Propane Oxidation and Ammoxidation

The direct ammoxidation of propane into acrylonitrile by its reaction with oxygen and ammonia is an alternative route to the conventional propylene ammoxidation catalytic reaction to acrylonitrile. The use of propane instead of propylene makes the reaction conditions more demanding because of the necessity of activating the propane C-H bond which is much stronger than the propylene methyl C-H bond. Different catalytic systems have been investigated for the ammoxidation of propane into acrylonitrile with vanadium-based catalysts proving particularly efficient, particularly the Sb-V-0 mixed oxide catalyst.  

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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. ... 

Table 6 Catalytic results in propane oxidation and ammoxidation over Mo-V-Te-Nb oxide catalyst ...

SCHEME 8.1 Propane oxidation and ammoxidation mechanism and active site (From Vedrine, J.C., Top. Catal. 2002, 21, 97-106. With permission.)... 

The ODH of propane over titanium and vanadium containing zeolites and nonzeolitic catalysts revealed that Ti-silicalite was the most active. The addition of water caused an increase in selectivity, probably due to a competitive adsorption on the active sites. The reaction is proposed to occur on the outer surface of the Ti-silicalite crystallites on Lewis acid sites, and a sulfation of the catalyst, which increases the acidity of these sites, results in a further increase of the catalytic activity. The maximum conversion obtained was 17% with a propene selectivity of up to 74% [65]. Comparison of propane oxidation and ammoxidation over Co-zeolites shows an increase in conversion and propene selectivity during ammoxidation. For a conversion of 14%, 40% propene selectivity was obtained with ammonia, whereas, at 10% conversion the propene selectivity was only 12% with oxygen. The increase in activity and selectivity can be due to the formation of basic sites via ammonia adsorption [38]. 

Further examples of attempts to replace olefins by alkanes as a starting materials, as in the maleic anhydride process, are the development of processes for selective oxidation and ammoxidation. Examples are processes for acrolein, acrylic acid (Table 2, entry 19) and acrylonitrile (Table 2, entry 20) using propane as a feedstock... 

Vanadyl pyrophosphate is widely considered to play an important catalytic role in the oxidation of -butane to MA, specifically the (100) face (Figure 18b), which is retained from the topotactic transformation (6,43,84—86) of the catalyst precursor phase (Figure 18a). Furthermore, this active phase has been reported to be an efficient catalyst for the oxyfimctionalization of light paraffins (a) for the oxidation of ethane to acetic acid (3,87), (b) for the oxidation and ammoxidation of propane to acrylic acid (88) and acrylonitrile (89,90), respectively, and (c) for the oxidation of n-pentane to maleic and phthalic anhydrides (90-102). 

We shall first show that it is still far fiom clear which are the families of catalysts to be used for the various reactions mainly oxidative dehydrogenation or oxidation to oxygen-containing molecules of ethane, propane or isobutane. Much research is still necessary for understanding the mechanisms leading to high selectivity. In this context, we shall suggest that many concepts inherited from the development in selective oxidation and ammoxidation of olefins are probably of little use. 

In the case of MoVTe(Sb)NbO-based catalysts, iiutially developed by Mitsubishi for the partial oxidation and ammoxidation of propane. 

Oliver, J., Lopez Nieto, J. and Botella, P. (2004). Selective Oxidation and Ammoxidation of Propane on a Mo-V-Te-Nb-O Mixed Metal Oxide Catalyst A Comparative Study, Catal. Today, 96, pp. 241-249. 

Shishido, T., Konishi, T., Matsuura, I., et al. (2001). Oxidation and Ammoxidation of Propane over Mo-V-Sb Mixed Oxide Catalysts, Catal. Today, 71, pp. 77-82. 

The most effective, from the listed catalysts till date, for propane oxidation to acrylic acid is Mo-V-Te-Nb mixed oxides, patented by Ushikubo et al. [56,57] and Lin and Linsen [59], which give more than 40% yield of acrylic acid. The catalyst with the same elemental composition appears to be very active and selective for propane ammoxidation reaction (58% yield of acrylonitrile at 89% propane conversion) [68]. This indicates that propane oxidation and propane ammoxidation share some fundamental reaction steps and active crystalline phases. 

Formation of active component of MoVTeNb oxide catalyst for selective oxidation and ammoxidation of propane and ethane... 

Most industrially desirahle oxidation processes target products of partial, not total oxidation. Well-investigated examples are the oxidation of propane or propene to acrolein, hutane to maleic acid anhydride, benzene to phenol, or the ammoxidation of propene to acrylonitrile. The mechanism of many reactions of this type is adequately described in terms of the Mars and van Krevelen modeE A molecule is chemisorbed at the surface of the oxide and reacts with one or more oxygen ions, lowering the electrochemical oxidation state of the metal ions in the process. After desorption of the product, the oxide reacts with O2, re-oxidizing the metal ions to their original oxidation state. The selectivity of the process is determined by the relative chances of... 

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). 

The multifunctionality is achieved through either the combination of two different compounds (phase-cooperation) or the presence of different elements inside a single crystalline structure. In antimonates-based systems, cooperation between the metal antimonate (having a rutile crystalline structure), employed for propane oxidative dehydrogenation and propene activation, and the dispersed antimony oxide, active in allylic ammoxidation, is made more efficient through the dispersion of the latter compound over the former. In metal molybdates, one single crystalline structure contains both the element active in the oxidative dehydrogenation of the hydrocarbon (vanadium) and those active in the transformation of the olefin and in the allylic insertion of the N H2 species (tellurium and molybdenum). 

More than a decade after the publication of the MoVNb catalyst system, scientists at Mitsubishi Chemical reported that modifying this family of mixed metal oxides with Te produced a catalyst for the amoxidation of propane to acrylonitrile [4] and the oxidation of propane to acrylic acid [5], Modification of the Union Carbide catalyst system with Te was probably not a random choice as it is a known propylene activator [5 b] and the molybdate phase TeMoO oxidizes propylene into acrolein and ammoxidizes propylene to acrylonitrile [6], a key intermediate in the commercial production of acrylic acid using Mo-based oxides. Significant efforts to optimize this and related mixed metal oxides continues for the production of both acrylic acid and acrylonitrile, with the main participants being Asahi, Rohm Hass, BASF, and BP. 

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. 

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

The mechanism for acrylonitrile formation via intermediate propylene is demonstrated by the results given in Figure 6, which compares the oxidation and the ammoxidation of propane over a V/Sb/0 mixed oxide catalyst (69). The conversion of propane and the selectivity to the main products are reported (acrolein was formed only in traces). 

In other examples, extensively studied by Delmon et al., SbaOa was used with M0O3 for isobutene oxidation to methacrolein [29, 30], and for the dehydration of N-ethyl formamide [46,47]. Antimony is one of the elements frequently found in selective oxidation catalysts, as in the pionneering work on uranium antimony oxides for ammoxidation of propene [48], and more recently in ammoxidation of propane on V-Sb-Al system [49]. 

This coherent reaction network clearly demonstrates the in ortance of the 30-40 kJ mole selectivity limit. When it is exceeded, as is the case with propane oxidation to acrolein, selectivity declines drastically. Similarly the accnmmulated data for propane and propene ammoxidation [27,28] to acrylonitrile indicate selectivities at 30% conversion of 50% and 85% respectively. These data are consistent with the 41 kJ mole difference in bond enthalpies shown in scheme 2 for propane and propene. 

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