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 degree s C) samples, respectively. The T-max shifts to lower temperatures, i. e. 380-400 degrees C, during the second reduction (of the oxide obtain ed from the oxidation of the first reduced Ni-4A samples). After the s econd reduction, the Ni-4A samples did not show sharp XRD peaks charac teristic of nickel metal, which confirms the absence of migration of n ickel to the external surface of the zeolite. At higher nickel loading s, the reduction step favours partial structure collapse of zeolite 4A , rather than promotion of the surface migration of nickel.