Iranian Polymer Journal18 (12), 2009, 969-979

metallocene;support;silica;Ga modification;copolymerization.

(*) To whom correspondence to be addressed.E-mail: bunjerd.j@chula.ac.th

A B S T R A C T

Key Words:

Ethylene/1-Octene Copolymerization overGa-modified SiO2-supported Zirconocene/MMAO

Catalyst Using In Situ and Ex Situ Impregnation Methods

Mingkwan Wannaborworn and Bunjerd Jongsomjit*

Center of Excellence on Catalysis and Catalytic Reaction Engineering, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn

University, Bangkok 10330, Thailand

Received 1 July 2009; accepted 18 November 2009

Ethylene/1-octene copolymerization over the SiO2-supported zirconocene/MMAOcatalysts was investigated. The silica support was modified by different batchesof 1.0 and 0.2 wt% Ga. It was found that Ga was well-dispersed all over the

silica granules which cannot be detected by XRD. Based on the EDX measurement, itwas revealed that Ga-modification increased the adsorption ability of MMAO on the silica support after impregnation. Thus, Ga-modification showed a promising way toenhance the rate of ethylene/1-octene copolymerization. Based on TGA study, lowerinteractions between MMAO and support caused by Ga-modification was another reason for increase in catalytic activity. Moreover, the Ga-modification increased Lewisacid centres or active species on the catalytic system. In addition, a comparative studyof polymerization was also conducted with in situ and ex situ impregnation methods. Itwas found that the in situ impregnation method of the MMAO onto silica exhibitedremarkable (almost 3 times) activity compared to ex situ method which is attributed tolower deactivation effect of the catalyst. 13C NMR analysis, however, indicated that onlyrandom copolymers were produced in all systems. These results could be related to thehigh degree of 1-octene insertion and thus amorphous copolymers were produced inall systems.

INTRODUCTION

Polyethylene (PE) is by volume oneof the largest commodity chemicalsproduced in the world. It is widelyused in films, house wares, bottles,containers, pipe, tubing, wire andcable insulations, conduits, andcoatings [1-3]. Ziegler-Natta catalysts, metallocene catalysts,and supported metal oxides (Philipsprocess) all are capable of produc-ing linear polyethylene [4].Metallocene catalysts activated bymethylaluminoxane (MAO) showvery high activity in ethylene poly-

merization and produce poly-ethylene with a narrow molecularweight distribution of approxi-mately 2.0 [5]. Many metallocenecatalysts have been supported oninorganic carriers, typically silica,alumina, and titania [6-9]. Thedevelopment of supported metal-locenes is crucial for industrialapplication because it enables theiruse in gas- and slurry-phaseprocesses and prevents reactor-fouling problems. It also enablesthe formation of uniform particles

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with narrow size distribution and high bulk density. Ithas been reported that silica is perhaps the most attractive support employed for supported metallocene catalysts, so far. However, the propertiesof silica itself may not be completely satisfied for allpurposes based on the polymerization activity andproperties of the obtained polymers. This is becausesome intrinsic undesirable properties, such as theacidity of various surface OH groups can lead to theformation of multiple active sites [10]. Many effortshave been made in order to generate the active supported metallocene species which are more activeand more stable.

In our previous study, the copolymerization of ethylene/1-octene via zirconia modification on the silica-supported metallocene catalyst was also studied [11]. It was found that the zirconia modification on the silica support can result inincreased polymerization activity. Besides modification with zirconia, other researchers havefocused on catalytic behaviour of acidic metal-modified support [12-14]. In addition, Rahiala et al.[15] found that the presence of acidic Al seems to beadvantageous for the formation of active centres andexhibits high polymerization activity level withoutthe use of any pretreatment of the support with MAO.Therefore, the use of support modified with acidicmetal including Ga is an interesting alternative forenhancement of catalytic performance and can be further developed for ethylene polymerization.

In addition to support modification, catalyst preparation method is the other important factor thatinfluences the catalytic activity. Two methods usedfor preparation of supported metallocene are ex situand in situ impregnations. For ex situ impregnation, itwas found that homogeneous system gives higheractivity than the supported system [16,17]. However,Campos et al. [18] observed the opposite trend within situ impregnation. They inferred that the in situ impregnation procedure could be a potentialapproach for the development of a heterogeneous system.

