Subscriber access provided by UNIV OF ARIZONA
Article
Tuning the Ti3+ and Al3+ synergy in a Al2O3/TiClx catalyst to modulate the grade of the produced polyethylene Alessandro Piovano, Elena Morra, Mario Chiesa, and Elena Groppo ACS Catal., Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 12, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
Tuning the Ti3+ and Al3+ synergy in a Al2O3/TiClx catalyst to modulate the grade of the produced polyethylene Alessandro Piovano, Elena Morra, Mario Chiesa,* Elena Groppo* Department of Chemistry, INSTM and NIS Centre, University of Torino, via Giuria 7, 10125 Torino (Italy). ABSTRACT: A multi-technique approach (comprising in situ FT-IR, DR UV-Vis and advanced EPR spectroscopies, coupled with DSC analysis) was employed to investigate the local structure and the activation of a heterogeneous ethylene polymerization catalyst obtained by grafting TiCl4 on a transitional alumina. The activation procedure was found to affect the electronic structure and the coordinative environment of the reduced Ti sites, as well as their interaction with the Al3+ sites, measured in terms of spin density transfer from Ti3+ to nearby Al3+ ions. It was found that the extent of interaction between the two metal sites correlates with the microstructures of the obtained polyethylene. Tuning the synergy between the Ti3+ and the Al3+ Lewis acid sites is proposed as an efficient way to modulate the polyethylene microstructure, switching from a high density polyethylene to a highly branched polyethylene.
KEYWORDS: olefin polymerization; tandem catalysis; EPR spectroscopy; operando FT-IR; DR UV-Vis spectroscopy.
1. INTRODUCTION Industrial research in polyolefins has long been oriented towards the development of ethylene polymerization catalysts for Linear Low Density Polyethylene (LLDPE) production using ethylene as the only feed. This process presents significant commercial advantages with respect to the traditional catalytic route to LLDPE, which requires the co-feeding of an α-olefin with ethylene.1 The great interest in this field relies on the advantageous disproportion between the cheapness of the ethylene monomer in comparison with the α-olefins and the high added value of the produced LLDPE, which has a large market share and broad applications.2 The largely employed “in situ branching” mechanism requires a bi-functional catalyst, where the first function is competent in the oligomerization of ethylene to short α-olefins, and the second is able to co-polymerize ethylene with the in-situ produced αolefins.1,3-8 Industrial bifunctional heterogeneous catalysts for LLDPE production are often obtained by modifying a catalyst highly efficient in ethylene polymerization (such as the traditional Phillips or Ziegler-Natta type catalysts) with an external modifier or activator (often belonging to the group of metal alkyls).9,10 However, the exact role of the activators and the structure of the modified catalysts have been rarely investigated in details. Recent works on the Phillips catalyst do suggest that activators like silanes or aluminum alkyls convert a fraction of the ethylene polymerization sites into ethylene oligomerization sites, having peculiar structural and electronic properties that favor chain termination over propagation.1,11-13 We recently reported on the synthesis of an original alumina-supported Ziegler-Natta catalyst, producing branched polyethylene using ethylene as the only feed, which does not require the use of any activator.14 The cat-
alyst exploits the synergy between supported TiClx species and proximal, uncoordinated, Al3+ Lewis acid sites at the surface of a δ-alumina. The acidic properties of alumina are well known since a long time.15 In the specific field of olefin conversion, alumina has been used for both oligomerization and isomerization processes.16,17 On one hand, Lewis acidity is considered the main responsible for olefin oligomerization,18 that occurs through a carbocationic mechanism.19 On the other hand, both Lewis and Brønsted acid sites are involved in olefin isomerization.2024 Clearly, a fine tuning of the surface properties of alumina might allow converting olefins to specific products. Our Al2O3/TiClx catalyst was obtained through thermal reduction of the Al2O3/TiCl4 pre-catalyst in H2, leading to a completely dehydroxylated and chlorinated alumina surface exposing highly acidic Al3+ sites and reduced TiClx species. The Al3+ Lewis acid sites oligomerize ethylene to branched oligomers via a carbocationic mechanism and activate the reduced titanium chloride species for copolymerizing the in situ produced branched oligomers with ethylene.14 In the present work we explore an alternative strategy to activate the Al2O3/TiCl4 pre-catalyst, that is the use of AlEt3, a classical co-catalyst in Ziegler-Natta catalysis. The formation of the catalyst is investigated by means of a series of complementary spectroscopic techniques aimed at identifying the electronic structure and the coordinative environment of the reduced Ti sites and their interaction with the Al3+ sites. The spectroscopic results are discussed in comparison to those recently reported by some of us14 for the H2-reduced Al2O3/TiClx catalyst. No cooperation between the reduced Ti sites and the Al3+ sites was detected for the catalyst activated by TEA, while a strong interaction is observed for the catalyst activated by H2. The two catalysts efficiently polymerize ethylene to HDPE
ACS Paragon Plus Environment
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
and LLDPE, respectively, indicating that tuning the synergy between the reduced Ti sites and the Al3+ sites is an effective method to change the grade of the produced polymer.
