Surface structure and thermodynamics of coals

Abstract

Our work has been the determination of the average micropore diameter of an Argonne Illinois No. 6 coal and a spherocarb sample by {sup 129}Xe NMR. {sup 129}Xe NMR spectroscopy has been used to determine pore sizes and surface characteristics in zeolites. The chemical shift of {sup 129}Xe was measured as a function of pressure and the average micropore diameter was calculated using established techniques. For the spherocarb, the calculated pore diameter of 15.4 CA agrees well with the 0 manufacturer's reported value of 15{Angstrom}. The Illinois No. 6 coal gave two {sup 129}Xe peaks. Extrapolation of the pressure dependent peak gave a micropore diameter of 5.2{Angstrom}. The other peak was not pressure dependent and is tentatively assigned to Xe dissolved in the coal. The NMR chemical shift of {sup 129}Xe is a useful probe of micropore (<20 {Angstrom} diameter) size and surface characteristics. This application was developed principally by Fraissard and coworkers to determine pore size distributions and the degree of crystallinity in zeolites. The observed {sup 129}Xe NMR chemical shift can be attributed to 4 factors: (1) the number of Xe-Xe collisions; (2) the electric field established by neighboring ions' in the solid sample, found to be negligiblemore » for monovalent cations in zeolites; (3) the number of Xe-wall collisions; and (4) the nature of the interaction of Xe with the pore wall. The chemical shift due to Xe-wall collisions can be determined by extrapolation of {sup 129}Xe NMR resonances to 0 Xe pressure.« less