Insight into the Synthesis Process of an Industrial ZieglerâNatta

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Kinetics, Catalysis, and Reaction Engineering

Insight into the Synthesis Process of an Industrial Ziegler-Natta Catalyst Antoine Klaue, Matthias Kruck, Nic Friederichs, Francesco Bertola, Hua Wu, and Massimo Morbidelli Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05296 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 24, 2018

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Industrial & Engineering Chemistry Research

Insight into the Synthesis Process of an Industrial Ziegler-Natta Catalyst

Antoine Klaue1, Matthias Kruck2, Nic Friederichs2, Francesco Bertola2, Hua Wu1*, Massimo Morbidelli1* 1

Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland.

2

SABIC, Technology & Innovation, P.O. Box 319, 6160 AH Geleen, The Netherlands.

* Corresponding authors: Email: [email protected]; [email protected]

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Industrial & Engineering Chemistry Research 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

ABSTRACT In Ziegler-Natta catalysis, the catalyst particle size has a strong influence not only on catalyst performance but also on the morphology and particle size distribution of the final polymer particles. Fundamental insight into the catalyst particle formation process is therefore of industrial importance when addressing specific requirements in the final products. In the present work, we fully characterize a single-step catalyst preparation process, which comprises a reactive precipitation of a MgCl2-supported Ziegler-Natta catalyst, through decomposition of the hetero bi-metallic complex, Mg(OR)2∙Ti(OR)4, by addition of ethyl aluminum dichloride (EADC). We track the evolutions of both the concentrations of the metals (Mg, Ti, Al) as well as Cl in the liquid phase and the size of the formed catalyst particles. It is observed that the liquid phase composition is governed by the EADC feed rate, under fully Cl-starved conditions. The process can be divided into two stages: The first stage is dominated by the precipitation of the Mg-based support, and the second stage involves complex adsorption-precipitation of the Ti species. The growth of the catalyst particle size occurs only in the first stage and is controlled by the aggregation and breakage events during the MgCl2 precipitation. It follows that the hydrodynamic stress in the reactor plays the essential role in controlling the catalyst size. In the second stage, no further particle growth occurs, not only because of the depletion of Mg in the liquid phase but also because the adsorbed Ti complex stabilizes the particles against aggregation. Finally, we have performed polymerization tests with the prepared catalysts and found that the size distribution of the polymer particles indeed closely replicates the one of the used catalyst particles.

Keywords: Ziegler-Natta catalyst, precipitation, breakage and aggregation, stability, ethylene polymerization, particle size distribution

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INTRODUCTION Within the polymer industry, polyolefins make up more than half of the global plastics demand 1 and their worldwide production is expected to keep increasing along with the gross world production.2 The production processes of these materials in the late 1930s were run without catalyst by free radical polymerization of ethylene to form low density polyethylene, requiring 200–300 °C and several thousands of bars.3 It was only in the 1950s with the discovery of the heterogeneous coordination catalysts 4-5 that the process could be run at milder conditions. The most used catalyst system in polyolefin industry remains the MgCl2-supported Ziegler-Natta catalysts (ZNC).6 They are composed of Ti complexes of the form, TiX3-4, with X being a chloride or an alkoxide group, which are chemisorbed onto a MgCl2 matrix.7 Usually a co-catalyst is used, comprising an aluminum alkyl of the form, AlR3, which results in the activation of the catalyst by alkylation of the Ti species.8 The chain growth during the polymerization occurs on the solid-bound active species, which leads to the replication of the morphology of the parent catalyst particle size distribution (SD) within the produced polymer particles.9 It has been shown theoretically by Floyd et al.10 and Kanellopoulos et al.11 that the catalyst size impacts the catalyst activity by affecting mass and energy transfer limitations. This is supported by experimental data from Schecko et al.12 and more recently from Philippaerts et al.13 during slurry polymerization. However, this conclusion remains in contrast with the studies of Dall’Occo et al.,14 who observed insignificant effect of particle size on catalyst activity. On the other hand, they pointed out that the morphology of the catalyst particle is of paramount importance in the polymer particle production processes based on slurry or gas phase technologies.15 As a matter of fact, the size and morphology of the ZNC particles are of primary importance in the production of polyolefins as they can affect the plant operation and the catalyst productivity. Up to now, although many preparative methods for 2 ACS Paragon Plus Environment


ZNCs have been developed as reviewed by Karol,16 only a very basic understanding of the phenomena controlling the particle size during the synthesis is available. One important preparation method relies on the use of Mg(OEt)2 as precursor for the formation of the support. Chumachenko et al.17 studied the formation of a ZNC starting with solid Mg(OEt)2 and following the chemical composition of the solid products through different reactions including chlorination, followed by donor addition and titanation. The authors reported that the ZNC morphology was dominated by the breakage of the initial solid Mg(OEt)2 leading to a broader particle size distribution of the final catalyst. As an alternative to solid Mg(OEt)2, soluble Mg sources may be used. These can either take the form of Grignard reagents 18-19

or other alkoxides.20 For the latter case, Redzic et al.20 studied the precipitation of MgCl2

