Abstract: High resolution solid state NMR techniques (such as MAS, CP/MAS and CRAMPS) were employed to study the nature of organic molecules adsorbed on porous solids.
A magic angle spinning system was achieved for sealed samples with a spinning speed from 2KHz to 4.2KHz. Using this technique, high resolution ¹H MAS spectra of organic molecules and H₂S adsorbed on charcoal were obtained. Experimental results suggest that for high coverage of adsorbed organic molecules, the spectral lines were resolved very well. But for low coverage, the spectral lines could not be separated completely. As the organic molecules condensed in pores (liquid-like phase) dominate for high coverage, the spectra could be easily resolved under lower spinning speed rate. While for low coverage, the portion of molecules adsorbed on solid surface (solid-like phase) is relatively larger and the resonance lines could not be narrowed even under higher spinning speed rate due to the increase in various kinds of interactions of adsorbed molecules. We could not obtain further narrowed spectrum by CRAMPS technique. The contribution to the residual spectral width was discussed.
A modified Goldman-shen pulse sequence was provided to measure the ¹H spin diffusion times of toluene (MB) and tetrahydrofuran (THF) adsorbed on a series of porous solids (charcoal, SiO₂ and Al₂O₃). In the experiment, the two peaks of THF and MB were selectively reversed to produce effective spin diffusion. The spin diffusion times of MB and THF adsorbed on different types of solids vary in a wide range (from several milliseconds to several hundreds of milliseconds). This implies that the motions of molecules adsorbed on different solids experience different degrees of hindrance owing to their molecular structures and adsorbed states. For THF and MB adsorbed on Al₂O₃,biexponential behavior was observed in the recovery processes of the negative peaks. This was attributed to the existence of two different kinds of spin diffusion processes. One was the intramolecular spin diffusion of adsorbed molecules, the other was the intermolecular one between molecules adsorbed on solid surface and acidic groups of Al₂O₃. It is owing to the adsorbed state the bi-exponential character was obvious in the cases of THF and MB adsorbed on Al₂O₃.It is possible that the oxygen atom of THF and aromatic ring of MB are closely connected to Lewis acid sites, the neighbour acidic groups of Bronsted sites (or a small tiny amount of coadsorbed water molecules) will affect the protons of those molecules adsorbed on surface and enhance their spin diffusion. In addition, the intermolecular ¹H spin diffusion between charcoal and adsorbed THF molecules was confirmed by the indirect measurement proposed by Tekely et al.
The ¹³C-¹³C spin diffusion between enriched methanol (30%) and charcoal was investigated by ¹³C CP/MAS combining with selective excitation methods. The experimental results and ¹³C T₁ measurement show that there is no obvious ¹³C-¹³C spin diffusion and the relaxation of methanol carbon appears to be bi-exponential recovery which was assigned to two different kinds of adsorbedstates.
DNP (Dynamic Nuclear Polarization) was used to study CO (natural abundance) adsorbed on Co-Mo/Al₂O₃ catalysts. After only several hundreds of accumulation, the enhanced ¹³C signal was obtained. But the experiment could not be reproduced. The experimental details were discussed and some suggestion for further investigation was provided.

Key words: adsorbed, spinning, coverage, SOLID, sealed, separated, solids, magic, condensed, resolved