Surface Organometallic Fragments Research Articles

Surface Organometallic Chemistry as a Versatile Strategy for Synthesizing Supported Bimetallic Cluster Catalysts

Bimetallic clusters anchored on thermally stable and high-surface-area supports have gained a wide range of applications in heterogeneous catalysis. Compared to monometallic clusters or large bimetallic nanoparticles, bimetallic clusters show unprecedented catalytic performances due to the modulated electronic and geometric effects arising from the high fraction of surface unsaturated–coordinated metallic atoms and the synergistic effects between two constituting metals. However, even after more than 60 years of efforts, the controlled synthesis of homogeneously distributed bimetallic clusters with well-alloyed structure between two constituting metals remains a tremendous challenge so far. Herein, we present a versatile strategy based on the surface organometallic chemistry concept for synthesizing supported bimetallic cluster catalysts, which is achieved via the hydrogenation of a so-called “double surface organometallic complex”. The cooperative decomposition of two surface organometallic fragments in the double surface organometallic complex and their strong interaction with the support enable the formation of well-alloyed bimetallic clusters uniformly dispersed at the surfaces of different supports. This approach can serve as a platform technique for producing a variety of bimetallic clusters with varied compositions on a wide range of supports, such as Al2O3, TiO2, and zeolite. The resulting bimetallic cluster catalysts exhibit remarkably enhanced catalytic performance in benzene hydrogenation as compared to their monometallic counterparts because of highly exposed surface atoms and synergistic effects between constituting metals.

Predicting the DNP-SENS efficiency in reactive heterogeneous catalysts from hydrophilicity.

Identification of surfaces at the molecular level has benefited from progress in dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS). However, the technique is limited when using highly sensitive heterogeneous catalysts due to secondary reaction of surface organometallic fragments (SOMFs) with stable radical polarization agents. Here, we observe that in non-porous silica nanoparticles (NPs) (dparticle = 15 nm) some DNP enhanced NMR or SENS characterizations are possible, depending on the metal-loading of the SOMF and the type of SOMF substituents (methyl, isobutyl, neopentyl). This unexpected observation suggests that aggregation of the nanoparticles occurs in non-polar solvents (such as ortho-dichlorobenzene) leading to (partial) protection of the SOMF inside the interparticle space, thereby preventing reaction with bulky polarization agents. We discover that the DNP SENS efficiency is correlated with the hydrophilicity of the SOMF/support, which depends on the carbon and SOMF concentration. Nitrogen sorption measurements to determine the BET constant (CBET) were performed. This constant allows us to predict the aggregation of silica nanoparticles and consequently the efficiency of DNP SENS. Under optimal conditions, CBET > 60, we found signal enhancement factors of up to 30.

Surface organometallic chemistry in heterogeneous catalysis.

The broad challenges of energy and environment have become a main focus of research efforts to develop more active and selective catalytic systems for key chemical transformations. Surface organometallic chemistry (SOMC) is an established concept, associated with specific tools, for the design, preparation and characterization of well-defined single-site catalysts. The objective is to enter a catalytic cycle through a presumed catalytic intermediate prepared from organometallic or coordination compounds to generate well defined surface organometallic fragments (SOMFs) or surface coordination fragments (SCFs). These notions are the basis of the "catalysis by design" strategy ("structure-activity" relationship) in which a better understanding of the mechanistic aspects of the catalytic process led to the improvement of catalyst performances. In this review the application of SOMC strategy for the design and preparation of catalysts for industrially relevant processes that are crucial to the energy and environment is discussed. In particular, the focus will be on the conversion of energy-related feedstocks, such as methane and higher alkanes that are primary products of the oil and gas industry, and of their product of combustion, CO2, whose efficient capture and conversion is currently indicated as a top priority for the environment. Among the main topics related to energy and environment, catalytic oxidation is also considered as a key subject of this review.

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.

An investigation of catalyst/cocatalyst/support interactions in silica-supported olefin polymerization catalysts based on Cp*TiMe3*

Some of the interactions in an organometallic catalyst/cocatalyst/support material have been studied quantitatively (by analysis of volatiles and surface organometallic fragments) and spectroscopically (by IR, 13C and 29Si solid state CP/MAS NMR). The initial product of grafting Cp*TiMe3 onto an Aerosil silica is the supported organometallic complex ≡SiOTiCp*Me2, 1, which is thermally unstable. Mild heating generates the supported tetramethylfulvene complex ≡ SiOTiFv*(Me), 3, via an intramolecular C–H activation process. Grafting of AlMe3 onto either 1 or 3 yields a methylene-bridged heterobinuclear complex, (≡SiO)2Cp*TiCH2AlMe2, 5, and is accompanied by extensive methylation of the silica surface. The direct reaction of AlMe3 with the unmodified silica leads predominantly to the supported organometallic fragment ≡ SiOAl2(CH3)3(CH2), 6, whose proposed structure includes a bridging methylene and a bridging siloxide ligand. Its reaction with Cp*TiMe3 generates methane and ≡ SiOAl2(CH2)2TiCp*Me4, 7.

