Energy & Fuels 1994,8,920-924
920
Modeling Coal Liquefaction. 3. Catalytic Reactions of Polyfunctional Compounds? Malvina Farcasiu,' Steven C. Petrosius,t Patricia A. Eldredge,$ Richard R. Anderson, and Edward P. Ladner Pittsburgh Energy Technology Center, U.S.Department of Energy, P.O. Box 10940, Pittsburgh, Pennsylvania 15236 Received November 8, 1993. Revised Manuscript Received March 21,1994'
Substituent effects relevant to coalchemistry were studied using polyfunctional aromatic compounds in catalytic reactions under coal liquefaction conditions. The compounds utilized were n(11), and 4-methoxyhexyldibenzothiophene (I),6-methyl-9-(l-methylethyl)dibenzothiophen-4-01 6-methyl-9-(l-methylethyl)dibenzothiophene (111). Reactions were performed in the presence of an H donor at temperatures of 350-430 "C; catalysts utilized in this study consist of carbon black, iron oxides (magnetite and ferrihydrite, activated by methylene chloride), and a complex iron-sulfur catalyst system. For all the catalysts studied, the presence of phenol or alkoxy groups on an alkylated aromatic ring system was found to enhance considerably the tendency for cleavage of the C-C bond connecting an alkyl group to the ring, as compared to compounds without phenol or alkoxy groups. Other reactions occurring with these models are dehydroxylation (or demethoxylation),isomerization of the starting material, and hydrogenation. The time dependence of the catalytic conversion of I1 is presented as an illustration of the contrasting behavior of the carbon and iron catalysts.
Introduction Molecules possessingfunctional groups common to coal are useful in evaluating catalysts for coal liquefaction and for understanding catalystfcoal interactions for specific processes such as hydrocracking, dehydroxylation, hydrogenation, H transfer, and isomerization. Pertinent to the modeling of specificreactions relevant to catalytic coal liquefaction, we have used high molecular weight hydrocarbons1T2to study C-C cleavage reactions and phenols3 to examine C-0 cleavage. A more comprehensive analysis of coal reactivity, in addition to examination of specific bond scission, needs to include the occurrence of intramolecular interactions due to the proximity of various chemical functionalities in the coal and coal liquids structure. The resonance and/or inductive effects of phenolic OH groups on aromatic systems, for example, result in reactivities very different from those of the corresponding nonphenolic structures. Also, aromatic moieties containing heterocyclicaromatic structures (such as dibenzothiophene) often have very different reactivity than those containing only carbon and hydrogen. To investigate these complex interactions, we have now studied catalytic reactions with three model compounds containing dibenzothiophene units: n-hexyldibenzothiophene (I), 6-methyl-9-(l-methylethyl)dibenzothiophen4-01 (11): and 4-methoxy-6-methyl-9-(l-methylethyl)dibenzothiophene (III).4 All these compounds are in the t Published in part in 1993 Conference on Coal Science Proceedings; Michaelian, K. H., Ed.; IEA Coal Research Ltd.; Canadian National Organizing Committee, 1993;p 597. Oak Ridge Institute for Science and Education Appointee, Postgraduate Research Training Program. *Abstract published in Advance ACS Abstracts, May 1, 1994. (1) Farcasiu, M.; Smith, C. M. Energy Fuels 1991,5,83. (2)Farcasiu, M.; Smith, C. M.; Hunter, E. A. 1991 International Conference on Coal Science Proceedings; IEA Coal Research Ltd.; Butterworth, Heinemann Ltd.: London, 1991;p 166. (3)(a)Farcasiu,M.; Petrosius, S. C.; Ladner, E. P. Presented at the 205th National ACS Meeting, Denver, CO, March 1993.(b)Farcasiu,M.; Petrosius, S. C.; Ladner, E. P. J. CataL1994,146,313and the references therein.
