γ-Al2O3 catalysts: Effect of calcination and activation over thiophene hydrodesulfurization

Catalysis Communications 8 (2007) 285–288 www.elsevier.com/locate/catcom

Highly active ReS2/c-Al2O3 catalysts: Effect of calcination and activation over thiophene hydrodesulfurization N. Escalona a

a,*

, F.J. Gil Llambias b, M. Vrinat c, T.S. Nguyen c, D. Laurenti c, A. Lo´pez Agudo d

Universidad de Concepcio´n, Facultad de Ciencias Quı´micas, Casilla160c, Concepcio´n, Chile b Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile c Institut de Recherches sur la Catalyse, 2 Av. Einstein, 69626 Villeurbanne Cedex, France d Instituto de Cata´lisis y Petroleoquı´mica, CSIC, Cantoblanco, 28049 Madrid, Spain Received 1 August 2005; accepted 29 May 2006 Available online 16 June 2006

Abstract The effect of activation conditions on alumina-supported rhenium sulfide catalysts on their activity for thiophene hydrodesulfurization has been studied. The results showed that activation using H2S/N2 leads to ReS2/c-Al2O3 catalysts with a higher activity than an industrial NiMo catalyst. Characterization by temperature-programmed reduction and X-ray photoelectron spectroscopy revealed that the higher activity of the catalysts activated by H2S/N2 is associated with a higher sulfur content and probably related to different ReSx phases. Ó 2006 Published by Elsevier B.V.

1. Introduction A number of earlier studies documented that ReS2 catalysts, unsupported as well as supported on alumina, carbon or silica, exhibit a high activity for hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and hydrogenation (HYD) reactions of model compounds [1–7]. Moreover, we have recently reported that alumina- and carbon-supported ReS2 catalysts are also highly active in the HDS of gas oil performed under close industrial conditions [8,9]. Therefore, in view of these interesting results and taking into account that ReS2 has been found to exhibit relatively high hydrogenation properties [2,6,10,11], which could be used for improvement or design of catalysts for deep HDS, it seems worthwhile to investigate more the effects of the preparation and activation conditions of these materials on their catalytic properties. While numerous studies about the influence of the activation on the structure and HDS performance of Mo or W *

Corresponding author. Tel.: +56 41207236; fax: +56 41245974. E-mail address: [email protected] (N. Escalona).

1566-7367/$ - see front matter Ó 2006 Published by Elsevier B.V. doi:10.1016/j.catcom.2006.05.053

catalysts have been reported [12–20], only two studies have provided some information on the activation of Re sulfide catalysts [7,21]. In the first one Arnoldy et al. [21] reported on the sulfiding behaviour of oxidic Re/c-Al2O3 catalysts by temperature-programmed sulfidation, and in the second one the same authors discussed about the effect of the Re content and the calcination temperature on the thiophene HDS activity of Re2O7 catalysts supported on c-Al2O3, SiO2 and activated carbon [7]. However, information about the possible effect of the composition of the sulfidation mixture on the catalytic performance of supported Re catalysts has not yet been reported, though the importance of this activation parameter on the activity for HDS of Mo and Ru based catalysts has been demonstrated in several studies [15–19]. Therefore, the present study focuses on the influence of the nature of the sulfiding mixture and the precalcination temperature on the activity of Re/c-Al2O3 catalysts for the HDS of thiophene. The second objective was the evaluation of the catalytic behaviour of the prepared catalysts, over a wide range of Re content, under such activation conditions to estimate the optimum loading and to compare

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with the activity of a reference NiMo/c-Al2O3 catalyst. Moreover, TPR and XPS characterizations were performed in order to understand catalytic activities variations.

348 K

835 K

2. Experimental

(e)

2.1. Catalyst preparation and sulfidation procedure

(d)

u.a

A series of alumina-supported Re catalysts with different Re content ranging from 5.9 to 24.0 wt.% of Re2O7 (corresponding to 0.76–3.33 atoms of Re per nm2) was prepared and analyzed as previously described [8]. The catalysts were sulfided at 623 K for 4 h with a flow of 10 vol.% H2S in either H2 or N2. Both sulfidation procedures were applied to uncalcined (dried) precursors or after calcination at 573 K in dry air for 0.5 h.

