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MCM hydrotreating catalysts
Applied Catalysis B: Environmental 65 (2006) 118–126 www.elsevier.com/locate/apcatb
Influence of the incorporation of palladium on Ru/MCM hydrotreating catalysts D. Eliche-Quesada, J.M. Me´rida-Robles, E. Rodrı´guez-Castello´n, A. Jime´nez-Lo´pez * Departamento de Quı´mica Inorga´nica, Cristalografı´a y Mineralogı´a (Unidad Asociada al Instituto de Cata´lisis del CSIC), Facultad de Ciencias, Universidad de Ma´laga, 29071 Ma´laga, Spain Received 10 November 2005; received in revised form 22 December 2005; accepted 16 January 2006 Available online 28 February 2006
Abstract Zirconium-doped mesoporous silica with a Si/Zr molar ratio of 5 has been used as a support for bimetallic Ru-Pd catalysts. The effect of palladium as a promoter in a catalyst containing ruthenium as active phase, and the influence of the Ru/Pd atomic ratio were studied in the hydrogenation and ring-opening reaction of tetralin. The results indicate the promotional effect of Pd on hydrotreating activity of catalyst since an increase is observed in the formation of hydrogenation products, especially trans-decalin at low temperatures for 5-RuPd(8/1) and 5-RuPd(4/1), while at higher temperatures, there is an enhacement in the production of hydrogenolysis/hydrocracking products. In contrast 5-RuPd(15/1) catalyst shows at low temperatures lower yields of hydrogenation products and higher yields of high molecular weight products (HMW) than 5-Ru catalyst while at higher temperatures than 315 8C only produces HMW products. The best catalytic performance is displayed by the 5-RuPd(8/1) catalyst, giving a conversion of tetralin of 98.5% as well as high yields of hydrogenation (20.8%) and ring-opening (70.1%) products under the following experimental conditions: temperature of reaction 350 8C; H2/tetralin molar ratio of 10; contact time 2.8 s and total pressure of 6 MPa (P(H2) = 4.5 MPa and P(N2) = 1.5 MPa). Moreover, this catalyst exhibits good thiotolerance in the presence of 600 ppm of dibenzothiophene in the feed, and an excellent tolerance to nitrogen compounds even with 2000 ppm of acridine. # 2006 Elsevier B.V. All rights reserved. Keywords: Bimetallic ruthenium-palladium catalysts; Hydrogenation; Hydrogenolysis/hydrocracking; Tetralin; MCM-41
1. Introduction The importance of decreasing the impact of diesel fuel on air quality has led to increasingly stringent regulations to reduce emissions containing particles originating from combustion of polyaromatic hydrocarbons and nitrogen and sulphur oxides. In order to achieve this, the improvement of middle distillates is desirable and can be achieved by: (i) decreasing the sulphur content, (ii) reducing the aromatic hydrocarbons content and (iii) increasing the cetane number with hydrogenolysis/ring-opening reactions [1]. To a certain extent, the latter two strategies are interrelated, i.e. the reduction of the aromatic content has a positive effect on increasing the cetane number [2].
* Corresponding author. Tel.: +34 952 13 18 76; fax: +34 952 13 75 34. E-mail address: [email protected] (A. Jime´nez-Lo´pez). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.01.003
For deep hydrogenation (HYD) of aromatics and deep hydrodesulphuration (HDS) in diesel fuel, single-stage and two-stage approaches, have been proposed. The single-stage hydrotreating process combines severe hydrodesulphuration with aromatic hydrogenation (HDA) using sulphur-tolerant CoMo, NiMo or NiW catalysts supported on alumina, but high temperature and high hydrogen pressures are necessary to achieve an acceptable level of aromatic reduction [3,4]. In the two-stage hydrotreating process, the conventional hydrotreating catalyst is used in the first reactor to reduce the level of sulphur to a few ppm and a noble-metal catalyst is used in the second reactor. These noble metal catalysts can achieve a high level of aromatic hydrogenation at low reaction temperatures and moderate hydrogen pressures. However, they display a low resistance to sulphur poisoning [5,6]. Interestingly, sulphur tolerance may be enhanced by modifying the physicochemical characteristics of the metal atoms by: (i) using acidic carriers [6], (ii) changing the metal particle size or (iii) alloying with
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other noble metals [7]. Sulphur tolerance has been shown to be related to the formation of electron-deficient metal sites, which in turn weaken the strength of the sulphur–metal bond [7–9]. Noble metals supported on large-pore-zeolites have received much attention for their use as aromatic hydrogenation catalysts [10,11]. Zeolite acidity, however, drastically increases amounts of undesirable cracking products, which accelerates the ratio of coke deposition and the yield in gases [12]. Furthermore, due to their pore size, the access to active sites is restricted to the largest feed molecules [13]. Hence, much effort has been devoted to research for finding new mesoporous solid supports. Recently, MCM-41 type materials have been studied in these catalytic processes, since they possess an ordered structure, high surface area and large pore sizes that allow the diffusion of aromatic molecules [14–18]. Moreover, as mesoporous MCM-41 silica has only a small number of acid sites strong enough to catalyse the hydrogenolysis/ring-opening reactions, the siliceous framework can be doped with heteroatoms such as Al, Ti or Zr to increase the acidity of mesoporous solids. Thus, zirconium containing MCM-41 type materials provide an excellent behaviour as acid catalysts, as previously reported [19,20]. We have recently reported [21] the results obtained for ruthenium supported on a zirconium-doped mesoporous catalyst in the hydrotreating of tetralin. The catalyst with a 5 wt% of ruthenium displays an excellent catalytic activity at 320 8C and maintains its catalytic performance in the presence of 600 ppm of dibenzotiophene (DBT). In the current work we have also considered the promoting effect of palladium metal on the ruthenium catalyst in the hydrotreating of tetralin. With this aim, we prepared bimetallic catalysts with different Ru/Pd atomic ratios and the resulting catalysts were assayed in the hydrotreating of tetralin in the presence of DBT and acridine. The latter were used to represent sulphur and nitrogen poisoning, respectively, since they are the main S- and Ncontaining compounds in diesel. The catalysts were characterized by X-ray diffraction (XRD), temperature programmed desorption (TPD) of NH3, X-ray photoelectron spectroscopy (XPS), H2-chemisorption, temperature programmed reduction with H2 (H2-TPR) and adsorption–desorption of nitrogen. 2. Experimental 2.1. Catalyst preparation Zirconium-doped mesoporous silica with a Si/Zr molar ratio of 5, was prepared by following the method described elsewhere [20,22], but with a reduction in the reaction time to only 24 h at room temperature. The material was calcined at 550 8C (1 8C min 1 heating rate) for 6 h and was denoted as SiZr-5. Bimetallic Ru-Pd catalysts were prepared by wet impregnation of the pelletized support (0.85–1.00 mm) using mixed aqueous solutions of hydrated ruthenium(III) chloride (RuCl3xH2O) and Pd(NO3)2. A total metal loading of 5 wt% was kept constant in all samples, whereas different molar ratios of Ru/Pd(x/y) were used (15/1, 8/1, 4/1). After impregnation with metallic salts, the samples were dried at 120 8C and reduced directly at 400 8C for 1 h without previous calcination.
