Influence of Nitrogen-Containing Compounds on the

Jan 9, 2007 - To compensate for their lower adsorption constants .... dine. Similar to piperidine, pyridine increased the initial DDS selectivity from...
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Ind. Eng. Chem. Res. 2007, 46, 4124-4133

Influence of Nitrogen-Containing Compounds on the Hydrodesulfurization of 4,6-Dimethyldibenzothiophene over Pt, Pd, and Pt-Pd on Amorphous Silica-Alumina Catalysts Adeline Niquille-Ro1 thlisberger and Roel Prins* Institute for Chemical and Bioengineering, ETH Zurich, 8093 Zurich, Switzerland

Pyridine and piperidine inhibited the hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT) over Pt, Pd, and Pt-Pd catalysts supported on amorphous silica-alumina. Pyridine was a stronger inhibitor at low pressure, and piperidine was a stronger inhibitor at high pressure of the nitrogen-containing compound, because pyridine adsorbs relatively more strongly on the metal and piperidine adsorbs relatively more strongly on the acidic support. Hydrogenation of partly hydrogenated sulfur-containing intermediates, as well as the removal of sulfur from the intermediates, was slowed down by piperidine and pyridine. The hydrogenation pathway in the HDS of 4,6-DM-DBT was inhibited much more than the direct desulfurization pathway. The hydrogenation of the desulfurized products 3,3′-dimethylcyclohexylbenzene and 3,3′dimethylbiphenyl over the Pt-Pd catalyst was suppressed by piperidine and pyridine. Piperidine and pyridine strongly decreased the ability of noble metal particles to convert refractory molecules like 4,6-DM-DBT. Because the bimetallic Pt-Pd catalyst was less sensitive to pyridine and piperidine, it still had an advantage over the monometallic Pt and Pd catalysts. Introduction In many countries the sulfur content of transportation fuels must be very low. To attain this low level, also the most refractory molecules, such as 4,6-dimethyldibenzothiophene (4,6-DM-DBT), must be removed from naphtha and diesel by deep hydrodesulfurization (HDS) technology.1 The steric hindrance by the methyl groups strongly suppresses the direct desulfurization of 4,6-DM-DBT to 3,3′-dimethylbiphenyl, and as a consequence the HDS of 4,6-DM-DBT occurs almost exclusively by hydrogenation followed by desulfurization.1-3 Good hydrogenation catalysts, such as metals, would therefore be potential catalysts for deep HDS, but unfortunately many metals quickly transform into less active metal sulfides in the presence of sulfur-containing molecules. The noble metals Pt and Pd are less susceptible to such a transformation than other metals, and they have been intensively investigated for the removal of aromatic compounds from diesel fuel.4-7 Because of their good hydrogenation properties, Pt and Pd are also good deep HDS catalysts.8,9 The HDS of 4,6-DM-DBT was faster over Pd/γ-Al2O3 than over Pt/γ-Al2O3, and the combination of Pd and Pt greatly enhanced the HDS activity and the hydrogenation properties of the alumina-supported catalyst.10 Not only the metal, but also the support can improve the HDS activity of catalysts. Acidic supports, such as amorphous silicaalumina (ASA) and zeolites, increase the conversion of dibenzothiophene (DBT)11 and DBT substituted in the 4- and 6-positions.8,12,13 Partial electron transfer from the metal particles to the acidic sites of the support leads to electron-deficient metal particles,14,15 which are believed to have a better resistance to sulfur poisoning by decreasing the interaction with H2S.14,16 The improved activity of metal particles on an acidic support has also been ascribed to the creation of a second hydrogenation pathway by spillover of hydrogen atoms from the metal particles to the aromatic sulfur-containing molecules that are adsorbed on acidic sites in the vicinity of the metal particles.17 While the * To whom correspondence should be addressed. Tel.: +41-446325490. Fax: +41-44-6321162. E-mail: [email protected].

metal particles become poisoned by sulfur, they can still dissociate hydrogen molecules and the hydrogenation pathway by spillover would still be possible.18 Oil fractions contain not only sulfur-containing molecules, but also nitrogen-containing molecules, and these are known to inhibit HDS.2,3 HDS studies of the inhibition of various nitrogen-containing molecules have been mainly reported for sulfided NiMo and CoMo catalysts and sulfur-containing compounds that can relatively easily be converted, such as dibenzothiophene (DBT).19-23 Only a limited number of inhibition studies of the more refractory 4-methyldibenzothiophene (4-M-DBT),22 4,6-DM-DBT,22-25 4-ethyl-6-methyldibenzothiophene (4-E-6-M-DBT),13 and 4,6-diethyldibenzothiophene26 molecules have been reported. Similar inhibition research is scarce for noble metal catalysts. Inhibiting effects were observed for nitrogen-containing molecules over Pt and Pd catalysts in the HDS of DBT,27,28 4,6-DM-DBT,13 and 4-E-6-M-DBT.8,27 Therefore, we investigated the influence of nitrogen-containing molecules on the HDS of 4,6-DM-DBT over Pt, Pd, and Pt-Pd catalysts supported on ASA by adding piperidine and pyridine to the feed. ASA is not as acidic as zeolites, and metal particles supported on ASA may not be as sulfur resistant as on a zeolite. On the other hand, metal particles on ASA will be less prone to catalyst deactivation by undesired side reactions such as cracking and coking, which lead to fast deactivation of the catalysts,26,27 and all metal particles in the mesopores of ASA are accessible to the reacting molecules. Previously, we showed that the acidic amorphous silica-alumina support strongly enhanced the HDS activity for 4,6-DM-DBT,29 but that piperidine and pyridine strongly inhibited the HDS over alumina-supported noble metal catalysts.30 Piperidine and pyridine are smaller than most nitrogen-containing molecules present in gas oil, but have the advantage that they react slower, and thus exercise their inhibiting influence in a whole range of conditions, and are easier to analyze. To compensate for their lower adsorption constants, we added piperidine and pyridine in higher concentrations than nitrogen-containing molecules in gas oil.

