The characterization of asphaltenes and preasphaltenes obtained under various coal liquefaction conditions

A CP-MAS ¹³C NMR study of asphaltenes and preasphaltenes obtained under various coal liquefaction conditions is reported. The carbon aromaticity, f_a, of the solid extracts from the reaction products has a close relation with the reaction conditions. By plotting f_a against the atomatic $\text{H}\text{C}$ ratio for these solid products on the characterization chart for model polycyclic aromatic compounds, the molecular structures with relation to the liquefaction pathway from coal to oil can be proposed.     

Chemical structure of the high-molecular-weight hydrogenation products of coal from the Zashulanskoe field

The results of experimental studies on the determination of the chemical structure of asphaltenes and preasphaltenes in the liquid products of the hydrogenation of coal from the Zashulanskoe field in the Chita oblast. It was found that, in the preasphaltenes, the value of f _a and the concentrations of oxygen groups (OH, COOH, and C=O) were greater and the concentrations of CH₂ and CH₃ groups were smaller that those in the asphaltenes. The highest concentration of CH₂ groups was found in substances soluble in n-hexane (oils). It is likely that the change in the character of high-molecular-weight hydrogenation products formed from the structural fragments of the organic matter of coal (OMC) largely depended on reaction conditions, namely, the rate of heating and the isothermal exposure time.     

Chemical structure of heavy oil from coal hydrogenation 1. Hydrogenation with zinc chloride catalyst

Oil product from the hydrogenolysis of a high-volatile bituminous coal was separated by solubility, fractionated by gel permeation chromotography and characterized by structural analysis. The average structural unit in the hexane-soluble, aromatic oil fraction consists of 1–3 aromatic rings with 0.3-0.5 of the ring carbons substituted by alkyl groups and oxygen containing groups. Molecular weights vary from 200 to 500. The larger molecular weight fractions have longer alkyl chains and lower carbon aromaticities. The molecules are mainly of single unit structures. The average structural units in asphaltene fractions contain from 2.5-4 aromatic rings, are of higher carbon aromaticities and contain shorter alkyl groups. The asphaltene molecules consist of two or more structural units, crosslinked together, and have molecular weights of 300–1400. The oxygen content of the fractions decreases with decreasing molecular weight. Increasing the amount of ZnCl₂ catalyst during hydrogenolysis resylts in an increased yield of lower-molecular-weight material, but no change in the structural properties of the product. This is interpreted to mean that ZnCl₂ is active in the scission of covalent bonds between structural units during liquefaction and that the hydrogenolysis reaction is mostly cleavage of crosslinks between structural units with minimal reaction of the units themselves.     

Characteristic diagrams of coal-derived liquids: the correlation between aromaticity and atomic HC ratios

Characteristic diagrams, which are very useful in studies of the reaction pathways in coal liquefaction processes and to characterize coal-derived materials, are presented. Using model compounds, aromatic carbon fraction, f_a, and aromatic proton fraction, f_aH, are plotted against atomic $\text{H}\text{C}$ ratio. The experimental results of an anthracene oil and its hydrogenated oils are plotted on these diagrams, and the usage and the validity of the charts are verified.     

Study of the preasphaltenes of coal liquefaction and its hydro-conversion kinetics catalyzed by SO4/ZrO2

The investigation of hydro-conversion behavior of the heavy intermediate products derived from coal direct liquefaction is advantageous to optimize the technological conditions of direct coal liquefaction and improve the oil yield. In this paper, the hydro-conversion of preasphaltenes catalyzed by SO₄²⁻/ZrO₂ solid acid was investigated based on the structural characterization of preasphaltenes and its hydro-conversion products, and the determination of products distribution and the kinetics of preasphaltenes hydro-conversion. The results indicated that the content of condensed aromatic rings increased, and the contents of hydrogen, oxygen and aliphatic side chains of preasphaltenes decreased with the increase of coal liquefaction temperature. The preasphaltenes showed higher hydro-conversion reactivity while SO₄²⁻/ZrO₂ solid acid was used as catalyst. Higher temperature and longer time were in favor of increasing the conversion and the oil + gas yield. The conversion of preasphaltenes hydro-conversion under 425 °C, for 40 min reached 81.3% with 51.2% oil + gas yield. SO₄²⁻/ZrO₂ solid acid was in favor of the catalytic cracking rather than the catalytic hydrogenation in the hydro-conversion of preasphaltenes. The activation energy of preasphaltenes conversion into asphaltenes was 72 kJ/mol. The regressive reactions were only observed at a higher temperature.     

