Energy & Fuels 1998, 12, 843-848
843
Influences of Exchanged Metal Ions on Coal Liquefaction Kenji Murakami,† Hiroyuki Shirato, Naoki Hanada, and Yoshiyuki Nishiyama* Institute for Chemical Reaction Science, Tohoku University, Sendai 980-77, Japan Received September 5, 1997. Revised Manuscript Received May 18, 1998
To clarify the effectiveness of ion exchange as a method of loading liquefaction catalysts, a subbituminous coal and three brown coals were loaded with iron catalysts by ion exchange, impregnation, or physical mixing and liquefied in a laboratory scale small autoclave. For Yallourn coal from Australia, the liquefaction yields of the iron-exchanged coal with added sulfur as promoter were higher than that of coals loaded by impregnation and physical mixing methods. For this coal, the amount of exchanged iron could be enhanced by a calcium preexchange treatment, leading to a further increase in liquefaction yields. In the cases of the other coals, the activities of the exchanged iron catalysts were nearly the same or lower than those of the pyrite powder physically mixed with coal. To explain this result, it is necessary to consider the fact that these coals have a small amount of cation exchange capacity and the exchanged iron is not act as effective as the same amount of the pyrite powder. Thus, the effectiveness of the ion exchange is strongly dependent on the structure of the coal. The activity of the iron catalyst was significantly enhanced by the addition of nickel.
Introduction It is well accepted that iron is the most practical catalyst for coal liquefaction. Previous research on coal liquefaction has revealed that the pyrrhotite is the active species to which iron catalyst is transformed by addition of sulfur as a promotor.1,2 Furthermore, some reports have demonstrated that the highly dispersed catalysts, e.g., catalysts soluble in the solvent,3-9 ultrafine sized catalysts,10-12 and catalysts prepared using cation exchange13-22 and impregnation methods,23-26 * To whom correspondence should be addressed. Tel.: +81-22-2175629. Fax: +81-22-217-5631. E-mail:
[email protected]. † Present address: Department of Materials Engineering and Applied Chemistry, Mining College, Akita University, Akita 010, Japan. Tel.: +81-188-89-2433. Fax: +81-188-37-0404. E-mail: murakami@ ipc.akita-u.ac.jp. (1) Trewhella, M. J.; Grint, A. Fuel 1987, 66, 1315-1320. (2) Montano, P. A.; Granoff, B. Fuel 1980, 59, 214-216. (3) Anderson, R. A.; Bockrath, B. C. Fuel 1984, 63, 329-333. (4) Watanabe, Y.; Yamada, O.; Fujita, K.; Takegami, Y.; Suzuki, T. Fuel 1984, 63, 752-755. (5) Suzuki, T.; Yamada, O.; Fujita, K.; Takegami, Y.; Watanabe, Y. Fuel 1984, 63, 1706-1709. (6) Suzuki, T.; Yamada, O.; Takahashi, Y.; Watanabe, Y. Fuel Process. Technol. 1985, 10, 33-43. (7) Suzuki, T.; Yamada, H.; Sears, P. L.; Watanabe, Y. Energy Fuels 1989, 3, 707-713. (8) Herrick, D. E.; Tierney, J. W.; Wender, I.; Huffman, G. P.; Huggins, F. E. Energy Fuels 1990, 4, 231-236. (9) Pradhan, V. R.; Tierney, J. W.; Wender, I.; Huffman, G. P. Energy Fuels 1991, 5, 497-507. (10) Andre`s, M.; Charcosset, H.; Chiche, P.; Davignon, L.; DjegaMariadassou, G.; Joly, J.-P.; Pre´germain, S. Fuel 1983, 62, 69-72. (11) Djega-Mariadassou, G.; Besson, M.; Brodzki, D.; Charcosset, H.; Huu, T. V.; Varloud, J. Fuel Process. Technol. 1986, 12, 143-153. (12) Pregermain, S. Fuel Process. Technol. 1986, 12, 155-162. (13) Joseph, J. T.; Forrai, T. R. Fuel 1992, 71, 75-80. (14) Hatswell, M. R.; Jackson, W. R.; Larkins, F. P.; Marshall, M.; Rash, D.; Rogers, D. E. Fuel 1983, 62, 336-341. (15) Miki, K.; Yamamoto, Y.; Inaba, A.; Sato, Y. Fuel 1992, 71, 825829. (16) Taghiei, M. M.; Huggins, F. E.; Ganguly, B.; Huffman, G. P. Energy Fuels 1993, 7, 399-405.
