Pd/HAP Catalyzed Synthesis of Adipic Acid from 1,6‐Hexanediol under Aerial Base‐free Conditions

We explored Pd/hydroxyapatite (Pd/HAP) catalyst for the selective aerial oxidation of biomass derived 1,6-hexanediol (HDO) to adipic acid (AA) at 70 °C in water under base-free conditions using air as an oxidant, where the basicity of support played an important role in tuning the catalyst activity and to achieve high yields of adipic acid with the long-term stability of the Pd/HAP catalyst.

Abstract

Adipic acid (AA), an important precursor to produce nylon-6, nylon-6,6, and polyurethanes, can be synthesized from biomass-derived 1,6-hexanediol (HDO). Herein, we examined the selective oxidation of 1,6-hexanediol to adipic acid over Pd/support (support- Rice husk ash (RHA), TiO₂, C, ZnO, Al₂O₃, La₂O₃, Mg(OH)₂, and hydroxyapatite (HAP)) catalyst in water at 70 °C under base-free conditions. Pd/HAP exhibited high catalytic activity among the catalysts explored, affording adipic acid (AA) selectively in 89 % yield with complete conversion of HDO. Investigating these Pd/support catalysts and several control experiments inferred that the basicity of the HAP support concertedly with Pd nanoparticle participated in achieving efficient aerial oxidation of HDO to AA, and hence avoided the need for any additional base additives. Advantageously, Pd/HAP exhibited high activity for gram-scale reactions and long-term stability during reusing the catalyst for several cycles without any considerable loss in the catalytic activity.

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