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.

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.

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.

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.

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. 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.

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.