Sustainable feedstocks and low-cost precursors are crucial for the development of adsorbents for CO2 capture. In this study, three different amine-modified adsorbents were synthesized using sustainable green feedstock, coconut shells, and physical impregnation of DETA, TETA, and TEPA at different loadings (30%, 30%, and 10%, respectively). Their potential as alternative adsorbents for CO2 capture was investigated. Compared to the original activated carbon, all three amine-modified adsorbents showed improved CO2 uptake, with the DETA-modified adsorbent showing the highest CO2 adsorption capacity. The influence of carbon chain length and amine types on CO2 capture was studied in detail. Density functional theory (DFT) calculations were performed to study the CO2 adsorption on the bare activated carbon and the amine-modified adsorbents. The results showed that shorter carbon chains and primary amines could improve the CO2 adsorption performance of the adsorbents regardless of whether the organic amines were dehydrogenated.