Ind. Eng. Chem. Res. 2002, 41, 1469-1476
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Ethylbenzene Isomerization on Bifunctional Platinum Alumina-Mordenite Catalysts. 2. Influence of the Pt Content and of the Relative Amounts of Platinum Alumina and Mordenite Components F. Moreau,† N. S. Gnep,† S. Lacombe,‡ E. Merlen,‡ and M. Guisnet*,† Faculte´ des Sciences, Catalyse en Chimie Organique, UMR 6503, 40 avenue du Recteur Pineau, 86022 Poitiers Cedex, France, and Institut Franc¸ ais du Pe´ trole, 1 et 4 avenue de Bois-Pre´ au, 92852 Rueil-Malmaison Cedex, France
Ethylbenzene transformation was carried out over intimate mixtures of Pt/Al2O3 (PtA) and HMOR zeolites under the following conditions: fixed-bed reactor, temperature 683 K, pressures of hydrogen and of ethylbenzene equal to 8 and 2 bar, respectively, weight hourly space velocity between 5 and 200. A significant number of these bifunctional PtA/HMOR catalysts were used, differing by the platinum content of PtA (0.5, 1.1 and 2.3 wt %), the framework Si/Al ratio of the mordenite (from 6.6 to 180), and especially by the relative proportions of the PtA and HMOR components. With all of the bifunctional catalysts, ethylbenzene undergoes isomerization into xylenes, disproportionation, dealkylation, secondary ethylbenzene-xylene transalkylation, hydrogenation followed by ethylcyclohexane isomerization, and secondary cracking of C8 naphthenes. The rate of isomerization which occurs through bifunctional catalysis was shown to depend mainly on the balance between hydrogenating and acid functions, taken here as the ratio between the concentration of accessible platinum atoms and of protonic acid sites (nPt/ nH+). The rate of disproportionation per acid site does not depend on the relative proportion of zeolite and of Pt/Al2O3 in the bifunctional catalysts but increases with the density of protonic sites in the mordenite component. This suggests that this bimolecular reaction requires more than one acid site for its catalysis. Ethylbenzene dealkylation does not depend on the density of protonic sites but seems very sensitive to their strength. For these three reactions, the apparent activity of the protonic sites is significantly greater when the HMOR components of the bifunctional catalysts are dealuminated, hence present mesopores; most of the protonic sites of these dealuminated mordenites would be active, whereas for mordenites without mesopores, it would be the case only for the protonic sites of the shell of the crystallites. High selectivities to xylenes (≈75% at 35% conversion) were shown to be obtained on bifunctional catalysts with values of nPt/nH+ sufficiently high to have the acid isomerization of olefinic intermediates as the limiting step of ethylbenzene isomerization. Introduction The isomerization of the C8 aromatic cut coupled with the separation of p-xylene by adsorption allows the production of this industrially important isomer. Indeed, p-xylene is oxidized into terephthalic acid which is the precursor of polyesters used for preparing major commercial fibers.1-4 The C8 aromatic cut which results from naphtha reforming and steam cracking contains not only xylenes but also ethylbenzene. The separation of ethylbenzene being more expensive than its preparation by benzene ethylation,1 the aim of the isomerization unit is to transform m- and o-xylenes into the thermodynamic equilibrium xylene mixture and ethylbenzene into valuable products, either benzene by dealkylation or xylenes by isomerization. Whereas xylene isomerization can occur through acid catalysis,2 bifunctional metal/acid catalysis is required for complete dealkylation as well as for isomerization of ethylbenzene.2,5-7 * To whom correspondence should be addressed. Tel: (+33) 5 49 45 39 05. Fax: (+33) 5 49 45 37 79. E-mail:
[email protected]. † UMR 6503. ‡ Institut Franc ¸ ais du Pe´trole.
In a previous paper,8 ethylbenzene isomerization was carried out over bifunctional catalysts constituted by intimate mixtures of 0.5 wt % Pt/Al2O3 (75 wt %) and of HMOR zeolites (25 wt %) with Si/Al ratios from 6.6 to 180 (PtA/HMOR). From this study, conclusions could be drawn concerning the influence of acidity (density and strength of the protonic sites), of the balance between the metallic and acidic functions, and of the zeolite porosity (presence or not of mesopores) on the rates of the desired isomerization and of secondary reactions, especially ethylbenzene disproportionation and dealkylation. In particular, the balance between the metal and acid functions, taken as the ratio between the concentrations of accessible Pt and protonic acid sites nPt/nH+, was shown to determine the rate and selectivity of ethylbenzene isomerization as could be expected from the bifunctional mechanism. However, when the Si/Al ratio of the zeolite is changed, only a limited range in the concentration of protonic sites and especially in the nPt/nH+ values of the bifunctional catalyst can be obtained. The consequence is that the relations established between the physicochemical and catalytic properties of the PtA/HMOR catalysts deserve to be confirmed and to be made less
10.1021/ie010505d CCC: $22.00 © 2002 American Chemical Society Published on Web 02/21/2002
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Table 1. Characteristics of the Mordenite Samplesa pore volume (cm3 g-1)
acidity (µmol g-1)
sample
unit cell formula
EFAL
micro
meso
nH
HMOR6.6 HMOR10 HMOR20 HMOR30 HMOR90 HMOR120 HMOR180
Na0.14H6.16A16.30S41.70O96 Na0.01H3.68Al3.69 Si44.31O96 Na0.05H1.95Al2Si46O96 Na0.006H1.344Al1.35Si46.65O96 Na0.003H0.507Al0.51 Si47.49O96 Na0H0.35Al0.35Si47.65O96 Na0H0.21Al0.21 Si47.79O96
0 0.35 0.20 0.05 0 0 0
0.14 0.19 0.18 0.18 0.14 0.15 0.14
0.01 0.04 0.11 0.13 0.13 0.13 0.14
1111 883 413 218 97 80 52
+
nL
nH+/nth
0 63 109 51 21 0 0
0.52 0.69 0.61 0.47 0.56 0.66 0.71
a EFAL: extraframework aluminum species. nth: concentration of protonic sites estimated from the unit cell formula. n + and n : H L concentrations of protonic sites (H+) and of Lewis sites (L) able to retain pyridine adsorbed at 423 K.
