Ind. Eng. Chem. Fundam. 1982, 21 I 236-242
236
Hydrogen Transfer Reactions of Model Compounds Typical of Coal Sudhlr V. Panvelker and Yatl8h T. Shah Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 7526 1
Donald C. Cronauer' Gulf Research & Development Company, Pittsburgh, Pennsylvania 15230
The hydrogen transfer reactions invoking some model compounds, representing the bonds and functional groups
typically found in coals, namely dibenzyl, dibenzyl ether, benzyl phenyl sulfide, acetophenone, and benzaldehyde, have been studied. The hydrogen donating capabilities of tetralin, tetrahydroquinoline, and molecular hydrogen, and the catalytic effect of SRC residue have been examined. Hydrogen c a n be abstracted more easily from tetrahydroquinolinethan from tetralin. Hydrogen pressure has a marked effect on the rates of conversion of dibenzyl ether, benzyl phenyl SUW, acetephenone,and benzaldehyde. SRC residue catalyzes the hydrogenation of carbonyl groups to a significant degree and the conversion of dibenzyl ether and benzyl phenyl sulfide to a lesser degree. The rate of cracking of dibenzyl is not affected by the presence of SRC residue.
Introduction During liquefaction of coal in the presence of a solvent, large coal molecules are fragmented, and the reactive coal fragments abstract hydrogen from the surrounding donor solvent, hydrogen-rich portions of coal, or react with dissolved hydrogen to yield a host of products. Coal has a highly complex molecular structure and, therefore, a convenient way to study the reactions of various types of bonds is to simulate the structure using a variety of model compounds. Benjamin et al. (1978) did an extensive study of thermal cleavage of numerous types of bonds in tetralin. Whitehurst et al. (1977) studied the reactions of bridged polycyclic hydrocarbons as models for possible skeletal structures of coals, and also the reactions of ethers, sulfides, carbonyls, aldehydes, etc. Cronauer et al. (1978, 1979) studied the kinetics of cracking of dibenzyl in a variety of solvents and also examined the reactions of various other compounds, such as ethers, aldehydes, ketones, etc. Kamiya et al. (1979) studied the thermal treatment of coalrelated aromatic ethers in tetralin. Patzer et al. (1979) studied the hydrogenation of methylnaphthalene in the presence of various catalysts. Vernon (1980) studied the effect of molecular hydrogen on dibenzyl cracking, whereas Brucker and Kolling (1965) examined the role of tetrahydroquinoline as hydrogen donor. Many other investigators have analyzed the catalytic effect of mineral matter on the hydrogenation of model compounds and coal itself (Mukherjee and Chowdhury, 1976; Guin et al., 1978, 1979a,b;Garg et al., 1979; Gray, 1978; Gangwer and Prasad, 1979; Rottendorf and Wilson, 1979). Messenger and Attar (1979) analyzed the thermodynamics of oxygen and sulfur containing functional groups during liquefaction. In this paper, we report a study on hydrogen donating capabilities of tetralin, tetrahydroquinoline and molecular hydrogen to dibenzyl, benzyl phenyl sulfide, dibenzyl ether, acetophenone, and benzaldehyde, under noncatalytic conditions and also in the presence of SRC residue, under coal liquefaction and preheater conditions. Coals and coal liquids contain a large amount of heterocyclic compounds such as quinoline. Some of the constituents of coal mineral matter are capable of catalyzing hydrogenation of such compounds and, hence, the reaction of quinoline to tetrahydroquinoline can effectively shuttle molecular hy0 196-43 131821i02i-0236$01.25/0
