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9-Fluorenemethanol: an internal electron donor to fine tune olefin polymerization activity Edwin S. Gnanakumar,a Eswara Rao Chokkapu,b Shrikant Kunjir,c T. G. Ajithkumar,c P. R. Rajamohanan,c Debashis Chakrabortyb and Chinnakonda S. Gopinath*a,d A new MgCl2 based molecular adduct has been synthesized with 9-fluorenemethanol (9FM) as a novel internal electron donor (IED), along with ethanol (EtOH) (MgCl2·n9FM·xEtOH). The above molecular adduct has been subjected to a variety of structural, spectroscopic and morphological characterization techniques. The results of the solid state 13C CPMAS NMR technique suggests the coordination of 9FM to MgCl2. Observation of a low angle diffraction peak at 2θ = 5.7° (d = 15.5 Å) underscores the coordination of 9FM along the z-axis, and ethanol in the molecular adduct. Active Ziegler–Natta catalysts were prepared by two different synthesis methods; the conventional method to obtain a high surface area active catalyst, and other one with 9FM as an integral part of the active catalyst in order to study the influence of 9FM as an IED over the active sites. The active catalysts were also characterized thoroughly with different analytical tools. The XRD results show (003) facets of δ-MgCl2 (α-MgCl2) for the conventional (non-
Received 17th March 2014, Accepted 19th March 2014
conventional) titanated catalyst. Results of the ethylene polymerization activity study reveals that the
DOI: 10.1039/c4dt00793j
conventionally prepared highly porous active catalyst shows 1.7–2.5 times higher activity than the nonconventional prepared catalyst; however, the latter shows a low molecular weight distribution and
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confirms the role of the Lewis base as an IED.
1.
Introduction
The industrial production of polyolefin was ignited by the remarkable discovery of the Ziegler–Natta (Z–N) olefin polymerization catalyst in the 1950s.1–3 However, the invention of MgCl2 as an effective support for the Z–N catalyst by Kashiwa and co-workers overwhelmed the industrial significance of the polyolefin catalyst system.4–6 Most of the polyolefin demand in the present market is met by the heterogeneous Z–N catalytic system.6 The multi component heterogeneous Z–N catalyst comprised of TiCl4 as the active part, alkyl aluminum (R3Al) as cocatalyst, and MgCl2 as the active support. However, to finally increase the activity of the above catalyst system, Lewis bases or electron donors (ED) such as alcohols, ethers and esters have been added.5–7 The polymer properties
a Catalysis Division, CSIR-National Chemical Laboratory, Dr Homi Bhabha Road, Pune, 411-008, India. E-mail: [email protected]; http://nclwebapps.ncl.res.in/ csgopinath/; Fax: +91-20-2590 2633 b Department of Chemistry, Indian Institute of Technology, Madras, Chennai 600-036, India c Central NMR Facility, CSIR-National Chemical Laboratory, Dr Homi Bhabha Road, Pune, 411-008, India d Centre of Excellence on Surface Science, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
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can be further tuned by altering the morphology and porosity of the active Z–N catalyst.8–10 Although olefin polymerization using the Z–N catalyst for the production of polymers has attained a technologically advanced and/or probably a saturated stage, the molecular level understanding of the catalytic system is still in its infancy due to the complexities present in the above multi component system.11–14 A number of experimental and theoretical research projects have been carried out on the heterogeneous Z–N catalyst to uncover the function of the active sites, electron donors and co-catalysts.14–20 In order to design and develop the next generation catalytic system, a basic understanding of each component present in the complex catalytic system and how they are interacting with each other is essential.21 Insights into the structure and electronic structure aspects of the molecular adduct and active catalyst towards the polymerization activity would help to rationalize understanding in this relatively unchartered territory. It would be a mammoth task for surface science, spectroscopy, and computational methods to derive the insights of the MgCl2 supported Z–N catalytic system.11–20,22–25 In modern high mileage MgCl2 supported Z–N catalysis, electron donors play a vital role in the polymerization of olefins. Many research efforts have been invested to explore the influence of different electron donors on the activation of the MgCl2 surface, but very few findings are presented in the
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literature.13,20,26,27 In fact, certain crystallographic planes exposed on the MgCl2 surface, such as (110) lateral cuts of MgCl2, is active for olefin polymerization.9,11 Characterization techniques such as XRD, NMR, IR, Raman, SEM, TEM, AFM and XPS have been used by different research groups to unravel and understand the MgCl2 supported Z–N catalyst.17,28–31 Among many MgCl2 based alcohol molecular adducts, only very few single crystal data such as MgCl2·6EtOH (MgEtOH) and MgCl2·6BzOH are present in the literature.32,33 Recently, we reported a few MgCl2 based molecular adducts derived from various alcohols such as methanol, isopropanol, isobutanol, 2-butanol, benzyl alcohol and cyclohexanol.34–39 We have followed a systematic approach towards increasing the complexity of the catalyst by adding one component at a time and subjecting it to a detailed evaluation; this approach is elaborated in our earlier publications.27,34–39 In general, the “super active” support is synthesized through the removal of alcohol from a MgCl2 based molecular adduct and concurrent introduction of TiCl4 or TiClx (x = 2–4) into the MgCl2 structure to obtain a porous nanostructure of the active catalyst. The polymerization activity of the Z–N catalyst varies with the nature of alcohol being used in the preparation of the molecular adduct, though negligible amount of alcohols would be present in the final active Z–N catalyst.36 Only very few Lewis bases, such as, 1,3 diethers, can be used as an internal electron donor (IED) in the Z–N catalyst support, which can influence the active sites without an external donor and control the polymer property.26 1,3-Diether shows chemical stability towards TiCl4, and cannot be removed from the support. However, there are only few reports available with other simple donors which show the above mentioned functions. In this article, we used 9FM as a Lewis base to synthesize the molecular adduct with MgCl2 along with ethanol as a second Lewis base for preparative reasons. To the best of our knowledge, none of studies reported in the literature describe the use of 9FM as an IED in a MgCl2 supported Z–N catalyst, except a few fluorenyl based diethers.26 It is reported in the literature that fluorenyl ligands increase the activity of homogeneous olefin polymerization catalysts.40 The aim of the present work was to study the influence of 9FM over the active sites in the heterogeneous Z–N catalyst. For this, a MgCl2·n9FM·xEtOH molecular adduct (Mg-9FM-EtOH) was synthesized. In order to study the role of 9FM as a simple Lewis base and as an IED over the active sites of the Z–N catalyst, two active catalysts namely Ti-9FM-EtOH-1 and Ti-9FM-EtOH-2 have been synthesized through different methods. The molecular adduct and active catalysts were subjected to detailed characterization and screened for ethylene polymerization.
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Partially hydrated (∼5% H2O) MgCl2, titanium tetrachloride, ethanol, 9FM (all from Sigma Aldrich), trimethylaluminium (TMA, 1.0 M solution in heptane), triethyl aluminium (TEA, 0.6 M solution in heptane), and triisobutylaluminium (TIBA, 1.1 M solution in toluene) (all from Acros Organics) were used as received. Chlorobenzene (Sigma-Aldrich) was used after drying over anhydrous calcium hydride. n-Hexane and toluene (from Merck) were dried by refluxing with sodium prior to use. Ethylene ( purity of 99.99%) was obtained from a commercial plant and used without further purification for polymerization. 2.1.
Preparation of MgCl2·xEtOH·n9FM
Partially hydrated MgCl2 (0.05 M) was added with 2 g of 9FM, 2 mL of dried ethanol and 25 mL of toluene in a 200 mL round-bottom flask. The above reaction mixture was refluxed under stirring for 3 h at 110 °C. Subsequently, the solution was kept at 0 °C for 2 h for crystallization of the Mg-9FM-EtOH adduct. The white precipitate was washed with 500 mL of hexane, dried at room temperature under vacuum for 30 min, and stored in vacuum desiccator. MgCl2·6EtOH (MgEtOH) adduct preparation procedure details are available in literature.17,27,33 2.2.
Synthesis of active catalysts
2.8 g of Mg-9FM-EtOH adduct was taken with 22 mL of chlorobenzene and stirred for 1 h at 110 °C. Subsequently, 22 mL of TiCl4 was added over a period of 10 min and stirred further for 1 h. The resulting solid product was washed with two 22 mL portions of TiCl4. Finally, the solid catalyst was filtered and washed several times with dry hexane at 60 °C until all the physisorbed Ti-species was removed.36,39 Then, the obtained Ti-9FM-EtOH-1 catalyst was dried under vacuum and stored in a dry N2 atmosphere. This was denoted as the conventional catalyst, for brevity. The titanium content in the active catalyst was found to be 12.5 wt% (0.264 mmol). In order to understand the influence of 9FM on the polymerization activity, a second active catalyst was also synthesized without the washing step. For comparison, an equal amount of titanium was taken (as found in Ti-9FM-EtOH-1) to prepare the Ti-9FM-EtOH-2 active catalyst. The synthesis procedure for Ti-9FM-EtOH-2 is as follows; 2.8 g of Mg-9FM-EtOH was taken in dry hydrocarbon and 12.5 wt% of the titanium precursor (TiCl4) was added and stirred for 48 h without any further washing and filtering. The Ti-9FM-EtOH-2 catalyst was dried under vacuum and stored in a dry N2 atmosphere. Although it is presumed that TiCl4 is incorporated in a similar way in the above catalysts, it cannot be ruled out that there is a difference in the nature of TiClx species on both the above active catalysts. This second catalyst was denoted as the nonconventional catalyst.
2. Experimental section
2.3.
All manipulations involving air or moisture sensitive reactions were carried out by following standard procedures under an ultrahigh pure N2 atmosphere using standard Schlenk techniques.
Ethylene polymerization was carried out in a Büchi Glas Uster glass polyclave reactor fitted with a thermocouple, an automatic temperature control unit and stirring speed of 500 rpm.
