From citrus waste to value: optimizing sulfonated carbons for limonene upcycling into value-added products†

Abstract

Limonene extracted from orange essential oil represents one of the globally prevalent and low-cost terpenes. Through the isomerization of limonene, it is possible to obtain a variety of high-added-value terpenic compounds such as terpinolene, α-terpinene, and γ-terpinene, as well as p-cymene. These products have diverse applications in the food, cosmetic, polymer, and chemical industries. The present study focused on developing a sustainable approach to producing valuable chemicals from renewable resources such as limonene, particularly via the isomerization of limonene over modified sulfonated carbons as bio-based catalysts. The synthesis of sulfonated carbons from glucose was optimized through a Central Composite Rotatable Design (CCRD), which enabled the identification of correlations between synthesis conditions and catalytic performance. Thus, sulfonated carbon catalysts with larger surface areas and smaller pore diameters lead to higher results in limonene isomerization. Various characterization techniques were employed to elucidate the physicochemical properties of the synthesized carbons, confirming the presence of acidic surface groups and showing the influence of textural characteristics on the limonene isomerization. After a 2 hour reaction at 150 °C, a 96% conversion of limonene was achieved, resulting in a good overall yield of the major products, ranging from 40% to 50%, namely α-terpinene, γ-terpinene, and terpinolene. The obtained findings highlight that the use of sulfonated carbons has the potential to drive the sustainable transformation of limonene into valuable compounds. In particular, the sustainability approach of this work includes (i) using a minimal amount of concentrated H₂SO₄ acid in the catalyst synthesis, (ii) employing bio-based and metal-free carbon catalysts, (iii) exploring a renewable substrate, and (iv) conducting the reaction process without added solvents.