(242e) Selective Ethylene Dimerization over Heterogenized Cobalt Ammine Complexes: Nature of Active Sites and Reaction Kinetics

The global linear alpha olefins (LAO) market is expected to exceed $12 billion by 2025, underlining its importance in the chemicals sector. Presently, commercial plants produce a wide array of LAOs (C_4-30) using ethylene oligomerization through full-range processes. However, a gap in demand growth between comonomer grade LAOs (C_4-8) and higher fraction LAOs (C_10-30) causes market volatility and supply chain pressures for C_4-8 LAOs. Additionally, the prevalent use of homogeneous catalytic routes in full-range processes leads to high operational costs, significant environmental impacts, and poor selectivity for comonomer grade LAOs. Therefore, there is strong interest in developing heterogeneous catalysts for producing C_4-8 LAOs on-demand through selective ethylene dimerization/oligomerization.

Herein, we report on the molecular spectroscopic characterization and differential reaction kinetics of a selective ethylene dimerization (and oligomerization) catalyst comprised of heterogenized cobalt ammine complexes (Co(NH₃)_x/support), as a function of varying the catalyst support (activated carbon vs. g-C₃N­₄ vs. BN vs. SiO₂ vs. Al₂O₃ vs. high SAR zeolites). We identify unique interaction between heterogenized Co(NH₃)_x molecular complexes and the activated carbon support, yielding catalytic sites that are active, selective, and stable. Evidence suggests that defects unique to the carbon support allow for the formation of low coordination catalytic sites during the thermal pre-treatment stage. On the contrary, when Co(NH₃)_­x­ complexes are heterogenized on non-carbon supports, they form sites that can activate ethylene, but unselectively, leading to coking and rapid deactivation. Differential reaction kinetics evince cobalt loading dependent properties including: apparent activation barrier ranging from +115 to – 40 kJmol^-1, and reaction orders in ethylene ranging from 2 to 1. Moreover, Constable- Cremer relationship was also observed as the cobalt loading was increased in the Co(NH₃)­_x/activated carbon catalysts. Collectively, our results suggest the possibility of the “compensation” phenomenon occurring during ethylene dimerization over these unique heterogenized catalysts.