NaY Catalyst and the Effect of Sulfur

J. Phys. Chem. C 2010, 114, 18501–18508

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Influence of Mo Species on the Pt/NaY Catalyst and the Effect of Sulfur Content on the Hydrodenitrogenation Reaction Rita de Ca´ssia Colman,† Maria Auxiliadora Scaramelo Baldanza,‡ and Martin Schmal*,‡ UniVersidade Federal Fluminense (UFF), Rua Passo da Patria 156, bloco D, sala 240, Rio de Janeiro/RJ, Brazil, and NUCAT-PEQ-COPPE, UniVersidade Federal do Rio de Janeiro, Rio de Janeiro/RJ, Brazil ReceiVed: May 21, 2010; ReVised Manuscript ReceiVed: September 19, 2010

The effect of Mo species and sulfur content on the hydrodenitrogenation was investigated for monometallic Pt/NaY and bimetallic PtMo/NaY catalysts. These catalysts were prepared by successive impregnation on NaY zeolite and characterized by different techniques. The catalysts were tested with quinoline. Results showed that Mo species are formed and highly dispersed. The dispersion of Pt increased 2 times on the bimetallic catalyst compared to the monometallic Pt catalyst. The conversion of Pt catalyst increased with maximum conversion of 30%, but deactivated quickly. The bimetallic catalyst stabilized after 10 h up to the end. Results show also that the conversion of the bimetallic 1Pt1Mo is 4-fold compared to the 1Pt catalyst for similar conditions. The selectivity on the bimetallic catalyst is twice that of the monometallic one. The deactivation was not significant for sulfur contents up to 500 ppm. 1. Introduction The oil industry is coming under increasing pressure from legislators to improve the quality of diesel fuels, in order to reduce exhaust emissions, especially NOx. Organic nitrogen compounds are removed catalytically by the hydrodenitrogenation (HDN) process. It is essential for many refining processes, because the presence of nitrogen compounds in the exit load is highly undesirable, causing instability and final products of lower quality. According to the literature,1-3 heterocyclic nitrogen compounds, mainly nonbasic ones (such as pyrrole and indole derivatives), cause instability of middle distillates on petroleum processing. Nonbasic nitrogen compounds undergo easily to auto-oxidation (by molecular oxygen) and condensation, presenting high-molecules of colored bodies and sediments. Indeed, distillates from destructive processing that contain high nitrogen contents (even above 1000 wt ppm) showed high instability, in contrast to straight-run middle distillates containing few nitrogen. Regarding the quality of products based on middle distillates, such as commercial diesel fuel or domestic fuel oils, maximum nitrogen content (as opposed to sulfur content) is not imposed. However, in the near future, the nitrogen content will be regulated to even below 100 ppm, which would ensure sufficient stability of middle distillate fuels. Catalytic hydrotreating may be considered as a convenient way to remove nitrogen from middle petroleum distillates. There are other new processes, such as special selective adsorbents4 that do not find commercial applications. It must be stressed that the amounts of adsorbent would be very large (or would be regenerated very often) considering the high nitrogen content in many distillates (1000 ppm of the total nitrogen corresponding to 1.5 wt % organic nitrogen compounds). * To whom correspondence should be addressed. Current address: Av. Hora´cio Macedo 2030, Centro de Tecnologia, Bloco G, Sala G-115 CEP: 21941-914, Caixa postal 68502, Cidade Universita´ria/Ilha do Funda˜o, Rio de Janeiro/RJ, Brazil. Phone: 2562-8151. Fax: 2590-7135. E-mail: [email protected]. † Universidade Federal Fluminense. ‡ Universidade Federal do Rio de Janeiro.

