Xuelian He

Phillips Cr/Silica Catalyst for Ethylene Polymerization

The Phillips Cr/silica catalyst, discovered by Hogan and Banks at the Phillips Petroleum Company in the early 1950s, is one of the most important industrial catalysts for polyethylene production. In contrast to its great commercial success during the past half-century, academic progress regarding a basic understanding of the nature of the active sites and polymerization mechanisms is lagging far behind. During the last decade, increasing research efforts have been performed on the Phillips catalyst through various approaches, including spectroscopic methods, polymerization kinetics, heterogeneous model catalysts, homogeneous model catalysts, and molecular modeling. Much deeper mechanistic understanding, together with successive catalyst innovations through modifications of the Phillips catalyst, has been achieved.

DFT Functional Benchmarking on the Energy Splitting of Chromium Spin States and Mechanistic Study of Acetylene Cyclotrimerization over the Phillips Cr(II)/Silica Catalyst

In this work, a two-state reaction mechanism for the acetylene cyclotrimerization over a cluster model for the Phillips Cr(II)/silica catalyst were systematically investigated using density functional theory (DFT). Since spin crossover phenomenon was confirmed in the catalytic cycle, an accurate prediction of the energy gap between low- and high-spin states is crucial for the description of a reaction involving a two-state reactivity. Therefore, a massive DFT functional benchmarking test has been conducted on the cluster model by taking a CASPT2 energy gap as a reference. Consequently, B3PW91* with 28% Hartree-Fock exchange energy was selected for the following mechanistic investigation. Each of the possible potential energy surface including singlet, triplet, and quintet surfaces was explored. On the quintet surface, the reaction begins with a coordination of an acetylene on the chromium center to generate a π-coordinated complex. The following oxidative coupling through further coordination with a second acetylene was predicted to be a two-step reaction to generate a chromacyclopentadiene species. This transformation was found to be energetically prohibitive by the presence of the transition state (5)TS[C-E] (ΔG(‡) = 31.1 kcal/mol). On the triplet surface, however, the coordination of an acetylene generates a chromacyclopropene species without showing any activation barrier. The second acetylene incorporation proceeding via a coordination on the chromium center followed by an insertion into a Cr-C σ-bond of the chromacyclopropene was predicted to be a facile reaction pathway (ΔG(‡) = 10.2 kcal/mol). The third acetylene was captured by the cluster model through the formation of a hydrogen bond. The later transformation on the triplet surface was found to be an intermolecular [4 + 2] cycloaddition to finish the cyclization. The lack of the aromaticity of the benzene ring in (3)L results in an uncompleted reaction pathway on a single triplet surface. Consequently, a two-state reaction pathway that is connected by two low-lying minimum-energy crossing points (MECPs) of the two surfaces is thus described. It is worthy of note that the third acetylene in the tri(acetylene)chromium complex captured by the cluster model only through the formation of a hydrogen bond rules out the [2 + 2 + 2] concerted one-step reaction pathway proposed by Zecchina et al. [Phys. Chem. Chem. Phys.2003, 5, 4414]. The singlet reaction profile is far higher in energy compared with that proceeded on the triplet and quintet surfaces.

Active Site Transformation During the Induction Period of Ethylene Polymerization over the Phillips CrOx/SiO2 Catalyst

In spite of the great importance of the Phillips catalyst in commercial polyethylene production and long‐term research efforts, the initiation mechanism of polymerization still remains unclear. The effect of formaldehyde desorption on the active site transformation during the induction period of the Phillips catalyst is investigated over cluster models by using DFT. No reaction can be initiated over the cluster model coordinated with two formaldehyde molecules, owing to steric hindrance and electronic donation. The first reaction over cluster models, on which either one or no formaldehyde molecule is adsorbed, follows the metallacyclic mechanism into chromacyclopentane. Subsequent dimerization to 1‐butene and metathesis to propylene and ethylene are more favorable over the cluster model adsorbed with one formaldehyde molecule. Only after a complete desorption of formaldehyde does further ring expansion to chromacycloheptane followed by 1‐hexene formation become preferential. Spin state crossing from quintet diethylene–CrII complex to triplet chromacyclopentane with a spin acceleration effect is revealed.

