Energy & Fuels 1997, 11, 483-490
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New Approach to Coal Structure through Its Evolution during Dry Catalytic Hydrogenation A. M. Mastral,*,† M. C. Mayoral,† J. Rivera,‡ and F. Maldonado‡ Instituto de Carboquı´mica, CSIC, P.O. Box 586, 50080 Zaragoza, Spain, and Departamento Quı´mica Inorga´ nica, Universidad de Granada, 18071 Granada, Spain Received August 7, 1996X
Coal structure and its evolution in catalytic hydrogenation are studied in this work. The hydrogenation experiments were performed in tubing bomb reactors, in the absence of solvent, with an initial pressure of 10 MPa of H2, and for a reaction time of 30 min; iron sulfide was the catalyst precursor. The characterization of the solid residues was performed by different techniques, especially solvent swelling in pyridine and surface and porosity studies. The swelling values are related to the topological structure rather than to the density in noncovalent crosslinkages. The evolution of that structure is monitored by calculating the average number of carbon atoms between cross-link points, finding that at mild hydrogenation conditions (350 °C for low-rank coals, 400 °C for bituminous coals) the solid residues present a structure with longer chains of aromatic clusters than in the parent coals. This would seem to be due to the release of heteroatom-containing moieties and the prevention of cross-linking reactions by the catalyst. The textural characterization showed that there is a relationship between the increase of the skeletal density and chain length; the structure becomes more compact and ordered in the absence of low molecular weight or noncovalently bonded material.
Introduction Coal Structure. Significant progress in gaining a basic understanding of coal structure and reactivity has been made in the past decade. There are in the literature very deep efforts to review the state-of-the-art of coal structure and reactivity studies.1-5 It is generally agreed that a major part of coal consists of a three-dimensional macromolecular network, made up of fused aromatic ring clusters (which are described by their molecular weight), many of which contain substituents. The clusters are supposed to be linked by aliphatic or heteroaliphatic bridges, some of them relatively weak. The relative sizes of the clusters, bridges, and substituents vary depending on rank. Nowadays attention is centered on characterizing the aromatic cluster ring sizes and their functionality. In recent years, another view of coal physical structure has been proposed, based on observed slow and rapid proton relaxation rates in NMR experiments. In this view, the macromolecular network is an “immobile phase” and is “host” to a number of many presumably smaller, mobile elements. The manner in which the smaller molecules are associated with the network may involve both physical and chemical forces, such as covalent bonds, desinterpersion forces, and H-bonding and could also involve physical entrapment. It is doubtful whether any †
CSIC. Universidad de Granada. Abstract published in Advance ACS Abstracts, January 15, 1997. (1) Gorbaty, M. L. Fuel 1994, 73 (12), 1819-1828. (2) Solomon, P. R.; Fletcher, T. H.; Pugmire, R. J. Fuel 1993, 72 (5), 578-597. (3) Haenel, M. W. Fuel 1992, 71, 1211-1223. (4) van Krevelen, D. W. Coal; Elsevier; Amsterdam, 1994. (5) Derbyshire, F.; Marzec, A.; Sculten, H. R.; Wilson, M. A.; Davis, A.; Tekely, P.; Delpuech, J. J.; Jurkiewicz, A.; Bronnimann, C. E.; Wind, R. A.; Maciel, G. E.; Narayan, R.; Bartle, K.; Sugre, C. Fuel 1989, 68, 1091-1106. ‡
X
S0887-0624(96)00125-9 CCC: $14.00
single technique can distinguish between these different attachments sufficiently clearly to establish a precise boundary.1-5 Detailed structural characterization has been found to be extremely difficult, so that coal structure research is still a challenging task and continues to be pursued intensively. One approach to this subject is the use of spectroscopic techniques directly applied to coal, but some researchers have applied semidegradative processes to the study of coal structure and its inferences in coal reactivity.1,2,5 When heated, the network decomposes to produce small fragments; a competitive process with the bridge-breaking is the retrogressive process of cross-linking2. It was proposed that as long as coal conversions are low, the liquefaction residues are related more to the original structure than to a side product derived by condensation of reactive fragments. For these kinds of studies Derbyshire et al.6,7 proposed that the absence of vehicle solvent simplifies the fundamental information because the physical properties of the modified coals and insoluble residues can be measured directly. Moreover, low-temperature catalytic hydrogenation suppresses the tendency of formation of cross-links, promoting the cleavage of existing crosslinkages. Catalytic precursors are used to guarantee a high degree of hydrogenation provided that there is no hydrogen donor solvent in the reaction medium. The most widely used precursor is ammonium heptamolybdate, with the sulfide as the thermodynamically stable species under reductive conditions.6-10 Iron precursors have been as well used as hydrogenation catalysts from (6) Derbyshire, F. J.; Terrer, M. T.; Davis, A.; Lin, R. Fuel 1988, 67, 1029-1035. (7) Derbyshire, F. J.; Davis, A.; Lin, R. Energy Fuels 1989, 3, 432437. (8) Bockrath, B. C.; Illig, E. G.; Wassell-Bridger, W. D. Energy Fuels 1987, 1, 227-228.