same range of molecular weight and have chemical structures reasonable for coal liquids. Due to their molecular size, all three were in liquid phase at the reaction temperatures, an experimental condition that is important for modeling coal liquefaction. Three catalytic systems have been investigated: a high-surface-area carbon black (Cabot BP2000), iron oxides (a moderate surface-area magnetite and high-surface area ferrihydrite) activated by methylene chloride, and a high-surface-areairon-sulfur catalytic system previously described.6
OH
I
I1
Y OCH3
111
Experimental Section All reactions were performed in sealed Pyrex tubes following a previously described procedure.' Specific reaction conditions (temperatureand reagent amounts) are presented in Tables 1-4. For iron oxide activation, the reaction tube ends, before sealing, were cleaned with cotton swabs saturated in methylene chloride; this procedure allows for traces of CHzClz to react with the iron (4)Firsan, S. J.; Eisenbraun E. J. submitted for publication in J . Heterocycl. Chem., 1993. (5)Farcasiu, M.; Eldredge P. A.; Petrosius, S. C. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. , 1993,38,53.
088'7-0624194f 2508-0920$04.50/0 0 1994 American Chemical Society
Energy & Fuels, Vol. 8, No. 4, 1994 921
Modeling Coal Liquefaction oxide and form the active catalyst. Analysis of reaction products was accomplishedby gas chromatography,usingan HP5890 Series I1 gas chromatograph equipped with a J&W SE-52 column (60 m). Identification of products was obtained with a Hewlett Packard 6988A gas chromatograph/mass spectrometer system employing 70-eV ionizing voltages and a quadrupole mass filter. i.d. fused-silicacapillary column, Lee Scientific A 30-m X 0.2" SB-Phenyl-5, with a 0.3-rm film thickness was used to perform the separation. Helium was usedas the carrier gas with an average h e a r velocity of 31.2 cm/s at 80 OC. The column temperature was held at 80 OC for 2 min and then programmed from 80 to 310 "C at 4 OC/min with a 30-min hold at 310 "C. The mass spectrometer was scanned from 40 to 400 amu every 1.3 8. In addition to the identification of products by GC-MS, highresolution gas chromatographyemploying a dual-flameionization detector and a sulfur-selectiveflame photometric detector (FID/ FPD) was performed by using a Tracor 570 GC equipped with a Spectra-Physics Model 4270 integrator and a Westronics dualpen recorder. A 25m X 0.32-mm i.d. fused-silica capillary column, Chrompack CP-Sil-8 CB, with a film thickness of 1.2 Mm was used with He carrier gas having 26 cm/s average linear velocity. The column temperature program was the same as that used in the GC-MS analysis. Further details of the reaction procedures and analyses are presented elsewhere.' a n
10
The reaction products have also been fractionated by liquid chromatography, and selected fractions were analyzed by 13C NMR and 1H NMR on a Bruker MSL300 (300 MHz) spectrometer. Two-dimensional 'H-W and 1H-lH COSY analyses were performed to obtain detailed structural information about the products. Of particular interest in the NMR examinations were the products 2- and 3-(l-methylethyl)-9,lO-dihydrophenanthrene. Their lgC and lH chemical shifts are reported in the Appendix. Compound I was synthesized at Mobil Corp., Research and Development Division.* Compounds I1 and I11were synthesized by Dr. E. J. Eisenbraun's group at Oklahoma State University.' The carbon black, BP2000, was donated by Cabot Corp. and its properties have been previously reported.' Magnetite (Fe304) was donated by Miles, Inc. (Control No. EB5240VNB) and has a surface area of 37 m2/g. The iron-sulfur catalyst system is prepared in situ by reacting ferrihydrite, 5Fe~Or9H20(30 A particle size; donated by Mach I), with sulfur in the presence of a hydrogen donor. Details of the preparation and characterization of this iron-sulfur catalyst are presented in a separate report) The hydrogen donor used for reactions with I was 1,2,3,6,7,8hexahydropyrene. In all the other experiments, the H donor was 9,lO-dihydrophenanthrene (9,lO-DHP). Both hydrogen donor compounds were obtained from Aldrich Chemical Co.
Results and Discussion Thermal and catalytic reactions of compounds I,11,and I11 were performed in the presence of an H donor, at temperatures of 350-430 "C,and for reaction times of 20 min to 2 h.
Under these experimental conditions, n-hexyldibenzothiophene (I) was unreactive at temperatures lower than 430 OC after 1 h. A t 430 OC,conversion of I was 2% for the thermalreaction and 8 % for the carbon black-catalyzed reaction; only the dealkylation reaction was observed. Under identical catalytic reaction conditions, however, I1 reacts readily (Table 1). The reactions observed are (6) Farcaeiu, M.; Forbus, T. R.; LaPierre, R. SOC.,Diu. Petr. Chem. 1983, 28, 279.