are more effective for coal liquefaction than the conventional method of mixing catalyst powder in the coalsolvent slurry. In the present study, the effectiveness of adding iron catalyst for coal liquefaction by ion exchange method is examined as a function of the amount of loading and coal structure. There are several reports on the catalytic activity of exchanged cations. Joseph et al. evaluated the catalytic activity of the exchangeable cations: sodium, potassium, calcium, and iron.13 They reported that alkali and alkaline earth metal cations reduced conversion and the catalytic activity of iron deposited by ion exchange was far superior to iron species deposited by slurry mixing. Hatswell et al.14 and Miki et al.15 have also observed the high liquefaction activity for an exchanged iron catalyst. Taghiei et al. reported that a significant enhancement of the liquefaction yields was achieved by incorporating iron by ion exchange process16,17 and that added iron was initially present in bimodal form: the majority of the iron was present (17) Taghiei, M. M.; Huggins, F. E.; Mahajan, V.; Huffman, G. P. Energy Fuels 1994, 8, 31-37. (18) Agnew, J. B.; Jackson, W. R.; Larkins, F. P.; Rash, D.; Rogers, D. E.; Thewlis, P.; White, R. Fuel 1984, 63, 147-152. (19) Hatswell, M. R.; Jackson, W. R.; Larkins, F. P.; Marshall, M.; Rash, D.; Rogers, D. E. Fuel 1980, 59, 442-444. (20) Marshall, M.; Jackson, W. R.; Larkins, F. P.; Hatswell, M. R.; Rash, D. Fuel 1982, 61, 121-123. (21) Cassidy, P. J.; Hertan, P. A.; Jackson, W. R.; Larkins, F. P.; Rash, D. Fuel 1982, 61, 939-946. (22) Taghiei, M. M.; Huggins, F. E.; Ganguly, B.; Huffman, G. P. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1993, 38, 149-155. (23) Garg, D.; Givens, E. N. Fuel Process. Technol. 1983, 7, 59-70. (24) Garg, D.; Givens, E. N. Fuel Process. Technol. 1984, 8, 123134. (25) Weller, S.; Pelipetz, M. G.; Friedman, S.; Storch, H. H. Ind. Eng. Chem. 1950, 42, 330-334. (26) Weller, S.; Pelipetz, M. G. Ind. Eng. Chem. 1951, 43, 12431246.
