Immobilization of ZieglerâNatta Catalyst for Ethylene Polymerization

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Immobilization of Ziegler-Natta Catalyst for Ethylene Polymerization on Macropores SiO2 with An Open-framework Structure Bing Xue, Lei Hui, Huaqin Yang, Yulai Zhao, Linxi Hou, and Wei Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03993 • Publication Date (Web): 20 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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Industrial & Engineering Chemistry Research

Immobilization of Ziegler-Natta Catalyst for Ethylene Polymerization on Macropores SiO2 with An Open-framework Structure Bing Xue,† ‡ Lei Hui, ‡ Huaqin Yang, ‡ Yulai Zhao,† Linxi Hou†* and Wei Li‡* †

Department of Materials-Oriented Chemical Engineering, School of Chemical

Engineering, Fuzhou University, Fuzhou, 350100, Fujian, P.R. China ‡

Department of Polymer Science and Engineering, School of Material Science and

Chemical Engineering, Ningbo University, Ningbo, 315211, Zhejiang, P.R. China

1

ACS Paragon Plus Environment


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Abstract 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 δ-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 (i.e., 12.1×106 g PE·(mol Ti·h)-1 or 17.8×kg PE·(g cat·h)-1) than that of the traditional 955-SiO2 supported catalyst (i.e., 2.0×106 g PE·(mol Ti·h)-1 or 2.7×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. Keywords Macro-SiO2, Ziegler-Natta catalyst, ethylene polymerization, morphology.

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1 Introduction Heterogeneous Ziegler-Natta catalysts (Z-N) play a significant role in olefin polymerization processes because of its low cost and high efficiency.1-5 Today, the Z-N catalysts allow a precision of polyolefin structures with a high activity based on the designation of suitable reaction environment at the nanoscale.6-7 The polymerization process, occurring inside the supported catalysts, refers to the micro- and mesoscale.7-8 The microscale relates to the influences of chemical structure of active sites (e.g., donor effects, the structure of Ti species, and catalyst stereospecificity, etc.).8 The mesoscale focuses on the effects of the agglomerations of active units (e.g., titanium complexes/internal donor/MgCl2).9-11 It is undoubtedly necessary to disclose the catalysis mechanism in the microscale. Because this structural features are at the atomic scale, relating to the performance of active sites and the primary structures of synthesized polymer.10-12 In this decade, the mechanisms had been well studied in the microscale, such as the reaction between an alcohol and the MgCl2,13-15 structural and electronic properties of surface species (Ti (IV)),12, 16 and the coordinated mechanism of Ti active sites.12, 16-18 The results are helpful to understand how the activated surface of the MgCl2 influences the properties of active sites. However, the reaction, occurring in the mesoscale, is also crucial, especially for determining the polymerization kinetics and the polymer particle morphology6, 9. The hierarchical agglomeration of active units, of which dimensions are believed to be from nm to µm,19-20 leads to a formation of a range of porosity from micro to macropores.21-22 These hierarchical pores affect the diffusion 3



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behaviors of the reaction medium and also the fragmentation process of the supports.13, 23 It is demonstrated that the highly dispersed MgCl2 is critical for achieving the highly active Z-N catalyst.13,

24

However, the frailty of MgCl2 is a problem for

controlling the polymerization kinetics, and also the morphology of the synthesized polymer.25 To overcome this issue, the MgCl2 is always anchored on the silica owing to the porous structure, well spherical morphology, high surface area, and mechanical stability of the silica.25-27 Somehow, the catalyst activity decreased since the existence of hydroxyl groups on the surface of silica. Many types of research had focused on the chemical treatment to reduce the content of OH groups, which can increase the activity.25, 28-29 It shall be mentioned that the silica, which is widely used in the immobilization of polyolefin catalysts (e.g., 955 silica or 2485 silica), contains large numbers of confined pores (i.e., micro- and mesopores). These constrained environments can take significant effects on the dispersion or immobilization of the MgCl2 and the Ti compounds, and also can control the diffusion of reaction medium during the polymerization.21 Taniike et al. used ill-defined hierarchical Mg(OEt)2 particles with micro-, meso-, and macropores to synthesize the Z-N catalysts. The mesopores and macropores were reported to be positive to the incorporation of comonomer.9 Silveira et al. investigated the influence of micropores and mesopores of silica on the activity of metallocene. Their results suggested that the large pores were beneficial for the diffusion of reactants, presenting a high activity on the ethylene polymerization.21, 30 4


