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Ziegler–Natta catalysts supported on crystalline and amorphous MgCl2/THF complexes
Abstract
Two MgCl2/THF/TiCl4 (THF = tetrahydrofuran) catalysts were prepared with MgCl2/THF supports having an amorphous (Sup-A) or a molecular compound structure, [Mg3Cl5(THF)4Bu]2 (Bu = n-butyl) (Sup-B). Amorphous catalyst A had a normal supported Ziegler–Natta-type structure with low donor and titanium contents (the molar ratios, THF/Mg = 0.14 and Ti/Mg = 0.17), while catalyst B had a molecular type of composition with high donor and titanium contents (the molar ratios, THF/Mg = 0.84 and Ti/Mg = 0.80). Despite the different composition, Cat-A and Cat-B had some structural similarities as evidenced from the powder X-ray diffraction data and nuclear magnetic resonance and infrared spectroscopy results. Polymerization studies showed that these catalysts are highly active in copolymerization of ethylene and 1-butene. Although there were differences in the composition and the structure between the catalysts, they produced almost similar copolymers as indicated by the molecular weight, molecular weight distribution, comonomer content, melting point, and crystallinity of the copolymers.
... 2,3,27 Solvent based methods or rather tedious thermochemical procedures are necessary to produce pure, anhydrous MgCl 2 . 3 Besides the rich hydrate chemistry of MgCl 2 , it also forms different organometallic complexes, which are of great importance in organic chemistry (such as Grignard reagents or for Ziegler-Natta catalysis) [30][31][32][33] and have more recently even sparked interest in the battery community. 34 These complexes are formed by the inclusion of organic molecules such as pyridine (Py), dimethoxyethane (DME), tetrahydrofuran (THF) etc., and a lot of research studies are devoted to their structural analysis, especially to understand the coordination between metals and organic molecules. ...
... 34 These complexes are formed by the inclusion of organic molecules such as pyridine (Py), dimethoxyethane (DME), tetrahydrofuran (THF) etc., and a lot of research studies are devoted to their structural analysis, especially to understand the coordination between metals and organic molecules. 30,[33][34][35][36][37][38] Moving on from halides to pseudo-halides, the previously mentioned coordination chemistry between metals and organic molecules is still a rather unexplored field for crystalline, solid thiocyanates. Some examples of solvent complexes with dimethoxyethane (DME), tetrahydrofuran (THF) and acetonitrile (AN) can be found for rare earth metal thiocyanates and Ca(SCN) 2 . ...
Mg(SCN)2·4H2O can be converted into previously unknown compounds Mg(SCN)2·(4 - x) H2O·xTHF with x = 0, 2 and 4 by multiple recrystallization in tetrahydrofuran (THF). The phases were characterized by infrared spectroscopy (IR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), and their crystal structures were solved from X-ray powder diffraction (XRPD) data. In the crystal structures isolated Mg(NCS)2(H2O)4-x(THF)x units form layered motifs. The thermal behavior of Mg(SCN)2·4H2O and Mg(SCN)2·4THF was investigated by temperature dependent in situ XRPD, where Mg(SCN)2·4THF was found to acquire a room temperature (α-form) and high temperature modification (β-form). The phase transformation is associated with an order-disorder transition of the THF molecules and with a reversion of the stacking order of the layered motifs. Further heating eventually leads to the formation of Mg(SCN)2·2THF. There thiocyanate related sulfur atoms fill the voids in the coordination sphere of magnesium, which leads to the formation of one dimensional electroneutral ∞[Mg(NCS)2/2(SCN)2/2(THF)2] chains. All investigated Mg(SCN)2·(4 - x) H2O·xTHF phases exhibit a remarkable anisotropic thermal expansion, and Mg(SCN)2·4H2O and Mg(SCN)2·2THF were found to show both positive and negative thermal expansion coefficients.
... MgCl2-ethanol adducts have also been gaining the privilege as effective Z-N supports, since these adducts tend to lose the alcohol group on interacting with TiCl4, thereby resulting in a high surface area [14][15][16]. Other supports have also been used in the synthesis of Z-N catalysts, such as: MgCl2/tetrahydrofuran (THF) complexes that are reported with good catalytic activity [17]. The recrystallization method of Z-N catalyst synthesis using TiCl4/TiCl3 has also been an industrially recognized process for the catalysis of the commercial polymerization process [5,18]. ...
... The diffractogram has characteristic peaks of MgCl2 at 2θ = 15ׄ°, 30°, 35°, 50°, and 55° [6,14,34]. The additional reflections between 11° to 30° might probably be due to the formation of the Ti complexes during the synthesis of the catalyst [17]. MgCl2 is easi-ly dissolved in alcohols, and during quick precipitation, many defects are produced in MgCl2 and lead to a porous structure [11,14]. ...
