The use of chemical dispersants is a well-established approach to oil spill remediation where surfactants in an appropriate solvent are contacted with the oil to reduce the oil–water interfacial tension and create small oil droplets capable of being sustained in the water column. Dispersant formulations typically include organic solvents, and to minimize environmental impacts of dispersant use and avoid surfactant wastage it is beneficial to use water-based systems and target the oil–water interface. The approach here involves the tubular clay minerals known as halloysite nanotubes (HNTs) that serve as nanosized reservoir for surfactants. Such particles generate Pickering emulsions with oil, and the release of surfactant reduces the interfacial tension to extremely low values allowing small droplets to be formed that are colloidally stable in the water column. We report new findings on engineering the surfactant-loaded halloysite nanotubes to be stimuli responsive such that the release of surfactant is triggered by contact with oil. This is achieved by forming a thin coating of wax to stopper the nanotubes to prevent the premature release of surfactant. Surfactant release only occurs when the wax dissolves upon contact with oil. The system thus represents an environmentally benign approach where the wax coated HNTs are dispersed in an aqueous solvent and delivered to an oil spill whereupon they release surfactant to the oil–water interface upon contact with oil.

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This work develops the concepts of particle-stabilized emulsions using tubular natural clays known as halloysites to attach to the oil–water interface and stabilize oil-in-water emulsions. Such halloysite nanotubes (HNT) serve as reservoirs for surfactants and can deliver surfactants to the oil–water interface and thus lower the oil–water interfacial tension. This two-step concept of surfactant delivery and droplet stabilization by particles has significant implications to oil spill remediation. However, to deliver surfactant loaded HNTs in a water-based solvent slurry, it is important to stopper the nanotubes to prevent premature release of the surfactant. This work focuses on the use of an environmentally benign two-dimensional metal–organic framework formed by coordinating Fe­(III) with a polyphenolic as a stoppering agent. Such metal–phenolic networks (MPN) form a skin around the HNTs, thus providing a way to effectively sequester surfactant cargo for controlled release. Cryo-scanning electron microscopy (Cryo-SEM) shows that these HNTs and HNT bundles attach to the oil–water interface with side-on orientation. Inverted drop tensiometry was used to characterize the dynamic interfacial tension resulting from the release of a model surfactant (Tween 80) from the HNTs and indicates that the MPN stoppers are effective in sequestering the surfactant cargo for extended periods at neutral pH values. Release triggered by MPN disassembly at acidic pH values can be performed just prior to delivery to oil spills. The concepts and scalability of this process have significant implications for oil spill remediation, enhanced oil recovery, and biomedical and pharmaceutical applications.

A facile aerosol-assisted technique was employed for the design of a new class of composite zeolite catalyst material with spherical morphology. This technique enables the one-step encapsulation of zeolite microcrystals into the matrix of spherical mesoporous silica particle supports such as MCM-41. By introducing presynthesized zeolite microcrystals into precursor solutions containing the templating surfactant and the silica precursor followed by aerosolization through nozzles larger than the microcrystals, it is possible to entrain the microcrystals in the aerosol droplet. Transport of the droplet through the heated zone of the furnace leads to hydrolysis and condensation of the silica precursor (tetraethyl orthosilicate, TEOS) in each droplet and the formation of spherical particles of MCM-41 containing embedded zeolite microcrystals. This bottle-around-a-ship procedure to make zeolite-MCM-41 composites is extremely effective and can be easily scaled up. Detailed characterization of these composite particles reveals that up to 75 wt % of ZSM-5 zeolite can be embedded in MCM-41 microspheres with no loss of fidelity in particle morphology. To verify access of the reactants to the zeolite, we impregnated the ZSM-5 with nickel (Ni@ZSM-5) prior to encapsulation in MCM-41, and have shown the feasibility of the system to the model reaction of the liquid phase catalytic reduction of 4-nitrophenol to 4-aminophenol. While the reaction proceeds efficiently, there are diffusional restrictions to the transport of 4-nitrophenol resulting in a composite catalyst effectiveness factor of 0.4. The encapsulation of zeolite crystals within a micrometer-sized mesoporous MCM-41 shell provides structural stability to the zeolites and could reduce the pressure drop across a fixed bed tubular reactor due to the increased particle size of the composite. Potential applications of such composite particles include the ability of the MCM-41 to act as sacrificial adsorbents for coke and catalyst poisons, thus extending the life of the active material.

In many porous catalyst supports, the accessibility of interior catalytic sites to reactant species could be restricted due to limitations of reactant transport through pores comparable to reactant dimensions. The interplay between reaction and diffusion in porous catalysts is defined through the Thiele modulus and the effectiveness factor, with diffusional restrictions leading to high Thiele moduli, reduced effectivess factors, and a reduction in the observed reaction rate. We demonstrate a method to integrate ceramic nanostraws into the interior of ordered mesoporous silica MCM-41 to mitigate diffusional restrictions. The nanostraws are the natural aluminosilicate tubular clay minerals known as halloysite. Such halloysite nanotubes (HNTs) have a lumen diameter of 15–30 nm, which is significantly larger than the 2–4 nm pores of MCM-41, thus facilitating entry and egress of larger molecules to the interior of the pellet. The method of integrating HNT nanostraws into MCM-41 is through a ship-in-a-bottle approach of synthesizing MCM-41 in the confined volume of an aerosol droplet that contains HNT nanotubes. The concept is applied to a system in which microcrystallites of Ni@ZSM-5 are incorporated into MCM-41. Using the liquid phase reduction of nitrophenol as a model reaction catalyzed by Ni@ZSM-5, we show that the insertion of HNT nanostraws into this composite leads to a 50% increase in the effectiveness factor. The process of integrating nanostraws into MCM-41 through the aerosol-assisted approach is a one-step facile method that complements traditional catalyst preparation techniques. The facile and scalable synthesis technique toward the mitigation of diffusional restrictions has implications to catalysis and separation technologies.