CO_2/H_2O绿色溶剂体系的特性及对催化加氢反应的作用

Characteristics of CO_2/H_2O green solvent systems and their functions playing in the catalytic hydrogenation

H2O与超临界CO2(scCO2)作为绿色溶剂取代传统挥发性有机溶剂在催化反应中的应用研究取得了较大的进展.本文主要介绍CO2/H2O绿色溶剂体系的特性及其在催化加氢反应中的作用,从体系的特性及H2O分子、CO2分子与反应物分子、中间产物、产物分子间的相互作用,阐述CO2/H2O体系对催化加氢反应速率、产物选择性的影响及促进作用.

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 （scCO2） 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, scCO2 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 scCO2 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 H2O with the substrate molecules could accelerate reaction rate. Therefore, for the catalytic hydrogenation reaction, the combination of CO2 and H2O not only shows each own advantages, but also present their combining merits. Such as, the molecular interaction between CO2 or H2O with the functional groups of reactant, higher H2 concentration in the reaction system, and the acidity of the CO2/H2O system, etc. which will improve the reaction rate and product selectivity. Herein, we will review the characteristics of the CO2/H2O green solvent systems and their promoting functions played in the catalytic hydrogenation. The roles of CO2 and H2O in affecting the reaction rate and product selectivity was revealed from the characteristics of the CO2/H2O systems and the molecular interactions among H2O, CO2 and reactant, intermediate or product molecules.