γ-Al2O3 catalysts

Catalysis Communications 7 (2006) 1053–1056 www.elsevier.com/locate/catcom

Effect of the hydrogen spillover on the selectivity of dibenzothiophene hydrodesulfurization over CoSx/c-Al2O3, NiSx/c-Al2O3 and MoS2/c-Al2O3 catalysts N. Escalona

a,*

, R. Garcı´a a, G. Lagos a, C. Navarrete a, P. Baeza b, F.J. Gil-Llambı´as a

b

Universidad de Concepcio´n, Facultad de Ciencias Quı´micas, Casilla 160c, Concepcio´n, Chile b Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chile Received 6 March 2006; received in revised form 2 May 2006; accepted 3 May 2006 Available online 10 May 2006

Abstract Dibenzothiophene (DBT) hydrodesulphurization (HDS) reaction at 3 MPa and 325–375 °C on Mo/c-Al2O3 single-bed and Me/cAl2O3//SiO2//Mo/c-Al2O3 (Me = Co or Ni) double-bed catalysts were investigated. Results indicate that ratio cyclohexylbenzene (CHB)/biphenyl (BP) or selectivity is higher when using double-beds rather than a single-bed. Synergy in dibenzothiophene hydrodesulphurization on Co//Mo and Ni//Mo double-beds is also detected. Changes in selectivity and conversion are attributed to the action of spillover hydrogen (Hso) formed in the first bed that reaches the second bed. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Spillover hydrogen; Selectivity; Hydrodesulfurization; Dibenzothiophene

1. Introduction The most accepted mechanism for the dibenzothiophene (DBT) hydrodesulphurization reaction on Co(or Ni)–Mo/ c-Al2O3 catalysts is shown in the Fig. 1 [1,2]. This mechanism proposes that C–S bond cleavage of DBT in the hydrodesulphurization may occur either by a direct desulphurization route (DDS), leading to biphenyl, or by a second reaction pathway in which dibenzothiophene is first hydrogenated (HYD) and then desulfurized conducing to cyclohexylbenzene (CHB). The promoter role on selectivity and activity in Me–Mo/c-Al2O3 (Me = Co or Ni) catalysts for the hydrodesulphurization (HDS) reaction is still a controversial point [3–7]. The synergism of Co–Mo and Ni– Mo on HDS activity has been explained mainly by the formation of mixtures, such as the ‘‘CoMoS’’ phase [4,6] and by the so called remote control (RC) model [5,7].

*

Corresponding author. E-mail address: [email protected] (N. Escalona).

1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.05.011

Recently, we detected synergism between two singlebeds of Co/SiO2 and Mo/SiO2 catalysts separated by 5 mm of SiO2, in the HDS of gas-oil at high-pressure in a continuous-flow micro-reactor [8]. Additionally, the synergism in Co//Mo and Ni//Mo pairs in gas-oil HDS between double-beds of Me/c-Al2O3 and Mo/c-Al2O3 monometallic catalysts separated by 5 mm of c-Al2O3, was also recently detected in our laboratory [9]. Considering that both beds were separated in these studies, the interaction between monometallic catalysts is not possible. More precisely the CoMoS or NiMoS phases cannot be formed under these conditions; consequently the synergism in Co//Mo and Ni//Mo pairs could be more adequately explained by the mechanism proposed by the remote control model [5,7]. The remote control model attributes the synergism to the hydrogen spillover species (Hso), which migrates, from a donor (D) phase (CoSx or NiSx) to an acceptor (A) phase (MoS2). Under our experimental conditions [8,9], Hso is formed on CoSx or NiSx in the first bed and probably displaces across the SiO2 or c-Al2O3 surface to MoS2 in the second bed, increasing HDS activity of the Mo/c-Al2O3

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Fig. 1. Reaction network for the hydrodesulfurization of dibenzothiophene (DBT) (BP = biphenyl, TH–DBT = tetrahydrodibenzothiophene, HH– DBT = hexahydrodibenzothiophene and CHB = cyclohexylbezene).

catalyst. In that same study [9], we informed that if SiC is used as separator instead SiO2 or c-Al2O3, no synergism is detected, proving that a hypothetic succession of two reactions (a partial reaction on the Co/c-Al2O3 bed and a second reaction on the Mo/c-Al2O3 beds) does not occur. More recently, we reported the influences of the distance between Co9S8 and MoSx and the separator’s nature (cAl2O3, SiO2, carbon, MgSiO3 and two silica–aluminas) on the synergism. The results prove that Co//Mo and Ni//Mo synergism increases when the distance between the donor and acceptor phases decreases. Moreover, the results show that an inverse correlation exists between the value of the isoelectric point (IEP) of the separator surface and the enhancement of HDS activity [10]. On the other hand, the new European Union environmental specifications [11] for diesel fuels have generated interest in investigating new catalysts for deep hydrogenation (HYD) and deep hydrodesulphurization (HDS) [12,13]. Considering that preparation of these new catalysts can be optimized if the role of the Hso in the HYD and HDS mechanism is better understood, the aim of the present work was to study the influence of Hso on the cyclohexylbenzene (CHB)/biphenyl (BP) selectivity in the desulphurization of DBT. With this purpose, the selectivity of promoted and un-promoted Mo systems were compared, using a single-bed (SB) of monometallic Mo/cAl2O3 catalyst and composite beds (CB) consisting in two separated single-beds of Me (Co or Ni)/c-Al2O3 and Mo/ c-Al2O3 monometallic catalysts. 2. Experimental 2.1. Catalyst preparation As in previous studies [8,9], Mo/c-Al2O3, Co/c-Al2O3 and Ni/c-Al2O3 monometallic samples were prepared by wet impregnation, dried overnight at 100 °C and calcined at 550 °C for 4.0 h. Cobalt nitrate (Merck PA), nickel nitrate (Merck PA) and ammonium heptamolybdate (Merck PA) were used as precursors. The support was a c-Al2O3 Girdler T-126 (N2 BET 190 m2 g1 and pore vol-

