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As an atom-efficient strategy for the large-scale interconversion of olefins, heterogeneously catalysed olefin metathesis sees commercial applications in the petrochemical, polymer and speciality chemical industries1. Notably, the thermoneutral and highly selective cross-metathesis of ethylene and 2-butenes1 offers an appealing route for the on-purpose production of propylene to address the C3 shortfall caused by using shale gas as a feedstock in steam crackers2,3. However, key mechanistic details have remained ambiguous for decades, hindering process development and adversely affecting economic viability4 relative to other propylene production technologies2,5. Here, from rigorous kinetic measurements and spectroscopic studies of propylene metathesis over model and industrial WOx/SiO2 catalysts, we identify a hitherto unknown dynamic site renewal and decay cycle, mediated by proton transfers involving proximal Brønsted acidic OH groups, which operates concurrently with the classical Chauvin cycle. We show how this cycle can be manipulated using small quantities of promoter olefins to drastically increase steady-state propylene metathesis rates by up to 30-fold at 250 °C with negligible promoter consumption. The increase in activity and considerable reduction of operating temperature requirements were also observed on MoOx/SiO2 catalysts, showing that this strategy is possibly applicable to other reactions and can address major roadblocks associated with industrial metathesis processes. ISSN:0028-0836 ISSN:1476-4687

Rh-based catalysts modified by transition metals have been intensively studied for CO2 hydrogenation due to their high activity. However, understanding the role of promoters at the molecular level remains challenging due to the ill-defined structure of heterogeneous catalysts. Here, we constructed well-defined RhMn@SiO2 and Rh@SiO2 model catalysts via surface organometallic chemistry combined with thermolytic molecular precursor (SOMC/TMP) approach to rationalize the promotional effect of Mn in CO2 hydrogenation. We show that the addition of Mn shifts the products from almost pure CH4 to a mixture of methane and oxygenates (CO, CH3OH, and CH3CH2OH) upon going from Rh@SiO2 to RhMn@SiO2. In situ X-ray absorption spectroscopy (XAS) confirms that the Mn-II is atomically dispersed in the vicinity of metallic Rh nanoparticles and enables to induce the oxidation of Rh to form the Mn-O-Rh interface under reaction conditions. The formed interface is proposed to be key to maintaining Rh+ sites, which is related to suppressing the methanation reaction and stabilizing the formate species as evidenced by in situ DRIFTS to promote the formation of CO and alcohols. ISSN:2041-6520 ISSN:2041-6539

A cost-efficient DFT methodology was designed to calculate 1H chemical shifts of antiferromagnetically coupled metal dimeric. This method allowed us to assign the 1H chemical shifts observed for the industrial-like UC catalyst.

The olefin metathesis activity of silica-supported molybdenum oxides depends strongly on metal loading and preparation conditions, indicating that the nature and/or amounts of the active sites vary across compositionally similar catalysts. This is illustrated by comparing Mo-based (pre)catalysts prepared by impregnation (2.5–15.6 wt % Mo) and a model material (2.3 wt % Mo) synthesized via surface organometallic chemistry (SOMC). Analyses of FTIR, UV–vis, and Mo K-edge X-ray absorption spectra show that these (pre)catalysts are composed predominantly of similar isolated Mo dioxo sites. However, they exhibit different reaction properties in both liquid and gas-phase olefin metathesis with the SOMC-derived catalyst outperforming a classical catalyst of a similar Mo loading by ×1.5–2.0. Notably, solid-state 95Mo NMR analyses leveraging state-of-the-art high-field (28.2 T) measurement conditions resolve four distinct surface Mo dioxo sites with distributions that depend on the (pre)catalyst preparation methods. The intensity of a specific deshielded 95Mo NMR signal, which is most prominent in the SOMC-derived catalyst, is linked to reducibility and catalytic activity. First-principles calculations show that 95Mo NMR parameters directly manifest the local strain and coordination environment: acute (SiO–Mo(O)2–OSi) angles and low coordination numbers at Mo lead to highly deshielded 95Mo chemical shifts and small quadrupolar coupling constants, respectively. Natural chemical shift analyses relate the 95Mo NMR signature of strained species to low LUMO energies, which is consistent with their high reducibility and corresponding reactivity. The 95Mo chemical shifts of supported Mo dioxo sites are thus linked to their specific electronic structures, providing a powerful descriptor for their propensity toward reduction and formation of active sites. ISSN:0002-7863 ISSN:1520-5126