qualitative. That is why other ways to change the physicochemical characteristics of the PtA/HMOR catalysts were used in this work: change in the platinum content of the PtA component and in the relative proportions of the PtA and HMOR components. The large range of nPt/nH+ values which was obtained (from 0 to 12 instead of from 0 to 2 in the previous paper8) will allow us to specify the characteristics of optimal (active and selective) catalysts for ethylbenzene isomerization. Furthermore, the activity of the protonic sites for disproportionation is shown here to be practically independent of the relative amounts of HMOR and PtA in the bifunctional catalysts. The hydrogenating function has no effect on the rate of ethylbenzene disproportionation, contrary to what was found by several authors.9,10 The positive effect the density of protonic sites in the HMOR component has on the rate of disproportionation, hence the demanding character of this reaction (two acid sites required for its catalysis), is confirmed. Experimental Section Catalysts. Mordenite samples will be called HMOR followed by the approximate value of the total Si/Al. HMOR10 was a commercial product from Tosoh, Amsterdam, The Netherlands, and HMOR20 from PQ Zeolites B.V., Amsterdam, The Netherlands. HMOR30 and HMOR90 were prepared by dealumination of HMOR20 by HNO3 solutions at reflux for 4 h with HNO3 normalities of 6 and 10, respectively. The volume of a HNO3 solution/catalyst weight ratio was equal to 10. The unit cell formula of the HMOR samples used here and in ref 8, the pore volume distribution, and the concentrations of protonic and Lewis sites estimated by pyridine adsorption followed by IR spectroscopy8 are reported in Table 1. The morphology of HMOR10, -20, and -90 was determined by scanning electron microscopy; the three samples were mainly constituted of small crystallites (100-200 nm) and of plates of approximately 1 µm both associated in aggregates of 50-70 µm (HMOR10) and of 10-40 µm (HMOR20 and -90). Pt/Al2O3 (PtA) was prepared by ionic exchange of γ-Al2O3 with a hexachloroplatinic acid solution followed by calcination at 773 K under dry air for 4 h. The Pt contents were equal to 0.53, 1.1, and 2.3 wt %. The Pt dispersion was determined by H2-O2 titration after reduction under hydrogen flow (30 mL min-1) at 693 K for 12 h. The bifunctional Pt/Al2O3 (75-95 wt %) + HMOR (525 wt %) catalysts were prepared by milling the mixture of the two components (crystallite size of HMOR < 1 µm, particle size of alumina < 100 µm), then pelletizing at a pressure of 1.4 ton/cm2, and sieving at 0.2-0.4 mm.
Ethylbenzene Transformation. The transformation of ethylbenzene (Fluka puriss product percolated on silica gel before use) was carried out in a fixed-bed stainless steel reactor under the following conditions: temperature ) 683 K, total pressure ) 10 bar, H2/ ethylbenzene molar ratio ) 4, WHSV (weight hourly space velocity) ) 5-200 (g of ethylbenzene) h-1 (g of catalyst)-1. Before use, the catalysts were treated in situ under H2 (10 bar and 1 mL min-1) at 753 K for 4 h. Reaction products were analyzed online by flame ionization detector gas chromatography using two fused silica J&W capillary columns: a 30 m DB-Wax and a 60 m DB-1. The first one was used for the separation of ethylbenzene and xylene isomers, and the second one was used to obtain the distribution of all of the products. Results Ethylbenzene transformation was first carried out over various bifunctional catalysts constituted by HMOR10 or -90 (5-25 wt %) and Pt/Al2O3 (75-95 wt %) with platinum contents of 0.53, 1.1, or 2.3 wt %. The platinum dispersion was close to 100% for 0.53 and 1.1 wt % Pt/Al2O3 and 75% for 2.3 wt % Pt/Al2O3. Various other catalysts with HMOR20 and -30 as acidic components were also used. 1. Influence of the Catalyst Characteristics on the Rate of Ethylbenzene Transformation. With all of the catalysts, ethylbenzene transformation was first carried out for 10-15 h at the same weight hourly space velocity (WHSV ) 15 h-1), then at different WHSV values (hence different contact times), and then finally at the initial value of WHSV. Various