An FTIR spectroscopic view of the initiation of ethylene polymerization on Cr/SiO2 catalyst

doi:10.1016/0021-9517(92)90164-D

Journal of Catalysis

Volume 137, Issue 2, October 1992, Pages 368-376

https://doi.org/10.1016/0021-9517(92)90164-D Get rights and content

Abstract

The onset of polymerization of ethylene on Cr/SiO₂ catalyst activated by high-temperature treatment was studied at room temperature using fast-scanning FTIR spectral measurements. Initial rapid adsorption of molecular ethylene on the pre-activated catalyst preceded the onset of polymerization. A time-delay between adsorption and polymerization was observed. The experimental evidence suggests that initiation of polymerization follows dissociation of ethylene, presumably on two Cr sites. Propagation and other mechanistic steps then follow as suggested earlier.

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Another possible proton transfer reaction can generate Cr(IV) vinyl hydride site 3A10 (Fig. 3). Such a structure was proposed in the literature as an active intermediate [33,37]. Geometry analysis suggests that the carbon atoms in 3A1 have sp3 hybridization rather than sp2, in contrast to the π-complex 5A1.
The nature of the active sites and the mechanism of ethylene polymerization over the Phillips CrO_x/SiO₂ catalyst are still under debate. In this work, a number of potential initiation, propagation and termination mechanisms for ethylene polymerization are investigated using density functional theory and cluster models. Cr(II), Cr(III), Cr(III)OH and Cr(V) oxide species are considered as the active site precursors. It is predicted that the oxachromacycle ring expansion mechanism is more probable than the routes involving Si(OH)Cr(II)-vinyl or Si(OH)Cr(III)-vinyl sites. We also show that the hydroxylated Cr(III)OH species, further transforming into the active Cr(III) vinyl sites, can be effective for ethylene polymerization. Additionally, we propose that defect sites in the amorphous silica framework can play a role in low-temperature transformation of the Cr(II) species into the active Cr(III) sites. A potential polymerization mechanism involving minor Cr(V) oxide species is also calculated.
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The Phillips supported chromium catalyst is used to produce some 40–50% of the world's high-density polyethylene. The chemistry of this important catalyst is reviewed from an industrial perspective, on the basis of more than half a century of commercial experience, as well as published research. Polymer characteristics are used as an analytical tool to probe and better understand the active-site population on the catalyst.
Infrared Spectroscopy of Transient Surface Species
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After the creation of Y2 groups, the mechanism proceeds via insertion of ethene, with formation of the intermediate represented in square brackets. This model explains well the experimentally observed first-order behavior of the polymerization reaction with respect to the monomer partial pressure (218–224). The aim of this Section is to discuss the experimental methods, problems, and recent improvements in the determination of the exact nature of the precursor species Y1, Y2, Y3, etc. (i.e., the determination of the initiation mechanism for the ethene polymerization).
A full understanding of a catalytic reaction requires a thorough identification of precursor and intermediate species. In this regard spectroscopies, in general, and infrared (IR) spectroscopy, in particular, can play an important role. However, the spectroscopic identification of precursor and intermediate species is often difficult owing to their transient nature. In this review we show how control of experimental parameters such as temperature, equilibrium pressure, and reactant–catalyst contact time allows experimentalists to alter the rates of dynamic processes and consequently to modify appreciably the relative concentrations of precursor, intermediate, and product species present under reaction conditions. In this context, time-resolved FTIR spectroscopy (with constant temperature and pressure during the experiment) and also temperature-time resolved (with both temperature and time changing simultaneously in a controlled way during the experiment) can be useful for kinetic investigations of several types of reactions. In this review several examples demonstrating the potential value of FTIR spectroscopy for the identification of surface transient species are discussed. These are classified in four main categories: (i) adsorption processes and transformations in the adsorbed state; (ii) proton-catalyzed oligomerization and polymerization of alkenes and unsaturated molecules in protonic zeolites; (iii) oligomerization reactions catalyzed by basic surface sites, and (iv) oligomerization and polymerization of alkenes on catalysts incorporating supported transition metal ions. All these reactions are discussed in the framework of a few common potential energy profiles in which the evolution from the precursor through the intermediate species to the final products is governed by the relative height of the corresponding activation energy barriers.
