Ind. Eng. C h e m . Res. 1994,33, 203-210
203
Inhibiting Effect of H2S on Toluene Hydrogenation over a MoS2/A1203 Cata1yst S l a v i k Kasztelan' and Denis Guillaume Division Cinbtique et Catalyse, Znstitut Franqais d u Pbtrole, B.P. 311, 92506 Rueil-Malmaison Cedex, France
The influence of hydrogen sulfide partial pressure on the activity of a model MoSz/y-A1203catalyst for toluene hydrogenation has been investigated over a wide range of hydrogen sulfide partial pressure, from 0 to 0.3 MPa, under a total pressure of 6 MPa, reaction temperature between 320 and 410 "C, and liquid hourly space velocity between 0.5 and 7.5 h-l. A moderate inhibiting effect of H2S on the hydrogenation activity is found for H2S partial pressure up to 60 000 Pa with an order of reaction relative t o H2Scomprised between 0 and -1/2. For H2Spartial pressure higher than 60 000 Pa, no apparent inhibiting effect of H2Son the hydrogenation activity is detected. The results have been best interpreted in term of a kinetic model whereby the reaction occurs on a surface containing two sites, one unsaturated Mo ion and one stable sulfur ion host of the proton generated by the heterolytic dissociative adsorption of hydrogen and hydrogen sulfide. Introduction The ability of molybdenum-based hydroprocessing catalysts to hydrogenate unsaturated compounds in the presence of sulfur and nitrogen compounds makes these catalysts of practical importance in refining of petroleum feedstocks (Gates et al., 1979; Le Page et al., 1978,1987). These feedstocks must be hydrogenated in order to improvetheir H/C ratio by olefinicand aromatic compound saturation and must be purified by hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) (Girgisand Gates (1991)and references therein). These two latter reactions involve also hydrogenation steps; however, for HDS, the hydrogenation step is usually considered to be not critical (Girgis and Gates, 1991; Massoth, 1982; Vrinat, 1983) whereas it is critical for HDN (Girgis and Gates, 1991;Ho, 1988; PBrot, 1991). In hydroprocessing conditions, the catalyst works in the presence of H2S and also in some cases in the presence of NH3 and strongly adsorbed nitrogen-containing compounds which are known to be inhibitors of the hydrogenation function. In recent reviews several authors have stressed the importance of inhibiting effects in hydrotreating reactions and especially HDN (Girgis and Gates, 1991; Ho, 1988; PBrot, 1991). In particular, Girgis and Gates (1991) concluded that kinetics data are represented in an oversimplified manner and indicated that H2S has a complex effect on the HDS, HDN, and hydrogenation reactions suggesting that the effect of such a simple reactant on the various hydrotreating reactions over a sulfide catalyst is not clearly rationalized. Among the many difficulties limiting the determination of more general kinetic models is the lack of knowledge of the details of the reaction mechanisms for hydrogenation and C-S and C-N bond cleavage reactions over sulfide catalysts. From both a practical and an academic point of view it is of interest to establish kinetic laws for the reactions involved in hydroprocessing valid over a wide range of operating conditions and accounting well for the inhibiting effect of strongly adsorbed reactants. In the case of hydrogen sulfide, this effect is usually interpreted by the competitive adsorption of unsaturated compounds and hydrogen sulfide for the same site (Gates et al., 1979; Le Page et al., 1978, 1987; Massoth, 1982). Kinetic models
* To whom correspondence should be addressed. 0888-58~5/94/2633-0203$04.50/0
for hydrotreating reactions differ in general by the use of one type or two types of sites and either the molecular adsorption of hydrogen or the homolytic dissociation of hydrogen. Thus one well-established feature of the kinetics of aromatic hydrogenation over sulfided hydroprocessing catalysts is the order 1dependence of the rate of hydrogenation relative to both hydrogen and the aromatic in the presence of H2S (Ahuja, 1967;Ahuja et al., 1970; Gates et al., 1979; Gultekin et al., 1984; Le Page et al., 1978,1987; Vrinat, et al., 1980;Vrinat, 1983). Several different kinetic models can account for these two orders of reaction, which are therefore not sufficient to select a reaction mechanism. Noticeably, other types of kinetic models such as redox models or models based on heterolytic dissociation have not been considered. In this work an investigation of the effect of H2S on toluene hydrogenation activity of a model MoSz/A1203 catalyst over a large range of H2S