Propane Ammoxidation Research Articles

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.

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.

Effect of the oxide support on the propane ammoxidation with Sb–V–O-based catalysts

The role of Nb 2O 5 and γ-Al 2O 3 oxide supports on the ammoxidation of propane on supported mixed Sb–V oxide at different Sb+V surface coverages is studied. Sb and V oxide species on alumina and on niobia support show different structural features that reflect in different performance during the ammoxidation of propane to acrylonitrile. Niobia-supported catalysts are much more selective to acrylonitrile than alumina-supported ones. Alumina interacts weakly with the supported oxides while niobia forms new phases through solid state reactions with the supported oxides during catalytic operation that must account for its higher selectivity values towards acrylonitrile and higher specific rate of acrylonitrile formation per vanadium site.

Cr/V/Sb mixed oxide catalysts for the ammoxidation of propane to acrylonitrile: Part I: Nature of the V species

Rutile-type Cr/V/Sb mixed oxides, catalysts for the ammoxidation of propane to acrylonitrile, were prepared and characterized. For atomic ratios between components equal to Cr/V/Sb 1/ x/1 and 1/ x/2 the systems were monophasic, but different types of compounds formed depending on the ratio between the three metals. The compositional parameter which most affected the nature of the compound formed was the (Cr+V)/Sb atomic ratio. When this ratio was between 2 and ≈1, a rutile Cr 3+/V 4+/Sb 5+ mixed oxide of composition Cr 1V x Sb 1O 4+2 x developed (0< x<1), which in practice corresponds to a solid solution between 1 CrSbO 4 and x VO 2. When the (Cr+V)/Sb ratio was between 0.5 and ≈1, a rutile Cr 3+/V 3+/Sb 3+/Sb 5+ mixed oxide of composition CrV x Sb 1+ x+2 z O 4+4 x+4 z developed (0< x<1), which corresponds to a solid solution between 1 Cr 3+Sb z 3+Sb 1+ z 5+O 4+4 z and x VSbO 4. The distinction between the two classes of compounds was not clear-cut, and when the (Cr+V)/Sb atomic ratio was around 1, mixed oxides containing both V 3+ and V 4+ formed. Values of the (Cr+V)/Sb atomic ratio lower than ≈0.5 led to the additional formation of antimony oxide.

Activation of propane and butanes over niobium- and tantalum-based oxide catalysts

Niobium and tantalum are important elements for the activation of alkanes in the viewpoints of acidic property and the formation of unique mixed metal oxides. And the difference of the ability of alkane activation between niobium- and tantalum-based oxide catalysts is studied. Although hydrated niobium and tantalum oxides show strong acid property, only hydrated tantalum oxide is activated to a solid superacid by the treatment with sulfuric acid, and isomerizes n-butane to isobutane at room temperature. The sulfuric acid treated tantalum oxide activates P–Mo–V heteropolyacid compounds for the selective oxidation of isobutane to methacrolein (MAL) and methacrylic acid (MAA). The difference of ability of alkanes activation between niobium and tantalum is studied by using surface science technique. Mo–V–Nb–Te mixed metal oxide catalysts are active for the ammoxidation of propane to acrylonitrile (AN). However, Mo–V–Ta–Te mixed metal oxide is less active. The effect of catalyst preparation condition is studied. Mo–V–Nb–Te mixed metal oxide catalysts are also active for the oxidation of propane to acrylic acid (AA).

