Ind. Eng. Chem. Res. 2000, 39, 1705-1712
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Kinetics of the Hydrodenitrogenation of Indole S. C. Kim and F. E. Massoth* Department of Chemical and Fuels Engineering, University of Utah, Salt Lake City, Utah 84112
The hydrodenitrogenation (HDN) reactivity of indole (IND) was determined over a NiMo/Al2O3 catalyst and a CoMo/Al2O3 catalyst using a fixed-bed reactor at 631 K and 3.5 MPa pressure. Conversions and product distribution data were obtained at various feed compositions of indole and space times. The kinetics of the individual steps in the HDN pathways were determined by application of Langmuir-Hinshelwood kinetics to a reaction model proposed by Zhang and Ozkan. All reaction paths were inhibited by indole and three different active sites were required to correlate the data. Under comparable conditions, the CoMo catalyst was more active for the initial conversion of indole, but the NiMo catalyst gave higher HDN conversion. The former catalyst gave higher yields of N-containing intermediates than the latter. An increase in hydrogen sulfide concentration had little effect on the total conversion and decreased the HDN conversion for the NiMo catalyst, whereas total conversion increased while HDN conversion was unaffected for the CoMo catalyst. Introduction
NiMo and CoMo catalyst, and assess the effect of hydrogen sulfide on reactivities.
Catalytic hydrotreating has become an important process for the removal of sulfur and nitrogen from petroleum because of increasing environmental constraints. Heterocyclic compounds containing sulfur and nitrogen are relatively stable structures because of their aromatic character. Nitrogen compounds are more resistant to hydrotreating than sulfur compounds. Although the importance of hydrodenitrogenation is gaining importance, less quantitative information is available on the catalytic chemistry of hydrodenitrogenation than that of hydrodesulfurization. The hydrodenitrogenation (HDN) of heterocyclic nitrogen compounds generally involves the following reactions:1 (1) hydrogenation of nitrogen heterorings, (2) hydrogenation of aromatic rings, and (3) hydrogenolysis of C-N bonds. Several studies using indole as a model compound for the HDN reaction have invoked various reaction pathways involving a sequence of coupled hydrogenation and hydrogenolysis steps.2-5 Recently, Zhang and Ozkan6 proposed a more complex network, as shown in Scheme 1. A rapid hydrogenation of indole (IND) to indoline (HIN) is followed by two different paths, that is, direct hydrogenation of the aromatic ring of indoline to form octahydroindoline (OHIN), which proceeds rapidly to another intermediate, ethylcyclohexylamine (OECHA), and C-N bond rupture to form the intermediate orthoethylaniline (OEA). The latter then reacts further by two paths, that is, direct denitrogenation leading to ethylbenzene (EB) or aromatic ring hydrogenation to ethylcyclohexylamine. The latter, formed from two paths, is very reactive and rapidly forms ethylcyclohexene (ECHE) and ethylcyclohexane (ECH). The difference between this scheme and previous ones is the proposal for reaction paths through the intermediates OHIN and OECHA, which were detected under certain reaction conditions.7 The aims of the present study were to determine the kinetics of the HDN of indole, compare the results of a * To whom correspondence should be addressed.
