Communication pubs.acs.org/EF
Efficient Direct Brown Coal Liquefaction with Sulfided Co/SiO2 Catalysts Martin Trautmann and Yvonne Traa* Institute of Chemical Technology, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany S Supporting Information *
several brown coals by ion exchange of Co with coal carboxyl groups. This procedure caused a high cobalt dispersion with high oil yields (58 wt %) and high conversions (97%), but the catalyst could not be recovered.7 Song et al. used soluble Mo− Co−S complexes impregnated onto the coal with high metal dispersion in brown coal liquefaction and reached oil yields up to 50 wt %.8 Mochida et al. pointed out the importance of catalyst recovery and applied a magnetic gradient for recovery of carbon/ferrite composites and Fe3Al powder from direct brown coal liquefaction residue.9 Sakanishi and co-workers employed Ketjen Black as a nanosized carbon support with low density for Fe, Ni, Mo, and combinations of these metals and claim that the catalyst can be recovered by gravimetric separation. They also successfully tested bottom recycle, thus reducing the amount of fresh catalyst needed.10,11 In our studies, iron, nickel, and especially cobalt as hydrogenation metals were interesting because of their ferromagnetism, which provides possibilities for separation by magnetic gradient. Our inspiration was to use cheap and nanostructured SiO2 (fumed silica) as a novel, thermostable coal dissolution support with a high surface area (352 m2 g−1) to maximize the possible contact area between the catalyst and the coal (fragments). This support has not been used for direct coal liquefaction before. The reactant brown coal from a mine in Leipzig, Germany, possesses a high natural H/C molar ratio of 1.25, which simplifies coal to liquid transformations. The high aliphatic content (at least 90% of the carbon in the coal is aliphatic) could be shown by attenuated total reflectance (ATR) and solid-state nuclear magnetic resonance (NMR) (see the Supporting Information). The ultimate analysis for the moisture- and ash-free (maf) coal was 73.6 wt % carbon, 7.7 wt % hydrogen, 0.3 wt % nitrogen, 1.7 wt % sulfur, and 16.7 wt % oxygen (by difference). Our brown coal yields 4.66 wt % ash on a dry basis. The higher heating value (HHV) of the brown coal was calculated with the Dulong equation12 (eq 1), where ZC, ZH, ZS, and ZO are the weight percentages of carbon, hydrogen, sulfur, and oxygen related to the maf coal mass, to 33.1 MJ kg−1, respectively.
The rapid depletion of the reserves and resources of crude oil and the increasing price per barrel enforces the consideration of alternatives for the fuel sector. One solution could be coal conversion, because coal is more abundant and globally wider distributed than crude oil.1 The production of liquid fuels from coal is possible via direct coal liquefaction (DCL, coal hydrogenation) or indirect coal liquefaction (coal gasification followed by Fischer−Tropsch synthesis). The former technique is preferred with respect to the overall energy consumption. However, the associated technology (DCL) is a catalytic process with severe conditions up to 30 MPa hydrogen pressure and 763 K with a process-derived solvent.2−5 The reaction encompasses two stages: dissolution of coal (usually H/C molar ratio ≈ 0.8) to primary coal liquids in the first stage and upgrading by conventional techniques of crude oil refining to synthetic crude oil in the second stage (H/C molar ratio ≈ 2.0).3 The coal liquids of the first stage, which are soluble in pentane, are defined as oils.4 The first step is the most difficult step, because the organic macromolecules in coal must be cleaved. In the presence of a