Boehmite-supported vanadium oxide catalysts

Abstract

Two crystalline boehmites obtained by hydrothermal synthesis at 393 and 473 K have been studied as model supports for vanadium oxide, deposited by the equilibrium adsorption method. Supports and catalysts have been characterized by X-ray Diffraction, TGA, TEM, UV-Visible and Near Infrared Diffuse Reflectance Spectroscopy, FT-IR, XPS and 27Al NMR MAS spectroscopy. It was found that well-defined boehmite crystals are formed only at the higher synthesis temperature. Typical crystal dimensions for the two samples were approximately 8 and 40 nm, respectively. Based on the XPS and textural data, a simple model has been developed for the surface of fully hydrated crystalline boehmite. It is proposed that: (i) the (020) basal face is formed from non-reactive bridged hydroxyl groups, as in the bulk structure; (ii) both (100) and (101) faces expose Al-coordinated water molecules; (iii) reactive three-coordinated oxide ions on the (100) face and one-coordinated hydroxyl groups on the (101) face are also available. In qualitative agreement with these results, a maximum coverage of 38% vanadium equivalent monolayer was attained by adsorption at pH 2. At ambient temperature, when hydrated, the degree of condensation of the supported vanadia species has been evaluated from the UV-Visible absorption edge energy and FT-IR spectra. It was found that vanadium polymerization follows the trend observed with alumina supports, increasing essentially with surface vanadium concentration.