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

The transformation of a-limonene into p-cymene over oxide catalysts (gamma-Al2O3, TiO2, SiO2, MCM-41) has been studied under H-2 and He atmosphere in the temperature range 200-300 degrees C using a fixed bed micro-reactor working under atmospheric pressure. The catalysts were characterized using various physicochemical techniques (N-2 adsorption-desorption, NH3-TPD, XRD, TPR, HR-TEM). We demonstrated that an almost complete transformation of a-limonene into p-cymene can be obtained at 300 degrees 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 isomerizationand 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. (C) 2013 Elsevier B.V. All rights reserved.