化解生物可降解聚酯废料,实现高效、可持续的增值产品循环利用

IF 7.4 Q1 ENGINEERING, ENVIRONMENTAL
Xin Gao, Huayi Shen, Chun-Ran Chang
{"title":"化解生物可降解聚酯废料,实现高效、可持续的增值产品循环利用","authors":"Xin Gao, Huayi Shen, Chun-Ran Chang","doi":"10.1021/acsestengg.4c00376","DOIUrl":null,"url":null,"abstract":"It is well-known that conventional disposable plastics, such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polystyrene (PS), are causing “white pollution” and becoming one of the greatest challenges to the natural environment worldwide. To overcome severe environmental pollution, policy makers have introduced a series of regulations to reduce and replace the utilization of conventional nonbiodegradable plastics, for instance, guiding plastic manufactories to produce biodegradable (or compostable) plastics instead of conventional nonbiodegradable plastics, banning markets from using or selling conventional nonbiodegradable plastics, and calling on citizens to use and even reuse biodegradable plastics for various applications (including shopping bags and boxes, catering materials, agricultural mulching film, medical devices, etc.). By far, the most common state-of-the-art biodegradable polyester plastics in markets are polylactic acid (PLA), polyhydroxyalkanoates (PHA), polybutylene adipate coterephthalate (PBAT), polyglycolic acid (PGA), polycaprolactone (PCL), polybutylene succinate (PBS), and polypropylene carbonate (PPC) (Figure 1). It is noteworthy that these novel polyesters usually contain sizable ester groups. On the basis of the different physical properties (e.g., melting point, stretchability, percentage of elongation, gas resistance, etc.) and chemical properties (e.g., molecular structures, molecular weights, oxygen and carbon contents, etc.) of these novel polyesters, their functionalities could be adopted in a wide range of industrial and consumer sectors. Figure 1. State-of-the-art emerging polyester plastics and their chemical structures, biological degradation process, and chemolytic valorizations. Theoretically, there should be no further concerns about the end life of biodegradable plastic waste because these types of polyesters are expected to be biologically and completely decomposed quickly into small molecules (e.g., water, carbon dioxide, and methane). However, practically, the realistic situation is that such biological degradation (biodegradation) of biodegradable plastics is strictly conditional, where suitable biodegradation factors must be reached, including temperature, humidity, quality and quantity of microorganisms, large-scale industrial or homemade composting plant, intrinsic degradation properties, etc. In other words, biological decomposition is not spontaneous. Therefore, the utilization of biodegradable plastics cannot guarantee that the plastic pollution issue can be readily and automatically resolved. Considering the fast-growing momentum of biodegradable polyester plastic utilization and the subsequent rapid increase in the amount of biodegradable plastic waste, the current implementation status of the treatment facilities for biodegradable plastics (i.e., industrial composting plant) still lags behind the growth in the use of biodegradable plastics, and therefore, the facilities do not possess effective capabilities and capacities to confront the rapid increase in the amount of biodegradable plastic waste. Thus, although conducting biological decomposition in a composting environment is supposed to be efficient, we are still not ready to completely rely on composting plants for biodegradable plastic waste treatment now or in the short to middle term. If there are no back-up actions to implement, the accumulated biodegradable plastic waste will continue to make “white pollution” in a manner similar to that seen for conventional nonbiodegradable plastic waste for the foreseeable future. It will also lead to the substantial loss of carbon resources. Moreover, another significant issue is the deficiency of the circular economy (large value differences of biodegradable plastics before and after use). The emerging biodegradable polyester plastics are still much more expensive than conventional nonbiodegradable plastics; however, the used biodegradable plastics are directly treated as plastic waste without demonstrating additional application value. In other words, there is a substantial economic gap for biodegradable plastics before and after utilization. Hence, the circular economy of such emerging biodegradable polyester plastics is quite poor. For this purpose, upgrading the polyester plastic waste to value-added products would be a strategic and promising choice. Altogether, especially in the short to middle term, rethinking the appropriate treatment of biodegradable plastics to avoid the vast accumulation of biodegradable plastic waste and loss of carbon resources, and to improve the circular economy, is an imperative and urgent task. Chemical upcycling of biodegradable plastic