Direct coal liquefaction (DCL) is an effective method of utilizing coal to produce clean liquid fuels, especially for those countries that have limited oil resources but are rich in coal, such as China [1-3]. Oil yield is a key performance index for the DCL process, and 1% increase in oil yield can result in 20 000 tons of additional oil production or approximately 100 million RMB of additional profit for an industrial-scale DCL plant with a one million ton annual production capacity. A high-performance DCL catalyst can accelerate the pyrolysis of coal and promote the hydrocracking of macromolecular fragments, resulting in higher coal conversion and increased liquefied oil yield [4-6].

Iron-based materials applied as disposal catalysts during DCL have been widely employed due to their lower costs and moderate activities [7-11]. Pyrrhotite (Fe_1-xS), which is generated from iron-based catalysts during liquefaction, has been identified as the catalytically-active phase [12-15]. Red mud, natural iron ore, synthetic iron sulfide, synthetic iron oxides and oil-or water-soluble iron-containing materials have all been used in DCL. However, variations in properties such as crystalline phase, structure or dispersion will lead to pronounced differences in the resulting pyrrhotite and thus the catalytic activity [16-18].

Because DCL is a complicated three-phase reaction, it is very difficult to determine the relationship between catalytic precursors and oil yields. Linehan et al. [19] studied 11 different iron oxides, each having a single crystalline phase, during the catalytic cracking of a model coal compound, and found that the crystal structure was the key factor affecting the performance of the catalyst precursor. Kaneko et al. [20, 21] assessed the physical properties and sulfidation activities of α-FeOOH, γ-FeOOH, ferrihydrite and two iron ores, and demonstrated that smaller pyrrhotite crystallite sizes correlate with higher oil yields. The particle size and extent of dispersion of the precursor are also important, and so ultrafine iron catalysts have been developed by numerous researchers [13, 22]. Zhao et al. [23, 24] has suggested that it is also important for the catalyst to maintain its dispersion at high temperatures, based on a comparative study of an ultrafine catalyst (with an average particle diameter of 3 nm) and two binary iron oxide catalysts (Si/ferrihydrite and Al/ferrihydrite, with average particle diameters of 5 and 10 nm, respectively). Cugini [25] reported that liquefaction feed coal not only promoted the generation of ultrafine FeOOH but also assisted in dispersing the Fe_1-xS that was generated in situ during DCL. It is worth noting that ultrafine γ-FeOOH catalysts supported on coal have been successfully applied in the first megaton DCL industrial demonstration plant, operated by the Shenhua Group [26-28].

Many important results have been reported concerning the relationship between precursor catalyst properties and catalytic liquefaction activity, but these data have certain limitations because of variations in preparation methods as well as the compositions or sources of the catalyst precursors. In the present study, six unsupported iron–oxygen compounds having different microstructures were prepared by the same method and using the same raw materials, and the relationship between the microstructures of the catalyst precursors and their catalytic behaviors during coal liquefaction was systematically studied. In addition, a series of coal-supported catalyst precursors was prepared. The effects of the coal carrier and synthesis temperature on the formation of iron–oxygen compounds were assessed, with the aim of obtaining information of use during the future applications of industrial catalysts.

The coal sample employed in experimental trials was obtained from the Shenhua Shangwan coal mine, which provides the feedstock for the Shenhua DCL demonstration plant. The coal was first ground to obtain particle sizes below 150 μm, dried under vacuum at 100 ℃ for 6 h, and then stored in an airtight container. The results of the proximate, ultimate and petrographical analyses of the Shenhua Shangwan coal are shown in Table 1, while the properties of the recycle solvent produced by the demonstration plant are summarized in Table 2. The proton donor quality index (PDQI), as developed by Tanabe [29], reflects the hydrogen-donating ability of the solvent and is expressed in units of milliequivalents of β-naphthenic hydrogen per gram. The PDQI value of 18.88 mg/g in Table 2 indicates superior hydrogen-donating ability. The aromaticity (f_a) value of the solvent (0.49) was calculated based on its ¹H nuclear magnetic resonance (NMR) spectrum and elemental composition. These two parameters were calculated using the equations below [4, 29, 30].

Here, H_Nβ, H_α, H_γ, H_β and H_t are the intensities of the β-naphthenic hydrogen, α-hydrogen, γ-hydrogen, β-hydrogen and total hydrogen peaks in the ¹H NMR spectrum, respectively, while w(H) and w(C) are the H and C mass concentrations in the recycle solvent, respectively.

