Isolated Ti(III) Species on the Surface of a Pre-active Ziegler Natta Catalyst
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Applied Magnetic Resonance
ORIGINAL PAPER
Isolated Ti(III) Species on the Surface of a Pre‑active Ziegler Natta Catalyst Leonora Podvorica1 · Enrico Salvadori1 · Fabrizio Piemontesi2 · Gianni Vitale2 · Giampiero Morini2 · Mario Chiesa1 Received: 27 June 2020 / Revised: 8 September 2020 / Accepted: 13 September 2020 © The Author(s) 2020
Abstract The nature of Ti(III) species, introduced in working models of industrial Ziegler Natta catalyst precursors, consisting of MgCl2/TiCl4 binary systems, eventually containing different Lewis basis, are studied by a combination of X- and Q-band CW and pulse EPR spectroscopy. In Ziegler Natta catalysts, Ti(III) play the double role of active catalytic species and unconventional spin probes. On the binary system, two dominant Ti(III) species, characterized by distinctively different EPR spectra, are observed. 35,37Cl Q-Band HYSCORE spectra allow estimating the hyperfine and nuclear quadrupole interactions of directly coordinated Cl, characterized by a hyperfine dipolar contribution of the order of 5 MHz and nuclear quadrupole interactions of the order of e2qQ/h = 9 MHz. Interestingly, the two dominant EPR active species are selectively suppressed by the presence of different Lewis bases, indicating the possibility to address the long standing issue of the influence of Lewis bases in driving specific morphological configurations and influencing the catalytic properties of Ti(III) active sites.
1 Introduction “It is structure that we look for whenever we try to understand anything”. This famous quote by Linus Pauling [1] underpins the issues which we are facing in understanding—more than 60 years after its discovery—the fine details of the Ziegler–Natta (ZN) olefin polymerization process. Ziegler–Natta olefin polymerization is probably the most effective and atomeconomical large-volume industrial chemical process and an almost unique example in chemistry of a development that arose from the synergy between fundamental * Mario Chiesa [email protected] 1
Department of Chemistry, University of Torino, Via Giuria 9, 10125 Torino, Italy
2
Basell Poliolefine Italia S.R.L. LyondellBasell Group, G. Natta R&D Center, Piazzale G. Donegani 12, 44122 Ferrara, Italy
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research and industrial efforts. However, despite the economic relevance of polyolefins, with a yearly market exceeding a billion dollar, a detailed understanding of the structure of the active sites in the heterogeneous catalyst is still missing. This limited knowledge is a consequence of the complex structure of heterogeneous ZN catalysts and the elusive nature of the open-shell Ti(III) active species, formed during pre-catalyst activation. Efforts have been dedicated to characterize model systems [2–5], but this has only partially provided an insight into the complex, polycrystalline working catalysts at molecular level. Modern industrial ZN catalysts are obtained by activating a pre-catalyst, consist gCl2, with an Al-alkyl (AlR3), typically ing of T iCl4 adsor
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