Oil foams stabilized by POSS/organosilica particle assemblies: application for aerobic oxidation of aromatic alcohols

Abstract

A novel amphiphilic polyhedral oligomeric silsesquioxane (POSS) with surfactant-like behavior was synthesized. By combining this new POSS, used as a frother, with surface-active catalytic organosilica particles, used as a stabilizer, we designed a dual particle system able to generate foams in pure organic solvents. Tunable foamability and foam stability were achieved in a variety of organic solvents by simply adjusting the POSS concentration. As a result, the catalytic activity was drastically boosted in the aerobic oxidation of pure aromatic alcohols under 1 bar O₂ pressure. Particles were conveniently recycled with high foamability and the catalytic efficiency was maintained for at least 7 consecutive runs.

---

1. Introduction

Foams are widely used in industrial applications such as food manufacturing processes, cosmetics, healthcare products, fire extinguishing, wastewater treatment (froth flotation), and enhanced oil recovery.^1–4 Recently, foams have been formulated for more elaborate applications such as the manufacture of porous materials⁵ and catalysis.⁶ Particle-stabilized (or Pickering) foams are often more attractive compared to surfactant-based counterparts due to their much higher stability under harsh conditions (e.g., shear stress and high temperature). Particle-stabilized aqueous foams have been thoroughly studied using a variety of stabilizers such as polymers,⁷ proteins,⁸ particles⁹ and crystals.¹⁰ However, few studies have focused on non-aqueous/oil foams owing to the low surface tension of organic solvents, restricting particle adsorption at the gas–liquid interface. Nonetheless, there is room to design particle-stabilized non-aqueous/oil foams for gas-liquid-solid microreactors comprising water-insoluble reactants.

The formation of foams depends on three main factors. First, the particles need to be dispersed in the solvent and agglomeration should be limited. Second, stable foams can be generated only if the interfacial contact angle of particles lies in the “stability window” range. Contrary to Pickering emulsions, the range of contact angles for foam formation is much narrower,¹¹ making the design of surface-active particles challenging and with limited scope for accessible organic solvents. Third, owing to the much larger size of particles compared to surfactants, the kinetics of particle adsorption at the gas–liquid interface is notably slower, leaving enough time for bubble coalescence before sufficient surface coverage by particles can be achieved. Overall, ideal particles that promote foamability and foam stability should be highly dispersible in the solvent, exhibit an interfacial contact angle in the “stability window” range, and display fast adsorption kinetics. Nonetheless, particles meeting all these requirements simultaneously for a given solvent are scarce. In practice, particle-stabilized foams are often prepared using co-solvents¹² or surfactants,^13,14 to tune the particle dispersion and interfacial contact angle. The use of co-solvents, however, complicates further product separation and purification steps, whereas the use of surfactants is limited to aqueous foams only,¹⁵ as particle–surfactant interactions are driven by hydrophobic forces not existing in organic systems. Moreover, separation and purification in the case of surfactants is not straightforward.

Here we designed a dual particle system for foam stabilization in a broad range of organic reagents, and engineered efficient gas–liquid–solid multiphase microreactors for the aerobic oxidation of alcohols. The system combines surface-active oleophobic (fluorinated) silica particles incorporating Pd nanoparticles, used as stabilizers, and fluorinated anisotropic polyhedral oligomeric silsesquioxanes (POSSs), used as frothers. POSS are very small silica nanoparticles with surfactant-like behavior.^16,17 They can stabilize foams and tune the wettability of large particles similarly to silica/surfactant mixtures in aqueous systems. Unlike surfactants, POSS can be separated by simply adjusting the liquid composition. POSS are also good candidates for catalytic applications owing to their thermal¹⁸ and chemical stability.^19–21

