Selective Oxidation Of Propane Research Articles

Structure and properties of molybdenum oxide nitrides as model systems for selective oxidation catalysts

Molybdenum oxide nitride (denoted as Mo(O,N)3) was obtained by ammonolysis of α-MoO3 with gaseous ammonia. Electronic and geometric structure, reducibility, and conductivity of Mo(O,N)3 were investigated by XRD, XAS, UV-Vis spectroscopy, and impedance measurements. Catalytic performance in selective propene oxidation was determined by online mass spectrometry und gas chromatography. Upon incorporation of nitrogen, Mo(O,N)3 maintained the characteristic layer structure of α-MoO3. XRD analysis showed an increased structural disorder in the layers while nitrogen is removed from the lattice of Mo(O,N)3 at temperatures above ~600 K. Compared to regular α-MoO3, Mo(O,N)3 exhibited a higher electronic and ionic conductivity and an onset of reduction in propene at lower temperatures. Surprisingly, α-MoO3 and Mo(O,N)3 exhibited no detectable differences in onset temperatures of propene oxidation and catalytic selectivity or activity. Apparently, the increased reducibility, oxygen mobility, and conductivity of Mo(O,N)3 compared to α-MoO3 had no effect on the catalytic behavior of the two catalysts. The results presented confirm the suitability of molybdenum oxide nitrides as model systems for studying bulk contributions to selective oxidation.

On the Role of the Vanadium Distribution in MoVTeNbO x Mixed Oxides for the Selective Catalytic Oxidation of Propane

The selective oxidation of propane to acrylic acid is studied over a series of nearly pure M1-phase MoVTeNbO x catalysts. Quantitative analysis of the reaction network shows that the ratio of the rate constants for propane oxidative dehydrogenation to propene and for the further oxidation of propene is constant. The rates towards acrolein and acetone, however, vary subtly with the concentration of vanadium and the location of its substitution. The reaction of acrolein to acetic acid and carbon oxides, associated with accessible metal cations, contributes two-thirds towards the non-selective pathway. The other third is associated with acetone formation. Vanadium is first substituted selectively at sites that are inactive for propane activation. Depending on the selectivity of this substitution two groups of materials have been identified, which show a distinctly different dependence on the concentration of vanadium. Statistic distribution of vanadium in the M1 phase appears to be the most promising strategy to improve the performance of MoVTeNbO x catalysts for a given vanadium concentration.

Towards an Understanding of the Reaction Pathways in Propane Ammoxidation Based on the Distribution of Elements at the Active Centers of the M1 Phase of the MoV(Nb,Ta)TeO System

The M1 phase of the MoV(Nb,Ta)TeO system is one of the most effective catalysts for the ammoxidation and selective oxidation of propane to acrylonitrile (AN) and acrylic acid, respectively. The active centers of the M1 phase reside on the ab planes of this crystalline material (i.e., the (001) lattice face). Early on we proposed that the thus located active centers contain all key catalytic elements strategically placed for the conversion of propane to AN. These seven element comprising active centers contain: five metal oxide octahedra (2 V0.325+/Mo0.686+, 1 V0.624+/Mo0.386+, 2Mo0.56+/Mo0.55+) and two Te4+—oxygen sites. In this contribution we analyze the various compositional probabilities of the seven element active centers and their additional eight element surroundings and conclude that there are 32 possible compositional arrangements of this 15 element assembly. From the diverse structural arrangements, diverse catalytic properties can be assigned to the individual sites, leading to diverse propane reaction pathways. We conclude that there are 22% AN forming, 22% propylene, 10% waste and 46% inert sites. After normalization these sites are deemed to lead to the following product yields: 41% AN, 41% propylene and 18% waste. The highest experimentally attained AN yield from propane is 42%, employing M1 phase only, which coincides with the predicted value of a concerted mechanism. Higher AN yields are, however, anticipated, up to a lofty upper limit of 82%, by allowing also for a consecutive mechanism (C3° → C3=→AN). This possibility can be rationalized on the basis of the existence of vicinal C3° → C3=/C3=→AN sites whose presence is plentiful on the catalytically important ab planes of M1. The placement and efficiency of these sites is, however, not perfect; therefore the upper AN yield limit is not realized in practice. Our analysis of the elemental distribution at the active centers and their immediate surroundings provides us with new insights into the relationship between structure and catalytic reaction mechanisms of the M1 phase and might serve as a guide towards a redesign of the M1 composition, so as to attain higher AN yields from propane. It provides a challenging task for the synthetic chemist.