through the addition of AlEtCl2 to a toluene solution of magnesium 2-ethyl-hexanolate with focus onto the MgCl2 crystallite size within the produced solid support. Soluble hetero bimetallic alkoxides21 involving both Ti and Mg complexes may be used. The addition of a chlorinating agent to such compounds leads to the decomposition of the initial metal complex and the formation (precipitation) of the catalyst. Studies on the decomposition reaction of such compounds are very scarce, mainly due to the low solubility of magnesium alkoxides in hydrocarbon solvents and a proper description of the involved mechanisms is still elusive or very limited.22-23 In general, the particle size obtained through fed-batch precipitation processes is the resultant of the primary processes of nucleation and growth, but very often, secondary processes such as aggregation and breakage also affect the morphological characteristics of the final product.24 This was observed by Söhnel and Mullin25 for the crystallization of strontium molybdate and by Ilievski26 in the case of aluminum hydroxide. The interplay between shear forces and the solute concentrations27 (e.g. agglomerate bond formation) might also play a significant role.


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Clearly, the understanding of the particle formation mechanism requires the combination of contingent information about the chemical reactions and the evolution of particle size. In the present work, we propose a methodology to study and fully characterize the particle formation process during ZNC preparation, using a single-step process28 as an example. This proceeds through the decomposition of a hydrocarbon soluble hetero bi-metallic complex of the type, Mg(OR)2∙Ti(OR)4, by addition of AlEtCl2 in fed-batch mode. This process requires only one reactive step, and is of primary importance in the frame of the production of polyolefins.6 The identification of the reactions affecting the particle size is a challenge, and the decomposition of the precursor complex is not described in the open literature. The identification of the relevant precipitation reactions and the evaluation of their kinetics would indeed contribute to understand the catalyst formation process. With the proposed methodology, this is achieved by monitoring the composition of the liquid phase during the ZNC preparation via ICP-OES and by monitoring the size growth of the ZNC particles through small angle static light scattering. The combination of these two techniques provides a powerful tool for the understanding of this process and opens new insight into the actual control over the catalyst characteristics and consequently on the final properties of the corresponding final polymer particles. MATERIALS AND METHODS All reactant manipulations, reactions and measurements were done under nitrogen atmosphere by using a glove box or the Schlenk technique. Materials Commercial magnesium diethoxide (Fluka) was used without any further purification. Titanium tetrabutoxide (Fluka) was purified by distillation before use (140 °C, 1×10-3 mbar according to Armarego and Chai29). Commercially available ethyl aluminum dichloride (EADC) in solution (50 wt% in hexane) was used (Chemtura). All reactions and particle size 4 ACS Paragon Plus Environment


measurements were done in hexane (HPLC grade, Fisher Scientific). Hexane was dried over molecular sieves (4 Å, Acros) to maintain a water content below 3 ppm. The water content of the solvent was checked by means of a Karl Fisher titrator (Titrino Coulometer, Metrohm). To minimize oxygen concentration, nitrogen was bubbled through the solvent tank for 24 h before each reaction and the tank was stored under nitrogen atmosphere. Preparation of the Mg-supported Ziegler-Natta catalyst The preparation of the catalyst is based on the procedure described by Friederichs et al.28 Briefly, magnesium diethoxide, titanium tetrabutoxide and ethyl aluminum dichloride are reacted in two steps. In the first one, magnesium diethoxide and titanium tetrabutoxide are mixed to form a hydrocarbon soluble precursor complex. In the second step, EADC is fed continuously to this precursor under intense agitation. Finally, the formed dispersion is put under reflux conditions to form the final catalyst.

Preparation of the catalyst precursor complex The hydrocarbon soluble Mg- and Ti-containing complex is synthesized following the procedure described in reference28 using 73 g of Mg(OEt)2 powder, 105 g of Ti(OBu)4 and 733 g of hexane to reach a weight fraction of the complex of about 20%. The resulting solution was clear, with a small amount of grey precipitate that was filtered out or decanted before use. Synthesis of the ZNC The synthesis of the ZNC particles was carried out in a thermostated 1 dm3 glass reactor, which was equipped with a condenser, three baffles and a paddle-type agitator as depicted in Figure S1 in the supplementary information. To keep track of the temperature within the reactor, a K-type thermocouple was fitted to the setup. All reactions were carried out under nitrogen atmosphere at constant pressure. Before each synthesis, the reactor inner surface was heated up with the help of a heat gun to eliminate traces of water. Subsequently, it was closed and put under nitrogen flow. 5 ACS Paragon Plus Environment

