Ti bimetallic Ziegler-Natta Catalysts

Journal Pre-proof Ethylene polymerization over novel organic magnesium based V/Ti bimetallic ZieglerNatta Catalysts Yi Zhou, Rui Zhang, He Ren, Xuelian He, Bao Liu, Ning Zhao, Boping Liu PII:

S0022-328X(19)30509-1

DOI:

https://doi.org/10.1016/j.jorganchem.2019.121066

Reference:

JOM 121066

To appear in:

Journal of Organometallic Chemistry

Received Date: 26 September 2019 Revised Date:

25 November 2019

Accepted Date: 4 December 2019

Please cite this article as: Y. Zhou, R. Zhang, H. Ren, X. He, B. Liu, N. Zhao, B. Liu, Ethylene polymerization over novel organic magnesium based V/Ti bimetallic Ziegler-Natta Catalysts, Journal of Organometallic Chemistry (2020), doi: https://doi.org/10.1016/j.jorganchem.2019.121066. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

1

For Journal of organometallic chemistry

2

Ethylene polymerization over novel organic magnesium based V/Ti bimetallic

3

Ziegler-Natta Catalysts

4

Yi Zhou1, Rui Zhang3, He Ren3, Xuelian He1*, Bao Liu1, Ning Zhao1, Boping Liu2*

5

1

6

University of Science and Technology, Shanghai 200237, China

7

2

8

510642, China

9

3

Shanghai Key Laboratory of Multiphase Material Chemical Engineering, East China

College of Materials and Energy, South China Agricultural University, Guangzhou

Daqing Petrochemical Research Center of PetroChina, Daqing 163714, China

10 11

Corresponding Authors

12

Xuelian He (E-mail: [email protected])

13

Boping Liu (Email: [email protected])

14 15

Author Lists:

16

Yi Zhou: [email protected]

17

Rui Zhang: [email protected]

18

He Ren: [email protected]

19

Bao Liu: [email protected]

20

Ning Zhao: [email protected]

1

Abstract

2

A series of novel V/Ti bimetallic polyethylene catalysts were developed successfully

3

through the introduction of vanadium species into the (SiO2/MgR2/MgCl2)·TiClx

4

Ziegler-Natta Catalyst. The organometallic heterogeneous catalysts combined

5

advantages of reducibility of magnesium alkyl on the activation of secondary metal

6

active sites (vanadium active sites), which successfully enhanced average molecular

7

weight in homopolymer products and produced dominantly bimodal molecular weight

8

distribution in copolymer products. Catalyst preparation conditions including

9

calcination temperature and magnesium dosage were investigated to explicit the effect

10

on the relative performance of different metal active centers. Moreover, the factors

11

including hydrogen and ethylene/1-hexene copolymerization were systematically

12

studied. The bimetallic catalysts exhibit 63.9 % enhancement on activity, 37.3% better

13

hydrogen response than the original (SiO2/MgR2/MgCl2)·TiClx organic magnesium

14

Ziegler-Natta catalyst. Moreover, a simplified schematic model was proposed to

15

elaborate on the reason why the bimetallic catalysts showed much higher activity than

16

the original one, and each active species performance well. Vanadium active sites react

17

quickly in the initial reaction time on the external surface of the support, which helps

18

titanium active sites crack the catalyst particles together in the fragmentation and

19

activation stage; then the released internal surface active sites, which were accessible to

20

the monomer in the slurry phase, speed up the polymerization reaction during the latter

21

reaction process.

1

Keywords: Polyethylene, bimetallic Ziegler-Natta, alkyl magnesium, vanadium

2

modification, accelerated activation process.

3

1

Introduction

2

Polyethylene (PE) holds the highest market share among all the polymers due to plenty

3

of advantages including chemical resistance, high impact strength, and stiffness in

4

extreme chill conditions. Molecular weight (MW) and molecular weight distribution

5

(MWD) influence the polymer property greatly. The high MW part shows a great

6

advantage in mechanical ability but is hard to process. In turn, the low MW part is ideal

7

for process capability with the sacrifice of mechanical properties [1-3].

8

Ziegler-Natta catalyst is one of the most famous catalysts in the polyolefin area. Since

9

the discovery of Ziegler-Natta catalysts, massive works were conducted to upgrade

10

the catalyst system. Kashiwa [4, 5] and Galli [6], independently in 1968, discovered

11

MgCl2/TiCl4 catalyst system that greatly improved catalyst activity and obtained

12

polymer properties, which directly promoted the development of polyolefin industry.

13

Böhm [7] discovered another type of Ziegler-Natta catalyst using Mg(OEt)2 as the Mg

14

source. Besides, the industry also carried out many research works. UCC company [8]

15

patented the Ti/Mg Ziegler-Natta catalyst for the UNIPOL gas-phase polyethylene

16

process. Chamla and BP company [9] synthesized Si/Mg bi-supported catalyst

17

through reacting SiO2 with MgR2 and TiCl4, successively.

18

Although the organometallic compounds were introducing into the Ziegler-Natta

19

system, those organic magnesium reagents were mainly served as magnesium

20

resources. Many organometallic heterogeneous catalysts in the synthesis area ideally

21

inherited the advantage of organic metallic complex and function well with high

1

activity [10-14]. In polyolefin area, the -OH group on the surface of silica provides the

2

possibility to load vanadium oxide with impregnation and calcination. The vanadium

3

oxide on the support could be reduced by organometallic compound to activate well.

4

The alkyl magnesium type Ziegler-Natta catalyst lacks attention due to the use of

5

air-sensitive magnesium reagent makes the process costly. Until recently, as the

6

pre-reduction effect reported in Phillips catalysts system in our previous work [15, 16],

7

it was noticed that alkyl magnesium reagent could reduce the optimum amount of

8

expensive cocatalyst consumption in the successive polymerization process, which

9

could offset the alkyl magnesium reagent cost and makes the catalyst of great industrial

10

potential. Further, the silica gel support in this system produced the potential to use the

11

secondary metal active center to meliorate the performance of the series catalyst. In

12

general, the magnesium alkyl reagent serves as magnesium chloride precursor [9],

13

hydroxyl

14

Vanadium magnesium catalyst (VMC) is the most common vanadium type catalyst,

15

containing VOCl3 or VCl4 as the active species on MgCl2 support [18-23]. Although

16

the low activity showed by VMC catalysts, Zakharov [19, 20, 24-26] reported that the

17

polymer obtained from VMC showed much wider PDI (usually between 14-21) than

18

polymer produced by titanium magnesium catalysts (TMC). Meanwhile, the VMC

19

showed great advantages in hydrogen response and commoner incorporation. In 1996,

20

Czaja [21] supported VOCl3 on the surface of Mg(THF)2 adduct, discovered a catalyst

21

with over 80 % vanadium sites activated. Wang [23] dispersed the VCl4 into the MgCl2

group scavenger and the reducing agent for a secondary active sites [17].

1

surface through Grignard reagent, and made the catalyst with an even broader PDI and

2

better methyl branching extent. Echevskaya [26] investigated the effect of hydrogen on

3

molecular weight distribution with the MgCl2/VCl4 catalyst, which verified the wide

4

PDI feature of vanadium catalysts and its good response to hydrogen.

