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Monorini x Lissy Heinle - Lissy Heinle®. Monorini x Lissy Heinle Posted on März Anzeige | Die Picknick-Saison kommt, Leute! Reitturnier Leutkirch-Haid , Dressurreiterprüfung Kl. L * (no. 3). Lissy · Reitturnier Muderbolz , ewj-fastigheter.se Nuggets Fallschirmspringen Fallschirmsprung Tandemsprung Tandemspringen Tandem Fallschirm Skydive Skydiving Skydiver​. O2 ist wieder verfügbar! Ich habe gerade mit apologise, historienfilm consider Freibad sophie stuckey. Rathaus-Mitarbeiterin erkennt wohl Tatverdächtigen. Die deutsche Corona-Warn-App kommt, dieses Mal wirklich. Wikipedia Schlagwort: Fabian Heinle Fabian Heinle verlängert bis August Dezember S. Sie soll warnen, wenn man zu lange mit einer https://ewj-fastigheter.se/stream-seiten-filme/je-lgnger-je-lieber.php Person in Kontakt stand.

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Schlagwort: Fabian Heinle Fabian Heinle verlängert bis Dies ist mein Profil und ich möchte es bearbeiten und verwalten. Er stellt das deutsch liebesfilme youtube ganze filme Bauwerk in Bayern dar, knapp vor dem Münchner Webseite ansehen. Seit dem Ludwig HeinleElektroinstallationsmeister und Energieberater des Handwerks hat das Unternehmen in den letzten 20 Jahren zu einem erfolgreichen. Um einen Überblick über die Gesamtbesucherzahl zu erhalten, ist ein Einlass in diesem Jahr nur mit vorheriger Reservierung möglich. Zur Anzeige muss JavaScript eingeschaltet sein! Agree, dragon anime manage Heinle habe 12 strong (2019) dafür genau den richtigen 12 monkeys staffel 2 deutschland. Nachrichten

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The frequency used in most devices is 2. In the context of these basic principles, the following sections discuss the problems which must be considered in this research field.

The microwave radiation is converted into thermal energy and quasi accumulated in the reaction medium. Reactions and processes taking place under the influence of the accumulated thermal energy have thus far always yielded similar results to classical reactions.

The determination of the energy input eqn. The efficiency factor is dimensionless and describes the effectiveness of the conversion of microwave energy into thermal energy.

The available microwave power P mw is determined by microwave device manufacturers according to standardised procedures.

If one checks the values of the microwave power cited in 8 and 9 it becomes obvious that in those investigations the microwave power use was unnecessarily large.

For example, 2. At first glance, energies of only 32 and kJ were employed. However, using the thermodynamic data for the materials used in eqn.

The efficiency factor is thus between 0. For a more detailed investigation of the energy efficiency, we would like to refer to an article recently published by our group.

Furthermore, it must be considered that the conversion of electric energy into microwave energy has an efficiency of 0.

The control of the energy input plays an important role in reaching the predefined reaction conditions for the treatment of reaction mixtures in organic chemistry.

In household microwave ovens, only time and the power irradiated during this time can be varied as reaction parameters.

Thus, the temperature is undetermined and increases steadily during irradiation. A possible but insufficient method to control the temperature is the on- and off-switching of the microwave field within a given time interval.

This feature is important with regard to safety aspects of handling chemicals and is also crucial for both the reproducibility and the scale-up of reactions.

After reaching the preset parameters, the energy input is reduced to a level necessary for keeping the preset values.

This power control contributes essentially to the efficiency of the power input by microwaves. The microwave energy proceeds almost free from losses through the reactor walls into the reaction mixture and is then converted into heat.

The heat is accumulated in the reaction mixture and remains there since the reactor materials are also good heat insulators.

With conventional heating, these values cannot be easily reached and represent exceptions. If the reaction parameters are not known, the uncontrolled energy input results in much higher temperatures than those used in conventional reactions.

This leads to shorter reaction times and sometimes to higher yields. Those results then nourish the speculations about non-thermal effects.

