Grafting of L-proline and L-phenylalanine amino acids on kaolinite through synthesis catalyzed by boric acid

Abstract

The amidation reaction catalyzed by boric acid between the amino acids proline and phenylalanine and the alkoxide 3-aminopropyltriethoxysilane was studied. Afterwards, the modified alkoxide was used to functionalize the clay mineral kaolinite. The hybrid material was characterized by powder X-ray diffraction, molecular absorption spectroscopy in the infrared region, thermal analysis and scanning electron microscopy, to understand the influence of the reaction time on the kaolinite intercalation process, in order to improve this functionalization route for both amino acids. The reaction times evaluated varied from 0 to 24 h, keeping the reflux temperature fixed at 180 °C, being then reacted with kaolinite previously intercalated with dimethyl sulfoxide (DMSO). After 24 h of reaction the modified alkoxide interacted effectively, replacing the DMSO molecules and resulting in the formation of covalent bonds between the modified alkoxide and the clay. The material showed greater dispersion of the organic phase along the layered matrix, and significantly increased its thermal stability, with maximum loss at 320 °C. The sample containing phenylalanine was more agglomerated than that containing proline, with 2.4 times in mol number. In all cases, the crystallinity of the material decreased by the interaction of the modified alkoxide with the kaolinite matrix, the modified alkoxide being partially outside the interlayer space because of its large molecular size.