© 1997 American Chemical Society
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the early stages of coal research to the present day spectroscopic techniques.11-15 Research in recent years has focused on adapting solvent swelling as applied to polymers to defining coal macromolecular weigh between cross-links. This vital parameter could be used to follow the extent of coal network “depolymerization” during processing and for determining how many bonds can be cleaved to reduce coal from a solid macromolecular network to the desired liquid products. Swelling Mechanisms. The solvent swelling technique has been widely used to gain insight into coal structure,16-18 especially the volumetric method developed by Liotta,19 which gives a reliable and reproducible measure of network swelling under the selected solvent. Pyridine is usually selected for this kind of structural study, due to its high electron-donor number and polarity, which guarantees a high degree of interaction with the substrate.20,21 The scientific contributions of this technique have not been only in the field of coal structure research but also in the study of coal reactivity and structural evolution under different treatments. The common inferences from solvent swelling experiments in coal structure research are related to the crosslinking density and solvent interactions with coals and chars. The degree of penetrant swelling of a cros-linked macromolecular network can be used to determine the coal penetrant thermodynamic interaction parameter and the effective number-average molecular weight between cross-links of the macromolecular coal structure as long as appropriate molecular theories are available. On the basis of the equations designed to model the swelling phenomenon in polymer networks, statistical theories of polymer network elasticity, it is possible to find different approaches to measure the cross-link density of coals. Nelson22 used a modification of the Flory-Rehner equation for application to the highly cross-linked non-Gaussian macromolecular network of coal through the calculation of solvent-polymer interaction parameters, and Lutch23 tried the same by analysis of equilibrium data by application of a modified Gaussian network equation. They gave semiempirical information about the structure of coals. A comprehensive study of the work done on the subject24,25 examined its strengths and limitations by considering the ther(9) Mastral, A. M.; Izquierdo, M. T.; Mayoral, C.; Pardos, C. Fuel Process. Technol. 1994, 37, 87-97. (10) Mastral, A. M.; Rubio, B.; Izquierdo, M.; Mayoral, C.; Pardos, C. Fuel 1994, 73, 925-928. (11) Bacaud, R.; Besson, M.; Djega-Mariadassou, G. Energy Fuels 1994, 8, 3-9. (12) Pradhan, V. R.; Hu, J.; Tierney, J. W.; Wender, I. Energy Fuels 1993, 7, 446-454. (13) Mastral, A. M.; Mayoral, C.; Palacios, J. Energy Fuels 1994, 8, 94-99. (14) Mastral, A. M.; Mayoral, C.; Izquierdo, M. T.; Rubio, B. Energy Fuels 1995, 9, 753-759. (15) Mastral, A. M.; Mayoral, C.; Izquierdo, M. T.; Pardos, C. Fuel Process. Technol. 1993, 36, 177-184. (16) Larsen, J. W.; Cheng, J. C.; Pan, C. S. Energy Fuels 1991, 5, 57-59. (17) Larsen, J. W.; Pan, C. S.; Shawver, S. Energy Fuels 1989, 3, 557-561. (18) Marzec, A.; Kisielow, W. Fuel 1983, 62, 977. (19) Liotta, R.; Brons, G.; Isaacs, J. Fuel 1983, 62, 781-785. (20) Hall, P. J.; Marsh, H.; Thomas, M. Fuel 1988, 67, 863-866. (21) Suuberg, E. M.; Otake, Y.; Langer, M. J.; Leng, K. T.; Milosavijevic, I. Energy Fuels 1994, 8, 1247-1262. (22) Nelson, J. R. Fuel 1983, 62, 112-116. (23) Lutch, L. M.; Peppas, N. A. Fuel 1987, 66, 803-809. (24) Painter, P. C.; Graf, J.; Coleman, M. M. Energy Fuels 1990, 4, 379-384.
