Supported metallocenes produced by a non-hydrolytic sol-gel process: Application in ethylene polymerization

Colloids and Surfaces A 584 (2020) 124020

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Supported metallocenes produced by a non-hydrolytic sol-gel process: Application in ethylene polymerization

T

Arthur A. Bernardesa, Guilherme L. Schefflera, Claudio Radtkea, Dirce Pozebona, ⁎ João H.Z. dos Santosa, , Zênis N. da Rochab a b

Universidade Federal do Rio Grande do Sul, Instituto de Química, Av. Bento Gonçalves, 9500, Porto Alegre, CEP 91500-000, Brazil Universidade Federal da Bahia, Instituto de Química, Rua Barão de Geremoabo, 147, Salvador, CEP 40170-115, Brazil

G R A P H I C A L A B S T R A C T

Entrapped metallocenes in silica and roughnesses of the materials according to SAXS.

A R T I C LE I N FO

A B S T R A C T

Keywords: Non-hydrolytic sol-gel process Supported metallocenes Ethylene polymerization Silica SAXS

The immobilization of Cp2ZrCl2 was performed by entrapment within the binary oxides SiO2-CrO3, SiO2-MoO3 and SiO2-WO3 using a non-hydrolytic sol-gel route. Catalyst performance was evaluated in ethylene polymerization. The highest catalyst activity was found with a lower metal chloride:Si ratio and catalyst bearing W moieties. The catalyst and oxide matrices were characterized by complementary techniques (nitrogen porosimetry, small angle X-ray scattering, Fourier Transform infrared spectroscopy, X-ray photoelectron spectroscopy, and differential pulse voltammetry). The interaction between the binary oxide support and the immobilized metallocene reduced the electron density on the Zr centers, enabling the activation reaction to occur with lower concentrations of the methylaluminoxane (MAO) co-catalyst (Al/Zr = 500). The catalytic activity showed a direct relationship with the size and shape of the mass fractal. The results demonstrate that the entrapping method allows for the generation of a catalyst system in which part of the activation process may be attributed to the support.

⁎

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

https://doi.org/10.1016/j.colsurfa.2019.124020 Received 11 August 2019; Received in revised form 24 September 2019; Accepted 25 September 2019 Available online 28 September 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.

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1. Introduction

has many advantages over hydrolytic sol-gel in supporting air-sensitive catalysts, few studies have been reported on the development of catalysts for the polyolefin industry. The development of non-aqueous sol-gel routes to metal oxides has become a very attractive area within the broad field of materials research, opening up great opportunities to access a wide variety of binary, ternary and doped metal oxide particles with high crystallinity and well-defined particle morphologies - important parameters towards their application in catalysts synthesis. Firstly, in a previous work several types of binary oxides were synthesized by the non-hydrolytic sol-gel process. These materials were later characterized by a series of analytical techniques in which the texture, structure and morphology of the supports were evaluated. It was observed not the influence of the type of metal chloride, but the effect of its proportion on the silicon species [14]. As an extension of that study, the present article describes the immobilization of the zirconocene Cp2ZrCl2 within the binary oxides SiO2-CrO3, SiO2-MoO3 and SiO2-WO3 using a non-hydrolytic solgel route for entrapment as a potential method for tuning the electronic effects that the support has on the immobilized metallocene.

In spite of more than twenty years of research on the development of supported catalysts, this topic is still an active area of study. Recent publications have described several approaches such as the production of self-assembly polypropylene nanofibrils produced by metallocene on silica nuclei produced via Stöber sol-gel method [1], or the immobilization of metallocene on layered double hydroxides [2], on silica produced by rice husk ash [3], on sodium clay [4], or aluminum containing dendrimeric silica nanoparticles [5], just to mention a few. The quest of new polymerization catalysts has been motivated by the possibility to tailoring the steric and electronic environment of the catalytic complex by modification of surface of many supports, thereby directly influencing the catalyst performance, as well as the resulting polymer properties. In terms of heterogeneization techniques, the majority of the studies have dealt with grafting reactions in which ligands from the metallocene react with silanol or other chemical groups introduced on the surface of the support. In a less exploited approach, the immobilization of the metallocene by trapping it within the oxide matrix based on nonhydrolytic sol-gel approach has been investigated by Fisch et al. [6–8]. The heterogeneity of the metallocene encapsulation is performed while the support is syntheszed by a suitable sol-gel method. Non-hydrolytic sol-gel process is the condensation polymerization reaction of an alkoxide and an acid chloride, which may be the same metal or not, in the presence of a Lewis acid which serves as catalyst. Non-hydrolytic sol-gel process involves condensation reactions and various non-aqueous media, which significantly affects the properties of texture, homogeneity and surface of the resultant materials [9]. Some features of this process include: (i) the complex is kept within the silica matrix in the absence of a covalent bond to the oxide surface, and therefore, it may preserve the characteristics of a homogeneous catalyst; (ii) it is possible to immobilize a higher load of metallocene than with the grafting method because there is no dependence on the availability of silanol groups, and the metallocene will likely be well distributed along the particle radius. To summarize, heterogenization by the entrapment route attempts to combine the advantages of using a homogeneous catalyst with those of a supported one [10]. Metallocenes encapsulated in silica and applied to the polymerization of olefins showed catalytic activities similar to those obtained via grafting with activation performed with MAO (Al/Zr = 1000 to 2000) [6,7]. Zirconium immobilized in acidic binary oxides, such as SiO2-WO3, exhibits good activity with low concentrations of MAO (Al/Zr = 50–500) [8]. An interesting feature of encapsulation systems obtained via nonhydrolytic sol-gel processes is the space constraint because of the narrow pore diameter and reduced surface area. In this environment, immobilization of the metallocene occurs through interactions with the surface without chemical binding. These interactions are responsible for changes in the electronic structure of the transition metal complex such that the electron deficiency of the metal center enables a reduced amount of MAO. The sol-gel synthesis allows for easy control of the type and amount of heteroatoms that can be combined with the synthesis of binary oxide. A balance in the acidity of the transition metal center can be achieved by adjusting these parameters. The results reported by Fisch et al. [8]. Suggest the need for an acid for activation in the absence of MAO, but this would not guarantee high catalytic activities. The works developed by Fisch et al. comprised the synthesis of catalysts by the non-hydrolytic sol-gel process and their application in ethylene polymerization. These catalysts were prepared from TEOS with TiCl4 or WCl6 generating different acid species on the silica surface. The influence of the nature of the support on the catalytic activity and ethylene polymerization conditions and on the microstructural properties of the polymers has been investigated. Materials prepared by NHSG chemistry have found useful application in many fields: Brønsted acid catalyst [11], metathesis of olefins [12], photocatalyst [13]. Although the non-hydrolytic sol-gel process

