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SBA-15 catalysts on selectivity of DBT hydrodesulfurization
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.
Effect of citrate addition in NiMo/SBA-15 catalysts on selectivity of DBT hydrodesulfurization Diego Valencia,a Isidoro García-Cruz,b Tatiana Klimovaa* a
Facultad de Química, Universidad Nacional Autónoma de México (UNAM), Cd. Universitaria, Coyoacán, México D.F., 04510, México b Programa de Ingeniería Molecular, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152,Col. San Bartolo Atepehuacán, México D.F., 07730, México
Abstract NiMo catalysts supported on SBA-15 were prepared by coimpregnation and successive impregnation methods with the addition of citric acid in the impregnation solutions. Addition of citric acid resulted in an increase in both catalysts’ activity in dibenzothiophene HDS and selectivity towards the direct desulfurization route, which was due to an increase in the MoS2 dispersion and in the amount of Ni-Mo-S species. Keywords: NiMo catalysts, SBA-15, citric acid, hydrodesulfurization, dibenzothiophene
1. Introduction Nowadays, the need to improve the removal of sulfur from gasoline and diesel oil by means of deep hydrodesulfurization (HDS) is managed by new environmental legislations regarding fuel specifications. Most common HDS catalysts are molybdenum disulphide (MoS2) nanocrystallites, promoted by Co or Ni atoms and deposited on a high specific surface area support. In general, SiO2-supported catalysts show low HDS activity. However, recently we reported interesting results obtained with NiMo and NiW catalysts using SBA-15 silica [1,2]. The aim of the present work is to study the effect of the addition of citric acid (CA) during the preparation of NiMo catalysts supported on SBA15 on their activity and selectivity in hydrodesulfurization of dibenzothiophene (DBT).
2. Experimental NiMo catalysts were prepared by incipient wetness impregnation of SBA-15 with aqueous solutions of ammonium heptamolybdate and Ni(II) nitrate. Catalysts were prepared by two methods, coimpregnation of Mo and Ni in presence of citric acid (NiMoCA/SBA-15(C) catalyst) and successive impregnation of Mo (first) and then of Ni-citrate solution (NiMoCA/SBA-15(S) sample). In both cases, the molar ratio Ni:Mo:CA = 0.5:1:1 and pH = 9 were kept. After impregnation, NiMoCA/SBA-15(C) and NiMoCA/SBA-15(S) catalysts were air-dried at 100oC for 12 h without calcination. For comparison, three reference NiMo catalysts supported on SBA-15 and γ-alumina were prepared without CA using coimpregnation (C) or succesive impregnation (S) methods. Reference catalysts were air-dried (100oC, 12 h) and calcined (500oC, 4 h). Nominal metal charges in all catalysts were 12 wt. % MoO3 and 3 wt. % NiO. Prepared catalysts were characterized by N2 physisorption, small-angle and powder XRD, UVVis DRS, thermal analysis (TGA/DTG/DTA), TPR and HRTEM, and tested in the DBT HDS reaction. The catalytic activity tests were performed in a batch reactor at 300oC and 7.3 MPa total pressure for 8 h. Before the activity tests, the catalysts were sulfided ex situ in a tubular reactor at 400oC for 4 h in a stream of 15 vol. % of H2S in H2.
