Cation exchange in metal-organic frameworks (MOFs): The hard-soft acid-base (HSAB) principle appraisal

Systematic Study on the Removal of Per- and Polyfluoroalkyl Substances from Contaminated Groundwater Using Metal–Organic Frameworks

“…As per Figure S1, they are the fastest for NU-1000 where the equilibrium of PFAS adsorption was reached within the first minute regardless of the charge state of PFAS as previously observed for anionic PFAS . This performance significantly outperforms the commercial GAC (equilibrium time >48 h, Figure S2), IX resins, and other reported adsorbents. , However, it should be noted for reference that all sorbents studied exhibit different particle sizes as listed in Table S6. It is not considered in the discussion below, but these differences might also influence the sorbets’ adsorption kinetics.…”

“…In addition, most PFAS studies generally probe the performance of adsorbents for anionic PFAS (e.g., PFOS and PFOA) removal, but there are only limited reports on the adsorption of non-ionic PFAS. ,, We observed non-ionic FASA exhibiting higher removal compared to PFSA with the same perfluorocarbon chain length (e.g., C4, C6, and C8; 68 and 91% in IDW 6 for C6 PFHxS and C6 FHxSA, respectively). This preferential adsorption might be induced by FASA’s amine group, which is considered by Pearson as a hard base resulting in strong interactions with NU-1000’s Zr 6 -node (a hard acid). ,, Thus, the higher removal for FASA compared to PFSA in this study indicates that the mechanism for non-ionic PFAS adsorption (with a sulfonamide functional group) is based on the acid–base interactions while hydrophobic interactions also play a role. Furthermore, Figure B shows that the PFAS removal capacities for FASA exceed that for FASAA (e.g., in IDWs 1, 10, and 11).…”

Mechanochemistry as a Reconstruction Tool of Decomposed Metal–Organic Frameworks

“…To date, diverse research approaches have been explored to improve the stability of MOFs. For example, introducing hydrophobic groups to the ligands inhibits water molecules from contacting the framework. , In addition, the metal–ligand bond strength can be greatly enhanced by tailoring the design and synthesis of MOFs according to the hard and soft acid and base theory, such as the coupling of low-valency metal ions with azolate-based ligands [e.g., zeolitic imidazolate frameworks (ZIFs)] or high-valency metal ions with carboxylates (e.g., UiO series). , These efforts to develop synthetic strategies for more stable MOFs have significantly expanded the practical applications of MOFs. Nevertheless, these “stable MOFs” are still less stable than other industrially viable porous materials, including zeolites and activated carbons .…”

Present and Perspectives of Photoactive Porous Composites Based on Semiconductor Nanocrystals and Metal-Organic Frameworks

“…Strong coordination bonds should be rationally designed for MOFs to be stable enough and avoid side reactions. In light of hard/soft acid-base (HSAB) theory, there is a preference for hard acids to bind with hard bases, providing additional stability in terms of bond dissociation energy [ 60 ]. Hence, high-valent metal ions (e.g., Zr 4+ , Al 3+ , Cr 3+ , Ti 4+ , Fe 3+ ) usually possess high charge densities and coordination numbers, thus benefitting the formation of strong coordination bonds and rigid structures when binding to hard bases [ 61 ].…”