2. Results and discussion

2.1. Preparation of surface-active POSS particles

Phenyl-based POSS (i.e. Ph₇-POSS) was synthesized by hydrolytic condensation of triethoxylphenylsilane under basic conditions (Fig. 1a) (see the ESI† for experimental details).²² The sample was further functionalized with fluorinated chains by reaction with trichloro(1H,1H,2H,2H-perfluorooctyl)silane (i.e. Ph₇/F₁₃-POSS), allowing corner-capping of Si–ONa groups. The ²⁹Si NMR MAS spectrum of Ph₇/F₁₃-POSS (Fig. 1b) exhibits two bands at −68.7 and −78.6 ppm. These bands appear within the T3 range and evidence the closure of the POSS cage after the corner-capping reaction.^23,24 The Si atom bound directly to the benzene ring has a lower chemical shift due to the higher electron density of phenyl groups.²⁵ The ¹³C NMR MAS spectra of the samples before and after capping (Fig. S1†) show a complex band in the range of 125–138 ppm belonging to aromatic groups.²⁶ Two additional bands are visible at 2.5 and 25 ppm in Ph₇/F₁₃-POSS that can be assigned to CH₂ groups in fluorinated chains.²⁷ Besides, a broad and complex band appears in the range of 110–120 ppm that can be assigned to CF₃ and CF₂ groups in fluorinated chains.²⁸ Tetrahydrofuran (THF), which was used as solvent for Ph₇-POSS synthesis, is visible in the corresponding spectrum.²⁷ This band vanishes after grafting the fluorinated chains, pointing out the full capping of all Si–ONa groups in Ph₇-POSS. The ¹⁹F NMR MAS spectrum of Ph₇/F₁₃-POSS displays a sharp signal at 80.7 ppm that can be ascribed to CF₃ groups (Fig. S2†).²⁸ Additional sharp bands are visible in the range of 115–128 ppm that can be ascribed to internal CF₂ groups.²⁸

Fig. 1 (a) Synthesis of Ph₇/F₁₃-POSS. (b) ²⁹Si NMR MAS spectra of Ph₇/F₁₃-POSS. (c) FT-IR of Ph₇-POSS and Ph₇/F₁₃-POSS. (d) TGA of Ph₇-POSS and Ph₇/F₁₃-POSS.

Ph₇-POSS and Ph₇/F₁₃-POSS were also analyzed by FT-IR spectroscopy (Fig. 1c). Typical bands of silica appear at 1100 cm⁻¹ and 800 cm⁻¹ that can be assigned to asymmetric stretching and bending vibrations of Si–O–Si bonds, respectively.²⁹ A broad band appears in the range of 3000–3500 cm⁻¹ for Ph₇-POSS belonging to Si–ONa groups interacting with adsorbed water. This band vanishes for Ph₇/F₁₃-POSS, which confirms the capping of Si–ONa groups. Characteristic bands ascribed to the fluorocarbon chain appear at 913, 1171 and 1237 cm⁻¹ that can be assigned to stretching modes of C–Si and C–F (CF₂/CF₃) bonds, respectively.²⁹ Additional bands at 710 and 670 cm⁻¹ are attributed to the symmetric stretching bands of CF₃ groups.³⁰ Bands ascribed to the aromatic skeleton appear at 3100 cm⁻¹ for asymmetric stretching modes of C–H groups, and at 1620 cm⁻¹, 1430 cm⁻¹ and 750 cm⁻¹, that can be attributed to C–H stretching and in-place and out-plane bending vibrations, respectively.³¹ Additional overtones ascribed to aromatic groups appear in the range of 1750–2000 cm⁻¹.³¹

TG analysis was used to explore the thermal stability of Ph₇-POSS and Ph₇/F₁₃-POSS (Fig. 1d). Ph₇-POSS exhibits a small weight loss until 100 °C (about 5%) that is attributed to water desorption. Phenyl groups start to decompose at 380 °C with a weight loss of about 37% matching the theoretical value (38.4%). After corner-capping of –ONa groups, Ph₇/F₁₃-POSS shows almost no weight loss below 400 °C, but has a higher overall weight loss as compared to Ph₇-POSS, up to 600 °C.

The XRD pattern of Ph₇/F₁₃-POSS (Fig. S3†) exhibits two diffraction bands at 2θ values of ca. 8.3° and 18.9° corresponding to a d-spacing of ca. 11 and 5 Å, respectively, that can be attributed to the overall dimension and body diagonal of the POSS cage, respectively.³² The sample shows a semicrystalline nature which is known to be related to the choice of solvent and lattice stabilization conditions.²⁷

The elementary particle size and aggregation behavior of Ph₇/F₁₃-POSS was inspected in different solvents by dynamic light scattering (DLS). In toluene and THF (Fig. S4a and b†), which are good solvents for POSS with phenyl groups,³³ the average particle size is below 10 nm at low POSS concentration (≤0.1 wt%). This value is in line with the theoretical particle size of Ph₇/F₁₃-POSS (1–3 nm),³⁴ with only partial aggregation. However, at a Ph₇/F₁₃-POSS concentration above 0.3 wt%, large aggregates are observed in toluene. In contrast, in benzyl alcohol (Fig. S4c†), Ph₇/F₁₃-POSS is present as aggregates even at extremely low concentration (0.001 wt%).