2. Experimental 2.1. Materials The catalysts were prepared using tetraethoxysilane (TEOS, ACROS, > 98%) and silicon tetrachloride (SiCl4, Sigma Aldrich, > 99%). FeCl3 (Sigma Aldrich, > 98%) was previously treated under vacuum. Chromium trichloride (CrCl3, 99.9%), molybdenum pentachloride (MoCl5, > 99.9%), tungsten hexachloride (WCl6, 99.9%) and bis(cyclopentadienyl) zirconium dichloride (Cp2ZrCl2, 98%) purchased from Sigma-Aldrich were used as received. All procedures were performed under inert atmosphere (Schlenk technique) using Ultra Pure Argon (Air Liquid, 99.999%). 2.2. Synthesis of catalysts The catalysts were synthesized via a non-hydrolytic sol-gel process based on the studies of Hay and Raval [15] and Bourget et al. [16]. A typical non-hydrolytic sol-gel reaction involves the condensation polymerization of an alkoxide and a chloride, which may be the same or different metal in the presence of a Lewis acid, which acts as the catalyst. Nine different routes were tested to study the possible implications on the final catalyst using three types of metal chloride (CrCl3, MoCl5 and WCl6). The catalysts were synthesized using metal chloride:TEOS molar ratios of 0.1:2, 0.2:2 and 0.4:2 for each metal chloride and a 1:2 M ratio of SiCl4: TEOS. The catalysts were synthesized with their molar ratios to see if the lower or higher amount of metal chloride would affect the catalyst performance, both with regard to the catalytic process and the properties of the resulting polymers. In a typical experiment, 15 mg of FeCl3 and a metal chloride (CrCl3, MoCl5 or WCl6) in the molar ratios described above were mixed in a Schlenk flask, then TEOS (2 cm3, 8.95 mmol) and 0.55 cm3 of SiCl4 (4.47 mmol) were added, and finally the metallocene (0.15 g dissolved in 2 cm3) was added to this solution. The materials were stirred under argon at 70 °C until gelification occurred (ca. 24 h). The catalysts were dried under vacuum and ground up under inert atmosphere to obtain a controlled particle size of 53 μm. The resulting xerogels were labeled as SiMyZr, where M = the Cr, Mo, W and "y" amounts in moles of metal chloride. Scheme 1 depicts the sol-gel routes employed for the xerogel synthesis. 2.3. Ethylene polymerization The ethylene polymerization reactions using the catalyst Cp2ZrCl2 immobilized on different supports were performed in a glass reactor with a 0.3 L capacity equipped with a water circulator for temperature control and a magnetic stirrer (450 rpm). In a typical experiment, the 2

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(900–1300 cm−1) were independently deconvoluted into Gaussian components using a nonlinear least-squares fitting method. The percentage of six-member rings (%(SiO)6) was estimated as the ratio of fitted areas shown in Eq. (1):

A(LO6) + A(TO6) ) ⎤ × 100 %(Sio)6 = ⎡ ⎢ ⎦ ⎣ A(LO6) + A(TO4 ) + A(TO4 ) + A(TO6) ⎥

(1)

X-ray photoelectron spectroscopy was performed in an OmicronSPHERA station using Al/Kα radiation (1486.6 eV). The detection angle of the photoelectrons (Q) with respect to the sample surface (emission angle) was fixed at 0° for all measurements. All spectra were fitted assuming a Shirley background. Lines were fitted by a combination of Gaussian and Lorentzian functions, setting a value of full width at half maximum for each line. Differential pulse voltammetry measurements (DPV) were taken with a potentiostat/galvanostat (PARC, model 273). All experiments were carried out using a conventional three electrode cell. The working electrode consisted of a PVC body containing a graphite disk, which supported the carbon paste. The carbon paste was prepared by mixing high purity graphite (Fisher Scientific) and the samples in a 9:1.5 (w:w) ratio with a few drops of mineral oil. Ag/AgCl was used as reference electrode and platinum wire as the auxiliary electrode. All measurements were performed under high purity argon. Metal content was measured by inductively coupled plasma optical emission spectrometer from Perkin Elmer (OptimaTM 2000 DV). Argon (99.996%) was used as auxiliary, nebulizer and principal gas, nitrogen (99.996%) was used as purge gas of the optical system, and compressed air was used as chear gas. More details are reported elsewhere [8].