5

Nowadays, great efforts were conducted on developing bimetallic catalysts that could

6

possibly combine the merits of both parental catalysts and improve polymer

7

performance.

8

Our group developed several bimetallic catalysts by introducing the secondary metal

9

active sites into monometallic polyethylene catalysts [15, 16, 27-34]. These series of

10

bimetallic catalysts are (1) SiO2-supported silyl-chromate(Cr)/ vanadium-oxide(V)

11

bimetallic catalyst; (2) SiO2-supported silyl-chromate(Cr)/ imido-vanadium(V)

12

bimetallic catalyst; (3) SiO2-supported chromium oxide(Cr)/ vanadium oxide(V)

13

bimetallic catalyst; (4) chromium/vanadium modified inorganic magnesium

14

Ziegler-Natta catalyst; (5) TiCl4/VOCl3 hybrid Ziegler-Natta catalyst; (6) CrOx/

15

(SiO2/MgR2/MgCl2)·TiClx organic magnesium Ziegler-Natta Catalyst. These novel

16

catalysts showed several advantages of both metal active sites. Among them, the

17

Cr/imido-V catalyst (2) in the above list were synthesized by reacting the secondary

18

vanadium active sites with imido reagent and highly activated the vanadium active sites,

19

which produce high molecular weight polymer. Bimodal high-density polyethylene,

20

which combined with mechanic ability and processability, was produced with this

21

organic compound treated bimetallic catalyst. Triggered by the organic treatment in

1

their Cr/V bimetallic catalyst, magnesium alkyl was chosen as the magnesium source

2

for developing vanadium modified silica-supported organic magnesium-based

3

(SiO2/MgR2/MgCl2)·TiClx Ziegler-Natta catalyst. As a result, organic magnesium

4

Ziegler-Natta catalyst in this work showed tremendous enhancement on the average

5

molecular weight while early inorganic magnesium catalyst (5) were not effective

6

enough to do so.

7 8

Fig. 1. Typical preparation procedure of the V-Ti bimetallic catalyst and the

9

corresponding monometallic catalysts

10

Figure 1 showed a typical preparation method of inorganic magnesium based

11

VOx·(SiO2/MgO/MgCl2)·TiClx Ziegler-Natta catalyst and organic magnesium based

12

VOx·(SiO2/MgR2/MgCl2)·TiClx Ziegler-Natta catalyst. The role of magnesium alkyl

13

reagent in the catalyst preparation process is not only the support for loading titanium

14

active species. More importantly, the magnesium alkyl reagent reduces the vanadium

15

active sites into proper valence state and consumes the main extra hydroxyl group on

1

the silica support, which can strengthen the secondary metal active center performance

2

and enhance the overall activity while lower the cocatalyst consumption.

3

Silica gel was an ideal material due to its high loading ability with a porous structure,

4

chemical stability and low mass transfer resistance. Some recent works reported some

5

heterogeneous catalysts based on silica gel with better separation and reserved the

6

merits of organometallic compounds [35-37]. According to Nowlin [38], the residual

7

SiO2 surface hydroxyl groups (-OH) directly influence the magnesium loading in the

8

catalyst preparation process. Unlike SiO2 supported inorganic magnesium

9

Ziegler-Natta catalysts [29], it should be noticed that almost all magnesium

10

components in the organic magnesium based Ziegler-Natta catalysts were loaded on the

11

surface hydroxyl groups with minority washed as dissociated magnesium. On one hand,

12

magnesium content feeding affects the amount of MgCl2 that in situ formed in the TiCl4

13

reflux process, which would sway the titanium active species amount. Thus, the

14

hydroxyl groups would indirectly affect the magnesium content and then influence

15

titanium active species. Besides, hydroxyl groups were found to directly influence the

16

catalyst activity [2, 39-41] and polymer property [2, 17, 40, 42] in many silica

17

supported polyolefin catalysts. On the other hand, as a considerable way to influence

18

the active sites, the impregnation amount of magnesium metal component would also

19

affect both catalyst activity and polymer property.

20

In this work, an elaborated study focus on this series of novel V/Ti bimetallic catalysts

21

were carried out by delving into how the V-Ti catalyst response to different preparation

1

conditions in shifting the dominance of each metal active center type in its catalytic

2

process and polymer properties. The magnesium content and calcination temperature

3

were

4

(SiO2/MgR2/MgCl2)·TiClx Ziegler-Natta catalysts focus on calcination temperature

5

and metal content were prepared to compare with the monometallic Ti and V catalyst.

6

These catalysts exhibited greatly improved activities and the obtained polymer

7

properties exhibit a clear relationship with the variation of two metal species amount.

8

The vanadium active species in this catalyst fairly activated and do affect the catalyst

9

reaction rate and polymer feature. The results also showed how the pre-reduction effect

selected

to

demonstrate

the

trend.

These

vanadium

modified

10

from alkyl metal reagent influences the catalyst modification effect.

11

2. Experimental Section

12

2.1. Materials

13

In this work, silica gel was provided by Qilu Branch Co., SINOPEC. Ammonium

14

metavanadate (99%, NH4VO3) were purchased from Sinopharm Chemical Reagent Co..

15

Dibutyl Magnesium (MgBu2, 1.0 M in heptane) and Triisobutylaluminum (TIBA, 1.0

16

M in toluene) were purchased from Aladdin Co. Ltd , stored and used as received in dry

17

box. Hydrochloric acid (HCl, AR grade) and titanium chloride (TiCl4, 99 wt%) were

18

purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd.. n-hexane (AR grade,

19

Sinopharm Chemical Reagent Co.) and n-heptane (AR grade, Sinopharm Chemical

20

Reagent Co.) as solvent were distilled with sodium metal slices until the indicator

21

diphenyl ketone showed pure blue. The purified solvent was stored in a stainless-steel

1

storage tank under purified nitrogen. The ultrahigh purity nitrogen (≥99.999%) was

2

dehydrated through passing one column of 4A molecular sieves (purchased from

3

Sinopharm Chemical Reagent Co.) and deoxidized through passing one column of

4

sliver molecular sieves (28 wt% of silver (I) oxide on alumina, purchased from

5

Sigma-Aldrich). All the gas including hydrogen (≥99.999%), ultrahigh purity nitrogen

6

(≥99.999%), high-purified dry air (99.99%) and ethylene monomer (polymer-grade)

7

were purchased from Shanghai Wetry Criterion Gas Co., Ltd.. Ethylene monomer was

8

dehydrated and deoxidized by passing through columns of 4A molecular sieves, Q-5

9

reactant catalyst (13 wt% of copper (II) oxide on alumina, purchased from Sigma

10

Aldrich) and 13X molecular sieves (purchased from Sinopharm Chemical Reagent Co.,

11

Ltd.). 1-Hexene (total purity 97%) as commoner was purchased from J&K Chemical

12

Co. and purified as above solvent before use.

