Microwave initiated reactions: Pechmann coumarin synthesis, Biginelli reaction, and acylation

An energy-efficient protocol has been developed for solvent-free reactions that are mildly exothermic but not spontaneous. The exothermic reaction mixture—on several g-scale—is exposed for about 30 s to low power (about 200 W) microwaves and then the microwave oven is switched off. After this short burst of energy, the exothermic reaction gets initiated and proceeds on its own to completion. A number of coumarins were synthesized by the Pechmann reaction using this strategy.     

Microwave extraction in analytical chemistry of pollutants: Polychlorinated biphenyls

The microwave energy produced by a high intensity microwave oven was used to extract polychlorinated biphenyls (PCB) from water samples. This static method was compared with liquid-liquid extraction to determine the optimum and limiting conditions in extraction processes. The sample size and sample container shape determine the temperature profiles in the sample. Using a 500 mL sample size, the detection limit is in the low ng/L range when an electron capture detector is employed.     

DTA-untersuchung der Umwandlungen des Amorphen Schwefels

DTA was applied to investigate amorphous sulfur samples remelted at different temperaturesT _f. For the sample remelted atT _f<159 °C, an exothermic process I occurs in the range 30–40°C. Transformation of orthorhombic to monoclinic sulfur, melting and polymerization ofS ₈ rings was observed at higher temperatures. For the sample remelted atT _f > 159 °C, a new, fast exothermic process II occurs at ambient temperature, followed by an exothermic process III at slightly higher temperature. The next, also exothermic process IV is detected in the vicinity of the melting point. The heats of the thermal effects for all the above-mentioned processes, and the part of sulfur insoluble in CS₂, were determined. An attempt was made to evaluate the mechanism of the transformations.     

Thermodynamic and kinetic studies on reversible liquid-phase Diels Alder reaction between maleic anhydride and 2-methyl furan

The liquid phase reaction between maleic anhydride and 1-methyl-furan leads to an exothermic cycloaddition. The reaction enthalpy, the rate constant and the equilibrium constant are evaluated with good precision by a method of analysis using thermal data and a dynamic model based on physical properties. The mathematical models that are presented here in a simple way (first-order system) represent the reaction accurately and are useful for studying other reactions and even industrial processes.     

On-line analysis of complex hydrocarbon mixtures using comprehensive two-dimensional gas chromatography

This paper discusses the first setup for on-line qualitative and quantitative comprehensive two-dimensional gas chromatography (GC × GC) of complex hydrocarbon mixtures. A built-in 4-port 2-way valve allows switching between flame ionization detection (FID) and time-of-flight mass spectrometry (TOF-MS) between runs, without the need to cool down and vent the MS. Proper selection of GC carrier gas flow rates enables maximal agreement between the obtained chromatograms in both configurations. For on-line analysis of reactor effluents, a dedicated sampling system allows automatic sampling of the hot reactor effluent gases and immediate injection of the sample on the GC × GC. To determine a complete effluent composition in a single run of the GC × GC, a subzero oven starting temperature was employed. Modulation is started when the oven temperature reaches 40 °C, thus dividing the chromatogram in a conventional 1D and a comprehensive 2D part. This work illustrates the mature and robust character of GC × GC, extending its capabilities from mere laboratory use to on-line routine analysis for industrial processes in the (petro-)chemical industry.     

Gas chromatographic characterization of vegetable oil deodorization distillate

Because of its complex nature, the analysis of deodorizer distillate is a challenging problem. Deodorizer distillate obtained from the deodorization process of vegetable oils consists of many components including free fatty acids, tocopherols, sterols, squalene and neutral oil. A gas chromatographic method for the analysis of deodorizer distillate without saponification of the sample is described. After a concise sample preparation including derivatization and silylation, distillate samples were injected on column at 60 degrees C followed by a gradual increase of the oven temperature towards 340 degrees C. The temperature profile of the oven was optimized in order to obtain a baseline separation of the different distillate components including free fatty acids, tocopherols, sterols, squalene and neutral oil. Good recoveries for delta-tocopherol, alpha-tocopherol, stigmasterol and cholesteryl palmitate of 97, 94.4, 95.6 and 92%, respectively were obtained. Repeatability of the described gas chromatographic method was evaluated by analyzing five replicates of a soybean distillate. Tocopherols and sterols had low relative standard deviations ranging between 1.67 and 2.25%. Squalene, mono- and diacylglycerides had higher relative standard deviations ranging between 3.33 and 4.12%. Several industrial deodorizer distillates obtained from chemical and physical refining of corn, canola, sunflower and soybean have been analyzed for their composition.     