Scheme 1. Synthetic route to the immobilized metallocenes.

reactor (taken from the oven) was mounted under argon to remove the air. 0.15 L of dry toluene were transferred into its interior and added 5 mL of TEA (10% Al), for washing. The washing step was performed at 60 °C for a period of 30 min. After draining the wash solution was added 0.15 L of dry toluene. The ethylene pressure (1 bar) connected through the top of the reactor pipe and the temperature of reaction were and 60 °C, respectively. In order to keep a constant reactor temperature of 60 °C circulator thermostatic water bath was used. In a typical experiment, the reactor was assembled and heated under argon to remove air; then, 0.15 L of dry toluene and an MAO solution (10 wt.% Al) as a function of the calculated ratio Al/Zr were added, followed by a sufficient catalyst volume for the 7.5 × 10−6 mol of substrate used in all reactions. After 30 min, the reactor was drained into a solution of acidified ethanol (10% v/v HCl) in a glass beaker, for deactivation of the catalyst system and precipitation of the polymer. The mixture was filtered and washed with acetone several times until the toluene and the catalytic residues were eliminated. The polymer retained on the filter was dried until a constant mass was achieved.

2.5. Polymer characterization The polymer melting points (Tm) and crystallinity contents (Xc) were determined using a differential scanning calorimeter (DSC Q20) from TA Instruments with heat treatment of 20– 160 °C, maintaining that temperature for 5 min and then cooled to −20 °C, and again heated to 160 °C and then cooled to 20 °C at a heating rate of 10 °C/min with a N2 flow rate of 50 mL/min. The melting temperatures and the crystallinity of the polymers were measured using the second heating ramp. The melting temperature and crystallinity were determined from the second heating data. The degree of crystallinity was calculated relative to a heat of fusion of a standard considered 100% crystalline polyethylene (293 J/g). The Tm assigned to each sample corresponds to the minimum temperature in the range of 100 and 140 °C, observed in the thermograms. The molar mass (Mw) and polidispersity (MwD) of the polymers were determined by gel permeation chromatography (GPC). The equipment used was a Model 220 PL liquid chromatograph equipped with a refractive index (IR) detector and a viscometer detector. For calibration of the equipment, a series of monodisperse polystyrene standards were used to create the calibration curve. The molar mass values and theoretical polydispersion of the standard NBS 1475 and obtained are described in Supplementary Table 1

2.4. Characterization of the catalysts N2 adsorption isotherms were performed on a Micromeritics Gemini 2375. The samples were pre-heated at 80 °C for 24 h under vacuum. SAXS experiments were carried out on a D11A beamline at the Brazilian Synchrotron Light Laboratory (LNLS, Campinas, Brazil) using a wavelength (λ) of 1.488 nm. The dried samples were sandwiched between two Kapton®sfoils, and the collimated X-ray beam was passed through a chamber containing the stainless steel sample holder. All measurements were performed at room temperature. Silver behenate powder was used as a standard to calibrate the sample-to-detector distance, the detector tilt and the direct beam position. SAXS data analysis was performed using the Irena evaluation routine implemented in Igor Pro Software (WaveMetrics, Portland, USA) [17]. A multi-level unified fit was used to describe one or two levels of structural organization evident in the scattering data [18,19]. Fourier transform infrared spectroscopy (FTIR) were performed on a Varian FT-IR spectrophotometer model Varian (32 scans with a resolution of 4 cm−1). The region of the silica spectrum at lower wavenumbers provides information about the structural characteristics of these materials corresponding to the asymmetric stretching from SieOeSi moieties and its two components: transversal optic (TO) and longitudinal optic (LO) modes. For a detailed structural analysis, the νas((SieO)eSi) and the νs((SieO)eSi) spectral regions

2.6. Statistical analysis SPSS software (SPSS for Windows, version 19, IBM®) was used to analyze the data. All statistical tests were performed at the P < 0.05 or P < 0.01 levels of significance. 3. Results and discussion The set of catalyst systems was initially evaluated in ethylene polymerization using MAO as the co-catalyst. The resulting polyethylenes were characterized in terms of molar mass measurements and thermal characteristics. In a second step, the performance of such systems was correlated to structural and textural characteristics of the heterogeneized catalysts. 3

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polymer showed a fibrous microstructure, obtained by extrusion of the growing chain through the pore, due to space constrain [8]. The replication of the polymers presented in this work has not been evaluated. As observed in Table 1, the amount of the metal within the silica network also affects the Mw of the resulting polymer. For both Cr and Mobased systems, increasing the nominal metal content increases the molar mass of polymers. This increase also occurred in the W-based systems, which afforded the highest Mw value (381.000 Da). For this system, the higher W content (SiW04Zr) resulted in a significant reduction in the final Mw. For this material, it is likely that the increase in the percentage of siloxane bridges will promote greater encapsulation of metallocene and thus more bimolecular deactivation reactions are occurring which explains the lower activity between these systems. The reduction in Mw of the generated polymers is due to an increase in chain termination reactions that can be caused due to less interaction between the metallic center and the monomer. As observed in the ethylene polymerization results and GPC deconvolution, there was no broadening in molecular weight distribution, however, it is possible to observe a significant reduction in Mw value when analyzing the polymers synthesized with SiW04Zr catalyst. It is likely that, with increasing W concentration in the silica network, it will not only affect the electron density on the metallocene metal center, but also chain transfer reactions possibly caused by hydrogen generation which in turn may lead to polymer chain termination by separating it from the metallic center and finally reducing the values of Mw and Mn. This behavior also provided an increase in molar mass distribution [21]. The polydispersity of the synthesized polymers was between 1.8 and 3.3. No clear bimodality could be observed. Nevertheless, the shape of the chromatogram peak can be described as a sum of a Schulz-Flory distribution, which in turn may provide kinetic information about the catalysts centers. Fig. 1 illustrates the deconvolution using a stationary Schulz-Flory distribution for the polyethylene produced by SiW04Zr. It was necessary to include four active sites to provide a suitable description of the molar mass distribution. The distributions were deconvoluted using the method proposed by Soares and Hamielec [21], wherein iterations are made on the number of site families until the residual error between the measured and the predicted molar mass distribution is less than a given value (χ2), which in these cases, values of χ2 ranged from 0:01 to 0:08. As the support is fragmented occurs release of the metallocene to the reaction medium after the start of polymerization will form distinct polymers with different molar mass. Table 2 shows the results of the deconvoluted GPC curve in terms of potential active sites. As observed in Table 2, most of the supported systems appear to have four active sites. Increased contents of Cr, Mo and W in the entrapped catalysts caused an increase in the average molar mass values within each site. Sites I and II and III appear to be most responsible for the majority of the polymer production. The synthesized catalysts produced polymers with weight average molar mass (Mw) greater than that obtained with homogeneous catalyst. The increase in Mw may be attributed to the internal morphology of the catalyst, which has low specific surface area and thereby hindering the occurrence of chain termination reactions, especially those via beta-elimination. When metallocene is supported, encapsulated within the support particle, polymerization reactions occur in a confined environment, very different from that in solution. This requires that the environment around the metallocene immobilized on the support has electronic interactions and steric factors that may favor high catalytic activity and give the desired properties to the resulting polymer. Thus, it is possible that heterogenization may produce an increase in PDI by changing the electronic environment in which the single site catalyst is immobilized. According to the results of gel permeation chromatography it was possible to observe by deconvolution of GPC chromatograms the presence of polymers with higher molecular weights. However, these Mn fractions are being produced in small quantity [22]. In this case, there is

