In Situ Generation of Active Sites in Olefin Metathesis

Highly Efficient Low-loaded PdOx/AlSiOx Catalyst for Ethylene Dimerization

Ethylene dimerization is an industrial process that is currently carried out using homogeneous catalysts. Here we present a highly active heterogeneous catalyst containing minute amounts of atomically dispersed Pd. It requires no co-catalyst(s) or activator(s) and significantly outperforms previously reported catalysts tested under similar reaction conditions. The selectivity to C4- and C6-hydrocarbons was about 80 % and 10 % at 42 % ethylene conversion at 200 °C using an industrially relevant feed containing 50 vol % ethylene, respectively. Our kinetic and catalyst characterization experiments complemented by density functional theory calculations provide molecular insights into the local environment of isolated Pd(II)Ox species and their role in achieving high activity in the target reaction. When the developed catalyst was rationally integrated with a Mo-containing olefin metathesis catalyst in the same reactor, the formed butenes reacted with ethylene to propylene with a selectivity of 98 % at about 24 % ethylene conversion.

Thermal Formation of Metathesis-Active Tungsten Alkylidene Complexes from Cyclohexene

A 7-tungstabicyclo[4.3.0]nonane complex forms slowly upon addition of cyclohexene to the ethylene complex, W(NAr)(OSiPh3)2(C2H4), at 22 °C. A single-crystal X-ray study showed its structure to be closest to a square pyramid (τ = 0.23). At 22 °C, loss of cyclohexene or ring contraction of the 7-tungstabicyclo[4.3.0]nonane complex is slow. Above ∼80 °C, cyclohexene is ejected to give W(NAr)(OSiPh3)2(C2H4), but a sufficient amount of 7-tungstabicyclo[4.3.0]nonane complex remains in the presence of cyclohexene and the ring contracts to yield methylenecyclohexane and a methylidene complex or ethylene and a cyclohexylidene complex. Other complexes that have been observed include an 8-tungstabicyclo[4.3.0]nonane complex formed from 1,7-octadiene, a 7-tungstabicyclo[4.2.0]octane complex (formed from a methylidene complex and cyclohexene), and a methylenecyclohexane complex. 13C-Labeling studies show that the exo-methylene group in methylenecyclohexane and the α positions in the 8-tungstabicyclo[4.3.0]nonane come from ethylene. An alternative ring contraction of a tungstacyclopentane made from two molecules of cyclohexene cannot be excluded when concentrations of ethylene are low. A cyclohexylidene complex could also form from two cyclohexenes via a newly proposed "alkyl/allyl" mechanism. The results reported here are the first experimental confirmations that a tungstacyclopentane can ring-contract thermally at a substituted WCα position to form a tungstacyclobutane and therefore metathesis-active alkylidenes.

Propylene Metathesis over Molybdenum Silicate Microspheres with Dispersed Active Sites

In this work, we demonstrate that amorphous and porous molybdenum silicate microspheres are highly active catalysts for heterogeneous propylene metathesis. Homogeneous molybdenum silicate microspheres and aluminum-doped molybdenum silicate microspheres were synthesized via a nonaqueous condensation of a hybrid molybdenum biphenyldicarboxylate-based precursor solution with (3-aminopropyl)triethoxysilane. The as-prepared hybrid metallosilicate products were calcined at 500 °C to obtain amorphous and porous molybdenum silicate and aluminum-doped molybdenum silicate microspheres with highly dispersed molybdate species inserted into the silicate matrix. These catalysts contain mainly highly dispersed MoOx species, which possess high catalytic activity in heterogeneous propylene metathesis to ethylene and butene. Compared to conventional silica-supported MoOx catalysts prepared via incipient wetness impregnation (MoIWI), the microspheres with low Mo content (1.5-3.6 wt %) exhibited nearly 2 orders of magnitude higher steady-state propylene metathesis rates at 200 °C, approaching site time yields of 0.11 s-1.