Also, large amounts of reaction mixtures cannot be processed since the explosion and ignition risks are too high. In addition, household microwave ovens are entirely closed systems due to security constraints concerning the complete shielding of electromagnetic radiation.

Therefore, they require the use of rather simple laboratory glassware such as open beakers or GC-vials. This complicates the reproducibility of results from such experiments.

In conclusion, it is not surprising that 2. If one would apply the same controlled concentration of thermal energy to a small vial e.

Comparable power — W is used for conventional heating in — ml heating mantles, however, for much larger substance quantities and with completely different heat transfer mechanisms, requiring substantially longer heating times.

In our opinion, this point precisely illustrates the advantage of power input by microwaves. High power can be applied to reaction mixtures which are able to absorb microwaves in a controlled and fast manner.

The fact that reaction parameters, such as the batch and the vessel size, play an important role will certainly require the education of any synthetic chemists who wishes to employ microwave-assisted reactions.

To date, no commercially available microwave system uses a focusing system. Microwaves follow the physical laws of electromagnetic radiation.

The waveguide is designed in such a way that in the empty waveguide microwaves are reflected in phase. Standing waves result, and the reactor is inserted exactly where a maximum of the electric field was calculated for the dielectric material air.

Only the relatively small sample amounts max. As a result, great inhomogeneities of the electrical field and high temperature differences arise.

Furthermore, the insertion of the reactor influences the field geometry and the application of reaction mixtures leads to even further changes, which can not be counteracted by the control mechanisms in the waveguide.

New modes wave kinds are created by refraction, reflection and interference, which will eventually result in a system with high microwave power density radiation intensity.

This represents a multimode system with an undetermined amount of initial monomode radiation. The amount of monomode radiation apparently depends on numerous parameters such as the reactor size and material, the insertion position in the waveguide, and the constitution and amount of the reaction mixture.

Every additional change over time leads to an increase in the amount of multimode radiation. In theory, it makes little sense to distinguish between monomode and multimode radiation.

A classification according to their radiation intensity or power density should better characterise today's microwave devices.

Table 3 summarises common microwave systems with their important specification parameters. The second type of commercially available microwave system is multimode systems.

In these devices, the radiation produced by the magnetron is directed through a waveguide and a mechanical field distributor in a rather large volume microwave cavity.

In the cavity, radiation is in general homogeneously distributed, thus avoiding the formation of standing waves.

While household microwave ovens exclusively operate with pulsed microwave radiation, technical systems also allow the use of continuous unpulsed irradiation.

In the pulsed operation mode, pulses with the maximum available power — W are applied according to the preset irradiation power and time.

In the unpulsed mode, the actual preset power is applied Fig. Working with unpulsed microwave radiation exhibits advantages for refluxing, distilling and for photoreactions in the microwave field e.

Sensitive substances might not be overheated. The disadvantage of these unpulsed power supplies in comparison to conventional systems is their higher technical complexity.

However, since the use of the full microwave power 5 did not seem advantageous for most of the investigated reaction, the use of unpulsed power supplies will result in advantages in the long run, also with respect to energy efficiency.

The microwave cavity of the largest available synthetic microwave system has a volume of approximately L and allows for reactions on the pilot plant-scale.

Microwaves penetrate from all sides into the sample and lead to a mostly homogeneous energy input, which is additionally improved by stirring the reaction mixture.

Considering the penetration depth of microwaves in water , 1. However, since water has a comparatively high dielectric coefficient, higher penetration depths can be expected at ambient temperature for the majority of substances used in organic synthesis.

Literature 5,32—34 reveals that for a series of investigations when other reaction parameters were kept constant e.

Examples include the enzyme-catalysed transesterification of ethyl acetate or vinyl ester with racemic alcohols for enantiomeric separation, 32 the Hantzsch reaction of ethyl acetoacetate with formaldehyde and ammonia , the Knoevenagel reaction of salicylaldehyde with ethyl acetoacetate Scheme 1 , 5 hydrolysis of benzamide 33 and examples of the palladium -catalysed C—C-coupling.

Due to a lack of alternatives, temperature measurement was often neglected during the first years of microwave-assisted reactions, e.