There are several reviews summarizing processes and catalysts.5-7 The key points in the HDN process are the natures and structures of the active sites and their relationship to the reactivity. The structures and functionalities of the catalysts and the promoters in HDS have been reviewed,7 but not those in HDN. There are studies showing that Pt supported on zeolite is very active for hydrodenitrogenation, although Pt can be easily poisoned by the presence of sulfur.8 The aim of this work is to study the mono- and bimetallic Pt-Mo on NaY zeolite catalyst and to determine the role of Mo and the influence of different sulfur contents for the HDN reaction. The activity and selectivity were evaluated by using quinoline as model reaction. 2. Experimental Section 2.1. Catalysts Preparation. The catalysts were prepared using a commercial NaY zeolite (SAR 5) from Petrobras and the precursor salts (NH4)6Mo7O24 · 4H2O (Merck) and Pt(NH3)4Cl2 (ACROS Organics). The zeolite was calcined at 500 °C with air at a flow rate of 1000 mL min-1. The monometallic Pt on the NaY zeolite was prepared by dry impregnation for a metal loading of 1 wt %, drying at 110 °C for 24 h, and then calcination in air-flow (1000 mL/min) at 350 °C for 2 h. Molybdenum (1 wt %) was prepared by wet impregnation on the NaY zeolite, using as precursor (NH4)6Mo7O24 · 4H2O, drying at 110 °C for 24 h, and calcination under air-flow (1000 mL/min) at 500 °C for 2 h. The bimetallic catalyst Pt-Mo was prepared by successive impregnation of Pt on the Mo/NaY sample, drying and similar calcination. The samples were codified as 1Pt, 1Mo, and 1Pt1Mo. 2.2. Catalysts Characterization. Hydrogen chemisorption measurements were performed in the ASAP 2010C equipment (Micromeritics). The sample was pretreated with He flux at 300 °C for 1 h. The reduction was performed with pure H2 at 30 mL/min, at 10 °C/min up to 500 °C, held for 2 h at this temperature, and cooled to 25 °C in He flow. The H2 uptakes

10.1021/jp104698s  2010 American Chemical Society Published on Web 10/11/2010

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were obtained at 100 °C, varying pressures from 0.07 to 0.43 bar. Molybdenum catalyst does not chemisorb hydrogen. The platinum dispersion was calculated by assuming a ratio of Pt:H 1:1. Temperature programmed reduction (TPR) was performed as described elsewhere.10 The samples are pretreated at 300 °C with Ar flow before reduction and switched to a mixture of 1.65% H2/Ar flow at 30 mL/min, raising the temperature at 10 °C/min up to 1000 °C. The exit gas was detected by TCD. Temperature-programmed desorption (NH3 TPD) was performed in homemade equipment11 coupled to a mass spectrometer. Before analysis, the 200 mg mass was purged with He flow (30 mL/min) from 25 to 350 °C at 5 °C/min, cooled to room temperature, reduced with H2 flow (30 mL/min) at 5 °C/ min up to 350 °C (for 1Pt and 1Pt1Mo) or 500 °C (for 1Mo) for 1 h, purged with He flow for 30 min, and cooled to room temperature. An ammonia mixture of 4% NH3/He was admitted (60 mL/min) at 150 °C for 1 h and then switched to He flow to remove physisorbed ammonia. Thermodesorption was performed under He flow (30 mL/min), raising the temperature from 150 to 700 °C at 20 °C/min. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were conducted on a Nexus 470 (Thermo Nicolet) FTIR equipped with an in situ diffuse reflection chamber and high-sensitivity MCT/A detector cooled by liquid nitrogen and a high-temperature chamber fitted with ZnSe windows (Spectra-Tech). Before analyses, the sample was treated with He flow (30 mL min-1) at 400 °C for 30 min and cooled to room temperature (30 °C) under helium, which was then switched to pure H2 (30 mL min-1) for reduction at 300 °C for 1 h. The chamber was cleaned with He flow and cooled to room temperature (30 °C). CO was admitted flowing at 1%CO/He (30 mL min-1) in the chamber for 1 min and closed for 15 min. Physisorbed gas was removed under He flow for 10 min, and the reference spectrum was recorded. Finally, desorption spectra were recorded at 150 and 300 °C. Diffuse reflection spectroscopy (UV-DRS) measurements were conducted in a Varian Cary 5 spectrophotometer (Harrick Scientific) with a diffuse reflectance accessory of “praying mantis” geometry and windows of silicon oxide. The samples 1Mo and 1Pt1Mo were set in holds with 2 mm thickness. The NaY supports were used as reference compound. The spectra were recorded in the range between 190 and 800 nm with a scanning speed of 1800 nm min-1. In situ X-ray diffraction (XRD) measurements were performed on a Rigaku equipment DMAX 2500 PC with copper radiation (Cu KR λ ) 1.5488 Å). The size of Pt crystallites with reference plane (111) in the structure was evaluated by using the Scherrer equation with the fwhm (full width at halfmaximum) of the NaY zeolite, Pt, and PtO located at the values for the Bragg angle 2θ ) 6.2, 39.7, and 48.4, respectively,

d)

