A One-Step Facile Encapsulation of Zeolite Microcrystallites in Ordered Mesoporous Microspheres

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.