A MOF-supported chromium catalyst for ethylene polymerization through post-synthetic modification

Journal of Molecular Catalysis A: Chemical 387 (2014) 63–68

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A MOF-supported chromium catalyst for ethylene polymerization through post-synthetic modification Bing Liu, Suyun Jie ∗ , Zhiyang Bu, Bo-Geng Li State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China

a r t i c l e

i n f o

Article history: Received 26 January 2014 Received in revised form 23 February 2014 Accepted 25 February 2014 Available online 3 March 2014 Keywords: MOF Chromium Catalyst Alkylaluminum Ethylene polymerization

a b s t r a c t Isoreticular metal-organic framework-3 (IRMOF-3) has been post-synthetically modified to generate a Cr(III)-based heterogeneous catalyst (IRMOF-3-SI-Cr) for ethylene polymerization, which has been characterized by a variety of physical methods. The XRD analysis indicated that the structure integrity of the final solid was preserved after the functionalization with the imine and the subsequent coordination to chromium. The BET surface area of the final solid was slightly reduced as determined by N2 adsorption–desorption experiments. The material exhibited a unique behavior for ethylene polymerization upon activation with various alkylaluminium co-catalysts, and the polyethylenes formed featured high molecular weights and broad molecular weight distributions.

1. Introduction Metal-organic frameworks (MOFs) [1–4], a class of porous materials have aroused great interest for their unique properties, which consist of metal ions (or clusters) and multidentate organic molecules. On one hand, their pore size, shape, dimensionality, and chemical environment can be well controlled by the judicious selection of the structural unit (metal and organic linker) [5]. On the other hand, the presence of strong metal–ligand interactions can confer the permanent porosity, so it is possible for appropriate guest molecule to freely get in and out without structure collapse [6]. Over the past decade, MOFs possess a wide array of potential applications in chemical engineering, chemistry, and materials science [7–15], especially in heterogeneous catalysis [16–18]. In general, the coordination sphere of the metal ions in MOFs is completely blocked by the organic linkers; therefore there are no free positions available to interact/activate with the reactants, which could limit the possibilities of MOFs for catalysis [19]. The new synthetic strategies have been developed to overcome the above drawbacks and prepare MOFs with unsaturated metal sites, such as the introduction of labile ligands during the synthesis of MOFs [19–21]. Another approach is available to prepare MOFs with active metal sites by using metal complexes as linkers that further react with a second metal to form frameworks [22,23].

∗ Corresponding author. Tel.: +86 571 87951515; fax: +86 571 87951612. E-mail addresses: [email protected], [email protected] (S. Jie). http://dx.doi.org/10.1016/j.molcata.2014.02.028 1381-1169/© 2014 Elsevier B.V. All rights reserved.

© 2014 Elsevier B.V. All rights reserved.

In recent years, the covalent post-synthetic modification of MOFs has received great interest [24–26]. IRMOF-3 [27], as a derivative of the canonical MOF-5, consists of octahedral Zn4 O clusters linked by 2-aminoterephthalate ligands, in which the aromatic amino groups do not participate in binding to the tetranuclear Zn4 O nodes and thereby provide the opportunities for the post-synthetic modification. The amino groups of IRMOF-3 were firstly modified with alkyl anhydrides [28]. The condensation reaction between amino groups in IRMOF-3 and salicylaldehyde afforded the formation of a salicylidene moiety (R N C C6 H4 OH), which activated the framework toward metal sequestering, demonstrated by heterogenizing a vanadyl complex (IRMOF-3-Vsal ) for the oxidation of cyclohexene with t-BuOOH [29]. Similarly, a MOF-containing gold(III) Schiff-base complex lining the pore walls was prepared by the post-synthetic modification and found to be highly active, selective and reusable for domino coupling and cyclization reactions in liquid phase [30]. The observed catalysis did provide a proof of principle that, via the sequential functionalization, porous MOFs can be chemically transformed to introduce useful ligating groups, activating the material toward metal complex binding and producing catalytically active MOFs. The heterogenization of homogeneous catalysts for gas and slurry polymerization of olefins is a highly desirable objective because of slower deactivation of catalysts, less co-catalyst required, better polymer morphology, avoidance of reactor fouling in heterogeneous polymerization [31]. Many materials have been used as catalyst supports, such as silica and alumina, magnesium chloride, zeolites, polymers, and so on [32]. Recently, MOFs appear