The main objective of the present study was to further develop a better understanding on how both different impregnation methods of cocatalyst and Ga modifications on the silica support would simultaneously affect the catalytic activity and poly-

mer properties. No such study has been investigatedso far. It was found that both Ga modification and different impregnation methods can play importantroles on the catalytic activities of the supported metallocene catalysts.

EXPERIMENTALS

MaterialsAll chemicals and polymerizations were manipulatedunder an argon atmosphere, using a glove box and/orSchlenk techniques. Toluene was dried over dehydrated CaCl2 and distilled over sodium/benzophenone before use. The rac-ethylenebis(indenyl)zirconium dichloride (rac-Et[Ind]2ZrCl2)was supplied from Aldrich. Modified methylalumi-noxane (MMAO) in hexane was donated by Tosoh(Akso, Japan). Trialkylaluminium (TMA, 2 M intoluene) was supplied by Nippon Aluminum Alkyls,Ltd., Japan. Ultrahigh purity argon was further purified by passing it through columns that werepacked with BASF catalyst R3-11G (molecular-sieved to 3 Å), sodium hydroxide (NaOH), and phosphorus pentoxide (P2O5) to remove traces of oxygen and moisture. Ethylene gas (99.96% pure)was donated by the National Petrochemical Co. Ltd.,Thailand. 1-Octene (d = 0.715) was purchased fromAldrich.

Preparation of Ga-modified Silica SupportThe Ga modification of the silica support was prepared by the incipient-wetness impregnationmethod according to the procedure described previously [19]. The Ga source in the present casewas Ga(NO3). Ga was impregnated onto silica gel(Cariact Q-50) by either of 0.2 or 1.0 wt% of Ga. Thesupport was dried in oven at 110ºC for 12 h and thencalcined in air at 400ºC for 2 h.

Preparation of Dried-MMAO (dMMAO)Removal of TMA from MMAO was carried out according to the reported procedure [20]. The toluenesolution of MMAO was dried under vacuum for 6 hat room temperature to evaporate the solvent, TMA,and Al(iBu)3 (TIBA). Then, MMAO was dissolved in 100 mL of heptane and the solution was evaporated

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Iranian Polymer Journal / Volume 18 Number 12 (2009)970

under vacuum to remove the remaining TMA andTIBA. This procedure was repeated 4 times and thewhite powder of dried MMAO (dMMAO) wasobtained.

Preparation of Supported MMAOEx Situ Impregnation MethodDue to the very low adsorption ability of MMAO ontothe silica support, the removal of TMA and TIBA ofMMAO to obtain dMMAO was needed prior toimpregnation. The silica-supported dMMAO (ex situ)was prepared by reacting 0.1 g of each of thermallytreated unmodified and Ga-modified silica supports(at 400ºC under vacuum for 4 h) with the desiredamount of dMMAO in 10 mL toluene at room temperature for 30 min. The solid part was separatedand washed once with 20 mL of toluene and 3 timeswith 20 mL hexane, followed by drying in vacuum atroom temperature. A white powder of supportedcocatalyst (dMMAO/SiO2) was then obtained.

In Situ Impregnation Method and PolymerizationSamples (0.2 g ) of the unmodified and Ga-modified silica supports were allowed in contact with 1.14 mLof MMAO ([Al]MMAO/[Zr]cat = 1135) for 30 min ina reactor with magnetic stirring. After this period oftime, the suspension underwent the clarified liquidtest to confirm that all MMAO was immobilized onthe support. Then, 1 mL of the clarified liquid test wasinjected into the polymerization reactor, where adesired amount of zirconocene was already present.By formation of any noticeable amount of polymer itbecame evident that this clarified liquid still containsMMAO. Therefore, the fixation of cocatalyst onto thesupport was not completed. To ensure that MMAOwas completely impregnated onto the support, theMMAO/support ratio was then reduced until no polymer was formed in the reactor [21].