Oxidative dehydrogenation and ammoxidation of ethane and propane over pentasil ring Co-zeolites

The activity of Co-zeolites with MFI, BEA, FER, MOR topologies was studied in ethane and propane oxidative dehydrogenation and ammoxidation in a through-flow arrangement and temperature range 350–500 °C. The products of oxidative dehydrogenation of paraffins were corresponding olefins, CO and CO 2, in the propane reaction only a small part was cracked to ethylene and methane. Besides CO and CO 2, the ammoxidation of ethane yielded ethylene and acetonitrile, and of propane brought propylene and acetonitrile too. Presence of ammonia increased substantially the paraffin conversion as well as selectivity and yield of the sum of olefin and nitrile for CoH-BEA and CoH-MFI zeolites, while with CoNa-zeolites the paraffin conversion and selectivity were similar. The sequence in activity in both the oxidative dehydrogenation and ammoxidation (in TOF values per Co ion in conversion of paraffins and yields of olefin and olefin+acetonitrile) of Co-zeolites (more pronounced for ammoxidation) Co-BEA>CoMFI⪢Co-MOR>Co-FER is ascribed to the location of prevailing amount of Co ions in β cationic position of pentasil ring zeolites, i.e. at the exposed site at the intersection of straight and sinusoidal channel in ZSM-5, and in open channels of beta-zeolite, in contrast to Co-β ions in MOR and FER structures occurring in the narrow interconnected channels and in eight-member ring channel, respectively. Formation of acetonitrile at propane ammoxidation can be attributed to interaction of ammonia with double bond of propylene at β position, as suggested in literature, as well as to the observed easy decomposition of acrylonitrile to acetonitrile in the presence of ammonia.

Mechanism of Selective Oxidation and Ammoxidation of Propene on Bismuth Molybdates from DFT Calculations on Model Clusters

In this paper, we use first principles quantum mechanical methods (B3LYP flavor of Density Functional Theory) to examine the mechanism of selective oxidation and ammoxidation of propene by BiMoO_x catalysts. To do this, we use finite clusters chosen to mimic likely sites on the heterogeneous surfaces of the catalysts. We conclude that activation of the propene requires a Bi(V) site, whereas all subsequent reactions involve di-oxo Mo(VI) sites adjacent to the Bi. We find that two such Mo sites are required for the most favorable reactions. These results are compatible with current experimental data. For ammoxidation, we conclude that ammonia activation would be easier on Mo(IV) rather than on Mo(VI). Ammonia would be activated more easily for more reducing condition. Because ammonia and propene are reducing agents, higher partial pressures of them could accelerate the ammonia activation. This is consistent with the kinetic model of ammoxidation proposed by Grasselli and co-workers that imido sites (Mo═NH) are more abundant in higher partial pressures of feed. Our calculations also indicate that allyl groups produced as a result of the hydrogen abstraction from propenes would be adsorbed more easily on imido groups (Mo═NH) than on oxo groups (Mo═O) and that the spectator oxo effect is larger than spectator imido effect. Thus, we propose that the best site for ammoxidation (at least for allyl adsorption) is the imido group of the “oxo-imido” species.

Operando Raman study of alumina-supported Sb-V-O catalyst during propane ammoxidation to acrylonitrile with on-line activity measurement.

Operando Raman spectra during propane ammoxidation show partially reversible structural transformations of the active phases as a function of reaction environment.

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.

Effect of Sb/V Ratio and of Sb + V Coverage on the Molecular Structure and Activity of Alumina-Supported Sb–V–O Catalysts for the Ammoxidation of Propane to Acrylonitrile

The effect of the total coverage of Sb + V oxides and the effect of the Sb/V atomic ratio on the ammoxidation of propane to acrylonitrile on alumina-supported V and Sb oxide catalysts is reported. The fresh and used catalysts are characterized by XRD and in situ Raman spectroscopy. Comparison with binary V–Al–O and Sb–Al–O catalysts shows that the presence of both Sb and V oxides strongly enhances the rate of propane ammoxidation to acrylonitrile on alumina-supported Sb–V oxide catalysts. The stability and structural changes during on-stream operation prior to reach steady-state operation originates from a close interaction between Sb and V oxides. The Sb–V interaction depends on total Sb + V coverage on alumina. Below the dispersion limit, SbVO4 phases are not stable under reaction and break into the individual oxides. At Sb + V loading beyond dispersion limit, SbVO4 phases are stable under reaction conditions while Sb and V oxides that did not combine during calcination of the precursor recombine into SbVO4 phases. This solid-state reaction accounts for a higher propane conversion and selectivity to acrylonitrile. Comparison of the performance and molecular structures of fresh and used catalysts further suggests that Sb–V–O phases are necessary for this reaction. The specific formation of acrylonitrile per vanadium site reaches a maximum at an atomic Sb/V ratio of 2. It is likely that a moderate excess of antimony may be necessary for an efficient ammoxidation of propane to acrylonitrile.