Table 1 Catalytic activity and polymer properties obtained with the homogeneous and supported catalysts. Catalysts

Activity (Kgpol/ molZr.h)

Tm (°C)

Tc (°C)

Xc (%)

Mw (Da)

Mn (Da)

MwD

Cp2ZrCl2 SiCr01Zr SiCr02Zr SiCr04Zr SiMo01Zr SiMo02Zr SiMo04Zr SiW01Zr SiW02Zr SiW04Zr

2300 1493 67 0 1680 320 0 2240 1653 1200

133 133 136 – 135 136 – 136 135 132

110 111 110 – 112 110 – 108 112 113

64 72 52 – 58 55 – 52 47 67

120.000 57.000 267.000 – 231.000 324.000 – 119.000 381.000 57.000

53.500 17.000 148.000 – 102.000 141.000 – 66.000 153.000 17.000

2.2 3.3 1.8 – 2.3 2.3 – 1.8 2.5 3.3

Polymerization conditions: Catalyst concentration of 7.5 × 10−6 M (toluene solvent). MAO activation (Al/Zr = 500), under Ar and ethylene at atmospheric pressure. Polymerization temperature (Tp) = 60 °C, reaction time (h) = 0.5.

3.1. Catalyst performance in ethylene polymerization Table 1 shows the polymerization conditions and results in terms of catalytic activity and polymer characteristics, including melting temperature (Tm), crystallinity (Xc), weighted-average molar mass (Mw), and polidispersity (MwD). Based on the data in Table 1, the most active catalysts were those that were synthesized with the lowest metal chloride:Si ratio, with SiCr01Zr, SiMo01Zr and SiW01Zr showing activities of 1493, 1680 and 2240 (Kgpol/molZr.h), respectively. These values are still lower than those obtained with homogeneous Cp2ZrCl2 under comparable polymerization conditions (2300 Kgpol/molZr.h). Similar differences have been reported in the literature and have been attributed to features usually found by comparing homogeneous and supported catalysts, specifically the potential total availability of the homogeneous catalyst versus the steric effect of the surface in the supported catalyst, which acts as a voluminous ligand that reduces the access of the monomer to the metal centers. In addition, several surface/support generated species may not be active for the polymerization. These characteristics and comparisons have been widely discussed in the literature [20]. Among the supported catalysts (see Table 1), as the metal content within silica matrix increases, the activity decreases drastically. In addition to the silicon species generated during encapsulation, other species are also generated such as tetracoordinated, pentacoordinated and hexacoordinated species of chromium, molybdenum and tungsten, respectively. Some of these species may make it difficult for the co-catalyst to access the active metallocene centers, thereby reducing their catalytic activity. In the case of the formed tungsten species, it can be inferred that these species make the active catalyst center more accessible to both monomer and MAO. Through the FTIR deconvolution, an increase in the percentage of species bearing (SiO)6 was observed in tungsten-based catalysts, which in turn may facilitate access the to active sites. This behavior may be a consequence of structural and textural characteristics that will be discussed later. As shown in Table 1, the resulting polymers exhibited melting temperature between 132 and 136 °C, which is typical of high density polyethylenes. The crystallinities ranged from 52 to 72% and higher molar mass when compared to the homogeneous catalyst (ca. 40.000 Da). Fig. 1 shows the GPC chromatograms for the PEs obtained by Cp2ZrCl2 and SiW02Zr. It appears that the presence of this metal in the silica network may affect the nature of the catalyst site, which in turn may influence the polymerization kinetics that determine the molar mass of the resulting polymer. Several studies have reported the production of polyethylene from metallocene complex immobilized on inorganic supports, such as silicatitania by non-hydrolytic sol-gel method and in these studies, the 4

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Fig. 1. Chromatograms of PEs produced by Cp2ZrCl2 and SiW02Zr.

