Study On Catalytic Fast Pyrolysis Characteristics Of Polyethylene Terephthalate(PET)

The environmental pollution caused by plastic waste has always been an urgent problem that plagues the healthy development of society and cities.Scientists from all over the world are trying to find ways to efficiently dispose of plastic waste.Plastic waste was once treated as waste,and only considered harmless treatment.Now it is gradually regarded as an effective resource,which can be converted into high-value fuel or chemicals through rational use.In this paper,by conducting separate/catalytic/co-pyrolysis experiments on waste PET plastics,using existing detection technologies to qualitatively and quantitatively analyze and identify pyrolysis products,and explore the reaction mechanism and pathways of PET under different pyrolysis conditions.According to the fact that the pyrolysis product contains a large amount of oxygen-containing acid products in the absence of catalyst,it is proposed to use ZSM-5 as a catalyst to upgrade the product.In addition,a co-pyrolysis experiment was carried out on PET and lignocellulose.The effect of introducing PET as a "hydrogen source" into biomass on the distribution of pyrolysis products,and summarized the characteristics of ZSM-5,such as easy coking,deactivation and poisoning in the catalytic process,has been upgraded and upgraded.Finally,the pyrolysis mechanism and reaction path of PET in the above several pyrolysis modes is summarized.The main conclusions of the study are as follows:1.Through the analysis of the pyrogravimetric data of PET alone,the effects of heating rate,air flow rate,and final temperature on the pyrolysis of PET during the pyrolysis process are studied.Studies have found that the pyrolysis of PET will produce a large amount of waxy products.Waxy products are the most important products at any temperatures,accounting for more than 50wt% of the output.The heating rate has a significant effect on the thermal cracking of PET.The faster the heating rate,the higher corresponding temperature related to the fastest reaction rate.But the effect of air flow rate on the thermal cracking of PET is almost negligible.In addition,infrared spectroscopy results show that there are obvious characteristic peaks of hydroxyl(-OH),carbonyl(C=O),and ester group(-COO-)functional groups in the waxy product,indicating that the main components of the waxy product may be acids and esters.Other oxygen-containing substances,GC-MS results further revealed that benzoic acid,terephthalic acid and their derivatives are the main components of waxy products.Through analysis,it is found that the oxygen-containing acidic substance is related to the breaking of the ester bond during the pyrolysis of PET,the subsequent olefin end group products and its quaternary cycle transformation state.2.When using ZSM-5 molecular sieve and NiCl₂ as a catalyst,it is found that there are two obvious stages in the catalytic pyrolysis of PET,namely thermal cracking stage? catalysis stage.The two stages are completed at different temperatures,and the addition of catalyst hardly affects the pyrolysis of the first stage.In the second stage,the two catalysts exhibited completely different catalytic properties under different temperature conditions.For example,ZSM-5 has no significant effect on the yield of solid residues at 450°C,which means that ZSM-5 hardly participates in PET heat.The first stage of cracking,but the solid residue reached the maximum yield when the mass ratio of ZSM-5 to PET was 2 under high temperature conditions,and thereafter it dropped sharply.NiCl₂ is the opposite of ZSM-5,which is conducive to the further degradation of solid residues to generate more gas products.The NMR analysis of the waxy product shows that both ZSM-5 and NiCl₂ catalysts can greatly reduce the carbonyl content in the product.With the increase in the amount of ZSM-5 molecular sieve used,the proportion of aromatic ether substituted carbon in the product decreases significantly.This confirms that ZSM-5 as a catalyst has a positive deoxidation effect,but there are also problems such as coking affecting the life of the catalyst.3.The fast catalytic co-pyrolysis experiment was carried out using bagasse(lignocellulose)and PET.According to the high deoxidation efficiency and easy-coking characteristics of ZSM-5 catalyst,the problem of catalyst deactivation caused by coking during the co-pyrolysis of biomass and PET was improved by mixing Al2O3 loaded with Na+.The study found that the increase in the original biomass ratio is conducive to the generation of pyrolysis oil,and the increase in the ratio of ZSM-5 in the mixed catalyst can facilitate the production of aromatics,benzene and their homologs in the pyrolysis liquid product.The catalytic co-pyrolysis of PET and lignocellulose to produce aromatic hydrocarbons is mainly accomplished through two ways.One is the transfer of hydrogen elements in the hydrocarbon products produced by the pyrolysis of PET,which leads to the weakening of the carbon polymerization reaction and avoids the formation of coke;the other is Diels-Alder reaction in the co-pyrolysis process,and the addition reaction of olefins in the zeolite molecular sieve produces a large amount of aromatic products.In addition,the synergistic effect of Na,hydrocarbyl and alkoxy also improves the deoxygenation efficiency of the catalyst.