Grafting titanate nanotubes with 3-aminopropyltriethoxysilane for enhanced CO₂ adsorption

Amino-functionalized trititanate nanotubes (TTNTs) were prepared and evaluated for CO₂ capture under conditions suitable for direct air capture (DAC) for the first time. 3-Aminopropyltriethoxysilane (APTES) was successfully grafted onto mesoporous TTNTs via a solution-phase method conducted in 95% ethanol at room temperature. Chemical analyses indicated that surface saturation was achieved at approximately 1.1 mmol of APTES per gram of grafted material. X-ray powder diffraction (XRPD), transmission electron microscopy coupled with energy-dispersive spectroscopy (TEM/EDS), and N₂ sorption analyses confirmed the preservation of the nanotubular architecture and a homogeneous distribution of the grafted aminosilane species throughout the material. A concomitant reduction in the specific surface area and pore volume by 25-35% was observed. Systematic variation of the APTES concentration in the solution-phase method revealed a pronounced influence on the binding mechanism of the aminosilane molecules to the hydroxylated TTNT surface at the constant content of the grafted molecule. Elemental (CHN), thermogravimetric (TGA), Fourier-transform infrared (FTIR), and X-ray photoelectron spectroscopic (XPS) analyses verified the complete hydrolysis of APTES, yielding silanol groups that were involved in competing surface condensation pathways. The preferred configuration involved covalent Ti-O-Si bond formation, which preserves terminal -NH₂ functionalities for CO₂ chemisorption. These linkages were established either through direct condensation of silane triol species with surface Ti-OH groups (anchoring mode) or through partial self-condensation, leading to Si-O-Si crosslinking within an oligomeric or polymeric network (crosslinking mode). At very low pressures, approaching the partial pressure of CO₂ in ambient air conducive to DAC, all functionalized samples exhibited significantly enhanced CO₂ adsorption capacities compared with the pristine TTNT sorbent. A direct correlation was established between the CO₂ chemisorption capacity and both the relative abundance of free -NH₂ groups (quantified using XPS N 1s) and the degree of Si-O-Si polymerization (estimated using FTIR). The highest content of free -NH₂ groups, corresponding to 57% of the total nitrogen present in the grafted materials, was achieved by adding 2.7 mmol g^-1 APTES (2.5 times higher than the saturation content), leading to the best DAC capacity. The absolute CO₂ chemisorption capacity of all grafted TTNT samples, measured under static conditions and below 0.1 mmol g^-1, was lower than that typically reported for other solid sorbents, including APTES-modified oxides such as mesoporous alumina and silica. The present study provides a detailed understanding of the APTES grafting mechanism on TTNTs and establishes a direct correlation between the CO₂ chemisorption capacity under DAC-relevant conditions and the nature of APTES grafting.