Alkene oligomerization catalyzed by Ni
2+ 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 C
2H
4) 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 Ni
2+ (without observing Ni
+, NiO, or Ni
0) 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 intermediates
2. 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,
