1098
Znd. Eng. Chem. Res. 1994,33, 1098-1106
Catalytic Hydroprocessing of Simulated Heavy Coal Liquids. 1. Reactivities of Aromatic Hydrocarbons and Sulfur and Oxygen Heterocyclic Compounds Michael J. GirgistJ and Bruce C. Gates**+ Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, and Central Research Laboratory, Mobil Research and Development Corporation, Princeton, New Jersey 08543
The hydroprocessing of mixtures simulating a coal liquid without organonitrogen compounds was investigated with a once-through flow reactor operated with liquid-phase reactants a t 350 "C and 171atm. The catalyst was sulfided Ni-Moly-AlzOB. The reactants included pyrene, phenanthrene, The products fluoranthene, dibenzothiophene, dibenzofuran, and 5,6,7,8-tetrahydro-1-naphthola formed from each reactant were determined, and each reaction was modeled as first order in the organic reactant. The reactivity of fused six-ring aromatics increases with the number of rings, but the change from one member of the family to another is less than the order-of-magnitude increase in reactivity from benzene to naphthalene. Fluoranthene must be considered in a separate compound class from fused six-membered-ring aromatics because it is hydrogenated more rapidly. Dibenzothiophene gives biphenyl selectively. Dibenzofuran reacts very slowly,whereas 5,6,7,8-tetrahydro1-naphthol reacts very rapidly. The results reported here, in combination with the reaction networks developed in the sequel, are the first quantitative evaluation of reactivities of components in a mixture simulating a hydroprocessing feedstock that take account of competitive reactions and the formation of intermediate products.
Introduction Catalytic hydroprocessing continues to be one of the most important routes in fossil fuel upgrading, but there is still much to be learned about the reaction chemistry, which involves simultaneous hydrogenation of aromatic hydrocarbons and heterocyclic compounds, hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and hydrodeoxygenation (HDO). Hydrocracking also occurs if the catalyst has an acidic function or if the conditions are severe. The complicated patterns of competitive reaction and inhibition by reactants and products are not well resolved. Hydroprocessing investigations with reactants ranging from single compounds to fractions of fossil fuels have been undertaken to help elucidate the reaction chemistry. The literature includes reports of quantitative reaction networks for several individual compounds (Girgis and Gates, 1991) but hardly any informationregarding relative rates of different reaction types (e.g., HDS vs HDN). Investigations with binary and ternary mixtures (Girgis and Gates, 1991) have provided the basis for a first step in the modeling of reaction inhibition, which has usually been represented by Langmuir-Hinshelwood rate expressions; however, the functional group distributions in the simple reaction mixtures have not been representative of commercial feeds. Experiments with acidic, basic, and neutral fractions of a coal liquid (Katti et al., 1983,1986; Grandy et al., 1986; Li et al., 1985b) determined rates of disappearance of a few of the compounds in relatively complex mixtures, but the simplificationprovided by the fractionation of the feed eliminated the possibility of providing practicallyvaluable information about inhibition effectsbecause of the chemical similarity of the compounds making up each fraction and because of the complexity of the feed. To develop a sound quantitative representation of hydroprocessing chemistry that includes competitive of Delaware. Mobil Research and Development Corp.
+ University
t
0888-5885/94/2633-1098$04.50/0
reaction and inhibition effects, results are needed to characterize the reactions of mixtures of compounds that represent the important classesof reactants. Our objective was to make a first step toward this goal. In this paper and its sequels, we report an investigation of the hydroprocessing reactions of mixtures of eight to nine compounds that simulate a coal liquid from the SRC-I1process. The functional group distribution of the feeds was chosen to be roughly representative of the whole coal liquid. The number of reactant compounds in the feed was chosen to be small enough to allow identification of almost all the products and a determination of the reactivities of all the feed components; the primary goal was development of approximate reaction networks of the reactants and quantitative modeling of the dominant inhibition effects. Such a degree of quantitative detail has not yet been reported for mixtures of similar complexity; earlier treatments were limited to determination of reactant conversions or rates of their disappearance (Girgis and Gates, 1991). A secondary goal was to determine the degree of quantitative detail that could be extracted from the data for mixtures as complex as those used. The specific objectives of the research included the following: 1. Determination of reactivities of the reactants in the simulated coal liquid in the absence of the strongest inhibitors. The data were intended to represent the reactivities of the key functional groups and lay a foundation for the remainder of the work. The results are summarized in the present paper. 2. Identification of all the products and approximate reaction networks for individual reactants. The results were intended to provide a detailed quantitative representation of the reactivitiesof the simultaneously reacting components. The results are summarized in the sequel (Girgis and Gates, 1994a). 3. A simplified representation of the inhibition arising from competitive adsorption, including identification of 0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994 1099 Table 1. Components of Simulated Coal Liquid and Concentrations of Corresponding Functional Groups in the SRC-I1 Heavy Distillate ~ _ _ _ ~ ~ ~ _ _ ~ ~ ~ ~ ~ _ _ ~
compound
~
structure
_
concn, mo1/100 g 0.0145
quinoline
functional group quinoline (basic nitrogen)
acridine
quinoline (basic nitrogen)
0.0145
indole
carbazole (nonbasic nitrogen)
0.021
phenol
0.063
dibenzofuran
ether bridge
0.035
dibenzothiophene
dibenzothiophene
0.019
phenanthrene (three fused ring aromatic)
0.128
naphthalene (two fused ring aromatic)
0.193
pyrene (four fused ring aromatic)
0.064
H
phenanthrene
08 0
fluoranthene
0
p
Pyrene
key inhibitors and modeling of the inhibition. This work is to be reported in part 3 (Girgis and Gates, 1994b).
