Study Of Hydrothermal Transformation Of Carbon Dioxide Into Value-added Products

The combustion of organics and fossil fuel have led to higher greenhouse gas carbon dioxide production and making global warming a pressing issue.Conversion of carbon dioxide into value-added chemicals and fuels seems to be more attractive and promising from the viewpoints of reducing carbon dioxide emission by utilizing it as a feedstock and storing renewable energy in the form of high energy density fuels.The main purpose of this research is to achieve efficient hydrothermal transformation of carbon dioxide and sodium bicarbonate into hydrocarbon?methane,ethane,and propane?and formate by water splitting using non-precious metal reductants aluminum powder and ring-pull cans,respectively.Meanwhile,the hydrothermal method could avoid the disadvantage of deactivation caused by carbon deposition as the process occurred in liquid phase.In chapter 2,methane,ethane,and propane could be obtained when using aluminum as reductant and cobalt as catalyst under hydrothermal conditions.The in situ hydrogen formed by water splitting with aluminum can react with carbon dioxide efficiently with the assistance of cobalt nanosheets to produce hydrocarbon.The highest yield of methane is ca.80%and the total yield of ethane and propane is ca.5%.The optimal condition for the highest yield is that the amount of aluminum reductant is 40 mmol?H₂/CO₂=4.5?,the pressure of carbon dioxide is 1.5 MPa,the amount of cobalt catalyst is 20 mmol,the pH is adjusted to 13.5 by sodium hydroxide,the reaction temperature is 300oC,the reaction time is 3 h,and the water filling is 50%.The pH was the most critical one of all the parameters investigated.The system was sensitive to pH and the hydrocarbon could be obtained in very narrow pH range.The hydrogen was the main product below the optimal pH while formate was gradually the main product in higher pH solution.In chapter 3,the formate was generated when using ring-pull cans as reductant and in-situ formed iron oxide as catalyst.The in situ hydrogen formed by water splitting with ring-pull cans could react with sodium bicarbonate with the assistance of in situ formed iron oxide to produce formate.The highest yield of formate is ca.13%.The optimal condition for the highest yield is that the amount of ring-pull cans is 12 mmol,the concentration of sodium bicarbonate is 2 mol/L,the reaction temperature is300oC,the reaction time is 2 h,and the water filling is 50%.In chapter 4,we investigated the possible reaction pathways of the carbon dioxide and sodium bicarbonate hydrogenation to hydrocarbon and formate,respectively.In the cobalt-catalytic hydrocarbon formation case,the bulk cobalt particles transformed into cobalt nanosheets under hydrothermal conditions and more active sites were generated.Then the carbon dioxide molecule adsorbed on the surface of nanosheets and reacted with in situ hydrogen to form carbon monoxide.The continuous hydrogenation of carbon monoxide generated methyl and methylene groups.The methyl could desorbed from the surface to form methane or reacted with methylene to generate ethane and propane.In the sodium bicarbonate hydrogenation case,the ring-pull cans were oxidized to form iron oxide which was covered by in situ hydrogen by water splitting and bicarbonate.The hydrogen attacked bicarbonate to form new C-H bond and the formate was generated with the release of hydroxyl group.Therefore,hydrothermal transformation is an effective method for carbon dioxide hydrogenation and provides a new opportunity toward carbon dioxide utilization as building block.