Structural and catalytic properties of several potassium-doped nickel/α-alumina solids

Abstract

Studies of the chemical preparation, activation energies of reduction, temperature-programmed reduction (TPR), X-ray diffraction (XRD), X-ray photoelectron (XP) spectra and catalytic activities of several potassium-doped nickel/α-alumina catalysts have been carried out for the catalytic hydrogenation of hexanedinitrile, in a continuous process at 1 atm pressure, 443 K, and in the absence of ammonia. Activation energies of reduction for the potassium-doped non-stoichiometric NiO/α-alumina precursors were higher than those obtained for the potassium-free NiO/α-alumina and for the unsupported non-stoichiometric NiO. TPR results show that there is an inhibiting effect of α-alumina and potassium on NiO reduction. XRD measurements reveal the presence of α-alumina, NiO and Ni phases, the latter being present in larger quantities at higher reduction temperatures and lower potassium contents. XRD spectra also show fairly constant nickel particle sizes for all catalysts tested, XPS results confirm the inhibiting effect of potassium upon the reducibility of NiO. The XPS intensity ratios of surface Ni : NiO match the BET area ratios Ni : NiO of the samples. Conversions of the catalysed reactions decrease with increasing potassium content of the catalysts, probably because of poisoning of the latter with a Thorpe cycling formation of an enamine at high basic K₂O contents. Selectivities towards 6-aminohexanenitrile (which may reach 100% at 85% conversion, thereby of potential interest for the manufacture of nylon-6,6) increase with potassium content of the catalysts. The less selective catalysts also yield 1,6-diaminohexane and azacycloheptane. A lack of oligomerization reactions is observed, and a mechanism is proposed.