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Each experiment consisted of the following steps: The Mg-Ti precursor solution (134 mL) was placed in the reactor and diluted with 289 mL of hexane. 98 mL of the original EADC solution (50 wt%) were placed in a 250 mL Schlenk tube and diluted with 56 mL of hexane. The diluted EADC solution was fed continuously to the diluted precursor solution under agitation with the help of a syringe pump (Lambda) equipped with a stainless steel syringe (Harvard Apparatus) and a feed system composed of stainless steel tubing and a three-way valve (Swagelok). After all EADC had been added, the reactor was heated up to 69 °C to reach reflux conditions. The dispersion was kept under reflux conditions for 2 h. Finally, the particles were washed over a filter frit (P10 type, 10 µm pores) using 2 L of hexane. SEM micrographs The SEM micrographs were collected on a Leo 1530 (Zeiss). In order to avoid the contact to ambient air, the samples were plated on a doped silicon wafer support and subsequently protected by a rubber cover. The microscope was equipped with a pre-chamber, where the pressure could be reduced before introducing the samples under the electron beam. The rubber covers could be removed by using the pressure difference between the pre-chamber and the sample below the rubber cover. This was done to ensure minimal exposure of the samples to ambient air. The micrographs were acquired with an acceleration voltage of 5 kV using an aperture of 30 μm. For the detection, both an inlens and SE2 detectors were used. Composition analysis Liquid phase composition analysis The liquid phase composition was measured over time during the process. For this purpose, aliquots of 5 mL of dispersion were sampled with the aid of a syringe and using a stainless steel needle. Each sample was immediately weighted and filtered over a PTFE syringe filter with 0.2 μm pores (Macherey Nagel), whereby the solid was discarded. The liquid phase was poured 6 ACS Paragon Plus Environment


into a 20 mL glass vial, which was immediately closed and weighted. The solvent in the liquid samples was evaporated under nitrogen, and a 5 M sulfuric acid solution (Fluka) was added to the remaining precipitate to completely dissolve it. The obtained solutions were diluted with deionized water to reach an approximate analyte concentration of 20 ppm. The measurements were performed on an Arcos ICP-OES (Arcos, Spectro Analytical Instruments Inc.). Final catalyst composition analysis The composition of the final catalyst was assessed by XRF according to Bichinho et al.30 for Mg, Ti, Al and Cl. For this analysis, the catalyst samples were filtered and dried under high vacuum (1×10-3 mbar). The same protocol was implemented for the determination of the residual Ti content in the polyethylene particles subsequently produced in the polymerization reactor. The content of ethanol and butanol was assessed by GC-FID after quenching the samples. Particle size measurements The particle size measurement during and after their preparation was performed off-line by a small angle static light scattering (SASLS) instrument, Mastersizer 2000 (Malvern), using a flow-through cell. After cleaning, the flow channel was heated up through the cell casing with a heat gun, in order to minimize the presence of water, and the cell window was put in an oven at 140 °C. Subsequently, the flow-through cell was reassembled and flushed with nitrogen before being filled with dry hexane. The sampling was performed with a polypropylene syringe (Braun Injekt) fitted with a stainless steel needle (length = 25 cm; inner diameter = 1.2 mm). The sample volume was typically in the range from 0.1 to 1 mL, depending on the ZNC concentration, and then diluted in a Schlenk flask equipped with a stirrer bar and filled with 100 mL dry hexane to minimize the effect of multiple scattering. The dilution is also needed to prevent possible aggregation of catalyst particles. The total time required for sampling, dilution and injection into the flow cell was less than 30 s. 7 ACS Paragon Plus Environment

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As discussed later, the catalyst can be considered as compact fractal-like clusters composed of primary particles of 100 nm in diameter. Thus, the scattered intensity, I(q), can be expressed as follows31:

I (q) = I (0)  S (q)  P(q)

(1)

where I(0) the scattered intensity at q = 0, q is the magnitude of scattering wave vector, defined as

q=

4 n    sin    2

(2)

with n the refractive index of the solvent, λ the wavelength and θ the scattering angle. In Eq. 1, P(q) is the form factor of the primary particles, and S(q) is the structure factor, representing the arrangement of the primary particles inside the cluster. When the ZNC particles are considered to be fractal objects, their average morphology is characterized by two quantities, the average radius of gyration, Rg, and the fractal dimension, df. The Rg value was estimated using the Guinier plot:

 ( q Rg  )2  I ( q) = exp  −  I (0) 3  

for qRg < 1

(3)

The df value could be estimated directly from the power-law region of I(q), because of the substantially large size of the ZNC particles with respect to that of the primary particles:

I ( q)  S ( q)  q

−d f

for qRg >> 1

(4)

In addition, for the ZNC particles as well as the corresponding polymer particles obtained after polymerization, we have used the SASLS data to estimate their size distribution based on the Mie theory of spherical particles. Catalyst tests Polymerization tests were performed with the produced catalyst particles to evaluate their activity and the particle size distribution of the corresponding polymer powder. To this purpose, semi-batch slurry polymerization of ethylene was carried out28. These reactions were carried 8 ACS Paragon Plus Environment


out using hexane as solvent. The further operating conditions are summarized in Table 1. To obtain the polymer yield, the residual Ti concentration inside the polymer mass was measured by XRF. Table 1 Summary of the polymerization conditions of the catalytic tests.