B.Prepr.-Am. Chem.
Table 1. Thermal and Catalytic Conversion of I1 in the Presence of 9,lO-Dihydrophenanthrene. catalyst
none BP2000 magnetitee
iron-sulfw systemdr ferrihydrite, 3 nm none BP2000 magnetite
iron-sulfw system ferrihydrite, 3 nm
none BP2000 magnetite
iron-sulfw system ferrihydrite, 3 nm none BP2000 magnetite
selectivity ( % ) (%) dealkyln dehydroxyln Reaction at 350 O C 0 25.9 100 0 24.3 97.8 4.4 63.0 98.8 3.7 5.9 100 0 Reaction at 380 "C
conversion
0 58.9 98.5 40.2 98.5 67.0 94.3 28.5 98.5 Reaction at 410 O C 3 94.7 98 67.3 99 67.3 95 64.1 100 Reaction at 430 O C 13.8 100 95.9 96.4 77.5 99 78.9 91.5 74.8 100
~~~~~0~
I1 ( % ) b
0 8.1 63.4 0
-
5.2 4.5 45.9 3.0
0 11.8 73.1 18.7
-
-
16.0 3.3 46.4 2.9
0 64.0 72.5 59.6
0 25.4 7.7 42.5 8.6
4.6 0 72.8 76.8 74.8
iron-sulfw system ferrihydrite, 3 nm a Reaction conditions: 1 h reaction time, sealed glass tube, weight ratio 9,lO-DHPIkcatalyt = 41:0.1. * Isomerization is presented as a percentagerelative to unconverted 11. Low surfacearea (37 m2/g) commercial magnetite, activated. d The ironaulfur catalyst is obtained by heating equal parta ferrihydrite (3 nm) and sulfur in 9,lODHP for 1 h at 200 O C (see ref 5). .If weight ratio IIcatalyet is changed to 1:1, conversion increases slightly (to 67.9%) but dehydroxylation selectivity increases substantially (to 44.6%). Scheme 1
non
OH
'Iv
-w
+
*
P
VI
dealkylation (only deisopropylation), dehydroxylation, hydrogenation as well as isomerization of t h e unconverted 11. In some cases, dealkylation and dehydroxylation selectivitiestotalmore than 100% because certain products (4-methyldibenzothiophene,IV,for example) are the result of both dealkylation and dehydroxylation reactions and therefore fall into both selectivity categories. As seen in Table 1,dealkylation is by far the most prevalent reaction (withselectivityatorclose to 100%). Thedehydroxylation reaction is much slower in all cases and more catalyst dependent (Table 1). Scheme 1illustrates the likely pathways for t h e reaction of 11. It is based on the identified reaction products, their yields as a function of time (Table 2), and previous literature data:3v7 In the case of the ether I11 (Table 31, its reactivity is very similar to that of t h e phenol I1 because of the ether
Farcasiu et al.
922 Energy & Fuels, Vol. 8, No. 4, 1994 Table 2. Time Dependence of Compound I1 Conversion and Product Yields, Reaction at 380 OC.
