Znd. Eng. Chem. Res. 1995,34, 101-117
101
Polynuclear Aromatic Hydrocarbons Hydrogenation. 1. Experimental Reaction Pathways and Kinetics Styliani C. Korret and Michael T. Klein’ Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
Richard J. Quann Mobil Research and Development Corporation, Paulsboro Research Laboratory, Paulsboro, New Jersey 08011
The relationship between molecular structure and hydrogenation reactivity in heavy oil hydroprocessing was sought via the elucidation of the controlling reaction pathways and kinetics of one-, two-, three-, and four-fused ring compounds. Hydrogenation reactions of o-xylene, tetralin, naphthalene, phenanthrene, anthracene, pyrene, and chrysene and their multicomponent mixtures were studied in cyclohexane solvent using a presulfided CoMo/AlzOs catalyst in an l-L batch autoclave a t PH,= 68.1 atm and T = 350 “C.Quantitative network analysis allowed estimation of 45 hydrogenation rate parameters and a n equal number of equilibrium ratios for 36 aromatic and hydroaromatic compounds. These values were then used in the evaluation of five adsorption parameters for aromatic ring number-based lumps from the multicomponent mixture experiments. Three classes of hydrogenation were discerned, based on the magnitude of numerator rate parameters. The adsorption parameters clearly increased with increasing aromatic ring number.
Introduction The increased attention paid to catalytic hydroprocessing in recent years is due in part to stricter environmental legislation and the public demand for cleaner fuels. Hydroprocessing technology is unique in that it can handle the sulfur and nitrogen heteroatoms in heavy feeds while increasing the hydrogen to carbon ratio and thus the fuel value of the oil. Hydroprocessing chemistry involves bifunctional catalysts, where a metal function promotes hydrogenation and an acidic function promotes isomerization, ring opening, and dealkylation. The hydrogenation function is important from the point of view of product hydrogen to carbon ratio and in the establishment of the balance of olefin (aromatic) and alkane (naphthenic) characters of the molecule. Heavy oils can contain an appreciable amount of polynuclear aromatic hydrocarbons (PNAs). This has motivated keen interest in the discernment of hydrogenation pathways, kinetics, and mechanisms through the use of “model” or “pure component” experiments. Moreover, since hydrogenations are exothermic, higher temperatures applied to enhance acid center transformations, for example, benefit dehydrogenations more than hydrogenations. It is therefore of interest to examine not only rate but also saturation equilibrium issues in heavy oil catalytic hydroprocessing. The field has been recently reviewed (Moreau and Geneste, 1990; Girgis and Gates, 1991). A large body of information for single-ring aromatics hydrogenation exists (Aubert et al., 1988; Moreau et al., 1990). Naphthalene (Qader and Hill, 1972; Qader, 1973; Salim and Bell, 1982; Zeuthen et al., 1987), phenanthrene (Qader et al., 1973; Qader and McOmber, 1975; Wu and Haynes, 1975; Huang et al., 1977; Shabtai et al., 1978; Haynes et al., 1983; Lemberton and Guisnet, 1984; Salim and Bell, 19841, and anthracene (Qader and Hill,
* Author to whom correspondence should be addressed (
[email protected]). + Present address: E x o n Research and Engineering Company, P.O.Box 101,Florham Park, NJ 07932.
1972; Qader, 1973; Qader and McOmber, 1975; Salim and Bell, 1984) hydrogenation have been examined in considerable detail as well. Pyrene hydrogenation has been analyzed in the context of its hydrogen-donating ability for coal liquefaction (Qader and Hill, 1972; Shabtai et al., 1978; Haynes et al., 1983; Stephens and Chapman, 1983; Johnston, 1984; Stephens and Kottenstette, 1985). Polynuclear aromatic hydrocarbons containing five-membered rings (fluoranthene, fluorene) were examined more recently under hydrocracking conditions by Lapinas and co-workers (Lapinas et al., 1987; Lapinas, 1989). Finally, the equilibrium-related thermochemistry of these molecules has also been examined (Frye, 1962; Frye and Weitcamp, 1969; Shaw et al., 1977; Stein et al., 1977). In spite of this impressive literature base, the available data are often fragmented and catalyst dependent. Few generalizations with respect to reactivity can be made, since the impact of different conditions is frequently unclear. For example, there appears to be agreement that hydrogenation reactivity increases with the number of aromatic rings (Girgis and Gates, 1991; Neurock and Klein, 1993), but the effect of alkyl substituents and/or naphthenic rings is not clear. Opposing suggestions have been offered (Shabtai et al., 1978; Moreau and Geneste, 1990). The effect of competitive adsorption and subsequent inhibition (Bhinde, 1979; LaVopa and Satterfield, 1988) has not been pursued at length. There is often controversy surrounding the pathways of even the most extensively studied molecules, such as the direct conversion of dito tetrahydrophenanthrene. Finally, the results reported often do not contain detailed quantitative kinetics. Thus, the field is far from settled in terms of reactivity information. Quantitative networks are needed under consistent reaction conditions for a wide array of aromatics and hydroaromatics. This motivated the current study of hydrogenation of compounds bearing from one to four aromatic rings, such as o-xylene, tetralin, naphthalene, phenanthrene, anthracene, pyrene,
0888-5885/95/2634-0101$09.00/0 0 1995 American Chemical Society
102 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995
and chrysene. Hydrogenation yields the respective hydroaromatics, and carefhl product identification has enabled estimation of their rate and equilibrium parameters from experiments with the aromatics as reactants. Finally, multicomponent mixture experiments enabled the evaluation of adsorption constants for aromatic ring number-based lumps.
Experimental Procedure Materials and Equipment. Hydrogenation experiments were performed in a l-Lbatch autoclave, equipped with a spinning catalyst basket, reactant loader for good zero time determination, on-line data acquisition, and continuous sampling capabilities. A detailed description is available elsewhere (Landau, 1991). Reactions took place in the presence of 420 g of cyclohexane as a solvent and 68.1 atm of Hz at 350 "C. The total pressure, including cyclohexane vapor pressure at 350 "C, was 191.6 atm. Hydrogen back-pressure was kept constant throughout the reaction. The selected conditions were intended to emphasize hydrogenation rate measurements. Thus, sampling concentrated on the early hydrogenation stages. Nevertheless, long-time experiments were also scheduled to provide hydrogenation equilibrium information. The reacting mixture was in a one-phase supercritical state in all cases, as verified both by calculations employing the Virial equation of state (Reid et al., 1987) and also from the actual experimental vapor pressure data. The excellent material balance closures further attest to the absence of crystallization. Cyclohexane was subject to limited isomerization (x .e 2 x to methylcyclopentane during heatup, which continued during the typical reaction time of 3 h without any apparent effect of the presence of the aromatic reactant. An average additional conversion of x = 8 x was observed over the course of the reaction. The catalyst was 10 g of CoMo/AlzOs (1.7% Co, 7.0% Mo) in particles of 0.3 cm and was presulfided in the autoclave for 135 min at 400 "C in a stream of 10%HzS in Hz. CSz (1mL) was added with each experiment to keep the catalyst in a sulfided state. The catalyst was equilibrated for 10 h at 350 "C and 68.1 atm of Hz during three naphthalene hydrogenation experiments in cyclohexane (Landau, 1991). All experiments were performed using the same catalyst charge, and the steady-state catalytic activity was periodically verified by naphthalene hydrogenation kinetics (Korre, 1994). Routine sample analysis was performed immediately after sampling on a Hewlett-Packard 5880A flame ionization gas chromatograph employing a J&W Scientific DB-5 fused silica capillary column. A HewlettPackard 5970 mass selective detector connected t o the outlet stream of a Hewlett-Packard 5890 gas chromatograph was also used for product identification. Yields less than 0.1% could be safely detected at all cases. Response factors were established for the commercially available reactants and several of the products. For the bulk of the hydroaromatics, response factors were set equal to that of the aromatic with the same number of rings. Parameter Estimation. Nonlinear parameter estimation was performed using a Simplex optimization algorithm (Press et al., 1986) and the LINPAC DASSL predictor/corrector method ODE solver. The objective function F was the sum of the squares of the differences between experimental and calculated yields:
nmes i=l
The Langmuir-Hinshelwood-Hougen-Watson (LHHW)kinetics formalism was used to account for the competitive adsorption of all aromatic hydrocarbons (Froment and Bischoff, 1990). The rate law reflected the model of a reversible bimolecular surface reaction as the rate-determining step (Qader, 1973; Landau, 1991). Nevertheless, since there was no statistical difference between models for a unimolecular and a bimolecular surface reaction, the simpler case of unimolecular reaction was adopted. The reaction order in hydrogen was not considered explicitly, since its pressure was the same for all experiments and was kept constant throughout a run (Bhinde, 1979). Individual estimation of adsorption parameters for each compound examined was statistically uncertain because of the relatively low initial concentration of reactants combined with the multitude of products. Thus the adsorption parameters, Km, were lumped according t o the number 0 I1 I 4 of aromatic rings (Neurock and Klein, 1993). This required estimation of only five adsorption parameters, KL, for the whole series of experiments. Implicit is the approximation that the adsorption parameter is weakly dependent on the alkyl substituents, at least for the narrow range examined. This approximation leads to the inevitable loss of detail during the estimation of numerator rate parameters, k ~ .They thus implicitly contain an adsorption constant dependence. The validity of this approximation is assessed in detail in a following publication (Korre et al., 1995). Equation 2 summarizes the foregoing discussion in terms of the operative LHHW rate law used to reduce the experimental data to rate and adsorption parameters:
In eq 2, r~ (mol kg,,t-l s-l) is the rate of conversion of compound i to compoundj, Ci (mol L-l) is the concentration, k~ (L kg,,-l s-l) is the numerator rate parameter, including surface reaction and adsorption contributions, KU (molj/mol i) is the equilibrium ratio, Km (L mol-l) is the adsorption parameter of individual compounds, and KL(L mol-l) is the adsorption parameter of aromatic ring-number based lumps. It follows that the overall equilibrium constant is K,$ = K~~/(PHJ~, where a is the reaction stoichiometry w t h respect t o hydrogen. This is because the forward rate parameters contain a hydrogen partial pressure dependence. The overall rate of production of a given compound Ci was obtained as the summation of all the conversion rates involving species i. The rate parameters and equilibrium ratios were regressed individually for each compound, with the constraint of non-negativity. Equilibrium ratios were further constrained during parameter estimation by application of the Denbigh rule concerning equivalent pathways in a network. The parameter estimation strategy examined both relative and absolute rates, in turn. For a given compound, pure component and mixture experimental
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 103 Table 1. Experimental Plan for Pure Compound Hydrogenation NA' 1 1 2 3 3 4 4 a
reactant o-xylene tetralin naphthalene phenanthrene anthracene PPene chrysene
loading (g) 34.18 29.75 23.90 15.18 2.06 5.75 2.0
(h) 5.993 3.000 3.147 3.000 3.000 3.000 4.733
tmax
N A = number of aromatic rings.
Table 2. Initial Aromatics Distribution (wt %) for Mixture Experiment@ NA* reactant GAUS LCO SR TCFD EQUIL 1 2 3 4 4
o-xylene naphthalene phenanthrene pyrene chrysene loading(g) tmax (h)
13.00 20.04 33.96 20.11 12.99 20.02 6.939
13.01 61.51 22.02 3.50
20.00 14.01 14.00 32.00 0.00 20.01 20.01 20.00 7.000 7.638
24.00 19.00 26.00 19.01 12.00 20.00 6.295
0.00 27.80 27.56 27.92 16.72 17.94 9.574
a GAUS = Gaussian, LCO = light cycle oil, SR = straight run. TCFD = thermally cracked flashed distillate, EQUIL = equilibrium run. NA = number of aromatic rings.
data (where available) were used simultaneously. As a first step, parameter estimation was performed using the product yield vs reactant conversion data involving a particular compound (ri VS.XI). These relative rates eliminated the LHHW denominators and provided rate and equilibrium parameters that were independent of the mixture composition (eq 3):
In eq 3, CO(mom) is the initial reactant concentration. The rate parameters k~ and equilibrium ratios KG regressed by eq 3 were then held constant, and eq 2 was used for the estimation of adsorption parameters, Kl, using the yield vs time data, simultaneously for all experiments involving a particular compound and for all compounds in the mixture. This assured maximum agreement and between experiments with varying initial compositions (Korre, 1994). Experimental Design. The hydrogenation experimental plan is outlined in Tables 1 and 2. Purecompound experiments were complemented by polynuclear aromatics mixture experiments at different initial loadings. The model compounds of Table 1were selected so as to represent fundamental structural characteristics encountered in heavy oils. The structural parameter varied for the reactant selection was the aromatic ring number (one to four aromatic rings). Hydrogenation of
those structures yielded the respective hydroaromatics. Specifically, the hydrogenation reactions of o-xylene (Aldrich, 99%), tetralin (Aldrich, 99%), naphthalene (Aldrich, 99+%), phenanthrene (Aldrich, 98+%), anthracene (Aldrich, 97%), pyrene (Aldrich, 99+%), and chrysene (Aldrich, 98%) were studied. The mixture experiments summarized in Table 2 allowed quantitative assessment of competitive adsorption and inhibition interference. Single-ring aromatics were represented by o-xylene, two-ring aromatics by naphthalene, three-ring aromatics by phenanthrene, and four-ring aromatics by pyrene and chrysene. The initial distributions for the PNA mixture experiments summarized in Table 2 were selected so as to mimic aspects of the aromatic distributions in both straightrun and processed oil fractions (van der Eijk et al., 1990). The LCO distribution is triangular, centered at two-ring aromatics (i.e., naphthalene). The SR distribution was approximated by an inverted triangular distribution. The TCFD and EQUIL distributions had approximately equal amount of PNA's with two to four rings, but EQUIL did not include single-ring aromatics. The gaussian (GAUS) distribution was centered at three-ring aromatics (Korre, 1994). The cumulative wt % distribution is also of interest. The SR distribution had the highest content of four-ring aromatics, with 52.01 wt %, and the EQUIL distribution had the highest combined content in three- and fourring aromatics, with 72.20 wt % (Table 2). The LCO distribution had the lowest content in both cases (3.50 wt % four-ring aromatics, 25.52 wt % three- and fourring aromatics combined). These experiments are considered below on a compound by compound basis. Product identification issues are addressed first. Then the product rank in the network, as provided mainly by the yield and selectivity vs conversion information (Delplots), is considered (Bhore et al., 1990). Subsequent parameter estimation using the kinetics data with the proposed network affords rate, equilibrium, and adsorption parameters, as discussed above.