9144 | Dalton Trans., 2014, 43, 9143–9151
Ethylene polymerization
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In a typical polymerization, 0.5 L of dry hexane was added to the reactor at 75 °C, followed by alkyl aluminium (solution in n-heptane), and the catalyst was introduced into the reactor under a dry N2 stream and then evacuated. Ethylene (5 bar) was then fed at a constant pressure. The polymerization was carried out for 1 h at 75 °C. 2.4.
Characterization methods
The X-ray diffraction patterns were recorded on a Philips X’Pert Pro powder X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å) with a flat sample stage in the Bragg–Brentango geometry. The diffractometer was equipped with a Ni filter and X’celerator as the detector. All the samples were scanned between the range of 2θ = 5–70°. A thin layer of Nujol on the sample surface was applied before recording the diffraction pattern to avoid the degradation of the sample by reaction with the atmosphere.35–39 Thermal analysis of the adduct and its titanated catalyst was carried out using a Perkin-Elmer Diamond’s thermogravimetry (TG) and differential thermal analysis (DTA) instrument using alumina as an internal standard.41 A high-resolution FEI QUANTA 200 3D Environmental SEM was used to measure the surface morphology.42 Nova 1200 Quantachrome equipment was used to measure the surface area by using the Brunauer–Emmett–Teller (BET) method via nitrogen adsorption.43 All the solid state NMR experiments were carried out on a Bruker Avance 300 spectrometer operating at a static field of 7.04 T, resonating at 75.5 MHz for 13C using a 4 mm double resonance MAS probe. Samples in the form of a fine powder were packed into a 4 mm o.d. zirconia rotor under a nitrogen atmosphere and spun at 8 or 10 kHz. 13C CPMAS measurements were performed using a standard ramped-amplitude cross-polarization pulse sequence44 with a recycle delay of 4 s and a contact time of 2.5 ms. Chemical shifts were referred to the CH2 carbon of adamantane (38.48 ppm) for 13C. Typically, 4500 scans of transients were collected, and the sensitivity of the raw data was improved by exponential multiplication using a line broadening factor of 50 Hz. Molecular weight distribution and polydispersity of polyethylene materials were determined using GPC (Waters 150-CALC/GPC) at 135 °C in 1,2,4-trichlorobenzene as solvent. μ-Styragel columns were used, and the peaks were calibrated with polystyrene. A 0.3–0.4% w/v solution was used at a flow rate of 1 mL min−1.36,38
3. Results and discussion 3.1.
Characterization of the Mg-9FM-EtOH adduct
Fig. 1 shows a comparison of the powder XRD patterns of anhydrous MgCl2, MgEtOH, 9FM and Mg-9FM-EtOH. A rhombohedral cubic close packing structure is exhibited by anhydrous MgCl2 and it gives characteristic strong diffraction peaks for the (003), (006) and (110) planes at 2θ = 15.1°, 30.2° and 50.4°, respectively. The XRD pattern of MgEtOH shows a high intensity and characteristic (001) reflection at 2θ = 9°;35 however, Mg-9FM-EtOH shows a high intensity diffraction
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Fig. 1 Comparison of the powder XRD patterns of (a) anhydrous MgCl2, (b) 9FM, and (c) MgEtOH, and (d) Mg-9FM-EtOH adducts.
peak at 2θ = 5.7° (d(001) = 15.49 Å). The high intensity reflection of the (001) plane clearly shows that the growth of crystals, in azeotropic distillation preparation method,34,35 occurs along the (00z) planes.45 However, the MgEtOH molecular adduct and anhydrous MgCl2 exhibits d001 = 9.838 and d003 = 5.741 Å, respectively. The higher d value for Mg-9FM-EtOH compared to that of MgEtOH and MgCl2 suggests adduct formation with bulky 9FM as the Lewis base along the z-axis. Since 9FM is highly bulky, the Cl–Mg–Cl triple layer structure is strongly disrupted when the Lewis base interacts with Mg2+. Indeed, the size of the 9FM molecule is approximately 8.8 × 7.3 Å2, which is much bigger than EtOH or other alcohol molecules employed as the Lewis base. Hence, Mg-9FM-EtOH exhibits the higher d-value among other known adducts.27,34–39 Fig. 2 shows the TG and DTA results obtained from the Mg9FM-EtOH adduct. The temperature of the adduct was ramped from ambient to 350 °C at a rate of 5 °C min−1 under a flow of ultrapure dry nitrogen (99.999%) at 40 mL min−1. Well defined weight loss features were observed in the thermal analysis and they are attributed to the successive loss of ethanol as well as 9FM molecules from the molecular adduct. Similarly, the welldefined DTA curve also confirms the successive dealcoholation process of the Mg-9FM-EtOH adduct.35,36 Due to the complex nature of the molecular adduct, i.e., two different Lewis bases (EtOH and 9FM) being been used, it is difficult to find the dealcoholation temperature of each entity present in the molecular adduct. From the TG-DTA analysis of the MgEtOH adduct,27 it is expected that EtOH molecule should dissociate first from the adduct due to the lower boiling point (78 °C) than that of 9FM. It is also concluded that the relatively low
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Fig. 3 75.5 MHz 9FM-EtOH.
Fig. 2
13
C CP-MAS NMR spectra of (a) 9-FM and (b) Mg-
TG-DTA analyses of the Mg-9FM-EtOH adduct.
weight loss up to ∼170 °C is attributed to the loss of EtOH, and 9FM dissociation could occur between 150 and 260 °C. The stoichiometry of the molecular adduct containing 9FM and EtOH was also determined separately by liquid state NMR14 using a known quantity of the adduct in 1 mL of deuterated acetonitrile containing a known amount of a reference compound, methanol. The mole fraction of 9FM and EtOH was estimated from the 1H integrals and used for the calculation of the absolute amount of 9FM and EtOH in the adduct, vide infra, and found that the stoichiometry was MgCl2(9FM)1.1-(EtOH)2.3. The expected total weight loss of the above stoichiometry, found by NMR for the MgCl2-9FM-EtOH adduct, matches well with the experimental weight loss within ±1% error. The absence of any weight loss above 260 °C indicates the presence of only MgCl2, after dissociation of all the alcohol molecules. Fig. 3 shows the 13C CPMAS solid state NMR spectra of pure 9-FM and Mg-9FM-EtOH. Pure 9-FM gives a –CH2–OH peak at 62.3 ppm, >CH– (five membered) peak at 46.9 ppm and aromatic peaks between 118 and 144 ppm. Ethanol molecules present in Mg-9FM-EtOH show peaks at 16.3 and 58.5 ppm for CH3– and –CH2–OH, respectively.27 The 9FM moieties present in Mg-9FM-EtOH exhibit peaks at 46.8, 64.5 and between 117 and 145 ppm for >CH– (five membered), –CH2–OH and aromatic carbons, respectively. After a closer look at the –CH2–OH peak of the 9FM present in the Mg-9FM-EtOH adduct at 64.5 ppm, it is clear that the peak has been shifted downfield compared with pure 9FM at 62.3 ppm. This downfield shift of 2.2 ppm clearly demonstrates the coordination of 9FM to MgCl2 through the hydroxyl group. The splitting observed for the >C–H (five membered in 9FM) and –CH2–OH peak of ethanol in the Mg-9FM-EtOH spectrum should be noted. Further, aromatic signals show somewhat better resolved features on the adduct than on virgin 9FM. This not only hints at
9146 | Dalton Trans., 2014, 43, 9143–9151
Fig. 4
SEM images of the Mg-9FM-EtOH adduct.
the interaction of 9FM and EtOH, but also the relatively free molecular motion of 9FM in the molecular adduct. Nonetheless, it confirms the formation of the molecular adduct of 9FM with MgCl2. The SEM images of the Mg-9FM-EtOH molecular adduct are shown in Fig. 4. A needle-like morphology is observed in the present case, unlike the typical spherical morphology observed in the adducts derived from methanol, ethanol, cyclohexanol, benzyl alcohol.35–39 The average length of a needle is approximately 100 μm. The needle morphology indicates that the growth of particles is unidirectional in nature. However, most of the needles were agglomerated. An increase in the molecular size from EtOH to 9FM, and their presence in a single molecular adduct, has differently effects on the interactions with MgCl2 and leads to a different morphology of the adduct and catalyst. 3.2.
