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ammoxidation reactions
Science and Technology in Catalysis 2002 Copyright 9 2003 by Kodansha Ltd.
189
35 Formation of New Re Clusters in HZSM-5 and
Their Catalytic Property in Propene Selective Oxidation/Ammoxidation Reactions
N. Viswanadham, T. Shido, T. Sasaki, Y. Iwasawa* Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Abstract
A catalyst prepared by chemical vapor deposition (CVD) of CH3ReO3 (MTO) on H-ZSM-5 and subsequent treatment at 673 K was active for selective oxidation/ammoxidation of propene. Presence of ammonia was prerequisite for the catalytic selective oxidation of propene to acrolein. The structural change of the Re species during the pretreatment and catalytic reaction was investigated by SS-NMR, and XAFS. MTO interacted with protons of HZSM-5 at 333 K, and the subsequent treatment at 673 K formed tetrahedral [ReO4] monomer species. Ammonia promoted h
the formation of a new [Re60~7] cluster at 673 K. A proposed cluster structure constituted an octahedral Re6 framework with terminal and bridge oxygen atoms, which were relevant to the selective oxidation. The active [Re6017] clusters were converted to the inactive [ReO4] monomers in the absence of ammonia.
1. INTRODUCTION Selective oxidation/ammoxidation of light hydrocarbons is one of the important reactions in both fundamental research and industrial process [1,2]. In the previous studies, we investigated propene oxidation reactions on several zeolite-supported Re catalysts and found the remarkable effects of precursor (MTO or NH4ReO4), preparation method (CVD of MTO, impregnation of NH4ReO4, or physical mixing of NH4ReO4) and type of zeolite (HZSM-5, NaZSM-5, and HY) [3,4]. Among the examined catalysts, the Re/HZSM-5 catalyst prepared by CVD of MTO exhibited highest activity and selectivity. The conversion and selectivity of prepene to oxidation /ammoxidation products were 10-20% and ca. 80%, respectively at 673 K. What is interesting is that presence of ammonia is required to propene oxidation reaction to form acrolein, even ammonia is not a reactant of this reaction [4].
190 N. Viswanadhamet aL In the present study, we have characterized the catalyst during the pretreatment and catalytic reaction by XAFS, SS-NMR, and XRD to understand the genesis of the promoting effect of ammonia.
2. EXPERIMENTAL
Preparations of the catalysts have been reported in the previous papers [3,4]. Briefly, HZSM-5 zeolite which was pre-evacuated at 673 K was exposed to MTO at room temperature (HZmto-l.2), where the loading of Re was 1.2 wt%. Then the sample was heated under He flow at 673 K (HZcvd-l.2). HZcvd-l.2 was used for catalytic oxidation/ammoxidation reaction. The HZcvd-l.2 sample was also exposed to NH3 (HZmt3-1.2) and to a mixed gas of 02 + C3H6 (H7~3--+o2-1.2). Solid-state (SS) magic angle spinning (MAS) 29Si M R
spectra were recorded on a Chemagnetics
CMX-300. The spectra were acquired at 59.68 MHz by cross-polarization (CP). Solid-state 27AI MAS M R
spectra were recorded with 78.2 MHz spinning frequencies at a pulse length of 1 gs,
which corresponds to rd12 pulse for non-selective excitation. XAFS spectra were measured at BL-9A and 12C of KEK-IMSS-PF. Re Lm and Lj edge XAFS spectra were measured at I0 K in a transmission mode. The samples were transferred from a Shlenk tube to XAFS cells in a glove box, in which they were sealed with wax without exposure to air. EXAFS spectra were analyzed by the UWXAFS package [5]. The phase shift and amplitude functions were calculated by the FEFF8 code [6]. The k-range for the Fourier transformation and fitting R-range were 30-140 nm 1 and 0.1-0.32 rim, respectively.
3. RESULTS AND DISCUSSION
Figure
shows Fourier transformed EXAFS functions (k3x(k)) at Re LIII edge for the
Re/HZSM-5 samples. For HZmto-1.2, Re-O and/or Re-C contributions were observed at 0.171 and 0.201 nm with coordination numbers (CNs) of 3.0 and 1.1, respectively, which suggests that MTO retained the structure after the exposure.