Kinetics and Thermodynamics Results o-Xylene. Reaction of o-xylene in 420. g of cyclohexane solvent was a t 68.1 atm of H2 and 350 "C. The conditions for the experiments involving o-xylene are presented in Tables 1-3. Reaction duration was 4.637 tf < 7.638 h, and the ultimate conversion was 0.138 < XM < 0.183. The major products were truns-1,2dimethylcyclohexane, cis-1,2-dimethylcyclohexane, mxylene, and p-xylene. Traces of cis- and truns-1,3dimethyl- and -1,4-dimethylcyclohexaneswere also detected. All products were identified by co-injections and mass spectral information. The kinetics of conversion corresponding t o the mixture experiments in Table 2 are presented in Figure 1. The highest conversion for a particular time was observed for experiments with the LCO distribution
Table 3. Mass Balances and Maximum Conversions for Compounds in the Mixture Experiments
Pure Gauss LCO
SR TCFD EQUIL
o-xylene MB closure (%) 0.984f 2.07 0.997 f 2.01 0.989 0.79 0.994f 1.34 1.005 f 1.67 -
*
xmax
0.183 0.151 0.156 0.155 0.145
-
naphthalene phenanthrene PFene MB closure (%) xmax MB closure (%) xmax MB closure (%) 0.981 f 3.101 0.952 0.990 f 2.564 0.977 0.945f 1.900 0.976f 3.447 0.984 0.973f 1.381 0.959 0.966 f 4.585 0.958f 3.987 0.976 0.988f 3.676 0.959 0.993f 6.536 0.978f 4.007 0.982 0.970f 3.517 0.953 0.944f 4.707 0.985 f 7.062 0.989 0.994f 7.297 0.959 0.973f 5.546 0.972f 4.763 0.998 0.978f 7.923 0.996 0.952f 8.866
xmax 0.858 0.832 0.851 0.855 0.887 0.915
chrvsene MB closure (%) 0.969f 2.826 0.950 f 7.035 0.962 f 3.834 0.956f 10.68 0.976i 7.067
xmax 0.999 0.992
-
0.984 0.992 1.000
104 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995
e 0.14 0
'E
0.12
El
:: 0.1 u" 0.08
=,Ic 0.06 8
0.04
8
0.02
0 0
120
240
480
360
Time (min)
Figure 1. Effect of reacting mixture composition on o-xylene conversion kinetics. ((0)GAUS; (+) LCO; (0) SR; (A)TCFD; (-1 Est.) 0.16
1i1
I
0.14 0.12
0
0. I
.I0.08
+
0.06
the selectivity vs conversion Delplots (Bhore et al., 1990) of Figure 2(ii). A replicate experiment produced identical results, as is depicted in Figure 2(ii). Thus the apparent zero initial selectivity to hydrogenation, relative to isomerization, is telling. Perhaps subtleties in the differences in adsorption of the xylene isomers are manifested as apparent autocatalytic kinetics. This would have caused the dimethylcyclohexaneproduction rate to increase as the o-xylene concentration decreased. However, the maximum conversion is not likely enough to account for the observed production acceleration. This is reflected in a small regressed value of adsorption parameter (maximum 1.364 m3 kmol-I, with eq 21, which cannot account for the observed kinetics. The apparent secondary rank of the dimethylcyclohexanes could also result from the autocatalytic action of hydrogen transfer from the hydrogenated product, according to the combined unimolecular and bimolecular rate expression of eq 4. The plausibility of this scheme derives from the several conjugated structures possible after abstraction of a tertiary hydrogen. This somewhat empirical scheme fit the experimental data extremely well. This is demonstrated by the continuous lines of Figure 2(i).
0.04 0.02
n 0
120
60
180
240
300
360
Time (min)
(4)
1
0.8
2! *
-
.- 0.6 .-*
-g
0.4
m Y
0.2 0 0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
ortho-xylene conversion x
Figure 2. Kinetics of o-xylene hydrogenation. (i) Yield vs time and (ii) selectivity vs conversion plots. (i) Solid curves represent the network correlation with eq 4.Dotted curves represent the estimated network correlation of Figure 4.F = 1.636 x deviation = 5.720 x ((+) o-xylend7; (0)cis- and truns-l,2dimethylcyclohexane; (W) metu- and para-xylene; (A) 1,3-dimethyland 1,4-dimethylcyclohexane.)(ii) Open symbols represent replicate experiment; (0,0) truns-l,2-dimethyl- plus cis-1,2-dimethylcyclohexane; (W, 0 ) m-plus p-xylene; (A,A) 1,3-dimethyl- plus 1,4-dimethylcyclohexane.
because of the lower loading in aromatics with higher ring numbers. Equivalently, the lowest conversions were observed for the SR hydrogenation, with the GAUS and TCFD distributions inhibiting in similar degree. The evolution of products with time for the reaction of pure o-xylene is presented in Figure 2(i). Clearly, m- and p-xylene were primary products, and their concentrations reached constant values after 30 min. cis- and truns-l,2-dimethylcyclohexanes were the major hydrogenation products observed, in very similar yields. Relative to the behavior of the xylene products, the dimethylcyclohexanes, including both cis- and truns-1,2dimethyl, formed with what appeared to be zero slope at t = 0 in Figure 2(i). This is exposed more clearly by
However, the quantitative aspects of eq 4 undermined the hydrogen-transfer scenario. Note that the best fit equilibrium ratio is less than 1 (0.175). This implied that o-xylene would be more abundant than 1,2-dimethylcyclohexane at equilibrium, contradicting previous studies that reported equilibrium ratios for singlering hydrogenations well over 10 at 350 "C(Frye, 1962; Frye and Weitcamp, 1969). This suggested that the critical slopes in Figure 2(i) that led to the hydrogentransfer hypothesis were overemphasized a t the low conversions. This suspicion motivated a set of experiments aimed a t broadening the range of conversions to probe the hydrogen-transfer hypothesis further. A series of experiments at conditions identical to those for o-xylene were thus performed where o-xylene was reacted in a mixture with 1,2-dimethylcyclohexanes. Figure 3a,b presents results from 0 wt %, 50 wt %, and 80 wt % 1,2-dimethylcyclohexanesin o-xylene. The net 1,2dimethylcyclohexaneyields (Figure 3a) exhibited similar behavior at t = 0 for all three experiments. Their subsequent evolution with time, however, was much different than the equilibrium predicted based on the autocatalytic rate law (eq 4) and its parameters. Parameter estimation on eq 2 was thus performed for all three experiments simultaneously. The results are summarized in the numerical values of the combined unimolecular and bimolecular rate expression of eq 5 and the fair fit of Figure 3a,b. -dy1,2 - r3.16 x
dt
10-*(Yortho - y1,2/5.61 x
lo4) + 2.69 x
0.14 0.12
0
60
0.12
0.1
.
120 180 240 300 360
0
/
2 0 * 0 0.06 . O 8 i m n , 0.04
m n I30
0 0
60
., 0.4
0.6
0.8
1
Figure 4. Proposed network for xylenes hydrogenation. (k in L/(kg,t s). Numerator rate parameters underlined).
Yield
Time (min)
0.02
0.2
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 106
,
120
~
~
240 Time (min)
180
300
., 360
1
420
Figure 3. Initial composition dependence of xylene hydrogenation product distributions. (a) Net 1,2-dimethylcyclohexaneyield vs time for different initial compositions. (Curves represent paramestimated eter estimation results with eq 5. F = 2.439 x deviation = 1.030 x low2.(0,0 ) 100 w t % o-xylene;(A) 50 w t % o-xylene, 50 wt % 1,2-dimethylcyclohexanes; (H) 20 w t % o-xylene, 80 wt % 1,2-dimethylcyclohexanes).(b)Reaction time as calculated from o-xylene hydrogenation yields for different initial compositions. (Curves represent parameter estimation results using eq 5: (W) o-xylene; (A) 1,2-dimethylcyclohexanes.)(c) 1,3-Dimethylcyclohexane yield from m-xylene for different initial compositions ((HI Reactant, pure m-xylene;(0) reactant, 50 w t % m-xylene, 50 w t % 1,2-dimethylcyclohexanes.)
Note that the bimolecular rate parameter is two orders of magnitude lower than in eq 4, and the equilibrium ratio is much larger than 1. These results essentially nullified the hydrogen-transfer hypothesis. The bestfit total yield profiles are presented in Figure 3b as the curves, along with the experimental data. The time for a desired conversion was calculated from eq 5, and was 28.0 h for 0.470 conversion and 67.2 h for 0.818 conversion. Note that, on the scale of Figure 3b, the “autocatalytic behavior” at very low conversions is at most a “boundary layer” phenomenon. The absence of hydrogen-transfer reactions was also demonstrated by two additional experiments involving m-xylene as the reactant, with and without the candidate hydrogen transfer additive 1,2-dimethylcyclohexane. Figure 3c demonstrates that the yields of 1,3dimethylcyclohexane were almost identical in both cases. Thus, the hydrogen-transfer hypothesis as an explanation of the apparent secondary rank of the dimethylcyclohexanes was abandoned. The bifunctional catalyst thus promoted both isomerization and hydrogenation reaction pathways. The isomerizations to m- and p-xylene were substantially faster than hydrogenation; recall that the products of the latter reaction appeared with a low initial rate. Although the xylene isomers yields were low-the maximum yield to m- and p-xylene was 0.0131-they were achieved within the first 30 min of reaction. This suggested that the rates were high. Indeed, Figure 2b indicated that the initial selectivity to m- and p-xylene was close to unity. This motivated construction of the network of o-xylene reactions presented in Figure 4. o-Xylene hydrogenates to 1,2-dimethylcyclohexanesin parallel with its isomerization to m- and p-xylene. The latter in turn hydrogenate to 1,3-dimethyl- and 1,4-dimethylcyclohexanes.
