Effects of the thermal treatments on dispersion, reducibility and activity of Ni/Al2O3 and Ni-Sn/Al2O3 catalysts

Abstract

Effects of the calcination and oxychirination on cyclohexane dehydrogenation activity of Ni/Al2O3 and Ni-SnAl2O3 catalysis were studied. Four Cl-free catalysts were prepared, two monometallic catalysts with 1.0 and 5.0% Ni by impregnation, and two bimetallic catalysts with 1.0%Ni-0.5%Sn and 5.0%Ni-2.5%Sn by step impregnation, both with Sn/Ni atomic ratio = 0.25. Calcination (in air) and oxychlorination (in air + CHCl3) treatments were carried out at 500°C for 1 h. The catalysts were characterized by temperature-programmed reduction (TPR). CO chemisorption al 22°C and oxygen titration at 400°C. The reaction was performed using a flow reactor al 300°C. All the catalysts presented very low Ni dispersions. The thermal Treatments, particularly the oxychlorination, caused a decrease in the dispersion and variations in the degree of reduction. These effects were dependent on the Ni content and the presence of Sn. The TPR profiles suggest the formation of bimetallic particles of Ni and Sn in the 5.ONi-2.5Sn catalyst subjected to calcination or oxychlorination. This catalyst showed a low activity, which was independent of the treatment. For the remaining catalysts. the oxychlorination produced more active catalysts than the calcination. E1ectronic effects of Sn on Ni explain the lower activity of bimetallic ensemblcs.

Keywords

• calcination
• oxychlorination
• Ni/Al2O3 catalysts
• Ni-SnAl2O3 catalysts
• dehydrogenation