2. EXPERIMENTAL 2.1 Catalysts synthesis The synthesis procedure for the δ-Al2O3-600/TiCl4 precatalyst was discussed in our previous work.14 Briefly, a fumed δ-Al2O3 (Aeroxide Alu C, from Evonik-Degussa), having an average primary particle size of 13 nm and a specific surface area of 100 m2g–1, was used as support and treated in dynamic vacuum at 600 °C for prolonged time. The activation procedure significantly reduces the amount of surface OH groups to an approximate value of 4 OH/nm2.25 A controlled titanation of the activated alumina was achieved by dosing vapors of TiCl4 (pure, from Sigma-Aldrich) at room temperature. The final amount of titanium in the pre-catalyst is expected to be about 2 wt%, by considering the involvement of all the surface OH groups in grafting the TiClx species, as demonstrated by FT-IR spectroscopy. The activation of the δ-Al2O3600/TiCl4 pre-catalyst was accomplished according to two different procedures: a) by treatment in H2 at 400 °C, followed by degassing at the same temperature, as discussed in our previous work,14 or b) through reaction with triethylaluminum (TEA, pure, from Sigma-Aldrich) vapors at room temperature. Hereafter we will indicate the two catalysts as δ-Al2O3-600/TiCl4/H2-400 and δ-Al2O3-600/TiCl4/TEA respectively, where the temperature of each treatment (if not room temperature) is reported as subscripts. All the synthesis steps were carried out directly inside the cells used for the spectroscopic measurement or inside the quartz reactor adopted for the catalytic tests, in order to avoid catalyst poisoning. 2.2 Characterization techniques FT-IR spectroscopy. FT-IR spectra were collected at a resolution of 2 cm–1 with a Bruker Vertex70 instrument equipped with a MCT detector. The samples were measured in the form of thin self-supporting pellets (surface density ca. 10 mg cm–2) placed inside a quartz cell with two KBr windows, which allows performing thermal treatments and measurements in the presence of gases, and can be interfaced with the spectrophotometer to monitor in situ the evolution of the spectra. Diffuse Reflectance (DR) UV-Vis-NIR spectroscopy. DR UV-Vis-NIR spectra were collected in diffuse reflectance mode with a Varian Cary5000 spectrophotometer, equipped for reflectance measurements. The samples were measured in the form of thick pellets (surface density ca. 150 mg cm–2) placed in a cell equipped with an optical window (quartz suprasil), which allows performing thermal treatments and measurements in the presence of gases. All the spectra were collected in reflectance mode and successively converted into Kubelka-Munk units. Electron Paramagnetic Resonance. X-band continuouswave (CW)-EPR spectra were detected with a Bruker EMX spectrometer (microwave frequency 9.75 GHz) equipped with a cylindrical cavity. A microwave power of 1 mW, a modulation amplitude of 0.2 mT and a modulation fre-
Page 2 of 7
quency of 100 kHz were used. Pulse EPR experiments were performed on an ELEXYS 580 and SuperQ FT-EPR Bruker spectrometer operating at Q-band frequency (34 GHz), equipped with a liquid-helium cryostat from Oxford Inc. The magnetic field was measured by means of a Bruker ER035 M NMR gauss meter. Hyperfine Sublevel Correlation (HYSCORE)26 experiments were carried out with the pulse sequence π/2–τ–π/2–t1–π–t2–π/2–τ–echo, applying a eight-step phase cycle for eliminating unwanted echoes. Microwave pulse lengths tπ/2 = 16 ns, tπ = 32 ns, and a shot repetition rate of 0.5 kHz were used. The t1 and t2 time intervals were incremented in steps of 8 ns, starting from 200 ns giving a data matrix of 250 x 250 points. The time traces of the HYSCORE spectra were baseline corrected with a third-order polynomial, apodized with a Hamming window and zero filled. After two-dimensional Fourier transformation, the absolute value spectra were calculated. Spectra with different τ values were recorded, which are specified in the figure captions. The spectra were added for the different τ values in order to eliminate blind-spot effects. All of the EPR spectra were simulated employing the Easyspin package.27 Differential Scanning Calorimetry. DSC measurements on the obtained polymers were performed with a TA Q200 instrument. The polymer was extracted by dissolving the corresponding catalyst with fluoridric acid, and successively removing the fraction soluble in heptane at 50 °C. Each DSC measurement consists of two consecutive heating and cooling temperature ramps in the 50-150 °C range at a heating rate of 2°C/min. The values for the polymer melting temperature (Tm) were taken from the second heating ramp, so that the measures were not affected by the thermal history of the polymer.
