Enhancing selective CO2 adsorption via chemical reduction of a redox-active metal–organic framework

Chanel F Leong,Deanna M D'Alessandro,Cameron J Kepert,Aaron W Thornton,Peter Turner,Thomas B Faust,Pavel M Usov,Ravichandar Babarao

doi:10.1039/c3dt00083d

Abstract

A new microporous framework, Zn(NDC)(DPMBI) (where NDC = 2,7-naphthalene dicarboxylate and DPMBI = N,N'-di-(4-pyridylmethyl)-1,2,4,5-benzenetetracarboxydiimide), containing the redox-active benzenetetracarboxydiimide (also known as pyromellitic diimide) ligand core has been crystallographically characterised and exhibits a BET surface area of 608.2 ± 0.7 m(2) g(-1). The crystallinity of the material is retained upon chemical reduction with sodium naphthalenide (NaNp), which generates the monoradical anion of the pyromellitic diimide ligand in the framework Zn(NDC)(DPMBI)·Na(x) (where x represents the molar Na(+)/Zn(2+) ratio of 0.109, 0.233, 0.367 and 0.378 from ICP-AES), as determined by EPR, solid state Vis-NIR spectroelectrochemistry and UV-Vis-NIR spectroscopy. The CO2 uptake in the reduced materials relative to the neutral framework is enhanced up to a Na(+)/Zn(2+) molar ratio of 0.367; however, beyond this concentration the surface area and CO2 uptake decrease due to pore obstruction. The CO2 isosteric heat of adsorption (|Q(st)|) and CO2/N2 selectivity (S), obtained from pure gas adsorption isotherms and Ideal Adsorbed Solution Theory (IAST) calculations, are also maximised relative to the neutral framework at this concentration of the alkali metal counter-ion. The observed enhancement in the CO2 uptake, selectivity and isoteric heat of adsorption has been attributed to stronger interactions between CO2 and both the radical DPMBI ligand backbone and the occluded Na(+) ions.

Highlights

Metal–organic frameworks (MOFs) have been the subject of significant interest due to their promise in the fields of gas separation and storage, sensing, catalysis, guest exchange and drug delivery.[1]
The new porous framework Zn(NDC)(DPMBI) with a moderate surface area of 608.2 ± 0.7 m2 g−1 retains its crystallinity upon chemical reduction with NaNp
With increasing Na+ ion content, the CO2 uptake of the framework is enhanced up to a molar Na+/Zn2+ ratio of 0.367, where the CO2 adsorption is maximised at 1 bar; beyond this dopant concentration, the CO2 uptake is significantly lowered

Summary

Introduction

Metal–organic frameworks (MOFs) have been the subject of significant interest due to their promise in the fields of gas separation and storage, sensing, catalysis, guest exchange and drug delivery.[1]. Le Bail refinement of the PXRD pattern obtained from a bulk sample of the as-synthesised Zn(NDC)(DPMBI) framework at 298 K was in good agreement with the single crystal data, whereby the fitted unit cell parameters of a = 25.893(7), b = 14.470(4) and c = 20.672(7) Å are very similar to those determined from the single crystal X-ray measurement at 150 K.

Results

Conclusion