Surface acidity-basicity

Mg/Me (Me=Al, Fe) mixed oxides prepared from hydrotalcite precursors were compared in the gas-phase m-cresol methylation in order to find out a relationship between catalytic activity and physico-chemical properties. It was found that the regio-selectivity in the methylation is considerably affected by the surface acid-basic properties of the catalysts. The co-existence of Lewis acid sites and basic sites leads to an enhancement of the selectivity to the product of ortho-C-alkylation with respect to the sole presence of basic sites. This derives from the combination of two effects, (i) The H+-abstraction properties of the basic site lead to the generation of the phenolate anion, (ii) The coordinative properties of Lewis acid sites, through their interaction with the aromatic ring, make the mesomeric effect less efficient, with predominance of the inductive effect of the -O species in directing the regio-selectivity of the C-methylation into the ortho position. 

Thus, the probable mechanism of catalytic redox transformation of the substrate within one catalytic domain is described in the framework of the BRC theory. It is implemented owing to two-proton transfer to the acidic-basic carrier (A1203) groups with electron transfer to active site perFTPhPFe(III)OH. In this context, of special attention are data on studies of qualitative and quantitative parameters of surface acidic-basic sites of inorganic matrices, including A1203 [8],... 

Interesting feature Is that oxides are now available with zero points of charge spanning seven or more pH units (see appendix 3, table b). Chemically speaking, this means that oxides are at our disposal with widely differing surface acidity/ basicity. By comparing (T°(pH) curves on chemically different oxides It Is possible to discriminate between oxlde-speclflc and generic double layer features. (For mercury a similar approach Involves comparison with other metals.)... 

Surface Acidity/Basicity. - Van Veen and co-workers were one of the first groups to discuss the importance of surface acidity/basicity as it relates to the interactions that favor decorating metal oxide surfaces with certain metal complexes. We alluded to the acidity/basicity of the surface vide supra) in describing how a metal complex may become subject to a reaction with the surface. Van Veen, et al. were interested in decorating surfaces with a wide range of metal acetylacetonate complexes to include Pt and Pd precursors. They showed that metal complexes that were susceptible to attack by acids were likely to decorate the surface of an acidic surface oxide by loss of a ligand to form a firm attachment with the surface. 

Surface acidity, basicity and redox sites measured by spectrophotometric method are presented in Tables 2 3 which indicate the highest acidity of 600°C calcined samples. But further increase in calcination temperature decreases the acidity[15]. Among all the samples VOAIPO4 shows highest acidity, whereas pure AIPO4 shows the lowest. However, all the samples possess very few number of redox sites. 

Figure 9. Modification of surface acidity/basicity by alkali addition.

Eor oxides the p.z.c. is a measure of the surface acidity/basicity. Silica is an acid oxide it has a low pHo, implying that its surface is negative over almost the entire pH range. Hematite is a basic oxide its pHo is high so that the surface tends to be positively charged. Tables of critically evaluated pHos are available in the literature see the general reference [2]. 

As a partial conclusion, we have made available a parameter, Ath, which varies with stoichiometry and with valence, coordination, spin state of cation(s) in the oxide structure, and which is calculable for any oxygen-containing solid. Scales of bulk and of surface acidity/basicity are proposed by using optical basicity as a suitable parameter, which allows to rank oxides, and which can be used for several applications particularly in catalysis as seen further. 

Spinel ferrite nanoparticles have been intensively studied in the recent years, because of their typical ferromagnetic properties, low conductivity, high electrochemical stability and catalytic behavior. These materials are widely used in large-scale applications (i) in electric and electronic devices, (ii) in H2O, CO2 and alcohols decomposition and in CO and CH4 oxidation [1, 2]. Several routes are used for the preparation of NiFe204 catalysts such as co-precipitation, hydrothermal, sol gel, combustion [3-6] etc. However, the structural and textural properties of ferrite spinel are strongly influenced by the preparation methodology used in their synthesis and may influence the catalytic activity of these materials when used as catalysts. Hence, the effect of the preparation method on the surface acid-basic properties and therefore on the catalytic activity is a very interesting subject. 

Gas-phase methylation of catechol by methanol was studied on y -AI2O3 modified by the basic elements K, Li, Mg and Ca. Addition of 7.5 at.% Mg to y-AljOa was optimal and increased the 3-methyl catechol selectivity from 0.26 to 0.65. X-ray diffraction experiments showed the diffusion of Li and Mg cations into the y -AI2O3 bulk. This induces a change in the surface species (XPS data) and the surface acid-base properties (TPD experiments). Ca and K addition to y-alumina was ineffective due to formation of basic oxide layers on the sur ce. 

For the studied catechol methylation reaction the catalyst structure and surface properties can explain the catalytic behaviour As mentioned above, the reaction at 260-350°C has to be performed over the acid catalysts. Porchet et al. [2] have shown, by FTIR experiments, the strong adsorption of catechol on Lewis acid/basic sites of the Y-AI2O3 surface. These sites control the reaction mechanism. 

Due to the formation of Ca/Al mixed oxide on the surface, the Ca -modified alumina has a completely different structure compared to the spinel one This leads to a different type of surface Lewis acid/basic sites, rendering the catalyst 30 times less active. 

Busca, G. (2006) The surface acidity and basicity of solid oxides and zeolites, Chemical Industries (Boca Raton, FL, United States), 108 (Metal Oxides), 247. 

Hagelin, J., T. Brinck, M. Berthelot, J. S. Murray, and P. Politzer. 1995. Family Dependent Relationships Between Computed Molecular Surface Quantities and Solute Hydrogen Bond Acidity/Basicity and Solute-Induced Methanol O-H Infrared Frequency Shifts. Can. J. Chem. 73, 483. 

In all above mentioned applications, the surface properties of group IIIA elements based solids are of primary importance in governing the thermodynamics of the adsorption, reaction, and desorption steps, which represent the core of a catalytic process. The method often used to clarify the mechanism of catalytic action is to search for correlations between the catalyst activity and selectivity and some other properties of its surface as, for instance, surface composition and surface acidity and basicity [58-60]. Also, since contact catalysis involves the adsorption of at least one of the reactants as a step of the reaction mechanism, the correlation of quantities related to the reactant chemisorption with the catalytic activity is necessary. The magnitude of the bonds between reactants and catalysts is obviously a relevant parameter. It has been quantitatively confirmed that only a fraction of the surface sites is active during catalysis, the more reactive sites being inhibited by strongly adsorbed species and the less reactive sites not allowing the formation of active species [61]. 

Tanabe [78] defined a solid acid as a solid on whose surface a basic indicator changes its color or a base is chemically adsorbed. Several acidic properties... 

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