Selection of Components of Simulated Coal Liquid The simulated coal liquid components were selected to represent the heavy distillate fraction (288-482 "C boiling range) of a liquid produced in the SRC-I1 process. This liquid was characterized thoroughly (Petrakis et al., 1983b,c) and represented quantitatively by 25 functional groups (Petrakis et al., 1983a; Allen et al., 1984; 1985). The feed consisted of aromatic hydrocarbons and compounds incorporating basic nitrogen, nonbasic nitrogen, heterocyclic oxygen, naphtholic (phenolic) oxygen, and heterocyclic sulfur. The compounds chosen to represent each compound class and the corresponding concentrations in the SRC-I1 heavy distillate are summarized in Table 1. The mixture was simple enough to facilitate the analysis of the products of the catalytic reactions. Only those functional groups present in significant amounts in the whole feedstock and undergoing important hydrotreating reactions (Le., aromatic hydrogenation, HDS, HDN, and HDO, but not hydrocracking) were included. Thus ketones, carboxylicacids, saturated hydrocarbons (except for the solvent; see below), and hydroaromatics were excluded. No alkyl substituents were included on the aromatic rings; consequently, the reactant mixture represents coal liquids better than it does petroleum (Allen et al., 1985). To simplify the analysis,the amine functional group was not included, although anilines are formed in quinoline HDN (Girgis and Gates, 1991). Quinoline and acridine were chosen to represent the basic nitrogen and
indole to represent the nonbasic nitrogen. The hydrocarbons included two-ring, three-ring, and four-ring components; naphthalene, although it is an obvious choice for a two-ring aromatic hydrocarbon, was omitted because it is a product of 5,6,7,8-tetrahydro-l-naphtholHDO (Li et al., 1985a), and its inclusion in the feed would have complicated the product analysis. Fluoranthene was included because it contains a monoaromatic moiety and a five-carbon-memberedring, which has a unique reactivity, at least for hydrocracking (Lapinas et al., 1987). The flu0ranthene:phenanthrene:pyreneratio was set equal to 3:2:1 (molar) (vs 7.61.81 in the SRC-I1 heavy distillate) to reduce the monoaromatic concentration to avalue closer to that of the heavy distillate. The feed composition shown in Table 1 was used in scoping experiments, the results of which show (Girgis, 1988)that organonitrogen compounds were the dominant inhibitors, and the reaction networks of the remaining reactants were determined in the absence of these inhibitors. Experiments conducted with the organonitrogen compounds in the feed and modeling of the strong inhibition effects are reported in part 3 (Girgis and Gates, 1994b). Concentrations of the non-nitrogen-containing reactants were kept the same in almost all the experiments.
Experimental Section Materials. The catalyst was American Cyanamid HDS9A, a commercial NiO-MoOsly-AlzOs supplied as 1/16in. extrudate. The catalyst was ground and sieved to 80100 mesh (149-177 pm) particles. Properties of the asreceived &e., oxide) form of the catalyst are given in Table 2. Fisher "RR" alundum (90 mesh size) was used as an
_
1100 Ind. Eng. Chem. Res,, Vol. 33, No. 5, 1994 Table 2. Properties of American Cyanamid HDS-SA Catalyst# component wt% NiO 3.1 Moo3 18.3 wo3
NazO surf. area, m2/g pore vol, cm3/g 0
0.04 0.05 149
0.51
The reported values were determined by the catalyst supplier.
inert packing in the reactor. All the organic compounds were used as received. Suppliers and purities are listed elsewhere (Girgis, 1988). Reaction Experiments. Experimentswere carried out with a high-pressurefixed-bed flow reactor (Girgis,1988). The feed consisted of the organic reactants (Table 1) in cyclohexane solvent. The use of a solvent allowed the use of small amounts of the reactants and obviated heating the feed system and transfer lines. Only a small amount of catalyst was used (typically, 50 mg). In contrast to the procedure in typical hydroprocessing operations with trickle-bed reactors, H2 was introduced by dissolving it entirely in the liquid feed at room temperature. This method offered several advantages over trickle-bed operation: (1) the organic reactants and H2 were all present in the liquid phase and thus their concentrations in the reactor were well-defined; (2) the catalyst particles were fully wetted; (3) no volatile reaction products were lost to a gas phase. Nearly plug flow was achieved (Girgis, 1988). A disadvantage of introducing the H2 in this manner is the requirement of low concentrations of the organic reactants to achieve a stoichiometric excess of Hz. The limitation is imposed by the low solubility of H2 in organic solvents at room temperature (Berty et al., 1966). Maintenance of an excess of Ha allowed experiments with a nearly constant H2 concentration, which was not varied in the experiments. The organic reactants constituted only about 0.25w t % of the reactants. The concentrations of aromatic reactanta in the reactor were thus much smaller than those in industrial feeds, and consequently only inhibition associted with the strongest inhibitors could be observed in the experiments. Two l-L autoclaves served as feed reservoirs. After purging of the feed with H2 for 2 h, it was pressurized with H2 at 137 atm to saturate it, giving a mole fraction of H2 of 0.05 (Katti, 1984). Such a high pressure was needed to introduce a sufficiently high H2 concentration to avoid significant Hz depletion as a result of conversion. The molar ratio of Hz to organic reactants was more than 33, and the maximum conversion of H2 was less than 3 % (Girgis, 1988). The reactant solution was pumped through the reactor with a liquid chromatograph pump; with two feed reserviors, operation could be continued for several days, as the reactant solution was loaded into one autoclave while being fed from the other. Reactor pressure (typically 171 atm) was maintained with a back-pressure regulator. Because Hzsolubilityincreases with temperature (Sandler, 1979) and the reactor pressure was always greater than the saturation pressure, there was no possibility of degassing of the H2 as the feed was pumped into the hot reactor. The reactor effluent flowed to a 500-mL reservior from which the liquid product was withdrawn. Prior to startup, the catalyst was sulfided by passing a mixture of 105% H2S in H2 over it for 2 h at 400 "C at 30-50 cm3(NTP)/h. The catalyst was maintained in the sulfided
