Effect of amine surface density on CO₂ adsorption behaviors of amine-functionalized polystyrene

Ethylenediamine was grafted onto a polystyrene surface to study the relationship between amine density and amine efficiency. Amine-tethered polystyrene was synthesized in two steps: (1) chloromethylation and (2) amination. The products were characterized by argentometry, N₂ physisorption, thermogravimetric analysis, X-ray diffraction, and FT-IR spectroscopy. The amine efficiency declined drastically when the amine density was less than 1 amine molecule per nm², because formation of ammonium carbamate is difficult under such conditions. A double-functionalization method was introduced to improve the CO₂ uptake and amine efficiency of the amine-grafted sorbent by impregnating more amino groups into the porous structure. The combination of grafted and mobile amino groups led to a synergic effect, resulting in a CO₂ uptake of up to 20.2 wt% at 30 °C and 1 bar CO₂. It was concluded that double-functionalization with tetraethylenepentamine was advantageous for enhancing the amine efficiency, whereas double-functionalization with polyethyleneimine was advantageous for enhancing the cyclic stability of the sorbent.