Chromium Species as Captors of Sulfur Molecules on Nickel-Based

Energy & Fuels 2009, 23, 101–110

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Chromium Species as Captors of Sulfur Molecules on Nickel-Based Hydrotreating Catalysts M. Go´mez-Cazalilla, A. Infantes-Molina, J. Me´rida-Robles, E. Rodrı´guez-Castelloń, and A. Jimeńez-Lo´pez* Departamento de Quı´mica Inorgańica, Cristalografıá y Mineralogıá (Unidad Asociada al ICP-CSIC), Facultad de Ciencias, UniVersidad de Ma´laga, Campus de Teatinos, 29071 Ma´laga, Spain ReceiVed September 4, 2008. ReVised Manuscript ReceiVed October 27, 2008

Three nickel-chromium catalysts supported on aluminated mesoporous silica (SBA-15) were prepared with a fixed nickel loading of 15 wt % and differing chromium contents, which ranged between 2.6 and 5.2 wt %. They were tested in the hydrotreating of tetralin in the presence of dibenzothiophene (DBT). Moreover, a monometallic nickel catalyst was also prepared and tested, for comparison. Information regarding the structure of the calcined, reduced, and used catalysts was obtained by several physical-chemical techniques such as X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR), temperature-programmed desorption of ammonia (NH3-TPD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and elemental chemical analysis. The presence of small amounts of Cr3+ ions not only modifies the nickel dispersion, leading to the formation of a more active catalyst in the hydrogenation of tetralin, but also has a strong influence on the stability of these catalysts in the presence of 425 ppm DBT in the feed. The Cr3+ ions seem to act as a sulfur trap, thus preventing sulfur poisoning of nickel particles.

1. Introduction Diesel fuel amelioration is necessary to facilitate the widespread introduction of clean diesel vehicles that comply with both current and future environmental legislation. The requirements to be fulfilled include the cetane number, useful for cold starts as air pollution and “white smoke” during startup; a reduction in aromatic content, which will reduce air pollution; and a reduction in the particulate matter emissions. High levels of aromatics can lead to the premature plugging of pollution control equipment, thereby reducing fuel economy and air quality benefits. Environmental concerns are the driving force behind the research into diesel fuels, searching for products that are friendly to both the planet and mankind. Hydroprocessing technology for diesel fuel production has focused its research on hydrotreating catalysts capable of attaining the quasi-elimination of sulfur and nitrogen present in the fuel along with a deep saturation of aromatics to match with future stringent environmental specifications. So far, the two-stage hydrotreating process, currently considered the best alternative, can achieve deep levels of aromatic hydrogenation under mild operating conditions (low temperatures and moderate hydrogen pressures).1 However, pressure due to environmental problems keeps provoking new research into this field of catalysis. For the second-stage hydrotreating process, supported nickel catalysts are suitable systems for hydrotreating reactions. When monometallic nickel catalysts are studied in the hydrogenation and hydrogenolysis/hydrocracking of tetralin, good results have * Corresponding author: phone (+34) 952131876; fax (+34) 952137534; e-mail [email protected]. (1) Fujikawa, T.; Idei, K.; Ebihara, T.; Mizuguchi, H.; Usui, K. Aromatic hydrogenation of distillates over SiO2-Al2O3-supported noble metal catalysts. Appl. Catal., A 2000, 192, 253–261.

been attained.2,3 However, their susceptibility to poisoning in the presence of sulfur-containing molecules is well-known. This has been explained by the strong adsorption of the S-compound on nickel particles4 along with a sintering of metal particles, which is induced by a decrease in metal-support interaction in the presence of sulfur-containing molecules.5 This has prompted the use of promoter agents to enhance nickel catalytic activity. In relation to this, Laine et al.6 pointed out that a promoter agent increases the number of active sites as well as having a significant effect on the metal dispersion and pore dimensions. Eliche-Quesada et al.7 studied the addition of tungsten to nickel catalysts in the hydrotreating of tetralin reaction. In the presence of dibenzothiophene, a tungsten(IV) sulfide with hydrogenating properties is formed, maintaining high conversion values. Chromium oxide-based catalysts have been widely studied in polymerization,8,9 hydrogenation,10 dehydrogenation of light (2) Hernańdez-Huesca, R.; Me´rida-Robles, J.; Maireles-Torres, P.; Rodrı´guez-Castelloń, E.; Jimeńez-Lo´pez, A. Hydrogenation and ringopening of tetralin on Ni and NiMo supported on alumina-pillared R-zirconium phosphate catalysts. A thiotolerance study. J. Catal. 2001, 203, 122–132. (3) Eliche-Quesada, D.; Me´rida-Robles, J.; Maireles-Torres, P.; Rodrı´guez-Castelloń, E.; Jimeńez-Lo´pez, A. Hydrogenation and ring opening of tetralin on supported nickel zirconium-doped mesoporous silica catalysts. Influence of the nickel precursor. Langmuir 2003, 19, 4985–4991. (4) Bartholomew, C. H.; Agrawal, P. K.; Katzer, J. R. Sulfur poisoning of metals. AdV. Catal. 1982, 31, 135–242. (5) Simon, L. J.; Kooyman, P. J.; van Ommen, J. G.; Lercher, J. A. Effect of Co and Ni on benzene hydrogenation and sulfur tolerance of Pt/ H-MOR. Appl. Catal., A 2003, 252, 283–293. (6) Laine, J.; Brito, J.; Gallardo, J.; Severino, F. The role of nickel in the initial transformations of hydrodesulfurization catalysts. J. Catal. 1985, 91, 64–68. (7) Eliche-Quesada, D.; Me´rida-Robles, J.; Maireles-Torres, P.; Rodrı´guez-Castelloń, E.; Busca, G.; Finocchio, E.; Jimeńez-Lo´pez, A. Effects of preparation method and sulfur poisoning on the hydrogenation and ring opening of tetralin on NiW/zirconium-doped mesoporous silica catalysts. J. Catal. 2003, 220, 457–467.

10.1021/ef800741n CCC: $40.75  2009 American Chemical Society Published on Web 12/19/2008

