CO ₂ deactivation of supported amines: Does the nature of amine matter?

Adsorption of CO ₂ was investigated on a series of primary, secondary, and tertiary monoamine-grafted pore-expanded mesoporous MCM-41 silicas, referred to as pMONO, sMONO, and tMONO, respectively. The pMONO adsorbent showed the highest CO ₂ adsorption capacity, followed by sMONO, whereas tMONO exhibited hardly any CO ₂ uptake. As for the stability in the presence of dry CO ₂, we showed in a previous contribution [J. Am. Chem. Soc.2010, 132, 6312-6314] that amine-supported materials deactivate in the presence of dry CO ₂ via the formation of urea linkages. Here, we showed that only primary amines suffered extensive loss in CO ₂ uptake, whereas secondary and tertiary amines were stable even at temperature as high as 200 °C. The difference in the stability of primary vs secondary and tertiary amines was associated with the occurrence of isocyanate as intermediate species toward the formation of urea groups, since only primary amines can be precursors to isocyanate in the presence of CO ₂. However, using a grafted propyldiethylenetriamine containing both primary and secondary amines, we demonstrated that while primary amines gave rise to isocyanate, the latter can react with either primary or secondary amines to generate di- and trisubstituted ureas, leading to deactivation of secondary amines as well.