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Preparation and characterization of agglomerated porous hollow silica supports for olefin polymerization catalyst
Journal of Non-Crystalline Solids 353 (2007) 1030–1036 www.elsevier.com/locate/jnoncrysol
Preparation and characterization of agglomerated porous hollow silica supports for olefin polymerization catalyst Jian-Feng Chen a
a,b
, Ji-Rui Song a, Li-Xiong Wen
a,*
, Hai-Kui Zou b, Lei Shao
b
Key Lab for Nanomaterials, Ministry of Education, Beijing University of Chemical Technology, Bei San Huan Dong Road 15#, Beijing 100029, China b Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, PR China Received 6 April 2006; received in revised form 29 December 2006
Abstract Self-prepared porous hollow silica (PHS) nanoparticles were agglomerated with an oil (kerosene)–ammonia method and used as a novel support of metallocene catalysts for olefin polymerization. Transmission electron microscopy (TEM), scanning electron microcopy (SEM), small-angle X-ray diffraction (SA-XRD) and nitrogen adsorption–desorption were employed to characterize the morphologies and mesostructures of the PHS particles and agglomerated PHS particles. It was found that the agglomerated PHS particles had an average diameter between 15 and 30 lm, a specific surface area of 214 m2/g, a pore diameter of 11–18 nm and a pore volume of 1.41 mL/g. Metallocene catalysts prepared with the agglomerated PHS as support demonstrated a very high activity of 8350 g PE/gCata. for ethylene polymerization due to their particular morphology and mesostructures. Analysis with Differential Scanning Calorimetry (DSC), SEM, TEM and molecular weight analysis indicated that the polyethylene product was composed of tiny spherical particles with good crystalline and narrow molecular weight distribution, and the morphology and structure of the individual PHS support particle remained unchanged in polyethylene following the polymerization. Ó 2007 Published by Elsevier B.V. Keywords: Chemical properties; Catalysis; Oxide glasses; Silica
1. Introduction In recent years, there has been a rapid growth in the polyolefin production and the yield of polyolefin is about 35% in the polymer industry. Since metallocene catalysts play a key role in the olefin polymerization, the demands for the catalysts increase significantly with the increasing production of polyolefin. Therefore, metallocene catalysis could be considered as a major breakthrough in polyolefin technology [1–3]. The metallocene catalysts and co-catalysts are usually prepared with the utilization of support in order to get monodispersed active sites and to reduce the amount of consumed catalysts and co-catalysts. At present, most attentions have been paid to the use of sup*
Corresponding author. Tel.: +86 10 64443614; fax: +86 10 64434784. E-mail address: [email protected] (L.-X. Wen).
0022-3093/$ - see front matter Ó 2007 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2007.01.024
ported metallocene complexes [4–8]. Many different materials such as SiO2, MgCl2, MCM-41, zeolite, carbon, SiO2/ MgCl2 and polymer have been used as support for metallocene catalysts in olefin polymerization [9–17]. In contrast to the traditional understanding of catalysts, polyolefin catalysts (catalyst, co-catalyst and support) remain within the products following the reaction. This offers a number of new possibilities with regard to the polymer properties, including the formation of polymer nanocomposites [18–20]. In order to obtain a clear, colorless materials (e.g. for film application) without degrading the mechanical properties of the polymer, both the active catalyst and the support particles must be distributed uniformly during the polymerization process following the support fragmentation, as shown in Fig. 1. The agglomerated support is broken up during the polymerization by the increasing hydraulic pressure caused by the growing poly-
J.-F. Chen et al. / Journal of Non-Crystalline Solids 353 (2007) 1030–1036
olefin
1031
olefin
agglomerate support
polymerization
olefin
Al O Al O
O
O
Al Al
Al O Al
Al
polyolefin
O
Metallocene + Methylaluminoxane (MAO) Fig. 1. Scheme of support agglomeration, catalyst formation and ethylene polymerization.