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as particularly good candidates for catalytic heterogeneous polymerization [33–37], as they can exhibit well-defined catalytic sites (accessible metal sites) and pores with various sizes and shapes. For instance, the Nd-based MOFs had been used as precatalysts for the selective polymerization of isoprene in the presence of Al-based co-catalyst and the residual Nd material with unchanged structure was found in the polymer [36,37]. Therefore, MOFs have been considered to be introduced as supports into heterogeneous catalysts for ethylene polymerization. In this study, we prepared a MOFsupported chromium(III) phenoxy-imine complex (IRMOF-3-SI-Cr) as an active heterogeneous catalyst for ethylene polymerization on treatment with various alkylaluminium co-catalysts, producing polyethylenes with high molecular weights and broad molecular weight distributions. 2. Experimental 2.1. General procedures All manipulations of air- and/or moisture-sensitive compounds were performed under nitrogen using standard Schlenk and glovebox techniques. The solvents for synthesis and polymerization, THF, n-hexane, diethyl ether, and toluene were refluxed over metallic potassium/sodium with benzophenone as an indicator and distilled under nitrogen prior to use. Methylaluminoxane (MAO, 1.46 M in toluene) was purchased from Akzo Nobel Corp. Trimethylaluminum (TMA, 1.0 M in heptane), triethylaluminum (TEA, 1.0 M in hexane), triisobutylaluminium (TIBA, 1.1 M in toluene) were purchased from Acros Chemicals. 2-Aminoterephthalic acid (H2 ATA), Zn(NO3 )2 ·6H2 O, aniline, salicylaldehyde and CrCl3 (THF)3 were obtained from J&K Scientific Ltd. and used as received. Polymerization-grade ethylene (TOPGRAND Petrifaction Industry Gas. Ltd.) was purified by passing it through columns of CuO catalyst and 3 A˚ molecular sieves. All other chemical chemicals were obtained commercially and used without further purification unless otherwise stated. 2.2. Synthesis of IRMOF-3-SI-Cr, {Zn4 O[SITA-CrCl2 (THF)2 ]0.09 (ATA)2.91 } IRMOF-3, [Zn4 O(ATA)3 ] and IRMOF-3-SI, [Zn4 O(SITA)0.09 (ATA)2.91 ] (SITA = 2-salicylideneimine terephthalate) were prepared according to the literature [30]. The IRMOF-3-SI (2 g) was dispersed in THF (30 mL) after it was degassed under vacuum (50 ◦ C, 6 h). To this slurry a solution of CrCl3 (THF)3 (164 mg, 0.43 mmol) in THF (30 mL) was dropwise added at room temperature and the mixture was stirred for an additional hour. The yellowish green solid (2.06 g) was collected by filtration, washed once with THF and twice with CH2 Cl2 , and dried in vacuo. Elemental analysis for Zn4 O[SITACrCl2 (THF)2 ]0.09 (ATA)2.91 (C4 H8 O) (corresponding to 3% amine functionalization and quantitative chromium uptake): Calculated: C 38.21%, H 2.54%, N 4.96%. Found: C 38.39%, H 2.56%, N 4.95%. The amount of chromium in the final solid was determined by ICP-AES. Calculated: Cr 0.51%. Found: Cr 0.47%. 2.3. Ethylene polymerization Ethylene polymerization was carried out in 250 mL glass reactor with a mechanical stirrer under atmospheric pressure. After it was dried under vacuum, purged 3 times with nitrogen and replaced twice with ethylene, the reactor was full of ethylene, and then placed into a water bath set at the operating temperature. Toluene (100 mL) and the required amount of co-catalyst were introduced to the reactor. Subsequently a prescribed amount of catalyst was added through a connected ampoule to start the