ChemInform Abstract: The Importance of Site Isolation and Phase Cooperation in Propane Ammoxidation on Rutile‐Type Vanadia Catalysts

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The Structure of Oxide Ga–Sb–Ni–P–W–O/SiO2 Catalyst and Its Catalytic Properties in Propane Ammoxidation

The structure of the multicomponent catalyst Ga1Ni1P2W0.5Sb6Ox /SiO2 and its catalytic properties in propane ammoxidation are studied. The catalyst is nanostructured and consists of noncoherently spliced blocks of a multiply promoted phase with a structure of gallium antimonate, which covers SiO2 particles with a thin layer. In the multiply promoted compound with a structure of gallium antimonate, Ni2+ ions partially substitute for Ga3+ and W6+ ions partially substitute for Sb5+. This leads to an increase in the crystalline lattice parameters a and c. Phosphate ions are stabilized in the region of block interfaces. The catalyst is characterized by high efficiency in propane ammoxidation.

Active Sites Characterization in Mixed Vanadium and Iron Antimonate Oxide Catalysts for Propane Ammoxidation

Ammoxidation of propane was investigated over FeVSbO mixed oxide catalysts. The results obtained show that in VSbO4 mixed oxide catalyst systems, substitution of vanadium by small quantities of iron has a strongly positive effect on the activity of the catalysts. With such a system, the selectivity to acrylonitrile decreases while the selectivity to propene increases. At high levels of vanadium substitution by iron, the activity was low but the selectivity towards acrylonitrile was enhanced. The characterization of the catalysts carried out using several techniques, such as X-ray diffraction, Mössbauer spectroscopy, X-ray absorption, and electron microscopy, has shown that a continuous solid solution was formed between VSbO4 and FeSbO4. At low iron loading, Fe3+ substituted for V4+ in the cationic-deficient structure V4+0.64V3+0.28Sb5+0.92♢0.16O4, leading to a structure with both cationic and anionic vacancies, Fe3+xV3+0.33V4+0.41−xSb5+0.92♢0.18O4−x, and generating very active sites for alkane activation. At high iron loading, only V3+ was present in the bulk of the solids and only Fe3+ contributed to the isolation of the vanadium sites, which are selective to acrylonitrile.

A 121 Sb Mössbauer Study of the Chemical State of Antimony in V-Sb-O Mixed-Oxide Catalysts for the Ammoxidation of Propane

The structural changes of two representative samples of a group of V-Sb-oxide catalysts, a vanadium-rich and an antimony-rich specimen, are investigated by 121Sb Mossbauer spectroscopy after treatment under propene or hydrogen at 673 K. The as-prepared catalysts contain both Sb(V) and Sb(III) as crystalline and microcrystalline α-Sb2O4 and VSbO4, as well as an additional amorphous V5+ oxide phase. The oxidation state of antimony does not change upon exposure to propene, whereas a partial reduction can be obtained by a more intense reduction under hydrogen. The Mossbauer results indicate that the antimony in VSbO4 can be reduced more easily than that in α-Sb2O4, which appears to be more stable. These results are discussed in view of the observed depletion of vanadium at the surface of the catalyst particles under reaction conditions.