ume 0.365 cm3 g1) and the separator of both beds was SiO2 BASF D11-10 (BET 154 m2 g1 and pore volume 0.270 cm3 g1). The metallic contents of Co, Ni and Mo were 3.9, 3.9 and 11.9 g of CoO, NiO and MoO3 per 100 g of c-Al2O3, respectively. The Mo and Co or Ni contents were determined by inductive couple plasma (ICP) in a Perkin–Elmer optima 3300DV using the respective 102.301, 228.616 and 231.604 nm emission lines. 2.2. Reaction condition The reactions were carried out in a continuous-flow stainless steel micro-reactor. The composite bed was made as follows: the first bed consisted in 0.2 g de Co/c-Al2O3 or Ni/c-Al2O3 and the second bed in 0.2 g of Mo/c-Al2O3 monometallic catalyst, diluted 1:1 with SiO2. These beds were separated by 2 mm of SiO2. The remaining space in the reactor was filled with SiC particles. The particle size of all catalysts, SiO2 and SiC, between 0.42 and 0.85 mm. The composite beds are indicated as Co/cAl2O3//SiO2//Mo/c-Al2O3 or Ni/c-Al2O3//SiO2//Mo/cAl2O3 and the synergism via Hso are indicated as Co// Mo and Ni//Mo. Prior to the reaction, the catalysts received an in situ sulfidation treatment during 4 h, at 350 °C, total pressure 3 MPa, using 7% CS2 dissolved in decaline. The HDS reaction temperatures were 325, 350 and 375 °C; the total pressure was 3 MPa; the liquid feed of 2000 ppm of DBT dissolved in decaline; and, the space velocities liquid were LHSV = 30 h1 and H2 GHSV = 3600 h1. The products were collected after 10 h of reaction and analyzed by GC with TCD (autosystem XL, Perkin–Elmer) equipped with a column CPSIL-5 CB. In addition to unreacted DBT, biphenyl (BP) and cyclohexylbenzene (CHB) were the only composed detected. The conversion was expressed as HDS (%), calculated as the disappearance of DBT. Selectivity towards BP and CHB are given by Eqs. (1) and (2), respectively, S BP ¼ ðX BP =ðX BP þ X CHB ÞÞ  100 S CHB ¼ ðX CHB =ðX BP þ X CHB ÞÞ  100

ð1Þ ð2Þ

N. Escalona et al. / Catalysis Communications 7 (2006) 1053–1056

where Xi is the degree of conversion for components i in the mixture.

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1.0

Table 1 shows the dibenzothiophene HDS conversion at 325, 350 and 375 °C using simple beds of Ni/c-Al2O3 (Test 2), Co/c-Al2O3 (Test 1), and Mo/c-Al2O3 (Test 3) catalysts as well as composite beds of Co//Mo (Test 4), and Ni//Mo (Test 5). Table 1 shows that Ni(3.9)/c-Al2O3 (Test 2) and Co(3.9)/c-Al2O3 (Test 1) catalysts are not active in dibenzothiophene HDS under our experimental conditions. Additionally, in Table 1, it can be seen that for all reaction temperatures, the Co//Mo and Ni//Mo composite beds are more active than the single-bed. As in previous studies, the Co//Mo and Ni//Mo synergism occurs even when both beds are separated by 2 mm of SiO2. Under these conditions the corresponding Co–Mo–S and Ni–Mo–S phase can not occurs, confirming so that the Co//Mo and Ni// Mo synergism observed for the gas-oil HDS [8,9] is a general behavior. It is very important to note that both monometallic Co/ c-Al2O3 and Ni/c-Al2O3 catalysts used in HDS of DBT (Test 1 and Test 2), do not present any activity. Thus compound detected at reactor outlet was uniquely DBT. This result confirms our previous conclusions [9] that Me//Mo synergism is not due to the succession of two catalytic reactions, but rather to a partial reaction in the Ni/c-Al2O3 or Co/c-Al2O3 bed and a second reaction in the Mo/c-Al2O3 bed. Fig. 2 show that composite beds have a ratio cyclohexylbenzene (CHB)/biphenyl (BP) selectivity higher than a single-bed. The selectivity reported in this work is at 325 ° C. At higher reaction temperatures, the synergism Co//Mo and Ni//Mo via Hso decreases as was previously reported [8,9] and ratified in Table 1, consequently the changes of selectivity are not significant (not showed here). Nevertheless, at 325 °C this interesting increase in the HYD route in the DBT mechanism shown in Fig. 1 must be caused by the hydrogen spillover. This behavior has been previously suggested by Delmon and co-workers [14–17], whose studies were performed using supported and unsupported MoS2 and CoSx catalysts prepared by mechanical mixing. They found a strong synergism and an increment in the HYD