After this test, the suspension was mixed withdesired amounts of rac-Et[Ind]2ZrCl2 and TMA([Al]TMA/[Zr]cat = 2500). Then, toluene (to make atotal volume of 30 mL) was introduced into the reactor. The reactor was frozen in liquid nitrogen tostop reaction and then 0.018 mol of 1-octene wasinjected into the reactor. The reactor was evacuated toremove argon and then it was heated up to polymeri-zation temperature (70ºC). The polymerization started

Scheme I. Diagram of in situ and ex situ impregnationmethods.

by feeding ethylene gas (total pressure 50 psi in thereactor) until the concentration of ethylene reached0.018 mol (6 psi at pressure gauge). The polymeriza-tion reaction was completely terminated by additionof acidic methanol. The reaction time was recordedfor purpose of calculating the activity. The precipitat-ed polymer was washed with methanol and dried atroom temperature. In order to give a better understanding, these different impregnation methodsare shown in Scheme I.

Polymerization on the Support Catalyst with Ex Situ ImpregnationThe ethylene/1-octene copolymerization reaction wasperformed in a 100 mL semi-batch stainless steelautoclave reactor equipped with a magnetic stirrer. In the glove box, the desired amounts of rac-Et[Ind]2ZrCl2 and TMA were mixed and stirredfor 5 min for aging. Then, toluene (to make a totalvolume of 30 mL) and 0.2 g of dMMAO/SiO2 ordMMAO/Ga-modified SiO2 ([Al]MMAO/[Zr]cat =1135) were introduced into the reactor. The mixture ofrac-Et[Ind]2ZrCl2 (5×10-5 M) and TMA (3.75×10-3 M corresponding to [Al]TMA/[Zr]cat = 2500) was mixed in the reactor and stirred for 5 min aging at room temperature, separately, and it was then injected into the reactor. From this point, a similar procedure was performed as describedabove.

971Iranian Polymer Journal / Volume 18 Number 12 (2009)

Ethylene/1-Octene Copolymerization ...Wannaborworn M et al.

Characterization of Supports and CatalystPrecursorsX-ray Diffraction A Siemens D-5000 X-ray diffractometer (Germany)with CuKα (l = 1.54439 Å) was employed to determine the bulk crystalline phases of the samples.The spectra were scanned at a rate of 2.4 degree/minin the range of 2θ = 20-80º.

SEM and Energy Dispersive X-ray SpectroscopyA Jeol mode JSM-6400 SEM (Japan) was used todetermine the morphologies of the sample granulesand an EDX with Link Isis series 300 program toobserve their complete elemental distribution.

N2 PhysisorptionBET surface area, average pore diameter and poresize distribution were determined by N2 physisorp-tion method using a Micromeritics ASAP 2000 automated system (USA).

Thermogravimetric AnalysisTGA was performed using a TA Instrument SDT Q600 analyzer (USA). The samples of 10-20 mg and a temperature range between 30 to 400ºC at 2ºC min-1

were used in the operation with N2 UHP carrier gas.

Characterization of Polymer13C NMR Spectroscopy13C NMR spectroscopy was used to determine thetriad distribution and 1-octene insertion indicating thecopolymer microstructure. Chemical shifts were referenced internally to the CDCl3 and calculatedaccording to the method described by Randall [22].Sample solution was prepared by dissolving 50 mg ofcopolymer in 1,2,4-trichlorobenzene and CDCl3. 13C NMR spectra were taken at 60ºC using BrukerAvance II 400 (Germany) operating at 100 MHz withan acquisition time of 1.5 s and a delay time of 4 s.

Differential Scanning CalorimetryThe melting temperature of ethylene/1-octenecopolymer products was determined with a Perkin-Elmer diamond DSC (USA). The analyseswere performed at the heating rate of 20ºC/min in the temperature range of 50-150ºC. The heating cyclewas run twice. In the first scan, samples were heated,

Table 1. BET surface areas of unmodified and Ga-modifiedsilica supports.

and then cooled to room temperature. In the secondscan, samples were reheated at the same rate, but onlythe results of the second scan were reported becausethe first scan was influenced by the mechanical andthermal history of the samples.

