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Energy & Fuels 1989, 3,444-448
Infrared Spectroscopic Study of Coal Depolymerization Catalysis by Metal Chlorides+ H. Paul Wang, Ben A. Garland, and Edward M. Eyring* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 Received October 14, 1988. Revised Manuscript Received April 3, 1989
Diffuse reflectance infrared spectroscopy was used to monitor in situ reactions in which ether linkages undergo cleavage during the depolymerizationof metal chloride impregnated Wyodak coal at elevated temperatures. Subtraction procedures were used to obtain difference spectra that reveal small net changes in the IR band intensity between 2000 and 1000 cm-l. The spectra suggest that the ZnC12-impregnatedcoal samples could be depolymerized in an inert argon atmosphere. However, the extent to which the ZnC12-catalyzedether linkages are cleaved under an inert atmosphere is only about 30% of that for a high-pressure hydrogen atmosphere (68 atm). Phenyl ethers of the form Ph-0-CH2-R are cleaved catalytically by metal chloride, and the oxygen is retained as a phenolic group. Both the FeC13 and CrC1, catalysts have a higher reactivity and selectivity for the cleavage of ether linkages than does the ZnC1, catalyst. A higher degree of catalytic cleavage of phenyl ether is obtained when the ZnC12-impregnatedcoal is prepared in methanol as compared to samples prepared in water and acetone.
Introduction Production of useful chemicals from coal liquefaction depends upon the development and application of effective catalysts.'V2 Better catalysts can lead to more favorable processes by increasing the conversion rate and the product selectivity at reduced operating temperatures and pressures. Many coal conversion catalysts have been investigated, and these studies have primarily involved two types of catalytic materials: metal sulfides and acid cata l y s t ~ .Acid ~ ~ ~catalysts tend to have low hydrogenation activities.2 Since they cannot stabilize the cracking products adequately, the depolymerization reactions are accompanied by condensation reactions leading to the formation of high molecular weight products. The metal sulfides (such as those of molybdenum) are believed to function as hydrogenation catalysts while the metal halides are thought to promote bond cleavage of the linkages connecting the coal structural units.4 Because of the extremely complex structure of coal, the mechanisms of catalyzed coal liquefaction are still not well understood. Ether oxygen is thought to play an important role in linking the macromolecular units in coal. Wachowska and Pawlak6 have indicated that ether groups represent the main linkages between aromatic clusters. Breaking the bridging bonds between the oxygen and the methylene is thought to initiate thermal conversion and liquefaction of coal. Earlier work6 has shown that acid catalysts facilitate the cleavage of linkages that connect the coal structural units. Metal halides are among the many interesting acid catalysts used for coal depolymerization. For example, ZnC12 has a relatively low melting point and thus a significant vapor pressure at temperatures below the temperatures at which coal pyrolyzes.' A high metal chloride vapor pressure facilitates its penetration and dispersion into the coal matrix. Mobley and Bell8 used model compounds to study the effect of zinc chloride on the cleavage of ether structures at reaction temperatures of 225 and 325 "C. They showed Presented at the Symposium on Coal Liquefaction, 196th National Meeting of the American Chemical Society, Los Angeles, CA, Sept 25-30, 1988.
0887-0624/89/2503-0444$01.50/0
that both cyclic and noncyclic ethers react readily in the presence of zinc chloride provided that the ether oxygen is adjacent to at least one methylene group. They proposed that the active catalytic species for bond cleavage is a B r ~ n s t e dacid formed by the zinc chloride. Depolymerization reactions proceed by an ionic mechanism where the driving force for the reaction is determined by the acidity of the catalyst. For stronger acid catalysts, the rate of the reaction can be accelerated and the reaction temperature can be reduced. The average chemical properties of coal have been widely studied by using infrared (IR) ~pectroscopy.~ Recent improvements in the IR spectroscopic techniques have made it possible to measure the IR spectra of coal powders in situ by the diffuse reflectance (DR) m e t h ~ d . ~ Other advantages of FT-IR spectroscopy are the digital storage of spectra and the availability of many computerized data analysis routines that permit operations such as base-line corrections, spectral synthesis, and factor analysis. The present work was undertaken to determine the extent to which the cleavage of ether linkages is catalyzed by metal chlorides.
