An innovative organocatalyst for the chemical recycling of commodity polymers

Abstract

Over the past century, synthetic plastics have become ubiquitous in our daily life, occupying an ever-expanding range of uses. Their global production has exponentially increased in the past half-century, from 15 to 311 million tons between 1964 and 2014, and is expected to double again by 2035. Many of those materials have extremely short lifetimes and the direct consequence is the tremendous quantities of plastic wastes accumulating in the environment for years. Produce, buy, use and dispose, this linear way of consuming more and more plastics is nowadays raising concerns, not only from governments, interstate institutions and companies but also by citizens themselves. The treatment of plastic wastes is a global problem which requires innovative solutions to collect, sort, degrade, and re-process these materials. Thus, recycling is a crucial matter from an environmental point of view but also taking into account the plastic production and the tremendous income recycling could be for the global economy. Currently, most of the recycled plastics are by means of mechanical methods that involve grinding and re-processing of the material into lower value plastic products. The structural deteriorations lead to recycled product which does not share the same properties as the virgin polymer and also rapidly ends up as waste. Another approach relies on their direct conversion into high calorific value fuels through pyrolysis, but this thermal deterioration only postpones their unsustainable end-of-life since the resulting combustible will typically be burnt releasing mainly green-house gases such as CO2 and potentially affecting to the global warming. In comparison, chemical recycling involves the depolymerisation of polymers into monomers or oligomeric fragments that can then be subsequently polymerised to yield recycled materials, it represents an attractive long-term strategy to create a sustainable polymer supply chain. Recently, the chemical recycling of polymers has attracted a lot of attention among the scientific community, mainly driven by the current public awareness of the plastic pollution problem. However, as a consequence of the high stability of most polymers, depolymerisation processes are generally conducted in very harsh conditions and in the presence of catalysts, principally organometallics, which can present several drawbacks: possible presence of metal in the final product, low monomer yields or challenging purification procedures. Organocatalysts are promising “green” substitutes to classic organometallic catalysts. Although they are currently widely investigated for various polymerisation techniques, they have been much less explored in depolymerisation processes. One of the main reason behind this is that typically organic catalysts show poor thermal stability at temperatures that would be practical for recycling reactions. Thus, the partial or full degradation of the catalyst hinders the perspective of reusing it for several reactions and entails colouration of the final products, low conversion or undesirable side-reactions. In Chapter 1, an innovative series of acid and base mixtures have been explored as catalyst for depolymerisation reactions. Not only these acid-base mixtures displayed unique thermal stability, a tremendous advantage compared to most of organocatalysts which usually degrade at relatively low temperatures, but also reveals very good abilities for the depolymerisation of commodity polymers. Indeed, both poly(ethylene terepthalate) (PET) and Bisphenol A-based polycarbonate (BPA-PC) have been depolymerised using an equimolar mixture of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and methanesulfonic acid (MSA) as catalyst in a solvent-free procedure. The comparison with already reported procedures have demonstrated the superior control of the reaction employing the present organocatalyst. Chapter 2 has explored the influence of different parameters on the PET glycolysis catalysed by TBD:MSA (1:1). Using the adequate amount of reagent and catalyst, over 90% of Bis(hydroxyethyl)terephtalate (BHET) is obtained and easily recovered. Kinetics have emphasised the high selectivity of the reaction to form the desired monomer compared to well-known organocatalyst. Both the reagent and the catalyst can be easily recycled, demonstrating no loss of catalytic activity even after 6 cycles. Finally, it was demonstrated that this catalyst could even be used in the self-condensation of BHET to obtain recycled PET exhibiting good thermal and physical properties, closing the polymer to monomer to polymer loop. In a similar way, Chapter 3 has investigated the same procedure for the depolymerisation of BPA-PC into both Bisphenol A (BPA), its industrial monomer, and valuable building blocks. By wisely choosing the starting reagent and tuning the reaction conditions, 5- and 6-membered cyclic carbonates were obtained in reasonable to excellent yields (up to 97 %), constituting a phosgene-free, 100% atom economy procedure for the ringclosing of valuable carbonates widely reported for the synthesis of highperformance materials. Similarly, innovative linear carbonates and ureas were obtained. Density functional theory (DFT) methodology was employed for determining the mechanisms involved for both reactions – with PET and with BPA-PC. The obtained pathways exhibited similar chemical interactions but with a large energetic difference, inspiring the possibility for these two polymers to be recycled selectively. Thus, in Chapter 4, using different reagents and different reaction conditions investigated in the previous chapters, the simultaneous depolymerisation of BPA-PC and PET was explored using different reagents and in the presence of other plastics (i.e. polyolefins)