| Reaction at supercritical conditions is conducted in the presence of a supercritical fluid that plays a role of a solvent or reactant. Because of the unique physical and chemical properties of supercritical fluids, a reaction at supercritical conditions always exhibits different thermodynamic and kinetic features. Some reactions even proceed well at supercritical conditions without the addition of catalysts that are essential under ambient conditions. In this dissertation, supercritical technology was applied to the traditional aldol reactions. The synthesis of isophorone (IP) via self-condensation of acetone and the synthesis of 4-hydroxy-2-butanone (HB) via cross aldol condensation of formaldehyde and acetone were studied at supercritical conditions. The influences of temperature, catalyst amount and residence time were discussed respectively, and the corresponding mechanisms and kinetic models were proposed.For the synthesis of IP via self-condensation of acetone catalyzed by aqueous KOH at supercritical conditions, the product mainly consisted of 14 compounds including acetone’s dimers, trimers and overcondensates (tetramers and pentamers), among which structures of 4 kinds of overcondensates were first identified. Acetone itself was weak acidic and could underwent acid-catalyzed self-condensation to generate mesityl oxide, diacetone and isomesityl oxide at 285℃ and 25 MPa. When aqueous KOH was added, the reaction was catalyzed by KOH and the amount of IP increased significantly. Experiments were conducted at different temperatures (250℃,265℃,275℃ and 285℃) at 25 MPa, and the result showed that IP was more selective than reversible and irreversible overcondensates respectively at higher temperatures. The experimental data was fitted with a reasonably simplified kinetic model. Most deviations between experimental and calculated data were less than 10% and the R2 was 0.9993. The fitting result showed that the apparent activation energy of reverse reaction was greater than the one of forward reaction for most aldol reactions. Thus, raising the temperature favored reversion of aldol reactions, which explained the decrease of reversible overcondensates at higher temperatures.For the synthesis of HB from formaldehyde and acetone at supercritical conditions without addition of any catalyst, the liquid-phase product mainly consisted of HB, methyl vinyl ketone, methanol, formic acid, diacetone and mesityl oxide, among which formic acid contributed most to reaction mixture’s acidity, and the gas-phase product mainly consisted of CO, H2, CO2 and CH4. Experiments proved that formaldehyde underwent Cannizzaro reaction under hydrothermal conditions producing formic acid which subsequently underwent decarboxylation, dehydration and hydride transfer reaction producing methanol, CO, H2 and CO2. As the synthesis of HB proceeded, the mass fraction of formic acid increased sharply to the order of 10-4 and then decreased slowly. The experiment with additional formic acid in feed proved that formic acid played a role of catalyst in the formation and dehydration of HB. Experiments were conducted at different temperatures (230℃,240℃,250℃,260℃ and 270℃) at 17 MPa to access the influence of temperature. With a mechanism-based detailed chemical kinetics model, the rate constants at different temperatures were fitted. The activation energies for forming and dehydration of 4-hydroxy-2-butanone were 97.5±3.5 kJ/mol and 120.6±5.8 kJ/mol, respectively. The maximum yield of HB reported in this dissertation exceeded 90% at appropriate reaction temperatures. The problem of low HB yield (generally<78%) and alkalic waste in traditional base-catalyzed HB production was solved. |
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