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2.2. Catalyst characterization Powder X-ray diffraction patterns were recorded on a Siemens D5000 diffractometer, equipped with a graphite monochromator and using Cu Ka radiation. X-ray photoelectron spectra were collected using a Physical Electronics PHI 5700 spectrometer with non-monochromatic Mg Ka radiation (300 W, 15 kV, 1253.6 eV) and with a multichannel detector. Charge referencing was measured against adventitious carbon (C 1 s, 284.8 eV). A PHI ACCESS ESCAV6.0 F software package was used to record and analyze the spectra. A Shirley-type background was subtracted from the signals. Recorded spectra were always fitted using Gauss– Lorentz curves in order to determine the binding energy of the different element core levels more accurately. The reduced samples, before the XPS analysis, were kept in n-hexane to avoid contact with air. The reducibility of the catalysts was studied by temperatureprogrammed reduction with hydrogen (H2-TPR). Experiments were carried out between 50 and 700 8C, using a flow of Ar/H2 (40 cm3 min 1, 10 vol% of H2) and a heating rate of 10 8C min 1. The effluent gas was passed through two traps before passing into the thermal conductivity detector, a solid CaO trap to remove HCl, and a cold trap ( 80 8C) to remove water. The acidic properties of reduced catalysts were analyzed by temperature-programmed desorption of chemisorbed ammonia (NH3-TPD). Before the adsorption of ammonia at 100 8C, the samples were reduced at 400 8C using a flow of H2 (60 cm3 min 1). The ammonia desorbed between 100 and 550 8C (heating rate of 10 8C min 1) was analyzed by an on line gas chromatograph (Shimadzu GC-14A) provided with a TC detector. The specific surface areas of the solids were evaluated from the nitrogen adsorption–desorption isotherms at 196 8C in a MICROMERITICS ASAP 2020 apparatus, after degassing at 200 8C and 1.3 10 2 Pa for 24 h. Hydrogen chemisorption was performed in a MICROMERITICS ASAP 2010 apparatus, after the in situ reduction of samples at 400 8C (15 8C min 1) for 1 h, under a flow of H2. After reduction, the catalysts were degassed at 10 4 Pa for 10 h at the same temperature and cooled at 35 8C, to carry out the chemisorption of H2. The range of pressure studied in chemisorption was 0.013–0.04 MPa and the amounts of hydrogen chemisorbed were calculated by extrapolation of the isotherms to zero pressure. Dispersion data have been calculated by assuming a stoichiometry H/Ru and H/Pd = 1. CNHS chemical elemental analysis was used for determine the content of coke and sulphur in the spent catalyst and was carried out by using a LECO CNHS 932 instrument. 2.3. Catalytic activity measurement The hydrogenation of tetralin was performed in a highpressure fixed-bed continuous-flow stainless steel catalytic reactor (9.1 mm i.d. and 230 mm length) operated in the downflow mode at different temperatures. The reaction temperature
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was measured with an interior placed thermocouple in direct contact with the top part of the catalyst bed. The organic feed consisted of a solution of tetralin in n-heptane (10 vol%) and was supplied by means of a Gilson 307SC piston pump (model 10SC). A fixed volume of catalyst (3 cm3 with particle size of 0.85–1.00 mm) without dilution was used in all cases. Prior to the activity test, the catalysts were reduced in situ at atmospheric pressure with H2 (flow rate 60 cm3 min 1) at 400 8C for 1 h, with a heating rate of 15 8C min 1. Catalytic activities were measured at different temperatures, under 6.0 MPa (4.5 MPa H2 and 1.5 MPa N2), and a liquid hourly space velocity (LHSV) of 6.0 h 1. The catalytic reaction was studied by collecting liquid samples after remaining 60 min at each reaction temperature, and kept in sealed vials for posterior analysis by both gas chromatography (Shimadzu GC-14B, equipped with a flame ionization detector and a capillary column, TBR-1, coupled to a Shimadzu AOC-20i automatic injector) and mass spectrometry (Hewlett-Packard 5988A). The performance of the microreactor and the accuracy of the analytical method were studied by feeding a solution of tetralin in n-heptane (10 vol%) into the reactor filled with 3 cm3 of SiC, operating at 300 8C and 6.0 MPa. No formation of foreign products was detected with a recovery percentage of the tetralin feed of 95%. To investigate the effects of sulphur and nitrogen poisoning on the catalytic performance, the organic feed was mixed with different concentrations of dibenzothiophene (DBT) (between 300 and 1000 ppm wt%), or with different concentrations of acridine (between 500 and 2000 ppm wt%). In previous experiments, a variation in the amount of catalyst and gas flow rate while maintaining the space velocity constant were also carried out in order to discard the possible existence of diffussional limitations under the experimental conditions used, led to no modification of conversion values. Particle diameter was found to have no influence either. The catalysts were tested in tetralin hydrogenation to evaluate their potential behaviour in the hydrogenation of aromatic hydrocarbons in diesel fuels. Total conversion of
tetralin was evaluated as tetralin converted with respect to tetralin in the feedstock. A large number of products were detected by gas chromatography analysis. After identification of the majority of them, they were classified, as recently reported [23–25], into the following groups: (i) volatile compounds (VC) that include non-condensable C1–C6 products whose presence was calculated from the carbon balance of the reaction, (ii) hydrogenation products that include trans- and cis-decalin, (iii) high-molecular-weight compounds (HMW) that include primary products such as toluene, ethylbenzene, oxylene, 1-ethyl-2-methylbenzene, 1-propenyl-2-methylbenzene, n-propylbenzene and iso-propylbenzene and secondary products, which are derived from ring-opening reactions such as polyalkylolefins, decadiene and cyclohexene-1-butylindene and (iv) naphthalene (Scheme 1). In the HMW group, are also included the products formed by isomerization of tetralin and decalins that can lead to the formation of a large number of C10 isomers, as was found by other authors [26,27]. 3. Results and discussion 3.1. Catalyst characterization The XRD profiles at high angles do not show diffraction peaks corresponding to the noble metal salts or metal aggregated phases for the precursor and the reduced catalysts, respectively. This fact indicates a very good and stable dispersion of the phases for all compositions. Thus, for all catalysts the metal crystal sizes must be below the XRD limit of detection. The N2 adsorption–desorption isotherms of the supported precursors are characteristic of mesoporous materials and exhibit the same shape as that of the supported SiZr-5 and can be considered as Type IV in the IUPAC classification. When ruthenium–palladium salts were dispersed on the support (Table 1) the BET surface area and the pore volume undergo a slight decrease which could be due to a partial blockage of
Scheme 1. Products distribution in the hydrogenation and hydrogenolysis/hydrocracking of tetralin.
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mesopores by ruthenium–palladium salts. The pore size distributions are very narrow with a maximum centered in the region of mesoporous materials (5.4–5.6 nm). Temperature-programmed reduction is a powerful technique to study the reduction behaviour of oxide phases or salts, as well as providing information regarding the degree of interaction between metal–metal and metal–support. The profiles of monometallic Ru (5 wt%) and Pd (1 wt%) catalysts are included as a reference in Fig. 1 where the profile of 5RuPd(8/1) is shown as representative of this family of catalysts. In the case of the monometallic Pd sample, a maximum centred at 152 8C is observed which is assigned to the reduction of the Pd precursor, Pd(NO3)2. The monometallic Ru sample exhibits a main peak centred at 146 8C with a small shoulder at 102 8C. The widening of this peak is possibly due to the reduction of Ru3+ located in different environments. This main peak can be assigned to Ru3+/Ru0 single step reduction [21,28,29]. The profile also exhibits a small and ill defined band of hydrogen consumption between 275 and 350 8C which could correspond to a small amount of ruthenium oxide or oxychloride formed by the exposition and drying in air at 120 8C [30–32]. All bimetallic Ru–Pd samples display an essentially singular reduction peak with a maximum at temperatures between 134 and 151 8C. Thus, the H2-TPR profile of the bimetallic 5RuPd(8/1) catalyst (Fig. 1) shows a single peak at 140 8C, i.e. at a lower temperature than those of the monometallic catalysts. However, as the maximum reduction peaks of the monometallic catalysts are very close to those of the monometallic ones and ruthenium is in great excess as compared to Pd, it is very difficult to establish the formation of any Ru–Pd alloy. The simultaneous reduction of the two salts could however indicate that ruthenium and palladium must be in close interaction with each other, suggesting the formation of bimetallic Ru–Pd particles. In any case, the reduction of Ru salts seems to be easier in the presence of Pd because the peak at 350 8C almost disappears in the bimetallic catalysts. The TPD of adsorbed ammonia was used to determine the surface acidity of the reduced catalysts. The desorption temperature is a measure of the strength of the corresponding acid sites, while the total amount of ammonia desorbed after saturation coverage permits the quantification of the number of acid sites at the surface. The total surface acidity of the support and reduced catalysts between 100 and 550 8C are given in Table 1. The total acidity of the bimetallic catalysts is always higher than that of the support, but it is much lower than that of
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Fig. 1. H2-TPR profiles for the 5-Ru, 5-RuPd(8/1) and 1-Pd catalysts.
