The Reduction-Coupled Oxo Activation (ROA) Mechanism Responsible for the Catalytic Selective Activation and Functionalization of n-Butane to Maleic Anhydride by Vanadium Phosphate Oxide

Oxygen-Atom Defect Formation in Polyoxovanadate Clusters via Proton-Coupled Electron Transfer

The uptake of hydrogen atoms (H-atoms) into reducible metal oxides has implications in catalysis and energy storage. However, outside of computational modeling, it is difficult to obtain insight into the physicochemical factors that govern H-atom uptake at the atomic level. Here, we describe oxygen-atom vacancy formation in a series of hexavanadate assemblies via proton-coupled electron transfer, presenting a novel pathway for the formation of defect sites at the surface of redox-active metal oxides. Kinetic investigations reveal that H-atom transfer to the metal oxide surface occurs through concerted proton-electron transfer, resulting in the formation of a transient VIII-OH2 moiety that, upon displacement of the water ligand with an acetonitrile molecule, forms the oxygen-deficient polyoxovanadate-alkoxide cluster. Oxidation state distribution of the cluster core dictates the affinity of surface oxido ligands for H-atoms, mirroring the behavior of reducible metal oxide nanocrystals. Ultimately, atomistic insights from this work provide new design criteria for predictive proton-coupled electron-transfer reactivity of terminal M═O moieties at the surface of nanoscopic metal oxides.

Reaction Mechanism and Strategy for Optimizing the Hydrogen Evolution Reaction on Single-Layer 1T' WSe2 and WTe2 Based on Grand Canonical Potential Kinetics

Transition-metal dichalcogenides (TMDs) in the 1T' phase are known high-performance catalysts for hydrogen evolution reaction (HER). Many experimental and some theoretical studies report that vacant sites play an important role in the HER on the basal plane. To provide benchmark calculations for comparison directly with future experiments on TMDs to obtain a validated detailed understanding that can be used to optimize the performance and material, we apply a recently developed grand canonical potential kinetics (GCP-K) formulation to predict the HER at vacant sites on the basal plane of the 1T' structure of WSe₂ and WTe₂. The accuracy of GCP-K has recently been validated for single-crystal nanoparticles. Using the GCP-K formulation, we find that the transition-state structures and the concentrations of the four intermediates (0-3 H at the selenium or tellurium vacancy) change continuously as a function of the applied potential. The onset potential (at 10 mA/cm^-2) is -0.53 V for WSe₂ (experiment is -0.51 V) and -0.51 V for WTe₂ (experiment is -0.57 V). We find multistep reaction mechanisms for H₂ evolution from Volmer-Volmer-Tafel (VVT) to Volmer-Heyrovsky (VH) depending on the applied potential, leading to an unusual non-monotonic change in current density with the applied potential. For example, our detailed understanding of the reaction mechanism suggests a strategy to improve the catalytic performance significantly by alternating the applied potential periodically.

Crystal Imperfections of Industrial Vanadium Phosphorous Oxide Catalysts

This study presents information about crystal imperfections in the main phase of industrial vanadium phosphorous oxide catalysts that are used to catalyze the oxidation of n-butane to maleic anhydride, being an important intermediate in the chemical industry. The mechanism of this reaction is still debated, and the catalytically active and selective surface centers have not yet been identified. The results presented are based on X-ray diffraction data obtained by both laboratory-scale and synchrotron powder diffraction experiments, as well as laboratory-scale single-crystal diffraction experiments. It has been proven that pronounced Bragg reflection broadening effects found in laboratory-scale powder diffraction patterns of industrial VPO catalysts are real and not due to an insufficient 2-θ resolution of the apparatus. In the framework of this work, a powder diffraction full profile fitting strategy was developed using the TOPAS software, which was applied to analyze the X-ray diffraction data of four differently activated industrial catalyst samples, originating from one batch after they had been catalytically tested. It was found that the reflection broadening is mainly caused by an anisotropic crystal size, which results in platelet-shaped crystallites of vanadyl pyrophosphate. A further contribution to the reflex broadening, especially for (111), was found to be a result of stacking faults perpendicular to the a direction in the crystal structure of vanadyl pyrophosphate. These results were used to elaborate on possible correlations between structural proxies and catalytic performance. A direct correlation between the extension of coherently scattering domains in the z direction and the catalyst’s selectivity could be proven, whereas the activity turned out to be dependent on the crystallite shape. Regarding the phase contents, it could be shown that sample catalysts containing a higher amount of β-VO(PO3)2 showed increased catalytic activity.

