scholarly journals Spectroscopic Signature and Structure of the Active Sites in Ziegler–Natta Polymerization Catalysts Revealed by Electron Paramagnetic Resonance

2021 ◽  
Author(s):  
Anton Ashuiev ◽  
Matthieu Humbert ◽  
Sébastien Norsic ◽  
Jan Blahut ◽  
David Gajan ◽  
...  
Keyword(s):  
Electron Paramagnetic Resonance ◽  
Active Sites ◽  
Polymerization Catalysts ◽  
Paramagnetic Resonance ◽  
Electron Paramagnetic

scholarly journals Spectroscopic Signature and Structure of Active Sites in Ziegler-Natta Polymerization Catalysts revealed by Electron Paramagnetic Resonance

2021 ◽  
Author(s):  
Anton Ashuiev ◽  
Matthieu Humbert ◽  
David Gajan ◽  
Sebastien Norsic ◽  
Jan Blahut ◽  
...  
Keyword(s):  
Electron Paramagnetic Resonance ◽  
Ethylene Polymerization ◽  
Active Sites ◽  
Active Species ◽  
Atomic Scale ◽  
Polymerization Catalysts ◽  
Paramagnetic Resonance ◽  
Spectroscopic Signatures ◽  
Electron Paramagnetic ◽  
Ziegler Natta Catalysts

Despite decades of extensive studies, the atomic-scale structure of the active sites in heterogeneous Ziegler-Natta (ZN) catalysts, one of the most important processes of the chemical industry, remains elusive and a matter of debate. In the present work, the structure of “active sites” of ZN catalysts in the absence of ethylene, referred to as “dormant active sites”, is elucidated from magnetic resonance experiments, carried out on samples reacted with increasing amounts of BCl<sub>3</sub> so as to enhance the concentration of active sites and observe clear spectroscopic signatures. Using EPR and NMR spectroscopies, in particular 2D HYSCORE experiments complemented by DFT calculations, we show that the activated ZN catalysts contain bimetallic alkyl-Ti(III),Al species whose amount is directly linked to the polymerization activity of MgCl<sub>2</sub>-supported Ziegler-Natta catalysts. This connects those spectroscopic signatures to the active species formed in the presence of ethylene, and enables us propose an ethylene polymerization mechanism on the observed bimetallic alkyl-Ti(III),Al species based on DFT computations<br>

scholarly journals CH4-to-CH3OH on Mononuclear Cu(II) Sites Supported on Al2O3: Structure of Active Sites from Electron Paramagnetic Resonance

2021 ◽  
Author(s):  
Jordan Meyet ◽  
Anton Ashuiev ◽  
Gina Noh ◽  
Mark Newton ◽  
Daniel Klose ◽  
...  
Keyword(s):  
Electron Paramagnetic Resonance ◽  
Organometallic Chemistry ◽  
Active Sites ◽  
Spectroscopic Characterization ◽  
Paramagnetic Resonance ◽  
Transition Alumina ◽  
Selective Conversion ◽  
Conversion Of Methane ◽  
Electron Paramagnetic ◽  
Alumina Supports

The selective conversion of methane to methanol remains one of the holy grails of chemistry, where Cu-exchanged zeolites have been shown to selectively convert methane to methanol under stepwise conditions. Over the years, several active sites have been proposed, ranging from mono-, di- to trimeric Cu(II). Herein, we report the formation of well-dispersed monomeric Cu(II) species supported on alumina using surface organometallic chemistry and their reactivity towards the selective and stepwise conversion of methane to methanol. Extensive studies using various transition alumina supports combined with spectroscopic characterization, in particular electron paramagnetic resonance (EPR), show that the active sites are associated with specific facets, which are typically found in gamma- and eta-alumina phase, and that their EPR signature can be attributed to species having a tri-coordinated [(Al<sub>2</sub>O)Cu<sup>II</sup>O(OH)]<sup>-</sup>,T-shape geometry. Overall, the selective conversion of methane to methanol, a two-electron process, involve two of these isolated monomeric Cu(II) sites that play in concert.

scholarly journals Studies by electron-paramagnetic-resonance spectroscopy and stopped-flow spectrophotometry on the mechanism of action of turkey liver xanthine dehydrogenase

1976 ◽  
Vol 153 (2) ◽  
pp. 297-307 ◽  
Author(s):  
M J Barber ◽  
R C Bray ◽  
D J Lowe ◽  
M P Coughlan
Keyword(s):  
Electron Paramagnetic Resonance ◽  
Redox Potential ◽  
Active Sites ◽  
Electron Paramagnetic Resonance Spectroscopy ◽  
Xanthine Dehydrogenase ◽  
Resonance Spectroscopy ◽  
Stopped Flow ◽  
Paramagnetic Resonance ◽  
Flow Measurements ◽  
Electron Paramagnetic