Characterization of the active catalyst
Fig. 5 shows the powder XRD patterns of the active catalysts, namely Ti-9FM-EtOH-1 and Ti-9FM-EtOH-2, along with that of MgCl2 and 9FM for comparison. Significantly broad and low intensity diffraction features are observed for the titanated active catalysts. This is in contrast to the narrow and intense features observed for MgCl2 and the Mg-9FM-EtOH adduct,
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Fig. 6
Fig. 5 Powder X-ray diffraction patterns of (a) Ti-9FM-EtOH-2, (b) Ti9FM-EtOH-1 active catalysts, (c) anhydrous MgCl2, and (d) 9FM. The α and δ forms of MgCl2 is indicated with crystallographic facets. Trace (b) is multiplied by a factor of 3. Dash-dot line is to indicate the observation of some of the 9FM features on both active catalysts.
indicating that the active catalysts are short range ordered in nature. Among both active catalysts, the Ti-9FM-EtOH-1 catalyst exhibits relatively broad peaks, indicating that the crystallite size is smaller than that of Ti-9FM-EtOH-2. A broad feature observed between 2θ = 17 and 35° for Ti-9FM-EtOH-1 originates from the glass substrate, and the same was confirmed independently. For the synthesis of the Ti-9FM-EtOH-1 catalyst, the addition of TiCl4 and subsequent washing predominantly removes 9FM molecules and TiCl3/TiCl4 is incorporated into the lattice of MgCl2. This rigorously decreases the crystallinity of MgCl2. Ti-9FM-EtOH-1 exhibits peaks at 2θ = 18° for (003) and broad peaks around 27°, 32°, and 51°. Stacking of the Cl–Mg–Cl triple layer in δ-MgCl2 is evidenced by the presence of a peak at 18°.9,46 Significantly, (101) facets of MgCl2 is observed suggesting a high catalytic activity.38 Similar XRD pattern has been reported earlier for a titanated catalyst and it corresponds to the structurally disordered δ-MgCl2 crystal structure.36,38 However, in the case of Ti-9FM-EtOH-2, the absence of a peak at ∼18° indicates that there is no disordered δ-MgCl2 crystal structure available. This could be due to the non-conventional synthesis method followed during the preparation of Ti-9FM-EtOH-2. However, the typical (003) facet that corresponds to the α-MgCl2 structure seems to appear in the above case, as in anhydrous MgCl2. The 9FM diffraction features are observed for both active catalysts; however, a high intensity of 9FM features on Ti-9FM-EtOH-2 indicates its presence on the surfaces. Hence, it is evident that both catalysts
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13
C CPMAS NMR spectra of the titanated active catalyst.
are different in terms of exposing (003) facets from different forms of MgCl2 and different amounts of 9FM. It is a known fact that upon loading TiCl4, on the molecular adducts, HCl formation occurs due to interaction between chlorides from TiCl4 and protons from hydroxyl groups. This could dislodge 9-FM from the molecular adduct; however, 9FM was removed in the conventional synthesis by subsequent washing (Ti-9FM-EtOH-1), while it is retained on the surfaces when the non-conventional procedure (Ti-9FM-EtOH-2) was followed. In fact, the latter method would resemble the adding of electron donors directly to the catalyst and just before polymerization. Fig. 6 shows 13C CPMAS NMR of active catalysts derived from Mg-9FM-EtOH. Ti-9FM-EtOH-1 catalyst shows peak at 25.1, 44.7, 77.5 ppm and broad peak around 129 ppm. The aromatic peak around 129 ppm could be due to the presence of chlorobenzene, which has been used as a solvent for the synthesis of the active catalyst, or the presence of 9FM moieties in the active catalyst. The peak at 25 ppm confirms the presence of aliphatic hydrocarbons, i.e., hexane used during the washing step in the synthesis of active catalyst. Peaks at 77.5 ppm and a small broad peak around 44.7 could be due to ether or other organic moieties formed during the titanation and others harsh conditions involved in the synthesis of the active catalyst. For the Ti-9FM-EtOH-2 catalyst, broad peaks around 49 and 68 ppm confirm the presence of 9FM moieties in the active catalyst. The –CH2OH feature around 68 ppm is shifted downfield compared to virgin 9FM and Mg-9FM-EtOH, possibly indicating a stronger interaction. The feature around 25 ppm is attributed to the presence of hexane used in the synthesis. Peaks between 120 to 144 ppm, as in Mg-9FM-EtOH, also supports the presence of 9FM moieties in the active catalyst. In conclusion, solid state NMR characterization confirms the presence of 9FM moieties as an integral part of Ti-
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Fig. 7
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TG-DTA analysis of active Ziegler–Natta catalysts.
9FM-EtOH-2 and is in agreement with the XRD results. However, no ethanol features are observed, indicating the removal of the same on both catalysts. The TG and DTA analysis results of the titanated active catalysts are shown in Fig. 7. Among the two active catalysts synthesized, there is a significant difference in the weight loss observed. The catalyst prepared by the conventional synthesis (Ti-9FM-EtOH-1) shows a weight loss of 48%, compared to the larger weight loss of 74% from Ti-9FM-EtOH-2. The lower weight loss associated with Ti-9FM-EtOH-1 could be due the presence of a smaller amount of 9FM compared to the other catalyst. During the conventional synthesis of the active catalyst, most of the organic moieties are expected to be removed in the hexane washing step, and hence the weight loss is small (48%). However, in the case of Ti-9FM-EtOH-2, a 74% weight loss indicates the presence of more organic moieties in the active catalyst. In fact, the presence of integrated 9FM moieties in Ti-9FM-EtOH-2 was supported by the 13C solid state NMR spectra in Fig. 6. However, in the case of Ti-9FM-EtOH-1, the catalyst contains mostly hydrocarbons, such as chlorobenzene, and other organic residues. The relatively smaller weight loss observed above 150 °C suggests that the amount of 9FM moieties is likely to be minimum on Ti-9FM-EtOH-1 compared to the other active catalyst. Fig. 8 shows the SEM images of the active catalysts. The images were recorded after the sonication of the dispersed active catalyst in the triblock copolymer and toluene solution to avoid any agglomeration and to prevent any atmospheric degradation. The SEM images clearly show the highly porous nature of the Ti-9FM-EtOH-1.38 The particle size of the active catalyst was observed to be about 500 nm. The Ti-9FM-EtOH-2 catalyst exhibits a spherical morphology with a relatively smooth surface. The size of the particles were measured to be between 1–5 μm. The harsh conditions employed in the syn-
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Fig. 8 SEM images of the active catalysts. (a) and (b) for Ti9FM-EtOH-1, and (c) and (d) for Ti-9FM-EtOH-2.
Fig. 9 Adsorption–desorption isotherms and pore size distributions of the active catalysts. (a and b) Ti-9FM-EtOH-1, and (c and d) Ti-9FM-EtOH-2 catalyst.
thesis of Ti-9FM-EtOH-1 could be the reason for the smaller particle size compared to Ti-9FM-EtOH-2. The N2 adsorption–desorption isotherms and pore size distributions of the active catalysts are shown in Fig. 9. The BET
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Table 1
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Activity results of the catalysts in polymerization reactionsa
S. No
Support
Ti wt% (mmol)
Co-catalyst
Condition
PE yield (g g−1 of catalyst)
1 2 3 4 5 6 7
Ti-9FM-EtOH-1 Ti-9FM-EtOH-1 Ti-9FM-EtOH-1 Ti-9FM-EtOH-2 Ti-9FM-EtOH-2 Ti-9FM-EtOH-2 MgEtOH26
12.5 (0.264) 12.5 (0.264) 12.5 (0.264) 12.5 (0.264) 12.5 (0.264) 12.5 (0.264) 11 (0.23)
Me3Al Et3Al iBu3Al Me3Al Et3Al iBu3Al Et3Al
75 °C, 1 atm 75 °C, 1 atm 75 °C, 1 atm 75 °C, 5 atm 75 °C, 5 atm 75 °C, 5 atm 75 °C, 5 atm
2685 4841 5813 1870 3303 2356 1300
a
PE yield (g mmol−1 of Ti)
Mn (Mw) g mol−1
MWD
1039.9 1875 2251.5 724.3 1279.3 912.5 572
17 196 (166 809) 14 451 (154 631) 19 323 (237 681) 26 051 (187 570) 20 057 (150 430) 33 696 (215 660) 22 173 (255 010)
9.7 10.7 12.3 7.2 7.5 6.4 11.5
Catalyst quantity = 0.1 g; Al/Ti = 100 and 50 for Mg-9FM-EtOH and MgEtOH, respectively.
method was employed to calculate the surface area of the active catalysts. Ti-9FM-EtOH-1 shows an extraordinary high surface area of 410 m2 g−1, which is far higher than any commercial polyolefin catalyst. The average pore diameter was calculated and found to be 69 nm with a pore volume of 0.711 cc g−1. However, Ti-9FM-EtOH-2 exhibits a surface area of 75 m2 g−1 and an average pore diameter of 50 nm with a pore volume of 0.095 cc g−1. The high surface area of Ti-9FM-EtOH-1, compared to Ti-9FM-EtOH-2, could be partly due to the removal of the bulky 9FM organic moieties due to the thorough washing step in the synthesis of the active catalyst. The former active catalyst shows a type-IV isotherm with a H3 hysteresis loop and the latter active catalyst shows a type-II isotherm. Macropore contribution in the active catalysts is evident from the pore size distribution in both catalysts. 3.3.
Ethylene polymerization
Ethylene polymerization reactions have been carried out with both active catalysts derived from Mg-9FM-EtOH. Co-catalysts with three different alkyl groups, namely, methyl, ethyl, and isobutyl (R3Al; R = CH3, –CH2CH3, and –CH2CH(CH3)2) have been employed to activate the catalytic active sites. For each cocatalyst, reactions were carried out at 5 atm pressure and 75 °C. Average results were taken after carrying out three sets of polymerizations for each set of conditions. Activity results are shown in Table 1. Three important points are worth highlighting and given below: (1) the overall polymerization activity of Ti-9FM-EtOH-1 is higher than that of Ti-9FM-EtOH-2. (2) Among the ethylene polymerization reactions, entry 3 (Table 1) shows the best activity of Ti-9FM-EtOH-1, when (iBu)3Al was used as co-catalyst. This activity is ∼4.5 times higher than the Z–N catalyst based on MgCl2·6EtOH adduct (Table 1, entry 7). (3) The overall MWD of the polymer obtained using Ti-9FM-EtOH-2 was better than Ti-9FM-EtOH-1 and entry 6 (Table 1) shows the lowest MWD of polyethylene among all the polymers obtained. The overall higher activity of the Ti-9FM-EtOH-1 catalyst could be due to the higher surface area (410 m2 g−1) compared with that of Ti-9FM-EtOH-2 (75 m2 g−1) and many other commercial catalysts.6 The highest polymerization activity for the combination of Ti-9FM-EtOH-1 with iBu3Al is attributed to the
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highly porous nature associated with the active catalyst. A relatively large number of pores and comparatively lower lability of iBu3Al (compared with other alkyl aluminium used) might have influenced the reduction of the TiClx species in the pores of the active catalyst to Ti3+ and thereby led to the highest activity.38 The overall lower MWD of the PE for the Ti9FM-EtOH-2 catalyst could be due to the influence of 9-FM as IED present on the surfaces of active catalyst. Absence of 9FM and the high porosity associated with Ti-9FM-EtOH-1 makes all sites equally available for polymerization and hence the MWD is also large. In contrast, the availability of 9FM regulates the polymerization within a regime and hence the MWD is significantly reduced, which emphasizes the role of 9FM as an IED for polyolefin synthesis.
