Catalytic Pyrolysis Of Polyethelene Waste By Metakaolin-based Geoplymer Microspheres

In the new situation of increasingly serious plastic pollution and rising crude oil prices,the cracking and oiling technology of plastics has attracted interest of researchers.Molecular sieve catalysts with high silicon-to-aluminum ratio showed high activity in plastic cracking,while catalysts with low silicon-to-aluminum ratio were studied less.Geopolymer is an alkali metal aluminosilicate inorganic material of amorphous or semi-crystalline formed by the reaction of solid aluminosilicate with a high concentration of alkali metal hydroxide,silicate solution or strong acid solution.Through the selection of different raw materials,preparation conditions and curing conditions,geopolymers can exhibit various excellent properties:high compressive strength,high temperature resistance and low thermal conductivity,etc.In the fields of fireproof materials,concrete materials and inorganic coatings,etc.Many researches on different preparation methods and applications of geopolymers,but the practical application for catalysis is still in its infancy.In this paper,the metakaolin-based geopolymer microspheres（MGM）,molybdenum-loaded amorphous geopolymer microspheres（MMGM）and NaA-type molecular sieve metakaolin-based geopolymers prepared with different ratios and preparation conditions under low silicon-aluminum ratio were investigated.The physicochemical properties of polymer microspheres（NMGM）were analyzed.The above microspheres were applied to the reaction of thermal catalytic cracking of waste low density polyethylene（LDPE）and catalyst recycling.The effect of microsphere mesh,reaction time,reaction temperature and other conditions on the yield and hydrocarbon composition of liquid phase products from thermal catalytic cracking of low density polyethylene（LDPE）was studied,and the stability of the catalyst was tested.The main results were as follows:（1）The ratio of silicon to aluminum（SiO₂/Al₂O₃）was 2.4 and water to sodium ratio（H₂O/Na₂O）was 22,0.05wt.%hydrogen peroxide（H₂O₂）and 0.1wt.%sodium dodecyl sulfate（K12）were added,the MGM with hierarchical pore structure was prepared by hydrothermal treatment at 200℃ for 24 h.The prepared MGM was applied to the thermal catalytic cracking of waste LDPE,and the reaction was carried out at 400℃ for 1 h under normal pressure,and the liquid-phase yield was up to 72.86wt.%.The liquid phase product consists of alkanes,alcohols,alkenes and aromatics.The increase of the ratio of SiO₂/Al₂O₃ and H₂O/Na₂O ratios of MGM is not conducive to the formation of aromatics in the liquid product.The yield of carbon deposition after four cycles of reaction was2.69wt.%,and the yield of liquid remained at 64.00wt.%.（2）Under the conditions of a silicon-aluminum ratio（SiO₂/Al₂O₃）of 2.4 and a water-to-sodium ratio（H₂O/Na₂O）of 22,hydrothermally at 200℃ for 24 h,MGM was prepared as a carrier,molybdenum-supported metakaolin-based geopolymer microspheres（MMGM）loaded with 15wt.%molybdenum prepared by hydrogenation reduction at 500℃.Using it as a catalyst for thermal catalytic cracking of waste LDPE,under the conditions of a reaction temperature of 400℃ and a reaction time of 0.5 h,the yield of liquid products reached 81.48wt.%,of which the content of diesel components was 67.76%,and the content of aromatics was up to 3.31%.After four cycles,the carbon deposition yield was 9.25wt.%,and the liquid yield remained at 54.47wt.%.（3）When the sodium-silicon ratio（Na₂O/SiO₂）of the catalyst was 0.8,metakaolin-based NaA molecular sieve geopolymer microspheres（NMGM）were prepared after heating in a water bath at 80℃ for 1 d,and the total acid content of the microspheres increased from 45.01μmol/g to 951.09μmol/g.The optimal reaction conditions for NMGM thermal catalytic cracking of LDPE into fuel oil were as follows:thermal catalytic cracking of waste LDPE at a reaction temperature of 380℃ for 1 h,the yield of liquid phase products was 89.36wt.%.Among them,the content of diesel components was 80.97%,and the content of aromatic components was up to 8.08%.The yield of carbon deposition after four cycles of reaction was 1.34wt.%,while the yield of liquid phase remained at74.64wt.%.