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A comparative study of the catalytic properties for the oxidation of C2-C3 alkanes and olefins has been carried out over unpromoted and M-promoted NiO catalysts (Me = K, La, Ce, al, Zr, Sn, Nb). The catalysts have been characterized by several physico-chemical techniques (UV Raman, Visible Raman, FTIR of adsorbed CO and XPS). The characteristics of promoter elements are of paramount importance, since they are able to modify both the nature of the active nickel and the concentration of electrophilic O2−/O− oxygen species. Thus, a relatively high acidity and valence of the promoter oxide (with oxidation state higher than + 3) are necessary to achieve high selectivity to olefins during the oxidative dehydrogenation (ODH) of C2–C3 alkanes. In addition, an inverse correlation between the selectivity to the corresponding olefin and the concentration of electrophilic oxygen species has been observed, although the selectivity to propene during propane ODH is lower than the selectivity to ethylene achieved during ethane ODH. On the other hand, a very low influence of alkane conversion on the selectivity to the corresponding olefins is observed. This behaviour can be explained by considering that the reaction rate for olefin combustion is lower than the reaction rate for alkane oxidation. However, the comparative study of the oxidation of alkanes and olefins suggest that the differences observed between the ODH of propane and ethane are not related to the reactivity of olefins, but to the different number and reactivity of C–H bonds in both alkanes. A discussion on the importance of the concentration of active sites and the characteristics of the alkanes fed on the selectivity to olefin during the alkane ODH is also presented. [ABSTRACT FROM AUTHOR]

The need for on-purpose techniques for the conversion of cheaper and abundant light alkanes to petrochemical products has revitalized research interests on light alkanes aromatization. Here ethane/propane aromatization and ethylene oligomerization over the representative Zn-HZSM-5 and Pt/HZSM-5 catalysts have been studied by the step-perturbation transients to provide insight into the kinetics and mechanisms leading to higher olefins and aromatics formation from ethane aromatization. The time-dependent catalytic behavior during the build-up and back-transient between ethane and inert, ethane and propane, as well as ethylene and inert, has been extensively discussed. We suggested that the hydrocarbon-pool mechanism be involved once ethylene was produced from the dehydrogenation of ethane. The oligomerization/cracking, cyclization, and dehydrogenation/hydride transfer reactions involved with the hydrocarbon-pool species reach the thermodynamic equilibrium quickly. The initial ethane dehydrogenation and the final formation of aromatics from their corresponding intermediates are slow surface-reactions. The rate constants k for benzene, toluene, and xylene formation from the "lumped hydrocarbon-pool" have been evaluated based on the first-order kinetic model of the back-transient. The rate constants k for aromatics over the Pt0 clusters/particles in the Pt/HZSM-5 are ⁓ 20–30% higher than that over the Zn (II) cations in the Zn-HZSM-5 catalyst. [ABSTRACT FROM AUTHOR]

The aim of work was to analyse and discuss the effect of cerium on the catalytic behaviour of different types of nickel species in the oxidative dehydrogenation of ethane. Ni-Al and Ni-Ce-Al catalysts were prepared from cerium nitrate and nickel nitrate or nickel acetate as a precursor. As-prepared materials were characterized by using of XRD, DR UV-Vis spectroscopy and H-TPR. It was found that the addition of cerium affects both the reducibility of nickel species and the population of nickel species. Firstly, the addition of cerium led to the significant decrease of the reduction peak at 755-785 °C to about 40 °C due to the Ni-Ce interaction at both Ni-Ce-Al-NO and Ni-Ce-Al-ac catalysts. Secondly, the addition of cerium affected the relative population of nickel species in tetrahedral and octahedral coordination, which population originated from used nickel precursor. The presence of cerium in Ni-Ce-Al-NO catalysts led to the decrease in the relative population of Ni(T) species, while the crystal size of NiO in Ni-Ce-Al-ac catalysts decreased with increasing Ce/Ni ratio. The highest ethylene productivity was achieved on catalysts with Ce/Ni molar ratio 0.13. Catalysts exhibited an ethylene productivity up to 1.0 gC=·g·h with a selectivity of 82 % (Ni-Ce-Al-NO) and 1.3 gC=·g·h with a selectivity of 75 % (Ni-Ce-Al-ac). [ABSTRACT FROM AUTHOR]