4.
Conclusions
A single phase Mg-9FM-EtOH molecular adduct has been synthesized and subjected to detailed spectroscopic and structural investigations. In order to determine the effect of 9-FM molecules as an IED, the active catalysts were synthesized conventionally (Ti-9FM-EtOH-1) and with the predominant fixing of TiCl4 over the support (Ti-9FM-EtOH-2); both catalysts were subjected to detailed investigations. The textural properties of the active catalysts shows a higher surface area for the conventionally synthesized catalyst than the unconventional. Ethylene polymerization reactions were measured with different cocatalysts and showed that iBu3Al was the best cocatalyst for the above supported Z–N catalytic system. The polymerization activity results suggest that the conventionally synthesized catalyst exhibited ∼4.5 times better activity than the Z–N catalyst prepared from the MgEtOH adduct, and still better than merely TiCl4 fixed over the Mg-9FM-EtOH support. However, the latter catalytic system (Ti-9FM-EtOH-2) shows the lowest MWD of PE obtained, confirming the role of 9-FM as and IED in the catalytic system. Detailed studies confirm that the lower lability of iBu3Al and the contribution of macropores are the reason for the conventionally synthesized catalyst giving a higher polymerization activity. The XRD results exhibit the (003) facets of α-MgCl2 (δ-MgCl2) on the merely fixing TiCl4 (conventionally) prepared catalyst with a significant (low) crys-
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tallinity and large (small) amount of 9FM on the surfaces was also a reason for the different polymerization activities.
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Acknowledgements We thank Dr S. Bhaduri for many suggestions and long-standing collaboration. ESG thanks the CSIR, New Delhi, for the senior research fellowship. Partial financial support from the CSIR, New Delhi for CSC0404 project is acknowledged.
Notes and references 1 K. Ziegler, E. Holzkamp, H. Breil and H. Martin, Angew. Chem., 1955, 67, 541. 2 K. Ziegler, E. Holzkamp, H. Breil and H. Martin, Angew. Chem., 1955, 67, 426. 3 G. Natta, J. Polym. Sci., 1955, 16, 143. 4 N. Kashiwa, H. Fujimura and Y. Tokuzumi, JP Patent 1031698, 1968. 5 N. Kashiwa, Polym. J., 1980, 12, 603. 6 N. Kashiwa, J. Polym. Sci., Part A: Polym. Chem., 2004, 42, 1. 7 N. Kashiwa and J. Yoshitake, Makromol. Chem. Rapid Commun., 1983, 4, 41. 8 J. Boor Jr., Ziegler-Natta Catalysts and Polymerization, Academic Press, New York, 1979. 9 F. Auriemma and C. De Rosa, Chem. Mater., 2007, 19, 5803. 10 M. Rerraris, F. Rosati, S. Parodi, E. Giannetti, G. Motroni and E. Albizzati, U.S. Pat 4,399,054, 1983. 11 P. Sobota, Coord. Chem. Rev., 2004, 248, 1047. 12 (a) C. R. Tewell, F. Malizia, J. W. Ager III and G. A. Somorjai, J. Phys. Chem. B, 2002, 106, 2946; (b) S. H. Kim and G. A. Somorjai, J. Phys. Chem. B, 2002, 106, 1386. 13 A. Andoni, J. C. Chadwick, S. Milani, J. W. Niemantsverdriet and P. Thüne, J. Catal., 2007, 247, 129. 14 P. Sozzani, S. Bracco, A. Comotti, R. Simonutti and I. Camurati, J. Am. Chem. Soc., 2003, 125, 12881. 15 M. Boero, M. Parrinello and K. Terakura, Surf. Sci., 1999, 438, 1. 16 D. V. Stukalov and V. A. Zakharov, J. Phys. Chem. C, 2009, 113, 21376. 17 M. Seth, P. M. Margl and T. Ziegler, Macromolecules, 2002, 35, 7815. 18 G. Monaco, M. Toto, G. Guerra, P. Corradini and L. Cavallo, Macromolecules, 2000, 33, 8953. 19 H. Mori, M. Sawada, T. Higuchi, K. Hasebe, N. Otsuka and M. Terano, Macromol. Rapid Commun., 1999, 20, 245. 20 E. Groppo, K. Seenivasan and C. Barzan, Catal. Sci. Technol., 2013, 3, 858. 21 (a) F. Malizia, A. Fait and G. Cruciani, Chem. - Eur. J., 2011, 17, 13892; (b) V. Subramanian, E. S. Gnanakumar, D.-W. Jeong, W.-B. Han, C. S. Gopinath and H. S. Roh, Chem. Commun., 2013, 49, 11257.
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22 E. Magni and G. A. Somorjai, Surf. Sci., 1996, 345, 1. 23 S. H. Kim, R. T. Craig and G. A. Somorjai, Langmuir, 2000, 16, 9414. 24 H. Mori, K. Hasebe and M. Terano, Macromol. Chem. Phys., 1998, 199, 2709. 25 E. Puhakka, T. T. Pakkanen and T. A. Pakkanen, J. Phys. Chem. A, 1997, 101, 6063. 26 D. Xu, Z. Liu, J. Zhao, S. Han and Y. Hu, Macromol. Rapid Commun., 2000, 21, 1046. 27 K. S. Thushara, E. S. Gnanakumar, R. Mathew, R. K. Jha, T. G. Ajithkumar, P. R. Rajamohanan, K. Sarma, S. Padmanabhan, S. Bhaduri and C. S. Gopinath, J. Phys. Chem. C, 2011, 115, 1952. 28 V. D. Noto, A. Marigo, M. Viviani, C. Marega, S. Bresadola and R. Zannetti, Makromol. Chem., 1992, 193, 123. 29 J. C. J. Bart, J. Mater. Sci., 1995, 30, 2809. 30 A. Andoni, J. C. Chadwick, J. W. Niemantsverdriet and P. Thüne, J. Catal., 2008, 257, 81. 31 M. Terano, M. Saito and T. Kataoka, Makromol. Chem. Rapid Commun., 1992, 13, 103. 32 G. Valle, G. Baruzzi, G. Paganetto, G. Depaoli, R. Zannetti and R. Marigo, Inorg. Chim. Acta, 1989, 156, 157. 33 V. D. Noto, S. Bresadola, R. Zannetti and M. Z. Viviani, Kristallografiya, 1993, 204, 263. 34 K. S. Thushara, R. Mathew, T. G. Ajithkumar, P. R. Rajamohanan, S. Bhaduri and C. S. Gopinath, J. Phys. Chem. C, 2009, 113, 8556. 35 E. S. Gnanakumar, K. S. Thushara, D. S. Bhange, R. Mathew, T. G. Ajithkumar, P. R. Rajamohanan, S. Bhaduri and C. S. Gopinath, Dalton Trans., 2011, 40, 10936. 36 (a) E. S. Gnanakumar, R. R. Gowda, S. Kunjir, T. G. Ajithkumar, P. R. Rajamohanan, D. Chakraborty and C. S. Gopinath, ACS Catal., 2013, 3, 303; (b) C. S. Gopinath and F. Zaera, J. Catal., 2001, 200, 270. 37 (a) K. S. Thushara, E. S. Gnanakumar, R. Mathew, T. G. Ajithkumar, P. R. Rajamohanan, S. Bhaduri and C. S. Gopinath, Dalton Trans., 2012, 41, 11311; (b) K. S. Thushara, T. G. Ajithkumar, P. R. Rajamohanan and C. S. Gopinath, J. Phys. Chem. A, 2014, 118, 1213. 38 E. S. Gnanakumar, K. S. Thushara, R. R. Gowda, S. K. Raman, T. G. Ajithkumar, P. R. Rajamohanan, D. Chakraborty and C. S. Gopinath, J. Phys. Chem. C, 2012, 116, 24115. 39 K. S. Thushara, T. G. Ajithkumar, P. R. Rajamohanan and C. S. Gopinath, Appl. Catal., A, 2014, 469, 267. 40 H. G. Alt and E. Samuel, Chem. Soc. Rev., 1998, 27, 323. 41 (a) T. Mathew, M. Vijayaraj, S. Pai, B. B. Tope, S. G. Hedge, B. S. Rao and C. S. Gopinath, J. Catal., 2004, 227, 175; (b) M. Mapa and C. S. Gopinath, Chem. Mater., 2009, 21, 351. 42 K. Sivaranjani, A. Verma and C. S. Gopinath, Green Chem., 2012, 14, 461. 43 T. Mathew, K. Sivaranjani, E. S. Gnanakumar, Y. Yamada, T. Kobayashi and C. S. Gopinath, J. Mater. Chem., 2012, 22, 13484.
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C. S. Gopinath, S. Bhaduri and G. K. Lahiri, J. Phys. Chem. C, 2008, 112, 9428. 46 H. L. Ronkko, H. Knuutilla, P. Denifl, T. Leinonen and T. Venalainen, J. Mol. Catal. A: Chem., 2007, 278, 127.
