Nickel-zeolite
Y of various parent cation types, NiNa-A and NiNa-M were prepared. The
reducibilities of nickel ions in these zeolites were evaluated by
hydrogen consumption measurement. The nickel ions were reduced markedly
by hydrogen in NiCa-Y, NiSr-Y and NiBa-Y in contrast to that of NiNa-Y,
whereas the degrees of nickel reduction in NiH-Y and NiH-M were slight.
It was assumed that the reducibilities were mainly affected by both the
occupying sites of nickel ions and the acidic hydroxyl groups in
zeolites.
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 Tmax 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 Tmax 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.