A Comparative Study of AlCl3 and FeCl2-Modified TiCl4/MgCl2/THF Catalytic System in the Presence of Hydrogen for Ethylene Polymerization

Niyomthai, Thanyathorn; Ratchadaphet, Aniroot; Jongsomjit, Bunjerd; Praserthdam, Piyasan

doi:https://doi.org/10.1155/2016/6289326

International Journal of Polymer Science

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Abstract Introduction Experimental Results and Discussion Conclusion Additional Points Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2016 | Article ID 6289326 | https://doi.org/10.1155/2016/6289326

A Comparative Study of AlCl₃ and FeCl₂-Modified TiCl₄/MgCl₂/THF Catalytic System in the Presence of Hydrogen for Ethylene Polymerization

Thanyathorn Niyomthai,¹Aniroot Ratchadaphet,¹Bunjerd Jongsomjit,¹and Piyasan Praserthdam¹

Academic Editor: Atsushi Sudo

Received28 Apr 2016

Revised06 Jul 2016

Accepted14 Jul 2016

Published06 Sept 2016

Abstract

Ethylene homopolymerization over TiCl₄/MgCl₂/THF catalysts modified with different metal halide additives (AlCl₃ and FeCl₂) with and without hydrogen was investigated based on catalytic activity and polymer properties. Lewis acid modification can improve activity because it can remove the remaining THF in the final catalyst, which can poison the catalyst active sites via the ring-opening of THF that was confirmed by XRD measurements. Moreover, the activity enhancement was due to the formation of acidic sites by modifying the catalysts with Lewis acids. Thus, FeCl₂ doped catalyst (Fe-THF) exhibited the highest activity followed by AlCl₃ doped catalyst (Al-THF) and undoped catalyst (None-THF). In H₂/C₂H₄ molar ratio of 0.08, Fe-THF showed a better hydrogen response than Al-THF due to more titanium cluster distribution. Fe-THF is considered to have more clustered Ti species than Al-THF. As a consequence, it led us to obtain more possible chances to precede chain transfer reaction by hydrogen. The molecular weight, melting temperature, and crystallinity of obtained polymers were investigated by GPC and DSC measurement, respectively.

1. Introduction

The present generation of heterogeneous catalytic systems for polyolefin production based on TiCl₄/MgCl₂ supported catalyst is referred to Ziegler-Natta (ZN) catalyst. It is one of the most advanced industrial and academic applications of catalysis. One of the important steps to create the highly active ZN catalyst is the preparation method of the precatalyst prior to the activation with alkylaluminum [1]. Lewis base, normally an alcohol likes ethanol or an ether likes tetrahydrofuran (THF) [1–3], plays an important role in creating bimetallic complexes with magnesium dichloride [3, 4]. Even though ethanol is more preferable choice as Lewis base, there are still many researches employing THF in the process of catalyst preparation. This is due to its easiness to prepare, which leads to the lower in cost. It also provides good hydrogen response and the moderate activity and increases the molecular weight of polymer [4–6]. Besides, Sobota [7] revealed that THF is the most mature as a selectivity control agent in ethylene polymerization. Therefore, THF complexes of MgCl₂ and TiCl_x are frequently employed as the precursors. Another important factor that industries are concerned with is controlling of the molecular weight distribution of obtained polymers because it affects the final mechanical and processing properties of the polymer [8–11]. Polymer having short chains provides good processability, while polymer having high molecular weight fraction provides good mechanical properties. Hence, polymers having both short and long chains are required to produce polymers with a broad molecular weight distribution [12]. However, narrow or board molecular weight distribution depends on the desired application. As it is widely noticed, hydrogen is the key agent usually used to control molecular weight of polymer. Effect of hydrogen on activity is complicated to forecast because it depends on several factors such as type of catalyst system, monomer, donor systems, and polymerization conditions [13]. For example, hydrogen typically decreases activity in ethylene polymerization but increases in propylene polymerization for ZN catalyst [14]. Nevertheless, Hair and Hertl [15] revealed that the addition of a small amount of hydrogen can improve the activity in ethylene copolymerization. However, hydrogen effect on ZN catalyst systems is still ill-defined; however it is very significant. Thus, in this work, a comparative study of the AlCl₃ and FeCl₂ modified TiCl₄/MgCl₂/THF catalytic system on the activity and polymer properties in the presence of hydrogen for ethylene polymerization was investigated.

