Periodic Reporting for period 3 - MICHELANGELO (MultiphasIc NanoreaCtors for HEterogeneous CataLysis via SmArt ENGinEering of TaiLored DispersiOns)
Periodo di rendicontazione: 2021-10-01 al 2023-03-31
Gas-liquid-solid reactions (G-L-S) are widespread in catalytic processes. Conventional G-L-S reactors (Figure 1A) usually suffer from resilient mass/heat transfer limitations due to their low G/L and L/S specific interface areas. In practice, co-solvents, surfactants and G/L phase-transfer catalysts (e.g. molten salts) can be employed to promote the G/L contact and distribute the catalyst between the phases, affecting the economy and eco-efficiency of the process. Alternatively, continuous flow microreactors and catalytic membrane contactors have been considered for increasing the G/L interface area. Nonetheless, these systems require complex equipment and still do not guarantee an efficient L/S contact at the catalyst surface. In this context, for a major improvement on current systems in terms of cost efficiency and energy savings, G-L-S reactors operating at the nanoscale are required.
The aim of MICHELANGELO is to design robust particle-stabilized G/L dispersions (i.e. micro/nano-bubbles and liquid marbles) as highly efficient G/L/S nanoreactors for conducting catalytic reactions at mild conditions (Figure 1B). To meet this aim, there are 5 objectives:
O1. Preparation of NPs with defined sizes, shapes, hydrophilic-lipophilic balance (HLB), catalytic functions;
O2. Generation of particle-stabilized bubbles and liquid marbles affording highly active and selective reactions at the G/L/S interface and NP recycling after each catalytic cycle;
O3. Imaging of reaction/diffusion profiles and NP adsorption/desorption dynamics at the G/L interface using external stimuli when necessary (hyperthermia, light, radiofrequency, ultrasound);
O4. Rationalization of the interplay between the NP assembly at the G/L interface and the catalytic properties along the reaction by marrying simulations with well-designed experiments; and
O5. Reengineering of G/L/S multiphasic reactors using particle-stabilized nanoreactors to achieve a high catalytic performance at milder operation conditions compared to conventional reactors while keeping a high degree of stability and flexibility at reduced layouts.
The aim of MICHELANGELO is to design robust particle-stabilized G/L dispersions (i.e. micro/nano-bubbles and liquid marbles) as highly efficient G/L/S nanoreactors for conducting catalytic reactions at mild conditions (Figure 1B). To meet this aim, there are 5 objectives:
O1. Preparation of NPs with defined sizes, shapes, hydrophilic-lipophilic balance (HLB), catalytic functions;
O2. Generation of particle-stabilized bubbles and liquid marbles affording highly active and selective reactions at the G/L/S interface and NP recycling after each catalytic cycle;
O3. Imaging of reaction/diffusion profiles and NP adsorption/desorption dynamics at the G/L interface using external stimuli when necessary (hyperthermia, light, radiofrequency, ultrasound);
O4. Rationalization of the interplay between the NP assembly at the G/L interface and the catalytic properties along the reaction by marrying simulations with well-designed experiments; and
O5. Reengineering of G/L/S multiphasic reactors using particle-stabilized nanoreactors to achieve a high catalytic performance at milder operation conditions compared to conventional reactors while keeping a high degree of stability and flexibility at reduced layouts.
During the evaluation period, we have concentrated prepared low-surface energy particles for stabilizing organic foams. Versatile foams based on aromatic solvents were prepared using surface-active colloidal particles based on organosilicas. A family of organosilicas was built using bridged silsesquioxane precursors being structurally similar to aromatic solvents (e.g. benzyl alcohol). By modifying the organic architecture and surface properties of the materials, oil foams were prepared by simple hand shaking. The particle structure was directly related to its foamability: both the presence of biphenyl rings and short alkyl side chains (ethoxy groups) was essential for benzyl alcohol foaming, while longer chains prevented foam formation. The contact angle revealed as key parameter controlling foaming with an optimal range found between 32° and 53°, and a maximum foamability being achieved around 45°. At the same time, the surface tension of the solvent should lie in the range 35-44 mN·m-1.
We also prepared organic foams stabilized by oleophobic (fluorinated) silica particles, which were rendered catalytic by impregnation and further reduction of a palladium precursor. Foamability increased with the particle concentration and stirring rate. High foam stability was achieved for benzyl alcohol / xylene mixtures even at very low particle concentration (<1 wt%) for contact angles in the range 41-73°. The catalytic performance of the particles was strongly affected by their foaming properties. Indeed, the catalytic activity of foam systems was much higher than for non-foam systems (five times in air). Intermediate foam stability was required to achieve high catalytic activity. In contrast, low or high foam stability decreased both the interfacial area generated and the gas exchange rate, respectively. Preferential location of the catalytic particles at the G-L interface was found even more important than foamability for enhancing the catalytic activity. Particles could be recycled with high foamability and catalytic efficiency kept for at least 7 consecutive runs. Besides xylene, other solvents with surface tension lower than that of the substrate were used to tune the particle wettability, enhancing the foamability and catalytic performance in the oxidation of a panel of alcohols. In a further step, we conducted catalytic reactions at the G-L interface for pure organic foams based on benzyl alcohol (e.g. without cosolvent) using fluorinated silica particles combined with surfactant-like fluorinated polyhedral oligomeric silsesquioxanes (POSS) with small particle sizes (<10 nm).
