Catalytic Pyrolysis Of Waste Plastics To Enhance Aromatic Hydrocarbons Selectivity Based On Ga-modified HZSM-5 And Its Combinations

The environmental pollution caused by waste plastics is becoming more and more serious.Recycling waste plastics into industrial raw materials by pyrolysis can not only solve environmental pollution,but also alleviate resources to a certain extent.HZSM-5 zeolite was used as the best aromatization catalyst for preparation of aromatic hydrocarbons by plastic pyrolysis.However,there were still problems such as large amount of catalysts,low yield of liquid phase products,and low selectivities of aromatic hydrocarbons,monocyclic aromatic hydrocarbons(MAHs)and benzene,toluene,ethylbenzene and xylene(BTEX)in pyrolysis oil.In order to improve liquid phase products yield of plastics catalytic pyrolysis and improve aromatics selectivity of pyrolysis oil,this paper carried out metal modification on HZSM-5,which introduced cracking catalyst CaO combination with Ga/ZSM-5 into different catalytic modes.Besides,HZSM-5 was modified by alkali treatment and metal composite,so as to catalytic pyrolysis waste plastics to prepare liquid-phase products with high aromatic hydrocarbons selectivity.1.In order to improve aromatic hydrocarbons selectivity of HZSM-5 catalytic pyrolysis oil,a two-stage fixed-bed reactor was used to conduct catalytic pyrolysis waste plastics experiments when the ratio of raw material to catalyst was 5:1.Different metal-modified HZSM-5 zeolite molecular sieve catalysts were prepared,and the effects of pyrolysis temperature,metal species and metal loading on gas-liquid twophase product distribution of catalytic pyrolysis waste plastics were investigated.It was found that optimum pyrolysis temperature for fast catalytic pyrolysis of LLDPE was500℃,and optimum loading of metal varied with types of loads.Optimum loading of Ce/ZSM-5 zeolite catalyst was 3 wt.%,The optimal loading of Ga/ZSM-5 zeolite catalyst was 7 wt.%.Ga/ZSM-5 was better than Ce/ZSM-5,and aromatics selectivity of 7%Ga/ZSM-5 can reach 94.02%,compared with HZSM-5,both liquid product yield and aromatic hydrocarbon selectivity were significantly improved.2.In accordance with low proportion of monocyclic aromatic hydrocarbons in Ga/ZSM-5 pyrolysis liquid phase products,CaO was introduced to crack polycyclic aromatic hydrocarbons into monocyclic aromatic hydrocarbons.Catalytic pyrolysis of LLDPE was conducted using Ga/ZSM-5 sequential with CaO mode,CaO sequential with Ga/ZSM-5 mode and mixed mode in order to explore the impact of such three catalytic modes and ratios of Ga/ZSM-5 to CaO on selectivity of aromatic hydrocarbons in liquid products.It was found that the optimal selectivity of benzene,toluene,ethylbenzene and xylene(BTEX)was obtained when ratio of CaO to Ga/ZSM-5 was1:2.In addition,synergistic mechanism of Ga/ZSM-5 and CaO varied.Aromatic hydrocarbons were inhibited under the CaO sequential with Ga/ZSM-5 mode and facilitated under the other two modes.The mixed mode had the best catalytic effect,where LLDPE pyrolytic gas was randomly and repeatedly exposed to Ga/ZSM-5 and CaO.By synergistic effect between Ga/ZSM-5 and CaO,selectivities of monocyclic aromatic hydrocarbons(MAHs)and BTEX were 72.85% and 56.21%,which was a significant improvement compared with that of 45.37% and 29.18% when using unmodified HZSM-5.3.The alkali-treated and metal composite modified HZSM-5 catalyst were prepared,according to the influences of catalyst acid sites,catalyst pores and metal active sites,which had advantages of both cracking and aromatization catalysts.Using different concentrations of Na OH solution for desilicated HZSM-5,it was found that A0.1M HZSM-5 and A0.4M HZSM-5 had better comprehensive regulation effect on acidic sites and pores,which make monocyclic aromatic selectivity to 58.45% and59.54%,respectively.High-concentration 0.4M alkali treatment would sacrifice acidic sites of HZSM-5 and reduce aromatization effect,but low-concentration 0.1M alkali treatment had poor pore-enlarging effect on micropore channels.The optimal loading of Ga modified with desilicated HZSM-5 zeolite was 5 wt.%.The loading of Ga could not only introduce metal active sites into desilicated HZSM-5,but also slightly increase the amount of strong acid sites.The proportion of MAHs in A0.4M 5%Ga/ZSM-5pyrolysis oil was 79.95%,which was 4.13% higher than that in Ga/ZSM-5:CaO mixed catalytic mode,which was 75.82%.