When the HZmto-1.2 sample was heated in He at 673 K
(HZcvd-l.2), one shell (Re-O) fitting reproduced the observed EXAFS data. The Re-O distance and CN were determined to be 0.17 nm and 4.4, respectively, which shows the formation of a tetrahedral [ReO4] species during the treatment of HZmto- 1.2 at 673 K for 4 h. Ammonia treatment caused a significant change in ReOx species. When the HZcvd-l.2 sample was exposed to NH3 at 673 K, two different Re-O bondings were observed at 0.172 and 0.203 nm. Besides a new Re-Re contribution was observed at 0.276 nm. The observation of Re-Re bonds indicates the formation of Rr
clusters. The structural parameters are entirely different from those
191 for R~-~<)2(Re-O = 0.194 nm (CN = 4), Re-O = 0.211 nm (CN = 2), Re-Re = 0.261 nm (CN = 2), Re-O = 0.312 nm (CN = 2) by XRD crystallography). The phenomenon of ReOx cluster formation was also observed under the catalytic selective oxidation in the presence of NH3. Further treatment of HZNm-l.2 with a mixture of propene and 02 in the absence of ammonia (HZcs=+o2-1.2) exhibited the reformation of the tetrahedral [ReO4] species. The Re-O distance and CN were 1.73 nm and 3.9, respectively, which are similar to those for HZcvd-l.2
27A1NMR data suggest that the Re species in HZmto-1.2, HZcvd-l.2, and HZcs=+o2 -1.2 were bound to Al~ of zeolite framework, became the NMR peak due to framework aluminum which is typically observed at 60 ppm shifted to 50 ppm and became broad. 29Si NMR data reveal that the [ReO4] species interacts with Si(O-Al)5 pentagonal ring of HZ [4]. The additional interaction between Re species and Al~ disappeared by the NH3 treatment.
2O 10
-10
-20
0
2
40
2
40
2
40
2
4
R/IO-1 nm
Figure.
Fourier transformed k3-weighted EXAFS functions for Re/HZSM-5 after various
treatment. Scheme shows structural transformation of the Re species during pretreatmem and catalytic reaction suggested by EXAFS and 29Si and 27AI NMR [4]. When HZSM=5 was exposed to the vapor of MTO, MTO molecules interact with retention of the structure of MTO as suggested by EXAFS (species If). When species II was heated under He at 673 K, the MTO molecules reacted with the protons to form Rr
(species Ill).
After that, when species iIi was exposed to ammonia,
[Re6019] clusters (IV) were formed as proposed in the scheme. The clusters were converted to species HI when the species IV were exposed to O2+CsI-I6 again. The detailed characterization of Re species revealed that the formation of Re6 clusters in HZ was the genesis of the activity for the selective oxidation reaction of propene [4]. The coexistence of NI-I3 was prerequisite for the catalyst performance.
192 N. Viswanadhamet aL
CH3
""
? 11
o/S~ ~ ~ ,
~
major ~VD,
(,)
333 K
O~S.~.
i
/s~, /s~ (it)
minor \ "CH4~
s! S=
-CH4 673 K He
i s?~A, .s,
O,:: ~ 0
o~
...
.~o 3 '
R~---~-//~Re ----u'-~Si
"9
/" "AI Sf (IV)
~i Si
with NH3
~o
O~J~e~ 0
without NH3 673 K (ill)
Scheme. Structural changes of the Re/HZSM-5 catalyst preparation and the catalvtic reaction conditions. References [1] S. Albonetti, E Cavani, and E Trifiro, Catal. Rev.-Sci. Eng. 38 (1996)413. [2] R. K. Grasselli, Catal. Today 49 (1999) 141. [3] N. Viswanadham, T. Shido, and Y. Iwasawa, Appl. Catal. A General
219 (2001) 223.
[4] N. Viswanadham, T. Shido, T. Sasaki, and Y. Iwasawa, J. Phys. Chem. B in press. [5] E. Stern, Phys. Revs. B 48 (1993) 9825. [6] J. J. Rehr, R. Albers, Phys. Rev. B 41 (1990) 8139.