Performance Descriptors for Catalysts Based on Molybdenum, Tungsten, or Rhenium Oxides for Metathesis of Ethylene with 2-Butenes to Propene

The metathesis of ethylene with 2-butenes to propene is an established large-scale process. However, the fundamentals behind in situ transformation of supported WOx , MoOx , or ReOx species into catalytically active metal-carbenes and the intrinsic activity of the latter as well as the role of metathesis-inactive cocatalysts are still unsolved. This is detrimental for catalyst development and process optimization. In this study, we provide the required essentials derived from steady-state isotopic transient kinetic analysis. For the first time, the steady-state concentration, the lifetime, and the intrinsic reactivity of metal carbenes were determined. The obtained results can be directly used for the design and the preparation of metathesis-active catalysts and cocatalysts, thereby opening up possibilities for optimizing propene productivity.

Active Site Descriptors from 95Mo NMR Signatures of Silica-Supported Mo-Based Olefin Metathesis Catalysts

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.

Promoting active site renewal in heterogeneous olefin metathesis catalysts

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.

Data-Centric Heterogeneous Catalysis: Identifying Rules and Materials Genes of Alkane Selective Oxidation

Artificial intelligence (AI) can accelerate catalyst design by identifying key physicochemical descriptive parameters correlated with the underlying processes triggering, favoring, or hindering the performance. In analogy to genes in biology, these parameters might be called "materials genes" of heterogeneous catalysis. However, widely used AI methods require big data, and only the smallest part of the available data meets the quality requirement for data-efficient AI. Here, we use rigorous experimental procedures, designed to consistently take into account the kinetics of the catalyst active states formation, to measure 55 physicochemical parameters as well as the reactivity of 12 catalysts toward ethane, propane, and n-butane oxidation reactions. These materials are based on vanadium or manganese redox-active elements and present diverse phase compositions, crystallinities, and catalytic behaviors. By applying the sure-independence-screening-and-sparsifying-operator symbolic-regression approach to the consistent data set, we identify nonlinear property-function relationships depending on several key parameters and reflecting the intricate interplay of processes that govern the formation of olefins and oxygenates: local transport, site isolation, surface redox activity, adsorption, and the material dynamical restructuring under reaction conditions. These processes are captured by parameters derived from N₂ adsorption, X-ray photoelectron spectroscopy (XPS), and near-ambient-pressure in situ XPS. The data-centric approach indicates the most relevant characterization techniques to be used for catalyst design and provides "rules" on how the catalyst properties may be tuned in order to achieve the desired performance.

Polyfunctional catalysis in conversion of light alkenes

Light alkenes are among the main petrochemical intermediate products, the consumption of which is steadily growing. Using ethylene as an example, the possibilities of using polyfunctional heterogeneous catalysts for carrying out practically important reactions of its oligomerization, alkylation, and metathesis were considered. Particular attention was paid to catalysts for the conversion of ethylene to propylene.

Thermal Alteration in Adsorption Sites over SAPO-34 Zeolite

Zeolites have found tremendous applications in the chemical industry. However, the dynamic nature of their active sites under the flow of adsorbate molecules for adsorption and catalysis is unclear, especially in operando conditions, which could be different from the as-synthesized structures. In the present study, we report a structural transformation of the adsorptive active sites in SAPO-34 zeolite by using acetone as a probe molecule under various temperatures. The combination of solid-state nuclear magnetic resonance, in situ variable-temperature synchrotron X-ray diffraction, and in situ diffuse-reflectance infrared Fourier-transform spectroscopy allow a clear identification and quantification that the chemisorption of acetone can convert the classical Brønsted acid site adsorption mode to an induced Frustrated Lewis Pairs adsorption mode at increasing temperatures. Such facile conversion is also supported by the calculations of ab-initio molecular-dynamics simulations. This work sheds new light on the importance of the dynamic structural alteration of active sites in zeolites with adsorbates at elevated temperatures.