A tool for predicting the integrity of Y zeolite crystalline structure

The purpose of this work was to employ the differential thermal analysis technique (DTA) to compare variations in the collapse energy of the Y zeolite crystalline structure in a fresh sample and in the sample after temperature treatment and impregnated with 3,000 ppm of vanadium and nickel. A small exothermic signal in the DTA curve at 950–1,150 °C indicated the collapse of the crystalline structure. The areas of the exothermic signals in the DTA curves of the samples indicated a 20% reduction in the exothermic area peak of sample treated 600 °C for 3 h and 25% reduction in same peak in the metal impregnated Y zeolite. These results were compared with X-ray data leading to the conclusion that metal impregnation affects the Y zeolite crystalline structure and that the DTA technique is a potentially useful tool for measuring the integrity of Y zeolite in catalysts.     

Thermal behavior of holo- and hemicellulose obtained from rice straw and bagasse

Thermal gravimetric analysis, and differential thermal behavior of holo- and hemicelluloses obtained from rice straw and bagasse were investigated. Degradation was found to be of first order reaction. The activation energy values and the rate constants were calculated from the kinetic of weight loss. Hemicellulose was found to be less stable than holocellulose, and the stability of the samples was arranged in the order, rice straw holo- > bagasse holo- > rice straw hemi- > bagasse hemicellulose. Degradation of rice straw (holo- as well as hemicellulose) was performed via two exothermic processes, whereas bagasse hemicellulose combustion was completed through three exothermic processes. The magnitude and the shape of the exotherms depend, mainly, on the chemical composition of the sample.     

Sulfur speciation in hard coal by means of a thermal decomposition method

A new method for the determination of organic and pyritic sulfur in hard coal is presented. The method is based on controlled thermal decomposition of coal sample in oxygen-free and oxygen atmospheres. The results for sulfur liberated in an argon atmosphere at temperatures up to 773 K were close to organic sulfur contents (Sorg), although owing to the definition of 'organic sulfur' the values were not directly comparable. Sorg contents are calculated from the difference between total sulfur content in coal and contents of this element in the form of sulfides, sulfates and pyrites. Sulfur contents, found in the second stage of analysis, were close to pyritic sulfur contents. The difference between total sulfur content and the sum of sulfur values obtained in stages I and II corresponded to sulfur contents in those samples which were neither decomposed nor oxidized at temperatures up to 1173 K. Although not comparable with such conventional concepts for industrial purposes these data are attractive due to the ease and rapidity of the new method for the control of sulfur streams in industrial processes.     

A systematic solution to the problem of sample background correction in DSC curves

The problem of sample background correction in differential scanning calorimetric curves is addressed in this paper. An equation is derived for the heat capacity of the system, which corresponds to the sample background. Thereby, it is assumed that during the thermal event the system is a two-component mixture of the initial substance and the final product. According to this model, the variation in heat capacity of the system is due both to the increase in the partial concentration of the product at the expense of the initial substance and to the physical change in the specific heats of the two components, resulting from the temperature increase. The final result of the derivation is an integral equation which can be solved by means of a numerical technique. The algorithm used is presented in detail. The model is general, and can be applied to diverse exothermic or endothermic processes. The melting of a semi-crystalline polymer and the cure process of a thermoset are given as demonstrative examples. The method improves the reliability and the reproducibility of the data.     

Nature of the first exothermic peak on the DSC curve of linear polydicyclopentadiene obtained with a special catalytic system

A DSC and TGA study of linear polydicyclopentadiene (L-PDCPD) shows that the thermal behaviour of this polymer is characterized by an endothermic and two exothermic processes. In previous work it was demonstrated that the endothermic process is a glass transition and the second exotherm represents the thermo-oxidation of the polymer. In this work the first exothermic process is shown to be a possible thermal polymerization of the double bonds in the polymer chains.     