λk β cos θ

where β is the width of the peak at half-maximum intensity of a specific phase (hkl) in radians, k is a constant varying with the method of taking the breadth (0.89 < k < 1) and assumed to be 0.9 in the case of cubic particles, λ is the wavelength of incident X-rays, and θ is the center angle of the peak. The analyses were performed in situ under the same reduction conditions used for the catalytic tests. Results were analyzed at room temperature and at 400 °C using Rietveld’s refinement method.

de Ca´ssia Colman et al.

Figure 1. Reduction profile of (a) 1Pt, (b) 1Mo, and (c) 1Pt1Mo catalysts.

TABLE 1: Degree of Reduction from TPR Results for Different Catalysts catalyst

Pt reduction (%) Pt2+ to Pt0

Mo reduction (%) Mo6+ to Mo4+

1Pt 1Pt1Mo

75 90

86

The morphology of 1Pt1Mo catalyst was analyzed by scanning electron microscopy (SEM), using a Quanta 400 field emission gun scanning electron FEG microscope (FEG-SEM from FEI Co.) equipped with an energy dispersive X-ray spectrometer (EDS). The sample, without previous treatment, was observed in high vacuum mode, at accelerating voltages of 10 and 20 kV. Elemental mapping was obtained for Pt and Mo. 2.3. Catalytic Tests. Regarding the catalytic experiments, 200 mg of catalyst was reduced with pure H2 at 300 °C for 2 h and then evaluated by using quinoline as reaction model at 400 °C. The reactions were performed with a H2 flow of 1000 mL/ min and 50 bar. The feed mixture consisted of 2% w/w quinoline in cyclohexane + 100 ppm sulfur (CS2), with a total time on stream reaction of about 50 h. The feed and the products were analyzed by gas chromatography (Chrompack with FID detector, capillary column, and automatic injection). The stability of the bimetallic catalyst was verified under severe sulfur conditions, by increasing the sulfur content in the feed from 150 to 500 ppm for a total time on stream reaction of about 75 h. 3. Results and Discussion 3.1. Temperature Programmed Reduction. Figure 1 displays the TPR profiles for 1Pt (a), 1Mo (b), and 1Pt1Mo (c) on NaY, and Table 1 presents the reduction degrees. The TPR profile of the Pt/NaY sample showed peaks at 233, 338, and 410 °C, which according to Lieske et al.9 and Silva et al.10 are assigned to the reduction of [Pt(OH)xCly]S and [PtOxCly]S. species. According to Raddi et al.,11 the peak at 150 °C can be assigned exclusively to the reduction of PtO. The degree of reduction indicates 75% of platinum reduction in this region. The peak at 338 °C indicates the reduction of Pt2+ to Pt0, while the peak at 410 °C is assigned to the reduction of Pt4+ species. The reduction of molybdenum occurs at higher temperature, as shown in curve b. It displays broad peaks around 641 and 931 °C. Thomas et al.12 and Regalbuto et al.13 attributed the peak around 627 °C to the reduction of MoO3 to MoO2, but it