drogen to the hydrogen acceptors in that tetrahydroquinoline can act as a donor. We also report a study on the catalytic effect of mineral matter (SRC residue) for the reactions of hydrogen acceptors with molecular hydrogen. Experimental Section The hydrogen transfer reactions were studied in a 1-L stainless steel autoclave (Autoclave Engineers) equipped with a magnadrive. In all cases, the hydrogen acceptor together with the main donor solvent, if any, was dissolved in a poor hydrogen donating medium, such as mesitylene, n-dodecane, or n-hexadecane. The choice of this solvent was made after evaluating the temperature range of interest for the given system. Most of the solvent was charged to the reactor along with the catalyst, if any, and the reactor was pressurized to 2.4 MPa with nitrogen or hydrogen depending upon the conditions being studied. This mixture was then heated to 20 K above the reaction temperature. At this time, the remaining solvent containing all of the donor solvent and acceptor was injected into the autoclave under pressure. The pressure was then regulated to the desired value and the samples were withdrawn at prespecified time intervals. Mesitylene and tetralin were obtained from Fisher Scientific and Chemical Samples Co., respectively. Dibenzyl, dibenzyl ether, benzyl phenyl sulfide, benzaldehyde, acetophenone, n-hexadecane, and n-dodecane were obtained from Aldrich Chemical Co. SRC residue, the solid recovered after coal liquefaction and subsequent extraction with tetrahydrofuran, was obtained from the liquefaction of Kentucky Coal at the Ft. Lewis SRC pilot plant. The SRC residue was made up of 73.8% ash (26.2% loss on ignition) with the oxides of silicon, aluminum, and iron being the principle components at metal levels (emission spectroscopy) of about 25, 10, and 15 wt 70, respectively. Hydrogen and nitrogen were obtained from Air Producis Co. The samples were analyzed on a gas chromatograph. Reaction products were identified by comparison of retention times with those of pure samples. Products from dibenzyl, dibenzyl ether, acetophenone, and benzaldehyde were analyzed on an HP 5750 GC using a 16-ft glass column packed with Carbowax-20m and a 20-ft glass column packed with OV-101. The produces from benzyl phenyl 0 1982 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 21, No. 3, 1982 I
I
I
I
I
I
I
93
1
-I
I
I
I
t
I t
I
I
4.17% Tetrahydroquinoline
[3 4.17% Tetralin t 4.17%
A
cl
0
237
Tetrahydroquinoline
A 4.17% Tetralin +
A
D [ I I +
A
+ z
Y 8
[II
+
0 t
4
A
a cc
F
z w 0 z a
v
5
+
A
0 1
>
+ NO S R C Residue
0 0.13% 21
0
I
I
50
100
N
Z
W
S R C Residue
I
I
I
I
150
200
250
300
I
ma
TIME (MINUTES)
Figure 1. Conversion of dibenzyl with and without the presence of SRC residue, at 700 K, 10.34 MPa; feed concentration of dibenzyl was 0.9%.
sulfide were analyzed on an HP 5880A using an 11-m fused silica capillary column coated with methyl silicones. Results and Discussion The kinetics of all systems examined here could be represented fairly accurately by pseudo-fit-order kinetic models. The rate constants were evaluated by linear regression plots of log of concentration vs. time. All the kinetic plots presented here are with dimensionless concentration, unless stated otherwise, obtained by dividing the actual concentration by the concentration at zero time as predicted by linear regression of first-order kinetic model. This was done because it was difficult to obtain an accurate initial concentration in the reactor due to a limited residual holdup of reactants in the charge tank after injection and delays necessary for effective mixing in the reactor prior to sampling. Dibnzyl. The kinetics of the cracking of dibenzyl have been studied in detail (Cronauer et al., 1978). One important conclusion of this study was that the rate of dibenzyl cracking is independent of the nature of the hydrogen donor and catalyst. However, the latter factors affect the product distribution. We confirmed this by reacting dibenzyl dissolved in n-hexadecane under hydrogen pressure of 10.34 MPa with and without the presence of mineral matter (SRC residue) at 700 K. Figure 1 shows the first-order kinetic plot for both the cases. A rate constant of 2.23 X min-l can describe the plot fairly closely (this value is consistent with that reported by Cronauer et al., 1978). Dibenzyl could have abstracted hydrogen from either the solvent (n-hexadecane) or molecular hydrogen. Although all the products were not identified, toluene was the major product. The amount of stilbene produced (less than 6% of initial dibenzyl content) was small for both cases. The cracking of dibenzyl has been described by a free radical mechanism (Cronauer et al., 1978). Ar-CH,-CH,-Ar 2Ar-CH2. Ar-CH,.
