Studies of cyclodextrin functionalised silica materials

Abstract

Mesoporous silica materials containing microporous cavities provided by covalently bound β-cyclodextrin (CD ICS) were synthesized by co-condensation of a β-CD functionalized triethoxy silane (CD ICL) with tetraethyl orthosilicate (TEOS) by using neutral amine surfactants as structure directing agents (SDA). CD ICL was prepared by reacting β-CD with 3-isocyanatopropyltriethoxysilane. IR spectroscopy of CD ICL showed complete disappearance of isocyanato group at 2270 cm-1. 1H NMR results indicate an average of four isocyanate linkers covalently attached to random hydroxyl substituents of each molecule of β-CD. Nine different CD ICS materials were synthesized using dodecylamine, tetradecylamine or hexadecylamine with β-CD (2, 4, and 6 mol %) with respect to TEOS. The incorporation of β-CD within the mesoporous framework was supported by IR, Raman, MALDI TOF MS, solid state 13C NMR CP-MAS and TGA results. Small angle X-ray diffraction results showed a peak at 2θ ≈ 2.2⁰, supporting the presence of an ordered silica mesostructure framework. For materials with same CD loading, the surface area and pore volume doubled as the surfactant from dodecylamine to hexadecylamine. However, as the CD loading increased from 2% to 6%, the surface area decreases by a factor of ~ 1.5. MALDI TOF mass spectrometry showed two peaks at m/z 1157 a.m.u. and 1173 a.m.u. for [β-CD + Na]+ and [β-CD + K]+ respectively due to desorption of β-CD from the walls of the silica matrix. The 13C NMR CP MAS results showed 13C signals in the region δ=60-110 ppm due to the nuclei of β-CD. CD ICS materials were found to be effective as a sorbent in both gas and aqueous phases, respectively. The sorption capacity (mmol/g) of p-nitrophenol increased from 61% to 84% with an increase of CD loading from 2% to 6% and as the alkyl chain length of the SDA increases from dodecylamine to hexadecylamine. The adsorption isotherm of CH3Cl in the gas phase and that of p-nitrophenol in the aqueous phase at ambient temperature adopts a multilayer model of adsorption.