Polyethylenimine functionalized mesoporous silica is a potential alternative of amine based absorption process for CO2 capture from flue gas. By impregnation of high molecular weight polyethylenimine (PEI) into three dimensional mesoporous KIT-6, a novel sorbent was prepared for highly CO2/N-2 selective separation at moderate temperature and low partial pressure with high stability. TEM micrograph shows that, pore is covered with PEI and also a layer of PEI is formed on the surface of adsorbent without disturbing its original structure. TGA analysis confirmed that with increasing molecular weight of PEI, thermal stability of adsorbent was increased. The sorption capacity was increased with increasing the temperature form 30 to 120 degrees C and the adsorption mechanism is elucidated with the help of the nature of CO2 equilibrium isotherms. The very high CO2/N-2 selective adsorbent follows the order K/250PEI < K/12PEI < K/8PEI with maximum operating sorption capacity 3.0 mmol CO2/g of K/8PEI at 105 degrees C and 100 kPa. Adsorbent showed stable sorption performance at moderate temperature (90-105)A/100D degrees C during 20 adsorption/desorption cycles. However, K/250PEI gives the maximum benefits in terms of stable sorption performance (2.4 mmol CO2/g adsorbent at 105 degrees C and 100 kPa) and the necessary thermal stability for practical application. (C) 2016 Elsevier B.V. All rights reserved.