mer chains, leaving behind fragments that may influence polymer properties like mechanical behavior and film formation characteristics. Therefore, it is extremely important to obtain a catalyst system where the active sites are distributed evenly and not just simply immobilized at the external surface of the support [21]. Since the polyolefin catalysts (catalyst, co-catalyst and support) remain within the product, the amount of the used catalyst and support can affect the product properties. The yield of olefin polymerization will be undesirable if the amount of catalyst is not sufficient. However, the mechanical properties of polyolefin will be deteriorated due to the excessive ash if the catalyst amount (including support) is beyond the polymerization demand. Hence, an efficient catalyst loading on the support is desirable. Traditional catalyst supports are agglomerated solid SiO2, MgCl2, SiO2/ MgCl2 or other solid materials; therefore, the active sites can be only loaded on the external surface of the supports, leading to a limited amount of active sites. To enhance the yield of polyolefin, the amount of active sites need to be increased by utilizing more polyolefin catalysts, which, however, will lead to the deteriorated properties of product. If some porous hollow materials are applied as the catalyst supports, the active sites will be immobilized on the external surface, the pore channels as well as the internal surface of the supports, and as a result many more active sites can be supported on the same amount of porous hollow supports as compared with their solid counterparts. In this way, a high yield of polyolefin can be achieved with an acceptable amount of support and catalyst, which will not weaken the mechanical properties of polyolefin due to the limited ash produced in the olefin polymerization. In addition, some added properties for the polyolefin, such as sound absorbability and heat insulation, may be induced if the porous hollow structure of the support remains perfect in the products, as shown in Fig. 1. In this paper, porous hollow silica (PHS) nanoparticles were synthesized with nano-CaCO3 as template and
sodium silicate as the silica source. The PHS was then agglomerated with an oil–ammonia method. Both PHS and agglomerated PHS were characterized with transmission electron microscopy (TEM), scanning electron microscope (SEM), Little-angle X-ray Diffraction (L-XRD), specific surface area (BET) and pore diameter distribution (PSD). And the agglomerated PHS was used as the support for metallocene catalyst and tested in olefin polymerization. 2. Experimental 2.1. Chemicals Nano-CaCO3 (average diameter of 30–40 nm, cubic) was prepared with a high gravity technology in our lab as reported previously [22]. Sodium silicate (Na2SiO3 Æ 9 H2O), BF3, hydrochloric acid (HCl) and ammonia (NH3 Æ H2O) were purchased from the indicated suppliers. All chemicals were of analytical grade and used without any further purification. Deionized water was adopted throughout the experiment. 2.2. Preparation of agglomerated PHS PHS was prepared via the hydrolysis of Na2SiO3 Æ 9 H2O with HCl solution by adopting nano-CaCO3 particles as the structure-directing template, and more detailed procedures were described in our previous work [23,24]. Ten grams of the as-prepared PHS was put into 100 mL of deionized water and the formed suspension was stirred for 30 min using a magnetic stirrer. The suspension was then added dropwisely at a rate of 5 mL/min into a selfdesigned tube, which was filled with oil (kerosene)–ammonia in advance. The produced precipitate, i.e., agglomerated PHS, was separated by filtration and heated to 378 K with ramp 2 K/min and dried for 6 h. The dried powder was further calcined in air at 923 K (with ramp 1 K/min)
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for 4 h to remove the remained kerosene in PHS and to solidify the agglomerations. 2.3. Characterization of agglomerated PHS Morphologies of both PHS and agglomerated PHS were observed by an H-800 transmission electron microscopy (TEM, Japan) and S250MK3 scanning electron microscopy (SEM, Japan). The samples were dispersed in ethanol and deposited onto a carbon coated copper grid for TEM analysis. SEM samples were attached to the sample holder using adhesive carbon tape and sputtered with gold in vacuum for 15 min to ensure the transmission of electrons. Small-angle X-ray diffraction (SA-XRD) measurement was performed by a Rigaku D/Max 2500 VB2+/PC X-ray diffractometer (Rigaku, Japan) using nickel-filtered CuKa radiation of 0.15405 nm in wavelength at 40 kV, 40 mA. Diffraction data were recorded between 1° and 10° 2h at an interval of 0.01 2h. A scanning speed of 1°/ min was adopted (automatic slit system). Nitrogen adsorption–desorption isotherms for PHS and agglomerated PHS samples were measured with an ASAP 2010 surface area analyzer (Micromeritics Instrument Corporation, USA) at 77 K over a wide range of relative pressures from 0.01 to 0.995. Prior to the analyses, each sample was degassed at 473 K for 1 h in vacuum. The surface area was calculated from the adsorption curve according to the Brunauer-Emmettm-Teller (BET) method. The specific pore volume was calculated according to Gurwitsch at a p/p0value of 0.975 of the desorption branch. The pore size distribution was determined from the adsorption branch of the isotherm by the Barrett-Joyner-Halenda (BJH) method [25]. 2.4. Preparation of metallocene catalysts and ethylene polymerization Prior to being used as the catalyst support, agglomerated PHS was dried at 383 K for about 72 h in nitrogen in order to remove the adsorbed water. The dried PHS particles were then treated with a chemical method, in which BF3 solution (dissolved in ether) was mixed with agglomerated PHS at room temperature with stirring for about 4 h and the mixture was then filtrated and washed with toluene for three times, to settle the surface silanol groups. To prepare the metallocene catalysts, methylaluminoxane (MAO) and metallocene catalysts were immobilized on agglomerated PHS as follows: MAO (15 wt% in toluene) solution was mixed with treated agglomerated PHS with stirring in N2 ambience for about 6 h, and the mixture was filtrated and dried in vacuum at 328 K. Such MAO-treated agglomerated PHS was then mixed with metallocene solution (dissolved in dichloromethane) with stirring in N2 ambience for about 8 h and the supported catalysts were finally filtrated and dried in vacuum for about 72 h to obtain the olefin polymerization catalysts.