Hexane volume (L)

5

Catalyst (mg):

20

Co-catalyst TIBA (mmol/mL):

0.8

H2/Ethylene (mol/mol):

3

T (°C):

85

t (min):

120

pEthylene (bar):

1.2

RESULTS AND DISCUSSION To monitor the catalyst formation, we sampled the reaction mixture at fixed time points during the precipitation reaction, and the particle size, as well as the elemental composition of the liquid phase was determined. With this approach, elemental information on the decomposition reactions of the precursor complex could be linked to the evolution of the ZNC particle size during the process. Time evolution of the liquid phase composition Effect of the EADC feeding rate The concentrations of species in the liquid phase were determined as a function of time at two EADC feed rates, Q = 5 and 10 mL/min, for a total feed time of tf = 30 and 15 min, respectively. For Q = 10 mL/min, the concentrations of the soluble Mg and Ti species normalized by their initial concentration are shown by the empty symbols in Figure 1a. It is 9 ACS Paragon Plus Environment

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seen that the Mg concentration in the liquid phase decays rapidly and drops below the detection limit after about 6 min. This process is characterized by the appearance of white precipitates soon after the first droplets of the EADC solution enters into the reactor, whereas the filtrate remains colorless. The Ti concentration decays much slower than the Mg concentration, and it reaches a local minimum at approximately 10 min, after which the Ti concentration starts increasing till the EADC feed is complete, i.e., at about 15 min. The corresponding Al concentration during the feeding is shown in Figure 1c, which is normalized by its maximal hypothetical concentration, i.e., based on total fed EADC in the system, as given in Table S2 in the Supporting Information (SI). It increases with time, following a linear trend, and then reaches unity, i.e., its maximal hypothetical concentration. This indicates that during feeding the Al consumption in the solid phase is negligible. The Cl concentration shown in Figure 1c is also normalized by the maximal hypothetical concentration of Al.

Figure 1 Normalized concentrations of Mg (triangles) and Ti (squares) (a and b) and Al (squares) and Cl (triangles) (c and d) measured by ICP-OES at the EADC feed rate of 5 mL/min (full symbols) and



10 mL/min (empty symbols), respectively.

The results corresponding to the EADC feed rate of Q = 5 mL/min, are shown by full symbols in Figure 1, and they are similar to those at Q = 10 mL/min but with a delayed development of the behavior described above. When replotting all data as a function of time,  = t/tf, renormalized using the total feed time, tf, all data overlap as shown in Figure 1b and 1d. This result indicates that the process is controlled by the feed rate and not by the chemical reaction. In order to better investigate this behavior, a further experiment was done with an EADC feed rate, Q = 5 mL/min, but the feed was stopped after 10 min or  = 0.31, after which samples were continuously taken for analysis. The results can be found in Figure S2 in the supplementary information (SI). It is seen that after the EADC feeding is stopped, the concentrations of all species in the liquid phase do not change with time. This further supports that the process in the reactor is fully controlled by the EADC feed rate, or in the other words, the characteristic time of involved reactions is much smaller than the feeding time.

Figure 2 Normalized concentrations of Ti (a) and Cl (b) during the stage of heating-up to reflux conditions (69 °C), starting from room temperature and reaching 69 °C within 10 min.

It is worth noting in Figure 1c and d, that initially the Cl concentration is practically equal to the Al concentration, and after 10 min, it becomes larger and reaches a maximum at the end of the feeding stage. This means that Cl is always available in the liquid phase, while the process is controlled by the EADC feeding rate. This allows us to conclude that only one Cl atom in 11 ACS Paragon Plus Environment

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EADC is reactive, and participates in the precipitation. This phenomenon can be understood when considering that EADC is a Lewis acidic and its structure can be stabilized by forming dimers. According to a comparison of experimental and calculated IR spectra,32 the dimer has the forms depicted in Figure 3. It appears plausible that among the four Cl atoms in the dimer only the non-bridging two can participate to the Cl transfer. The Cl transfer leads to the eventual formation of MgCl2, which is lipophobic and insoluble in hexane. Furthermore, it is seen that Al remains soluble throughout the process, since it undergoes a ligand exchange reaction with soluble Mg complexes as follows: Mg(OR)2 + AlEtCl2 → Mg(OR)2-xClx + AlEt(OR)xCl2-x

(5)

Figure 3 Structure of EADC according to Tarazona et al.32

The peculiar evolution of the Ti concentration The behavior of Ti concentration in liquid phase shown in Figure 1 needs some more discussion. Unlike MgCl2, chlorotitanium alkoxides, i.e., [TiCl(OEt)3] and [TiCl(OBu)3], as well as their di-chlorinated analogues, are known to be viscous liquids and soluble in hexane.33 This has been further verified by reacting the same EADC solution with only [Ti(OBu)4]. Although the reaction mixture turned yellow, no formation of any solid could be observed, thus confirming negligible precipitation of Ti-bearing species during the EADC addition. On the other hand, it was already demonstrated in the literature that Ti(IV) complexes such as TiCl4 can adsorb on sufficiently Lewis-acidic crystal cuts of MgCl2, more specifically on the (110) face.7, 34-36 Thus, in Figure 1, the initial decay of Ti in the liquid phase with time can be explained by adsorption of chloro titanium alkoxides to the MgCl2 crystallite faces. It follows that both Mg and Ti species contribute to the disappearance of Cl from the liquid phase during the initial stage. 12 ACS Paragon Plus Environment