Transition metal carbides have numerous applications and are known to excel in terms of hardness, thermal stability and conductivity. In particular, the Pt-like behavior of Mo and W carbides has led to the popularization of metal carbides in catalysis, ranging from electrochemically-driven reactions to thermal methane coupling. Herein, we show the active participation of carbidic carbon in the formation of C2 products during methane coupling at high temperature that is associated with the dynamics of Mo and W carbides. A detailed mechanistic study reveals that the catalyst performance of these metal carbides can be traced back to its carbon diffusivity and exchange capability upon interaction with methane (gas phase carbon). A stable C2 selectivity over time on stream for Mo carbide (Mo2C) can be rationalized by fast carbon diffusion dynamics, while W carbide (WC) shows loss of selectivity due to slow diffusion leading to surface carbon depletion. This finding showcases that the bulk carbidic carbon of the catalyst plays a crucial role and that the metal carbide is not only responsible for methyl radical formation. Overall, this study evidences the presence of a carbon equivalent to the Mars–Van Krevelen type mechanism for non-oxidative coupling of methane., Chemical Science, 14 (22), ISSN:2041-6520, ISSN:2041-6539

The development of an efficient heterogeneous catalyst for storing H-2 into CO(2 )and releasing it from the produced formic acid, when needed, is a crucial target for overcoming some intrinsic criticalities of green hydrogen exploitation, such as high flammability, low density, and handling. Herein, we report an efficient heterogeneous catalyst for both reactions prepared by immobilizing a molecular iridium organometallic catalyst onto a high-surface mesoporous silica, through a sol-gel methodology. The presence of tailored single-metal catalytic sites, derived by a suitable choice of ligands with desired steric and electronic characteristics, in combination with optimized support features, makes the immobilized catalyst highly active. Furthermore, the information derived from multinuclear DNP-enhanced NMR spectroscopy, elemental analysis, and Ir L-3-edge XAS indicates the formation of cationic iridium sites. It is quite remarkable to note that the immobilized catalyst shows essentially the same catalytic activity as its molecular analogue in the hydrogenation of CO2. In the reverse reaction of HCOOH dehydrogenation, it is approximately twice less active but has no induction period., Inorganic Chemistry, 61 (27), ISSN:0020-1669, ISSN:1520-510X

Zeolites, although key materials used in industrial processes, remain poorly understood on a molecular level despite their well-defined crystal lattices. In fact, obtaining a direct spectroscopic signatures and resolving the structure of Lewis acid sites (LAS) has remained a challenge. In this work, thanks to 1D and 2D

International audience; The structural characterization of supported molecular catalysts is challenging due to the low concentration of surface sites and the presence of several organic/organometallic surface groups resulting from the often complex surface chemistry associated with support functionalization. Here, we provide a complete atomic-scale description of all surface sites in a silica-supported iridium-N-heterocyclic carbene (Ir-NHC) catalytic material, at all stages of its synthesis. By combining a suitable isotope labelling strategy with the implementation of multi-nuclear dipolar recoupling DNP enhanced NMR experiments, the 3D structure of the Ir-NHC sites, as well as that of the synthesis intermediates were determined. As a significant fraction of parent surface fragments does not react during the multi-step synthesis, site-selective experiments were implemented to specifically probe proximities between the organometallic groups and the solid support. The NMR-derived structure of the iridium sites points to a well-defined conformation. By interpreting extended x-ray absorption fine structure (EXAFS) spectroscopy and chemical analysis data augmented by computational studies, the presence of two coordination geometries is demonstrated: Ir-NHC fragments coordinated by a 1,5-cyclooctadiene and one Cl ligand, as well as, more surprisingly, a fragment coordinated by two NHC and two Cl ligands. This study demonstrates a unique methodology to disclose individual surface structures in complex, multi-site environments, a long-standing challenge in the field of heterogeneous/supported catalysts, while revealing new, unexpected structural features of metallo-NHC supported substrates. It also highlights the potentially large diversity of surface sites present in functional materials prepared by surface chemistry, an essential knowledge to design materials with improved performances.

