Mixed oxides acid-base properties

The acidity of materials can be controlled by mixing oxides with different properties and in various ratios. If phase separation does not occur, substitution of cations of the minor component oxide in the host oxide lattice results in bulk phases of true mixed oxides. Another method used to modify the acid-base properties of oxide... 

Di Cosimo et al. investigated the structural requirements and reaction pathways in condensation reactions of alcohols, using Mg/Al mixed oxides obtained by decomposition in N2 at 673 K for 4 h of LDH precursors with a wide range of composition [53], and found that the mechanistic pathway of the condensation reactions is affected not only by the catalyst acid-base properties but also by the chemical nature of the alcohols as well as steric factors. 

Shen, J., Kobe, J. M., Chen, Y. and Dumesic, J. A. (1994). Synthesis and surface acid/base properties of magnesium-aluminium mixed oxides obtained from hydrotalcites. Langmuir 10, 3902. 

In order to try to clarify the different types of mechanism involving either redox cycles and/or acid-base properties, a study of the surface chemistry of single, doped and mixed oxides is of much interest. The calorimetric technique, by allowing heat transfer measurements, can provide very informative data on the thermodynamics of solid-gas interactions and for the study of the surface and reactivity of these metal oxides. 

The acid-base properties of mixed metal oxides have been found to change with the nature of the constituents, with their relative concentrations and with the preparation and pre-treatment procedures [81]. Accordingly, mixed oxides can be used to obtain catalysts with the desired acid-base characteristics by appropriately choosing the above-mentioned variables. 

The acid-base properties of ceria-zirconia solid solutions [82] and ceria-lanthana co-precipitated mixed oxides have been investigated by Cutrufello and... 

Thermal treatments induce dehydration, dehydroxylation and loss of the charge-compensating anions, resulting in mixed oxides with the MgO-type structure. Hydrotalcites are consequently a class of precursors useful for the preparation of catalytically active oxides showing basic properties [94], The acid-base properties of Mg-Al mixed oxides are governed by the Mg Al molar ratio, calcination temperature and preparation conditions. The study of the influence of the acid-base properties and chemical composition on the catalytic performance of calcined hydrotalcites is thus of interest. 

The main objective of this article is to present a survey of theoretical and applied aspects of microcalorimetry to heterogeneous catalysis with particular emphasis on the determination of acid-base properties of metal oxides and mixed metal oxides. This review is not meant to be comprehensive but to provide an overview of recent work done in the area. Additional applications can be found in recent reviews 1-4). 

In most recent calorimetric studies of the acid-base properties of metal oxides or mixed metal oxides, ammonia and n-butylamine have been used as the basic molecule to characterize the surface acidity, with a few studies using pyridine, triethylamine, or another basic molecule as the probe molecule. In some studies, an acidic probe molecule like CO2 or hexafluoroisopropanol have been used to characterize the surface basicity of metal oxides. A summary of these results on different metal oxides will be presented throughout this article. Heats of adsorption of the basic gases have been frequently measured near room temperature (e.g., 35, 73-75, 77, 78,81,139-145). As demonstrated in Section 111, A the measurement of heats of adsorption of these bases at room temperature might not give accurate quantitative results owing to nonspecific adsorption. 

The acid-base properties of amorphous mixed metal oxides can be varied by choosing different metal oxide constituents at diflerent concentrations and by changing the treatment of the sample (44). Thus, it appears that, by properly choosing the aforementioned variables, mixed oxides could be used to develop new catalysts with desired acid-base properties. The use of micro-calorimetric adsorption measurements to quantify the acid-base properties of metal oxides and mixed metal oxides has been limited, to date, to a few systems. However, for some of these solids, for example, silica, alumina, and silica-alumina, several investigations have led to a satisfactory description of their acidity and acid strength. We present here a compendium of those measurements and discuss some of the important properties observed. 

