Sébastien Norsic

“Hydro‐metathesis” of Olefins: A Catalytic Reaction Using a Bifunctional Single‐Site Tantalum Hydride Catalyst Supported on Fibrous Silica (KCC‐1) Nanospheres

We thank ESCPE-Lyon, the CNRS, and the KAUST for financial and logistic support and Anne Baudouin for NMR spectra acquisition.

Polymerization of Ethylene by Silica-Supported Dinuclear CrIIISites through an Initiation Step Involving CH Bond Activation

The insertion of an olefin into a preformed metal-carbon bond is a common mechanism for transition-metal-catalyzed olefin polymerization. However, in one important industrial catalyst, the Phillips catalyst, a metal-carbon bond is not present in the precatalyst. The Phillips catalyst, CrO3 dispersed on silica, polymerizes ethylene without an activator. Despite 60 years of intensive research, the active sites and the way the first CrC bond is formed remain unknown. We synthesized well-defined dinuclear Cr(II) and Cr(III) sites on silica. Whereas the Cr(II) material was a poor polymerization catalyst, the Cr(III) material was active. Poisoning studies showed that about 65 % of the Cr(III) sites were active, a far higher proportion than typically observed for the Phillips catalyst. Examination of the spent catalyst and isotope labeling experiments showed the formation of a Si-(μ-OH)-Cr(III) species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at Cr(III) O bonds.

Proton transfers are key elementary steps in ethylene polymerization on isolated chromium(III) silicates

Significance The Phillips catalyst—CrOx/SiO2—produces 40–50% of global high-density polyethylene, yet several fundamental mechanistic controversies surround this catalyst. What is the oxidation state and nuclearity of the active Cr sites? How is the first Cr–C bond formed? How does the polymer propagate and regulate its molecular weight? Here we show through combined experimental (infrared, ultraviolet-visible, X-ray near edge absorption spectroscopy, and extended X-ray absorption fine structures) and density functional theory modeling approaches that mononuclear tricoordinate Cr(III) sites immobilized on silica polymerize ethylene by the classical Cossee–Arlman mechanism. Initiation (C–H bond activation) and polymer molecular weight regulation (the microreverse of C–H activation) are controlled by proton transfer steps. Mononuclear Cr(III) surface sites were synthesized from grafting [Cr(OSi(OtBu)3)3(tetrahydrofurano)2] on silica partially dehydroxylated at 700 °C, followed by a thermal treatment under vacuum, and characterized by infrared, ultraviolet-visible, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopy (XAS). These sites are highly active in ethylene polymerization to yield polyethylene with a broad molecular weight distribution, similar to that typically obtained from the Phillips catalyst. CO binding, EPR spectroscopy, and poisoning studies indicate that two different types of Cr(III) sites are present on the surface, one of which is active in polymerization. Density functional theory (DFT) calculations using cluster models show that active sites are tricoordinated Cr(III) centers and that the presence of an additional siloxane bridge coordinated to Cr leads to inactive species. From IR spectroscopy and DFT calculations, these tricoordinated Cr(III) sites initiate and regulate the polymer chain length via unique proton transfer steps in polymerization catalysis.

Non-Oxidative Coupling Reaction of Methane to Ethane and Hydrogen Catalyzed by the Silica-Supported Tantalum Hydride: (≡SiO)2Ta−H

Silica-supported tantalum hydride, (SiO)2Ta-H (1), proves to be the first single-site catalyst for the direct non-oxidative coupling transformation of methane into ethane and hydrogen at moderate temperatures, with a high selectivity (>98%). The reaction likely involves the tantalum-methyl-methylidene species as a key intermediate, where the methyl ligand can migrate onto the tantalum-methylidene affording the tantalum-ethyl.