partial pressure is reported. It is postulated that such a study should help to choose among different kinds of kinetic models for the hydrogenation reaction, in particular on the basis of the order of reaction relative to H2S. Among the various types of reaction mechanisms considered, the reaction mechanisms whereby hydrogen and H2S are heterolytically dissociated on the MoS2 surface have been extensively analyzed. Experimental Section The M003/A1203catalyst used in this work was prepared by impregnating y-alumina in the form of extrudates (RMne-Poulenc, surface area = 240 m2 g',pore volume 0.56 cm3/g) with a solution of ammonium heptamolybdate ((NH&M0702~4H20 from Merck). The catalyst was dried at 100 "C and calcined for 4 h at 500 "C. The loading, measured by X-ray fluorescence, was found to be 10.2 wt % MO (15.2 wt % MOOS). Toluene hydrogenation tests have been performed in a high pressure fixed bed continuous flow "Catatest" unit from VINCI Technologies. The experimental conditions employed were total pressure of 6 MPa, reaction temperature between 320 and 410 "C,liquid hourly space velocity (LHSV) between 0.5 and 7.5 h-l, and 40 cm3 of catalyst. The liquid feed was composed of toluene (20 wt %), thiophene (between 0 and 13wt %), and cyclohexane (for balance). Tests referred to as performed under zero H2S partial pressure correspond to a feed containing no 0 1994 American Chemical Society
204 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994
0.00 0.00
Y
0.50
1 .oo
1.50
2.00
2.50
1/LHSV (h)
Figure 1. Activity versus contact time for toluene hydrogenation over a MoS2/A1203 catalyst at 6 MPa, 350 O C , and (A) 42000, (m) 2500, and (e) 500 Pa H2S.
thiophene, and in that case the initial gas-phase composition was 4 MPa Hz, 0.37-MPa toluene, and 1.64-MPa cyclohexane. Thiophene was used as a HzS generator. It has been found completely hydrogenated into butane and HzS in the experimental conditions employed as checked by gas chromatographic (GC) analysis. When 13 wt % thiophene was used in the feed, the initial gas-phase composition was 3.3-MPa Hz, 0.32-MPa HzS, 0.32-MPa butane, 0.44-MPa toluene, and 1.62-MPa cyclohexane. The experimental conditions mentioned in the text as being “reference conditions” were 6-MPa total pressure, 350 O C , LHSV = 2 h-l, and feed composition of 20 wt % toluene, 2 wt % thiophene, and 78 wt 5% cyclohexane leading to an initial gas phase composition of 3.9-MPa Hz, 0.042-MPa HzS, 0.042-MPa butane, 0.383-MPa toluene, and 1.63-MPa cyclohexane. Prior to catalytic tests, the samples were sulfided in situ by passing a feed containing 2 wt % dimethyl disulfide in cyclohexane over the catalyst at 6 MPa, from room temperature to 350 “C with a ramp of 2 OC/min followed by a 4-h stand at 350 OC. The liquid products of the reaction were analyzed by gas chromatography using a 50-m PONA column at 60 OC and a flamme ionization detector. A first-order kinetic law has been used to compute the hydrogenation rate coefficient k in mol/(gh) from the conversionof toluene into hydrogenated products (toluene conversion in the range 2%-50%). A correction of the rate coefficient has been performed assuming first order relative to Hz and to toluene to account for the variation of the hydrogen partial pressure and the toluene partial pressure due to the consumption of HZby the thiophene decomposition reaction. This correction is important for high thiophene content in the feed. Then all hydrogenation activities are referred to the HZand toluene partial pressure of the test with no HzS, i.e., 4-MPa Hz and 0.37MPa toluene, respectively. Results Prior to the study of the effect of HzS on toluene hydrogenation, the effect of the contact time, defined here as l/LHSV, on the toluene hydrogenation activities has been measured in the reference conditions at three different HzS partial pressures. Toluene conversion levels varied between 0 and 30%, and it has been found that first-order hydrogenation activities vary linearly with the contact time (Figure 1). This indicates that the toluene hydrogenation reaction follows a first-order reaction kinetics in our experimental conditions in good agreement with literature data (Ahuja, 1967; Ahuja et al., 1970; Galiasso, 1970;Gates et al., 1979; Girgis and Gates, 1991; Gultekin et al., 1984; Le Page et al., 1978, 1987; Linero,
1
10
100
1000
10000
100000
H2S Partial Pressure (Pa)
Figure 2. Effect of hydrogen sulfide partial pressure in the range 04.1 MPa on toluene first-orderhydrogenation activity of a MoSd A1203 catalyst at 6 MPa for (m) 320 , (e)350 , (e)370, and (A)410 OC.
