Energy & Fuels 1987,1, 363-366
363
On the Mechanism of Hydrogen Transfer from Decalin to Coal. Pressure Effect and Kinetic Isotope Effect Janusz Pajab* Institute of Petroleum and Coal Chemistry, Polish Academy of Sciences, 44-100 Gliwice, Poland
Kay R. Brower* Chemistry Department, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801 Received March 9, 1987. Revised Manuscript Received May 19, 1987
In order to study the mechanism of the reaction of bituminous coal with decalin, the molar activation volume and H/D kinetic isotope effect have been measured. The activation volume a t 400 "C is -70 f 5 cm3/mol. This result suggests a mechanism in which ionization accompanies or precedes the formation of the transition state. It is inconsistent with the concept that the rate is determined by homolysis of the coal molecule. The H/D kinetic isotope effect for the coal/decalin reaction is 2.3 f 0.2. This result reinforces the conclusions above. Studies on effects of pressure and isotropic substitution on the rate of reaction of some model compounds with decalin were also performed. The anthracene/decalin reaction, with AV* = -55 f cm3/mol and KIE = -2.1 f 0.2, seems to proceed through the same mechanism as the coal/decalin reaction. Introduction The process of coal hydrogenation, believed to consist of a series of competitive and consecutive reactions, is additionally complicated by the fact that recycle oil, serving as a hydrogen donor solvent, is a mixture of many individual compounds. There is no reason to believe that different donor compounds will react with coal by the same mechanism, particularly taking into account the complexity of coal composition. In order to get some basic knowledge of coal hydrogention, we must start with studies of the mechanism of the reaction of coal with individual donor solvents and then further substitute model organic compounds for coal. Much work has been done to elucidate the details of the reaction of coal and model compounds with tetralin. An excellent review of the recent research has been provided by St0ck.l Though the radical capping mechanism proposed by Gorin2 seems still to be the most popular view on coal hydrogenation, other reaction pathways have gained increased attention. We recently showed that thermal homolysis of the organic matter of coal is not a rate-determining step of the reaction of a subbituminous coal with tetralin at 335 0C.374 It might be possible, however, that at higher temperatures coal thermolysis is rate-determining, particularly for less reactive donor solvents. This paper will present the results of studies of reaction of bituminous coal and some model compounds with decalin. Decalin has been selected for these studies for the following reasons: (a) Decalin is reactive toward free radicals as shown by the bibenzyl/decalin reaction described by Cronauer et al.6 (b) Preliminary studies have shown that decalin is stable in a stainless-steel vessel at temperatures up to 415 "C. (1) Stock, L.M. In Chemisty of Coal Conversion; Schlosberg, R. H., Ed;Plenum: New York, 1985; p 253. (2) Curran, G.P.Struck, R. T.;Gorin E.Znd. Eng. Chem. Process Des.
Deu. 1967,6, 166. (3) Brower, K. R.J . Org. Chem. 1982,47, 1889. (4) Brower, K. R.; Pajak, J. J. Org. Chem. 1984,49, 3970. (5) Cronauer, D.C.; Jewell, D. M.; Shah,Y. T.;Kueser, K. A. Znd. Eng. Chem. Fundam. 1978,17, 291.
0887-0624/87/2501-0363$01.50/0
(c) With respect to molecular structure, decalin is related to tetralin, which is frequently used as a solvent for investigation of coal hydrogenation. On the ansumption that the slow step in reaction of coal with decalin is dominated by a single type of process, for example homolysis, molecular disproportionation, or hydride transfer, we believe it is useful to measure activation volume (AV*) and the H/D kinetic isotope effect (KIE). Details of the activation volume measurements and interpretation may be found in a recent book! Briefly, the method relies on measurement of the pressure effect on the reaction rate constant and calculation of the activation volume, AV*, from eq 1. Bond breaking in the transition AV* = -RT(d(ln k ) / d P ) T
(1)
state increases the volume, whereas bond making decreases the volume. Ionization is usually characterized by a large negative activation volume. The kinetic isotope effect is a widely used tool to elucidate mechanisms of organic reactions.' In the present case, the existence of a primary KIE, indicating the participation of decalin in the slow step of reaction, would be inconsistent with the concept of rate-determining homolytic decomposition of coal.
