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70 Mobility of sulfur in molybdenum sulfide clusters encaged in zeolite during hydrodeselenium reaction as studied by XAFS
Science and Technology in Catalysis 2002 Copyright 9 2003 by Kodansha Ltd.
331
70
Mobility of Sulfur in Molybdenum Sulfide Clusters Encaged in Zeolite during Hydrodeselenium Reaction as Studied by XAFS Takeshi KUBOTA, Naoto HOSOMI, Yuya HAMASAKI and Yasuaki OKAMOTO Department of Material Science, Shimane University, Matsue 1060, 690-8504, Japan
Abstract The S-Se exchange during the hydrode-selenium (HD-Se) reaction of selenophene was investigated over MoS2/AI203 and intrazeolite Mo sulfide clusters. The local structure of Se incorporated into the Mo sulfide catalysts was studied by means of Se K-edge XAFS.
It was found
that the extent of S-Se exchange during the HD-Se reaction, rate constant and dynamic behaviors of Se incorporation strongly vary with the cluster size of Mo sulfide. i.
INTRODUCTION
Hydrodesufurization (HDS) is used to remove sulfur atoms from petroleum feedstocks. Currently, Co-Mo sulfide catalysts are used in industry for HDS reaction [I]. mechanism of HDS on Mo and Co-Mo catalysts is not well understood.
But the reaction
One of the key reaction
steps is the migration of the sulfur atoms of the metal sulfide catalysts during the HDS reaction. Leliveld et al. [2] studied hydrode-selenium (HD-Se) reaction of selenophene, a homolog of thiophene, at 673 K.
However, the decomposition of H2Se made the reaction system complex.
In
the present study, we investigated the local structure of Se on Mo sulfide catalysts during HD-Se of selenophene at 523 K, where no decomposition of H2Se occurs, by means of Se K-edge XAFS to estimate the dynamic behavior of initial HDS reaction steps.
We also examined the effect of the
cluster size of Mo sulfides on the catalytic property of HD-Se. 2.
EXPERIMENTAL
NaY zeolite-supported Mo sulfide catalysts were prepared by a CVD method using Mo(CO), as a precursor [3-5].
A NaY zeolite used here was supplied by the Catalysis Society of Japan as a
reference catalyst (JRC-Z-Y5.5, Si/AI=2.8).
Mo(CO)6/NaY was sulfided in a 10% H2S/H 2 flow
332 T. Kubota et al.
(100 mL min 1) at 673 K for 1.5 h. MoSx/NaY.
The Mo sulfide catalyst (10wt% Mo) thus prepared is denoted
A conventional MoS2/AI20 a sulfide catalyst (8.7wt% Mo) was prepared by
impregnating AI2Oa (177 m 2 g-l) using (NH4)6MOTO~ ~
and calcined at 773 K for 5 h, followed
by sulfidation at 673 K for 1.5 h. The HD-Se reaction of selenophene was performed using a closed circulation system at 523 K. A vapor pressure of selenophene at 273 K was used and the C4I-I4Se/H2 ratio was kept almost constant during the reaction.
The reactants and products (H2S, H2Se and C4 compounds) were
analyzed by gas chromatography. The Se K-edge XAFS spectra for the catalysts used for the HD-Se reaction were measured at BL-10B of KEK-IMSS-PF in a transmission mode (proposal number: 99G-082). radiation was monochromatized by a Si(311) monochromator.
The synchrotron
The catalyst was evacuated at 673
K for 30 min afar the HD-Se reaction and transferred to an in situ XAFS cell without exposure to air. The detailed analysis procedure for XAFS data has been reported elsewhere [4]. _
3.
RSULTS AND DISCUSSION With both catalysts, MoS2/AI203 and MoSx/NaY, it was found by quadrupole mass analysis that
only H2S was produced during the HD-Se reaction of selenophene.
This result indicates that Se is
incorporated into the catalyst by the reaction and that S atoms of the catalyst are replaced by Se atoms during the reaction.
Figure 1 shows typical profiles of H2S production during the HD-Se
reaction on the catalysts.
The amount of produced H2S increases and gradually levels off with the
elapse of reaction time.
