Zeolites ammoxidation

Vinyl chloride is an important monomer for polyvinyl chloride (PVC). The main route for obtaining this monomer, however, is via ethylene (Chapter 7). A new approach to utilize ethane as an inexpensive chemical intermediate is to ammoxidize it to acetonitrile. The reaction takes place in presence of a cobalt-B-zeolite. 

Several other important commercial processes need to be mentioned. They are (not necessarily in the order of importance) the low pressure methanol process, using a copper-containing catalyst which was introduced in 1972 the production of acetic add from methanol over RhI catalysts, which has cornered the market the methanol-to-gasoline processes (MTG) over ZSM-5 zeolite, which opened a new route to gasoline from syngas and ammoxidation of propene over mixed-oxide catalysts. In 1962, catalytic steam reforming for the production of synthesis gas and/or hydrogen over nickel potassium alumina catalysts was commercialized. 

Some companies are successfully integrating chemo- and biocatalytic transformations in multi-step syntheses. An elegant example is the Lonza nicotinamide process mentioned earlier (.see Fig. 2.34). The raw material, 2-methylpentane-1,5-diamine, is produced by hydrogenation of 2-methylglutaronitrile, a byproduct of the manufacture of nylon-6,6 intermediates by hydrocyanation of butadiene. The process involves a zeolite-catalysed cyciization in the vapour phase, followed by palladium-catalysed dehydrogenation, vapour-pha.se ammoxidation with NH3/O2 over an oxide catalyst, and, finally, enzymatic hydrolysis of a nitrile to an amide. 

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

The high activity of Cr-Cl catalyst may result probably from the formation of more active species such as Cr02+ complex cation during the ammoxidation reaction. Some authors [7] assumed that Cr(V) species occurred inside the zeolite structure as complex cation such as Cr02+ coordinated to two framework oxygen atoms and suggest the following two step process as the most probable pathway of chromate formation ... 

Ammoxidation of propane over Fe-zeolites effect of reaction variables, catalyst composition and catalyst structure... 

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. 

In the direct ammoxidation of propane over Fe-zeolite catalysts the product mixture consisted of propene, acrylonitrile (AN), acetonitrile (AcN), and carbon oxides. Traces of methane, ethane, ethene and HCN were also detected with selectivity not exceeding 3%. The catalytic performances of the investigated catalysts are summarized in the Table 1. It must be noted that catalytic activity of MTW and silicalite matrix without iron (Fe concentration is lower than 50 ppm) was negligible. The propane conversion was below 1.5 % and no nitriles were detected. It is clearly seen from the Table 1 that the activity and selectivity of catalysts are influenced not only by the content of iron, but also by the zeolite framework structure. Typically, the Fe-MTW zeolites exhibit higher selectivity to propene (even at higher propane conversion than in the case of Fe-silicalite) and substantially lower selectivity to nitriles (both acrylonitrile and acetonitrile). The Fe-silicalite catalyst exhibits acrylonitrile selectivity 31.5 %, whereas the Fe-MTW catalysts with Fe concentration 1400 and 18900 ppm exhibit, at similar propane conversion, the AN selectivity 19.2 and 15.2 %, respectively. On the other hand, Fe-MTW zeolites exhibit higher AN/AcN ratio in comparison with Fe-silicalite catalyst (see Table 1). Fe-MTW-11500 catalyst reveals rather rare behavior. The concentration of Fe ions in the sample is comparable to Fe-sil-12900 catalyst, as well as... 

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

Table 8.5 Performance of ReOx/Zeolite Catalysts in Selective Oxidation/Ammoxidation of Propene on a CVD HZcvd Catalyst, an Impregnated HZimp Catalyst, and a Physically Mixed HZphys Catalyst at 673 K...

The alkylation of naphthalene and 2-methylnaphthalene with methanol and their ammoxidation were investigated by F r a e n k e 1 et al. [22-25] on zeolites ZSM-5, mordenite and Y. In the alkylation over HZSM-5 - unlike on H-mordenite or HY - the slim isomers, namely 2-methylnaphthalene as well as 2,6- and 2,7-dimethylnaphthalene, again clearly predominated. These authors suggest that such shape selective reactions of naphthalene derivatives occur at the external surface of zeolite ZSM-5, in so-called "half-cavities" [22, 24, 25]. D e r o u a n e et al. [26,27] went even further and generalized the concept of shape selectivity at the external surface. Based, in part, on Fraenkel s experimental results, Derouane [26] coined the term "nest effect". This whole concept, however, is by no means fully accepted and has recently been severely questioned in the light of results obtained in catalytic studies with a much broader assortment of ten-membered ring zeolites [28]. 