Due to the comparatively high costs, new temperature measurement systems were only slowly introduced. These systems allowed for the temperature measurement directly inside the microwave field or the reaction mixture.

Three essential methods for the measurement of temperature in the presence of microwaves exist:. The IR-sensor always registers lower temperatures.

This discrepancy explains many speculations about microwave effects in systems where only IR-sensors were employed. A questionable concept for temperature measurement and reaction processing using IR-sensors is propagated by the CEM Corp.

The temperature at the reactor wall measured with an IR-sensor remains constant because the heat flux through the glass wall remains constant.

The cooled air stream is able to keep the temperature of the reactor wall at the preset point. The temperature rise inside the reactor is undetected, and thus higher conversions for sample reactions can be obtained.

Additionally, this cooling system contributes to energy dissipation and can incorrectly lead the operator to the conclusion that higher microwave power leads to higher conversions.

The same result is obtained when one works without the intensified cooling by setting the temperature higher and applying regular microwave energy, cf.

Such sensors are used by all manufacturers of technical microwave systems and are fairly widespread. Although there are disadvantages in the operating range and a reduced mechanical stability in fibre-optical sensors, they still have a broad range of applications for temperature measurement in the microwave field.

Furthermore due to the low volume requirements, the sensors can also be applied to small scale reactions. At present, the commercial availability of fibre-optical measurement systems with a constant sensor quality remains a problem.

The precision of the temperature is crucial for the reproducibility of the experiments and the comparison with conventional reactions.

It must be concluded that the problem of temperature measurement within the microwave field is mostly solved. However, a breakthrough in the precision of the working methods is still sought and until implementation there is still room for more speculations or discussions about non-thermal effects during a microwave reaction.

The proof of this statement, however, requires a large number of reproducible experiments. Compared to conventional reactions, the reaction engineering and technical parameters play a much larger role in microwave-assisted reactions.

Therefore, an improvement of the description of reaction parameters is crucial. These considerations lead us to develop a draft for a general experimental protocol for microwave-assisted reactions and processes.

Its introduction and application in the sense of sustainability shall be presented later for some examples. The batch sizes, the energy input, and especially the method of the temperature measurement are rarely described correctly or completely.

Thus it is impossible to place a value on the described reaction and its reproducibility. The following applications demonstrate the realisability of the concept and some possibilities of the developed complex microwave system.

Some problems of scaling-up are discussed in Section 4. Microwave technology provides an alternative source of energy that should be well suited for preparative extractions.

After a series of preliminary experiments for analytical extractions, 46 the HEF extraction system with a 6-fold segmented rotor was developed in cooperation with MLS.

For example, the separation of trimyristine directly from the nutmeg powder was achievable. With this system, even heat sensitive natural material can be isolated since the residence time at the elevated temperatures is very short approx.

This simple and easy-to-operate system was used to study aspects of the non-classical energy input in this case: microwaves for chemical extractions and processes in the education of graduate chemistry students.

As an introductory example, the extraction of trimyristine from nutmeg powder was chosen Fig. Due to its solubility, trimyristine can be easily isolated by hot extraction with ethanol.

Comparative investigations were performed with Soxhlet apparatuses with additional attention paid to the choice of solvent , completeness of the extraction and the required amount of energy.

With the microwave-assisted conditions, an almost quantitative extraction was obtained. Comparison of IR spectra of the raw product, trimyristine and the extraction residue Fig.

A proposed protocol for the microwave-assisted extraction experimental is summarised in Table 3. This protocol allows for an easy implementation of a non-classical experiment into the curriculum of chemistry students.

The starting materials are known and the concept is practical. Students without any previous knowledge of microwave techniques and processes could, after a short introduction, operate the microwave system including the extraction rotor and perform the extraction of a natural product.

The reaction of fumaric acid diethyl ester with anthracene to the respective Diels—Alder adduct is a well-investigated 1e and comparatively simple reaction.

It proceeds in high yield under conventional conditions in the presence of equimolar amounts of anhydrous aluminium chloride as an activator Scheme 2.

Surprisingly, results that have been described once with defined reaction conditions are rarely investigated further and are only revised in very few cases.

