Hydrocarbon Oxidation Reactions Research Articles

Dispersions of transition-metal-based phases in mesostructured silica matrixes: preparation of high-performance catalytic materials

Abstract The finding that ordered mesostructured silica (OMS) matrixes can stabilize guest nanocrystals has transformed the preparation of advanced catalytic materials. Today, it is possible to tailor the materials in such a way that high loading of guest catalytic phases (CPs) may be achieved at different locations in the host mesoporous matrix and, in some cases, to combine several different chemical functionalities in a single nanocomposite. The conditions for the preparation of host-guest catalytic composites have been elaborated, as have the preparation and testing of representative nanocrystal ensembles, i.e. WS2, MoS2, WO3, ZrO2 (tetragonal), TiO2 (anatase), Cs2.5H0.5PW12O40, clusters of alumina phase and molecular species of H3PW12O40, inserted in an OMS (SBA-15 and MCM-41) matrix at 35–80 wt.% loading. Special fixation methods provide uniform distribution of nanoparticles in the silica nanotubes and at the outer surface of the OMS microcrystals at high loadings. CPs with high thermostability as a result of their stabilization in the OMS matrix demonstrated superior performance by a factor of 1.5–5 in hydrotreating, acidic and oxidation reactions of hydrocarbons compared with conventional reference catalysts. Diffusion limitations in catalytic reactions performed with CP/OMS composites were estimated to be negligible. The only factor limiting the accessibility of CP to reacting molecules was partial plugging of the OMS mesopores.To cite this article: M.V. Landau et al., C. R. Chimie 8 (2005).

A comparison of ceria and Sm-doped ceria for hydrocarbon oxidation reactions

The oxidation of methane, ethane, propane, and n-butane has been studied over CeO 2 and Samaria-doped ceria (SDC) catalysts. The rates for methane and ethane were found to be indistinguishable over the two catalysts, while the rates for propane and n-butane were much higher on ceria compared to SDC. The difference between n-butane oxidation over ceria and SDC is shown to result from a low-temperature rate process on ceria that is not present on SDC. Measurements using CD 4 and C 4D 10 show that both low- and high-temperature rates exhibit a similar kinetic-isotope effect; however, the low-temperature process is half-order in O 2, while the high-temperature process is zero-order. Pulse studies demonstrate that oxygen from the bulk becomes accessible for reaction at approximately the same temperature as that at which the high-temperature rate process becomes important. The implications of these results for understanding the effect of doping on reactions over ceria, and for characterization of oxides using standard test reactions, are discussed.

Proton transfer reaction rate constants between hydronium ion (H3O+) and volatile organic compounds

We report proton transfer reaction rate constants between the hydronium ion (H3O+) and selected atmospherically important volatile organic compounds (VOCs). The quantum chemical method was used to determine the structures of the organic species employing the density function theory-B3LYP. The ion–molecule reaction rates were determined using the average-dipole-orientation theory, along with the permanent dipole moment and polarizability of the organic species predicted from the quantum chemical calculations. The theoretical results are compared to available literature data of the permanent dipole moment, polarizability, and ion–molecule reaction rate. The newly calculated proton transfer rate constants facilitate the use of the proton transfer reaction mass spectrometry (PTR-MS) technique in applications of laboratory investigation of photochemical hydrocarbon oxidation reactions and field measurements of the abundance of VOCs.

Selective oxidation of ethane over a VO x/γ-Al 2O 3 catalyst – investigation of the reaction network