Recently, some other oxidation reactions using N20 oxidant such as the ammoxida-tion and epoxidation were successfully conducted. The ammoxidation of propane proceeds with rather high selectivity over FeZSM-5 zeolite [151]. Remarkably, the reaction most effectively proceeds in the presence of a N20-02 mixture. 

As time moved on, the Mossbauer studies of iron-containing zeolites started to involve different aspects of zeolite chemistry, most notably in the area of catalysis. In addition, other new preparations were revealed as well as experiments that were designed to obtain superparamagnetic iron oxide particles in zeolites. Garten, Gallard-Nechschein and Boudart (5) studied iron mordenites in the reverse of the water-gas shift reaction and the ammoxidation of propylene. Collins and Mulay (6) reported that Fe(C0)5 and FeClj when thermally treated in air would form fine particles of Fe2<)3. 

Thus, in ammonia synthesis, mixed oxide base catalysts allowed new progress towards operating conditions (lower pressure) approaching optimal thermodynamic conditions. Catalytic systems of the same type, with high weight productivity, achieved a decrease of up to 35 per cent in the size of the reactor for the synthesis of acrylonitrile by ammoxidation. Also worth mentioning is the vast development enjoyed as catalysis by artificial zeolites (molecular sieves). Their use as a precious metal support, or as a substitute for conventional silico-aluminaies. led to catalytic systems with much higher activity and selectivity in aromatic hydrocarbon conversion processes (xylene isomerization, toluene dismutation), in benzene alkylation, and even in the oxychlorination of ethane to vinyl chloride. 

Synthetic zeolites have gained importance as industrial catalysts for cracking and isomerization processes, because of their unique pore structures, which allow the shape-selective conversion of hydrocarbons, combined with their surface acidity, which makes them active for acid-catalyzed reactions. Many attempts have been made to introduce redox-active TMI into zeolite structures to create catalytic activity for the selective oxidation and ammoxidation of hydrocarbons as well as for SCR of nitrogen oxides in effluent gases (69-71). In particular, ZSM-5 doped with Fe ions has attracted attention since the surprising discovery of Panov et al. (72) that these materials catalyze the one-step selective oxidation of benzene to phenol... 

A different type of catalyst is conshtuted by Co ion-exchanged zeolites and mesoporous materials (MCM-49). Co-ZSM-5 was found to be selective in ethane ammoxidation [135]. A good correlahon between the acidity and the catalytic activity was observed. The strength of ammonia bonding to the catalyst appears to have a crucial effect on the activity of Co-ZSM-5. Li and Armor [136] reported that dealuminated zeolite was achve for the ammoxidation of ethane to acetonitrile. Pan and coworkers [137] instead studied ion-exchanged Co-Na-MCM-49 and Co-H-MCM-49 materials for the same reaction, reporting that the presence of ammonia in the feed considerably improved the selectivity and total yield of ethylene and acetonitrile. 

Since 1980, the applications zeolites and molecular sieves in the speciality and fine chemicals increased enormously. Zeolites are being used in the various types of reactions like cyclization, amination, rearrangement, alkylation, acylations, ammoxidation, vapour and liquid phase oxidation reactions. Zeolites and molecular sieves have also been used to encapsulate catalytically active co-ordination complexes like ship-in-bottle and as a support for photocatalytic materials and chiral ligands. Redox molecular sieves have been developed as an important class of liquid and vapour phase oxidation and ammoxidation reactions. We have discussed few typical recent examples of various types of reactions. 

In view of these two major drawbacks, it is essential that the acidity be taken care of. This might be achieved if a basic reagent were to take part in the reaction. Hence ammoxidation of olefins might be contemplated in the case of TMI-Zeolites as a promisingly selective reaction. A recent report on the ammoxidation of propane over vanadium MFI did not prove promising (62). However the MFI structure severely hinders mass transfer which is essential in order to achieve decent selectivities. 

The Nb-Sb-0 ethane ammoxidation catalyst, when supported on alumina, gives about 50% to 55% selectivity to acetonitrile at around 30% conversion of ethane (172,173). Coproducts are ethylene (less than 10% selectivity), CO, and CO2. Selectivity to acetonitrile is close to that obtained with a Co-zeolite catalyst. However, the Nb-Sb-0 catalyst gives more coproducts CO and CO2 and less ethylene than Co-zeolite catalysts. Thus, with recycle of the coproduct ethylene along with the unreacted ethane, the Co-zeolite catalysts is expected to provide higher recycle jdelds of acetonitrile than the AI2O3-supported Nh-Sb-0 catalyst system. 

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