Renewable aromatics from the degradation of polystyrene under mild conditions

Catalytic routes towards polystyrene recycling

Polystyrene (PS) is one of the most popular plastics due to its versatility, which renders it useful for a large variety of applications, including laboratory equipment, insulation and food packaging. However, its recycling is still a challenge, as both mechanical and chemical (thermal) recycling strategies are often cost-prohibitive in comparison to current disposal methods. Therefore, catalytic depolymerization of PS represents the best alternative to overcome these economical drawbacks, since the presence of a catalyst can improve product selectivity for chemical recycling and upcycling of PS. This minireview focuses on the catalytic processes for the production of styrene and other valuable aromatics from PS waste, and it aims to lay the ground for PS recyclability and long-term sustainable PS production.

A Thermo-Catalytic Pyrolysis of Polystyrene Waste Review: A Systematic, Statistical, and Bibliometric Approach

Global polystyrene (PS) production has been influenced by the lightness and heat resistance this material offers in different applications, such as construction and packaging. However, population growth and the lack of PS recycling lead to a large waste generation, affecting the environment. Pyrolysis has been recognized as an effective recycling method, converting PS waste into valuable products in the chemical industry. The present work addresses a systematic, bibliometric, and statistical analysis of results carried out from 2015 to 2022, making an extensive critique of the most influential operation parameters in the thermo-catalytic pyrolysis of PS and its waste. The systematic study showed that the conversion of PS into a liquid with high aromatic content (84.75% of styrene) can be achieved by pyrolysis. Discussion of PS as fuel is described compared to commercial fuels. In addition, PS favors the production of liquid fuel when subjected to co-pyrolysis with biomass, improving its properties such as viscosity and energy content. A statistical analysis of the data compilation was also discussed, evaluating the influence of temperature, reactor design, and catalysts on product yield.

Catalytic Pyrolysis of Polystyrene Waste in Hydrocarbon Medium

The fast catalytic pyrolysis of polystyrene in the hydrocarbon medium (light and heavy cycle oil) over zeolite catalysts at 450-550 °C was investigated. The influence of reaction conditions (medium, temperature, vapor residence time, polystyrene concentration) on polymer conversion and product distribution was studied. It was found that the polymer conversion is close to 100%, while ethylbenzene, benzene, and toluene are the main products of its transformation. The maximum yield of ethylbenzene (80%) was achieved at 550 °C, vapor residence time 1-2 s, polystyrene concentration 10%, and heavy cycle oil as the medium. The influence of zeolite topology on product distribution was explored. The possible mechanism of polystyrene pyrolysis was proposed.

Metal-Catalyzed, Photo-Assisted Selective Transformation of Tertiary Alkylbenzenes and Polystyrenes into Carbonyl Compounds

Every year, thousands of tons of polystyrene are produced and discarded, filling landfills and polluting the marine environment. Although several degradation alternatives have been proposed, the need for an effective procedure for the chemical recycling of polystyrene still remains. Here, a vanadium-catalyzed reaction, assisted by visible light, promoted the direct, selective conversion of tertiary alkylbenzenes into acetophenone and other ketone derivatives. Likewise, standard polystyrene samples as well as polystyrenes from insulation and packaging waste could be chemically recycled into acetophenone in a scalable way regardless of their molecular weight, polydispersity, or form. Preliminary mechanistic investigations revealed the participation of singlet oxygen, superoxide, and hydroxyl radical species in this homogenously catalyzed process. Acetophenone could be used as an additive to accelerate the reaction and to increase the yields in some cases.

Insights into the removal of polystyrene nanoplastics using the contaminated corncob-derived mesoporous biochar from mining area

Nanoplastic has become a prominent threat to the aquatic ecosystem, and the cost-effective technologies for controlling that are still insufficient. The aim of this study is to use contaminated corncobs collected in mining area to prepare functional mesoporous biochar (MBC) and to investigate its ability to remove polystyrene nanoplastics (PSNPs) from water. The adsorption of PSNPs by MBC could be better described by the Sips isotherm and followed the second-order kinetics, with the theoretical maximum adsorption capacity of MBC for PSNPs was 56.02 mg·g^-1. Then the PSNPs adsorbed on MBC could be hydrothermally degraded and the biochar could be simultaneously regenerated. The ability was affected by various factors, including oxygen-containing functional groups, metallic components, superoxide radicals and holes. The degradation products were dominated as low-molecule-weight oligomers and the main possible pathways involved scission, hydrolysis and radical reaction. The findings highlight the great potential of biochar prepared using contaminated biowaste in mining area to remove the nanoplastic pollutants in the aqueous environment.

Thermo-catalytic pyrolysis of polystyrene in batch and semi-batch reactors: A comparative study

Thermo-catalytic pyrolysis is considered as a promising process for the chemical recycling of waste polymeric materials aiming at converting them into their original monomers or other valuable chemicals. In this regard, process parameters and reactor type can play important roles for an enhanced recovery of the desired products. Polystyrene (PS) wastes are excellent feedstocks for the chemical recycling owing to the capability of PS to be fully recycled. In this respect, the present work deals with the thermo-catalytic pyrolysis of PS in batch and semi-batch reactor setups. The main goal was to perform a comprehensive study on the depolymerisation of PS, thereby investigating the effect of reactor type, catalyst arrangement, feed to catalyst ratio and residence time on the yields of oil and styrene monomer (SM). A further goal was to identify the optimum operating conditions as well as reactor type for an enhanced recovery of oil and SM. It was demonstrated that the semi-batch reactor outperformed the batch reactor in terms of oil and SM yields in both thermal (non-catalytic) and catalytic tests performed at 400°C. Furthermore, it was shown that the layered arrangement of catalyst (catalyst separated from PS) produced a higher amount of oil with higher selectivity for SM as compared to the mixed arrangement (catalyst mixed with PS). Moreover, the effect of carrier gas flowrate on the product distribution was presented.

