Flexible lanthanide MOFs as highly selective and reusable liquid MeOH sorbents

Abstract

A series of new 3-D Ln³⁺ metal–organic frameworks (MOFs), which are based on the semi-rigid ligand H₃CIP [H₃CIP = 5-(4-carboxybenzylideneamino)isophthalic acid], [Ln₂(CIP)₂(DMF)_4−x(H₂O)_x] (Ln³⁺ = La³⁺, Ce³⁺, Pr³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺; x = 0–2) are reported. Magnetic susceptibility data for selected compounds indicated the presence of antiferromagnetic interactions, while the photoluminescence properties of the MOFs revealed emission peaks that are red-shifted compared to the emission (405 nm) of the uncoordinated H₃CIP ligand, whereas the Eu³⁺ and Tb³⁺ analogues showed emission peaks typical of these lanthanide ions. Single-Crystal-to-Single-Crystal (SCSC) coordinating solvent exchange experiments with acetone and methanol for the Ce³⁺ analogue afforded compounds that are isostructural with the pristine material and contain one acetone and one water or 1.25 methanol and 0.75 water ligated molecules per Ce³⁺ respectively. The capability of activated Ce³⁺ MOF to absorb liquid MeOH was evaluated by means of ¹H NMR spectroscopy. The results of the sorption experiments indicated a maximum absorption capacity of 96(2) mg g⁻¹ and fast kinetics, while the sorbent is reusable and is also capable of highly selective sorption of MeOH over EtOH. Overall this work emphasizes the multifunctional nature of lanthanide MOFs and provides insight into the liquid-phase sorption of small organic molecules by such materials demonstrating their high potential as sorbents.