catalyst, the cleavage of connecting linkages in the coal is promoted by the addition of free H atoms to the ipso positions; thereafter, the weakened linkages can be thermolytically cleaved. The catalyst serves as a pump to provide free H atoms. If solvents are present, the dispersion of the catalyst is facilitated, and solvent−catalyst reactions can improve H transfer.6 High hydrogen pressures are required because the catalyst is mostly a cheap disposable (oncethrough) catalyst with moderate activity, such as FeS2 (NEDOL process) or red mud (Kohleöl process). Normally, catalyst recovery from the liquefaction residue is not economical.3 To make direct coal liquefaction economically more attractive, a more sustainable catalyst with a higher activity should be developed, so that the high hydrogen pressures, which are a major cost factor, could be reduced. However, more active catalysts generally employ more active and more expensive metals, so that, on the one hand, high metal dispersions are desirable and, on the other hand, catalyst recovery is usually necessary. High metal dispersions can be achieved with supported catalysts, which are not widely used for the primary coal liquefaction stage, because the coal fragments are not thought to enter the pores efficiently.3−5 One of the few exceptions is the single-stage H-Coal process by Hydrocarbon Technologies, Inc., where Ni−Mo/Al2O3 and Co−Mo/Al2O3 are used as catalysts. The process is operated at 723 K and hydrogen pressures of 20 MPa. Spent catalyst particles can be partially removed from the ebullated bed reactor by gravity separation and can be returned to the process.3,5 Sugano et al. introduced cobalt as a catalyst metal on © 2013 American Chemical Society
HHV (MJ kg −1) = 0.3389ZC + 1.433Z H + 0.094ZS − 0.179ZO (1)
We performed brown coal liquefaction in a 250 mL batch reactor. In preliminary experiments under sub-optimal conditions, we compared Fe, Ni, and Co impregnated onto Received: May 22, 2013 Revised: August 27, 2013 Published: August 28, 2013 5589
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Table 1. Results of Direct Brown Coal Liquefaction Reactionsa entry
catalyst,b reaction time
active metal usedc (wt %)
1 2 3 4 5 6
-, 2 h Co/SiO2, 2 h -, 1 h Co/SiO2, 1 h S−Co/SiO2,g 1 h FeS2, 1 h
1.8 1.8 1.8
A B C
Fe/SiO2, 2 h Ni/SiO2, 2 h Co/SiO2, 2 h
0.9 0.9 0.9
1.8
oil yieldd (wt %)
gas yieldd (wt %)
R + A + Pd,e (wt %)
conversionf (%)
46 55 47 52 55 47 Preliminary Experimentsh 44 48 53
32 44 29 35 32 32
22 1 24 13 13 21
78 99 76 87 87 79
43 34 36
13 18 11
87 82 89
a Reaction conditions: temperature, 673 K; initial H2 pressure, 10 MPa; maf brown coal, 10 g; and solvent, tetralin (50 g, 378.78 mmol). bSupported catalysts (1 g) with about 18 wt % Co on SiO2 were used for that purpose. For FeS2, 0.388 g (3.07 mmol) was employed to reach the same active metal weight. cTotal mass of active metal related to the original maf coal mass. dProduct yields related to the original maf coal mass. The standard deviation between oil yields from reproduction experiments was ±0.5 wt %. eR + A + P = residue, asphaltenes, and preasphaltenes; because the amounts of this fraction were rather small, they were looked upon as residue. For entries 3 and 5, the sum of asphaltenes and preasphaltenes amounted to 21 and 11 wt %, respectively, thus showing that this is a pessimistic approximation. fConversions were calculated by defining R + A + P as unconverted coal. gPresulfidation of Co/SiO2 in the batch reactor with an excess of elemental sulfur for 2 h under reaction conditions. hIn preliminary experiments, only half of the amount of catalyst was used, i.e., 0.5 g of supported catalyst with about 18 wt % metal on SiO2.
