The Role of Nickel and Brønsted Sites on Ethylene Oligomerization with Ni-H-Beta Catalysts

Abstract

The present work studies the adsorption of ethylene on Ni-H-Beta particles to unravel the roles of nickel and Brønsted sites in the catalytic oligomerization of ethylene. Three models (i.e., two based on the Cossee–Arlman mechanism and one based on the metallacycle mechanism) are examined in terms of the nature of the active sites and the adsorption mechanism involved in the ethylene coordination step. The results are consistent with the participation of two active sites in the formation of [Ni(II)-H]+ Cossee–Arlman centers and also suggest that ethylene dissociates upon adsorption on [Ni(II)-H]+ sites. Further characterization of Ni-H-Beta catalysts prepared at different nickel loadings and silica-to-alumina ratios reveals that highly dispersed Ni2+ exists on the catalyst surface and interacts with the catalyst’s lattice oxygen and free NiO crystals. At the same time, the kinetic results indicate that Brønsted sites may form isolated nickel-hydride ([Ni(II)-H]+) centers on the catalyst surface. In addition, the presence of residual, noncoordinated Ni2+ and Brønsted sites (not involved in the formation of [Ni(II)-H]+ sites) shows a reduced probability of the formation of nickel-hydride sites, hindering the conversion rate of ethylene. A mechanism for forming [Ni(II)-H]+ centers is proposed, involving ethylene adsorption over Ni2+ and a Brønsted site. This research has important implications for improving ethylene oligomerization processes over nickel-based heterogeneous catalysts.

Ethylene Oligomerization: Unraveling the Roles of Ni Sites, Acid Sites, and Zeolite Pore Topology through Continuous and Pulsed Reactions

“…Moreover, as with the uptake (Figure 5a), Figures 5b and c demonstrate that the residence times and variances reach a pseudo‐steady state after approximately pulse #15 for all catalysts. This agrees with Seufitelli et al ., [15] who also examined ethylene adsorption over an Ni/BEA catalyst and indicated the presence of two types of site (Ni and acid sites) on the catalyst surface.…”

“…Incorporating Ni into the zeolite results in the partial substitution of BAS with LAS formed by Ni‐acid sites [15] . This enables a new reaction pathway for the oligomerization of ethylene wherein the Ni sites and acid sites act as catalysts and co‐catalysts for the Cossee‐Arlman mechanism.…”

“…Incorporating Ni into the zeolite results in the partial substitution of BAS with LAS formed by Ni-acid sites. [15] This enables a new reaction pathway for the oligomerization of ethylene wherein the Ni sites and acid sites act as catalysts and co-catalysts for the Cossee-Arlman mechanism. Isolated Ni 2 + species can also be grafted on silanol groups with a slower deactivation rate than Ni 2 + in ion exchange positions.…”

“…Attempts to overcome these constraints have driven interest in gas-phase heterogeneous catalysis, wherein the zeolites have occupied a prominent place [5] due to their high thermal stability and tunable acidity and morphology. [6][7][8] Some of the most commonly reported zeolites for ethylene oligomerization are ZSM-5 [6,[9][10][11] and beta [11][12][13][14][15][16][17][18][19] zeolites due to their relatively low deactivation rates. [20][21] The ZSM-5 zeolite (MFI) contains 10-membered rings, giving it medium-sized pores intersecting straight and sinusoidal channels with cross-sections of 5.5 Å.…”

NOx and Hydrocarbon Trapping and Conversion in a Sequential Three-Zone Monolith: Spatiotemporal Features

“…Likewise, van den Berg et al provided infrared evidence that ethylene is readily polymerized on H-ZSM-5 at room temperature. A recent study investigated several mechanisms of ethylene oligomerization on the Ni/BEA catalyst and found that ethylene adsorbs over two sites, cationic Nickel and Brønsted acid sites (BAS). Protonation of ethylene occurs at BAS which leads to the formation of a primary carbonium ion Z –+ CH 2 CH 3 , the forerunner of oligomerization forming products such as 1-butene, 2-butene, and isohexene …”