Alkane Activation Research Articles

Activation of Light Alkanes over Zinc Species Stabilized in ZSM-5 Zeolite: A Comprehensive DFT Study

The mechanism of alkane dehydrogenation over Zn/ZSM-5 zeolite was studied by means of density functional theory using the cluster modeling approach. Three types of active sites were considered: Zn2+ ions stabilized in conventional ion-exchange sites (ZnZs), Zn2+ ions stabilized in cation sites with distantly placed aluminum ions (ZnZd), or as binuclear [ZnOZn]2+ cations (ZnOZn). A comparison of the computed energetics of various reaction paths for ethane indicates that the catalytic reaction proceeds most easily over the ZnZd sites. The enhanced Lewis acidity of these sites facilitates the subsequent heterolytic C−H bond cleavage. The most favorable proton-accepting sites are not those of the framework ring to which Zn2+ is attached, but O sites of a neighboring ring. Although the reactivity of isolated Zn2+ ions strongly depends on the distribution of framework aluminum in ZSM-5 zeolite, the adsorption of alkane molecules is only slightly influenced by this factor. After the heterolytic C−H bond cleavag...

The selective oxidative activation of light alkanes. From supported vanadia to multicomponent bulk V-containing catalysts

V-containing materials have been proposed as active and selective catalysts in the selective oxidative transformation of C2–C4 alkanes to the corresponding olefins, oxygenates and nitriles. Although vanadium ions are generally assumed as the active sites in the oxidative activation of short chain alkanes, the coordination, the oxidation state and the environment of V atoms strongly influence both the catalytic activity and the nature of the reaction products. A comparative study on the catalytic behaviour of V-containing catalysts, i.e. metal vanadates, supported vanadium oxides, V-containing microporous/mesoporous materials and multicomponent bulk Mo–V–Te(Sb)–Nb mixed metal oxides, on the alkane oxidation is presented. The influence of the structure (including the V-coordination and V-environment) and the physico-chemical properties (especially redox and acid–base characteristics) of the catalysts on their behaviour in the selective oxidation of paraffins and alkenes is discussed.

Effects of hydrogen and oxygenated hydrocarbons on the activity and SO2-tolerance of Ag/Al2O3 for selective reduction of NO

De-NOx performance and SO2-tolerance of Ag/Al2O3 catalyst for selective catalytic reduction of NO by hydrocarbon (HC-SCR) and HC-SCR in the presence of H2 (H2-HC-SCR) were investigated using 10 types of reductant (Et2O, ETBE, EtOH, 1-propanol, 2-propanol, t-butanol, 1-propanal, ethyl acetate, acetone, C3H8). Among various reductant tested, Et2O was most effective for achieving high de-NOx activity for HC-SCR and H2-HC-SCR. For all the reductants, de-NOx activity at low temperature region was increased by H2 co-feeding. Degree of activity enhancement was higher for C3H8 than for oxygenates, which is consistent with our previous proposal that H2 enhances de-NOx activity by promoting the oxidative activation of alkane into partially oxidized hydrocarbon intermediates. Effects of reductants and H2 co-feeding on the SO2-tolerance were examined using Et2O, ETBE, and EtOH. The result showed that use of oxygenates as well as H2 co-feeding resulted in the higher SO2-tolerance.

Mechanism of Alkane C−H Bond Activation by Copper and Silver Homoscorpionate Complexes

DFT Becke3LYP calculations are applied to the computational study of the activation of alkane C−H bonds by metallocarbene homoscorpionate complexes. A total of 16 different combinations of metallocarbene complex and alkane are explored, defined by the use of TpAgC(H)(CO2CH3), TpBr3AgC(H)(CO2CH3), TpCuC(H)(CO2CH3), and TpBr3CuC(H)(CO2CH3) species as metallocarbene and methane, ethane, propane, and butane as alkane. The reaction is found to be under kinetic control, and the selectivity is decided in a step with a low-barrier transition state where the key bond-breaking and bond-forming processes take place in a concerted way. This transition state has several possible conformations, which are systematically explored to find the one with lowest energy for each reaction. Variations of the energy barrier as a function of the nature of metal, ligand, and alkane are analyzed and discussed.