the mixed-oxide support and corresponding system bearing the encapsulated metallocene was observed. Pore diameter and volume diameter do not appear to be specifically affected, but rather, the number of pores is affected. It is very likely that this reduction in surface area is due to the presence of the metallocene catalyst within the silica matrix; a slight decrease in surface area value is observed with an increase in metal content. The amount of adsorbed gas (N2), especially in catalysts with higher chloride content is so low that it was not possible to obtain BET and BJH data for these samples. The zero catalytic activity of some catalysts can be explained due to this significant reduction in specific area and probable material pore closure, making access to active sites difficult. As can be seen from the N2 adsorption and desorption measurements, the silicas are basically non-porous. The low specific area and reduced pore volume may be related to the low activity of these catalysts, which makes it difficult for MAO to enter the catalyst pores. Thus, it can be inferred that the encapsulated metallocene is more labile and contacts the MAO on the catalyst surface. During the catalyst leaching tests, no monomer was used and thus it is likely that the monomer itself will contribute to support fragmentation by releasing the metallocene into the reaction medium and thus being activated by the MAO. Therefore, the relatively good catalytic activity of the supported systems cannot be attributed to the surface area of the resulting encapsulated metallocenes. Nevertheless, a strong Spearman’s correlation (rSp = 0.880 for p < 0.01) [23] was found between catalyst activity and pore diameter. The catalyst leaching tests were performed to verify the possible extraction of the encapsulated complex by the action of MAO, or even by the solvent medium itself. The test is performed with the same mass of the catalyst and the same amount of MAO described above at 60 °C for 1 h. To determine the leaching of Cr, Mo, W and Zr to the reaction medium a reaction under the same polymerization conditions was carried out but without the use of the monomer. Aliquots were collected at different times (5, 15, 30 and 60 min), dried and acidified for their determination by ICP OES. The roughness of the immobilized catalysts in terms of leaching from MAO co-catalyst was evaluated through measuring Zr in the eluted fraction. According to ICP OES analyses, Zr metal content in solution was shown to lain between 0.03-0.14%. Therefore, leaching process during polymerization was considered very negligible (see Supplementary Table 2). In the present study, the SAXS curves of these materials had a structure formed by two levels of organization comprising a Guinier region and a Power Law region (see Fig. 3). The former provides an estimation of the Guinier radius of gyration (Rg), while the latter provides details about the organization of the system. The formation of pores in the mixed catalysts is related to the mechanism of aggregation of primary particles. The unified fit was used to interpret the

Table 2 Polyethylene GPC deconvolution data: number of active sites; percentage and polymer Mn. Catalysts

Cp2ZrCl2a SiCr01Zr SiCr02Zr SiMo01Zr SiMo02Zr SiW01Zr SiW02Zr SiW04Zr a

Site I

Site II

Site III

Site IV

%

Mn

%

Mn

%

Mn

%

Mn

χ2

100 22 54 58 50 84 25 13

53500 5100 89000 64000 86000 47300 76800 3700

39 26 22 38 14 45 32

16100 127000 140000 230000 121000 128500 10500

– 28 15 12 8 2 22 42

– 33850 272000 213000 680000 250000 335000 27000

– 11 5 8 4 – 8 13

– 81750 478000 437000 760000 – 1160000 70000

– 0.01 0.02 0.07 0.06 0.08 0.07 0.03

(homogeneous catalysts).

inhibition of the allyl deactivation mechanism (common in more active catalysts), an, in addition, the different formed sites have distinct kinetics and the higher Mw centers are less active, which in turn cleaves the chain by beta elimination reaction. Thus, the growth kinetics is under a regime that favors its growth, leading to higher values of Mw [23]. To better understand the performance of such catalysts, these systems were further analyzed via a series of instrumental techniques.

3.2. Characterization of the catalysts Table 3 shows the textural properties of the supported catalysts synthesized by the non-hydrolytic sol-gel method. For comparison reasons, data from the supported mixed oxides were also included. According to Table 3, a drastic reduction in surface area between Table 3 Textural properties of silica-mixed oxides and their supported metallocenes. Catalysts

SBET (m2 g−1)

Dp (Å)

Vp (cm3 g−1)

SiCr01Zr SiCr02Zr SiCr04Zr SiMo01Zr SiMo02Zr SiMo04Zr SiW01Zr SiW02Zr SiW04Zr SiO2

12 11 – 8.5 2.0 – 10 2.6 – 39

14 10 – 20 11 – 16 14 – 13

0.02 0.02 – 0.03 0.02 – 0.02 0.02 – 0.02

SBET = specific area; Dp = pore diameter; Vp = pore volume. 5

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accommodation of the particles. Regarding to the organization of primary particles in terms of P (Level 2), values between 1.0 and 4.0 were observed. For SiMo and SiW, secondary particles with less condensed structures (mass fractals) were formed, with P values (level 2) between 1.0 and 2.2. For SiCrZr systems, the formation of secondary particles with mass fractal characteristics was observed, and the values decreased with increasing oxide concentration in the final support. Additionally, the Power-law decay shown in Table 4 indicates that these elementary silica particles had a smooth surface (q−4) in the case of SiCr04Zr (P = 4). The Power-law regime at low -q of the SAXS curve can be attributed to a second level of organization, which is attributed to large order structures. Thus, it is likely that this structural level is formed by small elementary particles that aggregate to form larger ones. For the catalyst, as the concentration of metal chloride in the catalyst synthesis increases, the exponential decay value increases. Mass fractals characterize particles with P values lower than 3. The secondary particles with mass fractal geometry polymeric structures are more open, and the active sites in the system are more accessible to the coordination and insertion of ethylene molecules. Therefore, for these materials with more branched clusters, higher catalytic activity is expected. The lower activity values were observed for the systems that had the highest P values. For these materials, denser structures may hinder the access of condensed ethylene molecules to the active sites within the pores. Fig. 3 shows the relationship of the catalytic activity and surface roughness of the catalyst. As shown in Fig. 3, after the SAXS measurements, a relationship between the size of primary particles of silica (Rg1) and the size (Rg2) and shape of the cluster with catalytic activity became apparent. A strong indirect Spearman correlation [24] (rSp = − 0.833 for p < 0.01) was found between Rg1 and Rg2. The catalytic activity decreased with as the size of the primary particles increased. The decrease in cluster size yielded a decrease in the catalytic activity of the catalysts. Furthermore, it was observed that as the fractal roughness decreased, there was a decrease in catalytic activity, most likely due to the formation of catalysts with dense silica networks, which may hinder both MAO and monomer access to the active sites of the metallocene. In both cases, the decrease in the catalytic activity resulted in an increase in the molar mass of the polymer and consequently an increase in the average size of the chains (Mn). In fact, strong Spearman’s correlations were found between Rg1 and Mw (rSp = 0.782 for p < 0.01) and Mn (rSp = 0.748 for p < 0.01), indicating that the support texture might have affected the polymerization kinetics [24]. To investigate the structural characteristics of the catalysts, the systems were further characterized by FT-IR. Vibrational spectra of silica-based materials have previously been used to obtain information on structural aspects of the silica network [25]. Table 5 shows the main detected bands and their assignments for the supported catalysts. The FT-IR spectrum of catalyst SiW01Zr (see Fig. 4) includes a band centered at 1087 cm−1, which is attributed to the asymmetric stretching vibrations, νas(Si-O), of the siloxane groups (SieOeSi). The band centered at 958 cm−1 can be attributed to the symmetric stretching, νs(Si-O). The angular deformation band, δ(Si-O-Si), of the