After decreasing until a local minimum at τ = 0.7 in Figure 1b, the liquid phase Ti concentration starts to increase upon further feeding of EADC. Simultaneously, the Cl concentration in the liquid phase increases and becomes larger than the Al concentration. This behavior can be related to what is commonly referred to as catalyst leaching,15, 35 where the active Ti based complex is detached from the surface upon extended contact time with a Lewisacid, which in the present case is further fed EADC. The formation of soluble Ti(IV) complexes upon further addition of EADC can be explained by considering that alkoxy-chloro complexes of Ti(IV) can undergo complexation equilibria with EADC.36 Through a molar balance of Cl and Ti getting soluble in this stage, the dissolution of Ti and Cl species occurs with a molar ratio of 1:2. This indicates that a complex with a possible formula of [TiCl2(OR)2] has been released from the solid to the liquid phase. The stage after completing the EADC feeding After completion of the EADC feed, the system was equilibrated for ten minutes. Subsequently, the reactor was heated up to 69 °C within 10 min, which was held for 2 hours. In this stage, as shown in Figure 2a and 2b, both the Ti and Cl concentrations in the liquid phase decrease with time. In particular, the Ti concentration reduces practically to zero after about 15 min, while meantime the normalized Cl concentration reaches unity, i.e., equal to that of Al. It is obvious that now the soluble Ti species react with the Cl species to precipitate to the solid phase. Furthermore, a color change to red was observed, which indicates the reduction of Ti(IV). Commonly, the Ti(IV) reduction occurs through transfer of an alkyl group to the Ti center and subsequent elimination of an alkyl radical, leading to the reduced form of Ti(III), Ti(OR)3-xClx, which are insoluble in hydrocarbons.36 In the study of Cucinella et al., it is concluded that under large excess of EADC, solid TiCl3 is formed. According to our liquid phase concentration measurements, the molar ratio of the precipitated Cl to the precipitated Ti


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during the heating step has a value of 3.75, which is in agreement with the study by Cucinella et al.36 Time evolution of the ZNC particle size Particle Growth Mechanism The particle size evolution in terms of the root mean square gyration radius (〈𝑅𝑔 〉) is shown in Figure 4 as a function of time, at the stirring speed of 1400 rpm and three different EADC feed rates, 2.5, 5.0 and 10.0 mL/min. Let us first consider the case at the EADC feed rate of 5.0 mL/min in Figure 4a. The EADC addition was completed after 30 min, followed by subsequent heating. It shows that the 〈𝑅𝑔 〉 value is around 1 µm after 1 min and very quickly reaches a maximum value within 10 min, and then in the remaining EADC feed time the particle size remains constant. In the subsequent time no measurable change of the catalyst particle size is observed, suggesting that all the reactions responsible for the particle growth are occurring during the feed of the EADC solution.

Figure 4 The 〈𝑅𝑔 〉 value as a function of time (a) and dimensionless time (b) for the EADC feed rate at 2.5 mL/min (circles), 5.0 mL/min (triangles) and 10.0 mL/min (squares), respectively. The stirring speed is set at 1400 rpm.

When the results at the EADC feed rate of 5.0 mL/min are compared to those at the feed rate of 2.5 mL/min and 10.0 mL/min in Figure 4a, it becomes clear that the faster the feed rate, the quicker the particles reach their final size. One may also conclude that the feed rate has no 14 ACS Paragon Plus Environment


significant effect on the final particle size under the tested conditions. On the same figure, it can be noticed that the particle size increases dramatically at a certain point in the process. In analogy to the composition analysis, this size evolution is also reported against the dimensionless time , in Figure 4b. It is found again that the three curves practically overlap. This indicates that the particle growth in the three cases follows the same mechanism, which is closely related to the measured liquid phase composition and therefore, to the EADC feeding rate. To get further insight into the particle growth mechanism, we have assessed the colloidal stability of the particles along their growth. For this, we kept the sample for one hour in a Schlenk tube under gentle agitation by a magnetic stirring bar and then measured again the Rg. The measured values are compared in Figure 5a. It is seen that one hour after sampling, the Rg value increases quite a bit for samples taken at short times. Such a difference decreases however as time proceeds and becomes not significant after about  = 0.5. This indicates that the initially formed particles have poor stability in hexane, and significant aggregation occurs during the one hour storage. In Figure 5b, the normalized scattering intensity curves corresponding to the first measurement point in Figure 5a are compared. With one hour delay for the measurement, in addition to the much larger 〈𝑅𝑔 〉, the value of the fractal dimension (df = 2.5) is also much smaller than that (df = 2.9) without delay. The smaller df value indicates a more open morphology and/or a broader particle size distribution, thus further supporting the occurrence of aggregation among the formed catalyst particles during the one hour storage. It is evident that the stability of the particles improves along the EADC feeding (i.e., along the particle growth), because the Rg increase during the one hour storage tends to vanish. In addition, to explore the strength of the clusters formed during the one hour storage, we have sonicated the samples after the storage in an ultrasonic bath (Branson) for 5 min, and the 15 ACS Paragon Plus Environment

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〈𝑅𝑔 〉 values were re-measured. The results, shown in Figure 5a by squares, show that the 〈𝑅𝑔 〉 value reduces and approaches very close the original one, before the one hour storage. This indicates that the bonding among the particles in the aggregates formed during the one hour storage is not as strong as that formed within the reactor and can be easily destroyed with a short ultrasonic treatment. In addition, the fact that there is no significant difference in the 〈𝑅𝑔 〉 values after  ≈ 0.5 for the three cases indicates that the finally formed particles in the reactor are stable in hexane. The final ZNC particles are in fact stable even after storage ofor several months, as reported in Table S3 in SI.

