Hydrodesulfurization of Dibenzothiophene over MCM-41-Supported

Jul 13, 2012 - 3000 analyzer. ... 300 °C and 1.0 MPa with a 30 N mL/min hydrogen flow for 2 h. After ... an FID detector using a commercial HP-5 colu...
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Hydrodesulfurization of Dibenzothiophene over MCM-41-Supported Pd and Pt Catalysts Xiang Li,†,‡ Feng Zhou,† Anjie Wang,*,†,‡ Linying Wang,† and Yao Wang‡ †

State Key Laboratory of Fine Chemicals, Dalian University of Technology, No. 2 Linggong Road, Dalian 116024, P. R. China Liaoning Key Laboratory of Petrochemical Technology and Equipments, Dalian University of Technology, Dalian 116024, P. R. China

‡

S Supporting Information *

ABSTRACT: Three series of aluminosilicate MCM-41 (Al-MCM-41) were synthesized using different aluminum sources, including aluminum isopropoxide (AlM-I), pseudoboehmite, and aluminum sulfate, by a hydrothermal method. The hydrodesulfurization (HDS) performance of the Al-MCM-41-supported Pd and Pt catalysts prepared with chlorided precursors were evaluated with dibenzothiophene (DBT) as the model sulfur-containing molecule, in comparison with those supported on a siliceous MCM-41 (SiM). Pd/SiM and Pt/SiM were not promising for DBT HDS because of their relatively low activities and the rapid irreversible deactivation. Pd and Pt supported on the acidic Al-MCM-41 materials showed higher dispersion and enhanced HDS performances. AlM-I, which possessed the strongest acidity, was the most promising among the mesoporous materials investigated. The deactivated Pd/AlM-I and Pt/AlM-I can be reversibly regenerated by H2 reduction. DBT HDS over the Pd catalysts predominantly took the hydrogenation (HYD) pathway, whereas the direct desulfurization (DDS) pathway and HYD pathway were comparable for the Pt catalysts. Increasing the support acidity had no positive effect on the DDS activity of Pd but significantly enhanced its HYD activity, while the increase in the rate constant of DDS pathway was close to that of the HYD pathway for Al-MCM-41-supported Pt catalysts. The effect of the acid properties of the supports on the HDS performance of Pd and Pt catalysts was discussed by considering the formation of “electronic-deficient” particles and the hydrogen spillover process.

1. INTRODUCTION The catalytic hydroprocessing to remove heteroatoms such as sulfur, nitrogen, and oxygen from petroleum feedstocks is a critical step in the refining process. To meet the increasingly stringent regulations for the reduction in sulfur emissions, considerable research has been focused on the development of more active hydrodesulfurization (HDS) catalysts. Dibenzothiophene (DBT) and its alkylated derivatives such as 4methyldibenzothiophene and 4,6-dimethyldibenzothiophene (4,6-DMDBT) are the most refractory sulfur-containing molecules to desulfurize because of steric hindrance.1 The HDS of these bulky polyaromatic sulfur-containing compounds occurs by two parallel reaction pathways: direct desulfurization (DDS) and hydrogenation (HYD). DDS leads to the formation of biphenyls, while HYD yields tetrahydro (TH), hexahydro (HH), and dodecahydro (DH) sulfur-containing intermediates, which are desulfurized to cyclohexylbenzenes and bicyclohexyls.2 The hydrogenation of the aromatic ring, which releases the steric hindrance, will remarkably enhance the HDS reactivity of DBT and its alkylated derivates.3 The supported noble metals are much better hydrogenation catalysts than conventional bimetallic sulfides and thus might be good candidates for deep HDS.4 Nevertheless, the HDS activity of noble metals is not stable because they are sensitive to H2S and sulfur compounds contained in diesel fuel. Either alloyed or as monometallic catalysts, Pt and Pd have shown good HDS performance and are less susceptible than other metals to transformation into inactive sulfides.5,6 Pt and Pd exhibit different behaviors in DBT and 4,6-DMDBT HDS © 2012 American Chemical Society

reactions. Pt is highly active and selective to DDS, whereas Pd has a lower activity but possesses a high hydrogenation ability, especially for the refractory 4,6-DMDBT.4,7 Kabe et al. investigated the amount of sulfur accommodated and the behavior of sulfur on the sulfided noble metal catalysts in the HDS reaction using a 35S radioisotope pulse tracer method.8 The results indicated that the Pt−S and Pd−S bonds are weak. The thermodynamic equilibrium data for the sulfidation of bulk phases of Pt and Pd show that the affinity of metal to sulfur decreases in the order of Pt−S > Pd−S, indicating that Pd is more resistant to sulfur than Pt.9 Actually, Pd has been reported to have the best resistance against H2S.10 The sulfur tolerance of the supported Pt and Pd HDS catalysts and their intrinsic activity can be further improved by alloying,9,11 adding a second metal,12,13 or by the use of an acidic support such as amorphous aluminosilicate, zeolite, or the zeolite-containing supports.14−16 Nevertheless, the relatively large molecules such as DBT and 4,6-DMDBT cannot enter the pores of microporous zeolite and reach the active sites inside. Moreover, their excessively strong acidity can result in strong hydrocracking activity, not only lowering the liquid yield but also inducing severe deactivation by coking.17 Some mixed oxide supports have also been investigated.18−20 The incorporation of secondary oxides such as TiO2, ZrO2, or MgO in SiO2 or γ-Al2O3 was supposed to alter the interactions Received: April 24, 2012 Revised: July 11, 2012 Published: July 13, 2012 4671

