H₂ effect in Cr/PNP-catalyzed ethylene tetramerization: a DFT study

Fangqian Yin, Tingchun Zhu, Bin Li, and Le Zhang*

Hebei Key Laboratory of Heterocyclic Compounds Handan University Handan 056005, China E-mail: josephgo@sina.com

Supporting information for this article is given via a link at the end of the document.

Abstract: It is well noticed that hydrogen promotes catalyst activity in Cr/PNP-catalyzed ethylene tetramerization, but the mechanism of this boost is unclear. A density functional theory (DFT) study devoted to exploring this effect was conducted, and conformation changes were carefully taken into consideration to build a clear reaction pathway. Three components in the catalytic cycle was examined in detail: the production of 1-hexene from the metallacycloheptane, the production of 1-octene from metallacyclononane, and the formation of active center on the catalyst. The result indicates that the formation of active center on the catalyst becomes more favorable upon imposition of hydrogen, where hydrogen function as a second ligand. This easing effect could be the key factor leading to the outperformed catalyst activity.

Keywords: ethylene oligomerization • Cr-PNP catalyst • DFT calculation • Hydrogen promoting effect • active center

Introduction

Driven by the emerging polyethylene, particularly the linear low-density polyethylene (LLDPE) market, the demand for α -olefin proliferate rapidly, especially the low carbon number components such as 1-butene, 1-hexene, and 1-octene^[1]. Compared to the fast development of selective ethylene trimerization^[2]. which first commercialized as early as 2003 by Chevron Phillips Chemical Company LP^[3], the selective ethylene tetramerization catalyst still suffer from relatively low activity and selectivity^[4]. The first selective ethylene tetramerization catalyst that has the potential to be commercialized was reported by Sasol at 2004^[5], using methylaluminoxane(MAO)-activated Cr/PNP catalyst system to give a 1-octene selectivity up to 70%, which is also the most studied catalyst system up till today. It was noticed then that addition of 2.5 bar of hydrogen to the system alleviated the polyethylene formation without much compromise on selectivity and productivity, but the mechanism is unclear. Further investigation regarding this hydrogen promoting effect remained scarce until 2016 when Jiang et al. ^[6] confirmed that there is a significant improvement of PNP/Cr(III)/MAO catalyst activity on ethylene tetramerization under hydrogen pressure ranging from 0.03 to 0.50 Mpa. A DFT study of hydrogen effect on the Chevron-Phillips trimerization system was conducted by Bahri-Laleh et al. at 2017^[7], in which hydrogen is believed to participate into the reaction and leads to the formation of alkanes. However, ethylene tetramerization experiments conducted by Jianget al. showed little alkane product in the presence of hydrogen, indicating further investigation is required. Liu et al. ^[8] investigated the effects of hydrogen in PNP/Cr catalytic system using DFT analysis at 2018, in which he proposed that the increase of catalyst activity under hydrogen addition is due to the lowered energy barrier to form metallacyclopentane. However, the energy required for the oxidation addition from quartet bis(ethylene)Cr complex to quartet heterocyclic Cr complex actually increases from 6.6 kcal/mol to 7.0 kcal/mol upon addition of hydrogen, seemingly insufficient to account for the significant increase of catalyst activity. Hydrogen effect on the following reactions leading to the product was also investigated where hydrogen function solely as a second ligand. Based on these previous studies, here we compare competitive reactions from metallacycloalkanes where hydrogen either participate into the reaction to produce alkene, or function as a second coordinated ligand, with conformation changes taken into consideration, to further explore how hydrogen might have affect the ethylene oligomerization process using DFT calculation. We also explore possible hydrogen effect outside the catalytic cycle, namely the formation of catalyst active center to gain the whole picture.