Ind. Eng. Chem. Fundam., Vol. 18, No. 2, 1979
Nomenclature a = attraction parameter in the Peng-Robinson equation AC,, = constant-pressure molar heat capacity change on fusion f = fugacity h = molar enthalpy Ahfu8= molar heat of fusion hE = excess partial molar enthalpy k = binary interaction parameter for the Redlich-Kwong equation n = number of moles P = total pressure R = gas constant T = absolute temperature V = total volume u = molar volume Au = molar volume change of fusion = partial molar volume Greek Letters 4 = fugacity coefficient y = activity coefficient w = acentric factor 6 = binary interaction parameter for the Peng-Robinson equation Subscripts 1,2 = components c = critical property m = melting point
153
Superscripts c = condensed phase F = supercritical fluid phase = infinite dilution 0 = reference state OL = pure liquid OS = pure solid SAT = saturation
Literature Cited Chueh, P. L., Prausnitz, J. M., Ind. Eng. Chem. Fundam., 6 , 492 (1967). Diepen, G. A. M., Scheffer, F. E. C., J . Phys. Chem., 57, 575 (1953). Fredenslund, Aa., Jones, R . L., Prausnitz, J. M., AIChE J . , 21, 1086 (1975). Kaul, 5. K., Prausnitz, J. M.. Ind. Eng. Chem. Fundam., 18, 335 (1977). Kaul, B. K., Prausnitz, J. M., AIChE J., 24, 223 (1977). Oellrich, L., Plocker, U., Prausnitz, J. M., Knapp, H., Chem. Ing. Tech., 40, 955 (1977). Peng, D.-Y., Robinson, D. B., Znd. Eng. Chem. Fundam., 15, 59 (1976). Prausnitz, J. M., NBS Technical Note 3 76, (July 1965). Quinn, E. L., J . Am. Chem. SOC., 50, 672 (1928). Redlich, O.,Kwong, J. N. S.,Chem. Rev., 44, 233 (1949). Sandler, S.I., "Chemical and Engineering Thermodynamics", Wiley, New York, N.Y., 1977. Tsekhanskaya, Y. V., Iomtev, M. B., Mushkina, E. V., Russ. J . Phys. Chem., 38, 1173 ( ,964).
Received f o r review July 5 , 1978 Accepted January 4, 1979
The authors gratefully acknowledge the financial support provided for this work by the National Science Foundation under Grant NO. ENG78-05584.
Mechanism and Kinetics of Selected Hydrogen Transfer Reactions Typical of Coal Liquefaction Donald C. Cronauer," Douglas M. Jewell, Yatish T. Shah,* and Rajiv J. Modi Gulf Research & Development Company, Pittsburgh, Pennsylvania 15230
A series of hydrogen transfer reactions has been done using model compounds (donors and acceptors) at reaction conditions consistent with coal liquefaction. Emphasis was placed upon acceptors having C-C linkages and oxygen compounds with functionality likely to be present in coal. The cracking of dibenzyl was shown to be faster than that of diphenylbutane, diphenylmethane, and I-phenylhexane at 400-450 O C . With oxygen-containing compounds, the relative order of reactivity was: furans < phenols < ketones < aldehydes < chain ethers. A study of hydrogen transfer using either a deuterium atmosphere or deuterium-tagged donors was undertaken with the oxygen-containing compounds. It was shown that much of the hydrogen necessary to stabilize free radicals comes from the donor solvent or intramolecular rearrangement and not from dissolved gas.
