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Enhanced dibenzothiophene desulfurization over NiMo catalysts simultaneously impregnated with saccharose
10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.
Enhanced dibenzothiophene desulfurization over NiMo catalysts simultaneously impregnated with saccharose José Escobar,* José A. Toledo, Ana W. Gutiérrez, María C. Barrera, María A. Cortés, Carlos Angeles, Leonardo Díaz Instituto Mexicano del Petróleo, Eje Central L. Cárdenas 152, San Bartolo A., G. A. Madero, México, D.F., 07730, México
Abstract High surface area Al2O3 (Sg=307 m2 g-1) was impregnated by pore-filling with a solution prepared from MoO3 digestion in presence of H3PO4. Nickel hydroxycarbonate was further added resulting in Ni and P concentrations corresponding to Ni/(Ni+Mo)=0.29 and P2O5/(NiO+MoO3)=0.1, respectively. Saccharose (SA) at various SA/Ni molar ratios (1, 2 and 3) was further dissolved in impregnating solutions. TPR profiles of noncalcined precursors suggested that highly stable Mo-complexes were formed in SAmodified impregnating solutions. Hydrotreating catalysts obtained by sulfiding precursors at 400 °C were tested in dibenzothiophene hydrodesulfurization which was enhanced by the organic modifier. However, that beneficial effect depended on SA concentration, maximum activity being observed at SA/Ni=1. By HR-TEM, SA addition resulted in MoS2 particles of lower stacking but slightly increased length. Keywords: saccharose, NiMo catalyst, hydrodesulfurization, organic additive
1. Introduction Methodology of impregnation of Co-Mo or Ni-Mo phases plays a major role on dispersion, sulfidability and promotion degree in final sulfided hydrodesulfurization (HDS) catalysts. Various effects have been related to using either chelating [1] or nonchelating [2] organic agents, due to their interaction with metals to deposit. In formulations prepared in presence of those additives the mechanism provoking increased “M1M2S” (where M1= Ni or Co and M2= Mo4+ or W4+) formation is not completely unveiled. Promoter chelation by organic additives could contribute to delayed sulfidation of those species which could be then properly integrated to already formed MoS2 crystals [1]. Also, due to their interaction with the carrier non-chelating additives could contribute to preserve (during support impregnation) species where Mo and promoter atoms could coexist, that fact resulting in enhanced formation of either CoMoS or NiMoS phases after sulfiding [2,3]. However, other factors (carbon deposition, type II sites formation, etc.) could result in catalysts of increased HDS activity. Thus, deeper studies are pertinent. In this regard, we prepared P-doped NiMo/Al2O3 catalyts modified by saccharose (SA) at various SA/Ni ratios. Materials were studied by TPR and HR-TEM and tested in dibenzothiophene (DBT) HDS.
2. Experimental 2.1. Catalyst synthesis High surface area alumina was obtained by calcining (under static air) a commercial boehmite (Versal 200, Euro Support) at 500 ºC (5 h). Al2O3 textural properties were
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J. Escobar et al.
Sg=307 m2 g-1, Vp=0.9 cm3 g-1 and average pore diameter (from 4×Vp/Sg) of ~12 nm. Pore-filling simultaneous impregnation (of support previously dried at 120ºC, 2 h) was carried out by an aqueous solution (pH ~1.9) prepared from digestion (at ~80ºC) of MoO3 (99.5 wt % PQM) in presence of H3PO4 (85.3 wt% Tecsiquim). After 2 h, a yellow transparent solution was observed. 2NiCO3·3Ni(OH)2·4H2O (Sigma-Aldrich) was then added, heating being maintained (2 h). Ni and P concentration corresponded to Ni/(Ni+Mo)=0.29 and P2O5/(NiO+MoO3)=0.1 (mass ratio), respectively [2]. Transparent emerald green solutions were thus obtained. Saccharose (SA, J.T. Baker) at various SA/Ni molar ratio (1, 2 and 3) was added to impregnating solutions which pH was essentially unaltered (pH ~2.0) after organic agent dissolution. After impregnation materials were dried at 120ºC (2 h), calcining being avoided to preserve organic additive integrity. Nominal Mo, Ni and P loadings corresponded to 12, 3 and 1.6 wt% in final catalyst, respectively. Samples were identified by the SA(x) key where “x” stands for SA/Ni ratio. A reference material with no organic additive was also synthesized (SA(0). Sulfided catalysts were obtained by submitting impregnated precursors to treatment at 400°C (heating rate 6ºC/min) under H2/H2S (Praxair) at 50/6 (ml/min)/ (ml/min) during a 2 h period of time.