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In this work, the macropores SiO2 with an open-framework structure and thin walls are synthesized to immobilize the Z-N catalyst. This open skeleton materials will generate an unconstrained environment for the catalysts immobilization, reactants diffusion, and also ethylene polymerization. We aim to show the influence of this open-frame structure on the immobilization of Z-N catalyst and the ethylene polymerization behavior, expecting to achieve a highly active catalyst and an excellent morphology of polyethylene particles.

2 Experimental

2.1 Materials All manipulations of air-sensitive and moisture-sensitive compounds were conducted under an inert nitrogen atmosphere using standard Schlenk techniques or in a glovebox. Epoxy resin (E50) was purchased from the Wuxi Resin Corporation. Polyethylene glycol (PEG 1000 and PEG 2000), and tetraethoxysilane (TEOS) were obtained from the Chinese Medicine Group Shanghai Chemical Reagent Co., Ltd. The 955 silica

(955-SiO2, average pore diameter=22.6 nm, surface area=264 m2/g,

average particles size=40 µm), was purchased from the W. R. Grace, USA. The silica was activated at 600 °C for 5 h with nitrogen flow before use. The titanium (IV) chloride (TiCl4, 99.9 wt%) was supplied by the Acros Organics. The anhydrous magnesium chloride was purchased from the Aladdin Chemical Reagent. The 1,4-butanediol (BD) was obtained from the Aladdin Chemical Reagent, and was dried over molecular sieves for 2 days before use. The triethylaluminium (TEA) (1 mol/L in

5



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hexane) was purchased from the J&K Chemical Corp. The tetrahydrofuran (THF), n-heptane and n-hexane were distilled over the sodium/diphenyl ketone before use.

2.2 Preparation of Macropore Silica (Macro-SiO2) Blends of epoxy resin (4.0 g) and polyethylene glycol (6.5 g of PEG 1000 and 1.5 g of PEG 2000) were heated to be molten and stirred to generate a solution. Then, 1.25 g of diethylenetriamine was added within 1 min. The solution was introduced into a mold and kept at 70 °C for 3 h. The obtained solids were washed with the hot water (80 °C) to remove the PEG. A white 3-dimensional skeletal polymer was synthesized after drying, and was used as a templet in the next procedure. The obtained 3-dimensional skeletal polymers were merged into 5.0 g TEOS for 3 h to adopt the silicon source. Then, the sample was put in NH3·H2O at 50 °C for 12 h to reduce the TEOS. The synthesized compound was placed at 60 °C for 2 h to remove the residual alcohol and NH3·H2O. Finally, the composite was calcined at 800 °C for 30 min under the atmosphere to remove the skeletal polymer, obtaining the Macro-SiO2. This Macro-SiO2 was milled, and then activated at 600 °C for 5 h under nitrogen before use.

2.3 Preparation of Heterogeneous Catalysts Macro-SiO2/MgCl2/TiCl4: 0.5 g of MgCl2 was stirred with 30 ml of THF at 60 °C until the complete dissolution. The 0.5 g of Macro-SiO2 was mixed with 0.25 ml of BD and 20 ml of THF in another Schlenk flask at 40°C for 2 h. The prepared MgCl2/THF solution was then added into the Macro-SiO2/BD solution. The MgCl2 6


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can mainly recrystallize on the surface of Macro-SiO2 because of the strong coordination between the MgCl2 and the BD (See Figure S1).27 The mixture was stirred for another 2 h. The obtained solids were filtered and washed three times with 30 mL of n-hexane, and dried under vacuum for 3 h, achieving the Macro-SiO2/MgCl2. Subsequently, 30 mL of n-hexane and 5 mL of TEA were mixed with 1.0 g of Macro-SiO2/MgCl2 to remove the coordinated BD. The slurry was stirred at 60 °C for 2 h and washed with 30 mL of n-hexane three times. Finally, 30 mL of n-hexane and 1 mL of TiCl4 were added. The mixture was stirred for another 2 h at 60 °C. The catalyst (Macro-SiO2/MgCl2/TiCl4) was washed by three times with 30 mL of n-hexane to remove the extra TiCl4 and then dried under vacuum for 3 h at 50 °C. The 955-SiO2, containing significant amounts of mesopores and macropores, is widely used in the preparation of Z-N catalyst both the academy and industry. The 955-SiO2/MgCl2 and 955-SiO2/MgCl2/TiCl4 were synthesized as the above procedure. The 955-SiO2 systems were used as a benchmark in this work.

