Polymeric amine encapsulation
in high surface area MCM-41
particles
for CO2 capture is well established but has the drawback
of leaching out the water-soluble polymer upon exposure to aqueous
environments. Alternatively, chemical (covalent) grafting amine functional
groups from an alkoxysilane such as 3-aminopropyltriethoxysilane
(APTES) on MCM-41 offer better stability against this drawback. However,
the diffusional restriction exhibited by the narrow uniform MCM-41
pores (2–4 nm) may impede amine functionalization of the available
silanol groups within the inner mesoporous core. This leads to incomplete
amine functionalization and could reduce the CO2 adsorption
capacity in such materials. Our concept to improve access to the MCM-41
interior is based on the incorporation of nanostraws with larger inner
diameter (15–30 nm) to create a hierarchical porosity and enhance
the molecular transport of APTES. Halloysite nanotubes (HNT) are used
as tubular straws that are integrated into the MCM-41 matrix using
an aerosol-assisted synthesis method. Characterization results show
that the intrinsic structure of MCM-41 remains unaltered after the
incorporation of the nanostraws and amine functionalization. At an
optimal APTES loading of 0.5 g (X = 2.0), the amine-functionalized
composite of MCM-41 with straws (APTES/M40H) has a 20% higher adsorption
capacity than the amine-modified MCM-41 (APTES/MCM-41) adsorbent.
Furthermore, the CO2 adsorption capacity APTES/M40H doubles
that of APTES/MCM-41 when normalized based on the composition of MCM-41
in the composite particle with straws. The facile integration of nanostraws
in MCM-41 leading to hierarchical porosities could be effective toward
the mitigation of diffusional restriction in porous materials with
potential for other catalytic and adsorption technologies.