2. Experimental

2.1. Materials

All materials and operation steps were conducted under nitrogen atmospheres using a standard glove box and Schlenk techniques. Polymerization grades of ethylene, hydrogen, and nitrogen were purchased from Linde Co., Ltd. Tetrahydrofuran (THF), aluminum trichloride (AlCl₃), and iron (II) chloride (FeCl₂) were purchased from Sigma-Aldrich and used without further purification. Titanium (IV) chloride (TiCl₄) was supplied from Merck. Anhydrous MgCl₂, n-hexane, and triethylaluminum (TEA) were donated by Thai Polyethylene Co., Ltd.

2.2. Catalyst Preparation Methods

2.2.1. TiCl₄/MgCl₂/THF Catalyst [2, 16]

A mixture of 2 g of anhydrous MgCl₂ (and Lewis acid if any) and 150 mL of tetrahydrofuran (THF) was added into the 500 mL three-necked round bottom flask equipped with magnetic stirrer. Then, 2 mL (0.87 mole) of TiCl₄ was injected dropwise. The reaction was heated up to 70°C and held for 3 h. After that, the reaction was cooled down to 40°C. The washing steps were carried out for 7 times with n-hexane prior to obtain the None-THF catalyst.

TiCl₄/AlCl₃/MgCl₂/THF and TiCl₄/FeCl₂/MgCl₂/THF were denoted as Al-THF and Fe-THF, respectively, as shown in Table 1. Lewis acid was directly added to MgCl₂ in THF solution. All modified catalysts were synthesized by the same procedure as used for the preparation of the unmodified catalyst (None-THF) excepting for adding anhydrous AlCl₃ for Al-THF and FeCl₂ for Fe-THF, with 0.07 mol/mol of Lewis acid/MgCl₂.

2.3. Polymerization Reaction

2.3.1. Effect of H₂/C₂H₄ Molar Ratio

Ethylene polymerization was performed in 2 L autoclave reactor connected with n-hexane liquid feed line and nitrogen, hydrogen, ethylene gas feed lines. Purification of reactor and all feed lines were conducted by evacuation and purging with nitrogen many times. After that, 1 L of n-hexane, triethylaluminum, and catalyst slurry (Al/Ti molar ratio = 140) were introduced into the reactor. Then the reaction was heated up to 67°C. At this temperature, 1 bar of nitrogen was filled and followed by the desired amount of hydrogen (H₂/C₂H₄ molar ratio = 0, 0.08, 0.20, and 0.30). Finally, the reactor was pressurized with the total pressure of 8 bars with ethylene at 75°C. The polymerization temperature and time were 80°C and 1 h, respectively. The obtained polymer was dried at room temperature overnight.

2.4. Characterizations

2.4.1. Catalyst

Inductively Coupled Plasma (ICP). Inductively coupled plasma optical emission spectrometer (ICP-OES optima 2100 DV from Perkin Elmer) was employed to determine element content in bulk. The catalyst sample was digested with 5 mL of hydrochloric acid and then diluted with DI water.

Fourier Transform Infrared Spectroscopy (FT-IR). The Fourier transforms infrared spectroscopy was applied with Nicolet 6700 FT-IR spectrometer with transmittance mode. The catalyst powder was prepared as a thin film on NaCl disk and kept under argon atmosphere during the analysis. The measurement was carried out in the range of scanning between 400 and 4000 cm⁻¹ for 400 times.

X-Ray Diffraction (XRD). XRD was performed to identify phases and crystallinity of the catalyst and support samples using Bruker of D8 Advance. The sample was placed on holder under argon atmosphere to prevent air and moisture during the experiment. Diffraction patterns were recorded with the scanning speed of 0.3 second/step and step size 0.02 in the 2θ range of 10–80 degrees.

Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX). SEM was used to identify the morphologies of catalyst, while EDX was applied to investigate the elemental distribution. The SEM of Hitachi mode S-3400N was employed. The EDX was performed using Link Isis series 300 program.

2.4.2. Polymer

Gel Permeation Chromatography (GPC). Molecular weight and molecular weight distributions of polymers were determined by using a high temperature GPC (Waters 150°C) equipped with a viscometric detector. The characterization was performed at 160°C and used 1,2,4-trichlorobenzene as a solvent.

Scanning Electron Microscopy (SEM). SEM was applied to determine the morphologies of the resultant polymers. The same equipment (as aforementioned) was carried out.

Differential Scanning Calorimetry (DSC). The melting temperature () and percentage of crystallinity () of polyethylene were determined by DSC-TGA with TA Instruments SDT Q600 V8.1 Build 99. The characterization was performed at a heating rate of 10°C/min in the range of temperature = 30–600°C. The crystallinity of polymers was calculated by the following equation:where is heat of fusion of polyethylene sample.

is that the heat of fusion of linear polyethylene equals to 290 J/g [16].

3. Results and Discussion

3.1. Catalyst Characterization

Table 2 reported the chemical compositions of all catalysts determined by ICP and EDX measurements. It was found that although the addition of TiCl₄ amount of all catalysts was fixed, the titanium contents in bulk, which were determined by ICP, were slightly changed compared with None-THF due to the presence of different Lewis acids on the support. This might be the fact that the Lewis acid modification can reduce free-vacancies of MgCl₂ crystallization and Ti insertion [17, 18]. Thus, the titanium content of Al-THF slightly decreased. According to this result, it is also in good agreement with the report by Coutinho and Xavier [19]. They observed that the addition of PCl₃ in TiCl₄/MgCl₂ via ball milling method decreased the Ti incorporation due to decrease of free-chloride crystal vacancies in the surface of milled MgCl₂ caused by the complexation of PCl₃. Furthermore, increasing of PCl₃ content can be attributed to the reduction of catalyst active sites resulting in drop in Ti incorporation. Nonetheless, Ti content in bulk of Fe-THF increased compared with None-THF. This is because the radius of iron (II) ions (Fe²⁺) and the crystal compound structures (FeCl₂) were similar to Mg²⁺ and MgCl₂. Moreover, Tanase et al. [20] reported that the catalyst contained metal dichloride as a carrier material exhibited relatively high activity compared with MgCl₂. Therefore, FeCl₂ was considered as one of effective supports for anchoring TiCl₄ resulting in the improvement of Ti content. However, from the EDX result, it was found that all catalysts showed higher Ti content on catalyst surface than Ti content measured by ICP technique. This indicated that most Ti active sites were located on the surface of catalyst more than in the catalyst pores. In addition, the Lewis acid contained catalysts showed a higher Ti/Mg ratio than that of unmodified catalyst. Fe-THF displays the highest Ti/Mg ratio. It indicated that a lot of Ti atoms can incorporate at MgCl₂ support.

3.1.1. FT-IR Spectroscopy of Catalysts

FT-IR spectroscopy was applied to define the interaction of TiCl₄ and MgCl₂ support as presented in Figure 1. Typically, IR spectra of the TiCl₄/MgCl₂ complex with THF were in the range of 1100–700 cm⁻¹, indicating the most sensitive to THF complexes with a metal center [21]. Many researchers reported that pure THF shows the symmetrical and asymmetrical C-O-C stretching bands around 913 cm⁻¹ and 1071 cm⁻¹, respectively [21, 22]. However, TiCl₄/MgCl₂/THF catalysts modified with different Lewis acids (Al-THF and Fe-THF) exhibited a slight shift to the lower wavenumbers compared with None-THF. None-THF, Al-THF, and Fe-THF display the symmetrical C-O-C stretching bands at 883.4, 874.6, and 873.6, respectively, and its asymmetrical C-O-C stretching bands at 1032.6, 1025.1, and 1024.6, respectively. It indicated that metal halides incorporated with the Ti-Mg-THF bimetallic complex resulting in the lower frequencies of the C-O-C stretching bands of THF. Therefore, this result may cause an increase in the Lewis acidity of the active center [2, 3]. All catalysts also displayed C-H stretching band around 2885–2976 cm⁻¹. In addition, a neutral alkoxy Ti species, which are inactive, via the ring-opening of THF were also found [1]. These IR spectra were observed in the range of 3000–3600 cm⁻¹. These results are probably due to the fact that the interaction of MgCl₂ and THF can change the nature of Ti species through creating a transient cationic Ti mononuclear species, which finally leads to a neutral alkoxy Ti species via the ring-opening of THF [1].