state by inclusion of 0.1 w t 5% CS2 in the feed; the CSZ reacted rapidly with Hzto give H2S and CH4 under reaction conditions. A catalyst break-in period of about 72 h was typically observed. The reported data represent the period following the break-in. Mass and heat-transfer effects on the reaction rates were absent, as shown by the results of standard calculations (Girgis, 1988). Except for the scoping experiments, all the reactions were carried out at 350 "C. A separate catalyst charge was used in each experiment. Conversionswere varied by changing the feed flow rate. The flow rate was typically maintained at a given value for at least 6 h to allow enough time for the conversions to become time invariant. Catalyst stability was monitored by periodically checking the conversions under standard conditions. Product Analysis. Feed and liquid product samples were analyzed with a Tracor 560 gas chromatograph equipped with a 6O-m-long, 0.25 mm4.d. wall-coated open tubular capillary column with a0.5-pm film thickness (the stationary phase was 95 % dimethylpolysiloxane and 5 ?6 diphenylpolysiloxane) and a flame ionization detector. A cold on-column injector was used to give good reproducibility in the analyses of high-boiling three- and four-ring components. Peak integration was performed with a Hewlett-Packard 3388A integrator. n-Decane was used as an internal standard. Some samples were analyzed by gas chromatography1 mass spectrometry (GC/MS) with a Hewlett-Packard 5970Bmass-selectivedetector (70 eV ion source) connected to a Hewlett-Packard 5890 gas chromatograph. The identities of several compounds were confirmed by comparing their mass spectra with reference spectra (Heller and Milne, 1981).
Results and Discussion Scoping Experiments and Strategy. Scoping experiments were conducted with the full slate of reactants listed in Table 1. The results of these experiments, presented in detail elsewhere (Girgis, 19881, identified the organonitrogen compounds and 5,6,7,8-tetrahydro-lnaphthol as possible inhibitors of aromatic hydrogenation and dibenzothiophene HDS. Representative results are given in Table 3. Noncatalytic reactions were insignificant; in an experiment with only presulfided alundum present in the reactor at 400 "C, conversions of all reactants were 10
reasonable: the limited information in the literature shows that adsorption parameters characteristic of aromatic hydrocarbons, sulfur-containing heterocyclics, oxygencontaining heterocyclics, and phenolic oxygen-containing compounds are equal, within a factor of 5 (Girgis and Gates, 1991); the near equality of the adsorption parameters coupled with the small amount of cracking observed in this work should result in nearly constant values of denominators in Langmuil-Hinshelwood rate expressions, which would then reduce them to pseudo-first-order kinetics expressions. Moreover, the data are well approximated by this assumption, as shown in the companion paper (Girgis and Gates, 1994a). Reactivities of the Feed Components. Table 5 is a list of the pseudo-first-order rate constants for the disappearance of the individual reactants in experiment B1 (Table 4). The reactivities span 4orders of magnitude, with dibenzofuran being the least reactive compound by far, a result that had been anticipated on the basis of the results of the scoping experiments (Table 3). Hydrogenation of pyrene, which has four fused rings, was approximately 3 times faster than hydrogenation of phenanthrene, which has three fused rings. This result is consistent with the generally observed trend (Girgis and Gates, 1991) of increasing hydrogenation rate with an increasing number of rings in aromatic hydrocarbons. However, the difference is much smaller than the orderof-magnitude difference between the reactivities of naphthalene and benzene (Sapre and Gates, 1981). Thus, a large increase in hydrogenation rate was not observed as the number of fused aromatic rings increased beyond two. Sapre and Gates (1981) rationalized the higher reactivities of two-fused-ring aromatics (naphthalene, 2-phenylnaphthalene) relative to those of monoaromatics (benzene, biphenyl) by suggesting that the transition state for hydrogenation was similar to that of an aromatic electrophilic substitution. The higher reactivity of naphthalene was then explained on the basis of the greater stability of the transition state involving the protonated naphthalene. This suggestion is consistent with the higher reactivity of pyrene than of phenanthrene because the positive charge on the cation formed by protonating pyrene would be distributed over a larger condensed-ring aromatic moiety than that for the analogous phenanthrene cation. Fluoranthene is more reactive than phenanthrene or pyrene (Table 5 ) . The reaction occurs predominantly in the diaromatic moiety to give 1,2,3,1Ob-tetrahydrofluoranthene. Experiments with naphthalene and l-phenylnaphthalene reactants demonstrated that the fluoranthene hydrogenation rate was substantially higher than the rates of hydrogenation of either of these (Table 6). Concentrations of aromatic reactants are included in the table to give an estimate of the contribution of inhibition effects on hydrogenation rates. The data show that the fluoranthene diaromatic moiety is about 3 times more reactive than naphthalene and about 4 times more reactive than 1-phenylnaphthalene. Thus, the relatively high reactivity
1102 Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994 Table 6. ComDarison of Fluoranthene Hydrogenation Reactivity with Those of Naphthalene Moieties pseudo-first-order rate const, reaction L/(g of catalyst-h) fluoranthene + 2Hz 4,5,9,10b-tetrahydrofluoranthene 1.73 1-DhenvlnaDhthalene+ 2Hz 1-phenyltetralin, 2-phenyltetralin 0.380 0.578 naphthalene + 2Hz tetrain ~~
- -
