G Model
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Y. Xie et al. / Chinese Chemical Letters xxx (2018) xxx–xxx
necessary to overcome harsh reaction conditions and avoid
rehydration of the HMF generated. Therefore, the synthesis of
highly efficient and stable heterogeneous catalyst with highly
selective for the conversion of fructose into HMF is very significant.
Schiff bases have been drawn attention because they contain
the C N bond of Schiff base, indicating the formation of the Schiff
base complex.
¼
According to Fig. S2 (Supporting information), it was not
difficult to find that pure MCM-41 presented spherical particles
with uniform size and diameter at the nanometer level. As shown
in Fig. 1a, Zr(IV)-salen-MCM-41 generated aggregation because of
the incorporation of organic functional groups, but it still
maintained a spherical structure. As seen from Fig. 1b, the TEM
image of Zr(IV)-salen-MCM-41 confirmed that the material
presented longer range order and dimensional pores, similar to
the pure silicon MCM-41.
electron-donating group (C N), which can be coordinated with
¼
numbers of metal ions [24]. Transition metal Schiff base complexes
have been widely used as homogeneous catalysts for various
reactions, such as hydrogenation and Heck reaction [25,26]. More
and more studies have focused on immobilizing Schiff base
complexes onto various supports to obtain different heteroge-
neous catalysts. The common catalyst carriers are mesoporous
molecular sieves, montmorillonite, polymers, Fe3O4, carbon nano-
materials and so on.
The XRD patterns of MCM-41, Zr(IV)-salen-MCM-41 were
shown in Figs. 1c and d. In the Low angle XRD pattern of pure
MCM-41, a very strong reflection at 2
u
= 1.89ꢀ for d (100) and two
In this paper, we first synthesize the mesoporous silica MCM-41
and then introduce aminopropyl to the surface of MCM-41. Finally,
the Zr-Schiff base is combined with the modified MCM-41 to form a
heterogeneous catalyst and used for the dehydration of fructose.
Mesoporous silica MCM-41 and the modified mesoporous silica
MCM-41 were prepared according to the literature method with a
slight modification [27–29]. Zr(IV)-salen-MCM-41 preparation
other weaker reflections at 2
u
= 3.79ꢀ and 4.35ꢀ for d (110) and d
(200) were observed, which could be indexed to a well-ordered
mesoporous material of the hexagonal symmetry [25]. However,
the characteristic reflection peaks (d (110) and d (200)) in the low-
angle XRD pattern of Zr(IV)-salen-MCM-41 were significantly
reduced to barely visible, which possibly due to the deterioration of
MCM-41 after symmetry modification. The results provided
further evidence that functionalization mainly occurred inside
the mesopore channels and the catalyst remained structural
ordering of the MCM-41 channels [30]. Moreover, in the wide angle
XRD pattern of the Zr(IV)-salen-MCM-41, the only hump was due
to the amorphous form of silica, indicating the absence of other
crystalline phases.
The N2 adsorption-desorption isotherms of MCM-41 and Zr(IV)-
salen-MCM-41 were shown in Fig. S3 (Supporting information).
MCM-41 belonged to typical type IV adsorption isotherms with H1
hysteresis loop according to IUPAC classification, which indicated
the presence of the mesoporosity [31]. On the contrary, Zr-salen-
MCM-41 belonged to the typical type III adsorption isotherms
according to IUPAC classification. Textural properties and acid/base
sites density of the samples were summarized in Table S1
process was as follows: ZrOCl2 8H2O (2 mmol) was added to
ꢁ
imine-MCM-41 (1 g) in acetonitrile (30 mL) at 80 ꢀC under N2
atmosphere. After stirring for 6 h, the mixture was filtered and
washed with water, and then dried overnight to obtain a peach
powder. All the materials, characterizations used in this work and
the detailed preparation methods were presented in the Support-
ing information.
The dehydration reaction was carried out in a 5 mL reaction vial
equipped with magnetic stirrer. A typical procedure for dehydra-
tion of fructose was as follow: fructose (100 mg), catalyst (50 mg)
and DMSO (2 mL) were added into the reaction vial. The reaction
mixture was heated to desirable temperatures with an oil bath
under strong stirring for a specific time. After the reaction, the
catalyst was separated by centrifugation, then the sample was
diluted with deionized water and analyzed by high-performance
liquid chromatography (HPLC). The specific analysis method was
shown in the Supporting information.
Infrared spectroscopy was used as a key method to character-
ize the functional groups of the samples. The FT-IR spectra of
MCM-41, NH2-(CH2)3-MCM-41 and Zr(IV)-salen-MCM-41 were
showed in Fig. S1 (Supporting information). The prominent peaks
at 1089 and 802 cmꢂ1 were assigned to asymmetric and
symmetrical stretching vibration of Si-O-Si. The bands at 3428,
3401 and 3414 cmꢂ1 in the FTIR spectra of MCM-41, NH2-(CH2)3-
MCM-41 and Zr(IV)-salen-MCM-41 respectively indicated Si–OH
(Supporting information). MCM-41,
a common mesoporous
material, showed high surface areas and pore volume. Noticeably,
the surface area of the Zr(IV)-salen-MCM-41 decreased a lot, which
indicated that some pendant group on the surface of the catalyst
had blocked the adsorption of nitrogen molecules [28]. Such
significant decrease of textural properties of porous materials on
grafting had been reported [32]. In addition, acid and base density
of the Zr(IV)-salen-MCM-41 were determined by NH3-TPD and
CO2-TPD respectively, and the results were presented in Table S1
and Fig. S4 (Supporting information). As shown in Table S1, there
were a large number of acid-base sites in the catalyst, and the acid
and base sites were derived from Zr4+ and aniline groups,
respectively. Moreover, phenolic hydroxyl groups could also
provide some basic site.
stretching vibration. There were
a symmetrical stretching
vibration of –NH2 at 1565 cmꢂ1 and a stretching vibration of
–CH2 at 2934 cmꢂ1 in curve b (Fig. S1), suggesting that the
coupling agent had been successfully attached to the surface of
MCM-41. The absorption peak at 1622 cmꢂ1 could be attributed to
The TG curve of the synthesized materials was presented in Fig. S5
(Supporting information). The TG curve of the Zr(IV)-salen-MCM-41
Fig. 1. SEM image (a), TEM image (b), wide angle XRD pattern (c) of the catalyst Zr(IV)-salen-MCM-41 and low angle XRD patterns of the MCM-41 and Zr(IV)-salen-MCM-41.
Please cite this article in press as: Y. Xie, et al., Zirconyl schiff base complex-functionalized MCM-41 catalyzes dehydration of fructose into 5-