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44 V. Sreeja, T. S. Smitha, D. Nand, T. G. Ajithkumar and P. A. Joy, J. Phys. Chem. C, 2008, 112, 14737. 45 (a) A. V. Murugan, A. K. Viswanath, C. S. Gopinath and K. Vijayamohanan, J. Appl. Phys., 2006, 100, 074319; (b) N. Maity, P. R. Rajamohanan, S. Ganapathy,
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PAPER
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9-Fluorenemethanol: an internal electron donor to fine tune olefin polymerization activity Edwin S. Gnanakumar,a Eswara Rao Chokkapu,b Shrikant Kunjir,c T. G. Ajithkumar,c P. R. Rajamohanan,c Debashis Chakrabortyb and Chinnakonda S. Gopinath*a,d A new MgCl2 based molecular adduct has been synthesized with 9-fluorenemethanol (9FM) as a novel internal electron donor (IED), along with ethanol (EtOH) (MgCl2·n9FM·xEtOH). The above molecular adduct has been subjected to a variety of structural, spectroscopic and morphological characterization techniques. The results of the solid state 13C CPMAS NMR technique suggests the coordination of 9FM to MgCl2. Observation of a low angle diffraction peak at 2θ = 5.7° (d = 15.5 Å) underscores the coordination of 9FM along the z-axis, and ethanol in the molecular adduct. Active Ziegler–Natta catalysts were prepared by two different synthesis methods; the conventional method to obtain a high surface area active catalyst, and other one with 9FM as an integral part of the active catalyst in order to study the influence of 9FM as an IED over the active sites. The active catalysts were also characterized thoroughly with different analytical tools. The XRD results show (003) facets of δ-MgCl2 (α-MgCl2) for the conventional (non-
Received 17th March 2014, Accepted 19th March 2014
conventional) titanated catalyst. Results of the ethylene polymerization activity study reveals that the
DOI: 10.1039/c4dt00793j
conventionally prepared highly porous active catalyst shows 1.7–2.5 times higher activity than the nonconventional prepared catalyst; however, the latter shows a low molecular weight distribution and
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confirms the role of the Lewis base as an IED.
1.
Introduction
The industrial production of polyolefin was ignited by the remarkable discovery of the Ziegler–Natta (Z–N) olefin polymerization catalyst in the 1950s.1–3 However, the invention of MgCl2 as an effective support for the Z–N catalyst by Kashiwa and co-workers overwhelmed the industrial significance of the polyolefin catalyst system.4–6 Most of the polyolefin demand in the present market is met by the heterogeneous Z–N catalytic system.6 The multi component heterogeneous Z–N catalyst comprised of TiCl4 as the active part, alkyl aluminum (R3Al) as cocatalyst, and MgCl2 as the active support. However, to finally increase the activity of the above catalyst system, Lewis bases or electron donors (ED) such as alcohols, ethers and esters have been added.5–7 The polymer properties
a Catalysis Division, CSIR-National Chemical Laboratory, Dr Homi Bhabha Road, Pune, 411-008, India. E-mail: [email protected]; http://nclwebapps.ncl.res.in/ csgopinath/; Fax: +91-20-2590 2633 b Department of Chemistry, Indian Institute of Technology, Madras, Chennai 600-036, India c Central NMR Facility, CSIR-National Chemical Laboratory, Dr Homi Bhabha Road, Pune, 411-008, India d Centre of Excellence on Surface Science, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
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can be further tuned by altering the morphology and porosity of the active Z–N catalyst.8–10 Although olefin polymerization using the Z–N catalyst for the production of polymers has attained a technologically advanced and/or probably a saturated stage, the molecular level understanding of the catalytic system is still in its infancy due to the complexities present in the above multi component system.11–14 A number of experimental and theoretical research projects have been carried out on the heterogeneous Z–N catalyst to uncover the function of the active sites, electron donors and co-catalysts.14–20 In order to design and develop the next generation catalytic system, a basic understanding of each component present in the complex catalytic system and how they are interacting with each other is essential.21 Insights into the structure and electronic structure aspects of the molecular adduct and active catalyst towards the polymerization activity would help to rationalize understanding in this relatively unchartered territory. It would be a mammoth task for surface science, spectroscopy, and computational methods to derive the insights of the MgCl2 supported Z–N catalytic system.11–20,22–25 In modern high mileage MgCl2 supported Z–N catalysis, electron donors play a vital role in the polymerization of olefins. Many research efforts have been invested to explore the influence of different electron donors on the activation of the MgCl2 surface, but very few findings are presented in the
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literature.13,20,26,27 In fact, certain crystallographic planes exposed on the MgCl2 surface, such as (110) lateral cuts of MgCl2, is active for olefin polymerization.9,11 Characterization techniques such as XRD, NMR, IR, Raman, SEM, TEM, AFM and XPS have been used by different research groups to unravel and understand the MgCl2 supported Z–N catalyst.17,28–31 Among many MgCl2 based alcohol molecular adducts, only very few single crystal data such as MgCl2·6EtOH (MgEtOH) and MgCl2·6BzOH are present in the literature.32,33 Recently, we reported a few MgCl2 based molecular adducts derived from various alcohols such as methanol, isopropanol, isobutanol, 2-butanol, benzyl alcohol and cyclohexanol.34–39 We have followed a systematic approach towards increasing the complexity of the catalyst by adding one component at a time and subjecting it to a detailed evaluation; this approach is elaborated in our earlier publications.27,34–39 In general, the “super active” support is synthesized through the removal of alcohol from a MgCl2 based molecular adduct and concurrent introduction of TiCl4 or TiClx (x = 2–4) into the MgCl2 structure to obtain a porous nanostructure of the active catalyst. The polymerization activity of the Z–N catalyst varies with the nature of alcohol being used in the preparation of the molecular adduct, though negligible amount of alcohols would be present in the final active Z–N catalyst.36 Only very few Lewis bases, such as, 1,3 diethers, can be used as an internal electron donor (IED) in the Z–N catalyst support, which can influence the active sites without an external donor and control the polymer property.26 1,3-Diether shows chemical stability towards TiCl4, and cannot be removed from the support. However, there are only few reports available with other simple donors which show the above mentioned functions. In this article, we used 9FM as a Lewis base to synthesize the molecular adduct with MgCl2 along with ethanol as a second Lewis base for preparative reasons. To the best of our knowledge, none of studies reported in the literature describe the use of 9FM as an IED in a MgCl2 supported Z–N catalyst, except a few fluorenyl based diethers.26 It is reported in the literature that fluorenyl ligands increase the activity of homogeneous olefin polymerization catalysts.40 The aim of the present work was to study the influence of 9FM over the active sites in the heterogeneous Z–N catalyst. For this, a MgCl2·n9FM·xEtOH molecular adduct (Mg-9FM-EtOH) was synthesized. In order to study the role of 9FM as a simple Lewis base and as an IED over the active sites of the Z–N catalyst, two active catalysts namely Ti-9FM-EtOH-1 and Ti-9FM-EtOH-2 have been synthesized through different methods. The molecular adduct and active catalysts were subjected to detailed characterization and screened for ethylene polymerization.
Dalton Transactions
Partially hydrated (∼5% H2O) MgCl2, titanium tetrachloride, ethanol, 9FM (all from Sigma Aldrich), trimethylaluminium (TMA, 1.0 M solution in heptane), triethyl aluminium (TEA, 0.6 M solution in heptane), and triisobutylaluminium (TIBA, 1.1 M solution in toluene) (all from Acros Organics) were used as received. Chlorobenzene (Sigma-Aldrich) was used after drying over anhydrous calcium hydride. n-Hexane and toluene (from Merck) were dried by refluxing with sodium prior to use. Ethylene ( purity of 99.99%) was obtained from a commercial plant and used without further purification for polymerization. 2.1.
Preparation of MgCl2·xEtOH·n9FM
Partially hydrated MgCl2 (0.05 M) was added with 2 g of 9FM, 2 mL of dried ethanol and 25 mL of toluene in a 200 mL round-bottom flask. The above reaction mixture was refluxed under stirring for 3 h at 110 °C. Subsequently, the solution was kept at 0 °C for 2 h for crystallization of the Mg-9FM-EtOH adduct. The white precipitate was washed with 500 mL of hexane, dried at room temperature under vacuum for 30 min, and stored in vacuum desiccator. MgCl2·6EtOH (MgEtOH) adduct preparation procedure details are available in literature.17,27,33 2.2.
Synthesis of active catalysts
2.8 g of Mg-9FM-EtOH adduct was taken with 22 mL of chlorobenzene and stirred for 1 h at 110 °C. Subsequently, 22 mL of TiCl4 was added over a period of 10 min and stirred further for 1 h. The resulting solid product was washed with two 22 mL portions of TiCl4. Finally, the solid catalyst was filtered and washed several times with dry hexane at 60 °C until all the physisorbed Ti-species was removed.36,39 Then, the obtained Ti-9FM-EtOH-1 catalyst was dried under vacuum and stored in a dry N2 atmosphere. This was denoted as the conventional catalyst, for brevity. The titanium content in the active catalyst was found to be 12.5 wt% (0.264 mmol). In order to understand the influence of 9FM on the polymerization activity, a second active catalyst was also synthesized without the washing step. For comparison, an equal amount of titanium was taken (as found in Ti-9FM-EtOH-1) to prepare the Ti-9FM-EtOH-2 active catalyst. The synthesis procedure for Ti-9FM-EtOH-2 is as follows; 2.8 g of Mg-9FM-EtOH was taken in dry hydrocarbon and 12.5 wt% of the titanium precursor (TiCl4) was added and stirred for 48 h without any further washing and filtering. The Ti-9FM-EtOH-2 catalyst was dried under vacuum and stored in a dry N2 atmosphere. Although it is presumed that TiCl4 is incorporated in a similar way in the above catalysts, it cannot be ruled out that there is a difference in the nature of TiClx species on both the above active catalysts. This second catalyst was denoted as the nonconventional catalyst.
2. Experimental section
2.3.
All manipulations involving air or moisture sensitive reactions were carried out by following standard procedures under an ultrahigh pure N2 atmosphere using standard Schlenk techniques.
