Vanadium-magnesium Catalysts Research Articles

Ethylene Polymerization over Supported Vanadium-Magnesium Catalysts with Different Vanadium Content: The Effect of Hydrogen on Molecular Weight Characteristics of the Produced Bimodal Polyethylene

Data are presented on the activity of supported vanadium-magnesium catalysts (VMCs) with different vanadium content in ethylene polymerization and the molecular weight characteristics of the produced polyethylene. The VC1 catalyst, with a very low vanadium content (0.12 wt.%), showed a sixfold higher activity per unit weight of vanadium than the VC2 catalyst with a high-vanadium content (4.0 wt.%). Additionally, the total activity of VC2 (kg PE/g cat·h) was fivefold higher when compared to VC1. The introduction of hydrogen in polymerization leads to a considerable decrease in the activity of both catalysts. The polyethylene obtained in the presence of hydrogen over both catalysts has a broad bimodal molecular weight distribution (MWD) with a distinct shoulder in the high-molecular region (Mw ≥ 106 g/mol). Decomposition of the MWD curves of bimodal polyethylene into two fractions (high- and low-molecular fractions) made it possible to determine for the first time the ratio of the reaction rate constants of chain transfer with hydrogen (KtrH) and polymer chain propagation (Kp) for two groups of the VMC active sites producing low- and high-molecular fractions of bimodal polyethylene.

Dynamic mechanical analysis of ethylene/1‐hexene copolymers: The effect of the catalyst type on the short‐chain branching distribution and properties of the amorphous and crystalline phases

ABSTRACTDynamic mechanical analysis was used to study ethylene/1‐hexene copolymers with different compositions, molecular weight distributions, and profiles of short‐chain branching (SCB) versus molecular weight. These copolymers were produced over a highly active supported titanium–magnesium catalyst (TMC), a highly active supported vanadium–magnesium catalysts (VMC), and a supported zirconocene catalyst. A higher fraction of the crystalline phase in the copolymers prepared with VMC was shown to result in higher elastic modulus values. β relaxation was found to be sensitive to the SCB distribution versus the molecular weight. The copolymers prepared with the zirconocene catalyst and VMC were characterized by more uniform SCB distributions and higher temperatures of β relaxation compared to the copolymers prepared with TMC. The mobility of the polymer chains at room temperature in the amorphous phase obtained by the spin‐probe method rose with increasing branch content in the copolymers and was not sensitive to different SCB distribution profiles. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44638.

Study of the compositional heterogeneity of ethylene–hexene-1 copolymers by thermal fractionation technique by means of differential scanning calorimetry

The successive self-nucleation/annealing technique (SSA) by differential scanning calorimetry has been applied to study the heterogeneity of ethylene–hexene-1 copolymers produced with supported catalytic systems of different compositions: highly active supported Ziegler–Natta (Z–N) catalysts—a titanium–magnesium catalyst TiCl4/MgCl2 (TMC) and a vanadium–magnesium catalyst VCl4/MgCl2 (VMC), a supported zirconocene catalyst Me2Si(Ind)2ZrCl2/SiO2 (MAO), and a chromium-oxide catalyst CrO3/SiO2. Comparative data by SSA technique with the same temperature program were obtained for copolymers differed by MWD from narrow to very broad (Mw/Mn = 2.4–54) and short chain branching distribution from narrow (zirconocene catalyst) to very broad (TMC and chromium oxide catalysts). It is demonstrated that copolymers produced with the zirconocene catalyst have the narrowest melting range and do not contain thick lamellae. The widest lamella thickness distribution has been found for a copolymer produced with the chromium-oxide catalyst. Copolymers produced with the supported Z–N catalysts are ranked in the middle with a more narrow lamella thickness distribution for copolymer prepared with VMC as compared with the one produced with TMC. The SSA results are compared with the data on copolymer fractionation by TREF. It is shown that these methods give a good correlation for copolymers with narrow short-chain branching distribution produced with the supported zirconocene catalyst. In the case of copolymers produced with TMC, TREF yields a higher content of the high-branched fractions.