1974; Sapre and Gates, 1982; Surjit, 1972; Vrinat et al., 1980; Vrinat, 1983). A determination of the order of reaction relative to hydrogen and toluene has not been performed in this work. Such experiments have been reported by Gultekin et al. (1984), Le Page et al., (1978 and 19871, Sapre and Gates (1981), and Vrinat et al. (1980) for different types of hydrotreating catalysts (CoMo, NiMo, NiW on alumina, silica, etc.) and different types of unsaturated hydrocarbons (olefins, aromatics). It will be considered in the following that the order of reaction relative to toluene and hydrogen is 1 and is independent from the H2S partial pressure. Thus first-order activities, corrected from the variation of the hydrogen partial pressure, have been reported throughout this work. The effect of the HzS partial pressure on the toluene hydrogenation activities has been investigated over a wide range of HzS partial pressures and at different reaction temperatures. Three series of experiments have been performed which are described in the following. In the first series of experiments, catalytic tests have been performed at 60 bar, LHSV = 6 h-l, and successively at 320,350, and 380 “C starting from a high H2S partial pressure (42 000 Pa) to end up witha test with no thiophene in the feed. For all reaction temperatures or H2S partial pressure used, the steady state of hydrogenation activity has been reached within 4 h. Toluene conversion levels varied between 2 and 50%. The steady state hydrogenation activities are reported in Figure 2. A second series of testa has been performed on the same load of catalyst at 410 “C, LHSV = 7.4 h-I, from a H2S partial pressure of 2500 Pa to no thiophene in the feed. This series of tests was undertaken to explore the high reaction temperature range. Toluene conversion levels varied between 20 and 50%, and the first-order hydrogenation activities are also reported in Figure 2. As can be noted in Figure 2, the hydrogenation activity seems to stabilize at high H2S partial pressure. In order to check this observation, a third series of tests has been undertaken to explore the large H2S partial pressure range, from 10 000 Pa to 320 000 Pa, at 6-MPa total pressure, 350 O C and LHSV = 0.5 h-l. For the sake of clarity, the results are reported separately in Figure 3 to emphasize the change of the order of reaction relative to H2S. From Figures 2 and 3 emerges an unusual behavior of the catalyst for toluene hydrogenation activity in the presence of H2S. Although the curves activity versus H2S partial pressure are continuous, three domains of H2S partial pressure can be distinguished, a first domain from 0 to about 50 Pa, a second domain from 50 to 60 000 Pa,
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 205 0.5
5
I I
0.45
5c-o
e = X
003: 0.3 0.25
0.2 0.15 0.1
0.05
_ .
0 10000
100000
1000000
300
H2S Partlal Pressure (Pa)
360
380
400
420
Figure 5. Plot of toluene hydrogenation activity versus reaction no H2S in feed, temperature for various H2S partial pressures: (0) ( 0 )50 Pa, (A)100 Pa, (A)500 Pa, (e)2500 Pa, ( 0 )10 OOO Pa, and (D) 42 OOO Pa. Table 2. Apparent Activation Energies (&) of Toluene Hydrogenation at Different Has Partial Pressures Estimated from Figure 5
2
>
340
Reaction Temperature ("C)
Figure 3. Effect of hydrogen autfide pressure in the range 0.01-0.32 MPa on toluene hydrogenation activity of a MoS21Al203 catalyst at 6 MPa and 350 "C.