Variable temperature FTIR study on the surface acidity of variously treated sulfated zirconias

Abstract The acidity of three related sulfated zirconias has been compared by IR spectroscopy using CO and H 2 as probe molecules. The parent material (SZ) was obtained by calcination of a commercial sulfated zirconium hydroxide. The other two samples, SZ-WW and SZ-SO 3 , were obtained from SZ by water washing and by sulfation with gaseous SO 3 , respectively. The labile sulfate groups responsible for alkane activation and associated with an IR band at 1406 cm −1 are removed by water washing and increased by SO 3 -sulfation. No remarkable differences in the strength of either Lewis or Bronsted acid sites have been observed between SZ and SZ-SO 3 . Water washing strongly weakens Bronsted acidity but only slightly weakens Lewis acidity.

Alkane Metathesis Catalyzed by a Well‐Defined Silica‐Supported Mo Imido Alkylidene Complex: [(SiO)Mo(NAr)(CHtBu)(CH2tBu)

A two-functions-in-one catalyst precursor allows an olefin metathesis catalyst to be turned into an alkane metathesis catalyst. A silica-supported alkyl alkylidene molybdenum complex containing an ancillary imido ligand is a highly active olefin metathesis catalyst and also acts as a catalyst precursor for alkane metathesis. This system involves a single metal with dual properties and shows that ancillary ligands are compatible with alkane metathesis reactions.

In situ generation of highly active olefin metathesis initiators

A ruthenium based benzylidene complex bearing an O, N-bidentate Schiff base ligand exhibits poor olefin metathesis activity but generates a highly active catalyst system when treated with acidic cocatalysts. Various Lewis acids were able to boost the ring opening metathesis polymerization (ROMP) of cycloocta-1,5-diene (COD). The best results were obtained with trichlorosilane (HSiCl 3), which was also found to improve metathesis activity of the Grubbs second generation catalyst [RuCl 2( CHPh)(H 2IMes)(PCy 3)] in the ROMP of COD and the ring closing metathesis (RCM) of diethyl diallylmalonate.

Activation of Light Alkanes on Sulfated Zirconia

AbstractThe present contribution reviews recent experiments of our group towards the synthesizing and understanding of highly reactive sulfate‐modified zirconia. It is shown that in the absence of Pt, the catalytic activity depends upon the ability to oxidize a small fraction of the alkanes to alkenes. The alkenes form sec‐carbenium ions with Brønsted acid sites and isomerize via carbenium ion/alkoxy intermediates. Alkane activation/conversion propagates through hydride transfer from n‐butane to the tert‐butyl carbenium ion. A kinetic model accounting for the elementary steps is proposed and catalyst preparation strategies are reported.

Supported gold catalysts for the total oxidation of alkanes and carbon monoxide

Gold catalysts on oxide supports prepared by coprecipitation (CoO x , MnO x , CuO, Fe 2O 3 and CeO 2), deposition-precipitation (TiO 2) and impregnation (CoO x and MnO x ) have been characterised and tested for the total oxidation of methane, ethane and propane, and the catalytic activity was contrasted with their performance for oxidation of CO at ambient temperature. Many of the oxides selected as supports are active alkane activation catalysts at high temperature (e.g. MnO x and CoO x ). The addition of gold to these active materials for the alkane combustion leads to a marked increase in the catalytic activity in all cases investigated. All the supports were active for the oxidation of alkanes but the addition of gold leads to a decrease in the temperature at which oxidation is first observed. The most effective catalyst for alkane oxidation was Au/CoO x which was found to retain constant high activity for a 48 h test period, and the highest activity catalysts were prepared by coprecipitation rather than impregnation. Interestingly, no correlation between alkane oxidation and CO oxidation was apparent in these studies indicating that different mechanisms of oxygen activation and/or different active sites are involved. This is discussed in terms of recent observations concerning selective versus total oxidation with gold catalysts.

