Abstract
Limonene is a renewable cyclic monoterpene that is easily obtainable from citrus peel and it is commonly used as a nutraceutical ingredient, antibacterial, biopesticide and green extraction solvent as well as additive in healthcare, fragrance and food and beverage industries for its characteristic lemon-like smell. Indeed, the lack of toxicity makes limonene a promising bio-alternative for the development of a wide range of effective products in modern biorefineries. As a consequence, industrial demand largely exceeds supply by now. Limonene can be also used as starting substrate for the preparation of building block chemicals, including p-cymene that is an important intermediate in several industrial catalytic processes. In this contribution, after reviewing recent advances in the recovery of limonene from citrus peel and residues with particular attention to benign-by-design extractive processes, we focus on the latest results in its dehydrogenation to p-cymene via heterogeneous catalysis. Indeed, the latest reports evidence that the selective production of p-cymene still remains a scientific and technological challenge since, in order to drive the isomerization and dehydrogenation of limonene, an optimal balance between the catalyst nature/content and the reaction conditions is needed.
Highlights
Other important platform chemicals can be extracted from biomass-derived wastes and residues and used as biobased building blocks for the preparation of value-added intermediates, products, renewable energy and biofuels, supporting and slowly replacing the well-assessed technologies that gave a great contribution to these fields [6,7,8,9,10]
In recent years, coupling microwave-assisted extraction (MAE) with other technologies, such as microwave-assisted hydro distillation (MADH) [23] and microwave Steam Distillation (MSD) [14], is of particular interest due to the reduced time of the extraction process and its allowing the recovery of essential oil without causing any change in the composition of the oil gas
US was successfully used to intensify the extraction of d-limonene from the Citrus limetta peel by Khandare et al As evidenced in Figure 3, the ultrasoundassisted extraction (EAU) under optimized conditions allows yields to be achieved similar to those obtained by using Soxhlet, close to 100%, in very short extraction times, due to the physical and structural changes on the substrate surface and the consequent improvement of extraction efficiency [69]
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Other important platform chemicals can be extracted from biomass-derived wastes and residues and used as biobased building blocks for the preparation of value-added intermediates, products, renewable energy and biofuels, supporting and slowly replacing the well-assessed technologies that gave a great contribution to these fields [6,7,8,9,10] In this context, limonene (1-methyl-4-(1-methyl phenyl) cyclohexene), the main constituent of citrus essential oil (around 68–98% w/w) is industrially derived from the citrus industry from orange processing waste [11]. We highlight recent advances in the extraction of limonene from citrus processing waste with particular attention to benign-by-design extractive processes, and its catalytic dehydrogenation for the production of p-cymene
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