Experimental Section A powdered sample of Polish bituminous coal was demineralized with hydrofluoric and hydrochloric acid by the method of Radmacher? Characteristic data (wt %, daf basis) for the coal sample are as follows: C, 77.6; H, 5.0; 0 + N, 17.4; ash, 1.8; vitrinite, 60; exinite, 12; inertinite, 28. Decalin-d18and other chemicals were purchased from Aldrich Chemical Co. Reactions of coal with decalin (ratio 1:1, usually 100 mg:100 pL) were carried out at 400 O C under pressures up to 58.6 MPa in a stainless-steel microreactor consisting of a 15-em length of 2-mm-i.d.pressure tubing. After emplacement of the reaction (6) Isaacs, N. S. Liquid Phase High Pressure Chemistry; Wiley: New York, 1981. (7) Melander, L.; Saunders, W. H., Jr., Reaction Rates of Isotopic Molecules; Wiley: New York, 1980. (8) Radmacher, W.; Mohrbauer, P. Brennst.-Chem. 1956,37,353.
0 1987 American Chemical Society
Pajak and Brower
364 Energy & Fuels, Vol. 1, No. 4, 1987 Table I. Naphthalene and Tetralin Formation in Coal/Decalin Reactions 4.1 MPa 35.2 MPa 58.6 MPa amt formed, amt formed, amt formed, time, mol % time, mol % time, mol % min ClOH8 CldIlZ min CldI8 ClZHlZ min Cl&8 CldIlZ 45 75 135 195 255 300 435 720
Table 11. Relative Reaction Times for Coal/Decalin Reactions under Pressure hydrogen transfer, 4.1 MPa 35.2 MPa 58.6 MPa g/100 g of coal time, min t O / t p time, min t o / t p time, min to/t, 2.3 1.5 50 0.30 (1.0) 75 115 2.0 1.5 110 0.40 (1.0) 150 220 1.8 1.6 175 0.50 (1.0) 205 320 1.9 1.5 240 0.60 (1.0) 305 470 Table 111. Reaction of Coal with Deuteriated Decalin time, min 105 165 255 345 435 675 3.9 6.6 4.4 5.5 1.9 mol % of C& 1.5 1.8 0.9 1.3 1.6 mol % of CIJllz 0.7 0.8
41 MPa 35.2 MPa 0 58.6 MPa
0 '
A
0
I
100
200
300
400
the (minl
Figure 1. Hydrogen transfer from decalin to coal under various pressures. mixture, pressure was generated by pumping nitrogen into the reactor, which was then placed in a preheated aluminum block for a chosen time. Since thermal equilibration is attained within 5 min after insertion of the microreactor in the heater and since quenching is even faster, no time scale adjustments were made. Reaction products were extracted with hexane and analyzed by capillary GC (a Tracor instrument with FID detector) and GC/MS (a Hewlett-Packard 5890A with 5790 MS detector). Reactions of model compounds with decalin were carried out in a similar way. For the anthracene/decalin reaction the molar ratio was 1:5.8 (typically 40 mg and 200 rL);for the benzophenone/decalin reaction the molar ratio was 1:1.5; and for anthraquinone, 1-naphthol, and 2-methyl-1-naphthol it was 1:l.
Results Pressure Effect on the Coal/Decalin Reaction. The formation of naphthalene and tetralin in the reaction of coal with decalin is presented in Table I. In addition, trace amounts of octahydronaphthalene were detected. Assuming that all hydrogen from decalin was transferred to coal (no other decalin-derived producta were found), the amount of hydrogen transferred to 100 g of daf coal was calculated, and the results are shown in Figure 1. The activation volume, AV*, is obtained from eq 1. It is not reasonable to expect for such a complicated substance as coal any simple rate law, but it is still possible to obtain a meaningful AV* even for a reaction with an unknown rate law. On the assumption that only k is a function of P, the pressure effect for a certain degree of reaction is equal to t o / t , (k,/ko = ratio of rate constants at high and low pressure = t O / t p= ratio of times required for the same degree of reaction). Table ll shows the results extracted in this way from Figure 1. Substitution in eq 1 gives the molar activation volume -70 f 5 cm3/mol. Kinetic Isotope Effect for the Coal/Decalin Reaction. The kinetic isotope effect for the reaction of coal with decalin can be obtained from kinetic measurements in the same way as the activation volume. Table I11
Figure 2. Hydrogen transfer from decalin and perdeuteriodecalin t o coal. 1 % Hydrogmtion 0
222018 ..
