Maia Stefanova

Low rank coals sulphur functionality study by AP-TPR/TPO coupled with MS and potentiometric detection and by XPS

Abstract Atmospheric pressure-temperature programmed reduction (AP-TPR) and X-ray photoelectron spectroscopy (XPS) techniques were applied to low rank coals sulphur study. Coal samples were pyrolysed in a flow of water vapor (WV). It was demonstrated that this treatment influenced mainly aliphatic sulphur. Samples were characterised by two methods and data were interpreted within the limits of the techniques. XPS measurements registered sulphur 2p spectra with two main signals for organic and inorganic sulphur compounds. The AP-TPR set-up, with potentiometric detection of the formed H2S as S2− using an ion selective Ag2S-electrode, gives quantitative data about the presence of different sulphur species. The AP-TPR equipment on-line coupled with a mass spectrometer (MS) gives extra qualitative information about different reductive and oxidative organic sulphur forms. Using MS not only H2S but also SO2, COS, CS2, and all other volatile sulphur and organic compounds can be monitored, giving more information for the initial presence of the different sulphur forms and to the mechanisms involved in the pyrolytic process. This AP-TPR-MS experiment is subsequently followed by AP-TPO-MS measurement (in an oxidated atmosphere) to study sulphur presence in the residue (tar and char) in the reactor. Comparing all these AP-TPR profiles results in a better assignment of the different signals to specific sulphur functionalities.

Structural study of coals by means of directly-prepared potassium-coal adducts☆

Abstract Directly-prepared potassium adducts (PA) of different rank coals and of an anthracite are studied by X-ray diffraction and e.s.r.-spectroscopy. The reactivity of the PAs towards butyl iodide and methanol are also examined. Complete disruption of the lamellar coal structure (i.e. disappearance of the 002 line in the X-ray diffraction patterns) is observed after reaction with potassium. E.s.r.-spectroscopic parameters are not substantially changed by the potassium except for coals with C, 85–90 wt%, for which the concentration of paramagnetic centres is significantly increased. These coals are also capable of significant butylation and reduction (thus becoming soluble). A correlation between the number of unpaired electrons and the number of alkyl groups introduced is established. Anthracite is not capable of butylation or reduction in this manner. With all coals (including anthracite) the layer stacking indicated by the 002 diffraction line is partially recovered after the treatment of the PA with butyl iodide or alcohol. Also, the diffraction intensities in the γ-band region are increased depending on the degree of the butylation or reduction occurred. The results are interpreted with respect to the similarity of PA of different rank coals with the intercalation compounds of graphite and with the ionic adducts of polycyclic aromatic hydrocarbons with alkali metals.

E.s.r. spectrometric study of directly prepared potassium-coal adducts

Abstract E.s.r. spectral parameters (concentrations of paramagnetic centres, g values and linewidths) of two vitrains of different rank (C, 89.2 and 75.8 wt%) and of their potassium adducts prepared by a direct method, as well as of the reductively alkylated high-rank vitrain are reported. The potassium adduct of the high-rank vitrain is characterized by the accumulation of a rather high concentration of unpaired electrons, 7.7 spins per 100 C atoms (a 120-fold enhancement). With the adduct of the low-rank vitrain the concentration is 0.14 spins per 100 C atoms (a 17-fold enhancement). The adduct of the low-rank vitrain is negligibly alkylated by alkyl halides in THF, unlike the adduct of the high-rank vitrain which can be significantly alkylated. The g values and linewidths of the spectra of the vitrains and the corresponding potassium adducts are very similar. The experimental results are interpreted mainly with respect to the formation of ionic potassium-coal adducts of coals of different rank and their alkylation.

Low-molecular-weight fractions of Bulgarian lignite humic acids

Abstract The molecular weight distribution is investigated of low-molecule-weight fractions of gel chromatographic separations of lignitic humic acids. The lignites are from ‘Maritza-lztok’, Bulgaria. The homogeneity of the fractions was assessed by disc gel electrophoresis in the presence of detergent (SDS) and denaturant (urea). The behaviour of the low-molecular-weight fractions on gel filtration was compared with that of standard proteins and peptides. The mean values for M n (8600, 5460, 3600 and 2900) are in agreement with the results of osmometric measurements. Extensive chromatographic separation gives rise to discrete fractions with reduced polydispersity.