Nickel ion-exchanged zeolite-4A: reduction behaviour of nickel and structure stability

Abstract

The structural stability of ion-exchanged Ni-4A samples prepared at 25 and 90 °C was examined by thermogravimetric, X-ray diffraction (XRD), degree of reduction and temperature-programmed reduction (TPR) studies. The crystallinity of the zeolite-4A framework decreased with an increase in the nickel loading and the temperature of ion-exchange. The collapse of the structure is more significant in the reduced Ni-4A samples. The TPR profile of the fresh Ni-4A samples showed a broad reduction peak with a T_max at high temperatures, i.e. 480–500 °C and 440–450 °C for Ni-4A (25 °C) and Ni-4A (90 °C) samples, respectively. The T_max shifts to lower temperatures, i.e. 380–400 °C, during the second reduction (of the oxide obtained from the oxidation of the first reduced Ni-4A samples). After the second reduction, the Ni-4A samples did not show sharp XRD peaks characteristic of nickel metal, which confirms the absence of migration of nickel to the external surface of the zeolite. At higher nickel loadings, the reduction step favours partial structure collapse of zeolite 4A, rather than promotion of the surface migration of nickel.