Tuning the Ti3+ and Al3+ Synergy in an Al2O3/TiClx Catalyst To Modulate the Grade of the Produced Polyethylene

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[Piovano et al., ACS Catal., 2017, pagg 4915–4921]

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[

http://pubs.acs.org/doi/abs/10.1021/acscatal.7b01284

]

Tuning the Ti

_{and Al}

_{synergy in a Al}

2

O

/TiCl

cata-lyst to modulate the grade of the produced

polyeth-ylene

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

spectro-scopies, 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 Al}3+ _{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 Al}3+ _{Lewis acid sites is proposed as an efficient way to modulate the polyethylene}

mi-crostructure, 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 tradi-tional 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 eth-ylene monomer in comparison with the α-olefins and the high added value of the produced LLDPE, which has a large market share and broad appli-cations.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 α-olefins.1,3-8 _Industrial

bi-functional heterogeneous catalysts for LLDPE pro-duction are often obtained by modifying a cata-lyst highly efficient in ethylene polymerization (such as the traditional Phillips or Ziegler-Natta type catalysts) with an external modifier or acti-vator (often belonging to the group of metal alkyls).9,10 _{However, the exact role of the}

activa-tors and the structure of the modified catalysts have been rarely investigated in details. Recent works on the Phillips catalyst do suggest that

ac-tivators 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 catalyst exploits the}

syn-ergy between supported TiClx species and

proxi-mal, 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 isomer-ization processes.16,17 _{On one hand, Lewis acidity}

is considered the main responsible for olefin oligomerization,18 _{that occurs through a}

carboca-tionic mechanism.19 _{On the other hand, both}

Lewis and Brønsted acid sites are involved in olefin isomerization.20-24 _{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 chlo-rinated alumina surface exposing highly acidic Al3+ _{sites and reduced TiCl}

x species. The Al3+

oligomers via a carbocationic mechanism and ac-tivate the reduced titanium chloride species for co-polymerizing 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 cata-lyst is investigated by means of a series of com-plementary spectroscopic techniques aimed at identifying the electronic structure and the coor-dinative environment of the reduced Ti sites and their interaction with the Al3+ _{sites. The}

spectro-scopic 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 cat-alyst activated by H2. The two catalysts efficiently

polymerize ethylene to HDPE and LLDPE, respec-tively, indicating that tuning the synergy between the reduced Ti sites and the Al3+ _{sites is an}

effec-tive method to change the grade of the produced polymer.

2. EXPERIMENTAL

2.1 Catalysts synthesis

The synthesis procedure for the δ-Al2O3-600/TiCl4

pre-catalyst was discussed in our previous work.14

Briefly, a fumed δ-Al2O3 (Aeroxide Alu C, from

Evonik-Degussa), having an average primary par-ticle size of 13 nm and a specific surface area of 100 m2_g–1_{, was used as support and treated in}

dy-namic 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

tem-perature. The final amount of titanium in the pre-catalyst is expected to be about 2 wt%, by con-sidering the involvement of all the surface OH groups in grafting the TiClx species, as

demon-strated by FT-IR spectroscopy. The activation of the δ-Al2O3-600/TiCl4 pre-catalyst was accomplished

according to two different procedures: a) by

treat-ment in H2 at 400 °C, followed by degassing at

the same temperature, as discussed in our previ-ous work,14 _{or b) through reaction with}

triethylalu-minum (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-Al2O 3-600/TiCl4/TEA respectively, where the temperature

of each treatment (if not room temperature) is re-ported 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 continuous-wave (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 frequency 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 /2––/2–t1–

–t2–/2––echo, applying a eight-step phase cycle

for eliminating unwanted echoes. Microwave pulse lengths t/2 = 16 ns, t = 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  values were recorded, which are specified in the figure captions. The spectra were added for the different  values in order to eliminate blind-spot effects. All of the EPR spectra were simulated employing the Easyspin package.27

Differential Scanning Calorimetry. DSC

measure-ments on the obtained polymers were performed with a TA Q200 instrument. The polymer was ex-tracted 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 δ-Al2O3-600 (spectrum 1), of the δ-Al2O3-600/TiCl4

pre-catalyst (spectrum 2) and of the δ-Al2O 3-600/TiCl4/TEA and δ-Al2O3-600/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}

summa-rized in the following. Activated δ-Al2O3-600

ex-poses 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}

accompa-nies the dehydroxylation of the surface,31-35 _which

may thus affect the whole activity of the sup-ported 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.}

Figure 1. Part A) FT-IR spectra of δ-Al2O3-600 (spectrum

1), δ-Al2O3-600/TiCl4 pre-catalyst (spectrum 2) and

δ-Al2O3-600/TiCl4/TEA and δ-Al2O3-600/TiCl4/H2-400 catalysts

(spectra 3a and 3b, respectively). Part B) FT-IR spec-tra of δ-Al2O3-600/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 com-parison 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-Al2O 3-600/TiCl4 in H2 at 400 °C leads to the complete

dis-appearance of the chemisorbed [–O−HCl−Al≡] species and of any residual surface hydroxy groups. Indeed, the FT-IR spectrum of δ-Al2O 3-600/TiCl4/H2-400 (spectrum 3b in Figure 1A) is

com-pletely 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 δ-Al2O3-600/TiCl4/TEA catalyst