Chromium Catalysts for Ethylene Polymerization and Oligomerization

Abstract Chromium-based catalysts are the most important ethylene polymerization and oligomerization catalysts widely applied for industrial production of polyethylene and 1-hexene. Phillips chromium catalyst is a well-known heterogeneous catalyst for commercial production of HDPE products, which accounts for more than 40% of world production annually. The Chevron-Phillips Cr-based homogeneous catalyst system is the first commercialized catalyst for the production of 1-hexene through selective ethylene oligomerization. Although a great success with these Cr-based catalysts has been achieved in industrial applications, there are still many debates in the academic field concerning the precise structure of active chromium species, the oxidation states of chromium center, the effects of cocatalysts/ligands and the catalytic mechanisms. During the last decades, a step-forward mechanistic understanding has been achieved through extensive and successive investigations on these Cr-based catalysts for ethylene polymerization/oligomerization. In addition, the progress in mechanistic understanding on alkyne cyclotrimerization by the same Phillips catalyst and ethylene polymerization over Mo-based catalyst are also covered. The later might be served as an alternative green catalyst for the industrial production of polyethylene.

Molecular dynamics study of the isothermal crystallization mechanism of polyethylene chain: the combined effects of chain length and temperature

A molecular level understanding of the polyethylene (PE) crystallization process was elucidated by molecular dynamics simulation of three states, with varying chain length and temperature. The process can be classified into the following three states: (1) nucleation controlled state, (2) competitive state of crystal growth process and new nuclei formation, and (3) crystal growth controlled state, which could be quantified by the evolution of nuclei number. With increasing chain length, two phenomena occur: the single crystallization mechanism changes from state (1) to (3), and the crystal size increases while the b/a axial ratio in the lateral surface decreases. These changes can be explained from a thermodynamic point of view, in that the van der Waals (vdW) interaction per CH2 unit is strengthened and more nucleation sites are generated for longer chain. Size effect (meaning different surface fractions when the chain collapses into a globule) was an important factor determining vdW energy per unit and the crystallization states of a single PE chain. On the other hand, the crystallization states were independent of chain length for short chains systems with the same size effect. In both conditions, a long chain generates multi-crystal domains, and a short chain prefers a single crystal domain. Our results not only provide molecular level evidence for crystallization states but also clarify the influence of chain length on the crystallization process.

Improvement of Mechanical Properties and Ultraviolet Resistance of Polyethylene Pipe Materials Using High Density Polyethylene Matrix Grafted Carbon Black

In recent years, high grade high density polyethylene (HDPE) pipe materials are being more and more widely used for water and gas supply. Carbon black (CB) is usually used as an anti-UV-light reagent for pipe materials. However, homogeneous dispersion of CB in the HDPE matrix and modification of the interface has always been a great challenge. In this work, HDPE matrix grafted CB (HDPE-g-CB) was successfully prepared through HDPE radicals formation by a thermo-mechanical method and subsequent radical capture by the CB surface. The weight percentage of grafted HDPE approached 10 wt% and the modification sharply reduced the surface free energy of the CB. The SEM (scanning electron micrographs) and TEM (transmission electron microscopy) results showed that HDPE-g-CB was uniformly dispersed in the HDPE pipe materials and the domain size of the dispersed phase was remarkably decreased from that in HDPE/CB. Therefore, compared with the HDPE/CB, the mechanical properties and ultraviolet (UV) resistance of HDPE/HDPE-g-CB were significantly improved, positively influencing the expected life span of pipelines.

Modification of the (SiO2/MgO/MgCl2)·TiClx Ziegler-Natta polyethylene catalysts using the third metal elements

Various (SiO2/MgO/MgCl2)·TiClx Ziegler-Natta catalysts modified by the third metal elements were synthesized by the co-impregnation of water-soluble magnesium and the third metal salts. Several key factors including the electronegativity of the third metal elements, catalyst performances in ethylene homo-polymerization, ethylene/1-hexene copolymerization and hydrogen response were systematically investigated. Both the catalyst performance and the polymer properties are influenced by the introduction of the third metal elements. Compared with the unmodified (SiO2/MgO/MgCl2)·TiClx Ziegler-Natta catalyst, activity and 1-hexene incorporation are enhanced by the introduction of zirconium, vanadium, aluminum and chromium, while deteriorated by the addition of ferrum, nickel, molybdenum and tungsten. Correlations of the catalyst activities and 1-hexene incorporation ability with the electronegativity of the third metal elements are discovered. It is found that the lower electronegativity of the third metal elements leads to the catalyst with higher activity and higher α-olefin co-polymerization ability. The polyethylene produced by a nickel modified catalyst showed broad molecular weight distribution (MWD) and the lowest average molecular weight (MW), while by using a ferrum modified catalyst, the resulting polyethylene had the highest MW, reaching the ultra-high MW area. Vanadium and chromium modified catalysts demonstrated the best hydrogen response.

