Maleic anhydride, formation using vanadium oxidation

Ortho-xylene may be oxidized directly by air in vapor phase over vanadium pentoxide catalysts under conditions resembling those used in oxidation of naphthalene to phthalic anhydride. The stability of the cyclic anhydride structure of phthalic anhydride at the temperatures required and in the presence of oxidizing conditions is, of course, the distinctive feature. Since the oxidation of o-xylene to phthalic anhydride requires the theoretical interaction of only six atoms of oxygen relative to the nine required by naphthalene, the amount of heat generated per unit of product is less, and the volume of diluent gases in the product stream may be lower. Because of decreased formation of quinones and color bodies, product purification should be easier. Very little is available by way of information relative to commercial operating conditions. Some laboratory results of early work showed a maximum conversion to total acids of 18.2 per cent when commercial xylene was oxidized in vapor phase over unfused vanadium oxide catalyst. Recent work with o-xylene showed a conversion of 42.7 per cent to phthalic anhydride over unfused vanadium oxide catalyst and conversions up to 61.7 per cent to phthalic anhydride plus fi.6 per cent to maleic... 

Diacetyl (DA) is used as a flavour enhancer in the food industry and is currently manufactured from methyl ethyl ketone (MEK) in homogeneous systems via an oxime intermediate (ref.1). In principle, DA can also be manufactured by the selective oxidation of MEK and several reports have appeared in the literature which apply heterogeneous catalysts to this task (refs. 2-4). A number of reports have specified the importance of basic or weakly acidic sites on the catalyst surface for a selectively catalysed reaction and high selectivities to DA at moderate conversions of MEK have been reported for catalysts based on C03O4 as a pure oxide and with basic oxides added conversely scission reactions have been associated with acidic oxide additives (refs. 2-4). Other approaches to this problem have included the application of vanadium phosphorus oxide (VPO) catalysts. Ai (ref. 5) has shown that these catalysts also catalyse the selective oxidation of MEK to DA. Indeed this catalyst system, used commercially for the selective oxidation of n-butane to maleic anhydride (ref.6), possesses many of the desired functionalities for DA formation from MEK, namely the ability to selectively activate methylene C-H bonds without excessive C-C bond scission. 

Vanadium phosphates have been established as selective hydrocarbon oxidation catalysts for more than 40 years. Their primary use commercially has been in the production of maleic anhydride (MA) from n-butane. During this period, improvements in the yield of MA have been sought. Strategies to achieve these improvements have included the addition of secondary metal ions to the catalyst, optimization of the catalyst precursor formation, and intensification of the selective oxidation process through improved reactor technology. The mechanism of the reaction continues to be an active subject of research, and the role of the bulk catalyst structure and an amorphous surface layer are considered here with respect to the various V-P-O phases present. The active site of the catalyst is considered to consist of V and V couples, and their respective incidence and roles are examined in detail here. The complex and extensive nature of the oxidation, which for butane oxidation to MA is a 14-electron transfer process, is of broad importance, particularly in view of the applications of vanadium phosphate catalysts to other processes. A perspective on the future use of vanadium phosphate catalysts is included in this review. 

Partially reduced vanadium oxide catalysts have been examined in the selective oxidation of pentane and pentene to phthalic and maleic anhydride. The anhydride selectivity has been shown to be a strong function of catalyst pre-reduction and reaction temperatures. Controlled-atmosphere, postreduction and post-reaction surface characterization experiments have shown the most selective catalyst surface to.be comprised of VgO,j, V.,09, and VOg species. In this phase of the research, dicyclopentadiene has been used as a probe molecule to elucidate the reaction network for the formation of phthalic and maleic anhydrides. 

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