20 mg of the supported catalysts and 600 g hexane solvent were added into a 2 L polymerization reactor, which was equipped with a magnetic stirrer and protected under a stream of N2. Hydrogen was fed into the reactor immediately after stopping the N2 flow, and ethylene was fed into the reactor afterwards at 2 MPa and 333 K with stirring at 500 rpm to start the reaction. The polymerization reaction was terminated after 2 h by adding an excess of dilute hydrochloric acid solution in methanol, and the resulting polymer was isolated and dried. The morphology, structure and properties of polyethylene (PE) were then characterized with SEM and Differential Scanning Calorimetry (DSC). DSC was performed by a TG-DTA/DSC apparatus (STA 449 C JupiterÒ, NETZSCH-Gera¨tebau GmbH, Germany) in a temperature range of 303–1073 K at a heating rate of 10 K/min. The morphology of PHS particles in the PE product was characterized with TEM. 3. Results 3.1. Characterizations of PHS particles TEM image of PHS particles is shown in Fig. 2. It can be seen from the image that the morphology of PHS particles was nearly spherical with an empty core and the particle size was about 70–90 nm with the wall thickness of about 10 nm. It can also be observed that a part of PHS particles was aggregated together in a loose style, due to the very high surface energy of nano-particles. Fig. 3 shows the SA-XRD pattern of the PHS particles. It indicated that there was an intense peak at about 2° of 2h, which is the characteristic of mesoporous materials with a pore structure lacking long-range order [26]. Fig. 4 demonstrates the N2 adsorption–desorption isotherm and BJH pore size distribution of the PHS particles. The adsorption isotherm was a typical type-IV isotherm of mesoporous materials and exhibited little hysteresis, showing that the pores in the wall of the particles may not be
Fig. 2. TEM image of PHS particles.
J.-F. Chen et al. / Journal of Non-Crystalline Solids 353 (2007) 1030–1036
6000
Intensity (Counts)
5000 4000 3000 2000 1000 0 0
2
4
6
8
10
2θ (Deg.) Fig. 3. SA-XRD pattern of PHS particles.
Adsorption Desorption
120 100 0.0030
40 20
Pore Volume (cm3 /g)
Volume Adsorbed (cm3/g)
140
60
the aggregated PHS particles, which can hold metallocene catalyst and MAO co-catalyst easily in addition to the interior empty cores of PHS particles, resulting in further improved supporting capacity for the catalysts. The effects of the calcining temperature on the specific surface area of the agglomerated PHS were also studied. As can be seen from Fig. 6, the calcining temperature affected the BET surface area dramatically. The specific surface area reduced almost linearly with the temperature increasing from 625 to 825 K. The PHS particles will be sintered if the calcining temperature is too high, leading to the reduced specific surface area. However, if the calcining temperature is not high enough, the oil within the agglomerated PHS introduced during the kerosene–ammonia agglomeration process will remain in the particles. Therefore, the best calcination for the agglomerated PHS was achieved at 673 K. 3.3. Characterization of PE
160
80
1033
0.0025 0.0020 0.0015 0.0010 0.0005 0.0000 10
100
1000
Pore Diameter (A)
0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
Relative Pressure (P/P0) Fig. 4. N2 sorption isotherms and BJH pore size distributions of PHS particles.
well ordered. The pore size distribution curve showed that the pore size in the wall of PHS was about 4 nm. BET analysis indicated that the specific surface area was about 700 m2/g. Therefore, the agglomerated PHS with high BET surface area and enormous pores is suitable for supporting metallocene for olefin polymerization. 3.2. Characterization of agglomerated PHS Particles SEM images of agglomerated PHS particles, which were produced through the kerosene–ammonia method, are shown in Fig. 5. It can be seen that the size of most agglomerated PHS particles was ranged between 15 and 30 lm with many PHS particles aggregated together. Fig. 5(c) shows a single agglomerated PHS particle and Fig. 5(d) is the local part zoom of the particle shown in Fig. 5(c). It demonstrated clearly that the agglomerated PHS particles are formed of many tiny single PHS particles. There are many large pores produced by the inter-space between
PE was fabricated with agglomerated PHS as metallocene catalyst support, and the specific activity of the catalyst was found to be 8350 gPE/gCata. This specific activity was very high as compared with the industrial requirement. Fig. 7 is the DSC thermogram of PE, which shows that PE has only one melting temperature at about 415.8 K, indicating that the PE product has good crystalline and narrow molecular weight distribution. The molecular weight analysis demonstrated that the molecular weight distribution (MWD) of PE is very narrow with poly dispersity about 4.90. The produced fine PE suggested that the agglomerated PHS could be used as metallocene catalyst support for ethylene polymerization. The morphology of PE was characterized with SEM as shown in Fig. 8. It indicated that the PE particles were composed with many tiny particles with a nearly spherical shape and a diameter of 1 lm. The tiny PE particles might grow on individual PHS particles that broke down from the agglomerated PHS and functioned as the seeds during the polymerization, and TEM analysis on the PE slice samples revealed that the structure of individual PHS particles remained unchanged in PE with their hollow morphology, as shown in Fig. 9. Therefore, some added properties for the PE product, such as sound absorbability and heat insulation, may be induced due to the untouched hollow structure of the entrapped PHS particles. 4. Discussion The agglomeration mechanism of single PHS particles is that PHS particles have different surface tensions in different solutions. When the PHS aqueous slurry was dropped into kerosene, the slurry drops shrank and the PHS particles within each slurry drop agglomerated together due to the surface tensions. When these slurry drops went through the oil/water interface of the kerosene–ammonia system and into the ammonia solution, the change of surface ten-
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Fig. 5. SEM images of agglomerated PHS particles.
