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[Barzan et al., 59, Topics in Catalysis, 2016, pagg 1732–1739]
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1 2 3 4 5 6 7 8 9 10 11 12Reactivity of hydrosilanes with the Cr
II/SiO
2
Phillips catalyst: observation of
intermediates and properties of the modified Cr
IIsites
Caterina Barzan1, Silvia Bordiga1, Elsje Alessandra Quadrelli2* and Elena Groppo1*
1University of Torino, Department of Chemistry, NIS Centre and INSTM, Via G. Quarello 15A, I10135, Italy. e-mail: elena.groppo@unito.it (*), caterina.barzan@unito.it, silvia.bordiga@unito.it
2Universite´ de Lyon, CNRS—CPE Lyon—Universite´ Lyon 1 (C2P2 UMR 5265), Bât 308F, 43 B. du 11 Novembre 1918, 69616, Villeurbanne, France. e-mail: alessandra.quadrelli@cpe.fr (*)
Abstract
The reaction of hydrosilanes (both silane and triethylsilane) with CrII/SiO2 catalyst has been investigated in detail by analysis of the gaseous by-products, temperature- and pressure- resolved FT-IR spectroscopy and deuterium exchanges. We found that the reaction proceeds via two steps, passing through intermediates characterized by elongated Si-H bonds and transient Cr-hydride species leading to the release of H2 in the gas phase. These experimental evidence allowed us to advance an hypothesis of the reaction mechanism, which validates our previous proposal for the structure of the modified chromium sites. Furthermore, based on the intermediates of the reaction mechanism, we have also tested the ability of the modified “homogeneous-like” CrII sites toward H2 (D2) activation, demonstrating that, contrarily to the unmodified CrII species, such reactivity is present.
Keywords
Phillips catalyst, hydrosilanes, tandem catalysis, chromium hydride, FT-IR spectroscopy
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1. Introduction
In the field of industrial ethylene polymerization, tandem catalysis is often adopted to yield in a single reactor a polymer that otherwise would not be accessible by the individual catalysts. This method is used for the one-pot production of specific linear low density polyethylenes (LLDPEs), avoiding the preparation, purification and transport of the comonomer to the polymerization site [1, 2]. Basically, this approach involves the cooperative action of two active sites co-present in the same reactor: the first oligomerizes ethylene to the desired comonomer and the second copolymerizes ethylene and the comonomer, to give a co-polymer with specific properties [1]. More in detail, tandem catalysts can be constituted of two separate systems composed of a highly selective tri/tetramerization catalyst and a polymerization one [3-8]. However, some difficulties might arise due to the interference between the two catalysts (usually one homogenous and one heterogeneous) and the matching of the two activities, which is fundamental to control the ethylene-comonomer concentration. Another possibility to obtain a tandem catalyst is the selective modification of an heterogeneous polymerization catalyst with an external agent, leading to the co-presence in the same catalyst of sites active either in ethylene oligomerization or in polymerization [9-12]. This approach, called in-situ branching, is highly advantageous since in-situ produced α-olefins are incorporated about two to four times more efficiently than in ordinary catalytic copolymerization with α -olefins added from external sources [13].
Modified Cr/SiO2 heterogeneous Phillips catalysts account for the production of several unique low-density commercial polyethylenes manufactured by using the in-situ branching technology [14-17]. To this purpose, the most employed modifying agents are metal alkyls (such as AlR3, BR3 and ZnR2) and hydrosilanes [13]. AlR3 was recently [18] employed to develop a different reactivity on a Cr/Ti/SiO2 Phillips catalyst, leading to the in-situ production of α-olefins which are incorporated into the polyethylene chains with a reverse branching distribution (more branched at high molecular weight). The effect of hydrosilanes on the structure of the chromium sites at a molecular level has been recently investigated by us for the CO-reduced CrII/SiO2 catalyst [19]. We found that the in-situ branching is promoted by the co-presence of two different CrII sites, as shown in Scheme 1. A large fraction of modified CrII sites, having a “homogenous-like” character and displaying a flexible ligand sphere of general formula [OCrIIOSiR3] (with R = H or Et), account for the oligomerization of ethylene. The remaining unmodified CrII sites, highly unsaturated and less flexible, copolymerize the in-situ produced comonomers and ethylene.
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Although the molecular structure of the modified CrII sites has been defined, no hypothesis on their generation mechanism was advanced yet.