(spec-trum 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}

co-existence of alkylated TiClxRy species, reaction

by-products (such as AlRxCly) and unreacted AlR3,

al-though a distinction among all the different species is not possible. Figure 1B compares the FT-IR spectrum of δ-Al2O3-600/TiCl4/TEA catalyst

(af-ter subtraction of the spectrum of δ-Al2O 3-600/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 blue-shift of the}

ν(CHx) absorption bands suggests that TEA

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 synthe-sis 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 δ-Al2O3-600/TiCl4 (spectrum 2) is dominated by

three intense bands at about 42000, 35000 and 28000 cm–1_{, which were previously assigned}14 _to

ligand (either Cl or O) to metal (either 6-fold or 4-fold Ti4+ _{sites) charge-transfer transitions, as}

fol-lows: Cl

→

6c at 28000 cm–1, Cl

→

and O

→

6c at 35000 cm–1, and O

→

Ti4+

4c at 42000 cm–1).48-50 A color change is

ob-served upon activation: the δ-Al2O3-600/TiCl4/H2-400

catalyst was light blue, while the δ-Al2O 3-600/TiCl4/TEA catalyst was dark brownish, a clear

Figure 2. Part A) DR UV-Vis spectra of δ-Al2O3-600

(spectrum 1), δ-Al2O3-600/TiCl4 pre-catalyst (spectrum

2) and δ-Al2O3-600/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 δ-Al2O3-600/TiCl4/TEA

(spectrum 3a) and δ-Al2O3-600/TiCl4/ H2-400 (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 dif-ferent local environment in the two cases. The spectrum of the δ-Al2O3-600/TiCl4/H2-400 catalyst

(spectrum 3b in Figure 2A) is characterized by a well-defined absorption band (having a d-d

char-acter) centered at 13000 cm–1_{, which is the}

un-equivocal 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 par-tial charge transfer character, involving two Ti3+

sites bridged by a Cl– _{ligand, as previously}

re-ported for bulk TiCl3 polymorphs.51 This can be

considered as a proof for the formation of re-duced 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 of isolated Ti3+ _sites,

similar to those present in the δ-Al2O3-600/TiCl4/H 2-400 catalyst.

A detailed characterization of the paramag-netic Ti3+ _(3d1_{) sites in the two catalysts was}

ob-tained 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 Ti}3+

species characterized by similar g values account for 35% and 65% of the experimental signal (Ta-ble 1). These g values agree with typical values

reported for MgCl2-based Ziegler Natta catalysts

activated with alkyl aluminium compounds.14,56 _In

particular, g-values reaching 1.97 have been sug-gested to be associated to surface-alkylated Ti3+

species in supported titanium–magnesium cata-lysts activated by AlR3.56 In Figure 2B comparison

is set to the EPR spectrum of δ-Al2O3-600/TiCl4/H2-400

catalyst (spectrum 3b), which reveals that the two spectra, although similar, are characterized by slightly different g values and an approxi-mately reverse percentage composition (Table 1), suggesting a different environment of the Ti3+

species in the two cases.

Table 1. g-matrix components for Ti3+

species in δ-Al2O3-600/TiCl4/TEA, δ-Al2O 3-600/TiCl4/H2-400. Data for the δ-Al2O3-600/TiCl4/H 2-400 system are taken from Ref. 14.

Catalyst Ti3+ _g 1 g2 g3 (%) δ-Al2O3-600/ TiCl4/TEA (1) 1.977_0.003  1.945_0.005  1.90 _0.01 35 (2) 1.955_0.003  1.928_0.005  1.88 _0.01 65 δ-Al2O3-600/ TiCl4/H2-400 (1) 1.963_0.003  1.948_0.005  1.89 _0.01 80 (2) 1.942  0.003 1.9400.005  1.88 0.01 20 Such differences can be effectively probed by means of HYSCORE experiments, which allow de-tecting hyperfine interactions - with sub-MHz res-olution - of Ti3+ _{paramagnetic species with nearby}

magnetically active nuclei. In particular, the hy-perfine interaction with 27_{Al nuclei (I=5/2) is}

par-ticularly 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-Al2O3-600/TiCl4/TEA catalyst at

different magnetic field settings are shown in Fig-ure 3A and compared with spectra reported in Ref. 14 for the δ-Al2O

3-Figure 3. Experimental (blue) and simulated (red)