189
35 Formation of New Re Clusters in HZSM-5 and
Their Catalytic Property in Propene Selective Oxidation/Ammoxidation Reactions
N. Viswanadham, T. Shido, T. Sasaki, Y. Iwasawa* Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Abstract
A catalyst prepared by chemical vapor deposition (CVD) of CH3ReO3 (MTO) on H-ZSM-5 and subsequent treatment at 673 K was active for selective oxidation/ammoxidation of propene. Presence of ammonia was prerequisite for the catalytic selective oxidation of propene to acrolein. The structural change of the Re species during the pretreatment and catalytic reaction was investigated by SS-NMR, and XAFS. MTO interacted with protons of HZSM-5 at 333 K, and the subsequent treatment at 673 K formed tetrahedral [ReO4] monomer species. Ammonia promoted h
the formation of a new [Re60~7] cluster at 673 K. A proposed cluster structure constituted an octahedral Re6 framework with terminal and bridge oxygen atoms, which were relevant to the selective oxidation. The active [Re6017] clusters were converted to the inactive [ReO4] monomers in the absence of ammonia.
1. INTRODUCTION Selective oxidation/ammoxidation of light hydrocarbons is one of the important reactions in both fundamental research and industrial process [1,2]. In the previous studies, we investigated propene oxidation reactions on several zeolite-supported Re catalysts and found the remarkable effects of precursor (MTO or NH4ReO4), preparation method (CVD of MTO, impregnation of NH4ReO4, or physical mixing of NH4ReO4) and type of zeolite (HZSM-5, NaZSM-5, and HY) [3,4]. Among the examined catalysts, the Re/HZSM-5 catalyst prepared by CVD of MTO exhibited highest activity and selectivity. The conversion and selectivity of prepene to oxidation /ammoxidation products were 10-20% and ca. 80%, respectively at 673 K. What is interesting is that presence of ammonia is required to propene oxidation reaction to form acrolein, even ammonia is not a reactant of this reaction [4].
190 N. Viswanadhamet aL In the present study, we have characterized the catalyst during the pretreatment and catalytic reaction by XAFS, SS-NMR, and XRD to understand the genesis of the promoting effect of ammonia.
2. EXPERIMENTAL
Preparations of the catalysts have been reported in the previous papers [3,4]. Briefly, HZSM-5 zeolite which was pre-evacuated at 673 K was exposed to MTO at room temperature (HZmto-l.2), where the loading of Re was 1.2 wt%. Then the sample was heated under He flow at 673 K (HZcvd-l.2). HZcvd-l.2 was used for catalytic oxidation/ammoxidation reaction. The HZcvd-l.2 sample was also exposed to NH3 (HZmt3-1.2) and to a mixed gas of 02 + C3H6 (H7~3--+o2-1.2). Solid-state (SS) magic angle spinning (MAS) 29Si M R
spectra were recorded on a Chemagnetics
CMX-300. The spectra were acquired at 59.68 MHz by cross-polarization (CP). Solid-state 27AI MAS M R
spectra were recorded with 78.2 MHz spinning frequencies at a pulse length of 1 gs,
which corresponds to rd12 pulse for non-selective excitation. XAFS spectra were measured at BL-9A and 12C of KEK-IMSS-PF. Re Lm and Lj edge XAFS spectra were measured at I0 K in a transmission mode. The samples were transferred from a Shlenk tube to XAFS cells in a glove box, in which they were sealed with wax without exposure to air. EXAFS spectra were analyzed by the UWXAFS package [5]. The phase shift and amplitude functions were calculated by the FEFF8 code [6]. The k-range for the Fourier transformation and fitting R-range were 30-140 nm 1 and 0.1-0.32 rim, respectively.
3. RESULTS AND DISCUSSION
Figure
shows Fourier transformed EXAFS functions (k3x(k)) at Re LIII edge for the
Re/HZSM-5 samples. For HZmto-1.2, Re-O and/or Re-C contributions were observed at 0.171 and 0.201 nm with coordination numbers (CNs) of 3.0 and 1.1, respectively, which suggests that MTO retained the structure after the exposure.
