Degradation of polyethylene and polypropylene into fuel oil by using solid acid and non-acid catalysts

Abstract

The thermal and catalytic degradation of plastic polymers, polyethylene (PE) at 430°C and polypropylene (PP) at 380°C into fuel oil were carried out by batch operation. The catalysts employed were acid-catalysts silica–alumina (SA-1, SA-2), zeolite ZSM-5 and non acidic mesoporous silica catalysts (silicalite, mesoporous silica gel and mesoporous folded silica (FSM). The yields of product gas, liquid and residues; recovery rate of liquid products, and boiling point distribution of liquid products by catalytic degradation were compared with those of non-catalytic thermal degradation. The present work is divided into three sections: (1) a study of effect of catalytic contact mode and (2) a study of effect of types of catalysts on plastic degradation, and (3) the evaluation of catalysts during the degradation of PE and PP by repeating batch operation. For PP degradation in liquid phase contact with SA-1, the yield of liquid hydrocarbons was obtained with 69 wt.%, and the boiling point (bp) of the oil ranged between 36 and 270°C, equivalent to the bp of normal paraffins n-C 6 to n-C 15. The liquid products from catalytic degradation have a carbon number distribution very similar to commercial automobile gasoline. For vapor phase contact, the yield of liquid products was much lower (54%) and the rate of liquid recovery (or formation) was much slower. Catalysts possessing strong acid sites such as zeolite ZSM-5 accelerated the degradation of PP and PE into gases which resulted in low liquid yields. For FSM, which possesses no acid sites, the initial rates of PP and PE degradation into liquid were as fast as that over an acid catalyst (SA-1) and the liquid yields were higher. The liquid products from catalytic degradation over FSM have a carbon number distribution similar to a mixture of kerosene and diesel oil. Upon repeated use SA-1 deactivated very rapidly due to coke deposition on the catalyst, whereas FSM deactivated much more slowly. These findings concerning the FSM catalyst strongly suggest that the mesopores surrounded by the silica sheet may act as reservoir for radical species and the radical species accelerate the degradation of plastic melt.