Fluorescence spectroscopy in the characterization of coal-derived liquids

The use of fluorescence spectroscopy as a qualitative analytical technique for the analysis of polyaromatic hydrocarbons in coal-derived liquids is reported. Examples of fluorescence emission and fluorescence excitation spectra obtained from aromatic ring systems in coal hydrogenation oils and solvent extracts are shown. Twelve aromatic ring systems were identified. The separation scheme was simple and, therefore, the method has potential for routine analysis.     

An h.p.l.e. and m.s. analysis of distillate fractions of neutral oil from hydrogenated Akabira coal to elucidate chemical structures

Akabira coal-derived neutral oil was separated into 25 narrow boiling range fractions covering 183–423 °C, and subsequently separated into compound class fractions : alkanes, monoaromatics, naphthalene-type diaromatics, fluorene-type diaromatics and tri- and/or tetraaromatics, by high performance liquid chromatography (h.p.l.c.). The compound type analyses of the distillate/h.p.l.c. fractions were performed using electron impact mass spectroscopy (e.i.m.s.) or field ionization mass spectroscopy (f.i.m.s.). Aromatic/hydroaromatic compound types and the alkyl side-chain carbon distribution of the distillate/h.p.l.c. fractions were clarified, based on the separation behaviour of h.p.l.c. and the type analyses according to Z value by m.s. By the distillation/h.p.l.c./m.s. method, coal-derived oil was characterized in terms of the distribution of the numbers of aromatic rings, naphthenic rings and carbons of alkyl groups attached to these rings. The variations in chemical structure in a compound class with distillation temperature are discussed in terms of these chemical structural factors.     

The composition of the bases in a supercritical gas extract

Average structure data for the basic fraction from a supercritical toluene extract of Liddell coal are reported based on ¹³C- and ¹H-n.m.r. spectroscopy combined with elemental, functional group and molecular weight analyses. This data indicated an average aromatic ring size of two condensed rings with about a third of the possible aromatic sites substituted. The ¹³C-n.m.r. spectrum showed the presence of unsubstituted alkyl chains >C₈. A number of nitrogen heterocycles, ranging from one to five condensed rings, were identified by g.c.-m.s. and fluorescence and spectroscopy.     

Electrochemical analysis of higher molecular mass coal-derived materials

The factors affecting the analysis of asphaltenes and preasphaltenes from coal-derived liquids by reductivemode differential-pulse voltammetry at the hanging mercury drop electrode have been investigated. Reprecipitation and size-exclusion chromatography reveals a bias towards signals originating from low molecular mass material. Diffusion coefficients of model compounds measured by convolution potential-sweep voltammetry, and DPV of narrow preparative s.e.c. fractions, suggest that the voltammetric response of coal-derived molecules falls as molecular mass increases up to ≈ 450, and then levels off. The dependence of diffusion coefficient on MM for condensed aromatic molecules is not governed by Stokes' law. Comparison of voltammograms of asphaltenes and preasphaltenes with those of specially synthesized model compounds allows the aromatic nuclei linked together in the higher MM structures to be identified. These structures depend on the extraction conditions, but are also characteristic of those in the original coal. Extraction of coal with hydrogenated pyrene allowed linked fluoranthene structures to be identified in the extract and hence in the coal.     

Spectroscopic study of cyanine dyes interacting with the biopolymer,DNA

Dyes intercalated into DNA strands or bound to grooves show fluorescence intensity changes and aggregate formation depending on the conditions. In order to establish some empirical rules concerning dye intercalation, spectroscopic studies for the effects of DNA on several series of cyanine dyes with different aromatic rings, conjugated chain length and alkyl substituents were made. Absorption spectra, fluorescence intensity and circular dichroism spectra showed strong dependence on the species of dyes. Combination of preceding studies and these present results indicates that cyanine dyes tend to intercalate into DNA strand if their polymethine bridge was composed of only one carbon. For molecules with the longer chains irregular aggregates were formed by small amounts of DNA, which transformed into complexes composed of multiple dye and DNA strands. These results would serve as a useful guideline for designing of optical functional materials and devices utilizing DNA complex.     