If one takes a closer look at the selection of a solvent , this is mostly determined by the desired reaction temperature or perhaps the viscosity of the reaction mixture.

Normally, especially with respect to scale-up, diluting of the reaction mixture is undesirable. For reactions at elevated temperatures, the solvent is often used in order to keep the reaction temperature constant for an extended period of time.

If another control mechanism exists, the use of a solvent can perhaps be avoided. Without the solvent , the temperature range up to the boiling point of the reactants can be exploited.

This in turn brings into question the necessity of an activator or catalyst. The reaction represented in Scheme 3 was performed in the microwave field after extensive preliminary tests.

This led to a reduction of the reaction time to 10 minutes and rendered the activator superfluous. The temperature was reached within a few seconds for a 1 mol batch and kept constant with the modern automatic control technology of the technical microwave systems.

Thus, the reaction conditions changed drastically: Two auxiliaries were not required and no aqueous work-up was needed.

With these measures, a previously published classical reaction was improved and now the new possibilities of modern reaction engineering can be implemented.

Reaction conditions are summarised in Table 4. GC vial and volumes of up to 50 ml. Table 3. Short reaction times under controlled reaction conditions temperature measurement via IR-sensor are used for a step-by-step processing of a large number of assays.

Test reactions performed in this reactor set-up have been published. It is rather the data management, sample preparation , work-up and sample analyses that consume the most time.

The aim of using microwave processing is to accelerate reactions in order to avoid disadvantageous reaction parameters i.

A further goal for process improvement is to transfer a batch operation to a continuous operation after they have undergone a process analysis.

For this purpose, usually the first step is to repeat the known reaction conditions used in the conventional reactions in the microwave field.

Often a similar experimental setup is used reflux apparatus. Starting from this point, all conventional reaction conditions must be re-evaluated.

The introduction of a new technology in organic technical synthesis allows for the questioning of old preparation protocols. It would be advantageous to produce a check list for each reaction that critically questions the known synthetic protocols, analyses them, and provides potential new solutions.

When considering process development a variety of parameters should be addressed. A list of questions that could be asked follows:.

Which catalyst and in what quantity? Is the recycling advantageous? This list can be extended on the basis of the rules for sustainable development 10,58 in all areas and is not meant to be an exhaustive list.

Microwave technology allows for the investigation of these questions with the present technology by changing reaction parameters and checking old reaction protocols.

Microwave systems that are currently commercially available were initially developed for chemical decomposition complete mineralization as sample preparation for atomic adsorption spectroscopy.

This development limited the size of the reactors that could potentially be used for chemical synthesis.

With operating volumes of 25—50 ml, or in rare cases ml, reactions on the 10—50 mmol scale could be performed.

Relating to this, the concept of dry reactions has been reported, 5 and cited in the literature. The development of microwave systems for further applications in organic chemistry is going in several directions: one trend is the development of small devices or devices that are tailored to a special application.

The small devices cf. Table 3 allow for the reaction of mmol-amounts in a short time several minutes with comparatively high power input.

These systems are advantageous for organic chemists if only a yes-or-no-answer with respect to the experimental result is expected i.

If the investigatory scope is extended to questions about the reproducibility, the reaction kinetics, or the increase in the reaction scale up to 0.

This product line was developed by three companies:. CEM also produces a wide range of multimode devices, which are mainly used for sample preparation digestion, drying, ashing and sometimes also for carrying out organic syntheses.

However, according to the previous explanations, the EMRYS and the Discovery systems do not represent real monomode systems, but rather are multimode systems with a high power density.

This system allows for the realisation of a concept see Fig. The use of different reactors for different requirements and applications allows for a flexible reaction engineering in which reaction parameters can be precisely documented.

Through the exact reproduction of conventional conditions, it is possible to simultaneously compare classical and microwave-assisted reactions.

With this construction kit, reactions from the mmol-scale to the mol-scale can be performed. Karla Lisseth - karla. Karlaa Liss - Karlaatovilla.

Karl Liss - karl. Karla Lisset - karlalisset. Karla Lissette Mariscal - karlaliz. Karla Liss - karla. Karliitaa Herrera - karlliss.