New Cs-containing Mo–V 4+ based oxides with the structure of the M1 phase—Base for new catalysts for the direct alkane activation

The high catalytic performance of Mo 3.5V 4+W 0.5O 14 for the selective oxidation of acrolein (ACR) to acrylic acid (AA) is strongly connected to its semi-crystalline tunnel structure of an oxidic bronze. The real structure in the crystallographic ( a and b)-plane is characterized by an intergrowth of different channel sizes typical for oxidic bronze structures without long-range order in the ( a and b)-plane. The growth direction of the needle-shaped crystals is perpendicular to the ( a and b)-plane and parallel to the crystallographic c-axis. In this direction, in contrast to the high disorder of different tunnel motifs in the ( a and b)-plane, a strong crystallographic order of corner-shared MO 6 octahedra (M = Mo, V, W) is observed. A structural analogue in the family of the block or shear structures is M-Nb 2O 5. This structure is characterized by an intimate intergrowth of ReO 3-blocks with very different block sizes and without any long-range order in the ( a and b)-plane, and by a high order of corner-shared NbO 6 octahedra in [ c]. The semi-crystalline oxidic bronze structure of Mo 3.5V 4+W 0.5O 14 is not unique. Many ternary and quaternary oxides of transition metals, which crystallize at high temperatures with the crystal structure of oxidic bronzes, at low calcination temperatures exhibit a semi-crystalline bronze structure. Mo 10V 2 4+Nb 2TeO 42− x with the crystal structure of Cs 0.5[Nb 2.5W 2.5O 14] (M1 phase) exhibits its best catalytic properties in the direct oxidation of propane to acrylic acid if no impurity of M2 phase is present. Phase-pure M1 phase can be prepared in a highly reproducible way by avoiding any precipitation by continuous and instantaneous spray-drying of aqueous solutions. The resulting amorphous precursor is calcinated in air (<300 °C) and in N 2 (300–650 °C) and post-treated with aqueous acids to remove any impurity of undesired phases (e.g. the M2 phase). New Mo–V based oxides with the composition A 0.5[Mo 5− a− b V a 4+X b O 14] (A = Rb, Cs, X = no element, Nb, Ta, W, Sb, Bi, Se, Te, …) and with the structure of the M1 phase can be prepared. Due to the Cs-content, the new compounds exhibit a low catalytic activity in the direct propane oxidation. By treating Cs 0.5[Mo 3.7V 1.2 4+Bi 0.1O 13.9] with HClO 4 50% of the Cs could be removed, and the catalytic activity was increased. Therewith the mentioned new preparation concept for tailor-made new Mo–V based M1 phases with limited Cs-content and with flexible composition may enable new and well-defined M1 phases with (e.g. Bi containing) active sites to overcome the catalytic difficulties in the step of the oxidative transformation of propene to acrolein within the direct oxidation of propane to AA.

Kinetics of Oxidative Dehydrogenation of C2‐C3 Alkanes on Oxide Catalysts

This review is a survey of the main facts and concepts appeared in the last years on kinetics of oxidative dehydrogenation (ODH) of ethane, propane, butane and isobutane. The accent is put on the mechanisms and the kinetic models used for the description of this reaction. The main catalysts for ODH of light alkanes and the specific features of each type of alkane are discussed. Influence of the type of supports, promoters, acid–base properties, type of oxygen on the performance of ODH reaction is also discussed. No clear interpretation regarding the mode of alkane activation of each stage of this reaction can be derived from the survey of the literature and the problems which are still waiting for the solution or need further studies are described.