13C1H Cross-polarization nuclear magnetic resonance spectra of macerals from coal

The carbon aromaticities (f_a) of vitrinite, exinite, micrinite and fusinite from a high-volatile A bituminous coal have been determined by ¹³C¹H cross-polarization nuclear magnetic resonance spectrometry. Values of f_a for the four macerals were found to decrease in the order: fusinite > micrinite ≈ vitrinite > exinite. Estimates of the average ring size using the f_a value and the elemental composition of each macerai indicated that the fusinite contained the largest polynuclear condensed aromatic ring system (> 5 rings) whereas the mean structural unit of the vitrinite contains 3–4 condensed rings.     

Catalytic liquefaction of various coals using a mixture of carbon monoxide and water

In a batch-autoclave, twenty coals were liquefied using a cobalt-molybdenum oxide catalyst with a mixture of CO and H₂0 at 400 °C with or without vehicle oil. Furthermore, lignite and peat were liquefied on tungsten oxide catalyst at 300 °C in the absence of CO. The reactivity of coal in this liquefaction is found to depend strongly on its rank. The vehicle oil significantly influences the extent of the water-gas shift reaction, especially when bituminous coals are liquefied, by dissolving such coals. Liquefaction of coal by this process is considered to take place via three routes: hydrogenolysis by the nascent hydrogen produced from the shift reaction; dissolution of coal into the vehicle oil which is an initial stage of hydrogenolysis; and a solvolytic reaction with H₂O, such as hydrolysis.     

Application of new 13C n.m.r. techniques to the study of products from catalytic hydrodeoxygenation of SRC-II liquids

A middle-heavy SRC-II distillate (b.p. 230–455 °C), containing 3.0 wt% of oxygen, has been studied by means of ¹³C n.m.r. at 75, 100 and 125 MHz. The magnetization refocussing techniques INEPT and J-resolved two-dimensional Fourier transform have been utilized to demonstrate methods by which resonance line multiplicities may be determined in complex liquid mixtures. Products derived from the above coal liquid by hydrodeoxygenation at temperatures from 200 to 370 °C, using sulphided Co—Mo and Ni-W catalysts, were also examined. The fraction of aromatic carbon in the hydrotreated liquids was found to correlate directly with their C/H atomic ratio and inversely with the hydrogen content. Comparison of O/C atomic ratios with f_a values for these liquids indicates that hydrogen uptake < 260 °C is associated primarily with hydrogenolytic oxygen removal without attendant ring hydrogenation, while at temperatures between 260 and 350 °C hydrodeoxygenation is accompanied by ring hydrogenation and dealkylation reactions.     

Kinetics of coal liquefaction during heating-up and isothermal stages

Direct liquefaction of Shenhua bituminous coal was carried out in a 500 ml autoclave with iron catalyst and coal liquefaction cycle-oil as solvent at initial hydrogen of 8.0 MPa, residence time of 0–90 min. To investigate the liquefaction kinetics, a model for heating-up and isothermal stages was developed to estimate the rate constants of both stages. In the model, the coal was divided into three parts, easy reactive part, hard reactive part and unreactive part, and four kinetic constants were used to describe the reaction mechanism. The results showed that the model is valid for both heating-up and isothermal stages of liquefaction perfectly. The rate-controlled process for coal liquefaction is the reaction of preasphaltene plus asphaltene (PAA) to oil plus gas (O + G). The upper-limiting conversion of isothermal stage was estimated by the kinetic calculation.     

