Advantages of chemical reactions in supercritical fluids
Supercritical fluids have high diffusivity and low viscosity similar to gases, but also have similar dissolution capabilities as liquids.For example, certain compounds that are difficult to dissolve or insoluble at normal temperature and pressure can be dissolved in supercritical fluids, and, With the change of temperature and pressure, the solvation ability of supercritical fluid changes accordingly. The above characteristics provide feasibility for the application of supercritical fluid in chemical reactions3.The chemical reactions in supercritical fluids can be divided into two main categories, one is the use of supercritical fluids as solvents, and the other is that supercritical fluids not only serve as solvents, but also participate in chemical reactions themselves. In general, the application of supercritical fluids in chemical reactions has many advantages
(1) Strengthen the delivery process
In the supercritical state, the fluid has better diffusion properties and lower viscosity than the liquid. For the gas-liquid two-phase reaction controlled by the transfer process, the reaction in supercritical fluid can effectively improve the reaction rate. Compared with other gases, supercritical fluids have a higher heat capacity. Therefore, when chemical reactions are carried out in supercritical fluids, heat transfer can be enhanced and the reaction heat can be transferred in time. Therefore, it is advantageous to carry out the reaction in a supercritical fluid reaction system for a reaction process that is sensitive to temperature changes
(2) Regulate reaction selectivity
Since the solubility of the substance is related to the density of the supercritical fluid, the solubility of the supercritical fluid to the substance can be changed by adjusting the density of the system. Therefore, for some equilibrium reactions, the temperature and pressure of the system can be adjusted to change the product in the reaction system. Solubility, so that the product precipitates in time, prompting the reaction to continue in the positive direction, effectively improving the conversion rate and selectivity
(3) Process enhancement
Compared with traditional solvents, the chemical reaction in supercritical fluids has a fast reaction rate, high selectivity, and the reaction products in supercritical fluids can be separated from the reaction system in time. Therefore, the reaction system can be applied to the continuous reaction process in. Moreover, compared with the traditional continuous reaction system, the supercritical fluid reaction system can not only provide a safe and mild reaction environment for the reaction, but also effectively improve the conversion rate and selectivity in the reversible reaction system.
In recent years, people have conducted research on chemical reactions in supercritical fluids in many fields. For example, selective oxidation in supercritical carbon dioxide, selective reduction of nitro compounds, carbonylation, hydrogenation, polymerization, etc.