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 Ti3+ centers and strong Lewis acid Al3+ 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 Al3+ ions promote ethylene oligomerization through a carbocationic mechanism and activate the Ti3+ sites for the traditional ethylene coordination polymerization.

Chromium Environment within Cr-Doped Silico-Aluminophosphate Molecular Sieves from Spin Density Studies

“…The best simulation of the experimental HYSCORE spectrum ( Figure 4 b) was obtained by summing up individual simulated HYSCORE spectra (all with the same weight) obtained considering 21 Al with a iso values distributed within three ranges, namely, 27 Al(1) with a iso = 0–2 MHz, 27 Al(2) with a iso = 4–9 MHz, and 27 Al(3) with a iso = 13–17 MHz (see also Table 1 ). Such a dispersion of isotropic hyperfine coupling constants, i.e., “ a -strain,” has been observed previously in both solid-state 16 , 39 and molecular systems. 40 , 41 It results from structural fluctuations of the ligand environment around the paramagnetic metal center, which, in our case, can be associated with different locations of Cr 5+ in the lattice, where it experiences slightly different coordination geometries as indicated by DFT calculations (vide infra).…”

“…Considering that the value of a 0 = 3367.76 MHz for the unit spin density in the 27 Al 3s orbital, 42 the corresponding spin density in the Al 3s orbital is in the range ≈ 0.06–0.5%, in good agreement with other d 1 transition-metal ions involving M(3 d 1 )–O–Al linkages. 16 , 39 It is interesting to note that a similar degree of spin density transfer has been observed for the isoelectronic Ti 3+ and V 4+ (3d 1 ) toward 31 P and 29 Si in microporous aluminophosphate and silicalites, and it appears to be a distinctive feature of isomorphous framework substitution. 43 Moreover, a ridge with a maximum extension of about 2 MHz centered at the 31 P Larmor frequency is observed.…”

Engineering Poly(ethylene-co-1-butene) through Modulating the Active Species by Alkylaluminum

“…From the point of view of catalyst itself, the clear presence of aluminum species on the catalyst surface has demonstrated the capability of alkylaluminum to form complexes with catalyst components. [23][24][25] Recently, Chiesa, Groppo and their colleagues through advanced electron paramagnetic resonance (EPR) methods indirectly observed the aluminum species in the proximity of titanium in an industrial fourth-generation ZNC 26 and thereafter recorded a direct Fermi contact between Ti(III) and Al via chlorine bridge in a model catalyst 27 .…”