sviluppo di materiali mesoporosi a base silice per la degradazione catalitica del polietilene

Polyolefins (POs) are hardly replaceable materials that revolutionized everyday life, combining unrivalled functionalities with low cost, low weight and excellent barrier properties. However, POs generate significant externalities along their whole life-cycle. In particular, their production still relies on the use of fossil resources and is responsible for the emission of greenhouse gases. Moreover, at the end of their lifetime only about 30% of PO wastes are recycled, while the remaining are often mismanaged causing the well-known degradation of natural systems. Given the increasing amount of POs waste generated, new and more efficient methods to manage their end-of-life are needed. Today, the vast majority of PO wastes are mechanically recycled, reprocessed into other plastic goods by thermal remoulding. However, mechanical recycling of plastics cannot be considered as a circular process, since the mechanically recycled plastics progressively lose their quality. Hence, recycled POs are deemed to exit the recycling circle when their properties degrade below a sufficient technical grade. The most usual exit is through incineration with energy recovery, taking advantage of the high calorific power of the polyolefins. In this case, the energy value of plastics is used to produce electricity and heat. However, incineration implies the loss of material, the emission of greenhouse gases and pollutants. For this reason, recently different types of recycling methods have gained high attention. Among them, catalytic recycling is a wide investigated process because it is easy to apply and it has a high potential in converting PO waste into monomers, fuels or chemicals. This process is favoured with respect to mechanical recycling because it allows to produce recycled materials with the same properties and applications as the initial ones. For polyolefins, however, it is almost impossible to recover the initial monomers. The goal then is to produce something that can be reinserted in the market, such as transportation fuels. To do this the properties of the catalysts need to be carefully selected to tune the product distribution and quantity.One of the most promising approaches is catalytic pyrolysis. This process converts long-chain polymers into small molecules (gases, liquids and aromatics) in the presence of a solid acid catalyst (either Lewis or Brønsted) at high temperature (above 300 °C) and proceeds through a carbocationic mechanism. A large number of silica-alumina-based catalysts, as well as microporous and mesoporous aluminosilicates have been investigated. Acidity and porosity are the main drivers for activity and selectivity, although also the reactor design, the pressure, the temperature, the heating rate, and the catalyst loading play a role, in such a way that it is often difficult to compare results. The production of liquid hydrocarbon fuels from POs waste is technically feasible and has been already implemented at an industrial level. However, elevated temperatures make the pyrolysis process economically unsound and raise the challenge of selectivity control and catalyst fast deactivation. This thesis is inserted in this context. In particular, we have investigated the effect of different types of metal-substituted ordered mesoporous silicas with acidic properties on the catalytic pyrolysis of polyethylene, trying to correlate the different activity and selectivity with the properties of the materials.