Mastral et al.
modynamics of mixing coals with solvents, extending the application of the Flory lattice models so as to include the effect of hydrogen bonding. The theories of swelling that have been utilized are well-known to be severely limited in their application to coal, but have nevertheless been considered useful as a qualitative and comparative probe of structure. The relatively stiff and presumably short nature of the chains present in coals is a characteristic that precludes any application of the ideal network theories. Nevertheless, this characteristic allows the application of an alternative approach.26 The starting point is that coals consist of cross-linked macromolecular networks made up of chains that have very limited flexibility. On the basis of studies about swollen polymers, it was proposed that swelling was associated with topological reorganization of the network, where the cross-link points essentially rearrange their positions with only minor perturbations to the chain dimensions, a process that the authors labeled desinterspersion. Painter et al.26 proposed that this is the predominant mechanism by which coal can swell, and the degree of swelling is then defined largely by the geometry of the system, as significant chain extension or deformation does not occur. A relationship for the molecular weight between cross-link points can thus be established. In this way, an equation was proposed that related the number of carbon atoms between cross-link points with the average number of carbon atoms per average repeat aromatic unit from which the chains are made for. Solvent Swelling Measurements Compared with Other Techniques. The changes in the macromolecular structure of a lignite and a bituminous coal during rapid pyrolysis in a broad temperature range were described by Suuberg, monitoring the extent of crosslinking27,28 in the chars in pyridine. Since then, the pyrolysis of coals has been investigated by different means, but even the most modern and comprehensive studies2 combine and compare new sophisticated techniques and instrumentation with the results obtained by the solvent swelling technique. Nuclear magnetic resonance experiments (cross polarization-MAS-dipolar dephasing) were performed to determine an average molecular weight for the ring clusters and also the average number of attachments per ring cluster,29 and the results showed that both techniques offered similar trends in cross-linking variation with thermal treatment. The proton magnetic resonance thermal analysis30 technique can observe the evolution with temperature, like melting, bridge-breaking, and cross-linking. It was found2 that there was a complete correspondence of this evolution with the information obtained by solvent swelling. Coal Physical Structure. There are works in the literature that compare the results obtained by solvent swelling technique with physical characterization such (25) Painter, P. C.; Park, Y.; Sobkowiak, M.; Coleman, M. M. Energy Fuels 1990, 4, 384-393. (26) Painter, P. C.; Graf, J.; Coleman, M. M. Energy Fuels 1990, 4, 393-397. (27) Suuberg, E. M.; Lee, D.; Larsen, J. W. Fuel 1985, 64, 16681671. (28) Suuberg, E. M.;Unger, P. E.; Larsen, J. W. Energy Fuels 1987, 1, 305-308. (29) Solum, M. A.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989, 3, 187-193. (30) Barton, W. A.; Lynch, L. J. Energy Fuels 1989, 3, 402-411.
Coal Structure
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as Gieseler plastometry and microdilatometry31 and by many other means, but, surprisingly, the characterization of the surface and density of the residues has not been as widely used as the techniques mentioned. There are works in the literature that accomplish a continuous debate about the description of coal porous structure.32-37 In those studies the authors have not reached an agreement about coal pore structure. Some of them claim that coal is comprised by a rigid framework holding a set of interconnected pores;37 meanwhile, some others propose that coals behave as organic macromolecular systems and not as rigid inorganic rocks33 with closed pores as isolated bubbles in a solid, only reachable by diffusion through the solid. In any case, and to avoid the problems raised by the use of CO2 as adsorvate, some authors have proposed the He and Hg densities and porosimetry as the most suitable method to describe the porous structure of coals.35,37 The porous structure of coals has been related with the reactivity,38 and its evolution under several treatments has also been studied, such as partial solubilization39 and solvent extraction,40 but there are in the literature few works that study the evolution of pore structure and active surface areas of coal and char during hydrogenation at harsh41 and mild42-44 conditions. The aim of the present work is to achieve a new contribution to coal structure theories through the characterization of mild coal hydrogenation residues by different techniques, especially comparing the results from solvent swelling in pyridine with the textural parameters obtained by surface and porosity studies. Experimental Section Coals. Six coals denoted S13, S16, and S18 (Andorra-Arin˜o, Spain), B19 (Illinois No. 6, U.S.A.), B22 (Zollverein, Germany), and B25 (Bagworth, U.K.), were studied. Table 1 shows their characteristics. Coals were ground to pass though a 0.25 mm sieve (-60 mesh, Tyler scale) and stored in argon atmosphere until use. Catalysts. The catalytic precursor used was