Photon Factory Activity Report 2014 #32 (2015) B

BL-9A/2014G514

Fine Structure around Chromium Species Incorporated into MCM-41
Using Template Ion-exchange Procedure
Takuya Ehiro,1 Yusuke Seno,1 Toshihiro Moriga,2 and Shigeru Sugiyama*2,3
Department of Chemical Science and Technology, Tokushima University, Minamijosanjima,Tokushima-shi, Tokushima 770-8506, Japan Photon Factory, Tsukuba 305-0801, Japan2
Department of Advanced Materials, Institute of Technology and Science, Tokushima University,
Minamijosanjima, Tokushima-shi, Tokushima 770-8506, Japan3
Department of Resource Circulation Engineering, Center for Frontier Research of Engineering,Tokushima University, Minamijosanjima, Tokushima-shi, Tokushima 770-8506, Japan
1

1 Introduction
Previously, we reported that Cr-doped FSM-16 (#16
folded sheets of mesoporous materials) showed the great
catalytic activity for the oxidative dehydrogenation(ODH) of isobutane [1]. In the present study, templateion exchange (TIE) method [2] was applied to MCM-41(#41 Mobil Composition of Matter) [3] to examine therelationship between the catalytic activity and finestructure around Cr specie in the catalysts.
2 Experiment
In the preparation of MCM-41, the hydrothermal
synthesis method was applied [2]. After the hydrothermal
synthesis at 413 K for 48 h without stirring, the obtained
white wet solid was filtered, washed with distilled water,and dried at 333 K. Chromium-doping was carried out
via the TIE method [3]. The catalytic activity tests werecarried out in a fixed-bed continuous flow reactor atatmospheric pressure. Each catalyst (0.25 g, 0.85-1.70mm) was pretreated with 12.5 mL/min of O2 at 723 K.
After the pretreatment, catalytic activity test was started
by flowing 15 mL/min of He, i-C4H10 and O2. The partialpressures were adjusted to P(He) = 74.6 kPa, P(i-C4H10) =14.4 kPa, and P(O2) = 12.3 kPa, respectively. Under
these conditions, stable activity was observed for up to 6h on-stream. The structural information on the Cr species
were evaluated via X-ray absorption fine structure(XAFS) performed with Synchrotron radiation at thebeam-line NW9A station of the Photon Factory in theHigh Energy Accelerator Research Organization(Tsukuba, Japan).
3 Results and Discussion
The catalytic activity of MCM-41 was quite low andthe conversion of i-C4H10 and the yield of i-C4H8 were 8.8and 0.9%, respectively. To improve this low catalytic
activity, Cr-doping into MCM-41 was conducted via theTIE method, resulting in the conversion and the yield to20.6 and 6.8%, respectively. Unfortunately, XRD did notafford any information on Cr species. Therefore 2.2 wt%
Cr-MCM-41 was analyzed using XAFS. Figure 1 showsthe effect of the reaction (Figs. 1 (a) and (c) for those
before and after the reaction, respectively) and thepretreatment using 12.5 mL/min of O2 gas flow at 723 Kfor 2 h (Fig. 1 (b)) on Cr K-edge XANES spectra of the Cr-MCM-41. As shown in Figures 1 (a) and (b), the intensityof the pre-edge peak was increased by O2 treatment. Thepre-edge peak at ca. 5991 eV represents tetrahedrally
coordinated Cr6+ species [4]. The coordination statearound Cr species was similar to that around the Si in
MCM-41. However, the pre-edge peak was disappeared
after the ODH of i-C4H10 (Fig. 1 (c)). Although our CrMCM-41 was not deactivated during 6 h on-stream, it canbe concluded that the reduction rate of active chromium
species during the ODH of i-C4H10 might have been fasterthan that of their regeneration by oxygen, since the preedge peak disappeared.

Fig. 1 Cr K-edge XANES spectra of (a) 2.2 wt% Cr
MCM-41 (a) before ODH of isobutane, (b) after thepretreatment with O2 flow and (c) after ODH of isobutane.

References[1] S. Sugiyama, Y. Okada, Y. Kosaka, K. Nakagawa, M.
Katoh, Y. Katou, S. Akihara, T. Yasukawa and W.
Ninomiya, J. Chem. Eng. Japan, 46 (9), 620 (2013).
[2] M. Iwamoto, T. Abe and Y. Tachibana, J. Mol. Catal.
A Chem., 155, 25 (2000).
[3] M. Iwamoto and Y. Tanaka, Catal. Surv. Japan, 5 (1),25 (2001).
[4] K. Takehira, Y. Ohishi, T. Shishido, T. Kawabata, K.
Takaki, Q. Zhang and Y. Wang, J. Catal., 224, 404(2004).
* sugiyama@tokushima-u.ac.jp