Regular growth of MOF crystals in restricted networks to prepare chitosan/polyvinylpyrrolidone/ZIF-8 gel beads for adsorbing CO2

Abstract

Chitosan (CS) and polyvinylpyrrolidone (PVP) were crosslinked by glutaraldehyde and Zn²⁺ to fabricate polymer networks. Subsequently, CS/PVP sols were dropped into an alkaline solution, resulting in deionization of hydrophilic groups and contraction of polymer networks to form CS/PVP beads with restricted networks. ZIF-8 crystals were incubated in restricted networks to prepare ZIF-8@CS/PVP gel beads (CPZ_xB). The growth of ZIF-8 crystals in the restricted networks showed a more regular morphology. Meanwhile, the size of ZIF-8 crystals in beads was modulated by the molar ratio of 2-MeIM to Zn²⁺. Specifically, the size of ZIF-8 crystals decreased with an increase in the ratio of 2-MeIM to Zn²⁺. Additionally, the equilibrium adsorption capacity of beads for CO₂ was 3.13 mmol g⁻¹ under an initial pressure of 400 kPa at 288.15 K, and increased by 23% compared to ZIF-8 crystals. The adsorption mechanism was explored using the kinetic model and Langmuir model, and the adsorption thermodynamic parameters were calculated using the van't Hoff equation. The results indicated that the adsorption of CO₂ on gel beads primarily depended on chemisorption and was a spontaneous exothermic process. As a result, growing MOF crystals in confined networks can be an effective strategy to obtain more regular morphology.