iron(II) sulfide prepared by bubbling H2S for 15 min through an aqueous FeSO4‚7H2O solution (Merck, with purity >99.5%); the neutrality of the solution was maintained by adding some drops of NaOH concentrated solution. The conversion of the salt takes place quickly and is accompanied by a color change. Then, and after filtration and washing with distilled water, the precipitate was added to an aqueous slurry of coal and the mixture was magnetically stirred for 30 min. After that, the slurry was freeze-dried to remove the water at 0 °C to avoid (31) Stansberry, P. G.; Lin, R.; Terrer, M. T.; Lee, C. W.; Davis, A.; Derbyshire, F. J. Energy Fuels 1987, 1, 89-93. (32) Walker, P. L.; Mahajan, O. P. Energy Fuels 1993, 7, 559-560. (33) Larsen, J. W.; Hall, P.; Wernett, P. C. Energy Fuels 1995, 9, 324-330. (34) Gorbaty, M. L.; Mraw, S. C.; Gethuer, J. S.; Brenner, D. Fuel Process. Technol. 1986, 12, 31-49. (35) Walker, P. L.; Verma, S. K.; Rivera-Utrilla, J.; Khan, M. R. Fuel 1988, 67, 719-726. (36) Larsen, J. W.; Wernett, P. Energy Fuels 1988, 2, 719-720. (37) Mahajan, O. P. Carbon 1994, 29 (6), 735-742. (38) Wells, W. F.; Smoot, L. D. Fuel 1991, 70, 454-458. (39) Parkash, S.; Moschopedis, S. E. Fuel 1983, 62, 1231-1234. (40) Medeiros, D.; Petersen, E. Fuel 1979, 58, 531-533. (41) Cypre`s, R.; Planchon, D.; Braekmen-Danheux, C. Fuel 1985, 64, 1375-1378. (42) Maldonado-Ho´dar, F. J.; Rivera-Utrilla, J.; Mastral, A. M. Fuel 1995, 74 (6), 823-829. (43) Maldonado-Ho´dar, F. J.; Rivera-Utrilla, J.; Mastral, A. M.; Izquierdo, M. T. Fuel 1995, 74 (6), 1709-1715. (44) Rivera-Utrilla, J.; Maldonado-Ho´dar, F. J.; Mastral, A. M.; Mayoral, M. C. Energy Fuels 1995, 9, 310-323.
Table 1. Characterization of the Parent Coals coal proximate (% wt, dry) mineral matter ash volatile matter fixed carbon ultimate C (% wt, daf) H (% wt, daf) N (% wt, daf) Stot (% wt, dry) maceral (% vol) vitrinite exinite inertinite
S13
S16
S18
B19
23.48 13.60 35.37 50.79
21.60 12.75 33.89 53.01
27.51 17.17 33.05 49.77
17.78 13.79 32.66 53.54
B22
B25
12.08 13.19 12.07 8.80 25.88 39.51 60.95 51.68
67.45 64.82 67.40 76.98 83.30 79.40 4.98 5.26 5.01 5.03 5.05 3.37 0.40 0.64 0.48 1.45 1.71 1.34 7.23 7.32 7.19 4.05 0.99 1.22 71.8 3.1 25.1
74.7 1.6 23.7
75.4 1.0 23.6
75.4 1.0 23.6
67.0 9.0 19.0
72.0 5.0 23.0
altering the original catalyst dispersion. In this way, the coal was the catalytic support. The catalyst loading was Fe 5 wt % on daf basis. Hydrogenation Procedure. About 10 g (dmmf) of catalystloaded coal was charged into the reactor, tubing bomb type of 160 cm3 capacity. The reactor was pressurized with hydrogen to the initial pressure of operation (10 mpa) and placed in a holder hung from the oscillation system. It was immersed in a preheated sand bath at the temperature reaction for 30 min. At the end of the process, the reactor was cooled by quenching in water. Product Workup. The gases were vented from the cold reaction vessels into a gas sampling bag and yields in CO, CO2, H2S, and C1-C4 were determined by GC. The reactor content was transferred into a Soxhlet extraction thimble for extraction with tetrahydrofuran for 24 h. Then, the THF solubles were fractionated with n-hexane into oils and asphaltenes, which were characterized by ultimate analysis (C, H, N) in a Leco CHN. Oils were also characterized by chromatographic fractionation by TLC in a Iatroscan. Solid Residues Characterization. The THF insolubles, which comprise the solid residue object of this research, were analyzed by elemental analysis in a LECO CHN, infrared spectroscopy, and 13C CP-MAS NMR. The volumetric swelling ratio for each residue was calculated following Liotta’s procedure,19 with the correction for mineral matter (the percentage of mineral matter was calculated by low-temperature ashing, and the values of densities were those obtained by mercury porosimetry) as described previously.10 To compare the volumetric swelling ratio of the hydrogenation residues with the swelling of parent coals, they were Soxhelt extracted in THF prior to the swelling test. Surface areas were determined from CO2 adsorption at 273 K in a home-made adsorption apparatus. The Dubinin-Radushkevich equation was used for calculation of surface areas, and an expression developed by the same authors was used to compute the micropore volume Vo. Mercury porosimetry measurements were carried out in a Quantachrome Autoscan 60 instrument, which reaches a pressure of 4200 kg cm-2, at which the skeleton density was determined for each sample. The data on the pore distribution, collected by mercury penetration as a function of pressure, were corrected by taking into account the mineral matter percentage of each sample, the porosity of which was considered negligible compared to that of the organic fraction. To express the density values on a dry, mineral-matter-free basis, a density of 2.7 g cm-3 was assumed for mineral matter. The CO2 adsorption data were corrected to the dry, mineral-matterfree basis by considering that the mineral matter present had an average surface area of 15 m2 g-1. More details about the experimental procedure are given elsewhere.13,14,43,44
Results and Discussion Coal Hydrogenation. The present work is framed inside a broader research, in which the role of iron
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Figure 1. Product distribution form coal hydrogenation at different temperatures: (a) B19 bituminous, (b) S18 subbituminous. Catalytic precursor: 5% Fe loaded as iron sulfide.
