Synthesis of Bio‐Based Methacrylic Acid by Decarboxylation of Itaconic Acid and Citric Acid Catalyzed by Solid Transition‐Metal Catalysts

Abstract

Methacrylic acid, an important monomer for the plastics industry, was obtained in high selectivity (up to 84%) by the decarboxylation of itaconic acid using heterogeneous catalysts based on Pd, Pt and Ru. The reaction takes place in water at 200-250 °C without any external added pressure, conditions significantly milder than those described previously for the same conversion with better yield and selectivity. A comprehensive study of the reaction parameters has been performed, and the isolation of methacrylic acid was achieved in 50% yield. The decarboxylation procedure is also applicable to citric acid, a more widely available bio-based feedstock, and leads to the production of methacrylic acid in one pot in 41% selectivity. Aconitic acid, the intermediate compound in the pathway from citric acid to itaconic acid was also used successfully as a substrate.

Highly selective one-step dehydration, decarboxylation and hydrogenation of citric acid to methylsuccinic acid

“…14 In addition, NaOH was added in order to reduce unwanted side reactions, such as hydration of the generated double bond. 13 In general, citric acid is very reactive; full conversions are readily obtained in all reactions. Scheme 1 gives an overview of the different products observed via 1 H-NMR.…”

“…Pd or Pt. 13 The catalyst support (at least in case of BaSO 4 ) appears to have no influence on the reaction (Table 1, entry 26). Via this first decarboxylation, itaconic acid ( 2 ) is formed (Scheme 2).…”

“…infra ). 13,15 In addition, in the absence of H 2 2-hydroxyisobutyric acid is formed with a selectivity of 12% (Table 1, entry 3). This side product most probably stems from the hydration of mesaconic or citraconic acid ( 3 , 4 ), followed by decarboxylation.…”

“…Pd/C, Pd/Al 2 O 3 or Pd/MgAl 2 O 4 ; see ESI†), 19,20 side or consecutive reactions are occurring somewhat more prominently. Examples are the further decarboxylation of itaconic acid and its isomers ( 2–4 ) on Pd/C or Pd/Al 2 O 3 (entries 4, 8), 13 or the hydrogenation to propane-1,2,3-tricarboxylic acid ( 10 ), even before the first decarboxylation can occur, as in the reaction with Pd/MgAl 2 O 4 (entry 6).…”

“…12 Sanders and co-workers recently reported the use of citric acid as a platform chemical for the synthesis of methacrylic acid. 13 The dehydration of citric acid was followed by a double decarboxylation, catalyzed by Pd/Al 2 O 3 ; this reaction sequence resulted however in a selectivity of only 41% at full conversion with 44% selectivity to volatile degradation products.…”