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alkanes,11,12 and oxidation-reduction reactions.13 Besides, chromia-pillared phosphate materials have shown good performance in the hydrodesulfurization of thiophene due to considerable autosulfurization, as SH groups are formed around active sites that have a synergic effect on the metallic sites.14 Cr2O3 is also used as a promoter having hydrogenation-dehydrogenation and dehydrocyclization properties.15 Thus, by adding chromium to nickel catalysts, it has been found that Cr forms an alloy with Ni, altering the electronic properties of the Ni atoms in such a way that the encapsulation of nickel by inactive carbon filaments is reduced.16 Hu et al.17 reported the enhancement of sulfur resistance by adding Cr as a promoter of Pd catalysts in the hydrogenation of toluene in the presence of 3000 ppm thiophene. On the other hand, the large size of some aromatic molecules present in real feeds has provoked, in recent years, interesting research into new mesoporous and macroporous materials other than alumina, amorphous silica-alumina or zeolites, which have traditionally been used for the preparation of hydrotreating catalysts. A material with a high specific surface and a sizable pore diameter together with proper acidic functions has been the main goal of many researchers in this area. To date, the discovery of MCM-41 mesoporous silica has proven to be the material of choice.18-20 However, their poor thermal stability has posed serious limitations on their practical applications.21,22 New mesoporous silica-type materials have been developed in recent years, intending to optimize the synthesis methods to obtain mesoporous supports similar to MCM-41. So far, it seems

that SBA-15 is the best candidate to replace MCM-41, so long as one can minimize the limitations found when thermal and hydrothermal stability are considered but retain the advantages of the materials that they intend to replace.23 Compared to MCM-41 mesoporous silica, better activity with SBA-15 has been reported in ethylene polymerization reactions.24 The incorporation of heteroatoms in the silica framework has become a very attractive subject for the researchers, since the acidity and redox properties of such materials can be easily modified. With regard to this approach, a great variety of mesoporous systems such as Zr-MCM-41,25 Zr-MSU,26 Ti-SBA15,27 Fe-SBA-15,28 Mn-MCM-41,29 Al-MCM-41,30 and organosulfonic acid-functionalized mesoporous silica materials (SBA15)31 have been prepared and tested in several reactions. In one case, Chmielarz et al.32 deposited transition metal oxides such as Cu, Cr, and Fe on various mesoporous silicas (MCM-48, SBA-15, MCF, and x-MSU); the silica support was a key factor to consider in the activity of the resulting materials. The aim of this study is to evaluate the effect of addition of chromium to nickel catalysts supported over an Al-SBA-15 mesoporous material in the reaction of hydrotreating tetralin, trying to improve the robustness and resistance of nickel catalysts to sulfur-containing feeds. The role of chromium content in the catalytic activity will be the main issue considered in this work.

(8) Gaspar, A. B.; Perez, C. A. C.; Dieguez, L. C. Characterization of Cr/SiO2 catalysts and ethylene polymerization by XPS. Appl. Surf. Sci. 2005, 252, 939–949. (9) Liu, B.; Fang, Y.; Terano, M. High resolution X-ray photoelectron spectroscopic analysis of transformation of surface chromium species on Phillips CrOx/SiO2 catalysts isothermally calcined at various temperatures. J. Mol. Catal., A 2004, 219, 165–173. (10) Yokoyama, T.; Fujita, N. Hydrogenation of aliphatic carboxylic acids to corresponding aldehydes over Cr2O3-based catalysts. Appl. Catal., A 2004, 276, 179–185. ´ lcańtara-Gonza´lez, M.; Rodrı´guez-Castelloń, (11) Me´rida-Robles, J.; A E.; Santamarıá-Gonza´lez, J.; Maireles-Torres, P.; Jimeńez-Lo´pez, A. Chromium-impregnated mesoporous silica as catalysts for the oxidative dehydrogenation of propane. Stud. Surf. Sci. Catal. 2000, 130, 1865–1870. (12) Gorriz, O. F.; Corte´s Corberań, V.; Fierro, J. L. G. Propane dehydrogenation and coke formation on chromia-alumina catalysts: Effect of reductive pretreatments. Ind. Eng. Chem. Res. 1992, 31, 2670–2674. (13) Ko¨hler, K.; Engweiler, J.; Viebrock, H.; Baiker, A. Chromium oxide supported on titania: Preparation of highly dispersed Cr(III) systems by grafting. Langmuir 1995, 11, 3423–3430. (14) Pe´rez-Reina, F. J.; Rodrı´guez-Castelloń, E.; Jimeńez-Lo´pez, A. Chromia-pillared R-zirconium phosphate materials as catalysts for thiophene hydrodesulfurization. Langmuir 1999, 15, 2047–2054. (15) Hensley, A.-L.; Munster, Tr.; Thomas, D.-N. U.S. Patent 3,893,908, 1975. (16) Bangala, D.-N.; Abatzoglou, N.; Chornet, E. Steam reforming of naphthalene on Ni-Cr/Al2O3 catalysts doped with MgO, TiO2, and La2O3. AIChE J. 1998, 44, 927–936. (17) Hu, L.; Xia, G.; Qu, L.; Li, C.; Xin, Q.; Li, D. Strong effect of transitional metals on the sulfur resistance of Pd/HY-Al2O3 catalysts for aromatic hydrogenation. J. Mol. Catal., A 2001, 171, 169–179. (18) Wang, A.; Wang, Y.; Kabe, T.; Chen, Y.; Ishihara, A.; Qian, W. Hydrodesulfurization of dibenzothiophene over siliceous MCM-41-supported catalysts I. Sulfided Co-Mo catalysts. J. Catal. 2001, 199, 19–29. (19) Ramı´rez, J.; Contreras, R.; Castillo, P.; Klimova, T.; Za´rate, R.; Luna, R. Characterization and catalytic activity of CoMo HDS catalysts supported on alumina-MCM-41. Appl. Catal., A 2000, 197, 69–78. (20) Klimova, T.; Rodrı´guez, E.; Martıńez, M.; Ramı´rez, J. Synthesis and characterization of hydrotreating Mo catalysts supported on titaniamodified MCM-41. Microporous Mesoporous Mater. 2001, 44/45, 357– 365. (21) Li, Z.; Gao, L.; Zheng, S. Investigation of the dispersion of MoO3 onto the support of mesoporous silica MCM-41. Appl. Catal., A 2002, 236, 163–171. (22) Kooyman, P. J.; Waller, P.; van Langeveld, A. D.; Song, C.; Reddy, K. M.; van Veen, J. A. R. Stability of MCM-41-supported CoMo hydrotreating catalysts. Catal. Lett. 2003, 90, 131–135.

2.1. Preparation of Catalysts. The support material used to prepare bimetallic nickel-chromium catalysts was an aluminumdoped mesoporous silica (Al-SBA) with Si/Al molar ratio of 10, prepared by postsynthesis alumination, as described elsewhere.33 The bimetallic catalysts were prepared by the incipient wetness impregnation method with subsequent impregnation of both precur-