10.1021/ie0610849 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/09/2007

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Figure 1. Total conversion (A) and HDS conversion (B) in the HDS of 4,6-DM-DBT over Pt/ASA as a function of weight time at different piperidine pressures ([, 0 Pa; ], 20 Pa; f, 100 Pa; g, 500 Pa).

Experimental Section Catalyst Preparation. As support for the noble metals, we used amorphous silica-alumina (ASA) (Shell, Si/Al ratio 4.3, surface area 560 m2/g, pore volume 0.8 mL/g) that was milled and sieved to 120-170 mesh size (90-125 µm), dried at 120 °C for 4 h, and calcined at 500 °C for 4 h. Pt and Pd were pore volume impregnated with aqueous solutions of Pt(NH3)4(NO3)2 (Aldrich, 99%) and Pd(NH3)4(NO3)2 (Alfa, 5 wt % solution), respectively. After impregnation, the catalysts were dried in air at room temperature for 3 h and then at 120 °C for 4 h, and finally calcined at 500 °C for 4 h (heating rate 5 °C/min). Monometallic 0.45 wt % Pt/ASA and 0.49 wt % Pd/ASA catalysts and a bimetallic 0.22 wt % Pt- 0.25 wt % Pd/ASA catalyst were prepared. These catalysts will be referred to hereafter as Pt/ASA, Pd/ASA, and Pt-Pd/ASA, respectively. An almost 1:1 weight ratio of Pt to Pd (1:2.08 molar ratio) was used in the bimetallic catalyst, because Fujikawa et al. had shown that a surplus of Pd gives higher hydrogenation activity.7 Catalytic Experiments. All the HDS experiments were performed at 300 °C and 5 MPa total pressure in a microflow reactor over a mixture of 10 mg of ASA-supported noble metal catalyst with 8 g of SiC to ensure good heat conductivity and plug-flow conditions. The catalysts were reduced in situ at 300 °C.29 The reactivity of 4,6-DM-DBT (Acros, 95%) was studied over the Pt/ASA, Pd/ASA, and Pt-Pd/ASA catalysts with a gas-phase feed consisting of 1 kPa 4,6-DM-DBT, 130 kPa decane (solvent), 8 kPa dodecane (internal GC standard), ∼4.85 MPa hydrogen, and 20, 100, or 500 Pa piperidine (Fluka, 99%) or pyridine (Merck, 99%). After condensation of the reaction products, off-line samples were collected and analyzed by means of a Shimadzu GC-14A gas chromatograph equipped with a DB-5 fused silica capillary column (J&W Scientific, length 30 m, inner diameter 0.25 mm, film thickness 0.25 µm) and a flame ionization detector. Every series of HDS experiments over a freshly in situ reduced noble metal catalyst started with a stabilization period of at least 1 night at the highest weight time (lowest flow rate), to diminish a possible influence of catalyst deactivation. Then, experimental data were collected with increasing flow rates of the sulfur feed and hydrogen (at constant ratio), thus with decreasing weight time. For each measuring point, we let the system stabilize for several hours (longer with low flow rates, shorter with high flow rates) after the change of flow conditions. We also checked for a possible deactivation of the catalysts by performing experiments with decreasing as well as increasing weight time. The difference in the conversion was only a few percent over the 2-3 weeks of the whole run with one catalyst. This means that stable conversions and selectivities were obtained after a few hours. Because of the acidity of ASA, isomerization and cracking occurred in the HDS of 4,6-DM-DBT over the ASA-supported

Table 1. Pseudo-First-Order Rate Constants k (in mol/min‚gcat) for the Reaction of 4,6-DM-DBT over Pt, Pd, and Pt-Pd on ASA Catalysts in the Presence of Piperidine (pip) or Pyridine (py) catalyst, pip or py

0

Pt/ASA, pip Pt/ASA, py Pd/ASA, pip Pd/ASA, py Pt-Pd/ASA, pip Pt-Pd/ASA, py

1.28 1.28 3.76 3.76 4.34 4.34

k (mol/min‚gcat.) for pip or py (Pa) 20 100 500 0.46 0.11 0.90 0.50 1.41 1.23

0.18 0.08 0.48 0.43 0.90 0.82

0.07 0.05 0.24 0.27 0.37 0.57

noble metal catalysts. Isomers with the methyl groups located at different positions were observed for each product. One isomer of 4,6-DM-DBT could be detected in trace amount, but like for the reaction products, the presence of nitrogencontaining molecules completely suppressed the isomerization. Although we confirmed the identity of the molecules corresponding to the gas chromatographic peaks by mass spectrometry, it was impossible to determine the exact chemical structure of each compound. Therefore, the many isomers of each species were collected under the same class name, without any indication of the positions for the methyl groups that had migrated around the rings by isomerization. Results Effect of Piperidine and Pyridine over Pt/ASA. The overall yields and selectivities demonstrated that cracking was significant over Pt/ASA. The selectivity to cracked products was 8% at high weight time (1.8 g‚min/mol), and an even larger amount of isomerized compounds was formed, leveling off at 32% selectivity at high weight time. Piperidine and pyridine completely suppressed the cracking and strongly reduced the isomerization reactions over all ASA-supported noble metal catalysts. Over Pt/ASA, in the presence of 20 Pa piperidine, the cracking selectivity was 0% and the isomerization selectivity only 3%, and it was even lower at higher piperidine pressure. The total conversions and HDS conversions (conversions to sulfur-free products) of 4,6-DM-DBT over Pt/ASA at 0, 20, 100, and 500 Pa piperidine are presented in Figure 1. The data at low conversion were used to calculate the pseudo-first-order rate constant k for the consumption of 4,6-DM-DBT (Table 1). The rate constant decreased monotonically with increasing piperidine pressure. The yields of the first two intermediates in the HDS of 4,6-DM-DBT (Scheme 1), dimethyltetrahydrodibenzothiophene (DM-TH-DBT) and dimethylhexahydrodibenzothiophene (DM-HH-DBT), decreased with increasing piperidine pressure, and the maxima in their yield-time curves shifted to higher weight time (Figure 2). Dimethylperhydrodibenzothiophene (DM-PH-DBT) was not observed over Pt/ ASA.