RESULTS AND DISCUSSION

Characteristics of Catalyst SupportIn this study, silica was used as a support for the zirconocene/MMAO catalytic system because it hasbeen one of the most widely used supports for metallocene catalysts, so far. However, the propertiesof silica itself may not be suitable for supporting themetallocene catalysts due to the presence of variousfunctional groups on the surface. Therefore, the modification of silica is important in order to improvethe linkage between silica and metallocene catalyst[23]. Here, we used Ga to modify the silica support

Figure 1. XRD patterns of different supports.

Iranian Polymer Journal / Volume 18 Number 12 (2009)972

Ethylene/1-Octene Copolymerization ... Wannaborworn M et al.

Support BET surface area(m2/g)

Pore volume(mL)

SiO2

SiO2-Ga-0.2%SiO2-Ga-1.0%

70.970.668.8

0.260.250.23

for that purpose. After modification with Ga loadingof 0.2 wt% (SiO2-Ga-0.2%) and 1 wt% (SiO2-Ga-1.0%), the silica and Ga-modified silica supports werecharacterized by means of N2 physisorption, XRD,and SEM/EDX. As shown in Table 1, it can beobserved that the surface areas of the unmodified andGa-modified silica supports were in the range of 68-71 m2/g which indicate no significant change inthe surface area after Ga modification due to its efficient dispersion.

The XRD patterns of the supports are shown inFigure 1. All supports exhibit similar XRD patternsshowing only the broad peaks between 20-30º as seentypically for the conventional amorphous silica [19].No peak of Ga compounds was detected on the modified samples. This was an indication of the crystallite size of Ga being smaller than 3 nm (as inwell-dispersed form) [11].

SEM was used in order to determine the morphologies of catalyst supports before and aftermodification with Ga. SEM images of different supports are shown in Figure 2. It was found that afterGa modification, the support was slightly agglomerated. However, the agglomeration of support would have no effect on other properties asproven by XRD and N2 physisorption.

In the ex situ impregnation method, the cocatalyst,e.g., MMAO has to be deposited on the support. First,the impregnation of MMAO was used. However, itwas found that the adsorption ability of MMAO onthese specified supports was very low. Therefore, theremoval of TMA and TIBA was necessary to obtainthe dried MMAO (dMMAO). After impregnationwith dMMAO, the different supports were determinedby SEM and EDX to study the changes in morphologies and distribution of dMMAO in terms of[Al]dMMAO distribution.

The observed morphologies and [Al]dMMAOdistribution are shown in Figure 3. Based on the EDXmapping, it is revealed that different supports exhibitgood distribution all over the support granules.Besides, EDX was also used to measure the amount of[Al]dMMAO present on different supports as the typical EDX spectrum of dMMAO/ support is shownin Figure 4. The amounts of [Al]dMMAOpresent in different supports is presented in Table 2. Itwas found that the average amounts of [Al]dMMAO on

Figure 2. SEM micrographs of different supports beforedMMAO impregnation: (a) SiO2, (b) SiO2 -Ga -0.2%, and (c)SiO2 -Ga -1.0%.

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Iranian Polymer Journal / Volume 18 Number 12 (2009) 973

(a)

(b)

(c)

SiO2, SiO2-Ga-0.2%, and SiO2-Ga-1.0% were 13.1,15.2, and 17.7 wt%, respectively. Apparently, Gamodification on silica can result in increased amountsof [Al]dMMAO present on the Ga modified support.This can be attributed to the increased adsorption abil-ity of [Al]dMMAO and silica by Ga modification.Then, the different supports with and without Gamodification having the dMMAO impregnation wereused for polymerization (ex situ impregnationmethod). In order to compare the catalytic activity, the

Figure 4. A typical spectrum of EDX analysis for thedMMAO/support.

in situ impregnation of supports with and without Gamodification (without dMMAO impregnation) wasalso conducted further.

Catalytic ActivityThe catalytic activities for all different supports andimpregnation systems are listed in Table 3.Considering the homogeneous (run 1) and ex situimpregnation system (runs 3, 5, and 7) it can be seenthat the supported system exhibited lower activitiesdue to supporting effect as mentioned elsewhere[7,11,24]. However, for the supported system, it wasfound that activities increased with Ga modification.In general, increased activities can be attributed to

Table 2. the amounts of [Al]dMMAO present in different supports.

