Phillips Catalyst Research Articles

Polyolefins, a Success Story.

This paper reports the principal discoveries which have played a major role in the polyolefin field and have positioned polyolefins as the most produced plastics. The early development of polyolefins covering the production of LDPE (Low density polyethylene) at ICI (Imperial Chemical Industries) and the discovery of Phillips or Ziegler-Natta catalysts are highlighted in the first section. In the second part, the impact of the implementation of molecular catalysts on the research in polyolefins is discussed together with the most recent advances leading to high-performance tailor-made resins.

High-resolution 29Si CP/MAS solid state NMR spectroscopy and DFT investigation on the role of geminal and single silanols in grafting chromium species over Phillips Cr/silica catalyst

Phillips Cr/silica catalyst is industrially important in ethylene polymerization. The high-resolution solid state 1H MAS NMR and 29Si CP/MAS NMR allowed the identification of the type and amount of silanols: geminal vs. single (isolated and vicinal) silanols on Phillips catalysts calcined at different temperatures, which were compared with those of the bare silica gel counterparts. The residual silanols on the catalyst and silica gel samples were all decreased with increasing calcination temperatures from 120 to 800°C. For the catalysts treated at temperatures lower than 300°C, the amount of residual silanol groups were much lower than those of the silica gel counterparts. It suggested that the chromium species were mainly grafted on the silica gel through esterification reaction with surface silanols below 300°C. The geminal silanols almost disappeared on the catalysts at 120°C, while that for the silica gel occurred at 300°C. Further increasing the calcination temperatures from 300 to 800°C, the amount of single silanols were slower decreased for the catalysts than that for the silica gel samples. It indicated that the presence of the grafted chromate species obstructed further removal of the residual single silanols. The role of silanols on the formation of surface chromate species on the well-defined polyoligomericsilsesquioxane (POSS) models containing various types of silanols was theoretically studied by density functional theory (DFT) method. It was shown that one silanol of the geminal pair and another adjacent single silanol was the most thermodynamically favored for grafting chromium species. The priority of the reaction between chromium species and different types of surface silanol groups during calcination for Phillips catalysts were experimentally and theoretically elucidated for the first time.

A Novel SiO2‐Supported Fluorine Modified Chromium–Vanadium Bimetallic Catalyst for Ethylene Polymerization and Ethylene/1‐Hexene Copolymerization

AbstractPhillips catalyst is one of the most significant industrial ethylene polymerization catalysts. Chemical modifications have been carried out to tune the Phillips catalyst performance and improve the polyethylene properties. After the modification of the catalyst by fluorine, the polyethylene product with higher molecular weight (MW) and narrower molecular weight distribution (MWD) is suitable for producing automobile fuel tanks. Vanadium containing Phillips catalyst enhances α‐olefin incorporation and MW regulation. In present work, fluorine modified and unmodified chromium–vanadium (Cr–V) bimetallic catalysts are prepared and explored. Compared with the fluorine‐free catalyst, the activities of F‐modified bimetallic catalysts slightly decrease with the increasing MW of the product and the hydrogen response increases slightly. Due to the synergistic effect of the chromium, vanadium and fluorine on the silica gel support, the short‐chain branch distribution (SCBD) of copolymers from F‐modified Cr–V bimetallic catalyst (Cr–V–F)600 is more beneficial than that of Cr–V bimetallic catalyst (Cr–V)600 and F‐modified Cr–V bimetallic catalyst (Cr–V–F)500. The fluorination of Cr–V bimetallic catalysts has not only preserved the high polyethylene activity of bimetallic active sites but also produced the advantage of the high MW ability from fluorine. image

Strain effect and dual initiation pathway in CrIII/SiO2 polymerization catalysts from amorphous periodic models

Heterolytic CH bond activation of ethylene on CrIIIO has been proposed as the initial step in olefin polymerization on (SiO)3CrIII active sites of molecularly-defined analogues of the Phillips catalyst. Here, by using realistic amorphous periodic models that account for structural complexity, strain, and active site heterogeneity of well-defined silica-supported CrIII-based polymerization catalysts, we show that this activation step is significantly favored due to the strain present on highly dehydroxylated silica. Furthermore, we find that initiation by insertion of ethylene into the CrO bond is even more favorable, especially for more strained sites, while both mechanisms can compete for less strained and thereby less active sites. Our results suggest a competing dual pathway for ethylene polymerization on CrIII sites and are consistent with a distribution of active sites, the experimentally observed broad distribution of polymer molecular weight, and the increased polymerization activity upon high-temperature calcination in Phillips catalysts.

