Increasing awareness of the endocrine disrupting properties of ortho-phthalate plasticizers has led to serious restrictions on these compounds and a growing interest in safe alternatives. With the ongoing tendency to explore renewable resources for the production of chemicals, the potential of biobased plasticizers is increasingly researched. Here, the focus is on the production of tricarballylate plasticizers directly from the widely abundant and renewable citric acid. These tricarballylate plasticizers possess excellent plasticizing properties, comparable to or even better than those of traditional ortho-phthalates. Recently, a one-pot dehydration-hydrogenation of citric acid to tricarballylic acid was developed by applying a H-Beta zeolite together with small amounts of Pd/C in water as a solvent (Route 1). Although high yields of tricarballylic acid were obtained, citric acid also elicited Al leaching from the zeolite framework, resulting in loss of activity over time. In this work, Nb2O5.nH2O was found to be an equally performant but also a highly stable catalyst for the dehydration of citric acid. Interestingly, a strong involvement of the hydroxyl group of citric acid with the Nb2O5.nH2O surface was observed by 13C MAS NMR. Next, bifunctional Pd/Nb2O5.nH2O were synthesized via a low-temperature reduction method in propylene carbonate. By this approach, well-dispersed Pd-nanoparticles were obtained and the acid site density of the Nb2O5.nH2O support was preserved, which was evidenced by XAS, HAADF-STEM imaging and pyridine adsorption FTIR respectively. In presence of 0.6 wt% Pd/Nb2O5.nH2O, tricarballylic acid yields of &gt;90% were achieved over multiple runs. After esterifying tricarballylic acid, tricarballylate plasticizers are obtained. However, the esterification and dehydration-hydrogenation processes can also be reversed, which shifts the challenge to the dehydration-hydrogenation of citrate esters to tricarballylate esters (Route 2). To our delight, the developed Pd/Nb2O5.nH2O catalyst is also efficient to convert a variety of citrate esters to tricarballylate esters in high yields (up to 93%) in methyl cyclohexane as a solvent.

Development of highly stable, bifunctional Pd/Nb2O5.nH2O catalysts for the sustainable production of tricarballylate plasticizers from citric acid

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ACS Fall 2022

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