Electrostatic Purification of Mixed-Phase Metal-Organic Framework Nanoparticles

Although macroscopic metal-organic framework (MOF) single crystals have been routinely synthesized, undesired impurity phases are sometimes obtained in MOF nanoparticle (NP) syntheses, where purification remains challenging. Herein we report an electrostatic adsorption strategy to separate mixed phases of MOF NPs on the basis of their metal cluster-dependent surface charge differences. As a proof of concept, two groups of mixed-phase MOF NPs were synthesized and subsequently separated on the basis of their different Coulombic attraction to negatively charged magnetic beads (MBs). Different frameworks form on the basis of the conditions used. In the first group, a combination of three possible iron-terephthalate frameworks were evaluated: MIL-53, MIL-88B, and MIL-101 (MIL = Material Institute Lavoisier). In the second group, two zirconium-terephthalate frameworks were separated: MIL-140A and UiO-66 (UiO = University of Oslo). MIL-53 and MIL-140A are not positively charged and do not adsorb onto the MBs. The extraction of adsorbed MIL-88B, MIL-101, and UiO-66 MOF NPs from the MBs was achieved by adding 4-hydroxybenzophosphonate, a surface-capping ligand that neutralizes the charge of the MOF NPs and therefore results in their desorption from the MBs. The phase purities of the isolated NPs were verified by powder X-ray diffraction as well as scanning electron microscopy. This straightforward purification strategy provides quick access to phase-pure MOF NPs, which is important for their use as biological probes, catalysts, and building blocks for colloidal crystal engineering strategies.