Interconversion between 1,2- and 1,3/1,4-dimethyl cyclohexanes is also included. p-Xylene yields were less than 10% of m-xylene yields and thus not always quantitatively detectable. Consequently, m- and p xylene were lumped into one component. The same was true for 1,Cdimethyl- and 1,3-dimethylcyclohexanes. Parameter estimation to this network exploited the range of available experimental compositions. Thus, the pure o-xylene data, the 50 w t % data, the 80 w t % 1,2dimethylcyclohexane in o-xylene data, as well as the m-xylene hydrogenation experimental data combined to give the parameter set of Figure 4. Isomerization of oto m-xylene was approximately 1 order of magnitude slower than hydrogenation of o-xylene to 1,a-dimethylcyclohexanes. Hydrogenation of o-xylene proceeded with a higher rate parameter than hydrogenation of the m- and p-xylene lump; both reactions were virtually irreversible. Finally, the rate parameter for isomerization of 1,2-dimethyl- to 1,3-and 1,4-dimethylcyclohexanes was negligible. This parameter set provided the calculated yield profiles corresponding to conversion interval 0-0.181 presented as the dotted curves in Figure 2(i). The reaction patterns discovered above were repeated for o-xylene hydrogenation in the PNA mixture (Table 2). No shifts in selectivity were observed, although o-xylene conversion was inhibited in the presence of higher aromatics. This allowed for evaluation of adsorption parameters for the aromatic ring number-based lumps, as discussed below. Tetralin. Reaction of 29.75 g of tetralin took place in 420. g of cyclohexane solvent at 68.1 atm of Hz and 350 “C. Conversion after 3 h was 0.137. The major products were trans-decalin, cis-decalin, and naphthalene, which accounted for 0.991 f 0.011 material balance closure. The balance consisted of traces of decalin isomers, 1-and 2-methylindans and ring-opening products (total max. 1.1%).Including those products, the material balance closure was 1,000 f 0.009. All products were identified by coinjection and mass spectral information. Figure 5 summarizes the evolution of product yields and selectivities with tetralin conversion. Naphthalene was the product with the highest initial selectivity, followed by trans- and cis-decalin. Notice the similarities between the Delplots for tetralin in Figure 5 and those for o-xylene in Figure 2. Decalins appeared to be fairly unreactive under these conditions, while naphthalene hydrogenated back into tetralin. All three major products were thus considered primary in the network of Figure 6. The interconversion between cisand trans-decalin was also included for completeness. Parameter estimation to the network of Figure 6 using the tetralin yield vs conversion data and eq 3 is summarized as the smooth curves in Figure 5. The equilibrium ratio for isomerization of trans- t o cisdecalin was obtained as the ratio of the two hydrogenation equilibrium ratios, according to Denbigh’s rule, and is denoted in italics.
106 Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995 0.08 0.07 0.06
.
-
.
0.04 . 0.05
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
og
x Tetralin
I
0
240 Time (min)
120
360
480
Figure 7. Effect of reacting mixture composition on naphthalene conversion kinetics. ((0) GAUS; (+) LCO; (0)SR (A)TCFD; (A) EQ; (-1 Est.)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
x Tetralin
Figure 5. Kinetics of tetralin hydrogenation. Yield (i) and selectivity (ii) vs conversion plots. (Curves represent the parameter
0
-x. c3 1 1 u
0.6
0.8
1
0
0.2
x Naphthalene
estimation results of Figure 6 ((0)trans-decalin; ( 0 )cis-decalin; (W naphthalene).) trans
0.2 0.4
0.4
0.8
1
0.i 0.i
i
0.6
x Naphthalene
0.25
e
.-
? .m
0.361
h
0.2 0.15 0.1
0.05
0 0
m
0.2 0.4 0.6
0.8
x Naphthalene
1
o
0.2
oi
x Naphthalene
Figure 8. Kinetics of naphthalene hydrogenation. Yield (i) and
L/(kg,,t 9). Numerator rate parameters underlined. F = 1.75 x estimated deviation = 0.009 35.)
selectivity (ii) vs conversion plots for tetralin (a) and cis-decalin (b). (Curves represent the parameter estimation results of Figure 9. (W Pure; (0) GAUS; (+I LCO; ( 0 )SR (A)TCFD; (A) EQ; (-) Est.)
Figure 6 also summarizes the kinetics. The overall rate parameter for hydrogenation of tetralin was slightly higher than that for hydrogenation of o-xylene. Considering the similar position of the alkyl substituents with respect to the aromatic ring, this enhancement may be attributed to the presence of the saturated ring, either by additional inductive effect or by improved adsorption properties. These results also show that the naphthalenic moiety hydrogenated 2 orders of magnitude faster than the benzenic moiety. To quantify this effect further, naphthalene was employed as a reactant. Naphthalene. Reaction of naphthalene in 420.g of cyclohexane solvent was at 68.1 atm of H2 and 350 "C. The conditions for the experiments involving naphthalene are presented in Tables 1-3. Reaction duration was 3.147 < tf < 9.574 h, and maximum conversion varied between 0.952 < XM 0.998. The major hydrogenation products were tetralin, trans-decalin, and cisdecalin, accounting for more than 95% of the initial weight of naphthalene. Traces of methylindans, alkylbenzenes, and alkylcyclohexaneswere detected as well. Figure 7 summarizes the kinetics of conversion for the mixture experiments of Table 2. The highest conversion for a particular time was observed for experiments with the LCO composition distribution; the
lowest conversions were observed for the SR and EQUIL hydrogenation experiments. This may be attributed to the fact that although EQUIL distribution had a lower content in four-ring aromatics than SR, it had a higher combined content of three- and four-ring aromatics. These inhibition effects imply a higher adsorption parameter for the three-ring than for the two-ring aromatics lump. The reaction network was revealed by the selectivity vs conversion Delplots of Figure 8(ii). The initial selectivities show that tetralin was the sole primary product, whereas cis- and trans-decalins were clearly secondary. Note that the data of Figure 8 coincide for all experiments. This shows that the numerator rate and equilibrium parameters are concentration-independent and attests t o the validity of eq 3 and the parameter estimation approach. Parameter estimation to the implied network of Figure 9 was performed using the rate expression of eq 3. The equilibrium ratio for isomerization of trans- to cis-decalin was constrained as above. Figure 8 shows that the resulting best-fit numerator parameters of Figure 9 provide good agreement between model and data. Note that the kinetic parameters are quite close
Figure 6. Proposed network for tetralin hydrogenation. (k in
