The Effect of Noncatalytic Cations on the Activity and Selectivity of Nickel-Exchanged X Zeolites for Propene Oligomerization

A series of alkali metal or alkaline earth-exchanged NiX (X=faujasite zeolite, Si/Al=1.2) zeolites containing approximately 0.6wt% Ni as Ni2+ cations were examined as catalysts for propene oligomerization at 453K and 5bar (500kPa). In the presence of propene, the activity of alkali metal-exchanged zeolites (NiLiX, NiNaX, and NiKX) increased with time on stream, reached a maximum, then decreased, and finally reached steady state. In contrast, the activity of alkaline earth-exchanged zeolites (NiMgX, NiCaX, and NiSrX) was high initially and then decreased until steady-state activity was achieved. The primary product formed in all cases was hexene (90%), and nonene was the only other product observed (10%). Both the rate of propene dimerization and the ratio of branched to linear hexene isomers were determined to depend on the identity of the charge-compensating cation, with the dimerization rate and degree of dimer branching increasing with increasing free volume in the zeolite supercages. The apparent activation energy for trimer formation was greater than that for dimer formation, which suggests that steric constraints imposed by the zeolite may inhibit the growth of larger oligomers.