Synthesis of tungsten-containing MCM-41 catalysts and their activity for olefin metathesis

Abstract

Two methods for synthesis of tungsten-containing MCM-41 catalysts were studied: direct synthesis and impregnation. The methods of direct synthesis were attempted in both acidic and basic conditions to obtain the good-quality product of W-MCM-41 with maximum tungsten content. W-MCM-41 was directly synthesized by hydrothermal crystallization from a mixture of starting substance including tungsten. To define an appropriate synthesis method several parameters were varied such as acidity of the gel, types of templates (cetylpyridinium bromide or cetyltrimethylammonium bromide), type of tungsten source (ammonium tungstate or sodium tungstate), and silicon to tungsten ratios in gel (4-120). The products synthesized were characterized using X-ray diffraction, nitrogen adsorption, inductively coupled plasma-emission, Fourier transform infrared, Raman scattering, and diffuse reflectance UV-Visible techniques. White solids with the highly ordered structure of MCM-41 were achieved from crystallization from the gel in basic condition at the temperature of 100 ℃ for 4 days with pH adjustment daily. Effect of temperatures on catalytic activity of W-MCM-41 was studied using a feed of 30.5% 1-hexene vapor in nitrogen gas. To prepare WO₃/MCM-41 catalysts, a solution of required amount of sodium tungstate was impregnated on MCM-41 using the incipient wetness method. The activities of W-MCM-41 and Wo₃/MCM-41 catalysts with the same Si/W ratios in 1-hexene metathesis were compared at the same conditions. The gas products are mainly obtained from metathesis of 1-hexene with more than 90% conversions of 1-hexene over W-MCM-41 while WO₃MCM-41 provides very low conversions of 1-hexene (13-61%) at the same conditions. Product selectivity to propylene is independent from any parameter. Formation of butenes is favored at the temperature of 300 ℃while further conversion of butenes to ethylene takes place effectively at higher temperatures. Methane is formed in a significant amount for all cases especially at high temperatures. The directly synthesized W-WCM-41 gives much higher selectivity to ethylene than WO₃/MCM-41 from impregnation. Highly dispersion of tungsten on the catalyst surface is accounted for the high catalytic activity of W-MCM-41.