Surface chromium single sites: open problems and recent advances
0
Department of Chemistry, NIS Centre of Excellence and INSTM, University of Torino
,
Torino
,
Italy
The results of decades of studies on the Phillips chromium (Cr)/silica polymerization catalyst are briefly summarized. The application of several characterization methods has allowed a detailed knowledge of the structure and reactivity of Cr centres to be obtained. In particular, many aspects of this apparently simple single-site catalyst, including the heterogeneity, the modification of the ligand sphere upon interaction with many molecules and the initiation mechanism of the ethylene polymerization reaction, have been clarified. It is shown that based on the acquired knowledge, it is now possible to proceed further towards the intelligent modification of the ligand sphere with the scope to increase the reaction rate and selectivity. It is also illustrated that, besides polymerization/oligomerization reactions, it is possible to extend the study of CrII reactivity towards new reactions.
1. Single-site heterogeneous catalysts: a definition
Following the usual definition, a single-site heterogeneous catalyst is constituted
by metal atoms, ions or small clusters of atoms, held to a rigid framework through
surface ligands; these sites are isolated inside the hosting structure (Thomas &
Thomas 1996; Zecchina et al. 2011). In some cases, the hosting matrix has
crystalline character, and the position of the hosted centres is (consequently)
well known. This is the case of the popular titanium silicalite (TS-1) catalyst
where isolated titanium (Ti) centres occupy tetrahedral positions in substitution
of silicon atoms in the mordenite framework inverted (MFI) structure (Ricchiardi
et al. 2001). In the presence of reactants, the Ti centres assume a more open
structure, where the metal atoms display a full ability to bind substrates, and
where bonds are broken and formed.
However, in numerous and industrially relevant catalysts, the supporting solid
(or rigid framework) is amorphous or, when crystalline, it exposes several different
faces. In such cases, several anchoring situations are possible, characterized
One contribution of 14 to a Special feature Recent advances in single-site heterogeneous catalysis.
by different surface ligand arrangements whose structure is usually not well
known. Heterogeneity of sites differentiates the heterogeneous single-site catalysts
from the homogeneous counterparts, whose ligand sphere is, in principle, more
accurately engineered. As a consequence, heterogeneous catalysts are often
considered less defined than homogeneous ones. Nevertheless, many reactions
catalysed by homogeneous single-site catalysts are characterized by an induction
period, which suggests that the employed monometallic complexes are simply
precursor species, whereas the real catalyst is formed successively only through
interaction of these precursors with reagents. In other words, as mentioned for
TS-1, also in homogeneous single-site catalysts, only the structure of the precursor
species is well known; on the contrary, the structure of the active species formed
in the presence of reactants is usually more the result of reasonable hypotheses on
the nature of the reactions occurring during the promotion/induction phase, than
of direct determination by means of structural methods (Zecchina et al. 2007).
Elucidation of the structure of active centres in single-site catalysts
(both heterogeneous and homogeneous) requires the application of accurate
characterization methods, such as vibrational (IR and Raman), UVvis
nearIR, X-ray absorption near edge spectroscopy (XANES) and extended X-ray
absorption fine structure spectroscopy, magnetic resonance (electron spin
resonance and nuclear magnetic resonance) and electron (electron energy loss
spectroscopy, X-ray photoelectron spectroscopy and others) spectroscopies. The
characterization of the active sites could be complex because of the presence of
many types of sites (instead of only one) and because the active centres can
represent a small fraction of the total species present in solution (homogeneous
catalysts) or on the surface (heterogeneous catalysts). The choice of the best
physical methods to be used depends upon the type of catalyst and reaction
under investigation. The only general rule which can be advanced is that the
characterization method must be able to distinguish between spectator and
active species.
2. The chromium/silica Phillips catalyst for ethylene polymerization
(a) The structure of the active chromium sites
The Cr/SiO2 Phillips catalyst for ethylene polymerization is by far one of the
simplest single-site heterogeneous catalysts (McDaniel 1985; Groppo et al. 2005b;
McDaniel 2010). The common synthesis receipt is very simple, as shown in
figure 1. The surface hydroxyl groups of amorphous silica (figure 1a) react with a
Cr precursor (such as chromic acid) forming surface chromate species (figure 1b).
The Cr loading is very low, usually lower (...truncated)