2.2. Foamability of Ph₇/F₁₃-POSS

The foamability of Ph₇/F₁₃-POSS was tested in benzyl alcohol (BnOH), anisole and toluene, with a surface tension of 39, 35 and 28.4 mN m⁻¹ at 20 °C, respectively. In all cases, foams are generated after handshaking of a 2 mL sample (Fig. 2a), and the foam height increases with the particle concentration (Fig. 2b). In benzyl alcohol, foams are stable for at least 30 min above 0.3 wt% Ph₇/F₁₃-POSS, whereas they destabilize within 1 min in anisole and toluene (Fig. 2c). The foam volume increases with the Ph₇/F₁₃-POSS concentration, but the foam stability does not improve (Fig. S5†), which may be explained by the low adsorption energy of Ph₇/F₁₃-POSS due to its small particle size. We extended our analysis to 39 other liquids (Table S1†) to assess the foaming scope of Ph₇/F₁₃-POSS (Fig. S6†). Ph₇/F₁₃-POSS can generate foams provided that the surface tension of the liquid is higher than 27 mN m⁻¹. No foams are produced in very polar liquids (diols and polyols, surface tension ≥45 mN m⁻¹) due to the poor dispersion of Ph₇/F₁₃-POSS.

Fig. 2 (a) Macroscopic image of foams in benzyl alcohol, anisole and toluene stabilized by 0.5 wt% Ph₇/F₁₃-POSS in 2 mL of liquid. (b) Effect of the Ph₇/F₁₃-POSS concentration on the foam height at time = 0, and foam lifetime. (c) Dynamic surface tension of benzyl alcohol at variable Ph₇/F₁₃-POSS concentrations.

Particle adsorption at gas–liquid and liquid–liquid interfaces is often regarded to have no or marginal effect on the surface tension.^14,35,36 However, POSS materials were shown to exhibit interfacial activity similar to surfactants.³⁷ To assess the surface activity of Ph₇/F₁₃-POSS at the gas–liquid interface, we measured the surface tension of benzyl alcohol at variable Ph₇/F₁₃-POSS concentrationusing the maximum bubble pressure method (Fig. 2d).³⁸ The surface tension of pure benzyl alcohol is 39.6 mN m⁻¹. Upon dispersing 0.1 wt% Ph₇/F₁₃-POSS, the surface tension decreases to 30 mN m⁻¹, and declines further to 27 mN m⁻¹ at 0.5 wt% concentration. The surface tension drops fast for a surface age within 50 000 ms. Beyond this value, it decreases smoothly due to continuous interfacial particle adsorption, and remains almost unchanged above 160 000 ms, especially at a POSS concentration above 0.4 wt%.

2.3. Foamability of Ph₇/F₁₃-POSS/fluorinated silica particle mixtures

We showed above that Ph₇/F₁₃-POSS nanoparticles exhibit surface-active behavior at the gas–liquid interface. However, except for benzyl alcohol, the stability of foams is poor. To promote foam stability, we combined Ph₇/F₁₃-POSS with larger oleophobic particles, which can irreversibly adsorb at the gas–liquid interface. A fluorinated organosilica (33 wt% F, 364 nm particle size) was synthesized by the Stöber method using 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES), 3-mercaptopropyltriethoxysilane (MPTES) and tetraethyl-orthosilicate (TEOS) precursors, with TEOS/MPTES and TEOS/PFDTES molar ratios of 16 and 4, respectively (see the ESI† for details). Catalytic Pd nanoparticles were further loaded by wet impregnation using an ethanolic solution of Pd(OAc)₂.The final particles were denoted as Pd@SiNP_F₁₇ (1.33 wt% Pd).

