Photon Factory Activity Report 2014 #32 (2015) B

BL-9C/2014G575

XAFS study on the local structure of mesoporous silica supported titanium oxide aswater-tolerant Lewis acid catalyst
Nobuyuki Ichikuni1,* Kiyotaka Nakajima2,3, Hiroshi Shintaku2 and Michikazu Hara2,4
1
Chiba University, Inage-ku, Chiba 263-8522, Japan
2
Tokyo Institute of Technology, Midori-ku, Yokohama 226-8503, Japan
3JST, PRESTO, Kawaguchi 332-0012, Japan
4JST, ALCA, Kawaguchi 332-0012, Japan

2 Experiment
Mesoporous silica SBA-15 was chosen for the supportand synthesized by using an block copolymer P123 as aSDA and tetramethyl orthosilicate as a silica source.
Titanium species was immobilized onto SBA-15 by apostgrafting method [2]. Obtained powder was calcined at823 K to give a TDMS. Ti loading was regulated as 2
atom%. Samples were dehydrated at 573 K under vacuum
to remove the physisorbed water. Dehydrated samples
were pressed into 20 mm diameter self-supporting discs
and sealed in polyethylene bags under nitrogen
atmosphere without contacting air and moisture. Solvent
exposed samples were prepared by dropping the solventonto discs at least 10 min before the XAFS measurements.
Ti K-edge XAFS data were collected at PF BL-9C
with Si(111) double crystal monochromator in atransmission mode. Higher harmonics was reduced bydetuning the double crystal at about 60 % intensity of theincident beam. Data analysis was carryied out by using
the program REX2000 (Rigaku Co.).
3 Results and Discussion
Ti K-edge XANES spectra are shown in Fig. 1. As
prepared TDMS sample (unexposed to an air) shows asharp pre-edge peak at around 4968.5 eV (Fig. 1 (c)),
which can be attributed to the Ti 1s to 3d mixed with O

2p of tetrahedrally coordinated titanium species. On theother hand, both anatase TiO2 and rutile TiO2 show
typical weak pre-edge peaks assignable to the transitionof octahedrally coordinated titanium species. Ti species
on TDMS seemd to have the TiO4 tetrahedra structure.
The TDMS catalyst shows the high activity toward theMukaiyama-aldol reaction in water comparable asSc(OTf)3 catalyst. The high activity seemed to come fromthis TiO4 tetrahedra. However, the spectrum was taken indehydrated state which was far from the reaction
condition.
Interestingly, by exposing TDMS sample to the solvent,such as benzaldehyde (Fig. 1 (d)) and water (Fig. 1 (e)),
the XANES spectra did not change from original one (Fig.
1 (c)). This suggests the high structural tolerance to waterand also benzaldehyde. One of the reasons for showing
the high activity is this highly stable Lewis acid site.

0.5

normalized absorption

1 Introduction
Lewis acid catalysts are one of the most importantmaterials for the synthesis of industrial chemicals.
However, the conventional Lewis acids work in strictly
anhydrous conditions because of their low tolerance towater. From the viewpoint of the Green Sustainable
Chemistry, the organic solvent system should be replaced
by water solvent system. For this sake, many efforts weredeveloped to create the solid Lewis acid catalysts. We
have recently reported the Nb2O5·nH2O can act as awater-tolerant Lewis acid catalyst [1]. TiO4 deposited
mesoporous silica (TDMS) also showed the high catalytic
performance for Lewis acid catalyzed reaction in water[2].
The active site of TDMS seemed to be the isolated
TiO4 tetrahedra, however, it is still unknown whether thelocal structure is changed or not in the solvent. In thisstudy, the local structure of Ti on TDMS in/out thesolvent was characterized by XAFS analysis.

0.4(e)
(d)

0.3

(c)

0.2

(b)
(a)

0.1
0.0

4960

4980
5000
5020
photon energy / eV
Fig. 1: Ti K-edge XANES spectra; (a) TiO2 (rutile), (b)TiO2 (anatase), (c) TDMS (as prepared), (d) TDMS withbenzaldehyde, (e) TDMS with water.
References[1] K. Nakajima et al., J. Am. Chem. Soc. 133, 4224
(2011).
[2] T. Shintaku et al., ACS Catal. 4, 1198 (2014).
* ichikuni@faculty.chiba-u.jp