(344n) Effects of Ni Site Density on Deactivation Rates and Mechanisms during Ethene Oligomerization on Ni-Beta Zeolites

Alkene oligomerization catalyzed by Ni²⁺ cations on zeolites proceeds via the coordination-insertion mechanism,¹ but rates decrease with time-on-stream due to deactivation^1,2 according to different mechanistic proposals^2,3. Deactivation phenomena are complicated by the presence of activation periods at initial times, and the behavior of residual Brønsted acid sites present. We synthesized Ni-Li-Beta materials with varying Ni density (Si/Al = 12, Ni/Al = 0.06-0.25), wherein Li⁺ replaces H⁺ sites. Ethene oligomerization rates (453 K, 0.01-1 kPa C₂H₄) were measured in a differential reactor, and combined with in situ X-ray absorption (XAS) to investigate Ni structure during deactivation. In situ XAS revealed that Ni is present as isolated Ni²⁺ (without observing Ni⁺, NiO, or Ni⁰) during deactivation, reflecting poisoning of Ni sites by heavier molecular weight intermediates. The molar selectivity toward oligomer products at fixed ethene conversion did not change as Ni-Li-Beta deactivated to different extents, indicating non-selective deactivation of each Ni site. Deactivation transients were modeled using an apparent n^th order deactivation model, where the order in Ni site concentration (n), the apparent deactivation constant (k_d), and the product formation rate are fitted parameters. Second-order deactivation rates are measured at high Ni density, suggesting two Ni sites are involved in the deactivation mechanism, as proposed via formation of Ni-alkyl-Ni intermediates². In contrast, first-order deactivation rates are measured at low Ni density, implicating a single-site deactivation mechanism. This quantitative approach allows modeling the loss in number of active Ni sites during deactivation of catalysts of different composition and at different reaction conditions.

References:

(1) Joshi, R.; Gounder, R. ACS Catal. 2018, 8,

(2) Mlinar, A. N.; Baur, G. B.; Getsoian, A. B.; Bell, A. T. Catal. 2012, 296, 156.

(3) Mlinar, A. N.; Shylesh, S.; Ho, O. C.; Bell, A. T. ACS Catal. 2014, 4,