Co-oxidation with hydrocarbons

Although it is necessary to consider the importance of individual reactions in particular systems, there is general agreement that the autoxidation of hydrocarbons (RH) in the presence of inhibitors (AH) may be represented by the overall scheme 

The oxidation of hydrocarbons, reactions (1), (la), (2)—(4), is inhibited to an extent that depends on the efficiency of chain termination, reactions (5), (6), (8)—(10), on the possibility of chain transfer and regeneration, reactions (lb), (5) and (7), and on the possibility of degradation of hydroperoxides to inert products, reaction (11). Amines and phenols are known to be efficient chain breaking inhibitors, while sulphides promote reaction (11). 

The relative importance of individual reactions depends on the nature of the system and the inhibitor. This importance is usually assessed by developing rate expressions on the basis of different assumptions, and comparing predictions with experimental results. The most satisfactory method of doing this was applied by Mahoney [3], who used a computer to handle a complex interactive reaction scheme. However, in order to identify and to discuss alternative reactions, the present article adopts the classical approach of considering inhibition reactions individually. Where the situation demands a more complex approach, individual reactions are considered in the light of possible interactions. 

This is particularly pertinent in the context of nitrogen-containing inhibitors, which can interfere with the autoxidation of hydrocarbons at various points. Indeed, provided that their redox potential is sufficiently low, they may even initiate oxidation via reaction (lb) [6]. Simple and complex inhibition is discussed below the influence of different factors on these reactions is discussed at the end of this section. 

Although inhibition of autoxidation by donation of hydrogen to peroxy radicals, reaction (5), is an important reaction, Boozer and Hammond [7] have suggested that inhibition by complex formation may also be important. Assuming that the major termination step involves reactions (9) and (10), and that the reaction is initiated by azobisisobutyro-nitrile (AIBN), then the rate of initiation is 

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The theory of chain co-oxidation of two reactants R H and R2H was described earlier in Chapter 5. The results of the study of ether co-oxidation with hydrocarbons and ether (1) + ether (2) are collected in Table 7.16. 

Parameters of Aldehydes Co-oxidation with Hydrocarbons, Alcohols, and Aldehydes... 

As discussed with reference to co-oxidation with hydrocarbons, sulphide oxidation chemistry is complicated by the further reactions of sulphoxide products. The autoxidation of sulphides in the absence of hydrocarbons is a free radical process [149] leading, in the first instance,... 

Co-oxidation of Hydrocarbons and Alcohols with Selective Inhibitor... 

Hydrocarbon oxidation in the presence of intermediate products proceeds as a chain reaction of co-oxidation of hydrocarbon with intermediate products of oxidation. The... 

The chain mechanism is complicated when two hydrocarbons are oxidized simultaneously. Russell and Williamson [1,2] performed the first experiments on the co-oxidation of hydrocarbons with ethers. The theory of these reactions is close to that for the reaction of free radical copolymerization [3] and was developed by several researchers [4-9], When one hydrocarbon R H is oxidized in the liquid phase at a sufficiently high dioxygen pressure chain propagation is limited only by one reaction, namely, R OO + R H. For the co-oxidation of two hydrocarbons R1 and R2H, four propagation reactions are important, viz,... 

CO-OXIDATION OF HYDROCARBONS AND ALCOHOLS WITH SELECTIVE INHIBITOR... 

In the initial period the oxidation of hydrocarbon RH proceeds as a chain reaction with one limiting step of chain propagation, namely reaction R02 + RH. The rate of the reaction is determined only by the activity and the concentration of peroxyl radicals. As soon as the oxidation products (hydroperoxide, alcohol, ketone, etc.) accumulate, the peroxyl radicals react with these products. As a result, the peroxyl radicals formed from RH (R02 ) are replaced by other free radicals. Thus, the oxidation of hydrocarbon in the presence of produced and oxidized intermediates is performed in co-oxidation with complex composition of free radicals propagating the chain [4], A few examples are given below. 

Aldehydes do not co-oxidize alkanes due to a huge difference in the reactivity of these two classes of organic compounds. Alkanes are almost inert to oxidation at room temperature and can be treated as inert solvents toward oxidized aldehydes [35]. Olefins and alkylaromatic hydrocarbons are co-oxidized with aldehydes. The addition of alkylaromatic hydrocarbon (R2H) to benzaldehyde (R1H) retards the rate of the initiated oxidation [36-39]. The rate of co-oxidation obeys the equation [37] ... 

Using the dendrimer route, it is possible to prepare supported catalysts not available via traditional routes. Dendrimer derived Pt-Au catalysts having compositions within the bulk miscibility gap can be prepared on several oxide supports. For all the supports studied, the bimetallic catalysts exhibited synergism with respect to mono- and cometallic catalysts for the CO oxidation and hydrocarbon NOx SCR reactions. The bimetallic Pt-Au catalysts also showed evidence of exchanging surface and subsurface atoms in response to strongly binding ligands such as CO. 