waste is an effective and sustainable approach for better resolving the speedy growth of the amount of biodegradable plastic waste with multiple key benefits, including environmental remediation, circular carbon, and circular economy. Essentially, because of the abundant storage of fragile ester groups in biodegradable plastics, chemolysis could be a promising upcycling strategy for depolymerizing emerging biodegradable polyester plastics into a wide range of value-added platform chemicals under mild conditions (e.g., low temperature) compared with the tough cracking of conventional nondegradable plastics that contain massive tough C–C and C–H bonds. In detail, there are different methods of chemolysis available for upgrading biodegradable polyester plastics, primarily involving hydrolysis, hydrogenolysis, ammonolysis, aminolysis, and alcoholysis (or transesterification) (Figure 1). Correspondingly, water, hydrogen, ammonia, amines, and alcohols (e.g., methanol, ethanol, and ethylene glycol) are needed to break the chemical bonds between carbon and oxygen in polyesters to produce different types of molecules, such as alcohols, acids, amines, amino acids, esters, etc. (1) In hydrolysis, biodegradable polyester plastics could be directly converted back to their corresponding monomeric precursors, for instance, (non)catalytic hydrolysis of PLA and PHB (polyhydroxybutyrate, a member of the PHA family) to LA (lactic acid) and 3-HBA (3-hydroxybutyric acid), respectively. The benefit of the hydrolytic process is that it is accessible to repolymerize the monomeric precursors back to biodegradable polyesters. It is noteworthy that the pH of the hydrolytic solution (e.g., acidic, basic, and neutral), the reacting atmosphere (e.g., inert gas or hydrogen), the temperature, and the properties of the catalysts are the key factors for optimizing the reaction efficiency. In hydrogenolysis, borrowing molecular hydrogen to first break the chemical bonds in biodegradable polyesters to form intermediate monomers and further to hydrogenate the corresponding monomers to value-added chemical products, such as alcohols, acids, and esters, is typical. (2) For instance, molecular Ru-catalyzed hydrogenolysis of PLA-based consumer products (e.g., granulate and beverage cup) and PCL afforded high yields of 1,2-propanediol and 1,6-hexanediol, respectively. (3) Alternatively, catalytic hydrogenolysis of PLA powders and straws in a solvent-free system could produce liquid blends that comprise alcohols and esters for use in biofuels. (4) In addition, hydrogenolysis of PGA and PBS might correspondingly produce ethylene glycol and 1,4-butanediol. (2) In ammonolysis and aminolysis, ammonia and amines, respectively, are generally employed to depolymerize biodegradable plastic molecules to synthesize various nitrogen-related monomers, including amino acids and amines. For instance, alanine could be produced from the ammonolysis of PLA in a NH<sub>3</sub>·H<sub>2</sub>O solution over solid Ru catalysts. (5) In alcoholysis (or transesterification), nucleophilic solvents (e.g., methanol, ethanol, and ethylene glycol) can facilely depolymerize the polyester waste. Methanol is the most frequently used alcohol for the alcoholysis of polyesters; this reaction is also commonly known as methanolysis. For instance, methanolysis of PLA-based consumer goods (e.g., PLA-made cups, toys, and three-dimensional printing materials) is conducted to synthesize methyl lactate on zinc-based catalysts, where the methanolysis mechanism involves dual steps: the step from PLA to the oligomer and the subsequent step from the oligomer to the methyl lactate monomer. (6,7) Methanolysis of PHB could produce methyl 3-hydroxybutyrate over acid catalysts (e.g., H<sub>2</sub>SO<sub>4</sub> and TsOH). (1) Catalytic ethanolysis of PLA forms ethyl lactate. (8) Catalytic alcoholysis of PLA with diols, such as ethylene glycol, 1,3-propanediol, and 1,4-butanediol, could synthesize various ester monomers. (9) In addition to being solvents in alcoholysis reactions, alcohols could be hydrogen donors for transfer hydrogenation due to their protolytic characteristics. For example, the product distribution of the methanol-mediated PLA methanolysis–hydrodeoxygenation process varies among methyl lactate, methyl propionate, and 1-propanol. Such a reaction system for PGA and PCL substrates can selectively produce methyl acetate and methyl hexanoate, respectively. (10) Despite a large number of research reports regarding chemical upcycling of plastic waste in recent years, most of the efforts mainly focus on the conversion of conventional nonbiodegradable plastic waste. In this respect, chemolysis of emerging biodegradable polyester plastics remains underdeveloped, especially in this fast-growing era of emerging polyester plastic utilization. Thus, more substantial research efforts on the chemolysis of emerging biodegradable polyester plastic waste are greatly desirable. In