530
D. Valencia et al.
3. Results and discussion 3.1. Catalyst characterization Results from textural characterization of the supports and NiMo catalysts (Table 1) indicate that the incorporation of Ni and Mo species on the supports’ surface produced a decrease in the textural properties, which was stronger for the samples prepared with the addition of citric acid. This result can be attributed to a higher loading of deposited species in dried NiMoCA catalysts compared with that in calcined NiMo samples. It can also be noted that NiMo catalysts prepared by coimpregnation method have better textural properties than their counterparts prepared by successive impregnation procedure. In addition, all catalysts supported on SBA-15 had a significantly higher surface area than that of the NiMo/γ-Al2O3 analog. Citric acid addition in NiMo/SBA15 catalysts almost did not affect the shape of the N2 adsorption-desorption isotherm characteristic for SBA-15 support (Fig. 1), as well as its pore arrangement (Fig. 2). Table 1. Textural characteristics* of supports and NiMo catalysts. SBET (m2/g) Sμ (m2/g) Vp (cm3/g) Vμ (cm3/g)
Sample
Dads (Ǻ)
SBA-15
850
140
1.09
0.056
85
NiMoCA/SBA15(C)
526
60
0.72
0.022
85
NiMoCA/SBA-15(S)
459
49
0.76
0.017
84
NiMo/SBA-15(C)
597
83
0.78
0.031
82
NiMo/SBA-15(S)
578
86
0.81
0.033
85
γ-Al2O3
200
-
0.48
-
115
NiMo/γ-Al2O3(S)
185
-
0.39
-
115
* SBET, specific surface area; Sμ, micropore area; VP, total pore volume; Vμ, micropore volume; Dads, pore diameter determined from the N2 adsorption isotherm by the BJH method. 800
(100)
600
(b) (c)
500 400 300
Intensity (a.u.)
Volume Adsorbed (cm3/g STP)
(a) 700
(110) (200)
200
(a) (b)
100
(c)
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Fig. 1. N2 adsorption-desorption isotherms of (a) SBA-15; (b) NiMo/SBA-15(C); and (c) NiMoCA/SBA-15(C).
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
2Θ (o)
Fig. 2. Small-angle XRD patterns of (a) SBA-15; (b) NiMo/SBA-15(C); and (c) NiMoCA/SBA-15(C).
5.0
Effect of citrate addition in NiMo/SBA-15 catalysts on selectivity of DBT HDS 35
(a)
531
345
30
(b) (c)
20 15
(d)
(a)
337
Signal (a.u.)
F(R)
25
(b)
400
10
(c) 376
5 0
(d) 200
250
300
350
λ (nm)
400
450
500
200
400
600
800
1000
Temperature (°C)
Fig. 3. UV-Vis DRS spectra of (a) NiMoCA/ SBA-15(S); (b) NiMoCA/SBA-15(C); (c) NiMo/ SBA-15(S); and (d) NiMo/SBA-15(C).
Fig. 4. TPR profiles of (a) NiMoCA/SBA15(S); (b) NiMoCA/SBA-15(C); (c) NiMo/ SBA-15(S); (d) NiMo/SBA-15(C).
Thermal analysis of CA-containing catalysts (not shown) revealed that CA decomposition occurs at about 200°C. Powder XRD patterns of NiMo and NiMoCA catalysts did not show the presence of any crystalline phase, pointing out a good dispersion of the deposited metal oxide species in all samples. However, DRS and TPR characterization results (Figs. 3 and 4) indicate that, as expected, citric acid addition increased the dispersion of oxidic Mo species on SBA-15 surface and made their reduction much easier and more complete. The dispersion of sulfided MoS2 particles was also improved by CA addition (Table 2). Table 2. Average length and layer number of the MoS2 crystallites determined by HRTEM and activity and selectivity of NiMo catalysts in HDS of DBT. Selectivity*
Morphology of MoS2 phase
Conversion (%)
Average length (Å)
Average stacking
4h
8h
NiMoCA/SBA-15(C)
32.1
2.96
54
96
2.8
NiMoCA/SBA-15(S)
33.4
3.18
46
83
1.5
NiMo/SBA-15(C)
36.0
3.43
40
76
0.9
NiMo/SBA-15(S)
37.6
3.37
43
75
0.6
NiMo/γ-Al2O3(S)
40 [3]
~2.0 [3]
55
95
2.0
Catalyst
*
BP/CHB ratio at 50% of DBT conversion; BP, biphenyl; CHB, cyclohexylbenzene.