The foaming properties of Ph₇/F₁₃-POSS and Pd@ SiNP_F₁₇ mixtures were first tested in benzyl alcohol using 0.1 wt% Ph₇/F₁₃-POSS and 2 wt% Pd@SiNP_F₁₇ (Fig. 3a and S7†). Pd@SiNP_F₁₇ alone, with a contact angle of 95° (Fig. S8†), is poorly dispersible at room temperature, and only some bubbles are generated. In parallel, Ph₇/F₁₃-POSS alone can make foams (contact angle of 58°, Fig. S9†), but their stability at 0.1 wt% is limited to 3 min. When both particles are combined, small amounts of Ph₇/F₁₃-POSS helps Pd@SiNP_F₁₇ to disperse (see DLS distributions in Fig. S10a†), resulting in modest foaming with a layer of bubbles remaining for 20 min. Increasing the Ph₇/F₁₃-POSS concentration to 0.3 wt% at 2 wt% Pd@SiNP_F₁₇ results in a higher foam height and 1 h stability. Further increase of the Ph₇/F₁₃-POSS concentration to 0.5 wt% results in a higher initial foam height, but with much lower stability, also encompassing lower particle dispersion.

Fig. 3 Foam height at room temperature with 2 wt% Pd@SiNP_F₁₇ in 2 mL of (a) benzyl alcohol, (b) anisole and (c) toluene at variable Ph₇/F₁₃-POSS concentrations.

Given these results, we extended our analysis to anisole and toluene (Fig. 3b, c, S7, S10b and c†). In analogy to BnOH, Pd@SiNP_F₁₇ is poorly wetted and cannot be dispersed in both solvents, resulting in large agglomerates with an average size of about 500 nm that sediment within 2 min. In both cases, Ph₇/F₁₃-POSS alone (0.1–0.5 wt%) generates poorly stable foams, which destabilize within 2 min. With 2 wt% Pd@SiNP_F₁₇ and 0.1–0.3 wt% Ph₇/F₁₃-POSS, foams are much more stable (for more than 1 h) with no obvious collapse even after several days (data not shown). Noteworthy, after adding Ph₇/F₁₃-POSS, Pd@SiNP_F₁₇ can be dispersed without sedimentation after 10 min, suggesting that Pd@SiNP_F₁₇ particles are surrounded by Ph₇/F₁₃-POSS, altering the wettability. The two-particle interactions are driven most likely by van der Waals forces.³⁹ However, the foam stability decreases down to 15 min when the Ph₇/F₁₃-POSS concentration is further increased to 0.5 wt%. At these conditions, larger agglomerates are again apparent, disfavoring dispersion. The foam stability at this high Ph₇/F₁₃-POSS concentration may be preserved by increasing the Pd@SiNP_F₁₇ concentration to 4 wt% and 6 wt% (Fig. S12†). Finally, to show universality of the approach, we obtain highly stable foams in 10 additional liquids using 2 wt% Pd@SiNP_F₁₇ and 0.1 wt% Ph₇/F₁₃-POSS (Fig. S13†).

To gain more insight into the interactions between Ph₇/F₁₃-POSS and Pd@SiNP_F₁₇ particles, we surveyed the effect of Ph₇/F₁₃-POSS on the surface tension of benzyl alcohol in the presence of Pd@SiNP_F₁₇ (Fig. 4a). At a low Ph₇/F₁₃-POSS concentration (0.1 wt%), the decline of surface tension is less prominent compared to that for Ph₇/F₁₃-POSS alone at the same concentration (Fig. 2d). In contrast, at higher concentration (0.3–0.5 wt%), at the overall behavior is comparable to that observed for Ph₇/F₁₃-POSS alone. This observation points out a competition between Ph₇/F₁₃-POSS and Pd@SiNP_F₁₇ for the gas–liquid interface which is similar to that observed in aqueous particles/surfactant systems.⁴⁰

Fig. 4 (a) Dynamic surface tension of benzyl alcohol at 1 wt% Pd@ SiNP_F₁₇ and variable Ph₇/F₁₃-POSS concentration. (b) Schematic representation of the synergy between Ph₇/F₁₃-POSS and Pd@SiNP_F₁₇ particles.

