Crystal engineering of a family of hybrid ultramicroporous materials based upon interpenetration and dichromate linkers

A new family of 2-fold interpenetrated primitive cubic (pcu) networks of formula [M(L)₂(Cr₂O₇)]_n (M = Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺; L = 4,4′-azopyridine), DICRO-3-M-i, has been synthesised and their structures, permanent porosity and gas sorption properties were comprehensively characterised. Molecular simulations indicate that CO₂ molecules occupy both of the two distinct ultramicropores that run through this isostructural series. The orientation of the Cr₂O₇^2- pillars is thought to contribute to high isosteric enthalpy of adsorption (Q_st) towards CO₂ and temperature programmed desorption experiments reveal that DICRO-3-Ni-i selectively adsorbs CO₂ from gas mixtures that simulate flue gas. Performance in this context is among the highest for physisorbents measured to date and these materials are readily regenerated at 50 °C.