(683c) Tuning Activity and Selectivity of Light Olefin Oligomerization in Ni-Exchanged Zeolites

Oligomerization of C₂-C₅ olefins offers a means for reincorporating light olefins created during the production of transportation fuels from the cracking of petroleum or during Fischer-Tropsch synthesis back into the final gasoline, diesel, and jet fuel product. It has been previously shown that Ni-exchanged aluminosilicates, including zeolites, are active for light olefin oligomerization and exhibit greater than 97% selectivity to oligomers [1, 2]; however, the specific role of the support and starting olefin on the activity and product selectivity is unclear. To this end, we have synthesized a series of Ni-exchanged zeolites, including FAU, LTL, MFI and MOR, and examined them as light olefin oligomerization catalysts for C₂-C₄olefins. Ongoing work in our research group has shown that the starting olefin as well as the zeolite framework, Si/Al ratio, and co-cation of the zeolite influence the rate of olefin oligomerization and the degree of branching in the final product. For example, the oligomerization of ethene at 453 K and 5 bar pressure using a Ni-Na-X catalyst with 0.6 wt% Ni produces mainly linear and monobranched oligomers up to the tetramer. In contrast, propene oligomerization under the same conditions formed only dimers and trimers with a larger degree of product branching, including small amounts of dimethylbutene. In addition, changing the co-cation during propene oligomerization at 453 K and 5 bar using 0.6 Ni wt% Ni-M-X zeolites was found to drastically influence the degree of branching in the dimer product, from 32% in Ni-K-X to 63% in Ni-Sr-X. The results of these experiments are rationalized using a proposed reaction pathway developed for propene dimerization in Ni-Na-X zeolites that explains the activation, deactivation, and dimerization regimes that occur with time on stream [2]. The conclusions of this presentation will highlight considerations important to the design of Ni-based heterogeneous catalysts for use in selectively producing gasoline, jet, and diesel fuel from light olefins.

[1] J. Heveling, C.P. Nicolaides, M.S. Scurrell, Appl. Catal. A-Gen. 173 (1998) 1-9.

[2] A.N. Mlinar, G.B. Baur, G.G. Bong, A. Getsoian, A.T. Bell, in preperation