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 107 The selectivity vs conversion plots of Figure ll(ii) provided information about the reaction network. Diand tetrahydrophenanthrenes were clearly primary products, the former appearing with higher initial selectivity (projected 0.7 vs 0.3). On the other hand, both octahydrophenanthrenes exhibited secondary behavior. Perhydrophenanthrene appeared very late in the reaction network, and only with low yields. cis The phenanthrene network contains an ambiguity Figure 9. Proposed network for naphthalene hydrogenation. (k concerning the hydrogenation pathway and reactivity in I.4kg-t s). Numerator rate parameters underlined. F = 0.123; estimated deviation = 0.0428.) of dihydrophenanthrene. This is still an open question in the literature. Lemberton et al. (Lemberton and Guisnet, 1984) and Wu et al. (Wu and Haynes, 1975) suggest interconversion of di- to tetrahydrophenanthrene. The driving force behind this reaction may be energetic, i.e., the recovery of the two fused aromatic ring system, in place of the two isolated aromatic rings. Additionally, the dihydrophenanthrene molecule is sterically strained, as the hydrogenation of the middle ring forces the aromatic rings out of coplanarity (Shabtai et al., 1978). In contrast, more recent studies (Girgis and Gates, 1991), employing dihydrophenanthrene as the reactant a t comparatively milder conditions than the 0% present study, suggest that its main reaction was 0 120 240 360 480 dehydrogenation to phenanthrene. Time (min) The foregoing motivated the present studies of the Figure 10. Effect of reacting mixture composition on phenancatalytic reaction of dihydrophenanthrene with CoMol threne conversion kinetics. ((0) GAUS;(+) LCO; (0) S R (A)TCFD A1203 and the standard conditions of 350 "C and 68.1 (A) EQ;(-1 Est.) atm of H2. As shown in Figure 12a(ii), the initial selectivity to phenanthrene approached unity at zero to those estimated from the experiment with tetralin conversion (0.58 conversion at the first 15 min), while as a reactant (Figure 6). the initial selectivity to tetra- and octahydrophenanPhenanthrene. Reaction of phenanthrene in 420. threnes approached zero. The underlying driving force g of cyclohexane solvent was a t 68.1 atm of H2 and 350 appeared to be thermodynamic, as demonstrated in "C. The conditions for the experiments involving phenanFigure 12b, where the evolution of the dihydrophenanthrene are presented in Tables 1-3. Reaction duration threndphenanthrene ratio with time was plotted for the tf < 9.574 h, and ultimate varied between 3.147 two experiments. In both cases, the ratio approached conversions of 0.953 < X M < 0.996 were observed. The a constant value of approximately 0.6, which was very identified products were dihydrophenanthrene, tetrahyclose to the equilibrium concentration ratio. Thus these drophenanthrene, symmetric and asymmetric octahyresults suggest that a reasonable phenanthrene network drophenanthrene, and perhydrophenanthrene. These would include a fast reversible reaction of phenanthrene accounted for more than 97% of the initial weight of to dihydrophenanthrene with no further reaction of phenanthrene at all cases. Traces of methylbiphenyls dihydrophenanthrene to deeper hydrogenation products. and biphenyl were also detected (total max. 3.5%). These observations led to the construction of the The kinetics of phenanthrene conversion at the mixnetwork of Figure 13. Hydrogenation proceeded in a ture compositions of Table 2 are presented in Figure ring-by-ring manner, phenanthrene t o tetra- to octa10. The inhibition due to the presence of higher and perhydrophenanthrene. Secondary hydrogenation aromatics is revealed in the lowest conversion for the of dihydrophenanthrene does not occur in Figure 13. SR and EQUIL hydrogenation mixtures and the highest Figure lla-c contains the results of parameter conversion for the LCO mixture. estimation as the smooth curves. The rate expression Resolution of the reaction pathways required careful of eq 3 and the phenanthrene network of Figure 13 were analysis of the product spectra. Identification of 9,lOused with all experimental data involving phenanthrene di-, 1,2,3,4-tetra-, and perhydrophenanthrene was and dihydrophenanthrene simultaneously. The asstraightforward, as they were the only GC peaks coroctahydrophenanthrene to perhydrophenanthrene equiresponding to molecular weight 180, 182, and 192, librium ratio was constrained by the three other equirespectively. Five GC peaks of molecular weight 186 librium ratios in the network to account for the reaction were detected, correspondingto octahydrophenanthrene path degeneracy and is denoted in italics in Figure 13, isomers. The three earlier-eluting GC peaks exhibited along with the best-fit parameters. There is very good identical MS fragmentation patterns, including fragagreement between experimental and calculated yield ments of molecular weight 43 and 57-signifying two profiles, as shown in Figures 11 and 12. fused saturated rings-and were associated with 1,2,3,4,The parameter estimation results invite scrutiny of 4a,9,10,10a-octahydrophenanthrene (as-octahydrophenanthrene) conformational isomers. The two laterqualitative structureheactivity relationships. Phenaneluting GC peaks exhibited a distinct MS fragmentation threne hydrogenation at the middle ring was kinetically favored over that at the terminal ring. This may be pattern with fragments of molecular weight 28-signifyattributed to the increased reactivity of the 9,lO-bridge ing a saturated ring fused with an aromatic ring-and were attributed to 1,2,3,4,5,6,7,8-octahydrophenan- (Moreau and Geneste, 1990). Notice that the equilibrium ratio for this reaction was less than one (0.5821, threne (s-octahydrophenanthrene)conformational isowhich indicates that more phenanthrene than dihydromers (McLafTerty, 1980). hWlS
M
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Figure 11. Kinetics of phenanthrene hydrogenation. Yield (i) and selectivity (ii) vs conversion plots for dihydrophenanthrene (a), tetrahydrophenanthrene (b), and as-octahydrophenanthrene(c). (Curves represent the parameter estimation results of Figure 13. (W) Pure; (0) GAUS; (+) LCO; (0) SR; (A)TCFD; (A) EQ; (-1 Est.)
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Figure 12. Kinetics of dihydrophenanthrene hydrogenation. (a) Yield (i)and selectivity (ii) vs conversion plots. (Curves represent the parameter estimation results of Figure 13. ( 0 )Phenanthrene; (A) dihydro-, (W) Tetrahydro-, (0)s-octahydro-, (0)as-octahydro-, (A) perhydrophenanthrene.) (b) Equilibrium between phenanthrene and dihydrophenanthrene. ((0)Dihydrophenanthrene reactant; ( 0 )phenanthrene reactant.)
phenanthrene is present at equilibrium. Rate parameters for hydrogenation of both two- and three-fused aromatic rings were 1 order of magnitude higher than
for the hydrogenation of a single aromatic ring. The presence of saturated rings also had an effect on reactivity. For example, the hydrogenation of octahydrophyanthrenes was much faster than that of either tetralin or o-xylene. These results are in qualitative agreement with those of Aubert et al. (19881, who suggested an anchoring effect of the naphthenic substituents on the catalyst surface. This is contrary to the results reported by Shabtai et al. (19781, who suggested that the bulkiness of the naphthenic rings would result in lower hydrogenation rates than less substituted molecules. Another revelation of the parameters in Figure 13 is the difference in hydrogenation rates between terminal and middle rings for the "ring-substituted" benzenic and naphthalenic moieties. The terminal ring in tetrahydrophenanthrene (naphthalenic moiety) hydrogenated faster than the middle ring. This may be attributed to inaccessibility of the internal ring t o hydrogenation due
Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995 109 1 - 1
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Figure 16. Effect of reacting mixture composition on pyrene conversion kinetics. ((0) GAUS (e)LCO; (0) S R (A)TCFD; (A) EQ; (-1 Est.)
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Figure 14. Kinetics of anthracene hydrogenation. Yield (i) and selectivity (ii) vs conversion plots. (Curves represent the parameter estimation results of Figure 15. (A) Dihydro-, (H) tetrahydro-, (0) s-octahydro-, (0) as-octahydro-, (A)perhydroanthracene.)
m Figure 15. Proposed network for anthracene hydrogenation. (k in L/(kg,,t s). Numerator rate parameters underlined. F = 0.0164; estimated deviation = 0.0158.)
