ABSTRACT
Solid oxides used in catalytic reactions belong to the whole periodic table. Their role in catalysis depends mostly on their acid-base and redox properties. Schematically, acidic (basic) oxides of the right (left) part of the periodic table are involved in cracking, isomerization, alkylation, etc., of hydrocarbons, and are used as supports of metals or of other oxides. Redox catalysts are mainly found among the transitionmetal oxides and aremore particularly involved inmild or total oxidation of hydrocarbons or of other molecules (e.g., alcohols). If parameters describing the catalytic activity are numerous and well-known, selectivity is always related to kinetics, for example, expressed as a ratio of the rates of one process comparedwith another. Finding parameters that are able to account for selectivity in mild oxidation has been one of our concerns, but until recently our attempts failed because of the lack of parameters accounting for the solid-state properties of transition metal oxides. Indeed, there are few answers when one tries to understand why only vanadium oxide supported on titania is able to selectively transform o-xylene into phthalic anhydride, or why Mo3VO11 is selective in the oxidation of acrolein to acrylic acid. Similarly, the fact that, for example, n-butane is selectively oxidized to butadiene, maleic anhydride, or CO2, if CoMoO4, (VO)2P2O7, or LaCrO3, are used as respective catalysts, is not explained. This means that well-defined properties of the catalyst are required to get selectivity in a given type of reaction. In these reactions, the surface lattice oxygens (O2−) of the metallic oxide are directly responsible for the selective formation of the required product. The commonly used two-stepped scheme proposed by Mars and van Krevelen [1] describes this participation:
[R-C-H] + 2KO→ [R-C-O] + H2O+ 2K (10.1) 2K + O2 → 2KO (10.2)
The first step is the transfer of surface lattice oxygen O to the molecules of the products (RCO and H2O) leaving the catalyst in its reduced form K. K is regenerated as KO by gaseous dioxygen which is, generally co-fed with the reactant [R-C-H] in usual reactors. Therefore, the properties of O2− species linked to metallic cations determine the catalytic properties, and particularly the selectivity to products. Among them, a key parameter is the nucleophilicity as noticed
“DK3029_C010” — #3
by Haber [2], who showed that nucleophilic (O2−) and electrophilic (O−2 , O 2− 2 )
oxygen species were responsible for mild and total oxidation, respectively. However, there was no scale of nucleophilicity/electrophilicity or, more generally, of basicity/acidity utilizable for transition metal oxides until recently. To account for selectivity as a general concept that allows the classification of reactions, it would be necessary to have available parameters accounting for a similar type of property in the gas (liquid) phase (organic chemical reaction) and in the solid phase (mineral catalyst). Moreover, thermodynamic parameters would be more appropriate than kinetic parameters, which depend strongly on experimental conditions: the initial state would be the reactant facing the oxidized KO form and the final state would be the required product facing the reduced form K of the “selective” catalyst. Since oxidation catalysis proceeds by an exchange of electrons that accompanies the exchange of O species, we have thought that the ionization energy of the organic molecules could be useful to account for the gas phase reaction. Finding a parameter of the same type for solid oxides was not so easy. Modern theories of reactivity propose to consider the oxidizing power, acidity, basicity, and reducing power, as steps along a same continuum, instead of distinct phenomena [3]. Among parameters, Zhang’s scale of electronegativity [4], optical electronegativity and optical basicity [5,6], electronic polarizability [7], Racah parameter [8], and ioniccovalent parameter (ICP) [9,10], were used recently to quantitatively describe the acid (base) or redoxbehavior of oxides in various applications. General correlations of catalytic activity, with, for example, Me-O bond strength, electronegativity, or oxygen partial charge, are numerous [11-14]. Auroux and Gervasini [15,16] used some of these parameters in order to predict heats of adsorption and acid strength. Differential heats of adsorption of probe molecules such as NH3 or CO2 as a function of coverage have been worked out for different simple oxides. Also, relationships between the charge/radius ratio or the percentage of ionic character and average heats of CO2 or ammonia adsorption, respectively, were obtained. Recently, Idriss and Seebauer [17] showed that the rates of oxidative dehydrogenation (ODH) of ethanol to acetaldehyde and of benzaldehyde esterification are correlated with the oxygen electronic polarizability αO.
