(25a) Bio/Catalytic Synthesis of Methacrylic Acid

2-methylpropenoic acid, also known as methacrylic acid, is an organic acid with one carboxyl group as the main functional group. This organic acid is a significant monomer in the polymer industry, due to its application as a precursor in production of methyl methacrylate (MMA) and poly methyl methacrylate (PMMA). Polymers synthesized from methacrylic acid and methyl methacrylate are chemically resistant and transparent such that they can be used as glass substitutes and in optical fibers. Industrially, methacrylic acid is produced using the hazardous acetone cyanohydrin process involving large amounts of sulfuric acid.

In this work, methacrylic acid was synthesized from the bio-based substrates citric acid, itaconic acid, and 2-hydroxyisobutyric acid. Hydrotalcite, a solid base catalyst, was employed to form methacrylic acid through decarboxylation of itaconic acid and citric acid. The effect of varying temperatures, catalyst mass, residence times and substrate concentrations on methacrylic acid concentration and yield was determined. Comparing yields of methacrylic acid in each series of reactions resulted in finding an optimum substrate to catalyst mass ratio where methacrylic acid yield reaches to its highest value (9.6 g-substrate/g-catalyst and 22.7% yield for citric acid, 6.4 g/g and 22.9% yield for itaconic acid). Catalyst reusability experiments resulted higher methacrylic acid yields. 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 (71.48 %) was achieved at 275 °C and a 1 minute residence time using 2-hydroxyisobutyric acid and subcritical water.