Fig. 4d shows a schematic representation of the Ph₇/F₁₃-POSS and Pd@SiNP_F₁₇ systems. Pd@SiNP_F₁₇ particles are predominantly wetted by the gas phase (contact angle >90°) and are therefore badly dispersed in the liquid, resulting in no foam formation. When a small amount of Ph₇/F₁₃-POSS nanoparticles is added, they adsorb on Pd@SiNP_F₁₇, embedding the fluorinated chains while exposing phenyl groups to the liquid phase (red curve in Fig. 3a). This results in better particle dispersion. At an optimal Ph₇/F₁₃-POSS/Pd@SiNP_F₁₇ ratio, the interfacial contact angle falls into the “foaming window”, resulting in stable foam formation (green curve in Fig. 3a). Further increase of the Ph₇/F₁₃-POSS concentration favors Ph₇/F₁₃-POSS adsorption at the gas–liquid interface, whereas Pd@SiNP_F₁₇ particles are pushed into the bulk liquid. Foamability continues to increase, while foam stability drops drastically (violet curve in Fig. 3a).

Ph₇/F₁₃-POSS can also be applied to phase inversion. A BnOH marble can be stabilized using solely Pd@SiNP_F₁₇ particles, while a mixture of these particles with Ph₇/F₁₃-POSS allows foam generation. Ph₇/F₁₃-POSS and Pd@SiNP_F₁₇ particles can be separated and isolated from the mixture by washing with THF (see more details in Section 2.5). Then, after drying, Pd@SiNP_F₁₇ can be again used for marble generation.

2.4. Catalysis in benzyl alcohol foams

With these results in hand, we explored the catalytic properties of Pd@SiNP_F₁₇ and Ph₇/F₁₃-POSS particle mixtures in the aerobic oxidation of pure benzyl alcohol (BnOH) at 80 °C in the presence of foam. This reaction has been explored in previous studies over transition metal catalysts showing high selectivity to benzaldehyde by avoiding autooxidation.^41,42 In these tests, we kept the Pd@SiNP_F₁₇ concentration fixed at 2 wt% while varying the Ph₇/F₁₃-POSS concentration (Fig. 5a). Without Ph₇/F₁₃-POSS, the benzaldehyde (BAH) yield is only 12% after 2 h, while it increases to 90% after addition of 0.1 wt% Ph₇/F₁₃-POSS. This is attributed to a better dispersion of Pd@SiNP_F₁₇ particles along with foam generation and transfer of catalytic particles from the bulk to the gas–liquid interface (Fig. 5c and S15†). Increasing the Ph₇/F₁₃-POSS concentration to 0.3–1.0 wt% results in a lower BAH yield due to foam instability (Fig. 5c and S16†). Meanwhile, the interface is mainly populated by Ph₇/F₁₃-POSS, while catalytic Pd@SiNP_F₁₇ particles are pushed into the bulk BnOH. A similar reaction pattern is observed at higher Pd@SiNP_F₁₇ concentration and a fixed Ph₇/F₁₃-POSS concentration of 0.1 wt% (Fig. 5b). The highest BAH yield (85%) is achieved at 2 wt% Pd@SiNP_F₁₇, while the highest turnover number (TON) at 2 h (10 700 mol mol⁻¹) is obtained with 1 wt% Pd@SiNP_F₁₇. The BAH yield decreases at the highest Pd@SiNP_F₁₇ concentration (4 wt%), encompassing the genesis of very stable foams. As a result, the ‘armored’ gas–liquid interface is expected to show lower permeability and in turn poorer O₂ renewal during the reaction.⁴³

Fig. 5 (a) Aerobic oxidation of BnOH over Pd@SiNP_F₁₇ at variable Ph₇/F₁₃-POSS concentration. Reaction conditions: 1.5 mL BnOH, O₂ flow [10 mL(STP) per min], 2 wt% Pd@SiNP_ F₁₇, 1500 rpm, 80 °C, 2 h. (b) Aerobic oxidation of BnOH over Pd@SiNP_F₁₇ at variable Pd@SiNP_F₁₇ concentration. Reaction conditions: 1.5 mL BnOH, O₂ flow [10 mL(STP) per min], 0.1 wt% Ph₇/F₁₃-POSS, 1500 rpm, 80 °C, 2 h. (c) Optical images of the foam system after the reaction corresponding to (a). (d) Microscopic image of BnOH foam stabilized by 0.1 wt% Ph₇/F₁₃-POSS and 2 wt% Pd@SiNP_F_17. (e) Fluorescence microscopy image of BnOH foam stabilized by Ph₇/F₁₃-POSS and rhodamine b isothiocyanate-loaded fluorinated silica particles.

