Prospective post-combustion CO2 capture sorbents
were
prepared by the immobilization of a low-molecular-weight, branched
polyethyleneimine (PEI) and 3-(aminopropyl)triethoxysilane (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 °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) ...