Browsing by Author "Alsharif, Aliyah"

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Selective dehydroisomerisation of cyclic monoterpenes to p-cymene over bifunctional metal-acid catalysts
(Saudi Digital Library, 2023-10-02) Alsharif, Aliyah; Kozhevnikov, Ivan
The dehydroisomerisation of α-pinene and limonene, naturally occurring abundant monoterpenes, is a clean sustainable route to produce p-cymene, an important intermediate in organic synthesis and ingredient in cosmetics and medicinal products. Currently, p-cymene is produced in a mixture with o- and m-isomers by the Friedel-Crafts alkylation of toluene with propene, followed by isomer separation, with an adverse effect on the environment. The aim of this work is to study the dehydroisomerisation of cyclic monoterpenes, such as α-pinene, β-pinene, limonene, α-terpinene, γ-terpinene and terpinolene, in the gas phase using silica-supported ZnO and CdO as new bifunctional metal-acid catalysts. It is demonstrated that all these monoterpenes can be converted to p-cymene with excellent yields of 90–100% using ZnO/SiO2 and CdO/SiO2 as the catalysts. The catalysts were prepared by wet impregnation of silica with metal nitrates from an aqueous solution followed by drying and calcination at 400–500 oC and characterised by BET, TGA, XRD, DRIFTS, H2-TPR and ICP-OES. The dehydroisomerisation reactions were carried out in a continuous flow fixed-bed microreactor with online GC analysis. It was found that dehydroisomerisation of α-pinene over ZnO/SiO2 produces p-cymene with 90% yield at 100% conversion at 370 oC and WHSV = 0.020 h-1. The reaction with limonene gives a 100% p-cymene yield at 325 oC and WHSV = 0.080 h-1. ZnO/SiO2 catalyst shows stable performance for over 70 h without co-feeding hydrogen to the reactor. The reaction over silica-supported ZnO catalysts with β-pinene produces a 100% p-cymene yield at 400 oC and WHSV = 0.080 h-1. The reaction with monocyclic terpenes such as limonene, α-terpinene, γ-terpinene and terpinolene gives a 100% p-cymene yield at 300–325 oC and WHSV = 0.16–0.08 h-1. The dehydroisomerisation of bicyclic monoterpenes, such as α-pinene and β-pinene, over CdO/SiO2 gives 91–95% p-cymene yields at 325–375 oC and WHSV = 0.010–0.020 h-1, whereas the more reactive monocyclic terpenes, such as limonene, α-terpinene, γ-terpinene, and terpinolene, give a 100% yield at 200–250 oC and WHSV = 0.040–0.080 h-1. This catalyst shows stable performance for over 25 h without co-feeding hydrogen. To the best of our knowledge, CdO/SiO2 has the highest efficiency in monoterpene-to-p-cymene dehydroisomerisation among the catalysts reported to date. The proposed mechanism of monoterpene dehydroisomerisation to p-cymene on bifunctional ZnO/SiO2 and CdO/SiO2 catalysts involves two steps: fast isomerisation of monoterpene reactant on Brønsted acid sites (silanol groups of silica support) to form p-menthadiene intermediates followed by their slow dehydrogenation on oxo-metal sites to p-cymene. The dehydrogenation is suggested to proceed through the abstraction of allylic hydrogen from the substrate by an oxo-metal site followed by the elimination of another hydrogen atom to form p-cymene π-bonded to metal ion (Zn(II) or Cd(II)). Then, the elimination of the p-cymene molecule and H2 closes the catalytic cycle.
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