Telechelic Polyethylene from Catalyzed Chain‐Growth Polymerization

Since the discovery of the Ziegler–Natta catalyst 2] for the coordinative polymerization of ethylene, continual chemical and process optimizations have led to a broad range of commodity polyolefins with enhanced properties. Despite these extensive efforts and numerous breakthroughs, telechelic polyethylenes (PEs), in which both chain ends feature the same functional group (X-PE-X) or chemically distinct groups (X-PE-Z), are yet to be accessed using catalytic ethylene polymerization. Telechelic polymers have important commercial applications as cross-linkers, chain linkers, or building blocks, highlighting the opportunities reliant on the development of telechelic PE production. In this context, catalytic polymerization of ethylene, producing many PE chains per transition-metal center, is the best route to overcome the cost limitation presented by other strategies, while reliably attaining the crystalline, insoluble, thermoplastic properties of high density PE. Previous methods to produce telechelic PE have involved polymerization of butadiene followed by functionalization and hydrogenation, partial hydrogenation of polybutadiene followed by metathesis degradation of the interior olefin groups, ring-opening metathesis polymerization of a cyclic olefin followed by functionalization and hydrogenation, and the living coordinative polymerization of olefins. 13] These techniques have produced valuable materials for the fundamental understanding of structure–property relationships. They are, however, either multistep processes, noncatalytic (using stoichiometric quantities of high-cost initiators), or employ monomers, such as butadiene or cyclic olefins, that are expensive compared to ethylene and consequently incompatible with the prerequisites of industrial production. In the field of catalytic ethylene polymerization, the scope of end-functional PE production using high-volume methods is limited by both the range of efficient, quantitative, and selective transformations of transition-metal-bound polymer chain ends and by competition from chain-transfer reactions, in particular b-hydrogen transfer, that deactivate the chain end. Approaches to overcoming these limitations have emphasized the exploitation of the reactivity of the polymer–metal bond present in living systems, in which chain transfer reactions are absent. The development of complexes that mediate catalyzed chain growth (CCG) of PE chains on a main-group metal has facilitated the introduction of PE end functions under catalytic conditions. In CCG polymerization, reversible PE chain transfer, which is rapid in comparison to propagation, occurs between a catalytic amount of a transition metal (on which the chains propagate) and a main-group metal used as the chain-transfer agent (CTA). A PE-Mg-PE intermediate can be produced by CCG on magnesium using [(C5Me5)2NdCl2Li(OEt2)2] in combination with a dialkyl magnesium as CTA. 21] The nucleophilic Mg C bonds of PE-Mg-PE can then be exploited to install functional polymer chain ends. Using this strategy, continuing investigations have revealed the potential of endfunctional, linear PE blocks (PE-Z) to form part of more complex architectures. Alternatively, end-group transformations with small-molecule reagents have been demonstrated, through which the reactivity, secondary structure, or surface properties of PE can be modified. The production of telechelic PE presents the additional challenge of introducing potential reactivity at both ends of the polymer chain, prior to or during polymerization. The established reactivity of PE-Mg-PE towards electrophiles and the behavior of dialkyl magnesium cocatalysts (MgR2) as CTAs in coordinative ethylene polymerization, offer a synthetic pathway to 1,w-difunctional PE, provided that R features a functional group that remains a spectator during ethylene polymerization. Bis(pentamethylcyclopentadienyl)neodymium catalysts have shown no propensity to incorporate a-olefins into predominantly PE chains, which raised the possibility of employing exo-alkenyl magnesium CTAs. We describe herein the use of a bis(exo-alkenyl) magnesium CTA in controlled CCG polymerization of ethylene, thereby installing a vinyl functionality prior to the polymerization. We then demonstrate the synthesis of 1,w-heterodifunctional, linear PE (X-PE-Z) by CCG polymerization of ethylene followed by single-step, in situ functionalization. Bis(3-butenyl)magnesium (B-3-BM) was produced by adaptation of a precedent Grignard disproportionation procedure. Reaction of a THF solution of 3-butenylmagnesium bromide with 1,4-dioxane caused the rapid precipitation of {MgBr2(O2C4H8)2}n, leaving a THF solution of B-3-BM upon filtration. To preclude a suppressive effect of THF, both on the polymerization activity of the Nd-based catalyst and on the level of functionality attainable upon post-polymerization functionalization, the THF solvent was removed and replaced [*] Dr. I. German, W. Kelhifi, Dr. S. Norsic, Dr. C. Boisson, Dr. F. D’Agosto Universit de Lyon, Univ. Lyon 1, CPE Lyon, CNRS UMR 5265 Laboratoire de Chimie, Catalyse, Polym res et Proc d s (C2P2), Equipe LCPP, Bat 308F 43 Bd du 11 novembre 1918, 69616 Villeurbanne (France) E-mail: boisson@lcpp.cpe.fr dagosto@lcpp.cpe.fr Homepage: http://www.c2p2-cpe.com

Divinyl‐End‐Functionalized Polyethylenes: Ready Access to a Range of Telechelic Polyethylenes through Thiol–Ene Reactions