Highly active limonite catalysts for direct coal liquefaction

The properties of limonite in Australia and Indonesia were examined by using X-ray diffraction, Mössbauer spectroscopy, Thermal gravimetric analysis (TGA) and TEM–EDX in relation to the catalytic activities in the liquefaction of low-rank coals. The molar ratio of H₂O/Fe in limonite was determined from the weight loss resulting from the dehydration reaction of FeOOH to Fe₂O₃ during heating from 120 to 400 °C in TGA. The H₂O/Fe molar ratio varied from 0.06 for hematite to 0.60 for limonite YY, depending on the hematite content. Results from the pulverization tests showed that the higher value of H₂O/Fe molar ratio resulted in lesser abrasion of medium balls. A unique limonite YY in Australia, containing no hematite, was easily pulverized to sub-micron particle size and showed an excellent oil yield in coal liquefaction. It appeared that H₂O/Fe molar ratio could be one of the most important factors to select the better limonite catalyst for coal liquefaction. Moreover, it was found that Ni containing limonite SO in Indonesia exhibited a higher liquefaction activity than YY catalyst, because of transformation into a smaller crystallite size of pyrrhotite (Fe_1−xS). The agglomeration of pyrrhotites may be suppressed by a strong interaction between FeOOH and Al(OH)₃ such as Fe–O–Al. TEM–EDX analysis suggests that Ni may be located near the Fe_1−xS structure. Oil yield was significantly increased from 43 to 62 wt% daf by CLB addition to the coal slurry in the liquefaction of Banko coal. Finely pulverized limonite catalyst (SO) can be advantageously used in a commercial plant for coal liquefaction in Indonesia due to the low catalyst cost and a high liquefaction activity to obtain a high oil yield.     

Chemical structure of asphaltenes and preasphaltenes obtained by coal hydrogenation under high-speed heating conditions

The results of experimental studies on the determination of the chemical structure of asphaltenes and preasphaltenes in the liquid hydrogenates of coal from the Zashulanskoe field in Transbaikalia are reported. These results were obtained under the conditions of high-speed heating (～200 K/min; reaction time, 10 min). It was found that the structural characteristics of asphaltenes and preasphaltenes obtained by high-speed hydrogenation at 425°C and 10 MPa exhibited a number of special features. At similar fractions of aromatic carbon, these products differed from the coal hydrogenates obtained at a slow rate of heating (5 K/min; reaction time, 120 min) in terms of the distribution of hydrogen and oxygen over structural groups. In them, the hydrogen content of CH₂ and CH_ar groups was smaller and that of CH₃ groups was greater. The highest concentration of total oxygen-containing groups was detected in the high-speed process preasphaltenes, whereas it was lower in the asphaltenes.     

Coal liquefaction by in-situ hydrogen generation.: 2. Zinc-water model compound reactions

Model compound studies were carried out to elucidate the reaction mechanisms taking place during the liquefaction of coal with the hydrogen produced from the reaction of zinc and water. In compounds of the type Ph-(CH₂)_n-Ph the splitting of the aliphatic bridge was easier with higher n values. Ether type compounds such as diphenylether were unreactive although the C-O bond in dibenzylether was easily cleaved. Condensed ring aromatic compounds gave low conversion with hydrogenation being facilitated by an increase in ring number. Phenolic compounds such as phenol did not react well, but the reactivity increased with increase in aromatic ring size. The cleavage of the aliphatic bridge was accelerated by the OH group, for example, in the case of 4-hydroxydiphenylmethane bond scission was about 15 times higher than that of diphenylmethane. Heterocyclic compounds were unreactive.     

Evaluation of the aromaticity in French coals by 13C-1H cross polarization,magic angle spinning and dipolar dephasing nuclear magnetic resonance spectroscopy

The aromaticity factor f_a of various French coals, as determined by ¹³C-¹H cross polarization and magic angle spinning, is reported. The f_a, values, which are estimated after a careful examination of the evolution of the ¹³C magnetization as a function of the contact time, are 0.63, 0.74, 0.82 and 0.85, respectively, for the Gardanne, Vouters, Méricourt and Escarpelle coals. The fraction of non-protonated aromatic carbons, fⁿ_a, as determined by dipolar dephasing experiments seems to decrease with increasing rank until a minimum value is reached for the low volatile bituminous coal from Méricourt, followed by an increase for the semi-anthracite coal from Escarpelle.     

Liquefaction reaction of coal: 2. Structural correlation between coal and its liquefaction products

The structural correlation between coal and its liquefaction products has been examined using cross-polarization, magic angle spinning (CP/MAS) ¹³C n.m.r. and field ionization mass spectrometry (f.i.m.s.). The CH₂/aromatic carbon ratios of all solid products (asphaltene, preasphaltene and residue) were close to the corrected +CH₂/aromatic carbon ratio for the coal. This suggests that the ring structure of the structural unit of each solid product is essentially similar to that of the parent coal, except for a difference in the degree of polymerization of the structural units. The CH₂/aromatic carbon ratios of aromatic ring-type oil fractions also correlated with the corrected ratio for the coal, although they were larger. The z series distribution obtained from the f.i.m.s. of oil fractions revealed that coal with a higher CH₂/aromatic carbon ratio produced an oil rich in naphthenic structures.     

