Activation entropy of degradation of cross-linked polymer in catalyzed and uncatalyzed slow pyrolysis

The motivation for this study was the development of technologically feasible method for processing waste cross-linked polyethylene (XLPE) by thermal decomposition using a ruthenium catalyst in the form of Ru/Al 2 O 3 . XLPE is widely used material which – unlike polyethylene – is very stable both chemically and mechanically. Therefore, its waste is difficult to process. A very promising way is slow pyrolysis catalyzed by Ru/Al 2 O 3 which allows the conversion of waste into hydrocarbons via degradation of rigid chemical structure of crosslinked material. High hydrocarbon yields (91–92 wt.%) were achieved in this way. It was shown that the catalyst promotes the decomposition of polymer structure by reducing the activation energy of decomposition, but the dependence of frequency factor on the reaction progress did not satisfactorily support this conclusion. In this work, the frequency factor was therefore transformed into the activation entropy based on the formal similarity of the Arrhenius and Eyring-Polányi equations for the rate constant. The obtained dependences of activation entropy changes on the reaction progress / temperature then provided a clear explanation of the effect of catalyst. In this way, it was found that in the initial phase of degradation, the activation entropy changes caused by the catalyst were significantly greater compared to the initial phase without catalyst. If changes in activation entropy without catalyst were −170–105 J K −1 mol −1 , with catalyst were significantly higher, −200–150 J K −1 mol −1 . This means that the catalyst significantly accelerated the onset of degradation. In the more advanced stages of polymer degradation, the activation entropy gradually increased to a maximum, meaning that the catalyst promoted radical formation and radical formation dominated recombination. As a result, the composition of the main product, hydrocarbons, changed significantly, as the content of low-molecular hydrocarbons C 6 –C 9 achieved with catalyst was up to 37–39 wt.%, while without catalyst it was zero. Conversely, the content of high-molecular hydrocarbons C 18 –C 35 decreased to up to 5 wt.%. Further, the hydrocarbons yield achieved with catalyst was 92 wt.%, but without the catalyst it was lower, 85 wt.%. Conversely, the solid residue yield was 10 wt.% without catalyst, and only 2 wt.% with catalyst (end temperature of 460 °C). These phenomena were explained based on the activation entropy approach. • Activation entropy of degradation of cross-linked polymer was determined. • The Arrhenius and Eyring-Polányi equations were used. • Using this approach, catalyzed and uncatalyzed slow pyrolysis were described and compared. • A clear explanation of the catalyst's effect was provided.