In this contribution, we investigate in detail the reaction between hydrosilanes (both silane and triethylsilane) and the CrII/SiO2 catalyst, with the final aim to identify the reaction intermediates and the mechanism leading to the formation of the modified sites. The reaction is monitored by means of gas chromatography (equipped with a thermal conductivity detector –TCD) of the gaseous by-products, temperature- and pressure-resolved FT-IR spectroscopy and deuterium exchanges. The complementary use of these techniques reveals to be fundamental in the identification of the intermediate species involved in the formation of the modified [OCrIIOSiR3] sites.
Scheme 1. Schematic representation of the structure and function of the dual active sites at the surface of a CrII/SiO 2
catalyst after modification with hydrosilanes at room temperature: the modified [OCrIIOSiR
3] sites oligomerize ethylene (mainly to 1-hexene), whereas the unmodified [OCrIIO] sites copolymerize the in-situ produced α-olefins and ethylene to yield LLDPE (“in-situ branching mechanism”).
2. Experimental
The parent Cr/SiO2 catalyst having a chromium loading of 1 wt% was prepared by wet-impregnation of a fumed silica–Aerosil 380 (specific surface area about 380 m2g-1) with a chromic acid solution [20-22]. The catalyst was then activated according to the following procedure: (i) slow heating up to 973 K in dynamic vacuum (< 10-7 bar) followed by calcination in O2 at the same temperature for 1 hour; (ii) reduction in CO at 623 K for 1 hour followed by evacuation at the same temperature; and (iii) cooling down to room temperature under dynamic vacuum. It was previously demonstrated that step (i) provides highly dispersed monochromates [22-24], which are stoichiometrically reduced to highly uncoordinated CrII sites by CO (step (ii)) [19, 20, 23]. In the following we will refer to this sample as CrII/SiO2 catalyst.
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Modification of the CrII/SiO2 catalyst with silane (SiH4) was performed by contacting the surface with a mixture of SiH4 (5000 ppm) in argon at room temperature (10 L gas bottle, 150 bar, SiH4 0.5 %, Ar 99.5 %). SiH4 was left in contact with the catalyst for about 45 minutes for a complete reaction. Modification with triethylsilane (TES from Sigma-Aldrich, 99% purity) was accomplished by dosing the vapour pressure of TES (about 20 mbar), and leaving it in contact with CrII/SiO2 for about 15 minutes. The excess was successively removed by degassing at room temperature.
Deuterium exchanges experiments were performed at room temperature after modification of the CrII/SiO2 catalyst with both hydrosilanes by contacting the surface with 20 mbar of pure D2. Deuterium gas phase was left in contact with the catalysts until an equilibrium is reached.
FT-IR experiments were performed in-situ on thin self-supported wafers placed inside an IR cell designed to allow thermal treatments in the 1000 – 77 K range. This procedure allows to activate the samples directly into the IR cell where FT-IR experiments are conducted, avoiding any catalyst poisoning. The FT-IR spectra were collected on a Bruker Vertex 70 spectrophotometer at 2 cm-1 resolution, and at a time resolution of 30 s. Temperature- and pressure-resolved FT-IR experiments were performed by cooling the sample down to about 100 K with liquid nitrogen and following the interaction of the hydrosilanes at increasing temperature from 100 to 298 K.
The gas phase analyses were performed with a gaschromatograph Agilent7890A, equipped with a HP-PLOT molecular sieve column (15 m, ID 0.32 mm, 25 µm film) at 328 K. The detection of H2 was performed with a TCD at 473 K.
3. Results and Discussion
3.1 Reactivity of hydrosilanes with CrII/SiO2: in-situ FT-IR spectroscopy
Figure 1a shows the FT-IR spectra of CrII/SiO2 before and after interaction with SiH4, where the SiH4:Cr ratio is 1. The spectrum of CrII/SiO2 (spectrum 0) is that typical of a highly dehydroxylated SiO2 sample and in particular it is characterized by: i) a narrow IR absorption band at 3745 cm-1 due to the ν(OH) of isolated silanol groups; ii) three absorption bands at 1980, 1868 and 1640 cm-1 due to the first overtones of the silica framework modes, and iii) a strong and out-of-scale absorption below 1300 cm-1 assigned to the vibrational modes of the bulk [21]. Upon interaction with SiH
4 (spectrum 1) several IR absorption bands gradually grow in the 2300-2000 cm-1 region, reaching their maximum intensity in about 45 minutes. The time evolution is mainly due to the slow diffusion of the highly diluted SiH4 (5000 ppm in Ar) in the cell. These absorption bands have been previously assigned [25-28] to the ν(Si-H) modes of two different types of SiHx species: a) the
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bands above 2200 cm-1 have been ascribed to SiHx species bonded to an oxygen atom [OSiHx] [26, 27]; b) the bands below 2200 cm-1 have been assigned to [SiH2]n and related oligomeric species [25]. No other changes are observed in the IR spectrum of CrII/SiO2, indicating that the reaction of SiH4 with CrII/SiO2 does not involve the consumption or the formation of surface OH groups.