2. Experimental Section

(23) Guo, W.; Li, X.; Zhao, X.-S. Understanding the hydrothermal stability of large-pore periodic mesoporous organosilicas and pure silicas. Microporous Mesoporous Mater. 2006, 93, 285–293. (24) Calleja, G.; Aguado, J.; Carrero, A.; Moreno, J. Ethylene polymerization over chromium supported onto SBA-15 mesoporous materials. Stud. Surf. Sci. Catal. 2005, 158B, 1453–1460. (25) Jones, D.-J.; Jimeńez-Jimeńez, J.; Jimeńez-Lo´pez, A.; MairelesTorres, P.; Olivera-Pastor, P.; Rodrı´guez-Castelloń, E.; Rozie`re, J. Surface characterisation of zirconium-doped mesoporous silica. Chem. Commun. 1997, 5, 431–432. (26) Infantes-Molina, A.; Me´rida-Robles, J.; Maireles-Torres, P.; Finocchio, E.; Busca, G.; Rodrı´guez-Castelloń, E.; Fierro, J. L. G.; JimeńezLo´pez, A. A new low-cost synthetic route to obtain zirconium containing mesoporous silica. Microporous Mesoporous Mater. 2004, 75, 23–32. (27) Newalkar, B.-L.; Olanrewaju, J.; Komarneni, S. Direct synthesis of titanium-substituted mesoporous SBA-15 molecular sieve under microwave - hydrothermal conditions. Chem. Mater. 2001, 13, 552–557. (28) Wang, X.-Q.; Ge, H.-L.; Jin, H.-X.; Cui, Y.-J. Influence of Fe on the thermal stability and catalysis of SBA-15 mesoporous molecular sieves. Microporous Mesoporous Mater. 2005, 86, 335–340. (29) Derylo-Marczewska, A.; Gac, W.; Popivnyak, N.; Zukocinski, G.; Pasieuna, S. The influence of preparation method on the structure and redox properties of mesoporous Mn-MCM-41 materials. Catal. Today 2006, 114, 293–306. (30) Liu, C.; Yu, X.; Yang, J.; He, M. Preparation of mesoporous AlMCM-41 with stable tetrahedral aluminum using ionic liquids as a single template. Mater. Lett. 2007, 61, 5261–5264. (31) Mbaraka, I.-K.; Shanks, B.-H. Acid strength variation due to spatial location of organosulfonic acid groups on mesoporous silica. J. Catal. 2006, 244, 78–85. (32) Chmielarz, L.; Kustrowski, P.; Kruszec, M.; Dziembaj, R.; Cool, P.; Vansant, E.-F. Nitrous oxide reduction with ammonia and methane over mesoporous silica materials modified with transition metal oxides. J. Porous Mater. 2005, 12, 183–191. (33) Go´mez-Cazalilla, M.; Me´rida-Robles, J.-M.; Gurbani, A.; Rodrı´guez-Castelloń, E.; Jimeńez-Lo´pez, A. Characterization and acidic properties of Al-SBA-15 materials prepared by post-synthesis alumination of a low-cost ordered mesoporous silica. J. Solid State Chem. 2007, 180, 1130– 1140.

Chromium Species as Captors of Sulfur Molecules sor salts, Ni(NO3)2 · 6H2O and Cr(NO3)3 · 9H2O. The nickel aqueous solution was first added to the pelletized support (0.85-1.00 mm), air-dried at 60 °C, and then impregnated with the chromium aqueous solution. After impregnation, the samples were again dried in air at 60 °C (12 h) and finally calcined at 550 °C for 6 h to obtain the corresponding catalyst precursors. The concentration of the precursor solutions was adjusted to the desired metal loading in each catalyst, 15 wt % for nickel, denoted by Ni; and 2.6, 3.9, and 5.2 wt % for chromium. The samples were labeled Ni15Crx, where x is the weight percentage of Cr. The corresponding nickel monometallic catalyst with the same nickel loading (15 wt %) was also prepared and labeled Ni15. 2.2. Characterization of Catalysts. X-ray powder diffraction (XRD) patterns for catalyst precursors and reduced catalysts were obtained by using a Siemens D5000 diffractometer (Cu KR source) provided with a graphite monochromator. The textural parameters of the catalyst precursors were evaluated from nitrogen adsorptiondesorption isotherms at -196 °C as determined by an automatic ASAP 2020 system from Micromeritics. Elemental chemical analysis of the spent catalysts was performed with a LECO CHNS 932 analyzer. Transmission electron micrographs (TEM) of the reduced samples were obtained by using a Philips CM 200 Supertwin-DX4 microscope. Samples were dispersed in ethanol, and a drop of the suspension was put on a Cu grid (300 mesh). Hydrogen temperature-programmed reduction (H2-TPR) experiments for precursor catalysts were carried out between 50 and 900 °C with a flow of 10% H2/Ar (48 mL min-1) and a heating rate of 10 °C min-1. Water produced in the reduction reaction was eliminated by passing the gas flow through a coldfinger (-80 °C). H2 consumption was controlled by an online gas chromatograph (Shimadzu GC-14A) provided with a thermal conductivity detector (TCD). Temperature-programmed desorption of ammonia (NH3-TPD) was carried out to evaluate the total acidity of the catalysts. Catalyst precursors were reduced at atmospheric pressure by flowing hydrogen (60 mL min-1) from room temperature to 500 °C with a heating rate of 10 °C min-1 and maintaining the sample at 500 °C for 60 min. After the sample was cleaned with helium and ammonia was adsorbed at 100 °C, NH3-TPD was performed between 100 and 550 °C with a heating rate of 10 °C min-1 by using a helium flow and maintained at 550 °C for 15 min. The evolved ammonia was analyzed by online gas chromatography (Shimadzu GC-14A) provided with a TCD. X-ray photoelectron spectra (XPS) were collected on a Physical Electronics PHI 5700 spectrometer with nonmonochromatic Al KR radiation (300 W, 15 kV, 1486.6 eV) with a multichannel detector. Spectra of catalyst precursors and reduced catalysts were recorded in the constant pass energy mode at 29.35 eV, with a 720 µm diameter analysis area. Charge referencing was measured against adventitious carbon (C 1s at 284.8 eV). A PHI ACCESS ESCAV6.0 F software package was used for acquisition and data analysis. A Shirley-type background was subtracted from the signals. Recorded spectra were always fitted to Gaussian-Lorentzian curves in order to determine the binding energy of the different element core levels more accurately. 2.3. Catalytic Test. Hydrogenation and hydrogenolysis/hydrocracking of tetralin was performed in a high-pressure fixed-bed continuous-flow stainless steel catalytic reactor (9.1 mm i.d. and 230 mm length) operating at a pressure of 6.0 MPa (4.5 MPa H2 and 1.5 MPa N2). Prior to the activity test, 3 cm3 of catalyst (particle size of 0.85-1.00 mm) was reduced in situ, at atmospheric pressure, by flowing hydrogen (60 mL min-1) from room temperature to 500 °C with a heating rate of 10 °C min-1 and maintained at 500 °C for 60 min. The N2 and H2 gases were then mixed with a solution of tetralin (THN) in n-heptane (10 vol%) supplied by means of a Gilson 307SC piston pump (model 10SC) [with a liquid hourly space velocity (LHSV) of 6.0 h-1] and then introduced into the reactor. A series of preliminary experiments consisting of varying the size of the sieved fractions, catalyst mass, and gas flow rate at constant space velocity (F/W) were also carried out in order to

Energy & Fuels, Vol. 23, 2009 103

Figure 1. X-ray diffractograms of catalyst precursors: (A) catalyst precursors and (B) reduced catalysts.

