Computational study of methane activation by mercury(II) complexes

A computational investigation of methane activation by Hg^II complexes is reported. Calculated geometries and energetics of Hg(II)-containing reactants and products are consistent with available experimental data for a wide range of diverse ligand types. Calculated reaction enthalpies and activation barriers for Hg^II complexes cover a wide range of values for different ligands. This diversity suggests that the kinetics and thermodynamics of methane activation by Hg(II) and related medium-valent complexes can be tailored through rational modification of the ligand environment. Calculated activation barriers and reaction enthalpies for methane activation by Hg(II) complexes indicates that harder, more electronegative, ligands are kinetically and thermodynamically preferred. Potential donor groups on the activating ligand can stabilize the transition state versus the ground state reactants and hence result in substantially lower methane activation barriers. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 902–911, 1998     

The specific surface area of methane hydrate formed in different conditions and manners

The specific surface area of methane hydrates, formed both in the presence and absence of sodium dodecyl sulfate (SDS) and processed in different manners (stirring, compacting, holding the hydrates at the formation conditions for different periods of time, cooling the hydrates for different periods of time before depressurizing them), was measured under atmospheric pressure and temperatures below ice point. It was found that the specific surface area of hydrate increased with the decreasing temperature. The methane hydrate in the presence of SDS was shown to be of bigger specific surface areas than pure methane hydrates. The experimental results further demonstrated that the manners of forming and processing hydrates affected the specific surface area of hydrate samples. Stirring or compacting made the hydrate become finer and led to a bigger specific surface area. Supported by the National natural Science Foundation of China (Grant Nos.20490207, 2076145, uo633003), Program for New Century Excellent Talents in University and National The National High Technology Research and Development Program of China Project.     

Quantum chemical study of C—H bond activation in methane molecule by AuIII aqua chloride complexes

The mechanism of the reactions of methane with the gold(III) complexes [AuCl_x(H₂O)_{4− x} ]^3−x (x = 2, 3, or 4) was studied by the DFT/PBE method with the SBK basis set. High activation barriers obtained for the reactions of [AuCl₄]⁻ and [Au(H₂O)Cl₃] with methane suggest these reactions cannot proceed under mild conditions. The reaction of the [Au(H₂O)₂Cl₂]⁺ complex with methane has a rather low energy barrier and proceeds through the formation of an intermediate complex. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 191–201, February, 2006.     

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.     

Reactivity of surface and bulk oxygen in La1-xCaxFeO3-y system with respect to methane oxidation

The reactivity and mobility of the surface and lattice oxygen in a La_1-xCa_xFeO_3-y system prepared via a ceramic route were studied by using CH₄-TPR. While the middle-temperature (400-700^oC) oxygen lattice mobility reaches maximum for samples with disordered anion vacancies, the high-temperature lattice oxygen flux appears to be controlled by a rapid transfer along disordered domain walls. In the high-temperature range, a high selectivity to syngas of methane oxidation by the lattice oxygen was achieved both for pure lanthanum ferrite and for samples with a microdomain structure. This revised version was published online in August 2006 with corrections to the Cover Date.     

Isolation of rhenium and ReOx species within ZSM5 channels and their catalytic function in the activation of alkanes and alkanols

Synthesis protocols, structures, and reactivity of Re-oxo species grafted onto H-ZSM5, and their subsequent conversion to Re-clusters through contact with H2 or CH4 were studied by using Raman, infrared, and X-ray absorption spectroscopies. Reactivity measurements by using alkane and alkanol reactants were also examined. Sublimation of Re(2)O(7) at 723 K led to a stoichiometric exchange with each ReO(x) species replacing one proton. Raman features for Re(2)O(7) disappeared during thermal treatment and Raman bands assigned to distorted-tetrahedral Si-O(f)ReO(3)-Al (O(f): zeolite-lattice oxygen atoms) species emerged; infrared bands for acidic OH groups in H-ZSM5 weakened concurrently. X-ray absorption near-edge and fine-structure spectra detected the formation of distorted-tetrahedral Re7+-oxo species during thermal treatment of Re(2)O(7)/H-ZSM5 mixtures in air, and their subsequent reduction to Re(0) in H2 or CH4 to form encapsulated Re metal clusters similar in diameter (approximately 8 A) to the channel intersections in ZSM5. Si-O(f)ReO(3)-Al species in ReO(x)-ZSM5 catalyzed the oxidative conversion of C(2)H(5)OH to acetaldehyde, acetal, and ethyl acetate with very low selectivity to CO(x) (<1 %). Unprecedented turnover rates were exhibited at temperatures much lower than previously found for ReO(x)-based catalysts, and without deactivation or sublimation processes ubiquitous in crystalline Re7+ compounds at temperatures required for catalysis. Encapsulated Re metal clusters formed by the reduction of Si-O(f)ReO(3)-Al precursors led to CH4 pyrolysis and C(3)H(8) dehydrocyclodimerization rates (per Re) that are higher than those previously reported for zeolite-based catalysts. The rate of CH4 conversion to benzene, by using Re-ZSM5, was approximately 30 % higher than that of the best reported catalysts, based on encapsulated MoC(x) clusters, whereas C(2)H(4) and C6+ arene selectivities were similar. C(3)H(8) activity and selectivity of Re-ZSM5 was significantly higher than that of Ga-ZSM5, the best reported catalyst for these reactions. Reaction rates (per Re) were independent of the Re/Al(f) (Al(f): aluminum framework) ratio for both Re and ReO(x) species. This is consistent with the uniform character of the structures formed during grafting of the ReO(x) species through sublimation and their ability to retain their homogeneity even after their reduction to encapsulated Re-clusters.     