particular, rather than the individual polyester compound (e.g., PLA and PBAT), chemolysis of complex biodegradable plastic waste [e.g., PLA/PBAT and polylactic-<i>co</i>-glycolic acid (PLGA)] is more applicable to real-world polyester waste treatment. In addition, to purposefully extend the product value chain of emerging biodegradable polyester plastic waste, a cascade reaction is one important strategy for addressing this expectation, for instance, alcoholysis with hydrogenation and hydrolysis with hydrogenation. The design and preparation of efficient catalysts (both homogeneous and heterogeneous) are highly beneficial for decreasing the chemolysis reaction barriers, tuning the product selectivity, and making reaction conditions milder (e.g., low temperature and low pressure) with less energy input. Considering the practices of the chemical industry with respect to product separation, catalyst reuse, and process cost, heterogeneous catalysts would be more feasible for industrial scale-up. Moreover, the economic analysis and process optimization of chemolysis demand more detailed investigations. Chun-Ran Chang is professor of chemical engineering at Xi’an Jiaotong University, Xi’an, China. He obtained his Ph.D. in chemistry from Tsinghua University, Beijing, China. His research areas mainly focus on heterogeneous catalysis, computational catalysis, and physical chemistry for a wide range of applications in energy conversion and environmental protection. His h-index (Google Scholar) is 37 with more than 7500 total citations. This work is supported by the National Key R&amp;D Program of China (2023YFA1506302), the Science and Technology Plan Fund of Yulin City (CXY-2022-141), the Qinchuangyuan “Scientists + Engineers” Team Construction Program of Shaanxi Province (Grant 2023KXJ-276), and the research program of Shaanxi Beiyuan Chemical Industry Group Co., Ltd. (Grant 2023413611014). X.G. acknowledges the financial support from the Foundation of State Key Laboratory of Clean and Efficient Coal Utilization, Taiyuan University of Technology, China (MINYSKL202309), the Natural Science Basic Research Program of Shaanxi Province at China (2024JC-YBQN-0071), and the Postdoctoral Research Project Fund of Shaanxi Province, China (2023BSHEDZZ22). This article references 10 other publications. This article has not yet been cited by other publications.","PeriodicalId":7008,"journal":{"name":"ACS ES&T engineering","volume":null,"pages":null},"PeriodicalIF":7.4000,"publicationDate":"2024-09-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Chemolysis for Efficient and Sustainable Upcycling of Biodegradable Polyester Waste to Value-Added Products\",\"authors\":\"Xin Gao, Huayi Shen, Chun-Ran Chang\",\"doi\":\"10.1021/acsestengg.4c00376\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"It is well-known that conventional disposable plastics, such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polystyrene (PS), are causing “white pollution” and becoming one of the greatest challenges to the natural environment worldwide. To overcome severe environmental pollution, policy makers have introduced a series of regulations to reduce and replace the utilization of conventional nonbiodegradable plastics, for instance, guiding plastic manufactories to produce biodegradable (or compostable) plastics instead of conventional nonbiodegradable plastics, banning markets from using or selling conventional nonbiodegradable plastics, and calling on citizens to use and even reuse biodegradable plastics for various applications (including shopping bags and boxes, catering materials, agricultural mulching film, medical devices, etc.). By far, the most common state-of-the-art biodegradable polyester plastics in markets are polylactic acid (PLA), polyhydroxyalkanoates (PHA), polybutylene adipate coterephthalate (PBAT), polyglycolic acid (PGA), polycaprolactone (PCL), polybutylene succinate (PBS), and polypropylene carbonate (PPC) (Figure 1). It is noteworthy that these novel polyesters usually contain sizable ester groups. On the basis of the different physical properties (e.g., melting point, stretchability, percentage of elongation, gas resistance, etc.) and chemical properties (e.g., molecular structures, molecular weights, oxygen and carbon contents, etc.) of these novel polyesters, their functionalities could be adopted in a wide range of industrial and consumer sectors. Figure 1. State-of-the-art emerging polyester plastics and their chemical structures, biological degradation process, and chemolytic valorizations. Theoretically, there should be no further concerns about the end life of biodegradable plastic waste because these types of polyesters are expected to be biologically and completely decomposed quickly into small molecules (e.g., water, carbon dioxide, and methane). However, practically, the realistic situation is that such biological degradation (biodegradation) of biodegradable plastics