Force-activated organocatalysts for plastic recycling

合成聚合物几乎应用于现代生活的各个方面。出于这个原因，它们也提出了一个重要的环境挑战。虽然一些聚合物可以很容易地回收（例如），但需要昂贵和/或具有挑战性的工艺。如果塑料是由混合聚合物组成和/或有目标的单体回收，则尤其如此。如果在塑料中加入一种添加剂，可以加速塑料在使用寿命结束时的降解，那么这一步就会变得更加容易。这一目标可以通过实施机械反应分子来实现，这种分子被称为机械团，在施加力的情况下发生反应，产生有用的化学实体。例如，力活化催化剂可用于促进解聚合。在这个项目中，我们将基于不同的nhc前体生产新的机械催化剂，并利用超声波和固态研究它们在溶液中的机械催化性能。最终，我们的目标是生产一种机械催化剂，能够在回收/加工条件下提高具有挑战性的商品聚合物（例如PET， PU）的可降解性/解聚性，例如：挤出或批量水解/氨解。只有少数机械催化剂被报道过，但它们都是金属基的，这使得它们昂贵且难以加工，因为它们的热稳定性和机械稳定性都很低。理想的机械催化剂只在需要时才被激活（而不是在产品的生产或正常使用过程中）。换句话说，它必须对pH值、温度和湿度的变化表现出最小的敏感性，但对所需的力水平做出反应。我们最近推出了一种热稳定且环保的替代品，基于力活化的n -杂环碳（NHC）前体（Nat. Chem. 2020, 12,826; JACS 2021, 143, 3033）。最近在CEAS中进行的多相催化工作表明，许多纯聚烯烃原料（聚乙烯、聚丙烯和聚苯乙烯）和这三者的共混物可以在较低的温度下成功地快速加氢裂化，产生以C3 - C9为主的碳氢化合物。化学。Res., 58 (45), 20601；EP专利2437886B1 -2019)。对缩合聚合物聚对苯二甲酸乙二醇酯（PET）的进一步研究表明，在220℃的条件下，在90分钟内，PET的转化率为80%，选择性约为80%。该项目将扩展目前的工作，目标是生产一种机械催化剂，能够在回收/加工条件下增加具有挑战性的商品聚合物（例如PET，聚氨酯（PUR））的解聚，例如挤出或批量水解/加氢裂化。目标1：溶液中催化剂的机械化学和催化性能的设计和优化目标2：固体/熔融状态下机械化学和催化性能的探索目标3：间歇式反应器中各种激活模式的研究（例如湍流，蒸汽诱导的拉长流）目标4:PET和PU水解/加氢裂化条件下机械化学和催化性能的优化。在英国，每年有超过40万吨的塑料垃圾被送往垃圾填埋场或焚烧（8%是PET和类似数量的PUR， plasticeurope, the Facts 2020），塑料回收的综合催化方法的发展将使我们能够为这一重要领域做出重大而持久的贡献。控制聚合物生命周期的能力将对塑料废物的通量产生巨大影响（Nature 2016, 540, 363）。嵌入的分子机制可以延长高性能聚合物的使用寿命。我们建议研究的机械催化剂可用于实现PET， PU或聚酰胺的自降解机制，以促进其在生命周期结束时的降解/解聚。

Synthetic polymers are used in almost every aspect of modern life. For this reason, they also present an important environmental challenge. Although some polymers can be easily recycled (e.g.), required costly and/or challenging processes. This is especially true if the plastic is composed of mixed polymers and/or if the recovery of the monomer is targeted. This step could be facilitated if plastic where embedded with an additive that could accelerate their degradation at the end of their life. This goal could be achieved by implementing mechanoresponsive molecules, called mechanophores, that react upon application of a force to generate a useful chemical entity. For example, a force-activated catalyst could be used to facilitate depolymerisation. In this project we will produce new mechanocatalysts based on varied NHC-precursors, and investigate their mechanocatalytic properties in solution, using ultrasound and in the solid state. Ultimately, we aim to produce a mechanocatalyst able to increase the degradability/depolymerisation of challenging commodity polymer (e.g. PET, PU) in recycling/processing conditions such as : extrusion or batch hydrolysis/aminolysis.Only a handful of mechanocatalysts have been reported but they are all metal-based, which make them expensive and difficult to process due to their low thermal and mechanical stability. The ideal mechanocatalyst is only activated when needed (and not for example during the production or the normal usage of the product). In other words, it must display a minimal sensitivity to a change in pH, temperature and humidity, but respond to a desired level of force. We have recently introduced a thermally-stable and environment-friendly alternative, based on a force-activated N-heterocyclic carbene (NHC) precursor (Nat. Chem. 2020, 12, 826; JACS 2021, 143, 3033). 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). Further work on the condensation polymer, polyethylene terephthalate (PET) using hydrolysis has led to conversions of 80% with around 80% selectivity to terephthalic acid at 220oC in 90 minutes. This project will extend this current work and will target the production of a mechanocatalyst able to increase the depolymerisation of challenging commodity polymer [e.g. PET, polyurethane (PUR)] in recycling/processing conditions such as, extrusion or batch hydrolysis/hydrocracking.Objective 1: Catalyst design and optimisation of the mechanochemical and catalytic properties in solution Objective 2: Exploration of the mechanochemical and catalytic properties in the solid state/meltObjective 3: Investigation of various modes of activation in batch reactor (e.g. turbulent flow, vapor-induced elongational flow)Objective 4: Optimisation of the mechanochemical and catalytic properties in hydrolysis/hydrocracking conditions for PET and PU.With over 4.0 Mte of plastic waste being sent to landfill or incinerated annually in the UK (8 % is PET and a similar amount of PUR, PlasticsEurope, The Facts 2020), the development of integrated catalytic approaches for plastic recycling would allow us to make a significant and lasting contribution to this important area. The ability to control the lifecycle of polymers would have a dramatic impact on the flux of plastic waste (Nature 2016, 540, 363). Embedded molecular mechanisms act to prolong the life of high-performance polymers. The mechanocatalysts we propose to investigate could be used to implement self-degrading mechanisms in PET, PU, or polyamides to facilitate their degradation/depolymerisation at the end of their lifecycle.