Abstract
For geochemical purposes, complex aluminum compounds in coal organic matter in different types of coal were identified by solid state nuclear magnetic resonance measurements of 27Al. Low ash samples of anthracites, bituminous coals and altered coals from the Czech Republic, Russia, Ukraine, China and Australia were tested; further, low ash lignite and xylite from the Czech Republic and gagatite from Poland were analyzed. In acquired 27Al MAS NMR spectra, two significant peaks at chemical shifts were recorded, at 3.5 - 4 and 13.5 - 15 ppm. It was found that the significant peak at chemical shift at 3.5 - 4 ppm in spectra of bituminous coals, lignite, gagatite and a thermally weakly altered coal corresponds to that obtained for triaquo-hydroxo-diphenoxido-Aluminum(III) complex. The existence of triaquo-triphenoxido-Aluminum(III) complex in the spec- tra of anthracites, some bituminous coals and another thermally altered coal can be approved by the chemical shift at 13.5 - 15 ppm. These findings indicate that at least two different Al complexes were identified in coal organic matter. Further it was found that these complexes are concentrated in vitrinite fraction (alicyclic-aromatic part of coal), notably in collotelinite (gelified and homogenous vitrinite constituent). Ways of Al complexes formation in coal are suggested and their thermal stability is discussed.
Highlights
Complexes in coal are among the least investigated compounds, despite the fact that their study could lead to the discovery of entirely new complex compounds and to an increase in knowledge about coal composition, espe-How to cite this paper: Straka, P. (2016) Characterization of Aluminum(III) Complexes in Coal Organic Matter
These findings prove that at least one, but probably two Al complexes with different properties were present in the coal organic matter of the samples; it is evident that petrographically characterized coal fractions can be leached and further analyzed with the 27Al MAS NMR method
In the case of anthracites, some bituminous coals, and lignite from Northern Bohemian Basin, the significant peak at chemical shift at 13.5 - 15 ppm corresponding to triaquo-triphenoxido-Aluminum(III) or an Al-catechin complex was observed
Summary
Complexes in coal are among the least investigated compounds, despite the fact that their study could lead to the discovery of entirely new complex compounds and to an increase in knowledge about coal composition, espe-How to cite this paper: Straka, P. (2016) Characterization of Aluminum(III) Complexes in Coal Organic Matter. No examples of the occurrence of Al complexes were discussed, mellite, present in coal, and its likely association with Al was mentioned This association was further investigated; according to Klika and Kolomazník [11], Al is one of the elements with significant affinity for organic matter in coal. The mechanisms of bonding and the linkages between salicylate anions and Al3+ octahedra at illite clay edges were investigated using Fourier transform infrared spectroscopy; molecular orbital calculations were performed, too. On this basis, the authors suggested that covalent Al-O-C linkages form between salicylate anions and Al3+ octahedra. Thompson and Botto [3] conducted MAS NMR experiments on coal and concluded that correlations of tetrahedral Al content with coal rank arise assuming that contribution from organo-chelated Al species is taken into account
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