Physicochemical characterization of organosilylated halloysite clay nanotubes

• Halloysite nanotubes (HNTs) were silylated successfully with several organosilanes.
• The silylation mechanism proceeded through reaction of alkoxy moieties from organosilane.
• Aminopropyltrimethoxysilane grafted into HNTs with the highest efficiency.
• Aminosilanes showed polymerisation side-reaction via NH2-silicon oligomerization as a parallel functionalisation pathway.

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 phenyltriethoxysilane (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 °C, bulk elemental analysis, X-ray photoelectron spectroscopy, thermogravimetry, Fourier transform infrared spectroscopy-attenuated total reflectance and 13C, 29Si and 27Al solid-state nuclear magnetic resonance.The techniques identified pristine HNTs as halloysite-7Å (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 29Si and 27Al 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 APTES, AEAPTMS and PhTES); in the case of APTES- and AEAPTMS-functionalized HNTs, a polymerization side-reaction was also evidenced, as a parallel functionalization pathway.

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