is strictly conditional, where suitable biodegradation factors must be reached, including temperature, humidity, quality and quantity of microorganisms, large-scale industrial or homemade composting plant, intrinsic degradation properties, etc. In other words, biological decomposition is not spontaneous. Therefore, the utilization of biodegradable plastics cannot guarantee that the plastic pollution issue can be readily and automatically resolved. Considering the fast-growing momentum of biodegradable polyester plastic utilization and the subsequent rapid increase in the amount of biodegradable plastic waste, the current implementation status of the treatment facilities for biodegradable plastics (i.e., industrial composting plant) still lags behind the growth in the use of biodegradable plastics, and therefore, the facilities do not possess effective capabilities and capacities to confront the rapid increase in the amount of biodegradable plastic waste. Thus, although conducting biological decomposition in a composting environment is supposed to be efficient, we are still not ready to completely rely on composting plants for biodegradable plastic waste treatment now or in the short to middle term. If there are no back-up actions to implement, the accumulated biodegradable plastic waste will continue to make “white pollution” in a manner similar to that seen for conventional nonbiodegradable plastic waste for the foreseeable future. It will also lead to the substantial loss of carbon resources. Moreover, another significant issue is the deficiency of the circular economy (large value differences of biodegradable plastics before and after use). The emerging biodegradable polyester plastics are still much more expensive than conventional nonbiodegradable plastics; however, the used biodegradable plastics are directly treated as plastic waste without demonstrating additional application value. In other words, there is a substantial economic gap for biodegradable plastics before and after utilization. Hence, the circular economy of such emerging biodegradable polyester plastics is quite poor. For this purpose, upgrading the polyester plastic waste to value-added products would be a strategic and promising choice. Altogether, especially in the short to middle term, rethinking the appropriate treatment of biodegradable plastics to avoid the vast accumulation of biodegradable plastic waste and loss of carbon resources, and to improve the circular economy, is an imperative and urgent task. Chemical upcycling of biodegradable plastic waste is an effective and sustainable approach for better resolving the speedy growth of the amount of biodegradable plastic waste with multiple key benefits, including environmental remediation, circular carbon, and circular economy. Essentially, because of the abundant storage of fragile ester groups in biodegradable plastics, chemolysis could be a promising upcycling strategy for depolymerizing emerging biodegradable polyester plastics into a wide range of value-added platform chemicals under mild conditions (e.g., low temperature) compared with the tough cracking of conventional nondegradable plastics that contain massive tough C–C and C–H bonds. In detail, there are different methods of chemolysis available for upgrading biodegradable polyester plastics, primarily involving hydrolysis, hydrogenolysis, ammonolysis, aminolysis, and alcoholysis (or transesterification) (Figure 1). Correspondingly, water, hydrogen, ammonia, amines, and alcohols (e.g., methanol, ethanol, and ethylene glycol) are needed to break the chemical bonds between carbon and oxygen in polyesters to produce different types of molecules, such as alcohols, acids, amines, amino acids, esters, etc. (1) In hydrolysis, biodegradable polyester plastics could be directly converted back to their corresponding monomeric precursors, for instance, (non)catalytic hydrolysis of PLA and PHB (polyhydroxybutyrate, a member of the PHA family) to LA (lactic acid) and 3-HBA (3-hydroxybutyric acid), respectively. The benefit of the hydrolytic process is that it is accessible to repolymerize the monomeric precursors back to biodegradable polyesters. It is noteworthy that the pH of the hydrolytic solution (e.g., acidic, basic, and neutral), the reacting atmosphere (e.g., inert gas or hydrogen), the temperature, and the properties of the catalysts are the key factors for optimizing the reaction efficiency. In hydrogenolysis, borrowing molecular hydrogen to first break the chemical bonds in biodegradable polyesters to form intermediate monomers and further to hydrogenate the corresponding monomers to value-added chemical products, such as alcohols, acids, and esters, is typical. (2) For instance, molecular Ru-catalyzed hydrogenolysis of PLA-based consumer products (e.g., granulate and beverage cup) and PCL afforded high yields of 1,2-propanediol and 1,6-hexanediol, respectively. (3) Alternatively, catalytic