Prospective post-combustion CO2 capture sorbents were prepared by the immobilization of a low-molecular-weight, branched polyethyleneimine (PEI) and 3-(aminopropyptriethoxysilane (APTES) within a commercially available porous PQ Corporation CS-2129 silica support to investigate (i) CO2 adsorption properties of the supported mixed-amine (PEI+APTES) sorbents in both pure CO2 environments and simulated flue gas conditions, (ii) their thermal and hydrolytic stability over numerous adsorption and desorption cycles, and (iii) their equilibrium and kinetic adsorption behavior. Initial CO2 adsorption desorption measurements via thermogravimetric analysis (TGA) were conducted in pure CO2 to measure dry, near-equilibrium CO2 adsorption capacities, together in calculating amine efficiencies, which was recognized in being a meaningful criterion in evaluating sorbent performance for selecting the "most favorable" mixed-amine (PEI+APTES) composition. The as-prepared materials containing various weight ratios of PEI to APTES showed less uptake of CO2, relative to the supported PEI-only impregnated material under investigated TGA experimental conditions. Nitrogen adsorption desorption isotherms in evaluating the physical properties of the synthesized mixed-amine (PEI+APTES) samples showed reduced values specific to surface area, and total pore volume largely predictable from the successful incorporation of PEI multilayers into the structure of the porous silica matrix, together with unreacted APTES moieties remaining behind after material synthesis. Breakthrough curves produced by (PEI-15-APTES-35)-PQCS2129, (PEI-25-APTES-25)-PQCS2129, (PEI-35-APTES-15)-PQCS2129, and (PEI-50)-PQCS2129 showed mean near-equilibrium CO2 adsorption capacities of 1.81 +/- 0.17, 2.43 +/- 0.26, 2.44 +/- 0.19, and 2.44 +/- 0.45 mol CO2/kg of sorbent, respectively, over multiple CO2 adsorption desorption cycles utilizing a 10% CO2, 8% H2O (balance, He stream) at 60 degrees C and 1.01 bar for adsorption; followed by regeneration in a He stream containing 90 vol% water vapor at 105 C. From these studies, (PEI-25-APTES-25)-PQCS2129 and (PEI-35-APTES-15)-PQCS2129 exhibited a higher CO2 capturing efficiency (absorbed amount of CO2 per gram of PEI), relative to (PEI-50)-PQCS2129, indicating the PEI/APTES interface (i.e., interaction between layers of surface alkyl chains associated with APTES and PEI) is perhaps contributing to improving the deposition/dispersion of PEI, thereby decreasing the diffusion resistance with regard to CO2 entering into the bulk of the PEI multilayers. Conversely, the lower amine efficiency of (PEI-50)-PQCS2129 can be ascribed to the possible clustering of the PEI molecules from the higher PEI loading, resulting in a decrease of accessible amine sites and creating a higher diffusional resistance in connection with CO2 molecules penetrating into the majority of layers of PEI. Near-equilibrium CO2 adsorption measurements of (PEI-25-APTES-25)-PQCS2129 in utilizing the laboratory-scale, fixed-bed flow reactor system located at ADA-ES (Littleton, CO) displayed ranges of 2.70-3.45 mol CO2/kg sorbent at 40 degrees C under different CO2 partial pressures. The (PEI-, 25-APTES-25)-PQCS2129 material showed a relatively stable performance over many adsorption desorption cycles (i.e. , >250 cycles) under humidified simulated flue gas conditions, along with a higher amine efficiency relative to the (PEI-50)-PQCS2129 sample ("PEI-only" sample). In fitting the experimental data of ADA-ES, the Langmuir isotherm model was determined to be an acceptable representation of the observed thermodynamics.