Zr-zeolite beta: A new heterogeneous catalyst system for the highly selective cascade transformation of citral to (±)-menthol

Abstract

The transformation of citral to menthols involves hydrogénation steps as well as cyclisation of the intermediate, citronellal. The ability of Zrzeolite beta to catalyse the cyclisation with high diastereoselectivity to (±)-isopulegol is the critical step in this cascade transformation. Bifunctional catalysts containing nickel or rhodium supported on Zr-zeolite beta gave menthols in yields of 87-89 % and an excellent diastereoselectivity of 94% for the desired (±)-menthol. Dual catalyst systems of Zr-zeolite beta and nano-dispersed Ni on an MCM-41 support were equally effective and have the added advantage that the rates of the acid and hydrogenation-catalysed steps can be independently varied. By applying a pressure ramp of 0.2-2 MPa, the yield of menthols could be increased to 95 %, with 94 % diastereoselectivity for (±)-menthol. The low initial pressure minimises the rates of competing hydrogenation reactions to byproducts such as citronellol and 3,7-dimethyloctanol. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.