The discovery and evolution of metallocene-based olefin polymerization catalysts

The development of the homogeneous metallocene/methylaluminoxane catalyst for the polymerization of olefin has widely increased the possibilities in controlling the polymer composition, polymer structure, tacticity and special properties with high precision compared to the heterogeneous Ziegler–Natta and Phillips catalysts. Metallocene catalysts allow the synthesis of isotactic, isoblock, syndiotactic, stereoblock or atactic polymers, as well as polyolefin composite materials with superior properties and low content of extractables. The homogeneous character of metallocene-based catalysts leads to a better understanding of the mechanism of the olefin polymerization and allows the synthesis of optically active olefin oligomers using chiral transition metal complexes. These single-site catalysts are able to copolymerize ethene and propene with short- and long-chained α-olefins, cyclic olefins, or polar vinyl monomers such as ethers, alcohols or esters. Such copolymers are suitable for blends of polyolefins with polyethers and other polar polymers because of an excellent adhesion of the two polymers. In the future, polyolefin nanocomposites and tailored copolymers open up the approach to new classes of materials with great property combinations such as improved stiffness, high gas barrier properties, significant flame retardancy, and high crystallization rates.

Reduction of the Phillips catalyst by various olefins: Stoichiometry, thermochemistry, reaction products and polymerization activity

The adsorption, reaction and desorption of ethylene or hexenes on Cr(VI)/SiO2 containing 1 or 3wt.% chromium were monitored by thermogravimetry, differential scanning calorimetry and mass spectrometry. Reaction of Cr(VI)/SiO2 with ethylene at a temperature of 200°C always included some oligomerization, making it impossible to identify the stoichiometry of the initial redox reaction. Reaction of Cr(VI)/SiO2 with 1-hexene or cyclohexene at 150°C indicated an adsorption stoichiometry of one olefin per chromium. Heats of reaction were consistent with oxidation of the olefin to an aldehyde or ketone, or even to a carboxylic acid or ester. Oxidation products were retained on the surface. Thermal desorption of surface products resulted in CO2 evolution at temperatures of 265–285 and 390–415°C, with the co-products indicating decomposition of formate and acetate species, respectively. Reduction of Cr(VI) with ethylene led to sites active for ethylene polymerization after thermal desorption of the redox products, whereas reduction with hexenes did not generate active sites irrespective of thermal desorption of products.

Activation and Deactivation of Phillips Catalyst for Ethylene Polymerization Using Various Activators

The present article reveals important roles of metal alkyl activators in tuning the performance of the Phillips catalyst in ethylene polymerization. The addition of aluminum alkyl aids the activation of the catalyst, while excess addition leads to the loss of the activity. The balance between the activation and deactivation depends on the type of employed aluminum alkyl, and tri‐n‐octylaluminum offers the most efficient catalyst usage by preferentially suppressing the deactivation. The passivation of aluminum trialkyl with a hindered phenol mildens not only the deactivation but also chain transfer reactions, leading to an increment of high molecular weight fractions.image

Ethylene polymerization by Cr(III) salts on acidic carriers

Chromium tris-2,4-pentanedionate becomes quite active as an ethylene polymerization catalyst when sublimed onto an acidic high-porosity carrier and in the presence of aluminum alkyl cocatalyst. The resultant catalyst produces a broad MW distribution of mostly linear polyethylene. Because the MW distribution reflects the active site distribution, it has been studied in this paper as the catalyst was probed by changing the organic ligand, the acidic carrier, the cocatalyst and reactor additives. The MW distribution was quite responsive to changes in the carrier and the cocatalyst, but not to changes in the organic ligand.These responses were then compared to those of a commercial relative, the Phillips chromium (VI) oxide catalyst system anchored on the same supports. Many common behaviors were observed, and also some contrasting ones. For example, the incorporation of 1-hexene, and the resulting branch placement within the MW distribution were identical between the two catalysts. On the other hand, the role of the support calcination temperature as a MW regulator was greatly reduced from the experimental catalyst, but the response to reactor H2 was much more pronounced than that of Phillips catalysts. Thus, the experimental catalyst offers many of the same features of commercial Cr catalysts, but with some environmental and operational advantages too.