Figure 5 (a) The Rg values as a function of the dimensionless feed time, measured immediately after sampling (triangles), after one hour storage (circles), and after one hour storage and then 5 min sonication (squares); (b) the normalized scattering intensity I(q)/I(0) as a function of q, corresponding to the points at τ = 0.16 in a). EADC feed rate = 10 mL/min; stirring speed = 1400 rpm. The continuous curves in b) are fitting using the Fisher-Burford expression.31

To explain the stability of the particles along their growth, we should consider that the very first particles formed during the (precipitation) reaction are very small and mainly composed of lipophobic MgCl2, as discussed in the previous section For these two reasons the particles are not very stable and exhibit a strong tendency towards aggregation. Thus, the particles formed inside the reactor at the initial times result from both precipitation and aggregation. This is also the reason why aggregation takes place during the storage of the samples after sampling 16 ACS Paragon Plus Environment


and dilution. The aggregation during storage is only due to van de Waals attraction, and the formed physical bonds are therefore weak and thus easy to break, as shown by the sonication experiment. On the other hand, the aggregation inside the reactor leads to stronger bonds, because the continuously fed EADC leads to precipitation inside the aggregates, thus strengthening the cohesive forces among the particles.37 This process is described by Mumtaz 27

based on the formation of a crystal neck between two surfaces of aggregating particles and

received some validation by Hounslow et al.38 for calcium carbonate and calcium oxalate monohydrate. The increased stability of the particles in the later stage of the precipitation could be explained by progressive formation of a lipophilic surface composition. It is found, from quenching experiments of the final ZNC particles with water, that significant quantities of ethanol and butanol are formed, as reported in Table S3. This confirms that alkoxy-ligands are indeed present on the surface of the final ZNC particles, and these are responsible for the improved stability. Effect of the shear rate on the particle size As discussed above, the initially formed particles inside the reactor result from both precipitation and aggregation. Then, it is expected that the shear rate generated by the stirring system would affect strongly the particle size, because it not only interferes with the aggregation but also breaks up the formed aggregates. Figure 6 shows runs at the EADC feed rate of 2.5 mL/min but at two different stirring speeds, 400 rpm and 1400 rpm, respectively. From the results, it is evident that the Rg value reaches its maximum at the same dimensionless time (around  = 0.33) for the two runs at different stirring speeds, but the maximum Rg value reached at 400 rpm is much larger than that at 1400 rpm. This clearly shows that breakage , which sets in at lower size for larger shear, plays a major role. Considering the knowledge gained above on the liquid phase composition, a further experiment was carried out to better identify the growth mechanism at different times. In particular, we set initially the stirring speed 17 ACS Paragon Plus Environment

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to 1400 rpm, and then at the EADC feed time of  = 0.33, the stirring speed was reduced to 400 rpm. We selected  = 0.33 because this is the typical location where Rg reaches its maximum and the depletion of soluble Mg is complete (see Figures 1 and 5). The obtained results are shown again in Figure 6. It is seen that there is no significant difference between the Rg evolutions obtained at 1400 rpm all the time and at 1400 rpm before  = 0.33 and then 400 rpm. Two conclusions may be drawn from this experiment.

Figure 6 The 〈𝑅𝑔 〉 values as a function of the dimensionless EADC feed time at the EADC feed rate of 2.5 mL/min and at the stirring speed, 1400 rpm (triangles), 400 rpm (squares), and 1400 rpm until  = 0.33 and then 400 rpm for the remaining time (diamonds).

First, it is further confirmed that the particle growth is strongly linked to the MgCl2 precipitation, and once this process is completed, the particles do not grow anymore, as shown in the scheme in Figure 7. In this period the stirring speed does affect the particle size through breakage. Second, as discussed above, the Ti species do not precipitate when contacted with EADC, but instead, they adsorb in the form of chloro titanium alkoxides to the MgCl2 crystallite surface. Thus, the results in Figure 6 confirm that it is the adsorbed chloro titanium alkoxides on the MgCl2 surface that stabilize the particles such that no further aggregation occurs inside the reactor in the later time, and therefore no influence of the stirring speed is observed. 18 ACS Paragon Plus Environment


In addition, it should be mentioned that the formation of crystal necks within the formed aggregates along the MgCl2 precipitation also contributes to the particle growth, because this permits the particles to better withstand the hydrodynamic stress caused by the stirrer. A similar mechanism was already proposed by Ilievski et al.26 for the case of Al(OH)3 crystallization in caustic aluminate solutions. Furthermore, the present observation is in line with the ones by Hartel et al.39 and Wòjcik et al.,40 where a dependency of agglomeration and breakage rate on the applied supersaturation was observed in the case of calcium oxalate and calcium carbonate respectively.

Figure 7 Scheme showing the particle formation process, which involves the initial precipitation, the interplay between aggregation and breakage, the subsequent precipitation on the formed aggregates, stabilization and TiCl3 precipitation after completion of the EADC feed.