2.4 Ethylene Polymerization Ethylene polymerization was carried out in a 1.0 L Buchi stainless steel autoclave reactor, equipped with a mechanical stirrer and a temperature control equipment. The reactor was heated above 130 °C under vacuum for more than 3 h and repeatedly purged with nitrogen before polymerization. Then, the reactor temperature was reduced to the polymerization temperature. 350 mL of n-heptane was added to the reactor. After the introduction of TEA as the cocatalyst, 15±2 mg of the catalyst was 7



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injected into the reactor. The polymerization was carried out under a continuous ethylene flow to meet 10 bars at a stirring rate of 450 rpm. The polymerization proceeded until the given time. At the end of the polymerization, the autoclave was quickly vented. The synthesized product was precipitated and washed with acidified ethanol (5 wt% hydrochloric acid), and dried at 60 °C under vacuum for 12 h.

2.5 Characterization The titanium content of the prepared catalysts was measured by a ultraviolet-visible (UV-vis) measurement through the hydrogen peroxide colorimetric method. The supported catalysts were dissolved in 5 mL of H2SO4 solution (1 mol/L), and 2 mL of H2O2, and then the solution was diluted with an H2SO4 solution to 25 mL. The UV measurement was performed by a TU-1901 spectrophotometer (PERSEA Corp, China). The intensity of the peak at 410 nm was used to quantify the titanium content. The morphology of the Macro-SiO2 was observed using a transmission electron microscopy (TEM, Tecnai F20, USA). The Macro-SiO2 was embedded in an epoxy resin-based gel, and was sliced into 70 nm of thickness. The morphology of catalysts, supports, and the synthesized polyethylene were observed using a scanning electron microscopy (SEM, Hitachi S-4700, Japan). The samples were sputter-coated with Pt before the measurement. Energy dispersive X-ray detector (EDX) was used to measure the element contents and the surficial element distribution of the catalysts. A Fourier transform infrared (FTIR) measurement was performed by using an IR spectroscopy (PROTEGE460 E.S.P, Nicolet) at a resolution of 4 cm-1. The thermally 8


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activated Macro-SiO2 was mixed with KBr, and then was pressed into a tablet. Thermogravimetric analysis (TGA) was performed with an SDT 2960-TGA thermogravimetric instrument. The Marco-SiO2 were heated from room temperature to 700 °C at a heating rate of 10 °C/min under the protection of nitrogen. The Marco-SiO2/MgCl2 and SiO2/MgCl2 supports were heated from room temperature to 400 °C at a heating rate of 10 °C/min under the protection of nitrogen. A N2 absorption/desorption experiment was conducted with the Micromeritics ASAP 2020 (USA) at 77 K. The surface area of 955-SiO2 was evaluated using the Brunauer-Emmett-Teller (BET) method, and the average pore size was calculated by the Barrett-Joyner-Halenda (BJH) method. Moreover, the pore size distribution of Macro-SiO2 was measured by an Automatic Mercury Porosimeter (PoreMaster-60, Quantachrome, USA) at a pressure range of 0.3-30,000 psi. The pore size was evaluated using the Washburn-Laplace equation. The contact angle and surface tensions were 130° and 0.480 N/m, respectively. A powder X-ray diffraction (XRD) measurement was carried out on a Bruker GADDS diffract meter with the Cu Kα radiation at 40 kV and 40 mA (λ = 0.154 nm, 5-60o). The step size was 0.02°. An X-ray photoelectron spectroscopy (XPS) measurement was carried out on Escalab 250Xi using Al Ka radiation as the X-ray source (300 W, 1486.3 eV). The vacuum chamber was about 5 10-9 Torr. The binding energies were calibrated to C1s peak at 284.8 eV. The Mg content in the Marco-SiO2/MgCl2 and SiO2/MgCl2 supports were measured 9



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by an inductively coupled plasma (ICP, 730-ES, Varian, USA). Samples were dispersed in 20 ml of nitric acid (10 wt%) before analysis. The concentration of Mg in the Marco-SiO2/MgCl2 and SiO2/MgCl2 are 8.1 wt% and 7.6 wt%, respectively. The weight-average molar mass (Mw) and the molecular weight distribution (MWD) were measured by a gel permeation chromatography (GPC) at 150°C with a PL-GPC-220

instrument

(Polymer

Laboratories,

Shropshire,

U.K.).