+ donor
--
products
The first step of bond cleavage is the rate-controlling step. As such, the rate of the second step does not influence the overall conversion of dibenzyl. If there are two different
4
0
50
100
150
200
250
TIME (MINUTES)
Figure 2. Conversion of dibenzyl in the presence of limiting amounts of donor solvents.
donor solvents present, the free radical reacts preferentially with the donor from which hydrogen is more easily abstracted. An alternative mechanism based upon a pericyclic hydrogen transfer between hydroaromatics (namely, 1,2-dihydro- and 2,3-dihydronaphthalenes)has been proposed by Virk (1979). In this case, the concentration of donor solvent is important. To compare the hydrogen donating capabilities of tetralin and tetrahydroquinoline, the concentrations of these donor solvents were monitored under different conditions. It is pointed out that the concentration of a hydroaromatic donor can decrease, under such conditions, either due to hydrogen transfer to any acceptor or due to thermal dehydrogenation to the corresponding aromatic species and molecular hydrogen. All runs were made with a low level of donor solvent so that the change in the quantity of donor solvent because of hydrogen transfer to benzyl radicals was more pronounced than that due to thermal dehydrogenation. These experiments were carried out in hexadecane under N2pressure of 10.74 MPa at 693 K. In the first run,6.25% (by weight) dibenzyl was reacted in the presence of 4.17% tetralin; in the second run, the same amount of dibenzyl was reacted with 4.17% tetrahydroquinoline; and in the third case, the same amount of dibenzyl was reacted with 4.17% tetralin and 4.17% tetrahydroquinoline. The concentration profiles of dibenzyl, as shown in Figure 2, demonstrate that the rate of dibenzyl cracking is not affected by the choice of donor. However, it can be seen from Figure 3 that the conversion of tetralin to naphthalene is significantly reduced by the presence of tetrahydroquinoline. The conversion of tetrahydroquinoline and the formation of quinoline (Figure 4) is not affected to an appreciable extent by the presence of tetralin. This implies that the benzyl radicals abstract hydrogen from tetrahydroquinoline with greater ease than from tetralin indicating that the former is a much better donor to the acceptor in the above series of experiments.
238 Ind. Eng. Chem. Fundam., Vol. 21, No. 3, 1982
Table I. Run Conditions and First-Order Rate Constants for the Reaction of Dibenzyl Ether. Amount of Dibenzyl Ether in Each Run = 2 g SRC
n-
a
run
temp, K
press., MPa
tetralin, g
DE-1 DE-2 DE-3 DE-4 DE-5 DE-6 DE-7 DE-8 DE-9 DE-10 DE-1 1 DE-12
648 638 658 648 63 8 658 648 648 64 8 648 638 658
10.34 N, 10.34 N, 10.34 N? 10.34 N, 10.34 N, 10.34 N, 10.34 H, 13.79 H, 10.34 H, 10.34 H, 10.34 H, 10.34 H,
20 20 20 0 0 0 20 20 0 0 0 0
THQ,* g dodecane, g residue, g
0 0 0 20 20 20 0 0 0 0 0 0
200 200 200 200 200 200 200 200 220 220 220 220
rate const, min-' 3.55 x 10-3 2.25 x 10-3 5.16 x 10-3 3.55 x 10-3 2.25 x 10-3 5.76 x 10-3 15.69 X 28.88 x 10-3 12.92 X 15.37 X lo-' 6.91 x 10-3 20.0 x 10-3
0 0 0 0 0 0 0 0 0 0.5 0.5 0.5
THQ = tetrahydroquinoline. 4.5
r
1
I
I
I
+ 4.17% Tetrahydrosuinoline
111 4.17% Tetrahydroquinollne
c.:
4.0
1
i
4.17% Tetraline
+
Tetrahydroquinoline
El L
2 k
a
LL
El
I 3.0
+
1
E
l
+ I
2.5
i 0
-g
W
z
Z
W
I tI
z
i
l
+
9
L 6
E
2.0
\
1
W
z
I
1.5
\
z
J
-
0
I
1
2c t W
t El
0 LL
ci
>I
a c W I-
0
i+
3 U
LL
0.0
1.5
z
50
100
150
200
250
0.5
1
0.0 €b 0
i
El
+
El
I
.b
I
I
I
I
50
100
150
200
I
250
TIME (MINUTES) Figure 3. Consumption of tetralin and formation of naphthalene for reaction with dibenzyl.
TIME (MINUTES) Figure 4. Consumption of tetrahydroquinoline and formation of quinoline for reaction with dibenzyl.
