Gas-Phase Hydrogenolysis of Benzene and Derivatives at Elevated Pressure; Methane Formation

Abstract

Benzene, in 20−30 bar of hydrogen, reacts smoothly in a quartz tubular flow reactor between 1163−1262 K, to give high yields of methane. With dwell times of 3−23 s, degrees of conversion ranged from 10 to 95%. Substituents such as Cl and CH3 are displaced more easily: ca. 350 times more rapidly at 1250 K. Mechanisms are discussed on a quantitative thermochemical‐kinetic basis. Of the alternatives considered, only a pathway through H atom addition, (reversible) isomerisation of C6H7· species, H‐transfer from H2 to give − for example − methylcyclopentadiene (F), followed by splitting into ·CH3 and cyclopentadienyl radical (G), would fully explain the observed rates of conversion of benzene.

Thermal Hydrogenolysis of Polyaromatic Hydrocarbons and Activated Carbon at Elevated Temperatures and Pressures

“…Resistance to ring-opening hydrogenolysis in highly condensed entities and limited accessibility for H 2 /H • together account for this difference. (4) Even in the TH of this relatively simple system with toluene and benzene, many more reactions occur at a lower level, but with increasing importance at more elevated temperatures. One is condensation by combination of two stabilized radicals, as in Equation (4).…”

“…(4) Even in the TH of this relatively simple system with toluene and benzene, many more reactions occur at a lower level, but with increasing importance at more elevated temperatures. One is condensation by combination of two stabilized radicals, as in Equation (4). Another is the degradation of benzene itself: traces of methane are already being produced at 500°C, [5] while its yield passes the percent level (based on total carbon) in 8 seconds at 1 bar of H 2 at 900°C .…”

“…[6] We have recently reported on the TH of benzene in a flow reactor under pressure, with up to 36 bar of hydrogen. [4] Methane was the only important product, and its output was as high as 70% of total C at, for example, 985°C /30 bar/21 s dwell time. Quantitative thermokinetic analysis led us to conclude that the ring-opening hydrogenolysis occurs through reversible addition of a H atom, reversible isomerisation of C 6 H 7 • radical species, H transfer from H 2 to give, for example, methylcyclopentadiene, which decomposes into • CH 3 and cyclopentadienyl radical (see Scheme 1 of ref.…”

“…Quantitative thermokinetic analysis led us to conclude that the ring-opening hydrogenolysis occurs through reversible addition of a H atom, reversible isomerisation of C 6 H 7 • radical species, H transfer from H 2 to give, for example, methylcyclopentadiene, which decomposes into • CH 3 and cyclopentadienyl radical (see Scheme 1 of ref. [4] ) Ϫ which is further hydrogenolysed into methane. Under these conditions the conversion of toluene to benzene [Equation (2)] is ca.…”

“…400 times faster than the breakdown of the benzene ring. [4] In a way, the gasification of benzene to methane is the reverse of condensation to aromatics beyond benzene and biphenyl: naphthalene or PAHs in general. Said higher homologues are expectedly also susceptible to hydrogenolysis, but as yet with unknown rates, products, and mechanisms.…”