hydrogenolysis of PLA powders and straws in a solvent-free system could produce liquid blends that comprise alcohols and esters for use in biofuels. (4) In addition, hydrogenolysis of PGA and PBS might correspondingly produce ethylene glycol and 1,4-butanediol. (2) In ammonolysis and aminolysis, ammonia and amines, respectively, are generally employed to depolymerize biodegradable plastic molecules to synthesize various nitrogen-related monomers, including amino acids and amines. For instance, alanine could be produced from the ammonolysis of PLA in a NH<sub>3</sub>·H<sub>2</sub>O solution over solid Ru catalysts. (5) In alcoholysis (or transesterification), nucleophilic solvents (e.g., methanol, ethanol, and ethylene glycol) can facilely depolymerize the polyester waste. Methanol is the most frequently used alcohol for the alcoholysis of polyesters; this reaction is also commonly known as methanolysis. For instance, methanolysis of PLA-based consumer goods (e.g., PLA-made cups, toys, and three-dimensional printing materials) is conducted to synthesize methyl lactate on zinc-based catalysts, where the methanolysis mechanism involves dual steps: the step from PLA to the oligomer and the subsequent step from the oligomer to the methyl lactate monomer. (6,7) Methanolysis of PHB could produce methyl 3-hydroxybutyrate over acid catalysts (e.g., H<sub>2</sub>SO<sub>4</sub> and TsOH). (1) Catalytic ethanolysis of PLA forms ethyl lactate. (8) Catalytic alcoholysis of PLA with diols, such as ethylene glycol, 1,3-propanediol, and 1,4-butanediol, could synthesize various ester monomers. (9) In addition to being solvents in alcoholysis reactions, alcohols could be hydrogen donors for transfer hydrogenation due to their protolytic characteristics. For example, the product distribution of the methanol-mediated PLA methanolysis–hydrodeoxygenation process varies among methyl lactate, methyl propionate, and 1-propanol. Such a reaction system for PGA and PCL substrates can selectively produce methyl acetate and methyl hexanoate, respectively. (10) Despite a large number of research reports regarding chemical upcycling of plastic waste in recent years, most of the efforts mainly focus on the conversion of conventional nonbiodegradable plastic waste. In this respect, chemolysis of emerging biodegradable polyester plastics remains underdeveloped, especially in this fast-growing era of emerging polyester plastic utilization. Thus, more substantial research efforts on the chemolysis of emerging biodegradable polyester plastic waste are greatly desirable. In particular, rather than the individual polyester compound (e.g., PLA and PBAT), chemolysis of complex biodegradable plastic waste [e.g., PLA/PBAT and polylactic-<i>co</i>-glycolic acid (PLGA)] is more applicable to real-world polyester waste treatment. In addition, to purposefully extend the product value chain of emerging biodegradable polyester plastic waste, a cascade reaction is one important strategy for addressing this expectation, for instance, alcoholysis with hydrogenation and hydrolysis with hydrogenation. The design and preparation of efficient catalysts (both homogeneous and heterogeneous) are highly beneficial for decreasing the chemolysis reaction barriers, tuning the product selectivity, and making reaction conditions milder (e.g., low temperature and low pressure) with less energy input. Considering the practices of the chemical industry with respect to product separation, catalyst reuse, and process cost, heterogeneous catalysts would be more feasible for industrial scale-up. Moreover, the economic analysis and process optimization of chemolysis demand more detailed investigations. Chun-Ran Chang is professor of chemical engineering at Xi’an Jiaotong University, Xi’an, China. He obtained his Ph.D. in chemistry from Tsinghua University, Beijing, China. His research areas mainly focus on heterogeneous catalysis, computational catalysis, and physical chemistry for a wide range of applications in energy conversion and environmental protection. His h-index (Google Scholar) is 37 with more than 7500 total citations. This work is supported by the National Key R&amp;D Program of China (2023YFA1506302), the Science and Technology Plan Fund of Yulin City (CXY-2022-141), the Qinchuangyuan “Scientists + Engineers” Team Construction Program of Shaanxi Province (Grant 2023KXJ-276), and the research program of Shaanxi Beiyuan Chemical Industry Group Co., Ltd. (Grant 2023413611014). X.G. acknowledges the financial support from the Foundation of State Key Laboratory of Clean and Efficient Coal Utilization, Taiyuan University of Technology, China (MINYSKL202309), the Natural Science Basic Research Program of Shaanxi Province at China (2024JC-YBQN-0071), and the Postdoctoral Research Project Fund of Shaanxi Province, China (2023BSHEDZZ22). This article references 10 other publications. 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引用次数: 0

摘要

