Propane Ammoxidation Research Articles

Site isolation and cooperation effects in the ammoxidation of propane with VSbO and Ga/H-ZSM-5 catalysts

The performances of a conventional mixed oxide catalyst, VSbyOx, and of a novel zeolite-based catalyst, Ga-modified H-ZSM-5 (Ga2Z27), are compared for the ammoxidation of propane. In both cases, the effects of reaction temperature and ammonia to propane ratio were investigated. Increasing productivity to acrylonitrile is observed when the temperature is raised and the ammonia to propane ratio increased. The maximum yields in acrylonitrile are 8.5 and 11.4% for the unpromoted VSbyOxand Ga2Z27 catalysts, respectively, at comparable propane conversions (ca. 30%). The activity and productivity to acrylonitrile of Ga2Z27 are about 50% of those of VSbyOx, at 500 °C, in the absence of any additional promoters. As reported previously, propene and carbon oxides, CO and CO2, were observed to be major by-products when using VSbyOxcatalysts, 10–20 and 20–40%, respectively. By contrast, the Ga2Z27 catalyst shows a very low selectivity to carbon oxides (1% or less) and a rather high and interesting selectivity to C4hydrocarbons, consisting mainly of isobutane and isobutene. These observations can be explained by differences in the reaction mechanisms describing the ammoxidation of propane on these two systems. For VSbyOx, the classical mechanism applies. Propene is the primary reaction product which is easily oxidised to carbon oxides. For Ga2Z27, propane is initially activated to a protonated pseudo-cyclopropane (PPCP) transition state on bifunctional sites involving dispersed gallium species and Bronsted acid sites of the zeolite. The PPCP transition state can evolve in several ways leading to propene, C2–C4hydrocarbons, and N-containing compounds. Isolation of gallium species is necessary for Ga/H-ZSM-5 in order to generate the bifunctional, Ga3+–H+, active sites, whereas vanadium sites isolation is required to avoid overoxidation for VSbyOx. Phase cooperation is necessary for the VSbyOxcatalysts, as reported previously, whereas site cooperation is required for Ga-modified H-ZSM-5.

The importance of site isolation and phase cooperation in propane ammoxidation on rutile-type vanadia catalysts

Propane ammoxidation to acrylonitrile over rutile-type vanadia catalysts is discussed regarding phase cooperation and site isolation effects. Compared with the pure phases, biphasic catalysts with both ∼SbVO4and α-Sb2O4are considerably more selective to the formation of acrylonitrile. It is demonstrated that cooperation between the phases during the calcination of the catalyst and the use in propane ammoxidation results in spreading of antimony species from free antimony oxide to the surface of ∼SbVO4, forming Sb5+–V3+/V4+supra-surface sites being involved in the formation of acrylonitrile. Dilution and isolation of the vanadium centers in ∼SbVO4through the partial replacement with, e.g., Al, Ti and W improves the catalytic properties. Structure–reactivity correlations using data for a nominal Sb0.9V0.9-xTixOyseries indicate that the activation of propane occurs on a V3+site and the activation of ammonia requires an Sb5+site.

Involvement of Redox Sites in Propane Ammoxidation on V-Sb-Oxide Catalysts.

V-Sb-Oxide catalysts were prepared by SS (solid solution) and CP (co-precipitation) methods, and the involvement of redox sites and acid sites in the ammoxidation of propane were investigated by BET, XRD, XPS, NARP (Nitric oxide-Ammonia Rectangular Pulse) and NH3-TPD methods. Both series of catalysts mainly consisted of the SbVO4 phase with minor components of V2O5 and α-Sb2O4 phases. The reaction rate of propane ammoxidation increased linearly with the increase in the number of redox sites measured by NARP technique. There was no significant effect of strength and amount of acid sites on the activity of propane ammoxidation at low conversion levels. The number of redox sites is the determining factor for ammoxidation activity. V-Sb-Oxide catalysts with excess α-Sb2O4 phase exhibited higher turnover frequency.

Selective oxidation and ammoxidation of propene on bismuth molybdates, ab initio calculations

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 BiMoOxcatalysts. 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 while 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. Since 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 coworkers 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.

