Hydrodesulfurization of benzothiophene catalyzed by molybdenum Sulfide cluster encapsulated into zeolites

J.W. Hightower, W.N. Delgass, E. Iglesia and A.T. Bell (Eds.) l l t h I n t e r n a t i o n a l C o n g r e s s on Catalysis - 40th Anniversar 3,

Studies in Surface Science and Catalysis, Vol. 101 9 1996 Elsevier Science B.V. All rights reserved.

107

H y d r o d e s u l f u r i z a t i o n of B e n z o t h i o p h e n e Catalyzed by M o l y b d e n u m Sulfide Cluster E n c a p s u l a t e d into Zeolites M. Taniguchi, a S. Yasuda, ' Y. Ishii, b T. Murata, b M. Hidai b and T. Tatsumi ~ aEngineering Research Institute, Faculty of Engineering, The University of Tokyo, Yayoi, Tokyo 113, Japan bDepartment of Chemistry and Biotechnology, Faculty of Engineering, The University of Tokyo, Hongo, Tokyo 113, Japan

A cationic molybdenum sulfide cluster [Mo354(H20)9] 4§ with incomplete cubane-type structure and a cationic nickel-molybdenum mixed sulfide cluster [Mo3NiS4CI(H20)9]3§ with complete cubane-type structure were introduced into zeolites NaY, HUSY and KL by ion exchange. Stoichiometry of the ion exchange was well established by elemental analyses. The UV-visible spectra and EXAFS analysis data exhibited that the structure of the molybdenum cluster remained virtually intact after ion exchange. MoNi/NaY catalyst prepared using the molybdenum-nickel sulfide cluster was found to be active and selective for benzothiophene hydrodesulfurization.

1. INTRODUCTION The great practical significance of the processes of hydrodesulfurization (HDS) of petroleum feedstocks has developed a keen research interest in the structure and synthesis method of the active component of HDS catalysts. All HDS processes have been performed on catalysts of the same type, based on molybdenum (or tungsten) sulfide supported on high surface area 7-aluminas with cobalt or nickel added as a promoter. In recent years much attention has been focused on the deep HDS of light oil, since the reduction in sulfur concentration in fight oil is effective in reducing particulate and NOx in the exhaust gas of Diesel engines. The main problem in the deep HDS of fight oil is the conversion of alkyldibenzothiophenes, e.g., 4,6-dimethyldibenzothiophene, which is one of the most difficult compounds to desulfurize among dibenzothiophene derivatives [1]. One approach to this problem is to rearrange and/or to eliminate alkyl groups by using catalysts with enhanced acidity. Zeolite-supported molybdenum with nickel or cobalt as a promoter is promising in this regard. Molybdenum/zeolite catalysts prepared by impregnating zeolites with ammonium heptamolybdate solution generally give rise to poor dispersion of molybdenum [2]. In contrast, ion exchange would be an ideal method for loading active metal species onto supports. Few cationic forms are available as simple salts of molybdenum of high oxidation

108

state, however. Furthermore, most of them exist only in strongly acidic solutions where many zeolites are unstable and where exchangeable cations must compete with protons. Therefore few studies have succeeded in introducing cationic molybdenum compounds into zeolites by ion exchange [3-5]. Molybdenum/Y-zeolites were prepared by aqueous ion exchange with Mo2(ethylenediamine)4Ch [3] and MoO2CI: [4]. Lunsford et al. also reported solid-state exchange of HY and ultrastable form of HY (HUSY) with MoC15 [5]. A molybdenum sulfide cluster, [Mo3S4(H20)9] 4§ (1) with incomplete cubane-type structure was recently prepared [6-8]. It is stable in water and air and easily forms mixed metal clusters with cubane-type Mo3MS4 cores (M = Fe, Co, Ni, Pd, etc.) [9]. Mixed metal clusters with a Mo3PdS4 core have been derived from 1, showing intriguing reactivities at the Pd site toward alkenes, CO, isonitriles, and alkynes [10,11]. A molybdenum-nickel cluster with a Mo3NiS4 core has also been found to uptake CO stoichiometrically to give a new cluster where one CO molecule combines with the nickel atom [12]. Thus these clusters are of considerable interest in connection with potential application to a variety of catalytic reactions. It occurred to us that the molybdenum and mixed metal clusters could be incorporated into zeolites by ion exchange and promote unique catalytic reactions. Preparation of molybdenum/zeolites using the molybdenum sulfide cluster cation as a precursor by ion exchange is considered to have several advantages. First, molybdenum species could be loaded with high dispersion. Secondly, molybdenum is loaded on zeolites as sulfide, not as oxide; presulfiding is unnecessary before HDS reactions. To our best knowledge, there is no such report that succeeded in introducing molybdenum species into zeolites in its sulfide state and applying the resulting catalyst to HDS reactions. Thirdly, by loading molybdenum-nickel cluster with a Mo3NiS, core into zeolites, molybdenum and nickel could be introduced in the homogeneously well-mixed state. This paper describes the successful incorporation of molybdenum and molybdenum-nickel clusters into zeolites with 12-membered ring by aqueous ion exchange and application of the resulting materials to HDS reaction of benzothiophene. Stoichiometry of the ion exchange was examined by elemental analysis. UV-visible spectroscopy and EXAFS measurements were carried out to investigate the structure of molybdenum species loaded on zeolites.

