Biocatalytic CO Hydration for Industrial CO Off-Gas to Formate

Abstract

The steel industry contributes approximately 7% of global CO₂ emissions (IEA, 2020), underscoring its significant role in greenhouse gas production. Valorizing industrial off-gases presents a viable strategy to mitigate emissions while generating valuable chemicals. Enzymatic catalysis for CO conversion has emerged as a compelling alternative to traditional metal catalysts due to its lower energy and resource demands. However, industrial adoption is constrained by challenges such as the high costs of enzyme immobilization matrices and purification processes. This study introduces a cost-effective whole-cell catalytic system for converting CO gas into formate. Operational parameters were optimized under physiochemical conditions relevant to industrial settings, and the long-term production of formate was evaluated. The system leverages O₂-tolerant carbon monoxide dehydrogenase (ChCODH-II) from Carboxydothermus hydrogenoformans and formate dehydrogenase (MeFDH-I) from Methylobacterium extorquens AM1, overexpressed in Escherichia coli BL21(DE3) and M. extorquens AM1, respectively. A 100 mL gas bubble-column reactor was utilized for whole-cell-based formate production under nearambienttemperature and pressure, employing a simulated gas mixture of CO and CO₂ in a 50:50 ratio. Key reaction parameters, including gas flow rate, pH, electron mediator concentration, and whole-cell catalyst ratios, were optimized. A 140-hour long-term performance test achieved a high formate concentration of 0.9 M in the reactor with continuous CO/CO₂ gas feeding. However, the study observed reduced productivity at higher formate concentrations due to mass transfer limitations and product inhibition. This work highlights the potential of biocatalysis to drive carbon neutrality in the steel industry, demonstrated by the successful implementation of the whole-cell catalyst at POSCO Steel in South Korea.