Reactivity of Hydrosilanes with the CrII/SiO2 Phillips Catalyst: Observation of Intermediates and Properties of the Modified CrII Sites

The reaction of hydrosilanes (both silane and triethylsilane) with CrII/SiO2 catalyst has been investigated in detail by analysis of the gaseous by-products, temperature- and pressure- resolved FT-IR spectroscopy and deuterium exchanges. We found that the reaction proceeds via two steps, passing through intermediates characterized by elongated Si–H bonds and transient Cr-hydride species leading to the release of H2 in the gas phase. These experimental evidence allowed us to advance an hypothesis of the reaction mechanism, which validates our previous proposal for the structure of the modified chromium sites. Furthermore, based on the intermediates of the reaction mechanism, we have also tested the ability of the modified “homogeneous-like” CrII sites toward H2 (D2) activation, demonstrating that, contrarily to the unmodified CrII species, such reactivity is present.

Models for heterogeneous catalysts: studies at the atomic level

A systematic approach to model heterogeneous catalyst material and characterization at the atomic level is presented. Two examples are used to illustrate the concepts derived from those studies. They document the problems arising on the way to create such model systems and they also indicate possible solutions. The first example is connected with activation of CO2 at the rim of electron rich MgO (thin film-supported Au islands), and the second example aims at the creation of a model system for the Phillips catalyst for ethylene polymerization and, in particular, the creation of a hydroxylated silica support.

One-Electron-Redox Activation of the Reduced Phillips Polymerization Catalyst, via Alkylchromium(IV) Homolysis: A Computational Assessment

In ethylene polymerization by the Phillips catalyst, inorganic Cr(II) sites are believed to be activated by reaction with ethylene to form (alkyl)CrIII sites, in a process that takes about 1 h at ca. 373 K. The detailed mechanism of this spontaneous self-initiation has long remained unknown. It must account both for the formation of the first Cr–C bond and for the one-electron oxidation of Cr(II) to Cr(III). In this study, we used density functional theory to investigate a two-step initiation mechanism by which ethylene oxidative addition leads first to various (organo)CrIV sites, and subsequent Cr–C bond homolysis gives (organo)CrIII sites capable of polymerizing ethylene. Pathways involving spin crossing, C–H oxidative addition, H atom transfer, and Cr–C bond homolytic cleavage were explored using a chromasiloxane cluster model. In particular, we used classical variational transition theory to compute free energy barriers and estimate rates for bond homolysis. A viable route to a four-coordinate bis(alkyl)CrIV site was found via spin crossing in a bis(ethylene)CrII complex followed by intramolecular H atom transfer. However, the barrier for subsequent Cr–C bond homolysis is a formidable 209 kJ/mol. Increasing the Cr coordination number to 6 with additional siloxane ligands lowers the homolysis barrier to just 47 kJ/mol, similar to reported homolysis paths in molecular [CrR(H2O)53+] complexes. However, siloxane coordination also raises the barrier for the prior oxidative addition step to form the bis(alkyl)CrIV site. Thus, we suggest that hemilability in the silica “ligand” may facilitate the homolysis step while still allowing the oxidative addition of ethylene.

Toward the Understanding of the Comonomer Effect on CrII/SiO2 Phillips Catalyst

The origin of the “comonomer effect” (i.e., the enhancement of ethylene polymerization rate in the presence of α-olefin comonomer) on CrII/SiO2 Phillips catalyst was investigated by means of in situ FT-IR and Diffuse Reflectance UV–vis spectroscopies. A nonpolymerizing olefin, cyclohexene, was chosen as “comonomer” and introduced on the catalyst prior the monomer injection in order to produce only homopolymers. Under our experimental conditions, α-olefin (ethylene, propene, and 1-hexene) homopolymerization is enhanced up to a factor of 5. Our experimental data provide the first direct evidence that the comonomer strongly interacts with all the CrII sites and that it is responsible for their structural and electronic rearrangement at a molecular scale, as proposed for the “trigger mechanism”. We also proved that the fraction of Cr actives sites do not retain the comonomer during the olefin polymerization, meaning that the increased reactivity is due to a modification influencing all the catalyst Cr sites.

Pre-reduction of the Phillips CrVI/SiO2 catalyst by cyclohexene: A model for the induction period of ethylene polymerization

The pre-reduction of the industrially relevant CrVI/SiO2 Phillips catalyst by cyclohexene at room temperature was investigated by means of operando XANES, Diffuse Reflectance UV–Vis and transmission FT-IR spectroscopies. It was found that cyclohexene efficiently reduces the surface chromates sites (CrVI) at room temperature to give mainly divalent chromium sites and aldehyde by-products, which are rapidly converted in ester species. These by-products remain in strong interaction with the reduced chromium sites, defining a complex and tunable ligand sphere, which controls the entrance of incoming molecules, including ethylene. Unlike the CrVI/SiO2 catalyst, the cyclohexene-reduced Cr/SiO2 catalyst polymerizes ethylene already at room temperature with no induction time. The reactivity of cyclohexene with the CrVI species mimics that of ethylene during the induction period on CrVI/SiO2 catalyst, simplified by the absence of the further polymerization step. Hence, the results discussed in this work are potentially useful to understand what happens during the induction period in the presence of ethylene.