The fluorescence of suspensions of powdered coals

The fluorescence spectra of aqueous suspensions of powders of polystyrene, two phenol–formaldehyde resites and a humic acid have been shown to be similar to the fluorescence spectra of their solutions in polar solvents. Fluorescence occurs both from individual aromatic structures and from excimers caged in the solid powders. Accordingly, the synchronous fluorescence (Δ=20 nm) of aqueous suspensions of a range of powdered coals showed the majority of the aromatic structures present in the coals to consist of substituted mono- and bi-aromatic rings. Excimers and exciplexes were also present. Excitation spectra of the aqueous suspensions confirm this interpretation. The fluorescence of powdered coals suspended in chloroform arises from the material extracted into the solvent. Accordingly the fluorescence spectra are dominated by the excimers and exciplexes formed in the solution; this fluorescence is enhanced by energy transfer from the smaller aromatic systems.     

Characterization of catalytically hydrotreated coal liquid produced by solvolytic liquefaction in petroleum asphalt

A hydrotreated coal liquid produced by solvolytic liquefaction of a feed originally consisting of 66% petroleum asphalt and 34% coal liquid was characterized. The hydrotreatment was carried out over a CoMo catalyst in order to upgrade moderately and selectively the coal-derived fraction. The present process was found very effective to depolymerize rather selectively the coal-derived fraction, originally benzene insoluble, to be soluble in benzene in the dominant presence of petroleum asphalt, while 20% of the asphalt was converted to be distillable with α-methylnaphthalene (the hydrotreatment solvent). Structural analyse of the fractions before and after hydrotreatment were compared to reveal what kinds of chemical reactions had taken place during hydrotreatment. The major reactions of the coal derived fraction are hydrodeoxygenation and hydrodealkylation, both of which are effective for depolymerization. No significant hydrogenation on aromatic rings was observed. Hydrotreated liquid was further pyrolyzed to obtain oils and cokes of high quality.     

Evidence of poly-condensed aromatic rings in a Victorian brown coal

An attempt was made for obtaining UV/VIS absorption and fluorescence spectra of a whole Victorian brown coal and then examining the presence of aromatic ring systems (ARSs) with poly-condensed rings in the coal. Loy Yang brown coal was subjected to an alkali-promoted depolymerization in an aqueous solution of sodium hydroxide at 473 K and then dissolved nearly completely into the solution. The solution of the solubilized coal was analyzed by UV/VIS absorption and fluorescence spectroscopies. The spectra of the solubilized coal were compared with those of a tar that was produced from the rapid pyrolysis of the coal in a wire-mesh reactor. Absorbance of the solubilized coal per unit molar concentration of aromatic carbon and unit light path length was averaged over 50 nm wavelength intervals in a range from 250 to 600 nm. The averaged absorbance of the solubilized coal was much greater than those of mono-aromatic compounds and lignin at intervals from 400 to 600 nm while comparable to those of tetra- to hexa-aromatic compounds, suggesting that the coal is abundant of ARSs with poly-condensed rings. The fluorescence spectra of the solubilized coal also give indication of the presence of ARSs with three to six condensed rings. Furthermore, the presence of even larger ARSs is evidenced from appreciable difference in apparent quantum yield of the solubilized coal and that of the tar. By comparing the absorption and fluorescence spectra of the solubilized coal with those of the tar, it was revealed that the tar is richer in relatively small poly-ARSs than the solubilized coal while the larger poly-ARSs are abundant in the solubilized coal. Smaller poly-ARSs were thus released from the coal preferentially to larger poly-ARSs during the pyrolysis.     