polymerization. After the desired time, the acidified ethanol (1 wt% HCl) was injected into the reactor to terminate the polymerization. The large amount of acidified ethanol was added to the mixture and stirred overnight. The polymers were collected, washed with ethanol, and dried in vacuo overnight at 60 ◦ C. The polymerization reactions under high pressure of ethylene were carried out in a 500 mL stainless steel autoclave with a mechanical stirrer and a temperature controller. Except for 200 mL of toluene used, the same procedure was employed as that under atmospheric pressure. 2.4. Compared experiments IRMOF-3-SI-Cr was allowed to react with 300 equiv. of TIBA in 100 mL of toluene at 50 ◦ C for 30 min. The mixture was filtered, and yellowish green precipitation was washed three times with toluene for testing. The filtrate was collected for ethylene polymerization under atmospheric pressure. 2.5. Characterization Elemental analysis was performed on a Flash EA1112 microanalyzer. The crystallinity of the materials was characterized by powder X-ray diffraction (XRD) on a PANalytical B.V.-Empyrean diffractometer using Cu-K␣ radiation source. Thermogravimetric analysis (TG) was carried out using Perkin-Elmer Pyris 1 TGA instrument in flowing air with a heating rate of 10 ◦ C min−1 . Nitrogen adsorption–desorption isotherms were measured at 77 K using a Quantachrome Autosorb-1-C instrument. Prior to the BET analysis, the samples were degassed at 100 ◦ C for 24 h. ICP were performed on an IRIS Intrepid 2 XSP. Samples were first slowly heated to 500 ◦ C, dissolved in hydrochloric acid after 5 h, and finally diluted to the preset volume. The molecular weight and molecular weight distribution of PE samples were measured at 150 ◦ C in 1,2,4-trichlorobenzene with a PL-GPC220 coupled with an inline capillary viscometer. The melting temperatures of PE samples were measured with a Perkin-Elmer DSC 7 instrument in a standard mode. The samples were first heated to 150 ◦ C at 20 ◦ C min−1 to eliminate thermal history and held constant for 2 min at 150 ◦ C. The samples were then cooled to 30 ◦ C at 10 ◦ C min−1 and held constant for 2 min at 30 ◦ C, before heated to 150 ◦ C at 10 ◦ C min−1 . 3. Results and discussion 3.1. Synthesis and characterization The starting material (IRMOF-3) was synthesized from 2aminoterephthalic acid (H2 ATA) and Zn(NO3 )2 ·6H2 O in the presence of triethylamine at room temperature according to a reported procedure [30]. The covalent post-synthetic modification of IRMOF-3 with salicylaldehyde afforded the formation of salicylideneimine (IRMOF-3-SI), in which ca. 3% of the total NH2 groups were functionalized without losing the framework integrity. Subsequently, the desolvated IRMOF-3-SI was treated with the excess of CrCl3 (THF)3 to generate the MOF-supported Cr(III) catalyst (IRMOF3-SI-Cr). During the reaction, a color change of the solid was observed from yellow to yellowish green. On the contrary, the mixture of IRMOF-3 and CrCl3 (THF)3 under the same conditions remained unchanged, which indicated that the coordination reaction occurred between chromium and nitrogen atom of imine group rather than amine group (Scheme 1). The XRD patterns of IRMOF-3 and IRMOF-3-SI were coincident with those reported in the literature [30]. In comparison, no apparent loss of crystallinity was observed after the coordination to chromium, although the incorporation of chromium resulted in a slight variation in the intensity of the diffraction peaks (Fig. 1).

B. Liu et al. / Journal of Molecular Catalysis A: Chemical 387 (2014) 63–68

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Scheme 1. The covalent post-synthetic modification of IRMOF-3 to yield a MOF-supported Cr(III) catalyst (IRMOF-3-SI-Cr).

IRMOF-3 IRMOF-3-SI IRMOF-3-SI-Cr

(Figure S1), whereas it was smaller than the reported IRMOF-3-SIAu (380 m2 g−1 ) [30] because of the coordinated THF molecules. According to ICP and EA results, the amount of chromium in the sample (IRMOF-3-SI-Cr) was ca. 0.47 wt%, so the complexation of chromium to the IRMOF-3-SI was almost quantitative and all the corresponding imine groups have been occupied. This result was suitable for the heterogeneous olefin polymerization.

3.2. Ethylene polymerization

10

15

20

25

30

35

40

Diffraction angle/2θ Fig. 1. XRD patterns of IRMOF-3 (black), IRMOF-3-SI (red) and IRMOF-3-SI-Cr (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

TG analysis of IRMOF-3-SI-Cr showed that a first weight loss observed between 50 and 125 ◦ C due to the removal of residual solvent and coordinated THF, and a second weight loss due to the combustion of organic part of the framework between 325 and 460 ◦ C (Fig. 2). The final residue was a mixture of ZnO and Cr2 O3 ; therefore, its weight fraction was larger than that of IRMOF-3-SI (only containing ZnO). The nitrogen adsorption–desorption experiments showed the BET surface area of IRMOF-3-SI-Cr was also preserved (277 m2 g−1 )