to sterics. The same observations hold for as- vs s-octahydrophenanthrene hydrogenation. Anthracene. Reaction of 2.06 g of anthracene took place in 420. g of cyclohexane solvent at 68.1 atm of Hz and 350 "C. The major products were dihydroanthracene, tetrahydroanthracene, symmetric and asymmetric octahydroanthracene, and perhydroanthracene. These accounted for 0.968 f 0.043 of the material balance closure. The balance consisted of traces of hydrophenanthrene isomers, whose yields brought the material balance closure to 1.000 & 0.048. Product identification followed the reasoning detailed above for phenanthrene; the retention times and order were very similar. The kinetics of anthracene hydrogenation are summarized in Figure 14. Di- and tetrahydroanthracenes were clearly primary products, the former apparently appearing in higher initial selectivity, while octahydroanthracenes exhibited secondary behavior. This suggested the anthracene hydrogenation network to be as shown in Figure 15. Parameter estimation using eq 3 in the network of Figure 15 and the yield vs conversion data of Figure 14 provided a quantitative summary of the kinetics. As was the case for phenanthrene, the equilibrium ratio for as-octahydroanthracene hydrogenation was constrained, according to Denbigh rule. The resulting rate
parameters for the initial part of the anthracene network are larger than for phenanthrene, possibly due to anthracene's decreased resonance stabilization energy (Moreau and Geneste, 1990). The equilibrium ratio for the hydrogenation of the middle of the three rings is larger than in the phenanthrene network, but still less than unity. Deeper in the network, tetrahydroanthracene hydrogenation is overall slower than tetrahydrophenanthrene hydrogenation. The single-aromaticring hydrogenation rate parameters observed from octahydroanthracenes' hydrogenation are also lower than those regressed from phenanthrene network. The preferential hydrogenation of a terminal vs an internal ring in one- and two-aromatic ring systems was also seen in both tetra- and octahydroanthracenes. Overall, Figure 14 shows that the results of parameter estimation to the anthracene hydrogenation network proposed above are quite good. Pyrene. Reactions of pyrene took place in 420. g of cyclohexane solvent at 68.1 atm of Hz and 350 "C. The conditions for experiments involving pyrene are presented in Tables 1-3. Reaction duration was 3.147 < tf < 9.574 h, which afforded maximum conversions between 0.831 < XM < 0.915. The major identified products were dihydropyrene, tetrahydropyrene, symmetric and asymmetric hexahydropyrene, decahydropyrenes A and B, and perhydropyrene. Traces of alkylphenanthrenes and alkylbiphenyls were also detected. Figure 16 summarizes the kinetics of pyrene conversion for the mixture experiments of Table 2. A timeinvariant plateau was established in all cases after approximately 4 h. Pyrene conversion rates also decreased with increasing heaviness of the feed. The rate was highest for the lightest feed (LCO) and lowest for the heaviest feeds (SR and EQUIL). Pyrene conversion was more inhibited in the EQUIL composition distribution than in the SR distribution. Resolution of the reaction pathways required careful analysis of the product spectra. The identification of pyrene, 4,5-dihydropyrene, and perhydropyrene was straightforward from the mass spectra, as they were the only possible products with molecular weights 202,204, and 218, respectively. The two hexahydropyrene isomers were identified by co-injection of available shexahydropyrene (1,2,3,6,7,8-hexahydropyrene, Aldrich 94%). The presence of 4,5,9,10-tetrahydropyrenewas deduced as it was the only stable tetrahydropyrene, and in addition it eluted earlier than both hexahydropyrenes, signifying the presence of single aromatic rings. Three GC peaks corresponding to molecular weight 212
110 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 0.8
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Figure 17. Kinetics of pyrene hydrogenation. Yield (i) and selectivity (ii) vs conversion plots for dihydropyrene (a),tetrahydropyrene (b), and as-hexahydropyrene (c). (Curves represent the parameter estimation results of Figure 18. (H)Pure; (0) GAUS; (e)LCO; (0) SR (A) TCFD; (A) EQ; (-1 Est.)
were detected and attributed to decahydropyrene isomers. The earlier eluting isomer exhibited an MS fragmentation pattern distinctly different from the later two, and was identified with 1,2,3,3a,4,5,9,10,10a,lObdecahydropyrene by the NBS-REVE mass spectral library (decahydropyrene A), leaving the latter two GC peaks for 1,2,3,3a,4,5,5a,6,7,8-decahydropyrene (decahydropyrene B) conformational isomers. Selectivity data for hydrogenation of pyrene and hydropyrenes are presented in Figure 17a-c. The selectivity vs conversion plots show clearly that dihydropyrene appeared as a primary product. Tetra-, and s- and as-hexahydropyrenes exhibited mixed primary and secondary behavior. Finally, deca- and perhydropyrenes were clearly of tertiary or higher rank. These observations led t o the construction of the network of Figure 18. The interconnectivity of the network rendered four equilibrium ratios redundant, and they were calculated in terms of the other equilibrium ratios in the network. They are presented in italics in Figure 18. Subsequent parameter estimation provided an indication of structurelreactivity trends. Pyrene hydrogenation at the middle ring was faster than hydrogenation a t the terminal ring. Further hydrogenation of the phenanthrenic moiety in dihydropyrene at the middle ring proceeded with a rate parameter very close to that for phenanthrene itself. The equilibrium ratio for both middle-ring hydrogenations was less than unity. As was the case for phenanthrene, this indicates that
Figure 18. Proposed network for pyrene hydrogenation. (k in L/(kg,, 9). Numerator rate parameters underlined. F = 0.153; estimated deviation = 0.0513.)
middle-ring hydrogenations were influenced by the equilibrium limitation, with the parent aromatic more abundant than the hydroaromatic at equilibrium. Hydrogenation of dihydropyrene at the terminal ring to as-hexahydropyrene was faster than the formation of tetrahydrophenanthrene from phenanthrene. asHexahydropyrene production directly from pyrene was negligible. s-hexahydropyrene was a minor primary hydrogenation product of pyrene, as well as an isomerization product of as-hexahydropyrene. This is in agreement with the pathways involving consecutive rearrangements proposed in previous works (Johnston, 1984; Girgis, 1988). The secondary reactions of s-hexahydropyrene were notably slow. The rate constant for its further hydro-
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 111 1
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Figure 19. Effect of reacting mixture composition on chrysene GAUS; (0)S R (A)TCFD; (A) EQ; (-) conversion kinetics. ((0) Est.)
genation to B-decahydropyrene was small compared to values for similar moieties in other host molecules. The location of the aromatic rings in internal parts of the molecule could adversely affect hydrogenation reactivity. Indeed, in the as-hexahydropyrene isomer, hydrogenation of the equivalent internal ring also proceeds much more slowly than hydrogenation of the terminal ring. A possible explanation for these trends could involve steric as well as thermodynamic arguments. Assuming that s- and as-hexahydropyrene need to adsorb flatly on the catalyst for the addition of all four hydrogen atoms to proceed to the innermost loa- and 10bpositions, the availability of sites would be reduced (compared to tetrahydrophenanthrene or naphthalene) due to the size and bulkiness of naphthenic substituents. In that respect, the sterics of their saturation would approximate single-ring saturations and should therefore proceed with equivalent rate constants. The equilibrium ratios in both unfavored hydrogenations are also less than unity. This indicates a strong dehydrogenation trend of the resulting single-ring hydroaromatic. This is distinctly different from all other naphthalenic moieties and can be used to explain the equilibrium limitations in pyrene hydrogenation suggested in the kinetics data of Figure 16. Rate constants for the hydrogenations of the benzenic moieties in the decahydropyrenes were slightly higher than for the benzenic moieties in phenanthrene. It can be speculated that the presence of two more methylene groups further enhanced hydrogenation reactivity. Terminal ring hydrogenations were again preferred to internal ring hydrogenations. Figure 17a-c shows the goodness of fit of the optimal rate and equilibrium parameters to the network of Figure 18. Overall, the parameter estimation results, represented by the smooth curves of Figures 16 and 17, represent the observed pyrene conversion kinetics well. Chrysene. Reactions of chrysene in 420. g of cyclohexane solvent were at 68.1 atm of Hz and 350 "C. The conditions for these experiments are presented in Tables 1-3. Reaction duration was 3.147 < tf < 9.574 h, and this produced maximum conversions of 0.962 < X M 1.000. The major identified products were dihydrochrysene, tetrahydrochrysene, hexahydrochrysene, symmetric and asymmetric octahydrochrysene, dodecahydrochrysenes A and B, and perhydrochrysene. These major products accounted for more than 95% of the initial weight of chrysene in all cases. Traces of perhydrochrysene isomers, alkylphenanthrenes, and alkylbiphenyls were also detected.
