Study On Catalytic Cracking Of Polyolefin Plastics By Hierarchical Y Zeolite

Polyolefin plastics are widely used in packaging,lubrication,preservation and other fields because of its good corrosion resistance.However,the long carbon chain structure of polyolefin makes it difficult to degrade.The catalytic cracking can convert polyolefin plastics into liquid petroleum hydrocarbons,which not only solves environmental pollution,but also means the reuse of solid waste resources.However,the catalytic degradation of plastics still faces the problems such as low liquid product yield,catalyst coke deactivation.The paper chose Y zeolite as research object.The high-temperature calcination and acid-base treatment were used to modify pore structure of zeolite.Then,a suitable process was selected to prepare catalysts with different pore structures.The catalysts were chosen to crack blended HDPE/PP in several cycling times.This paper mainly reported the influence of post-synthesis on the framework of Y zeolite,the relationship between the structure of zeolite and the effect of catalytic pyrolysis on polyolefin,etc.,and studies the factors of coke formation in Y molecular sieves.The main achievements of this paper are as follows:By exploring the effect of calcined processing on zeolite framework elements,the transforming rule of silicon aluminum species was clarified.Besides,the process of modifying zeolite with optimal acidity sites,pore volume and pore size distribution was obtained.The article confirmed that when the calcined zeolite has acidic sites retained in good cases,following acid-base treatment could improve the characteristics of the molecular sieve framework.After the improvement of the process,the prepared alkali-leaching desilication molecular sieves have good specific surface area(1357 m~2/g),large pore volume(1.591 cm~3/g)and relatively concentrated pore size distribution(2-15 nm),which can be used as catalysts for plastic degradation.The catalytic cracking of polyolefin plastics by modified zeolite Y with different structures was studied and the gas-liquid products were analyzed.The results showed that the catalytic deactivation of the three catalysts occurred after repeated cyclic cracking.Calcined zeolite without acid-base treatment showed high gas selectivity but poor cycling stability.The liquid yield obtained by desilication zeolite is the most stable(78-81%).The relationship between the structure of modified catalyst and catalytic cracking reaction was investigated.Two aspects of increasing pyrolysis conversion and reducing gas yield were discussed.The changes about the yield of calcined zeolite and desilicated zeolite in multiple using cycles were summarized.The results showed that by reducing the proportion of micropores in the catalyst,both mesoporous materials and non-porous materials can achieve high yield of polyolefin degradation in liquid phase.The changes on zeolite molecular sieve pore structure and acidity level distribution before and after cracking cycles were used to assess the coke deposition of zeolite.The results showed that cage structure in Y zeolite was against large molecules diffusion.acid and alkali modification can destroy the cage structure of zeolite to improve molecular mass transfer efficiency and improve the coke deposition on molecular sieves.In addition,there is a correlation between the distribution density of acid points on the zeolite catalyst surface and the existence mode of coke.