The ratios of produced H2S to Mo atom are 0.5 and 1.5 after 4 h for
MoS2/AI203 and MoSx/NaY, respectively.
These results show that the S/Mo ratio of exchangeable
i
1.5
MoSx/NaY
c,=
r162
1.0
I
.. i=,,. ~i
....
M~ I ......... Se ..... M o S , / N a Y ]
r162
t
0.5 t~
E L_ 0.C0
0
100 200 Reaction time / min
z
I
12660 12680 Photon energy / eV
12700
Figure 1. Amount of H2S production
Figure 2. Se K-edge XANES spectra
during the HD-Se reaction at 523 K as
for reference compounds of Se and
a function of reaction time.
MoSx/NaY after the I-ID-Se reaction.
333 sulfur atoms in Mo sulfides depends on the catalyst.
This may be attributed to the difference in the
cluster size of the Mo sulfide species in MoS2/AI203 and MoSx/NaY.
It is estimated from the Mo-
Mo coordination number that with MoS2/AI203, the Mo sulfide particles on AI203 consist of ca. 10 atoms of Mo on average.
On the other hand, with MoS~/NaY, the formation of highly dispersed
Mo sulfide dimer clusters was confirmed in conformity with previous XAFS studies [3-5].
With
MoS2/AI203, the HD-Se activity was not restored in the second reaction after evacuation at 673 K, following the first HD-Se reaction.
The rate equation of the reaction was well represented by
assuming a first order reaction with respect to the amount of produced H2S. After the HD-Se reaction on MoS2/AI203, aDS of thiophene was carried out at 623 K.
It was found that the HDS
activity was significantly suppressed by the Se-incorporation, indicating that the active sites for the HDS are also involved in the HD-Se reaction.
In addition, the total amount of exchangeable sulfur,
(S/Mo)t=| was linearly correlated to the amount of NO adsorption capacity, NO/Mo. The chemical state and local structure of the incorporated Se atoms were studied by Se K-edge XAFS as a function of reaction time.
Figure 2 shows XANES spectra for reference compounds of
Se and MoSx/NaY after HD-Se reaction (2 h).
A comparison of the Se K-edge XANES spectra
reveals that the spectrum for MoSx/NaY is distinctly different from that of elemental Se or selenophene adsorbed on NaY zeolite and is close to that of MoSe2. Accordingly, it is concluded that the Se atoms incorporated into the catalyst bind with Mo atoms, and that they are accommodated in the Mo sulfide phase during the reaction.
Taking into account the findings in
Figure 1, it is inferred that the incorporated Se atoms remain at the active sites of the Mo sulfide phase and that the active sites exchanged by Se lose the HD-Se activity.
I
I
i
Figure 3 shows the
i
......... 5 min ..... 30 min ---- 120 min
O
2
,...~ f=
g
t
~=
1-
II
MoS~AI20a
9 MOSx/NaY 0 0 0 0.0
0.1
0.2 R
Figure 3.
0.3
0.4
,,
I
,, 9
40 Reaction
0.5
l 80
.
l 120
.
time / min
/rim
Fourier transforms of k3-weighted
Figure 4.
Relation between the coordination
EXAFS oscillations of Se K-edge for the
number of a Se-Mo shell calculated by EXAFS
MOSx/NaY catalysts after the HD-Se reaction.
analysis and the reaction time of HD-Se for
MoS2/A]203and MoS~/NaY.
334 T. Kubota et
al.
Fourier transforms (FI') of the k3-weighted EXAFS oscillations for the MoS~/NaY catalysts after the HD-Se reaction. bonding.
A curve fitting analysis was carried out by assuming a single wave of Se-Mo
Relatively large Debye-Waller factors (0.0075-0.0087 nm) were obtained from the
analysis, this indicating the generation of structural distortions around Se atoms exchanged with S atoms because of a small difference between Mo=Se and Mo-Se bond distances.
The coordination
number (CN) of the Se-Mo shell calculated by EXAFS analysis is plotted in Figure 4 as a function of the reaction time for MoS2/AI203 and MoS~/NaY. The coordination numbers of the Se-Mo shell (bond distance: 0.258 + 0.001nm) increase with the reaction time and approach the values of 2 and 3 for MoS~/NaY and MoS2/AI203, respectively. Extrapolation of the lines to reaction time = 0 suggests that the Se-Mo coordination numbers at the initial reaction are 1.3 and 2 for MOSx/NaY and MoS2/AI203, respectively.