1,2,4-trichlorobenzene was used as the solvent. A differential scanning calorimetry measurement (DSC) was performed by a DSC-Q2000 instrument (TA Instruments Corp, SUA) under a nitrogen atmosphere to measure the melting point and crystallinity of polyethylene. Samples were first heated from 50 to 160 °C at a rate of 10 °C/min and then cooled to 50 °C at the same speed. Finally, the samples were heated again to 160 °C at a rate of 10 °C/min. The melting points were determined at the peak of the curve. The crystallinity was determined in comparison with the melting enthalpy of a 100% crystalline polyethylene, 287.3 J/g.

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3 Results and Discussions

3.1 Morphology of Supports

Figure 1 Structures of Marco-SiO2: (a) TEM morphology, (b) SEM morphology, (c) pore size distribution detected by automatic mercury porosimeter, (d) FTIR spectra of activated Macro-SiO2 and 955-SiO2. Figure 1 shows the structure of Macro-SiO2. The Macro-SiO2 presents a typical macroporous structure with an average pore size of 0.91 µm (See Figure 1a-c). The porosity can reach 90.8% (measured by mercury intrusion porosimetry) which indicates that the polymer templets are removed by heating. This result can be further proved by the FTIR spectra of the supports, where a typical resonance of SiO2 is shown (See Figure 1d). The peak at 3455 cm-1 is related to the OH bonds from Si-OH groups, indicating the presence of hydroxyl groups on the surface of Macro-SiO2. The bands at 1096 and 466 cm-1 are assigned to the vibration from Si-O-Si groups. The similar resonance of FTIR spectra for the Macro-SiO2 and the 955-SiO2 demonstrates their similar chemical structures. The TG-DTG curves of the Macro-SiO2 was further 11



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shown in the supporting information (See Figure S2). No additional weight-loss peak can be found except the peak of the adsorbed water (around 90 °C), which indicates that the polymer templets are removed completely. It is thus demonstrated that the silica can be assemble to an entirely macroporous structure in the Macro-SiO2 system. This macroporous material shows a uniformed pore size distribution (See Figure 1c and Figure S3) and also a considerable specific surface area (132 m2/g).

Figure 2 The morphology of supports: (a) Macro-SiO2 after milling and (b) 955-SiO2. (c) The absorption isotherm of curves of Macro-SiO2 and (d) 955-SiO2 (the inset is the corresponding pore size distribution curve). Figure 2 a,b shows the morphology of Macro-SiO2 (i.e., the particles after milling) and 955-SiO2. Comparing with that of Macro-SiO2, the surface of 955-SiO2 is more closure, and contains some micro- and mesopores. The fraction of the macropores is less (See Figure 2b,d). N2 adsorption isotherm and pore size distribution of Macro-SiO2 and 955-SiO2 are shown in Figure 2c,d. The adsorption isotherm of Macro-SiO2 (Figure 2c) exhibits a gradual adsorption kinetic, and has no adsorption saturation, reflecting the Type-IV (H3) isotherm.24 In addition, the Macro-SiO2 shows a fast adsorption and desorption rate at a relatively low pressure, indicating that the 12


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resistance to N2 transfer is less inside the Macro-SiO2 particles. However, the 955-SiO2 exhibits a sharp hysteresis loop, showing the Type-IV isotherm with H1 hysteresis loop.31 This result suggests that the 955-SiO2 contains a significant amount of confined pores with which can generate notable transfer resistance to the N2.

