Xylenes heterogeneous oxidation

Heterogeneous oxidative processes operate at high temperatures (250-450 6C) and are useful for the synthesis of acrolein and acrylic acid from propylene over bismuth molybdate catalysts, the synthesis of maleic and phthalic anhydrides from the oxidation of benzene (or C4 compounds) and naphthalene (or o-xylene) respectively over vanadium oxide,101 arid the synthesis of ethylene oxide from ethylene over silver catalysts.102... 

For the characteristic differences between the low and high activity states we suggest the following explanations. When the reaction starts in the low activity state, palladium is in metallic form and hence, the catalyst must be trained to develop a surface structure characteristic of high activity state in the total conversion of m-xylene. In this state the combustion occurs in a typical heterogeneous oxidation, so the conversion increases with temperature in an exponential manner. 

Attenlion should be drawn to ihe use of tin oxide systems as heterogeneous catalysts. The oldest and mosi extensively patented systems are the mixed lin-vanadium oxide catalysis for the oxidation of aromatic compounds such as benzene, toluene, xylenes and naphthalene in the. synthesis of organic acids and acid anhydride.s. More recenily mixed lin-aniimony oxides have been applied lo the selective oxidaiion and ammoxidaiion of propylene to acrolein, acrylic acid and acrylonilrile. 

The IR study performed in static controlled atmospheres in the IR cell allowed us to identify a number of adsorbed intermediate and secondary products, together with the main reaction products in oxidation and ammoxidation of toluene and the three xylene isomers. Surface reactions schemes are proposed that account for most of the mechanistic features of the heterogeneously-catalyzed industrial reactions. Our data support the following conclusions ... 

Benzylic oxidation of aromatic side-chains is also a well established technology in the bulk chemicals arena, e. g. toluene to benzoic acid and p-xylene to ter-ephthalic acid [1,2]. These processes involve homogeneous catalysis by, e. g., cobalt compounds, however, and also fall outside the scope of this book. Ammoxi-dation of methyl-substituted aromatic and heteroaromatic compounds is performed over heterogeneous catalysts in the gas phase but this reaction is treated elsewhere (Section 9.5). Transition metal-substituted molecular sieves have been widely studied as heterogeneous catalysts for oxidation of aromatic side-chains in the liquid phase, but there are serious doubts about their heterogeneity [5,6]. Here again, a cursory examination of the literature reveals that supported palladium seems to be the only heterogeneous catalyst with synthetic utility [4]. 

Petal, J., Ollis, D., 1992, Heterogenous photocatalytic oxidation of gas phase organics for air purification acetone, 1-butanol, biityraldehyde, formaldehyde, m-xylene oxidation, J. of Cataly.sis, 136 554-565... 

In the first half of this chapter, only three components (acetic acid, water, and the entrainer) in the heterogeneous distillation column will be assumed to be present. However, in the production of aromatic acid, acetic acid is used as a solvent in the terephthalic acid or iso-phthalic acid processes. A terephthalic acid, or isophthahe acid slurry in acetic acid solvent, is produced by oxidizing p-xylene (PX) or m-xylene (MX), removing water by evaporation of a stream of mixture with water and acetic acid, and returning acetic acid to the oxidation step (Hindmarsh et al. and Lee et al. ). To recover acetic acid as a cyclic solvent stream, an acetic acid dehydrating column is necessary. However, a tiny amount of one reactant (PX or MX) may also enter into this acetic acid dehydration column through the feed stream. The... 

The heterogeneously catalyzed selective gas phase oxidation of o-xylene in a micropacked bed reactor was investigated, using the benefits of explosive regime [158]. 

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