2.2. Materials characterization Temperature-programmed reduction experiments were conducted in an Altamira 2000 equipment. As-made (dried) Ni–Mo–P/Al2O3 samples, either with or without glycol, were put in a quartz reactor. Circa 50 mg of materials ground at particle size that passed through U.S. Mesh 80 (178 μm) were heated from 30 to 850°C (at 5°C min-1, heating rate) under a 30 ml min-1 flow of an Ar/H2 90/10 (vol/vol) mixture. High resolution transmission electron microscopy studies of sulfided catalysts were performed in a JEM-2200FS at 200 kV accelerating voltage. The apparatus was equipped with a Schottky-type field emission gun and an ultra-high resolution (UHR) configuration (Cs = 0.5 mm; Cc = 1.1 mm; point-to-point resolution, 0.19 nm) and in-column omega-type energy filter. Samples to be analyzed were ground, suspended in isopropanol and dispersed with ultrasonic agitation. Then, some drops of the suspension were deposited on a 3 mm diameter lacey carbon copper grid. GATANTM software was used during statistical determination of slab length and stacking of supported MoS2 particles.
2.3. HDS reaction test HDS activity of synthesized catalysts was studied in a tri-phasic slurry batch reactor (Parr 4575). The reaction mixture was prepared by adding ∼ 0.3 g of dibenzothiophene (99 mass %, from Aldrich) and ∼0.2 g of sieved catalyst (80-100 U.S. mesh) in 100 cm3 of n-hexadecane (99 mass %, from Aldrich) Operating conditions, carefully chosen to avoid external diffusion limitations, were P= 5.59 ± 0.03 MPa, T= 320 ± 3°C and 1000 RPM. Samples taken periodically were analyzed by gas chromatography (Agilent 6890N, flame ionization detector and Econocap-5 capillary column (from Alltech). HDS kinetic constants were calculated assuming a pseudo-first order model referred to organo-S compound concentration and zero order with respect to excess H2.
3. Results and discussion 3.1. HDS activity test It was observed that original emerald green impregnating solutions changed to cobaltblue after aging. This phenomenon was function of saccharose concentration, the change in coloring taking place in about two days (at room temperature) for solution at SA/Ni=3 and longer time for the others. From Figure 1, it was clear that saccharose
Enhanced dibenzothiophene desulfurization over NiMO catalysts
769
Intensity (a.u.)
-4
-1
-1
k × 10 (L g s )
addition resulted in catalysts of increased HDS activity. Notably, this effect was much more pronounced when cobalt-blue solutions were used during impregnation.
2.5 2.0 1.5 1.0
SA(3) SA(2) SA(1)
SA(0)
0.5 0.0
SA(0)g SA(1)g SA(1) SA(2) SA(3)
Catalyst Figure 1. DBT HDS pseudo first order kinetic constants for catalysts tested. Subindex “g” indicates materials impregnated with green solution.
200
400
600
800
Temperature (°C) Figure 2. TPR profiles corresponding to alumina-supported NiMo samples prepared with saccharose as additive.
The organic additive could be possibly converted to an chelating compound during aging in the impregnating solution. It is known that saccharose could be transformed to saccharic acid in presence of some ions [4]. Saccharose in aqueous solution decomposes in two monosaccharides, glucose and fructose. Then, these cyclic compounds could be oxidized (by ions present in the impregnating solution), generating linear carboxylic acids [5]. These species could effectively chelate Ni2+ or Mo6+ cations, that fact being possibly related to the color change observed. Interestingly, during separate experiments where Ni nitrate was dissolved in H3PO4 solution, folllowed by SA addition (at SA/Ni=1) no color change (due to blue-shifted Ni2+ d-d transitions bands) was observed. This indicated that water molecules (in Ni[(H2O)6]2+) were not substituted by organic moieties (stronger ligands in spectroscopic series). Nevertheless, when a sulfur-yellow solution obtained by MoO3 digestion in H3PO4 (Mo, P and SA added in similar ratio to that in SA(1)) was left aging for some days, a drastic color change to cobalt-blue was registered. That suggested that phosphomolybdates originally present in solution [2] could be complexated by organic moieties. However, Ni2+ complexes formation in our SA-modified impregnating solutions could not be ruled out. In any case, it seemed that Mo chelation could be partially responsible for HDS activity trend in Figure 1.
