Optimum relative humidity enhances CO2 uptake in diamine-appended M2(dobpdc)

Abstract

Diamine-appended M2(dobpdc) metal–organic frameworks (MOFs) are a promising class of CO2 adsorbents, but the impact of relative humidity (RH) on their adsorption performance has yet to be carefully studied. We evaluated the CO2 uptake of (2-ampd)2M2(dobpdc) at varying relative humidities and associated water loadings using gravimetric and breakthrough adsorption methods. A significant enhancement in uptake is observed between 20 and 40 % RH, with an optimum of 30 % RH, at which a maximum CO2 uptake of 5.7 ± 0.2 mmol/g is achieved at 0.044 atm of CO2. These uptakes – which are above 1 CO2:diamine – suggest that humidity promotes an additional sorption mechanism beyond the well-known cooperative insertion between the metal and diamine. Temperature-programmed desorption and IR spectroscopy also indicated a second adsorption site accounting for the additional uptake, hypothesized to be carbamic acid pairs on the free amine within the MOF channel. Further increasing the humidity to 50 % RH resulted in a dramatic loss of capacity due to water-filled pores at these high humidity conditions. These results reveal the potential for a mixed mechanism in (2-ampd)2M2(dobpdc) promoted by humidity and highlight an optimum humidity range that will maximize the MOF’s utilization in CO2 capture applications.