A
55.2 MPa 4.5 MPa 4.5 MPa,deuterated
Figure 3. Hydrogenation of anthracene with decalin.
presents the formation of naphthalene and tetralin in the reaction of coal with perdeuteriated decalin, and Figure 2 shows the plot of hydrogen (deuterium) transfer vs. time. The reaction times required for a given transfer of hydrogen are the reciprocals of the relative rates, and they are equal to the Km as conventionally expressed (kH/kD). If X represents hydrogen (deuterium) transfer in g/100 g of coal, some values of t ~ /are t 2.3 ~ (X= 0.30),2.1 (X = 0.40),and 1.9 (X = 0.50). Since a small hydrogen/ deuterium exchange between decalin and coal was observed (for example, after 11 h Cl,,Dls exchanged about 2.3% of its deuterium as indicated by GC/MS analysis) the 'true" KIE value is closer to values obtained for lower degrees of reaction, and is therefore taken to be 2.3 i 0.2. Reaction of Anthracene with Decalin. Figure 3 shows the plot of hydrogenation of anthracene with decalin vs. time. Since 9,lO-dihydroanthracene disproportionates for the to anthracene and 1,2,3,4-tetrah~droanthracene,~ (9) King, H. H.; Stock, L.
M.Fuel
1982, 61, 257.
Pressure and Kinetic Isotope Effects
Energy & Fuels, Vol. 1, No. 4, 1987 365 Scheme I
Table IV. Reaction of Organic Compounds with Decalin temp, compd benzophenone
400
2-methyl-1-naphthol
400
1-naphthol
400
O C
anthraquinone
400
anthraquinone
350
identified products diphenylmethane benzhydrol tetralin naphthalene 1-naphthol dimethyl-1-naphthol dimethylnaphthalene methylnaphthalene naphthalene tetralin naphthalene dihydro-1-naphthol tetrahydro-1-naphthol tetralin anthrone anthracene naphthalene naphthalene tetralin
change of rate with pressure 0.67
0.59
If**t
not measd
tetralin not measd 0.83
purpose of calculating percent hydrogenation it was assumed that in the reaction product 1 mol % of tetrahydronapthalene is the equivalent of 2 mol % of dihydroanthracene. Extracting the relative reaction times from the plot in a similar way as for the coal/decalin reaction and comparing them, we obtain AV* = -55 f 5 cm3/mol; KIE = 2.1 f 0.2. Detailed kinetic data (not presented here) also indicate significant acceleration of disproportionation of 9,lO-dihydroanthracene with increase in pressure. Pressure Effect on Other Model Compounds. The results of pteliminary tests of pressure effect on reaction of some organic compounds with decalin are presented in Table IV. The number describing the change of rate with pressure expresses the estimated k J k o for P = 55 MPa. Measured under low (4 MPa) pressure, the rates of conversion of benzophenone, 1-naphthol, and 2-methyl-lnaphthol were about the same in decalin as in tetralin. Error Analysis. Errors in evaluation of activtion volumes and kinetic isotope effects can best be judged by reference to Figures 1 and 2. The data used in the calculations are the reaction times corresponding to any arbitrary position on the ordinate. On both f w e s the ratios obtained in this way can vary by as much as &lo%. When the upper and lower limits of the ratio obtained from Figure 1are inserted in eq 1,the result is a 5-mL uncertainty in activation volume. A similar procedure applied to Figure 2 gives an uncertainty of 10% in the kinetic isotope effect.
Discussion It seems useful to start here with a discussion of the hydrogen transfer cracking reaction of bibenzyl, which could have been suggested as a coal model compound in some respects. It is well established that homolysis of the central C-C bond is the rate-determining step of bibenzyl thermolysis.10 For this reaction pathway the reaction rate should be independent of the amount or type of hydrogen donor solvent, and accordingly, a similar rate of bibenzyl conversion in decalin and in tetralin has been found in spite of different reactivities of these solvents toward free radicals? The use of activation volume and kinetic isotope effect techniques confirmed the homolytic mechanism. The activation volume a t 395 "C is + 31 cm3/mol,11and (10) Miller, R. E. Stein, S. E. J. Phys. Chem. 1981,85, 580. (11)Brower, K. R. J. Org. Chem. 1980,46, 1004.
