Accessing Realistic Models for the WO3–SiO2 Industrial Catalyst through the Design of Organometallic Precursors

On the use of 17O NMR for understanding molecular and silica-grafted tungsten oxo siloxide complexes

¹⁷O-labelled tungsten siloxide complexes [WOCl₂(OSitBu₃)₂] (1-Cl) and [WOMe₂(OSitBu₃)₂] (1-Me) were prepared and characterized by ¹⁷O MAS NMR, with input from theoretical calculations of NMR parameters. Guidelines linking ¹⁷O NMR parameters and the coordination sphere of molecular and silica-grafted tungsten oxo species are proposed. The grafting of 1-Me on SiO_2-700 afforded material 2, with surface species [(SiO)WOMe₂(OSitBu₃)] as shown by elemental analysis, IR and ¹H and ¹³C MAS NMR. The DFT calculations of the grafting mechanism are in line with the observed reactivity. They indicate the occurrence of several isomeric species of close energy for the grafted W centers, precluding efficient ¹⁷O MAS NMR studies. The lack of catalytic activity in olefin metathesis and ring-opening olefin metathesis polymerization indicates that initiation by α-H elimination is not operative in 2, contrary to related tungsten surface species, which illustrates the crucial influence of the nature of the metal coordination sphere.

Olefin metathesis: what have we learned about homogeneous and heterogeneous catalysts from surface organometallic chemistry?

Since its early days, olefin metathesis has been in the focus of scientific discussions and technology development. While heterogeneous olefin metathesis catalysts based on supported group 6 metal oxides have been used for decades in the petrochemical industry, detailed mechanistic studies and the development of molecular organometallic chemistry have led to the development of robust and widely used homogeneous catalysts based on well-defined alkylidenes that have found applications for the synthesis of fine and bulk chemicals and are also used in the polymer industry. The development of the chemistry of high-oxidation group 5-7 alkylidenes and the use of surface organometallic chemistry (SOMC) principles unlocked the preparation of so-called well-defined supported olefin metathesis catalysts. The high activity and stability (often superior to their molecular analogues) and molecular-level characterisation of these systems, that were first reported in 2001, opened the possibility for the first direct structure-activity relationships for supported metathesis catalysts. This review describes first the history of SOMC in the field of olefin metathesis, and then focuses on what has happened since 2007, the date of our last comprehensive reviews in this field.

Single-Atom Catalysts across the Periodic Table

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

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.

Grafting of a new bis-silylamido aluminum species on silica: insight from solid-state NMR into interactions with the surface

The new bisamido aluminum species [AlCl{N(SiMe3)2}2(THF)] (1) was prepared and fully characterized by 27Al and 35Cl solid-state NMR, along with X-ray diffraction studies. 1 was grafted on silica partially dehydroxylated at 700 °C, affording silica-supported Al species. The resulting material (2) was characterized by IR, elemental analysis and 1H, 13C and 27Al solid-state MAS NMR. The 1D and 2D 27Al MAS NMR studies showed the occurrence of two types of species, where the Al center adopts a tetracoordinated coordination sphere, with as an additional coordinated Lewis base, either a THF ligand or a silica-surface siloxane moiety. DFT calculations allowed understanding the grafting mechanism and the spectroscopic properties of the material.

Heterogeneous catalysts for gas-phase conversion of ethylene to higher olefins

The reduced availability of propylene and C4 products from steam crackers continues to provoke on-purpose technologies for light olefins such that almost 30% of propylene in 2025 is predicted to be supplied from unconventional sources. Furthermore, the recent discoveries of natural gas reservoirs have urged interest in the conversion of surplus alkanes and alkenes, especially ethane and ethylene. The direct conversion of ethylene to propylene or a combination of value-added chemicals, including butylenes and oligomers in the range of gasoline and diesel fuel, provides the capability of responding to the fluctuations in the balance between supply and demand of the main petrochemicals. A comprehensive review of heterogeneous catalysts for the gas-phase conversion pathways is presented here in terms of catalytic performances (ethylene conversion and product selectivities), productivities, lifetimes, active sites, physicochemical properties, mechanisms, influence of operating conditions, deactivation and some unresolved/less-advanced aspects of the field. The addressed catalysts cover both zeolitic materials and transition metals, such as tungsten, molybdenum, rhenium and nickel. Efforts in both experimental and theoretical studies are taken into account. Aside from the potential fields of progress, the review reveals very promising performances for the emerging technologies to produce propylene, a mixture of propylene and butenes, or a liquid fuel from ethylene.