Special cooling system for the on-column injector in capillary gas chromatography eliminating discrimination of sample compounds

On-column injection into a hot gas chromatograph oven involves the danger that the bottom part of the needle is warmed up above the boiling point of the solvent in the sample. Consequently parts of the sample are evaporated out of the needle instead of being introduced as a liquid into the capillary column. This causes losses and hence discrimination of the less volatile parts of the sample which remain in the needle. A technical solution is proposed using a system for cooling the capillary during the injection, down to the injection point in the oven.     

Development of a new pyrolyzer for thermal desorption and/or pyrolysis gas chromatography of polymeric materials

A new vertical microfurnace-type pyrolyzer for thermal desorption and/or pyrolysis-gas chromatography has been developed. The pyrolyzer consists of two independent temperature-controlled ovens. Initially, in the desorption process, a sample cup containing the polymeric sample of interest is inserted into an oven at 300°C; the sample is then re-positioned at the upper part of the pyrolyzer where the temperature is maintained at room temperature. The resulting vaporized components such as residual solvents and additives give a desorption chromatogram. The relative peak intensities of desorbed plasticizers in acrylonitrile butadiene-rubber gave a relative standard deviation (RSD) of less than 2%. Subsequently, pyrolysis of the remaining polymer is conducted by dropping the sample cup into the second, pyrolyzing, oven at 55°C; at this stage the pyrogram is recorded. The resulting two chromatograms of desorbed components and pyrolysis products make it easier to characterize the polymer formulation than the complicated pyrogram obtained by an ordinary, single-step pyrolysis.     

Hyphenation of thermogravimetry and soft single photon ionization–ion trap mass spectrometry (TG–SPI–ITMS) for evolved gas analysis

Thermal processes are part of many industrial treatments; therefore, it is of great interest to gain more insight of these processes. Evolved gas analysis (EGA) is the most straightforward way to make chemical reactions in thermal processes accessible for on-line investigations. The sample matrix of evolved off gas e.g., from coffee roasting is a permanently changing and complex mixture of a multitude of substances that have to be analyzed simultaneously for real on-line investigations without any sample trapping or separation device. Therefore, a measurement system as an ion trap mass spectrometer (ITMS) with soft ionization is required with its tandem mass spectrometry capability to provide distinct substance identification unperturbed by the remaining matrix. The presented novel system setup is based on a thermogravimetric device (TG) to simulate the thermal treatment as in industrial processes combined with an ITMS with soft single photon ionization (SPI) to achieve the required substance information. Hence it is possible to gain single mass spectrometric information of expected substances for process control. More comprehensive than that are the two-dimensional MS data which are required for research and process development purposes though. The conducted analyses show that this novel setup is able to provide distinct substance identification in evolved gas of roast and ground coffee powder. To our knowledge, this is the first TG–SPI–ITMS setup with successful application in verifying the identity of different mass traces within a single run.     

Small scale adiabatic dewar techniques to determine exothermic onset temperatures

The heightened focus on safety in the chemical process industry has resulted in a growing trend to provide reliable thermochemical information before process scale-up. The exothermic onset temperature of compounds is of key concern. The small scale adiabatic dewar age (SSAD) test, developed at Merck Research laboratories, is used to determine exothermic onset temperatures. These tests are conducted under nearly adiabatic conditions using specially designed dewar sample cells and commercially available programmable calorimeters. The dewar cell method offers a rapid and accurate method to enhance standard test procedures and allows measurement of auto catalytic behavior using small samples.The SSAD technique is applicable to both solids and liquids and has the following advantages: 1) small sample size, 2) ability to handle compounds in which exothermic activity is accompanied by large pressure increases without damaging experimental equipment, 3) accurate determination of exothermic onset temperature, 4) ease of experiment set-up, 5) ease of data interpretation and 6) rapid experimental turn around time.The determination of the exothermic onset temperature using the SSAD technique will be presented and compared to data obtained using standard and isothermal age techniques.     