Surface organometallic chemistry on metals: Influence of the presence of functional groups grafted at the surface of Rh particles on the competitive hydrogenation of terminal and internal double bonds of unsaturated primary alcohols

Careful hydrogenolysis of organometallic compounds with group VIII metallic surfaces lead to surface organometallic fragments directly grafted on the metallic particles with well-defined structures. The presence of such surface organometallic fragments was demonstrated by various physical and chemical techniques (EXAFS, magnetic measurements, IR spectroscopy, Mössbauer, thermoreduction, etc.). Surface organometallic complexes with an average formula M s[M′R x ] y (M: Rh, Pt, Ni; M′: Sn, Ge, Si, Se; R: Me, Et, Bu) were proposed. It was recently demonstrated that organometallic fragments with functional groups can also be grafted on a platinum or rhodium surface. In this work, we demonstrate that organo-tin fragments with functional groups, grafted on a silica-supported rhodium surface can strongly modify the regio-selectivity of the hydrogenation of unsaturated primary alcohols. The rate of hydrogenation of trans-2-hexenol and 5-hexenol was determined at room temperature under 20 bar of hydrogen in heptane solution for Rh/SiO 2 catalysts which were either used as they were or were modified with Bu 3Sn(CH 2) 3OH or Bu 2Sn[(CH 2) 2(O(CH 2) 2) 2OMe] 2. The rate of hydrogenation of the terminal double bond is slightly modified by the presence of functional groups, while that of the internal unsaturation decreases by a factor of 4. These results are explained by the formation of a hydrophilic layer around the metallic particle which inhibits the adsorption of the unsaturated alcohol by its internal double bond.

Surface Organometallic Chemistry on Metals Applied to the Environment: Hydrogenolysis of AsPh3 with Nickel Supported on Alumina

The reaction of AsPh3 with Ni/Al2O3 at 100 to 200°C under 6 to 30 bars of hydrogen has been followed by kinetic analysis of both the consumption of AsPh3 and the evolution of benzene and cyclohexane as well as magnetic and XRD measurements of the metallic and intermetallic phase(s). The reaction proceeds via complete hydrogenolysis of the As–Ph bonds, with final formation of a well-characterized NiAs alloy (XRD). The mechanism of alloy formation is complex: at low coverage of the nickel surface, stepwise hydrogenolysis of the As–Ph bonds leads to the formation of the surface organometallic fragments Nis[AsPhx]y (kinetic studies and analytical data). Further hydrogenolysis of these fragments progressively leads to the formation of arsenic “adatoms” which quickly migrate inside the nickel particles to form NiAs0.45 amorphous intermetallic phases (XRD and magnetic measurements). The reaction then proceeds slowly up to the As/Ni ratio of unity with formation of the crystalline NiAs “nickeline” phase (XRD).

A Kinetic Study of Ethylene and 1-Hexene Homo- and Copolymerization Catalyzed by a Silica-Supported Cr(IV) Complex: Evidence for Propagation by a Migratory Insertion Mechanism

The surface organometallic fragment (⋮SiO)2CrCHCMe3 initiates the polymerization of ethylene and 1-hexene at room temperature in the absence of organoaluminum or other activators. The kinetics at the gas−solid interface were studied by in situ IR spectroscopy in a constant volume, variable pressure reactor. At low pressures, the reaction is first order in P(C2H4), first order in the number of moles of chromium, and linearly dependent on the Cr loading on silica. The second-order rate constant for polymerization is (177 ± 3) s-1 (mol Cr)-1 at 21 °C, and is virtually unchanged upon subsequent additions of ethylene to the catalyst. The kinetic isotope effect, k(C2H4)/k(C2D4), is a meagre 1.29. The temperature dependence of the second-order rate constants yields ΔH‡obs = (30.2 ± 0.9) kJ/mol and ΔS‡obs = (−99 ± 3) J/(K·mol). These values are consistent with a propagation mechanism of agostically assisted migratory insertion at an alkylchromium(IV) active site. They are inconsistent with an alternating alkylidene/metallacycle mechanism requiring rate-determining C−H cleavage. The rate of polymerization is accelerated by a factor of 3.2 in the presence of 18 mol % H2. Homopolymerization of 1-hexene is only 4.7 times slower than that of ethylene, an effect attributed to weaker preequilibrium binding of the substituted olefin which is partially compensated by faster migratory insertion. Copolymerization of ethylene and 1-hexene is random. It results in slower incorporation of ethylene but faster incorporation of 1-hexene relative to their respective homopolymerization rates.

