(225g) Conceptual Design of Nano-Straws in Spherical Porous Particles for Improved Adsorption Capacity for Carbon Capture

Amine functionalization of high surface area MCM-41 particles for CO₂ capture, using polymeric amines, is well established but has a drawback of the timely degradation and leeching of the polymer when exposed to moisture or wet environment. Alternatively, chemical (covalent) grafting amine functional groups from an alkoxysilane such as 3-Aminopropyl triethoxysilane (APS) on MCM-41 offers better stability against these drawbacks. However, the diffusional restriction exhibited by the narrow uniform MCM-41 pores (2 – 4 nm) impedes the effective amine functionalization of all available silanol groups within the inner mesoporous core. This leads to incomplete amine functionalization and thus, the CO₂ adsorption capacity is low. To solve this problem, we have integrated 40 wt% of halloysite nanotubes (HNTs) as nano-straws into the spherical MCM-41 spheres (M40H) using a one-step facile aerosol-assisted synthesis technique. This ship-in-a-bottle approach is such that, as the silica growth occurs in the heating zone due to evaporation induced hydrolysis and condensation, and the HNTs are integrated into the mesoporous spherical matrix with a considerable part of their length protruding as straws. Afterwards, the particles are functionalized with varying volume of APS in Toluene at 80 ℃ for 24 h. Characterization results shows that the structural properties of MCM-41 remained unaltered after the incorporation of the nano-straws and amine functionalization. We showed the CO₂ capture at atmospheric pressure and 35 ℃ at dry conditions, using a thermogravimetric analysis setup. The results showed that the APS functionalized M40H (APS/M40H_2) has a higher adsorption capacity of 1.81 mmol/g which is 18.3% higher than that of APS functionalized MCM-41 (APS/MCM-41_2). When normalized based on 60 wt% of MCM-41 in M40H, the APS/M40H_2 has an adsorption capacity of 3.02 mmol/g which doubles that of APS/MCM-41_2 (1.53 mmol/g). This shows that the incorporation of HNT as nano-straws leads to enhanced CO₂ capture due to (1) improved accessibility (and diffusion) of APS into the inner core of MCM-41 spheres for enhanced amine functionalization by covalent grafting and (2) enhanced CO₂ capture via improved molecular transport of CO₂ molecules to access the amine sites within the MCM-41 framework. This unique composite morphology of nano-straws in spherical porous particles has potential for other applications to yield similar effect.