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Characterization of oxygenated species in coal liquefaction products: an overview FOREWORD
As mentioned in the introduction, identification and quantification of oxygen-containing compounds in coal-derived liquids is of considerable importance to understand their behaviours in further processing. However these species have not been characterized as fully as the predominant hydrocarbon components.
This first chapter surveys the analytical tools investigated for the separation, selective identification, and quantification of oxygenated compounds in coal oils. Although liquid and gas chromatography emerge as the most widespread techniques, many other spectroscopic techniques like FTIR (Fourier Transform Infrared), NMR, and mass spectrometry enabled to improve the understanding of these species. The state-of-the-art of preparative fractionation is also presented. Furthermore, the advantages and limitations of each technique are discussed and the potential contribution of multidimensional chromatographic systems to the analysis of complex matrices is highlighted. Considering the global energetic context, diversifying fuels is of growing importance and many new alternatives are promising. Coal liquefaction products definitely appear among the new generation substitutes. These comes from two fundamental process schemes: direct coal liquefaction (DCL) based on research pioneered by Friedrich Bergius in the beginning of the twenties, and indirect liquefaction based on Fischer and Tropsch work [1, 2]. Coal upgrading into a synthetic fuel always emerged in particular geopolitical contexts. Indeed, during World War II, Germany faced crude restrictions imposed by the allies by launching an industrial production of coal liquids (DCL). A few years later, it is South Africa’s turn to reply to crude embargo during the Apartheid by producing nearly 190,000 barrels of DCL a day in 2010. The 1st modern DCL unit is being started in Inner Mongolia in China by Shenhua, with a first DCL train producing 20 000 barrels per day of fuel, scheduled to be extended up to 60 000 barrels per day of fuel in the near future. Nowadays, petroleum decline and the rise of developing countries needs explain the special interest granted to coal-derived oils [3, 4]. Figure 1-1 shows the direct coal liquefaction conversion process used to transform coal into coal oil.
Before processing, DCL product characteristics are quite far from fuel specifications and upgrading must be applied to the gasoline and atmospheric gas oil cuts. In fact, they are mainly composed of naphthenes, polycondensed aromatic structures and heteroatomic compounds (Nitrogen and Oxygen) [1, 5-9]. To envisage their use as an alternative fuel, it is more than necessary to study their chemical and physical properties. Except hydrocarbons, the needs in terms of molecular characterization enhancement concern oxygenated compounds which belong to many different chemical families and are present in high concentrations before hydrodeoxygenation (HDO). In order to find an adapted process scheme, a detailed characterization of oxygenates families must be carried out.
This chapter gives an overview of the analytical schemes developed in the literature focusing on the separation and selective detection of oxygenated compounds in DCLs. Liquid and gas chromatographies appear to be the most widespread solutions. However, several other techniques have been used in the literature such as FTIR, NMR, Mass spectrometry and the elucidation of the oxygenated matrix composition can be approached by achieving a retrospective of all these studies. Limitations of the previous techniques for the analysis of oxygenated compounds in Coal-derived liquids will be discussed throughout this chapter and the contribution of a multidimensional chromatographic system emphasized.
Properties of coal-derived liquids
The chemical composition of coal liquefaction products is very different from conventional oil fractions ones obtained by crude distillation. While classical petroleum fractions are usually rich in paraffinic compounds and in sulphur, coal products are mainly composed of aromatics, unsaturated species, and heteroatomic species (Nitrogen and Oxygen-containing molecules). Chemical properties of coal-derived liquids are also influenced by the origin and the maturity of the raw material. In fact, as geological processes apply pressure to biological derived material, it is successively transformed into lignite, sub-bituminous coal, bituminous coal, anthracite which finally turns into graphite. Thus, according to the type of coal used in the liquefaction process, the elemental compositions found in the literature are quite different. (Table 1-1)
It is difficult to clearly conclude about the elemental composition of products described in Table 1-1 since different raw materials and different liquefaction processes including hydrotreatment step are used. Nevertheless, it clearly appears that globally, oxygen content is lower than those of products derived from the conversion of lignocellulosic biomass. Moreover coal liquefaction products exhibit lower H contents when compared to conventional petroleum distillation cuts.
When compared to physical properties of any processed crude oils, the key characteristics of these coal-derived distillates, before any subsequent hydrotreating or hydrocracking step, have very high densities (0.8-1) and poor combustion properties: low smoke point (10-15 mm) and low cetane number (20-30). This essentially results from the extremely low paraffin content and the high content of poly-naphthenic and naphtheno-aromatic structures [1].