discard the possible existence of diffusional limitations under the experimental conditions used. The thiotolerance of the catalysts was evaluated by adding 425 ppm of dibenzothiophene (DBT) to the feed. The reactant and products were studied by collecting liquid samples. One sample was taken after 1 h on stream, when the temperature had increased to the desired value at a heating rate of 5 °C min-1. This allowed us to study the influence of the temperature. Subsequently, samples were taken at hourly intervals, allowing us to study the influence of the reaction time. All samples were analyzed by gas chromatography [Shimadzu GC-14B, equipped with a flame ionization detector (FID) and a TRB-1 capillary column, coupled to a Shimadzu AOC-20i automatic injector]. After analysis, they were classified into the following groups: (i) volatile compounds (VC), including noncondensable C1-C6 products whose carbon content was calculated from the carbon balance of the reaction; (ii) hydrogenation products, including trans- and cisdecalin (HYD); (iii) hydrocracking, hydrogenolysis, and hydroisomerization compounds (HHC) that include primary products such as toluene, ethylbencene, o-xylene, 1-ethyl-2-methylbenzene, 1-propenyl-2-methylbenzene, n-propylbenzene, and isopropylbenzene and secondary products derived from ring-opening reactions such as polyalkyolefins, decadiene, and cyclohexene-1-butylidene; and (iv) naphthalene. Products heavier than decalins were not found. It should be noted that high yields of hydrogenation products, and especially cracking compounds, give rise to an increase in the cetane number of fuels.

3. Results and Discussion 3.1. Catalyst Characterization. 3.1.1. X-ray Diffraction and Transmission Electron Microscopy. X-ray powder diffraction measurements were carried out in order to identify the species formed on the samples. The XRD patterns of all calcined samples are shown in Figure 1A. In all cases, two strong lines arising from the presence of nickel(II) oxide are noticeable at 2θ ) 37.3° and 43.3°. Looking at the diffractograms of the samples with chromium, it can be observed the appearance of a faint shoulder in the line at 37.3° can be observed that is more prominent in the diffractogram of the sample with the highest amount of chromium, Ni15Cr5.2, where it is easily distinguishable. This reflection line at 35.7° corresponds with the most



Figure 2. TEM micrographs of reduced catalysts: (A) Ni15 and (B) Ni15Cr5.2.

intense diffraction line of the NiCr2O4 spinel, revealing the formation of the compound. However, the formation of crystalline chromium oxide (Cr2O3) cannot be confirmed so long as no diffraction lines of such compound are noticeable in the diffraction patterns. Once the precursors are reduced at 500 °C (Figure 1B), the lines corresponding to nickel(II) oxide disappear as well as that due to NiCr2O4. Meanwhile, two new signals located at 2θ ) 44.5° and 51.8° appear, which reveal the formation of metallic nickel after the reduction process. It is important to note the broadness and decrease in the intensity of these two lines in the highest chromium loading sample, which could indicate a higher dispersion of these metallic nickel particles due to the presence of considerable amounts of chromium. The presence of Cr2O3 particles avoids the formation of high particles of Ni0. This fact reveals, at the same time, that Ni0 species are located near or are surrounded by particles of Cr2O3. With regard to the average Ni particle size for the reduced catalysts, this is calculated from the full width at halfmaximum (fwhm) of the Ni diffraction line located at 2θ ) 44.5°, by use of Scherrer’s equation. It decreases with higher amounts of chromium, ranging from 57.5 nm for the Ni15 sample to 18 nm for the catalyst with the highest chromium loading, Ni15Cr5.2. These results agree well with those obtained from TEM of the reduced catalysts, where a change in the size particle is noticeable (Figure 2), confirming the decrease of the nickel particle size when chromium is present in the sample. Besides, in the Ni15 catalyst two different Ni particles are clearly distinguishable: those located in the interior of the pores and those sited on the external surface of Al-SBA support, with larger size. 3.1.2. Nitrogen Adsorption-Desorption Isotherms at 77 K. The textural properties of the support and the catalyst precursors, evaluated from nitrogen adsorption-desorption isotherms at 77 K, are gathered together in Table 1. In all cases the isotherms are of type IV, typical of mesoporous solids,

Table 1. Textural Properties of Support and Calcined Materials sample

SBET, m2 g-1

Vp, cm3 g-1

dp (av), Å

Al-SBA Ni15 Ni15Cr2.6 Ni15Cr3.9 Ni15Cr5.2

359.0 250.0 143.7 138.9 132.0

0.354 0.238 0.129 0.129 0.131

40.6 39.5 37.9 38.5 42.3

similar to that of the material support. As could be expected, the specific surface areas of catalyst precursors are much lower than that obtained for the bare Al-SBA support. For the Ni15Crx samples, the surface area as well as the pore volume decreases with increasing chromium loading. This could be related to blocking of the pores caused by the deposition of oxide particles, which are bigger than the pore size of the support. The average pore diameter of the calcined samples and the support, calculated from the pore size distribution by the Cranston and Inkley method for cylindrical pores, is virtually the same as that obtained from the material support. Any slight differences could be due to the formation of an additional porosity in the nickel and chromium species located on the surface of the support, as has previously been detected for other supported phases.34-36 3.1.3. Temperature-Programmed Desorption of Ammonia. The acidic properties of this family of catalysts were evaluated by NH3-TPD, and the corresponding results are plotted in Figure (34) Dumeignil, F.; Sato, K.; Imamura, M.; Matslubayashi, N.; Payen, E.; Shimada, H. Characterization and hydrodesulfurization activity of CoMo catalysts supported on sol-gel prepared Al2O3. Appl. Catal., A 2005, 287, 135–145. (35) Infantes-Molina, A.; Me´rida-Robles, J.; Rodrı´guez-Castelloń, E.; Fierro, J. L. G.; Jimeńez-Lo´pez, A. Effect of molybdenum and tungsten on Co/MSU as hydrogenation catalysts. J. Catal. 2006, 240, 258–267. (36) Pawelec, B.; Castanõ, P.; Arandes, J. M.; Bilbao, J.; Thomas, S.; Penã, M. A.; Fierro, J. L. G. Factors influencing the thioresistance of nickel catalysts in aromatics hydrogenation. Appl. Catal., A 2007, 317, 20–33.

Chromium Species as Captors of Sulfur Molecules


Figure 3. Histogram of ammonia desorbed as a function of temperature for reduced catalysts. Figure 5. H2-TPR profile of Ni15Cr5.2 deconvoluted. Table 2. H2 Consumption for Each Band of H2-TPR Profilesa µmol of H2 Ni15 Ni15Cr2.6 Ni15Cr3.9 Ni15Cr5.2

band A

band B

band C

band D

total

49.0 17.9 17.2 18.4

13.8 11.5 9.3

69.4 80.3 83.6

148.8 129.4 134.2 141.4

197.8 230.5 243.1 252.6

a Band A, reduction of NiO located at the external surface; band B, reduction of CrO3; band C, reduction of Ni2+ forming NiCr2O4; band D, reduction of NiO located at the internal surface along with the reduction of Cr3+ to Cr2+ ions.

Figure 4. H2-TPR profiles of the catalyst precursors.

