A 27Al MAS, MQMAS and off-resonance nutation NMR study of aluminium containing silica-based sol-gel materials.

Aluminium containing hybrid materials were prepared via the sol-gel method using aluminium sec-butoxide complexed with ethylacetoacetate (Al(OBus)2EAA or Al(OBus)3/EAA mixtures). As silanes, phenyltrimethoxysilane (PhTMS) or phenyltriethoxysilane (PhTES), 3-glycidoxypropyl trimethoxysilane (Glymo) and tetraethylorthosilicate (TEOS) were used. After room temperature drying of the samples the 27Al single pulse excitation (SPE) magic angle spinning (MAS) NMR shows that octahedral (5 ppm) and tetrahedral (55 ppm) coordinated aluminium species are present in the materials. The relative amount of these two species depends on the preparation method. However, the Al(IV)/Al(VI) ratio is lower than 3 (typically 2.3) in all materials, indicating the presence of a small amount of an aluminate phase. Annealing of the samples at 100, 150 and 200 degrees C results in the formation of an extra signal at 30 ppm (peak maximum measured at 11.7 T). Based on the resonance frequency this signal is generally assigned to a pentahedrally coordinated aluminium species. Hydration/dehydration processes of annealed samples were studied with 27Al SPE MAS NMR, multiple-quantum MAS NMR (MQMAS) and off-resonance nutation NMR. Upon hydration of the annealed sample the signal intensity around 30 ppm decreases in intensity and at the same time the intensity of the signal around 55 ppm increases by the same amount (tetrahedrally coordinated aluminium). The MQMAS spectra reveal that the signal around 30 ppm is not caused by a fivefold-coordinated aluminium species but mainly by tetrahedrally coordinated aluminium species in a distorted environment, experiencing large quadrupole induced shifts and small chemical shifts due to conformational changes in the polymeric network. From the MQMAS NMR spectra it can be concluded that the linebroadening observed in the 27Al MAS NMR spectra is due to both a distribution in isotropic chemical shifts and a distribution in quadrupole coupling constants (Cqcc = e2qQ/h). Hydration of the sample results in a decrease of the average Cqcc for the tetrahedrally coordinated aluminium from 6 to 4 MHz, whereas the average Cqcc of the octahedrally coordinated aluminium is hardly influenced (4 MHz). These MQMAS results are confirmed by off-resonance nutation experiments.