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Catalytic Liquefaction of Coal with Highly Dispersed Fe2S3 Impregnated in-Situ† Haoquan Hu,*,‡,§ Jinfeng Bai,‡ Hejun Zhu,‡ Yong Wang,‡ Shucai Guo,‡ and Guohua Chen| Institute of Coal Chemical Engineering, Dalian University of Technology, 129 street, Dalian 116012, P.R. China, State Key Lab of Coal Conversion, Institute of Coal Chemistry, Chinese Academy Science, Taiyuan, P.R. China, and Department of Chemical Engineering, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, P.R. China Received October 16, 2000. Revised Manuscript Received April 9, 2001
Daliuta subbituminous coal, from Shenfu, Shanxi province of China, was liquefied in a 50 mL micro-autoclave apparatus at 440 °C, initial hydrogen pressure of 6.0 MPa, soaking time of 30 min, using a mixture of tetralin and cyclohexane as solvent. The experiments were carried out to investigate the effects of in-situ impregnated Fe2S3 on the liquefaction conversion, oil and gas yields of the coal, and the aromatic, aliphatic, and polar compounds fraction content in the oil. The effect of surfactant treatment during catalyst impregnation was also studied using hexadecyltrimethylammonium bromide. Gradient elution chromatography (GEC), GC, and GC-MS were used to characterize and quantify the coal liquids. XRD and TEM were used to characterize the catalyst. The results indicate that without catalyst the conversion and oil yield are 43.2 and 37.4 wt %, respectively. At the same reaction condition but with the addition of 1.0 wt % Fe (based on daf coal), the conversion and oil yield reach 62.6 and 54.2 wt %, respectively. When the surfactant was used during the 1 wt % Fe catalyst impregnation, the conversion and oil yield became 68.8 and 59.5 wt %, respectively. The catalyst is dispersed in nanometer size particles in amorphous phase that transforms to pyrrholite phase during the liquefaction. The addition of 9.2 × 10-4 M surfactant changes the ζ-potential of the coal particles from -15 mV to +29 mV, decreases the size of catalyst particle on the coal surface from 30-40 to 15-20 nm. The oil products are complex aromatics with 2 to 3 rings.
Introduction Coal is the most abundant fossil fuel in the world. Its effective utilization will play an important role in sustainable development. Liquefaction is one of the challenging technologies in comprehensive utilization of coal. To date, direct coal liquefaction is still difficult to compete with petroleum economically. That is why such a process has no commercial implementation yet. Although this trend will remain for a certain period of time, there is a niche market, however, for the utilization of the polycyclic aromatic structure in coals because the high value aromatic hydrocarbon chemicals and polar compounds are difficult to obtain from petroleum processing. For this purpose, several researchers have carried out some investigations on this topic.1-5 † Part of the work has been presented in the 17th Annual International Pittsburgh Coal Conference, Sept. 11-15, 2000, Pittsburgh, PA. * Corresponding author. Tel.:+86-411-3631333-3250. Fax: +86-4113646633. E-mail:
[email protected] (Haoquan Hu). ‡ Dalian University of Technology. § Institute of Coal Chemistry, Chinese Academy Science. | The Hong Kong University of Science & Technology. (1) Song, C. S.; Schobert, H. H. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37 (2), 524. (2) Song. C. S.; Schobert. H. H. Fuel Process. Technol. 1993, 34, 157. (3) Song, C. S.; Saini, A. K. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39 (4), 1103. (4) Nomura, M.; Moritaka, S.; Miura, M. Energy Fuels 1995, 9, 936.
