Collaborative Research: Biocatalytic Alcoholysis of PET in Nonaqueous Solvents for Polymer Recycling

随着塑料在环境中的积累成为当今最紧迫的社会挑战之一，开发改善塑料回收的方法变得越来越重要。塑料回收的一个主要挑战是开发具有成本效益的工艺，并生产具有与原始塑料相同或更好性能的聚合物。虽然使用生物催化剂进行聚合物回收和再循环已经引起了极大的兴趣，但研究一直局限于在水中发生的反应。由于水作为此类反应的介质的局限性，需要使用基于生物的方法来提高塑料废物的效率和转化率的新方法。这项工作的总体目标是探索使用天然酶在没有大量水的情况下解构常见的一次性塑料，如聚对苯二甲酸乙二醇酯（PET -通常用于服装纤维和液体/食品容器）。这项工作将特别利用酶在“干燥”环境中显着改善的特性，包括催化一系列在水介质中不可能的反应的能力。这项研究将研究可能以新的方式降解PET的酶，这些酶可以促进反应产物的再聚合，这对回收至关重要。这一努力将导致对影响酶将PET废物转化为有用的再生塑料构件的效率的反应参数的详细了解。这项研究将通过让不同的本科生和高中生参与这项研究计划来扩大对STEM研究的参与。参与者将通过各种计划招募，包括夏季多元文化访问研究培训（SMART）和在CU博尔德和迈阿密大学的STEM路线计划。此外，将开发有意义的教学模块，可以纳入K-12课程的材料和生物催化的生物降解。本提案的目的是开发一种新的生物催化方法，用于解构聚对苯二甲酸乙二醇酯（PET）塑料的基础上，在聚合物主链中的酯键的醇解脂肪酶。特别感兴趣的是PET醇解的动力学与解聚反应的热力学以及溶剂性质相关，这使得醇解的速率能够被微调。这种方法相对于PET水解的主要优点是使用非水介质可以促进聚合物的溶胀，从而增加聚合物链的塑性。这反过来将增加聚合物主链中酯键的可及性，因此可以显著提高生物催化转化的速率。此外，使用非水溶剂比使用水更容易分离反应产物，并且可以增强酶的热稳定性。这些优点潜在地将消除对高反应温度以及能量密集型预处理步骤的需要，例如当前PET回收方法中使用的熔融挤出和微研磨。这项工作将具体测试的假设，酯酶可以催化PET在非水介质中的醇解，并且醇解的速率可以通过改变反应的热力学平衡水活度以及溶剂的性质和选择醇作为亲核试剂来控制。这些研究将使用脂肪酶，包括角质酶，作为模型酯酶，因为它已经很好地建立了脂肪酶可以催化无水介质中的醇解反应，并结合到PET。此外，为了将PET醇解的动力学与反应的热力学相关联，将使用盐水合物来控制水活性。最后，作为这项工作的一部分，将制定合理的策略，以减少脂肪酶在非水溶剂中的扩散限制，包括与表面活性剂的离子配对。虽然这项工作将集中在PET回收，所提出的方法可能适用于回收其他合成聚酯。此外，该研究项目对解聚反应机理的基本理解也可能导致化学改性聚酯以提高其实用性和可回收性的新方法。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

With the accumulation of plastics in the environment being one of the most pressing societal challenges today, the development of methods to improve plastics recycling continues to grow in importance. A major challenge to plastics recycling is developing processes that are cost effective and yield polymers with properties that are identical to or better than those of the original plastic. While the use of biological catalysts for polymer recycling and upcycling has generated significant interest, research has been restricted to reactions that take place in water. Due to the limitations of water as a medium for such reactions, new approaches that improve the efficiency and conversion of plastic waste using biologically based methods are needed. The overall aim of this work is to explore the use of natural enzymes to deconstruct common single-use plastics such as polyethylene terephthalate (PET – commonly used in clothing fibers and liquid/food containers) in the absence of bulk water. This work will specifically leverage the dramatically improved properties of enzymes in “dry” environments, including the ability to catalyze a range of reactions that are not possible in aqueous media. This research will investigate enzymes that potentially can degrade PET in new ways that facilitate the repolymerization of the reaction products, which is critical for recycling. This effort will lead to a detailed understanding of the reaction parameters that impact enzyme efficiency in converting PET waste into useful, recycled plastics building blocks. This research will broaden participation in STEM research by engaging a diverse group of undergraduate and high school students in this research program. Participants will be recruited through a variety of programs, including the Summer Multicultural Access to Research Training (SMART) and STEM Routes programs at CU Boulder and Miami University. Moreover, meaningful teaching modules will be developed that can be incorporated into K-12 curricula on the biodegradation of materials and biocatalysis.The objective of this proposal is to develop a novel biocatalytic process for the deconstruction of polyethylene terephthalate (PET) plastics based on the alcoholysis of ester bonds in the polymer backbone by lipase. Of specific interest is correlating the kinetics of PET alcoholysis with the thermodynamics of the depolymerization reaction as well as the solvent properties, which enable the rate of alcoholysis to be fine-tuned. A major advantage of this approach over the hydrolysis of PET is that the use of nonaqueous media may promote swelling of the polymer, thereby increasing plasticity of the polymer chains. This in turn will increase the accessibility of the ester linkages in the polymer backbone and thus may significantly enhance the rate of biocatalytic conversion. Additionally, the use of nonaqueous solvents over water increases the ease of separation of the reaction products and can enhance the thermostability of the enzymes. These advantages potentially will eliminate the need for the high reaction temperatures as well as energy intensive pretreatment steps, such as melt extrusion and microgrinding, used in current PET recycling approaches. This work will specifically test the hypothesis that esterases can catalyze the alcoholysis of PET in nonaqueous media, and that the rate of alcoholysis can be controlled by varying the thermodynamic equilibrium water activity of the reaction as well as the solvent properties and choice of alcohol as the nucleophile. These studies will use lipase, including cutinase, as a model esterase since it is already well established that lipases can catalyze alcoholysis reactions in anhydrous media and bind to PET. Additionally, to correlate the kinetics of PET alcoholysis with the thermodynamics of the reaction, water activity will be controlled using salt hydrates. Finally, as part of this effort, rational strategies will be developed to reduce diffusional limitations of lipase in nonaqueous solvents, including ion pairing with surfactants. While this effort will focus on PET recycling, the proposed approach may be applicable in recycling of other synthetic polyesters. Moreover, the fundamental understanding of depolymerization reaction mechanisms developed by this this research program also may lead to new ways to chemically modify polyesters to improve their utility and recyclability.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.