Unearthing a Well-Defined Highly Active Bimetallic W/Ti Precatalyst Anchored on a Single Silica Surface for Metathesis of Propane

Bimetallic Sites for Catalysis: From Binuclear Metal Sites to Bimetallic Nanoclusters and Nanoparticles

Heterogeneous bimetallic catalysts have broad applications in industrial processes, but achieving a fundamental understanding on the nature of the active sites in bimetallic catalysts at the atomic and molecular level is very challenging due to the structural complexity of the bimetallic catalysts. Comparing the structural features and the catalytic performances of different bimetallic entities will favor the formation of a unified understanding of the structure-reactivity relationships in heterogeneous bimetallic catalysts and thereby facilitate the upgrading of the current bimetallic catalysts. In this review, we will discuss the geometric and electronic structures of three representative types of bimetallic catalysts (bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles) and then summarize the synthesis methodologies and characterization techniques for different bimetallic entities, with emphasis on the recent progress made in the past decade. The catalytic applications of supported bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles for a series of important reactions are discussed. Finally, we will discuss the future research directions of catalysis based on supported bimetallic catalysts and, more generally, the prospective developments of heterogeneous catalysis in both fundamental research and practical applications.

Small molecule activation with bimetallic systems: a landscape of cooperative reactivity

There is growing interest in the design of bimetallic cooperative complexes, which have emerged due to their potential for bond activation and catalysis, a feature widely exploited by nature in metalloenzymes, and also in the field of heterogeneous catalysis. Herein, we discuss the widespread opportunities derived from combining two metals in close proximity, ranging from systems containing multiple M-M bonds to others in which bimetallic cooperation occurs even in the absence of M⋯M interactions. The choice of metal pairs is crucial for the reactivity of the resulting complexes. In this context, we describe the prospects of combining not only transition metals but also those of the main group series, which offer additional avenues for cooperative pathways and reaction discovery. Emphasis is given to mechanisms by which bond activation occurs across bimetallic structures, which is ascribed to the precise synergy between the two metal atoms. The results discussed herein indicate a future landscape full of possibilities within our reach, where we anticipate that bimetallic synergism will have an important impact in the design of more efficient catalytic processes and the discovery of new catalytic transformations.

Titanium methyl tamed on silica: synthesis of a well-defined pre-catalyst for hydrogenolysis of n-alkane

Alkylation of Ti(CH₃)₂Cl₂1 by MeLi gives the homoleptic Ti(CH₃)₄2 for the first time in the absence of any coordinating solvent. The reaction of 2 with silica pretreated at 700 °C (SiO_2-700) gives two inequivalent silica-supported Ti-methyl species 3. Complex 3 was characterized by IR, microanalysis (ICP-OES, CHNS, and gas quantification), and advanced solid-state NMR spectroscopy (¹H, ¹³C, DQ, TQ, and HETCOR). The catalytic activity of the pre-catalyst 3 is investigated in low-temperature hydrogenolysis of propane and n-butane with TONs of 419 and 578, respectively.

Metal Alkyls with Alkylidynic Metal-Carbon Bond Character: Key Electronic Structures in Alkane Metathesis Precatalysts

The homologation of alkanes via alkane metathesis is catalyzed at low temperatures (150 °C) by the silica-supported species (≡SiO)WMe₅ and (≡SiO)TaMe₄ , while (≡SiO)TaMe₃ Cp* is inactive. The contrasting reactivity is paralleled by differences in the ¹³ C NMR signature; the former display significantly more deshielded isotropic chemical shifts (δ_iso ) and almost axially symmetric chemical shift tensors, similar to what is observed in their molecular precursors TaMe₅ and WMe₆ . Analysis of the chemical shift tensors reveals the presence of a triple-bond character in their metal-carbon (formally single) bond. This electronic structure is reflected in their propensity to generate alkylidynes and to participate in alkane metathesis, further supporting the role of alkylidynes as key reaction intermediates. This study establishes chemical shift as a descriptor to identify potential alkane metathesis catalysts.

Docking of tetra-methyl zirconium to the surface of silica: a well-defined pre-catalyst for conversion of CO2 to cyclic carbonates

The metal complex (Zr(CH3)4(THF)2) has been fully synthesized, characterized and grafted onto partially dehydroxylated silica to give two surface species [([triple bond, length as m-dash]Si-O-)Zr(CH3)3(THF)2] (minor) and [([triple bond, length as m-dash]Si-O-)2Zr(CH3)2(THF)2] (major) which have been characterized by SS NMR, IR, and elemental analysis. These supported pre-catalysts exhibit the best conversion of CO2 to cyclic carbonates, as compared to the previously reported SOMC catalysts.

The Comparison between Single Atom Catalysis and Surface Organometallic Catalysis

Single atom catalysis (SAC) is a recent discipline of heterogeneous catalysis for which a single atom on a surface is able to carry out various catalytic reactions. A kind of revolution in heterogeneous catalysis by metals for which it was assumed that specific sites or defects of a nanoparticle were necessary to activate substrates in catalytic reactions. In another extreme of the spectrum, surface organometallic chemistry (SOMC), and, by extension, surface organometallic catalysis (SOMCat), have demonstrated that single atoms on a surface, but this time with specific ligands, could lead to a more predictive approach in heterogeneous catalysis. The predictive character of SOMCat was just the result of intuitive mechanisms derived from the elementary steps of molecular chemistry. This review article will compare the aspects of single atom catalysis and surface organometallic catalysis by considering several specific catalytic reactions, some of which exist for both fields, whereas others might see mutual overlap in the future. After a definition of both domains, a detailed approach of the methods, mostly modeling and spectroscopy, will be followed by a detailed analysis of catalytic reactions: hydrogenation, dehydrogenation, hydrogenolysis, oxidative dehydrogenation, alkane and cycloalkane metathesis, methane activation, metathetic oxidation, CO₂ activation to cyclic carbonates, imine metathesis, and selective catalytic reduction (SCR) reactions. A prospective resulting from present knowledge is showing the emergence of a new discipline from the overlap between the two areas.

