Adsotption And Thermal Condensation Mechanisms Of Glutamic Acid And Alanine On Al₂O₃

A series of adsorption experiments of glutamic acid and alanine on alumina were performed in aqueous solution at different initial concentrations and pH values (acidic/netural/basic). The samples were dried at room emperture, and then characterized by several techniques of XRD,TG/DTA,DRIFT and in-situ DRIFT.(1) The results of the adsorption experiment indicate that the adsorption performances are prominently influenced by the equilibrium concentration and pH value of glutamic acid solution. When the equilibrium concentration is below 0.388 mol/L, the adsorption isotherm meets Langmuir model; at higher equilibrium concentration (>0.388 mol/L), the multi-layer adsorption phenomena are observed, which leads to the precipitation and crystallization ofβ-glutamic acid on alumina surface. The lower the pH value is, the more glutamic acid adsorbed on the support. The adsorbed glutamic acid molecule is bidentately binding to the Lewis-acid site on alumina surface throughα-carboxyl group,making its symmetric stretching band shifting from 1376 cm-1 to 1348 cm-1 . At about 200℃, the cyclic anhydride bond (N-H stretching band at 3317 cm-1) is formed via the intra-molecular condensation reaction and the main product is pyroglutamic acid, which bidentately binds to the surface of Al2O3 to form aluminium pyroglutamate species with high thermal stability. The formation of"surface acyls", previously proposed for related systems, has not been evidenced here.(2) The adsorption isotherm of alanine on alumina meets Langmuir model. The lower pH value is, the more alanine adsorbed on the support. Alumina could decrease the temperature of condensation reactions. It is more obvious when the alanine aqueous solution is lower. Alanine is specifically adsorbed on the surface of alumina, probably with moiety ofα-COO- to the surface through H-bonding interaction,no shift of–NH3+ absorption peaks is observed in the DRIFT spectra. Thermal treatments induce cyclodimerization of the adsorbed alanine molecules from 100℃～200℃. No Ala-Ala product is detected.