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Scheme 1. Reaction Network of the HDS of 4,6-DM-DBT over Noble Metal Catalysts

Piperidine strongly decreased the yields of the two desulfurized hydrocarbon products dimethylcyclohexylbenzene (DMCHB) and dimethylbicyclohexyl (DM-BCH), but had only a small effect on the yield of dimethylbiphenyl (DM-BP) (Figure 2). As a result, the yield of the desulfurized products decreased from 39% in the absence of piperidine to 5% in the presence of 500 Pa piperidine at the highest weight time studied (Figure 1B). The shape of the yield-time curves of the three desulfurized compounds changed in the presence of piperidine. In the absence of piperidine, the yields of DM-BP, DM-CHB, and DM-BCH leveled off at higher weight time, meaning that their rates of formation decreased with increasing weight time. These

decreases are due to the cracking of the hydrocarbons on the acidic ASA support.29 In the presence of piperidine, however, the rates of formation of DM-CHB and DM-BCH were about constant at all weight times (as demonstrated by the linear yield-time curves), while the rate of formation for DM-BP decreased slightly with time (Figure 2), because piperidine suppressed the cracking of the hydrocarbons. Largely because of the strong decrease of the yields of DM-CHB and DM-BCH (Figure 2), not only the selectivities of these hydrocarbons decreased, but also the selectivities to DM-BP and dimethyltetrahydrodibenzothiophene (DM-TH-DBT) increased when piperidine was added to the feed (Figure 3). The shape of the selectivity-time curves of DM-CHB and DM-BCH changed in the presence of piperidine (Figure 3). The DM-BCH curve showed a positive slope in the presence of piperidine, instead of a negative one in the absence of piperidine. This change in slope is due to the suppression of cracking by piperidine. For the same reason, the selectivity curve of DM-CHB became steeper, especially at high weight time. To learn more about the influence of piperidine, we compared the product distributions obtained at different piperidine pressures at constant conversion (5%) of 4,6-DM-DBT (Table 2). The selectivity of DM-BCH decreased strongly and that of DMBP increased strongly with increasing piperidine pressure. The selectivity of DM-HH-DBT decreased slightly with increasing piperidine pressure, while that of DM-CHB increased slightly. The selectivity of DM-TH-DBT increased with the addition of 20 Pa piperidine and decreased with further increase of the

Figure 2. Product yields in the HDS of 4,6-DM-DBT over Pt/ASA as a function of weight time at different piperidine pressures ([, 0 Pa; ], 20 Pa; f, 100 Pa; g, 500 Pa).

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Figure 3. Selectivities in the HDS of 4,6-DM-DBT in the absence (A) and presence of 500 Pa (B) piperidine over Pt/ASA as a function of weight time (4, DM-BP; 1, DM-TH-DBT; right-pointing 4, DM-HH-DBT; b, DM-CHB; 9, DM-BCH). Table 2. Product Selectivities and Weight Time (τ) at 5% Conversion in the HDS of 4,6-DM-DBT over Pt/ASA at Different Piperidine Pressures selectivities (%) at piperidine pressure (Pa) product DM-BP DM-CHB DM-BCH DM-TH-DBT DM-HH-DBT τ5% (g‚min/mol)

0

20

100

500

8 31 22 32 7.0

15 31 13 36 4.6

20 34 9 33 4.0

27 33 6 31 2.9

0.05

0.15

0.40

1.25

piperidine pressure. As a result of all these effects, piperidine enhanced the direct desulfurization (DDS) route (DM-BP) from 8 to 27%, increased the weight time needed to reach 5% conversion from 0.05 to 1.25 g‚min/mol, and decreased the proportion of sulfur-containing compounds from 39 to 34%. Pyridine gave results comparable to those of piperidine. The main difference was that pyridine was a much stronger inhibitor of the HDS of 4,6-DM-DBT especially at low concentration. Thus, 20 Pa pyridine decreased the pseudo-first-order rate constant k by 91%, while piperidine decreased it by only 64%. Further increases in the pyridine pressure induced smaller inhibiting effects than piperidine (Table 1). Increasing pressures of piperidine and pyridine affected the yields of all the reaction products similarly, but the inhibition by pyridine was always much stronger than that by piperidine, particularly at low pressure. Hence, the HDS conversion declined from 39% in the absence of pyridine to 2% in the presence of 500 Pa pyridine. To compare the product distributions at constant conversion (5%) of 4,6-DM-DBT in the presence of pyridine, the product yields at 500 Pa pyridine had to be determined by extrapolation, because at that pyridine pressure the inhibition was too strong to attain 5% conversion within the weight time range used. The selectivities behaved slightly differently with pyridine than with piperidine. The selectivity to DM-TH-DBT remained constant