Influence of Mo Species on the Pt/NaY Catalyst may also suggest the reduction of Mo6+ to Mo4+, according to Gutie´rrez et al.14 The peak at 758 °C is assigned to the reduction of Mo species such as Mo6+ to Mo4+ 10 and the reduction of Mo6+ tetrahedral coordinated species with strong interaction with the support. Finally, the peak at 931 °C is assigned to the reduction of dispersed tetrahedral coordinated species.10 Profile c displays the bimetallic system, which is different from the profiles of the monometallic samples Pt and Mo systems (b) after the addition of Pt. When compared to the monometallic Pt profile, the peaks at 233, 338, 410 °C are shifted to 265 °C with a little shoulder at 204 and 423 °C, respectively, and when compared to the Mo profile the main peak at 931 is shifted to 847 °C. As seen, the line shapes of this profile changed compared to the monometallic ones. The peak at 204 °C is probably attributed to the reduction of superficial Pt species containing chlorine as well as the reduction of PtO, followed by the reduction of Pt2+ to Pt0 at 265 °C.15 The degree of reduction was 90% below 400 °C, as shown in Table 1. On the other hand, the reduction of Mo6+ to Mo4+ in the bimetallic Pt-Mo catalyst occurs at much lower temperature around 423 °C when compared to the monometallic Mo sample, where this reduction occurred at 641 °C. Moreover, the reduction of dispersed Mo species occurred also at lower temperature, 847 °C, when compared to the monometallic Mo sample at 931 °C. Jao et al.16 observed similar results for the Ni-Pt catalyst supported on mordenite. According to the literature, the facilitated reductions of the PtO and Mo species of the bimetallic catalysts, compared to the monometallic ones, are attributed to the catalytic effect of reduced metals or oxide species, where the reduced Pt species facilitate the reduction of Mo species, and vice versa, the reduced Mo species help to reduce the bimetallic Pt-Mo system.17,18 3.2. Infrared of Pyridine and TPD of NH3. The FTIR spectra of pyridine adsorption show nicely the Lewis and Bro¨nsted acid sites in figure 2. There are three bands at 1442, 1490, and 1595 cm-1 together with small shoulders at 1573 and 1613 cm-1. The band at 1442 cm-1 is assigned to weak Lewis acid sites, which disappears after desorption at 300 °C (curve c). On the other hand, the band at 1490 cm-1 indicates the contribution of Lewis and Bro¨nsted acid sites, which according to Rosenthal et al.19 and Wang et al.20 are proportional. However, the absorbance of the band at 1490 cm-1 is relatively small compared to the band at 1442 cm-1 that is the evidence of the major presence of Lewis acid sites. The addition of Pt and Mo did not change significantly the acidity of NaY, suggesting that the nature of the acid sites did not change. The acid strength was determined by TPD of ammonia (Table 2) using a mass spectrometer. Results show different amounts of NH3 desorption when adding Pt or Mo: 20, 133, and 803 (µmol NH3/g cat.) for NaY, 1Pt, and 1Mo, respectively. Therefore, the acid strengths are different, but according to the pyridine adsorption results, the nature of the acid sites is similar. On the other hand, the bimetallic catalyst showed a different IR spectrum, with smaller intensity and with the appearance of a new band at 1548 cm-1, which is assigned to Bro¨nsted acid sites. It agrees with the TPD result of NH3 for the bimetallic sample (600 µmol NH3/g cat.), corresponding to the desorption of ammonia concentrated on moderate and strong acid sites in the temperature range of 355 -410 and 465-530 °C, associated with Bro¨nsted acid sites. 3.3. Diffuse Reflectance of Infrared Spectroscopy (DRIFTS). DRIFTS analyses of CO for the Pt, Mo, and PtMo catalysts are displayed in Figures 3-5. Figure 3 shows the spectrum for the Pt catalyst displaying bands at 1640, 2090,

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Figure 2. IR spectra of adsorbed pyridine of catalysts and support after (a) pyridine adsorption, (b) desorption at 200 °C, and (c) desorption at 300 °C.

TABLE 2: TPD of NH3 and Acid Strengths acid strength (%) sample NaY 1Pt/NaY 1Mo/NaY 1Pt1Mo/NaY

TPD of NH3 peak 1, peak 2, peak 3, (µmol/g cat.) 300-330 °C 355-410 °C 465-530 °C 20 133 803 600

60 100 -

40 100

100 -

2170, 2355, 3645, and 3695 cm-1. The band at 2090 cm-1 reveals linear CO adsorption on metallic Pt0, according to Zafeiratos et al.21 and Silva et al.10 The band at 2170 cm-1 is assigned to CO adsorption on Ptn+, according to Silva et al.10 and Yu et al.22 This band is not stable and disappears after fluxing He at room temperature. The hydroxyl band is displayed around 3000 cm-1 and the carbonate at 1640 cm-1, which is associated with the CO adsorption on the support.23 The band localized at 2355 cm-1 is, according to Bischoff et al.,24 attributed to the CO adsorption on Pt2+ ions on the Pt/ NaY catalyst. In contrast, Mattos et al.25 claim that this band corresponds to the interaction of CO with the zeolite structure or to the adsorption of CO2. The 1Mo catalyst displays a visible band of carbonates at 1640 cm-1 and bidentate band at 2120 and 2165 cm-1, corresponding to the CO adsorption on Moδ+, according to Silva et al.10 It suggests that the CO adsorption occurs on different valences of Mo ions, with predominance on Mo6+ at 2120 cm-1, besides adsorption band at 2165 cm-1 due to the CO adsorption on Moδ+. These bands are not stable, disappearing after fluxing with He at 150 °C. The bimetallic Pt-Mo catalyst shows the CO and carbonate adsorption bands, as shown in Figure 5. The band at 2090 cm-1