5-Ru (1628 mmol NH3 g 1). Because the high acidity of the ruthenium catalyst is essentially due to the marked trend of this metal to adsorb ammonia molecules, the dramatic decrease in the acidity could be due to a modification of the superficial nature of the ruthenium particles by the addition of palladium. Therefore, palladium atoms can probably be said to be located on the ruthenium metallic particles instead of forming an alloy. On the other hand, all catalysts studied display a mild acidity because they desorbed the main quantity of NH3 in the temperature range of 100–300 8C. The metallic properties of the reduced catalysts are compiled in Table 2. The degree of reduction, a, was 100% for all samples. The dispersion degree (D), the metallic surface area and the particle size were obtained from H2 chemisorption data. It is observed that the dispersion is higher for the highest Ru/Pd molar ratio. The catalyst with the highest ruthenium content, 5RuPd(15/1), shows the greatest amount of chemisorbed
Table 1 Textural properties of 5-RuPd(x/y) precursors and acidic properties of 5-RuPd(x/y) reduced catalysts Sample
SiZr-5 5-RuPd(15/1) 5-RuPd(8/1) 5-RuPd(4/1) 5-RuPd(8/1) spent DBT 5-RuPd(8/1) spent acridine
SBET (m2 g 1)
515 443 456 425 449 424
Vp (cm3 g 1)
0.63 0.61 0.63 0.58 0.60 0.54
Acidity (mmol NH3 g 1) 100–300 8C
300–400 8C
400–550 8C
Total
293 443 427 433 – –
125 131 111 157 – –
113 79 102 163 – –
531 653 640 754 – –
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hydrogen, and hence the lowest metallic particle size (2.0 nm) and the highest dispersion degree (43.6%). These values are similar to those of the catalyst containing only 5 wt% of ruthenium [21]. However, the catalysts with a ruthenium– palladium molar ratio of 8/1 and 4/1 show lower dispersion values of 21.6 and 11.4%, respectively, and metallic particle sizes of 4.0 and 7.4 nm, respectively. These results indicate that the dispersion of these palladium promoted catalysts is only higher than that of 5-Ru catalyst for the 5-RuPd(15/1) catalyst which contains the lowest loading of promoted palladium, only a 0.33 wt%. Photoelectron spectra of Si 2p, Zr 3d, Ru 3p and Pd 4f core levels were recorded for precursors and reduced samples. The binding energies of Si 2p and Zr 3d core electrons are practically constant and similar to those of the support. The XPS spectra of the core level region of Ru 3p for all ruthenium– palladium precursors exhibit a symmetrical peak at 462.8– 462.3 eV, which coincides with the binding energy of Ru 3p3/2 in the impregnation salt, RuCl3 [33]. After reduction, this signal is slightly shifted to a lower B.E. at 462.0–462.1 eV, indicating the presence of Ru(0) (Fig. 2) [33]. The spectrum of the Ru-5 catalyst after reduction shows a small shoulder at higher binding energies, which could be due to the presence of some remaining oxychlorides, which, as shown by TPR results, are seen to be more difficult to reduce. This shoulder disappeared for the mixed catalysts (Fig. 2), indicating that the incorporation of Pd favoured the reduction of ruthenium. However, the formation of alloys between both metals is not apparent from XPS data since the B.E.s of Ru in mixed catalysts are very close to that of the Ru catalyst. The direct measurement of the position and area of the Pd 3d5/2 peak is difficult to establish due to the overlapping of the Pd 3d5/2 core level with the intense peak of the Zr 3p3/2 component of the substrate centred at 333.4 eV. The B.E. of the Pd 3d5/2 level for the precursors appears at 336.7–337.2 eV corresponding to Pd(II) found in the Pd(NO3)2 salt [33]. After reduction, the B.E. of the Pd 3d is shifted to 335.1 eV and is overlapped completely with the Zr 3p signal, therefore the study of metallic palladium is not possible. 3.2. Catalytic results 3.2.1. Influence of the temperature of reaction and ruthenium–palladium molar ratio The ruthenium–palladium family of catalysts was tested in the hydrogenation and hydrogenolysis–isomerization–hydro-
Fig. 2. Ru 3p core level spectra of (a) 5-Ru and (b) 5-RuPd(8/1) reduced catalysts.
cracking of tetralin under pressure at different reaction temperatures (275–375 8C). The conversion of tetralin and the yields of different products for the three catalysts described above are shown in Fig. 3. The results obtained with a ruthenium catalyst (5-Ru) on the same support are included for comparison [21]. In general, all the conversion values are very high for the range of temperatures studied (>85%), but the distribution of products depends on the reaction temperature. At low temperatures the formation of decalins is favoured due to the exothermic character of this reaction [34,35]. Thus, in all cases, the main product at low reaction temperatures (Fig. 3) was trans-decalin. Moreover, this yield and the trans/cis ratio increase when the molar Ru/Pd ratio decreases. With respect to the 5-Ru catalyst, at low temperatures (T = 315 8C) palladium containing catalysts have a promotional effect: not only is the conversion increased but also the formation of hydrogenation products is increased too, especially that of trans-decalin, except for 5-RuPd(15/1) catalyst which has the maximum yield of HMW products (39%) at this low temperature. When the temperature is raised, the yield of HMW products increases. This is due to the hydrogenolysis/hydrocracking of tetralin and decalins involving C–C bond cleavage which is an endothermal process [36], but is detrimental to decalin formation. At 350 8C
Table 2 Metallic characteristics for the reduced ruthenium–platinum supported catalysts Catalysts
Ru (wt%)
Pd (wt%)
D (%)
2
m 5-Ru 5-RuPd(15/1) 5-RuPd(8/1) 5-RuPd(4/1) 5-RuPd(8/1) (spent 1000 ppm DBT) 5-RuPd(8/1) (spent 1000 ppm acridine)
5.00 4.67 4.42 3.96 4.42 4.42
– 0.33 0.58 1.04 0.58 0.58
D is the metallic dispersion and d is the average diameter of the metallic crystallite.
35.2 43.6 21.6 11.4 11.0 20.2
d (nm)
Smet
6.4 8.1 4.1 2.2 2.1 3.8
gcat1
m
2
gRu1-Pd
128.6 161.7 81.0 43.3 41.2 75.7
2.4 2.0 4.0 7.4 7.8 4.2
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Fig. 3. Evolution of the conversion of tetralin hydrogenation/hydrogenolysis–hydrocracking and yield of the different reaction products as a function of the reaction temperature on Ru/Pd catalysts: (*) conversion, () trans-decalin, (&) cis-decalin, (*) HCC, (~) naphthalene and ( ) VC. Experimental conditions: H2/THN molar ratio = 10.1, P(H2) = 4.5 MPa, P(N2) = 1.5 MPa, contact time = 2.8 s.
all catalysts show considerable HMW yields. The 5-RuPd(15/1) catalyst shows the highest yield of HMW (67.9%), while the yield of decalins is only 1.7%. The behaviour of the 5-RuPd(8/ 1) and 5-RuPd(4/1) catalysts is very interesting because they show similar values with a good balance for hydrogenation products (22.0 and 30.0%, respectively) and HMW products (65.0 and 59.4%, respectively). Therefore, a new enhancement of conversion and formation of HMW products is observed in both palladium-doped catalysts, while the formation of VC and naphthalene is very small. Palladium has been used as promoter in cobalt catalysts for heavy oil upgrading [37]. The 5-RuPd(15/1) catalyst shows higher dispersion values, and therefore has lower particle sizes, so the metal centres are very close to the acid sites of the support and the molecules adsorbed onto these acid sites can receive hydrogen spilloved from the metal surface more easily, and thus produce the greatest volume of HMW products at 350 8C. The formation of a high quantity of VC products at 375 8C confirms the accessibility to acid sites of this catalyst. Since the total acidity of the mixed catalysts is lower than that of the 5-Ru catalysts, the increase in the formation of HMW products and the preferential formation of trans-decalin in the mixed catalysts could only be explained taking into account that the incorporation of palladium not only increases the degree of reducibility of ruthenium, which has an important hydrogenolysis activity, but at the same time the superficial Pd increases the adsorption capability of organic molecules to produce the isomerization of cis-decalin [18] and the formation of HMW, which takes place in successive steps.
