Ultrasound-assisted conversion of tannic acid to gallic acid as a strategy to obtain value-added products

In this work, ultrasound was applied for the conversion of tannic acid into gallic acid using only diluted H2O2 as reagent. Experiments were carried out using several types of ultrasonic horns operating at 20 kHz (VC750W processor). The following experimental conditions were evaluated: H2O2 concentration (0.2 to 8.5 mol L−1), horn type (10 to 25 mm of diameter), ultrasound amplitude (20 to 70%), sonication time (10 to 45 min), tannic acid concentration (170 to 1360 mg L−1), and reaction temperature (50 to 90 °C). Gallic acid production was monitored with ultra-performance liquid chromatography with high-resolution time-of-flight mass spectrometry (UPLC-ToF-MS). The isolated gallic acid was confirmed with nuclear magnetic resonance (1H and 13C NMR). It is important to emphasize that this study was developed as a proof of concept to demonstrate the potential of ultrasound for tannic acid conversion into gallic acid using just diluted H2O2. Under selected conditions gallic acid production yield was 128 ± 4 mg g−1 of initial tannic acid (using 170 mg L−1 of tannic acid as starting material). Reaction time was set as 30 min, which was carried out using 1 mol L−1 H2O2 and ultrasound amplitude of 50% at 90 °C. At silent conditions (mechanical stirring, from 100 to 1000 rpm), gallic acid production was halved (less than 78 ± 4 mg g−1 of initial tannic acid).