2. EXPERIMENTAL

2.1. Catalyst preparation The chloride salt of cluster 1 was synthesized by the reported method [8]. As supports, NaY (Nikka Seiko, SK-40; Si/AI = 2.3), HUSY (Tosoh, HSZ-330HUA; Si/A1 = 3.1) and KL (Tosoh, TSZ-500KOA; Si/AI = 3.1) zeolites with 12-membered ring were used. To a suspension of zeolite (4.58 g) in water (91.6 g) was added dropwise a 0.01 M aqueous solution of the chloride salt of 1 (86.9 ml) with vigorous stirring at 313 K. In the case of NaY the color of zeolite changed from white to brown and the solution was colorless. For HUSY and KL, brown-colored species remained in the solution after ion exchange. The metal cluster-containing zeolite was separated by filtration and washed with distilled water, followed by drying at 313 K under reduced pressure. Resulting materials are referred to as Mo/NaY, Mo/HUSY and Mo/KL, respectively. The chloride salt of cluster [Mo3NiS4CI(H20)9]3+ (2) was also synthesized according to the reported method [9] and

109 similarly treated with NaY to afford MoNi/NaY. MoNi/A1203 was also prepared by impregnating A1203 (JRC-ALO-4) with the solution of chloride salt of 2. 2.2. Characterization Molybdenum and sodium concentrations in the filtrate were determined by ICP on a Nippon Jarrell-Ash ICAP-575 spectrophotometer in order to estimate molybdenum loadings after ion exchange and to confirm the stoichiometry of ion exchange. Sulfur contents were determined by the LECO method. Chlorine contents of the catalysts were determined by ion chromatography. Powder X-ray diffraction patterns of the catalysts were collected on a Rigaku RINT 2400 X-ray diffractometer. Crystallinity of the zeolites were determined by the average of the relative intensities of selected seven intense peaks, using starting material NaY, HUSY and KL as the reference. UV-visible spectra were recorded on a Hitachi U-4000 spectrophotometer. Mo K-edge X-ray absorption spectra were collected on a R.igaku R-EXAFS 2100S (30 kV, 280 mA) instrument using a Ge(400) crystal monochromator at room temperature in air atmosphere. Four samples were applied to the EXAFS measurement: (a) 5% physical mixture of cluster 1 and NaY, (b) Mo/NaY, (c) sample (b) treated in flowing helium at 373 K for 0.5 h and (d) sample (b) treated in flowing helium at 573 K for 0.5 h. The sample (a) was set on a sample holder with 0.3 mm thickness, and the samples (b)-(d) were pressed into self-supporting wafers. The method of EXAFS data analysis utilized REX, an EXAFS data analysis software provided by Rigaku. The sample (a) was used as a standard for Mo-S, Mo-O and Mo-Mo interactions. Crystallographic data (N and R of Mo-S, Mo-O and Mo-Mo, where N is the coordination number and R the radial distance from the absorber to the backscatter atom) of the cluster 1 [13] were used in order to determine the EXAFS parameters o and AE0 (where o is the Debye-Waller factor and AEo the inner potential correction of the edge position) which were used for the samples (b)-(d). Once a close fit was obtained for the parameters N and R with k3-weighting, the fit was optimized by allowing a few of the parameters to be fitted while the rest of the parameters were held fixed. 2.3. HDS reaction The catalytic activities of the zeolite- and alumina-supported Mo and Mo-Ni catalysts for benzothiophene HDS were measured in a flow system incorporating a microreactor made of stainless steel tube containing 0.2 g of catalyst. The catalysts were pretreated in flowing helium by ramping the temperature from room temperature to 573 K at a rate of 3.3 K/min and then in flowing hydrogen for 0.5 h at 573 K. When presulfided, the catalysts were treated in flowing 5% H2S/H2 mixed gas for 2 h at 573 K following hydrogen pretreatment. Benzothiophene in decane (0.5% or 5% as sulfur) was supplied to the reactor at a rate of 4.5 ml/h. The reaction conditions were 573 K, 3.0 MPa, H2/benzothiophene = 379 or 3790 (mol/mol) and W/F = 38.2 or 382 g-cat.h/mol-benzothiophene.