Experimental Section The tetrahydrofuran (THF) preextracted Wyodak coal samples (Argonne coal, >lo0 mesh) were impregnated with 10% by weight of either ZnC12, FeC13, or CrC1, from methanol. T h e Wyodak coal was also impregnated with 10% by weight ZnClz from two other solvent media, water and acetone. T h e metal chloride impreg(1) Lumpkin, R. E. Science 1988, 239, 873.
(2)Derbyshire, F. J. Catalysis in Coal Liquefaction; IEA Coal Research: London. 1988. (3) Derbyshire, F. J. P r e p . Pap-Am. Chem. Soc., Diu. Fuel Chem. 1988, 33(3), 188. (4)Mazumdar, B. K.;Banerjee, D. D.; Ghosh, G. Energy Fuels 1988, 2, 224. (5)Wachowska, H.; W. Pawlak, H. Fuel 1977,56, 422. (6) Wood, R. E.; Wiser, W. H. Id. Erg. Chem. Process Des. Deu. 1976, 15. . .I
144. ----
(7) Wang, H. P.; Wann, J.; Eyring, E. M. P r e p . Pap.-Am. Soc., Diu. Fuel Chem. 1988, 33(3), 259.
Chem.
(8)Mobley, D. P.; Bell, A. T. Fuel 1978,58, 661. (9)Fuller, M. P.;Hamadeh, I. M.; Griffiths, P. R.; Lowerhaupt, D. E. Fuel 1982, 61, 529.
0 1989 A m e r i c a n C h e m i c a l Society
IR Study of Coal Depolymerization
Energy & Fuels, Vol. 3, No. 4, 1989 445
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Figure 1. Small changes in the infrared band intensity are revealed by a difference reflectance spectrum. Spectra of ZnC12-impregnatedWyodak coal were measured at temperatures of (a) 317 and (b) 9 5 "C. The shaded area indicates negative features calculated from spectrum a - spectrum b. nated coal samples were then dried at 80 "C for 16 h in a vacuum oven. The hydrotreatment of the coal samples was carried out in a vertical tubing bomb reactor under elevated (68 and 122 atm) hydrogen pressures and at elevated temperatures. Infrared spectra were obtained on a Digilab FT-IR spectrometer (FTS-40) with a 64-scan data accumulation a t a resolution of 4 cm-'. Subtraction procedures (Digilab 3200 software) were used to obtain the reported difference spectra by subtracting one spectrum from the other with a weighting factor of unity. The IR spectra showed coal to have little or no absorption in the 2600-2000-cm-' spectral region. Thus, this region was used to define the zero point of the difference spectra. By linear extrapolation, this zero point was extended to the spectral regions of interest. The difference spectra allow a more effective comparison of spectra measured under various conditions as well as the ability to observe small changes in band intensities. Figure 1 illustrates the ability of subtraction routines to reveal small changes in IR band intensity over the 4000-400-cm-' spectral range. Figure 1 also indicates that an almost identical base line was obtainable for the IR spectra, which enhances the accuracy of the calculation for small spectroscopic changes between different samples using the subtraction method. The low signal/noise (SIN)ratios in the spectra are primarily caused by specular reflectance in the infrared diffuse reflectance cell. The consistency of the infrared spectral results nevertheless lends credibility to conclusions drawn from these comparatively noisy spectra. Approximately 50 mg of each coal sample were dried at 100 "C for 1 h in a flowing-argon (30 mL/min) environmentally controlled chamber (Spectra Tech Inc., Model No. 0030-025). This cell was designed for diffuse reflectance Fourier transform infrared (DR FT-IR) spectroscopy at elevated temperatures. Coal samples were studied by in situ IR spectroscopy at temperatures ranging up to 450 "C.