Alkane Bromination Revisited: “Reproportionation” in Gas-Phase Methane Bromination Leads to Higher Selectivity for CH3Br at Moderate Temperatures

The reaction of methane and bromine is a mildly exothermic and exergonic example of free radical alkane activation. We show here that the reaction of methane and bromine (CH4:Br2 > or = 1) may yield either a kinetically or a thermodynamically determined bromomethane product distribution and proceeds in two main phases between 450 and 550 degrees C under ambient pressure on the laboratory time scale. This is in contrast to the highly exothermic methane fluorination or chlorination reactions, which give kinetic product distributions, and to the endergonic iodination of methane, which yields an equilibrium distribution of iodomethanes. The first phase of reaction between methane and bromine is a relatively rapid consumption of bromine to yield a kinetic methane bromination product distribution characterized by low methane conversion, low methyl bromide selectivity, and higher polybromomethane selectivity. In the second slower phase CHxBr(4-x) reproportionation leads to significantly higher methane conversion and higher methyl bromide selectivity. For methane bromination at 525 degrees C, CH4 conversion and CH3Br selectivity reach 73.5% and 69.5%, respectively, after ample (60 s) time for reproportionation. The high selectivity and simple configuration make this pathway an attractive candidate for scale-up in halogen-mediated methane partial oxidation processes.

Handbook of C-H transformations: applications in organic synthesis

Volume 1. Preface. List of Contributors. I. General. 1. What is C-H Bond Activation? (Bengu Sezen and Dalibor Sames). 1.1 Introduction. 1.2 Activation or 1.3 The Origin and Historical Context of the Organometallic Definition. 1.4 What Do We Do With Two Definitions? 1.5 Conclusion. 2. C-H Transformation in Industrial Processes (Leslaw Mleczko, Sigurd Buchholz, Christian Munnich) 2.1 Introduction. 2.2 Alkane Activation. 2.3 C-H Transformation at Olefins. 2.4 Basic Chemicals from Aromatic Hydrocarbons. 2.5 Fine Chemicals. II. C-H Transformation at sp-Hybridized Carbon Atoms. 1. C-H Transformation at Terminal Alkynes. 1.1 Recent Developments in Enantioselective Addition of Terminal Alkynes to Aldehydes (Tobias Ritter and Erick M. Carreira). 1.2 The Sonogashira Coupling Reaction (Herbert Plenio and Anupama Datta). 1.3 Glaser Homocoupling and the Cadiot-Chodkiewicz Heterocoupling Reaction (Peter Siemsen and Beatrice Felber). 1.4 Dimerization of Terminal Alkynes (Emilio Bustelo and Pierre H. Dixneuf). 1.5 anti -Markovnikov Addition to Terminal Alkynes via Ruthenium Vinylidene Intermediates (Christian Bruneau). 2. Asymmetric Hydrocyanation of Alkenes (Jos Wilting and Dieter Vogt). 2.1 Introduction. 2.2 Mechanism. III. C-H Transformation at sp 2 -hybridized Carbon Atoms. 1. C-H Transformations at Arenes. 1.1 Direct Oxidation of Arenes to Phenols and Quinones (Vsevolod V. Rostovtsev). 1.2 Metalation of Arenes. 1.3 Alkylation and Vinylation of Arenes. 1.4 Aryl-Aryl Coupling Reactions. 2. C-H Transformation at Alkenes. 2.1 The Heck Reaction (Lukas Gooben and Kathe Baumann). 2.2 Wacker Oxidation (Lukas Hintermann). 3. C-H Transformation at Aldehydes and Imines. 3.1 Inter- and Intramolecular Hydroacylation (Chul-Ho Jun and Young Jun Park). Volume 2. IV. C-H Transformation at sp 3 -hybridized Carbon Atoms. 1. C-H Transformation at Functionalized Alkanes. 1.1 C-H Transformation in the Position &alpha to Polar Functional Groups. 1.2 C-H Transformation in the Allylic and Benzylic Positions. 1.3 C-H Transformation at Functionalized Alkanes via Palladacycles (Gerald Dyker). 2. C-H Transformation at Unfunctionalized Alkanes. 2.1 C-O Bond Formation by Oxidation. 2.2 Radical Halogenations of Alkanes (Peter R. Schreiner and Andrey A. Fokin). 2.3 Preparative SET C-H Transformations of Alkanes (Andrey A. Fokin and Peter R. Schreiner). 2.4 Photochemical Processes. 2.5 Heterogeneous Catalysts for the C-H Transformation of Unfunctionalized Alkanes (Robert Schlogl). 2.6 Transition-metal Catalyzed Carboxylation of Alkanes (Yuzo Fujiwara and Tsugio Kitamura). 2.7 Photochemical and Thermal Borylation of Alkanes (John F. Harwig and Joshua D. Lawrence). 2.8 Preparation of Olefins by Transition Metal-catalyzed Dehydrogenation (Alan S. Goldman and Rajshekhar Ghosh). 2.9 Rhodium-catalyzed Enantioselective Carbene Addition (Huw M.L. Davies). Index.

