Framework Effects on Activation and Functionalisation of Methane in Zinc-Exchanged Zeolites

The first selective oxidation of methane to methanol is reported herein for zinc-exchanged MOR (Zn/MOR). Under identical conditions, Zn/FER and Zn/ZSM-5 both form zinc formate and methanol. Selective methane activation to form [Zn-CH₃]⁺ species was confirmed by ¹³C MAS NMR spectroscopy for all three frameworks. The percentage of active zinc sites, measured through quantitative NMR spectroscopy studies, varied with the zeolite framework and was found to be ZSM-5 (5.7 %), MOR (1.2 %) and FER (0.5 %). For Zn/MOR, two signals were observed in the ¹³C MAS NMR spectrum, resulting from two distinct [Zn-CH₃]⁺ species present in the 12 MR and 8 MR side pockets, as supported by additional NMR experiments. The observed products of oxidation of the [Zn-CH₃]⁺ species are shown to depend on the zeolite framework type and the oxidative conditions used. These results lay the foundation for developing structure–function correlations for methane conversion over zinc-exchanged zeolites.     

Methane Activation by Platinum: Critical Role of Edge and Corner Sites of Metal Nanoparticles

Complete dehydrogenation of methane is studied on model Pt catalysts by means of state‐of‐the‐art DFT methods and by a combination of supersonic molecular beams with high‐resolution photoelectron spectroscopy. The DFT results predict that intermediate species like CH₃ and CH₂ are specially stabilized at sites located at particles edges and corners by an amount of 50–80 kJ mol^?1. This stabilization is caused by an enhanced activity of low‐coordinated sites accompanied by their special flexibility to accommodate adsorbates. The kinetics of the complete dehydrogenation of methane is substantially modified according to the reaction energy profiles when switching from Pt(111) extended surfaces to Pt nanoparticles. The CH₃ and CH₂ formation steps are endothermic on Pt(111) but markedly exothermic on Pt₇₉. An important decrease of the reaction barriers is observed in the latter case with values of approximately 60 kJ mol^?1 for first C? H bond scission and 40 kJ mol^?1 for methyl decomposition. DFT predictions are experimentally confirmed by methane decomposition on Pt nanoparticles supported on an ordered CeO₂ film on Cu(111). It is shown that CH₃ generated on the Pt nanoparticles undergoes spontaneous dehydrogenation at 100 K. This is in sharp contrast to previous results on Pt single‐crystal surfaces in which CH₃ was stable up to much higher temperatures. This result underlines the critical role of particle edge sites in methane activation and dehydrogenation.     

On the Activation of Methane and Carbon Dioxide by [HTaO]+ and [TaOH]+ in the Gas Phase: A Mechanistic Study

The thermal reactions of [Ta,O,H]⁺ with methane and carbon dioxide have been investigated experimentally and theoretically by using electrospray ionization mass spectrometry (ESI MS) and density functional theory calculations. Although the activation of methane proceeds by liberation of H₂, the activation of CO₂ gives rise to the formation of [OTa(OH)]⁺ under the elimination of CO. Computational studies of the reactions of methane and carbon dioxide with the two isomers of [Ta,O,H]⁺, namely, [HTaO]⁺ and [Ta(OH)]⁺, have been performed to elucidate mechanistic aspects and to explain characteristic reaction patterns.     

On the Role of Vanadia Species for VOx／SiO2 in the Selective Oxidation of Methane

Various VOx/SiO2 catalysts were prepared by the methods of physical mixing, conventional wetness impregnation and ultrasonication-assistant impregnation. The catalysts were characterized by XRD, UV-Vis DRS, Raman, TPR, ESR and TPSR techniques and the nature of the vanadium species were correlated to their catalytic performance in the reaction of direct conversion of methane to formaldehyde. It is concluded that highly dispersed monomeric and low oligomeric vanadia species are formed on the sample prepared with both traditional wetness impregnation method and ultrasonication-assistant impregnation, whereas in the latter case, the amount of oligomeric vanadia species is much smaller. The V2O5 microcrystallines are the dominant species on the material prepared by physical mixing method. During the selective oxidation of methane, Vv species are reduced to V^IV paramagnetic species and both microcrystalline V205 species and oligomeric vanadia species are found to further disperse and transform into tetrahedral vanadia species. Based on the results of UV Raman spectroscopy and TPSR, C02 is suggested to be formed via two different routes, in which one is from the sequence reaction of CH4→ HCHO→ CO → CO2 over monomeric vanadia species, and the other is from the direct oxidation of methane to CO2 over oligomeric vanadia species. Oligomeric vanadia species is more active than monomeric vanadia species for methane activation.     