precursor in terms of yield and products selectivity,13 influence of dispersion, and chemical state and activity of the catalytic species14 as well as the influence of the intrinsic parameters of each coal to their performance in hydrogenation15,44 have been studied. Coal reactivity is measured as a function of conversion into soluble products in hydrogenation experiments, performed in the absence of catalyst (blank tests). The effect of reaction temperature on the conversion yields and product distribution is shown in Figure 1, as examples of the behavior of high-rank (Figure 1a) and low-rank (Figure 1b) coals under catalytic hydrogenation. The conversions achieved at 350 °C were in the range of 3040%, and higher conversions were obtained at 400 °C (70-80% for low-rank coals). Bituminous coals exhibited similar trends but with a lesser extent in reactivity. A blank test performed at higher temperature (425 °C) for bituminous coals did not imply an increase of conversion but a lower ratio THF solubles/gas. The effect of the catalyst precursor addition was studied in a broader range of temperatures. Its addition involved only slight increases in conversion values and in product selectivity. There was as well an increase in oil production (between 2% and 10% depending on the coal and the process conditions), although their composition, analyzed by TLC, proved to be more dependent on the temperature than on the presence of catalyst. Solid Residue Characterization. The more interesting effect of the catalyst appeared to happen in the solid residue composition. The elemental analysis (corrected for mineral matter values) showed that for all coals, the solid residue obtained at each temperature in the catalyzed experiments presented higher percentages of carbon, with the corresponding decrease in heteroatom content (a decrease of about 10% O+Sdmmf in low-rank coals), than the residues obtained in the blank tests. This is a clear indication of what was expected from an iron catalyst: a deoxygenation effect and the removal of heteroatom groups by the thermal treatment. As a matter of fact, this behavior could be expected as well under pyrolysis conditions, so there is a need of different analytical techniques to monitor the
Figure 2. Solvent swelling ratios of the solid residues obtained in catalyzed experiments: (a) Qv vs conversion percentages, low-rank coal residues and B25 coal; (b) Qv vs temperature of treatment, B19 and B22 coals.
effect of hydrogenation. It is known that pyrolysis involves the increase in cross-link density of the chars parallel to thermal treatment.2 As mentioned in the Introduction, this cross-link density can be followed by a number of methods, the volumetric swelling ratio being one of the most straightforward tests to do. Following Liotta’s method, the ratio was calculated by letting the solid be in contact with the solvent. A period of 1 month was found to be enough for these coals to reach equilibrium. In Figure 2a the values of Qv for those residues obtained in catalytic hydrogenation of the low-rank coals are displayed. The trend is similar for the three of them: low conversions involve a slight decrease of cross-link density with respect to the original (preextracted in THF) coal. At higher conversions, achieved at 350 °C, the residues obtained show a decrease in the number of crosslinkages, which allows the structure to achieve a new equilibrium configuration that is more easily swelled by the solvent. At higher conversion, there is a notable change in the properties of the solids: the structure is highly cross-linked. It is clear that under the conditions studied, these coals do not show the decrease in Qv which is characteristic in pyrolysis studies. In previous
Coal Structure
work, the decreasing profile was obtained,9,10 but the catalyst used in that work was a sulfided form of Mo. It conferred to the coal higher activity, so the conversions achieved even at 350 °C were well over the range of 30-50%. On the other hand, in the present work, the swelling test of blank experiment residues showed that for those chars obtained in the absence of catalyst, the higher the conversion, the more cross-linked the structure is, a behavior expected in pyrolysis conditions, with an increase in CO and CO2 production in gases. With respect to the bituminous coals, the trend was similar, although a display similar to Figure 2a would give a high degree of dispersion. Nevertheless, the Qv evolution with temperature, Figure 2b, shows that these coals also present a maximum in Qv values at moderate conversions. From these results, it can be inferred that the hydrogenation reactions have been more effective in the presence of the catalyst, because the formation of cross-linkages by repolymerization or condensation reactions has been minimized. The iron sulfide was able to produce an atmosphere rich in hydrogen radicals available to stabilize those hydrocarbon radicals formed by thermal treatment. Higher temperature drives to a highly cross-linked residue. The results suggest that at 400 °C, the hydrogenation activity of the catalyst is insufficient to fully suppress condensation reactions. Another important inference from these results is that Qv data are not related to the density in noncovalent cross-linkages, that is to say, with hydrogen bonds. The elemental