This work describes the functionalization of mesoporous Si-MCM-41 with different loadings of polyethylenimine (PEI), their characterization and detailed study of their CO2 adsorption performance. A series of Si-MCM-41 samples functionalized with 10�50 wt PEI was synthesized by wet impregnation method. The physicochemical properties such as pore structural ordering and textural properties of the samples were characterized by X-ray diffraction and N2 adsorption/desorption analysis, respectively. The XRD analysis of pristine Si-MCM-41 showed well-defined four diffraction peaks of (100), (110), (200) and (210) planes. Upon loading of 10�50 wt PEI, intensity of these diffraction peaks reduced gradually due to decrease in scattering contrast. Also, N2 adsorption/desorption analysis showed change in textural properties from porous to non-porous material as the loading of PEI increased from 10 to 50 wt. To optimize the loading of PEI for maximum CO2 adsorption, samples were screened at 25 °C and 0�1 bar using a gravimetric method. The sample containing 50 wt PEI showed the highest CO2 uptake of 47.93 mg/g due to high content of CO2�affinity sites and was further used to investigate the effect of temperature ranging from 25 to 125 °C on CO2 adsorption performance. The best CO2 adsorption (99.44 mg/g) was obtained at 100 °C and 1 bar due to flexible behavior of PEI. At this optimized temperature, the Si-MCM-41 containing 50 wt PEI was used to evaluate the effect of pressure ranging from 0 to 20 bar where it showed increase in CO2 adsorption uptake of 156 mg/g. Si-MCM-41 containing 50 wt PEI showed strong affinity for CO2 compared to N2 and H2. In a regeneration study, a gradual decline in CO2 uptake was observed as the number of adsorption/desorption cycles increased. Infrared spectroscopy revealed carbamates formation confirming the interaction of CO2 with the amino groups of PEI incorporated into Si-MCM-41 containing 50 wt PEI via chemisorption. © 2017 Elsevier B.V.