Analysis of a coal-derived liquid obtained by mild hydrogenation: 3. Acid,base and polar fractions

Polar fractions of a coal-derived liquid obtained by mild hydrogenation of Japanese Taiheiyo coal were investigated. The acid fraction was mainly composed of dimethyl or trimethyl phenols with a minor content of indanols. Z = − 7 N (tetrahydroquinoline) and − 9 N (octahydro-phenanthridine/acridine) series were dominant in the base fraction. Neutral oil was separated into six fractions (DS01–06) by vacuum distillation and each fraction was further separated into saturate, aromatic and polar fractions by liquid chromatography. The polar sub-fractions of DS01 (b.p. 186–281 °C) and DS02 (b.p. 281–305 °C) were analysed by f.i.m.s. and g.c.-m.s. Phenols and tetralinols (or indanols) both with long alkyl chains were major components of these fractions. Those of DS03–DS06, which were too complex to analyse by g.c. only, were repeatedly hydrogenated to eliminate the functional groups and then dehydrogenated to simplify the subsequent analysis. The components in the hydrogenated/dehydrogenated products were identified by g.c. and g.c.-m.s. and compared with the type analysis data obtained from f.i.m.s. of each polar subfraction assuming that those aromatic nuclei having hydroxyl functional groups are partially hydrogenated, if at all. Thus a quantitative analysis of the neutral polar fractions was calculated, which shows them to be composed of only seven kinds of aromatic nuclei.     

Composition of an SRC-II coal-derived liquid: Changes in composition resulting from hydrotreatment at different levels of severity

An SRC-II coal-derived liquid fuel and eight upgraded coal liquids derived from it were separated into chemical class-type fractions by preparative liquid chromatography. Mass and infrared spectroscopy, gas chromatography and non-aqueous titration were used, together with the mass balance data from the l.c. separations, to describe changes in composition of the liquids as a function of severity of hydrotreating conditions. The relative abundance of the following classes of compounds was determined in each of the liquids : saturates, monoaromatics, diaromatics, polyaromatics, total acids, total bases, hydroxyarenes, strong acids, pyrrolic benzologs, carbonyl compounds (amides), basic diarylamines, basic monoarylamines, azaarenes, and strong bases. The results are discussed in the light of known reaction pathways for aromatic ring saturation and heteroatom removal.     

Adsorption of naphthalene and indole on F300 MOF in liquid phase by the complementary spectroscopic,kinetic and DFT studies

Metal–organic frameworks (MOFs) are the promising functional materials for adsorption in liquid phase. It is important to understand the details of chemical bonding between the aromatic and heteroaromatic adsorbates and major structural units in the MOF sorbent—metal coordinatively unsaturated sites (CUS) and organic linkers. In this paper, we report the mechanistic studies of adsorption of naphthalene and indole on F300 Basolite MOF by two complementary spectroscopic methods and density functional theory (DFT) calculations. Fluorescence spectra, the near-UV/visible diffuse reflectance spectroscopy (near-UV/VIS DRS) and DFT calculations suggest that naphthalene forms an adsorption complex with F300 MOF where naphthalene is quantum confined within the cavity of the F300, weakly electronically bound to the Fe(III) CUS, and strongly dispersively stabilized by side interactions with the benzene rings of the linker of F300. On the other hand, indole forms an adsorption complex with F300 MOF in which indole is electronically bound to the Fe(III) CUS and dispersively stabilized by the side interactions with benzene rings of the linker. Coordination bonds between indole and F300 MOF in the adsorption complex are detected by geometry optimization using the DFT method, and electronic spectra are calculated by the time-dependent-DFT method. The direct spectroscopic proof of the formation of adsorption complex with coordination bonds between indole and F300 MOF is provided by the complementary near-UV/VIS DRS spectroscopy (new absorption bands at 460–660 nm), the wavelength-dependent fluorescence spectroscopy (new fluorescence bands at 410 and 430 nm) and by the time-dependent fluorescence spectroscopy.     

