Isobutyric and n-butyric acid production from H2 and CO2 by Clostridium luticellarii in a hollow fiber membrane biofilm reactor: Reactor operation and process modelling

Clostridium luticellarii is an acetogen uniquely capable of producing acetic, n-butyric and isobutyric acid from H₂ and CO₂. Since n-butyric and isobutyric acid are currently produced from petrochemically derived propylene, establishing a direct production route from CO₂ and H₂ could greatly improve their sustainability as platform chemicals while simultaneously capturing carbon. Here, we demonstrate for the first time the production of these acids from H₂ and CO₂ by C. luticellarii in a continuously operated hollow fiber membrane biofilm reactor (HFMBR). The HFMBR was operated for 80 days while gradually increasing the H₂:CO₂ ratio from 2.5 to 5.0. Concentrations of acetic, n-butyric and isobutyric acid of respectively 8.93 ± 0.22 g L⁻¹, 1.56 ± 0.04 g L⁻¹ and 1.51 ± 0.06 g L⁻¹ were achieved at a dilution rate of 0.25 d⁻¹. Subsequently, a holistic process model was developed to gain deeper insights into the performance of the system and how to optimize its operation. The model described the autotrophic production of the acids, as well as mass transfer of H₂ and CO₂ through the membrane, biofilm diffusion and convective movement of particulates, and biofilm growth and detachment. Calibration against experimental data confirmed the model accuracy in reflecting the production dynamics (NRMSE of products < 0.1). Scenario analyses highlighted the importance of biofilm thickness control to minimize pH gradients and revealed that increased membrane surface area and optimized dilution rates are critical in enhancing volumetric production rates and steering product selectivity. Overall, these combined experimental results and model-generated insights can inform future optimization of HFMBRs for the production of added-value products from H₂ and CO₂.