Experimental Section The catalysts used were Topsøe TK-554, which consisted of 4.1% CoO and 20.5% MoO3 supported on alumina (220 m2/g), and Topsøe TK-555, which consisted of 3.8% NiO and 24% MoO3 supported on alumina containing 2% phosphorus (160 m2/g). The 1.27-mm extrudates were crushed and sieved to 40-60-mesh particles. All runs were carried out in a fixed-bed reactor at 613 K and 3.5 MPa under vapor-phase conditions. Either a 0.25- or 0.1-g sample of catalyst, mixed with 5 cm3 of glass beads, was presulfided with 10% H2S-90% H2 mixture by volume under atmospheric pressure and 673 K for 2 h. The liquid feed consisted of 0.25-0.75 wt % indole (IND) and 1.0 or 4.0 wt % dimethyl disulfide (DMDS), in n-heptane solvent. The hydrogen flow rate and liquid flow rate were varied proportionally so that a constant feed concentration was maintained throughout the run; hence, the hydrogen partial pressure was constant at 3.1 MPa and the total pressure at 3.5 MPa. After the catalyst was aged for 2 days under reaction conditions, catalyst activity remained essentially constant over 300 h. Liquid samples taken at various space times were analyzed by gas chromatography (0.32 × 36.6-cm stainless steel column packed with 6% OV-17 on 100-120-mesh WHP Chromosorb) and use of a flame ionization detector with temperature programming of 10 K/min. The identity of individual products was determined by comparison with pure reference samples and GC/MS analysis. Molar GC factors were determined using available samples of IND and its reaction products. The major compounds found in the samples from runs with indole were indole (IND), indoline (HIN), orthoethylaniline (OEA), two isomers of ethylcyclohexene (ECHE), ethylcyclohexane (ECH), and ethyl benzene (EB). Small amounts of 2-phenylethylamine were also detected, which were combined with OEA. Both IND and HIN were considered reactant.4,5 Total conversion (TOT) of IND was calculated from the sum of the mole fractions of products, while HDN conversion from the
10.1021/ie9906518 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/29/2000
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Scheme 1. Suggested Reaction Pathways for the HDN of Indole, Adapted from Zhang and Ozkan6
sum of the mole fractions of the non-nitrogen-containing products. Space time, τ, was defined as the weight of catalyst divided by the total gas flow (hydrogen plus vaporized liquid feed) at STP. Further details are provided elsewhere.8 Results 1. Preliminary Results. Figure 1 displays product distribution data versus space time for the NiMo catalyst at a feed of 0.5 wt % IND. Maxima in the OEA and ECHE yields signify that both are intermediates in the reaction pathway, whereas the increasing yields of ECH and EB with increasing space time indicate that these are secondary reaction products. These results are consistent with the model of Zhang and Ozkan6 (Scheme 1). Similar results were obtained for the CoMo catalyst. In one run, separate feeds of IND or HIN were tested at three space velocities.Within experimental error, the same conversion and product distribution values were obtained whether the run was started with IND or HIN. In kinetic runs discussed later, the HIN/IND mole ratio was consistently about 0.17, regardless of IND feed composition and space time. This value is in reasonable agreement with literature data,2,4 that is, about 0.14 for our reaction conditions. These results demonstrate that, under our reaction conditions, rapid equilibrium between IND and HIN is achieved before subsequent HDN reactions proceed.
Figure 1. Product distributions (mole fraction) vs space time (τ) for NiMo catalyst (K-9). Feed: 0.25% IND; 0.25 g of catalyst.
To evaluate the effect of the intermediate OEA on the conversion and product distribution of IND, tests were carried out at the end of several runs with added OEA or orthopropylaniline. It was necessary to keep the concentration of IND constant because of its strong inhibition effect on its conversion and product distribution. The total conversion of IND with added OEA was calculated by
x(TOT) ) (yIND0 - yIND)/yIND0
(1)