Thus, among this set of catalysts, 5-RuPd(8/1) and 5RuPd(4/1) appear to be the most effective in the reaction of hydrogenation and hydrogenolysis/hydrocracking of tetralin. Considering that the economic aspect is of considerable importance for industrial and commercial applications, the 5RuPd(8/1) catalyst should be employed to optimise the experimental conditions in order to obtain a better performance since Pd is a more expensive metal than Ru. 3.2.2. Influence of the experimental conditions Evaluating the influence of the contact time and the H2/ tetralin molar ratio on the selectivity for hydrogenation and the conversion of HMW products completed this catalytic study. To modify the contact time, both GHSVand LHSV were first changed. Conversion and yield for the different products on the 5-RuPd(8/1) catalyst as a function of the contact time are displayed in Fig. 4a. The reaction was carried out at 350 8C because at this temperature a good balance between hydrogenation and HHC products was found. The conversion is not affected by the contact time, however the yield of the different products is highly modified. When contact time is increased, a decrease in decalins and a concomitant increase in HMW products take place. Thus, the increase in the contact time improves the tetralin hydrogenation and the successive isomerization and hydrogenolysis/hydrocracking reactions. With a long contact time, 3.6 s, this catalyst produces mainly HMW products with a very low yield of hydrogenated products and decalins. The formation of naphthalene and VC became more considerable.
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feedstock. Although the organic sulphur and nitrogen compounds are removed catalytically by the hydrodesulphuration (HDS) and hydrodenitrogenation (HDN) processes, small amounts of sulphur and nitrogen are present in the feedstocks in industrial practise. Therefore, the resistance of these catalysts to sulphur and nitrogen compounds was studied.
Fig. 4. Evolution of the conversion and yield of the different reaction products of the 5-RuPd(8/1) catalyst as an function of (a) contact time and (b) H2/tetralin molar ratio. Experimental conditions: T = 350 8C, P(H2) = 4.5 MPa, P(N2) = 1.5 MPa.
3.2.3.1. Thio-resistance. The thiotolerance of the 5-RuPd(8/ 1) catalyst was evaluated with a feed containing between 300 and 1000 ppm of DBT on stream for 2 h, using optimal experimental conditions, i.e. 350 8C, contact time 2.8 s, H2/ tetralin molar ratio of 15. As can be observed in Fig. 5, with 300 ppm of DBT this catalyst is not deactivated, maintaining the value of conversion but changing the pattern of yields of different products, showing an increase in the yield of hydrogenation products and with a decrease in the yield of HMW products. When increased amounts of sulphur are introduced into the feed (up to 750 ppm) a gradual decrease in the conversion takes place. Under these experimental conditions the decrease in the yield of HMW products is more pronounced but the yield of hydrogenation products is maintained. This observation could result from the sulphur produced in the HDS reaction of DBT that is not directly released as H2S but rather could be present on the catalyst structure [38]. So, palladium or ruthenium metals are modified by the presence of sulphur, possibly forming palladium or ruthenium sulphide with hydrogenation properties. Under more severe operating conditions, i.e. by feeding 1000 ppm of DBT the conversion fell to 32.8% and the catalyst not only looses its cracking activity, but also its hydrogenation activity, increasing the formation of naphthalene (5.2%), a dehydrogenation product. This deactivation is also irreversible because the regeneration by treating with H2 was not effective, retrieving a conversion of only 33.5%. In any case, with 600 ppm of DBT in the feed, this catalyst shows a similar conversion to that of 5-Ru catalyst [21], however since the number of DBT molecules fed to the reactor per surface metal atom is higher for 5-RuPd(8/1)
Once the optimal contact time was established (2.8 s), the H2/tetralin molar ratio was also studied since the hydrogenation of tetralin is an exothermic reaction and high pressures of hydrogen seem to be necessary to overcome the thermodynamic limitations [35]. Fig. 4b shows the variation of the conversion and yield as a function of the molar ratio. It can be observed that the increase in this ratio from 5 to 15 gives rise to a substancial increment in the tetralin conversion of 60.9 and 98.5%, respectively. This increase is due in particular to the substantial enhancement of the isomerization/hydrogenolysis/ hydrocracking reactions which lead to the formation of HMW products: 70.1% in the case of a H2/tetralin molar ratio of 15. This trend confirms that this catalyst requires in excess of three to five times the stoichiometric ratio (i.e. H2/tetralin = 3:1, mol/ mol) to counteract the loss of activity. 3.2.3. Sulphur- and nitrogen-tolerance In the pretreatment of feeds for catalytic cracking and for hydrodearomatization, the first objective is to reduce the amount of organic sulphur and nitrogen compounds in the
Fig. 5. Variation of tetralin conversion and yield of the different reaction products of the 5-RuPd(8/1) catalyst after 2 h with varying DBT concentrations in the feed at 350 8C. Experimental conditions: H2/THN molar ratio = 15, P(H2) = 4.5 MPa, P(N2) = 1.5 MPa, contact time = 2.8 s.
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(6.39 10 6 molecules/surface metal atom) than for 5-Ru (3.15 10 6 molecules/surface metal atom) this indicates that the mixed catalyst shows a better thiotolerance. To understand this behaviour, the measurement of microchemical analysis and particle size by chemisorption of hydrogen of the spent catalyst were carried out. The content of sulphur and carbon on the catalyst, determined by microchemical analysis was 0.83 and 0.88%, respectively, showing a partial poisoning by sulphur-compounds and formation of coke. Also, the hydrogen chemisorption revealed sintering of ruthenium–palladium particles as illustrated by the increase in particle size from 4.0 in the fresh catalyst to 7.8 nm in the spent one. Thus, formation of sulphur–ruthenium or sulphur–palladium as well as the agglomeration of the metallic particles takes place [39]. 3.2.3.2. Resistance of nitrogen compounds. According to certain studies, the most powerful N-compound inhibitor of catalysts is acridine, whereas anilines, quinolines and benzoquinolines have only a moderate inhibiting effect [40,41]. Therefore, the effect of a basic compound, acridine (0–2000 ppm), on the activity of the 5-RuPd(8/1) catalyst in the hydrogenation of tetralin was studied under the following experimental conditions: 350 8C, contact time = 2.8 s; H2/ tetralin molar ratio of 15. Fig. 6 shows the experimental data found for the conversion and yields of tetralin as a function of the concentration of acridine in the feed. The catalyst showed a slight decrease in conversion when increasing amounts of acridine were added to the feed. The conversion of tetralin decreased from 98.5%, in the absence of acridine, to 80.7% when the feed contained 2000 ppm of acridine. The yield of hydrogenation products decreased as a consequence of the presence of acridine in the feed. However, the yield in hydrogenolysis/ring-opening was maintained and the yield of naphthalene increased. The partial deactivation of this catalyst is due to the formation of coke as was shown by microchemical analysis; the spent catalyst
revealed a carbon content of 2.09%. The formation of coke seems to take place on the metallic sites, thus lowering the hydrogenation activity. This catalyst shows a light decrease in the specific area values due to the adsorption of heavy compounds (tar) (Table 1). Because the formation of HMW products is hardly modified by the presence of acridine, the strong adsorption of this basic molecule on the acid sites must be ruled out [42]. 4. Conclusions The incorporation of palladium metal into the Ru-MCM catalyst has a promoting effect on its performance because it favours the total reduction of ruthenium catalysts with a high hydrogenation capacity at low temperatures for 5-RuPd(8/1) and 5-RuPd(4/1) catalysts. Also, the presence of palladium seems to increase the capacity of adsorption of organic molecules, and therefore the mixed catalysts display an excellent performance in the formation of HMW products at high temperatures. Further, the thiotolerance of 5-RuPd(8/1) is very close to that of 5-Ru and the resistance to nitrogen compounds is excellent. Acknowledgements This research was performed under Contract No. GR5D2001-0537 of the European Union. We also wish to thank the CICYT (Spain, project MAT03 2986) for financial support. DEQ also thanks the Ministerio de Ciencia y Tecnologı´a (Ministry of Science and Technology) for a fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Fig. 6. Variation of tetralin conversion and yield of the different reaction products of the 5-RuPd(8/1) catalyst after 2 h of reaction with varying acridine concentrations in the feed at 350 8C. Experimental conditions: H2/THN molar ratio = 15, P(H2) = 4.5 MPa, P(N2) = 1.5 MPa, contact time = 2.8 s.