Analysis of upgraded SRC-11 and H-coal liquid products

Upgraded coal-derived liquids obtained from catalytic hydroprocessing of SRC-11 and H-coal syncrudes have been studied by i.r., p.m.r., g.c.-m.s., and silica gel chromatography. With increase in residence time, nitrogen, oxygen and aromatics decrease, while naphthenes increase substantially. All the upgraded liquids show low viscosity at 298 K (1.3–1.4 mN s m^?2), even though saturate and aromatic fractions varied with processing severity. In the aromatic-1 fraction, 1 -ring aromatics increase, and 3-ring aromatics decrease, with increase in severity of hydroprocessing. G.c.-m.s. analyses indicate a marked qualitative similarity for saturate and aromatic fractions irrespective of syncrude source. Only the heavier end of the aromatic-1 fraction is noticeably different. Tentative identifications based on m.s. and g.c. retention times are made for most of the significant components. 600 M Hz p.m.r. spectra of the upgraded SRC-11 and H-coal liquids appear identical, but the n.m.r. difference technique revealed slight differences between the two liquids in concentrations of certain species.     

Group-structural composition of resinous-asphaltene substances of high-boiling fractions of Azerbaidzhan crude oils

• 1.1. The physicochemical characteristics of resinous-asphaltene substances isolated from >450°C vacuum residues of different Azerbaidzhan crude oils been determined; integral structural analysis of molecules of the resins has been carried out.
• 2.2. The molecules of the vacuum residue resins are based on compounds with a hybrid structure that are similar in skeleton to the structure of high-boiling aromatic fractions. The resin molecules contain about two polycyclic blocks built from an average of two condensed highly substituted benzene rings and (with the exception of highly paraffinic Sangachal'sk crude oil) four or five naphthenic rings, irrespective of the depth of occurrence and the site of oil production.

     

Study of the chemical nature of petroleum components of high molar mass from Russian fields

• 1.1. Adsorption chromatography on wide-pore adsorbent-Silochrome-was used to segregate components of native petroleum asphaltenes into fractions with very different oxygen and sulphur contents, average molecular weights and aromaticity.
• 2.2. Russian petroleum resin molecules contain aromatic (tri- or tetracyclic on average) blocks containing one or more nitrogen atoms. Polar substances which form the main part of resins contain long, slightly branched aliphatic chains. Low-polarity compounds form a smaller proportion of resin components, their molecules probably contain large, polycyclane fragments which are not condensed with aromatic nuclei.
• 3.3. Asphaltene components, on the whole, are more aromatic than the resinous components and independent of the molar mass of the fractions, contain aromatic nuclei of similar average dimensions, differing mainly in the number of such nuclei in molecules.

     

Application of chemometric methods to synchronous UV fluorescence spectra of petroleum oils

Fluorescence spectroscopy offers the possibility to make qualitative and quantitative analysis of crude oils. Its sensitivity and selectivity permit its use for oils exploration and production. The study describes a novel approach for identification and discrimination of oils by chemometric treatment of synchronous ultraviolet fluorescence (SUVF) spectra which are usually studied by indexes method.Clusters of samples are obtained according to their geographic origin on a 3D plane using SUVF indexes and in the space of principal components for PCA method. Evaluation of oils composition is performed with MCR-ALS algorithm by extracting spectra of families representing aromatic structures.A PLS calibration, built with spectra intensities as input and oils origin as output allows to identify oils chosen for the prediction. Results show that mathematical exploration of full spectral region leads rapidly to classify and predict some oils properties according to their geochemistry.     

Method to evaluate benzonaphthenic carbons and donatable hydrogens in fossil fuels

Inspection of the n.m.r. spectra of liquid fuels has enabled an estimate to be made of various types of aromatic carbons: protonated, methyl- or alkyl-substituted, condensed carbons including both internal (at the junction of two or three aromatic rings) and benzonaphthenic (at the junction of an aromatic and a fused six-membered naphthenic ring) carbons. In this work a methodology is presented to estimate benzonaphthenic carbons apart from bridgehead carbons using the assumptions of the Williams' model. These values yield in turn an estimate of the number of donatable hydrogens, of hydroaromatic rings and of bridging alkyl chains. The propagation of numerical errors arising from integrated signal areas is estimated in each case. Two typical applications are given to illustrate the method using three petroleum asphalts and a coal liquid as representative samples.     

