Studies On The Adsorption And Thermal Condensation Of Glycine On Kaolinite And Montmorillonite

A series of adsorption experiments of glycine on kaolinite and montmorillonite were performed in aqueous solution at different initial concentrations and pH values (acidic/netural/basic). The samples were dried at room temperture, and then characterized by several techniques of XRD,TG/DTA,FT-IR,DRIFT and in-siu DRIFT.(1) The results of the adsorption experiments of glycine on kaolinite indicate that the weakly acidic solution favors the adsorption most, and gives the largest adsorption amount; however, the adsorption isotherm does not meet the Langmuir model. FT-IR results show that in the strongly acidic,weakly acidic and basic solutions, the glycine exists in the form of cation,zwitterion and anion, respectively. In the weakly acidic solution, the hydrogen bond interaction between -NH3+ and≡S-O- groups should be the driving force of the adsorption, the hydroxylation of≡S-O- group in strongly acidic solution and the conversion of -NH3+ to -NH2 in the basic solution are the main reason for the decrease of adsorption amount. The in-situ DRIFT results reveal that the linear dimer of glycine is formed during 110~160oC region. Up to 210 oC, the dimer undergos further intra-molecular condensation to form the cyclodimer diketopiperazine (DKP). During the reaction, no any evidence of the formation of R-CO-O-S (S=Si or Al) intermediates from the esterification reaction is found. The reaction should proceed by the self-condensation mechanism promoted by the hydrogen bond interaction between glycine and kaolinite. The presence of the kaolinite has remarkably decreased the temperature of the condensation reaction.(2) The adsorption isotherm of glycine on montmorillonite does not meet the Langmuir model. The lower pH value is, the more adsorption amount adsorbed on the support; In the strongly acidic solutions, the glycine exists in the form of cation, which can exchange with Na+ in montmorillonite, and enter into the inter-layer of montmorillonite. This should be the main reason for the high adsorption amout; in weakly acidic and basic solutions, the glycine exists in the form of zwitterion and anion respectively, and the capability of cation exchange decreases. The static repulsion between -COO- and≡S-O- groups is the main reason for the decrease of adsorption amount. At high adsorption amount, the multi-layer adsorption phenomena are observed, which leads to the precipitation and crystallization ofγ-glycine on montmorillonite. The results of TG/DTA indicate that the presence of montmorillonite could decrease the temperature of condensation reactions. The lower the adsorption amount is, the lower the reaction temperature is. The in-situ DRIFT results reveal that the cyclodimer diketopiperazine is formed at about 160oC. During the reaction, no any evidence for the formation of R-CO-O-S (S=Si or Al) intermediates from the esterification reaction and the linear dimer of glycine are found.