(569d) Kinetic Model Describing Self-Limiting CO2 Diffusion in Supported Amine Adsorbents

Abstract: A reaction-diffusion shrinking core model describing the decay in diffusivity of supported amine sorbents upon CO₂ sorption under both direct air capture and point source capture conditions is described. The decay in CO₂ diffusivity is explained by crosslinking in the aminopolymer samples and general pore blockage in the amino-silane derived samples, which occurs as CO₂ is adsorbed. The model is used to extract four kinetic parameters that govern the CO₂ uptake kinetics and working capacity: an apparent reaction rate constant, an initial effective diffusivity, and two dimensionless decay parameters. Ideally, an initially reaction limited system would allow for direct determination of the intrinsic reaction rate constant; however, sorption experiments suggest mass transfer resistances related to gas mixing, external boundary layers and intraparticle diffusion are present. Reaction rate constants are determined and agree well with theoretically values predicted with the Eyring equation. The kinetic performance is expressed as the average effective diffusivity as a function of average conversion, which can be correlated to the dispersion of sorption sites on the support and the morphology of the active sorbent phase. Four supports were impregnated or grafted with amines, SBA-15, single-walled zeolite nanotubes (ZNT), syloid SiO₂, and γ-Al₂O₃. Due to its pore structure, γ-Al₂O₃ supported amines can be modeled at the μm scale or at the nm scale, where the shell balance is on the μm-sized macroporous particle aggregate or on the nm-sized amine film on the surface of the Al₂O₃ nanoparticles, which comprise the spherical particle aggregates. Faster diffusion rates are maintained under 400 ppm rather than 10% CO₂ due to a slower reaction rate giving a slower decay in diffusivity. Analysis of the difference in diffusion rates under ultra-dilute compared to concentrated CO₂ concentrations suggests a structure-property relationship with the morphology of the active sorbent phase. When the diffusion rates decrease at 10% CO₂, it is likely that amine plugs are present in the pores. This work provides a first principles kinetic analysis of CO₂ sorption where previous models are semi-empirical and use arbitrary kinetic parameters.