Comparison of ReaxFF, DFTB, and DFT for Phenolic Pyrolysis. 1. Molecular Dynamics Simulations

TT Qi and CW Bauschlicher and JW Lawson and TG Desai and EJ Reed, JOURNAL OF PHYSICAL CHEMISTRY A, 117, 11115-11125 (2013).

DOI: 10.1021/jp4081096

A systematic comparison of atomistic modeling methods including density functional theory (DFT), the self-consistent charge density-functional tight-binding (SCC-DFTB), and ReaxFF is presented for simulating the initial stages of phenolic polymer pyrolysis. A phenolic polymer system is simulated for several hundred picoseconds within a temperature range of 2500 to 3500 K. The time evolution of major pyrolysis products including small-molecule species and char is examined. Two temperature zones are observed which demark cross-linking versus fragmentation. The dominant chemical products for all methods are similar, but the yields for each product differ. At 3500 K, DFTB overestimates CO production (300-400%) and underestimates free H (similar to 30%) and small CmHnO molecules (similar to 70%) compared with DFT. At 3500 K, ReaxFF underestimates free H (similar to 60%) and fused carbon rings (similar to 70%) relative to DFT. Heterocyclic oxygen-containing five- and six- membered carbon rings are observed at 2500 K. Formation mechanisms for H2O, CO, and char are discussed. Additional calculations using a semiclassical method for incorporating quantum nuclear energies of molecules were also performed. These results suggest that chemical equilibrium can be affected by quantum nuclear effects at temperatures of 2500 K and below. Pyrolysis reaction mechanisms and energetics are examined in detail in a companion manuscript.

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