at 32%, and that to DM-CHB first increased from 31 to 37% upon addition of 20 Pa pyridine and then decreased to 31% at 500 Pa pyridine. As with piperidine, the selectivity of DM-BP increased from 8 to 27%, and the selectivities of DM-BCH and DM-HH-DBT decreased similarly as in the presence of piperidine. Similar to piperidine, pyridine increased the initial DDS selectivity from 8 to 24%, substantially increased the weight time to reach 5% conversion from 0.05 to 2.85 g‚min/mol, and decreased the fraction of sulfur-containing compounds from 39 to 36%. Effect of Piperidine and Pyridine over Pd/ASA. Piperidine strongly inhibited the total conversion and HDS conversion of 4,6-DM-DBT over Pd/ASA (Figure 4). The pseudo-first-order rate constant k decreased by 76% by the addition of 20 Pa piperidine and less strongly (about 50%) by further increase of the piperidine pressure to 100 and 500 Pa (Table 1). Most product yields were decreased by piperidine; only the (very small) yield of DM-BP increased (Figure 5), as a result of the suppression of the cracking of DM-BP. Once this cracking was eliminated by 20 Pa piperidine, further increase of the piperidine pressure had no effect on the yield-time curve of DM-BP. The strongest yield decrease occurred for DM-BCH. Whereas its yield was higher than that of DM-CHB in the absence of piperidine, it decreased so drastically that it became lower than that of DM-CHB at 100 Pa piperidine. Piperidine affected the three sulfur-containing intermediates in a different way from the hydrocarbon products, because it not only decreased their yields but also changed the shapes of their yield-time curves (Figure 5). While in the absence of piperidine a maximum in the yield-time curves was present at short weight time, in the presence of piperidine the yields increased continuously. This means that the maxima had shifted to higher weight times, outside the measuring range. The initial rates of formation (slopes of the yield-time curves) of DM-TH-DBT, DM-HHDBT, and DM-PH-DBT decreased with increasing piperidine pressure. Piperidine influenced the yield-time curves of DMTH-DBT and DM-HH-DBT in the same way, but suppressed the rate of formation of DM-PH-DBT much more than those of DM-TH-DBT and DM-HH-DBT (Figure 5). Because the DM-BCH yield decreased much stronger than those of the sulfur-containing intermediates, the HDS conversion decreased from 42% in the absence of piperidine to 4% in the presence of 500 Pa piperidine at high weight time (Figure 4). For the same reason, the selectivities to DM-BCH and DM-PH-DBT decreased with increasing piperidine pressure, while that to DMCHB remained constant and the selectivities to DM-TH-DBT, DM-HH-DBT, and DM-BP increased (Figure 6). Thus, DMTH-DBT and DM-HH-DBT became the main reaction products already at 20 Pa piperidine. To determine the product distributions at constant conversion (10%) of 4,6-DM-DBT, the data of the experiment in the absence of piperidine had to be extrapolated. The selectivities to DM-BP and DM-CHB increased with increasing piperidine pressure (Table 3). On the other hand, those to DM-BCH and DM-PH-DBT decreased substantially in the presence of 20 Pa piperidine, but only weakly when the piperidine pressure was further increased. The selectivity to DM-TH-DBT increased significantly with the addition of 20 Pa piperidine and decreased weakly at higher piperidine pressure. The initial hydrogenation (HYD) selectivity decreased slightly from 99.8 to 98.4% with increasing pressure of piperidine. At the same time, a 10 times longer weight time (0.5 instead of 0.05 g‚min/mol) was required to reach 10% conversion and a small reduction of the fraction

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Figure 4. Total conversion (A) and HDS conversion (B) in the HDS of 4,6-DM-DBT over Pd/ASA as a function of weight time at different piperidine pressures ([, 0 Pa; ], 20 Pa; f, 100 Pa; g, 500 Pa).

Figure 5. Product yields in the HDS of 4,6-DM-DBT over Pd/ASA as a function of weight time at different piperidine pressures ([, 0 Pa; ], 20 Pa; f, 100 Pa; g, 500 Pa).

of sulfur-containing compounds (from 91 to 86%) was observed when the piperidine pressure was increased from 0 to 500 Pa. Pyridine had effects similar to those of piperidine on the product yields and selectivities in the HDS of 4,6-DM-DBT over Pd/ASA, but with different intensities. It had a strong inhibiting effect at 20 Pa, but further increases in pressure had only a small influence on the product yields and even less on their selectivities. The pseudo-first-order rate constant k decreased by 87% with the addition of 20 Pa pyridine, but much less with the subsequent 5-fold increase in the pyridine pressure (Table 1). Both piperidine and pyridine suppressed the formation of the reaction intermediates and products in a comparable way, pyridine being the stronger inhibitor at low pressure. The conversion to the desulfurized products decreased from 42%

in the absence of pyridine to 3% in the presence of 500 Pa pyridine at high weight time, which is almost the same as with piperidine. The product distributions at 10% conversion of 4,6-DM-DBT showed an increase in the selectivities to DM-BP and DM-CHB with increasing pyridine pressure, a decrease in the selectivity to DM-PH-DBT, and small decreases in the selectivities to DMHH-DBT and DM-BCH. The selectivity to DM-TH-DBT increased from 48 to 56% at 20 Pa pyridine and then remained constant at 56% in the presence of 100 and 500 Pa pyridine, a behavior similar to that in the case of piperidine. A small decrease of the initial HYD selectivity from 99.8 to 99.0% was observed with increasing pyridine pressure. However, the enhancement of the DDS route was less important than with