of the fluoranthene diaromatic moiety cannot be attributed to electron-donation effects from its monoaromatic moiety. We therefore propose that the high fluoranthene reactivity is related to its strained five-memberedring and suggest that hydrogenation of the naphthalene moiety alleviates the five-membered ring strain in the transition state, resulting in greater hydrogenation reactivity. We further propose that the high reactivity of fluoranthene relative to that of pyrene is not a consequence of stronger adsorption, since the sizes of these two molecules are similar. The results of Tables 5 and 6 provide evidence that fused-ring aromatic compounds that include a fivemembered ring are hydrogenated more rapidly than aromatic compounds having a similar number of fused rings but lacking a five-membered ring. This result implies that the five-membered ring species should be considered as a separate compound class for lumping purposes, in agreement with the conclusion of Lapinas et al. (19871, who showed that the hydrocracking pathways of fluoranthene-like molecules are distinct from those of fused-ring aromatics. Dibenzofuran is an order of magnitude less reactive than dibenzothiophene (Table 5). The following simplified explanation is suggested for the relative lack of reactivity of dibenzofuran. In the HDS mechanism for thiophene proposed by Kwart et al. (1980), the first step involves coordination of the C1-C2 bond with a catalyst surface cation at a sulfide anion vacancy with simultaneous interaction of the sulfur atom with a surface sulfide anion. The bonding between the c1-C~bond and the Mo cation changes the electronic distribution about the sulfur atom in dibenzothiophene, making it more electron deficient and thus promoting its interaction with the surface sulfide anion. Because oxygen is a much less polarizable atom than sulfur (Streitwieser and Heathcock,1976),we propose that the electron distribution about the oxygen atom is changed to a much smaller degree than that about sulfur when the bonding between the C& and the cation occurs, making the interaction of the oxygen in dibenzothiophene with the surface sulfide anion weaker. We suggest that the relatively weak ability of the oxygen to coordinate with the surface retards hydrogen addition to the ring and ultimately the hydrogenolysis reaction. The low dibenzofuran reactivity implies that relatively severe conditions are required for its HDO (specifically, high H2 concentrations), as were used in reaction network investigations (Krishnamurthy et al., 1981; LaVopa and Satterfield, 1987). Except for trace amounts of 1,2,3,4-tetrahydrodibenzothiophene, the only products of dibenzothiophene conversion were hydrocarbons and H2S. The pseudo-firstorder rate constant for dibenzothiophene HDS shown in Table 5 is in good agreement with the values determined by Katti et al. (Katti et al., 1983; Katti, 1984) at 350 "C for dibenzothiophenein the neutral oils fraction of a SRCI1 coal liquid (0.21) and in a pure compound mixture simulating the latter fraction (0.201, where the units of the rate constants are those of Table 5. The near equality of these rate constants for dibenzothiophene HDSvalidates
concn of arom reactants, mmol/L 5.39 1.37 1.31
Table 7. Feed Composition in Kinetics Experiments with Quinoline in the Feed. concn, mmol/L reactant expt 6 3 expt Q4 quinoline 1.04 2.07 5,6,7,8-tetrahydro-l-naphthol 0.667 0.676 dibenzofuran 0.380 0.381 dibenzothiophene 0.205 0.207 phenanthrene 1.38 1.38 fluoranthene 2.07 2.06 0.687 0.691 pyrene 0 Experimental conditions: 350 O C , 171 atm reactor pressure, 136 atm saturator pressure.
our assumption of weak inhibition in the experimentswith the base-case reactants (Table 4). Only an order of magnitude estimate could be obtained for the reactivity of 5,6,7,8-tetrahydro-l-naphthol. Since its organic products were all found to be hydrocarbons, conversion is identical the 5,6,7,8-tetrahydro-l-naphthol to its HDO conversion. The HDO rate constant given in Table 5 was estimated from thenonzero 5,6,7,%tetrahydro1-naphthol concentration observed at the lowest space velocity in an experiment (Q4,Table 7)in which quinoline, a strong inhibitor, was included in the feed. Possible reasons for the high 5,6,7,8-tetrahydro-l-naphthol reactivity are addressed below. The HDO rate constant in the base-case experiments is inferred to be higher than that stated in Table 5, since quinoline and its products inhibited network. reactions in the 5,6,7,8-tetrahydro-l-naphthol The general conclusion is that HDO of naphtholic and phenolic compounds in coal liquids is very rapid. Implications of the Reaction Networks. We now examine the networks of the different feed components reacting simultaneously and draw conclusions about the reactivities of the reactants and some of the intermediates. The networks of the base-case reactants are given in Figure 1;the determination of these networks is reported in the sequel (Girgis and Gates, 1994a). We emphasizethat the comparison of the rate constants in Figure 1as a basis for assessment of the relative reactivities is appropriate because the results were obtained with all the reactions occurring simultaneously and thus with all the reactants subject to the same competitive adsorption phenomena. Phenanthrene and pyrene are hydrogenated selectively at the bridging positions, viz., the 9,lO-positionin phenanthrene and the 4,5-position in pyrene (Figure 1). Hydrogenation at the terminal ring is especially slow for pyrene. Hydrogenation of the triaromatic 4,5-dihydroppene also occurs selectively at a bridging position, giving 4,5,9,10tetrahydropyrene. However, the diaromatic 1,2,3,4-tetrahydrophenanthrene is not hydrogenated at a bridging position, since the product would be an unstable partially hydrogenated aromatic. The greater selectivity in the conversion of tri- and tetraaromatics to give products hydrogenated at bridging positions is consistent with the previously reported anthracene pathways (Wiser et al., 1970) and is well-known for hydrogenation and electrophilic substitution reactions generally (Streitwieser and Heathcock, 1976). The selective hydrogenation at bridging positions of fused-ring aromatics having at least three rings could have
Ind. Eng. Chem. Res., Vol. 33, No. 5,1994 1103
08
(A)
(D) 0.0225
o-fJ1.l,
0.196
Singl,Rlng Products
11
2.74
(C)
0.228
tIo.073a
0.137
ItO.854
cradting Products
Figure 1. Reaction networks for hydroprocessingof (A) phenanthrene, (B)pyrene, (C) fluoranthene, (D)dibenzofwan and dibenzothiophene, determined in the absence of organonitrogencompounds in experiment B1. The feed compoeition and and (E) 5,6,7,8-tetrahydro-l-naphthol reaction conditions are given in Table 4. The numbers over the arrows represent first-order rate constants in units of L/(g of catalyst-h).
