Silica-supported metallocene catalyst poisoning: The effect of surface modification on the efficiency of the catalytic system

Colloids and Surfaces A 565 (2019) 36–46

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Silica-supported metallocene catalyst poisoning: The effect of surface modification on the efficiency of the catalytic system Marcius A. Ullmann, Arthur A. Bernardes, João H.Z. dos Santos

T

⁎

Universidade Federal do Rio Grande do Sul, Instituto de Química, Av. Bento Gonçalves, 9500, Porto Alegre, CEP 91500-000, Brazil

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Supported metallocenes Silica Catalyst poison Non-hydrolytic sol-gel Grafting

Cp2ZrCl2 was immobilized on silica by two methods: direct grafting (GS) and via entrapment within silica matrix, produced by the non-hydrolytic sol-gel method (NH). In the latter case, the surface was further modified by metal salts (Cr, Mo and W). The supported catalysts (three GS and four NH systems) were evaluated in ethylene polymerization, having methylaluminoxane (MAO) as the cocatalyst. Almost all of them resisted to at least 0.2 μg g−1 acetone (used as probe molecule for oxygenated poisons). NH systems bearing W or Mo (3 wt.%) on its structure were shown to be more productively efficient against poisoning. NH containing W in its structure tended to keep its catalyst activity even after the addition of the poison. Therefore, metal doping may be a suitable strategy to keep or increase the efficiency of entrapped zirconocene systems against oxygenated poisons. SAXS analyses show likewise that the smaller the primary spheres, the more protected will be the active site. Texture and microstructure of the catalytic systems and properties of the yielded polyethylenes reveal that the nature of the immobilization route affects the polymer properties (Mw and Tm).

1. Introduction According to IUPAC, catalyst poison is an inhibitory substance (present in the feed stream, for instance) characterized by its propensity to attach very strongly (e.g. covalent bond) to the surface atoms or ions ⁎

which constitutes the catalytically active sites [1]. These intrusive molecules may act simply by blocking an active site, by altering the interaction of the substrate with the catalyst site due to electronic effect or by modifying the nature of the active sites, resulting in formation of new compounds, thus changing catalyst performance. While catalyst

Corresponding author. E-mail address: [email protected] (J.H.Z. dos Santos).

https://doi.org/10.1016/j.colsurfa.2018.12.039 Received 28 May 2018; Received in revised form 29 November 2018; Accepted 18 December 2018 Available online 27 December 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.

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activity loss is one of the main problems related to heterogeneous catalysts within the so-called time-on-stream, poisoning tolerance/ roughness represents an important issue on the development of catalysts. Recent studies have shown the relevance on the development of stronger catalyst system, which would be less prone to catalyst deactivation. For instance, a functional membrane coated with Mn-La-Ce-NiOx catalyst for selective reduction of NO by NH3 was shown to possess good anti-poisoning ability against SO2 and water [2]. Cyclopalladated arylimine immobilized on graphene oxide nanosheet, active for the Suzuki-Miyaura coupling reaction was shown to be resistant for poisons such as Hg, PPh3 and thiophene [3]. The mechanism of poisoning effect on K and Mg on the Mn/TiO2 for the catalytic reduction of NOx has also been recently reported in the literature [4]. The presence of surface (or subsurface) carbon was shown to enhance the poison tolerance of encapsulated Pd catalysts in the selective hydrogenation of cinnamaldehyde [5]. A catalyst resulting from the polymerization of 1,5-diaminonaphtalene, followed by pyrolysis, was shown to be tolerant to a wide range of substance that poison precious metal based catalysts [6]. It exemplifies that catalyst strength is related to the capability to keep active the system in front of a physical variable or chemical agent that prevents the catalysed reaction [7]. Specifically, in the case of polymerization catalysts (Ziegler-Natta catalysts, metallocenes), Lewis basis (water, methanol, acetone, ethyl acetate, for instance) represents potential poisons [8]. Phillips Cr/silica catalysts are strongly inhibited by alcohols, organic amines and sulfides [9]. Cr(VI)/SiO2 systems yield ketones in olefin polymerization on its own surface, depending on olefin-Cr stoichiometry by reduction of chromium species [10]. McDaniel [11] discussed how electron-withdrawing Lewis acids may increase catalyst activity in Cr/silica and Cr/ silica-titania structures as well as in metallocenes: Heterogeneous catalysts can be chemically treated in order to afford Lewis acidity, which in turn may affect polymer chain termination reaction rate due to agostic β-hydrid coordination. Likewise, heterogeneous catalysts were titrate by poisoning (e.g., CO) for active sites quantification, indicating strong affinity by active polymerization sites and poisoning molecules at these sites. Nucleophiles compete with the monomer (ethylene) to the catalyst active site in polymerization reactions [12–14], that is why carbonylated compounds coordinate to a variety of metals and play a role of poison [15]. When metallocenes are applied to olefin polymerization, they are especially sensitive to compounds with high electron density (e.g., acetone, O2 and CO2) due to its cationic electrophilic centre of the transition metal [13], as shown by recent ketone reactions through zirconacarborane alkyls to generate new complex bonds [16]. Studies have evaluated the effect of contaminating molecules in polymerization processes with homogeneous metallocene systems [13] and, lately, MgCl2/TiCl4 type Ziegler-Natta systems [14,17]. Classical Ziegler-Natta systems, such as MgCl2/TiCl4/SiO2 and bare silica, were functionalized with Ag and Cu for chemisorption of contaminants (e.g. acetone) [18,19]. Sol-gel technology is a powerful approach to produce solids bearing different organic moieties or metal doping sites. Particularly in the case of water- and oxygen-sensitive metallocene, the non-hydrolytic sol-gel system represents the versatile process to produce silica-entrapped metallocene catalysts, in which the support is produced around the catalyst. In such system, silica formation occurs in anhydrous organic solvents (and under inert atmosphere), in which a metal halide reacts with silicon alkoxides in presence of Lewis acids [20,21]. The zirconocene is dissolved in toluene and added to the sol-gel sources [22]. Zirconocene single-site catalyst heterogeinization aims to adequate immobilized on silica systems to industrial reality, which is currently designed to work with classical heterogeneous Ziegler-Natta catalysts [23]. In a previous study, silica bearing Cr, Mo or W has been produced by the non-hydrolytic sol-gel: texture characteristics and network structure were shown to be highly dependent on the type of metal incorporated into silica [24]. As an extension of this study, a series of supported

