Heterogeneous nucleation and growth of MOF crystals on polymer substrate to fabricate chitosan/ZIF-8 hydrogels for efficient capture of CO2

Abstract

A chitosan/ZIF-8 hydrogel (CGsZx) was fabricated by in situ nucleation and growth of ZIF-8 crystals with a tunable morphology in chitosan hydrogel (CG) networks, and used as an ideal adsorbent for CO₂. The CG has heterogeneous nucleation sites and was then soaked sequentially in a methanolic solution of Zn(NO₃)₂·6H₂O and then a methanolic solution of 2-methylimidazole (2MeIM), and then combined with Zn²⁺ by forming a Zn–N coordination bond, and then the ZIF-8 crystals were formed on the CG. The growth and distribution of the ZIF-8 crystals in the CG network were achieved by regulating the molar ratio of Zn²⁺ to the glucosamine of the CS. In particular, the ZIF-8 crystals with spherical, cubic, tetrahedral, and cuboid shapes on the CGsZx were tuned by adjusting the molar ratio of Zn²⁺ to the glucosamine of CS. In addition, the ZIF-8 crystal with a petaloid morphology on the CG was obtained by using sodium tripolyphosphate (STPP) as a crosslinker. The synergistic effect of heterogeneous nucleation and coordination modulation were the main factors for the change of morphology and size of ZIF-8. The CGsZx exhibited a 441.7% adsorption capacity for CO₂ which was higher than that of CG, and 65.3% higher than that of ZIF-8. After recycling five times, the adsorption capacity of the composite for C₂ remained at 89.6%. The kinetics simulation indicates that the adsorption behaviour of CGsZx for CO₂ was physical adsorption.