3.2. Catalytic behavior Catalytic activity tests (Table 2) show that citric acid addition in NiMo/SBA-15 catalysts leads to an increase in the activity in DBT HDS and selectivity towards the direct desulfurization (DDS) route of the reaction. A comparison of the catalytic behavior of the NiMoCA/SBA-15 catalysts prepared by coimpregnation (C) and successive impregnation (S) methods indicates that the catalyst prepared by the C method resulted to be more active and more selective for the DDS route than its counterpart prepared by the S procedure. In general, activity and selectivity trends
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D. Valencia et al.
observed inside a series of the SBA-15-supported samples can be attributed to differences in the dispersion of oxide and sulfided Mo species. Results from textural characterization, DRS, TPR and HRTEM showed that citric acid addition in the impregnation solutions leads to a better dispersion of the active phase explaining its higher activity, and to a decrease in the stacking degree of MoS2 particles giving rise to an increase in the selectivity towards direct desulfurization of DBT. This conclusion is well in line with previous report [4] in which higher hydrogenation ability in DBT HDS was found for Mo catalysts with more stacked MoS2 particles and vice versa. In addition, another effect produced by CA addition can be noted when comparing catalytic behavior of the NiMoCA/SBA-15(C) catalyst, the best catalyst among SBA15-supported samples, and the NiMo/γ-Al2O3 reference. Both catalysts showed similar activity in HDS of DBT, but the selectivity for BP formation was higher for the catalyst prepared with CA, although the stacking degree of MoS2 particles in it was higher than in the alumina-supported reference (Table 2). The explanation for this fact can be made considering that not only the stacking degree of the MoS2 crystallites determines the catalyst’s selectivity for the direct desulfurization route. Also, the formation of Ni-Mo-S species is necessary to provide the catalyst ability for the cleavage of the C-S bond in the DBT molecule [5]. According to this, it can be supposed that citric acid addition during the impregnation of Ni and Mo species on SBA-15 surface also leads to an increase in the number of formed Ni-Mo-S active sites enhancing by this means the rate of the DDS route. Similar increase in the formation of the Co-Mo-S structure has been reported recently when citric acid was added to the impregnation solution during preparation of CoMo catalysts [6].
4. Conclusions Addition of citric acid in the impregnation solutions during preparation of NiMo catalysts supported on SBA-15 resulted in an increase in both catalyst activity in hydrodesulfurization of DBT and selectivity towards the direct desulfurization route. Coimpregnation of Ni and Mo species in presence of citric acid seems to be a better way for catalyst preparation than their successive impregnation. Addition of citric acid resulted in an increase in the MoS2 dispersion and formation of a larger amount of Ni-Mo-S species.
Acknowledgements Financial support by CONACyT-Mexico (grant 100945) is gratefully acknowledged. The authors thank I. Puente Lee, C. Salcedo Luna, M. Aguilar Franco and M. Portilla for technical assistance with HRTEM, XRD and TGA/DTG/DTA characterizations.
References 1. 2. 3. 4. 5. 6.
L. Lizama, J.C. Amezcua, R. Reséndiz, S. Guzmán, G.A. Fuentes, T. Klimova, Stud. Surf. Sci. Catal., 165 (2007) 799. L. Lizama and T. Klimova, Appl. Catal. B: Env., 82 (2008) 139. T. Klimova, D. Solís Casados, J. Ramírez, Catal. Today, 43 (1998) 135. E.J.M. Hensen, P.J. Kooyman, Y. van der Meer, A.M. van der Kraan, V.H.J. de Beer, J.A.R. van Veen, R.A. van Santen, J. Catal., 199 (2001) 224. F. Bataille, J.L. Lemberton, P. Michaud, G. Pérot, M. Vrinat, M. Lemaire, E. Schulz, M. Breysse, S. Kasztelan, J. Catal., 191 (2000) 409. T. Fujikawa, Top. Catal., 52 (2009) 872.