In a representative procedure, unsupported iron-oxygen compounds were synthesized by adding an ammonia solution (1.5 mol/L) dropwise to 500 mL of an aqueous ferrous sulfate solution (0.3 mol/L) with vigorous stirring until the pH value reached 7.5 ± 0.1. Subsequently, the precipitate was obtained and oxidized by bubbling air (600 mL/min) for 30 min, maintaining the pH value at 7.5 ± 0.1 with an ammonia solution. The temperature of the reaction solution was controlled so as to remain constant during both the precipitation and oxidation processes. The resulting precipitate was separated by centrifugation at 5000 r/min for 10 min and then dried at 120 ℃ under air for 24 h. Finally, the iron–oxygen compounds were ground to a particle size of less than 80 μm and stored under nitrogen in preparation for use. The unsupported iron–oxygen compounds synthesized at 20, 30, 40, 50, 60 and 70 ℃ are denoted herein as Fe-20, Fe-30, Fe-40, Fe-50, Fe-60 and Fe-70, respectively.

The coal-supported iron–oxygen catalyst precursors were prepared in a similar manner to the unsupported materials, except that 150 g of the dried coal was first added to the 500 mL FeSO₄ solution to form a slurry prior to the precipitation. The coal-supported iron–oxygen compounds synthesized at 30, 45, 60 and 75 ℃ are denoted as Fe/C-30, Fe/C-45, Fe/C-60 and Fe/C-75, respectively.

The Fe concentrations in the unsupported iron–oxygen compounds were determined from the mass of ash after calcination at 900 ℃ for 3 h. The Fe levels in the coal-supported iron-oxygen samples were obtained using an ultraviolet spectrophotometer (UNICO UV-2800A, USA). The samples were initially calcined at 900 ℃ for 3 h, after which hydrochloric acid (37 wt%) was added and then diluted by adding a mixture of ascorbic acid, acetic acid, sodium acetate and phenanthroline. The absorbance of the resulting solution at 510 nm was measured and compared to a standard calibration curve. The specific surface areas and pore volumes of the unsupported iron–oxygen compounds were obtained from N₂ adsorption/desorption isotherms determined at liquid N₂ temperature (–196 ℃) using an automatic analyzer (Micromeritics TristarⅡ3020, USA). Prior to these adsorption measurements, each fresh sample (100 mg) was degassed under vacuum at 100 ℃ for 6 h. The total pore volumes were calculated from the amount adsorbed at a relative pressure of 0.99. The specific surface areas were calculated by the Brunauer-Emmett-Teller (BET) method and the total pore volumes were calculated via the Barrett-Joyner-Halenda (BJH) method. Powder X-ray diffraction (XRD) patterns of the samples were acquired with a D/max-RA X-ray diffractometer (Rigaku, Japan) using Cu K_α radiation (λ = 0.154 nm), operated at 40 kV and 200 mA. Standard powder XRD cards, compiled by the Joint Committee on Powder Diffraction Standards (JCPDS), were used to identify the iron phases of the precursors and sulfided catalysts. Scanning electron microscopy (SEM) images were obtained with S-4800 and S-4700 microscopes (Hitachi, Japan) operated at 5 and 15 kV, respectively. Thermogravimetric (TG) analysis was performed using a thermal analysis instrument (SDT-Q600, USA). During these trials, the mass loss was measured while applying a programmed 10 ℃/min ramp to 200 ℃, followed by a 4 ℃/min ramp to 300 ℃ and a 10 ℃/min ramp to 800 ℃ under nitrogen (100 mL/min). Temperature-programmed reduction by H₂ (H₂-TPR) experiments were performed using a chemisorption analyzer (Chembet PULSAR, Quantachrome, USA). Each sample (15 mg) was pretreated by calcination at 100 ℃ for 1 h and subsequently cooled to 50 ℃ under a He flow (30 mL/min). Thereafter, the catalyst bed was subjected to a 5% H₂–95% Ar flow (30 mL/min) at 50 ℃ for 20 min, followed by heating at a rate of 10 ℃/min up to 900 ℃. Finally, the reduction sample was cooled under a He flow (30 mL/min). The H₂ consumption as a function of the reduction temperature was continuously monitored using a thermal conductivity detector (TCD).

Sulfidation of the unsupported iron–oxygen compounds was conducted with a 0.5 L autoclave (Parr4575, USA). During sulfidation, a quantity of the unsupported iron–oxygen compound (containing 0.42 g Fe) was combined with 0.48 g of sulfur as a sulfidation agent (at a S/Fe molar ratio of 2) and 60 g of recycle solvent in the autoclave. The autoclave was subsequently flushed with hydrogen three times and then pressurized with hydrogen to 10.0 MPa at room temperature. The autoclave was heated to 300 ℃ at a rate of 20 ℃/min in conjunction with stirring of the reaction solution at 500 r/min. At the point at which the reaction solution temperature reached 300 ℃, or after holding at 300 ℃ for 1 h, the autoclave was cooled rapidly to room temperature using a high-powered blower. The sulfided catalyst was washed with tetrahydrofuran (THF), followed by filtration and vacuum drying at 80 ℃ for 4 h to remove the THF, after which the catalyst was stored under nitrogen.

