Transformation of α-limonene into p-cymene over oxide catalysts: A green chemistry approach

The transformation of α-limonene into p-cymene over oxide catalysts (γ-Al2O3, TiO2, SiO2, MCM-41) has been studied under H2 and He atmosphere in the temperature range 200–300 °C using a fixed bed micro-reactor working under atmospheric pressure. The catalysts were characterized using various physicochemical techniques (N2 adsorption–desorption, NH3-TPD, XRD, TPR, HR-TEM). We demonstrated that an almost complete transformation of α-limonene into p-cymene can be obtained at 300 °C over a high surface area anatase (Alfa Aesar) in helium atmosphere. The high surface area titania was also proved a very stable catalyst. The high catalytic performance of this oxide could be attributed to a good balance between the acidity and surface reducibility exhibited by this material. The aforementioned properties are, respectively, relevant to the isomerization and disproportionation/dehydrogenation steps involved in the mechanism of title reaction. Based on the yields obtained for p-cymene and by-products we proposed a mechanistic scheme for the title reaction over the high surface area titania.