Aleix Comas-Vives

Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities

Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities Christophe Copeŕet,*,† Aleix Comas-Vives,† Matthew P. Conley,† Deven P. Estes,† Alexey Fedorov,† Victor Mougel,† Haruki Nagae,†,‡ Francisco Nuñ́ez-Zarur,† and Pavel A. Zhizhko†,§ †Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1−5, CH-8093 Zürich, Switzerland ‡Department of Chemistry, Graduate School of Engineering Science, Osaka University, CREST, Toyonaka, Osaka 560-8531, Japan A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str. 28, 119991 Moscow, Russia

NMR signatures of the active sites in Sn-β zeolite.

Dynamic nuclear polarization surface enhanced NMR (DNP-SENS), Mössbauer spectroscopy, and computational chemistry were combined to obtain structural information on the active-site speciation in Sn-β zeolite. This approach unambiguously shows the presence of framework Sn(IV)-active sites in an octahedral environment, which probably correspond to so-called open and closed sites, respectively (namely, tin bound to three or four siloxy groups of the zeolite framework).

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.

A computational study of the olefin epoxidation mechanism catalyzed by cyclopentadienyloxidomolybdenum(VI) complexes.

A DFT analysis of the epoxidation of C(2)H(4) by H(2)O(2) and MeOOH (as models of tert-butylhydroperoxide, TBHP) catalyzed by [Cp*MoO(2)Cl] (1) in CHCl(3) and by [Cp*MoO(2)(H(2)O)](+) in water is presented (Cp*=pentamethylcyclopentadienyl). The calculations were performed both in the gas phase and in solution with the use of the conductor-like polarizable continuum model (CPCM). A low-energy pathway has been identified, which starts with the activation of ROOH (R=H or Me) to form a hydro/alkylperoxido derivative, [Cp*MoO(OH)(OOR)Cl] or [Cp*MoO(OH)(OOR)](+) with barriers of 24.9 (26.5) and 28.7 (29.2) kcal mol(-1) for H(2)O(2) (MeOOH), respectively, in solution. The latter barrier, however, is reduced to only 1.0 (1.6) kcal mol(-1) when one additional water molecule is explicitly included in the calculations. The hydro/alkylperoxido ligand in these intermediates is eta(2)-coordinated, with a significant interaction between the Mo center and the O(beta) atom. The subsequent step is a nucleophilic attack of the ethylene molecule on the activated O(alpha) atom, requiring 13.9 (17.8) and 16.1 (17.7) kcal mol(-1) in solution, respectively. The corresponding transformation, catalyzed by the peroxido complex [Cp*MoO(O(2))Cl] in CHCl(3), requires higher barriers for both steps (ROOH activation: 34.3 (35.2) kcal mol(-1); O atom transfer: 28.5 (30.3) kcal mol(-1)), which is attributed to both greater steric crowding and to the greater electron density on the metal atom.

CO2-to-Methanol Hydrogenation on Zirconia-Supported Copper Nanoparticles: Reaction Intermediates and the Role of the Metal-Support Interface

Methanol synthesis by CO2 hydrogenation is a key process in a methanol-based economy. This reaction is catalyzed by supported copper nanoparticles and displays strong support or promoter effects. Zirconia is known to enhance both the methanol production rate and the selectivity. Nevertheless, the origin of this observation and the reaction mechanisms associated with the conversion of CO2 to methanol still remain unknown. A mechanistic study of the hydrogenation of CO2 on Cu/ZrO2 is presented. Using kinetics, in situ IR and NMR spectroscopies, and isotopic labeling strategies, surface intermediates evolved during CO2 hydrogenation were observed at different pressures. Combined with DFT calculations, it is shown that a formate species is the reaction intermediate and that the zirconia/copper interface is crucial for the conversion of this intermediate to methanol.

Heterolytic Activation of C-H Bonds on Cr-III-O Surface Sites Is a Key Step in Catalytic Polymerization of Ethylene and Dehydrogenation of Propane

We describe the reactivity of well-defined chromium silicates toward ethylene and propane. The initial motivation for this study was to obtain a molecular understanding of the Phillips polymerization catalyst. The Phillips catalyst contains reduced chromium sites on silica and catalyzes the polymerization of ethylene without activators or a preformed Cr-C bond. Cr(II) sites are commonly proposed active sites in this catalyst. We synthesized and characterized well-defined chromium(II) silicates and found that these materials, slightly contaminated with a minor amount of Cr(III) sites, have poor polymerization activity and few active sites. In contrast, chromium(III) silicates have 1 order of magnitude higher activity. The chromium(III) silicates initiate polymerization by the activation of a C-H bond of ethylene. Density functional theory analysis of this process showed that the C-H bond activation step is heterolytic and corresponds to a σ-bond metathesis type process. The same well-defined chromium(III) silicate catalyzes the dehydrogenation of propane at elevated temperatures with activities similar to those of a related industrial chromium-based catalyst. This reaction also involves a key heterolytic C-H bond activation step similar to that described for ethylene but with a significantly higher energy barrier. The higher energy barrier is consistent with the higher pKa of the C-H bond in propane compared to the C-H bond in ethylene. In both cases, the rate-determining step is the heterolytic C-H bond activation.