experiments with different samples of the same catalyst were carried out to estimate the reproducibility. Quasi identical values of conversion ((5%) were obtained. With all of the catalysts, there is, during the first period, a decrease in conversion followed by a plateau, and the final value of the conversion at WHSV ) 15 h-1 is very close to the value obtained at the plateau of the first period. The stability is practically independent of the catalyst composition. Indeed, with all of the catalysts, the ratio between the conversions at the plateau and those at short time on stream (45 min) was between 0.75 and 0.9. As was previously shown,8 the reaction products result from six different transformations of ethylbenzene and/or of the primary products: (1) the desired isomerization of ethylbenzene into xylenes, (2) disproportionation of ethylbenzene into benzene and diethylbenzenes, (3) dealkylation of ethylbenzene into benzene and ethylene totally transformed into ethane, (4) ethylbenzene/ xylenes transalkylation, which leads to ethyltoluenes (ET) and toluene or to dimethylethylbenzenes (DMEB) and benzene, (5) hydrogenation of C8 aromatics (with
Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002 1471 Table 2. Activity [10-3 mol h-1 (g of Catalyst)-1] of the Bifunctional PtA/HMOR10 Catalysts in Ethylbenzene Transformation (Total) and in Isomerization (Isom), Disproportionation (Disp), Dealkylation (Dealk), and Production of C8 Naphthenes (Hydrog) and of Ethylcyclohexane (ECH) activity catalyst
total
isom
disp
hydrog
ECH
95/5 93/7 90/10 85/15 75/25
48.0 49.3 64.0 73.2 111.5
0.5 PtA/HMOR10 10 10.5 1.0 11.7 16.2 1.3 12.5 23.3 1.9 15.5 28.9 2.4 14.5 69.5 5.7
dealk
26.5 20.1 26.3 26.4 21.8
18.1 10 14.5 12.6 7.5
95/5 90/10
56.3 69.5
1.1 PtA/HMOR10 12.2 13.6 1.2 14.8 22 2.2
29.3 30.5
17.9 15
95/5 90/10 75/25 50/50
59.5 74.4 117.1 122.9
2.3 PtA/HMOR10 15 13.5 1.4 19.4 20 2.1 34.7 40.2 4.7 19.8 70 8.2
29.6 32.9 37.5 24.9
17.9 17.9 14.2 7.5
Table 3. Activity [10-3 mol h-1 (g of Catalyst)-1] of the Bifunctional PtA/HMOR20, PtA/HMOR30, and PtA/ HMOR90 Catalysts in Ethylbenzene Transformation (Total) and in Isomerization (Isom), Disproportionation (Disp), Dealkylation (Dealk), and Production of C8 Naphthenes (Hydrog) and of Ethylcyclohexane (ECH) activity catalyst
total
isom
hydrog
ECH
95/5 75/25
48.5 151
0.5 PtA/HMOR20 10.6 20.1 2.1 7.3 115 13.3
disp
dealk
15.7 15.4
7.9 6.2
95/5 90/10 75/25
34.2 49.7 60.9
0.5PtA/HMOR30 7.1 5.6 0.7 13.2 12.1 1.5 11.1 28.6 3.1
20.8 22.9 18.1
13.5 11.3 7.7
95/5
51.9
1 PtA/HMOR30 11.6 6.8 1.0
32.5
21.6
95/5 90/10 75/25 50/50
26.9 42.1 50.2 52.7
0.5 PtA/HMOR90 6.3 2.8 0.4 10 5.4 0.5 13.6 10.4 1.2 8.4 25.9 3.2
17.4 26.2 25.0 15.2
12.8 18.5 17.6 7.0
95/5
37
1 PtA/HMOR90 6.2 3.8 1.2
25.8
18.9
isomerization of the hydrogenation products), (6) cracking of C8 naphthenes (N8) into C3-C6 alkanes. For all of the catalysts, the total conversion of ethylbenzene and its conversion through each of the reactions were plotted as a function of contact time τ (taken here as the reverse of WHSV). These curves confirm the primary kinetic nature of products of reactions 1-3 and 5 and the secondary nature of reactions 4 and 6 previously established:8 no direct formation of the products of these latter reactions from ethylbenzene. The rate of ethylbenzene transformation and the rates of reactions 1-3 and 5 were estimated from the slope of the tangent to the curves at zero conversion. The values obtained are reported in Tables 2 and 3. It should be emphasized that, for hydrogenation (reaction 5) as well as for ethylcyclohexane production, a plateau in conversion is obtained for low values of contact time,8 which makes the estimation of the corresponding rates relatively imprecise. Underestimated values of hydrogenating activities are probably obtained, with this underestimation increasing with the platinum content.
Figure 1. Ethylbenzene transformation over PtA/HMOR10 75/ 25. Product distribution: isomers (b), disproportionation products (]), dealkylation products (0), cracking products (+), transalkylation products (s), hydrogenation products (×) as a function of ethylbenzene conversion (X EB).