8
320
1
P H (Pa) ~
z g 0 A -1
-2 0
2
4
6
8
10
12
14
Ln(H2S Partlal Pressure (Pa))
Figure 4. Log-log plot of toluene hydrogenation activity versus hydrogen sulfide partial pressure for (m) 320 , (e and 0 ) 350,( 0 ) 370,and (A)410 "C. Table 1. Order of the Toluene Hydrogenation Reaction over a MoS,/AlsOs Catalyst Relative to Has at Different Reaction Temwratures (ComDuted from Figure 4)
42 OOO 20 OOO 10 OOO 2 500 500 250 100 0
temp range ("C) 320-410 320-410 320-410 320-410 320-410 320-3501380-410 320-3501380-410 320-350/380-410
ET (kcallmol) 20 16 15 16 15 1610
171-1 161-5
temperatures 320 and 350 "C; hence the rate law (3)
Noteworthy, in the absence of H2S in the feed, a high hydrogenation activity is obtained which depends on the reaction temperature. It has been checked that this nIH2S activity is stable over several days and reversible after reaction temp (OC) 100-2500 Pa 10 000-42 OOO Pa reexposure of the catalyst to a feed containing thiophene. 320 4-50 -0.48 It can be seen in Figure 2 that the effect of the reaction -0.50 -0.23 350 temperature on the hydrogenation activity is strongly 370 -0.43 -0.14 influenced by P H ~This . is observed in Figure 5, where 410 -0.41 the hydrogenation activities are plotted versus the reaction and a third domain for HzS partial pressure higher than temperature for PH~s. For P H ~>S 500 Pa, the apparent 60 000 Pa. A log-log plot of the hydrogenation activity activation energies for the reaction can be computed versus the H2S partial pressure reported in Figure 4 also according to an Arrhenius law and values between 10 and emphasizes these three domains. 20 kcal/mol, increasingwith the increase of P H a , are found The results of Figure 3 clearly confirm the premises of (Table 2). In Figure 5, at reaction temperature higher Figure 2: i.e., HzS has no more effect on the hydrogenation than 360 "C and for < 100 Pa, a decrease of the activity for a partial pressure higher than 60 000 Pa. This hydrogenation activities is observed which is not due to is an indication that the order of reaction relative to H2.S a thermodynamic limitation. For example, at 380 "C and is 0 in these conditions as shown in Figure 4. Hence, at 410 "C the toluene conversion is 53% and 45%, assuming an order 1 relative to Hz and toluene for P H ~ respectively, whereas thermodynamic equibrium com> 60 000 Pa, the rate of hydrogenation can be written puted from the data of Stull et al. (1969)leads to 96 5% and 80% of the hydrogenated form, respectively. rHYD = k ' H Y & i 2 p h , p o H f i (1) The effect of the reaction temperature on the hydrogenation activities at PH~S < 100 Pa results from the In the range 50-60000 Pa of HzS (Figure 2), the negative apparent activation energy of the rate law (eq 3) inhibiting effect of HzS on the hydrogenation activity is in that range of PH# The variation of the order of reaction evident. A log-log plot, reported in Figure 4, gives a curve relative to H2S and of the apparent activation energies of rather than a linear relationship. For the sake of clarity the reaction suggest that there is a complex rate law and estimation of the order relative to HzS has been given in possibly changes of the rate-determining step. Equations Table 1for two subdomains: 100-2500 and 10 000-42 000 1-3 represent therefore simplified forms of the rate law Pa. In the first subdomain the order of reaction is about valid in a limited range of PH~s. -1/2. It is however influenced by the reaction temperature. In the second subdomain values comprised between -112 Discussion and 0 are obtained. In the range 50-60000 Pa of H2S partial pressure the rate law can then be written The hydrogenation or hydrodesulfurization activity of MoSz/AlzOs-based hydroprocessing catalysts is known to r m = k ' H y $ ~ z ~ ~ o ~ ~ with H z s -112 < ')' < 0 (2) be inhibited by H2S (Gates et al., 1979; Girgis and Gates, 1991; Le Page et al., 1978, 1987; Massoth, 1982). The In the range 0-50 Pa, the hydrogenation activityremains results of the present study confirm this effect but indicate stable with the increase of P H ~at, least for reaction
206 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 Table 3. Kinetic Models for Hydrogenation Based on Molecular Adsorption or Homolytic Dissociation of Hz and HzS (ai= KPd Model 1 Hz + *-V c* *-H2 KHa HzS + *-V *-SH2 KHls R *-V *-R KR
--
+
*-R
+ *-HZ
4
--
-
*-RH
+ *-H
+
CYH,
+ a&$
KH~ KHls
KR
c*
--
"R~H,
(1 + "R
Model 2
Hz + *-V *-Hz HzS + *-V *-SH2 *-R R + *-V
+ +
r1 = k[*-VIo
+ RHzt
2*-V
Model 3
Hz 2*-V 2*-H HzS + 2*-V *-H *-SH R *-V *-R *-R + *-H c* *-V + *-RH
+
2*-V
+ RHzf
Hz + 2.-V c* 20-H HzS + 2*-V c* *-H + *-SH R *-V *-R *-R + 0-H c* *-RH + .-V
+
*-RH
.+
+ *-H
-
*-V
Model 4 KHa KHls
KR KH
+ *-V + RHzt
