Transformation of limonene into p-cymene over acid activated natural mordenite utilizing atmospheric oxygen as a green oxidant: A novel mechanism

Production of Industrially Useful and Renewable p‐Cymene by Catalytic Dehydration and Isomerization of Perillyl Alcohol

“…However, these conditions resulted in accumulation of even more oligomers (99%). Previous studies that utilized heterogeneous catalysts such as zeolites and acid‐activated clays also encountered significant oligomerization, which was attributed to regions of strong Brønsted acidity on the catalytic surface (Du et al, 2005; Fernandes et al, 2007; Golets et al, 2015; Linnekoski et al, 2014; Lycourghiotis et al, 2018; Makarouni et al, 2018). Thus, we speculated that the strong Brønsted acidity of p TsOH (pKa −1.3; Berkowitz and Grunwald, 1961) favored oligomerization over dehydroisomerization, thereby resulting in production of oligomers as opposed to the production of 1 .…”

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

“…A 2-step mechanism was proposed by Makarouni et al starting from the limonene isomerization over activated natural mordenites and then its isomerization into p-cymene in a non-catalytic process, using atmospheric oxygen as a green oxidant (Fig 7) The acid treatment with sulfuric acid aqueous solutions of natural mordenite causes the removal of sodium oxide from its micropores, which drastically increases the specific surface and acidity, making natural mordenite very active in the catalytic transformation of limonene into p-cymene, and causing a significant enhancement in both the limonene conversion and in the amount of p-cymene obtained in the reaction mixture. A rather high p-cymene yield (63%) at 140 °C, with a limonene/catalyst ratio of 15 and a reaction time of 7 hours is obtained [35]. Another achievement of the same group was the use of aqueous solutions of various acids (CH3COOH, HCl, H2SO4, HNO3) to further improve the surface area and the acidity of natural mordenite with a significant increase in the conversion of limonene to p-cymene up to 65% [80].…”

“…Indeed, in an alternative process, p-cymene can be produced via isomerization or hydrogenation, as well as by the direct dehydrogenation of limonene [75]. Reports on the transformation of limonene into p-cymene by homogeneous catalysts are quite limited since low yields and separation drawbacks generally occur thus driving the interest towards heterogeneous systems (Table 2), able to ensure good performances and, at the same time, an easy separation from the post-reaction mixture [35,76].…”

“…In further detail, the FC alkylation is performed at high temperatures (200-450 °C) in the presence of AlCl3, BF3 or H2SO4 as catalysis, producing significant amounts of by-products (especially o-and m-cymene along with other multiple alkylation products) with an overall yield of p-cymene that usually does not exceed 50%. Finally, since the conventional p-cymene production occurs in a liquid phase, the separation of the reactants from the catalyst is an energy-intensive process [33][34][35]. Alternative methodologies for the production of p-cymene have been widely investigated including the use of renewable limonene also in consideration that a relatively low amount of p-cymene (4000 tonnes) is manufactured yearly [36][37][38][39][40].…”

“…The reaction mechanism of the two-step conversion of limonene into p-cymene. Reproduced with permission from Makarouni et al[35]. Copyright (2018) Elsevier ltd.…”