Mechanistic insights into hydrogen production from CaO-catalyzed lignin gasification: A ReaxFF MD and DFT study

Calcium oxide (CaO)-catalyzed hydrogen production via biomass steam gasification has attracted considerable attention, while elucidating the underlying catalytic mechanism remains crucial for advancing this technology. In this study, based on the molecular structure of lignin, the most significant and widespread typical component of biomass, the mechanism of CaO-catalyzed lignin gasification for hydrogen production was elucidated through Reactive Molecular Dynamics (ReaxFF MD) and Density Functional Theory (DFT) calculations. The results show that temperature exhibits a nonlinear modulation effect, with increased temperature driving a surge in the frequency of H 2 O dissociation; however, high-temperature agglomeration leads to an 18 % decrease in H 2 yield—defined as the percentage of molecules produced in the system with CaO relative to those without CaO—with subsequent references to ‘yield’ following this definition. Steam to biomass mass ratio (S/B) increase promotes H 2 generation through enhanced water gas reaction (WGS), but the adsorption capacity of CaO is prone to saturation under high-temperature and high S/B conditions. In-depth mechanistic analysis revealed that the binding of Ca 2+ to H 2 O led to a 3.37 eV decrease in the O–H bond dissociation energy, which induced the cleavage of H 2 O to generate H + /hydroxyl intermediates, thereby providing a hydrogen source for H 2 formation. Frontier molecular orbital calculations revealed that Lewis acid-base interactions of Ca 2+ with oxygenated organic precursors (e.g., C 2 HO, CH 3 O) narrowed their HOMO-LUMO energy gaps by 91 %, which weakened the electron density of the C–H bonds through electrostatic polarization effects and prompted C–H/C–O bond rearrangement through electrostatic polarization effects, accelerating tar cracking. This study provides theoretical guidance for the design of highly efficient carbon-resistant CaO-based biomass gasification catalysts. • The mechanism of CaO-Catalyzed lignin gasification to produce hydrogen was revealed. • Increasing S/B ratio promotes H 2 production, while high S/B lead to CaO adsorption saturation. • Ca 2+ and H 2 O binding reduces the O–H energy by 3.37 eV, inducing its cleavage to produce H + . • The interaction of Ca 2+ with organic precursors triggers electron rearrangement, promoting tar cracking.