Catalytic conversion of polystyrene over HMCM-41, HZSM-5 and amorphous SiO2-Al2O3:: comparison with thermal cracking

HMCM-41, amorphous SiO2-Al2O3 and HZSM-5 zeolite have been studied as catalysts in the degradation of polystyrene (PS) at 375 degrees C. HMCM-41 is the only catalyst that shows a slight increase in activity with regard to the thermal cracking, whereas the other two materials present similar or even lower conversions. This fact has been explained by the existence of competitive cross-linking reactions promoted by the acid catalysts. Two types of behaviour can be clearly distinguished according to the product distribution. Thermal cracking and catalytic degradation over HZSM-5 yield mainly styrene (similar to 50 wt.%). On the contrary, the main products resulting from the catalytic cracking over HMCM-41 and SiO2-Al2O3 are benzene, ethylbenzene and cumene but in proportions lower than 20 wt.%. The results obtained over HZSM-5 zeolite are attributed to both its microporous structure and acid features. External acid sites are practically the only ones active for PS degradation, since this polymer is too bulky to enter the zeolite micropores. In addition, competitive cross-linking reactions also take place, which are highly promoted by the strength and Bronsted nature of the zeolite acid sites. Finally, the superior activity obtained over HMCM-41 is explained in terms of its uniform mesoporous structure and its medium acid strength. (C) 2000 Elsevier Science B.V. All rights reserved.