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and pseudoboehmite were labeled as AlM-S, AlM-I, and AlM-B, respectively. The supported Pt and Pd catalysts were prepared by an incipient wetness impregnation method using an aqueous solution of H2PtCl6 or appropriate dilute hydrochloric acid solution of PdCl2 at room temperature. After impregnation, the samples was dried at 120 °C for 12 h and calcined at 500 °C in air for 3 h. For each catalyst, the loading of Pd or Pt was 1.0 wt %. 2.3. Characterization. The X-ray diffraction (XRD) patterns of the supports were measured on a Rigaku D/MAX-2400 diffractometer using nickel-filtered Cu Kα radiation at 40 kV and 100 mA. Nitrogen physisorption measurements were performed using a Tristar II 3020 adsorption analyzer. The aluminum contents of AlM-S, AlM-I, and AlM-B were determined by inductively coupled plasma optical emission spectrometry (ICP, Thermo Jarrell Ash IRIS/AP). The pyridine-adsorbed FT-IR spectra were recorded using an Equinox 55 spectrophotometer. Before the pyridine adsorption, the sample was subjected to vacuum in the sample holder at 450 °C until a pressure of 7 × 10−4 Pa was reached. Pyridine vapor was added in doses at ambient temperature until the catalyst surface was saturated and then desorbed until a pressure of 7 × 10−4 Pa was reached to ensure that there was no more physisorbed pyridine on the sample. The sample containing chemisorbed pyridine was subjected to thermal treatment at 300 °C, and then the IR spectrum was recorded. NH3 temperatureprogrammed desorption (NH3-TPD) was performed on a Chembet3000 analyzer. Prior to the NH3-TPD measurement, a 0.2 g sample was first heated in He for 2 h at 500 °C and then cooled to 40 °C and exposed to NH3 for 30 min. After the reactor was purged in a He flow, the temperature was raised at 10 °C/min to 500 °C. Transmission electron microscopy (TEM) images of the supported Pd and Pt catalysts were taken using a JEM-2100 microscope. The H2 chemisorption measurements of the catalysts were carried out using a dynamic pulse method.28 About 0.05 g of sample was reduced in a H2 flow at 300 °C for 1 h. The temperature was kept for another 1 h in an Ar stream to remove any H2 adsorbed. The catalyst was then cooled to 70 °C, and H2 chemisorption was performed. After pretreatment, 20− 30 μL of H2 was injected into a flow of Ar (40 mL/min), and the H2 uptake was measured using a thermal conductivity detector. H2 pulses were repeatedly injected until the response from the detector showed no further H2 uptake after consecutive injections. The volume of H2 adsorbed by the catalyst, the metal dispersion, and the average particle diameters of Pt and Pd were calculated based on the equations given by Aramendiá et al.29 2.4. HDS of DBT. The HDS of DBT was conducted in a trickle bed stainless-steel tubular reactor (8.0 mm i.d.). The catalyst sample was pelleted and then crushed and screened to 20−40 mesh. To improve the thermal conductivity, 0.05 g of catalyst diluted with 1.8 g of inert particles of quartz sand was charged for each run. The HDS activities of the prepared catalysts were evaluated with 0.8 wt % DBT in decalin as the model fuel. Prior to HDS reaction, the catalyst was reduced at 300 °C and 1.0 MPa with a 30 N mL/min hydrogen flow for 2 h. After that, the total pressure was increased to 5.0 MPa by H2. The reaction conditions for the HDS of DBT were as follows: temperature 300 °C, total pressure 5.0 MPa, H2/feed ratio 850 N m3/m3, and WHSV 54 h−1. The reaction products were introduced into a gas−liquid separator to collect the liquid products. The feed and liquid products were analyzed by an Agilent 6890N gas chromatograph equipped with an FID detector using a commercial HP-5 column. Because biphenyl (BP) is the only product of the DDS pathway and the hydrogenation of BP to CHB is negligible in the presence of DBT, BP selectivity (SBP) is used as a measure of the DBT DDS pathway selectivity, while (1 − SBP) represents the selectivity to the HYD pathway. The HYD pathway leads to the formation of DBT hydrogenated sulfur-containing intermediates, such as tetrahydrodibenzothiphene (TH-DBT) and hexahydrodibenzothiophene (HHDBT), as well as their desulfurized products, cyclohexylbenzene (CHB) and bicyclohexyl (BCH). Over acidic catalysts, the hydrocracking (HYC) of CHB and BCH may take place, producing benzene, cyclohexane, and the other hydrocarbons with the carbon number no more than 6. Therefore, HDS conversion (xHDS) and HYC