Thermal and stability study of tetracene using differential scanning calorimetry

A thermal analysis study was made of tetracene using differential scanning calorimetry (DSC). The effect of different scan speeds was investigated. At scan speeds of 0.625 to 10°C min^?1 two large rounded exothermic peaks were produced. The peaks occurred at an increasingly high temperature as the scan speed increased (for example, the peaks occurred at 128 and 130°C at a scan speed of 0.625°C min^?1 and at 148 and 150°C at a scan speed of 10°C min^?1. When tetracene was heated at a scan speed of 80°C min^?1 only one large sharp exothermic peak was produced. It is believed that the two peaks obtained at scan speeds of 0.625 to 10°C min^?1 represent decomposition of the tetracene in two successive stages, while the one peak obtained at 80°C min^?1 represents an explosion. A stability test for tetracene is proposed that involves heating of the tetracene in aluminum pans from the DSC apparatus in ovens at 100, 75, and 60°C, removing the pans and samples at intervals of 30 min, 24 h, and 7 days, respectively, subjecting the samples to DSC at 1.25°C min^?1, and noting the time interval in the oven that produces a DSC curve that shows obliteration of the second peak. Two lots of tetracene made by different processes showed marked differences in stability characteristics.     

Proposed performance qualification and calibration method for evaporative light scattering detectors

Summary A method for the performance testing of Sedex 55 Evaporative Light Scattering Detectors (ELSD) has been developed as part of our regulatory compliance programme. The method uses a readily available chromatographic column, caffeine as the test probe and can be completed in less than one hour.     

KDNBF形貌控制及其对热分解行为和感度的影响(英文)

通过7-羟基-4,6-二硝基-5,7-二氢苯并呋咱(DNBF)的钠盐与硫酸钾在水溶液中反应,制备得到DNBF的钾盐(KDNBF).在制备过程中加入不同表面活性剂,得到了不同晶形的KDNBF样品.运用X射线粉末衍射分析了单种晶形控制剂对样品晶形的影响.采用差示扫描量热(DSC)和热重(TG)分析技术研究了不同晶形样品的热分解特性,利用Kissinger方法和Ozawa-Doyle方法研究了样品第一放热分解峰的分解动力学过程.结果表明球形样品与其它晶形的样品相比,具有较低的分解温度、较高的活化能;同时发现,球形样品具有较高的摩擦和撞击感度与较低的火焰感度.     

Digestion of plastic materials for the determination of toxic metals with a microwave oven for household use.

A rapid sample-digestion method for the determination of toxic metals, cadmium, chromium, and lead, in polyethylene and polyvinyl chloride has been developed by using a microwave oven for household use. An appropriate amount of the sample taken in a PTFE decomposition vessel was mixed with nitric acid or nitric and sulfuric acids. The vessel was heated in a microwave oven by a predetermined operating program. The digested sample was diluted to a definite volume with water after evaporating most of the nitric acid. The precipitate, if formed, was filtered off by a membrane filter. The metals were determined by ICP-AES. The sample digestion required 5 min (for 20-mg sample) to 25 min (for 60-mg sample). The analytical results obtained for cadmium, chromium, and lead in a polyethylene certified reference material, BCR-680, digested with nitric acid, were in good agreement with the certified values.     

Relationship between the growth regime of polyelectrolyte multilayers and the polyanion/polycation complexation enthalpy

The alternate deposition of polyanions and polycations leads to the formation of films called polyelectrolyte multilayer films (PEMs). Two types of growth processes are reported in the literature, leading to films that grow either linearly or exponentially with the number of deposition steps. In this article we try to establish a correlation between the nature of the growth process and the heat of complexation between the polyanions and the polycations constituting the PEM film. Isothermal titration microcalorimetry experiments performed on several polyanion/polycation systems seem to indicate that an endothermic complexation process is characteristic of an exponential film growth, whereas a strongly exothermic process corresponds to a linear growth regime. Finally, weakly exothermic processes seem to be associated with weakly exponentially growing films. These results thus show that exponentially growing processes are mainly driven by entropy. This explains why the exponential growth processes are more sensitive to temperature than the linear growing processes. This temperature sensitivity is shown on the poly-L-glutamic acid/poly(allylamine) system which grows either linearly or exponentially depending on the ionic strength of the polyelectrolyte solutions.