where yIND is the mole fraction of IND in the product and yIND0 is the mole fraction of IND in the feed. The data in Table 1 show that TOT conversions were about the same with or without added OEA and indicates that the effect of OEA is negligible. It is also seen that the HDN conversion was lower with added OEA. This implies that the conversion of OEA f ECHE is slower than that of IND f ECHE; that is, less OEA is converted compared to IND converted in IND f ECHE. In the same table, the lower mole fractions of products ECHE and ECH with added OEA compared to those without OEA do not indicate that less of these are formed on the basis of total feed, that is, IND + OEA. Their mole fractions must be multiplied by the total moles of feed, which of course gives higher yields of ECHE and ECH compared to the case without added OEA. Orthopropylaniline (OPA) was also used to check the inhibition from OEA on the basis of the assumption that the HDN reaction pathway of OPA is similar to that of OEA. In addition, products from OPA do not overlap with products from IND in the GC analysis, so that conversions from each feed can be analyzed independently. Table 1 shows a comparison of IND and OPA products for three different mixtures, 0.5% indole, 0.5% indole + 0.2% OPA, and OPA alone over the CoMo catalyst, where separate analyses were performed for both IND and OPA conversions. In a comparison of the results with IND alone and with IND + OPA, it is evident that OPA did not affect the conversion of IND. Therefore, the effect of OPA on the sites responsible for IND f CNH products is negligible compared to that of IND. However, IND greatly suppressed the conversion of OPA. Thus, IND not only strongly inhibits the conversion of IND but also inhibits secondary reactions of OEA and ECHE. An experiment to test for inhibition by NH3 on the IND reaction was carried out. In practice, the inhibition adsorption constants of NH3 can only be properly
Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 1707 Table 1. Conversions and Product Mole Fractions for Tests with Added OEA and OPA K-16, NiMo, 0.25 g feed % IND % OEA τ, kg‚min/m3 IND conv. TOT HDN IND prods. OEA EB ECHE ECH
0.5 0 1.11
0.5 0.25 1.11
0 0.5 1.11
0.5 0 1.47
0.5 0.25 1.47
0.5 0 0.74
0.5 0.25 0.74
0.573 0.421
0.712 0.349
0.965
0.723 0.553
0.79 0.431
0.405 0.284
0.589 0.206
0.153 0.023 0.146 0.251
0.364 0.021 0.132 0.196
0.035 0.091 0.044 0.831
0.17 0.034 0.149 0.371
0.359 0.028 0.125 0.279
0.122 0.015 0.141 0.128
0.383 0.012 0.107 0.087
K-18, CoMo, 0.1 g feed % IND % OPA τ, kg‚min/m3 IND conv. CNH HND IND prods. OEA EB ECHE ECH OPA conv. HDN OPA prods. PB PCHE PCH
0.5 0 0.444
0.5 0.2 0.444
0.171 0.064 0.107 0.003 0.029 0.032
0 0.2 0.444
0.5 0 0.634
0.5 0.2 0.634
0.174 0.066
0.232 0.094
0.108 0 0.032 0.034
0.138 0.006 0.030 0.058
0 0.2 0.634
0.5 0 0.887
0.5 0.2 0.887
0.253 0.097
0.363 0.159
0.36 0.149
0.156 0 0.038 0.059
0.204 0.013 0.042 0.104
0.211 0 0.045 0.104
0 0.2 0.887
0.106
0.237
0.093
0.345
0.14
0.502
0.011 0.062 0.033
0.047 0.05 0.139
0.012 0.025 0.056
0.073 0.038 0.233
0.026 0.02 0.094
0.103 0.043 0.356
determined at high HDN conversions when the relative concentration of NH3 in the reaction stream is relatively high. Only under this condition will the inhibition effect of NH3 have a significant influence on the HDN reaction. Ammonia precursor, urea (H2NCONH2), was added to the IND feed to simulate a condition of high NH3 concentration. Under these conditions, urea is easily decomposed into NH3 and if any inhibition from ammonia exists then, it should affect the product distribution of the indole HDN reaction. The results showed that ammonia formed from urea decomposition did not affect IND conversion or the product distribution. These experiments confirm that N-containing compounds from reaction of IND, such as OEA and ammonia, do not influence the reaction paths. This means that the adsorption constants of the IND reaction products, OEA and NH3, are negligible compared to those of IND. 2. Kinetic Model. Figure 2 displays first-order plots of -ln(1 - x), where x is total conversion versus space time (tau) at three different feed concentrations of IND for the NiMo catalyst. The nonlinear behavior of the curves indicates that the reaction of indole does not follow first-order kinetics. The data show an inverse effect of IND concentration on the conversion of IND, indicating a strong inhibition of IND on the overall reaction. Similar results were obtained for the CoMo catalyst. Product distribution data shown in Figure 1 indicate the following: (1) OEA and ECHE are primary products (from positive slopes at τ ) 0) and are intermediates (from maxima in slopes versus τ). (2) ECH and EB are secondary products (from initial slopes close to zero). (3) As a consequence of the above, two parallel reaction paths of IND exist, that is, IND f OEA and IND f ECHE. These findings are consistent with the proposed reaction sequence given in Scheme 1 and the simplified
Figure 2. First-order plots for the CoMo catalyst (K-18). Numbers are % IND in feed.
