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Zr–Zeolite Beta: A New Heterogeneous Catalyst System for the Highly Selective Cascade Transformation of Citral to (±)‐Menthol
Author(s) -
Nie Yuntong,
Jaenicke Stephan,
Chuah GaikKhuan
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200801776
Subject(s) - citral , catalysis , zeolite , citronellal , citronellol , bifunctional , chemistry , rhodium , menthol , organic chemistry , chromatography , essential oil , geraniol
Minty green : Zr–zeolite beta (Zr‐beta) directs the one‐pot catalytic cascade transformation of citral to menthols with high diastereoselectivity. The solid catalyst, a bifunctional Ni/Zr‐beta or a composite Zr‐beta–Ni/MCM‐41 system, is easily recovered and reused in this green synthetic method (see figure).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.