The activity values for the other reactions are more accurate ((10%). The total rate of ethylbenzene transformation increases significantly with the percentage of mordenite in the bifunctional catalysts, hence when the percentage of PtA is decreasing (Tables 2 and 3). It should be remarked that when the percentage of mordenite is multiplied by 5 (from 5 to 25%), the percentage of Pt/ Al2O3, and hence the hydrogenating activity, is divided by only 1.27 (from 95 to 75%). Therefore, it is most likely that the change in rate is essentially due to the change in the acid activity. In agreement with that, the increase in the percentage of platinum causes (at isocontent in mordenite) a small increase in the total activity and not a large decrease. Furthermore, there is a small decrease in the total activity when the Si/Al of the mordenite increases from 10 to 90 (Tables 2 and 3). The increase in activity with the mordenite content is due to a large increase in the rates of disproportionation and dealkylation; the isomerization rate passes through a maximum and the rate of hydrogenation is practically constant. Furthermore, the increase in the percentage of platinum has practically no effect on the rate of disproportionation and a positive effect on the rates of isomerization, dealkylation, and hydrogenation (Tables 2 and 3). 2. Influence of the Catalyst Characteristics on Product Distribution. The distribution of the products grouped by reactions responsible for their formation was determined for a large range of conversions obtained by operating at different contact times. As shown in Figure 1 for 0.5 PtA/HMOR10 [75/25 (wt %)], the product distribution depends very much on conversion: a significant increase with conversion in the percentages of isomerization and dealkylation, a significant decrease in disproportionation and hydrogenation, and the appearance at high conversion of the secondary products of cracking and transalkylation. The decrease in hydrogenation and disproportionation products is due to the establishment of the thermodynamic equilibrium for relatively low values of contact time, hence of conversion, and also to secondary transformations: dealkylation of diethylbenzenes and cracking of C8 naphthenes. The desired products (xylenes) undergo also a secondary reaction of transalkylation with ethylbenzene. However, this secondary transformation is slow in comparison to their formation, and the percentage of xylenes increases significantly at the expense of hydrogenation and disproportionation products (Figure 1). The product distributions obtained at 35% ethylbenzene conversion on all of the PtA/HMOR catalysts are
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Table 4. Distribution (wt %) of the Reaction Products Extrapolated at 35% Conversion in Isomerization (Isom), Disproportionation (Disp), Dealkylation (Dealk), Cracking Products (Crack), of Production of C8 Naphthenes (Hydrog)a distribution catalyst
isom
disp dealk cracking hydrog transalk
95/5 93/7 90/10 85/15 75/25
62.4 (71.7) 58.2 (66.4) 54.3 (62.1) 51.9 (60.0) 28.4 (31.6)
0.5 PtA/HMOR10 15.1 3.6 3.2 16.8 5.0 4.2 19.9 5.0 4.7 18.7 7.0 6.0 37.1 12.5 7.4
13.0 12.4 12.6 13.5 10.2
2.7 3.4 3.5 2.9 4.4
95/5 90/10
1.1 PtA/HMOR10 65.4 (72.7) 13.3 5.1 3.3 57.1 (65.3) 17.7 5.4 4.1
10.1 12.6
2.8 3.1
95/5 90/10 75/25 50/50
66.2 (75.5) 59.6 (68.0) 47.9 (55.1) 27.7 (31.2)
2.3 PtA/HMOR10 12.3 3.5 3.1 16.4 4.7 3.8 24.1 6.7 5.1 37.2 12.5 7.1
12.3 12.3 13 11.3
2.6 3.2 3.2 4.2
95/5 75/25
0.5 PtA/HMOR20 47.4 (53.8) 23.0 8.0 5.6 6.9 (7.3) 47.0 28.1 8.2
11.9 5.4
4.1 4.4
95/5 90/10 75/25
0.5 PtA/HMOR30 63.1 (70.5) 15.7 4.5 3.4 53.0 (60.1) 19.9 6.7 5.1 31.0 (34.7) 32.9 15.5 8.2
10.5 11.8 9.0
2.8 3.5 3.4
95/5
1 PtA/HMOR30 65.9 (73.5) 13.0 4.5 3.4
10.4
2.8
95/5 90/10 75/25 50/50
67.5 (76.2) 65.9 (75.6) 53.1 (60.1) 23.0 (25.4)
0.5 PtA/HMOR90 11.5 3.1 3.7 11.9 3.3 3.3 21.2 5.3 4.8 37.1 16.2 10.7
11.4 12.8 11.6 9.6
2.8 3.0 4.0 3.4
95/5
1 PtA/HMOR90 65.1 (72.4) 11.1 7.1 3.8
10.2
2.7
a
The percentage of isomers indicated in parentheses was calculated without considering C8 naphthenes in the products.
reported in Table 4. The low amounts of C6, C7 naphthenes, and C10+ products were not considered in the distributions. A large increase in the selectivity to xylenes (Isom, Table 4) is observed when increasing the PtA/HMOR ratio. Thus, with the series of 0.5 PtA/ HMOR10 catalysts, the percentage of xylenes in the products passes from 28.4% to 62.4% when this ratio passes from 75/25 to 95/5. If C8 naphthenes, those in isomerization units, are recycled with the C8 aromatic cut and are not considered in the products, the percentage of xylenes passes from 31.6% to 71.7% (Table 4). This increase is due to a significant decrease in the selectivity to disproportionation products (from 37.1 to 15.1%), to dealkylation products (from 12.5 to 3.6%), and to cracking products (from 7.4 to 3.2%) (Table 4). Furthermore, the increase in the platinum content of PtA causes also an increase in the selectivity to isomers at the expense of the selectivities to disproportionation, dealkylation, and cracking products. The effect of the Si/Al ratio of mordenite is more complex, with a minimum in the selectivity to isomers for the PtA/ HMOR20 catalysts (Table 4). Discussion Ethylbenzene undergoes four main primary transformations: isomerization (reaction 1) into xylenes, which occurs through a bifunctional catalysis, and disproportionation (reaction 2) and dealkylation (reaction 3), which occur through acid catalysis and hydrogenation over the platinum sites (reaction 5). The activity of the
Figure 2. Turnover frequency values TOF (h-1) for ethylbenzene isomerization of the protonic sites of the bifunctional PtA/HMOR10 catalysts as a function of nPt/nH+, the ratio between the concentration of accessible platinum sites and of protonic sites able to retain pyridine adsorbed at 423 K.