It is considered from the results in Figure 4 that with MoSJNaY, Se
atoms move from an on-top site (CN=I) to a bridge site (CN=2) as the HD-Se reaction proceeds. On the other hand, with MoS,/AI203, Se atoms are incorporated into the unsaturated edge sites of MoS2 to form a Mo-Se-Mo bridge bond (CN=2) and then move to more stable triply coordinated sites (CN=3) in MoS2 particles. 4.
SUMMARY
In summary, it was found that the extent of S-Se exchange during the HD-Se reaction of selenophene on Mo sulfide catalysts depends strongly on the cluster size of Mo sulfide.
The XAFS
analysis showed that the Se atoms incorporated into the catalysts bind with the Mo atoms and that different dynamic behaviors of Se (S) incorporation are operative in HD-Se (HDS) reaction between MoSx/NaY and MoS2/AI203. 5.
ACKNOWLEDGEMENT
The present work was supported by Grant-in-Aid for Scientific Research (13450334) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
This work was also
supported by The Japan Petroleum Institute commissioned by the Japan Corporation Center, Petroleum with the subsidy of the Ministry of Economy, Trade and Industry. 6.
REFERENCES
[1] H. Topsr
B. S. Clausen, F.E. Massoth,
"Hydrotreating Catalysis" Catalysis Science and
Technology, Springer-Verlag, Berlin, 1996. [2] B. R. G. Leliveld, J. A. J. van Dillen, J. W. Geus, D. C. Koningsberger and M. de Boer, J. Phys. Chem. B i01 (1997) 11160. [3] Y. Okamoto, H. Okamoto, T. Kubota, H. Kobayashi and O. Terasaki, J. Phys. Chem. B 103 (1999)7160. [4] Y. Okamoto, H. Okamoto, T. Kubota, H. Kobayashi and O. Terasaki, J. Phys. Chem. B 103 (1999)7160. [5] Y. Okamoto and T. Kubota, Catal. Surv. Jpn. 5 (2001) 3.
331
70
Mobility of Sulfur in Molybdenum Sulfide Clusters Encaged in Zeolite during Hydrodeselenium Reaction as Studied by XAFS Takeshi KUBOTA, Naoto HOSOMI, Yuya HAMASAKI and Yasuaki OKAMOTO Department of Material Science, Shimane University, Matsue 1060, 690-8504, Japan
Abstract The S-Se exchange during the hydrode-selenium (HD-Se) reaction of selenophene was investigated over MoS2/AI203 and intrazeolite Mo sulfide clusters. The local structure of Se incorporated into the Mo sulfide catalysts was studied by means of Se K-edge XAFS.
It was found
that the extent of S-Se exchange during the HD-Se reaction, rate constant and dynamic behaviors of Se incorporation strongly vary with the cluster size of Mo sulfide. i.
INTRODUCTION
Hydrodesufurization (HDS) is used to remove sulfur atoms from petroleum feedstocks. Currently, Co-Mo sulfide catalysts are used in industry for HDS reaction [I]. mechanism of HDS on Mo and Co-Mo catalysts is not well understood.
But the reaction
One of the key reaction
steps is the migration of the sulfur atoms of the metal sulfide catalysts during the HDS reaction. Leliveld et al. [2] studied hydrode-selenium (HD-Se) reaction of selenophene, a homolog of thiophene, at 673 K.
However, the decomposition of H2Se made the reaction system complex.
In
the present study, we investigated the local structure of Se on Mo sulfide catalysts during HD-Se of selenophene at 523 K, where no decomposition of H2Se occurs, by means of Se K-edge XAFS to estimate the dynamic behavior of initial HDS reaction steps.
We also examined the effect of the
cluster size of Mo sulfides on the catalytic property of HD-Se. 2.
EXPERIMENTAL
NaY zeolite-supported Mo sulfide catalysts were prepared by a CVD method using Mo(CO), as a precursor [3-5].
