Surface organometallic chemistry: A new approach to heterogeneous Catal.ysis ?

M-Ge-Si thermolytic molecular precursors and models for germanium-doped transition metal sites on silica

The synthesis, thermolysis, and surface organometallic chemistry of thermolytic molecular precursors based on a new germanosilicate ligand platform, -OGe[OSi(OtBu)3]3, is described. Use of this ligand is demonstrated with preparation of complexes containing the first-row transition metals Cr, Mn, and Fe. The thermolysis and grafting behavior of the synthesized complexes, Fe{OGe[OSi(OtBu)3]3}2 (FeGe), Mn{OGe[OSi(OtBu)3]3}2(THF)2 (MnGe) and Cr{OGe[OSi(OtBu)3]3}2(THF)2 (CrGe), was evaluated using a combination of thermogravimetric analysis; nuclear magnetic resonance (NMR), ultraviolet-visible (UV-Vis), and electron paramagnetic resonance (EPR) spectroscopies; and single-crystal X-ray diffraction (XRD). Grafting of the precursors onto SBA-15 mesoporous silica and subsequent calcination in air led to substantial changes in transition metal coordination environments and oxidation states, the implications of which are discussed in the context of low-coordinate and low oxidation state thermolytic molecular precursors.

Oxidative Grafting for Catalyst Synthesis in Surface Organometallic Chemistry

The development of new methods of catalyst synthesis with the potential to generate active site structures orthogonal to those accessible by traditional protocols is of great importance for discovering new materials for addressing challenges in the evolving energy and chemical economy. In this work, the generality of oxidative grafting of organometallic and well-defined molecular metal precursors onto redox-active surfaces such as manganese dioxide (MnO2) and lithium manganese oxide (LiMn2O4) is investigated. Nine molecular metal precursors are explored, spanning groups 4-11 and each of the three periods of the transition metal series. The byproducts of the oxidative grafting reaction, a mixture of protodemetalation and ligand homocoupling for several organometallic precursors, was found to provide insights into the mechanism of the grafting reaction, suggesting oxidation of both the metal d-orbitals, as well as the metal-carbon σ-bonds, resulting in ejection of the ligand radical fragment. Analysis of the supported structures and oxidation state by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) suggests that several of the chemisorbed metal ions are intercalated into interstitial vacancies of the surface structure while other complexes form intact molecular fragments on the surface. Proof of concept for the use of this metalation protocol to generate diverse, metal-dependent catalytic performance is demonstrated by the application of these materials in the conversion of cyclohexane to K/A oil (cyclohexanol and cyclohexanone) with tert-butyl hydroperoxide, as well as in the low-temperature (T ≤ 50 °C) oxidation of carbon monoxide to carbon dioxide.

Heterogenization of molecular catalysts within porous solids: the case of Ni-catalyzed ethylene oligomerization from zeolites to metal-organic frameworks

The last decade has seen a tremendous expansion of the field of heterogenized molecular catalysis, especially with the growing interest in metal-organic frameworks and related porous hybrid solids. With successful achievements in the transfer from molecular homogeneous catalysis to heterogenized processes come the necessary discussions on methodologies used and a critical assessment on the advantages of heterogenizing molecular catalysis. Here we use the example of nickel-catalyzed ethylene oligomerization, a reaction of both fundamental and applied interest, to review heterogenization methodologies of well-defined molecular catalysts within porous solids while addressing the biases in the comparison between original molecular systems and heterogenized counterparts.

Single-Atom Catalysts on Covalent Triazine Frameworks: at the Crossroad between Homogeneous and Heterogeneous Catalysis

Heterogeneous single-site and single-atom catalysts potentially enable combining the high catalytic activity and selectivity of molecular catalysts with the easy continuous operation and recycling of solid catalysts. In recent years, covalent triazine frameworks (CTFs) found increasing attention as support materials for particulate and isolated metal species. Bearing a high fraction of nitrogen sites, they allow coordinating molecular metal species and stabilizing particulate metal species, respectively. Dependent on synthesis method and pretreatment of CTFs, materials resembling well-defined highly crosslinked polymers or materials comparable to structurally ill-defined nitrogen-containing carbons result. Accordingly, CTFs serve as model systems elucidating the interaction of single-site, single-atom and particulate metal species with such supports. Factors influencing the transition between molecular and particulate systems are discussed to allow deriving tailored catalyst systems.