XRD and SEM were used to characterize the sulfided catalyst, and the average crystallite size of the Fe_1-xS was calculated via the Scherrer equation based on the full width at half-maximum of the (200) XRD peaks. The samples heated at 300 ℃ for 0 h and for 1 h are denoted herein as Fe-x-S/0 and Fe-x-S/1, respectively (where x represents the synthesis temperature).

Catalytic hydroliquefaction trials using the Shenhua Shangwan coal were performed in a 0.5 L autoclave reactor (Parr4575, USA). In the case of the unsupported catalyst, 28.0 g of dry coal, 42.0 g of recycle solvent, 0.32 g of elemental sulfur (such that the S/catalyst atomic ratio was 2/1) and a quantity of the catalyst (containing 0.28 g Fe, such that the Fe/coal mass ratio was 1/100, based on the Fe concentrations in Table 3) were added to the autoclave. In the case of the coal-supported catalysts, the amounts of recycle solvent and sulfur were the same, as was the Fe/coal ratio, while the amount of added coal was 28.0 g minus the mass of coal in the coal-supported catalyst. The Fe and coal concentrations in the coal-supported catalysts are provided in Table 3. After being flushed three times with hydrogen, the autoclave was pressurized with hydrogen to 10.0 MPa at room temperature then heated to 450 ℃ at a rate of 20 ℃/min with stirring at 500 r/min. After holding at 450 ℃ for 1.0 h, the autoclave was cooled rapidly to room temperature using a high-powered blower. The gaseous reaction products were analyzed using a gas chromatograph (Agilent 7890A), with the hydrocarbons detected with a flame ionization detector in conjunction with an Al₂O₃ capillary column, and the N₂, CO, CO₂, H₂ and H₂S detected with a TCD, using a 13X molecular sieve packed column. All liquid and solid products were recovered via sequential Soxhlet extraction with hexane and THF over 48 h. The hexane-soluble substances were defined as oils, while the hexane-insoluble, THF-soluble substances were defined as preasphaltene and asphaltene. Ash values were obtained by calcination of the THF-insoluble substances at 815 ℃ for 6 h in a muffle furnace. The coal conversion (X), oil yield (O), gas yield (G), preasphaltene and asphaltene yield (PAA), hydrogen consumption (H) and water yield (W) were calculated on a dry, ash-free (daf) basis using the following equations.

Here, F_daf is the mass of coal on a moisture-free and daf basis (g), H₀ is the mass of hydrogen added to the reactor (g), H₁ is the mass of hydrogen recovered after the reaction (g), G₁ is the mass of gaseous products (g), HI is the mass of hexane-insoluble substances (g), TI is the mass of THF-insoluble substances (g), Ash is the mass of ash (g), O₀ is the mass of elemental oxygen in the feed coal (g), O₁ is the mass of elemental oxygen in the gaseous products (g), and O₂ is the mass of elemental oxygen in the liquid and solid products (estimated to be 10.73% of the elemental oxygen in the feed coal [31], g).

It was assumed that the total mass of ash was equal to that of the THF-insoluble substances. The coal liquefaction data reported herein represent the average of three replicate trials, during which the relative deviations in the conversion and oil yield values were found to be within 0.5% and 1%, respectively.

The results obtained from catalytic coal liquefaction using unsupported and coal-supported iron–oxygen compounds are shown in Table 3. It can be seen that the catalyst precursors synthesized at lower temperatures exhibited better catalytic activity, such that the Fe-20 and Fe-30 gave the highest coal conversion and oil yield. The oil yield from the Fe-70 was decreased by 4.6% compared to that of the Fe-20, along with 0.4% lower hydrogen consumption and a 1.6% increase in asphalt yield. Among the coal-supported iron–oxygen compounds, the Fe/C-30 exhibited the highest coal conversion and oil yield, with values of 88.7% and 57.4%, respectively. The oil yield from the Fe/C-45 was slightly decreased (by 0.2%), while those obtained from the Fe/C-60 and Fe/C-75 were much lower.