In the current study, Ziegler-Natta (Z-N) catalyst was synthesized via recrystallization method using MgCl2 as a support, AlCl3 as an activator and TiCl4 as a transition metal source. The TiCl4 used in the study was derived from Malaysian ilmenite through a sequential pyrometallurgical and hydrometallurgical process of ilmenite concentrate conversion to TiCl4. The recrystallization method of synthesis of the heterogeneous Z-N catalyst was studied by varying the synthesis parameters, such as the combined amount of MgCl2 and AlCl3, temperature, and amount of TiCl4, using statistical design of experiments. The investigation aimed at determining the best conditions for synthesizing the heterogeneous Z-N catalyst. The synthesis conditions posed a significant influence on the Ti content present in the catalyst product. The morphological and elemental analysis of SEM-EDX showed good spherical nature of the prepared catalysts. The XRD phase analysis detected the peaks of MgCl2, MgCl2-Ethanol, MgCl2/TiClx, and TiO2. The IR spectra confirmed the presence of the Mg-Cl bond at 1635 cm−1 and Ti-Cl bonds at 602 cm-1 and 498 cm-1. The produced catalyst contained a small amount of TiO2, which could be due to the seepage of moisture during the analysis or storage of the sample. The most favourable combination of the studied parameters was determined based on the Ti content in the catalyst product. Therefore, the best conditions for synthesizing the heterogeneous Z-N catalyst with high Ti content (181.1 mg/L) was at a combined amount of 2 g of MgCl2 for 6 g of AlCl3, crystallization temperature of 80 °C, and 2 mL dosage of TiCl4. Copyright © 2020 BCREC Group. All rights reserved
... In fact, experimental studies pointed out a more "direct" effect of the donors on the active site, including improving the stereoselectivity of already selective sites, the possible transformation of non-selective into stereoselective sites, variations of molecular mass and molecular mass distribution (see Table 1), and selectivity in the first monomeric unit insertion [113,114]. Overall, despite the numerous experimental [23,[114][115][116][117][118] and theoretical [119][120][121][122][123][124] studies on electron donors, how they increase catalyst productivity and isotacticity is still matter of debate. Theoretical studies on donor interactions with MgCl 2 surfaces confirm that the donor preferentially coordinates to the (110) facet [30,94,125,126], which suggested that this preferential coordination could possibly prevent TiCl 4 adsorption onto the (110) surface of MgCl 2 , considered to be feebly stereoselective. ...
Since 1963, when Karl Ziegler and Giulio Natta were jointly awarded the Nobel Prize for their discoveries of the catalytic polymerization of olefins with Ti-chlorides and Al-alkyls, heterogeneous Ziegler-Natta (ZN) catalysts have become the main catalysts for the industrial production of polyolefins. Despite of the relevance of ZN catalysts for the large-scale production of polyolefins, a clear mechanistic understanding of these catalysts is still incomplete due to the elusive nature of the active site structures. Over the last two decades, researchers have used density functional theory (DFT) methods to clarify the polymerization mechanisms and to identify the nature of the active sites, unraveling the influence of supports, cocatalysts, and the effect of internal and external donors on the polymerization processes. Major efforts were dedicated to understanding the origin of stereoselectivity in α-olefin polymerization as well as the termination reactions mechanisms, and the role that impurities can play in heterogeneous ZN catalysis. Here, we review the DFT studies on heterogeneous ZN catalysts and suggest promising areas for future research.
... SEM pictures allow the size of the nanoparticles in a Ziegler-Natta catalyst to be estimated [14,16,27]. For example, Busico et al. [28]. ...
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.
... In our studies, MgCl 2 supports have been synthesized with Grignard-Wurtz reaction in the presence of different ethers as elec- tron donors [20][21][22]. In the case of diethers, we have observed a cleavage of the C O bond during the Grignard-Wurtz synthesis. ...
Diethers are an important group of electron donors in Ziegler–Natta catalysts. A simple diether, 1,3-dimethoxypropane was studied as an electron donor in Grignard–Wurtz synthesis of a MgCl2–donor adduct. 1,3-Dimethoxypropane was unexpectedly found to undergo a cleavage reaction during the synthesis producing methoxy groups (OCH3). Each mole of 1,3-dimethoxypropane produced approximately 2 moles of methoxy groups, which are probably bound to magnesium chloride as methoxymagnesium chloride. A Grignard reagent, BuMgCl formed in the Grignard–Wurtz reaction most likely causes the cleavage of the ether bonds in 1,3-dimethoxypropane and there seem to be at least two parallel reaction paths taking place and producing at least two different by-products. The first step in the cleavage of 1,3-dimethoxypropane is a Grignard reagent (BuMgCl) induced elimination of OCH3, which gives 3-methoxy-1-propene. This intermediate product reacts further in a substitution reaction caused by the Grignard reagent producing 1-heptene as the by-product. The cleavage of the ether bond in 3-methoxy-1-propene and formation of OCH3 can also occur through another reaction path, which produces propene as the by-product.
This study focuses on gas-phase polymerization of ethylene using the titanium-based Ziegler–Natta catalysts prepared from different magnesium sources including MgCl2 (Cat A), magnesium powder (Cat B), and Mg(OEt)2 (Cat C). During polymerization, different cocatalysts were also used. It was found that Cat C with triethylaluminum as a cocatalyst exhibited the highest activity. This was likely attributed to optimal distribution of active sites on the catalyst surface. It can be observed by increased temperature in the reactor due to highly exothermic reaction during polymerization. By the way, the morphologies of the polymer obtained from this catalyst were spherical, which is more preferable. Besides the catalytic activity, crystallinity and morphology were also affected by the different magnesium sources used to prepare the catalysts.
