Al2O3 for hydrodesulfurization of dibenzothiophene

Spillover and Migration of Surface Species on Catalysts Can Lj and Qin Xin, editors 0 1997 Elsevier Science B.V. AI1 rights reserved.

Study

on the role

of platinum

171

in PtMo/A1203

for hydrodesulfurization

of

dibenzothiophene J. Wanga, W. -2. Li” Zhanga

l

, G. Perotb, J. L. Lembertonb,

C. -Y. Yua, C. Thomasb and Y. -2.

aDalian Institute of Chemical Physics, Chinese Academy of Sciences, 116023 Dalian, P. R. China bLaboratoire de Catalyse en Chimie Organique, URA CNRS 350, Universite de Poitiers, 40 avenue du Recteur Pineau, 86022 Poitiers Cedex, France.

Hydrodesulfurization (HDS) of dibenzothiophene (DBT) over PtMo/AlzOj was studied in the presence of H2S. Pt was found to be an effective promoter for this reaction and the Ptcontaining catalyst was sulfur tolerance. Data of the Hz-D* exchange revealed that both the amount of retained hydrogen and the initial rate of H-D exchange on the sulfided catalyst were increased by the introduction of a slight amount of Pt into the catalyst, and corresponded very well to the catalytic activities for HDS. The reduction and sulfidation of MO sites was also facilitated very much by Pt, which was found by the results of the pulse reduction and sulfidation technique. It is proposed that, during the pretreatment of PtMo/AlzOj, Hz is first dissociated into hydrogen atoms on Pt sites, which then spillover onto MO sites, providing a large amount of coordinately unsaturated MO sites for the HDS reaction, and resulting in the higher HDS activity than the unpromoted one.

1. INTRODUCTION In recent years, the more and more strict environmental legislation and the need for processing heavy fractions require studies of the new generation of deep hydrotreating catalysts, as well as the better understanding of the surface structure of catalysts and the nature

’ To whom

correspondence

should

be addressed.

172 of promoters. One of them is the try of using noble metal-containing catalysts with sulfur and/or ammonia resistance character as one of the oil processing catalysts [ 1-5]. For example, Hirschon and coworkers [2] found RuMo/AI203 exhibited much high catalytic activity and aromatic selectivity for hydrodenitrogenation of quinoline than Mo/AI203. Synergy effect was also found in the mechanical mixture of bulk MoS2 (or WS2) with noble metal supported phases for the HDS of thiophene and hydrogenation of cyclohexene [6]. However, the role of the noble metal in these catalysts remains still open to be studied. It has been revealed that [7], in various metal - (n-type) semiconductor catalytic systems ( Pt(Pd)/TiO2(SnO2, ZnO2, CeO2)), and in reductive atmosphere, the noble metal could act as an intermediate to accelerate the charge and hydrogen atom transfer between hydrogen molecule and the n-type semiconductor. Considering both molybdenum oxide and molybdenum sulfide are typical n-type semiconductors, in the present study, platinum is chosen as the promoter for molybdenum based hydrotreating catalysts, with a view to obtain new indications of hydrogen behaviors in noble metal promoted catalysts.

2. E X P E R I M E N T A L The Mo(9.3 wt.%)/A1203 catalyst was prepared from 7-A1203 and ammonium molybdate using the conventional impregnation method. Pt(0.2 wt.%)/A1203 and Pt(0.2 wt.%)Mo(9.3 wt. %)/A1203 catalysts were prepared with 7-A1203 and Mo/AI203 as the support respectively, impregnated in the aqueous solution of chloro-platinic acid, followed by evaporation and drying in air at 393K for 12 hours. MoO3 was obtained by decomposition of ammonium molybdate, and Pt(0.5 wt. %)/MOO3 was prepared with MoO3 as the support using the same method as Pt/A1203. Catalytic tests were carried out in a fixed bed, high pressure, continuous flow reactor. DBT

(from

Fluka)

and

dimethyldisulfide

(DMDS)

(from

Fluka),

dissolved

in

decahydronaphthalene (DHN) (from Aldrich), were injected into the reactor by a pump with hydrogen being the carrier gas. The catalyst was presulfided in situ by DMDS at 350~ for 10 hours before the reaction. T - 340~

40bar,

All the experiments were measured at the same conditions:

0 = 0.5s, P m = 30bar, PDBT= 0.2bar, Pros = 0.5bar, PCH4 = 0.5bar, Pmn~ = 0.8bar.

The H2-D2 exchange reaction was carried out using the apparatus described in the previous report [8]. The reaction was carried out in a 72 cm 3 recycling reactor. After sulfidation in situ in a flow of H2 (90%) and H2S (10%) at 40~

for 15 hours under

atmospheric pressure, the catalyst (0.250g) was cooled down to the reaction temperature of 80~ and swept with helium (1 bar). The reactant mixture (0.50 bar H2 plus 0.50 bar D2) was

173 then introduced into the reactor. The recycling pump (Masterflex) was started and adjusted so as to obtain a flowrate of 6 dm3h-~. The composition of H2 and D2 before and during the reaction was monitored by gas chromatography. The pulse reduction and sulfidation of the catalyst was tested by injecting a pulse containing 0.7ml HzS/H2 mixture via the six pore valve into the reactor with the 200mg catalyst at atmospheric pressure. The effluent was measured by the gas chromatography with a thermoconductor detector. The reactor was heated to the desired temperature while He at the flow rate of 30ml/min was introduced.

