Activation energies, hydrocarbon

The two possible initiations for the free-radical reaction are step lb or the combination of steps la and 2a from Table 1. The role of the initiation step lb in the reaction scheme is an important consideration in minimising the concentration of atomic fluorine (27). As indicated in Table 1, this process is spontaneous at room temperature [AG25 = —24.4 kJ/mol (—5.84 kcal/mol) ] although the enthalpy is slightly positive. The validity of this step has not yet been conclusively estabUshed by spectroscopic methods which makes it an unsolved problem of prime importance. Furthermore, the fact that fluorine reacts at a significant rate with some hydrocarbons in the dark at temperatures below —78° C indicates that step lb is important and may have Httie or no activation energy at RT. At extremely low temperatures (ca 10 K) there is no reaction between gaseous fluorine and CH or 2 6... 

When [NO] temperature dependence, an activation energy of 316 kj/mol (75.5 kcal/mol). Unfortunately, the rate becomes appreciable just in the range of typical hydrocarbon—ak flame conditions. If it is also assumed that [O] = [O], the observed rate in most lean-flame products in which N2 is roughly 75 mol % of the gas can be approximated by... 

The polycyclic aromatic hydrocarbons such as naphthalene, anthracene, and phenan-threne undergo electrophilic aromatic substitution and are generally more reactive than benzene. One reason is that the activation energy for formation of the c-complex is lower than for benzene because more of the initial resonance stabilization is retained in intermediates that have a fused benzene ring. 

Huonne specifically weakens the carbon-carbon bond opposite the carbon atom beanng fluonne by about 4-5 kcal/mol per fluonne atom [ 25] It has been shown expenmentally that isomenzation of cir-l, l-difluoro-2,3-dimethylcyclopropane to the trans isomer has an activation energy )of 49 7 kcal/mol [126], which is about 10 kcal/mol lower than that of the parent hydrocarbon [127] (equation 26)... 

A plof of fhe real part of the relative heat release response for three Lewis numbers is shown in Figure 5.1.10. This plot was calculated for a reduced activation energy y3 = 10 and a burnf gas femperafure of 1800 K, represen-fative of a lean hydrocarbon-air flame. Note fhaf fhe order of magnitude of fhe relative response of fhe flame is only a little more than unity. This is a relatively weak response. For example, a sound pressure level of 120 dB corresponds to a relative pressure oscillation p /p = 2 X10 so fhe fluctuation in the heat release rate will be of fhe same order of magnifude. 

Since the interaction of linear hydrocarbons is dominated by the van der Waals interaction with the zeohte, the apparent activation energies for cracking decrease hnearly with chain length. In some cases, differences in the overall rate are not dominated by differences in the heat of adsorption but instead are dominated by differences in the enthrones of adsorbed molecules. 

The activation energy of polypropylene degradation was lowered with ferrierite catalyst. The yield of i-butene as well as the yield of olefin over ferrierite was higher than that over HZSM-5. In the case of liquid product, main product over ferrierite is C hydrocarbon, while products were distributed over mainly C -C over HZSM-5. The amount of gaseous product increased with increasing ferrierite/PP ratio. Ferrierite showed high catalytic stability for the polypropylene degradation. 

Linear relations between the activation energies and heats of adsorption or heats of reaction have long been assumed to be valid. Such relations are called Bronsted-Evans-Polanyi relations [N. Bronsted, Chem. Rev. 5 (1928) 231 M.G. Evans and M. Polanyi, Trans. Faraday Soc. 34 (1938) 11]. In catalysis such relations have recently been found to hold for the dissociation reactions summarized in Pig. 6.42, and also for a number of reactions involving small hydrocarbon fragments such as the hydro-... 

The promotor effect of SO2 increases with the amount added to the reaction medium (Fig.3). An effect of the addition of sulfur dioxide has also been observed on the oxidation of decane with an increase of the activation energy expected for such a poisoning. This addition leads to a noticeable decrease of the rate of oxidation at low temperature, where Cu sulfate is stable, but the effect becomes negligible at about 600 K. At this temperature, the conversion of decane estimated by the evolution of the peak e/m = 57, characteristic of the hydrocarbon, is close to 100% with CufTi02 catalysts in presence or not of SO2 (Figure 4). With Cu/Zr02 SO2 inhibits decane oxidation below 640 K. At 640 K a conversion of about 60% is observed in both the presence or absence of additive and an acceleration of oxidation is noticed at higher temperatures. 

These reactions do not occur at lower temperatures because of activation energy barriers and because H2 becomes the dominant form of hydrogen. Aromatic species are produced initially from acetylene via Diels-Alder type processes, in which a two-carbon and a four-carbon hydrocarbon condense into an aromatic species. Once PAHs are synthesized, they may continue to grow to form carbonaceous small grains. 

Fig. 5. Apparent activation energies of the ethane hydrogenolysis and cyclopropane hydrogenation reactions on the group VIII noble metals. The activation energies were determined at hydrogen and hydrocarbon partial pressures of 0.20 and 0.030 atm, respectively (63).

---