100

100

90

Weight loss (%)

80

80

70

70

60

60

50

50

40

40

30

100

200

300

400

500

600

700

Weight loss (%)

90

IRMOF-3-SI IRMOF-3-SI-Cr

30 800

o

Temperature ( C) Fig. 2. TG data of IRMOF-3-SI (black) and IRMOF-3-SI-Cr (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

The substituted phenoxy-imine chromium complexes had been reported as active catalysts for ethylene polymerization; however, the chromium analogs without any substituents were inactive [38–40]. Having this MOF-supported chromium complex in hand, we wondered its potential as a heterogeneous catalyst for ethylene polymerization. The application of CrCl3 (THF)3 without further processing in the in situ polymerization [41–43] demonstrated that the coordinated THF molecules would not affect the catalytic performance of catalysts. In order to avoid the homogeneous polymerization of ethylene, the catalyst was washed many times with THF and CH2 Cl2 prior to its usage for catalytic ethylene reactivity. Some special experiments were also carried out to testify whether the metal and/or metal complexes were leaching out from the catalyst during the polymerization reaction. The IRMOF-3-SI-Cr/TIBA mixture was stirred in toluene for 30 min without ethylene and then filtered, or the mixture was filtered after the polymerization proceeded for 15 min. Both filtrates were colorless and inactive for ethylene, so there were no homogeneous active centers during the polymerization. As expected, the polymerization reaction did not proceed in the presence of IRMOF-3 and IRMOF-3-SI even if the co-catalyst was combined. After the IRMOF-3-SI-Cr solid was treated with the excess of triisobutylaluminium (TIBA), the XRD patterns (Figure S2) of the residue were consistent with IRMOF-3-SI-Cr, which indicated that the MOF structure could not be destroyed by alkylaluminium. The BET surface area was 291 m2 g−1 and slightly larger than IRMOF-3-SI-Cr (Figure S1), which probably ascribed to the variation of coordination environment around chromium centers. The unchanged IR spectra (Figure S3) also demonstrated that the MOF structure was rather stable in the presence of alkylaluminum. Methylaluminoxane (MAO) and alkylaluminium (TMA, TEA, TIBA) co-catalysts were used to activate IRMOF-3-SI-Cr for ethylene reactivity at 50 ◦ C (Table 1). Different from the similar homogeneous catalysts [40], MAO is not a satisfied activator for this heterogeneous catalyst because of its intrinsic frameworks of the MOF-supported compound. For this MOF-supported Cr(III) catalyst, the active centers are considered to be located in the holes of MOFs, and only the co-catalyst molecules diffuse into the holes could activate the metal centers generating the active species. In addition, MAO solution consists of two different components [44]: primary MAO polymers which are cages containing about 30–60 Al ˚ and residual TMA. Most of MAO polyatoms (radius = 12.0 ± 0.3 A), mers cannot diffuse into the holes because of their larger structures than the pore sizes of IRMOF-3-SI-Cr (Fig. 3), and the small TMA

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Table 1 Ethylene polymerization with IRMOF-3-SI-Cr/various co-catalyst.a Entry

Co-cat.

1 2 3 4 5 6 7 8 9 10 11 12 13 14d 15e

MAO MAO MAO TMA TMA TEA TEA TEA TEA TIBA TIBA TIBA TIBA TIBA TIBA

Al/Cr 500 1000 2000 60 300 30 60 120 300 60 120 300 600 300 300

Activity (104 g mol−1 (Cr) h−1 )

Mn b (kg/mol)

Mw b (kg/mol)

PDIb

Tm c (◦ C)

1.93 1.82 2.05 1.13 1.22 0 3.11 3.29 3.44 0 3.98 5.91 5.93 8.43 6.00

10.0 9.1 7.5 14.8 9.5 – 14.0 14.3 10.3 – 29.5 23.2 20.1 29.8 24.4

318.9 281.3 279.5 297.7 316.3 – 469.8 390.9 382.4 – 370.1 374.6 333.5 401.8 431.8

