Microreactors in Organic Chemistry and Catalysis, Second Edition (2013)
9. Gas–Liquid Reactions
9.6. Other Applications
9.6.1 Segmented Gas–Liquid Flow for Particle Synthesis
Segmented gas–liquid Taylor flow was used for particle synthesis within the liquid slugs. Tetraethylorthosilicate in ethanol was hydrolyzed by a solution of ammonia, water, and ethanol (Stöber synthesis) [166]. The resulting silicic acid monomer Si(OH)4 is then converted by polycondensation to colloidal monodisperse silica nanoparticles. These particles have industrial application, for example, in pigments, catalysts, sensors, health-care, antireflective coatings, and chromatography.
More recently, gold nanoparticles were synthesized from HAuCl4 with rapid reduction by NaBH4 [91, 167]. It is demonstrated that it is the slip velocity between the two fluids and internal mixing in the continuous-phase slugs that govern the nature of the particle size distribution.
The Taylor flow microreactor comprised a micromixer for mixing of the precursors for the particle synthesis followed by a gas inlet for separating this continuous mixed liquid stream into segments separated by gas bubbles [165–167].
Silica nanoparticles had a diameter of 200–500 nm [167], while gold particle sizes was 3.8 ± 0.3, 4.6 ± 2.1, and 4.9 ± 3.0 nm at residence times of 10, 20, and 40 s, respectively [167]. This is explained by the well-defined residence time with reduced axial dispersion of the liquid segments [165–167]. In addition, by moving the liquid segments through a tube recirculation flow sets in which is very effective in liquid mixing. Moreover, a careful choice of continuous and dispersed phases is necessary to control the nanoparticle size and size distribution.
9.6.2 Catalyst Screening
In three-phase asymmetric hydrogenations, the asymmetric catalytic system consists of a traditional hydrogenation catalyst, Pt/Al2O3, and a chiral inductor that adsorbs on the Pt surface giving chiral catalytic sites [168].
Screening of chiral modifiers to find the best catalyst is very important and in microstructured reactors it can be done faster than in traditionally used packed bed reactors.
A continuous microstructured reactor equipped with a perforated (5 μm) membrane is used for the investigation of the gas–liquid–solid asymmetric hydrogenation of ethylpyruvate on a Pt/γ-Al2O3 catalyst modified with chiral inductors under high hydrogen pressure (45 bar) [169]. Reactants and catalysts are sequentially injected through the microreactor, generating consecutively collected fractions for evaluation and classification of the different catalysts. Gas–liquid–solid microstructured contactor was used for that purpose [59]. It demonstrated for the first time the use of a gas–liquid–solid high-pressure microstructured reactor for screening applications and deactivation studies [59].