Structures, Mechanisms, and Kinetics of Ammoxidation and Selective Oxidation of Propane Over the M2 Phase of MoVNbTeO Catalysts

We report here first-principles-based predictions of the structures, mechanisms, and activation barriers for propane activation by the M2 phase of the MoVNbTeO multi-metal oxide catalysts capable of the direct conversion of propane to acrylonitrile. Our approach is to combine extensive quantum mechanical (QM) calculations to establish the mechanisms for idealized representations of the surfaces for these catalytic systems and then to modify the parameters in the ReaxFF reactive force field for molecular dynamics (MD) calculations to describe accurately the activation barriers and reaction mechanisms of the chemical reactions over complex mixed metal oxides. The parameters for ReaxFF are derived entirely from QM without the use of empirical data so that it can be applied to novel systems on which there is little or no data. To understand the catalysis in these systems it is essential to determine the surface structures that control the surface chemistry. High quality three-dimensional (3D) Rietveld structures are now available for the M1 and M2 phases of the MoVNbTeO catalysts. However the details of the chemical mechanisms controlling selectivity and activity have remained elusive because the catalytically important sites in these Rietveld structures are occupied by mixtures of Mo and V atoms, obscuring the actual distributions of the metals and oxides at the active sites. To solve this problem we use a supercell of the Rietveld structure sufficiently large that all atoms can be whole, then we use Monte Carlo techniques based on ReaxFF to resolve these partial occupations into the optimum configuration of whole atoms still consistent with the X-ray data. We will report the ReaxFF resolved 3D structures for the M2 phase of the MoVNbTeO system. Using the resolved 3D structures we consider the distribution of sites on the important surfaces and carry out ReaxFF Reactive Dynamics (RD) calculations to follow the initial steps of the reactions. Such studies provide insights into the chemical reaction steps on MMO catalysts that should be useful in designing more selective and more active systems. We find that this suggests the critical role of the Te^(IV) oxo chains for activating propene but not propane in the M2 phase. This suggests a new mechanism for this phase.

Reflux method as a novel route for the synthesis of MoVTeNbO x catalysts for selective oxidation of propane to acrylic acid

This work reports the applicability of reflux to the synthesis of Mo–V–Te–Nb–O metal oxide catalysts for the first time. The well-formed and defined orthorhombic M1 phase, i.e. Te 2 M 20O 57 ( M = Mo, V, Nb), was found to be better developed when a longer time was used to reflux the mixed metal clusters. The catalysts efficient for propane selective oxidation to acrylic acid, reaching ca. 60% of selectivity to acrylic acid at propane conversion of ca. 50% and at a reaction temperature of 693 K. In addition, a parallel between catalytic activity and catalyst reducibility is also concluded.

Influence of Graphite as a Shaping Agent of Bi Molybdate Powders on Their Mechanical, Physicochemical, and Catalytic Properties

The influence of using graphite (G) as a shaping agent for bismuth molybdate (BiMo) catalysts was analyzed. Shaping was done by tableting, with addition of different loadings of graphite (0, 0.5, 1, 3, 5, 7, and 10 wt %). The use of graphite during the pressing of bismuth molybdate powders eases tableting because of the lubricating properties of the former. Furthermore, the compressive strength of BiMo-G tablets is higher than that of pure BiMo. Concerning the physicochemical properties of BiMo-G, XRD and XPS showed that graphite changes neither the relative distribution of the crystallographic phases of bismuth molybdate nor the oxidation state of bismuth and molybdenum in the tableted powders. Consequently, the shaped BiMo-G catalysts displayed similar, or slightly better, performances in the selective oxidation of propylene to acrolein. TGA analysis of samples of BiMo-G confirmed the thermal stability of the catalysts under oxidative conditions. Graphite was observed to experience crystallization into the hexagonal 2H phase during the catalytic tests. The results reported herein demonstrate that graphite is an effective shaping agent for bismuth molybdate powders.

Lignocellulosic-derived catalysts for the selective oxidation of propane

Abstract The formation of dispersed VPO phases on a lignocellulosic-based activated carbon results in a catalyst that is selective for the partial oxidation of propane, and stable under oxidizing conditions. The use of a stable activated carbon as a catalytic support for active VOx species during the partial oxidation of propane is described for the first time.