(a)

(b)

3.1.2. XRD Measurement of Catalysts

XRD patterns of synthesized catalysts are reported in Figure 2. The peaks at 2θ = 14.9°, 30.2°, 34.8°, and 50.3° represent the high disorders of δ-form of MgCl₂ species, which are active for polymerization. The characteristic peaks of MgCl₂/THF complex were located at 2θ = 20.4° and 32.0° [16, 17]. The XRD peak around 2θ = 18.1° was also found and assigned as the creation of TiCl₄/MgCl₂/THF complex [23]. Moreover, the characteristic peaks of TiCl₄/THF complex and TiCl₃/THF complex were noticed at 2θ = 11.1° and 17.8°, respectively [16, 17, 23]. We did not observe the Lewis acid characteristic peak of Lewis acid modified catalysts because Lewis acid completely participated in TiCl₄/MgCl₂/THF structure [24, 25]. It can be suggested that the radius of Fe²⁺ (0.61 Å) is closer to Mg²⁺ (0.65 Å) than that of Al³⁺ (0.50 Å). Thus, the compatibility of FeCl₂ with MgCl₂ would be better than that of AlCl₃ with MgCl₂ [16, 26]. Due to the fact that Lewis acid can remove THF from MgCl₂/THF complex, therefore, Lewis acid presumably destroys the crystalline structure of MgCl₂/THF complex. This results in the lowering or loss in magnitude of XRD peaks at 2θ = 20.4° and 32.0°.

3.1.3. SEM Images of Catalysts

Typical SEM images are shown in Figure 3. It can be seen that None-THF is in irregular shape while modified catalysts with Lewis acid (Al-THF and Fe-THF) are in shape of bar.

(a)

(b)

(c)

(d)

(e)

(f)

3.2. Catalytic Activity

3.2.1. Effect of Lewis Acids Modification on the Catalytic Activity in the Absence of Hydrogen

According to Table 3, Lewis acid modification by adding AlCl₃ and FeCl₂ can improve the activity around 17% and 86% increase, respectively, compared with the activity of unmodified catalyst. This is because of the formation of acidic sites by modifying the catalysts with Lewis acids [2]. In addition, AlCl₃ and FeCl₂ could also remove THF in catalyst resulting in enhancement in the activity of ethylene polymerization. The remaining THF of MgCl₂·THF structure in the final catalyst leads to the reduction in catalytic activity because the remaining THF can poison the catalyst active sites [5]. THF occupies the coordination sites of Ti to make Ti a neutral alkoxy Ti species, which is an inactive center, via the ring-opening of THF [1]. Moreover, TEA can also coordinate with THF and remove some THF from the Ti center and then provide higher catalytic activity [5]. Chen and Fan [27] stated that the active center distribution of TiCl₄/MgCl₂ could change by doping AlCl₃, which can be observed by the polymer GPC curves through multiple Flory components. From the formation of more types of active center, Al-Mg-THF complex [28] leads to an improvement of catalyst performance. As mentioned earlier about the similar ionic radii of Fe and Mg, it could then be stated that FeCl₂ was more efficient to incorporate with MgCl₂ than AlCl₃. Therefore, Fe-THF gave higher activity than Al-THF.

