<p>Over the past decades, the considerable increase in greenhouse gas emissions has caused alarming issues such as global warming. Numerous investigations have been carried out to enhance CO₂ capture. One of the promising methods is to functionalize the MWCNTs, but different conditions and factors for functionalizing have significant effects on the adsorption. In this study, raw multi-wall carbon nanotubes were functionalized in two stages. First, they were modified using a mixture of 5 molars of sulfuric acid and nitric acid. Then, the carboxylated MWCNTs were functionalized with 1,3 Diaminopropane solutions. To obtain the optimum adsorbent parameters, the ratio of amine to solvent concentration and the reflux time of amine solution were changed. FTIR, FESEM, TGA, and nitrogen adsorption/desorption analyses were used to determine the characterizations of optimum adsorbents. Based on the experimental results, the maximum capacity equal to 3.37 mmolg^− 1, obtained under conditions of 303&#xa0;K, and an initial pressure of 18.5&#xa0;bar when the amine to ethanol concentration ratio was 60% w/w. Furthermore, the effect of the MWCNTs diameter on adsorption capacity was investigated as well. Results proved that by increasing the MWCNTs diameter in the raw and amine-functionalized samples, the adsorption capacity increased. Additionally, the adsorption isotherms were evaluated by Langmuir and Freundlich models, and the isosteric heat of adsorption, the adsorption mechanism and the adsorption capacity were measured. Ultimately, the regeneration cycles of optimal adsorbent was performed in five stages, indicating that the adsorbent was stable enough for the regeneration process.</p>