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

Abstract

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.

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

“…[103] On the contrary, a few changes are observed upon titanation of MgCl 2-250 in the MIR region (spectrum 2 in Figure 5b). [110] It is worth noticing that the DR UV-Vis spectra shown in Figure 5a neither exclude the presence of isolated Ti 3 + sites (that should contribute at around 12000 cm À1 , [108] or higher if alkylated, [111,112] but with a much weaker intensity), nor the presence of over-reduced Ti 2 + sites (that should contribute in an extended spectral range, but with a very weak intensity). Such a reaction is also accompanied by the appearance of a very weak band at around 1100 cm À1 (clearly visible in the inset), which is attributed to the formation of Ti alkoxide byproducts.…”

“…[108] The slight blue-shift to higher wavenumbers of the MgCl 2-250 /TiCl 4-90 /TEA spectrum with respect to that of bulk TiCl 3 can be explained in terms of the smaller sizes of the TiCl x aggregates, but it is also compatible with the presence of alkyl ligands in the coordination sphere, [109] in accordance with the spectrochemical series of the ligands. [110] It is worth noticing that the DR UV-Vis spectra shown in Figure 5a neither exclude the presence of isolated Ti 3 + sites (that should contribute at around 12000 cm À1 , [108] or higher if alkylated, [111,112] but with a much weaker intensity), nor the presence of over-reduced Ti 2 + sites (that should contribute in an extended spectral range, but with a very weak intensity). [113][114][115] We are tempted to exclude Ti over-reduction on account of the low amount of TEA used during the experiment (Al : Ti = 2 : 1), [116] but the co-presence of both isolated TiCl x species and Ti clusters, each one expressing a specific activity, [117] is plausible and cannot be discarded at all.…”

Low‐Coordinated Titanium(III) Alkyl—Molecular and Surface—Complexes: Detailed Structure from Advanced EPR Spectroscopy

“…With a d 1 electronic configuration, EPR spectroscopy appears as an ideal tool to identify Ti III species and to probe their coordination environment, especially with the emergence of advanced EPR methods . In fact, recent studies allowed the identification of Ti III sites in ZN catalyst activated with Et 3 Al or even to unravel the local structures of the Ti 3+ sites immobilized on chlorinated alumina …”

Low‐Coordinated Titanium(III) Alkyl—Molecular and Surface—Complexes: Detailed Structure from Advanced EPR Spectroscopy

“…[3] In fact, recent studies allowed the identification of Ti III sites in ZN catalyst activated with Et 3 Al [4] or even to unravel the local structures of the Ti 3+ sites immobilized on chlorinated alumina. [5] Silica-supported metal hydrides prepared by surface organometallic chemistry (SOMC), [6] display unprecedented reactivities such as the low-temperature activation and catalytic conversion of alkanes [2c, 7] as well as ethylene polymerization and depolymerization. [2a,b] In the case of titanium, it has been proposed that the surface species are constituted of titanium(IV) hydrides as well as titanium(III) species.…”