Study On The Co-pyrolysis Of Biomass And Plastics Catalyzed By Metal/nonmetal Co-doped Porous Carbons

In recent years,the process of catalytic co-pyrolysis of biomass and plastic waste to aromatics has received increasing attention.Activated carbon（AC）has shown great advantages in the field of catalytic pyrolysis due to its high specific surface area,rich surface functional groups,well-developed pores and tunable pore structure.However,AC suffers from poor catalytic deoxygenation and low selectivity for high value-added products such as aromatics.In order to further enhance its catalytic activity,a metal/non-metal heteroatom co-doped porous carbon catalyst was constructed and applied to the catalytic co-pyrolysis of biomass and plastics to aromatics.The effects of heteroatom doping,temperature and catalyst dosage on the content and selectivity of co-pyrolysis aromatics were investigated to provide a theoretical basis for the efficient thermochemical conversion and high value utilization of solid wastes such as biomass and plastics.The main studies and results are as follows:Firstly,boron-doped porous carbon catalysts（BAC）with different loading gradients（1.0,2.0 and 3.0 wt%）were prepared by the isovolume impregnation method.The specific surface area,pore capacity,surface functional groups and acidity of the catalysts were characterized by BET,FTIR and NH₃-TPD,and the crystal structure and presence morphology of boron before and after the use of BAC were investigated by XRD and XPS.It was found that with the increase of boron doping,the specific surface area and pore size of BAC gradually decreased,the amount of strong and weak acids increased significantly,and the BC₃ bond diffraction peak changed to B-C bond diffraction peak in the BAC after use.The BAC-catalyzed co-pyrolysis of corn stover（CS）and high-density polyethylene（HDPE）was carried out in a fixed-bed reactor,and the effects of boron doping,pyrolysis temperature and catalyst to feedstock mass ratio on the product yield,composition and distribution were investigated by the central combination design method.The results showed that the relative peak area content of monocyclic aromatic hydrocarbons（MAHs）increased and then decreased with the increase of boron doping,reaching a maximum value of 52.95%at 1.0 wt%of doping,which indicated that the appropriate boron doping provided sufficient active sites to accelerate the aromatization reaction.In addition,the relative content of MAHs to polycyclic aromatic hydrocarbons（PAHs）gradually increased with increasing temperature and catalyst/feedstock mass ratio,while the opposite was true for the relative peak area content of oxides.Secondly,nitrogen-doped porous carbon catalysts（NAC）with different urea to coconut shell mass ratios（0.1,0.4,0.8 and 1.6）were prepared by isovolumetric impregnation method.It was concluded that the specific surface area and pore size of NAC catalysts gradually increased with the increase of nitrogen doping,however,the pore structure of the catalysts was destroyed when the urea/coconut shell mass ratio was higher than 0.8.In addition,the doping of nitrogen resulted in a change in the surface morphology of the catalyst,introducing nitrogen-containing species such as pyridine-N and pyrrole-N on the catalyst surface,which promoted the generation of MAHs.This was attributed to the interaction between the doped nitrogen and the oxygen-containing functional groups on the surface of the carbon material,as well as the use of pyridine-N as an active site to facilitate the catalytic aromatization reaction.The high temperature and the NAC catalyst to feedstock mass ratio both promote the decarboxylation,decarbonylation and aromatization of the co-pyrolysis intermediates,which lead to the conversion to aromatic compounds.The relative peak area content of MAHs reached a maximum of 62.41%at a urea to coconut mass ratio of 0.8,a co-pyrolysis temperature of 550°C and a NAC catalyst to feedstock mass ratio of 2.0.Finally,porous carbon catalysts with different species of metals（Fe,Co,Ni and Zn）co-doped with nitrogen were prepared by an isovolume co-impregnation method.It was found that metal and nitrogen co-doping formed metal-N species,with the 10Fe/N-AC catalyst having the highest specific surface area and a predominantly microporous pore structure.The co-doping of metal with nitrogen achieved the co-production of aromatics and H₂,with 10Ni/N-AC showing high selectivity for H₂ production,while 10Fe/N-AC effectively promoted the production of MAHs.With increasing Fe doping,the relative peak area content of MAHs increased and then decreased,reaching a maximum at 10.0wt%Fe doping,while the opposite was true for PAHs.This indicates that the appropriate amount of Fe doping provides more active sites for the reaction and effectively promotes the deoxygenation and aromatization reactions of the co-pyrolysis volatiles.