众所周知,聚乙烯(PE)、聚丙烯(PP)、聚对苯二甲酸乙二酯(PET)、聚氯乙烯(PVC)和聚苯乙烯(PS)等传统一次性塑料正在造成 "白色污染",成为全球自然环境面临的最大挑战之一。为了解决严重的环境污染问题,决策者出台了一系列法规来减少和替代传统不可降解塑料的使用,如引导塑料生产厂家生产可生物降解(或可堆肥)塑料来替代传统不可降解塑料,禁止市场使用或销售传统不可降解塑料,呼吁市民在各种应用领域(包括购物袋和包装盒、餐饮材料、农用地膜、医疗器械等)使用甚至重复使用可生物降解塑料。到目前为止,市场上最常见的最先进的生物降解聚酯塑料是聚乳酸(PLA)、聚羟基烷酸酯(PHA)、聚己二酸丁二醇酯对苯二甲酸酯(PBAT)、聚乙二醇酸(PGA)、聚己内酯(PCL)、聚丁二酸丁二醇酯(PBS)和聚碳酸丙烯酯(PPC)(图 1)。值得注意的是,这些新型聚酯通常含有大量酯基。根据这些新型聚酯的不同物理性质(如熔点、拉伸性、伸长率、抗气性等)和化学性质(如分子结构、分子量、氧和碳含量等),它们的功能可广泛应用于工业和消费领域。图 1.最先进的新兴聚酯塑料及其化学结构、生物降解过程和化合价。从理论上讲,生物可降解塑料废弃物的最终使用期限应该不会再有问题,因为这些类型的聚酯预计会很快被生物完全分解成小分子(如水、二氧化碳和甲烷)。但实际上,可降解塑料的这种生物降解(生物降解)是有严格条件的,必须达到合适的生物降解因素,包括温度、湿度、微生物的质量和数量、大型工业或自制堆肥厂、内在降解特性等。换句话说,生物分解不是自发的。因此,利用可生物降解塑料并不能保证塑料污染问题能够立即自动解决。考虑到生物降解聚酯塑料利用的快速增长势头以及随之而来的生物降解塑料垃圾数量的快速增长,生物降解塑料处理设施(即工业堆肥厂)的实施现状仍然落后于生物降解塑料使用的增长,因此,这些设施不具备有效的能力和容量来应对生物降解塑料垃圾数量的快速增长。因此,尽管在堆肥环境中进行生物分解应该是有效的,但在目前或中短期内,我们还不能完全依赖堆肥厂来处理可降解塑料废物。如果没有后备措施,在可预见的未来,累积的可降解塑料垃圾将继续造成 "白色污染",其方式与传统的不可降解塑料垃圾类似。这也将导致碳资源的大量流失。此外,另一个重要问题是循环经济的不足(生物降解塑料在使用前后的价值差异很大)。新兴的可生物降解塑料比传统的不可生物降解塑料仍然昂贵得多;然而,使用过的可生物降解塑料被直接作为塑料垃圾处理,没有显示出额外的应用价值。换句话说,生物降解塑料在使用前后存在巨大的经济差距。因此,这种新兴的生物可降解塑料的循环经济性很差。为此,将聚酯塑料废料升级为高附加值产品将是一个具有战略意义和前景的选择。总之,特别是在中短期内,重新思考如何妥善处理可降解塑料,避免可降解塑料废弃物的大量积累和碳资源的流失,提高循环经济水平,是当务之急。对可降解塑料废弃物进行化学升级再循环处理是一种有效的、可持续的方法,能更好地解决可降解塑料废弃物数量快速增长的问题,具有环境修复、循环碳和循环经济等多重关键效益。
本文章由计算机程序翻译,如有差异,请以英文原文为准。

Chemolysis for Efficient and Sustainable Upcycling of Biodegradable Polyester Waste to Value-Added Products

Chemolysis for Efficient and Sustainable Upcycling of Biodegradable Polyester Waste to Value-Added Products
It is well-known that conventional disposable plastics, such as polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polystyrene (PS), are causing “white pollution” and becoming one of the greatest challenges to the natural environment worldwide. To overcome severe environmental pollution, policy makers have introduced a series of regulations to reduce and replace the utilization of conventional nonbiodegradable plastics, for instance, guiding plastic manufactories to produce biodegradable (or compostable) plastics instead of conventional nonbiodegradable plastics, banning markets from using or selling conventional nonbiodegradable plastics, and calling on citizens to use and even reuse biodegradable plastics for various applications (including shopping bags and boxes, catering materials, agricultural mulching film, medical devices, etc.). By far, the most common state-of-the-art biodegradable polyester plastics in markets are polylactic acid (PLA), polyhydroxyalkanoates (PHA), polybutylene adipate coterephthalate (PBAT), polyglycolic acid (PGA), polycaprolactone (PCL), polybutylene succinate (PBS), and polypropylene carbonate (PPC) (Figure 1). It is noteworthy that these novel polyesters usually contain sizable ester groups. On the basis of the different physical properties (e.g., melting point, stretchability, percentage of elongation, gas resistance, etc.) and chemical properties (e.g., molecular structures, molecular weights, oxygen and carbon contents, etc.) of these novel polyesters, their functionalities could be adopted in a wide range of industrial and consumer sectors. Figure 1. State-of-the-art emerging polyester plastics and their chemical structures, biological degradation process, and chemolytic valorizations. Theoretically, there should be no further concerns about the end life of biodegradable plastic waste because these types of polyesters are expected to be biologically and completely decomposed quickly into small molecules (e.g., water, carbon dioxide, and methane). However, practically, the realistic situation is that such biological degradation (biodegradation) of biodegradable plastics is strictly conditional, where suitable biodegradation factors must be reached, including temperature, humidity, quality and quantity of microorganisms, large-scale industrial or homemade composting plant, intrinsic degradation properties, etc. In other words, biological decomposition is not spontaneous. Therefore, the utilization of biodegradable plastics cannot guarantee that the plastic pollution issue can be readily and automatically resolved. Considering the fast-growing momentum of biodegradable polyester plastic utilization and the subsequent rapid increase in the amount of biodegradable plastic waste, the current implementation status of the treatment facilities for biodegradable plastics (i.e., industrial composting plant) still lags behind the growth in the use of biodegradable plastics, and therefore, the facilities do not possess effective capabilities and capacities to confront the rapid increase in the amount of biodegradable plastic waste. Thus, although conducting biological decomposition in a composting environment is supposed to be efficient, we are still not ready to completely rely on composting plants for biodegradable plastic waste treatment now or in the short to middle term. If there are no back-up actions to implement, the accumulated biodegradable plastic waste