27_{Al Q-band HYSCORE spectra of δ-Al}

2O3/TiCl4/TEA

(part A) and δ-Al2O3/TiCl4/H2-400 (part B) catalysts. The

magnetic field settings at which the HYSCORE spec-tra were recorded are indicated in the corresponding ESE spectra (top). The spectra reported in part B have been already discussed in Ref. 14, but repro-duced 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 27_Al

nuclear Larmor frequency, indicating the pres-ence of nearby Al nuclei. However, those signals display a considerably different extent of the HYSCORE correlation pattern for the two cata-lysts, which reflects the entity of the hyperfine coupling. For δ-Al2O3-600/TiCl4/TEA (Figure 3A) a

maximum ridge extension of about 6 MHz is ob-served, 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 selection, only part of the cor-relation pattern may be observed, therefore, ex-periments 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 domi-nated by the isotropic Fermi contact term. Com-puter simulation of the HYSCORE spectra (red patterns in Figure 3) indicate that the ridge exten-sion of the δ-Al2O3-600/TiCl4/TEAcatalyst can be

re-produced 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 27_{Al 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

den-sity transfer on the 27_{Al 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 re-flects 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 chem-ical 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}

indi-cates a small degree of interaction between the two metal sites, while a strong interaction is present in the case of the δ-Al2O3-600/TiCl4/H2-400

catalyst. In this last case, Al ions are uniquely pro-vided by the alumina, indicating that supported TiClx molecular fragments experience a strong

in-teraction with the support after the pre-catalyst activation. In contrast, the reaction with the alkyl aluminium activator seems to induce a substan-tial reorganization of the pre-catalyst. Surpris-ingly, although for δ-Al2O3-600/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 27_{Al hyperfine coupling is much}

smaller, but also the overall intensity of the 27_Al

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

drasti-cally altered by the reaction with TEA, as also in-dicated by the absorption band in the DR UV-Vis spectrum associated to the formation of TiClx

clustered species. In other words, 27_{Al HYSCORE}

experiments allow quantifying the degree of chemical interaction between Al3+ _{and Ti}3+ _sites

and reveal a substantial decrease of such interac-tion in δ-Al2O3-600/TiCl4/TEA with respect to δ-Al2O 3-600/TiCl4/H2-400. In the following, we discuss the

im-pact of this state of affairs on the reactivity of the two catalysts and the final polymer microstruc-ture.

3.2. Performances in gas-phase ethylene polymerization

Both the catalysts turned out to be active towards gas-phase ethylene polymerization, even in very mild conditions (25 °C, PC2H4 = 100 mbar).

Ethyl-ene 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 spec-tra in the ν(CHx) and δ(CHx) regions collected

dur-ing ethylene reaction over the two catalysts. Al-ready at a first glance, it is evident that the two catalysts have a different reactivity. Upon ethyl-ene 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 ν}

asym(CH2) and

νsym(CH2), respectively. Even in the very first

spec-trum, 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-Al2O3-600/TiCl4/H2-400 catalysts (part B). The

spectra are reported after subtracting that of the ac-tivated catalyst. Dotted spectra (vertically translated for clarity) refer to the ATR-IR spectra of the poly-mers after removal of the catalysts and of the lower molecular weight fractions. Part C) differential scan-ning calorimetry (DSC) analysis of the polymers pro-duced by δ-Al2O3-600/TiCl4/TEA (curves 1) and δ-Al2O 3-600/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 with those in part A.

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(CH}

2) spectral

re-gion two absorption bands are observed at 1472 and 1463 cm–1_{, which are related to the}

crys-talline and amorphous phases of HDPE, respec-tively.60 _{Since the former band is much more}

in-tense than the latter, it can be stated that δ-Al2O 3-600/TiCl4/TEA mostly catalyzes the production of a

highly crystalline HDPE. This conclusion is vali-dated 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 character-istic 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 carboca-tionic 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 distribu-tion, where the lower molecular weight fraction is basically a polyethylene wax constituted by the branched oligomers produced through the carbo-cationic 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 ex-tracted 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

re-sults are consistent with a branched polyethyl-ene.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 pre-catalyst 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

coex-isting with chemisorbed HCl species, responsible for the production of a highly crystalline HDPE. In contrast, thermal activation in H2 leads to the

for-mation of isolated Ti3+ _{sites (responsible for olefin}

polymerization via coordination) on an exten-sively chlorinated alumina surface exposing strongly acidic Al3+ _{sites (promoting a}

carboca-tionic ethylene oligomerization): the result is a branched polyethylene. Advanced EPR techniques clearly reveal the synergistic cooperation be-tween Ti3+ _{and Al}3+ _{sites, allowing to}

quantita-tively probing the degree of interaction between the two metal sites as a function of the activation pathway. We observe different degrees of spin density transfer from Ti3+ _{to nearby Al}3+ _{ions in}

the two catalysts, which correlate with the differ-ent polyethylene microstructures.

Such a multi-technique approach opens inter-esting perspectives not only in the field of Ziegler-Natta catalysis, but also more in general in the in-vestigation of tandem heterogeneous catalysts, where the rational introduction of a second metal offers a range of new synthetic possibilities to-wards modulating and optimizing catalytic perfor-mances. AUTHOR INFORMATION Corresponding Authors * E-mail: mario.chiesa@unito.it * E-mail: elena.groppo@unito.it 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 Bor-diga for useful discussion.

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