metallocene (Cp2ZrCl2) catalysts were prepared via encapsulation through silica synthesis by the non-hydrolytic sol-gel method, in which the silica network was concomitantly modified by group VI metal chlorides. In spite of having the possibility of adding the scavenger in separated phase, the intended concept here was to evaluate the feasibility in incorporating in one sole support the catalyst centre and the potential centres for poison adsorption. Lewis acidic sites are recently investigated for hydrogenation processes in heterogeneous catalysts [25] and could indicate some alternative applications for the showed structures, that could save steps where unit operations can configure deactivation paths. The supported catalysts were evaluated in ethylene polymerization, having methylaluminoxane (MAO) as the cocatalyst. Taking into account high temperature behaviour of Cr/SiO2 catalysts, that produce ketone in the presence of ethylene by Cr-reduction [10], our goal was testing potential tolerance of group VI metal-modified catalysts systems in the presence of traces of acetone (taken as probe molecule of oxygenated poison) under polymerization conditions. For comparative reasons, Cp2ZrCl2 was also directly grafted on commercial silica. 2. Materials and method 2.1. Chemicals Sylopol® 948 (GRACE) was used as support for the grafting reactions. Tetraethoxysilane (TEOS, ACROS, 98%) and silicon tetrachloride (SiCl4, Sigma Aldrich, 99%) were employed for the entrappment procedures. FeCl3 (Neon, 98%) was previously treated under vacuum for ca. 10 h. Chromium trichloride (CrCl3, Merck, 99.9%), molybdenum pentachloride (MoCl5, Sigma Aldrich, 99.9%), and tungsten hexachloride (WCl6, Sigma Aldrich, 99.9%) were also submitted to vacuum treatment. For the polymerization reactions, toluene (Nuclear, 98.5%) was dried by refluxing with sodium and benzophenone, followed by distillation under Argon and collected over 3 Å molecular sieve just before use. Acetone (Fmaia, 99.5%), used as poisoning agent, was dried by refluxing, at least, for 6 h with anhydrous potassium carbonate (K2CO3), followed by distillation under Argon, collected and stored over 3 Å molecular sieve. Both, methylaluminoxane (MAO, Sigma Aldrich, 10 wt.-% in toluene) and bis(η5-ciclopentadienil)zirconium dichloride (Cp2ZrCl2, Sigma Aldrich, 98%) were used as received. Ethylene (White Martins, 99.5%) was also used as received. All procedures were performed under ultra-pure Argon (Air Liquid, 99.999%) inert atmosphere, using the Schlenk technique. 2.2. Synthesis of immobilized metallocenes The supported metallocenes were prepared by two routes: direct grafted on silica surface and entrapment within silica-based matrix by non-hydrolytic sol-gel process. 2.2.1. Synthesis of grafted supported catalysts Silica was activated under vacuum (P < 10−4 bar) for 16 h at 450 °C and cooled to room temperature under Argon. In a typical experiment, a Cp2ZrCl2 toluene solution, corresponding to a 0.5 wt.-% Zr/SiO2, was added to ca. 1.0 g of the pre-activated silica and stirred for 1 h at room temperature [26,27]. Before this procedure, zirconocene and silica were, respectively, solubilized and suspended in toluene for 30 min, also at room temperature. Solvent was removed under vacuum through a fritted disk. The resulting solid was washed with 10 × 2.0 cm−3 aliquots of toluene and dried under vacuum for, at least, 4 h. Grafted Cp2ZrCl2 on activated silica systems were labelled as GS. For comparison, two analogous systems were prepared: One by previously grafting WCl6 (1 wt.-% W/SiO2) under similar conditions, followed by calcination. Heating rate was 22.5 °C min−1 up to 500 °C, stand 3.5 h at this temperature and cooled at 20 °C min−1 up to 300 °C, prior to metallocene grafting (corresponding to 0.3 mol-% of SiO2). GS37

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W refers to this W-doped commercial silica. Another one was yielded by grafting both metallocene and WCl6 (0.3 mol-% of SiO2) concomitantly, after dehydroxylation procedure with a turbomolecular pump. It was labelled GS-(W + Zr).

using a wavelength of λ = 1.488 nm. The X-ray beam was monochromated using a silicon monochromator. The incident ray was detected at two different sample-to-detector distances (491 and 1605 mm) to increase the scattering vector q (q = (4 / ) × sin ; 2 = scattering angle) range. The dried samples were fixed between two Kapton® foils, and the collimated X-ray beam was passed through a chamber that contained the stainless steel sample holder. Analysis of the SAXS data was performed using the Irena evaluation routine as implemented in the Igor Pro software package (WaveMetrics, Portland, USA) [29]. A multilevel unified fit was used to describe levels of structural organization evident in the scattering data. In this method, the scattering provided by each structural level is the sum of a Guinier exponential form and a structurally limited power-law tail [30,31].

