Optimization of molecular dynamics in hydrogen production through phenol gasification in supercritical water

Abstract

Phenol serves as a crucial intermediate in the biomass conversion process, and its solubility or conversion rate in the aqueous phase is a determining factor for the prerequisite conditions and rate of biomass gasification for hydrogen production. In this study, we utilized the ReaxFF reactive force field in conjunction with density functional theory to investigate the optimized molecular dynamics of phenol gasification for hydrogen production in supercritical water. The research systematically explored phenol gasification for hydrogen production in supercritical water under varying reaction temperatures, residence times, and relative concentrations, comprehensively analyzing the gasification mechanisms. Results indicate that under conditions of 4000 K, 0.4644 g mL−1, and 750,000 steps residence time, the hydrogen gasification rate reaches a maximum of approximately 80 %. The reaction temperature exhibits a positive correlation with gasification efficiency, while residence times and relative concentration also impact hydrogen yield. Furthermore, phenol in supercritical water predominantly exhibits two ring-opening pathways: one involves the initial dehydrogenation by removing a hydrogen atom connected to phenol itself, followed by subsequent ring-opening; the other entails the prior removal of a hydrogen atom attached to the hydroxyl group, leading to eventual ring-opening. The energy disparity for the final ring-opening in these pathways is not significant, both approximately at 880 Ha. In this system, H2O not only serves as the reaction medium but also provides essential free radical ions for the reaction, ultimately resulting in hydrogen gas generation. This study offers theoretical guidance for enhancing hydrogen production in biomass supercritical water processes.