Basic Research On Microwave Carbonization-directional Pyrolysis Of Discarded Disposable Medical Mask

The global spread of the COVID-19 epidemic has produced a large number of discarded disposable medical masks（DMMs）.Landfill and incineration are not only prone to toxic secondary pollutants,but also do not meet the long-term goal of carbon neutrality.Discarded DMMs are plastic wastes rich in carbon and hydrogen,and carbonization and pyrolysis can not only kill potential viruses but also convert them into high value-added functional carbon materials and hydrogen-rich gases.However,the traditional carbonization method has low yield,long process,and activation requires a large amount of alkali or metal salts.Conventional pyrolysis also faces difficulties such as high energy consumption and wide product distribution.Therefore,a clean and efficient discarded DMMs recycling method is urgently needed.In this paper,waste DMMs were converted into high value-added functional carbon materials and hydrogen-rich gases by clean and efficient microwave solvent thermal cross-linking carbonization and microwave pyrolysis.The mechanism of microwave solvothermal cross-linking and self-activation was studied in detail.The electrochemical properties of multi-element doped DMMs-based porous carbon were explored.The mechanism of microwave-assisted iron-based catalyst pyrolysis and the improvement of microwave absorption capacity of iron-based catalysts through defect engineering were revealed.The dehydrogenation efficiency of iron-based catalysts and iron-based catalysts doped with Co,Ni and Cu was explored,which provided theoretical support for the recycling of waste DMMs.（1）Study on microwave solvothermal cross-linking carbonization DMMs and energy storage performance.In the microwave field,concentrated sulfuric acid could crosslink with the linear parent chain of polypropylene in DMMs and spontaneously stack into aromatic carbon.The carbonized DMMs powder was obtained by controlling the microwave solvent heat treatment time.Microwave solvothermal treatment not only improved the thermal stability of DMMs（1000°C,53.9 wt.%）,but also achieved the co-doping of S and O atoms（21.56 at.%,1.57 at.%）.The sulfur cathode assembled by activated DMMs-based porous carbon had a first specific capacity of 1242.6 m Ah·g^-1at a current density of 0.1 C.After 400 cycles,it still maintained 35.2%of the initial specific capacity.Prolonging the microwave treatment time had little effect on the electrochemical performance of porous carbon,but significantly increased the processing cost.（2）Study on multi-element doped DMMs-based porous carbon and energy storage performance.In the process of microwave solvothermal cross-linking carbonization,S,N and O in-situ co-doped DMMs-based carbon materials was prepared by adding heterogeneous atomic source-urea,the doping rates of S,N and O were 1.68 at.%,3.15at.%and 23.16 at.%,respectively.The doped DMMs-based carbon material maintained51.2 wt.%of the initial mass at 1000℃.DMMs-based porous carbon materials with high specific surface area were obtained by controlling the self-activation temperature in an inert gas with low flow rate.The lower self-activation temperature（700℃）was not enough to produce sufficient active substances such as H₂O and H₂,resulting in a lower specific surface area of the sample,and the higher self-activation temperature（1100℃）led to the loss of doped atoms.In contrast,the porous carbon material prepared by self-activation at 900℃not only had a higher surface area(787.25 m²·g^-1),but also retained more heteroatoms.The assembled sulfur cathode had a first specific capacity of 1459.8 m Ah·g^-1 at a current density of 0.1 C and retained 56.8%of the initial specific capacity after 400 cycles.（3）Study on the microwave pyrolysis of DMMs by hydrogen reduction iron-based catalyst.The cheap iron-based catalyst is the preferred catalyst for the preparation of hydrogen-rich gas by microwave pyrolysis of waste plastics.In order to explore the microwave pyrolysis mechanism of iron-based catalysts,iron-based catalysts were prepared by microwave combustion method.It was found that the amorphous iron and hematite in the iron-based catalyst had poor microwave absorption capacity after oxidation and reduction,while the reduced iron-based catalyst had strong microwave absorption capacity.Among the iron-based catalysts,Fe₃O₄ had the highest microwave heating efficiency,followed by elemental iron,and Fe₂O₃ was the worst.Waste DMMs was cracked into 59.28 wt.%non-condensable gas and 26.57 wt.%carbon nanotubes within 10 min under the catalysis of the iron-based catalyst reduced at 650℃,the yield of H₂ was 25.43 mmol H₂·g_DMMs^-1.With the increase of the reduction temperature of the catalyst,the content of elemental iron in the iron-based catalyst increased continuously,which affected the deposition rate of carbon atoms during the microwave pyrolysis process,resulting in a decrease in the content of H₂ and carbon nanotubes,and also made the multi-walled carbon nanotubes into bamboo-like carbon nanotubes.（4）Study on Co,Ni,Cu doped iron-based catalysts for pyrolysis of DMMs.The doping of Co,Ni and Cu into the iron-based catalysts not only resulted in the structural and electronic rearrangement of the catalysts to form stable bimetallic catalysts,but also produced more lattice defects and oxygen vacancies during the lattice reconfiguration process,which improved the microwave absorption capacity of the catalysts.Compared with the undoped iron catalyst,the doped bimetallic catalyst increased the efficiency of absorbing microwave into internal energy by nearly 5 times.In particular,when the polypropylene in DMMs melted,the volume of the mixture collapsed,and the mixture of iron-based catalysts doped with Co and Ni and DMMs rose from 200℃to 900℃in2 min.Under the microwave pyrolysis of bimetallic catalyst,DMMs rapidly pyrolyzed into carbon nanotubes and hydrogen-rich gases.Compared with Ni and Cu doping,Co-doped iron-based catalysts had good dehydrogenation efficiency,the content of non-condensable gas after pyrolysis products was 62.63 wt.%,and the H₂ yield was as high as 39.74 mmol H₂·g_DMMs^-1.