Figure 3. Turnover frequency values TOF (h-1) for ethylbenzene isomerization of the protonic sites of various bifunctional PtA/ HMOR as a function of nPt/nH+, the ratio between the concentration of accessible platinum sites and of protonic sites able to retain pyridine adsorbed at 423 K.
protonic sites accessible to pyridine at 423 K (i.e., the turnover frequency TOF) for reactions 1-3 was determined. The discussion will first deal with the change of TOF values with the physicochemical characteristics of the bifunctional PtA/HMOR catalysts: hydrogenating activity (actually the concentration of accessible platinum atoms), acidity, and porosity of the mordenite component. In the second part of the discussion, the emphasis will be placed on the influence of these characteristics on the selectivity of the catalysts to xylenes. 1. Influence of the Catalyst Characteristics on the Primary Transformations of Ethylbenzene. 1.1. Ethylbenzene Isomerization. The following mechanism was proposed to explain the isomerization of ethylbenzene over bifunctional catalysts:2,4,11-15 2H2, Pt
H+
-2H2, Pt
EB y\z ECHE y\z DMCHE y\z X hydrogenation of ethylbenzene (EB) into ethylcyclohexene isomers (ECHE), isomerization of ethylcyclohexenes into dimethylcyclohexenes (DMCHE) over the acid component of the catalyst, dehydrogenation of dimethylcyclohexenes into xylenes (X). It should be emphasized that, under the operating conditions, the maximum amount of olefinic intermediates (i.e., at thermodynamic equilibrium) is very low but that their reactivity over protonic sites is very high.11 It is well-known that the balance between hydrogenating and acid functions determines for a large part the rate and selectivity of reactions occurring through bifunctional catalysis.16-19 That is why we have plotted in Figures 2 and 3 the TOF (activity per accessible protonic site) values for isomerization as a function of
Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002 1473
nPt/nH+, the ratio between the concentrations of accessible platinum atoms and of protonic sites able to retain pyridine molecules adsorbed at 423 K as pyridinium ions (Figures 2 and 3). Indeed, this ratio was previously shown to be representative of the balance between hydrogenating and acid functions. Figure 2 shows on the example of catalysts with HMOR10 as acidic component that, as expected from the bifunctional mechanism, TOF in isomerization first increases with nPt/nH+ and then becomes constant above nPt/nH+ ) 1. Therefore, for low values of nPt/nH+, the limiting step would be the hydrogenation of ethylbenzene into ethylcyclohexenes on the platinum sites; hence, TOF increases with nPt/nH+. For nPt/nH+ > 1, the limiting step would be the isomerization of ethylcyclohexenes into dimethylcyclohexenes over the acidic sites; hence, TOF (per acidic site) is independent of nPt/nH+. The same type of curve is obtained for the series of catalysts with PtA/HMOR90 as the acidic component (curve b, Figure 3). However, the value of TOF at the plateau in activity is approximately 4 times greater. The lower value obtained with HMOR10 catalysts was previously ascribed8 to diffusion limitations in the skeletal isomerization of ethylcyclohexene intermediates within the crystallites of HMOR10 which, contrary to HMOR90, presents practically no mesopores. However, another possibility could be the formation and retention of coke in the core of the zeolite crystallites. In the shell of these crystallites, coke formation would be inhibited by hydrogen activated on Pt sites.20 Indeed, the first step of coke formation is most likely the intramolecular dehydrogenating coupling of the diphenylethane intermediates of disproportionation:21
In the core of the crystallites, coke formation would be much faster. As has been previously shown for methanol conversion into hydrocarbons,22 the presence of mesopores in mordenite decreases significantly the deactivating effect of coke. Indeed, in the absence of mesopores (e.g., HMOR10) the formation and retention of one coke molecule in a channel cause blockage of the access of the reactant molecules to all of the inner protonic sites. With mordenites presenting mesopores, the diffusion of reactant molecules is quasi tridirectional and the formation of one coke molecule causes only blockage of the access to a fraction of the channel and hence to a small number of protonic sites (Figure 4). Whatever be the explanation, only part of the protonic sites of HMOR10 participate in ethylcyclohexene isomerization; i.e., the effectiveness factor η is lower than 1. By admission of a value of η equal to 1 for HMOR90, the value of η for HMOR10 is equal to the ratio of the TOF values at the plateau, i.e., to approximately 0.25. The values of TOF obtained with PtA/HMOR20 and -30 catalysts are between those of the PtA/HMOR90 and -10 catalysts. Therefore, some of the protonic sites do not participate in ethylbenzene isomerization. However, this part is less significant than that with PtA/HMOR10 catalysts: the value of η was estimated to be equal to 0.7 instead of 0.25 with PtA/HMOR10 catalysts. 1.2. Ethylbenzene Disproportionation. Influence of Acidity and Porosity. Ethylbenzene disproportionation is a typical acid reaction. With large pore zeolites,
Figure 4. Schematic representation of the pore structure of mordenite (A: HMOR6.6 and -10) and of dealuminated mordenite (B: HMOR120-180). Influence of coke (b) on the diffusion of organic molecules inside the crystallite. *: protonic site.
