Alkane ammoxidation reactions

Bulk Mixed-Metal Oxide Catalysts for Alkane Ammoxidation Reactions... 

This class of ammoxidation reactions can be fiirther divided into three subclasses wherein R is an alkene, an aromatic, or hydrogen (H). The latter is the ammoxidation of methane to hydrogen cyanide. Alkanes are also ammoxidized to unsaturated nitriles, but the saturated alkane molecule must first undergo dehydrogenation to the corresponding alkene before the ammoxidative... 

Ammoxidation reactions are irreversible and highly exothermic. Table 2 gives thermodynamic information for some selective ammoxidation reactions. As is illustrated in the table, alkane ammoxidation is more exothermic than alkene ammoxidation, which, in turn, is more exothermic than ammoxidation of an aldehyde. Table 2 also shows that nonselective reactions that produce the complete oxidation products, CO2, N2, and H2O, are thermodynamically more favorable than the selective reactions. Selective ammoxidation to nitriles in useful yields, thus, necessitates the use of a suitable catalyst to enhance the rate of the selective ammoxidation reactions relative to the nonselective complete oxidation reactions. 

The synthesis of intermediates and monomers from alkanes by means of oxidative processes, in part replacing alkenes and aromatics as the traditional building blocks for the chemical industry [2]. Besides the well-known oxidation of n-butane to maleic anhydride, examples of processes implemented at the industrial level are (i) the direct oxidation of ethane to acetic acid, developed by Sabic (ii) the ammoxidation of propane to acrylonitrile, developed by INEOS (former BP) and by Mitsubishi, and recently announced by Asahi to soon become commercial (iii) the partial oxidation of methane to syngas (a demonstration unit is being built by ENI). Many other reactions are currently being investigated, for example, (i) the... 

Acetonitrile can be produced by catalytic ammoxidation of ethane and propane over Nb-Sb mixed oxides supported on alumina, with selechvities to acetonitrile of about 50-55% at alkane conversions of around 30% [133]. In both cases, CO forms in approximately a 1 1 molar ratio with acetonitrile, owing to a parallel reaction from a common intermediate. When feeding n-butane, the selectivity to acetonitrile halves. Bondareva and coworkers [134] also studied ethane ammoxidation over similar types of catalyst (V/Mo/Nb/O). 

Important classes of reactions not included in the above list, because they are not yet used on a commercial scale, are (i) the oxidative dehydrogenation of C2-C5 alkanes, (ii) the selective oxidation of alkanes, such as the synthesis of maleic and phthalic anhydride from n-pentane and methacrolein or methacrylic acid from isobutene, and (iii) propane ammoxidation to acrylonitrile [317-319]. 

Supported metal oxides are currently being used in a large number of industrial applications. The oxidation of alkanes is a very interesting field, however, only until recently very little attention has been paid to the oxidation of ethane, the second most abundant paraffin (1). The production of ethylene or acetaldehyde from this feed stock is a challenging option. Vanadium oxide is an important element in the formulation of catalysts for selective cataljdic reactions (e. g. oxidation of o-xylene, 1-3, butadiene, methanol, CO, ammoxidation of hydrocarbons, selective catalytic reduction of NO and the partial oxidation of methane) (2-4). Many of the reactions involving vanadium oxide focus on the selective oxidation of hydrocarbons, and some studies have also examined the oxidation of ethane over vanadium oxide based catalysts (5-7) or reviewed the activity of vanadium oxide for the oxidation of lower alkanes (1). Our work focuses on determining the relevance of the specific oxide support and of the surface vanadia coverage on the nature and activity of the supported vanadia species for the oxidation of ethane. 

