| The polyolefin materials that widely used in the field of packaging seriously polluted the environment,the degradation and reuse of waste plastics is still a worldwide problem and research hotspot.The chemical recycling of waste plastics was considered to have a broad application prospect,which refers to the recycling of waste plastics by pyrolysis or catalytic pyrolysis to converting them into small molecular products or monomers.The commonly used catalysts for catalytic pyrolysis are molecular sieves(natural zeolite,MCM-41,etc.),which belong to solid acids(Blount acid or Lewis acid),and it depends on its abundant pores and large specific surface area to realized the contact with polymer melt.However,the polymer melt has a high viscosity,and is not easy to fully diffuse into the pore channels of catalyst.Besides,the coking products in the catalytic pyrolysis process would block the catalyst channels and deactivate the catalyst.Therefore,there are still some problems in the pyrolysis of waste plastics,such as high pyrolysis temperature,large dosage of catalyst,easier deactivation and uneven diffusion of polymer molecules in the catalyst pore.In view of the above problems,this project proposes to use AlCl3(a typical Lewis acid)as the main catalyst agent to catalyze the pyrolysis reaction of polyolefin.NaCl and AlCl3 can form liquid eutectic molten salt to reduce the sublimation of AlCl3 while maintaining its catalytic activity.In the pyrolysis reaction system,the AlC13-NaCl eutectic salt was liquid state,which can avoid the deactivation of catalyst that caused by coking products and improve the temperature uniformity of the reaction system.According to analysis results,the optimal molar ratio of AlCl3 to NaCl was 3:2.Finally,the molar ratio of AlCl3 in stable liquid eutectic salt formed in the temperature range of 300-500℃ was between 51%-54.9%.The AlCl3-NaCl eutectic salt with a molar ratio of 3:2 was used to catalyze the pyrolysis reaction of waste polyethylene(WPE),waste polypropylene(WPP)and waste polystyrene(WPS).Three kinds of collected waste plastics(WPE,WPP,WPS)were pretreated and pelleted.The results showed that the polystyrene was an amorphous polymer.The crystallinity of polypropylene and polyethylene were 45.51%and 54.57%,respectively.The thermal decomposition temperatures of the three kinds of polyolefins were 360℃,420℃ and 440℃,respectively,which affected by the side groups and crystallinity.The polystyrene was easiest to decompose,while polyethylene with tight arrangement of molecular chains was most difficult to decompose.The apparent activation energies of the three polyolefin pyrolysis reactions were 206.23 kJ·mol-1,227.07 kJ·mol-1 and 250.61 kJ·mol-1,respectively.The results showed that the AlCl3-NaCl eutectic salt catalyst can increase the ratio of coking products and gas products,while the liquid yield decreases,and the content of light components in liquid products increased.AlCl3-NaCl eutectic salt can significantly accelerate the pyrolysis reaction rate at the same temperature.In addition,AlCl3-NaCl eutectic salt can significantly reduce the initial pyrolysis temperature of the three kinds of waste polyolefins and the apparent activation energy of the pyrolysis reaction.When using 30 wt%eutectic salt,the apparent activation energy of the three kinds of waste polyolefins decreased to 92.32 kJ·mol-1,48.76 kJ·mol-1 and 122.16 kJ·mol-1,respectively.Due to the electrophilic effect of AlCl3 and the electron-donating effect of methyl groups,the apparent activation energy of polypropylene reduced most significant when the catalyst was used,and the drop of apparent activation energy of polystyrene was roughly equivalent to that of polyethylene,about 50%.By analyzing the valence layer electronic arrangement of Al3+,a catalytic mechanism based on coordination catalysis was proposed.The carbocation was formed by the electrophilic action of AlCl3-NaCl eutectic salt.Through electrophilic action of AlCl3-NaCl eutectic salt,carbocation was formed firstly,which formed pyrolysis products of various structures through hydrogen transfer reaction,isomerization,cyclation reaction and aromatization reaction.The composition of the liquid products was analyzed by gas chromatography-mass spectrometry(GC-MS)and flourier transform infrared spectrometer(FTIR).The structure and composition of the coking products were characterized by scanning electron microscopy(SEM),FTIR and energy dispersive spectrometer(EDS).The results indicated that the liquid pyrolysis products of polyethylene and polypropylene were mainly chain aliphatic hydrocarbons,and contains a small amount of cyclic structural hydrocarbons and aromatic hydrocarbons.The liquid pyrolysis products of polystyrene were mainly benzene series compounds such as toluene,ethylbenzene,styrene,polycyclic aromatic hydrocarbons and a small amount of polycyclic aromatic hydrocarbons.By analyzing the functional groups of the coking products,the coking products were considered to be produced by the direct dehydrogenation of polyolefin into carbon catalyzed by AlCl3-NaCl eutectic salt.The carbonized char has a porous structure and the application potential in the fields of adsorption and electrochemistry,the main elements of carbonized coking products were C element.Moreover,AlCl3-NaCl eutectic salt can reduce or prevent the generation of olefin products of the liquid products by the catalytic alkylation reaction and direct dehydrogenation of polyolefin into carbon,thus increase the content of saturated hydrocarbon in the liquid products. |
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