Ethylene polymerization was carried out in a Büchi Glas Uster glass polyclave reactor fitted with a thermocouple, an automatic temperature control unit and stirring speed of 500 rpm.
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Ethylene polymerization
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In a typical polymerization, 0.5 L of dry hexane was added to the reactor at 75 °C, followed by alkyl aluminium (solution in n-heptane), and the catalyst was introduced into the reactor under a dry N2 stream and then evacuated. Ethylene (5 bar) was then fed at a constant pressure. The polymerization was carried out for 1 h at 75 °C. 2.4.
Characterization methods
The X-ray diffraction patterns were recorded on a Philips X’Pert Pro powder X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å) with a flat sample stage in the Bragg–Brentango geometry. The diffractometer was equipped with a Ni filter and X’celerator as the detector. All the samples were scanned between the range of 2θ = 5–70°. A thin layer of Nujol on the sample surface was applied before recording the diffraction pattern to avoid the degradation of the sample by reaction with the atmosphere.35–39 Thermal analysis of the adduct and its titanated catalyst was carried out using a Perkin-Elmer Diamond’s thermogravimetry (TG) and differential thermal analysis (DTA) instrument using alumina as an internal standard.41 A high-resolution FEI QUANTA 200 3D Environmental SEM was used to measure the surface morphology.42 Nova 1200 Quantachrome equipment was used to measure the surface area by using the Brunauer–Emmett–Teller (BET) method via nitrogen adsorption.43 All the solid state NMR experiments were carried out on a Bruker Avance 300 spectrometer operating at a static field of 7.04 T, resonating at 75.5 MHz for 13C using a 4 mm double resonance MAS probe. Samples in the form of a fine powder were packed into a 4 mm o.d. zirconia rotor under a nitrogen atmosphere and spun at 8 or 10 kHz. 13C CPMAS measurements were performed using a standard ramped-amplitude cross-polarization pulse sequence44 with a recycle delay of 4 s and a contact time of 2.5 ms. Chemical shifts were referred to the CH2 carbon of adamantane (38.48 ppm) for 13C. Typically, 4500 scans of transients were collected, and the sensitivity of the raw data was improved by exponential multiplication using a line broadening factor of 50 Hz. Molecular weight distribution and polydispersity of polyethylene materials were determined using GPC (Waters 150-CALC/GPC) at 135 °C in 1,2,4-trichlorobenzene as solvent. μ-Styragel columns were used, and the peaks were calibrated with polystyrene. A 0.3–0.4% w/v solution was used at a flow rate of 1 mL min−1.36,38
3. Results and discussion 3.1.
Characterization of the Mg-9FM-EtOH adduct
Fig. 1 shows a comparison of the powder XRD patterns of anhydrous MgCl2, MgEtOH, 9FM and Mg-9FM-EtOH. A rhombohedral cubic close packing structure is exhibited by anhydrous MgCl2 and it gives characteristic strong diffraction peaks for the (003), (006) and (110) planes at 2θ = 15.1°, 30.2° and 50.4°, respectively. The XRD pattern of MgEtOH shows a high intensity and characteristic (001) reflection at 2θ = 9°;35 however, Mg-9FM-EtOH shows a high intensity diffraction
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Fig. 1 Comparison of the powder XRD patterns of (a) anhydrous MgCl2, (b) 9FM, and (c) MgEtOH, and (d) Mg-9FM-EtOH adducts.
peak at 2θ = 5.7° (d(001) = 15.49 Å). The high intensity reflection of the (001) plane clearly shows that the growth of crystals, in azeotropic distillation preparation method,34,35 occurs along the (00z) planes.45 However, the MgEtOH molecular adduct and anhydrous MgCl2 exhibits d001 = 9.838 and d003 = 5.741 Å, respectively. The higher d value for Mg-9FM-EtOH compared to that of MgEtOH and MgCl2 suggests adduct formation with bulky 9FM as the Lewis base along the z-axis. Since 9FM is highly bulky, the Cl–Mg–Cl triple layer structure is strongly disrupted when the Lewis base interacts with Mg2+. Indeed, the size of the 9FM molecule is approximately 8.8 × 7.3 Å2, which is much bigger than EtOH or other alcohol molecules employed as the Lewis base. Hence, Mg-9FM-EtOH exhibits the higher d-value among other known adducts.27,34–39 Fig. 2 shows the TG and DTA results obtained from the Mg9FM-EtOH adduct. The temperature of the adduct was ramped from ambient to 350 °C at a rate of 5 °C min−1 under a flow of ultrapure dry nitrogen (99.999%) at 40 mL min−1. Well defined weight loss features were observed in the thermal analysis and they are attributed to the successive loss of ethanol as well as 9FM molecules from the molecular adduct. Similarly, the welldefined DTA curve also confirms the successive dealcoholation process of the Mg-9FM-EtOH adduct.35,36 Due to the complex nature of the molecular adduct, i.e., two different Lewis bases (EtOH and 9FM) being been used, it is difficult to find the dealcoholation temperature of each entity present in the molecular adduct. From the TG-DTA analysis of the MgEtOH adduct,27 it is expected that EtOH molecule should dissociate first from the adduct due to the lower boiling point (78 °C) than that of 9FM. It is also concluded that the relatively low
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Dalton Transactions
Fig. 3 75.5 MHz 9FM-EtOH.
Fig. 2
13
C CP-MAS NMR spectra of (a) 9-FM and (b) Mg-
TG-DTA analyses of the Mg-9FM-EtOH adduct.
weight loss up to ∼170 °C is attributed to the loss of EtOH, and 9FM dissociation could occur between 150 and 260 °C. The stoichiometry of the molecular adduct containing 9FM and EtOH was also determined separately by liquid state NMR14 using a known quantity of the adduct in 1 mL of deuterated acetonitrile containing a known amount of a reference compound, methanol. The mole fraction of 9FM and EtOH was estimated from the 1H integrals and used for the calculation of the absolute amount of 9FM and EtOH in the adduct, vide infra, and found that the stoichiometry was MgCl2(9FM)1.1-(EtOH)2.3. The expected total weight loss of the above stoichiometry, found by NMR for the MgCl2-9FM-EtOH adduct, matches well with the experimental weight loss within ±1% error. The absence of any weight loss above 260 °C indicates the presence of only MgCl2, after dissociation of all the alcohol molecules. Fig. 3 shows the 13C CPMAS solid state NMR spectra of pure 9-FM and Mg-9FM-EtOH. Pure 9-FM gives a –CH2–OH peak at 62.3 ppm, >CH– (five membered) peak at 46.9 ppm and aromatic peaks between 118 and 144 ppm. Ethanol molecules present in Mg-9FM-EtOH show peaks at 16.3 and 58.5 ppm for CH3– and –CH2–OH, respectively.27 The 9FM moieties present in Mg-9FM-EtOH exhibit peaks at 46.8, 64.5 and between 117 and 145 ppm for >CH– (five membered), –CH2–OH and aromatic carbons, respectively. After a closer look at the –CH2–OH peak of the 9FM present in the Mg-9FM-EtOH adduct at 64.5 ppm, it is clear that the peak has been shifted downfield compared with pure 9FM at 62.3 ppm. This downfield shift of 2.2 ppm clearly demonstrates the coordination of 9FM to MgCl2 through the hydroxyl group. The splitting observed for the >C–H (five membered in 9FM) and –CH2–OH peak of ethanol in the Mg-9FM-EtOH spectrum should be noted. Further, aromatic signals show somewhat better resolved features on the adduct than on virgin 9FM. This not only hints at
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Fig. 4
SEM images of the Mg-9FM-EtOH adduct.
the interaction of 9FM and EtOH, but also the relatively free molecular motion of 9FM in the molecular adduct. Nonetheless, it confirms the formation of the molecular adduct of 9FM with MgCl2. The SEM images of the Mg-9FM-EtOH molecular adduct are shown in Fig. 4. A needle-like morphology is observed in the present case, unlike the typical spherical morphology observed in the adducts derived from methanol, ethanol, cyclohexanol, benzyl alcohol.35–39 The average length of a needle is approximately 100 μm. The needle morphology indicates that the growth of particles is unidirectional in nature. However, most of the needles were agglomerated. An increase in the molecular size from EtOH to 9FM, and their presence in a single molecular adduct, has differently effects on the interactions with MgCl2 and leads to a different morphology of the adduct and catalyst. 3.2.
Characterization of the active catalyst
Fig. 5 shows the powder XRD patterns of the active catalysts, namely Ti-9FM-EtOH-1 and Ti-9FM-EtOH-2, along with that of MgCl2 and 9FM for comparison. Significantly broad and low intensity diffraction features are observed for the titanated active catalysts. This is in contrast to the narrow and intense features observed for MgCl2 and the Mg-9FM-EtOH adduct,
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Fig. 6
Fig. 5 Powder X-ray diffraction patterns of (a) Ti-9FM-EtOH-2, (b) Ti9FM-EtOH-1 active catalysts, (c) anhydrous MgCl2, and (d) 9FM. The α and δ forms of MgCl2 is indicated with crystallographic facets. Trace (b) is multiplied by a factor of 3. Dash-dot line is to indicate the observation of some of the 9FM features on both active catalysts.
