Selective Oxidation Of Light Alkanes Research Articles

Mechanistic Insights into Radical-Induced Selective Oxidation of Methane over Nonmetallic Boron Nitride Catalysts

Boron-based nonmetallic materials (such as B2O3 and BN) emerge as promising catalysts for selective oxidation of light alkanes by O2 to form value-added products, resulting from their unique advantage in suppressing CO2 formation. However, the site requirements and reaction mechanism of these boron-based catalysts are still in vigorous debate, especially for methane (the most stable and abundant alkane). Here, we show that hexagonal BN (h-BN) exhibits high selectivities to formaldehyde and CO in catalyzing aerobic oxidation of methane, similar to Al2O3-supported B2O3 catalysts, while h-BN requires an extra induction period to reach a steady state. According to various structural characterizations, we find that active boron oxide species are gradually formed in situ on the surface of h-BN, which accounts for the observed induction period. Unexpectedly, kinetic studies on the effects of void space, catalyst loading, and methane conversion all indicate that h-BN merely acts as a radical generator to induce gas-phase radical reactions of methane oxidation, in contrast to the predominant surface reactions on B2O3/Al2O3 catalysts. Consequently, a revised kinetic model is developed to accurately describe the gas-phase radical feature of methane oxidation over h-BN. With the aid of in situ synchrotron vacuum ultraviolet photoionization mass spectroscopy, the methyl radical (CH3•) is further verified as the primary reactive species that triggers the gas-phase methane oxidation network. Theoretical calculations elucidate that the moderate H-abstraction ability of predominant CH3• and CH3OO• radicals renders an easier control of the methane oxidation selectivity compared to other oxygen-containing radicals generally proposed for such processes, bringing deeper understanding of the excellent anti-overoxidation ability of boron-based catalysts.

Low Temperature Oxidation of Ethane to Oxygenates by Oxygen over Iridium-Cluster Catalysts.

Direct selective oxidation of light alkanes, such as ethane, into value-added chemical products under mild reaction conditions remains a challenge in both industry and academia. Herein, the iridium cluster and atomically dispersed iridium catalysts have been successfully fabricated using nanodiamond as support. The obtained iridium cluster catalyst shows remarkable performance for selective oxidation of ethane under oxygen at 100 °C, with an initial activity as high as 7.5 mol/mol/h and a selectivity to acetic acid higher than 70% after five in situ recycles. The presence of CO in the reaction feed is pivotal for the excellent reaction performance. On the basis of X-ray photoelectron spectroscopy (XPS) analysis, the critical role of CO was revealed, which is to maintain the metallic state of reactive Ir species during the oxidation cycles.

Synthesis and performance of vanadium-based catalysts for the selective oxidation of light alkanes

Vanadium compounds have attracted much attention because they have widely been used for homogeneous and heterogeneous catalysis field, especially in the selective oxidation of light alkanes, where vanadium-based catalysts were announced to be one of the most efficient catalysts. The present mini-review analyzed the recent developments for the preparation using new synthesis approaches of vanadium based catalysts, and their catalytic performances in the selective oxidation reactions of light alkanes. The influences of several synthesis strategies on the catalytic performances were illustrated in detail, while the samples characterizations were briefly presented.

Redox State Dynamics at the Surface of MoVTe(Sb)NbO M1 Phase in Selective Oxidation of Light Alkanes

The influence of Te or Sb on the catalytic properties of the M1 phase of MoVTe(Sb)NbO catalysts in the oxidation of propane or oxidative dehydrogenation of ethane has been investigated. The results show that Te and Sb affect the intrinsic activity of the catalyst for ethane and propane oxidation in the same way and that the Te-containing M1 phase was significantly more active than the Sb-containing phase, even when they contained the same amount of vanadium, which is known to be the active species for the reactions. This feature has been explained by a strong modification of the relative number of available V5+ sites, which is related to the fact that Sb tend to stabilize vanadium in active site, in a reduced state. Sb and Te were also shown to affect the selectivity in the case of propane oxidation but not in the case of ethane oxidative dehydrogenation. This demonstrated that they should be involved directly in the further transformation of the alkene molecules that are formed as an intermediate and presumably play a role in the α-hydrogen abstraction from these molecules, as proposed earlier. Blocking problems related to the substitution of Te by Sb in view of industrialization are also discussed.