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Figure 6. Selectivities in the HDS of 4,6-DM-DBT in the absence (A) and presence of 500 Pa (B) piperidine over Pd/ASA as a function of weight time (4, DM-BP; 1, DM-TH-DBT; right-pointing 4, DM-HH-DBT; left-pointing 2, DM-PH-DBT; b, DM-CHB; 9, DM-BCH). Table 3. Product Selectivities and Weight Time (τ) at 10% Conversion in the HDS of 4,6-DM-DBT over Pd/ASA at Different Piperidine Pressures selectivity (%) at piperidine pressure (Pa) product DM-BP DM-CHB DM-BCH DM-TH-DBT DM-HH-DBT DM-PH-DBT τ10% (g‚min/mol)

0

20

100

500

0.5 2.5 6.0 48 26 17

0.6 4.4 4.3 58 26 7

1.0 6.1 4.0 57 26 6

1.5 8.9 3.9 55 25 6

0.05

0.15

0.25

0.50

piperidine. Moreover, longer weight times (from 0.05 to 0.65 g‚min/mol) were necessary to attain 10% conversion and a slightly decreasing fraction of sulfur-containing compounds (from 91 to 87%) was obtained with increasing pyridine pressure. Effect of Piperidine and Pyridine over Pt-Pd/ASA. Piperidine inhibited the total conversion and HDS conversion of 4,6-DM-DBT over Pt-Pd/ASA (Figure 7). The initial rate constant k decreased by 68% after the addition of 20 Pa piperidine and decreased further at higher piperidine pressure (Table 1). Piperidine inhibited the yield of DM-BCH more than that of DM-CHB (Figure 8). As a result, the yield of DM-BCH became smaller than that of DM-CHB already at 20 Pa piperidine and DM-TH-DBT was the major species at 500 Pa piperidine. The higher yield of DM-CHB at high weight time in the presence of 20 Pa piperidine than in the absence of piperidine is due to the suppression of the cracking and of the further hydrogenation of DM-CHB to DM-BCH. The same explanation holds for the higher yield of DM-BP with increasing piperidine pressure. As a consequence of the inhibition of DMBCH and DM-CHB, the HDS conversion decreased from 73% in the absence of piperidine to 14% in the presence of 500 Pa piperidine at high weight time (Figure 7).

The main influence of piperidine on the yields of the sulfurcontaining intermediates was to shift their maxima gradually to higher weight time, most of all for DM-PH-DBT, and to lower their maximum yield. The selectivities to DM-TH-DBT and DM-HH-DBT increased with the piperidine pressure, while those to DM-BCH and DM-PH-DBT decreased (Figure 9). Piperidine resulted in a more steeply increasing selectivitytime curve of DM-CHB. The selectivity of DM-BP increased with the piperidine pressure, because of its increasing yield and the decreasing yields of the main products. The product distributions obtained at 20% conversion of 4,6DM-DBT show that the selectivities of DM-BCH, DM-HHDBT, and DM-PH-DBT decreased with increasing piperidine pressure, while those of DM-BP and DM-CHB increased (Table 4). The selectivity of DM-TH-DBT increased slightly with the addition of 20 Pa piperidine and decreased at higher piperidine pressure. The initial HYD selectivity decreased from 99.5 to 95.5%. Simultaneously, a much higher weight time was required to achieve 20% conversion (from 0.05 to 0.80 g‚min/mol) and the fraction of sulfur-containing compounds decreased from 79 to 63%. The addition of pyridine led to results similar to those obtained with piperidine. As for the other catalysts, pyridine was a stronger inhibitor than piperidine at low pressure (20 Pa), but further increase of the pyridine pressure hardly changed the product yields and selectivities. The addition of 20 Pa pyridine decreased the rate constant k by 72% and further increase in the pyridine pressure caused smaller decreases (Table 1). Also, the yields of the reaction intermediates and products behaved similarly with pyridine as with piperidine. The HDS conversion at high weight time decreased from 73% in the absence of pyridine to 14% in the presence of 500 Pa pyridine. The product selectivities at 20% conversion of 4,6-DM-DBT were also similar. As with piperidine, an increase of the DDS selectivity by pyridine was observed (from 0.5 to 2.9%), but it was less pronounced than with piperidine. At the same time, the fraction

Figure 7. Total conversion (A) and HDS conversion (B) in the HDS of 4,6-DM-DBT over Pt-Pd/ASA as a function of weight time at different piperidine pressures ([, 0 Pa; ], 20 Pa; f, 100 Pa; g, 500 Pa).

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Figure 8. Product yields in the HDS of 4,6-DM-DBT over Pt-Pd/ASA as a function of weight time at different piperidine pressures ([, 0 Pa; ], 20 Pa; f, 100 Pa; g, 500 Pa).

Figure 9. Selectivities in the HDS of 4,6-DM-DBT in the absence (A) and presence of 500 Pa (B) piperidine over Pt-Pd/ASA as a function of weight time (4, DM-BP; 1, DM-TH-DBT; right-pointing 4, DM-HH-DBT; left-pointing 2, DM-PH-DBT; b, DM-CHB; 9, DM-BCH).