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Figure 3. DRIFTS spectra of CO for 1Pt catalyst after (a) CO flux for 1 min, (b) closed chamber for 15 min, (c) He flux for 10 min, (d) desorption at 150 °C, and (e) desorption at 300 °C.

Figure 4. DRIFTS spectra of CO for 1Mo catalyst after (a) CO flux for 1 min, (b) closed chamber for 15 min, (c) He flux for 10 min, (d) desorption at 150 °C, and (e) desorption at 300 °C.

Figure 5. DRIFTS spectra of CO for 1Pt1Mo catalyst after: (a) CO flux for 1 min, (b) closed chamber for 15 min, (c) He flux for 10 min, (d) desorption at 150 °C, and (e) desorption at 300 °C. 0

-1

n+

indicates the CO adsorption on Pt and at 2170 cm on Pt , which disappeared with increasing temperature.18,20 Comparing the CO adsorption on Mo with the monometallic catalyst (Figure 4) one observes that the band at 2120 cm-1, assigned to Mo6+, disappeared in the bimetallic system and that the band at 2165

Figure 6. IR diffuse reflectance spectra of 1Mo and 1Pt1Mo catalysts and after transformation to [F(R)hν]2 vs hν [where F(R) is the Kubelka-Munk function and hν is the energy of incident photon in electronvolts].

cm-1, which is assigned to CO on Moδ+ species, overlapping the band 2170 cm-1 of the Ptn+, probably exists, suggesting the presence of these species at the surface of the bimetallic system. CO also disappears with increasing temperature. These results confirm the existence of metallic Pt0 and ionic Ptn+ and Moδ+ species at the surface forming the bimetallic system. The Moδ+ species have high mobility, combining with Ptn+ species at the surface. TPR results corroborate with this assignment, suggesting the formation of bimetallics. Zafeiratos et al.21 studied the influence of the Mo oxidation state on the formation of stable Pt molybdenum species systems. 3.4. Diffuse Reflectance Spectroscopy (DRS). Figure 6 displays the DRS spectra of the catalysts.26 Both catalysts show large bands with maximum intensity around 250 nm. The band around 340 nm, attributed to the MoO3, was not observed, in accordance with DRIFTS results. Reference spectra of molybdenum species, such as Na2MoO4 · 2H2O and MoO3 (Giordano et al.27) indicate that bands between 290 and 330 nm values are assigned to Mo6+ in octahedral coordination, while bands between 250 and 290 nm are assigned to Mo6+ in tetrahedral coordination. Li et al.28 assigned the band at 250 nm to Mo6+ in the tetrahedral coordination. Abello29 and Xiong30 claim that bands between 250 and 280 nm correspond to the adsorption of ModO molybdate species in tetrahedral coordination. On the other hand, Fournier et al.31 showed that the cluster sizes and distances influence significantly the band position and not the local symmetry of Mo. According to Weber,26 this interpretation can be solved by plotting (F(R)hν)2 versus energy (eV), as shown in the upper detail of Figure 6. The edge energy inferring the neighboring Mo clusters was calculated and is equal 4 eV, in accordance with Leocadio et al.32 The calculated value for the bimetallic catalyst was also around 4 eV, but a second band appeared at 3.7 eV. The Moδ+ species are well-dispersed as small clusters similar to the [Mo2O7]2- anions, agreeing with DRIFTS results. Thus, the TABLE 3: H2 and CO Chemisorptions, Dispersions, and CO/H2 Ratio catalyst

H2 chemisorption (µmol/g cat.)

DH2 (%)

CO chemisorption (µmol/g cat.)

CO/H2

1Pt 1Pt1Mo

3.2 6.7

36.2 65.2

10.7 47.3

3.3 1.5

Influence of Mo Species on the Pt/NaY Catalyst

Figure 7. Verification of H2 spillover on the support for 1Pt1Mo catalyst.