Metal oxide promoted NiO catalysts with a Ni/(Me + Ni) atomic ratio 0.92 have been investigated for the oxidative dehydrogenation of ethane. These materials have been characterized by several techniques (N-adsorption, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transformed infrared spectroscopy of adsorbed CO and ethylene). The nature of surface sites is strongly influenced by the valence and the acid/base characteristics of the metal oxide promoters, which have a great impact on the selectivity to ethylene. Accordingly, a clear correlation between selectivity to ethylene and the valence of the promoter has been observed in the present work. Additionally, the acidity of the catalyst also enhances the selectivity to ethylene. [ABSTRACT FROM AUTHOR]

The effect of water onto the rate and selectivity of methane and ethane oxidation in the conditions of oxidative coupling of methane (OCM) is studied. The effect of water strongly depends on the OCM catalyst composition: whereas PbO/AlO undergoes an irreversible deactivation, no effect of water is observed on both reaction rate and OCM selectivity over La/MgO. Over NaWMn/SiO catalyst both rate of reaction and selectivity are enhanced by water addition to the feed at low conversions during methane oxidation. In the case of ethane oxidation, the rate of reaction is strongly affected by water addition, whereas selectivity to ethylene does not change at equal conversions. At increasing concentration of water in the feed gas its relative effect onto the methane oxidation substantially decreases, so the increasing concentration of water above ~8 vol% does not further enhance the rate of methane oxidation. The observed effects are explained by the participation of water in the active site turnover, namely in its re-oxidation by shifting the prevailing re-oxidation route from filling surface oxygen vacancies to oxidative dehydrogenation of surface hydroxy-groups. [ABSTRACT FROM AUTHOR]

The activity of Ni-Al mixed oxides obtained by the thermal pre-treatment of Ni-Al hydrotalcite-like precursors was studied in the ODH of ethane and propane. The activity of the Ni-Al mixed oxide catalysts was studied with respect to (i) the role of Ni content and (ii) the role of temperature during Ni-Al HTs thermal pre-treatment. The structure analysis and the activity of Ni-Al mixed oxides were discussed in three groups; (A) the catalysts pre-treated at 500 °C, (B) the catalysts pre-treated at 600 °C and (C) the Ni2-Al catalyst with constant Ni content pre-treated at 500-900 °C. Ni-Al mixed oxides were active and selective catalysts in the ODH of ethane even at 450 °C. On the other hand, the catalysts posses low selectivity to propene. It is supposed that the interaction of NiO with alumina phase plays the critical role in the active and selective catalysts. The Ni-Al mixed oxides were characterized by XRD, H-TPR and diffuse reflectance spectroscopy. [ABSTRACT FROM AUTHOR]

Ca-doped ThO, synthesized by solution combustion method was tested for dehydrogenation of ethane with CO. Doping ThO with Ca resulted in the creation of oxide ion vacancies and an increased conversion of ethane compared to pure ThO. On ThCaO selectivity to ethene was 97 at 46% ethane conversion at 725 °C. Well-known reference catalysts like 5%Cr/TS-1 or OMS-2 showed significantly lower selectivity, but the former was more active under the same conditions. [ABSTRACT FROM AUTHOR]

The valorization of light alkanes via catalytic oxidative dehydrogenation (ODH) and selective oxidation is, with a few exemptions, still not solved. Oxide catalysts play a foremost role in these reactions. The control of the nanostructure brings new ways to tune their catalytic properties, but to date this has been little explored for alkane activation. This paper offers an overview of the applications of nanostructured oxide catalysts to oxidative activation of alkanes. Relevant examples of their unusual performance, the improvement of activity and selectivity attained by these oxides, and the new features brought by ordered mesoporous oxides, are discussed. Application of nanotechnology to oxides brings both new challenges and opportunities for catalytic applications. To make the most of it, a broad multidisciplinary approach, and bridging the lack of communication among the various research areas (electronics, materials, catalysis) involved, are needed. [ABSTRACT FROM AUTHOR]

