Synthesis of Methacrylic Acid by Catalytic Decarboxylation and Dehydration of Carboxylic Acids Using a Solid Base and Subcritical Water

Methacrylic acid was synthesized from the biobased substrates citric acid, itaconic acid, and 2-hydroxyisobutyric acid (2-HIBA). Hydrotalcite, a solid base catalyst, was employed to form methacrylic acid (MAA) through decarboxylation of itaconic acid and citric acid. The effect of temperature, catalyst mass, residence time, substrate concentration, and fermentation media, on carboxylic acid conversion and methacrylic acid yield was determined. Optimum MAA yields occurred at a substrate to catalyst mass ratio of 9.6 g-substrate/g-catalyst and 21% for citric acid and 6.4 g/g and 23% for itaconic acid (250 °C, 15 min). Catalyst reusability experiments resulted in higher methacrylic acid yields for both citric and itaconic acid. Methacrylic acid was also formed from 2-hydroxyisobutyric acid in a single-step dehydration reaction. Among these three substrates, the highest yield of methacrylic acid (71.5%) was achieved at 275 °C (1 min) using 2-HIBA and subcritical water. Finally, we tested conversion of these three acids in a simulated residual fermentation broth (0.1 M NaOH, 0.04 M Na₂SO₄, 0.04 M Na₂HPO₄, 0.06 M glucose, 0.12 wt % albumin) and MAA yields from itaconic acid and citric acid using hydrotalcite increased in the presence of these fermentation “impurities” and decreased slightly from 2-HIBA