3.2 Dispersion of MgCl2 and Catalyst in The Supports

Figure 3 XRD patterns of the supports and the catalysts: (a) Macro-SiO2, (b) 955-SiO2, (c) Macro-SiO2/MgCl2, (d) 955-SiO2/MgCl2, (e) Macro-SiO2/MgCl2/TiCl4, (f) 955-SiO2/MgCl2/TiCl4 and (g) MgCl2. The XRD patterns of MgCl2, Macro-SiO2, and 955-silica are shown in Figure 3. The anhydrous MgCl2 exhibits a cubic close packing structure where strong XRD peaks at 2θ= 15° (003), 30.3° (012), 35° (004), and 50.5° (110) are given. This demonstrates that the anhydrous MgCl2 presents α-MgCl2 crystals (29.1 nm (004), 19.3 nm (003)).13,

19

This type of MgCl2 crystal is unsuitable for further

immobilization of Ti complex on account of its regular structure and small specific surface area.19 Some new peaks appear on the XRD curves of the Macro-SiO2 and the 955-silica after the recrystallization of MgCl2. The peaks at 2θ = 10.8° (001) with 13



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other reflections at 15.0° (003), 22.5° (002) and 35° (004) are the patterns of MgCl2/BD adducts.32-33 The selectively high intensity of these planes indicates that the cubic stacking structure of MgCl2 is damaged. The adducts are preferentially oriented growth along the z-axis to form octahedral coordinated molecular adducts.15, 33

It is noteworthy that the notable intensity at 2θ =10.8° (001) is present in the

Macro-SiO2/MgCl2/BD, compared with that of 955-SiO2/MgCl2/BD. This indicates an ehhanced interaction between the MgCl2 and the BD inside the Macro-SiO2.13, 33 This result can be assigned to the unconstrained environment of the Macro-SiO2, of which is positive to the growth of the MgCl2/BD agglomeration along the 001 directions.9,14 After the treatment of TiCl4, the characteristic peaks for MgCl2/BD adducts disappear, and the peaks around 15°, 29-32° and 50.5° appear (corresponding to the resonance of δ-MgCl2 form). It suggests that a notable change occurs because of the formation of TiClx-MgCl2 catalysts.13 Interestingly, a higher intensity around 29-32° (104) and 50.5° (110) is shown in the Macro-SiO2/MgCl2/TiCl4 (see Figure 3e), indicating a vast existence of defective surfaces of the MgCl2 crystals.12-13 These defective surfaces are favor to immobilizing the Ti compounds.12 Figure 4 shows the TGA curves of Marco-SiO2/MgCl2 and SiO2/MgCl2 supports. The observed weight loss feature before 130 °C can be assigned to the adsorbed THF in the adducts. 24, 32 The second and third weight loss (i.e., between 130 to 300 °C) can be attributed to the coordination between the MgCl2 and the BD. 24, 32 Interestingly, the temperatures of the second and the third weight loss in the Marco-SiO2/MgCl2 (i.e., 233.5 and 290.0 °C) are higher than those of the SiO2/MgCl2 supports (i.e., 227.3 14


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and 289.0 °C), indicating a stronger interaction inside the Marco-SiO2/MgCl2/BD adducts.24 This result shows a correspondence with that of the XRD results. Furthermore, the mole ratio of [BD]/[MgCl2] is found to be similar and close to 0.8 in these two kinds of supports (the weight loss of BD are 25.8 wt% for the Marco-SiO2/MgCl2, and 23.8 wt% for the 955-SiO2/MgCl2).

Figure 4 the TGA curves of Marco-SiO2/MgCl2 and SiO2/MgCl2 supports. 3.3 Chemical Structure of The catalysts

Figure 5 XPS analysis of the support and catalysts: (a) XPS spectra over the Si 2p for Macro-SiO2/MgCl2, (b) XPS spectra over the Ti(IV) 2p for Macro-SiO2/MgCl2/TiCl4 and 955-SiO2/MgCl2/TiCl4. Figure 5 and Table 1 show the XPS results of the supports and catalysts. The surface Si element is examined by the XPS analysis. The Si 2p spectrum shows two signals at 103.3 and 101.7 eV (See Figure 5a) which correspondents to the 15



