Carbon oxides, formation

Naphthalene (qv) from coal tar continued to be the feedstock of choice ia both the United States and Germany until the late 1950s, when a shortage of naphthalene coupled with the availabihty of xylenes from a burgeoning petrochemical industry forced many companies to use o-xylene [95-47-6] (8). Air oxidation of 90% pure o-xylene to phthaUc anhydride was commercialized ia 1946 (9,10). An advantage of o-xylene is the theoretical yield to phthaUc anhydride of 1.395 kg/kg. With naphthalene, two of the ten carbon atoms are lost to carbon oxide formation and at most a 1.157-kg/kg yield is possible. Although both are suitable feedstocks, o-xylene is overwhelmingly favored. Coal-tar naphthalene is used ia some cases, eg, where it is readily available from coke operations ia steel mills (see Steel). Naphthalene can be produced by hydrodealkylation of substituted naphthalenes from refinery operations (8), but no refinery-produced napthalene is used as feedstock. Alkyl naphthalenes can be converted directiy to phthaUc anhydride, but at low yields (11,12). 

Since the reachons generating by-products (carbon oxide formation and ammonia combushon) are themselves highly exothermic, the total exothermicity of the reachon is around 530-660 kj mol , making control of the reachon temperature crihcal. 

Figure 6. Experimental and predicted carbon oxides formation at three different temperatures vs. carbon diooxide partial pressure (P jj = 0.2 atm. and Pq2 = 0.01 atm. in the feed).

Lehmann and Baerns [18] have reported different reaction rate expressions based on a number of mechanisms to predict the hydrocarbon formation rate and carbon oxide formation rate in terms of PcH4 nd PQ2- None of the earlier studies included the dependence of R2 and Rj on Pc02- and co-workers [19] developed expressions for RcH4 function... 

Under steady state reaction conditions, the effects of CO2 on the methane coupling reaction over Li/MgO catalyst were quantitatively determined. Poisoning effects of CO2 on carbon oxide formation rate, C2 formation rate, and methane conversion were observed for all methane to oxygen ratios and all temperatures. However, C2 selectivity is relatively unaffected by CO2 partial pressure. The mechanism described here accounts for important elementary steps, especially the effects of carbon dioxide. Under the low conversion conditions used in this study, further oxidation of C2 products to CO and CO2 is assumed negligible. These reactions will become more important at high conversions. Rate expressions derived from the mechanism match well the experimental conversions and selectivities. 

In what concerns to products distribution, the effect of temperature is similar with both catalysts and corresponds to an increase in butane conversion the butenes selectivity decreases as the butadiene selectivity increases and the carbon oxides formation also increases, specially CO. These effects are more pronounced with the Cs doped sample. Butane partial pressure does not affect the products distribution with NiMo04 but increases the C4 s selectivity (specially butenes) decreasing mainly the CO2 formation with the 3% Cs doped catalyst. The effects of increasing P02 are the same for both catalysts. A decrease of C4 S selectivities and an increase of COx formation at low pressures is mainly observed. It is noteworthy that the main effects of Cs doping in the selectivities are increase in I-butene selectivity and decrease in CO formation. 

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. 

The active metals seem to exhibit a common characteristic they can cycle between at least two oxidation states. Although there are differences 1n select1v1t1es 1n C2-format1on and carbon oxides formation, no correlation seems to exist with the free-energy changes 1n the oxidation states. A possible mechanism for C2 format1on from methane 1s proposed. 

Steam-Carbon Reaction. The reaction C + H2O = CO + H2 was significant only at temperatures above 1700°F., and increased with temperature. At 1695°F. (Run HT-80), 50% of the feed steam decomposed, but at 1825°F. (Run HT-72), 70% was decomposed. Carbon oxides formation was related directly to the steam fed and to the steam decomposition. As much as 5.5 std. cu. ft. of carbon oxides/lb. of coal were... 

Figure 16, Experimental and calculated integral rates of carbon oxides formation for gasification in 2-, 4-, and 6-inch id fluid-bed reactors (IGT studies)...

Sananes, M. T., Hutchings, G. J., Volta, J. C., 1995, n-Butane Oxidation to Maleic Anhydride and Furan with no Carbon Oxide Formation Using a Catalyst Derived from V0(H2P04)2, j. Chem. Soc., Chemical Commun., 243. 

Teng, H., and Suuberg, E.M., Chemisorption of nitric oxide on char Irreversible carbon oxide formation, Ind. 

Carbon oxide formation. The rate of carbon oxide formation was shown to be 0.5th order in both methane and in oxygen [26] (for illustration see Figure 18a and b) ... 

Figures 18a and b. Dependence of carbon oxide formation rate on methane and oxygen partial pressures.

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