Benzene formation, pressure dependence

Theoretical calculations support the expectation that the preferred site of initial OH attack is ortho to the methyl group (Andino et al., 1996), but addition to the other positions also occurs. If the OH-aromatic adduct, which contains 18 kcal mol-1 excess energy, is not stabilized, it decomposes back to reactants, reaction ( — 62). The existence of the adduct in the case of the OH-benzene reaction has been observed spectroscopically (Fritz et al., 1985 Knispel et al., 1990 Markert and Pagsberg, 1993 Bjergbakke et al., 1996). As expected for such a mechanism, the rate constants at temperatures below 300 K exhibit a pressure dependence at lower pressures. At higher temperatures, the rate of decomposition of the excited adduct back to reactants is higher, so the net contribution of adduct formation to the overall reaction is small compared to H-abstraction. 

The mechanism of benzene and higher polymer formation remains uncertain with further work on isotopic mixtures needed to help determine the processes which occur. It is interesting to note that, in the P- and X-ray radiolysis of C2H2-C2D2 mixtures, Mains et observe all benzenes do to in the products and conclude that a C-H rupture must occur in the radiolysis. Dorfman and Wahl have shown that in the helium-sensitized radiolysis of acetylene, where only ionized states of acetylene are formed, there is no formation of benzene. The strong pressure-dependence of the benzene formation in the direct photolysis still provides the strongest evidence for a molecular mechanism such as given in the reaction scheme. 

The main conclusion to be drawn from the application of the benzene photosensitization method to the decomposition of cyclobutanone is that the Cj-hydrocarbons originate from the low-lying triplet state of the ketone. However, use of this method in the investigation of cyclopentanone decomposition indicated that reactions I, II and III (if it is a separate primary process) occur from the first excited state of the ketone. This conclusion was based on the quantitative agreement found between the pressure dependence of the decarbonylation-product formation and the fluorescence quenching by cyclopentanone. 

The vapor-phase nitration of benzene with NO2 was investigated by Suzuki et al. using polyorganosiloxanes bearing sulfonic acid groups and silica-supported benzenesulfonic acid catalysts [31]. Phenylsulfonic polysiloxane was the most active among the polysiloxanes tested, its activity was not related to the concentration of acid sites as determined by a titration method, however. From the partial pressure dependence of the reaction rate it was concluded that the formation of NO as the active species was the rate-limiting step [32]. 

Various kinetic models to represent catalytic reforming have been reported in the literature, which have different levels of sophistication [2-4]. The kinetic model of Krane et al [3] is one of the more elaborate models which considers all possible reactions for each individual hydrocarbon. However, the temperature and pressure dependency on the rate constants was not reported. In addition, this model does not consider the formation of the main benzene... 

Hydrogenolysis of epoxides to yield alcohols has been much reported in the patent literature, because of its importance as an industrial process, but studies on reactivity and selectivity have not been done systematically. The selectivity is highly dependent on the substituents, as in the case of reduction using metal hydrides. As a metal catalyst, Raney Ni was intensively examined in the early stage. It usually requires high pressures (ca. 100 atm) and temperatures (100 C), as shown in Table 10. Alcohols, benzene, THF and even water have been used as solvents. Accordingly, a hydroxy group in the epoxides remains intact, and hydrocarbons are formed only as by-products. In some cases by-product formation can... 

As in the case of ethylene and acetylene W, plasma polymerization of benzene produced either a powder or film depending on reaction conditions. A typical condition in which thin film with the required property was produced (the RO membrane condition) is shown in Table 1, coded as Condition B, while that for poor quality film formation is designated A. Conditions for powder formation are designated C and E in the table. Generally speaking, film formation was observed at high benzene flow rates, and powder formation was observed at low pressures and low benzene flow rates, as in the case of ethylene and acetylene ( ). However, the RO membrane conditions do not correspond to either a unique point on the pressure (P) versus benzene flow rate (Q(Bz)) plane nor do they correspond to the conditions in which a lot of polymer was produced. This means that the quality of the film cannot be correlated directly to the macroscopic reaction conditions. 

Another important factor in catalysis is the selectivity of a catalytic reaction. So far, however, information on the atom-by-atom evolution of this astonishing catalytic selectivity is still lacking. In this example, we illustrate such a size-dependent selectivity with the polymerization of acetylene on palladium nanocatalysts [46]. This reaction over supported Pd particles reveals a direct correspondence between reactivities observed on model systems and the behavior of industrial catalysts under working conditions [66]. In ultra-high vacuum (UHV) [67] as well as under high pressure, large palladium particles of typically thousands of atoms show an increased selectivity for the formation of benzene with increasing particle size [66]. In contrast, small palladium particles of typically hundreds of atoms are less selective for the cyclotrimerization, and catalyze butadiene and butene as additional products [66]. 

Trimerization of alkynes to derivatives of benzene is promoted by [Co(pyridine)6]+ BPh4. In the presence of hydrogen the catalyst induces the formation of a mixture of the dienes 565-567 and the tributylbenzenes 568 and 569 from 1 -hexyne the composition of the mixture depends on the hydrogen pressure. ... 

This reaction involves collision of the radicals, resulting in the abstraction of an atom, usually hydrogen, by one radical from the other. This leads to the formation of two stable molecules, with the atom abstracted being p to the radical center, for example, the disproportionation of two phenylethyl radicals to give styrene and ethyl benzene. The disproportionation reaction derives its driving force from the formation of two new strong bonds and from the fact that the P-CH bonds in radicals are usually weak. The ratio of disproportionation to combination is dependent on the structural features of the radicals involved and may be affected, for example, by solvent, pressure, and temperature (reviewed in Gibian and Corley, 1993). 

These clusters complicate the interpretation of the mass spectra. Depending on the pressure and the E/N value, the (H20) clusters can be present in the drift tube and react with the trace gas compounds. Since the PA of the clusters is higher than the PA of water, the PTR with a water cluster is more selective. This reaction can be equally efficient as the PTR, depending on the dipole moment of the neutral R. For nonpolar molecules like benzene, cluster reactions will not take place. Therefore, the sensitivity or detection efficiency of a molecule like benzene can be humidity dependent, since the amount of water clusters depends on humidity. The formation of these clusters with PTR-MS techniques can be limited and controlled by increasing the electric field applied over the reaction region or lowering the pressure. 

Hofmann and coworkers (327-330) have reported a series of studies on the deactivation kinetics for the heterogeneously catalyzed disproportionation of ethyl benzene to benzene and diethyl benzene under SCF conditions. Kinetic studies have been conducted in both a loop reactor using a protonated Y-faujasite (Z-14) catalyst (327) and in a continuous concentration-controlled recycle reactor using an HY-zeolite (HYZ) (329,330) and USY-zeolite, H-ZSM-5, and H-mordenite (328) under supercritical conditions T > 373 C, P > A5 bar). Coke extraction by SCFs was found to be strongly dependent on the type of catalyst used, and the Lewis acid centers were determined to play an important role in the coke formation and activity of the catalysts. A simple kinetic model for the catalyst deactivation was proposed (329) for SCF conditions and high ethyl benzene concentration. Based on the relatively high estimated deactivation energy of about 147 kJ/mol and first-order deactivation, the authors concluded that the catalyst deactivates much slower under SCF conditions than under atmospheric pressure. 

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