Preparation of Porous Spherical MgCl2/SiO2 ComplexSupport as Precursor for Catalytic PropylenePolymerization

Tao Jiang,1,2,3 Wei Chen,2 Feng Zhao,2 Yuexiang Liu,2 Ruen Wang,2 Hongbin Du,2 Tianyi Zhang2

1Department of Material Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China2Sinopec Beijing Research Institute of Chemical Industry, Beijing 100013, China3Department of Chemical Engineering, Daqing Petroleum Institute, Daqing 163318, China

Received 7 July 2004; accepted 15 February 2005DOI 10.1002/app.22263Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: The MgCl2/SiO2 complex support was pre-pared by spray drying using alcoholic suspension, whichcontained MgCl2 and SiO2. The complex support reactedwith TiCl4 and di-n-butyl phthalate, giving a catalyst forpropylene polymerization. The catalyst was spherical andporous with high specific surface area. TEA was used as acocatalyst, and four kinds of alkoxysilane were used asexternal donors. The bulk polymerization of propylene wasstudied with the catalyst system. The effect of the reactionconditions and external donor on the polymerization were

investigated. The results showed that the catalyst had highactivity, high stereospecificity, and sensitive hydrogen re-sponsibility. Polypropylene has good grain morphology be-cause of duplicating the morphology of the catalyst. © 2005Wiley Periodicals, Inc. J Appl Polym Sci 98: 1296–1299, 2005

Key words: alkoxysilane; complex supports; poly(pro-pylene) (PP); silica/magnesium chloride; Ziegler–Natta po-lymerization

INTRODUCTION

It is well known that support and donor have veryimportant impact on Ziegler–Natta catalyst systemsfor propylene polymerization. More and more noveldonors have been found and used in an MgCl2 basedcatalyst system in which the internal donors are ethylbenzoate, dialkyl phthalate, diether, and succinate es-ter, and the external donor is alkoxysilane.1 A varietyof metal halides and oxides have been reported assuitable supports for the catalysts.2 Inorganic oxides,such as SiO2 and Al2O3, have also been used as cata-lyst supports because of their high specific surfaceareas and good morphology.3,4 However, SiO2-sup-ported titanium catalysts often show very low activityand yield polypropylene with low isotacticity.5 Anhy-drous MgCl2 has been known as a preferred supportof highly efficient Ziegler–Natta catalysts for olefinpolymerization, because it can stabilize the active cen-ters and increase the activity of the catalysts. But thepreparation process of spherical MgCl2 is rather com-plicated, and the catalyst is apt to be broken up andproduces fine powder during polymerization.6 SiO2/MgCl2 supported TiCl4 catalysts have been used forthe polymerization of ethylene and propylene and for

their copolymerization. In general, the complex sup-ports were often prepared by codeposition or dip-molding.7 In this study, we assembled SiO2/MgCl2 bya spray drying technology and obtained a complexsupport. The complex support reacted with TiCl4, andan internal donor gave the catalyst. The behavior ofthe catalyst for polymerization of propylene was in-vestigated using propylene bulk polymerization andalkoxysilane as the external donor.

EXPERIMENTAL

Materials

Polymerization-grade propylene and high pure nitro-gen were obtained from Beijing Yanshan Petrochemi-cal Co., Ltd. (BYPC), and used after passage through a4A molecular sieve. Cyclohexylmethyldimethoxysi-lane (CHMDMS), dicycloamyldimethoxysilane (DCP-DMS), Di-i-butyldimethoxysilane (DIBDMS), di-i-pro-pyldimethoxysilane (DIPDMS), and triethylaluminum(TEA) were received from Aldrich. TS610 SiO2 (diam-eter is 0.02 � 0.1�m and specific surface area is 200� 50m2/g) was supplied by Cabot Corp. and wascalcined at 400°C under N2 for approximately 4h be-fore use. Titanium tetrachloride, di-n-butyl phthalate,n-hexane, and anhydrous magnesium chloride wereobtained from Beijing Chemical Reagents Co., Ltd.(Beijing, China).

Correspondence to: W. Chen (chenwei@brici.ac.cn).