3.2.2. Effect of Lewis Acids Modification on Hydrogen Response for Ethylene Polymerization

TiCl₄/MgCl₂/THF catalysts modified with Lewis acids (AlCl₃ and FeCl₂) were conducted in ethylene polymerization in the presence of various hydrogen pressures for comparative study on catalytic activities. The polymerization activities results of TiCl₄/Lewis acid/MgCl₂/THF catalysts are summarized in Table 3. Catalysts modified with Lewis acid presented the higher hydrogen response than unmodified catalyst, especially Fe-THF. Hydrogen response is high even in the system that has a small amount of hydrogen. Only small additional amount of hydrogen at H₂/C₂H₄ molar ratio of 0.08 results in 34% decrease in activity for Al-THF and 67% decrease in activity for Fe-THF. This indicates high hydrogen response for these catalysts in this H₂/C₂H₄ molar ratio period. From these results, this is probably due to increasing in acidic sites [2] by doping Lewis acid. Therefore, addition of Lewis acid causes the formation of more clustered active center leading to obtaining more chances to proceed chain transfer reaction by hydrogen. This result is in good agreement with article reported by Kouzai et al. [29]. They revealed that chain transfer reactions by hydrogen will occur when hydrogen dissociation sites are located in the instant surrounding of polymerization sites. Thus, the more the clustered active species, the higher the hydrogen response. In part of higher hydrogen amount at H₂/C₂H₄ molar ratio of 0.2–0.3, the increment of hydrogen pressure in the system was conducted. Activity of all catalysts did not change much compared with its activity in the less-hydrogen system (H₂/C₂H₄ molar ratio of 0.08). It could be implied that most of active sites were deactivated by hydrogen. The reason is that the creation of titanium-hydride bond, which is less active, was higher. Moreover, only the uncoordinated Ti-C₂H₅ bond is capable of reaction with ethylene monomer to generate the chain growth reaction [30–32]. This results in the reduction in activity and lower hydrogen response. Fe-THF is considered as a more clustered catalyst than Al-THF because it has higher Ti content than Al-THF. Moreover, catalyst having less titanium content, Al-THF, exhibited a less deactivation effect than the catalyst having higher titanium content, Fe-THF, due to the less amount of surface active site being able to react with TEA. Therefore, Fe-THF indicated the higher hydrogen response which showed rapidly dropped activity than Al-THF. However, a higher Ti content has a higher stereospecificity than a lower Ti content forming an isolated Ti site [29, 33, 34]. As a consequence, one of the possible reasons was the aggregation of Ti species which caused the reduction of active site concentration, resulting in remarkable depression of the monomer consumption [33] and finally decrease in activity. This result is in good agreement with the report of Taniike et al. [33]. They investigated the catalytic performance through the isolated and clustered Zeigler-Natta catalysts in ethylene and propylene homopolymerization. They also found that the isolated Ti species located on the MgCl₂ surface which acted as the loss of Ti-Ti interaction led to higher activity in both ethylene and propylene polymerization with TiCl₃/MgCl₂ model catalyst.

3.3. Polymer Characterization

The morphologies of obtained polyethylene are shown in Figure 4. It could be reported that all catalysts, including with and without Lewis acid modification, provided the similar morphology of polymer. Not only in the absence of hydrogen case, but also in the presence of hydrogen, the morphologies of resultant polymer were also similar.

(a)

(b)

(c)

(d)

(e)

(f)

3.3.1. Effect of Lewis Acids Modification on Polyethylene Properties

According to Table 4, polymers producing from Al-THF and Fe-THF without additional hydrogen exhibited a slight increase in melting temperature, but Lewis acid modification (Al-THF and Fe-THF) decreases crystallinity when compared with None-THF. These are the results from utilizing of the different types of Lewis acids. All catalysts produce polymers having the similar Mw/Mn, which are quite narrow Mw/Mn. This is because the doped and undoped catalysts had basically the same types of active sites [26].