There was a clear upper limit in terms of selectivity-conversion beyond which experimental studies have not advanced for many selective oxidation reactions. These limits have been achieved through detailed catalyst design and reactor optimization. This work shows that active sites on oxidation and ammoxidation catalysts are capable of selectively activating, typically, a C-H bond in a reactant, rather than a similar C-H or C-C bond in the product provided that the bond dissociation enthalpy of the weakest bond in the product is no more than 30-40 kJ mole weaker than the bond dissociation enthalpy of the weakest bond in the reactant. When these limits are exceeded selectivity falls drastically. This work also indicates that primary activation of alkanes is through C-H bonds although the corresponding C-C bonds are much weaker. Cleavage of a C-C bond in the primary activation step leads directly to carbon oxide formation, but this step is less favoured because steric Victors make it difficult for the C-C bonds to be accommodated at the active site. 

Ammoxidation refers to the formation of nitriles by oxidation of hydrocarbons with oxygen in the presence of ammonia (Figure 1) [1]. Ammoxidation is best conducted with olefins, or with aromatic or heteroaromatic compounds, containing a readily abstractable H atom (allylic or benzylic intermediates are formed), although the ammoxidation of alkanes (e. g. propane to acrylonitrile [e. g. 2-4] or ethane to acetonitrile [e. g. 5]) is also possible. An exceptional example is the ammoxidation of methane to hydrogen cyanide by the Andrussov reaction [6]. 

Combinatorial chemistry can perhaps help discover new catalyst formulations for reactions presently of particular interest, such as oxidations or ammoxidation, and generally all reactions of alkanes. Reactions traditionally made in different kinds of processes are frequently shown to be also activated by heterogeneous catalysts (e.g., epoxidations). Reactors of unexpected design allow surprisingly selective reactions (e.g., monoliths for the oxidative dehydrogenation of light alkanes). However, the distance often remains long between these discoveries and the manufacture of active and selective catalysts adequately structured for particular use in an industrial reactor inserted in an industrial plant. 

The low pifa values for alcohols ( 16 for methanol), compared to about 30 and 44 for alkenes and alkanes, respectively, along with the acid-base character of metal oxides, allows this step to be easily facilitated by a number of (amm)oxidation catalysts. These characteristics of the reaction allow selective ammoxidation of alcohols to be conducted under milder reaction conditions than ammoxidation of alkenes and alkanes, generally at temperatures below 400°C. This promotes selectivity for the ammoxidation of the alcohol to the corresponding nitrile product by lessening the oxidation activity of the catalyst for complete oxidation to COg. 

As with methanol ammoxidation, yield and selectivity to the nitrile product are relatively high compared to ammoxidation of an alkene or alkane substrate. Using alumina-supported V-P-Sb-0 catalysts, selectivity to acetonitrile of 96% is obtained at 84% conversion of ethanol (81% acetonitrile yield) at 400°C (101), whereas a silicoaluminophosphate molecular sieve gives a reported 99% yield of acetonitrile at complete conversion of acetonitrile at 350°C (102). Both catalysts possess a relatively high level of surface acidity, mainly because of the presence of phosphorus and aluminum oxide moieties. These are expected to promote the initial step of the reaction—surface alkoxy formation by, for example, an equilibrium with surface hydroxyl groups. 

Recently, vanadium phosphate catalysts have been found to be effective catalysts for the oxidation of other alkanes, for example, propane ammoxidation and pentane oxidation to phthalic anhydride and maleic anhydride. However, these reactions are not commercialized and the oxidation of n-butane to maleic anhydride represents the only industrial, large-scale selective oxidation of an alkane currently in operation. 

Indeed, more complex catalysts are required for partial oxidation reactions. Although several catalytic systems have been studied in the last twenty years, a very limited number of catalysts have been reported for industrial or pre-industrial use. In fact, in addition to V-P-0 catalysts (based on vanadyl pyrophosphate), the unique catalyst used for an alkane oxidation industrialized process, only V-Sb- and MoVTe(Sb)NbO-based mixed-metal oxides have been proposed as sufficiently effective catalysts for the propane ammoxidation process. In both cases pilot plants using the latter catalysts have been announced on the bases of their catalytic results. 

---