Ammoxidation propane

Competitive Reaction Pathways in Propane Ammoxidation over V-Sb-Oxide Catalysts an IR and Flow Reactor Study... 

The surface transformations of propylene, allyl alcohol and acrylic acid in the presence or absence of NHs over V-antimonate catalysts were studied by IR spectroscopy. The results show the existence of various possible pathways of surface transformation in the mechanism of propane ammoxidation, depending on the reaction condition and the surface coverage with chemisorbed NH3. A surface reaction network is proposed and used to explain the catalytic behavior observed in flow reactor conditions. 

The catal5dic behavior in propane ammoxidation of Sb V=1.0 and 3.0 is summarized in Fig. 1. The tests were carried out using a propane concentration of about 8% and oxygen as the limiting reactant, because these experimental con-dit-ions agree with those indicated as preferable in the patent literature [12] and from the analysis of the reaction kinetics [9,10]. 

Ammonia also reacts with the acrolein intermediate, via the formation of an imine or possibly oxime intermediate which transforms faster to the acrylonitrile than to the acrylamide intermediate. This pathway of reaction occurs at lower temperatures in comparison to that involving an acrylate intermediate, but its relative importance depends on the competitive reaction of the acrolein intermediate with the ammonia species and with catalyst lattice oxygens. NH3 coordinated on Lewis sites also inhibits the activation of propane differently from that absorbed on Brsurface reaction network in propane ammoxidation. 

Catalysts tested for the reaction of n-hexane ammoxidation are reported in Table 40.1. Samples with composition SnA /Nb/Sb (atomic ratios between components) equal to x/0.2/1/3 were prepared and characterized. The atomic ratio between V, Nb and Sb was fixed becanse it corresponds to the optimal one for the active components when these catalysts are used for propane ammoxidation (10). 

The catalytic behavior of Fe-MTW zeolites in the direct ammoxidation of propane was investigated. The obtained catalytic results are compared with behavior of Fe-silicalite catalysts whose activity in propane ammoxidation was recently published. It was found that Fe-MTW catalysts exhibit the similar activity as Fe-silicalites but the selectivity to acrylonitrile was substantially lower. On the other hand, Fe-MTW catalysts produce higher amount of propene and have better acrylonitrile-to-acetonitrile ratio. 

The increasing volume of chemical production, insufficient capacity and high price of olefins stimulate the rising trend in the innovation of current processes. High attention has been devoted to the direct ammoxidation of propane to acrylonitrile. A number of mixed oxide catalysts were investigated in propane ammoxidation [1]. However, up to now no catalytic system achieved reaction parameters suitable for commercial application. Nowadays the attention in the field of activation and conversion of paraffins is turned to catalytic systems where atomically dispersed metal ions are responsible for the activity of the catalysts. Ones of appropriate candidates are Fe-zeolites. Very recently, an activity of Fe-silicalite in the ammoxidation of propane was reported [2, 3]. This catalytic system exhibited relatively low yield (maximally 10% for propane to acrylonitrile). Despite the low performance, Fe-silicalites are one of the few zeolitic systems, which reveal some catalytic activity in propane ammoxidation, and therefore, we believe that it has a potential to be improved. Up to this day, investigation of Fe-silicalite and Fe-MFI catalysts in the propane ammoxidation were only reported in the literature. In this study, we compare the catalytic activity of Fe-silicalite and Fe-MTW zeolites in direct ammoxidation of propane to acrylonitrile. 

Table 1 Catalytic activity of Fe-zeolites in propane ammoxidation at 540 °C...

V-containing silicalite, for example, has been shown to have different catalytic properties than vanadium supported on silica in the conversion of methanol to hydrocarbons, NOx reduction with ammonia and ammoxidation of substituted aromatics, butadiene oxidation to furan and propane ammoxidation to acrylonitrile (7 and references therein). However, limited information is available about the characteristics of vanadium species in V-containing silicalite samples and especially regarding correlations with the catalytic behavior (7- 6). 

Recently, amorphous high surface area vanadium aluminium oxynitrides have been reported as active catalysts for propane ammoxidation to yield acrylonitrile (AC) at atmospheric pressure. Optimal performance was achieved at 500°C using a C3Hg 02 NH3 molar ratio of 1.25 3 1 (see Tables 4 and 5). The space time yields of these catalysts have been reported to be much higher than for other catalysts reported in the literature. 

Processes based on propane ammoxidation to manufacture acrylonitrile have also been developed,915 966 and BP has announced commercialization.966 Dehydrogenation at high reaction temperature (485-520°C), which is about 100°C higher than for propylene ammoxidation, results in the formation of propylene, which subsequently undergoes normal ammoxidation. Despite higher investments and the markedly lower selectivity (30-40%), the process can be economical because of the price difference between propylene and propane.966 Better selectivites can be achieved at lower (40-60%) conversions. 

Figure 9.4 Reaction scheme of propane ammoxidation to acrylonitrile.

Figure 9.5 Simplified flow-sheet of the Mitsubishi/BOC process for propane ammoxidation.

In systems developed by Rhodia [31] the main component is Sn02 (cassiterite), which is inactive in the reaction of propane ammoxidation, while it acts as the carrier for the active components V/Sb/O and SbOx. Tin oxide facilitates the dispersion of the active components, and yields a multifunctional catalyst in which the various species can effectively cooperate in the reaction. 

