Catalysts For Propane Ammoxidation Research Articles

Tunning the Nb addition to Sb-V-O catalysts for an efficient promotion of the ammoxidation of propane to acrylonitrile

The effect of niobium dopant on Sb-V oxide catalysts supported on alumina is reported with the purpose of understanding the promoting effect of Nb and preventing the non-desired inert SbNbO 4 phase. Two different catalyst syntheses have been tested: the most common in which Sb is added as Sb 2O 3 suspension and a novel one in which it is molecularly dissolved as a tartrate complex. The effect of adding Nb to alumina-supported Sb-V oxide catalysts depends on the preparation method. The preparation via Sb 2O 3 suspension affords segregated Sb oxide, which results in lower Sb-V interaction. The preparation via Sb tartrate complex affords molecularly dissolved Sb species that combine more easily with V or Nb species. The higher dispersion of Sb in the tartrate method also promotes its interaction with Nb dopant. The Sb tartrate preparation method is preferred for binary Sb-V supported oxide catalyst, while the Sb 2O 3 suspension method is preferred for Nb-promoted Sb-V supported oxide catalyst, which results in a very efficient catalyst for propane ammoxidation.

Role of Nb in rutile-type Cr/V/Sb/Nb mixed oxides, catalysts for propane ammoxidation to acrylonitrile

Rutile-type Cr/V/Sb/Nb mixed oxides were prepared by coprecipitation from ethanolic solutions and calcination at 700 °C. They were then tested as catalysts for the gas-phase ammoxidation of propane. The addition of increasing amounts of Nb to the rutile Cr/V antimonate led to a considerable increase in the selectivity to acrylonitrile and to a lower selectivity to N 2 derived from ammonia overoxidation. But this effect was evident only when excess Sb was present with respect to the stoichiometric requirement for the formation of the rutile compound. Evidence was obtained for the development of rutile-type mixed Cr/V antimonate/niobate; this multicomponent rutile had a highly defective structure, with cationic vacancies in relation to excess Sb 5+ with respect to the stoichiometric composition. The progressive increase of Nb concentration in this mixed oxide with increasing Nb loading led to a partial segregation of Sb oxide in the form of Sb 2O 4. The excess Sb in rutile provided the active sites for allylic ammoxidation on intermediate adsorbed propylene. The contemporaneous presence of Nb in the lattice improved the efficiency of these sites and was responsible for the better catalytic performance with respect to the Cr/V/Sb/O systems.

Rutile-type metal (Cr, V) niobates as catalysts for propane ammoxidation to nitriles: In situ characterization and operando reactivity

Rutile-type Cr/Nb and V/Nb mixed oxides were prepared by co-precipitation from ethanolic solutions, and calcination at 700 °C. The development of the rutile structure during the thermal treatment of the precursor was investigated by in situ TP–Raman spectroscopy. It was found that graphite-like carbon deposits build-up during calcinations in inert. Their decomposition provides the reducing agent that leads to the formation of rutile VNbO 4 at relatively mild conditions. Rutile Cr and V niobates were tested as catalysts for the ammoxidation of propane, under both hydrocarbon-rich and hydrocarbon-lean conditions. Catalysts were active but non-selective to acrylonitrile. The catalyst modifications occurring during reaction were followed by operando Raman–GC methodology. VNbO 4 transforms into a catalyst active and selective for acetonitrile under leaner hydrocarbon feed.

Effect of titanium substitution on the structure of VSbO 4 catalysts for propane ammoxidation

The effect of titanium substitution on vanadium antimony oxide propane ammoxidation catalysts was investigated by UV Raman spectroscopy and X-ray diffraction. UV Raman spectra exhibit bands in the region of 450–1000 cm −1, which are associated with the rutile structure, a cation vacancy site, and V 2O 5. Calculations of the vibrational frequencies made with the use of CASTEP optimized slab models provide good agreement with experimental results. With titanium substitution, the rutile bands shift and broaden. Comparison of freshly prepared and ground samples shows that crystalline V 2O 5 forms upon grinding in air. Titanium stabilizes vanadium in the vanadium antimonate rutile structure and thus prevents the formation of crystalline V 2O 5. An in situ Raman characterization of titanium-containing vanadium antimony oxide catalysts was made in reducing and oxidizing atmospheres. The band associated with two-coordinate oxygen at cation vacancy sites disappears under reduction conditions, then returns after reoxidation in air. This indicates that this lattice oxygen moiety is key to ammonia activation. The titanium-substituted sample shows additional characteristics that indicate titanium improves the reduction and oxidation capability of the vanadium antimonate catalyst.

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.

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.

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.

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.

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.

New synthesis route for Mo–V–Nb–Te mixed oxides catalyst for propane ammoxidation

Several methods for preparing Mo–V–Nb–Te mixed oxides were examined. Hydrothermal treatment gives a precursor of ammoxidation catalyst which shows twice as high activity after calcination as the catalyst prepared by the known dry-up method. Mixed oxides prepared by solid state reaction give rather poor activity. It has been suggested that the higher activity of hydrothermal treated catalyst is related primarily to its higher surface area.

Artificial neural network aided design of catalyst for propane ammoxidation

An artificial neural network (BP network) is applied to design a VSbWSn (P, K, Cr, Mo)/SIAL catalyst for acrylonitrile synthesis via propane. The conversion of propane and selectivity of acrylonitrile can be calculated as functions of the catalyst components by the BP network to be trained. After training, the network can simulate the catalytic system very well. If one takes the conversion of propane and selectivity of acrylonitrile as the two sub-objectives, a model for this catalytic system can be given as:Max(y1=X C3), Max(y2=S ACN), y= F(W,X in), 0≤y1≤1.0, 0≤y2≤1.0. A better catalyst could be found through optimization for propane ammoxidation to acrylonitrile. The best yield of acrylonitrile is 55.0%, which is higher than those reported in the literature.

Effect of ammonia chemisorption on the surface reactivity of V-Sb-oxide catalysts for propane ammoxidation

The influence of ammonia on the reactivity of a V—Sb-oxide catalyst in propane ammoxidation to acrylonitrile (ACN) was studied by infrared spectroscopy and flow reactor tests. Ammonia coordinates as such or as ammonium ions on the catalyst surface in relative amounts which depend on both the temperature at the time of contact and the hydroxylation state of the catalyst. The two chemisorbed species influence both the surface reactivity and the pathways of the reaction in propane ammoxidation. NH 3 chemisorbed on Lewis acid sites inhibits the activation of propane differently from that chemisorbed on Brønsted sites. The formation of the latter species, however, inhibits the reactivity of the Brønsted sites in catalyzing various side reactions which decrease the selectivity to ACN. The results of the infrared study are in good agreement with the change in surface reactivity observed in flow reactor tests as a function of the concentration of ammonia in the feed. Both the nature and the amount of the chemisorbed ammonia species thus determine the intrinsic surface reactivity and selectivity of V—Sb-oxide catalysts in the synthesis of acrylonitrile.