3.2. TPR and high resolution TEM From TPR profiles (Figure 2 and Table 1) of non-calcined impregnated precursors it could be determined that the signal related to reduction of Mo6+ species to Mo4+ shifted to higher temperature in samples prepared with saccharose, that effect being more remarkable in materials of augmented organic agent content. Similar phenomenon was observed for the peak related to Mo4+ reduction to Mo0. Those facts suggested the existence of highly stable Mo-complexes originally formed in impregnating solutions. On the other hand, Ni2+ reduction seemed to be shifted to lower temperature in materials prepared with saccharose as additive. Similarly to the case where ethyleneglycol was used as organic modifier, the corresponding signal appeared to merge with that related to Mo6+ reduction [2]. According to those results and in agreement with that previously reported regarding ZrO2-TiO2-supported NiMo catalysts synthesized with citric acid [6],
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J. Escobar et al.
presence of Ni2+-complexes of lower stability than those of Mo6+ is strongly suggested. By statistical analysis of HR-TEM micrographs of sulfided catalysts prepared, SAaddition resulted in MoS2 slabs of lower stacking but slightly increased size. Table 1. Temperature of maximum in TPR profiles of alumina-supported NiMo samples prepared with saccharose as additive. n.d.: not determined. Sample
SA(0) SA(1) SA(2) SA(3)
Tmax (ºC) Mo6+ → Mo4+ 400 418 425 460
Tmax (ºC) Ni2+ → Ni0 500 n.d. n.d. n.d.
Tmax (ºC) Mo4+ → Mo0 576 591 613 640
Stacking
Slab length (nm)
7 4 3 4
3 5 4 4
Finally, strongly diminished HDS activity in material prepared at higher SA concentration (SA/Ni=3) remains to be explained. We hypothesize that some surface carbon could remain after organics decomposition (during sulfiding). Enhanced amount of carbonaceous deposits could be formed in solids of increased SA concentration. Presumably, that C could provoke partial plugging of porous network, hindering access of reactant molecules to surface active sites. Clarification of this point is clearly needed.
4. Conclusions In uncalcined P-doped NiMo/alumina precursors modified by saccharose addition existence of highly stable Mo-complexes is strongly suggested. Corresponding sulfided hydrotreating catalysts had enhanced activity in dibenzothiophene conversion. Nevertheless, that beneficial effect depended on SA concentration, maximum activity being registered for materials prepared at equimolar SA/Ni ratio. SA addition resulted in MoS2 particles of lower stacking but slightly increased length.
References [1] A.J. van Dillen, R.J.A.M. Terörde, D.J. Lensveld, J.W. Geus, K.P. de Jong, 2003, Synthesis of supported catalysts by impregnation and drying using aqueous chelated metal complexes, J. Catal. 216, 257-264. [2] J. Escobar, M.C. Barrera, J.A. Toledo, M.A. Cortés-Jácome, C. Angeles-Chávez, S. Núñez, V. Santes, E. Gómez, L. Díaz, E. Romero, J.G. Pacheco, 2009, Effect of ethyleneglycol addition on the properties of P-doped NiMo/Al2O3 HDS catalysts: Part I. Materials preparation and characterization, Appl. Catal. B. 88, 564-575. [3] D. Nicosia, R. Prins, 2005, The effect of glycol on phosphate-doped CoMo/Al2O3 hydrotreating catalysts, J.Catal., 229, 424-438. [4] J.-S. Girardon, E. Quinet, A. Griboval-Constant, P.A. Chernavskii, L. Gengembre, A.Y. Khodakov, 2007, Cobalt dispersion, reducibility, and surface sites in promoted silicasupported Fischer–Tropsch catalysts, J. Catal. 248, 143-157. [5] E.A. Souza, J.G.S. Duque, L. Kubota, C.T. Meneses, 2007, Synthesis and characterization of NiO and NiFe2O4 nanoparticles obtained by a sucrose-based route, J. Phys. Chem. Solids 68, 591-599. [6] J. Escobar, M.C. Barrera, J.A. De los Reyes, J.A. Toledo, V. Santes, J.A. Colín, 2008, Effect of chelating ligands on Ni-Mo impregnation over wide-pore ZrO2-TiO2, J. Molec. Catal. A 287, 33-40.