Apart from zeolites and ion-exchange resins (Sections 4 and 5), the main classes of catalyst which are active for dehydration are salts (usually sulphates, phosphates, and to a lesser extent, carbonates) and oxides (single and mixed). A large proportion of research has been devoted to a correlation of acid-base properties with activity for dehydration and dehydrogenation of alcohols, the two reactions being closely associated. 

The data on the state of copper-containing phases and acid—base properties of active sites occurring at the surface of mixed cement systems, which were presented above, enable us to conclude that these catalysts can be employed in the oxidative ehlorination of ethane. 

The present chapter reviews the results of an investigation devoted to the study of the photodecarboxylation of ethanoic acid adsorbed over various solids. A series of pure insulator and semiconductor oxides covering a wide range of acid-base properties and a series of mixed insulator-insulator and insulator-semiconductor oxides of various composition were used for performing reactivity studies in a continuous photoreactor working in gas-solid regime. The nature of the species adsorbed over the various solids was monitored by IR spectra. 

Interactions between the precious metal and support influence the performance of the catalyst. Beil (1987) has defined metal-support interaction as depending on contact between the metal particle and the support which can be a dissolution of the dispersed metal in the lattice. The interaction could also depend on the formation of a mixed metal oxide, or the decoration of the metal particle surface with oxidic moieties derived from the support. It is possible that in this study, the differences in catalytic performance of the same active material supported on different washcoats can be attributed to any of these phenomena. Another explanation could be that the support materials exhibit different acid-base properties. According to the Bronsted and Lewis definitions, a solid acid shows a tendency to donate a proton or to accept an electron pair, whereas a solid base tends to accept a proton or to donate an electron pair. The tendency of an oxide to become positively or negatively charged is thus a function of its composition, which is affected by the preparation method and the precursors used. Refer to the section Catalyst characterization for further discussion on the influence of support material on catalyst performance. To thoroughly examine the influence of the support... 

With the aim of studying the acid-base properties of Mg-Fe mixed oxides we chose the alkylation of m-cresol with methanol as a model reaction. The distribution of products obtained may take account of the different surface properties of the catalysts [3,4]. The catalytic performances were compared with the acid and basic sites distributions, as determined by pyridine and CO2 adsorption and desorption, respectively. 

Lopez, T Gomez. R Llanos. ME Lopez-Salinas, E. Acidic-base properties of silica-magnesia sol-gel mixed oxides use of 2-butanol as test reaction. Materials Letters, 1999 38, 283-288. 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

When basic LDH is mixed or intercalated with acidic solid component, such as a zeolite, the composite will show a dual functional adsorption for CO2 and NH3. Okada et al. investigated the adsorption behaviors of MgAl-LDH/alumi-nosilicate composites and found that the adsorptivity of the composites for both of the probe molecules is dependent on the preparation pathways. The composite prepared via sol/precipitation shows a superior adsorption for both acidic and basic gases to those prepared via mechanical mixing and reconstruction methods (360). This is quite similar to the case of the dual acid/base properties of LDH-derived oxides, especially the LDH-polyoxometallate-derived oxides, which have been carefully examined by a similar adsorption method (361). 

Satsuma A, Katagiri M, Kakimoto S, Sugaya S, Shimizu K (2011) Effects of calcination temperature and acid-base properties on mixed potential ammonia sensors modified by metal oxides. Sensors 11 2155-2165 SbervegUeri G (1992) Qassical and novel techniques for the preparation of SnO thin-film gas sensors. Sens Actuators B 6 239-247... 

The selection of an efiicient catalyst and the choice of the reaction conditions are the key steps to realize an ideal oxidation procedure. Many of the heterogeneous catalysts used in liquid-phase oxidation are mixed oxides with one or more transition metak. For these catalysts, the active site is directly involved in successive redox cycles, which underlines the fundamental role of the electronic fiic-tor [30]. Transition metal oxides also exhibit surface acid-base properties. Many authors have attempted to relate these properties with the activity or selectivity in oxidation reactions [31] and photo-oxidative degradation of organic compounds. 

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