The oxidation of ethane was studied over a VO x /γ-Al 2O 3 catalyst (1.4% V) in a laboratory fixed bed reactor. Ethylene, CO and CO 2 were observed as the main products. To analyze the reaction network the oxidation of ethylene and CO was investigated in the same range of operation conditions. Based on the results obtained, a reaction network for ethane oxidative dehydrogenation can be proposed consisting of five partial reactions. Ethane reacts in two parallel reactions to ethylene (1) and directly to CO 2 (2). From the formed ethylene two further parallel pathways lead to CO (3) and CO 2 (4). Thus, ethylene over-oxidation is the source for CO observed during ethane oxidation. Finally, the consecutive oxidation of CO to CO 2 (5) is a part of the network. To understand the role of the catalyst, experiments were carried out comparing the performance of the VO x /γ-Al 2O 3 catalyst with that of the pure γ-alumina support and a FeO x /γ-Al 2O 3 catalyst. From these experiments it can be concluded that ethane conversion is correlated with the Broenstedt basic properties of the catalyst. Ethylene selectivity reveals an opposite trend. Ethylene formation from ethane consumes lattice oxygen, which can only be provided by redox sites of the catalyst. In contrast, deep oxidation reactions of both hydrocarbons (reactions 2–4) do not depend on the presence of redox sites, but only on the presence of gas phase oxygen. These deep oxidation reactions can be considered as surface reactions. Finally, CO oxidation (reaction 5) occurs primarily again under consumption of lattice oxygen, but under oxygen excess conditions also through surface reactions. In addition, two sources of carbon formation were identified, ethylene pyrolysis under oxygen absence and Boudouard reactions. For the network of the five main reactions, kinetic parameters were estimated. Reaction 1 was described by a Mars van Krevelen mechanism, the reactions 2–5 by Langmuir Hinshelwood equations. Applying this model a satisfactory fit of all experimental data was obtained.

A density functional study of O-O bond cleavage for a biomimetic non-heme iron complex demonstrating an Fe(v)-intermediate.

Density functional theory using the B3LYP hybrid functional has been employed to investigate the reactivity of Fe(TPA) complexes (TPA = tris(2-pyridylmethyl)amine), which are known to catalyze stereospecific hydrocarbon oxidation when H(2)O(2) is used as oxidant. The reaction pathway leading to O-O bond heterolysis in the active catalytic species Fe(III)(TPA)-OOH has been explored, and it is shown that a high-valent iron-oxo intermediate is formed, where an Fe(V) oxidation state is attained, in agreement with previous suggestions based on experiments. In contrast to the analogous intermediate [(Por.)Fe(IV)=O](+1) in P450, the TPA ligand is not oxidized, and the electrons are extracted almost exclusively from the mononuclear iron center. The corresponding homolytic O-O bond cleavage, yielding the two oxidants Fe(IV)=O and the OH. radical, has also been considered, and it is shown that this pathway is inaccessible in the hydrocarbon oxidation reaction with Fe(TPA) and hydrogen peroxide. Investigations have also been performed for the O-O cleavage in the Fe(III)(TPA)-alkylperoxide species. In this case, the barrier for O-O homolysis is found to be slightly lower, leading to loss of stereospecificity and supporting the experimental conclusion that this is the preferred pathway for alkylperoxide oxidants. The difference between hydroperoxide and alkylperoxide as oxidant derives from the higher O-O bond strength for hydrogen peroxide (by 8.0 kcal/mol).

Spin state tuning of non-heme iron-catalyzed hydrocarbon oxidations: participation of FeIII–OOH and FeVO intermediatesBased on the presentation given at Dalton Discussion No. 4, 10–13th January 2002, Kloster Banz, Germany.

We have found a family of non-heme iron complexes [FeII(L)(CH3CN)2] (L = tetradentate pyridine containing ligand) with cis labile sites that catalyze highly stereoselective hydrocarbon oxidations using H2O2 as oxidant. The hydrocarbon oxidation reactivity patterns of this family of catalysts divide them into two subgroups: Category A catalysts which carry out highly stereoselective alkane hydroxylation, olefin epoxidation, and olefin cis-dihydroxylation via low-spin FeIII–OOH, FeVO intermediates and Category B catalysts which form high-spin FeIII–OOH intermediates and strongly favor olefin cis-dihydroxylation in which both diol oxygen atoms derive from H2O2. 6-Methyl substituents on the ligands play an important role in tuning the spin states of the iron centers to afford a family of non-heme iron complexes that catalyze a remarkable array of hydrocarbon oxidation reactions.