Surface organometallic chemistry: some fundamental features including the coordination effects of the support

The study of the various kinds of reactions between organometallic complexes and the surfaces of inorganic oxides, metals or zeolites constitutes a new aspect of the coordination chemistry on surfaces. In this non-exhaustive and short review article, we would like to try to answer a few questions regarding this area of coordination (or organometallic chemistry) which may have some future impact in the field of catalysis. The questions that we would like to answer are the following: Are the basic rules of molecular organometallic and coordination chemistry valid when one tries to apply them to surfaces? One can wonder whether or not the functionalities which are present at the surface of an oxide, M x O y (M–OH groups, strained M–O–M groups and MO, aso) have a chemical reactivity which can be predicted on the basis of molecular chemistry. A few selected examples will be given about the reactivity of tin, rhenium or zirconium alkyls with the silanol groups of partially dehydroxylated silica. Can we obtain reliable and precise informations when some selected tools of surface science and molecular organometallic chemistry are applied simultaneously to elucidate the structure of surface organometallic fragments? One can reasonably expect that the way the surface organometallic fragments coordinate to the surface can be rationalized on the simple rules of coordination chemistry (electron counting, formal oxidation state). A few examples will be given regarding the surface structure of silica-supported zirconium hydrides or rhodium allyls. Is it possible that a well chosen surface organometallic fragment represents an intermediate in heterogeneous catalysis? If one can study the reactivity of a well chosen surface organometallic fragment, then one is in a position to demonstrate some elementary steps of heterogeneous catalysis. In this review we shall consider the surface reactivity of supported rhodium allyls or tin alkyls. What kind of mobility can we expect from surface organometallic fragments? In sharp contrast with discrete ligands of molecular chemistry, surfaces of oxides obviously provide a so called “pool of oxygens” which binds the surface organometallic fragments in a localized manner. However, due to its almost infinite structure, such a “pool” is obviously responsible for surface mobility, which is also a key parameter in certain catalytic processes (sintering, diffusion processes, reconstructions, leaching,⋯). Examples will be given on the mobility of Rh I(CO) 2 grafted onto a silica surface. The organometallic fragments are also mobile around the metal carbon bonds and this phenomenon can be evidenced by solid-state nuclear magnetic resonance (NMR) and can have applications in molecular separations and on the reactivity of the organometallic complexes. In each case, the role of the support as a coordinating ligand is a key factor of this chemistry.

Ligand Exchange and Oxidative Addition on a Silica-Supported Rhodium Complex, trans-[(⋮SiO)Rh(PMe3)2(CO)

The surface organometallic fragment [RhI(P(CH3)3)2(CO)], 1, was prepared by the reaction of trans-[CH3Rh(P(CH3)3)2(CO)] with partially dehydroxylated silica. 1 is attached to the silica surface by a covalent bond to oxygen. The reactions of 1 with organic and inorganic substrates were followed by transmission infrared and solid-state NMR spectroscopies. Ligand exchange reactions were observed when 1 was treated with 13CO and P(CH(CH3)2)3. In the presence of excess PR3 (R is methyl or isopropyl), cationic [Rh(PR3)3CO]+ was formed. In the presence of HCl/DCl, Cl2, CCl3Br, CH3I, and CH2CHCH2Cl, 1 underwent oxidative addition to give Rh(III) surface species, characterized by a shift of the ν(CO) modes ca. 100 cm-1 to higher frequency and displacement of the 31P chemical shifts to lower field. In the case of HCl/DCl, ν(Rh−H/D) modes were also observed. Oxidative addition of CH3I to 1 occurs without CO insertion, but the formation of acyl complexes was induced by addition of excess P(CH3)3 or CO. The reaction of 1 with CH2CHCH2Cl exhibits biphasic kinetic behavior, implying consecutive elementary reactions of oxidative addition followed by spontaneous CO insertion into the metal−carbon bond. The demonstration of these reactions on an oxide surface supports the idea that, under carefully controlled conditions, surface reactivity can be interpreted by using concepts from organometallic chemistry.