Fig. 2. SAXS plots of catalysts a) SiCr01Zr b) SiW01Zr c) SiMo01Zr and respective unified fittings.

Fig. 3. Relationship of the catalytic activity and surface roughness of the catalyst. Table 4 Experimental radii of gyration and level 2 Porod of supported catalyst determined from the fit of SAXS curves. Catalysts

SiCr01Zr SiCr02Zr SiCr04Zr SiMo01Zr SiMo02Zr SiMo04Zr SiW01Zr SiW02Zr SiW04Zr SiO2

Level 1

Level 2

Rg (nm)

P

Rg (nm)

P

2.5 7.8 2.2 2.9 8.7 3.1 3.6 8.1 6.8 6.5

4 4 4 4 4 4 4 4 4 4

76.5 31.9 81.2 65.2 22.7 58.8 41.9 28.6 10.8 33.6

1.0 3.7 4.0 1.5 1.4 2.2 1.0 1.5 2.0 2.6

organization in different length scales of the hierarchical structure. The details are described elsewhere [18,19]. Based on the SAXS plots of SiCr01Zr, SiMo01Zr and SiW01Zr, shown in Fig. 2 and the calculated data in Table 4, it can be observed that an increase in the size of the cluster corresponds to an increase in the radius (1.85, 2.01 and 2.02 Å) of Cr, Mo and W, respectively. The results obtained from the unified set of SAXS curves for the catalysts are presented in Table 4. As shown in Table 4, the radius of gyration (Rg) of the primary particles in the investigated systems remained between 2.2 and 8.7 nm. Furthermore, a correlation between the metal content of the xerogels and the Rg was observed (see insert in Fig. 2): The smaller the size of the primary particles, the larger the aggregate size due to better

Table 5 Assignments in the infrared region of 2000-400 cm-1.

6

Wavenumber (cm−1)

Assigments

1064 943 958 799 900 914 905 1443/1483 1370/1393

νas(SieO) de (SieOeSi) νs (SieO) de SiOH νs(SieOe(H…H2O) bending δ(SieOeSi) (Geminal) νas(SieOeCr) νas(SieOeMo) νs(SieOeW) νs(CeC) of the Cp ring νs(CeH) of the Cp ring

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Table 6 XPS measurements of binding energies of Zirconium 3d and silicon 2p in (eV) homogeneous and supported catalysts. Sample

Zr 3d5/2

Si 2p

Cp2ZrCl2 SiCr01Zr SiMo01Zr SiW01Zr

Ratio Zr/Si

BE (eV)

FWHM (eV)

BE (eV)

FWHM (eV)

– 102.9 104.0 103.1 103.8 103.6 104.6

– 2.09 2.22 1.87

182.1 182.4 182.5 183.1

1.54 2.71 2.83 2.49

184.6 184.7 184.5 185.6

0.03 0.03 0.05

the olefin, which results in higher catalyst activity. As shown in Table 6, there was an increase in Zr BE as the size of the radius of the metal (Cr, Mo and W) increased. According to the data in Table 4, higher catalyst activity was achieved with the W-based mixed oxide system, in which Zr presented the highest Zr BE. As noted before, the lower electron density on the catalyst metal center guarantees better catalytic activity. The correlation of BE and catalyst activity has previously been reported in the literature [28,29], and systems bearing higher BE have shown to have higher catalytic activity. In other words, the acid character of the support may interfere in the Zr coordination sphere, increasing, in turn, the cationic character of the metal center. The deconvolution of the Si 2p spectra showed the presence of a peak at approximately 103.0 eV is typical of the silica matrix and is attributed to the SieO species. The second (in lower percentage) may result from the SieO species that is bound to oxide moieties which results in Si centers that are less rich in oxide density electrons (not shown). This increase in binding energy around Si is probably due to the presence of electrophilic groups (chlorides) attached to the silica network. The amount of encapsulated zirconocene species within the uppermost particle surface appears that is roughly the same (Zr/ Si = 0.03-0.05) for the three investigated binary oxides. The interaction of the metallocene with the silica-based matrices was also investigated using differential pulse voltammetry (DPV). The voltammogram of the modified electrode [Cp2ZrCl2] in aqueous solution is characterized by negative cathode potential signals at −1.5, −1.8 and −2.1 mV (pH = 1). At pH = 4, the first two signals were slightly shifted to -1.8 and −1.9 V vs Ag/AgCl (see Supplementary Fig. 1). In analogy to studies performed in an acetonitrile solution, the mechanism of the electrode is represented by Eqs. (2)–(5) [30].

