Formation Of Acrylonitrile Research Articles

Effect of Titanium Substitution in ≈SbVO4 Used for Propane Ammoxidation

Catalysts of the nominal composition Sb0.9V0.9−xTixOy, 0.0 ≤ x ≤ 0.9, were prepared and characterized with X-ray diffraction, Fourier transform-Raman spectra, diffuse reflectance infrared Fourier transform spectra, transmission electron microscopy, electron diffraction, and X-ray microanalysis. The catalysts were used for the ammoxidation of propane to give acrylonitrile. Compared with the pure ≈SbVO4 (Sb0.92V0.92O4) it is observed that the activity decreases while the selectivity to acrylonitrile formation is improved when the Ti : V ratio of the preparation is increased. The characterization of the catalysts shows the formation of a rutile-type phase in all preparations with vanadium. Additionally, α-Sb2O4 is formed in an amount that increases with the amount of titanium in the sample. X-ray microanalysis data confirm that two substitution mechanisms occur in parallel, namely, one Ti4+ substitutes for one V4+ and two Ti4+ substitute for one V3+ and one Sb5+, forming the solid solution series Sb5+(0.92−z/2) V3+(0.28−z/2) V4+(0.64−u) Ti4+(z+u) □0.16 O4 (□ is a cation vacancy). Quantitative model calculations considering data obtained by the characterization methods give the content of cations in the unit cell of the rutile phase. It is demonstrated that the activity can be Hcorrelated with the content of V3+ in the unit cell, indicating that the activation of propane occurs on a V3+ center. Moreover, the selectivity to acrylonitrile can be correlated to the Sb5+/V3+ ratio, which indicates that the ammonia is activated on a Sb5+ moiety. Considering both activity and selectivity, the best performance for propane ammoxidation is obtained for a sample with the nominal composition Sb0.9V0.3Ti0.6Oy. The selectivity to acrylonitrile is 53% at 25% propane conversion to be compared with 12% at 15% propane conversion for the nonsubstituted ≈SbVO4. The improved selectivity can be explained by isolation of the vanadium centers in the active phase, which gives fewer bridging V-O-V moieties that are active for degradation and combustion of propane and the subsequently formed propylene.

Catalytic behavior of BiPO4 in the multicomponent bismuth phosphate system on the propylene ammoxidation

We found that the multicomponent bismuth phosphate (MBP) with multiphase activates catalytically the ammoxidation reaction of propylene and consists of mainly BiPO4 and metal molybdates, etc. The MBP has been characterized by several techniques such as DTA, XRD, and BET surface area and total pore volume measurements. The DTA curve clearly shows that there are phase transitions with sharp breaks of the curve. The specific surface area and total pore volume decrease with increasing of the BiPO4 phase content, showing that the BiPO4 phase remains essentially on the catalyst surface and blocks the active sites and catalyst pores. It would mean that the catalytic behavior in the MBP depends on the catalyst surface covered with the BiPO4 phase adequately. The pathway of the selective oxidation due to the controlled amounts of the BiPO4 phase favors the formation of acrylonitrile. The improvement of catalytic performances of the mixture of phases is explained by the synergy effect.

Effect of CO2 addition as an oxidant on the catalytic activity of K/MgO for acrylonitrile synthesis

The catalytic behavior of magnesium oxide doped with potassium oxide was investigated for acrylonitrile formation from acetonitrile and methanol. CO2 was applied as a mild oxidant. Under this oxidative condition, the reaction proceeded via an oxidative dehydrogenation pathway resulting in the increase of both acetonitrile conversion and acrylonitrile selectivity compared with the reaction under a non‐oxidative condition.

