Physicochemical characterization of organosilylated halloysite clay nanotubes

Halloysite nanotubes (HNTs) were functionalized with several organosilanes with different functional groups, by a post-grafting methodology, in aprotic and anhydrous conditions: 3-aminopropyltriethoxysilane (APTES), N-2-aminoethyl-3-aminopropyltrimethoxysilane (AEAPTMS), (3-mercaptopropyl)trimethoxysilane (MPTMS), (3-bromopropyl)-trimethoxysilane (BrTMS), vinyltrimethoxysilane (VTMS) and phenyl-triethoxysilane (PhTES). The pristine and silylated clay minerals were characterized by transmission and scanning electron microscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffraction, nitrogen adsorption-desorption isotherms at -196 degrees C, bulk elemental analysis, X-ray photoelectron spectroscopy, thermogravimetry, Fourier transform infrared spectroscopy-attenuated total reflectance and C-13, Si-29 and Al-27 solid-state nuclear magnetic resonance. The techniques identified pristine HNTs as halloysite-7 angstrom (dehydrated form) and proved their successful silylation without the disruption of the nanotubes structure. The silylated HNTs showed bulk Si and C contents up to 7.30 and 1.92 mmol/g, respectively, with the APTES functionalized material containing the highest bulk and surface Si and C loadings, confirming its highest silylation efficiency. Some insights into the silylation reaction and mechanism were also provided by the techniques used. Combination of Si-29 and Al-27 MAS NMR and XRD data suggested that silylation reaction occurred at Al-OH groups from the inner lumen surface, as well as the Al-OH and Si-OH groups at the edges or external surface defects; no evidence was found for the existence of functionalization in the interlayer Al-OH groups. The silylation mechanism was found to proceed through reaction of the alkoxy moieties from the organosilane with the referred surface groups from the HNTs in a 3-fold (for VTMS and BrTMS), or 2-fold covalent grafting (for MPTMS) or a mixture of both approaches (for AN ES, AEAPTMS and PhTES); in the case of APTES- and AEAPTMS-functionalized HNTs, a polymerization side-reaction was also evidenced, as a parallel functionalization pathway. (C) 2015 Elsevier Inc. All rights reserved.