Preparation of acetic acid from crop residues via hydrolysis coupled with oxidation in subcritical water

In this study, a green and effective method for the production of acetic acid from crop residues via a two-step process (hydrolysis followed by oxidation) in subcritical water was proposed. For the first step, CO₂ is used to catalyze the hydrolysis of crop residues to obtain intermediates. Next, the intermediates were oxidized in the presence of O₂ to obtain acetic acid. The effects of crop residue type on the yield of acetic acid were first studied. Among all the crop residues selected, the corn cobs provided the highest acetic acid yield, which reached 16.70 wt% at 270 °C for 20 min; 0.11 mol CO₂/corn cob (g) was added for the first step, and 250 °C was added for 50 min, with 0.15 mol O₂/corn cob (g) added for the second step. By employing corn cobs as feedstock, the effects of the corn cob/water mass ratio, temperature, time, and CO₂ loading in the first step and the effects of temperature, time, and O₂ loading in the second step on the yield of acetic acid were examined. Among all the parameters examined, the corn cob/water mass ratio had the greatest influence on the acetic acid yield. The highest acetic acid yield of 25.80 wt% was obtained at a corn cob/water mass ratio of 0.0016:1. Under fixed reaction conditions, recycling the product mixture can increase the concentration of acetic acid in the final product and reduce the cost of subsequent separation and purification.