Heterogeneous Ziegler-Natta Catalysts Research Articles

Gas-phase polymerization of propylene at low reaction rates: a precise look at catalyst fragmentation

In this work, we reported the development of a mini-reactor experimental setup for synthesizing of polypropylene with heterogeneous Ziegler–Natta catalysts in gas-phase. Use of pro-activated 4th generation of Ziegler–Natta catalyst and preheated monomer feed enabled the polymerization reaction to be carried out at constant temperature. Evaluation of monomer consumption with high precision (0.01 bar pressure drop) allowed the detection of polymerization yield at low reaction rates. In this regard, polymerization yield, particle morphology and catalyst fragmentation were studied, as well. The results of melt microscopy showed that catalyst fragmentation was developed during the reaction, and was not restricted to the initial rupture of catalyst particles. The rate determination showed a peak during the polymerization (not necessarily at the initial stage). The results showed that depending on the reaction condition, this peak could be either a consequence of a major catalyst fragmentation or overheating. Low reaction yield, large fragments of catalyst and agglomeration of particles were considered as evidence of particle overheating and polymer local melting. As we imposed the results of melt microscopy for the polymerization conditions, a layer-by-layer fragmentation of the catalyst was found to be the main fragmentation process, at least at the beginning of the polymerization reaction.

Phosphine-Iminoquinoline Iron Complexes for Ethylene Polymerization and Copolymerization

The synthesis and olefin polymerization behavior of a series of new phosphine-iminoquinoline iron complexes is described. Upon activation, the iron complexes are highly active for ethylene polymerization, producing low molecular weight (MW) linear polyethylene (PE) with activities up to 108 g of PE/((mol of Fe) h). This activity is comparable to that of the most active bis(imino)pyridine iron catalysts and heterogeneous Ziegler–Natta catalysts. The MWs and molecular weight distributions (MWDs) of the resulting polymers can be controlled by the modification of the catalyst structures. In addition, upon activation the iron complexes are capable of copolymerizing ethylene with 1-octene, giving copolymers with α-olefin incorporation up to 8.8%.

Poly1‐hexene: New impact modifier in HIPS technology

ABSTRACTPoly1‐hexene was prepared using a conventional heterogeneous Ziegler–Natta catalyst and its stereoregularity was characterized using 13C‐NMR analysis. New kind of high impact polystyrene (HIPS) was prepared by radical polymerization of styrene in the presence of different amounts of synthesized poly1‐hexene (PH) as impact modifier (HIPS/PH) and compared with conventional high impact polystyrene with polybutadiene (HIPS/PB) as rubber phase. Scanning electron microscopy (SEM) revealed that the dispersion of poly1‐hexene in polystyrene matrix was more uniform compared with it in HIPS/PB. The impact strength of HIPS/PH was 29–79% and 80–289% higher than that in HIPS/PB and neat polystyrene, respectively. FTIR was used to confirm more durability of HIPS/PH samples toward ozonation. To study the effect of rubber type and amount on the Tgs of polystyrene, differential scanning calorimetry was employed. Results obtained from TGA demonstrated higher thermal stability of HIPS/PH sample in comparison with conventional HIPS/PB one. Our obtained results suggest new high impact polystyrene that in all studied aspects has better performance than the conventional HIPS. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43882.

Optimal Catalyst and Cocatalyst Precontacting in Industrial Ethylene Copolymerization Processes

In industrial-scale catalytic olefin copolymerization processes, catalyst and cocatalyst precontacting before being introduced in the polymerization reactor is of profound significance in terms of catalyst kinetics and morphology control. The precontacting process takes place under either well-mixing (e.g., static mixers) or plug-flow (e.g., pipes) conditions. The scope of this work is to study the influence of mixing on catalyst/cocatalyst precontacting for a heterogeneous Ziegler-Natta catalyst system under different polymerization conditions. Slurry ethylene homopolymerization and ethylene copolymerization experiments with 1-butene are performed in a 0.5 L reactor. In addition, the effect of several key parameters (e.g., precontacting time, and ethylene/hydrogen concentration) on catalyst activity is analyzed. Moreover, a comprehensive mass transfer model is employed to provide insight on the mass transfer process and support the experimental findings. The model is capable of assessing the external and internal mass transfer limitations during catalyst/cocatalyst precontacting process. It is shown that catalyst/cocatalyst precontacting is very important for the catalyst activation as well as for the overall catalyst kinetic behavior. The study reveals that there is an optimum precontacting time before and after which the catalyst activity decreases, while this optimum time depends on the precontacting mixing conditions.

