L-Serine functionalized clays: Preparation and characterization

Abstract

l-Serine derivatives were covalently immobilized onto three clays with different nature, morphology or crystallite size: K10, which is a chemically-treated montmorillonite; laponite, which is a synthetic clay; and halloysite, which is a natural clay. To achieve that goal, the l-serine derivatives were firstly silylated with the organosilane 3-(triethylsilyl)propyl isocyanate, characterized by mass spectrometry, Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) and then covalently grafted to the clays. The resulting materials were characterized by elemental analyses (N, C and Si), FTIR and 13C solid-state NMR which confirmed the successful grafting of l-serine derivatives to the clays with increasing loading in the following order: halloysite nanoscrolls<laponite<K10. Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) were also performed for all the materials, which showed that no significant structural changes occurred in K10 and halloysite upon functionalization, but in the case of Laponite disruption of the pristine particles organization occurred after functionalization. Furthermore, 29Si solid-state NMR data provided evidence for the predominance of the bidentate grafting mechanism of the l-serines onto the clays, involving the reaction between two ethoxy groups of the l-serine based organosilanes and the clay edge/surface silanol groups of laponite and K10. For halloysite, the results suggested that the grafting involved the inner surface Al-OH groups at the internal walls or the Si-OH groups at the edges or external surface defects. It was also found that the remaining ethoxy groups of the T2-grafted organosilanes in K10 were partially hydrolyzed, resulting in free silanol groups or in lateral condensation.