analysis of the residues showed a decrease in O and S contents, so the possibilities of hydrogen bonding are reduced. It is common to consider hydrogen bonds to be additional cross-links in the coal network system, but they should be treated as specific interactions and not in the same way as covalent cross-links or junctions.45 From this viewpoint, the topological model can be used to explain the swelling mechanism rather than those only related to thermodynamic solvent-network interactions. Calculations with the Swelling Ratio. Apart from the thermodynamic theories, there are works in the literature that consider the topological approach as a suitable one for describing coal and char structures. Painter et al. developed a method for determining the number of carbon atoms between cross-link points in coals based on the geometry of swelling.26 The model follows the calculation of Bastide et al.,46 in which an expression between the number of statistical aromatic units and the volume fraction of the swollen network is obtained, regarding the volume of chains present in a sphere surrounding a particular branch point. Faulon47 proposed a correction for this desinterspersion model, based upon the Painter model and considering the formula of Euler applied to polymer networks, which distinguish the average repeat aromatic units from the actual links that form the chains and involve accounting for the cycle rank of the polymer network. The Faulon corrections for the Painter model suggest a new formulation of the equation, changing the concept of average repeat unit for average repeat connecting link, which gives higher values of chain lengths, more (45) Painter, P. Energy Fuels 1992, 6, 863-864. (46) Bastide, J.; Picot, C.; Candau, S. J. Macromol. Sci. Phys. 1981, B18, 13. (47) Faulon, J. L. Energy Fuels 1994, 8, 1020-1023.
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in accord with results from the classical thermodynamic approach to swelling phenomenon. In this way, an expression of the Painter model (with Faulon’s modification) that relates the average number of carbon atoms per “cluster” or average repeat unit, defined here to be the aromatic unit, with the number of carbon atoms between cross-link points can be reached. The independent variables are the swelling ratio, and values of sample characterization such as %Cdaf and its density.
[ ]
3fQvVc 1 ) nc 4πac3
1/(3v-1)
Nc(1-3v)/[3(v-1)] +
(2/f) - 1 (1) Nc
In eq 1 nc is the average number of carbon atoms per average repeat aromatic unit, f is the functionality of the cross-linking points, to be of the order of 3 or 4, Qv is the volumetric swelling ratio in pyridine calculated by Liotta’s method, and Vc is the molar volume of the nonswollen coal per carbon atom, which is given by4
Vc ) 1200/%Cdmmf F
(2)
where F is density, ac is the average virtual bond length per carbon atom, ac) 0.5 Å, v is an exponent statistic factor that has values of 0.5 for a chain following random walk statistics and 0.6 for a self-avoiding walk (it would be 1 for an extended rigid rod), and Nc is the number of carbon atoms between cross-link points. For the present calculations, a functionality of f ) 3 has been assumed on the basis of the work of Solum et al.29 They investigated eight Argonne Premium coals by 13C solid state NMR spectroscopy to determine their carbon skeletal structure, finding a value of 3 as the number of bridges and loops in each aromatic cluster, a concept similar to the f defined here. On the other hand, the values of density have been calculated by an equation of group distribution from the elemental analysis of each sample,4 because we had not helium densities available for them. The skeleton densities (from mercury porosimetry) seemed too low to be suitable for these calculations In Figure 3 it is possible to see how the PainterFaulon model applied to our results describes the chain length of coal S18 and the solid residues obtained by thermal treatment of the coal under H2 atmosphere. In Figure 3a the raw coal is described to have between 3 and 4 aromatic units between cross-links, supposing those aromatic units of 10 carbon atoms (i.e., predominantly benzene rings with a few naphthalene and heterocyclic species). The residue obtained at 350 °C (Figure 3b) seemed to be slightly more cross-linked. In a first approach, the model describes how the structure is developing a cross-linked network with thermal treatment. For a highly cross-linked sample (obtained at 400 °C, Figure 3c), the model described an almost collapsed structure (i.e., the number of carbon atoms per cluster is similar to the number between cross-link points) as it would imply a connected mesh that presumably could not swell at all. Assuming the aromatic cluster size to be of 10 carbons, two different values of Nc are obtained in the plots, one in the line of v ) 0.6 and the other in the line of v ) 0.5, the extreme possibilities of walk statistics, describing a range in which the real Nc should be, always, under the model. The results can be displayed
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Figure 5. Range of Nc calculated by eq 1 for nc ) 10, f ) 3, and ac ) 0.5 in high-rank coals: (0) raw coal; (shaded circle) residues from catalyzed experiments; (bar) residues from blank experiments; (a) B19 coal; (b) B22 coal; (c) B25 coal.