will continue to make “white pollution” in a manner similar to that seen for conventional nonbiodegradable plastic waste for the foreseeable future. It will also lead to the substantial loss of carbon resources. Moreover, another significant issue is the deficiency of the circular economy (large value differences of biodegradable plastics before and after use). The emerging biodegradable polyester plastics are still much more expensive than conventional nonbiodegradable plastics; however, the used biodegradable plastics are directly treated as plastic waste without demonstrating additional application value. In other words, there is a substantial economic gap for biodegradable plastics before and after utilization. Hence, the circular economy of such emerging biodegradable polyester plastics is quite poor. For this purpose, upgrading the polyester plastic waste to value-added products would be a strategic and promising choice. Altogether, especially in the short to middle term, rethinking the appropriate treatment of biodegradable plastics to avoid the vast accumulation of biodegradable plastic waste and loss of carbon resources, and to improve the circular economy, is an imperative and urgent task. Chemical upcycling of biodegradable plastic waste is an effective and sustainable approach for better resolving the speedy growth of the amount of biodegradable plastic waste with multiple key benefits, including environmental remediation, circular carbon, and circular economy. Essentially, because of the abundant storage of fragile ester groups in biodegradable plastics, chemolysis could be a promising upcycling strategy for depolymerizing emerging biodegradable polyester plastics into a wide range of value-added platform chemicals under mild conditions (e.g., low temperature) compared with the tough cracking of conventional nondegradable plastics that contain massive tough C–C and C–H bonds. In detail, there are different methods of chemolysis available for upgrading biodegradable polyester plastics, primarily involving hydrolysis, hydrogenolysis, ammonolysis, aminolysis, and alcoholysis (or transesterification) (Figure 1). Correspondingly, water, hydrogen, ammonia, amines, and alcohols (e.g., methanol, ethanol, and ethylene glycol) are needed to break the chemical bonds between carbon and oxygen in polyesters to produce different types of molecules, such as alcohols, acids, amines, amino acids, esters, etc. (1) In hydrolysis, biodegradable polyester plastics could be directly converted back to their corresponding monomeric precursors, for instance, (non)catalytic hydrolysis of PLA and PHB (polyhydroxybutyrate, a member of the PHA family) to LA (lactic acid) and 3-HBA (3-hydroxybutyric acid), respectively. The benefit of the hydrolytic process is that it is accessible to repolymerize the monomeric precursors back to biodegradable polyesters. It is noteworthy that the pH of the hydrolytic solution (e.g., acidic, basic, and neutral), the reacting atmosphere (e.g., inert gas or hydrogen), the temperature, and the properties of the catalysts are the key factors for optimizing the reaction efficiency. In hydrogenolysis, borrowing molecular hydrogen to first break the chemical bonds in biodegradable polyesters to form intermediate monomers and further to hydrogenate the corresponding monomers to value-added chemical products, such as alcohols, acids, and esters, is typical. (2) For instance, molecular Ru-catalyzed hydrogenolysis of PLA-based consumer products (e.g., granulate and beverage cup) and PCL afforded high yields of 1,2-propanediol and 1,6-hexanediol, respectively. (3) Alternatively, catalytic hydrogenolysis of PLA powders and straws in a solvent-free system could produce liquid blends that comprise alcohols and esters for use in biofuels. (4) In addition, hydrogenolysis of PGA and PBS might correspondingly produce ethylene glycol and 1,4-butanediol. (2) In ammonolysis and aminolysis, ammonia and amines, respectively, are generally employed to depolymerize biodegradable plastic molecules to synthesize various nitrogen-related monomers, including amino acids and amines. For instance, alanine could be produced from the ammonolysis of PLA in a NH3·H2O solution over solid Ru catalysts. (5) In alcoholysis (or transesterification), nucleophilic solvents (e.g., methanol, ethanol, and ethylene glycol) can facilely depolymerize the polyester waste. Methanol is the most frequently used alcohol for the alcoholysis of polyesters; this reaction is also commonly known as methanolysis. For instance, methanolysis of PLA-based consumer goods (e.g., PLA-made cups, toys, and three-dimensional printing materials) is conducted to synthesize methyl lactate on zinc-based catalysts, where the methanolysis mechanism involves dual steps: the step from PLA to the oligomer and the subsequent step from the oligomer to the methyl lactate monomer. (6,7) Methanolysis of PHB could produce methyl 3-hydroxybutyrate