Catalysts For Ammoxidation Research Articles

Characterization of Ca−Bi−Mo Oxide Catalyst for Selective Propane Ammoxidation, Using XRD, XPS, TPRX/TPRO, and IR/Raman

The physicochemical properties of highly active and selective Ca−Bi−Mo oxide for ammoxidation of propane to acrylonitrile were investigated by XRD (powder), IR/Raman, XPS, and TPRX/TPRO (temperature programmed reaction/temperature programmed reoxidation) techniques. It was found that the phases in the Ca−Bi−Mo oxide varied with the composition of the oxide. The modified γ-bismuth molybdate (γ-Bi2MoO6) and defective Ca−Bi−Mo phases produced by addition of Bi oxide played an important role in propane ammoxidation to acrylonitrile. XPS results revealed that the mole ratio of elements on the surface was different from that in the bulk. The concentration of bismuth decreased from the surface to the inner layer of CaxBi12-xMo12 oxide, while the concentration of calcium increased from the surface to the inner layer. The Ca−Bi−Mo oxide showed good catalytic performance when an appropriate amount of calcium oxide was present on the surface along with Bi and Mo oxides. The addition of Ca oxide into Bi−Mo oxide decreased the number of active sites for complete oxidation and increased the ability of reoxidation (O2 consumption) of reduced catalysts.

Active centers in Mo–V–Nb–Te–Ox (amm)oxidation catalysts

The catalytically active centers of the MoVNbTeOx system for propane ammoxidation to acrylonitrile have been identified. The catalytic system is comprised of three crystalline phases: orthorhombic Mo7.8V1.2NbTe0.94O28.9 (M1) (Pba2: a=21.1337Å; b=26.6440Å; c=4.01415Å; z=4), pseudo-hexagonal Mo4.67V1.33Te1.82O19.82 (M2) (Pmm2: a=12.6294Å; b=7.29156Å; c=4.02010Å; z=4) and a trace of monoclinic TeMo5O16 (P21/C: a=10.0349Å; b=14.430Å; c=8.1599Å; β=90.781°; z=1). The catalytically active and selective centers reside on the surface of the basal plane of the M1 phase and are comprised of an assembly of five metal oxide octahedra (2V0.325+/Mo0.686+, 1V0.624+/Mo0.385+, 2Mo0.56+/Mo0.55+) and two tellurium–oxygen sites (2Te0.944+), which are stabilized and structurally isolated from each other (site isolation) by four Nb5+ centers, each surrounded by five molybdenum–oxygen octahedra. The V5+ surface sites, distinguished through their (V5+O↔4+VO) resonance structure, are the paraffin activating sites capable of methylene-H abstraction; the Te4+ sites (lone pair of electrons) for the α-H abstraction of the chemisorbed propylene molecule, once formed; and the adjacent Mo6+ sites for the NH insertion into the π-allylic surface intermediate. Herewith, all key catalytic elements needed to transform propane directly to acrylonitrile are contained, strategically arranged and within bonding distance of each other, at the active center of the M1 phase.Based on the metal site distribution probabilities at the active center 44% of them are computed to be active and selective for propane ammoxidation, 46% inactive for propane but active for propylene ammoxidation and 10% are waste forming sites. A maximum theoretical acrylonitrile selectivity of 81% is predicted on this premise (not achieved as yet experimentally). Under mild operating conditions, the M1 phase alone suffices to effectively convert propane directly to acrylonitrile. Under demanding conditions symbiosis between the M1 and M2 phases occurs, with the latter serving as a co-catalyst or mop-up phase to the former, transforming unconverted, desorbed propylene to acrylonitrile. The M2 phase is incapable of propane activation, lacking V5+ sites, but is a good propylene ammoxidation catalyst. A maximum acrylonitrile yield from propane of 61.8% (86% conversion, 72% selectivity at 420° C) was achieved with a nominal catalyst composition of Mo0.6V0.187Te0.14Nb0.085Ox, identified by combinatorial methodology, and is comprised of 60% M1, 40% M2 and a trace of TeMo5O16.

