ESR of Radical Trapped in Coal

Abstract

Spin probe technique has been applied to study pore structure and functional groups of coal through the interaction between a nitroxide radical and the coal surface. A hexane solution of 2, 2, 6, 6-tetramethyl-4-piperidinol-1-oxyl (TEMPOL) radical was added to Morewell (MO), Wandoan (WA), Liddell (LI) coals and MO's heat-treated at 200, 300, 400°C. The TEMPOL radical trapped in the coal gave an anisotropic triplet ESR signal at room temperature because of hindered rotation of the nitroxide molecule. However, the anisotropic pattern was averaged out at high temperatures as a result of the liberated motion. The extent of the interaction between the radical and the coal surface was evaluated from the temperature range at which the anisotropic spectrum was converted to isotropic one.
The following results were obtained:
(1) The molecular motion of the TEMPOL radical in WA was hindered up to higher temperatures. The result is considered to be associated with diffusion of the radical into small pores.
(2) LI coal contains surface functional groups which own different interaction with the radical doped. Therefore, the spectral shift occurred at the wide temperature range.
(3) The water molecules occluded in raw MO coal brought about the constrained motion to high temperatures, presumably because of hydrogen bonding. When the radical was doped to heat-treated MO coals the transition range from anisotropic to isotropic pattern was shifted to low temperature and also became wider. It suggests that the heterogeneous sites with various functionalities emerged owing to the heat-treatment of MO.