Ammoxidation catalysts

Most, if not all, of the acetonitrile that was produced commercially in the United States in 1995 was isolated as a by-product from the manufacture of acrylonitrile by propylene ammoxidation. The amount of acetonitrile produced in an acrylonitrile plant depends on the ammoxidation catalyst that is used, but the ratio of acetonitrile acrylonitrile usually is ca 2—3 100. The acetonitrile is recovered as the water azeotrope, dried, and purified by distillation (28). U.S. capacity (1994) is ca 23,000 t/yr. 

The active site on the surface of selective propylene ammoxidation catalyst contains three critical functionalities associated with the specific metal components of the catalyst (37—39) an a-H abstraction component such as Sb ", or Te" " an olefin chemisorption and oxygen or nitrogen insertion component such as Mo " or and a redox couple such as Fe " /Fe " or Ce " /Ce" " to enhance transfer of lattice oxygen between the bulk and surface... 

Intensive studies on the mechanism of oxidation and ammoxidation have been carried out.900 939957-960 All selective oxidation and ammoxidation catalysts are... 

A commercial, iron-promoted ammoxidation catalyst (Fe/Sn/Sb = 0.25/1/4) was investigated by Crozat and Germain [93] using a flow reactor at 350—480°C, atmospheric pressure and a C3H6/02 ratio of 1/10. The... 

The kinetics of the toluene oxidation over Bi2Mo06 and a commercial Bi—Mo—P—O ammoxidation catalyst were investigated by Van der Wiele and Van den Berg [348]. A flow reactor was used at 450—550°C, 1—3 atm and varying feed rate, toluene and oxygen partial pressures. Benzaldehyde formation and combustion reactions are the main process the parallel-consecutive scheme which applies is... 

Almost all other studies on elements other than iron are performed in absorption mode. For example, Bussiere and co-workers used the Mbssbauer effect to study the state of tin in supported Pt-Sn [37] and Ir-Sn [38] reforming catalysts, and of tin and antimony in mixed Sb-Sn oxides for the selective oxidation of propylene [39]. Also of note are Millet s investigations using the 125Te isotope to characterize the state of the tellurium promoter in multicomponent ammoxidation catalysts [40],... 

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. 

Other characterization of catalysts have dealt with isotopes such as Te, Ir, Ru and Eu. The tellurium-containing catalysts were MoVTeNbO ammoxidation catalysts (175) and Te-doped Cr203 hydration-dehydration catalysts (176) and the... 

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

The considerations reported above suggest that the mechanism of reaction might not be the same as the well known allylic insertion of a nucleophilic NH " species onto the activated hydrocarbon, to generate the precursor of the cyano group. Indeed, none of the elements included in catalyst formulation is able to produce the M=NH species (which is generated by Mo and Sb in propene ammoxidation catalysts). 

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). 

CoUeuille and coworkers [122] investigated catalysts for butadiene ammoxidation which are similar to those also studied in the ammoxidation of benzene (see below). Table 20.5 summarizes the results reported. The main products were fumaronitrile and maleonitrile, cro to nitrile (the unsaturated mononitrile, 1-cyano-propene, with the two trans and cis isomers) and CO with traces of acrylonitrile and furan. The residence time used was very low in this case the best performance was obtained with a typical propene ammoxidation catalyst, made of Bi/Mo/P/O under conditions of low butadiene conversion. 

Brazdil, J.F. Cavalcanti, A.P. Padolewski, J.P. Method for Preparing Vanadium Antimony Oxide Based Oxidation and Ammoxidation Catalysts US Patent 5,693,587, Dec 2, 1997 [assigned to The Standard Oil Company of Ohio (Sohio/BP)]. 

All selective oxidation and ammoxidation catalysts possess redox properties. They must be capable not only of reduction during the formation of acrolein or acrylonitrile, but also subsequent catalyst reoxidation in which gaseous oxygen becomes incorporated into the lattice as to replenish catalyst vacancies (Scheme 2). As mentioned earlier, the incorporation of such redox properties into solid state metal oxides was one of the salient working hypotheses on which the development of the Sohio ammoxidation process was based (2). Later, Keulks (70) confirmed the involvement of lattice oxygen in propylene oxidation by using as a vapor phase oxidant. The results showed that the incorporation of O into the acrolein (and CO2) increases with time (Fig. 11), which is consistent with the above redox mechanism. 

The complex solid state relations of the cerium-molybdenum- tellurium oxide system were studied to determine the boundaries of single phase regions and phase distributions of a typical multicomponent ammoxidation catalyst. Between 400 and 600 C in air the (Ce,Mo,Te)0 system contains the following phases ... 

It has been described in patents that ammoxidation catalysts can be activated by treatment with ammonia and/or hydrogen (12) or with carbon monoxide (13). Therefore, both precursors and H reduced catalysts will be considered in this presentation. It will be shown that the performance of the catalysts are related to their characteristics. The adsorbed state of reactants will also be discussed. 

Scheme 3. Proposed Mechanism of Propane Ammoxidation using a Paraffin Activating Catalyst, e.g.,NiaCobMcMoOx and a Multicomponent Mixed Metal Molybdate Olefin Ammoxidation Catalyst, e.g., CSaKbNicMgdMeBif SbgMOhOx, where M = Ce, Cr, or Fe [15].

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. 

Modem industrial catalysts may incorporate a large number of components, e.g., Co, Fe, Bi, Mo, Na, Ba, P, and O. The first industrial ammoxidation catalysts were simpler, the critical constituents being Bi, Mo, and O. A later generation of catalysts consisted of U, Sb, and O . 

A size distribution of particles is always desired rather than a single size in a fluidized bed. The two-phase theory of fluidized-bed operation is suspect when a bed contains appreciable lines, and models based on uniform particles should be used with caution. The dense phase in such cases should really be regarded as consisting of two phases emulsion and clusters of lines (d < 40 pm). Indeed, the results of Yadav et al. (1994) on commercial propylene ammoxidation catalyst clearly show that the lines agglomerate. A critical level of lines (30%) was found in terms of bed expansion, aeratability, and cluster size at which fluid-bed behavior is optimum. They proposed a model that takes the two dense phase components (emulsion and cluster) into account. Adding lines widens the limits of operable gas velocities and minimizes the segregation of particles. 

This work shows the acquired experience in the preparation at pilot-scale of a novel propane ammoxidation catalyst based on a partially nitrided V-Al mixed oxide obtained hy co-precipitation. A systematic investigation of the different parameters controlling the preparation of the catalyst via a co-precipitation route at different scales was carried out. At lab-scale (50 to 100 g), the preparation parameters optimized were precipitation pH, V/Al atomic ratio, V concentration in solution and nitridation conditions, while at pilot-scale (1 kg), the optimized parameters were precipitation and ageing time, solution/solid ratio during the washing step, drying and calcination conditions, and extrusion parameters. Our results show that the optimum preparation conditions for the VAION system are pH = 5.5, V/Al atomic ratio = 0.25, concentration of V species in solution = 30.10 M. This catalyst shows the highest selectivity and yield in acrylonitrile. The samples prepared at different scales show the same activity profile in the propane ammoxidation reaction. 

In conclusion, these results proved the benefit of incorporating a structure-directing agent, such as CTAB, to prepare complex mixed oxide catalysts with larger smface area. They also confirmed the fact that a proper mixture of the elements is required to obtain an effective propane ammoxidation catalyst. The optimum molar ratio was found to be Mo V Nb Te = 1 0.30 0.15 0.30. The atmosphere in which the catalysts are activated was also shown to be very important, and it was found that the calcination in nitrogen enhances the catalytic performance. 

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