Effect of surface hydroxylation on the catalytic activity of a Cr(II)/SiO2 model system of Phillips catalyst
Graphical abstract
Introduction
The Cr/SiO2 Phillips catalysts were commercialized for the market of plastics more than a half century ago [1] and still preserve their importance; nowadays, they account for more than 50% of the annual HDPE production and produced different grades of LLDPE as well [2]. Prior to ethylene polymerization, the Phillips-type catalysts have to be “activated” by calcination in dry air (or oxygen) at elevated temperature. During this step, chromium ions in the oxidized state (+6) react with the hydroxyl groups at the silica surface and remain anchored in the form of chromates or, less probably, dichromates and polychromates [3]. Simultaneously, molecular water is released through the condensation of surface hydroxyl groups and the formation of strained siloxane bridges; the dehydroxylation process increases with the rise of temperature. It has been proved that the activation temperature has a strong influence on the catalyst activity toward ethylene polymerization [4]: activity increases with the calcination temperature up to 1173 K and decreases shortly after 1198 K due to the sintering of the silica support. Hence, citing McDaniel, “calcination does something more than just anchor the chromium to the silica surface”; rather, it influences the interplay between the chromium sites and their local environment, which is composed of siloxane rings and terminal hydroxyl groups.
A clear relationship between silica hydroxylation degree, properties of the chromium sites, and catalytic activity is still missing. In the present work, we report a systematic experimental study on the influence of the dehydroxylation state of the silica support on both the catalytic activity of Cr/SiO2 and the properties of the grafted chromium sites, after activation (outgassing and oxidation in O2 at elevated temperature to obtain Cr(VI)/SiO2 system) and a successive reduction in CO atmosphere at 623 K (Cr(II)/SiO2). This second step, which affords a stoichiometric reduction of Cr(VI) to Cr(II) sites, is often employed in academic laboratory for characterization purposes, but it is also an industrial practice to pre-reduce the chromium sites before ethylene polymerization, thus shortening the induction period. Notice that Cr(II) sites are simply the precursors of the active species which derive from Cr(II) by oxidative interaction with ethylene. This means that the valence state of Cr in active sites is larger than (II). It is possible that, like found by Coperet et al. [5], [6] also, Cr(III) can be the precursor of catalytic centers. However, a comparison between the Cr(II)/SiO2 system and the systems described in [5], [6] must be made with care because of their entirely different structure. The experiments were performed on a Cr-doped glass monolith (obtained by sol–gel method) with chromium loading of 0.1 wt% (i.e., 10 times lower than the usual 1 wt% loading), activated at three different temperatures: 823, 923, and 1023 K. This very low chromium loading, which represents a very innovative aspect of this study, was chosen as the safest method to minimize clustering and hence to enhance the concentration of genuinely isolated Cr(II) sites. Although the method does not fully guarantee that residual nearby Cr(II) sites can still be present, it ensures that their contribution is minimal. Transparent silica xerogels of high surface area, produced through a sol–gel process, were already employed as hosts for transition metal ions [7]. Recently, we demonstrated that mesoporous Cr/SiO2 monoliths are good models for the industrial Phillips catalysts, with the additional advantage to increase of some order of magnitude the sensitivity of characterization techniques, such as UV–Vis and FT-IR spectroscopies of adsorbed species [8]. Increased sensitivity is mandatory when dealing with such a low concentration of active sites. In this work, Cr(II) sites obtained on CO-reduced parent Cr(VI)/SiO2 systems (obtained in the 823–1023 K interval activation temperatures) have been investigated by means of UV–Vis spectroscopy and FT-IR of adsorbed CO. As catalytic activity is also a sensitive (although indirect) probe of the structure and properties of the catalytic centers, the activity of Cr(II) sites in ethylene polymerization on samples activated at temperatures in the 823–1023 K interval has also been examined.
The experimental results were then used for validating theoretical models of the Cr/SiO2 catalyst based on a cluster approach. Theoretical calculations provided important details on the geometrical, vibrational and electronic features of anchored chromates, surface Cr(II) species, and Cr(II)⋯CO adducts, which are useful to correlate the properties of the chromium sites with the observed catalytic activity. Previous studies were based both on periodic (mainly referring to Cr(VI)/SiO2 system) [9], [10] on hydroxylated surfaces or cluster (devoted to the study of Cr(II)/SiO2 system) [11], [12], [13], [14] approaches. To the best of our knowledge, this is the first time that the whole set of experimental data obtained on Cr(II)/SiO2 systems characterized by an exceptionally low concentration of chromium sites is compared with computational results. The main issue is thus the definition, by using the experimental data as sole reference, of the accuracy of adopted computational scheme (including the ONIOM embedding approach), which is planned to be employed for further studies of the first stages of ethylene polymerization on reduced model Cr/SiO2 catalyst.
Section snippets
Experimental
The Cr(VI)/SiO2 samples containing 0.1 wt% of Cr were synthesized in the form of xerogels by the acid-catalyzed sol–gel method; details were already reported elsewhere [8]. This loading is 5–10 times lower than that of industrially employed Phillips catalysts. After gelation and respective thermal treatment, dry xerogel was stabilized at 873 K. The monolith is homogeneously colored in yellow and optically transparent. A pure silica monolith was also synthesized as a reference following the same
Kinetics of ethylene polymerization on Cr(II)/SiO2
The kinetics of ethylene polymerization at 100 mbar of equilibrium pressure at room temperature were studied on three samples of Cr(II)/SiO2 activated at 823, 923, and 1023 K, respectively, by collecting FT-IR spectra in transmittance mode with 1-min time resolution. The whole set of results obtained within the first 10 min of reaction is shown in Fig. 2 in the ν(CH2) stretching region. The spectra were background subtracted and normalized to the thickness of the samples. It is clear that the
Conclusions
Mesoporous high surface area glass monolith doped with 0.1 wt% of chromium (a figure 10–20 times lower than the chromium loading characteristic of the Phillips catalysts) is an ideal system for fundamental studies on the structure and activity of chromium catalytic centers, for at least three main reasons: (i) the chromium sites are genuinely isolated and with mononuclear character; (ii) the silica surface area does not decline with activation temperature in the 823–1023 K range; and (iii) due to
Acknowledgment
The work has been financially supported by FIRB (RBAP115AYN).
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