Olefin Metathesis Catalysts Generated In Situ from Molybdenum(VI)-Oxo Complexes by Tuning Pendant Ligands

Tailored molybdenum(VI)-oxo complexes of the form MoOCl2 (OR)2 (OEt2 ) catalyse olefin metathesis upon reaction with an organosilicon reducing agent at 70 °C, in the presence of olefins. While this reactivity parallels what has recently been observed for the corresponding classical heterogeneous catalysts based on supported metal oxide under similar conditions, the well-defined nature of our starting molecular systems allows us to understand the influence of structural, spectroscopic and electronic characteristics of the catalytic precursor on the initiation and catalytic proficiency of the final species. The catalytic performances of the pre-catalysts are determined by the highly electron withdrawing (σ-donation) character of alkoxide ligands, Ot BuF9 being the best. This activity correlates with both the 95 Mo chemical shift and the reduction potential that follows the same trend: Ot BuF9 >Ot BuF6 >Ot BuF3 .

Olefin-Surface Interactions: A Key Activity Parameter in Silica-Supported Olefin Metathesis Catalysts

Molecularly defined and classical heterogeneous Mo-based metathesis catalysts are shown to display distinct and unexpected reactivity patterns for the metathesis of long-chain α-olefins at low temperatures (<100 °C). Catalysts based on supported Mo oxo species, whether prepared via wet impregnation or surface organometallic chemistry (SOMC), exhibit strong activity dependencies on the α-olefin chain length, with slower reaction rates for longer substrate chain lengths. In contrast, molecular and supported Mo alkylidenes are highly active and do not display such dramatic dependence on the chain length. State-of-the-art two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy analyses of postmetathesis catalysts, complemented by Fourier transform infrared (FT-IR) spectroscopy and molecular dynamics calculations, evidence that the activity decrease observed for supported Mo oxo catalysts relates to the strong adsorption of internal olefin metathesis products because of interactions with surface Si-OH groups. Overall, this study shows that in addition to the nature and the number of active sites, the metathesis rates and the overall catalytic performance depend on product desorption, even in the liquid phase with nonpolar substrates. This study further highlights the role of the support and active site composition and dynamics on activity as well as the need for considering adsorption in catalyst design.

Olefin metathesis over supported MoOx catalysts: influence of the oxide support

Supported MoOx catalysts on oxide supports (Al2O3, TiO2, ZrO2, SiO2) were synthesized for propylene metathesis, characterized with in situ spectroscopies (DRIFTS, Raman, UV-vis) and chemically probed with propylene-TPSR, ethylene/2-butene titration.

Tuning metathesis performance of molybdenum oxide-based catalyst by silica support acidity modulation and high temperature pretreatment

Molybdenum oxide-based catalysts containing 5 wt. % of Mo obtained by simple impregnation of silica mesoporous support were studied in olefin metathesis reaction at 50 °C. Effect of support modification...

Fundamentals and application potential of the synergy effect between ZnO and Mo/SiO2 for propene production in the metathesis of ethylene and trans-2-butene

The enhancing effect of ZnO on the activity of Mo/SiO2 in the metathesis of ethylene with 2-butene is investigated. The strength of the effect depends on the reaction temperature and the manner in which ZnO and Mo/SiO2 are located in the reactor.

Synthesis and Structure-Activity Characterization of a Single-Site MoO2 Catalytic Center Anchored on Reduced Graphene Oxide