13

2.2. Catalyst Preparation

14

All the experiments were conducted with standard Schlenk, vacuum-line, or

15

inert-atmosphere box techniques under a nitrogen atmosphere. Firstly, 5.5 g silica was

16

pretreated with 300 °C purified nitrogen in a fluidized bed reactor for at least 1 h before

17

use. Secondly, about 4.5 g of calcined silica was impregnated in aqueous solution with

18

the calculated amount of ammonium metavanadate at 50 °C for 5 h before drying under

19

atmosphere at 120 ℃ overnight. Then, the chromium-impregnated silica was calcined

20

under highly purified air in fluidized-bed quartz reactor for 4 h at various temperature

21

(300 ℃, 400 ℃, 500 ℃, and 600 ℃) and stored at dry box. Thirdly, under nitrogen, about

1

3.5 g of catalyst precursor prepared above were transferred into a three-necked flask

2

with stir, then needed amount dibutyl magnesium solution (1 wt%, 2 wt%, 4 wt%, and

3

10 wt% ) were injected with 60 ml purified n-heptane. The mixtures were fully mixed at

4

about 55 ℃ for 2 h and washed with fresh purified heptane. Before drying, the precursor

5

was swept with nitrogen for 1.5 h. Then, the precursor was vacuum dried for 30 min.

6

The dry loaded silica powder was refluxed with 30 ml titanium chloride (TiCl4, 99 wt%)

7

at 140 ℃ for 2 h. Then, the precipitates were washed 5 times with purified heptane. The

8

residual solvent was removed with nitrogen flow swept for 1 h and was dried in vacuum

9

for 0.5 h. Finally, the catalysts stored in the glove box after cooling.

10

According to our previous work [27, 32, 33], 2 wt% vanadium content and 2 wt%

11

magnesium content were selected as the model catalyst composition. This metal

12

content would be in the proper range, according to some even earlier work [2, 38, 40],

13

and helps remain a good active site performance.

14

Therefore, bimetallic Ziegler-Natta catalyst ((VOx/SiO2/MgR2/MgCl2)·TiClx) with 2

15

wt% of V content which calcined at different temperature including 300 ℃，400 ℃，

16

500 ℃ and 600 ℃ were prepared and denoted as 3VMT, 4VMT, 5VMT, 6VMT,

17

respectively. Further, 600℃were chosen with best vanadium active sites performance

18

as the fixed temperature to investigate how the magnesium content affects catalyst

19

performance,

20

(VOx/SiO2/MgR2/MgCl2) ·TiClx catalysts at 600 ℃ were further prepared and denoted

21

as 6VM1T, 6VM4T, 6VM10T. For comparison, unmodified monometallic

1

wt%,

4

wt%

and

10

wt%

magnesium

content

1

(SiO2/MgR2/MgCl2) ·TiClx catalysts, calcined at 400 and 600 ℃ were prepared in the

2

same way and donoted as 4MT and 6MT. And monometallic VOx catalyst calcined at

3

600 ℃ and pretreated with 2 wt% MgBu2 reagent were denoted as 6VM, for

4

comparison.

5

2.3 Ethylene and ethylene/1-hexene polymerization

6

The slurry polymerization reaction was carried out under atmospheric pressure. Firstly,

7

about 100 mg catalyst was weighed and sealed in a small tube in the glove box. The

8

tube was connected but isolated temporary to a 250 mL three-necked flask with a 70 ℃

9

thermostatic oil bath. Prior to use, the reactor was purged by nitrogen and ethylene

10

monomer successively to ensure the system was fully occupied with dry ethylene for

11

about 0.12 MPa. Secondly, a certain amount of cocatalyst TIBA, 80 mL n-heptane and

12

10 mL hydrogen (if necessary) or a certain amount of 1-hexene (if necessary, 1 vol%, 3

13

vol% and 5 vol% of n-heptane) were injected into the flask and mixed fully before

14

reaction. After feeding, the system was saturated at the ethylene pressure of 0.15 MPa

15

for 5 min, and the polymerization started after opening the sealed catalyst tube to mix

16

the catalyst into the slurry. The polymerization was reacted at 70 ℃ for 1 h. Last, the

17

reaction was stopped and the polymer was treated with 200 mL ethanol/HCl solution.

18

Obtained polymers were washed with ethanol, filtered and dried under vacuum before

19

weighing.

20

2.3 Characterization of Catalyst and Polymer

21

ICP spectrometer (Varian 710-ES, Varian Company, USA) were used to test the

1

magnesium and chromium loading of the prepared catalysts. The inductively coupled

2

plasma spectrometry (ICP) test was conducted under the power of 1.10 kW. The flow

3

rates of auxiliary gas and plasma gas were 1.50 and15.0 L/min. The pump speed was 13

4

rpm with nebulizing gas pressure at 200 kPa.

5

To determine titanium content of the prepared catalyst precisely, ultraviolet-visible

6

spectrophotometer (UV-vis) through hydrogen peroxide colorimetric method [43] was

7

preferred. The catalyst was pretreated with enough 2 M sulfuric acid and was diluted to

8

25 mL with deionized water. To form peroxotitanium complex [TiO(H2O2)] to detect,

9

the solution was treated with H2O2. UV-vis measurements were carried out on a

10

Mapada UV-3200 spectrophotometer (Mapada UV-3200, Mapada company, UK). The

11

titanium content was quantified according to the intensity of a peak at 409 nm.

12

The value of specific surface area (SBET) of catalyst

13

Brunauer-Emmett-Teller (BET) method. The catalyst average pore size (d) and pore

14

volume (Vp) were tested using the Barrett-Joyner-Halenda (BJH) method. Automatic

15

physisorption analyzer (Micromeritics ASAP 2020, Micromeritics company, USA) was

16

conducted to do these tests with nitrogen adsorption/desorption experiments at 77 K .

17

To study the polymer properties, Differential Scanning Calorimetry (DSC), High

18

Temperature Gel Permeation Chromatography (HT-GPC) were employed. For polymer

19

thermal properties, a 5 mg polyethylene sample was tested in an aluminum sample cell

20

in a DSC analyzer (DSC Q200, TA Company, USA). To remove thermal history, the

21

aluminum test carrier cell was heated to 160 ℃ at the rate of 10 ℃/min and remained for

was

determined

1

5 min. Secondly, the cell cooled down to 40 ℃ at 10 ℃/min and reheated to 160 ℃ at

2

10 ℃/min. The second heating curve and melting temperature (Tm) was recorded and

3

the enthalpy of fusion (∆Hf) of each sample can also be calculated by the thermal curve.

4

HT-GPC (Agilent PL-220, Agilent Company, UK) were used to characterize the MW

5

and MWD of produced polyethylene with two PL gel-mixed B columns at 160 ℃. The

6

solvent was 1,2,4-trichlorobenzene (TCB) and the standard sample was polystyrene

7

(PS). The polymers were dissolved and were filtered at 160 ℃ in TCB at a

8

concentration of 1.0 mg/mL before injection to the chromatography with the machine

9

flow rate 1.0 mL/min.

10

X-ray diffusion (XRD) and Differential thermogravimetric analysis (DTA) methods

11

and results were summarized in Supporting Information Part S2 and S3, respectively.

12

3. Result and discussion

13

3.1 Characterization of the catalyst

14

The catalysts were characterized by ICP and UV-Vis to measure elements contents. The

15

N2 absorption/desorption measurements were taken to determine pore structure

16

parameters.

