An e.p.r. study of pore accessibility in high volatile bituminous coal: solvent dependence

Abstract

Pore size and functional group distribution in high volatile bituminous coals were studied as a function of swelling and washing solvents using an e.p.r. spin-probe method. Spin probes with different shapes and sizes (I–IX) were selected. The influence of swelling solvents (such as toluene and pyridine) and washing solvents (such as ethanol and cyclohexane) on the pore structure of studied coals is discussed. Ethanol used as a washing solvent swells coal and makes the long shaped pores more accessible for cylindrical spin probes. On the other hand, the concentration of spherical and long chain spin probes as well as basic and acidic sites decreases one order of magnitude compared with the concentrations observed when the non-swelling cyclohexane solvent was used. Swelling properties of pyridine for Pittsburgh No. 8 coal obtained by e.p.r. spin-probe methods are in agreement with previously reported results obtained using other methods. A variable temperature study indicates that small spherical spin probes VI–IX trapped in the pores undergo librational motion (average E a = 6.0 ± 1 kJ mol −1) completely masking the hydrogen bonding effects detected by bulky long chain spin probes IV and V (average E a = 14.8 kJ mol −1).