over acid catalysts (e.g., H2SO4 and TsOH). (1) Catalytic ethanolysis of PLA forms ethyl lactate. (8) Catalytic alcoholysis of PLA with diols, such as ethylene glycol, 1,3-propanediol, and 1,4-butanediol, could synthesize various ester monomers. (9) In addition to being solvents in alcoholysis reactions, alcohols could be hydrogen donors for transfer hydrogenation due to their protolytic characteristics. For example, the product distribution of the methanol-mediated PLA methanolysis–hydrodeoxygenation process varies among methyl lactate, methyl propionate, and 1-propanol. Such a reaction system for PGA and PCL substrates can selectively produce methyl acetate and methyl hexanoate, respectively. (10) Despite a large number of research reports regarding chemical upcycling of plastic waste in recent years, most of the efforts mainly focus on the conversion of conventional nonbiodegradable plastic waste. In this respect, chemolysis of emerging biodegradable polyester plastics remains underdeveloped, especially in this fast-growing era of emerging polyester plastic utilization. Thus, more substantial research efforts on the chemolysis of emerging biodegradable polyester plastic waste are greatly desirable. In particular, rather than the individual polyester compound (e.g., PLA and PBAT), chemolysis of complex biodegradable plastic waste [e.g., PLA/PBAT and polylactic-co-glycolic acid (PLGA)] is more applicable to real-world polyester waste treatment. In addition, to purposefully extend the product value chain of emerging biodegradable polyester plastic waste, a cascade reaction is one important strategy for addressing this expectation, for instance, alcoholysis with hydrogenation and hydrolysis with hydrogenation. The design and preparation of efficient catalysts (both homogeneous and heterogeneous) are highly beneficial for decreasing the chemolysis reaction barriers, tuning the product selectivity, and making reaction conditions milder (e.g., low temperature and low pressure) with less energy input. Considering the practices of the chemical industry with respect to product separation, catalyst reuse, and process cost, heterogeneous catalysts would be more feasible for industrial scale-up. Moreover, the economic analysis and process optimization of chemolysis demand more detailed investigations. Chun-Ran Chang is professor of chemical engineering at Xi’an Jiaotong University, Xi’an, China. He obtained his Ph.D. in chemistry from Tsinghua University, Beijing, China. His research areas mainly focus on heterogeneous catalysis, computational catalysis, and physical chemistry for a wide range of applications in energy conversion and environmental protection. His h-index (Google Scholar) is 37 with more than 7500 total citations. This work is supported by the National Key R&D Program of China (2023YFA1506302), the Science and Technology Plan Fund of Yulin City (CXY-2022-141), the Qinchuangyuan “Scientists + Engineers” Team Construction Program of Shaanxi Province (Grant 2023KXJ-276), and the research program of Shaanxi Beiyuan Chemical Industry Group Co., Ltd. (Grant 2023413611014). X.G. acknowledges the financial support from the Foundation of State Key Laboratory of Clean and Efficient Coal Utilization, Taiyuan University of Technology, China (MINYSKL202309), the Natural Science Basic Research Program of Shaanxi Province at China (2024JC-YBQN-0071), and the Postdoctoral Research Project Fund of Shaanxi Province, China (2023BSHEDZZ22). This article references 10 other publications. This article has not yet been cited by other publications.
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来源期刊
ACS ES&T engineering
ACS ES&T engineering ENGINEERING, ENVIRONMENTAL-
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期刊介绍: ACS ES&T Engineering publishes impactful research and review articles across all realms of environmental technology and engineering, employing a rigorous peer-review process. As a specialized journal, it aims to provide an international platform for research and innovation, inviting contributions on materials technologies, processes, data analytics, and engineering systems that can effectively manage, protect, and remediate air, water, and soil quality, as well as treat wastes and recover resources. The journal encourages research that supports informed decision-making within complex engineered systems and is grounded in mechanistic science and analytics, describing intricate environmental engineering systems. It considers papers presenting novel advancements, spanning from laboratory discovery to field-based application. However, case or demonstration studies lacking significant scientific advancements and technological innovations are not within its scope. Contributions containing experimental and/or theoretical methods, rooted in engineering principles and integrated with knowledge from other disciplines, are welcomed.
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