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[Display omitted] • Amino modified MWCNT sorbents are prepared with different structural properties. • Amino modified materials show CO 2 selectivity on CO 2 /N 2 adsorption-separation tests. • Higher N content and microporosity is linked with the best CO 2 up-take profiles. • The best amino-modified MWCNT material is water-resistant and multicycle reusable. 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 CO 2 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 N 2 -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 CO 2 , 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 CO 2 adsorption capacity, with the maximum CO 2 uptake (2.11%) for COOH-MWCNT functionalized with DETASi. The selectivity for CO 2 /N 2 separation was also improved after DETASi grafting reaction, with the highest value (1.89%) observed for the standard-MWCNT material modified with DETASi. In general, the increase of the DETASi amount and of the microporosity in the functionalized MWCNT sorbents is accompanied by an improvement of the CO 2 adsorption-separation properties. The water vapor adsorption indicated that the standard-MWCNT sample modified with DETASi is resistant to the presence of water and its presence (water vapor of ~3.2 kPa) can improve the CO 2 uptake. [ABSTRACT FROM AUTHOR]