Promoted activity of the selective catalytic reduction of NO x with propene by H 2 addition over a metal-monolithic anodic alumina-supported Ag catalyst

The effect of H 2 addition on the selective catalytic reduction of NO x with propene (C 3H 6-SCR of NO x ) was investigated using a metal-monolithic anodic alumina-supported Ag catalyst. The de-NO x activity of the Ag/Al 2O 3 catalyst at low temperature was markedly enhanced by the addition of a small amount of H 2; the enhancement was associated with the improved propene activation at low temperatures. The addition of H 2 greatly promoted the nitrate decomposition at low temperatures, which was believed to alleviate nitrate poisoning and to initiate the SCR reaction at low temperatures, because of the formation of free silver sites. Furthermore, it is suggested that a small amount of H 2 addition will further reduce the freed Ag + species into Ag n δ+ and will eventually lead to the improved de-NO x activity, because Ag n δ+ is more active for the propene activation than Ag + at low temperatures. However, excess H 2 and high Ag loading mainly promoted the nonselective combustion of propene with oxygen, rather than the selective oxidation of propene with NO x . Additionally, a combination of this H 2 promotion effect and electrical heating produced a favorable de-NO x system with a quick response, a wide temperature-window and a high de-NO x activity.

Selective oxidation of propylene to acrolein by silica-supported bismuth molybdate catalysts

Silica-supported bismuth molybdate catalysts have been prepared by impregnation, structurally characterized and examined as improved catalysts for the selective oxidation of propylene to acrolein. Catalysts with a wide range of loadings (from 10 to 90 wt%) of beta bismuth molybdate (β-Bi2Mo2O9) were studied to provide a better understanding about the distribution of active sites, and to elucidate the role of lattice oxygen in the reaction. The catalyst containing 50 wt% of beta bismuth molybdate on SiO2 was found to possess good distribution of active sites and sufficiently high lattice oxygen, which resulted in an extraordinary increase of the catalytic activity.

Catalytic Selective Oxidation of Propane to Acrylic Acid in a Fixed-Bed Reactor with an O2-Distributor

Propane selective oxidation to acrylic acid has been carried out by sequential oxidative dehydrogenation of propane and selective oxidation to acrylic acid of propylene steps in one reactor loaded with a two-catalyst bed. The propane ODH over a Ni-based oxide catalyst was active at relative low temperatures, which is near to those of the propylene SOA over a mixed metal oxides catalyst. In the combined catalytic reaction system, the propane conversion as well as the AA selectivity was correlative with the catalyst bed configurations. A novel approach to improving the acrylic acid selectivity has been presented. Based on the use of an O2-distributor, the reaction system of propane to acrylic acid could provide a ∼80% of acrylic acid selectivity and a ∼19% of propane conversion at 340 °C and C3H8/O2/N2 (mol %) = 11.0/20.9/68.1 of feed gases.

A Metal-Monolithic Anodic Alumina-Supported Ag Catalyst for Selective Catalytic Reduction of NOx with Propene in the Presence of Hydrogen

The effect of H2 addition on the selective catalytic reduction of NOx with propene (C3H6-SCR of NOx) has been investigated using a metal-monolithic anodic alumina-supported Ag catalyst. The low temperature de-NOx activity of the Ag/Al2O3 catalyst is markedly enhanced by the addition of a small amount of H2, which is associated with the improved propene activation at low temperatures. The addition of H2 greatly promotes the nitrate decomposition at low temperatures, which is believed to alleviate nitrate poisoning and initiate the SCR reaction at low temperatures, because of the formation of free silver sites. Furthermore, it is suggested that a small amount of H2 addition will further reduce the freed Ag+ species into Agnδ+ and eventually lead to the improved de-NOx activity, because Agnδ+ is more active in propene activation than Ag+ at low temperatures. However, excess H2 and high Ag loading mainly promoted the nonselective combustion of propene with oxygen, rather than the selective oxidation of propene with NOx.