Solid-state NMR of quadrupolar nuclei for investigations into supported organometallic catalysts: scope and frontiers

The scope, limitations and outlooks of half-integer quadrupolar nuclei NMR as applied to supported catalysts characterization are discussed.

Density Functional Theory Study of the Reaction between d0 Tungsten Alkylidyne Complexes and H2O: Addition versus Hydrolysis

The reactions of early-transition-metal complexes with H₂O have been investigated. An understanding of these elementary steps promotes the design of precursors for the preparation of metal oxide materials or supported heterogeneous catalysts. Density functional theory (DFT) calculations have been conducted to investigate two elementary steps of the reactions between tungsten alkylidyne complexes and H₂O, i.e., the addition of H₂O to the W≡C bond and ligand hydrolysis. Four tungsten alkylidyne complexes, W(≡CSiMe₃)(CH₂SiMe₃)₃ (A-1), W(≡CSiMe₃)(CH₂^tBu)₃ (B-1), W(≡C^tBu)(CH₂^tBu)₃ (C-1), and W(≡C^tBu)(O^tBu)₃ (D-1), have been compared. The DFT studies provide an energy profile of the two competing pathways. An additional H₂O molecule can serve as a proton shuttle, accelerating the H₂O addition reaction. The effect of atoms at the α and β positions has also been examined. Because the lone-pair electrons of an O atom at the α position can interact with the orbital of the proton, the barrier of the ligand-hydrolysis reaction for D-1 is dramatically reduced. Both the electronic and steric effects of the silyl group at the β position lower the barriers of both the H₂O addition and ligand-hydrolysis reactions. These new mechanistic findings may lead to the further development of metal complex precursors.

Silica supported copper nanoparticles prepared via surface organometallic chemistry: active catalysts for the selective hydrogenation of 2,3-dimethylbutadiene

Mesitylcopper reacts with silica to give supported clusters, then reduced to nanoparticles. These catalysts allow the selective hydrogenation of 2,3-dimethylbutadiene to 2,3-dimethyl-1-butene. Materials were characterized in detail.

Well-defined silica supported bipodal molybdenum oxo alkyl complexes: a model of the active sites of industrial olefin metathesis catalysts

A novel well-defined supported bipodal molybdenum alkyl oxo species for “modelling MoO₃/SiO₂ industrial catalysts” that efficiently catalyzes olefin metathesis has been unveiled.

Solvent-Free Ring-Opening Metathesis Polymerization of Norbornene over Silica-Supported Tungsten-Oxo Perhydrocarbyl Catalysts

Ring opening metathesis polymerization (ROMP) of bicyclo[2.2.1]hept-2-ene (norbornene) is carried out over silica-supported catalysts based on tungsten complexes bearing an oxo ligand (1: [(SiO)W(O)(CH₂ SiMe₃ )₃ , 2: [(SiO)W(O)(CHCMe₂ Ph)(dAdPO)], dAdPO  2,6 diadamantyl-4-methylphenoxide, 3: [(SiO)₂ W(O)(CH₂ SiMe₃ )₂ ]). The evaluation of the catalytic activities of the aforementioned materials in ROMP indicates that at low reaction time (0.5 min), the highest polymer yield is obtained with catalyst 2. However, for longer reaction time (>2 min), complex 3, a model of the industrial catalyst, exhibits a better monomer conversion. The polymers obtained are characterized. Moreover, these catalysts are shown to be rather preferentially selective to give the cis polynorbornene (>65%), characterized by high melting points (≈300 °C). The experimental values of the average molecular weight (M_n ) of polynorbornenes are found to be close to the theoretical ones for the polymers prepared using catalyst 2 and higher for those originated from catalyst 3.

17O MAS NMR studies of oxo-based olefin metathesis catalysts: a critical assessment of signal enhancement methods

We assessed the DFS parameters for robust and optimal signal enhancement in ¹⁷O NMR studies of silica-supported catalysts.

Modification of silica-supported tungsten neosilyl oxo precatalysts: impact of substituted phenol on activity and stability in olefin metathesis

Development of a novel and convenient strategy to access a large variety of highly active tungsten oxo olefin metathesis pre-catalysts.