Ni-H-Beta Catalysts for Ethylene Oligomerization: Impact of Parent Cation on Ni Loading, Speciation, and Siting

Abstract

Ni-H-Beta catalysts for ethylene oligomerization (EO) were prepared by ion exchange of NH4-Beta and H-Beta zeolites with aqueous Ni(NO3)2 and characterized by H2-temperature-programmed reduction (TPR), NH3-temperature-programmed desorption (TPD), and diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS). Quadruple exchange of NH4-Beta at 70 °C resulted in 2.5 wt.% Ni loading corresponding to a Ni2+/framework aluminum (FAl) molar ratio of 0.52. [NiOH]+ and H+ are the primary charge-compensating cations in the uncalcined catalyst, as evidenced by TPR and DRIFTS. Subsequent calcination at 550 °C in air yielded a Ni-H-Beta catalyst containing primarily bare Ni2+ ions bonded to framework oxygens. Quadruple exchange of H-Beta at 70 °C gave 2.0 wt.% Ni loading (Ni2+/FAl = 0.41). After calcination at 550 °C, the resulting Ni-H-Beta catalyst comprises a mixture of bare Ni2+ ions: [NiOH]+ and NiO species. The relative abundance of [NiOH]+ increases with the number of exchanges. In situ pretreatment at 500 °C in flowing He converted the [NiOH]+ species to bare Ni2+ ions via dehydration. The bare Ni2+ ions interact strongly with the Beta framework as evidenced by a perturbed antisymmetric T-O-T vibration at 945 cm−1. DRIFT spectra of CO adsorbed at 20 °C indicate that the Ni2+ ions occupy two distinct exchange positions. The results of EO testing at 225 °C and 11 bar (ethylene) suggested that the specific Ni2+ species initially presented (e.g., bare Ni2+, [NiOH]+) did not significantly affect the catalytic performance.

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

“…[50] By comparison, Ni impregnation results in the appearance of acid-site signals at approximately 550 °C, with some reduction in the lower-temperature acid sites. Some studies have attributed the latter result to Lewis acidity provided by isolated Ni particles, [17,51] Deconvolution of the TCD signals reveals that the formation of strong acid sites after nickel impregnation is accompanied by a reduction in the weak acid sites, therefore indicating that nickel interacts with the weak acid sites to form strong acid sites. This is further confirmed by the pyridine-FTIR spectra (which is a more accurate measure of acidity) in Figure 1b, where both the MFI and BEA frameworks exhibit a greater increase in the peak intensity of the Lewis acid sites (LAS) at 1440-1460 cm À 1 relative to the decrease in peak intensity for the Brønsted acid sites (BAS) at 1530-1560 cm À 1 upon impregnation with Ni (Table 1).…”

“…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.…”

“…[26][27] Moreover, since their initial proposal in 2013, [13] the Ni/BEA-based catalysts have been the focus of many studies. [12,[14][15][16][17][18][19][28][29] These are considered to function via a bi-functional model which assumes that Ni sites activate the ethylene to produce butene, and the acid sites contribute to further oligomerization and side reactions such as cracking via β-scission and isomerization. [1,30] The relatively wide beta zeolite pores overcome the frequent limitations of microporous materials by facilitating the diffusion of bulkier species, making this zeolite highly selective for producing liquid C 5 À C 12 hydrocarbons, [28,31] and more resistant to deactivation via coke deposition.…”

“…Moreover, it has been shown that Ni interacts differently under different acidities (in terms of the Si/Al ratio) even in the same type of zeolite [34] . The performance and activation of Ni on heterogeneous catalysts can be affected by parameters such as the presence of various Ni species (Ni 0 , Ni + , Ni 2+ , or NiOH + ), [16,26,35] the pore topology of the zeolite, the pretreatment conditions, [36–38] and the Ni incorporation method [1,17,39] . Isolated Ni 2+ species can be present as acid site exchanged positions or grafted on silanol groups.…”

“…[34] The performance and activation of Ni on heterogeneous catalysts can be affected by parameters such as the presence of various Ni species (Ni 0 , Ni + , Ni 2 + , or NiOH + ), [16,26,35] the pore topology of the zeolite, the pretreatment conditions, [36][37][38] and the Ni incorporation method. [1,17,39] Isolated Ni 2 + species can be present as acid site exchanged positions or grafted on silanol groups. The former, with a stronger Lewis acidity, deactivates very fast.…”