3. RESULTS AND DISCUSSION 3.1. Investigation at a molecular level of the catalyst formation Figure 1A shows the FT-IR spectra of the activated δAl2O3-600 (spectrum 1), of the δ-Al2O3-600/TiCl4 pre-catalyst (spectrum 2) and of the δ-Al2O3-600/TiCl4/TEA and δAl2O3-600/TiCl4/H2-400 catalysts (spectra 3a and 3b, respectively). Spectra 1, 2 and 3b have been commented in our previous work,14 and only the main features are summarized in the following. Activated δ-Al2O3-600 exposes surface OH groups of different acidity (main absorption bands at 3733, 3776, 3729 and 3693 cm-1), as widely documented in the literature.25,28-30 A strengthening of the Lewis acidity accompanies the dehydroxylation of the surface,31-35 which may thus affect the whole activity of the supported catalytic sites.36 All the surface hydroxy groups are involved in the successive reaction with TiCl4, as demonstrated by the FT-IR spectrum of δ-Al2O3-600/TiCl4 (spectrum 2). Indeed, in the ν(OH) vibrational region, only a very broad band centered at 3470 cm–1 is observed. This band was previously assigned14 to the ν(OH) vibration of [–O−H⋅⋅⋅Cl−Al≡] species, which originate from the chemisorption of HCl (released upon the reaction of TiCl4 with the alumina hydroxy groups)37-42 onto the Al3+–O2– acid-base couples at the surface.
ACS Paragon Plus Environment
Page 3 of 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis species on the catalyst are adsorbed as monomers onto alumina surface.47 Additional details on the electronic properties of the Ti sites at each step of the catalyst synthesis are revealed by DR UV-Vis. Figure 2A shows the DR UV-Vis spectra of activated δ-Al2O3-600 (spectrum 1), of the δ-Al2O3-600/TiCl4 pre-catalyst (spectrum 2) and of the two catalysts (spectra 3a and 3b, respectively). The DR UV-Vis spectrum of δAl2O3-600/TiCl4 (spectrum 2) is dominated by three intense bands at about 42000, 35000 and 28000 cm–1, which were previously assigned14 to ligand (either Cl or O) to metal (either 6-fold or 4-fold Ti4+ sites) charge-transfer transitions, as follows: Cl Ti4+6c at 28000 cm–1, Cl Ti4+4c and O Ti4+6c at 35000 cm–1, and O Ti4+4c at 42000 cm–1).4850 A color change is observed upon activation: the δ-Al2O3600/TiCl4/H2-400 catalyst was light blue, while the δ-Al2O3600/TiCl4/TEA catalyst was dark brownish, a clear indi
Figure 1. Part A) FT-IR spectra of δ-Al2O3-600 (spectrum 1), δAl2O3-600/TiCl4 pre-catalyst (spectrum 2) and δ-Al2O3600/TiCl4/TEA and δ-Al2O3-600/TiCl4/H2-400 catalysts (spectra 3a and 3b, respectively). Part B) FT-IR spectra of δ-Al2O3600/TiCl4/H2-400 and δ-Al2O3-600/TiCl4/TEA catalysts after subtraction of spectrum 2, compared to the FT-IR spectrum of liquid TEA collected in ATR mode. Spectra 1, 2 and 3b are the same discussed as in Ref. 14, and reproduced here for sake of comparison with spectrum 3a.
The fate of the grafted TiClx species as well as of the chemisorbed [–O−H⋅⋅⋅Cl−Al≡] species in the final catalysts depends on the method employed for the activation. Thermal reduction of δ-Al2O3-600/TiCl4 in H2 at 400 °C leads to the complete disappearance of the chemisorbed [–O−H⋅⋅⋅Cl−Al≡] species and of any residual surface hydroxy groups. Indeed, the FT-IR spectrum of δ-Al2O3600/TiCl4/H2-400 (spectrum 3b in Figure 1A) is completely flat in the ν(OH) vibrational region. This evidence implies an extensive chlorination of the alumina surface, that may easily spread across the sub-surface layers,43-45 and the presence of highly exposed Al3+ sites, which turned out to play a fundamental role in ethylene conversion as strong Lewis acid sites.14 In contrast, in the FT-IR spectrum of δAl2O3-600/TiCl4/TEA catalyst (spectrum 3a in Figure 1A) the absorption band due to the chemisorbed [– O−H⋅⋅⋅Cl−Al≡] species is only slightly affected by reaction with TEA, meaning that TEA mainly reacts with the grafted TiClx species, without substantially affecting the chemisorbed HCl. The absorption bands in the ν(CHx) (2950 – 2800 cm-1) and δ(CHx) (1500 – 1350 cm-1) vibrational regions account for the coexistence of alkylated TiClxRy species, reaction by-products (such as AlRxCly) and unreacted AlR3, although a distinction among all the different species is not possible. Figure 1B compares the FT-IR spectrum of δ-Al2O3-600/TiCl4/TEA catalyst (after subtraction of the spectrum of δ-Al2O3-600/TiCl4), with that of liquid TEA recorded in ATR mode. While in the liquid phase TEA is known to be stable as a dimer,46 the slight blueshift of the ν(CHx) absorption bands suggests that TEA
Figure 2. Part A) DR UV-Vis spectra of δ-Al2O3-600 (spectrum 1), δ-Al2O3-600/TiCl4 pre-catalyst (spectrum 2) and δ-Al2O3600/TiCl4/TEA and δ-Al2O3-600/TiCl4/H2-400 catalysts (spectra 3a and 3b, respectively). Parts B) experimental (bold line) and simulated (thin line) X-band CW EPR spectra of the δAl2O3-600/TiCl4/TEA (spectrum 3a) and δ-Al2O3-600/TiCl4/ H2400 (spectrum 3b) catalysts. The EPR spectra were recorded at 77 K. Spectra 1, 2 and 3b are the same discussed as in Ref. 14, and reproduced here for sake of comparison with spectrum 3a.