Synthesis, characterization and evaluation as catalysts for propane ammoxidation of VMoSbO systems with rutile-type structure

Monophasic, rutile-type Mo/V/Sb mixed oxides were synthesized with the co-precipitation procedure; Mo6+ replaced for V4+ in the lattice of V antimonate, and the (Mo+V)/Sb ratio remained approximately constant. The addition of Mo increased the catalytic activity in propane ammoxidation, but the catalysts became less selective to acrylonitrile, due to the increased contribution of combustion, and more active in ammonia oxidation to molecular nitrogen. This was attributed to an enhanced Lewis acidity, which increased the interaction of the catalyst with ammonia and favored its unselective transformation.

Active centers in Mo?V?Nb?Te?Ox (amm)oxidation catalysts

The catalytically active centers of the MoVNbTeO x system for propane ammoxidation to acrylonitrile have been identified. The catalytic system is comprised of three crystalline phases: orthorhombic Mo 7.8 V 1.2 NbTe 0.94 O 28.9 (M1) ( Pba 2: a =21.1337 Å; b =26.6440 Å; c =4.01415 Å; z =4), pseudo-hexagonal Mo 4.67 V 1.33 Te 1.82 O 19.82 (M2) ( Pmm 2: a =12.6294 Å; b =7.29156 Å; c =4.02010 Å; z =4) and a trace of monoclinic TeMo 5 O 16 ( P 2 1/C : a =10.0349 Å; b =14.430 Å; c =8.1599 Å; β =90.781°; z =1). The catalytically active and selective centers reside on the surface of the basal plane of the M1 phase and are comprised of an assembly of five metal oxide octahedra (2V 0.32 5+ /Mo 0.68 6+ , 1V 0.62 4+ /Mo 0.38 5+ , 2Mo 0.5 6+ /Mo 0.5 5+ ) and two tellurium–oxygen sites (2Te 0.94 4+ ), which are stabilized and structurally isolated from each other (site isolation) by four Nb 5+ centers, each surrounded by five molybdenum–oxygen octahedra. The V 5+ surface sites, distinguished through their ( V 5+  O ↔ 4+ V  O ) resonance structure, are the paraffin activating sites capable of methylene-H abstraction; the Te 4+ sites (lone pair of electrons) for the α-H abstraction of the chemisorbed propylene molecule, once formed; and the adjacent Mo 6+ sites for the NH insertion into the π-allylic surface intermediate. Herewith, all key catalytic elements needed to transform propane directly to acrylonitrile are contained, strategically arranged and within bonding distance of each other, at the active center of the M1 phase. Based on the metal site distribution probabilities at the active center 44% of them are computed to be active and selective for propane ammoxidation, 46% inactive for propane but active for propylene ammoxidation and 10% are waste forming sites. A maximum theoretical acrylonitrile selectivity of 81% is predicted on this premise (not achieved as yet experimentally). Under mild operating conditions, the M1 phase alone suffices to effectively convert propane directly to acrylonitrile. Under demanding conditions symbiosis between the M1 and M2 phases occurs, with the latter serving as a co-catalyst or mop-up phase to the former, transforming unconverted, desorbed propylene to acrylonitrile. The M2 phase is incapable of propane activation, lacking V 5+ sites, but is a good propylene ammoxidation catalyst. A maximum acrylonitrile yield from propane of 61.8% (86% conversion, 72% selectivity at 420 ° C) was achieved with a nominal catalyst composition of Mo 0.6 V 0.187 Te 0.14 Nb 0.085 O x , identified by combinatorial methodology, and is comprised of 60% M1, 40% M2 and a trace of TeMo 5 O 16 .

Structural aspects of the M1 and M2 phases in MoVNbTeO propane ammoxidation catalysts

Abstract The structures of M1 and M2 in MoVNbTeO propane ammoxidation catalysts have been solved using a combination of TEM, neutron powder diffraction, and synchrotron X-ray powder diffraction. The unit cell of M1 is Pba2 (No. 32) with a = 21.134(2) Å, b = 26.658(2) Å, c = 4.0146(3) Å and Z = 4. The formula unit is Mo7.8V1.2NbTe0.937O28.9. The unit cell of M2 is Pmm2 (No. 25) with a = 12.6294(6) Å, b = 7.29156(30) Å, c = 4.02010(7) Å and Z = 4. The formula unit is Mo4.31V1.36Te1.81Nb0.33O19.81. Tellurium sites in hexagonal channels of both phases are displaced toward vanadium-occupied framework sites, whereas Te in the heptagonal channel of M1 is near the channel center. The chemical topology resulting from oxidation states and Madelung site potentials presents active moieties for the ammoxidation of propane in M1 and propene in M2. EPR confirmed the presence of V4+ and possibly Mo5+ in M1 and V4+ in M2.