New generation of supported Ziegler-type catalysts for polyethylene production

On the basis of new methods of synthesis of magnesium-containing carriers with the use of soluble organomagnesium compounds a wide spectrum of supported titanium-magnesium and vanadium-magnesium catalysts of various composition with controllable optimal morphology for obtaining versatile brands of polyethylene by suspension and gas-phase methods was elaborated. The usage of various modifications of these catalysts allows production of high amount (up to 1000 kg/g Ti) of polyethylene of high, medium, and low density with narrow and wide molecular weight distribution (including bimodal) as well as different brands of polyethylene ultrahigh molecular weight.

Study of Multi‐Site Nature of Supported Ziegler‐Natta Catalysts in Ethylene‐Hexene‐1 Copolymerization

AbstractSummary: Heterogeneity of active centers (AC) of titanium‐magnesium catalysts (TMC) and vanadium‐magnesium catalyst (VMC) in ethylene‐hexene‐1 copolymerization has been studied on the base of data of polymer molecular weight distribution (MWD) deconvolution technique and copolymer fractionation onto narrow fractions. It was found that 3 and 4 Flory components (groups of active centers) are required to describe experimental MWD curves of copolymers produced over TMC with different Ti content. In the case of VMC MWD of homopolymer and copolymer are characterized by set of 5 Flory components (5 groups of AC).Different character of inter‐relationship between MW and short chain branching (SCB) was found for ethylene‐hexene‐1 copolymers produced over different catalysts: heterogeneous type in the case of TMC and more uniform for copolymer prepared over VMC. The content of Ti affects on the slope of that profile in copolymers produced over TMC.The results indicated that TMC and VMC are different greatly on the heterogeneity of active centers to the formation of polymers with different molecular weights and to formation of copolymers with different inter‐relationship between MW and short chain branching. TMC produces polymers with more narrow MWD but it contains highly heterogeneous centers regarding comonomer reactivity ratios. VMC produces polymers with broad and bimodal MWD but it contains more homogeneous centers regarding comonomer reactivity ratios.

SUPPORTED ZIEGLER-NATTA CATALYSTS FOR ETHYLENE SLURRY POLYMERIZATION AND CONTROL OF MOLECULAR WEIGHT DISTRIBUTION OF POLYETHYLENE

The effect of chemical composition of highly active supported Ziegler-Natta catalysts with controlled morphology on the MWD of PE has been studied. It was shown the variation of transition metal compound in the MgCl2-supported catalyst affect of MWD of PE produced in broad range: Vanadium-magnesium catalyst (VMC) produce PE with broad and bimodal MWD (Mw/Mn = 14−21). MWD of PE, produced over titanium-magnesium catalyst (TMC) is narrow or medium depending on Ti content in the catalyst (Mw/Mn = 3.1−4.8). The oxidation state of the initial titanium compounds in TMC has only slight effect on MWD of PE produced. Based on MWD data of PE heterogeneity of active centers of TMC and VMC was studied. The results of resolution of experimental GPC curves into Flory components indicated three Flory components are sufficient to describe MWD curve of PE, produced with TMC; six Flory components are required in the case of VMC. In the case of copolymerization of ethylene with 1-hexene over TMC the addition of 1-hexene leads to decrease of MW and to slight effect on Mw/Mn values. On the contrary the strong effect of 1-hexene on MWD of PE produced over VMC was found: the introduction of 1-hexene results in considerable broadening of MWD due to the shifting of the main MWD peak to low MW region. At that comonomer doesn't affect the position of high molecular weight shoulder. The results indicate that some of active centers of VMC producing high MW polymer are not active in the reaction of chain transfer with comonomer.

