| Abstract
| - Autocatalytic, lower-temperature (≤1100 K) methane pyrolysis has defied mechanistic explanation for almostthree decades. The most recent attempt (by Dean in 1990) invoked the chemically activated addition of anallyl radical to acetylene, leading to a cyclopentadiene/cyclopentadienyl chain-branching system that promptedthe observed autocatalysis. However, newer, more accurate thermochemical data for the cyclopentadienylradical render that explanation untenable. A new model for methane pyrolysis is constructed here, using anovel mechanism generation approach that automatically computes any needed rate constants k(T,P) forchemically or thermally activated pressure-dependent reactions. The computer-generated mechanism accuratelypredicts the observed autocatalysis and concentration profiles without any adjustable parameters. Radical-forming reverse disproportionation reactionswhich involve propyne, allene, and fulveneaccount for atleast half of the experimentally observed autocatalytic effect. Many of these reverse disproportionations wereneglected in previous studies. The cyclopentadienyl radical is also important, but it is formed primarily bythe chemically activated reaction of propargyl with acetylene. New rate estimates for unimolecular ring-closure reactions of unsaturated radicals are also presented. This approach is the first to incorporate pressure-dependent reactions generally and systematically during computerized mechanism construction. It successfullyidentifies complex but critical chemical-reaction pathways and autocatalytic loops missed by experiencedkineticists.
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