Carbon oxidation mechanisms

Because the synthesis reactions are exothermic with a net decrease in molar volume, equiUbrium conversions of the carbon oxides to methanol by reactions 1 and 2 are favored by high pressure and low temperature, as shown for the indicated reformed natural gas composition in Figure 1. The mechanism of methanol synthesis on the copper—zinc—alumina catalyst was elucidated as recentiy as 1990 (7). For a pure H2—CO mixture, carbon monoxide is adsorbed on the copper surface where it is hydrogenated to methanol. When CO2 is added to the reacting mixture, the copper surface becomes partially covered by adsorbed oxygen by the reaction C02 CO + O (ads). This results in a change in mechanism where CO reacts with the adsorbed oxygen to form CO2, which becomes the primary source of carbon for methanol. 

Shift Conversion. Carbon oxides deactivate the ammonia synthesis catalyst and must be removed prior to the synthesis loop. The exothermic water-gas shift reaction (eq. 23) provides a convenient mechanism to maximize hydrogen production while converting CO to the more easily removable CO2. A two-stage adiabatic reactor sequence is normally employed to maximize this conversion. The bulk of the CO is shifted to CO2 in a high... 

Activated alumina and phosphoric acid on a suitable support have become the choices for an iadustrial process. Ziac oxide with alumina has also been claimed to be a good catalyst. The actual mechanism of dehydration is not known. In iadustrial production, the ethylene yield is 94 to 99% of the theoretical value depending on the processiag scheme. Traces of aldehyde, acids, higher hydrocarbons, and carbon oxides, as well as water, have to be removed. Fixed-bed processes developed at the beginning of this century have been commercialized in many countries, and small-scale industries are still in operation in Brazil and India. New fluid-bed processes have been developed to reduce the plant investment and operating costs (102,103). Commercially available processes include the Lummus processes (fixed and fluidized-bed processes), Halcon/Scientific Design process, NIKK/JGC process, and the Petrobras process. In all these processes, typical ethylene yield is between 94 and 99%. 

Oxidation of higher fatty acids was first studied in 1904 by Knoop who fed animals with phenyl-substituted fatty acids and analyzed the products in the urine. He showed that the fatty acid oxidation results in the successive cleavage of two carbon moieties from the carboxyl end. Knoop coined the fatty acid oxidation mechanism as n-oxidation. As has been established by Kennedy and Lehninger in 1948-1949, oxidation of fatty acids occurs in the mitochondria only. Lynen and coworkers... 

Based on these observations and several other experimental results with cofeeding of ethene and 1-alkene,9 the selectivity of branched hydrocarbons,11 and the different promoter effects of Li-, Na-, K-, and Cs-carbonate/oxide,1213 a novel mechanism has been proposed that is consistent with these various experimental results.14 The formulation of this mechanism follows the knowledge of analogous reactions in homogeneous catalysis and gives a detailed insight in the crucial step of C-C linkage formation. The aim of this work is to discuss in detail these experiments and their relationship to the proposed mechanism. 

Synthesis of 63 and 64 supports the olefin oxidation mechanisms in Fig. 16. These mechanisms have several important and noteworthy points about Ptm chemistry (1) olefins coordinate to Ptm at the axial position, which is contrasted to the -coordination of olefins perpendicular to the square-planar coordination plane of Ptn. Olefin coordination to Pt(III) should also be contrasted to the fact that olefins do not coordinate to Pt(IV). (2) Platinum111 is strongly electron-withdrawing, and the coordinated olefins receive nucleophilic attack. (3) The alkyl ce-carbon on the Ptm undergoes nucleophilic attack in aqueous solution, whereas in aprotic solvent, aldhyde (and possibly also ketone in other cases) is produced by reductive elimination. 

An alumina-based catalyst will be bound, for the purpose of mechanical strength, with carbon. The alumina-carbon mixture is essentially a composite support for adsorbing the Pt precursor. If it is desired that all metal go onto the alumina phase, which type of carbon (oxidized or unoxidized) and what type of Pt complex should be used and why A sketch of the surface potential vs. pH for alumina and the carbon binder will help. 

The higher-order hydrocarbons, particularly propane and above, oxidize much more slowly than hydrogen and are known to form metastable molecules that are important in explaining the explosion limits of hydrogen and carbon monoxide. The existence of these metastable molecules makes it possible to explain qualitatively the unique explosion limits of the complex hydrocarbons and to gain some insights into what the oxidation mechanisms are likely to be. 