A strategy to convert propane to aromatics (BTX) using TiNp4 grafted at the periphery of ZSM-5 by surface organometallic chemistry

The direct conversion of propane into aromatics (BTX) using modified ZSM-5 was achieved with a strategy of "catalysis by design". In contrast to the classical mode of action of classical aromatization catalysts which are purely based on acidity, we have designed the catalyst associating two functions: One function (Ti-hydride) was selected to activate the C-H bond of propane by σ-bond metathesis to further obtain olefin by β-H elimination and the other function (Brønsted acid) being responsible for the oligomerization, cyclization, and aromatization. This bifunctional catalyst was obtained by selectively grafting a bulky organometallic complex of tetrakis(neopentyl)titanium (TiNp4) at the external surface (external silanol ([triple bond, length as m-dash]Si-OH) group) of [H-ZSM-5300] to obtain [Ti/ZSM-5] catalyst 1. This metal was chosen to activate the C-H bond of paraffin at the periphery of the ZSM-5 while maintaining the Brønsted acid properties of the internal [H-ZSM-5] for oligomerization, cyclization, and aromatization. Catalyst 2 [Ti-H/ZSM-5] was obtained after treatment under H2 at 550 °C of freshly prepared catalyst 1 ([Ti/ZSM-5]) and catalyst 1 was thoroughly characterized by ICP analysis, DRIFT, XRD, N2-physisorption, multinuclear solid-state NMR, XPS and HR-TEM analysis including STEM imaging. The conversion of propane to aromatics was studied in a dynamic flow reactor. With the pristine [H-ZSM-5300] catalyst, the conversion of propane is very low. However, with [Ti-H/ZSM-5] catalyst 2 under the same reaction conditions, the conversion of propane remains significant during 60 h of the reaction (ca. 22%). Furthermore, the [Ti-H/ZSM-5] catalyst shows a good and stable selectivity (55%) for aromatics (BTX) of time on stream. With 2, it was found that the Ti remains at the periphery of the [H-ZSM-5] even after reaction time.

Metathesis of Functionalized Alkane: Understanding the Unsolved Story

For the first time, we developed a method which enables a functionalized alkane to be metathesized to its lower and higher homologues. For this metathesis reaction, we used [(≡Si-O-)W(CH3)5] as a catalyst precursor and 9-hexyl-9H-carbazole as a reactant.

Single-atom catalysis: a new field that learns from tradition

Much of industrial chemical processing (in the petrochemicals industry, for example), and a great deal of laboratory chemical synthesis, involves catalysts that both lower the energy barrier to reaction and may help steer a reaction along a particular path. Traditionally, catalysts have come in two classes: heterogeneous, typically meaning that the catalyst is an extended solid; and homogeneous, where the catalyst is a small molecule that shares a solvent with the reactants. In heterogeneous catalysis, the reaction generally takes place on a surface, involving molecules attached there by covalent bonds. Homogeneous catalysts are often organometallic compounds, in which a metal atom or small cluster of atoms supplies the active site for reaction.

 In recent years, these distinctions have become somewhat blurred thanks to the advent of single-atom catalysis, where the catalytic site consists of a single atom (as in many homogeneous catalysts) attached to or embedded in a surface. The emergence of this field might be regarded as the logical conclusion of the use of ‘supported metal clusters’—small metal particles of nanometer scale and below, containing perhaps hundreds, tens or just a few atoms. It has became clear that such clusters can sometimes provide greater product selectivity and activity than macro-sized particles or powders of the same metal, partly because the active sites might be atoms at particular locations (such as edges and corners) in the nanoscale particles. By reducing their scale down to the level of single atoms, one can optimize these properties. At the same time, the potential uniformity of the atoms’ environments makes such catalysts more amenable to rational design and modeling to understand mechanism.

 This field represents an appealing blend of fundamental chemistry and physics—from the quantum-mechanical level upwards—and applied research aimed at producing many of the products vital to society, such as fuels and materials. Researchers in China have been strongly active in this field in recent years (see, for example, refs [1–5]). Jean Marie Basset of the King Abdullah University of Science and Technology in Thuwal, Saudi Arabia, is one of the leading practitioners in the area, and National Science Review spoke to him about the development and prospects of the field.

Titanium Insertion into CO Bonds in Anionic Ti-CO2 Complexes

We explore the structures of [Ti(CO₂) _y]^- cluster anions using infrared photodissociation spectroscopy and quantum chemistry calculations. The existence of spectral signatures of metal carbonyl CO stretching modes shows that insertion of titanium atoms into C-O bonds represents an important reaction during the formation of these clusters. In addition to carbonyl groups, the infrared spectra show that the titanium center is coordinated to oxalato, carbonato, and oxo ligands, which form along with the metal carbonyls. The presence of a metal oxalato ligand promotes C-O bond insertion in these systems. These results highlight the affinity of titanium for C-O bond insertion processes.

Surface organometallic chemistry in heterogeneous catalysis

Surface organometallic chemistry has been reviewed with a special focus on environmentally relevant transformations (C–H activation, CO₂conversion, oxidation).