Enhanced dibenzothiophene desulfurization over NiMo catalysts simultaneously impregnated with saccharose José Escobar,* José A. Toledo, Ana W. Gutiérrez, María C. Barrera, María A. Cortés, Carlos Angeles, Leonardo Díaz Instituto Mexicano del Petróleo, Eje Central L. Cárdenas 152, San Bartolo A., G. A. Madero, México, D.F., 07730, México
Abstract High surface area Al2O3 (Sg=307 m2 g-1) was impregnated by pore-filling with a solution prepared from MoO3 digestion in presence of H3PO4. Nickel hydroxycarbonate was further added resulting in Ni and P concentrations corresponding to Ni/(Ni+Mo)=0.29 and P2O5/(NiO+MoO3)=0.1, respectively. Saccharose (SA) at various SA/Ni molar ratios (1, 2 and 3) was further dissolved in impregnating solutions. TPR profiles of noncalcined precursors suggested that highly stable Mo-complexes were formed in SAmodified impregnating solutions. Hydrotreating catalysts obtained by sulfiding precursors at 400 °C were tested in dibenzothiophene hydrodesulfurization which was enhanced by the organic modifier. However, that beneficial effect depended on SA concentration, maximum activity being observed at SA/Ni=1. By HR-TEM, SA addition resulted in MoS2 particles of lower stacking but slightly increased length. Keywords: saccharose, NiMo catalyst, hydrodesulfurization, organic additive
1. Introduction Methodology of impregnation of Co-Mo or Ni-Mo phases plays a major role on dispersion, sulfidability and promotion degree in final sulfided hydrodesulfurization (HDS) catalysts. Various effects have been related to using either chelating [1] or nonchelating [2] organic agents, due to their interaction with metals to deposit. In formulations prepared in presence of those additives the mechanism provoking increased “M1M2S” (where M1= Ni or Co and M2= Mo4+ or W4+) formation is not completely unveiled. Promoter chelation by organic additives could contribute to delayed sulfidation of those species which could be then properly integrated to already formed MoS2 crystals [1]. Also, due to their interaction with the carrier non-chelating additives could contribute to preserve (during support impregnation) species where Mo and promoter atoms could coexist, that fact resulting in enhanced formation of either CoMoS or NiMoS phases after sulfiding [2,3]. However, other factors (carbon deposition, type II sites formation, etc.) could result in catalysts of increased HDS activity. Thus, deeper studies are pertinent. In this regard, we prepared P-doped NiMo/Al2O3 catalyts modified by saccharose (SA) at various SA/Ni ratios. Materials were studied by TPR and HR-TEM and tested in dibenzothiophene (DBT) HDS.
2. Experimental 2.1. Catalyst synthesis High surface area alumina was obtained by calcining (under static air) a commercial boehmite (Versal 200, Euro Support) at 500 ºC (5 h). Al2O3 textural properties were
768
J. Escobar et al.
Sg=307 m2 g-1, Vp=0.9 cm3 g-1 and average pore diameter (from 4×Vp/Sg) of ~12 nm. Pore-filling simultaneous impregnation (of support previously dried at 120ºC, 2 h) was carried out by an aqueous solution (pH ~1.9) prepared from digestion (at ~80ºC) of MoO3 (99.5 wt % PQM) in presence of H3PO4 (85.3 wt% Tecsiquim). After 2 h, a yellow transparent solution was observed. 2NiCO3·3Ni(OH)2·4H2O (Sigma-Aldrich) was then added, heating being maintained (2 h). Ni and P concentration corresponded to Ni/(Ni+Mo)=0.29 and P2O5/(NiO+MoO3)=0.1 (mass ratio), respectively [2]. Transparent emerald green solutions were thus obtained. Saccharose (SA, J.T. Baker) at various SA/Ni molar ratio (1, 2 and 3) was added to impregnating solutions which pH was essentially unaltered (pH ~2.0) after organic agent dissolution. After impregnation materials were dried at 120ºC (2 h), calcining being avoided to preserve organic additive integrity. Nominal Mo, Ni and P loadings corresponded to 12, 3 and 1.6 wt% in final catalyst, respectively. Samples were identified by the SA(x) key where “x” stands for SA/Ni ratio. A reference material with no organic additive was also synthesized (SA(0). Sulfided catalysts were obtained by submitting impregnated precursors to treatment at 400°C (heating rate 6ºC/min) under H2/H2S (Praxair) at 50/6 (ml/min)/ (ml/min) during a 2 h period of time.