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974 Iranian Polymer Journal / Volume 18 Number 12 (2009)

(a1) (b1) (c1)

(a2) (b2) (c2)

Figure 3. SEM micrographs (top row) and EDX mappings (bottom row) of Al distribution on different supports forthe ex situ impregnation method: (a1 and a2) SiO2, (b1 and b2) SiO2-Ga-0.2%, and (c1 and c2) SiO2-Ga-1.0%

Catalyst precursor [Al]dMMAO on the support(wt%)

dMMAO/SiO2

dMMAO/SiO2-Ga-0.2%dMMAO/SiO2-Ga-1.0%

13.115.217.7

increased amount of [Al]dMMAO present on differentsupport and the interaction of [Al]dMMAO and support. The former factor can be eliminated by usingthe same ratios of [Al]dMMAO/[Zr]cat = 1135 in eachrun. As determined by EDX, the amounts of[Al]dMMAO present on different supports wereunequal due to the different adsorption ability of eachsupport. As a result, changes in activity did not causeby the different amounts of [Al]dMMAO. Hence, theeffect of interaction would be the case here. In orderto prove the interaction between the dMMAO andsupport, TGA measurement was performed. Asknown, the connection of the support and cocatalyst

Figure 5. TGA profiles of dMMAO on different supports.

occurred via the Osupport-Alcocatalyst linkage [25].The TGA can provide the useful information on thedegree of interaction for dMMAO bound to silica interms of weight loss and removal temperature [26-30]. Thus, too strong interaction can result in it,makes it more difficult for dMMAO bound to be supported to react with the zirconocene catalyst during the deactivation process. It was then lead tolower catalytic activity for polymerization. The TGAprofiles of [Al]dMMAO on various supports are shownin Figure 5. It was observed that the weight loss of[Al]dMMAO present on various supports were in the order of SiO2 (9%) < SiO2-Ga-0.2% (15%) <SiO2

-Ga-1.0% (16%). It also corresponds to thedecomposition temperature at 10% weight loss (Td10%) of 420, 205 and 178ºC, respectively. This indicates that [Al]dMMAO present on SiO2 exhibitedthe strongest interaction among other supports, resulted in the lowest activity obtained.

In order to compare the different impregnationmethod, the in situ impregnation of MMAO as shownin Scheme I was also conducted. The catalytic activities of supports with and without Ga modifications (run 4, 6, and 8) were also shown inTable 2. Considering the support with Ga modifica-tion, it was found that its effect on the catalytic activities was in similar trend as seen for those of theex situ impregnation method as mentioned before.Hence, it was confirmed that Ga modification apparently resulted in increased activity. In addition,

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Iranian Polymer Journal / Volume 18 Number 12 (2009) 975

Table 3. Catalytic activities of various supports and different impregnation methods.

Sample Run Methoda Yield(g)

Reaction time(s)

Catalytic activityb

(kg of pol/mol Zr h)

Homogeneous

SiO2

SiO2-Ga-0.2%

SiO2-Ga-1.0%

12

34

56

78

n.a.n.a.

EIII

EIII

EIII

1.4861.534

1.2251.495

1.2981.666

1.3751.756

130123

18488

17690

16276

27,433c

29,803d

15,978c

40,405d

17,700c

44,426d

20,370c

55,329d

(a) EI: ex situ impregnation method, II: in situ impregnation method; (b) activities were measured at polymerization temperature of 70ºC, [ethylene] = 0.018 mole, [1-octene] = 0.018 mole, [Al]MMAO/[Zr]cat =1135, [Al]TMA/[Zr]cat = 2500, in toluene with total volume = 30 mL, and [Zr]cat = 5×10-5 M; (c) with dMMAO;(d) with MMAO.

the effect of Ga in polymerization activity can be alsoexplained based on the work reported by Campos etal. [18]. They revealed that the introduction of Ga,even in small amount, strongly improves the ability ofthe MCM-41 supports to immobilize metallocene catalysts. Based on our present study, for the in situimpregnation, the activities increased about 1.5 timeswith Ga modification of SiO2. However, with the Gamodification on MCM-41 support, activities reportedby Campos et al. [18] increased almost 2.5 times. Itshould be mentioned that different increased activitieswere attributed to different types of metallocene,cocatalyst and metallocene ratios employed.