Catalysts affect the direct coal liquefaction significantly. The development of novel catalysts with high activity, low cost, and environmentally benign characteristics receive considerable attention recently. One of the preferred catalysts is the ultra-fine and highly dispersed iron-based catalyst.5-8 Liu et al. developed an in-situ impregnated iron sulfide catalyst by mixing a coal-containing Na2S solution with an FeCl3 solution.7 The catalyst so made produces nanometer sized iron sulfides precipitated on coal surface. With this method, high coal conversion and high hydrogenation activity were achieved.8 Catalyst dispersion on coal surface is dependent upon the nature of the catalyst and the coal surface. Pretreated coal surface with an appropriate surfactant can enhance catalyst dispersion.9 In the present study, in-situ impregnated iron sulfide catalysts were prepared by mixing a coal-containing Na2S solution with an Fe2(SO4)3 solution. A surfactant, (5) Satou, M.; Endoh, H.; Chiba, T.; et al. Proc. 9th Int. Conf. Coal Sci., Essen, Germany, Sept. 1997, 1537. (6) Kaneko ,T.; Tazawa, K.; Okuyama, N.; Tamura, M.; Shimasaki, K. Fuel 2000, 79, 263-271. (7) Liu, Z. Y.; Yang, J. L.; Zondlo, J. W.; Stiller, A. H.; Dadyburjor, D. B. Fuel 1996, 75 (1), 51-57. (8) Liu, Z. Y.; Yang, J. L.; Zondlo, J. W.; Stiller, A. H.; Dadyburjor, D. B. Energy Fuels 1995, 9, 673. (9) Abotsi, G. M. K.; Bota, K. B.; Saha, G.; Mayes, S. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 1996, 41 (3), 984-987.
10.1021/ef000227f CCC: $20.00 © 2001 American Chemical Society Published on Web 06/13/2001
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Energy & Fuels, Vol. 15, No. 4, 2001 831 Table 1. Reproducibility of Experimental Resultsa no catalyst
measured average standard variance
1% Fe as catalyst
O+G
conversion
aromatics
polar compounds
37.05 37.40 37.90 37.45 0.43
42.35 43.20 43.96 43.17 0.80
9.12
9.15
7.74 8.43 0.98
7.33 8.24 1.29
O+G
conversion
aromatics
polar compounds
53.66 54.25 54.72 54.21 0.53
61.64 62.70 63.58 62.64 0.97
13.69
14.13
13.72 13.70 0.02
12.31 13.22 1.29
a Liquefaction conditions: 440 °C, 30 min, 6 Mpa H , 4 g of coal sample, 2 mL tetralin + 6 mL cyclohexane as solvent, impregnation 2 of catalyst without using surfactant.
hexadecyl-trimethylammonium bromide (HTAB), was used to improve the coal surface properties during the catalyst impregnation. The effects of catalyst and surfactant on coal liquefaction were investigated in terms of the liquefaction conversion and the fraction yield. Experimental Section Coal Sample and in-Situ Impregnation of Fe2S3. The coal used is Daliuta subbituminous coal (abbr. DL) from Shenfu, Shanxi province of China. Its moisture, ash, volatile matter, and fixed carbon contents are 6.04, 4.34, 39.95, and 49.67 wt %, respectively, on an as received basis. Ultimate analysis gives 79.91 wt % carbon, 5.24 wt % hydrogen, 1.05 wt % nitrogen, and 13.80 wt % sulfur plus oxygen (by difference) on dry ash-free basis. Coal samples, with a particle size of -100 mesh, were dried at 100 °C for 8 h under vacuum before liquefaction. Then the in-situ impregnation of catalyst was carried out as follows: 4 g coal samples were mixed with certain 0.24 M Na2S solution followed with the addition of some 0.08 M Fe2(SO4)3 solution while stirring. The amount of 0.24 M Na2S solution and 0.08 M Fe2(SO4)3 solution was added to the system according to the required Fe loading. The Na2S will chemically react with Fe2(SO4)3 according to the following equation and form Fe2S3.
