Benzene pyrolysis

Pyrolysis. Benzene undergoes thermal dehydrocondensation at high temperatures to produce small amounts of biphenyls and terphenyls (see Biphenyl AND terphenyls). Before the 1970s most commercial biphenyl was produced from benzene pyrolysis. In a typical procedure benzene vapors are passed through a reactor, usually at temperatures above 650°C. The decomposition of benzene iato carbon and hydrogen is a competing reaction at temperatures of about 750°C. Biphenyls are also formed when benzene and ethylene are heated to 130—160°C ia the presence of alkaH metals on activated AI2O3 (33). 

It was mentioned previously that, unlike most arylnitrenes, polyfluorinated arylnitrenes have bountiful bimolecular chemistry. Banks and Sparkes found that pyrolysis of 4-azido-2,3,5,6-tetrafluoropyridine in cyclohexane produced the product of formal CH insertion in 45% yield. Tetrafluoropyridyinitrene was also captured by alkenes, dimethyl sulfoxide and even benzene. Pyrolysis of perfluorophenyl azide (15e) yields a product of formal insertion with a CH-bond of benzene and the formation of adducts with dimethyl sulfoxide.Abramovitch et demonstrated that... 

In addition to naphthalene and biphenyl, about 1 % each of biphenylene and triphenylene was found in the hydrocarbon portion of the phthalic anhydride pyrolysate. Neither was found in the benzene pyrolysis. These results indicated that phthalic anhydride gave benzyne on pyrolysis, and the benzyne subsequently reacted with benzene ... 

To confirm reaction scheme (4) for the formation of naphthalene and biphenyl, phthalic anhydride was allowed to react with benzene-dx (Fields and Meyerson, 1966c). Barring an appreciable isotope effect, two-thirds of the naphthalene should contain a deuterium atom and one-third only protium. Biphenyl arises in two ways insertion of benzyne into a C—H bond of benzene and pyrolysis of benzene. Biphenyl from benzyne insertion should form with retention of the deuterium atom biphenyl from benzene pyrolysis should—again ignoring any isotope effect—be d0, dlt and dz on the statistical basis of losing two, one, or zero deuterium atoms from a total of twelve protiums and deuteriums in the over-all reaction of two benzene molecules. The amount of biphenyl-... 

Recent thermochemical determinations suggest that A = 14.21 kcal/mol and B 17.21 kcal/mol (llg). To obtain, for instance, benzylic C-X bond strengths, the stabilization energy calculated from equation 9 is subtracted from C-X bond strengths for corresponding saturated species. Recent studies of alkyl-benzene pyrolysis (117) imply that within current uncertainties in relative bond strengths this formula applies to secondary and... 

Figure 2. Hydrogen concentration in product gas stream during benzene pyrolysis in various reactors. 12% benzene feed ( J) Vycor, (Q) Incoloy...

By using potassium as a carbon gasification catalyst, it is possible to obtain activated carbons of large adsorption capacity (large micropore volume), but with micropores of small dimensions. Nevertheless, these materials could not be converted into carbon molecular sieves by carbon deposition from benzene pyrolysis. Success was achieved with chars which were activated only to a limited extent [16]. 

One of the proposed theories applied successfully to the formation of gas-phase carbon from benzene is the droplet condensation mechanism. This "condensation theory", revised recently by Lahaye et al. (2), explains the formation of soot aerosols during benzene pyrolysis. The authors obtained an excellent agreement between the theory and experimental results. 

Figure 3. Effect of coke on product distribution for benzene pyrolysis...

Diphenyl. During hydrogenolysis at 930 °F and 200 atm H2, benzene was the sole product. The products of pyrolysis, besides coke, are not clear because the diphenyl results are derived from the benzene pyrolysis data of Kinney and Delbel (5). 

Since coke is the terminal product of the aromatic pyrolysis pathway, it is of interest to explore the formation mechanism. Insight into this process in the range 800° to 1100°C is provided by the benzene pyrolysis data of Kiney and Delbel (5) in a flow reactor. The diphenyl concentration vs. time behavior reported is characteristic of an intermediate in a sequential reaction A B C where A (benzene) decreases and C... 

Data of Kinney and Delbel (5) for benzene pyrolysis to carbon may be modeled by the above scheme of two sequential reactions (4 and 5) simplified such that (a) both reactions are kinetically limited in the forward directions and (b) Reaction 4 is at equilibrium while Reaction 5 is forward-kinetics controlled. Case (b) appears the more plausible for the bulk of the data, but some of the experimental trends at low conversions at the lower temperatures are qualitatively as well predicted by case (a). 

The first task is to formulate a function which defines /, If we examine the benzene pyrolysis example, Example 4.5, a suitable function is... 

A theoretical analysis of Dewar-benzene pyrolysis to give triplet benzene297 and a report of the direct and sensitized photoreactions of benzvalene have appeared.298... 

Jayasankar M, Chand R, Gupta SK, Kunzru D, Vapor-grown carbon-fibers from benzene pyrolysis, Carbon, 33(3), 253-258, 1995. 

Increase in Weight of Saran and PVDC Charcoals after Benzene Pyrolysis for Different Intervals of Time at 600°c... 

FIGURE 4.9 Adsorption isotherms of heptane vapors on Saran charcoal before and aflerpore blocking by benzene pyrolysis. (After Sharma, N., Ph.D. thesis submitted to Panjab Univ.,... 

On the basis of the preceding discussion, we proposed the following formation mechanism of OMCs [166], At OOO C, the products of benzene pyrolysis are known to be polycyclic aromatic hydrocarbon species, such as biphenyl, m-, p-, and o-terphenyls, and anthracene [155]. Such species first adsorb on the pore surfaces of SBA-15. Subsequently, further aromatization and carbonization of the adsorbed species takes place to form the first layer of carbon film [167,168], A layer-by-layer stacking then follows to create stacked graphene sheets. Thns, the favored direction of stacked graphene sheets is parallel to the long axis of the nanorods. Carbon deposition in the pores of the mesoporous silica template will stop when the pore size becomes smaller than the kinetic diameter of the benzene... 

The second step assumes that the reactivity of the ensemble is dominated by a few selected functionalities. The task then is to determine the reaction kinetics for each of the functional groups. Here the art of lumping applies in order to keep the number of kinetic lumps small. Information on reaction pathways and kinetics can be independently obtained from experiments using representative model compounds. For example, butyl benzene pyrolysis may serve as a model system for the pyrolysis of alkyl aromatics moieties in resids. 

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