(373d) Engineering Nanostraws for MCM-41 Microparticles Towards Enhancing the Adsorption of Guest Molecules: Application in CO2 Capture.

Ordered mesoporous silica particles such as MCM-41 have wide use in molecular adsorption and reaction catalysis due to their well-defined structure, porosity, and high surface areas. Spherical particles of MCM-41 are easily synthesized using a one-step facile aerosol assisted technique. However, the tight 3 nm pore channels of MCM-41 pose a challenge to molecules penetrating the interior of the pellet and this diminishes the efficiency of this material in adsorption and catalytic applications. We report new finding on introducing nanostraws into MCM-41 pellets to mitigate this diffusional restriction. We demonstrate the use of the aerosol synthesis to systematically incorporate clay nanotubes known as halloysites into MCM-41 pellets in one step. The morphology of the composite material is such that the nanotubes extend from the interior and protrudes through the surface of the pellet like straws. These ceramic nanotubes are 0.5 - 3 µm long and their lumen diameter ranges from 15 - 40 nm which are much larger than the pores of MCM-41. Thus, the nanotube lumen should provide a larger pathway for molecules to access the interior of the MCM-41 pellets. Halloysite is an environmentally benign material that is abundant and inexpensive, therefore it does not add significant costs to the production of this composite material. The composite MCM-41 particles with nanostraws are highly reproducible and detailed characterization showed that the intrinsic structural properties of the MCM-41 remain unchanged with the incorporation of the nanostraws. To illustrate the concept of improved molecular transport into MCM-41 pellets enabled by the nanostraws, the composite material was studied as a solid adsorbent for the impregnation of polymeric amines for CO₂ capture. In comparison to MCM-41, the composite with the nanostraws showed improved loading of polymeric amine macromolecules and consequently doubled the CO₂ capture capacity of the impregnated amines.