Investigation of Ni-catalyzed hydropyrolysis of hemicellulose via ReaxFF-MD simulation

Abstract

Catalytic hydropyrolysis represents a crucial method in the conversion of biomass, particularly hemicellulose with diverse components and an ambiguous structure, into high-value chemical derivatives. The catalytic mechanism and the role of H₂ remain challenging to fully elucidate due to the inherent complexity of the system. The mechanism of Ni-catalyzed hydropyrolysis of xylan at the molecular level is elucidated using ReaxFF molecular dynamics (ReaxFF-MD) simulations. The results show that the Ni catalyst not only provides strong chemical adsorption enabling reactants and H₂ to interact through active Ni surface sites, but also reduces the global activation energy of xylan hydropyrolysis from 37.12 kcal mol⁻¹ to 24.44 kcal mol⁻¹, significantly enhancing both pyrolysis and hydrodeoxygenation (HDO) reactions. Additionally, the Ni catalyst further optimizes the product distribution by guiding the rearrangement and cracking reactions leading to the enhanced formation of alkenes, cycloalkanes, and furan derivatives. The effective HDO reactions result in a substantial reduction in the oxygen content of the xylan hydropyrolysis products, with the oxygen content of the light condensable products decreasing to 4.07 wt%, the overall oxygen content dropping to 12.8 wt%, and the deoxygenation degree reaching 74.23%. Moreover, the hydrogenation process effectively suppresses carbon deposition over the Ni catalyst attributed to the synergistic interaction between Ni and H₂, but excessive H₂ may lead to over-reduction of the catalyst surface, thereby diminishing its catalytic activity. This research could provide a valuable approach to elucidate the intricate mechanisms underlying biomass catalytic hydropyrolysis.