Innovative process for the synthesis of vanadyl pyrophosphate as a highly selective catalyst for n-butane oxidation

Vanadyl pyrophosphate (VPO) catalysts are used for the selective oxidation of light alkanes, which are based on vanadyl hydrogen phosphate hemihydrate (VOHPO 4·0.5H 2O) as the precursor. Catalyst precursor with well-defined crystal size has been successfully synthesized for the first time, using a simple one-step high-pressure autoclave that was surfactant-free and water-free method with significantly lower temperature and shorter reaction time. VOHPO 4·0.5H 2O was prepared from V 2O 5 using an isobutanol, 1-pentanol, 1-heptanol and 1-dectanol as both solvent and reducing agent at elevated temperatures (100, 120, and 150 °C). This new method significantly reduced the preparation time and lowered production temperature (50%) of catalyst precursor (VOHPO 4·0.5H 2O) when compared to conventional hydrothermal synthesis methods. VOHPO 4·0.5H 2O can be obtained at temperature far below 150 °C. It was found that the presence of 1-heptanol and 1-decanol in the reaction mixtures is crucial for obtaining a well-defined crystal size of precursor phase and solely generated impurity [VO(H 2PO 4) 2]. Our findings show that both the phase composition and morphology of vanadium phosphate can be influenced by the use of different reducing agents and temperatures during the preparation process. This new methodology produces catalysts with a much higher surface area ( ca. 23 m 2 g −1) compared with those materials prepared by slow hydrothermal synthesis ( ca. 9.5 m 2 g −1). Finally, the yield of maleic anhydride was significantly increased from 21.3% for conventional catalyst to 37.9% for the new solvothermal catalyst.

Relationship between the Structures of Metal Oxide Catalysts and Their Properties in Selective Oxidation of Light Alkanes

It is of great importance to study the selective oxidation of light alkanes on metal oxide catalysts from a fundamental and industrial point of view.Clarification of the relationship between the structure of the oxide catalysts and their catalytic properties is vital for the design of efficient catalysts and for the realization of efficient oxidative conversion of light alkanes.In this review,we summarize some recent progress in the understanding of the structures and catalytic properties of several catalysts including V-P-O,V-Sb-O,Mo-V-Te-Nb-O,and V-Mg-O metal oxides.These oxides as well as the Re-Sb-O catalysts developed by us have been extensively studied for the selective oxidation of light alkanes.We then revisit several key aspects including multi-functionality,site isolation,and phase cooperation.These aspects are required for catalysts that are efficient in the selective oxidation of light alkanes and will provide insights into the design and synthesis of metal oxide catalysts and their efficient application to the oxidative conversion of light alkanes.

Innovative membrane based process for the selective oxidation of light alkanes assisted by the Fenton system

AbstractSelective oxidation of light alkanes under mild conditions (temp, 80–120 °C; pressure, 140 kPa) mediated by Fe2+/H2O2 Fenton system on Nafion based composite membranes is studied in a multifunctional three phase catalytic membrane reactor (3PCMR). The role of textural properties and acid functionality on the catalytic performance of the Nafion composite membranes is outlined. The features of the multifunctional 3PCMR, enabling reaction along with product separation and recovery, are discussed. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.

DEVELOPING SELECTIVE OXIDATION CATALYSTS OF LIGHT ALKANES: FROM FUNDAMENTAL UNDERSTANDING TO RATIONAL DESIGN

Selective oxidation of light alkanes remains to be a great challenge for the wider use of alkanes as feedstocks. To achieve high activity and at the same time high selectivity, some key issues have to be addressed: (1) the stability of the desired products with respect to the reactants; (2) the roles of the active components in the catalysts, the structure and the functionality of the active centers; (3) the reducibility of the metal cations, the Lewis acid sites and their synergic effects with the basic sites of the lattice oxygen anions; (4) spatial isolation of the active centers; and (5) the mechanisms for the formation and transformation of the intermediates and their kinetic controls. In this contribution, we took selective oxidation of propane to acrolein as our target reaction, and reviewed mainly our own work, trying to provide some thinking and answers to these five questions.

Novell Ternary and Polynary Vanadium(IV) Phosphates as Catalysts for Selective Oxidations of Light Hydrocarbons

AbstractTernary and polynary phosphates of transition metals offer various chemical and structural properties that make them interesting as catalyst materials for the selective oxidation of light alkanes. Redox potentials, oxygen decomposition pressures, acid‐base behavior, and structural chemistry of anhydrous transition metal phosphates may be varied over a wide range. Thus, for selective oxidation reactions of various light hydrocarbons Cn (1 ≤ n ≤ 4) suitable ternary or polynary vanadium phosphates were found. In this contribution, our recent results concerning synthesis, crystallographic characterization and catalytic properties of novell ternary and polynary vanadium(IV) phosphates [(MIIIVIV3O3(PO4)3 and MIII2(VIVO) (P2O7)2 (MIII: V, Cr, Fe), MIIVIV2O2(PO4)2 (MII: Co, Ni, Cu), and FeIIFeIII(VIVO)(PO4)(P2O7)] in the selective oxidation of n‐butane and 1‐butene to maleic acid anhydride are summarized.