Selective Oxidation Performance of Propane over Supported Vanadium Oxide Catalysts

A series of SBA-15 supported vanadium oxide catalysts with different active components were prepared by the method of incipient-wet impregnation. The structures of the catalysts were characterized by N2 adsorption, X-ray diffraction (XRD), ultraviolet (UV)-Raman, Fourier transform infrared (FTIR), and ultraviolet-visble diffuse reflectance spectroscopy (UV-Vis DRS) techniques, and their catalytic performances for the selective oxidation of propane were investigated. The results showed that SBA-15 was a better support in the catalyst system than SiO2 for the selective oxidation of propane to aldehydes. The SBA-15 supported low loading catalyst is a highly dispersed catalyst system and the SBA-15 supported vanadium oxide samples with low loading (n(V)/n(Si)<2.5%) have ordered hexagonal mesostructures. For the VOx/SBA-15 catalysts, isolated vanadyl species with tetrahedral coordination are the active sites for aldehyde formation at very low loadings of vanadium (n(V)/n(Si)<0.1%). The polymeric vanadyl species with octahedral coordination and microcrystalline vanadium oxide are active sites for the oxidative dehydrogenation or deep oxidation of propane when the loading of vanadium (n(V)/n(Si)) is higher than 2.5%.

A Combined Bond-Valence and Periodic DFT Study of the Active Sites in M1 Phase of MoVTeNbO Composite Oxide Catalyst

A combined bond-valence and periodic DFT study was carried out to interpret the nature of the d electrons in the active sites of the MoVTeNbO composite oxide catalyst (M1 phase). On investigating all the possible distribution of the coupled d electrons and oxygen vacancies, the most efficient combination of M-M-Te (M = Mo or V) active sites for propane selective oxidation has been argued. Besides, MoO6 octahedra primarily stabilizes the crystal structure of M1 phase by attaining strong covalent Mo–O bonding in the equatorial plane, while VO6 octahedra plays a minor role. Nb species that has been considered to stabilize the M1 phase was found to have mainly ionic inter-actions with the framework and act as a structure-directing agent in forming the pentagonal channels in the M1 phase.

Structural Investigations of Molybdenum Oxide Based Catalysts Supported on Nanostructured Magnesium Oxide

Molybdenum oxides are active in the heterogeneously catalyzed selective oxidation of light alkenes and alkanes. In order to investigate binary molybdenum oxides other than -MoO3 under relevant reaction conditions, we have chosen supported molybdenum oxides. In particular nanostructured oxide materials constitute suitable support systems for model oxide catalysts. Here, we present investigations of the structure and the catalytic performance of molybdenum oxide species supported on nanostructured MgO for the selective oxidation of propene.

Roles of Vanadium Substitution of Monoclinic TeMo&lt;SUB&gt;5&lt;/SUB&gt;O&lt;SUB&gt;16&lt;/SUB&gt; Catalyst for Propane Selective Oxidation

Roles of Vanadium Substitution of Monoclinic TeMo&lt;SUB&gt;5&lt;/SUB&gt;O&lt;SUB&gt;16&lt;/SUB&gt; Catalyst for Propane Selective Oxidation

Structure and properties of PVMo 11O 40 heteropolyoxomolybdate supported on silica SBA-15 as selective oxidation catalyst

Investigations into structure and properties of heteropolyoxomolybdates (HPOM) supported on nanostructured silica SBA-15 for the selective oxidation of propene are presented. H 4[PVMo 11O 40] Keggin ions supported on SBA-15 (PVMo 11-SBA-15) were prepared by incipient wetness of silica SBA-15 with suitable precursor solutions. Structure and properties of the resulting material were studied ex situ by X-ray diffraction and nitrogen physisorption measurements and in situ by combined X-ray absorption spectroscopy (XAS) and mass spectrometry under various reaction conditions. The characteristic structure of PVMo 11-SBA-15 and its evolution under reactive gas atmosphere are compared to that of bulk H 4[PVMo 11O 40]⋅ xH 2O. Structural investigations of as-prepared PVMo 11-SBA-15 have shown that the respective Keggin ions can be readily supported on silica SBA-15. In situ XAS measurements under reducing or oxidizing conditions revealed a pronounced support interaction effect. This effect resulted in a further decreased thermal stability of the supported Keggin ions compared to bulk H 4[PVMo 11O 40]⋅ xH 2O. Apparently, no stable HPOM Keggin ions could be obtained on silica SBA-15 under reaction conditions. However, in spite of their low thermal stability, HPOM supported on SBA-15 remain suitable well-defined precursors for preparing mixed oxide model catalysts for selective oxidation reactions.