Heterogeneous Ziegler–Natta catalysts with various sizes of MgCl2 crystallites: synthesis and characterization

A MgCl2-based Ziegler–Natta catalyst was characterized using X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and IR spectra. We focused on the XRD reflection at 2θ = 50° to determine the thickness of MgCl2 crystals, and validated these results with TEM pictures. SEM pictures were taken in order to measure the size of the nanoparticles formed by the MgCl2 crystals. Several compounds were synthesized for comparison and to aid interpretation of the infrared (IR) spectra. The catalysts were prepared by precipitating MgCl2, which was used as support material and subsequently treated with TiCl4. The thickness of the catalyst crystals was calculated from the XRD reflection at 2θ = 50°. Changing the precipitation temperature within a range from 40 to 90 °C altered the thickness of the MgCl2 crystal plates. The maximum thickness of 7 nm was achieved at a precipitation temperature of 60 °C. The SEM pictures showed that the nanoparticles had a diameter of ~200 nm. A crystal base unit had a volume that corresponded to that of a sphere of 3.5 nm radius. Thus, we estimated that a typical catalyst particle with a diameter of 20 μm contained about one million nanoparticles, each of which consisted of about 25,000 MgCl2 crystal units.

A Submicron Spherical Polypropylene Prepared by Heterogeneous Ziegler–Natta Catalyst

A novel type of spherical nano Ziegler–Natta catalyst was prepared by loading TiCl4 on nanosized magnesium chloride support for propylene polymerization to form nanosized spherical polypropylene (PP) particles. Polyvinylpyrrolidone (PVP), a common macromolecular surfactant, was used to ensure sphericity and inhibit aggregation of the nanosized particles. The morphology of the support (60 nm), catalyst (80 nm), and PP particles (500 nm) were spherical, and the size distribution is uniform which is confirmed by scanning electron microscopy (SEM) and transmission electron microscope (TEM), respectively. The laser granulometer results showed that the size of the prepared PP particles was in the range of 10–110 um, and the average diameter was 36 um, which had potential application in 3D printing. In addition, the obtained polypropylene had low bulk density (0.170 g/mL).

XAS and XES Techniques Shed Light on the Dark Side of Ziegler–Natta Catalysts: Active‐Site Generation

AbstractThe local structure and the electronic properties of the active Ti sites in heterogeneous Ziegler–Natta catalysts, generated in situ by interaction of the precatalyst with different Al‐alkyl activators, were investigated by combining X‐ray absorption and valence‐to‐core X‐ray emission spectroscopy (XAS and vtc‐XES), coupled with UV/Vis, FTIR, and DFT theoretical calculations. Irrespective of the activator used, the active system was found to be a highly dispersed TiCl3‐like phase in which the Ti sites are surrounded, not only by bridged chlorine ligands (with the same bond length of bulk TiCl3), but also by terminal chlorine ligands, at a much shorter distance. These results set Ziegler–Natta catalysts in the category of complex nanomaterials. Despite the observation that the investigated catalysts polymerize ethylene, cutting‐edge XAS and XES techniques do not yet offer unequivocal proof for the presence of any alkyl chain attached to the Ti sites, as a consequence of the small fraction of the active sites.

Highly stable anion exchange membranes based on quaternized polypropylene

“Side-chain-type” and crosslinkable quaternized polypropylene was prepared by heterogeneous Ziegler–Natta catalyst mediated polymerization for use in highly stable anion exchange membranes.