Molecularly derived single-site heterogeneous catalysts can bridge the understanding and performance gaps between conventional homogeneous and heterogeneous catalysis, guiding the rational design of next-generation catalysts. While impressive advances have been made with well-defined oxide supports, the structural complexity of other supports and the nature of the grafted surface species present an intriguing challenge. In this study, single-site Mo(═O)₂ species grafted onto reduced graphene oxide (rGO/MoO₂) are characterized by XPS, DRIFTS, powder XRD, N₂ physisorption, NH₃-TPD, aqueous contact angle, active site poisoning assay, Mo EXAFS, model compound single-crystal XRD, DFT, and catalytic performance. NH₃-TPD reveals that the anchored MoO₂ moiety is not strongly acidic, while Mo 3d_5/2 XPS assigns the oxidation state as Mo(VI), and XRD shows little rGO periodicity change on MoO₂ grafting. Contact angle analysis shows that MoO₂ grafting consumes rGO surface polar groups, yielding a more hydrophobic surface. The rGO/MoO₂ DRIFTS assigns features at 959 and 927 cm^-1 to the symmetric and antisymmetric Mo═O stretching modes, respectively, of an isolated cis-(O═Mo═O) moiety, in agreement with DFT computation. Moreover, the Mo EXAFS rGO/MoO₂ structural data are consistent with isolated (C-O)₂-Mo(═O)₂ species having two Mo═O bonds and two Mo-O bonds at distances of 1.69(3) and 1.90(3) Å, respectively. rGO/MoO₂ is also more active than the previously reported AC/MoO₂ catalyst, with reductive carbonyl coupling TOFs approaching 1.81 × 10³ h^-1. rGO/MoO₂ is environmentally robust and multiply recyclable with 69 ± 2% of the Mo sites catalytically significant. Overall, rGO/MoO₂ is a structurally well-defined and versatile single-site Mo(VI) dioxo heterogeneous catalytic system.

Propane to olefins tandem catalysis: a selective route towards light olefins production

On-purpose synthetic routes for propylene production have emerged in the last couple of decades in response to the increasing demand for plastics and a shift to shale gas feedstocks for ethylene production. Propane dehydrogenation (PDH), an efficient and selective route to produce propylene, saw booming investments to fill the so-called propylene gap. In the coming years, however, a fluctuating light olefins market will call for flexibility in end-product of PDH plants. This can be achieved by combining PDH with propylene metathesis in a single step, propane to olefins (PTO), which allows production of mixtures of propylene, ethylene and butenes, which are important chemical building blocks for a.o. thermoplastics. The metathesis technology introduced by Phillips in the 1960s and mostly operated in reverse to produce propylene, is thus undergoing a renaissance of scientific and technological interest in the context of the PTO reaction. In this review, we will describe the state-of-the-art of PDH, propylene metathesis and PTO reactions, highlighting the open challenges and opportunities in the field. While the separate PDH and metathesis reactions have been extensively studied in the literature, understanding the whole PTO tandem-catalysis system will require new efforts in theoretical modelling and operando spectroscopy experiments, to gain mechanistic insights into the combined reactions and finally improve catalytic selectivity and stability for on-purpose olefins production.

Room-Temperature Metathesis of Ethylene with 2-Butene to Propene Over MoOx-Based Catalysts: Mixed Oxides as Perspective Support Materials

We investigated the effect of supports based on ZrO2, TiO2, Al2O3, and SiO2 on the rate of propene formation in the metathesis of ethylene with 2-butene at 50 °C over Mo-containing catalysts possessing highly dispersed MoOx. Large improvements in this rate were achieved when using supports composed of mixed oxides (ZrO2–SiO2, ZrO2–PO4, TiO2–SiO2; Al2O3–SiO2) rather than of individual oxides (ZrO2, TiO2, Al2O3, SiO2). Although previous literature studies dealing with the metathesis reaction over Al2O3- or SiO2-suppported catalysts at higher temperatures suggest the importance of redox or acidic properties of supported MoOx species for catalyst activity, we were not able to establish any general direct correlation in this regard. Contrarily, the rate of propene formation can be significantly enhanced when promoting supports with an oxide promoter. We suggest that the created support lattice defects may facilitate the transformation of MoOx to Mo carbenes under reaction conditions or improve the intrinsic activity of the latter.

