ELECTRON-SPIN-RESONANCE STUDIES OF ETHYLENE DIMERIZATION CATALYZED BY NICKEL SPECIES ON Y-ZEOLITES

Electron spin resonance studies of CaNi-Y zeolites reduced thermally in H2 indicate the formation of species A (g∥ = 2.824, g⊥ = 2.096) and C (g1 = 2.540, g2 = 2.314, g3 = 2.011) assigned to Ni+ ions in the β-cage and the α-cage, respectively, complexed with H2. Species C disappears completely and species A partially after evacuation of the sample at 100°C for 10 min. Simultaneously, a new species (g∥ = 2.151, g⊥ = 2.061), assigned to isolated Ni+ ions in the α-cage, is seen and gives species E (g1 = 2.637, g2 = 2.511, g3 = 1.968) rapidly due to coordination with ethylene on exposure of the sample to ethylene or butenes, suggesting that the Ni+ species migrate into the α-cage where the olefin molecules coordinate with them by replacing H2. Subsequently, various species B1 (g∥ = 2.109, g⊥ = 2.061), B2 (giso = 2.048), and B3 (g∥ = 1.965, g⊥ = 2.620) are observed with simultaneous butene formation. The catalytic activity for ethylene dimerization increases with the reduction of Ni2+ to Ni+ ions; however, it decreases under high-temperature reduction due to enhanced formation of inactive Ni0. The results suggest that Ni+ is the precursor for the reaction. © 1990 American Chemical Society.