Table 4. Dealkylation Reactions Catalyzed by Carbon Black. ~~
yield ( % ) b isomerizn time conversion (min) (%) dealkvln dehvdroxsln hvdrom (%IC BP2000 0 22.5 0 0 20 22.5 47.1 2.6 1.2 0 40 48.1 1.5 0 58.0 3.1 60 58.9 2.7 0 72.1 5.0 80 73.1 4.3 0 84.4 7.5 120 85.8 Magnetite 14.2 1.0 1.0 2.1 20 15.2 33.9 1.8 1.8 8.2 40 34.8 1.8 1.8 11.8 39.6 60 40.2 56.7 1.3 1.7 53.1 80 57.3 120 58.4 1.7 1.7 54.4 59.3 Iron-Sulfur System 59.3 49.3 14.0 10.9 20 51.6 81.9 40 28.5 67.3 26.6 68.5 73.1 25.0 63.2 45.9 60 67.0 67.0 26.8 64.8 32.5 80 68.6 82.7 32.7 67.3 42.0 120 73.0 Ferrihydrite, 3 nm 12.1 0.5 20 12.4 0.3 7.3 21.1 1.0 40 21.6 0.5 8.2 60 0.4 18.7 28.1 0.9 28.5 40.8 1.6 80 41.7 0.7 34.5 120 47.2 0.6 40.3 46.7 1.7 Conditions: Sealed glass tube, weight ratio I1to DHP to catalyst = 1:40.1. bProduct yield is d e f i e d as: (percent selectivity) X (conversion). Isomerization is presented as a percentage relative to unconverted 11. (I
Table 3. Thermal and Carbon Black-Catalyzed Reactions of 1114 yieldb ( % ) catalyst conversion (%) dealkyln demethoxyln ether cleavageC 0 0 44.3 none 44.3 87.3 7.0 32.8 BP2000 93.5 Reaction Conditions: 1 h, 410 OC, 9,lO-dihydrophenanthrene: III:BP2000 = 4:l:O.l. b Yield is defined as (percent selectivity) X (conversion). Conversion of the other I11 into phenol 11. (I
cleavage to phenol. The demethylation of the ether to the phenol seems to be exclusively thermal. It should be mentioned that the formation of the corresponding phenols from methyl ethers has been previously observed in the thermolysis of methoxynaphthalenes.s Homolytic cleavage of the O-CH3 bond is the primary step in phenol formation. The reaction is, in this case, independent of either C-C cleavage or demethoxylation (in fact, dehydroxylation) reactions. Table 3 illustrates this point for the carbon black catalyst, given that the yield of phenol product is approximately the same with or without the catalyst. The following discussionaddresses the various reactions of 1-111 that take place during the catalytic reactions based on observed products and the proposed scheme above. Dealkylation Reactions. As mentioned above, nhexyldibenzothiophene (I) is unreactive as compared to the other compounds examined. Under conditions where limited activity is observed (430 "C), the selectivity is exclusivelyfor cleavage of the a bond. This is consistent with our previous finding with alkylnaphthalenes and alkylphenanthrenes.2 In the case of compounds I1 and 111, the only dealkylation reaction observed was deisopropylation. No demethylation was observed in any of the experiments. The reactivity of I1 toward catalytic dealkylation as compared with I is markedly increased (Table 1)due to
compound
I IIb 1-n-hexylnaphthalene(V1I)c 1-(2-hexyl)naphthalene (VIII)'
~
~~
% dealkylation thermal catalytic 0 0 3 75 4 17 7 23
Reaction conditions: 1h reaction time, 410 O C , weight ratio H donor:compound:BP2~= 4l:O.l. H donor was 9,lO-DHP for I1 and hexahydropyrene for I, VII, and VIII. b Dealkylation was exclusively deisopropylation;no demethylation was observed. Data from ref 2. (I
the presence of the hydroxyl group in the molecule. The fact that I1 is substituted with an isoalkyl group and that I possesses an n-alkyl substituent is not likely to have an important impact in the catalytic reactions if the relative reactivity of 1-n-hexyl- and 1-(2-hexyl)naphthaleneis an indication (Table 4). It should also be mentioned that small quantities of l-(l-methylethyl)-4-methyldibenzothiophene (VI and its partially hydrogenated derivative (VI) are always observed in the reaction products of 11. While the rates of dehydroxylation are lower than those of dealkylation, the small quantities of V which are formed from the direct dehydroxylation of I1 are not further dealkylated, but rather are partially hydrogenated into VI (Table 2, Scheme 1). This result is an indication of the activating influence of the hydroxyl group on the dealkylation activity of these compounds. Comparing the results with I to the results with alkylnaphthalenes, it is apparent that the dibenzothiophene moiety decreasesthe propensity for dealkylation reactions as compared with that of alkylnaphthalenes. However, the presence of phenolic OH in I1 increases