From FTIR analysis [17], there are the characteristic peaks of Lewis acid centre at 1457,1492 and 1621 cm-1 in the case of Ga-MCM-41,while MCM-41 does not exhibit these peaks. Thisresult suggested that the interaction between zirconocene and the Lewis acid centers derived fromthe introduction of Ga in the support seems to play animportant role in the formation of the active speciesand the optimization of Ga of the MCM-41 support.

When compared the catalytic activities of the ex situ and in situ impregnation methods (runs 3 and4, runs 5 and 6, and runs 7 and 8), it was found thatthe in situ impregnation exhibited remarkable activities compared to the ex situ impregnation. Thiscan be attributed to the partial entering of MMAO intothe pores of the support leading to the loss of activespecies for the ex situ impregnation method, whereasmost of the MMAO is present on the external surfaceof the support in case of in situ impregnation method.In addition, the multiple steps for the ex situ impregnation method would also be the cause fordeactivation of active sites.

It is worth noting that based on the in situ impregnation method, the heterogeneous system surprisingly exhibited higher catalytic activity thanthe homogeneous one (runs 2 and 4). However, basedon the work done by Jungling et al. [31] a good bulkdensity can improve the distribution of active site. Inaddition, it can reduce reactor fouling which is resulted from the adhesion of polymer to the reactor.Here, the heterogeneous system has exhibited higherbulk density than the homogeneous system under thisspecified reaction condition. Meanwhile, it can beproposed that the SiO2 support might inhibit the

formation of ZrCH2CH2Zr species [32,33] which areformed via a bimolecular process as it minimizes thesteric hindrance effect of the system.

Polymer CharacteristicsThe melting temperatures (Tm) of copolymer evaluated by differential scanning calorimeter (DSC)cannot be observed for all polymer samples, whichindicate that non-crystalline polymers are produced inthis specified polymerization system. The non-crystalline polymers were attributed to the highdegree of 1-octene insertion, which can be confirmedby 13C NMR. The quantitative analysis of triad distribution for all copolymers was conducted on theassignment of the 13C NMR spectra of ethylene/1-octene (EO) copolymer which was calculatedaccording to the method of Randall [22]. The typicalcharacteristics of 13C NMR spectra for all copolymersas shown in Figure 6 were similar indicating the formation of ethylene/1-octene copolymer. The EOtriad distribution of all polymers is shown in Table 4.Ethylene incorporation in all systems gave copolymers with similar triad distribution. Thus, onlythe random copolymers can be produced in all systems. It is shown that Ga modification tended toincrease the 1-octane incorporation to a small extent.

Figure 6. Typical 13C NMR spectra of ethylene/1-octenecopolymers obtained with (a) SiO2 and (b) SiO2-Ga-1.0%support.

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Ethylene/1-Octene Copolymerization ... Wannaborworn M et al.

The densities of copolymer were also measured usingdensitometer. They were in the range of 0.891-0.901 g/cm3. Thus, no significant change in densitywas found upon Ga modification.

CONCLUSION

In summary, the catalytic activities of ethylene/1-octene copolymerization with silica-supported zirconocene/MMAO catalyst can be enhanced withGa modification on the silica support. It was proposedbased on TGA measurement that Ga modification canresult in lower interaction between the Osupport-Alcocatalyst linkages. By comparing different impregnation methods, we found that in situ impregnation exhibited remarkably higher catalyticactivities compared to ex situ impregnation, which isdue to increased bulk density of the polymerizationsystem resulting in high dispersion of active sites anddecreased reactor fouling. The microstructure ofcopolymers was of a random type for both in situ andex situ impregnation systems showing that these twoimmobilized systems did not affect on the copolymermicrostructure as confirmed by 13C NMR.

ACKNOWLEDGEMENT

The authors thank the Thailand Research Fund (TRF)for the financial support of this project.

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Table 4. 13C NMR analysis of ethylene/1-octene copolymer.

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System MethodTriad distribution of copolymer 1-Octene insertion

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0.2360.091

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1427

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