Detection of a new trigonal bipyramidal copper species in Cu–CaX zeolite by electron spin resonance and electron spin echo modulation analysis

Electron spin resonance and electron spin echo modulation analysis are used to identify the location and coordination of Cu2+ in hydrated and dehydrated CaX zeolite. A new, trigonal bipyramidally coordinated Cu2+ is found to be the dominant copper center in this zeolite. It is located in a six ring window (site SII) and coordinated to three six ring oxygens and two hydroxyls, one in the α cage and the other in the β cage of the zeolite. This Cu(Oz)3(OH)2 species is found to be stable for dehydration up to 180 °C. At higher temperatures the Cu2+ species seems to be reduced irreversibly. However, a Cu2+ species can be regenerated by oxidation and hydration but most of it is only trigonally coordinated.