Results and Discussion Reactions with model compounds showed that ZnC12can catalyze the cleavage of linkages connecting the coal structural units.8 Furthermore, Gilbert and Gajewski'O showed that model aromatic ethers can be thermally cleaved via p-scission reactions. One of the model compounds used (phenethyl phenyl ether) was found t o yield equal amounts of phenol and styrene at temperatures just above 350 "C. In the present work the C-0 stretching vibrations of the phenyl ethers between 1265 and lo00 cm-l were investigated by in situ IR spectroscopy since the -C-0- moiety typifies the linkages between aromatic clusters. The band assignment for phenyl ether (Ph-O(10) Gilbert, K. E.; Gajewski, J. J. J. Org. Chem. 1982, 47, 4899.
2000
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Figure 2. Effect of hydrotreatment on Wyodak coal depolymerization. Difference IR spectra were measured between the original uncatdyzed coal sample and the (a) nonhydrotreated and (b) hydrotreated ZnC1,-impregnated coal samples.
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CH,-R) in the 1030-cm-l region was suggested by Hsieh" as well as others.', Other species (such as S-0, Si-0, and metal oxides) have vibrational frequencies in this region of study, but would not interfere with the difference spectra results. Their vibrational contributions would cancel upon subtraction since they are inert to the reaction conditions. The depolymerization of coal samples was investigated in the presence of externally added hydrogen (68 atm). Figure 2 shows a comparison of the difference spectra for both the hydrotreated (68 atm of hydrogen) and nonhydrotreated coal samples (ZnC12impregnated from acetone) at a depolymerization temperature of 300 "C. Difference IR spectra reveal the net changes in the C-0 stretching region and indicate that the ZnC12catalyst facilitates bond cleavage of ether linkages under a high pressure of hydrogen (68 atm) as compared to an inert argon atmosphere. The apparent ether cleavage for the argon atmosphere is about 30% of that for the hydrogen atmosphere. This suggests that condensation reactions may predominate under conditions of low hydrogen availability. It has been established5that the hydrogenation reaction results in coal degradation by cleaving the C-0 bonds. Phenyl ethers of the form Ph-O-CH2-R are catalytically cleaved by metal chloride, and the oxygen is retained as a phenolic group. The phenolic oxygen was founda to be unaffected by temperatures up to 325 "C. Thus, one would (11) Hsieh, F. Ph.D. Dissertation, University of Utah, 1987. (12) Solomon, P. R.;Hamblen, D. G.; Carangelo, R. M., In Coal and Coal Products: Analytical Characterization Techniques; Fuller, E. L., Jr., Ed.; ACS Symposium Series 205, American Chemical Society: Washington, DC, 1982; p 77; Painter, P. C., Snyder, R. W., Starsinic, M.; Coleman, M. M.; Kuehu, D. W.; Davis, A. Ibid., p 47.
Wang et al.
446 Energy & Fuels, Vol. 3, No. 4, 1989
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Figure 3. Formation of phenolic species during catalytic depolymerization of Wyodak coal at a temperature of 305 "C. Difference lR spectra were measured between the original uncatalyzed coal sample and the (a) ZnClz-, (b) FeC13-, and (c) CrC13-im-
pregnated coal samples. The shaded area indicates negative features in the infrared difference spectra.
2000
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Figure 4. Infrared difference spectra for Wyodak coal depolymerization catalyzed by ZnClz. The difference spectra were measured between the original uncatalyzed coal sample and the ZnC12-impregnatedcoal samples at temperatures of (a) 402, (b) 317, and (c) 220 "C. The shaded area indicates negative features in the infrared difference spectra.
