Effect of regeneration temperature and time on the activity of HUSY and HZSM5 zeolites during the catalytic pyrolysis of polyethylene

The effect of the temperature and time of regeneration on the activity of HZSM5 and HUSY zeolites employed in the catalytic pyrolysis of polyethylene have been studied by thermogravimetric analysis. The behaviour observed for both zeolites is completely different. After regeneration HUSY zeolite completely recovers the ability to reduce the decomposition temperature of the polymer, independently of the regeneration conditions, while HZSM5 is very sensitive to temperature and time. The type of polymer also affects the behaviour of HZSM5 since the activity is greater with LDPE than with HDPE, but after regeneration at high temperatures, the activity recovered is lower with LDPE. Fresh and regenerated zeolites were characterized by X-ray diffraction, nitrogen adsorption-desorption isotherms, ammonia temperature desorption, 27Al MAS-NMR and scanning electron microscopy to explain the changes observed after regeneration. As expected there were no significant changes in the HUSY properties. With respect to HZSM5, a progressive dealumination implying the loss of a greater number of strong acid sites (HT) than weak acid sites (LT) was observed. These results justify the changes in activity of HZSM5 after regeneration, and also the differences found in the activity recovered with the LDPE and HDPE samples.