I 1,2,3.4-tetrahydroanthracene
the rate of toluene formation is not affected by isotopic substitution at the a-or @-positionof tetralin.12 In the present study some model compounds, namely benzophenone, 2-methyl-1-naphthol, and 1-naphthol behave in a manner similar to bibenzyl. The reaction rate is the same for tetralin and decalin. The rate of reaction in decalin at 400 "C decreases with pressure, indicating a positive activation volume near 30-40 cm3/mol. It has been recently proposed by Stock13that the first step of the benzophenone/tetralin reaction is a radical process leading to benzhydrol and dihydronaphthalene. We think in all these cases the rate-detemining step is a radical process, but the details are still uncertain and require further investigation. Our sample of bituminous coal behaves quite contrary to bibenzyl when subjected to reaction with donor solvents. The reaction of coal with decalin is much slower than that with tetralin, and both reactions are accelerated by pressure, which seems to rule out the mechanism in which the generation of free radicals is rate-controlfing. The activation volume for coal/decalin reaction, -70 cm3/mol, is probably the greatest negative value of AV* reported so far. One of the reasons is that high temperature magnifies AV*, and only a few measurements have been done above 250 O C ? Ionic reactions are the type most likely to be characterized by large negative activation volumes. Ionic displacement reactions of piperidine with bromonaphthalenes at temperatures near 200 O C have activation volumes of about -60 cm3/moL1* As one possibility, we propose a hydride transfer mechanism as a pathway of hydrogen transfer from decalin to coal according to Scheme I. A portion of this scheme operating in the reverse direction has been demonstrated in the conversion of octaIin to decalin via 9-decalyl cation.IS An interesting feature of this reaction is that it forms chiefly cis-decalin which is less stable than trans-decalin. No rearrangement of the 9-decalyl cation was observed. Another possibility that would fit the observations is that radical ions are generated in a preequilibrium step: A +B A'- + B'+. The rate-controllingstep would then be a bimolecular reaction between decalin and the radical cation. The observed activation volume would be the sum of the reaction volume for radical ion-pair production (-45 mL) and the activation volume for the bimolecular step (-15 d). At present we are unable to propose any method for detecting or trapping any of the postulated intermediates. Whatever the details may be, it seems nearly certain that the formation of ions is responsible for the
-
(12) Pajak, J.; Brower, K. R. J. Org. Chem. 1985 50, 2210. (13) Choi, C.; Stock,L. M. J . Org. Chem. 1984,49, 2871. (14) Brower, K. R. J . Am. Chem. SOC.1958,80, 2105. (15) Carlson, R. M.; Hill, R. H. J. Org. Chem. 1969, 34, 4178.
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Energy & Fuels 1987,1, 366-376
amazing acceleration of these reactions by pressure. Kinetic isotope effect studies confirm the conclusion based on AV* measurements. The value of 2.3 at 400 "C indicatesa large primary deuterium isotope effect showing one hydrogen in transit in the activated complex. Similar effecta were observed for hydride transfer from 2-propanol to bromine.ls Also in quinone oxidations of triphenylm e t h a n e ~ ~and ' dihydroaromatic hydrocarb~ns'~J~ the (16)Swain, C. G.; Wiles, R. A,; Bader, R. F. W. J. Am. Chem. SOC. 1961,&3, 1945. (17)Lewis, E. S.;Perry, J. M.; Grinstein, R. H. J. Am. Chem. SOC. 1970,92,899. (18)Braude, E. A.; Jackman, L. M.; Linstead, R. P. J. Chem. SOC. 1964,3548. (19)Braude, E.A.; Brook, A. G.; Linstead, R. P. J.Chem. SOC.1954, 3569.
strongest evidence supporting a hydride transfer mechanism was large deuterium and tritium kinetic isotope effects. Another objective of this study was to identify a class of model compounds that resemble coal in their pattern of isotope effect in hydrogen transfer reactions with decalin. It is apparent that anthracene, with AV* = -60 cm3/m01and KIE = 2.1 is such a compound. This not only suggests a similarity of the reaction mechanism but also implies that anthracene (and maybe other poIycyclic aromatic hydrocarbon) units are the part of the coal structure that is reactive in decalin at high temperature.