Similar ν(Si-H) absorption bands are observed when a larger amount of SiH4 is dosed on CrII/SiO2, but with a different relative intensity. The inset in Figure 1a shows the FT-IR spectrum, in the ν(Si-H) region, of CrII/SiO2 in interaction with SiH4, where the SiH4:Cr ratio is 5 (spectrum 2), compared to that discussed above for a SiH4:Cr ratio of 1 (spectrum 1). The IR absorption bands below 2200 cm-1, assigned to oligomeric [SiH2]n species [25], are more intense than in the previously discussed case, while those above 2200 cm-1 (assigned to –[OSiHx] species [26, 27]) have almost the same intensity. These data indicate that the amount of –[OSiHx] species is already very large for a SiH4:Cr ratio of 1 and is unvaried upon addition of more reactant. On the basis of these results, the IR absorption bands at 2212 and 2195 cm-1 are assigned to the symmetric and antisymmetric stretching modes of –[OSiH3] species belonging to the modified CrII sites [19]. Indeed, we have reported that the reaction of CrII/SiO2 with silane leads ultimately to the formation of a large portion of [OCrIIOSiH3] species (85% of the total chromium content [19]). When SiH4 is dosed in excess with respect to chromium, the SiH4 leftover rapidly oligomerizes in the form of [SiH2]n like species, indicating the occurrence of a secondary reaction at the surface of the catalyst. To confirm that the reactivity observed involves the chromium sites, blank experiments were performed on pure SiO2 in presence of SiH4,revealing no reactivity at all (see SI Figure S1).
Interaction of CrII/SiO2 with large excess vapors of TES occurs much faster with respect to SiH4, since TES is not diluted. In presence of a large excess of TES, the FT-IR spectrum (spectrum 1 in Figure 1b) is dominated by the vibrational manifestations of physisorbed TES. In particular, the IR absorption bands around 2900 cm-1 and 1400 cm-1 are due to the vibrations of the ethyl groups (νasym(CH3) at 2960 cm-1, νsym(CH3) at 2878 cm-1, νasym(CH2) at 2915 cm-1, νsym(CH2) at 2855 cm-1, δasym(CH3) at 1465 cm-1, δsym(CH3) 1380 cm-1 and δ(CH2) at 1416 cm-1), while the sharp band at 2100 cm-1 is assigned to the ν(Si-H) of the Si-H group in interaction with Si-OH surface groups (whose absorption band shifts from 3745 cm-1 to around 3650 cm-1). Contrarily to SiH4, TES does not oligomerize. As a consequence, the IR spectrum after removal of TES in excess (spectrum 2 in Figure 1b) does not show anymore the ν(Si-H) absorption band, but only those characteristic of the
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ethyl groups. Contrarily, when TES is dosed on pure SiO2 it is only physisorbed and upon removal no absorption bands of ethyl groups are present (see SI, Figure S2). Thus, the IR absorption bands of ethyl groups remaining on the modified catalyst were related to the modified [OCrIIOSiEt3] species formed upon TES reaction [19].
Figure 1. Part a): FT-IR spectra of CrII/SiO
2 before (spectrum 0) and after (spectrum 1) interaction with SiH4 (SiH4:Cr = 1) at room temperature (contact time = 45 minutes). The inset in part a) reports the background subtracted spectra in the ν(Si-H) region of CrII/SiO
2 modified by SiH4 in a SiH4:Cr ratio of 1:1 (spectrum 1) and 5:1 (spectrum 2). Part b): FT-IR spectra of CrII/SiO
2 before (spectrum 0), after interaction with vapors of TES at room temperature (spectrum 1, contact time = 15 minutes) and after degassing the physisorbed TES (spectrum 2).