3.3.2. Effect of Hydrogen on Polyethylene Properties

As presented in Table 4, the molecular weight of polymer decreased when hydrogen increased because chain termination reaction by hydrogen occurred. As a consequence, the melting temperature decreases due to the decrease in molecular weight of polyethylenes. In addition, the increase in hydrogen pressure tends to increase the crystallinity of all polymers [8]. This is because the polymerization rate is reduced by hydrogen resulting in increase in crystallization rate [8]. Mw/Mn of all polymers did not change much at the different levels of hydrogen. This result is in agreement with that of Li Pi Shan et al. [13].

4. Conclusion

In conclusion, Lewis acid modification by using metal halide (AlCl₃ and FeCl₂) can improve activity due to the formation of acidic sites and the elimination of the remaining THF in the final catalyst as proven by XRD. In addition, FeCl₂ showed higher efficiency to remove the remaining THF in MgCl₂ structure than AlCl₃. Thus, the activity was in the order of Fe-THF > Al-THF > None-THF. In the presence of hydrogen, Fe-THF is considered to have more clustered Ti species than Al-THF. Therefore, Fe-THF showed a higher hydrogen response than Al-THF due to the titanium distribution. The increase in hydrogen results in the decrease in melting temperature, but it tends to increase the crystallinity of all polymers.

Additional Points

Research Highlights. (i) Active center distribution has a strong effect on TiCl₄/MgCl₂/THF performances. (ii) The study on the active center distribution has not been yet clarified because of the limitation of characterization apparatus. (iii) Hydrogen response is considered to be one of the most effective ways to describe how the active sites dispersed in Ziegler-Natta catalysts. (iv) Lewis acid modification can improve activity due to the formation of acidic sites and removing the remaining THF in the catalyst. (v) Fe-THF is considered as a more clustered Ti species than Al-THF. (vi) FeCl₂-doped catalyst showed a higher hydrogen response than AlCl₃ doped catalyst due to its Ti distribution.

Competing Interests

The authors declare that there are no competing interests regarding the publication of this paper.

Acknowledgments

This research was funded by the Royal Golden Jubilee Ph.D. scholarship from Thailand Research Fund (PHD/0236/2553) and the Ratchadapisek Sompoch Endowment Fund (2015), Chulalongkorn University (CU-58-027-AM). The authors gratefully thank Thai Polyethylene Co., Ltd., for GPC measurement.

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Copyright

Copyright © 2016 Thanyathorn Niyomthai et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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International Journal of Polymer Science

A Comparative Study of AlCl3 and FeCl2-Modified TiCl4/MgCl2/THF Catalytic System in the Presence of Hydrogen for Ethylene Polymerization

Abstract

1. Introduction

2. Experimental

2.1. Materials

2.2. Catalyst Preparation Methods

2.2.1. TiCl4/MgCl2/THF Catalyst [2, 16]

2.3. Polymerization Reaction

2.3.1. Effect of H2/C2H4 Molar Ratio

2.4. Characterizations

2.4.1. Catalyst

2.4.2. Polymer

3. Results and Discussion

3.1. Catalyst Characterization

3.1.1. FT-IR Spectroscopy of Catalysts

3.1.2. XRD Measurement of Catalysts

3.1.3. SEM Images of Catalysts

3.2. Catalytic Activity

3.2.1. Effect of Lewis Acids Modification on the Catalytic Activity in the Absence of Hydrogen

3.2.2. Effect of Lewis Acids Modification on Hydrogen Response for Ethylene Polymerization

3.3. Polymer Characterization

3.3.1. Effect of Lewis Acids Modification on Polyethylene Properties

3.3.2. Effect of Hydrogen on Polyethylene Properties

4. Conclusion

Additional Points

Competing Interests

Acknowledgments

References

Copyright

A Comparative Study of AlCl₃ and FeCl₂-Modified TiCl₄/MgCl₂/THF Catalytic System in the Presence of Hydrogen for Ethylene Polymerization

2.2.1. TiCl₄/MgCl₂/THF Catalyst [2, 16]

2.3.1. Effect of H₂/C₂H₄ Molar Ratio