Post-synthetic modulation of the charge distribution in a metal–organic framework for optimal binding of carbon dioxide and sulfur dioxide

Abstract

Modulation of pore environment is an effective strategy to optimize guest binding in porous materials. We report the post-synthetic modification of the charge distribution in a charged metal-organic framework, MFM-305-CH₃, [Al(OH)(L)]Cl, [(H₂L)Cl = 3,5-dicarboxy-1-methylpyridinium chloride] and its effect on guest binding. MFM-305-CH₃ shows a distribution of cationic (methylpyridinium) and anionic (chloride) centers and can be modified to release free pyridyl N-centres by thermal demethylation of the 1-methylpyridinium moiety to give the neutral isostructural MFM-305. This leads simultaneously to enhanced adsorption capacities and selectivities (two parameters that often change in opposite directions) for CO₂ and SO₂ in MFM-305. The host-guest binding has been comprehensively investigated by <i>in situ</i> synchrotron X-ray and neutron powder diffraction, inelastic neutron scattering, synchrotron infrared and ²H NMR spectroscopy and theoretical modelling to reveal the binding domains of CO₂ and SO₂ in these materials. CO₂ and SO₂ binding in MFM-305-CH₃ is shown to occur <i>via</i> hydrogen bonding to the methyl and aromatic-CH groups, with a long range interaction to chloride for CO₂. In MFM-305 the hydroxyl, pyridyl and aromatic C-H groups bind CO₂ and SO₂ more effectively <i>via</i> hydrogen bonds and dipole interactions. Post-synthetic modification <i>via</i> dealkylation of the as-synthesised metal-organic framework is a powerful route to the synthesis of materials incorporating active polar groups that cannot be prepared directly.