Mixed Ni–W–O catalysts (with a W/(Ni + W) atomic ratio of 0.3) supported on γ-Al2O3 or on mesoporous alumina have been prepared, characterized and tested in the oxidation of ethane. For comparison unsupported and supported NiO as well as bulk Ni–W–O mixed oxides catalysts have also been studied. Supported Ni–W–O materials show interesting catalytic performances in the oxidative dehydrogenation of ethane. They show similar catalytic activities than the corresponding unsupported Ni–W–O catalysts. However, the selectivity to ethylene over supported catalysts was higher than that achieved over unsupported samples (the selectivity to ethylene followed the trend: mesoporous-supported > γ-Al2O3-supported > unsupported Ni–W–O). In addition, it has also been observed that Ni–W–O catalysts are more efficient than the corresponding W-free NiO catalysts. The discussion of the catalytic results will be undertaken on the basis of the modification of active sites of NiO when incorporating WO3 and/or metal oxide supports. [ABSTRACT FROM AUTHOR]

The microkinetic reaction network of the oxidative dehydrogenation of ethane to ethene over Li/Dy/Mg/O and Li/Dy/Mg/O/Cl catalysts was investigated. With Li/Dy/Mg/O catalysts, the reaction kinetics is compatible with a heterogeneous-homogeneous radical based reaction mechanism. The formation of ethyl radicals on the surface is concluded to be the rate-determining step. In contrast, the reaction kinetics for Li/Dy/Mg/O/Cl is in line with a purely surface catalyzed reaction mechanism. However, also in this case, alkane activation is rate determining. [ABSTRACT FROM AUTHOR]

The oxidative dehydrogenation of ethane was studied over Pt–Sn impregnated monoliths at 1 bar, 600–900 °C and with different contents of oxygen, hydrogen and steam in the feed gas. As expected a decrease in oxygen in the feed led to a decrease in the conversion of ethane due to lower temperatures in the reactor. Adding steam to the feed showed no effect on the ethane conversion or the ethene selectivity. When the hydrogen/ethane ratio in the feed was varied from 0 to 0.5 at 700 and 850 °C, it resulted in a significant increase in the selectivity to ethene while the ethane conversion remained relatively unchanged. At 700 °C the selectivity increased from about 50% to 93% (carbon basis) with only a small decrease in the conversion of ethane. The results clearly show that both Pt and Sn have a catalytic effect. Pt caused the ethane conversion to rise and addition of Sn resulted in much better ethene selectivity. However, even though Sn alone showed some catalytic effect at lower temperatures, it cannot explain the great difference between the Pt and Pt–Sn catalysts. A reasonable assumption is therefore that there exist interactions between Pt and Sn that gives the Pt–Sn catalysts excellent properties for oxidative dehydrogenation of ethane, in particular upon addition of hydrogen. [ABSTRACT FROM AUTHOR]

A comparative study on the effects of alkali metal on the structures, physico-chemical properties and catalytic behaviors of silica-supported vanadium catalysts for the selective oxidation of ethane and the complete oxidation of diesel soot was reported. [ABSTRACT FROM AUTHOR]

At 600 °C, NO x catalyzes the partial oxidation of both methane and ethane by dioxygen to form formaldehyde. The yield of oxygenates from methane is over 11. The yield increases to over 16 when 0.7% of ethane is added to the gas mixture. The yield of oxygenates from ethane is over 24. A catalytic cycle involving NO2 as the C-H activating species is proposed. [ABSTRACT FROM AUTHOR]

Oxidative dehydrogenation of ethane to ethylene was studied using variously prepared Li/Dy/Mg/Cl mixed metal oxides as catalysts. The catalytic performance was found to be strongly dependant on the method of preparation and the LiCl content of the solids. Ethylene yields of up to 77% were obtained with catalysts prepared by precipitation of the catalyst precursors with an equimolar mixture of NH4Cl and HCl, and subsequent calcination in synthetic air (i.e., absence of CO2). Both highest ethylene yields and best long-term stability were achieved with catalysts having the highest chloride loading. Based on kinetic data and high-temperature XRD measurements (under controlled atmosphere), a new reaction mechanism is proposed wherein the active sites of the catalytic system are postulated to reside in molten LiCl, supported on Dy2O3/MgO. Oxygen is solved dissociatively in the LiCl melt forming the catalytically active hypochlorite OCl-. With increasing temperature, OCl- decomposes to O• + Cl- or O- + Cl•. The two radical species are highly oxidative and can readily activate an alkane by homolytic hydrogen abstraction. The so-created alkane radicals react further with OH to form an olefin and H2O. At low temperatures, a regime of high apparent activation energy has been determined for high chloride loadings, while at high temperatures and low chloride loadings, a second regime with lower activation energy was found. It is suggested that the first regime is controlled by reaction kinetics, whereas the second regime is diffusion-controlled. Which of the two regimes predominates is strongly dependent on the reaction temperature and the structure and composition of the catalyst. [ABSTRACT FROM AUTHOR]