Activation of Methane and Ethane as Mediated by the Triatomic Anion HNbN−: Electronic Structure Similarity with a Pt Atom

Investigations of the intrinsic properties of gas‐phase transition metal nitride (TMN) ions represent one approach to gain a fundamental understanding of the active sites of TMN catalysts, the activities and electronic structures of which are known to be comparable to those of noble metal catalysts. Herein, we investigate the structures and reactivities of the triatomic anions HNbN^? by means of mass spectrometry and photoelectron imaging spectroscopy, in conjunction with density functional theory calculations. The HNbN^? anions are capable of activating CH₄ and C₂H₆ through oxidative addition, exhibiting similar reactivities to free Pt atoms. The similar electronic structures of HNbN^? and Pt, especially the active orbitals, are responsible for this resemblance. Compared to the inert NbN^?, the coordination of the H atom in HNbN^? is indispensable. New insights into how to replace noble metals with TMNs may be derived from this combined experimental/computational study.     

Role of the Support in Catalysis: Activation of a Mononuclear Ruthenium Complex for Ethene Dimerization by Chemisorption on Dealuminated Zeolite Y

A set of supported ruthenium complexes with systematically varied ratios of chemisorbed to physisorbed species was formed by contacting cis‐[Ru(acac)₂(C₂H₄)₂] ( I ; acac=C₅H₇O₂^?) with dealuminated zeolite Y. Extended X‐ray absorption fine structure (EXAFS) spectra used to characterize the samples confirmed the systematic variation in the loadings of the two supported species and demonstrated that removal of bidentate acac ligands from I accompanied chemisorption to form [Ru(acac)(C₂H₄)₂]⁺ attached through two Ru? O bonds to the Al sites of the zeolite. A high degree of uniformity in the chemisorbed species was demonstrated by sharp bands in the infrared (IR) spectrum characteristic of ruthenium dicarbonyls that formed when CO reacted with the anchored complex. When the ruthenium loading exceeded 1.0 wt % (Ru/Al≈1:6), the additional adsorbed species were simply physisorbed. Ethene ligands on the chemisorbed species reacted to form butenes when the temperature was raised to approximately 393 K; acac ligands remained bonded to Ru. In contrast, ethene ligands on the physisorbed complex simply desorbed under the same conditions. The chemisorption activated the ruthenium complex and facilitated dimerization of the ethene, which occurred catalytically. IR and EXAFS spectra of the supported samples indicate that 1) Ru centers in the chemisorbed species are more electron deficient than those in the physisorbed species and 2) Ru–ethene bonds in the chemisorbed species are less symmetric than those in the physisorbed species, which implies the presence of a preferred configuration for the catalytic dimerization.     

Investigation of Fe−Ni Mixed‐Oxide Catalysts for the Reduction of NO by CO: Physicochemical Properties and Catalytic Performance

A series of Fe?Ni mixed‐oxide catalysts were synthesized by using the sol–gel method for the reduction of NO by CO. These Fe?Ni mixed‐oxide catalysts exhibited tremendously enhanced catalytic performance compared to monometallic catalysts that were prepared by using the same method. The effects of Fe/Ni molar ratio and calcination temperature on the catalytic activity were examined and the physicochemical properties of the catalysts were characterized by using XRD, Raman spectroscopy, N₂‐adsorption/‐desorption isotherms, temperature‐programmed reduction with hydrogen (H₂‐TPR), temperature‐programmed desorption of nitric oxide (NO‐TPD), and X‐ray photoelectron spectroscopy (XPS). The results indicated that the reduction behavior, surface oxygen species, and surface chemical valence states of iron and nickel in the catalysts were the key factors in the NO elimination. Fe_0.5Ni_0.5O_x that was calcined at 250 °C exhibited excellent catalytic activity of 100 % NO conversion at 130 °C and a lifetime of more than 40 hours. A plausible mechanism for the reduction of NO by CO over the Fe?Ni mixed‐oxide catalysts is proposed, based on XPS and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analyses.     

Catalytic Mechanisms of Transfer Hydrogenation of Azobenzene with Ammonia Borane by Pincer Bismuth Complex: Crucial Role of C=N Functional Group on the Pincer Ligand

Transfer hydrogenation of azobenzene with ammonia borane mediated by pincer bismuth complex 1 was systematically investigated through density functional theory calculations. An unusual metal-ligand cooperation mechanism was disclosed, in which the saturation/regeneration of the C=N functional group on the pincer ligand plays an essential role. The reaction is initiated by the hydrogenation of the C=N bond (saturation) with ammonia borane to afford 3^CN , which is the rate-determining step with Gibbs energy barrier (ΔG^≠) and Gibbs reaction energy (ΔG) of 25.6 and −7.3 kcal/mol, respectively. 3^CN is then converted to a Bi−H intermediate through a water-bridged pathway, which is followed up with the transfer hydrogenation of azobenzene to produce the final product N,N′-diphenylhydrazine and regenerate the catalyst. Finally, the catalyst could be improved by substituting the phenyl group for the tert-butyl group on the pincer ligand, where the ΔG^≠ value (rate-determining step) decreases to 24.0 kcal/mol.     