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Figure 5. Effect of phenyl ether linkage cleavage in Wyodak coal
catalyzed by ZnClz. Difference IR spectra were measured between the original uncatalyzed coal sample and the ZnC12-impregnated coal samples at temperatures of (a) 305, (b) 246, and (c) 199 "C. The shaded area indicates negative features in the infrared difference spectra.
expect to see an increase in the OH groups resulting from the cleavage of ether linkages. Figure 3 shows the difference infrared spectra (4000-3300 cm-') obtained from coal depolymerization catalysis by ZnCl,, FeC13,and CrC13 under a high pressure of hydrogen (122 am) and at a temperature of 305 "C. Note the negative features in the -3545-cm-' region of the difference spectra of Figure 3. These negative features result from the increase in OH groups arising from coal depolymerization. This is one of the strongest evidences for the present interpretation of the experimental data. The reaction temperature effect on the cleavage of ether linkages under an inert-gas atmosphere is of some interest. Figure 4 shows the difference spectra obtained for ZnCl,-impregnated coal samples at three different reaction temperatures (220,317, and 402 "C) under an inert argon atmosphere. Each spectrum is the difference between the IR spectrum obtained from the original uncatalyzed Wyodak coal and the ZnC1,-impregnated (from acetone) coal samples. In this case, difference spectra should reveal the net changes of the phenyl ether linkages during the catalytic coal depolymerization. As a result, the negative features between 1265 and 1000 cm-' in Figure 4 suggest a cleavage of ether linkages catalyzed by ZnC12. Bond cleavage of ether linkages occurred at temperatures as low as 220 O C , which is below the melting point of the catalyst. This suggests a possibility of a low-temperature coal liquefaction process catalyzed by ZnC1, under an inert-gas atmosphere. The negative features between 1350 and 1500 cm-' are partly due to CH, and CH3 bending.13 The negative feature occuring near 1740 cm-' may be attributable to the carbonyl (C=O) stretching groups.lS (13) Cooke, N.E.;Fuller, 0.M.; Gaikwad, R. P.Fuel 1986,65,1254.
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Energy &Fuels, Vol. 3, No. 4, 1989 447
Coal Depolymerization
8 f
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Figure 6. Effect of phenyl ether linkage cleavage in Wyodak coal catalyzed by FeC13. Difference E t spectra were measured between
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Figure 7. Effect of phenyl ether linkage cleavage in Wyodak coal
catalyzed by CrCls. DifferenceIR spectra were measured between
the original uncatalyzed coal sample and the FeC13-impregnated coal samples at temperatures of (a) 305, (b) 246, and (c) 199 OC. The shaded area indicates negative features in the infrared difference spectra.
the original uncatalyzed coal sample and the CrC13-impregnated coal samples at temperatures of (a) 305, (b) 246, and (c) 199 "C. The shaded area indicates negative features in the infrared difference spectra.
Under the Lewis acid-base definition, metal halides are acids and are generally known as Friedel-Crafts catalysts. In order to compare the relative activities or to evaluate the selectivity of metal chloride catalysts, a given reaction system must be specified. Figure 5 shows the effect that reaction temperature has on coal depolymerization when the hydrogen pressure is fixed. Each spectrum in Figure 5 is the difference spectrum obtained by subtracting the IR spectrum of Wyodak coal without ZnC1, from that of the ZnC12-impregnated (from methanol) Wyodak coal samples at the indicated temperatures. The negative features in the 1265-1000-cm-' region indicate that the bond cleavage of ether linkages was catalyzed by ZnC1,. Advancing the use of acid catalysts in coal depolymerization requires the ability to control catalyst acidity. To better understand the effect of catalyst acidity on coal depolymerization, similar studies were done using FeC1, (Figure 6) and CrC1, (Figure 7). The results indicate that both FeC13 and CrC13 catalysts have a higher selectivity and reactivity for the bond cleavage of ether linkages than does the ZnClpcatalyst. This is again confirmed by growth in the phenolic OH peak at 3545 cm-' (not shown in Figures 6 and 7.) However, these data do not permit a choice between FeC1, and CrC1, as the more effective coal depolymerization catalyst. A careful comparison of both Figures 6 and 7 shows that within the limit of low S I N ratios the two catalysts perform quite similarly. These results can possibly be attributed to the acidities of the metal chloride catalysts used. Both FeC1, and CrC1, are considered to be strong Lewis acids with similar acidities. Because of its smaller charge/size ratio and its ability to accept fewer electron pairs, ZnCl, is a weaker Lewis acid than the other two metal chloride catalysts. The above data appear to give a direct correlation between the cat-
