Tungsten-substituted mesoporous SBA-15 materials have been successfully synthesized by using a direct co-condensation sol-gel method. These resultant materials were characterized by X-ray diffraction (XRD), nitrogen adsorption, scan electron microscopy (SEM), transmission electron microscopy (TEM), 29Si MAS NMR and Raman spectroscopy. These results indicate that these materials have high surface area, uniform hexagonal channels and thick framework walls which are similar to that of SBA-15 and the tungsten species is highly dispersed in the silica-based framework structure. The tungsten-substituted mesoporous SBA-15 catalysts show excellent catalytic performance in the butene-1 metathesis to high value olefins. The strong interaction between tungsten species and silica as well as the pore structure of the tungsten-substituted mesoporous SBA-15 catalysts are crucial for the excellent catalytic behavior.
Maximizing the value of the steam cracker or refinery based C4 stream is a major objective for most companies. Olefin metathesis has become a valuable reaction for producing higher value products demanded from many available olefins. Tungsten oxide supported on silica is a well-known metathesis catalyst. In our previous studies, we found that the activity of butene- 1 metathesis over WO3/SiO2 prepared by traditional impregnation method was low which is due to the weak.
interaction of silica and tungsten oxide. SBA-15 has been used for catalyst support extensively for its excellent properties. As we know, the active sites in the molecular sieves are often from hetero-atoms. Therefore, it is greatly significant to introduce heteroatoms into the framework of SBA-15. In order to improve tungsten species dispersibility, alleviate aggregation of WO3 and strengthen interactions between tungsten species and silica, here we synthesized tungsten-substituted mesoporous SBA-15 (MWS) catalysts by using direct co-condensation sol-gel method, which exhibits excellent catalytic performance for the butene-1 metathesis.
The MWS materials were prepared using Pluronic P123 triblock polymer (EO20PO70EO20, Mav = 5800, Aldrich) as template under acidic conditions. The catalytic performances of the samples were carried out in a 20Ч650 mm stainless steel microreactor.
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Fig.1 shows that all the exhibited XRD patterns with a very intense diffraction peak and two weak peaks, which are characteristic of 2-D hexagonal (P6mm) structure with excellent textural uniformity. There were no significant changes upon increasing the amounts of tungsten oxide except for the expected change in XRD peak intensity. It was confirmed that the nitrogen adsorption isotherms as shown in Fig.2 are of Type IV in nature and exhibited H1 hysteresis loop, which is typical of mesoporous solids. BET surface area, pore volume and pore diameter results obtained from N2-adsorption measurements indicate that the introduction of tungsten oxide does not affect on the pore structures of the tungstensubstituted SBA-15 materials except for causing the decrease of surface area and increase of pore diameter with the increasing of tungsten oxide content. Fig.3 shows the TEM image of the MWS sample (Si/W=30). It has a 2-D hexagonal structure as does SBA-15.