Pyrolysis of polystyrene waste for recovery of combustible hydrocarbons using copper oxide as catalyst

The present work is focused on pyrolysis of polystyrene waste for production of combustible hydrocarbons. The experiments were performed in an indigenously made furnace in the presence of a laboratory synthesised copper oxide. The pyrolysis products were collected and characterised. The Fourier transform infrared spectra showed that the liquid fraction contains C-H, C-O, C-C, C=C and O-H bonds, which correspond to various aliphatic and aromatic compounds. Gas chromatography-mass spectrometry traced compounds ranging from C₁ to C₄ in the gaseous fraction, whereas in the liquid fraction 15 components ranging from C₃ to C₂₄ were detected. From the results it has been concluded that CuO as a catalyst not only increased the liquid yield but also reduced the degradation temperature to great extent. Fuel properties of the pyrolysis oil were determined and compared with standard values of commercial fuel oil. The comparison suggested potential application of pyrolysis oil for domestic and commercial use.

Fuel production from waste polystyrene via pyrolysis: Kinetics and products distribution

In the present study polystyrene waste (PS) was collected from a drop off site in a local market and pyrolyzed at heating rates of 5, 10, 15 and 20 °C/min and temperature range 40-600 °C under nitrogen condition. The apparent activation energy (Ea) and pre-exponential factor (A) were determined using 6 different kinetic methods. Activation energy and pre-exponential factor were found in the range of 82.3 - 202.8 kJmol^-1 and 3.5 × 10⁶-7.6 × 10¹⁴ min^-1 respectively. The results demonstrated that the calculated values of Ea and A vary with fraction of conversion, heating rates and the applied model. Moreover, pyrolysis of waste polystyrene was carried out in an indigenously manufactured furnace at temperatures ranging from 340 to 420 °C. The composition of liquid and gaseous fractions was determined using gas chromatography-mass spectrometry. Temperature and reaction time were optimized and the results revealed that temperature of 410 °C and exposure time of 70 min are the best conditions for maximum fuel oil production. Methane and ethane were found as the main products in the gas phase constituting about 82% of the gaseous fraction. The liquid products composed of broad range of C₂ - C₁₅ hydrocarbons depending on the pyrolytic parameters. A comparison of the composition of pyrolysis oil with standard parameters of diesel, gasoline and kerosene oil suggested that pyrolysis oil from polystyrene waste holds great promise for replacing fuel oil.

Selective conversion of polystyrene into renewable chemical feedstock under mild conditions

The aim of this work is to prepare catalysts for energy efficient conversion of polystyrene (PS) and its waste into valuable products with high conversion at 250 °C. The FeCo/Alumina bimetallic catalyst was synthesized by aqueous impregnation and structurally determined using scanning-transmission electron microscopy, temperature programmed desorption, X-ray diffraction, and X-ray photoelectron spectroscopy. Successfully, we have achieved up to 91% liquid yield with selectivities for styrene monomer (SM) up to 45 wt% and ethylbenzene (EB) up to 55 wt%, depending on the exposure time at 250 °C by FeCo/Alumina which is comparable to those of reactions at high temperatures (≥350 °C). Further increase of catalyst loadings from 200 to 400 mg also led to the decrease in styrene yield and increase in ethylbenzene yield. The analysis of the resulting clear liquid by gas chromatography/mass spectrometry (GC/MS) indicates the generation of products in the gasoline range.

A novel approach of solid waste management via aromatization using multiphase catalytic pyrolysis of waste polyethylene

A new and innovative approach was adopted to increase the yield of aromatics like, benzene, toluene and xylene (BTX) in the catalytic pyrolysis of waste polyethylene (PE). The BTX content was significantly increased due to effective interaction between catalystZSM-5 and target molecules i.e., lower paraffins within the reactor. The thermal and catalytic pyrolysis both were performed in a specially designed semi-batch reactor at the temperature range of 500 °C-800 °C. Catalytic pyrolysis were performed in three different phases within the reactor batch by batch systematically, keeping the catalyst in A type- vapor phase, B type- liquid phase and C type- vapor and liquid phase (multiphase), respectively. Total aromatics (BTX) of 6.54 wt% was obtained for thermal pyrolysis at a temperature of 700 °C. In contrary, for the catalytic pyrolysis A, B and C types reactor arrangement, the aromatic (BTX) contents were progressively increased, nearly 6 times from 6.54 wt% (thermal pyrolysis) to 35.06 wt% for C-type/multiphase (liquid and vapor phase). The pyrolysis oil were characterized using GC-FID, FT-IR, ASTM distillation and carbon residue test to evaluate its end use and aromatic content.