Titanium(IV) Surface Complexes Bearing Chelating Catecholato Ligands for Enhanced Band-Gap Reduction

Protonolysis reactions between dimethylamido titanium(IV) catecholate [Ti(CAT)(NMe₂)₂]₂ and neopentanol or tris(tert-butoxy)silanol gave catecholato-bridged dimers [(Ti(CAT)(OCH₂tBu)₂)(HNMe₂)]₂ and [Ti(CAT){OSi(OtBu)₃}₂(HNMe₂)₂]₂, respectively. Analogous reactions using the dimeric dimethylamido titanium(IV) (3,6-di-tert-butyl)catecholate [Ti(CATtBu₂-3,6)(NMe₂)₂]₂ yielded the monomeric Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂ and Ti(CATtBu₂-3,6)[OSi(OtBu)₃]₂(HNMe₂)₂. The neopentoxide complex Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂ engaged in further protonolysis reactions with Si-OH groups and was consequentially used for grafting onto mesoporous silica KIT-6. Upon immobilization, the surface complex [Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂]@[KIT-6] retained the bidentate chelating geometry of the catecholato ligand. This convergent grafting strategy was compared with a sequential and an aqueous approach, which gave either a mixture of bidentate chelating species with a bipodally anchored Ti(IV) center along with other physisorbed surface species or not clearly identifiable surface species. Extension of the convergent and aqueous approaches to anatase mesoporous titania (m-TiO₂) enabled optical and electronic investigations of the corresponding surface species, revealing that the band-gap reduction is more pronounced for the bidentate chelating species (convergent approach) than for that obtained via the aqueous approach. The applied methods include X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and solid-state UV/vis spectroscopy. The energy-level alignment for the surface species from the aqueous approach, calculated from experimental data, accounts for the well-known type II excitation mechanism, whereas the findings indicate a distinct excitation mechanism for the bidentate chelating surface species of the material [Ti(CATtBu₂-3,6)(OCH₂tBu)₂(HNMe₂)₂]@[m-TiO₂].

Design of Single-Atom Catalysts and Tracking Their Fate Using Operando and Advanced X-ray Spectroscopic Tools

The potential of operando X-ray techniques for following the structure, fate, and active site of single-atom catalysts (SACs) is highlighted with emphasis on a synergetic approach of both topics. X-ray absorption spectroscopy (XAS) and related X-ray techniques have become fascinating tools to characterize solids and they can be applied to almost all the transition metals deriving information about the symmetry, oxidation state, local coordination, and many more structural and electronic properties. SACs, a newly coined concept, recently gained much attention in the field of heterogeneous catalysis. In this way, one can achieve a minimum use of the metal, theoretically highest efficiency, and the design of only one active site-so-called single site catalysts. While single sites are not easy to characterize especially under operating conditions, XAS as local probe together with complementary methods (infrared spectroscopy, electron microscopy) is ideal in this research area to prove the structure of these sites and the dynamic changes during reaction. In this review, starting from their fundamentals, various techniques related to conventional XAS and X-ray photon in/out techniques applied to single sites are discussed with detailed mechanistic and in situ/operando studies. We systematically summarize the design strategies of SACs and outline their exploration with XAS supported by density functional theory (DFT) calculations and recent machine learning tools.

Controlled Growth of Platinum Nanoparticles on Amorphous Silica from Grafted Pt-Disilicate Complexes

Supported platinum nanoparticles are currently the most functional catalysts applied in commercial chemical processes. Although investigations have been performed to improve the dispersion and thermal stability of Pt particles, it is challenging to apply amorphous silica supports to these systems owing to various Pt species derived from the non-uniform surface structure of the amorphous support. Herein, we report the synthesis and characterization of amorphous silica-supported Pt nanoparticles from (cod)Pt-disilicate complex (cod = 1,5-cyclooctadiene), which forms bis-grafted surface Pt species regardless of surface heterogeneity. The synthesized Pt nanoparticles were highly dispersible and had higher hydrogenation activity than those prepared by the impregnation method, irrespective of the calcination and reduction temperatures. The high catalytic activity of the catalyst prepared at low temperatures (such as 150 °C) was attributed to the formation of Pt nanoparticles triggered by the reduction of cod ligands under H₂ conditions, whereas that of the catalyst prepared at high temperatures (up to 450 °C) was due to the modification of the SiO₂ surface by grafting of the (cod)Pt-disilicate complex.