17

Run

1 2 3 4 5

Table 1 Characterization results of different catalysts Elements Contents Pore Structurese Tid Mgb Mgc Va Vb Vc Catalyst SBET Vp d [wt [wt [wt [wt [wt [wt 2 3 [m /g] [cm /g] [nm] %] %] %] %] %] %] 3VMT 3.88 1.5 1.3 1.6 1.0 0.8 206.7 0.96 19.6 4VMT 3.65 1.3 1.2 1.6 1.2 1.0 208.5 0.97 19.1 5VMT 3.13 1.3 1.2 1.5 1.3 1.2 213.4 0.98 19.5 6VMT 2.81 1.2 1.2 1.6 1.5 1.4 208.1 1.01 20.5 6VM1T 2.70 1.1 0.8 1.6 1.5 1.4 214.0 1.02 21.0

1 2 3

6 6VM4T 3.34 2.3 1.7 1.6 1.3 1.2 210.0 0.92 18.7 7 6VM10T 3.61 3.8 3.3 1.6 1.2 1.0 200.8 0.75 15.8 8 4MT 3.96 1.4 1.3 / / / 222.7 1.23 24.2 9 6MT 2.99 1.3 1.3 / / / 224.6 1.24 23.7 10 6VM / / 1.6 1.6 / 1.6 230.3 1.23 23.8 a) Measured just after calcination by ICP; b)Measured before reflux with TiCl4 by ICP; c) catalyst Measured by ICP; d) catalyst Measured by UV-vis; e) catalyst Measured by nitrogen adsorption/desorption experiments.

4

According to an earlier study by Strobel [44], the Grignard compound would react with

5

hydroxyl groups to fix the magnesium group onto the silica surface. For catalyst

6

precursor, Table 1 showed that the magnesium content of catalyst precursor that stored

7

before the TiCl4 reflux reaction generally increases along with the decreasing

8

calcination temperature as well as the rising hydroxyl group. After the TiCl4 reflux

9

reaction, the Mg content of catalysts showed no significant difference among catalysts,

10

but the titanium content showed a clear trend. The loss of magnesium groups may

11

attribute to the titanium reflux process.

12

It clearly showed that the titanium content negatively related to the calcination

13

temperature. To be more precise, as the calcination temperature went up, the surface

14

hydroxyl groups decreased gradually[38, 40]. According to the ICP result, the

15

magnesium loading on the higher calcination temperature precursor declined by up to

16

20% than the lower calcination temperature did. In supported Ziegler-Natta titanium

17

catalyst system, magnesium load would further responsible for loading titanium active

18

sites. Consequently, the titanium loading in the higher temperature of calcination

19

catalyst 6VMT was 27.5% lower than that of low calcination temperature catalyst

20

3VMT. This may indicate that the relatively stable relationship between the titanium

1

and the magnesium content.

2

The vanadium contents in the catalyst precursor, firstly exhibited no difference after the

3

calcination process, constantly decreased during the catalyst preparation process. In

4

addition, the vanadium component was decreased with the dropping calcination

5

temperature. This can be explained by that the more hydroxyl groups increased the

6

magnesium loading and further rise the titanium load, and the amount of metal load had

7

a noticeable competition with the vanadium content. It was worth mentioning that in

8

the high calcination temperature catalyst, the loss of vanadium content as high as 61 %

9

compared with the 30 % in the 300 ℃ catalyst 3VMT. This reinforced that the

10

deficiency of hydroxyl groups in the high temperature exacerbated the heavily loading

11

sites competition [15].

12

The nitrogen absorb/desorb result was shown in Table 1. The structure parameter of

13

catalysts exhibited that the supported catalysts have lower SBET, pore volume and pore

14

diameter than original silica support, due to the loading of metal species. These

15

parameters decreased to some extent as the loading process, which is consistent with

16

abundant earlier work [2, 40, 43, 45, 46].

17

3.2 Ethylene homo-polymerization

18

3.2.1 Effect of calcination temperature

19

Plenty of early works reported that the calcination temperature of support material

20

would influence the reaction system [47-49]. In the polyolefin field, silica gel was one

21

of the most general support that could reinforce the polymer structure, increase the

1

activity and benign reproduction of good morphology [50]. The calcination

2

temperature of silica gel correlated negatively with the hydroxyl groups on the surface

3

of the support [38].

4

To investigate the relationship between calcination temperature and polymerization

5

performance on vanadium modified organic magnesium based Ziegler-Natta catalyst, a

6

series of 2 wt% vanadium modified organic magnesium based Ziegler-Natta catalysts

7

with different calcination temperature were synthesized. In this part, four V-Ti

8

bimetallic catalysts with varying calcination temperature (300 ℃，400 ℃，500 ℃，600 ℃，

9

respectively) were used in ethylene homopolymerization, labeled 3VMT, 4VMT,

10

5VMT, and 6VMT, respectively. TIBA was employed as the cocatalyst for

11

polymerization. The ethylene polymerization activity and product properties of various

12

catalysts were listed in Table 2.

13

Table 2 Ethylene homo-polymerization activities of different catalysts and

14

polymer properties. Activity Run

Catalyst

1 2 3 4c 5 6 7 8 9c 10

3VMT 3VMT 3VMT 3VMT 3VMT 4VMT 4VMT 4VMT 4VMT 4VMT

Al/Ti

[g PE/g Cat.·h-1]

5 10 20 20 30 2.5 5 10 10 20

54.3 56.6 57.4 56.7 52.3 79.4 84.5 88.5 70.1 70.0

Activity [kg PE/mol Ti.·h-1] 66.99 69.83 70.82 69.95 64.53 104.13 110.82 116.07 91.94 91.81

Tma

△Hf a

MW b

[oC]

[J/g]

[×105g/ mol]