50
415.8K
45 Heat Flow Endo Up (mW)
Specific Surface Area (m2/g)
240 220 200 180 160 140
40 35 30 25 20 15
600
650
700 750 800 850 Temperature (K)
900
950
Fig. 6. Effects of calcining temperature on the specific surface area of agglomerated PHS.
sion and the NHþ 4 in ammonia solution would solidify the agglomerated PHS drops to obtain the stable agglomerated PHS particles. The pore shape, pore size, pore size distribution, and pore connectivity of the supports are the most important factors for the immobilization of the active components onto the supports. Fig. 10 shows the N2 adsorption–
300 320 340 360 380 400 420 440 460 480 500 Temperature (K) Fig. 7. DSC thermogram of polyethylene produced by catalysts supported by agglomerated PHS.
desorption isotherms and BJH pore size distributions of the agglomerated PHS particles. It indicated that the agglomerated PHS particles had pores with a wide size range. The small pores with a size of about 4 nm were those formed originally on the walls of individual PHS particles during their synthesizing process, while the large holes with
J.-F. Chen et al. / Journal of Non-Crystalline Solids 353 (2007) 1030–1036
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Adsorption Desorption
800 0.0025
600 400
Pore Volume (cm /g)
Volume Adsorbed (cm3/g)
1000
18nm 11nm
0.0020
4nm 0.0015 0.0010 0.0005 0.0000
200 0 0.0
10
100
1000
Pore Diameter (A)
0.2
0.4 0.6 0.8 Relative Pressure (P/P0 )
1.0
Fig. 10. N2 sorption isotherms and BJH pore size distributions of agglomerated PHS particles. Fig. 8. SEM images of polyethylene produced by catalysts supported by agglomerated PHS.
the products may have some new properties, such as sound absorbability and heat insulation. 5. Conclusion Porous hollow silica can be synthesized with nanoCaCO3 as template and sodium silicate as silica resource. The PHS particles have a large specific surface area of about 700 m2/g and a pore diameter of about 4 nm. With the oil (kerosene)–ammonia method, the PHS particles can be formed into agglomerated PHS, which has a large pore diameter of about 11–18 nm, a particles size of about 15–30 lm and a large pore volume of about 1.41 mL/g. Used as metallocene catalyst support for ethylene polymerization, the agglomerated PHS shows a high specific activity and the hollow morphology of the individual PHS particles remains unchanged within the PE product, which may introduce some new properties, such as sound absorbability and heat insulation, to the polymerization product. Fig. 9. TEM images of PHS particles in polyethylene.
sizes larger than 10 nm were those produced by the interspace between the aggregated PHS particles. BET surface areas of about 214 m2/g and pore volumes of about 1.41 mL/g for the agglomerated PHS were also obtained from the N2 adsorption–desorption isotherms, which illustrated the suitability of the agglomerated PHS as the catalyst supports. Olefin monomers will enter the supports through the holes and polymerize on the catalysts during the polymerization. The PHS particles remain in the polyolefin products as the seeds for the polymerization. When polyolefin is formed, the chains of polyolefin molecule will grow and break the agglomerated PHS particles into small PHS particles or ultrafine fragments. Because PHS particles were calcined at 973 K during preparation, the hollow structure of PHS will be kept very well within polyolefin,
Acknowledgements The authors gratefully acknowledge the financial support provided by National ‘973’ program of China (Nos. 2003CB615807 and 2004CB217804), National Natural Science Foundation of China (Nos. 20506001 and 20325621), the Program for New Century Excellent Talents in University (No. NCET-04-0123) and Beijing Municipal Commission of Education (JD100100403). References [1] H.H. Brintzinger, D. Fischer, R. Mu¨hlhaupt, B. Rieger, R.M. Waymouth, Angew. Chem. 34 (1995) 1143. [2] T. Repo, M. Klinga, I. Mutikainen, Y. Su, M. Leskela¨, M. Polamo, Acta Chem. Scand. 50 (1996) 1116. [3] C. Janiak, B. Rieger, Die Angew. Macromol. Chem. 215 (1994) 47. [4] K. Soga, M. Kamimaka, Makromol. Chem. Phys. 195 (1994) 1369. [5] S. Collins, W.M. Kelly, D.A. Holden, Macromolecules 25 (1992) 170.
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[6] J.C. Chien, H.J. Dawei, Polym. Sci. Part A 29 (1991) 1603. [7] K. Soga, H. Kim, T. Shiono, Makromol. Chem. Phys. 195 (1994) 3341. [8] M.R. Ribeiro, A. Deffieux, M.F. Portela, Ind. Eng. Chem. Res. 36 (1997) 1224. [9] B.L. Moroz, N.V. Semikolenova, A.V. Nosov, V.A. Zakharov, S. Nagy, N.J. O’Reilly, J. Mol. Catal. A: Chem. 130 (129) 121. [10] S. Rahul, A. Govindaraj, C.N.R. Rao, Chem. Phy. Lett. 267 (1997) 276. [11] M. Michelotti, A. Altomare, F. Ciarfelli, E. Roland, J. Mol. Catal. A: Chem. 129 (1998) 241. [12] H. Rahiala, I. Beurroies, T. Eklund, K. Hakala, R. Gougeon, P. Trens, J.B. Rosenholm, J. Catal. 188 (1999) 14. [13] G.M.B. Anthony, R.M. Yolanda, Tetrahedron 58 (2002) 3785. [14] H.S. Cho, J.S. Chung, W.Y. Lee, J. Mol. Catal. A: Chem. 159 (2000) 203. [15] H.S. Cho, W.Y. Lee, J. Mol. Catal. A: Chem. 191 (2003) 155. [16] F. Ciardelli, A. Altomare, M. Michelotti, Catal. Today 41 (1998) 149.