Selective oxidation of methane to methanol by molecular oxygen is a fascinating route for upgrading abundant methane resource and represents one of the most challenging reactions in chemistry due to the overwhelmingly higher reactivity of the product versus the reactant. Here, we report that monometallic gold nanoparticles loaded on mordenite zeolite efficiently catalyze the selective oxidation of methane to methanol by molecular oxygen in the presence of carbon monoxide in aqueous medium. The methanol productivity reaches 1300 μmol gcat−1 h−1 or 280 mmol gAu-1 h-1 with 75% selectivity at 150 °C, outperforming most of those reported under comparable conditions. Both hydroxyl radicals and hydroperoxide species participate in the activation and conversion of methane; the lower affinity of methanol on gold mainly accounts for higher methanol selectivity.

Au-Zn catalysts have previously been shown to promote hydrogenation of CO2 to methanol, but their active state is poorly understood. Here, silica-supported bimetallic Au-Zn alloys, prepared by Surface Organometallic Chemistry (SOMC), are shown to be proficient catalysts for hydrogenation of CO2 to methanol. In situ X-ray absorption spectroscopy (XAS), in conjunction with gas-switching experiments, is used to amplify subtle changes occurring at the surface of this tailored catalyst during reaction. Consequently, an Au-Zn alloy is identified, and is shown to undergo subsequent reversible redox changes under reaction conditions according to multivariate curve resolution alternating least-squares (MCR-ALS) analysis. These results highlight the role of alloying and de-alloying in Au-based CO2 hydrogenation catalysts and illustrate the role of these reversible processes in driving reactivity.

The catalytic activity of silica-supported molybdenum oxides for olefin metathesis depends strongly on the metal loading and preparation conditions indicating that the nature and/or amounts of the active sites vary across catalysts. This is illustrated by comparing Mo-based (pre)catalysts prepared by impregnation with different metal loadings (2.5-15.6 wt% Mo) and a well-defined model material (2.3 wt% Mo) prepared via a surface organometallic chemistry (SOMC) synthetic approach. Analyses of FTIR, UV-vis, and Mo K-edge X-ray absorption spectra provide strong evidence that all the (pre)catalysts are composed predominantly of similar isolated Mo dioxo sites; however, they exhibit very different proportions of reducible surface sites and catalytic reaction properties. Specifically, the SOMC-derived catalyst is more active for liquid and gas-phase olefin metathesis conditions than a classical catalyst of similar Mo loading by a factor of 1.5-1.9, depending on precise reaction conditions. Most notably, solid-state 95Mo NMR spectra of these catalysts show distinct features, particularly evident under state-of-the-art high-field (28.2 T) measurement conditions where at least four distinct types of surface Mo dioxo sites are resolved, the distribution of which depends on the preparation methods. In particular, the presence of Mo sites with a specific deshielded 95Mo NMR signal correlates with the catalysts reducibility and metathesis activity; such sites are most prominent in the SOMC-derived catalyst. First-principles calculations show that the 95Mo NMR parameters, specifically the isotropic chemical shift and quadrupolar coupling constant, are good descriptors for local strain and coordination environment: acute (SiO-Mo(O)2-OSi) angles and low coordination numbers at the Mo sites leads to highly deshielded iso chemical shifts and small CQ values, respectively. Orbital and natural chemical shift analyses indicate that the deshielded 95Mo iso values of strained species are directly related to low LUMO energies, consistent with their higher reducibility and corresponding reactivity. Overall, this study shows that solid-state NMR is particularly powerful for the identification of distinct supported Mo dioxo species and that their 95Mo chemical shifts are related to their specific local electronic structures, providing a powerful descriptor for the propensity of Mo sites towards reduction and the formation of active sites.