Oxydehydrogenation of ethane as a route to ethylene has the attractive feature of removing the thermodynamic equilibrium conversion limitation of the simple dehydrogenation. For example, in the dehydrogenation of ethane to ethylene, the maximum conversion possible at 1000 °C is 51%, while essentially complete (100%) conversion is possible even at ambient conditions. The best catalysts discovered to date are those from Union Carbide's work (in the late 1970s and early 1980s), which operate at 300-400 °C. These reducible Mo–V–Nb oxide catalysts are thought to react via a surface ethoxide intermediate on a Mo or V site that can then undergo a β-elimination process to form ethylene. On the other hand, the surface ethoxide can be oxidized further to form surface acetate, which leads to acetic acid on hydrolysis with water. Aside from these low-temperature reducible catalysts, many catalysts containing reducible metal oxides and non-reducible metals are known to convert ethane to ethylene at 500-800 °C. It is proposed that these catalysts are essentially dehydrogenation catalysts where the H2 formed from straight dehydrogenation or during a surface intermediate stage after H-abstraction is converted to H2O, thereby shifting the dehydrogenation equilibrium. Therefore, the big question, the challenge, and the opportunity remains as to whether true oxydehydrogenation is possible at relative low-to-moderate temperatures? This challenge/opportunity will be discussed in the backdrop of some of the recent advances in alkane activation. [ABSTRACT FROM AUTHOR]

Acid silicoaluminophosphate SAPO-34 catalysts with a chabasite-related (CHA) structure were tested for the oxidative dehydrogenation of ethane in the temperature range 550-700 °C achieving very interesting catalytic performances (about 70% C2H4 selectivity at 45% ethane conversion) which were related to both Lewis and Brønsted acid sites, as found by a NH3-TPD study. [ABSTRACT FROM AUTHOR]

The dehydroaromatization of methane over Mo/HZSM-5 catalyst was investigated at 3 atm and 725 °C. The rate of benzene formation was significantly enhanced with the addition of a few percent of ethane. A higher formation rate of benzene at 1930 nmol C/g-cat/s was obtained for a CH4 + 6.3% C2H6 co-feed system. XRD results showed that the zeolite framework was intact after the reaction in different feeds. [ABSTRACT FROM AUTHOR]

Nozzles fabricated variously from nickel, molybdenum, iron, palladium, and quartz were utilized to produce longer-chain hydrocarbons CmHn (m≥3, n≤m) from C2 (ethane, acetylene) and C1 (methane) reactants at nozzle temperatures in the range 900-1150C. A combination of pyrolysis and expansion to a supersonic molecular beam is shown to be very effective in the conversion of light aliphatic hydrocarbons to heavier oligomers. The conversion of ethane is close to 100% at Tnozzle = 1000C, while that of methane reaches 70% at Tnozzle = 1100-1150C. The contact time in the nozzle is in the range 1-100 ms enabling at least simultaneous kinetic and thermodynamic control. Major products are acetylene and benzene and its homologues. Free radicals are also detected in the product distribution. The reaction mechanism involves formation of free radicals and, possibly, coupling reactions at the nozzle surface followed by desorption to the gas phase and expansion to a supersonic beam. [ABSTRACT FROM AUTHOR]