S0887-0624(97)00162-X CCC: $15.00 © 1998 American Chemical Society Published on Web 07/17/1998
844 Energy & Fuels, Vol. 12, No. 5, 1998
Murakami et al. Table 1. Analyses of Coals Used
coal (code)
moisture (wt %)
ash (wt %, db)
C
Yallourn (YL) Borodino (BD) Banko (BK) Tanitoharum (TH)
60.4 16.4 21.0 10.9
2.1 9.7 1.9 4.8
65.7 69.8 71.3 75.9
a
elemental analysis (wt %, daf) H N O(diff) 4.8 4.6 5.4 5.6
0.5 0.9 1.2 1.5
29.0 24.7 22.1 17.0
These values were determined by the sodium titration method described in our previous research.27,
carboxyl groups (mmol/g)a 3.7 1.8 1.5 0.64
28
as large oxyhydroxide particles, while the smaller fraction was molecularly dispersed. Their results also showed that the iron was rapidly transformed to pyrrhotite (Fe1-xS) during liquefaction with sufficient sulfur present in the system. The purpose of the present study is to clarify the effectiveness of the ion exchange method for loading a liquefaction catalyst for four different coals. To do so, we first examined the ion exchange behavior to see whether the method is suitable to control the amount of catalyst loading. Then, the liquefaction yield is compared among three loading methods: ion exchange, impregnation, and physical mixing. For one coal, Tanitoharum, enhancement of the ion exchange capacity by surface treatment was done to see whether such a method brings additional liquefaction conversion or not. In addition, the liquefaction of iron ion-exchanged coal was carried out in the presence of secondary exchanged metal cations such as sodium, nickel, and zinc to explore possible promotion of exchanged iron by a secondary element. Experimental Section Samples. The samples used were three brown coals, Yallourn coal (YL) from Australia, Borodino coal (BD) from Russia, and Banko coal (BK) from Indonesia, and one subbituminous coal, Tanitoharum coal (TH) from Indonesia. The analyses of these coals are listed in Table 1. These coals were pulverized, sieved to under 200 mesh size particles ( YL-2.8Fe(Mix) > YL2.8Fe(Im, C). Liquefaction of Tanitoharum, Borodino, and Banko Coals. Pretreatment of TH Coal for Ion Exchange. For TH-raw coal, the amount of exchanged iron is limited to about 1 wt %. Then sulfuric acid treatment, hydrogen peroxide treatment, and calcium preexchange treatment were carried out. The results of these treatments are quite satisfactory, and the iron loading can increase up to 2.0-4.8 wt %. The most effective treatment is the calcium preexchange. Comparison of the Conversions in Liquefaction. Figure 5 demonstrates the coal conversion for TH coal. The conversion for TH-raw is 58.5%, and this value is higher by 10% than YL-raw. In short, TH coal is easier to liquefy than Yallourn. Introduction of about 0.9% of iron by ion exchange resulted in an increase of conversion by about 30%, but the activity of the exchanged iron is not so much higher than that of the mixed iron. Moreover, the activity does not increase by increasing the exchanged iron with the pretreatment (sulfuric acid
Figure 7. The FT-IR spectra of the raw and the ironexchanged samples (Ex) for BD, BK, and TH coals. These spectra were measured by the KBr pellet method at 16 scan.
treatment, hydrogen peroxide treatment, or calcium preexchange, up to 4.8 wt %). Figure 6 shows the results of liquefaction of BD and BK coals. Although, the ion-exchanged iron species apparently affects the liquefaction, the activity of the mixed pyrite is higher than that of the exchanged iron. The ratio of the amount of iron ion-exchanged to the total amount loaded was suspected to be low, and infrared spectra were examined to determine whether ion exchange occurred or not. The spectra shown in Figure 7 indicated that the absorption of carboxyl groups (about 1700 cm-1) of three coals, BD-raw, BKraw, and TH-raw, hardly decreased by the ion exchange process. It might be that most of the iron species loaded by ion exchange are not at the carboxylic site as exchanged cation because the carboxyl groups in these coals are hard to exchange with iron ion. Alternatively, acidic groups in these coals were occupied by other cations at first and may have been replaced by iron. The
Exchanged Metal Ions on Coal Liquefaction
Energy & Fuels, Vol. 12, No. 5, 1998 847 Table 2. Results of Cation Exchange Capacities of Yallourn Coal from the Solutions of Various Iron-Nickel Concentrations metal contents in initial solution (mmol)
metal loaded in coal (wt %) Fe
Ni
fixing ratea (%)
Fe
Ni
Fe
Ni
2.0 0.8 0.6 0.4
2.0 3.2 3.4 3.6
High Concentration 4.76 0.27 3.80 2.43 2.61 2.85 2.05 3.89
43 85 78 92
2 13 14 18
0.12 0.20 0.15 0.10
0.60 0.20 0.15 0.10
Low Concentration 0.75 0.42 1.42 0.95 0.78 0.60 0.73 0.51
∼100 ∼100 93 ∼100
69 81 68 87
a The percentage of the exchanged cations against the initial concentration of cations present in solution before exchange reaction.