The identification of di-, tetra-, and perhydrochrysene was straightforward from the mass spectra, as they were the only possible products with molecular weights 230, 232, and 246, respectively. The abundance of a single peak a t mle 234, which could be either a hexahydrochrysene or a butylphenanthrene, was too low to allow decisive discrimination. However, the formation of butylphenanthrene was considered unlikely, because of the low acidity of the catalyst and the lack of further cracking products such as phenanthrene. Moreover, the mle 234 peak appeared early in the network. This suggested a hexahydrochrysene product and, on the basis of the pattern of selective terminal vs internal ring hydrogenation established so far, 1,2,3,4,5,6-hexahydrochrysene was selected against 5,6,11712,14,17-hexahydrochrysene. The latter would have lower stability, as a product of two consecutive internal ring hydrogenations. The two octahydrochrysene isomers were differentiated by their fragmentation patterns: as-octahydrochrysene was characterized by a prominent propylenic fragment (236/193). The dodecahydrochrysene isomers also exhibited two distinct fragmentation patterns: one contained a propylenic fragment (240/197) in addition to the pronounced presence of an ethylenic fragment (240/212),and the other an M - 82 (2401158) peak in the absence of an ethylenic fragment. The first pattern implicated dodecahydrochrysene B, because of the presence of one saturated ring attached t o an aromatic. This left the other pattern for dodecahydrochrysene A. The kinetics of chrysene conversion for the conditions of Table 2 are presented in Figure 19. Chrysene conversion rates appeared t o be the fastest and most insensitive to the reactant mixture composition of all previously examined PNAs. This may imply that chrysene is the most strongly adsorbed of all model compounds examined. The conversion dependence of the yields and selectivities of the initial chrysene hydrogenation products are summarized in Figure 20a-c. Di- and tetrahydrochrysenes were primary products, with approximately equal projected initial selectivity (0.45),while hexahydrochrysene exhibited mixed primary and secondary behavior. Octa-, dodeca-, and perhydrochrysenes were clearly of rank 2 or higher. The richness of the product spectrum and the possibilities opened by the secondary hydrogenation of hexahydrochrysene complicated the chrysene network. Thus, using the phenanthrene and naphthalene networks as guides, all possible reaction pathways were included in the chrysene reaction network. Parameter estimation t o the resulting network of Figure 21 using the relative rate expression of eq 3 reduced some of the complexity caused by the phenanthrene-like reaction path degeneracy. Six of the equilibrium ratios were redundant due to the high degree of interconnectivity in the network and were constrained by the other equilibrium ratios. They are denoted in italics in Figure 21. Scrutiny of the rate law parameters of the chrysene hydrogenation network led to reactivity trends analogous to those drawn from the preceding experiments. Middle-of-threeand middle-of-fourring hydrogenations in tetrahydrochrysene and chrysene proceeded with equilibrium ratios less than 1, as was the case for the corresponding moieties in phenanthrene and pyrene (i.e., chrysene t o dihydrochrysene and tetrahydrochrysene to hexahydrochrysene). Hydrogenation at the
112 Ind. Eng. Chem. Res., Vol. 34,No. 1,1995
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Figure 21. Proposed network for chrysene hydrogenation. ( K in L/(kg,t s). Numerator rate parameters underlined. F = 0.40912; estimated deviation = 0.1013.)
middle ring was faster than hydrogenation at the terminal ring for tetrahydrochrysene and of the same order of magnitude as that for chrysene. Based on these values, chrysene was the most reactive of all molecules examined. The reactivity of dihydrochrysene was essentially limited to hydrogenation of its naphthalenic moiety to hexahydrochrysene. Hydrogenation of the benzenic moiety in dihydrochrysene and further reactions of hexahydrochrysene proceeded with very small rate
parameters and equilibrium ratios larger than unity, which is typical of the benzenic moieties. Hydrogenation of the terminal ring in hexahydrochrysene was preferred to hydrogenation of the internal ring. The similar reactivities of the s- and as-octahydrochrysenes was also similar to or higher than the reactivity of other naphthalenic moieties. The A- and B-dodecahydrochryseneswere fairly reactive for singlearomatic-ring hydrogenation, with A-dodecahydrochrysene the most reactive molecule in the benzenic class.
Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995 113 Table 4. Aromatic Ring Number-Based L u m p Adsorption Constants from Parameter Estimation to Eq 2 for the Emeriments in Table 2 K (Umol) 38.5 38.5 17.5
four ringdpyene four ringdchrysene three-ring lump two-ring lump one-ring lump saturates lump
7.7 7.4 3.9
The general pattern appears to confirm the conclusion that reactivity increases with the addition of naphthenic rings. Figure 20a-c shows, as the curves, the results of rate and equilibrium parameter estimation to the network of Figure 21. The parameter estimation results, represented by the smooth curves of Figures 19 and 20, represent the observed chrysene conversion kinetics well.
Adsorption Results The compound-by-compound kinetic information presented thus far allowed the exploration of detailed hydrogenation reaction pathways. The application of eq 3 resulted in the evaluation of 45 LHHW numerator rate parameters and 45 numerator equilibrium ratios, 14 of which were constrained. This parameter set satisfactorily represented the yields of products with respect to reactant conversion. The effect of the reacting mixture composition on reactant conversion provides the remaining piece of the quantitative rate law that will in turn allow predictions with respect to time or space velocity. To this end, parameter estimation to eq 2 was performed using all available data simultaneously for the time dependence of component yields. This utilization of all the experiments of Table 2 represented 1.2
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regression to approximately 1700 experimental data points. Anthracene and tetralin data were not used, as these components were not present in the mixture experiments. cis- and trans-decalins were considered as one lump in the naphthalene network, as were all dimethylcyclohexanes and all xylenes in the o-xylene network. These approximations reduced the number of numerator rate and equilibrium parameters to 68. These were held constant, as the five adsorption parameters of the aromatic ring number-based lumps were optimized t o the 1700 data points. No statistical difference was observed between LHHW models with denominator exponents n = 1 and n = 2 (eq 2). Therefore, the simpler case of n = 1was adopted. The results are presented in Table 4 and their implications are illustrated in Figure 22a-b. Table 4 shows that the value of the adsorption parameters increased with the number of aromatic rings, i.e., it was lowest for the saturates lump and highest for the fourring compounds lump. A manifestation of this is demonstrated in Figure 22a, where the four-, three-, and two-aromaticring number-based lump yields are plotted with respect to time for all the initial compositions. Although the absolute value of the yield for a particular time depends on the initial composition, it is clear that the rates are essentially independent of the initial composition of the mixture for the four-aromatic ring lump. The rates for the three-aromatic ring lump decreased with increases in the heaviness of the composition. This effect is even more pronounced for the two-aromatic ring lump. The parameter estimation results are shown as the smooth curves in Figure 1 for xylene, Figure 7 for naphthalene, Figure 10 for phenanthrene, Figure 16 for pyrene, and Figure 19 for chrysene. In Figure 22b a typical example of the results for the reactants of the straight run composition experiment is presented (SR,
0.6
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300
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Figure 22. Adsorption parameter estimation results. (a) Aromatic ring number-based lump yields with respect to time for the fouraromatic ring lump (i), three-aromatic ring lump (ii), and two-aromatic ring lump (iii).(Curves represent results with the adsorption GAUS; (e) LCO; (0) SR (A)TCFD; (A) EQ; (-) Est.) (b) Reactant yields vs time for the straight run distribution parameters of Table 4. (0) (SR).(Curves represent results with the adsorption parameters of Table 4. (W) o-Xylene; (0) naphthalene; ( 0 )phenanthrene; (A) pyrene; (A)chrysene; (--) Est.)
114 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 I
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Figure 23. Rate parameters for hydrogenation of polynuclear aromatics. (Shade indicates the ring being saturated. k in L/(kgCts).)
Table 3). The parameter set thus represents the yield vs time experimental data very well.
Discussion Qualitative Hydrogenation Reactivity Trends. This experimental plan yielded reactivity information for a wide range of aromatic and hydroaromatic molecules under the same set of reaction conditions. Overall, 45 ring saturation reactions were probed. The regressed numerator rate parameters suggest the following qualitative trends: (1) PNA hydrogenation proceeded in a ring-by-ring manner; (2) hydrogenation reactivity increased with the number of aromatic rings; (3) for groups with the same number of fused aromatic rings, hydrogenation reactivity increased with the presence of alkyl substituents and naphthenic rings; (4) for moieties with one and two aromatic rings, hydrogenation of a ring located at the end of the molecule was faster than hydrogenation of an internal ring. These trends suggest that three broad categories of saturation, differentiated by numerator rate parameter magnitude and driving force, are operative. These categories are illustrated in Figure 23. The first category, single aromatic ring hydrogenation, was termed benzenic hydrogenation (six hydrogen atoms added). Hydrogenation of one out of two fused aromatic rings was the naphthazenic hydrogenation class, where four hydrogen atoms were added. Saturation of the terminal of three- or four-hsed aromatic ring compounds has also been included in this group. The unique hydrogenation of an aromatic ring fused between aromatic rings defines the phenanthrenic hydrogenation category, where two hydrogen atoms are added. There are several possible causes for separation into distinct hydrogenation classes based on the number of fused aromatic rings. As the number of fused aromatic rings increases, the resonance stabilization energy per aromatic ring decreases (Dewar, 1969) and the highest electronic density increases (Neurock and Klein, 1993). Both these factors can account for the observed increase in numerator rate parameter magnitude (Korre et al., 1995).