Fig. 4. FT-IR spectra of SiW01Zr in KBr pellets. Deconvolution of the spectra in the infrared region of 850-1000 cm−1, νs(Si-O-W) and 1000-1400 cm−1 νs(Si-OSi).

siloxane groups corresponds to the band detected at 793 cm−1. The literature indicates that the maximum centers and relative intensities of the longitudinal optic (LO) and transversal optic (TO) modes of Si–O–Si asymmetric stretching are shifted with the introduction of chemical groups or organic molecules into the silica network [26]. According to the FT-IR spectra of the catalysts, most of the systems seem to have higher percentages of six-membered rings than four-membered ones (see Supplementary Table 3). In general, a correlation can be found between the formation of six-membered rings, which have less tension and thus provide better accommodation of the oxides formed. To verify the presence of Si-O-Metal, deconvolution of the spectra was performed in the infrared region of 850-1000 cm−1; it was possible to observe the symmetric stretching vibrations related to SieOeW in 900 cm−1. The presence of SieOeMo (912 cm−1) and SieOeCr (900 cm−1) bonds were also observed in the SiO2MoO3 and SiO2CrO3 spectra, indicating the insertion of metals into the silica network as described by Lee and Wachs [27]. The doublet bands at 1443/1483 cm−1, which appear clearly in the catalysts, are attributed to the ν(CeC) of the cyclopentadienyl ring (Cp) in the centrally π-bonded complexes according to Fisch et al. [8]. This result suggests a weaker coordination of Cp to Zr in these catalysts, most likely due to some type of interaction between the support and the Cp ligand. This weaker coordination might engender a symmetry change in the complex, which in turn might promote an alteration in the allowed and forbidden bands in the infrared spectrum. The doublet bands at 1370/1393 cm−1, which also appear more clearly in the catalysts, are related to the ν(CeH) of the Cp ring. The same vibration appears at 1377 and 1396 cm−1 in the spectrum of the neat [Cp2ZrCl2]. However, the presence of 1370 and 1393 cm−1 vibrations in the spectrum of these catalysts was not observed as demonstrated by Fisch [8]. The catalysts were further analyzed by XPS. A typical high resolution spectrum of Zr, 3d core level spectrum, is characterized by the presence of two signals due to spin-orbit coupling of the 3d electrons of Zr: ca. 183 eV (3d5/2) and 185 eV (3d3/2). Typical zirconocene XPS spectra can be found elsewhere [28,29]. For the sake of simplicity, Table 6 presents only the results concerning the peak of Zr 3d5/2. Comparing data from free Cp2ZrCl2 to the encapsulated ones reveals that there is an increase in BE (binding energy) for the systems in which the metallocene was encapsulated. It appears that the acidity of the support affects the zirconium centers, rendering them more cationic. According to XPS measurements it can be observed binding energies for Zr smaller when this is in the form homogeneous when compared to the binding energies of Zr for situations in which it is encapsulated. The observed increase in Zr 3d5/2 BE for the encapsulated systems suggests that the catalyst metal center is more cationic due to a lower electron density. Consequently, there is a greater facility for the coordination of

[(Cp)2ZrIVCl2] + e−⇆ [(Cp)2ZrIIICl2]− −

−

[(Cp)2Zr Cl2] + e ⇆ [(Cp)2Zr Cl2] III

−

II

−

[(Cp)2Zr Cl2] ⇆ [(Cp)Zr Cl2] + L III

III

−

[(Cp)Zr Cl2] + e ⇆ [(Cp)Zr Cl2] III

2−

II

−

(2) (3) (4) (5)

The differential pulse voltammograms for the support SiCr01 (i.e., without metallocene) show a cathodic peak at approximately +400 mV and a shoulder at −275 mV. At pH = 5, a cathodic peak around +185 mV with a low current intensity and another peak in −80 mV versus Ag/AgCl are observed. The first cathodic peaks +400 and −80 mV are related to the reduction to Cr6+/n+ (Supplementary Fig. 2), similar to SiO2-TiO2 [31]. The pH dependence of the reduction process shows that the redox reaction involves the coordination of H3O+ ion sites in Cr. To gather information about the zirconocene entrapped on SiCr01, DPV of the corresponding modified electrode with SiCr01Zr was performed in the potential range of 1 to −2.0 V (versus Ag/AgCl), but no cathodic processes other than those observed for the modified electrode with SiCr01 could be detected. DPV of SiMo01 also showed that this system is pH dependent. At pH = 1, a cathodic signal is observed at +465 mV versus Ag/AgCl. For higher pH values, the resolution of this signal decreases. At pH = 1, the cathode has a signal at +185 mV. The signal at −260 mV (cathodic 7

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A.A. Bernardes, et al.

