The role of porosity and acidity in the catalytic upcycling of polyethylene

Plastics have become fundamental and widespread materials in our everyday life. Nowadays, the most demanded plastics are polyolefins, accounting for nearly half of the Global plastic production. However, the end-of-life disposal of these materials is an issue inevitably connected to their increased production. Among chemical recycling techniques, the most thoroughly investigated process is the catalytic pyrolysis, showing a vast panorama of different set-ups optimized for the major production of either gaseous or liquid compounds. In this contribution a series of mesoporous silica-based materials, either pure SiO2 or substituted with Zr and Al, were systematically synthetized varying both porous framework and surface acidity, in the attempt of rationalizing the effect of each parameter (deeply characterized by physico-chemical methods) on the catalytic pyrolysis of linear low-density polyethylene (LLDPE). The synthetized materials together with some zeolites and aluminas reference samples were tested towards LLDPE degradation in a batch reactor set-up optimized for the formation and accumulation of the gaseous com-pounds, coupled with IR spectroscopy for in situ monitoring the evolution of the most abundant species (mostly propylene and branched C4-C6 olefins, together with a minor fraction of ethylene and short hydrocarbons). This work points out the necessity of the presence of Bronsted acid sites for a good activity (in terms of both the yield of gaseous products and lowering of the process temperature), but it also reveals a non-negligible role of the Lewis acid sites and it warns about the importance of controlling not only the amount of acid sites but also their strength.