Study On The Catalytic Reaction Mechanism Of Lignin Degradation By Supercritical Water Gasification Based On ReaxFF Force Field

As the main industrial waste in pulp and paper industry,the direct emission of lignin will cause serious negative impact on the environment.The catalytic supercritical water gasification（SCWG）technique can realizing the double benefits of environment and energy,which could not only reduce the industrial pollution of the pulp and paper industry,but also turn waste into treasure.It is of great significance to study the degradation behavior and catalytic mechanism of lignin in supercritical water（SCW）for process optimization,reactor design and catalyst development.However,SCW reaction is a high temperature,high pressure and high reaction rate process,which involves the formation and consumption of a large number of reaction intermediates,and the reaction mechanism is extremely complex.Due to lack of the in-situ detection methods,it is difficult to study the micro mechanism of catalytic SCWG for lignin at the molecular level.Therefore,molecular dynamics method based on Reax FF reaction force field was used in this paper to explore the catalytic reaction mechanism of SCWG technology for lignin treatment.In the process of lignin catalyzed SCWG by Ni nanoclusters with different sizes（2.0nm,3.0 nm and 4.0 nm）,the depolymerization of lignin involved the breaking ofβ-O-4’bond,which mainly included three reaction pathways,and the size of Ni catalyst affected the breaking procrss.During ring opening,Ni catalyst can accelerate the cleavage of C-O bond,destroy the conjugatedπbond of aromatic ring and accelerate the process of ring opening.The formation of H₂ completely occured on the surface of Ni catalyst.H radicals gradually approached each other through the transformation of adsorption sites,in which diffusion of H radical was the rate limiting step of H₂ formation,especially in the initial stage.2.0 nm catalyst had larger surface area and more active sites,which promoted the cleavage of C-C,C-O,C-H and O-H bonds and showed good catalytic activity.In addition,due to the lower surface oxidation degree,2.0 nm Ni catalyst possessed higher stability.During lignin degradation,the lower the oxidation state of Fe possessed the stronger the catalytic ability,and zero-valent Fe had the highest catalytic activity.Meanwhile,Fe catalyst had unique advantages in catalytic hydrogen production,and the yield of H₂ and CH₄ in the system was obviously higher than that in the iron oxide system.In addition,it was found that Fe based catalysts with low oxidation state（Fe and Fe O）were beneficial to the formation of CO,while Fe based catalysts with high oxidation state（Fe₃O₄ and Fe₂O₃）were beneficial to the formation of CO₂.During the reaction,the lattice oxygen in the catalyst was consumed and replenished.Therefore,not only the original lattice oxygen but also the oxygen from H₂O molecules and lignin molecules was present in the ferrite compounds after the reaction.