Engineered bio- and heterogeneous catalysts for improved polymer circularity

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The misuse of manmade plastics has resulted in an accumulation of non-degradable materials in the biosphere. It is now widely recognized that these plastics pose a serious global pollution threat, especially in marine ecosystems (Science 2010, 329, 1185-1188. Science 2015, 347, 768-771). Consequently, there is a pressing demand for new catalytic strategies to efficiently deconstruct these anthropogenic contaminants to minimize and reverse their environmental impact and to allow valuable raw materials to be recovered in an environmentally sustainable manner. In this studentship we will develop integrated and scalable catalytic processes for the efficient recycling of two abundant plastics, poly(ethylene terephthalate) (PET) and polystyrene, using a combination of heterogeneous catalysis, biological catalysis and chemical engineering bio- and batch reactor technology. PET is one of the most abundantly produced synthetic polymers (ca. 40 million tonnes produced globally in 2014) and is accumulating in the environment at an alarming rate. Microbes are beginning to adapt to the presence of PET in the environment by evolving catabolic pathways its deconstruction, and several 'PETase' enzymes have now been characterized that are able to degrade PET through hydrolysis of the polyester backbone (Science 2016, 351, 1196-1199.) Unfortunately PETases suffer from poor thermostability and low catalytic efficiency which preclude their use as biocatalysts for commercial PET degradation. This low efficiency likely reflects the recent emergence of PETases in response to anthropogenic contaminants, as their catalytic functions have not been honed through prolonged periods of Natural evolution. Over the past 12 months, our lab has developed automated high-throughput directed evolution workflows to engineer enzymes that are able to operate on insoluble polymeric materials. We have exploited these workflows to engineer a highly efficient and thermostable PETase, thus taking strides towards commercially viable biodegradation of plastic waste. Armed with this engineered biocatalyst, we are now poised to develop a scalable process for PET recycling which will be explored within this studentship. We will develop optimized plastic pre-treatment conditions, process operating conditions and protocols for downstream isolation of recycled monomeric building blocks. Further rounds of enzyme evolution will subsequently be performed to develop a bespoke biocatalyst specialised to operate under the optimal conditions for commercial PET recycling.Recent heterogeneous catalysis work in CEAS has demonstrated that a number of pure polyolefin feedstocks (polyethylene, polypropylene and polystyrene) and blends of these three can be successfully hydrocracked rapidly at much reduced temperatures yielding a predominantly C3 - C9 hydrocarbons (Ind. Eng. Chem. Res., 58 (45), 20601; EP Patent 2437886B1 -2019). We aim through careful optimisation of the heterogeneous catalysts (noble metal on zeolites and NiMo on alumina), adjustment of the operating parameters of time, temperature and H2 pressure using batch reactors (Figure 1), and, through kinetic modelling to achieve low temperature hydrocracking of polystyrene (PS) with over 90% selectivity to ethyl benzene. We will subsequently discover and engineer biocatalysts, including desaturases and/or oxygenases in combination with dehydratases, for the desaturation of ethyl benzene to produce styrene as a monomeric building block for further polymerizations, thus making a significant contribution to the development of a circular plastics economy.The project takes an innovative approach to merge the fields of biocatalysis, heterogeneous catalysis and chemical engineering and thus is perfectly aligned to the strategic priorities of the iCAT network.