Figure 5. Turnover frequency values TOF (h-1) of the protonic sites for ethylbenzene disproportionation versus nH+, the concentration of protonic sites in the bifunctional PtA/HMOR catalysts: PtA/HMOR10 (×); PtA/HMOR20 (0); PtA/HMOR30 (4); PtA/ HMOR90 (O).
this reaction was shown to occur through a carbocation chain mechanism involving benzylic carbocations and diarylmethane intermediates:7,24
It should be emphasized that activated hydrogen was shown to decrease the rate of disproportionation of xylenes, which was explained by a reaction of hydrogen with the benzylic carbocation intermediates25 or by a decrease in the acidity of the zeolite.10 However, as shown in Tables 2 and 3, no decrease in the rate of disproportionation is observed when the percentage of platinum on alumina, and hence the concentration of platinum sites, is increased. Figure 5 shows that TOF in ethylbenzene disproportionation is practically independent of the concentration of protonic sites (nH+ per gram of catalyst) in the bifunctional PtA/HMOR10 catalysts (the small difference between the samples is probably due to imprecision in the estimation of the reaction rates and in the amount of the HMOR component in the catalysts). The same observation can be made for the other series of catalysts: PtA/HMOR20, -30, and -90. This seems in contradiction with the quasi proportionality which was found between TOF in disproportionation and the
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Figure 6. Turnover frequency values TOF (h-1) of the protonic sites for ethylbenzene disproportionation over various PtA/HMOR catalysts with different proportions of PtA and HMOR and different framework Si/Al ratios of the zeolite versus the concentration of protonic sites nH+ per gram of zeolite: PtA/HMOR6.6 (b); PtA/HMOR10 (×); PtA/HMOR20 (0); PtA/HMOR30 (4); PtA/ HMOR45 (9); PtA/HMOR90 (O); PtA/HMOR120 (+); PtA/HMOR180 (]). For the series of PtA/HMOR10, -20, -30, and -90 samples, the average value (×) was used for the plot of the straight line.
concentration of protonic sites in the bifunctional catalysts containing 75 wt % of 0.5 PtA and 25 wt % of HMOR with framework Si/Al ratios8 from 20 to 180 (Figure 6). This apparent contradiction can be easily explained. On the one hand, the change in the Si/Al ratio causes a change in the distance between the protonic sites of the mordenite, and hence has a significant effect on the rate of demanding reactions such as ethylbenzene disproportionation which, most likely, requires more than one protonic site for its catalysis.8 On the other hand, the change in the relative proportions of HMOR and PtA in the bifunctional catalysts causes a change in the distance between mordenite crystallites and no change in the distance between the protonic sites. Therefore, in this latter case, because there is no change in the local density of protonic sites, no effect on the rate of ethylbenzene disproportionation can be expected. Furthermore, all of the values of TOF obtained for the PtA/HMOR10 and -6.6 catalysts are largely located below the straight line found for the other catalysts: the acid sites of HMOR10 and -6.6 are approximately 10 and 35 times less active than those of the other mordenite samples. As was proposed above for isomerization, this large difference in TOF can be related to large differences in the porosity of the mordenites: a monodimensional pore system in HMOR6.6 and -10 and a quasi tridimensional pore system due to the presence of mesopores in the other HMOR samples. However, differences in acid strength could also play a role, explaining that the difference in TOF between PtA/ HMOR10 and -6.6 and the other bifunctional catalysts is more significant for disproportionation than for isomerization. Ethylbenzene disproportionation was, furthermore, shown to require (at least at 473-523 K) very strong acid sites for its catalysis.26 1.3. Ethylbenzene Dealkylation. Over acid catalysts, ethylbenzene dealkylation (which is the reverse reaction of ethylbenzene synthesis) occurs through the following mechanism:
Figure 7. Turnover frequency values TOF (h-1) of the protonic sites for ethylbenzene dealkylation versus nH+, the concentration of protonic sites in the bifunctional PtA/HMOR catalysts. PtA/ HMOR10 (×); PtA/HMOR20 (0); PtA/HMOR30 (4); PtA/HMOR90 (O).
Figure 8. Turnover frequency values TOF (h-1) of the protonic sites for ethylbenzene dealkylation over various PtA/HMOR catalysts with different proportions of PtA and HMOR and different framework Si/Al ratios of the zeolite versus the concentration of protonic sites nH+ per gram of zeolite: PtA/HMOR6.6 (b); PtA/HMOR10 (×); PtA/HMOR20 (0); PtA/HMOR30 (4); PtA/ HMOR45 (9); PtA/HMOR90 (O); PtA/HMOR120 (+); PtA/HMOR180 (]). For the series of PtA/HMOR10, -20, -30, and -90 samples, the average value (×) was used for the plot of the horizontal line.
Figure 7 shows that the TOF in dealkylation does not depend on the concentration of protonic sites in the bifunctional PtA/HMOR10 catalysts, and hence on their composition. The same trend can be observed for the other series of catalysts, with the values obtained for PtA/HMOR30 and -90 catalysts being similar and twice as high as those for PtA/HMOR10 catalysts and 2.5 times lower than those for PtA/HMOR20 catalysts (Figure 7). Furthermore, contrary to what was observed for the bimolecular reaction of disproportionation, quasi identical values of TOF for this monomolecular reaction were found for all of the catalysts, except with PtA/ HMOR20 (higher values of TOF) and with PtA/HMOR10 and -6.6 (lower values of TOF) (Figure 8). The higher values of TOF found with PtA/HMOR20 catalysts were previously ascribed to the presence in this mordenite sample of very strong acid sites able to retain pyridine adsorbed at 723 K,8 and hence particularly active in this difficult reaction. Indeed very unstable ethylcarbenium ions are involved as intermediates. The lower values of TOF found with PtA/HMOR10 and PtA/HMOR6.6 catalysts can be explained, like for isomerization and disproportionation, by the monodirectional diffusion of reactant molecules in these zeolites which do not present mesopores. 1.4. Formation of C8 Naphthenes. The formation of C8 naphthenes involves various reactions: hydrogenation of ethylbenzene into ethylcyclohexane followed by isomerization into dimethylcyclohexanes, methyl-
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Figure 9. Turnover frequency values TOF (h-1) of the metallic sites for N8 formation versus nH+, the concentration of protonic sites in the bifunctional PtA/HMOR catalysts.
