Energy & Fuels 1989,3, 398-402
398
Unsubstituted Polycyclic Aromatic Hydrocarbons Produced by Catalytic Hydropyrolysis of Coal Liquids Susumu Yokoyama,* Hideji Mishima, Masaaki Satou, and Yuzo Sanada Faculty of Engineering, Hokkaido University, Sapporo 060, Japan Received March 11, 1988. Revised Manuscript Received March 16, 1989
A primary coal hydrogenated liquid, SRC I1 heavy distillate, was upgraded by secondary hydroprocessing to a useful chemical feedstock, simple aromatic components without substitution on their rings. Upgrading reactions were performed with Mo03-A1203 catalyst under an initial hydrogen pressure of 20 kg/cm2 for two sets of reaction conditions: (1) 1 h a t temperatures between 500 and 600 OC; (2) various time spans a t 550 "C. The compound classes in products were separated by NH2 column HPLC. Structural analysis of each compound class by 'H NMR spectroscopy indicated a vigorous cracking of alkyl groups that were substituted onto various aromatic rings, increasing with reaction severity in both experimental series. Shorter and fewer alkyl groups on aromatic rings were observed in the upgraded oil, and bare-ring aromatics in various compound classes were mainly produced. Some cleavage of aromatic rings occurred with unacceptable levels a t 575 and 600 OC. A t 550 OC for 30 min, most aromatic compounds were stripped of their alkyl groups.
Introduction Primary coal hydrogenation liquids require secondary hydroprocessing reactions to be usable as alternative petroleum resources, carbonaceous materials, and chemical feedstocks. Coal liquids are an especially promising source to fill the growing demand for aromatic chemicals. Coal liquids consist predominantly of numerous aromatic compound species of varying aromatic ring size, with one to several condensed rings, on which alkyl, naphthenic ring, and functional groups are substituted or condensed. Unprocessed coal liquids are complicated mixtures, with limited use. Studies of the upgrading of coal liquids have concentrated on hydroprocessing to remove nitrogen and sulfur and for degradation to lighter hydrocarbons for liquid fuel,'-' while upgrading to chemicals has been less thoroughly investigated. The British National Coal Board (now British Coal) supported research on the hydropyrolysis of coal liquids to obtain BTX and acetylene, but the results were disappointing.8 Pregermaing upgraded coal liquids to BTX, ethane, and methane. Upgrading with dealkylation of coal oils was accomplished by hydrocracking'O and thermal pyrolysis" with methanol, producing various feedstock-qualityaromatic ring compounds. Upgrading by dealkylation of aromatic compounds in coal (1) Sullivan, R. F., Ed.; Upgrading Coal Liquids; ACS Symposium Series 156; American Chemical Society: Washington, DC, 1981. (2) Hara, T.; Tewari, K. C.; Li, N. C.; Fu, Y. C. Prepr. Pap.-Am
Chem. Soc., Diu. Fuel Chem. 1979,24, 215-223. (3) van der Watt, J. G.; Heenop, P. J. Fuel Process. Technol. 1986,11,
101-112. (4) Li, C.-L.; Xu, 2-ren; Gates, B. C.; Petrakis, L.Ind. Eng. Chem. Process Des. Dev. 1985,24, 92-97. (5) Green, 3. B.; Grizzle, P. L.;Thomson, J. S.; Hoff, R. J.; Green, J. A. Fuel 1985, 64, 1581-1590. (6) Yoshida, R.; Yoshida, T.; Narita, H.; Maekawa, Y. Fuel 1986,65, 425-428. (7) Ozawa, S.; Sugiura, N.; Ogino, Y. Fuel 1987, 66, 1219-1224. (8) Bernhardt, R. S.; Ladner, W. R.; Newman, J. 0. H.; Sage, P. W. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1980,25, 188-198. (9) Pregermain, S. Fuel Process. Technol. 1986,14, 125-135. (10) Matsuda, T. Development of Coal Resource and Liquefaction Technology; Saiensu-Fouramu: Tokyo, 1981; pp 414-439. (11) Ouchi, K.; Ozaki, Y.; Daigo, A.; Itoh, H.; Makabe, M. Fuel 1987, 66, 731-734. (12) Haw, J. F.; Glass, T. E.; Dom, H. C. Anal. Chem. 1983,55,22-29.