Introduction In a recent paper, Cronauer et al. (1978) reported the mechanism and kinetics of reactions between dibenzyl and a variety of hydrogen donor solvents. The major conclusion of this study was that the breakage of the carbon-carbon bond in dibenzyl occurs purely thermally and its rate is independent of the nature of the hydrogen donor solvent present during reaction. Coal liquefaction is accomplished by a combination of the dissolution of low molecular weight species and the cracking of larger species. The resulting free radicals are stabilized by the abstraction of hydrogen from a donor solvent or from the coal liquids themselves. The reaction of model hydrogen acceptors and donors similar to those 0019-7874/79/1018-0153$01 .OO/O
during coal liquefaction is the subject of this study. In this paper, we first briefly extend our previous work on the reactions between hydrogen donor solvents and a compound having C-C bonds. The role of catalyst in cracking of dibenzyl and the reactions of a good donor solvent with other types of C-C bond compounds (e.g., stilbene and noncondensed analogues of dibenzyl) are examined. Subsequently, we report the mechanism and kinetics of reactions between hydrogen donor solvents (e.g., tetralin) and a variety of oxygen compounds with functionality likely to be present in coal. Experimental and Analytical Procedures The kinetic experiments were performed in a manner similar to that of the previous study (Cronauer et al., 1978). 0 1979 American Chemical Society
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Ind. Eng. Chem. Fundam., Vol. 18, No. 2, 1979
,
I
SOLID CATALYST
1
A 0 0
A
,
t
0
NON-CATALYTIC &OTHERS
\
'"I c
,
I
-
NALCOMO KAOLIN Ni 4303 ( N I W I CARBON
Conditions 5 % s o l i d s loading 10% D i b e n z y l in Tetralin at 4 2 5 ° C t e m p e r a t u r e
40
,
10% Dibenryl with 40% Tetralin and 50% Mesitylene 10% Phenylhexane w i t h 40% Tetraiin and SOW Mesitylene 1 0 ~ 1 . 4Diphenylbutane w i t h 4 0 2 T e l r a l i n and5Ou M e s i tylene 10Y.Diphenylmelhane w i l h 4 0 ~ T e l r a l i n andSO%Mesitylene
I -
-ok= 0003min.l
4
,"/
--
.'
/y /
o
40
/
80
120
160
200
TIME,Min.
Figure 1. Dibenzyl cracking in the presence of catalytic solids at 425 "C.
In summary, the runs were made in a 300-cm3batch stirred autoclave with a 300-cm3 heated feed tank to permit the injection of reactants a t elevated temperatures under pressure. The typical run was of 3 h duration starting a t the time of injection of the reactants. An initial reactor temperature drop of 5 to 10 "C was observed, but it recovered within 3 min. Liquid samples were taken periodically during the course of the run. The hydrocarbon reaction products were analyzed on a high resolution capillary GLC with a 100-ft SCOT column coated with OV 101. Samples containing oxygenated compounds were analyzed using conventional liquid chromatography. Specifically, a Hewlett-Packard 7620 (FID) with a 12-ft X l/s-in. column packed with Carbowax 20M on Chromosorb W was used with temperature programming to separate the dibenzyl ether, alcohol, aldehyde system. A short column (6 ft) packed with 5% loading of Dexsil300 on Chromosorb W was used for tetralone and naphthol systems. The xylenol systems were analyzed using a 150-ft capillary column with a wall coated with didecyl phthalate/phosphoric acid pretreatment. The products from the deuterium experiments were analyzed using conventional GLC. In addition, samples were also analyzed using a medium resolution GLC interfaced with a duPont 21-491 mass spectrometer. The level of deuterium of individual compounds was calculated by measuring the M+ and M+ + 1 peak heights and correcting for 13C natural abundance in the completely unlabeled system.
Results and Discussion Role of Catalyst on Cracking of Dibenzyl. To test the possible role of an external catalyst on the dibenzyl/tetralin reaction, a series of runs was made at 425 "C using the following catalysts or supports: (1)Nalcomo 477 (CoMo on A1203)-Nalco Chemical Co.; (2) Ni 4303 (NiW on A1,03)-Harshaw Chemical Co.; (3) colloidal kaolin-Fisher Scientific Co.; and (4) Nuchar activated carbon-Westvaco Chemical Division. The runs were made with a charge level of 5 wt % of solid to the reactor. As shown in Figure 1, there was no enhancement of the dibenzyl cracking reaction with the introduction of these solids when the runs were made in a nitrogen atmosphere. A low conversion of dibenzyl with
40
80
120
160
200
T I M E Min
Figure 2. Conversion of dibenzyl analogues at 450 "C.