Journal of Applied Polymer Science, Vol. 98, 1296–1299 (2005)© 2005 Wiley Periodicals, Inc.

Synthesis and characterization of catalyst

All operations were carried out under dry nitrogenusing standard Schlenk techniques. The SiO2/MgCl2complex support was prepared by spray drying, usinga B-290 dryer (Buchi, Switzerland); then, the complexsupport reacted with TiCl4 and an internal donor ac-cording to the method in literature,8 giving the cata-lyst. The content of the titanium in the catalyst was2.34%, determined by a 722 spectrophotometer. Parti-cle size analysis results showed that the D50 of thecatalyst was 28.3�m. The total specific surface area ofthe catalyst was 306.5 m2/g and total pore volume was0.39 mL/g, obtained from nitrogen adsorption mea-surements (Carlo Erba Strumentazione, Sorptomatic1800).

Propylene polymerization

Propylene bulk polymerizations were carried out asfollows: Stoichiometric TEA, an external donor, ann-hexane slurry of solid catalyst, quantitative hydro-gen, and 1L liquid propylene were added to a 2L cleanstainless steel autoclave. The autoclave was heated tothe desired polymerization temperature within a fewminutes to perform the main polymerization for theprescribed polymerization time. The polymerizationwas terminated by opening the vent valve, allowingthe unreacted monomer to evaporate quickly. Afterflashing and flushing with nitrogen several times, theproduct was discharged from the valve at the bottomof the autoclave and dried 2 h in a vacuum oven at80°C.

Polymer characterization

The isotacticity index (II) of polypropylene was ob-tained by boiling heptane extraction for 6 h throughconventional methods. The molecular weight and mo-lecular weight distribution (MWD) of the polypro-pylene were determined by a Waters AllianceGPCV2000 gel permeation chromatograph (GPC)equipped with a refractive index detector, using three

Polymer Laboratory MIXED-B columns and 1,2,4-tri-chlorobenzene as solvent at 150°C. The number-aver-age and weight average molecular weight (Mn andMw, respectively) were determined with reference toa polystyrene standard calibration. The pore charac-terization of the propylene was determined by Mer-cury intrusion (Micromeritics, Autopore 9410).

RESULTS AND DISCUSSION

The morphology of the catalyst and polypropylene

The SEM picture of the catalyst and polypropylene areshown in Figures 1 and 2, respectively. From Figure 1we can see that the catalyst is spherical and welldistributed. The polypropylene grains in Figure 2were also spherical and well distributed because ofduplicating the morphology of the catalyst. Figures 3and 4 give the surface SEM picture of the complexsupport and polypropylene. From Figure 3 we can seethat the complex support was built from many spher-ical SiO2 and MgCl2 particles. We can see from Figure4 that the polypropylene grains were porous andspherical. Mercury intrusion determined the total spe-cific surface area is 224.6m2/g and total pore volume is0.46g/L.

Effect of TEA concentration on polymerization

Table I shows the effect of TEA concentration on thepolymerization reaction. The increase of the Al/Ti

Figure 1 SEM picture of the catalyst grain.

Figure 2 SEM picture of the polypropylene grain.

Figure 3 SEM picture of the complex support surface.

POROUS SPHERICAL MGCL2/SIO2 COMPLEX SUPPORT 1297

molar ratio from 100 : 1.0 to 200 : 1.0 increased theproductivity of the catalyst, whereas with a furtherincrease in the ratio the productivity declined. This isbecause TEA was used as cocatalyst in the propylenepolymerization, and it can alkylate the active centersof the catalyst and reduce Ti4� to Ti3� and Ti2�. Ti3�

is the active center for propylene polymerization.When the concentration of TEA was increased beyondthe optimum value, Ti4� was reduced to Ti2�, result-ing in the decrease of activity. The isotactic index (II)value of the polypropylene slightly decreased with anincreasing ratio Al/Ti. This is because with the in-crease of TEA concentration, the internal donor dis-placed from the surface of the solid component byTEA increased, which resulted in the decrease of the IIvalue of polypropylene. Molecular weight (Mw) de-creased with the increase of the Al/Ti molar ratiobecause of chain transfer to Al. Similar results werereported by Zohuri and coworkers.9

