielectric heating and conventional thermal heating in order to probe the existence of specific or nonthermal microwave effects. All transformations were conducted in a dedicated reactor setup that allowed accurate internal reaction temperature measurements using fiber-optic probes. In addition, the concept of simultaneous external cooling while irradiating with microwave power was also applied in all of the studied cases. This method allows a higher level of microwave power to be administered to the reaction mixture and, therefore, enhances any potential microwave effects while continuously removing heat. For all of the five studied (S)-proline-catalyzed asymmetric Mannich- and aldol-type reactions, the observed rate enhancements were a consequence of the increased temperatures attained by microwave dielectric heating and were not related to the presence of the microwave field. In all cases, in contrast to previous literature reports, the results obtained either with microwave irradiation or with microwave irradiation with simultaneous cooling could be reproduced by conventional heating at the same reaction temperature and time in an oil bath. No evidence for specific or nonthermal microwave effects was obtained.     

Controlled microwave heating in modern organic synthesis

Although fire is now rarely used in synthetic chemistry, it was not until Robert Bunsen invented the burner in 1855 that the energy from this heat source could be applied to a reaction vessel in a focused manner. The Bunsen burner was later superseded by the isomantle, oil bath, or hot plate as a source for applying heat to a chemical reaction. In the past few years, heating and driving chemical reactions by microwave energy has been an increasingly popular theme in the scientific community. This nonclassical heating technique is slowly moving from a laboratory curiosity to an established technique that is heavily used in both academia and industry. The efficiency of "microwave flash heating" in dramatically reducing reaction times (from days and hours to minutes and seconds) is just one of the many advantages. This Review highlights recent applications of controlled microwave heating in modern organic synthesis, and discusses some of the underlying phenomena and issues involved.     

Stable van der Waals-induced deformations of the air-water interface. Theoretical predictions and a suggestion for an experiment

This article concerns the stability of the air-water interface subjected to a 2D attractive van der Waals stress. The physical problem models the setup of a Wilhelmy plate experiment prior to three-phase contact line formation. We present and employ an unambiguous condition to quantify the stability limit in terms of the distance of closest approach of a solid cylindrical plate of parabolic cross section to the fluid surface as a function of the strength of the van der Waals surface force and plate geometry. A numerical study spanning 4 orders of magnitude of the Hamaker constant and nearly 6 orders of magnitude of solid geometry characterizes the dependence of the stability limit on these physical parameters. Comparisons are also made with a previously published analytical condition guaranteeing a stable deformation of the fluid interface. A possible experiment for testing the theory is also described. Used together with the theory, the technique could be used as an independent means of determining system properties such as the surface tension or Hamaker constant.     

Optimization of digestion methods for sewage sludge using the Plackett-Burman saturated design

The Plackett-Burman saturated factorial design was used to select optimized dissolution conditions for sewage sludge samples. Three different digestion methods were applied: i) microwave oven digestion in a domestic oven with Parr-type reactors; ii) microwave oven digestion with controlled-pressure reactors; iii) pressure bomb reactor heated on a hot plate. The three methods were validated by statistically comparing the metal contents found with the certified ones of the sewage sludge sample (BCR 145R). No significant differences were obtained and the RSD values were lower than 3% in all cases. The metals were determined by flame-AAS. The variables studied were the following: microwave power; digestion time; predigestion; volume of hydrochloric acid; volume of hydrofluoric acid; volume of nitric acid. The operative advantages offered by microwave digestion with controlled-pressure reactors were also considered. Received: 23 July 1997 / Revised: 31 October 1997 / Accepted: 5 November 1997     

Optimization of digestion methods for sewage sludge using the Plackett-Burman saturated design

The Plackett-Burman saturated factorial design was used to select optimized dissolution conditions for sewage sludge samples. Three different digestion methods were applied: i) microwave oven digestion in a domestic oven with Parr-type reactors; ii) microwave oven digestion with controlled-pressure reactors; iii) pressure bomb reactor heated on a hot plate. The three methods were validated by statistically comparing the metal contents found with the certified ones of the sewage sludge sample (BCR 145R). No significant differences were obtained and the RSD values were lower than 3% in all cases. The metals were determined by flame-AAS. The variables studied were the following: microwave power; digestion time; predigestion; volume of hydrochloric acid; volume of hydrofluoric acid; volume of nitric acid. The operative advantages offered by microwave digestion with controlled-pressure reactors were also considered. Received: 23 July 1997 / Revised: 31 October 1997 / Accepted: 5 November 1997     