Comparatively few values have been measured for liquid-phase co-oxidations of hydrocarbon mixtures. With the exception of the cumene-Tetralin system, the reported values are all surprisingly low even for other systems giving tertiary and secondary (or primary) peroxy radicals. For example, at 60°C. values of 0.7 (36) and 1.3 (4) have been reported for the co-oxidation of cumene and ethylbenzene kt = 2.0 X 107 at 30°C. (14)], and a value of 1.4 (2) has been reported for the co-oxidation of cumene and 1-hexene [which gives mainly primary peroxy radicals with kt probably 1.3 X 108 at 30°C. (15)]. The confirmation that the present work provides for a relatively large value in the cumene-Tetralin system suggests that the other > systems deserve a close and careful reinvestigation. 

Butadiene has been co-oxidized with a number of aralkyl hydrocarbons and cyclic olefins. The order of increasing reactivity toward butadieneperoxy radicals (X—02—C H602) is cumene, sec-butylbenzene < cyclooctene < cyclohexene... 

Hence, in co-oxidations of hydrocarbons at constant rate of initiation in the presence of sufficient oxygen, it will be easy to depress oxidation rates of the relatively few hydrocarbons with slow termination constants, but otherwise oxidation rates will not differ much from a linear function of composition. When rates of chain initiation are not known or controlled, other relations may appear. 

In general, liquid phase autoxidations on hydrocarbons after the initial stages take place, may be considered as co-oxidations with aldehydes, alcohols, ketones, carboxylic acids, etc. Often aldehydes or ketones are deliberately added to hydrocarbon autoxidations in order to promote the reaction. For example, in the cobalt-catalyzed oxidations of alkylaromatics (see Section II.B.3.b), aldehydes, or methyl ethyl ketone are usually added in commercial processes in order to attain high rates and eliminate induction periods. 

By carrying out a number of co-oxidations with various hydrocarbons, it is possible to compare the termination rate coefficient of these hydrocarbons and thereby group them accordingly [140,141]. Although more direct and more precise methods of measuring termination rate coefficients are available, this technique is an effective qualitative method for estimating these coefficients. 

Cross-termination parameters, 0, for co-oxidation of hydrocarbons with cumene... 

The vapor cloud of evaporated droplets bums like a diffusion flame in the turbulent state rather than as individual droplets. In the core of the spray, where droplets are evaporating, a rich mixture exists and soot formation occurs. Surrounding this core is a rich mixture zone where CO production is high and a flame front exists. Air entrainment completes the combustion, oxidizing CO to CO2 and burning the soot. Soot bumup releases radiant energy and controls flame emissivity. The relatively slow rate of soot burning compared with the rate of oxidation of CO and unbumed hydrocarbons leads to smoke formation. This model of a diffusion-controlled primary flame zone makes it possible to relate fuel chemistry to the behavior of fuels in combustors (7). 

Fire refining, the final smelting operation, removes further impurities and adjusts the oxygen level ia the copper by air oxidation followed by reduction with hydrocarbons, ammonia, or reformed gas (CO + H2). 

Steam reforming is the reaction of steam with hydrocarbons to make town gas or hydrogen. The first stage is at 700 to 830°C (1,292 to 1,532°F) and 15-40 atm (221 to 588 psih A representative catalyst composition contains 13 percent Ni supported on Ot-alumina with 0.3 percent potassium oxide to minimize carbon formation. The catalyst is poisoned by sulfur. A subsequent shift reaction converts CO to CO9 and more H2, at 190 to 260°C (374 to 500°F) with copper metal on a support of zinc oxide which protects the catalyst from poisoning by traces of sulfur. 

In addition to carbon monoxide (CO) and unbumed hydrocarbons (UHC), the most significant products of combustion are the oxides of nitrogen (NOx). At high temperatures, free oxygen not consumed during combustion reacts with nitrogen to form NO and NO2 (about 90% and lO /i of total NOx, respectively). 

Molybdenum hexacarbonyl [Mo(CO)6] has been vised in combination with TBHP for the epoxidation of terminal olefins [44]. Good yields and selectivity for the epoxide products were obtained when reactions were performed under anhydrous conditions in hydrocarbon solvents such as benzene. The inexpensive and considerably less toxic Mo02(acac)2 is a robust alternative to Mo(CO)6 [2]. A number of different substrates ranging from simple ot-olefms to more complex terpenes have been oxidized with very low catalytic loadings of this particular molybdenum complex (Scheme 6.2). The epoxidations were carried out with use of dry TBHP (-70%) in toluene. 

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