Development and Application of Oxygen Permeable Membrane in Selective Oxidation of Light Alkanes

In this paper, oxygen permeable membrane used in membrane reactor for selective oxidation of alkanes will be discussed in detail. The recent developments for the membrane materials will be presented, and the strategy for the selection of the membrane materials will be outlined. The main applications of oxygen permeable membrane in selective oxidation of light alkanes will be summarized, which includes partial oxidation of methane (POM) to syngas and partial oxidation of heptane (POH) to produce H2, oxidative coupling of methane (OCM) to C2, oxidative dehydrogenation of ethane (ODE) to ethylene and oxidative dehydrogenation of propane (ODP) to propylene. Achievements for the membrane material developments and selective oxidation of light alkanes in membrane reactor in our group are highlighted.

Novel approaches for the improvement of selectivity in the oxidative activation of light alkanes

Selectivity is the key parameter for the practical application of oxidative activation of light alkanes, improving energy and raw materials utilisation efficiency and reducing CO 2 formation and emission. The intrinsic process complexity and the catalyst multifunctionality imply the need of a close control of many parameters (active centres nature, reactant composition, reaction mechanism, etc.) to improve the selectivity. Novel approaches to get this goal along three complementary directions, say: oxide nanocatalysts preparation by non-conventional routes (to tune the nature of the active centre), oxidant selection (to avoid overoxidation), and catalyst arrangement (to take advantage of the reaction mechanistic features), are presented and discussed by means of representative examples of their application. These include ODH of propane on nanosized molybdates, ODH of ethane with CO 2 on ceria- and MCM-41-based catalysts, and selective oxidation of light alkanes to unsaturated oxygenates over transition metal substituted MCM-41 catalysts.

Novel Selective Oxidation of Light Alkanes Using Carbon Dioxide. Oxidized Diamond as a Novel Catalytic Medium.

Carbon dioxide is an effective oxidant for the conversion of alkanes to chemical feedstocks. Oxidized diamond is useful as a novel catalytic support material. Oxidized diamond provided catalytic sites for the activation of alkanes under CO2 atmosphere. The present findings also suggest that the surface-properties of oxidized diamond may facilitate unique catalytic reactions, such as alkanes conversion in the presence of CO2.

Novel Selective Oxidation of Light Alkanes Using Carbon Dioxide. Oxidized Diamond as a Novel Catalytic Medium

Abstract Carbon dioxide is an effective oxidant for the conversion of alkanes to chemical feedstocks. Oxidized diamond is useful as a novel catalytic support material. Oxidized diamond provided catalytic sites for the activation of alkanes under CO2 atmosphere. The present findings also suggest that the surface-properties of oxidized diamond may facilitate unique catalytic reactions, such as alkanes conversion in the presence of CO2.

Synthesis of Molybdenum and Vanadium‐Based Mixed Oxide Catalysts with Metastable Structure: Easy Access to the MoVNbTe(Sb)Ox Catalytically Active Structure Using Reductant and Oxoacid.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis of Molybdenum and Vanadium-Based Mixed Oxide Catalysts with Metastable Structure: Easy Access to the MoVNbTe(Sb)Ox Catalytically Active Structure Using Reductant and Oxoacid

This work presents a simple synthesis route for MoVNbTe(Sb)Ox with the metastable structure that is catalytically active for the selective oxidation of light alkanes. The addition of an oxoacid to the partially reduced raw material mixture assisted in the formation of the active structure around 600 °C in an O2-excluded atmosphere. This simple synthesis using a reductant and oxoacid is remarkable since it allows a combinatorial approach to diversify Mo- and V-based mixed oxides such as catalytic materials.

A substrate-versatile catalyst for the selective oxidation of light alkanes: II. Catalyst characterization