The oxidative dehydrogenation and cracking of ethane and propane over LiDyMg mixed oxides is reported. High yields of olefins and only moderate formation of carbon oxides was observed. Both are primary products that hardly interconvert under the reaction conditions used. Addition of chloride increases the rate of reaction, while slightly decreasing the selectivity to olefins. The addition of carbon dioxide strongly decreases the rate of reaction, the negative order of 0.5 indicating that two active Li+sites are blocked by the adsorption of one CO2molecule. The reaction proceeds at low oxygen pressure primarily via elimination of dihydrogen, while at higher oxygen partial pressure the hydrogen elimination occurs via water formation. It is speculated that dehydrogenation and cracking involve Li+and a rather nucleophilic oxygen site. [ABSTRACT FROM AUTHOR]

Nanostructured cerium oxides, pure or doped with CaO, prepared from amorphous acetate precursors obtained by the freeze drying method, exhibit high surface areas and are active and selective catalysts for the oxydehydrogenation of ethane (ODE) using CO2as an oxidant. The incorporation of Ca into a solid solution in the ceria framework reduces the activity but improves markedly the selectivity to ethene and the efficiency of CO2. Under reaction conditions, the pure ceria was stable even at 1023 K, while the surface area of the Ca-containing sample decreased markedly. This change was accompanied by a drastic increase in the selectivity to ethene, allowing to obtain yields of ethene up to 22% with 91% selectivity. [ABSTRACT FROM AUTHOR]

From consideration of several examples of catalytic oxide systems, it appears that the mild oxidation of light alkanes to oxygenates is controlled by the local properties of the surface of oxides which favour the isolation of the active cation or of a controlled number of active sites. This appears to be quite general on massic mixed oxides as well as on supported oxides. Two examples are given for n-butane oxidation to maleic anhydride on the VPO catalyst and for ethane oxidation to acetic acid on the VPMoO/TiO2catalyst. This conclusion was reached by using several physicochemical techniques which are complementary and may distinguish the surface and the bulk properties of the catalytic oxides. As a consequence, the improvement of the performance of a catalytic oxide system and the discovery of new generation of catalysts will stem from the modification at short distance of the local environment of the active site. [ABSTRACT FROM AUTHOR]

The dynamics of ethane trapping on Pt(111)-p(2×2)-O were investigated by supersonic molecular beam techniques at a surface temperature of 100 K. The initial trapping probability was measured in the range of incident energy from 10 to 45 kJ/mol and incident angles from 0° to 60°. A broad angular distribution of scattered ethane and total energy scaling (ET cos 0.2θ) for ethane trapping indicated a corrugated gas–surface potential. Stochastic trajectory simulations employing a potential developed from the trapping of ethane on Pt(111) gives quantitative agreement of the measured initial trapping probabilities over entire ranges of incident energies and angles. Calculations of energy transfer for ethane after the first bounce on Pt(111) and Pt(111)-p(2×2)-O clearly indicate that interconversion of parallel and perpendicular momentum and energy transfer to lattice vibrations account primarily for the differences in trapping probabilities between ethane on the two surfaces. At glancing incidence trapping is not significantly reduced on the oxygen-covered Pt(111) because the parallel momentum appears to be transferred partially to phonons. [ABSTRACT FROM AUTHOR]

Different complementary physicochemical tools have been used to explain the improvement of the catalytic performances for ethane oxidation into acetic acid induced by the addition of molybdenum as phosphate to vanadium phosphate deposited on TiO2-anatase, at a coverage below the monolayer. Electron microscopy techniques have shown that the elements are dispersed on the support. Electron spin resonance, laser Raman and UV-visible spectroscopies have evidenced that the short range order around vanadium is modified by the presence of molybdenum. 51V NMR has shown that molybdenum favours the octahedral symmetry of vanadium. The acidic properties of the catalyst are improved by the adding effect of molybdenum and by the addition of water. This should explain a better desorption of acetic acid and the improvement of the corresponding yield. This confirms the importance of the atomic environment of vanadium-based oxides to control the mild oxidation of light alkanes. [ABSTRACT FROM AUTHOR]

The relative reactivities of the lower alkanes in hydrogenolysis on a Pt/AlO catalyst depend on the H pressure used, as do those of a Ru/AlO catalyst, pretreated in various ways, for propane hydrogenolysis. Apparent activation energies also vary with H pressure. No single rate measurement adequately represents 'catalytic activity', which is properly defined as the rate constant for the slow step. [ABSTRACT FROM AUTHOR]