Agenda de l'ENS de Lyon

Gas-liquid-solid reactions (G-L-S) are widespread in chemical, petrochemical,
biochemical and environmental catalytic processes. Conventional technologies
usually suffer from resilient mass/heat transfer limitations due to their low G/L and L/S
specific interface areas, especially when dealing with fast reactions. In this thesis, we
designed robust particle-stabilized G/L dispersions (i.e. micro/nano-bubbles) as highly
efficient G-L-S nanoreactors for conducting catalytic reactions low gas pressures.
In this thesis, we first prepared non-aqueous foams stabilized by Pd-supported
surface-active fluorinated silica particles. Foamability increased with both the particle
concentration and stirring rate. High foam stability was achieved in benzyl alcohol /
xylene mixtures even at low particle concentration (<1 wt%) for a contact angle in the
range 41-73°. The catalytic activity was much higher than in non-foam systems (five
times in air, eight times in pure O2). Intermediate foam stability was required to
achieve good catalytic activity. In contrast, low or high foam stability exerted a
negative effect on the interfacial area generated and gas exchange rate due to a
higher permeability of the adsorbed particle layer. The foamability and catalytic
activity kept unchanged for at least 7 consecutive runs. Besides xylene, other solvents
with surface tension lower than that of the substrate could be implemented, enhancing
the foamability and catalytic performance in aerobic oxidation reactions of a panel of
alcohols, demonstrating the universality of our approach.
Next, we redesigned our catalytic foam system to afford the aerobic oxidation of pure
alcohols. To this aim, we synthesized a new polyhedral oligomeric silsesquioxane
(POSS) with asymmetric shape as co-foaming agent. High foamability was obtained
by combining POSS with fluorinated silica particles for a broad scope alcohols with a
surface tension >26 mN/m except polyols due to their high viscosity. As a result, a
very high activity was achieved in the aerobic oxidation of pure benzyl alcohol
compared to a non-foam system. Noteworthy, stable foams could be generated for
ethanol and n-octane.

Finally, we studied catalytic oxidation at the level of a single bubble stabilized by
fluorinated silica particles. To this aim, we developed a microfluidic trap to produce
and store particle-stabilized bubbles in organic solvents at elevated temperatures.
The solvent composition was used for in situ adjustment of the contact angle of the
particles, and trapping of bubbles could be achieved with traps of simple design. We
then showed that fluorescent probe 2’,7’-dichlorodihydrofluorescein diacetate can be
used to monitor an oxidation reaction in the organic phase after deesterification with
organic base. Large bubbles loosely covered by catalytic particles showed high
reactivity while monodisperse armored bubbles remained inert.