Studies by e.p.r. (electron-paramagnetic-resonance) spectroscopy and by stopped-flow spectrophotometry on turkey liver xanthine dehydrogenase revealed strong similarities to as well as important differences from the Veillonella alcalescens xanthine dehydrogenase and milk xanthine oxidase. The turkey enzyme is contaminated by up to three non-functional forms, giving molybdenum e.p.r. signals designated Resting I, Resting II and Slow. Slow and to a lesser extent Resting I signals are like those from the Veillonella enzyme, whereas Resting II is very like a resting signal described by K. V. Rajagopolan, P. Handler, G. Palmer & H. Beinert (1968) (J. Biol. Chem. 243, 3784-3796) for aldehyde oxidase. Another non-functional form that gives the Inhibited signal is produced on treatment of the enzyme with formaldehyde. Stopped-flow measurements at 450 nm show that, as for the milk enzyme, reduction by xanthine is rate-limiting in enzyme turnover. The active enzyme gives rise to Very Rapid and Rapid molybdenum(V) e.p.r. signals, as well as to an FADH signal. That these signals are almost indistinguishable from those of the milk enzyme, confirms the similarities between the active sites. There are two types of iron-sulphur centres that give signals like those in the milk enzyme, though with slightly different parameters. Quantitative reduction titration of the functional enzyme with xanthine revealed two important differences between the turkey and the milk enzymes. First, the turkey enzyme FADH/FADH2 system has a redox potential sufficiently low that xanthine is incapable of reducing the flavin completely. This finding presumably explains the very low oxidase activity. Secondly, whereas the Fe/S II chromophore in the milk enzyme has a relatively high redox potential, for the turkey enzyme the value of this potential is lower and similar to that of its Fe/S I chromophore.

scholarly journals CH4-to-CH3OH on Mononuclear Cu(II) Sites Supported on Al2O3: Structure of Active Sites from Electron Paramagnetic Resonance

2020 ◽  
Author(s):  
Jordan Meyet ◽  
Anton Ashuiev ◽  
Gina Noh ◽  
Mark Newton ◽  
Daniel Klose ◽  
...  
Keyword(s):  
Electron Paramagnetic Resonance ◽  
Organometallic Chemistry ◽  
Active Sites ◽  
Spectroscopic Characterization ◽  
Paramagnetic Resonance ◽  
Transition Alumina ◽  
Selective Conversion ◽  
Conversion Of Methane ◽  
Electron Paramagnetic ◽  
Alumina Supports

The selective conversion of methane to methanol remains one of the holy grails of chemistry, where Cu-exchanged zeolites have been shown to selectively convert methane to methanol under stepwise conditions. Over the years, several active sites have been proposed, ranging from mono-, di- to trimeric Cu(II). Herein, we report the formation of well-dispersed monomeric Cu(II) species supported on alumina using surface organometallic chemistry and their reactivity towards the selective and stepwise conversion of methane to methanol. Extensive studies using various transition alumina supports combined with spectroscopic characterization, in particular electron paramagnetic resonance (EPR), show that the active sites are associated with specific facets, which are typically found in gamma- and eta-alumina phase, and that their EPR signature can be attributed to species having a tri-coordinated [(Al<sub>2</sub>O)Cu<sup>II</sup>O(OH)]<sup>-</sup>,T-shape geometry. Overall, the selective conversion of methane to methanol, a two-electron process, involve two of these isolated monomeric Cu(II) sites that play in concert.

scholarly journals Spectroscopic Signature and Structure of Active Centers in Ziegler-Natta Polymerization Catalysts revealed by Electron Paramagnetic Resonance

2020 ◽  
Author(s):  
Anton Ashuiev ◽  
mathieu humber ◽  
sebastien norsic ◽  
jean blahut ◽  
david gajan ◽  
...  
Keyword(s):  
Active Sites ◽  
Atomic Scale ◽  
Paramagnetic Species ◽  
Polymerization Catalysts ◽  
Active Centers ◽  
Paramagnetic Resonance ◽  
Joint Application ◽  
Spectroscopic Signatures ◽  
Electron Paramagnetic ◽  
Ziegler Natta Catalysts

Despite decades of extensive studies, the atomic-scale structure of active sites in heterogeneous Ziegler-Natta (ZN) catalysts remains elusive and a matter of debate. Here, the structure of polymerization ZN catalysts is elucidated from magnetic resonance experiments carried out on samples reacted with increasing amounts of BCl<sub>3</sub> so as to enhance the concentration of active sites and observe clear spectroscopic signatures. Notably, we show that EPR and NMR spectroscopy of the activated ZN catalysts enables to observe paramagnetic species whose amount increases in conjunction with the catalytic activity. The joint application of 2D HYSCORE experiments and DFT calculations reveals the presence of bimetallic alkyl-Ti(III),Al complexes that are assigned to the catalytic centers of MgCl<sub>2</sub>-supported Ziegler-Natta catalysts

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