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2009, 113, 8556–8559 Published on Web 04/28/2009
MgCl2 · 4(CH3)2CHOH: A New Molecular Adduct and Super Active Polymerization Catalyst Support K. S. Thushara,† Renny Mathew,‡ T. G. Ajithkumar,‡ P. R. Rajamohanan,‡ Sumit Bhaduri,*,§ and Chinnakonda S. Gopinath*,† Catalysis DiVision, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India, Central NMR Facility, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India, and Reliance Industries Limited, V. N. PuraV Marg, Chembur, Mumbai 400 071, India. ReceiVed: March 24, 2009; ReVised Manuscript ReceiVed: April 20, 2009
A new molecular adduct, MgCl2 · 4(CH3)2CHOH, has been synthesized and characterized for structural aspects and demonstrated for super active ethylene polymerization activity with TiCl4 to ultrahigh molecular weight polyethylene in high yield. The polyolefin industries began in the 1950s, but the discovery of MgCl2-supported TiCl4 catalyst in 1968 brought about a breakthrough that led to a spectacular improvement in the productivity of the catalysts.1,2 However, despite its enormous technological advancement, science of polyolefin making suffers from a serious drawback of poor molecular level understanding of the catalyst and support and their role in polymerization.3 Right support material with an excess of TiCl4 can make it as “super active” catalyst. A common method of synthesizing these type of catalysts, commonly known as Ziegler-Natta catalysts, involves the treatment of an alkanol adduct of MgCl2 with an excess of TiCl4 with or without an additive such as an ester.2 The actual support in Ziegler-Natta catalysts such as the one described in this work is MgCl2. If an alkanol adduct of MgCl2 is reacted with excess TiCl4, the alcohol reacts with TiCl4 to give TiCl3(OR), which under optimum conditions is almost completely replaced by TiCl4. The ethanol adducts, MgCl2 · xEtOH (1 e x e 6), have been studied significantly,4,5 but only MgCl2 · 6EtOH (MgEtOH) has been characterized by single crystal X-ray crystallography. Recent studies with solid state NMR has shown the presence of mixed phases in materials formulated as MgCl2 · xEtOH (x ) 1-3).6 Thus isolation and unambiguous structural characterization of any other alkanol adduct as a single phase solid with extended lattice that displays polymerization activity comparable to commercial catalyst, is an important problem that remains a challenge. We report here the preparation of an isopropanol adduct of MgCl2, namely MgCl2 · 4(CH3)2CHOH (MgiPrOH), characterized by X-ray diffraction (XRD), solid-state NMR, thermal analysis, and optical microscopy. It may be noted that until date, no MgCl2 adduct with a secondary alcohol has been reported. In addition, MgiPrOH displays an unusual hemispherical particle morphology with a uniform diameter of ∼20 µm and on treatment with TiCl4 gives a highly active ethylene * To whom correspondence should be addressed. E-mail: cs.gopinath@ ncl.res.in. Phone: 0091-20-25902043. Fax: 0091-20-2590 2633. Website: www.ncl.org.in/csgopinath;
[email protected]. † Catalysis Division, National Chemical Laboratory. ‡ Central NMR Facility, National Chemical Laboratory. § Reliance Industries Limited.
10.1021/jp9026546 CCC: $40.75
polymerization catalyst of productivity comparable to many other literature reported systems.2,7 All the chemicals used were AR grade from Sigma-Aldrich. MgiPrOH adduct was prepared by azeotropic distillation method. Partially hydrated MgCl2 (5% H2O, 0.1 M), isopropanol (2 M), and required toluene were refluxed in a 500 mL RB flask under continuous azeotropic distillation of water for 3 h. The above solution was cooled to 0 °C, and the crystalline solid was washed with n-hexane, dried, and stored in a vacuum desiccator. MgEtOH adduct was also prepared by the above method with ethanol.7,8 XRD patterns were performed on a Rigaku Geigerflux instrument equipped with Ni-filtered Cu KR radiation (λ ) 1.5405 Å).9 Optical microscopic images of the MgiPrOH adducts were recorded on a Olympus make (BX50, Japan) optical microscope. Triblock copolymer and adduct is added to toluene and sonicated. A thin liquid layer from the above solution was employed for recording images, mainly to avoid particle agglomeration. Thermal analysis of MgiPrOH adducts were recorded on Perkin-Elmer Diamond TG/DTA with alumina as the internal standard.9 Solid-state NMR experiments were carried out on a Bruker Avance 300 spectrometer operating at a static field of 7.04 T, resonating at 300 MHz for 1H and 75.5 MHz for 13C, equipped with 4 and 2.5 mm double resonance MAS probes. The samples were spun at a spinning speed of 8 kHz in 4 mm probe for the cross polarization magic angle spinning (CP/MAS) experiments and 33 kHz in the 2.5 mm probe for the 1H MAS experiments. For the 2D exchange experiment, 512 experiments were collected with 16 scans in each experiment with a recycle delay of 2.5 s. Ethylene polymerization was carried out in a pressure reactor at 75 °C for 1 h under 5 atm of ethylene without H2 in 500 mL of hexane. The catalyst was prepared by a general procedure reported in innumerable patents with the following modifications.8 A suspension of MgiPrOH (2.8 g) was stirred in chlorobenzene (22 mL) for 1 h at 110 °C as TiCl4 (22 mL, 200 mmol) was added over the course of 10 min. The resulting solid was filtered hot and washed for 10 min each with two 10 mL portions of TiCl4 at 110 °C and filtered again followed by several 2009 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 113, No. 20, 2009 8557
Figure 2. Optical microscopic image of MgCl2 · 4(CH3)2CHOH adduct shows uniform hemispherical morphology, and a diameter around 20 µm.
Figure 1. XRD patterns of (a) anhydrous MgCl2, (b) MgCl2 · 6EtOH, and (c) MgCl2 · 4(CH3)2CHOH adducts.
washes with isopentane. The resultant catalyst (Ti, 11.5%, Mg 15.2%, Cl-, 73%) did not show detectable levels of isoporopoxide. Polymerizations of ethylene were carried out with different amounts of catalyst and (Et)3Al as the cocatalyst. In a typical run, 10 mg of catalyst (∼0.025 mmol Ti) and 160 mg of Et3Al with a Al/Ti mole ratio of 55 was used to give 140 g of polymer. A catalyst prepared from MgEtOH under identical conditions gave 49 g polymer. Figure 1 shows the XRD patterns of anhydrous MgCl2, MgiPrOH, and MgEtOH. Samples were protected with a thin layer of nujol or handled strictly under dry N2 atmosphere to avoid any degradation due to interaction with atmospheric components. MgCl2 exhibits a cubic close packing structure that gives a strong XRD peaks at 2θ ) 15.1° (003), 35°(004), and 50.4°(110).4,10 XRD pattern of MgEtOH shows a high intensity and characteristic (001) reflection at 9°.5-7,10 Our preparation method8 leads to preferential (00z) plane oriented MgEtOH crystallites, as evident from the selective high intensity reflections for 001, 002, and 004 planes. XRD pattern of MgiPrOH adduct shows a characteristic low angle peak at 9.95°, and it is attributed to 001 reflection. High intensity (001) reflections 100 °C indicates a relatively strong binding character of iPrOH in adduct with possibly some molecular reorganization. iPrOH/MgCl2 ratio was also identified to be 4 from the weight loss of MgiPrOH adduct due to loss of iPrOH molecules (∼72%). No weight loss was observed above 245 °C, and it is reasonable to assume that only MgCl2 remains >245 °C. The thermal analysis data indicates that the iPrOH molecules that dissociate
