Direct Synthesis and Structural Characteristics of Ordered SBA-15 Mesoporous Silica Containing Tungsten Oxides and Tungsten Carbides

Abstract

A series of WO3-SBA-15 materials with different Si/W ratios have been hydrothermally synthesized using tetraethyl orthosilicate (TEOS) as silica precursor, ammonium paratungstate as tungsten precursor, and EO20PO70EO20 (P123) as structure-directing reagent. After temperature-programmed carburization (TPC) in flowing CH4/H2 (20/80 v/v mixture), the materials were converted to the corresponding WxC-SBA-15 materials. The structure of the oxide and carbide materials has been characterized using X-ray diffraction (XRD), X-ray fluorescence (XRF), nitrogen adsorption−desorption measurements, 29Si magic-angle spinning (MAS) NMR spectroscopy, Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), and thermogravimetric and differential scanning calorimetric analysis (TG-DSC) measurements. The results show that after hydrothermal synthesis using different amounts of tungsten and subsequent carburization, the materials retain the mesopore structure of SBA-15. When Si/W = 30−15, the majority of the tungsten is dispersed in the channels of SBA-15 with the remainder being incorporated into the framework of SBA-15 with the formation of Si−O−W bonds. The tungsten carbide exists as a single W2C phase after carburization. At higher tungsten content (Si/W = 7.5), the amount of tungsten in the framework of SBA-15 increases with the formation of both Si−O−W bonds and W−O−W bonds. The tungsten carbide formed after carburization exists as a mixture of W2C and WC phases. A model for the distribution of tungsten in SBA-15 is proposed involving three different tungsten species: α-W inside SBA-15 channels, β-W embedded in the internal surfaces of the SBA-15 channels, and γ-W inside the framework of SBA-15. After temperature-programmed carburization, α-W sites are transformed into W2C, whereas β-W sites afford WC; in contrast, γ-W sites show little change after carburization.