Selective Oxidation of Citronellol over Titanosilicate Catalysts

Daifallah Aldhayan,Květa Kalíková,Mohammed Rafi Shaik,Jan Přech,Mohammed Rafiq H Siddiqui

doi:10.3390/catal10111284

Daifallah Aldhayan, Květa Kalíková + Show 3 more

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https://doi.org/10.3390/catal10111284

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Abstract

Citronellol is one of the most widely used fragrances for bouquetting purposes and it is a starting material for synthesis of several other terpenoids. Nevertheless, few data have been reported on citronellol selective oxidation. Accordingly, we report our findings on the selective oxidation of citronellol with hydrogen peroxide using a set of titanosilicate catalysts with different morphologies and textural properties—conventional titanium silicalite 1 (TS-1), mesoporous TS-1, layered TS-1 and silica-titania pillared TS-1 and also studying the effect of the solvent used. Epoxidation of C6=C7 double bond was the main primary reaction in this system and trace signals of C5 allylic oxidation products were observed without formation of citronellal. Due to the presence of post-synthesis introduced additional Ti sites, the silica-titania pillared TS-1 (TS-1-PITi) provided the highest conversion among the tested catalysts; nevertheless, citronellol was oxidized over all the studied catalysts including conventional TS-1; therefore, showing that it penetrates even into MFI micropores (0.55 nm in diameter). When using acetonitrile as a solvent, the conversion was proportional to the titanium content of the catalyst. When studying the effect of the solvent, acetonitrile provided the highest epoxide selectivity (55%) while in methanol, 2-propanol and 1,4-dioxane, ring opening reactions caused epoxide decomposition.

Highlights

Terpenes and terpenoids are important natural substances derived from isoprene (2-methylbuta-1,3-diene)
We report on our findings on selective oxidation of citronellol with hydrogen peroxide using a set of hierarchical and layered titanium silicalite 1 (TS-1) catalysts
The aluminum-free titanosilicate catalyzed citronellol oxidation with hydrogen peroxide was investigated using a set of TS-1 catalysts with different morphologies

Summary

Introduction

Terpenes and terpenoids are important natural substances derived from isoprene (2-methylbuta-1,3-diene). Some of them are abundant and isolated from natural products, others are rare or difficult to isolate They exhibit a high structural variability and typically contain a number of multiple bonds and oxygen functional groups. Terpenoids find their applications as fragrances or flavors as well as drug and other fine chemicals building blocks [1]. Selective oxidation is one of the important transformations of the terpenoids, leading to epoxides, polyols and other compounds of industrial importance. These derivatives are reaction intermediates in several applications such as cosmetics, perfumes, food, dyestuffs, and pharmaceutical industries

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