Investigation On Processing Of Waste Plastics Into Carbon Materials In Supercritical Carbon Dioxide System

The "white pollution" caused by waste plastic brings heavy pollution and a variety of potential hazards on the ecological environment and human survival. Recycling of waste plastic will be, significant in savings energy and materials, helpful to reduce the ecological pressure and achieve sustainable development. As the complexity of the plastic mixtures, current sorting techniques are inefficient for economically feasible recycling. Therefore, a general approach which is compatible with different kinds of plastics to generate value-added products is important. Here in this dissertation, supercritical carbon dioxide (SC-CO2) was used in processing waste plastics (mainly including PET, PP and PE), as well as discarded oil (cooking oil) to produce novel carbon materials.A simple chemical route to prepare carbon microspheres by pyrolyzing PET waste in a SC-CO2system has been illustrated. The graphitization of carbon products is mainly determined by the reaction time and temperature. A higher temperature tends to generate a higher yield of solid carbon product. The spherical particles were obtained with a carbon yield as high as28%at650℃for3h. It is found that the SC-CO2system favors the dissociation of the PET precusor to aromatic hydrocarbons, which further decompose to yield carbon spheres. The material exhibits a first discharge capacity of504.9(mA h)/g at a constant current density of100mA/g and maintains a capacity retention of40%after the20th cycle. The acid-mixture-sonicate process could improve22%higher of the capacity. An annealing-oxidation at1500℃leads to the microspheres from PET converting into two distinctive structures, layer-by-layer stacking and porous golf ball structure.Carbon microspheres were also synthesized by pyrolyzing PP or PE wastes in a SC-CO2system. The yield of Carbon microspheres was42.0%and43.5%, respectively. An annealing-oxidation treatment at1500℃results in carbon microspheres from PP or PE converting into layer-by-layer stacking structure. The layer-by-layer stacking carbon microspheres were further changed into layer-by-layer stacking carbon microellipses by treatment via a Hummers-Method.A simple route for efficient production of carbon microspheres by direct pyrolysis of discarded oil in a scCO2system was demonstrated. The solid carbon product was obtained in spherical, filament-or egg-like morphologies. The yield of solid carbon reached42.2wt%when the treatment was conducted at600℃using14g dry ice. Moreover, microspheres formed by layer-by-layer stacking were obtained after further vacuum annealing at1300-1500℃. The optimal conditions for producing high-yield layer-structured carbon spheres were600℃for6h, followed by annealing at1500℃for1-2h. Also, microporous carbon spheres have potential for use in electrodes, chemical energy storage, separation technologies, and lubricants.It was found that the carbon materials formed by reduction of scCO2with alkali metals (Na, K, and Li) demonstrate porous structure with an average pore size of3.88-3.94nm and surface area of110.9-571.6m2g-1. A formation mechanism was proposed that nanodroplets of alkali metals take the roles both as reductants for CO2reduction to carbon and as templates for the production of porous carbon. It was found the actual reaction temperature was much higher than the oven temperature, it helps to get deeper understanding of the reaction of alkali metals-scCO2system. Porous carbon as anode material of lithium ion batteries was tested. The first discharge capacity for the porous carbon reaches1704mAh/g, which is ascribed to its large pore volume providing abundant space to catch Li ions. After20discharge-charge cycles, the discharge capacity stabilizes at200mAh/g, the large irreversible capacity is mainly due to the formation of SEI film on the surface of the electrode material during the first charge-discharge process. It is suggested that further graphitization of the porous carbon materials could lead to better electrochemical performance.