Reduction of Carbon Dioxide in Hydrothermal Cracking of Polymer Wastes

Abstract

T he Earth's environment is seriously threatened by an increase in atmospheric carbon dioxide (CO 2 ) concentrations. To support sustainable development in the 21st century, controlling the global warming caused by the increasing atmospheric CO 2 levels has become a main scientific and technological challenge. 1 To prevent catastrophic events related to atmospheric CO 2 , the emission of CO 2 should be reduced as soon as possible. One of the most promising solutions to overcome this challenge is the conversion of CO 2 into valuable chemicals, thus contributing to the reduction of the atmospheric CO 2 load. This process is regarded as one of the most ideal solutions for the "greenhouse effect" problem. Potential techniques have been studied extensively, including FischerÀTropsch reactions, photochemical or electrochemical conversions, catalytic hydrogenation, and producing valuable compounds, such as methanol and methane. 2À5 In recent decades, hydrothermal reactions have received more attention for the treatment of organic wastes and biomass conversion because of the unique inherent properties of hightemperature water (HTW) that include a high ion product (K w ) and a low dielectric constant, which are favorable for promoting reactions without catalysts. 6À9 In our previous studies, it was found that hydrogen can be generated during the hydrothermal cracking of bitumen and polymer wastes, such as polyethylene (PE) and sulfur-containing rubber. 10À14 A cheap source of hydrogen has become the primary challenge in the conversion of CO 2 . Hydrogen is currently produced by reforming hydrocarbons, which is an energy-intensive process. If the hydrogen produced during the hydrothermal cracking of polymer wastes could be directly used to reduce CO 2 , then an efficient process for CO 2 conversion and polymer waste use would be realized.In this study, we examined the possibility of CO 2 reduction during the hydrothermal cracking of polymer wastes. PE, as a representative of plastic waste, and ethylene propylene diene monomer (EPDM), as a model for sulfur-containing waste rubber, were used as test materials. The molecular structure of EPDM can be found in the Supporting Information. Experiments were conducted at temperatures varying from 300 to 450°C in a bomb-type batch reactor with a Hastelloy C-276 inner wall and an inner volume of 42 mL. The reactor was equipped with a highpressure valve to allow for pressure measurements, CO 2 input, and gas sampling. The experimental setup has been described in detail elsewhere. 15 After reactions, gas and liquid samples were collected and analyzed by gas chromatographyÀthermal conductivity detector (GCÀTCD) and gas chromatographyÀmass spectroscopy (GCÀMS), respectively. A more detailed description of experimental procedures and the gas collection method can be found in the Supporting Information.First, experiments with PE were conducted with various temperatures (300À450°C), reaction times, and initial pH values with or without CO 2 . No significant differences were observed in th...

A Potentially Useful Technology by Mimicking Nature—Rapid Conversion of Biomass and CO2 into Chemicals and Fuels under Hydrothermal Conditions

“…Previous research has demonstrated that hydrogen can be produced from water in supercritical-water up-gradation of organic waste, such as polyethylene (PE) and sulfur-containing rubber, If the hydrogen produced during the hydrothermal cracking of polymer wastes could be directly used to reduce CO 2 , then an efficient process for CO 2 conversion and polymer waste utilization would be realized. Thus, we studied the poetential of the reduction of CO 2 under hydrothermal conditions in the presence of organic waste …”

Applications of supercritical technologies to CO2 reduction: Catalyst development and process intensification

“…In particular, biomass residues, glycerin [50] or residues of Sulphur-containin polymers have been used to reduce the metal and reutilize it in the process [56]. Additionally, some authors have proved that CO 2 reduction can be carried out in hydrothermal media by oxidation of an alcohol, without addition of any metals [57].…”

2Hydrothermal CO2 conversion using zinc as reductant: Batch reaction, modeling and parametric analysisof the process

“…However, these technologies still face important limitations, especially with respect to the selectivity and the productivity of the conversion reaction, and new investigations regarding process intensification and energy integration are needed to make these technologies technically and economically feasible [6] Among the different carbon dioxide conversion technologies, the hydrothermal reduction of CO 2 stands out since it has already shown the potential to selectively convert carbon dioxide into valuable products such as formic acid, methane and methanol, favored by the increased reactivity of CO 2 in hydrothermal conditions [7][8][9][10]. In this process, gaseous CO 2 , or CO 2 captured in a basic aqueous solution as bicarbonate, is reduced using zero-valent metals [10][11][12] or organics [11,[13][14][15] as reductants, eliminating in this way the need of using hydrogen gas with all its economical and safety issues. In many of these processes water acts not only as a solvent but also as a hydrogen source, and, what it is more, the reduction performed with the hydrogen so generated is faster than the reaction using gaseous hydrogen [11,16].…”