Scheme 2. Simplified Scheme for Kinetic Analysis
kinetic model of Scheme 2. In the latter, the hydrogenated species octahydroindole (OHIN) and orthoethylcyclohexylamine (OECHA) are omitted because neither was detected in the products and are assumed to react fast under the given reaction conditions. The above
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species can thus be treated by the steady-state assumption and do not enter into the kinetic scheme. Also, HIN was shown to be at equilibrium with IND and, therefore, its formation from IND is not rate-limiting. As for ECHE, the ratio of the two isomers of ECHE was consistently about three, independent of reaction conditions and catalyst. This indicates that the rate of hydrogenation of the two isomers to ECH is essentially the same, and thus it is only necessary to model the kinetics with the sum of the ECHE’s. Also, from the data of Table 1, IND strongly inhibits reaction of OEA, and presumably of ECHE. Therefore, it appears that IND inhibits all reaction paths in the reaction network of Scheme 2. The results discussed above show that OEA does not affect IND conversion or secondary reactions. The adsorption of OEA is therefore negligible on all paths. Furthermore, the separate test with added urea showed that ammonia has no effect on the reaction steps. Finally, the adsorption of ECHE and ECH are considered to be negligible, as these are weakly adsorbed. A multisite, Langmuir-Hinshelwood rate model was assumed in the kinetic modeling. Rate equations were developed for each step of Scheme 2. Initial trials showed that assuming only one type of active site, that is, one inhibition term for all steps, fit the data poorly. Therefore, according to Scheme 2, two types of sites were designated, a CNH site (S1) for IND f OEA (step 1) and a HYD site (S2) for IND f ECHE (step 2). This led to different adsorption constants for each step. For OEA f ECHE (step 3), the same site (S2) as that for step 2 was adopted because step 3 involves hydrogenation. The data fit was further improved if another site (S3) was assumed for ECHE f ECH (step 4). Finally, OEA f EB (step 5) was assumed to occur on site S1, as CNH is involved, and the very small values of EB were not sufficient to accurately model this step with a different site. On the basis of the above, the set of equations given in the Appendix were developed. There are five rate constants and three adsorption constants to be evaluated, a total of eight independent parameters. It is noted that each rate constant, ki, includes an adsorption constant, Ki, and a hydrogen pressure term. The adsorption constants could not be extracted separately, as the adsorption constant for each species (except IND) could not be obtained. The hydrogen pressure was constant in all runs and is included in ki. 3. Kinetic Parameters. The kinetic study with the NiMo catalyst (K-20) was performed using both sets of data for indole alone and for a mixture of indole and orthoethylaniline as feed. These data were obtained at four or five space times and three indole feed concentrations, resulting in a total of 90 data points (data available in Supporting Information). The purpose of making this run with a small amount of catalyst (0.1 g) was to keep the conversion low, thus obtaining good estimates of the primary rate constants, k1 and k2. The object of the additional tests with added OEA was to obtain better values for the secondary rate constants because high conversions with IND alone could not be achieved because of equipment limitations. Kinetic parameters were first regressed using the data with only indole in the feed. Then, the parameters obtained were used as initial values for the complete data set. Table 2 summarizes the values of the parameters obtained. An example of the fit with added OEA is given
Table 2. Kinetic Parameters for NiMo and CoMo Catalysts NiMo
CoMo
parametera
K-20
C.I.b
K-18
C.I.b
k1 k2 k3 k4 k5 K1 K2 K3
0.61 2.57 1.23 5.01 0.17 2.04 4.96 1.76
0.04 0.12 0.12 0.40 0.04 0.14 0.21 0.27
1.03 0.97 0.49 6.66 0.37 1.99 3.51 0.84
0.05 0.07 0.13 0.54 0.04 0.10 0.26 0.29
a
Units: k in m3/kg‚min; K in m3/mol. b 95% confidence interval.
Figure 3. Representative data fit to model for NiMo catalyst (K20). Feed: 0.25% IND; 0.1 g of catalyst.
Figure 4. Parity plot of predictive vs experimental mole fractions for NiMo catalyst (K-20).
in Figure 3. The data correlation for all the data is shown in the parity plot of Figure 4, comparing experimental versus predicted mole fractions. The correlation is quite good in this case. Values of the parameters are in the order
k4 > k2 > k3 > k1 > k5; K2 > K1 > K3 Thus, adsorption of IND on the sites is S2 > S1 > S3. Although K1 and K3 are approximately the same, this is probably a coincidence because they are considerably
Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 1709 Table 3. Conversions and Product Mole Fractions for 1% DMDS vs 4% DMDS in Indole Feed catalyst NiMo (K-20) wt % DMDS H2S part. press., kPa conv. TOT HDN prods. OEA ECH ECHE EB
Figure 5. Representative data fit to model for CoMo catalyst (K18). Feed: 0.25% IND; 0.1 g of catalyst.
