<p>The present study suggests a microwave-assisted detemplation technique to synthesize 3-D mesostructured siliceous foam (MSF) support that is functionalized using polyethylenimine (PEI) to use the prepared sorbent for postcombustion CO₂ capture. The bare support and PEI-functionalized sorbent are characterized systematically using various analytical techniques. CO₂ adsorption performance and multicyclic stability of the amine-modified sorbents are studied using a simulated flue gas mixture of 9.7% CO₂/N₂. Over 86.13% template removal is achieved under microwave detemplation conditions of 700&#xa0;W 60&#xa0;min, resulting in maximum CO₂ uptake of 3.7 mmol/g at 75&#xa0;°C using 70 wt% amine loading owing to its large pore size and higher concentration of amine groups. The equilibrium CO₂ adsorption isotherm and kinetic model are also proposed to provide the dynamics and mechanism of the CO₂ uptake process using the synthesized PEI-functionalized sorbents over a wide range of temperatures. For the kinetics of CO₂ adsorption, the fractional order kinetic model shows a good correlation to the experimental values with an AAD % of 5.85, while good agreement between the experimental and model predicted equilibrium CO₂ uptake with an AAD value of 4.65% indicates that a temperature-dependent form of the Toth isotherm model can successfully account for the CO₂ isotherm on MMw700-60/70PEI sorbent in the temperature range of 50–75&#xa0;°C. Furthermore, the functionalized sorbent showed very good cyclic stability with an average working CO₂ uptake of 3.05 mmol/g when tested over 50 consecutive temperature-swing adsorption/desorption cycles for 3000&#xa0;min.</p> Graphical abstract <p></p>