An Investigation of the Al–Sb–V–W–Oxide System for Propane Ammoxidation

Al–Sb–V–W–oxide catalysts were prepared and used for propane ammoxidation to give acrylonitrile. The preparations were characterized, both as freshly prepared and after use in propane ammoxidation, using XRD, XPS, Raman spectroscopy, and transmission electron microscopy together with electron diffraction and X-ray microanalysis. Activity tests of the freshly prepared samples reveal a remarkable activation behaviour with time-on-stream under influence from the reaction medium. During the first 15–50 h on-stream, a considerable increase in the activity and the selectivity to acrylonitrile is observed in parallel with a decrease in the selectivity to waste products. The duration of the activation period depends on the W-content and increases with increase of the latter. Characterization with TEM, electron diffraction, and X-ray microanalysis show that the activation of the catalyst precursor under reaction conditions results in the formation of an ≈ Sb(V, W)O4 phase of rutile-type, which is active and selective to acrylonitrile formation. The aluminum in the catalysts is present in form of δ-Al2O3, which serves as a catalyst support. Analysis of the catalysts and their precursors with XPS shows coverage of the alumina surface with oxides with Sb, V, and W, and the Raman spectra show the presence of tetrahedral and octahedral tungsten oxide species being attached to the alumina surface. Both XPS and X-ray microanalysis results indicate that the surfaces and the external layers of the precursors are enriched with antimony. Activity test of an Al–Sb–V–W–oxide catalyst with Sb:V:W=5:1:2 and 50 wt% Al2O3, using a feed which is stoichiometric with respect to acrylonitrile formation, shows that the selectivity to acrylonitrile is 48% at 77% propane conversion, corresponding to a yield of 37%. Comparison with Sb–V–oxide and Al–Sb–V–oxide preparations reveals a substantial improvement of the catalytic properties with the addition of Al and W to Sb–V–oxide.

An FTIR Study on the Mechanism of the Reaction between Nitrogen Dioxide and Propene over Acidic Mordenites

Nitrosonium ions, NO+, are formed upon reaction between NO, NO2, and the Brønsted acid sites of H–MOR. It is shown that the NO+ ions are highly reactive towards propene, forming propenal oxime at 120°C. At temperatures above 170°C, propenal oxime is dehydrated to acrylonitrile. A mechanism is proposed to explain the acrylonitrile formation. The nitrile can be hydrolysed to yield adsorbed ammonium ions, which are known to be efficient in reducing nitrogen oxides to nitrogen. The dehydration of propenal oxime appears to be the rate determining step in nitrile formation.

A new ternary mixed oxide catalyst for ammoxidation of propane: Sn/V/Sb

The catalytic properties of a Sn/V/Sb mixed oxide for propane ammoxidation to acrylonitrile are studied in this paper. In particular the antimony and vanadium amounts were changed in order to optimize the relative atomic ratio. The ternary sample with the best catalytic performance was compared with the analogous binary samples (Sn/Sb and Sb/V). The results obtained indicate that vanadium is responsible for the paraffin activation, nevertheless much vanadium produces a lot of carbon oxides. Antimony is responsible for the insertion of nitrogen in the molecule (it can be considered as an acrylonitrile selectivity modulator); nevertheless, too much antimony deactivates the catalyst. Tin does not act only as a dispersive matrix for the active sites, but leads to the formation of a polyfunctional catalyst, increasing the rate of acrylonitrile formation from the intermediate propene. From a structural point of view, this catalytic system can be described as a homogeneous system, containing microfields of rutile type oxide (SnO2), promoted with antimony and vanadium in substitutional solid solution, dispersed in an excess of amorphous antimony oxide.

Characterizations and catalytic behaviors of vanadium-antimony oxide catalysts in propane ammoxidation

The ammoxidation of propane to acrylonitrile was investigated over a series of V/Sb mixed oxide catalysts. Characterizations of catalysts were carried out by X-ray diffraction, temperature-programmed desorption of ammonia and infrared spectroscopy. The rutile-type of VSbO 4 is found to be highly selective for the formation of acrylonitrile due to its enhancing effect of reoxidation. Correlation of the XRD patterns and NH 3-TPD profiles indicates that the ammonia desorption peak appearing at about 90°C is caused by the ammonia adsorption on the VSbO 4 rutile-type surface. Ammonia inhibits the formation of carbon oxides and acrylonitrile, and therefore the competition between ammonia and oxygen takes place on the same kind of active site. Propane is found to be adsorbed on another site. With regards to the reaction mechanism, it is proposed that the adsorption of propane on the surface occurs first through a formation of propylene-like adsorption intermediate, which appears to react with oxygen to form an oxidized site. The further reaction with ammonia forms a second oxidized site and finally, acrylonitrile is produced from the second oxidized site. A Langmuir-Hinshelwood model fits fairly the experimental data.

