Heterogene transfer-dehydrogenatie voor katalytische upcycling van polyethyleen

Since the 1950s, plastic materials, and polyolefins in particular, have become ubiquitous in many sectors due to their low cost and versatility. To keep plastic use in modern society sustainable, proper waste management focused on recycling is essential. However, current recycling technologies for polyolefins suffer from major drawbacks. Mechanical recycling inevitably causes some degree of material deterioration, while pyrolysis is energy-intensive and produces a broad range of products that are difficult to separate. In this work, a method for chemical recycling of polyolefins, based on catalytic transfer dehydrogenation and olefin metathesis, is presented. In lieu of complex and expensive homogeneous transfer dehydrogenation catalysts, simple heterogeneous Pt catalysts were studied, allowing for facile synthesis and greater recoverability. In Chapter 2, the heterogeneous transfer dehydrogenation of alkanes was explored over Pt/C catalysts. Using tert-butyl ethylene and ethylene as hydrogen acceptors to provide a thermodynamic driving force, the desired alkenes were formed with appreciable turnover numbers, alongside alkyl aromatics as the only major byproducts. Pt-M/C catalysts, promoted with various metals, were also investigated, with notably improved performance for Pt-Ni and Pt-Sn catalysts. In Chapter 3, a more in-depth study of alkane dehydrogenation over Pt/C and PtSn/C using ethylene as a hydrogen acceptor was conducted. The Sn-promoted catalyst showed markedly improved deactivation resistance, alongside a significant decrease in olefin selectivity in favor of alkylaromatics. The effect of ethylene pressure, a critical parameter for the reaction, was thoroughly studied. Excess ethylene was found to inhibit the reaction, with olefin yields increasing with decreasing ethylene pressure. To minimize excess ethylene pressure, ethylene was fed slowly, approximately at the expected reaction rate, leading to significantly improved performance, especially for the unpromoted Pt/C, which appears highly sensitive to deactivation due to ethylene. Product olefins, in concentrations similar to the achieved yields, were also found to inhibit the reaction, and lead to deactivation even with minimal ethylene present. In Chapter 4, various support materials were tested to further improve the catalyst, with Pt/ZnO showing the best performance, having both excellent activity and olefin selectivity. In batch reactions, ethylene was found to reach an apparent kinetic steady state with ethane, likely indicating reversibility of the ethylene hydrogenation due to a more desirable balance in adsorption strength between alkanes and alkenes on the catalyst. To adjust for this, a continuous flow of ethylene was passed through the reactor, constantly removing the ethane formed, leading to a 50% increase in yield compared to simple batch reactions. The transfer dehydrogenation was then applied to various polyolefin feeds. To resolve issues arising from the high viscosity of these feeds, polyolefins were mixed with hexadecane. A degree of dehydrogenation of approximately 1 bond per 350 carbon atoms was achieved for a model polyethylene. Catalysts were found to be recoverable from these reactions and reusable after re-calcination at least 4 times without loss of activity. Next, the dehydrogenated polymers were subjected to olefin metathesis. Ethyl acrylate proved to be a suitable metathesis partner, yielding full conversion of all polyethylene feeds to ester-terminated telechelic macromonomers, however, metathesis of a dehydrogenated polypropylene feed was unsuccessful. Finally, hydrogenation of the remaining double bonds after metathesis was conducted in the absence of an additional hydrogenation catalyst, utilizing the hydrogenation activity of the residual Ru metathesis catalyst. Full hydrogenation of all ester-terminated macromonomers was achieved. The findings of this work show that it is feasible to upcycle waste polyethylene to telechelic macromonomers, with applications in specialty polymers and compatibilizers, promoting circularity and sustainability in the plastic industry.