Figure 11. Selectivity for ethylbenzene isomerization at 35% conversion for PtA/HMOR catalysts (calculated without considering C8 naphthenes in the products): PtA/HMOR20 (0); PtA/ HMOR30 (4); PtA/HMOR45 (s); PtA/HMOR90 (O); PtA/HMOR120 (b); PtA/HMOR120 (]).
Figure 10. Selectivity for ethylbenzene isomerization at 35% conversion for PtA/HMOR10 catalysts (calculated without considering C8 naphthenes in the products).
ethylcyclopentanes, and trimethylcyclopentanes through bifunctional catalysis and also hydrogenation of xylene products followed by bifunctional isomerization. Furthermore, because of the rapid establishment of thermodynamic equilibrium for hydrogenation, especially for high platinum contents, the values of activity, and hence of TOF, per metallic site cannot be determined with great accuracy. That is probably why the TOF value found for 2.3 PtA/HMOR catalysts is lower than that for 0.5 PtA/HMOR catalysts (Figure 9). 2. Influence of the Catalyst Characteristics on the Selectivity. Figure 10 shows with the series of PtA/ HMOR10 samples that the balance between hydrogenating and acid functions has a significant effect on the selectivity to xylenes at high ethylbenzene conversion (35%). At low value of nPt/nH+, the selectivity to xylenes is low, and for nPt/nH+ > 0.6, a plateau at a value of 7375% (when C8 naphthenes are not considered) is obtained. It should be remarked that this change in selectivity is the same as the one in TOF (Figure 2), indicating that high values of selectivity are obtained with catalysts on which the limiting step is the acid isomerization of ethylcyclohexene intermediates. However, the limit value of nPt/nH+ for constant TOF is lower: 0.6 (Figure 10) instead of 1 (Figure 2). The same change in selectivity to xylenes with nPt/ nH+ is obtained with all of the other catalysts used in this work and for the catalysts containing HMOR components with Si/Al ratios > 20 used in a previous work (Figure 11). However, the plateau (at 74-76%) is obtained for a higher value of nPt/nH+ (approximately 2.5). This could be expected because with HMOR10 samples only the acid sites of the crystallite shell participate in ethylbenzene transformations. Consequently, the ratio between nPt and the concentration of active protonic sites is much higher than the nPt/nH+ values reported in Figure 10.
Figure 12. Selectivity for ethylbenzene disproportionation, dealkylation, and cracking at 35% conversion for PtA/HMOR10 catalysts (calculated without considering C8 naphthenes in the products).
Figure 12 shows on the example of PtA/HMOR10 catalysts that the increase with nPt/nH+ in the selectivity to xylenes is mainly related to a decrease in the disproportionation and dealkylation products and to a less extent in the cracking and transalkylation products. All of this is in good agreement with the effect of the hydrogenating activity and of the acidity on the rates of the various reactions (Tables 1 and 2). 3. Optimal Catalysts for Ethylbenzene Isomerization. All of the bifunctional PtA/HMOR catalysts with values of nPt/nH+ sufficiently high to obtain the acid rearrangement of ethylcyclohexenes as a limiting step of ethylbenzene isomerization present a high selectivity to xylenes (≈75%). This high value of selectivity is obtained with PtA/HMOR10 catalysts [without mesopores but very acidic (Table 1)], which are more active than the other catalysts, e.g., 60 × 10-3 mol h-1 g-1 with 2.3 PtA/HMOR10 (95/5; Table 2) instead of 40 × 10-3 mol h-1 g-1 with 0.5 PtA/HMOR90 (90/10; Table 3). Therefore, PtA/HMOR10 catalysts would be better for ethylbenzene isomerization than bifunctional catalysts with mesoporous mordenites, with another advantage being the easier preparation, and hence the lower cost of the HMOR10 sample. It should, however, be remarked that additional work on the transformation of the xylene/ethylbenzene mixture and on long-time deactivation and regeneration is necessary to make a definitive choice of the optimal catalyst. Conclusions From this comparison of the activity and selectivity in ethylbenzene transformation of a large series of bifunctional PtA2O3/HMOR catalysts differing by the Si/
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Al ratio of the HMOR component, the platinum content of Pt/Al2O3 (PtA), or the relative proportions of Pt/Al2O3 and HMOR, the following conclusions can be drawn: 1. Ethylbenzene undergoes four apparent direct transformations: bifunctional isomerization into xylenes, acid disproportionation into benzene and diethylbenzenes, acid deethylation, and hydrogenation on platinum sites. 2. For ethylbenzene isomerization, the activity per protonic site (TOF) changes with the balance between hydrogenating and acid functions, as is expected from the bifunctional mechanism proposed for this reaction. TOF depends also significantly on the secondary porosity of the mordenite component; when the zeolite does not present mesopores (HMOR10), TOF is very low, suggesting that only some of the protonic sites participate in the reaction. 3. For ethylbenzene disproportionation, the TOF per acid site does not depend on the relative proportions of the catalyst components, and hence on the concentration of protonic sites in the bifunctional catalysts and on the hydrogenating function. Therefore, the linear dependence of TOF with the concentration of protonic sites in a series of catalysts containing dealuminated mordenites (Si/A1 from 20 to 180) presenting mesopores8 is only due to the demanding character of disproportionation. The very low values of TOF obtained with PtA/ HIMOR10 catalysts confirm diffusion limitations or blockage of channels by coke deposits in mordenites without mesopores. 4. For the monomolecular reaction of dealkylation, the TOF per acid site does not depend on the concentration of protonic sites when this one is modified by changing either the Si/Al ratio of the mordenite or the percentage of mordenite in the bifunctional catalysts. A positive effect of the acid strength is observed for this reaction that is difficult to catalyze. Again, for catalysts presenting no mesopores, the low values of TOF which are found can be related to diffusion limitations or to blockage of channels by coke deposits. 5. A high selectivity to xylenes: 75% at 35% conversion (Table 5) can be observed for the PtA/HMOR catalysts in which the limiting step of isomerization is the rearrangement of ethylcyclohexene intermediates on the protonic sites of the mordenite components. The corresponding bifunctional catalysts containing HMOR10 (which does not present mesopores but has the largest concentration of protonic sites) are slightly more active in isomerization than those containing HMOR with a secondary porosity. Literature Cited (1) Franck, H.-G.; Stadelhofer, J. W. Industrial Aromatic Chemistry, Raw Materials, Process, Products; Springer-Verlag: Berlin, Germany, 1988; p 283.