When the HZmto-1.2 sample was heated in He at 673 K
(HZcvd-l.2), one shell (Re-O) fitting reproduced the observed EXAFS data. The Re-O distance and CN were determined to be 0.17 nm and 4.4, respectively, which shows the formation of a tetrahedral [ReO4] species during the treatment of HZmto- 1.2 at 673 K for 4 h. Ammonia treatment caused a significant change in ReOx species. When the HZcvd-l.2 sample was exposed to NH3 at 673 K, two different Re-O bondings were observed at 0.172 and 0.203 nm. Besides a new Re-Re contribution was observed at 0.276 nm. The observation of Re-Re bonds indicates the formation of Rr
clusters. The structural parameters are entirely different from those
191 for R~-~<)2(Re-O = 0.194 nm (CN = 4), Re-O = 0.211 nm (CN = 2), Re-Re = 0.261 nm (CN = 2), Re-O = 0.312 nm (CN = 2) by XRD crystallography). The phenomenon of ReOx cluster formation was also observed under the catalytic selective oxidation in the presence of NH3. Further treatment of HZNm-l.2 with a mixture of propene and 02 in the absence of ammonia (HZcs=+o2-1.2) exhibited the reformation of the tetrahedral [ReO4] species. The Re-O distance and CN were 1.73 nm and 3.9, respectively, which are similar to those for HZcvd-l.2
27A1NMR data suggest that the Re species in HZmto-1.2, HZcvd-l.2, and HZcs=+o2 -1.2 were bound to Al~ of zeolite framework, became the NMR peak due to framework aluminum which is typically observed at 60 ppm shifted to 50 ppm and became broad. 29Si NMR data reveal that the [ReO4] species interacts with Si(O-Al)5 pentagonal ring of HZ [4]. The additional interaction between Re species and Al~ disappeared by the NH3 treatment.
2O 10
-10
-20
0
2
40
2
40
2
40
2
4
R/IO-1 nm
Figure.
Fourier transformed k3-weighted EXAFS functions for Re/HZSM-5 after various
treatment. Scheme shows structural transformation of the Re species during pretreatmem and catalytic reaction suggested by EXAFS and 29Si and 27AI NMR [4]. When HZSM=5 was exposed to the vapor of MTO, MTO molecules interact with retention of the structure of MTO as suggested by EXAFS (species If). When species II was heated under He at 673 K, the MTO molecules reacted with the protons to form Rr
(species Ill).
After that, when species iIi was exposed to ammonia,
[Re6019] clusters (IV) were formed as proposed in the scheme. The clusters were converted to species HI when the species IV were exposed to O2+CsI-I6 again. The detailed characterization of Re species revealed that the formation of Re6 clusters in HZ was the genesis of the activity for the selective oxidation reaction of propene [4]. The coexistence of NI-I3 was prerequisite for the catalyst performance.
192 N. Viswanadhamet aL
CH3
""
? 11
o/S~ ~ ~ ,
~
major ~VD,
(,)
333 K
O~S.~.
i
/s~, /s~ (it)
minor \ "CH4~
s! S=
-CH4 673 K He
i s?~A, .s,
O,:: ~ 0
o~
...
.~o 3 '
R~---~-//~Re ----u'-~Si
"9
/" "AI Sf (IV)
~i Si
with NH3
~o
O~J~e~ 0
without NH3 673 K (ill)
Scheme. Structural changes of the Re/HZSM-5 catalyst preparation and the catalvtic reaction conditions. References [1] S. Albonetti, E Cavani, and E Trifiro, Catal. Rev.-Sci. Eng. 38 (1996)413. [2] R. K. Grasselli, Catal. Today 49 (1999) 141. [3] N. Viswanadham, T. Shido, and Y. Iwasawa, Appl. Catal. A General
219 (2001) 223.
[4] N. Viswanadham, T. Shido, T. Sasaki, and Y. Iwasawa, J. Phys. Chem. B in press. [5] E. Stern, Phys. Revs. B 48 (1993) 9825. [6] J. J. Rehr, R. Albers, Phys. Rev. B 41 (1990) 8139.