Advanced Glycerol Oxidation to Formic Acid in a Multiphasic Jet Loop Reactor Using Polyoxometalate Catalysts

Glycerol is a common byproduct of commercial biodiesel production and can be used for the production of green platform chemicals such as biogenic formic acid (FA). Biogenic FA is industrially produced via selective catalytic oxidation in the OxFA process using various biomass in conventional stirred-tank reactors (STR). However, the reaction is limited by the low oxygen solubility in the aqueous reaction media that typically requires high oxygen pressures of 10-30 bar. This study aims to implement the multiphasic selective oxidation of glycerol to FA in a jet loop reactor (JLR), highlighting the economic and mass transfer advantages compared to the conventional used STR. The multiphasic approach was catalyzed by the homogeneous H₅PV₂Mo₁₀O₄₀ (HPA-2) polyoxometalate catalyst, already established in the commercial OxFA process. The reactor characterization indicates an efficient and high gas-liquid mass transfer, achieving volumetric mass transfer coefficient values (k _l · a values) ranging from 51 to 173 h^-1. Afterward, the multiphasic glycerol oxidation reaction to FA was implemented and compared in both reactor concepts under identical reaction conditions. The JLR achieved very high FA space-time-yields (STY) of up to 30.0 g_FA L_R ^-1 h^-1, highlighting its improved mass transfer and favorable economics already at 5 bar oxygen pressure in a simple glass setup. Determination of the kinetic parameters in the JLR resulted in reaction orders of 0.83 for glycerol and 0.54 for oxygen underlining the importance of an efficient gas-liquid mass transfer. Moreover, the activation energy was determined to be 78.3 kJ mol^-1, which is well in line with previous studies carried out for the OxFA process in a STR. The calculated Hatta number of 0.014 for the multiphasic glycerol oxidation in the JLR indicates that the reaction is in the kinetic regime already at low oxygen pressures of 5 bar demonstrating the high potential of the reactor concept for future studies.