Polyethers as potential electron donors for Ziegler–Natta ethylene polymerization catalysts

Poly(ethylene glycol) (PEG) and poly(tetrahydrofuran) (PTHF) were studied as possible electron donors for heterogeneous Ziegler–Natta catalysts employed for ethylene polymerization. Two synthetic routes were applied for the preparation of the catalysts by mixing the polyether and TiCl4 with δ-MgCl2 support in the same step or at different stages of the catalyst synthesis. The Fourier transform infrared spectroscopy (FT-IR) studies revealed a clear interaction between PEG and δ-MgCl2 inducing changes in the chain conformation of the polyether. If PEG was added at the same stage with TiCl4, a yellow PEG/TiCl4 complex was formed and the activity of the catalyst was decreased. A partial decomposition of PTHF to tetrahydrofuran (THF) in contact with δ-MgCl2 was evidenced from the FT-IR and nuclear magnetic resonance spectroscopy data. The decomposition could be induced by the Lewis acid sites or by possible organomagnesium compounds present in the synthesized δ-MgCl2. However, the decomposition did not have a negative effect on the polymerization behavior of the prepared catalysts, when compared to the reference catalysts where THF was initially used as a donor. The catalysts prepared with polyethers as electron donor showed good activity in ethylene (around 1000kgP/molTih) and in ethylene/1-hexene (around 1600kgP/molTih) polymerizations.

Basicity of Stereoregulating Electron-Donor Compounds in Ziegler–Natta Catalysts: A Study by Infrared Spectroscopy and Chemical Exchange Reactions

The Lewis basicity of typical electron donor compounds used to modify heterogeneous Ziegler–Natta catalysts, such as diakyl ethers, silyl ethers, esters, and ketones, was characterized by determining their effects on the IR frequency of surface silanol groups of silica gel, which correlate with the heat of this specific interaction and with their “donicity”, i.e., with the heat of formation of their complexes with SbCl5. In this manner, ethers and silyl ethers are found to be more basic than ketones and esters. These differences in basicity are likely to affect the stereoregulating properties of such electron donor compounds in Ziegler–Natta polymerization catalysis. By consecutively adsorbing electron donors with various strengths on a MgCl2 support, it was shown that weakly basic donors, such as ethyl benzoate or dibutyl phthalate, are partially displaced by donors with higher basicity, such as valerophenone and propyltrimethoxysilane. This indicates that the displacement of inner donors of the ester type from a titanium–magnesium catalyst upon addition of typical external donors, such as silyl ethers, is controlled by the relative basicities of these donor molecules.

EPR investigation of TiCl3 dissolved in polar solvents--implications for the understanding of active Ti(III) species in heterogeneous Ziegler-Natta catalysts.

Multi-frequency continuous-wave and pulsed EPR techniques are employed to investigate Ti(III)-chloro complexes obtained by dissolving TiCl3 in anhydrous and hydrated methanol. Two distinctly different species, characterized by different g matrices are observed in the two cases. Hyperfine sublevel correlation (HYSCORE) spectroscopy is found to be a powerful method to identify the type of nuclei surrounding the Ti(3+) ion. For the first time, the hyperfine and nuclear quadrupole data of Ti(III)-bound (35/37)Cl nuclei are reported together with (1)H and (13)C hyperfine data of the coordinated methanol molecules. DFT modelling allows interpreting the measured spin Hamiltonian parameters in terms of microscopic models of the solvated species. The theoretical observable properties (g matrix, (35/37)Cl, (1)H and (13)C hyperfine tensors) are in quantitative agreement with the experiments for two families of complexes: [TiCln(CH3OH)6-n]((3-n)+) (with n ranging from 1 to 3) and [Ti(CH3OH)5(OH)](2+) or [Ti(CH3OH)5(OCH3)](2+). The first complex is observed in anhydrous methanol, while the second type of complex is observed when water is added to the solution, the presence of OH(-) and/or CH3O(-) species being promoted by water hydrolysis. The results obtained for the frozen solutions are critically compared to EPR spectra recorded for a MgCl2-supported Ti-based Ziegler-Natta model catalyst.