2.2.2. Synthesis of entrapped catalysts Cp2ZrCl2 was entrapped into a silica matrix by the non-hydrolytic sol-gel method [28] bearing with Cr, Mo or W moieties in its structure, such as described by Bernardes et al. [24]. Final silica molar proportion of precursors and metal chlorides was: 3 SiCl4:5 TEOS:0.1 FeCl3:0.3 MClx, where M is the metal (Cr, Mo or W) and x is the respectively valence for each metal in each employed salt, i.e. 3, 5 or 6. In a typical experiment, about 30 mg of FeCl3 and 0.45 mmol of metal chloride (CrCl3, MoCl5 or WCl6), in the molar ratios described above, were mixed in a Schlenk flask, followed by the addition of 2 cm−3 of TEOS (8.8 mmol) and 0.6 cm−3 of SiCl4 (5.2 mmol). Cp2ZrCl2 toluene solution corresponding to ca. 15 wt.-% of total amount of SiO2 produced in the reaction was placed in the Schlenk. Usually, this quantity corresponds to 150 mg (0.50 mmol) of the organometallic compound. When all chemicals were mixed in the flask, it was dipped in liquid nitrogen and kept in this way, under Argon, until freezing. Schlenk was removed from N2(l) and reaction was deaerated under vacuum for around 10 min. The materials were stirred in the flasks containing inert atmosphere at 70 °C up to gelification (ca. 48 h). Thereafter, samples were dried under dynamic vacuum (average 8 h), ground with mortar in a Ar atmosphere-modified glove box until 53 μm granulomere. FeCl3 was employed as the Lewis acid catalyst in TEOS:FeCl3 molar ratio equal to 50. NH stands for silica produced by non-hydrolytic route, followed by the metal (W, Mo, Cr) employed in the synthesis. For instance, NH-Mo means encapsulated metallocene mixed metal oxide synthesized by non-hydrolytic route, with molybdenum and 3 mol-% of Zr/SiO2. For comparative reasons, silicas without the presence of Cr, Mo or W (blanks) were also synthesized and labelled only as NH.

2.4.3. Attenuated total reflection – Fourier transform infrared spectroscopy (ATR-FTIR) Infrared measurements were performed on a Bruker ALPHA-P FTIR spectrometer, using Attenuated Total Reflection (ATR) on a single bonce diamond. Spectral resolution was 4 cm−1 by coadding 32 scans. The scanned range was 500 cm−1 to 4000 cm−1 at absorbance mode. Powders were measured once at dry medium and twice in wetted systems surfaces by dripped acetone over them. A time was intentionally lapsed between acetone drop and measurement. 2.5. Characterization of the polyethylenes The molar masses and molar mass distributions were investigated with a Waters CV plus 150C high-temperature Gel-Permeation Chromatography (GPC) instrument, equipped with a viscosimetrical detector, and three Styragel HT-type columns (HT3, HT4 and HT6E) with an exclusion limit of 1 × 107 for polystyrene. The solvent was 1,2,4-trichlorobenzene at a flow rate of 1 cm3 min−1, and the analyses were performed at 140 °C. The columns were calibrated with polystyrenes having a standard narrow molar mass distribution and with linear low-density polyethylenes and polypropylenes. Deconvolution of data was done by the Flory distribution model [32]. The polymer melting points (Tm) were determined on a TA Instrument DSC 2920 Differential Scanning Calorimeter connected to a thermal analyst 5000 integrator and calibrated with indium, using a heating rate of 20 °C min−1 in the temperature range of 0–160 °C. The heating cycle was performed twice, but only the results of the second scan are reported, because the first scan could be influenced by polymer mechanical and thermal history.

2.3. Polymerization reactions Polymerizations were performed in toluene (150 cm−3) in a 300 cm−3 Pyrex glass reactor connected to a constant temperature circulator, magnetic stirring and inlets for Argon and monomers. For each experiment, a mass of the catalyst system corresponding to 5.10−5 mol L−1 of Zr was suspended in 10 cm−3 of toluene and transferred into the reactor under Argon. The polymerizations were performed at atmospheric pressure of ethylene at 60 °C for 30 min at Al/Zr = 500, using MAO as the cocatalyst. MAO was added as a solution (10 wt.-% in toluene) directly to the reactor. To test the tolerance of each catalytic system, a quantity of acetone (poison) was stepwise added into the reactor. Each new polymerization was loaded with higher amount of acetone until total deactivation of catalytic system. Acidified (HCl) ethanol was used to quench the processes. The reaction products were separated by filtration, washed with ethanol, and dried at room temperature.

2.6. Statistic treatment

rSp ) rP ) and non-parametric (Spearman Parametric (Pearson correlations are presented throughout the study for statistically treatment the experimental data and characterizations results. The analysis was processed by comparing all vs. all data using SPSS® software. Strong correlations are there that coefficient (rPou rSp ) was ± 0.7 . Significant correlations (positive or negative) were considered at the level of p < 0.01or p < 0.05.

2.4. Characterization of the supported metallocene catalysts 2.4.1. N2 porosimetry N2 adsorption isotherms were obtained using a Micromeritics TriStar II 3020. The samples were pre-heated at 70 °C for 24 h under vacuum. The surface retention capacity was determined using the Brunauer–Emmett–Teller (BET) method at −196 °C in the range 0.01 < P/Patm < 0.35. Adsorption isotherm was considered for estimating the average pore diameter. T-plot and isotherm pattern of Harkins and Jura were used for determine porous capacities.

3. Results and discussion 3.1. Effect of poison addition order Fig. 1 shows influence of the addition order of the reactants used in ethylene polymerization, namely the catalyst (Cp2ZrCl2), the cocatalyst (MAO) and the poison (acetone). For comparative reasons, catalyst activity with the systems in the absence of poison was also included. According to Fig. 1, in the situation (I), MAO plays its role as scavenger by interacting with acetone prior to generate the active species: The result is a reduction of ca. 40% in catalyst activity if compared to the system in the absence of poison. For situation (II), in which the

2.4.2. Small angle X-ray scattering (SAXS) SAXS experiments were conducted on D2A and D11 A beamlines at the Brazilian Synchrotron Light Laboratory (LNLS, Campinas, Brazil) 38

