PA-43
Microwave hydrothermal synthesis of bimetallic (Zr-Ti) ions modified MCM-41 for epoxidation of styrene
Guangjian Wanga,b, Zhengwang Liub, Yuran Wangc, Yiwu Liub, Zhenxing Yangb, Mingxia Xua, Lei Wangb
a School of materials science and technology, Tianjin University, Tianjin 300072 b Department of Chemistry, Huaibei Coal Normal Collage, Huaibei 235000, Anhui
c School of Chemistry and Chemical Engineering, Shanghai Jiaotong University, Shanghai 200030
Abstract:
The successful syntheses of the M41S family of ordered mesoporous materials in 1992 sparked worldwide interest in their synthesis and applications, specially in heterogeneous catalysis and separation science. In order to improve the catalytic activity of the mesoporous materials, especially to create redox active sites, active sites were incorporated into the silica framework or grafted on the surface. Transition metal ions such as Ti, Zr, Mn, and V have been successfully incorporated into the MCM-41 framework. The incorporation of two different metal ions might therefore create new hybrid materials with different or original redox and acid properties.
However, only few studies concerned the synthesis and catalysis of bimetallic ions modified mesoporous molecular sieves based on synergistic effect of two metal ions.
The microwave-assisted synthesis of molecular sieves was a relatively novel area of research. It offered many distinct advantages over conventional synthesis. They included rapid heating to crystallization temperature and eventually a shorter crystallization time compared to conventional autoclave heating. In This paper, Mesoporous molecular sieves MCM-41 modified by single (Ti) or bimetal (Zr–Ti) ions with highly ordered hexagonal arrangement of their cylindrical channels were prepared by direct synthesis under microwave–hydrothermal (M–H) conditions at 403K. Characterizations with XRD, FT-IR, N2 adsorption– desorption, SEM, EDS, TEM, photoluminescence (PL) and XPS showed that Zr and Ti was introduced into MCM-41 under M-H conditions and Zr-Ti bond was formed. Due to the content of high metal ions, the (110) and (200) peaks of XRD were overlap. The intensity of XRD peaks (d100) of Zr-Ti-MCM-41(50) samples by microwave irradiation 2h was very close to that of the samples conventional oven 48h. The intensity of the XRD peaks for calcined Zr-Ti-MCM-41 was decreased and the XRD peaks shifted to lower 2θ values, compared with calcined. The higher d-spacing and unit cell parameter values observed with increasing metal content suggested an incorporation of metal in the framework locations occured because of its (Zr-O(~ 0.171160 nm) longer bonding length with oxygen than that of Si-O (~ 0.15090 nm). The SEM image showed rather uniform and fine particles (ca. 0.3 µm) with round-like shape which was the typical morphology of normal MCM-41 synthesized under basic conditions. The PL was due to the charge transfer transition between the singly occupied nonbonding 2p σ-orbital of nonbridging oxygen and the lone-pair 2p orbital of one of the nearest bridging oxygens. In combination with the results of FTIR and XPS, it seemed reasonable to ascribe the PL from the samples to the defect emission (NBOHCs) at the Mesoporous molecular sieves surface. The vacancy will be filled by Zr, leading to the decrease in the photoluminescence effect. The modified materials were active and selective in the epoxidation of styrene at 343 K in comparison with single-functional MCM-41.
Moreover, compared to conventional method, the presented microwave hydrothermal synthesis of molecular sieves greatly improved the selectivity to styrene oxide, e.g., it
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reached 50.5% at styrene conversion of 11.2% over Zr-Ti-MCM-41 (50). The results clarify the synergistic effect of Zr and Ti on epoxidation of styrene with H2O2.
Keywords: Mesoporous molecular sieves; Zr-Ti-MCM-41; Epoxidation; Styrene;
XPS
*Corresponding author: Tel./fax: +86 561 3803233; E-mail address: [email protected] (Guangjian Wang)
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