Structure-Oriented Metal-Organic Framework Activation via Proximal Oligoalkyl Quaternary Ammonium Grafting Enhances Long-Chain PFAS Sorption.

Abstract

A structure-oriented design of zirconium-based metal-organic frameworks (Zr-MOFs) for long-chain per- and polyfluoroalkyl substances (PFAS) is critically needed, as current efficacy faces limitations, including restricted micropore functionalization, limited coordination sites, and weak host-guest interactions. Here, we present a surface-localized oligomeric grafting strategy to functionalize UiO-66-NH2 with proximally arranged oligoalkyl-quaternary ammonium (PAOQ) motifs, yielding UiO-66-L3 sorbent. Unlike conventional grafting approaches, the PAOQ motifs containing repetitive functional groups predominantly extend beyond surface pore layers, circumventing intrinsic grafting limitations of the MOF backbone and achieving a 2.2-fold increase in group loading. This tailored architecture enables strong synergistic interactions through precise spatial alignment of quaternary ammonium moieties and conformationally flexible oligoalkyl chains, promoting dual-mode coordination with long-chain PFAS. Consequently, UiO-66-L3 exhibits rapid kinetics (equilibrium within 5 min) and high sorption capacities (403-1872 mg g-1) for long-chain PFAS and derivatives, outperforming alkylated/quaternized analogs by up to 13.3-fold. Density functional theory (DFT) calculations reveal that electronic redistribution induced by oligoalkyl segments enhances the electrostatic potential at adjacent ammonium centers, enabling chain-length-dependent selectivity with sorption coefficients ranging from 2.3 to 4.9. Our work highlights the critical role of surface chemistry modulation in Zr-MOFs and advances structure-oriented design of high-performance sorbents for long-chain PFAS and derivatives.