3. As can be observed, it is clear that the acidity, in relation to surface area, is directly dependent on the chromium content, which indicates a higher proportion of acid sites on the surface when greater amounts of this element are present. A loading of chromium higher than 3.9 wt % does not lead to higher acidity, owing to the formation of bigger particles of the oxide and a corresponding lower number of chromium ions exposed to the basic molecules. The acidic properties resulting from the presence of Crn+ ions in the catalysts has been previously reported by other authors,37 who have pointed out that Cr3+ and Cr6+ species, existing on the surface of catalysts, were responsible for the formation of strong acid sites which are highly active in cracking reactions. The sample with the highest acidity is Ni15Cr3.9. 3.1.4. Hydrogen Temperature-Programmed Reduction. To study the reducibility of these samples, H2-TPR experiments were carried out and the corresponding results are depicted in Figure 4. Regarding the Ni15 profile, the presence of two H2 consumption peaks at ca. 330 and 533 °C are noticeable, these being associated with the presence of Ni2+ species in different environments. The first, which is located at the same temperature as that corresponding to the reduction of bulk NiO (340 °C),2 could be assigned to NiO particles located on the external surface, whose interaction with the support is much fainter than that of NiO located inside the pores. This being so, the reduction of these Ni2+ species (inside the pores) would be associated with the asymmetric band at the higher temperature, and the maximum centered at 533 °C, since the inner surface of the pores is lined by Al3+ ions incorporated postsynthesis on the silica SBA-15, and these Ni2+ ions strongly interact with these aluminum species. A trial was carried out by preparing the same nickel catalyst (Ni15) with the SBA support without (37) Sohn, J.-R.; Ryu, S.-G.; Kim, H.-W. Acidic property and catalytic behavior of chromium oxide-zirconia catalyst. J. Mol. Catal., A 1998, 135, 99–106.

aluminum (denoted as Ni15SBA), to gain a better understanding of the Ni15 H2-TPR profile. When the two profiles are compared (Figure 4), they are seen to possess the same two bands. Notwithstanding, over the pure silica support, the reduction of the Ni2+ species located inside the pores occurs at lower temperatures, shifting the band from 533 °C (Al-SBA) to the lower value of 441 °C (SBA). This is due to the absence of aluminum in this sample, giving rise to a reduced interaction of such nickel ions with the support. On the other hand, by considering chromium-doped sample profiles, the presence of chromium in the sample provokes, as expected, a drastic change in the H2-TPR profiles, indicating newly created chemical environments of the Ni particles upon the addition of chromium. Two main bands are also noticeable. The first one, previously found at 330 °C for the nickel catalyst, is now much fainter and shifts to lower temperatures (250 °C), indicating a lower proportion of nickel oxide being formed on the external surface with a smaller particle size than in the Ni15 sample, and therefore with better reducibility. Conversely, the band at 533 °C becomes much broader, ranging from ca. 400 to 650 °C. In order to make the discussion and assignment of such profiles easier, they have been deconvoluted and the hydrogen consumption quantified to gain further insight into the reduction pattern of the species on the surface. Figure 5 displays, as an example, the deconvolution of the Ni15Cr5.2 profile, and Table 2 compiles the hydrogen consumption for each band and each sample. From this figure, four reduction processes can be observed. As previously mentioned, the first band (A) is assigned to the reduction of external nickel oxide particles. However, three other contributions are now noticeably at higher temperatures; at ca. 370 (B), 470 (C), and 570 °C (D), suggesting that the reduction of nickel, forming a phase different from NiO, along with chromium ion reduction are also involved in this broad band. The faint band located at ca. 350 °C (band B) could correspond to the reduction of isolated Cr6+ (38) Jimeńez-Lo´pez, A.; Rodrı´guez-Castelloń, E.; Maireles-Torres, P.; Dıáz, L.; Me´rida-Robles, J. Chromium oxide supported on zirconium- and lanthanum-doped mesoporous silica for oxidative dehydrogenation of propane. Appl. Catal., A 2001, 218, 295–306.



Table 3. Spectral Parameters Obtained by XPS Analysis binding energy (eV) Ni 2p3/2

Cr 2p1/2

sample

NiO

Ni0

Cr3+

Cr6+

Ni15 Ni15Cr2.6 Ni15Cr3.9 Ni15Cr5.2

855.0 855.0 855.0 855.3

Catalyst Precursors 857.2 857.1 857.1 857.1

576.6 576.3 576.8

579.1 579.0 579.1

Ni15 Ni15Cr2.6 Ni15Cr3.9 Ni15Cr5.2

855.4 854.9 855.1 855.1

Reduced Catalysts 857.7 852.8 857.5 852.5 857.7 852.7 857.8 852.6

577.1 577.0 576.9

ions,38,39 which have been reported to be formed after the hydrolysis and calcination of Si-O-Cr and Al-O-Cr bonds,40 these being quite dispersed over the surface and not detectable by XRD. However, its presence can be confirmed by XPS measurements, lately exposed, where Cr3+ and Cr6+ ions are both detected. The total amount of H2 consumed to reduce Cr6+ to Cr3+ (band B) decreases with higher chromium loading. This has been previously reported by other authors, who related this fact to a lower presence of Cr6+ due to increasing chromium content.41 Formation of the NiCr2O4 spinel was revealed from the XRD measurements. Therefore, reduction of the nickel forming the spinel is responsible for one of the two remaining bands. It should be observed from data compiled in Table 2 that the hydrogen involved on each band is on the rise with the chromium content. If it is considered that nickel forming NiCr2O4 located on the surface, as deduced from XRD studies, is easier to reduce than Ni2+ ions strongly interacting inside the pores, then the band at the lower temperature (470 °C) and with a lower hydrogen consumption corresponds to the reduction of Ni2+ forming this spinel. The intensity of this band increases with chromium loading. On the other hand, the band with a maximum at 570 °C is associated with both reduction of Ni2+ located on the inner surface and reduction of Cr3+ to Cr2+.42 For this reason, the hydrogen consumption increases insofar as the chromium present in the sample does. By dwelling on the hydrogen consumed in each reduction process (Table 2), the amount of H2 used to reduce nickel ions in the Ni15 sample is close to the theoretical value of 204 µmol; therefore, in samples containing chromium, the difference from this value is due to the hydrogen required to reduce chromium species. If the hydrogen involved in reducing nickel species (micromoles of H2 total in Ni15 sample) and that used to reduce the Cr6+ to Cr3+ (band B) is subtracted from the total amount of hydrogen consumed in the chromium-containing samples, the hydrogen uptake for reducing the Cr3+ species to Cr2+ is obtained. The observed values are 18.7, 33.0, and 45.0 µmol of H2 for Ni15Cr2.6, Ni15Cr3.9, and Ni15Cr5.2, respectively, and are close to the theoretical values (39) Aguado, J.; Calleja, G.; Carrero, A.; Moreno, J. One-step synthesis of chromium and aluminium containing SBA-15 materials. New Phillips catalysts for ethylene polymerization. Chem. Eng. J. 2008, 137, 443–452. (40) Kim, C.-S.; Woo, S.-I. Characterization of Cr/silica ethylene polymerization catalyst by TPO/TPR and FT-IR. J. Mol. Catal., A 1992, 73, 249–263. (41) Pradier, C.-M.; Rodrigues, F.; Marcus, P.; Landau, M.-V.; Kaliya, M.-L.; Gutman, A.; Herskowitz, M. Supported chromia catalysts for oxidation of organic compounds: The state of chromia phase and catalytic performance. Appl. Catal., B 2000, 27, 73–85. (42) Santamarıá-Gonza´lez, J.; Me´rida-Robles, J.; Alcańtara-Rodrı´guez, M.; Maireles-Torres, P.; Rodrı´guez-Castelloń, E.; Jimeńez-Lo´pez, A. Catalytic behaviour of chromium supported mesoporous MCM-41 silica in the oxidative dehydrogenation of propane. Catal. Lett. 2000, 64, 209–214.