As an alternative, a benzyl radical may abstract a hydrogen from tetralin, preferably from the a-position, resulting in an a-tetralyl radical. This, in turn, may give up another hydrogen going to dihydronaphthalene, or it may abstract a hydrogen from tetrahydroquinoline thereby regenerating a tetralin molecule. Considering that dihydronaphthalene is an excellent donor (Virk, 1979 and Whitehurst et al., 1977) and that it would rapidly donate hydrogens to go to naphthalene, it appears most a-tetralyl radicals preferentially react to reform tetralin. From an analysis of the data given in Figures 2 through 4, it appears that in excess of 80% of the hydrogen to satisfy the benzyl radicals comes directly or indirectly from the tetrahydroquinoline. It is noted that both tetrahydroquinoline and tetralin dehydrogenate thermally, but the rate is affected by the level of free radicals, so it is not possible to accurately differentiate hydrogen transfer from dehydrogenation. Dibenzyl Ether. The results of the dibenzyl ether experiments are shown in Table I. Runs DE-1 and DE-6
were conducted to observe the conversion of dibenzyl ether at three temperatures with tetralin, as well as tetrahydroquinoline, as solvent. Figure 5 shows some typical results from these runs. There is no appreciable difference between the conversion levels with the two donor solvents. The activation energy for the conversion of dibenzyl ether in these solvents is 163 kJ/mol, which is comparable to 152 kJ/mol reported by Cronauer et al. (1979). As with dibenzyl, the rate of conversion of dibenzyl ether was independent of the type of solvent. For dibenzyl, the rate was also independent of any molecular H, present in the system (Cronauer et al., 1978). However, for the case of dibenzyl ether, there was a strong dependence of rate on the nature of atmosphere in the reactor and the pressure as seen from runs DE-1, DE-7, and DE-8 (see Table I). Changing from Nz to H2 at a pressure of 10.3 MPa results in an increase of rate of conversion by a factor of 4.4. A further increase of pressure to 13.79 MPa results in an increase by a factor of 1.8. It has been postulated
Ind. Eng. Chem. Fundam., Vol. 21, No. 3, 1982 239
Table 11. Run Conditions and First-Order Rate Constants for the Reaction of Benzyl Phenyl Sulfide. Amount of Benzyl Phenyl Sulfide in Each Run = 2 g nSRC run temp, K press., MPa tetralin, g THQ,a g dodecane, g residue, g rate const, min-' BPS-1 BPS-2 BPS-3 BPS-4 BPS-5 BPS-6 BPS-7 BPS-8 BPS-9 BPS-10 BPS-11 BPS-12 a
10.34 N, 10.34 N, 10.34 N, 10.34 N, 10.34 N, 10.34 N, 10.34 H, 13.79 H, 10.34 H, 10.34 H, 10.34 H, 10.34 H,
638 628 618 638 628 618 628 618 628 618 608 62 8
20 20 20
0
200 200 200 200 20 0 200 200 200 220 220 220 220
0 0 20 20 20 0 0 0
0 0 0
20 20 0 0 0
0 0 0
0
lo-' 10" lo-' lo-' lo-' lo-' lo-'
6.03 X 3.09 X 1.61 X 6.87 X 3.71 X 1.94 X 8.90 X 4.84 x 6.22 X 3.51 X 2.65 X 5.20 X
0 0 0 0 0 0
0 0 0.5 0.5 0.5 0
10lo-'
lo-' lo-' lo-'
THQ = tetrahydroquinoline. 3,
I
I \
100 0
I
I
I
I
1
I
I
T e t r a l i n r u n s under n i t r o g e n no SRC r e s i d u e
D M o l e c u l a r hydrogen w i t h SRC r e s i d u e ( i n t h e absence of a donor s o l v e n t )
2 '
8
I
+
k=2. Z ~ X ~ Omin- ~
0
0 v
?
7
-
0 Y
z
41
6
!-
a
z w
S -
r \
8 -
.-I
IT I-
10-2 -
z
w
7 -
Y
5 k=5.76~10-~min-~
0
?,
z
0 0
IT W I t
w
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k
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v)
z
0
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INDEX OF SYMBOLS
W
*-
Lia
Solvent
mc2
Temp.(K) 656 648 636
21
0
Tetralin
X
0
0
0 A
+
in-3
I
I
I
50
100
150
A.4
200
TIME (MINUTES) Figure 5. Conversion of dibenzyl ether in tetralin and tetrahydroquinoline under nitrogen atmosphere.