of sulfur-containing compounds decreased from 79 to 67% and the weight time necessary to reach 20% conversion increased from 0.05 to 0.60 g‚min/mol. Discussion Comparison of Piperidine and Pyridine. Piperidine and pyridine inhibited the HDS of 4,6-DM-DBT over the monometallic Pt and Pd and bimetallic Pt-Pd catalysts supported on ASA, due to the strong competition between the sulfur- and nitrogen-containing compounds for adsorption on the active sites. Piperidine and pyridine decreased the pseudo-first-order

initial HDS rate constant of the bimetallic Pt-Pd/ASA catalyst relatively less than the rate constants of the monometallic Pt/ ASA and Pd/ASA catalysts (Table 1). This weaker inhibition cannot be attributed to the particle size, because the strong hydrogen chemisorption measurements showed that all three catalysts had similar metal dispersion, 38% for Pt/ASA, 47% for Pd/ASA, and 38% for Pt-Pd/ASA.29 As over the alumina-supported catalysts,30 pyridine was always a stronger inhibitor than piperidine at low pressure, while at high pressure the reverse was true over Pd and Pt-Pd. An inversion in the inhibition of piperidine and pyridine was not

Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007 4131 Table 4. Product Selectivities and Weight Time (τ) at 20% Conversion in the HDS of 4,6-DM-DBT over Pt-Pd/ASA at Different Piperidine Pressures

Table 5. Turnover Frequencies (TOFs, in mol/min‚mmol) for ASA and Alumina-Supported Pt, Pd, and Pt-Pd Catalysts in the Absence and Presence of 500 Pa Piperidine or Pyridine

selectivity (%) at piperidine pressure (Pa) product DM-BP DM-CHB DM-BCH DM-TH-DBT DM-HH-DBT DM-PH-DBT τ20% (g‚min/mol)

0

20

100

500

0.5 14 6.5 41 21 17

1.6 20 5.9 43 20 9

2.3 23 5.6 42 20 7

3.5 28 5.6 39 18 6

0.05

0.20

0.30

0.80

observed over Pt/ASA, although a lower activity in the presence of piperidine than of pyridine might have been reached above 500 Pa piperidine and pyridine. The stronger inhibition by pyridine is unexpected, because saturated nitrogen-containing compounds are more basic than aromatic nitrogen-containing compounds and have a stronger inhibition due to their stronger adsorption.19,20,24,31 That means that the inhibition by piperidine should always be stronger than that by pyridine, which is, however, not the case (Table 1). Reaction of pyridine to piperidine and of piperidine to pentane was not responsible for this behavior. The reason is twofold. First, the conversion of pyridine to piperidine during the HDS of 4,6-DM-DBT was only high at high weight time, and piperidine was hardly converted. Second, at the low weight time used to determine the rate constants (Table 1), hardly any pyridine had reacted to piperidine. This was exactly the reason why we used the initial rates to compare the inhibiting effects. The crossover between pyridine, which inhibits more strongly at low pressure, and piperidine, which inhibits more strongly at high pressure, could be due to a competition between adsorption of the nitrogen-containing compounds on the support and on the metal. The more basic piperidine20 might adsorb stronger on the support than on the metal, and the reverse might be true for pyridine, which can be π bonded to the metal. This would explain the very strong inhibition by 20 Pa pyridine (about 90% for Pt and Pd), which would largely adsorb on the metal, and the much more moderate increase in the inhibition when the pyridine pressure is increased from 20 to 100 and 500 Pa. Piperidine, on the other hand, might initially adsorb preferentially on the acidic ASA support, and only at higher piperidine pressure would enough free piperidine be available to adsorb on the metal. The fact that piperidine is the stronger inhibitor at high pressure must mean that piperidine binds more strongly to the metal than pyridine. This would be in accordance with the fact that, whereas piperidine adsorbs in σ mode, pyridine adsorbs at high pressure in σ mode as well, to allow more molecules to adsorb. In the σ mode, however, the bonding of the metal is always stronger with piperidine than with the less basic pyridine. In this way, one can also understand why the crossover between pyridine and piperidine occurs at lower pressure for alumina30 than for ASA. Alumina is a weaker acid than ASA, and thus more piperidine is available for adsorption on the metal surface already at lower piperidine pressure. Increasing pressures of piperidine and pyridine inhibited both reaction pathways, HYD and DDS, in the HDS of 4,6-DMDBT over the noble metal catalysts supported on ASA, as was observed on alumina.30 To compare these pathways, turnover frequencies (TOFs) were calculated from the initial pseudofirst-order rate constants k (Table 1), the HYD and DDS selectivities, and the metal dispersions.10,29,30 The resulting TOFHYD and TOFDDS (Table 5) showed that TOFHYD decreased with increasing pressure of pyridine and piperidine over all

ASA catalyst, pip or py Pt Pt, pip Pt, py Pd Pd, pip Pd, py Pt-Pd Pt-Pd, pip Pt-Pd, py