the introduction of carbon was observed, for which no explanation can be given. In addition, runs were made with Nalcomo and kaolin in a hydrogen atmosphere (1500 psig total pressure). Again, no reaction enhancement was observed. This indicates that cracking of dibenzyl is purely a thermal reaction. Hydrogen-Transfer of Noncondensed Analogues of Dibenzyl. To evaluate the effect of chain length between noncondensed aromatic hydrocarbons, three additional hydrocarbons were reacted with tetralin a t 450 "C. These were diphenylmethane, 1,4-diphenylbutane, and 1phenylhexane. As shown in Figure 2, the cracking of 1,4-diphenylbutane and 1-phenylhexane appeared to follow a first-order rate expression a t 450 "C. Each compound was more stable than dibenzyl. Diphenylmethane was essentially unreactive. 1-Phenylhexane converted a t a lower rate than either dibenzyl or diphenylbutane. This lower rate is attributed to the presence of only one activating (phenyl) group. Ethylbenzene and toluene were the sole liquid products from the hydrogen transfer cracking of the noncondensed aromatics. No evidence was found for propyl- or butylbenzene in the liquid products from the runs with 1,4diphenylbutane and 1-phenylhexane. The major gaseous products were n-butane and methane in the latter run. This is consistent with a step-wise reaction as follows
Ips The experiments with both phenylhexane and diphenylbutane indicate that aromatic resonance stabilization of free radicals does not extend beyond the p position of chain substituents. Experimentation with Stilbene. Two hydrogen transfer runs were made using stilbene (due to its structural similarity to dibenzyl) to study the role of olefins, as intermediates in cracking. The stilbene/tetralin runs were made a t 400 and 450 "C. The plots of the concentrations of stilbene, dibenzyl, and toluene are given
Ind. Eng. Chem. Fundam., Vol. 18,
No. 2, 1979 155
n
Figure 5. Gas-liquid chromatograph of dibenzyl ether hydrogentransfer cracking products.
Figure 3. Concentration curves for the conversion of stilbene to dibenzyl and toluene at 400 "C.
-3
I
0
20
40
60
BO
100 120 140 160
180
T I M E Mln
Figure 4. Concentration curves for the conversion of stilbene to dibenzyl and toluene at 450 "C.
in Figures 3 and 4, respectively. The following react,ion sequence was assumed
.@-!73 \
\
+
\
/
/
2
0
:
H
/
(3)
(4)
The rate constant of disappearance of stilbene was best fit by the expression d c, = -h 1 csc t dt
--
where Csand C, are stilbene and tetralin concentrations in weight percent. Equation 5 fits the experimental data well. The rate constants for the reduction of stilbene a t 400 and 450 "C were found to be 1.3 X lo-* and 6.0 X g of liquid/g of tetralin min, respectively. The activation energy was calculated as 28 400 cal/g-mol. The dependence of the reaction rate upon tetralin concentration is an indication that the transfer may occur via a concerted four-centered reaction, and a free-radical intermediate may not be necessary. This may be particularly true a t temperatures where hydrogenation proceeds smoothly but cracking does not occur. However, a t present, insufficient data are available to confirm this mechanism. Hydrogen Transfer to Oxygenated Compounds. Preliminary experiments indicated that a heteroatom in an aromatic ring did not substantially increase the molecular activity with respect to hydrogen transfer. For example, dibenzofuran, dibenzothiophene, and carbazoles are essentially inert to tetralin a t temperatures up to 475 "C. They did show that if a sulfur or oxygen were not part of the aromatic system, they would generally enhance reactivity. Based on this observation, experimentation with dibenzyl ether and other compounds containing various carbonyl and hydroxyl groups was undertaken. The results of this study are briefly described below. 