Effect of H2 loading on polymerization

The effects of H2 concentration on polymerization be-havior of the complex support catalyst are listed inTable II. Different amounts of H2 were used as achain-transfer agent during propylene polymeriza-tion. The optimum productivity of the catalyst wasobtained with 4.0NL of H2 loading. The catalyst pro-ductivity, compared with the polymerization in theabsence of hydrogen, increased by four times with the

addition of hydrogen (Table II). Molecular weight andII value of the polymer decreased with the increase ofH2. Several hypotheses have been proposed to accountfor the enhancement of the polymerization rate causedby hydrogen. Okura and colleagues10 suggested thathydrogen and the cocatalyst interact to further reducethe catalytic surface, forming new active sites notavailable when the cocatalyst is the single reducingagent. The formation of additional active sites has alsobeen proposed by Buls and Higgins.11 There is anotherexplanation for this behavior, that H2 activates somedormant centers in the catalyst. Hydrogen is the mostwidely used chain-transfer agent for molecular weightcontrol in industrial practice with Ziegler–Natta sys-tems. The rate of chain-transfer reaction increases withthe increasing of hydrogen, resulting in Mw decreas-ing. A considerable activity enhancement effect of hy-drogen has been observed in the polymerization ofpropylene with an MgCl2-supported Ziegler–Nattacatalyst and metallocene/MAO catalyst systems.12,13

Effect of temperature on polymerization

The propylene bulk polymerizations were carried outfrom 50 to 80°C with the optimum H2 concentrationand Al/Ti molar ratio. Table III shows the effect oftemperature on the polymerization of propylene. Thehighest yield occurred at 70°C (see Table III). Molec-ular weight and II decreased with the increase of thetemperature. An increase in the values of the propa-gating rate constant with increasing temperature and

Figure 4 Surface SEM picture of the polypropylene.

TABLE IEffect of TEA Concentration on Polymerization

[A1]/[Ti]molar ratio

Activity kgPP/g cat II (%) MI2.16 Mw 104 MWD

1000 15.1 96.64 53.58 16.68 7.23600 15.0 97.21 42.34 19.66 5.92400 24.9 97.75 33.40 21.40 5.56200 26.3 98.09 25.53 24.41 8.10100 10.3 98.46 9.81 31.96 11.95

Temperature � 70°C; catalyst loading � 10mg; polymer-ization time � 60min; H2 � 4.0NL; [A1]/[CHMDMS] � 10.

TABLE IIEffect of H2 Loading on Polymerization

H2 loading(NL)

Activity kgPP/g cat II (%) MI2.16 Mw 104 MWD

6.0 19.5 95.29 84.75 15.91 7.394.0 24.9 97.75 33.40 21.40 5.562.0 24.4 97.97 11.43 30.42 4.291.0 16.5 98.26 2.68 43.16 4.850 5.7 98.52 0.11 150.45 6.09

Temperature � 70°C; catalyst loading � 10mg; [A1]/[Ti]� 400; polymerization time � 60min; [A1]/[CHMDMS] �10.

TABLE IIIEffect of Temperature on Polymerization

Temperature(°C)

Activity kgPP/g cat II (%) MI2.16 Mw 104 MWD

50 12.1 98.25 29.64 22.61 5.5660 16.4 98.21 30.01 22.23 5.9670 24.9 97.75 33.40 21.40 5.5680 21.7 96.87 35.87 17.81 6.63

Catalyst loading � 10mg; polymerization time � 60min;[A1]/[Ti] � 400; [A1]/[CHMDMS] � 10; H2 � 4.0 NL.

1298 JIANG ET AL.

an irreversible deactivation of active sites at high tem-peratures were reported early.14 The solubility of H2decreased at high temperatures, and this could affectthe polymerization rate as well.