Figure 8. Rietveld’s method refining of NaY zeolite.

bimetallic catalyst presents [Mo2O7]2- anions in tetrahedral coordination. The Moδ+ ions with Ptn+ ions at the surface promote distinct active sites from metallic Pt0 at the surface of the bimetallic Pt-Mo system. 3.5. H2 and CO Chemisorption. Hydrogen chemisorption results are presented in Table 3. The H2 uptake on Pt catalyst

J. Phys. Chem. C, Vol. 114, No. 43, 2010 18505 was 3.2 µmol of H2/g and 2-fold that on 1Pt1Mo (6.7 µmol of H2/g). The calculated dispersions were 36% and 70%, respectively. The increasing amount of H2 uptake can be explained by the higher mobility of molybdenum during the calcination after addition of Pt, favoring the Pt dispersion, according to Leclercq et al.33 A second possibility is the H2 spillover on the support of the 1Pt1Mo sample, according TPD analysis with He after reduction at 300 °C (Figure 7). The same effect occurred with CO adsorption; however, CO can adsorbs on metallic Pt as well as on reduced Mo oxide.10 Then, CO chemisorbed on metallic Pt0 facilitates the reduction of Mo oxide at the surface when in contact with Pt. DRS results confirm the presence of Mo ions in tetrahedral coordination of great mobility and low interaction with the support. These species favor the reduction, H2 spillover, and the CO adsorption. 3.6. In Situ XRD. The in situ XRD difractograms of NaY and of the catalysts after reduction at 300 and 400 °C are displayed in Figures 8-10. The zeolite is stable after inclusion of platinum; however, after reduction the diffratogram presents one line at 39.7°, assigned to metallic Pt 〈(111)〉. The intensity of this line increases after reduction at 400 °C. Table 4 presents the calculated parameters, based on the Rietveld analyses. The a0Z parameter changed after reduction from 24.69° to 24.80°, and the SAR number changed also from 5.0 to 3.9 before and after treatment, suggesting modification of the structure. On the other hand, the parameter relative to Pt was equal the theoretical value a0Pt ) 3.9231 Å. The calculated quantity of Pt after reduction at 300 °C was 0.55 and increased to 1.01 after reduction at 400 °C, close to the nominal value, suggesting migration of platinum from the bulk to the surface. The parameter (L111) of Pt after reduction at 300 and 400 °C changed from 0.6029 to 0.4444 rad, respectively, and the corresponding crystalline is sized from 139 to 188 Å, which is due to sintering. The diffratograms of the bimetallic 1Pt1Mo catalyst (Figures 9b and 10b) showed very similar patterns to the monometallic Pt catalyst. However, Table 3 presents significant differences. For reduction at 300 °C, the concentration of Pt was 0.32 and increased to 0.90 after reduction at 400 °C. Moreover, there is an enlargement of the line at midheight (L111) equal to 0.5553 rad at 400 °C and smaller crystallite sizes of 150 Å, suggesting higher dispersion of Pt at the surface in the bimetallic catalyst. Also the crystallite plane (111) of platinum is significantly different on the mono- and bimetallic catalysts. For reduction at 300 °C, the corresponding lines of (111) are larger on the

Figure 9. Rietveld’s method refining of (a) 1Pt catalyst after reduction at 300 °C and (b) of 1Pt1Mo catalyst after reduction at 300 °C.

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Figure 10. Rietveld’s method refining of (a) 1Pt catalyst after reduction at 400 °C and (b) of 1Pt1Mo catalyst after reduction at 400 °C.

TABLE 4: In Situ Parameters and Crystallite Sizes 1Pt a0z a0Pt surface Pt (%) L(111)(Pt) crystallite sizes (Å)

1Pt1Mo

300 °C

400 °C

300 °C

400 °C

24.8027 3.9360 0.55 0.6029 139

24.8053 3.9409 1.01 0.4444 188

24.8046 3.9404 0.32 0.3736 224

24.8072 3.9415 0.90 0.5553 150

bimetallic catalyst compared to the monometallic catalyst, confirming sintering. On the opposite, for reduction at 400 °C, there are smaller crystallites in the bimetallic catalyst. Molybdenum oxide was not observed in these analyses, confirming DRS results that only small molybdenum ions are dispersed at the surface, which probably hinder sintering of Pt

