Pyrolysis Simulations Of Various Lignin Molecular Models By ReaxFF Molecular Dynamics

Pyrolysis-based technologies are promising methods to convert lignin into biochemicals,biomaterials,and biofuels.Deep understanding of the molecular mechanisms involved in lignin pyrolysis is important for the development of the efficient lignin utilization technologies.Due to the complexity of lignin structure and pyrolysis process,it is still difficult to obtain the dynamic evolution scenario of the complex lignin pyrolysis reactions with the available experimental methods and QM calculation methods.By taking advantage of the new methodology of large scale ReaxFF MD simulations,this work investigates the complicated lignin pyrolysis process by simulations of large-scale lignin models containing 11800-77200 atoms.The simulations were performed using the GPU-enabled high performance code of GMD-Reax.The underlying reaction mechanisms of lignin pyrolysis were discovered with aid of VARxMD,a unique tool for visualization and reaction analysis for ReaxFF MD simulations.The evolution of linkages,intermediates,functional groups,and main products in the pyrolysis process of softwood,hardwood and Kraft lignin was systematically studied.The differences and similarities in the pyrolysis of different types of lignin were unraveled.The main results obtained in this work are as follows:(1)7 molecular models of softwood,hardwood,and Kraft lignin were constructed with a coverage of different plant sources,different separation methods,and different viewpoints on monomer polymerization of lignin.The models include branched polymer models of Adler and Freudenberg for softwood lignin,linear polymer model of Ralph-SL for softwood lignin,branched polymer model of Nimz for hardwood lignin,linear polymer model of Ralph-HL for hardwood lignin,and branched polymer models of Marton and Crestini for Kraft lignin.These 3D models span 7 types of oxygen-containing functional groups and 20 types of linkages.All the models contain more than 10,000 atoms.The large-scale lignin models can provide a better description of the structure diversity of branched or linear lignin polymers,thus facilitating the investigation of the reactivity interactions between linkages or monomer units in lignin pyrolysis.(2)A reaction analysis strategy was proposed to identify the specific linkage involved reactions by using a combined query structure of one linkage together with its two connected monomer rings.The dominant reaction pathways in the initial conversion of the 11 linkages and their linked monomers were obtained.C?/C? ether bond cracking is the dominant pathway for the consumption of ?-O-4,?-O-4,?-O-4&?-5,?-O-4&?-O-4&5-5,and ?-1&?-O-a linkages and their linked monomer aryl rings.Both the C?-O ether bond cracking and the monomer aryl ring opening are equally important for the consumption of the ?-?&?-O-? linkage and its linked monomer aryl rings.The aryl ring opening reactions are the dominant pathways for the consumption of other 4-0-5,5-5,?-1,?-2,and ?-5 linkages and their linked monomer aryl rings.In addition,the reactivity interactions of neighboring linkages and their monomer aryl rings were observed.The reactivity of a less reactive linkage(such as 4-O-5 linkage)and its monomer aryl rings can be activated by the phenoxy radicals generated by the ether bond breaking of its neighboring active linkages of ?-O-4 or ?-O-4.(3)It is found from ReaxFF MD simulations that the benzene ring can undergo ring-opening reactions,which is consistent with what suggested by previous QM calculations and shock wave experiments.This work further reveals for the first time the four pathways and three important intermediates that activate and convert the thermally stable aromatic rings into other ring structures in the pyrolysis of lignin.The aromatic rings can be converted into:(?)an oxygen-containing seven-membered heterocyclic ring induced by phenoxy radicals,(?)a five-membered aliphatic ring activated by the bridged 3-and 5-membered aliphatic rings,(?)a 3-membered aliphatic ring activated by the bridged 3-and 5-membered aliphatic rings,and into(?)a 7-membered alicyclic ring with the activation of the bridged 3-and 6-membered aliphatic rings(4)The differences and similarities in the pyrolysis behavior of softwood,hardwood and Kraft lignin models were further unraveled.The differences are;(?)The profile of the thermogravimetric curve is closely related to the content of C-O ether bonds.With the highest content of C-O ether bond,hardwood lignin has the highest thermal reactivity,lower of the softwood lignin,and the lowest thermal reactivity of Kraft lignin correspondingly.