[17] J.S. Chung, H.S. Cho, Y.G. Ko, W.Y. Lee, J. Mol. Catal. A: Chem. 144 (1999) 61. [18] R. Mu¨lhaupt, F. Stricker, Kunstsoffe 87 (1997) 482. [19] P. Wlater, D. Ma¨der, P. Reichert, J. Mcaromol. Sci. A 36 (1999) 1613. [20] P. Dubois, M. Alexandre, R. Je´rome, Macromol. Symp. 194 (2003) 13. [21] C.F. Hohenesche, K.K. Unger, T. Eberle, J. Mol. Catal. A: Chem. 221 (2004) 185. [22] J.F. Chen, Y.H. Wang, F. Guo, X.M. Wang, C. Zheng, Ind. Eng. Chem. Res. 39 (2000) 948. [23] J.F. Chen, H.M. Ding, J.X. Wang, L. Shao, Biomaterials 25 (2004) 723. [24] Y. Le, J.F. Chen, J.X. Wang, L. Shao, W.C. Wang, Mater. Lett. 58 (2004) 2105. [25] C.F. Cheng, H. He, W. Zhou, J. Klinowski, J.S. Concalves, L.F. Gladden, J. Phys. Chem. 100 (1996) 390. [26] S.A. Bagshaw, E. Prouzet, T.J. Prinnavaia, Science 269 (1995) 1242.
Preparation and characterization of agglomerated porous hollow silica supports for olefin polymerization catalyst Jian-Feng Chen a
a,b
, Ji-Rui Song a, Li-Xiong Wen
a,*
, Hai-Kui Zou b, Lei Shao
b
Key Lab for Nanomaterials, Ministry of Education, Beijing University of Chemical Technology, Bei San Huan Dong Road 15#, Beijing 100029, China b Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, PR China Received 6 April 2006; received in revised form 29 December 2006
Abstract Self-prepared porous hollow silica (PHS) nanoparticles were agglomerated with an oil (kerosene)–ammonia method and used as a novel support of metallocene catalysts for olefin polymerization. Transmission electron microscopy (TEM), scanning electron microcopy (SEM), small-angle X-ray diffraction (SA-XRD) and nitrogen adsorption–desorption were employed to characterize the morphologies and mesostructures of the PHS particles and agglomerated PHS particles. It was found that the agglomerated PHS particles had an average diameter between 15 and 30 lm, a specific surface area of 214 m2/g, a pore diameter of 11–18 nm and a pore volume of 1.41 mL/g. Metallocene catalysts prepared with the agglomerated PHS as support demonstrated a very high activity of 8350 g PE/gCata. for ethylene polymerization due to their particular morphology and mesostructures. Analysis with Differential Scanning Calorimetry (DSC), SEM, TEM and molecular weight analysis indicated that the polyethylene product was composed of tiny spherical particles with good crystalline and narrow molecular weight distribution, and the morphology and structure of the individual PHS support particle remained unchanged in polyethylene following the polymerization. Ó 2007 Published by Elsevier B.V. Keywords: Chemical properties; Catalysis; Oxide glasses; Silica
1. Introduction In recent years, there has been a rapid growth in the polyolefin production and the yield of polyolefin is about 35% in the polymer industry. Since metallocene catalysts play a key role in the olefin polymerization, the demands for the catalysts increase significantly with the increasing production of polyolefin. Therefore, metallocene catalysis could be considered as a major breakthrough in polyolefin technology [1–3]. The metallocene catalysts and co-catalysts are usually prepared with the utilization of support in order to get monodispersed active sites and to reduce the amount of consumed catalysts and co-catalysts. At present, most attentions have been paid to the use of sup*
Corresponding author. Tel.: +86 10 64443614; fax: +86 10 64434784. E-mail address: [email protected] (L.-X. Wen).
0022-3093/$ - see front matter Ó 2007 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2007.01.024
ported metallocene complexes [4–8]. Many different materials such as SiO2, MgCl2, MCM-41, zeolite, carbon, SiO2/ MgCl2 and polymer have been used as support for metallocene catalysts in olefin polymerization [9–17]. In contrast to the traditional understanding of catalysts, polyolefin catalysts (catalyst, co-catalyst and support) remain within the products following the reaction. This offers a number of new possibilities with regard to the polymer properties, including the formation of polymer nanocomposites [18–20]. In order to obtain a clear, colorless materials (e.g. for film application) without degrading the mechanical properties of the polymer, both the active catalyst and the support particles must be distributed uniformly during the polymerization process following the support fragmentation, as shown in Fig. 1. The agglomerated support is broken up during the polymerization by the increasing hydraulic pressure caused by the growing poly-
J.-F. Chen et al. / Journal of Non-Crystalline Solids 353 (2007) 1030–1036
olefin
1031
olefin
agglomerate support
polymerization
olefin
Al O Al O
O
O
Al Al
Al O Al
Al
polyolefin
O
Metallocene + Methylaluminoxane (MAO) Fig. 1. Scheme of support agglomeration, catalyst formation and ethylene polymerization.