Graphic Abstract

Induced Active Sites by Adsorbate in Zeotype Materials

There has been a long debate on how and where active sites are created for molecular adsorption and catalysis in zeolites, which underpin many important industrial applications. It is well accepted that Lewis acidic sites (LASs) and basic sites (LBSs) as active sites in pristine zeolites are generally believed to be the extra-framework Al species and residue anion (OH^-) species formed at fixed crystallographic positions after their synthesis. However, the dynamic interactions of adsorbates/reactants with pristine zeotype materials to "create" sites during real conditions remain largely unexplored. Herein, direct experimental observation of the establishment of induced active sites in silicoaluminophosphate (SAPO) by an adsorbate is for the first time made, which contradicts the traditional view of the fixed active sites in zeotype materials. Evidence shows that an induced frustrated Lewis pair (FLP, three-coordinated framework Al as LAS and SiO (H) as LBS) can be transiently favored for heterolytic molecular binding/reactions of competitive polar adsorbates due to their ineffective orbital overlap in the rigid framework. High-resolution magic-angle-spinning solid-state NMR, synchrotron X-ray diffraction, neutron powder diffraction, in situ diffuse reflectance infrared Fourier transform spectroscopy, and ab initio molecular dynamics demonstrate the transformation of a typical Brønsted acid site (Al(OH)Si) in SAPO zeolites to new induced FLP structure for hetereolytic binding upon adsorption of a strong polar adsorbate. Our unprecedented finding opens up a new avenue to understanding the dynamic establishment of active sites for adsorption or chemical reactions under molecular bombardment of zeolitic structures.

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.

DFT studies of selective oxidation of propene on the MoO3(010) surface

Selective oxidation of propene to acrolein and acrylic acid has been applied in industry for many years. In this work, the density functional theory plus U (DFT+U) method was performed to study the hydrogen abstraction of propene on the MoO3(010) surface. From the most stable chemisorbed propene (di-σ propene), the allyl intermediate is difficult to produce on a perfect MoO3(010) surface because of the high barrier. In general, the barriers of the second hydrogen abstraction are much lower than those of the first one. The conclusion from our slab model calculations is consistent with the experimental results. It is found that the (3 + 2) mechanism exhibits lower barriers than the (5 + 2) mechanism. Oxygen defects facilitate the first dehydrogenation significantly, and π-allyl converts to σ-allyl favorably on defects, in agreement with a previous experimental study. The present study indicates that increasing the surface oxygen defects might be an effective way to promote the activity of MoO3 to propene oxidation.

Enormous passivation effects of a surrounding zeolitic framework on Pt clusters for the catalytic dehydrogenation of propane

The significant passivation effect of the zeolitic framework on the catalytic performance of Pt clusters for dehydrogenation of propane to propylene is displayed. Pt/NaX shows 1100% enhanced TOFs and largely improved selectivity compared with Pt@NaX.

Propylene synthesis via isomerization–metathesis of 1-hexene and FCC olefins

Highly efficient conversion of 1-hexene and FCC mixture to propylene via isomerization–metathesis (ISOMET) catalyzed by a HBEA–MoOx/Al2O3 system.

Imaging the Heterogeneity of the Oxygen Evolution Reaction on Gold Electrodes Operando: Activity is Highly Local

Understanding the mechanism of the oxygen evolution reaction (OER), the oxidative half of electrolytic water splitting, has proven challenging. Perhaps the largest hurdle has been gaining experimental insight into the active site of the electrocatalyst used to facilitate this chemistry. Decades of study have clarified that a range of transition-metal oxides have particularly high catalytic activity for the OER. Unfortunately, for virtually all of these materials, metal oxidation and the OER occur at similar potentials. As a result, catalyst surface topography and electronic structure are expected to continuously evolve under reactive conditions. Gaining experimental insight into the OER mechanism on such materials thus requires a tool that allows spatially resolved characterization of the OER activity. In this study, we overcome this formidable experimental challenge using second harmonic microscopy and electrochemical methods to characterize the spatial heterogeneity of OER activity on polycrystalline Au working electrodes. At moderately anodic potentials, we find that the OER activity of the electrode is dominated by <1% of the surface area and that there are two types of active sites. The first is observed at potentials positive of the OER onset and is stable under potential cycling (and thus presumably extends multiple layers into the bulk gold electrode). The second occurs at potentials negative of the OER onset and is removed by potential cycling (suggesting that it involves a structural motif only 1–2 Au layers deep). This type of active site is most easily understood as the catalytically active species (hydrous oxide) in the so-called incipient hydrous oxide/adatom mediator model of electrocatalysis. Combining the ability we demonstrate here to characterize the spatial heterogeneity of OER activity with a systematic program of electrode surface structural modification offers the possibility of creating a generation of OER electrocatalysts with unusually high activity.

