Force-activated organocatalysts for plastic recycling

合成聚合物几乎应用于现代生活的方方面面。出于这个原因，它们也构成了一个重要的环境挑战。虽然有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的和/或如果单体的回收是有针对性的，这一点尤其正确。如果塑料中嵌入了一种添加剂，可以加速其在寿命结束时的降解，这一步骤可能会更容易实现。这一目标可以通过实现被称为机械团的机械反应分子来实现，这种分子在作用力的作用下发生反应，产生有用的化学实体。例如，可以使用力激活催化剂来促进解聚。在这个项目中，我们将基于不同的NHC-前体制备新的机械催化剂，并研究它们在溶液中、使用超声波和固体中的机械催化性能。最终，我们的目标是生产一种机械催化剂，能够提高具有挑战性的商品聚合物(如PET、PU)在回收/加工条件下的降解性/解聚性，如挤出或间歇水解/氨解。目前只有少数几种机械催化剂被报道，但它们都是金属基的，这使得它们价格昂贵，而且由于其较低的热稳定性和机械稳定性而难以加工。理想的机械催化剂只有在需要时才被激活(而不是在产品的生产或正常使用期间)。换句话说，它必须对pH、温度和湿度的变化表现出最小的敏感度，但对所需的力水平做出反应。我们最近推出了一种热稳定性和环境友好的替代方案，基于一种力激活的N-杂环卡宾(NHC)前体(NAT)。化学。2020，12,826；JACS 2021,143,3033)。CEAS最近的多相催化研究表明，许多纯聚烯烃原料(聚乙烯、聚丙烯和聚苯乙烯)及其混合物可以在较低的温度下成功地快速加氢裂化，生成主要的C3-C9烃(IND。英语。化学。决议，58(45)，20601；EP专利2437886B1-2019年)。对缩聚聚合物聚对苯二甲酸乙二醇酯(PET)的进一步研究表明，在220℃下，在90分钟内转化率为80%，选择性约为80%。这个项目将扩展目前的工作，目标是生产一种机械催化剂，能够增加具有挑战性的商品聚合物[例如，聚酯、聚氨酯(PUR)]在回收/加工条件下的解聚，例如挤出或间歇水解/加氢裂解。目标1：催化剂的设计和溶液中机械力化学和催化性能的优化目标2：探索固态/熔体中的机械力化学和催化性能目标3：研究间歇反应器中的各种活化模式(例如，湍流，目标4：优化PET和PU在水解/加氢裂解条件下的机械力化学和催化性能。英国每年有超过4.0MTE的塑料垃圾被送往垃圾填埋场或焚烧(8%是PET，类似数量的PUR，PlatticsEurope，Fact 2020)，塑料回收综合催化方法的开发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垃圾的流量产生重大影响(自然2016,540,363)。嵌入的分子机制可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在PET、PU或聚酰胺中实现自降解机制，以促进它们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.