Propene heterogeneous oxidation

The oxidation of propene is at present the most extensively studied gas phase heterogeneous oxidation process. The selective production of acrolein over cuprous oxide has been known for a very long time. However, the discovery of bismuth molybdates as highly active and selective catalysts for the oxidation to acrolein, and particularly the ammoxidation to acrylonitrile, has opened a new era in oxidation catalysis. 

Wiessmeier,G., Untersuchungenzur heterogen katahysierten Oxidation von Propen in einem Mikrostrukturreaktor, Masterthesis, Karlsruhe ( 1992). 

The oxidation of propene to propene oxide is considered an essential practice in industrial chemistry [1]. Haruta et al. showed that this process can be led by heterogeneous catalysis with gold supported over titania [15, 16]. Another goal in the gold catalysis sequence is the selective oxidation of some alcohols and carbohydrates with molecular oxygen, as studied by Prati and Rossi [17]. 

The epoxidation of propene is analogous to that of ethylene catalyzed by silver. However, the selectivity is much lower. Due to the pronounced oxidation sensitivity of the allyl CH3-group, excessive combustion occurs as a side reaction. The heterogeneous process has no practical significance, therefore, as it has to compete with a highly selective liquid phase epoxidation process. 

Zeolite catalysts may also be regarded as mixed oxides, but the crystallographic structures differ from the solids discussed above in that active sites for catalysis occur within the open lattice framework. In consequence, rate data are not directly comparable with similar observations for other heterogeneous reactions since the preexponential factors are calculated and reported on a different basis. For completeness, however, it is appropriate to mention here that instances of compensation behavior on zeolite catalysts are known. Taylor and Walker (282) described such an effect for the decomposition reactions of formic acid and of methyl forma te on cation-exchanged 13X molecular sieves, and comparable trends may be found in data reported for reactions of propene on similar catalysts (283). 

Oxo-metal complexes also intervene as active species in the heterogeneous gas-phase oxidation of hydrocarbons over metal oxide or mixed metal oxide catalysts at high temperatures. Characteristic examples are the bismuth molybdate-catalyzed oxidation of propene to acrolein and the V205-catalyzed oxidation of benzene to maleic anhydride (equations 17 and 18).SJ... 

As already mentioned, the processes for the homogeneously catalyzed carbony-lation of acetylene have opened up the way for acrylic acid to become a mass product for which worldwide production capacities are currently two million tonnes per annum. Acrylic acid and its esters are important monomers for polymer dispersions, whose use is widespread. Since the mid-1960s, however, the availability of propene, a less expensive feedstock than acetylene, has led to the development of an even more advantageous production process the heterogeneously catalyzed gas-phase oxidation of propene [21, 22]. Nowadays, acrylic acid is produced almost exclusively by this process (cf. Chapter 1). The Reppe acrylic acid plant at BASF is now the only one left in the world which still uses acetylene as feedstock. 

Meth)acrylic acid and esters are large-volume industrial chemical intermediates for the production of co- and homopolymers. Acrylic acid (AA), with a worldwide production of approx. 1.5 X 10 t/a, finds its main use in the manufacture of superabsorbent polymers and various acrylate esters. The most important production process for AA involves the two-stage oxidation of propene (via acrolein) in the presence of a large excess of steam by heterogeneous catalysts. Selectivities to AA for the overall process reach 85-90% based on propene (cf. Chapter 1, Introduction). 

The applied nickel catalyst, promoted by copper halides, required rather severe reaction conditions T = 220 °C, F = 10 MPa), but gave good AA yields up to 90% based on acetylene. This so-called catalytic Reppe process was commercially operated in Germany, the USA, and Japan. Due to the limited availability of cheap acetylene as feedstock and the severe reaction conditions involved in the carbonylation process, this process has lost the competition with (heterogeneously catalyzed) oxidation of readily available propene, even though a perfect selectivity to AA is not achieved in the latter process. 

Two variants of the Oxirane process are used (Figure 1) for the commercial production of propene oxide (PO) [29]. They differ in the hydrocarbon (isobutane or ethylbenzene (EB)) that is the precursor of the hydroperoxide, and, hence, in the alcohol co-product. ARCO operates both processes using a homogeneous molybdenum catalyst. Shell, in contrast, operates only the EB variant and uses a heterogeneous Ti /Si02 catalyst. 

TBHP vide supra). The autoxidation of EB is performed at 120-160 °C and 1- bar. MBA and acetophenone (ACP) are formed as by-products via the facile termination of the secondary 1-methylbenzylperoxy radicals. In order to minimize by-product formation by further oxidation of MBA and ACP, the autoxidation is carried out to only low conversions (< 12 %). This solution (ca. 10 %) of EBHP in EB is used in the epoxidation step, i.e., EB is the solvent for the latter step. A high propene/EBHP molar ratio is used and reaction conditions are similar to those of the TBHP process vide supra). The PO selectivity is reported to be 90 % at 92 % EBHP conversion [30] but in practice it may be higher. For comparison the heterogeneous Ti /SiOa catalyst in fixed-bed operation reportedly gives 93-94 % PO selectivity at 96 % EBHP conversion [11]. The products are separated by distillation and MBA is dehydrated to styrene in the vapor phase over a Ti02 catalyst. 

The allylic oxidation of propene typifies the so-called bimetallic heterogeneous catalysis [4], a terminus technicus to emphasize cooperative effects in catalytic conversions (for multicomponent homogeneous catalysis, see Section 3.1.5). Nevertheless, the SOHIO-type oxidation is included in this book because one can imagine a number of mechanistic implications on a molecular platform, too. Studies on organometallic model compounds and reactions are available in ref. [2]. 

More than 5 million tons of acrylonitrile are made annually. It is synthesized industrially by the gas-phase heterogeneous ammoxidation of propene. New catalysts based on Bi-Mo or V-Sb oxides may lead to manufacture of this important compound from propane. Although the new process has a considerably lower selectivity to acrylonitrile, the lower cost of the alkane makes it economically interesting (the propane method can cut at least 20 % from the production costs of the propene route). Nevertheless, increases in the selectivity of the catalysts, especially at higher conversions, will be necessary for this process to compete with the usual process of acrylonitrile synthesis [24]. 

For the commercial production of vinyl acetate, a procedure with a heterogeneous fixed-bed catalyst is exclusively applied today. The catalysts usually consist of palladium salts, mostly the acetate, or palladium metal together with alkali acetate supported on a carrier such as alumina, silica, or carbon without any additional oxidant. This process avoids the formation of larger amounts of by-products. Thus, from ethylene vinyl acetate and from propene, allyl acetate is obtained exclusively. 

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