Optical inspection of the foams after reaction stabilized by a combination of Pd@SiNP_F₁₇ and Ph₇/F₁₃-POSS particles reveals the formation of bubbles with a size ranging from 50 to 250 μm (Fig. 5d). By decorating Pd@SiNP_F₁₇ particles with rhodamine b isothiocyanate groups, preferential particle adsorption is observed at the gas–liquid interface (Fig. 5e), confirming the interfacial location of the catalytic particles during the reaction. Indeed, a brighter layer of fluorescent particles is clearly visualized at the boundary of bubbles.

2.5. Particle recycling

The recyclability and reuse of combined 2 wt% Pd@ SiNP_F₁₇ and 0.1 wt% Ph₇/F₁₃-POSS particles were studied for the aerobic oxidation of BnOH in seven consecutive runs (see experimental details in the ESI†). After each run, the Pd@SiNP_F₁₇ and Ph₇/F₁₃-POSS particle mixture was separated by centrifugation and washed with THF (Fig. 6a). The solid at the bottom of the centrifugation tube was Pd@SiNP_F₁₇, while Ph₇/F₁₃-POSS remained in the supernatant. Pd@SiNP_F₁₇ was further separated by decantation, whereas Ph₇/F₁₃-POSS was recovered after THF evaporation. The Ph₇/F₁₃-POSS nanoparticles were dried at 80 °C for 4 h before further use in the subsequent run after remixing with Pd@SiNP_F₁₇. Overall, the particles can be reused at least seven times without an appreciable loss of catalytic activity and foamability (Fig. 6b). Ph₇/F₁₃-POSS separation is however limited to liquids with polar groups such as BnOH. For non-polar solvents such as toluene, Ph₇/F₁₃-POSS can hardly be separated by centrifugation due to their good dispersion. No Pd leaching is observed during the reaction, as inferred by ICP-OES.

Fig. 6 (a) Schematic representation of the recycling process. (b) Recyclability and reuse of Pd@SiNP_F₁₇ and Ph₇/F₁₃-POSS for the aerobic oxidation of BnOH over seven consecutive runs. Reaction conditions: 1.5 mL BnOH, 2 wt% Pd@SiNP_F₁₇, 0.1 wt% Ph₇/F₁₃-POSS, O₂ (flow), 1500 rpm, 80 °C, 2 h.

2.6. Scope of aromatic alcohols

We further examined the generality of aromatic alcohol oxidation in foam systems stabilized by a combination of Pd@SiNP_F₁₇ and Ph₇/F₁₃-POSS particles in an O₂ flow (Table 1). The results obtained in the presence of Ph₇/F₁₃-POSS (0.1 wt%) clearly demonstrate higher activity of this system in the oxidation of aromatic alcohols compared to experiments without Ph₇/F₁₃-POSS. For 1-phenylethanol, the acetophenone yield is 50% after 2 h reaction at 100 °C in the foam system containing Ph₇/F₁₃-POSS. This value is about 9 times higher than the yield obtained without Ph₇/F₁₃-POSS. The conversion of methylbenzyl alcohol into the corresponding aldehyde in the presence of foam stabilized by Ph₇/F₁₃-POSS is 79% at 120 °C after 2 h, while the yield is only 30% without Ph₇/F₁₃-POSS. The oxidation of cinnamyl alcohol to cinnamaldehyde shows a yield of 46% at 80 °C after 1 h under foam in the presence of Ph₇/F₁₃-POSS, whereas the yield without Ph₇/F₁₃-POSS is only 12%. Finally, the conversion of benzyl alcohol to benzaldehyde at 100 °C is 61% and 87% after 0.5 and 1 h, respectively.