3Na2S + Fe2(SO4)3 f Fe2S3V +3Na2SO4
cyclohexane followed by tetrahydrofuran (THF). The THFinsolubles were dried in a vacuum oven at 100 °C for 8 h. The conversion was defined as the percentage of coal converted into THF-solubles and gases on dry and ash-free (daf) basis, which is calculated by (Wcoal - Wresidue)/Wcoal × 100%, where Wcoal is the dry ash-free coal weight in the reactor and Wresidue is the dry ash-free residue weight after coal liquefaction and Soxhlet extraction with THF, i.e., THFinsolubles. The cyclohexane-solubles were defined as oil while the cyclohexane-insolubles but THF-solubles as asphaltene and preasphaltene (A + PA). Oil and gas (O + G) yield was determined as the percentage of coal converted into cyclohexane-solubles and gases on daf basis. The oil fraction was further separated into aliphatics, aromatics, and polar compounds through sequential rinsing in silica column chromatograph by cyclohexane, benzene, and methanol, respectively, according to ASTM D 2549-81. Characterization of Coal, Catalyst, and Liquid Products. The ζ-potential values of the coals were measured at room temperature using a Brockhaveny Zetaplus ζ-meter. TEM of in-situ impregnated Fe2S3 in the coal samples was determined with HITACHI H-800. XRD patterns of different samples were obtained on a Dmax-rA X-ray diffraction meter with a Cu KR radiation operated at 35/40 kV and 40/70 mA. The fractions of the oil were quantified by HP 6980/5973 GC-MS with a column of HP-5 and CARLO ERBA model 4200 GC with a 30 m column of SE-54.
Under alkali condition, some Fe(OH)2 may also be formed as
Results and Discussion
Fe2+ + 2OH- f Fe(OH)2
Reproducibility of Experimental Results. In this work, all the experimental results are the mean value of at least two replicates. As an example, Table 1 shows reproducibility of the conversion, yields of O + G, aromatics, and polar compounds obtained in different experiments but at the same conditions. It can be seen that the standard variance for the conversion and yields of the O + G, aromatics, and polar compounds in oil fractions are less than 0.6, 1.0, 1.0, and 1.5, respectively. The difference of measured values between maximum and minimum is less than 2%. Effect of Catalyst Loading. Figure 1 presents the liquefaction conversion, O + G and A + PA yield of insitu Fe2S3-impregnated coals. It can be seen that with increase of Fe2S3 addition, the conversion and O + G yield increase significantly until Fe content is about 1 wt % (daf) while the A + PA yield does not change appreciably. This indicates that the highly dispersed insitu impregnated catalyst has a high catalytic activity on the coal liquefaction. The in-situ impregnated catalyst can make the free-radical fragment produced during the liquefaction process combined quickly with the active hydrogen because of the close contact of active metal with coal surface. Hence the retrogressive reaction is suppressed, thus the liquefaction conversion and small molecular weight products are increased.10 The
After reaction, the mixture was filtered and the cake was dried in the oven for 8 h at 50 °C under vacuum. For the Fe2S3impregnated coal sample with surfactant pretreatment, the same procedure as above was used except the Na2S solution with surfactant HATB was used in the place of Na2S solution. Liquefaction Experiment. The coal liquefaction experiments were carried out in a parallel 50 mL bomb reactor. Four grams of the dried coal loaded with catalyst was charged into the reactor together with 8 mL of solvent (2 mL tetralin and 6 mL cyclohexane). The less hydrogen donor solvent used in this study is selected in order to separate oil easily using a silica column chromatograph for the analysis of fractions of the oil by gas chromatography. The cyclohexane is to improve the mixing of coal with solvent. In addition, cyclohexane is in the supercritical state during the liquefaction process therefore may enhance the mass transfer. Before the liquefaction experiment, the reactor was sealed and flushed several times with hydrogen followed by pressuring the system to the desired initial pressure of 6.0 MPa with hydrogen. The reactor, agitated vertically at 120 times/min, was heated to 440 °C in 5 min by a fluidized sand-bath furnace and maintained at that temperature for 30 min. Then the reactor was quenched to ambient temperature in a water bath before the overhead pressure in the reactor was released slowly. The solid and liquid phases that remained in the reactor were washed out with cyclohexane and extracted in a Soxhlet extractor with
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Figure 1. Effect of catalyst loading on conversion and yields (conditions: 440 °C, 30 min, 6 Mpa H2, 4 g of coal sample, 2 mL tetralin + 6 mL cyclohexane as solvent).