between the active phases of Co(Ni)Mo sulfides and the supports. However, although the Al2O3−B2O3-supported CoMo sulfide showed enhanced asphaltene conversion, its deactivation rate was faster than that of the Al2O3-supported catalyst probably because of enhancement in acidic sites.20 Therefore, the mesoporous materials with large pore size, high surface area, and tunable acidity have attracted much attention as potential supports for noble metal HDS catalysts. The reported mesoporous materials for supporting the noble metal HDS catalysts can be roughly classified into three types: the mesoporous molecular sieves,21 the micromesoporous composite molecular sieves,22 and the mesoporous zeolites.23 As one of the most representative mesoporous materials, the HDS performance of MCM-41-supported noble metals has been reported. The studies indicated that Pt and Pd supported on the siliceous MCM-41 (Si-MCM-41)24 or aluminosilicate MCM-41 prepared by postsynthesis alumination of Si-MCM4121 are promising for the HDS of relatively small sulfurcontaining molecules such as thiophene and benzothiophene. It has been reported that the type of aluminum compounds used in the hydrothermal synthesis is one of the important factors affecting the acidity and the aluminum content of aluminosilicate MCM-41 (Al-MCM-41).25 In the present work, to gain further insight into the influence of the physical and chemical properties of MCM-41 on the HDS activities, selectivities, and stabilities of the supported noble metal catalysts, the HDS performance of Pd and Pt supported on Si-MCM-41 and the hydrothermally synthesized Al-MCM-41 using different aluminum sources was studied with DBT as the model sulfur-containing molecule.

2. EXPERIMENTAL SECTION 2.1. Materials. Sodium silicate hydrate, cetyltrimethylammonium bromide (CTABr), palladium chloride (PdCl2), and chloroplatinic acid (H2PtCl6) were all of A.R. grade. Aluminum sources used for the synthesis of Al-MCM-41 were aluminum sulfate, aluminum isopropoxide, and pseudoboehmite. Dibenzothiophene (DBT) was synthesized using biphenyl and sulfur according to the method in the literature.26 Decalin was a product of Shanghai Chemical Reagents Co. and was used as solvent without further purification. A cylinder of H2 was supplied by Dalian Institute of Chemical Physics Special Gas Co. Prior to being introduced into the reactor, the H2 was deoxygenated by flowing through a catalyst bed and then dried through an adsorption column. 2.2. Support and Catalyst Preparation. Si-MCM-41 was prepared following the procedure reported previously using CTABr as the template and silicate hydrate as the silica source.27 Al-MCM-41 materials were synthesized from a mixture of reactants with the following molar composition: 1 SiO2:0.008Al2O3:0.5Na2O:0.24CTABr:60H2O. In a typical synthesis, 5.2 g of sodium silicate hydrate were dissolved in 25 mL of deionized water under stirring. Then the CTABr aqueous solution was added to the mixture, and the pH was adjusted to 11.5 by adding 6 M H2SO4. After that, a given amount of aluminum source was added. The resulting mixture was stirred for 2 h and then charged into an autoclave which was kept at 130 °C for 12 h. The solid product was dried at 120 °C overnight and calcined at 600 °C for 12 h in N2 followed by 540 °C for 6 h in air. Before the catalyst preparation, the obtained mesoporous materials were ion-exchanged with a 0.5 M HNO3 aqueous solution (1 g of MCM-41 in 10 mL of 0.5 M HNO3 solution) at 50 °C under stirring to remove Na+ ions. The HNO3 solution was renewed every 2 h for a total of 4 h, followed by filtration, washing, drying at 120 °C overnight, and finally calcined at 500 °C for 3 h. The resulting Si-MCM-41 was denoted as SiM, while Al-MCM-41 materials synthesized using aluminum sulfate, aluminum isopropoxide, 4672

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conversion (xHYC) were used to present the desulfurization and hydrocracking activity of supported Pd and Pt catalysts respectively:

x HDS =

C0 − C DBT − C TH − C HH × 100% C0

x HYC =

C0 − C DBT − C TH − C HH − C BP − CCHB − C BCH C0

(1)

(2)

× 100%

where C0 and CDBT are the concentrations of DBT in the feed and product, while CTH, CHH, CBP, CCHB, and CBCH are the concentrations of TH-DBT, HH-DBT, BP, CHB, and BCH in the product, respectively. The relative errors of all the data were within 2%.