Relaxation of 13C and 15N nuclei by solvent-refined coal

The effects of changing various spectroscopic parameters on the solid-state and solution ¹³C-nuclear magnetic resonance (n.m.r.) spectra of solvent-refined coal have been investigated. Solution spectra were obtained with the use of broad band decoupling, inverse gated decoupling and coupling techniques. Conventional relaxation reagent (Cr(acac)₃) was sometimes added to the solvent-refined coal. The effects of pulse delay on the total signal intensity, and on the intensity of the signal from aromatic carbon have been measured. The results show that inverse gated decoupling with pulse delays of 10 s is needed for complete relaxation of solvent-refined coal, but pulse delays of 6 s can give accurate estimations of aromaticity. However, it is recommended that conventional relaxation reagent such as Cr(acac)₃ be added to ensure relaxation if shorter pulse delays are used. The effect of solvent-refined coal on relaxation of some pure compounds in solution has also been studied. Solvent-refined coal acts as a relaxation reagent on ¹³C nuclei in benzene, toluene and ethylbenzene. It can also relax ¹⁵N nuclei in aniline, N,N-dimethylaniline and nitromethane. Spin-lattice relaxation times (T₁'s) of selected nuclei have been measured and the contribution of solvent-refined coal to relaxation (T₁SRC) has been calculated. Solid-state 13C-n.m.r. spectra have been obtained using the cross-polarization (CP) technique with magic-angle sample spinning (MASS). A variety of cross-polarization times and recycle times have been used. The results show that no serious errors in measurement of aromaticity (f_a) are caused by using a contact time as short as 1 ms and a recycle time of 0.3 s. There is good agreement between f_a's obtained by solution and solid-state n.m.r., and as solid-state spectra can be obtained in only a fraction of the time needed to obtain a solution spectrum (≈$\text{1}\text{20th}$), CP-n.m.r. is the method of choice for analysis of f_a of solvent-refined coal. The results also show that CP-MASS n.m.r. can be used to estimate the fraction of aromatic carbon which is unprotonated in solvent-refined coal and, hence, indirectly, the fraction of hydrogen which is aromatic.     

Formation of anatase from titanium(IV) preasphaltenes complexes under coal liquefaction conditions

Solvent refined coal preasphaltenes from stable complexes with bis(cyclopentadienyl) Ti(IV) dichl-oride. On heating at 450 °C under 6.9 MPa H₂ atmosphere in tetralin, partial conversion of the BTD-preasphaltenes complex to anatase, Ti0₂ is observed. Preasphaltenes complexes of Ti may be involved in hydrodesulphurization catalyst deactivation by Ti0₂ in coal liquefaction processes.     

Average structure of coal: Structural analysis of pyridine extract of zinc-water-coal reaction product

Six coals, of 67.5–91.5 wt%C (daf), were hydrogenated with zinc and water under 8 MPa of nitrogen for 30–60 min at 445 °C. This reaction utilizes the hydrogen generated in situ by the reaction Zn+H₂0 = ZnO + 2H₂. The influence of reaction time was examined using the lowest-rank coal and 30 min was found to be the optimum for this coal. The conversion of the various coals was 80.7–100% with the exception of the highest-rank coal, which gave only 36.5% conversion. Structural analysis, carried out for the pyridine-soluble fractions using the Brown—Ladner equations, showed that the aromatic ring size increased from 1 for the lower-rank coals to 4 for the higher-rank coals.     