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Influence of the incorporation of palladium on Ru/MCM hydrotreating catalysts D. Eliche-Quesada, J.M. Me´rida-Robles, E. Rodrı´guez-Castello´n, A. Jime´nez-Lo´pez * Departamento de Quı´mica Inorga´nica, Cristalografı´a y Mineralogı´a (Unidad Asociada al Instituto de Cata´lisis del CSIC), Facultad de Ciencias, Universidad de Ma´laga, 29071 Ma´laga, Spain Received 10 November 2005; received in revised form 22 December 2005; accepted 16 January 2006 Available online 28 February 2006
Abstract Zirconium-doped mesoporous silica with a Si/Zr molar ratio of 5 has been used as a support for bimetallic Ru-Pd catalysts. The effect of palladium as a promoter in a catalyst containing ruthenium as active phase, and the influence of the Ru/Pd atomic ratio were studied in the hydrogenation and ring-opening reaction of tetralin. The results indicate the promotional effect of Pd on hydrotreating activity of catalyst since an increase is observed in the formation of hydrogenation products, especially trans-decalin at low temperatures for 5-RuPd(8/1) and 5-RuPd(4/1), while at higher temperatures, there is an enhacement in the production of hydrogenolysis/hydrocracking products. In contrast 5-RuPd(15/1) catalyst shows at low temperatures lower yields of hydrogenation products and higher yields of high molecular weight products (HMW) than 5-Ru catalyst while at higher temperatures than 315 8C only produces HMW products. The best catalytic performance is displayed by the 5-RuPd(8/1) catalyst, giving a conversion of tetralin of 98.5% as well as high yields of hydrogenation (20.8%) and ring-opening (70.1%) products under the following experimental conditions: temperature of reaction 350 8C; H2/tetralin molar ratio of 10; contact time 2.8 s and total pressure of 6 MPa (P(H2) = 4.5 MPa and P(N2) = 1.5 MPa). Moreover, this catalyst exhibits good thiotolerance in the presence of 600 ppm of dibenzothiophene in the feed, and an excellent tolerance to nitrogen compounds even with 2000 ppm of acridine. # 2006 Elsevier B.V. All rights reserved. Keywords: Bimetallic ruthenium-palladium catalysts; Hydrogenation; Hydrogenolysis/hydrocracking; Tetralin; MCM-41
1. Introduction The importance of decreasing the impact of diesel fuel on air quality has led to increasingly stringent regulations to reduce emissions containing particles originating from combustion of polyaromatic hydrocarbons and nitrogen and sulphur oxides. In order to achieve this, the improvement of middle distillates is desirable and can be achieved by: (i) decreasing the sulphur content, (ii) reducing the aromatic hydrocarbons content and (iii) increasing the cetane number with hydrogenolysis/ring-opening reactions [1]. To a certain extent, the latter two strategies are interrelated, i.e. the reduction of the aromatic content has a positive effect on increasing the cetane number [2].
* Corresponding author. Tel.: +34 952 13 18 76; fax: +34 952 13 75 34. E-mail address: [email protected] (A. Jime´nez-Lo´pez). 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2006.01.003
For deep hydrogenation (HYD) of aromatics and deep hydrodesulphuration (HDS) in diesel fuel, single-stage and two-stage approaches, have been proposed. The single-stage hydrotreating process combines severe hydrodesulphuration with aromatic hydrogenation (HDA) using sulphur-tolerant CoMo, NiMo or NiW catalysts supported on alumina, but high temperature and high hydrogen pressures are necessary to achieve an acceptable level of aromatic reduction [3,4]. In the two-stage hydrotreating process, the conventional hydrotreating catalyst is used in the first reactor to reduce the level of sulphur to a few ppm and a noble-metal catalyst is used in the second reactor. These noble metal catalysts can achieve a high level of aromatic hydrogenation at low reaction temperatures and moderate hydrogen pressures. However, they display a low resistance to sulphur poisoning [5,6]. Interestingly, sulphur tolerance may be enhanced by modifying the physicochemical characteristics of the metal atoms by: (i) using acidic carriers [6], (ii) changing the metal particle size or (iii) alloying with
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other noble metals [7]. Sulphur tolerance has been shown to be related to the formation of electron-deficient metal sites, which in turn weaken the strength of the sulphur–metal bond [7–9]. Noble metals supported on large-pore-zeolites have received much attention for their use as aromatic hydrogenation catalysts [10,11]. Zeolite acidity, however, drastically increases amounts of undesirable cracking products, which accelerates the ratio of coke deposition and the yield in gases [12]. Furthermore, due to their pore size, the access to active sites is restricted to the largest feed molecules [13]. Hence, much effort has been devoted to research for finding new mesoporous solid supports. Recently, MCM-41 type materials have been studied in these catalytic processes, since they possess an ordered structure, high surface area and large pore sizes that allow the diffusion of aromatic molecules [14–18]. Moreover, as mesoporous MCM-41 silica has only a small number of acid sites strong enough to catalyse the hydrogenolysis/ring-opening reactions, the siliceous framework can be doped with heteroatoms such as Al, Ti or Zr to increase the acidity of mesoporous solids. Thus, zirconium containing MCM-41 type materials provide an excellent behaviour as acid catalysts, as previously reported [19,20]. We have recently reported [21] the results obtained for ruthenium supported on a zirconium-doped mesoporous catalyst in the hydrotreating of tetralin. The catalyst with a 5 wt% of ruthenium displays an excellent catalytic activity at 320 8C and maintains its catalytic performance in the presence of 600 ppm of dibenzotiophene (DBT). In the current work we have also considered the promoting effect of palladium metal on the ruthenium catalyst in the hydrotreating of tetralin. With this aim, we prepared bimetallic catalysts with different Ru/Pd atomic ratios and the resulting catalysts were assayed in the hydrotreating of tetralin in the presence of DBT and acridine. The latter were used to represent sulphur and nitrogen poisoning, respectively, since they are the main S- and Ncontaining compounds in diesel. The catalysts were characterized by X-ray diffraction (XRD), temperature programmed desorption (TPD) of NH3, X-ray photoelectron spectroscopy (XPS), H2-chemisorption, temperature programmed reduction with H2 (H2-TPR) and adsorption–desorption of nitrogen. 2. Experimental 2.1. Catalyst preparation Zirconium-doped mesoporous silica with a Si/Zr molar ratio of 5, was prepared by following the method described elsewhere [20,22], but with a reduction in the reaction time to only 24 h at room temperature. The material was calcined at 550 8C (1 8C min 1 heating rate) for 6 h and was denoted as SiZr-5. Bimetallic Ru-Pd catalysts were prepared by wet impregnation of the pelletized support (0.85–1.00 mm) using mixed aqueous solutions of hydrated ruthenium(III) chloride (RuCl3xH2O) and Pd(NO3)2. A total metal loading of 5 wt% was kept constant in all samples, whereas different molar ratios of Ru/Pd(x/y) were used (15/1, 8/1, 4/1). After impregnation with metallic salts, the samples were dried at 120 8C and reduced directly at 400 8C for 1 h without previous calcination.
