Analysis of Atomic Scale Chemical Environments of Boron in Coal by 11B Solid State NMR

Atomic scale chemical environments of boron in coal has been studied by solid state NMR spectroscopy including magic angle spinning (MAS), satellite transition magic angle spinning (STMAS), and cross-polarization magic angle spinning (CPMAS). The 11B NMR spectra can be briefly classified according to the degree of coalification. On the 11B NMR spectra of lignite, bituminous, and sub-bituminous coals (carbon content of 70−90mass%), three sites assigned to four-coordinate boron [4]B with small quadrupolar coupling constants (≤0.9 MHz) are observed. Two of the [4]B sites in downfield are considered organoboron complexes with aromatic ligands, while the other in the most upper field is considered inorganic tetragonal boron (BO4). By contrast, on the 11B NMR spectra of blind coal (carbon content >90mass%), the [4]B which substitutes tetrahedral silicon of Illite is observed as a representative species. It has been considered that the organoboron is decomposed and released from the parent phase with the advance of coal maturation, and then the released boron reacts with the inorganic phase to substitute an element of inorganic minerals. Otherwise boron contained originally in inorganic minerals might remain preserved even under the high temperature condition that is generated during coalification.