mer chains, leaving behind fragments that may influence polymer properties like mechanical behavior and film formation characteristics. Therefore, it is extremely important to obtain a catalyst system where the active sites are distributed evenly and not just simply immobilized at the external surface of the support [21]. Since the polyolefin catalysts (catalyst, co-catalyst and support) remain within the product, the amount of the used catalyst and support can affect the product properties. The yield of olefin polymerization will be undesirable if the amount of catalyst is not sufficient. However, the mechanical properties of polyolefin will be deteriorated due to the excessive ash if the catalyst amount (including support) is beyond the polymerization demand. Hence, an efficient catalyst loading on the support is desirable. Traditional catalyst supports are agglomerated solid SiO2, MgCl2, SiO2/ MgCl2 or other solid materials; therefore, the active sites can be only loaded on the external surface of the supports, leading to a limited amount of active sites. To enhance the yield of polyolefin, the amount of active sites need to be increased by utilizing more polyolefin catalysts, which, however, will lead to the deteriorated properties of product. If some porous hollow materials are applied as the catalyst supports, the active sites will be immobilized on the external surface, the pore channels as well as the internal surface of the supports, and as a result many more active sites can be supported on the same amount of porous hollow supports as compared with their solid counterparts. In this way, a high yield of polyolefin can be achieved with an acceptable amount of support and catalyst, which will not weaken the mechanical properties of polyolefin due to the limited ash produced in the olefin polymerization. In addition, some added properties for the polyolefin, such as sound absorbability and heat insulation, may be induced if the porous hollow structure of the support remains perfect in the products, as shown in Fig. 1. In this paper, porous hollow silica (PHS) nanoparticles were synthesized with nano-CaCO3 as template and
sodium silicate as the silica source. The PHS was then agglomerated with an oil–ammonia method. Both PHS and agglomerated PHS were characterized with transmission electron microscopy (TEM), scanning electron microscope (SEM), Little-angle X-ray Diffraction (L-XRD), specific surface area (BET) and pore diameter distribution (PSD). And the agglomerated PHS was used as the support for metallocene catalyst and tested in olefin polymerization. 2. Experimental 2.1. Chemicals Nano-CaCO3 (average diameter of 30–40 nm, cubic) was prepared with a high gravity technology in our lab as reported previously [22]. Sodium silicate (Na2SiO3 Æ 9 H2O), BF3, hydrochloric acid (HCl) and ammonia (NH3 Æ H2O) were purchased from the indicated suppliers. All chemicals were of analytical grade and used without any further purification. Deionized water was adopted throughout the experiment. 2.2. Preparation of agglomerated PHS PHS was prepared via the hydrolysis of Na2SiO3 Æ 9 H2O with HCl solution by adopting nano-CaCO3 particles as the structure-directing template, and more detailed procedures were described in our previous work [23,24]. Ten grams of the as-prepared PHS was put into 100 mL of deionized water and the formed suspension was stirred for 30 min using a magnetic stirrer. The suspension was then added dropwisely at a rate of 5 mL/min into a selfdesigned tube, which was filled with oil (kerosene)–ammonia in advance. The produced precipitate, i.e., agglomerated PHS, was separated by filtration and heated to 378 K with ramp 2 K/min and dried for 6 h. The dried powder was further calcined in air at 923 K (with ramp 1 K/min)
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for 4 h to remove the remained kerosene in PHS and to solidify the agglomerations. 2.3. Characterization of agglomerated PHS Morphologies of both PHS and agglomerated PHS were observed by an H-800 transmission electron microscopy (TEM, Japan) and S250MK3 scanning electron microscopy (SEM, Japan). The samples were dispersed in ethanol and deposited onto a carbon coated copper grid for TEM analysis. SEM samples were attached to the sample holder using adhesive carbon tape and sputtered with gold in vacuum for 15 min to ensure the transmission of electrons. Small-angle X-ray diffraction (SA-XRD) measurement was performed by a Rigaku D/Max 2500 VB2+/PC X-ray diffractometer (Rigaku, Japan) using nickel-filtered CuKa radiation of 0.15405 nm in wavelength at 40 kV, 40 mA. Diffraction data were recorded between 1° and 10° 2h at an interval of 0.01 2h. A scanning speed of 1°/ min was adopted (automatic slit system). Nitrogen adsorption–desorption isotherms for PHS and agglomerated PHS samples were measured with an ASAP 2010 surface area analyzer (Micromeritics Instrument Corporation, USA) at 77 K over a wide range of relative pressures from 0.01 to 0.995. Prior to the analyses, each sample was degassed at 473 K for 1 h in vacuum. The surface area was calculated from the adsorption curve according to the Brunauer-Emmettm-Teller (BET) method. The specific pore volume was calculated according to Gurwitsch at a p/p0value of 0.975 of the desorption branch. The pore size distribution was determined from the adsorption branch of the isotherm by the Barrett-Joyner-Halenda (BJH) method [25]. 2.4. Preparation of metallocene catalysts and ethylene polymerization Prior to being used as the catalyst support, agglomerated PHS was dried at 383 K for about 72 h in nitrogen in order to remove the adsorbed water. The dried PHS particles were then treated with a chemical method, in which BF3 solution (dissolved in ether) was mixed with agglomerated PHS at room temperature with stirring for about 4 h and the mixture was then filtrated and washed with toluene for three times, to settle the surface silanol groups. To prepare the metallocene catalysts, methylaluminoxane (MAO) and metallocene catalysts were immobilized on agglomerated PHS as follows: MAO (15 wt% in toluene) solution was mixed with treated agglomerated PHS with stirring in N2 ambience for about 6 h, and the mixture was filtrated and dried in vacuum at 328 K. Such MAO-treated agglomerated PHS was then mixed with metallocene solution (dissolved in dichloromethane) with stirring in N2 ambience for about 8 h and the supported catalysts were finally filtrated and dried in vacuum for about 72 h to obtain the olefin polymerization catalysts.