In situ FTIR spectroscopy of key intermediates in the first stages of ethylene polymerization on the Cr/SiO<inf>2</inf> Phillips catalyst: Solving the puzzle of the initiation mechanism?
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Several spectroscopic techniques have been adopted to highlight the coordination environment of the surface Cr species. From UV–vis DRS [5–9], FTIR spectroscopy of adsorbed probe molecules (CO, NO, CO2, N2O, pyridine, and, more recently, H2 and N2) [5,10–24], Raman spectroscopy [5,9,25,26], resonant or preresonant Raman spectroscopy [27,28], XAS [5,8,29], and XPS [5,30,31], an extremely complex and heterogeneous scenario has emerged, reflecting the high heterogeneity of the silica support [5]. Recently, we demonstrated a precise relationship between the structure of the precursors of the Cr active sites, the polymerization activity, and the properties of the resulting polymers [32].
We report here the first experimental observation, by means of in situ FTIR spectroscopy, of key intermediate species in the ethylene polymerization on the Cr(II)/SiO₂ Phillips catalyst. We demonstrate that by adopting suitable strategies, it is possible to shed light on one of the remaining questions concerning the system responsible for one-third of the worldwide polyethylene production: the puzzle of the initiation mechanism. “Anomalous” bands in the CH₂ stretching region are observed during the first steps of the polymerization reaction and assigned to small cycles on the Cr(II) sites, characterized by a structural strain decreasing with increasing ring dimension. These intermediate species are stable only in presence of a sufficiently high C₂H₄ pressure and show a peculiar reactivity toward strong ligands, such as NO and O₂. These results allow us to prove that the initiation mechanism follows a metallacycle route, similar to what occurs for several ethylene trimerization and tetramerization catalysts.
Tuning the structure, distribution and reactivity of polymerization centres of Cr(II)/SiO<inf>2</inf> Phillips catalyst by controlled annealing
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The catalytic activity of two Cr(II)/SiO₂ samples obtained after two different controlled annealing procedures are discussed and compared. The combined application of FTIR spectroscopy and microgravimetric determinations demonstrated that the two Cr(II)/SiO₂ samples are characterized by a different relative population of the families of Cr(II) sites present on the silica surface and by a different catalytic activity toward ethylene polymerization. The different polymerization activity of the two catalysts is explained by supposing the presence of a distribution of Cr sites, all active but characterized by different turnover frequencies in ethylene insertion, which is related to the ability of Cr sites to insert up to three ligands into their coordination spheres. It is thus clear that the activation procedure plays an important role in determining the catalytic activity and the local structure of the active sites in the Phillips catalyst, which are strictly related. In this sense, the obtained results are of general validity, because identification of the structure–activity relation is a challenge common to many catalytic systems.
In situ, Cr K-edge XAS study on the Phillips catalyst: Activation and ethylene polymerization
2005, Journal of Catalysis
Citation Excerpt :
The structure of Cr(II) and, to a lesser extent, the average valence state of the reduced chromium on the silica surface have been widely investigated in the past with several spectroscopic and chemical techniques. Using XPS, UV–Vis, and IR spectroscopies, several authors [3,5,7,12,13,18–27] found that the average oxidation state of Cr in the model catalyst is just above 2; here the catalyst comprises mainly anchored Cr(II) and a variable amount of pseudo-octahedral Cr(III) species (presumably in the form of α-Cr2O3). From this brief introduction, it is clear that the problem of the structure of chromium on the Phillips catalyst is still an open question.
In this in situ EXAFS and XANES study on the Phillips ethylene-polymerization Cr/SiO₂ catalyst, two polymerization routes are investigated and compared. The first mimics that adopted in industrial plants, where ethylene is dosed directly on the oxidized catalyst, while in the second the oxidized catalyst is first reduced by CO at 623 K. On this reduced catalyst C₂H₄ polymerization has been investigated at room temperature and at 373 K. To allow experiments in transmission mode, a Cr loading of 4 wt% has been adopted. At this loading a fraction of clustered Cr₂O₃ particles has been observed and quantified. The use of a third-generation synchrotron radiation source has allowed us to improve the energy resolution and signal-to-noise ratio of the XANES data, allowing us to determine the fraction of Cr sites involved in the polymerization reaction. This number represents an upper limit of the active sites. Preliminary ReflEXAFS experiments have been performed on a model catalyst prepared by impregnation of Cr on a flat Si(100) substrate covered by a thin layer of amorphous silica. Experiments have been performed ex situ on the grafted catalyst (i.e., after impregnation and thermal activation) and at the end of the polymerization stage.