Telechelic α,ω-iodo-vinyl-polyethylenes (Vin-PE-I) were obtained by catalytic ethylene polymerization in the presence of [(C5 Me5 )2 NdCl2 Li(OEt2 )2 ] in combination with a functionalized chain-transfer agent, namely, di(10-undecenyl)magnesium, followed by treatment of the resulting di(vinylpolyethylenyl)magnesium compounds ((vinyl-PE)2 Mg) with I2 . The iodo-functionalized vinylpolyethylenes (Vin-PE-I) were transformed into unique divinyl-functionalized polyethylenes (Vin-PE-Vin) by simple treatment with tBuOK in toluene at 95 °C. Thiol-ene reactions were then successfully performed on Vin-PE-Vin with functionalized thiols in the presence of AIBN. A range of homobifunctional telechelic polyethylenes were obtained on which a hydroxy, diol, carboxylic acid, amine, ammonium chloride, trimethoxysilyl, chloro, or fluoroalkyl group was installed quantitatively at each chain end.

Low temperature hydrogenolysis of waxes to diesel range gasoline and light alkanes: Comparison of catalytic properties of group 4, 5 and 6 metal hydrides supported on silica–alumina

A series of metal hydrides (M = Zr, Hf, Ta, W) supported on silica–alumina were studied for the first time in hydrogenolysis of light alkanes in a continuous flow reactor. It was found that there is a difference in the reaction mechanism between d0metal hydrides of group 4 and d0 ↔ d2metal hydrides of group 5 and group 6. Furthermore, the potential application of these catalysts has been demonstrated by the transformation of Fischer–Tropsch wax in a reactive distillation set-up into typical gasoline and diesel molecules in high selectivity (up to 86 wt%). Current results show that the group 4 metal hydrides have a promising yield toward liquid fuels.

Toward Anisotropic Hybrid Materials: Directional Crystallization of Amphiphilic Polyoxazoline-Based Triblock Terpolymers

We present the design and synthesis of a linear ABC triblock terpolymer for the bottom-up synthesis of anisotropic organic/inorganic hybrid materials: polyethylene-block-poly(2-(4-(tert-butoxycarbonyl)amino)butyl-2-oxazoline)-block-poly(2-iso-propyl-2-oxazoline) (PE-b-PBocAmOx-b-PiPrOx). The synthesis was realized via the covalent linkage of azide-functionalized polyethylene and alkyne functionalized poly(2-alkyl-2-oxazoline) (POx)-based diblock copolymers exploiting copper-catalyzed azide-alkyne cycloaddition (CuAAC) chemistry. After purification of the resulting triblock terpolymer, the middle block was deprotected, resulting in a primary amine in the side chain. In the next step, solution self-assembly into core-shell-corona micelles in aqueous solution was investigated by dynamic light scattering (DLS) and transmission electron microscopy (TEM). Subsequent directional crystallization of the corona-forming block, poly(2-iso-propyl-2-oxazoline), led to the formation of anisotropic superstructures as demonstrated by electron microscopy (SEM and TEM). We present hypotheses concerning the aggregation mechanism as well as first promising results regarding the selective loading of individual domains within such anisotropic nanostructures with metal nanoparticles (Au, Fe3O4).

Improved direct production of 2,3-dimethylbutenes and 3,3-dimethylbutene from 2-methylpropene on tungsten hydride based catalysts

2-Methylpropene in the presence of W–H/Ni1%–Al2O3-(500) is transformed in high selectivity into a mixture of 2,3-dimethylbutenes (2,3-DMBs = DMB-1 and DMB-2) and neohexene. 2,3-DMBs arise from the unfavoured 2-methylpropene self-metathesis reaction whereas the neohexene originates from a cascade reaction: 2-methylpropene dimerisation followed by cross metathesis.

Efficient Hydrogenolysis of Alkanes at Low Temperature and Pressure Using Tantalum Hydride on MCM‐41, and a Quantum Chemical Study

Hydrogenolysis of hydrocarbons is of considerable technological importance for applications such as the hydroprocessing of petrochemical feedstocks to generate high‐value and useful chemicals and fuels. We studied the catalytic activity of tantalum hydride supported on MCM‐41 for the hydrogenolysis of alkanes at low temperature and low atmospheric pressure in a dynamic reactor. The reactions proceed with good turnover numbers, and the catalyst could be reused for several times, which makes the overall catalytic process sustainable. We derived the plausible mechanism by using DFT calculations and identified the preferred pathways by the analysis of potential energy surface. Our results and the proposed reaction mechanism demonstrate the viability of the “catalyst‐by‐design” approach.