Gates et al. [14] also improved the knowledge of coal oils by determining the relative concentrations of the functional groups. This structural characterization uses elemental analysis and NMR data and was applied on a heavy distillate into 9 fractions. The content of each functional group in the whole heavy distillate is given in Table 1-2.
Characterization by One-dimensional Gas Chromatography
Gas Chromatography has been widely used for the characterization of coal-derived products. O-FID is probably the first detector one thinks about for oxygen speciation. This detector was created to enable the selective identification of oxygen-containing species in hydrocarbon matrices. It appeared in 1980 and only a few articles mention its use. Most of these papers concern the ASTM D5599 which enables to quantify O-species at concentrations up to 0.1%w/w. The detector is composed of two microreactors and a Flame Ionization Detector installed in series. With this configuration, the oxygenated compounds eluted from the column enter the first reactor (cracking reactor with Pt/Rh catalyst) where they are selectively converted into CO. The methanizer installed downstream then converts this into methane. CH4 is finally detected by the Flame Ionization Detector.
This detection device involves two main limitations as it is not compatible with products containing water and the presence of sulphur (>10ppm) poisons the catalyst. Hence, even if this detection is appealing for oxygen speciation it is understandable that it was never applied to coal-derived matrices. Therefore Atomic Emission Detection and Mass spectrometry are much more widespread and a review of different applications of gas chromatography coupled to these detectors is summarized in Table 1-3.
Selective characterization of oxygenates in coal-derived liquids by GC-AED
A few papers mention the coupling between Gas Chromatography and Atomic Emission Detector (AED) for the analysis of coal-derived products. This device is a multielement detector that can be used to measure up to 23 different elements. GC-AED played an important role in the detection of nitrogen and sulphur species in hydrocarbon matrices [30-32]. Thus, it enabled the identification of benzothiophenes, dibenzothiophenes, indols, and carbazoles in coal–derived liquids. Applications to oxygenated compounds have not been as widespread and concern mainly the petroleum field. However, Murti et al. in 2002, and in 2005 used this selective tool to analyze kerosene-gas oil cuts respectively derived from the liquefaction of a sub-bituminous coal and South Banko coal [10, 25]. They highlighted the presence of many oxygenated compounds: alkylated phenols, benzofurans, naphthols and dibenzofurans [10, 25] (Figure 1-2).
Quantification could also be established for O-species which represent 3.7% of the fraction compared to 677ppm for S-species, 8400ppm for N-species, and 84.97%w/w for hydrocarbons. Among detected oxygenated compounds, phenols content is 51.16%w/w, compared to 33.07 %w/w for benzofurans, and 8.23 %w/w for dibenzofurans. Only 5.12% of oxygenated compounds are unknown which involves that phenols and benzofurans are the most predominant oxygen containing compounds. Bartle et al. also demonstrated in 2009 the potential of GC-AED for the analysis of oxygen-containing polycyclic aromatic compounds in coal-derived liquids [25]. Figure 1-3 shows the identified oxygenated species in an oil obtained from Samca coal treatment at 400 °C with a process-derived hydrogen donor solvent. It emphasizes the presence of dibenzofuran and its alkylated derivatives. Quantification was performed using a parent of benzonaphthofuran as an external standard. Oxygenated Poly Aromatic Cyclic (O-PACs) represent 0.03–0.3% in the tars, and 0.2– 0.1% in the neutral Poly Aromatic Cyclic (PAC) fraction of the pentane soluble product. Besides, in a recent study, this technique allowed the identification of phenyl-dibenzofuran, benzobisbenzofurans, triphenyleno[1,12-bcd]furan and 6-Oxa-12thia-indenol[1,2-b] fluorene [17] in coal tars, pitches and related materials. Other types of detectors are however preferred because AED device has a weak robustness and its sensibility to oxygen is not as appropriate as the one for carbon, hydrogen and sulphur. In fact, Gurka et al. studied detection limits for heteroatomic species i.e. hydrogen, nitrogen, oxygen, chlorine, and sulphur detection limit ranges are 0.17−3.0, 1.0−5.0, 0.65−11, 0.07−3.0, and 0.023−0.028 ng, respectively. This indicates that the order of increasing sensitivity to molecular structure is O < N < H < Cl < S.