(?)Using the proposed strategy to identify lignin pyrolysis stages based on the distinguished five lumped categories of lignin pyrolysates and their evolution characteristics,the pyrolysis process of softwood and hardwood lignin was found showing significant three-stage characteristics,while Kraft lignin pyrolysis exhibits only the latter two stages.(?)Different structural environments of active substituents of carbonyl,methoxy,and adjacent ?-O-4 linkages on ?-O-4 linkage structures will affect its conversion rate in different types of lignin models.With the highest portions of active substituents,the Nimz hardwood lignin model has the fastest conversion rate of the ?-O-4 linkages,faster for the Adler and Freudenberg softwood lignin models,and the slowest for Marton Kraft lignin model.Similar substituent effects also exist in the conversion of the ?-&y-O-a linkages.The ?-P&?-O-a linkages containing carbonyl groups(C?=O)in the Marton Kraft lignin model and the active ether bond(C?-O-4 and-OCH3)in the Nimz hardwood lignin model begin to react earlier.In addition,it was found that lignin models containing more inactive Ph-(C)n-Ph bonds and less active PhO-C bonds have the slower conversion rate of carbon-carbon linkages and their linked monomers.The pyrolysis similarity of softwood,hardwood,and Kraft lignin lies in the aromatic ring structures of lignin monomers that can be converted into six-membered aliphatic ring,oxygen-containing seven-membered heterocyclic ring,five-membered aliphatic ring,or seven-membered aliphatic rings by the activation of phenoxy functional groups,oxygen-containing functional groups on aryl rings,or benzyl groups in the pyrolysis of different types of lignin.Moreover,similar evolution trends of the ring structure intermediates were observed in the pyrolysis of different types of lignin.(5)This work investigated the possible influence of the branched or linear polymer structures of the same type of lignin on their lignin pyrolysis behavior.The results showed that the temperature for the six main small molecule products begining to generate and their maximum amount produced are closely associated to the content of the functional groups or linkage structures involved in their formation.The branched or linear polymer structures does not seem to be significant in generation of these small molecule products.(6)The Solid-Py/SR-VUV-PI-TOF-MS experiments of lignin samples were also carried out in this work.The comparison between the experimental results and the simulation results showed that 16 kinds of C0-C3 products detected in the experiments can be obtained in the simulations.Less pyrolyzates of benzene,phenol and their derivatives were obtained from the pyrolysis simulations of different lignin models than from the experimental results.The ReaxFF MD simulations of larger-scale Kraft lignin model can better describe the evolution behavior of benzene,phenol,and its derivatives pyrolysis products.The consistent evolution trends with temperature were observed between the experiments and simulations for the C0-C3 pyrolyzates of ethylene,formaldehyde,methanol,hydrogen sulfide and methyl mercaptan,as well as the aromatic pyrolyzates of benzene,phenol,o-methylphenol,catechol,and guaiacol.The consistent evolution trends over time at different temperatures were also obtained between the experiments and ReaxFF MD simulations for CH2O,CH3OH,benzene,phenol,and guaiacol.Their generation rates increase with the increasing temperatures.The higher the temperature,the faster approaching to their maximum yields.Although the time scale and temperature range between the Solid-Py/SR-VUV-PI-TOF-MS experiments and the ReaxFF MD simulations of lignin pyrolysis are quite different,the consistent evolution trends of the pyrolysis products over temperature and reaction time were obtained between the experiments and simulations.This work demonstrates that the large scale ReaxFF MD simulation method is a promising method not only for qualitatively predicting the evolution trend of lignin pyrolyzates with temperature and time,but also for explaining what predicted with the unraveled underlying mechanisms.It can provide deep insight into the molecular reaction mechanisms and theoretical supports for the technology development in the effective valorisation of lignin.