Origin of broad molecular weight distribution of polyethylene produced by Phillips-type silica-supported chromium catalyst

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Abstract

The origin of the broad molecular weight distribution of polyethylene produced by Phillips-type silica-supported chromium (Cr/SiO2) catalysts was studied by density functional calculations using active site models with various coordination environments. Difference of the coordination environment of chromium showed remarkable variations for both of the insertion and the chain transfer energies, resulting in a broad range of molecular weight from 102 to 1010 g/mol at 350 K. The results clarified that the special catalytic property of Cr/SiO2 for broad molecular weight distribution is attributed to the heterogeneity of the coordination environment of the chromium species.

Graphical abstract

Active site structures for broadening the molecular weight distribution of polyethylene by Phillips-type silica-supported chromium (Cr/SiO2) catalysts were disclosed by density functional calculations using active site models with various coordination environments.

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Highlights

► Properties of Cr/SiO2 were sensitive to the coordination environment. ► A ligand at an equatorial position led to a lower molecular weight. ► A ligand at an axial position led to a higher molecular weight.

Introduction

Phillips catalysts [1] composed of CrOx supported on SiO2 (Cr/SiO2) have long maintained their industrial importance after the discovery in 1950s in the polyolefin manufacture to produce nearly 7 million tons of a special grade of high density polyethylene (HDPE) per year over the world. The specialty of the Cr/SiO2 catalysts is an ability to produce HDPE with fine mechanical properties such as elasticity and impact resistance, and with superior moldability due to its high melt viscosity. They come from both broad molecular weight distribution (MWD) and adequate amounts of short and long chain branches, which are automatically produced in ethylene homopolymerization [2]. As finer controls of polymer structures with the Cr/SiO2 catalysts have been continuous demands for further multi-purpose materials, significant efforts still deposit obscurities in the mechanistic origin of the unique catalytic features.

The broad MWD has been believed to be mainly of a chemical origin, while some physical influences such as monomer concentration gradients within catalyst particles were also reported [3]. The chemical origin has been regarded as the heterogeneity of supported Cr species. Specifically, the conventional catalyst synthesized by an impregnation method makes a mixture of several types of potentially active Cr(VI) species and inactive Cr2O3 clusters [4], [5], [6], [7], [8], [9]. In past years, many researchers have tried to investigate structures of Cr(VI) species. Applying UV–vis diffuse reflectance, Raman and X-ray adsorption spectroscopies (XAS), Weckhuysen et al. [10] found mono- and dichromate with tetrahedral symmetry mainly present on the surface. Dichromate species are dominant among Cr(VI) at high Cr loadings and/or high calcination temperatures. However, since surface structures can be sensitive to the surface chemistry of oxide supports, the nuclearity of Cr(VI) is still under debate [11]. Furthermore, reduced Cr species, which are transient states between Cr(VI) and lower valence active species, exhibit a variation in the bonding and the interaction with surface oxygen. Such heterogeneity of coordination environment around Cr leads to a variety of symmetry and oxidation states of reduced Cr species. For example, UV–vis spectroscopy and XAS showed the presence of pseudo tetrahedral Cr(II), pseudo octahedral Cr(II) and pseudo octahedral Cr(III) on a reduced catalyst [12], [13]. A recent report by Gianolio et al. [14] showed a direct evidence of the coordination of surface siloxane ligands to reduced Cr species by the EXAFS, which is believed to be crucial for giving a variety of coordination environments around reduced Cr species. In an IR study by Zecchina et al. [15], [16], [17], three types of reduced species were identified (named as species A, B and C) through the adsorption of CO. Thus, Cr species on SiO2 can be roughly classified in terms of two kinds of heterogeneities: (1) nuclearity and (2) coordination environment.

Several Philips-type model catalysts with uniform Cr structures have been designed in order to reduce the above-mentioned heterogeneity. For example, Amor Nait Ajjou and Scott [18], [19], [20] supported dialkylated Cr(IV) mononuclear species on SiO2 by chemical vapor deposition (CVD) of CrNp4 (Np = Neopentyl). Ikeda and Monoi reported a model catalyst with monoalkylated Cr(III) mononuclear species supported on SiO2, using a molecular precursor of Cr(CH(SiMe3)2)3 [21]. Both of the model catalysts showed quite high activity for ethylene polymerization, and the MWDs of produced HDPEs were surprisingly very broad comparable with that obtained from impregnated catalysts, in spite of the almost uniform Cr nuclearity and oxidation state. It is believed that the heterogeneity of Cr species exists for these model catalysts due to the difference of the coordination environment around Cr species. On the other hand, Nenu et al. reported that an impregnated Cr/SiO2 catalyst behaved like a single-site catalyst, giving very narrow MWD close to 2, when 1,3,5-tribenzylhexahydro-1,3,5-triazine was added as a coordinative ligand to Cr species [13], [22], [23]. Obviously, all these results suggest much larger contribution of the coordination environment of Cr to broaden MWD than that of the nuclearity.

In this contribution, we present the first molecular-level investigation for influences of coordination environments of Cr on the MW of PE. The apparent free energies of activation for the ethylene insertion to growing chain and chain transfer (CT) to coordinated ethylene were calculated by density functional theoretical (DFT) calculation for various active site models. The results obtained here have disclosed active site structures of Cr for broadening MWD of PE, and have provided valuable insights in the catalytic properties of Cr/SiO2 catalysts as well as in molecular chemistry of these elemental reactions. To the best of our knowledge, DFT investigation on the origin of unique structures of produced PE by Phillips catalysts has never been undertaken so far.

Section snippets

Numerical details

All the DFT results were obtained using DMol3 as implemented in the Materials Studio (Version 4.2) [24]. The exchange-correlation functional of Perdew–Burke–Ernzerhof [25] (PBE), which is one of the most frequently used functional on the study of catalytic reactions on heterogeneous metal oxide catalysts, was employed. The basis set was the double numeric with d-polarization functions except for hydrogen (DND) combined with effective core potentials (ECP) [26], [27]. The real space cutoff

Results and discussion

Effects of the coordination environments on MW were mainly investigated for monoalkyl-Cr(III) and -Cr(IV) (alkyl: methyl or n-propyl) active species. Although some recent experimental results suggested the possibility of dialkyl-Cr(IV) active species for the propagation [18], [19], [20], [32], we found that a π-complex was not formed at both dimethyl-Cr(IV) and chromacyclopentane(IV) species, and the activation energies for the ethylene insertion into the dimethyl-Cr(IV) and

Conclusions

The heterogeneity of the interaction of Cr with SiO2 supports generates a variety of Cr species situated in different coordination environments in industrial Cr/SiO2 ethylene polymerization catalysts. In this density functional study, we have investigated the effects of the coordination environment around Cr active species on the molecular weight of produced polyethylene. On the basis of chromasiloxane having two Cr–O–Si bonds, various kinds of alkyl-Cr species were modeled by adding suitable

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