Intermediate Q from soluble methane monooxygenase hydroxylates the mechanistic substrate probe norcarane: evidence for a stepwise reaction.

Norcarane is a valuable mechanistic probe for enzyme-catalyzed hydrocarbon oxidation reactions because different products or product distributions result from concerted, radical, and cation based reactions. Soluble methane monooxygenase (sMMO) from Methylosinus trichosporium OB3b catalyzes the oxidation of norcarane to afford 3-hydroxymethylcyclohexene and 3-cycloheptenol, compounds characteristic of radical and cationic intermediates, respectively, in addition to 2- and 3-norcaranols. Past single turnover transient kinetic studies have identified several optically distinct intermediates from the catalytic cycle of the hydroxylase component of sMMO. Thus, the reaction between norcarane and key reaction intermediates can be directly monitored. The presence of norcarane increases the rate of decay of only one intermediate, the high-valent bis-mu-oxo Fe(IV)(2) cluster-containing species compound Q, showing that it is responsible for the majority of the oxidation chemistry. The observation of products from both radical and cationic intermediates from norcarane oxidation catalyzed by sMMO is consistent with a mechanism in which an initial substrate radical intermediate is formed by hydrogen atom abstraction. This intermediate then undergoes either oxygen rebound, intramolecular rearrangement followed by oxygen rebound, or loss of a second electron to yield a cationic intermediate to which OH(-) is transferred. The estimated lower limit of 20 ps for the lifetime of the putative radical intermediate is in accord with values determined from previous studies of sterically hindered sMMO probes.

Model for the origin of carboxylic acids in basinal brines

Carboxylic acids are ubiquitous in basinal brines from petroleum-producing sedimentary basins. Although the absolute concentrations of individual short-chain acids vary systematically over many orders of magnitude, relative abundances are characterized by approximately constant ratios. Laboratory experiments have demonstrated that oxidation of aqueous n-alkanes proceeds through a sequence of reactions involving alkene, alcohol, ketone, and carboxylic acid reaction intermediaries. The highly specific nature of these reactions allows the relative distribution of carboxylic acids produced during oxidation of n-alkanes to be predicted quantitatively as a function of petroleum composition. A model based on the results of laboratory experiments was developed to account for the thermogenic production of aqueous carboxylic acids in sedimentary basins.Model predictions are highly consistent with the distribution of carboxylic acids observed in nature. This result suggests that hydrocarbon oxidation reactions may be pervasive during petroleum maturation in sedimentary basins. Likely oxidizing agents include ferric iron-bearing aluminosilicates, oxides, and hydroxides, pyrite, sulfate-bearing minerals, and water. Such chemical interactions allow inorganic sedimentary components to act as sources of oxygen for the formation of oxygenated organic alteration products. Accordingly, the absolute amount and timing of carboxylic acid generation may not be limited by the compositional evolution of kerogen, as suggested by previous models.

Mechanism of dihydrogen cleavage by high-valent metal oxo compounds: experimental and computational studies.

