Preparation of hierarchically porous PVP/ZIF-8 in supercritical CO2 by PVP-induced defect-formation method for high-efficiency gas adsorption

The fabrication of structural stable metal organic frameworks (MOFs) with large pore size by facile, rapid and environmentally friendly synthetic method remains a serious challenge. Herein, it is the first time to develop a novel synthetic strategy that rapidly enlarges the pore size of microporous ZIF-8 by employing polyvinyl pyrrolidone (PVP) as a modulator to induce defect formation in environmentally friendly supercritical CO2 (scCO2). Notably, the high solubility and low mass transfer resistance of reactants in scCO2 and low surface tension nature of scCO2 are the key factors for the rapid preparation of structural stable PVP/ZIF-8 with hierarchically pores. And the PVP modulator as a weak ligand not only facilitates pore size but also perturbs coordination to form coordination defects, exposing more coordination metal sites. These characteristics are beneficial to fast diffusion and adsorption of CO2 molecules. In addition, the influences of reaction conditions on the morphology, pore parameter and CO2 adsorption performance of PVP/ZIF-8 are investigated. The result indicates that the PVP/ZIF-8 prepared with 1:1 of Zn/PVP mass ratio at 90 °C and 30 MPa in supercritical environment have the highest CO2 adsorption equilibrium capacity (2.02 mmol CO2/g sorbent) and great cycle stability, which should be attributed to hierarchically porous structure with interconnected macropores and micropores.