Al2O3

TOFHYD

TOFDDS

TOFHYD

TOFDDS

135 6 4 173 11 13 327 27 42

11 2 1 0.2 0.2 0.1 2 1 1

15 2 4 22 7 11 45 10 12

2.7 1.4 1.7 0.2 0.1 0.2 0.5 0.3 0.2

catalysts, whereas TOFDDS decreased over the monometallic Pt/ ASA and remained constant over the Pd/ASA and Pt-Pd/ASA catalysts. Furthermore, TOFHYD and TOFDDS were influenced in the same way as the overall TOF: a stronger inhibition by pyridine at low initial pressure and by piperidine at high initial pressure. Over all the ASA-supported catalysts, the HYD pathway was more inhibited by the nitrogen-containing compounds than the DDS pathway, just as over the aluminasupported catalysts.30 The decrease in the initial HYD selectivity with increasing pressure of piperidine or pyridine was always more pronounced for piperidine than for pyridine, and stronger over Pt/ASA. These results are in good agreement with other studies, in which a stronger inhibition of various nitrogencontaining molecules on hydrogenation than on desulfurization was reported in different HDS reactions.11,25,32 Kwak et al. reported that the HYD route was more inhibited by carbazole and quinoline than the DDS route in the HDS of DBT, but that a stronger inhibition of the DDS route took place in the HDS of 4-M-DBT and 4,6-DM-DBT.22 It is not clear if this different behavior is caused by the fact that they performed their studies in an autoclave, while all other studies were performed in flow reactors. Influence on Cracking, Isomerization, and Hydrogenation. Piperidine and pyridine completely suppressed the cracking and drastically decreased the isomerization in the HDS of 4,6-DMDBT over the ASA-supported catalysts. This must be due to neutralization of the acidic sites of the support by these nitrogencontaining compounds.33 In the same way, quinoline inhibited the formation of cracking byproducts in the hydrogenation of tetralin over a zeolite-supported Pt-Pd catalyst34 and quinoline and carbazole suppressed the methyl-migration isomerization around the rings of 4-M-DBT and 4,6-DM-DBT.22 Piperidine and pyridine not only eliminated the cracking and diminished the isomerization, they also inhibited the further hydrogenation of the desulfurized products over the bimetallic catalyst. Thus, the yield curve of DM-CHB leveled off at high weight time in the absence, but not in the presence, of piperidine (Figure 8). This cannot be attributed to the suppression of cracking only, as cracking was already low over Pt-Pd/ASA in the absence of nitrogen-containing compounds. Similarly, the yield of DM-BP became higher in the presence of piperidine and pyridine, which suggests the inhibition of the subsequent transformation of DM-BP. Suppression of the hydrogenation of BP to CHB by 2-methylpiperidine and 2-methylpyridine was also observed in the HDS of DBT over sulfided catalysts.25 Another reason for the higher yield of DM-BP in the presence of piperidine and pyridine may be that, owing to the inhibition of the HYD pathway, a larger amount of 4,6-DM-DBT is available for the DDS route. This argument was used to explain the promotion of the BP formation in the HDS of DBT over sulfided catalysts.11,19,25

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The yields of the main desulfurized products, DM-CHB and DM-BCH, decreased steadily with increasing pressure of the nitrogen-containing compounds. Simultaneously the ratio of DM-BCH to DM-CHB decreased, but more with piperidine than with pyridine. The hydrogenation of the sulfur-containing intermediate DM-HH-DBT to DM-PH-DBT was also inhibited, more strongly by piperidine than by pyridine, like the hydrogenation of 4,6-DM-DBT. At the same time, the fraction of sulfur-containing compounds declined gradually with increasing pressure of the nitrogen-containing molecules. Again, this trend was more pronounced with piperidine than with pyridine, and with Pt-Pd/ASA than with the monometallic catalysts. Hence, the hydrogenation reactions were more inhibited by nitrogencontaining components than the sulfur-removal reactions in the HDS network of 4,6-DM-DBT over the ASA-supported catalysts. Moreover, piperidine was a stronger poison for hydrogenation than pyridine, like over the alumina-supported catalysts.30 Similarly, Matsui et al. concluded that the loss in the hydrogenation activities of the aromatic rings of the sulfur-containing compounds was responsible for the decrease in the HDS activity of Pt and Pd catalysts in the presence of nitrogen-containing molecules.13 Comparison of ASA and Alumina. The noble metal catalysts supported on ASA were much more strongly inhibited by piperidine and pyridine than those on alumina. This is most clearly seen from the TOFs. Piperidine and pyridine reduced the TOFHYD by a factor of about 21 for Pt/ASA, 15 for Pd/ ASA, and 8-12 for Pt-Pd/ASA (Table 5). The same factors for the alumina-supported catalysts are much smaller, about 5, 2-3, and 4, respectively. Similarly, Pt/ASA was more affected by ammonia than Pt/SiO2 in the HDS of thiophene35 and butylamine was a stronger inhibitor for Pt and Pd supported on zeolite than on silica in the HDS of 4,6-DM-DBT.13 The nitrogen-containing compounds will interact with the acidic sites of the support, neutralize their acidity, and thus decrease the number of highly active electron-deficient noble metal particles on the catalyst surface. They also adsorb directly on the electrondeficient noble metal species and reduce their activity.14,33 However, the ASA-supported catalysts maintained higher turnover frequencies than the alumina-supported catalysts in the presence of nitrogen components. Nevertheless, the TOFs of the monometallic Pt and Pd catalysts supported on alumina and ASA were rather close in the presence of 500 Pa pyridine. This suggests that almost all the acidic sites of Pt/ASA and Pd/ASA were neutralized under these conditions and that the benefit of the acid support disappears in the presence of high pressure of nitrogen-containing molecules. In contrast, the bimetallic PtPd/ASA catalyst was more resistant, because even at 500 Pa piperidine and pyridine the TOF of Pt-Pd/ASA was 3-4 times higher than that of Pt-Pd/Al2O3 (Table 5). Conclusions Piperidine and pyridine strongly inhibited the HDS of 4,6DM-DBT over amorphous silica-alumina supported noble metal catalysts. The bimetallic Pt-Pd/ASA catalyst was always more resistant than the monometallic Pt/ASA and Pd/ASA catalysts. Like over the alumina-supported catalysts, a stronger inhibition was found with pyridine at low initial pressure and with piperidine at high initial pressure. Increasing pressures of nitrogen components progressively inhibited both reaction pathways of the HDS of 4,6-DM-DBT, but the HYD route was more affected than the DDS pathway. Piperidine and pyridine suppressed the cracking reactions completely, decreased the isomerization drastically, and inhibited the further hydrogenation of the desulfurized products. Probably

