Characterization of Acid-Base Sites in Oxides

Dipartimento di Chimica, Universita degH Studi di Milano, Milan, Italy e-mail antonella.gervasini unimi.it 

Springer Series in Materials Science 154, DOl 10.1007/978-3-642-11954-5 8, Springer-Verlag Berlin Heidelbeig 2013 

It may be taken into consideration that the proton affinity, PA, of a anion or of a neutral atom or molecule is a measure of its gas-phase basicity. It is the energy released in the following reactions A - - H+ HA or B + H+ BH+. For any species, the higher the proton affinity, the stronger the base and the weaker the conjugate acid in the gas phase. Recently, the relationships between the proton affinity and atomic charge and with the transfer of charge have been reexamined [7]. 

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The quantum-chemical cluster models of acid base sites of binary oxides have recently been reviewed by Zhidomirov and Kazansky.Concerning the Brensted acid sites of the binary oxide silica-alumina, they summarize as follows. The acid sites acting in typical acid-catalyzed reactions are bridged hydroxyl groups and water molecules coordinated on a trigonal aluminum atom. These centers are characterized by approximately equail surface densities and atomic catalytic activities. 

Isomerization of jS-isophorone to a-isophorone has been represented as a model reaction for the characterization of solid bases 106,107). The reaction involves the loss of a hydrogen atom from the position a to the carbonyl group, giving an allylic carbanion stabilized by conjugation, which can isomerize to a species corresponding to the carbanion of a-isophorone (Scheme 9). In this reaction, zero-order kinetics has been observed at 308 K for many bases, and consequently the initial rate of the reaction is equal to the rate constant. The rate of isomerization has been used to measure the total number of active sites on a series of solid bases. Figueras et al. (106,107) showed that the number of basic sites determined by CO2 adsorption on various calcined double-layered hydroxides was proportional to the rate constants for S-isophorone isomerization (Fig. 3), confirming that the reaction can be used as a useful tool for the determination of acid-base characteristics of oxide catalysts. 

Considerable evidence exists indicating that the acidity of an oxide surface can vary according to the pretreatment. For example, Finlayson and Shah [12] used flow microcalorimetry to characterize the oxidized surfaces of three aluminum specimens that had received different pretreatments. They found that the surface chemistry of the three samples was considerably different but was dominated by Lewis base sites in all cases. The peel strength of ethylene/acrylic acid copolymers laminated against the substrates increased as the basicity of the substrate and the acrylic acid content of the co-polymer increased. 

Characterization of Acidity/Basicity. The acid-base properties of a metal oxide play a crucial part in catalysis and are generally a function of pretreat-ment/preparation variables and catalyst morphology. It is important to be able to fully characterize the surface in terms of the strength and number of acid and base sites. Frequently, TPD methods involving probe molecules have been applied. However, caution must be exercised to simple TDP measurements, as it is often not clear whether the probe molecule is behaving in the acid-base manner assumed. An illustrative example of this is provided in a study of desorption of NH3 from CaO, in which the high temperature of desorption may lead one to believe that CaO is a strong acid (280). The explanation for this... 

While our discussion will mainly focus on sifica, other oxide materials can also be used, and they need to be characterized with the same rigorous approach. For example, in the case of meso- and microporous materials such as zeolites, SBA-15, or MCM materials, the pore size, pore distribution, surface composition, and the inner and outer surface areas need to be measured since they can affect the grafting step (and the chemistry thereafter) [5-7]. Some oxides such as alumina or silica-alumina contain Lewis acid centres/sites, which can also participate in the reactivity of the support and the grafted species. These sites need to be characterized and quantified this is typically carried out by using molecular probes (Lewis bases) such as pyridine [8,9],... 

The aim of this study is to develop model reaction for the characterization of the acidity and basicity of various transition aluminas, the experimental conditions being close to that for catalysis use. Among various model reactions, the transformation of cyclopentanol and cyclohexanone mixture was chosen for this work. Indeed, this reaction was well known for estimating simultaneously the acid-base properties of oxide catalysts [1], Two reactions take place the hydrogen transfer (HT) on basic sites and the alcohol dehydration (DEH) on acid sites. The global reaction scheme is shown in Figure 1. 

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