Sparse ab initio x-ray transmission spectrotomography for nanoscopic compositional analysis of functional materials

The performance of functional materials is either driven or limited by nanoscopic heterogeneities distributed throughout the material's volume. To better our understanding of these materials, we need characterization tools that allow us to determine the nature and distribution of these heterogeneities in their native geometry in 3D. Here, we introduce a method based on x-ray near-edge spectroscopy, ptychographic x-ray computed nanotomography, and sparsity techniques. The method allows the acquisition of quantitative multimodal tomograms of representative sample volumes at sub-30 nm half-period spatial resolution within practical acquisition times, which enables local structure refinements in complex geometries. To demonstrate the method's capabilities, we investigated the transformation of vanadium phosphorus oxide catalysts with industrial use. We observe changes from the micrometer to the atomic level and the formation of a location-specific defect so far only theorized. These results led to a reevaluation of these catalysts used in the production of plastics.

Phosphorus-Based Ionic Liquid as Dual Function Promoter Oriented Synthesis of Efficient VPO Catalyst for Selective Oxidation of n-butane

Abstract

Vanadium phosphorus oxide (VPO) catalysts promoted by phosphorus-based ionic liquids (ILs) as structure directing agent and promoters have been innovatively synthesized and investigated for selective oxidation of n-butane to maleic anhydride (MA). The catalytic performances showed that the IL addition notably improved the n-butane conversion and MA selectivity, during which the optimized 3%IL-VPO catalyst exhibited the maximum MA yield of 59.2% that is much better than that of blank VPO with 49.4% MA yield under the same reaction conditions. XRD, FI-IR, Raman, SEM, TG, BET, XPS and H2-TPR techniques were utilized combinatorically to elaborate the synergistic effect of cations and anions of ILs. Results demonstrated that IL-cations oriented synthesis of VPO precursor showing a vertically intercrossed slice structure morphology having smaller lamellar thickness. Correspondingly, it notably enhanced the specific surface area of the VPO catalysts and exposed the more active surface of (VO)2P2O7 after activation. Meanwhile, IL-anions as promoters largely modulated the P/V ratio, valence state of V and oxygen species amounts on the surface of VPO catalyst, etc. All of these influence factors were subsequently discussed in detail and correlated to the catalytic performance of VPO catalysts.

Graphic Abstract

Isolable cyclic (alkyl)(amino)carbene-phosphonyl radical adducts

Phosphonyl radicals ([R₂P[double bond, length as m-dash]O]˙) and their adducts are proposed as intermediates in a number of important chemical and biological processes. Despite the great interest in these species, there are no examples of stable, isolated phosphonyl radicals or their adducts reported in the literature. Here we report the synthesis, EPR and theoretical study of stable, isolable cyclic (alkyl)(amino)carbene (cAAC)-phosphonyl radical adducts, [cAAC-P(O)R₂]˙ (R = OPrⁱ, Ph).

An untethered C3v-symmetric triarylphosphine oxide locked by intermolecular hydrogen bonding

A C_3v-symmetric triarylphosphine oxide locked into conformation by H-bonding and displaying an extended MOF-like solid-state structure is reported.