the nitrogen-containing molecules neutralized the acidic sites of the support, which reduced the number of highly active electron-deficient noble metal particles on the catalyst surface. The hydrogenation reactions were more inhibited by the nitrogen-containing compounds than the sulfur-removal steps, and more by piperidine than by pyridine. The noble metal catalysts supported on ASA were much less resistant to nitrogen components than those supported on alumina. This was mainly attributed to the strong interaction of piperidine and pyridine with the acidic sites, which decreases the acidity of the support and its beneficial influence on the activity of the electron-deficient metal particles. Nevertheless, the ASA-supported catalysts, especially the bimetallic catalyst, remained more active than the alumina-supported catalysts. Literature Cited (1) Whitehurst, D. D.; Isoda, T.; Mochida, I. Present state and future challenges in the hydrodesulfurization of polyaromatic sulfur compounds. AdV. Catal. 1998, 42, 345. (2) Girgis, M. J.; Gates, B. C. Reactivities, reaction networks, and kinetics in high-pressure catalytic hydroprocessing. Ind. Eng. Chem. Res. 1991, 30, 2021. (3) Kabe, T.; Ishihara, A.; Qian, W. Hydrodesulfurization and Hydrodenitrogenation; Wiley-VCH: Weinheim, 1999. (4) Song, C. S.; Schmitz, A. D. Zeolite-supported Pd and Pt catalysts for low-temperature hydrogenation of naphthalene in the absence and presence of benzothiophene. Energy Fuels 1997, 11, 656. (5) Qian, W.; Yoda, Y.; Hirai, Y.; Ishihara, A.; Kabe, T. Hydrodesulfurization of dibenzothiophene and hydrogenation of phenanthrene on alumina-supported Pt and Pd catalysts. Appl. Catal., A 1999, 184, 81. (6) Navarro, R. M.; Pawelec, B.; Trejo, J. M.; Mariscal, R.; Fierro, J. L. G. Hydrogenation of aromatics on sulfur-resistant PtPd bimetallic catalysts. J. Catal. 2000, 189, 184. (7) Fujikawa, T.; Idei, K.; Ebihara, T.; Mizuguchi, H.; Usui, K. Aromatic hydrogenation of distillates over SiO2-Al2O3-supported noble metal catalysts. Appl. Catal., A 2000, 192, 253. (8) Reinhoudt, H. R.; Troost, R.; van Schalkwijk, S.; van Langeveld, A. D.; Sie, S. T.; Schulz, H.; Chadwick, D.; Cambra, J.; de Beer, V. H. J.; van Veen, J. A. R.; Fierro, J. L. G.; Moulijn, J. A. Application of ASA supported noble metal catalysts in the deep hydrodesulfurization of diesel fuel. Stud. Surf. Sci. Catal. 1997, 106, 237. (9) Ishihara, A.; Dumeignil, F.; Lee, J.; Mitsuhashi, K.; Qian, E. W.; Kabe, T. Hydrodesulfurization of sulfur-containing polyaromatic compounds in light gas oil using noble metal catalysts. Appl. Catal., A 2005, 289, 163. (10) Niquille-Ro¨thlisberger, A.; Prins, R. Hydrodesulfurization of 4,6dimethyldibenzothiophene and dibenzothiophene over alumina-supported Pt, Pd, and Pt-Pd catalysts. J. Catal. 2006, 242, 207. (11) Nagai, M.; Sato, T.; Aiba, A. Poisoning effect of nitrogen compounds on dibenzothiophene hydrodesulfurization on sulfided NiMo/ Al2O3 catalysts and relation to gas-phase basicity. J. Catal. 1986, 97, 52. (12) Reinhoudt, H. R.; Troost, R.; van Langeveld, A. D.; van Veen, J. A. R.; Sie, S. T.; Moulijn, J. A. Testing and characterization of Pt/ASA and PtPd/ASA for deep hydrodesulfurization reactions. Stud. Surf. Sci. Catal. 1999, 127, 251. (13) Matsui, T.; Harada, M.; Toba, M.; Yoshimura, Y. Effect of the coexistence of nitrogen compounds on the sulfur tolerance and catalytic activity of Pd and Pt monometallic catalysts supported on high-silica USY zeolite and amorphous silica. Appl. Catal., A 2005, 293, 137. (14) Gallezot, P. The state and catalytic properties of platinum and palladium in faujasite-type zeolites. Catal. ReV.sSci. Eng. 1979, 20, 121. (15) Sachtler, W. M. H.; Stakheev, A. Y. Electron-deficient palladium clusters and bifunctional sites in zeolites. Catal. Today 1992, 12, 283. (16) Cooper, B. H.; Donnis, B. B. L. Aromatic saturation of distillates: an overview. Appl. Catal., A 1996, 137, 203. (17) Lin, S. D.; Vannice, M. A. Hydrogenation of aromatic hydrocarbons over supported Pt catalysts. I. Benzene hydrogenation. II. Toluene hydrogenation. III. Reaction models for metal surfaces and acidic sites on oxide supports J. Catal. 1993, 143, 539, 563, 554. (18) Simon, L. J.; van Ommen, J. G.; Jentys, A.; Lercher, J. A. Sulfurtolerant Pt-supported zeolite catalysts for benzene hydrogenation. I. Influence of the support. J. Catal. 2001, 201, 60. (19) Nagai, M.; Kabe, T. Selectivity of molybdenum catalyst in hydrodesulfurization, hydrodenitrogenation, and hydrodeoxygenation: Effect of additives on dibenzothiophene hydrodesulfurization. J. Catal. 1983, 81, 440.

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ReceiVed for reView August 16, 2006 ReVised manuscript receiVed November 28, 2006 Accepted November 29, 2006 IE0610849