Effect of citrate addition in NiMo/SBA-15 catalysts on selectivity of DBT hydrodesulfurization Diego Valencia,a Isidoro García-Cruz,b Tatiana Klimovaa* a
Facultad de Química, Universidad Nacional Autónoma de México (UNAM), Cd. Universitaria, Coyoacán, México D.F., 04510, México b Programa de Ingeniería Molecular, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152,Col. San Bartolo Atepehuacán, México D.F., 07730, México
Abstract NiMo catalysts supported on SBA-15 were prepared by coimpregnation and successive impregnation methods with the addition of citric acid in the impregnation solutions. Addition of citric acid resulted in an increase in both catalysts’ activity in dibenzothiophene HDS and selectivity towards the direct desulfurization route, which was due to an increase in the MoS2 dispersion and in the amount of Ni-Mo-S species. Keywords: NiMo catalysts, SBA-15, citric acid, hydrodesulfurization, dibenzothiophene
1. Introduction Nowadays, the need to improve the removal of sulfur from gasoline and diesel oil by means of deep hydrodesulfurization (HDS) is managed by new environmental legislations regarding fuel specifications. Most common HDS catalysts are molybdenum disulphide (MoS2) nanocrystallites, promoted by Co or Ni atoms and deposited on a high specific surface area support. In general, SiO2-supported catalysts show low HDS activity. However, recently we reported interesting results obtained with NiMo and NiW catalysts using SBA-15 silica [1,2]. The aim of the present work is to study the effect of the addition of citric acid (CA) during the preparation of NiMo catalysts supported on SBA15 on their activity and selectivity in hydrodesulfurization of dibenzothiophene (DBT).
2. Experimental NiMo catalysts were prepared by incipient wetness impregnation of SBA-15 with aqueous solutions of ammonium heptamolybdate and Ni(II) nitrate. Catalysts were prepared by two methods, coimpregnation of Mo and Ni in presence of citric acid (NiMoCA/SBA-15(C) catalyst) and successive impregnation of Mo (first) and then of Ni-citrate solution (NiMoCA/SBA-15(S) sample). In both cases, the molar ratio Ni:Mo:CA = 0.5:1:1 and pH = 9 were kept. After impregnation, NiMoCA/SBA-15(C) and NiMoCA/SBA-15(S) catalysts were air-dried at 100oC for 12 h without calcination. For comparison, three reference NiMo catalysts supported on SBA-15 and γ-alumina were prepared without CA using coimpregnation (C) or succesive impregnation (S) methods. Reference catalysts were air-dried (100oC, 12 h) and calcined (500oC, 4 h). Nominal metal charges in all catalysts were 12 wt. % MoO3 and 3 wt. % NiO. Prepared catalysts were characterized by N2 physisorption, small-angle and powder XRD, UVVis DRS, thermal analysis (TGA/DTG/DTA), TPR and HRTEM, and tested in the DBT HDS reaction. The catalytic activity tests were performed in a batch reactor at 300oC and 7.3 MPa total pressure for 8 h. Before the activity tests, the catalysts were sulfided ex situ in a tubular reactor at 400oC for 4 h in a stream of 15 vol. % of H2S in H2.