(2) Beck, J. S.; Haag, W. O. In Handbook of Heterogeneous Catalysis; Ertl, G., Ko¨zinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, Germany, 1997; Vol. 5, p 2136. (3) Sie, S. T.; de Vries, A. F.; Mesters, C. M. A. M.; Boon, A. Q. M.; Bottenberg, K.; Trautrims, B. Erdo¨ l, Erdgas, Kohle 1996, Nov, 463. (4) Guisnet, M. Actual. Chim. 1989; Apr, 9. (5) Chen, N. Y.; Garwood, W. E.; Dwyer, F. G. In Shape Selective Catalysis in Industrial Applications, Chemical Industries; Heinemann, H., Ed.; Marcel Dekker: New York, 1989; Vol. 36, p 203. (6) Silva, J. M.; Ribeiro, M. F.; Ribeiro, F. R.; Benazzi, E.; Guisnet, M. Appl. Catal. 1995, 125, 1. (7) Silva, J. M.; Ribeiro, M. F.; Ribeiro, F. R.; Benazzi, E.; Guisnet, M. Appl. Catal. 1995, 125, 15. (8) Moreau, F.; Bernard, S.; Gnep, N. S.; Lacombe, S.; Merlen, E.; Guisnet, M. J. Catal. 2001, 202, 402. (9) Gnep, N. S.; Martin de Armando, M. L.; Guisnet, M. Stud. Surf. Sci Catal. 1983, 17, 309. (10) Karge, H. G.; Sarbak, Z.; Hatada, K.; Weitkamp, J.; Jacobs, P. A. J. Catal. 1983, 82, 236. (11) Weisz, P. B. Adv. Catal. 1962, 13, 179. (12) Gnep, N. S.; Guisnet, M. Bull. Soc. Chim. Fr. 1977, nos. 5-6, 429. (13) Gnep, N. S.; Guisnet, M. Bull. Soc. Chim. Fr. 1977, nos. 5-6, 435. (14) Ro¨bschla¨ger, K. H.; Christoffel, E. G. Ind. Eng. Chem. Prod. Res. Dev. 1979, 18, 347. (15) Nitta, M.; Jacobs, P. Catalysis by Zeolites. Stud. Surf. Sci. Catal. 1980, 5, 251. (16) Weitkamp, J. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 550. (17) Guisnet, M.; Perot, G. In Zeolites: Science and Technology; Ribeiro, F. R., Rodrigues, A. E., Rollmann, L. D.; Naccache, C., Eds.; NATO series E; Martinus Nijhoff: The Hague, 1984; Vol. 80, p 397. (18) Weitkamp, J.; Ernst, S. In Guidelines for Mastering Properties of Molecular Sieves. Relationship between the Physicochemical Properties of Zeolitic Systems and Their Low Dimensionality; Barthomeuf, D., Derouane, E. G., Ho¨lderich, W., Eds.; NATO ASI Series B; Plenum: New York, 1990; Vol. 221, p 343. (19) Guisnet, M.; Alvarez, F.; Giannetto, G.; Perot, G. Catal. Today 1987, 1, 415. (20) Guisnet, M.; Magnoux, P. Catal. Today 1998, 36, 477. (21) Guisnet, M.; Magnoux, P. Appl. Catal. A 2001, 212, 83. (22) Gnep, N. S.; Roger, P.; Cartraud, P.; Guisnet, M.; Juguin., B.; Hamon, C. C. R. Acad. Sci. Paris 1989, 309, Se´rie II, 17431747. (23) Fernandes, L. D.; Monteiro, J. L. F.; Sousa-Aguiar, S. F.; Martinez, A.; Corma, A. J. Catal. 1998, 177, 363. (24) Karge, H. G.; Ladebeck, J.; Sarbak, Z.; Hatada, K. Zeolites 1982, 2, 94. (25) Guisnet, M. J. Catal. 1984, 88, 249. (26) Karge, H. G.; Hatada, K.; Zhang, Y.; Fiedorow, R. Zeolites 1983, 3, 13.
Received for review June 8, 2001 Revised manuscript received October 24, 2001 Accepted October 30, 2001 IE010505D
