Adsorptive removal of an eight-component volatile organic compound mixture by Cu-, Co-, and Zr-metal-organic frameworks: Experimental and theoretical studies

Abstract

To effectively mitigate hazardous volatile organic compounds (VOCs) in air, adsorption techniques for their complex mixtures, rather than individual species, are of great demand in light of the scarcity of relevant data. Four types of broadly studied metal-organic frameworks (MOFs) (i.e., Cu- (MOF-199 (also known as HKUST-1)), Co- (Co-CUK-1), Zr- (UiO-66 (U6), and UiO-66-NH2 (U6N)) were effectively synthesized, characterized, and studied as the adsorbents along with a microporous activated carbon (AC) as a reference. Further, a multiple target system that include eight VOCs (four aromatics (benzene, toluene, styrene, m-xylene) and four oxygen-containing aliphatic compounds (methyl ethyl ketone (MEK), methyl isobutyl ketone (MiBK), butyl acetate (BuAc), and isobutyl alcohol (iBuAl))) was explored both as single and multi-component system. The MOFs adsorbed polar VOCs favorably, with the M199 outperforming the rest. The 10% breakthrough volume (L atm g−1) for M199 was noted to decrease in the following order: MiBK (7659) > MEK (4772) > iBuAl (1954) > styrene (1775) > m-xylene (316) > BuAc (206) > toluene (50) > benzene (36). In case of U6, the presence of amine groups (U6N) was seen to promote the adsorption of all eight VOCs. The experimental and density functional theory (DFT)-based modelling supported the enhanced adsorption performance of M199 over other MOFs in terms of rigid structure. The results further suggested apparent competition for the surface sorption sites between VOCs in a multicomponent system. In case of AC, the π-π interactions and van der Waals forces were the predominant -controls on the adsorption of VOCs. In contrast, the uncoordinated metal centers (Lewis acidic sites) were the key to the adsorption of polar VOCs by MOFs.