Preparation of Ni/ZrO₂-SO₄/^2- catalysts by incipient wetness method: Effect of nickel on the isomerization of n-butane

A series of Ni-promoted sulfated zirconia catalysts with different nickel concentration (from 1 to 9.6 wt.%) were prepared by incipient wetness method. Ni and SO₄^2- promoters were co-impregnated to a parent zirconium hydroxide by a solution of Ni(NO₃)₂ · 6H₂O in H₂SO₄. After calcination at 948 K, the solids isomerized n-butane at 338 K. Up to 4.5 wt.% Ni content, nickel increases activity of ZrO₂-SO₄^2-, afterwards, the catalytic activity decreases. 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 isomerizating 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 enhancement 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-SO₄^2- in 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- SO₄^2- exhibited also the usual induction period observed on nickel- promoted sulfated zirconia catalysts. Then, this bimolecular mechanism for the n-butane isomerization could also apply in the unpromoted zirconia sulfate catalyst.