We also prepared organic foams stabilized by oleophobic (fluorinated) silica particles, which were rendered catalytic by impregnation and further reduction of a palladium precursor. Foamability increased with the particle concentration and stirring rate. High foam stability was achieved for benzyl alcohol / xylene mixtures even at very low particle concentration (<1 wt%) for contact angles in the range 41-73°. The catalytic performance of the particles was strongly affected by their foaming properties. Indeed, the catalytic activity of foam systems was much higher than for non-foam systems (five times in air). Intermediate foam stability was required to achieve high catalytic activity. In contrast, low or high foam stability decreased both the interfacial area generated and the gas exchange rate, respectively. Preferential location of the catalytic particles at the G-L interface was found even more important than foamability for enhancing the catalytic activity. Particles could be recycled with high foamability and catalytic efficiency kept for at least 7 consecutive runs. Besides xylene, other solvents with surface tension lower than that of the substrate were used to tune the particle wettability, enhancing the foamability and catalytic performance in the oxidation of a panel of alcohols. In a further step, we conducted catalytic reactions at the G-L interface for pure organic foams based on benzyl alcohol (e.g. without cosolvent) using fluorinated silica particles combined with surfactant-like fluorinated polyhedral oligomeric silsesquioxanes (POSS) with small particle sizes (<10 nm).
Aqueous foams and “armored bubbles” can be stabilized by a variety of particles In contrast, only few reports are available on particle-stabilized non-aqueous foams, consisting of low-surface energy fluoropolymers/oligomers and particles. This limited scope arises from the low surface tension of organic liquids, restricting particle adsorption at the G-L interface. Besides, particle-stabilized foams differ from emulsions by a significant difference in density between the phases, which is similar for L-L systems, but can vary orders of magnitude for G-L systems. As a consequence, the gas phase needs to be continuously renewed in catalytic foams to avoid a stoichiometric deficit of gas during the reaction.
In the first half of the project, we have succeeded the first example of interfacial reaction in an organic foams based on the benzyl alcohol (BnOH)-xylene/air(O2) stabilized by surface-active oleophobic (fluorinated) silica particles incorporating catalytic Pd nanoparticles. The foams exhibited excellent catalytic properties in the aerobic oxidation of benzyl alcohol to benzaldehyde, and could be transposed to a library of alcohols and cosolvents. In a further step, for the first time, we have conducted catalytic reactions at the G-L interface for organic foams based on pure reactants (e.g. benzyl alcohol) without cosolvent using fluorinated silica particles combined with surfactant-like fluorinated polyhedral oligomeric silsesquioxanes (POSS) showing small particle sizes (<10 nm).
With the knowledge acquired on organic foams, we are now ready to tackle the second half of the project included in WP3-WP5. In particular, we foresee to study in detail the particle assembly, reaction/diffusion profiles at the G/L interface and particle adsorption/desorption dynamics at the level of a single arrested bubble (WP3). The current results will also be useful for building meso- and microscale simulation studies by DPD, MD and GCMC targeting the in silico design catalytic particles for reactions at the interface of bubbles and marbles (WP4). Finally, with the particles in hand, we will build a demonstrator based on a bubble-column milifluidic reactor under continuous flow implementing particle-stabilized bubbles coupled with online IR/UV/-GC detectors for running oxidation and hydrogenation reactions and measuring reaction kinetics (WP5). In parallel, in collaboration with the company Solvay, we plan to carry out specific developments targeting the direct synthesis of H2O2 in O2 or H2 foams, as well as amination reactions in NH3 foams, targeting reactions at lower pressure due to enhanced G-L-S contact and mutual G/L solubility of the reagents at the nanoscale level.
In the first half of the project, we have succeeded the first example of interfacial reaction in an organic foams based on the benzyl alcohol (BnOH)-xylene/air(O2) stabilized by surface-active oleophobic (fluorinated) silica particles incorporating catalytic Pd nanoparticles. The foams exhibited excellent catalytic properties in the aerobic oxidation of benzyl alcohol to benzaldehyde, and could be transposed to a library of alcohols and cosolvents. In a further step, for the first time, we have conducted catalytic reactions at the G-L interface for organic foams based on pure reactants (e.g. benzyl alcohol) without cosolvent using fluorinated silica particles combined with surfactant-like fluorinated polyhedral oligomeric silsesquioxanes (POSS) showing small particle sizes (<10 nm).
With the knowledge acquired on organic foams, we are now ready to tackle the second half of the project included in WP3-WP5. In particular, we foresee to study in detail the particle assembly, reaction/diffusion profiles at the G/L interface and particle adsorption/desorption dynamics at the level of a single arrested bubble (WP3). The current results will also be useful for building meso- and microscale simulation studies by DPD, MD and GCMC targeting the in silico design catalytic particles for reactions at the interface of bubbles and marbles (WP4). Finally, with the particles in hand, we will build a demonstrator based on a bubble-column milifluidic reactor under continuous flow implementing particle-stabilized bubbles coupled with online IR/UV/-GC detectors for running oxidation and hydrogenation reactions and measuring reaction kinetics (WP5). In parallel, in collaboration with the company Solvay, we plan to carry out specific developments targeting the direct synthesis of H2O2 in O2 or H2 foams, as well as amination reactions in NH3 foams, targeting reactions at lower pressure due to enhanced G-L-S contact and mutual G/L solubility of the reagents at the nanoscale level.