Mechanism of ammoxidation of propane on a Sb/V/O system

The mechanism of the direct ammoxidation of propane is investigated on a Sb/V mixed oxide catalyst (atomic ratio 5/1) in order to understand the sequence of the intermediates and the role of ammonia in the formation of acrylonitrile. The sample was characterized by different techniques (BET, FTIR, XRD and XPS) and tested in different conditions (oxidation and ammoxidation of propane and propene). It is proposed that propane is first converted to propene and hence to ``oxidation products'' (which could be acrolein or an intermediate absorbed on the catalyst surface, as allyl radical). In the absence of ammonia these ``oxidation products'' are converted to carbon oxide. But in its presence they are converted into acrylonitrile, which is appreciably more stable and undergoes minor degrees of oxidative degradation.

Catalysis and structure of the SbVO 4/Sb 2O 4 system for propane ammoxidation

Pure Sb 0.9V 0.9O 4 and various preparations with excess of either vanadia or antimony oxide, including mechanical mixtures, have been investigated for propane ammoxidation to acrylonitrile. The catalysts were characterized before and after use in catalysis by various methods, including electron microscopy, infrared spectroscopy and XPS. The catalytic data show that preparations with ≈~ SbVO 4 and α-Sb 2O 4, compared with the single phases, are more selective to acrylonitrile formation on the condition that the excess antimony oxide is present while synthesising the ≈~ SbVO 4 phase. Considering the catalytic data together with the results from the characterisations, various possibilities are discussed to explain the role of excess α-Sb 2O 4 in propane ammoxidation. Possibilities that can be excluded on rational grounds are catalysis on two phases, or, at grain boundaries, an influence on the morphology of ≈~ SbVO 4 from α-Sb 2O 4, the formation of α-Sb 2O 4 containing vanadium, defect formation, creation of active sites by the spillover of oxygen, and formation of VSb 2O 5. Instead, the observed synergy effect is due to the formation of ≈~ SbVO 4 enriched with antimony at the surface, creating isolation to a suitable level of the V-centres. The explanation is consistent with several observations including catalytic data for a series of vanadium compounds with different vanadium content, showing that structural isolation of the vanadium is necessary for obtaining high selectivity to acrylonitrile.

Ammoxidation of propane to acrylonitrile on VSbO catalysts. Role of ammonia in the reaction pathways

Transient experiments in the vacuum on the ammoxidation of propane over VSb 5O x- and VSb 5O x (30 wt%)/Al 2O 3 catalysts were performed. Additionally, the interaction of ammonia with the catalysts was investigated by means of TPD and DRIFT spectroscopic studies. The reported results reveal that short lived NH x .-species, most probably NH 3,ads or NH + 4, are active species in the formation of acrylonitrile from propane. These species are also involved in the formation of the non-selective side-product N 2, which is formed via the intermediates NO and N 2O.

The Al–Sb–V–Oxide System for Propane Ammoxidation: A Study of Regions of Phase Formation and Catalytic Role of Al, Sb, and V

Syntheses in the Al–Sb–V–O system, which is a candidate catalyst system for propane ammoxidation to acrylonitrile, were characterised by X-ray powder diffraction, electron diffraction, energy dispersive X-ray microanalysis, high-resolution electron microscopy, infrared spectroscopy, and catalytic measurements. The objective was to study phase formation as well as to identify the phase, or phases, which are active and selective for propane ammoxidation and to conclude about the catalytic role of the various types of metal ion centres. Calcination in air at 680°C of mixtures of Al(OH)3, Sb2O3, and V2O5showed the formation of α-Sb2O4, V2O5, AlVO4, and two rutile-related phases Sb0.9V0.9O4and Al1−xSbVxO4, 0<x<0.5. The latter presents a trirutile-type superlattice as shown by electron diffraction and high-resolution transmission electron microscopy. Formation of Al1−xSbVxO4was not observed starting from the phase composition, but required an excess of alumina (>60 at.% Al). Without alumina in excess, Sb0.9V0.9O4is formed. Activity measurements over phases belonging to the Al–Sb–V–O system revealed that Al1−xSbVxO4is the most selective toward acrylonitrile formation, and it was identified as the active phase in a sample of patent type with Al : Sb : V=21 : 5 : 1. Rationalisation of activity and selectivity data for V2O5, Sb2O4, Al2O3, Sb0.9V0.9O4, AlVO4, AlSbO4, Sb0.9V0.9O4/α-Sb2O4, β-Sb1.9V0.1O4, and Al1−xSbVxO4showed that in oxides with vanadium, it is the V-centres which are active and determine the specific activity. To have a catalyst that is selective to acrylonitrile and to avoid combustion, it is necessary to have structurally isolated V-centres which are surrounded by Al and/or Sb.