368 K

786 K (c)

2.2. Catalytic tests The catalytic activity was determined using the thiophene HDS, in a fixed bed microreactor operated in the dynamic mode at atmospheric pressure. Thiophene was introduced into the reactor by flowing H2 through a saturator maintained at the temperature of 273 K, with a total flow of 3 L/h and thiophene pressure 2.4 kPa. Reaction products were analysed in an online gas chromatograph. The specific activity (As) was determined at 573 K after of 16 h on stream in the pseudo stationary state according to the following equation: As ¼ F 0 X =m; where As is the specific activity (mol s1 g1), F0 is the molar flow rate of thiophene, X is the conversion of reactant and m is the weight of catalyst (g). 2.3. Catalyst characterization Temperature-programmed reduction (TPR-S) and XPS measurements of sulfided catalysts was performed as previously described [15,8]. 3. Results 3.1. Catalyst characterization 3.1.1. TPR The TPR-S profiles of unsupported ReS2, Re(0.76)/cAl2O3 and Re(2.75)/c-Al2O3 sulfided in a H2S/H2 or H2S/N2 mixture without previous calcinations are shown in Fig. 1. These results clearly show a distinct behaviour of these samples toward hydrogen reduction. Bulk ReS2 leads to a H2S production in the 800–1100 K temperature range with a peak maximum at about 1024 K, while supported ReSx was reduced at lower temperature with two peaks at about 368 K and 786 K for Re(0.76)/c-Al2O3 and 348 K and 835 K for Re(2.75)/c-Al2O3. Such variations indicate that supported ReSx species are well dis-

(b) 1024 K

(a) 400

600

800

1000

1200

1400

Temperature (K) Fig. 1. TPR spectra of unsupported ReSx (a), Re(0.76)/c-Al2O3 sulfided under H2S/H2 (b) and H2S/N2 (c) mixtures, and Re(2.75)/c-Al2O3 sulfided under H2S/H2 (d) and H2S/N2 (e) mixtures.

persed on c-Al2O3, and possibly different than bulk ReS2. From the comparison of the low temperature peaks of the two Re(0.76)/c-Al2O3 and Re(2.75)/c-Al2O3 samples, it is clearly observed that the amount of H2S evolved is higher using H2S/N2 than H2S/H2. This behaviour suggests that the samples sulfided by H2S/N2 presents an easier reducibility and a higher amount of surface sulfur atoms weakly bound, where sulfur vacancies are created by reduction. A shift of the peaks to the reduction temperature from 368 K to 348 K for Re(0.76)/c-Al2O3 and Re(2.75)/c-Al2O3 samples suggests a easier reduction of the ReS2 species present in this last sample. Moreover, calculation of the total H2S evolved during the reduction gave a Stotal/Re ratio of 1.8 for Re(0.76)/c-Al2O3 and 1.5 for Re(2.75)/c-Al2O3 when they are sulfided with H2S/H2, and 2.2 and 1.7, respectively, for the H2S/N2 activated samples. According to these Stotal/Re values, the H2S/H2 activated samples reach a lower degree of sulfidation than the H2S/N2 activated samples. That the H2S/N2 activated Re(0.76)/c-Al2O3 sample gave a Stotal/Re ratio higher than the value 2 expected for ReS2 particles points to the formation of non-stoichiometric sulfur species or elemental sulfur during the sulfiding treatment [21]. This is why XPS measurements were performed to determine the degree of sulfidation.

N2

350 300 250

H2

200

N2

150 100

-1 -1 -8

N2/H2S

120 100

H2/H2S

80 60 40 20 0

C

NC

C

Fig. 3. Effect of the calcination before sulfidation over thiophene HDS activity.

suggests that Re dispersion in this catalyst has not significantly changed during the calcination, probably due to a relatively strong interaction of Re species with alumina in both dried and calcined state. The effect of the Re loading on the thiophene HDS activity was therefore studied over uncalcined Re/cAl2O3 catalysts sulfided with a H2S/N2 mixture. The results presented in Fig. 4 show that the specific rate increases linearly with the Re content up to a value closed to about 1.5 Re at./nm2 and then increases more slowly for higher Re loadings. Even for 3.33 Re at./nm2, apparently the maximum does not appears to be reached. When compared to a commercial NiMo/c-Al2O3 catalyst containing 2.75 Mo at./nm2and a ratio Ni/[Ni + Mo] = 0.3, the thiophene specific rate of the comparable Re(2.75)/c-Al2O3 catalyst was about 1.6-fold higher than that of the NiMo/c-Al2O3 (Fig. 4). 4. Discussion The results show that the catalytic activity of aluminasupported Re catalysts depends strongly on the sulfiding mixture. As a general result, the use of the H2S/N2 mixture

300

-8

-8

-1 -1

Thiophene HDS (10 mol g s )

Fig. 2 shows that the HDS activity was superior by sulfiding with the H2S/N2 than by the H2S/H2 mixtures: 44% and 66% higher for Re(0.76)/c-Al2O3 and Re(2.75)/ c-Al2O3 catalysts, respectively. Activity over uncalcined and calcined Re(0.76)/c-Al2O3 precursors sulfided by either H2S/H2 or H2S/N2 show (Fig. 3) that calcination at 573 K has no significant effect on the HDS activity, which is in good agreement with the results of Arnoldy et al. [7]. It