cation that the reduced Ti sites experience a different local environment in the two cases. The spectrum of the δAl2O3-600/TiCl4/H2-400 catalyst (spectrum 3b in Figure 2A) is characterized by a well-defined absorption band (having a d-d character) centered at 13000 cm–1, which is the unequivocal proof of reduced Ti3+ species in a 6-fold coordination, having both Cl and O as ligands.14 In contrast, the spectrum of δ-Al2O3-600/TiCl4/TEA (spectrum 3a in Figure 2A) is dominated by a very intense and broad band centered around 22800 cm–1, with a pronounced tail at low wavenumbers. This band, which is much more intense than an usual d-d transition absorption band, is attributed to an inter-site d-d transition enhanced by a partial charge transfer character, involving two Ti3+ sites bridged by a Cl– ligand, as previously reported for bulk TiCl3 polymorphs.51 This can be considered as a proof for the formation of reduced TiClx clusters induced by TEA, as previously observed for a similar Ziegler-Natta like catalyst supported on silica.52 The broadness of the band indicates the presence of a variety of reduced Ti sites, characterized by a slightly different local environment. The extended tail is compatible with the presence of a fraction
ACS Paragon Plus Environment
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 7
of isolated Ti3+ sites, similar to those present in the δAl2O3-600/TiCl4/H2-400 catalyst. A detailed characterization of the paramagnetic Ti3+ (3d ) sites in the two catalysts was obtained by means of combined CW and pulse EPR experiments, which allow revealing fine details concerned with the electronic and geometrical structure of such species.53,54 The CW-X-band EPR spectrum of the TEA reduced catalyst is reported in Figure 2B (spectrum 3a) and is characterized by the typical powder pattern of Ti3+ species in a pseudo octahedral coordination.53-55 Two Ti3+ species characterized by similar g values account for 35% and 65% of the experimental signal (Table 1). These g values agree with typical values reported for MgCl2-based Ziegler Natta catalysts activated with alkyl aluminium compounds.14,56 In particular, gvalues reaching 1.97 have been suggested to be associated to surface-alkylated Ti3+ species in supported titanium– magnesium catalysts activated by AlR3.56 In Figure 2B comparison is set to the EPR spectrum of δ-Al2O3600/TiCl4/H2-400 catalyst (spectrum 3b), which reveals that the two spectra, although similar, are characterized by slightly different g values and an approximately reverse percentage composition (Table 1), suggesting a different environment of the Ti3+ species in the two cases. 1
Table 1. g-matrix components for Ti3+ species in δAl2O3-600/TiCl4/TEA, δ-Al2O3-600/TiCl4/H2-400. Data for the δ-Al2O3-600/TiCl4/H2-400 system are taken from Ref. 14. Catalyst δ-Al2O3-600/ TiCl4/TEA
δ-Al2O3-600/
Ti3+
g1
(1)
1.977 0.003
±
1.945 0.005
(2)
1.955 0.003
±
(1)
1.963 0.003
(2)
1.942 0.003
TiCl4/H2400
g3
(%)
±
1.90 ± 0.01
35
1.928 0.005
±
1.88 ± 0.01
65
±
1.948 0.005
±
1.89 ± 0.01
80
±
1.940 0.005
±
1.88 ± 0.01
20
g2
27
Such differences can be effectively probed by means of HYSCORE experiments, which allow detecting hyperfine interactions - with sub-MHz resolution - of Ti3+ paramagnetic species with nearby magnetically active nuclei. In particular, the hyperfine interaction with 27Al nuclei (I=5/2) is particularly important in this context as it provides direct insights into the proximity and degree of chemical interaction between the two active metal sites. The Q band HYSCORE spectra recorded for the δ-Al2O3600/TiCl4/TEA catalyst at different magnetic field settings are shown in Figure 3A and compared with spectra reported in Ref. 14 for the δ-Al2O3-
Figure 3. Experimental (blue) and simulated (red) Al Qband HYSCORE spectra of δ-Al2O3/TiCl4/TEA (part A) and δAl2O3/TiCl4/H2-400 (part B) catalysts. The magnetic field settings at which the HYSCORE spectra were recorded are indicated in the corresponding ESE spectra (top). The spectra reported in part B have been already discussed in Ref. 14, but reproduced here for sake of comparison with the spectra reported in part A. 600/TiCl4/H2-400 system (Figure 3B). Both series of spectra are characterized by signals centered at (+13.42, +13.42) MHz, corresponding to the 27Al nuclear Larmor frequency, indicating the presence of nearby Al nuclei. However, those signals display a considerably different extent of the HYSCORE correlation pattern for the two catalysts, which reflects the entity of the hyperfine coupling. For δ-Al2O3600/TiCl4/TEA (Figure 3A) a maximum ridge extension of about 6 MHz is observed, while for the H2 activated catalyst (Figure 3B) the maximum extension is of about 21 MHz. The extension of the ridge perpendicular to the diagonal corresponds to the maximum hyperfine coupling |2T+aiso| at a given observer position. Due to orientation
