Reversible CO2 Fixation and Release by a Trinuclear Zn(II) Cryptate Complex and Operando Analysis of the Complex Structure.

Abstract

Metal complexes inspired by carbonic anhydrase (CA), which is a metalloenzyme containing Zn(II), have been investigated as alternatives for CO2 fixation systems operating under ambient temperature and pressure conditions. In this study, we designed a trinuclear Zn(II) cryptate complex (Zn3L) and demonstrated rapid CO2 fixation with carbonation of CO2 using Zn3L. The CO2 fixation performance of Zn3L surpassed that of a standard CO2 absorbent (KOH(aq) solution) under conditions of the same solute concentration. In addition, the reaction achieved operation without support with addition of a base, which has been often required in systems of CA-inspired complexes. Fixed CO2 was released by protonating polyazacryptate ligand (L) and breaking the complex structure, and deprotonation of L induced the reconstruction of Zn3L, allowing it to refix CO2. This reaction mechanism was proposed based on the analysis of operando extended X-ray absorption fine structure spectroscopy. Zn3L also demonstrated the ability to capture dilute CO2 from air, and the volume of CO2 captured by Zn3L was approximately 2.6 times that captured by the KOH(aq) solution. Our Zn3L exhibited three valuable properties: rapid CO2 fixation without a base, reversibility, and ability to capture dilute CO2; thus Zn3L is a promising candidate as CO2 fixatives.