3. RESULTS 3.1. Characterization. Strong reflections at 2θ = 2.2° were observed in the XRD patterns of all the mesoporous supports, indicating that meso-structures were well developed (Figure S1 in Supporting Information). Compared with SiM, the intensity of this peak was decreased for all the Al-MCM-41 samples. This is in agreement with previous studies that the introduction of Al on the walls of Si-MCM-41 during the synthesis decreases the order in the material.30 The results of N2 physisorption (Table S1 in Supporting Information) demonstrated that all the prepared MCM-41 samples possessed a large specific surface area (>700 m2/g) and mesopores with a diameter around 3.3 nm. The aluminum contents and the SiO2/Al2O3 molar ratios determined by ICP for SiM, AlM-S, AlM-I, and AlM-B are summarized in Table 1. The aluminum content in Al-MCM-41

Figure 1. Infrared spectra of pyridine adsorbed on SiM, AlM-B, AlM-I, and AlM-S.

bonded pyridine and pyridine coordinated to acid sites can be clearly seen in the spectrum of AlM-B. In the NH3-TPD profiles (Figure S2 in Supporting Information), SiM and AlM-B only showed broad and asymmetric peak with a maximum at ca. 210 °C, indicating that these two materials only possessed weak acid sites. The intensity of this peak increased remarkably for AlM-I and AlM-S (Figure S2 in Supporting Information). Besides, a shoulder at ca. 410 °C assigned to strong acid sites was also detected in the spectra of AlM-I and AlM-S (Figure S2 in Supporting Information). A combination of the FT-IR and NH3-TPD results indicated that the acidity of the supports decreased in the following order: AlM-I > AlM-S > AlM-B > SiM, which was correlated with the aluminum contents of the samples. The measured H2 uptakes as well as the calculated dispersion and the particles sizes for the supported Pd and Pt catalysts are listed in Table 2. An increase in the metal dispersion and thus a

Table 1. Al2O3 Contents and the SiO2/Al2O3 Molar Ratios Determined by ICP for SiM, AlM-S, AlM-I, and AlM-B sample

Al2O3 contents (wt %)

SiO2/Al2O3

SiM AlM-S AlM-I AlM-B

0.28 1.82 2.59 1.28

564 92 64 129

Table 2. The H2 Chemisorption Results for the Supported Pd and Pt Catalysts

samples decreased in the following order: AlM-I > AlM-S > AlM-B. AlM-B had a SiO2/Al2O3 ratio which was almost identical to the corresponding feed ratios (125), while an aluminum-enrichment was observed for AlM-I and AlM-S. Similar results had been reported by Reddy et al.25 that the SiO2/Al2O3 ratio of Al-MCM-41 was strongly dependent on the aluminum source. The reason may be attributed to the different reactivities of aluminum species in the gel prepared from different aluminum sources. A small amount of Al2O3 was detected in SiM (Table 1), which should be introduced as impurities in the silica source. Figure 1 shows the FT-IR spectra of pyridine adsorbed on MCM-41 samples in the region 1400−1700 cm−1 at 300 °C. SiM showed two distinct bands due to hydrogen-bonded pyridine via surface OH groups at 1445 and 1600 cm−1, which are the only bands found in pure siliceous material.31 Due to the presence of small amount of Al2O3, the bands attributed to Lewis-bonded pyridine (1620 cm−1), Brönsted-bonded pyridine (1545 and 1640 cm−1), and the band at 1490 cm−1 which can be assigned to pyridine associated with both Brönsted and Lewis acid sites were observed with low intensities in the spectrum of SiM.32 Only the bands related to the acid sites were detected for AlM-I and AlM-S. Because of the low aluminum content, all the bands associated with the hydrogen-

catalyst

H2 uptake (μmol/g·cat.)

dispersion (%)

particle size (nm)

Pd/SiM Pd/AlM-S Pd/AlM-I Pd/AlM-B Pt/SiM Pt/AlM-S Pt/AlM-I Pt/AlM-B

21 30 33 32 20 22 23 23

22 32 36 34 39 44 45 46

5.4 3.5 3.2 3.3 2.6 2.3 2.3 2.2

decrease in the particles sizes were observed when Pd and Pt were supported on Al-MCM-41. The TEM images of the supported Pd and Pt catalysts are shown in Figure 2. Pd and Pt particles were heterogeneously distributed over SiM with particle sizes ranging from 16 to 4 nm and 15 to 3 nm, respectively. The heterogeneous distribution of the particle sizes was also found for Pd/AlM-B and Pt/AlM-S, whereas small and homogeneously distributed particles were observed for AlM-I- and AlM-S-supported Pd or AlM-I- and AlM-Bsupported Pt catalysts. A good dispersion of either Pd or Pt was achieved over AlM-I. The ordered mesoporous channels of all the catalysts can be directly seen from the TEM images, indicating that the meso-structure of the supports was well preserved after the loading of the active components. 4673