Figure 6. Ni 2p core level spectra for Ni15Cr5.2 catalyst: (A) catalyst precursor and (B) reduced catalyst.

of 20, 30, and 40, respectively. Accordingly, the reduction of Cr3+ to Cr2+ takes place in the band at 570 °C, which is why this band is larger and more intense than that of Ni15 at 533 °C. 3.1.5. X-ray Photoelectron Spectroscopy. The surface analysis of the precursor and reduced catalysts was performed by XPS with the aim of determining the chemical state of nickel and chromium species. Table 3 shows the Ni 2p and Cr 2p binding energy values for both the catalyst precursors and reduced catalysts. With regards to binding energy values, the incorporation of chromium into the samples and the reduction process hardly modify the binding energy values of the Ni 2p signal. If we look at the Ni 2p3/2 core level spectrum for the Ni15Cr5.2 sample, considered as representative of these systems and depicted in Figure 6, the spectrum of the precursor catalyst (Figure 6A) shows a broad and asymmetric band, characteristic of Ni(II). This band can be deconvoluted into two components at 855.1 and 857.2 eV, which are related to Ni2+ interacting in a different way with the material support.43 Given that the binding energy values probably decrease when the metal covalence increases,44 due to a decrease in the interaction of Ni with the support, the former (855.1 eV) reveals the presence of NiO located at the external surface,45 whereas the position of the second one could be due to Ni2+ located inside the pores and strongly interacting with the support. This has been reported previously in H2-TPR results, albeit it is also very close to the data reported in the literature for the NiCr2O4 spinel. In fact, this is confirmed by the satellite shift value, ∆E ) 5.2 eV, which is very close to that reported when nickel forms NiCr2O4.46 The signal located at 857.2 eV therefore involves the photoelectron lines that are derived from these two types of nickel species. After the reduction process, analysis of this signal (Figure 6B) reveals the appearance, as expected, of a new component located (43) Rodrı´guez-Castelloń, E.; Dıáz, L.; Braos-Garcıá, P.; Me´rida-Robles, J.; Maireles-Torres, P.; Jimeńez-Lo´pez, A.; Vaccari, A. Nickel-impregnated zirconium-doped mesoporous molecular sieves as catalysts for the hydrogenation and ring-opening of tetralin. Appl. Catal., A 2003, 240, 83–94. (44) Vedrine, J.-C.; Hollinger, G.; Minh, O.-T. Investigations of antigorite and nickel supported catalysts by X-ray photoelectron spectroscopy. J. Phys. Chem. 1978, 82, 1515–1520. (45) Parizotto, N. V.; Rocha, K. O.; Damyanova, S.; Passos, F. B.; Zanchet, D.; Marques, C. M. P.; Bueno, J. M. C. Alumina-supported Ni catalysts modified with silver for the steam reforming of methane: Effect of Ag on the control of coke formation. Appl. Catal., A 2007, 330, 12–22. (46) Sloczynski, J.; Zio´lkowski, J.; Grzybowska, B.; Grabowski, R.; Jachewicz, D.; Wcislo, K.; Gengembrey, L. Oxidative dehydrogenation of propane on NixMg1-xAl2O4 and NiCr2O4 spinels. J. Catal. 1999, 187, 410– 418.



Table 4. Superficial Atomic Ratios for Reduced Catalysts Ni15 Ni15Cr2.6 Ni15Cr3.9 Ni15Cr5.2

Ni/Si

Cr/Si

Ni/Cr

0.09 0.16 0.13 0.20

0.08 0.12 0.24

2.03 1.04 0.83

at ca. 852.5 eV arising from the formation of Ni0. Both peaks at 855.1 and 857.2 eV persisted because the samples were reduced at 500 °C, and according to the TPR profiles, the total reduction of Ni2+ only takes place at approximately 675 °C. The peak at 857.2 eV decreases in intensity due to the reduction of Ni2+ from NiCr2O4. However, the intensity of the band corresponding to NiO located on the external surface (855.1 eV) is high after the reduction process, contrary to what one would expect. This may be due to the impossibility of transporting the reduced samples from the reduction reactor to the spectrometer without exposure to air; therefore a partial surface oxidation of the catalyst masks the real amount of remaining nickel oxide at the external surface. Quantitative XPS data show that, for reduced catalysts (Table 4), the Ni/Si atomic ratio increases with chromium content. This seems to indicate that the presence of chromium favors the exposure of nickel atoms at the external surface. As deduced from the XRD data, the presence of Cr3+ ions hinders the sinterization of Ni0, thus avoiding the formation of big particles. Therefore, the presence of a high number of smaller metallic particles increases the nickel dispersion. The analysis of the signal of Cr 2p for Ni15Cr5.2 is plotted in Figure 7, for (A) unreduced and (B) reduced samples, respectively. The Cr 2p3/2 band corresponding to the unreduced sample (catalyst precursor) possesses two contributions at ca. 576.5 and 579.0 eV, revealing two oxidation states of the chromium species. The most intense peak, with a binding energy value of 576.5 eV, is associated with the presence of Cr3+ ions,8,47 found in the NiCr2O4 spinel. It also confirms the results previously obtained from the XRD measurements, although the formation of Cr2O3 cannot be ruled out. On the other hand, the band located at ca. 579.0 eV due to the presence of Cr6+ ions, mainly in the form of CrO3,48 corroborates the results previously obtained from H2-TPR experiments. Notwithstanding, they must be quite highly dispersed to go undetected by XRD. After the reduction process at 500 °C (Figure 7B), the Cr 2p3/2 signal becomes more symmetrical and intense, with the band at 576.5 eV being the only noticeable one. Hence, the vast majority of chromium ions are in the form of Cr3+. 3.2. Catalytic Study. Hydrogenation and hydrogenolysis/ hydrocracking of tetralin was the catalytic reaction carried out to evaluate the activity of these catalysts. The evolution of THN conversion as a function of temperature on these catalysts is depicted in Figure 8. Summarizing the activity data, one might note the following: (i) The presence of chromium in the catalysts improves the catalytic behavior of nickel catalysts, as demonstrated by considering the conversion and yields of the hydrogenation products, mainly in the case of catalysts with the higher chromium content, such as Ni15Cr5.2. (ii) The yields of HHC compounds are better with the nickel monometallic catalyst, Ni15. These results could be explained by considering the role of Cr3+ ions in the catalysts. The bifunctional nature of these catalysts, where both metal and acid sites take part in the reaction, explains the role of chromium in (47) Sohn, J.-R.; Ryu, S.-G. Redox and catalytic behaviors of chromium oxide supported on zirconia. Catal. Lett. 2001, 74, 105–110. (48) Merryfield, R.; McDaniel, M.; Parks, G. An XPS study of the Phillips Cr/silica polymerization catalyst. J. Catal. 1982, 77, 348–359.