Catalytic properties of mixed Mo-V-Sb-Nb-O oxides catalysts for the ammoxidation of propane to acrylonitrile

The bulk structure and catalytic behavior of bulk mixed Mo-V-Sb-Nb-O catalysts in propane ammoxidation were investigated. These mixed oxides were prepared via a redox reaction between V 5+ and Sb 3+ in the presence of Mo 6+ and Nb 5+. The catalytic performance of these model catalysts exhibited a strong dependence on the phases present. The catalysts showing significant amounts of α-MoO 3 phase exhibit low selectivity/yield in propane ammoxidation to acrylonitrile, while the presence of SbVO 4 phase resulted in high activity and selectivity to acrylonitrile. It was demonstrated that the SbVO 4 is the active phase in the formation of acrylonitrile while it is inefficient for propane oxidation to acrylic acid.

Influence of the reaction conditions on the activity properties of vanadium–aluminium oxynitride propane ammoxidation catalyst

The influence of different reaction conditions on the catalytic activity of vanadium–aluminium oxynitride “VAlON” in propane ammoxidation is investigated. This new catalytic material exhibits amorphous character, high specific surface area, basic-redox properties and remarkable acrylonitrile production per hour and amount of catalyst. Optimal performance is achieved when the reaction temperature is 500 °C and the molar ratio of propane, oxygen, and ammonia in the gas feed is 1.25:3:1. Under these optimal reaction conditions, the catalyst showed a propane conversion level of 60%, an acrylonitrile selectivity of 56%, and an acrylonitrile productivity of 812 l/kg h. The formation of a propylene intermediate compound as well as hydrogen cyanide was detected only for those reaction conditions where the NH 3:C 3H 8 ratio was lower than 1. This suggests differences in the reaction mechanism with respect to conventional V–Me-oxide propane ammoxidation catalysts. In propane ammoxidation over VAlON catalysts, ammonia seems to play a double role: (i) as a reagent necessary for the thermal nitridation and generation of nitrogen species, (ii) participating in N-insertion step for the acrylonitrile formation from propane. The catalytic performances depend on the generation of nitrogen species as a function of the reaction conditions, which determine the level of propane conversion and selectivity to the reaction products.

Propane ammoxidation catalyst based on vanadium‐aluminum oxynitride

In this communication, we show the competitive advantage of vanadium aluminum oxynitride catalysts (VAlON) in acrylonitrile production, as compared with propane ammoxidation systems reported in the patent literature.

Cr/V/Sb mixed oxides, catalysts for the ammoxidation of propane to acrylonitrile: Part II. Effect of catalyst composition on catalytic performance

Rutile-type, Cr/V/Sb mixed oxides having different atomic ratio between components were studied as catalysts for propane and propylene ammoxidation to acrylonitrile. Catalysts were more active than Cr/Sb and V/Sb mixed oxides; this was attributed to (i) the higher specific surface area of Cr/Sb/O and Cr/V/Sb/O with respect to V/Sb/O, and (ii) the preferential formation of V 4+ in Cr/V/Sb/O. The nature of the V species and the catalytic performance of Cr/V/Sb/O samples were functions of the (Cr+V)/Sb atomic ratio. When the latter was higher than ≈1, the prevailing species was V 4+; the catalysts were very active but poorly selective to acrylonitrile (selectivity lower than 20%) because of the prevailing formation of carbon oxides and propylene. This was due to the absence of sites able to transform the unsaturated intermediate to acrylonitrile. When the (Cr+V)/Sb ratio was between ≈1 and ≈0.5, catalysts reached a selectivity to acrylonitrile between 20 and 30%, and to propylene lower than 10%. In these samples, the presence of intra-crystalline Sb gradients in the rutile lattice provided a Sb surface enrichment and the development of allylic ammoxidation sites, able to transform the unsaturated intermediate to acrylonitrile. When the (Cr+V)/Sb ratio was lower than ≈0.5, i.e. in systems having excess Sb, the prevailing species was V 3+; the selectivity to acrylonitrile was higher than 30%, with low formation of carbon oxides and of propylene. In this case additional sites for allylic ammoxidation were provided by excess antimony oxide dispersed over the rutile surface.