1. Dibenzyl Ether. The study of the hydrogen transfer cracking of dibenzyl ether is an obvious extension of the experimentation with dibenzyl due to the relative ease of breaking the C-0 bond, the means available for product analysis, and the known presence of ethers in coals, particularly those of low rank. Since preliminary screening experiments showed a rapid conversion of ethers above 400 "C, a temperature range of 300-400 "C was chosen for the kinetic study. This study was made using tetralin and mesitylene as solvents. Since tetralin was essentially stable to rearrangement below 400 "C, this provided a system to evaluate hydrogen transfer with a minimum of ring contraction to methyl indane. Assuming that dibenzyl ether will thermally cleave a t the /3 position, similar to dibenzyl, the initial intermediates are expected to be Ph-CH2. and Ph-CH20. radicals. If hydrogen is abstracted from a good donor solvent, toluene and benzyl alcohol would be the stable end products. These sole products had been observed in the literature (Brucker and Kolling, 1965; and Reifarth, 1977) using tetrahydroquinoline or tetralin as the solvent. However, our analyses (GLC, MS, IR) have shown that benzaldehyde was formed in place of benzyl alcohol. Figure 5 is an example of the GLC separation indicating benzaldehyde (mass spectrometer detector); this was
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Ind. Eng. Chem. Fundam., Vol. 18, No. 2 , 1979
confirmed by co-injection techniques and infrared spectra, as shown in Figure 6. Calculation of the major component concentrations from these experiments confirmed that benzaldehyde was a reaction intermediate, which a t higher temperatures reacted with tetralin and, as confirmed by subsequent experiments with pure benzaldehyde, was converted primarily to toluene with only little conversion to the alcohol. The data also indicated that an insignificant change in tetralin concentration occurred in the 300-365 "C range and only a limited amount of naphthalene was formed by direct hydrogen transfer. The reaction also produced a small amount of benzene. Two dibenzyl ether experiments were done with mesitylene as a solvent a t 350 "C using both nitrogen and hydrogen atmospheres. These runs showed that conversion to toluene and benzaldehyde again resulted without hydrogen transfer but at a slightly slower rate than that observed with tetralin. No polymerization of mesitylene was observed. These various experiments suggest that conversion can occur by two routes: (1)intermolecular hydrogen transfer and ( 2 ) intramolecular hydrogen transfer. The overall reactions are proposed as follows. (1) Intermolecular: thermal cleavage to yield free radicals is shown in reactions 6 and 7 . ( 2 ) Intramolecular hydride transfer: generate H
H
FREQUENCY LCM ' I 10000
3000
50004000
001
I '
'
!
2 00
2000 1800
1600
1200
1400
l'i--rrrr-
,",''I
I
l
l
I
06 08
Figure 6. Dibenzyl ether products at 400 "C after 30 min reaction time.
1
k.Toluene + Benzaldehyde
Dibenzylelher
rate
*
-k6CDBE
Benialdahyde+Telralin
Toluene +Dihydrolelralin
1 i
r a t e -k,CBCT
c
-
REACTION SYSTEM 10% DBE 9 0 % Telralin
+
10% DEE
+ 9 0 % Telralin
1018A+90xTelralin
H
H
~
1O31 145
toluene and benzyloxy anion by the thermal cleavage of the C-0 bond which is then stabilized by rearrangement to the aldehyde as in reactions 8 and 9.
7
7
H
- QJi::"
150
155
160
+
I
lo3
165
170
175
1
( K.I)
RECIPROCAL TEMPERATURE
Figure 7. Arrhenius plots for the dibenzyl ether and benzaldehyde reactions.
hanced by tetralin a t any temperature by providing a source of hydrogen and by the fact that benzaldehyde can react as follows.
I
I
+
H
A satisfactory fit of the data to the following rate equation was achieved This can also be illustrated by a simple hydride transfer without a free-radical intermediate (reaction 10). The
intramolecular rearrangement is favored in poor (nondonor) solvents and a t low temperatures (