The oxidation of dihydrogen by metal tetraoxo compounds was investigated. Kinetic measurements of the oxidations of H(2) by MnO(4)(-) and RuO(4), performed by UV-vis spectroscopy, showed these reactions to be quite rapid at 25 degrees C (k(1) approximately (3-6) x 10(-2) M(-1) s(-1)). Rates measured for H(2) oxidation by MnO(4)(-) in aqueous solution (using KMnO(4)) and in chlorobenzene (using (n)Bu(4)NMnO(4)) revealed only a minor solvent effect on the reaction rate. Substantial kinetic isotope effects [(k(H)2/k(D)2 = 3.8(2) (MnO(4)(-), aq), 4.5(5) (MnO(4)(-), C(6)H(5)Cl soln), and 1.8(6) (RuO(4), CCl(4) soln)] indicated that H-H bond cleavage is rate determining and that the mechanism of dihydrogen cleavage is likely similar in aqueous and organic solutions. Third-row transition-metal oxo compounds, such as OsO(4), ReO(4)(-), and MeReO(3), were found to be completely unreactive toward H(2). Experiments were performed to probe for a catalytic hydrogen/deuterium exchange between D(2) and H(2)O as possible evidence of dihydrogen sigma-complex intermediates, but no H/D exchange was observed in the presence of various metal oxo compounds at various pH values. In addition, no inhibition of RuO(4)-catalyzed hydrocarbon oxidation by H(2) was observed. On the basis of the available evidence, a concerted mechanism for the cleavage of H(2) by metal tetraoxo compounds is proposed. Theoretical models were developed for pertinent MnO(4)(-) + H(2) transition states using density functional theory in order to differentiate between concerted [2 + 2] and [3 + 2] scissions of H(2). The density functional theory calculations strongly favor the [3 + 2] mechanism and show that the H(2) cleavage shares some mechanistic features with related hydrocarbon oxidation reactions. The calculated activation energy for the [3 + 2] pathway (DeltaH(++) = 15.4 kcal mol(-1)) is within 2 kcal mol(-1) of the experimental value.

Design of a high-pressure single pulse shock tube for chemical kinetic investigations

A single pulse shock tube has been designed and constructed in order to achieve extremely high pressures and temperatures to facilitate gas-phase chemical kinetic experiments. Postshock pressures of greater than 1000 atmospheres have been obtained. Temperatures greater than 1400 K have been achieved and, in principle, temperatures greater than 2000 K are easily attainable. These high temperatures and pressures permit the investigation of hydrocarbon species pyrolysis and oxidation reactions. Since these reactions occur on the time scale of 0.5–2 ms the shock tube has been constructed with an adjustable length driven section that permits variation of reaction viewing times. For any given reaction viewing time, samples can be withdrawn through a specially constructed automated sampling apparatus for subsequent species analysis with gas chromatography and mass spectrometry. The details of the design and construction that have permitted the successful generation of very high-pressure shocks in this unique apparatus are described. Additional information is provided concerning the diaphragms used in the high-pressure shock tube.

Competitive Reaction of Axial Ligands during Biomimetic Oxygenations

Nitrogenous bases have commonly been employed as axial ligands for metalloporphyrins in biomimetic model compounds and catalytic oxygenation chemistry. The addition of bases such as pyridines or imidazoles to metalloporphyrin-catalyzed hydrocarbon oxidation reactions is known to affect catalyst selectivity and turnover rate; this effect has been correlated with the electron-donor ability of the ligand. We have found that the role of pyridine in these reactions is far more involved than that of a simple axial ligand: pyridine is a competitive substrate and is converted in high yield to the N-oxide. Subsequently, both of these species act as ligands to the metal center. Thus, catalytic systems containing oxidizable pyridines involve complex equilibria with multiple forms of ligated catalyst, and kinetic results should be interpreted with caution. Alternatives to free pyridine were tested, including a pyridine “tail” which is covalently attached to the porphyrin.

Synthesis and characterization of zeolite-encapsulated metalloporphyrins

Metalloporphyrins of Fe III and Cu II were prepared inside the large pores of the zeolite NaY by a process of sequential introduction of components followed by assembly inside the void space of the zeolite. The appropriate process chosen for the porphyrin synthesis was using the propionic acid solvent for reaction between pyrrole and benzaldehyde and this solvent was not destructive for zeolite. The powder X-ray diffraction data confirmed that the crystallinity of the zeolite was maintained. The resulting materials were purified by Soxhlet extractor. The zeolite-included metalloporphyrins were identified for studies using UV-Vis, FTIR and EPR Spectroscopy, CA (chemical analysis), AAS (atomic absorption spectroscopy), TG/DSC, XRD, SEM and 13C-NMR techniques. The catalytic activity of these products was examined and the catalyst showed to be a promising catalytic system to aliphatic hydrocarbon oxidation reactions.