Multifunctionality of Active Centers in (Amm)oxidation Catalysts: From Bi–Mo–Ox to Mo–V–Nb–(Te, Sb)–Ox

Catalytic centers in selective (allylic) oxidation and ammoxidation catalysts are multimetallic and multifunctional. In the historically important bismuth molybdates, used for propylene (amm)oxidation, they are composed of (Bi3+)(Mo6+)2 complexes in which the Bi3+ site is associated with the α-H abstraction and the (Mo6+)2 site with the propylene chemisorption and O or NH insertion. An updated reaction mechanism is presented. In the Mo–V–Nb–Te–Ox systems, three crystalline phases (orthorhombic Mo7.5V1.5NbTeO29, pseudohexagonal Mo6Te2VO20, and monoclinic TeMo5O16) were identified, with the orthorhombic phase being the most important one for propane (amm)oxidation. Its active centers contain all necessary key catalytic elements (2V5+/Mo6+, 1V4+/Mo5+, 2Mo6+/Mo5+, 2Te4+) for this reaction wherein a V5+ surface site (V5+ = O ↔ 4+V•–O•) is associated with paraffin activation, a Te4+ site with α-H abstraction once the olefin has formed, and a (Mo6+)2 site with the NH insertion. Four Nb5+ centers, each surrounded by five molybdenum octahedra, stabilize and structurally isolate the catalytically active centers from each other (site isolation), thereby leading to high selectivity of the desired acrylonitrile product. A detailed reaction mechanism of propane ammoxidation to acrylonitrile is proposed. Combinatorial methodology identified the nominal composition Mo0.6V0.187Te0.14Nb0.085Ox for maximum acrylonitrile yield from propane, 61.8% (86% conversion, 72% selectivity at 420 °C). We propose that this system, composed of 60% Mo7.5V1.5NbTeO29, 40% Mo6Te2VO20, and trace TeMo5O16, functions with a combination of compositional pinning of the optimum orthorhombic Mo7.5V1.5±xNb1±yTe1±zO29±δ phase and symbiotic mop-up of olefin intermediates through phase cooperation. Under mild reaction conditions, a single optimum orthorhombic composition might suffice as the catalyst; under demanding conditions this symbiosis is additionally required. Improvements in catalyst performance could be attained by further optimization of the elemental distributions at the active catalytic center of Mo7.5V1.5NbTeO29, by promoter/modifier substitutions, and incorporation of compatible cocatalytic phases (preferably epitaxially matched). High-throughput methods will greatly accelerate the rational catalyst design processes.

Combinatorial Heterogeneous Catalysis: Oxidative Dehydrogenation of Ethane to Ethylene, Selective Oxidation of Ethane to Acetic Acid, and Selective Ammoxidation of Propane to Acrylonitrile

The application of combinatorial methods to three reactions catalyzed by multimetal oxides is described. Catalysts for the oxidative dehydrogenation of ethane to ethylene were tested using a 121- or 144-channel scanning mass spectrometer primary screening reactor and a 48-channel fixed bed secondary screening reactor; catalysts for the selective oxidation of ethane to acetic acid were tested using a 256-channel massively parallel microfluidic reactor primary screen alone, and catalysts for the selective ammoxidation of propane to acrylonitrile were tested using the massively parallel microfluidic reactor and an eight-channel fixed bed secondary/tertiary screening reactor. The details regarding catalyst design, synthesis, and screening are presented. This work has resulted in both the confirmation of published results and the generation of new lead materials for all three chemistries.