Characterization of paramagnetic compounds, in particular regarding the detailed conformation and electronic structure, remains a challenge - still today it often relies solely on the use of X-ray crystallography, thus limiting the access to electronic structure information. This is particularly true for lanthanide elements that are often associated with peculiar structural and electronic features in relation to their partially filled f -shell. Here, we showcase the use of state-of-the-art magnetic resonance spec- troscopy (EPR and solid-state NMR) and computational approaches as well as magnetic susceptibility measurements to determine the structure of a paramagnetic Yb(III) alkyl complex, Yb(III)[CH(SiMe3)2]3, that features a notable structure according to X-ray crystallography. Each of these techniques revealed specific information about the geometry and electronic structure of the complex; taken together, they provide a detailed understanding of this paramagnetic compound. Namely, this complex displays a three-centre-two-electron Yb-γ-Me-β–Si secondary metal-ligand interaction, whose NMR spectroscopic signature was acquired for the first time for a lanthanide paramagnetic species. The electronic configuration of Yb(III)[CH(SiMe3)2]3 is demonstrated to be close to the one of the free Yb(III) ion, with the partially filled f -shell of the Yb atom having little influence on its bonding properties and with minimal delocalization of f -electron density from Yb to the directly bonded carbons.

The Union Carbide (UC) ethylene polymerization catalysts, based on chromocene dispersed on silica, show distinct features from the Phillips catalysts, but share the same heated debate regarding the structure of their active sites. Based on a combination of IR, EPR spectroscopies, labeling experiments, and DFT modeling, we identified monomeric surface-supported Cr(III) hydrides, ( SiO)Cr(Cp)-H, as the active sites of the UC catalyst. These sites are formed in the presence of grafted and adsorbed chromocene as well as residual surface OH groups, only possible at high Cr loading, and involve a C-H activation of the Cp ring. These Cr-hydrides initiate polymerization, yielding Cr(III)alkyl species that insert ethylene through a Cossee-Arlman-type mechanism, as evidenced by spectroscopic studies. These insights inspired the design of a well-defined analog, CpCr(CH(SiMe3)(2))(2) grafted on partially dehydroxylated silica, that shows similar spectroscopic and polymer structure to the UC catalyst, further supporting the proposed active site structure., Chemical Science, 13 (37), ISSN:2041-6520, ISSN:2041-6539

Cationic d0 group 6 olefin metathesis catalysts have been recently shown to display in most instances superior activity in comparison to their neutral congeners. Furthermore, their catalytic performance is greatly improved upon immobilization on silica. In this context, we have developed the new family of molecular cationic molybdenum oxo alkylidene complexes stabilized by N-heterocyclic carbenes of the general formula [Mo(O)(CHCMe3)(IMes)(OR)[X−]] (IMes = 1,3-dimesitylimidazol-2-ylidene; R = 1,3-dimesityl-C6H3, C6F5; X− = B(3,5-(CF3)2C6H3)4−, B(ArF)4, tetrakis(perfluoro-t-butoxy)aluminate (PFTA)). Immobilization of [Mo(O)(CHCMe3)(IMes)(O-1,3-dimesityl-C6H3)+B(ArF)4−] on silica via surface organometallic chemistry yields an active alkene metathesis catalyst that shows the highest productivity towards terminal olefins amongst all existing molybdenum oxo alkylidene catalysts., Chemical Science, 13 (29), ISSN:2041-6520, ISSN:2041-6539

Polymer-supported catalysts have been of great interest in organic syntheses, but have suffered from the difficulty in obtaining direct structural information regarding the catalyst species embedded in the polymer due to the limitations of most analytical methods. Here, we show that dynamic nuclear polarization (DNP)-enhanced solid-state NMR is ideally positioned to characterize the ubiquitous cross-linked polystyrene (PS)-supported catalysts, thus enabling molecular-level understanding and rational development. Ammonium-based catalysts, which show excellent catalytic activity and reusability for the transesterification of methyl esters with glycidol, giving glycidyl esters in high yields, were successfully characterized by DNP 15N NMR spectroscopy at 15N natural abundance. DNP 15N NMR shows in particular that the decomposition of quaternary alkylammonium moieties to tertiary amines was completely suppressed during the catalytic reaction. Furthermore, the dilute ring-opened product derived from glycidol and NO3- was directly characterized by DNP 15N CPMAS and 1H-15N and 1H-13C HETCOR NMR using a 15N enriched (NO3) sample, supporting the view that the transesterification mechanism involves an alkoxide anion derived from an epoxide and NO3-. In addition, the detailed analysis of a used catalyst indicated that the adsorption of products on the cationic center is the major deactivation step in this catalysis., Chemical Science, 13 (16), ISSN:2041-6520, ISSN:2041-6539

