Identification of the structure of Ni active sites for ethylene oligomerization on an amorphous silica-alumina supported nickel catalyst

doi:10.1016/S1872-2067(21)63827-5

Chinese Journal of Catalysis

Volume 42, Issue 12, December 2021, Pages 2181-2188

https://doi.org/10.1016/S1872-2067(21)63827-5 Get rights and content

Abstract

The structure of Ni active sites supported on amorphous silica-alumina supports with different contents of Al₂O₃ loadings in relation to their activities in ethylene oligomerization were investigated. Two kinds of Ni sites were detected by in situ FTIR-CO and H₂-TPR experiments, that are Ni²⁺ cations as grafted on weak acidic silanols and Ni²⁺ cations at ion-exchange positions. The ethylene oligomerization activities of these Ni/ASA catalysts were found an ascending tendency as the Al₂O₃ loading decreased, which could be attributed to the enriched concentration of Ni²⁺ species on acidic silanols with a weaker interaction with the amorphous silica-alumina support. These Ni²⁺ species were more easily to be evolved into Ni⁺ species, which has been identified to be the active sites of ethylene oligomerization. Thus, it seems reasonable to conclude that Ni²⁺ species grafted on acidic silanols were the precursors of active sites.

Graphical Abstract

Introduction

Since the Shell Higher Olefin Process (SHOP) has been industrialized, nickel-based catalysts that being utilized in the field of ethylene polymerization has become a research hotspot at home and abroad [1, 2]. Johnson et al. [1] has reported that reducing the steric bulk of the diimine ligand of Ni(II) catalysts can reduce the molecular weight and degree of branching of polyethylene and even synthesize α-olefins. This discovery has spurred a rapid development of nickel-based catalysts for ethylene oligomerization. In recent years, researches on ethylene oligomerization over nickel-based catalysts have been continuously deepened, varying from homogeneous molecular complexes to heterogeneous supported components. Homogeneous nickel catalysts have stood out in that they display high activities, lifetimes and highly selective in controlling product distributions, albeit the catalysts are not easy to be recycled, and are sensitive toward polar functionalities arised from solvents or impurities. In this regard, nickel species supported on acidic porous aluminosilicates, such as mesoporous amorphous silica-alumina (ASA) [2, 3, 4, 5], Al-doped ordered mesoporous silicas [6, 7, 8, 9, 10, 11] and zeolites [12, 13, 14, 15], have attracted tremendous research interests, as they potentially offer a more sustainable alternative to the homogeneous process.

The catalytic performance of heterogeneous Ni catalysts could be evaluated by the amount of accessible active Ni sites. However, there still remains elusive in understanding the real active sites of Ni species. For instance, the presence of hydroxyls and other groups on the surface of silica-alumina (SA) supports may lead to a diversity in the formation of Ni specious with different heterogeneous environments after ion-exchanged procedures [16]. In these SA supported nickel catalysts, cationic Ni species occupying the exchange positions have been postulated as the active sites for the activation of ethylene [10, 17, 18]. However, it seems questionable to the authors who recently elucidated that isolated Ni²⁺ cations grafted on acidic silanols were the most likely active species in a working condition rather than the widely accepted ion-exchanged Ni²⁺ cations [19, 20]. In view of our recent study [21], monovalent nickel ions produced from the reduction of Ni²⁺ in a N₂-pretreated atmosphere, were identified as the active sites. As such, further determination on the evolution of Ni⁺ species from what kind of Ni²⁺ precursor is of particular importance.

Definitively solving these fundamental questions will be crucial to accelerate the discovery of improved nickel-based heterogeneous catalysts for ethylene oligomerization with better chance for industrial application. To this end, a group of Ni²⁺ ion-exchanged ASA catalysts with different contents of Al species in the supports were firstly fabricated and examined in the ethylene oligomerization reaction. With the combination of catalytic results and a set of in situ Fourier transform infrared CO adsorption (FTIR-CO) spectroscopies, the structure of Ni species during its precursor stage were determined. It is concluded that Ni²⁺ species grafted on acidic silanols act as the precursors of active sites, which can be evolved into Ni⁺ active sites more easily. C₂H₄-TPD and H₂-TPR experiments were further taken to derive a plausible reason for the superior performance of the Ni²⁺ species adsorbed on acidic silanols rather than that at exchange positions for ethylene oligomerization.

Section snippets

Chemicals

Chemicals were reagent-grade and were obtained from Aldrich Chemical Co., Milwaukee, WI. Deionized water was used in all experiments. All chemicals were used as received without further purification.

Catalyst preparation

ASA materials: All ASA materials were prepared by a co-precipitation method. In a typical synthesis, a required amount of Al(NO₃)₃·9H₂O was dissolved in deionized water to form an aqueous solution, with an Al³⁺ molar concentration of 0.2 mol L⁻¹. This solution was added dropwise (3 mL min⁻¹) into a

Elucidation of the active nickel sites in Ni/ASA catalysts

In view of one of our recent studies, monovalent nickel ions, obtained on a Ni/ASA catalyst as pretreated in a N₂ flow (denoted as N₂-Ni/ASA), were identified as the most likely active sites for ethylene oligomerization [21]. For the Ni/ASA sample pretreated in air (denoted as Air-Ni/ASA), there was almost no activity, which has been attributed to the low concentration of Ni⁺ species. To examine the differences of specific features between Ni/ASA catalysts as pretreated in N₂ and air, DRIFT

Conclusions

In summary, the effect of Al₂O₃ loading on the evolution of structure changes of Ni active sites and the resultant catalytic performance of Ni/ASA system in the ethylene oligomerization reaction was investigated. Two kinds of Ni sites were detected by in situ FTIR-CO and H₂-TPR experiments, i.e., Ni²⁺ species grafted on acidic silanols and ion-exchange positions. It was found that when the Al₂O₃ loading was increased from 1% to 15%, the amount of Ni²⁺ species at ion-exchange positions was

Electronic supporting information

Supporting information is available in the online version of this article.

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This work was supported by the National Key R&D Program of China (2016YFA0202804), The Strategic Priority Research Program of the Chinese Academy of Sciences (XDB36030200), the National Natural Science Foundation of China (21978286, 21925803), the Youth Innovation Promotion Association (CAS) and the China Postdoctoral Science Foundation (2018M631835).

Available online 10 September 2021

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ArticleIdentification of the structure of Ni active sites for ethylene oligomerization on an amorphous silica-alumina supported nickel catalyst

Abstract

Graphical Abstract

Introduction

Section snippets

Chemicals

Catalyst preparation

Elucidation of the active nickel sites in Ni/ASA catalysts

Conclusions

Electronic supporting information

App. Catal. A

App. Catal. A

J. Catal.

J. Catal.

J. Catal.

Chem. Eng. J.

App. Catal. A

App. Catal. A

App. Catal. A

Catal. Today

Catal. Today

J. Catal.

App. Catal. A

Microporous Mesoporous Mater.

J. Catal.

Microporous Mesoporous Mater.

J. Catal.

Article
Identification of the structure of Ni active sites for ethylene oligomerization on an amorphous silica-alumina supported nickel catalyst