530
D. Valencia et al.
3. Results and discussion 3.1. Catalyst characterization Results from textural characterization of the supports and NiMo catalysts (Table 1) indicate that the incorporation of Ni and Mo species on the supports’ surface produced a decrease in the textural properties, which was stronger for the samples prepared with the addition of citric acid. This result can be attributed to a higher loading of deposited species in dried NiMoCA catalysts compared with that in calcined NiMo samples. It can also be noted that NiMo catalysts prepared by coimpregnation method have better textural properties than their counterparts prepared by successive impregnation procedure. In addition, all catalysts supported on SBA-15 had a significantly higher surface area than that of the NiMo/γ-Al2O3 analog. Citric acid addition in NiMo/SBA15 catalysts almost did not affect the shape of the N2 adsorption-desorption isotherm characteristic for SBA-15 support (Fig. 1), as well as its pore arrangement (Fig. 2). Table 1. Textural characteristics* of supports and NiMo catalysts. SBET (m2/g) Sμ (m2/g) Vp (cm3/g) Vμ (cm3/g)
Sample
Dads (Ǻ)
SBA-15
850
140
1.09
0.056
85
NiMoCA/SBA15(C)
526
60
0.72
0.022
85
NiMoCA/SBA-15(S)
459
49
0.76
0.017
84
NiMo/SBA-15(C)
597
83
0.78
0.031
82
NiMo/SBA-15(S)
578
86
0.81
0.033
85
γ-Al2O3
200
-
0.48
-
115
NiMo/γ-Al2O3(S)
185
-
0.39
-
115
* SBET, specific surface area; Sμ, micropore area; VP, total pore volume; Vμ, micropore volume; Dads, pore diameter determined from the N2 adsorption isotherm by the BJH method. 800
(100)
600
(b) (c)
500 400 300
Intensity (a.u.)
Volume Adsorbed (cm3/g STP)
(a) 700
(110) (200)
200
(a) (b)
100
(c)
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
Fig. 1. N2 adsorption-desorption isotherms of (a) SBA-15; (b) NiMo/SBA-15(C); and (c) NiMoCA/SBA-15(C).
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
2Θ (o)
Fig. 2. Small-angle XRD patterns of (a) SBA-15; (b) NiMo/SBA-15(C); and (c) NiMoCA/SBA-15(C).
5.0
Effect of citrate addition in NiMo/SBA-15 catalysts on selectivity of DBT HDS 35
(a)
531
345
30
(b) (c)
20 15
(d)
(a)
337
Signal (a.u.)
F(R)
25
(b)
400
10
(c) 376
5 0
(d) 200
250
300
350
λ (nm)
400
450
500
200
400
600
800
1000
Temperature (°C)
Fig. 3. UV-Vis DRS spectra of (a) NiMoCA/ SBA-15(S); (b) NiMoCA/SBA-15(C); (c) NiMo/ SBA-15(S); and (d) NiMo/SBA-15(C).
Fig. 4. TPR profiles of (a) NiMoCA/SBA15(S); (b) NiMoCA/SBA-15(C); (c) NiMo/ SBA-15(S); (d) NiMo/SBA-15(C).
Thermal analysis of CA-containing catalysts (not shown) revealed that CA decomposition occurs at about 200°C. Powder XRD patterns of NiMo and NiMoCA catalysts did not show the presence of any crystalline phase, pointing out a good dispersion of the deposited metal oxide species in all samples. However, DRS and TPR characterization results (Figs. 3 and 4) indicate that, as expected, citric acid addition increased the dispersion of oxidic Mo species on SBA-15 surface and made their reduction much easier and more complete. The dispersion of sulfided MoS2 particles was also improved by CA addition (Table 2). Table 2. Average length and layer number of the MoS2 crystallites determined by HRTEM and activity and selectivity of NiMo catalysts in HDS of DBT. Selectivity*
Morphology of MoS2 phase
Conversion (%)
Average length (Å)
Average stacking
4h
8h
NiMoCA/SBA-15(C)
32.1
2.96
54
96
2.8
NiMoCA/SBA-15(S)
33.4
3.18
46
83
1.5
NiMo/SBA-15(C)
36.0
3.43
40
76
0.9
NiMo/SBA-15(S)
37.6
3.37
43
75
0.6
NiMo/γ-Al2O3(S)
40 [3]
~2.0 [3]
55
95
2.0
Catalyst
*
BP/CHB ratio at 50% of DBT conversion; BP, biphenyl; CHB, cyclohexylbenzene.