110 3. RESULTS AND DISCUSSION

3.1. Chemical composition The results of elemental analysis of the molybdenum and molybdneum-nickel/zeolite catalysts are shown in Table 1. After ion exchange of NaY with the solution of 1, almost all molybdenum was found to be transferred from the solution to NaY; the resulting Mo/NaY contained 5.0 wt% of molybdenum, which corresponds to the presence of 0.41 Mo3S4 cores per a supercage of the FAU type structure. The size of cluster 1 was estimated to be 0.72 nm, smaller than the diameter of the aperture (0.74 nm) [14] of supercages of the FAU type structure. The C1/Mo ratio of Mo/NaY was 0.11, suggesting that most of the cluster 1 acted as a tetravalent cation in the ion exchange. Fled sodium ion per loaded Mo cluster in the filtrate was 4.6 times (ideally 4 times) as much as the loaded cluster 1. The slight excess of lost sodium over the supported cluster may be due to the acidity of the cluster salt. The S/Mo ratio was maintained after ion exchange. In the preparation of Mo/HUSY, the cluster 1 amounting to 2.5 wt% (as molybdenum metal) of HUSY was added to the suspension of HUSY; 92% of the molybdenum was loaded onto HUSY. The CI/Mo ratio of Mo/HUSY was found to be 0.34, suggesting that in ion exchange the cluster 1 acted as a trivalent cation on the average. These findings indicate that the protons in HUSY are less exchangeable by the cluster cation than the Na cations in NaY. In the preparation of Mo/KL, the addition of 1 was stopped when pH of the solution was lowered to about 3. The resulting Mo/KL contained only 2.1 wt% (74% of added molybdenum clusters) of molybdenum. Chlorine was absent, which indicates that the cluster 1 acted as a tetravalent cation. The LTL structure is characterized by a monodimensional system of channels, whose diameter (0.70 nm) [14] is close to the size of the cluster 1. It is conceivable that, once the cluster 1 was incorporated into an LTL main channel and present at a site near the external surface, the sites deep in the channel are no more accessible to another cluster. The cluster 2 was also introduced into NaY by ion exchange. After the ion exchange, 99% of molybdenum and nickel were loaded on NaY; the Ni/Mo ratio did not change. The CI/Mo ratio suggests that excess chlorine was present on the catalyst.

Table 1 Elemental analysis of Mo and Mo-Ni / zeolite catalysts Catalyst

Si/A1 ratio

Loaded % loaded % loaded Na CI/Mo Mo (wt%) Mo a Ni" released b ratio

Mo / NaY

2.3

5.0

Mo / HUSY

3.1

2.3

Mo / KL

3.1

2.1

MoNi / NaY

2.3

5.0

99

100

S/Mo ratio

-

4.6

0.11

92

-

-

0.34

n.d.c

74

-

-

0.00

n.d.C

4.2

0.96

n.d.c

99

a Fraction of Mo or Ni loaded based on Mo or Ni added to the solution. b Mol Na fled into the solution per mol cluster loaded. c Not determined.