A NaY zeolite used here was supplied by the Catalysis Society of Japan as a
reference catalyst (JRC-Z-Y5.5, Si/AI=2.8).
Mo(CO)6/NaY was sulfided in a 10% H2S/H 2 flow
332 T. Kubota et al.
(100 mL min 1) at 673 K for 1.5 h. MoSx/NaY.
The Mo sulfide catalyst (10wt% Mo) thus prepared is denoted
A conventional MoS2/AI20 a sulfide catalyst (8.7wt% Mo) was prepared by
impregnating AI2Oa (177 m 2 g-l) using (NH4)6MOTO~ ~
and calcined at 773 K for 5 h, followed
by sulfidation at 673 K for 1.5 h. The HD-Se reaction of selenophene was performed using a closed circulation system at 523 K. A vapor pressure of selenophene at 273 K was used and the C4I-I4Se/H2 ratio was kept almost constant during the reaction.
The reactants and products (H2S, H2Se and C4 compounds) were
analyzed by gas chromatography. The Se K-edge XAFS spectra for the catalysts used for the HD-Se reaction were measured at BL-10B of KEK-IMSS-PF in a transmission mode (proposal number: 99G-082). radiation was monochromatized by a Si(311) monochromator.
The synchrotron
The catalyst was evacuated at 673
K for 30 min afar the HD-Se reaction and transferred to an in situ XAFS cell without exposure to air. The detailed analysis procedure for XAFS data has been reported elsewhere [4]. _
3.
RSULTS AND DISCUSSION With both catalysts, MoS2/AI203 and MoSx/NaY, it was found by quadrupole mass analysis that
only H2S was produced during the HD-Se reaction of selenophene.
This result indicates that Se is
incorporated into the catalyst by the reaction and that S atoms of the catalyst are replaced by Se atoms during the reaction.
Figure 1 shows typical profiles of H2S production during the HD-Se
reaction on the catalysts.
The amount of produced H2S increases and gradually levels off with the
elapse of reaction time.
The ratios of produced H2S to Mo atom are 0.5 and 1.5 after 4 h for
MoS2/AI203 and MoSx/NaY, respectively.
These results show that the S/Mo ratio of exchangeable
i
1.5
MoSx/NaY
c,=
r162
1.0
I
.. i=,,. ~i
....
M~ I ......... Se ..... M o S , / N a Y ]
r162
t
0.5 t~
E L_ 0.C0
0
100 200 Reaction time / min
z
I
12660 12680 Photon energy / eV
12700
Figure 1. Amount of H2S production
Figure 2. Se K-edge XANES spectra
during the HD-Se reaction at 523 K as
for reference compounds of Se and
a function of reaction time.
MoSx/NaY after the I-ID-Se reaction.
333 sulfur atoms in Mo sulfides depends on the catalyst.
This may be attributed to the difference in the
cluster size of the Mo sulfide species in MoS2/AI203 and MoSx/NaY.
It is estimated from the Mo-
Mo coordination number that with MoS2/AI203, the Mo sulfide particles on AI203 consist of ca. 10 atoms of Mo on average.
On the other hand, with MoS~/NaY, the formation of highly dispersed
Mo sulfide dimer clusters was confirmed in conformity with previous XAFS studies [3-5].
With
MoS2/AI203, the HD-Se activity was not restored in the second reaction after evacuation at 673 K, following the first HD-Se reaction.
The rate equation of the reaction was well represented by
assuming a first order reaction with respect to the amount of produced H2S. After the HD-Se reaction on MoS2/AI203, aDS of thiophene was carried out at 623 K.
It was found that the HDS
activity was significantly suppressed by the Se-incorporation, indicating that the active sites for the HDS are also involved in the HD-Se reaction.
In addition, the total amount of exchangeable sulfur,
(S/Mo)t=| was linearly correlated to the amount of NO adsorption capacity, NO/Mo. The chemical state and local structure of the incorporated Se atoms were studied by Se K-edge XAFS as a function of reaction time.
Figure 2 shows XANES spectra for reference compounds of
Se and MoSx/NaY after HD-Se reaction (2 h).