SEM micrographs of the unsupported iron–oxygen compounds are presented in Fig. 1. The Fe-20 and Fe-30 particles were evidently rod-like or needle-like in shape, while a few spherical-like particles appeared in the Fe-40. These spherical particles became more common as the synthesis temperature was increased to 70 ℃, along with increases in the particle size (from 80–100 to 100–300 nm). These changes in morphology indicate that the growth rates of the crystal faces were affected by the synthesis temperature differently, and that the growth of the iron–oxygen crystals was primarily determined by thermodynamics.

A gradual transformation in the crystalline phase of the unsupported iron–oxygen compounds with increases in the synthesis temperature can be seen in Fig. 2. Initially, γ-FeOOH peaks are generated by the Fe-20, while the intensity of these peaks gradually weakens in the case of the Fe-30 and Fe-40. In addition, α-FeOOH peaks are first observed in the Fe-30 pattern, become more prominent in the Fe-40 pattern, then weaken and finally vanish in the Fe-50 and Fe-60 patterns. In addition, γ-Fe₂O₃ peaks were first generated by the Fe-40 and are more intense in the Fe-60 and Fe-70 patterns. The variations in the crystalline phases of these iron–oxygen compounds suggest that the synthesis temperature had an impact on the iron–oxide coordination and the connecting structural units in these materials. Lower temperatures were conducive to the formation of hydrogen bonds in γ-FeOOH, while moderate temperatures promoted the formation of α-FeOOH structures with regular channels, and higher temperatures favored the appearance of a cubic γ-Fe₂O₃ structure [32].

Fig. 3 presents the results of TG analyses of the unsupported iron–oxygen compounds. An obvious mass loss can be seen from 150–300 ℃ in each case, attributed to the conversion of FeOOH to Fe₂O₃. The Fe-20 and Fe-30 both had mass loss values of 9.4% in this region (Table 4), which is close to the theoretical value expected for the decomposition of FeOOH to Fe₂O₃ (10.11%). In contrast, the Fe-60 and Fe-70 showed values of less than 1.4%. The decreased mass loss of these compounds indicates a lower FeOOH content with increases in the synthesis temperature, in accordance with the XRD results.

Moisture content is thought to be an important factor that should be controlled when attempting to improve the catalytic activity of iron oxide powders [19], and has been used as a standard for selecting limonite catalysts for coal liquefaction [33]. The oil yields in the present work are plotted as a function of the unsupported iron–oxygen compound moisture contents in Fig. 4. These data demonstrate that a high moisture level improves the oil yield. In this study, the moisture content was correlated with the proportion of the FeOOH phase in the precursors, suggesting that the higher catalytic activity can actually be attributed to a greater amount of FeOOH. However, it was not possible to confirm whether the γ-FeOOH or α-FeOOH precursors had a stronger effect on the oil yield.

The BET data acquired from the unsupported iron-oxygen compounds are shown in Table 4, where it is evident that increases in the synthesis temperature decrease both the surface area and pore volume. The Fe-20 had the largest surface area of 72.3 m²/g and pore volume of 0.30 mL/g, while the surface areas of the Fe-60 and Fe-70 were only 3.8 and 1.5 m²/g, respectively, along with significantly decreased pore volumes. Increasing the synthesis temperature thus may reduce the surface area and pore volume and limit the transformation of the precursors to the active phase during coal liquefaction. Fig. 5 presents a plot of the oil yield as a function of the surface areas of the iron–oxygen precursors. A general trend is apparent in which higher surface area precursors produce higher oil yields. However, the initial rapid rise of this plot and subsequent plateau suggest that the initial surface area of the iron–oxygen compound precursor is not the only factor affecting the catalytic activity.

Information regarding the relative difficulty in transforming the precursor to the active phase can be obtained from H₂-TPR. Each of the unsupported iron–oxygen compounds show two hydrogen consumption peaks in Fig. 6. The peaks in the low temperature region (300 to 450 ℃) correspond to the reduction of Fe₂O₃ to Fe₃O₄, while peaks in the high temperature region (450 to 800 ℃) are due to the reduction of Fe₃O₄ to FeO and FeO to Fe [34]. A slight shift of the low temperature peaks to higher temperatures is evident (Fig. 6, arrow B), indicating that increases in the synthesis temperature impede the reduction of Fe₂O₃ to Fe₃O₄. In addition, the small shoulder peaks (Fig. 6, arrow A) associated with the reduction of fine crystallites of Fe₂O₃ to Fe₃O₄ are also shifted to higher temperatures. This result demonstrates that there were fewer fine Fe₂O₃ crystallites in the precursors made at higher temperatures. The difficulty in reducing Fe₂O₃ to Fe₃O₄ noted above would lower both the quantity and activity of active oxygen species in the iron-oxygen compounds, implying a more difficult transformation to the active phase (pyrrhotite) during coal liquefaction.