indicating that the active catalysts are short range ordered in nature. Among both active catalysts, the Ti-9FM-EtOH-1 catalyst exhibits relatively broad peaks, indicating that the crystallite size is smaller than that of Ti-9FM-EtOH-2. A broad feature observed between 2θ = 17 and 35° for Ti-9FM-EtOH-1 originates from the glass substrate, and the same was confirmed independently. For the synthesis of the Ti-9FM-EtOH-1 catalyst, the addition of TiCl4 and subsequent washing predominantly removes 9FM molecules and TiCl3/TiCl4 is incorporated into the lattice of MgCl2. This rigorously decreases the crystallinity of MgCl2. Ti-9FM-EtOH-1 exhibits peaks at 2θ = 18° for (003) and broad peaks around 27°, 32°, and 51°. Stacking of the Cl–Mg–Cl triple layer in δ-MgCl2 is evidenced by the presence of a peak at 18°.9,46 Significantly, (101) facets of MgCl2 is observed suggesting a high catalytic activity.38 Similar XRD pattern has been reported earlier for a titanated catalyst and it corresponds to the structurally disordered δ-MgCl2 crystal structure.36,38 However, in the case of Ti-9FM-EtOH-2, the absence of a peak at ∼18° indicates that there is no disordered δ-MgCl2 crystal structure available. This could be due to the non-conventional synthesis method followed during the preparation of Ti-9FM-EtOH-2. However, the typical (003) facet that corresponds to the α-MgCl2 structure seems to appear in the above case, as in anhydrous MgCl2. The 9FM diffraction features are observed for both active catalysts; however, a high intensity of 9FM features on Ti-9FM-EtOH-2 indicates its presence on the surfaces. Hence, it is evident that both catalysts
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13
C CPMAS NMR spectra of the titanated active catalyst.
are different in terms of exposing (003) facets from different forms of MgCl2 and different amounts of 9FM. It is a known fact that upon loading TiCl4, on the molecular adducts, HCl formation occurs due to interaction between chlorides from TiCl4 and protons from hydroxyl groups. This could dislodge 9-FM from the molecular adduct; however, 9FM was removed in the conventional synthesis by subsequent washing (Ti-9FM-EtOH-1), while it is retained on the surfaces when the non-conventional procedure (Ti-9FM-EtOH-2) was followed. In fact, the latter method would resemble the adding of electron donors directly to the catalyst and just before polymerization. Fig. 6 shows 13C CPMAS NMR of active catalysts derived from Mg-9FM-EtOH. Ti-9FM-EtOH-1 catalyst shows peak at 25.1, 44.7, 77.5 ppm and broad peak around 129 ppm. The aromatic peak around 129 ppm could be due to the presence of chlorobenzene, which has been used as a solvent for the synthesis of the active catalyst, or the presence of 9FM moieties in the active catalyst. The peak at 25 ppm confirms the presence of aliphatic hydrocarbons, i.e., hexane used during the washing step in the synthesis of active catalyst. Peaks at 77.5 ppm and a small broad peak around 44.7 could be due to ether or other organic moieties formed during the titanation and others harsh conditions involved in the synthesis of the active catalyst. For the Ti-9FM-EtOH-2 catalyst, broad peaks around 49 and 68 ppm confirm the presence of 9FM moieties in the active catalyst. The –CH2OH feature around 68 ppm is shifted downfield compared to virgin 9FM and Mg-9FM-EtOH, possibly indicating a stronger interaction. The feature around 25 ppm is attributed to the presence of hexane used in the synthesis. Peaks between 120 to 144 ppm, as in Mg-9FM-EtOH, also supports the presence of 9FM moieties in the active catalyst. In conclusion, solid state NMR characterization confirms the presence of 9FM moieties as an integral part of Ti-
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Fig. 7
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TG-DTA analysis of active Ziegler–Natta catalysts.
9FM-EtOH-2 and is in agreement with the XRD results. However, no ethanol features are observed, indicating the removal of the same on both catalysts. The TG and DTA analysis results of the titanated active catalysts are shown in Fig. 7. Among the two active catalysts synthesized, there is a significant difference in the weight loss observed. The catalyst prepared by the conventional synthesis (Ti-9FM-EtOH-1) shows a weight loss of 48%, compared to the larger weight loss of 74% from Ti-9FM-EtOH-2. The lower weight loss associated with Ti-9FM-EtOH-1 could be due the presence of a smaller amount of 9FM compared to the other catalyst. During the conventional synthesis of the active catalyst, most of the organic moieties are expected to be removed in the hexane washing step, and hence the weight loss is small (48%). However, in the case of Ti-9FM-EtOH-2, a 74% weight loss indicates the presence of more organic moieties in the active catalyst. In fact, the presence of integrated 9FM moieties in Ti-9FM-EtOH-2 was supported by the 13C solid state NMR spectra in Fig. 6. However, in the case of Ti-9FM-EtOH-1, the catalyst contains mostly hydrocarbons, such as chlorobenzene, and other organic residues. The relatively smaller weight loss observed above 150 °C suggests that the amount of 9FM moieties is likely to be minimum on Ti-9FM-EtOH-1 compared to the other active catalyst. Fig. 8 shows the SEM images of the active catalysts. The images were recorded after the sonication of the dispersed active catalyst in the triblock copolymer and toluene solution to avoid any agglomeration and to prevent any atmospheric degradation. The SEM images clearly show the highly porous nature of the Ti-9FM-EtOH-1.38 The particle size of the active catalyst was observed to be about 500 nm. The Ti-9FM-EtOH-2 catalyst exhibits a spherical morphology with a relatively smooth surface. The size of the particles were measured to be between 1–5 μm. The harsh conditions employed in the syn-
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Fig. 8 SEM images of the active catalysts. (a) and (b) for Ti9FM-EtOH-1, and (c) and (d) for Ti-9FM-EtOH-2.
Fig. 9 Adsorption–desorption isotherms and pore size distributions of the active catalysts. (a and b) Ti-9FM-EtOH-1, and (c and d) Ti-9FM-EtOH-2 catalyst.
thesis of Ti-9FM-EtOH-1 could be the reason for the smaller particle size compared to Ti-9FM-EtOH-2. The N2 adsorption–desorption isotherms and pore size distributions of the active catalysts are shown in Fig. 9. The BET
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Table 1
Paper
Activity results of the catalysts in polymerization reactionsa
S. No
Support
Ti wt% (mmol)
Co-catalyst
Condition
PE yield (g g−1 of catalyst)
1 2 3 4 5 6 7
Ti-9FM-EtOH-1 Ti-9FM-EtOH-1 Ti-9FM-EtOH-1 Ti-9FM-EtOH-2 Ti-9FM-EtOH-2 Ti-9FM-EtOH-2 MgEtOH26
12.5 (0.264) 12.5 (0.264) 12.5 (0.264) 12.5 (0.264) 12.5 (0.264) 12.5 (0.264) 11 (0.23)
Me3Al Et3Al iBu3Al Me3Al Et3Al iBu3Al Et3Al
75 °C, 1 atm 75 °C, 1 atm 75 °C, 1 atm 75 °C, 5 atm 75 °C, 5 atm 75 °C, 5 atm 75 °C, 5 atm
2685 4841 5813 1870 3303 2356 1300
a
PE yield (g mmol−1 of Ti)
Mn (Mw) g mol−1
MWD
1039.9 1875 2251.5 724.3 1279.3 912.5 572
17 196 (166 809) 14 451 (154 631) 19 323 (237 681) 26 051 (187 570) 20 057 (150 430) 33 696 (215 660) 22 173 (255 010)
9.7 10.7 12.3 7.2 7.5 6.4 11.5
Catalyst quantity = 0.1 g; Al/Ti = 100 and 50 for Mg-9FM-EtOH and MgEtOH, respectively.
method was employed to calculate the surface area of the active catalysts. Ti-9FM-EtOH-1 shows an extraordinary high surface area of 410 m2 g−1, which is far higher than any commercial polyolefin catalyst. The average pore diameter was calculated and found to be 69 nm with a pore volume of 0.711 cc g−1. However, Ti-9FM-EtOH-2 exhibits a surface area of 75 m2 g−1 and an average pore diameter of 50 nm with a pore volume of 0.095 cc g−1. The high surface area of Ti-9FM-EtOH-1, compared to Ti-9FM-EtOH-2, could be partly due to the removal of the bulky 9FM organic moieties due to the thorough washing step in the synthesis of the active catalyst. The former active catalyst shows a type-IV isotherm with a H3 hysteresis loop and the latter active catalyst shows a type-II isotherm. Macropore contribution in the active catalysts is evident from the pore size distribution in both catalysts. 3.3.
Ethylene polymerization
Ethylene polymerization reactions have been carried out with both active catalysts derived from Mg-9FM-EtOH. Co-catalysts with three different alkyl groups, namely, methyl, ethyl, and isobutyl (R3Al; R = CH3, –CH2CH3, and –CH2CH(CH3)2) have been employed to activate the catalytic active sites. For each cocatalyst, reactions were carried out at 5 atm pressure and 75 °C. Average results were taken after carrying out three sets of polymerizations for each set of conditions. Activity results are shown in Table 1. Three important points are worth highlighting and given below: (1) the overall polymerization activity of Ti-9FM-EtOH-1 is higher than that of Ti-9FM-EtOH-2. (2) Among the ethylene polymerization reactions, entry 3 (Table 1) shows the best activity of Ti-9FM-EtOH-1, when (iBu)3Al was used as co-catalyst. This activity is ∼4.5 times higher than the Z–N catalyst based on MgCl2·6EtOH adduct (Table 1, entry 7). (3) The overall MWD of the polymer obtained using Ti-9FM-EtOH-2 was better than Ti-9FM-EtOH-1 and entry 6 (Table 1) shows the lowest MWD of polyethylene among all the polymers obtained. The overall higher activity of the Ti-9FM-EtOH-1 catalyst could be due to the higher surface area (410 m2 g−1) compared with that of Ti-9FM-EtOH-2 (75 m2 g−1) and many other commercial catalysts.6 The highest polymerization activity for the combination of Ti-9FM-EtOH-1 with iBu3Al is attributed to the
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highly porous nature associated with the active catalyst. A relatively large number of pores and comparatively lower lability of iBu3Al (compared with other alkyl aluminium used) might have influenced the reduction of the TiClx species in the pores of the active catalyst to Ti3+ and thereby led to the highest activity.38 The overall lower MWD of the PE for the Ti9FM-EtOH-2 catalyst could be due to the influence of 9-FM as IED present on the surfaces of active catalyst. Absence of 9FM and the high porosity associated with Ti-9FM-EtOH-1 makes all sites equally available for polymerization and hence the MWD is also large. In contrast, the availability of 9FM regulates the polymerization within a regime and hence the MWD is significantly reduced, which emphasizes the role of 9FM as an IED for polyolefin synthesis.
