Upcycling Plastic Wastes into Value‐Added Products by Heterogeneous Catalysis

Abstract

Plastics are playing essential roles in the modern society. The majority of them enter environment through landfilling or discarding after turning into wastes, causing severe carbon loss and imposing high risk to ecosystem and human health. Currently, physical recycling serves as the primary method to reuse plastic waste, but this method is limited to thermoplastic recycling. The quality of recycled plastics gradually deteriorates because of the undesirable degradation in the recycling process. Under such background, catalytic upcycling, which can upgrade various plastic wastes into value-added products under mild conditions, has attracted recent attention as a promising strategy to treat plastic wastes. This Review highlights recent advances in the development of efficient heterogeneous catalysts and useful strategies for upcycling plastics into liquid hydrocarbons, arene compounds, carbon materials, hydrogen, and other value-added chemicals. The functions of catalysts and the reaction mechanisms are discussed. The key factors that influence the catalytic performance are also analyzed.

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

“…However, diffusional limitations due to the micropores and faster catalytic deactivation among the existing challenges, similar to many other zeolites catalyzed processes discussed in this review. The different zeolites have been screened for the valorization of plastic waste, including zeolite Y, ZSM-5, beta, natural zeolites, and fluid catalyzed cracking (FCC) catalysts, which preferentially yield gasoline and diesel-ranged hydrocarbons with lower olefin content, light olefins and aromatics, gaseous products with reduced liquid fraction, (relatively unselective) liquid and gaseous products, and liquid oils, respectively. ,− In the case of doped zeolites, the preferential product selectivity depends on the type of metals and metal–support interactions based on the dispersion of metal: for example, the addition of Ni into zeolite favored the formation of shorter olefinic species, while iron doping on ZSM-5 improved the yield of monoaromatic species. , Similarly, activating hydrogen by Pd over Pd-doped zeolites promoted hydrogenation, dehydrogenation, and hydro-isomerization reactions to preferentially deliver saturated hydrocarbons. , Herein, we refer to three recent reviews on this topic, where the status of the current literature on plastic waste valorization over zeolite/heterogeneous catalysts has been appropriately discussed. ,, Although the carbenium-based mechanistic proposal seems reasonable in the zeolite-mediated plastic valorization, their experimental evidence is scarce. Herein, this research field could draw inspiration from the zeolite-catalyzed biomass conversion to deliver precise mechanistic information, which could be helpful in maturing this chemical reaction.…”

“…143,144 Herein, we refer to three recent reviews on this topic, where the status of the current literature on plastic waste valorization over zeolite/heterogeneous catalysts has been appropriately discussed. 18,20,145 Although the carbenium-based mechanistic proposal seems reasonable in the zeolite-mediated plastic valorization, their experimental evidence is scarce. Herein, this research field could draw inspiration from the zeolite-catalyzed biomass conversion to deliver precise mechanistic information, which could be helpful in maturing this chemical reaction.…”

Understanding the Structure–Activity Relationships in Catalytic Conversion of Polyolefin Plastics by Zeolite-Based Catalysts: A Critical Review

“…Pyrolysis or thermolysis is a thermochemical anaerobic process at a high reaction temperature (usually above 500 °C). It converts polyolefins to a mixture of gas, liquid, and solid-state hydrocarbons. ,, In comparison, catalytic cracking of polyolefins could be carried out at a relatively lower temperature within a shorter reaction time, forming hydrocarbon products with a relatively narrower molecular distribution . Although the products’ compositions obtained from both thermal and catalytic cracking are affected by the nature of polymers, reactors, and reaction conditions, such as heating rate, flow rate, and residence time, ,,, the selectivity difference mainly originates from different reaction mechanisms of the above two processes.…”

“…In the following decades, the undegradable property has made plastics competitive materials and been promoted worldwide. − As a result, the rapid accumulation of plastic waste has brought about serious environmental pollution problems, caused massive damage to the ecosystem, and threatened human health. − In contrast to the increasing amount of plastic waste generated yearly, only a small proportion of plastics are recycled. Most plastic waste is disposed of by landfill or incineration for energy recovery accompanied by toxic byproduct formation. , Besides, mechanical recycling, especially applicable to thermoplastics such as PET bottles and some polyolefin resins, will convert the plastic waste into the former plastic but will generally lead to degradation of thermal, physical, and mechanical properties. , Based on the current situation mentioned above, more eco-friendly and economical recycling of plastic waste needs to be explored urgently. Emerging as promising alternatives, chemical recycling and upcycling offer exciting opportunities to transform plastics into a series of value-added products, such as high-purity monomers for repolymerization, hydrogen and light cracking feed oil for fuels, value-added chemicals including light olefins and aromatic compounds, functional carbon materials etc. − Among all kinds of plastics, polyolefins, mainly consisting of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), and polypropylene (PP), are of particular significance, since they are the most consumed plastics in the world, accounting for 57% of total municipal solid waste. , …”

Circular Upcycling of Bottlebrush Thermosets

“…However, closed‐loop recycling of crosslinked thermosets is not possible due to irreversible, covalent crosslinks [10–12] . To provide end‐of‐life solutions for these recalcitrant materials, upcycling approaches have been developed involving polymer functionalization and/or depolymerization [13–16] to convert a post‐consumer product into a new material of higher value [16–23] …”