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 Cr2O72− pillars is thought to contribute to high isosteric enthalpy of adsorption (Qst) 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.