微波在柴油机滤烟器催化再生中的应用 Ⅰ.滤烟器在氮气中的微波加热温升行为

利用柴油机台架装置及微波加热装置考察了柴油机催化滤烟器的微波加热温升效应． 结果表明,过滤体上捕集的烟碳量对催化滤烟器在氮气中的微波加热温升至关重要,烟碳量的微小增加,可引起滤烟体温度的显著升高． 另外,当微波加热过程中隔离氧气时,滤烟体上出现了明显的金属氧化物催化剂被碳还原的现象．     

A novel design approach for lithium-sulphur batteries

A novel microwave synthesis path for sulphur-carbon compounds for lithium batteries was successfully developed. Model electrodes built from these materials showed an excellent and highly stable electrochemical performance over several hundred of cycles with a remarkable initial specific charge of about 1300 A h kg^?1 and a remaining specific charge of 1012 A h kg^?1 (at 1 C) after about 500 cycles. In addition, a silicon-containing anode setup was used to investigate the highly beneficial influence of silicon nanowires on the electrochemical properties of lithium-sulphur batteries.     

Mo-MCM-48中孔分子筛微波辐射合成的研究

在微波辐射下快速合成含Mo离子的MCM-48分子筛,考察了微波反应釜压力和反应时间对Mo-MCM-48合成的影响,并用XRD,TEM,FT-IR和N2吸附进行表征。结果表明：微波辐射下能快速合成结晶良好的Mo-MCM-48分子筛;合成时以0.481MPa压力、反应2h为宜;Mo离子进入MCM-48分子筛的骨架;焙烧结果表明Mo-MCM-48热稳定性较好。苯乙烯催化氧化结果表明Mo-MCM-48具有一定的催化氧化活性。     

Magnetic field dependence of electron spin polarization generated through radical-triplet interactions

The magnetic field dependence of electron spin polarization (ESP), generated in free radicals when they encounter photoexcited triplets, was measured experimentally and analyzed theoretically. The time-resolved electron paramagnetic resonance measurements were performed with a microwave setup consisting of low-loss dielectric ring resonators with tunable microwave frequencies and the corresponding magnetic fields. The ESP of the radical was found in the magnetic field range of 170-370 mT, and the results of the calculation based on the numerical solution of the stochastic Liouville equation were found to be in line with the experimental data showing that ESP decreases when the magnetic field increases.     

Microwave-assisted degradation of acid orange using a conjugated polymer,polyaniline, as catalyst

Microwave-assisted photocatalytic degradation of dyes is one of the emerging technologies for waste water remediation. Microwave effectively accelerates photocatalytic degradation, when microwave electrodeless lamp (MEL) substitutes traditional UV lamp as light source. This setup can be extremely simplified if MEL and photocatalyst can be replaced by a catalyst which can work under microwave irradiation in the absence of any light source. The present work reports for the first time degradation of acid orange 7 (AO) under microwave irradiation using polyaniline (PANI) as catalyst in the absence of any UV lamp as light source. The degradation/decolourization was carried out in neutral acidic and basic media and was monitored spectrophotometrically to evaluate the ability of microwave irradiation to degrade AO. Microwave irradiation showed excellent performance as it completely decolourizes AO dye solution in 10 min. With the advantages of low cost and rapid processing, this novel catalyst is expected to gain promising application in the treatment of various dyestuff wastewaters on a large scale.     

New approaches for the separation and determination of americium in soil samples using short column chromatography and alpha spectroscopy

This paper describes new approaches to digestion, accurate separation and determination of americium in soil samples by alpha spectrometry. The soil samples were obtained from surface and at a depth of 40?cm in a residential area. They were digested on a hot plate or in closed vessels heated in a microwave oven. The effect of decomposition methods on accuracy and reproducibility has been investigated. An extraction chromatography column is used to separate the americium from other actinide elements and interfering substances in the soil matrix. Prior to the determination of very low amounts of americium (100?ng?g^?1), electrodeposition at a current of 800?mA and a plating time of 150?min in the pH range of 2?C3 has been applied. The typical recovery of Am from the samples is 88?% when dissolution occurs in a microwave oven. This is higher than the typical recovery of 83?% that is observed when the samples are heated on a hot plate.     