Three-dimensional and synchronous fluorescence spectroscopy: A new approach applied to coalification studies

Three-dimensional excitation/emission fluorescence and synchronous fluorescence spectroscopy offer possibilities for rapid and precise characterization of coals as a rank parameter. In studying five coals of wide rank and differing markedly in their maceral composition, fluorescence spectra of dichloromethane extracts were unaffected by maceral composition and reflect only coal rank. The method may be applicable to different types of sedimentary organic matter.     

Absorption and emission spectra of Athabasca asphaltene in the visible and near ultraviolet regions

Absorption, excitation and emission spectra in the visible and ultra-violet region were recorded at room temperature for Athabasca (Alberta, Canada) asphaltene and five of its fractions separated according to molecular weight (MW) by gel permeation chromatography (g.p.c.) on Bio-Beads SX-1. Strong concentrations effects were observed on the profiles of all the excitation and emission spectra at concentrations higher than 5.0 mg l⁻¹. The proper excitation and emission spectra were taken at concentrations lower than 5.0 mg l⁻¹ where the profiles are independent of concentration. All the spectra obtained were broad, structureless bands with the exception of that of the lowest MW fraction, which shows the sharp Soret band characteristic of the vanadyl porphyrin complex. The absorbance of the whole asphaltene was found to be equal to the weighted sum of absorbances of the fractions at a given wavelength and a given weight concentration. This additive property was not observed among the intensities in the emission spectra. The profiles of all the excitation and absorption spectra are identical throughout the asphaltene fractions, suggesting that the basic absorbing chromospheres are the same throughout the fractions and independent of MW of the fractions. In addition the absorption spectra are not significantly affected by the Tyndall scattering. From the examination of absorptivity and the position of the absorption band, the average number of aromatic rings in the average absorbing chromophores throughout the fractions was found to consist of approximately two to three. The fluorescence spectra, on the other hand, show a very rapid decrease of intensity at a constant weight concentration and a gradual shift of the band maxima toward longer wavelength with increasing number average MW. The origin of the fluorescence band is considered to be exciplexes. The average rate of non-radiative processes decreases steadily with decreasing MW from fraction 1 to fraction 5. The influence of various solvents was explored on the absorption, excitation and emission spectra of the lowest MW (1200) asphaltene fraction.     

Hydrogen bonding of the acid and base fractions separated from SRC II middle distillates

It has been demonstrated that hydrogen bonding is important in defining the high viscosity of coal-derived liquids. The aim of this study was to investigate the nature and strength of hydrogen bonding between the acid and base fractions of SRC II middle distillates. Amberlite IRA-904 and Amerlyst A-15 were used as anion- and cation-exchange resins respectively, to separate coal-derived products into various acid and base fractions. Formation constants of hydrogen bonding between the acid and base fractions were determined by i.r. spectroscopy. It appears that the acid-II and base-II fractions behave as hydrogen bond donors and acceptors, comparable to p-cresol (unhindered phenol) and quinoline-type nitrogen bases respectively.     

Average structures of petroleum pitch fractions by n.m.r. spectroscopy

Ashland A-240 pitch was fractionated by sequential solvent extraction and hypothetical average molecular structures for the whole pitch and its fractions were determined by using ¹H and ¹³C n.m.r. spectroscopy. The average molecules contain condensed aromatic ring systems substituted by short alkyl side chains in both A-240 pitch and the pitch fractions. This result conflicts with Seshadri et al's claim that the Ashland pitch fractions consist of molecules with uncondensed aromatic ring systems linked by aliphatic bridges. It is shown that Seshadri et al's results are probably based on nonquantitative ¹³C n.m.r. data due to the use of a low concentration of paramagnetic relaxation agent.