Acknowledgment. J.P. thanks the Research Division of the New Mexico Institute of Mining and Technology for financial support.
Influence of Weathering and Low-Temperature Preoxidation on Oil Shale and Coal Devolatilizationt M. Rashid Khan Morgantown Energy Technology Center, US.Department
of Energy,
Morgantown, West Virginia 26505 Received February 6, 1987. Revised Manuscript Received May 5, 1987 The influences of preoxidation on the composition of oil shale and the yield of shale processing are not well-known. In this study, a western (Colorado) and an eastern (Kentucky) oil shale and their corresponding organic fractions (and a Pittsburgh seam coal, for comparison) were preoxidized under various conditions. Detailed pyrolysis behavior of these materials was subsequently determined by using techniques including thermogravimetric analysis combined with maas spectrometry (TGA/MS) , microdilatometry, and Fourier transform infrared spectroscopy. The feedstock H/C atomic ratios, weight loss during pyrolysis, kerogen fluidity, and hydrocarbon product yield and quality are significantly reduced by preoxidation, while the yield of oxygenated products (H20and COJ increased. These findings led to the hypothesis that preoxidation of oil shale organics introduces oxygen cross-linking bridges and/or reduces the aliphatic hydrogen contents of the shale samples. The hypothesis was tested by Fourier transform infrared spectroscopy, which confirmed that preoxidation of the shale organics introduces oxygenated functional groups such as carbonyl and carboxylic while there is a marked reduction in the aliphatic hydrogen content of the samples. The results suggest that part of the oxygen functional groups undergo decomposition and evolve during subsequent pyrolysis as H20, COP,and heavier liquids at the expense of desirable hydrocarbon fuels and overall weight loss (conversion) during pyrolysis.
Introduction and Background Previous work on weathering and low-temperature preoxidation has been reported for coals (refer to Lowry' and Van Krevelen2 for earlier work in coal and Khan and Jenkins3t4for recent developments in coal), but information for oil shale and kerogen is relatively sparse. Coburn and Ganeson5 studied oxygen uptake by the eastern and western shales and noted significantly more oxygen sorption by an eastern shale compared to that taken by a western shale. Leythaeusers monitored organic carbon and soluble organic matter (in common solvents) contents for Utah shales at various bed depths and reported significant variations in shale composition at various depths. The shale at shallow bed was considered more "weathered" (as was evident by a lower organic carbon and a soluble organic 'Presented in part, by invitation, at the 193rd National Meeting of the American Chemical Society, Denver, CO, April 1987.
matter content) compared to that observed at the deeper regions. Relatively little additional data are reported on the influence of weathering on shale composition and devolatilization mechanisms. Therefore, confusion still exists in this area. For example, Shaffer, Leininger, and Ennis' advocated weathering as a means to increase oil yield from shales. (1)Lowry, H.H.,Ed. Chemistry of Coal Utilization; Wiley: New York, 1963. (2)Van Krevelen, D. W. In Coal: Typology-Chemistry-Physics-Constitution; Elsevier: Amsterdam, New York, 1961. (3)Khan, M. R.;Jenkins, R. G. Fuel 1985,65,189. (4)Khan, M.R.;Jenkins, R. G. Fuel 1985,65,1.Also, see: Given, P. H.; Stockman, W.; Davis, A,; Zoeller, J.; Jenkins, R.; Khan, M. R. Fuel 1984,63,1655. (5)Coburn,T.T.;Ganeaon,P. Liquid Fuels Technol. 1983,I , 173-198. (6)Leythaeuser, D. Geochim. Cosmochim. Acta 1973,37, 113-120. (7)Shaffer, N. R.; Leininger, R. K.; Ennis, M. V. 1984 Eastern Oil Shale Symposium Proceedings; Institute for Mining and Minerals Research Lexington, KY, 1984;pp 401-412.
This article not subject to U.S.Copyright. Published 1987 by the American Chemical Society
