Characteristics of CO_2/H_2O green solvent systems and their functions playing in the catalytic hydrogenation
CHENG HaiYang;ZHAO FengYu;State Key Laboratory of Electroanalytical Chemistry,Changchun Institute of Applied Chemistry,Chinese Academy of Sciences;Key Laboratory of Green Chemistry and Process of Jilin Province,Changchun Institute of Applied Chemistry,Chinese Academy of Sciences;
A large amount of volatile organic solvents often be used in traditional chemical process. These solvents are flammable,explosive and toxic,and the use of these solvents often causes the pollution and the waste of resources. Therefore,the use of non-toxic solvents to replace of volatile organic solvents has become an important research issue in green chemistry. Supercritical CO2(sc CO2) and water are cheap,non-toxic and incombustible,can be used as green solvents to replace traditional toxic and volatile organic solvents in the catalytic organic synthesis reaction. sc CO2 has a characteristic feature that is tunable chem-physical properties,which is incomparable by the conventional organic solvents. For example,its viscosity,density,coefficient of mass and heat transfer,dielectric constant can be adjusted by altering pressure or temperature slightly near the supercritical point. An advantage or merit of CO2 solvent presenting in the catalytic hydrogenation is that H2 and organic substrates could dissolve into the sc CO2 to form a homogeneous phase,or the CO2 carrying H2 dissolve into the organic substrates to form an expansive phase,resulting in an increase of H2 concentration in the reaction systems. Thus,the properties of the reaction solution were changed,such as reducing kinetic viscosity,improving mass transfer and diffusion rate,thereby the hydrogenation reaction rate is improved. Moreover,the interaction between CO2,substrate and intermediates could adjust reaction pathways and so to improve the selectivity of the target product. The participant of water and/or the hydrogen bonding between H2 O with the substrate molecules could accelerate reaction rate. Therefore,for the catalytic hydrogenation reaction,the combination of CO2 and H2 O not only shows each own advantages,but also present their combining merits. Such as,the molecular interaction between CO2 or H2 O with the functional groups of reactant,higher H2 concentration in the reaction system,and the acidity of the CO2/H2 O system,etc. which will improve the reaction rate and product selectivity. Herein,we will review the characteristics of the CO2/H2 O green solvent systems and their promoting functions played in the catalytic hydrogenation. The roles of CO2 and H2 O in affecting the reaction rate and product selectivity was revealed from the characteristics of the CO2/H2 O systems and the molecular interactions among H2 O,CO2 and reactant,intermediate or product molecules.