particles and favor the redispersion of particles, in agreement with chemisorptions measurements. 3.7. Microscopy Analysis. SEM analyses are presented for the bimetallic 1Pt1Mo catalyst. The images are shown in Figures 11a,b. The red and green dots show characteristic X-ray signals of Pt and Mo, respectively. As can be seen in Figure 11, both Pt and Mo particles are well-dispersed on the zeolite surface. However, this element mapping should be interpreted with caution, as the amount of Pt and Mo in the catalyst is around 1% (which does not provide accurate quantitative analysis). However, this homogeneous distribution of Pt and Mo particles is in good accordance with DRS and in situ XRD results. As observed in the figures, Mo and Pt are distributed as isolated particles.

Figure 11. MEV results for 1Pt1Mo catalyst with two different scales, (a) 2 µm and (b) 10 µm. The red points corresponding to Pt atoms and green points to Mo atoms.

TABLE 5: Conversion, Selectivity and Product Distribution for HDN Reaction product distribution (%) catalyst

TReaction (°C)

quinoline conversion (%)

HDN select. (%)

PCH

PBz

PCHE

DHQ

5,6,7,8-THQ

others

1Pt 1Pt1Mo

400 400

31 35

46 98

66.8 58.3

18.8 34.6

0 0

0 1.3

14.4 4.1

0 1.7

Influence of Mo Species on the Pt/NaY Catalyst

Figure 12. Conversion versus time comparative graphic.

3.8. Catalytic Tests. The hydrodenitrogenation of quinoline was used as the model reaction for the evaluation of these catalysts. Results are summarized in Table 5 and Figure 12, where the HDN selectivity represents the ratio of molar concentrations of hydrocarbon formation (propylbenzene, propylcyclehexane, and propylcyclehexene) and all reaction products. Figure 12 presents the conversion versus time for 1Pt and 1Pt1Mo catalysts. The conversion on Pt catalyst increased, with a maximum conversion of 30%, but the catalyst deactivated quickly. The Pt sites are poisoned due to the presence of sulfur in the feed, as observed by Chang et al.34 The Mo catalyst was inactive. The bimetallic catalyst presented an opposite behavior, where the conversion increased with time on stream and stabilized after 10 h up to the end. Results show also that the conversion of the bimetallic 1Pt1Mo is 4-fold compared to the 1Pt catalyst for similar conditions. The reduction temperature was 300 °C, but the reaction temperature was 400 °C. There are two possibilities: first, the presence of hydrogen in the feed stream running at 400 °C increased the reduction degree, and second, exposing the catalyst to CS2 in the feed stream favors the sulfiding of the Mo species at the surface with time on stream. The reduction may also affect the structure, in particular the Pt particle sizes and the interaction with Mo species in the bimetallic system. Therefore, the reduction was performed separately with H2 at 300 and 400 °C and analyzed via in situ XRD. The results are presented in Table 4. The Pt crystallites of the Pt catalyst increased from 139 to 188 Å, respectively, at 300 and 400 °C, which suggests sintering. On the other hand, the Pt crystallites for the bimetallic PtMo catalyst decreased from 224 to 150 Å after reduction at 300 and 400 °C, respectively. It suggests redispersion of Pt crystallites. The temperature may also influence the interaction of the Moδ+ species at the surface with the sulfide reductant and, consequently, favor the bindings with sulfur species. The sulfur in the feed stream would preferentially sulfide the molybdenum species at the surface. Sulfur will also be accessible to the metallic and ionic Pt sites, in competition with the Mo species. However, sulfur poisons Pt sites and deactivates the catalyst as observed on the Pt catalyst. On the contrary, catalytic results showed that the activity for the Pt-Mo increased and therefore it is strong evidence that sulfur is preferentially accessed on MoO3, transforming to MoS2, which increases the activity, not disregarding the access to Pt sites. 3.9. Selectivity. Table 5 presents the selectivity for both catalysts as well the product distribution. These values were

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Figure 13. Influence of sulfur content on conversion versus time.