As discussed above, during the particle growth, breakage induced by the hydrodynamic stress of the stirring system plays a dominant role. It is known that whether an aggregate is broken or not is determined by two opposite factors, the hydrodynamic stress and the cohesive strength within the aggregate. The breakage rate is a monotonically increasing function of the particle size,41 such that a given hydrodynamic stress is associated to a critical aggregate size, beyond which the aggregate would break.42 It is evident that the critical aggregate size decreases as the hydrodynamic stress increases. In the literature, the relationship between the critical aggregate size and the hydrodynamic stress is typically expressed in terms of a power-law, 〈𝑅𝑔 〉 ∝ 𝜀 −𝛾 , where  is the energy dissipation rate and the exponent, , has a typical value around 0.3 43-47. For example, Soos et al.42 found a scaling exponent of 0.26 in breaking aggregates formed from 19 ACS Paragon Plus Environment

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destabilized polystyrene nanoparticles. For our system, we have estimated the mean  values, , based on the power input: 〈𝜀〉 =

𝑃

(6)

𝜌𝑉

where 𝜌 and 𝑉 are the density and volume of the fluid, and 𝑃 is the power input rate, which can be estimated using the following correlation:48 𝑃 = 𝑁𝑃 𝑁 3 𝐷5 𝜌

(7)

Here, 𝑁 is the stirring speed, 𝐷 the paddle diameter, and 𝑁𝑃 the power number. For the specific stirrer used in this work, considering that the applied stirring speeds are in the turbulent regime (Reynolds number between 36000 and 130000), we can use a value of 2 for 𝑁𝑃 .49 Therefore, we have prepared the final ZNC particles at the EADC feed rate of 10 mL/min at three stirring speeds, 400, 900 and 1400 rpm, about which the Rg evolutions with time are reported in Figure S3 in SI. The Rg values of the final ZNC particles are plotted in Figure 8a as a function of  in a log-log plane, and the estimated value for 𝛾 from the plot is 0.27, which is consistent with the typical values obtained in the literature mentioned above for breakage processes. This result further confirms that the formation of the ZNC particles is controlled by the breakage process. It may be of practical interest to mention that a 𝛾 value of 0.27 implies to multiply the energy dissipation rate by a factor of 8 in order to decrease the particle size by a factor of 2.



Figure 8 (a) The Rg values of the final ZNC particles, prepared at the EADC feed rate of 10 mL/min and three stirring speeds, 400, 900 and 1400 rpm, potted as a function of estimated average energy dissipation rate, ; (b) the corresponding normalized scattering intensity curves, where the continuous curves are fit from the Fisher-Burford expression.31

Figure 8b shows the normalized scattering intensity curves corresponding to the prepared final ZNC particles. It is seen that the slope of the power-law regime, i.e., the fractal dimension (df) of the particles, increases from 2.5 to 2.9 as the stirring speed increases from 400 to 1400 rpm. This means that the compactness of the ZNC particles increases as the stirring speed in the reactor increases. This result is different from the typical results obtained from breakage of aggregates formed from polymeric nanoparticles.42 In the latter cases, the fractal dimension of the aggregates is independent of the shear rate (or stirring speed). This difference may be related to the continuous precipitation occurring in our system, which leads to continuous strengthening of the cohesive forces in the aggregates, thus affecting both breakage and restructuring. Figure 9 shows the SEM micrographs of the ZNC particles prepared at the stirring speed of 400, 900 and 1400 rpm, and they confirm the SASLS results that the catalyst size decreases as the stirring speed increases. On the other hand, no significant difference in the compactness of the catalyst particles can be observed, probably because the fractal dimension is relatively high in all cases.


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Figure 9 SEM micrographs of the ZNC particles obtained at the stirring speed of 1400 rpm (a,b), 900 rpm (c, d) and 400 rpm (e, f).

As a further test of the role played by aggregation and breakage, we have also assessed the effect of the reactant concentration by preparing the catalyst particles by diluting both the catalyst precursor complex and the EADC feed by a factor, of 1, 5 and 25. The corresponding Rg value of the final ZNC particles is 4.6, 3.7 and 3.0 m, respectively, i.e., the size of the final ZNC particles decreases as the concentrations of the reactants decrease. This is in contrast to classical crystallization systems, where a lower supersaturation is associated with an increased growth rate as reviewed extensively by Aslund et al.50 for the crystallization of benzoic acid. Conversely, this is the typical behavior observed for systems exhibiting strong 22 ACS Paragon Plus Environment


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aggregation and agglomeration such as in the case of calcium oxalate39 and calcium carbonate.40 In a stirred vessel, aggregation and breakage typically occur simultaneously. However, since aggregation is a second-order process, while breakage is first-order, dilution reduces the rate more for aggregation than for breakage, thus shifting the aggregate size towards a smaller value. Furthermore, the formation of crystal necks within the formed aggregates becomes faster at higher supersaturation, leading to mechanically stronger agglomerates.51 Polymerization test of the prepared ZNC particles The semi-batch slurry polymerization of ethylene was used to test the prepared ZNC particles under the conditions summarized in Table 1. As a measurement of the catalyst productivity, the XRF measurements of the residual Ti mass fraction in the polymer mass are provided in Table 2. Due to the fact that the mass of catalyst initially injected is orders of magnitude smaller than the final polymer mass, the catalyst yield (CY) can be computed from the residual Ti mass 𝐶 𝑃 fraction within the polymer 𝑥𝑇𝑖 and the one in the initial catalyst 𝑥𝑇𝑖 with

𝐶𝑌 =

𝐶 𝑥𝑇𝑖

(8)

𝑃 𝑥𝑇𝑖

Table 2 Concentration of residual Ti in polymer measured by XRF, which was produced using the ZNCs prepared in this work at different stirring speeds, as well as the catalyst yield, CY, and replication factor, Φ.

