Solid-state NMR studies of absorption onto activated carbon

Abstract

This is a study of adsorption on three activated carbon substrates using solid- state NMR. The adsorbates used as probe molecules included a range of phosphates, phosphonates and deuterated water. High-resolution (^31)p and (^2)H NMR spectra have been obtained using magic-angle spinning and a single-pulse regime. The deuterium results include the generation of an adsorption isotherm. The traditional gravimetric analysis and NMR experiments were run concurrently. These results showed that the NMR technique was qualitatively and quantitatively accurate, while the proven adsorption isotherm theory could be applied to the NMR results. The additional information given by the (^2)H NMR results showed evidence of two distinct adsorption sites. Initial adsorption in the micropores gives a peak shifted by 6 ppm to low frequency of the liquid (^2)H(_2)O. This peak was broadened due to restricted motion in the micropores. The second peak was observed only at high relative humidities and was attributed to adsorption on the external surface or in macropores. The chemical shift was similar to that of the pure liquid. The (^31)p NMR results were used to directly observe the adsorption of phosphates with a range of molecular sizes. The NMR data were used to calculate the micropore accessibility for each phosphate. Differences in the adsorption mechanism were recorded, and direct comparison for each carbon gave some structural information. It was possible to follow competition reactions over time periods of 1 min to 24 hours. A battlefield simulation was studied, with (^2)H(_2)O and a phosphate competing for the adsorption sites. The NMR results showed that the phosphate was preferentially adsorbed into the micropores, displacing the (^2)H(_2)O However, the addition of (^2)H(_2)O to a carbon saturated with a phosphate enabled more phosphate to be adsorbed into the micropores via a cooperative mechanism. Measurements of the transverse relaxation for adsorbed molecules suggest that the broad micropore signal consists of some overlapping peaks. The peaks width similar chemical shift are attributed to adsorption in pores with differing dimensions. The natural linewidth involves broadening caused by restricted anisotropic motion within the micropores.