An insight into the impact of mixed-internal donors, comprised of bis-methoxymethyl-fluorene (BMF) and 2,2-diisopropyl succinate (DIS), on the crystalline structure of MgCl2 supported Ziegler-Natta catalyst are developed. The propylene polymerization performance of catalysts is studied and compared with related single internal electron donor systems. The study of the crystalline structure of the catalysts reveals that the increase of DIS-content leads to a relatively higher α-form over β-form in the catalyst matrix. Catalyst's performance is explained through the coordination nature and composition of internal donors. The molecular weight analysis shows that the molecular weight distribution of these systems is within the range of 2.8-3.3. Meanwhile, in these systems increase of DIS content is conducive to improve not only relative content but also lamella thickness of the polymer chains. A promising feature of obtained results is providing a guideline to synthesis desired polymer properties through the change in the mixture of internal donors in the catalysts.
Ziegler-Natta precatalysts were synthetized from Lewis-base-modified-MgCl2 supports and treated by various Lewis acids, prior to activation by triethylaluminum, in order to increase their activity in ethylene polymerization. BCl3 provided the highest increase in activity. Interestingly, polymerization results showed no substantial modification of polymer properties, which is consistent with that Lewis acid only promotes the creation of new active sites, after activation by TEA, possessing very similar features to the original ones achievable with conventional precatalysts (i.e. without Lewis-acid treatment).
Ziegler‐Natta catalysts for olefin polymerization are intrinsically complex, multi‐component systems. The genesis of the active sites involves several simultaneous and sequential steps, making the individual steps and interconnections difficult to be unraveled in an unambiguous manner. In this work, we combine X‐ray diffraction and spectroscopy to probe each step of the birth and life of a MgCl2‐based Ziegler‐Natta catalyst, namely the formation of high surface area MgCl2 by dealcoholation of an alcoholate precursor, the TiCl4 grafting, and the subsequent activation by triethylaluminum as co‐catalyst. The so‐prepared catalyst was tested towards ethylene polymerization, leading to the production of mainly crystalline high‐density polyethylene. The use of operando characterization techniques allowed for probing the transient details that are difficult to be dissected in the aftermath, but can affect the overall catalytic process.
In this study, crystalline magnesium chloride-electron donor complexes were prepared by recrystallization of δ-MgCl2 in the presence of chelating electron donors, including two diethers (1,2-dimethoxyethane; DME and 1,3-dimethoxypropane; DMP) and one diamine (N,N'-diethylethylenediamine; DEEDA). The syntheses and crystal structures of such magnesium chloride complexes with chelating ligands have been rarely reported, even though they can provide important information for the selection of electron donors for stereoselective MgCl2-supported Ziegler-Natta catalysts. The synthesized complexes were characterized using single-crystal X-ray diffraction, and FTIR and CP/MAS (13)C NMR spectroscopy methods. A polymeric complex [MgCl2(DME)]n and molecular complexes [Mg2Cl4(DMP)2(H2O)] and [MgCl2(DEEDA)2] were formed in recrystallizations. In all complexes, the bidentate electron donors are bound in chelate binding mode. The [MgCl2(DME)]n complex, the structure of which consists of a helical polymeric MgCl2 backbone chain and one DME molecule coordinated to each Mg atom, can be considered as a structural model for layered MgCl2. The crystal structure of the [Mg2Cl4(DMP)2(H2O)] complex is composed of a tetrahedral Mg atom with four Cl ligands and a distorted octahedral Mg atom with two DMP molecules, one water molecule, and one Cl ligand. The two types of Mg atoms are connected to each other with a bridging Cl ligand. In the [MgCl2(DEEDA)2] complex, magnesium is octahedrally coordinated by two chloride ligands trans to each other and two DEEDA molecules. The structures of the obtained magnesium chloride-electron donor complexes clearly show how diether and diamine electron donors can dictate the crystal structure of MgCl2.
A macroporous SiO2 (Macro-SiO2), which has uniformed macropores, an open-framework structure, and thin walls, was synthesized to support Ziegler Natta catalysts. The immobilization behaviors of the Ziegler-Natta catalysts inside the Macro-SiO2 were investigated. Notable agglomerations of 8-MgCl2/titanium complex were observed because of the unconstrained environment of the Macro-SiO2. A highly superficial Ti content and a heterogeneously chemical structure of Ti active sites were thus achieved inside the Macro-SiO2. The ethylene polymerization results revealed that the Macro-SiO2/MgCl2/TiCl4 exhibited much higher catalytic activity 12.1 x 10(6) g PE.(mol Ti-h)(-1) or 17.8 X kg PE.(g cat.h)(-1)) than that of the traditional 955-SiO2 supported catalyst (i.e., 2.0 X 10(6) g PE-(mol Ti.h)(-1) or 2.7 X kg PE.(g cat.h)(-1)). Finally, the fragmentation behavior of Macro-SiO2/MgCl2/TiCl4 was investigated during the polymerization. This unconstrained environment of Macro-SiO2 afforded fewer resistances to the diffusion of reactants and also the polymer growth.
Ethylene homopolymerization by two types of Ziegler-Natta catalyst including ZN-THF and ZN-EtOH catalysts was compared. The influences of hydrogen pressure and reaction time on catalytic activity using different types of catalyst were investigated. The EDX analysis and surface area measurement confirmed that ZN-EtOH catalyst had better active center distribution than that of ZN-THF. Thus, the ZN-EtOH catalyst could retard the effect of hydrogen and show higher activity with increased hydrogen pressures. However, catalytic activity was lower with high hydrogen pressure. Moreover, polymerization time also has an influence on activity. Both catalysts showed the similar result where an increase in polymerization time can improve yield of polymer, but it decreased activity. In addition, it is seen that the increase in hydrogen pressure leads to decrease the melting temperature for polyethylenes, but it increases the crystallinity.
