A AL-27 MAS, MQMAS AND OFF-RESONANCE NUTATION NMR-STUDY OF ALUMINUM-CONTAINING SILICA-BASED SOL-GEL MATERIALS

Aluminium containing hybrid materials were prepared via the sol-gel me thod using aluminium sec-butoxide complexed with ethylacetoacetate (Al (OBus)(2)EAA or Al(OBus)(3)/EAA mixtures). As silanes, phenyltrimethox ysilane (PhTMS) or phenyltriethoxysilane (PhTES), 3-glycidoxypropyl tr imethoxysilane (Glymo) and tetraethylorthosilicate (TEOS) were used. A fter room temperature drying of the samples the Al-27 single pulse exc itation (SPE) magic angle spinning (MAS) NMR shows that octahedral (5 ppm) and tetrahedral (55 ppm) coordinated aluminium species are presen t in the materials. The relative amount of these two species depends o n the preparation method. However, the Al(IV)/Al(VI) ratio is lower th an 3 (typically 2.3) in all materials, indicating the presence of a sm all 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 p pm (peak maximum measured at 11.7 T). Based on the resonance frequency this signal is generally assigned to a pentahedrally coordinated alum inium species. Hydration/dehydration processes of annealed samples wer e studied with Al-27 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 s pecies in a distorted environment, experiencing large quadrupole induc ed shifts and small chemical shifts due to conformational changes in t he polymeric network, From the MQMAS NMR spectra it can be concluded t hat the linebroadening observed in the Al-27 MAS NMR spectra is due to both a distribution in isotropic chemical shifts and a distribution i n quadrupole coupling constants (C-qcc = e(2)qQ/h). Hydration of the s ample results in a decrease of the average C-qcc for the tetrahedrally coordinated aluminium from 6 to 4 MHz, whereas the average C-qcc of t he octahedrally coordinated aluminium is hardly influenced (4 MHz). Th ese MQMAS results are confirmed by off-resonance nutation experiments. (C) 1997 Elsevier Science B.V.