ABSTRACT
Metal-organic frameworks (MOFs) show promise for the capture of greenhouse gases. To be used at a large scale in fixed-bed processes, their shaping under a hierarchical structure is mandatory and remains a major challenge, while keeping available their high specific surface area. For that purpose, we propose herein an original method based on the stabilization of a paraffin-in-water Pickering emulsion by a fluorinated Zr MOF (UiO-66(F4)) with polyHIPEs (polymers from high internal phase emulsions) strategy consisting of the polymerization of monomers in the external phase. After polymerization of the continuous phase and elimination of the paraffin, a hierarchically structured monolith is obtained with the UiO-66(F4) particles embedded in the polymer wall and covering the internal porosity. To avoid the pore blocking induced by the embedment of the MOF particles, our strategy was to modify their hydrophilic/hydrophobic balance with a controlled adsorption of hydrophobic molecules (perfluorooctanoic acid, PFOA) on the UiO-66(F4) particles. This will induce a displacement of the MOF position at the paraffin-water interface in the emulsion and then make the particles less embedded into the polymer wall. This leads to the formation of hierarchically structured monoliths integrating UiO-66(F4) particles with higher accessibility, maintaining their original properties and allowing their application in fixed-bed processes. This strategy was demonstrated by N2 and CO2 capture, and we believe that such original strategy could be applied to other MOF materials.
ABSTRACT
HYPOTHESIS: Stabilizing Pickering emulsions with metal-organic frameworks (MOFs) is a known way to incorporate them into hierarchically porous materials. Studies generally focus on their final properties and emulsion microstructures are rarely precisely described. Our hypothesis was that characterizing the microstructural and rheological properties of Pickering emulsions stabilized solely by Al-based MOFs (MIL-96) particles would provide insights into how to control their stability and workability for potential industrial applications. EXPERIMENTS: MIL-96(Al) particles, obtained from Li-ion battery waste were used to stabilize paraffin-in-water Pickering emulsions. The influence of the formulation parameters (paraffin/water volume ratio and MIL-96(Al) content) were investigated and the emulsions were analysed using optical microscopy, cryo-scanning electron microscopy and rheological measurements. FINDINGS: MIL-96(Al) efficiently stabilized paraffin-in-water emulsions with up to 80% of internal phase. The emulsions with a low paraffin volume fraction had large droplets and a fluid gel-like texture. The emulsions with higher paraffin volume fractions were more compact and had two-step flow curves. In this system, excess MIL-96(Al) particles aggregated in the continuous phase as flocs interact with particles adsorbed at the paraffin-water interface, creating a secondary network that has to be broken for flow to resume. This behaviour may be interesting to investigate in other MOF-stabilized emulsions.
ABSTRACT
As a step toward synthesizing zeolite-based porous materials, this study demonstrates for the first time the feasibility of stabilizing oil-in-water (O/W) high internal phase emulsions (HIPEs) using a cationic surfactant (tetradecyltrimethylammonium bromide, TTAB) and "homemade" submicronic Linde type A zeolite particles. The zeolite particles are hydrophilic and therefore do not attach to dodecane-water interfaces, but surface tension measurements and electrochemical data show that their surface can be activated by the electrostatic and subsequent hydrophobic adsorption of TTAB. Comparing the adsorption isotherm of TTAB and zeta potential of the particles with the droplet sizes and rheological properties of the emulsion shows that the stabilization mechanism depends on the TTAB/zeolite weight ratio. At low TTAB/zeolite weight ratios (≤0.2 wt %), gel-like O/W Pickering HIPEs form, but at intermediate TTAB concentrations, the zeolite particles become more hydrophobic, leading to phase inversion and the stabilization of W/O emulsions. At high TTAB/zeolite weight ratios (>1.25 wt %), a second phase inversion occurs and creamy O/W HIPEs form through a different stabilization mechanism. In this case indeed, the zeolite particles are fully covered by a bilayer of TTAB and remain dispersed in the aqueous phase with no adsorption to the dodecane-water interface. The emulsion is stabilized by electrostatic repulsion between the highly positively charged zeolite particles and the cationic surfactant adsorbed at the dodecane-water interface.
ABSTRACT
Vacuumable gels have been developed in the nuclear industry to decontaminate solid surfaces with little or no mechanical operations and without producing any liquid effluents. These gels can be spread on the contaminated surface and rapidly trap the radio-contaminants by sorption after corroding the substrate down to several tens of microns if necessary. The gel then dries and eventually fractures into a non-powdery solid that is easily removable by brushing or vacuum cleaning. This process was initially developed for the nuclear decontamination of metallic surfaces but innovative formulations are being developed for wide range of applications. This paper introduces two such formulations designed to remove sticky organic layers, bitumen in this study. We show that adding an organic bio-solvent, limonene, allows bitumen layers to be dissolved, absorbed in the liquid state into the gel matrix, and then removed from the substrate as solid waste after drying. Substituting an organic co-solvent (ethanol) for some of the limonene improves the overall efficiency of the process by decreasing the drying time of the gel and limiting the diffusion of the dissolved bitumen without affecting the dissolving power of the gel. Finally, the performance of these gels is demonstrated for the removal of nuclear contaminated (137Cs) bitumen stains.