a more complex behavior than foreseen. Indeed the hydrogenation of a monoaromatic molecule can be considered to be a simple reaction network free of complications from consecutive or parallel reactions. Therefore a rather clear-cut inhibiting effect of H2S would have been expected. At low H2S partial pressure a stable hydrogenation activity is found. Such an effect is different from that reported for CoMo catalyst showinga promoting effect probably related to sulfurization of the catalyst (Yamada, 1988). A promotional effect of low H2S partial pressure is however also known in HDN of some nitrogen compounds (Girgis and Gates, 1991; Ho, 1988; PBrot, 1991; Satterfield and Gultekin, 1981; Vivier et al., 1991). Of particular importance in the results presented above is that, at very high H2S partial pressure, H2S has no more effect on the rate of hydrogenation, an effect noted in a recent work by Van Gestel et al. (1992) in the case of 2,6diethylaniline HDN. Indeed if classical Langmuir-Hinshelwood rate laws for one-site or two-site hydrogenation are considered, the strong inhibiting effect of H2S should lead to orders of reaction relative to P H ~equal S to -1 or -2. The observed effect of H2S on the hydrogenation activities will be assigned in this work to kinetic effects rather than to structural modification of the active phase. Both the change of the order of reaction relative to H2S and the change of the apparent activation energy suggest changes in the reaction mechanisms or changes of the ratedetermining step of the reaction. In order to analyze these results, a limited number of simple models based on different reaction mechanisms and in particular mechanisms involving heterolytic dissociation of hydrogen and hydrogen sulfide have been
analyzed. Both the set of elementary reactions considered and the rate laws computed for the rate-determining steps chosen (using the notation ai = KiPi) are presented in Tables 3 and 4. In Table 3, the models chosen are those often cited in the literature dealing with kinetic studies of hydrogenation and hydrodesulfurization reactions by sulfide catalysts (Gates et al., 1979;Girgis and Gates (1991) and references therein; Le Page et al., 1978, 1987; Sapre and Gates, 1982; Vrinat, 1983). They involve either one or two sites and either molecular adsorption or homolytic dissociation of hydrogen and hydrogen sulfide. The first model (model 1) is a one-site model with molecular adsorption of all reactants. Model 2 is a twosite model with one site for the molecular adsorption of hydrogen and a second site for the molecular adsorption of all other reactants. Models such as model 2 are often considered in the literature as they give a good account for the effect of the hydrogen and hydrocarbon reactant partial pressure (Gates et al., 1979;Girgis and Gates, 1991; Le Page et al., 1978, 1987; Sapre and Gates, 1982; Satterfield and Gultekin, 1984;Vrinat et al., 1980;Vrinat, 1983). It must be emphasized however that under the reaction temperature and the hydrogen pressure employed the molecular adsorption of hydrogen and hydrogen sulfide is not very likely. Model 3 is a one-site model and model 4 is a two-site model, both involving homolytic dissociation of hydrogen and hydrogen sulfide. If the term LYHB = KH&'H~sis large compared to 1 and the other a = KP terms in the denominator, it is clear that an order of reaction of -1 or -2 relative to the H2S partial pressure should be obtained. Thus none of these models could explain the absence of inhibiting effect at high HzS partial pressure.
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 207 Table 4. Kinetic Models for Hydrogenation Based on Heterolytic Dissociation of Hz and Has (ai= Kp,) Model 5
*-RH-
+ *-SH-
+
*-V
+ *-S" + RHzt Model 6
*-SRH-
+ *-H- *-V + *-S" + RHzf -+
Model 7
Model 8
In Table 4 new models are presented based on heterolytic dissociation of hydrogen and hydrogen sulfide. These models are worth being considered in view of the proposed presence of a hydride species on the surface of MoS2 made by several authors (Kasztelan et al., 1987; Komatsu and Hall, 1991; Tanaka and Okuhara, 1977; Tanaka, 1985; Wambeke et al., 1988 ) and the observation by 'H NMR of a hydride species on RuS2 (Lacroix et al., 1992), confirmed by a neutron diffusion study (Jobic et al., 1993). In addition, a theoretical study of the interaction of hydrogen with MoS2 has shown that heterolytic dissociation of hydrogen is a strong possibility (Anderson et al., 1988). However, no physicochemical evidence of the existence of such a species on MoS2 has been reported so far in particular by 1H NMR (Komatsu and Hall, 1991). Models 5 and 6 in Table 4 are one- site models for which hydrogen and hydrogen sulfide dissociates heterolytically according to the two reactions (Kasztelan, 1992)
where *-V is a coordinatively unsaturated site.
