Improved CO2 adsorption properties through amine functionalization of multi-walled carbon nanotubes

Abstract Multi-walled carbon nanotubes (MWCNT) with different functionalities (standard-; OH-; and COOH- functionalized) are chemically modified with N1-(3-trimethoxysilylpropyl)diethylenetriamine (DETASi) aiming to enhance their CO2 adsorption/separation properties. The grafting of the DETASi groups to the surface of the MWCNT samples is studied by X-ray photoelectron spectroscopy (XPS), operando thermogravimetric analysis - infrared spectroscopy (TGA-IR), –196 °C N2-sorption isotherms, and scanning electron microscopy (SEM). A comprehensive overview of the reaction mechanism and samples structure/composition is provided. It was found that the aforementioned functionalities highly influence the physical-chemical and the textural properties of the DETASi modified-MWCNT materials, impacting in the DETASi functionalization degree and its distribution on the sample. Amount (by TGA-IR) and homogeneity (by SEM) of the organosilicon group in the functionalized samples increases with the increase of the functionalities in the parent MWCNT materials. Standard-MWCNT and OH- functionalized MWCNT showed to be unable to adsorb CO2, while COOH- modified MWCNT adsorbed just 0.1 % at 1 bar and 30 °C. At the same adsorption conditions, DETASi modification of the MWCNT materials enhanced the CO2 adsorption capacity, with the maximum CO2 uptake (2.11%) for DETASi functionalized COOH-MWCNT. The selectivity for CO2/N2 separation was also improved after DETASi grafting reaction, with the highest value (1.89%) observed for the sample DETASi functionalized standard-MWCNT. In general, the increase of the DETASi amount and of the microporosity in the functionalized MWCNT sorbents is accompanied by an improvement of the CO2 adsorption-separation properties. The water vapor adsorption indicated that the DETASi-modified standard-MWCNT is resistant to the presence of water and its presence (water vapor of ∼ 3.2 kPa) can improve the CO2 uptake.