A highly active and selective catalyst for light alkane oxidation that is composed of a pyridine salt of niobium-exchanged molybdo(vanado)phosphoric acid (NbPMo 11(V)pyr) is characterized using TGA–DSC, 31P MAS NMR, and in situ powder XRD, XAS, and XPS. The presence of both niobium and pyridinium species strongly influences structural and redox properties of the polyoxometalate. Activation of the catalyst by heating to 420 °C in an inert atmosphere removes all of the organic species present in the solid, and structural rearrangement of the starting heteropolyanion occurs at 420 °C as evidenced by 31P NMR and EXAFS. XRD shows that activated NbPMo 11Vpyr consisted of a mostly amorphous molybdenum oxide phase, the formation of which is strongly related to the composition of the catalyst. The presence of niobium as an exchange cation (NbO) 3+ or a framework atom PMo 11NbO 40 4− in the Keggin unit is verified by EXAFS for NbPMo 11Vpyr and (VO)PMo 11Nbpyr, respectively. During activation of either catalyst, niobyl species migrate and most likely coordinate to molybdenum oxide octahedra. Comparison of near-edge electronic spectra (XANES) for as-made NbPMo 11Vpyr and after activation that removes the pyridinium ions suggests reduction of Mo 6+ to Mo 5+ and Nb 5+ to Nb 4+. Under hydrocarbon-rich reaction mixtures molybdenum and niobium remain in their reduced state.

A substrate-versatile catalyst for the selective oxidation of light alkanes: I. Reactivity

The reactivity of niobium and pyridine-exchanged molybdophosphoric (NbPMo 12pyr) and molybdovanadophosphoric (NbPMo 11Vpyr) acid catalysts are investigated for the selective oxidation of light alkanes. Productivities and selectivities are presented for the reactions of propane to acrylic acid and n-butane to maleic acid. Pretreatment of the material to 420 °C in an inert atmosphere is required to form an active catalyst, and the combination of both niobium and pyridine is necessary for obtaining maximum activity. Vanadium is not essential for the selective formation of maleic acid from butane, but does appear to affect the selective formation of acrylic acid from propane. Other metal cations can be used instead of niobium; however, none are as effective as niobium. Comparison of activity results to those from other molybdate-based structures indicates that high activity requires the use of a reducible molybdenum species. In addition, the central phosphorus heteroatom of the polyoxometalate is important for high activity. Structures without phosphorus or with other heteroatoms have lower activity. The catalysts are effective under both hydrocarbon-rich and oxygen-rich conditions. For butane, the productivity to maleic acid exceeds that of the current industrial standard vanadium phosphorus oxide. With propane, the productivity to acrylic acid is above that of the MoVNbTe mixed-metal oxide. In addition to efficiently catalyzing the selective oxidation of propane and butane, the catalysts are active with feeds containing other substrates such as ethane, isobutane, and toluene.

Selective oxidation of light alkanes to aldehydes over silica catalysts supporting mononuclear active sites — acrolein formation from ethane

The oxidation of light alkanes, particularly ethane, was examined over Cs–M/SiO 2 catalysts (M=Fe, V, Bi). Turn-over-frequency (TOF) and product selectivities were strongly dependent on the loading amounts of M. When the loading amounts of M were less than 0.1 at.% to Si, very high TOF and relatively high selectivities to acetaldehyde and acrolein were observed. The isolated site or mononuclear structure of Fe, V or Bi on the Cs/silica surface is vital for the high aldehyde yields. Cesium was also important for the high reactivity of ethane and the high aldehyde selectivities. The reaction pathways giving acrolein from ethane and the roles of catalytic components are discussed.

Effects of additives on catalytic performance of heteropoly compounds for selective oxidation of light alkanes

Effects of the addition of various transition metals to heteropoly compounds on the catalytic performance for oxidation of light alkanes were reviewed. The catalytic selective oxidation of isobutane, propane, and ethane proceeded and the performance was variously changed by the additives. The addition of iron effectively enhanced the catalytic performance under both oxygen-rich and -poor conditions, while copper was found to most enhance the catalytic performance under oxygen-poor conditions. It is suggested that the enhancement by the iron addition under oxygen-rich conditions is due to the promotion of reduction of catalyst and that the enhancement by the copper and iron addition under oxygen-poor conditions is due to the promotion of the reoxidation of the catalyst.

On the potential of the multifunctional three phase catalytic membrane reactor in the selective oxidation of light alkanes by Fe 2+–H 2O 2 Fenton system

Selective oxidation of light alkanes under mild conditions ( T R, 80–120°C; P R, 140 kPa) is attained in a multifunctional three phase catalytic membrane reactor (3PCMR). The functionalisation of C 1–C 3 alkanes to the corresponding oxygenates on superacid catalytic membranes mediated by Fe 2+/H 2O 2 Fenton system is achieved. Among the various superacid catalytic agents, Nafion-based ones revealed the best performance. The features of the 3PCMR in enabling simultaneous reaction and product separation are presented. The influence of the operating conditions ( T R, [H 2O 2], [Fe 2+] or [Fe 3+]) on the catalyst activity is outlined. A general reaction pathway accounting for the activation of the C–H bond of the alkane molecule on the superacid sites and the subsequent reaction of the activated alkane with primary reactive intermediates, generated from the Fe 2+/H 2O 2 system, is proposed.