Use of hydrotalcites as catalytic precursors of multimetallic mixed oxides. Application in the hydrogenation of acetylene

The effect of the Ni/Zn molar ratio on the activity, selectivity, and coke formation of NiO . ZnO . Al2O3 catalysts (modified with Fe3+ or Cr3+) durin g acetylene hydrogenation has been studied. Coke formation is decreased sig nificantly in the presence of ZnO, and a similar effect is also found when the catalysts are doped with Cr3+ instead of with Fe3+. An optimum Ni/Zn ra tio for activity, selectivity and coke formation performance has been found . The existence of this maximum implies the necessity of adding ZnO to the support in order to modulate the catalytic properties of Ni. Furthermore, i f the Ni concentration is increased, the conversion, selectivity, and yield to ethylene not only fails to increase, but actually decreases, while coke formation simultaneously increases. The existence of the above-mentioned o ptimum is the consequence of a minimum concentration of the hydrogenolytic (naked) metallic sites, the majority being hydrogenating metallic sites cov ered by a monolayer of ethylidines. A kinetic model of coke growth is propo sed assuming the existence of two types of coke associated with the hydroge nolitic and hydrogenating sites respectively. (C) 1999 Elsevier Science B.V . All rights reserved.