Models 5 and 6 differ by the sequence of the surface hydrogen addition reaction to R, the unsaturated hydrocarbon. For model 5 the sequence of hydrogen addition is
R + H- -,RH- + H+ -,RH,
(6)
and for model 6 the sequence of hydrogen addition is
R + H+ -,RH+ + H- -,RH,
(7)
Two-site models need also to be considered. Indeed it is known that the surface sulfur species concentration has a strong influence on the hydrogenation activity for dienes, especially strongly bonded sulfur removed only after hydrogen treatment at 500 "C (Kasztelan et al., 1987; Komatsu and Hall (1991)and referencestherein; Wambeke et al., 1988 1. Then the presence of a strongly bonded sulfur species, stable in the experimental conditions, at least in a first approximation, may be postulated. Such a sulfur species may be the host of protons produced by heterolytic dissociation of hydrogen or hydrogen sulfide according to the reactions
208 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 Table 5. Orders of the Toluene Hydrogenation Reaction Relative to Toluene, Hydrogen, and Hydrogen Sulfide (~TOL,IIH:, na,s) Determined for Kinetic Models 1-8 of Tables 3 and 4 (ai= KjF'j) kinetic model
( ~ T O L ,nHz, ~ aH2S
r r r r
f f f f
=0
> 01R > ffHz
+4 H -
(9)
aH&
HB)
>> CYR > CfHz >> 1
aH,S
>> 1 >> aR > QH2
O O O O
r#O r f O r=O r=O r=O r=O r#O r f O r f O r#O
HzS + *-V
1 >> ffH&
-
+ 4'-
*-SH-
where one site is the unsaturated Mo ion of the MoS2 surface (*-V) and the second site is a strongly bonded sulfur ion ( *-S2-). Models 7 and 8 are the two-site models corresponding respectively to the sequence of hydrogen addition reactions of eqs 6 and 7. Rate laws have been calculated only for surface reactions being the rate-determining steps, as it is believed, a priori, that adsorption and desorption steps are not likely ratedetermining steps at the reaction temperature employed. The Langmuir-Hinshelwood-Hougen-Watson (LHHW) theory has been used to compute the rate laws. In the case of models involving heterolytic dissociation, the computations have also been made using the LHHW method but with the addition of an electrostatic charge conservation. For the sake of clarity, the description of the method of rate law calculation for one-site models 5 and 6 (Kasztelan, 1992) and two-site models 7 tand 8 (Kasztelan, 1993) are presented in Appendixes I and 11, respectively. Tables 3 and 4 summarize the reaction schemes and the rate laws obtained for the rate-limiting steps considered. The selection of a kinetic model is made hereafter on the basis of the orders of reaction rather than using a fitting procedure to a chosen model because of the too large number of parameters involved in the rate laws. The following criteria have been used for the selection of a model: first, according to literature data, the rate law must lead to an order 1 with respect to Hz and toluene when P H ~isSlarge. Second at P H ~=S0 the rate of hydrogenation should not be nil. Third, the order of the reaction relative to HzS found experimentally, i.e., from about -1/2 at P H ~ S > 500 Pa to 0 for > 60 000 Pa must be accounted for by the same reaction network, i.e. model, but eventually for different rate-determining steps. In Table 5, the three orders of reactions relative to toluene, hydrogen, and hydrogen sulfide are reported for all the kinetic models considered in this work, making three different approximations concerning the ai = KiPi. In the first case (Table 5, column 3), the relative values of the ai are 1 >> C Y H ~ > S CYR > C Y H ~ .A t the reaction temperature used in this work the hypothesis 1>>CYH~Sis realistic and, in addition, HzSis considered to be strongly adsorbed compared to hydrogen and toluene. However, for the usual models 1-4, an inhibiting effect of H2S can only be obtained if the term CYHB= K H ~ & +is S larger than one. The second case considered in Table 5 is therefore the case CYH~S>> CYR> C Y H>> ~ 1, and the third one is the intermediate situation C Y H ~>> S 1 >> CUR> C Y H ~In . column 2 of Table 5 the zero or nonzero value of the rate at P H ~ S = 0 is indicated. Although there are many different
