Micro Process Technology and Novel Process Windows –Three Intensification Fields Svetlana Borukhova and Volker Hessel - Process Intensification for Green Chemistry - page 118

After demonstrating the initial feasibility, a major push towards the use of high-T
superheated flow technology was given by research taking a similar approach in batch
microwave technology, using sealed vials. Extensive investigations demonstrated order-of-
magnitude decreases in reaction time [115]. Seeing the limitations of scaling up microwave
equipments, especially under such harsh conditions, a paradigm shift occurred - mainly
initiated by Kappe - in which high-p,T microwave reactions were transformed into
high-p,T flow reactions, almost all being done in capillaries. There are numerous examples
demonstrating such transformations and the associated massive speed up in reactivity
under superheated conditions [116,117].
4.3.3.2 Application Examples
Jensen et al. constructed a silicon/Pyrex microreactor (shown in Figure 4.15) that can
withstand temperatures up to 450 C and pressures up to 250 bar [191]. Silicon etching
techniques, combined with bonding to Pyrex, allowed tight thermal control, good chemical
compatibility and high visibility. Since connections capable of withstanding high T,p are
rare, a circumventing solution was provided by incorporating a 'mild'-conditions platform
separate from the high-T zone platform. Reactor was operated under supercritical
conditions. Formation of one supercritical fluid (SCF) from two immiscible liquid-liquid
phases was visualized for a water/hexane mixture; synthesis of iron oxide nanoparticles in
liquid toluene was accomplished at 100 bar and 300 C; and supercritical water oxidation
of methanol was performed by hydrogen peroxide at 250 bar and 400 C. At a fixed
residence time of 2 seconds, conversion increased from 36.5 at 20MPa and 300 Cto87at
25MPa and 380 C [192].
The same group also used another high-p,T chip microreactor made pressure-tight by the
aid of solder techniques known to give strong, irreversible joints between microstructured
plates [129]. This microreactor was applied in the Heck aminocarbonylation reaction at
pressures up to 100 bar. The reaction had dual reaction paths: inserting one molecule of
Figure 4.15 (a) Schematic illustration of a high-T,p reactor assembled from six parts: (1)
compressionplatform; (2) coolingfluid inlet/ outlet; (3) compression part; (4) O-rings and grooves;
(5) Pyrex plate; and (6) microreactor. (b) Photograph of the assembled reactor. Permission
granted by American Chemical Society, [192].