Applications of GC-MS to characterize oxygenates in coal liquids
Furthermore, mass spectrometry detection was used to identify and quantify phenolic compounds in 175-425°C fractions of coal-derived distillates. In order to concentrate this fraction, a solvent extraction was used by Pauls et al. [18]. It consists on isolating an acidic concentrate by extraction with sodium hydroxide and in neutralizing the solution by acid addition. This strategy was also used in 1984 by Uchino et al. in order to separate the basic fraction and the acidic fraction from the hydrocarbon matrix [33]. In a phenol-containing fraction that Pauls et al. managed to separate, four types of ring structures with different short alkylated chains were characterized by GC-MS: phenols, indanols, naphthols, and biphenylols (Table 1-4). The recovery of phenol in the fraction of interest is only 42% whereas recovery of the other compounds up to C3 is 70%. The quantification of these species is given in Table 1-4 for the atmospheric flash distillates of Illinois N ° 6 coal-derived oil obtained by a two stage liquefaction process. Another study using GC-MS also demonstrates that the phenols are essentially monocyclic and that methyl groups are the main substituents in an Irati shale Oil AGO cut [26]. It was found that phenols represent 4 w/%w/w using GC/MS. This product is however quite different from the matrix of interest.
As phenolic compounds are the most abundant oxygenated species in coal products, many authors focused on their identification by GC-MS. With non-polar phases, separations demonstrated important peak tailing that can be overcome by converting phenols into methyl [34], acetyl [35] or trimethylsilyl [36] derivatives as reviewed by Charlesworth et al. [37]. After isolation of the phenolic fraction of a SRC-II middle distillate, the use of a Superox-20M column enabled White and Norman to identify 29 compounds via chromatography with authentic standards and matching mass spectra [23]. In 1976, another study showed the advantages of using
a tris- (2,4-xylenyl) phosphate stationary phase for the separation phenol alkyl-derivatives [38]. In fact nearly 40 phenolic compounds were identified in a coal tar (Table 1-5). Many other works reveal the presence of similar species [19, 22, 39, 40].
Furthermore GC-MS was used to compare the composition of coal macerals liquefaction extracts. Macerals are to coal what minerals are to rock. These organic substances exhibit particular chemical and physical properties. Coal petrographers separate the macerals into three groups: liptinite, vitrinite, and inertinite [41]. Brodzki et al. carried out a very interesting study about the molecular composition of liquids derived from concentrates of each of these groups [27]. While many researches highlight hydrocarbons analysis in macerals using Py-GC-MS which enables the characterization of non-volatiles and intractable macromolecular complexes [8, 42], Brodzski et al focused on dibenzofuran and its alkylated derivatives and showed that they are present in higher content in fractions derived from inertinite than in liptinite or vitrinite extracts. Phenols are also identified in the three fractions but appear to be much less abundant in the inertinite extract. These findings lead to a better understanding of the liquefaction scheme.
To conclude, GC has considerably improved the knowledge of oxygenated compounds in direct coal liquefaction products. However, there remain some limitations considering the complexity of the matrices of interest and the relatively low peak capacity of the technique. Therefore, two-dimensional gas chromatography has extensively been used.
1.4 Applications of Two-dimensional Gas Chromatography to unravel oxygenated structures in DCL
One-dimensional gas chromatography rests on only one separation criterion and is not sufficient if the vapour pressures of many analytes of a mixture are too close [43]. Separation of coeluted species requires the integration of another separation criterion. Hyphenated to a mass spectrometer or flame ionization detector or a specific detector of oxygen, two-dimensional gas chromatography can offer outstanding separations and appears as a very useful tool for the analysis of complex mixtures such as coal-derived products. Even if a few studies used this technique to characterize coal-derived products [44-46], as far as we know, by 2010, only one paper gives information about oxygenated compounds by GC×GC [1].
In fact, Bertoncini et al. focused on Direct Coal Liquefaction distillates and carried out two different analyses of oxygenated species by GC×GC-ToF/MS: one applied to the kerosene cut and the other to the atmospheric gas oil cut [1]. For quantification purposes, FID detection was also used. Modulation was carried out using by a thermal nitrogen modulator with a frequency of 50 Hz. Results as well as chromatographic conditions are displayed in Figure 1-4 concerning the kerosene cut. In a nutshell, the analysis of the kerosene cut gives structured chromatograms. As illustrated, elution zones of saturates, mono-aromatics, naphtheno-aromatics and oxygenates are delimited. These works are very successful in the separation of oxygenated compounds from the hydrocarbon matrix. As far as the analysis of the AGO cut is concerned, it enables the identification of 250 oxygenated molecular structures belonging mainly to the two families mentioned before i.e. phenols and benzofurans. These species are however not completely separated from the paraffins, naphthenes and aromatics. The column set on stake was PONA (10mx0.2mmx0.5µm) x BPX-50 (0.8×0.1mmx0.1µm). Moreover, a quantification of hydrocarbons with a classification by group type was carried out for naphtha, kerosene, and AGO cuts. However, quantitative information about oxygenated compounds is given only for the naphtha cut using a one-dimensional PIONA analysis. Additionally, another study used GC×GC ToF/MS to characterize hydrocracking products. It was lead by Hamilton et al. in 2007 [44] and showed a comparative study between GC-MS and GC×GC-ToF/MS. Although one-dimensional results enabled the identification of more than a hundred compounds, the combination of two polarities clearly performs a separation between alkanes and aromatics, but no specific oxygenates zone was highlighted. The combination of HP-5 and DB-17 columns enabled the identification of many hydrocarbons with a good resolution. The use of the selective m/z ratio option allowed the identification of single ion m/z specific families. Apart from aromatic and paraffinic structures, this work highlights the presence of benzonaphtofuran (Figure 1-5). Many hydrogen donors structural isomers were also detected in the recycle solvent. Nevertheless, the maximal reachable mass of 210 units does not give access to heavier molecules highlighted by SEC analyses.
