Alkenes ammoxidation

Therefore, the co-ordinating properties (intrinsic Lewis acidity) and the redox properties (electron transfer) of the metal ion are important for the first step, and for the transformation of the adsorbed hydrocarbon into the nitrile rather than to carbon oxides and HCN. It is also worth noting that when a conventional alkene ammoxidation catalyst is used, such as Bi/Mo/O, the performance is much worse (yield 4.5% at 450°C). 

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 scientific and technological breakthrough of alkene ammoxidation resulted from the subsequent discovery by SOHIO and Distillers of the one-step conversion of propylene to acrylonitrile (5-7). It was found that acrylonitrile is produced in one step when ammonia and propylene are fed directly to a reactor containing a suitable oxidation catalyst such as the SOHIO bismuth... 

Surface Reaction Mechanism. The mechanism of catalytic alkene ammoxidation is invariably linked to allylic oxidation chemistry. Allylic oxidation is the selective oxidation of an alkene at the allylic carbon position. Selective allylic oxidation and ammoxidation proceed by abstraction of the hydrogen from the carbon positioned a to the carbon-carbon double bond. This produces an allylic intermediate in the rate-determining step. In the case where propylene is the hydrocarbon, the reaction is as follows ... 

Table 3. Correlation of Electronic Structure and Catalytic Function for Selective Alkene Ammoxidation Catalysts...

Alkene ammoxidation technology also encompasses several other allylic alkenes, most notably isobutene. When isobutene is the reactant, the selective nitrile product is methacrylonitrile. 

Another facet of surface organometallic chemistry involves modelling of the mechanisms of surface reactions on the basis of the reactivity of molecular models. For example, the reactivity of metal-imine complexes of molybdenum is considered by CHAN, who proposes elementary steps constituting the catalytic cycle of the surface-catalyzed alkene ammoxidation reaction, which is of great industrial importance. HERRMANN provides some very fine examples of molecular models of the rhenium oxide catalysts used commercially in the alkene metathesis reaction. 

Light hydrocarbons consisting of oxygen or other heteroatoms are important intermediates in the chemical industry. Selective hydrocarbon oxidation of alkenes progressed dramatically with the discovery of bismuth molybdate mixed-metal-oxide catalysts because of their high selectivity and activity (>90%). These now form the basis of very important commercial multicomponent catalysts (which may contain mixed metal oxides) for the oxidation of propylene to acrolein and ammoxidation with ammonia to acrylonitrile and to propylene oxide. 

Oxidation of the allylic carbon of alkenes may lead to allylic alcohols and derivatives or a, 3-unsaturated carbonyl compounds. Selenium dioxide is the reagent of choice to carry out the former transformation. In the latter process, which is more difficult to accomplish, Cr(VI) compounds are usually applied. In certain cases, mixture of products of both types of oxidation, as well as isomeric compounds resulting from allylic rearrangement, may be formed. Oxidation of 2-alkenes to the corresponding cc,p-unsaturated carboxylic acids, particularly the oxidation of propylene to acrolein and acrylic acid, as well as ammoxidation to acrylonitrile, has commercial importance (see Sections 9.5.2 and 9.5.3). 

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... 

A number of bismuth/metal oxides are used in the catalytic oxidation of hydrocarbons, and in the oxidation and ammoxidation of alkenes These transformations are described briefly in Bismuth Inorganic Chemistry. 

Figure 1. Formation of nitriles by ammoxidation of alkanes, alkenes, methyl aromatic and heteroaromatic compounds.

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... 

The yields and selectivities for the ammoxidation of substrate molecules to the corresponding nitriles generally increase as a function of the reactant type in the following order alkane < alkene < alcohol < aldehyde (see table below). 

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. 

A significant economic incentive exists to utilize alkanes in place of alkenes for the production of nitrile compounds by ammoxidation catalysis because of the historically large price differential between the two feedstocks. Despite the historical feedstock cost advantage of alkanes versus the corresponding alkenes, the yield and selectivity to product well in excess of 50 mol% are required for economic viability of an alkane-based process (108). 

Due to their synthetic utility and pharmaceutical applications, a number of synthetic routes to a,P-unsatuiated nitriles have been developed. Classical methods include dehydration of amides [15] or aldoximes [16], Wittig- [17] and Peterson-type [18, 19] olefination processes, ammoxidation of alkenes [20], and carbocyanation of alkynes [21], These approaches suffer variously from high waste generation, low yields and poor stereoselectivity. As such the transition metal-catalysed cyanation of vinyl halides is an attractive route for the synthesis of alkaiyl nitriles. Whilst several instances of such transformations have been reported in the literature, the area remains underdeveloped, particularly in comparison to analagous cyanations of aryl halides. 

Lopez Nieto, J., Botella Asuncion, P. and Solsona Espriu, B. (2003). Worldwide Patent 2003008096 Al, Catalyst for the Selective Oxidation and Ammoxidation of Alkanes and/or Alkenes, Particularly in Processes for Obtaining Acrylic Acid, Acrylonitrile and the Derivatives Thereof (CSIC-UPV, Spain). 

The charge of the heteroatom that is doubly bonded to the metal decreases by going towards the top right of the periodic table. With the complexes [OSO4] and [0s(=NR)03], the O and NR ligands react with the double bond of non-activated alkenes to undergo dihydroxylation, ammoxidation and diamination of the olefins. 

The traditional transformations towards aryl nitriles usually require harsh conditions. One of the groundbreaking researches in the direct transformation of alkenes to nitriles was reported by Denton et al. in 1950 (Scheme 4.12) [98]. In this alumna supported molybdic oxide catalyzed ammoxidation, alkyl and alkenyl aromatic hydrocarbons were converted to aromatic nitriles at high temperatures (524-552 °C). 

---