Figure 3. Relation between ζ-potential of coal and surfactant concentration.
Figure 2. Effect of surfactant (9.2 × 10-4 M HTAB) pretreatment on conversion and yields at different catalyst loading (conditions: 440 °C, 30 min, 6 Mpa H2, 4 g of coal sample, 2 mL tetralin + 6 mL cyclohexane as solvent).
Figure 4. Effect of surfactant concentration on conversion and yields at catalyst loading of 1 wt % Fe.
conversion and O + G yield reach 62.6 and 54.2 wt % daf at 1 wt % Fe impregnation which are, respectively, 19.5 and 16.8 wt % daf higher than those obtained with raw coal, i.e., without any catalyst. Effect of Surfactant. Figure 2 shows the liquefaction conversion, O + G and A + PA yield of in-situ Fe2S3impregnated coals with surfactant (9.2 × 10-4 M HTAB) treatment. It can be seen that when the catalyst was treated with surfactant during preparation, the conversion and O + G yield are, respectively, 68.8 and 59.5 wt % daf at 1 wt % Fe impregnation, representing 6.2 and 5.3 wt % daf increases over those with the same amount of Fe catalyst but without surfactant treatment. This may be explained from the variation of electrical properties and surface tension of the coal surface after addition of surfactant. To verify the hypothesis, the ζ-potential values of coal in different HTAB concentration solutions with pH value slightly higher than 7 were measured and shown in Figure 3. It can be seen that the ζ-potential of coal increases greatly with the increase of HTAB concentration. The ζ-potential of coal is -15.4 mV without addition of surfactant and is +29.1 mV when the concentration of surfactant is near the critical micelle concentration (CMC) of HTAB, 9.2 × 10-4 M. (10) Suzuki, T. Energy Fuels 1994, 3, 341-347.
Without using surfactant the coal surface is negatively charged because of the existence of polar function, such as -COOH, -CdO, and -S-. With the addition of surfactant, HTAB, the coal surface becomes positively charged which is beneficial for the adsorption of S2- and successively Fe3+. Meanwhile with the addition of surfactant, the surface tension of solution decreases from 72 × 10-5 N/cm to 34 × 10-5 N/cm at CMC of HTAB. This will also be beneficial for Fe2S3 to disperse on the coal surface which in turn promotes coal liquefaction. Figure 4 shows the conversion and yield of insitu Fe2S3-impregnated coal with different concentration of HTAB. It can be seen that with the increase of HTAB concentration, the conversion and O + G yield increase. Furthermore, in-situ impregnated Fe2S3 in water solution and HTAB solution show different particle sizes as illustrated by the TEM images in Figure 5. First of all, the Fe2S3 catalyst dispersed on the coal surface as nanometer sized particles. The particle sizes are 3040 nm when the coal surface was not treated with surfactant. When HTAB was used, the impregnated Fe2S3 particles have sizes between 15 and 20 nm. The finer particle size, noted in Figure 5b, provides much larger active surface areas for catalysis at the given weight of active metal loading. XRD analysis was conducted to further characterize the catalyst, Figure 6. The XRD pattern of Fe2S3impregnated coal in water solution does not show any peaks from 10 to 80° of 2θ except for at 21, 26, and 29
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Energy & Fuels, Vol. 15, No. 4, 2001 833
Figure 5. TEM of in-situ impregnated Fe2S3 in coal: (a) without surfactant; (b) with surfactant.