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3.2. HDS of DBT. Figure 3 shows the variations of HDS conversion, HYC conversion, and the selectivities to DDS and HYD pathways with time on-stream in DBT HDS for the supported Pd catalysts. The HDS activity and the stability of the four catalysts increased in the following order: Pd/SiM < Pd/AlM-B < Pd/AlM-S < Pd/AlM-I. Pd/SiM showed the lowest initial HDS activity and deactivated quickly. The HDS conversion of DBT over Pd/SiM decreased from 59% to 24% in 8 h after the start of reaction. DBT HDS over Pd/SiM is preferably desulfurized through the HYD pathway because the selectivity to the HYD pathway was ca. 80% under the conditions studied. Pd catalysts supported on the three Alcontaining MCM-41 materials showed enhanced stability, HDS activity, and HYD pathway selectivity. Pd/AlM-I exhibited the highest activity and the best stability in DBT HDS. The HDS conversion of DBT over Pd/AlM-I was as high as 89% and only decreased slightly to 84% in 8 h. The HYD selectivity was increased to more than 90% for Pd/AlM-S, Pd/AlM-I, and Pd/ AlM-B, indicating that the HYD pathway was more enhanced than the DDS pathway after the introduction of Al to the framework of MCM-41. Accompanied with the enhanced HDS performance and HYD pathway selectivity, an obvious hydrocracking reaction took place in DBT HDS over the AlMCM-41-supported Pd catalysts. This is the result of the bifunctional catalysis involving both metal and acid centers. Therefore, Pd/AlM-I showed the highest hydrocracking activity among the catalysts due to its strong acidity. The results of DBT HDS over the Pt catalysts are presented in Figure 4. Similar to the Pd catalysts, Pt/SiM showed a low HDS activity and a rapid deactivation, but its HDS performance (the HDS conversion and the stability) and hydrocracking activity were enhanced with the introduction of Al species to Si-

Figure 2. TEM images of the supported Pd and Pt catalysts. (a) Pd/ SiM, (b) Pd/AlM-S, (c) Pd/AlM-I, (d) Pd/AlM-B, (e) Pt/SiM, (f) Pt/ AlM-S, (g) Pt/AlM-I, (h) Pt/AlM-B.

Figure 3. Variations of xHDS (■), xHYC (◇) and the selectivities to DDS (●) and HYD (▲) pathways with time on-stream in DBT HDS over the supported Pd catalysts. 4674

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Figure 4. Variations of xHDS (■), xHYC (◇) and the selectivities to DDS (●) and HYD (▲) pathways with time on-stream in DBT HDS over the supported Pt catalysts.

4. DISCUSSION 4.1. HDS of DBT. Figures 3−5 suggest that the siliceous MCM-41 is not a promising support for Pd and Pt HDS catalysts. Guo et al.17 reported a similar result that the activity of aluminum-free SBA-15-supported Pt catalyst in 4,6-DMDBT HDS was much lower than that of Pt/γ-Al2O3 at equal Pt loading, even though SBA-15 has a much higher specific surface area and wider pores. Because the SBA-15 and γ-Al2O3 possessed a similar number of acid sites and similar acid strength, they suggested that the low HDS activity of Pt/SBA15 was due to its poor Pt dispersion. However, this should not be the case in our study. For example, Pt/AlM-B (Figure 2h) showed a better metal dispersion than Pt/AlM-S (Figure 2f), but its HDS performance was lower (Figure 4). On the other hand, a close relationship between the acidity of MCM-41 and the HDS performance of the catalyst is observed. The HDS activity, HYD selectivity, hydrocracking activity, and the stability of Pd and Pt catalysts were significantly enhanced when acidic Al-MCM-41 was used as the support instead of SiMCM-41. The best HDS performance was obtained for Pd or Pt supported on AlM-I, which possessed the strongest acidity among the mesoporous materials investigated. The “electron-deficient” and “hydrogen spillover” are usually used to interpret the enhanced HDS and hydrogenation activities of the noble metals or metal sulfides supported on the acidic supports. The term “electron-deficient” applied to supported metal particles was first introduced by Dalla Betta and Boudart to account for higher hydrogenation activity of Pt in Y zeolite.33 The close contact between the strong acid site and the small cluster of metal atoms made it possible for the electrons to be withdrawn from the noble metal, thus creating an electron-deficient metal particle. These electron-deficient metal particles exhibit high activity in hydrogenation of

