Zeolitic imidazolate frameworks with different organic ligands as carriers for Carbonic anhydrase immobilization to promote the absorption of CO2 into tertiary amine solution

Abstract

Carbonic anhydrase (CA) enzyme-based absorption processes represent a promising approach to flue gas CO2 capture owing to their rapid absorption rates and low energy penalties. However, CA enzymes exhibit poor stability and activity under typical flue gas conditions, thus constraining their industrial utilization. Enzyme immobilization has been proposed as a potential means of improving associated stability and activity levels, thus overcoming these limitations. Herein, a variety of zeolitic imidazolate frameworks including ZIF-8, ZIF-11, and ZIF-90 with a range of organic ligands were synthesized as adsorption-based carriers for CA enzyme immobilization. In a series of experiments, CA immobilized on ZIF-8 (CA/ZIF-8) was found to retain higher activity levels that were approximately 86% of those of the free enzyme, as compared to 44% and 16% for CA/ZIF-11 and CA/ZIF-90, respectively. Immobilized CA and associated carrier frameworks were characterized via XRD, SEM, and BET. Variations in CA enzyme secondary structural characteristics in the context of immobilization were monitored via FTIR and CD to explore the molecular basis for variable activity levels associated with different carriers, revealing more hydrophilic materials to be conducive to the preservation of enzymatic activity. In addition, carrier precursor selection should take both hydrophilicity and the potential inhibitory effects of functional groups on the CA enzyme into account.