Production of Polyolefins by Metallocene Catalysts and Their Recycling by Pyrolysis

SummaryPolyolefins are the main industrial used polymers as they are sustainable materials. They show a wide variety of properties, consist of only carbon and hydrogen atoms, can efficiently be produced from easy available monomers and can be recycled. Beside Ziegler‐Natta and Phillips catalysts, metallocenes and other transition metal complexes, activated by methylaluminoxane allow to tailor the structure, the tacticity, and the stereoregularity of polyolefins in a way which has not been reached before. Metallocene catalysts are able to copolymerize ethene or propene with different olefins, cyclic olefins, polar monomers as well as with macromers to form polymer short or long chain branched polymers with new properties. Polyolefin wastes can easily be recycled by extrusion moulding or by pyrolysis to gas and oil. While for extrusion moulding pure polymers have to be used the pyrolysis can take mixed and contaminated polyolefins. In a fluidized bed process, indirectly heated by fire tubes, different products are obtained in dependence of the pyrolysis temperature. By 500°C about 90 % aliphatic waxes are formed, by 700°C mainly olefins and aromatics. By addition of Lewis acids such as titanium or aluminum chlorides to the fluidized bed it is possible to decreasing the pyrolysis temperature by 100 °C.

Reprint of: Effect of Mo-modification over Phillips CrOx/SiO2 catalyst for ethylene polymerization

Phillips CrOx/SiO2 catalyst is an important catalyst for industrial production of polyethylene. In this work, a Mo-modified Phillips catalyst for ethylene polymerization (defined as CrMo/Si) was developed. The characterizations of the Phillips catalyst (defined as Cr/Si) and the CrMo/Si catalyst by temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) showed an obvious interaction between the chromium and molybdenum components on the silica support. The ethylene polymerization performances at different conditions, in terms of H2 responds, polymerization temperatures, and the copolymerization with 1-hexene were systematically investigated. Although the polymerization activity of the CrMo/Si catalyst was lower, the copolymer presented a higher molecular weight (MW) and a broader molecular weight distribution (MWD). The Mo-modified Phillips catalyst (CrMo/Si) exhibited 67% higher 1-hexene incorporation ability as determined by 13C NMR. Moreover, the short chain branch distribution (SCBD) of the copolymer obtained via the CrMo/Si catalyst was also improved as confirmed by temperature rising elution fractionation (TREF) cross successive self-nucleation and annealing (SSA) methods. The results of TREF and SSA showed that the amount of short chain branches for the copolymer was increased in the high MW part, which would benefit for the long term mechanical property of polyethylene.

Polyethylene with Reverse Co‐monomer Incorporation: From an Industrial Serendipitous Discovery to Fundamental Understanding

A triethylaluminium(TEAl)-modified Phillips ethylene polymerisation Cr/Ti/SiO2 catalyst has been developed with two distinct active regions positioned respectively in the inner core and outer shell of the catalyst particle. DRIFTS, EPR, UV-Vis-NIR DRS, STXM, SEM-EDX and GPC-IR studies revealed that the catalyst produces simultaneously two different polymers, i.e., low molecular weight linear-chain polyethylene in the Ti-abundant catalyst particle shell and high molecular weight short-chain branched polyethylene in the Ti-scarce catalyst particle core. Co-monomers for the short-chain branched polymer were generated in situ within the TEAl-impregnated confined space of the Ti-scarce catalyst particle core in close proximity to the active sites that produced the high molecular weight polymer. These results demonstrate that the catalyst particle architecture directly affects polymer composition, offering the perspective of making high-performance polyethylene from a single reactor system using this modified Phillips catalyst.