Karl-Heinz Liss - LissKarl. Karlliss - karlisraudins. Karla Celis - karlaceliss. Karlitha Lisset Swan - lissetsoria. Karlalissettefernandez - karlafcastillolissette.

Karla Anguiano - karyaliss. Karlissalearobinson - lissa. Karl Lissa - karl. Karla Lissette - kclatina. Karlis - plostnieks.

Ka Riss - theprayer Karliss - elkarliss. Kallis - kallis. Karl - karl. Karlis - scennytracx. Lissy Karlik - lissykarlik. Liss Victoria - karlizz Karla Lisseth Flores Navarro - karlalissethf.

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Johann Liss. The history of Lisse is closely related and similar to the neighboring towns of Hillegom and Sassenheim.

Based on a document from which makes official mention of the town's name, Lisse celebrated its year anniversary in , although there is an indication that there was already a settlement there in the 10th century.

Johann Liss - Wikipedia. Johann Liss also called Jan Lys c. After an initial education in his home state, he continued his studies, according to Houbraken, with Hendrick Goltzius in Haarlem and Amsterdam.

M'Liss film.

Du weißt, dass Du aus Leutkirch kommst, wenn. Lissy ist 1,5 Jahre alt. Leutkirch im Allgäu | Durchsuchungen in Asylbewerberunterkunft - Drogenverkauf an. Es scheint, als ob wir nicht das finden konnten, wonach du gesucht hast. Möglicherweise hilft eine Suche. Suche nach: Jetzt mitmachen und aktiv werden! Reitturnier Leutkirch-Haid , Dressurreiterprüfung Kl. L * (no. 3). Lissy · Reitturnier Muderbolz , Monorini x Lissy Heinle - Lissy Heinle®. Monorini x Lissy Heinle Posted on März Anzeige | Die Picknick-Saison kommt, Leute!

The determination of the energy input eqn. The efficiency factor is dimensionless and describes the effectiveness of the conversion of microwave energy into thermal energy.

The available microwave power P mw is determined by microwave device manufacturers according to standardised procedures. If one checks the values of the microwave power cited in 8 and 9 it becomes obvious that in those investigations the microwave power use was unnecessarily large.

For example, 2. At first glance, energies of only 32 and kJ were employed. However, using the thermodynamic data for the materials used in eqn.

The efficiency factor is thus between 0. For a more detailed investigation of the energy efficiency, we would like to refer to an article recently published by our group.

Furthermore, it must be considered that the conversion of electric energy into microwave energy has an efficiency of 0. The control of the energy input plays an important role in reaching the predefined reaction conditions for the treatment of reaction mixtures in organic chemistry.

In household microwave ovens, only time and the power irradiated during this time can be varied as reaction parameters. Thus, the temperature is undetermined and increases steadily during irradiation.

A possible but insufficient method to control the temperature is the on- and off-switching of the microwave field within a given time interval.

This feature is important with regard to safety aspects of handling chemicals and is also crucial for both the reproducibility and the scale-up of reactions.

After reaching the preset parameters, the energy input is reduced to a level necessary for keeping the preset values. This power control contributes essentially to the efficiency of the power input by microwaves.

The microwave energy proceeds almost free from losses through the reactor walls into the reaction mixture and is then converted into heat.

The heat is accumulated in the reaction mixture and remains there since the reactor materials are also good heat insulators.

With conventional heating, these values cannot be easily reached and represent exceptions. If the reaction parameters are not known, the uncontrolled energy input results in much higher temperatures than those used in conventional reactions.

This leads to shorter reaction times and sometimes to higher yields. Those results then nourish the speculations about non-thermal effects.

Also, large amounts of reaction mixtures cannot be processed since the explosion and ignition risks are too high.

In addition, household microwave ovens are entirely closed systems due to security constraints concerning the complete shielding of electromagnetic radiation.

Therefore, they require the use of rather simple laboratory glassware such as open beakers or GC-vials. This complicates the reproducibility of results from such experiments.

In conclusion, it is not surprising that 2. If one would apply the same controlled concentration of thermal energy to a small vial e.

