Oxygen conversion data

CO conversion data relative to (N1 SI ) and (ThNl Fe, series were taken from ref. ( ) and (,9), respectively. Catalytic measurements were obtained for oxygen treated N1 Th Intermetallics. Prior to each run, a sample mixture (50 mg cata ys + 50 mg ground quartz) was reduced In H. at 275 C for 16 hours. CO hydrogenation was carried out at 275 C using H /C0 ratio 9. More experimental details are given elsewhere (10). 

In the absence of TCE and chlorine, the possible active species are holes (h+), anion vacancies, or anions (02 ), and hydroxyl radicals (OH ). At constant illumination and oxygen concentration, we may expect h+, and O2 concentrations to be approximately constant, and the dark adsorption to be a dominant variable. If kh+, or ko2- does not vary appreciably with the contaminant structure, the rate would depend clearly on the contaminant coverage as shown in Figme 2a, and the reaction would therefore occur via Langmuir-Hinshelwood mechanism. (Note only rates with conversions below 95% are correlated here (filled circles), as the 100% conversion data contains no kinetic information). This rate vs. d>r LH plot is smoother than those for koH or koH suggesting that non-OH species (holes, anion vacancies, or O2 ) are the active species reacting with an adsorbed contaminant. 

The batch reactor, above described, permits both to operate at quasi-zero conversion per pass and to evaluate the cat ytic activity at finite values of the reagents conversion. A typical test performed on Si02 catalyst at 600°C is presented in Figure 1. It is remarkable how in our approach the product selectivity is unaffected by the methane conversion. A special care was taken to avoid oxygen-limiting conditions and, hence, methane conversion data obtained for oxygen conversions below 20% only have been used for the calculation of reaction rates. 

Centi and coworkers [37] have proposed that the reaction proceeds via a series of redox couples. The mechanism is based on experimental data obtained at high butane concentrations and total oxygen conversions. Under these conditions the catalyst undergoes a partial reduction and many sites are formed. The activation of butane requires a V —couple, while the subsequent conversion to maleic anhydride needs a V couple. The alkenes from butane dehydrogenation can desorb if no oxidizing sites are available. When the oxidizing sites are available, the alkenes are adsorbed and react very quickly to give maleic anhydride. 

Reaction rate based on oxygenates conversion calculated on a CH2 basis [3] at WHSV=417 h" for methanol, 600 h for DME and 417 h for propene. 2) Selectivity to coke=deposited coke(g)/ hydrocarbons foimed(g). 3) Initial data was obtained from the first pulse. 

The conversion of triene 77 to 13, and of 13 to 12 could not be photosensitized by acetophenone, nor quenched by oxygen. These data suggest that the reversible photochemical 1,7-hydrogen shift in 77 and 13 involves a singlet state species. Orbital symmetry arguments 14> 15> also suggest that the interconversion of 77 and 13 occurs via excited electronic states. Hence, the reactive state of 77 and 13 is most likely the lowest excited singlet state of each triene. 

In the second series of experiments, the products from the photo-oxidation of diethyl ether, carried out in a Teflon bag reactor at ppm and ppb levels, have been determined by withdrawing vapour samples and monitoring by gas chromatography, HPLC and by chemiluminescence analysis. The major reaction products which have been measured are ethyl formate, ethyl acetate, acetaldehyde, formaldehyde, PAN, methyl nitrate and ethyl nitrate. The products observed arise from the decomposition reactions of the 1-ethoxyethoxy radical and from its reaction with oxygen. The data enable the establishment of a quantitative mechanism for the photo-oxidative reaction. In addition the rate of conversion of NO to NO2, determined by chemiluminescence analysis, shows that for each molecule of ether reacted only one molecule of NO is converted to NO2. In further end-product analyses experiments, the OH radical initiated photo-oxidation of n-hexane or the photolyses of 2- or 3-hexyl nitrites were studied to examine the... 

The formation of higher hydrocarbons as well as the dehydrogenation of ethane was assumed to proceed in the gas phase only. For the latter reaction this was experimentally proven for temperatures above 973 K [32]. The dependencies of oxygen conversion on contact time and of product selectivities on oxygen conversion were calculated using the above mentioned model for conditions for which experimental data were available. The procedures of simulation and of fitting of the rate constants of the various surface reaction steps is documented in [5a] and will be described elsewhere [42]. 

