Control Of Products And Mechanism Of Degradation During Catalytic Degradation Of Polyolefin

Recently the recycling of waste plastics has received much attention all over the world because of serious environmental problems caused by waste plastics as well as their potential for use as resources. Among the four plastics recycling methods defined by ASTM, the tertiary recycling, in which waste plastics is converted into useful chemicals, is thought to be the most promising method. In particular, tertiary recycling of polyolefin, which in quantity terms represents the largest group of plastics, has attracted much work from many researchers. However, because of chemical and technical problems as well as economic and legal factors, all processes for tertiary recycling of polyolefin have not been cost effective until now.Large-scale tertiary recycling of plastic waste will require efficient catalytic degradation of waste polyolefin. Therefore, detailed knowledge of catalytic degradation mechanism is necessary. In this dissertation we systematically studied the behaviors and mechanism of catalytic degradation of polyolefin in a stirred batch reactor, including polyethylene (PE), polypropylene(PP), polystyrene(PS) and Polyvinyl chloride(PVC), which are main components in municipal waste plastics. It aims to improve their catalytic degradation ability and at the same time control potential harmful products produced during catalytic degradation. Furthermore, It expects to provide strong theoretical basis for recycling waste polyolefin.First, the catalytic degradation of single polyolefin was investigated. ForLDPE and PP, improved activities were observed using Zeolite (ZSM-5 and USY) as catalysts. The order of activities observed suggested that the main factor influencing the PE degradation was the acidity rather than the pore size of the catalysts. For PP, however, the main factor influencing degradation was the pore size of the catalysts.For polystyrene, it was concluded that different catalysts had different effects on its degradation behaviors. Solid bases (BaO) were more effective catalysts than solid acids (ZSM-5 and USY), which could be explained by their different catalytic degradation mechanism. When using acid catalysts, the degradation behavior was mainly affected by the acid properties. Besides, the pore structure also had certain effect on the degradation behavior. On the other hand, the degradation temperature was also important. With the increase of the temperature, the degradation rate, styrene yield, and liquid yield increased, and the residue yield decreased.Next, a new modified catalyst, DeLaZSM-5, was synthesized. This catalyst was characterized by SEM, XRD, NH3-TPD, ICP and SBET, it was observed that the structures and surface properties were similar with those of ZSM-5, but the pore volume and the number and strength of weak acid sites increased, and the strong acid sites decreased.Further study revealed that for LDPE, DeLaZSM-5 showed much higher catalytic activity than ZSM-5; For LDPE and PP, DeLaZSM-5 exhibited obvious shape-selective effect. The pore structure, acid properties of the catalyst and the reaction temperature are key to this shape-selective effect.To date, most waste streams contain a mixture of polymers, which are costly to segregate, therefore catalytic degradation of polyolefin mixtures were studied in this dissertation. For the mixtures without PVC, their catalytic degradation behaviors using different catalysts were compared. It was found that different polyolefin mixtures had different catalytic degradation behaviors. For LDPE/PP, degradation activities were improved when using catalysts, and the order of activities was: DeLaZSM-5 > ZSM-5 > USY > no catalyst. For LDPE/PS, different catalysts showed opposite catalytic effects: ZSM-5 and DeLaZSM-5 increased its degradationactivity, while USY decreased its degradation activity. The order of activities was: DeLaZSM-5 >ZSM-5 >no catalyst>USY. For PP/PS, different catalysts also had opposite catalytic effects. However, the order of activities was different from that of LDPE/PS: USY > no cataly...