ACS Paragon Plus Environment
Page 5 of 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
selection, only part of the correlation pattern may be observed, therefore, experiments at different magnetic field settings were performed. The resulting spectra (Figure 3 a, b and c) show only little orientation dependence, indicating that the hyperfine coupling is dominated by the isotropic Fermi contact term. Computer simulation of the HYSCORE spectra (red patterns in Figure 3) indicate that the ridge extension of the δ-Al2O3-600/TiCl4/TEA catalyst can be reproduced assuming an aiso values of 6 ± 1 MHz, and a dipolar coupling ([–T –T +2T]) in the order of T = 2 ± 0.5 MHz. Considering the value of a0 = 3367.76 MHz for unit spin density on the 27Al 3s orbital,57 the corresponding spin density in the Al 3s orbital is ca. 0.18%. The same analysis for the δ-Al2O3-600/TiCl4/H2-400 catalyst indicates a spin density transfer on the 27Al 3s orbital of the order of 0.6%.14 Such spin density transfer can be taken as a direct measure of the chemical interaction between the two metallic sites. This not only reflects the proximity of the two metal centers, but is also a faithful reporter of the way in which the two metal cations are connected through a chemical bond. The amount of spin density transfer is then expected to depend markedly on both bond angle and distance of the Ti-L-Al fragment (with L either Cl or O ions), making the Fermi contact term a sensitive structural probe. The lower spin density transfer between Ti3+ and Al3+ sites observed upon TEA activation indicates a small degree of interaction between the two metal sites, while a strong interaction is present in the case of the δ-Al2O3600/TiCl4/H2-400 catalyst. In this last case, Al ions are uniquely provided by the alumina, indicating that supported TiClx molecular fragments experience a strong interaction with the support after the pre-catalyst activation. In contrast, the reaction with the alkyl aluminium activator seems to induce a substantial reorganization of the pre-catalyst. Surprisingly, although for δ-Al2O3600/TiCl4/TEA the probed Al sites are provided both by the Al2O3 support and the Al-alkyl activator, the extent of the Ti3+-Al3+ interaction is weak in any case. We stress that not only the 27Al hyperfine coupling is much smaller, but also the overall intensity of the 27Al HYSCORE spectrum is significantly lower than in the previous case, despite the higher intensity of the EPR spectrum of the TEA activated sample. A similar behavior was observed in industrial Ziegler-Natta catalysts.58 This suggests that the structure of the TiClx chemisorbed layer is drastically altered by the reaction with TEA, as also indicated by the absorption band in the DR UV-Vis spectrum associated to the formation of TiClx clustered species. In other words, 27Al HYSCORE experiments allow quantifying the degree of chemical interaction between Al3+ and Ti3+ sites and reveal a substantial decrease of such interaction in δ-Al2O3600/TiCl4/TEA with respect to δ-Al2O3-600/TiCl4/H2-400. In the following, we discuss the impact of this state of affairs on the reactivity of the two catalysts and the final polymer microstructure. 3.2. Performances in gas-phase ethylene polymerization Both the catalysts turned out to be active towards gasphase ethylene polymerization, even in very mild condi-
tions (25 °C, PC2H4 = 100 mbar). Ethylene polymerization was followed by means of operando FT-IR spectroscopy, with the aim to monitor the initial steps of the reaction, while the functional and thermal properties of the polymers obtained after prolonged polymerization time were analyzed by FT-IR spectroscopy in ATR mode and by DSC. Figure 4 shows the evolution of the FT-IR spectra in the ν(CHx) and δ(CHx) regions collected during ethylene reaction over the two catalysts. Already at a first glance, it is evident that the two catalysts have a different reactivity. Upon ethylene reaction over the δ-Al2O3-600/TiCl4/TEA catalyst (Figure 4A), the ν(CHx) region is dominated by only two sharp absorption bands at 2919 and 2851 cm–1, which are assigned to νasym(CH2) and νsym(CH2), respectively. Even in the very first spectrum, the position of the two
