Microwave heating as a new way to induce localized enhancements of reaction rate. Non-isothermal and heterogeneous kinetics

Stuerga, D.; Gaillard, P.

doi:10.1016/0040-4020(96)00241-4

Cited by 132 publications

(75 citation statements)

References 14 publications

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“…Evidence of strong thermal gradients has been obtained (for ferrites, 50 mm À1 ). The author has shown that these hot spots or areas could induce localized reaction rate enhancement [145]. These results have shown that a very small density of superheating areas is sufficient to induce a consequent rate enhancement (2% of hot spots are sufficient to increase yield by 60%), even if their effects on averaged temperatures are not detectable.…”

Section: Hot Spots and Heterogeneous Kineticsmentioning

confidence: 87%

Microwave‐Material Interactions and Dielectric Properties, Key Ingredients for Mastery of Chemical Microwave Processes

Stuerga¹

2006

Microwaves in Organic Synthesis

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The main focus of the revised edition of this first chapter is essentially the same as the original -to explain in a chemically intelligible fashion the physical origin of microwave-matter interactions and, especially, the theory of dielectric relaxation of polar molecules. This revised version contains approximately 60% new material to scan a large range of reaction media able to be heated by microwave heating. The accounts presented are intended to be illustrative rather than exhaustive. They are planned to serve as introductions to the different aspects of interest in comprehensive microwave heating. In this sense the treatment is selective and to some extent arbitrary. Hence the bibliography contains historical papers and valuable reviews to which the reader anxious to pursue particular aspects should certainly turn.It is the author's conviction, confirmed over many years of teaching experience, that it is much safer -at least for those who rate not trained physicists -to deal intelligently with an oversimplified model than to use sophisticated methods which require experience before becoming productive. Because of comments about the first edition, however, the author has included more technical material to enable better understanding of the concepts and ideas. These paragraphs could be omitted, depending on the level of comprehension of the reader. They are preceded by two different symbols -k for TOOLS and j for CONCEPTS.After consideration of the history and position of microwaves in the electromagnetic spectrum, notions of polarization and dielectric loss will be examined. The orienting effects of electric fields and the physical origin of dielectric loss will be analyzed, as also will transfers between rotational states and vibrational states within condensed phases.Dielectric relaxation and dielectric losses of pure liquids, ionic solutions, solids, polymers and colloids will be discussed. Effect of electrolytes, relaxation of defects within crystals lattices, adsorbed phases, interfacial relaxation, space charge polarization, and the Maxwell-Wagner effect will be analyzed. Next, a brief overview of 1 thermal conversion properties, thermodynamic aspects, and athermal effects will be given.

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Section: Hot Spots and Heterogeneous Kineticsmentioning

confidence: 87%

Microwave‐Material Interactions and Dielectric Properties, Key Ingredients for Mastery of Chemical Microwave Processes

Stuerga¹

2006

Microwaves in Organic Synthesis

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“…Both differential coupling abilities of materials and distribution of electromagnetic fields may result in localized temperature distribution in catalytic beds, but the contribution of these effects is difficult to quantify. Stuerga and Gaillard 6,7 have given a thorough theoretical analysis which has conclusively demonstrated that specific athermal effects are implausible given the small energies associated with the microwave quanta and the classical nature of the heating phenomenon and have suggested that localized enhancements of the reaction rates may be responsible for non-isothermal and heterogeneous kinetic phenomena. The majority of the experimental studies on catalytic reactions under microwave conditions have dwelled on the kinetic aspects and noted that the rates of reaction and product distributions are more consistent with a temperature 300-400 K higher than that measured for the bulk of the catalyst bed.…”

mentioning

confidence: 99%

Apparent equilibrium shifts and hot-spot formation for catalytic reactions induced by microwave dielectric heating

Zhang¹,

Hayward

Mingos³

1999

Chem. Commun.

152

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Microwave dielectric heating of the gas phase decomposition of H 2 S catalysed by metal sulfides on a g-Al 2 O 3 support results in significant apparent shifts in the equilibrium constant, which have been attributed to the development of hot-spots in the catalytic beds; X-ray diffraction and electron microscopy measurements have indicated the formation of hot-spots with dimensions of 90-1000 mm and which involve not only the active phase, but also the support.The acceleration of heterogeneous catalytic processes under microwave dielectric heating conditions has attracted both academic and industrial interest. [1][2][3][4][5] The majority of systems studied involve a catalyst which preferentially absorbs the microwaves relative to the support material and the reasons for the observed rate enhancements have been the subject of some controversy. The measurement of temperature under conditions where there are strong electric fields is problematical, but may be overcome, for example, by using optical fibre technology. Such measurements are still only capable of providing average temperatures. Both differential coupling abilities of materials and distribution of electromagnetic fields may result in localized temperature distribution in catalytic beds, but the contribution of these effects is difficult to quantify. Stuerga and Gaillard 6,7 have given a thorough theoretical analysis which has conclusively demonstrated that specific athermal effects are implausible given the small energies associated with the microwave quanta and the classical nature of the heating phenomenon and have suggested that localized enhancements of the reaction rates may be responsible for non-isothermal and heterogeneous kinetic phenomena. The majority of the experimental studies on catalytic reactions under microwave conditions have dwelled on the kinetic aspects and noted that the rates of reaction and product distributions are more consistent with a temperature 300-400 K higher than that measured for the bulk of the catalyst bed. However, for supported metal catalysts, calculations 8,9 have shown that the rate of heat transfer at 1 atm gas pressure is so high that the formation of hot-spots localised at the catalytically active sites are implausible. Here we report significant circumstantial evidence for the formation of hotspots in the microwave experiments and have demonstrated for the first time that these hot-spots are not localised exclusively on the active catalyst, but also involve the support material. We have also estimated the dimensions of these hot-spots.The catalytic conversion of H 2 S into hydrogen and sulfur, which is commercially important for the coal and petrochemical industry, 10,11 has recently been investigated in our laboratories using parallel microwave and conventional heating conditions. The experimental set-up is illustrated schematically in Fig. 1 and the reactions were performed under continuous flow conditions using quartz reactors. The temperature in the microwave cavity was monitored using an Accufibre optica...

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“…The corresponding yields of MGCA were 62 microwave heating and 47 oil-bath heating after a 30 min heating period without solvent. Microwave heating can, in principle, induce localized hot spots that can lead to localized reaction rate enhancements 18 , and thus to uneven heating at the microscopic level as predicted from the dielectric factors. However, no microwave-specific effect s was apparent even though the reaction solution contained substrates of different dielectric characteristics when heated in homogeneous 2-propanol solvent by microwave dielectric heating.…”

Section: Reactionmentioning

confidence: 99%

Microwave Synthesis, Extraction, Improvement and Degradation in Oil Chemistry

Sumi

Horikoshi

2013

J. Oleo Sci.

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Microwave heating as a new way to induce localized enhancements of reaction rate. Non-isothermal and heterogeneous kinetics

Cited by 132 publications

References 14 publications

Microwave‐Material Interactions and Dielectric Properties, Key Ingredients for Mastery of Chemical Microwave Processes

Microwave‐Material Interactions and Dielectric Properties, Key Ingredients for Mastery of Chemical Microwave Processes

Apparent equilibrium shifts and hot-spot formation for catalytic reactions induced by microwave dielectric heating

Microwave Synthesis, Extraction, Improvement and Degradation in Oil Chemistry

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