4.
Conclusions
A single phase Mg-9FM-EtOH molecular adduct has been synthesized and subjected to detailed spectroscopic and structural investigations. In order to determine the effect of 9-FM molecules as an IED, the active catalysts were synthesized conventionally (Ti-9FM-EtOH-1) and with the predominant fixing of TiCl4 over the support (Ti-9FM-EtOH-2); both catalysts were subjected to detailed investigations. The textural properties of the active catalysts shows a higher surface area for the conventionally synthesized catalyst than the unconventional. Ethylene polymerization reactions were measured with different cocatalysts and showed that iBu3Al was the best cocatalyst for the above supported Z–N catalytic system. The polymerization activity results suggest that the conventionally synthesized catalyst exhibited ∼4.5 times better activity than the Z–N catalyst prepared from the MgEtOH adduct, and still better than merely TiCl4 fixed over the Mg-9FM-EtOH support. However, the latter catalytic system (Ti-9FM-EtOH-2) shows the lowest MWD of PE obtained, confirming the role of 9-FM as and IED in the catalytic system. Detailed studies confirm that the lower lability of iBu3Al and the contribution of macropores are the reason for the conventionally synthesized catalyst giving a higher polymerization activity. The XRD results exhibit the (003) facets of α-MgCl2 (δ-MgCl2) on the merely fixing TiCl4 (conventionally) prepared catalyst with a significant (low) crys-
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tallinity and large (small) amount of 9FM on the surfaces was also a reason for the different polymerization activities.
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Acknowledgements We thank Dr S. Bhaduri for many suggestions and long-standing collaboration. ESG thanks the CSIR, New Delhi, for the senior research fellowship. Partial financial support from the CSIR, New Delhi for CSC0404 project is acknowledged.
Notes and references 1 K. Ziegler, E. Holzkamp, H. Breil and H. Martin, Angew. Chem., 1955, 67, 541. 2 K. Ziegler, E. Holzkamp, H. Breil and H. Martin, Angew. Chem., 1955, 67, 426. 3 G. Natta, J. Polym. Sci., 1955, 16, 143. 4 N. Kashiwa, H. Fujimura and Y. Tokuzumi, JP Patent 1031698, 1968. 5 N. Kashiwa, Polym. J., 1980, 12, 603. 6 N. Kashiwa, J. Polym. Sci., Part A: Polym. Chem., 2004, 42, 1. 7 N. Kashiwa and J. Yoshitake, Makromol. Chem. Rapid Commun., 1983, 4, 41. 8 J. Boor Jr., Ziegler-Natta Catalysts and Polymerization, Academic Press, New York, 1979. 9 F. Auriemma and C. De Rosa, Chem. Mater., 2007, 19, 5803. 10 M. Rerraris, F. Rosati, S. Parodi, E. Giannetti, G. Motroni and E. Albizzati, U.S. Pat 4,399,054, 1983. 11 P. Sobota, Coord. Chem. Rev., 2004, 248, 1047. 12 (a) C. R. Tewell, F. Malizia, J. W. Ager III and G. A. Somorjai, J. Phys. Chem. B, 2002, 106, 2946; (b) S. H. Kim and G. A. Somorjai, J. Phys. Chem. B, 2002, 106, 1386. 13 A. Andoni, J. C. Chadwick, S. Milani, J. W. Niemantsverdriet and P. Thüne, J. Catal., 2007, 247, 129. 14 P. Sozzani, S. Bracco, A. Comotti, R. Simonutti and I. Camurati, J. Am. Chem. Soc., 2003, 125, 12881. 15 M. Boero, M. Parrinello and K. Terakura, Surf. Sci., 1999, 438, 1. 16 D. V. Stukalov and V. A. Zakharov, J. Phys. Chem. C, 2009, 113, 21376. 17 M. Seth, P. M. Margl and T. Ziegler, Macromolecules, 2002, 35, 7815. 18 G. Monaco, M. Toto, G. Guerra, P. Corradini and L. Cavallo, Macromolecules, 2000, 33, 8953. 19 H. Mori, M. Sawada, T. Higuchi, K. Hasebe, N. Otsuka and M. Terano, Macromol. Rapid Commun., 1999, 20, 245. 20 E. Groppo, K. Seenivasan and C. Barzan, Catal. Sci. Technol., 2013, 3, 858. 21 (a) F. Malizia, A. Fait and G. Cruciani, Chem. - Eur. J., 2011, 17, 13892; (b) V. Subramanian, E. S. Gnanakumar, D.-W. Jeong, W.-B. Han, C. S. Gopinath and H. S. Roh, Chem. Commun., 2013, 49, 11257.
9150 | Dalton Trans., 2014, 43, 9143–9151
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22 E. Magni and G. A. Somorjai, Surf. Sci., 1996, 345, 1. 23 S. H. Kim, R. T. Craig and G. A. Somorjai, Langmuir, 2000, 16, 9414. 24 H. Mori, K. Hasebe and M. Terano, Macromol. Chem. Phys., 1998, 199, 2709. 25 E. Puhakka, T. T. Pakkanen and T. A. Pakkanen, J. Phys. Chem. A, 1997, 101, 6063. 26 D. Xu, Z. Liu, J. Zhao, S. Han and Y. Hu, Macromol. Rapid Commun., 2000, 21, 1046. 27 K. S. Thushara, E. S. Gnanakumar, R. Mathew, R. K. Jha, T. G. Ajithkumar, P. R. Rajamohanan, K. Sarma, S. Padmanabhan, S. Bhaduri and C. S. Gopinath, J. Phys. Chem. C, 2011, 115, 1952. 28 V. D. Noto, A. Marigo, M. Viviani, C. Marega, S. Bresadola and R. Zannetti, Makromol. Chem., 1992, 193, 123. 29 J. C. J. Bart, J. Mater. Sci., 1995, 30, 2809. 30 A. Andoni, J. C. Chadwick, J. W. Niemantsverdriet and P. Thüne, J. Catal., 2008, 257, 81. 31 M. Terano, M. Saito and T. Kataoka, Makromol. Chem. Rapid Commun., 1992, 13, 103. 32 G. Valle, G. Baruzzi, G. Paganetto, G. Depaoli, R. Zannetti and R. Marigo, Inorg. Chim. Acta, 1989, 156, 157. 33 V. D. Noto, S. Bresadola, R. Zannetti and M. Z. Viviani, Kristallografiya, 1993, 204, 263. 34 K. S. Thushara, R. Mathew, T. G. Ajithkumar, P. R. Rajamohanan, S. Bhaduri and C. S. Gopinath, J. Phys. Chem. C, 2009, 113, 8556. 35 E. S. Gnanakumar, K. S. Thushara, D. S. Bhange, R. Mathew, T. G. Ajithkumar, P. R. Rajamohanan, S. Bhaduri and C. S. Gopinath, Dalton Trans., 2011, 40, 10936. 36 (a) E. S. Gnanakumar, R. R. Gowda, S. Kunjir, T. G. Ajithkumar, P. R. Rajamohanan, D. Chakraborty and C. S. Gopinath, ACS Catal., 2013, 3, 303; (b) C. S. Gopinath and F. Zaera, J. Catal., 2001, 200, 270. 37 (a) K. S. Thushara, E. S. Gnanakumar, R. Mathew, T. G. Ajithkumar, P. R. Rajamohanan, S. Bhaduri and C. S. Gopinath, Dalton Trans., 2012, 41, 11311; (b) K. S. Thushara, T. G. Ajithkumar, P. R. Rajamohanan and C. S. Gopinath, J. Phys. Chem. A, 2014, 118, 1213. 38 E. S. Gnanakumar, K. S. Thushara, R. R. Gowda, S. K. Raman, T. G. Ajithkumar, P. R. Rajamohanan, D. Chakraborty and C. S. Gopinath, J. Phys. Chem. C, 2012, 116, 24115. 39 K. S. Thushara, T. G. Ajithkumar, P. R. Rajamohanan and C. S. Gopinath, Appl. Catal., A, 2014, 469, 267. 40 H. G. Alt and E. Samuel, Chem. Soc. Rev., 1998, 27, 323. 41 (a) T. Mathew, M. Vijayaraj, S. Pai, B. B. Tope, S. G. Hedge, B. S. Rao and C. S. Gopinath, J. Catal., 2004, 227, 175; (b) M. Mapa and C. S. Gopinath, Chem. Mater., 2009, 21, 351. 42 K. Sivaranjani, A. Verma and C. S. Gopinath, Green Chem., 2012, 14, 461. 43 T. Mathew, K. Sivaranjani, E. S. Gnanakumar, Y. Yamada, T. Kobayashi and C. S. Gopinath, J. Mater. Chem., 2012, 22, 13484.
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C. S. Gopinath, S. Bhaduri and G. K. Lahiri, J. Phys. Chem. C, 2008, 112, 9428. 46 H. L. Ronkko, H. Knuutilla, P. Denifl, T. Leinonen and T. Venalainen, J. Mol. Catal. A: Chem., 2007, 278, 127.
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44 V. Sreeja, T. S. Smitha, D. Nand, T. G. Ajithkumar and P. A. Joy, J. Phys. Chem. C, 2008, 112, 14737. 45 (a) A. V. Murugan, A. K. Viswanath, C. S. Gopinath and K. Vijayamohanan, J. Appl. Phys., 2006, 100, 074319; (b) N. Maity, P. R. Rajamohanan, S. Ganapathy,
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