the reactivity for C-C cleavage above that observed for both n-alkyl and isoalkyl-substituted naphthalenes (VI1 and VIII). Increased reactivity due to the presence of phenolic groups was previously reported for cleavage of C-C bonds adjacent to aromatic systems in hydmxydiphenylmethanea and hydroxydinaphthylmethane by McMillen and coworkers.s Moreover, Wender et al.1° have recently found an increased reactivity toward dealkylation of dodecylphenol as compared with dodecylbenzene in reactions catalyzed by sulfated zirconia. Concerning the dealkylation reaction of 11, iron compounds and carbon black catalyze the dealkylationreaction in substantially different ways. Initially, while various iron catalysts exhibit differing rates of reaction (Figure 1 and Table 2), in all cases the dealkylation conversion reaches a plateau; iron catalysts with higher initial rates reach the maximum in a shorter time. Secondly, in each instance of iron catalyst use, the deisopropylation activity appears to result from a transalkylation reaction to the 9,lO-DHP. Indeed, in every reaction using an iron catalyst, the formation of 2- and 3-(l-methylethyl)-9,1O-dihydrophenanthrene (and also the phenanthrene analogs) was observed and molar amounts of these products are comparable to the amount of dealkylated products from 11. The identity of these products was confirmed by NMR analysis,ll and chemical shift data for these compounds are reported in the Appendix. No transalkylation was (7) Schlosberg, R. H.; Kurs,A. J . Org. Chem. 1984,49, 3032. (8) Schlosberg, R. H.; Dupre, G. D.; Kurs,A.; Szajowski, P. F.; Ashe, T. R.; Pancirov, R. J. J. Liq. Fuels Technol. 1983,1, 115. (9) McMillen, D. F.; Ogier, W. C.; Ross, D. S. 1981 International Conference on Coal ScienceProceedings (Dwseldorf); Verlag Gluckauf: Essen, Germany, 1981; p 104. (10)Wender, I., personal communication.
Modeling Coal Liquefaction
Energy & Fuels, Vol. 8, No. 4, 1994 923
100
BP2oO0 /,"
80-
./'
,.' ,/'
,,.,.,''
,.I,'
Figure 1. Time dependence of compound I1 conversion with various catalysts at 380 OC.
observed when using BP2000 as the catalyst. As mentioned above (see also Table l),the iron catalysts also catalyze extensive isomerization of unreacted 11. Both the transalkylation and isomerization reactions are indicativeof an ionic ( i e . ,acid-catalyzed)mechanism. Acidcatalyzed reactions would be expected for methylene chloride activated iron oxides but their occurrence is surprising for the iron-sulfur system. Unactivated iron oxides and CH2Clz alone both show no transalkylation activity under the same conditions. The absence of both isomerization (Table 1) and transalkylation activity with BP2000 indicates that the carbon functions differently than the above described iron catalysts. Also, the presence of an H donor is not necessary for C-C cleavage in the presence of carbon black, which therefore does not require H transfer as the first step. The presence of the H donor for cleavage reactions is necessary for fragment stabilization after scission takes place. It has been reported previously' that, if no H donor is present, the carbon black catalyzed cleavage of Caromatic-Calipbtic bonds takes place with the same selectivity as in the presence of an H donor, but higher molecular weight compounds are also observed, arising from fragment recombination. This same observation is true for the carbon black-catalyzed dealkylation of 11. At 400 OC and 1 h reaction time, in the absence of an H donor and in the presence of 10 w t % of BP2000, conversion of I1 into dealkylated compoundswas 56 % . No reaction takes place under these conditions in the absence of carbon black. Also, no plateau is observed in the variation of compound I1conversion with time in the presence of the carbon black catalyst, and the overall dealkylation yield is much higher than for any iron catalyst at temperatures of 380 OC and higher (Tables 1 and 2 and Figure 1). Dehydroxylation Reactions. The carbon blackcatalyzed dehydroxylation reactions of polycyclicaromatic phenols have recently been r e p ~ r t e d ;the ~ proposed (11) The NMR analyses and product identification were performed byDr.TonisPehk,InstituteofChemietryandPhysica,EetonianAcademy
of Sciences.
