Highly Hydrophilic ZIF-8 as a Carbonic Anhydrase Mimetic Catalyst for Promoting CO2 Absorption

Abstract

Carbonic anhydrase (CA) is an enzyme with unparallele d catalytic activity in CO2 hydration. However, its high cost and low stability seriously restrict its industrial application in CO2 absorption. Constructing CA mimics is a promising strategy to overcome the above limitations. However, to date, a majority of mimics reported are suffering from laborious synthetic processes, harsh synthetic conditions, and costly ligand materials. Therefore, developing novel CA mimics with higher applicability remains a high priority. Due to the structural similarity between zeolite imidazolate framework (ZIF)-8 and natural carbonic anhydrase (CA), ZIF-8 may be considered as a potential CA-mimetic catalyst for CO2 absorption. To validate this assumption, ZIF-8 and hydrophilic ZIF-8-TA were synthesized and the results from subsequent CO2 absorption experiments indicated that the synthesized catalysts could significantly enhance CO2 absorption in water and common alkaline solutions. The addition of 1.0 wt % ZIF-8 in water and different alkaline solutions respectively increased the average CO2 absorption rate by 42.35 and 6.78–24.51%, while the addition of 1.0 wt % of ZIF-8-TA increased the CO2 absorption rate by 81.80 and 18.16–34.39%, respectively. Besides, both catalysts possessed excellent reusability that they retained high performance even after five cycles of absorption and regeneration. Through esterase experiments, it was confirmed that ZIF-8 and ZIF-8-TA exhibited higher CA-mimetic activity than many reported CA mimics. Through density functional theory (DFT) calculations, it was indicated that the catalytic CO2 hydration pathway of ZIF-8 primarily involves four steps, including nucleophilic attack, rearrangement, replacement, and deprotonation, which is similar to that of natural CA, while ZIF-8-TA does not undergo the rearrangement step. Moreover, the electron-donation effect and van der Waals interaction of tannic acid endow ZIF-8-TA with higher catalytic activity.