Tubular Clay Nano-Straws in Ordered Mesoporous Particles Create Hierarchical Porosities Leading to Improved CO2 Uptake

The use of solid adsorbents for CO2 capture is of importance in the development of technologies to remove this greenhouse gas. High surface area mesoporous materials are used to encapsulate or functionalize amines that selectively capture CO2. The MCM-41 class of hexagonally ordered mesoporous silicas have surface areas between 1000 and 1500 m2/g, which in principle allows a significant degree of capture when amines such as polyethyleneimine (PEI) are encapsulated within the pores and occupy the entire surface area of the materials. However, the tight 3 nm pore channels of MCM-41 create a challenge to the infiltration of PEI throughout the pore volume, resulting in much lower CO2 uptake at high PEI concentrations. This work describes the introduction of clay nanotubes (halloysite) with an inner diameter of 15–30 nm into MCM-41 pellets to mitigate such diffusional restrictions. The introduction of the nanotubes is through a one-step ship-in-a-bottle approach to synthesis in an aerosol-assisted system where MCM-41 is synthesized in droplets containing the halloysite. The morphology of the composite material is such that several nanotubes extend from the interior and protrude through the surface of the pellet like straws. These ceramic nanotubes are 0.5–2 μm in length, and their lumen diameter ranges from 15 to 30 nm, thus providing an improved entry pathway for molecules to access the interior of the MCM-41 pellets. The concept is used to enhance PEI loading in MCM-41, which leads to a significant increase in CO2 uptake levels. MCM-41 composites with halloysite nano-straws (MCM-41/HNT) show a doubling of CO2 uptake levels (7.1 wt %) and uptake kinetics (k = 0.11 s–1) in comparison to MCM-41 sorbents (3.9 wt % and k = 0.05 s–1) at 50 wt % PEI loading. This indicates the potential validity of the nano-straws to improve access to the interior of MCM-41, thus coupling enhanced molecular transport with a high surface area material. The generality of the concept indicates further applications to adsorption and catalysis.