3.2 Mechanism of reaction between hydrosilanes and CrII/SiO2
The gas-phase by-products formed during the reaction of hydrosilanes with CrII/SiO2 have been analyzed by gas chromatography: only molecular H2 was detected. This observation combined with the stepwise study reported above suggests that the reaction proceeds through two consecutive steps. The first one (Step 1 in Scheme 2) involves the reaction of a hydrosilane molecule with the [OCrIIO] sites, leading to the formation of the [OCrIIOSiR3] (with R = H or Et) modified sites and of a [SiH] surface species. This latter species can successively react with a second hydrosilane molecule (Step 2 in Scheme 2) to give a [Si-SiR3] moiety and a H2 molecule. When SiH4 is present in excess, further reactions may occur between gaseous SiH4 and the surface species, leading to the formation of silanes oligomeric species, as for example [SiH2]n, with the concomitant release of one molecule of H2 for each insertion event. Hence, the term SiH4 dehydro-oligomerization is more appropriate to indicate the reaction of SiH4 with CrII/SiO2.
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Scheme 2. The two consecutive steps in the reaction between hydrosilanes and the CrII/SiO
2 catalyst.
More details on the reaction mechanism are obtained by following the reaction below room temperature [29, 30]. This method allows to slow down the reaction, enabling the observation of intermediate species. In-situ FT-IR spectroscopy at low temperature and in controlled pressure conditions has been successfully applied in the past to obtain important insights on the ethylene polymerization mechanism on the CrII/SiO2 catalyst [31-34] and also on the hydrosilane modified catalysts objects of this work [28]. In the latter case, this experimental approach allowed the observation of the in-situ formed -olefins before their incorporation in the growing polymer chains.
Figure 2 shows the temperature and pressure-resolved FT-IR spectra, in the ν(Si-H) region, collected during interaction of SiH4 with CrII/SiO2. The experimental procedure was as followed. A pellet of CrII/SiO2 is cooled down to about 100 K. When SiH4 is introduced in the cell it immediately condenses at the coldest part of the cell (metallic sample holder) and no detectable amount of gas reaches the sample (spectrum 0 in Figure 2a). At this point, the temperature is allowed to gradually increase. Under such conditions, two main thermodynamic variables play a role: temperature and pressure. In the 100 K - 160 K range SiH4 slowly vaporizes and condenses at the surface of CrII/SiO2, starting to react with it slowly. The FT-IR spectra collected at increasing temperature are characterized by three series of absorption bands which evolve in a different way: 1) a series of bands in the 2280 – 2150 cm-1 region (maxima at 2245, 2220, and 2188 cm-1), most of them present also in the spectrum of CrII/SiO2 modified by SiH4 at room temperature; 2) a complex IR absorption band around 1600 cm-1 (maxima at 1645, 1618 and 1600 cm-1); and 3) a second very broad absorption around 2000 cm-1 (with maxima at 2083, 1988, 1960 and 1896 cm-1). The latter
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two groups of absorption bands, which are not observed in the spectrum of CrII/SiO2 modified by SiH4 at room temperature (spectrum 1 in Figure 1a), grow simultaneously and reach a maximum for a temperature around 160 K (spectrum 1 in Figure 2a). Then they gradually decrease in intensity in a parallel way, until they disappear at room temperature (spectrum 2 in Figure 2b). With increasing temperature also the IR absorption band at 2245 cm-1 gradually disappears, in concomitance with the growth of the bands in the 2200-2100 cm-1 range. The final FT-IR spectrum is analogous to that collected for CrII/SiO2 modified by SiH4 at room temperature.
The transient behavior of the two groups of bands around 1600 and 2000 cm-1 indicates that they are associated to the vibrational manifestations of intermediate species in the reaction between SiH4 and CrII/SiO2. In particular, the absorption bands around 1600 cm-1 are assigned to the vibrations of pseudo Cr-H species [35, 36], whereas those around 2000 cm-1 are assigned to the vibrations of elongated Si-H bonds [37]. The complex nature of both absorptions indicates the formation of a multitude of similar intermediate species. On the contrary, the IR absorption band at 2245 cm-1 is assigned to the vibration of surface [Si-H] species [38] formed upon the cleavage of the O-Cr bond. These species are observed only at low temperature.
