Efficient two-step production of biobased plasticizers: dehydration-hydrogenation of citric acid followed by Fischer esterification

Abstract

We report the production of biobased plasticizers starting from citric acid (CA) by a two-step process comprising dehydration-hydrogenation of CA followed by a Fischer esterification. The use of citric acid based plasticizers is well-known in PVC. However, citrate esters tend to leach out of the PVC material over time. This problem is currently tackled by acetylating the tertiary hydroxyl group of CA via complex and environment polluting processes. Our alternative strategy consists in the removal of the tertiary hydroxyl group, resulting in propane-1,2,3-tricarboxylic acid (PTA). First, dehydration reactions of a large amount of CA (20 mmol) were performed at relatively mild reaction conditions (150 °C and 20 bar H₂) using Al₂(SO₄)₃ as a homogeneous catalyst and water as a green solvent. The catalytic system was proven to be robust in time and in the presence of other organic functionalities (e.g. amino acids with different functional groups, diacids). In a second step, the reaction mixture was transferred to a Dean–Stark setup in order to perform a Fischer esterification with n-butanol, during which the previously used Al₂(SO₄)₃ could be recuperated. This resulted in an overall yield of 90% tributyl propane-1,2,3-tricarboxylate (TBPTC), which can be used as a plasticizer in PVC.