Figure 3. Plots of nc vs Nc for f ) 3 and ac ) 5: ([) selfavoiding walk (v ) 0.6); (9) random walk (v ) 0.5); (a) S18 coal, preextracted in THF; (b) solid residue from blank tests at 350 °C; (c) solid residue from blank tests at 400 °C.
Figure 6. Aromaticity factor and carbon speciation for S13 coal and its solid residues from catalytic hydrogenation.
Figure 4. Range of Nc calculated by eq 1 for nc ) 10, f ) 3, and ac ) 0.5 in low-rank coals: (0) raw coal; (shaded circle) residues from catalyzed experiments; (bar) residues from blank experiments; (a) S13 coal; (b) S16 coal; (c) S18 coal.
in the way that Figures 4 and 5 show, where it is possible to see the range of number of aromatic clusters between cross-links, both in raw coals and in catalyzed runs. In these figures it is possible to see how the model describes the chain length in raw coals preextracted in THF. As was expected, subbituminous coals present a structure of 5-9 rings, being high-rank coals structure more condensed, of 3-4 rings. B19 coal seems to be an exception, but this result has been contrasted with that described by Painter with the same Illinois No. 6.26 The limits correspond to the range of aromatic cluster units between cross-links for blank tests. For the three low-
rank coals studied, the evolution with temperature of the solid residues obtained without catalyst is similar to that described in Figure 2. The striking result corresponds to the characterization of the solid residues obtained in catalyzed runs. A temperature of 300 °C involves a decrease in chain length, which means a preponderance of self-stabilization reactions over hydrogenation pathways. Higher temperatures involve the formation of the catalytically active species,14 pyrrhotite, which is almost complete at 400 °C. A preponderance of hydrogenation reactions over pyrolysis, as was deduced from Qv values above, can be observed at these temperatures. Organic Matter Evolution. According to the discussion above, the catalytic mild hydrogenation under the described experimental conditions produces a solid residue having a structure that is less cross-linked than the original coal. Complementary information can be obtained about the kind of species that remain in the structure. First, the values of fraction of aromaticity (fa ) aromatic carbon/total carbon), obtained by 13C solid state NMR, showed that the residues are progressively more aromatic with treatment (Figure 6). Nevertheless, through a simple material balance to carbon with the fa, the conversion percentages, and the elemental analysis, it is possible to see how the different types of carbon evolve throughout reaction. In Figure 6 the raw S13 coal presents 66% of aromatic carbon. As the figure shows, the fa increases because the aliphatic fraction is
Coal Structure
disappearing during the reaction, but the aromatic fraction remains in the same proportion. The numerical difference between the conversion percentage at 350 °C (35.6%) and the decrease in bulk carbon (24%) is comprised by the removal of other components such as O and S, mainly in the gas fraction. The presence of partially reacted matter in the remaining aromatic fraction, or insoluble vitroplast, was discarded by taking into account the results obtained with optical microscopy: the residues, before and after extraction in THF, were studied in the fluorescence mode, finding that the previtroplast formed at 300 °C and the vitroplast formed at 350 °C were not observed after THF extraction. With increasing extract yields at 400 °C, the reflectivity of the vitrinite and vitrinite-derived intermediates (insoluble in this case) is enhanced, which suggests that the thermal condensation and covalent cross-linking occurred at 400 °C. These results could indicate that the materials released during the hydrogenation at mild conditions are part of that component termed mobile in the two-component model of coal structure, and the remaining solid residue is in its structure close to that of the original coal. Actually, the conversion values (40% by weight in low-rank coals, 25% in bituminous ones) are in the range of the mobile phase defined by Marzec.5 In any case, the solid residues obtained in this way, although liberated of the easily releasable materials, are very unlikely to be completely representative of the network phase of each coal, due to possible alterations along the treatment. Nevertheless, they are very useful to monitor the evolution of the physical structure by textural characterization, because changes in the structure can be considered minimum, due to the low severity of the process and, in this way, an approach to coal structure can be done. Porous Structure. As the hydrogenation process progresses, pore aperture is induced by the release of products. This is translated into an increase in the wider pores, which leads to the appearance of a significant volume of meso- and microprores. This evolution is shown in Figure 7, where the values of meso (pore volume contained in pores