Abstract
This study reports substantial improvement in the process for oxidising α-pinene, using environmentally friendly H2O2 at high atom economy (∼93%) and selectivity to α-pinene oxide (100%). The epoxidation of α-pinene with H2O2 was catalysed by tungsten-based polyoxometalates without any solvent. The variables in the screening parameters were temperatures (30–70 °C), oxidant amount (100–200 mol%), acid concentrations (0.02–0.09 M) and solvent types (i.e., 1,2-dichloroethane, toluene, p-cymene and acetonitrile). Screening the process parameters revealed that almost 100% selective epoxidation of α-pinene to α-pinene oxide was possible with negligible side product formation within a short reaction time (∼20 min), using process conditions of a 50 °C temperature in the absence of solvent and α-pinene/H2O2/catalyst molar ratio of 5 : 1 : 0.01. A kinetic investigation showed that the reaction was first-order for α-pinene and catalyst concentration, and a fractional order (∼0.5) for H2O2 concentration. The activation energy (Ea) for the epoxidation of α-pinene was ∼35 kJ mol−1. The advantages of the epoxidation reported here are that the reaction could be performed isothermally in an organic solvent-free environment to enhance the reaction rate, achieving nearly 100% selectivity to α-pinene oxide.
Highlights
Waste biomass-derived terpenes such as a-pinene are important sources of avours, fragrances and pharmaceutical applications.[1,2,3,4,5,6,7] a-Pinene is a central component of turpentine oil, a useful by-product of wood and paper industries
Following reagents were utilised in this study: tungstic acid sodium salt dihydrate (Na2WO4$2H2O), hydrogen peroxide (H2O2) (30% wt in H2O), phosphoric acid (H3PO4) 42.5%, sulphuric acid (H2SO4) 48.5%, 0.02 M potassium permanganate (KMNO4) solution (0.1 N), anhydrous sodium sulphate (Na2SO4, $99%), anhydrous sodium chloride (NaCl, $99%), a-pinene (98%), p-cymene (99%), acetonitrile (99%), toluene (99%), 1,2dichloroethane (99%), Adogen 464® phase transfer catalyst. aPinene oxide (97%), pinanediol (99%), trans-sobrerol (99%), campholenic aldehyde ($96%), verbenol (95%) and verbenone ($99%) were used for product identi cation
The yield of verbenol increases from 4–13% with an increase in oxidant amount from 125–200 mol%, which subsequently increases the yield of verbenone. These results are in accord with an existing study, which states that an increase in H2O2 concentration generates more radicals and favours the allylic oxidation route, which in turn oxidises verbenol to verbenone.[46]
Summary
Waste biomass-derived terpenes such as a-pinene are important sources of avours, fragrances and pharmaceutical applications.[1,2,3,4,5,6,7] a-Pinene is a central component of turpentine oil, a useful by-product of wood and paper industries. The sustainable utilisation of waste biomass-derived a-pinene is bene cial, as it decreases waste and maintains CO2 neutrality.[8] The oxidation of a-pinene generates many valuable products (Fig. 1), such as a-pinene oxide, verbenol and verbenone,[9,10] the last of which is an important intermediate in many processes and has many applications.[11] The isomerisation of a-pinene oxide into products for instance campholenic aldehyde is useful for the production of fragrances.[12]. A-Pinene oxide can rearrange to form campholenic aldehyde (4) and experience allylic oxidation to produce In the presence of H+ and H2O, the a-pinene oxide can undergo hydrolytic decomposition to form pinanediol (3) and sobrerol (5). a-Pinene oxide can rearrange to form campholenic aldehyde (4) and experience allylic oxidation to produce
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