Catalytic CO₂ Hydration and Sequestration by Carbonic Anhydrase-Mimetic Nanotubes

Promotion of the conversion of CO₂ into HCO₃^- is necessary for the efficient sequestration of CO₂ into metal carbonate minerals, a promising CO₂ utilization strategy. Carbonic anhydrase (CA), a ubiquitous enzyme, exhibits high activity in CO₂ hydration and suffers from inherent instability as a biological disadvantage. Consequently, there is a demand for a CA-mimetic catalyst that is stable at high temperatures and can be easily prepared. In this study, we have developed a new CA-mimetic nanotube (CMNT) catalyst, demonstrating remarkable catalytic activity in CO₂ hydration. The CMNT catalyst is easily prepared by the layer-by-layer (LbL) method, which involves the sequential infusion of polyelectrolyte and polypeptide solutions into a track-etched polymeric membrane. Multiple catalytic Zn-histidyl imidazole complexes were built on the inner channel of nanotubes, on which CO₂ hydration takes place. CMNT follows Michaelis-Menten kinetics with a catalytic activity (k_cat/K_M) of 8.2 M^-1 s^-1 at pH 7.5, comparable to those of other CA-mimetic catalysts. Remarkably, CMNT maintains its catalytic activity at 70 °C, a temperature that destabilizes CA. Sequestration of CO₂ into calcites was demonstrated with extensive characterization of CMNT and catalytic activity. Moreover, the CMNT-integrated membranes could be applied to continuous CO₂ hydration. This work underlines the straightforward preparation of a CA-mimetic catalyst using an established technique, offering the potential for future modification or incorporation of additional catalytic functions.