important processing implications. If a cracking function could be incorporated into the catalyst, 2 mol of monoaromatics might be produced per mole of either phenanthrene or pyrene with minimal Hz consumption.
The rate constants determined for the various reactions of the different networks permit quantitative statements of reactivity for the various fused-ring aromatic hydrocarbons. Hydrogenation of pyrene, which contains three
1104 Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994
fused rings, is 2-3 times faster than hydrogenation of phenanthrene, which contains three fused rings. [However, the hydrogenation rate constant characterizing the three-fused-ring 4,5-dihydropyrene is apparently greater than that of pyrene, but we discount this value because of the large experimental uncertainty (more than i50% ) (Girgis and Gates, 1994a).] Furthermore, hydrogenation of the triaromatic phenanthrene is more rapid than that of diaromatics appearing in its network (e.g., 1,2,3,4tetrahydrophenanthrene). Although naphthalene hydrogenation was apparently faster (Table 6),the concentration of polynuclear aromatics was lower (1.37 vs more than 5 mmol/L in experiment Bl). Hydrogenation of the monoaromaticswas very slow, as shown by the absenceof decalins and phenyldecalins in experiments with naphthalene and phenylnaphthalene as reactants (Table 6) and indirectly by the absence of perhydro products in all the hydrocarbon networks (Figure 1). We can summarize the reactivities as follows, with rate constants (in the standard units) given in parentheses:
-
(1.1-1.5)
four-fused-ring aromatics
-
(0.42445)
three-fused-ring aromatics
(0.24)
two-fused-ring aromatics
(a. 0.01)
monoaromatics
saturates (1)
Fluoranthene is hydrogenated more rapidly than either phenanthrene or pyrene. Hydrogenation of the 1,2,3,lob-fluoranthene, the primary hydrogenation product formed in greatest yield, is also rapid, giving substantial yields of decahydrofluoranthenes (Figure 1). The hydrogenation rate of the nonplanar 1,2,3,10b-tetrahydrofluoranthene, which consists of two unconjugated monoaromatic moieties and can be thought of as a hindered biphenyl, is much faster than those of monoaromatics. This result shows clearly the effect of the strained fivemembered ring in increasing reactivity and suggests that hydroprocessing of fluoranthenes would require high HP consumptions. The unique reactivity pattern of the five-membered ring also manifests itself in the formation of the ring-opening products l-phenylnaphthalene and 5-phenyltetralin detected as fluoranthene products and grouped as cracking products in Figure 1. Ring-opening reactions are expected to be acid catalyzed (Pines, 1981). However, neither ringopening nor isomerization products of phenanthrene or pyrene that contained methylcyclopentane rings were detected. We conclude that the Ni-Mo catalyst is insufficiently acidic to catalyze the latter reaction and that formation of the five-membered ring cracking products is a consequence of the unique reactivity of the fivemembered ring, possibly involving catalysis by weakly acidic sites. The hydrogenations of the aromatic hydrocarbons are all reversible. The equilibrium limitation is especially severe for pyrene. Higher H2 concentrations than those used in this work would be needed to increase the conversions of the fused-ring hydrocarbons. It is anticipated that in commercial reactors, in which high liquidphase H2 concentrations are present, the equilibrium limitation would be much less severe than in the experiments described here. Furthermore, the strong equilibrium limitation for pyrene hydrogenation implies that data allowing comparison of conversions should be interpreted with caution because, at high space velocities, the pyrene
conversion was higher than that of phenanthrene, whereas the reverse held at lower space velocities. Dibenzothiophene HDS gives biphenyl selectively and thus occurs via the path of least H2 consumption. Under conditions more representative of commercial practice, the H2 consumed in dibenzothiophene HDS will be larger because of the larger contribution of the pathway forming cyclohexylbenzene. The HDO of dibenzofuran was too slow to be measured accurately. The rapid reaction of 5,6,7,8-tetrahydro-l-naphthol precluded the determination of rate constants characterizing its disappearance. The estimated rate constants for the remaining reactions in the network (Figure 1) are characterized by large