2D-QSPR/DFT studies of aryl-substituted PNP-Cr-based catalyst systems for highly selective ethylene oligomerization

Abstract1-Hexene and 1-octene are important comonomers for the synthesis of high performance polyolefins. Recently, various N-substituted Cr-bis(diphenylphosphino)amine (PNP-Cr) catalysts show the potential as excellent candidates for highly selective ethylene trimerization/tetramerization. In this work, a series of aryl-substituted PNP-Cr catalysts were studied by two-dimensional quantitative structure–property relationship (QSPR) method based on density functional theory (DFT) calculations. The heuristic method (HM) and best multi-linear regression (BMLR) were used to establish the best linear regression models to describe the relationship between selectivities and catalyst structures. Both Cr(I) and Cr(II) active site models for ethylene trimerization/tetramerization were considered. It was found that 1) the relativity and stability of the models were increased by using self-defined descriptors based on DFT calculations; 2) Cr(I)/Cr(III) centers were the most plausible active sites for ethylene trimerization, while Cr(II)/Cr(IV) active sites were most possibly responsible for ethylene tetramerization; and 3) the skeleton structures of the PNP-Cr system with good complanation and symmetry were crucial for achieving excellent catalytic selectivity of 1-octene, while the PNP-Cr backbone with a large steric effect on N atom would benefit ethylene trimerization. Six new PNP ligands with high selectivity toward ethylene trimerization/tetramerization were predicted based on descriptor analysis and the best linear regression models providing a good basis for further development of novel catalyst systems with better performance. FigureAryl-substituted PNP-Cr-based catalyst systems

Exploring Si/Mg Composite Supported Ziegler-Natta Ti-based Catalysts for Propylene Polymerization

A series of (SiO2/MgO/ID/MgCl2)·TiClx Ziegler-Natta catalysts for propylene polymerization has been prepared with a new method. These catalysts were synthesized using soluble Mg-compounds as the Mg-source and the preparation progress was relatively simple. The catalyst could copy the spherical shape of the carrier very well. The propylene polymerization results showed that the catalyst revealed the best activity with 9,9-di(methoxymethyl)fluorene (BMMF) as internal donor at 50 °C with the optimal molar ratio Al/Ti = 5, which was much lower than what the industrial polypropylene catalyst used (at least molar ratio Al/Ti = 100), resulting in great cost saving. Additionally, the polymerization kinetics of the catalyst exhibited very stable property after achieving a relatively high value. These catalysts possessed rather high activity and good hydrogen response. The isotactic index (II.) value of the PP products could be higher than 98% in the presence of both internal and external electron donors. Moreover, temperature rising elution fractionation method was used to understand the influence of donors and H2 on the properties of the PP products.

Effect of F‐modification over Phillips Cr/SiO2 catalyst for ethylene polymerization

In this work, a fluoride‐modified Phillips catalyst was investigated by using combined experimental and computational methods. The addition of fluoride to the Phillips Cr/SiO2 catalyst can increase the activity of the catalyst calcined at a low temperature and the molecular weight of the polyethylene product. DFT calculations were performed to show that the difference of the Gibbs free energy barriers between chain transfer and chain propagation increased after the introduction of the fluoride onto the silica surface, which is in accordance with the increased molecular weight of the polyethylene produced by the fluoride‐modified Phillips Cr/F‐SiO2 catalyst. The computational results also reflect the increase of the activity of the ethylene/1‐hexene copolymerization after the introduction of fluoride, although the modification has little effect on the regioselectivity of the produced ethylene/1‐hexene copolymer. Moreover, copolymers produced by the fluoride‐modified Phillips catalyst with more short‐chain branches (SCBs) in the high‐molecular‐weight fraction and a significant enhancement of the environmental stress crack resistance can be explained from the results of the DFT calculations.