Atomic Pt-Catalyzed Heterogeneous Anti-Markovnikov C-N Formation: Pt10 Activating N-H for Pt1δ+-Activated C═C Attack

C-N formation is of great significance to synthetic chemistry, as N-containing products are widely used in chemistry, medicine, and biology. Addition of an amine to an unsaturated carbon-carbon bond is a simple yet effective route to produce new C-N bonds. But how to effectively conduct an anti-Markovnikov addition with high selectivity has been a great challenge. Here, we proposed a strategy for highly regioselective C-N addition via hydroamination by using supported Pt. It has been identified that atomic-scale Pt is the active site for C-N addition with Pt₁²⁺ for Markovnikov C-N formation and atomic Pt (Pt₁^δ+ and Pt₁⁰) contributing to anti-Markovnikov C-N formation. A selectivity of up to 92% to the anti-Markovnikov product has been achieved with atomic Pt in the addition of styrene and pyrrolidine. A cooperating catalysis for the anti-Markovnikov C-N formation between Pt₁^δ+ and Pt₁⁰ has been revealed. The reaction mechanism has been studied by EPR spectra and in situ FT-IR spectra of adsorption/desorption of styrene and/or pyrrolidine. It has been demonstrated that Pt₁⁰ activates amine to be electrophilic, while Pt₁^δ+ activates C═C by π-bonding to make β-C nucleophilic. The attack of nucleophilic β-C to electrophilic amine affords the anti-Markovnikov addition. This strategy proves highly effective to a variety of substrates in anti-Markovnikov C-N formation, including aromatic/aliphatic amines reacting with aromatic olefins, aromatic/aliphatic olefins with aromatic amines, and linear aliphatic olefins with secondary aliphatic amines. It is believed that the results provide evidence for the function of varied chemical states in monatomic catalysis.

Site-averaged kinetics for catalysts on amorphous supports: an importance learning algorithm

We combine importance sampling and kernel regression techniques to efficiently predict site-averaged kinetics for isolated catalyst sites on amorphous supports.

Well-Defined Silica-Supported Tungsten(IV)-Oxo Complex: Olefin Metathesis Activity, Initiation, and Role of Brønsted Acid Sites

Despite the importance of the heterogeneous tungsten-oxo-based olefin metathesis catalyst (WO₃/SiO₂) in industry, understanding of its initiation mechanism is still very limited. It has been proposed that reduced W(IV)-oxo surface species act as precatalysts. In order to understand the reactivity and initiation mechanism of surface W(IV)-oxo species, we synthesized a well-defined silica-supported W(IV)-oxo species, (≡SiO)WO(OtBuF₆)(py)₃ (F6@SiO_2-700; OtBuF₆ = OC(CH₃)(CF₃)₂; py = pyridine), via surface organometallic chemistry (SOMC). F6@SiO_2-700 was shown to be highly active in olefin metathesis upon removal of pyridine ligands through the addition of tris(pentafluorophenyl)borane (B(C₆F₅)₃) or thermal treatment under high vacuum. The metathesis activity toward olefins with and without allylic C-H groups, namely β-methylstyrene and styrene, respectively, was investigated. In the case of styrene, we demonstrated the role of surface OH groups in initiating metathesis activity. We proposed that the presence of strong Brønsted acidic OH sites, which likely arises from the presence of adjacent W sites in the catalyst as revealed by ¹⁵N-labeled pyridine adsorption, can assist styrene metathesis. In contrast, initiation of olefins with allylic C-H groups (e.g., β-methylstyrene) is independent of the surface OH density and likely involves an allylic C-H activation mechanism, like the molecular W(IV)-oxo species. This study indicates that initiation mechanisms depend on the olefinic substrates and reveals the synergistic effect of Brønsted acidic surface sites and reduced W(IV) sites in the initiation of olefin metathesis.