Figure 8. The effect of the secondary metal on the coal conversion of YL coals with iron catalyst. The catalysts in all samples were loaded by the ion exchange method, and the code “(Ex)” was omitted. Top: influences of the several exchanged cations on the liquefaction yields. Bottom: influences of the various Fe-Ni loadings with sulfur on the liquefaction yields.
reason for the ineffectiveness of “exchanged” iron on these coals is not clarified and needs further study. However, because the ion-exchanged iron was not very attractive for these coals, we did not pursue this problem further. Addition of a Secondary Element as Promoter. The ion exchange method would be useful to prepare a binary catalyst system mixed at an atomic level by a single loading procedure. We were interested in the possible promotion of exchanged iron species by addition of a secondary element for catalysis in liquefaction, and a preliminary investigation was undertaken. Here, sodium, nickel, and zinc were added by simultaneous ion exchange as additives to evaluate the activity of the secondary metal in liquefaction. The results of liquefaction for the YL coals loaded with various catalysts are illustrated in the top of Figure 8. The results indicate that among the three systems investigated the Fe-Ni system seems to work synergistically in the presence of sulfur. Sodium cannot catalyze liquefaction nor promote the action of iron. Zinc by itself is known to be an effective catalyst for liquefaction, but it does not work as a cooperative component. Cation Exchange Properties from Solution Containing Iron and Nickel Ions. The ECE of YL coal from solutions of various iron-nickel concentrations was measured to see the possibility of simultaneous loading of a binary system. The results for iron-nickel are summarized in Table 2. The fixing rate in this table indicates the percentage of cations present in solution that are exchanged into the coal. When the total concentration of cations in solution is high enough, iron was preferentially exchanged so that some regulation of concentration in the exchange solution is required. The cation exchange from a dilute solution of iron and
nickel ions proceeded independently and can be regarded as simultaneous exchange from the solution of single metal ions, and both ECEs increase with increasing concentration of metal ions in solution. Thus, we can prepare a catalyst of various iron-nickel compositions by changing the initial concentration of metal ions in solution. However, when concentrated mother solutions were used, the interaction of two metal species in exchange was observed and iron obviously is the preferred species to nickel. The Effect of the Iron and Nickel Binary Catalysts on the Liquefaction. Further experiments were carried out with the Fe-Ni catalysts, and some results are included in the bottom of Figure 8. As seen in Figure 8, the iron and nickel binary catalysts show extremely high activity. Because of the high coal conversion, the influence of composition is not clear here, and further study of the effect of composition of Fe-Ni binary system will be reported later. Discussion Loading of Liquefaction Catalysts by the Ion Exchange Method. It is necessary for a catalyst loading method to be able to control the amount of loading. Yallourn coal is abundant in acidic functional groups but is not sufficient for loading of liquefaction catalyst up to the level of 3 wt %. Several treatments were tried in order to enhance the exchanged iron ions. The most effective method was the calcium preexchange method. Schafer29 and the present authors27 reported that the sodium preexchange also increased the iron loading. In the present experiment, the pH of the iron exchange solution increased for calcium preexchanged samples, and the iron loading increased almost linearly with the amount of calcium preexchanged. When the metal ions exchange with protons of carboxyl groups on coal, the carboxyl groups dissociate as shown in eq 1.
coal-COOH f coal-COO- + H+
(1)
However, the carboxyl groups are hard to dissociate below pH 4 where the exchange of iron ions is performed. In the case of the cation preexchanged coals, on the other hand, the protons were replaced easily in the first stage with calcium ions since it is conducted at a high
848 Energy & Fuels, Vol. 12, No. 5, 1998
Murakami et al.
pH. The preexchanged cations mostly dissociate even below pH 4, as in eq 2.