(Cronauer et al., 1979; Kamiya et al., 1979; Schlosberg et al., 1981a,b) that dibenzyl ethejjfirst undergoes a reaction to form benzaldehyde and toluene by intramolecular rearrangement. Toluene, benzaldehyde, and benzene (which can be formed by cracking benzaldehyde) were observed in the products. However, according to the above mechanism, the rate of conversion of the ether should not be dependent on the concentration of the hydrogen donating species. Since the rate is strongly dependent upon hydrogen pressure, there could be another primary reaction of dibenzyl ether, the rate of which is dependent upon hydrogen pressure. Runs DE-9 through DE-12 were made to demonstrate the effect of molecular hydrogen and SRC residue. A comparison of DE-9 and DE-10 shows a small degree (20%) of catalytic action of the SRC residue. As shown in Figure 6, the activation energy for the dibenzyl ether reaction made with tetralin under a nitrogen atmosphere in the absence of SRC residue was 164 kJ/mol. In the absence of a donor solvent, but with the addition of SRC
I
I
I
I
I
I
I
I
1.50 1.51 1.52 1.531.541.551.561.57
RECIPROCAL OF TEMPERATURE
I
I
1.581.59 1.60 x1 0-3
(
1 /K 1
Figure 6. Arrhenius plot for conversion of dibenzyl ether. residue and a hydrogen atmosphere, the reaction rate was lower than the above by a factor of 4. The indicated activation energy was 186 kJ/mol; however, due to the scatter of the three points, these two activation energies may be essentially equal. Cronauer et al. (1979) reported rapid conversion of dibenzyl ether in the presence of commercial CoMo/A120a, and Kamiya et al. (1979) reported a catalytic effect of coal ash on the conversion of dibenzyl ether in the presence of tetralin. The lower catalytic activity of this SRC residue could be due to the low content of acidic component and to a coating of the coal mineral matter by carbonaceous deposits. Benzyl Phenyl Sulfide. The run conditions and corresponding first order rate constants of the hydrogen transfer reactions of benzyl phenyl sulfide (MH,-S-Ar) are given in Table 11. Mesitylene was used as the bulk solvent. Benzene and toluene were observed as products but no benzyl mercaptan (ArCH,-SH) or thiophenol (Ar-SH) was deteded. In all cases the kinetics of hydrogen transfer could be described by pseudo-first-order rate constants. Figure 7 shows the first-order kinetic plot for
240
Id.Eng. Chem. Fundam., Vol. 21, No. 3, 1982
+
10-1 5PS-3
(-I-
a =w-_ J 2 6
K
- ( ~6 i r
K
~
Q
P
C
[3 T e t r a l l n , Nitrogen Atmospher No SRC Residue
KI
9
A Tetrahydroquinoline, Sitrogen
8
Atmosphere, No SRC Residue
+
7
Hydrogen Atmosphere (10.3411Pa With S R i Residue
6
-,T.
5
Z
-
.-I
135.2 kJ/mo?
4
t
z
a
t 0 Z 0 0
3
W
t
= 2 0 6 . 3 kJ/mol
a
IT
2 E = 2 1 6 . 5 kJ/m
10-2
1
0
I
I
I
I
I
20
40
60
80
100
10-2 1.551.561.571.581.591,601.611.621.631,641,65
120
Figure 7. Rate of conversion of benzyl phenyl sulfide with tetralin.