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2.2. Catalyst characterization Powder X-ray diffraction patterns were recorded on a Siemens D5000 diffractometer, equipped with a graphite monochromator and using Cu Ka radiation. X-ray photoelectron spectra were collected using a Physical Electronics PHI 5700 spectrometer with non-monochromatic Mg Ka radiation (300 W, 15 kV, 1253.6 eV) and with a multichannel detector. Charge referencing was measured against adventitious carbon (C 1 s, 284.8 eV). A PHI ACCESS ESCAV6.0 F software package was used to record and analyze the spectra. A Shirley-type background was subtracted from the signals. Recorded spectra were always fitted using Gauss– Lorentz curves in order to determine the binding energy of the different element core levels more accurately. The reduced samples, before the XPS analysis, were kept in n-hexane to avoid contact with air. The reducibility of the catalysts was studied by temperatureprogrammed reduction with hydrogen (H2-TPR). Experiments were carried out between 50 and 700 8C, using a flow of Ar/H2 (40 cm3 min 1, 10 vol% of H2) and a heating rate of 10 8C min 1. The effluent gas was passed through two traps before passing into the thermal conductivity detector, a solid CaO trap to remove HCl, and a cold trap ( 80 8C) to remove water. The acidic properties of reduced catalysts were analyzed by temperature-programmed desorption of chemisorbed ammonia (NH3-TPD). Before the adsorption of ammonia at 100 8C, the samples were reduced at 400 8C using a flow of H2 (60 cm3 min 1). The ammonia desorbed between 100 and 550 8C (heating rate of 10 8C min 1) was analyzed by an on line gas chromatograph (Shimadzu GC-14A) provided with a TC detector. The specific surface areas of the solids were evaluated from the nitrogen adsorption–desorption isotherms at 196 8C in a MICROMERITICS ASAP 2020 apparatus, after degassing at 200 8C and 1.3 10 2 Pa for 24 h. Hydrogen chemisorption was performed in a MICROMERITICS ASAP 2010 apparatus, after the in situ reduction of samples at 400 8C (15 8C min 1) for 1 h, under a flow of H2. After reduction, the catalysts were degassed at 10 4 Pa for 10 h at the same temperature and cooled at 35 8C, to carry out the chemisorption of H2. The range of pressure studied in chemisorption was 0.013–0.04 MPa and the amounts of hydrogen chemisorbed were calculated by extrapolation of the isotherms to zero pressure. Dispersion data have been calculated by assuming a stoichiometry H/Ru and H/Pd = 1. CNHS chemical elemental analysis was used for determine the content of coke and sulphur in the spent catalyst and was carried out by using a LECO CNHS 932 instrument. 2.3. Catalytic activity measurement The hydrogenation of tetralin was performed in a highpressure fixed-bed continuous-flow stainless steel catalytic reactor (9.1 mm i.d. and 230 mm length) operated in the downflow mode at different temperatures. The reaction temperature
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was measured with an interior placed thermocouple in direct contact with the top part of the catalyst bed. The organic feed consisted of a solution of tetralin in n-heptane (10 vol%) and was supplied by means of a Gilson 307SC piston pump (model 10SC). A fixed volume of catalyst (3 cm3 with particle size of 0.85–1.00 mm) without dilution was used in all cases. Prior to the activity test, the catalysts were reduced in situ at atmospheric pressure with H2 (flow rate 60 cm3 min 1) at 400 8C for 1 h, with a heating rate of 15 8C min 1. Catalytic activities were measured at different temperatures, under 6.0 MPa (4.5 MPa H2 and 1.5 MPa N2), and a liquid hourly space velocity (LHSV) of 6.0 h 1. The catalytic reaction was studied by collecting liquid samples after remaining 60 min at each reaction temperature, and kept in sealed vials for posterior analysis by both gas chromatography (Shimadzu GC-14B, equipped with a flame ionization detector and a capillary column, TBR-1, coupled to a Shimadzu AOC-20i automatic injector) and mass spectrometry (Hewlett-Packard 5988A). The performance of the microreactor and the accuracy of the analytical method were studied by feeding a solution of tetralin in n-heptane (10 vol%) into the reactor filled with 3 cm3 of SiC, operating at 300 8C and 6.0 MPa. No formation of foreign products was detected with a recovery percentage of the tetralin feed of 95%. To investigate the effects of sulphur and nitrogen poisoning on the catalytic performance, the organic feed was mixed with different concentrations of dibenzothiophene (DBT) (between 300 and 1000 ppm wt%), or with different concentrations of acridine (between 500 and 2000 ppm wt%). In previous experiments, a variation in the amount of catalyst and gas flow rate while maintaining the space velocity constant were also carried out in order to discard the possible existence of diffussional limitations under the experimental conditions used, led to no modification of conversion values. Particle diameter was found to have no influence either. The catalysts were tested in tetralin hydrogenation to evaluate their potential behaviour in the hydrogenation of aromatic hydrocarbons in diesel fuels. Total conversion of
tetralin was evaluated as tetralin converted with respect to tetralin in the feedstock. A large number of products were detected by gas chromatography analysis. After identification of the majority of them, they were classified, as recently reported [23–25], into the following groups: (i) volatile compounds (VC) that include non-condensable C1–C6 products whose presence was calculated from the carbon balance of the reaction, (ii) hydrogenation products that include trans- and cis-decalin, (iii) high-molecular-weight compounds (HMW) that include primary products such as toluene, ethylbenzene, oxylene, 1-ethyl-2-methylbenzene, 1-propenyl-2-methylbenzene, n-propylbenzene and iso-propylbenzene and secondary products, which are derived from ring-opening reactions such as polyalkylolefins, decadiene and cyclohexene-1-butylindene and (iv) naphthalene (Scheme 1). In the HMW group, are also included the products formed by isomerization of tetralin and decalins that can lead to the formation of a large number of C10 isomers, as was found by other authors [26,27]. 3. Results and discussion 3.1. Catalyst characterization The XRD profiles at high angles do not show diffraction peaks corresponding to the noble metal salts or metal aggregated phases for the precursor and the reduced catalysts, respectively. This fact indicates a very good and stable dispersion of the phases for all compositions. Thus, for all catalysts the metal crystal sizes must be below the XRD limit of detection. The N2 adsorption–desorption isotherms of the supported precursors are characteristic of mesoporous materials and exhibit the same shape as that of the supported SiZr-5 and can be considered as Type IV in the IUPAC classification. When ruthenium–palladium salts were dispersed on the support (Table 1) the BET surface area and the pore volume undergo a slight decrease which could be due to a partial blockage of
Scheme 1. Products distribution in the hydrogenation and hydrogenolysis/hydrocracking of tetralin.
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mesopores by ruthenium–palladium salts. The pore size distributions are very narrow with a maximum centered in the region of mesoporous materials (5.4–5.6 nm). Temperature-programmed reduction is a powerful technique to study the reduction behaviour of oxide phases or salts, as well as providing information regarding the degree of interaction between metal–metal and metal–support. The profiles of monometallic Ru (5 wt%) and Pd (1 wt%) catalysts are included as a reference in Fig. 1 where the profile of 5RuPd(8/1) is shown as representative of this family of catalysts. In the case of the monometallic Pd sample, a maximum centred at 152 8C is observed which is assigned to the reduction of the Pd precursor, Pd(NO3)2. The monometallic Ru sample exhibits a main peak centred at 146 8C with a small shoulder at 102 8C. The widening of this peak is possibly due to the reduction of Ru3+ located in different environments. This main peak can be assigned to Ru3+/Ru0 single step reduction [21,28,29]. The profile also exhibits a small and ill defined band of hydrogen consumption between 275 and 350 8C which could correspond to a small amount of ruthenium oxide or oxychloride formed by the exposition and drying in air at 120 8C [30–32]. All bimetallic Ru–Pd samples display an essentially singular reduction peak with a maximum at temperatures between 134 and 151 8C. Thus, the H2-TPR profile of the bimetallic 5RuPd(8/1) catalyst (Fig. 1) shows a single peak at 140 8C, i.e. at a lower temperature than those of the monometallic catalysts. However, as the maximum reduction peaks of the monometallic catalysts are very close to those of the monometallic ones and ruthenium is in great excess as compared to Pd, it is very difficult to establish the formation of any Ru–Pd alloy. The simultaneous reduction of the two salts could however indicate that ruthenium and palladium must be in close interaction with each other, suggesting the formation of bimetallic Ru–Pd particles. In any case, the reduction of Ru salts seems to be easier in the presence of Pd because the peak at 350 8C almost disappears in the bimetallic catalysts. The TPD of adsorbed ammonia was used to determine the surface acidity of the reduced catalysts. The desorption temperature is a measure of the strength of the corresponding acid sites, while the total amount of ammonia desorbed after saturation coverage permits the quantification of the number of acid sites at the surface. The total surface acidity of the support and reduced catalysts between 100 and 550 8C are given in Table 1. The total acidity of the bimetallic catalysts is always higher than that of the support, but it is much lower than that of
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Fig. 1. H2-TPR profiles for the 5-Ru, 5-RuPd(8/1) and 1-Pd catalysts.