140

-1 -1

3.2. Catalytic activity

287

NC

Thiophene HDS (10 mol g s )

3.1.2. XPS XPS spectra (not shown) of Re(2.75)/c-Al2O3 catalyst after sulfidation using H2S/H2 and H2S/N2 mixtures indicated that in both cases the Re 4f spectra could be fitted to a single doublet with the more intense Re 4f7/2 component at 41.6 eV, which corresponds to the value reported for ReS2 [2,8,9], suggesting in principle a complete sulfidation of Re. However, a more detailed examination of the spectra revealed that the spectrum of the sample sulfided by the H2S/H2 mixture showed a higher broadening towards low binding energy values. This asymmetry of the spectrum of the sample sulfided by the H2S/H2 mixture could be attributed to the formation of a small amount of rhenium species with lower oxidation state, most probably Re0 species, which have a Re 4f7/2 binding energy close to 40.5 eV [22]. It should be noted that the identification of Re0 by XPS is very difficult because its binding energy is very close to that of ReS2. The XPS Re/Al atomic ratio for both the H2S/H2 and H2S/N2 sulfided Re(2.75)/c-Al2O3 samples was found to be approximately equal to 0.060, suggesting that in both cases the dispersion of Re species was similar, and that there is some aggregation of ReS2. However, the S/Re ratio values was 2.2 and 3.1 for the Re(2.75)/c-Al2O3 sample sulfided with H2S/H2 and H2S/N2 mixtures, respectively. In line with the TPR results, the XPS results also reveal an excess of sulfur in the surface of the ReS2 crystallites and especially using the H2S/N2 mixture.

Thiophene HDS (10 mol g s )

N. Escalona et al. / Catalysis Communications 8 (2007) 285–288

H2

50 0 0.76

0.76

2.75

2.75 -2

Re content (atoms nm ) Fig. 2. Effect of the nature of the sulfiding mixture over thiophene HDS for low and high rhenium loading.

200

Ni-Mo/γ−Al2O3 100

0 0

1

2

3

4

-2

Re content (atoms nm ) Fig. 4. Influence of the rhenium content over HDS activity for Re/cAl2O3 catalysts sulfided under a H2S/N2 mixture.

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leads to a higher HDS activity compared to the H2S/H2 mixture. This higher efficiency of the H2S/N2 treatment seems be related to the higher degree of sulfidation obtained by the catalyst under such sulfiding conditions, which leads to the creation of a larger amount of sulfur vacancies under the reaction conditions. This is in line with the fact that both TPR and XPS gave S/Re values (considered as parameter related to the degree of sulfidation of Re species) relatively greater for the catalysts sulfided using H2S/N2 than using H2S/H2. Using H2S/H2 activation, the degree of sulfidation of Re is lower because the reduction by H2 of the oxidic Re species to Re0 competes with their sulfidation to ReS2. This effect would be greater at high Re content since crystallites of NH4ReO4 are present, and due to the much easier diffusion of H2, as compared with H2S, through a dense ReS2 shell initially formed, Re metal would be formed inside of the ReS2 shell [21]. Indeed, the above XPS results suggest the presence of a few Re metal in the H2S/H2 activated sample. Similar effect has been previously observed for some of us [15] in the case of RuS2/c-Al2O3 catalysts. For this system, a well sulfided and highly active catalyst was obtained only if H2 was avoided during the sulfidation treatment. It appears that some parallelism could be made between Re and Ru based sulfided catalysts. However in both cases the S/Re ratio is higher than 2, suggesting that the amount of Re0 exists only in a small amount. Therefore we believe that, depending of the nature of the sulfiding mixture, different ReSx phases could be formed. Indeed, Schwarz et al. [23] reported that rhenium sulfide could exists as ReS2, Re2S7 and Re2S4, these two later materials being members of a continuum of Re(S)1.5–1(S2)1–1.5 structures that differ in terms of their 2 S2 ratio. In this case the S/Re ratio could vary 2 =S between 2 and 4 without variation of the binding energy of Re, as observed by XPS. Variation in the amount of such rich sulfur content phase would depend not only on the nature of the sulfiding mixture, but also on the loading of Re over the support, since our results show (Fig. 2) that the effect of the activation sulfiding mixture on the HDS specific rate was greater at high Re content. Moreover these proposals are in line with the high catalytic activity reported for Re2S7, possibly in connection with the functionality of (S2 2 ) entities [24,25]. More characterizations of these catalysts by HREM are in progress in order to evidence some possible change in the structure of the ReSx phase with the sulfiding mixture. 5. Conclusions The sulfidation of Re/c-Al2O3 catalysts with H2S/N2 instead of H2S/H2 leads to highly active catalysts in thio-