the conversion of benzyl phenyl sulfide using tetralin as solvent. The fit of the data with tetrahydroquinoline as solvent is of similar consistency. From Table 11, it can be seen that the fist-order rate constants for runs made with tetrahydroquinoline are only slightly greater than those made with tetralin. Arrhenius plots for selected benzyl phenyl sulfide runs are given in Figure 8. The activation energy for the reaction in the presence of tetrahydroquinoline as donor is equivalent to that for tetralin, within experimental accuracy. Figure 9 shows the first-order kinetic plots for the reactions of benzyl phenyl sulfide with molecular hydrogen as donor in the presence of mineral matter at three temperatures and in the absence of SRC residue for one temperature. Examination of the rate constants for BPS-9 and BPS-12 from Table I1 shows that there is some catalytic effect of SRC residue at 628 K (the rate constant increases by about 20%). The activation energy for the catalytic reaction is much lower (135 kJ/mol) than that for thermal reaction with donor solvents, as expected. To confirm that benzyl phenyl sulfide can react with the molecular hydrogen, two runs (BPS-7 and BPS-8) were made using a hydrogen atmosphere to be compared to those (BPS-2 and BPS-3) made with a nitrogen atmosphere without changing the liquid commition. As shown in Table 11, the presence of hydrogen gas at 10.34 MPa over a mixture of tetralin and mesitylene increased the rate constants by a factor of about 3. Therefore, the rate of hydrogen transfer to benzyl phenyl sulfide is strongly dependent upon the atmosphere and the concentration of donor species. In conclusion, benzyl phenyl sulfide reacts with tetrahydroquinoline only slightly faster than tetralin when both the present in equal quantity. The rate of reaction is dependent upon the concentration of donor species (tetralin and hydrogen). The sulfide also reads with molecular
x10-3
RECIPROCAL TEMPERATURE ( 1 / K )
T I M E (MINUTES)
Figure 8. Arrhenius plot for the conversion of benzyl phenyl sulfide. I
100
I
I
I
X BPS-12, Hydrogen, no SRC Residue 6 2 5
+
A BPS-9, Hydrogen, SRC Residue 626
I \\\
-
13 BPS-19,
K
Rydrogen, SRC Residue 616 I
t-
2+
2 -
Z
W
0
z
0
0
W 0
10-1
I
t _I
3 0
i t
5 -
z w
I IL i
t N
z
%
2 -
10-2
I
I
I
1
hydrogen even when tetralin is present in excess. SRC residue catalyzes the conversion of benzyl phenyl sulfide, under the experimental conditions, to a small extent although the activation energy is apparently changed by a large amount. It is obvious from the above that the de-
Ind. Eng. Chem. Fundam., Vol. 21, No. 3, 1982 241
Table 111. Run Conditions and First-Order Rate Constants for the Reaction of Acetophenone and Benzaldehyde SRC residue, g
n-
run
press., MPa
tetralin, g
THQP g dodecane, g
A-1 A-2 A-3 A-4 A-5 A-6 A-7
674 685 674 685 685 68 5 685
a. Acetophenone Runs; Amount of Acetophenone in Each Run = 2 g 200 0 10.34 N, 20 0 200 0 10.34 N, 20 0 200 0 10.34 N, 0 20 200 0 10.34 N, 0 20 200 0 10.34 H, 20 0 200 0 13.79 H, 20 0 200 0 17.24 H, 20 0
B-1 B-2
638 638 638 638 638
b. Benzaldehyde Runs; Amounts of Benzaldehyde in Each Run = 2 g 10.34 N, 20 0 200 0 10.34 H, 20 0 200 0 20 200 0 10.34 N, 0 10.34 H, 0 20 200 0 10.34 H, 0 0 220 0
B-3 B-4 B-5 a
temp, K
rate const, min-’ 0.37 x 0.54 x 1.60 x 2.40 x 4.90 x 8.29 x 11.91 x
10-3 10-3 10-3
10-3 10-3 10-3 10-3
2.88 x 1 0 - ~
6.72 x
io-)
9.70 x 10-3 12.31 X 1 O - j 5.42 x 10-3
THQ = tetrahydroquinoline.
composition of the sulfide does not proceed by a single mechanism. Acetophenone and Benzaldehyde. Coal contains a limited amount of oxygen in the form of carbonyl groups (Ruberto and Cronauer, 1978). These functionalities are more reactive than those considered above. Compounds like benzaldehyde can also be formed as intermediates from the reactions of ethers like dibenzyl ether. Table I11 shows the run conditions employed and the corresponding firsborder rate constants for a series of runs with acetophenone and benzaldehyde. The reaction with tetrahydroquinoline present as donor solvent is faster than that with tetralin by a factor of more than 4. This difference in rates is due to the preexponential factor in that the activation energy for the reaction with tetrahydroquinoline is 140 kJ/mol while that for the reaction with tetralin 135 kJ/mol. This difference is not significant considering the scatter of the data and the availability of data a t only two temperatures. The difference in rates for different donor solvents is much larger for acetophenone than for benzyl phenyl sulfide. This could be due to hydrogen bonding between tetrahydroquinoline and an oxygen atom of acetophenone. It would be anticipated that less strong hydrogen bonding would occur between tetrahydroquinoline and sulfur-containing species. Figure 10 shows the dependence of rates of the hydrogen pressure. Neglecting the contribution due to tetralin, the rate is proportional to a power of 1.5-2 of the hydrogen pressure. Reactions of benzaldehyde are similar to that of acetophenone in the aspects mentioned above. However, as a minor pathway, benzaldehyde can thermally crack to benzene without any hydrogen transfer from the donor. Benzaldehyde was reacted with tetralin and tetrahydroquinoline under both N2 and H2 pressure in order to analyze the mode of conversion of benzaldehyde. An examination of the rate constants from Table I11 reveals that the rate is higher with tetrahydroquinoline than with tetralin, and the effect of changing N2 atmosphere to H2 atmosphere is more pronounced when tetralin is present. This again suggests that there is a hydrogen-bonding interaction between the tetrahydroquinoline and the carbonyl group.