MWD b

133 133 134 134 133 132 132 134 133 134

140.9 140.3 139.1 155.7 142.4 136.5 142.5 145.0 155.1 144.5

15.91 14.74 14.33 8.06 13.88 20.56 19.42 19.27 8.90 19.56

11.59 12.23 13.95 17.45 15.93 9.77 10.79 12.59 19.79 9.72

1 2 3 4

11 5VMT 2.5 52.2 79.83 132 136.8 21.87 7.75 12 5VMT 5 68.7 105.07 132 136.9 22.19 8.68 c 13 5VMT 5 47.1 72.03 134 157.2 12.53 13.56 14 5VMT 10 63.0 96.35 134 141.6 21.52 9.01 15 5VMT 20 52.5 80.29 133 137.6 20.84 12.75 16 6VMT 1.25 54.2 92.33 132 135.1 24.32 6.84 17 6VMT 2.5 55.7 94.89 132 137.9 24.16 6.73 c 18 6VMT 2.5 48.2 82.11 134 157.4 15.34 9.74 19 6VMT 5 54.7 93.18 133 135.0 23.86 6.39 20 6VMT 10 40.0 68.14 133 135.9 23.47 7.29 21 4MT 2.5 43.0 51.98 133 144.0 17.29 3.90 22 4MT 5 49.0 59.23 133 144.7 17.02 3.79 23 4MT 10 54.0 65.28 133 148.1 16.85 4.23 c 24 4MT 10 42.0 50.77 134 159.0 11.11 4.99 25 4MT 20 46.2 55.61 133 144.8 16.19 4.78 26 6VM1T 1.25 39.3 69.15 134 124.3 25.34 5.15 27 6VM1T 2.5 42.6 75.53 134 124.3 27.35 6.52 c 28 6VM1T 2.5 40.1 71.10 134 142.7 16.61 14.65 29 6VM1T 5 36.3 64.36 134 122.3 26.94 7.49 30 6VM1T 10 31.3 55.49 135 118.3 25.81 7.24 31 6VM4T 2.5 49.2 70.52 135 125.4 24.04 5.79 32 6VM4T 5 50.3 72.09 134 126.7 23.89 4.15 33 6VM4T 10 50.8 72.81 135 123.7 22.82 4.89 c 34 6VM4T 10 42.0 60.20 135 147.2 15.55 10.32 35 6VM4T 20 48.2 69.08 135 125.9 21.96 5.09 36 6VM10T 2.5 44.0 58.35 134 121.7 22.43 6.57 37 6VM10T 5 43.6 57.82 135 122.1 21.81 4.36 38 6VM10T 10 46.9 62.19 135 125.0 20.89 6.18 c 39 6VM10T 10 42.8 56.75 135 140.0 15.21 5.83 40 6VM10T 20 40.1 53.17 135 125.0 20.16 6.29 41 6MT 2.5 41.2 65.64 134 127.8 19.05 3.16 42 6MT 5 45.1 72.05 134 126.4 17.84 4.23 c 43 6MT 5 40.0 64.04 135 145.0 12.87 4.23 44 6MT 10 43.3 68.84 135 127.8 16.89 4.17 45 6MT 20 40.4 64.04 134 126.6 16.64 6.19 d 46 6VM / 10.0 / 136 127.2 29.71 3.54 Other polymerization conditions: catalyst 100 mg, ethylene 0.15 MPa, n-heptane 80 mL, TIBA, 70 °C, 1 h; a Tm and ∆Hf by DSC thermograms; b MW and MWD (MW/Mn) measured by HT-GPC; c polymerization with 10ml H2; d same amount of cocatalyst amount as run 18.

1 2

Fig. 2. Ethylene homo-polymerization activities of calcination temperature group

3

catalysts with different dose of TIBA. Other polymerization conditions: catalysts

4

100 mg, ethylene pressure 0.15MPa, heptane 80ml, 70 , 1h.

5

Table 2 and Figure 2 showed that the ethylene polymerization activities increase

6

sharply to a maximum value at 400 ℃, and then drop steadily, with the increase of

7

calcination temperature.

8

On one hand, in the relatively low temperature (below than 400 ℃), the calcination

9

temperature increases remarkably declined the surface hydroxyl groups, which may

10

serve as one of the main impurity in the polymerization. Thus, the relatively low

11

activity of 300 ℃ catalyst 3VMT could be explained by too much hydroxyl groups on

12

the surface of silica gel, which has already under full investigation in many earlier

13

literatures [51, 52]. On the other hand, after 400 ℃, a steadily decline on activity could

1

be noticed during the further boost in calcination temperature. According to the ICP

2

results in Table 1, along with the calcination temperature increasing, the magnesium

3

content of catalysts declined smoothly. Hence, the activity decrease was attributed to

4

the fall of titanium concentration, which directly influence the active site amount.

5

In addition, from Table 2 and Figure 2, the optimum Al/Ti ratio increased as the

6

calcination temperature decreased. It was well reported that the cocatalyst could reduce

7

the active species into proper valence state and alkylate the transition metal to activated

8

condition for initiating the first polymerization chain, and served as impurities

9

scavenge [53]. Consequently, the relatively low cocatalyst consumption in high

10

calcination temperature catalyst was due to the pre-reduction of organic magnesium

11

reagent and decreasing surface -OH groups which is one of the key impurities.

12

By comparing the highest activity evaluated by gPE/gCat h of each catalyst, all the

13

modified catalysts showed a higher activity than unmodified one, which indicated the

14

vanadium modification is quite satisfactory. Run 2 showed the highest activity of 88.5

15

gPE/gCat h, 63.9% higher than unmodified catalyst 4MT, which verified the great

16

boost in activity with the introduction of secondary metal active species.

1 2

Fig. 3. SEM result of polymer and catalyst particle in different times. (a-d) 0 s;

3

(e-h) 60 s; (i-l) 120 s. Catalyst in polymerization: (a, b, e, f, i, j) 4MT; (c, d, g, h, k, l)

4

4VMT

5

Figure 3 showed the SEM result of the polymerization process on 4MT and 4VMT. It

6

clearly showed that all the catalyst particles remain in good sphere shape. After the

7

reaction goes for 60 seconds, catalyst 4MT showed minor polymer dotted on the

8

relatively intact catalyst surface while the support of catalyst 4VMT had already started

9

to crack into the microscale cluster. In 120 s last row, 4MT start to crack into

10

sub-particles while the 4VMT had almost accomplished the fragmentation process.

11

What is behind the phenomenon can be explained that the inner active sites access to

12

the monomer much faster for V-Ti bimetallic catalysts, that is why the reaction activity

13

of the vanadium modified catalyst is much higher than unmodified Ti one [48]. Based

1

on the activity enhancement and fragmentation difference depicted by SEM, a

2

simplified schematic model is presented in Figure 4 to depict this accelerate activation

3

process to the regard of dual supported bimetallic Ziegler-Natta catalyst.

4 5

Fig. 4. Schematic model of the two-stage accelerating activation process depicted

6

how the pre-reduced vanadium active sites involved in the acceleration of the

7

weak point fracture and make the buried inner surface active sites accessible to

8

the monomer.

9

Figure 4 showed the two-step accelerating activation process of the bimetallic catalyst.

10

The first step was that the catalyst particle fragmentized quickly with the help of

11

vanadium active sites. Then, the inner active species were accessible to the monomer in

12

the slurry phase and start to react quickly. With the pre-reduction effect of magnesium

13

alkyl reagent, the catalyst exhibited high activity and better performance in MWD,

14

which would discuss below.

15

The importance and relationship of morphology and activity had long been studied in

16

both experimental and theoretical aspects. McKenna [54] conducted a theoretical

17

study with the polymeric flow model in the slurry phase reactor, which based on the

18

morphological data from the experiment. The result showed that the fragmentation of

1

the catalyst, with more voids in the system, could significantly increase the activity.

2

Soares [55] reported the particle morphology evolution and growth. The monomer

3

diffused into the pore and formed polymer at the same time, and the generated

4

polymer would build up the stress and the fragmentation forms at the weak point

5

under the accumulated stress.

6

Other practical theoretical studies [56, 57] on Random pore polymeric flow model

7

also described the relationship between the morphology evolution and intraparticle

8

monomer concentration, diffusion coefficient, as following: = 1− ,

9

⋅

= 1−

Where

+ ∙

1+3 ,

,

+

,

and

represent the monomer concentration at the

10

external surface of particle, at the polymer phase intraparticle and at the pore inside

11

the particle.

12

surface, polymer phase and pore pathway inside the particle. ε represent the porosity

13

of the catalyst/polymer particle and the

14

tortuosity.

