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Effect of Ni Site Density on the Deactivation of Ni-Beta Zeolite Catalysts During Ethene Oligomerization
thesisposted on 13.04.2021, 14:51 authored by Arunima SaxenaArunima Saxena
This work studies ethene oligomerization on Ni-zeolites and is focused on understanding the effects of Ni active site density on experimentally measured activation and deactivation phenomena and oligomer product selectivity. Ethene oligomerization rates are measured at 453 K on Ni-Beta zeolites with varying isolated Ni site density (Si/Al = 12, Ni/Al = 0.06–0.25). Oligomerization rates (per Ni) are observed to have convoluted effects of activation and deactivation kinetics, which depend on Ni site density. The catalyst deactivation mechanism is hypothesized to occur through poisoning of Ni sites by higher molecular weight oligomers. For the sample of highest Ni density (Ni/Al = 0.25), the butene isomer selectivity at a given ethene conversion did not change through the course of deactivation (453 K, 0.8 kPa C2H4, 0.76–1.85 mol Ni s (mol C2H4)-1), indicating that the decrease in rate (per Ni) with time-on-stream reflects a decrease in site-contact time caused by non-selective deactivation of Ni active sites. However, for the sample of lowest Ni density (Ni/Al = 0.06), the butene isomer selectivity at a given ethene conversion varied with site-contact time (453 K, 0.8 kPa C2H4, 0.80–4.60 mol Ni s (mol C2H4)-1), indicating selective deactivation of distinct types of Ni sites. A quantitative approach for assessing deactivation is used to model the decrease in Ni sites in catalysts of different compositions and at different reaction conditions. Deactivation rates are second-order in Ni site density for the sample of highest Ni density (Ni/Al = 0.25), consistent with formation of Ni-alkyl-Ni intermediates that lead to deactivation. In contrast, deactivation rates are first-order in Ni site density for samples of lower Ni density (Ni/Al = 0.06, 0.09), implicating a single-site deactivation mechanism. Dihydrogen co-feed experiments (453 K, 0.4–0.8 kPa C2H4, and 10–100 kPa H2) are performed on the sample of lowest Ni site density (Ni/Al = 0.06) to eliminate the activation period, which has been previously attributed to conversion of Ni(II) cations to Ni(II)-H intermediates in the coordination-insertion mechanism. In the presence of H2, deactivation rates become second-order in Ni density, suggesting the participation of two Ni sites in deactivation. The transition from first- to second-order deactivation behavior is hypothesized to be caused by Ni aggregation, eventually forming metallic Ni that increases the rate of ethene hydrogenation in parallel.
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