Plastic plays an important role in our daily lives due to its versatility, light weight and low production cost. Plastics became essential in many sectors such as construction, medical, engineering applications, automotive, aerospace, etc. In addition, economic growth and development also increased our demand and dependency on plastics which leads to its accumulation in landfills imposing risk on human health, animals and cause environmental pollution problems such as ground water contamination, sanitary related issues, etc. Hence, a sustainable and an efficient plastic waste treatment is essential to avoid such issues. Pyrolysis is a thermo-chemical plastic waste treatment technique which can solve such pollution problems, as well as, recover valuable energy and products such as oil and gas. Pyrolysis of plastic solid waste (PSW) has gained importance due to having better advantages towards environmental pollution and reduction of carbon footprint of plastic products by minimizing the emissions of carbon monoxide and carbon dioxide compared to combustion and gasification. This paper presents the existing techniques of pyrolysis, the parameters which affect the products yield and selectivity and identify major research gaps in this technology. The influence of different catalysts on the process as well as review and comparative assessment of pyrolysis with other thermal and catalytic plastic treatment methods, is also presented.

The catalytic degradation of high-density polyethylene to hydrocarbons was studied over different zeolites. The product range was typically between C3 and C15 hydrocarbons. Distinctive patterns of product distribution were found with different zeolitic structures. Over large-pore ultrastable Y, Y, and β zeolites, alkanes were the main products with less alkenes and aromatics and only very small amounts of cycloalkanes and cycloalkenes. Medium-pore mordenite and ZSM-5 gave significantly more olefins. In the medium-pore zeolites secondary bimolecular reactions were sterically hindered, resulting in higher amounts of alkenes as primary products. The hydrocarbons formed with medium-pore zeolites were lighter than those formed with large-pore zeolites. The following order was found regarding the carbon number distribution:  (lighter products) ZSM-5 < mordenite < β < Y < US-Y (heavier products). A similar order was found regarding the bond saturation:  (more alkenes) ZSM-5 < mordenite < β < Y < US-Y (more alkanes). Dependent upon the chosen zeolite, a variety of products was obtained with high values as fuel, confirming catalytic degradation of polymers as a promising method of waste plastic recycling.

Configurational-bias MonteCarlo simulations in the grand-canonical ensemble are employed to compute adsorption isotherms of methane, ethane, propane, butane and binary mixtures of methaneethune, methanepropane, ethanepropane, and methanebutane in the zeolite silicalite. Comparison of the simulation results with the limited expetiniental data uuailuble shows good agreement. For ethane at room temperature, a small inflection point in the isotherm was observed due to a surprising ordering of the ethane molecules in the zeolite. For the simulation of a methane-ethane mixture, at low pressure ethane is preferentially adsorbed, while at high pressures methane replaces ethane due to entropic effects.

The catalytic cracking of polyethylene has been studied over two natural clays and their pillared analogues with a view toward assessing their suitability in a process for recycling plastic waste to fuel. Although these clays were found to be less active than US-Y zeolite around 600 K, at slightly higher process temperatures, they were able to completely decompose polyethylene. Their yields to liquid products were around 70%, compared to less than 50% over US-Y zeolite. Moreover, the liquid products obtained over the clay catalysts were heavier. Both of these facts are attributed to the milder acidity of clays, as the very strong acidity characterizing zeolites leads to overcracking. Furthermore, this milder acidity leads to significantly lower occurrence of hydrogen-transfer secondary reactions compared to US-Y zeolite, and as a consequence, predominantly alkenes were the products over the clay catalysts. An additional advantage of these catalysts is the considerably lower amount of coke formed.