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contributions of Si atoms in bulk and at the external surface, respectively.34 The Macro-SiO2/MgCl2 has a higher amount of surface Si (2p at 101.7 eV), compared to the 955-SiO2/MgCl2. This higher concentration of Si atoms is caused by the open-framework structure of the Macro-SiO2 where more Si atoms are exposure to the environment. The amount of surface Si is decreased after immobilizing the TiCl4, suggesting that the surface of silica is covered by the agglomeration of active species.35 The binding energy of Mg 2p for the Macro-SiO2/MgCl2 (51.2 eV) is higher than that of 955-SiO2/MgCl2 (48.9 eV). This indicates an enhancement on the electron deficient of Mg atoms as a result of the coordination between the MgCl2 and the BD.36 The XPS results provide another evidence for the enhanced interaction between the MgCl2 and the BD in the Macro-SiO2. Further immobilization of TiCl4 shows a similarly increased variance of BE for Mg 2p and also the same value for Ti(IV) 2p in the Macro-SiO2 and 955-SiO2 systems. This similar response indicates that the immobilization mechanism of Ti complex is not changed.36 However, the full wave at half maximum (FWHM) of the Ti(IV) 2p is widened in the Macro-SiO2 systems compared with that of 955-SiO2 systems (i.e., 2.0 vs. 1.5 eV for Ti(IV) 2p3/2; and 3.6 vs. 2.6 eV for Ti(IV) 2p1/2). This broad FWHM suggests a notably heterogeneous structure of Ti(IV) atoms inside the Macro-SiO2/MgCl2/TiCl4, probably owing to the hierarchical agglomeration of δ-MgCl2 (See Figure 3e)

9

. The hierarchical

agglomeration of active species can be further evidenced by the BET data of the Macro-SiO2/MgCl2/TiCl4, where a notable hysteresis loop presents (See Figure S3 in the supporting information). 16


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Table 1 XPS results of the supports and catalysts Ti(IV) 2p3/2

Si 2p

Ti(IV) 2p1/2

Mg 2p Sample

101.7 eV (eV) (wt%)

BE

FWHM

BE

FWHM

(eV)

(eV)

(eV)

(eV)

Macro-SiO2/MgCl2

13.2

51.2

/

/

/

/

Macro-SiO2/MgCl2/TiCl4

8.3

51.6

458.6

2.0

464.3

3.6

955-SiO2/MgCl2

8.8

48.9

/

/

/

/

955-SiO2/MgCl2/TiCl4

5.1

50.9

458.7

1.5

464.3

2.6

“/” means the value cannot be measured. Table 2 shows the chemical compositions of catalysts where a similar bulk Ti content (i.e., around 6.9 wt%) can be found in the Macro- and 955-SiO2 supported catalysts. This may be owing to the similar chemical dosage of the reactants during the catalyst preparation. However, the surface concentration of Ti, which is measured by EDX, is much higher in the Macro-SiO2/MgCl2/TiCl4 (i.e., 8.2 wt%). This is because the Macro-SiO2 has a high porosity (i.e., 90.8%), a large pore size and an open-framework structure. This unconstrained structure is helpful to the growth of the MgCl2/BD agglomerations. More defects of the MgCl2 layers are generated after the agglomerations are destroyed by removing the BD. Thus, the supports can immobilize the Ti atoms more efficiently.9, 12-13

3.4 Ethylene Polymerization

3.4.1 Catalyst Activity of Macro-SiO2/MgCl2/TiCl4 17



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Table 2 Chemical composition of catalysts and ethylene polymerization results a Ti Catalyst

a

Ti b

c

Act.d Act.e

Mwf

MWDg

(wt%)

(wt%)

Macro-SiO2/MgCl2/TiCl4

6.9

8.2

12.1

17.8

36.2

3.3

955-SiO2/MgCl2/TiCl4

6.5

6.6

2.0

2.7

70.0

2.4

Polymerization condition: 10 bar, 60 °C, [Al]/[Ti]=100, 30 min. b Bulk content of Ti was measured by UV-vis.

c

Surface content of Ti was measured by EDX.

d

Act.

means the activity with the unit of×106 g PE·(mol Ti·h)-1. e Act. means the activity with the unit of kg PE·(g cat·h)-1.