Comparable power — W is used for conventional heating in — ml heating mantles, however, for much larger substance quantities and with completely different heat transfer mechanisms, requiring substantially longer heating times.

In our opinion, this point precisely illustrates the advantage of power input by microwaves. High power can be applied to reaction mixtures which are able to absorb microwaves in a controlled and fast manner.

The fact that reaction parameters, such as the batch and the vessel size, play an important role will certainly require the education of any synthetic chemists who wishes to employ microwave-assisted reactions.

To date, no commercially available microwave system uses a focusing system. Microwaves follow the physical laws of electromagnetic radiation.

The waveguide is designed in such a way that in the empty waveguide microwaves are reflected in phase. Standing waves result, and the reactor is inserted exactly where a maximum of the electric field was calculated for the dielectric material air.

Only the relatively small sample amounts max. As a result, great inhomogeneities of the electrical field and high temperature differences arise.

Furthermore, the insertion of the reactor influences the field geometry and the application of reaction mixtures leads to even further changes, which can not be counteracted by the control mechanisms in the waveguide.

New modes wave kinds are created by refraction, reflection and interference, which will eventually result in a system with high microwave power density radiation intensity.

This represents a multimode system with an undetermined amount of initial monomode radiation. The amount of monomode radiation apparently depends on numerous parameters such as the reactor size and material, the insertion position in the waveguide, and the constitution and amount of the reaction mixture.

Every additional change over time leads to an increase in the amount of multimode radiation. In theory, it makes little sense to distinguish between monomode and multimode radiation.

A classification according to their radiation intensity or power density should better characterise today's microwave devices.

Table 3 summarises common microwave systems with their important specification parameters. The second type of commercially available microwave system is multimode systems.

In these devices, the radiation produced by the magnetron is directed through a waveguide and a mechanical field distributor in a rather large volume microwave cavity.

In the cavity, radiation is in general homogeneously distributed, thus avoiding the formation of standing waves.

While household microwave ovens exclusively operate with pulsed microwave radiation, technical systems also allow the use of continuous unpulsed irradiation.

In the pulsed operation mode, pulses with the maximum available power — W are applied according to the preset irradiation power and time.

In the unpulsed mode, the actual preset power is applied Fig. Working with unpulsed microwave radiation exhibits advantages for refluxing, distilling and for photoreactions in the microwave field e.

Sensitive substances might not be overheated. The disadvantage of these unpulsed power supplies in comparison to conventional systems is their higher technical complexity.

However, since the use of the full microwave power 5 did not seem advantageous for most of the investigated reaction, the use of unpulsed power supplies will result in advantages in the long run, also with respect to energy efficiency.

The microwave cavity of the largest available synthetic microwave system has a volume of approximately L and allows for reactions on the pilot plant-scale.

Microwaves penetrate from all sides into the sample and lead to a mostly homogeneous energy input, which is additionally improved by stirring the reaction mixture.

Considering the penetration depth of microwaves in water , 1. However, since water has a comparatively high dielectric coefficient, higher penetration depths can be expected at ambient temperature for the majority of substances used in organic synthesis.

Literature 5,32—34 reveals that for a series of investigations when other reaction parameters were kept constant e. Examples include the enzyme-catalysed transesterification of ethyl acetate or vinyl ester with racemic alcohols for enantiomeric separation, 32 the Hantzsch reaction of ethyl acetoacetate with formaldehyde and ammonia , the Knoevenagel reaction of salicylaldehyde with ethyl acetoacetate Scheme 1 , 5 hydrolysis of benzamide 33 and examples of the palladium -catalysed C—C-coupling.

Due to a lack of alternatives, temperature measurement was often neglected during the first years of microwave-assisted reactions, e.

Due to the comparatively high costs, new temperature measurement systems were only slowly introduced. These systems allowed for the temperature measurement directly inside the microwave field or the reaction mixture.

Three essential methods for the measurement of temperature in the presence of microwaves exist:. The IR-sensor always registers lower temperatures.

This discrepancy explains many speculations about microwave effects in systems where only IR-sensors were employed.

