Base strength superbases

It can be expected that solid bases could be successful for commercializing the alkylation of toluene with methanol as a route to styrene, or for selective alkene coupling. There is no doubt that achieving success in several important commercial processes will boost the field of solid base catalysis. Because it appears to be difficult to achieve superbasic organic resins, much more attention should be paid to enhancement of the base strengths of solid superbases. Further work should be done on supported alkali metals and mixed metal oxides. Development of new solid superbases will be improved by increasing our understanding of how alkali metal clusters (302-304) interact with supports and become stabilized. 

Solid superbases, which have been defined as materials with base strengths, H > 26, represent a highly desirable class of solid materials. This is because they offer the possibility of activation of relatively unreactive species under mild reac-hon conditions. 

A suitable solid base must have the appropriate base strength for the reaction under investigation. If the initial reaction step is the removal of a proton from a reactant of the form R1-CH2-R2 then the acidity of the proton to be removed depends on the identity of the R) and R2 groups (Table 1). The solid base selected should have sufficient base strength to carry out the reaction but should not have excessive base strength as this may lead to rapid catalyst deactivation or to side-product formation. For aldehyde and ketone condensation reactions therefore with a p.Ka of 19.7 - 20 a strong base is required but not a superbase material. Caustic can be used to carry out reactions with reactants with the removable proton having a pA a of up to around 20. 

On the other hand, a solid superbase is defined as a solid whose base strength expressed by the basicity function, //, is higher than -h 26. The basis of the definition has been described in the literature. The kinds of solid superbases are shown in Table 1.4 together with their preparation method and pretreatment temperature. 

Addition of alkali metals to certain types of oxides resulted in the formation of very strong base sites. Materials which possess base sites stronger than H- = —26 are caUed superbases. The H- value 26 proposed to be set in conformity vrith the definition of superacid. The critical Ho value for superacid is ca. —12, whidi differs by 19 Ho units from Ho=7, the neutral acid-base strength. The H- value 26 differs from H- = 7 for neutral acid-base strength by 19 H- units. Althou alkali and alkaline earth oxides show superbasicity without the addition of alkali metals, as described in Sections 3.1.1 and 3.1.2, this section deals only with alkali metal-added materials showing strong basicity. The materials which show superbasicity by the addition of alkali metals are limited to alkaline earth oxides and alumina. 

In this Chapter, which is devoted to very strong solid bases, the term superbase is used to denote a base that is so strong that it can detach the proton from a hydrocarbon molecule. It should deprotonate triphenyl-methane, — 33, and at the upper limit methane, pK = 40. The basic strength of a superbase should therefore lie in the range 40 < H < 33. 

Methylformate decomposition to CO and H2 is effectively catalyzed by solid super-base. The Na —MgO catalyst prepared by heating a mixture of MgO and sodium azide shows much higher activity than MgO alone. Although the basic strength was not measured, the high activity of Na —MgO is considered to be due to superbasicity of the catalyst. 

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