Table 1 Substrate scope expansion for catalytic tests in an O₂ flow^a

Reactant	Pd@SiNP_F17/wt%	Ph7/F13-POSS/wt%	T/°C	t/h	Yield/%
a Reaction conditions: 1.5 mL substrate, 1500 rpm, O2 flow rate [20 mL(STP) per min], and 1 bar O2.
	2	—	100	2	5.6
	2	0.1	100	2	50
	2	—	120	2	30
	2	0.1	120	2	79
	1	—	80	1	12
	1	0.1	80	1	46
	2	0.1	100	0.5	61
	2	0.1	100	1	87

---

3. Conclusions

In summary, we prepared oil foams stabilized by a dual particle system combining surface-active catalytic organosilica particles, used as stabilizer, and a novel type of amphiphilic polyhedral oligomeric silsesquioxane (POSS) with surfactant-like properties, used as frother. Both the foamability and foam stability in aromatic solvents are promoted in the presence of a low concentration of POSS, favoring the dispersion of catalytic organosilica particles driven by the interaction between the fluorinated chains of organosilica and POSS particles. As a result, the catalytic activity is drastically enhanced in the presence of POSS with a 9-time increase of the benzaldehyde yield in the oxidation of benzyl alcohol in pure O₂ compared to a catalytic test without POSS. Intermediate foam stability, affording a high gas exchange rate, was required to achieve high catalytic activity. The particles were conveniently recycled with high foamability and the catalytic efficiency was maintained for at least 7 consecutive runs. The dual particle catalytic system was successfully applied to a range of aromatic alcohols, demonstrating the versatility of our approach.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was funded by the ERC grant Michelangelo (contract number #771586).