Reactions of Tetraalkylchromium(IV) with Silica: Mechanism of Grafting and Characterization of Surface Organometallic Complexes

The reactions of CrR4 (1, R is neopentyl or trimethylsilylmethyl) with the surfaces of partially dehydroxylated silicas lead to the formation of discrete mononuclear surface organometallic fragment...

Surface organometallic chemistry of tin: Thermal transformation, in vacuo, of SiνOνSnMe3 supported on silica; in situ IR, 13C CP MAS NMR and TPD experiments

The thermolysis of Si–O–SnMe 3 grafted onto partially dehydroxylated silicas has been studied by in situ IR, 13 C CP MAS NMR and TPD. The only detected gaseous product was methane, showing that the thermolysis process passes through a simple hydrolysis reaction between the grafted organometallic fragment and silanol groups. This reaction leads to the formation of (Si–O) 2 SnMe 2 , (Si–O) 3 SnMe, and finally tin compounds without organic ligands. The relative proportions of these surface organometallic fragments have been quantitatively determined by three independent methods which gave concordant results. Due to the higher density of hydroxy groups on SiO 2 200, the reactions proceed at a lower temperature on this surface. After thermal treatment at 400 °C, no more methyl ligands remain on the surface and all tin species are probably present as tin(IV) compounds.

Supramolecular approach to metal-support interactions: reactivity of silica-supported bis(allyl)rhodium(III) and the influence of surface hydroxyl groups

The structure and stoichiometric reactivity of a surface organometallic fragment, bis(allyl)rhodiuim(III) on silica, has been studied in order to identify concepts of molecular chemistry which can be applied to mechanisms of heterogeneous catalysis. The reactions with electrophiles and Lewis bases are examined. The similarity of the ‘supramolecular’ surface chemistry to molecular chemistry was limited since the surface hydroxyl groups of silica were found to participate. A hydroxyl group in the coordination sphere of rhodium is a source of electrophilic protons, and neighbouring hydroxyl groups facilitate surface mobility of grafted organometallic fragments.

Surface Organometallic Chemistry of Tin: Grafting Reactions on Highly Dehydroxylated Silica

Abstract A new grafting method of organotin complexes with the surface of highly dehydroxylated silica is reported. Characterisation was achieved by surface microanalysis, in situ IR, solid state 13C CP-MAS and 119Sn MAS NMR. Bu3Sn-O-SnBu3 reacts with the strained siloxanes and opens these bridges at room temperature. The reaction occurs also with the remaining hydroxyl groups and with less strained siloxane bridges, leading in all cases to the same surface organometallic fragment ≡Si-O-SnBu3.

Surface organometallic chemistry of tin. Controlled thermal transformation in vacuo of Sn(n-C4H9)3fragments grafted on the surface of alumina

The thermal transformation in vacuo of the well defined surface organometallic fragments [AlO]xSn(n-C4H9)3(x= 1,2) 1 grafted on various partially dehydroxylated aluminas (α, γ, η) has been investigated. The surface reactions were followed by qualitative and quantitative determination of the evolved gases including IR spectroscopy, 13C cross-polarisation magic angle spinning and 119Sn magic angle spinning NMR studies. On all aluminas, comparable transformations of 1 were identified showing that the ratio of octahedral: tetrahedral aluminium sites in the aluminas is not an important parameter. Solvolysis reactions of 1 led to the formation of the surface multigrafted species (AlO)x+zSnBu3-z(x= 1, 2; z= 1, 2, 3), while β-H elimination of but-1-ene followed by reductive elimination led to inorganic tin(II) compounds on the surface of the alumina. These transformations occur at temperatures lower than those observed with SiOSnBu3 grafted on the surface of silica. This is possibly related to the more acidic properties of alumina or to the higher concentration and/or mobility of Al hydroxy groups with respect to Si hydroxy groups.

