Enhancing The Production Of Monocyclic Aromatics From Mixed Plastic Wastes Via Tandem Catalytic Pyrolysis Coupled With Hydrothermal Dechlorination Pretreatment

The massive accumulation of plastic wastes in the natural environment has caused the serious waste of resources and environmental pollution.Pyrolysis,which is one of the most promising high-value utilization technologies for waste plastics,can convert it into basic chemicals such as monocyclic aromatic hydrocarbons.However,the actual industrial recycling mainly consists of chlorine-containing mixtures with multiple types of waste plastics（polyethylene,polypropylene,polystyrene,polyvinyl chloride,etc.）.It is difficult to remove the chlorinated pollutants,and the yield of monocyclic aromatic hydrocarbons（MAHs）is low.Thus,a novel method of hydrothermal dechlorination pretreatment of mixed waste plastics coupled with dual-catalysts tandem catalysis was proposed.Efficient dechlorination under mild conditions and high-yield recovery of monocyclic aromatics were achieved.Firstly,the dechlorination characteristics of hydrothermal pretreatment were explored,and a new method for obtaining high-yield chlorine-free pyrolysis oil via hydrothermal-pyrolysis was proposed.A novel high-performance catalyst for the fabrication of nitrogen-doped carbon-coated nano-metal particles by a one-step carbonization method was designed.Finally,the economic and practical industrial application feasibility was explored based on the optimized design of the hydrothermal pretreatment-tandem catalytic system.The production of chlorine-free oil from mixed plastic wastes via hydrothermal pretreatment coupled with fast pyrolysis was investigated.The dechlorination characteristics of mixed waste plastics via hydrothermal pretreatment with the effects of hydrothermal temperature and alkali concentration were investigated.The effect of pretreatment on the distribution of fast pyrolytic products was further analyzed.The results showed that HCl was removed from PVC through elimination reaction in the hydrothermal pretreatment,and the dechlorination efficiency was improved by high hydrothermal temperature and alkali solution concentration.Under mild hydrothermal pretreatment conditions of 240℃for 2 h,the dechlorination efficiency of 99.9%was achieved by 0.2 M Na OH.Compared with fast pyrolysis,the yield of condensable products was improved from 75.31 wt%to 82.38 wt%by hydrothermal-fast pyrolysis,and the selectivity of CH₄was remarkably increased from 32.87%to 50.68%.High pyrolysis temperature,high carrier gas flow rate,and small particle size can promote the migration and conversion of organic chlorine in condensable products to inorganic chlorine in gaseous products.The production of MAHs via dual-catalyst catalytic pyrolysis of mixed waste plastics was investigated.A method of tandem catalytic pyrolysis which combined the in-situ and ex-situ catalysis of mixed plastic wastes over dual catalysts was proposed.The catalytic effect of various catalysts in tandem catalysis was investigated.Based on this,a novel method of carbon-based/zeolite-based dual-catalyst tandem catalysis was developed.The synergistic mechanism of carbon-based catalysts coupled with microporous zeolite catalysts for catalysis was revealed.The results showed that the selectivity of MAHs was significantly improved by the tandem catalytic method coupling MCM-41in-situ and HZSM-5 ex-situ,and the in-situ pre-reforming of MCM-41 effectively inhibited the coke deposition on the surface of HZSM-5.Compared with single-catalyst catalysis,the acid strength of micro-mesoporous catalysts could be better preserved by tandem catalysis.The utilized acidity of micro-porous catalysts was positively correlated with the BTEX yield.The synergistic effects of multi-metals in the carbon-based catalyst（Fe/Ni/Mo@N/C）effectively improved the nitrogen doping content of the catalyst as well as the defect sites dominated by pyridine nitrogen,thus,stronger in-situ cracking was exhibited compared to MCM-41.In the tandem catalysis of Fe/Ni/Mo@N/C coupling with HZSM-5,the selectivity of benzene,toluene,ethylbenzene,and xylene（BTEX）was68.72%,and the total yield of which was 303.33 mg/g.Hydrothermal dechlorination pretreatment of mixed plastic wastes coupled with tandem catalytic pyrolysis was investigated,and the reaction system was designed.Firstly,the pyrolytic kinetics of the feedstocks obtained after hydrothermal pretreatment was studied.The synergistic mechanism of hydrothermal and catalysis was established.The stability of the long-term operation of the system and the economic analysis were investigated.The results showed that the homogenization effect of hydrothermal pretreatment on plastics was the key to improving its catalytic effect.Higher pretreatment temperature effectively weakened the branch-chain bond energy of the polyolefin molecules and dramatically reduced its pyrolytic activation energy.At the same time,theβ-scission and alkylation reactions in the catalytic process were promoted by the pretreatment,and the yields of toluene and xylene were significantly improved through catalysis.After hydrothermal-tandem catalysis,the selectivity of MAHs and BTEX reached 84.09%and 71.29%,respectively.The yield of BTEX was 351.97 mg/g,which was 1.90 times that of ex-situ catalysis over the commercial HZSM-5.After the catalytic reaction of 360 min and processing the mixed plastic wastes 18 times the catalyst mass,the coke amount of the microporous catalyst was still lower than 4 wt%.The technical and economic analysis revealed that the system had a short payback period and reasonable profitability.The prospects of industrial application were promising.