A crystalline SbRe2O6 catalyst active for selective ammoxidation of isobutylene and propene

This paper reports the first performance of a crystalline SbRe2O6 catalyst, which is a new family of Re mixed oxide, for the selective ammoxidation reactions of isobutylene to methacrylonitrile and propene to acrylonitrile. The SbRe2O6 with alternate (Re2O6)3– and (SbO)+ layers showed much better performance than two other known Re–Sb–O compounds SbOReO4sdot2H2O and Sb4Re2O13 with tetrahedral (ReO4)– anions and cationic (SbO)+ layers. The SbRe2O6 catalyst was also much more active than a coprecipitated SbRe2Ox catalyst, a supported Re2O7/Sb2O3 catalyst, and Re oxides such as Re2O7, ReO3 and ReO2 for the selective ammoxidation. Rhenium is prerequisite to the ammoxidation catalysis of the Re–Sb mixed oxides. Sb oxides such as Sb2O3 and Sb2O4 were inactive, but Sb in the SbRe2O6 catalyst contributes positively to the methacrylonitrile and acrylonitrile syntheses. Neither change nor modification of the surface composition, crystallinity and morphology of SbRe2O6 occurred under the ammoxidation reaction conditions, where the presence of NH3 stabilized the crystalline SbRe2O6 structure.

Rutile-type Cr/Sb mixed oxides as heterogeneous catalysts for the ammoxidation of propane to acrylonitrile

Cr/Sb mixed oxides with a rutile-type structure were synthesized by calcining (700 °C) a mixture of oxohydrates obtained by coprecipitation from an alcohol solution containing the required amount of the components. The samples were characterized using X-ray powder diffraction (XRD), FT-IR spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). In the Cr/Sb/O compound prepared with Cr/Sb 1/0.8 (atomic ratio), an almost stoichiometric CrSbO4develops (no antimony or chromium oxides are found), which however is characterized by enrichment in Sb in outmost rutile atomic layers, and thus by a non-homogeneous intracrystalline distribution of the two components. For Cr/Sb ratios lower than 1, the amount of Sb in the rutile-type structure exceeds the stoichiometric value. The reactivity of Cr/Sb/O catalysts with increasing Sb contents (from Cr/Sb 1/0.8 to 1/2.8) is slightly affected by the Cr/Sb atomic ratio. In particular, the selectivity increases with increasing Sb content, while the catalytic activity is higher for the samples having higher Cr/Sb ratios. The Cr/Sb/O system presents considerable differences with respect to the V/Sb/O rutile system; these differences are discussed in reference to the properties of the transition metal components of the rutile mixed oxides.

Crystal chemistry and phase composition of the MoVTeNbO catalysts for the ammoxidation of propane

Unit cell parameters and space group symmetry have been determined for the two main phases of the MoVTeNbO catalysts; models of their structures are proposed based on electron micrographes, EDX data and comparison with other crystal structures.

A pilot plant study and 2-D dispersion-reactor model for a high-density riser reactor

The radial concentration profiles of reactant concentrations in a pilot plant high-density riser propylene ammoxidation reactor were sampled and analyzed and the extreme maldistribution of the profiles were found. A two-dimensional dispersion-reactor model was proposed to simulate the selective oxidization that taken place in the riser, which considered the influences of axial and lateral gas mixing, the non-uniformity of radial solids concentration and gas-velocity profiles on the reactor. The model can successfully predict the axial and radial profiles of concentrations sampled in the pilot plant. Simulation results indicate that the non-uniformity in radial catalyst concentration and gas-velocity distribution as well as insufficient lateral gas mixing result in a significant loss of selectivity and yield of acrylonitrile. The scale-up effect and the interdependence of the reaction rate, the non-uniformity of catalyst concentration and the requirement of gas mixing are discussed in details.

Propane ammoxidation over gallium-modified MFI zeolites

This is the first report indicating that the presence of Brønsted sites, acting in synergy with redox sites in Ga-modified MFI zeolites, can catalyse the ammoxidation of propane. Two catalysts with gallium loadings of 2 and 0.3 wt.% were prepared by ion-exchange of MFI zeolites with Si/Al ratios equal to 33 and 150, respectively. The integrity of the zeolites was essentially preserved following ion-exchange, as demonstrated by surface area, porosity, and crystallinity measurements. In situ 13C and 15N MAS NMR studies, using propane-2- 13C and 15NH 3 as labelled reactants suggest that Ga-modified H-MFI catalysts promote the formation of C–N bonds when propane is reacted in the presence of ammonia and oxygen (C 3H 8:NH 3:O 2 molar ratio=1 :3 :2 , T=673 K ). Conventional microcatalytic reactor results confirm the conclusions of the in situ MAS NMR investigations. Propane is converted to mainly propene and acrylonitrile, with minimal production of CO x (CO and CO 2), in the temperature range 723–773 K. Increasing the temperature to 773 K worsens the selectivity to acrylonitrile, whereas increasing the molar ratio NH 3:C 3H 8 from 2 to 4 increases the selectivity to acrylonitrile and decreases conversion. Both the conversion and the selectivity to acrylonitrile increase when the molar ratio O 2:C 3H 8 is increased from 0.3 (sub-stoichiometric) to 2 (stoichiometric). A new reaction pathways is proposed in which a cyclic protonated pseudo-cyclopropane (PPCP) transition state is responsible for the low temperature activation of propane.