15

It was noteworthy that the monomer concentration and diffusion coefficient in the

16

pore pathway or void would be 3-5 times higher than that in the polymer phase, where

17

monomer must endure the mass transfer resistance of amorphous polymer and the

18

impenetrability of crystallized polyethylene [56]. Based on the above equation, the

19

monomer concentration and the diffusion coefficient would increase when the

,

,

,

and

,

indicates the diffusion coefficient at the external

is the constant related to the particle

1

polymer particle breaks and introduces more voids and fractures, and more accessible

2

for the inner surface active sites. Hence, the monomer concentration and diffusion

3

coefficient would help soar the catalyst activity and the active sites performance.

4 5

Fig. 5. GPC curves of polyethylene produced by catalysts: (a) 3VMT; (b) 4VMT;

6

(c) 5VMT; (d) 6VMT, at different TIBA dose. Other polymerization conditions:

7

catalysts 100 mg, ethylene pressure 0.15MPa, heptane 80ml, 70 , 1h.

8

Further looking into the influence of various catalysts on homo-polymerization, the

9

polymers were characterized under the same condition with HT-GPC and DSC. The

10

results were listed in Table 2 above. GPC results of each catalyst under different

11

cocatalyst condition were showed in Figure 5. Combined with Table 2, the molecular

12

weight and MWD was not changed much with various cocatalyst content. In addition,

13

the alkyl metal content in the high calcination temperature catalyst was remarkably

14

lower when compared with conventional inorganic magnesium based Ziegler-Natta

1

catalyst [29]. This unique advantage favored by industrial society may result from

2

organic magnesium reagent, that is, the characteristics of the pre-reduction catalyst, as

3

our lab reported before [16]. The alkyl metal reagent would reduce the catalyst active

4

site into proper valent state and lower the consumption of cocatalyst. However, for low

5

calcination temperature catalyst below than 400℃, the cocatalyst consumption is

6

similar to traditional inorganic magnesium based Ziegler-Natta catalyst for

7

overwhelming hydroxyl groups serve as an impurity on the surface of catalyst. The

8

molecular weight and its distribution were similar for the same catalyst, indicating the

9

catalysts were quite insensitive to the cocatalyst concentration.

10

For various calcination temperature catalysts, we could find that the average molecular

11

weight (MW) was between that of the monometallic Ti and V model catalyst.

12

Meanwhile, the activity was higher than the two single catalysts. This indicated that

13

two active sites were well activated in the polymerization and the polymer showed both

14

features that we would discuss in detail below.

15

From plenty of earlier work [15, 30-32], the monometallic vanadium catalysts showed

16

high molecular weight with middle board molecular weight distribution. The result

17

showed that the molecular weight of 3VMT at low calcination temperature was below

18

15×105 g/mol. With the increasing temperature of calcination, a higher V/Ti species

19

ratio value in the bimetallic catalyst almost doubled the molecular weight into

20

24.16×105 g/mol. Moreover, the polymer property of 4MT and 6MT that listed in Run

21

23 and Run 42 of Table 2 showed that the calcination temperature would not have a

1

noticeable change on the performance of titanium active species, a similar result can be

2

obtained when comparing with some early work from our group [29]. Combine these

3

together, it showed that the benign chemical condition of vanadium active sites in this

4

bimetallic catalyst, especially in the high calcination temperature catalyst.

5 6

Fig. 6. GPC curves of polyethylene produced by catalysts at optimum TIBA dose.

7

Other polymerization conditions: catalysts 100 mg, ethylene pressure 0.15 MPa,

8

heptane 80 ml, 70

9

Figure 6 showed the molecular weight and its distribution (MWD) of different

10

calcination temperature catalysts. The MWD curves of low calcination temperature

11

catalyst 3VMT were clearly boarder than others. As the calcination temperature went

12

up, the low molecular part (about 105 g/mol) shrank quickly and the high molecular part

13

(over 106 g/mol) uplifted significantly.

, 1 h.

1

The unique trend of MWD curves resulted from the shifting balance of two active sites

2

that influenced much by calcination temperature. Firstly, for low calcination

3

temperature catalysts, the catalyst 3VMT showed the lowest molecular weight among

4

the catalysts with high titanium content. Meanwhile, the lowest vanadium content in

5

the catalyst also explained the polymer features more close to unmodified titanium

6

catalyst in low calcination temperature. As the calcination temperature went up, the

7

hydroxyl groups on the silica surface dropped smoothly which lower the magnesium

8

loading and further decline the titanium loading. Less titanium loading weakens the

9

competition of loading and leaves more vanadium active sites loaded. Combined with

10

less titanium active sites and more vanadium species, the molecular weight of the

11

polymer increase 68.6% to 24.16×105 g/mol. This also clearly seen from relative

12

shrinking the low MW part and uplifting the high MW part.

13

Further, from Table 1, for a given catalyst, the polymer MW generally decreased as the

14

cocatalyst dose went up, while the MWD was becoming boarder as cocatalyst dose rise.

15

This was explained by the strong chain transfer ability of alkyl metal catalyst as many

16

literature already reported [58, 59].

17

3.2.2 Effect of magnesium content

18

In the last part, the calcination temperature was identified as an efficient way to adjust

19

the balance between two metal active sites. Catalyst characterization, polymerization

20

activity and polymer features (MW and MWD) verified that the hydroxyl group

21

amountcontrolled by calcination temperature matters in the bimetallic V-Ti catalyst

1

performance. To further investigate the relationship between those two active species,

2

magnesium dosage was selected to straightly regulate the catalyst performance.

3

In this part, a series of organic magnesium based bimetallic V-Ti Ziegler-Natta catalysts

4

calcined at 600 ℃ was synthesized with various magnesium addition at 1 wt%, 2 wt%, 4

5

wt% and 10 wt%, labeled as 6VM1T, 6VMT, 6VM4T, 6VM10T, respectively. In

6

addition, the monometallic vanadium catalyst 6VM which sealed before this precursor

7

reacts with TiCl4 to obtain catalyst 6VMT and monometallic titanium catalyst 6MT

8

calcined at 600 ℃ were made as a comparison. TIBA was employed as a cocatalyst for

9

polymerization. The ethylene polymerization activities and product properties of

10

various catalysts were listed in Table 2.

11 12

Fig. 7. Ethylene homo-polymerization activities of catalysts with different dose of

13

TIBA. Other polymerization conditions: catalysts 100 mg, ethylene pressure

1

0.15MPa, heptane 80ml, 70 , 1h.

2

Figure 7 demonstrated that catalyst activity as a function of catalyst type and

3

cocatalyst addition. The activities went up at first when magnesium content added to 2

4

wt%, then the activity slowly declined with the Mg addition increase. The first

5

increase process in the relative low Mg content (lower than 2 wt%) was caused by the

6

introduction of more high activity titanium active species. However, an extra dose of

7

magnesium content may make the magnesium atoms in poor agglomeration condition,

8

which may be harmful for titanium loading, and decrease the activity in turn. This

9

would be explained by the fact that the surface hydroxyl groups deficiency on the

10

support would reach under the high temperature of calcination (600℃), makes no

11

space for too many magnesium sites to disperse well. Besides, the over-reduction of

12

vanadium sites may occur with too much magnesium alkyl reagent also possibly

13

responsible for the activity drops.

