Chemistry Reference
In-Depth Information
Figure 4.11 (a) Structure of functionalized magnetic particle. (b) Flow tube reactor filled with
catalyst-coated magnetic particles, shown within an inductor. Permission granted by 2008
Wiley-VCH, Weinheim, [121].
The same thing is found in the field of safety and explosions as a whole. Thermal
explosions take place when a system generates positive feedback in response to the reaction
mechanism. There are two possible types of explosion: kinetic and thermal. Kinetic
explosions take place in gas phase-containing systems, whenever propagation leads to a
generation of more highly reactive radicals. Thermal explosions occur when heat evolved
in exothermic reactions facilitates such reactions further through the temperature-depen-
dent rate coefficients [123].
In order to quench radical formation and extensive heat generation, quenching materials,
inerts and special solvents can be utilized on a macroscale. However, these measures are
not in line with green chemistry principles, and further are not completely reliable, due to
fluctuations in local temperature and concentrations within reactors. Microreactors, as
discussed earlier, facilitate the homogeneous distribution of both chemical species and heat
within the stream.
Fischer et al. demonstrated a method of determining the maximum safe capillary
diameter for microreactors in the oxidation process of ethane by pure oxygen [11]. A
stoichiometric amount of oxygen was safely used to afford 88% selectivity in the oxidation
of cyclohexane under 16 bar and 200
C reaction conditions within a capillary microreactor
[124]. Struempel et al. produced benzoic acid methyl ester in a 75% yield by reacting
benzoic acid with diazomethane in the temperature range 0-50
C [125]. Maleic anhydride
[126] and ethylene oxide [127,128] are further successful examples of safety-relevant
direct oxidations in microchannels. In both reactions, full oxidation to carbon dioxide can
set in, which releases much more heat than the already exothermic selective oxidation -
almost an order of magnitude more. Highly pyrophoric trimethylaluminium was used to
produce a yield of 49% of the antiobesity drug rimonabant, in 2 minutes' reaction time
[129]. Roberge et al. demonstrated an improved selectivity and yield in the nitration of
aromatic compounds, rising to 79 and 77%, respectively, under almost solvent-free
conditions for particular compounds [95]. While formerly prohibited due to safety hazards,
the utilization of ethyl diazoacetate was enabled through microreactor technology by
Zhang et al. in a reaction with N-Boc-4-piperidine (yield: 49%, resolution time: 1.8
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