Photon Factory Activity Report 2017 #35 (2018)

BL-9C/2016G518

XANES Evidence on Active Species over MCM-48 Doped with Chromium
Yuki KATO1 and Shigeru SUGIYAMA2,*Otake R&D Center, Mitsubishi Chemical Corporation, 20-1, Miyuki-cho, Otake-shi,
Hiroshima 739-0693, Japan2
Department of Applied Chemistry, Graduate School of Technology, Industrial and Social Science,
Tokushima University, Minamijosanjima, Tokushima-shi, Tokushima 770-8506, Japan
12 Experiment
Testing of catalytic performances was carried out witha fixed-bed continuous flow reactor under atmosphericpressure at 723 K, using 0.25 g of pelletized and sieved
catalyst (0.85-1.07 mm). The partial pressures of thereactant gases were adjusted to P(He) = 74.6 kPa, P(O2) =12.3 kPa and P(iso-C4H10) = 14.4 kPa. No homogeneous
reaction was detected under these conditions. The
preparation method of MCM-48 introduced withchromium was described elsewhere [1]. The surface
property of the catalysts was analyzed using XPS (PHI5000VersaProbe II, ULVAC-PHI Inc.). The fine-structurechanges around the chromium species were estimatedusing XAFS measurement carried out via synchrotron
radiation using the BL9C station at the Photon Factory of
the High Energy Accelerator Research Organization. The
storage ring energy and current were 2.5 GeV and 450mA, respectively. The X-rays were monochromatizedusing a Si(111) water-cooled double crystal.
3 Results and Discussion
On MCM-48, isobutane was barely converted. The
yields of isobutene were 1.9% for MCM-48. However,the conversion of isobutane and the selectivity toisobutene were greatly improved by introducing only asmall amount of chromium into MCM-48 (Cr-MCM-48).
The maximum catalytic performance up to 50% or greaterof the yield of isobutene was detected when using CrMCM-48 (Si/Cr ratio = 1,000). When Cr-MCM-48 (Si/Crratio = 100 or 1,000) were analyzed using XPS, anyevident XPS peaks due to Cr3+ and Cr6+ were not detectedeven though the integration number was extensivelyincreased. However, chromium species from Cr-MCM-48(Si/Cr ratio = 100 or 1,000) were detected using XANES
as shown in Fig. 1. The XANES spectra of both CrMCM-48 before the reaction showed a pre-edge
absorption at ca. 5,990 eV, indicating that the chromium
species possesses a tetrahedral coordination structure
consisting the Cr6+. After the reaction, the absorption peak
was shifted by ca. 3.6 eV to a lower energy level,
followed by the disappearance of the pre-edge absorption
peak. The XANES spectra of the catalysts after thereaction were similar to that of Cr2O3, indicating that thechromium species had an octahedrally coordinatedstructure. These XANES information revealed that thestructure of chromium species changed from tetrahedral
to octahedral coordination, while the chromium species
was reduced from Cr6+ to Cr3+ during the reaction.

(e)(d)
Absorption [a. u.]

1 Introduction
Although isobutene is a key compound for theproduction of various chemicals, it is mainly supplied
from ethylene and FCC plants via a supply route that isundoubtedly insufficient. Under these circumstances,oxidative dehydrogenation (ODH) of isobutane toisobutene have been investigated in our laboratory. In thepresent study, the ODH of isobutane to isobutene wasexamined using MCM-48 introduced with chromium via
the template ion exchange method. It was found out thatthe catalytic performance of MCM-48 was greatly
improved by the introduction of a small amount ofchromium. Although various characterization procedures
were used to clarify the role that chromium played in theimprovement, only XAFS returned the most valuable
information. In the present report, XAFS results weredescribed comparing to those obtained via XPS.

(c)
(b)
(a)

5950

6000

6050
6100
Energy [eV]6150

Fig. 1: Cr K-edge XAFS spectra of (a) Cr2O3, (b) CrMCM-48 with a Si/Cr ratio = 1,000 before the reaction,(c) Cr-MCM-48 with a Si/Cr ratio = 1,000 after thereaction, (d) Cr-MCM-48 with a Si/Cr ratio = 100 before
the reaction, and (e) Cr-MCM-48 with a Si/Cr ratio = 100
after the reaction.
References[1] Y. Kato, S. Nitta, S. Shimazu, M. Kurashina, M.
Katoh, W. Ninomiya and S. Sugiyama, Submitted to J.
Chem. Eng. Japan (2018).
* sugiyama@tokushima-u.ac.jp