1.3

111

Table 2 Crystallinity of Mo and Mo-Ni / zeolite catalysts Catalyst

Si/A1 ratio

pH after ion exchange

Crystallinity (%)

Mo / NaY Mo / HUSY Mo / KL

2.3 3.1 3.1

4.6 4.2 3.2

75 85 93

MoNi / NaY

2.3

4.7

72

3.2. Crystallinity of zeolites It is reported that introduction of molybdenum cations into zeolites often gives rise to destruction of the zeolite skeleton because of their acidity [5,15]. The crystaUinity of the ion exchanged zeolites are shown in Table 2. Although the solutions during ion exchange were slightly acidic (pH = 3.2 - 4.7), the X-ray powder diffraction patterns were only slightly attenuated, indicating the zeolites kept their crystallinity without significant destruction. The decrease in crystallinity was not related to the pH after ion exchange but to the Si/AI ratio of the zeolites.

(a)

O

< (

I

200

300

I

1

1

I

400 500 600 700 Wavelength (nm)

1

800

900

Figure 1. UV-visible spectra of (a) physical mixture of NaY and the chloride salt of 1, (b) Mo/NaY, (c) Mo/HUSY and (d) Mo/KL.

112 3.3. Structure of the clusters UV-visible spectra of the cluster 1 and molybdenum/zeolite catalysts are shown in Figure 1. The cluster 1 showed bands at 300, 390 and ca. 650 nm. Similar bands were observed for the spectrum of each molybdenum/zeolite catalyst, suggesting that the structure of cluster 1 was practically unchanged after ion exchange. In order to obtain more structural information about the molybdenum species in Mo/NaY, EXAFS measurements of the cluster 1 and Mo/NaY were carried out. The Fourier transforms of the EXAFS data are shown in Figure 2. Structural parameters (Table 3) showed no change of the Mo-O, Mo-S and Mo-Mo distances, suggesting that there is no significant structural difference between the cluster 1 and the molybdenum compound in the Mo/NaY. From these EXAFS parameters and the UV-visible spectra, it is considered the structure of cluster 1 remained virtually intact after ion exchange.

14

14 (a)

12

(b)

12 .~10

~10

...a

'~

8

6 a~ 4

u2 4

0

0.1

0.2

0.3 0.4 R (nm)

0.5

0

0.6

0.1

0.2

0.3 0.4 R (nm)

0.5

0.6

14

14

(c)

12

(d)

12

~10

.~10

'2

'~ e~

8

8

6

6

a~ 4

4

0

0 0

0.1

0.2

0.3 R (nm)

0.4

0.5

0.6

0

0. I

0.2

0.3

0.4

0.5

0.6

R (nm)

Figure 2. Fourier transforms of EXAFS data of (a) the chloride salt of 1, (b) Mo/NaY, (c) sample (b) treated in flowing He at 373 K and (d) sample (b) treated in flowing He at 573 K.

113

Table 3 Structural parameters from EXAFS of the Mo/NaY catalysts Mo-S Sample Cluster I

Treatment -

N~

Mo-Mo R (nm) b

(3.0) ~ (0.230) ~

Na

Mo-O R (nm) b

(2.0) c (0.274) c

Na

R (nm) b

(3.0) ~ (0.218) ~

3.5

0.232

2.0

0.276

3.9

0.217

Mo / NaY

He, 373 K

3.1

0.232

1.7

0.277

3.3

0.218

Mo / NaY

He, 573 K

1.2

0.238

0.69

0.276

1.3

0.214

Mo / NaY

-

"Coordination number. b Radial distance from the absorber to the backscatter atom. ~ Taken from ref. 13 and treated as fixed parameters.

After treatment at 373 K in helium flow, the coordination numbers of Mo-O, Mo-S and Mo-Mo were not largely different from those in the cluster 1 and the distances of Mo-O, Mo-S and Mo-Mo were hardly changed. After treatment at 573 K in He flow, however, the color of NaY changed from brown to black and the curve fitting results of the EXAFS data exhibited lower coordination numbers of all interactions than those of the cluster 1. The decrease in the coordination number seems to be not due to the decrease in sulfur amount in the catalyst but to the disordering of each interaction since the S/Mo ratio hardly decreased after thermal treatment. These results show that the structure of the cluster 1 loaded on NaY was maintained at 373 K, but lost at 573 K.