1 11
4 44
CoMo (K-21) 1 11
4 44
0.269 0.188
0.268 0.138
0.220 0.096
0.357 0.099
0.081 0.067 0.110 0.011
0.130 0.048 0.086 0.004
0.124 0.046 0.046 0.003
0.258 0.051 0.043 0.006
sion compared to NiMo, which showed significantly reduced HDN conversion with an increase in H2S partial pressure. The effect of H2S on product distributions is also given in Table 3. The higher partial pressure of H2S at 4% DMDS favored the formation of OEA, that is, the difference between TOT and HDN conversion, over that at 1% DMDS. This effect was especially pronounced with the CoMo catalyst. The effect of the increase in H2S concentration was to decrease hydrogenated product yields (ECH and ECHE) for the NiMo catalyst, while it had little effect for the CoMo catalyst. Although the aromatic product (EB) appeared to decrease at a higher H2S concentration for the NiMo catalyst, while it increased for the CoMo catalyst, these results are uncertain because of the larger analytical error for the low values of EB. Discussion
Figure 6. Parity plot of predictive vs experimental mole fractions for CoMo catalyst (K-18).
different for the CoMo catalyst, as discussed below. Further, according to our model, K3 represents a HYD site and might be expected to be the same as K2, also a HYD site, but not K1, a CNH site. The kinetic run with the CoMo catalyst (K-18) produced a total of 14 sets (70 data points) for this run (data available in Supporting Information). Values of the parameters obtained are given in Table 2. The order of the parameters is
k4 > k1 > k2 > k3 ∼ k5; K2 > K1 > K3 The order in the rate constants is similar to that for the NiMo catalyst, except for k1. Data fits for this run are shown in Figures 5 and 6, again demonstrating good fits to the model. 4. Effect of H2S on the HDN of Indole. The effect of H2S on the kinetics of the HDN of indole with both NiMo and CoMo catalysts was studied by increasing the amount of dimethyl disulfide (DMDS) from 1% to 4% in the indole feed. The effect of H2S partial pressure on conversions for both catalysts is shown in Table 3. With an increasing partial pressure of H2S, the total conversion of IND showed no change for the NiMo catalyst, whereas it increased for the CoMo catalyst. However, the CoMo catalyst displayed no change in HDN conver-
1. Kinetic Model. In view of the differences among reactions taking place in the HDN network of indole, more than one type of catalyst site is indicated. Although the possibility of a separate site for each reaction cannot be excluded, this assumption would lead to a model with a large number of adsorption parameters, which would hardly be useful. For this reason, separate sites only for the main reactions of the HDN were considered. Selectivity changes during the initial deactivation period also suggest that the different behaviors of the hydrogenation and hydrogenolysis functionalities are associated with different catalyst sites. With the CoMo catalyst, a 15% drop in the HDN conversion compared to only a 5% drop in the total conversion indicates that the hydrogenation site is more susceptible to deactivation than the hydrogenolysis site. Hadjiloizou et al.9 reported that, in the HDN of pyridine, catalyst deactivation occurred in two distinct regimes, a rapid initial step followed by a more gradual activity decline. They attributed this to the deactivation of two types of catalyst sites with different activities. Kinetic analysis using a power of 1 for the inhibition (denominator) terms in the rate expressions (Appendix) gave poor fits to the data, thus, the application of the square of the inhibition terms in the kinetic equations. This may be rationalized by reaction of the adsorbed reactant species either with a vacancy site or with adsorbed H2 or H on an adjacent site. An additional complicating factor for reactions involving the formation of various intermediates is that these intermediates may have different adsorption characteristics. Depending on the strength of adsorption, it may be possible to omit some of the parameters which
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would not influence the adsorption term of the kinetic model. For example, in the simultaneous reaction of IND and OPA or OEA, it was found that the presence of the latter had no influence on the HDN conversion of IND, showing that the adsorption of OEA is weak and can be neglected in the rate equations. Callant et al.10 reported that aniline did not affect the conversion of indole. The adsorption constants of IND on sites 1 and 2 are different (Table 2). This justifies the distinction of different sites for these two reaction paths. In a previous paper,11 we postulated that adsorption on site 1 (CNH path) was through the N-atom and on site 2 (HYD path) via π bonding of the aromatic ring. From theoretical calculations, Ruette et al.12 showed both σ- and π-adsorption modes are