Parallel microwave chemistry in silicon carbide reactor platforms: an in-depth investigation into heating characteristics

The heating behavior of silicon carbide reaction platforms under 2.45 GHz microwave irradiation was investigated with the aid of online thermoimaging cameras and multiple-channel fiber-optic probe temperature sensors placed inside the wells/vials of the silicon carbide microtiter plates. Microwave irradiation leads to a rapid and homogeneous heating of the entire plate, with minimal deviations in the temperature recorded at different positions of the plate or inside the wells. In temperature-controlled experiments using dedicated multimode reactors, solvents with different microwave absorption characteristics can be heated in parallel in individual wells/vials of the silicon carbide plate reaching the same set temperature. Due to the large heat capacity and high thermal conductivity of silicon carbide, the plates are able to moderate any field inhomogeneities inside a microwave cavity. Although the heating of the plates can be performed extremely efficiently inside a microwave reactor, heating and synthetic applications can alternatively be carried out by applying conventional conductive heating of the silicon carbide plates on a standard hotplate. Due to the slower heating of the silicon carbide material under these conditions, somewhat longer reaction times will be required.     

迫呫吨并呫吨的快速合成及其光电性能

用微波碳浴法,以2-萘酚为原料,快速合成了迫呫吨并呫吨.优化后的反应条件:醋酸铜(Cu(OAc)₂)做氧化剂;Cu(OAc)₂与2-萘酚的量的比为3:1;加热时间为210s左右。在氯仿溶液中,其吸收光谱与发射光谱呈镜面对称关系,最大发射峰波长约为445nm,且呈现较高的荧光量子效率。循环伏安法测得该化合物的最高占有分子轨道(HOMO)和最低未占有分子轨道(LUMO)分别为5.45eV,3.00eV。     

Microwave-assisted Mannich-type three-component reactions

Mannich-type three-component reactions have been performed successfully using microwave heating in conjunction with the use of ionic liquids as heating agents. Good product yields and short reaction times have been achieved.     

红外拟合光谱分析微波对重晶石浮选的影响机理

目前浮选提高重晶石品位的方法通常采用新型浮选药剂从低品位重晶石矿中进行分选,采用微波加热预处理矿物可提高浮选回收率,但微波对浮选药剂与矿物的作用机理还不清楚。微波加热技术近年来用于矿物加工、冶金与材料制备等领域,具有反应速度快、产品指标高等优点。以油酸钠为捕收剂,对微波预处理后的重晶石纯矿物进行浮选,并对不同微波作用时间下的重晶石浮选样品进行红外光谱检测,通过红外拟合平滑光谱和二阶导数光谱计算分析,研究微波对重晶石浮选的影响机理。浮选试验结果表明,未经微波预处理的重晶石,在油酸钠用量为55 mg·L-1、pH值为8.0的条件下,浮选指标最佳,回收率为91.41%;而对比之下,经微波作用后的重晶石进行浮选,随着微波处理时间的增加浮选指标逐渐提高,且在微波作用60s时的回收率最高,达95.27%。基于浮选试验的红外光谱分析表明,重晶石未经微波预处理进行浮选,与油酸钠作用,在波数为3 004 cm-1处-CH₂-的对称伸缩振动峰、2 953 cm-1处-CH₃的反对称伸缩振动峰、1 119和1 077 cm-1处SO2-₄的非对称伸缩振动峰均发生了红移,说明油酸钠在重晶石表面发生化学吸附;而经微波预处理后的重晶石浮选时,在波数为2 853 cm-1处-CH₂-对称伸缩振动峰、2 923 cm-1处-CH₂-反对称伸缩振动峰、2 958 cm-1处-CH₃反对称伸缩振动峰、1 181,1 122和1 086 cm-1处SO2-₄非对称伸缩振动峰、982 cm-1处SO2-₄对称伸缩振动峰、635和610 cm-1处SO2-₄弯曲振动峰,其峰位并未发生红移,但峰强随着微波作用时间的增加明显加强,且微波作用60 s时其峰强增加最明显;对微波处理后的红外光谱进行拟合平滑光谱和二阶导数光谱计算发现,在波数为2 958,2 923,2 853,1 181,1 122,1 086,982,635和610 cm-1处峰面积均有不同程度的增加,且在微波作用60s时峰面积分别增加了1.84%,259.12%,761.15%,235.72%,145.61%,198.50%,641.16%,549.67%和744.97%,表明微波预处理并未诱发重晶石表面发生化学反应,但强化了捕收剂油酸钠与重晶石矿之间的化学吸附,使其与重晶石表面的化学吸附更加致密,吸附量增加,因此重晶石回收率增加,浮选指标提高。     