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metallocene. Furthermore, due to steric effects, the capability to incorporate new monomer units into the polymer chain is also disfavoured. Systems containing tungsten present differences in activity depending on the immobilization method. NH-W is more active than GS-(W + Zr) because the latter was obtained by the concomitant addition of W salt and Cp2ZrCl2. In this condition, it is possible that both components compete for the silanol groups on the silica surface. Furthermore, grafting implies a washing step, which promotes leaching of the unreacted material from its surface. NH-W structure immobilizes metallocene by entrapment and there is no washing step. Therefore, NH-W contains the nominal amount of added Cp2ZrCl2. Previous works, conducted at our group, show that similar grafted silica [26,33] and the same entrapped zirconocene hybrid material [34,35] have negligible leaching effect, even in specific leaching test and no longer leaching processes were observed during polymerization. Previous studies have reported that less than 1% of Cp2ZrCl2 leaches out of the material [36]. It suggests that all exchanged MAO interactions in order to activate the Cp2ZrCl2 occurs on silica surface. In addition, it is worth mentioning that Fisch et al. [35] showed that SiO2WO3 systems without zirconocene do not polymerize, suggesting that all catalyst activity becomes from Cp2ZrCl2, in which WO3 plays a role of tuning electronic effect of support by doping quantities. Bernardes et al. [24] showed that the W-, Mo- or Cr- oxides are incorporated in silica network. This work extends tuning polymerization concept to group VI oxides and validate the idea that surface Lewis acids, generated in non-hydrolytic synthesis, can play a role as contaminant adsorbents to protect active sites. There is no doubt that Cr, Mo or W, incorporated into the silica, modify it structurally. Metal periodic properties, such as higher atomic radius, higher density or different electronic environment (higher Pauling electronegativity [37]) seems to affect. Therefore, activity increases in the same direction of the periodic properties of the metals incorporated in the silica, namely: NH-W > NH-Mo > NH-Cr NH. Finally, GS-W system was not active, but was kept in the scope of the work in an attempt to find structural, morphological or textural characteristics that distinguish it from other systems and justify its inactivity. Polymer properties in Table 1 suggest that silica surface affects the catalyst centre during polymer chain growing steps, as pointed out by Fisch et al. [22,28], who also noted that immobilized systems generate polymers with higher Mw than those produced with the homogeneous one. The role of catalyst modification is, among others, increasing catalyst activity and change of polymer granular composition, which is achieved by changes on supports that are used. Therefore, active centre inhomogeneities is a well-known feature of heterogeneous catalysts which may be used for tailoring polymer properties (e.g., Mw) [38]. In the present study, at least, polyolefins synthesized with heterogeneous catalysts (e.g., NH systems) have shown products composed by mixtures of macromolecules which differ in molecular weight and stereoregularity [36,38,39], i.e., polyethylenes produced on chemically similar catalytic centres could show differences in Mw because the monomer diffusion kinetic over catalyst support is, already, affected by its own surface features (e.g., specific surface area, porosity, acidity, etc.) [40,41]. Bashir et al. [40] pointed difficulties to isolate synergistically effects (e.g., particle size, texture of catalysts) that affect Mw. Systems that have shown activity below 90 kgPE molZr1 h−1 (Table 1) could not have the polymer product analysed by GPC due to the mineral filler, which prevented their complete solubilisation in the solvent. In Table 1, a low increasing trend for PDI at immobilized catalysts was observed. Examining grafted systems, a great number of isolated OH groups, generated by thermal pre-treatment at 450 °C, could produce more uniform set of active sites, which lead to lower polydispersity [42]. Entrapped systems show PDI near to that reported in literature for non-hydrolytic sol-gel synthesized silicas [34–36]. It is possible that active sites inhomogeneities, caused by Lewis acids, is accountable for most differences in Mw, despite lower increasing in PDI. The single

Fig. 1. Effect of the order of reactant addition process in catalyst activity of Cp2ZrCl2 in ethylene polymerization.

addition of the poison is just after the addition of the cocatalyst, a 57% of catalyst activity loss was then observed. The difference between both situations is that at (I) remains more MAO to generate the active complex system with zirconocene, because the time lapsed among acetone and MAO addition allows ketone volatilization before the cocatalyst introduction. This is why (II) consumes more MAO in scavenging process. The most drastic loss in catalyst activity (ca. 75%) is observed when acetone was added after the other reactants, i.e., after the generation of the active – complexed with MAO – species. Therefore, the order of deactivation situations was: (I) < (II) < (III). In the continuation of the present study, situation (III) was chosen as the model for testing the supported catalyst tolerance because of its most drastic behaviour among the three investigated cases. 3.2. Catalysts activity and the nature of the supported catalysts 3.2.1. Catalyst activity and polymer features in the absence of contaminant Table 1 shows the catalyst activity in ethylene polymerization of the different supported catalyst. For comparative reasons, data concerning the homogeneous system was also included. Data about melting point (Tm), weight-average molecular weights (Mw) and polydispersity index (PDI) also are shown in Table 1, labelled as polymer properties. SEMEDX results, as M/Si ratio, about Lewis acid NH systems from previous work [24] was included, indicating W has higher amount incorporated to silica. According to Table 1, as expected, the homogeneous catalyst system was the most active one. For the other ones: immobilization method seems to affect the resulting catalyst activity. For instances, for NH, active sites occluded within the silica structure during non-hydrolytic sol-gel synthesis have hindered the access of the cocatalyst to the Table 1 Catalyst activity of the supported metallocenes in ethylene polymerization and features of obtained polyethylenes. System

Cp2ZrCl2 GS GS-(W+Zr) GS-W NH NH-W NH-Mo NH-Cr

Catalyst Activitya 1 (kg PE ·mol ·h 1 ) Zr

Polymer properties Tm (°C)

Mw (Kg mol−1)

PDI

M/Si ratio (%)b

1815 960 735 ― 90 1560 1135 480

133 137 142 ― 141 135 140 140

50 127 204 ― ― 87 173 178

1.9 2.0 2.4 ― ― 2.1 3.5 2.9

― ― ― ― 0.85 ± 12 0.085 ± 4 0.029 ± 4

a [Zr] = 5∙10−5 mol L−1; [Al/Zr] = 500; T = 60 °C; V = 0.15 L (toluene); t = 0.5 h; P = 1 bar (ethylene). b Data from [24]; M = transition metal.

