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Coal Conversion in CO/Water. 2. Oxygen Loss and the Conversion Mechanism David S. ROSS,*Thomas K. Green, Riccardo Mansani, and Georgina P. Hum Physical Organic Chemistry Department, Fuel Program, SRI International, Menlo Park, California 94025 Received September 4, 1986. Revised Manuscript Received December 22, 1986
The fate of oxygen in Illinois No. 6 coal has been studied in CO/water and corresponding CO/N2 conversions at 400 OC. Oxygen determinations were made directly. The results showed that about half of the starting oxygen in the coal is lost thermally, whether or not conversion takes place. The liberated oxygen is not phenolic but rather comes from the unaccounted for oxygen pool. We assert that this oxygen cannot be ether oxygen, on the basis of the ease with which it is liberated, and that most likely the lost 0 is due to tightly bound water in the starting coal.
Introduction The connection between conversion of coal to soluble products and oxygen loss through the cleavage of ether links in the coal is a commonly accepted route to conversion, most recently summarized by Youtcheff and Given.' In that account and in earlier accounts, research is described in which several coals were converted in tetralin/hydrogen, and the results were then correlated both with the types of oxygen functionalities in the coals and coal products and with the loss of oxygen during the
conversion^.^^^ It was found that roughly 40% of the oxygen in the coals was phenolic oxygen, with the remainder referred to as unaccounted oxygen. The unaccounted oxygen was surmised to be etheric, and it was noted further that the major oxygen loss during conversion was from the unaccounted oxygen pool, with considerably smaller losses of phenolic oxygen. In work over a broad temperature range providing spreads of both conversion and oxygen loss, it was found that both processes responded similarly to the varying thermal treatment. The increased conversion to soluble products followed the loss of oxygen. No unambiguous mechanistic connection between two processes can be made on the basis of such a correspondence, as Given and Szladow carefully point Nonetheless it has been tempting to presume a direct connection between conversion and loss of unaccounted oxygen. This presumption has then been coupled with a second presumption that unaccounted oxygen is ether oxygen. The resulting view of conversion is one that includes thermal ether cleavage as an important route to liquids. A test of the connection between conversion and ether cleavage is reported on in this work. Our work includes conversion studies on a bituminous coal in CO/water, the results of which allow us to test both the connection between oxygen loss and conversion and the source of the liberated oxygen. We have pointed out that the CO/water system is particularly useful in an examination of these points because it allows a very broad range of conversion (1)Youtcheff, J.; Given, P. H. Prepr. Pup-Am. Chem. SOC.,Diu.Fuel Chem. 1984, 29(5), 1-8 and references therein. (2) Youtcheff, J.; Given, P. H. Fuel, 1982, 61, 980-987. (3) Szladow, A. J.; Given, P. H. Prepr. Pup.-Am. Chem. Soc., Diu. Fuel Chem. 1978,23(4),161-168.
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Table I. Analytical Data (%) for Illinois No. 6 Coal (PSOC 1098)'
carbon hydrogen nitrogen sulfur (organic) oxygen (diff) mineral matter
81.56 5.61 1.60 1.87 9.36 17.31b
Presented on a dmmf basis. Calculated according to MM = 1.13(ash) + O.47Sp,where S, is percent pyritic sulfur.