Catalysts for new tasks: preparation and applications of tunable ruthenium catalysts for olefin metathesis

A continuous survey across structures, made over the past decades, has led to the development of highly active olefin metathesis catalysts for sophisticated synthetic tasks and for polymer technology. In this paper, our efforts toward novel and improved ruthenium complexes with even better performance in olefin metathesis are described. Oxygen ether derivatives 3, pioneered by Hoveyda, exhibit high activity and possess excellent functional group tolerance. We have successfully fine-tuned catalyst 3b to increase its activity and applicability by the introduction of electron-withdrawing groups to diminish the donor properties of the oxygen atom. As a result, the stable and easily accessible nitro-substituted catalyst 6 has found a number of successful applications in various research and industrial laboratories. We were intrigued by the possibility to further fine-tune the Hoveyda-type catalysts by combining two activating effects-steric and electronic-in a single catalyst. This was possible to achieve in so-called scorpio carbenes, which are currently under investigation in our laboratory. These modifications can be used not only to control the catalyst activity, but also to alter its physical-chemical properties, such as solubility in a given medium or an affinity to silica gel. An example of immobilization strategy based on this concept is presented.

Synthesis of highly active sulfated zirconia by sulfation with SO 3

Highly active sulfated zirconia (SZ) materials were prepared by sulfation of crystalline zirconia with gaseous SO 3. This direct method circumvents the use of liquid sulfation agents, as well as a final calcination step. The amount of sulfur species retained by SO 3-SZ samples was directly proportional to the fraction of monoclinic phase and the concentration of Brønsted acid sites. However, the catalytic activity in n-butane isomerization at 373 K was proportional to the concentration of labile chemisorbed SO 3, most likely in the form of pyrosulfate species showing a characteristic IR band at 1404 cm −1. Tetragonal zirconia favors the formation of these labile sulfur species, playing a key role in light alkane activation.

C–H activation of alkanes on Rhn+ (n=1–30) clusters: Size effects on dehydrogenation

The rate coefficients for the dehydrogenation of ethane, propane, and isobutane with cationic rhodium atoms Rh+ and clusters Rh+ n of up to 30 atoms were measured under single-collision conditions in a Fourier-transform ion cyclotron resonance mass spectrometer. The reaction rates are cluster size dependent and parallel for all the three alkanes. While the reactions proceed close to the theoretical collision rates for a large number of clusters, characteristic minima are observed for Rh+ (5/6/9/19/28). The degree of dehydrogenation varies with the cluster size with maxima for 10< or =n< or =15 for the three alkanes and for n=3 and 2-4 in the cases of ethane and propane, respectively. However, complete dehydrogenation is only observed for the reaction of Rh+ 11 with propane. Dehydrogenation is remarkably selective and no other neutral products than H2 are observed. The results are interpreted in terms of likely cluster geometries.

