Phosphorus-31 NMR studies of adsorption onto activated carbon

Abstract

Magic-angle spinning 31P NMR spectra have been obtained for trimethylphosphate, triethylphosphate and dimethylmethyl phosphonate on three activated carbons for a variety of loading conditions up to 60 w/w%. At low loadings a broad resonance band is seen at ca. 8 ppm to low frequency of the signal for the neat liquid adsorbate. This is ascribed to overlap of bands from adsorbate molecules in different micropores, with proximity of the adsorbate to aromatic carbon rings. At high loadings a second band appears, close to the resonance for neat liquid adsorbate. It may be assigned to molecules weakly adsorbed at surfaces and particle interfaces relatively remote from aromatic molecular fragments. Variations with the nature of the adsorbate, of the carbon type and of loading are discussed. Quantitative data are presented. A significant process of slow adsorbate transport was found, indicating some subtle re-arrangement of the adsorbate–adsorbent system with time.