2.2. Materials characterization Temperature-programmed reduction experiments were conducted in an Altamira 2000 equipment. As-made (dried) Ni–Mo–P/Al2O3 samples, either with or without glycol, were put in a quartz reactor. Circa 50 mg of materials ground at particle size that passed through U.S. Mesh 80 (178 μm) were heated from 30 to 850°C (at 5°C min-1, heating rate) under a 30 ml min-1 flow of an Ar/H2 90/10 (vol/vol) mixture. High resolution transmission electron microscopy studies of sulfided catalysts were performed in a JEM-2200FS at 200 kV accelerating voltage. The apparatus was equipped with a Schottky-type field emission gun and an ultra-high resolution (UHR) configuration (Cs = 0.5 mm; Cc = 1.1 mm; point-to-point resolution, 0.19 nm) and in-column omega-type energy filter. Samples to be analyzed were ground, suspended in isopropanol and dispersed with ultrasonic agitation. Then, some drops of the suspension were deposited on a 3 mm diameter lacey carbon copper grid. GATANTM software was used during statistical determination of slab length and stacking of supported MoS2 particles.
2.3. HDS reaction test HDS activity of synthesized catalysts was studied in a tri-phasic slurry batch reactor (Parr 4575). The reaction mixture was prepared by adding ∼ 0.3 g of dibenzothiophene (99 mass %, from Aldrich) and ∼0.2 g of sieved catalyst (80-100 U.S. mesh) in 100 cm3 of n-hexadecane (99 mass %, from Aldrich) Operating conditions, carefully chosen to avoid external diffusion limitations, were P= 5.59 ± 0.03 MPa, T= 320 ± 3°C and 1000 RPM. Samples taken periodically were analyzed by gas chromatography (Agilent 6890N, flame ionization detector and Econocap-5 capillary column (from Alltech). HDS kinetic constants were calculated assuming a pseudo-first order model referred to organo-S compound concentration and zero order with respect to excess H2.
3. Results and discussion 3.1. HDS activity test It was observed that original emerald green impregnating solutions changed to cobaltblue after aging. This phenomenon was function of saccharose concentration, the change in coloring taking place in about two days (at room temperature) for solution at SA/Ni=3 and longer time for the others. From Figure 1, it was clear that saccharose
Enhanced dibenzothiophene desulfurization over NiMO catalysts
769
Intensity (a.u.)
-4
-1
-1
k × 10 (L g s )
addition resulted in catalysts of increased HDS activity. Notably, this effect was much more pronounced when cobalt-blue solutions were used during impregnation.
2.5 2.0 1.5 1.0
SA(3) SA(2) SA(1)
SA(0)
0.5 0.0
SA(0)g SA(1)g SA(1) SA(2) SA(3)
Catalyst Figure 1. DBT HDS pseudo first order kinetic constants for catalysts tested. Subindex “g” indicates materials impregnated with green solution.
200
400
600
800
Temperature (°C) Figure 2. TPR profiles corresponding to alumina-supported NiMo samples prepared with saccharose as additive.
The organic additive could be possibly converted to an chelating compound during aging in the impregnating solution. It is known that saccharose could be transformed to saccharic acid in presence of some ions [4]. Saccharose in aqueous solution decomposes in two monosaccharides, glucose and fructose. Then, these cyclic compounds could be oxidized (by ions present in the impregnating solution), generating linear carboxylic acids [5]. These species could effectively chelate Ni2+ or Mo6+ cations, that fact being possibly related to the color change observed. Interestingly, during separate experiments where Ni nitrate was dissolved in H3PO4 solution, folllowed by SA addition (at SA/Ni=1) no color change (due to blue-shifted Ni2+ d-d transitions bands) was observed. This indicated that water molecules (in Ni[(H2O)6]2+) were not substituted by organic moieties (stronger ligands in spectroscopic series). Nevertheless, when a sulfur-yellow solution obtained by MoO3 digestion in H3PO4 (Mo, P and SA added in similar ratio to that in SA(1)) was left aging for some days, a drastic color change to cobalt-blue was registered. That suggested that phosphomolybdates originally present in solution [2] could be complexated by organic moieties. However, Ni2+ complexes formation in our SA-modified impregnating solutions could not be ruled out. In any case, it seemed that Mo chelation could be partially responsible for HDS activity trend in Figure 1.