experimental uncertainties, as indicated by the poor run-to-run reproducibility (Girgis, 1988). Only the relative values of the rate constants (selectivities) were determined with satisfactory reproducibility. The network shown in Figure 1is only a rough representation of the data. The rapidity of the HDO of 5,6,7,8-tetrahydro-l-naphthol points to one of the limitations of our approach of attempting to determine reactivities and reaction networks simultaneously for species of widely varying reactivity. The HDO of 5,6,7,8-tetrahydro-l-naphthol evidently proceeds by two paths, with tetralin produced in one path and octalins in the other (Girgis and Gates, 1994a). The octalins are postulated to be formed from dehydration of a perhydro intermediate (l-decalol). Some of the octalins were converted to give tetralin or naphthalene. It was assumed that all of the octalins were converted solely to tetralins because this was the simplest assumption giving a good representation of the data. Thus, the higher H2 consumption pathway involving octalin formation is partially offset because the octalins are dehydrogentated to give tetralin (and possibly naphthalene) to some extent. The hydroxylgroup is an activating ortho-para director in electrophilic substitution reactions (Streitwieser and Heathcock, 1976). Formation of the postulated l-decalol from 5,6,7,8-tetrahydro-l-naphthol is a hydrogenation reaction, not an electrophilic substitution. But electrophilic addition reactions to olefins are well-known reactions in organic chemistry. We speculate that the initial hydrogenation of the 5,6,7,8-tetrahydro-l-naphthol proceeds by protonation at the ring containing the hydroxyl substituent at a position ortho or para to the latter, yielding four resonance structures, including a stable oxonium ion (Streitwieser and Heathcock, 1976). Hydrideaddition may then follow, forming a cyclic diolefin (a dihydro intermediate), which may react by two different paths: (1) rapid hydrogenation to give the l-decalolor (2) dehydration to give tetralin. The relative rates of the two postulated pathways would depend on the coverage of the catalyst surface with hydrogen, which is influenced by the H2 concentration in the fluid phase. Inhibition by 5,6,7,8-Tetrahydro-l-naphthol.Inhibition by 5,6,7,8-tetrahydro-l-naphthol (or the water formed from it) of the reactions in the networks of the three hydrocarbons and dibenzothiophene was very weak (Tables 8-11]. Consequently,the complete HDO of 5,6,7,8tetrahydro-l-naphthol is inferred to be the result of an intrinsically high reactivity rather than a high surface coverage by this organo oxygen compound. We caution, however, that because of the low reactant concentrations used in this work, the observed inhibition effects are likely to be weaker than those observed in trickle-bed reactors with higher reactant concentrations. Inhibition of aromatic hydrogenation by either water or organooxygen compounds has not been reported. But
Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994 1105 Table 8. Reproducibility of Pseudo-First-Order Rate Constants in the Phenanthrene Network (Figure 1) and Weak Inhibition by 5,6,7,8-Tetrahydro-l-naphthola ~
- - + + -
reaction 9,10-dihydrophenanthrene 9,lO-dihydrophenanthrene phenanthrene + Hz phenanthrene + 2Hz 1,2,3,4-tatrahydrophenanthrene 1,2,3,4-tetrahydrophenanthrene phenanthrene + 2H2 1,2,3,4-tetrahydrophenanthrene 2Hz sym-octahydrophenanthrene 1,2,3,4-tetrahydrophenanthrene 2Hz asym-octahydrophenanthrene
phenanthrene + Hg
~~
~~
pseudo-fist-order rate const, L/(g of catalyst-h) expt B1 expt B2 expt A1 0.288 0.264 0.296 1.19 1.09 1.20 0.157 0.153 0.155 0.243 0.232 0.0979 0.202 0.197 0.216 0.0430 0.0503 0.0771
Feed compositions are given in Table 4.
0
Table 9. Reproducibility of Pseudo-First-Order Rate Constants in Proposed Pyrene Network (Figure 1) and Weak Inhibition by S,6,7,8-Tetrahydro-1-naphthola pseudo-first-order rate const, L/(g of catalyst-h) reaction expt B1 expt B2 expt AI 1.33 1.11 0.932 pyrene + Hz 4,bdihydropyrene 4,Bdihydropyrene pyrene + Hz 5.04 4.30 3.33 4,5-dihydropyrene + Hz 4,5,9,10-tetrahydropyrene 2.87 4.71 4.40 4,5,9,10-tetrahydropyrene 4,5-dihydropyrene + H2 25.3 26.9 28.0 0.196 0.187 0.195 pyrene + 3H2 sym-hexahydropyrene sym-hexahydropyrene pyrene + 3Hz 2.74 2.47 2.43 sym-hexahydropyrene asym-hexahydropyrene 17.4 17.5 11.3 asym-hexahydropyrene sym-hexahydropyrene 24.1 22.8 14.3
--- --
0
Feed compositions are given in Table 4.