reaction scheme for dehydroxylation of I1 (Scheme 1) is based on that work and the literature cited therein. It has been shown that, in contrast to the carbon black-catalyzed C-C cleavage reaction, no dehydroxylation of condensed polycyclic phenols occurs in the absence of an H donor. The first step in the case of the dehydroxylation reaction is transfer of hydrogen to the aromatic ring of the phenol from an H donoI.9 or from high-pressure H2.' The dehydroxylation activity with BP2000 was found to require substantially higher temperature than C-C cleavage for the corresponding alkylated aromatics. Table 1illustrates that this effect is also present for alkylated phenols; the C-C cleavagereaction is predominant at lower temperature whereas dehydroxylation becomes relatively more prevalent at higher temperature. For the methylene chlorideactivated iron oxides, which exhibit minimal H transfer or hydrogenation activity, very limited dehydroxylation is observed even at high temperatures (Table 1). The iron-sulfur system is more active at higher temperature and is also the most active dehydroxylation catalyst in the group of catalysts reported here. Other work with this catalyticsystem also demonstrates its activity in H-transfer reactions? which is consistent with the connection between hydrogenation and dehydroxylation activity. Isomerization Reactions. Isomerization of I1 is observed with all the studied iron catalysts but not in carbon black-catalyzed reactions. As there is no net bond cleavage or formation to obtain the isomerized product, we do not include this reaction when determining conversions or selectivity of the products. The isomerization is reported as the 9% of isomerized I1 in the unconverted starting material. The structures of the isomers of I1have not yet been conclusively determined, but a number of possibilities are suggestedfrom the present results. Figure 2 shows a section of the gas chromatograph output in the GC-MS analysis of the reaction of I1 with the iron-sulfur catalyst at 400 OC (based on retention time, the center peak is assigned to the starting material), and the mass spectra for the three isomers. The fragmentation patterns are all extremely similar, which indicates that the compounds are most likely positional isomers. A reversal of the C-C or C-O cleavage reaction would result in such positional isomer. An alternative structure for the phenol isomer is the keto tautomer; however, on the basis of the stability of the aromatic enol structure and the lack of IR carbonyl absorption, this possibility is unlikely. Deactivation of Iron Catalysts. The isomerization of I1 may be a part of the explanation for the deactivation of the iron catalysts (see Figure 1and Table 2). Considering that the isomerization of the starting compound appears to be concurrent with the observed loss in activity for the iron catalysts, there is a possibility that the two results are related. After 40 min at 380 OC with the ironsulfur catalyst, the isomerization reaches a maximum (ca. 82 % ). Similarly, magnetite shows a maximum in isomerization after 80 min (ca. 53%). Carbon black, which does not isomerize 11, shows no deactivation. These results may be due to the decreased reactivity of the isomers of I1 as compared with compound I1 itself. In a detailed study of hydroxydiphenylmethane C-C bond cleavage reactivity, McMillen and co-workers'2 show that 0 - and p- (hydroxypheny1)phenylmethane undergo scissionof the ArCHzphenyl bond at 400 "C in tetralin, whereas the meta isomer is unreactive under these conditions, as is (12) McMillen, D. F.; Ogier, W. C.; Rose,D.S.J. Org. Chem., 1981,46, 3322.
Farcasiu et al.
924 Energy & Fuels, Vol. 8, No. 4, 1994 Isomer of I1 (a)
i
11 27 0
I1 Isomer of I1 (b)
2s
26 0
30 0
0
Retention Time (min) 241
2; 1
256
\
Isomer of I1 (b)
I'2'
82
1:s
7' 7'
171
174 497
2?4
'F6
diphenylmethane. The mechanism developed to account for the reactivity enhancement seen for only the ortho and para derivatives involves enol tautomerization to the keto tautomer, which results in a weakening of the ArCH2phenyl bond by ca. 40 kcal/mol. The apparent loss of activity upon isomerization in our study of compound I1 is consistent with this mechanism and indicates that the keto tautomer form of I1may be a transient species in the reaction. It should be mentioned, however, that other factors are also known to be responsible for the deactivation of iron catalysts, especially their propensity to agglomerate and form lower surface area catalysts. In addition, the extent of conversion of the ionic reactions catalyzed by the studied iron catalysts may be influenced by other factors, such as the acid strength and the molar ratio of the donor (11)to the acceptor (9,lO-DHP). In a previous work,13 we have shown that, because the equilibrium constant of the transalkylation reactions is equal to 1, the final conversion is dependent on the molecular ratio between the donor and acceptor. All the above-mentioned factors can contribute to the observed plateau obtained for conversion of I1 in the presence of the iron catalysts. Relevance of the Results to the Modeling of Coal Liquefaction. The results presented here regarding the influence of phenol groups on C-C cleavage may help to understand the differences in liquefaction reactivity for various coals and may contribute to better selection of catalysts for the first stage of coal liquefaction. It has (13) Farcasiu, M.; Scott, E. J. Y.; LaPierre, R. B. Prepr.-Am. Chen. SOC.,Diu. Petr. Chem., 1985,30,672.