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run no. 1-1 1-2
1-3 1-4
Table I. Reaction Conditions reacn reacn temp, OC time, min catalyst 500 60 60 525 60 550 60 575
1-5
600
2-1 2-2
550 550 550
2-3 2-4 3-1 3-2 4-1
550
550 550 550
60 0" 15
30 45 0"
60 60
H2 pressure, kg/cm2 20 20 20 20 20 20 20 20 20 20 20 40
" When the reaction temperature of 550 OC was reached, the autoclave was cooled immediately. liquids has the potential to provide usuable aromatic chemicals. In this paper, upgrading of an SRC I1 coal liquid by dealkylation to produce aromatic materials with some short alkyl groups attached is studied and the successful production of feedstock-quality chemicals is described.
Experimental Section Upgrading Reaction. The feed material, SRC I1 heavy distillate (bp 288-454 "C), was produced by a coal liquefaction plant of The Pittsburgh & Midway Coal Mining Co. Elementary analysis of SRC I1 is as follows: C, 90.0; H, 7.8; N, 1.1; S, 0.5; 0, 0.5 (by difference). Upgrading reactions were performed with a no-dead-volume 50-mL shaking type autoclave (Makino High Pressure Chemical Lab.). The vessel was charged with 1g of coal liquid and 0.5 g of Mo03/A1203catalyst under an initial hydrogen pressure of 20 kg/cm2 a t varying temperatures from 500 to 600 "C for 60 min and for varying times at 550 "C. Reaction conditions are summarized in Table I. Separation of Products. The volume of the gaseous products was measured, and the component gases were determined by gas chromatography. The reaction contents were recovered with an n-hexane wash and separated by filtration into the catalyst with an n-hexane-insoluble part (HI) and an n-hexane-soluble part (HS). Upgraded oils, HS, were separated further by HPLC with an NH2 column into saturates (Fr-P), monoring aromatics (Fr-M), naphthalene type diaromatics (Fr-Dl), biphenyl type diaromatics (Fr-D2), triaromatics and larger aromatics (Fr-T1 and Fr-T2), 0 - 1989 American Chemical Societv
Energy & Fuels, Vol. 3, No. 3, 1989 399
Catalytic Hydropyrolysis of Coal Liquids 100
- 80
Extractlon wlth n-hexane
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Table 11. Chemical ProDerties of SRC 11 distribn of compd struct params classes (wt 9% of SRC I1 HS base) f. ADS n SRC I11 0.75" Fr-P (5.5) Fr-M (4.3) 0.52 3.5 2.6 Fr-D1 (21.5) 0.68 2.5 1.9 Fr-D2 (8.2) 0.79 1.6 1.6 0.88 1.3 1.5 Fr-T1 (18.0) Fr-T2 (14.8) 0.89 1.5 1.6 Fr-PP (27.9)
20
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.-
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M, 159 192 196 204
,
260
"By 13CNMR analysis. and polar compounds (Fr-PP)with the procedure reported prev i o ~ s l y(see ~ ~Figure 1). Analyses of Upgraded Oil. Distillation curves for feed and upgraded oils were measured with simulated distillation gas chromatography (SD-GC, Shimazu GC-7AG). The procedure used for SD-GC analysis was described in detail in a previous report.13 lH NMR and 13C NMR spectra were obtained with a Varian XL-200 instrument. Samples for 'H NMR and 13C NMR spectroscopy were prepared with solution of deuteriochloroform,with TMS as a reference.
Results and Discussion Chemical Characteristics of the SRC I1 Liquid. Table I1 shows the chemical properties of SRC 11. Saturated carbon substituted or condensed to aromatic ring as alkyl groups and naphthenic rings corresponds to 0.25 of the total carbon (1- f a ) , according to the 13CNMR spectrum. The quantities of compond classes are shown in Table 11with their main structural parameters as proposed by Haw et a1.12 The yields of polar compound (Fr-PP) reached the considerable amount of 28 wt %. The aromatic carbon fraction, fa, increased in rough proportion to the number of aromatic rings, in the order of Fr-M, Fr-D1, Fr-D2, Fr-T1, and Fr-T2. Thus, large saturated chains and naphthenic rings tend to be substituted onto smaller aromatic ring structures, while large ring structures took on fewer and shorter saturated chains. Reaction Products as a Function of Temperature (60 min). Initially, product differences a t various temperatures from 500 to 600 "C at 25 "C intervals were observed while other reaction conditions were held a t constant values (time = 60 min). Figure 2 shows the distribution changes of products. Liquid products soluble in n-hexane (HS = upgraded oil) decreased gradually from 74.6 wt % a t 500 "C to less than 40 wt % a t 600 "C, indicating that vigorous degradation reactions occurred in the coal liquid accompanyingthe evolution of low molecular weight components designated as loss. Hydrocarbon gases (C,-C,) and losses, calculated by material balances, (13) Uchino, H.; Yokoyama, S.; Satou, M.; Sanada, Y. Fuel 1985,64, 842.