Ni–Nb–O mixed oxides as highly active and selective catalysts for ethene production via ethane oxidative dehydrogenation. Part I: Characterization and catalytic performance

This work demonstrates the high potential of a new class of catalytic materials based on nickel for the oxidative dehydrogenation of ethane to ethylene. The developed bulk Ni–Nb–O mixed oxides exhibit high activity in ethane ODH and very high selectivity (∼90% ethene selectivity) at low reaction temperature, resulting in an overall ethene yield of 46% at 400 °C. Varying the Nb/Ni atomic ratio led to an optimum catalytic performance for catalysts with Nb/Ni ratio in the range 0.11–0.18. Detailed characterization of the materials with several techniques (XRD, SEM, TPR, TPD-NH 3, TPD-O 2, Raman, XPS, electrical conductivity) showed that the key component for the excellent catalytic behavior is the Ni–Nb solid solution formed upon the introduction of niobium in NiO, evidenced by the contraction of the NiO lattice constant, since even small amounts of Nb effectively converted NiO from a total oxidation catalyst (80% selectivity to CO 2) to a very efficient ethane ODH material. An upper maximum dissolution of Nb 5+ cations in the NiO lattice was attained for Nb/Ni ratios ⩽0.18, with higher Nb contents leading to inhomogeneity and segregation of the NiO and Nb 2O 5 phases. A correlation between the specific surface activity of the catalysts and the surface exposed nickel content led to the conclusion that nickel sites constitute the active centers for the alkane activation, with niobium affecting mainly the selectivity to the olefin. The incorporation of Nb in the NiO lattice by either substitution of nickel atoms and/or filling of the cationic vacancies in the defective nonstoichiometric NiO surface led to a reduction of the materials nonstoichiometry, as indicated by TPD-O 2 and electrical conductivity measurements, and, consequently, of the electrophilic oxygen species (O −), which are abundant on NiO and are responsible for the total oxidation of ethane to carbon dioxide.

NaIO4‐Mediated C—H Activation of Alkylbenzenes and Alkanes with LiBr.

AbstractFor Abstract see ChemInform Abstract in Full Text.

The effect of voids and dilution on n-hexane oxidation over a VMgO catalyst

The conversion of n-hexane has been studied over magnesium orthovanadate (Mg 3V 2O 8). This work illustrates some of the problems associated with alkane activation and subsequent conversion to more valuable products with respect to location of voids and the catalyst in the reactor. The effect of differently packed reactors and dilution factors on the product profile was investigated at a constant gas hourly space velocity (GHSV) of 3000 h −1. The products included propane, propene, butane, benzene, carbon oxides and isomers of butene and hexene. Cracked products, propane and propene were favored in an empty reactor. Benzene production in an empty steel and glass reactor suggested that its formation was non-catalytic. Benzene production below 400 °C was catalytically driven, whilst production above 400 °C was thermodynamically driven. The product-selectivity profile was investigated with respect to catalytic products from the conversion of n-hexane over Mg 3V 2O 8 and non-catalytic products from blank reactor and inert packed reactor experiments.

Oxidative Activation of n-Butane on Sulfated Zirconia

Catalytic activation and conversion of light alkanes by sulfated zirconia is unequivocally shown to be initiated by producing small concentrations of olefins. This occurs via stoichiometric oxidative dehydrogenation of butane by SO3 or pyrosulfate groups to butene (present mostly as alkoxy groups), water, and SO2. Thermal desorption and in situ IR spectroscopy have been used to determine all three reaction products. The concentration of butene formed determines both the catalytic activity of sulfated zirconia as well as the deactivation via formation of oligomers. The thermodynamics of the oxidative dehydrogenation of n-butane by different SZ surface structures has been examined by density functional (DFT) calculations. The calculations show that pyrosulfate or re-adsorbed SO3 species have the highest oxidizing ability.