Maya Stefanova

Polar aromatic biomarkers in the Miocene Maritza-East lignite, Bulgaria

Aromatic and polar compounds of lignite lithotype bitumens (Maritza-East) were analyzed. Diterpenoid phenols and ketophenols, i.e., mainly ferruginol, sugiol, hinokione, and products of their diagenetic transformations, were identified as the dominant biomarkers. A variety of sesquiterpenoids was present as well. These compounds provided evidence for the assumption that members of the Cupressaceae s. 1. are the coal precursors. Polar triterpenoids, i.e., lupeol, lupenone, adiantone and hopanones were present in subordinate quantities in the humoclarain sample. These biomarkers support an input of deciduous (angiosperm) vegetation to the peat swamp source matter. Three diagenetic transformation schemes for the diterpenoid phenols/ketophenols are proposed: (1) ferruginol and/or sugiol alteration through dehydrogenation, demethylation and ring-B-cleavage; (2) hinokione aromatization to simonellite; and (3) 7-ketototarol change through 7-ketototar-5-enol to 20-nor-3-oxo-totara-5(10),6-dienol. The identified diterpenoids demonstrated the presence of Gymnospermae in the coal forming community. Furthermore, the phenolic diterpenoids indicated a specific input of organic matter from conifers of the Cupressaceae/Taxodiaceae or Podocarpaceae as the most abundant trees in the coal forming swamp.

Biomarker assemblage of some Miocene-aged Bulgarian lignite lithotypes

Abstract Two homogeneous (humovitrain and xylain) and one heterogeneous (humoclarain) lithotype macerals, separated from a lignite, were subjected to mild reductive treatment by NaH. The extensive chromatographic separation of the decalin-soluble fraction revealed some structural peculiarities. A strong predominance of α-phyllocladane was characteristic for all macerals. Series of n-, iso-, and anteiso-alkanes, regular isoprenoids, alkylbenzenes and 1-methyl-4-n-alkylcyclohexanes were identified by mass spectrometry. An angiosperm source input was represented by friedelane, lupane and seco-hopane structures. Products of des-A and des-E-ring cleavages and C-10 demethylation were registered by mass spectrometry. The main difference was in the hopane distribution pattern. A strong prevalence of extended ββ-hopanes was found in the humoclarain sample. The mass spectral data revealed the existence of a basic difference in hopane precursors—diploptent or diplopterol for the homogeneous lithotypes and bacteriohopanetetrol for the heterogeneous lithotype.

Composition of the extract from a Carboniferous bituminous coal: 1. Bulk and molecular constitution

Abstract A procedure for the extraction, separation and characterization of coal bitumen is described. A Carboniferous coal was subjected to exhaustive (single-step) extraction with dimethylacetamide (DMAA) at ambient temperature. The treatment solubilized ∼24 wt% of the organic matter. The bitumen was fractionated further and some fractions were separated into normal and branched/cyclic components. By g.c. and g.c.-m.s., n -alkanes, hopanes, benzohopanes, steranes, bicyclic sesquiterpanes, seco -hopanes, polycyclic aromatic hydrocarbons, and their alkylated homologues were identified. Number-average structural analysis was applied to the polar fractions and revealed some peculiarities of the structural units, i.e. simple molecular aggregates.

The composition of lignite humic acids

Abstract Humic acids from lignite deposits in Bulgaria were characterized with respect to the concentration of oxygen-containing functional groups. The following ranges were determined, in mequiv g −1 : total acidity 5.1–6.8; carboxylic groups 3.5–4.5; phenolic OH groups 1.6–2.3; ketone groups 1.0–2.0; quinone groups 1.0–1.3; and methoxyl groups 0.3–1.3. Yields of high- and low-molecular fractions were monitored by g.p.c. on Sephadex G-50. The molecular mass for the lower molecular fraction was ~ 4000u. On the basis of the elemental composition, molecular mass and distribution of oxygen in functional groups, a concept for lignite humic acid structure was developed.

Composition of the extract from a Carboniferous bituminous coal. 2. Compound-specific isotope analyses

Abstract Hydrocarbon subfractions of a Carboniferous bituminous coal extract were subjected to compound-specific isotope analysis (CSIA). The carbon isotope compositions of individual n -alkanes, isoprenoids, aromatic hydrocarbons and triterpenoids were determined to provide supplementary confirmation for the biomarker sources as deduced from their molecular structures. The total bitumen extract has a δ 13 C value of −25.03‰. The δ 13 C data for the biomarkers fit an origin from higher plants with a minor contribution from microbial activity.

