Abstract
In this work, we have explored the catalytic activity of Keggin-type heteropolyanions PMo12−nVnO40(3+n)− (n = 0, 1, 2, or 3) in the form of sodium salts in green oxidation routes of terpene alcohols with hydrogen peroxide. Nerol was the model molecule selected to assess the impacts of the main reaction parameters, such as temperature, catalyst load, and stoichiometry of reactants. The impacts of the presence of vanadium at different proportions (i.e., V1, V2, and V3 loads/per anion) in the structure of phosphomolybdate catalysts were assessed. All the catalysts were characterized by various techniques such as powder X-ray diffraction, attenuated diffuse reflectance infrared spectroscopy, ultraviolet-visible spectroscopy, thermogravimetric analysis, isotherms of adsorption–desorption of N2 measurements of surface area, scanning electronic microscopy, energy-dispersive X-ray spectroscopy, and n-butylamine potentiometric titration. Among the catalysts assessed, Na4PMo11VO40 was the most active and selective toward epoxides. The efficiency of this catalyst in the epoxidation of different terpene alcohols was investigated. Special attention was dedicated to correlating the composition and properties of the vanadium-doped phosphomolybdic catalysts with their catalytic activity.
Highlights
To address this demand and make the oxidation reactions more benign environmentally, the green and inexpensive oxidants molecular oxygen or hydrogen peroxide have been used in many catalytic processes to oxidize terpenic alcohols, generating water as the only by-product.[7,8,9]
In the initial catalytic tests, we have studied the activity of pristine phosphomolybdic acid, and their undoped and vanadium-doped sodium salts in the oxidation reactions of nerol, the selected model molecule (Fig. 11)
When one vanadium atom was introduced in the Keggin phosphomolybdate anion, there was a signi cant enhancement in the conversion and selectivity toward epoxide of oxidation reaction of nerol (Fig. 12)
Summary
The development of catalysts that can achieve more selective and environmentally friendly oxidation routes of terpenic compounds has received attention due to economic and environmental reasons.[1,2] Terpenic alcohols are an abundant natural origin feedstock and occur in many plants; they are relevant platform molecules to produce key intermediates for the perfumery, avoring, ne chemicals, and pharmaceutical industries, being used as ingredients for the formulation of cosmetics and household products.[3,4] The oxidation of terpenic alcohol is a synthetic route of interest, leading to the formation of valuable compounds such epoxides, through epoxidation of the ole nic double bond, or carbonylic compounds, a er the oxidation of hydroxyl groups.[5,6] most of the oxidative processes that are industrially used still consume hazardous metal stoichiometric oxidants, which should be disposed of into the environment a er use. Cs+ and Na+ cations lacunar HPA salts were successfully used as catalysts in the oxidation of terpenic compounds with hydrogen peroxide.[35,36,37] The second modi cation is to ll the vacancy of lacunar HPA salts with a transition metal cation.[38] Metal doped phosphotungstic acid salts were effective catalysts in the oxidation reactions of benzylic and terpenic alcohols.[39,40,41] Another modi cation that may improve the activity of Keggin HPAs is to exchange one or more molybdenum atoms by vanadium in the primary structure of catalysts.[42] Such modi cation accelerates the steps of oxidation–reduction, which results in the improvement in activity and selectivity in oxidation reactions.[43,44,45,46]. As far as we know, vanadium-containing sodium phosphomolybdates salts were not used yet as catalysts in the oxidation reaction of terpenic alcohols
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