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.