Modification of the surface reactivity of vanadium antimonate catalysts during catalytic propane ammoxidation

The changes that take place in a vanadium antimonate catalyst during catalytic propane ammoxidation were studied by transient reactivity experiments and infrared spectroscopy. The investigation was focused on both the change in catalytic behaviour of a fresh sample and the change in reactivity with stepwise increases in the concentration of one or more reagents while maintaining the concentration of the other reactants constant. The results obtained show that two main types of modifications are present: (1) A change in the reactivity from the fresh to the activated samples due to the surface formation of a VSbO 4 phase by reaction of vanadium and antimony oxides that have not reacted during calcination; this modification leads to formation of carbon oxides and a decrease in the rate of propane depletion as well as a considerable increase in the rate of acrylonitrile formation. (2) A change in the reactivity of the activated sample which is a function of the redox characteristics of the reagent mixture; this modification does not change the rate of propane depletion, but rather only results in differences in the relative selectivity to products. The results also show that (i) in the steady state, the activated catalyst is partially reduced in comparison with the same sample after treatment with oxygen and (ii) the rate of acrylonitrile formation is lower. Infrared results confirm the formation of oxygen vacancies in the catalyst upon interaction with ammonia and propene as well as the presence of these defects in the sample after the catalytic tests.

Ammoxidation of Propane over Antimony Vanadium-Oxide Catalysts

Catalysts belonging to the Sb-V-O system were prepared with various Sb/V ratios and were used for propane ammoxidation to acrylonitrile. XRD patterns of freshly prepared samples show those with excess vanadia to consist of V2O5 and SbVO4, while SbVO4 and α-Sb2O4 are constituents in the samples with a Sb/V ratio above unity. High rate and selectivity for propylene formation at low conversion are characteristic for samples with excess vanadia and considering XRD, Raman, infrared, and XPS results, this is explained by formation of amorphous vanadia spread over the surface of SbVO4. Catalysts with both α-Sb2O4 and SbVO4 phases are the most selective for acrylonitrile formation, a function that is linked to their ability to selectively transform intermediate propylene. XPS data suggest this function to be associated with the formation of suprasurface antimony sites on SbVO4 as a result of migration of antimony from α-Sb2O4 during the catalytic process. Raman and infrared spectral features revealed that compared with SbVO4, the samples with both α-Sb2O4 and SbVO4 are more efficiently reoxidised during propane ammoxidation. Rate dependences on the partial pressures of reactants over a sample with excess α-Sb2O4 show that the adsorption of propane is the rate limiting step for propylene formation, and that acrylonitrile and carbon oxides are predominantly formed from the intermediate propylene in routes comprising nonequilibrated steps. Addition of water vapour results in an increase of rate and selectivity for acrylonitrile formation. The kinetic dependences indicate that for acrylonitrile formation it is advantageous to have a feed rich in propane and to use recirculation for obtaining high productivity.

Surface characterization and reactivity in ammoxidation reactions of vanadium antimonate catalysts

Unsupported vanadium antimonate catalysts with Sb/V ratios of 1 and 5 and samples with the latter ratio supported on alumina were studied in toluene and propane ammoxidation to benzonitrile and acrylonitrile, respectively, and were characterized by X-ray photoelectron spectroscopy (XPS) analysis before and after catalytic tests. Activity data for toluene ammoxidation suggest that excess antimony with respect to the stoichiometric amount required for forming the VSbO 4 rutile phase affects the dispersion of the latter phase giving smaller particles. Vanadium sites are involved both in the activation of toluene and in the insertion of nitrogen in this reaction, whereas antimony does not play a specific role in the reaction mechanism. In propane ammoxidation, on the other hand, due to a higher reaction temperature with respect to toluene (500°C vs. 370°C), free vanadia on the surface of the catalyst has a negative influence on the selectivity because it promotes the conversion of ammonia to nitrogen, decreasing the surface nitrogenous species required for the selective formation of acrylonitrile. Excess antimony is thus necessary for completing the reaction between antimony and vanadium oxides, but antimony also participates in the reaction mechanism. In propane ammoxidation, in fact, XPS data show that both vanadium and antimony sites are reduced. Tentatively, vanadium sites are involved in the activation of propane, while antimony sites insert nitrogen. The differences between the toluene and propane ammoxidation mechanisms are interpreted to be primarily related to the different reaction temperatures.

ChemInform Abstract: Formation of Propionitrile (III) and Acrylonitrile (IV) from Acetonitrile (I) and Methanol (II).