In reality, it is believed that the oxidation of carbonaceous surfaces occurs through adsorption of oxygen, either immediately releasing a carbon monoxide or carbon dioxide molecule or forming a stable surface oxygen complex that may later desorb as CO or C02. Various multi-step reaction schemes have been formulated to describe this process, but the experimental and theoretical information available to-date has been insufficient to specify any surface oxidation mechanism and associated set of rate parameters with any degree of confidence. As an example, Mitchell [50] has proposed the following surface reaction mechanism ... 

However, with different pretreatment of the carbon, a different spatial arrangement of the groups seems to result during oxidation. These relationships deserve further studies. The effect of oxidation catalysts or inhibitors on the formation of the functional groups has not been studied yet. A definite influence is to be expected since the oxidation mechanism is certainly changed by additives such as water vapor or chlorine (87,88). 

Electrolytic or anodic oxidation is fast, uniform and best suited to mass production. This process is most widely used for treatment of commercial carbon fibers. The oxidation mechanism of most carbon fibers is characterized by simultaneous formation of CO2 and degradation products that are dissolved in the electrolyte of alkaline solution or adhere onto the carbon fiber surface in nitric acid. Only minor changes in the surface topography and the surface area of the fiber are obtained with a small weight loss, say, normally less than 2%. 

The R—0—B bonds are hydrolysed in the alkaline aqueous solution, generating the alcohol. The oxidation mechanism involves a series of B-to-0 migrations of the alkyl groups. The stereochemical outcome is replacement of the C—B bond by a C—O bond with retention of configuration. In combination with the stereospecific syn hydroboration, this allows the structure and stereochemistry of the alcohols to be predicted with confidence. The preference for hydroboration at the least substituted carbon of a double bond results in the alcohol being formed with regiochemistry which is complementary to that observed in the case of direct hydration or oxymercuration, that is, anti-Markownikoff. 138... 

The gas-phase selective oxidation of o-xylene to phthalic anhydride is performed industrially over vanadia-titania-based catalysts ("7-5). The process operates in the temperature range 620-670 K with 60-70 g/Nm of xylene in air and 0.15 to 0.6 sec. contact times. It allows near 80 % yield in phthalic anhydride. The main by-products are maleic anhydride, that is recovered with yields near 4 %, and carbon oxides. Minor by-products are o-tolualdehyde, o-toluic acid, phthalide, benzoic acid, toluene, benzene, citraconic anhydride. The kinetics and the mechanism of this reaction have been theobjectof a number of studies ( 2-7). Reaction schemes have been proposed for the selective pathways, but much less is known about by-product formation. 

The purpose of the present paper is to offer a contribute to the understanding of the mechanisms of these reactions by using an IR spectroscopic method and well-characterized "monolayer" type vanadia-titania (anatase) as the catalyst. We will focus our paper in particular on the following subjects i) the nature of the activation step of the methyl-aromatic hydrocarbon ii) the mechanism of formation of maleic anhydride as a by-product of o-xylene synthesis iii) the main routes of formation of carbon oxides upon methyl-aromatic oxidation and ammoxidation iv) the nature of the first N-containing intermediates in the ammoxidation routes. 

Much less studied has been the role of V species in n-pentane oxidation to MA and PA. Papers published in this field are aimed mainly at the determination of the reaction mechanism for the formation of PA (2-4,11). Moreover, it has been established that one key factor to obtain high selectivity to PA is the degree of crystallinity of the VPP amorphous catalysts are not selective to PA, and the progressive increase of crystallinity during catalyst equilibration increases the formation of this compound at the expense of MA and carbon oxides (12). Also, the acid properties of the VPP, controlled by the addition of suitable dopants, were found to play an important role in the formation of PA (2). 

Spectroscopic techniques, namely, in situ IR investigations and vibrational spectroscopies, allowed investigators to acquire information of the adsorbed species involved in the hydrogenation of carbon oxides.8,35 Although different interpretations exist with respect to the role of surface formates, there is agreement that a bifunctional mechanism is operative namely, C02 adsorbs mainly on Zr02 and hydrogen adsorbs and dissociates on Cu. 

Although other experimental evidence also exists for a formate to methoxy mechanism, Baiker argues that formate intermediates are not involved in the reduction of carbon oxides to methanol on Zr02-based catalysts.8,35,624 Rather, formates are intermediates in methanation (see Section 3.2.1). 

In the present work, therefore, a comparative study of the production of O-heterocycles during the cool-flame combustion of three consecutive n-alkanes—viz., n-butane, n-pentane, and n-hexane—was carried out under a wide range of reaction conditions in a static system. The importance of carbon chain length, mixture composition, pressure, temperature, and time of reaction was assessed. In addition, the optimum conditions for the formation of O-heterocycles and the maximum yields of these products were determined. The results are discussed in the light of currently accepted oxidation mechanisms. 

---