Zr-zeolite beta: a new heterogeneous catalyst system for the highly selective cascade transformation of citral to (+/-)-menthol.
The transformation of citral to menthols involves hydrogenation steps as well as cyclisation of the intermediate, citronellal. The ability of Zr-zeolite 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.
Nie
Yuntong
Y
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Kent Ridge, Singapore 117543.
Jaenicke
Stephan
S
Chuah
Gaik-Khuan
GK