MCM-41. The variations of the HYD and DDS selectivities of DBT HDS with time on-stream over different Pt catalysts indicated that the HYD pathway was more enhanced than the DDS pathway when Pt was deposited on the Al-containing materials. The best HDS performance was achieved with Pt supported on AlM-I. Although Pt catalysts had a high initial HYD selectivity, it decreased dramatically and became comparable to the DDS pathway selectivity in a few hours after the reaction started, indicating that HYD and DDS pathways are competitive in the HDS of DBT over the Pt catalysts. 3.3. Regenerability of Pd and Pt Catalysts. To evaluate the regenerability of the noble metal catalysts, a consecutive HDS/reduction/HDS reaction was carried out for Pd and Pt supported on SiM and AlM-I under the relatively harsh reaction conditions: 300 °C and WHSV = 114 h−1. After 3 h HDS reaction, the catalyst was flushed and rereduced with H2 for 1 h before the second stage of HDS reaction. The results are illustrated in Figure 5. A fast and irreversible deactivation was observed for SiM-supported catalysts. The initial DBT HDS conversions for Pd/SiM and Pt/SiM decreased from 46% and 43% in the first stage of reaction to 19% and 22% in the second stage of reaction, respectively. AlM-I-supported Pd and Pt were much more stable than Pd/SiM and Pt/SiM during DBT HDS, and the initial DBT HDS conversions for Pd/AlM-I and Pt/ AlM-I in the second stage of reaction (48% and 53%, respectively) were almost identical to those of the first stage (49% and 54%, respectively), suggesting that the HDS activity of these two catalysts can be reversibly regenerated by reduction with hydrogen. 4675

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Figure 5. Variations of xHDS with time on-stream for the SiM- and AlM-I-supported Pd (a) and Pt (b) catalysts in HDS/reduction/HDS reactions.

aromatics.34 The adsorption of the aromatic compounds on the metal surface has been shown to be a flat adsorption via πbonds involving an electron transfer from the aromatic rings to the unoccupied d-metal orbitals.35 As a consequence, the aromatic compounds would be expected to be more strongly adsorbed on the active sites possessing electron deficient character. Moreover, the sulfur tolerance of noble metal catalysts increases with electron deficiency because the bonding energy between the electron-deficient metal and the electronacceptor sulfur is weakened.36 However, it is suggested that the electron deficiency is the property of only rather small metal particles; otherwise, any positive charge induced on the metal particle of a large size would be shared between numerous metal atoms forming the particle.34 According to Reyes et al.,37 the electronic effect is essentially valid only for a metal particle smaller than 1.5 nm. Ichikuni et al. studied the dependency of d electron density of Pt in Pt/SiO2 catalysts on the particle size by means of in situ XANES under vacuum.38 They found that the d electron density of the Pt particles was almost constant in particle sizes larger than 1.5 nm, whereas the smaller the particle size in the region below 1.5 nm, the more the Pt particle became electron deficient. Taking into account the relatively large particle sizes of the Pd and Pt catalysts determined by TEM and H2 chemisorption in the present study, explanations other than the formation of “electrondeficient” structure should be considered for the enhanced HDS performance of Pd and Pt supported on the acidic AlMCM-41.

Many researchers have suggested that hydrogen spillover plays an essential role in the the HDS reaction. Taking bimetallic sulfides for example, the remote control model postulates that a small fraction of hydrogen gets dissociated to spillover hydrogen on the group VIII metal sulfide, which will migrate onto the surface of the group VI metal sulfide to create the coordination unsaturated active sites.39 Ojeda et al.40 observed a synergy between beds of Mo/SiO2 and Co/SiO2 separated by 5 mm of SiO2 in the HDS of gas oil, which they suggested can be explained by the remote control model through hydrogen spillover. Because the spillover takes place over several millimeter distances, a direct interaction between the two separated beds is unnecessary for the occurrence of the synergy.39,40 Both the stability and the amount of spillover hydrogen can be enhanced by increasing the acidity of the support catalyst.41 Besides, some researchers assume that the acid site on the surface of the support could be an additional active site for HDS and hydrogenation reactions. Simon et al.42 considered two reaction routes for benzene hydrogenation over the Pt/mordenites: (1) the monofunctional hydrogenation of benzene on the metal itself and (2) the hydrogenation of Brönsted acid bonded benzene using hydrogen dissociated on the close metal surface. In the presence of sulfur-containing compounds, the activity of solely metal-catalyzed route ceased, while the route involving Brönsted acid sites was more sulfur tolerant.39 The catalyst deactivation for the reactions under hydrogen atmosphere could be inhibited by hydrogen spillover. The hydrogen spillover might be involved in the cleaning of the deactivated catalyst surface in HDS processes.41,43 We also 4676

