Hydrogen-bonded networks and N2O/N2 adsorption separation performance of pyridinecarboxylate guanidinium HOFs
Efficient capture of the greenhouse gas nitrous oxide (N2O) is critical for climate change mitigation and resource recovery. In this study, two guanidinium-based hydrogen-bonded organic frameworks (HOFs) with pyridyl nitrogen site isomerism, namely G-5,5'-BPyDC and G-4,4'-BPyDC, were constructed using 2,2'-bipyridine-5,5'-dicarboxylic acid and 2,2'-bipyridine-4,4'-dicarboxylic acid as ligands. The effects of ligand structure on hydrogen-bonded network, pore environment, and N2O/N2 adsorption separation performance were systematically investigated via single-crystal X-ray diffraction, thermogravimetric analysis, Hirshfeld surface analysis, and gas adsorption experiments. Both frameworks are built via N-H...O hydrogen bonds. The asymmetric unit of G-5,5'-BPyDC contains two methanol molecules, resulting in larger free volume and surface area compared to G-4,4'-BPyDC, which exhibits more compact packing. Both materials show decomposition temperatures above 290°C, indicating good thermal stability. Hirshfeld surface analysis reveals that the total contribution of O-H/H-O and N-H/H-N hydrogen bonds in G-5,5'-BPyDC (32.0%) is higher than that in G-4,4'-BPyDC (29.7%). At 25°C and 4.0 MPa, the N2O adsorption capacity of G-5,5'-BPyDC is 2.32 mmol/g, surpassing that of G-4,4'-BPyDC (2.02 mmol/g). IAST calculations show that the selectivities of G-5,5'-BPyDC for N2O/N2 (50:50 and 10:90) mixtures reach 29.26 and 111.32, respectively, significantly superior to those of G-4,4'-BPyDC (6.61 and 17.03). Pyridyl nitrogen site isomerism effectively optimizes N2O/N2 adsorption and separation by modulating pore polarity and hydrogen-bonded network, offering a new strategy for isomer design.