The Limonene Biorefinery: From Extractive Technologies to Its Catalytic Upgrading into p-Cymene

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.

Hydrothermal Carbonization as Sustainable Process for the Complete Upgrading of Orange Peel Waste into Value-Added Chemicals and Bio-Carbon Materials

“…The distribution of products depends on the process conditions and on the feedstock used, but in general 50-80% of the original biomass is present in the solid product, 5-20% in the aqueous phase which contains inorganic and organic matter, and 2-5% in the gas phase which is mainly composed of CO 2 [12]. Hydrothermal methods are largely used in petroleum-based refineries and are getting more and more attention from both scientific and industrial researchers since their process parameters can be easily translated into modern biorefineries aimed at the upgrading of lignocellulosic residues and wastes [13]. With respect to other (bio)refinery processes, HT protocols present several advantages in terms of sustainability since they generally adopt mild reaction conditions (e.g., temperature and pressure) without any homogeneous or heterogeneous catalysts and in the presence of water as a green reaction solvent (used as such or in combination with simple aliphatic alcohols) [13].…”

Dehydroisomerisation of α-Pinene and Limonene to p-Cymene over Silica-Supported ZnO in the Gas Phase

“…Dehydroisomerisation of limonene to p-cymene. p-Cymene can also be produced by dehydroisomerisation of limonene occurring through double bond migration on acid sites followed by dehydrogenation on metal or oxo-metal sites (Scheme 2), the latter probably being the rate-limiting step [20][21][22][23][24][25]. This reaction occurs easier than the α-pinene-to-p-Cymene conversion because it does not include C-C bond breaking.…”

“…This reaction occurs easier than the α-pinene-to-p-Cymene conversion because it does not include C-C bond breaking. A number of heterogeneous catalysts have been reported for this reaction in liquid and gas phases [20][21][22][23][24][25], including Ti/SBA-15 (liquid phase, 160 • C, 56% p-Cymene yield) [21], Pd/HZSM-5 (liquid phase, 260 • C, 8 bar pressure, 82% yield) [22], Pd/Al 2 O 3 (supercritical EtOH, 300 • C, 65 bar pressure, 80% yield) [23], TiO 2 (gas phase, 300 • C, 90% yield) [24], Pd/SiO 2 (gas phase, 300 • C, in H 2 flow, 99% yield) [25] and others [20]. Pd/SiO 2 gives the highest yield of p-Cymene in the gas phase, however, it requires continuous H 2 supply to prevent catalyst deactivation [25] as in the case of α-pinene dehydroisomerisation [1].…”

Development of an Optimized Method to Obtain a Limonene‐Rich Concentrate from the Discarded Lemon Peels

“…[3] It has been also recommended as alternative solvent to petroleum-based solvents. [4] Its antiviral property was also observed in disease-causing pathogenic microbes such as Herpes simplex and influenza virus (Gupta et al, 2021). Its action against Covid-19 also demonstrates that it might be a candidate antiviral agent if confirmed by further studies.…”