Figure 2. Temperature- and pressure-resolved FT-IR spectra collected during the reaction of SiH4 with CrII/SiO2 in the 100 – 160 K temperature range (part a) and from 160 K to room temperature (part b). Spectra 0, 1 and 2 have been collected at about 100 K, 160 K (i.e. when the intensity of the IR absorption bands due to intermediate species is maximized) and close to room temperature. All spectra have been subtracted from the spectrum of the starting CrII/SiO 2. 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228
The temperature- and pressure-resolved FT-IR data discussed above provide an evidence that the reaction between SiH4 and CrII/SiO2 proceeds through the formation of intermediates characterized by an elongated Si-H bond and a pseudo Cr-H bond. At least two types of four-center intermediates may account for the experimental results. Intermediate Si-H in Scheme 3 justifies the observation of an elongated Si-H bond. It may directly evolve to the modified [OCrIIOSiH3] sites, through the breaking of a [CrIIOSi] bond and the formation of a [SiH] surface species (band at 2245 cm-1). On the other hand, intermediate Cr-H in Scheme 3 may account for the observation of a pseudo Cr-H bond, in similarity to σ-bond metathesis occurring on M-O bonds [39-42]. This intermediate might also evolve in the thermodynamically more stable [OCrIIOSiH3] species according to non-direct rearrangements. The presence of at least two types of reaction intermediates having similar characteristics explains the complexity of the FT-IR spectra. Both intermediates are stable only at low temperature and are observed simultaneously to the formation of the modified sites.
Scheme 3. Hypothesized intermediate species involved in the first step of the reaction between SiH4 and CrII/SiO2. In red are indicated the species giving transient IR absorption bands around 2000 cm-1 (elongated Si-H species) and 1600 cm-1 (Cr-hydride). Both intermediates can be observed only at low temperature. Intermediate Si-H evolves directly to the final product ([OCrIIOSiH
3] + [O3SiH]), while intermediate Cr-H rapidly evolves and rearranges to give the product thermodynamically most stable.
Transient species similar to those discussed above were observed also during reaction of CrII/SiO2 with TES, although with a different relative intensity. Figure 3 parts a) and b) show the FT-IR spectra, in the ν(Si-H) region, collected at three different temperatures during a
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and pressure-dependent FT-IR experiment: i) around 100 K (spectrum 0), when TES is almost completely condensed at the sample-holder; ii) around 230 K (spectrum 1), when the absorption bands of the intermediate species are maximized; and iii) close to room temperature (spectrum 2). Spectrum 1 is characterized by a sharp band at 2100 cm-1 due to the ν(Si-H) mode of physisorbed TES, and by a very broad absorption in the 2000-1850 cm-1 region, witnessing the presence of a large heterogeneity of elongated Si-H species. A very weak band is also observed around 1600 cm -1, indicating the formation of very little Cr-hydride species. Upon increasing temperature, the bands in the 2000-1850 cm-1 region and that around 1600 cm-1 decrease in intensity until they disappear. The band at 2245 cm-1, testifying the presence of surface [Si-H] species, is not observed in this case, indicating that its lifetime is much shorter than in presence of SiH4. This observation suggests that the side-reaction involving the catalyst surface (step 2 in Scheme 2) proceeds faster. The spectrum collected at room temperature (spectrum 2) is the same as that collected upon dosing TES on CrII/SiO2 at room temperature (spectrum 2 in Figure 1). Hence, the data shown in Figure 3 allow to conclude that TES reacts with CrII/SiO2 following a mechanism similar to that discussed for SiH4, but involving a lower amount of Cr-hydride intermediate species.
Figure 3. Temperature- and pressure-resolved FT-IR spectra collected during the reaction of TES with CrII/SiO
2 from 100 K to around 230 K (part a) and from 230 K to room temperature (part b). Spectra 0, 1 and 2 have been collected at about 100 K, 230 K (i.e. when the intensity of the IR absorption bands due to intermediate species is maximized) and at room temperature. All spectra have been subtracted from the spectrum of the starting CrII/SiO
2.
3.3 The modified CrII sites activate the D2 (H2) molecule
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The observation of Cr-hydride intermediates and the production of molecular H2 during the reaction between hydrosilanes and CrII/SiO2, suggested the possibility for the modified [OCrIIOSiR3] sites to activate the H2 molecule. This was verified by dosing D2 ( P
D2 = 20 mbar)
at room temperature on a CrII/SiO2 catalyst previously modified by SiH4 (SiH4:Cr = 5:1). Figure 4 shows the FT-IR spectra of the modified catalyst (spectrum 1) and of the same sample after twenty minutes of reaction with D2 (spectrum 2). The absorption bands below 2200 cm-1, assigned to silanes oligomeric species, as for example [SiH2]n, are gradually consumed in favor of new IR bands in the 1675-1500 cm-1 region. These bands, which are red-shifted of about 600 cm-1 with respect to the ν(Si-H) bands, are assigned to ν(Si-D) of [SiD2]n and similar species, in agreement with the isotopic ratio typical of deuterium substitutions (1.35-1.41). It is interesting to observe that the isotopic substitution involves only the oligomeric [SiH2]n species and not the [OCrIIOSiH3] modified sites. This observation indicates that an equilibrium is present for the dehydro-oligomerization of SiH4, since the reaction proceeds in the reverse direction in presence of gaseous D2. These experiments demonstrate that the [OCrIIOSiH3] modified sites are able to activate the D2 (H2) molecule, contrarily to the unmodified -[O-CrII-O]- sites.