A comparison of the Se K-edge XANES spectra
reveals that the spectrum for MoSx/NaY is distinctly different from that of elemental Se or selenophene adsorbed on NaY zeolite and is close to that of MoSe2. Accordingly, it is concluded that the Se atoms incorporated into the catalyst bind with Mo atoms, and that they are accommodated in the Mo sulfide phase during the reaction.
Taking into account the findings in
Figure 1, it is inferred that the incorporated Se atoms remain at the active sites of the Mo sulfide phase and that the active sites exchanged by Se lose the HD-Se activity.
I
I
i
Figure 3 shows the
i
......... 5 min ..... 30 min ---- 120 min
O
2
,...~ f=
g
t
~=
1-
II
MoS~AI20a
9 MOSx/NaY 0 0 0 0.0
0.1
0.2 R
Figure 3.
0.3
0.4
,,
I
,, 9
40 Reaction
0.5
l 80
.
l 120
.
time / min
/rim
Fourier transforms of k3-weighted
Figure 4.
Relation between the coordination
EXAFS oscillations of Se K-edge for the
number of a Se-Mo shell calculated by EXAFS
MOSx/NaY catalysts after the HD-Se reaction.
analysis and the reaction time of HD-Se for
MoS2/A]203and MoS~/NaY.
334 T. Kubota et
al.
Fourier transforms (FI') of the k3-weighted EXAFS oscillations for the MoS~/NaY catalysts after the HD-Se reaction. bonding.
A curve fitting analysis was carried out by assuming a single wave of Se-Mo
Relatively large Debye-Waller factors (0.0075-0.0087 nm) were obtained from the
analysis, this indicating the generation of structural distortions around Se atoms exchanged with S atoms because of a small difference between Mo=Se and Mo-Se bond distances.
The coordination
number (CN) of the Se-Mo shell calculated by EXAFS analysis is plotted in Figure 4 as a function of the reaction time for MoS2/AI203 and MoS~/NaY. The coordination numbers of the Se-Mo shell (bond distance: 0.258 + 0.001nm) increase with the reaction time and approach the values of 2 and 3 for MoS~/NaY and MoS2/AI203, respectively. Extrapolation of the lines to reaction time = 0 suggests that the Se-Mo coordination numbers at the initial reaction are 1.3 and 2 for MOSx/NaY and MoS2/AI203, respectively.
It is considered from the results in Figure 4 that with MoSJNaY, Se
atoms move from an on-top site (CN=I) to a bridge site (CN=2) as the HD-Se reaction proceeds. On the other hand, with MoS,/AI203, Se atoms are incorporated into the unsaturated edge sites of MoS2 to form a Mo-Se-Mo bridge bond (CN=2) and then move to more stable triply coordinated sites (CN=3) in MoS2 particles. 4.
SUMMARY
In summary, it was found that the extent of S-Se exchange during the HD-Se reaction of selenophene on Mo sulfide catalysts depends strongly on the cluster size of Mo sulfide.
The XAFS
analysis showed that the Se atoms incorporated into the catalysts bind with the Mo atoms and that different dynamic behaviors of Se (S) incorporation are operative in HD-Se (HDS) reaction between MoSx/NaY and MoS2/AI203. 5.
ACKNOWLEDGEMENT
The present work was supported by Grant-in-Aid for Scientific Research (13450334) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
This work was also
supported by The Japan Petroleum Institute commissioned by the Japan Corporation Center, Petroleum with the subsidy of the Ministry of Economy, Trade and Industry. 6.
REFERENCES
[1] H. Topsr
B. S. Clausen, F.E. Massoth,
"Hydrotreating Catalysis" Catalysis Science and
Technology, Springer-Verlag, Berlin, 1996. [2] B. R. G. Leliveld, J. A. J. van Dillen, J. W. Geus, D. C. Koningsberger and M. de Boer, J. Phys. Chem. B i01 (1997) 11160. [3] Y. Okamoto, H. Okamoto, T. Kubota, H. Kobayashi and O. Terasaki, J. Phys. Chem. B 103 (1999)7160. [4] Y. Okamoto, H. Okamoto, T. Kubota, H. Kobayashi and O. Terasaki, J. Phys. Chem. B 103 (1999)7160. [5] Y. Okamoto and T. Kubota, Catal. Surv. Jpn. 5 (2001) 3.