Raman spectroscopy during catalytic operations with on-line activity measurement (operando spectroscopy): a method for understanding the active centres of cations supported on porous materials

Raman spectroscopy with on-line activity measurement (operando Raman spectroscopy) is used to reach a molecular understanding of the structure–activity relationship of supported vanadium oxide catalysts during alkane selective oxidation and ammoxidation reactions. The advantage of the operando methodology is that the molecular structures are recorded during true catalytic operation, thus affording information about the structures relevant to the catalytic performance. It is shown that efficient propane ammoxidation requires both surface mono-oxo vanadium oxide species and SbVO4 phases.

Oxidation and ammoxidation of propane over Mo–V–Sb mixed oxide catalysts

Catalytic oxidation and ammoxidation of propane to acrolein and acrylonitrile, respectively, were carried out over Mo–V–Sb mixed oxide catalysts. V–Sb mixed oxide showed the activity for the oxidative dehydrogenation of propane to propene, and the selectivity to propene remarkably increased with increasing the concentration of cation vacancy in VSbO 4 phase. It is likely that the oxidative dehydrogenation of propane on the VSbO 4 phase is initiated via H-abstraction by acid–base concerted mechanism. The selectivity to acrolein and acrylonitrile increased by the addition of molybdenum species to V–Sb mixed oxide catalyst. Among a series of Mo–V–Sb oxide catalysts, Mo 1V 1Sb 10O x exhibited the highest selectivity to acrolein and acrylonitrile at 430 and 480°C, respectively. The highly dispersed molybdenum suboxide was formed together with the both phases of VSbO 4 and α-Sb 2O 4 in the Mo 1V 1Sb 10O x . The high catalytic activity of Mo 1V 1Sb 10O x can be explained by the bifunctional mechanism of the highly dispersed molybdenum suboxide and the VSbO 4 phases as follows: the oxidative dehydrogenation of propane proceeds on the VSbO 4 phase followed by the oxidation of propene into acrolein or the ammoxidation into acrylonitrile on the molybdenum suboxide phase. When large size of MoO 3 crystallites were formed, cracking reaction, i.e., C–C bond cleavage, occurred leading to non-selective total oxidation, resulting in decreasing the selectivities to acrolein and acrylonitrile.

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.

Characterization of Antimony-Doped TIN Oxide Catalysts

Antimony-doped tin oxide (ATO) catalysts are used for the oxidation of propylene to acrolein, the ammoxidation of propylene to acrylonitrile and the oxidative dehydrogenation of butanes to 1,3- butadiene. The distribution and valence states of Sb in ATOs are key in determining their catalytic activities. While these materials have been subjects of intensive studies for more than 20 years, X-ray photoelectron spectroscopy, Mössbauer spectrometry, and X-ray absorption spectroscopy4 have so far provided only indirect data for the distribution of Sb and its valence states. in particular, while has been hypothesized that the tin (IV) oxide contains Sb (V) within the bulk lattice and Sb (III) located at surface sites, no direct experimental evidence for this has been provided.Here we use electron energy loss spectroscopy (EELS) combined with Z-contrast imaging in a JEOL 2010F field emission STEM/TEM operating at 200 KV to analyze ATO catalysts.

Iron antimony oxide catalysts for the ammoxidation of propene to acrylonitrile: comments on the method of preparation of tellurium promoted catalysts

Three methods of catalyst preparation for tellurium promoted iron antimony oxide catalysts are investigated: (a) co-precipitation, (b) impregnation and (c) reaction of solid oxides. For all three methods, the addition of Te increases the selectivity for acrylonitrile significantly. For catalysts prepared by co-precipitation, this effect is only apparent at low Te concentrations whereas, for the other two preparation methods, the positive effect on selectivity is maintained at higher doping levels. The catalysts are characterised using X-ray diffraction and X-ray photoelectron spectroscopy. For the Fe 30Sb 60Te 2O x formulations prepared using the three methods, a similar Te/Sb surface atomic ratio was observed which is consistent with the similar specific activity observed with all three catalysts, indicating that the three methods of preparation can prepare similar surfaces following calcination and catalyst testing. However, the co-precipitation method gives catalyst surface areas approximately three times higher than the other two methods and, consequently, gives the best catalyst performance.

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.