Zeolite shape selectivity impact on LDPE and PP catalytic pyrolysis products and coke nature

Abstract

Catalytic pyrolysis of plastic waste can offer an economical process for plastic waste conversion due to the capability to produce high-quality products with high yields. A simple stirred tank reactor was used for two significant types of polyolefin (low-density polyethylene and polypropylene) under thermal and catalytic conditions, i.e., promoted by HZSM-5, HBEA, and HY. Complete conversion of the polymers into light gases, aromatic coke, and liquid products (mainly in the gasoline range) was achieved in the presence of all catalysts. The detailed analysis of the gas, liquid, and coke deposited inside the zeolite framework was used to unravel the mechanism responsible for converting LDPE and PP over the different zeolites. LDPE and PP particular polymer structures significantly impacted the reaction pathway, with PP yielding more reactive and stereochemically cumbersome intermediates. Furthermore, secondary reactions, particularly hydrogen transfer, secondary cracking, cyclization, and oligomerization, significantly impacted the product distribution. The higher acidity and particular framework shape of HZSM-5 benefited secondary cracking and hydrogen transfer reaction leading to higher selectivity towards gas and aromatic compounds. Further, HBEA and HY, respectively, favored isomerization and cyclization instead. Additionally, the spent catalysts' physicochemical properties combined with the information obtained from the analysis of coke composition were used to differentiate the processes responsible for the deactivation of the different zeolites. The presence of polyaromatic coke promoted the deactivation of the catalysts, with HZSM-5 retaining more than 50% of its acidity after the reaction. In contrast, more than 80% of HY and HBEA acid sites were lost under the same conditions.