39

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nature of the zirconocene might have been preserved in each isolate system, mainly for NH-W and NH-Cr. Most active catalysts seemed to produce polyethylenes with low Mw, which is a trend typically attributed to chain transfer reactions that determines molecular weight of polymers [43,44]. Less active catalysts have metal centres that interact with a low amount of growing polymer chains and generate less molecular hydrogen, therefore it shows increasing in molar masses, i.e., polymer chain keeps to grow [40]. This approach explains the lowest molar mass for GS, NH-W and Cp2ZrCl2, respectively. Highest molar mass values were observed for GS(W + Zr), NH and NH-Cr, which are low catalyst activity systems. Despite higher activity, NH-Mo produces polyethylenes with higher Mw, which could be due to complex structural synergistic effects in silica by precursor MoCl5 volatile nature. Tm logically follows the trend in Mw increasing because of the chain length enlargement effect. In order to help to rationalize relationships between Mw and system activities, literature review allows us to identify two most recurring explanations to possible behaviours: (i) A survey crossing more than eighty activities data (including this work) of random systems or system conditions in some researches [22,27,28,34,35,39,45–47] with obtained polyolefin Mw show a sharp tendency to low molecular weights with increasing of activities (Fig. 2a). All systems were applied in ethylene polymerization and contain Cp2ZrCl2 or (n-BuCp)2ZrCl2 heterogenized in a silica structure or polymer support. Fig. 2a shows that 49% of systems with activity up to 1000 kgPE molZr1 h−1 have Mw up to 250 Kg mol−1; still just 15% of those with activity above cited activity have higher molar masses. This trend is explained by in situ H2 generation by metallocene β-hydride elimination (i.e., if the catalyst is high active, more it interacts with growing polymer chains, eliminating H2 and forming an allyl species. Inert allyl species impair chain growth mechanism and lead to low Mw) [40,48]. Despite, H2 is a significant driver to tailoring molecular weight in polyethylenes [28]. (ii) Conversely, when a specific dataset (Fig. 2b inset) show an opposite trend, it is attributed to support features, that hinder metallocene deactivation step, resulting in more growth of polymer chain [27,42]. In this case, β-elimination transfer between two metallocene centres is hampered by the solid support that lead to higher Mw. Capeletti et al. [36] endorsed the concept by concluding: “that a greater number of monomer units are inserted in the same chain when the number of available active sites is smaller”.

Fig. 3. Catalyst loss activity of the supported metallocenes with increasing acetone (poison) concentration.

3.2.2. Catalyst activity of the supported metallocenes in the presence of contaminant Poisoning experiments with the different catalytic systems are shown in Fig. 3, which indicate the loss of catalyst activity by stepwisely acetone addition. According to Fig. 3, NH systems decrease catalyst activity in order: NH-W > NH-Mo > NH-Cr. Systems with tungsten have a low level of decreasing when 0.1 μg g−1 and 0.2 μg g−1 of acetone was added to the reaction in comparison to the sharp drop of homogeneous catalysts, but it highly deactivates with higher amounts of acetone. In comparison, homogeneous and heterogeneous systems withstand up to the same concentration of acetone. Notwithstanding that 57% of the active systems could resist up to 0.3 μg g−1 of acetone, the way that these systems resist of different poison concentrations is unrelated. For example, analysis of the slope of the steps of acetone poisoning indicates that are many pronounced differences between productivity falling of all systems. Fig. 4 shows, likewise, the parameter 1 log activ , represented by bars, y˜

which rationalizes the experimental results observed in Fig. 3 as activ (slope of the linear fitting to each set of data) normalized by the medians of the activities (y˜) , that means, the sum of all stepwisely acetone addition for each system (overlaid in Fig. 4). When

( ) activ

y˜

tends to unity it implies that deactivation rate cost and catalyst activity benefit are more balanced, that is, catalyst has more productive efficiency towards the contaminant. The concept here presented entails

Fig. 2. Literature activities [22,27,28,34,35,39,45–47] of randomly selected systems and its trend with polyethylene weight-average molecular weights (Mw).

Fig. 4. Comparison of productive efficiency by poisoning of each system. 40

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that there is no 100% efficient catalyst. Cp2ZrCl2 is more prone poisoning (Fig. 4). Higher yields without contamination in the first instants of polymerization increase activity median, deactivation is high yet and reduce the efficiency to the smallest value for homogeneous systems. Comparing the four encapsulated systems, NH has the lowest productive efficiency, suggesting that the presence of Lewis acids from W, Mo or Cr species, improve catalyst productivity due to the more anchored poison molecules on the sacrifice metal. NH-W and NH-Mo both have the lower productively decay. On the other hand, they are systems whose median of activity remains high during the successive additions of the contaminant. Silica shell could protect the encapsulated active site by steric effect, which makes the system more acetone-tolerant, such as core-shell structures development for catalytic oxidation/reduction of NO over vehicle catalysts, often exposed to SO2 and H2O contaminating flows in diesel combustion engines [49,50]. It seems that the metal (i.e., Mo or W) keeps significant activity by probably playing the role of sacrificial component due to its affinity with the carbonyl groups. The interaction WeC]O is implicit, for example, in tungsten-based nanomaterials that were recently synthesized and applied as acetone gas-phase sensor [51,52]. For detecting acetone at high temperatures, molybdenum oxide-based chemical electrodes are as well development [53]. The metal-acetone interaction is also expected by the development of chemical electrodes for atmosphere acetone detecting on chromium oxidebased materials [54]. M-Acetone stoichiometry, where M is Cr, Mo or W, shows that silica surface needs to incorporate ca. 15 mol of metals for adsorbing one mol of acetone – independent from GS or NH systems. NH-Cr needs higher quantities (ca. 18 mol) of metal for adsorb the same amount of acetone than NH-W and NH-Mo, which adsorb the maximum amount of acetone and become inactive around 12 mol of metal content. Systems without metals incorporated in their structure, or that were obtained by the grafting of the metallocene on the surface of the silica, have shown to be less active or to bear higher productivity fall.