possible for pyridine adsorbed on a model MoS system. Cleavage of the C-N bond is an indispensable step in the HDN process, which usually takes place through saturated amine intermediates. The kinetic results indicate that the majority of the C-N bond cleavage reactions in the HDN of IND takes place through C(sp3)-N bond cleavage. There can be two ways that C(sp3)-N bond cleavage can occur, that is, elimination and hydrogenolysis. By studying a series of amines, Portefaix et al.13 demonstrated that an E2 Hoffman-type elimination mechanism is responsible for the C-N bond cleavage in molecules which contain hydrogen atoms in the β-C position. In the case of indole, the reaction of ECHA to ECHE is an elimination, and this process is the major route to forming ECHE. It has been shown that the removal of NH3 from aliphatic amines with hydrogen atoms on the carbon atoms in the β position is easy and that the rate is higher for molecules with more β-H atoms.13 Jian et al.14 suggested that direct C(sp2)-N bond cleavage takes place on a different catalytic site than hydrogenation reactions in the catalytic hydroprocessing over sulfided NiMo/Al2O3 catalysts. Therefore, the distinction was made between these catalytic sites in the kinetic modeling. Also, to simplify the kinetic calculations, it was assumed that reaction of OEA to ECHE (path 3) uses the same catalytic sites as reaction path 2. Hydrogenation of olefins, however, has been demonstrated to take place on a different catalytic site than that of ring opening and N-elimination reactions of piperidine15 and decahydroquinoline.16 Ethylbenzene was not observed in a separate experiment with ethylcyclohexane as feed, thus confirming that dehydrogenation does not occur under the reaction conditions studied. Also, the hydrogenation of ethylbenzene alone showed that, even at the highest space time, only small amounts ( CoMo NiMo < CoMo
for CNH (k1 and k5) for HYD (k2 and k3) for HYD (k4)
The first two items are consistent with NiMo being a better hydrogenation catalyst and CoMo a better CNH catalyst.18 To compare actual rates, the inhibition terms need to be considered. Initial rates for path 1 (CNH path) and path 2 (HYD path) were determined from the following equations derived from the Appendix:
rCNH,0 )
rHYD,0 )
k1CIND,0
(2)
[1 + CIND,0K1]2 k2CIND,0
(3)
[1 + CIND,0K2]2
where yA ) 1 when τ ) 0 and CIND,0 ) 0.339 mol/m3 (0.5% IND feed). Parameters from Table 2 were entered giving the following initial rates (mol/kg‚min): CHN (path 1) HYD (path 2)
NiMo (K-20)
CoMo (K-18)
0.072 0.121
0.125 0.069
It is evident that the NiMo catalyst gives lower initial C-N bond cleavage, but higher HYD than the CoMo catalyst, while the sum of the two paths is essentially the same. Again, the NiMo catalyst favors HYD over CNH, whereas the opposite is true of the CoMo catalyst. Although the major difference in responses between the catalysts is mostly likely because of the nature of the promotor (Ni versus Co), some contribution could be because of differences in catalyst formulations, that is, Mo loading and presence or absence of phosphorus. The lower k4 for the NiMo catalyst compared to that of the CoMo catalyst (Table 2) appears to be anomalous, in that path 4 (ECHE f ECH) is a HYD path and we would expect the NiMo catalyst to be better, as observed for paths 2 and 3. Again, the inhibition term needs to be included in evaluating the rate of a path. For example, using the data of K-20 and K-18 for 0.5% IND and a τ of 0.444 and the appropriate parameters of Table 2 and eq A-4 of the Appendix, the specific rates for step 4 were 0.087 and 0.067 mol/kg‚min for the NiMo and CoMo catalysts, respectively. Hence, the HYD rate to ECH is actually higher with the NiMo catalyst, as expected. This is also reflected in the aromatic-tohydrocarbon product ratios obtained, that is, 0.05 for NiMo and 0.11 for CoMo. Satterfield and Yang19 reported that for a NiMo catalyst the hydrogenolysis rate constants in quinoline HDN were lower than those of the hydrogenation rate constants, in agreement with our results for IND (Table 2). The overall results from the two catalysts in the present study show that the slow step for HDN of indole
Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 1711
with the NiMo catalyst is the hydrogenolysis step (HIN f OEA), whereas the aromatic hydrogenation step (OEA f OECHA) is the slow step with the CoMo catalyst. Shih et al.20 showed that, for the HDN of quinoline, the hydrogenation rate constants were higher for a NiMo catalyst than for a CoMo catalyst, whereas the opposite occurred for the hydrogenolysis constants. From the data of Table 3, the following trends on the effect of increasing H2S concentrations are observed:
TOT conv. HDN conv. OEA ECHE + ECH
NiMo
CoMo
) V v V
v ) v )
Acknowledgment The support of Haldor Topsøe A/S for this research is gratefully appreciated.
