Catalytic behavior of carbonic anhydrase enzyme immobilized onto nonporous silica nanoparticles for enhancing CO₂ absorption into a carbonate solution

Nonporous, nano-sized support materials can offer a large external surface area for enzyme immobilization and improve the activity of the immobilized enzyme by eliminating internal diffusion of a substrate. In this study, nonporous silica nanoparticles with different sizes were synthesized by a flame spray pyrolysis (FSP) method and used as support materials for immobilization of the carbonic anhydrase (CA) enzyme for a CO₂ capture application. The immobilization conditions, such as reaction time, pH, and initial enzyme concentration, were investigated to obtain both high enzyme loading and activity. The effects of temperature and pH on the immobilized enzyme's catalytic activity were investigated. Through immobilization, the CA enzymes remained active at the elevated temperature and pH conditions expected during CO₂ absorption from the flue gases. At 50°C and pH 10.5, the activities of the immobilized enzymes were about three times that of the free enzyme. Classic Danckwerts theory for absorption with reaction was applied to calculate the enzymatic kinetics for CO₂ absorption. The enzyme's thermal stability also was significantly improved by the immobilization. Compared to its free enzyme counterpart, the half-life time of the immobilized enzyme was increased by up to 4.4 times over a 30-day test period at 50°C.