alyst acidity and the extent that ether linkages are cleaved during coal depolymerization. Mobley and Bell8suggested that the Lewis acid form of ZnC1, may activate ether cleavage by forming a complex with the ether oxygen or it may involve the presence of a strong Brransted acid formed by the metal halide. Taylor and BeF4have studied the role of Lewis acid catalysts in the cleavage of both aliphatic and aryl-aryl linkages. They found that the rate of alky-aryl bond cleavage was dependent on the Srransted acidity of the active catalyst. This type of reasoning may also hold true for the cleavage of ether linkages in the present study. As one would expect, all three catalysts showed a more pronounced cleavage of the ether linkages in the coal matrix at higher reaction temperatures. Some organic solvents (such as methanol, acetone, THF) induce the swelling of coal during density measurements and surface area measurements, as well as during the extraction of coal.15 This phenomenon is related to the characteristics of coal as a cross-linked polymeric system.I5 Before depolymerization can take place, the catalyst and the hydrogen must diffuse through the pore systems of the coal network to the reacting groups in the coal. Solvents that cause the coal to swell may enhance the porosity and thus the reactivity of some coals under milder reaction conditions. The effect that different solvent media, used for the ZnCl, impregnation of the coal, had on the depolymerization of these coal samples is shown in Figure 8. The difference spectra were obtained by subtracting the IR spectrum of the original uncatalyzed coal, measured at 31 7 "C with a flowing argon atmosphere, from the spectra of (14) Taylor, N. D.; Bell, A. T. Fuel 1980,59, 499. (15)Marzec, A.; Kisielow, W. Fuel 1983, 62, 977.
448 Energy & Fuels, Vol. 3, No. 4, 1989
Wang et al. acetone and water. We attribute this negative feature in the spectra to the cleavage of phenyl ether linkages. Evidently, dispersion of active centers (e.g. ZnClz) in the coal matrix is dependent upon the solvent medium used for the catalyst impregnation procedure. In this case, methanol appears to disperse the ZnClz better than the other two solvents. These data are consistent with the results of Wann and Bodily who showed the relative dispersion of ZnC12 or FeC13 from methanol media in Wyodak coal to be above 80%.l6
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Figure 8. Effect that the solvent media used for ZnCl, impregnation of Wydak coal samples have on coal depolymerization. Difference IR spectra were measured between the original uncatalyzed coal sample and the ZnC1,-impregnated coal (a) from methanol, (b) from water, and (c) from acetone. The shaded area indicates negative features in the infrared difference spectra.
ZnC12-impregnated coal samples, which were prepared in water, acetone, and methanol. The ZnClz-impregnatedcoal sample prepared from methanol shows a larger negative amplitude in the 1200-1000-cm-’ region than do the correspondin spectra obtained for coal samples prepared from
Summary Several conclusions follow from this work: 1. The subtraction routines used to obtain the difference spectra can reveal small net changes in the highly complex IR spectra of coal. 2. Hydroxy group (-3545 cm-’) formation accompanies the loss of ether linkages in coal depolymerization. 3. The extent of cleavage of ether linkages catalyzed by ZnC1, in an inert atmosphere is about 30% of the cleavage obtained in a high-pressure hydrogen atmosphere. 4. Both FeC13 and CrC13catalysts cause greater cleavage of ether linkages in coal than does the ZnClz catalyst. 5. ZnC12-impregnatedcoal prepared in methanol gives rise to greater cleavage of ether linkages than similar samples impregnated from water and acetone.
Acknowledgment. Informative discussions with Professor D. M. Bodily and J. Wann (Fuels Engineering Department, University of Utah) are gratefully acknowledged. Financial support by the US. Department of Energy, Fossil Energy Division, through the Consortium for Fossil Fuel Liquefaction Science, Contract No. UKRF-4-2100386-24, is also acknowledged. Registry No. ZnC12, 7646-85-7; FeCl,, 7705-08-0; CrCl,, 10025-73-7;methanol, 67-56-1. (16) Wann, J. M.S. Thesis, University of Utah, 1987.