20 mg of the supported catalysts and 600 g hexane solvent were added into a 2 L polymerization reactor, which was equipped with a magnetic stirrer and protected under a stream of N2. Hydrogen was fed into the reactor immediately after stopping the N2 flow, and ethylene was fed into the reactor afterwards at 2 MPa and 333 K with stirring at 500 rpm to start the reaction. The polymerization reaction was terminated after 2 h by adding an excess of dilute hydrochloric acid solution in methanol, and the resulting polymer was isolated and dried. The morphology, structure and properties of polyethylene (PE) were then characterized with SEM and Differential Scanning Calorimetry (DSC). DSC was performed by a TG-DTA/DSC apparatus (STA 449 C JupiterÒ, NETZSCH-Gera¨tebau GmbH, Germany) in a temperature range of 303–1073 K at a heating rate of 10 K/min. The morphology of PHS particles in the PE product was characterized with TEM. 3. Results 3.1. Characterizations of PHS particles TEM image of PHS particles is shown in Fig. 2. It can be seen from the image that the morphology of PHS particles was nearly spherical with an empty core and the particle size was about 70–90 nm with the wall thickness of about 10 nm. It can also be observed that a part of PHS particles was aggregated together in a loose style, due to the very high surface energy of nano-particles. Fig. 3 shows the SA-XRD pattern of the PHS particles. It indicated that there was an intense peak at about 2° of 2h, which is the characteristic of mesoporous materials with a pore structure lacking long-range order [26]. Fig. 4 demonstrates the N2 adsorption–desorption isotherm and BJH pore size distribution of the PHS particles. The adsorption isotherm was a typical type-IV isotherm of mesoporous materials and exhibited little hysteresis, showing that the pores in the wall of the particles may not be
Fig. 2. TEM image of PHS particles.
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6000
Intensity (Counts)
5000 4000 3000 2000 1000 0 0
2
4
6
8
10
2θ (Deg.) Fig. 3. SA-XRD pattern of PHS particles.
Adsorption Desorption
120 100 0.0030
40 20
Pore Volume (cm3 /g)
Volume Adsorbed (cm3/g)
140
60
the aggregated PHS particles, which can hold metallocene catalyst and MAO co-catalyst easily in addition to the interior empty cores of PHS particles, resulting in further improved supporting capacity for the catalysts. The effects of the calcining temperature on the specific surface area of the agglomerated PHS were also studied. As can be seen from Fig. 6, the calcining temperature affected the BET surface area dramatically. The specific surface area reduced almost linearly with the temperature increasing from 625 to 825 K. The PHS particles will be sintered if the calcining temperature is too high, leading to the reduced specific surface area. However, if the calcining temperature is not high enough, the oil within the agglomerated PHS introduced during the kerosene–ammonia agglomeration process will remain in the particles. Therefore, the best calcination for the agglomerated PHS was achieved at 673 K. 3.3. Characterization of PE
160
80
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100
1000
Pore Diameter (A)
0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
Relative Pressure (P/P0) Fig. 4. N2 sorption isotherms and BJH pore size distributions of PHS particles.
well ordered. The pore size distribution curve showed that the pore size in the wall of PHS was about 4 nm. BET analysis indicated that the specific surface area was about 700 m2/g. Therefore, the agglomerated PHS with high BET surface area and enormous pores is suitable for supporting metallocene for olefin polymerization. 3.2. Characterization of agglomerated PHS Particles SEM images of agglomerated PHS particles, which were produced through the kerosene–ammonia method, are shown in Fig. 5. It can be seen that the size of most agglomerated PHS particles was ranged between 15 and 30 lm with many PHS particles aggregated together. Fig. 5(c) shows a single agglomerated PHS particle and Fig. 5(d) is the local part zoom of the particle shown in Fig. 5(c). It demonstrated clearly that the agglomerated PHS particles are formed of many tiny single PHS particles. There are many large pores produced by the inter-space between
PE was fabricated with agglomerated PHS as metallocene catalyst support, and the specific activity of the catalyst was found to be 8350 gPE/gCata. This specific activity was very high as compared with the industrial requirement. Fig. 7 is the DSC thermogram of PE, which shows that PE has only one melting temperature at about 415.8 K, indicating that the PE product has good crystalline and narrow molecular weight distribution. The molecular weight analysis demonstrated that the molecular weight distribution (MWD) of PE is very narrow with poly dispersity about 4.90. The produced fine PE suggested that the agglomerated PHS could be used as metallocene catalyst support for ethylene polymerization. The morphology of PE was characterized with SEM as shown in Fig. 8. It indicated that the PE particles were composed with many tiny particles with a nearly spherical shape and a diameter of 1 lm. The tiny PE particles might grow on individual PHS particles that broke down from the agglomerated PHS and functioned as the seeds during the polymerization, and TEM analysis on the PE slice samples revealed that the structure of individual PHS particles remained unchanged in PE with their hollow morphology, as shown in Fig. 9. Therefore, some added properties for the PE product, such as sound absorbability and heat insulation, may be induced due to the untouched hollow structure of the entrapped PHS particles. 4. Discussion The agglomeration mechanism of single PHS particles is that PHS particles have different surface tensions in different solutions. When the PHS aqueous slurry was dropped into kerosene, the slurry drops shrank and the PHS particles within each slurry drop agglomerated together due to the surface tensions. When these slurry drops went through the oil/water interface of the kerosene–ammonia system and into the ammonia solution, the change of surface ten-
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Fig. 5. SEM images of agglomerated PHS particles.
