RELATIONSHIP BETWEEN REDUCED NICKEL AND ACTIVITY FOR BENZENE HYDROGENATION ON NI-USY ZEOLITE CATALYSTS

Nickel-containing ultrastable Y zeolites (NixUSY, x = 0.76-9.01) have been prepared with variable amounts of nickel and tested for the reaction of benzene hydrogenation. All catalysts were prereduced at 673 and 873 K and then tested for the reaction in the temperature range 373-423 K, but only those with the highest nickel content appeared active. The hydrogen-prereduced catalysts have been characterized by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FT-IR) of chemisorbed carbon monoxide. Both techniques revealed that only the catalysts x = 4.98 and 9.01 show Ni0 clusters after reduction, in the other preparations with lower nickel contents, the Ni2+ ions appear to be strongly stabilized by the zeolite lattice. The large XPS Ni/Si intensity ratios found on the x = 4.98 and 9.01 zeolites indicate that the NiO clusters are located at the outer surface and the large cavities of the zeolite. This same finding was confirmed by the appearance of an infrared band at 2100 cm-1 of chemisorbed carbon monoxide on degassed zeolites. All data support the previous conclusion that the hydrogenation of benzene is proportional to the extent of nickel reduction, and hence to the number of reduced nickel atoms.