Multimode-ultrasound and microwave assisted natural ternary deep eutectic solvent sequential pretreatments for corn straw biomass deconstruction under mild conditions

Mild and effective pretreatments are essential to deconstruct lignocellulosic biomass so as to reuse cellulose content for value-added products. In this study, sequential multimode-ultrasound and microwave with natural ternary deep eutectic solvent (NATDES) pretreatments were used to deconstruct corn straw and optimized factors such as NATDES, ultrasonic, and microwave parameters. Results indicated that the ultrasound-NATDES or microwave-NATDES pretreatment could remove 37.86% and 52.36% lignin, respectively. When using sequential multimode-ultrasound and microwave assisted NATDES pretreatment, the delignification efficiency increased to 61.50%, and the cellulose content increased from 34.70% to 76.08%. In addition, the delignification of sequential multimode-ultrasound and microwave assisted NATDES pretreatment (under the mild conditions of microwave heating at 60 °C and 60 min) increased to 57.39%, and the cellulose content increased to 59.98%, too. This highlighted the effect of the combined ultrasound and microwave technology. Finally, the microstructural changes of mercury intrusion porosimeters, scanning electron microscopy, thermogravimetric, X-ray diffraction and Fourier transform mid-infrared spectroscopy were conducted to confirm the effectiveness of this method to deconstruct corn straw. A mechanism of the deconstruction of corn straw biomass in NATDES with the assistance of the sequential multimode-ultrasound and microwave heating was proposed. This research could open a window for future use of biomass energy by deconstructing lignocellulosic biomasses using environmentally friendly pretreatment methods.     

Microwave assisted IMDAF# reaction: microwave irradiation applied with success to cycloaddition reaction of N-propargyl-N-p-tolyl-N-2-furfurylamines

The new tertiary furfurylamine with triple bond as a dienophylic part i.e. N-(5-methyl-2-furfuryl)-N-prop-2-ynyl-p-toluidine (1) was prepared and the intramolecular Diels-Alder reaction of the amine (1) was performed under microwave irradiation conditions and by heating a benzene solution of the amine under nitrogen. Comparing the results of the usual thermal and the MAOS reaction, we confirmed our expectations that MAOS could promote the outcome of IMDA reaction of the suitably N-substituted tertiary 2-furfuryl-amines. In the present example, N-p-tolyl-5-methyl-5,7a-dihydro-5,7a-epoxyisoindoline was obtained in much better yield and of higher purity.     

Microwave-assisted Pd(0)-catalyzed amination of aryl halides on solid support

In this study, the microwave-assisted Pd(0)-catalyzed coupling of aryl halides to Rink resins is compared to a conventional heating protocol. While yields and purities of the coupling products were comparable under both conditions, microwave heating allows a significant acceleration of the reaction. The coupling of electron-deficient aryl halides proceeded within 10-15 minutes, while under conventional conditions 20 h of heating were necessary for complete conversion.     

Arylboronic acids as versatile coupling partners in fast microwave promoted oxidative Heck chemistry

The useful and selective reactivity of arylboronic acids makes them favourite building blocks for many modern organic chemistry applications like the metal-mediated formation of C-C, C-O, C-N, and C-S bonds. This report describes oxidative Heck coupling reactions of arylboronic acids and olefins, which were conveniently and rapidly (5-30 min) carried out under air with temperature-controlled microwave heating. Different reaction conditions were investigated with regard to both microwave heating capability and chemical productivity. Copper(II) acetate was identified as a microwave compatible reoxidant of Pd(0). The scope and limitations of this high-speed chemistry protocol with diverse olefins and organoboronic acids are discussed.     

Research on pyrolysis of toluene under microwave heating by using ReaxFF molecular dynamics simulations

ReaxFF molecular dynamics (MD) simulations are performed to study high-temperature pyrolysis of toluene under microwave heating. It is observed that the temperature of the reaction system under microwave heating has a rapidly rising stage, which is similar to the phenomenon of thermal runaway often appeared in reactions under microwave heating. Simulations indicate that the consumption rate of toluene and generating rates of H₂ and CH₄ obtained under microwave heating are always lower than those obtained under conventional heating at the early stage. Analyses of the pyrolysis of toluene show that ReaxFF MD simulations can provide an efficient way to study chemical reactions under microwave heating.     