Rutile-type Cr antimonate, CrSb04, is fairly active and selective in propane ammoxidation in catalysts studied by Snamprogetti, adding V considerably improves... 

An important dopant for rutile-type mixed oxides is Nb oxide [46a]. Banares et al. [49] found that when used as the support for V/Sb/O, Nb205 formed new phases by reaction with V and Sb under catalytic reaction conditions these phases, of unclear nature, affected the catalytic performance in propane ammoxidation. When instead Nb was added as a promoter for the alumina-supported V/Sb/O system, the interaction between the active components led to an improvement of catalytic performance with respect to the undoped V/Sb/O. Nb also forms rutile-type mixed... 

Flego [1] recommends the use of micro devices for automated measurement and microanalysis of high-throughput in situ characterization of catalyst properties. Murphy et al. [5] stress the importance of the development of new reactor designs. Micro reactors at Dow were described for rapid serial screening of polyolefin catalysts. De Bellefon ete al. used a similar approach in combination with a micro mixer [6], Bergh et al. [7] presented a micro fluidic 256-fold flow reactor manufactured from a silicon wafer for the ethane partial oxidation and propane ammoxidation. 

The main route to ACRN is the one-step propylene ammoxidation process. In this process propylene, ammonia and air reacted in a fluidized bed reactor to produce ACRN with acetonitrile and hydrogen cyanide as by-products. New technology based on propane ammoxidation has been developed by BP, Mitsubishi (in conjunction with BOC) and Asahi Kasei with claims of a 30% production cost advantage over the propylene route276. However no plans have been announced to build a propane-based plant as of first quarter 2004 297. 

Centi, G., Graselli, R. K., Trifiro, F., Propane ammoxidation to acrylonitrile, an overview, Catal. Today, 661-666, 1992... 

DeSanto P, Jr., Buttrey DJ, Grasselli RK, Lugmair CG, Volpe AF, Jr., Toby BH, Vogt T. Structural aspects of the Ml and M2 phases in MoVNbTeO propane ammoxidation catalysts. Zeitschrift fuer Kristallographie. 2004 219(3) 152-165. 

Popova GY, Andrushkevich TV, Aleshina GI, Plyasova LM, Khramov MI. Effect of oxalic acid content and medium of thermal treatment on physicochemical and catalytic properties of MoVTeNb oxide catalysts in propane ammoxidation. Applied Catalysis A General. 2007 328(2) 195-200. 

Mul et al., 2003). More reducible oxide moieties (e.g., alumina-supported chromia) exhibited partial reduction during reaction (Puurunen and Weckhuysen, 2002 Puurunen et al., 2001). The average oxidation state of a particular catalyst will depend on the hydrocarbon-to-C>2 ratio in the feed (Garcfa-Cortez and Banares, 2002) and on the presence of other reactants. For example, a typical propane ammoxidation feed, which also contains ammonia, results in a more reduced catalyst than a propane ODH feed (Guerrero-Perez and Banares, 2002). 

FIGURE 16 Raman spectra recorded during propane ammoxidation on alumina-supported nanocrystalline V-Sb-O system and simultaneous activity data determined by online gas chromatography (Guerrero-Perez M.O., and Banares, M.A. Chem. Commun. 1292 (2002), Operando Raman study of alumina-supported Sb-V-O catalyst during propane ammoxidation to acrylonitrile with on line activity measurement, reproduced with permission of the Royal Society of Chemistry) (Guerrero and Banares, 2002). 

Few reports have discussed the structures of Mo V Te Nb oxide catalysts in relation to propane oxidation and ammoxidation. Some reports indicate that not only the elemental composition but also preparative variables greatly affect the structure and performance of Mo-V-Te Nb oxide catalysts. Among the preparative variables, methods for precursor preparation appear to be critical. One example is Mo V Te-Nb oxide, which when prepared by a sohd-state method from corresponding oxides of each element is a mixture of M0O3 and (Mo-X)50i4 (X is other cations) and is inactive for the propane ammoxidation. However, Mo-V-Te-Nb oxide prepared by a hydrothermal reaction method from the same oxide by the solid-state method is a mono-phasic oxide with an orthorhombic layered structure, which selectively catalyzes propane to acrylonitrile. ... 

Structural Characterization of the Ml phase in the MoVNbTeO Propane Ammoxidation Catalyst... 

Figure 20.4 Feed composition in propane ammoxidation claimed by different companies.

Active and selective in propane oxidation to acrylic acid propane ammoxid. to acrylonitrile ethane oxidation to ethylene/acetic acid... 

Two main preparation methods have been used to synthesize Mo/V/Te/(Nb)/0 catalysts active in propane ammoxidation (a) the dry-up and (b) the hydrothermal synthesis. The dry-up method involves mixing aqueous slurries of metal oxide precursors followed by a gradual evaporation of the combined aqueous slurry. Solvent evaporation leads to nucleation and growth of precursor metal oxide phases, which require further heat treatment to obtain active catalysts. 

Figure 20.9 (a) Unit cell of the Ml (orthorhombic) phase along the c axis with the regions considered to be the active centers in propane ammoxidation shown in rectangular boxes, (b) Enlarged view of the proposed active center in Ml phase (Mo7 5Vi.5NbTe039) in [001] projection with indication (numbers) of the different atoms summarized below in the scheme of the active site, (c) Elaborated from [6a, 45]. 

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