Comparing the data of bare acetone ( (C = O) = 1710 cm 1) (Fig. 5a) and (C = O) for acetone dripped onto grafted and entrapped systems, it can be seen a shift to lower wavenumber values. Those shifts are greater in entrapped systems than in grafted ones. This behaviour can be assigned to a reduction in the bond order of ketone carbonyl [18], which is due to coordination onto the metallic centre (Lewis acid sites). Nonhydrolytic route employs FeCl3, which is a Lewis acid catalyst that is not eliminated from silica network, as reported in the literature [55,56]. It leads to an acidic surface that acetone have affinity, reflecting the (C = O) shift of NH as shown in Fig. 5b. Shifts shown in Fig. 5b are, so forth:

NH

Mo

(C = O )

NH

W

(C = O )

> NH

(C = O )

NH

Cr

(C = O )

It suggests that Mo and W contribute for surface polarizability, which yield a higher acetone-affinity material. Cr-based systems shifted (C = O) close to NH systems, which indicates that Cr does not contribute more than the synthesis itself for surface polarizability. Observed shifts at acetone adsorption render the surface of NH systems comparable to the surface of traditional Lewis acids, such as chrysotile, alumina or Ag/ Cu-modified silicas [18,19]. Fig. 5a shows spectra without shift values for GS-(W + Zr) and GSW modified silicas. Compared to GS, it is possible that lability acquired by the silica surface silanol groups (Si-OH) – subsequent occupied sites by the W halide – affected the surface polarizability in the way that the acetone becomes inert to these surfaces by which it formerly had affinity. 3.4. Catalyst systems texture and structure 3.4.1. Textural characteristics Fig. 6 shows the resulting isotherms for catalyst systems N2 porosimetry. According to Fig. 6, resulting isotherms profiles was shown to be dependent of the immobilization method. The first set (Fig. 6a), composed solely by NH systems, shows a Type II isotherm profile, according to BDDT classification [57]. This kind of isotherm is usually attributed to non-porous or macroporous solids. However, a plateau formed in P/ Po range of 0.2 to 0.8 is close to the Type I isotherm, assigned to microporous materials. Some samples could not categorically be characterized by its isotherms, whereas authors [57,58] state the existence of the features of one type of isotherm in another (such as the saturation plateau joint to Type II and Type IV isotherms). Therefore, systems will resemble to Type I isotherm according to: NH-W > NH-Cr > NH

3.3. Metal-Acetone interaction Acetone adsorption process is affected by specific surface area, the nature of the surface sites and silanol concentration introduced by chemical modification [18]. Interaction between acetone and silica surface could by easy measured by monitoring (C = O) stretching – in ATR experiments – when the contaminant adsorbs on GS or NH systems (Fig. 5).

Fig. 5. Acetone adsorption on (a) grafted (GS) and (b) entrapped (NH) systems, monitored by 41

(C = O ) .

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Fig. 6. Systems N2-physissorption isotherms. Column (a) entrapped NH systems. Column (b) grafted GS systems. Detail in each column is an illustration of pore geometry based on its hysteresis shape.

NH-Mo. This order agrees with the order in specific surface increasing (95 m2 g-1 < SBET < 305 m2 g−1) for NH-Cr, NH and NH-Mo, may be due to exposure of surface. Hysteresis that extends to the lowest attainable pressure is an indicative of presence of micropores in NH systems, increasing in the same order as described. Hysteresis, for these systems, are associated with the swelling of non-rigid pore walls or with the entrapment of probe molecules in pores having diameters near to the adsorbate. Assigning NH systems hysteresis is H4, according to IUPAC [58] recommendations, since the range of P/Po which viewed hysteresis is greater than 0.7, with horizontal and parallel shape. H4 is an extreme case of porous shape, attributed to narrow slit pores, whose diameter can be less than 1.3 nm [58]. Fig. 6a shows, likewise, increase in system micropore capacity (nmicro) . That is a parameter of comparison between the systems, calculated by the difference between the retention capacity of the solid (nmBET ) and the content of the monolayer, out to the micropores, attained by the “t-plot” method, which curve was defined by the Harkins and Jura equation. Rouquerol et al. [59] criteria and recent IUPAC recommendation were satisfied [60]. NH, NH-Cr and NH-Mo have significant micropore capacity, confirming by continuous hysteresis down to the lowest pressures. NH-W system, as well as, all GS systems have very reduced micropore capacity (nmicro 5 cm3 g 1) (Fig. 6b), and could, therefore, be disregarded. Despite the smaller pore diameter of NH systems (20 Å < Dp < 50 Å) in comparison to GS system one (215 Å < Dp < 290 Å), which could occlude the catalyst and prevent cocatalyst or monomer diffusion, polymerization satisfactorily occurred (Table 1). Micropores could prevent the entrance of MAO into the structure and may reduce activity [61,62], as suggested by the lowest activity of NH-Cr and highest activity for NH-W. Polymerization with silica-entrapped zirconocenes produced by non-hydrolytic sol-gel process that has mesoporous (under 30 Å) were well described by Fisch et al. [63] who suggested that 20 Å diameter

porous is sufficient to allows MAO to perpass and initiate the wellknown heterogeneous catalysts fragmentation [64–68]. Such approach was proposed by Babushkin et al. and Fisch et al. that estimated the theoretical particle diameter of MAO under 15 Å [22,69]. Regarding the second set in Fig. 6b, composed only by GS systems, shows a Type IV isotherm profile: Mesoporous materials according to BDDT [57]. It is consistent with commercial silica (Silopol 948®) used for grafting zirconocene on its surface. IUPAC [58] classification for hysteresis shape is H1, another utmost of current categorization, since P/Po window that exhibit the hysteresis curve have a 0.2 gap, featuring a vertical parallel plot. H1 is shaped by channels and slits formed due to the relatively uniform compact agglomeration of sphere-shaped material [58]. There was a minimum change in structure and pore distribution of GS materials post-grafting due to its similar isotherm profiles. Pore volume of such systems is extremely affected by the immobilization route: NH systems present very low pore volumes (Vp 0.15 cm3 g−1) compared to those in GS systems (1.0 cm3 g-1 < Vp < 2.0 cm3 g−1). Fig. 7 shows the relation between the productivity falling ( ativ ) and the parameter C, obtained from BET model [58,60,70]. Parameter C is indicative of surface polarity [70,71] and has already been satisfactorily employed to establish relationships between the surface polarity of silica supports and the relative concentration of zirconium (Zr/SiO2) or Zr-O distance of surface immobilized metallocenes by grafting techniques [33,72]. Inserted in Fig. 7 is the table that shows constant C values for each investigated system. A high C value (C ≳ 150), associated with the narrow micropore filler, is observed for NH and NH-Cr, confirming isotherms interpretation. NH-Mo was disregarded for interpretations due to the divergent and low C value. GS systems have a range of 75 < C < 115, according to mesoporous materials [60,73]. A non-linear correlation between the C parameter and the slope of deactivation curves was verified (Fig. 7) (rSp = 0.94 |p < 0.01) . That 42