levels to be utilized, with no changes necessary in the conversion times and temperatures! Specifically the work deals with conversions conducted at 400 "C on an Illinois No. 6 coal in a stirred, batch autoclave. Elemental analyses were conducted on both the starting coal and the products, and included direct analyses for carbon, hydrogen, oxygen, and nitrogen. The phenolic contents of the coal and products were also determined. Experimental Section The coal used in this work was an Illinois No. 6 coal, PSOC 1098, supplied by The Pennsylvania State University. The coal was ground and sieved under dry Nz to pass -60 Tyler mesh and then dried under vacuum at 105 "C overnight. The conversions were conducted as described elsewhere.4b Elemental Analyses of Conversion Products. Carbon, hydrogen, and nitrogen analyses of coal products were determined in our laboratory on a Control Equipment 241 microanalyzer. The analysis is presented in Table I. The starting coal and several products were submitted for direct oxygen analysis to Galbraith Laboratories, who performed the analyses using the procedure described by culm^.^ It involves pyrolysis of samples in an inert atmosphere a t 950 "C over platinized carbon. The treatment yields CO from the organic oxygen, with the CO subsequently oxidized to C02. The C02 is then determined quantitatively. Water itself under these conditions yields CO C
+ HzO
-
CO
+ H2
and under conditions used in the analysis, the production of CO from water is quantitative.6 It can be seen therefore that should any sample yield CO, COz,or HzO from any source, these products (4) (a) Rosa, D. S.; Blessing,J. E.; Nguyen, Q.Prepr. Pap.-Am. Chem.
Soc., Diu.Fuel Chem. 1981,26(2),149-158. (b) Ross, D. S.; Green, R. K.;
Mansani, R.; Hum, G. P. Energy Fuels, preceding paper in this issue. (5)Culmo, R. Mikrochim.Acta (Wien),1968, 811-815. (6) Culmo, R., private communication. 0 1987 American Chemical Society
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Coal Conversion i n COIWater Table 11. Product Data for a Series of Runs with PSOC 1098 in Aqueous Conversion Media % toluene % element recovereda medium init pH(D) solubility C H N 0 N2/H20 7.0 7 90 70 99+ 52 29 91 77 98 34 CO/H20 7.0 82 86 40 CO/D20 11.5 45 88 13.0 13.4
60 60
89 94
95 93
95 95
38 38
Calculations are based on the molar quantities of the respective elements in the starting coal and the sums of the quantities in both the toluene-soluble and' toluene-insoluble product fractions. As described in the text, the oxygen values are from direct oxygen determinations. will ultimately be recognized as a fraction of the total analytical oxygen in the sample. Youtcheff and Given have commented on this aspect of direct oxygen analysis, noting that the clays and carbonates in the mineral fraction of the coal could contribute respectively H 2 0 and COz, ultimately contributing to the determined analytical oxygen value.* 0-Acetylation of Hydroxyl Groups. Given's modification of Blom's method of 0-acetylation was used to determine the hydroxyl contents of the starting coal and coal products? About 0.5 g of coal was heated for 4 days at 80 "C in a sealed glass vial containing 6 mL of dried pyridine and 6 mL of a 1:9 mixture of acetic anhydride and dried pyridine. The product was mixed with 10 mL of toluene and 10 mL of water and left overnight. The mixture was transferred to a 150-mL flask. Acetic acid was titrated with 0.35 N NaOH to a p H of 9.0 by using a combination glass calomel electrode. The phenols were determined in terms of milliequivalents of OH per gram of product and were calculated through the expression mequiv of OH/g = d ( a - b ) / c where a and b are the number of milliliters of titrant for the blank and sample, respectively, c is the weight of the sample, and d is the normality of the titrant.
Results Approach. Our approach to the problem was developed from the following points. We are aware of no firm experimental evidence demonstrating that unaccounted oxygen is ether oxygen. Ethers such as diary1 ethers are highly stable thermally, and while some special cases such as benzyl phenyl ether and dibenzyl ether may indeed cleave thermally under conversion conditions,' it is unlikely that conversion requires such specialized, fragile links. Indeed if such ethers were significant as critical links in the macromolecular coal structure, their propensity for engaging in radical-chain cleavage would result in coal's being readily degraded by the addition of chain initiators at temperatures as low as 200 oC.s It, of course, is not. Moreover, if aryl ether links and their cleavage were significant to conversion, then the cleavage would yield phenols, There would be an accordant increase in phenol oxygen in the products, whereas in fact a small decrease is reported.2 Finally, while the observed correlation between conversion and oxygen loss may be consistent with a connection between the two processes, it clearly is not compelling. The two could very well be unconnected, but with similar temperature coefficients. To test the ether cleavage conjecture, we conducted conversions of bituminous coals in CO/water, where a wide range of conversions are attainable simply with changes in the initial pH of the m e d i ~ m Moreover, .~ runs can be (7) Siskin, M.; Aczel, T. Fuel, 1983, 62, 1321-1326. (8)McMillen, D.,personal communication.
