Regio- and stereoselective terpene epoxidation using tungstate-exchanged takovites: a study of phase purity, takovite composition and stable catalytic activity

Abstract

Nanocrystalline tungstate-exchanged layered double hydroxides of the takovite type, [Ni_xAl_1−x(OH)₂](NO₃)_0.9−x(WO₄)_0.05·mH₂O, have been prepared with changing values of x. The morphology–texture parameters of the synthesized materials were characterized by using scanning electron microscopy and nitrogen adsorption techniques. X-Ray diffraction, diffuse reflectance spectroscopy, ²⁷Al MAS NMR and infrared spectroscopy were applied to study the structure and the phase purity of the tungstate exchanged materials. Identification of the tungsten state was attempted by infrared and Raman spectroscopy. The catalytic properties of the pure tungstate exchanged takovites were tested in the bromide-assisted epoxidation of terpenic olefins using H₂O₂ as environmental benign oxidant. The bromide-assisted oxidation shows very interesting chemo-selectivity for various substrates with unique regio- and stereo-selectivity often opposite to that of many traditional methods. The activity of the tungstate catalyst was observed to highly depend on the Al content in the takovite support, most likely due to efficient charge shielding at the catalyst’s surface. Tungstate exchanged on Al-rich takovite has a remarkable oxidation activity in terms of turnover frequency. Moreover, the takovite catalyst was found to be stable under reaction conditions and recyclable.