Influence of nanocrystalline HZSM-5 external surface on the catalytic cracking of polyolefins

Catalytic cracking of both LDPE and HDPE has been studied using three different samples of nanocrystalline HZSM-5 zeolite as catalysts. The zeolite samples were synthesized under different conditions in order to obtain materials with different crystal sizes in the nanometer range (10-60 nm) and high external surface areas (78-242 m(2)/g). The plastic cracking reactions were carried out at 340 degrees C for 2 h using a plastic/ catalyst mass ratio of 100. Despite these really mild conditions, high activities were observed over the three catalysts. The plastic conversion and the turnover frequency increased strongly with the amount of external surface area of the catalysts, showing the importance of the non-sterically hindered external acid sites for the conversion of the bulky plastic molecules. The main products obtained in the cracking reactions over the three catalysts are light hydrocarbons (C-3-C-5 fraction) with a considerable amount of olefins, which indicates the possibility of addressing the zeolite selectivity towards hydrocarbons useful as raw chemicals. These products are formed via an end-chain cracking pathway of the polymer molecules over the zeolite acid sites. The occurrence of this mechanism has been confirmed by DSC analysis of the LDPE and HDPE residues remaining within the reactor after the catalytic tests, showing they still consist of plastic molecules with melting temperatures similar to those of the raw polymers. (C) 2005 Elsevier B.V. All rights reserved.