Changes in composition of solvent-refined coal with severity of hydrogenation

The effects of mild and severe hydrogenation on the chemical composition of solvent-refined coal (SRC) produced from Wyodak subbituminous coal in the direct coal liquefaction SRC-I process were investigated. The yields of solvent-derived fractions of ‘oils’ and ‘asphaltenes’ increased with increasing severity of hydrogenation at the expense of ‘preasphaltenes’. Further separation of ‘oils’ and ‘asphaltenes’, each into three compound-class fractions, revealed more compositional changes. Concentrations of hydrocarbons, nitrogen compounds and hydroxyl aromatics in ‘oils’ increased with increasing severity of hydrogenation. ‘Asphaltenes’, containing nitrogen compounds and hydroxyl aromatic fractions but almost no hydrocarbons, showed an increase in nitrogen-compound concentration with increasing severity of hydrogenation. Hydroxyl aromatic concentration in ‘asphaltenes’ increased under mild but decreased under severe hydrogenation conditions. High-performance liquid chromatography followed by field-ionization mass spectrometry analysis of the hydrocarbon subfractions revealed a complex picture of structural transformations. Over fifty homologous series of aromatic and hydroaromatic hydrocarbons covering a carbon number range from about C₁₂ to C₅₀ were identified and approximate concentrations obtained. Small amounts of partly aromatized pentacyclic triterpane ‘biomarkers’ and their hydrogenation products were found.     

Solvent de-ashing from heavy product of brown coal liquefaction using toluene:: 2. concentration and separation of ash with a continuous de-ashing system

The brown coal liquefaction (BCL) process is a two-stage liquefaction (hydrogenation) process developed for Victorian brown coal in Australia. The BCL process has a solvent de-ashing step to remove the ash and heavy preasphaltenes from the heavy liquefaction product (vacuum residue) derived from the coal in primary hydrogenation and named CLB (coal liquid bottom). This solvent de-ashing step uses toluene or coal-derived naphtha as a de-ashing solvent (DAS). After dissolving the CLB into the solvent (CLB/solvent ratio, 1/8–1/4, w/w) under high temperature (200–290°C) and high pressure (4–5 MPa), insoluble solid particles which consist of ash and heavy preasphaltenes are settled by gravity and separated from the solution as an ash-concentrated slurry. The ash-concentrated slurry and the de-ashed solution are withdrawn from the settler as an underflow and overflow, respectively. The de-ashed heavy product is recovered from the solution by eliminating the solvent and is further hydrogenated in secondary hydrogenation. The authors have reported on the solubility of CLB in toluene and the settling velocity (V) of the boundary of ash content in the settler under de-ashing conditions. This paper discusses the effects of de-ashing conditions on ash concentration in the settler bottom and the operating conditions of a continuous de-ashing system. The ash content in underflow (C_UF, kg/kg or wt.%) at the settler bottom was found to increase with temperature and to decrease with the rate (flux) of downward flow (underflow). The maximum C_UF, Z, is expressed by the equation: Z=B_CLB(FL/0.35)^−0.32(T/523)^4.26, where B_CLB, FL and T are the characteristic parameters of organic CLB (kg/kg or wt.%), flux of underflow in the settler (kg/m² s) and temperature (K), respectively. B_CLB is also expressed by using the analytical results of organic insolubles in the CLB under de-ashing conditions. Finally, stable operating conditions of a continuous de-ashing system are confirmed to be determined as the following qualifications: |V_u|<|V|, W_UF>W_SA/C_UF and Z>C_UF, where |V_u|, |V|, W_SA and W_UF are the upward velocity of the solution in the settler (mm/s), settling velocity of the ash boundary (mm/s) in the settler, flow rate of ash in the feed slurry (kg/h) and flow rate of underflow (kg/h), respectively. Under these qualified conditions, the 50 t/d pilot plant constructed in Australia was operated under stable conditions for 3700 h using toluene as a DAS.     

Liquefaction mechanism of Wandoan coal using tritium and 14C tracer methods: 2. Liquefaction using 3H labelled gaseous hydrogen

Tritium labelled gaseous hydrogen was used to clarify the role of gaseous hydrogen in coal liquefaction. Wandoan coal was hydrogenated under 5.9 MPa (initial pressure) of ³H-labelled hydrogen and in unlabelled solvents such as tetralin, naphthalene and decalin at 400 °C and for 30 min in the presence or absence of NiMoAl₂O₃ catalyst. Without a catalyst, liquefaction proceeded by addition of the hydrogen from donor solvent. The NiMoAl₂O₃ catalyst enhanced both hydrogen transfer from gas phase to coal and hydrocracking of coal-derived liquids. With NiMoAl₂O₃ catalyst, liquefaction in naphthalene solvent proceeded through the hydrogen-donation cycle: naphthalene → tetralin → naphthalene. The amount of residues showed that this cycle was more effective for coal liquefaction than the direct addition of hydrogen from gas phase to coal in decalin solvent. The ³H incorporated in the coal-derived liquids from gas phase was found to increase in the following order: oil < asphaltenes < preasphaltenes < residue.