Mechanism of Initiation in the Phillips Ethylene Polymerization Catalyst: Redox Processes Leading to the Active Site

The detailed mechanism by which ethylene polymerization is initiated by the inorganic Phillips catalyst (Cr/SiO2) without recourse to an alkylating cocatalyst remains one of the great unsolved mysteries of heterogeneous catalysis. Generation of the active catalyst starts with reduction of CrVI ions dispersed on silica. A lower oxidation state, generally accepted to be CrII, is required to activate ethylene to form an organoCr active site. In this work, a mesoporous, optically transparent monolith of CrVI/SiO2 was prepared using sol–gel chemistry in order to monitor the reduction process spectroscopically. Using in situ UV–vis spectroscopy, we observed a very clean, stepwise reduction by CO of CrVI first to CrIV, then to CrII. Both the intermediate and final states show XANES consistent with these oxidation state assignments, and aspects of their coordination environments were deduced from Raman and UV–vis spectroscopies. The intermediate CrIV sites are inactive toward ethylene at 80 °C. The CrII sites, wh...

Effect of Mo-modification over Phillips CrOx/SiO2 catalyst for ethylene polymerization

Phillips CrOx/SiO2 catalyst is an important catalyst for industrial production of polyethylene. In this work, a Mo-modified Phillips catalyst for ethylene polymerization (defined as CrMo/Si) was developed. The characterizations of the Phillips catalyst (defined as Cr/Si) and the CrMo/Si catalyst by temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS) showed an obvious interaction between the chromium and molybdenum components on the silica support. The ethylene polymerization performances at different conditions, in terms of H2 responds, polymerization temperatures, and the copolymerization with 1-hexene were systematically investigated. Although the polymerization activity of the CrMo/Si catalyst was lower, the copolymer presented a higher molecular weight (MW) and a broader molecular weight distribution (MWD). The Mo-modified Phillips catalyst (CrMo/Si) exhibited 67% higher 1-hexene incorporation ability as determined by 13C NMR. Moreover, the short chain branch distribution (SCBD) of the copolymer obtained via the CrMo/Si catalyst was also improved as confirmed by temperature rising elution fractionation (TREF) cross successive self-nucleation and annealing (SSA) methods. The results of TREF and SSA showed that the amount of short chain branches for the copolymer was increased in the high MW part, which would benefit for the long term mechanical property of polyethylene.

Computational Kinetic Discrimination of Ethylene Polymerization Mechanisms for the Phillips (Cr/SiO2) Catalyst

The mechanism of ethylene polymerization by the widely used Phillips catalyst remains controversial. In this work, we compare initiation, propagation, and termination pathways computationally using small chromasiloxane cluster models for several previously proposed and new mechanisms. Where possible, we consider complete catalytic cycles and compare predicted kinetics, active site abundances, and polymer molecular weights to known properties of the Phillips catalyst. Prohibitively high activation barriers for propagation rule out previously proposed chromacycle ring expansion and Green–Rooney (alternating alkylidene/chromacycle) mechanisms. A new oxachromacycle ring expansion mechanism has a plausible propagation barrier, but initiation is prohibitively slow. On sites with adjacent bridging hydroxyls, either ≡Si(OH)CrII-alkyl or ≡Si(OH)CrIII-alkyl, initiated by proton transfer from ethylene, chain growth by a Cossee–Arlman-type mechanism is fast. However, the initiation step is uphill and extremely slow, so essentially all sites remain trapped in a dormant state. In addition, these sites make only oligomers because when all pathways are considered, termination is faster than propagation. A monoalkylchromium(III) site without an adjacent proton, (≡SiO)2Cr-alkyl, is viable as an active site for polymerization, although its precise origin remains unknown.

The Effect of the Electronic Nature of Spectator Ligands in the C-H Bond Activation of Ethylene by Cr(III) Silicates: An ab initio Study.

The Phillips catalyst, chromium oxides supported on silica, is one of the most widely used catalysts for the industrial production of polyethylene (PE). We recently synthesized a well-defined mononuclear Cr(III) silicate as active site model of the Phillips catalyst. The catalytic activity of this well-defined catalyst was similar to the industrial Phillips catalyst. We proposed that C-H bond activation of ethylene over a Cr-O bond initiates polymerization in this Cr(III) catalyst. Our results also showed that the presence of a second ethylene olefin in the coordination sphere of Cr decreases the intrinsic energy barrier of the C-H activation of ethylene. In order to understand the effect of this additional ligand in the C-H activation of ethylene by the Cr(III) catalyst, we evaluated the energetics of this step with different spectator ligands (C2H4, C2F4, N2 and CO) coordinated to the Cr center. The Charge Decomposition Analysis (CDA) of the bonding interactions between the Cr(III) catalyst and the ligands showed that the intrinsic energy barrier for the C-H activation of ethylene decreases with the increasing electron-donor properties of the spectator ligand.