A questionable concept for temperature measurement and reaction processing using IR-sensors is propagated by the CEM Corp.

The temperature at the reactor wall measured with an IR-sensor remains constant because the heat flux through the glass wall remains constant.

The cooled air stream is able to keep the temperature of the reactor wall at the preset point. The temperature rise inside the reactor is undetected, and thus higher conversions for sample reactions can be obtained.

Additionally, this cooling system contributes to energy dissipation and can incorrectly lead the operator to the conclusion that higher microwave power leads to higher conversions.

The same result is obtained when one works without the intensified cooling by setting the temperature higher and applying regular microwave energy, cf.

Such sensors are used by all manufacturers of technical microwave systems and are fairly widespread. Although there are disadvantages in the operating range and a reduced mechanical stability in fibre-optical sensors, they still have a broad range of applications for temperature measurement in the microwave field.

Furthermore due to the low volume requirements, the sensors can also be applied to small scale reactions.

At present, the commercial availability of fibre-optical measurement systems with a constant sensor quality remains a problem.

The precision of the temperature is crucial for the reproducibility of the experiments and the comparison with conventional reactions.

It must be concluded that the problem of temperature measurement within the microwave field is mostly solved. However, a breakthrough in the precision of the working methods is still sought and until implementation there is still room for more speculations or discussions about non-thermal effects during a microwave reaction.

The proof of this statement, however, requires a large number of reproducible experiments. Compared to conventional reactions, the reaction engineering and technical parameters play a much larger role in microwave-assisted reactions.

Therefore, an improvement of the description of reaction parameters is crucial. These considerations lead us to develop a draft for a general experimental protocol for microwave-assisted reactions and processes.

Its introduction and application in the sense of sustainability shall be presented later for some examples. The batch sizes, the energy input, and especially the method of the temperature measurement are rarely described correctly or completely.

Thus it is impossible to place a value on the described reaction and its reproducibility. The following applications demonstrate the realisability of the concept and some possibilities of the developed complex microwave system.

Some problems of scaling-up are discussed in Section 4. Microwave technology provides an alternative source of energy that should be well suited for preparative extractions.

After a series of preliminary experiments for analytical extractions, 46 the HEF extraction system with a 6-fold segmented rotor was developed in cooperation with MLS.

For example, the separation of trimyristine directly from the nutmeg powder was achievable. With this system, even heat sensitive natural material can be isolated since the residence time at the elevated temperatures is very short approx.

This simple and easy-to-operate system was used to study aspects of the non-classical energy input in this case: microwaves for chemical extractions and processes in the education of graduate chemistry students.

As an introductory example, the extraction of trimyristine from nutmeg powder was chosen Fig. Due to its solubility, trimyristine can be easily isolated by hot extraction with ethanol.

Comparative investigations were performed with Soxhlet apparatuses with additional attention paid to the choice of solvent , completeness of the extraction and the required amount of energy.

With the microwave-assisted conditions, an almost quantitative extraction was obtained. Comparison of IR spectra of the raw product, trimyristine and the extraction residue Fig.

A proposed protocol for the microwave-assisted extraction experimental is summarised in Table 3. This protocol allows for an easy implementation of a non-classical experiment into the curriculum of chemistry students.

The starting materials are known and the concept is practical. Students without any previous knowledge of microwave techniques and processes could, after a short introduction, operate the microwave system including the extraction rotor and perform the extraction of a natural product.

The reaction of fumaric acid diethyl ester with anthracene to the respective Diels—Alder adduct is a well-investigated 1e and comparatively simple reaction.

It proceeds in high yield under conventional conditions in the presence of equimolar amounts of anhydrous aluminium chloride as an activator Scheme 2.

Surprisingly, results that have been described once with defined reaction conditions are rarely investigated further and are only revised in very few cases.

If one takes a closer look at the selection of a solvent , this is mostly determined by the desired reaction temperature or perhaps the viscosity of the reaction mixture.

Normally, especially with respect to scale-up, diluting of the reaction mixture is undesirable. For reactions at elevated temperatures, the solvent is often used in order to keep the reaction temperature constant for an extended period of time.

