Application of Zr-Cluster-Based MOFs for the Adsorptive Removal of Aliphatic Aldehydes (C-1 to C-5) from an Industrial Solvent

Abstract

Metal-organic frameworks (MOFs) are recognized as advanced sorbents for the effective removal and recovery of various hazardous pollutants in liquid and gaseous environments. In this research, the potential applicability of two Zr-based MOFs (UiO-66 (U6) and its amine counterpart Ui0-66-NH2 (U6N)) was investigated relative to activated carbon (AC, tested as a reference adsorbent) for the purification of industrial organic solvents (e.g., methanol) from six different carbonyl impurities (CCs (C-1 to C-5): formaldehyde (FA, CH2O), acetaldehyde (AA, CH3CHO), propionaldehyde (PA, C3H6O), butyraldehyde (BA, C4H8O), isovaleraldehyde (IA, C5H10O), and valeraldehyde (VA, C5H11O)). In the sorptive removal of these CCs (both individually and in binary mixtures with FA), U6N showed higher efficacy in capturing all of the target CCs than U6 and AC. The adsorption selectivity of U6N toward single CC compounds was in the order of PA (165.1 mg g(-1)) > BA (158.9 mg g-') > IA (154 mg g(-1)) > AA (136 mg g(-1)) > VA (131.5 mg g(-1)) > FA (120 mg g(-1)). In all binary mixtures, U6N selectively captured FA over the heavier CCs (C-2-C-5) by 1.5-3.3 times due to the steric hindrance of the C-2-C-5 aliphatic tails in the pore diffusion mechanism. The preferential adsorption of FA onto U6N can also be accounted for by the contribution of chemical bonding (Schiff base interaction) between the -NH2 groups in U6N and the C=O functionalities (aldehyde molecules) and physisorption, as confirmed by density functional theory (DFT) calculations. Theoretical DFT simulations also revealed that the competition between aldehyde molecules for Bronsted acidic sites (mu(3)-OH of Zr-clusters) created minor distortions in the U6/U6N frameworks.