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

Polymeric amine encapsulationin high surface area MCM-41particlesfor CO2 capture is well established but has the drawbackof leaching out the water-soluble polymer upon exposure to aqueousenvironments. Alternatively, chemical (covalent) grafting amine functionalgroups from an alkoxysilane such as 3-aminopropyl-triethoxysilane(APTES) on MCM-41 offer better stability against this drawback. However,the diffusional restriction exhibited by the narrow uniform MCM-41pores (2-4 nm) may impede amine functionalization of the availablesilanol groups within the inner mesoporous core. This leads to incompleteamine functionalization and could reduce the CO2 adsorptioncapacity in such materials. Our concept to improve access to the MCM-41interior is based on the incorporation of nanostraws with larger innerdiameter (15-30 nm) to create a hierarchical porosity and enhancethe molecular transport of APTES. Halloysite nanotubes (HNT) are usedas tubular straws that are integrated into the MCM-41 matrix usingan aerosol-assisted synthesis method. Characterization results showthat the intrinsic structure of MCM-41 remains unaltered after theincorporation of the nanostraws and amine functionalization. At anoptimal APTES loading of 0.5 g (X = 2.0), the amine-functionalizedcomposite of MCM-41 with straws (APTES/M40H) has a 20% higher adsorptioncapacity than the amine-modified MCM-41 (APTES/MCM-41) adsorbent.Furthermore, the CO2 adsorption capacity APTES/M40H doublesthat of APTES/MCM-41 when normalized based on the composition of MCM-41in the composite particle with straws. The facile integration of nanostrawsin MCM-41 leading to hierarchical porosities could be effective towardthe mitigation of diffusional restriction in porous materials withpotential for other catalytic and adsorption technologies.