f

Weight average molecular weight (Mw): ×104

g/mol. g Molecular weight distribution. The Macro-SiO2/MgCl2/TiCl4 shows the highest activity (i.e., 12.1×106 g PE·(mol Ti·h)-1) for ethylene polymerization at 60 °C and 100 of [Al]/[Ti] molar ratio (See the supporting information, Figure S4). Compared to the 955-SiO2/MgCl2/TiCl4, the Macro-SiO2/MgCl2/TiCl4 exhibits a notably high activity (i.e., more than 6 times, See Table 2). This super high activity can be assigned to the higher surficial concentration of Ti atoms, and also less transfer resistance for the small molecules in the Macro-SiO2 (See Figure 2).13, 25, 37 The enhanced monomer transfer behaviors can increase the local concentration of ethylene around the active sites with which can accelerate the chain termination. Thus, the molecular weight (Mw)

of the

synthesized polyethylene is reduced.38 Moreover, the molecular weight distribution (MWD) of the synthesized polyethylene is much broader in the Macro-SiO2 system, because of the increased heterogeneity of the Ti atoms (See Figure 5b).39 18


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3.4.2 Evolution of The Synthesized Polymer Structures in The Macro-SiO2 Supported Catalyst Table 3 Ethylene polymerization results for different time a Activity

DSC results

Time (min)

a

Act.b

Act.c

1d m

2e m

1d c

GPC results 2e c

T

T

X

X

(°C)

(°C)

(%)

(%)

Bulk density Mwf

MWDg

(g PE/cm-3)

5

3.6

5.2

141.2

135.3

66.5

52.8

13.4

2.7

0.144

10

4.1

6.0

141.7

135.4

65.7

51.9

16.8

2.8

0.152

20

9.0

13.2

140.7

136.0

62.2

54.0

35.9

3.2

0.170

30

12.1

17.8

138.4

136.4

64.1

61.6

36.2

3.3

0.226

40

10.4

15.3

139.4

136.2

63.1

58.0

36.5

3.7

0.251

Polymerization conditions: 10 bar, 60 °C, Al/Ti=100, 350 mL of n-heptane.

means the activity with the unit of×106 g PE·(mol Ti·h)-1.

c

f

Act.

Act. means the activity

with the unit of kg PE·(g cat·h)-1. d The first melting scan of DSC. melting scan of DSC.

b

e

The second

Weight average molecular weight (Mw): ×104 g/mol.

g

Molecular weight distribution. Table 3 demonstrates the results of ethylene polymerization using the Macro-SiO2/MgCl2/TiCl4 at different times. The dependence of the activity on the polymerization time is shown in Figure S5. The activity increases with the polymerization time up to 30 mins and then declines with a further increase in time. This polymerization kinetic can be assigned to the fragmentation of support.38,

40

During the initial polymerization (less than 30 mins), the support is fragmented by the hydrodynamic force of the propagation chains. This will generate more active sites, with which can contact with the reactants, resulting in an increment of the activity.38, 19



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41

However, the propagated chains will cover the active sites, hindering the

transference of ethylene, and reducing the catalyst activity thereof.41 This can be further proved by the evolution of polyethylene microstructure. It can be found that the melting point (Tm), crystallinity (Xc) (the second heating scans of DSC), the Mw and the MWD notably increased in the initial 20 mins of polymerization. However, these data are changeless, when the polymerization time is longer than 20 mins (See Figure S6 for the DSC curves, and Figure S7 for the GPC curves). The catalyst has multi groups of active centers. The first group forms rapidly which produces the polymer material with a relatively low Mw and a slightly lower Tm. The formation of this kind of active centers is easily activated by the TEA in the initial stage of polymerization. The second group of centers activates slower. It produces the polyethylene with a higher Mw and a slightly higher Tm. However, all the active centers are activated after 20 mins of polymerization, which makes the microstructure of the synthesized polymers stable.

38,40,41

Interestingly, the nascent polymer shows a

high melting point and crystallinity. This high melting point and crystallinity lost on the second heating scan (See Figure S6). Especially, the polymers synthesized in the initial 20 mins of polymerization have a notably high melting point (i.e., ≥ 140.7 °C). This high melting point is usually found in the case of ultra-high molecular weight polyethylene because the long chains can form thick crystal lamella.42-43 Considering the low Mw of the polyethylene in this work, this unusually high melting point (assigning to the large crystal lamella44) may be caused by the unconstrained environment of the Macro-SiO2 support. The macropores and open-framework 20


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structures of the Macro-SiO2 can afford the unconstrained environment for the crystal growth of polyethylene segments during the polymerization.9,