Table 10. Reproducibility of Pseudo-First-Order Rate Constants in Proposed Fluoranthene Network (Figure 1) and Weak Inhibition by 5.6.7.8-Tetrah~dro-l-na~hthol* pseudo-fist-order rate const, L/(g of catalyst-h) expt B1 expt B2 expt A1 reaction 1.73 1.72 1.74 fluoranthene + 2Hz 1,2,3,1Ob-tetrahydrofluoranthene 1,2,3,1Ob-tetrahydrofluoranthene fluoranthene + 2Hz 1.81 1.83 1.89 0.0740 0.0798 0.0854 fluoranthene + 3Hz hexahydrofluoranthene hexahydrofluoranthene fluoranthene + 3Hz 0.228 0.250 0.189 0.387 0.373 0.391 1,2,3,10b-tetrahydrofluoranthene + 3Hz decahydrofluoranthene decahydrofluoranthene 1,2,3,10b-tetrahydrofluoranthene + 3Hz 0.365 0.356 0.297 0.137 0.135 0.143 1,2,3,10b-tetrahydrofluoranthene + nHz cracking products + nHz cracking products 1,2,3,1Ob-tetrahydrofluoranthene 0.854 0.661 0.245
- --
0
Feed compositions are given in Table 4.
Table 11. Reproducibility of Pseudo-First-Order Rate Constants in Dibenzothiophene and Dibenzofuran Reaction Networks and Weak Inhibition by 5,6,7,8-Tetrahydro-l-naphthola pseudo-first-order rate const, L/(a of catalyst-h) reaction expt B1 expt B2 expt A1 0.226 0.210 dibenzothiophene + 2Hz biphenyl + HzS 0.231 0.0343 0.0315 0.0520 dibenzothiophene + 5Hz cyclohexylbenzene + HzS 0.0208 0.0192 dibenzofuran + nH2 single-ring products 0.0460 ~
0
--
Feed compositions are given in Table 4.
in examining the effect of water on the reactions of the dibenzothiophene reaction network, Krishnamurthy and Shah (1982)found that the pseudo-fiist-order rate constant for the hydrogenation of biphenyl to give cyclohexylbenzene decreased by 1order of magnitude upon addition of 1wt % water to the feed. However, this apparent strong inhibition of biphenyl hydrogenation is inconsistent with their observation that the pseudo-first-order rate constant for dibenzothiophene conversion to give cyclohexylbenzene increased by about 20% upon addition of the same amount of water, since cyclohexylbenzene formation from dibenzothiophene proceeds by slow hydrogenation of dibenzothiophene. Krishnamurthy and Shah's results are therefore regarded as inconclusive in this regard. Mild inhibition of HDS reactions by phenolic oxygen compounds has been reported. Obedbunmi and Ollis (1983) expressed inhibition of benzothiophene HDS by rn-cresol at 375 "C by a Langmuir-Hinshelwood expression. The results of Odebunmi and Ollis are thus consistent with the weak inhibition observed here (Girgis, 1988).
Conclusions The results reported here, in combination with the reaction networks developed in the sequel (Girgis and Gates, 1994a),constitute the first quantitative evaluation of reactivities in catalytic hydroprocessing that takes account of competitive reactions and the formation of intermediate products. The reactivities of the individual compounds span 4 orders of magnitude. Only order-ofmagnitude estimates could be determined for the most reactive and least reactive feed components. The reactivity of fused six-ring aromatics increases with the number of rings, but the change is smaller than the order-of-magnitude difference in reactivity between benzene and naphthalene. Phenanthrene and pyrene are hydrogenated selectively to give dihydro species having hydrogenated central rings. Fluoranthene must be considered in a separate compound class from fused sixmembered-ring aromatics (e.g., naphthalene, phenanthrene, and pyrene) because it is hydrogenated more
1106 Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994
rapidly owing to the strain in its five-membered ring. Dibenzothiophene HDS gives biphenyl selectively and thus occurs via the path of least H2consumption;in commercial practice, when the Hz concentration is higher, the contribution of a pathway leading to the formation of cyclohexylbenzene is expected to be greater. The HDO of dibenzofuran, the least reactive of the feed components, requires higher H2 concentrations. In contrast, the HDO is very rapid, proceeding of 5,6,7,8-tetrahydro-l-naphthol by two parallel pathways, one of which is postulated to involve a perhydro intermediate. Neither 5,6,7,8-tetrahydro-1-naphthol nor the water formed from it significantly inhibits dibenzothiophene HDS or hydrogenation of the aromatic hydrocarbons under the conditions examined.
Literature Cited Allen, D. T.; Petrakis, L.; Grandy, D. W.; Gavalas, G. R.; Gates, B. C. Determination of Functional Groups of Coal-derived Liquids by NMR and Elemental Analysis. Fuel 1984,63,803-809. Allen, D. T.; Grandy, D. W.; Jeong, K.-M.; Petrakis, L. Heavier Fractions of Shale Oils, Heavy Crudes, Tar Sands, and Coal Liquids: Comparison of Structural Profiles. Znd. Eng. Chem. Process Des. Dev. 1985,24,737-742. Berty, T. E.; Reamer, H. H.; Sage, B. H. Phase Behavior in the Hydrogen-Cyclohexane System. J. Chem. Eng. Data 1966, 11, 25-30. Fishel, N. A,; Auvil, S.R.; Gross, D. E.; o-Phenylphenol from Phenol: A Two-step Selective Substitution Process. Catalysis in Organic Syntheses; Academic Press: New York, 1980. Girgis, M. J. Reaction Networks, Kinetics, and Inhibition in the Hydroprocessing of Simulated Heavy Coal Liquids. Ph.D. Thesis, University of Delaware, Newark, 1988. Girgis, M. J.; Gates, B. C. Reactivities, Reaction Networks, and Kinetics in High-pressure Catalytic Hydroprocessing. Znd. Eng. Chem. Res. 1991,30,2021-2058. Girgis, M. J.; Gates, B. C. Accepted for publication in Znd. Eng. Chem. Res., 1994a. Girgis, M. J.; Gates, B. C. Unpublished results, 1994b. Grandy, D. W.; Li, C.-L.; Gates, B. C.; Petrakis, L. Catalytic Hydroprocessing of SRC-I1Heavy Distillate Fractions 5. Conversion of the Acidic Fractions Characterized by Gas Chromatography/ Mass Spectrometry. Ind. Eng. Chem. Process Des. Deu. 1986,25, 40-48. Heller, S. R.; Milne, G. W. A. EPAINZH Mass Spectral Data Base; National Bureau of Standards, U.S. Department of Commerce: Washington, DC, 1981. Katti, S. S. Catalytic Hydroprocessing of the Neutral-Oils, Basic, and Neutral-Resin Fractions Obtained from Hydroliquefied Coal. Ph.D. Thesis, University of Delaware, Newark, 1984. Katti, S.S.;Westerman, D. W. B.; Gaten,B. C.; Y ounglees,T.; Petrakis, L. Catalytic Hydroprocessingof SRC-II Heavy Distillate Fractions. 3. Hydrodesulfurization of the Neutral Oils. Znd. Eng. Chem. Process Des. Deu. 1983,23, 773-778. Katti, S. S.;Gates, B. C.; Petrakis, L. Catalytic Hydroprocessing of SRC-I1 Heavy Distillate Fractions. 6. Hydroprocessing of the Bases and Neutral Resins. Znd. Eng. Chem. Process Des. Deu. 1985,26,618-626.