SBA-15 as a Support for Effective Olefin Metathesis Catalysts

Olefin metathesis is the catalytic transformation of olefinic substrates, finding a wide range of applications in organic synthesis. The mesoporous molecular sieve Santa Barbara Amorphous (SBA-15) has proven to be an excellent support for metathesis catalysts thanks to its regular mesoporous structure, high BET area, and large pore volume. A survey of catalysts consisting of (i) molybdenum and tungsten oxides on SBA-15, and (ii) molybdenum and ruthenium organometallic complexes (Schrock and Grubbs-type carbenes) on SBA-15 is provided together with their characterization and catalytic performance in various metathesis reactions. The comparison with catalysts based on other supports demonstrates the high quality of the mesoporous molecular sieve SBA-15 as an advanced catalyst support.

Silica-Supported Molybdenum Oxo Alkylidenes: Bridging the Gap between Internal and Terminal Olefin Metathesis

Grafting a molybdenum oxo alkylidene on silica (partially dehydroxylated at 700 °C) affords the first example of a well-defined silica-supported Mo oxo alkylidene, which is an analogue of the putative active sites in heterogeneous Mo-based metathesis catalysts. In contrast to its tungsten analogue, which shows poor activity towards terminal olefins because of the formation of a stable off-cycle metallacyclobutane intermediate, the Mo catalyst shows high metathesis activity for both terminal and internal olefins that is consistent with the lower stability of Mo metallacyclobutane intermediates. This Mo oxo metathesis catalyst also outperforms its corresponding neutral silica-supported Mo and W imido analogues.

A chemical titration method for quantification of carbenes in Mo- or W-containing catalysts for metathesis of ethylene with 2-butenes: verification and application potential

We introduce and experimentally validate a simple titration method for quantifying the number of carbenes acting as active sites in the metathesis of ethylene with 2-butenes over Mo- or W-containing catalysts.

Active Sites in Heterogeneous Catalytic Reaction on Metal and Metal Oxide: Theory and Practice

Active sites play an essential role in heterogeneous catalysis and largely determine the reaction properties. Yet identification and study of the active sites remain challenging owing to their dynamic behaviors during catalysis process and issues with current characterization techniques. This article provides a short review of research progresses in active sites of metal and metal oxide catalysts, which covers the past achievements, current research status, and perspectives in this research field. In particular, the concepts and theories of active sites are introduced. Major experimental and computational approaches that are used in active site study are summarized, with their applications and limitations being discussed. An outlook of future research direction in both experimental and computational catalysis research is provided.

C-H Activation and Proton Transfer Initiate Alkene Metathesis Activity of the Tungsten(IV)-Oxo Complex

In alkene metathesis, while group 6 (Mo or W) high-oxidation state alkylidenes are accepted to be key reaction intermediates for both homogeneous and heterogeneous catalysts, it has been proposed that low valent species in their +4 oxidation state can serve as precatalysts. However, the activation mechanism for these latter species-generating alkylidenes-is still an open question. Here, we report the syntheses of tungsten(IV)-oxo bisalkoxide molecular complexes stabilized by pyridine ligands, WO(OR)₂py₃ (R = CMe(CF₃)₂ (2a), R = Si(O tBu)₃ (2b), and R = C(CF₃)₃ (2c); py = pyridine), and show that upon activation with B(C₆F₅)₃ they display alkene metathesis activities comparable to W(VI)-oxo alkylidenes. The initiation mechanism is examined by kinetic, isotope labeling and computational studies. Experimental evidence reveals that the presence of an allylic CH group in the alkene reactant is crucial for initiating alkene metathesis. Deuterium labeling of the allylic C-H group shows a primary kinetic isotope effect on the rate of initiation. DFT calculations support the formation of an allyl hydride intermediate via activation of the allylic C-H bond and show that formation of the metallacyclobutane from the allyl "hydride" involves a proton transfer facilitated by the coordination of a Lewis acid (B(C₆F₅)₃) and assisted by a Lewis base (pyridine). This proton transfer step is rate determining and yields the metathesis active species.

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.