Substrate‐Dependent H/D Kinetic Isotope Effects and the Role of the Di(μ‐oxo)diiron(IV) Core in Soluble Methane Monooxygenase: A Theoretical Study

Soluble methane monooxygenase (sMMO) is an enzyme that converts alkanes to alcohols using a di(μ‐oxo)diiron(IV) intermediate Q at the active site. Very large kinetic isotope effects (KIEs) indicative of significant tunneling are observed for the hydrogen transfer (H‐transfer) of CH₄ and CH₃CN; however, a relatively small KIE is observed for CH₃NO₂. The detailed mechanism of the enzymatic H‐transfer responsible for the diverse range of KIEs is not yet fully understood. In this study, variational transition‐state theory including the multidimensional tunneling approximation is used to calculate rate constants to predict KIEs based on the quantum‐mechanically generated intrinsic reaction coordinates of the H‐transfer by the di(μ‐oxo)diiron(IV) complex. The results of our study reveal that the role of the di(μ‐oxo)diiron(IV) core and the H‐transfer mechanism are dependent on the substrate. For CH₄, substrate binding induces an electron transfer from the oxygen to one Fe^IV center, which in turn makes the μ‐O ligand more electrophilic and assists the H‐transfer by abstracting an electron from the C?H σ orbital. For CH₃CN, the reduction of Fe^IV to Fe^III occurs gradually with substrate binding and H‐transfer. The charge density and electrophilicity of the μ‐O ligand hardly change upon substrate binding; however, for CH₃NO₂, there seems to be no electron movement from μ‐O to Fe^IV during the H‐transfer. Thus, the μ‐O ligand appears to abstract a proton without an electron from the C?H σ orbital. The calculated KIEs for CH₄, CH₃CN, and CH₃NO₂ are 24.4, 49.0, and 8.27, respectively, at 293 K, in remarkably good agreement with the experimental values. This study reveals that diverse KIE values originate mainly from tunneling to the same di(μ‐oxo)diiron(IV) core for all substrates, and demonstrate that the reaction dynamics are essential for reproducing experimental results and understanding the role of the diiron core for methane oxidation in sMMO.     

Highly Dispersed Ni Nanoparticles on Anhydrous Calcium Silicate (ACS) Nanosheets for Catalytic Dry Reforming of Methane: Tuning the Activity by Different Ways of Ni Introduction

Three kinds of nickel‐loaded anhydrous calcium silicate nanocatalysts (ACS), including Ni‐ACS‐Dop, Ni‐ACS‐Iex and Ni‐ACS‐Im, were prepared by introducing Ni species through doping in the synthesis of calcium silicate hydrate (CSH) nanosheets, ion‐exchange with premade CSH nanosheets and deposition on calcined ACS nanosheets, respectively. Although Ni species were introduced in different ways, all the Ni‐ACS catalysts showed similar chemical compositions and microstructures, where Ni nanoparticles were highly dispersed on the ultrathin ACS nanosheets with a large surface area and good thermal stability. However, the differences in the way of Ni introduction did produce Ni with different electronic states. The Ni‐ACS‐Iex catalyst with “surface Ni” as a dominant form had more electrons enriched on the surface of Ni, which led to the highest activity in the dry reforming of methane (DRM) reaction among the three catalysts, whereas the Ni‐ACS‐Dop catalyst with “lattice Ni” as a dominant form showed an electron‐deficient property and lowest activity. Different from the introduction of a more favourable nanostructure or chemical component to the catalyst system, this work controlled the chemical environment of metal precursors and created metal catalysts with a preferred surface electronic state during synthesis, which could be a new strategy to improve the catalytic activity.     

Perovskites as Precursors for Ni/La2O3 Catalysts in the Dry Reforming of Methane: Synthesis by Constant pH Co‐Precipitation,Reduction Mechanism and Effect of Ru‐Doping

LaNiO₃ perovskite is an interesting precursor for Ni/La₂O₃ catalysts for the dry reforming of methane at high temperatures. Precursors have been synthesized by co‐precipitation without, with 2.5 at %, and with 5 at % Ru doping. The presence of Ru leads to a stabilization of the perovskite structure and hinders the decomposition into NiO and Ruddlesden‐Popper mixed oxides La_n+1Ni_nO_3n+1, which was observed for the Ru‐free sample upon calcination at 1000 °C (n = 3). Upon reduction in hydrogen, a mechanism involving at least two steps was observed and the first major step was identified as the partial reduction of the precursor leading to a LaNiO_2.5‐like intermediate. The second major step is the reduction to Ni metal supported on La₂O₃ independent of the Ru content of the catalyst. In the presence of Ru, indications for Ni‐Ru alloy formation and for a higher dispersion of the metallic phase were found. The catalytic activity in DRM of the catalyst containing 2.5 % Ru was superior to the catalysts with more or without Ru. Furthermore, the propensity of coke formation was reduced by the presence of Ru.