ACID-BASE AND CATALYTIC PROPERTIES OF MIXED OXIDES IN SELECTIVE HETEROGENEOUS OXIDATION REACTIONS: DESCRIPTION OF ACTIVE SITES

摘要

In this paper some actual views are presented on heterogeneous catalytic selective oxidation reactions of alkenes and aromatics into aldehydes, acids, nitriles, epoxides, etc. and of alkenes and alkanes to olefins. After reminding the pioneered works by Brazdil, Burrington and Grasselli in the early 1980s, emphasis is borne on more recent processes in selective (ammo)-oxidation reactions and recent ideas concerning the active sites and their acid-base and catalytic properties. Some emphasis is also borne on the importance of preparation and activation of catalysts and their characterization, in conditions as close as actual reaction conditions. Some reactions and catalysts have been chosen to illustrate our views on the nature and properties of catalytic sites, such as VPO catalyst for butane to maleic anhydride and MoVNb(Sb,Te)-O for propane to acrylonitrile or acrylic acid. Emphasis is also put on reactor configurations, including structured reactors, and microstructured catalysts to improve performances and sustainability. For selective oxidation reactions, the reaction may proceed via radical intermediates or, more often, via a Mars and van Krevelen5 mechanism, as schematised below and which involves cations of variable oxidation state, such as Cr, Cu, Fe, Mo, V, etc.: 2[CatO] + R-CH2 ? 2[Cat] + R-C=O + H2O and 2[Cat] + O2 (gas) ? 2[CatO]. Here, [CatO] represents the oxidised catalyst surface and [Cat] its reduced state, rred the rate of reduction by reactant and rox rate of re-oxidation by co-fed oxygen, R-CH2 and R-C=O the reactant and the product. The kinetic equation involves the relative concentration of reduced (θ_(red)) and oxidised (θ_(ox)) sites of the catalyst. One writes: kred pHC (1-θ) = k_(ox) pO2 θ, pHC, pO2 being the partial pressures in HC and O2, and k_(red), k_(ox) the rate constants for reduction of KO and reoxidation of K, respectively. At steady state, r = r_(red) = rox and and 1/r = [1/k_(red) p_(HC)] + [1/kox pO2]. The relative rate value of rred and rox is important for the selectivity in the product and involves lattice oxygen anions, which may be incorporated in the reactant and the corresponding vacancy created is then replenished by gaseous oxygen in the re-oxidation step. If k_(red) p_(HC) k_(ox) pO2, θ 1, reoxidation of the surface is the rate determining step, then r ~ k_(ox) pO2 (zero order in HC), as for propene oxidation on MoO3 and molybdates. If k_(red) p_(HC) k_(ox) PO2, θ 1, reduction of the surface is the rate determining step: r ~ kred PHC (zero order in oxygen), as for o-xylene oxidation over V based catalysts, such as V2O5/TiO2. It is clear that reducibility and reoxidability of the cation may give different kinetic expressions based on mechanistic studies6. According to this mechanism the substrate is oxidised by the catalysts and not directly by the gaseous oxygen. The role of oxygen is to regenerate or maintain the oxidised state of the catalyst. The lattice oxygen is introduced in the substrate or in H20 for ODH reaction. This involves two active sites: an active cationic site and a site active for oxygen reduction. The process starts with the abstraction of a proton, accompanied by a two electron transfer which reduces the transition metal cations, followed by a nucleophilic addition of an oxide ion from the catalyst to the oxidised hydrocarbons with formation of oxygen vacancies7. Then