31.9 30.9 37.3 20.2 33.2 – 33.6 27.4 37.0 – 12.5 16.2 16.6 13.5 17.7

134.6 133.6 133.2 135.1 134.8 – 135.1 134.3 134.2 – 135.3 135.4 135.0 135.2 134.8

a Polymerization conditions: 5 ␮mol of Cr, 100 mL of toluene, 50 ◦ C, 1 bar of ethylene, 30 min, 400 rpm. MAO: methylaluminoxane, TMA: trimethylaluminum, TEA: triethylaluminum, TIBA: triisobutylaluminium. b Determined by GPC, PDI = Mw /Mn . c Determined by DSC. d Reaction time: 15 min. e Reaction time: 60 min.

molecules actually play roles. This hypothesis was demonstrated by the similar activities and properties of polyethylene obtained from the experiments using TMA as co-catalyst (entries 1–5). Comparing the results obtained by different alkylaluminium co-catalysts, the activities in the order of TIBA > TEA > TMA were observed and TIBA was most efficient for the current MOF-supported heterogeneous catalyst. Another feature of ethylene polymerization catalyzed by this heterogeneous catalyst was that the molecular weight and PDI of PEs were affected by the type and concentration of co-catalysts, and the different co-catalyst may yield the special results (Fig. 4). For example, TMA, the smallest co-catalyst used, produced the lowest Mn and the broadest PDI of PEs because the smaller volume allowed TMA molecules to diffuse into the extra smaller holes, which increased the types of active centers; however, these smaller holes limited the growth of polymer chains and resulted in the lower Mn and broader PDI. With regard to the largest TIBA, the opposite results were achieved, the highest Mn and the narrowest PDI. The increase in the concentration of co-catalyst would increase the probability of its diffusion into the smaller holes, yielding the lower Mn and broader distribution. As for TIBA, when the Al/Cr molar ratio increased from 120 to 600, the Mn reduced from 29.5 to

20.1 kg/mol and the corresponding PDI broadened from 12.5 to 16.6 (entries 11–13). The type and amount of co-catalysts did not affect the melting points of PEs in the range of 133.2–135.4 ◦ C, which were fully consistent with the linear polyethylenes [45]. When the reaction time was shortened to 15 min, a higher activity was obtained because of the lower deactivation rate. However, the activity changed slightly when the reaction time was extended to 60 min from 30 min, indicating the longer life-time of the catalytic system. This was probably because the longer time induced the greater chance of co-catalyst entering smaller holes and activating chromium to generate more active centers. However, the number of active centers did not increase greatly, which resulted in the slight change of Mn and PDI. The ethylene polymerizations were also carried out over the temperature range of 0–75 ◦ C using TEA or TIBA as the co-catalyst (Table 2 and Fig. 5). In general, both the reaction rate constant (Kp ) and the deactivation rate constant (Kd ) increase with temperature, so the activities increase firstly and then decrease. In the current heterogeneous system, although it led to faster inactivation rate, the high temperature may enhance mass transfer process and allow more co-catalyst molecules to diffuse into the holes to activate

30

MAO TMA TEA TIBA

Mn (kg/mol)

25

20

15

10

5

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

500 1000 2000 60

300

60 120 300

120 300 600

Al/Cr

Pore width (nm) Fig. 3. Pore diameter distributions of IRMOF-3-SI-Cr.

Fig. 4. The influence of the type and amount of co-catalysts on the Mn of PEs (Table 1).

B. Liu et al. / Journal of Molecular Catalysis A: Chemical 387 (2014) 63–68

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Table 2 Ethylene polymerization with IRMOF-3-SI-Cr/TEA or TIBA at different temperature.a Entry

T (◦ C)

Co-cat.

Al/Cr

Activity (104 g mol−1 (Cr) h−1 )

Mn b (kg/mol)

Mw b (kg/mol)

PDIb

Tm c (◦ C)

16 17 8 18 19 20 12 21

0 25 50 75 0 25 50 75

TEA TEA TEA TEA TIBA TIBA TIBA TIBA

120 120 120 120 300 300 300 300

0.85 1.96 3.29 2.29 1.41 3.73 5.91 6.80

16.3 14.0 14.3 14.3 101.6 128.3 23.2 19.2

505.2 595.8 390.9 428.0 581.6 670.4 374.6 418.5

30.9 42.5 27.4 29.9 5.7 5.2 16.2 21.8

134.7 135.5 134.3 134.3 132.7 133.8 135.4 135.0

a b c

Polymerization conditions: 5 ␮mol of Cr, 100 mL of toluene, 50 ◦ C, 1 bar of ethylene, 30 min, 400 rpm. Determined by GPC, PDI = Mw /Mn . Determined by DSC.