3.2. Catalytic behavior Catalytic activity tests (Table 2) show that citric acid addition in NiMo/SBA-15 catalysts leads to an increase in the activity in DBT HDS and selectivity towards the direct desulfurization (DDS) route of the reaction. A comparison of the catalytic behavior of the NiMoCA/SBA-15 catalysts prepared by coimpregnation (C) and successive impregnation (S) methods indicates that the catalyst prepared by the C method resulted to be more active and more selective for the DDS route than its counterpart prepared by the S procedure. In general, activity and selectivity trends
532
D. Valencia et al.
observed inside a series of the SBA-15-supported samples can be attributed to differences in the dispersion of oxide and sulfided Mo species. Results from textural characterization, DRS, TPR and HRTEM showed that citric acid addition in the impregnation solutions leads to a better dispersion of the active phase explaining its higher activity, and to a decrease in the stacking degree of MoS2 particles giving rise to an increase in the selectivity towards direct desulfurization of DBT. This conclusion is well in line with previous report [4] in which higher hydrogenation ability in DBT HDS was found for Mo catalysts with more stacked MoS2 particles and vice versa. In addition, another effect produced by CA addition can be noted when comparing catalytic behavior of the NiMoCA/SBA-15(C) catalyst, the best catalyst among SBA15-supported samples, and the NiMo/γ-Al2O3 reference. Both catalysts showed similar activity in HDS of DBT, but the selectivity for BP formation was higher for the catalyst prepared with CA, although the stacking degree of MoS2 particles in it was higher than in the alumina-supported reference (Table 2). The explanation for this fact can be made considering that not only the stacking degree of the MoS2 crystallites determines the catalyst’s selectivity for the direct desulfurization route. Also, the formation of Ni-Mo-S species is necessary to provide the catalyst ability for the cleavage of the C-S bond in the DBT molecule [5]. According to this, it can be supposed that citric acid addition during the impregnation of Ni and Mo species on SBA-15 surface also leads to an increase in the number of formed Ni-Mo-S active sites enhancing by this means the rate of the DDS route. Similar increase in the formation of the Co-Mo-S structure has been reported recently when citric acid was added to the impregnation solution during preparation of CoMo catalysts [6].
4. Conclusions Addition of citric acid in the impregnation solutions during preparation of NiMo catalysts supported on SBA-15 resulted in an increase in both catalyst activity in hydrodesulfurization of DBT and selectivity towards the direct desulfurization route. Coimpregnation of Ni and Mo species in presence of citric acid seems to be a better way for catalyst preparation than their successive impregnation. Addition of citric acid resulted in an increase in the MoS2 dispersion and formation of a larger amount of Ni-Mo-S species.
Acknowledgements Financial support by CONACyT-Mexico (grant 100945) is gratefully acknowledged. The authors thank I. Puente Lee, C. Salcedo Luna, M. Aguilar Franco and M. Portilla for technical assistance with HRTEM, XRD and TGA/DTG/DTA characterizations.
References 1. 2. 3. 4. 5. 6.
L. Lizama, J.C. Amezcua, R. Reséndiz, S. Guzmán, G.A. Fuentes, T. Klimova, Stud. Surf. Sci. Catal., 165 (2007) 799. L. Lizama and T. Klimova, Appl. Catal. B: Env., 82 (2008) 139. T. Klimova, D. Solís Casados, J. Ramírez, Catal. Today, 43 (1998) 135. E.J.M. Hensen, P.J. Kooyman, Y. van der Meer, A.M. van der Kraan, V.H.J. de Beer, J.A.R. van Veen, R.A. van Santen, J. Catal., 199 (2001) 224. F. Bataille, J.L. Lemberton, P. Michaud, G. Pérot, M. Vrinat, M. Lemaire, E. Schulz, M. Breysse, S. Kasztelan, J. Catal., 191 (2000) 409. T. Fujikawa, Top. Catal., 52 (2009) 872.