Camphor synthesis through the metal-doped phosphotungstic acid lacunar salt-catalyzed borneol green oxidation with hydrogen peroxide

Abstract

A new green oxidation procedure was developed using metal-doped cesium phosphotungstate lacunar salts as catalysts and hydrogen peroxide as an oxidant for the conversion of borneol to camphor. A series of metal-doped cesium undecaphosphotungstate lacunar salts with the general formula Cs_7−xPW₁₁M^x+O₃₉ (M^x+ = Zn²⁺, Cu²⁺, Fe³⁺, Co²⁺, Mn²⁺, and Cu²⁺) were synthesized and characterized by FT-IR, Raman, and DRX analyses and potentiometric titration. Among the salts evaluated, zinc-doped cesium undecaphosphotungstate was the most active and selective toward camphor formation from borneol. Its highest activity among the salts was attributed to its greater Lewis and Pearson acidity. The main reaction parameters were assessed. Based on experimental data, a reaction pathway was proposed. Borneol oxidation was carried out over a solid catalyst, providing camphor in a 95% yield. This environmentally friendly route uses a green oxidant and gives water as a byproduct. The solid catalyst can be easily recovered and reused without loss of activity.