Figure 7. Cr 2p core level spectra for Ni15Cr5.2 catalyst: (A) catalyst precursor and (B) reduced catalyst.

the hydrogenation reaction. From the results extracted from the acidity experiments, the amount of chromium present directly correlates to the total acidity, whereby the samples with the highest chromium content have the highest number of acid sites. In the literature, the enhancement of activity in hydrogenation reaction due to acid catalysts is related to the metal-support interface region, as the concentration of hydrogen is at its highest in the vicinity of the metal particles. In this regard, the hydrogenation of tetralin involves the adsorption of such molecules onto the acid sites followed by its hydrogenation by hydrogen, which spills over from metal sites, where its dissociation takes place.49,50 The presence of chromium increases the total amount of acid centers, with the aromatic molecules possibly being adsorbed onto the chromium ions, later to be hydrogenated by hydrogen coming from nickel particles. The acidity also has a strong influence in hydrocracking reactions; but the higher acidity of chromium-doped samples does not improve the formation of such products. Chromium ions seem to generate neither strong acid sites capable of carrying out hydrocracking reactions nor active sites for hydrogenolysis reactions. On the other hand, a nickel catalyst gives better HHC yields regardless of its lower acidity. Hydrogenolysis reactions on nickel particles could be responsible for such behavior, as reported for other metallic phases.51,52 In fact, Hu et al.17 reported that the hydrogenolysis of methylcyclohexane on Pd catalysts is a structure-sensitive reaction and needs multicenter metal sites. The addition of a second metal has an influence on the structure of the active metal clusters on the surface, decreasing the fraction of Pd ensembles with atoms in the correct geometry for hydrogenolysis/hydrocracking reactions. The presence of chromium could have the same effect on nickel particles. The role of chromium on nickel catalysts is highlighted by studying the behavior in the presence of a sulfur-containing molecule acting as a poisoning agent. The low thiotolerance of nickel in the presence of sulfur compounds is due to a strong (49) Lin, S.-D.; Vannice, M.-A. Hydrogenation of aromatic hydrocarbons over supported Pt catalysts. II. Toluene hydrogenation. J. Catal. 1993, 143, 554–562. (50) Infantes-Molina, A.; Me´rida-Robles, J.; Rodrı´guez-Castelloń, E.; Fierro, J. L. G.; Jimeńez-Lo´pez, A. Pt, Ir and Pd promoted Co/MSU catalysts for hydrotreating of tetralin: A thiotolerance study. Appl. Catal., B 2007, 73, 180–192. (51) Nyleń, U.; Frontela Delgado, J.; Ja¨rås, S.; Boutonnet, M. Low and high-pressure ring opening of indan over 2 wt. % Pt, Ir and bi-metallic Pt25Ir75/boehmite catalysts prepared from microemulsion systems. Appl. Catal., A 2004, 262, 189–200. (52) Xiao, T. C.; York, A. P. E.; Al-Megren, H.; Williams, C. V.; Wang, H. T.; Green, M. L. H. Preparation and Characterisation of Bimetallic Cobalt and Molybdenum Carbides. J. Catal. 2001, 202, 100–109.



Figure 8. Plot of conversion and yields vs temperature for hydrogenation hydrogenolysis/hydrocracking of tetralin. Experimental conditions: H2/THN molar ratio ) 10, pH2 ) 4.5 MPa, pN2 ) 1.5 MPa, LHSV ) 6 h-1, GHSV ) 1300 h-1, contact time ) 2.8 s. Figure 10. Influence of contact time: plot of conversion and yields vs temperature for hydrogenation hydrogenolysis/hydrocracking of tetralin. Experimental conditions: H2/THN molar ratio ) 10, pH2 ) 4.5 MPa, pN2 ) 1.5 MPa.

Figure 9. Evolution of conversion and yields on time on stream for hydrogenation hydrogenolysis/hydrocracking of tetralin in the presence of 425 ppm of DBT. Experimental conditions: pH2 ) 4.5 MPa, pN2 ) 1.5 MPa, LHSV ) 6 h-1, GHSV ) 1300 h-1, H2/THN molar ratio ) 10, contact time ) 2.8 s, Ta ) 315 °C.

adsorption of S compounds on nickel atoms and because of the high affinity of nickel for sulfur adsorption. This has been fully documented in the literature.4,53 Figure 9 displays the catalytic activity of these samples with time on stream by feeding 425 ppm of DBT to the feed at 315 °C. Only the Ni15Cr5.2 catalyst maintains a high level of conversion as well as high yields of hydrogenation products. Similarly to the temperature test, the yield of HHC compounds is very low for all the chromiumcontaining catalysts. The deactivation of nickel was expected, although better behavior of the other two chromium-containing (53) Mare`cot, P.; Paraiso, E.; Dumas, J.-M.; Barbier, J. Deactivation of nickel catalysts by sulphur compounds I. Benzene hydrogenation. Appl. Catal., A 1992, 80, 79–88.

samples, Ni15Cr2.6 and Ni15Cr3.9, was also expected. For a better understanding of such behavior, surface analysis of the samples used in the hydrotreating of tetralin in the presence of dibenzothiophene was performed in order to determine and confirm the presence of sulfur in the spent catalysts. XPS measurements of the S 2p spectrum of Cr-containing samples reveals the presence of sulfur in the sample with the highest Cr content, that is, Ni15Cr5.2 displaying an S 2p3/2 BE of 162.2 eV (not shown). This value is slightly lower than that found for Cr2S3 (162.5 eV) and slightly higher than the value reported for S2- ions (161.0-161.7 eV),54,55 possibly indicating that the formation of a superficial mixed oxide-sulfide of chromium(III) is formed. In the presence of DBT, chromium ions act as tramp of such molecules forming a superficial sulfide, which inhibits the negative effect of the interaction of sulfur compound on nickel catalysts; that is, these nickel metallic particles are surrounded with Cr2O3 ones, and the latter are transformed into superficial CrS in the presence of DBT, an active phase in HDS reactions that protects the Ni0 phase from S poisoning. In this sense, Pawelec et al.36 reported that the presence of Pd or Li in nickel catalysts did not improve the thioresistance by electronic modifications since no change of the Ni 2p3/2 binding energy was observed. Either a coke coating was formed on the active sites or the protection of metallic nickel resulted from other phases such as Li2O or even Ni2+, which prevented these sites from S poisoning. In our case, from the data listed in Table 5 relating to the elemental chemical analysis of catalysts used in the presence of DBT, one can observe a decrease in the carbon percentage when the chromium content increases; therefore these (54) De Jong, A.-M. Thesis, Eindhoven University of Technology, Eindhoven, The Netherlands, 1994. (55) Bouwer, S. M. A.; van Zon, F. B. M.; van Dijk, M. P.; van der Kraan, A. M.; de Beer, V. H. J.; van Veen, J. A. R.; Koningsberger, D. C. On the structural differences between alumina-supported comos type I and alumina-, silica-, and carbon-supported comos type II phases studied by XAFS, MES, and XPS. J. Catal. 1994, 146, 375–393.