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mesoporous structure (Dp = 50 Å) showed high activity without poisoning (Table 1). Fisch et al. [35] have found textural results comparable to ours with similar W-entrapped structures that showed high activity when low MAO quantities were applied in polymer synthesis. Parameters shown in Table 2, such as Rg1, provide evidence about formation of pores in silica-based mixed oxides through primary particles aggregation [24]. Particle radius (Rp) is proportional to Rg (Rp = 5/3 × Rg ). Power law exponent (I q P ) determines particle agglomeration, which is the fractal dimension (P). If 1.0 < P < 3.0 , the particles at that level have a mass fractal aggregate. While 3.0 < P < 4.0 , the aggregates of this level have surface fractal geometry (dense particles with high roughness). However, P = 4.0 , means that the level has a particle with dense nucleus and a uniform surface [31,82]. Table 2 shows that GS systems tend to P2 > P3, as a result the first level particles tend to be structured into surface fractals at the subsequent level and these particles, in turn, tend to aggregate exclusively into mass fractals at the last level, corroborating larger pore volumes (Vp), showed by textural properties in section 3.4.1. NH systems tend to P2 < P3, as a result particles of the first level tend to structure surface fractals at the subsequent level and keep on with this structure in the last level of organization, but more dense and less roughness, validating low Vp (v. section 3.4.1). Statistically, by adding 0.1 μg g−1 of acetone, productivity fall (highest deactivation slope) trend with increase of the radii of primary particles of the catalytic systems (Rp1) is observed (rP = 0.82p < 0.05) . It suggests that heterogeneous catalytic system will be deactivated by contaminant with enlargement of particle radius of the first hierarchical level, namely the smallest spherical units from the support. It is possible that an amount of occluded Cp2ZrCl2 in the larger spheres of the NH systems (which may be an inaccessible active site e to the monomer) has higher deactivation if compared with those that present reduced sizes and available sites, since the poison could easily deactivate sites out of the spheres. GS exposed sites deactivation could be due to a diffusion of the poison on the surface of the larger particles. Therefore, the smaller the primary spheres, the more protected the active site will be. When Rp2 increases, it leads to a decrease in N2 adsorbed Vp (rSp = 0.80 |p < 0.05) , which may indicates a densification of system, that is why a reduction in Vp is observed, with increase in P3 [(rSp = 0.85 |p < 0.05) ; (rP = 0.91 |p < 0.01)]. Considering that P values increase with the prevalence of dense and less rough particles and decrease with increasing roughness and structure collapse. Therefore, porosity reduces as each system radius increases in the second level of aggregation. Activity when acetone was adding (0.1 μg g−1) and ds exhibit significant positive correlation (rP = 0.96p < 0.05) . The nature of the element incorporated in the silica network may affect the distance of the primary particles and that entails increasing, to some extent, both the activity and the system productive efficiency, since efficiency order is NH < NH-Cr < NH-Mo < NH-W; the same order of increasing atomic radius, atomic number and molar mass of each added transition metal.

Fig. 7. Relation between C constant (BET model) and productivity falling.

behaviour suggests that catalytic systems containing higher polarity surface deactivate less (lower productivity falling). Acetone preference will be given by the more polar surface, which should have the surface with the highest concentration of Lewis acid sites, keeping catalytic active site intact. Silveira et al. [33] proposed that silicas obtained by the non-hydrolytic sol-gel method increase the C parameter due to the use of Lewis acids in its synthesis, which was used to promote the reaction between SiCl4 and Si(OEt)4. It incorporates metallic residues into the silica and lead to C(NH ) C(GS ) . However, the group VI metal employed in the synthesis modifies the surface characteristics through its electronic nature that the feature reverts, overcoming the influence that any residue of the synthesis can exhibit, and C(NH W ) < C(GS (W + Zr )) . The addition of the group VI metal tends to reduce the magnitude of the C parameter at the same synthetic route. Thereby to non-hydrolytic solgel: C(NH ) C(NH Cr ) > C(NH W ) . For grafted systems, C varies in order: C(GS ) > C(GS (W + Zr )) > C(GS W ) . 3.4.2. Microstructural features The systems were further investigated by SAXS, which may elucidate multiscale particle structures assuming a hierarchically fractal geometry [74,75]. Fig. 8 depicts SAXS profile of some investigated systems, which shows curves with and without W for each: grafted (Fig. 8a) and encapsulated (Fig. 8b) synthetic route. Each heterogeneous system show hierarchical structure composed of three levels of organization. NH systems have a strong correlation between primary particles which behaviour not affects GS systems. This strong correlation was due to the presence of the neighbouring particles that influence the scattering. It allows estimating the average distance (ds) between them using the maximum scattering vector (qmax ) of the maximum scattering region (I (q)max ) with [76]: dS = 2 /qmax . The detail in Fig. 8b is a SAXS curves magnification of entrapped systems (NH) by section of the first hierarchical level. The shoulder indicates particles correlation. Observed shift for the sample NH-W in Fig. 8b (detail), reflects a sharp change in the average distance of primary particles in this specific system in comparison to other NH systems. The behaviour reflects on activity, keeping this system active with addition of acetone. Distance between primary particles and their aggregation for forming a second fractal structure is well known [79,80]. Non-hydrolytic sol-gel synthesis surrounds entrapped metallocenes by primary particles deep inside the silica matrix [63]. Void pores formed by channels between the primary particles as well as between the clusters influence catalyst polymerization properties [81]. It endorse us to conclude that distance of NH-W primary particles has allowed monomer better diffusion and a greater access to the active sites of the Cp2ZrCl2 catalyst, notwithstanding competition between CH2CH2 and poison. Therefore, NH-W

