Ethene Dimerization Mediated By the Coordination-Insertion Mechanism at Nickel Cations Supported Onto Molecular Sieves

Dimerization is an entry step in oligomerization and chain growth pathways to convert light alkenes derived from shale gas hydrocarbons into chemical intermediates and liquid fuels [1]. Nickel cations supported on various inorganic supports are reported to catalyze alkene dimerization in the absence of externally supplied activators or co-catalysts [2], which are often required in the case of homogeneous nickel-halide complexes [3]. The mechanistic origin of alkene dimerization on Ni sites located on heterogeneous aluminosilicate supports, however, has been ascribed to both coordination-insertion and metallacycle-based cycles. Conflicting conclusions appear to reflect attempts to reconcile observations that externally supplied activators or co-catalysts are not required to initiate the catalytic cycle (which is characteristic of the metallacycle mechanism), and that products are also formed from alkene reaction pathways occurring in parallel (e.g., oligomerization, isomerization) at H⁺ sites located on these supports.

Here, ethene dimerization (453 K) was studied in absence of externally supplied activators on Beta zeolites synthesized to contain exchanged Ni²⁺ cations, according to site balances determined by cation exchange, and to CO infrared, UV-Visible, and Ni K-edge X-ray absorption spectroscopies. The catalytic contributions of residual H⁺ sites on aluminosilicate supports were suppressed by selectively poisoning them with Li⁺ cations or NH₃ base titrants, or by weakening them using a zincosilicate support. Beta zeolites with only H⁺ sites form linear butene isomers (1-butene, cis-2-butene, trans-2-butene) in thermodynamically-equilibrated ratios, in addition to isobutene and products of subsequent oligomerization-cracking cycles; thus, isobutene formation serves as a kinetic marker for the presence of H⁺ sites. On the other hand, after residual H⁺ sites on Ni-zeolites deactivate during initial reaction times or when they are suppressed prior to reaction, linear butene isomers form in non-equilibrated ratios that are invariant with ethene site-time velocity, reflecting primary butene double-bond isomerization events catalyzed at Ni²⁺-derived active intermediates. Further, in situ X-ray absorption spectroscopy showed that Ni cations retain their 2+ oxidation state during ethene dimerization. Also, Ni-zeolites pretreated in oxidative environments (5 kPa O₂, 773 K) show transient activation periods during initial reaction times at dilute ethene pressures (<0.4 kPa), but not at higher ethene pressures (>0.4 kPa) or in the presence of co-fed hydrogen (5 kPa). This behavior is consistent with in situ ethene-assisted formation of Ni²⁺-H intermediates, which isotopically scramble H_­2-D₂ mixtures (453 K) and are quantified from surface H/D exchange reactions (453 K).

Taken together, these findings provide unambiguous evidence for the coordination-insertion mechanism as the dominant route for alkene dimerization at Ni²⁺ cations exchanged onto molecular sieves. This implies that kinetic factors determine the isomer distribution in alkene dimers, providing opportunities for altering product isomer selectivity based on changes to the coordination or confining environment around Ni centers.

References -

[1] C.P. Nicholas, Applications of light olefin oligomerization to the production of fuels and chemicals, App. Catal., A, 543 (2017) 82-97.

[2] A. Finiels, F. Fajula, V. Hulea, Nickel-based solid catalysts for ethylene oligomerization - a review, Catal. Sci. Technol., 4 (2014) 2412-2426.

[3] K.P. Bryliakov, A.A. Antonov, Recent progress of transition metal based catalysts for the selective dimerization of ethylene, Journal of Organometallic Chemistry, 867 (2018) 55-61.