-
400,
0
0
20
40
60
80
100
Distillate ( w t % 1
Figure 3. Distillation curves by SD-GC for feed and products.
increased with temperature and reached 33 and 29 wt %, respectively, at 600 "C. Losses correspond to low boiling molecules such as benzene, toluene, and xylene, which cannot be recovered during the treatment of products due to volatilization. Cokes (hexane-insoluble material = HI) form less than 3 w t %. The effect of the catalyst is evident from a comparison with the result from the experiments performed without it, presuming less dealkylation (solid symbols in Figure 2). Distillation curves derived by SD-GC for feed and HS products are shown in Figure 3. The distillation curves shift remarkably to lower temperatures and have a variety of ranges at different constant temperatures, representing the concentration of characteristic components by upgrading reactions. These flat temperatures correspond to the boiling points of naphthalene (217.9 "C), biphenyl (254 "C), acenaphthene (279 "C), fluorene (295 "C), and phenanthrene (340 "C), respectively. Distribution and Constitution of Compound Classes. In Figure 2, the distribution of the contents of various compound classes in the upgraded oil are shown with the reaction temperature. Changes of content distributions are significant, with an accompanying disappearance of Fr-P, Fr-M, and Fr-PP up to 550 "C. From these results, hydrocracking of alkyl groups substituted onto various aromatic ring compound classes and paraffin
Yokoyama et al.
400 Energy & Fuels, Vol. 3, No. 3, 1989
hydrocarbon seem to occur at a significant rate, with some emissions of C1-C4 organic gas. Fr-M was difficult to recover completely for conversion to smaller molecules such as BTX, which was derived from dealkylation of the larger alkyl benzenes. The content of Fr-PP consisted of polar compound in feed diminished extremely a t 500 "C (from 27.9 to 4.2 w t %), due to the occurrance of hydrodeoxygenation and denitrogenation of polar compounds. Deheterogenation of polar compounds may produce hydrocarbons corresponding to their carbon skeletal structure but with the heteroatoms removed. Changes in the chemical structure of the products during the upgrading reaction were compared with average structural parameters derived from a structural analysis proposed by Haw et a1.12 for respective aromatic ring compound classes. These parameters are the aromatic carbon fraction, fa, the average degree of substitution, ADS, and the average molecular weight, M,. During the dealkylation reaction, the values of fa of each compound class (except for Fr-D2) gradually approach 1,while the values of ADS approach zero, corresponding to their respective parent aromatic compounds; i.e., their molecular weights converge to the constant values for each bare aromatic ring compound. Raising the temperature of the upgrading reaction ruptures alkyl groups and produces aromatic ring compounds with few or no alkyl groups attached. Therefore, aromatic ring compounds without alkyl substitution, including naphthalene, biphenyl or fluorene, phenanthrene, and chrysene, can be derived from Fr-D1, Fr-D2, Fr-T1, and Fr-T2 compound class fractions, respectively, with extreme dealkylation. For Fr-D2, the skeleton structure consists mainly of fluorene types as they are close to its structure parameters (fa= 0.92, ADS = 0 and M , = 166). Reaction Condition for Upgrading. In the above study, undesirable side reactions, such as hydrocracking of aromatic rings with the subsequent formation of lighter fractions (as losses) and gaseous products, may accompany the desired main reaction of dealkylation and deheterogenation. The total weight percent of saturated groups (Sa)in the feed, which is due to alkyl groups, naphthenic groups, and paraffin compounds, calculated by structural analyses by 'H and 13C NMR spectroscopy is 29% as indicated in Figure 2. If dealkylation were carried out exclusively a t saturated groups, with consideration of the results of structural analysis for various compound classes, cumulative gas evolution should closely correspond to the amount of the saturated group equal to 29 w t % (indicated by dotted line in Figure 2). When the reaction temperature was raised to 600 "C, the volume of measured C144 gas increased gradually and, a t 560 O C , slightly exceeded the line corresponding to the actual content of saturated groups (Sa)in feed. Although gas evolution by hydrocracking in some ring structure parts occurred over 560 OC, this was not significant. However, the total contents of each compound class decreased with temperature to values significantly lower than the original aromatic ring wt % content (Ar in Figure 2) in the feed. Considerable rupture of aromatic rings may occur within the range of the reaction temperatures used, corresponding to the high losses due to easy volatilization. Subsequent study for selection of optimum conditions is desirable. Reaction Products as a Function of Time (550 "C). The reaction temperature of 550 OC (holding other conditions constant) may be the optimum one for dealkylation with a minimum rupture of aromatic rings. Therefore experiments with different reaction times from 0 to 60 min a t a constant temperature of 550 "C were performed.