Figure 4. Evolution of the FT-IR spectra in the ν(CHx) and δ(CHx) regions during the reaction of C2H4 (25 °C, PC2H4 = 100 mbar) over δ-Al2O3-600/TiCl4/TEA (part A) and δ-Al2O3600/TiCl4/H2-400 catalysts (part B). The spectra are reported after subtracting that of the activated catalyst. Dotted spectra (vertically translated for clarity) refer to the ATR-IR spectra of the polymers after removal of the catalysts and of the lower molecular weight fractions. Part C) differential scanning calorimetry (DSC) analysis of the polymers produced by δAl2O3-600/TiCl4/TEA (curves 1) and δ-Al2O3-600/TiCl4/H2-400 (curves 2) catalysts, after dissolving the catalysts in HF. The operando spectra reported in part B have been already discussed in Ref. 14, but reproduced here for sake of comparison
ACS Paragon Plus Environment
ACS Catalysis with those in part A.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
bands already coincide with the values reported for “infinite” polymeric chains, indicating that the reaction is so fast that it is not possible to witness the conformational disorder typical of the first short polymeric chains.59 In the δ(CH2) spectral region two absorption bands are observed at 1472 and 1463 cm–1, which are related to the crystalline and amorphous phases of HDPE, respectively.60 Since the former band is much more intense than the latter, it can be stated that δ-Al2O3-600/TiCl4/TEA mostly catalyzes the production of a highly crystalline HDPE. This conclusion is validated by the analysis performed on the polymer extracted from the catalyst. The polymer melts above 130 °C (curve 1 in Figure 4C) and the FT-IR spectrum (dotted curve in Figure 4A) is characteristic of a HDPE.61-63 In contrast, the FT-IR spectra collected during ethylene reaction over the δ-Al2O3-600/TiCl4/H2-400 catalyst (Figure 4B) are much more complex. A careful analysis of the spectra reveal the simultaneous occurrence of a carbocationic ethylene oligomerization promoted by the Al3+ Lewis acid sites and of olefin polymerization via coordination catalyzed by the Ti3+ centers.14 In agreement with that mechanism, size-exclusion chromatography indicated that the produced polymer has a bimodal molecular weight distribution, where the lower molecular weight fraction is basically a polyethylene wax constituted by the branched oligomers produced through the carbocationic mechanism, and the high molecular weight fraction is characterized by long linear chains of polyethylene in which a few branched oligomers are occasionally enchained.14 This is confirmed by the analysis performed on the extracted polymer, after removing the catalyst and the lower molecular weight fraction. Indeed, the polymer melts at 128 °C (as determined by DSC, curve 2 in Figure 4C), and the corresponding FT-IR spectrum (dotted in Figure 4B) clearly shows the absorption bands due to the CH3 groups: both results are consistent with a branched polyethylene.62-64
4. CONCLUSIONS A multi-technique spectroscopic approach was used to study the structure and activation of a heterogeneous ethylene polymerization catalyst obtained by grafting TiCl4 over a transitional Al2O3 Particular emphasis was placed in understanding the crucial step of the precatalyst activation, the exact role of the activators and the structure of the activated catalysts. Activation by TEA induces the formation of small reduced TiClx clusters coexisting with chemisorbed HCl species, responsible for the production of a highly crystalline HDPE. In contrast, thermal activation in H2 leads to the formation of isolated Ti3+ sites (responsible for olefin polymerization via coordination) on an extensively chlorinated alumina surface exposing strongly acidic Al3+ sites (promoting a carbocationic ethylene oligomerization): the result is a branched polyethylene. Advanced EPR techniques clearly reveal the synergistic cooperation between Ti3+ and Al3+ sites, allowing to quantitatively probing the degree of interaction between the two metal sites as a function of the activation pathway. We observe different degrees of spin
Page 6 of 7
density transfer from Ti3+ to nearby Al3+ ions in the two catalysts, which correlate with the different polyethylene microstructures. Such a multi-technique approach opens interesting perspectives not only in the field of Ziegler-Natta catalysis, but also more in general in the investigation of tandem heterogeneous catalysts, where the rational introduction of a second metal offers a range of new synthetic possibilities towards modulating and optimizing catalytic performances.