Krishnamurthy, S.; Shah, Y. T. Interactions Between Dibenzothiophene,7,8Be~uinoline,and OxygenCompoundeDuring Heteroatom Removal. Chem. Eng. Commun. 1982,16,109-117. Krishnamurthy, S.; Panvelker, S.; Shah, Y. T. Hydrodeoxygenation of Dibenzofuran and Related Compounds. AZChE J. 1981,27, 994-1001. Kwart, H.; Schuit, G. C. A.; Gates, B. C. Hydrodesulfurization of Thiophenic Compounds: The Reaction Mechanism. J. Catal. 1980,61,128-134. Lapinas, A. T.; Klein, M. T.; Gates, B. C.; Macris, A,; Lyons, J. E. Catalytic Hydrogenation and Hydrocracking of Fluoranthene: Reaction Pathways and Kinetics. Znd. Eng. Chem. Res. 1987,26, 1026-1033. LaVopa, V.; Satterfield, C. N. Catalytic Hydrodeosygenation of Dibenzofuran. Energy Fuels 1987, I, 323-331. Li, C. L.; Xu, 2.;Cao, Z.-A.; Gates, B. C.; Petrakis, L. Hydrodeoxygenation of 1-Naphthol Catalyzed by Sulfided Ni-MolyAlzO,. AZChE J. 1985a, 31,170-174. Li, C.-L.; Xu, Z.-R.; Gates, B. C.; Petrakis, L. Catalytic Hydroproceasing of SRC-I1 Heavy Distillate Fractions. 4. Hydrodeoxygenation of Phenolic Compounds in the Acidic Fractions. Znd. Eng. Chem. Process Des. Dev. 198Sb, 24,92-97. Odebunmi, E. 0.;Ollis, D. F. Catalytic Hydrodeosygenation 11. Interactions between Catalytic Hydrodeoxygenation of m-Cresol and Hydrodesulfurization of Benzothiophene and Dibenzothiophene. J. Catal. 1983, 80,6575. Petrakis, L.; Allen, D. T.; Gavalas, G. R.; Gates, B. C. Analysis of Synthetic Fuels for Functional Group Determination. Anal. Chem. 1983a, 54, 1557-1564. Petrakis, L.; Ruberto, R. G.; Young, D. C.; Gates, B. C. Catalytic Hydroprocessing of SRC-II Heavy Distillate Fractions. 1. Preparation of the Fractions by Liquid Chromatography. Znd. Eng. Chem. Process Des. Dev. 1983b, 22, 292-298. Petrakis, L.; Young, D. C.; Ruberto, R. G.; Gates, B. C. Catalytic Hydroprooeesingof SRC-I1Heavy Distillate Fractions. 2. Detailed Structural Characterizations of the Fractions. Znd. Eng. Chen. Process Des. Dev. 1983c, 22, 298-305. Pines, H. The Chemistry of Catalytic Hydrocarbon Conversions; Academic Press: New York, 1981. Sandler, S. I. Chemical and Engineering Thermodynamics; Wiley: New York, 1979. Sapre, A. V.; Gates, B. C. Hydrogenation of Aromatic Hydrocarbons Catalyzed by Sulfided CoO-Mo09/y-A1203. Znd. Eng. Chem. Process Des. Dev. 1981,20, 68-73. Satterfield, C. N.; Yang, S. H. Catalytic Hydrodenitrogenation of Quinoline in a Trickle-Bed Reactor. Comparison withVapor Phase Reaction. Znd. Eng. Chem. Process Des. Dev. 1984,23, 11-19. Streitwieser, A., Jr.; Heathcock, C. H. Introduction to Organic Chemistry; MacMillan Publishing Co.: New York, 1976. Wiser, W. H.; Singh, S.; Qader, S. A.; Hill, G. R. Catalytic Hydrogenation of Multiring Aromatic Coal Tar Constituents. Ind. Eng. Chem. Prod. Res. Deu. 1970, 9, 350-357. Received for review September 27, 1993 Revised manuscript received January 31, 1994 Accepted February 16, 1994. ~~
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Abstractpublishedin Advance ACSAbstracts, April 1,1994.