The numerator rate parameters of Figure 23 implicitly contain a hydrogen pressure dependence, which, if extracted, would further enhance the separation of the three hydrogenation classes. That is, division of the rate parameters by the hydrogen pressure raised to the hydrogen stoichiometric coefficient, a,would decrease the rate constant most for the benzenic class (a= 3) and least for the phenanthrenic hydrogenation class (a = 1).
The numerator rate parameters also encompass the kinetically possible symmetries. For example, hydrogenation of phenanthrene at the terminal ring can occur a t two possible positions, while hydrogenation of tetrahydrophenanthrene at the terminal ring can occur at only one position. Other examples of such symmetries include hydrogenations of pyrene and chrysene a t the middle ring and hydrogenations of s-hexahydropyrene and s-octahydrochrysene. All these factors need to be taken into account for the quantitative interpretation of numerator rate parameter information (Korre, 1994; Korre et al., 1995). HydrogenatiodDehydrogenationEquilibria Although the selected conditions and most measurements were intended to emphasize the hydrogenation rate, the longer-time mixture experiments allowed regression of reliable dehydrogenation rate parameters for each step in each network. This was revealed in the form of equilibrium ratios, as shown in eqs 6 and 7. For hydrogenation of an aromatic, (A), to a hydroaromatic (HA),with n mol of H2, the equilibrium parameter, Kes, can be obtained by dividing the equilibrium ratio, K, by the hydrogen pressure to the nth power:
(7) The equilibrium ratio may be viewed as the equilibrium constant for 1 atm of hydrogen pressure. It also possessed intuitive bounds. For example, an equilibri-
Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995 115 I
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Figure 24. Equilibrium concentration ratios for hydrogenation of polynuclear aromatics. (Shade indicates the ring being saturated. K = KePHzn-I
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um ratio K < 1implies more aromatic than hydroaromatic at equilibrium, and thus indicates a comparatively unfavorable reaction at the given conditions. In the present work the equilibrium ratio was of order unity or higher. The best-fit equilibrium ratios K are listed in Figure 24. Several qualitative trends are apparent. For hydrogenation of an isolated aromatic ring, such as in benzenic hydrogenation, where three hydrogen molecules are added, the equilibrium ratios are usually much larger than unity. This implies that the fully saturated (perhydrogenated) molecule is much more abundant than the parent single-ring aromatic a t equilibrium. The best-fit equilibrium ratio is very large for xylene (K > 1000) and gradually decreases as more naphthenic rings are added. Thus it appears that the more complex the hydroaromatic structure, the lower the equilibrium ratio. Equilibrium ratios larger than unity were also found for hydrogenation of a terminal aromatic ring fused to
only one additional aromatic ring, such as in the series naphthalene, tetrahydrophenanthrene, octahydrochrysene, etc. Note that 2 mol of H2 are added in these instances. These equilibrium ratios also dropped as more naphthenic (and aromatic)rings were added to the unit sheet. Indeed, the value for s- and as-hexahydropyrene hydrogenation a t an internal ring was less than unity. This implies that the hexahydropyrenes are more abundant than the decahydropyrenes a t equilibrium and explains the equilibrium limitations observed in the overall pyrene network. Comparatively small equilibrium ratios K < 1were also observed for the hydrogenation of the middle-ofthree or -four aromatic rings, such as in the addition of 1 mol of H2 to phenanthrene, pyrene, dihydropyrene, chrysene, and tetrahydrochrysene. Figure 25 compares calculations of the equilibrium ratios K based on experimental work by Frye and coworkers (Frye, 1962; Frye and Weitcamp, 1969) against those found in the present experimental work for
116 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 reactions in the naphthalene and phenanthrene network at 350 "C and 68.1 atm of Ha. An excellent agreement persists over about 4 orders of magnitude, lending credibility to the values reported here and summarized in Figure 24. Adsorption. The adsorption constant trends based on aromatic ring number are consistent with an assumption of acidhase interactions between the catalyst and the adsorbed molecule. Indeed, the observed increase in adsorption constants with increasing aromatic ring number may be attributed to the concurrent increasing basicity of the PNAs. This has been revealed by both experimental measurements of gas-phase basicity (LaVopa and Satterfield, 1988)) as well as computational chemistry calculations of proton affinity (Neurock and Klein, 1993). The adsorption equilibrium constants for the aromatic ring number-based lumps listed in Table 4 are essentially an average of the molecules present in mixtures of different compositions that contained the same number of aromatic rings. This renders them a very good estimate of the competitive inhibition in a mixture; nonetheless, they must yet be considered as devoid of rigorous microscopic structural significance (i.e., adsorption energetics or other adsorption fundamentals). It is thus reasonable to suspect that significant differences could be observed on a microscopic scale within a lump. They would depend on the number of saturated rings, their position with respect to the aromatic ring(s), and the nature of their adsorption on a given catalytic site. A bulky hydroaromatic molecule, such as B-decahydropyrene, could adsorb flatly on the catalytic surface, and thus reduce the number of sites available to other molecules. Even with the same energetics of its one aromatic ring adsorption lump, it could thus appear in kinetics experiments to have a higher adsorption constant than o-xylene, for example. Such differences could be observed even between isomers, such as A- and B-decahydropyrenes. Further elaboration of these ideas could reveal more information and will be pursued in a following publication (Korre et al., 1995; Korre, 1994).
Conclusions An extensive experimental plan provided a consistent data base of quantitative reactivity information for 36 aromatic and hydroaromatic compounds containing up to four aromatic rings subject to 45 reactions at 350 "C and 68.1 atm of Hz. o-Xylene hydrogenation proceeded with relatively slow rates, inhibited by the presence of other aromatics. Tetralin hydrogenation to cis- and tram-decalin was equally slow, while dehydrogenation to naphthalene occurred until equilibrium concentrations were reached. Phenanthrene hydrogenation proceeded through tetrahydro- and octahydrophenanthrenes to complete saturation, while experiments with dihydrophenanthrene established it as a "dead end" in the network, its only reaction being dehydrogenation t o phenanthrene. Anthracene pathways were similar to those for phenanthrene. Pyrene hydrogenation pathways were equilibrium limited, probably due to the high dehydrogenation reactivity of the intermediate tetra- and hexahydropyrenes; perhydrogenated products were detected, though. Finally, chrysene hydrogenation was the fastest and the least affected by equilibrium or inhibition considerations.
Three classes of hydrogenation were discerned, based on the magnitude of saturation numerator rate parameters: 1,benzenic (single isolated aromatic ring); 2, naphthalenic (two isolated aromatic rings or terminal of three- and four-aromatic ring systems); 3, phenanthrenic (middle-of-three or four fused-aromatic rings). Hydrogenation equilibrium ratios were much larger than unity for the benzenic hydrogenation class, generally larger than unity for the naphthalenic hydrogenation class, and smaller than unity for the phenanthrenic hydrogenation class. There was excellent agreement between the values reported here and those published by Frye and co-workers (Frye, 1962) for the same conditions. Adsorption parameters were evaluated for aromatic ring number-based lumps. They clearly increased with increasing aromatic ring number, which is consistent with an acid-base interaction between catalyst and adsorbed molecule.
Acknowledgment The authors acknowledge the financial support of Mobil Research and Development Corp. (Paulsboro Research Laboratory) and the State of Delaware, as authorized by the State Budget Act of Fiscal Years 1990-1992. Mr. Dennis Kalaygian's help with the laboratory experiments is greatly appreciated.
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Received for review March 17,1994 Revised manuscript received August 10,1994 Accepted August 26,1994* @
Abstract published in Advance A C S Abstracts, November
1, 1994.