Stirring speed used in

Residual Ti concentration in

the ZNC preparation

polymer (ppm)

400 rpm

CY (kg/g)

Φ

9.9

10.5 ± 0.2

14.6

900 rpm

10.7

9.81 ± 0.09

20.9

1400 rpm

9.7

10.8 ± 0.2

22.7

As reported in Table 2, the CY value remained constant for all three ZNCs prepared using three different stirring speeds. This indicates that the productivity of these ZNCs is less sensitive 23 ACS Paragon Plus Environment

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to their size variations than for other systems studied in the literature,13 where a higher yield was obtained for the ZNC with smaller particles in a slurry polymerization process. This may be due to the fact that the ZNCs were synthesized using a different synthetic route and tested under different polymerization conditions. In additon, as shown in Figure 8b, all the ZNCs formed in this work are typical fractal objects. Though the fractal dimension increases as the stirring speed increases, the accessibility to the catalytic sites is always excellent for fractal objects. Figure 10a and 10b show the catalyst SD and the corresponding polymer SD, respectively, estimated based on the SASLS measurements and the Mie theory by assuming spherical shape. It is seen that the size of the polymer particles scales strictly with that of the corresponding catalyst particles. Since the catalyst yield is constant, it follows that the larger the catalyst size, the larger the polymer particle size. Second, the shape of the polymer SD reflects the catalyst SD. The variance of the distributions scales with the mean value, as the broadest SD is obtained from the largest catalyst. If one defines a replication factor, according to Casalini et al. 52, as 𝑃 𝐶 the ratio of the polymer to the catalyst particle size, Φ = 𝑑50 /𝑑50 , from the  values reported 𝑃 in Table 2, it appears that the polymer particle size 𝑑50 does not correlate linearly to the catalyst 𝐶 size 𝑑50 . Similar behavior was also found by Philippearts et al.13 A reason could be the stronger

tendency of breaking up for larger catalysts during or at the very beginning of the polymerization process, as already pointed out elsewhere.53 In this case, the larger the initial catalyst particles, the larger the importance of the phenomenon, leading to a non-linear relation that should lead to a maximum polymer particle size for given hydrodynamic conditions.47 A further reason for this non-linear correlation in size between catalyst particle and polymer particle at equal productivity could be related to the differences in the polymer porosity, related to the porosity of the catalyst prepared at different stirring speeds.54



Figure 10 (a) The SDs of the final ZNC particles prepared at the EADC feed rate of 10 mL/min and at the stirring speed of 1400 rpm (squares), 900 rpm (circles) and 1400 rpm (triangles), and (b) the corresponding SD of the polyethylene particles.

Concluding remarks The decomposition of the hetero bi-metallic complex, Mg(OR)2∙Ti(OR)4 (with R being ethyl and n-butyl groups), by addition of EADC to form the Ziegler-Natta Catalyst (ZNC), has been investigated by measuring both the liquid phase composition via ICP-OES and the solid particle size via SASLS. The results from the liquid phase characterization show that the initial precipitation reaction involves a ligand exchange with EADC leading to the decomposition of the complex characterized by the complete precipitation of the initially charged Mg in the form of MgCl2. In this stage, about 30 % of the initially fed Ti moieties leave the liquid phase as well. Further addition of EADC causes part of the Ti in the solid phase to resolubilize, but the subsequent increase of reactor temperature leads to the complete precipitation of all Ti left in solution in the form of TiCl3 through its reduction. The SASLS measurements during the formation process of the ZNCs show that the size of the ZNC particles increases with the addition of EADC up to the point where the liquid phase does not contain soluble Mg anymore. It is demonstrated that the particle growth is controlled not only by the precipitation reactions but also by the aggregation-breakage events. For this reason, the catalyst particle size can be effectively controlled by changing the shear rate applied 25 ACS Paragon Plus Environment

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in this initial stage. After this, the further reactions do not affect the final particle size because the steady Ti complex adsorption on the particle surface stabilizes the particles against aggregation. Due to the observed close match in the shapes of the SDs of the catalyst and polymer particles, it is concluded that the quality of the final polymer particles can be controlled by the shear rate applied in the growth stage of the ZNC particles preparation process.



ASSOCIATED CONTENT Supporting Information

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (H.W.) * E-mail: [email protected] (M.M.)

ACKNOWLEDGEMENTS Financial support from the Swiss National Science Foundation (Grant No. 200020_165917) is greatly acknowledged. The authors would like to thank Dr. Anna Beltzung and Dr. Baptiste Jaquet for the support and fruitful discussions. For the polymerization tests, the authors thank Erik Janssen for his precious assistance.


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ABSTRACT GRAPHIC


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Insight into the Synthesis Process of an Industrial ZieglerâNatta

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