Controlled microwave heating in modern organic synthesis: highlights from the 2004–2008 literature

Direct and rapid heating by microwave irradiation in combination with sealed vessel processing in many cases enables reactions to be carried out in a fraction of the time generally required using conventional conditions. This makes microwave chemistry an ideal tool for rapid reaction scouting and optimization of conditions, allowing very rapid progress through hypotheses–experiment–results iterations. The speed at which multiple variations of reaction conditions can be performed allows a morning discussion of “What should we try?” to become an after-lunch discussion of “What were the results” Not surprisingly, therefore, many scientists both in academia and industry have turned to microwave synthesis as a front-line methodology for their projects. In this review, more than 220 published examples of microwave-assisted synthetic organic transformations from the 2004 to 2008 literature are discussed. An additional ca. 500 reaction schemes are presented in the Electronic Supplementary Material, providing the reader with an overall number of ca. 930 references in this fast-moving and exciting field.     

Complete inheritance of fractal properties during first-order phase transition

We present experimental evidence of complete fractal properties inheritance in the course of first-order phase transition from amorphous to monoclinic or tetragonal zirconia under hydrothermal conditions. This phenomenon takes place either under rapid microwave heating or conventional heating regardless of starting fractal dimension value. Exactly the same effect is observed for hafnia. The similarity of the local structures of amorphous and crystalline zirconia as well as relatively soft crystallization conditions could be the definite reasons for the conservation of the mesostructure in the course of phase transition.     

Effect of Lewis acids on the Diels–Alder reaction in ionic liquids with different activation modes

The Diels–Alder reaction has been examined in room temperature ionic liquids with high molar concentrations of Lewis acids under various conditions. A molar ratio of 10% catalyst gave a large increase in the selectivity and the yield of the reaction. The effect of catalysts on reaction rates was also examined under 100 MPa of pressure which leads to modest improvements in reaction rates. Ultrasound and microwave dielectric heating were also shown to improve the rate and, to a minor extent, selectivity of the examined reactions. Copyright © 2008 John Wiley & Sons, Ltd.     

微波辐照对胶原蛋白三股螺旋结构的影响

胶原蛋白的三股螺旋结构是其不同于其他蛋白质的特殊结构，也是其具有特殊功能的基础，然而，胶原的三股螺旋结构易在外界条件的影响下被破坏。目前微波已被越来越多的应用于胶原蛋白的提取和改性过程，但是关于微波辐照对胶原蛋白结构影响的研究还相对较少。首先从牛跟腱中提取胶原蛋白，然后采用0.5 mg·mL-1的胶原蛋白溶液在30 ℃下以微波辐照保温为实验样，水浴加热和未经加热处理为对比样，最后采用紫外-可见光谱、傅里叶变换红外光谱、圆二色谱以及荧光发射光谱等方法，对不同加热方法中胶原蛋白的三股螺旋结构和超分子结构进行表征，研究了微波辐照对胶原蛋白结构的影响。实验结果表明，在低于胶原变性温度的条件下，无论是微波辐照还是水浴加热都不会破坏胶原蛋白的三股螺旋结构，也不会使胶原蛋白变性。但是，与水浴加热相比，微波辐照会对胶原蛋白的聚集行为产生抑制作用。微波辐照对胶原蛋白的作用既有与常规加热相同的热效应，又有常规加热过程中不存在的非热效应，非热效应表现为抑制胶原蛋白的聚集行为。研究结果可为微波场中胶原蛋白结构和性质的变化提供科学依据。     

A Kinetic Model for the Reactions of Co and H2 to CH4 and C2H2 in a Flow Microwave Discharge Reactor

A kinetic model was developed to describe the reactions of CO and H2 to CH4 and C2H2in a microwave plasma. The experimental system consisted of a 24 mm I.D. tubular quartz reactor which passed through a microwave cavity. A variable-incident power waveguide system could supply up to 800 watts of incident microwave power to the cavity. The reactant gas mixture of H2 and CO flowed through the reactor, where a plasma was maintained under pressures of 20 - 100 mm Hg. The reactor effluent was analyzed by IR spectroscopy for CH4 and C2H2. Conversions of up to 5.3% CO to C2H2 and 7.2% CO to CH4 were observed. A 26-reaction kinetic model was developed and fitted to the experimental data. The plasma reactor was modeled in two zones: a discharge zone where electron-impact dissociations produce H, C, and O, and a downstream recombination zone where the atomic species from the discharge recombine. The discharge zone was modeled as a well-mixed reactor, and the recombination zone was modeled as a plug-flow reactor. The model was able to explain the asymptotic shape of the observed conversion versus residence time data; the effect is due to a kinetic limitation. This also explains why the conversions obtained in the plasma cannot be predicted by thermodynamic equilibrium.     

