Carbon dioxide capture using diaminoalkane-grafted MIL-101(Cr)s: Critical role of geometric configuration of loaded amines

Preventing the increase of CO₂ content in the atmosphere is crucial to mitigating climate change. In this study, we investigated the adsorption of CO₂ using isostructural Cr-based MOFs, MIL-101 and MIL-100, after grafting diaminoalkanes with various chain lengths to determine the effect of the geometric configuration of the amines and to develop a competitive adsorbent. The performance of CO₂ adsorption (e.g., adsorption capacity, selectivity, and adsorption heat) over MIL-101 increased as the size of diaminoalkanes increased, topped with 1,4-butanediamine, and declined with longer diaminoalkanes. The favorable contribution of grafted 1,4-butanediamine in increasing the performances in CO₂ adsorption could be explained by the adequate size of the amines for the optimized configuration in the pore window of the MIL-101. However, interestingly, none of the MIL-100s grafted with diaminoalkanes showed a similar positive effect probably because of the smaller pore window of MIL-100 than that of MIL-101. To the best of our knowledge, this is the first study to demonstrate the importance of geometric configuration (or, the chain length of amines) in amine-grafted MOFs for the CO₂ adsorption that follows the general mechanism that requires interactions between two uncoordinated amines for the formation of ammonium carbamates. Moreover, one of the modified MIL-101 grafted with 1,4-butanediamine exhibited remarkable performance in CO₂ adsorption with 4.0 times the adsorption capacity of CO₂ at 15 kPa and 20.6 times CO₂/N₂ selectivity (based on ideal adsorbed solution theory, at 100 kPa) compared to the virgin MIL-101.