Sulfidation is an important step for the transformation of iron-containing precursors into Fe_1-xS, which acts as the active phase during coal liquefaction. As shown in Fig. 7, the morphologies of the sulfide generated in this work were similar to the morphologies of their precursors, but the manner by which each precursor was transitioned to the active phase was different. The rod-or needle-like FeOOH particles in catalysts synthesized at 20 to 40 ℃ were found to shrink or fracture to give small, thin hexagonal Fe_1-xS particles. In contrast, the spherical precursors made at 50 to 70 ℃ tended to transform directly into regular sheets of Fe_1-xS.

As shown in Fig. 8, each of the precursors was transformed into Fe_1-xS after sulfidation at 300 ℃ for either 0 or 1 h. The peak at 2θ= 26° indicates the presence of graphite (PDF #89-8487) due to a carbon deposition reaction involving the solvent, with graphite deposited on the catalyst surfaces. The average crystallite sizes of the Fe_1-xS were calculated using the Scherrer formula, and the (1-x) mean values were determined using an empirical formula developed by Djega-Mariadassou and Lambert et al. [35, 36]. The results are shown in Table 5.

The Fe-20-S/0, Fe-30-S/0 and Fe-40-S/0 had similar crystallite sizes and (1–x) value (crystallite sizes from 14.1–14.4 nm, (1–x) values from 0.855–0.860), but there were obvious increases in these parameters for the Fe-50-S/0, Fe-60-S/0 and Fe-70-S/0. It was also found that the crystallite sizes and (1–x) values of the sulfided catalysts increased slightly following sulfidation at 300 ℃ for 1 h. The crystallite size trend was similar to that exhibited by the (1–x) values, indicating a decrease in the coordination number of sulfur in the pyrrhotite with the crystallite size growth. An approximately linear correlation between the oil yield and the pyrrhotite crystallite size can be seen in Fig. 9, suggesting that smaller crystallites are associated with higher oil yields. The crystallite size of the pyrrhotite could therefore be one of the main parameters determining the activity of the catalyst.

The micrographs of the coal-supported iron–oxygen compounds are shown in Fig. 10. The iron–oxygen compounds synthesized at different temperatures were well dispersed on the surface of the carrier coal, and these compounds were similar to the unsupported catalysts in morphology. Moreover, the particle sizes of the iron–oxygen compounds on the coal were much smaller than those of the unsupported catalysts. The carrier coal played a role by inhibiting secondary polymerization of precipitated grains in the synthesis process. Although the carrier coal decreased the particle size and improved the dispersion of the iron–oxygen compounds, some aggregation still occurred. This suggests that the process should be further optimized. Fig. 11 shows XRD patterns acquired from the coal supported iron–oxygen compounds. The effects of synthesis temperature on the crystalline phases were similar to those observed with the unsupported materials, indicating a lack of obvious chemical interactions between the carrier coal and the precursors.

Although the addition of a coal support did not significantly affect the crystal structure and morphology of the iron–oxygen compound precursors, the catalytic activities of the coal-supported iron–oxygen compounds were superior to those of the unsupported materials synthesized at similar temperatures, because of the smaller particle sizes. In addition to improving the dispersion of the precursors, the carrier coal could potentially inhibit aggregation of the Fe_1–xS during coal liquefaction.

The performance of the catalyst during DCL is the result of many factors, and so it is difficult to identify a single parameter that directly correlates with the catalytic activity or oil yield. In this work, unsupported iron–oxygen compound precursors synthesized at lower temperatures and composed primarily of γ-FeOOH or α-FeOOH crystalline phases, also having high specific surface areas, elevated moisture levels and more active oxygen, readily transformed to fine pyrrhotite. These materials also exhibited superior catalytic activity during DCL. The type of pyrrhotite obtained was found to be closely related to the properties of the precursors, and an approximately linear correlation was established between the oil yield and the pyrrhotite crystallite size. The crystalline phase may be the most important attribute of the iron–oxygen compound precursors because this factor is closely associated with other important attributes, such as the texture, morphology and particle size of the precursor.

When used as a carrier for the catalyst, liquefaction coal does not modify the crystalline phase or morphology, but can improve the catalytic activity significantly. The carrier coal appears to effectively inhibit secondary polymerization and particle size changes of the iron–oxygen compounds. Both the particle size and the dispersion of the precursor are equally important to the formation of a crystalline phase. Therefore, using liquefaction feed coal as a carrier for the catalyst is a facile and effective means of improving catalytic performance during coal liquefaction.