Figure 1. X-ray powder diffraction patterns of (a) freshly activated Co/SiO2, (b) Co9S8/SiO2 after reaction, and (c) ex situ sulfided S−Co/SiO2.
qualitatively that the largest fraction of the oils are C13−C24 aliphatic hydrocarbons. With low effective concentrations of cobalt (dispersed on SiO2) of 1.8 wt % related to maf coal, significant increases in oil yields and conversions can be achieved in comparison to noncatalytic runs (entries 1−4 in Table 1). With the same effective metal concentration of FeS2 (entry 6 in Table 1), which is the catalyst of the NEDOL process,3 the oil yields and coal conversions of cobalt-catalyzed reactions could not be reached. Hence, the Co/SiO2 catalyst employed in this work with its high metal dispersion on a nanostructured support has obviously better effects on the oil yields and the coal conversions than the conventional disposable catalyst FeS2. However, the magnetic properties of Co/SiO2 were lost during the reaction, indicating that a new phase on the support has formed, which is not ferromagnetic. By powder diffraction of the freshly activated Co/SiO2 catalyst and the catalyst after 2 h of liquefaction (entry 2 in Table 1), the new phase became evident (Figure 1). The broad peak at 2θ = 44.1° for catalyst a could be assigned to metallic cobalt with high dispersion, and the peaks for catalysts b and c indicating a new Co9S8 phase are at 2θ ≈ 29.6°, 31.0°, 39.3°, 47.3°, and 52.0° (for peak assignments, see the Supporting Information). The atomic
fumed silica. The metal particles are ferromagnetic and could be easily attracted by a common bar magnet before the reaction. As Table 1 (preliminary experiments, entries A−C) shows, Co is superior to Fe and Ni concerning the best oil yields and coal conversions. Thus, our further study focuses on Co/SiO2 catalysts. For all further reactions, the effective supported cobalt concentration was 1.8 wt % related to the maf coal. The primary quality features in coal liquefaction are coal conversion and oil yields. In many industrial (not currently running) processes, conversions range between 63 and 93% and oil yields range between 30 and 65 wt %.3 Hence, the conversions of 76−99% achieved in this work and the liquefaction oil yields between 44 and 55 wt % (Table 1) demonstrate that our liquefaction procedure with relatively mild reaction conditions is appropriate for catalyst development. Both, catalytic and non-catalytic runs (entries 1 and 2 in Table 1) yielded oils having almost the same elemental composition (±0.3 wt % standard deviation) of 86.1 wt % carbon, 11.5 wt % hydrogen, 0.2 wt % nitrogen, 0.4 wt % sulfur, and 1.8 wt % oxygen and higher heating values of about 45.4 MJ kg−1 (±0.2 MJ kg−1 standard deviation). The oils consisted of C5−C24 alkanes. By investigation of the product oils by gas chromatography/mass spectrometry (GC/MS), we found 5590
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retrograde reactions because they are poorly stabilized, which explains the lower coal conversions, the higher amount of organic residues, and the lower oil yields (Table 1). To discuss the quality of the obtained coal liquids, H/C and O/C molar ratios are plotted in a Van Krevelen diagram (Figure 2). Because the brown coal used has already a high
ratios of S and Co could be confirmed by energy-dispersive Xray elemental analysis. For catalyst b, there are also additional peaks at 2θ ≈ 27.0°, 50.0°, and 50.1°, which could indicate a modification of the Co9S8 phase. Obviously, the sulfur in the coal (1.7 wt %) is responsible for the formation of the Co9S8 phase on SiO2, because the sulfidation of Co/SiO2 with an excess of sulfur under the same conditions as in the reaction leads almost to the same phase (Figure 1c). This means that the thermodynamically most stable phase under the applied liquefaction conditions is Co9S8. Furthermore, the cobalt sulfides are also the more active catalytic phase, because the direct comparison of presulfided S−Co/SiO2 and Co/SiO2 led to a slightly higher oil yield of 55 wt % on the presulfided catalyst compared to 52 wt % (entries 4 and 5 in Table 1). These findings match with the results by Derbyshire,6,13 who also reports that, under liquefaction conditions, metal sulfides are not only the thermodynamically more stable but also the more active phases. The activities of both catalysts with and without presulfidation and non-catalytic runs were investigated closer by reactions with the coal model compound 4-methoxythiophenol. Instead of 10 g of brown coal, 10 g of the model compound were used, keeping the remaining reaction conditions constant. We chose this compound because it contains characteristic C−O bonds of brown coal5 and a sulfur group, which plays a crucial role in our reactions. Table 2 shows