3.4. HDS reactions The results of hydrodesulfurization of benzothiophene (0.5 wt% as sulfur) in decane are shown in Table 4. Over Mo/NaY and Mo/HUSY, selectivity for ethylbenzene was low and main products were dihydrobenzothiophene and alkylbenzothiophenes. The latter seems to be produced by the reaction of unreacted benzothiophene with alkenes generated from decane cracking on acid sites of the zeolite. For Mo/I-/USY, decane cracking and following alkylation of benzothiophene took place appreciably. Mo/KL showed high HDS activity compared to Mo/NaY in spite of its lower molybdenum loading. This is consistent with our supposition that molybdenum clusters were loaded on near external surface of KL, as described above. Since the KL zeolite has no strong acidity, decane cracking and following alkylation of benzothiophene proceeded only to a small extent. Involvement of Ni resulted in great enhancement of HDS activity; MoNi/NaY proved to be a highly active and selective catalyst for HDS of benzothiophene to ethylbenzene, and other products were scarcely observed. No change in the activity was observed during 4 hours. Table 5 shows HDS product distributions over several catalysts prepared by using the molybdenum-nickel cluster 2. Sulfur content in decane was adjusted to 5.0 wt% in these experiments. MoNi/NaY was found to be more active than MoNi/Al203. It is to be noted that during the high temperature pretreatment the original cluster structure would have been changed. However, the high activity of the MoNi/NaY catalyst for benzothiophene HDS is probably due to the formation of active sites derived from this particular mixed metal cluster,

114 Table 4 Effect of nickel and acidity of zeolites on the benzothiophene hydrodesulfurization ~ Selectivity (%) Catalyst

Conversion (%)

EB b

DHB"P

Others

Mo/NaY

53

8.3

76

16

Mo / HUSY Mo/KL

39 75

6.1 38

66 52

28 9.4

MoNi / NaY

97

88

1.2

5.7

a Reaction conditions: 573 K, 3.0 MPa, W/F = 382 g-cat.h/mol-benzothiophene, S content 0.5 wt%. b Ethylbenzene. Dihydrobenzothiophene.

Table 5 Catalytic performance of MoNi catalysts for benzothiophene hydrodesulfurization ~ Selectivity (%) Catalyst

Conversion (%)

EB b

DHBT:

Others

MoNi / NaY MoNi / NaY (presulfided)

36 19

51 23

39 55

11 4.7

MoNi / A1203 MoNi / A1203 (presulfided)

23 30

42 39

52 45

5.1 16

a Reaction conditions: 573 K, 3.0 MPa, W/F = 38.2 g-cat.h/mol-benzothiophene, S content 5.0 wt%. b Ethylbenzene. c Dihydrobenzothiophene.

together with high dispersion. The pore structure of the zeolite support may also play a role in the stabilization of active species reminiscent of the cluster structure. Presulfiding effect was quite different between two supports; although MoNi/AI203 was activated by presulfiding, MoNi/NaY was slightly deactivated by presulfiding, suggesting that the active species in MoNi/NaY is different from that in MoNi/A1203. The S/Mo ratio of the cluster 1 is 1.3. It is suspected that the increase in the HDS activity of MoNi/AI203 by sulfiding resulted from the increase in the sulfur content, in agreement with the generally accepted belief that the active component of A1203-supported conventional HDS catalysts is MoS2 [16], where the S/Mo ratio is 2. Study on applicability of MoNi/zeolite catalysts to the HDS reactions of other sulfur compounds is ongoing in our laboratory.

115 4. CONCLUSIONS A cationic molybdenum sulfide cluster 1 with incomplete cubane-type structure and a cationic molybdenum-nickel sulfide cluster 2 with complete cubane-type structure were employed as new precursors for Mo/zeolite and MoNi/zeolite catalysts. Mo/NaY, Mo/HUSY, Mo/KL and MoNi/NaY were prepared by aqueous ion exchange with the cluster 1 and 2. Stoichiometry of the ion exchange was well established by elemental analysis. Though pH of the solution during ion exchange was lowered, crystallinities of the zeolites were preserved. The UV-visible spectra and EXAFS analysis data exhibited that the structure of the cluster 1 remained virtually intact after ion exchange. In the HDS of benzothiophene Mo/HUSY and Mo/KL showed higher activity than Mo/NaY. The use of molybdenum-nickel mixed cluster resulted in a great enhancement of HDS activity. By presulfiding, MoNi/AlzO3 was activated, whereas MoNi/NaY was not. These results suggest that the active species in MoNi/NaY is different from that in MoNi/AI203.

ACKNOWLEDGMENT We thank Professor K. Asakura, Faculty of Science, The University of Tokyo, for his support with the EXAFS analyses.

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