Preparation, Characterization And Catalytic Performance Of Tungsten Carbides Confined In The Channels Of SBA-15 Mesoporous Silica

Transition metal carbides are promising catalysts which are comparable in performance to noble metals in many cases, and in some cases, such as deep hydrodesulfurization (HDS) of oil and gas, hydrodenitrogenation (HDN), hydrocarbon isomerization, and methane reforming to synthesis gas, have an even better performance. However, the catalytic activity of carbides catalysts is easily influenced with the defect of small specific surface area and agglomeration of the carbide components of transitional support under the high temperature reaction conditions involved. SBA-15, a kind of ordered mesoporous silica with large surface area and uniform hexagonal channels, has attracted considerable attention due to its potential applications in catalysis and adsorption. In this paper, the preparation of well-dispersed carbides catalysts in SBA-15 mesoporous silica is described with post-synthesis and direct-synthesis method. The formation mechanism and structure characteristics of carbides in SBA-15, the relationship between catalysts structure and catalytic activity are investigated, respectively.Firstly, a series of WO₃/SBA-15 (Si/W=30-7．5) materials have been prepared by impregnating the host material SBA-15 with aqueous ammonium paratungstate solutions and subsequently temperature-programmed reduction to WC/SBA-15(Si/W=30-7．5) . On the other hand, a series of WO-SBA-15(Si/W=30-7．5) materials have been hydrothermally synthesized using tetraethyl orthosilicate (TEOS) as silica precursor, ammonium paratungstate as tungsten precursor, EO₂₀ PO₇₀ EO₂₀ (P123) as structure directing reagent and subsequently temperature-programmed reduction to W_xC-SBA-15 (Si/W=30-7．5) .Secondly, the structure of as-prepared catalysts is characterized using X-ray diffraction, nitrogen adsorption-desorption measurements, FTIR, ²⁹ Si MAS NMR spectroscopy, TEM and TG-DSC measurements. The phases, pore structure, morphology, acid property and thermal stability of the catalysts are investigated in detail.As far as WO₃/SBA-15 and W₂C/SBA-15 are concerned, the ordered mesoporous structure of SBA-15 is still retained, however, the specific surface area, pore diameter, and total volume gradually decreases with increasing W contents. The phase of carbide is W₂C which has been confined into the channels of SBA-15．Quantitative ²⁹ Si Single Pulse Excitation MAS experiments and FTIR spectroscopy show that the incorporation of W₂C in the channels of SBA-15 is correlated with the formation of Si-O-W bonds. Some Si-O-W bonds are transformed into Si-O-H bonds after carburization. A model involving a discrete W₂C thin layer in the channels of SBA-15 is proposed on the basis of the NMR data.θ, correction factor, is defined as the coverage of hydroxyl group on the surface of SBA-15 for the first time. The calculated thickness of the discrete W₂C thin layer, 1．7～1．9nm, is consistent with value given by HRTEM. The NH₃-TPD analysis indicates that WO₃/SBA-15 materials have only the weak acid sites of SBA-15, whereas W₂C/SBA-15 catalysts have both the weak acid sites of SBA-15 and the strong acid sites of W₂C.As far as WO₃-SBA-15 and W_xC-SBA-15 are concerned, the ordered mesoporous structure of SBA-15 is still retained. The phase of carbide is W₂C with lower W contents, whereas are W₂C and WC with higher W contents. ²⁹ Si MAS NMR, FTIR, XRD and N₂ adsorption-desroption measurments indicate that there are three different W speices within SBA-15:α-W inside SBA-15 channels,β-W embedded in the internal surfaces of the SBA-15 channels, andγ-W inside the framework of SBA-15．Incorporation ofα-W,β-W andγ-W has different effects on the structure of SBA-15 because of their different chemical environments. The formation of WO₃ involvesα-W sites and the observed decrease in pore diameter can be attributed to the bonding ofα-W with Si sites on the internal surfaces of SBA-15．The presence ofβ-W sites favors the formation distortional hexagonal channels. The formation of framework tungsten sites (γ-W) is responsible for the observed increase in cell parameter. After temperature-programmed carburization,α-W sites are transformed into W₂C, whereasβ-W sites afford WC; in contrast,γ-W sites show little change after carburization. The NH₃-TPD analysis indicates that the acid of WO₃-SBA-15 is stronger than that of WO₃/SBA-15 and the acid of W_xC-SBA-15 is stronger than that of W₂C/SBA-15．Finally, the newly tungsten carbide catalysts show high catalytic activity for thiophene HDS under atmosphere condition. The promotion of Ni on the carbide catalysts is remarkable. In situ DRIFT spectra of tungsten carbide catalysts containing Ni indicate that there is the weaker CO line-type adsorption on the surface of W₂C/SBA-15 catalysts and the stronger CO line-type adsorption on the surface of W_xC-SBA-15 catalysts. It is significant for the research and development of high performance deep hydrodesulfurization catalysts.