The Ziegler-Natta polymerization is one major example of application of catalysis in industry. Since the first discovery of the Ziegler and Natta, several modifications of the catalyst have been developed, in order to improve its performances. Nowadays, a typical Ziegler-Natta catalyst for polyethylene synthesis consists of a precatalyst, composed by TiCl4 supported on MgCl2 in presence of a Lewis base, activated by organoaluminium. The atomic-scale characterization of the Ziegler-Natta catalyst is crucial for further improvement of the catalyst. Here, the precatalyst TiCl4-MgCl2 with EtOH as inter- nal Lewis base is characterized combining solid-state NMR spectroscopy and periodic density functional theory calculations. From total energy and NMR spectra, 8 surface species were proposed showing EtO– ligands on the Ti and EtOH/EtO– on the surface Mg species. These species lead to a complete interpretation of the NMR 2D spectra. Hence a detailed molecular scale description of the pre-catalyst was obtained.
The structural, vibrational, and electronic properties of silica-supported Ziegler-Natta catalysts industrially relevant for polyethylene production were investigated in detail by means of a multitechnical approach at each step of catalyst preparation, including precatalyst activation. In the precatalyst, the TiClx phase is grafted mainly to the silica surface and almost independent of the supported MgClx phase. However, the subsequent activation by means of an aluminum-alkyl compound causes important changes to both the supported MgClx phase and the TiClx phase. The resulting catalyst is entirely reconstructed so that most of the titanium sites are detached from the silica surface and in interaction with a highly dispersed MgCl2 phase, thus mimicking the most famous and highly investigated Ziegler-Natta catalysts (not silica-supported). For the first time, the catalyst performances were monitored by means of in situ FT-IR spectroscopy in transmission mode, simulating industrially significant polymerization conditions (i.e., ethylene is fed onto the catalyst in the presence of the activator and the solvent). These results demonstrate that it is now possible to achieve a complete description at a molecular level of all the constituents of Ziegler-Natta catalysts and at each step of the catalyst preparation.
A novel chromium-modified (SiO2/MgO/MgCl2)·TiClx Ziegler-Natta ethylene polymerization catalyst with improved catalytic properties is successfully developed. The catalyst is prepared through co-impregnation of water-soluble magnesium and chromium compounds with SiO2 followed by calcination in dry air and refluxing reaction with TiCl4 to synthesize magnesium dichloride in situ, and the supporting of titanium species is also accomplished simultaneously. By characterization of the catalysts and the polymers, several key factors such as cocatalyst concentration, the addition amount of chromium species, the effect of hydrogen and 1-hexene in ethylene polymerization are systematically investigated. Compared with the original (SiO2/MgO/MgCl2)·TiClx Ziegler-Natta catalyst, the novel Cr-modified catalyst with proper initial addition amount of chromium shows better catalytic performance with 13% higher activity, 40% higher hydrogen response, and 20% higher 1-hexene incorporation ability with better short chain branch distribution.
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 1000 kgP/molTi h) and in ethylene/1-hexene (around 1600 kgP/molTi h) polymerizations.
Heterogeneous Ziegler–Natta and Phillips-type olefin polymerization catalysts have the monopoly of isotactic polypropylene production and a large share in the market of high density polyethylene, respectively. Their high industrial impact and the relatively mild conditions under which they work explain why both catalysts have been the subject of an intense research. The industrially adopted strategy to improve catalyst's formulation is still based on a trial-and-error procedure; however, a rational design of new and more efficient catalysts (which is the key to produce polyolefins having a specific architecture) necessarily implies to achieve a detailed understanding of the structure of the active sites at a molecular level. Herein, it is shown that spectroscopic methods have this potential, especially when several complementary techniques are adopted and coupled with theoretical calculations. This is valid for both Phillips-type and Ziegler–Natta catalysts, because most of the problems encountered in their characterization and understanding are common, although for decades they were not considered to be closely related. The main advantages and disadvantages of several spectroscopies in the investigation of both categories of catalysts are critically analyzed, by discussing many examples taken from the recent literature.
High performance MgCl2 supported titanium catalyst having diisobutyl phthalate (DIBP) as internal donor has been synthesized. The organic components
present in the catalyst have been studied through FTIR, 1D and 2D NMR spectroscopy. The results indicate presence of diethyl
phthalate also in addition to DIBP. WAXD analysis has been done to study the features of MgCl2 crystallites. Impact of donor components on the catalyst preparation leading to reaction pathways and performance for propylene
polymerization has been evaluated.
The effect of the type of the comonomer (1-pentene, 1-hexene, 1-octene, 1-decene and 1-dodecene) on the copolymerization of ethylene with α-olefin over vanadium (VOCl3 and VCl4) and titanium (TiCl4) catalysts supported on MgCl2(THF)2 and activated by Et2AlCl was studied. The results show that the introduction of a longer α-olefin in the ethylene polymerization feed depresses the catalytic activity of all investigated catalysts. The catalyst activity does not depend on the type of the comonomer applied but changes with the comonomer concentration in the feed. The incorporation of α-olefin in the polymer chain was found to be dependent on the type and concentration of the comonomer in the feed as well as the type of catalyst. The incorporation of α-olefin increases with the increase of the comonomer concentration in the polymerization feed and the comonomer reactivity decreases with the increase of the size of the α-olefin chain, especially in the case of comonomers longer than octene. It should be stressed, however, that both vanadium catalysts clearly demonstrate higher activity in the copolymerization process than their titanium counterpart and also produce copolymers with higher comonomer content.The density, crystallinity and melting temperature of the copolymers decrease with increase of the amount of the comonomer incorporated, but the type of the comonomer does not have a strong influence on these properties.