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Figure 4. Distribution of reaction product for various reaction HS, (A)gas, (0) HI, (0) and loss; ( 0 )HS, times (550"C): (0) (A)gas, (V) HI, (0)and loss, no catalyst (run no. 3-1);( 0 )HS, (A)gas, (v)HI, (0)and loss, Hz pressure of 40 kg/cm2(run no. 4-1).
0
15
30 45 60 T i m e (min.)
Figure 5. Distribution of C,-C, gas evolution and predicted gas volume with reaction time (550 OC): (0) measured gas; ( 0 )
predicted gas.
Figure 4 shows product distributions of this experimental series. The products seem to be in better proportion than those from the first set of experiments, with a high yield of upgraded oil and a decline of gas and loss. The catalyst had some effect (a, 0 , A, and v in Figure 4), though the high pressure of hydrogen (40 kg/cm2) induced a large evolution of gas, revealing that vigorous hydrocracking occurred (a, 0 , A, and v in Figure 4). Figure 5 shows the distribution of C1-C4 hydrocarbon gases that evolve by the dealkylation of various aromatic ring classes and the cracking of paraffin components. Chemical Characteristics. Structural analyses were made with 'H NMR spectra of the various compound classes by using the above procedure. The structural parameters fa, ADS, and M, are shown in Figure 6. With longer reaction times, fa approached 1and ADS dropped to 0 for Fr-D1, Fr-T1, and Fr-T2, corresponding to naphthalene, phenanthrene, and pyrene or chrysene, and fa = 0.92 and ADS = 0 for Fr-D2, which corresponds to fluorene. For M,, Fr-D1 approached the molecular weight of naphthalene (M, = 128), Fr-D2 that of fluorene (M, = 1661, Fr-T1 that of phenanthrene (M,= 178), and Fr-T2 that of aromatic mixtures of four or more aromatic rings (chrysene, M , = 228). Dealkylation. The extreme chemical changes in each compound class with an upgrading reaction occurred
Energy & Fuels, Vol. 3, No. 3, 1989 401
Catalytic Hydropyrolysis of Coal Liquids
T
1
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a
0.5
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15
30
45
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mainly with dealkylation and deheterogenation, but the parent aromatic ring clusters for respective compound classes were assumed to be preserved under appropriate conditions by structural analysis. Evolution of organic hydrocarbon gases may be predicted with dealkylation of aromatic compound classes. Figure 7 shows the change in the distribution of carbon numbers for various bonding types of aliphatic carbons in Fr-D1 and Fr-D2, calculated by lH NMR structural analysis. The number of saturated carbons substituted onto aromatic rings decreased in the initial reaction period up to 15 min and the lowest values reached were 0.2. From these results, it can be presumed that rupture proceeded at random positions in the alkyl side chains because of the severe reaction temperature of 550 "C. For Fr-D2, values for diphenylmethane-type carbon (Ca2)do not change drastically, representing less degradation of the fluorenetype ring structure. The cumulative numbers of decreasing various bonding alkyl carbons on upgrading oils are expected to correspond with the amounts of hydrocarbon gases evolved by the dealkylation. Therefore, the cumulative organic gas evolutions predicted during respective reaction times were calculated by the summation of decreasing aliphatic carbon numbers measured by lH NMR analyses. Paraffin compound, Fr-P, may also cause thermal cracking and give rise to lighter and lighter hydrocarbon gases. The content decrease for paraffin, Fr-P (shown in Figure 2), supports the above explanation. Values of gas emissions corrected for the amounts of decreasing paraffin compound class were plotted in Figure 5. Satisfactory agreement was ob-
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15
30
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tained between both results for reaction times greater than 30 min. Efficiency of Upgrading Reactions. Dealkylation and deheterogenation are desirable for upgrading reactions, although hydrocracking of aromatic rings seems to occur along with main reactions, in view of the considerable "losses". The content distribution for various compound classes is shown in Figure 8, with reference to the levels of saturated structure (SA) in the feed material, which furnish
Energy & Fuels 1989,3, 402-411
402
an estimate of gas evolution after dealkylation. When the distribution of aromatic ring classes in the feed was calculated, it had been assumed that the distributions of aromatic ring numbers in Fr-PP are approximately the same as those in the feed SRC I1 itself. The observed amounts of all compound classes diminished gradually with longer reaction times. Amounts of organic gases evolved from respective compound classes were calculated by ‘H NMR as described above. These values were added to the respective weight percentage of the corresponding compound classes a t different reaction times and are shown with their cumulative values (shaded region in Figure 8 as organic gas). For two-ring naphthalene-type aromatics (Fr-Dl), the total weight percent of evolved gases calculated and upgraded oil were approximately constant, corresponding to the content of Fr-D1 in feed. For Fr-D1, dealkylation and deheterogenation of naphthalene derivatives are the main reactions, judging from the low value of “loss”, which means there was little or no serious hydrocracking of aromatic rings. Meanwhile, significant hydrocracking was presumed to occur in Fr-T1 and Fr-T2, and the amounts of oil products and gases do not coincide with the contents
of each compound classes. Some reactions cause slight rupture of aromatic rings, followed by their breakup into lighter, volatile molecular compounds (designated as loss in Figure 8). Larger aromatic ring compounds suffer some degradation of their ring structure.