Table 1 HDS conversion with single and composite beds Test

1 2 3 4 5

Beds

Co(3.9)/c-Al2O3 Ni(3.9)/c-Al2O3 Mo(11.9)/c-Al2O3 Co(3.9)/c-Al2O3//SiO2//Mo(11.9)/ c-Al2O3 Ni(3.9)/c-Al2O3//SiO2//Mo(11.9)/ c-Al2O3

(%) HDS 325 °C

350 °C

375 °C

0 0 31.1 39.1

0 0 55.5 60.1

0 0 70.6 73.8

37.1

58.6

72.7

(SCHB)/(SBP)

0.8

3. Results and discussion

0.6

0.4

CB SB

CB

0.2

0.0

MoO3

NiO

CoO

Fig. 2. Ratio of selectivity cyclohexylbezene (SCHB)/biphenyl (SBP) of a Mo(11.9)/c-Al2O3 single-bed (SB) and a (Ni or Co)(3.9)/c-Al2O3//SiO2// Mo(11.9)/c-Al2O3 composite bed (CB) (selectivity to 325 °C).

route for studied systems. However, in these studies [14– 17], the selectivity change cannot be exclusively assigned to the hydrogen spillover since the presence of a ‘‘CoMoS’’ phase in the contact points between CoSx and MoS2 particles cannot be unequivocally rejected. On the contrary under the experimental conditions used in the present study, the formation of a phase like ‘‘CoMoS’’ or ‘‘NiMoS’’ is not possible, therefore the increment of the hydrogenation selectivity must be exclusively due to Hso. On the other hand, as generally reported, the CoMo/ alumina and NiMo/alumina catalysts are 20 times more active than the Mo/alumina catalyst for the HDS of DBT [4]. This promoting effect was essentially due to the enhancement of the rate of the DDS pathway of DBT. In fact, the HYD pathway represented about 75% on the Mo/alumina catalyst but only 13–15% with the promoted catalysts [18]. The changes of selectivity (HYD/DDS) observed in the presence of the phases CoMoS or NiMoS are different of those observed in our experimental conditions. Suggesting, that the Hso generates changes in the catalytic centers different to the observed in NiMoS and CoMoS phases. The changes of selectivity observed in the composite beds involve an important information, that must be taken in to consideration to understand the complex reactions of HDS of the DBT in presence of Hso. Furthermore, the observed increment of the selectivity of HYD is additional information that can be used for the design of new catalysts. Particularly for those catalytic systems oriented to the HDS of molecules type 4, 6 DMDBT, in which the more important pathway is the benzenic ring hydrogenation [18,19]. The selectivity change can be explained by the model developed by Vrinat et al. [19,20], who proposed that two types of active centers (hydrogenation and desulfurization) exists in bimetallic hydrotreating catalysts, being located at a single active site. Thus, hydrogenation sites are formed by a vacancy associated to a SH group and a hydrogen atom adsorbed on a molybdenum atom. The sites involved in the direct desulphurization route, and more generally in the

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C–S bond cleavage, should be composed by two vacancies associated with its corresponding sulfur anion. Both centers could be considered basically identical except that adsorbed hydrogen should be considered as part of the hydrogenation center. In agreement with this, Delmon and Dallons [21] proposed that Hso creates and regenerates the active sites in the acceptor phase (in this case MoS2). Therefore, the Hso should be adsorbed in adjacent vacancies to a SH group, forming a hydrogenation center and increasing selectivity to CHB. Finally, Fig. 2 shows that HYD selectivity to cyclohexylbenzene (CHB)/biphenyl (BP) is higher using Co than using Ni as first bed. The higher selectivity change and higher HDS synergism of Co//Mo than Ni//Mo doublebeds are consistent with a greater Hso production in the Co/c-Al2O3 catalyst rather than in the Ni/c-Al2O3 catalyst, previously reported [9]. 4. Conclusions In the systems studied using double-beds like Co/cAl2O3//SiO2//Mo/c-Al2O3 and Ni/c-Al2O3//SiO2//Mo/cAl2O3 it was observed an increase in the HDS activity and in the ratio (CHB/BP) compared to that of the single-bed Mo/c-Al2O3 catalyst. In absence of mixtures phases like CoMoS or NiMoS, the changes in selectivity and activity can only be explained by the remote control model, through the formation and participation of hydrogen spillover. Acknowledgments Financial supports for the present study were received from DIUC-UDEC Grant No. 205.22.21-1.0, FONDECYT Grant No. 1020046 and CONICYT Grant AT 4040033.

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