AUTHOR INFORMATION Corresponding Authors * E-mail:
[email protected] * E-mail:
[email protected] Author Contributions All authors have given approval to the final version of the manuscript.
Funding Sources This work has been supported by the Progetto di Ateneo/CSP 2014 (Torino_call2014_L1_73).
ACKNOWLEDGMENT We are grateful to Adriano Zecchina and Silvia Bordiga for useful discussion.
REFERENCES (1) McDaniel, M. P. Adv. Catal. 2010, 53, 123-606. (2) Wittcoff, H. A.; Reuben, B. G.; Plotkin, J. S. Industrial Organic Chemicals: Third Edition; John Wiley and Sons, 2013. (3) Barnhart, R. W.; Bazan, G. C.; Mourey, T. J. Am. Chem. Soc. 1998, 120, 1082-1083. (4) Komon, Z. J. A.; Bazan, G. C. Macromol. Rapid Commun. 2001, 22, 467-478. (5) Frediani, M.; Fiel, C.; Kaminsky, W.; Bianchini, C.; Rosi, L. Macromol. Symp. 2006, 236, 124-133. (6) Frediani, M.; Bianchini, C.; Kaminsky, W. Kinetics A: Catal. 2006, 47, 207-212. (7) Climent, M. J.; Corma, A.; Iborra, S.; Sabater, M. J. ACS Catal. 2014, 4, 870-891. (8) Lohr, T. L.; Marks, T. J. Nat. Chem. 2015, 7, 477-482. (9) Yang, M.; Yan, W.; Hao, X.; Liu, B.; Wen, L.; Liu, P. Macromolecules 2009, 42, 905-907. (10) Vestberg, T.; Denifl, P.; Parkinson, M.; Wilén, C. E. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 351-358. (11) Barzan, C.; Groppo, E.; Quadrelli, E. A.; Monteil, V.; Bordiga, S. Phys.Chem.Chem.Phys. 2012, 14, 2239–2245. (12) Barzan, C.; Gianolio, D.; Groppo, E.; Lamberti, C.; Monteil, V.; Quadrelli, E. A.; Bordiga, S. Chem. Eur. J. 2013, 19, 17277-17282. (13) Cicmil, D.; Meeuwissen, J.; Vantomme, A.; Wang, J.; Van Ravenhorst, I. K.; Van Der Bij, H. E.; Muñoz-Murillo, A.; Weckhuysen, B. M. Angew.Chem. Int. Ed. 2015, 54, 13073-13079. (14) Piovano, A.; Thushara, K. S.; Morra, E.; Chiesa, M.; Groppo, E. Angew. Chem. Int. Ed. 2016, 55, 11203-11206. (15) Pines, H.; Haag, W. O. J. Am. Chem. Soc. 1960, 82, 2471-2483. (16) O'Connor, C. T.; Kojima, M. Catal. Today 1990, 6, 329-349. (17) Peereboom, M. J. Catal. 1984, 88, 37-42. (18) Fletcher, J. C. Q.; Kojima, M.; O'Connor, C. T. Appl. Catal. 1986, 28, 181-191. (19) Baird, M. C. Chem. Rev. 2000, 100, 1471-1478. (20) Corado, A.; Kiss, A.; Knözinger, H.; Müller, H. D. J. Catal. 1975, 37, 68-80. (21) Lunsford, J. H.; Zingery, L. W.; Rosynek, M. P. J. Catal. 1975, 38, 179-188. (22) Guisnet, M.; Lemberton, J. L.; Perot, G.; Maurel, R. J. Catal. 1977, 48, 166-176.
