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

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 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.