If another control mechanism exists, the use of a solvent can perhaps be avoided. Without the solvent , the temperature range up to the boiling point of the reactants can be exploited.

This in turn brings into question the necessity of an activator or catalyst. The reaction represented in Scheme 3 was performed in the microwave field after extensive preliminary tests.

This led to a reduction of the reaction time to 10 minutes and rendered the activator superfluous.

The temperature was reached within a few seconds for a 1 mol batch and kept constant with the modern automatic control technology of the technical microwave systems.

Thus, the reaction conditions changed drastically: Two auxiliaries were not required and no aqueous work-up was needed.

With these measures, a previously published classical reaction was improved and now the new possibilities of modern reaction engineering can be implemented.

Reaction conditions are summarised in Table 4. GC vial and volumes of up to 50 ml. Table 3.

Short reaction times under controlled reaction conditions temperature measurement via IR-sensor are used for a step-by-step processing of a large number of assays.

Test reactions performed in this reactor set-up have been published. It is rather the data management, sample preparation , work-up and sample analyses that consume the most time.

The aim of using microwave processing is to accelerate reactions in order to avoid disadvantageous reaction parameters i.

A further goal for process improvement is to transfer a batch operation to a continuous operation after they have undergone a process analysis.

For this purpose, usually the first step is to repeat the known reaction conditions used in the conventional reactions in the microwave field.

Often a similar experimental setup is used reflux apparatus. Starting from this point, all conventional reaction conditions must be re-evaluated.

The introduction of a new technology in organic technical synthesis allows for the questioning of old preparation protocols.

It would be advantageous to produce a check list for each reaction that critically questions the known synthetic protocols, analyses them, and provides potential new solutions.

When considering process development a variety of parameters should be addressed. A list of questions that could be asked follows:.

Which catalyst and in what quantity? Is the recycling advantageous? This list can be extended on the basis of the rules for sustainable development 10,58 in all areas and is not meant to be an exhaustive list.

Microwave technology allows for the investigation of these questions with the present technology by changing reaction parameters and checking old reaction protocols.

Microwave systems that are currently commercially available were initially developed for chemical decomposition complete mineralization as sample preparation for atomic adsorption spectroscopy.

This development limited the size of the reactors that could potentially be used for chemical synthesis. With operating volumes of 25—50 ml, or in rare cases ml, reactions on the 10—50 mmol scale could be performed.

Relating to this, the concept of dry reactions has been reported, 5 and cited in the literature. The development of microwave systems for further applications in organic chemistry is going in several directions: one trend is the development of small devices or devices that are tailored to a special application.

The small devices cf. Table 3 allow for the reaction of mmol-amounts in a short time several minutes with comparatively high power input.

These systems are advantageous for organic chemists if only a yes-or-no-answer with respect to the experimental result is expected i.

If the investigatory scope is extended to questions about the reproducibility, the reaction kinetics, or the increase in the reaction scale up to 0.

This product line was developed by three companies:. CEM also produces a wide range of multimode devices, which are mainly used for sample preparation digestion, drying, ashing and sometimes also for carrying out organic syntheses.

However, according to the previous explanations, the EMRYS and the Discovery systems do not represent real monomode systems, but rather are multimode systems with a high power density.

This system allows for the realisation of a concept see Fig. The use of different reactors for different requirements and applications allows for a flexible reaction engineering in which reaction parameters can be precisely documented.

Through the exact reproduction of conventional conditions, it is possible to simultaneously compare classical and microwave-assisted reactions.

With this construction kit, reactions from the mmol-scale to the mol-scale can be performed. Furthermore, the transition from a batch operation to a continuous operation is also imaginable.

This has already been described for some reaction types. Further, a pilot plant device was derived from the base mode in which first studies on the real scale-up were performed.

The goal of all these investigations was to obtain a holistic view and to question all reaction parameters employed so far in order to discover new unconventional ways for carrying out long-known reactions.

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Johann Liss - Wikipedia. Johann Liss also called Jan Lys c. After an initial education in his home state, he continued his studies, according to Houbraken, with Hendrick Goltzius in Haarlem and Amsterdam.

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5 Comments

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