ACS Paragon Plus Environment
Page 7 of 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
(23) Houžvička, J.; Ponec, V. Appl. Catal. A: Gen 1996, 145, 95-109. (24) Trombetta, M.; Busca, G.; Rossini, S. A.; Piccoli, V.; Cornaro, U. J. Catal. 1997, 168, 334-348. (25) Knoezinger, H.; Ratnasamy, P. Cat. Rev. - Sci. Eng. 1978, 17, 3170. (26) Höfer, P.; Grupp, A.; Nebenführ, H.; Mehring, M. Chem. Phys. Lett. 1986, 132, 279-282. (27) Stoll, S.; Schweiger, A. J. Magn. Reson. 2006, 178, 42-55. (28) Tsyganenko, A. A.; Filimonov, V. N. J. Mol. Struct. 1973, 19, 579-589. (29) Digne, M.; Sautet, P.; Raybaud, P.; Euzen, P.; Toulhoat, H. J. Catal. 2004, 226, 54-68. (30) Busca, G. Catal. Today 2014, 226, 2-13. (31) Morterra, C.; Chiorino, A.; Ghiotti, G.; Garrone, E. J. Chem. Soc., Faraday Trans. 1 1979, 75, 271-288. (32) Morterra, C.; Magnacca, G. Catal. Today 1996, 27, 497-532. (33) Liu, X.; Truitt, R. E. J. Am. Chem. Soc. 1997, 119, 9856-9860. (34) Lundie, D. T.; McInroy, A. R.; Marshall, R.; Winfield, J. M.; Jones, P.; Dudman, C. C.; Parker, S. F.; Mitchell, C.; Lennon, D. J. Phys. Chem. B 2005, 109, 11592-11601. (35) Liu, X. J. Phys. Chem. C 2008, 112, 5066-5073. (36) Komeili, S.; Ravanchi, M. T.; Taeb, A. Sci. Iran. Trans. C 2016, 23, 1128. (37) Kinney, J. B.; Staley, R. H. J. Phys. Chem. 1983, 87, 3735-3740. (38) Haukka, S.; Lakomaa, E. L.; Jylha, O.; Vilhunen, J.; Hornytzkyj, S. Langmuir 1993, 9, 3497-3506. (39) Haukka, S.; Lakomaa, E. L.; Root, A. J. Phys. Chem. 1993, 97, 5085-5094. (40) Kytokivi, A.; Haukka, S. J. Phys. Chem. B 1997, 101, 10365-10372. (41) Seenivasan, K.; Gallo, E.; Piovano, A.; Vitillo, J. G.; Sommazzi, A.; Bordiga, S.; Lamberti, C.; Glatzel, P.; Groppo, E. Dalton Trans. 2013, 42, 12706-12713. (42) McInroy, A. R.; Lundie, D. T.; Winfield, J. M.; Dudman, C. C.; Jones, P.; Parker, S. F.; Lennon, D. Catal. Today 2006, 114, 403-411. (43) Krzywicki, A.; Marczewski, M. J. Chem. Soc. Faraday I 1980, 76, 1311-1322. (44) Kytökivi, A.; Lindblad, M.; Root, A. J. Chem. Soc. Faraday Trans. 1995, 91, 941-948.
(45) Khaleel, A.; Dellinger, B. Environ. Sci. Technol 2002, 36, 16201624. (46) Benn, R.; Janssen, E.; Lehmkuhl, H.; Rufínska, A. J. Organomet. Chem. 1987, 333, 155-168. (47) Kvisle, S.; Rytter, E. Spectrochim. Acta A 1984, 40, 939-951. (48) Seenivasan, K.; Sommazzi, A.; Bonino, F.; Bordiga, S.; Groppo, E. Chem. Eur. J 2011, 17, 8648-8656. (49) Jorgensen, C. K. In Orbitals in Atoms and Molecules; Academic Press: London and New York, 1962, p 80-100. (50) Jorgensen, C. K. Progr. Inorg. Chem. 1970, 12, 101-157. (51) Clark, R. J. H. J. Chem. Soc. 1964, 417-425. (52) Piovano, A.; Martino, G. A.; Barzan, C. Rend. Fis. Acc. Lincei 2016. (53) Morra, E.; Maurelli, S.; Chiesa, M.; Giamello, E. Top. Catal. 2015, 58, 783-795. (54) Morra, E.; Giamello, E.; Chiesa, M. J. Magn. Reson. 2017. (55) Morra, E.; Maurelli, S.; Chiesa, M.; Van Doorslaer, S. Phys. Chem. Chem. Phys. 2015, 17, 20853-20860. (56) Koshevoy, E. I.; Mikenas, T. B.; Zakharov, V. A.; Shubin, A. A.; Barabanov, A. A. J. Phys. Chem. C 2016, 120, 1121-1129. (57) Fitzpatrick, J. A. J.; Manby, F. R.; Western, C. M. J. Chem. Phys. 2005, 122. (58) Morra, E.; Giamello, E.; Van Doorslaer, S.; Antinucci, G.; D'Amore, M.; Busico, V.; Chiesa, M. Angew. Chem. Int. Ed. 2015, 54, 4857-4860. (59) Groppo, E.; Lamberti, C.; Bordiga, S.; Spoto, G.; Zecchina, A. J. Catal. 2006, 240, 172-181. (60) Chelazzi, D.; Ceppatelli, M.; Santoro, M.; Bini, R.; Schettino, V. Nat. Mater. 2004, 3, 470-475. (61) Pagès, P.; Carrasco, F.; Saurina, J.; Colom, X. J. Appl. Polym. Sci. 1996, 60, 153-159. (62) Fonseca, C. A.; Harrison, I. R. Thermochim. Acta 1998, 313, 3741. (63) Munaro, M.; Akcelrud, L. J. Polym. Res. 2008, 15, 83-88. (64) Liu, T. M.; Harrison, I. R. Thermochim. Acta 1994, 233, 167-171.
ACS Paragon Plus Environment
