New inserts and low temperature—Two strategies to overcome the bottleneck in MAS NMR on wet gels

Abstract

MAS NMR experiments on wet and dry aluminium alkoxide fluoride gels are presented here for the first time. For the MAS studies on wet jelly like gels special inserts were developed which fit perfectly a 4 mm rotor and allow rotation frequencies up to 12 kHz. Six different insert materials were tested concerning their suitability including different organic polymers ((poly(methyl methacrylate) (PMMA), poly(ether ether ketone) (PEEK), poly(vinylchloride) (PVC) and poly(tetrafluorethylene) (PTFE)), glass as well as pure quartz (SiO 2). Alternatively, low-temperature MAS NMR experiments taken at 150 K with the wet gel frozen directly in the rotor led to comparable results. 27Al, 19F, 13C, 1H were used as nuclear spin probes to identify local structures in the wet as well as in the dry xerogels. Both wet and dry gels consist of a network structure of AlF 6− x O iPr x species ( x: 0–3). The main structural units of the dried gel are already preformed in the jelly like gel and change only little at aging and drying processes.