been found14that coal conversion in thermal liquefaction reaches a maximum for bituminous coals with 8 2 4 5 % carbon (DMMF) and specific O/C ratios. It is conceivable that a proper balance between the content of condensed polycyclic aromatic moieties (two or more rings) and their degree of substitution with OH groups (or other nucleophilic substituents) results in coals with increased reactivity toward C-C cleavage and, consequently, toward liquefaction. To observe such an effect, the rate of C-C cleavage should be higher than the rate of dehydroxylation. If this is the case, a good catalyst for first-stage coal liquefaction should exhibit higher rates for hydrocracking than for hydrogenation, based on the observation that the hydrogenation reaction is the first step toward dehydroxylation and, as such, possibly results in deactivation of coal toward hydrocracking. The solvent used in coal liquefactionin the presence of such catalysts should possess sufficient H-transfer properties to stabilize the fragments formed in the hydrocracking reaction, but should have very limited hydrogenation (dehydroxylation) activity in order not to deactivate the coal toward hydrocracking prematurely in the liquefaction process. Conclusions The use of polyfunctional compounds with chemical structures relevant to coal has shown that the presence of OH substituents on condensed aromatic rings increases dramatically the selectivecleavage of carbon-carbon bonds in the presence of iron and carbon-based catalysts and a hydrogen donor. A lower rate of dehydroxylation (hydrogenation) as compared with the rate of hydrocracking increases the overall reactivity of polyfunctional phenols, such as 11. These results have implications to coal liquefaction, where the presence of nucleophilic substituents on condensed aromatic rings may enhance the liquefaction reactivity of certain coal materials relative to that of less-functionalized coals. Acknowledgment. This research was supported in part by appointment to the Postgraduate Research Training program under contract DE-AC05-760R00033 between theU.S. DOE and Oak Ridge Institute for Science and P.A.E). We want to thank Dr. and Education (S.C.P. Reimer Holm (Miles Inc.) for a gift of well-characterized magnetite. Reference in the paper to any specific commercial product, process, or service is to facilitate understanding and does not necessarily imply its endorsement or favoring by the United States Department of Energy. Appendix The 13C and 'H chemical shifts (proton shifts for the hydrogens on the respective carbon atom are in parentheses) for the alkyl substituted dehydrophenantrenes are as follows: 2-(l-Methylethyl)-9,10-dihydrophenanthrene: C1, 126.20 (7.05);Cz, 148.14;Cs, 124.97 (7.13);C4,123.41(7.68); C4a, 134.56; C5a, 134.71; C ~ ~ 1 2 3 . (7.63); 63 c6,126.82 (7.24); C7, 126.92 (7.14); Cs, 128.01 (7.16); Cg, 29.20 (2.76); C10, 29.13 (2.76); ClOa, 137.45; C11 (i-propyl CH), 33.84 (2.87); CIZ (i-propyl CH3), 23.95 (1.27). 3-(l-Methylethyl)-9,1O-dihydrophenanthrene: C1, 128.08 (7.17);C ~ ~ 1 2 5 . 3(7.06); 7 C ~ ~ 1 4 7 . 4C4,121.81(7.61); 3; C4a, 134.80; Cba, 134.27; C6, 126.82 (7.24);C,, 127.19 (7.18); Cg, 128.02 (7.11); Ch, 137.25; Cg, 29.20 (2.76); C10, 28.63 137.08; C11 (i-propyl CHI, 34.09 (2.91); C12 (2.76); (i-propyl CH3), 24.12 (1.29). (14) Whitehurst,D. D.;Mitchell,T. O.;Farcasiu,M. CoaZLiquefaction; Academic Press: New York, 1980; pp 121-161 and references therein.