To conclude the use of new coupled techniques such as GC×GC would be of great interest to unravel oxygenated structures contained in these types of matrices. In fact GC×GC overcomes the limits of classical GC in terms of resolution and peak capacity. Recent advances also show the possibility to use an atomic emission detector (AED) coupled to a GC×GC device to improve the understanding of petroleum matrices.
Table of contents :
PART AOXYGEN SPECIATION IN COAL DERIVED LIQUIDS
CHAPTER 1. CHARACTERIZATION OF OXYGENATED SPECIES IN COAL LIQUEFACTION PRODUCTS: AN OVERVIEW
1.1 Introduction
1.2 Properties of coal-derived liquids
1.3 Characterization by One-dimensional Gas Chromatography
1.3.1 Selective characterization of oxygenates in coal-derived liquids by GC-AED
1.3.2 Applications of GC-MS to characterize oxygenates in coal liquids
1.4 Applications of Two-dimensional Gas Chromatography to unravel oxygenated structures in DCL
1.5 Characterization by Liquid Chromatography
1.5.1 Preparative Liquid Chromatography
1.5.2 High Performance Liquid Chromatography
1.6 Selective analysis of phenols and alcohols by NMR spectroscopy
1.7 Characterization of oxygenated compounds by FT-ICR/MS
1.8 Conclusion
CHAPTER 2. INVESTIGATING GC×GC TO OPTIMIZE THE SEPARATION OF OXYGENATED COMPOUNDS IN A DIRECT COAL LIQUEFACTION MIDDLE DISTILLATE
2.1 Introduction
2.2 Experimental
2.2.1 Samples
2.2.2 GC×GC-FID setup
2.2.3 GC×GC-ToF/MS
2.2.4 Data handling
2.2.5 Two-dimensional decisive factors
2.3 Results and discussion
2.3.1 Investigated configurations
2.3.2 Selection of the most adapted configuration
2.3.3 GC×GC-ToF/MS analysis of the selected configuration
2.4 Conclusion
CHAPTER 3. USING GAS CHROMATOGRAPHY TO CHARACTERIZE A DIRECT COAL LIQUEFACTION NAPHTHA
3.1 Introduction
3.2 Experimental
3.2.1 Materials
3.2.2 GC-ToF/MS setup
3.2.3 GC-GC-FID setup
3.2.4 GC×GC-FID
3.2.5 GC×GC-ToF/MS
3.3 Results and discussion
3.3.1 Speciation of oxygenated compounds by GC-ToF/MS
3.3.2 Towards a detailed characterization of oxygenates by GC-GC-FID
3.3.3 Speciation of oxygenated compounds using GC×GC
3.3.4 Comparison of the three techniques
3.4 Conclusion
CHAPTER 4. A NOVEL ANALYTICAL APPROACH FOR OXYGEN SPECIATION IN COAL-DERIVED LIQUIDS
4.1 Introduction
4.2 Materials and methods
4.2.1 Samples
4.2.2 Gas chromatography analysis
4.2.3 Mass spectrometry analysis
4.2.4 31P Nuclear Magnetic Resonance analysis
4.2.5 UV-visible spectroscopy analysis
4.3 Results and discussion
4.3.1 Methodology
4.3.2 Detailed quantification of alcohols and phenols by GC×GC
4.3.3 Identification of phenols and carboxylic acids by FT-ICR/MS in the coal derived AGO
4.3.4 Global quantification of alcohols, phenols and carboxylic acids by 31P NMR
4.3.5 Global quantification of ketones by UV-visible spectroscopy
4.3.6 Final quantitative assessment
4.4 Conclusion