(coal-COO)nM f ncoal-COO- + Mn+
(2)
Consequently, it is easy to load the iron ions in the second stage. Loading of the Iron-Nickel Binary System. From the result that the fixing rate of iron ions in Table 2 is almost 100% at low concentration, the iron ions exchange with protons preferably at the beginning, and then the residual exchange sites are used to exchange with nickel ions. Lafferty et al.32 and Baruah et al.33 investigated the cation exchange capacities from various solutions containing only single metal ions. Comparing the extents of cation exchange among cation species, they concluded that ionic selectivities followed the Irving-Williams order indicating the stability order for the first transition metal-ligand complex formation, i.e., Mn < Fe < Co < Ni < Cu > Zn. In our present experiments, however, we obtained different results. This discrepancy may be caused by the different conditions between the two cases or by the effect of other factors apart from the stability of the metal ionscarboxyl groups complex formation, such as mobility of hydrated ions through pores in coal. Catalysis of the Exchanged Iron in Liquefaction. The preparation of the highly dispersed catalysts has been the recent trend of research in liquefaction.34 In the present work, ion exchange, impregnation, and physical mixing were compared as catalyst loading methods. The ion exchange method is expected to be most effective to obtain a finely dispersed state. However, the advantages of ion exchange method appeared only for YL coal in the present study. One possible reason for the ineffectiveness of the other three coals would be poor ion exchange, as partly suggested by infrared spectra. It is generally believed that the highly dispersed catalysts on coal surfaces enhance the reactivity of liquefaction system, but the working state of catalyst is not clarified. Several studies indicate that pyrrhotite is the active form of the catalyst. If this is the case, then iron atoms finely dispersed on the coal surface (32) Lafferty, C. J.; Hobday, M. D. Fuel 1990, 69, 84-87. (33) Baruah, M. K.; Upreti, M. C. Fuel 1994, 73, 273-275. (34) Derbyshire, F. J. Catalysis in Coal Liquefaction: New Directions for Research; IEA Coal Research: London, 1988.
should, at first, leave the ion exchange site, migrate, and convert to a desired form. Previously, we examined the difference of influence on pyrolysis between the exchanged cations and the impregnated metals.35,36 These studies described the role of the exchanged cations during pyrolysis, where the exchanged cations stabilized carboxylate groups but decomposed other functional groups. Thus, there are two roles of a catalyst species in liquefaction. One is the influence on the decomposition of the functional groups and the other is hydrogenation of coal. To be active for the latter reaction, iron atoms have to transform through migration, sintering, and sulfidation. The surface state of coal and the atmosphere would have a significant effect on the course of iron transformation. Thus, the difference in effectiveness of exchanged iron for different coal species could be due to a combination of reasons. Further study is needed on this point. Conclusion The main results obtained are as follows. (1) The iron exchange capacity of a coal can be enhanced by the preexchange of more easily exchangeable cations such as calcium ions for coals with sufficient acidic sites. (2) For Yallourn coal, the coal conversion in the presence of the exchanged iron is larger than that obtained by impregnation of iron or the physical mixing of pyrite powder. (3) For Tanitoharum, Borodino, and Banko coals, the activity of the mixed pyrite is higher than or similar to that of the exchanged iron. Thus the applicability of the ion exchange method is quite selective to coal species. (4) In the case of YL coal, mixed iron and nickel loaded by simultaneous ion exchange showed very high activity, even when the total of the exchanged cations was less than 1 wt %. Acknowledgment. The authors thank the Nippon Brown Coal Liquefaction Co., Ltd. for providing coal specimens as well as synthetic pyrite. Part of this work is financially supported by New Energy and Industrial Technology Development Organization. EF970162P (35) Murakami, K.; Shirato, H.; Ozaki, J.; Nishiyama, Y. Fuel Process. Technol. 1996, 46, 183-194. (36) Murakami, K.; Shirato, H.; Nishiyama, Y. Fuel 1997, 76, 655661.