Oxidation of hydrocarbons in a barrier discharge reactor

The oxidation of n-hexane, cyclohexane, cumene (isopropylbenzene), and cyclohexene in a barrier discharge reactor under conditions ensuring effective product transport out of the discharge zone was studied. It was shown that hydrocarbon oxidation reactions can be carried out in barrier discharge plasma reactors with a fairly high selectivity. The oxidation of n-hexane, cyclohexane, and cumene primarily yields the hydroxy and carbonyl compounds alcohols, aldehydes and ketones. The major product of cyclohexene oxidation is epoxycyclohexane (~62 wt %).

Hydrogen assisted oxygen desorption from the V_2O_5(010) surface

Vanadium oxide surfaces are well known to play an active role as catalysts in hydrocarbon oxidation reactions where oxygen from different surface sites participates in the reaction. Due to the ubiquity of hydrogen in these systems, reaction steps involving (temporary) hydrogenation are possible and may influence the overall reaction scheme. This work examines structural and energetic consequences of hydrogen interacting with different oxygen sites at the V2O5(010) surface where the local surface environment is modeled by embedded clusters. The electronic structure and equilibrium geometries of the clusters are obtained by density functional theory (DFT) using gradient corrected functionals (RPBE) for exchange and correlation. Hydrogen is found to stabilize preferentially near oxygen sites forming surface OH and H2O species with binding energies of 0.5–2.3 eV per H atom depending on the site and species. Hydrogen adsorption weakens the binding of the surface oxygen with its vanadium neighbors considerably where the weakening is larger for H2O than for OH formation as evidenced by bond order analyses and results of the binding energetics. Thus, the studies suggest strongly that the presence of hydrogen at the oxide surface facilitates oxygen removal and, therefore, contributes to the enhanced yield of oxygenated products near vanadia based surfaces.

An Oxidation System for Green Chemistry: The Oxidation of Alcohols and Hydrocarbons in the Presence of Metal–Polymer Complexes

Environmental and economic factors make the use of harmful oxidants increasingly unacceptable except on a small scale. Accordingly, we have investigated the use of dioxygen and hydrogen peroxide as oxidants. For this oxidation reaction of hydrocarbons, transition metal complexes and polymer-bound transition metal complexes were effective as catalysts. Earlier investigations indicated that the catalytic site was a bi- or multinuclear complex. Thus, several binuclear complexes of Cu and Mn [Eqs. (9)–(11)] were designed and their effectiveness in oxidizing phenols to biphenols and benzoquinones and in monooxygenase activity was demonstrated. In the oxidation of phenols, the system did not produce poly(phenylene oxide) since the intermediate phenoxy radical underwent C–C coupling to biphenol or underwent attack by hydroxyl radical to a hydroquinone that could be oxidized to a quinone. A reaction mechanism involving the binuclear complex for the oxidation is proposed.

Enthalpies of elementary chain propagation steps in the oxidation of organic compounds

A calculation of the enthalpies of elementary steps of the intra- and intermolecular chain propagation for model oxidation reactions of ethers, esters, ketones and hydrocarbons has been carried out. The heats of the intermolecular and intramolecular transfer of free valence with participation of peroxy radicals and C−H bond of the oxygen-containing compounds are shown to be comparable.