3.2. TPR and high resolution TEM From TPR profiles (Figure 2 and Table 1) of non-calcined impregnated precursors it could be determined that the signal related to reduction of Mo6+ species to Mo4+ shifted to higher temperature in samples prepared with saccharose, that effect being more remarkable in materials of augmented organic agent content. Similar phenomenon was observed for the peak related to Mo4+ reduction to Mo0. Those facts suggested the existence of highly stable Mo-complexes originally formed in impregnating solutions. On the other hand, Ni2+ reduction seemed to be shifted to lower temperature in materials prepared with saccharose as additive. Similarly to the case where ethyleneglycol was used as organic modifier, the corresponding signal appeared to merge with that related to Mo6+ reduction [2]. According to those results and in agreement with that previously reported regarding ZrO2-TiO2-supported NiMo catalysts synthesized with citric acid [6],
770
J. Escobar et al.
presence of Ni2+-complexes of lower stability than those of Mo6+ is strongly suggested. By statistical analysis of HR-TEM micrographs of sulfided catalysts prepared, SAaddition resulted in MoS2 slabs of lower stacking but slightly increased size. Table 1. Temperature of maximum in TPR profiles of alumina-supported NiMo samples prepared with saccharose as additive. n.d.: not determined. Sample
SA(0) SA(1) SA(2) SA(3)
Tmax (ºC) Mo6+ → Mo4+ 400 418 425 460
Tmax (ºC) Ni2+ → Ni0 500 n.d. n.d. n.d.
Tmax (ºC) Mo4+ → Mo0 576 591 613 640
Stacking
Slab length (nm)
7 4 3 4
3 5 4 4
Finally, strongly diminished HDS activity in material prepared at higher SA concentration (SA/Ni=3) remains to be explained. We hypothesize that some surface carbon could remain after organics decomposition (during sulfiding). Enhanced amount of carbonaceous deposits could be formed in solids of increased SA concentration. Presumably, that C could provoke partial plugging of porous network, hindering access of reactant molecules to surface active sites. Clarification of this point is clearly needed.
4. Conclusions In uncalcined P-doped NiMo/alumina precursors modified by saccharose addition existence of highly stable Mo-complexes is strongly suggested. Corresponding sulfided hydrotreating catalysts had enhanced activity in dibenzothiophene conversion. Nevertheless, that beneficial effect depended on SA concentration, maximum activity being registered for materials prepared at equimolar SA/Ni ratio. SA addition resulted in MoS2 particles of lower stacking but slightly increased length.
References [1] A.J. van Dillen, R.J.A.M. Terörde, D.J. Lensveld, J.W. Geus, K.P. de Jong, 2003, Synthesis of supported catalysts by impregnation and drying using aqueous chelated metal complexes, J. Catal. 216, 257-264. [2] J. Escobar, M.C. Barrera, J.A. Toledo, M.A. Cortés-Jácome, C. Angeles-Chávez, S. Núñez, V. Santes, E. Gómez, L. Díaz, E. Romero, J.G. Pacheco, 2009, Effect of ethyleneglycol addition on the properties of P-doped NiMo/Al2O3 HDS catalysts: Part I. Materials preparation and characterization, Appl. Catal. B. 88, 564-575. [3] D. Nicosia, R. Prins, 2005, The effect of glycol on phosphate-doped CoMo/Al2O3 hydrotreating catalysts, J.Catal., 229, 424-438. [4] J.-S. Girardon, E. Quinet, A. Griboval-Constant, P.A. Chernavskii, L. Gengembre, A.Y. Khodakov, 2007, Cobalt dispersion, reducibility, and surface sites in promoted silicasupported Fischer–Tropsch catalysts, J. Catal. 248, 143-157. [5] E.A. Souza, J.G.S. Duque, L. Kubota, C.T. Meneses, 2007, Synthesis and characterization of NiO and NiFe2O4 nanoparticles obtained by a sucrose-based route, J. Phys. Chem. Solids 68, 591-599. [6] J. Escobar, M.C. Barrera, J.A. De los Reyes, J.A. Toledo, V. Santes, J.A. Colín, 2008, Effect of chelating ligands on Ni-Mo impregnation over wide-pore ZrO2-TiO2, J. Molec. Catal. A 287, 33-40.