situations possible, it has been found in Table 5 that these three cases are sufficient to select a model. In Table 5 it can be checked that none of models 1-4 satisfies all the selection criteria. Only the rate law 4b leads to order 1 relative to hydrogen and toluene and an order -1/2 relative to HzS when ( Y H ~ S>> 1 >> (YR > ( Y H ~ . However model 4 cannot account for an order nil relative to HzSat high P H g and therefore does not respect the third criterion. Among models 5-8, it appears in Table 5 that model 7 respects the selection criteria in the case 1 >> ( Y H ~ S> CYR > C Y H ~It. can be checked that model 5 respects the orders of reaction and the change of the order relative to HzS from -1/2 to 0 again for 1 >> ( Y H >~ CYR> CYH~.However for PH+= 0 the rate is nil as no *-S2- species remains on the surface to host the H+ necessary for the reaction and this model must be rejected. Thus the two-site model appears superior to the one-site models. Model 7 corresponds to the addition of the hydride ion to the adsorbed unsaturated hydrocarbon followed by the addition of the proton. Model 7a corresponds to the hydride addition being the rate-determining step and is valid at low HzS partial pressure. Model 7b corresponds to the proton addition being the rate-determining step and is valid at high H2S partial pressure. It is worth noting that at very low HzS partial pressure the rate law of model 7a leads to an order 0 relative to H2S which is experimentally found a t the reaction temperatures 320 "C and 350 "C in Figure 2 (see eq 3). At higher reaction temperatures, 380-410 "C, a maximum of the hydrogenation activity starts to appear which may indicate a beginning of surface reduction when no H2S is present. The interpretation of the kinetic results reported in this work indicates that models based on heterolytic dissociation are worth being considered in the case of hydrogenation over MoSz-based catalysts and should not be excluded a priori from the analysis of kinetic data. It must be recognized however that the apparent good fit of a rate law with a limited set of data cannot be taken as proof of a reaction mechanism. Thus more data on the effect of inhibiting compounds on the activity of MoS2based catalysts are needed to validate or invalidate the reaction mechanism proposed in this work. Conclusion
The present study has shown that toluene hydrogenation activity at 6 MPa of a model MoSz/A1203 catalyst is influenced by H2S in a complex manner depending on its partial pressure. At high HzS partial pressure H2S has surprisingly no effect on the hydrogenation activities. At medium H2S partial pressure, an inhibiting effect is
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 209 observed, whereas a t very small H2S partial pressure no effect is detected. Conventional kinetic models based on one-site and two-site surfaces and on either the molecular adsorption or homolytic dissociative adsorption of hydrogen and hydrogen sulfide have been been found unable to account for the zero order of reaction relative to HzS at high HzS partial pressure. A two-site model, i.e., one vacancy and one stable sulfur ion host of the protons generated by hydrogen and hydrogen sulfide heterolytic dissociation, has been found to account the best for the results if it is considered that the hydrogenation proceeds by addition of a hydride ion followed by addition of a proton to the aromatic molecule. At low hydrogen sulfide partial pressure the rate-determining step would be the addition of the hydride ion, whereas at high hydrogen sulfide partial pressure the rate-determining step would be the addition of the proton to the half-hydrogenated intermediate. It is proposed that heterolytic mechanisms should not be excluded from the analysis of kinetic data from catalytic reactions over sulfide catalysts.
