Alkyl-Aryl Amine-Rich Molecules for CO₂ Removal via Direct Air Capture

Alkyl-aryl amine-rich small molecules (Ph-X-YY) are prepared by one-step nucleophilic substitution from tri- (X = 3) and hexa-bromine-substituted (X = 6) aromatic cores with two different aliphatic diamines, ethylenediamine (YY = ED) and propylenediamine (YY = PD). The resulting Ph-X-YY molecules are impregnated into mesoporous silica support materials (SBA-15) at predetermined organic loadings (20%–60%). TGA and DSC analyses suggest that these sorbent materials have good thermal stability. An increase in the amine loading led to an increase in CO₂ adsorption capacity measured under simulated dry direct air capture (DAC) conditions. The Ph-3-YY based sorbents show superior CO₂ adsorption capacities compared to the Ph-6-YY-based sorbent homologues, likely due to the bulkier molecular structure of the Ph-6-YY family. The highest CO₂ capacity was demonstrated with the Ph-3-ED-based sorbent, 1.9 mmol/g_SiO2, and an amine efficiency of 0.13 mmol CO₂/mmol N at 35 °C under dry conditions. Temperature-swing adsorption/desorption cycles showed that the Ph-X-YY/SBA-15 had relatively stable performance that is comparable to that of the benchmark poly­(ethyleneimine) (PEI) and linear poly­(propyleneimine) (PPI) supported SBA-15 sorbents. The 60% Ph-3-ED/SBA-15 sample showed a 2-fold increase in the CO₂ adsorption capacity under humid conditions (2.9 mmol/g_SiO2 @ 30%, RH) and a 70% increase in amine efficieny (0.17 mmol CO₂/mmol N). Thus, these aromatic-based polyamine molecules offer good potential for incorporation into practical CO₂ sorbents targeting DAC technologies, and their structures expand the library of the amine-rich molecules that have been explored for DAC.