Effect of external donor on polymerization

Four kinds of alkoxysilane were used as external do-nor in the Mg/Si support TiCl4 based catalysts duringpropylene polymerization. The structures of the exter-nal donors are given in Figure 5. The results of pro-pylene polymerization are listed in Table IV. From itwe can draw a conclusion that the propylene polymer-ization productivity with CHMDMS as external donoris the highest, and with DIPDMS as external donor isthe lowest. The Mw and II were the highest usingDCPDMS as external donor. This is because the alkylof DCPDMS is a five member ring structure and hasthe highest bulk hindrance, which results in higherMw and II than the others. The alkyl of CHMDMS haslittle bulk hindrance and gives the polypropylene withlower II and Mw. Toward DIBDMS and DIPDMS, theresearch result is consistent with literature.15 Thoughi-C4H9 has bigger bulk than i-C3H7, the polypropyleneusing DIPDMS as external donor has high II and Mw.This is because the chain of i-C4H9 is more flexiblethan that of i-C3H7.

CONCLUSIONS

We have developed a novel spherical MgCl2/SiO2complex support using spray drying technology. The

MgCl2/SiO2 complex support has better particleshape, with high specific surface area and porosity.Porous spherical polypropylene can be produced us-ing the catalyst. It was found that the optimum Al/Timolar ratio with respect to the catalyst activity is 200.With the increase of H2 concentration within 4.0NL,the productivity of the catalyst increased, but a higherconcentration of H2 reduced the productivity. Thehighest productivity was obtained at 70°C. With theincrease of TEA, H2 concentrations, and temperature,the II and Mw declined. In pair with di-n-butyl phtha-late in MgCl2/SiO2/TiCl4/TEA systems for propylenepolymerization, the order of productivity is: CHM-DMS � DIBDMS � DCPDMS � DIPDMS. The orderof II and Mw are: DCPDMS � DIPDMS � DIBDMS� CHMDMS.

References

1. Chadwick, J. C. Macromol Symp 2001, 173, 21.2. Moore, E. P. Polypropylene Handbook; Hanser: Munich, 1996.3. Lu, H.; Xiae, S. Makromol Chem 2095 1993, 194.4. Soga, K.; Kaminaka, M. Macromol Chem Phys 1994, 195, 1369.5. Kratovchila, J.; Shiono, T.; Soga, K. Makromol Rapid Commun

1993, 14, 85.6. Chein, J. C. W.; Hu, Y. L. J Polym Sci Part A: Polym Chem 2003

1988, 26.7. Lu, H. L.; Xiao, S. J. Die Makromol Chem 1993, 194, 421.8. Mao, B. Q.; Yang, J. X.; Li, Z. L.; Yang, A. C.; Zheng, Y.; Xia, X. Z.

Chinese Patent CN 1,091,748A, 1994.9. Zohuri, G. H.; Azimfar, F.; Jamjah, R; Ahmadjo, S. J Polym Sci

Part A: Polym Chem 2003, 89, 1177.10. Okura, J.; Kojima, A.; Soga, K. J Polym Sci Part A: Polym Chem

1970, 8, 2717.11. Buls, V. W.; Higgins, J. L. J Polym Sci Part A: Polym Chem 1973,

12, 925.12. Samson, J. J. C.; Bosman, P. J.; Weickert, G.; Westerterp, K. R. J

Polym Sci Part A: Polym Chem 1999, 37, 219.13. Chien, J. C. W.; Nozaki, T. J Polym Sci Part A: Polym Chem 1991,

27, 1499.14. Jochem, T. M. P.; Weickertb, G.; Wim, P. M. S. Chem Eng Sci

2002, 57, 3461.15. Proto, A.; Oliva, L.; Pellecchia, C. Macromolecules 1990, 23,

2904.

Figure 5 Four kinds of external donor used in propylene bulk polymerization.

TABLE IVEffect of External Donor on Polymerization

Externaldonor

Activity kgPP/g cat II (%) Mw 104 MWD

CHMDMS 24.9 97.15 21.40 5.56DCPDMS 16.15 98.85 24.69 7.12DIPDMS 13.85 98.01 23.62 5.99DIBDMS 17.05 97.65 22.50 9.43

Catalyst loading � 10mg; polymerization time � 60min;[A1]/[Ti] � 400; [A1]/[external donor] � 10; H2 � 4.0NL;Temperature � 70°C.

POROUS SPHERICAL MGCL2/SIO2 COMPLEX SUPPORT 1299