Appendix I
+ *-V + *-s2HZS + *-V R
*-RH-
(1.9) with E = 2[*-VIo - 1-30, T = [*-VI0 - 1-30, ai = K&, 6 = (1+CYR), and p = ( C U H ~ LYH~S) a~fi-l'~. The solution of eq 1.9 is
+
[*-VI =
(TN' - 466)
+ (Pp4- 4 d p 2 ( 2-~e))'/' 26(p2 - 46)
(1.10)
Thus all surface species concentrations can be computed. A complex formula will be obtained for the rate law which will be a function of the two initial concentrations [*-VI0 and [-lo. ,One simplification is to consider that on the surface the number of negative charges is equal to the number of sites, i.e., [*-VI0 = 1-10, which should be respected in order to keep the electroneutralityof the active phase. Hence e = [*-VI,,, T = 0, and
(1.11) or [*-VI = [*-VIo x
The computation of rate laws for the hydrogenation of an unsaturated compound R over a sulfide catalyst possessing one type of sites, i.e., an unsaturated surface site symbolized by *-V and where both hydrogen and hydrogen sulfide dissociate heterolytically, is presented in this appendix. The computation is made according to the classical Langmuir-Hinshelwood-Hougen-Watson method to which an electrostatic charge conservation equation is added (Kasztelan, 1992). The hydrogenation of R into RH2 according to model 5a in Table 5 where the addition of H- is followed by the addition of H+is taken as an example to illustrate the method and underline the hypothesis made. The set of elementary reactions to be considered is H,
[*-VI26(p2-46) - [*-V](p% - 464 - 2 = 0
c*
*-H-
+ *-S"
+ *-V
+ *-SH--
*-R
*-V
KHz
2*-SH- KHZs
c*
c*
+ *-SH-
KR
(1.1) (1.2) (1.3)
+ *-S% + RH,t
CYHB1/2
+ aR)1/2(aHZ + a H # + 2aHz3li2(1 + aR)'/')
(1
(1.12)
The rate of the hydrogenation reaction described by eqs 1.1-1.5 (model 5a in Table 5 ) is then: r5* =
k[*-Vlo x
Appendix I1 The computation of the rate law for the hydrogenation of a reactant R over a sulfide catalyst possessing two types of sites is computed as follows (Kasztelan, 1993). The surface is composed of the coordinatively unsaturated surface Mo ion symbolized by *-V and the stable sulfur ion, symbolized by *-S2-, host of the protons generated by the hydrogen and hydrogen sulfide heterolytic dissociation (12). The example taken is model 7a in Table 5 for which the set of elementary reactions is
(1.5)
The addition of the first hydrogen (hydride), reaction 1.4, is taken as being the rate-determining step (rds), and the surface concentration of the half-hydrogenated intermediate RH- is neglected. The computation is made by writing the three equations for the equilibrium constants Ki and one conservation equation for the site: [*-VIo = [*-VI
+ [*-H-] + [*-SH-] + [*-S2-]
+ [*-R]
(1.6)
One more equation however is needed to get as many equations as unknown variables. This new conservation equation will be an electrostatic charge conservation equation established for the negative charges:
[-lo = 2[*-S2-]
+ [*-HI + [*-SH-]
(1.7) Resolution of this system of equations is obtained by solving the equation 26[*-Vl + p(6[*-Vl - T ) ~ " [*-VI'/'which can also be written in the form
E
=0
(1.8)
The addition of the first hydrogen (hydride),reaction 11.4, is considered to be the rate-determining step. Three equations can be written for the equilibrium constant Ki, and two conservation equations must written for the two types of sites, namely,
+
[*-Ssl0 = [*-SH-] [*-S2-] (11.7) An electrostatic charge conservation equation is however needed to get as many equations as unknown variables:
210 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994
1-10 = 2[*-S2-l + re-SH-1
+ [*-HI + [*-SH-]
(11.8)
The number of negative charges will be twice the number of S2- anions present on the initial surface, hence 1-30 = 2[*-S2-Io and a second degree equation is obtained: 6(p - 6)[*-V12
+ (266 - pT)[*-vl
- f2
=0
(11.9)
with T = [*-VI0 - [*-S2-lo, e = [*-Vlo, 6 = (1 + CQ), and = ( a ~+,~H,s). The solution of eq 11.9 is
P
All surface concentrations can be now computed as well as the rate law. However they are again dependent on two initial concentrations: [*-VIo, [*-S2-Io, or [-IO. One simplification is to consider that the concentrations of the two different types of sites are equal, i.e., [*-VI0 = [*-SzIo, as saturation of any of these two types of sites would suppress heterolytic dissociation. In addition, the number of negative charges is twice the initial concentration of sulfur ions, Le., [-IO = 2[*-S2-]0. This is equivalent to the rule that the number of negative charges is equal to the number of sites, Le., [*-VI, + [*-S2-1o = [-IO. Then T = 0 and
(11.11) or
[*-VI = [*-VIo x 1
(1
+ C X R ) ' / ~ ( ( ~ H+, aH,s)1/2+ (1+ aR)'l2)
(11.12)
and the rate law for the hydrogenation described by eqs 11.1-11.5 (model 7a of Table 5) is then
r,e = k[*-V],
X
%a~.
(11.13)
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Abstract published in Advance ACS Abstracts, December 15, 1993. @