50
415.8K
45 Heat Flow Endo Up (mW)
Specific Surface Area (m2/g)
240 220 200 180 160 140
40 35 30 25 20 15
600
650
700 750 800 850 Temperature (K)
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950
Fig. 6. Effects of calcining temperature on the specific surface area of agglomerated PHS.
sion and the NHþ 4 in ammonia solution would solidify the agglomerated PHS drops to obtain the stable agglomerated PHS particles. The pore shape, pore size, pore size distribution, and pore connectivity of the supports are the most important factors for the immobilization of the active components onto the supports. Fig. 10 shows the N2 adsorption–
300 320 340 360 380 400 420 440 460 480 500 Temperature (K) Fig. 7. DSC thermogram of polyethylene produced by catalysts supported by agglomerated PHS.
desorption isotherms and BJH pore size distributions of the agglomerated PHS particles. It indicated that the agglomerated PHS particles had pores with a wide size range. The small pores with a size of about 4 nm were those formed originally on the walls of individual PHS particles during their synthesizing process, while the large holes with
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Adsorption Desorption
800 0.0025
600 400
Pore Volume (cm /g)
Volume Adsorbed (cm3/g)
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18nm 11nm
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4nm 0.0015 0.0010 0.0005 0.0000
200 0 0.0
10
100
1000
Pore Diameter (A)
0.2
0.4 0.6 0.8 Relative Pressure (P/P0 )
1.0
Fig. 10. N2 sorption isotherms and BJH pore size distributions of agglomerated PHS particles. Fig. 8. SEM images of polyethylene produced by catalysts supported by agglomerated PHS.
the products may have some new properties, such as sound absorbability and heat insulation. 5. Conclusion Porous hollow silica can be synthesized with nanoCaCO3 as template and sodium silicate as silica resource. The PHS particles have a large specific surface area of about 700 m2/g and a pore diameter of about 4 nm. With the oil (kerosene)–ammonia method, the PHS particles can be formed into agglomerated PHS, which has a large pore diameter of about 11–18 nm, a particles size of about 15–30 lm and a large pore volume of about 1.41 mL/g. Used as metallocene catalyst support for ethylene polymerization, the agglomerated PHS shows a high specific activity and the hollow morphology of the individual PHS particles remains unchanged within the PE product, which may introduce some new properties, such as sound absorbability and heat insulation, to the polymerization product. Fig. 9. TEM images of PHS particles in polyethylene.
sizes larger than 10 nm were those produced by the interspace between the aggregated PHS particles. BET surface areas of about 214 m2/g and pore volumes of about 1.41 mL/g for the agglomerated PHS were also obtained from the N2 adsorption–desorption isotherms, which illustrated the suitability of the agglomerated PHS as the catalyst supports. Olefin monomers will enter the supports through the holes and polymerize on the catalysts during the polymerization. The PHS particles remain in the polyolefin products as the seeds for the polymerization. When polyolefin is formed, the chains of polyolefin molecule will grow and break the agglomerated PHS particles into small PHS particles or ultrafine fragments. Because PHS particles were calcined at 973 K during preparation, the hollow structure of PHS will be kept very well within polyolefin,
Acknowledgements The authors gratefully acknowledge the financial support provided by National ‘973’ program of China (Nos. 2003CB615807 and 2004CB217804), National Natural Science Foundation of China (Nos. 20506001 and 20325621), the Program for New Century Excellent Talents in University (No. NCET-04-0123) and Beijing Municipal Commission of Education (JD100100403). References [1] H.H. Brintzinger, D. Fischer, R. Mu¨hlhaupt, B. Rieger, R.M. Waymouth, Angew. Chem. 34 (1995) 1143. [2] T. Repo, M. Klinga, I. Mutikainen, Y. Su, M. Leskela¨, M. Polamo, Acta Chem. Scand. 50 (1996) 1116. [3] C. Janiak, B. Rieger, Die Angew. Macromol. Chem. 215 (1994) 47. [4] K. Soga, M. Kamimaka, Makromol. Chem. Phys. 195 (1994) 1369. [5] S. Collins, W.M. Kelly, D.A. Holden, Macromolecules 25 (1992) 170.
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