Figure 2. Van Krevelen diagram of different hydrocarbon sources for fuels and liquefaction oils obtained in this work and the “Kohleöl process”.14−17
Table 2. Reactions of Brown Coal Model Compound 1aa H/C molar ratio, comparably high H/C values of the product oils can be obtained in contrast to the oil from the German “Kohleöl process”. In addition, the oils obtained in this work are close to crude oil, so that proportionally less energetic and material resources are necessary for further upgrading. The best H/C molar ratio of 1.69 could be obtained with presulfided S− Co/SiO2 (entry 5 in Table 1). Furthermore, Figure 2 shows that the brown coal liquefaction oils of this work are superior to other oil-type liquids currently investigated as alternatives to crude oil, such as bitumen from oil sands, heavy crude oil, or bio-oil from pyrolysis of biomass. Because of the fact that 99% conversion could be achieved (entry 2 in Table 1), the catalyst could be recovered. For this purpose, we took the residue of the reaction and washed it with acetone to remove residual soluble carbon deposits. Elemental analysis and thermogravimetric analysis showed that about 25% of the remaining solid after washing was coke. The coke was burnt in flowing air (100 dm3 h−1) by heating slowly from 293 to 823 K (heating rate of 1 K min−1). After burning of the coke followed by reduction in hydrogen (see the Supporting Information), we were able to recover 70 wt % of the original catalyst mass for the reaction, which could be used for further liquefaction experiments. In a continuous process with an optimized procedure, higher percentages of the catalyst could certainly be recovered from the reaction mixture, e.g., by gravity separation. Altogether, we could find a good supported coal liquefaction catalyst, which is effective with the small metal concentration of 1.8 wt %. Because of the high H/C molar ratio and the low aromaticity of the coal, the brown coal used served as a perfect basis for direct liquefaction and high-quality product oils in good quantities could be obtained. Unfortunately, the catalyst lost its ferromagnetic properties during the reaction. However, because of the high conversion, the catalyst could be recovered by burning of the coke.
selectivityb (%) entry
catalyst
conversionb (%)
2a
2b
2c
2d
1 2 3 4
0 SiO2 Co/SiO2 S−Co/SiO2
91 91 100 100
4 2 3 5
33 49 78 75
63 49 13 18
0 0 6 2
a
Reaction conditions: see Table 1, besides the reactant being 10 g of compound 1a instead of 10 g of brown coal. bConversions and selectivities were calculated from GC analysis.
the results of these comparisons. In the non-catalytic reaction and the reaction with the metal-free support, the conversion was lower compared to both catalytic runs with full conversion (entries 1 and 2 in Table 2). The selectivities were higher for C1−C4 gases than for the other products. In contrast, the catalytic runs had higher selectivities to the liquid products (entries 3 and 4 in Table 2). Those results suggest that the stabilization of intermediates during the catalytic reactions of 4methoxythiophenol works better than in the non-catalytic run, where the organic radicals undergo consecutive reactions to C1−C4 gases. The metal of the catalysts can stabilize the intermediates with activated hydrogen. For both catalysts, the same diffraction pattern as in Figure 1c was obtained after the model reactions. When the model concepts are applied to the bigger fragments of coal occurring during the liquefaction reaction, it can be assumed that they could be saturated and/or further converted by activated hydrogen because of the active Co9S8 phase, which is also formed after the liquefaction (Figure 1b). In non-catalytic runs, the fragments are likely to undergo 5591
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details on materials, reactions, catalyst preparation, and characterization. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +49-711-685-64061. Fax: +49-711-685-64065. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS First, we want to thank MIBRAG for allocating the brown coal used in this work from Tagebau Schleenhain (Leipzig, Germany). Further, we thank C. Lauxmann of the Institute of Organic Chemistry, University of Stuttgart, for elemental analysis. Additional thanks go to T. Montsch for TEM and Prof. M. Hunger for NMR spectra.
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REFERENCES
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