Bulk and surface structure and composition of V–Sb mixed-oxide catalysts for the ammoxidation of propane

Vanadium–antimony mixed oxides, which are active and selective catalysts for the ammoxidation of propane to acrylonitrile, were obtained via different preparation routes and studied with a number of bulk and surface-sensitive techniques to elucidate the bulk composition of these complex materials and the character of their exposed surfaces. The V–Sb oxides were prepared via a redox reaction between NH 4VO 3 and Sb 2O 3 in an aqueous slurry, with subsequent calcination, or via a solid-state reaction between Sb 2O 3 and V 2O 5. The characterisation techniques employed were X-ray diffraction (XRD), N 2 physisorption, electron microscopy, potentiometric titration, Mössbauer, EPR, X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), ultraviolet photoelectron spectroscopy (UPS), and IR spectroscopy. It was found that samples obtained by the solid-state reaction were more homogeneous than those prepared via the slurry route. The former consisted of (non-stoichiometric) VSbO 4 for Sb/V=1, or a physical mixture of it with defective Sb 2O 4 for Sb/V=2. Their bulk Sb/V ratio was found also for the surface region, the outmost part of which was enriched in vanadium to a small extent. Materials prepared via the slurry route consisted of (non-stoichiometric) VSbO 4, Sb 2O 4, and V 2O 5 if Sb/V=1; the latter was missing for Sb/V=2 and Sb/V=5. While the character of the mixed surfaces was not determined by the V 2O 5 present, amorphous V oxide structures (highly dispersed species and aggregates) were supported on the Sb 2O 4 surface, which caused a significant vanadium excess in the overall surface composition in samples of Sb/V>1. The average oxidation degree of the surface V species was higher than 4+. Use of these catalysts in the propane ammoxidation reaction caused the surface V oxidation degree to approach 4+ and diminished the degree of surface enrichment in vanadium. This was due both to a disappearance of the dispersed V entities and a decreased detectability of V oxide aggregates (aggregate growth or solid-state reaction with supporting Sb 2O 4). No evidence is available for surface spreading of Sb species as discussed in the literature.

Characterization and Kinetic Studies on the Highly Active Ammoxidation Catalyst MoVNbTeOx

The catalytic performance and structures of multicomponent “MoVNbTe” oxides have been studied to find the correlation among the performance, active phases, and preparation conditions. The catalyst with the composition MoV0.4Nb0.1Te0.2Ox showed high activity and selectivity for propane ammoxidation reaction (ca. 50% selectivity to acrylonitrile at ca. 90% conversion) when it was pretreated at 830–900 K for 2 h under O2-free or less than 100 ppm O2-contaminated N2 or He flow conditions. Active phases were characterized by XRD, which gave 2θ=14.1°, 24.9°, 26.9°, 29.1°, 32.4°, 33.2°, and 35.4° in addition to the peaks at 22.1°, 28.2°, 36.2°, 44.7°, and 50.0° reported previously. XPS revealed the presence of Mo5+ in addition to Mo6+. In the presence of more than 1000 ppm O2 contamination during the pretreatment, α-MoO3 was formed at the surface region, resulting in reduction of the activity for propane ammoxidation. The activation energy for propane conversion was 32.0 kcal/mol, which was a little smaller than the 36.1 kcal/mol for propene ammoxidation. The reaction rate (v) for propane ammoxidation on the MoVNbTe oxide catalyst at 693 K is given by v=k [propane]0.8 [O2]0.4 [NH3]−0.4. The active phases and reaction mechanism are discussed on the basis of the characterization by XRD and XPS and the kinetic data.

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.

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.

Modeling of a circulating fluidized bed for ammoxidation of propane to acrylonitrile

The catalytic ammoxidation of propane over V–Sb–Al catalyst in a circulating fluidized bed (CFB) is investigated. A mathematical model for the system based on combining the kinetics of reaction and the hydrodynamics of the bed is used to simulate the reactor performance. Sensitivity analysis of the effect of operating conditions shows that the acrylonitrile yield increases as reaction temperature increases or as propane feed mole fraction decreases. The acrylonitrile yield decreases as solid circulation rate, and gas superficial velocity increase.

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.

