Immobilization of l-alanine into natural kaolinite via amidation catalyzed by boric acid for the development of biohybrid materials

The natural amino acid l-alanine was immobilized on the interlayer space and surface of the natural Brazilian São Simão’s kaolinite using two different routes. In the first 3-aminopropyltriethoxysilane was previously grafted into kaolinite interlayer space resulting into aminofunctionlized kaolinite that was further employed as reactive sites on kaolinite surfaces to bond l-alanine via amidation reaction. In the second route 3-aminopropyltriethoxysilane was first modified by l-alanine via amidation reaction catalyzed by boric acid and the modified alkoxide grafted into kaolinite using kaolinite-dimethylsulfoxide resulting on the biohybrid. In both strategies the amino acid was covalently bounded using the carboxylic acid from the amino acids and amine groups present on the kaolinite surfaces resulting in amide bonds. In both routes, the reaction was catalyzed by boric acid. Results confirms that the nature and loading of immobilized l-alanine changes depending of the route employed. X-ray diffraction confirmed the expansion of the interlayer spaces of the grafted solids resulting in 8.3 ​Å and 10.8 ​Å respectively for grafted materials routes one and two respectively. Reaction effectivity was kinetically evaluated by infrared vibrational spectroscopy that confirms the presence of amide bonds in both products due to the bands at 1560 ​cm−1, due to carbonyl (N–CO) stretching, and 1490 ​cm−1, ascribed to N–H plane vibrations and C–N stretching. The typical vibration of inner surface aluminol groups at 938 ​cm−1 (for purified kaolinite) is absent for both routes employed after grafting reactions confirming the attachment of the modified alkoxide into aluminol groups via covalent bonds resulting in bonds type Al–O–Si. Thermal analysis showed the typical reduction of dehydroxylation temperature from 510 ​°C on pure kaolinite to 480 and 450 ​°C, respectively, on the biohybrid materials and confirmed the functionalization in both routes, the amount of organic content was quantified and was 0.193 and 0.298 ​mol of organic unit per 1 ​mol of kaolinite minimal formulae using route of previous grafting of kaolinite with amine groups and direct grafting of modified alkoxide into kaolinite dimethyl sulfoxide respectively. Specific surface area analysis by N2 adsorption-desorption isotherms (BET method), cationic exchange capacity, and total specific surface area by the methylene blue method also confirmed the reduction of specific surface area from kaolinite grafted der