Ind. Eng. Chem. Res. 2007, 46, 4335-4340
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Effect of La2O3 in Carbon Deposition on Pt/Al2O3 during Methylcyclopentane Reaction D. Gonza´ lez, E. Lima, and N. Martı´n* UniVersidad Auto´ noma Metropolitana-Iztapalapa. Dpto. Quı´mica, AV. San Rafael Atlixco 186, 09340 D.F. Me´ xico
In this work, we have studied the effect of the addition of lanthanum oxide to Pt/Al2O3 catalysts on its catalytic properties and its resistance to coke deactivation. The catalysts used contained 1.0 wt % in Pt, dispersed on three different supports: γ-Al2O3 (Pt/A), La2O3 (Pt/L), and a mixture (50:50 wt %) of La2O3-γ-Al2O3 (Pt/ AL). All the samples were characterized by X-ray diffraction (XRD), BET analysis, and temperatureprogrammed reduction (TPR). TPR profiles evidence the presence of an important interaction between Pt and La2O3. The activity and selectivity of the catalysts were evaluated in the hydrogenolysis of methylcyclopentane (MCP) reaction. The specific initial rates showed the following order: Pt/A . Pt/AL ≈ Pt/L. Regarding selectivity, the three samples were all alike. The ratios of specific rates, final to initial, were: Pt/A 0.68; Pt/AL 0.50; Pt/L 0.52. The amount of carbon deposited on the Pt/AL catalyst was comparable to that on Pt/A, and both were smaller than the one deposited on Pt/L. The characterization of deactivated samples by TPO, Fourier transform infrared (FTIR), 27Al MAS (magic-angle spinning) NMR and 13C MAS NMR showed differences in the coke nature between the samples. The results point out lanthana sites with differences responses to carbonaceus residues. 1. Introduction Platinum-alumina-supported catalysts are widely used in the petroleum industry for naphtha reforming. It has been reported1 that lanthanide oxides modify the temperature-programmed reduction (TPR) profiles of supported noble metals, giving place to interesting effects regarding activity and selectivity in reforming reactions. In previous studies, it has been investigated the modification of the Al2O3 produced by addition of lanthanides. One of the reported effects is the modification in their acidic/basic properties.2 Besides, the activity of supported metal catalysts can be modified by metal-supported interaction effects.3 It has also been shown that the presence of lanthanides produces a diminution of metal particle size on alumina-supported catalysts.4-6 Addtionally, La+3 atoms have possitive effects as inhibitors on the sintering and phase transformations of alumina.7 The hydrogenolysis of methylcyclopentane (MCP) is a very well studied probe reaction.8 MCP ring opening results in the formation of n-hexane (n-Hex), 2-methylpentane (2MP), and 3-methylpentane (3MP) as well as the light ends C1-C5. The so-called selective hydrogenolysis leads mainly to 2MP and 3MP on the large metal particles while nonselective hydrogenolysis yields a mixture of 2MP, 3MP, and n-Hex on small particles.9 Usually, the catalysts used are bifunctional in nature. 2MP and 3MP are formed on catalyst metal sites while cyclohexane, hexane, and benzene require the presence of both the metal and acid sites. But when the support is nonacidic and the reaction is carried out at low temperature, hydrogenative “ring opening” prevails.10 The product distribution of this reaction depends on the catalyst treatment, the nature of metals, the amount of carbon deposition, and the sulfiding degree associated with the feed.11,12 However, this distribution is rather insensitive to the calcination temperature and metallic dispersion than to the acidity and nature of the support.13 Despite its importance to the activity and * To whom correspondence should be addressed. Phone: (52)(55)58044667. Fax: (52) (55) 58044666. E-mail:
[email protected].
selectivity of reforming reactions, modification of the support has received less attention. It was also observed that MCP is one of the most active molecules for coke formation.12 In view of the location of coke deposits, it has been shown that coke forned during naphtha reforming can occupy both metal centers and the support.11 In this work, we have studied the effect of La2O3 on the hydrogenolysis of methylcyclopentane as well as on the concomitant deposition of carbonaceous residues over aluminasupported Pt catalysts. 2. Experimental Pt/Al2O3 (Pt/A) catalyst, as well as Pt/La2O3 (Pt/L), and Pt/ Al2O3-La2O3 (Pt/AL) with 50 wt % in lanthana were used. The γ-Al2O3 (A) and γ-Al2O3-La2O3 (AL) supports were prepared in the laboratory by the coprecipitation method using aluminum sec-butoxide (Aldrich 98%) and lanthanum acetylacetonate (Aldrich 98%). Supports were dried at 393 K for 24 h and then calcined under air flow at 923 K for 4 h. The support of La2O3 (L) was obtained by direct calcination of La2(NO3)3. The platinum (1.0 wt % Pt) was incorporated into the catalyst by impregnation using solutions of Pt(NH4)4(NO3)2. The impregnated samples were dried at 393 K and calcined at 673 K for 3 h. Finally, they were reduced under H2 flow during 4 h at 673 K. BET specific surface areas of the catalysts were determined by nitrogen adsorption using a Quantachrome Autosorb-3B equipment. Samples were previously degassed in vacuum ( Pt/L, i.e., the Pt/A catalyst activity is substancially higher than the Pt/AL and Pt/L. This effect can be attributed to a blockage of oxygen sites in the lanthana that are linked strongly to the platinum, diminishing the accessibility of the metal active sites as it has been previously reported.17 Moreover, the surface specific area and metallic dispersion are lower on Pt/L than on Pt/A and Pt/AL catalysts. In the literature, it has been reported8 that the activity of MCP conversion is more reliable to obtain information on the metallic dispersion, provided that (i) the metal is fully reduced and (ii) the catalytic activity takes place on the metallic phase at a rate independent of the dispersion. On the first point, the presence of metallic character of platinum even at low-temperature reduction (300 °C) is reported.22 In the second point, it is important to note that the MCP reactivity occurs on the metallic particles, even if some interaction with the support may slightly deviate the selectivity. As reported elsewhere,8 the activity and selectivity of Pt-alumina in MCP hydrogenolysis is not structure-sensitive when particles sizes are higher than about 3 nm. This result is in agreement with the observations of the dispersion obtained from TEM and benzene hydrogenation with particle size ranging from ca. 6 nm on Pt/A and Pt/AL to more than ca. 10 nm on Pt/L.