21

However, more

polymers will congest inside the pores as the polymerization goes on, enhancing the confinement of the support (See Figure 6).38 Thus, the nascent polymers have a reduced difference of the melting point between the twice DSC heating scans. Figure 6 shows the morphology of PE-Macro-SiO2 obtained at various polymerization times. Importantly, the spherical morphology of polyethylene particles can be achieved, although the catalysts show a less regular morphology (See Figure S8 for the morphology of catalysts). Furthermore, the growth of polyethylene chains makes polymer particles more compactly, achieving an increment of the bulk density on the increase of polymerization time.40 At initial polymerization time, the PE-5 min (Figure 6a,b) and the PE-10 min (Figure 6c,d) show similar morphology with a worm-like structure. It suggests that the diffusion resistance of reactant is less, affording the propagated chains with an unconstrained environment.41 The growing chains can extend out from the pores. However, larger microparticles with the presence of polyethylene strings can be observed when the polymerization is longer than 20 mins. (See Figure 6e-6j). The absence of strings indicates the gradually noticeable diffusion resistance of reactant. 41 This result is in line with that of the DSC and the GPC. However, the PE-955-SiO2 (Figure 6k, l) has a different morphology: cauliflowers with many nanofibers, owing to the confined environment of 955-SiO2.45-46

21



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Figure 6 SEM morphology of PE-Macro-SiO2 obtained at various polymerization times and PE-955-SiO2. (a, b) 5 min, (c, d) 10 min, (e, f) 20 min, (g, h) 30 min, (i, j) 40 min and (k, l) PE-955-SiO2.

4 Conclusions A macroporous SiO2 with an open-framework structure and thin walls are synthesized to immobilize the Ziegler-Natta catalysts. In comparison to the conventional 955-silica, the Macro-SiO2 can achieve a low transfer resistance, high dispersion of MgCl2, and highly superficial Ti contents. The unconstrained environment of the Macro-SiO2 is helpful to the growth and agglomeration of the MgCl2/BD adducts along the 001 directions. After the removal of BD, notable amounts of defective MgCl2 crystals present with which are beneficial for the further immobilization of TiCl4. Thus, the Macro-SiO2/MgCl2/TiCl4 shows a high activity on the ethylene polymerization than that of the 955-SiO2/MgCl2/TiCl4. However, a heterogeneously chemical structure of Ti atoms is found in the Macro-SiO2 system, 22


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and can widen the MWD of the synthesized polyethylene. The synthesized polyethylene exhibits a well spherical morphology and a considerable bulk density. A high melting point (≥ 140.7 °C) and many worm-like structures can be found in the nascent polyethylene synthesized by the initial 20 mins of polymerization time, indicating the appearcene of unconstrained microenvironments for chain growths. The Macro-SiO2 supports are fragmented completely after 30 mins of polymerization time, making the microstructures of the synthesized polymer stable. The reported results provide a new strategy for the synthesis of highly effective polyolefin catalysts.

Associated Content

Supporting Information

(a) Elements distribution of the Si and Mg atoms on the Macro-SiO2/MgCl2 and SiO2/MgCl2 supports (Figure S1). (b) TG and DTG curves of activated Macro-SiO2 (Figure S2). (c) BET and automatic mercury porosimeter analysis of Macro-SiO2 and Macro-SiO2/MgCl2/TiCl4 (Figure S3). (d) Dependence of catalyst activity on temperature and [Al]/[Ti] mole ratio using Macro-SiO2/MgCl2/TiCl4 (Figure S4). (e) Dependence of catalyst activity on polymerization time (Figure S5). (f) DSC traces of polymers synthesized by Macro-SiO2/MgCl2/TiCl4 at different polymerization time (Figure S6). (g) GPC curves of the synthesized polyethylene (Figure S7). (h) The morphology of catalysts (Figure S8). This material is available free of charge via the internet at http://pubs.acs.org.

Author Information 23



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Corresponding Author: *Wei Li, Email: [email protected]; Linxi Hou, Email: [email protected]. Author Contributions The manuscript is written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgements The research was supported by the Natural Science Foundation of China (No. 21206078, 21376054), and the Project of Natural Science Foundation of Ningbo (2016A610048).

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