Molecular mass characteristics of polyethylene produced with supported vanadium‐magnesium catalysts

AbstractHighly active supported vanadium‐magnesium catalysts (VMC) produce polyethylene (PE) with broad and bimodal molecular mass distribution (MMD) in comparison with the famous titanium‐magnesium catalysts (TMC). The effect of hydrogen as an efficient chain‐transfer agent on the MMD of PE has been studied. Increasing hydrogen concentration causes a considerable broadening of MMD of PE due to the shift of the low molecular weight peak on the MMD curve. At the same time, the high molecular weight shoulder stays at the same position even at high hydrogen concentration. This means that VMC contain two types of active centre. One type is very reactive in the chain‐transfer reaction with hydrogen. These centres produce low molecular weight PE in polymerization in the presence of hydrogen. The other type of active centre is not active in chain transfer with hydrogen. These centres produce high molecular weight PE ((1–3) × 106) and hydrogen does not affect the position of the high molecular weight shoulder. MMD data were used to analyze the kinetics of the chain‐transfer reaction with hydrogen and to calculate the rate constants of this reaction. Copyright © 2005 Society of Chemical Industry

Ethylene Polymerization with Supported Vanadium‐Magnesium Catalyst: Number of Active Centers and Propagation Rate Constant

The data on the number of active centers in the ethylene polymerization with highly active supported vanadium-magnesium catalyst (130 kg-PE/g- V h bar) were obtained by using 13 CO as a radioactive inhibitor. The number of active centers was found to attain 0.06 mol/mol-V and the propagation rate constant equaled 2.2 - 10 4 L/mol - s (80°C). The effect of reversibl deactivation of the catalyst in ethylene polymerization in the presence of hydrogen was studied. It was found that a part of the active centers containing vanadium-polymer bonds transforms to a temporary inactive (dormant) state during the polymerization in presence of hydrogen that may explain the catalyst deactivation. As hydrogen is removed from the reaction medium, these centers transform to the active state again.

Ethylene Polymerization with Supported Vanadium-Magnesium Catalyst: Number of Active Centers and Propagation Rate Constant

Ethylene Polymerization with Supported Vanadium-Magnesium Catalyst: Number of Active Centers and Propagation Rate Constant

Ethylene Polymerization with Supported Vanadium-Magnesium Catalyst: Hydrogen Effect

Ethylene polymerization on supported vanadium-magnesium catalysts with the composition VCl4/MgCl2 and VOCl3/MgCl2 was studied in the presence of hydrogen as the chain transfer agent. Vanadium-magnesium catalysts (VMC) are known to exhibit a high reactivity towards hydrogen. This reactivity is manifested as a sharp decrease in polyethylene (PE) molecular weight accompanied by a dramatic (2–6-fold) loss of catalyst activity, when hydrogen is introduced. Catalyst deactivation in the presence of hydrogen is reversible. Catalyst activity is restored, when hydrogen is removed from the reaction medium. With this regard we have studied the effect of the concentration of hydrogen and of Al(i-Bu)3 and Al(i-Bu)2H, used as co-catalysts, on the activity of VMC. We have also identified the reaction products formed after polymerization reaction in the presence of hydrogen. Based on experimental data we suggest reaction the scheme leading to catalyst deactivation in the presence of hydrogen and its reactivation, when hydrogen is removed from the reaction medium. According to this scheme, catalyst deactivation is caused by dialkylaluminumhydride forming in the side reaction of AlR3 co-catalyst with the V-H bonds and then being adsorbed onto the catalyst active sites. It is shown that the deactivation process may be diminished, if some modifiers able to bind with alkylaluminumhydride are introduced into the catalyst composition. Magnesium dichloride introduced into the catalytic system or aluminum trichloride introduced into the catalyst composition may be used for modification. Using this approach we have obtained highly active supported VMC with an activity up to 50 kg/gV·h·bar C2H4 (80°C) at polymerization in the presence of hydrogen.