14

Evaluating the catalyst activity by gPE/gCata h, the activity of various V-Ti catalysts

15

clearly enhanced when compared with the monometallic V catalyst and Ti catalyst,

16

indicating the V modification was quiet benefit for polymerization activity. The

17

highest activity enhanced 23.8 % than the unmodified Ti catalyst. Meanwhile, the

18

relative low cocatalyst consumption may owing to limited hydroxyl groups at 600 ℃

19

and the pre-reduction effect from magnesium reagent.

1 2

Fig. 8. GPC curves of polyethylene produced by catalysts: (a) 6VM1T; (b) 6VMT;

3

(c) 6VM4T; (d) 6VM10T, at different TIBA dose. Other polymerization conditions:

4

catalysts 100 mg, ethylene pressure 0.15 MPa, heptane 80 ml, 70

, 1 h.

5 6

Fig. 9. GPC curves of polyethylene produced by catalysts at optimum TIBA dose.

1

Other polymerization conditions: catalysts 100 mg, ethylene pressure 0.15 MPa,

2

heptane 80 ml, 70

3

The DSC and HT-GPC method results were listed in Table 2，also were shown in

4

Figure 8 and Figure 9. From Table 1, Tm and △Hf of all polymers were similar. This

5

was owing to that in our ethylene homo-polymerization, there were very few short

6

chain branches in the polymer. Compared with lower calcination temperature, these

7

series of catalysts in 600 ℃ all shared a high MW and a narrow MWD.

8

According to the ICP result, the magnesium content in the final catalyst increased as the

9

addition increase and the vanadium content were similar. From Figure 9, it could be

10

found that the MW of polymers produced by different catalysts decreased as the

11

magnesium content increased slightly. This could be explained that the raise of titanium

12

content makes the balance shift to the titanium side, which represents relatively low

13

molecular weight. Alternatively, the overwhelming of magnesium content may ruin the

14

chemical environment of vanadium active sites.

15

Among the catalysts, the MW of 6VM1T product was highest among the samples, at

16

27.35×105g/mol, 53.3 % higher than single Ti catalyst 6MT and 7.9 % lower than

17

single vanadium oxide catalyst. While the MW of 6VM10T shows the lowest at

18

20.89 ×105 g/mol, 17.1 % higher than the unmodified Ti catalyst and 29.7 % lower than

19

V monometallic catalyst 6VM.

20

These discrepancies in Table 2 and Figure 9 indicated that the catalyst preparation

21

condition makes a great influence on the polymer feature. Compared with Figure 6, it

, 1 h.

1

was observed that the effect of magnesium content on polymer MW was not as

2

prominent as the effect of calcination temperature. Calcination temperature factor may

3

cause different dispersity environment of magnesium composition. In this way,

4

titanium active species would have a good chemical environment for polymerization in

5

proper calcination temperature and magnesium loading conditions. However, the

6

magnesium contents relied on deficient hydroxyl groups in the high calcination

7

temperature, which may cause the agglomeration that would make titanium sites

8

partially inactivated. The evidence for the deficient hydroxyl group also reinforced by

9

the fact that the gross loss of magnesium content was observed in the catalyst

10

preparation process.

11

3.3 Effect of hydrogen

12

In industry, olefin polymerization was carried out in the presence of hydrogen for

13

balancing the mechanical property and processing property of polymers produced. As

14

literature reported, the hydrogen introduction would lower the polymerization rate and

15

reduce molecular weight with boarder MWD [60-62]. The Ti-H bond generated in the

16

chain transfer process with hydrogen presented caused the low activity. However, this

17

explanation weakened by the finding that significantly enhanced ethylene

18

polymerization activity of surface titanium or zirconium hydride that is formed in

19

supported organometallic catalysts when treated by hydrogen. Further, a hypothesis

20

was proposed to explain the reduced reaction rate that hydrogen can accelerate the

21

formation of poorly active Ti-C2H5 complex, which adverse to the further ethylene

1

insertion due to β-H agostic effect was putting forward to attribute [63, 64]. Hitherto, to

2

the best of our knowledge, the existing of the chemical structure of titanium organic

3

complex still lacked collaborate evidences. Until now, the hydrogen effect was still

4

waiting to be studied in detail.

5 6

Fig. 10. Hydrogen effect on the catalyst activities and average molecular weight

7

(Mw) of the polymers (factor: calcination temperature). Other polymerization

8

conditions: catalyst 100 mg, ethylene 0.15 MPa, n-heptane 80 ml, TIBA, 70

, 1 h.

1 2

Fig. 11. Hydrogen effect on the catalyst activities and average molecular weight

3

(MW) of the polymers (factor: magnesium dose). Other polymerization conditions:

4

catalyst 100 mg, ethylene 0.15 MPa, n-heptane 80 ml, TIBA, 70

5

In this work, the hydrogen effect results were listed in Table 2 and were depicted as

6

Figure 10 (for calcination temperature) and Figure 11 (for magnesium content). The

7

activities of all the catalysts showed a noticeable drop in the presence of hydrogen.

8

GPC characterization indicated the molecular weight of polymers decrease

9

significantly with the MWD value doubled. In general, the vanadium modified catalyst

10

showed a much better hydrogen response than the unmodified Ti catalyst. In the

11

presence of hydrogen, the average MW of polymers produced by calcination

12

temperature groups 3VMT, 4VMT, 5VMT, 6VMT catalysts drops about 40 %,

13

compared with 4MT’s 36.1 %. With regard to different magnesium content under

14

600 ℃, the 6VM1T, 6VMT, 6VM4T, 6VM10T showed a decline of 38.3 %, 36.5 %,

, 1 h.

1

31.9 %, 27.1 %, respectively, while the unmodified 6MT decreased 27.9 %. This

2

obviously concluded that the introduction of vanadium species could better regulate the

3

polymer MW with sacrificing the activity, which was also consistent with the work of

4

Wang et al [32].

5

3.4 Ethylene/1-hexene copolymerization and characterization of the copolymers

6

Various copolymer takes advantage of better flexibility, lower viscosity, and favorable

7

processing ability when compared with homo-polymer, which is caused by the

8

introduction of commoner into polymer carbon backbone. C4-C8 α-olefins was

9

ordinarily used in the polyolefin industry for producing a diversity of commercial PE

10

products. Among them, 1-hexene was one of the most important commoners in the

11

industry. In that way, the ethylene/1-hexene copolymerization was carried out for

12

further investigating the copolymerization behavior with vanadium modified organic

13

magnesium Ziegler-Natta catalyst. Table 3 demonstrated the ethylene/1-hexene

14

copolymerization activity. Consequential enhancement on the activities were observed

15

with both V-Ti bimetallic catalysts and monometallic Ti monometallic catalyst. This

16

illustrated that the commoner was activated in the copolymerization of ethylene and

17

1-hexene. Meanwhile, like in the homopolymerization process, the catalyst 4VMT

18

showed the highest activity as well in the copolymerization. The activity enhancement

19

by the α-olefin had studied for long but not fully proved, named “commoner effect” [63,

20

65-70]. The explanations come from two angles: chemical effect and physical effect.