Efficient microwave assisted synthesis of optically active bicyclic 2-pyridinones via Δ2-thiazolines

A new efficient synthesis of optically active bicyclic 2-pyridiones has been developed using microwave irradiation. The synthesis is a two-step procedure via delta2-thiazolines, which only requires a 3 + 2 min reaction time compared to 2 days when using conventional heating. The optimized conditions proved to be suitable for the synthesis of a small library in excellent yields and with limited racemizations.     

A comparative study of high pressure versus other activation modes in the asymmetric Michael reaction of chiral imines

The Michael reaction of chiral imines under neutral conditions has emerged as an efficient method for the elaboration of quaternary carbon centers in the alpha position relative to a carbonyl group. It was established early on that this reaction could be accelerated by thermal activation without altering its remarkable features, namely its high regio- and stereoselectivities, Michael adducts being obtained typically in high yield and with 90–98% ee. In this article, a comparative study of high-pressure activation versus thermal, microwave or catalytic activation of this reaction will be reported, highlighting the specificity of each activation mode.     

Study of microwave plasma-assisted chemical vapor deposition of poly-and single-crystalline diamond films

We study conditions for microwave plasma-assisted chemical vapor deposition of high-quality single-crystal diamond films in a CVD reactor. These conditions are studied using the results of homoepitaxial growth of polycrystalline diamond films on diamond substrates and on the basis of numerical simulation of the microwave discharge in a CVD reactor. A high-quality single-crystal diamond layer is synthesized on a synthetic, type Ib diamond substrate. The properties of the obtained monolayer are studied by means of Raman and X-ray diffraction spectroscopy as well as optical and atomic-force microscopy.     

Phenomenological models of microwave heating of a flat coal mass with release of absorbed heat by the convection law

Phenomenological models of electrodynamics and heat transfer in application to microwave heating are constructed. Analytically rigorous solutions to problems of heating a flat coal mass under microwave radiation are obtained. The boundary conditions correspond to convection mechanism of absorbed heat release to the ambient medium. Mathematical models of dielectric heating for homogeneous boundary conditions are solved by a method of dual integral Laplace and Fourier transform. In the presence of inhomogeneities in the boundary conditions, a quite universal method of Green functions is used. The obtained formulas have a constraint associated with constancy of electro- and thermophysical characteristics of coal fuel, or when their piecewise constant approximation is admissible. The obtained dependences form the basis for scientific support of the microwave heating technology.     

三氧化硫低温磺化法合成聚苯乙烯磺酸新工艺

利用三氧化硫在低温条件下与聚苯乙烯合成聚苯乙烯磺酸,反应速度快,副反应少,后处理简单,是一种绿色合成新工艺.实验结果发现,最佳的反应条件为:常压下-20℃时,以氯仿为溶剂,三氧化硫与聚苯乙烯按0.8∶1的质量比混匀,低速搅拌反应时间30min,老化30 min.溶剂与产物分离后,测定纯净产品的磺化度为92.45%.通过...     

Susceptor-Assisted Enhanced Microwave Processing of Ceramics - A Review

Susceptor-assisted microwave processing is a rapidly growing technology due to its superiority over the conventional processing. In contrast to the conventional heating from the surface, the microwave heating occurs volumetrically via direct interaction with the material. Correspondingly, the microwave heating rates are in general much faster than the heating rates in the conventional furnaces, where heat has to be transferred from the heat sources to the material via conduction, convection and radiation. The need for the susceptor stems from the fact that the majority of the ceramics are low lossy materials and they cannot couple well with the microwave at room temperatures. The susceptor provides an easy and non-invasive technique to exploit the rapid microwave processing even for the highly microwave transparent ceramics, such as alumina, silicon nitride, quartz, etc. This article critically evaluates the susceptor-assisted microwave sintering and solid state synthesis of ceramics which have been reported over the last two decades. A wide range of ceramics has been considered and each case has been analyzed in terms of the enhancement of the processing rates and product qualities (grain structure, material properties, etc.) compared to the conventional processing. It has been shown that the susceptor-assisted microwave processing can greatly reduce the processing time while providing an easy pathway to achieve the desired product qualities. The use of the appropriate susceptor is the key to achieve the fast, smooth, and reliable microwave processing of ceramics and this article provides the required database for the appropriate design of the susceptor based on the process requirement.