Table 111. Hydroxyl Group Analysis of Illinois No. 6 Coal (PSOC 1098) and Products from Conversion in CO/Watera mmol of
9.tid
7.0d
1.28 1.49
3.25 3.35
1.9 2.2
1.8 1.6
8.3e 8.6O
a Values presented on a dry basis. * Starting coal. Total millimoles in 5.11 g of starting coal. dFor both runs 5.11 g of dry coal was used in 30 mL of water. The runs were for 20 min at 400 "C. eTotal millimoles in products.
conducted with N2 in place of CO. In this case, in the absence of any reducing potential but with all other conditions unchanged, the strictly thermal reactions of coal can be studied under conversion conditions. Findings. An important aspect of the study involves the elemental mass balances after conversion. Data for C, H, N, and 0 for runs in N2/H20,CO/H20, and CO/D20 are presented in Table 11. The conversions span toluene solubilities from 7% to 60%. The contributions of sulfur have been ignored. The work in the deuterio medium was conducted for mechanism and kinetics studies reported elsewhere,4band the isotopic switch is of no special consequence here. As is seen in the table, good carbon and nitrogen balances were obtained over the range of conversion levels. In contrast, considerable hydrogen was lost relative to the hydrogen content of the starting coal, except at the highest conversion levels. This result is similar to that reported recently by Bockrath et al.9 and will be discussed in subsequent accounts of this work. Of interest here are the oxygen recoveries, which are also low, uniformly falling in the range 35-50%. Even in the N2 run, with virtually no conversion, about half of the oxygen was lost. That result, plus the similarity in lost oxygen values, suggests strongly that the oxygen loss is a process separate from conversion and is due solely to the thermal treatment. The source of the lost oxygen is apparent from the data in Table 111, showing the phenolic oxygen contents for both the starting coal and products. The results are presented as millimoles of OH per gram of dry sample and are further translated into the quantities of OH in the 5.11 g of starting coal and in the products. The data show that the full quantity of phenolic groups was about 10.2 mmol in the starting coal, whereas the phenolic content of the total products was around 8.5 mmol. Thus 80435% of the starting phenolic functionality has been retained in the products, and the oxygen thermally liberated came primarily from the unaccounted for oxygen pool. This finding is similar to that of Youtcheff and Given for their THFsoluble products.2
Discussion The finding that half of the oxygen in coal is lost during heating, whether or not a conversion condition (i.e. a reduction potential) is present, demonstrates that oxygen loss is a thermal process. It has no direct link to conversion; the two processes apparently have somewhat similar temperature coefficients, but operate independently. The oxygen liberated in this process is not phenolic for the most part, but comes from some other structural feature or features. However, as stated above, there is no evidence (9) Bockrath, B. C.; Finseth, D. H.; Illig, E. G. Am. Chem. SOC. Diu. Fuel Chem. P r e p . 1985, 30,308-314.
