Application of micromixers - Micromixers Fundamentals Design and Fabrication - page 323

materials such as glass, photochemical [2 2] cycloaddition reaction that requires ultraviolet

(UV) light exposure can be carried out in microscale [12] . Compared to conventional batch
reactors, the residence time was significantly reduced to 2 hours. Production scale (few grams per
hour) of a key intermediate for furofuran lignans was achieved in a micromixer [13] .Inthis
work, tributyltin hydride-mediated radical reactions of organic halides were carried out with
a residence time of one minute. Production-scale elimination reaction was carried out in
micromixers for the synthesis of pristine, an adjuvant for monoclonal antibody production [14] .
Increasing conversion rate of 85-95% for dehydration of 1-hexanol to hexane was achieved in
a micromixer [15] . A conventional reactor can only provide 30% conversion for the same
reaction.
Coupling reactions are processes where two hydrocarbon fragments are coupled with the aid of
a metal catalyst. Coupling reactions are powerful tools for carbon-carbon bond formation. The two
types of coupling reactions are cross-couplings and homocouplings. Cross-couplings are reactions
between two different partners, while homocouplings link two identical partners.
Cross-coupling reactions were carried out in micromixers using heterogeneous Pd catalysts where
the Pd/SiO 2 catalyst bed was immobilized in a microchannel for the Suzuki-Miyaura reaction that
couples organic halides with organoboron compounds [16] . Ionic liquid with Pd catalyst was used in
a micromixer for the Mizoroki-Heck reaction, where an unsaturated halide (or triflate) reacts with an
alkene and a base and palladium catalyst to form a substituted alkene [17] . The microfluidic system
allows recycling of the ionic liquid and the Pd catalyst in a continuous manner. Microfabrication
allows making micromixers in optically transparent materials for photochemical coupling reactions.
Lu et al. carried out photochemical synthesis of benzopinacol in a micromixer made of silicon and
quartz [18] . The transparent quartz allows UV light to reach the mixing channel. Both optical trans-
parency and short residence time in a micromixer allow photochemical [2

þ

3] cycloaddition in
microscale [19] . In contrast to a conventional batch reactor, the reverse reaction is prevented by the
shorter residence time in a micromixer.
Oxidation and reduction are basic reactions for the synthesis of both organic and inorganic
compounds. Many oxidation reactions are exothermic. Thus, micromixers offer a safe reaction plat-
form with controlled conditions. Furthermore, the danger of explosions is minimized due to the small
amount of reactants involved. Hydrogen peroxide can be used as an effective liquid-phase oxidizing
agent. Yube and Mae carried out the oxidation of 2-methylnaphthalene with hydrogen peroxide to
2-methyl-1,4-naphthoquinonone, an antihemorrhagic vitamin [20] . Because of the shorter residence
time of 30 seconds, side reactions are minimized and a selectivity of 50% can be achieved for the
intended product.
Using microtechnology, electrodes can be easily integrated in micromixers. Therefore, electro-
chemical oxidation and reduction of organic compound can be achieved. The large surface-to-volume
ratio in microscale brings advantages of the higher mass transfer rate from and to the electrodes.
Mengeaud et al. [21] carried out oxidation of furans in an electrochemical microreactor made of
ceramic using H 2 SO 4 as the support.

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9.1.3 Gas-liquid reactions
Gas-liquid reactions can be carried out in micromixers in the same way as liquid-liquid micromixers.
The immiscible phases will form emulsion or slug flow in the mixing channel. The small scale in