Figure 11. Influence of H2/THN molar ratio: plot of conversion and yields vs temperature for hydrogenation hydrogenolysis/hydrocracking of tetralin. Experimental conditions: contact time ) 3.6 s, pH2 ) 4.5 MPa, pN2 ) 1.5 MPa. Table 5. Elemental Chemical Analysis for Spent Catalysts after feeding 425 ppm of DBT (τ ) 2.8 s and H2/THN ) 10) %C %H %S

Ni15

Ni15Cr2.6

Ni15Cr3.9

Ni15Cr5.2

3.00 0.92 0.00

1.26 0.92 0.03

1.24 0.88 0.10

1.07 0.89 0.15

coatings should be discarded. The binding energy value corresponding to the Ni 2p signal does not change. However, discarding any electronic modifications, the increase in the sulfur percentage and the appearance of the S 2p band in the XPS measurements for the sample with the highest chromium content point to the role of chromium ions acting as captors of sulfur that prevent poisoning. The two samples with 2.6 and 3.9 wt % of chromium CrO3 seem not to have enough chromium sites to prevent nickel particles being poisoned by DBT molecules. Changes in the experimental conditions were tested in order to optimize the catalyst performance. The Ni15Cr3.9 catalyst was chosen to try and improve its catalytic activity, especially in the presence of DBT in the feed. First, the gas and liquid hourly space velocities (GHSV and LHSV) were modified to change the contact time between reactant and catalyst. Figure 10 depicts the results when the contact time varies from 1.8 to 3.6 s; and at different temperatures. In general, at all temperatures studied, the longer the contact time, the better the catalytic results, as previously found for cobalt-based catalysts.56 It is important to remark on the sizable increase in the HHC yields at 350 and 375 °C with the contact time, which is to be expected so long as the longer the contact between the reactants and the active sites, the more favored these types of reactions become. (56) Infantes-Molina, A.; Me´rida-Robles, J.; Rodrı´guez-Castelloń, E.; Pawelec, B.; Fierro, J.-L.-G.; Jimeńez-Lo´pez, A. Catalysts based on Co/ zirconium doped mesoporous silica MSU for the hydrogenation and hydrogenolysis/hydrocracking of tetralin. Appl. Catal., A 2005, 286, 239– 248.

When the above results were taken into account, 3.6 s was the contact time chosen to further investigate the influence of the H2/tetralin molar ratio. Thus, this ratio was varied from 5 to 15. It is important to note that catalysts need a H2 excess of 2-3 times the stoichiometric ratio (i.e., H2/tetralin ) 3:1 mol/ mol) to counteract the thermodynamic limitations at higher temperatures. The corresponding results are displayed in Figure 11. With molar ratios of 10 and 15, an outstanding improvement is achieved in the conversion at temperatures above 275 °C. This is due to greater formation of both hydrogenation products (HYD) and hydroisomerization compounds (HHC). At 375 °C, the production of naphthalene from the direct dehydrogenation of tetralin is favored, giving yields close to 20%. The changes are more pronounced at 315 °C with a H2/tetralin molar ratio of 15, where a conversion close to 100% is attained, with a HYD yield value of 81%. Although the formation of the HHC products is not very high (12%), no formation of naphthalene and only a very low concentration of volatile compounds (VC) is found at this temperature. Therefore, a contact time of 3.6 s and a H2/THN molar ratio of 15 are the ideal conditions to be used in the new catalytic experiments. A new thiotolerance test was performed once the experimental conditions had been optimized with the main goal of improving the catalytic behavior of the samples with the lowest chromium content, Ni15Cr2.6 and Ni15Cr3.9. To this end, 425 ppm of DBT was added to the feed at a temperature of 315 °C. The results are plotted in Figure 12. The catalysts with the lowest chromium content are again slowly deactivated with time on stream under these new conditions, which seems to indicate that there is a very small amount of chromium in these samples capable of retaining the sulfur molecules. These same molecules could be retained over the nickel particles, causing a progressive loss of the catalytic activity by poisoning. Both the conversion and the yield of HYD products decrease with time on stream



these operating conditions. This occurs as the conversion and the yield of HYD product tend to the constant values of 82% and 70%, respectively. The sulfur molecules are retained by chromium species, forming a superficial sulfide, and do not exert their harmful effect on the nickel particles. This “chromium sulfide” phase is also active in hydrotreating reactions as reported by many authors.14,57 4. Conclusions The addition of chromium to nickel-supported catalysts on an aluminated mesoporous silica (SBA-15) gives rise to a new hydrotreating catalyst with improved sulfur resistance. In the presence of sulfur-containing molecules, a monometallic nickel catalyst is deactivated. However, by adding chromium to the catalysts, the sulfur molecules are retained by chromium species, which form a surface sulfide and prevent the poisoning of the nickel species. The Ni15Cr5.2 sample is the most active and stable in the hydrogenation of tetralin when operating with a feed containing 425 ppm of DBT at moderate temperatures. Figure 12. Evolution of conversion and yields on time on stream for hydrogenation hydrogenolysis/hydrocracking of tetralin in the presence of 425 ppm of DBT. Experimental conditions: pH2 ) 4.5 MPa, pN2 ) 1.5 MPa, LHSV ) 4 h-1, GHSV ) 1000 h-1, H2/THN molar ratio ) 15, contact time ) 3.6 s, Ta ) 315 °C.

for these two samples. This fact could be explained by considering nickel particles coverage by sulfur species. This coating would hinder the activation of the late hydrogen molecules spilling over and prevent them from carrying out hydrogenation reactions. On the other hand, the sample with the highest chromium content, Ni15Cr5.2, undergoes an amelioration in activity under

Acknowledgment. We gratefully acknowledge the Ministerio de Educacioń y Ciencia (Project MAT2003-02986 and MAT200602465). M.G.-C. also thanks the Ministerio de Educacioń y Ciencia (Spain) for a fellowship. A.I.-M. also thanks Junta Andalucıá for a postdoctoral contract (Excellence Proyect P06-FQM-01661). EF800741N (57) Sychev, M.; (San) de Beer, V. H. J.; Kodentsov, A.; van Oers, E. M.; van Santen, R. A. Chromia- and chromium sulfide-pillared clays: preparation, characterization, and catalytic activity for thiophene hydrodesulfurization. J. Catal. 1997, 168, 245–254.

Chromium Species as Captors of Sulfur Molecules on Nickel-Based

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