不均一系Ziegler-Natta触媒の非経験的設計に向けて－ドナーに関する構造性能相関－

不均一系Ziegler-Natta触媒の非経験的設計に向けて－ドナーに関する構造性能相関－

Melamine-Formaldehyde-Supported Titanium-Based Heterogeneous Ziegler-Natta Catalyst for Ethylene Polymerization in Slurry Process

Ethylene was polymerized at 40°C temperature and 1 atm. pressure using melamine-formaldehyde (M-F) supported titanium tetrachloride (TiCl4) catalyst. Polymer-supported catalysts were prepared with a different weight ratio of TiCl4/ M-F in hexane. The wt% titanium incorporated in the polymer matrix was determined by UV-visible spectroscopy. In addition, the resulting catalysts were characterized by SEM-EDX, XRD, and TGA. The catalytic productivity is found to be 4721.09 g PE/g Ti/h. The productivity of the catalysts also depends on the titanium content in the polymer matrix. The catalyst with titanium content 3.5 wt% showed maximum activity. These catalysts also showed good storability.

Precise Active Site Analysis for TiCl3/MgCl2 Ziegler-Natta Model Catalyst Based on Fractionation and Statistical Methods

In heterogeneous Ziegler-Natta catalysts for olefin polymerization, isolation of a single type of active sites is a kind of ambition, which would solve long-standing questions on the relationship between active site and polymer structures. In this paper, polypropylene produced by TiCl3/MgCl2 model catalysts with minimum Ti heterogeneity was analyzed by combined solvent fractionation and the two-site statistical model. We found that the active sites of the model catalysts were classified into only three types, whose proportions were dependent on the Ti dispersion state. The addition of external donors not only newly formed highly isospecific sites, but also altered the stereochemical nature of the other active sites.

A DFT study on the effect of hydrogen in ethylene and propylene polymerization using a Ti-based heterogeneous Ziegler–Natta catalyst

Hydrogenolysis of a series of model Ziegler–Natta (Z–N) catalysts to form Ti–H bond was studied within DFT. We focused our efforts on Ti species attached to the (110) lateral cut of MgCl2 which exist as different centres including Ti–C2H5, Ti–CH2CH(CH3)2, and Ti–CH(CH3)CH2CH3 in ethylene and propylene polymerization. In the next step, reactivity of Ti–H bond towards ethylene and propylene (1,2- and 2,1-) insertion was investigated. Results showed that insertion of ethylene and propylene into Ti–H bond has less barrier, in comparison with their insertion in Ti–C bond, however, ethylene and propylene 2,1- insertion lead to Ti–C2H5 and Ti–CH(CH3)2 centres respectively, which were stable due to strong β-agostic interactions. Finally, by considering different possible reactions of active centre, activity depression in ethylene polymerization and activity increase in propylene polymerization were explained in detail.

Synthesis of (co-)polyethylene with broad molecular weight distribution by the heterogenous Ziegler–Natta catalysts via one-pot strategy

A new method was presented for preparing (co-)polyethylene with broad molecular weight distribution (MWD) and high molecular weight employed by the novel heterogeneous Ziegler–Natta catalysts via one-pot strategy. The preparation of these catalysts involved the introduction of alkoxy silane compounds as electron donors. The influences of the electron donors structure, as well as polymerization conditions such as temperature, molar ratio of Al/Ti, ethylene pressure and the concentration of 1-hexene in feed on the polymerization performance for ethylene (co-)polymerization, were investigated. The morphology of the catalyst particles was characterized by SEM and Ti content of these catalysts was characterized by ICP. The GPC results showed that the obtained homopolyethylene and ethylene/1-hexene copolymer had the widest molecular weight distribution up to ca 50 when diethoxy-isopropoxy-(t–butoxy)-silane (ED3) was used as internal electron donor. The 13C NMR and FT-IR analysis indicated that these catalysts efficiently catalyzed the copolymerization of ethylene with 1-hexene.