in the diameter range 3.750 nm) and macro (diameter range 50-200 nm), obtained by mercury porosimetry, are displayed versus the conversion percentages for the solid residues obtained in catalyzed test for the six coals studied. Microporosity is obtained from CO2 adsorption isotherms, and its evolution is not as clear as in wider pores. Although the data in Figure 7 present a high dispersion, if they are displayed for each coal, the plots present a trend described in Figure 8, for a low-rank coal and a highrank one as examples. The main information that this figure gives is that microporosity varies in a narrower range for high-rank coal residues, but for both ranks, the evolution is the same: there is a minimum in the microporosity at intermediate conversions, in the same range in which we found the maximum in swelling ratio. There is a deep controversy about adsorption technique for this kind of carbonaceous samples, in which the possible interactions of the CO2 with the surface can overlap the accurate description of the porous structure. Although there are works in the literature which preclude that the interaction does not affect the mea-
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Figure 7. Evolution of the textural parameters for the residues as a function of conversion obtained.
Figure 8. Micropore volume (diameter < 2 nm) of the residues as a function of the conversions achieved (a) for S16 coal residues and (b) B19 coal residues.
Figure 9. Apparent density of the solid residues obtained in catalyzed experiments vs conversion percentages, low-rank coal residues.
surement,35,48 mercury porosimetry was chosen as a reliable technique of the porous structure. Among all of the parameters obtained by Hg porosimetry, we found that the skeletal density, obtained at 4200 kg cm-2 (which allows one to measure pores with a diameter >3.7 nm), gave interesting results. The values for FHg (corrected for mineral matter) are plotted versus the conversion percentages (Figure 9) for residues obtained (48) Amarasekera, G.; Scarlett, M. J.; Mainwaring, D. E. Fuel 1995, 74 (1), 115-118.
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materials. In Figure 10b, for high-rank coals, the trend is similar but less defined. Neither original coals nor residues from blank experiments showed this characteristic. Conclusions
Figure 10. Apparent density of the solid residues obtained in catalyzed experiments vs average number of aromatic units in a chain for (a) low-rank coal residues and (b) high-rank coal residues.
in catalyzed runs of low-rank coals. It is extremely similar to the trend presented in Figure 2 for the same residues, in which the swelling ratio values are displayed versus the conversion percentages. So, all of the discussion developed over those results can be applied to the density values: the hydrogenation process at mild conditions implies the removal of highly voluminous heteroatom moieties and the partial solubilization of the organic matter. In this way, the less cross-linked matter is reorganized in a more compact structure. Actually, chain lengths, calculated as described above, can be compared with the density values obtained by Hg porosimetry for the residues of catalyzed experiments. In Figure 10a, the trend for subbituminous coals shows that the longer the average chain in the residue, the higher the density is, suggesting that a more organized structure is reached once the network is liberated of the mobile phase and other easily releasable
Coal structure can be studied by dry catalytic hydrogenation of coals. The presence of an iron catalyst minimized the formation of cross-linkages, favoring the radical stabilization by hydrogenation pathways. Nevertheless, low temperatures (300 °C) involve the preponderance of self-stabilization reactions over the hydrogenation mechanism because the catalyst has not reached its active species. A higher extent of solubilization is achieved by increasing the experimental temperature (400, 425 °C), which involves fast thermal cracking of the structure and radical capping by the hydrogen available, but a certain degree of repolymerization is detected by the decrease in swelling ratio. However, the solubilization degrees obtained at 350 °C for subbituminous coals and at 400 °C for high-rank ones seem to be due to the removal of heteroatomcontaining moieties and easily releasable low molecular weight. The solvent swelling technique can be used to measure the average chain length of the macromolecular structure of the residues obtained, because the swelling phenomenon is more related to the topological organization rather than to the density in noncovalent cross-linkages. The textural characterization of the residues showed that there is a clear trend of the skeletal density to increase with chain length, with the structure becoming more compact and ordered than in the parent coals. Acknowledgment. This study was financed by EC Project 7220/EC/755 and by Spanish CICYT Project PB-413. EF960125I
