Metal oxides with Brpnsted acids

Solid superacids may be made by treating ordinary solid acid catalysts with strong Br0nsted or Lewis acids. For example, if freshly precipitated titanium hydroxide or zirconium hydroxide is treated with sulfuric acid and calcined in air at 500 °C. a very active solid acid catalyst results. The solids consist mainly of the metal dioxides with sulfate ions coordinated to the metal ions on the surface. Likewise, a superacid solid catalyst can be made by treating these metal oxides with antimony pentafluoride. Both catalysts contain both Brpnsted and Lewis acid sites, and they are sufficiently active to catalyze the isomerization of n-butane at room temperature.- ... 

An interesting variation on sulfated metal oxide type catalysts was presented by Sun et al. (198), who impregnated a dealuminated zeolite BEA with titanium and iron salts and subsequently sulfated the material. The samples exhibited a better time-on-stream behavior in the isobutane/1-butene alkylation (the reaction temperature was not given) than H-BEA and a mixture of sulfated zirconia and H-BEA. The product distribution was also better for the sulfated metal oxide-impregnated BEA samples. These results were explained by the higher concentration of strong Brpnsted acid sites of the composite materials than in H-BEA. 

There are thus two classes of acids on surfaces of metal oxides Lewis acids and Brdnsted acids (which are also termed proton acids). The weight of evidence (1-8) shows that strong Brpnsted acids are the primary seat of catalytic activity for skeletal transformations of hydrocarbons. In the solids under review, they consist of protons associated with surface anions. 

Enhanced acidity solids including Brpnsted and Lewis acid-modified metal oxides and mixed oxides, as well as metal salts complexed with Lewis acids. 

Transalkylation of alkylbenzenes, polyalkylbenzenes and other arenes can be brought about by a variety of catalysts including Lewis acids, Brpnsted acids and various zeolites and silicates with or without being doped with various transition metals or their oxides. There has been a particularly explosive growth in the volume of literature pertaining to the use of various natural and modified zeolites. Recent developments include the study and applications of shape-selective catalysis by zeolites. Much of the work is patented, and largely applies to industrial processes. 

In these cases the M-0 bond has a covalent character, which is true also for its oxide. As discussed, this applies for very electronegative elements at the upper right of the periodic table (P, S, CH"). As shown, however, this refers to the neutral oxo-hydroxo species. The Brpnsted acidity also applies with highly charged d metal cations such as Mn ". The critical electronegativity for the acidic dissociation is denoted X (A). 

Small amounts of sulfate ion on metal oxide catalysts can either enhance or decrease the activity, depending on the nature of the compound formed by the reaction of the sulfate with the metal oxide. The effect of SO2 or SO3 on metal oxide catalysts can also be to enhance the activity by increasing the Brpnsted acidity believed responsible for the active sites on these SCR catalysts 52. Large amounts of condensible sulfate can result in pore blockage. 

Extensive later investigations using only the oxides (as opposed to the salts) of alkali and alkaline earth metals produced evidence that Brpnsted acid sites are responsible for the SCR activity on supported V2O5 catalysts50-52. They found a direct correlation between the basicity of the alkali and the severity of the poisoning, and also observed that whenever the number or intensity of the Brpnsted acid sites was increased, e.g., by SO2 or HCl, the SCR activity increased, and vice versa with additives that reduced the Brpnsted acidity. 

Yang et al.52 and Chen and YangSO assessed the poisoning severity of As, P, and Pb relative to that of the alkali metals (as their oxides), as reported in the preceding section, and found the latter to be distinctly more severe, consistent with their greater basicity neutralizing the Brpnsted acidity believed responsible for the SCR activity. 

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