21

The insertion of ethylene into the Ti-H bond would produce the Ti-C2H5 complex which

1

becomes a dormant site, according to β-agostic effect. This complex site was relatively

2

not active in polymerization [63, 64]. However, the Ti-C6H13 was produced with the

3

insertion of 1-hexene and this stable complex showed high activity. In addition, the

4

commoner can cause the donor effect to increase the activity. From the physical effect

5

perspective, reduction of crystallinity caused by generated copolymers further

6

accelerates the monomer diffusion in that the ethylene was believed impenetrable in the

7

crystallized PE, according to many early works [56, 57].

8 9

Fig. 12. GPC curve of ethylene copolymerization using catalysts: (a) 3VMT; (b)

10

4VMT; (c) 5VMT; (d) 6VMT, at different amount of 1-hexene with optimum dose

11

of TIBA. Other polymerization conditions: catalysts 100 mg, ethylene pressure

12

0.15MPa, heptane 80ml, 70 , 1h.

1 2

Fig. 13. GPC curve of ethylene copolymerization using catalysts: (a) 6VM1T; (b)

3

6VMT; (c) 6VM4T; (d) 6VM10T, at different amount of 1-hexene with optimum

4

dose of TIBA. Other polymerization conditions: catalysts 100 mg, ethylene

5

pressure 0.15MPa, heptane 80ml, 70 , 1h.

6

HT-GPC and DSC were used to characterize the resulting copolymers. Figure 12 and

7

Figure 13 shows the GPC curve of V-Ti bimetallic catalysts with different commoner

8

concentration. In Figure 12, the bimodal distribution showed up in the polymer of

9

higher calcination temperature catalyst (5VMT and 6VMT). Similar bimodal results

10

can be found in the polymer of all magnesium content catalyst at 600 ℃ results in

11

Figure 13, these results are clear evidence that the successful activation of both active

12

sites with the introduction of the secondary vanadium active species into the catalyst

13

system.

14

The polymer characterization result was shown in Table 3, the copolymer melting point

1

and ∆Hf of the polymer decreased slightly as the commoner concentration in the

2

system increase. This was due to the introduction of 1-hexene into the polymer carbon

3

backbone. Meanwhile, the MW of polymers were observed a sharply decline. The

4

significant drop in MW and broaden in MWD was influenced by the strong chain

5

transfer ability as illustrated in early works [71, 72]. In addition, the preference

6

insertion of 1-hexene in the low molecular polymer produced more low-MW polymer

7

[63].

8

Table 3. Ethylene/1-Hexene copolymerization activities of different catalysts and

9

polymers characterization. 1-Hexene Run

Catalyst

1 3VMT 2 3VMT 3 3VMT 4 4VMT 5 4VMT 6 4VMT 7 5VMT 8 5VMT 9 5VMT 10 6VMT 11 6VMT 12 6VMT 13 6VM1T 14 6VM1T 15 6VM1T 16 6VM4T 17 6VM4T 18 6VM4T 19 6VM10T 20 6VM10T 21 6VM10T

[vol%] 1 3 5 1 3 5 1 3 5 1 3 5 1 3 5 1 3 5 1 3 5

Activity [g PE/g Cat.·h-1] 87.3 75.5 72.3 109.0 89.1 84.4 91.8 89.4 78.8 91.1 73.9 63.4 90.6 74.1 70.5 92.8 82.2 75.8 91.0 83.3 70.5

Tma

△Hf a

[oC]

[J/g]

124 122 123 125 122 122 125 123 122 125 121 120 126 125 125 125 123 122 126 123 122

98.4 91.9 72.9 127.7 63.5 50.0 111.4 75.6 64.1 117.6 65.1 37.1 109.8 78.9 64.7 104.6 71.3 65.9 95.3 51.5 52.3

Mw b [×105g/ mol] 7.42 7.04 6.40 8.98 8.17 7.27 10.42 9.16 9.04 16.27 15.33 15.00 17.63 16.13 15.79 14.54 13.85 13.44 13.66 9.86 8.98

MWD c 18.76 21.29 20.68 22.68 18.69 25.99 11.71 14.15 12.48 8.71 8.97 9.10 6.81 8.80 12.04 9.01 9.15 8.85 6,79 6.62 9.51

1 2 3 4

22 4MT 5 79.1 122 44.4 6.93 10.11 23 6MT 5 75.6 122 54.3 6.99 10.35 Other polymerization conditions: catalyst 100 mg, ethylene 0.15 MPa, n-heptane 80 mL, TIBA, 70 °C, 1 h; a Tm and ∆Hf by DSC thermograms; b MW estimated by HT-GPC in TCB, average molecular weight; c MWD (MW/Mn) measured by HT-GPC.

5

4. Conclusion

6

A series of novel V/Ti bimetallic polyethylene catalysts were developed successfully

7

through the introduction of Vanadium species into the (SiO2/MgR2/MgCl2)·TiClx

8

Ziegler-Natta Catalyst using organic magnesium reagent, and the homo-polymerization

9

and copolymerization were carried out. The reducibility of organometallic compound

10

MgR2 on calcined VOx remarkably exploited and improved the performance of the

11

vanadium site. As a result, the V-Ti bimetallic catalysts showed higher activity in

12

ethylene polymerization and the polymer exhibited up to 53.3 % higher MW and up to

13

63.9 % higher activity than that produced by the single Ti catalyst.

14

Calcination temperature and magnesium content were systematically investigated to

15

describe the effect on the polymer proprieties with the shift of balance between the two

16

metallic active centers. Lower calcination temperature is prone to the domination of

17

titanium active species performance in the polymerization. Besides, the magnesium

18

content would directly influence the amount of titanium species and exacerbate the

19

competition of load between metal species. Generally, the polymerization exhibited

20

consequential improvement with the introduction of the vanadium species.

21

Except as the precursor of MgCl2 support, the magnesium alkyl reagent could highly

22

activate the vanadium active sites by pre-reduction and consume the surface hydroxyl

1

groups to scavenge impurity. The activated vanadium sites not only produce high MW

2

part to increase polymer average MW but also help crack the silica support to make

3

more metal active species in the inner surface of support accessible to the monomer in

4

the slurry phase, which could highly increase the reaction activity, according to the

5

SEM result. Meanwhile, the vanadium modified bimetallic catalyst shows 37.3 %

6

higher hydrogen response and the copolymer sample even gets bimodal molecular

7

weight distribution with proper average molecular weight, which is favored by the

8

industry.

9

In this work, these vanadium modified (SiO2/MgR2/MgCl2)·TiClx Ziegler-Natta V/Ti

10

bimetallic catalysts are developed and the effect of active species dominant change on

11

the polymer property are fully discussed. In addition, a simplified model was proposal

12

to explain the high activity and good modification effect of our bimetallic catalyst in

13

terms of pre-reduction effect and two-stage initiation process.

14

1

Acknowledge

2

This work is financial supported by National Natural Science Funds of China

3

[201674036].

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Highlight: Introduce alkyl magnesium to activate secondary vanadium active sites and systematically study on the V-Ti bimetallic catalyst; A two-stage bimetallic catalyst crack model were proposed. Polymerization activity increased 63.9 %, MW increase over 30 %, bimodal MWD shows in copolymerization; Low cocatalyst consumption; Analyst the relative performance of two active sites with different preparation conditions;

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: