Oxidation in supercritical carbon dioxide
The oxidation reaction in the supercritical carbon dioxide system is generally a free radical reaction. When free radicals are dispersed in the supercritical fluid, the presence of the supercritical fluid in the reaction process may have a certain impact on the reaction path, which in turn affects the oxidation products. Selective.
Using supercritical carbon dioxide fluid as a solvent for oxidation reaction has the following advantages
① Carbon dioxide is the tail gas of industrial production, with large output and low price.
② Carbon dioxide is a non-toxic and harmless gas, and is environmentally friendly, in line with the development of modern chemistryneed.
③ Carbon dioxide is the ultimate oxidation product of carbon, and its chemical nature is stable.Participate in chemical reactions, thereby reducing side reactions caused by the oxidation of solvents
④ For the gas-liquid two-phase reaction, due to the special dissolution properties of supercritical carbon dioxide, it can be eliminatedThe gas-liquid interface makes the reaction system homogeneous, which is helpful to improve the conversion rate of the reaction. The supercritical carbon dioxide fluid has the properties of high diffusivity and low viscosity, which can effectively reduce the mass transfer resistance, especially in the heterogeneous catalytic reaction process, which can enhance the transfer process of reactants and products in the pores of the solid catalyst. Due to the special solubility properties of supercritical carbon dioxide, some products can also be extracted directly from the catalyst surface.
⑤ Carbon dioxide has a high thermal conductivity, which makes it act as a protective gas in the oxidation reaction, and can also transfer heat in a timely and efficient manner, thereby ensuring the safe progress of the reaction process
⑥ The state of existence of carbon dioxide mainly depends on the pressure.The product is separated from the solvent, and carbon dioxide can be reused after recompression.
⑦By adjusting parameters such as temperature and pressure, some physical properties of the system can be changed, and then the existence state of carbon dioxide can be adjusted, and finally the control of the reaction process can be realized. Studies have shown that the chemical reactions carried out in supercritical carbon dioxide are related to the state of existence of the system. In different states, the selectivity and conversion rate of chemical reactions will be quite different, so the study of the state of supercritical carbon dioxide reactant system has become the focus of research.
The supercritical carbon dioxide oxidation reaction system is generally composed of three phases of carbon dioxide, oxygen and organic reactants. Palakodaty et al. investigated the influence of phase behavior on cyclohexane oxidation reaction, selected nitrogen with similar properties instead of oxygen, drew the phase diagram of the three-phase system composed of nitrogen, cyclohexane and carbon dioxide, and the state of existence in the three-phase system sort
①Supercritical homogeneous system with high carbon dioxide content;
②Supercritical mixture system;
③ Gas-liquid two-phase system mainly based on gas phase;
The study found that the general supercritical carbon dioxide chemical reaction system is a homogeneous system formed by the mutual dissolution of reactants and carbon dioxide. In this system, carbon dioxide as a solvent will play a certain role in dilution, and as the pressure of carbon dioxide in the reaction system increases, the reaction conversion rate and target product yield will be significantly reduced.
Banker et al. applied spectroscopic techniques to study the state and phase change of the catalyst during the reaction in supercritical CO2. The study found that the active component Pd loaded on ALO3 mainly exists in the reduced state, and the catalytic activity of the active component during the reaction is related to the concentration of oxygen. When the oxygen concentration is low, the surface of the active metal component of the catalyst mainly exists in the reduced state; as the oxygen concentration increases, the metal surface will adsorb active oxygen, so that the oxidation reaction can proceed on the metal surface; continue to increase the oxygen concentration When, it will make the active metal component react with oxygen and form an oxide layer on the metal surface, making the catalyst inactive
Through the above research, it is found that the catalytic oxidation reaction mechanism in supercritical carbon dioxide system is similar to that in traditional solvents. Some special properties of supercritical carbon dioxide can be used to improve the defects such as low mass transfer efficiency or slow diffusion in the traditional catalytic reaction process, and improve the reaction conversion rate and target product selectivity.