Conclusions and Implications to Coal Liquefaction This study shows a wide difference in the behavior of selected coal-related functional groups in hydrogen transfer
100 A
9
Y
-
A
I
6 -
5 1
0
A
A-2.
1 0 . 3 4 MPa N2 p r e s s u r e or,
C]
A-5,
1 0 . 3 4 MF’a H2 p r e s s u r e on t e t r a l l n
f A-6,
1 3 . 7 9 MPa H2 p r e s s u r e o n t e t r a l l n
x
1 7 . 2 4 MPa H2 p r e s s u r e on t e t r a l i n
A-7,
I
I
I
1
50
100
150
200
TIME (MINUTES) Figure 10. Effect of hydrogen pressure on conversion of acetophenone at 685 K.
reactions. It also shows the difficulty in optimizing coal liquefaction conditions using these results as a frame of reference. Based on the studies on the relative reactivities of the cracking reactions of different bonds found in coal, it appears that coal first liquefies through the “depolymerization” of heterofunctional linkages. Subsequently, the cracking of alkyl linkages occurs. The effect of hydrogen pressure, type of donor solvent, and catalytic solids (such as mineral matter, SRC residue, etc.) upon these two broad categories of reactions is much different. Increased hydrogen pressure enhances the cracking of bonds represented by dibenzyl ether and benzyl phenyl sulfide. Cleavage of such bonds is important during the early stages of coal liquefaction. The positive effect of H, pressure on the cracking of such bonds can explain the
242
Ind. Eng. Chem. Fundam., Vol. 21, No. 3, 1982
positive effect of hydrogen on the conversion of Wyodak coal at short contact times, as observed by Whitehurst et al. (1977). The rate of cracking of bonds represented by dibenzyl is not affected by hydrogen pressure (Cronauer et al., 1981), although the product distribution can be affected (Vernon, 1980). The problem of optimizing the hydrogen pressure is complicated by its relationship with the rate of donor solvent hydrogenation and its direct reaction with coal functional groups. The concentration of the donor components should be maintained at a high level during all phases of liquefaction to prevent char formation. On the other hand, not all of the reactions involving hydrogen and coal species are desirable. Reactions like those of the reduction of aldehydes and ketones consume a sizable amount of hydrogen. Excessive hydrogenation of the donor components is also undesirable, e.g., decalin is a poorer donor than tetralin (Cronauer et al., 1978). Hydrogen from nitrogen-containingheterocyclics is more readily available to hydrogen acceptors than that from equivalent hydroaromatics as shown by the tetrahydroquinoline/ tetralin experiments. This is particularily true when the species contain oxygen (or to a lesser extent sulfur) in which there is an apparent interaction between the nitrogen and oxygen containing species. Again, this is important during the initial stages of liquefaction when ether and thioether types of linkages are being broken. During the latter stages of liquefaction, the cracking of alkyl linkages is of greater importance. At this stage, the presence of nitrogen-containing heterocyclics and high hydrogen pressure would appear to be of less significance to the cracking reactions as indicated by the experiments with dibenzyl. However, it the solvent becomes poor in hydrogen donors, the resulting free radicals are likely to undergo adduction-type reactions with a loss of solvent and distillate liquids. In summary, there is somewhat of a balance wherein the value of high hydrogen pressure apparently is diminished during the latter stages of liquefaction but its use is still desirable to avoid side reactions. Many components of coal mineral matter have catalytic properties for hydrogenation, in particular, rehydrogenation of the depleted donor solvent. Hence, the reaction sequence tetrahydroquinoline quinoline