The selectivity patterns in MCP reaction to n-hexane (n-Hex), n-hexene (nHx)), 2-methylpentane (2MP), 3-methylpentane (3MP), benzene (Bz), and methylpentenes (MPe)) are also reported in Table 2. At the beginning of the reaction, the characteristic behavior of a bifunctional catalyst was observed for all three samples, i.e., obtaining the products of the ring opening reaction (2MP, 3MP, and nHx) and light products18,19 as well as those of chain lengthening (cHx and Bz) and alkenes formation (MPe) and nHx)). Bz was the major product for all samples. Production of significant quantities of Bz requires contribution from both metallic and acid functions. Thus, the higher selectivity in Bz on catalysts supported on the Al2O3 and La2O3 could be explained with an increase in the number of active sites, but the platinum dispersion remains low on these catalysts; also, we observe low 2MP selectivity. Low selectivity to light products was observed for our catalysts, which corresponds to supports with low acidity as it has been reported.19 For Pt/A, the percentage of products corresponding to the ring opening was quite low (20%), which can be related with the method of preparation of the samples. The formation of alkenes, nHx) and MPe), was also observed. On Pt/AL, the amount of 2MP and MPe) at the beginning of the reaction was higher than in the other samples. This is an interesting result due to the high demand of alkenes for the manufacture of other chemicals products. After 6 h of reaction, the selectivity pattern of the catalysts changed. The production of Bz and alkylated compounds decreases and that of nHx) slightly increases for Pt/A and Pt/ L. On the other hand, Bz production increases for Pt/AL at this time. It could be explained by assuming that coke formation is given preferably on metallic particles. In Figure 3, one can observe that the conversion decreases significantly with time on stream due to the high deactivation rate. Moreover, Pt/A suffers a higher deactivation than the other samples but was more stable after 50 min. In Table 2, the ratio of final to initial specific rates (rf/r0) in the MCP reaction is shown. The following order is observed: Pt/A > Pt/AL = Pt/ L. The reaction of MCP involves metallic and acidic sites. The acidic sites are important coke precursors to the supports which promote more polymerized coke than the metallic sites.11 Therefore, it is expected that the alumina acidic support to be more severely altered by the coke deposition than the lanthana support, in contrast with our results. Table 2 shows that the percentages of carbon of all deactivated samples were lower than 2%. The percentage of deposited carbon decreases in the following order: Pt/L > Pt/AL = Pt/ A. This could be correlated with the formation of MPe), a wellknown coke precursor,20 as well as with the similar selectivity observed for Pt/A and Pt/AL. However, Pt/L does not show a high production of alkenes and it is deactivated 50% from its initial activity. This result indicates that coke is affecting lanthana sites rather than the alumina ones. The TPR profiles and the catalytic activity of Pt/L indicate a lower dispersion on Pt/L catalyst with respect to those obserrved in the Pt/A and Pt/AL. These results probably explains the higher coke content in Pt/L compared with the Pt/A catalyst. Lankhorst et al.21 showed that the Pt/SiO2 catalysts with a low metallic dispersion are more sensitive to autodeactivation than well-dispersed catalysts. Also under the conditions of catalytic reforming, the activity of the metallic function could be solely responsible for the decrease in the performance of the catalysts.22 When the dispersion of the metal decreases the coke coverage increases, however this fact cannot explain the main deactivation of the Pt/A and Pt/AL catalyst. Therefore,
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Ind. Eng. Chem. Res., Vol. 46, No. 12, 2007
Table 2. Specific Rate (r) and Percent Selectivity (S) for MCP Reaction at 673 K, H2/Hc ) 6, and Atmospheric Pressure r × 104 mmol g-1 s-1
samples Pt/A Pt/AL Pt/L
r0 rf r0 rf r0 rf
3.57 2.42 1.69 0.85 1.43 0.75
rf/r0a 0.68 0.50 0.52