phene hydrodesulfurization. For rhenium concentration higher than 1.5 atoms nm2 Re/c-Al2O3 samples are more active than a NiMo industrial reference. Such higher activities could be correlated with differences in the reducibility of the sulfided phases and to different S/Re ratios, probably due to new sulfide phases. Acknowledgements Financial support from Project 1020046, FONDECYT (Chile), Programme COFACAL CNRS (France). N.E is indebted to CNRS for his postdoctoral grant. References [1] T. Pecoraro, R.R. Chianelli, J. Catal. 67 (1981) 430. [2] J.P.R. Vissers, C.K. Groot, E.M. van Oers, V.H.J. de Beer, R. Prins, Bull. Soc. Chim. Belg. 93 (1984) 813. [3] M.J. Ledoux, O. Michaux, G. Agostini, J. Catal. 102 (1986) 275. [4] S. Eijsbouts, V.H.J. de Beer, R. Prins, J. Catal. 109 (1988) 217. [5] C. Sudhakar, S. Eijsbouts, V.H.J. de Beer, R. Prins, Bull. Soc. Chim. Belg. 96 (1987) 885. [6] M. Lacroix, N. Boutarfa, C. Guillard, M. Vrinat, M. Breysse, J. Catal. 120 (1989) 473. [7] P. Arnoldy, E.M. van Oers, V.H.J. de Beer, J.A. Moulijn, R. Prins, Appl. Catal. 48 (1989) 241. [8] N. Escalona, J. Ojeda, R. Cid, G. Alvez, A. Lo´pez Agudo, J.L.G. Fierro, F.J. Gil Llambı´as, Appl. Catal. A 234 (2002) 45. ´ vila, A. Lo´pez Agudo, J.L. Garcı´a, [9] N. Escalona, M. Yates, P. A Fierro, J. Ojeda, F.J. Gil Llambı´as, Appl. Catal. A 240 (2003) 151. [10] J. Ra¨ty, T.A. Pakkanen, Catal. Lett. 65 (2000) 175. [11] J. Quartaro, S. Mignard, S. Kasztelan, J. Catal. 192 (2000) 307. [12] H. Topsøe, B.S. Clausen, F.E. Massot, in: J.R. Anderson, M. Boudart (Eds.), Hydrotreating Catalysis, vol. 11, Springer-Verlag, Berlin, 1996. [13] R. Prada Silvy, J.M. Beuken, P. Bertrand, B.K. Hodnett, F. Delannay, B. Delmon, Bull. Soc. Chim. Belg. 93 (1984) 775. [14] R. Prada Silvy, J.L.G. Fierro, P. Grange, B. Delmon, Stud. Surf. Sci. Catal. 31 (1987) 605. [15] J.A. De Los Reyes, S. Go¨bo¨lo¨s, M. Vrinat, M. Breysse, Catal. Lett. 5 (1990) 17. [16] G. Geantet, S. Go¨bo¨lo¨s, J.A. De Los Reyes, M. Cattenot, M. Vrinat, M. Breysse, Catal. Today 10 (1991) 665. [17] L. Portela, P. Grange, B. Delmon, J. Catal. 243 (1995) 243. [18] H.R. Reinhoudt, A.D. van Langeveld, P.J. Kooyman, R.M. Stokmann, R. Prins, H.W. Zandbergen, J.A. Moulijn, J. Catal. 179 (1998) 443. [19] L.I. Merin˜o, A. Centeno, S.A. Giraldo, Appl. Catal. A 197 (2000) 61. [20] H.R. Reinhoudt, R. Troost, A.D. van Langeveld, J.A.R. van Veen, S.T. Sie, J.A. Moulijn, J. Catal. 203 (2001) 509. [21] P. Arnoldy, J.A.M. van den Heijkant, V.H.J. de Beer, J.A. Moulijn, Appl. Catal. 23 (1986) 81. [22] P.S. Kirlin, B.R. Strohmeier, B.C. Gates, J. Catal. 98 (1986) 308. [23] D.E. Schwarz, A.I. Frenkel, R.G. Nuzzo, T.B. Rauchfuss, A. Vairavamurthy, Chem. Mater. 16 (2004) 51. [24] C.M. Bolinger, T.D. Weatherill, T.B. Rauchfuss, A.L. Rheingold, C.S. Day, S.R. Wilson, Inorg. Chem. 2 (1986) 634. [25] D. Seyferth, R.S. Henderson, L.C. Song, Organometallics 1 (1982) 125.