Hdgm)
catalyst
acceptor
quinoline
tetrahydroquinoline
can effectively shuttle hydrogen from the gas phase to the hydrogen acceptors. We have found that in the presence of SRC residue the hydrogenation of quinoline to tetrahydroquinoline is much faster than that of naphthalene
to tetralin. Such a hyrogenation of heterocyclic compounds may be very important during the initial stages of liquefaction, particularily before coke-like deposits form on the particles of mineral matter. As a note, the activity of SRC residue toward the cracking of dibenzyl ether was much less than that observed by Kamiya et al. (1979)for coal ash. While this may be due to differences in compositions between the two materials, it is more likely due to a coating of the mineral particles by coke-like material. This coating of mineral particles has been effectively discussed by Jenkins (1982). It is noted that the SRC residue used herein contained about 20 w t % carbon. Acknowledgment Funding of this work was provided by the U.S.Department of Energy under Project DE-AC22-80PC30080. We wish to also acknowledge the comments of R. I. McNeil and A. B. King. Literature Cited Benjamin, B. M.; Raaen, V. F.; Maupln. P. H.; Brown, L. L.; Collins, C. J. Fuel 1978, 57,289. Brucker, R.; Kolllng, G. Brennst. Chem. 1985,46,41; 1969, 50. 1. Cronauer, D. C.; Jewell, D. M.; Shah, Y. T.; Modi, R. J. Ind. Eng. Chem. Fundem. 1979. 18, 153. Cronauer, D. C.; Jewel. D. M.;Shah, Y. T.; Kuesser, K. A. Ind. Eng. Chem. Fundem. 1978, 17, 291. Gangwer, T. F.; Prasad, H. Fuel 1979, 58,577. Garg, D.; Tarrer, A. R.; Curtls, C. W.; Lbyd, J. M.; Guin, J. A. Prep. Div. Fuel Chem. Am. ohem. Soc. 1979, 2 4 , 831. Gray, D. Fuel 1978, 57,213. Guin, J. A.; Tarrer, A. R.; Lee, J. M.;Van Brackle, H. F.; Curtis, C. W. Ind. Eng. Chem. PrOcessDes. Dev. 1979, 18, 831. Ouin. J. A.: Tamer, A. R.; Lee, J. M.; Lo, L.; Curtis, C. W. Ind. Eng. Chem. Process Des. D e v . 1979, 18, 371. Guin, J. A.; Tamer, A. R.; Pratter, J. W.; Johnson, D. R.; Lee, J. M. Ind. Eng. them. ~ o c e s s~ e so.e v . 1078, 17,118. Jenkins, R. G. “The Role of Retrogressive Reactions in Coal Liquefaction”, Chemistry of Coal Liquefaction Seminar, University of Utah, Feb 1982. (See also EPRI Annual Report AF-832: Dec 1978; Wakeley, L. D..et ai. Fuel 1979, 58,379.) Kamiya. Y.; Teo, T.; Okawa, S. Prepr. Div. Fuel Chem. Am. Chem. Soc. 1979, 24, 118. Messenger, L.; Attar, A. Fue/1979, 58. 655. Mukherjee, D. K.; Chowdhury, P. B. Fuel 1978, 55,4. Patzer, J. F., 11; Farranto, R. J.; Montagna, A. A. Ind. Eng. Chem. Process Des. Dev. 1979, 78, 825. Rottendorf, H.; Wilson, M. A. Fuel 1980, 59. 175. Ruberto, R. 0.;Crotlauer, D.C. “ACS Symposium Series No. 71”, Larsen, J. W., Ed., 1978; Chapter 2. Schlohrg, R. H.; Ashe, T. R.; Pancirov, R. J.; Donaidson, M., Fuel I 9 8 l e , 60, 155. Schlosberg. R. H.; Davls. W. H., Jr.; Ashe. T. R., Fue/ I 9 8 l b , 6 0 , 201. Vernon, L. W. Fuel 1980,59, 102. Vlrk, P. S.Fuel 1979, 58,249. Whitehurst, D. D.; Farcasiu, M.;Mitchell, T. 0.;Dickert, J. J., Jr. “The Nature and Origin of Asphaltenes in Processed Coals”; E m 1 Report AF-480, July 1977. Whitehust, D. D.; Mitchel. T. 0.;Farcaslu, M. “Coal Liquefaction”; Academic Press: New York, 1980; Chapters 2 and 9.
Received for review February 23, 1981 Accepted February 26, 1982