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that the structure is etheric. These results are in accord with observations by a number of investigators of the pyrolysis of coal, which topic has been recently reviewed by Howard.lo Particularly relevant findings are those by Suuberg et al. in a study of the rapid pyrolysis of a bituminous coal (Pittsburgh Seam No. 8) in helium, with heating rates up to 1000 "C/s." The pyrolytic products included organic tars and liquids, as well as substantial quantities of pyrolytically formed water a t temperatures up to about 400 "C. CO and C 0 2 were evolved at temperatures above 600 "C. The pyrolytic water was easily distinguished from the small quantity of surface moisture initially driven off and corresponded to about 65% of the total oxygen in the starting coal. With the activation parameters for the formation of this water provided in the account [E,, 35 kcal/mol; log A , 13.0 s-l (assigned)],it can be shown that at temperatures as low as 225 "C it is fully evolved in about 20 min; at 400 "C it is lost virtually instantaneously, and its source would then be exhausted. This water must correspond to unaccounted oxygen. It is of interest to point out that there has been no purely thermal ether chemistry identified as yielding water at temperatures as low as 250 "C. Conclusions During conversion, therefore, bituminous coal initially and rapidly loses water in thermally driven reactions not directly related to conversion. The chemistry surrounding conversion then operates on this initially formed product, (10) Howard, J. In Chemistry of Coal Utilization; Elliott, M. A,, Ed., Wiley: New York, 1981; Second Supplementary Volume, pp 665-784. (11) Suuberg, E. M.; Peters, W. A.; Howard, J. B. Seoenteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1979; pp 117-130.
breaking bonds and yielding soluble product material. Direct oxygen analyses conducted on both the starting coal and the upgraded products will show a net oxygen loss, corresponding to the quantity of water thermally liberated during conversion. However the oxygen source is not etheric. There are both organic and mineral sources that might be suggested as the oxygen source, but for various reasons they are not fully suitable. For example, Poutsma and Dyer have observed that naphthols dehydrate to form a collection of condensed products including ethers at 400 "C.12 However, the fact that in conversion the phenol levels are not highly affected rules out this chemistry as a primary source. Similarly, kaolinite, a major clay component in coal, dehydrates at around 400 "C.13 However, the quantities of water-yielding clays in most coals is insufficient to account for most of the water. As described in the Experimental Section, since oxygen from any source will be included in the direct oxygen analysis, we can speculate that it may be water itself that contributes to the oxygen count in the analyses of the starting coals. This water would of course have to be very tightly bound; however in the face of no known waterliberating chemistry operating with coal-related oxygen structures, water itself is the most reasonable candidate to consider as the source. Acknowledgment. We acknowledge the support of the U.S. Department of Energy for this work. We also thank Dr. D. McMillen for very useful discussion of the work. Registry No. CO, 630-08-0; HzO, 7732-18-5. (12) Poutama, M. L.; Dyer, C. W. J. Org. Chem. 1982,47,3367-3377. (13) Brindley, G. W.; Nakahira, M. J. Am. Chem. SOC.1959, 42, 314-318.
Kinetic Studies on the Hydroliquefaction of Coals Using Organometallic Complexes Toshimitsu Suzuki,* Toshihiro Ando, and Yoshihisa Watanabe Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Kyoto 606, Japan Received September 9, 1986. Revised Manuscript Received January 14, 1987
Kinetic studies were carried out on the hydroliquefaction reaction of three different rank coals by using Bu,Sn or Fe(CO), as catalyst precursors. A combined parallel and consecutive reaction scheme was employed to estimate first-order rate constants. Direct parallel processes to give oil and asphaltene from coal prevailed in the liquefaction of low-rank Yallourn coal, with both Sn and Fe catalysts. However, a bituminous coal (Mi-ike coal) was converted into oil via a series of reactions through the preasphaltene and asphaltene fractions. Both Sn and Fe catalysts seem to stabilize coal fragment radicals formed by thermal dissociation of coal and to transfer molecular hydrogen activated on the catalyst to roal fragments to give stable molecules. It seems probable that Sn and Fe catalysts promote stepwise processes. Introduction A large number of studies to elucidate the mechanism of the coal liquefaction reaction have been reported. Kinetic studies on thermal or catalytic liquefaction have
been carried out by many researchers, and several kinetic models were proposed. 0887-0624/87/2501-0294$01.50/0
Weller et a1.l found that the conversion of coal to oil could be described by the following sequential reaction scheme. (1) (a) Weller, S.; Pelipetz, M. G.; Friedman, S. Ind. Eng. Chem. 1951, 43, 1572. (b) Weller, S.; Pelipetz, M. G.; Friedman, S. Ind. Eng. Chem. 1951,43, 1575.
0 1987 American Chemical Society
