Ordered Macro–Microporous ZIF-8 with Different Macropore Sizes and Their Stable Derivatives for Lipase Immobilization in Biodiesel Production

Abstract

Enzyme immobilization in hierarchical macro–microporous metal–organic frameworks (MOFs) is greatly desirable but challenging due to the poor stability of MOFs in a practical biocatalysis process. Herein, we prepared a series of single-crystalline ordered macro–microporous zeolitic imidazolate framework-8 (SOM-ZIF-8) with different macropore sizes of 180, 270, and 360 nm for lipase immobilization and investigated their performance in a practical biodiesel production process. Under an ethanol-assisted infiltration strategy, the potential of SOM-ZIF-8 for enzyme immobilization was unleashed from the nonwettability arising from its exceptional surface hydrophobicity, with a significant increase by 123.8% in the loading capacity. However, a fatty acid-induced complete decomposition of ZIF-8 in a nonaqueous system of biodiesel production was observed. To address this challenge, a degradation mechanism coupling the intermolecular autoionization of carboxylic acid was proposed, and a template-assisted pyrolysis approach was adopted to prepare the SOM-ZIF-8-derived carbon material that not only well preserved the 3D-ordered structure of the precursor but also exhibited enhanced chemical stability. Moreover, the carbon derivative exhibited great potential for enzyme immobilization, with a 22.9% increase in loading capacity, a 35.9% increase in specific activity, and a 12.7% increase in activity recovery, compared with SOM-ZIF-8. It is anticipated that this study will shed new light on the realistic design and modification of MOFs for the immobilization of enzymes in biocatalysis and extensive applications.