NICKEL ION-EXCHANGED ZEOLITE-4A - REDUCTION BEHAVIOR OF NICKEL AND STRUCTURE STABILITY

The structural stability of ion-exchanged Ni-4A samples prepared at 25 and 90 degrees 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 degrees C and 440-450 degrees C for Ni-4A (25 degrees C) and Ni-4A (90 degrees C) samples, respectively. The T-max shifts to lower temperatures, i.e. 380-400 degrees 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.