Stoichiometry-controlled two flexible interpenetrated frameworks: higher Co₂ uptake in a nanoscale counterpart supported by accelerated adsorption kinetics

Abstract

Here, we report the synthesis, structural characterizations, and gas storage properties of two new 2-fold interpenetrated 3D frameworks, {[Zn₂(bpdc)₂(azpy)]·2H₂O·2DMF}_n (1) and {[Zn₃(bpdc)₃(azpy)]·4H₂O·2DEF}_n (2) [bpdc = 4,4′-biphenyldicarboxylate; azpy = 4,4′-azobipyridine], obtained from the same set of organic linkers. Furthermore, 1 has been successfully miniaturized to nanoscale (MOF1N) of spherical morphology to study size dependent adsorption properties through a coordination modulation method. The two different SBUs, dinuclear paddle-wheel {Zn₂(COO)₄} for 1 and trinuclear {Zn₃(µ₂-OCO)₂(COO)₄ }for 2, direct the different network topologies of the frameworks that render different adsorption characteristics into the systems. Both of the frameworks show guest induced structural transformations as supported by PXRD studies. Adsorption studies of 1 and 2 show CO₂ selectivity over several other gases (such as N₂, H₂, O₂, and Ar) under identical experimental conditions. Interestingly, MOF1N exhibits significantly higher CO₂ storage capacity compared to bulk crystals of 1 and that can be attributed to the smaller diffusion barrier at the nanoscale that is supported by studies of adsorption kinetics in both states. Kinetic measurement based on water vapor adsorption clearly distinguishes between the rate of diffusion of bulk (1) and nanospheres (MOF1N). The respective kinetic rate constant (k, s–1) for MOF1N (k = 1.29 × 10–2 s–1) is found to be considerably higher than 1 (k = 7.1 × 10–3 s–1) as obtained from the linear driving force (LDF) model. This is the first account where a new interpenetrated MOF has been scaled down to nanoscale through a coordination modulation method, and their difference in gas uptake properties has been correlated through a higher rate of mass diffusion as obtained from kinetics of adsorption.