Now, writing in Joule, Ung Lee and colleagues report a pilot-plant study for formic acid (FA) production from hydrogenated CO2 (C. Kim et al. Joule https://doi.org/10.1016/j.joule.2024.01.003; 2024). This study demonstrates optimization across several key elements of the production process. At the reactor level, consideration of crucial catalyst properties, such as catalytic efficiency, morphology, water solubility, thermal stability and large-scale resource availability, contributes to enhanced reactor performance while keeping the costs of the necessary raw materials low. Here, the authors employ a ruthenium (Ru) catalyst supported on a bipyridine–terephthalonitrile mixed-covalent triazine framework (known as Ru/bpyTNCTF). They optimize the selection of a suitable amine pair that allows for effective CO2 capture and conversion, opting for N-methylpyrrolidine (NMPI) as the reaction amine to capture CO2 and promote the hydrogenation reaction to produce formate, and N-butylimidazole (NBIM) as the separation amine that separates formate to further produce FA via trans-adduct formation. Furthermore, they refine the reactor operating conditions in terms of temperature, pressure and the H2/CO2 ratio to maximize CO2 conversion. In terms of process configuration, they developed a setup comprising a trickle-bed reactor followed by three sequential distillation columns. Residual bicarbonate is stripped in column one; NBIM is produced via trans-adduct formation in column two; and the FA product is obtained in column three. Material selection for the reactors and columns is also carefully considered: stainless steel (SUS316L) was chosen for most components and a commercial Zr-based material (Zr702) was chosen for the third column to mitigate reactor corrosion, given its resistance to FA corrosion, and relatively low cost.
Having carefully optimized the production process — selecting the ideal feeds, designing the trickle-bed reactor and three sequential distillation columns, carefully selecting the materials for both the column body and internal packing to mitigate corrosion, and fine-tuning the reactor operating conditions — the authors demonstrated a pilot plant with a production capacity of 10 kg per day of FA, maintaining stable operation for more than 100 hours. Through a rigorous techno-economic assessment and life-cycle analysis, this pilot plant showcased a 37% reduction in cost and 42% decline in global warming potential compared with conventional FA production processes. Moreover, with an overall process efficiency of 21%, the energy efficiency of this process rivals that of fuel-cell vehicles that use hydrogen as the fuel.
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