Hence, for the NiMo catalyst, increasing H2S has little effect on total conversion and decreases HDN as a result of lower hydrocarbon yields. The latter indicates that H2S suppresses secondary hydrogenation reactions. Similar results were reported by Callant et al.5 for a NiMo catalyst. On the other hand, for the CoMo catalyst, increasing H2S increases the overall conversion, but has little effect on HDN. This is in agreement with previous results with a CoMo catalyst, which showed a significant increase in indole conversion and a relatively small increase in HDN with an increase in H2S concentration.4 Because of limited data, detailed kinetic analyses were not performed on the higher H2S series. However, product mole fraction data at one IND concentration (1%) and 4 space times allowed estimation of initial rates. This was accomplished by plots of TOT conversion and OEA yield versus τ and obtaining the slopes as τ f 0.11 The initial slopes obtained had relative errors of less than 15%. To better compare results to the same run conditions with 1% DMDS, slopes of the latter were similarly estimated. Conversion of slopes to initial rates (mol/kg‚min) gave the following: NiMo (K-20) DMDS CNH (path 1) HYD (path 2)
1% 0.075 0.122
4% 0.080 0.119
CoMo (K-21) 1% 0.098 0.066
(2) The evidence from kinetic analysis indicates the existence of three types of active sites. (3) The NiMo catalyst is superior to the CoMo catalyst for HDN by virtue of its relatively lower intermediate N-compound yield. (4) An increase in H2S concentration has no effect on the total conversion of indole, while it decreases HDN conversion for the NiMo catalyst, whereas higher H2S results in increased total conversion but has no effect on HDN for the CoMo catalyst.
4% 0.194 0.108
The estimated rate values for the NiMo catalyst at 1% DMDS are in good agreement with those given above from the kinetic analyses. However, the estimated rates for the CoMo catalyst are slightly lower for this run (K21) as compared to those for run K-18 by kinetic analyses, probably because of the somewhat different degree of deactivation of the catalyst at steady-state line out. The above results show that increasing the H2S concentration from 1% to 4% DMDS has no noticeable effect on either CNH or HYD initial rates for the NiMo catalyst, in general agreement with the lack of H2S effect on total conversion seen in Table 3. On the other hand, the increase in total conversion at higher H2S concentration for the CoMo catalyst in Table 3 is reflected in both increased CNH and HYD initial rates; that is, higher H2S promoted both paths. Conclusions (1) The initial conversion of indole occurs by two parallel reaction paths, a CNH path leading to an intermediate N-compound and a HYD path leading to hydrogenated products.