Figure 6. XRD of Fe2S3-impregnated coal and its liquefaction residue.
of 2θ which is very similar to that of raw coal sample. This indicated that the Fe-containing species formed insitu are either amorphous or in very small sizes which agrees with the TEM of the samples. While the Fe2S3impregnated coal in surfactant, the XRD pattern shows some crystalline pyrrhotite phase that is widely believed to be an active phase for coal liquefaction.11-13 This could be partially the reason for the different conversion and yields between Fe2S3-impregnated coal with and without surfactant. The Fe compounds impregnated on coal are expected to transform during coal liquefaction. This expectation is confirmed by the appearance of peaks at 30 to 80 degrees of 2θ on the XRD of the residue that indicates the formation of crystalline pyrrhotite phase. The findings here are consistent with previous studies.14,15 (11) Mochida, L.; Sakanishi, K. Adv. Catal. 1994, 40, 39. (12) Derbyshire, F.; Hager, T. Fuel 1994, 73, 1087. (13) Cugini, A. V.; Krastman, D.; Lett, R. G.; Balsone, V. D. Catal. Today 1994, 19, 395. (14) Taghiei, M. M.; Huggins, F. E.; Ganguly, B.; Huffman, G. P. Energy Fuels 1993, 7, 399.
Figure 7. Effect of catalyst loading on oil fraction yields with and without surfactant pretreatment (conditions: 440 °C, 30 min, 6 Mpa H2, 4 g of coal sample, 2 mL tetralin + 6 mL cyclohexane as solvent).
Oil Fractions Yields. Figure 7 presents the aromatics, aliphatics, and polar compound yields obtained from different Fe2S3-impregnated coal with and without the usage of surfactant. It can be seen that the aromatics and polar compound yields increase with the increase of catalyst loading of Fe until 1 wt %. At the catalyst loading of 1 wt % Fe without use of surfactant, the aromatics and polar compound yields are 13.4 and 13.2 wt % daf, respectively, which are about 4 and 5 wt % daf higher than those obtained from liquefaction of the raw coal. When the surfactant was used during the catalyst impregnation, the aromatics and polar compound yields are further increased by 22 and 16.7% to have the values of 16.3 and 15.4 wt % daf, respectively. The aliphatics yield remains relatively constant at about 5 wt %. (15) Taghiei, M. M.; Huggins, F. E.; Ganguly, B.; Huffman, G. P. Energy Fuels 1994, 8, 31.
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The components of aromatics were analyzed by GC/ MS. The results suggest that the main components are 2 to 3 ring aromatic compounds and are very complex.16 Further processing is necessary to get simple aromatic compounds. Conclusion In-situ impregnated Fe2S3 catalyst presents high catalytic activity in Daliuta coal hydro-liquefaction. Loading of 1 wt % Fe is sufficient with the conversion and O + G yield being 1.20 and 1.17 times those of raw coal at initial cool hydrogen pressure of 6.0 MPa reacting at 440 °C for 30 min. The increase in oil yield with increasing catalyst loading is due mainly to the increase in the aromatic and polar fractions, without (16) Bai, J. F. Coal liquefaction for producing aromatic hydrocarbons. Dissertation, Dalian University of Technology, 2000.
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significant changes in the yield of saturate fraction. Insitu impregnated Fe2S3 coal with surfactant can get even higher conversion and O + G yield because of the increase of ζ-potential, decrease of surface tension, and thus decrease of catalyst particle sizes. In-situ impregnated Fe2S3 coal has high aromatics and polar compounds yield in its liquefaction oil fraction than raw coal. The addition of surfactant during catalyst impregnation results in an increase in these yields. Acknowledgment. This work was performed with the supports of the Nature Science Foundation of China (No. 29876005), the grant-in-aid for Science Research, the State Key Laboratory of Coal Conversion, Taiyuan, P.R China (Project 97-10) and Science & Technology Commission of Liaoning Province of P.R. China [PostDoc Starting Fund (1995)043]. EF000227F
