Alessandro Piovano

Unraveling the Catalytic Synergy between Ti3+ and Al3+ Sites on a Chlorinated Al2O3: A Tandem Approach to Branched Polyethylene

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.

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.

The Influence of Alcohols in Driving the Morphology of Magnesium Chloride Nanocrystals

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.

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.

Genesis of MgCl2-based Ziegler-Natta Catalysts as Probed with Operando Spectroscopy

Ziegler-Natta catalysts for olefin polymerization are intrinsically complex multi-component systems. The genesis of the active sites involves several simultaneous and sequential steps, making the individual steps and interconnections difficult to be unraveled in an unambiguous manner. In this work, we combine X-ray diffraction and spectroscopy to probe each step of the birth and life of a MgCl2 -based Ziegler-Natta catalyst, namely the formation of high surface area MgCl2 by dealcoholation of an alcoholate precursor, the TiCl4 grafting, and the subsequent activation by triethylaluminum as co-catalyst. The so-prepared catalyst was tested towards ethylene polymerization, leading to the production of mainly crystalline high-density polyethylene. The use of operando characterization techniques allowed probing the transient details that are difficult to be dissected in the aftermath, but can radically affect the overall catalytic process.