Cooperativity and Dynamics Increase the Performance of NiFe Dry Reforming Catalysts

The dry reforming of methane (DRM), i.e., the reaction of methane and CO2 to form a synthesis gas, converts two major greenhouse gases into a useful chemical feedstock. In this work, we probe the effect and role of Fe in bimetallic NiFe dry reforming catalysts. To this end, monometallic Ni, Fe, and bimetallic Ni-Fe catalysts supported on a MgxAlyOz matrix derived via a hydrotalcite-like precursor were synthesized. Importantly, the textural features of the catalysts, i.e., the specific surface area (172-178 m2/gcat), pore volume (0.51-0.66 cm3/gcat), and particle size (5.4-5.8 nm) were kept constant. Bimetallic, Ni4Fe1 with Ni/(Ni + Fe) = 0.8, showed the highest activity and stability, whereas rapid deactivation and a low catalytic activity were observed for monometallic Ni and Fe catalysts, respectively. XRD, Raman, TPO, and TEM analysis confirmed that the deactivation of monometallic Ni catalysts was in large due to the formation of graphitic carbon. The promoting effect of Fe in bimetallic Ni-Fe was elucidated by combining operando XRD and XAS analyses and energy-dispersive X-ray spectroscopy complemented with density functional theory calculations. Under dry reforming conditions, Fe is oxidized partially to FeO leading to a partial dealloying and formation of a Ni-richer NiFe alloy. Fe migrates leading to the formation of FeO preferentially at the surface. Experiments in an inert helium atmosphere confirm that FeO reacts via a redox mechanism with carbon deposits forming CO, whereby the reduced Fe restores the original Ni-Fe alloy. Owing to the high activity of the material and the absence of any XRD signature of FeO, it is very likely that FeO is formed as small domains of a few atom layer thickness covering a fraction of the surface of the Ni-rich particles, ensuring a close proximity of the carbon removal (FeO) and methane activation (Ni) sites.

Unraveling the pathway of gold(I)-catalyzed olefin hydrogenation: an ionic mechanism.

The reaction mechanism of olefin hydrogenation catalyzed by the bimetallic gold catalyst {(AuCl)(2)[(R,R)-Me-DuPhos]} was studied by means of density functional theory calculations. This catalyst is enantioselective for the homogeneous hydrogenation of olefins and imines. The reaction mechanism involves activation of the H(2) molecule. This process takes place heterolytically, generating a metal-hydride complex as the active species and releasing a proton (formally EtOH(2)(+)) and a chloride ion to the medium. The hydrogenation reaction proceeds through an ionic mechanism in which the gold catalyst provides a hydride and the proton comes from the solvent. The reaction mechanism ends up with H(2) coordination and subsequent heterolytic cleavage, regenerating the gold(I)-hydride active species. Significant differences were found in the reaction mechanism depending on the nature of the substrate (ethene, cyclohexene, or diethyl 2-benzylidenesuccinate) and the character of the catalyst (mono- or bimetallic). Our data suggest that for prochiral substrates, the step that determines the enantioselectivity within the ionic mechanism involves a proton transfer.

The Wacker Process: Inner- or Outer-Sphere Nucleophilic Addition? New Insights from Ab Initio Molecular Dynamics

The Wacker process consists of the oxidation of ethylene catalyzed by a Pd(II) complex. The reaction mechanism has been largely debated in the literature; two modes for the nucleophilic addition of water to a Pd-coordinated alkene have been proposed: syn-inner- and anti-outer-sphere mechanisms. These reaction steps have been theoretically evaluated by means of ab initio molecular dynamics combined with metadynamics by placing the [Pd(C(2)H(4))Cl(2)(H(2)O)] complex in a box of water molecules, thereby resembling experimental conditions at low [Cl(-)]. The nucleophilic addition has also been evaluated for the [Pd(C(2)H(4))Cl(3)](-) complex, thus revealing that the water by chloride ligand substitution trans to ethene is kinetically favored over the generally assumed cis species in water. Hence, the resulting trans species can only directly undertake the outer-sphere nucleophilic addition, whereas the inner-sphere mechanism is hindered since the attacking water is located trans to ethene. In addition, all the simulations from the [Pd(C(2)H(4))Cl(2)(H(2)O)] species (either cis or trans) support an outer-sphere mechanism with a free-energy barrier compatible with that obtained experimentally, whereas that for the inner-sphere mechanism is significantly higher. Moreover, additional processes for a global understanding of the Wacker process in solution have also been identified, such as ligand substitutions, proton transfers that involve the aquo ligand, and the importance of the trans effect of the ethylene in the nucleophilic addition attack.

Cooperativity between Al Sites Promotes Hydrogen Transfer and Carbon–Carbon Bond Formation upon Dimethyl Ether Activation on Alumina

The methanol-to-olefin (MTO) process allows the conversion of methanol/dimethyl ether into olefins on acidic zeolites via the so-called hydrocarbon pool mechanism. However, the site and mechanism of formation of the first carbon–carbon bond are still a matter of debate. Here, we show that the Lewis acidic Al sites on the 110 facet of γ-Al2O3 can readily activate dimethyl ether to yield CH4, alkenes, and surface formate species according to spectroscopic studies combined with a computational approach. The carbon–carbon forming step as well as the formation of methane and surface formate involves a transient oxonium ion intermediate, generated by a hydrogen transfer between surface methoxy species and coordinated methanol on adjacent Al sites. These results indicate that extra framework Al centers in acidic zeolites, which are associated with alumina, can play a key role in the formation of the first carbon–carbon bond, the initiation step of the industrial MTO process.