Appendix Symbols are as shown in Scheme 2. For IND Only Feed.
dyA/dτ ) -k1yA/DA,1 - k2yA/DA,2
(A1)
dyB/dτ ) k1yA/DA,1 - k3yB/DA,2 - k5yB/DA,1 (A2) dyE/dτ ) k2yA/DA,2 + k3yB/DA,2 - k4yE/DA,3 (A3) dyC/dτ ) k4yE/DA,3
(A4)
dyD/dτ ) k5yB/DA,1
(A5)
DA,1 ) (1 + K1C0AyA)2
(A6)
DA,2 ) (1 + K2C0AyA)2
(A7)
DA,3 ) (1 + K3C0AyA)2
(A8)
initial conditions: τ ) 0; yA ) 1; yB ) 0; yC ) 0; yD ) 0; yE ) 0 where yi is the mole fraction from GC data and C0A is the concentration of IND in the feed. For IND + OEA Feed. The above equations are the same, except all Ki are replaced by Ki/X0A, where X0A is the mole fraction of A in the feed.
initial conditions: τ ) 0; yA) X 0A; yB ) 1 - X 0A; yC ) 0; yD ) 0; yE ) 0 Supporting Information Available: Table of product mole fractions for kinetic runs K-18 and K-20. This material is available free of charge via the Internet at http://pubs.acs.org. List of Symbols CIND,0 ) feed concentration of IND, mol/m3 CNH ) carbon-nitrogen hydrogenolysis DMDS ) dimethyl disulfide EB ) ethylbenzene ECH ) ethylcyclohexane ECHE ) ethylcyclohexene HDN ) hydrodenitrogenation HIN ) indoline (dihydroindole) HYD ) hydrogenation IND ) indole k ) rate constant, m3/kg‚min K ) adsorption constant, m3/mol
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OEA ) orthoethylaniline OECHE ) ethylcyclohexylaniline OHIN ) octahydroindole OPA ) orthopropylaniline Si ) adsorption site tau(τ) ) space time, kg‚min/m3 TOT ) total conversion of indole x ) total conversion y ) mole fraction
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with the Hydrogenation Function of a NiMoP-γAl2O3 Catalyst. Bull. Soc. Chim. Belg. 1991, 100, 823. (11) Kim, C. S.; Massoth, F. E. Hydrodenitrogenation Activities of Methyl-Substituted Indoles. J. Catal. 2000, 189, 70. (12) Ruette, F.; Poveda, F. M.; Sierraalta, A.; Sanchez, M.; Rodriguez-Arias, E. N. Model Parametric Hamiltonians and Bonding Theoretical Tools in Simulation of Catalytic Reaction Steps. Hydrotreatment of Oil Components. J. Mol. Catal. 1997, 119, 335. (13) Portefaix, J. L.; Cattenot, M.; Guerriche, M.; Breysse, M. Mechanism of Carbon-Nitogen Bond Cleavage during Amylamine Hydrodenitrogenation over a Sulfided NiMo/Al2O3 Catalyst. Catal. Lett. 1991, 9, 127. (14) Jian, M.; Kapteijn, F.; Prins, R. Kinetics of the Hydrodenitrogenation of ortho-Propylaniline over NiMo(P)/Al2O3 Catalysts. J. Catal. 1997, 168, 491. (15) Jian, M.; Rico Cerda, J. L.; Prins, R. The Function of Phosphorus, Nickel and H2S in the HDN of Piperdine and Pyridine over NiMoP/Al2O3 Catalysts. Bull. Soc. Chim. Belg. 1995, 104, 225. (16) Jian, M..; Prins, R. Kinetics of the Hydrodenitrogenation of Deacahydroquinoline over NiMo(P)/Al2O3 Catalysts. Ind. Eng. Chem. Res. 1998, 37, 834. (17) Jian, M.; Prins, R. Determination of the Nature of Distinct Catalytic Sites in Hydrodenitrogenation by Competitive Adsorption. Catal. Lett. 1998, 50, 9. (18) Clausen, B.; Topsøe, H.; Massoth, F. E. Hydrotreating Catalysis. In Catalysis. Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1996; Vol. 11, p 1. (19) Satterfield, C. N.; Yang, S. H. Catalytic Hydrodenitrogenation of Quinoline in a Trickle-Bed Reactor. Comparison with Vapor Phase Reaction. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 11. (20) Shih, S.; Reiff, E.; Zawadski, R.; Katzer, J. R. Effect of Catalyst Composition on Quinoline and Acridine Hydrodenitrogention. Am. Chem. Soc., Fuel Div. Prepr. 1978, 23, 99.
Received for review September 1, 1999 Revised manuscript received December 2, 1999 Accepted December 8, 1999 IE9906518