Single-atom catalysis is recognized as a frontier of heterogeneous catalysis for its efficient utilization of metals and the possibility to engender unusual reactivity. Yet, despite the observation of single atoms, understanding their coordination structures and developing structure–property relationships remains challenging due to the structural complexity of support surfaces. Here, using single-crystalline MgO(111) two-dimensional nanosheets and a surface organometallic chemistry method, we describe the formation of highly dispersed Ir(III) sites (isolated at 0.1 wt%, and Ir pairs and trimers at 1 wt%) with well-defined coordination structures. These species display unique catalytic properties in the coupling reaction of benzene and ethylene to form styrene, a reactivity that contrasts with conventional homogeneous and heterogeneous iridium catalysts that yield ethylbenzene. The similar activities for high- and low-loading catalysts suggest that iridium sites, whether isolated or in the form of clusters (for example Ir3), have similar activity, consistent with the involvement of surface dynamics. The fine tuning of the interface between single atoms and their supports may open advantageous reactivity scenarios, although it remains challenging. Now, Liu, Coperet and colleagues address this problem by dispersing Ir(III) species on well-defined single-crystalline MgO(111) 2D nanosheets, achieving a unique reactivity for the coupling of benzene and ethylene.

Gallia–alumina (Ga,Al)2O3(x : y) spinel-type solid solution nanoparticle catalysts for propane dehydrogenation (PDH) were prepared with four nominal Ga : Al atomic ratios (1 : 6, 1 : 3, 3 : 1, 1 : 0) using a colloidal synthesis approach. The structure, coordination environment and distribution of Ga and Al sites in these materials were investigated by X-ray diffraction, X-ray absorption spectroscopy (Ga K-edge) as well as 27Al and 71Ga solid state nuclear magnetic resonance. The surface acidity (Lewis or Brønsted) was probed using infrared spectroscopy with pyridine and 2,6-dimethylpyridine probe molecules, complemented by element-specific insights (Ga or Al) from dynamic nuclear polarization surface enhanced cross-polarization magic angle spinning 15N{27Al} and 15N{71Ga} J coupling mediated heteronuclear multiple quantum correlation NMR experiments using 15N-labelled pyridine as a probe molecule. The latter approach provides unique insights into the nature and relative strength of the surface acid sites as it allows to distinguish contributions from Al and Ga sites to the overall surface acidity of mixed (Ga,Al)2O3 oxides. Notably, we demonstrate that (Ga,Al)2O3 catalysts with a high Al content show a greater relative abundance of four-coordinated Ga sites and a greater relative fraction of weak/medium Ga-based surface Lewis acid sites, which correlates with superior propene selectivity, Ga-based activity, and stability in PDH (due to lower coking). In contrast, (Ga,Al)2O3 catalysts with a lower Al content feature a higher fraction of six-coordinated Ga sites, as well as more abundant Ga-based strong surface Lewis acid sites, which deactivate through coking. Overall, the results show that the relative abundance and strength of Ga-based surface Lewis acid sites can be tuned by optimizing the bulk Ga : Al atomic ratio, thus providing an effective measure for a rational control of the catalyst performance., Coordination geometry and Lewis acidity of Ga and Al (bulk and surface) sites in mixed oxide gallia–alumina nanoparticles is correlated with the performance in propane dehydrogenation.

Generating or even retaining slow magnetic relaxationin surfaceimmobilized single-molecule magnets (SMMs) from promising molecularprecursors remains a great challenge. Illustrative examples are organolanthanidecompounds that show promising SMM properties in molecular systems,though surface immobilization generally diminishes their magneticperformance. Here, we show how tailored Lewis acidic Al-(III) siteson a silica surface enable generation of a material with SMM characteristicsvia chemisorption of (Cp-ttt)(2)DyCl ((Cp-ttt)(-) = 1,2,4-tri-(tert-butyl)-cyclopentadienide).Detailed studies of this system and its diamagnetic Y analogue indicatethat the interaction of the metal chloride with surface Al sites resultsin a change of the coordination sphere around the metal center inducingfor the dysprosium-containing material slow magnetic relaxation upto 51 K with hysteresis up to 8 K and an effective energy barrier(U (eff)) of 449 cm(-1),the highest reported thus far for a supported SMM., Journal of the American Chemical Society, 145 (23), ISSN:0002-7863, ISSN:1520-5126