Table 3 Ethylene polymerization with IRMOF-3-SI-Cr/TIBA at different pressure.a Entry

P (bar)

Activity (104 g mol−1 (Cr) h−1 )

Mn b (kg/mol)

Mw b (kg/mol)

PDIb

Tm c (◦ C)

12 22 23 24

1 10 20 30

5.91 27.6 38.5 62.4

23.2 129 137 134

374.6 1120 1182 1181

16.2 8.66 8.65 8.83

135.4 136.3 135.5 134.2

a b c

Polymerization conditions: 5 ␮mol of Cr, Al/Cr: 300, 200 mL of toluene except for entry 12, 50 ◦ C, 30 min, 400 rpm. Determined by GPC, PDI = Mw /Mn . Determined by DSC.

more Cr centers; therefore the activity synergistically depended on the above two factors. When smaller TEA used as co-catalyst, the intrinsic kinetics played a preponderant role at higher temperature and the highest activity was obtained at 50 ◦ C. In the case of TIBA, the diffusion process was strongly affected by temperature and the activity increased as the temperature was enhanced. This viewpoint could also be supported by the properties of PEs. For instance, the ethylene polymerization at lower temperature (0 and 25 ◦ C) reduced the accessibility of TIBA to the pores and gave the higher Mn (101.6 and 128.3 kg/mol) and the narrower PDI (5.7 and 5.2) (entries 19 and 20). In addition, the lower Mn and broader PDI were obtained at higher temperature (50 and 75 ◦ C) due to the enhanced diffusion (entries 12 and 21). In order to clarify the influence of ethylene concentration, the ethylene polymerizations were conducted with IRMOF-3-SICr/TIBA catalytic system at different pressure of ethylene (Table 3). The activity almost increased linearly with ethylene pressure. An interesting phenomenon was observed that the Mn of PEs firstly increased and then remained unchanged (Figure S4), so did PDI. Fixing the amount of co-catalyst (Al/Cr = 300) and reaction

temperature (50 ◦ C), the propagation rate constant (kp ), the chain transfer constant (kt ) and the active centers kept constant and the only variable was the concentration of ethylene. Therefore, one conclusion could be drawn that the polymerization was controlled by diffusion at low pressure (1 bar) because of the slower diffusion rate of ethylene than chemical reaction rate. The increase in ethylene concentration (from 1 to 10 bar of pressure) resulted in the higher Mn and narrower PDI. When the ethylene pressure was further enhanced, the diffusion effect can be almost ignored and the chemical reaction rate became a restricting factor of polymerization, so the polymerization was controlled by chemical kinetics at higher pressure of ethylene. Although the chain growth rate increased, chain transfer rate to monomer also increased. Thus, even if the ethylene pressure further increased, both Mn and PDI did not change any more. For Cr/SiO2 or Phillips-type catalysts, the ethylene pressure is also an important factor, and is usually kept between 20 and 30 bar. However, the higher the ethylene pressure in the polymerization reactor, the higher the Mn of the produced polyethylene [46]. 4. Conclusions

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A MOF-supported chromium catalyst was designed and synthesized by post-synthetic strategies and its structure integrity was preserved after the functionalization. On comparison with the inactive homogeneous analogs, the resulting heterogeneous IRMOF-3-SI-Cr catalyst exhibited an active behavior for ethylene polymerization. The type and amount of the co-catalysts had great influence on catalytic activity and properties of PEs. TIBA was proved to be the most efficient co-catalyst yielding PEs with high molecular weight and broad molecular weight distribution. The results demonstrated that the MOF structure in this heterogeneous system may play an important role in ethylene polymerization. Combined with traditional homogeneous catalysts, these emerging MOFs materials could be used for olefin polymerization with some unique advantages.

TEA TIBA

TEA TIBA

7

-1

Activity (10 g mol (Cr) h )

120 6 100

-1

Mn (kg/mol)

5

4

4

3

80

60

2

40

1

20 0

20

40

60

Temp (°C)

80

0

20

40

60

80

Temp (°C)

Fig. 5. The activities (left) and Mn (right) at different temperatures in the presence of TEA and TIBA (Table 2).

Acknowledgements The work was supported by the National Natural Science Foundation of China (No. 21006085), the National Basic Research

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