Tungsten–Oxo-Species Deposited on Alumina. II. Characterization and Catalytic Activity of Unpromoted W(vi)/γ-Al2O3Catalysts Prepared by Equilibrium Deposition Filtration (EDF) at Various pH's and Non-Dry Impregnation (NDI)

Abstract

A series of W(vi)/γ-Al2O3catalysts of varying W(vi)content in the range 3.3–21.2% WO3was prepared at various pH's using equilibrium deposition filtration (EDF).These catalysts were then characterized using various physicochemical techniques (BET, XPS, XRD, DRS, NO chemisorption, TPD of NH3, microelectrophoresis, and DTGA) and the surface characteristics achieved were related with the nature of the W(vi)species deposited on the γ-alumina surface during preparation. A close relation exists between the nature of the deposited W(vi)species and the structure and dispersity of the W(vi)-supported phase: Deposition through adsorption or reaction of themonomericWO42−species in the pH range 7.4–4.9 resulted in a well-dispersed, presumably monolayered, W(vi)phase relatively rich in tetrahedral W(vi). In contrast, deposition through adsorption of thepolymericHW6O20(OH)25−species at pH's 4.0 and 3.5 resulted in a supported phase with relatively low dispersity and a relatively small amount of tetrahedral W(vi). Supported WO3crystallites with size greater than 40 nm are formed in this case. Moreover, it was inferred that the mode of deposition of the monomeric WO42−species is reflected in the properties of the supported W(vi)phase of the calcined samples. Thus, TPR and NO chemisorption data strongly suggested that the change in the deposition mechanism (upon decreasing pH from 6.1 to 4.9) from deposition through surface reaction of the monomeric WO42−species with neutral hydroxyls of the support into deposition through adsorption of these species on protonated surface hydroxyls of the support brought about a decrease in the “supported phase-carrier” interactions as well as an increase in the surface of the supported W(vi). Three catalysts with W(vi)loading corresponding to that of the three EDF catalysts (3.3, 11, and 21.2% WO3/γ-Al2O3) were also prepared using the usual methodology of nondry impregnation (NDI) and characterized using the aforementioned techniques. It was found that NDI resulted in a supported phase with lower dispersity and less rich in tetrahedral W(vi)species than EDF. Moreover, catalytic tests, using the hydrodesulfurization of thiophene and the hydrogenation of cyclohexene as probe reactions, showed that EDF provides more active catalysts than NDI and this is another example showing that EDF with controlled regulation of the impregnation parameters is a promising methodology for preparing supported catalysts even with relatively low loadings. Finally, it was found that EDF provides more acidic samples than NDI.