3. R E S U L T S A N D D I S C U S S I O N 3.1. Catalytic performances of PtMo/AI2Oa in the HDS reaction of DBT The HDS reaction of DBT was carried out in the presence of H2S, imitating the industrially used conditions, over both Pt-containing and Pt-free catalysts. It is found that PtMo/A1203 exhibits sulfur resistance character, whose activity is quite stable within 12 hours of the time on stream like Mo/AI203. The products observed over Mo/AI203 and PtMo/A1203 contain both the S-extruded products: biphenyl (BiP), cyclohexylbenzene (CHB) and dicyclohexyl (DCH), and the S-bearing intermediates: tetrahydrodibenzothiophene (THDBT) and hexahydrodibenzothiophene (HHDBT). Only a small amount of S-extruded products occur over Pt/AI203. Table 1 shows that, the Pt-AI203 catalyst is almost inactive for HDS reaction, but the activity of Mo/A1203 is distinctively enhanced by the introduction of a slight amount of Pt. Table 1 Catalytic activity (A) and activity for S-extruded products (AHDs) on Pt promoted and unpromoted molybdenum catalysts for HDS of DBT Catalyst

Catalyst composition (wt%) Pt

A

AHDS

Mo

(m.mol/g.h)

(m.mol/g.h)

Pt/A 1203

0.2

-

0.04

0.04

Mo/A 120 3

-

9.3

1.14

0.46

PtMo/A120 3

0.2

9.3

1.57

0.91

174 Furthermore, when only S-extruded products are concerned,the activity of PtMo/A1203 is almost doubled as compared with Mo/A1203, which means that the selectivity of S-extruded molecules is also increased by Pt. This indicates clearly that platinum is an effective promoter in HDS of DBT. The detailed performances of PtMo/A1203 in the HDS reaction is displayed in Figure 1. On the one hand, BiP increases linearly with the raise of conversion, while THDBT and HHDBT pass through a maximum respectively; on the other hand, no CHB and DCH are observed at the very low conversion, but they increase significantly when conversion increases. Accordingly, BiP, THDBT, and HHDBT are primary reaction products, and CHB and DCH are secondary ones. 25

From the above observations, a

O

DCH

reaction network in scheme 1 can be

X7

HHDBT

presumed over PtMo/AI203, in which

_~N 20

0

CHB

two parallel reaction pathways coexist:

~

one is the prereduction of DBT leading

._0

to CHB and DCH, with THDBT and

..~

HHDBT as intermediates, and the other

one

is

the

direct

double

t ~

IS!

BiP

Z~

THDBT

10

.~

hydrogenolysis of DBT giving BiP. The route from BiP to CHB through hydrogenation can be excluded for the sake of the linearly increment of BiP with conversion.

This proposal

is

consistent with the previous literature [9] with Co(Ni) being the promoter. Figure

1

also

reveals,

by

9

0

.-

10

v,

.

20

.

.

30

.

40

50

C o n v e r s i o n of D B T (%) Figure 1. The product distribution for HDS over PtMo/AI203 (hollow) and Mo/AI203 (solid)

comparing the results of Mo/AI203 with that of PtMo/AI203, that the presence of Pt influences the product distribution remarkably. The molar percentage of BiP in the product mixture is enhanced greatly by a slight amount of Pt in PtMo/AI203. This indicates that, the direct hydrogenolysis route in Scheme 1 is most likely facilitated by the introduction of Pt, with the result that more BiP is created. A commercial NiMo/AI203 is also tested under the same conditions as PtMo/A1203, and the same products are observed with the exception of that no DCH is detected on the former, i.e., the Pt promoter allows the deep hydrogenation of monoaromatic, which is favor to the increase of the cetane index of diesel [ 10].

175

~/'2 H2 ~ s l ~ THDBT

DBT

~'H2 ~ s ~ HHDBT H21-H2S

(2) 2 H2~- H2S

/"

BiP

i/3H2

~

CHB

3H

DCH@ Scheme 1. The reaction network for HDS of DBT over PtMo/Al203 catalyst

3.2. Characterization results

It has been shown previously [8] that, the amount of hydrogen retained on the presulfided hydrotreating catalyst, and the initial rate of H-D exchange can be obtained from the data of H2-D2 exchange reaction. Table 2 shows that, only a trace of hydrogen is found retained on the Pt-Al2Oa catalyst, however, much more hydrogen is observed on Pt promoted catalyst than unpromoted one. Moreover, the initial rate of HD exchange is also accelerated remarkably in the presence of Pt. Table 2 Results of H2-D2 isotopic exchange reaction at 80~

over Pt promoted and unpromoted

catalysts. catalyst

nHab s

Initial rate of exchange

(10 .4 mol)

