Amorphous metal oxides, acid-base

The surface acid-base properties of bulk oxides can be conveniently investigated by studying the adsorption of suitably chosen basic-acidic probe molecules on the solid. Acidic and basic sites are often present simultaneously on solid surfaces. The two centers may work independently or in a concerted way, and the occurrence of bifunctional reaction pathways requiring a cooperative action of acidic and basic centers has also received considerable attention [39]. The acid-base properties of numerous amorphous metal oxides investigated by mrcrocalorime-try have been summarized in an extensive review by Cardona-Martinez and Dumesic [11]. 

This acid-catalyzed cleavage of the glycosidic bonds is rather complex and often suffers from a lack of selectivity mainly due to side dehydration or recombination reactions of monosaccharides. In the existing literature, four different classes of solid catalysts are reported (1) cation-exchange resins, (2) siliceous-based materials, (3) metal oxides, and (4) sulfonated amorphous carbons. 

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. 

Systematic studies carried out at C.F.R.I. pertainining to catalytic vapour phase synthesis of pyridine bases, namely 2 4-picolines through cyclodehydrogenation reaction of acetaldehyde and anunonia have been described. Metal oxide modified amorphous silica-aliunina and crystalline alumino-silicate namely ZSM-5 zeolites were found to be active and selective catalysts towards the formation of the lower pyridines bases. The method of preparation, pretreatment vis-a-vis the acidity have been found to affect the catalytic activity and selectivity. It is interesting to observe that metal modified crystalline alumino-silicate ZSM-5 is more selective than amorphous silica-alumina for the formation of 2 4-picoline. A plausible reaction mechanism based on the findings of this study has been proposed. 

Reducible mixed metal oxides (MMOs) catalysts are currently the most promising catalytic materials for the selective oxidation of propane to acrylic acid. They consist of two or more mixed oxides of transition metals, most typically based on Mo and V oxides. In general, MMO do not have well-defined structure as HPCs in fact, they are mixture of multiple crystallized and amorphous phases. Mixed oxides are typically prepared via calcination at high temperature, which gives them an excellent thermal stability. 

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