Conclusion The difficulty in utilizing coal liquids as chemical feedstock stems from the complexity of their chemical components. The object of this study was to modify coal liquids to a limited number of more or less pure chemical components in large concentrations. Upgrading of coal liquids by dealkylation of aromatic compounds is an effective procedure because their constitution consists mwtly of large numbers of aromatic components with varying attached saturated structures. This indicates a potential for providing simple and fairly pure aromatics, such as naphthalene, fluorene, phenanthrene, and pyrene as chemical feedstock easily and in great quantity. Acknowledgment. We express our thanks to Prof. K. Tanabe and Associate Prof. H. Hattori for the preparation of catalysts.
‘HNMR Spin-Lattice Relaxation in Bituminous Coals CSIRO Division of
Wesley A. Barton* and Leo J. Lynch Coal Technology, P.O.Box 136, North Ryde, NSW 2113, Australia
Received January 20, 1989. Revised Manuscript Received March 21, 1989
Measurements of proton nuclear magnetic resonance (‘HNMR) spin-lattice relaxation in a wide selection of Australian bituminous coals varying in rank and petrographic composition have been carried out. In all cases the relaxation behavior can be closely fitted by the sum of two exponential components. The results are compared with those of previous studies and discussed in terms of the composition, molecular structure, and properties of bituminous coals. The two spin-lattice relaxation components, which occur in very different proportions in vitrinite and inertinite macerals, may arise from structurally different regions in the coal or, as is more likely, from a combination of direct and spin diffusion-limited relaxation of proton magnetization by unpaired electrons. The overall relaxation rate is appreciably greater for inertinite than for vitrinite macerals, a difference attributed to higher concentrations of organic free radicals and/or inorganic paramagnetic species in the inertinites. The intermediate relaxation rate found for liptinites is probably determined by differences in average molecular mobility as well as in unpaired electron concentration between the macerals. For coals of similar maceral composition, there is a minimum in relaxation rate near 87% C (daf). The variations with rank in the relaxation rate for the vitrinite and inertinite macerals below 87% C are attributed largely to the effects of changes in oxygen functionality. The sharp increase in relaxation rate above 87% C is associated with the concomitant increase in free-radical concentration and growth of graphite-like structures.
Introduction Proton Spin-Lattice Relaxation in Organic Solids. In a hydrogen-containing material placed in a static magnetic field, the net alignment of the proton magnetic moments gives rise to a macroscopic magnetization M, which has a value Mo when the proton moments and the surrounding molecular lattice are in thermal equilibrium. If this equilibrium is disrupted by an external perturbation (e.g. a radiofrequency pulse), proton spin-lattice relaxation is the process whereby the equilibrium magnetization Mo 0887-0624/89/2503-0402$01.50/0
is restored. In homogeneous solids the magnetization recovery is usually exponential (sometimes after an initial nonexponential behavior-see below) with a time constant known as the spin-lattice relaxation time T l . In heterogeneous materials (such as coals) the intrinsic proton relaxation behavior and therefore the time constant would be expected to vary between different parts of the structure. Proton spin-lattice relaxation is induced by modulation of the magnetic interactions of the protons a t frequencies 0 1989 American Chemical Society
