Propene oligomerization over Ni-exchanged Na-X zeolites
The oligomerization of propene was investigated over a series of nickel ion-exchanged Na-X zeolites with varying Ni loadings. Catalyst characterization by temperature-programmed reduction, elemental analysis, and XANES indicates that all of the exchanged Ni is present as Ni2+ that charge-compensates two exchange sites. The selectivity to propene oligomers remained greater than 98% for all Ni-Na-X catalysts with dimers being the main product. In contrast, the activity of Ni-Na-X was determined to depend strongly on Ni loading. At low to intermediate Ni loadings, the catalyst activates, reaches a maximum activity, and then deactivates with further time on stream. The rates of activation and deactivation are functions of the Ni content in the zeolite and both increase with increasing Ni loading. Stable activity was achieved for low Ni loadings (<0.6 wt%) by rapidly activating and deactivating the catalyst in propene at elevated temperature. The rate of propene dimerization measured under steady state conditions is first order in propene and characterized by an activation energy of 45 kJ mol(-1). Activation of Ni-Na-X is attributed to migration of the Ni2+ cations from hexagonal prisms of the zeolite into the supercage where the cations form a catalytically active Ni2+-olefin complex. Deactivation is proposed to occur via the reaction of two nearby Ni-olefin complexes leading to the deactivation of both sites. A model for the dynamics of activation and deactivation and for the dimerization of propene to hexene is proposed. This model provides a satisfactory description of the effects of propene partial pressure and Ni loading on the rate of propene dimerization as a function of time on stream. (C) 2012 Elsevier Inc. All rights reserved.