Amine-impregnated millimeter-sized spherical silica foams with hierarchical mesoporous–macroporous structure for CO2 capture

Millimeter-sized spherical silica foam supports with hierarchical mesoporous–macroporous structure were prepared using agar addition, foaming, and drop-in-oil method, and they were modified with polyethyleneimine (PEI) by a wet impregnation process for CO2 capture. CO2 sorption capacity of the millimeter-sized spherical sorbents increased as amount of PEI impregnation increased. However, the CO2 sorption capacity of the sorbent with 70 wt% PEI impregnation loading did not show steady increase, and the maximum CO2 sorption capacity decreased. This was attributed to the channels (i.e., small macropores) in the sorbents being clogged by the amine that remained after the mesopores had been filled up, which decreased the inward diffusion of CO2 molecules and eventually reduced the accessibility of CO2 molecules to the activated sites. Furthermore, the CO2 sorption capacity of the mm-sized sorbents was shown to increase due to the bimodal macropore structure (small and large). Among the studied sorbents, SSF-PEI60 (at 56.5 wt% PEI loading) exhibited the highest CO2 sorption capacity of 188.3 mg g−1 of sorbent, corresponding to 333.2 mg g−1 of PEI. The CO2 sorption capacity of the spherical sorbents reported in this study does not supersede those of the particle-type sorbents reported in previous studies under the same experimental conditions. Nevertheless, these spherical sorbents are bulk-type sorbents with the best performance in terms of CO2 capture capacity per gram of PEI among all bulk-type sorbents reported so far. Sorption stability was confirmed through 100 cycles of CO2 sorption–desorption.