Probing Hydrogen Atom Transfer at a Phosphorus(V) Oxide Bond Using a "Bulky Hydrogen Atom" Surrogate: Analogies to PCET

Recent computational studies suggest that the phosphate support in the commercial vanadium phosphate oxide (VPO) catalyst may play a critical role in initiating butane C-H bond activation through a mechanism termed reduction-coupled oxo activation (ROA) similar to proton-coupled electron transfer (PCET); however, no experimental evidence exists to support this mechanism. Herein, we present molecular model compounds, (Ph₂N)₃V═N-P(O)Ar₂ (Ar = C₆F₅ (2a), Ph (2b)), which are reactive to both weak H atom donors and a Me₃Si^• (a "bulky hydrogen atom" surrogate) donor, 1,4-bis(trimethylsilyl)pyrazine. While the former reaction led to product decomposition, the latter resulted in the isolation of the reduced, silylated complexes (Ph₂N)₃V-N═P(OSiMe₃)Ar₂ (3a/b). Detailed analyses of possible reaction pathways, involving the isolation and full characterization of potential stepwise square-scheme intermediates, as well as the determination of minimum experimentally and computationally derived thermochemical values, are described. We find that stepwise electron transfer (ET) + silylium transfer (ST) or concerted EST mechanisms are most likely. This study provides the first experimental evidence supporting a ROA mechanism and may inform future studies in homogeneous or heterogeneous C-H activation chemistry, as well as open up a possible new avenue for main group/transition metal cooperative redox reactivity.

A Mono-, Di-, and Trivanadocene Phosphorus Oxide Series: Synthesis, Magnetism, and Chemical/Electrochemical Properties

In this Article, we outline the synthesis of B(C₆F₅)₃-coordinated mono-, di-, and trivanadocene phosphorus oxide complexes, Cp₂VOP(OB(C₆F₅)₃)Ph₂ (2), (Cp₂VO)₂P(OB(C₆F₅)₃)Ph (3), and (Cp₂VO)₃P(OB(C₆F₅)₃) (4), respectively (Cp = η⁵-cyclopentadienyl). The complexes were synthesized from the known reagents, Cp₂VF and Ph₂P(O)OSiMe₃ (for 2) or PhP(O)(OSiMe₃)₂ (for 3) or (Me₃SiO)₃PO (for 4), via Me₃SiF elimination and in the presence of B(C₆F₅)₃. The multimetallic complexes (3 and 4) could not be synthesized without the capping B(C₆F₅)₃ Lewis acid, whereas the uncapped version of 2, Cp₂VOP(O)Ph₂ (1), has previously been reported by us. Spectroscopic and crystallographic analyses of 2-4 support an increasingly Lewis basic P═O bond upon substitution of -Ph for -OVCp₂ fragments (2-4). The increased metal nuclearity also results in increasingly reducing complexes as evidenced by cyclic voltammetry (CV). Magnetic measurements (SQUID) further revealed high-spin complexes with negligible magnetic exchange between V centers. Chemical oxidation of 2 with 0.5 equiv of [Ag][B(C₆F₅)₄] resulted in a ligand rearrangement reaction producing the V^IV product, Cp₂V(OP(OB(C₆F₅)₃)Ph₂)₂ (7). In contrast, the oxidation of 4 with the trityl salt, [Ph₃C][B(C₆F₅)₄], resulted in the isolation of a mixed-valent V^III/V^IV dimetallic species, (Cp₂VO₂P(OB(C₆F₅)₃)OVCp₂ (9). Both oxidations likely produce the [Cp₂V][B(C₆F₅)₄] byproduct and evidence for its formation is presented. The synthesis and characterization of the mono- and dimetallic species, Cp₂VOP(OBPh₃)Ph₂ (8) and (Cp₂VO)₂P(OB(C₆F₅)₃)OSiMe₃ (5), is also reported.

Synthesis of vanadium phosphorus oxide catalysts promoted by iron-based ionic liquids and their catalytic performance in selective oxidation ofn-butane

Catalytic performance of undoped and Fe-doped VPO catalysts.

Highly crystalline vanadium phosphate catalysts synthesized using poly(acrylic acid-co-maleic acid) as a structure directing agent

Addition of poly(acrylic acid-co-maleic acid) (PAAMA) during the synthesis of vanadium phosphates changes the morphology and improves the catalyst performance.