Figure 4. FT-IR spectra, in the ν(SiHx) and ν(SiDx) regions, of CrII/SiO2 catalyst modified by SiH4: (SiH4:Cr = 5:1, spectrum
1), and of the same sample after reaction with D2 (PD2 = 20 mbar, contact time 20 minutes, spectrum 2). All spectra have been subtracted from the spectrum of the starting CrII/SiO
2. Arrows indicate the evolution of the absorption bands as a function of time.
Figure 5 shows the same experiment for a CrII/SiO2 catalyst previously modified by TES. It must be recalled that this catalyst does not present any residual Si-H groups that can be substituted by D2. Interestingly, in presence of D2 only some of the absorption bands assigned to the ethyl groups decrease in intensity. In particular, the bands at 2915 and 2855 cm-1 (due to the asymmetric and symmetric (CH2) modes) are substituted by three bands at 2185, 2149 and 2084 cm-1. The bands
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at 2185 and 2084 cm-1 are assigned to (CD2) modes, whereas that at 2149 cm-1 is attributed to the 2δ(CD2) mode (fundamental vibration around 1085 cm-1), indicating that only the CH2 groups of reacted TES are selectively exchanged by CD2 groups. A similar H/D exchange was observed by Coutant et al. [43] in presence of zirconium hydride complexes. The observation of H/D exchange on ethyl groups testifies that also the modified [OCrIIOSiEt3] sites are able to activate the D2 (H2) molecule.
Figure 5. FT-IR spectra, in the ν(SiHx), ν(SiDx) and δ(SiDx) regions, of CrII/SiO2 catalyst modified by TES (spectrum 1), and of the same sample after reaction with D2 (PD2 = 20 mbar, contact time 20 minutes, spectrum 2). All spectra have been subtracted from the spectrum of the starting CrII/SiO
2. Arrows indicate the evolution of the absorption bands as a function of time.
Conclusions
Hydrosilanes are commercially used to modify the Cr/SiO2 catalyst, which otherwise would produce a high-density polyethylene, for the manufacture of a unique low-density commercial polyethylene. It is known that the production of LLDPE proceeds through an in-situ branching mechanism: the modified chromium sites account for the oligomerization of ethylene to α-olefins (mainly 1-hexene), whereas the unmodified chromium sites copolymerize the in-situ produced α-olefins and ethylene [13, 19]. In this work we have investigated in detail the reaction of hydrosilanes (both SiH4 and TES) with the [OCrIIO] sites at the surface of CrII/SiO2 to give ultimately the modified [OCrIIOSiR3] sites (with R = H or Et). GC-TCD of the gaseous by-products revealed that H2 is released during the reaction. This experimental observation witnesses the occurrence of a side-reaction at the catalyst surface, subsequent to the modification of CrII, and involving the [Si-H] species nearby the modified [OCrIIOSiR3] sites. More important, temperature- and pressure-resolved FT-IR spectroscopy provided evidence for at least two types of intermediate species during the CrII modification step, which are characterized by elongated Si-H
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and pseudo Cr-H bonds, the latter more abundant when SiH4 is used. These intermediates are stable only at low temperature (below 230 K). At higher temperature they rapidly evolve (directly or through rearrangements) to the thermodynamically stable modified [OCrIIOSiR3] sites.
Finally, we have also demonstrated that the modified “homogeneous-like” CrII sites are able to activate the H2 (D2) molecule, contrarily to the unmodified CrII species. This latter finding might explain why CrII/SiO2 modified by hydrosilanes shows a good H2 response during ethylene polymerization, behaving “more like organochromium species” [13], while the unmodified CrII/SiO2 shows almost no H2 response.
Acknowledgements
The authors are grateful to the organizing committee of ISHHC Conference held in Utrecht in July 2015. The laboratory COMS and LCPP in Lyon are kindly acknowledged for the support in the collection of gas-chromatographic data. This work has been supported by the Progetto di Ateneo/CSP 2014 (Torino_call2014_L1_73) and by the Grant VINCI from Université Franco-Italienne. 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338
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