Preparation of Ordered Macroporous ZIF-8-Derived Magnetic Carbon Materials and Its Application for Lipase Immobilization

Metal–organic framework materials (MOFs) and their derivatives are considered ideal immobilization carrier materials because of their large specific surface area, high porosity and excellent structural designability. Among them, ZIF-8 has great potential for immobilization of enzymes due to mild synthesis conditions, and good biocompatibility. However, conventional ZIF-8 crystals have poor separation and recovery efficiency due to their small pore size and poor acid stability, greatly limiting their application in enzyme immobilization and further application. Although the carbonization of ZIF-8 by pyrolysis has been shown to be one of the approaches that can enhance its chemical stability, this still does not effectively solve the problem of the difficulty of recycling. Herein, we developed a strategy of pre-carbonization immersion (immersion in aqueous FeSO₄ solution before carbonization) to synthesize ordered macroporous ZIF-8-derived carbon materials with stable ferromagnetism (denoted as CZ-x-M-y, where x denotes the carbonization temperature and y denotes the concentration of the impregnated FeSO₄ solution) and used them to immobilize lipases for biodiesel production. XRD analysis showed that the magnetic properties in the materials came from Fe₃C species. We found that the magnetic carbon materials obtained by carbonization at 600 °C showed the best immobilization effect, where CZ-600-M-0.3 (using 0.3 mol·L⁻¹ FeSO₄ aqueous solution to soak ZIF-8 and carbonized at 600 °C) had the highest enzyme loading of 183.04 mg·g⁻¹, which was 49.7% higher than that of the non-magnetic CZ-600. In addition, CZ-600-M-0.5 maintained the highest enzyme activity, which was 81.9% of the initial activity, after five batches of reuse. The stable magnetic support materials reported in this study have promising potential for the industrial application of immobilized lipase.