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significantly enhanced its HYD activity. As a result, the kHYD/ kDDS for Pd/AlM-I increased to 50. However, the situation is quite different for Pt. The increase in the kDDS was close to that in the kHYD when Pt was supported on Al-MCM-41 so that the kHYD/kDDS for Pt was always around 1. The reason for the different HDS performance of the noble metal catalysts in the HDS of refractory sulfur-containing compounds is still less known,4 but our kinetics results suggest that DBT HDS over the MCM-41-supported Pd and Pt catalysts could follow different mechanisms and/or the adsorption configurations of DBT on the two catalysts are different. The adsorption states of DBT on the HDS catalysts are supposed to be in two ways: one is the end-on manner through the adsorption of sulfur atoms yielding the DDS pathway product (BP), while the other is the flat manner through the π electrons of DBT molecules leading to the formation of the HYD pathway products.2 4.2. Catalyst Deactivation. To quantify the catalyst deactivation, a simple model proposed by McCoy et al. was used in this study:46

studied the HDS performances of Pd/SiM and Pt/SiM catalysts diluted with AlM-I. To make results comparable, 0.05 g of Pd/ SiM or Pt/SiM was mixed with 0.05 g of AlM-I before being charged to the reactor. The catalysts are described as Pd/SiM +AlM-I and Pt/SiM+AlM-I. Either Pd/SiM+AlM-I or Pt/SiM +AlM-I showed intermediate HDS and hydrocracking activities between Pd/SiM and Pd/AlM-I or Pt/SiM and Pt/AlM-I, respectively (Figure S3 in Supporting Information). Nevertheless, the DDS and HYD pathway selectivities and the stabilities of the two mixed catalysts were close to those of the AlM-I-supported Pd and Pt catalysts (Figure S3 in Supporting Information). Thus, we cannot exclude that hydrogen spillover could play a role in DBT HDS over Al-MCM-41-supported Pd and Pt catalysts, enhancing their HDS and HYD activities and their sulfur tolerance. Figures 3 and 4 showed that the Pd catalysts exhibited a different HDS performance compared with the Pt catalysts. The HDS of DBT over the supported Pd catalysts occurred predominantly through the HYD pathway, whereas the DDS pathway and HYD pathway were comparable for the Pt catalysts. This is consistent with the reported HDS behavior of the supported Pt and Pd catalysts that Pt is highly selective to DDS, whereas HYD can be more competitive with DDS for Pd.7 To investigate further the influence of the acidity of MCM41 on the reaction pathways of the Pd and Pt catalysts, we studied the kinetics of DBT HDS. Thiophenic compounds follow pseudo-first-order kinetics during HDS and the reaction orders of the total reaction of DBT as well as of the DDS and HYD pathways are one.44,45 Therefore, the rate constant of the overall conversion of DBT (kDBT), the HYD pathway (kHYD), and the DDS pathway (kDDS) for different catalysts were calculated based on the data at the steady state (8 h after the start of the HDS reaction) with the pseudo-first-order plug-flow rate equation:44 −ln(1 − x)F kHDS = w

⎡ ⎤ ⎛ ⎞ 1 ln⎢(1 + ε)ln⎜ ⎟ − εx HDS⎥ = ln(k 0τ ) − kdt ⎢⎣ ⎥⎦ ⎝ 1 − x HDS ⎠ (6)

where ε is the volume expansion coefficient due to reaction, τ is the residence time, which for a heterogeneous packed bed reactor is calculated as w/F, k0 is the first-order rate constant for the fresh catalyst, kd is the deactivation rate constant, and t is time on-stream. Because of the large excess of H2 and the low DBT concentration, ε was negligible and thus eq 6 can be simplified to: ⎡ ⎛ ⎞⎤ 1 ln⎢ln⎜ ⎟⎥ = ln(k 0τ ) − kdt ⎢⎣ ⎝ 1 − x HDS ⎠⎥⎦

On the basis of the data of Figure 5 (the first stage of HDS reaction) and eq 7, the deactivation rate constants for AlM-Iand SiM-supported Pd and Pt catalysts were calculated. The obtained kd values for Pd/SiM and Pt/SiM were 1.1 and 0.61, respectively. For AlM-I-supported Pd and Pt catalysts, the deactivation rate constants were 0.22 and 0.13, respectively. The results indicate that SiM-supported Pd and Pt catalysts deactivate much faster than their AlM-I-supported counterparts. Coke and metal deposition should be considered for the catalyst deactivation during hydroprocessing of crude oil.47 However, this cannot be the case in our study because the catalysts were tested using the model fuel rather than a real feed. Hence, the deactivation of the supported noble metals must be the result of sulfur poisoning. There are two factors mainly responsible for the sulfur poisoning of noble metals: (1) The strong adsorption of sulfur-containing species or the partial sulfidation of the noble metals. It has been reported that Pd and Pt were in a partially sulfided state (PdS1−x and PtS1−x) under the typical working conditions.8 In a 35S radioisotope study, Qian et al. found that the sulfided noble metal species on Pd/γAl2O3 and Pt/γ-Al2O3 were presented in the form of PdSx and or PtSx (x = 0−0.25) under the HDS reactions.48 PdSx and PtSx can be reduced depending on the H2S/H2 ratio in the reactor.48 According to thermodynamic calculations presented by Mangnus et al.,49 the reduction of PtS to Pt on Pt/γ-Al2O3 catalysts may occur when the H2S/H2 ratio was less than 0.2. Therefore, the reversible or the temporary deactivation of Pd/

