Caterina Barzan

The potential of spectroscopic methods applied to heterogeneous catalysts for olefin polymerization

Heterogeneous Ziegler–Natta and Phillips-type olefin polymerization catalysts have the monopoly of isotactic polypropylene production and a large share in the market of high density polyethylene, respectively. Their high industrial impact and the relatively mild conditions under which they work explain why both catalysts have been the subject of an intense research. The industrially adopted strategy to improve catalysts formulation is still based on a trial-and-error procedure; however, a rational design of new and more efficient catalysts (which is the key to produce polyolefins having a specific architecture) necessarily implies to achieve a detailed understanding of the structure of the active sites at a molecular level. Herein, it is shown that spectroscopic methods have this potential, especially when several complementary techniques are adopted and coupled with theoretical calculations. This is valid for both Phillips-type and Ziegler–Natta catalysts, because most of the problems encountered in their characterization and understanding are common, although for decades they were not considered to be closely related. The main advantages and disadvantages of several spectroscopies in the investigation of both categories of catalysts are critically analyzed, by discussing many examples taken from the recent literature.

Ethylene polymerization on a SiH4-modified Phillips catalyst: detection ofin situ produced α-olefins by operando FT-IR spectroscopy

Ethylene polymerization on a model Cr(II)/SiO(2) Phillips catalyst modified with gas phase SiH(4) leads to a waxy product containing a bimodal MW distribution of α-olefins (M(w) < 3000 g mol(-1)) and a highly branched polyethylene, LLDPE (M(w) ≈ 10(5) g mol(-1), T(m) = 123 °C), contrary to the unmodified catalyst which gives a linear and more dense PE, HDPE (M(w) = 86,000 g mol(-1) (PDI = 7), T(m) = 134 °C). Pressure and temperature resolved FT-IR spectroscopy under operando conditions (T = 130-230 K) allows us to detect α-olefins, and in particular 1-hexene and 1-butene (characteristic IR absorption bands at 3581-3574, 1638 and 1598 cm(-1)) as intermediate species before their incorporation in the polymer chains. The polymerization rate is estimated, using time resolved FT-IR spectroscopy, to be 7 times higher on the SiH(4)-modified Phillips catalyst with respect to the unmodified one.

The Effect of Hydrosilanes on the Active Sites of the Phillips Catalyst: The Secret for In Situ α‐Olefin Generation

Polyolefins are among the most widespread plastics in use today; in 2010 their world production amounted to more than 120 Mt, 30 % of which was constituted by polyethylene. A dominant share of polyethylene market is based on the silica-supported chromium-based Phillips catalyst (Cr/SiO2), [2] whose largest industrial application is high-density polyethylene (HDPE) production (about 40–50 % of the total HDPE market). Modified versions of the Phillips catalyst account for the production of some linear-low-density polyethylene (LLDPE), either by ethylene/a-olefins copolymerization or directly from neat ethylene polymerization. This latter co-monomer-free industrial process presents several commercial advantages with respect to the co-polymerization route, such as: 1) lower feedstock cost (ethylene is cheaper than the typical co-monomers), 2) feasibility on production sites where the co-monomer is not available, and 3) reduced cost of loading, purification, storage, feeding, or downstream recycling in absence of comonomer. Consequently, modified Phillips catalysts capable of yielding LLDPE without the use of external a-olefins as co-monomers have become the basis for some unique lowdensity commercial polymer grades introduced during the early 1990s. In 1991 Martin et al. patented the use of hydrosilanes as modification agents for the Cr/SiO2 catalyst to produce LLDPE by co-monomer free ethylene polymerization. Among many co-catalysts (such as AlR3, BR3, MgR2, and ZnR2), [5d] hydrosilanes remain the most effective for specific LLDPE production. So far, mechanistic insight into such hydrosilane-induced modification has been gathered essentially from the analysis of the resulting reaction products. The presence of large quantities of a-olefins along with LLDPE was traced back to the ability of the hydrosilanemodified catalyst in oligomerizing ethylene to short a-olefins, which are then incorporated in the growing polymer chain through an “in situ branching” mechanism. More recently, the a-olefins produced in situ were detected by means of temperatureand pressure-resolved FT-IR spectroscopy before their incorporation in the polymeric chain. All the hydrosilane compounds presenting at least one Si H bond have been claimed to act as efficient modifiers of both the oxidized and reduced Phillips catalyst, whereas tetraalkylsilanes have no effect. This observation, combined with the analysis of the resulting polymer, led to the proposal of a chromium hydride as the catalytically active modified site (see structure a in Scheme 1), formed by the cleavage of a Cr O bond from the starting Cr species (structure 1 in Scheme 1). However, other possible structures can be hypothesized for the chromium modified sites, such as those shown in Scheme 1 (structures b and c).

A spectroscopic investigation of silica-supported TiClx species: a case study towards Ziegler–Natta catalysis

Since their discovery in the 1950s, several breakthroughs marked the evolution of Ziegler–Natta catalysts and contributed to improve their catalytic activity and selectivity, conditioning the whole polyolefin market. However, some fundamental questions are still open about the structural and functional properties of the active sites, whose understanding could open new perspectives in controlling the polymerization process. In this context, a SiO2-supported Ziegler–Natta catalyst was prepared following a step by step approach, with the simultaneous goal of synthesis and characterization, to investigate with a surface science approach each step of the catalytic process.

