Control of Microwave-Generated Hot Spots. 6. Generation of Hot Spots in Dispersed Catalyst Particulates and Factors That Affect Catalyzed Organic Syntheses in Heterogeneous Media

Abstract

This article revisits the formation of 4-methylbiphenyl by the Suzuki−Miyaura reaction to examine the formation of hot spots and the factors impacting product yields, such as (i) mass transfer of reactants to the Pd/AC catalyst, (ii) continuous versus pulsed microwave irradiation, (iii) presence of a standing wave versus a nonstanding wave, and (iv) microwave input power levels. Present results indicate that mass transfer and hot spots impact the catalytic process. The rate of stirring of the heterogeneous mixture impinges on the formation of hot spots and product yields. Continuous and pulsed microwaves have little effect, whereas both mass transfer and the presence or absence of a standing wave do affect the yields. Beyond a certain stirring rate (1500 rpm), mass transfer is no longer an issue as yields remain constant; below this value, however, mass transfer, hot spots, and microwave input power levels play a role in the extent of products formed.

In situ temperature measurements in microwave-heated gas-solid catalytic systems. Detection of hot spots and solid-fluid temperature gradients in the ethylene epoxidation reaction

“…The problem becomes more severe in liquid phase catalyzed reactions where the 3 catalyst acts simultaneously as a MW absorber 4 ,13-14 . However, even in these cases temperature inhomogeneity can be reduced by introducing forced convection in the form of a fast stirring of the liquid mass 14 . On the other hand, in gas-solid systems the induction of hot spots in the microwave-absorbing solid phase (catalyst) becomes most likely.…”

Avoiding hot-spots in Microwave-assisted Pd/C catalysed reactions by using the biomass derived solvent γ-Valerolactone

“…Thus hot spot represent one of the main limitations to the use of MW dielectric heating by some organic chemists. It is well documented 20 , 22 , 23 and we directly experienced 24 – 26 that hots spots are usually observed when solvents with low boiling points are used in the presence of a heterogeneous catalysts under high electric fields. Sometimes, this issue can be circumvented in lab scale microwave assisted reactions by the use of solvents with higher boiling points than the temperature requested by the process (e.g.…”

The electromagnetic wave energy effect(s) in microwave–assisted organic syntheses (MAOS)

“…The synthesis of 4-methylbiphenyl by the Suzuki Miyaura cross-coupling process (Fig. S-3) was carried out to compare the results with our earlier study 24 . Microwave power was fixed at 51 W for each generator.…”

“…6b ). The Pd/AC catalyst was prepared using the procedure reported in our earlier study 24 . The quantity of Pd on the activated carbon particulates support was ca.…”