signal) is shifted to −520 mV at pH = 1 and to −700 mV at pH = 3 and 4. There is also a sign of high current at −1.38 mV versus Ag/AgCl (see Supplementary Fig. 3). All these processes are attributed to the redox processes centered on Mo. The first cathodic peak positioned at +465 (pH = 1) can be attributed to the reduction of Mon+/ (n−1) or Mo(n-1)+/(n-2)+, corroborating the UV–vis diffuse reflectance spectroscopy data reported in the literature for silica-based mixed oxide bearing Mo6+ species [8]. The dependence of the peak position and peak current on pH indicates that the redox reaction involves the reduction of an Mo water complex [Mox(OH2)2] (see Supplementary Fig. 3). In the region of −1 to −2 mV, the cathode signals represent redox processes located on the metallocene with the contribution of Mo reduction. The difference in profile compared to bare metallocene reflects the interference of the support on the electrochemical processes of Cp2ZrCl2, as described in Eqs. (2)–(5). In the case of SiW01, the same pH dependence on the cathodic process could be observed. Differential pulse voltammogram of the modified aqueous solution at pH = 4, where the signal corresponding to first cathodic reduction of W is present, demonstrates that there is a reversible electrode. It is noteworthy that at pH 1–3, the same behavior was observed. At pH 1, the cathode has a signal at 210 mV, which is designated as Wn+/n−1. The existence of an isosbestic point at 310 mV (Supplementary Fig. 4) may be indicative of the formation of a single species, which may be the product containing coordinated water, as shown in the Scheme (inset), thus explaining the significant pH dependency. The catalyst system in which the metallocene was immobilized within a silica-tungsten matrix had a voltammogram profile with an offshoot in the more positive sign in the positive potential region, indicating matrix changes. As shown in Supplementary Fig. 1, in Cp2ZrCl2, there is a metal centered reduction process of the metallocene in the range of −1.5, −1.8 and −2.1 mV versus Ag/AgCl. In the SiW01Zr catalyst, there are signals in this region. However, there is no possibility of assigning with certainty because there are signals in the voltammogram of the cathode SiO2-WO3 matrix in this potential range. When a comparison is made, it is clear that in the range of −1.4 V to −1.8 V, the signals must be those corresponding to the contribution of the metallocene, i. e., the process in which the reduction occurs (Zr (IV)). As can be observed in the DPV results, the SiMo01 and SiW01 supports interact with the metal center of the metallocene, shifting it to a more positive potential and thus causing a change in the electron density on Zr, which corroborates the XPS results. The change in the oxidation potential towards more positive values, as in the cases of the catalysts SiMo01Zr and SiW01Zr, reflects the necessity of a larger current to move the electrode potential. Thus, the metal center of the metallocene is less susceptible to deactivation because it is more protected by the oxide support. Therefore, such systems exhibit high activity. The interaction between the acid support surface and the immobilized metallocene enables the activation reaction to occur with lower concentrations of MAO. In other words, the immobilization of metallocenes within acid supports leads to a significant reduction in electron density on the Zr, which is able to polymerize with lower concentrations of MAO to activate the catalysts.

termination reactions, especially those via beta-elimination. When the metallocene is encapsulated within the support particle, polymerization reactions occur in a confined environment, different from that in the absence of the support. This requires that the environment around the metallocene immobilized on the support has electronic interactions and steric factors that may favor a high catalytic activity and give the desired properties to the obtained polymer. Moreover, it is possible that heterogenization will produce an increase in PDI by changing the electronic environment in which the single site catalyst is immobilized. According to the results of gel permeation chromatography it was possible to observe by deconvolution of GPC chromatograms the presence of polymers with higher molecular weights, however, these Mn fractions are being produced in smaller quantity The catalytic activity was also shown to be directly related to the size and shape of the mass fractal; it is observed that by increasing the size of the primary particle, the access to catalyst centers may be hindered, affecting the polymerization kinetics. These results suggest the necessity of taking into account synthetics routes that guarantee small primary particles. The electronic, textural and structural characteristics of the binary oxides seem to affect the catalyst performance when a zirconocene is encapsulated within this silica network. The main features of this system are that the catalyst characterization showed that the entrapped complex exhibits lower Zr electronic density than the corresponding unsupported metallocene, which in turn is affected by the nature of the mixed oxide, according to the XPS and DPV results. The binding energy of Zr 3d3/2 of the synthesized catalysts is higher than those found for free metallocene. The increase in binding energy show that interaction of Cp2ZrCl2 with silica makes Zr electron-deficient and produces a zirconocene cation. The highest activities may be related to better stabilization of metallocene complex when entrapped in support. The results of differential pulse voltammetry showed changes in oxidation potentials to more positive values for SiMo01Zr and SiW01Zr catalysts. Changing the oxidation potential towards more positive values, reflects the need for greater power to move the electrode potential and thus the metal center of the metallocene is less susceptible to deactivation. The polymer generated from SiCr01Zr catalyst showed the lowest molar mass and this effect can be attributed to a larger number of chain termination reactions with β-H transfer to the transition metal. It can also be related to a smaller separation of the ion pair formed between the MAO and the metallocene complex and thus stabilization the cationic complex is not efficient. From the results of XPS, it was observed an increase in the FWHM of the high-resolution Zr (3d3/2) spectrum for the heterogeneous catalysts compared with the homogeneous catalyst. This increase can be attributed to a greater number of active sites in the catalyst, thus confirming the results of the GPC curves in which the presence a greater number of active sites and higher molar mass polymers synthesized with heterogeneous catalysts was observed.

Declaration of Competing Interest All the authors are aware of the submission and agree to its publication. We confirm that the present submission is original and not under consideration for publication elsewhere. There is no conflit of interest with other people or organization.

4. Final remarks The results demonstrate that the entrapping method allows the generation of a catalyst system in which part of the activation process is attributed to the support. An understanding of fundamental structureactivity relationships is important for future catalytic reaction studies because it allows for the determination of the fundamental factors affecting the reactivity and selectivity of the catalytic active sites in order to improve their catalytic activity. The increase in Mw may be attributed to the internal morphology and texture of the catalyst, which has low specific surface area and thereby hindering the occurrence of chain

Acknowledgements This project was partially financed by the CNPq (309002/2015-0) and FAPERGS (16/2551-0000470-6). A. Bernardes thanks CAPES for the grant, and the authors are thankful to the LNLS (Project D11ASAXS1-8691) for the measurements in the SAXS beamline.

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Appendix A. Supplementary data

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