Coadsorption model for first-principle description of roles of donors in heterogeneous Ziegler–Natta propylene polymerization

Systematic periodic density functional calculations were conducted to clarify the mechanism for donors to exert steric and electronics influences on propylene polymerization using heterogeneous Ziegler–Natta catalysts. It was concluded that TiCl4 preferentially adsorbs as mononuclear species on the MgCl2 (110) surface, and the coadsorption of donors with it is energetically viable. The coadsorption of donors on the (110) surface reinforces the electron density of the Ti mononuclear species and sterically transfers the underlying C2 symmetry to convert the originally aspecific mononuclear species into isospecific one. The nearest coadsorption of ethylbenzoate (EB) not only sterically induces the isospecificity of the Ti mononuclear species but also electrostatically improves the regiospecificity in propylene insertion. In addition, EB prevents sterically demanding chain transfer to propylene, increasing the molecular weight of the produced polypropylene. Thus theoretically derived “coadsorption model” is highly consistent with a variety of experimentally known facts and believed to be useful for the ab initio prediction of new donor structures.

Structure–performance relationship in Ziegler–Natta olefin polymerization with novel core–shell MgO/MgCl2/TiCl4 catalysts

The impact of support architectures on the olefin polymerization performance of heterogeneous Ziegler–Natta catalysts has been scarcely understood due to the complexity of pore system and the fragmentation-induced structural changes during polymerization. In this communication, a series of novel core–shell MgO/MgCl2/TiCl4 catalysts were synthesized by utilizing poreless single-crystal MgO nanoparticles as a non-fragmentable core material. Employing these catalysts, we have successfully established a structure–performance relationship (SPR) that the propylene polymerization activity is perfectly proportional to the catalyst surface area for the first time. On the other hand, polymer properties were found to be decided by active site natures independently of the surface area.

Validation of BET specific surface area for heterogeneous Ziegler-Natta catalysts based on αS-plot

Specific surface area based on the BET method in N2 adsorption has historically posed poor correlation with olefin polymerization activity of heterogeneous Ziegler-Natta (ZN) catalysts. We have investigated the validity of the BET surface area for ZN catalysts based on the αS-plot method, where non-porous core–shell MgO/MgCl2/TiCl4 catalysts were used as a reference material. The αS-plots for typical industrial ZN catalysts clarified the presence of two classes of micropores and resultant difficulty to isolate a multilayer adsorption region for the BET method. The results cast doubt on the application of the BET method to evaluate the surface area of heterogeneous ZN catalysts.

Investigation of the stereodefect distribution and conformational behavior of isotactic polypropylene polymerized with different Ziegler–Natta catalysts

AbstractTwo isotactic polypropylene (iPP) samples (PP‐A and PP‐B) were obtained by utilizing two different heterogeneous Ziegler–Natta catalysts in a given polymerization system. The molecular structure and conformational behavior of the samples were studied. The results of determination of xylene soluble material (XS) and 13C NMR showed that the average isotacticity of the samples were nearly same. However, the results of high‐resolution high‐temperature 13C NMR (HRHT 13C NMR) and successive self‐nucleation and annealing (SSA) fractionation revealed that the amount of high isotacticity of PP‐B was lower than that of PP‐A, and the amount of relative medium and low isotacticity of PP‐B was higher than PP‐A, indicating that the stereodefect distribution of PP‐B was more uniform than PP‐A. The calculation of average meso sequence length from SSA was found to be in good agreement with that calculated from the results of HRHT 13C NMR. Moreover, Fourier transformation infrared was utilized to study the influence of stereodefect distribution on the conformational behavior of iPP. The result suggested that the molecular conformation of the PP‐B was more disordered than PP‐A, the regularity of molecular structure for PP‐B was lower than that of PP‐A. The related action mechanism and the influences of which on crystallization behavior were discussed. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012