Catalytic conversion of lactic acid to commodity and specialty chemicals

Vapor phase lactic acid conversion has been performed using a fixed-bed, down-flow reactor. Lactic acid conversion conducted over sodium salts on low surface area silica catalysts produces mainly 2,3-pentanedione, acetaldehyde, and acrylic acid. Formation of acrylic acid from lactic acid is optimum at 23% yield over NaOH at 350{dollar}spcirc{dollar}C and 2-3 second residence time. This yield is relatively low as a result of acetaldehyde formation which is favored at high temperature. The optimum 2,3-pentanedione yield over these sodium salts is around 25% of theoretical at 300{dollar}spcirc{dollar}C and 3-4 seconds. Post-reaction FTIR spectra of catalysts after exposure to lactic acid vapor indicate the formation of sodium lactate as the dominant species on the surface at {dollar}260{lcub}-{rcub}320spcirc{dollar}C. It is thus concluded that the anions of initial sodium salts used do not participate in the condensation to 2,3-pentanedione; hence similar yields are obtained for all salts once they are fully converted to sodium lactate. Conversion of the sodium salt to lactate is favorable when the salt used has a low melting point and a volatile conjugate acid.; The results obtained from lactic acid conversion over potassium or cesium salts and bases on silica show that the yield of 2,3-pentanedione increases with decreasing Lewis acidity of the alkali metal. Detailed reaction studies over different alkali metal loadings indicate that the rate of 2,3-pentanedione formation varies linearly with loading up to saturation at 2 mmol of catalyst per gram of support. The acidity of the silica support also reduces the yield to 2,3-pentanedione because of competitive formation of acetaldehyde through acid-catalyzed decarbonylation. As such, the best 2,3-pentanedione yield obtained over a 2 mmol CsOH/g silica catalyst at 280{dollar}spcirc{dollar}C is 60% of theoretical with 80% selectivity. A kinetic model of the reaction system is developed along with a proposed mechanism for the formation of 2,3-pentanedione from the condensation of lactic acid catalyzed by alkali lactate.; Deactivation of the catalyst toward the formation of 2,3-pentanedione is not significant over forty hours. Afterward, the selectivity toward 2,3-pentanedione decreases slowly due to coking. A similar effect is also observed with feeds containing a small amount of glucose. However, conversion with feed containing ammonium lactate gives low 2,3-pentanedione yields, primarily as a result of the reaction between ammonium hydroxide and 2,3-pentanedione to form a low volatility product.