Computational study on the pyrolysis of 2,5-diketopiperazine: From electronic structure calculations to kinetic modeling

Pyrolysis is an important thermochemical conversion process for biomass and is conducted in the absence of oxygen at temperatures between 400 and 1000 ^∘C. Biomass pyrolysis yields cleaner combustion fuels by decreasing fuel-bound oxygen and nitrogen species, thus reducing NO_X formation and net CO₂ emissions. A structural model compound for cyclic peptides — important nitrogen-containing components in biomass — is 2,5-diketopiperazine (DKP). In this work, we apply an automated workflow that combines reactive molecular dynamics simulations with electronic structure calculations at different levels of theory to develop a detailed kinetic model for the pyrolysis of DKP at the level of elementary reaction steps. This complements previous studies that focused only on the net reaction scheme. The developed DKP kinetic submodel for pyrolysis is implemented in the kinetic modeling software OpenSMOKE++ . Under pyrolysis, DKP decomposes into hydrogen cyanide (HCN), carbon monoxide (CO) and hydrogen (H₂). Ammonia (NH₃) is not formed in primary decomposition steps but rather in secondary reactions involving the primary intermediates. The submodel qualitatively reproduces DKP pyrolysis products observed in a fluidized bed reactor under kinetically controlled conditions and provides a reliable basis for further studies on peptide decomposition. Beyond the specific kinetic submodel, this work proposes a general workflow for investigating thermal decomposition and combustion processes.