4. Conclusion The present study has shown that it is possible to synthesize a single silica device with Lewis acid sites and polymerization active site by an inert silica manufacturing. Non-hydrolytic sol-gel preserves metallocene sites by inert atmosphere manipulating, allow us to incorporate Lewis sites and enables to support the catalyst (by entrapping) in a single gelation step. The contaminant tolerance depends on the Cp2ZrCl2 heterogenization route. Higher tolerance was found for NH-W and NH-Mo, which resist up to 0.3 μg g−1 of contaminant and keep a high productive efficiency during poisoning. GS tolerates 0.4 μg g−1 of acetone, but activity was very low. Such results indicate the importance of the support 43

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Fig. 8. Typical SAXS profiles of heterogeneous catalytic systems obtained by (a) grafting - GS and (b) non-hydrolytic sol-gel - NH. Plots show curves with (△) and without (⬜) tungsten. Enclosed in (b) is a magnification of the scale window by the section of the first hierarchical level, comprised of NH smoothed curves. Smoothing by Savitzky-Golay routine [77], 44-point window, second-order polynomial [78].

reactions and to provide in situ ATR measurements. Future works, testing the showed method to rationalize catalyst efficiency on other systems and system conditions – such as changes in poison model (water, CO or CO2), will be performed.

Table 2 SAXS fit profiles parameters: particle radii of the first (Rp1) and second (Rp2) aggregation levels, fractal dimensions of the second (P2) and third (P3) levels and average distance between primary particles (ds). System

SAXS Level 1

GS GS-(W+Zr) GS-W NH NH-W NH-Mo NH-Cr

Level 2

Acknowledgments

Level 3

Rp1 (nm)

ds (nm)

P2

Rp2 (nm)

P3

1.4 4.7 0.7 0.8 1.9 1.0 2.0

— — — 1.44 1.66 1.43 1.39

4.0 2.4 3.8 3.6 3.6 3.0 3.9

6.0 10.1 6.4 9.5 13.5 8.4 25.5

1.2 2.8 1.1 3.9 4.0 3.7 3.7

M. A. Ullmann thanks CAPES for the Grant. Such project was partially supported by CNPq. Authors are thankful to the LNLS (Project D11A-SAXS1-8691) for the measurements at the SAXS beamline. FAPERGS (Project 16/2551-0000470-6) is also thanked for the complementary financial support. References [1] IUPAC, M. Nic, J. Jirat, B. Kosata, A. Jenkins (Eds.), The "Gold Book", Blackwell Scientific Publications, Oxford, 1997, p. 2006. [2] B. Yang, Y.S. Shen, Y. Su, P.W. Li, Y.W. Zeng, S.B. Shen, S.M. Zhu, Functionalmembrane coated Mn-La-Ce-Ni-Ox catalysts for selective catalytic reduction NO by NH3 at low-temperature, Catal. Commun. 94 (2017) 47–51. [3] Z.Q. Xue, P.P. Huang, T.S. Li, P.X. Qin, D. Xiao, M.H. Liu, P.L. Chen, Y.J. Wu, A novel "tunnel-like" cyclopalladated arylimine catalyst immobilized on graphene oxide nano-sheet, Nanoscale 9 (2017) 781–791. [4] R.T. Guo, S.X. Wang, W.G. Pan, M.Y. Li, P. Sun, S.M. Liu, X. Sun, S.W. Liu, J. Liu, Different poisoning effects of K and Mg on the Mn/TiO2 catalyst for selective catalytic reduction of NOx with NH3: a mechanistic study, J. Phys. Chem. C 121 (2017) 7881–7891. [5] S.J. Chen, L. Meng, B.X. Chen, W.Y. Chen, X.Z. Duan, X. Huang, B.S. Zhang, H.B. Fu, Y. Wan, Poison tolerance to the selective hydrogenation of cinnamaldehyde in water over an ordered mesoporous carbonaceous composite supported pd catalyst, ACS Catal. 7 (2017) 2074–2087. [6] D. Malko, T. Lopes, E. Symianakis, A.R. Kucernak, The intriguing poison tolerance of non-precious metal oxygen reduction reaction (ORR) catalysts, J. Mater. Chem. A 4 (2016) 142–152. [7] G. Rothenberg, Catalysis: Concepts and Green Applications, Wiley, Germany, 2008. [8] K. Tangjituabun, S.Y. Kim, Y. Hiraoka, T. Taniike, M. Terano, B. Jongsomjit, P. Praserthdam, Poisoning of active sites on ziegler-natta catalyst for propylene polymerization, Chin. J. Polym. Sci. 26 (2008) 547–552. [9] M.E. Grayson, M.P. McDaniel, Sulfide poisoning of ethylene polymerization over Philips Cr/silica catalysts, J. Mol. Catal. 65 (1991) 139–144. [10] K.C. Potter, C.W. Beckerle, F.C. Jentoft, E. Schwerdtfeger, M.P. McDaniel, Reduction of the Phillips catalyst by various olefins: stoichiometry, thermochemistry, reaction products and polymerization activity, J. Catal. 344 (2016) 657–668. [11] M.P. McDaniel, A review of the phillips supported chromium catalyst and its

as protective shell for the active specimen. ATR Results suggesting that W and Mo act as sacrificial components by chemisorbing the acetone (molecular probe), avoiding its interactions (deactivation) with the active MAO-Cp2ZrCl2 complex. Lability of the metal contributes to increase activity. Surface polarity hold deactivation, by acetone addition, slow due its adsorption on Lewis acid sites present on surface. The present research suggests that poison tolerance is a synergic complex effect of support microstructure, surface polarizability and nature of metal incorporation, which in turn is dependent on metallocene immobilization route. Although grafted systems have large porous, higher porous volume and higher surface retention capacity, entrapped systems show the highest activities. This could be attributed to Lewis acids on its surface. SAXS analysis show likewise that the smaller the primary spheres, the more protected the active site will be. The distance between the primary particles (ds) is quite significant for NH-W compared to the other NHSiO2 systems and may justify higher activities of this system. NH-W indicates slower deactivation kinetics between 0.1 μg g−1 and 0.2 μg g−1 of acetone addition and it has higher activity among the entrapped systems; exhibiting the highest productive efficiency. Acetone was an easy probe molecule to be introduced in slurry 44

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