Sustainable catalytic route to limonene from renewable 1,8-cineole: Fe–Al2O3 driven conversion and process optimisation

Abstract

Advancing sustainable catalytic routes for monoterpene production is critical for reducing dependence on the energy-intensive and resource-limited conventional synthesis of limonene. This study establishes a novel continuous-flow vapour-phase catalytic platform for the selective conversion of renewable eucalyptus-derived 1,8-cineole to limonene. A series of γ-Al₂O₃-supported transition-metal catalysts (Fe-, Cu-, Co- and Ni-based) were systematically evaluated to identify active site environments capable of directing endocyclic ether activation and controlled monoterpene rearrangement to selectively produce limonene. Among these, Fe–Al₂O₃ delivered the highest performance, achieving 78% 1,8-cineole conversion and 81% selectivity to limonene, attributed to highly dispersed, redox-active Fe species exhibiting strong metal–support interactions on γ-Al₂O₃, high surface area and a balanced distribution of weak-to-moderate Lewis-type acidity. A comprehensive process-optimisation study quantifying the effects of metal loading, catalyst amount, reaction temperature, feed flow rate and feed concentration identified temperature as the dominant control parameter, while lower metal loadings, reduced feed flow rates, and diluted feeds maximised limonene formation. Robust time-on-stream stability, catalyst reusability, and benchmarking against a previously reported system (Pd–Al₂O₃) confirmed the durability and performance of the optimised catalyst. Extensive structural characterisation clarified the key structure–activity relationships, linking textural properties, surface acidity profiles, and metal–support interactions to the observed selectivity patterns. This work establishes a sustainable materials framework for terpene upgrading, demonstrating how finely tuned surface acidity and structural features can be harnessed to enable efficient renewable molecular transformations.