Ballast gas for heat transfer control in fixed-bed reactors

Controlled oxidation reactions of hydrocarbons are practiced today in fixed-bed, fluid bed or transport bed reactors with oxygen or air as the oxidant. In fixed-bed reactors, due to the exothermicity of the oxidation reactions, heat removal and temperature control are critical in achieving safe and optimum reaction conditions that maximize conversion and selectivity. Industrial reactors often control hot spot formation by using an appropriate diluent (ballast) gas. Steam, nitrogen, methane and carbon dioxide are the most commonly used ballast gases. The scope of this paper is to elucidate the benefits of ballast gas use as a tool to optimize heat transfer and reduce the hot spot effect in controlled oxidations with fixed-bed reactors. Ballast gas does not participate in the reaction, but it is used to control heat removal and flammability. A homogeneous one-dimensional reactor model was used to study and compare two oxidation processes: (1) ethylene oxidation to ethylene oxide and (2) o-xylene oxidation to phthalic anhydride. For both processes hot spot temperature and conversion decreased as methane replaced nitrogen in the process ballast gas. Selectivity increased in the ethylene oxide case but decreased in the phthalic anhydride case, indicating that ballast gas effect on selectivity depends on the reaction mechanism. Thus, there is an optimum ballast gas composition that will optimize hot spot temperature and reactor yield.

Liquid-phase hydrocarbon oxidation using supported transition metal catalysts: Influence of the solid support

The cyclohexane oxidation into cyclohexanol and cyclohexanone using several zeolite-encapsulated and activated carbon supported iron-phthalocyanine catalysts was investigated in both batch and continuous reaction conditions. The catalytic activity and efficiency were shown to be strongly dependent on the polarity of the solid support material. A substantial increase in the Si/Al ratio of the NaY zeolite resulted in a increase in hydrocarbon oxidation reaction rate. Adsorption equilibrium of the different solid support materials was determined with an HPLC method. Initial reaction rate of the cyclohexane oxidation, obtained in a continuous reactor configuration, correlated well with these adsorption data. A rapid catalyst deactivation was observed under continuous reaction conditions.

Structure–reactivity relationships in oxidation of C 4 hydrocarbons on supported vanadia catalysts

Molecular vanadia species supported on the surface of metal oxides are used as catalysts for many hydrocarbon oxidation reactions. This paper discusses (i) the structure of the surface vanadia species in fresh and dehydrated catalysts, (ii) the structural and oxidation state changes during C4 hydrocarbon oxidation, and (iii) the roles of the oxide support, the surface oxygen species, and the acidity of the supported vanadia catalysts in selective oxidation catalysis. The molecular structure and reactivity information provides new fundamental insights into the catalytic properties of surface vanadia species during C4 hydrocarbon oxidation. On the other hand, the studies of the model supported vanadia catalysts shed new light on the origins of the catalytic activity of the bulk vanadium–phosphorus oxide (VPO) catalysts for partial oxidation of n-butane to maleic anhydride. The new fundamental structure–activity and selectivity relationships of surface vanadia species on oxide supports provide a foundation for the molecular engineering of supported catalysts for selective oxidation of hydrocarbons.

Catalytic and electrocatalytic properties of intrazeolitically prepared iron phthalocyanine

Iron–phthalocyanine (FePc) has been successfully synthesised inside the pores of the NaY zeolite as evidenced by various physicochemical characterisation methods such as scanning electron microscopy (SEM), X-ray diffraction (XRD), electron spin resonance spectroscopy (ESR), ultraviolet spectroscopy (UV), cyclic voltammetry (CV) and thermal methods of analysis. XRD analysis indicates the formation of FePc molecules inside the supercages. SEM pictures show that the original microporous nature of NaY is not affected by encapsulation of FePc. ESR spectrum shows two peaks with g values of 1.977 and 4.030 indicating the partial oxidation of FePc in the cavity, while IR spectra suggest that the molecule can undergo a saddle deformation inside the zeolite matrix. In UV–Vis spectrum, the Q band of the FePc molecule is shifted from 656 nm to 686 nm, although the soret band is less affected. Thermal analysis indicates that these molecules have enhanced thermal stability inside the zeolite cavities. Cyclic voltammetric studies show that the electrochemical behaviour of pure FePc is not much affected by the encapsulation. The encapsulated FePc molecules are found to catalyse the hydrocarbon oxidation reactions. In addition, modified electrodes prepared with this material indicate good electrocatalytic effects as demonstrated by a 260 mV shift in the peak potential for oxygen reduction in neutral medium. Considerable electrocatalytic behaviour is also observed for oxidation of hydrazine.