A new heterogeneous Ziegler-Natta (Z-N) catalyst support material, MgCl2 center dot 4(CH3CH(OH)CH2CH3) (Mg2BuOH) has been synthesized. 2-Butanol, a linear, secondary alcohol was chosen for the generation of an active MgCl2 support which results in a Z-N catalyst with TiCl4. Significant feature of this work is the formation of rod shaped molecular adduct with highly porous character. Adduct material is characterized by XRD, TG-DTA, Raman spectroscopy, solid-state NMR and SEM. Activity of the Z-N catalyst supported on Mg2BuOH for ethylene polymerization is comparable with that of commercially available heterogeneous Z-N catalyst. However, there is scope to improve the activity by optimizing textural properties.
In this research, ethylene polymerization was carried out in the presence of different additives (ZnCl2, SiCl4, and the combined ZnCl2-SiCl4) on TiCl4/MgCl2/THF catalytic system. The presence of ZnCl2-SiCl4 mixtures showed higher activity in ethylene polymerization when compared with the catalytic activity in the presence of single Lewis acids, ZnCl2, or SiCl4. The modified catalyst with ZnCl2-SiCl4 demonstrated the highest activity, which was more than three times the activity of the system without Lewis acid modification. The enhanced activity can be attributed to the reduction in the peak intensity of MgCl2/THF complexes with Lewis acid compounds as proven by XRD. This was reasonable because of some THF removal from the structure of MgCl2/THF by Lewis acid compounds. In addition to the effect of modification with additives on the partial elimination of THF, the catalytic activities could be increased due to the titanium atoms that have been locally concentrated on the surface as seen by energy dispersive X-ray spectroscopy measurement. On the basis of the in situ electron spin resonance measurement, the mixed metal chlorides (ZnCl2-SiCl4) addition could promote the amount of Ti3+after reduction with triethylaluminum. It revealed that the modification of TiCl4/MgCl2/THF catalytic system with mixed metal chlorides (ZnCl2-SiCl4) is very useful for ethylene polymerization. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1588–1594, 2013
The influence of the electron donor tetrahydrofuran (THF) on the formation of the crystalline morphology of MgCl2 was studied. MgCl2 was synthesized from magnesium and 1-chlorobutane in the presence of THF with the THF/Mg molar ratio ranging from 0.063 to 2.0. With the highest amounts of THF crystalline particles were formed, as evidenced from scanning electron microscopy images. The X-ray structure analysis of these single crystals revealed the new organometallic Mg compound [Mg3Cl5(THF)4Bu]2 (Bu = n-butyl), which has an open dicubane-like structure with four octahedrally and two tetrahedrally coordinated Mg atoms. The compound also contains two butyl groups directly bound to the Mg atoms at the opposite corners of the dicubane structure. With the lowest THF amounts, structurally disordered MgCl2/THF complexes were formed. The transition from single crystals toward particles with an amorphous morphology appeared to occur through an intermediate type of morphology with a three-dimensional network structure. The network structure is formed from a thin (30–100 nm) fiberlike MgCl2/THF material. Infrared and solid-state 13C nuclear magnetic resonance spectroscopy measurements revealed different types of Mg sites present in the MgCl2/THF complexes.
Study to get a high activity for olefin polymerization by a supporting Ziegler-type catalyst on MgCl2 was carried out. Only heating anhydrous MgCl2 with TiCl4, very little amount of TiCl4 was fixed because of high crystallinity and small surface of anhydrous MgCl2. A pretreatment by some organic electron donner such as butanol or methyl acetate before the reaction with TiCl4, resulted a considerable fixation of TiCl4 onto MgCl2 and gave high activity for ethylene polymerization by 70—90 times of conventional TiCl3 catalyst. The study of the TiCl4–MgCl2 comilling, the other supporting technique, was also investigated. There are efficient Ti and inefficient ones to participate in the formation of active species among the fixed Ti. The titanium fixed in the initial stage of milling at the active site of broken MgCl2 particle surface was rather efficient. Furthermore, The high stereospecificity was performed for propylene polymerization, by arranging some organic electron donner around Ti for the above mentioned TiCl4–MgCl2 catalyst.
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.
Adsorption of various internal electron donors (IDs) and coadsorption of these IDs with TiCl4 on activated MgCl2 have been studied by DRIFT and analytical techniques. As a result, the structures of the ID species on the (1 0 4) and (1 1 0) MgCl2 surfaces were proposed. Based on the ν(CO) band resolution data for the adsorbed ID, it was estimated that the activated MgCl2 surface contains ca. 90% of the five-coordinated Mg cations residing on the (1 0 4) surface and ca. 10% of the four-coordinated Mg cations residing on the (1 1 0) surface. The size of ID molecule was found to influence its adsorption more significantly on the (1 0 4) MgCl2 surface than on the (1 1 0) MgCl2 surface. Coadsorption of the various ID and TiCl4 demonstrated that TiCl4 formed greatly weaker surface complexes than the ID; obviously, upon coadsorption TiCl4 occupies mainly the adsorption sites that are inaccessible for the ID because of steric reasons.