Niobia-supported Sb–V–O catalysts for propane ammoxidation: effect of catalyst composition on the selectivity to acrylonitrile

The effect of the Sb/V atomic ratio and of the total Sb + V coverage for the ammoxidation of propane to acrylonitrile on niobia-supported V–Sb oxide catalysts is reported. In all cases, acrylonitrile is produced along with propylene and CO2. Depending on the Sb + V coverage two different activity behaviours are reported: below the Sb + V dispersion limit on niobia, the system is predominantly selective to acrylonitrile, this fact has been attributed to exposed Nb centres and to V–Nb–O mixed oxide phases promoted by Sb. At Sb + V loading above monolayer coverage, the system is also selective to acrylonitrile but product distribution is clearly different and characteristic of SbVO4 phases, affording selectivities to propylene similar to those to acrylonitrile.

Structural Characterization of the Orthorhombic Phase M1 in MoVNbTeO Propane Ammoxidation Catalyst

The structure of the orthorhombic phase in the MoVNbTeO propane ammoxidation catalyst system has been characterized and refined using a combination of TEM, synchrotron X-ray powder diffraction (S-XPD), and neutron powder diffraction (NPD). This phase, designated as M1 by Ushikubo et al. [1], crystallizes in the orthorhombic space group Pba2 (No. 32) with a = 21.134(2) A, b = 26.658(2) A, and c = 4.0146(3) A. The formula unit is Mo7.5V1.5NbTeO29. Bond valence sum calculations indicate the presence of d 1 metal sites neighbored by d 0 metal sites. The d 1 sites are occupied by a distribution of Mo5+ and V4+, whereas the d 0 sites are occupied by a distribution of Mo6+ and V5+. Out-of-center distortions in d 0 octahedra are consistent with the second-order Jahn–Teller effect and lattice effects. We argue that the V5+–O–V4+/Mo5+ moieties adjacent to Te4+ and Mo6+ sites in the [001] terminal plane provide a spatially isolated active site at which the selective ammoxidation of propane occurs.

Effect of alumina in multicomponent bismuth phosphate catalyst supported on silica-alumina for propylene ammoxidation reaction

The effect of compositions of silica-alumina support on multicomponent bismuth phosphate catalyst (MBP) was investigated for the propylene ammoxidation. It is appears that small amounts of Al2O3 cause retardation of the ammoxidation reaction, which would be due to activation of the selective oxidation pathway. It is understood from such results that the catalytic action of the MBP is essentially different with that of the multicomponent bismuth molybdate catalyst (MBM), which is generally used as propylene ammoxidation catalyst.

Defined vanadium phosphorus oxides and their use as highly effective catalysts in ammoxidation of methyl aromatics

The heterogeneous catalytic ammoxidation of methyl aromatics and methyl hetero aromatics is a preferred method for the synthesis of aromatic and hetero aromatic nitriles. The resulting nitriles are valuable intermediates for the production of dyestuffs, pesticides, pharmaceuticals and other chemical products. Usually, the ammoxidation is carried out using V-containing oxides (e.g. V/Ti, V/Sn, V/Mo) promoted by further transition metals as catalysts. However, we have shown recently that defined vanadium phosphates (VPO) revealed an excellent ammoxidation performance. The ammoxidation of toluene was used as model reaction. An intense characterisation of some VPO catalysts under pre-treatment and working conditions by the use of various in situ-methods has thrown more light in formation and stability of the used VPO solids as well as the effect of different reactants (ammonia, toluene) on VPO surface and bulk properties.

Characterization of tellurium in MoVTeNbO catalysts for propane oxidation or ammoxidation

The oxidation state and local geometry of tellurium in MoVTeNbO catalysts used in the ammoxidation or oxidation of propane were characterized by X-ray absorption, Mössbauer, and X-ray photoelectron (XPS) spectroscopies. The results obtained by Mössbauer, and XPS spectroscopies showed that the catalysts contained Te (IV) in the bulk and mainly Te (VI) at the surface. X-ray absorption fine structure (EXAFS) measurements allowed to determine that the tellurite entities corresponded to TeO 4E trigonal bipyramid in the hexagonal phase and to TeO 3E somewhat distorted trigonal pyramid in the orthorombic one. The completely determined environment appeared to correspond in both phases to crystallographic sites in hexagonal channels. These results allowed to determine the stoichiometries of the two phases which are TeM 3O 10 for the hexagonal phase and Te 2M 20O 57 for the orthorombic phase (M=Mo, V, Nb). 125Te Mössbauer isomer shift and pre-peak surface of the X-ray absorption spectra of the Te L III-edge have been correlated and contributions of the Te(5s) and Te(2p 3/2) to the structure have been analyzed.

