Effect of Boron-Doping on Gate-Opening CO2 Adsorption in Zinc-Benzimidazolate Coordination Networks

Flexible metal-organic frameworks (MOFs) have attracted much attention as selective gas adsorption and storage. This report describes boron doping in zeolitic imidazolate framework-7 (B-ZIF-7), which exhibits reversible phase transition during CO₂ adsorption/desorption. We have successfully prepared B-ZIF-7 coordination networks using boron-bridged benzimidazolate (B(bim)₄^-) as organic ligands. Powder X-ray diffraction (PXRD) measurements and infrared spectroscopy revealed that B-ZIF-7 has a crystal structure similar to that of ZIF-7 while containing boron bridging in the coordination network. Since B-ZIF-7 forms a cationic coordination network, the guest anions are encapsulated within the pore. CO₂ adsorption/desorption measurements at 300 K showed that B-ZIF-7(NO₃), which contains nitrate ions (NO₃^-) as guest anions in its pores, exhibits a S-shaped CO₂ adsorption/desorption isotherm, which is characteristic of gate-opening type MOFs. Compared with ZIF-7, B-ZIF-7(NO₃) has superior CO₂ adsorption capacity in the low-pressure and superior CO₂ storage capacity. The CO₂ adsorption and desorption behavior of B-ZIF-7(NO₃) was analyzed by in situ temperature-controlled PXRD measurements and thermogravimetric analysis under a CO₂ atmosphere, and a reversible phase transition was observed. We have also successfully prepared B-ZIF-7(Cl) and B-ZIF-7(OTf) (OTf = CF₃SO₃^-) with different guest anions. The CO₂ adsorption/desorption behaviors of B-ZIF-7(Cl) and B-ZIF-7(OTf) were significantly different from those of B-ZIF-7(NO₃) and ZIF-7. B-ZIF-7(Cl) showed gate opening at a higher pressure than ZIF-7, and B-ZIF-7(OTf) did not show S-shaped CO₂ adsorption isotherm and showed adsorption behavior in micropores. These results indicate that the CO₂ adsorption behavior of B-ZIF-7 depends on the interaction between the guest anions and CO₂ molecules or the cationic framework and the bulkiness of the guest anions. Boron doping in a coordination network with boron-bridged imidazolate ligands is a promising strategy to increase the gas adsorption capability of porous materials.