While Ziegler–Natta (ZN) polymerization is one of the most important catalytic industrial processes, the atomic-scale nature of the catalytically active surface species remains unknown. Coupling high-resolution solid-state NMR spectroscopy with periodic density functional theory (DFT) calculations, we demonstrate that the major surface species in the ZN precatalyst corresponds to an alkoxy Ti(IV) surface species, which probably results from the ring-opening of tetrahydrofuran (THF) on a cationic Ti(IV) species.
Extensive dry ball milling (up to 200 h in a vibratory mill) of the layer-lattice type MgCl2 structure (used as olefin polymerization support) results in a moderate reduction in the mean particle size and a more pronounced decrease in crystallite size in directions parallel and transverse to the hexagonal crystal axis. The relative reductions in crystallite size are understood on the basis of sliding of the Cl-Mg-Cl double layers by the action of shear. The effect is accompanied by a change from cubic or hexagonal close packed layer structures to arrangements with lattice disorder. The effect can be reversed completely by annealing above about 250 °C.
A good supporting material for Ziegler-Natta type catalysts was obtained by reaction between powdered metallic magnesium and 1-chloro-n-CmH2m+1 (m = 3–9). This reaction affords a highly disordered form of MgCl2 which was characterized by FT-IR spectroscopy and powder X-ray diffraction (XRD) analysis. This MgCl2 form shows a crystallographic disorder much higher than that exhibited by the products obtained following the commercial activation procedures of α-MgCl2, either mechanical or chemical. As shown by the XRD spectra, the synthetized MgCl2 is characterized by the typical structure of the δ-form. Therefore, this MgCl2 form, in view of its highly disordered structure, appears as a promising material for the preparation of active supported Ziegler-Natta type catalysts. Thus, by titanation of the obtained δ-MgCl2 we have prepared some supported catalysts which have been tested in the slurry propene polymerizations, showing high activity (136000–138000 gPP/gTi) and good stereoselectivity (82–88% I.I.).
Infrared vibrational spectroscopy has been used to reveal the conformations of the Ziegler−Natta catalysts internal electron donor 9,9-bis(methoxymethyl)-fluorene and its selectively deuterated derivative (9,9-bis(1,1-dideutero-methoxymethyl)-fluorene), in the solid state, complexed with TiCl4 and MgCl2 and in the catalyst precursor. The experimental spectra have been interpreted by means of spectroscopic correlations and theoretical results from quantum chemical calculations on selected molecular models. From experimental and theoretical data two chelating geometries of the donors with both oxygen directly interacting with Ti or Mg atoms turn out to be favorable. They can be identified by the four dihedral angles along the CH3−(O−CH2−R−CH2−O)−CH3 residue that take on conformations of approximately trans (T), gauche+ (G), or gauche− (G′), namely, TGGT and TGG′T. We propose the TGGT conformations for the crystal phase of the donor, the TGG′T when it is complexed with TiCl4, and both TGGT and TGG′T when complexed with MgCl2 along the [110] lateral cut. In the precatalysts, donors molecules are complexed along the [110] MgCl2 lateral cut and likewise the MgCl2 complex. A model for the catalytic sites consisting of TiCl4 species and donor molecules complexed along the [110] lateral cut is proposed.
A new molecular adduct, MgCl2·4(CH3)2CHOH, has been synthesized and characterized for structural aspects and demonstrated for super active ethylene polymerization activity with TiCl4 to ultrahigh molecular weight polyethylene in high yield.
Ziegler–Natta type catalysts for α-olefin polymerization supported on MgCl2–ZnCl2 mixtures were prepared and the effect of Zn doping on the catalyst performance was investigated. The supports were obtained by reacting a blend of powdered metallic magnesium and zinc with an excess of 1-Cl-n-butane under UV-visible radiations. Their treatment with TiCl4 yielded procatalysts, which were fully characterized and tested in propylene and ethylene polymerization. The effects of ZnCl2 as doping salt in MgCl2 supports were also investigated in dependence on the zinc concentration. In particular, in the propylene polymerization carried out at 70°C and 2 atm, the catalyst increased its activity on increasing the ZnCl2 content in the procatalyst up to 0.73 wt.% expressed as zinc. Going to a higher ZnCl2 content, it was found that the activity decreases. The isotactic index of polypropylene produced increases only slightly on increasing the zinc content up to a weight percentage of 0.73, but going to a higher zinc chloride content, the isotacticity decreases. In comparison with a reference commercial catalyst, the polydispersity was found to be very little affected, at least in the range of ZnCl2 content that we had investigated. The ZnCl2 doping was shown to cause a notable effect on the activity of the catalyst also in the ethylene polymerization (T=70°C, P=2 atm). In fact, in ethylene polymerization, a catalyst based on a support containing a doping amount of 0.73 wt.% of zinc exhibited an activity 2.5 times greater than that shown by the reference catalyst.
