Transformation of dl Limonene into Aromatic Compounds Using Supported Heteropolyacid Catalysts

Nickel and Cobalt Ilmenites-Based Catalysts for Upgrading Pyrolytic Oil during Pyrolysis of Waste Tires

Pyrolysis as a waste treatment method has gained relevance because it can generate higher value-added products in addition to reducing the environment’s secondary pollution. In this study, the catalytic pyrolysis of waste tires was evaluated using NiTiO3 and CoTiO3 ilmenites as catalysts and precursors of metal catalysts with the aim to produce an oil enriched in high-value hydrocarbons, such as benzene, toluene, a xylenes mixture, and products less-reported, such as p-cymene and p-cymenene. The experiments were performed in an analytical pyrolyzer coupled to GC/MS. The effect of the nature of the catalysts on the product distribution was compared with the uncatalyzed reaction. The main products of uncatalyzed pyrolysis were D, L-limonene (~60%), and isoprene (~25%) due to the depolymerization of natural rubber. Meanwhile, Ni-ilmenites-based catalysts favored the formation of target compounds to expense D, L-limonene. Moreover, the presence of metal in reduced-ilmenite sharply enhanced the selectivity by ~50% concerning oxidized ilmenite and above 80% compared to the uncatalyzed reaction for p-cymene and p-cymenene. By contrast, the Co-ilmenites-based catalysts showed a marginal effect on secondary reactions. Finally, the feasibility of forming the aromatic terpenes, p-cymene, and p-cymenene from limonene in the non-catalytic pyrolysis was evaluated.

Conversion of Limonene over Heterogeneous Catalysis: An Overview

The natural terpene limonene is widely found in nature. The (R)-limonene (the most abundant enantiomer) is present in the essential oils of lemon, orange, and other citrus fruits, while the (S)- limonene is found in peppermint and the racemate in turpentine oil. Limonene is a low-cost, low toxicity biodegradable terpene present in agricultural wastes derived from citrus peels. The products obtained from the conversion of limonene are valuable compounds widely used as additives for food, cosmetics, or pharmaceuticals. The conversion of limonene to produce different products has been the subject of intense research, mainly with the objective of improving catalytic systems. This review focused on the application of heterogeneous catalysts in the catalytic conversion of limonene.

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

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.

Waste Rubber Pyrolysis: Product Yields and Limonene Concentration

Tires, conveyor belts, floor mats, and shoe soles form a main-stream of rubber waste. The amount of these used materials continuously increases due to development of the rubber market. Therefore, pro-ecological utilization (i.e., energy recycling instead of burning) and recovering valuable and recyclable materials becomes an urgent necessity. In this regard, this work was devoted to the chemical recycling of selected used rubber products, and it especially explores the possibility of limonene production. Different types of waste rubber were characterized and pyrolyzed at microgram and laboratory scales, and the results were compared. Additionally, the pyrolysis of tires, the most significant stream of rubber waste, was also conducted in a semi-technical scale reactor. The effectiveness of limonene formation in the liquid fractions obtained from different types of waste rubber was compared.