Tuneable CO2 binding enthalpies by redox modulation of an electroactive MOF-74 framework

Abstract

The [M₂(dobdc)] series of frameworks (also known as MOF-74 and CPO-27, where M²⁺ = Zn, Mg, for example, and dobdc²⁻ = 4,6-dioxido-1,3-benzenedicarboxylate) have been the subject of enormous interest in respect to their attractive gas adsorption properties that arise from the ultra-high concentration of open metal sites. An interesting prospect with respect to future applications of these materials is the possibility of generating a ‘switchable’ framework in which the guest adsorption can be modulated by virtue of the redox state of the framework. Herein, we report a closely related metal–organic framework (MOF), [Zn₂(DSNDI)], where the redox state of the electroactive ligand DSNDI⁴⁻ (N,N′-bis(4-carboxy-3-hydroxyphenyl)-1,4,5,8-naphthalenetetracarboxydiimide) can be modulated using both in situ and ex situ methods. The material is stable to both chemical and electrochemical reduction of its organic DSNDI units without structural collapse or degradation, and remains porous to N₂, H₂ and CO₂. The chemically reduced frameworks demonstrate notable changes from the parent material, particularly with respect to CO₂ binding. The CO₂ isosteric heat of adsorption (Q_st) increases from 27.9 kJ mol⁻¹ in the neutral material to 43.9 kJ mol⁻¹ after reduction due to enhanced host–guest interactions, the latter being comparable to the best performing member of the MOF-74 series, [Mg₂(dobdc)]. The origin of the enhanced interaction was probed using Density Functional Theory (DFT) calculations which corroborated the presence of enhanced framework-CO₂ interactions for the chemically-reduced MOF. The tuneable CO₂ binding character provides an attractive mechanism to tune gas separations and capture.