Reactivity of V2O5/MgF2 Catalysts for the Selective Ammoxidation of 3-Picoline

V2O5/MgF2 catalysts with V2O5 contents ranging from 2.1 to 15.7 wt% were prepared, and the influence of the V2O5 content of the V2O5/MgF2 catalyst on the structure and activity for the ammoxidation of 3-picoline was investigated. XRD data indicate that V2O5 is in a highly dispersed state though segregation of V2O5 into tiny crystallites occurs at and above 8 wt% V2O5. The 3-picoline ammoxidation activity increased with an increase in V2O5 content due not only to the species arising out of interaction of V2O5 and MgF2, but also to the presence of V2O5 microcrystals in the catalysts.

Design of a Propane Ammoxidation Catalyst Using Artificial Neural Networks and Genetic Algorithms

Artificial neural networks (ANNs) and genetic algorithms (GAs) are applied to the optimal design of a catalyst for propane ammoxidation. The mole percentages of six components of a catalyst (P, K, Cr, Mo, Al2O3/SiO2, and VSb5WSn) are used as inputs, and the activity and the acrylonitrile selectivity serve as the two outputs. This trained optimal linear combination (OLC) network is used to evaluate the yield of new catalyst compositions generated during GA optimization. The best yield of acrylonitrile found after GA optimization is 79%, which is higher than the highest yield previously reported (64%). The OLC neural network, using the acrylonitrile yield (i.e., activity times selectivity) as the output, greatly improves the simulation of the catalyst system compared to a simple, single-network architecture. In particular, whereas single-network methods can all easily reproduce the experimental patterns used for training and validation, the OLC is markedly superior for generalizing to novel catalyst patterns.

Fluid-bed tin-based catalyst for propane ammoxidation

A fluid-bed tin-based catalyst for propane ammoxidation to acrylonitrile (ACN) was prepared by an original technique which combines incipient wetness impregnation (for the insertion of the metallic ions of the active phase inside the carrier) and neutralization (for the simultaneous co-precipitation of the different active sites inside the carrier). Tests were carried out using silica and alumina as the carrier. The silica-based catalyst was found to give the best performance. Different active phase loadings, vanadium concentrations and surface areas were tested in order to define the optimal composition of the Sn/V/Sb on SiO 2 catalyst.

Selective Ammoxidation of Isobutylene to Methacrylonitrile on a New Family of Crystalline Re–Sb–O Catalysts

The catalytic properties of a new family of crystalline Re–Sb–O compounds SbRe2O6, SbOReO4·2H2O, and Sb4Re2O13 in selective ammoxidation of isobutylene to methacrylonitrile (MAN) have been studied and compared with those of a coprecipitated SbRe2Ox catalyst, an Sb2O3-supported Re2O7 catalyst, bulk Re oxides, and bulk Sb oxides. The Re-based catalysts were more or less active for MAN synthesis with selectivities of 47.9–83.6% at 673 K, whereas bulk Sb oxides (Sb2O3 and Sb2O4) showed no activity. The results demonstrate that Re is prerequisite for the ammoxidation catalysis of Re–Sb–O systems. In catalytic systems the presence of Sb also contributes to the ammoxidation catalysis for MAN synthesis. Among these catalysts, SbRe2O6 was most active and selective (83.6%) for MAN formation at 673 K. No structural change in the bulk and surface of SbRe2O6 was observed after i-C4H8 ammoxidation by means of X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and confocal laser Raman microspectroscopy. The good performance of SbRe2O6 may be ascribed to its specific crystal structure composed of alternate octahedral (Re2O6)3− and (SbO)+ layers. Pulse reaction results suggested that adsorbed NH3 species on the SbRe2O6 catalyst facilitated the adsorption and subsequent activation of isobutylene. Increasing reaction temperature and decreasing GHSV did not give rise to increasing formation of by-products CO2 and acetonitrile, while increasing the i-C4H8 conversion. Thus the crystalline SbRe2O6 compound may be a new promising catalyst for ammoxidation of light hydrocarbons.