calculated on the basis of propylbenzene (PBz), propylcyclohexene (PCHE), and propylcyclohexane (PCH), according to Coccheto and Satterfield35 for isoconversion around 30%. The formation of 5,6,7,8-tetrahydroquinoline (5,6,7,8-THQ) and other products not identified was also observed. Results in Table 5 show that the selectivity on the bimetallic catalyst is twice that compared to the monometallic, 98% and 46%, respectively. Moreover, PCHE was not observed and decahydroquinoline (DHQ) only at very low concentration on the bimetallic catalyst. Besides the main products, others were observed, which presumes that other parallel reactions may occur. According to Prins et al.,7 it is possible that due to the hydrogenolysis of 1,2,3,4-tetrahydroquinoline (1,2,3,4-THQ), o-propylaniline (OPA), or 5,6,7,8-THQ these compounds were formed, which are competing with other reactions. The propylcyclohexane was the main product, but propylbenzene is also significant in the bimetallic catalyst. Since less 5,6,7,8-THQ being formed would indicate that, besides higher hydrogenation capacity, the bimetallic catalyst favors the hydrogenolysis, which can be attributed to the sulfided Moδ+ species and MoS2 sites, in accordance with the literature.7 On the other hand, the hydrogenation occurs due to the free Pt free sites in the reaction path, where less hydrogen is consumed. 3.10. Effect of Sulfiding Concentration. For sulfur contents of 100 ppm the 1Pt1Mo catalyst was very active. The effect of S concentration on the selectivity or activity was tested with 150, 200, and 500 ppm of S content during 75 h. The experiment was repeated at the end for the initial condition. Results are shown in Figure 13. The conversion increases during the first 20 h with time on stream when the sulfur content was 150 ppm without deactivation. The deactivation was not significant for sulfur contents of 200 and 500 ppm, decaying from 23 to 20 or 17%, respectively. After returning to the initial sulfur concentration, the conversion reached the same value, indicating that the process is reversible. One concludes that the Pt sites are not affected by sulfur contamination due to the presence of Mo sites, which retain preferentially the sulfur. The formation of sulfided Moδ+ species and MoS2 species are responsible for the higher activity on the bimetallic Pt-Mo catalyst. 4. Conclusions Results showed that Mo species are formed and highly dispersed as Mo6+ in tetrahedral coordination and reduced to

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Mo4+ with strong interaction with the support. The dispersion of Pt increased 2 times on the bimetallic catalyst compared to the monometallic Pt catalyst. The in situ XRD showed that for reduction at 300 °C the plane (111) is larger on the bimetallic compared to the monometallic catalyst, confirming sintering. In contrast, for reduction at 400 °C there are smaller crystallites in the bimetallic catalyst. The conversion of Pt catalyst increased with maximum conversion of 30%, but deactivated quickly. The bimetallic catalyst presented an opposite behavior, where the conversion increased with time on stream and stabilized after 10 h up to the end. Results show also that the conversion of the bimetallic 1Pt1Mo is 4-fold compared to the 1Pt catalyst for similar conditions. The formation of sulfided Moδ+ species are responsible for the higher activity on the bimetallic Pt-Mo catalyst. The selectivity on the bimetallic catalyst is twice that compared to the monometallic one. The deactivation was not significant for sulfur contents up to 500 ppm. Acknowledgment. R.d.C.C. acknowledges Conselho Nacional de Pesquisa e Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq) for the Doctoral Fellowship. We also express our thanks to Sonia Vasconcelos for the MEV analysis and to Carlos A. Perez for the discussion in the XRD “in situ” analysis. We acknowledge CNPq, Faperj, and Finep for the financial support. References and Notes (1) Kozai, S.; Kabashima, H.; Hattori, H. Fuel 2000, 79, 305–310. (2) Suchaneck, A. J. Oil Gas J. 1990, (7), 109. (3) Barbier, J.; Lamy-Pitara, E.; Marecot, P. AdV. Catal. 1990, 37, 279– 317. (4) Arribas, M. A.; Corma, A.; Dı´az-Caban˜as, M. J.; Martı´nez, A. Appl. Catal., A 2004, 273, 277–286. (5) Katzer, J. R.; Sivasubranian, R. Catal. ReV. Sci. Eng. 1979, 20, 155–208. (6) Ho, T. C. Catal. ReV. Sci. Eng. 1988, 30, 117–160. (7) Prins, R.; Jian, M.; Flechsenhar, M. Polyhedron 1997, 16, 3235– 3246. (8) Topsøe, H.; Clausen, B. S.; Topsøe, N. Y.; Pedersen, E. Ind. Eng. Chem. Fund. 1986, 25, 25–36.

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