Conversion of methane into C1 oxygenates by deep-UV photolysis on solid surfaces: influence of the nature of the solid and optimization of photolysis conditions

Deep-UV photolysis (either 165 or 185 nm) of surface hydroxy groups leads to homolytic O-H bond-cleavage with the generation of oxyl radicals that can initiate the room-temperature radical-chain methane activation. Whilst in the absence of oxygen, radical coupling reactions to give low-molecular-weight alkanes are observed in the gas phase, the presence of some oxygen quenches these radicals and increases the selectivity towards C1 oxygenates (methanol, formaldehyde, and formic acid species). The nature of the solid influences the efficiency of the photochemical process and the distribution between products in the gas and solid phases. Using Beta-, delaminated ITQ2 and ITQ6, and medium-pore ZSM5 zeolites, mesoporous MCM41 silicates, and non-porous TiO(2), we observed that confinement and porosity increased the proportion of C1 oxygenates adsorbed onto the solid and reduced the contribution of the gas-phase products. In addition, the presence of aluminum in the zeolite framework, which is responsible for the generation of acid sites, increased overoxidation of methanol and methoxy groups into formaldehyde and formic acids. For a given amount of methane and unchanged photolysis conditions, the conversion increased with the amount of the solid used as photocatalyst. In this way, methane conversions of up to 7% were achieved for the 185 nm photolysis of methane for 1 h with a 76 MJ mol(-1) energy consumption.     

Low-temperature activation of methane over rare earth metals promoted Zn/HZSM-5 zeolite catalysts in the presence of ethylene

At low temperature of 723 K, methane can be easily activated in the presence of ethylene in the feed, and converted to higher hydrocarbons (C2–C4) and aromatics (C6–C10), through its reaction over rare metals modified Zn/HZSM-5 zeolite catalysts without undesirable carbon oxides formation. Methane can get 37.3% conversion over the above catalysts under low temperature, and the catalysts show a longer lifetime than usual metal supported HZSM-5 zeolite catalysts without adding any rare earth metals. The effects of methane activation over various rare earth metal promoted Zn/HZSM-5 catalysts on the products and influences of several reaction conditions such as temperature, catalyst lifetime and molar ratio of CH4/C2H4 have been discussed.     

Prediction of the 13C NMR chemical shifts of organic species adsorbed on H-ZSM-5 zeolite by the ONIOM-GIAO method

The ONIOM-GIAO method has been used to accurately predict 13C NMR chemical shifts for a series of organic species adsorbed on H-ZSM-5 zeolite. This is useful for the spectroscopic identification of complicated catalytic systems.     

A mononuclear cyclopentadiene-iron complex grafted in the supercages of HY zeolite: synthesis, structure, and reactivity

The reaction of ferrocene with the acidic hydroxy groups in the supercages of zeolite HY dehydrated at 673 K and the reactivity of the resultant surface species towards CO and O(2) were investigated by temperature-programmed decomposition (TPD) and reduction (TPR) and IR, X-ray absorption fine structure analysis (XAFS), and X-ray photoelectron (XP) spectroscopy. In situ FTIR, TPD, TPR, and chemical analysis reveal that the Cp(2)Fe molecule adsorbed on the zeolite surface loses one cyclopentadienyl group under vacuum at 423 K, which leads to the formation of a well-defined mononuclear surface Fe-C(5)H(6) complex grafted to two acidic sites and one ([triple bond]Si-O-Si[triple bond]) unit, as confirmed by the lack of Fe-Fe contributions in the EXAFS spectra. Each iron atom is coordinated, on average, to three oxygen atoms of the zeolite surface with a Fe--O distance of 2.00 A and to five carbon atoms with a Fe--C distance of 2.09 A. IR spectra indicate that the cyclopentadiene-iron species grafted on the surface of the zeolite is quite stable in vacuo or under an inert or hydrogen atmosphere below 423 K, and is also relatively stable under oxygen at room temperature. However, the cyclopentadiene ligand readily reacts with CO to form a compound containing carbonyl at 323 K, and even at room temperature. The single carbonyl band in the IR spectra provides evidence for the nearly uniform formation of a cyclopentadiene-iron species on the surface of the zeolite.     

Study of the grafting reaction of SnMe4 on the surface of ZSM‐5 zeolite

The grafting reaction of tetramethyltin on the surface of ZSM‐5 zeolite (Si:Al = 55.0) was studied under vacuum conditions, and the chemical compositions, structure and properties of the resulting solid were characterized by in situ FTIR, ICP, XRD, XPS, UV–vis DRS, temperature programmed decomposition (TPD) and N₂ adsorption. The results show that the reaction occurs on the surface of ZSM‐5 zeolite at 223 K without destroying the zeolite framework. The BET surface area and the pore volume of the zeolite decrease and the surface properties change; however, the microporous structure is retained during the reaction and post treatment. Copyright © 2006 John Wiley & Sons, Ltd.