The structure of catalytically active vanadium sites in V–Sb oxides: an EPR study of asynthesized and tungsten promoted working catalysts

EPR investigation of complex V–Sb oxide catalysts used for the ammoxidation of propane to acrylonitrile (VSb y O x ( y=1, 2, 5), VSb 5WO x and 30 wt. % VSb 5WO x/Al 2O 3) indicated the presence of small amounts of isolated VO 2+ ions evidenced by characteristic hfs signals superimposed on broad singlets of interacting V 4+ centres. Almost equal g- and A-tensor parameters of the VO 2+ ions in the VSb y O x samples suggest their location in similar hosts, presumably in a XRD-amorphous V 5+O x -phase. Changes caused by tungsten doping indicate that W ions are incorporated in close neighbourhood to the vanadium ions. In samples with high antimony content additional V 4+–V 4+ pairs were detected. In situ measurements under reaction conditions (793 K, C 3H 8/N 2/O 2/NH 3) with a self-constructed in situ-EPR flow reactor revealed that not isolated VO 2+ centres but interacting V 4+ ions participate in the activation of propane while no changes of the vanadium signals occurred on addition of NH 3.

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.

Propane ammoxidation on an Al–Sb–V–W-oxide catalyst. A mechanistic study using the TAP-2 reactor system

The reaction mechanism of propane ammoxidation was studied on an Al–Sb–V–W-oxide catalyst using a TAP-2 reactor system. Analyses of the responses from both high-speed pulse transients with reactants and TPD experiments were performed. Since the ammoxidation process with three reactants proceeds from propane to acrylonitrile over propylene as an intermediate, the experiments comprised oxidation of propylene, oxidation of ammonia, ammoxidation of propylene, oxidation of propane, and ammoxidation of propane. The results show that propane is irreversibly adsorbed at the surface forming propylene, which desorbs. Propylene then readsorbs forming an intermediate allyl species, which reacts with lattice oxygen to give acrolein. Acrolein is unstable and some of it reacts further to produce either carbon oxides, or, acrylonitrile. Formation of the nitrile occurs by adsorbed acrolein reacting with an NH x species. The latter species is short-lived and reacts competitively to form N 2, N 2O and NO. Lattice oxygen plays an important role in the pathway to acrylonitrile. However, weakly adsorbed oxygen species are also present at the catalyst surface, and these species participate in degradation routes producing waste products. Consideration of the mechanistic scheme which is derived from the experimental results shows the possibility to achieve improvement of the ammoxidation process by using either a recirculating solids reactor, or a high propane/oxygen ratio in the feed.

Advances and future trends in selective oxidation and ammoxidation catalysis

A review of the current status and future trends in oxidation catalysis is presented. The topics entail the (amm)oxidation of propylene to acrylonitrile using complex multicomponent mixed metal molybdates and antimonates as catalysts, the ammoxidation of propane to acrylonitrile using V-Sb-oxides or V-Te-Nb-Mo-oxides, the oxydehydrogenation of light paraffins using Ni-Co-molybdates or Mg-V-Sb-oxides, dehydrogenation of propane combined with selective hydrogen oxidation using Group VIII elements, oxidation of propane to acrylic acid using complex molybdates, and the (amm)oxidation of xylenes to terepthalic acid or terepthalonitrile using Fe-Mo-modified ZSM-5 catalysts. The respective reaction mechanism of the prevailing selective catalytic oxidations are discussed, as are the specific functionalities of the various elements contained in the mixed metal catalysts. Some new approaches to the rational design and synthesis of improved catalysts are offered.

Improving conversion and selectivity of catalytic reactions in bubbling gas–solid fluidized bed reactors by control of the nonlinear bubble dynamics

In this paper a model is presented that is a dynamic extension of the classic two-phase reactor models used to predict conversion and selectivity of fluidized reactors. The most important part of the model is a dynamic discrete bubble model that can correctly predict bubble sizes and also exhibits chaotic dynamics. This bubble model is based on the discrete bubble models presented by Clift and Grace [AIChE Symp. Ser. 66 (105) (1970) 14; 67 (116) (1971) 23; in: J.F. Davidson, R. Clift, D. Harrison (Eds.), Fluidization, Academic Press, London, 1985, p. 73] and Daw and Halow [AIChE Symp. Ser. 88 (289) (1992) 61]. The latter showed that this type of models can exhibit chaotic behavior. By application of an extended version of Pyragas' control algorithm [K. Pyragas, Phys. Lett. A 170 (1992) 421] the bubble dynamics can be changed from chaotic to periodic in a `flow'-regime in which the model otherwise would predict chaotic behavior. Pyragas' control algorithm is used to synchronize a chaotic system with one of its periodic solutions using a feedback control loop. This results in smaller bubbles, thus enhancing mass transfer of the reactant gas in the bubbles to the catalyst particles. The model is used to predict the effect of the changed bubble dynamics on a catalytic reaction of industrial importance, viz. the ammoxidation of propylene to acrilnitril (Sohio process). It is shown that both conversion and selectivity are appreciably enhanced.