Surface vanadium compounds in supported vanadium–magnesium catalysts for ethylene polymerization: X-ray photoelectron and infrared diffusion reflectance spectroscopy studies

Adsorption of NO as a probe molecule in infrared diffusion reflectance spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS) have been used in order to identify surface vanadium species in vanadium–magnesium catalysts (VMC). The presence of V 3+ and V 4+ in VCl 4/MgCl 2 and V 4+ in VOCl 3/MgCl 2 was shown by XPS and DRIFTS. However, DRIFTS has some limitations due to oxidation of V 3+ by the reaction with NO; moreover, apparently ions V 5+ and V 2+ do not adsorb NO. We have also demonstrated the possible identification of surface ions V 5+ and V 2+ in VMC using XPS. Practically in all cases (VCl 2/MgCl 2, VCl 4/MgCl 2 and VOCl 3/MgCl 2) one can see a mixture of vanadium ions in different oxidation and coordination states.

Polymerization of ethylene in the presence of hydrogen over supported vanadium-magnesium catalysts

Polymerization of ethylene in the presence of hydrogen over supported vanadium-magnesium catalysts

Molecular structure of polyethylene produced with supported vanadium-magnesium catalyst

The molecular structure of polyethylene (PE) produced with supported titanium-magnesium (TMC) and vanadium-magnesium (VMC) catalysts has been studied by means of IR and 13C NMR spectroscopic methods. It has been shown that PE produced with VMC in the presence of hydrogen differs by a lower content of double bonds and higher branching compared with PE obtained with TMC. Furthermore, branching is observed both in the heptane-soluble low molecular weight fraction and in the high molecular weight fraction of PE. 13C NMR studies of the low molecular weight fraction of PE showed the presence of methyl branches. Intramolecular isomerization of the growing polymer chains involving hydrogen seems to be a possible reason for the formation of methyl branches.

Molecular structure of polyethylene produced with supported vanadium-magnesium catalyst

Molecular structure of polyethylene produced with supported vanadium-magnesium catalyst

Ethylene polymerization on vanadium-magnesium catalysts activated with organoaluminium compounds

Ethylene polymerization on vanadium-magnesium catalysts activated with organoaluminium compounds

Thermal and X‐ray investigations of ethylene–α‐olefin copolymers obtained with highly active supported Ti–Mg and V–Mg catalysts

AbstractCopolymers of ethylene with propylene, butene‐1 and hexene‐1 were prepared using a titanium–magnesium (TMC) or a vanadium–magnesium catalyst (VMC). The copolymers were examined for thermal stability by TGA, melting and crystallization behaviour by DSC and crystallinity by XRD. Fractionated samples of ethylene–hexene‐1 copolymers were also similarly characterized. Results indicate that VMC produces copolymers with a higher degree of crystallinity and greater compositional homogeneity than TMC.

Composition of the active component of supported vanadium-magnesium catalysts according to 51 NMR data

Vanadium-magnesium supported catalysts and model compounds (magnesium vanadates) have been studied with use of 51V NMR. It has been established that on the catalyst surface there exist isolated tetrahedral and octahedral vanadium species, their associated forms and magnesium orthovanadate. Their relative concentrations were determined by both the surface concentration of vanadium and the nature of modifying agents (such as SO 4 2−). The influence of CO 2 and H 2O adsorption and catalyst reduction with butene-1 on the vanadium coordination state has been studied.

Influence of water vapor on the dehydrogenation of ethylbenzene in the presence of various hydrogen acceptors

1. Water vapor influences the activity and selectivity of a vanadium-magnesium catalyst during the dehydrogenation of ethylbenzene in the presence of hydrogen acceptors, viz., oxygen, nitrobenzene, and sulfur dioxide, reducing the formation of carbon on the surface of the catalyst and promoting elongation of the operation of the catalyst without regeneration. 2. Water vapor provides for the formation of the optimally active catalyst surface, which contains vanadium ions in high oxidation states with octahedral coordination.