The chemistry of chemical recycling of solid plastic waste via pyrolysis and gasification: State-of-the-art, challenges, and future directions

Plastic pollution is ubiquitous in terrestrial and aquatic ecosystems. Plastic waste exposed to the environment creates problems and is of significant concern for all life forms. Plastic production and accumulation in the natural environment are occurring at an unprecedented rate due to indiscriminate use, inadequate recycling, and deposits in landfills. In 2019, the global production of plastic was at 370 million tons, with only 9% of it being recycled, 12% being incinerated, and the remaining left in the environment or landfills. The leakage of plastic wastes into terrestrial and aquatic ecosystems is occurring at an unprecedented rate. The management of plastic waste is a challenging problem for researchers, policymakers, citizens, and other stakeholders. Therefore, here, we summarize the current understanding and concerns of plastics pollution (microplastics or nanoplastics) on natural ecosystems. The overall goal of this review is to provide background assessment on the adverse effects of plastic pollution on natural ecosystems; interlink the management of plastic pollution with sustainable development goals; address the policy initiatives under transdisciplinary approaches through life cycle assessment, circular economy, and sustainability; identify the knowledge gaps; and provide current policy recommendations. Plastic waste management through community involvement and socio-economic inputs in different countries are presented and discussed. Plastic ban policies and public awareness are likely the major mitigation interventions. The need for life cycle assessment and circularity to assess the potential environmental impacts and resources used throughout a plastic product’s life span is emphasized. Innovations are needed to reduce, reuse, recycle, and recover plastics and find eco-friendly replacements for plastics. Empowering and educating communities and citizens to act collectively to minimize plastic pollution and use alternative options for plastics must be promoted and enforced. Plastic pollution is a global concern that must be addressed collectively with the utmost priority.

Impacts of Plastic Pollution on Ecosystem Services, Sustainable Development Goals, and Need to Focus on Circular Economy and Policy Interventions

https://pubs.acs.org/doi/pdf/10.1021/acssuschemeng.1c05013

Managing Plastic Waste─Sorting, Recycling, Disposal, and Product Redesign

In the present study, a comprehensive population balance model is developed to predict the dynamic evolution of the particle size distribution in high hold-up (e.g., 40%) non-reactive liquid–liquid dispersions and reactive liquid(solid)–liquid suspension polymerization systems. Semiempirical andphenomenological expressions are employedto d escribe the breakage andcoalescence rates of d monomer droplets in terms of the type and concentration of suspending agent, quality of agitation, and evolution of the physical, thermod ynamic andtransport properties of the polymerization system. The fixedpivot (FPT) numerical methodis appliedfor solving the population balance equation. The predictive capabilities of the present model are demonstrated by a direct comparison of model predictions with experimental data on average mean diameter and droplet/particle size distributions for both non-reactive liquid–liquid dispersions andthe free-rad ical suspension polymerization of styrene andVCM monomers. 2005 Elsevier Ltd. All rights reserved.

Application of a Generalized Population Balance Model for the Prediction of Particle Size Distribution in Suspension Polymerization Reactors

Hydrogenolysis of Polypropylene and Mixed Polyolefin Plastic Waste over Ru/C to Produce Liquid Alkanes

A tree-based kinetic Monte Carlo (kMC) model is presented that differentiates between 38 end-group pairs for isothermal degradation of poly(styrene peroxide) (PSP). The binary trees allow for fast and accurate calculation of reaction probabilities, with mass-weighted binary trees for the accurate sampling of peroxide bond fissions and hydrogen abstractions along chains. The kinetic parameters are tuned via artificial neural networks (ANNs) to successfully predict literature experimental data, among other lumped product yields. ANNs are also utilized for sensitivity analysis to unravel the effects of individual reactions on the time evolution of experimental responses and other simulation outputs, including the variations of the chain length distributions of the macrospecies. PSP degradation is characterized by three stages of degradation considering both instantaneous and time-averaged concentrations. The first stage features rapid unzipping and results in the fast production of major products benzaldehyde and formaldehyde, the second stage features the most significant level of hydrogen abstractions involving PSP and other macrospecies types, and the third stage exhibits the consumption of the remaining peroxide bonds toward oligomeric species in a wide time frame until the degradation process is finalized by the depletion of peroxide bonds. This proof-of-concept study based on unprecedentedly detailed analyses of the chemistry via kinetic Monte Carlo paves the way to further improve our understanding of chemical recycling of solid plastic waste.

Distribution changes during thermal degradation of poly(styrene peroxide) by pairing tree-based kinetic Monte Carlo and artificial intelligence tools

Valorization of pyrolytic lignin to fuels and chemicals is still poorly understood due to its ill-defined structure and the complexity of the decomposition chemistry. To shed some light on the dominant reaction pathways of lignin thermolysis, novel experimental and first-principles based calculations of its building blocks have been carried out. Pyrolysis chemistry of hydroxycinnamic acids is investigated in this work using a unique Py-GC × GC-FID/TOF-MS coupled with a customized GC to detect water and gases, to gain an understanding of the role of the branching ratios in lignin and its linkages with hemicellulose. Mean residence times of cinnamic and ferulic acids were estimated to be 12 and 21  s at 573  K, based on time-resolved experiments. Cinnamic acid undergoes a CO2 elimination reaction at temperatures higher than 873  K without an intermediate liquid phase. At temperatures as low as 573  K, –OH and –OCH3 substituted cinnamic acids underwent decarboxylation despite bearing similar BDEs for Cβ–Cγ scission. At these temperatures, p-coumaric and ferulic acids were converted into 4-vinylphenol and 4-vinylguaiacol by 40  wt% and 30  wt%, respectively. On the other hand, sinapinic acid converted nearly by 80  wt% at temperatures below its boiling point of 676  K. In conjunction with novel quantum chemical calculations, it could be ruled out that decarboxylation was not occurring via concerted unimolecular reactions at low temperatures. Instead, water-catalyzed reactions of hydroxycinnamic acids seem to be the primary cause for the CO2 elimination in the intermediate liquid phase via a 6-centered transition state.

Onur Dogu

Papers

The chemistry of chemical recycling of solid plastic waste via pyrolysis and gasification: State-of-the-art, challenges, and future directions

Distribution changes during thermal degradation of poly(styrene peroxide) by pairing tree-based kinetic Monte Carlo and artificial intelligence tools

On the primary thermal decomposition pathways of hydroxycinnamic acids

Primary Thermal Decomposition Pathways of Hydroxycinnamaldehydes

Bayesian tuned kinetic Monte Carlo modeling of polystyrene pyrolysis: unraveling the pathways to its monomer, dimers, and trimers formation