Phase formation and selective oxidation of propane over MoVTeNbO x catalysts with varying compositions

The selective oxidation of propane to acrylic acid on MoVTeNbO x catalysts with varying concentrations of vanadium, tellurium and niobium was investigated. Catalysts containing M1 phase were obtained over the compositional range of MoV 0.14–0.22Te 0.1–0.2Nb 0.1–0.2O x . Vanadium containing sites in the M1 phase are drastically more active for propane activation than in other materials studied. The catalytic activity is directly correlated to its fraction in the overall material and in particular the M1 phase. High concentrations of tellurium induce the formation of the M2 phase decreasing so the overall activity of the catalysts. The intrinsic activity of the M1 phase is, however, independent of the tellurium concentration. Although the presence of the M1 phase is not a stringent requirement for the oxidative dehydrogenation of propane to propene, it is required to oxidize the intermediately formed propene with high selectivity to acrylic acid. The active sites for propane activation and propene oxidation are structurally coupled, because the ratio between the rates of the two reactions was always 1:25. Oxygen defect sites in mixed oxides seem to enhance interaction with acrylic acid and lead to decarboxylation and total oxidation.

KINETIC INVESTIGATION OF PROPANE DISAPPEARANCE AND PROPENE FORMATION IN PROPANE OXIDATION ON DILUTED AND LEACHED MoVTeNb CATALYST

Reaction kinetics for the oxidation of propane over diluted-leached MoVTeNb is described. This paper is focused on the study of products selectivity profile and determination of the orders of propane disappearance and propene formation. The result shows that selective oxidation of propane to propene over this catalyst follows the Langmuir-Hinshelwood mechanism. The disappearance of propane is first order with respect to hydrocarbon and partial order (0.21) with respect to oxygen. The propene formation is first order with respect to hydrocarbon and not depending on oxygen concentration. Keywords: propane oxidation, propane disappearance, propene formation, kinetic, reaction order

Opposite effect of Al on the performances of MoO 3/SiO 2-Al 2O 3 catalysts in the metathesis and in the partial oxidation of propene

The selective oxidation and the metathesis of light alkenes are two important catalytic reactions for the petrochemical industry. This paper highlights the contrasting effect of alumina on the catalytic behaviour of silica and silica-alumina supported molybdenum oxide catalysts in these two reactions. Model MoO 3/SiO 2-Al 2O 3 catalysts with ca. 6% MoO 3 weight loading are prepared via wet impregnation of ammonium heptamolybdate on a set of amorphous mesoporous silica-alumina supports with silica weight content comprised between 100% and 75%. The samples are characterized by ICP-AES, N 2-physisorption, XRD, NH 3-chemisorption and XPS and are evaluated in the selective oxidation of propene to acrolein and in its self-metathesis to form butene and ethene. The addition of aluminium oxide into silicon oxide increases the acidity of the support and of the catalyst. It also affects the nature of the deposited species and the dispersion of Mo. Overall, the effect of Al is negative in the case of the partial oxidation of propene because it favours over-oxidation towards carbon oxides. On the contrary, the presence of Al is crucial for the metathesis reaction. It appears that the acidity, created by the presence of alumina in silica, is beneficial for the metathesis reaction at low temperature. An optimum of activity is found for the catalyst supported on the silica-alumina containing 15 wt% of Al 2O 3.

The effect of V in La 2Ni 1− xV xO 4+1.5 x+ δ on selective oxidative dehydrogenation of propane: Stabilization of lattice oxygen

In this study, the non-stoichiometric redox compounds La 2NiO 4+ δ and La 2Ni 0.9V 0.1O 4.15+ δ have been tested as an oxidant in selective oxidation of propane in pulse experiments at temperatures between 450 and 650 °C. The oxygen contents in the samples at different temperatures were calculated based on back-titration with O 2 pulses as well as on TGA. In the case of La 2Ni 0.9V 0.1O 4.15+ δ over-stoichiometric oxygen was the only active species, responsible for the formation of CO 2, C 3H 6, C 2H 4 and CH 4 while pulsing propane at all investigated temperatures. Contrary, the reactive species in La 2NiO 4+ δ changed from over-stoichiometric oxygen only at 450 °C, to both over-stoichiometric and lattice oxygen at 550 °C, to lattice oxygen only at 650 °C. The activation of lattice oxygen and the consequent formation of Ni metal were detrimental for selectivity to propylene as formation of CO x , CH 4 and coke mainly occurred and no olefins were formed.