5-Ru (1628 mmol NH3 g 1). Because the high acidity of the ruthenium catalyst is essentially due to the marked trend of this metal to adsorb ammonia molecules, the dramatic decrease in the acidity could be due to a modification of the superficial nature of the ruthenium particles by the addition of palladium. Therefore, palladium atoms can probably be said to be located on the ruthenium metallic particles instead of forming an alloy. On the other hand, all catalysts studied display a mild acidity because they desorbed the main quantity of NH3 in the temperature range of 100–300 8C. The metallic properties of the reduced catalysts are compiled in Table 2. The degree of reduction, a, was 100% for all samples. The dispersion degree (D), the metallic surface area and the particle size were obtained from H2 chemisorption data. It is observed that the dispersion is higher for the highest Ru/Pd molar ratio. The catalyst with the highest ruthenium content, 5RuPd(15/1), shows the greatest amount of chemisorbed
Table 1 Textural properties of 5-RuPd(x/y) precursors and acidic properties of 5-RuPd(x/y) reduced catalysts Sample
SiZr-5 5-RuPd(15/1) 5-RuPd(8/1) 5-RuPd(4/1) 5-RuPd(8/1) spent DBT 5-RuPd(8/1) spent acridine
SBET (m2 g 1)
515 443 456 425 449 424
Vp (cm3 g 1)
0.63 0.61 0.63 0.58 0.60 0.54
Acidity (mmol NH3 g 1) 100–300 8C
300–400 8C
400–550 8C
Total
293 443 427 433 – –
125 131 111 157 – –
113 79 102 163 – –
531 653 640 754 – –
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hydrogen, and hence the lowest metallic particle size (2.0 nm) and the highest dispersion degree (43.6%). These values are similar to those of the catalyst containing only 5 wt% of ruthenium [21]. However, the catalysts with a ruthenium– palladium molar ratio of 8/1 and 4/1 show lower dispersion values of 21.6 and 11.4%, respectively, and metallic particle sizes of 4.0 and 7.4 nm, respectively. These results indicate that the dispersion of these palladium promoted catalysts is only higher than that of 5-Ru catalyst for the 5-RuPd(15/1) catalyst which contains the lowest loading of promoted palladium, only a 0.33 wt%. Photoelectron spectra of Si 2p, Zr 3d, Ru 3p and Pd 4f core levels were recorded for precursors and reduced samples. The binding energies of Si 2p and Zr 3d core electrons are practically constant and similar to those of the support. The XPS spectra of the core level region of Ru 3p for all ruthenium– palladium precursors exhibit a symmetrical peak at 462.8– 462.3 eV, which coincides with the binding energy of Ru 3p3/2 in the impregnation salt, RuCl3 [33]. After reduction, this signal is slightly shifted to a lower B.E. at 462.0–462.1 eV, indicating the presence of Ru(0) (Fig. 2) [33]. The spectrum of the Ru-5 catalyst after reduction shows a small shoulder at higher binding energies, which could be due to the presence of some remaining oxychlorides, which, as shown by TPR results, are seen to be more difficult to reduce. This shoulder disappeared for the mixed catalysts (Fig. 2), indicating that the incorporation of Pd favoured the reduction of ruthenium. However, the formation of alloys between both metals is not apparent from XPS data since the B.E.s of Ru in mixed catalysts are very close to that of the Ru catalyst. The direct measurement of the position and area of the Pd 3d5/2 peak is difficult to establish due to the overlapping of the Pd 3d5/2 core level with the intense peak of the Zr 3p3/2 component of the substrate centred at 333.4 eV. The B.E. of the Pd 3d5/2 level for the precursors appears at 336.7–337.2 eV corresponding to Pd(II) found in the Pd(NO3)2 salt [33]. After reduction, the B.E. of the Pd 3d is shifted to 335.1 eV and is overlapped completely with the Zr 3p signal, therefore the study of metallic palladium is not possible. 3.2. Catalytic results 3.2.1. Influence of the temperature of reaction and ruthenium–palladium molar ratio The ruthenium–palladium family of catalysts was tested in the hydrogenation and hydrogenolysis–isomerization–hydro-
Fig. 2. Ru 3p core level spectra of (a) 5-Ru and (b) 5-RuPd(8/1) reduced catalysts.
cracking of tetralin under pressure at different reaction temperatures (275–375 8C). The conversion of tetralin and the yields of different products for the three catalysts described above are shown in Fig. 3. The results obtained with a ruthenium catalyst (5-Ru) on the same support are included for comparison [21]. In general, all the conversion values are very high for the range of temperatures studied (>85%), but the distribution of products depends on the reaction temperature. At low temperatures the formation of decalins is favoured due to the exothermic character of this reaction [34,35]. Thus, in all cases, the main product at low reaction temperatures (Fig. 3) was trans-decalin. Moreover, this yield and the trans/cis ratio increase when the molar Ru/Pd ratio decreases. With respect to the 5-Ru catalyst, at low temperatures (T = 315 8C) palladium containing catalysts have a promotional effect: not only is the conversion increased but also the formation of hydrogenation products is increased too, especially that of trans-decalin, except for 5-RuPd(15/1) catalyst which has the maximum yield of HMW products (39%) at this low temperature. When the temperature is raised, the yield of HMW products increases. This is due to the hydrogenolysis/hydrocracking of tetralin and decalins involving C–C bond cleavage which is an endothermal process [36], but is detrimental to decalin formation. At 350 8C
Table 2 Metallic characteristics for the reduced ruthenium–platinum supported catalysts Catalysts
Ru (wt%)
Pd (wt%)
D (%)
2
m 5-Ru 5-RuPd(15/1) 5-RuPd(8/1) 5-RuPd(4/1) 5-RuPd(8/1) (spent 1000 ppm DBT) 5-RuPd(8/1) (spent 1000 ppm acridine)
5.00 4.67 4.42 3.96 4.42 4.42
– 0.33 0.58 1.04 0.58 0.58
D is the metallic dispersion and d is the average diameter of the metallic crystallite.
35.2 43.6 21.6 11.4 11.0 20.2
d (nm)
Smet
6.4 8.1 4.1 2.2 2.1 3.8
gcat1
m
2
gRu1-Pd
128.6 161.7 81.0 43.3 41.2 75.7
2.4 2.0 4.0 7.4 7.8 4.2
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Fig. 3. Evolution of the conversion of tetralin hydrogenation/hydrogenolysis–hydrocracking and yield of the different reaction products as a function of the reaction temperature on Ru/Pd catalysts: (*) conversion, () trans-decalin, (&) cis-decalin, (*) HCC, (~) naphthalene and ( ) VC. Experimental conditions: H2/THN molar ratio = 10.1, P(H2) = 4.5 MPa, P(N2) = 1.5 MPa, contact time = 2.8 s.
all catalysts show considerable HMW yields. The 5-RuPd(15/1) catalyst shows the highest yield of HMW (67.9%), while the yield of decalins is only 1.7%. The behaviour of the 5-RuPd(8/ 1) and 5-RuPd(4/1) catalysts is very interesting because they show similar values with a good balance for hydrogenation products (22.0 and 30.0%, respectively) and HMW products (65.0 and 59.4%, respectively). Therefore, a new enhancement of conversion and formation of HMW products is observed in both palladium-doped catalysts, while the formation of VC and naphthalene is very small. Palladium has been used as promoter in cobalt catalysts for heavy oil upgrading [37]. The 5-RuPd(15/1) catalyst shows higher dispersion values, and therefore has lower particle sizes, so the metal centres are very close to the acid sites of the support and the molecules adsorbed onto these acid sites can receive hydrogen spilloved from the metal surface more easily, and thus produce the greatest volume of HMW products at 350 8C. The formation of a high quantity of VC products at 375 8C confirms the accessibility to acid sites of this catalyst. Since the total acidity of the mixed catalysts is lower than that of the 5-Ru catalysts, the increase in the formation of HMW products and the preferential formation of trans-decalin in the mixed catalysts could only be explained taking into account that the incorporation of palladium not only increases the degree of reducibility of ruthenium, which has an important hydrogenolysis activity, but at the same time the superficial Pd increases the adsorption capability of organic molecules to produce the isomerization of cis-decalin [18] and the formation of HMW, which takes place in successive steps.
