Utilization of metal-organic frameworks for the adsorptive removal of an aliphatic aldehyde mixture in gas phase
Considerable efforts have been undertaken in the domain of air quality management for the removal of hazardous volatile organic compounds, particularly carbonyl compounds (CCs). In this study, the competitive sorptive removal of six CCs (namely, formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, isovaleraldehyde, and valeraldehyde) was assessed using the selected metal-organic frameworks (MOFs: MOF-5, MOF-199, UiO-66, and UiO-66-NH2) and an inexpensive commercial activated carbon as a reference sorbent. The sorption experiments were conducted using a mixture of six CCs (formaldehyde and acetaldehyde at ~1 Pa and propionaldehyde, butyraldehyde, isovaleraldehyde, valeraldehyde at ~0.2 Pa) together with 15 Pa water and 2.6 Pa methanol in 1 bar nitrogen. For all of the carbonyl compounds other than formaldehyde, MOF-199 showed the best 10% breakthrough performance such ranging from 34 L g-1 and 0.14 mol kg-1 Pa-1 for acetaldehyde to 1870 L g-1 and 7.6 mol kg-1 Pa-1 for isovaleraldehyde. Among all sorbents tested, UiO-66-NH2 exhibited the best 10% breakthrough performance metrics towards the lightest formaldehyde which remains to be one of the most difficult targets for sorptive removal (breakthrough volume (285 L g-1) and partition coefficient (1.1 mol kg-1 Pa-1). Theoretical density functional theory (DFT)-based computations were also conducted to offer better insights into the adsorbate-adsorbent interactions. Accordingly, the magnitude of adsorption energy increased with an increase in CC molar mass due to an enhancement in the synergetic interaction between C=O groups (in adsorbate molecules) and the MOF active centers (open metallic centers and/or NH2 functionality) as adsorbent. Such interactions were observed to result in strong distortion of MOF structures. In contrast, weak van der Waals attractions between the hydrocarbon “tail” of CC molecules and MOF linkers were seen to play a stabilizing role for the sorbent structure. The presence of NH2 group in the MOF structure was suspected to play a key role in capturing lighter CCs, while such an effect was less prominent for heavier CCs. Overall, the results of this study provided a basis for the establishment of an effective strategy to enhance the sorption capacity of MOFs against diverse carbonyl species.