Figures 20a-c. Dependence of selectivities on oxygen conversion comparison of experimental data (symbols) and simulation (lines) (CH4/O2/N2 = 10 1 4 T = 1020 K Pjqj = 100 kPa).

Frequently, in evaluating catalyst performance emphasis is put on the selectivity of a desired product and not only on activity. In such cases account should be taken of the fact that selectivity depends on the degree of conversion of the key feed molecules. Under certain circumstances, e.g. selective oxidation of hydrocarbons, the flow conditions can be adjusted in such a way that the degree of oxygen conversion is 1 thus, the selectivity of all the catalysts can be compared. Otherwise it may be advisable to vary the total flow-rate to the multi-channel reactor in this way, selectivity can be obtained as a function of the degree of conversion. Comparable selectivity data for the different catalysts can be... 

We believe that the most feasible explanation for the presence of oxygen in the gaseous reaction products is the formation of secondary oxygen. This is most clearly illustrated by the data from [85], which show that, after reaching its maximum, the oxygen conversion then decreases with time (Fig. 2.12). 

The pressure dependence of the temperatures of onset of conversion and of complete oxygen conversion in a quartz reactor can be plotted (Fig. 3.1) based on the data from [45] and [68]. In addition, the figure shows the data from [91] at P = 80 atm. Although these works differ somewhat in experimental conditions, particularly, in the oxygen concentration and the... 

FIGURE 3.1 Dependence of the temperatures of ( ) onset of conversion and ( ) complete oxygen conversion on the pressure plotted based on the data from [45], [68], and [91]. 

With increasing temperature, the reaction-time shortens (the time of complete oxygen conversion, practically identical to the induction period of the reaction). The nearly ideal Arrhenius temperature dependence of the oxygen conversion displayed in Fig. 3.17 gives an effective activation energy of 46 kcal/mol. The heating of the mixture under these experimental conditions is close to adiabatic with a 43 °C increase of the temperature per each percent of oxygen in the reaction mixture. These experimental data were well reproduced by kinetic simulations of the process [89]. 

The paper [106] reports data on the effect of dilution with helium on the selectivity of methanol formation and complete oxygen conversion temperature for the DMTM reaction in a small d = l mm) fused silica capillary at P = 100 atm and = 10 s. At a constant total pressure and reaction time, the increase of the helium fraction (decrease in the concentrations of the reactants) in the mixture was accompanied by the monotonic growth of the complete oxygen conversion temperature and the reduction of the selectivity of methanol formation to zero at [He] = 90% (Fig. 4.10). 

The nature of the reactor surface affects not only the kinetics of the formation of the products during the reaction, but also their subsequent transformations, especially in experiments with a long residence time of the reactants in the reactor. Note, however, that, even at residence times not exceeding a few seconds, the selectivity and yield of methanol and, especially, formaldehyde decrease with increasing residence time after oxygen-conversion completion. However, the available data on the rate of decomposition of the products are rather contradictory, strongly dependent on the nature of the reactor surface material. 

Kinetic analysis shows that the growth of the partial pressure of the reactants should lead not only to an increase in the methane conversion rate, but also to an enhancement of the role of nonlinear gas-phase radical reactions. A consequence may be an increase in the selectivity of methanol formation. Figure 7.1 and Table 7.1 show the most reliable experimental data on the pressure dependence of the methanol deld. All the experiments were performed imder essentially similar conditions, optimal for achieving a high methanol deld. Only the experiments in which the oxygen conversion was close to 100% were taken into accoimt. Comparison was conducted for the methanol )deld, because it is the most reliable experimentally measured quantity. Due to a low degree of methane conversion, the selectivity of methanol formation is usually measured with great imcertainty, let alone that, in many practice-oriented studies, it was not determined at all. For the works [45] and [67], which report only the selectivity, we estimated the methanol )deld based on the relationship A[CH4]/A[Oi] = 1. This makes it possible to evaluate the methanol yield YmcOH as... 

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