Preparation of Ni/ZrO2-SO42- catalysts by incipient wetness method: effectof nickel on the isomerization of n-butane

A series of Ni-promoted sulfated zirconia catalysts with different nickel c oncentration (from 1 to 9.6 wt.%) were prepared by incipient wetness method . Ni and SO42- promoters were co-impregnated to a parent zirconium hydroxid e by a solution of Ni(NO3)(2) . 6H(2)O in H2SO4. After calcination at 948 K , the solids isomerized n-butane at 338 K. Up to 4.5 wt.% Ni content, nicke l increases activity of ZrO2-SO42-, afterwards, the catalytic activity decr eases. The temperature-programmed desorption of ammonia (TPD-NH3) and IR of pyridine adsorbed results show that enhanced activity cannot be completely explained in function of a higher acid strength. The increase of the isome rizating activity is better explained in terms of a bimolecular mechanism, as proposed by Guisnet et al. [M.R. Guisnet, Acc. Chem. Res. 23 (1990) 392] , involving olefins as intermediates. In this mechanism, Ni causes an enhan cement in the surface concentration of olefins. In spite of the relatively high Ni concentration, X-ray diffraction results showed no evidence of any NiO phase due to this oxide is well-dispersed on the surface of ZrO2-SO42- ill form of small particles. The inhibition of isomerizating properties of the catalysts when hydrogen was present in the reactor feed confirmed this bimolecular mechanism. Interestingly, unpromoted ZrO2-SO42- exhibited also the usual induction period observed on nickel-promoted sulfated zirconia ca talysts. Then, this bimolecular mechanism for the n-butane isomerization co uld also apply in the unpromoted zirconia sulfate catalyst. (C) 1999 Publis hed by Elsevier Science B.V. All rights reserved.