Aliphatic biomarkers from Miocene lignites desulphurization

Lignites of the Thracian basin, the main coal potential of Bulgaria have been subjected to an extensive study, as their high sulphur content (3–7%) creates ecological problems during combustion. The aim of the present study was to examine the effect of desulphurization (three techniques, i.e. hydrothermal, t-BuONa and n-BuLi/TMEDA treatments) on biomarkers assemblage, as sulphur bridge cleavage could release covalently attached species or trapped molecules. Results of the chromatographic analysis of the aliphatic portions of the chloroform extracts from initial and treated lignites have pointed out that during desulphurization treatments coal organic matter was rearranged and smaller fragments became soluble. Some peculiarities were registered: alkane fraction of Maritza-Iztok treated by t-BuONa was strongly dominated by “even” carbon numbered n-alkanes, n-C16, n-C18 and n-C20; in products of Elhovo lignite desulphurized by n-BuLi/TMEDA alkyl cyclopentanes were tentatively identified. After n-BuLi/TMEDA desulphurization, hopane distribution was largely affected in the case of the Maritza-Iztok sample: the higher homologues and ββ dominance from C29 to C31 hopanes have disappeared; “geo hopanes” prevailance was registered;—18α(H)-oleanane, 25-nor-17α(H)21β(H)-hopane, C29, and a suite of “non-hopanoid” C30 triterpanes were observed. Our data have demonstrated the negligible steric protection of macromolecular structure in the case of low rank coals as far as covalently-bound species were characterized by higher degree of maturation compared to the easily extractable species.

Preserved lignin structures in Miocene-aged lignite lithotypes, Bulgaria

Abstract Contents of preserved lignin structures in Miocene-aged lignite lithotypes were determined. Phenol aldehydes, ketones and acids released from lignin by CuO oxidative hydrolysis were separated by capillary zone electrophoresis (CZE). Products of degradation were separated and identified by GC–MS to confirm CZE data and to complement composition data. The following values for the contents of preserved lignin structures in lignite lithotypes were determined (in wt%): xylain, 8.5; humovitrain, 6.7; liptain, 6.4; and humoclarain, 3.3. Some fatty acids, negligible contents of benzene polycarboxylic acids and their hydroxy/methoxy counterparts were also observed using GC–MS.

Malthenes from solubilization of low rank coal by methanol/sodium hydroxide treatment

Abstract The possibility of O-methylation occurring during the treatment of low rank coals with methanol/sodium hydroxide is evaluated. Chemical and spectral data of malthene fractions confirm unequivocally the presence of methoxyl groups. Their content was found to correlate with the efficiency of treatment. Formation of methyl esters after saponification of ester bridges in lignites is also probable. Reactions of O-alkylation contribute to stabilization through methylation of reactive intermediate species formed after cleavage of ether or ester linkages in coal organic matter, thus improving solubilization.

Thermochemical transformation of the quinoline-soluble portion of coal tar pitch

Abstract The quinoline-soluble portion of coal tar pitch and products of its carbonization at 250, 350 and 450°C were subjected to chemical study. The structural transformations were followed by extraction and chemical and number-average analyses. The samples under study, i.e. asphaltenes + preasphaltenes and carboids, were non-reductively alkylated in the system n-BuLi-BuI. The modification improved the solubility, owing to addition of 6–7 butyl groups per 100 carbon atoms. The alkylation procedure was considered mild, in so far as it affected only the ‘acidic’ centres in the pitch and did not alter the condensed aromatic systems. A set of structural parameters was calculated. Chemical transformations for carbonization temperatures > 350°C were established as an abrupt increase in carboid content, an increase in aromaticity and degree of condensation of the aromatic structures, intense cleavage of ether bridges, their replacement by phenolic OH groups, and further rearrangement of oxygen in cyclic form.

Molecular mass distribution of modified coal extract

Abstract The chloroform soluble portion of dimethylacetamide extract from a subbituminous coal was subjected to different types of chemical modification i.e. reduction, reductive alkylation, non-reductive alkylation, ‘ionic’ oxidation with subsequent alkylation, etc. The products of the reactions were studied by spectral and Chromatographic methods. An increase in molecular mass, prevailing over degradation, was registered by gel permeation chromatography (g.p.c.) analysis.