(3)

where x is the conversion of DBT, w is the weight of the catalyst (g), and F is the feed rate of DBT (mmol/h). Because BP is the only product of the DDS pathway and its further reaction is negligible in the presence of sulfur-containing compounds, the rate constants of DDS and HYD pathways were calculated as: kDDS = kHDSSBP

(4)

kHYD = kHDS − kDDS

(5)

The results are given in Table 3. Increasing the support acidity had no positive effect on the DDS activity of Pd but Table 3. Pseudo-First-Order Rate Constants for DBT HDS over the Supported Pd and Pt Catalysts catalyst

kDBT (mmol/(g·h))

kDDS (mmol/(g·h))

kHYD (mmol/(g·h))

Pd/SiM Pd/AlM-S Pd/AlM-I Pd/AlM-B Pt/SiM Pt/AlM-S Pt/AlM-I Pt/AlM-B

1.2 2.3 5.1 2.3 0.8 3.5 4.7 2.6

0.3 0.2 0.1 0.3 0.4 1.4 2.0 1.3

0.9 2.1 5.0 2.0 0.4 2.1 2.7 1.3

(7)

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AlM-I and Pt/AlM-I should be the result of this reversible sulfidation of the noble metals. (2) The sulfur-poisoninginduced metal agglomeration. This kind of sulfur poisoning of noble metals has been attributed to the fact that the adsorbed H2S decreases the metal−support interaction and thus promotes the migration and growth of the metal particles.50 Another explanation is that the Pt agglomeration during sulfur poisoning can be attributable to the formation of platinum sulfides via the direct reaction of H2S with the Pt clusters.51 Platinum sulfides are much easier to agglomerate than Pt because platinum sulfides possess a lower melting point but a higher vapor pressure.51 This can be the reason for the irreversible deactivation of Pd/SiM and Pt/SiM. The role of acidic support is supposed to decrease the formation rate of metal sulfides, to inhibit the adsorption of H2S on the support,51 or to modify the metal sites so as to inhibit the adsorption of H2S by the formation of electron-deficient species,52 thereby retarding the metal agglomeration rate. Regarding the enhanced particle dispersion of Pd/AlM-I and Pt/AlM-I compared with those of Pd/SiM and Pt/SiM (Figure 2), it is also possible that Pd and Pt have a much stronger interaction with Al-MCM-41 than with Si-MCM-41, which significantly inhibits their migration and growth. However, due to the broad metal particle size distribution of Pd/SiM and Pt/ SiM (Figure 2), it is hard to measure the changes of the particle sizes for the two catalysts before and after HDS reaction. Further studies are still needed to clarify this hypothesis.

AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-411-84986121. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (20773020, 20973030, 21073022, 21173033, and U1162203), the Ph.D. Programs Foundation (MOE, 20100041110016), the Fundamental Research Funds for the Central Universities and Project “111”.

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5. CONCLUSION Siliceous MCM-41-supported Pd and Pt prepared using the chlorided precursors exhibited low activities and rapid irreversible deactivation in the HDS reaction. A close relationship between the acidity of the support and the HDS performance of the catalyst is observed. Pd and Pt supported on the acidic Al-MCM-41 showed enhanced metal dispersion and stability as well as high HDS, HYD, and hydrocracking activities. The best HDS performance was achieved for Pd or Pt supported on AlM-I, which possessed the strongest acidity among the materials investigated. The deactivated Pd/AlM-I and Pt/AlM-I can be regenerated by H2 reduction. Because the metal particle sizes of the MCM-41-supported Pd and Pt were larger than that required for the formation of the active sites possessing an electron deficient character, it is suggested that the hydrogen spillover could be one of the reasons responsible for the enhanced HDS performance of Al-MCM-41-supported noble metal catalysts. DBT HDS over the Pd catalysts predominantly took the pathway of HYD, whereas the DDS pathway and HYD pathway were comparable for the Pt catalysts. Increasing the support acidity had no positive effect on the DDS activity of Pd but significantly enhanced its HYD activity, while the increase in the kDDS was close to that in the kHYD when Pt was supported on Al-MCM-41. It appears that DBT HDS over Pd and Pt catalysts could follow different mechanisms, and/or the adsorption configurations of DBT on the two catalysts may be different.

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Article

ASSOCIATED CONTENT

* Supporting Information S

Table S1 and Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org. 4678

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