Activity of a titanium catalyst supported on a bimetallic magnesium–aluminium system, involving a Lewis base [MgCl2(THF)2/Al(C2H5)2Cl], was studied in ethylene polymerization, and the effect of the catalyst composition on the properties of the final polymer produced was investigated. Analysis and discussion of the findings covering also some part of the kinetic study, resulted in defining the roles for MgCl2 and a Lewis base in the third-generation Ziegler-Natta catalysts. MgCl2 forms a bimetallic complex with an organoaluminium compound, which involves also a Lewis base. Its reaction with TiCl4 yields a very active catalyst wherein the Mg : Ti molar ratio is close to 1. This means that MgCl2 is involved in creating catalytic active sites. The structure of these catalytic sites were suggested. Hence, the role for MgCl2 can be twofold: it is a component of a catalytic active site, and it is a support if Mg : Ti > 5. It was found in the study that the catalytic function of MgCl2 can be maintained while its supporting function can be omitted. Thus, MgCl2 can be substituted for Al2O3 and the catalytic system obtained will have the same activity. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1005–1011, 1998
In this work, we report on the structure and formation energy of uncovered MgCl2 crystallites of different shapes (hexagonal and square), sizes (up to crystallites composed of 157 MgCl2 units), and edges (crystallites presenting the (104) and (110) edges). Both uncovered crystallites and crystallites covered by dimethyl ether were considered. Our results indicate that the formation energy of uncovered crystallites, irrespective of shape, size, and edges, linearly depends on the density of vacancies (measured as the ratio between the number of Mg vacancies and the number of MgCl2 units in the crystallite) and that larger crystallites that present (104) edges are favored. In the case of crystallites completely covered by dimethyl ether, our results indicate that the formation energy of crystallites, again irrespective of shape, size, and edges, inversely depends on the dimethyl ether/Mg ratio. As opposed to uncovered crystallites, in the presence of dimethyl ether, smaller crystallites presenting (110) edges are favored. The knowledge acquired with both uncovered and dimethyl ether-covered crystallites was used to achieve insight into the behavior of carbon monoxide-covered crystallites by performing calculations on a limited number of small crystallites.
Two Ziegler−Natta catalysts supported on molecularadducts, namely, MgCl2·6EtOH (ME) and MgCl2·5EtOH·EtOOCPh (Est-ME), have been prepared. A systematic effort has been made to unravel the molecular level structure−property relationships of the catalysts and adducts. Ethylbenzoate is an internal electron donor, and its in situ formation through EtOH + PhCOCl coupling is successfully achieved. The above adduct has been treated with TiCl4, and the resultant catalyst (Ti/Est-ME) is evaluated for ethylene polymerization activity. IR and 13C CP/MAS NMR of Est-ME (Ti/Est-ME) show carbonyl features at 1730 (1680) cm−1 and 169 (170) δ, respectively, providing direct support for the presence of ester as an integral part. In spite of low surface area, Ti/Est-ME gives higher yield for ethylene polymerization than the one derived from ME. The results indicate that electronic environment is more important than surface area or any other single factor in determining the polymerization activity.
The ability of a MgCl 2 support to activate a transition metal catalyst has been found to depend both on the crystallographic structure of the support and on the nature of the catalyst. A high degree of crystallographic disorder can be very effective for the immobilization and activation of titanium and vanadium complexes, but is not necessarily effective for zirconocene activation. A highly disordered support prepared by the reaction of MgBu 2 with HCl gave high activity with TiCl 4 but low activity with ( n ‐PrCp) 2 ZrCl 2 . High polymerization activities with the zirconocene were only obtained with supports of type MgCl 2 /AlR n (OEt) 3− n prepared from the reaction of AlR 3 with MgCl 2 · 1.1EtOH. These supports are characterized by additional peaks in the X‐ray diffraction pattern, indicating the presence of a crystalline structure which is absent in the other supports and contains highly Lewis acidic sites able to generate the active metallocenium species.
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The use of tri iso butylaluminium and diethyl zinc as cocatalyst and chain transfer agent respectively in ethylene/1‐hexene copolymerisation, carried out with a Ziegler‐Natta catalyst of type MgCl 2 /TiCl 4 /di iso butyl phthalate, has been found to give significantly higher comonomer incorporation than was obtained when triethylaluminium was used in combination with diethyl zinc. Tri iso butylaluminium reacts readily with diethyl zinc, yielding metallic zinc, most likely via the formation of intermediate hydride species. In contrast, the use of 2,6‐dimethylpyridine as external donor in this system has been found to have relatively little effect on comonomer incorporation and distribution, indicating that a change from tendentially isospecific to syndiospecific propagation in propene polymerisation is not accompanied by a major improvement in comonomer distribution in ethylene/1‐hexene copolymerisation.
¹ H NMR spectrum of the liquid product of the reaction between tri iso butylaluminium and diethyl zinc.
image ¹ H NMR spectrum of the liquid product of the reaction between tri iso butylaluminium and diethyl zinc.