Hydroaminoalkylation of alkenes using transition metals complexes grafted on silica SBA15 as catalysts

The gas-phase hydroaminoalkylation reaction of propene catalyzed by group 4 (M = Ti, Zr and Hf) metal amido complexes [(≡Si-O-)(M(-NMe₂)₃] was investigated by using PBE0-D3/SVP//TZVP level of theory. The geometrical analysis traced the formation of the metallaaziridines and the azametallacyclopentanes as key intermediates in these reactions. The metallaaziridines were simulated through the activation of α-C-H bonds of the amido groups; while the azametallacyclopentanes were configured by slotting the propene double bond onto the M - C bonds of the metallaaziridines. The latter reaction was considered the rate-determining step. Thermochemical calculations showed that the order of catalytic activity is: Ti ≥ Zr > Hf; while the preference of the azametallacyclopentanes is: Hf > Zr ≥ Ti.

Yttrium tris(trimethylsilylmethyl) complexes grafted onto MCM-48 mesoporous silica nanoparticles

A series of tris(trimethylsilylmethyl) yttrium donor adduct complexes was synthesized and fully characterized by X-ray diffraction, ¹H/¹³C/²⁹Si/³¹P/⁸⁹Y heteronuclear NMR and FTIR spectroscopies as well as elemental analyses. Treatment of Y(CH₂SiMe₃)₃(thf)_x with various donors Do led to complete (Do = TMEDA, DMAP) and partial displacement of THF (Do = NHC^iPr, DMPE). Exceptionally large ⁸⁹Y NMR shifts to low field were observed for the new complexes. Complexes Y(CH₂SiMe₃)₃(tmeda) and Y(CH₂SiMe₃)₃(dmpe)(thf) were chosen to perform surface organometallic chemistry, due to a comparatively higher thermal stability and the availability of the ³¹P nucleus as a spectroscopic probe, respectively. Mesoporous nanoparticles of the MCM-48-type were synthesized and used as a 3^rd generation silica support. The parent and hybrid materials were characterized using X-ray powder diffraction, solid-state-NMR spectroscopy, DRIFTS, elemental analyses, N₂-physisorption, and scanning electron microscopy (SEM). The presence of surface-bound yttrium alkyl moieties was further proven by the reaction with carbon dioxide. Quantification of the surface silanol population by means of HN(SiHMe₂)₂-promoted surface silylation is shown to be superior to titration with lithium alkyl LiCH₂SiMe₃.

Cerium Pyrazolates Grafted onto Mesoporous Silica SBA-15: Reversible CO2 Uptake and Catalytic Cycloaddition of Epoxides and Carbon Dioxide

The activation and catalytic conversion of CO₂ is a current topic relating to molecular chemistry and materials science alike. As a transdisciplinary field of research, surface organometallic chemistry (SOMC) might be applicable to perform synergistically, thus striking a new path in sustainable chemistry. Both ceric and cerous rare-earth-metal pyrazolates, which were recently shown to reversibly insert CO₂ and to promote the catalytic cycloaddition of epoxides and carbon dioxide, were grafted onto large-pore mesoporous silica SBA-15₅₀₀, thermally pretreated at 500 °C. The obtained hybrid materials [Ce(Me₂pz)₄]₂@SBA-15₅₀₀, Ce(Me₂pz)₄(thf)@SBA-15₅₀₀, Ce₄(Me₂pz)₁₂@SBA-15₅₀₀, and [Ce(Me₂pz)₃(thf)]₂@SBA-15₅₀₀ (Me₂pz = 3,5-dimethylpyrazolato) were characterized by DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy), solid-state ¹H/¹³C NMR spectroscopy, elemental analysis, ICP/OES, and N₂ physisorption. The lanthanum(III)-based material [La(Me₂pz)₃(thf)]₂@SBA-15₅₀₀ was synthesized for better assessment of the cerous materials being highly sensitive to oxidation. To mimic ceric surface species, Ce[OSi(OtBu)₃]₃Cl was treated with 1 equiv of K(Me₂pz), generating the mixed pyrazolyl/siloxy complex KCe[OSi(OtBu)₃]₄(Me₂pz) featuring a cerium(IV)-bonded terminal pyrazolato ligand. All hybrid materials show efficient and reversible carbon dioxide uptake of maximum 20 wt % in the solid state. When combined with tetra-n-butylammonium bromide (TBAB), the hybrid materials catalyze the cycloaddition of CO₂ and epoxides, displaying good conversion of various epoxides and reusability.