Surface Organometallic Chemistry on Metals: Preparation of Bimetallic Catalysts by Controlled Hydrogenolysis of Sn(n-C4H9)4 on a Ni/SiO2 Catalyst

Selective hydrogenolysis of Sn(n-C4H9)4 on a Ni/SiO2 catalyst has been carried out at various temperatures and at various coverages of the metallic surface by the organotin(IV) compound. The surface reaction was followed by chemical analysis of the gaseous and surface products, electron microscopy, infrared spectroscopy, magnetic measurements, and EXAFS measurements. At room temperature, and in the absence of metallic particles, Sn(n-C4H9)4 is simply physisorbed on the silica surface and can be easily extracted by n-heptane. In the presence of metallic Ni, and provided that the amount of Sn introduced represents less than ca. a monolayer of the metallic surface, hydrogenolysis of Sn(n-C4H9)4 occurs exclusively on the Ni particles, as evidenced by STEM-EDAX measurements, infrared spectroscopy, and EXAFS measurements. Analysis of the gaseous products evolved at increasing temperatures, infrared measurements, and EXAFS measurements indicate a stepwise cleavage of Sn-alkyl bonds. At temperatures lower than 323 K, there is formation of relatively stable surface organometallic fragments which can be formulated as Nis[Sn(n-C4H9)x]y in which "Ni-s" represents surface nickel atoms (x = 2, 3). The stoichiometry of the surface reaction leading to stable surface organometallic fragments depends both on the surface coverage (y) of the nickel particle by the organometallic compound and on the temperature of the hydrogenolysis. For the sample obtained after hydrogenolysis at 323 K for 10 h and for a Sn/Nis ratio of 0.5, an average of 2.5 butyl groups remain on the surface. The EXAFS experiments (at the Sn K-edge) carried out on such a sample show that adsorbed Sn is surrounded on average by 2.5 light backscatterers (very probably C type) and by 1.3 heavy backscatterers (most probably Ni). Various possible structures have been proposed, taking into account the results of the analytical and EXAFS data, as well as the magnetic and chemisorption measurements. For reaction temperatures greater than 373 K, all butyl groups are hydrogenolysed mainly as butane. STEM-EDAX experiments show that, provided that the amount of Sn introduced is lower than a monolayer of the Nis, the signal of Sn is always associated with that of Ni, indicating the lack of migration of tin to silica. Electron microscopy indicates that the average metallic particle size on the surface increases by a value of about 1.5 nm. For a Sn/Nis ratio of 0.5, EXAFS studies show that each "naked" tin atom is surrounded by 4 Ni atoms of less, suggesting that tin atoms have not migrated into the nickel particles. Magnetic measurements suggest that each tin atom has then the same magnetic effect as 4.5 chemisorbed hydrogen atoms.

Surface organometallic chemistry on metals: a novel and effective route to custom-designed bimetallic catalysts

The synthesis, characterization and catalytic properties of new materials obtained by reaction of organometallic complexes of groups IIb, IVa, and VIa with the surface of metallic particles are reviewed. Two types of materials may be obtained by surface organometallic chemistry on metals: metal particles covered with organometallic fragments, and bimetallic particles of predetermined composition. Characterization of the organometallic fragments on the metal particles has demonstrated their thermal stability. These particles covered with surface organometallic fragments are new catalytic materials, highly selective in several reactions such as the hydrogenation of α,β-unsaturated aldehydes, ethyl pyruvate, nitrobenzene, acrylonitrile, and olefins. The bimetallic particles without organometallic fragments are also highly active and selective for a variety of reactions such as hydrogenolysis of various alkanes and hydrogenolysis of esters. For these systems, the concept of “site isolation” has been advanced to account for the high selectivity of the reactions.

Stoichiometric and catalytic reactivity of organometallic fragments supported on inorganic oxides

The reaction of some organometallic complexes with the surfaces of inorganic oxides leads to the formation of surface organometallic complexes, chemically bound to the surface yet retaining many features of their molecular structure. These surface organometallic complexes can therefore be considered to belong to both the molecular and solid states. In cases where such complexes have been structurally characterised, their reactivity can be interpreted with molecular concepts. In this review article, the stoichiometric and catalytic reactivity of some relatively well-defined surface organometallic fragments is surveyed. Many elementary steps which have precedent in molecular organometallic chemistry and homogeneous catalysis have now been demonstrated with surface organometallic fragments, including reversible ligand binding, oxidative addition, reductive elimination, protonation, heterolytic metal—carbon bond cleavage, electrophilic CH bond activation and insertion into metal—carbon bonds. In some cases, the supported organometallic complexes are highly effective low temperature catalysts, a phenomenon which is not always observed with molecular analogues nor with conventionally prepared heterogeneous catalysts. Applications of surface organometallic chemistry to catalytic alkane hydrogenolysis, olefin isomerisation and hydrogenation, the Fischer—Tropsch synthesis and the water—gas shift reaction are discussed. Proposed mechanisms for several representative catalytic cycles are presented.