Polyolefins represented by polyethylene (PE) and polypropylene (PP) are indispensable materials in our daily lives. TiCl3 catalysts, established by Ziegler and Natta in the 1950s, led to the births of the polyolefin industries. However, the activities and stereospecificities of the TiCl3 catalysts were so low that steps for removing catalyst residues and low stereoregular PP were needed in the production of PE and PP. Our discovery of MgCl2-supported TiCl4 catalysts led to more than 100 times higher activities and extremely high stereospecificities, which enabled us to dispense with the steps for the removals, meaning the process innovation. Furthermore, they narrowed the molecular weight and composition distributions of PE and PP, enabling us to control the polymer structures precisely and create such new products as very low density PE or heat-sealable film at low temperature. The typical example of the product innovations by the combination of the high stereospecificity and the narrowed composition distribution is high-performance impact copolymer used for an automobile bumper that used to be made of metal. These process and product innovations established these polyolefin industries. The latest MgCl2-supported TiCl4 catalyst is very close to perfect control of isotactic PP structure and is expected to bring about further innovations. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1–8, 2004
Some 60 addition compounds of the type MgCl2nEtOH (6 n 1.4), formed between magnesium chloride and ethanol, have been prepared and were characterized by means of X-ray powder diffraction. The spectra were interpreted on the basis of a subset of nine characteristic diffraction patterns, and a range of (partially) new products, MgCl2nEtOH with n=6, 4.5, 4, 3.33, 2.5, 1.67, 1.50 and 1.25, is described. MgCl2 adducts with alcohols can be converted to active olefin polymerization catalyst supports through the elimination of the alcohol molecules from the adducts by thermal desolvation.
The ethylene copolymerizations with 1-hexene or 1-octene in the presence of hydrogen using three catalysts, MgCl2(THF)2/VOCl3/Et2AlCl, MgCl2(THF)2/VCl4/Et2AlCl, MgCl2(THF)2/TiCl4/Et2AlCl, were investigated. It was found that the addition of hydrogen into the copolymerization feed reduces the molecular weight of the copolymers produced and decreases the activity of all the studied catalysts. The microstructure of the copolymers obtained was determined on the basis of 13C NMR investigations and the reactivity ratios of the comonomers were calculated. The lack of tendency of the olefin comonomers to the creation of the polymer block was confirmed. It was found that the products of the comonomers reactivity ratios are close to the value of 1 and they are independent of both the catalyst and comonomer type. This means that the structure of all the copolymers obtained is random.
The origin, the classification and the effect on catalytic performance of pore texture of heterogeneous catalysts are briefly examined. The techniques and the methods suitable for the determination of related properties (surface area, pore volume and pore size distribution) are reviewed. Particular attention is paid to the most widely used ones: vapour adsorption at low temperature, mercury porosimetry, incipient wetness impregnation and picnometry. Advantages, disadvantages and applicability of each method to different pore textures are pointed out. The best choice of a group of methods, allowing a complete characterization of catalysts, is proposed.
The crystal structures of three MgCl(2)·nEtOH complexes with n=1.5, 2.8, and 3.3 have been fully determined. Such complexes are the fundamental precursors for Ziegler-Natta polymerization catalysts used to produce polyolefins on a multimillion-ton scale worldwide. The ab initio structure solution showed that the structure of MgCl(2)·nEtOH complexes with n=1.5 and 2.8 are based on ribbons of metal-centered octahedra, whereas for n=3.3 this chainlike arrangement breaks into a threadlike structure of isolated octahedra linked by hydrogen bonds. A clear correlation between catalyst performance and the crystal structure of precursors has been found, and reveals the fundamental role of the latter in determining catalyst properties. The direct knowledge of building blocks in the precursor structures will help to develop more accurate models for activated catalysts. These models will not require the arbitrary and oversimplified assumption of locating the catalyst active sites on selected cut surfaces of the α-MgCl(2) crystal lattice.
X-ray powder diffraction (XRPD), Infrared, Raman, and UV/Vis spectroscopy have been used to investigate the structural, vibrational, and optical properties of Ti and Mg chloride tetrahydrofuranates as precursors of heterogeneous Ziegler-Natta catalysts for polyethylene production; as well as their interaction compound (pro-catalyst) and the final catalyst obtained after interaction with the AlR(3) activator. Although the structure of the precursors and of the pro-catalyst were well known, that of the catalyst (obtained by reaction of the pro-catalyst with AlR(3)) was not easily obtainable from XRPD data. IR and Raman spectroscopy provided important information on tetrahydrofuran (thf) coordination and on the ν(M-Cl) region; whereas UV/Vis spectroscopy gave the direct proof on both the formal oxidation state and the coordination environment of the active Ti sites. Those presented herein are among the first direct experimental data on the structure of the active Ti sites in Ziegler-Natta catalysts, and can be used to validate the many computational studies that have been increasing exponentially in the last few decades.
Ethanol associates easily with MgCl(2) to form adducts of complex architecture, but until now available characterization methods have failed to identify the pure stoichiometric compounds and their structures. To remedy this, we set about applying homonuclear and heteronuclear 2D correlated solid-state NMR spectroscopy to identify the pure compounds and the ethanol-to-magnesium coordination pattern. High spinning speed and Lee-Goldburg sequences were able to reduce the hydrogen spin-diffusion and homonuclear coupling in the crystalline solid, thus achieving high resolution also in the hydrogen domain. On this basis, the pure adducts, of interest as catalyst supports for Ziegler-Natta polymerization, were isolated for the first time. Magnesium coordination sites with given numbers of ligands and their multiplicity in the crystal cells were determined in the new-found stoichiometric complexes. Variable temperature and 2D carbon-carbon exchange NMR, as well as relaxation times in the fast motion regime, revealed the disordering phenomena generated by ethanol dynamics in the crystal. Decoding the intriguing polymorphism of the precursors permits to trace the genealogy of tailored MgCl(2) titanate granules, active as highly productive catalysts for the stereospecific polymerization of olefins.