Notes and references

1. E. Dickinson, Curr. Opin. Colloid Interface Sci., 2010, 15, 40–49.
2. R. Farajzadeh, A. Andrianov, R. Krastev, G. J. Hirasaki and W. R. Rossen, Adv. Colloid Interface Sci., 2012, 183, 1–13.
3. K. A. Matis and P. Mavros, Sep. Purif. Methods, 1991, 20, 163–198.
4. R. Höfer, F. Jost, M. J. Schwuger, R. Scharf, J. Geke, J. Kresse, H. Lingmann, R. Veitenhansel and W. Erwied, Foams and Foam Control, VCH, Weinheim, 1988, vol. 11.
5. R. Murakami and A. Bismarck, Adv. Funct. Mater., 2010, 20, 732–737.
6. J. Huang, F. Cheng, B. P. Binks and H. Yang, J. Am. Chem. Soc., 2015, 137, 15015–15025.
7. A. L. Fameau, A. Carl, A. Saint-Jalmes and R. Von Klitzing, ChemPhysChem, 2015, 16, 66–75.
8. A. R. Cox, D. L. Aldred and A. B. Russell, Food Hydrocolloids, 2009, 23, 366–376.
9. B. P. Binks and T. S. Horozov, Angew. Chem., Int. Ed., 2005, 44, 3722–3725.
10. B. P. Binks and H. Shi, Langmuir, 2020, 36, 991–1002.
11. T. N. Hunter, R. J. Pugh, G. V. Franks and G. J. Jameson, Adv. Colloid Interface Sci., 2008, 137, 57–81.
12. R. Murakami and A. Bismarck, Adv. Funct. Mater., 2010, 20, 732–737.
13. Y. Han, J. Yang, M. Jung, S. Han, S. Kim and H. S. Jeon, Langmuir, 2020, 36, 10331–10340.
14. A. Maestro, E. Rio, W. Drenckhan, D. Langevin and A. Salonen, Soft Matter, 2014, 10, 6975–6983.
15. L. R. Arriaga, W. Drenckhan, A. Salonen, J. A. Rodrigues, R. Íñiguez-Palomares, E. Rio and D. Langevin, Soft Matter, 2012, 8, 11085–11097.
16. Z. Wang, Y. Li, X. H. Dong, X. Yu, K. Guo, H. Su, K. Yue, C. Wesdemiotis, S. Z. D. Cheng and W. Bin Zhang, Chem. Sci., 2013, 4, 1345–1352.
17. K. Wu, M. Huang, K. Yue, C. Liu, Z. Lin, H. Liu, W. Zhang, C. H. Hsu, A. C. Shi, W. Bin Zhang and S. Z. D. Cheng, Macromolecules, 2014, 47, 4622–4633.
18. N. Ahmed, H. Fan, P. Dubois, X. Zhang, S. Fahad, T. Aziz and J. Wan, J. Mater. Chem. A, 2019, 7, 21577–21604.
19. L. A. Bivona, O. Fichera, L. Fusaro, F. Giacalone, M. Buaki-Sogo, M. Gruttadauria and C. Aprile, Catal. Sci. Technol., 2015, 5, 5000–5007.
20. C. H. Lu and F. C. Chang, ACS Catal., 2011, 1, 481–488.
21. P. Sangtrirutnugul, T. Chaiprasert, W. Hunsiri, T. Jitjaroendee, P. Songkhum, K. Laohhasurayotin, T. Osotchan and V. Ervithayasuporn, ACS Appl. Mater. Interfaces, 2017, 9, 12812–12822.
22. K. Koh, S. Sugiyama, T. Morinaga, K. Ohno, Y. Tsujii, T. Fukuda, M. Yamahiro, T. Iijima, H. Oikawa, K. Watanabe and T. Miyashita, Macromolecules, 2005, 38, 1264–1270.
23. V. Vij, T. S. Haddad, G. R. Yandek, S. M. Ramirez and J. M. Mabry, Silicon, 2012, 4, 267–280.
24. H. Araki and K. Naka, Macromolecules, 2011, 44, 6039–6045.
25. W. Wang, W. Ding, J. Yu, M. Fei and J. Tang, J. Polym. Res., 2012, 19, 9948.
26. M. Ye, Y. Wu, W. Zhang and R. Yang, Res. Chem. Intermed., 2018, 44, 4277–4294.
27. S. T. Iacono, A. Vij, W. Grabow, D. W. Smith and J. M. Mabry, Chem. Commun., 2007, 4992–4994.
28. L. Van Ravenstein, W. Ming, R. D. Van de Grampel, R. Van der Linde, G. de With, T. Loontjens, P. C. Thüne and J. W. Niemantsverdriet, Macromolecules, 2004, 37, 408–413.
29. G. D. Chukin and V. I. Malevich, J. Appl. Spectrosc., 1977, 26, 223–229.
30. S. M. S. Shahabadi, H. Rabiee, S. M. Seyedi, A. Mokhtare and J. A. Brant, J. Membr. Sci., 2017, 537, 140–150.
31. Spectroscopy of Aromatic Compounds, https://chem.libretexts.org/@go/page/31568.
32. D. B. Cordes, P. D. Lickiss and F. Rataboul, Chem. Rev., 2010, 110, 2081–2173.
33. A. J. Guenthner, K. R. Lamison, L. M. Lubin, T. S. Haddad and J. M. Mabry, Ind. Eng. Chem. Res., 2012, 51, 12282–12293.
34. W. Zhang, G. Camino and R. Yang, Prog. Polym. Sci., 2017, 67, 77–125.
35. A. P. Kotula and S. L. Anna, Soft Matter, 2012, 8, 10759–10772.
36. A. Stocco, E. Rio, B. P. Binks and D. Langevin, Soft Matter, 2011, 7, 1260–1267.
37. H. Hou, J. Li, X. Li, J. Forth, J. Yin, X. Jiang, B. A. Helms and T. P. Russell, Angew. Chem., Int. Ed., 2019, 58, 10142–10147.
38. V. B. Fainerman, V. D. Mys, A. V. Makievski and R. Miller, J. Colloid Interface Sci., 2006, 304, 222–225.
39. Y. Xue, H. Wang, Y. Zhao, L. Dai, L. Feng, X. Wang and T. Lin, Adv. Mater., 2010, 22, 4814–4818.
40. R. Pichot, F. Spyropoulos and I. T. Norton, J. Colloid Interface Sci., 2012, 377, 396–405.
41. M. Sankar, E. Nowicka, E. Carter, D. M. Murphy, D. W. Knight, D. Bethell and G. J. Hutchings, Nat. Commun., 2014, 5, 1–6.
42. S. Meher and R. K. Rana, Green Chem., 2019, 21, 2494–2503.
43. S. Zhang, D. Dedovets, A. Feng, K. Wang and M. Pera-Titus, J. Am. Chem. Soc., 2022, 144(4), 1729–1738.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d2ta00667g

---