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constant of supercritical water varies from 1 (i.e. nonpolar) to 30 at high p, in contrast to
being 80 at ambient conditions.
4.3.4.4 Solubility Effects
Finally, the solubility of gases and liquids increases with pressure. Thus, gas-liquid reactions
can ultimately be performed as a quasi single-phase medium with high concentrations of
dissolved gases. Miscibility tends to change as well, due to the change in density and polarity
of liquids.
Probably by virtue of such solubility effects, CO
2
was incorporated in n-benzylme-
thylamine to yield carbamic acid in 8 seconds at 300 bar using a specially arranged
microchip [211]. The low mechanical strength of the polymethylsiloxane microreactor
required it to be embedded into a high-pressure Parr reactor (45 bar), thereby showing a
way towards the use of high-p polymer microreactors. A 10-fold increase in the reaction
rate of cyclohexene hydrogenation over Pd catalyst was achieved upon going from
ambient conditions to 71
C and 51 bar [212]. No byproduct was formed at 20%
conversion of acetone to isopropanol on Ru/C catalyst [213].
Until now, high T has been used largely as an accelerating tool in continuous flows at the
microscale. However, with increasing technological abilities, equipment can be developed
for a higher-pressure performance that will assist in the investigation of the complexity of
the processes under high-p,T conditions.
Tiggelaar et al. tested the performance of several designs based on the in-plane
connection of the fibre interfaces to the glass chip [213]. Connections at the inlet/outlets
were shown to withstand pressures as high as 690 bar when connected in a tubular fashion.
The chip was tested in carbamation of N-benzylmethylamine up to pressures of 300 bar; at
higher pressures no product formation was observed, due to the lower residence time
available.
4.3.5 Alternative Reaction Media
Solvents mainly derived from petroleum [214] are used as bystanders in chemical reactions
to facilitate the encounter of reactants. They must either be reprocessed after each use,
which usually requires high energy consumption, or else sent to waste, which is less
environmentally friendly. Thus, eliminating the use of these solvents, or using alternatives
that require less energy consumption and are easier to recover seems to be a sound green
idea. Solvents such as water [215], ILs [216] and SCFs [217] are the most desirable options.
As has already been mentioned, microfluidics offers a tight control over operating
conditions, such as temperature, pressure and residence time, and create a platform for
the investigation and hopefully large-scale implementation of alternative solvents, such as
SCFs and ILs.
4.3.5.1 Supercritical Fluids
At conditions above their critical pressure and temperature, substances become SCFs.
The possibility of using microfluidics as an operating platform for supercritical
processing has recently been reviewed by S. Marre et al. [218]. Figure 4.16 illustrates
the operating conditions of recently developed microfluidic devices that can be operated
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