Catalysis by Design: Well-Defined Single-Site Heterogeneous Catalysts

Heterogeneous catalysis, a field important industrially and scientifically, is increasingly seeking and refining strategies to render itself more predictable. The main issue is due to the nature and the population of catalytically active sites. Their number is generally low to very low, their "acid strengths" or " redox properties" are not homogeneous, and the material may display related yet inactive sites on the same material. In many heterogeneous catalysts, the discovery of a structure-activity reationship is at best challenging. One possible solution is to generate single-site catalysts in which most, if not all, of the sites are structurally identical. Within this context and using the right tools, the catalyst structure can be designed and well-defined, to reach a molecular understanding. It is then feasible to understand the structure-activity relationship and to develop predictable heterogeneous catalysis. Single-site well-defined heterogeneous catalysts can be prepared using concepts and tools of surface organometallic chemistry (SOMC). This approach operates by reacting organometallic compounds with surfaces of highly divided oxides (or of metal nanoparticles). This strategy has a solid track record to reveal structure-activity relationship to the extent that it is becoming now quite predictable. Almost all elements of the periodical table have been grafted on surfaces of oxides (from simple oxides such as silica or alumina to more sophisticated materials regarding composition or porosity). Considering catalytic hydrocarbon transformations, heterogeneous catalysis outcome may now be predicted based on existing mechanistic proposals and the rules of molecular chemistry (organometallic, organic) associated with some concepts of surface sciences. A thorough characterization of the grafted metal centers must be carried out using tools spanning from molecular organometallic or surface chemistry. By selection of the metal, its ligand set, and the support taken as a X, L ligands in the Green formalism, the catalyst can be designed and generated by grafting the organometallic precursor containing the functional group(s) suitable to target a given transformation (surface organometallic fragments (SOMF)). The choice of these SOMF is based on the elementary steps known in molecular chemistry applied to the desired reaction. The coordination sphere necessary for any catalytic reaction involving paraffins, olefins, and alkynes also can thus be predicted. Only their most complete understanding can allow development of catalytic reactions with the highest possible selectivity, activity, and lifetime. This Account will examine the results of SOMC for hydrocarbon transformations on oxide surfaces bearing metals of group 4-6. The silica-supported catalysts are exhibiting remarkable performances for Ziegler-Natta polymerization and depolymerization, low temperature hydrogenolysis of alkanes and waxes, metathesis of alkanes and cycloalkanes, olefins metathesis, and related reactions. In the case of reactions involving molecules that do not contain carbon (water-gas shift, NH3 synthesis, etc.) this single site approach is also valid but will be considered in a later review.

Palladium stabilized by 3,4-dihydroxypyridine-functionalized magnetic Fe3O4nanoparticles as a reusable and efficient heterogeneous catalyst for Suzuki reactions

A Pd supported on 3,4-dihydroxypyridine (Py)-functionalized Fe₃O₄(Fe₃O₄/Py/Pd) hybrid material has been synthesized for the first time.

Heterogeneous catalysis

A heterogeneous catalyst is a functional material that continually creates active sites with its reactants under reaction conditions. These sites change the rates of chemical reactions of the reactants localized on them without changing the thermodynamic equilibrium between the materials.

A high-throughput reactor system for optimization of Mo–V–Nb mixed oxide catalyst composition in ethane ODH

75 Mo–V–Nb mixed oxide catalysts with a broad range of compositions were tested using a high-throughput reactor in order to optimize the composition for the ethane oxidative dehydrogenation reaction.

The use of a well-defined surface organometallic complex as a probe molecule: [(SiO)TaVCl2Me2] shows different isolated silanol sites on the silica surface

Ta^VCl₂Me₃ reacts with silica₍₇₀₀₎ and produces two different surface organometallic species, the heterogeneity of the silica surface was disclosed and studied with experimental and theoretical approach.

Rational design of single-site heterogeneous catalysts: towards high chemo-, regio- and stereoselectivity

The main methods for the design and preparation of single-site heterogeneous catalysts on inorganic oxide supports are described and reviewed. Catalytically active metal sites can be either introduced into the framework of porous materials via direct synthesis or added to a pre-existing support by post-synthesis techniques. Particular attention is paid to selected examples where the geometry, the nature and the chemical surroundings of the active single site is a key factor to obtain catalytic systems with enhanced chemo-, regio- and stereoselectivity. The ever-increasing capabilities of ‘nanoarchitecture’ at molecular level enable chemists to build ideal catalysts for the sustainable transformation of bulky and high added-value molecules.