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

Abstract

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.

Gram-scale ruthenium catalysts templated on halloysite nanotubes and MCM-41/halloysite composite for removal of aromatics from gasoline fraction

“…Therefore, it can be concluded that the HNT-supported catalyst has substrate selectivity for p -xylene thanks to its tubular structure with Ru particles embedded into the tubes, providing o - and m -xylenes with lower diffusivity and acting as nanoreactors. 27,35 Similar to the composite-based analogue, no isomerization or acid-catalyzed cracking was observed over the HNT-supported Ru catalyst, which is proved by the n -heptane content being constant over the whole temperature range.…”

Design of Nanostraws in Amine-Functionalized MCM-41 for Improved Adsorption Capacity in Carbon Capture

“…We also note that the introduction of APTES leads to small changes in peak position, which we attribute to the inclusion of Si atoms from APTES that results in a small but perhaps not negligible contribution to the X-ray scattering. This shows that the incorporation of HNT as straws into MCM-41 did not alter the mesoporous MCM-41 framework. , Upon amine functionalization, the intensity of the (100) peak decreases as the weight of APTES increases and the secondary peaks lose resolution. This can be attributed to the attached APTES chain within the silica framework. − The presence of silicon and oxygen in the organic APTES molecules increases the electron density of MCM-41 and M40H after amine functionalization.…”

“…We introduce HNT into MCM-41 as “nanostraws” with the hypothesis that the large pores of HNT will facilitate the entry of amines to the interior and thus improve amine loading. In our earlier work, we studied class I type systems of encapsulating polyethylenimine (PEI) and showed that the introduction of HNT into MCM-41 improved PEI loading by 40% and doubled CO 2 capture efficiencies . In this study, we extend this concept of improved access to the particle interior through the nanostraws to enhance the covalent grafting of APTES into the MCM-41 matrix and develop an improved class II adsorbent with higher CO 2 capture.…”

Architectural design of core–shell nanotube systems based on aluminosilicate clay

“…4 . 25 After spray-drying, separated microparticles were collected and calcined at 550 °C to remove organics ( Fig. 4a ).…”

“…The surface area could be moderately increased by alumina acid etching or drastically enhanced by selective adherence of smaller-pore silica, such as MCM-41 (Mobil Composite Material-41) or nanocarbon (Scheme 1, bottom right). 8,[23][24][25][26] Negatively charged nanotube outer surfaces were modied with cationic surfactants, e.g. quaternary ammonium salts such as cetyltrimethyl ammonium bromide.…”