Dehydration of sodium carbonate monohydrate with indirect microwave heating

In this study, dehydration of sodium carbonate monohydrate (Na₂CO₃·H₂O) (SCM) in microwave (MW) field with silicon carbide (SiC) as an indirect heating medium was investigated. SCM samples containing up to 3% free moisture were placed in the microwave oven. The heating experiments showed that SCM is a poor microwave energy absorber for up to 6 min of irradiation at an 800 W of microwave power. The heat for SCM calcination is provided by SiC which absorbs microwave. The monohydrate is then converted to anhydrous sodium carbonate on the SiC plate by calcining, i.e. by removing the crystal water through heating of the monohydrate temperatures of over 120 °C. The calcination results in a solid phase recrystallization of the monohydrate into anhydrate. In the microwave irradiation process, dehydration of SCM in terms of indirect heating can be accelerated by increasing the microwave field power.     

Microwave-assisted high-throughput derivatization techniques utilizing silicon carbide microtiter platforms

Parallel microwave-assisted gas chromatography (GC) derivatization protocols utilizing a silicon carbide (SiC)-based microtiter plate platform fitted with screw-capped GC vials were developed. For three selected standard derivatization protocols such as acetylation (exemplified for morphine), pentafluoropropionylation (for 6-monoacetylmorphine) and trimethylsilylation (for Δ⁹-tetrahydrocannabinol) complete derivatization was achieved within 5 min at 100 °C in a dedicated multimode microwave instrument using online temperature monitoring. Microwave irradiation leads to rapid and homogeneous heating of the strongly microwave-absorbing SiC plate, with minimal deviations in the temperature recorded at different positions of the plate. The current platform allows the simultaneous derivatization of 80 reaction mixtures under strictly controlled temperature conditions. Similar results can also be obtained using a standard hotplate as heating source, although heating to the target temperature of 100 °C is slightly slower. The results demonstrate that parallel microwave derivatization procedures can significantly reduce the overall analysis time and increase sample throughput for GC–MS-based analytical methods.     

电感耦合等离子体原子发射光谱(ICP-AES)法测定ADC12中硅

在微波消解仪中以氢氧化钠溶解试样,然后滴加过氧化氢,使得铸造铝ADC12中的硅完全溶解,并采用电感耦合等离子体原子发射光谱(ICP-AES)法测定,硅的质量百分数平均值为10.31％的情况下,标准偏差0.09％,相对标准偏差(RSD)为0.83％.对比了国家标准方法中加热板消解法与微波消解法对硅加标回收率的影响,加热板消解法的加标回收率低于微波消解法,微波消解法辅助前处理样品,促使样品在短时间内反应完全,实验步骤简便,结果满意.     

Nonthermal microwave effects revisited: on the importance of internal temperature monitoring and agitation in microwave chemistry

The concept of nonthermal microwave effects has received considerable attention in recent years and is the subject of intense debate in the scientific community. Nonthermal microwave effects have been postulated to result from a direct stabilizing interaction of the electric field with specific (polar) molecules in the reaction medium that is not related to a macroscopic temperature effect. In order to probe the existence of nonthermal microwave effects, four synthetic transformations (Diels-Alder cycloaddition, alkylation of triphenylphosphine and 1,2,4-triazole, direct amide bond formation) were reevaluated under both microwave dielectric heating and conventional thermal heating. In all four cases, previous studies have claimed the existence of nonthermal microwave effects in these reactions. Experimentally, significant differences in conversion and/or product distribution comparing the conventionally and microwave-heated experiments performed at the same measured reaction temperature were found. The current reevaluation of these reactions was performed in a dedicated reactor setup that allowed accurate internal reaction temperature measurements using a multiple fiber-optic probe system. Using this technology, the importance of efficient stirring and internal temperature measurement in microwave-heated reactions was made evident. Inefficient agitation leads to temperature gradients within the reaction mixture due to field inhomogeneities in the microwave cavity. Using external infrared temperature sensors in some cases results in significant inaccuracies in the temperature measurement. Applying the fiber-optic probe temperature monitoring device, a critical reevaluation of all four reactions has provided no evidence for the existence of nonthermal microwave effects. Ensuring efficient agitation of the reaction mixture via magnetic stirring, no significant differences in terms of conversion and selectivity between experiments performed under microwave or oil bath conditions at the same internally measured reaction temperatures were experienced. The observed effects were purely thermal and not related to the microwave field.