(10"7mol.HD/g.s)

Pt/AI203

trace

0.16

Mo/AI203

4.6

9.33

PtMo/A1203

6.4

10.20

176 It is known that the H2-D2 exchange reaction taking place over the sulfided molybdenum catalyst arises from the formation of SH groups on the surface of the catalyst during sulfidation, and there exists a equilibrium among dihydrogen, sulfur vacancies and SH groups on this catalyst [ 11 ]. SH groups are thought to play a crucial role in the catalytic activity of sulfided molybdenum based catalysts [ 12.13]. It is proposed in the course of the pretreatment of PtMo/ A1203 that, H2 is first dissociated into H atoms on Pt centers, and then these H atoms can spillover onto Mo sites, which behave as 'hydrogen reservoir'. The spiltover hydrogen is involved in the creating of SH groups, resulting in the more amount of retained hydrogen, as well as the higher initial rate of H-D exchange, detected by H2-D2 exchange reaction. Therefore, the activity of PtMo/A1203 for the HDS of DBT increases remarkably in the presence of Pt (Table 1), also owing to the formation of more amount of SH groups on its surface, which is indispensable for HDS active sites [ 14]. Figure 2 and Figure 3 display the results of pulse reduction and sulfidation over MoO3 and Pt/MoO3. It is found in Figure 2 that, at the temperature up to 400~

the injected H2/H2S

mixture is desorbed from the catalyst completely, which means no apparent reduction and sulfidation reaction between H2/H2S and the pure MoO3 is observed at this moment, and at the high temperature of 600~

the injected Hz/HzS is consumed partly by the catalyst, indicating

the occurrence of reduction and sulfidation reaction. By contrast, the initial consumption temperature of H2/H2S on PtMo/AlzO3 locates at about 100~ (Figure

3),

which

means

and it is used up at 400~

the

reduction and sulfidation reaction

H2

[

I

H2.q

could take place more easily on the Pt promoted catalyst than the

-'7-.

Pt-free one. It could be deduced from this phenomenon that, when

o

PtMo/AI203 and Mo/AI203 are pretreated

in H2/H2S at same

conditions,

more

coordinately

unsaturated Mo sites, which act as the active centers in HDS reaction,

15 300

350

400

450

500

550

could be achieved over the former catalyst, because of the more amount

of

spillover

Temperature range/~

hydrogen

resulted from the existence of Pt

Figure 2. Pulse reduction and sulfidation of

in this catalyst.

MoO 3 at different temperature range.

600

177

I

An important criterion for

2

[

I

noble metal-containing catalysts

H2S

is the sulfur poisoning tolerance character,

and

containing

noble

the

catalysts

metal

with

g

sulfur and/or ammonia resistance specifications for producing oil products with high quality attract interests of many researchers [ 10]. The present work indicates that PtMo/AI203

cannot

be

deactivated by sulfur hydrogen,

15 100 200 250 300 350 400 Temperature range/~

on the contrary, Pt is shown to be a promoter for the HDS reaction,

Figure 3. Pulse reduction and sulfidation of

and a constant conversion and

PtMo/AI203 at different temperature range.

product distribution are obtained. It has been verified that [15], Pt/TiO2 was much more resistant than Pt/AI203 to sulfur hydrogen poisoning, owing to the reaction of the active support, TiO2 with H2S, with the formation of hydrogen and sulfur, SOx or SO2 leaving the catalyst, by which a large part of the sulfur contamination on Pt sites could be autoregenerated. In our case, MoS2 on sulfided PtMo/AI203 is also very active for the decomposition of H2S [11 ], and one could assume that this could preserve Pt from poisoning by H2S. Further work would be done to obtane a better understanding of it. 4. C O N C L U S I O N S Pt is evidently an effective promoter in PtMo/A1203 catalyst for the HDS reaction of DBT, and shown to be sulfur hydrogen resistance. This reaction takes place over PtMo/AI203 through a two-pathway reaction network: one is the prereduction of DBT leading to CHB and DCH, and the other one is the direct double hydrogenolysis of DBT with the produce of BiP. It is proposed that, the very slight amount of Pt can enhance both,the reactivity and selectivity of this reaction, mainly by accelerating the direct hydrogenolysis route in Scheme 1 with more BiP produced. During the pretreatment of PtMo/AI203 under the reductive atmosphere of H2S/H2 mixture, H2 is first adsorbed on Pt centers and dissociated into H atoms, and then these H atoms can spillover into Mo sites easily, which behave as the 'hydrogen reservoir'. Accordingly, more coordinately unsaturated Mo sites could be obtained over PtMo/A1203 than

178 over Mo/AI203, as a result of the existence of the spillover hydrogen. By this, the reactivity of HDS reaction can be promoted by Pt. ACKNOWLEDGMENTS This work was carried out within the framework of the 'Programme International de Cooperation Scientifique du CNRS' (PICS n ~ 299), with the help of the 'Programme de Recherches Avancees de Cooperations Franco-Chinoises' (PRA E 94-5, Hydrotreatment Catalysts).

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