Catalytic activity of metal impregnated catalysts for degradation of waste polystyrene

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.

Carbon precursors in coal tar: Extraction and preparation of carbon materials

Coke resources are abundantly available worldwide and are a large by-product of tar production. Moreover, their utilization presents a series of environmental pollution problems. Common technologies for coal tar production applications urgently need to be upgraded because coal tar is listed as a national hazardous waste. This review associates coal tar development with deep processing technology for extracting environmentally beneficial compounds from coal tar, which have never been reported. Recent studies on the innovative approaches for extracting phenols and nitrogen-containing compounds from coal tar have been addressed, as well as a preparation method of carbon materials with high catalytic activity and a well-ordered structure by confined polymerization. Tremendous demand for further research and exploration of selectively extracted compounds from coal tar implies a new opportunity for polymerizing the resin and a great challenge for the current technology implemented for valorizing coal tar into ordered carbon materials. Consequently, more concerted efforts should be implemented to achieve a wide range of polymer resin applications and improve the quality of carbon precursors extracted from the coal tar, thus increasing the economic benefit and scientific value of coal tar.

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: A Review

The manufacturing of polystyrene around the globe has escalated in the past years due to its huge applications in various areas. The perpetual market needs of polystyrene led the polystyrene wastes accretion in the landfill causing environmental deterioration. The soaring need for polystyrene also led to the exhaustion of petroleum, a non-renewable energy source, as polystyrene is a petroleum-derived product. Researchers from around the world have discovered a few techniques to take care of the polystyrene scraps, namely recycling and energy recovery techniques. Nevertheless, there are demerits involved with recycling techniques, such as they call for huge labor expenses in the separation process and cause water pollution, thereby decreasing the process sustainability. Owing to these demerits, the researchers have focused their attention on the energy recovery technique. Since petroleum is the main ingredient of polystyrene synthesis, the restoration of liquid oil from polystyrene via the pyrolysis method is a promising technique as the recovered oil has greater calorific value as compared to commercially available fuel. The present paper surveys the pyrolysis technique for polystyrene and the important process parameters that control the end product, like oil, gas, and char. The chief process parameters that are discussed in this review paper include the type of reactors, temperature, residence time, pressure, catalyst types, type of fluidizing gases, and their flow rate. A more recent technique of utilizing a solvent to perform pyrolysis and the effect of various process conditions on the product yield have been discussed. Apart from this, various outlooks to optimize the liquid oil recovery from polystyrene are also reviewed.

Experimental investigation on liquefaction of plastic waste to oil in supercritical water

In order to solve the problem of low thermal conductivity and high viscous molten liquid reaction product in the process of plastic liquefaction, the experiments of high impact polystyrene (HIPS) plastic liquefaction were carried out in supercritical water. In this paper, the effects of different operating conditions (temperature, time, feedstock concentration and pressure) on liquid products were studied. It is found that the novel phenomenon that the liquid products of HIPS plastic were mainly toluene and ethylbenzene rather than styrene, which was a product of polystyrene. The experimental results showed that plastic first depolymerized to form styrene and 1,3-diphenylpropane, which were then converted to toluene and ethylbenzene. The increase in temperature promoted this transformation process and some traces of polycyclic aromatic hydrocarbons also produced. At 490 °C, the maximum carbon liquefaction rate of 77.0 wt% was obtained, which was 6 times higher than the conventional pyrolysis, and the content of toluene and ethylbenzene were 14 wt% and 51.3 wt%, respectively. Increasing the reaction pressure and prolonging the reaction time all promoted the progress of the plastic liquefaction reaction, while increasing the feedstock concentration caused the carbon liquefaction rate to increase first and then slightly decrease.

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.