Insights into Cr/SiO2 catalysts during dehydrogenation of propane: an operando XAS investigation

In situ and operando XAS spectroscopic methods were applied to monitor the variations in the oxidation state and in the local structure of the chromium sites in a 2.0Cr/SiO2-DHS catalyst during propane dehydrogenation under non-oxidative and different oxidative conditions. The spectroscopic data have been compared to the catalytic performances. Although the majority of the specialized literature ascribes the propane dehydrogenation activity mainly to CrIII species and only a few works report the presence of small amounts of CrII species, our data unequivocally demonstrate the co-presence of both CrIII and CrII species during the propane dehydrogenation reaction. The relative amount of the two chromium phases has been estimated by analyzing both the XANES and EXAFS spectra with a two-phase fitting approach. It was found that the CrIII-phase prevails when oxygen is present in the reaction mixture and when the reaction is performed at low flow or under static conditions. The amount of the CrIII-phase correlates well with the propane conversion. In contrast, the selectivity to propene correlates with the amount of the CrII-phase, at least when propane dehydrogenation is performed under oxidative conditions. Although other phenomena surely contribute to the overall catalytic performances of Cr/SiO2 catalysts, our results indicate that the relative proportion of the CrII and CrIII phases plays a role in influencing the selectivity of the propane oxidative dehydrogenation reaction.

Low temperature activation and reactivity of CO2 over a CrII-based heterogeneous catalyst: a spectroscopic study

A new heterogeneous catalyst for CO(2) activation was identified in the Cr(II)/SiO(2) Phillips catalyst, one of the most important catalysts used industrially for olefin polymerization. Interestingly, it was found that Cr(II)/SiO(2) strongly activates CO(2) already at room temperature, making it available for chemicals synthesis. A preliminary attempt in this direction was done by following the reaction of CO(2) with ethylene oxide at room temperature by means of FT-IR spectroscopy, which showed the formation of ethylene carbonate. Besides non-reductive CO(2) activation, Cr(II)/SiO(2) showed good performances in catalytic reduction of CO(2) to CO, when heated under mild conditions or irradiated with UV-Vis light. Both, in situ FT-IR and UV-Vis spectroscopy, were applied to highlight the redox process occurring at the Cr centres. These results open interesting perspectives to be developed in the field of CO(2) chemical fixation.

The Effect of Al-Alkyls on the Phillips Catalyst for Ethylene Polymerization: The Case of Diethylaluminum Ethoxide (DEALE)

Al-alkyls are often used in the industrial practice for modifying the Phillips catalysts for polyethylene production: they are not necessary to develop the activity, but they have relevant effects on the catalysis, decreasing the induction time, promoting the in situ branching, and enhancing the H2 sensitivity for the molecular weight regulation. Herein we investigate the effect of diethylaluminum ethoxide (DEALE) on Cr(II)/SiO2 (in a stoichiometric amount of Al:Cr = 2:1), focusing the attention on the modification of the Cr(II) sites at a molecular level. Diffuse reflectance UV–Vis and FT-IR spectroscopies, applied in the presence of CO and CD3CN as molecular probes, unequivocally demonstrate that: (1) DEALE modifies only a fraction of the Cr(II) sites (ca. 30%) even if dosed in excess with respect to the Cr sites; (2) DEALE reacts with the silica surface, forming Al-grafted species which are at least partially in interaction with the Cr(II) sites and hence act as ancillary ligands; (3) the modified Cr(II) sites are more acidic and likely mono-grafted to the silica surface; the presence of Al-grafted species nearby is essential for their stabilization. Finally, kinetic experiments indicate that the modified Cr(II) sites are ca. 60 times faster in inserting ethylene than the unmodified Cr(II) sites. The intrinsic higher activity of the modified Cr(II) sites is a consequence not only of their molecular structure, but of the whole series of effects listed above.

Photoinduced Ethylene Polymerization on Titania Nanoparticles

Herein, we report on the ability of H2‐photoreduced TiO2 nanoparticles to catalyze the polymerization of ethylene without the use of an alkylating agent. Even more, we demonstrate that the same reaction occurs directly on stoichiometric TiO2 in the presence of ethylene under UV‐light irradiation at room temperature and, thus, without any prereduction step. The synergic use of electron paramagnetic resonance and diffuse reflectance UV/Vis‐near‐IR spectroscopy allowed us to identify and correlate different Ti3+ EPR signals and UV/Vis‐NIR bands, whereas transmission FTIR spectroscopy (applied under in situ UV‐light irradiation) was fundamental to prove ethylene polymerization and to observe ethylene oxidation byproducts. The possibility to polymerize ethylene easily in a one‐step procedure can be a promising approach to increase the compatibility of TiO2 as an inorganic additive for polyethylene blends.

Reactivity of Hydrosilanes with the CrII/SiO2 Phillips Catalyst: Observation of Intermediates and Properties of the Modified CrII Sites

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.