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Synergistic Effects of Superbasic Catalysts on the Selective Formation of Acrylonitrile via Oxidative Methylation of Acetonitrile with Methane

The oxidative methylation of acetonitrile with methane to acrylonitrile occurs more actively and selectively over bialkali promoted CaO catalysts than over any monoalkali promoted system. The most effective catalytic systems are obtained with Li A + Cs A, Na A + Cs A, or K A + Cs A (A = SO 2− 4, OH −, C1 −, CH 3COO −, CO 2− 3, or NO − 3) supported on CaO, containing total alkali loadings of 10 mol% with equal molar amounts of both alkalis. At 750°C, under atmospheric pressure, at CH 4 : O 2 : CH 3CN : He partial pressure ratios of 5.0 : 1.0 : 1.5 : 6.5, and at a space velocity of 15,000 cm 3 g −1 V −1, the highest selectivity to acrylonitrile (70.0 mol%) and yield (25%) are obtained over (5 mol% Na + + 5 mol% Cs +)/ CaO (prepared from the sulfate precursors). Any bialkali-promoted system containing Rb was less effective, whereas the Li-containing systems, though active initially, gradually lost the activity due to its volatility. The performances of the effective bialkali systems, after an initial increase, remained almost unchanged for a period of 60 h. In contrast, the stability of any monoalkali promoted system with time-on-stream was very low and the maximum initial yield of acrylonitrile was only 11.5% under the aforementioned conditions. The synergistic increase in the catalytic performance of the bialkali promoted CaO is reflected in the synergistic increase of the surface basicity (leading to superbasicity) caused by the high enrichment of the surface layer with the alkali ions. The relationship between the catalytic performances and the physicochemical characteristics of the catalysts revealed by XPS, AAS, and basicity measurements is explored.

Ammoxidation of propene over antimony-vanadium-oxide catalysts

Antimony-vanadium-oxide catalysts were prepared with various Sb/V ratios and were used for propene ammoxidation. It was observed that antimony in excess of the amount required for forming SbVO4 was required to have a catalyst that is selective to acrylonitrile formation. Characterisation of catalysts with FTIR revealed partial reduction of the oxidised phase Sb0.92V0.92O4 upon use to form Sb0.95V1.05O4. XPS data showed the surfaces of most selective catalysts to be further enriched with antimony in course of the catalytic reaction, thereby creating a surface structure that is selective. For acrylonitrile formation a yield of 55% was obtained at 90% of conversion. In propene oxidation, on the other hand, the yield for acrolein formation was limited to 20% due to consecutive combustion of the aldehyde.

The kinetics of the formation of nitrile compounds in the atmospheres of Titan and Neptune

The nitrile compounds HCN and C2HCN (cyanoacetylene) were detected on Titan by Voyager 1. The ethylene‐analog nitrile to cyanoacetylene C2H3CN (acrylonitrile) remains undetected. More recently, ground‐based observers have detected HCN on Neptune and placed an upper limit on C2HCN. Kinetic aspects of the formation of acrylonitrile have been studied in a discharge‐flow mass spectrometric system at T = 298 K. The fractional yield for the formation of acrylonitrile from the fast gas phase reaction CN + C2H4 → C2H3CN + H has been determined to be (0.2 ± 0.1). The other possible gas phase reaction leading to the formation of acrylonitrile, HCN + C2H3 → C2H3CN + H, has been determined to be slow under planetary conditions at k(T = 298 K) = 2–7 × 10−14 cm3 molecule−1 s−1, but remains an important chemical sink for HCN. The planetary implications of the results are discussed in terms of the ratio of acrylonitrile to cyanoacetylene on Titan and Neptune.

Selective formation of acrylonitrile via oxidative methylation of acetonitrile with methane over superbasic catalysts

Promotion of CaO or MgO with various binary alkali metal compounds such as NaA + CsA, KA + CsA, LiA + CsA or LiA + NaA, where A = SO42–, OH–, Cl–, AcO–, CO32– or NO3–, leads to highly basic (superbasic) catalysts, which exhibit a noticeable synergistic effect compared with the effect produced by any monoalkali promoted system in the selective formation of acrylonitrile viaoxidative methylation of acetonitrile with methane.

Formation of propionitrile and acrylonitrile from acetonitrile and methanol

Formation of propionitrile and acrylonitrile from acetonitrile and methanol