Thus, among this set of catalysts, 5-RuPd(8/1) and 5RuPd(4/1) appear to be the most effective in the reaction of hydrogenation and hydrogenolysis/hydrocracking of tetralin. Considering that the economic aspect is of considerable importance for industrial and commercial applications, the 5RuPd(8/1) catalyst should be employed to optimise the experimental conditions in order to obtain a better performance since Pd is a more expensive metal than Ru. 3.2.2. Influence of the experimental conditions Evaluating the influence of the contact time and the H2/ tetralin molar ratio on the selectivity for hydrogenation and the conversion of HMW products completed this catalytic study. To modify the contact time, both GHSVand LHSV were first changed. Conversion and yield for the different products on the 5-RuPd(8/1) catalyst as a function of the contact time are displayed in Fig. 4a. The reaction was carried out at 350 8C because at this temperature a good balance between hydrogenation and HHC products was found. The conversion is not affected by the contact time, however the yield of the different products is highly modified. When contact time is increased, a decrease in decalins and a concomitant increase in HMW products take place. Thus, the increase in the contact time improves the tetralin hydrogenation and the successive isomerization and hydrogenolysis/hydrocracking reactions. With a long contact time, 3.6 s, this catalyst produces mainly HMW products with a very low yield of hydrogenated products and decalins. The formation of naphthalene and VC became more considerable.
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feedstock. Although the organic sulphur and nitrogen compounds are removed catalytically by the hydrodesulphuration (HDS) and hydrodenitrogenation (HDN) processes, small amounts of sulphur and nitrogen are present in the feedstocks in industrial practise. Therefore, the resistance of these catalysts to sulphur and nitrogen compounds was studied.
Fig. 4. Evolution of the conversion and yield of the different reaction products of the 5-RuPd(8/1) catalyst as an function of (a) contact time and (b) H2/tetralin molar ratio. Experimental conditions: T = 350 8C, P(H2) = 4.5 MPa, P(N2) = 1.5 MPa.
3.2.3.1. Thio-resistance. The thiotolerance of the 5-RuPd(8/ 1) catalyst was evaluated with a feed containing between 300 and 1000 ppm of DBT on stream for 2 h, using optimal experimental conditions, i.e. 350 8C, contact time 2.8 s, H2/ tetralin molar ratio of 15. As can be observed in Fig. 5, with 300 ppm of DBT this catalyst is not deactivated, maintaining the value of conversion but changing the pattern of yields of different products, showing an increase in the yield of hydrogenation products and with a decrease in the yield of HMW products. When increased amounts of sulphur are introduced into the feed (up to 750 ppm) a gradual decrease in the conversion takes place. Under these experimental conditions the decrease in the yield of HMW products is more pronounced but the yield of hydrogenation products is maintained. This observation could result from the sulphur produced in the HDS reaction of DBT that is not directly released as H2S but rather could be present on the catalyst structure [38]. So, palladium or ruthenium metals are modified by the presence of sulphur, possibly forming palladium or ruthenium sulphide with hydrogenation properties. Under more severe operating conditions, i.e. by feeding 1000 ppm of DBT the conversion fell to 32.8% and the catalyst not only looses its cracking activity, but also its hydrogenation activity, increasing the formation of naphthalene (5.2%), a dehydrogenation product. This deactivation is also irreversible because the regeneration by treating with H2 was not effective, retrieving a conversion of only 33.5%. In any case, with 600 ppm of DBT in the feed, this catalyst shows a similar conversion to that of 5-Ru catalyst [21], however since the number of DBT molecules fed to the reactor per surface metal atom is higher for 5-RuPd(8/1)
Once the optimal contact time was established (2.8 s), the H2/tetralin molar ratio was also studied since the hydrogenation of tetralin is an exothermic reaction and high pressures of hydrogen seem to be necessary to overcome the thermodynamic limitations [35]. Fig. 4b shows the variation of the conversion and yield as a function of the molar ratio. It can be observed that the increase in this ratio from 5 to 15 gives rise to a substancial increment in the tetralin conversion of 60.9 and 98.5%, respectively. This increase is due in particular to the substantial enhancement of the isomerization/hydrogenolysis/ hydrocracking reactions which lead to the formation of HMW products: 70.1% in the case of a H2/tetralin molar ratio of 15. This trend confirms that this catalyst requires in excess of three to five times the stoichiometric ratio (i.e. H2/tetralin = 3:1, mol/ mol) to counteract the loss of activity. 3.2.3. Sulphur- and nitrogen-tolerance In the pretreatment of feeds for catalytic cracking and for hydrodearomatization, the first objective is to reduce the amount of organic sulphur and nitrogen compounds in the
Fig. 5. Variation of tetralin conversion and yield of the different reaction products of the 5-RuPd(8/1) catalyst after 2 h with varying DBT concentrations in the feed at 350 8C. Experimental conditions: H2/THN molar ratio = 15, P(H2) = 4.5 MPa, P(N2) = 1.5 MPa, contact time = 2.8 s.
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(6.39 10 6 molecules/surface metal atom) than for 5-Ru (3.15 10 6 molecules/surface metal atom) this indicates that the mixed catalyst shows a better thiotolerance. To understand this behaviour, the measurement of microchemical analysis and particle size by chemisorption of hydrogen of the spent catalyst were carried out. The content of sulphur and carbon on the catalyst, determined by microchemical analysis was 0.83 and 0.88%, respectively, showing a partial poisoning by sulphur-compounds and formation of coke. Also, the hydrogen chemisorption revealed sintering of ruthenium–palladium particles as illustrated by the increase in particle size from 4.0 in the fresh catalyst to 7.8 nm in the spent one. Thus, formation of sulphur–ruthenium or sulphur–palladium as well as the agglomeration of the metallic particles takes place [39]. 3.2.3.2. Resistance of nitrogen compounds. According to certain studies, the most powerful N-compound inhibitor of catalysts is acridine, whereas anilines, quinolines and benzoquinolines have only a moderate inhibiting effect [40,41]. Therefore, the effect of a basic compound, acridine (0–2000 ppm), on the activity of the 5-RuPd(8/1) catalyst in the hydrogenation of tetralin was studied under the following experimental conditions: 350 8C, contact time = 2.8 s; H2/ tetralin molar ratio of 15. Fig. 6 shows the experimental data found for the conversion and yields of tetralin as a function of the concentration of acridine in the feed. The catalyst showed a slight decrease in conversion when increasing amounts of acridine were added to the feed. The conversion of tetralin decreased from 98.5%, in the absence of acridine, to 80.7% when the feed contained 2000 ppm of acridine. The yield of hydrogenation products decreased as a consequence of the presence of acridine in the feed. However, the yield in hydrogenolysis/ring-opening was maintained and the yield of naphthalene increased. The partial deactivation of this catalyst is due to the formation of coke as was shown by microchemical analysis; the spent catalyst
revealed a carbon content of 2.09%. The formation of coke seems to take place on the metallic sites, thus lowering the hydrogenation activity. This catalyst shows a light decrease in the specific area values due to the adsorption of heavy compounds (tar) (Table 1). Because the formation of HMW products is hardly modified by the presence of acridine, the strong adsorption of this basic molecule on the acid sites must be ruled out [42]. 4. Conclusions The incorporation of palladium metal into the Ru-MCM catalyst has a promoting effect on its performance because it favours the total reduction of ruthenium catalysts with a high hydrogenation capacity at low temperatures for 5-RuPd(8/1) and 5-RuPd(4/1) catalysts. Also, the presence of palladium seems to increase the capacity of adsorption of organic molecules, and therefore the mixed catalysts display an excellent performance in the formation of HMW products at high temperatures. Further, the thiotolerance of 5-RuPd(8/1) is very close to that of 5-Ru and the resistance to nitrogen compounds is excellent. Acknowledgements This research was performed under Contract No. GR5D2001-0537 of the European Union. We also wish to thank the CICYT (Spain, project MAT03 2986) for financial support. DEQ also thanks the Ministerio de Ciencia y Tecnologı´a (Ministry of Science and Technology) for a fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Fig. 6. Variation of tetralin conversion and yield of the different reaction products of the 5-RuPd(8/1) catalyst after 2 h of reaction with varying acridine concentrations in the feed at 350 8C. Experimental conditions: H2/THN molar ratio = 15, P(H2) = 4.5 MPa, P(N2) = 1.5 MPa, contact time = 2.8 s.
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