Activated
MgCl2 nanocrystals were prepared by controlled
dealcoholation of the MgCl2·6CH3OH adduct,
mimicking the routinely adopted methods to synthesize industrial heterogeneous
Ziegler–Natta catalysts. The effect of the alcohol in driving
the morphology of MgCl2 crystals, i.e., the type and extension
of the exposed surfaces, was investigated by integrating a detailed
structural, morphological, and surface characterization with a state-of-the-art
computational modeling. FT-IR spectroscopy of CO adsorbed at 100 K
emerged as a feasible, simple, and powerful method to characterize
the surface of structurally disordered MgCl2 and MgCl2-based Ziegler–Natta catalysts. Our computational morphological
analysis revealed that the (012), (015), and (110) surfaces are highly
stabilized by methanol as an electron donor, especially at the temperature
typically adopted in the preparation of the precatalysts. FT-IR spectroscopy
of adsorbed CO allows distinguishing these surfaces from the other
penta-coordinated ones and provides a clear experimental evidence
that TiCl4 binds to the (110) and (015) surfaces. The (015)
surface was never considered in the past and is characterized by an
unusual flexibility in the presence of adsorbates, which detach the
Mg cations from the Cl underneath, leaving a coordination vacancy
available for the binding of asymmetric titanium sites. Since the
recent literature identified a tetra-coordinated Mg as a site of election
for the deposition of the Ti species relevant in olefin polymerization,
the presence of two eligible Mg sites for Ziegler–Natta catalysis
is highly interesting.
Operando-sensitive spectroscopic techniques were employed for investigating the changes in the molecular structure of the Cr sites in the Cr/SiO Phillips catalyst during ethylene polymerization. Practically, the most arduous barrier to be overcome was the separation of the chromates reduction carried out by ethylene from the subsequent polymerization. By carefully tuning the experimental parameters we succeeded in observing these two events separately. We found that the sites involved in ethylene polymerization are mainly divalent Cr ions in a 6-fold coordination, in interaction with the oxygenated byproduct (mostly methylformate, generated from the disproportionation of two formaldehyde molecules). Unreduced Cr species are also present during ethylene polymerization as well as reduced Cr species (either Cr or Cr) acting as spectators. Our results challenge the old vision of "naked" chromium species (i.e., low coordinated) as the active sites and attribute a fundamental role to external (and flexible) oxygenated ligands that resemble the ancillary ligands in homogeneous polymerization catalysis.
In this work, we
summarize and critically compare some of the experimental
results recently published on the Phillips catalyst, in the attempt
to make the point on a few particularly debated questions that have
recently animated the specialized literature; in particular, we discuss
the structure of the active chromium sites and how ethylene polymerization
initiates on them. The data collected in this article unequivocally
demonstrate that the structural and electronic properties of the chromium
sites strongly depend on the strain of the silica surface, which in
turns is affected by both the activation treatment and the chromium
loading. This explains, at least partially, the differences of results
obtained in different research groups. Another fundamental message
is the need of applying the largest possible set of characterization
methods, including theoretical calculation on large and flexible models.
Our final purpose (and hope) is to promote a positive and constructing
discussion on this catalyst, as a premise to create a solid scientific
base useful to both the young researchers approaching this field and
the industrial researchers who daily work with it.
An original step-by-step approach to synthesize and characterize a bifunctional heterogeneous catalyst consisting of isolated Ti(3+) centers and strong Lewis acid Al(3+) sites on the surface of a chlorinated alumina has been devised. A wide range of physicochemical and spectroscopic techniques were employed to demonstrate that the two sites, in close proximity, act in a concerted fashion to synergistically boost the conversion of ethylene into branched polyethylene, using ethylene as the only feed and without any activator. The coordinatively unsaturated Al(3+) ions promote ethylene oligomerization through a carbocationic mechanism and activate the Ti(3+) sites for the traditional ethylene coordination polymerization.
MgCl2 nanocrystals prepared in the presence of methanol and ethanol were characterized by complementing surface‐science tools with a systematic quantum‐mechanical investigation of the stability order (in terms of Gibbs free energy) of different MgCl2 surfaces in the presence of the two alcohols. Both alcohols drastically change the overall stability and the stability order of the exposed surfaces, mainly as a consequence of the entropic contribution to the Gibbs free energy, hence inhibiting or promoting crystal growth in certain directions. The environment‐dependent occurrence of the MgCl2 surfaces may influence the structure and properties of the supported TixCl4−x sites in the MgCl2/TiCl4 precatalyst, which has important implications in the design of morphologically controlled Ziegler–Natta catalysts.
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